This article was published as part of the

2009 Metal–organic frameworks issueReviewing the latest developments across the interdisciplinary area of

metal–organic frameworks from an academic and industrial perspective Guest Editors Jeffrey Long and Omar Yaghi

Please take a look at the issue 5 table of contents to access the other reviews.

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Phosphonate and sulfonate metal organic frameworksw

George K. H. Shimizu,* Ramanathan Vaidhyanathan and Jared M. Taylor

Received 26th January 2009

First published as an Advance Article on the web 12th March 2009

DOI: 10.1039/b802423p

Recent progress in phosphonate and sulfonate MOFs is reviewed with an emphasis on open

frameworks. These two ligating functionalities are paired due to their structural analogy but the

review will show that their differences likely outweigh their similarities when it comes to their

framework structures and properties. Examples that are highlighted focus on new routes to open

structures, demonstrations of porosity and functionality, and examples with dynamic structures.

This critical review is geared to researchers interested in designing open framework solids

(134 references).

Introduction

As part of a special issue concerning coordination network

materials, we will not provide general background on MOFs

other than where specifically germane to phosphonate and

sulfonate frameworks. In the broad domain of coordination

polymer/metal organic framework research, networks based

on phosphonate and sulfonate ligation are, by and large, less

studied. However, both families show great potential for new

functional materials. Phosphonates have demonstrated

promise for robust porous solids while sulfonates generally

show more promise for dynamic materials where structural

pliancy is desired.

For phosphonates, from the authors’ perspective, there are

three points that have likely hindered broader proliferation of

their study as MOFs, say relative to carboxylates. The first is

the predisposition of simple metal phosphonates to a dense

layered motif that, while still offering opportunities for

function, makes forming high surface area materials a

challenge.1–3 The second reason is that growth of single

crystals with phosphonates is generally more difficult as they

often precipitate rapidly as less ordered, insoluble phases.

While this does not preclude interesting properties, it does

make structural characterization, a hallmark property of

MOFs, a challenge. The final point is that, again relative to

other more studied ligating groups (e.g. COO–, pyridyl), the

coordination chemistry of phosphonates is less predictable

owing to more possible ligating modes and three possible

states of protonation.

Sulfonate networks have been studied considerably less than

other classes of ligand but for different reasons than listed for

phosphonates. Coordination of sulfonate anions is relatively

weak.4–6 This facilitates formation of crystalline products but,

oftentimes, the networks are not sufficiently robust to sustain

permanent pores. That said, the weaker ligation does enable

(single-crystal to single-crystal) solid state dynamics that

are increasingly gaining interest in coordination polymer

chemistry.7–9 In parallel with phosphonates, the spherical

ligating ability of a sulfonate does again make a priori

Department of Chemistry, University of Calgary, Calgary, Alberta,Canada T2N 1N4. E-mail: gshimizu@ucalgary.ca;Fax: 1 403 289 9488; Tel: 1 403 220 5347w Part of the metal–organic frameworks themed issue.

George Shimizu

George Shimizu completed hisBSc and PhD at the Universi-ties of Winnipeg and Windsor(S. Loeb), respectively. Hethen undertook postdoctoralwork at the University ofBirmingham (J. F. Stoddart)and the National ResearchCouncil of Canada (J. Rip-meester and D. Wayner). Pre-sently, he is a Full Professorand his research concernsinorganic–organic materials forenergy applications includinggas storage/separation andproton conduction. A research

theme is network solids that demonstrate dynamic motion(possibly) reconciled with crystalline architectures.

Ramanathan Vaidhyanathan

Ramanathan Vaidhyanathangained his PhD in Chemistryfrom the Jawaharlal NehruCentre for Advanced ScientificResearch (India), under thesupervision of Prof. C. N. R.Rao and Prof. S. Natarajan.His PhD aimed at investigat-ing the synthesis and applica-tions of open-framework metalcarboxylates. He then joinedthe group of Prof. M. J.Rosseinsky at the Universityof Liverpool, where he investi-gated the magnetic propertiesof Co oxide-hydrides, and

chiral metal–organic frameworks (MOFs). Presently he workswith Shimizu investigating the proton conduction and gas cap-ture applications of MOFs.

1430 | Chem. Soc. Rev., 2009, 38, 1430–1449 This journal is �c The Royal Society of Chemistry 2009

CRITICAL REVIEW www.rsc.org/csr | Chemical Society Reviews

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coordination predictions a challenge although sulfonates

appear almost exclusively as monoanions (RSO3�). As we will

discuss, this lack of regular inorganic assemblies is not wholly

detrimental. New inorganic clusters can form in situ and the

structure-directing role of the organic linker can be enhanced.

In comparing phosphonate and sulfonate coordination

chemistry in framework materials, the contrasts likely

outnumber the similarities so, for clarity, the two families will

be discussed in sequence rather than in parallel beginning with

the phosphonate networks. We will highlight some of the

prominent examples of both families but the overall goal of

this paper will be to provide the reader with a sense of the

challenges and potential of these families relative to more

widely studied MOF components. Several of the more

interesting open-framework compounds presented have not

had gas sorption data reported. In most cases, it is not certain

whether this is because the material collapsed or whether the

experiments were not able to be performed. In this review, we

would prefer to merely state the facts given in the cited work

rather than presuppose any additional properties or lack

thereof.

A brief overview of layered metal phosphonates will

be followed by open-framework phosphonates, loosely

categorized by those with homoleptic phosphonate coordina-

tion and those with additional ligands. The sulfonate examples

will be presented first as layered solids, followed by open

frameworks, hydrogen-bonded coordination complexes, and

finally a separate heading for dynamic materials.

