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E-ISSN 1471–0676 PLATINUM METALS REVIEW A Quarterly Survey of Research on the Platinum Metals and of Developments in their Application in Industry www.platinummetalsreview.com VOL. 51 APRIL 2007 NO. 2 Contents Modelling Reactions at the Active Sites of Chiral Ruthenium 54 Catalysts Using Density Functional Theory By S. A. French The Electronic Structure of Platinum-Gold Alloy Particles 63 By Geoffrey C. Bond “The Separation and Refining Technologies of Precious Metals” 68 A book synopsis by Jianmin Yu New Frontiers in Metathesis Chemistry 69 A conference review by Ileana Dragutan and Valerian Dragutan “Metal Catalyzed Cascade Reactions” 76 A book review by Ron Grigg Strengthening of Platinum-5 wt.% Copper by Heat Treatment 78 By Chumani Mshumi and Candy Lang Successful Scale-Up of Catalytic Processes 83 A conference review by Chris Mitchell “Organic Light-Emitting Devices: Synthesis, 85 Properties and Applications” A book review by J. A. Gareth Williams Platinum Group Metal Perovskite Catalysts 87 By Thomas Screen “Combinatorial and High-Throughput Discovery 93 and Optimization of Catalysts and Materials” A selective book review by Dave M. Newman and M. Lesley Wears Abstracts 95 New Patents 99 Final Analysis: Casting Platinum Jewellery 102 – A Challenging Process By Neill Swan Communications should be addressed to: The Editor, Barry W. Copping, Platinum Metals Review, [email protected]; Johnson Matthey Public Limited Company, Orchard Road, Royston, Hertfordshire SG8 5HE

PLATINUM METALS REVIEW · Platinum Metals Rev., 2007, 51, (2) 56 energy barriers that are encountered. At each stage, the geometry of the system is optimised with respect to one constraint,

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E-ISSN 1471–0676

PLATINUM METALS REVIEWA Quarterly Survey of Research on the Platinum Metals and

of Developments in their Application in Industrywww.platinummetalsreview.com

VOL. 51 APRIL 2007 NO. 2

Contents

Modelling Reactions at the Active Sites of Chiral Ruthenium 54Catalysts Using Density Functional Theory

By S. A. French

The Electronic Structure of Platinum-Gold Alloy Particles 63By Geoffrey C. Bond

“The Separation and Refining Technologies of Precious Metals” 68A book synopsis by Jianmin Yu

New Frontiers in Metathesis Chemistry 69A conference review by Ileana Dragutan and Valerian Dragutan

“Metal Catalyzed Cascade Reactions” 76A book review by Ron Grigg

Strengthening of Platinum-5 wt.% Copper by Heat Treatment 78By Chumani Mshumi and Candy Lang

Successful Scale-Up of Catalytic Processes 83A conference review by Chris Mitchell

“Organic Light-Emitting Devices: Synthesis, 85Properties and Applications”A book review by J. A. Gareth Williams

Platinum Group Metal Perovskite Catalysts 87By Thomas Screen

“Combinatorial and High-Throughput Discovery 93and Optimization of Catalysts and Materials”

A selective book review by Dave M. Newman and M. Lesley Wears

Abstracts 95New Patents 99

Final Analysis: Casting Platinum Jewellery 102– A Challenging Process

By Neill Swan

Communications should be addressed to: The Editor, Barry W. Copping, Platinum Metals Review, [email protected]; Johnson Matthey Public Limited Company, Orchard Road, Royston, Hertfordshire SG8 5HE

54

This work addresses homogeneous asymmetrichydrogenation of C=O bonds, from ketones toalcohols, which has applications in the industrialproduction of pharmaceutical intermediates. Thecatalyst studied, [(S)-XylBINAP-RuH2-(S,S)-DPEN], is shown in Figure 1.

Each reaction mechanism may be understoodin terms of the energies of intermediates and theroles of ligands and additives, as determined bystate-of-the-art computational techniques. Theknowledge gained will then be exploited for thedesign and synthesis of ligands for improved cata-lyst systems. Advances in experimental techniqueswill allow rapid identification of lead ‘designed’catalysts by automated parallel screening. With

this scheme in mind, a consortium was assembledfrom leading experts (both industrial and academ-ic) in all areas of the workflow. The partners arethe Royal Institution of Great Britain, theUniversity of Liverpool, the University ofSouthampton, Johnson Matthey, AstraZeneca,GlaxoSmithKline and Pfizer. The aims of the pro-ject were to implement an evolutionaryimprovement in ligand/catalyst design strategies:

COMPUTATION ⇔ SYNTHESIS ⇔

ACCELERATED TESTING

This computation-guided approach for catalystdiscovery is expected to be more efficient, fasterdelivering and more revealing on the molecularaspects of a catalytic cycle than one-at-a-time syn-thesis or combinatory methodologies, whichusually screen catalysts at random (5); seeScheme I.

Since the project’s conception there has been astep change in the ability of industry to performhigh-throughput screening. This acceleration hasenormously reduced the time required to identifythe right catalyst for any desired transformationfrom a library of existing catalysts or ligands. Thepreparation of the library of ligands and catalystsremains, however, the bottleneck in this process.

Platinum Metals Rev., 2007, 51, (2), 54–62

DOI: 10.1595/147106707X180891

Fig. 1 The structure of [(S)-XylBINAP-RuH2-(S,S)-DPEN], asymmetric hydrogenation catalyst

Ru

PAr2 H H2

N H

Ru

P H N HAr2 H2

Modelling Reactions at the Active Sites ofChiral Ruthenium Catalysts Using DensityFunctional TheoryNEW APPROACH TO UNDERSTANDING OF CATALYTIC REACTIONS

By S. A. FrenchJohnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, U.K.; E-mail: [email protected]

Selectivity is a key success factor in the chiral catalyst technologies market. Understandingthe fundamental processes that occur when a reagent interacts with a homogeneous singlesite catalyst, both in its approach and at the active site, is therefore critical to the rationaldesign of new catalysts. Ruthenium-based asymmetric hydrogenation catalysts have beenconsidered as part of a collaborative research project. [(S)-XylBINAP-RuH2-(S,S)-DPEN],first developed by Noyori (1–3), is studied as the parent or prototype model of a series ofefficient hydrogenation catalysts, among them the catalysts based on the P-Phos, PhanePhosand ParaPhos ligand families (4).

For example, the selection of the correct sub-stituents at phosphorus in any family of bidentatephosphine ligands is largely a matter of trial anderror, with each component of the family requir-ing independent and often time-consumingsynthesis. The design and synthesis of new ligandbackbones is even more time-consuming, andthere is no certainty that the new ligands will beeffective in the desired transformation.Understanding the factors which govern the rela-tionship between the structure of the ligand andits efficacy in catalysis will accelerate the processof ligand design by evaluation through computersimulation (‘in silico’) of a large number of structur-al variations, among which only the mostpromising structures will actually be synthesised.

The project, supported financially by the U.K.Department of Trade and Industry’s‘Manufacturing Molecules Initiative’ (6), isfocused on two industrially important organicreactions: (a) the production of chiral alcohols viathe asymmetric reduction of ketones; and (b) C–Cbond forming reactions such as the Heck reaction.Molecular modelling has so far focused onReaction (a). Computer simulations have beenused, at the molecular-mechanical (7) and, asreported here, quantum-mechanical levels, toinvestigate the structures of proposed catalysts,and to probe the reaction mechanism.

Initially it was proposed to use activation ener-gies calculated by considering transition states(TS) between reactant and products to comparethe performance of catalysts. However, difficultiesin simulating the reactants and products correctlycaused the TS calculations to fail. Therefore analternative strategy was implemented; a geometricconstraint was applied and the reactant broughttowards the reactive centre, exploring the pathwayof the ketone molecule to the active site.Understanding of the correct relative positions of

reactants and products, and further understandingthe need to ‘lock’ conformations of ligands, hasled to the capability of performing TS calculationson ‘cut down’ (i.e. simplified) model catalysts, aswell as exploring the entry of reactants to a realcatalyst system.

MethodsThe processes of prime interest to us involve

the breaking and creation of bonds, which meansthat the electronic structure as well as the molecu-lar structure must be modelled. Traditionalmethods in electronic structure theory, in particu-lar Hartree-Fock theory and its descendants, arebased on the complicated many-electron wave-function. The main objective of density functionaltheory (DFT) is to replace the many-body elec-tronic wavefunction as the basic quantity by theelectronic density. Within this study we have usedthe DFT code DMol3 (8, 9) for both model TScalculations and constrained optimisations.

All the DFT investigations were performedusing the linear combination of atomic orbitalsapproximation, with a double numerical basis setaugmented by polarisation functions (with a 5.5 Åcut-off). The calculations employed the gradient-corrected Perdew-Becker-Ernzerhof (PBE)exchange-correlation functional. The fine accura-cy convergence criteria were used throughout forboth electronic structure and atomic optimisationcalculations. The criteria guarantee that the energyper bond, bond lengths and angles converge toapproximately 0.1 eV, 0.01 Å and 1º, respectively.

Constrained Optimisation CalculationsTo understand how the reactant molecule

approaches the metal centre and what restrictionsare placed on its passage, we have performed alarge number of simulations to compile a trajecto-ry of the path followed, and to compare the

Platinum Metals Rev., 2007, 51, (2) 55

Catalyst Modelling(Royal Institution)

Ligand Synthesis(Liverpool, Southampton

Universities)

Catalyst Screening(Johnson Matthey)

Commercial Application(Pharmaceutical

Companies)

Scheme I Roles of research consortium members

Platinum Metals Rev., 2007, 51, (2) 56

energy barriers that are encountered. At eachstage, the geometry of the system is optimisedwith respect to one constraint, namely the dis-tance between the hydride H on the rutheniumand the C of the carbonyl group in the ketone.(These species eventually become bonded to oneanother in the alcohol product.) The output fromone simulation is used to generate the initial con-figuration for the next. Initially we used the resultsto understand the interaction between reactantand catalyst, to identify the most relevant reac-tants and products for TS calculations, which aredescribed below. The application of the methodwas then extended to consider four ‘quadrants ofattack’ of the ketone to the active site, to probethe potential energy surfaces of the reaction. Thiswill provide vital information concerning theselectivity of the catalysts.

Transition State CalculationsThe TS calculations have required a workflow

applied to a cut down version of the catalyst(Figure 2), to arrive at a final model for the TS.The stages are as follows: (a) Relaxation of the reactant;(b) Construction of the product from the reac-

tant using ‘chemical intuition’ to moveatoms around;

(c) Constrained relaxation of the product, tak-ing care to avoid conformational changes inthe ring;

(d) Full relaxation of the product;(e) Linear synchronous transit (LST) method

to approximate reaction path and provideinput to full LST/quadratic synchronoustransit (QST) calculation with conjugategradient (CG) optimisation;

(f) Single-point calculation of TS with frequen-cy analysis;

(g) Animation of negative mode to checkwhich centres are involved;

(h) TS optimisation calculation where mode isfollowed;

(i) Single-point calculation of TS with frequen-cy analysis;

(j) Animation of negative mode.

Results and DiscussionAmong the models used to rationalise the

structure-activity relationship in asymmetrichomogeneous catalysis, the so-called ‘quadrantapproach’ is one of the simplest and most effec-tive.

The space around the reactive centre is dividedinto four volumes, across which the substrate canbind to the metal centre in a number of differentconformations. The ligand will prevent access tosome quadrants by simple steric interaction,thereby forcing the substrate to bind to the metalin a preferred conformation that, upon transfer ofthe hydride from the metal to the substrate,becomes the precursor to the favoured enan-tiomer of the product. Such a model, althoughvery simplistic, allows straightforward rationalisa-tion of the sense of stereoinduction obtainablewith a number of well-known hydrogenation cat-alysts such as Ru-BINAP, Rh-DuPhos andRh-BisP* (10, 11).

Starting from this simplistic approach (seeFigure 3) we have developed a more sophisticatedmodel that takes into account the whole trajecto-ry of the substrate into the ‘reactive pocket’ of thecatalyst. It is well known that subtle modificationsof the substituents at phosphorus can producevery significant changes in the activity and selec-tivity of the catalyst (one example of this being theso-called ‘meta-effect’). We suggest that the rea-son for these effects may reside not only inchanges at the transition state, but also in thedocking of the substrate into the reactive pocket,well before the bond breaking/bond forminginteractions are established.

Constrained Optimisations: Initial QuadrantInitially we have considered the catalyst

[(S)-XylBINAP-RuH2-(S,S)-DPEN] as an exem-

PH2

PH2

Ru

NH2

NH2

H

H HH

HH

H

H

H

H

Fig. 2 Cut down model of the Noyori catalyst

Platinum Metals Rev., 2007, 51, (2) 57

plar of the class of asymmetric materials that weare interested in understanding. As mentionedabove, initially we focused on understanding thepathway of the reactant to the active site. Withthis in mind we chose to model the quadrant andorientation known to lead to the preferred prod-uct. Determining the conformational changesforced on the reactant or catalyst during approachwould provide a better understanding of wherereactants and products should be sited for TS cal-culations. The Platinum Metals Review websiteincludes an animation (12) showing the finalstructure from each of the constrained optimisa-tion calculations, starting with a constraint (Ru–H - - - C=O) of 8 Å and reducing the separa-tion between the reactant and catalyst in steps of0.5 Å. It is clear from the animation and subse-quent analysis of the potential energy surface forthe pathway (Figures 4 and 5) that we can beginto understand the complexity of these systems.There are two distinct energy barriers that the

reactant must overcome before arriving at theactive site of the catalyst. The reactant must firstpush into a pocket of the catalyst, before arrivingat its final alignment. The C=O of the ketone andthe Ru–N bonds lie parallel, thereby maximisingorbital overlap with the hydrogen atoms thattransfer to form the alcohol. The advantage ofcomputational models is that the changes in geo-metrical structure as the ketone approaches canbe observed ‘frame by frame’. It is then possibleto follow the trajectory of approach, analyse theposition of the barriers, and view the correspond-ing changes in atomic structure.

When the reactant enters the pocket of thecatalyst, which begins to occur when the con-straint (Ru–H - - - C=O) is between 5 Å and 4 Å,the ketone-catalyst system stabilises. This isshown by the total energy of the system decreas-ing, before it has to overcome the largest barrierbetween 3.5 Å and 2.75 Å. The barrier is due tothe interaction of the phenyl ring of the ketone

Ru

PP

N NORu

PP

N NORu

PP

N NO

Ru

PP

N NO

Q1(R)-alcohol

Q2(S)-alcohol

Q3(S)-alcohol

Q4(R)-alcohol

Ru

PP

N NORu

PP

N NORu

PP

N NO

Ru

PP

N NO

Q1(R)-alcohol

Q2(S)-alcohol

Q3(S)-alcohol

Q4(R)-alcohol

Q1(R)-alcohol

Q2(S)-alcohol

Q3(S)-alcohol

Q4(R)-alcohol

Q1Product:

(R)-alcohol

Q2Product:

(S)-alcohol

Q3Product:

(S)-alcohol

Q4Product:

(R)-alcohol

Fig. 3 Quadrants of inter-est showing the stericinteractions between thereactant and ligands of thecatalyst

1 1.5 2 2.5 3 3.5 4 4.5 5 5.5

Entry intoPocket

Constraint, ÅRel

ativ

e E

nerg

y, k

J m

ol–1

– N

orm

alis

ed a

t 5 Å

Fig. 4 The potential ener-gy surface of the pathwayfor the reactant toapproach the catalystactive site. The black cir-cles show the positions onthe trajectory that areillustrated in Figure 5

5

3

1

–1

–3

–5

–7

Platinum Metals Rev., 2007, 51, (2) 58

with ligands of the catalyst; this interactionincreases as the ketone is pulled down onto theactive site. At a constraint of 2.75 Å there is a con-formational change of the reactant, with thephenyl ring tilting so that all the carbons of thering are no longer in the plane of the other atom-ic centres of the ketone. The driving force for thischange of conformation is the formation of ahydrogen bond, which holds the ketone as itmoves closer to the active site. The hydrogenbond distance reduces further as the reactant ispulled further towards the catalyst, but the posi-tion of the oxygen atom does not change greatly;the major movement is that of H(NH) of the catalyst.

For the reactant to leave the pocket there isthen another barrier, which requires the alignmentof the C=O bond of the ketone to the underlyingRu–N bond of the catalyst. To this end, the car-bon moves further down, changing from sp2 to sp3

hybridisation, until it is in the same plane as oxy-gen. This results in the C=O bond lying parallel tothe Ru–N bond, maximising overlap with the twohydrogen atoms to be transferred from the catalystto the reactant. Simultaneously, the Ru–H bondelongates; this would contribute to the barrier at a(Ru–H - - - C=O) constraint of 2.25 Å.

Constrained Optimisations: Other QuadrantsHaving considered the approach of the reac-

tant along the favoured quadrant, we thenaddressed the question of whether computationalcalculations contain sufficient detail to predict theselectivity of a specific catalyst. A catalyst thatforms products with a high enantiomeric excess(ee) is highly desirable, as these reactions areimportant in a pharmaceutical context. Here thephysiological reaction to one enantiomer may dif-fer greatly from that to another. Using dockingcalculations, the aim is to investigate the variousarrangements, and thereby provide insight intohow catalysts may be optimised.

The difference in energy barriers between thedifferent quadrants of approach has the potentialto provide such discrimination. From Figures 2and 6 it is obvious that two of the possible orien-tations are sterically ‘favourable’ (quadrants Q1and Q3) and two sterically ‘unfavourable’ (Q2 andQ4) (1). However, this is not sufficient to under-stand the selectivity, as Q1 and Q3 lead to (R) and(S) products, respectively. In fact what this simpleanalysis demonstrates is that the channels thatwould be followed by reactants approaching alongQ2 and Q4 should be closed. However, of mostimportance is to understand why certain catalysts

3 Å 2.75 Å

2.25 Å 1.75 Å

Fig. 5 Molecular modelsshowing stages of thereactants’ pathway to thecatalyst active site

Platinum Metals Rev., 2007, 51, (2) 59

produce much higher ee than others. To this endwe must understand the difference between Q1and Q3.

Figure 7 shows clearly that for [(S)-XylBINAP-RuH2-(S,S)-DPEN], Q1 possesses a much lower

barrier to approach and would therefore beexpected to show high ee. This is confirmedexperimentally where, depending upon ex-perimental conditions, an ee of around 97%is achieved.

Q1 Q2

Q3 Q4

Fig. 6 Starting configura-tions with a constraint of 5 Å showing the quadrants

-40

-30

-20

-10

0

10

20

30

40

1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8

Constraint (Angs)

Rel

E -

8 A

ngs

- kJ/

mol

X-Q1X-Q2X-Q3X-Q4

Rel

ativ

e E

nerg

y, k

J m

ol–1

– N

orm

alis

ed a

t 8 Å

Constraint, Å

Fig. 7 The energy profiles for approach of the reactant to the catalyst along different quadrants. (Numbers are nor-malised to 8 Å, as here the reactant and catalyst do not interact)

With an understanding of the pathway of thereactant to the catalyst active site and of the stableposition of the ketone along this path, we couldreturn to considering the calculations of TS andtherefore the activation energies for the hydro-genation processes. We are currently consideringother catalysts and reactants to ascertain whetherwe can correlate the barrier for entry into the cat-alyst pocket with the selectivity of known systems.

Transition StatesTo make the best use of available computer

resources, initial TS calculations were performedon a cut down model of a commercial catalyst(Figure 2). Acetone was initially considered as thereactant molecule. From our understanding of theapproach of the ketone to the catalyst, we havebeen able to determine a stable configuration forthe reactant. The constrained optimisation calcu-lations showed that with the ketone hydrogenbonded to the catalyst, (R2C=O - - - HNH) wasat a minimum energy position when the (Ru–H - - - C=O) was held at 2.75 Å, with thehydrogen bond holding the reactant in place. Wefurther optimised this structure to provide thestarting point for our TS calculations. The productstate has the alcohol physisorbed above the cata-lyst, and is stabilised by a hydrogen bond betweenCOH - - - NH.

The TS forms a six membered ring (Figure 8).The hydride bond is elongated from 1.7 Å to1.75 Å. It can be seen from Figure 9 that there isalso a change in the hybridisation of carbon,which moves from sp2 towards sp3. The TS has a (O)C - - - H(Ru) distance of 1.96 Å and (C)O - - - H(NH) of 1.86 Å. It is possible to visu-alise the imaginary mode that is associated withthe TS, and it is found that the two hydrogenatoms and carbon are the atomic centres thatmove the most. Figure 10 shows the energy pro-files for TS searches using the LST and QSTcalculation methods.

The activation energy that we have calculated is3 kcal mol–1, which is in agreement with previouscalculations on similar-sized models of the com-mercial catalyst, while the calculated reactionenergy is exothermic by 6 kcal mol–1. The hydro-

genation of acetone is therefore extremely facile,proceeding as follows: – Incipient bond formation is signalled by the

shortening of the O - - - H distance (from 2.11Å to 1.86 Å) in the N–H - - - O hydrogen bond,and of the Ru–H - - - C distance (from 3.02 Åto 1.95 Å);

– Small changes in the same direction areobserved in the other bond lengths (< 2%).The structure of the TS, therefore, resembles

much more that of the reactant complex[RuH2–acetone] than that of the product complex[RuH2–iPrOH]. This process is therefore a goodexample of the Hammond principle. This statesthat the structure of the transition state will resem-ble that of the product more closely than that ofthe reactant for endothermic processes, whereasthe opposite is true for exothermic reactions. A

Platinum Metals Rev., 2007, 51, (2) 60

Ru

NH2

NHH3P

H3P

H

H

H

C

O

H3C

H3C

Fig. 9 Molecular configuration for transition state inhydrogenation of acetone; EA = 3.07 kcal mol–1

Fig. 8 Valence bond representation of transition state inhydrogenation of acetone

previous computational study on a trans-dihydro(diamine)ruthenium(II) Noyori-typemodel catalyst has evaluated a reaction barrier forthe hydrogenation of acetone of 3.6 kcal mol–1 atB3LYP/6-31G** level (13). Our results are inapparent agreement with previous calculations onthe formaldehyde/methanol transformation bythe RuH(NH2CH2CH2NH)(η6-benzene) complexperformed at B3LYP (14) and generalised gradi-ent approximation (GGA) (15). This shows thatclassical reaction barriers computed with GGAfunctionals are smaller than those obtained withB3LYP by about 2 kcal mol–1.

