Metal carbonyls may be mononuclear or polynuclear

Metal carbonyls may be mononuclear or polynuclear

Characterization of metal carbonyls

IR spectroscopy M-C-O (C-O bond stretching modes)

Effect of charge

Effect of other ligands

PF3 weakest donor (strongest acceptor) PMe3 strongest donor (weaker acceptor)

Lower frequency, weaker CO bond(free CO) 2143 cm-1

Increasing elec donating ability of phosphines

The number of active bandsas determined by group theory

Typical reactions of metal carbonyls

Ligand substitution:

Cr(CO)6 + CH3CN Cr(CO)5(CH3CN) + CO

Always dissociative for 18-e complexes, may be associative for <18-e complexes

OC CO

COMn

OC

CH3

CO

OC CO

COMn

C

CO

H3C

OOC CO

COMn

C

CO

CO

H3C

OCO

Migratory insertion:

Metal complexes of phosphines

PR3 as a ligandGenerally strong donors, may be π-acceptor

strong trans effectElectronic and steric properties may be controlled

Huge number of phosphines available

M-PR3

M P

*

Metal complexes of phosphines

M-PR3

Basicity: PCy3 > PEt3 > PMe3 > PPh3 > P(OMe)3 > P(OPh)3

> PCl3 > PF3

Can be measured by IR using trans-M(CO)(PR3) complexesSteric properties:

P

R1

R2

R3

M

The cone angleRigid structures create chiral complexes

R2P

PR2

M

apex angle of a cone that encompassesthe van der Waals radii of the outermost

atoms of the ligand

Typical reactions of metal-phosphine complexes

Ligand substitution:

HCo(CO)4 + PBu3 HCo(CO)3(PBu3) + CO

HRh(CO)(PPh3)3 + C 2H4 HRh(CO)(PPh3)2(C2H4) + PPh3

Very important in catalysisMechanism depends on electron count

presence of bulky ligands (large cone angles)

can lead to more rapid ligand dissociation

Metal hydride and metal-dihydrogen complexes

Terminal hydride (X ligand)

Bridging hydride (-H ligand, 2e-3c)

Coordinated dihydrogen (2-H2 ligand)

Hydride ligand is a strong donor and the smallest ligand availableH2 as ligand involves -donation and π-back donation

M H

HMM

MH

H

Synthesis of metal hydride complexes

IrCl(CO)(PPh3)2 + H2 Ir(H)2Cl(CO)(PPh3)2

RuCl2(PPh3)3 + H 2Et3N

RuHCl(PPh3)3 + Et 3N.HCl

Co2(CO)8 + H 2 2 HCo(CO)4

[Fe(CO)4]2- + H+ [HFe(CO)4]-

Cp2ZrCl2 + NaBH4 Cp2ZrHCl

Characterize these kinds of reactions.

Characterization of metal hydride complexes

1H NMR spectroscopy

High field chemical shifts ( 0 to -25 ppm usual, up to -70 ppm possible)

Coupling to metal nuclei (101Rh, 183W, 195Pt) J(M-H) = 35-1370 Hz

Coupling between inequivalent hydrides J(H-H) = 1-10 Hz

Coupling to 31P of phosphines J(H-P) = 10-40 Hz cis; 90-150 Hz trans

IR spectroscopy

(M-H) = 1500-2000 cm-1 (terminal); 800-1600 cm-1 bridging(M-H)/(M-D) = √2Weak bands, not very reliable

Some typical reactions of metal hydride complexes

Transfer of H-

Cp2Zr(H)2 + 2CH2O Cp2Zr(OCH3)2

Transfer of H+

HCo(CO)4 H+ + [Co (CO)4]- A strong acid !!

Insertion

IrH(CO)(PPh3)3 + (C 2H4) Ir(CH2CH3)(CO)(PPh3)3

A key step in catalytic hydrogenation and related reactions

Bridging metal hydrides

2-e ligand 4-e ligand

bonding

Non-bonding

Anti-bonding

Metal dihydrogen complexes

H H

M

H

H

M

PiPr3

W

PiPr3

OC

COOC

H

H

If back-donation is strong, then the H-H bond is broken (oxidative addition)

Very polarized+, -

Characterized by NMR (T1 measurements)

back-donation to * orbitals of H2

the result is a weakening and lengthening of the H-H bond in comparison with free H2

Metal-olefin complexes

2 extreme structures

metallacyclopropane π-bonded only

sp3

sp2

Zeise’s salt

Net effect weakens and lengthens the C-C bond in the C2H4 ligand (IR, X-ray)

Effects of coordination on the C=C bond

Compound C-C (Å) M-C (Å)