Metal phosphonates

Layered metal phosphonates

As layered metal phosphate and phosphonates have already

been reviewed thoroughly,1–3,10 we will only provide brief

background on this large area. This field is highly relevant

to the core concept of a metal–organic framework. Metal

phosphates had been studied as layered inorganic solids but

evolved into the field of ‘‘hybrid inorganic–organic’’ solids by

appending organic groups off the rigid inorganic layers.11 This

made the interlayer, where guest molecules could be

introduced, much more tunable as it could range from hydro-

philic to hydrophobic; clearly there exists a parallel to MOF

chemistry. Pendant groups have included other functionalities

to impart chirality,12 photoactivity13 or even crown ethers.14

Guest intercalation in such systems is accompanied by layer

swelling and it is possible to even exfoliate the layers to

films;15 both phenomena are confirmation of a robust layered

architecture.

Several tactics exist to generate porosity in a metal-

phosphonate system. Within the layered structures, one can

replace some phosphonates with a non-pillaring group

(e.g. phosphate, phosphite or a small monophosphonate) in

order to create some interlayer space.16 This method was

introduced by Dines and co-workers.17 They prepared a

pillared zirconium phosphate/diphosphonate with rough

composition of Zr(O3PO(CH2)6OPO3)0.5(O3P(CH2)8PO3)0.5and hydrolyzed the hexamethylene ester moieties to obtain

porous materials with high surface areas. The problem with

this approach is that substitution is random so, even though a

porous phosphonate can be formed, structural characteriza-

tion and a narrow pore size distribution are challenges.

A second approach could broadly be classified as examples

where the geometry of the organic core in a polyphosphonate

disrupts the layered motif and necessitates an open framework.

Early work by Alberti and co-workers concerned zirconium

phosphite (3,30,5,50-tetramethylbiphenyldiphosphonate),18

where the pillar moiety enforced porosity. The specific surface

area as determined from BET analysis was 375 m2 g�1. A third

approach would be to employ a second functional group on

the ligand to chelate the metal ion and direct the structure

away from simple layers. This can result in an open phosphonate

framework though often these are heteroleptic structures, with

the second ligating group playing a substantial role. These

latter two approaches will be discussed in more detail as they

are germane to MOF chemistry.

Homoleptic open-framework metal phosphonate frameworks

While a layered motif is unquestionably the most common

observation for a simple metal phosphonate, an important

exception is the family of complexes observed with methyl-

phosphonic acid. The first 3D framework metal phosphonate

b-Cu(O3PCH3), shown in Fig. 1, a 1D channel system with the

methyl groups lining the pores, was reported by LeBideau and

co-workers in 1994.19 The distance between the opposite

methyl carbons was 5.97 A, leaving an effective pore size of

about 3 A. In the same year, Maeda and co-workers reported

the synthesis of polymorphs AlMepO-a and AlMepO-b, theframework compositions of which are Al2(O3PCH3)3,

and revealed their microporous nature by using gas

adsorption.20,21 Building blocks for both are composed of

one AlO6 octahedron, three AlO4 tetrahedra, and six methyl-

phosphonate units with different manners of connectivity. The

corner-sharing aluminates and phosphonate units formed 3D

open frameworks. Both Al materials possessed 1D channels

lined with methyl groups and both channels have triangular

cross sections about 7 A on edge. The nitrogen adsorption

isotherms showed that AlMepO-b was of Type I, as is normal

for zeolites and AlPOs, while that of AlMepO-a had a two-

step adsorption in the low-pressure region. This difference was

explained by the different triangular channel shapes of the twoJared Taylor

Jared Taylor gained his BScHons. in chemistry at theUniversity of Calgary. He iscurrently working towards hisPhD, as an Alberta IngenuityNanotechnology and NSERCof Canada Graduate Scholar,under the supervision ofProf. Shimizu. His researchfocuses on proton conducting,phosphonate-based metal–organic frameworks for lowto intermediate-temperaturefuel cell applications.

This journal is �c The Royal Society of Chemistry 2009 Chem. Soc. Rev., 2009, 38, 1430–1449 | 1431

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polymorphs. Propane, 2-methylpropane and 2,2-dimethyl-

propane gave Type 1 adsorption isotherms for both adsorbents

at 273 K. In contrast to a model where the inorganic layers

are absolute structure determinants with the organic groups

simple pendant, these structures show that inefficient

packing of the organic interlayer can direct the structure

away from the layered motif to that of 1-D channels.

Structural variation by this approach is quite limited as a small

phosphonate is essential.

After these reports, methylenediphosphonic acid was

successfully used to form a 3-D open framework,

Co2(O3PCH2PO3)�H2O, in 1997 by Lohse and Sevov.22 The

structure showed a 1-D inorganic channel lined with

methylene bridges. This spurred related efforts as several

phosphonate frameworks were made using ligands of the type

H2PO3(CH2)nPO3H2 (n = 1–4) and most of them had

inorganic frameworks with channels lined by the organic

groups.23–28 This approach hinged on the notion that a very

short linker between two phosphonate groups would disfavor

the default layered motif. As a representative example, a nickel

phosphonate [Ni4(O3PCH2PO3)2�(H2O)2], VSB-3, reported by

Cheetham and co-workers29 is presented (Fig. 2). The skeleton

of VSB-3 can be visualized as sheets of edge-sharing NiO6

trimers cross-linked in the third dimension by corner-sharing

NiO4 tetrahedra. This framework is then extensively decorated

by the bridging P–CH2–P units. The cross-linking of the layers

by the tetrahedral Ni centers generates narrow methylene-

lined pores (3.64 � 8.39 A). Completely dehydrated VSB-3

(VSB-4) shows excellent thermal stability up to 575 1C and

ferromagnetic interactions between Ni centers owing to its

three dimensional connectivities.