We certainly anticipated that the methodologyused would impact on the activation energy (EA),and we are currently evaluating the effect ofchanging the density functional. Initial resultsshow that increasing electron localisation by mov-ing from GGA via hybrid to meta functionalsleads to a slight increase in EA. We are also con-sidering other ketones, and attempting to build upa larger model of the catalyst system, so that wecan make direct comparison with experimentaldata for industrially relevant systems.

ConclusionsThe two complementary DFT simulation

methodologies of transition state searches andconstrained geometry optimisations are now yield-ing results that are of considerable importance to

understanding catalyst behaviour, potentially lead-ing to the prediction and design of new catalystsfor the ketone hydrogenation reaction.

AcknowledgementsThe author would like to thank R. Catlow, E.

Palin and D. Di Tommaso (Royal Institution); J.Xiao, Z. Chen, X. Wu and J. Ruan (University ofLiverpool); A. Danopoulos and N. Stylianides(University of Southampton); F. King, F. Hancockand A. Zanotti-Gerosa (Johnson Matthey); P.Hogan and M. Purdie (AstraZeneca); P.Ravenscroft (GlaxoSmithKline); and P. Levett andA. Pettman (Pfizer).

References1 R. Noyori, Angew. Chem. Int. Ed., 2002, 41, (12),

2008 – Nobel Lecture 2 R. Noyori and T. Ohkuma, Angew. Chem. Int. Ed.,

2001, 40, (1), 40 3 T. Ohkuma, M. Koizumi, K. Muñiz, G. Hilt, C.

Kabuto and R. Noyori, J. Am. Chem. Soc., 2002, 124,(23), 6508

4 A. Zanotti-Gerosa, W. Hems, M. Groarke and F.Hancock, Platinum Metals Rev., 2005, 49, (4), 158

5 A. Hagemeyer, B. Jandeleit, Y. Liu, D. M. Poojary,H. W. Turner, A. F. Volpe and W. H. Weinberg,Appl. Catal. A: Gen., 2001, 221, (1–2), 23

6 ‘Manufacturing Molecules Initiative (MMI): ASource of Funding’, URN 02/527, U.K.Department of Trade and Industry, London, 2002

7 E. J. Palin, G. A. Grasa and C. R. A. Catlow, Mol.Simulat., 2006, 32, (10–11), 901

Platinum Metals Rev., 2007, 51, (2) 61

–1349.93

–1349.94

–1349.95

–1349.96

–1349.97

–1349.98

–1349.99

–1350.00

–1350.01

–1350.02

Ene

rgy,

Ha

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0Path Coordinate

Energy vs. LST path 1 Energy vs. CG path 1 Energy vs. QST path 2Energy vs. CG path 2 Energy vs. QST path 3 Energy vs. CG path 3Transition state

Fig. 10 Energy profilesof TS searches in hydro-genation of acetone

Platinum Metals Rev., 2007, 51, (2) 62

8 B. Delley, J. Chem. Phys., 1990, 92, (1), 508 9 B. Delley, J. Chem. Phys., 2000, 113, (18), 7756

10 I. D. Gridnev and T. Imamoto, Acc. Chem. Res.,2004, 37, (9), 633

11 J. M. Brown, ‘Hydrogenation of FunctionalizedCarbon-Carbon Double Bonds’, in“Comprehensive Asymmetric Catalysis”, eds. E. N.Jacobsen, A. Pfaltz and H. Yamamoto, Springer,1999, Vol. 1, p. 121

12 http://www.platinummetalsreview.com/images/Q1_new.gif13 K. Abdur-Rashid, S. E. Clapham, A. Hadzovic, J. N.

Harvey, A. J. Lough and R. H. Morris, J. Am. Chem.Soc., 2002, 124, (50), 15104

14 M. Yamakawa, H. Ito and R. Noyori, J. Am. Chem.Soc., 2000, 122, (7), 1466

15 J.-W. Handgraaf, J. N. H. Reek and E. J. Meijer,Organometallics, 2003, 22, (15), 3150

The Author

Sam French holds a degree in Chemistry withMedicinal Chemistry from the University ofWarwick, U.K., as well as a Ph.D. from the RoyalInstitution of Great Britain (RI). At the RI, he workedunder the supervision of Prof. Richard Catlow FRS.He held three subsequent postdoctoral researchassistant positions at the RI. All involved largecollaborations with industrial input, specialising inQM/MM techniques and grid technology in

chemistry and catalysis. He joined Johnson Matthey as a SeniorScientist in November 2004, to initiate and lead the ComputationalChemistry Group.

Great success has been achieved in a variety ofcatalytic processes by combining two metallic ele-ments; examples that spring to mind include theplatinum-iridium (Pt-Ir) pair for petroleumreforming, the platinum-tin (Pt-Sn) pair for alkanedehydrogenation, and the nickel-gold (Ni-Au) sys-tem for steam-reforming of alkanes (1). The recentupsurge of interest in gold as a catalytic element,rather than as an inert component that somehowprotects the active one, has led to a series of obser-vations on gold-containing bimetallic combi-nations (2). In particular, the palladium-gold(Pd-Au) pair has been found more effective thaneither component by itself in a number of selectiveoxidations, including those of reducing sugars,alkenes, and hydrogen (to form hydrogen peroxiderather than water). The Pt-Au combination hasalso proved to be beneficial, performing betterthan platinum alone in oxygen reduction at the fuelcell cathode (3), in selective oxidation of reducingsugars and other polyols (2), in alkane isomerisa-tion when contained in the cages of the HY zeolitestructure (3), and in reactions of environmentalimportance such as nitric oxide reduction bypropene (2).

In the macroscopic state, palladium and goldform a continuous range of solid solutions, so itwould not be surprising if chemical methods forpreparing nanoscale bimetallic particles were alsoto lead to microalloy products. Paradoxically, how-ever, detailed structural examination has shownthat this is not always the case. Instead a ‘cherry’

structure often occurs, in which a gold core is sur-rounded by a palladium shell (4). The beneficialcatalytic effect is therefore obtained by the coreexerting some kind of modifying influence on thesurface atoms. While this has not yet received atheoretical explanation, the Pt-Au system alsoposes problems of interpretation that have yet tobe addressed; these are discussed here.

The Platinum-Gold SystemUnlike the Pd-Au system, the Pt-Au phase dia-

gram exhibits a very considerable miscibility gap(5), that is to say, the solubility of each metal in theother is strictly limited (Figure 1). In the region ofambient temperature, the limit of the solubility of

63

The Electronic Structure ofPlatinum-Gold Alloy ParticlesBETTER CATALYSTS FOR SELECTIVE OXIDATIONS

By Geoffrey C. Bond59 Nightingale Road, Rickmansworth WD3 7BU, U.K.; E-mail: [email protected]

Although the platinum-gold (Pt-Au) phase diagram shows a wide miscibility gap due to limitedmutual solubility of the components, small particles (< 3 nm) form homogeneous alloys becauseall atoms retain their atomic electronic structure, and rehybridisation due to band formationdoes not take place. Supported Pt-Au catalysts are often superior to those containing Pt alonefor low-temperature selective oxidations.

Platinum Metals Rev., 2007, 51, (2), 63–68

DOI: 10.1595/147106707X187353

liquid

liquid + alpha

alpha 1 alpha 1 + alpha 2

alpha 2

0 20 40 60 80 100[Au], %

1873

1673

1473

1273

1073

873

Tem

pera

ture

, K

Fig. 1 Phase diagram for the platinum-gold system

platinum in gold is only about 17%, rising to 20%at 973 K, while the solubility of gold in platinum iseven less (4% rising to 6%). Between these limitsthere will be a mixture of the two saturated solu-tions. The contrast with the Pd-Au system is atfirst sight surprising, because the palladium andplatinum atoms are almost identical in size. Thereason for the difference must lie in their differentelectronic structures; for palladium the outer elec-tron configuration is 4d 10, while for platinum it is5d 9 6s1. The elevation of a d electron to the s levelis a consequence of the relativistic stabilisation ofthe s and p levels in relation to the d and f levels (6, 7). Figure 2 shows calculated energy levels formolybdenum, tungsten and seaborgium, and wemay suppose that those for palladium, platinumand eka-platinum (darmstadtium) would be similar.The gap between the s- and d-levels is muchreduced with platinum, and this allows its d-elec-trons to be activated for bonding; this is why the

Pt(IV) state is readily accessible, but the Pd(IV)state is not (6, 7). It would be an interestingHonours level question for undergraduate studentsto predict the chemistry of eka-platinum.

Supported Platinum-Gold CatalystsIn view of this knowledge of the Pt-Au phase

diagram, it is unexpected to find a number ofpapers reporting the preparation of supported cat-alysts containing these elements, the very smallparticles apparently consisting of homogeneousmicroalloys of the two components. The word‘apparently’ is used advisedly, because very oftenno evidence for alloy formation is provided, andthe conjunction of the two components is onlyinferred from the catalytic behaviour. It is how-ever of interest to examine the methods that havebeen claimed to give homogeneous alloy particles.They may be classified as follows (2): (a) simulta-neous or sequential exchange of protons of Yzeolite with the ethylenediamine complexes of thetwo components (8); (b) simultaneous deposition-precipitation onto TiO2–SiO2 by hydrolysis of asolution of the mixed chloro-complexes (9); (c)adsorption of the organometallic complexPt2Au4(–C≡CtBu)4 from hexane solution ontoSiO2, followed by calcination to remove the ligands(10) (this method gives very small particles (2–3 nm, see Figure 3), shown by energy-dispersiveanalysis to be bimetallic); (d) deposition of Pt-Aucolloids onto carbon (11, 12); (e) use of dendrimer-stabilised colloidal particles to make PtAu/SiO2

and PtAu/TiO2 catalysts (13). Other proposed

Platinum Metals Rev., 2007, 51, (2) 64

Mo W Sg

NR Rel NR Rel NR Rel

5p

5s

4d

6p

6s

5d

7p

7s

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3/2

1/2

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–0.2

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nce

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.u.

Fig. 2 Calculated outer-most atomic energy levelsfor molybdenum, tungstenand seaborgium (6, 7)(Rel = relativistic; NR =non-relativistic)

50 nm

Fig. 3 Typical high-resolution electron micrograph ofPt2Au4/SiO2 catalyst (10)

methods involve deposition of either platinumonto gold particles or of gold onto platinum parti-cles (14, 15), and are therefore not likely to givehomogeneous products in the first instance.

In a number of cases these methods have beenshown to form small particles (< 5 nm), but it issignificant to observe that the classic method ofco-adsorption of chloro-complexes from solution,followed by chemical reduction, gives large parti-cles (> 20 nm) with severe phase separation (16).Co-reduction of solutions of precursors gives first‘frozen’ solid solutions that are approximatelyhomogeneous, but which on heating to 700 Kequilibrate into a mixture of two phases; this isconsequent on the increase in particle size that theheating produces (17). The use of alkanethiol-encapsulated colloids to prepare Pt-Au/C catalystswith various Pt:Au ratios results in particles thatare mainly 2 ± 1 nm in size, and their homo-geneity has been nicely demonstrated (11) by mea-suring their lattice parameters using X-raydiffraction. This shows that they exactly obeyVegard’s law; that is, the lattice parameters are alinear function of composition (Figure 4). This hasalso been recently confirmed by Rossi (18), at leastfor compositions of gold content ≥ 50% (see alsoFigure 4). We therefore have very clear evidencethat sufficiently small particles can form true solidsolutions, but that with larger particles phaseseparation is inevitable; they behave as does themacroscopic system. There is no indication in theliterature of the critical size at which the change ofbehaviour takes place; for this information weneed to consider other factors.

Before doing so, however, it is worth notingthat many scientists express no surprise that theirsmall particles do not behave as the macroscopicalloy does. This lack of curiosity is unfortunate, tosay the least, because knowledge of the bonding inhomogeneous alloy particles might help in under-standing their catalytic behaviour, and hence infurther improvement.

Changes in Electronic Structurewith Particle Size

It has long been appreciated that the electronicstructure of platinum in the macroscopic state

differs from that of the isolated atom (19); in thefree state it is 5d 9 6s1, but in the solid state it isapproximately 5d 9.46s0.6. This is because, as thenumber of atoms forming the particle increases,the electron energy levels of the free atom begin tobroaden, then overlap, and finally form a continu-ous electron band in which rehybridisation betweenthe s, p and d valence orbitals can readily occur.The spacing between adjacent energy levels, δ, isgiven to a first approximation by Equation (i):

δ = EF/n (i)

where EF is the Fermi energy level and n the num-ber of atoms in the particle. When δ exceeds thethermal energy kBT (where kB is the Boltzmannconstant), the levels begin to act independently,and the particle then resembles a giant molecule.For EF = 10 eV, a typical value, the critical num-ber at room temperature is about n = 400, but inthe transitional region particles become less metal-lic at low temperature. There is therefore acontinuous change from non-metallic to metallicbehaviour, without any abrupt transition. Manyoptoelectronic properties vary with particle size,but the minimum size for showing fully metalliccharacter seems to depend on the techniqueemployed (1). The nature of these changes hasbeen discussed in a recent book (1).

One way of looking at the continuous changefrom non-metallic to metallic behaviour is to

Platinum Metals Rev., 2007, 51, (2) 65

4.08

4.04

4.00

3.96

3.92

Latti

ce P

aram

eter

, Å

0 20 40 60 80 100[Au], %

Fig. 4 Vegard’s law plot of the lattice parameters ofplatinum-gold colloids (open points from (11); filledpoints from (18))

recognise that atoms on the surface partly resem-ble free atoms rather than those in the bulk,because they do not form so many bonds to neigh-bours. The electronic structure of the entireparticle is averaged over all the atoms, and there-fore depends on the fraction of atoms not havinga full complement of neighbours; physical proper-ties can then be correlated with the averagenumber of bonds formed between neighbours. Asa rough guide one may say that, moving downwardin size, one begins to notice changes at about 3 nm,and they become rapid at about 2 nm. A sphericalparticle of this size has about 230 atoms of which140 are on the surface.

In the case of platinum, therefore, one con-sequence of the rehybridisation of the energy levelsis a decrease in the number of d-band holes per atomas particle size is lowered, and this can be directlysensed by X-ray absorption near-edge spec-troscopy (XANES), which exhibits a decrease inthe intensity of the white line on the leading edgeof the LIII X-ray absorption band (20). While itdoes not appear possible to use this quantitatively,the effect, although small, is quite distinct.

It is less well appreciated that exactly the samephenomenon occurs with gold. Although in thefree atom there is a filled d-electron level, in thebulk state a small white line is seen, showing againthat there has been some rehybridisation, so thatthe electronic structure of the metal is actually5d 10–x6s1+x (21, 22), and the white line that revealsthis weakens as the size of the particle becomessmaller (20).

Effects of Particle Size Variation onCatalytic Activity

Much has been written on this subject (1), andonly a very brief note is appropriate here. There isa major conceptual difficulty in that, in the sizerange where the electronic structure of metal parti-cles is changing, there are also major changes in thegeometric arrangement of atoms on the surface, ifstructural models are to be believed. It is thereforeimpossible to assign the catalytic effects of parti-cles of size less than about 5 nm unambiguously toeither a geometric or an electronic factor. It is notpossible to have one without the other. If,

however, an effect persists above 5 nm, it is morelikely due to there being a need for an active cen-tre of specific geometry, as electronic variationswill have petered out by this stage. Of course,where the rate of a catalysed reaction is indepen-dent of particle size (as for example with alkenehydrogenation), there is no problem; where there isa progressive change, there have been manyattempts to connect this with a requirement forsurface atoms of specific coordination number (1),but simultaneous changes in electronic structureare often overlooked.

Electronic Structure of Platinum-Gold Alloys

The theoretical interpretation of bonding inmacroscopic alloys is a somewhat complicatedmatter, and various models have been used toexplain what is observed (5). In this short articlewe can only sketch what are likely to be the mainrelevant factors in the Pt-Au system.Experimentally it is found that each componentretains its individual band structure, although theenergies and the widths of the bands may alter. Forexample, for a low concentration of one compo-nent, its band widths will be narrower than for thebulk, because of limited overlap with the levels ofother like atoms. This means that there is noactual transfer of electrons from filled to partiallyvacant bands, as was originally thought, and a morerealistic model involves electron pairing, i.e. co-valent bond formation, between unpairedelectrons on the two types of atom.

Perhaps the easiest situation to understand isthe small alloy particle (of diameter < 2 nm) inwhich all the atoms remain atom-like, and there-fore retain their atomic electron structures. Pairingof the s electrons on each atom will allow Au–Au,Pt–Pt, and Pt–Au covalent bonds to be formed (asin the complex Pt2Au4(–C≡CtBu)4) (10). Thisprocess can occur at all Pt:Au ratios, and completemutual solubility is possible.

The situation with larger particles (diameter > 2 nm) or macroscopic forms is somewhat differ-ent. Gold then loses its strong electronegativecharacter that determines much of its chemistry,because of the rehybridisation of its valence

Platinum Metals Rev., 2007, 51, (2) 66

orbitals through electron band formation. Thenumber of unpaired 6s electrons on the gold atomsis lowered, the capacity for bond formation withthe 6s electron of platinum is decreased, and thesolubility of platinum in gold is therefore limited.A similar effect operates at the other end of thephase diagram; there are only 0.4 unpaired s elec-trons on the platinum atoms available for bondingwith the gold’s 6s1, and solubility is therefore verylimited.

The presence of a support to stabilise verysmall particles is an inevitable complication whenconsidering their structure and catalytic activity(1). Electron transfer across the metal-supportinterface is now largely discounted as a factor,because its occurrence would immediately createan opposing image potential; with reducible sup-ports, however, there may be some effect oflimited extent at the interface, and this has beensuggested as one of the causes of the StrongMetal-Support Interaction (1). Geometric distor-tion of the metal structure close to the interface isprobably more common, as the particle ‘struggles’to adapt to the structure of the support with whichit is in contact.

No doubt the true explanation (if indeed there

is one) will turn out to be far more complicated,and perhaps less easily understood. However,there does not appear to have been any theoreticalanalysis of Pt-Au bonding in small particles, soperhaps this article will alert theoreticians to theexistence of the problem, and attract them to workon it.

ConclusionThe many examples of superior catalytic prop-

erties shown by bimetallic catalysts containingpalladium or platinum with gold (2, 3, 5) at presentlack a theoretical foundation. An important step inunderstanding how they work is the realisationthat the Pt-Au pair are able to form homogeneousalloy particles, provided their size is not greaterthan about 3 nm, in spite of the fact that in thebulk state there is a wide miscibility gap. The prob-lem is compounded by the recent observation thattrimetallic catalysts (for instance PtNiFe, PtVFe)show interesting behaviour in electrocatalytic oxi-dations, relevant to the operation of fuel cells(23, 24). Theoretical work is urgently needed tounderpin these new developments in catalysisusing platinum or palladium with gold.

Platinum Metals Rev., 2007, 51, (2) 67

1 G. C. Bond, “Metal-Catalysed Reactions ofHydrocarbons”, Springer, New York, 2005 and references therein

2 G. C. Bond, C. Louis and D. T. Thompson,“Catalysis by Gold”, Catalytic Science Series,Volume 6, Imperial College Press, London, 2006

3 D. T. Thompson, Platinum Metals Rev., 2004, 48, (4),169 and references therein

4 J. K. Edwards, B. E. Solsona, P. Landon, A. F.Carley, A. Herzing, C. J. Kiely and G. J. Hutchings,J. Catal., 2005, 236, (1), 69

5 V. Ponec and G. C. Bond, “Catalysis by Metals andAlloys”, Studies in Surface Science and Catalysis,Volume 95, Elsevier, Amsterdam, 1995

6 G. C. Bond, J. Mol. Catal. A: Chem., 2000, 156, (1–2),1

7 G. C. Bond, Platinum Metals Rev., 2000, 44, (4), 1468 G. Riahi, D. Guillemot, M. Polisset-Thfoin, A. A.

Khodadadi and J. Fraissard, Catal. Today, 2002, 72,(1–2), 115

9 A. Zwijnenburg, M. Saleh, M. Makkee and J. A.Moulijn, Catal. Today, 2002, 72, (1–2), 59

10 B. D. Chandler, A. B. Schabel, C. F. Blanford and L.

H. Pignolet, J. Catal., 1999, 187, (2), 36711 J. Luo, M. M. Maye, V. Petkov, N. N. Kariuki, L.

Wang, P. Njoki, D. Mott, Y. Lin and C.-J. Zhong,Chem. Mater., 2005, 17, (12), 3086

12 N. Dimitratos and L. Prati, Gold Bull., 2005, 38, (2),73

13 R. W. J. Scott, C. Sivadinarayana, O. M. Wilson, Z.Yan, D. W. Goodman and R. M. Crooks, J. Am.Chem. Soc., 2005, 127, (5), 1380

14 H. Tada, F. Suzuki, S. Ito, T. Akita, K. Tanaka, T.Kawahara and H. Kobayashi, J. Phys. Chem. B, 2002,106, (34), 8714

15 P. Del Angel, J. M. Dominguez, G. Del Angel, J. A.Montoya, J. Capilla, E. Lamy-Pitara and J. Barbier,Top. Catal., 2002, 18, (3–4), 183

16 A. Vázquez-Zavala, J. García-Gómez and A.Gómez-Cortés, Appl. Surf. Sci., 2000, 167, (3–4), 177

17 H. C. de Jongste, F. J. Kuijers and V. Ponec, in“Preparation of Catalysts I”, eds. B. Delmon, P. A.Jacobs and G. Poncelet, Elsevier, Amsterdam, 1976,p. 207

18 M. Rossi, Università di Milano, 2006, personal com-munication

References

19 N. F. Mott and H. Jones, “The Theory of theProperties of Metals and Alloys”, DoverPublications Inc, New York, 1958

20 J. A. van Bokhoven, C. Louis, J. T. Miller, M.Tromp, O. V. Safonova and P. Glatzel, Angew. Chem.Int. Ed., 2006, 45, (28), 4651

21 L. F. Mattheiss and R. E. Dietz, Phys. Rev. B, 1980,22, (4), 1663

22 R. E. Benfield, D. Grandjean, M. Kröll, R. Pugin, T.Sawitowski and G. Schmid, J. Phys. Chem. B, 2001,105, (10), 1961

23 J. Luo, L. Wang, D. Mott, P. N. Njoki, N. Kariuki,C.-J. Zhong and T. He, J. Mater. Chem., 2006, 16,(17), 1665

24 J. Luo, N. Kariuki, L. Han, L. Wang, C.-J. Zhongand T. He, Electrochim. Acta, 2006, 51, (23), 4821

Platinum Metals Rev., 2007, 51, (2) 68

The Author

Geoffrey Bond held academic posts atLeeds and Hull Universities beforejoining Johnson Matthey PLC in 1962as Head of Catalysis Research. In 1970he was appointed Professor in BrunelUniversity’s Chemistry Department, andis now an Emeritus Professor at thatUniversity.