C2H4 1.337(2)

C2(CN)4 1.34(2)

C2F4 1.31(2)

K[PtCl3(C2H4)] 1.354(2) 2.139(10)

Pt(PPh3)2(C2H4) 1.43(1) 2.11(1)

Pt(PPh3)2(C2(CN)4) 1.49(5) 2.11(3)

Pt(PPh3)2(C2Cl4) 1.62(3) 2.04(3)

Fe(CO)4(C2H4) 1.46(6)

CpRh(PMe3)(C2H4) 1.408(16) 2.093(10)

C=C bond is weakened (activated) by coordination

Characterization of metal-olefin complexes

NMR 1H and 13C, < free ligand

X-rays C=C and M-C bond lengths indicate strength of bond

IR (C=C) ~ 1500 cm-1 (w)

Reactions of metal-olefin complexes

Metal alkyl, carbene and carbyne complexes

Main group metal-alkyls known since old times(Et2Zn, Frankland 1857; R-Mg-X, Grignard, 1903))

Transition-metal alkyls mainly from the 1960’s onward

W(CH3)6 Ti(CH3)6 PtH(CCH)L2

Cp(CO)2Fe(CH2CH3)6 [Cr(H2O)5(CH2CH3)6]2+

Why were they so elusive?

Kinetically unstable (although thermodynamically stable)

Metal-alkyl complexes

Reactions of transition-metal alkyls

LnM

R

XLnM + R-X

LnM R LnM+ + R-H+ H+

Blocking kinetically favorable pathways allows isolation of stable alkyls

Metal-carbene complexes

C

R

R: C

R

R

..

sp2 sp2

pzpz

singlet carbene triplet carbene

C

R

R:M

:C

R

RM

..

..

d

d d

d

Fischer carbene Schrock carbene

M C

R

OR

M C

R

R

M C

R

OR-+

L ligandLate metalsLow oxidation statesElectrophilic

X2 ligandEarly metalsHigh oxidation statesNucleophilic

Fischer-carbenes

Schrock-carbenes

Synthesis

Typical reactions

Np3Ta

t-Bu

X Y

O

Np3Ta O

X

Y

H

t-Bu+

+

+ olefin metathesis (we will speak more about that)

Compare to Wittig

Grubbs carbenes

Excellent catalysts for olefin metathesis

Metal cyclopentadienyl complexes

M

M

M

L L

M

LL L

Metallocenes(“sandwich compounds”)

Bent metallocenes

“2- or 3-leggedpiano stools”

Homogeneous catalysis:an important application of organometallic compounds

Catalysis in a homogeneous liquid phase

Very important fundamentally

Many synthetic and industrial applications

M H

M CO

M H

M PR3

M Cp

M

Reaction (FOS) (CN) (NVE)

Association-Dissociation of Lewis acids 0 ±1 0

Association-Dissociation of Lewis bases 0 ±1 ±2

Oxidative addition-Reductive elimination ±2 ±2 ±2

Insertion-deinsertion 0 0 0

Fundamental reaction of organo-transition metal complexes

Combining elementary reactions

MLn + H2 MLn

H H

(oxidative addition)

MLn

H H

+MLx

H H-L(ligand exchange)

MLx

H H

MLn

H C C H(insertion)

Completing catalytic cycles

(reductive elimination)

Olefin hydrogenation

MLx

H H

MLn

H C C H(insertion)

MLn

H C C H

MLn + C C

H H


H H

H C C

H3C H CH3

H

CH3

H H

CH3

H3CMLx

MLn

MLx

H HH C C H

MLx MLn

-H eliminationno net reaction

-H elimination resulting in C=C bond migration

Olefin isomerization


Olefin isomerization

H H

H C C

H3C H CH3

H

CH3

H H

CH3

H3CMLx

MLn

MLx

H H

MLx

MLx

H2

H H

H C CH H

H

MLx

MLn

MLx

H H

MLx

H2H2C CH2

H

HH3C CH3



Wilkinson’s hydrogenation catalyst

RhCl(PPh3)3

Very active at 25ºC and 1 atm H2

Very selective for C=C bondsin presence of other unsaturations

Widely used in organic synthesis

AcO

AcOH

H

H2 RhCl(Ph3)3

Prof. G. Wilkinson won the Nobel Prize in 1973

Other hydrogenation catalysts

[Rh(H)2(PR3)2(solv)2]+ With a large variety of phosphinesincluding chiral ones for enantioselective hydrogenation

RuII/(chiral diphosphine)/diamine

Extremely efficient catalysts for the enantioselective hydrogenationof C=C and C=O bonds

Profs. Noyori, Sharpless and Knowles won the Nobel Prize in 2001

Olefin hydroformylation

R

+ H2 + COcat

R

O

H

R

O

+

n-isomer i-isomer

Cat: HCo(CO)4; HCo(CO)3(PnBu3) HRh(CO)(PPh3)3; HRh(CO)(TPPTS)3

6 million Ton /year of products worldwideAldehydes are important intermediates towards plastifiers, detergents

(reductive elimination)


MLx

H H

MLn

H C C H(insertion)

MLn

H C C H

MLn + C C

H H

What else could happen if CO is present?