Varying the chain length in metal alkyldiphosphonates

alters the structures considerably. Compounds with –CH2–

chain lengths of n = 2–4 typically form pillared layered

structures23–28 as opposed to the open 3-D framework

above.20–22,29 No reports exist on n = 5–7 but these would

likely also form pillared layered motifs. When the alkyl chain

length of the diphosphonate was increased to n = 8, with

Co2+, a low-dimensional structure resulted composed of

cationic [Co(H2O)4(H4L)]2+ chains with charge-balancing

1,8-octylenediphosphonate clathrated in the structure.30

In this case, the pH of the complexation likely played a role

but the result is still surprising.

Trivalent vanadium has been complexed to members of the

same family of diphosphonate ligands used above,

[HO3P(CH2)nPO3H] (n = 2, 3).31–33 Five such frameworks

were reported which showed both 3-D and pillared layered

structural types. As these structures were not highly open,

their main interest was in the range of different anionic

vanadium clusters that resulted from the framework

formation. Other vanadium phosphonate structures have been

reported that include M2+-chelate subunits (e.g. [Cu(bipy)2]2+)

as templates and/or building units.34–37 These networks often

employ F� in place of O2� to generate frameworks with large

apertures (up to 19 A) where the guest metal complexes are

situated. In these studies it was shown that fluoride was

essential to effect mineralization and to induce crystallization

at the acidic pH of the reactions, although fluoride was not

incorporated into the structures in most cases.37 Appropriate

modification of reaction conditions could allow incorporation

of fluoride to produce oxyfluorinated materials.37

Use of polyfunctional phosphonate, N,N0-piperaziniumbis-

methylenephosphonate, with lanthanum chloride gave

rise to two polar three-dimensional open frameworks.38

Topologically similar inorganic ‘‘lanthanum phosphate’’

chains were linked in two different ways by the organic

ligands, the nature of which appeared to depend on the guest

Cl� ions. The frameworks were made up from one-dimen-

sional lanthanide phosphonate chains connected in three

dimensions by the piperazinium backbone. The piperazinium

cations imparted a macro-cationic nature to the framework.

The three-dimensional framework contained channels, lined

Fig. 1 Three-dimensional open-framework structure of copper

methylphosphonate19 showing the methylene decorated 1-D channels

running along the framework. Color scheme: Cu-cyan; P-purple;

O-red; C-grey.

Fig. 2 3-D open-framework structure of the nickel phosphonate,

Ni4(O3PCH2PO3)2�(H2O)2 (VSB-3).5 The layers (running perpendicu-

lar to the plane) are built up of trimeric edge-sharing NiO6 octahedra

while the cross-linking units (along the plane) are made up of NiO4

tetrahedra. Color scheme: Ni-cyan; P-purple; O-red; C-grey.

1432 | Chem. Soc. Rev., 2009, 38, 1430–1449 This journal is �c The Royal Society of Chemistry 2009

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by these piperazinium and phosphonate units, and occupied

by the Cl� ions. The genuine ‘‘polar’’ ordering of the

Cl-anions within the channels gave a polar framework topology.

No data was available on the porosity of the compound.

In a work related to earlier efforts pillaring phosphonate

layers with crown ethers, Clearfield reported the use of a

bis(phosphonate) derivative of a diazocrown ether to form

one-dimensional leaflet structures using both hydrogen-bond

interactions and covalent bonds.39 The partially deprotonated

form of the ligand was crystallized in the presence of ClO4�

ions to form hydrogen bonded 1-D chains consisting of

macrocyclic leaflet motifs (note: Co2+ ions from the

Co(ClO4)2 used were not incorporated in the structure). The

phosphonate groups from the adjacent ligand hydrogen

bonded among themselves to form a 1-D backbone. The

crown ether groups, which were attached to the phosphonate

groups via the N atoms, were arranged like leaflets around

these chains. The crown groups accommodated the solvent

water molecules and were aligned with respect to one another.

Complexing Cd(NO3)2 with this ligand formed 1-D Cd-N-

(phosphonomethyl)-aza-18-crown-6 chains also with a leaflet

like structure. In this case the Cd2+ centers were coordinated

by the crown phosphonate groups and also by nitrate anions

to again form a one-dimensional coordination polymer. The

author suggested that the work offered novel supramolecular

routes for grafting crown ethers into a polymeric matrix and

that such assemblies might exhibit distinct selectivity for guests

as compared to simple crown ethers in solution or the

solid state.

Another example of coupling phosphonate linkers with

crown ethers was reported but which also incorporated chirality.

Single crystals of homochiral lanthanide crown phosphonates

were prepared using an enantiomerically pure form of

2,2 0-pentaethyleneglycol-3,3 0-diphenyl-1,1 0-binaphthalene-

6,60-bis(phosphonic acid).40 The structure in this case was

made up of 1-D lanthanide phosphonate chains linked by

the binaphthyl backbones of the monoprotonated ligands,

thus creating a 2-D coordination network. The chiral crown

ethers decorate the interlayer spaces. Notably, the framework

was thermally and chirally stable to guest loss (including HCl).

An alternative approach to forming pores in metal phos-

phonates is to employ a large, multidirectional ligand that

would completely disfavor the formation of a layered inorganic

motif. The ligand, 1,3,5,7-tetrakis(4-phenylphosphonic acid)-

adamantane, was such a molecule as it possessed four

phosphonic acid moieties spaced by rigid phenyl groups from

an adamantane core. A number of reports of metal complexes

with this ligand have appeared. Neumann et al. reported two

papers41,42 concerning open, but less ordered, frameworks

with Ti4+ and V3+ and this ligand. No crystal structures were

obtained but the Ti sample41 showed broad features at 5.8 and

11.21 in the PXRD. This, with other supporting data, led to a

model being proposed with 22 � 9 A pores. Gas sorption

measurements gave a surface area of 557 m2 g�1. Using the

same ligand, the same group reported catalytic activity of a

mesoporous V framework42 for aerobic oxidation or benzylic

alcohols to aldehydes.