Combining theory with practice, this intro-ductory book provides detailed data obtained dur-ing studies on the refining technologies of theprecious metals in China and abroad. The book has10 chapters, organised by the refining technologiesof gold, silver, and the platinum group metals.

The main physical and chemical properties,compounds and complexes of precious metals areintroduced in Chapter 1. The separation methodsand technologies of the precious metals arereviewed in Chapter 2. The refining technologies,for gold, silver, palladium, platinum, rhodium, iridi-um, osmium and ruthenium comprise the most

important part of the book, and occupy Chapters 3to 9. The preparation methods of high purity matri-ces for spectroscopic analysis of the precious metalsare specially described in Chapter 10. There are twoAppendices which describe the memberships of theShanghai Gold Exchange and refining productstandards in the U.S.A. and Russia, respectively.

DOI: 10.1595/147106707X192708

“The Separation and Refining Technologiesof Precious Metals”

EDITED BY JIANMIN YU (Kunming University of Science and Technology, China), (in Chinese), Chemical Industry Press,

Beijing, China, 2006, 272 pages, ISBN (hardcover) 7-5025-9008-0, Yuan ¥45.00

The Editor of the BookJianmin Yu is a Professor in the Applied Chemistry Departmentat Kunming University of Science and Technology, Kunming,Yunnan, 650093, China. He is interested in extraction,separation, recovery and refining of precious metals.E-mail: [email protected]

A new scientific event in metathesis chemistry,the NATO Advanced Study Institute (NATOASI) on New Frontiers in Metathesis Chemistry:From Nanostructure Design to SustainableTechnologies for Synthesis of Advanced Materials(1) was held in Antalya, Turkey, from 4th to 16thSeptember 2006 (co-directors: Y. Imamoglu andV. Dragutan). This event is an appropriate sequelto the memorable Nobel Prize awarded tometathesis scientists Yves Chauvin, Robert H.Grubbs and Richard R. Schrock (Stockholm,November 2005) (2–4), and the XVIthInternational Symposium on Olefin Metathesis(Poznan, Poland, June 2005) (5).

Prominent scientists and young students fromtwelve NATO countries (Belgium, Bulgaria,Canada, Czech Republic, France, Hungary, Poland,Romania, Spain, Turkey, U.K. and U.S.A.) and sixNATO Partner countries (Armenia, Azerbaijan,Kazakhstan, Moldova, Russia and Ukraine), inter-ested in metathesis chemistry convened in Antalyafor two weeks under NATO sponsorship. Thepurpose was to debate on the newest trends inolefin metathesis and identify future perspectivesin this fascinating field of synthetic organic andorganometallic chemistry, where platinum groupmetals (pgms), especially ruthenium, are playing akey role.

Olefin metathesis, one of the most efficienttransition metal-mediated C–C bond forming reac-tions, has emerged during the last few years as apowerful synthetic strategy for obtaining finechemicals, biologically active compounds, architec-turally complex assemblies, new materials andfunctionalised polymers tailored for specific uses,including sensors, semiconductors and microelec-tronic devices. Metathesis reactions, such asring-closing metathesis (RCM), enyne metathesis,

cross-metathesis (CM) and ring-opening metathe-sis polymerisation (ROMP), have moved farbeyond their 20th century boundaries. This hasresulted in a broad diversification towards sustain-able technologies, and in new perspectives for awide range of industrial applications, from produc-tion of smart, nanostructured materials to themanufacture of new pharmaceuticals (6–8).

During the meeting, recent advances inmetathesis chemistry were disseminated among aselected audience of distinguished scientists andyoung researchers. Lectures, discussions andposter presentations, organised by a ScientificCommittee (Yavuz Imamoglu (HacettepeUniversity, Turkey), Valerian Dragutan (RomanianAcademy, Romania), Lajos Bencze (University ofVeszprém, Hungary), Ezat Khosravi (University ofDurham, U.K.) and Kenneth B. Wagener(University of Florida, U.S.A.)) in ten scientificmain sessions, dealt primarily with novel metathe-sis catalysts pertaining to the pgms and severalother late transition metals, and their application tokey metathesis reactions or tandem meta-thesis/non-metathesis processes of environmental,industrial and commercial relevance. Suchmetathesis reactions have profound implications inmaterials science, nanotechnology, and also inorganometallic, organic and polymer chemistry.

Developments in Ruthenium-BasedCatalysts

As expected, in lectures on catalyst-related top-ics emphasis was placed on the newestdevelopments concerning ruthenium-based com-plexes of high activity, selectivity and robustness,popular for their excellent tolerance toward a vari-ety of functional groups. A whole range ofalkylidene ruthenium complexes, both neutral and

69

New Frontiers in Metathesis ChemistryOLEFIN METATHESIS – TRENDS AND PERSPECTIVES IN ORGANIC SYNTHESIS AND RUTHENIUMCATALYSIS

Reviewed by Ileana Dragutan and Valerian Dragutan*Institute of Organic Chemistry, Romanian Academy, 202B Spl. Independentei, PO Box 35-108, 060023 Bucharest, Romania;

*E-mail: [email protected]

Platinum Metals Rev., 2007, 51, (2), 69–75

DOI: 10.1595/147106707X188956

ionic, which can be recovered and recycled, wereillustrated by Pierre Dixneuf (University ofRennes, France) in his lecture ‘RutheniumCatalysts for Alkene Metathesis’. He includedpreparation methods and selected applications. Inaddition to the broad applications of the classicalSchrock molybdenum catalysts and the Grubbs(1st and 2nd generation), Nolan or Hoveyda-typeruthenium catalysts, Dixneuf disclosed in a secondseminal lecture: ‘Recent Applications of AlkeneMetathesis for Fine Chemical and SupramolecularSystem Synthesis’, further significant utilisations ofother active catalyst precursors, such as the ruthe-nium allenylidene, 1, and the indenylidenepromoter, 2, derived from it. These applicationsrefer specifically to the synthesis of new macrocy-cles via RCM, to RCM in the synthesis of newligands, rotaxanes, catenanes and supramolecularsystems, and to CM in organic synthesis andsupramolecular system formation.

Elaborating on the essential role played by N-heterocyclic carbene (NHC) ligands in creatingthe most effective ruthenium metathesis catalysts(for example, 4 and 5 vs. 3), Steven P. Nolan(University of Tarragona, Spain) extended the con-cept of introduction of the valuable NHC moietyto other late transition metal complexes, as out-lined in his fascinating talk on ‘The Role of NHC

in Late Transition Metal Catalysis’.As a further topic in this series, the synthesis,

catalytic activity and pertinent mechanistic aspectsof some recently discovered NHC-endowed latetransition metal complexes of palladium, nickel,copper and gold were fully exemplified by Nolanfor a set of catalysed transformations other thanmetathesis (cross-coupling reactions, hydrothiola-tion of alkynes, ‘click chemistry’ for rapid synthesisof new compounds and combinatorial libraries,and hydrosilylation of carbonyl compounds, aswell as cycloisomerisation of polyunsaturated sys-tems) in his comprehensive and originalpresentation ‘NHC-Metal Complexes of Groups10 and 11. Recent Developments in Synthesisand Catalysis’.

New Insights into Catalyst DesignNew insights into catalyst design were provided

by Deryn Fogg (University of Ottawa, Canada) andby Natalia Bespalova (United Research &Development Centre, Russian Academy ofSciences).

In her lecture ‘New Insights in Ring-ClosingMetathesis: Catalyst Design and MALDI-MSAnalysis’, Fogg demonstrated that ruthenium cata-lysts containing electron-deficient aryloxide(‘pseudohalide’) ligands confer high activity at low

Platinum Metals Rev., 2007, 51, (2) 70

Ru C C CPh

PhClPCy3

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PCy3

1 2

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Cl

Cl

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catalyst loadings, while also expanding the struc-tural diversity of the ligand set and the capacity forsteric and electronic tuning of activity and selectiv-ity. She pointed out that by using thematrix-assisted laser desorption/ionisation massspectrometry (MALDI-MS) analysis technique,distinctions in the behaviour of the Grubbs vs.pseudohalide catalysts could be revealed. This pro-vides an in-depth understanding of the RCMmechanism, to be exploited in synthesis of largeand medium-sized rings.

In addressing ‘Catalyst Design for FunctionalOlefins Production via Olefin MetathesisReactions’, Bespalova dwelt on the modificationof well-defined ruthenium carbene catalyststhrough appropriate changing of the carbenoidmoiety and the imidazole ligand, which allows avariation in the catalytic properties. A comparisonbetween performances in CM of the new catalystswith those of the ill-defined catalysts based ontungsten was also presented.

Ligands for CatalysisFollowing up on the general notion of NHC

utilisation in catalysis, Lionel Delaude (Universityof Liège, Belgium), in his instructive presentations:‘Olefin Metathesis with Ruthenium-AreneCatalysts Bearing N-Heterocyclic Carbene’ and‘Studies on N-Heterocyclic Carbene LigandPrecursors’, dealt mainly with the best methodsfor obtaining symmetrically N,N-disubstitutedNHC ruthenium arene complexes and withROMP of cycloolefins promoted thereby. KarolGrela (Institute of Organic Chemistry, PolishAcademy of Sciences), in an interesting and infor-mative exposition on ‘Catalysts for New Tasks:Preparation and Applications of TunableRuthenium Catalysts for Olefin Metathesis’,extended the class of Hoveyda catalysts to relatedruthenium systems, showing that the catalyst activ-ity can be enhanced by using adequate electronwithdrawing groups (EWGs) as substituents onthe isopropoxy-benzylidene ligand. FrancisVerpoort (Ghent University, Belgium), in his com-prehensive accounts ‘Olefin Metathesis Mediatedby Schiff Base Ru-alkylidene Complexes’ and‘Rational Design and Convenient Synthesis of a

Novel Family of Ruthenium Complexes withO,N-Bidentate Ligands’, reported on new strate-gies in Kharasch addition, enol-ester synthesis andROMP of dicyclopentadiene (DCPD) using high-ly efficient Schiff-base ruthenium complexes.

Poster presentations demonstrated additionaldata in support of the role of NHC ligands inruthenium-based metathesis precatalysts (XavierSauvage, University of Liège, Belgium, andAdriana Tudose, University of Liège andRomanian Academy) and the unexpected activat-ing effect of strong acids on ruthenium complexesincorporating O,N-bidentate ligands (RenateDrozdzak and Nele Ledoux, Ghent University,Belgium). Providing interesting information aboutruthenium-mediated processes related to metathe-sis, Chloe Vovard (University of Rennes, France)briefly talked on ‘Ruthenium Catalysed Additionof Diazocompounds to Enynes: Synthesis ofBicyclic Compounds with VinylcyclopropaneMoiety’.

Nanoscience and MaterialsIn a very attractive lecture, ‘Molecular

Nanoscience and Catalysis’, Didier Astruc(University of Bordeaux, France), starting fromsynthesis of organometallics, catalysts and elec-tron-transfer agents and mechanisms involvingelectron transfers, dealt eloquently with such hottopics as:– dendrimers (catalysis, molecular electronics,

recognition and transport);– gold and palladium nanoparticles (sensors and

catalysts);– metathesis reactions with nano-objects;– ‘click chemistry’.In another creative subject, ‘Combining SimpleArene Activation with Ru-Catalysed OlefinMetathesis for the Assembly and Functional-ization of Nano-Objects’, Astruc revealed conve-nient routes to desirable supramolecular structuresand elaborate synthetic methods. In this extremelybroad context, several elegant applications ofruthenium dendritic structures were given (forexample, use of 6 in synthesis of polymers, 7).

New trends presently evolving in metathesischemistry were critically discussed by Hynek

Platinum Metals Rev., 2007, 51, (2) 71

Balcar (J. Heyrovský Institute of PhysicalChemistry, Academy of Sciences of the CzechRepublic). The new directions cited were:(a) Eco-friendly protocols for metathesis catalysts;(b)Supported catalysts and novel techniques for

immobilisation;(c) Applying to metathesis at room temperature

the recognised advantages of ionic liquidsthrough the creation of recyclable imidazoli-um-tagged catalysts;

(d)CM, enyne and RCM as key reaction steps in

organic synthesis yielding pharmaceuticals andnatural products (sugars, alkaloids, nucleosides,amino acids);

(e) Chiral catalysts enabling enantioselective meta-thetical pathways in precision synthesis.

In his talk on ‘Molecular Sieves as Supports forMetathesis Catalysts’, Balcar described howsiliceous sieves (MCM-41 and -48, SBA-15) andorganised mesoporous alumina were successfullyused in obtaining new catalysts (principally basedon molybdenum and rhenium oxides) and in het-

Platinum Metals Rev., 2007, 51, (2) 72

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C C

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A r

A r

A r

A r

A r

A r A r

A r A r

A r

A r

A r

n n

n

n n

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n

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n

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n n

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1600 eq

RT

erogenisation of some well-defined homogeneouscatalysts for olefin metathesis and metathesispolymerisation.

The details of extending NHC ligands to osmi-um, with effect on catalyst activity,chemoselectivity and stereoselectivity, have beenwidely evidenced by Ricardo Castarlenas(University of Zaragoza, Spain) in his talk on‘NHC Osmium-arene Catalysts for OlefinMetathesis Reactions’. Pertinent structural andmechanistic aspects regarding this recently intro-duced class of metathesis catalysts were alsoaddressed.

New applications of ruthenium-mediated tan-dem metathesis/non-metathesis processes werefully documented by Deryn Fogg in a presentationon ‘Tandem ROMP-Hydrogenation Catalysis inTissue Engineering Applications’. This drew atten-tion to a potentially important practical applicationof polymers obtained in this way (galactose-func-tionalised polynorbornenes), which have so farproved successful in clinical trials for corneal tis-sue engineering.

The advantages of another tandem combina-tion, RCM-hydrogenation, were presented byNatalia Bespalova in the lecture ‘Synthesis ofHigher Esters Using Tandem Olefin Metathesis-Hydrogenation Reactions’, bringing into pro-minence the production of valuable highly saturat-ed esters by means of the catalyst pairBu3SnH/2nd generation Grubbs.

‘Catalytic Cycloisomerisation of EnynesInvolving Various Activation Processes’, present-ed by Christian Bruneau (University of Rennes,France), extended the scope of ruthenium-basedmetathesis chemistry to a variety of initiating sys-tems of fundamental relevance in catalysis.Skeleton reorganisations, carried out with catalystsbased on palladium, cobalt, rhodium, iridium, plat-inum, gold and ruthenium and involvingelectrophilic activation, oxidative coupling andmetathesis reactions were described, along withapplications of metathesis for the transformationof some natural compounds. ‘Cross-metathesis ofVinyl-substituted Organosilicon Derivatives withOlefins in the Presence of Grubbs Catalysts’, pre-sented by Cezary Pietraszuk (Adam Mickiewicz

University of Poznan, Poland), evidenced howruthenium-catalysed metathesis can be fruitfullyused in synthesis of novel silicon-containingadvanced materials.

Macromolecular ChemistryEzat Khosravi (University of Durham, U.K.)

illustrated the versatility of ROMP in two comple-mentary lectures on ‘Ruthenium Initiators andOxygen Containing Norbornene Derivatives’ and‘Synthesis of Novel Polymers via ROMP’. In thefirst, he highlighted the importance of both thepresence and position of the oxygen substituent innorbornene derivatives subjected to ROMP, whilealso unambiguously establishing the identity of thepropagating alkylidene species. In his second lec-ture, by discussing two examples taken from theirongoing research: (i) polymers with ball-and-chainsequences synthesised by ROMP of dendronised(polycarbonate) monomers, and (ii) synthesis ofpolymeric bioresorbable materials based on graftcopolymers consisting of polyoxanorbornenebackbones with poly(hydroxyacid) side-chains,Khosravi demonstrated that ROMP is a powerfultool in macromolecular engineering, allowing syn-thesis of polymers with novel topologies.

Moreover, Eugene Finkelshtein (TopchievInstitute of Petrochemical Sciences, RussianAcademy of Sciences) in his outstanding lectureon ‘ROMP and Other Ring-Opening Processes asan Effective Route to New CarbosilaneMembrane Materials’, evidenced ways for produc-ing specialty polymers usable as gas-separatingmembrane materials when a particular combina-tion of high film-forming, permeability andseparation properties is met. Debating on the cor-relation between the polymer structure and the gastransport parameters, he pointed out the roleplayed by the occurrence, number and location ofMe3Si-groups in a variety of copolymers synthe-sised for this purpose, as further reinforced byMaria Gringolts in a subsequent talk.

The main aspects of the kinetics and mecha-nism of the carbonyl-olefin exchange reaction,having a formal similarity with olefin metathesis,were elaborated by Christo Jossifov (Institute ofPolymers, Bulgarian Academy of Sciences) in his

Platinum Metals Rev., 2007, 51, (2) 73

amply documented presentation ‘Carbonyl-olefinExchange Reaction and Related Chemistry’. Thistype of reaction could be successfully performedonly when the two functional groups (carbonylgroup and olefin double bond) are situated in thesame molecule and are conjugated as in α,β-unsat-urated carbonyl compounds (substitutedpropenones). Substituted polyacetylenes havingvaluable conducting and optical properties couldalso be obtained by this straightforwardmethodology.

Presentations from Ken Wagener’s group(University of Florida, U.S.A.) by two of his stu-dents, Emine Boz and Giovanni Rojas, on‘Correlating Precisely Defined Primary Structurewith Crystalline Properties in Halogen-ContainingPolyolefins’ and ‘Precision Polyolefin Structures’,respectively, showed how acyclic diene metathesis(ADMET) induced by Grubbs ruthenium catalystscan rigorously control the polymer microstructure,and ultimately the product properties.

Special attention was paid to addition polymeri-sation and copolymerisation of selected monomers(such as silyl-functionalised norbornenes), inducedby late transition metal catalysts (nickel and palla-dium), providing new materials with specialproperties (high thermal and chemical stability),for applications such as optical components, elec-trical insulators and photoresists. This wasconvincingly shown by Eugene Finkelshtein in hislecture on ‘Addition Polymerization of Silyl- andSome Other Functionalized Norbornenes’; byVictor Bykov (Topchiev Institute of PetrochemicalSciences, Russian Academy of Sciences) in hiscommunication on ‘Copolymerization of Ethylenewith Norbornene and Their FunctionalDerivatives on Nickel-ylide Catalysts’; and byMaria Gringolts (Topchiev Institute ofPetrochemical Sciences, Russian Academy ofSciences) in her talk on ‘The Influence of Presence,Number and Location of Alk3Si Groups inNorbornenes and Norbornadienes on TheirPolymerisation and Polymer Properties’.

Nanostructured materials and how they couldbe used in nanomachines and molecular clock-works were attractively described by Lajos Bencze(University of Veszprém, Hungary), in two com-

plementary lectures: ‘Long Range Transfer ofChiral Information in Rotalicene TypeNanomachines’ and ‘Molecular Clockworks asPotential Models for Biological Chirality’. Goingbeyond metathesis but in close correlation withthis reaction, comprehensive and fascinating lec-tures on ‘Smart Nanostructured Materials by AtomTransfer Radical Polymerization’ and ‘Environ-mental and Sensors by Atom Transfer RadicalPolymerization’, delivered by KrzysztofMatyjaszewski (Carnegie Mellon University,U.S.A.), illustrated the great potential of preciselycontrolled macromolecular structures obtained byatom transfer radical polymerisation (ATRP) to beassembled into smart materials, sensors and vari-ous molecular devices. A successful combinationof ATRP with ROMP to produce new materialswith valuable properties was also discussed. In histurn, Osama Musa (National Starch & ChemicalCo, U.S.A.) furnished in his attractive lecture‘Exploration of Novel Thermoset Resins: Faster,Higher and Stronger’, a wide range of applicationsof advanced materials prepared by different poly-merisation procedures.

Concluding RemarksThe social programme was organised in a

friendly and warm style, facilitating informal scien-tific discussion among renowned experts andyoung researchers, and strengthening contacts andthe exchange of information between researchgroups of different nationalities.

At the end of the event, there was a general dis-cussion on perspectives of future NATO ASImeetings on metathesis chemistry. Commentsfrom a number of participants converged to theidea that, in view of the current upward trend indevelopments in this field, organising furthermeetings would be both opportune and highlybeneficial, especially for the young generation ofscientists involved in metathesis research.