MLn

H C C H

OCCO MLn

H C C

C

O

HMLn +

H

C C

C

O

HCO insertion reductive elimination

R behaves as H did

Olefin hydroformylation

H H

H C C

H H

H

MLx

MLn

MLx

H H

MLx

H2H2C CH2

H

H

H3CH2C

H C C

O CH3

HMLn

H

CHO

CO

Catalysts for polyolefin synthesis

Polyolefins are the most important products of organometallic catalysis(> 60 million Tons per year)

•Polyethylene (low, medium, high, ultrahigh density) used in packaging, containers, toys, house ware items, wire insulators, bags, pipes.

•Polypropylene (food and beverage containers, medical tubing, bumpers, foot ware, thermal insulation, mats)

Catalytic synthesis of polyolefin

H2C CH2

H2C CH

CH3

isotactic

syndiotactic

atactic

Monomers

Polymerizationcatalysts

Polymers


H2C CH2

High density polyethylene (HDPE) is linear, d 0.96

“Ziegler catalysts”: TiCl3,4 + AlR3

Ti Cl + R3Al TiR

+

Electrophilic metal center

Vacant site

Coordinated alkyl

Insoluble (heterogeneous) catalyst


Isotactic polypropylene is crystalline

“Natta catalysts”: TiCl3 + AlR3

Ti Cl + R3Al TiR

+


Vacant site

Coordinated alkyl

Insoluble (heterogeneous) catalyst, crystal structure determines tacticity

H2C CH

CH3


“Kaminsky catalysts”


Vacant site

Coordinated alkyl

Soluble (homogeneous) catalyst, structural rigidity determines tacticity

H2C CH

CH3

+ MAO ZrR

+

XZr

R

+

X

Polymerization mechanism

M X + "R-Al" MR

initiation

MR

+ MR

M

R'

propagation

M

R' -HM H + P

+H2M H + P

+HXM X + P

termination

Olefin metathesisThe Nobel Prize 2005 (Chauvin,

Schrock, Grubbs)

RCH=CHR + R'CH=CHR' 2RCH=CHR'

N NRR

Ru

PCy3

PhCl

Cl N

Mo

H

CMe2Ph

O-C(CF3)2CH3H3C(F2C)2CO

Grubbs catalyst Schrock catalyst

(CH2)n

ring-closing (RCM)

ring-opening (ROM)

(CH2)n +

ADMET

n

n

ROMP

The metathesis mechanism (Chauvin, 1971)

Concepts and skil ls Unit 6 Chem 76/76.1/710G (Advanced Inorgan ic Chemistry)

Chapters 13-14. Organometallic chemistry and catalysis.

Main concepts Main skills Ligand classification and the 18-electron rule Oxidat ion states, coordination numbers Bonding of CO, alkenes, H2, carbenes to transition metals

To be ab le to identify L, X, LX, LnXm ligands and their electron count To be ab le to determine the number of valence electrons for a complex and to associate those values with stable, reactive and unstable complexes

Phospine ligands, electronic and steric par ameters Genera l features of metal complexes of hydride, phosphine, alkyl, alkene, carbene and cyclopentadienyl ligands. Fundamental react ions in organometallic chemistry: Lewis acid and Le wis base association dissociation, oxidative addition/reductive elimination, insert ion/deinsertion ( -H elimination). Attack on coordinated ligands. Reaction mechanisms Elements of homogeneous catal ysis: hydrogenation, hydroformylation, metathesis, polymerization, oxid ation

To qualitatively describe the bonding of metals to: ¹ -acid ligands (CO, alkenes, H2), carbenes To predict reactivity of metal complexes on the basis of fundamental reactions covered (ligand exchange, oxidative addition-reduc tive elimination, insert ion-deinsertion, attack on coordinated liigands) To explain stability/instability of metal alkyls (kinetic vs. thermodynamic stability) To select appr opriate characterization methods for various types of metal complexes To describe some important homogeneous catalysts and some general mechanisms of the catalytic reactions studied

Documents

Metal carbonyls may be mononuclear or polynuclear