Taylor et al. reported that the crystal structure of the

Cu2+ network of the same tetraphosphonate ligand.43

[Cu3(H3L)(OH)(H2O)3]�H2O�MeOH, consisted of trigonal

trinuclear copper clusters linked by the organic spacer into a

diamondoid net (Fig. 3). The cluster was composed of three

copper centers coordinated by three separate phosphonate

groups and capped by a fourth phosphonate group to create

a pseudo-tetrahedral arrangement of phosphonates. The other

side of the cluster contained a m3-hydroxyl group to

charge balance. The network was a doubly interpenetrated

diamondoid structure but the bulky copper clusters resulted in

voids filled with H2O/MeOH. CO2 sorption revealed a BET

surface area of 200 m2 g�1. Variable-temperature powder

X-ray diffraction showed even at 30 1C there was some loss

in crystallinity, which was pronounced by 280 1C. This result

indicated that there was some collapse of the structure upon

desolvation but the material did still remain porous. This

main outcome of this work was to show that, with metal

phosphonates, that the organic linker could in fact determine

the inorganic aggregate as new metal clusters were formed

as required by the diamondoid topology. Despite the trimeric

Cu core, the material showed efficient antiferromagnetic

coupling.

Another accessible core unit, analogous to trimesic acid,

would be 1,3,5-benzenetriphosphonate where the trigonal

substitution should again disfavor a simple layered motif.

A study on Cu2+ complexes of this ligand, with and without

additional N-donors, showed a range of results.44 The simplest

combination, {Cu6[C6H3(PO3)3]2(H2O)8}, was a highly

hydrated network with the ligand in the 6� state. Tetracopper

clusters were the building units. Porosity was not examined in

this material. The compounds formed with 4,40-bipy and 4,40-

dipyridylpropane incorporated the ligand in 2� and 4� states,

respectively. Of these two complexes, the dipyridylpropane

complex yielded a material with solvent-filled channels

of B9 � 14 A dimension. These were formed by the

pillaring of Cu(1,3,5-benzenetriphosphonate) layers by the

Fig. 3 Crystallographic representation of a diamondoid cage formed

from the 1,3,5,7-tetrakis(4-phenylphosphonic acid)adamantane tetra-

hedron and pseudo-tetrahedral tricopper clusters. The network is

two-fold interpenetrated and the material displays a BET surface area

of 198 m2 g�1. Color code: Cu-cyan; P-purple; O-red; C-grey.

This journal is �c The Royal Society of Chemistry 2009 Chem. Soc. Rev., 2009, 38, 1430–1449 | 1433

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dipyridylpropane. Gas sorption was not reported but again,

new inorganic clusters were formed.

Using a similar approach but with a larger core,

Vaidhyanathan et al. reported the ligand 1,3,5-tris(4-phosphono-

phenyl)benzene, a triphosphonate with a rigid D3h geometry.45

Upon hydrothermal reaction with Sr(OH)2 in the presence

of MeOH, the triphosphonate ligand formed a two-fold

interpenetrated, 3-D crystalline solid with the formula,

Sr2(H2L)(CH3OH)(H2O)4, shown in Fig. 4. This compound

contained 1-D SrO chains linked together by the trigonal

ligand. Each ligand bonded to three different chains and vice

versa, so a three-dimensional framework with large 1-D

channels along the a-axis resulted. In a single column, the

ligands were separated by 6.92(1) A, a distance suitable for

inclusion of another aromatic system. The 1-D inorganic

chains from one network filled the channels of the other to

form an interpenetrated structure. Though interpenetrated,

clefts remained between the three peripheral aryl rings. These

were the proposed sites of CO2 uptake as sorption isotherms

indicated that this material was porous with a BET surface

area of 146 m2 g�1. When left standing, it was observed that

the complex took up CO2 from the air, as confirmed by IR

spectroscopy. The topology of this network had design

implications as, with longer phenyl spacers, a third inter-

penetrating network would not be possible and increased

surface area would need to result.

A homochiral open-grid, layered solid has been reported for

the Ln complexes of a bisphosphonated binaphthyl ligand.46

In these structures, Ln atoms as vertices, each linked four

different binaphthyl ligands, to form a 2-D open rhombic grid

which then stacked in the third dimension (Fig. 5). Each Ln

vertex was eight-coordinate incorporating four molecules of

water in its coordination sphere. Upon dehydration, the

network transformed to an amorphous phase but notably,

the parent structure could be reformed upon exposure to water

vapor, illustrating that while long-range order was lost, the

local coordination environment was likely maintained. While

the nature of the amorphous material was not commented on

at length in this work, most likely, the structure shifted to

enable increased ligation of the Ln ions by the phosphonate

groups.

The next section of this review will deal with mixed ligand

phosphonates as a route to generate porosity. Our final

example47 on homoleptic systems actually concerns a complex

that was designed with the heteroleptic approach in mind. The

result was a Zn network with homoleptic PO3 coordination

and permanent porosity.47 The ligand, 1,4-dihydroxy-2,5-

benzenediphosphonic acid was prepared with the intent of

forming a chelating ring between the hydroxyl group and the

phosphonate. With Zn2+ in DMF, a phase-pure product,

Zn(H2L)(DMF)2, was formed with a structure of 1-D columns

of phosphonate-bridged Zn centers. From these columns, the

R groups protruded in four directions to form a square grid.

The role of the non-coordinating hydroxyl groups was not to

perturb the structure by chelation but rather simply by steric

hindrance; in comparison 1,4-benzenediphosphonic acid

readily forms layered structures with most metals.48 Through

PXRD and TGAmeasurements, it was determined that the Zn

complex was stable to loss of B80% of the included DMF

molecules, which inferred porosity. CO2 and N2 sorption

analysis gave BET surface areas of 216 and 209 m2 g�1,

respectively. Upon complete loss of DMF, a loss of order

was observed by PXRD, but order was regained upon resolva-

tion. The free hydroxyl group in this compound represented a

potentially reactive site for post-synthetic modification.