A major conclusion emerging from the lectures,posters and discussions at this Institute is that aprincipal focus in this highly challenging area ofresearch is the advantages of using new rutheniumcatalysts for a multitude of chemical transforma-tions. Further exploration of the metathesis

Platinum Metals Rev., 2007, 51, (2) 74

chemistry of other pgms such as osmium is also ofgreat interest. The application profile of the novelmetathesis catalysts is expanding rapidly, particu-larly in RCM, and CM. These methods may beexploited for the synthesis of therapeutic com-pounds, as well as in ROMP for the production ofspecialised and highly functionalised polymers.

Following the useful practice of previousNATO ASI meetings (see, for example, (9)),selected contributions including plenary lectures,short communications and posters will be com-piled in a special volume dedicated to thisoutstanding scientific meeting, and will be pub-lished by Springer Verlag in 2007 (10).

References1 NATO ASI: New Frontiers in Metathesis

Chemistry: http://www.nato-asi.tk/2 The Nobel Prize in Chemistry 2005: http://nobel-

prize.org/nobel_prizes/chemistry/laureates/2005/press.html

3 P. Ahlberg, ‘Development of the metathesis method

in organic synthesis’, Advanced information on theNobel Prize in Chemistry 2005, The Royal SwedishAcademy of Sciences, Stockholm, 2005;http://nobelprize.org/nobel_prizes/chemistry/laureates/2005/chemadv05.pdf

4 V. Dragutan, I. Dragutan and A. T. Balaban,Platinum Metals Rev., 2006, 50, (1), 35

5 B. Marciniec, J. Mol. Catal. A: Chem., 2006, 254,(1–2), 1

6 A. M. Thayer, Chem. Eng. News, 2007, 85, (7), 377 T. Netscher, G. Malaisé, W. Bonrath and M.

Breuningen, Catal. Today, 2007, 121, (1–2), 718 “Handbook of Metathesis”, ed. R. H. Grubbs,

Wiley-VCH, Weinheim, 20039 M. J. H. Russell, Platinum Metals Rev., 1989, 33, (3),

11710 “Metathesis Chemistry. From Nanostructure

Design to Synthesis of Advanced Materials”,Proceedings of the NATO Advanced StudyInstitute on New Frontiers in Metathesis Chemistryfrom Nanostructure Design to SustainableTechnologies for Synthesis of Advanced Materials,Antalya, Turkey, 4–16 September, 2006, eds. Y.Imamoglu and V. Dragutan, NATO Science SeriesII: Mathematics, Physics and Chemistry , Volume243, Springer, Berlin, Heidelberg, 2007

Platinum Metals Rev., 2007, 51, (2) 75

The Reviewers

Ileana Dragutan is a SeniorResearcher at the Institute ofOrganic Chemistry of theRomanian Academy. Herinterests are in stable organicfree radicals – syntheses andapplications as spin probes,olefin metathesis, Ru catalysis,transition metal complexes

with free radical ligands and their magnetic andcatalytic properties, azasugars and prostaglandin-related drugs.

Valerian Dragutan is a SeniorResearcher at the Institute ofOrganic Chemistry of theRomanian Academy. Hisresearch interests arehomogeneous catalysis bytransition metals and Lewisacids; olefin metathesis andROMP of cycloolefins;

bioactive organometallic compounds; andmechanisms and stereochemistry of reactions inorganic and polymer chemistry.

76

Cascade reactions, also known as domino reac-tions, are multibond-forming processes in whichthe first reaction creates the functionality/geome-try necessary for the second reaction to proceed,and so on. Volume 19 of the Topics inOrganometallic Chemistry series comprises eightchapters written by experts in the relevant areasand is heavily weighted towards palladium(0)-catal-ysed processes (5 chapters). Additionally, there isan excellent chapter on the Pauson-Khand reac-tion (mainly cobalt catalysis) and on metathesis(ruthenium).

Chapter 1 (by E. Negishi, G. Wang and G. Zhu)reviews Pd(0)-catalysed cyclisation-carbopallada-tion and acylpalladation cascades. A logicalorganisational framework enables systemisation ofthe sprawling literature, and adequate references topertinent reviews and early work are provided. Therole of proximal alkene and alkyne functionalitiesin facilitating oxidative insertion of Pd(0) intoaryl/vinyl C–halogen bonds is noted. Carbopalla-dation comprises the major part of this chapter,and the versatility of such processes is amplydemonstrated by processes involving polyenes,polyenynes, enylallenes and ynylallenes for theassembly of a bewildering array of fused, bridgedand spirocyclic ring systems. The majority ofthe cascades are one- or two-component pro- cesses involving the formation of up to five rings.

The following Chapter 2 (by P. von Zezschwitzand A. de Meijere) covers sequential and cascadecombinations of the Heck reaction with 6π-elec-trocyclisations or Diels-Alder and 1,3-dipolarcycloadditions. The importance of the relativerates of reaction of the substrates and of tempera-ture in cascade design involving three or more

components, which in adverse circumstances leadsto the sequential one-pot option, is noted. Thedevelopment of bicyclopropylidene as a cyclo-propyl-1,3-diene source in the three-componentHeck-Diels-Alder processes is well reviewed, as isthe use of allenes in three-component Heck-Diels-Alder and Heck-1,3-dipolar cycloaddition cas-cades. The latter provide access to a substantialarray of heterocycles. Catalytic cross-coupling withensuing thermal 6π-electrocyclisations, whichresults in the annulation of 6-membered carbo- orheterocyclic rings onto various core rings including[2,2]-paracylcophanes, is well exemplified.

A survey of Pd(0)-catalysed cascades involvingπ-allylpalladium(II) species is given in Chapter 3(by N. T. Patil and Y. Yamamoto), which is essen-tially a review of Yamamoto’s contributions to thearea. The major focus is on π-allyl generation fromallylic systems, including vinyl oxiranes, thiiranes,aziridines and 1,3-dienes. Generation of π-allylspecies from allenes is only fleetingly mentioned.The emphasis is on one- and two-componentprocesses and examples of both carbo- and hetero-cycle formation are given, including Yamamoto’sthree-component Pd(0)-Cu(I)-catalysed triazoleand tetrazole syntheses from allylic carbonates,trimethylsilyl azide (TMSN3) and an alkyne ornitrile.

Metal-promoted cyclisative cascade reactionswhich incorporate Michael addition as a key stepare the focus of Chapter 4 (by G. Balme, D. Bouyssi and N. Monteiro). The major emphasisis on Pd-promoted processes but examples of copper(I)-, scandium(III)-, yttrium(III)- and rhodium(I)-promoted processes are included. Thereview is nicely organised and mainly surveys for-mation of 5- and 6-membered carbocycles and

“Metal Catalyzed Cascade Reactions”TOPICS IN ORGANOMETALLIC CHEMISTRY, Volume 19

EDITED BY T. J. J. MÜLLER (Universität Heidelberg, Germany), Springer, Berlin, Germany, 2006, 339 pages,

ISBN 978-3-540-32958-9, £123.00, €159.95, U.S.$199.00

Reviewed by Ron GriggMIDAS Centre, University of Leeds, Leeds LS2 9JT, U.K.; E-mail: [email protected]

Platinum Metals Rev., 2007, 51, (2), 76–77

DOI: 10.1595/147106707X190403

heterocycles, with a strong emphasis on 5-mem-bered oxygen heterocycles in the latter case. BothMichael initiated and terminated sequences arereviewed and exemplified by unimolecular andtwo- and three-component cascades involving avariety of metal activated intermediates (π-allylpal-ladium, π-complexed alkynes, alkenes andenolates).

Chapter 5 (by T. J. J. Müller) reviews a series ofsequential Pd-catalysed processes initiated by Heckreactions, allylic substitution, amination,Sonogashira coupling, metallation (for example, insitu formation of Stille and Suzuki reagents, Pdmigration/insertion into C–H bonds) and cycloi-somerisation sequences. Given the huge andbuoyant literature, this chapter, of necessity, pre-sents a ‘bird’s-eye’ view of a dynamic field.

The 100% atom economic Pauson-Khand reac-tion (PKR), a formal [2+2+1]-cycloadditionreaction involving an alkyne, an alkene and carbonmonoxide, together with related processes, arecovered in Chapter 6 (by J. Pérez-Castells). Theconcise introduction benefits the non-specialistreader. The current limitations of the catalyticPKR are discussed and the wide range of metalcomplexes employed are illustrated, as are the var-ious approaches and strategies for chiral induction.The interfacing of the Nicholas reaction (alkylationof Co-stabilised α-carbocationic alkynes) with thePKR and bimetallic Pd-Rh catalysis to generateand process the PKR substrate in situ are illustrat-ed, as are in situ sources of carbon monoxide. Awide variety of cascade PKRs, including combina-tion with Diels-Alder and photochemical[2+2]-reactions, are also reviewed in this excellentchapter.

Access to complex polycyclic compounds fromacyclic precursors is the concern of Chapter 7 (byC. Aubert, L. Fensterbank, V. Gandon and M.Malacria). Inevitably there is some overlap with theother chapters, particularly Chapters 1 and 2. The

review is divided into two broad classes: thoseinvolving non-carbenoid intermediates and thoseinvolving metallo carbenoids. The non-carbenoidprocesses involve cycloaddition, cycloisomerisa-tion or ene-type reactions in which a simple changein temperature often drives the process to comple-tion. Topics covered include the Kinugasareaction, the Cu(I)-catalysed β-lactam synthesisfrom an alkyne plus a nitrone and the coupledRh(I)-catalysed alkylation of π-allyl species – carbocyclisations which provide a facile access tobicyclic systems. Examples accessing three or morerings include those involving nickel enolates,rhodium- or silver-catalysed Alder-ene processesand Co-catalysed Conia-ene reactions. Processesinvolving metallo carbenoids focus on α-diazocar-bonyl compounds and illustrate how rhodium,ruthenium, nickel and tungsten react with these togenerate bridged, fused and spirocyclic ring sys-tems. Other non-carbenoid routes to complexcyclopropanes are also illustrated.

The final Chapter 8 (by C. Bruneau, S. Dérienand P. H. Dixneuf) is concerned with cascade andsequential Ru-catalysed metathesis processes. Thecascade processes, which are only briefly reviewed,are enyne metathesis and alkene metathesis, andare largely examples involving Grubbs 2nd genera-tion Ru heterocyclic carbene catalyst andHoveyda’s catalyst. The metathesis area is so fastmoving that reviews have a short ‘shelf life’.

Overall, this would not be considered a volumefor personal libraries but is well worth consultingwhen appropriate. It is to be recommended on that basis.

Platinum Metals Rev., 2007, 51, (2) 77

The ReviewerProfessor Ron Grigg is Director of the Centre forMolecular Innovation, Diversity and AutomatedSynthesis (MIDAS Centre) at the University ofLeeds, U.K. His current interests include cascadereactions catalysed by palladium, iridium, rhodiumand ruthenium, and their applications to medicinalchemistry.

78

The mechanical properties of the widely usedjewellery alloy Pt-5% Cu have been investigatedby several researchers (see, for example, (1–3)) forthe cast, cold worked and annealed conditions. Itis however not widely known that platinum con-taining around 5 wt.% copper undergoes anordering transformation after low-temperatureheat treatment (4, 5). Recent work in the Centrefor Materials Engineering (6) showed that thePt7Cu ordered phase, which can form after heattreatment at temperatures as low as 200ºC, canimprove the hardness of Pt-5% Cu. If the alloy iscold worked before heat treatment, the increase inhardness after heat treatment is significant. If thealloy is first quenched from high temperatures, anincrease in hardness on heat treatment is measur-able but not as great.

Given the widespread use of Pt-5% Cu in plat-inum jewellery, it is surprising that there is littleinformation on this phenomenon in the technicalliterature. The hardness increase for this alloy hasimportant implications for the manufacture ofjewellery, since using low-temperature heat treat-ment as a final step in the manufacture of ajewellery piece can produce a better finish andimproved scratch resistance. Conversely, hardnessmay be inadvertently increased by heating duringmanufacture, leading to difficulty in subsequentworking of the piece. An understanding of theeffect of heating on strength accordingly allowsoptimal planning of the manufacturing route.

The effect of the Pt7Cu ordering transforma-

tion on mechanical properties other than hardnesshas not previously been investigated. In this paperwe investigate the effect on the strength and duc-tility of Pt-5% Cu of heat treatments which resultin ordering. The tensile mechanical properties ofplatinum and its alloys have not been studiedextensively, owing to the costs associated withtensile specimens of conventional (AmericanSociety for Testing and Materials (ASTM)) scale(7). These costs can be considerably reduced bymicrosample tensile testing, described previouslyin this Journal (3). Mechanical properties such asyield strength, tensile strength and ductility can bemeasured using specimens just 8 mm in length. Inthe present work the mechanical properties of Pt-5% Cu, before and after heat treatments whichresult in ordering, are fully characterised bymicrosample tensile testing and hardness testing.

Experimental ProcedureBars (50 g in mass) of Pt-5% Cu were cold

rolled to reduce thickness by 50%, beforehomogenising at 1000ºC for 12 hours in an argonatmosphere. A fully recrystallised grain structureresulted after homogenising. Composition andhomogeneity were assessed by scanning electronmicroscopy (SEM) and energy dispersive spec-troscopy (EDS). The thickness was then furtherreduced by 90%, and specimens were cut fromthe resulting sheet. The 8 mm long specimens formicrosample tensile testing were cut with the longaxis parallel to the rolling direction. Heat

Strengthening of Platinum-5 wt.% Copperby Heat TreatmentBy Chumani Mshumi and Candy Lang*Centre for Materials Engineering, University of Cape Town, Rondebosch 7701, South Africa; *E-mail: [email protected]

Heat treatment of platinum-5 wt.% copper (Pt-5% Cu) below 500ºC is known to result inan ordering transformation which can significantly increase the hardness of the alloy.Microsample tensile testing of Pt-5% Cu shows that low-temperature heat treatment ofpreviously cold-worked specimens results in an increase in yield strength and tensilestrength, with a maximum in strength occurring after heat treatment at 300ºC; but ductilityis unchanged.

Platinum Metals Rev., 2007, 51, (2), 78–82

DOI: 10.1595/147106707X187173

treatments were carried out at temperaturesbetween 100ºC and 1000ºC for 3 hours, followedby furnace cooling.

After heat treatment all specimens wereground, then polished to a 1 μm surface finish. AZwick microhardness tester with a standardVickers indenter was used at 100 gf load to mea-sure the hardness of the specimens for eachcondition. Metallography specimens were etchedelectrolytically in a solution of 25 g sodium chlo-ride, 20 cm3 hydrochloric acid 32% and 65 cm3

distilled water; an alternating current was used ata potential of 10 V, for an etching time of 40 to75 seconds, with a stainless steel anode and agraphite cathode used to complete the circuit.

Tensile specimens were ground and polished to a1 μm surface finish on both sides. Measuredgauge widths were between 440 μm and 510 μm,and gauge thicknesses were between 125 μm and350 μm. Tensile tests were carried out using amicrosample tensile tester at a strain rate of 10–3 s–1.

ResultsFigure 1 shows representative engineering

stress/engineering strain curves for Pt-5% Cu inthe cold worked condition and after heat treat-ment at several temperatures. Figure 2 is anextract from Figure 1, showing results for the coldworked condition and after heat treatment at

Platinum Metals Rev., 2007, 51, (2) 79

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35Engineering Strain

Eng

inee

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ss, M

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0

cw cw + 100°C cw + 200°C cw + 300°C cw + 400°Ccw + 500°C cw + 600°C cw + 700°C cw + 800°C cw + 1000°C

Fig. 1 Stress-strain curvesfor Pt-5% Cu (cw = coldworked)

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35Engineering Strain

cw cw + 300°C cw + 1000°C

1200

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Fig. 2 Extract from Figure1: Stress-strain curves forPt-5% Cu in the 90% coldworked condition and afterheat treatment at 300ºC(highest increase instrength) and 1000ºC (cw= cold worked)

Platinum Metals Rev., 2007, 51, (2) 80

300ºC (highest increase in strength) and 1000ºC.Relative to the cold worked condition (90%reduction in rolling), yield strength and tensilestrength are observed to increase after heat treat-ment at between 100ºC and 500ºC. Heattreatment at 600ºC and above results in a decreasein strength and an increase in ductility.

Figure 3 shows the yield strength, tensilestrength and hardness of Pt-5% Cu as a functionof heat treatment temperature. Strength and hard-ness consistently increase after heat treatmentbetween 100ºC and 400ºC, as seen in Figure 1.Heat treatment at 500ºC results in little change inthe properties relative to the cold worked condi-tion; heat treatment at higher temperatures resultsin a decrease in strength and hardness. Measuredvalues for hardness, yield strength, tensilestrength and ductility (percentage elongation after

fracture) are given in Table I.Figure 4 shows the microstructure of the

Pt-5% Cu alloy before and after heat treatment.After heat treatment at 300ºC (the temperaturewhich resulted in the greatest increase instrength), the heavily deformed and elongatedgrains from the cold working are unchanged.After heat treatment at 700ºC, a recrystallisedgrain structure is observed, and after heat treat-ment at 1000ºC grain growth has occurred.

DiscussionCold work followed by heat treatment below

500ºC results in an increase in the yield strength,tensile strength and hardness of Pt-5% Cu. Theseresults are consistent with a previous report (6) ofan increase in the hardness of this alloy due to anordering transformation. For this alloy the critical

Fig. 3 Mechanical properties ofPt-5% Cu vs. heat treatment temper-ature after heat treatment for 3 hours

(a) (b)

(c) (d)

200 μm 200 μm

200 μm 200 μm

Fig. 4 Micrograph of Pt-5% Cu: (a) 90% coldworked, then heat treatedfor 3 hours at: (b) 300ºC;(c) 700ºC; (d) 1000ºC

0 200 400 600 800 1000 1200Heat Treatment Temperature,°C

Stre

ngth

, MP

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Tensile Strength, MPa Yield Strength, MPa Hardness, HV

350

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Platinum Metals Rev., 2007, 51, (2) 81

ordering temperature, Tc, is around 500ºC (4, 6),which means that a transformation to an orderedstate can be expected to occur as a result of heattreatment below 500ºC. This does not result inany change in grain structure relative to the coldworked state, as seen in Figure 4(b); the observedchange in properties thus arises entirely from theordering transformation.

Hardness is expected to be proportional toyield strength (8); this is observed in the resultsshown in Figure 3. Hardness measurements alone,however, do not provide a complete characterisa-tion of mechanical properties, which requirestensile testing. It is of interest to note that,although heat treatment between 200ºC and400ºC results in very similar hardness increases,there is a clear maximum in yield strength and ten-sile strength after heat treatment at 300ºC.

Generally, ductility is expected to decrease asstrength increases and vice versa: Figures 1 and 2

and Table I show that when strength decreases asa result of recovery and recrystallisation, ductilityincreases as expected. Heat treatment of coldworked Pt-5% Cu below 500ºC, however, canresult in a significant increase in strength relativeto the cold worked value, but ductility remainsunchanged. The ordering transformation whichoccurs below 500ºC accordingly has an unexpect-ed effect on tensile properties, in that an increasein strength is achieved without further lossof ductility.

Heat treatment at temperatures above Tc doesnot result in an ordering transformation. In thisalloy, heat treatment at temperatures above 500ºCleads to recovery and recrystallisation, as shown inFigure 4(c), which results in a decrease in strength.Increasing heat treatment temperature above700ºC results in grain growth, and a consequentfurther decrease in strength and increase in ductil-ity are observed.

Table I

Mechanical Properties of Pt-5% Cu Alloy in the Cold Worked Condition, and After Heat Treatment forThree Hours

Heat treatment Vickers hardness, Yield strength, Ultimate tensile Ductility,temperature, ºC HV MPa strength, MPa %

No heat treatment; 259 ± 11 896 ± 40 910 ± 53 3 ± 190% cold worked 240 (3) 970 ± 100 (3) 990 ± 90 (3) 2 ± 1 (3)

241 (6)

100 292 ± 9 932 ± 48 972 ± 56 3 ± 1

200 320 ± 9 994 ± 89 1056 ± 91 2 ± 1360 (6)

300 322 ± 8 1048 ± 55 1153 ± 43 2 ± 1

400 320 ± 12 993 ± 60 1055 ± 61 2 ± 1

500 282 ± 11 887 ± 14 952 ± 18 3 ± 1

600 251 ± 16 711 ± 18 805 ± 18 4 ± 1

700 222 ±14 652 ± 13 749 ± 20 10 ± 3

800 172 ± 8 245 ± 40 477 ± 24 30 ± 7150 (6) 280 ± 30 (3) 530 ± 40 (3) 36 ± 9 (3)

1000 158 ± 7 228 ± 31 469 ± 27 31 ± 8

Values in italics are from References (3) and (6). Note that in Reference (3) heat treatment time is six hours

Platinum Metals Rev., 2007, 51, (2) 82

ConclusionsThe hardness and strength of cold worked

Pt-5% Cu can be significantly enhanced by heattreatment for three hours at temperaturesbetween 200ºC and 400ºC, without reducing duc-tility. A short, low-temperature heat treatmentcan thus significantly enhance the mechanicalproperties of Pt-5% Cu jewellery items whichhave been produced by cold working.

AcknowledgementThe financial support of the National Research

Foundation and the Sainsbury Trust is gratefullyacknowledged.

References1 G. Normandeau and D. Ueno, ‘Understanding

Heat Treatable Platinum Alloys’, 1999 PlatinumDay Symposium, Vol. 5, Platinum Guild

International USA, Los Angeles, U.S.A., 1999;http://www.pgi-platinum-tech.com/pdf/V5N7.pdf

2 R. Lanam and F. Pozarnik, ‘Platinum AlloyCharacteristics: A Comparison of ExistingPlatinum Casting Alloys With Pt-Cu-Co’, 1997Platinum Day Symposium, Vol. 3, Platinum GuildInternational USA, Los Angeles, U.S.A., 1999;http://www.pgi-platinum-tech.com/pdf/V3N1w.pdf

3 K. M. Jackson and C. Lang, Platinum Metals Rev.,2006, 50, (1), 15

4 A. Schneider and U. Z. Esch, Z. Elektrochem. Angew.Phys. Chem., 1944, 50, 290

5 R. Miida and D. Watanabe, J. Appl. Cryst., 1974, 7,(1), 50

6 M. Carelse and C. I. Lang, Scripta Mater., 2006, 54,(7), 1311

7 Standard Test Methods for Tension Testing ofMetallic Materials, ASTM 370, E8-93, pp. 130–149

8 D. A. LaVan and W. N. Sharpe, Exp. Mech., 1999,39, (3), 210

The Authors

ChumaniMshumi ispursuingdoctoralresearch atthe Centre forMaterialsEngineering,University of

Cape Town (UCT), South Africa.

Candy Lang isProfessor in theDepartment ofMechanicalEngineering,UCT. She isleader of theteam developingnovel platinum

alloys for the jewellery industry.

A one day meeting on the Successful Scale-Upof Catalytic Processes took place on the 5thOctober 2006. Organised by the Applied CatalysisGroup of the Royal Society of Chemistry (1), it washosted by Davy Process Technology Ltd (2) atStockton-on-Tees, U.K. In all, almost 60 delegatesattended the meeting, with over 80% being fromindustry. There were a total of six oral presenta-tions, of which three covered work using platinumgroup metal (pgm) catalysts, especially palladium.

Cost-Effective Palladium-CatalysedProcesses

Ian Archer (Ingenza Ltd, U.K.) described thescale-up of a novel process for the synthesis of pureenantiomers of amino acids and amines. Theprocess uses chemo-enzymatic deracemisation togenerate a single enantiomer from a racemic mixture(3). A biocatalysed enantioselective oxidation is usedto convert one enantiomer of the amino acid oramine to a non-chiral imine, which is then reducednon-selectively back to the amine, see Scheme I.

In order to develop commercially viableprocesses, Ingenza have needed to develop cost-effective pgm-catalysed reductions. For example,for the stereoinversion of D-2-aminobutyric acid to

L-2-aminobutyric acid, over 40 different pgm cata-lysts were screened for use in the reduction step.Initially, a Pd/C catalyst was identified as the mostpromising candidate, but the cost proved to be toohigh, even with reuse of the catalyst and recoveryof the Pd metal. Development of a second genera-tion Pd catalyst enabled the costs to be reduced toan economically attractive level.

David Johnson (Lucite International Ltd, U.K.)presented a paper which outlined the developmentand scale-up of their new route to methylmethacrylate (MMA). This novel process employstwo catalytic reaction steps:(a) carbonylation of ethene in methanol to produce

methyl propionate, using a homogeneous palla-dium-phosphine catalyst;

(b)condensation of methyl propionate withformaldehyde over a basic heterogeneous cata-lyst (Cs/SiO2).

There were several issues which needed to beaddressed during the scale-up of the overallprocess, including product separation and purifica-tion issues, catalyst manufacture, and thedemonstration of product quality.

Although the initial research had identified asuitable carbonylation catalyst with a turnover

83

Successful Scale-Up of Catalytic ProcessesPALLADIUM IN INDUSTRIAL CATALYSIS

Reviewed by Chris MitchellHuntsman Polyurethanes, Everslaan 45, B-3078 Everberg, Belgium; E-mail: [email protected]

Platinum Metals Rev., 2007, 51, (2), 83–84

DOI: 10.1595/147106707X193464

NH2

CO2HR1

biocatalyst(oxidase)

chemocatalyst(reductant )

Accumulatesduring reaction

NH2

CO2HR1

NH2

CO2HR1

NH2

CO2HR1

biocatalyst(oxidase)

chemocatalyst(reductant )

Accumulatesduring reaction

NH2

CO2HR1

NH2

CO2HR1

NH2

CO2HR1

NH2

CO2HR1

biocatalyst(oxidase)

chemocatalyst(reductant )

Accumulatesduring reaction

NH2

CO2HR1

NH2

CO2HR1

NH2

CO2HR1

NH2

CO2HR1

Biocatalyst(oxidase)

Chemocatalyst(reductant)

Accumulatesduring reaction

Scheme I Deracemisation of aminoacids by a chemo-enzymaticprocess, using a palladium catalystas reductant

number of ~ 50,000 and selectivity in excess of99.9%, the phosphine ligand was not commercial-ly available at that time and the existing synthesiswould have been prohibitively expensive. Thecompany therefore had to develop alternative syn-thesis routes in conjunction with Professor PeterEdwards at Cardiff University, U.K.

In order to achieve recovery of the methyl pro-pionate, the process was designed to operate onthe product-rich side of an azeotrope. This had theunfortunate effect of lowering catalyst activity.Further, it was discovered that carbon monoxidepoisoned the catalyst; this necessitated operatingthe process with a high ethene:CO ratio. However,it was then discovered that addition ofpolyvinylpyrrolidone was able to stabilise the cata-lyst activity and also improve the Pd recovery.

Optimised Palladium CatalystPerformance

Kevin Treacher (Reaxa Ltd, U.K.) presented anoverview of the use of Pd EnCatTM catalysts fororganic synthesis. A range of different catalysts areavailable based on palladium(II) acetate in combina-tion with a variety of phosphine ligands andencapsulated in porous polyurea beads. These cata-lysts can be used for a variety of synthetic reactionssuch as Suzuki coupling, Heck reactions and hydro-genations. A catalyst with encapsulated Pdnanoparticles is also available.

A case study of the Suzuki coupling of phenyl-boronic acid and 4-fluoro-1-bromobenzene to give4-fluorobiphenyl was presented. Initial screening ofdifferent bases in conjunction with a Pd(II)EnCatTM BINAP30 catalyst was carried out in abatch reactor. The best performing system fromthese tests was then evaluated in three different flowreactor configurations: a continuous stirred tankreactor (CSTR), a tubular reactor and two sequentialtubular reactors. With this latter configuration, opti-mised system productivity in excess of 150 g per gcatalyst was achieved; the catalyst still exhibited astable performance after 250 hours operation.

Other ApproachesIn addition to these three presentations which

included pgm catalysis, the other talks described the

approaches to catalytic process scale-up used bydifferent companies. Steven Colley (Davy ProcessTechnology Ltd, U.K.) outlined their approach toprocess scale-up, with the focus being on the use ofmini-plants to generate the data required forprocess design, and the issues that can be encoun-tered. Simon Froom (BP, U.K.) described thescale-up of BP’s Avada® process (4) for the manu-facture of ethyl acetate from acetic acid and ethene.Professor Wölfgang Holderich (RWTH-AachenUniversity, Germany) described the developmentof two new processes: the manufacture of caprolac-tam via the acid-catalysed Beckmann rearrangementof cyclohexanone oxime; and the production ofbiodegradable lubricants via the esterification ofnatural oils and glycerides.

ConclusionOverall, the meeting was a resounding success

and provided many valuable insights into the vary-ing methodologies which can be used in thescale-up of catalytic processes, whether for moretraditional large-scale manufacturing processes orfor smaller-scale speciality chemical synthesis. TheApplied Catalysis Group of the Royal Society ofChemistry plans to hold a one-day symposium onthe Challenges in Catalysis for Pharmaceuticals andFine Chemicals at the Society of Chemical Industry,London, U.K., on 6th November 2007 (1).

References1 RSC Applied Catalysis Group:

http://www.rsc.org/Membership/Networking/InterestGroups/catalysis/

2 Davy Process Technology Ltd:http://www.davyprotech.com/

3 I. Fotheringham, I. Archer, R. Carr, R. Speight andN. J. Turner, Biochem. Soc. Trans., 2006, 34, (2), 287

4 B. Harrison, Platinum Metals Rev., 2001, 45, (1), 12

Platinum Metals Rev., 2007, 51, (2) 84

The ReviewerChris Mitchell is the Catalytic ChemistryExpert for Huntsman Polyurethanes andworks within their Global Technologyorganisation, based in Belgium. His mainfocus is the development of heterogeneouscatalytic processes, includinghydrogenations, acid catalysis and

epoxidations/oxidations. He has particular interests in thecharacterisation of catalysts, reaction kinetics and catalystdeactivation. Among the platinum group metals, he has workedwith platinum, palladium and ruthenium.

Research into organic light-emitting devices(OLEDs) has been intense over the past 20 years,since the landmark report in 1987 of electricallygenerated light emission from aluminium tris(8-hydroxyquinoline) (1). Regarded by many asthe next generation of display screen technology,OLEDs may eventually rival liquid crystal displaysand conventional inorganic LEDs, and commer-cialisation has indeed recently begun. The keyhallmark of research in the field is its interdisciplinar-ity, embracing synthetic chemists, physicists,physical chemists, and electronic and optical engi-neers, and elegantly spanning fundamental andapplied science.

This new book, edited by two leading Germanpolymer chemists, brings together contributionsfrom a dozen of the leading research groups in thefield, representing all of the aforementioned disci-plines. The result is a coherent account of thefundamental principles behind the science ofOLEDs and of the materials required for their fab-rication. The book is not for the novice or theundergraduate student; on the contrary, the major-ity of the chapters assume familiarity with theunderlying physical concepts, and the book isprobably aimed primarily at those already workingin the field, or intending to enter into it seriously.Perhaps rather disappointingly for the platinumgroup metal chemist, coverage of iridium- andplatinum-based electrophosphorescent dopants islimited to just one chapter that focuses primarilyon the triplet energy transfer processes rather thanon the chemistry. This is despite the fact that theirdevelopment as ‘harvesting agents’ for the other-wise non-emissive triplet states has been a buoyantand high-profile area of research in the field over

the past six years or so. (Reviews and representa-tive papers in this field include References (2–5)).

The book is logically structured, appropriatelybeginning with a description of the science behindinorganic LEDs – already very well established –before turning to a sequence of chapters describingthe physical processes underlying charge injectionand light emission in conjugated polymers. This isfollowed by more chemistry-oriented chaptersdescribing the synthesis and properties of electro-luminescent polymers, charge-transporting/charge-blocking materials and dendrimeric sys-tems, and the use of crosslinking strategies inmaterials processing. The final three chaptersaddress hybrid inorganic-organic systems, includ-ing a useful discussion of nanocrystalline emission(colloidal quantum dots); promotion of tripletemission using phosphorescent dyes, such ascyclometallated iridium complexes and platinumporphyrins (see Figure 1 for representative struc-tures); and organic semiconductor lasers.

Any new book on such an interdisciplinary sub-ject as OLEDs faces the challenge of facilitatingmutual understanding of the different disciplines,and aiding scientists from one discipline to gain anappreciation of the issues facing those tackling thesubject from a different perspective. The bookdoes achieve this to an admirable extent, althoughsome chapters are, not surprisingly, more success-ful than others at doing so; the interested chemistmay find himself at sea with some of the physicschapters and perhaps vice versa. On the other hand,this is simply inevitable given the depth of cover-age: the volume overall is unified in structure andcomprehensive in scope.

The book is well produced, with clear diagrams,

85

“Organic Light-Emitting Devices:Synthesis, Properties and Applications”EDITED BY K. MÜLLEN (Max-Planck-Institute for Polymer Research, Mainz, Germany) AND U. SCHERF (Bergishe Universität

Wuppertal, Germany), Wiley-VCH, Weinheim, Germany, 2006, 426 pages, ISBN 978-3-527-31218-4, £100.00, €150.00,

U.S.$175.00

Reviewed by J. A. Gareth WilliamsDepartment of Chemistry, University of Durham, South Road, Durham DH1 3LE, U.K.; E-mail: [email protected]

Platinum Metals Rev., 2007, 51, (2), 85–86

DOI: 10.1595/147106707X192537

Platinum Metals Rev., 2007, 51, (2) 86

many of which are helpfully reproduced in colour.A few chapters, particularly some of those on syn-thetic aspects such as a key chapter on thesynthesis of electroluminescent polymers, were alittle disappointing in that they surveyed little post-2000 work. Given the rapid progress of the field,surely it is important to ensure that a volume ofthis sort is as up-to-date as possible at the time ofpublication. It was also a little surprising to findthat while nine of the twelve chapters employ acommon referencing style, three use two otherstyles. Though a minor peccadillo, one might rea-sonably have expected a unified style. I was alsobemused to read in the publishing and retailingpublicity accompanying the book that Ching Tangof Eastman Kodak was one of the contributors,even though there is no contribution from him oranyone else at Eastman Kodak.

Comparison with Other TextsThere are few comprehensive books available

on OLEDs. Kalinowski’s recent volume “OrganicLight-Emitting Diodes: Principles, Characteristicsand Processes” (6) is geared more exclusively to thephysics and engineering aspects and, while compre-hensive in its treatment of the fundamental physics,is a less tractable work. More closely comparable isShinar’s edited review “Organic Light-EmittingDevices: A Survey” (7), which covers some materi-al common to the present volume. Nevertheless,Müllen and Scherf’s contribution is an altogetherwider ranging survey, bringing in a more diverserange of systems, and has a fresher feel to it. Yet,from the admittedly biased perspective of a plat-inum group metal chemist, the rather cursorytreatment of only a very few iridium and platinum

complexes as phosphorescent dopants in Chapter11 does seem something of a lost opportunity. Theinorganic chemist hoping for a survey of the latestdevelopments with respect to this aspect wouldneed to look elsewhere, and in this context, perhapsa forthcoming review volume edited by Balzani andCampagna on the photophysics of metal complex-es may help to fill the gap (8).

ConclusionIn summary, “Organic Light-Emitting Devices”

provides a comprehensive, wide-ranging andauthoritative account of both the science behindOLEDs and of recent developments in the materi-als used to produce them. It will make a valuablecontribution to the research field for chemists,physicists and optical engineers alike.

References1 C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett.,

1987, 51, (12), 9132 M. A. Baldo, M. E. Thompson and S. R. Forrest,

Pure Appl. Chem., 1999, 71, (11), 20953 R. C. Evans, P. Douglas and C. J. Winscom, Coord.

Chem. Rev., 2006, 250, (15–16), 20934 A. Tsuboyama, H. Iwawaki, M. Furugori, T.

Mukaide, J. Kamatani, S. Igawa, T. Moriyama, S.Miura, T. Takiguchi, S. Okada, M. Hoshino and K.Ueno, J. Am. Chem. Soc., 2003, 125, (42), 12971

5 I. R. Laskar and T.-M. Chen, Chem. Mater., 2004, 16,(1), 111

6 J. Kalinowski, “Organic Light-Emitting Diodes:Principles, Characteristics and Processes”, MarcelDekker, New York, 2005

7 “Organic Light-Emitting Devices: A Survey”, ed. J.Shinar, Springer-Verlag, New York, 2004

8 “Photochemistry and Photophysics of CoordinationCompounds”, eds. V. Balzani and S. Campagna, tobe published in “Topics in Current Chemistry”,Springer, Berlin/Heidelberg, 2007

(a) (b) (c)

Fig. 1 Structures of some of the platinum group metal electrophosphorescent dopants discussed in the book:(a) fac-tris(phenylpyridine) iridium, Ir(ppy)3; (b) bis(2-(2′-benzothienyl)-pyridinato-N,C3′)iridium(acetylacetonate),Ir(btp)2(acac); (c) platinum(II) octaethylporphyrin, PtOEP

The role of the support in heterogeneous pgmcatalysis is frequently crucial. The choice of sup-port (usually from carbons, silicas, aluminas,zeolites or other inorganic compounds) caninfluence the selectivity, activity and longevity ofthe catalyst. Though catalysts of exceedingly highefficiency are regularly produced, the long-termstability of the pgm system, especially underharsh conditions, can be a problem. In addition,rising pgm demand and costs are incentivestowards achieving lower metal loadings and high-er activity. Some of these issues can be addressedusing pgm-containing perovskites.

The perovskite structure is a highly versatileand widespread mineral form, of great impor-tance in nature and technology (1). They werefirst described and named by the Russian miner-alogist Count Lev Aleksevich von Perovski, whodiscovered the naturally occurring mineral calci-um titanate (CaTiO3). The common feature ofperovskites is the generic structure ABO3, with Aand B drawn from a range of metals, subject tocertain size constraints. The range of possiblecation substitutions is limited by constraints onthermodynamic stability, as represented in termsof the Goldschmidt factor (1). The ideal per-ovskite has a cubic structure, with an octahedralconfiguration of the oxygen atoms at each corneraround the B atom (Figure 1) (2). The structurecan tolerate significant non-stoichiometry andpartial substitution. By varying the types ofatoms at the A and B sites, the resulting structure

will deviate from the ideal depending on the con-stituent properties (for example, ionic radius,valence and electronegativity). Fractional incor-poration of several different elements at specificsites can be achieved through careful design.With non-stoichiometric substitutions, oxygenion con- ductivity may be induced by oxygenvacancies (3). Control of substitution in the per-ovskite matrix is important for tuning theproperties of the material. For instance, catalyticactivity is apparently determined primarily by thespecies at the B site (4).

This review focuses on methods of prepara-tion of pgm-substituted perovskites, on theirperformance as catalysts, and on their principalcurrent and potential uses. As opposed to per-ovskites used as a support for a metal catalyst,the main consideration here is of perovskitescontaining the pgm within the mineral lattice; thelatter require initial synthesis.

87

Platinum Group Metal Perovskite CatalystsPREPARATION AND APPLICATIONS

By Thomas ScreenReaxa Ltd, Hexagon Tower, Blackley, Manchester M9 8ZS, U.K.; E-mail: [email protected]

Perovskites are a large class of minerals, both naturally occurring and synthetically produced,with important technological applications. In this article, platinum group metal (pgm) perovskitesare introduced as a relatively new catalyst material. Due to their high activity, versatile andstable structure and low pgm content, they can offer advantages over conventional catalysts.Some of the typical applications and preparation methods of pgm perovskites are reviewed,with particular focus on their potential use to address current challenges concerningautocatalysts, and in organic chemistry.

Platinum Metals Rev., 2007, 51, (2), 87–92

DOI: 10.1595/147106707X192645

Fig. 1 The perovskite structure (2)

A site (La)B site (Fe, Co)B site (Pd)Oxygen

A

OB

Platinum Metals Rev., 2007, 51, (2) 88

Preparation of PGM PerovskitesSynthetic perovskites may be prepared by a

range of methods. Routes similar to those for theproduction of other ceramics include oxide sinter-ing, combustion synthesis and sol-gel methods (5).Oxide sintering entails heating the powders of theconstituents together at high temperature. A vari-ant of this is combustion synthesis, where theignition of a combustible component provides theenergy for the formation of the perovskite miner-al. Aqueous combustion synthesis has been used toprepare platinum- and ruthenium-containing per-ovskites, which were subsequently investigated asanode catalysts in direct methanol fuel cells(DMFCs) (6), a promising technology for portablepower applications. Perovskite oxides could pro-vide an alternative to more expensiveplatinum-based catalysts and a solution to prob-lems of CO poisoning. It was found that SrRuO3

doped with platinum added at the combustion syn-thesis stage gave performance comparable to thatof standard platinum-ruthenium catalysts.

A versatile synthesis of perovskites is bycoprecipitation from soluble precursors of theconstituent metals in a suitable solvent, followedby solvent removal and heat treatment. S.Petrovic et al. prepared palladium-containing per-ovskites of the form LaTi0.5Mg0.5–xPdxO3 (0 < x <0.1) by annealing ethanol solutions of the precur-sors in a nitrogen flow at 1200ºC (7). It was foundthat at least a proportion of the palladium was notincorporated into the perovskite structure, butexisted as a separate metallic phase, which wasbelieved to influence the catalytic activity. Thesamples were tested in the catalytic combustionof methane, with perovskites with lower palladi-um loading (x = 0.05) showing higher activity attemperatures over 500ºC, attributed to the finerdispersion of the palladium in the lattice. Otherpgm-containing perovskites have been tested forthe combustion of light hydrocarbons, whichfinds important application in volatile organiccompound abatement. Perovskites have beensuggested as good candidate catalysts (8), forexample lanthanum/cobalt perovskitesLa1–xMxCoO3 (M = Ag, Pd, Pt; 0.08 ≤ x ≤ 0.2) forcatalytic methane combustion (9).

Use of PGM Perovskites inAutocatalysts

Reflecting the trend in pgm usage as a whole(10), the major current use of pgm perovskites is incatalytic converters for cars. Catalytic convertershave been in use since the 1970s (11) and usuallyconsist of supports of high surface area, coated withpgms. These systems have proved very successful atsimultaneously converting CO into CO2, unreactedhydrocarbons (HCs) into CO2 and water and reduc-ing nitrogen oxides (NOx) to nitrogen but, atpresent, the automotive industry faces severalchallenges.

The conventional means to meet tightening leg-islative emissions control targets is simply toincrease the amount of pgm in the autocatalyst. Theneed to guarantee catalyst performance over thetypical vehicle lifetime of 80,000 km also means thatexcess metal must be added, since the performanceof the catalyst drops off over time. In the harsh con-ditions experienced in the exhaust stream withtemperatures up to 1000ºC, the metal in the catalystis prone to deactivation by sintering, leading to areduction in surface area and hence catalytic activi-ty. This issue of ageing performance is expected tobecome even more important, as the Euro 5requirements (12) also include an extension of cata-lyst lifetime to 160,000 km. The robustness ofperovskites and the low pgm content typical inpgm-doped perovskites combine to provide a novelsolution to these problems.

The palladium-containing perovskiteLaFe0.77Co0.17Pd0.06O3, synthesised by coprecipitationof the metal nitrates and perovskite-supported pal-ladium (LaFe0.8Co0.2O3/Pd at the same palladiumloading), were compared for their preparation meth-ods and catalytic behaviour by K. Zhou et al. (13).Both showed excellent three-way catalytic activity,with the supported palladium somewhat better; thisis attributed to greater ease of reduction of the sur-face palladium to generate the active catalystcompared to the palladium in the perovskite, whichwas shown to be dispersed throughout the lattice byX-ray diffraction (XRD) and transmission electronmicroscopy (TEM) studies. Platinum-promoted lan-thanum manganate type perovskite catalysts,prepared by coprecipitation of the metal hydroxides

Platinum Metals Rev., 2007, 51, (2) 89

using aqueous ammonia, have also shown potentialfor automobile applications (14), and rutheniumperovskites LaySr1–yRuxCr1–xO3 (y = 0.7; 0.025 ≤ x≤ 0.100) have been tested as candidates for leanNOx automotive emission control (15).