Open-framework phosphonates from mixed ligand systems

The study of novel open-framework metal phosphonates was

accelerated by attaching functional groups to the phosphonic

Fig. 4 The two interpenetrating nets supported by 1-D Sr(PO3)

columns in the Sr2+ structure of tris-1,3,5-(4-phosphonophenyl)-

benzene. Clefts exist between the peripheral aryl rings of like nets.

Fig. 5 Homochiral framework of lanthanide phosphonate material

made using the bulky chiral ligand, 2,20-diethoxy-1,10-binaphthalene-

6,60-bisphosphonic acid. The largest channel has a cross section

of B12 A. The material is able to show interesting sorption and

catalytic activities arising from the microporous nature of the pores.

Color code: Gd-orange; P-purple; O-red; C-grey. The space-filled

model has been superimposed.

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acid ligands.49–52 A new functional group on the organo-

phosphonate ligand would perturb the layered structure in

metal diphosphonates with the result hopefully being a new

3-D open framework. The functional groups attached to the

phosphonic acid to create 3-D open frameworks have included

imino,51 hydroxyl53 carboxylic acid54,55 sulfonic acid56–58 and

pyridine.37,59 This is a large field where many structurally

characterized products with infinite metal ligand connectivity

have formed dense solids. Here, we will present a selected

sampling of those with either open-framework structures or

some interesting functionality.

Stucky and co-workers reported an intriguing open-

framework Zn-carboxyphosphonate, Zn3(O3PCH2COO)2-

(O3PCH2COOH)(NH3CH2CH2NH3)�(BTC).60 This had a

complex, interrupted tetrahedral zeolite-like structure

(Fig. 6) which was markedly different from structures

made with other aliphatic diphosphonates or phosphono-

carboxylates.25–28,31–33,38 The framework had large 24-ring

channels lined by the carboxylate end of the ligand,

H2O3PCH2COOH. The pores were occupied by free benzene-

tricarboxylate (BTC) molecules, which were used in the

synthesis. The structure-directing agents, diprotonated ethylene-

diamine molecules in this case, were not located at the center

of the 24-ring channels, but instead were within the wall that

separated the 24-ring channels. To further probe the effect of

BTC and the role of the carboxylates in the synthesis, three

different carboxylic acids were used in the reaction mixture:

1,2,4,5-benzenetetracarboxylic acid, cis-1,2-cyclohexanedi-

carboxylic acid and cis-1,3,5-cyclohexanetricarboxylic acid.

All of these carboxylic acids led to the formation of the

same framework with the 24-ring apertures. Interestingly, this

particular framework has only been reported in the presence of

templating polycarboxylates. Gas sorption data were not

available.

Ferey and co-workers61 reported the first open-framework

lanthanide-carboxyphosphonates, M4(H2O)7[O2CC5H10-

NCH2PO3]4�(H2O)5 MIL-84 (M = Pr, Y). These isostructural

lanthanide compounds had a three-dimensional open-

framework structure and showed significant thermal stability.

The structure possessed a one-dimensional inorganic sub-

network, built up from chains of edge-sharing rare-earth

polyhedra, interconnected via the organic acids to create an

open-framework structure with small water-filled pores.

X-Ray thermodiffractometry was used to show that the

MIL-84(Pr) was stable up to 523 K and possessed a reversible

hydration–dehydration capability. Gas sorption data were not

available.

Alternatively framework compounds have been made using

pyridyl ligands carrying phosphonate functionalities on their

ring.59,62,63 A series of lanthanide–transition metal compounds

were made using the unsymmetrical 2-pyridylphosphonate

ligands.64 Despite varying the lanthanide metal through

almost the entire lanthanide series the compounds formed

only two different 3-D frameworks. One type was a hydrate

with a chiral framework: Ln2Cu3(C5H4NPO3)6�4H2O I

(Ln = La, Ce, Pr, Nd) and the other was the anhydrous form:

Ln2Cu3(C5H4NPO3)6 II (Ln=Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho).

Structures I had a three-dimensional framework with a

3-connected 10-gon (10,3) topology resembling the Si net of

SrSi2, in which the Ln and Cu atoms are alternately linked by

the phosphonate oxygen atoms. Each 2-pyPO32� ion involved

in the framework served as a tetradentate chelating and as a

bridging ligand. The chiral framework consisted of helical

one-dimensional channels occupied by water molecules and

the solvent-accessible-volume per unit cell was calculated to be

8%. No gas sorption data were reported.

Anhydrous forms II had a complex structure,64 wherein the

Ln centers were connected into a chain exclusively by the

phosphonate groups, whilst the Cu centers coordinated to

both pyridyl and phosphonate groups. The entire three-

dimensional framework could be visualized as being built up

from the inter-linking of Ln chains by the Cu(2-pyPO3) units.

The resulting spaces in the framework were occupied by the

heterocyclic ring backbone of the ligand. Theoretical studies

were carried out to estimate the lattice energy of these

structures and their variation with the ionic radii of the

lanthanide. Interestingly, the correlations of the lattice

energies with the radii of metal ions followed the principle of

lanthanide contraction. Accordingly, the larger ions seemed to

prefer the open-structure capable of accommodating water

molecules, while the smaller ones adopted the denser

anhydrous structure. The calculations also indicated that to

some extent the coordinative versatility of the Cu2+ ion had a

role in determining the framework type.