Work by H. Tanaka and coworkers at DaihatsuMotors has produced a range of palladium per-ovskites from soluble alkoxide precursors (16). Thesalts of the constituent metals were combined intoluene in the desired proportions. Hydrolysis ofthe salts with water gave a precipitate which wasisolated and dried in air to obtain the perovskites.For example, palladium-containing perovskiteLaFe0.57Co0.38Pd0.05O3 was prepared from lan-thanum, cobalt and iron ethoxyethylates withaqueous palladium nitrate solution (16), andLaFe0.95Pd0.05O3 was synthesised from base metalethoxyethylates and palladium acetylacetonate (17).Recently, using the same method, platinum- andrhodium-containing perovskites CaTi0.95Pt0.05O3 andLaFe0.95Rh0.05O3 respectively, were also prepared(18, 19).

The use of these perovskites in autocatalystscentres around a novel mechanism which exploitsthe inherent fluctuations between reducing and oxi-dising atmospheres in the exhaust gases. In amodern three-way catalytic converter, the exhauststream over the catalyst is controlled to give optimalconditions for the required reactions. This isachieved by control of the air-to-fuel ratio in the

engine, using an oxygen sensor which continuallymonitors the exhaust gas composition and feedsinformation back to the air intakes. Inevitable timelags between sensing and adjustment lead to alter-nations between conditions which are oxygen richand oxygen poor relative to the ideal stoichiometry.

As synthesised, and under oxidising exhaustconditions, the palladium in LaFe0.95Pd0.05O3 existsas a solid solution dispersed throughout the per-ovskite lattice. However, under reducing conditionsand the high temperatures in the exhaust stream,the palladium segregates to form metallic nanopar-ticles (1 to 3 nm in size). This process was shown tobe reversible, with the palladium redispersing in thelattice on a return to oxidising conditions (Figure 2)(19). This phenomenon accounts for the excellentageing performance of the perovskite autocatalysts.In conventional autocatalysts using pgm dispersedon a support, sintering over time to ever largermetal particles at the prevailing elevated tempera-tures leads to a reduction in catalytic activity. In theperovskite catalyst, the oxidising/reducing cyclemaintains the catalytic activity by regenerating thepalladium metal nanoparticles and preventing metalparticle growth. This has led to the catalysts beingdubbed ‘intelligent’, due to their capacity to react totheir environment, resulting in greater efficiency(16). Recently, the same effect has been shown inplatinum and rhodium perovskites, extending theconcept to the full range of pgms commonly used

Dispersed on conventional ceramic

Restored toatomic level

Segregation of1 to 3 nm Pd particles

Atomic levelcomplex

Enlargement ofpgm

Further enlargementand deterioration

Perovskite Self-regeneration!

Oxidising environment(initial condition)

Reducingenvironment

Oxidisingenvironment

Passageof time

pgm

‘Intelligent’catalyst

Conventionalcatalyst

Fig. 2 Schematic of the operation of self-regenerating pgm perovskite autocatalysts (2)

Platinum Metals Rev., 2007, 51, (2) 90

in autocatalysts (18, 19).The perovskite-containing catalytic converters

are also highly active. A vehicle equipped with sucha high activity perovskite autocatalyst achieved theJ-ULEV (Japan Ultra Low Emissions Vehicle) emis-sions standard in 2002, demonstrating that pollutantlevels more than 50% below those required by cur-rent legislation were measured. Another majoradvantage of perovskite autocatalysts is the reducedmetal content compared with that of conventionalautocatalysts of similar activity. Reductions of 70 to90% have been reported possible (20), translatinginto potentially significant cost savings.

PGM Perovskites in OrganicSynthesis

Reactions catalysed by pgms, such as cross-cou-plings and hydrogenations, are becomingincreasingly prevalent in organic synthesis. Wherethe pgm-catalysed steps are part of the synthesis ofactive pharmaceutical intermediates (APIs), thereare stringent limits on permissible metal contamina-tion, driving a requirement to minimise the releaseof metals into the process (21). Conventionalapproaches to achieve this are to move from homo-geneous to heterogeneous catalysts, or to‘heterogenise’ catalysts via processes such as immo-bilisation (22) or microencapsulation (23).

This is another area where the stability, robust-ness and low pgm content of perovskite catalystsfacilitate their application. The perovskite mineralswhich are air-stable powders are well suited to use inthe chemical laboratory. Professor Steven Ley and

coworkers at the University of Cambridge, U.K.,have tested palladium-containing perovskites inorganic transformations which are otherwise carriedout with conventional palladium catalysts (24).

Use of perovskite catalysts in a standard Suzukicoupling, between an aryl bromide and aryl boronicacid, showed that the reaction was catalysed withsimilar rates by a wide range of perovskites contain-ing 5 at.% palladium (25). It also demonstrated thatpalladium was an essential component for success-ful conversion, and that the oxidised form of theperovskites worked better under the reaction condi-tions. Working with the best-performing catalyst,LaFe0.57Co0.38Pd0.05O3, M. D. Smith et al. extendedthe perovskite-catalysed Suzuki reaction to encom-pass a wide range of different substrates, includingaryl iodides and bromides, heteroaryl halides andaryl and alkenyl boronic acids (Scheme I) (26). Theapplication of microwave heating also enabled cou-pling to aryl chlorides.

The use of a copper-palladium perovskiteLaFe0.57Cu0.38Pd0.05O3 allowed the extension of per-ovskite-catalysed organic chemistry to theSonogashira coupling reaction of aryl halides andacetylenes (27), again giving good yield across arange of aryl bromide and iodides.

There has been considerable work on ascertain-ing the mechanism by which perovskites function inorganic reactions. The lower temperatures involved(typically 80ºC) preclude the type of self-regenera-tion seen in autocatalysts. Investigation focused onwhether the reaction proceeded via a homogeneousor heterogeneous mechanism, and evidence has

N

Br

Br

OMe

B(OH)2

B(OH)2

OMe

S I

B(OH)2

MeO

I B(OH)2

N

OMe

MeO

MeO

S

85 %, 18 h

89 %, 1 h

89 %, 18 h

92 %, 18 h

Scheme I A selection ofreactions catalysed by pal-ladium perovskite(Conditions:LaFe0.57Co0.38Pd0.05O3 (0.05mol% Pd), 3 eq. K2CO3, 1.5eq. boronic acid, 1:1IPA:H2O, 80ºC) (26)

92%, 18 h

85%, 18 h

89%, 1 h

89%, 18 h

+

+

+

+

Platinum Metals Rev., 2007, 51, (2) 91

been built up by several methods (25). Removal ofthe bulk catalyst by filtration at partial reaction,followed by returning the filtrate to the reactionconditions, showed that the reaction progressed tosignificantly higher conversions in the absence ofthe solid catalyst. This demonstrated that an activesolution palladium species was formed, a conclu-sion supported by solution and solid-phasecatalyst poisoning studies. Performance in a three-phase test, employing solution and solid-supported substrates, provides further evidence tosupport the hypothesis of an active solutionspecies, but also demonstrated that an aryl halidemust be present in the solution phase for the reac-tion to proceed.

Collation of the evidence (25) led to the pro-posed mechanism shown in Figure 3. The initialstep is a reduction of the Pd(III) or other high-valent palladium species in the perovskite, possiblyby the solvent, to form a surface-bound Pd(0)species. This is next taken into solution by oxida-tive addition to the aryl halide. The couplingreaction can now proceed through a fairly conven-

tional solution catalytic cycle, at the end of whichthe palladium either remains in solution to contin-ue the reaction or is readsorbed onto theperovskite surface.

The release of highly active palladium into solu-tion from the perovskite explains a very efficientcatalytic turnover, with loadings of less than 0.05mol% palladium sufficient. Recapture of the palla-dium by the perovskite at the end of the reactioncycle accounts for the extremely low residual pal-ladium levels found in the crude reaction products.Palladium contents of less than 2 ppm were foundin a Suzuki coupling product (26). The combinedbenefits of a highly efficient catalyst with low pgmcontent and very low levels of metal contamina-tion make these attractive catalysts for chemicalapplications, especially synthesis of pharmaceuticaland electronic materials, where exclusion of cata-lyst residues is essential. Furthermore, theperovskite catalysts have been shown to be recy-clable (26), leading to even greater potential costsavings over catalysts which must be disposed ofafter a single use.

Oxidised perovskite

catalyst

Reduced Pd/Cometallic

f.c.c. cluster

H2, 800ºC

O2, 800ºCPd(III)

Pd(III)

Pd(II)

Pd(0)

Pd(0)

Pd(0)

Pd(0)

Solid

Ar1 Br

Ar1- BrAr2B(OH)2

Ar1- Br

Ar1- Ar2

Fig. 3 Proposed mechanism for catalytic activity of pgm perovskites in organic synthesis (25)

ConclusionsThe pgm-containing perovskites constitute an

active and expanding area of research. The poten-tial and versatility of pgm-containing perovskites ascatalysts is shown by the range of applications inwhich they have been tested – from catalyticcombustion to organic synthesis. Their sturdymineral structure and stability offer advantageswherever high temperatures are involved and insome cases, such as self-regenerating autocatalysts,give distinct benefits where other metal supportsare deactivated over time. The high activity oftenassociated with pgm perovskites, combined withthe low loadings of pgms required, result in theiroffering significant potential savings in metal costs.In the organic chemistry laboratory, where the

stable, easily handled pgm perovskites work ashighly active and clean catalysts, a whole new appli-cation area may open in the near future,reinforcing their significance.

Platinum Metals Rev., 2007, 51, (2) 92

1 A. S. Bhalla, R. Guo and R. Roy, Mater. Res. Innov.,2000, 4, (1), 3

2 “DAIHATSU:News”, 11th July, 2002;http://www.daihatsu.com/news/n2002/02071101/

3 S. Kato, M. Ogasawara, M. Sugai and S. Nakata,Catal. Surv. Asia, 2004, 8, (1), 27

4 M. Misono, Catal. Today, 2005, 100, (1–2), 955 C. N. R. Rao and B. Raveau, “Transition Metal

Oxides”, Wiley-VCH Publishing Inc, New York,1995, pp. 289–324

6 K. Deshpande, A. Mukasyan and A. Varma, J. PowerSources, 2006, 158, (1), 60

7 S. Petrovic, L. Karanovic, P. K. Stefanov, M. Zdujicand A. Terlecki-Baricevic, Appl. Catal. B: Environ.,2005, 58, (1–2), 133

8 F. Gaillard, X. Li, M. Uray and P. Vernoux, Catal.Lett., 2004, 96, (3–4), 177

9 B. Kucharczyk, Przemysl Chemiczny, 2004, 83, (11), 56710 “Platinum 2006 Interim Review”, Johnson Matthey,

Royston, U.K., 2006;http://www.platinum.matthey.com/publications/1132062873.html

11 W. F. Libby, Science, 1971, 171, (3970), 49912 For information on the proposed Euro 5 limits see:

‘Clean cars: Commission proposes to reduce emis-sions’, Europa press releases, Brussels, 21/12/2005,Reference IP/05/1660;http://europa.eu/rapid/pressReleasesAction.do?ref-erence=IP/05/1660&format=HTML&aged=0&language=EN&guiLanguage=en

13 K. Zhou, H. Chen, Q. Tian, Z. Hao, D. Shen and X.Xu, J. Mol. Catal. A: Chem., 2002, 189, (2), 225

14 N. K. Labhsetwar, A. Watanabe, R. B. Biniwale, R.Kumar and T. Mitsuhashi, Appl. Catal. B: Environ.,2001, 33, (2), 165

15 R. Bradow, D. Jovanovic, S. Petrovic, Z. Jovanovicand A. Terlecki-Barcevic, Ind. Eng. Chem. Res., 1995,

34, (6), 192916 H. Tanaka, I. Tan, M. Uenishi, M. Kimura and K.

Dohmae, Top. Catal., 2001, 16–17, (1–4), 6317 Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M.

Uenishi, M. Kimura, T. Okamoto and N. Hamada,Nature, 2002, 418, (6894), 164

18 H. Tanaka, M. Taniguchi, M. Uenishi, N. Kajita, I.Tan, Y. Nishihata, J. Mizuki, K. Narita, M. Kimuraand K. Kaneko, Angew. Chem. Int. Ed., 2006, 45, (36),5998

19 H. Tanaka, M. Uenishi, M. Taniguchi, I. Tan, K.Narita, M. Kimura, K. Kaneko, Y. Nishihata and J.Mizuki, Catal. Today, 2006, 117, (1–3), 321

20 H. Tanaka, M. Taniguchi, N. Kajita, M. Uenishi, I.Tan, N. Sato, K. Narita and M. Kimura, Top. Catal.,2004, 30–31, (1), 389

21 EU metal contamination limits for APIs can befound in: ‘Note for Guidance on Specification Limitsfor Residues of Metal Catalysts’, European Agencyfor the Evaluation of Medicinal Products, London,2002;http://www.emea.eu.int/pdfs/human/swp/444600en.pdf

22 T. J. Colacot, E. S. Gore and A. Kuber,Organometallics, 2002, 21, (16), 3301

23 C. Ramarao, S. V. Ley, S. C. Smith, I. M. Shirley andN. DeAlmeida, Chem. Commun., 2002, (10), 1132

24 S. V. Ley, M. D. Smith, C. Ramarao, A. F. Stepan andH. Tanaka, U.S. Patent Appl. 2005/0,215,804

25 S. P. Andrews, A. F. Stepan, H. Tanaka, S. V. Ley andM. D. Smith, Adv. Synth. Catal., 2005, 347, (5), 647

26 M. D. Smith, A. F. Stepan, C. Ramarao, P. E.Brennan and S. V. Ley, Chem. Commun., 2003, (21),2652

27 S. Lohmann, S. P. Andrews, B. J. Burke, M. D.Smith, J. P. Attfield, H. Tanaka, K. Kaneko and S. V.Ley, Synlett, 2005, (8), 1291

References

The AuthorThomas Screen was born in London, U.K. Hestudied Natural Sciences (Chemistry) at theUniversity of Cambridge, U.K., where he wasawarded a B.A. in 1996. This was followed bytwo years working at Johnson Matthey CatalyticSystems Division in Royston before returning toacademia and the group of Professor Harry L.Anderson at the University of Oxford. Thomas

completed his D.Phil. there on the ‘Synthesis and Properties ofConjugated Porphyrin Polymers’ in 2002 before moving toGermany for a postdoctoral appointment with Professor KlausMüllen at the Max Planck Institute for Polymer Research in Mainz.On returning to the U.K., Thomas joined Peakdale Molecular Ltd inChapel-en-le-Frith, Derbyshire, at the end of 2003. Since 2005 hehas worked at the new catalyst technologies company Reaxa Ltd inManchester, U.K.

IntroductionA new book series begins with this volume,

which covers molecular diversity and combinator-ial chemistry, high-throughput discovery andassociated technologies including characterisationtechniques. Given the wide scope of this book, acomprehensive review is not possible. Thereforeparticular areas of interest having relevance to theplatinum group metals (pgms) have been selectedfor a series of reviews. Here, Dave Newman andLesley Wears of the University of Exeter, U.K.,present a review of two chapters: respectively,Chapter 16, titled ‘Innovation in Magnetic DataStorage Using Physical Deposition andCombinatorial Methods’, by Erik B. Svedberg(Seagate Technologies, U.S.A.); and Chapter 17,‘High-Throughput Screening of Next GenerationMemory Materials’, by Chang Hwa Jung, Eun JungSun and Seong Ihl Woo (Korea Advanced Instituteof Science and Technology, South Korea). Furtherchapters will be reviewed in future issues ofPlatinum Metals Review.

Thin Film Deposition for DataStorage Technology

Chapter 16 focuses closely on the thin film metaldeposition techniques that have delivered the com-plex and task-tailored multilayer thin film structuresthat underpin the continuing and often spectacularadvances in magnetic data storage technology.These are embodied in the hard disk drive nowubiquitous, not just in computer systems, but inreduced-size formats across the full spectrum of

consumer electronics. The importance of optimis-ing both material and multilayer performance, notonly for the recording medium supporting thestored data, but also for the complex structures thatcomprise the readout sensor heads is referenced,but unfortunately this section is all too brief, so thatthe real challenges that have been met and over-come are not readily appreciated in full.

It is the comprehensive, detailed and well writtendescriptions of the deposition geometries directedto specific ends that provide the real benefit of thework. This section clearly describes the variousdeposition geometries by which material composi-tion or thickness can be adjusted, to produce seriesof samples in which specific characteristics are var-ied in a highly controlled manner. It is supported byclear diagrams and photographs showing the rela-tive dispositions of components in such systems.Concepts such as the movement of substrates andmasks in complex patterns over multiply orientedsources are introduced and discussed, with refer-ence to what might be achieved by way ofcompositional variation and the control of charac-teristics. A useful and illustrative example is givenbased on the development of cobalt/platinum(Co/Pt) and cobalt-chromium/platinum (CoCr/Pt)multilayers and, importantly, considerable effort isdirected towards introducing the modelling of desir-able magnetic-dependent parameters on materialcomposition and thickness.

In summary, the chapter meets its stated aimsand what it covers is well referenced. Its narrownessof scope is, however, disappointing. The increasing

93

“Combinatorial and High-ThroughputDiscovery and Optimization of Catalystsand Materials”CRITICAL REVIEWS IN COMBINATORIAL CHEMISTRY, Volume 1EDITED BY RADISLAV A. POTYRAILO (General Electric Global Research Center, New York, U.S.A.) AND WILHELM F. MAIER

(Saarland University, Germany), CRC Press, Boca Raton, U.S.A., 2007, ISBN 978-0-8493-3669-0, £115.00, U.S.$199.95

A Selective Review by Dave M. Newman* and M. Lesley Wears**School of Engineering, Computer Science & Mathematics, Harrison Building, University of Exeter, Exeter EX4 4QF, U.K.;

E-mail: *[email protected]; **[email protected]

Platinum Metals Rev., 2007, 51, (2), 93–94

DOI: 10.1595/147106707X192672

importance of pgms to magnetic recording is notfully represented. No mention is made, for example,of the attempts to produce patterned media basedon platinum-cobalt (PtCo) or platinum-iron (PtFe)by chemical and biological processes (1–5).

Screening of Thin Film DataStorage Materials

Chapter 17 promotes the use of a thin film depo-sition/characterisation procedure developed toenable rapid parallel characterisation of data storagematerials. The conclusion, however, highlights thelimits of this technology when applied to memorymaterials, due to the unavailability of high-through-put characterisation techniques. The diagrams andtext are clear and concise, but the reader would ben-efit from the figures being close to the text thatdescribes them.

The authors give an overview of current materi-als and techniques used by the storage industry.However, they fail to address the imminent prob-lems facing the industry, which is surprising as thesesolutions involve pgms in particular. To increase theareal density of memory materials, the storageindustry has committed to perpendicular recording.Although this is alluded to in the final section,where magnetoresistive random access memory(MRAM) and cobalt-chromium-platinum-tantalum(CoCrPtTa) are discussed, the failure to includeheat-assisted magnetic recording (HAMR) (6), a

technique which involves the deposition of highanisotropic materials which include PtCo and PtFe,may be considered an oversight.

ConclusionChapters 16 and 17 together provide a basic

introduction to the often complex deposition tech-nologies and operational methodologies nowroutinely employed in the manipulation and combi-nation of material properties to a specifictechnological end. In this context they can be rec-ommended; however reference to the importanceof pgms is rather limited.

Platinum Metals Rev., 2007, 51, (2) 94

The ReviewersDave M. Newman, whoreviewed Chapter 16, isReader in InformationStorage Materials andTechnologies at theSchool of Engineering,Computer Science &Mathematics, Universityof Exeter, U.K.

M. Lesley Wears, whoreviewed Chapter 17, is aSenior Research Fellowat the School ofEngineering, ComputerScience & Mathematics,University of Exeter, U.K.

References1 S. Sun, C. B. Murray, D. Weller, L. Folks and A.

Moser, Science, 2000, 287, (5460), 19892 B. Warne, O. I. Kasyutich, E. L. Mayes, J. A. L.

Wiggins and K. K. W. Wong, IEEE Trans. Magn.,2000, 36, (5), 3009

3 T. J. Klemmer, C. Lui, N. Shulka, X. W. Wu, D.Weller, M. Tanase, D. E. Laughlin and W. A. Soffa,J. Magn. Magn. Mater., 2003, 266, (1–2), 79

4 S. Wang, S. S. Kang, D. E. Nikles, J. W. Harrell andX. W. Wu, J. Magn. Magn. Mater., 2003, 266, (1–2), 49

5 K. Elkins, D. Li, N. Poudyal, V. Nandwana, Z. Jin,K. Chen and J. P. Liu, J. Phys. D: Appl. Phys., 2005,38, (14), 2306

6 R. E. Rottmayer, S. Batra, D. Buechel, W. A.Challener, J. Hohlfeld, Y. Kubota, L. Li, B. Lu, C.Mihalcea, K Mountfield, K. Pelhos, C. Peng, T.Rausch, M. A. Seigler, D. Weller and X.-M. Yang,IEEE Trans. Magn., 2006, 42, (10), 2417

PROPERTIESProbing the Interface in Vapor-DepositedBimetallic Pd-Au and Pt-Au Films by COAdsorption from the Liquid PhaseD. FERRI, B. BEHZADI, P. KAPPENBERGER, R. HAUERT, K.-H.ERNST and A. BAIKER, Langmuir, 2007, 23, (3), 1203–1208

PVD was used to prepare Pt, Pd, Au, Pt-Au and Pd-Au films. Their surfaces were characterised by XPS,AFM and CO adsorption from liquid CH2Cl2 moni-tored by ATR-IR spectroscopy. The changesobserved in the IR frequency and in the shape of theCO signals upon adsorption indicated that morpho-logical changes occur in the Pd films when decreasingthe film thickness from 2 to 0.2 nm and when intro-ducing a 1 nm Au film. The Pt-Au surfaces were lesssensitive toward CO adsorption.