A relatively longer pyridyl-phosphonate linker was used to

form polar, non-centrosymmetric frameworks by Lin and co

workers.49 Three different compounds formed using ethyl-4-

[2-(3-pyridyl)ethenyl]phenylphosphonate) (L-Et), were

isostructural and crystallized in the noncentrosymmetric space

group Fdd2. They formed a complicated 3D framework

structure composed of [M2(L-Et)4(m-H2O)] building units,

Fig. 6 Open-framework of Zn carboxymethylphosphonate, posses-

sing large 24-membered channels and smaller 8- and 12-membered

rings. The largest channels are lined by the carboxylate end (dark grey)

of the ligand. The larger channels are occupied by free benzenetri-

carboxylate moieties (not shown). Also, the structure-directing

ethylenediamine molecules are within the wall that separates the

24-ring channels. Color code: Zn-cyan; P-purple; N-blue; O-red;

C-grey. The space-filled model has been superimposed.

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far from a layered motif. Within an [M2(L-Et)4(m-H2O)]

building unit, each metal center was connected to the other

metal center through two phosphonate groups and one

bridging water molecule. Each [M2(L-Et)4(m-H2O)] unit was

then linked to four adjacent [M2(L-Et)4(m-H2O)] units via

double L-Et bridges to form a 3D solid. Flexible ethoxy

groups on the L-Et ligands effectively filled all the void spaces

generated within the 3D framework, and these compounds did

not have any included solvent molecules. Although they

crystallized in polar space group Fdd2, all the L-Et ligands

adopted a centrosymmetric arrangement with all their dipoles

cancelling each other. Their polarity resulted from a parallel

arrangement of the bridging water molecules as well as

the phosphonate groups. All these polar compounds

exhibited a second harmonic generation higher than that of

potassium dihydrogen phosphate. In the same article, a

centrosymmetric solid, [Cd(HL)2], was also presented. This

Cd compound had a dense pillared-layered type framework

composed of sinusoidal Cd-phosphonate chains linked by the

pyridyl ligands. No adsorption data were reported on any of

these compounds.

Systematic high-throughput analysis was employed to

screen conditions for growth of a Ln sulfo-phosphonate

network.65 Accordingly, two high-throughput experiments

comprising 96 individual hydrothermal reactions were

performed to systematically investigate the influence of pH,

rare-earth ion, molar ratio of Ln3+ : H3L, and the counterion

in the system LnX3–H3L–NaOH–H2O with X = NO3�, Cl�

and CH3COO�. It was observed that under basic conditions

Ln(OH)3 was formed, while acidic reaction conditions lead to

nine isotypic compounds Ln(O3P–C2H4–SO3)(H2O) with Ln)

La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8) and

Dy (9). These dense pillared-layered 3-D structures were built

up from cross-linking of the Ln-sulfonato-phosphonate layers

by the organic groups. Surprisingly, the only result which

varied was the crystal size of the compounds; the crystal size

increased with decreasing pH and increasing ionic radii of the

lanthanide involved. No significant influence of the

counterions of the rare-earth salts was observed.

Two related Zr4+ compounds have been reported

with bis(aminodiphosphonate) ligands. Zr(HPO3CH2)2N–

C4H8–N(CH2PO3H)2�4H2O66 has a layered architecture

formed by the linking of one-dimensional Zr–PO3 subunits

by the organic spacers. The layers were aligned creating

one-dimensional tunnels (10 � 3.5 A) running along the

interlayer axes, which were occupied by guest water

molecules. An analogous compound where the central

linker was changed to a cyclohexyl ring gave

Zr(PO3CH2)2NC6H10N(CH2PO3)2Na2(H2O)�5H2O, a three-

dimensional open-framework compound. This had a

‘brick-wall‘ type framework formed once again by the linking

of inorganic chains by organic linkers (Fig. 7). The framework

consisted of rectangular channels (12 � 5 A), wherein guest

water molecules and charge balancing Na+ ions reside. This

3D compound showed reversible hydration and dehydration

properties. N2 adsorption studies on this and the butyl

derivative showed negligible surface areas which was attri-

buted to the organic linkers being insufficiently rigid to prevent

structural relaxations occurring during loss of guests.66

None of the compounds presented in this section to this

point have reported gas sorption data to confirm porosity.

There was no report of gas adsorption in non-layered metal

phosphonates until recently. In 2006, MIL-91 (Fig. 8)

which has the proposed formula MX(N,N0-piperazine-

bis(methylenephosphonate)�nH2O (n = 4.5 or 3; MX =

TiO, AlOH) was reported by Ferey.67 These 3-D open frame-

works were built from inorganic corner-sharing chains of TiO6

or AlO6 octahedra linked in two directions via the diphos-

phonate moieties. Small channels (3.5 � 4.0 A2), filled with

free water molecules, were formed in each case. Thermogravi-

metric analysis and X-ray thermodiffractometry of the samples

Fig. 7 Three-dimensional framework of Zr(PO3CH2)2NC6H10N-

(CH2PO3)2Na2(H2O)�5H2O, showing the rectangular channels

(12 � 5 A) present. Color code: ZrO6 octahedra-brown; PO3C

tetrahedra-purple; C-grey; N-blue. The charge-balancing Na+ ions

and guest water molecules in the channels have been removed for

clarity.

Fig. 8 Highly porous framework of the aluminum and isostructural

titanium diphosphonates (MIL-91) formed using N,N0-piperazinebis-

methylenephosphonic acid. View of channels along the c-axis. The

zeolitic structure renders high porosity to the material (B500 m2 g�1).

Color code: Al/Ti-olive green; P-purple; N-blue; O-red; C-grey. The

space-filled model has been superimposed.

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revealed that water was lost reversibly below 423 K and that

both structures were stable up to 463 K. Nitrogen sorption at

77 K gave Langmuir surface areas close to 500 m2 g�1. Strong

Ti–O or Al–O bonds and no interpenetration in the open

frameworks contributed to the thermal stability and relatively

high surface area of these two compounds.