Electric Field-Induced Modification of Magnetismin Thin-Film FerromagnetsM. WEISHEIT, S. FÄHLER, A. MARTY, Y. SOUCHE, C. POINSIGNONand D. GIVORD, Science, 2007, 315, (5810), 349–351

The magnetocrystalline anisotropy of ordered FePtand FePd intermetallic compounds can be reversiblymodified by an applied electric field when immersedin an electrolyte (propylene carbonate and Na+OH–).A voltage change of –0.6 V on 2 nm thick filmsaltered the coercivity by –4.5 and +1% in FePt andFePd, respectively. The modification of the magneticparameters is attributed to a change in the number ofunpaired d electrons.

Hydrogen-Induced Stress Relaxation in Thin PdFilms: Influence of Carbon ImplementationR. NOWAKOWSKI, P. GRZESZCZAK and R. DUS, Langmuir, 2007,23, (4), 1752–1758

The influence of C impurities on mechanical proper-ties of thin Pd film/H2 has been investigated in situ byAFM. The systems characterised by C incorporationfrom the two opposite sides of thin Pd film (HOPGsubstrate and HC fragments deposit from a gas-phasereached by preadsorption of ethylene) are compared.During PdHx decomposition, C impurities induce cre-ation of an organised network of cracks which dividesthe continuous film into separated domains.

Hydrogen Sorption Properties of TernaryIntermetallic Mg–(Ir,Rh,Pd)–Si CompoundsT. SPASSOV, S. TODOROVA, W. JUNG and A. BORISSOVA, J. AlloysCompd., 2007, 429, (1–2), 306–310

Mg–(Ir,Rh,Pd)–Si (1) with the highest Mg content(Mg15Ir5Si2 with channel-like structure) reveals thehighest electrochemical H capacity. Generally thecapacities of (1) studied are low. Refining the particlesize and microstructure of (1) did not result in any sig-nificant H-capacity increase.

CHEMICAL COMPOUNDSReactivity Studies of Rhodium Porphyrin Radicalwith Diazo CompoundsL. ZHANG and K. S. CHAN, Organometallics, 2007, 26, (3),679–684

Rh(II) tetramesitylporphyrin, Rh(tmp), reacted withethyl diazoacetate and (trimethylsilyl)diazomethane togive Rh(III) porphyrin alkyls. Mechanistic studiesshowed that Rh(tmp) was coordinated with a diazocompound, which then underwent a rapid H atomabstraction via C–H bond activation to giveRh(tmp)H. This subsequently reacted with a secondmolecule of the diazo compound in the rate-deter-mining step to give Rh(tmp) alkyl and N2.

Structural and Magnetic Study of N2, NO, NO2, andSO2 Adsorbed within a Flexible Single-CrystalAdsorbent of [Rh2(bza)4(pyz)]nC. KACHI-TERAJIMA, T. AKATSUKA, M. KOHBARA and S.TAKAMIZAWA, Chem. Asian J., 2007, 2, (1), 40–50

[Rh2(bza)4(pyz)]n (1) exhibits gas adsorbency. Thestructures obtained were characterised as (1)·1.5 N2

(298 K), (1)·2.5 N2 (90 K), and (1)·1.95 NO (90 K)under forcible adsorption conditions and (1)·2 NO2

(90 K) and (1)·3 SO2 (90 K) under ambient pressure.The NO inclusion crystal exhibited antiferromagneticinteraction between the NO molecules and paramag-netism arising from the NO monomer.

Ordered Arrays of Organometallic IridiumComplexes with Long Alkyl Chains on GraphiteJ. OTSUKI, T. TOKIMOTO, Y. NODA, T. YANO, T. HASEGAWA,X. CHEN and Y. OKAMOTO, Chem. Eur. J., 2007, 13, (8),2311–2319

fac-[Ir(ppy)3] complexes having long alkyl chainswere shown to form lamellar arrays at a 1-phenyloc-tane/HOPG interface. From STM images, it isconcluded that the molecules align with alkyl chainsbeing interdigitated. Similar lamellar arrays were alsoobtained at the air/HOPG interface upon drop-cast-ing of toluene solutions.

ELECTROCHEMISTRYRuthenium–Ligand Complex, an Efficient Inhibitorof Steel Corrosion in H3PO4 MediaM. BENABDELLAH, R. TOUZANI, A. DAFALI, B. HAMMOUTI andS. EL KADIRI, Mater. Lett., 2007, 61, (4–5), 1197–1204

The effect of a macrocycle Ru complex (1) on thecorrosion of steel in H3PO4 was investigated.Inhibition efficiency (E%) increased with concentra-tion of (1). Electrochemical impedance spectroscopyshowed that the dissolution process of the steeloccurs under activation control. Polarisation curvesindicate that (1) acts as a cathodic inhibitor.

ABSTRACTSof current literature on the platinum metals and their alloys

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PHOTOCONVERSIONPhosphorescent Dyes for Organic Light-EmittingDiodesP.-T. CHOU and Y. CHI, Chem. Eur. J., 2007, 13, (2), 380–395

Highly emissive, charge-neutral Pt, Ir, Os and Rucomplexes (1) with chelating C-linked 2-pyridylazo-late ligands were synthesised. The photophysicalproperties of (1) were investigated using spec-troscopy, relaxation dynamics, and theoreticalapproaches, from which the lowest-lying excitedstates, competitive radiative decay, and radiationlessprocesses were then analysed. The potential use of (1)in OLEDs was evaluated.

Tris(2,2'-bipyridyl)ruthenium(II) ChemiluminescenceEnhanced by Silver NanoparticlesB. A. GORMAN, P. S. FRANCIS, D. E. DUNSTAN and N. W.BARNETT, Chem. Commun., 2007, (4), 395–397

Mixtures of Ag(I) and citrate ions that are used toproduce Ag nanoparticles induce intense chemilumi-nescence with Ru(bpy)3

2+ and Ce(IV), which can beutilised for the determination of citrate ions and otheranalytes. Solutions of glycine, proline and tartaric acid(5 × 10–5 M) that contained AgNO3 (2.5 × 10–3 M)gave chemiluminescence with Ru(bpy)3

2+ and Ce(IV)that was ~ two orders of magnitude more intensethan for solutions without AgNO3.

SURFACE COATINGSHot Corrosion Behavior of Pt-Ir ModifiedAluminide Coatings on the Nickel-Base SingleCrystal Superalloy TMS-82+Y. N. WU, A. YAMAGUCHI, H. MURAKAMI and S. KURODA, J.Mater. Res., 2007, 22, (1), 206–216

Pt-Ir films (Ir = 0, 32, 46, 83, 100 at.%) weredeposited on TMS-82+ by magnetron sputtering.After annealing and aluminising, the Pt-Ir modifiedaluminide coatings (1) mainly consisted of PtAl2 andβ-(Ni,Pt,Ir)Al phases. The hot corrosion resistance of(1) was evaluated by exposure at 1173 K in the pres-ence of 90 wt.% Na2SO4 + 10 wt.% NaCl. The lowestmass gain (0.299 mg cm–2, after 100 h) was for Pt-46Irdue to formation of a dense, continuous protectiveAl2O3 scale. Phase transformation from β-(Ni,Pt)Alto γ'-(Ni,Pt)3Al and protection by Pt/Ir enriched layerhad important effects on the corrosion of (1).

Batch CVD Process for Depositing Pd ActivationLayersL. WANG and G. L. GRIFFIN, J. Electrochem. Soc., 2007, 154, (3),D151–D155

CVD was employed for depositing a Pd activationlayer for subsequent electroless Cu deposition. Theprocess uses a continuous Pd(hfac)2 precursor trans-fer step followed by a batch H2 reduction step. Theresulting layer contains both isolated Pd(0) clustersand dispersed Pd(II) species. Deposited Cu filmsshowed poor adhesion upon drying, which is attrib-uted to weak film attachment at the Pd(II) sites.

APPARATUS AND TECHNIQUEPlatinum Decorated Carbon Nanotubes for HighlySensitive Amperometric Glucose SensingJ. XIE, S. WANG, L. ARYASOMAYAJULA and V. K. VARADAN,Nanotechnology, 2007, 18, (6), 065503

Fine Pt nanoparticles were deposited on function-alised C MWNTs using a decoration technique. Anenzymatic Pt/C MWNTs paste-based mediated glu-cose sensor (1) was fabricated. Improved sensitivityfor glucose sensing was shown by (1) without usingany picoampere booster or Faraday cage. The calibra-tion curve exhibited a good linearity in the glucoseconcentration range of 1–28 mM.

A Room Temperature Si3N4/SiO2 Membrane-TypeElectrical Substitution Radiometer Using ThinFilm Platinum ThermometersG. ALLÈGRE, B. GUILLET, D. ROBBES, L. MÉCHIN, S. LEBARGYand S. NICOLETTI, Meas. Sci. Technol., 2007, 18, (1), 183–189

The temperature control of the title radiometer,using two control loops and a chopping procedure,was investigated. Sensing and heating elements werepatterned in a Pt thin film, deposited on a 1560 μm ×1560 μm membrane made of a 280 nm thickSi3N4/SiO2 bilayer. The sample was fabricated in a500 μm thick Si substrate by chemical anisotropicmicromachining and then passivated with a 1 μmthick SiO2 layer. The device was operated in a prima-ry vacuum chamber, with no coolant or large heatsink other than the sample holder itself.

Measurement and Modeling of HydrogenTransport through High-Flux Pd MembranesF. C. GIELENS, H. D. TONG, M. A. G. VORSTMAN and J. T. F.KEURENTJES, J. Membrane Sci., 2007, 289, (1–2), 15–25

H2-selective Pd membranes (1) were fabricated withmicrosystem technology. Permeation experimentswere carried out over 623–873 K at H2 feed partialpressures of 0.2–1.0 bar. At 823 K, a permeancebased on the free membrane area of 18 mol H2/m2 sbar0.58 was measured for (1) (thickness 0.5 μm). (1)were stable for a rather long period; however, SEManalysis showed the formation of a grain-structuredsurface. At 873 K the H2/He selectivity of (1)decreases rapidly, caused by the formation of holes.

Quartz Crystal Microbalance Sensor Based onNanostructured IrO2

T. W. CHAO, C. J. LIU, A. H. HSIEH, H. M. CHANG, Y. S. HUANGand D. S. TSAI, Sens. Actuators B: Chem., 2007, 122, (1), 95–100

Nanostructured IrO2 crystals (1) were grown on aAu-coated quartz substrate by MOCVD, and theirgas sensing properties studied by the quartz crystalmicrobalance (QCM) technique. Propionic acidadsorbed and desorbed reversibly on the IrO2 surfaceat room temperature. (1) with nanoblade and layered-column morphologies showed higher sensitivitiesthan (1) with incomplete-nanotube and square-nanorod morphologies. An IrO2 QCM sensor wassensitive to ppm-level propionic acid vapour.

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Characterization of a Glucose Sensor Prepared byElectropolymerization of Pyrroles Containing aTris-bipyridine Osmium ComplexM. TSUJIMOTO, T. YABUTANI, A. SANO, Y. TANI, H. MUROTANI,Y. MISHIMA, K. MARUYAMA, M. YASUZAWA and J. MOTONAKA,Anal. Sci., 2007, 23, (1), 59–63

A glucose sensor (1) was obtained by electrocopoly-merisation using pyrroles containing a tris-bipyridine(bpy) Os complex, pyrrole (py), pyrrole propanoicacid (PPA) and glucose oxidase (GOx). Tris-bipyri-dine Os pyrrole complexes (Os-py) with differentmethylene moieties were evaluated. The electrocat-alytic response of glucose was observed at electrodesmodified with Os-py, except for the one immobilisedwith Os-py containing the shortest methylene moiety.The electrocatalytic response to glucose of (1) with[Os(bpy)2(py(6)-bpy]2+/3+ was stable for > 100 days.

HETEROGENEOUS CATALYSISComparison of Two Palladium Catalysts onDifferent Supports during HydrogenationM. CIZMECI, A. MUSAVI, A. TEKIN and M. KAYAHAN, J. Am. OilChem. Soc., 2006, 83, (12), 1063–1068

Soybean oil was hydrogenated using 5% Pd/C and10% Pd/Al2O3, at various ratios in a reactor (at165ºC, 2 bar H2 and 500 rpm stirring rate). Reactionrate, trans isomer formation, selectivity ratios andmelting behaviours were monitored. The activity ofPd/C was ~ 10 times higher than that of Pd/Al2O3.

Molecular Level Dispersed Pd Clusters in theCarbon Walls of Ordered Mesoporous Carbon as aHighly Selective Alcohol Oxidation CatalystA.-H. LU, W.-C. LI, Z. HOU and F. SCHÜTH, Chem. Commun., 2007,(10), 1038–1040

Pd/ordered mesoporous C (1), where temperaturestable Pd clusters (< 1 nm) are uniformly embeddedin the C walls, can be synthesised by a nanocastingroute. The activity of (1) was tested in the oxidationof alcohols (benzyl alcohol, 1-phenylethanol, cin-namyl alcohol) using sc-CO2 as the reaction medium.The selectivity to the corresponding aldehyde was >99%. (1) are are stable and reusable.

The Fabrication of Reactive Hollow PolysiloxaneCapsules and Their Application as a RecyclableHeterogeneous Catalyst for the Heck ReactionH. WANG, X. ZHENG, P. CHEN and X. ZHENG, J. Mater. Chem.,2006, 16, (48), 4701–4705

4-(Triethoxysilyl)butyronitrile and dimethyldimethoxysi-lane monomers were consecutively cocondensed ontoa microemulsion of preformed polydimethylsiloxane.The templated polydimethylsiloxane was removed byexposure to solvents. The above product is thenreacted with Pd(OAc)2 in anhydrous toluene andreduced with KBH4 in EtOH to produce the hollowpolysiloxane capsule-supported Pd complex (1). (1) ishighly active and stereoselective for the Heck aryla-tion of alkenes. (1) can be retrieved and reused.

FT-IR Study on CO Hydrogenation to C2-Oxygenatesover Rh-Based CatalystW. CHEN, Y. DING, D. JIANG, L. YAN, T. WANG, H. ZHU and H.LUO, Chin. J. Catal., 2006, 27, (12), 1059–1062

Evolved species from Rh-Mn-Li-Ti/SiO2 (1) duringCO hydrogenation were investigated using in situ FT-IR spectroscopy. High pressure favoured theadsorption and activation of CO; high temperaturefavoured the dissociation of adsorbed CO. High pres-sure and high temperature promoted CO adsorptionon (1) and allowed CO dissociation. Enhanced COinsertion activity produced good performance for C2-oxygenate formation.

Effect of Ru Nanoparticle Size on Hydrogenationof Soybean OilB. XU, K. Y. LIEW and J. LI, J. Am. Oil Chem. Soc., 2007, 84, (2),117–122

Ru nanoparticles were used as catalysts (1) for theselective hydrogenation of soybean oil at 353 K andinitial pressure of 1.5 MPa. PVP-Ru-MeOH withmean size of 3.10 nm, which had the highest activity,produced the lowest cis isomer content, only 30.6% cisisomers remained. (1) with larger mean sizes of 9.06and 17.22 nm, which have lower activity, producedless trans isomer: 49 and 46%, respectively. However,(1) with the smallest size but the lowest hydrogena-tion activity, Ru-MeOH with mean size of 1.13 nm,was more active for the isomerisation.

HOMOGENEOUS CATALYSISPromoting Role of [PtI2(CO)]2 in the Iridium-Catalyzed Methanol Carbonylation to Acetic Acidand Its Interaction with Involved Iridium SpeciesS. GAUTRON, N. LASSAUQUE, C. LE BERRE, L. AZAM, R.GIORDANO, P. SERP, G. LAURENCZY, J.-C. DARAN, C. DUHAYON,D. THIÉBAUT and P. KALCK, Organometallics, 2006, 25, (25),5894–5905

The catalytic activity of the Ir complexes involved inMeOH carbonylation is enhanced when [PtI2(CO)]2

(1) is added. Under CO (1) readily gives [PtI2(CO)2].The turnover frequency value, which is 1450 h–1 for Iralone, reaches 2400 h–1 for a Pt/Ir = 3/7 molar ratio,under 30 bar of CO and at 190ºC. A catalytic cycle isproposed, which includes the cooperative effectbetween the Pt promoter and the Ir catalyst.

Catalysis by Ir(III), Rh(III) and Pd(II) Metal Ions inthe Oxidation of Organic Compounds with H2O2

P. K. TANDON, GAYATRI, S. SAHGAL, M. SRIVASTAVA and S. B.SINGH, Appl. Organomet. Chem., 2007, 21, (3), 135–138

PdCl2, RhCl3 and IrCl3 were used in the oxidation ofbenzaldehydes (unsubstituted, p-chloro, p-nitro, m-nitro, p-methoxy) and cinnamaldehyde; anthraceneand phenanthrene; cyclohexanol and benzyl alcoholby 50% H2O2. Traces of the chlorides catalyse theseoxidations, resulting in good to excellent yields. PdCl2is the most efficient catalyst. Oxidation in aromaticaldehydes is selective at the aldehyde group only.

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Platinum Metals Rev., 2007, 51, (2)

Asymmetric Transfer Hydrogenation of Ketonesand Imines with Novel Water-Soluble ChiralDiamine as Ligand in Neat WaterL. LI, J. WU, F. WANG, J. LIAO, H. ZHANG, C. LIAN, J. ZHU and J.DENG, Green Chem., 2007, 9, (1), 23–25

A H2O-soluble Rh(III) catalyst (1) was preparedfrom o,o'-aminated N-tosyl-1,2-diphenylethylenedi-amine and [Cp*RhCl2]2. (1) was efficient for thecatalytic asymmetric transfer hydrogenation (ATH) ofketones and imines with sodium formate as H donorin neat H2O. (1) can catalyse the ATH of α-bro-momethylaromatic ketones and imines besides simpleketones. High yields and enantioselectivities can beachieved within a few hours at 28ºC.

Stability of the First-Generation GrubbsMetathesis Catalyst in a Continuous Flow ReactorZ. LYSENKO, B. R. MAUGHON, T. MOKHTAR-ZADEH and M. L.TULCHINSKY, J. Organomet. Chem., 2006, 691, (24–25),5197–5203

Ethylene pretreatment of (PCy3)2Cl2Ru=CHPh (1)prior to cross-metathesis of ethylene and cis-2-buteneto form propylene in a continuous flow reactor (CFR)produced a direct effect on catalyst deactivation.Similar pretreatment of (1) with cis-2-butene causedfar less change in the catalyst activity. Continuousremoval of products in the CFR was important forseparating the effects of catalyst decay and catalystdeactivation caused by the terminal olefin, propylene.

FUEL CELLSDeposited RuO2–IrO2/Pt Electrocatalyst for theRegenerative Fuel CellY. ZHANG, C. WANG, N. WAN and Z. MAO, Int. J. HydrogenEnergy, 2007, 32, (3), 400–404

RuO2-IrO2/Pt (1) was prepared by even depositionof Ir hydroxide hydrate and Ru hydroxide hydrate onPt black and calcination in air. The RuO2-IrO2 waswell dispersed and deposited on the surface of Ptblack. URFC with deposited (1) showed better per-formance than that of URFC with mixed (1) catalyst.Cyclic performance of the URFC with deposited (1)was very stable during 10 cyclic tests.

Pt–Ir–IrO2NT Thin-Wall Electrocatalysts Derivedfrom IrO2 Nanotubes and Their Catalytic Activitiesin Methanol OxidationC.-C. SHAN, D.-S. TSAI, Y.-S. HUANG, S.-H. JIAN and C.-L. CHENG,Chem. Mater., 2007, 19, (3), 424–431

Lattice O of IrO2 nanotubes (IrO2NT) wasremoved under high-vacuum thermal annealing tofacilitate nucleation of 3–5 nm Ir grains and subse-quent synthesis of PtIr catalyst on the tube walls. Theamount of Ir being reduced, the Ir grain size, and thedeposited Pt size influence the surface area and thecatalytic activity. Pt-Ir-IrO2NT reduced at 500ºCexhibited higher activity than Pt-IrO2NT and Pt-IrNT in MeOH oxidation, and also a higher currentdensity than that of PtRu in the high potential region.

Fast Preparation of PtRu Catalysts Supported onCarbon Nanofibers by the Microwave-PolyolMethod and Their Application to Fuel CellsM. TSUJI, M. KUBOKAWA, R.YANO, N. MIYAMAE, T. TSUJI, M.-S.JUN, S. HONG, S. LIM, S.-H. YOON and I. MOCHIDA, Langmuir,2007, 23, (2), 387–390

PtRu alloy nanoparticles (24 ± 1 wt.%, Ru/Pt atom-ic ratios = 0.91–0.97) supported on C nanofibres(CNFs) were prepared by a microwave-polyol method.The DMFC activities of PtRu/CNF catalysts were:platelet > tubular > herringbone. The DMFC activitiesof PtRu/CNFs measured at 60ºC were higher than forstandard PtRu (29 wt.%, Ru/Pt atomic ratio = 0.92)catalyst loaded on C black (Vulcan XC72R).

The Effect of Heat Treatment on Nanoparticle Sizeand ORR Activity for Carbon-Supported Pd–CoAlloy ElectrocatalystsL. ZHANG, K. LEE and J. ZHANG, Electrochim. Acta, 2007, 52,(9), 3088–3094

An impregnation method was used for the synthe-sis of Pd-Co/C (1), in which NaBH4 was the reducingagent. (1) heat-treated at 300ºC had average particlesize of 8.9 nm, and the highest ORR catalytic activity.Electrocatalytic ORR activity was also examined in anacidic solution containing MeOH. (1) has MeOH tol-erant capabilities.