In 2008, Wright and Ferey reported three new phases of

Ni-N,N0-piperazine-bis(methylenephosphonate).68 Structures

of octahydrated (NiSTA-12), dihydrated and anhydrous forms

were determined using synchrotron PXRD measurements.

The structure, shown in Fig. 9, could be broadly described

as helical chains of edge-sharing NiO5N octahedra linked into

a honeycomb arrangement by the ligand. Dehydration of the

parent phase resulted in topotactic formation of the

anhydrous phase which possessed elliptical pores 8 � 9 A in

free diameter. The specific pore volume of dehydrated

NiSTA-12 was B0.21 cm3 g�1, which was comparable to

those in some of the large-pore zeolites (zeolite beta =

B0.22 cm3 g�1). Adsorption measurements were performed

on dehydrated NiSTA-12 using various gases including CO2,

H2, CH4 and CO. At 304 K and 1 atm, the network showed a

10-fold higher uptake for CO2 over CH4. IR measurements

coupled with low-temperature adsorption studies using H2 and

CO as probe molecules, indicated that the Ni2+ centers acted

as weak Lewis acid sites enhancing the interaction of the

adsorbed gases and PQO groups projecting into the pores

were also acting as adsorption sites. Finally, porosity was also

demonstrated using molecular absorption experiments, where

toluene, o-xylene and mesitylene were used as probes giving

uptake of 8.9, 6.2 and 5.8 wt%, respectively.

The use of amino acid derived phosphonates to form

porous architectures resulted in the formation a homochiral

metal-phosphonate solid.69 The chirally pure form (S)-proline,

N-(phosphonomethyl)proline was reacted with a series of

lanthanides to form isostructural solids. These compounds

comprised triple-stranded helical chains wherein the lanthanide

centers were connected via phosphonate units and the

carboxylate end running all around the chains and projecting

into the inter-chain spaces. The chains form strong hydrogen

bonds with other adjacent chains through the carboxylate

groups. Such stacking of the chains result in a three-

dimensional structure with 1D tubular channels of 4.32 �3.81 A free diameter. These channels are occupied by helically

arranged water molecules. The water molecules in the channels

could be removed without collapse of the framework. The

surface area of the Dy compound was determined using N2

adsorption to be 86 m2 g�1, and the compound was shown to

take up water and methanol in a reversible fashion. However,

no adsorption data is available for the other phases.

As the use of a wider R group can exclude the formation of

a simple layer, so can filling the coordination sites on the metal

centers with stronger ligands. The use of ancillary chelating

ligands such as 2,20-bipyridyl, terpyridyl or 1,10-phenathroline

with phosphonate have resulted in lower dimensional

structures.34–37 In particular, the group of Mao has been

studying the effect of ancillary ligation,55–57,63 with both

chelating and bridging ligands, on the networks formed by

metal phosphonates. This had led to the observation of new

inorganic clusters within frameworks.55–57,63 The main

impetus of this research is not so much the generation of

porosity but rather new physical properties stemming from

the inorganic aggregates. Magnetism has been a focus

and much of the research on lanthanide phosphonates is

targeting new luminescence properties.70 As an example, use

of the polyfunctional tetraphosphonic acid ligand, (H8L =

(H2O3PCH2)2NCH2CH2CH2CH2N(CH2PO3H2)2), in combi-

nation with oxalic acid gave rise two different types of

lanthanide-based luminescent three-dimensional frameworks.71

The first type was built up from zigzag LaOx chains

which were interconnected by the phosphonate groups in three

dimensions giving rise to a honeycomb-like open framework

structure with tunnels (4 � 6 A) that are occupied by the

carbon backbone of the organophosphonate ligand (Fig. 10).

The other type made using Nd and Eu were isostructural with

Fig. 9 Framework of nickelN,N0-piperazine-bis(methylenephosphonate)

showing the large open spaces (7–10 A) present within the structure.13

The framework is made up of the linking of homoleptic Ni-phospho-

nate chains by the piperazine backbone. The Co and Fe analogues are

isotypical with this. The pores are occupied by water molecules

(not shown for clarity) and the structure collapses to a denser phase

on dehydration. Color code: NiO6 octahedra-cyan; CPO3 tetrahedra-

purple; N-blue; O-red; C-grey.

Fig. 10 A lanthanum phosphonate/oxalate structure where channels

are defined by crosslinking oxalate and PO3 groups and filled by the

organic linker of the phosphonate.

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a much denser pillared–layered architecture. The La com-

pound with honeycomb-like structure exhibited blue lumines-

cence, while the Nd one showed luminescence in the near-IR

region and the Eu compound showed red luminescence. The

author identified that the use of oxalic acid as a second ligand

provided better control over the crystallization of the

lanthanide phosphonates. Water is also included in the

channel but porosity measurements were not reported.

Metal sulfonates

Metal sulfonates have been examined with transition metals as

potential analogues of metal phosphonates. Under anhydrous

conditions, layered solids reminiscent of a-Zr phosphonates

are obtained and IR spectroscopy can provide a useful

diagnostic for the sulfonate coordination mode.72 Under

hydrous conditions, with transition metals and most hard

metal ions, the most frequent observation is the formation

of ion pairs between fully hydrated metal cations and

sulfonate anions, that is, no true network is formed.73–76 A

key point is that the bonding tendencies of any functional

group in solution, either in molecular complexes or in discrete

assemblies, do not transfer linearly to its coordinative tenden-

cies in network solids. A network solid represents the extreme

case of cooperative bonding interactions between components,

so-called matrix effects, and so what may be a weak inter-

action in a discrete system in solution can have its stability

augmented dramatically in an infinite solid. That said, as will

be shown in the examples to follow, with softer metal ions or

in a hydrogen bonding role, sulfonates can play key roles in

the formation of stable and functional coordination polymers.