ELECTRICAL AND ELECTRONICENGINEERINGHeterostructured Magnetic Nanoparticles: TheirVersatility and High Performance CapabilitiesY. JUN, J CHOI and J. CHEON, Chem. Commun., 2007, (12),1203–1214

The recent advances in the development of magnet-ic nanoparticles (such as FePt, CoPt3) are reviewed,with a focus on multicomponent heterostructurednanoparticles including alloys, core–shells, and binarysuperlattices synthesised via nonhydrolytic methods.Their multifunctionality and high performance capa-bilities are demonstrated for applications in highdensity magnetic storage, catalysis, and biomedicalseparation and diagnostics. (94 Refs.)

Lithography-Free in Situ Pd Contacts to TemplatedSingle-Walled Carbon NanotubesM. R. MASCHMANN, A. D. FRANKLIN, A. SCOTT, D. B. JANES, T.D. SANDS and T. S. FISHER, Nano Lett., 2006, 6, (12),2712–2717

C SWNTs were synthesised from an embedded Fecatalyst in a modified porous anodic alumina (PAA)template (1). Pd is electrodeposited into (1) to formnanowires. Individual vertical channels of C SWNTsare created, each with a vertical Pd nanowire backcontact. Further Pd deposition resulted in annular Pdnanoclusters that form on portions of C SWNTsextending onto the PAA surface. Two-terminal elec-trical characteristics produce linear I–V relationships,indicating ohmic contact in the devices.

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METALS AND ALLOYSReflective AlloyTANAKA KIKINZOKU KOGYO KK

European Appl. 1,734,140A Ag alloy with excellent reflectance maintenance

can be used as a sputtering target. A first dopant element is chosen from Pt, Pd, Rh, Ru, Ir and Au. Asecond dopant is selected from Cu, Mn, Si, Cr, Ni,Co, Fe, Sc, Zr, Nb, Mo, Ta, W, In, Sn, Pb, Al, Ca, Ge,Ga, Bi, Sb, Sr, Hf, Gd, Sm, Nd, La, Ce, Yb and Eu.The total concentration of the two dopants is 0.01–5.0 at.%.

CHEMICAL COMPOUNDSNitrogen-Containing Metal Hydroxide ComplexesJOHNSON MATTHEY PLC World Appl. 2006/131,766

A substantially halide-free amine-metal hydroxidecomplex can be made by partitioning an aqueoussolution of a HCl or NaCl salt of one or more metalhalides, in the presence of a base, with an organic sol-vent system containing an amine. The metal isselected from Pt, Pd, Au, Ag, Cu and Ni. A reducingstep may further be carried out at ≤ 5ºC, to affordsubstantially halide-free metal nanoparticles.

PHOTOCONVERSIONOrganic Electroluminescent ElementKONICA MINOLTA HOLDINGS INC

U.S. Appl. 2006/0,280,966An organic electroluminescent element (1) includes

a light emitting layer containing an Ir or Pt guest com-pound, having an aromatic ring or heterocycle plus a5-membered aromatic or non-aromatic N-heterocy-cle. The host compound contains an alkyl, alkenyl,alkynyl, cycloalkyl, aromatic, aromatic heterocyclic orheterocyclic group. (1) can be made to emit blue orwhite light and can be used as an illuminator in adevice with a liquid crystal display.

Organometallic Electroluminescent DeviceSAMSUNG SDI CO LTD Japanese Appl. 2006-213,720

The title device can be made to emit light rangingfrom blue to red and contains an organometallic com-plex with a metal, M, selected from Ir, Pt, Pd, Ru, Os,Re or Pb. Ligands include a 3–60 C heterocyclicgroup bonded to M via N and linked to a 3–60 C arylgroup which is bonded to M via C; plus a heterocyclecontaining at least 2 N atoms and bonded to M via N.

Cyanophenylpyridine Iridium ComplexCHEMIPROKASEI KAISHA LTD

Japanese Appl. 2006-241,046A phenylpyridine Ir complex for use in an organic

electroluminescent device contains 0–4 electron with-drawing groups on the pyridine ring, plus 1–4 CNgroups and 0–2 F atoms on the phenyl ring. Light isclaimed to be emitted with shorter wavelength thanother phenylpyridine Ir complexes.

ELECTRODEPOSITION AND SURFACECOATINGSBright Rhodium ElectrodepositionR. J. MORRISSEY U.S. Appl. 2007/0,012,575

An electroplating solution for obtaining brightwhite Rh electrodeposits contains a soluble sulfate orphosphate compound of Rh with excess H2SO4,H3PO4 or a mixture. One or more N-containing het-erocyclic compounds with at least one N atom in a6-membered aromatic ring, such as pyridine, picoline,pyrimidine, pyridazine or pyrazine or derivativesthereof, is added as a brightening agent.

Platinum-Cobalt Alloy Plating SolutionTANAKA KIKINZOKU KOGYO KK

Japanese Appl. 2006-213,945PtCo alloy films can be formed using the claimed

plating solution, containing a bivalent Pt salt selectedfrom Na2[Pt(C2O4)2], K2[Pt(C2O4)2], [Pt(NH3)4]Cl2,[Pt(NH3)4]SO4, [Pt(NH3)4](NO3)2, [Pt(NO3)2(NH3)2]and K2PtCl4, at [Pt] = 1–30 g dm–3 and a bivalent Cosalt at [Co] = 1–60 g dm–3. An inorganic or carboxylicacid or a salt thereof, or a polyaminocarboxylic acid,at 1–200 g dm–3 is included as a buffer.

APPARATUS AND TECHNIQUEChromatographic Method of Separating Ruthenium ANGLO AMERICAN PLATINUM CORP LTD

U.S. Patent 7,163,570A chromatographic method separates Ru from a

feed solution containing chlorocomplexes of other Ptgroup metals including Ir and Rh, by converting Ruto a nitrosyl complex, which is temporarily retainedon the column. Subsequently Ru is eluted using anoxidising or reducing eluent.

Manufacture of Iridium Crucible TANAKA KIKINZOKU KOGYO KK

Japanese Appl. 2006-205,200A crucible made of Ir or Ir alloy is manufactured by

joining a cylindrical trunk portion to a circular base bywelding. A second step involves remelting and solidi-fying the welded portion at the inside bottom cornerusing a welding current of 150–180 A, lower than thatused for joining. The crucible resists leaks during use.

Analytical Reagent for Amino AcidsNAT. INST. ADV. IND. SCI. TECHNOL.

Japanese Appl. 2006-234,449An analytical reagent to detect amino acids, in par-

ticular histidine, methionine or cysteine, or peptidesand proteins containing these, by a colouring reac-tion, is based on a cyclopentadienyl Rh complex (1),plus a pigment such as an azo dye. Substituents on (1)may include 1–10 C straight or branched alkyl oralkoxy chains; phenyl, amino, nitro, thiol or hydroxylgroups; carboxylic or sulfonic acids, salts, esters oramides; or ketones, halogens or sugar residues.

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NEW PATENTS

Platinum Metals Rev., 2007, 51, (2), 99–101

DOI: 10.1595/147106707X186903

HETEROGENEOUS CATALYSISPlatinum Catalysts with Nanonetwork StructuresINST. NUC. ENERGY RES. European Appl. 1,728,618

A method for preparing Pt and Pt alloy catalysts (1)on supports with nanonetwork structures includesfirst dispersing nanospheres of a structure-directingmaterial (2), which may be an organic polymer orSiO2, of diameter 50–2000 nm, onto a support mate-rial to obtain a compact structure of 1–10 layers. (1)is then formed in the void spaces by chemical reduc-tion from aqueous solution or vacuum ion-sputtering.Finally, (2) is removed by thermal decomposition(polymer) or by chemical dissolution (SiO2). Uses mayinclude fuel cells or catalytic converters.

Perovskite-Type Composite OxideDAIHATSU MOTOR CO LTD European Appl. 1,728,766

A perovskite-type composite oxide (1) containing asolid solution of Pd has the formula AxB1–yPdyO3+δwhere A = a rare earth or alkaline earth element; B = a transition element (excluding rare earth ele-ments and Pd), Al or Si; x > 1; 0 < y ≤ 0.5; and δ represents an O excess. (1) can be used in a cat-alyst composition for exhaust gas purification or as acoupling reaction catalyst for organic synthesis.

Catalyst and Process for its ManufactureJOHNSON MATTHEY PLC World Appl. 2006/134,403

A catalyst, of average particle size 1–150 μm, con-tains at least one skeletal porous sponge metalselected from Ni, Co, Fe and Cu, with two promotermetals: a first selected from Pd, Pt, Ru, Rh, Os and Ir(0.01–5 wt.%); and a second from Fe, Ni, Co, Zn, V,Ce, Cu, W, Mo, Ti, Nb, Mg, Ag, Cd, Pr and Nd(0.01–5 wt.%). Processes for manufacture includeimpregnation or precipitation of promoter metalsfrom a solution onto the sponge metal. The catalystsmay be used for hydrogenation of an organic nitrocompound or nitrile to the corresponding amine.

Improvements in H2O2 Formation CatalystsUNIV. COLL. CARDIFF CONSULT. LTD

World Appl. 2007/007,075Catalysts for direct reaction of H2 and O2 to form

H2O2 consist of particles of Au, Pd or, preferably, Auand Pd, deposited on an acid-washed support such asSiO2, TiO2, Al2O3, Fe2O3, a stable zeolite or activatedC. Weight ratio of Au:Pd may be ~ 5.25:1. Reactionis carried out in H2O-MeOH at 2–40ºC. High selec-tivity to and production of H2O2 is observed, withlow decomposition. The catalysts have extended life.

Manufacture of Diesel Range HydrocarbonsNESTE OIL OYJ U.S. Appl. 2007/0,006,523

A process for converting vegetable oils to middledistillate hydrocarbons which can be used in dieselfuels is claimed. A feed containing > 20% triglycerideC12–C16 fatty acids, or fatty acid esters is hydrotreatedin the presence of a Pd, Pt, Ni, NiMo or CoMo cata-lyst on an Al2O3 and/or SiO2 support, to given-paraffins. These are isomerised to branched-chainparaffins in the presence of a supported catalystwhich contains one of Pt, Pd or Ni.

Recyclable Ruthenium Metathesis CatalystsZ.-Y. J. ZHAN U.S. Appl. 2007/0,043,180

A Ru catalyst (1) includes a substituted benzylideneligand having an electron withdrawing group; plus anelectron donating ligand such as a heterocyclic car-bene or a phosphine. Additionally, one of the ligandsin (1) may be chemically bound to the surface of apolymer, resin, PEG or silica gel support to give asupported Ru catalyst composition (2). Either (1) or(2) may be used for olefin metathesis reactions suchas RCM, CM, ROMP or for polymerisation reactions.(2) is recyclable.

Polymer Immobilised Platinum CatalystJAPAN SCI. TECHNOL. AGENCYJapanese Appl. 2006-198,491

A crosslinked polymer formed from monomerscontaining an aromatic side chain, a hydrophilic sidechain and a bridge formation radical is used to immo-bilise a Pt catalyst (1). Formation of (1) is carried outby forming ultrafine particles of Pt on the crosslink-able polymer in a solution containing a polar solventto form a micelle, followed by a crosslinking reaction,for example, by heating. (1) can be used for hydrosi-lylation, hydrogenation or boration reactions.

Catalyst for Removing NOx from Exhaust GasASAHI CHEMICAL CORP Japanese Appl. 2006-218,352

A catalyst for efficient removal of NOx from dieselexhaust gas in a lean-burn atmosphere (≥ 5% O2) at150–300ºC is claimed. Particles of Pt and/or Ir ofmean diameter 0.4–20 nm are coated with a layer ofrefractory material with a melting point ≥ 1000ºC,such as Mo, W, V, Fe, Ti or their oxides, to thickness0.1–1 nm, and supported on a hardly-soluble carriersuch as mesoporous SiO2, Al2O3, ZrO2 or CeO2-ZrO2

with specific surface area 100–1400 m2 g–1. A highcatalytic activity is claimed to be maintained evenafter catalyst regeneration.

Fuel Reforming CatalystNISSAN MOTOR CO LTD Japanese Appl. 2006-231,132

H2-rich gas is produced by reforming fuel in thepresence of a Rh-containing catalyst. Rh is carried onan inorganic monolith support in an upstream part ofthe system, with Co on a second inorganic carrier inthe downstream portion. Rh is present in 0.1–10 wt.% and Co in 0.5–20 wt.% of their respec-tive catalyst powders, and the mole ratio of Co:Rh isbetween 0.2–9.0.

HOMOGENEOUS CATALYSISPreparation of a SiloxaneSHIN ETSU CHEM. CO LTD U.S. Appl. 2007/0,037,997

A 1-(alkoxysilyl)ethyl-1,1,3,3-tetramethyldisiloxaneis prepared by adding a vinyl-containing alkoxysilanein portions to 1,1,3,3-tetramethyldisiloxane in thepresence of a Rh compound, which is free of P-con-taining ligands and may include a halide or a1,5-cyclooctadiene ligand. Reaction is carried out at atemperature between 0–60ºC.

Platinum Metals Rev., 2007, 51, (2) 100

Isolating Rhodium CatalystsBASF AG U.S. Appl. 2007/0,037,999

A distillation process is used to separate a com-pound (1) having at least 2 functional groups selectedfrom nitrile, carboxylic acid, carboxylic ester and car-boxamide groups, from a mixture containing a Rhcatalyst compound. (1) may be a monoolefinicallyunsaturated compound obtained by dimerising twoterminal olefins in the presence of a catalyst contain-ing Rh, Ru, Pd or Ni, preferably Rh, and may furtherbe hydrogenated in the presence of the same catalystto give (1) as a saturated compound. Distillation maybe carried out with an average mean residence time of1–45 min, at 50–200ºC and 0.05–50 kPa.

FUEL CELLSVoltage Cycling Durable Platinum CatalystsGENERAL MOTORS World Appl. 2007/005,081

An electrocatalyst layer (1) with increased voltagecycling durability is claimed. Pt or Pt alloy particlesare annealed at 800–1400ºC to reduce their surfacearea to < 80% of their pre-annealed state, have aver-age particle diameter 3–15 nm, and are deposited ona support structure such as C, activated C, graphite, Cnanotubes, ionomers, conductive oxides, conductivepolymers or a mixture. The electrocatalytically activesurface area of (1) is > 50% of its original extent after ~ 15,000 voltage cycles at 0.6–1.0 V.

Exhaust Gas Purification Method for Fuel Cell VehicleNISSAN MOTOR CO LTD U.S. Appl. 2006/0,292,051

A CH4 removal catalyst for accelerating the oxida-tion of CH4 in fuel cell vehicle exhaust to H2 and COincludes 1–10 wt.% of at least one of Rh, Pt and Pd,preferably Pd, on a porous carrier. A second catalystdownstream of the first, for conversion of H2 and COinto H2O and CO2, contains 0.1–3 wt.% of at leastone of Rh, Pt and Pd, preferably Pt. The system canbe used with fuel cell, compressed natural gas orhybrid vehicles.

Platinum-Palladium-Titanium CatalystSYMYX TECHNOL. INC U.S. Appl. 2007/0,037,696

A composition for use as a fuel cell catalyst is madefrom Pt, Pd and Ti or their oxides, carbides and/orsalts. The sum of the concentrations of Pt, Pd and Tiis > 90 at.%, and preferably > 94 at.%.Concentrations of each element are in the ranges (inat.%): 5–60 Pt, 5–50 Pd and 15–75 Ti.

Manufacturing Method for an ElectrodeTOPPAN PRINTING CO LTD Japanese Appl. 2006-236,881

A catalyst ink (1) contains particles of Pt on C sup-port, dispersed in a solution which includes a liquidproton conductive material such as Nafion. A catalystelectrode is manufactured by applying (1) in dropletform onto a conductive material such as a C materi-al, resulting in formation of a ternary phase interfacebetween the C/Pt/proton conductive material, thencarrying out thermal compression bonding.

Carbon Monoxide Oxidation CatalystNAT. INST. ADV. IND. SCI. TECHNOL.

Japanese Appl. 2006-261,086A catalyst composition (1) for electrochemical oxi-

dation of CO includes a Rh porphyrin compoundhaving up to eight substituents, selected from alkylgroups, H or halides, which may be supported on aconductive support such as C black. (1) can be usedin a CO sensor or for an anode in a SPFC.

ELECTRICAL AND ELECTRONICENGINEERINGMagnetic Film for a Magnetic DeviceFUJITSU LTD European Appl. 1,752,996

A multilayer magnetic film for a high-density mag-netic recording device includes alternately laminatedferromagnetic and Pd or Rh metal or alloy films,which are formed by a dry processing method such assputtering, vacuum deposition or chemical vapourdeposition. A Pd film layer has thickness ≥ 0.05 nm,or a Rh film layer, 0.1–0.4 nm. The ferromagneticfilm layer may be composed of an FeCo containingalloy which may also include one of Pd, Rh or Pt.

Magnetic Recording MediumHITACHI MAXELL LTD Japanese Appl. 2006-236,486

A high-density magnetic recording medium isclaimed which includes a magnetic layer of PtFe alloyfilm (1) having high magnetic anisotropy and finecrystal grains with small size distribution. An amor-phous inorganic compound such as an oxide of Si,Al, Ti, Ta, Zr or Zn is used as a substrate, to which alayer of Fe oxide is applied. Onto this, a layer of Feand then a layer of Pt are formed, each layer havingthickness 1–4 nm. These layers are heated to inducecounter diffusion between layers and produce (1).

MEDICAL USESTextured Iridium for Vascular DevicesMEDTRONIC VASCULAR INC World Appl. 2006/119,116

Vascular devices such as stents can be made fromtextured polycrystalline Ir. The method of texturingincludes cold/warm working Ir at 700–1100ºC tobreak up the polycrystalline structure and promotethe desired orientation. Ir is then recrystallised to givea majority of grains aligned in <110> direction.Lattice matched second phase particles inhibit recrys-tallisation in undesired orientation and may includeIr5Th, IrRu, IrTa, IrRh, IrV, IrTh, IrZr or IrW.

Platinum Complexes for Targeted Drug DeliveryUNIV. SOUTH FLORIDA World Appl. 2007/008,247

Biotin-containing Pt complexes for the treatmentof oncological or inflammatory disorders can also beused for the treatment or prevention of infections,and may include a further molecule such as an anti-body, a ligand or a receptor bound to the biotinmoiety. Synthesis consists of mixing cisplatin ortransplatin in H2O and an organic solvent such asdichloroethane or hexane, adding a biotin-containingligand, then treating the mixture with NO2(g).

Platinum Metals Rev., 2007, 51, (2) 101

Although rapid advances have been made inequipment and materials used in the process, cast-ing platinum for jewellery is still a challenge. Ataround 2000ºC, getting the melt temperatureabsolutely right for the size, type and number ofpieces being cast is a fine balancing act, as is judg-ing the correct mould temperature. Too low andthe result could be an incomplete fill; too high andshrinkage porosity may occur.

It is often the caster who is blamed for porosi-ty, but it should be remembered that a casting doesnot have the same structure as wrought material.All castings are porous to some extent, and it isunfair to compare any casting with a piece madefrom sheet or wire. Even poured ingots start outporous, but have the porosity ‘squeezed’ out ofthem during forging, rolling or drawing to sheetand wire.

Polishing platinum is very different from pol-ishing gold. The surface of gold ‘smears’ and canbe buffed to a high polish. Platinum, however,must be polished with abrasives, and since thewhole of a casting is porous it is impossible toachieve the same surface finish on an untreatedcast platinum product as could be obtained on ahandmade or stamped and machined platinumpiece.

An ‘as-cast’ structure is also softer and morebrittle than a worked structure, and untreated plat-inum castings can be more prone to denting anddeforming than pieces made from wrought plat-inum. Platinum-cobalt or platinum-rutheniumalloy systems are commonly used because theyprovide relatively hard castings (see Table I). Sojewellers should ensure that they use one of these,or an alloy of equivalent hardness.

Getting the alloy right is essential, but there areother measures that manufacturers and designerscan take to improve their cast platinum pieces andreduce the time and effort spent finishing them.

The first is to help reduce the amount of

porosity in a piece, both through its design and bypaying attention to how it is sprued. Rapid changesin cross-section and sprueing into fine sections cancause flow problems and result in poor castings.New designs should be discussed with the caster,seeking their input in the model making process.The second measure is further processing of thecasting after delivery. Much can be done toimprove the ‘as-cast’ structure on the surface ofthe casting by using a burnishing process (1, 2).

Burnishing a platinum casting squeezes outporosity near to the surface, and puts some workinto the piece. The hardened surface makes itmore resistant to denting and deformation.Although this is an extra process, it actually saveson finishing time by smoothing out the rough castsurface and reducing polishing time.

This is an abridged version of an article originally pub-lished as Reference (3).

NEILL SWAN

References1 J. Maerz, Jewellery in Britain, 2004, (19), 62 J. Maerz, Jewellery in Britain, 2004, (19), 73 N. Swan, Jewellery in Britain, 2004, (19), 5

Platinum Metals Rev., 2007, 51, (2), 102 102

FINAL ANALYSIS

Casting Platinum Jewellery – A Challenging Process

DOI: 10.1595/147106707X190061

Table I

Hardness of a Range of Platinum Alloys

Alloy Hardness, Hv

95% Pt-5% Co 13595% Pt-5% Ru 13095% Pt-5% Cu 10895% Pt-5% Ir 8095% Pt-5% Pd 68

The Author

Neill Swan is Sales and Marketing Managerwith Johnson Matthey Precious MetalsMarketing, and has worked for the companyfor 27 years. His responsibilities nowinclude managing the company’s platinumand palladium jewellery marketdevelopment activities in the U.S.A.,Switzerland, China and the U.K.