Layered and/or dense metal sulfonates

The structure of a prototypical hybrid inorganic–organic solid

is that of rigid inorganic layers that provide a scaffolding of

regular anchor points for pendant organic groups. The

structures of silver p-toluenesulfonate77,78 and silver

benzenesulfonate79 both fit this class of solid but, a closer

look showed that not only were the aryl groups oriented

differently, they were anchored to the layer at different points.

That is, the silver coordination and the layer structure were

fundamentally different in the two compounds. In the benzene-

sulfonate salt, the silver(I) ion adopted a six-coordinate

geometry and the SO3 groups each bridged six different silver

centers (i.e. a saturated m6 mode) whereas in the OTs salt

(OTs = 4-toluenesulfonate also known as tosylate), the

aromatic group was much more tilted and the SO3 group

bridged only five Ag ions. A key observation that was

extracted from these two examples, which diverged from the

pattern observed for metal phosphonates, was that the

inorganic backbone did not provide an inflexible skeleton

upon which the organic groups were merely pendant. In silver

(metal) sulfonates, the organic groups play a structure affecting

role, if not necessarily a structure determining one. The differ-

ences in the two layered solids arose simply from the presence

of the methyl group in the 4-position of the phenyl ring in

AgOTs. The larger inference of this small observation was that

packing and aryl–aryl interactions in the interlayer could affect

the coordination modes of both the silver ions and SO3 groups

in the layer itself.

Given the apparent role of the R group, Cote et al.

performed a study where the breadth of the R group on the

sulfonate was systematically increased and the effect on

structure determined crystallographically.80 To determine this

structural tolerance, it was first necessary to define an ideal

framework as a reference. For this purpose, Ag benzene-

sulfonate, with its m6 coordination mode of the SO3 group,

was chosen. The trace of the unit cell for this compound onto

the inorganic layer gave a rhombus with an area of 23.45 A2.

This was viewed as the area in a plane required for a sulfonate

group to coordinate to silver(I) ions in a simple layered motif.

Taking this area, 23.45 A2, and dividing this value by the

width of an aryl ring (3.66 A), gave 6.41 A. This value

represented, for silver sulfonates, the breadth of an aryl group

that could be accommodated while maintaining a lamellar

structure. This prediction was tested with four different

monosulfonate aromatic R groups, of varying breadth, and

silver(I) giving five complexes: [Ag(4-biphenylsulfonate)]N,

[Ag(2-naphthalenesulfonate)]N, [Ag(H2O)0.5(1-naphthalene-

sulfonate)]N, [Ag(1-naphthalenesulfonate)]N and [Ag(1-pyrene-

sulfonate)]N.80

The compounds with biphenyl and 2-naphthyl appendages,

respectively, had lateral breadths below the calculated

threshold and both formed simple layered solids. For the

1-naphthyl appendage, which exceeded the threshold by a

relatively small amount (6.93 vs. 6.41 A), the Ag sulfonate

backbone was displaced to allow the incorporation of a water

molecule or, under anhydrous conditions, shifted to allow the

formation of Ag–p interactions with half of the naphthyl

groups.80 For the 1-pyrene appendage, which significantly

exceeded the estimated critical breadth to allow a continuum

of SO3-bridged Ag ions (8.14 A vs. 6.41 A), the structure

adapted to a greater extent by forming cation–p interactions

between silver(I) centers and all the aromatic moieties. The

formation of p interactions with the appended arene and the

silver ion necessitated the conversion of 2-D AgSO3 layers into

1-D columns. Based on the structural prediction and the

obtained results, classification for silver sulfonates into three

families was proposed, referred to as Type 1, 2 and 3. Type 1

structures were those sustained exclusively by bonding

between Ag ions and sulfonate oxygen atoms. Type 2 struc-

tures involved a continuum of interactions between Ag ions

and sulfonate oxygen atoms but with ancillary ligation by

additional simple Lewis bases. Type 3 networks involve

coordination between Ag ions and sulfonate oxygen atoms

but with additional coordination of Ag by the p system of the

appended arene. Beyond providing a predictive reference for

the design of silver sulfonates, this work affirmed the structural

role of the organic groups in these networks.

A silver sulfonate was reported by Ma et al. in which

5-sulfosalicylic acid was used as a multifunctional ligand in

order to construct the framework.81 Two structures were

reported, in which both the sulfonate group and the

carboxylate group coordinate to the silver centers. In the first

structure, the 5-sulfosalicylic acid was doubly deprotonated

and in a 2 : 1 metal : ligand ratio, and this formed a 3-D

framework in which the sulfonate, carboxylate and hydroxyl

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group coordinated to Ag centers. The carboxylate groups

served to bridge two unique Ag ions, which were connected

through an Ag–Ag bond. The carboxylate bridged Ag ions

were further connected to two other unique Ag ions through

sulfonate coordination. The second reported structure was a

2-D framework where the metal : ligand ratio was now 1 : 1

and there was no coordination from the hydroxyl group. In

this framework, sulfonate groups served to link Ag centers

into a 1-D coordination polymer and these chains were linked

into two dimensions through bridging carboxylate groups.

Solid-state photoluminescent properties were investigated on

both structures.81

Studies have also been published which investigated the

structure directing role of neutral ligands on Ag sulfonate

frameworks. In one report by Li et al., three sulfonates:

1-naphthalene sulfonate, p-toluenesulfonate and 1,3,6,8-

pyrenetetrasulfonate were used in order to construct silver

frameworks, with pyrazine as the neutral linker.82 The

1-naphthalene sulfonate ligand was also used with hexamethyl-

enetetramine and b-picoline to construct silver frameworks.

It was found that for all the structures, the neutral ligand takes

on a structure directing role, for example the pyrazine bridges

silver centers into 1-D chains, where the chains are linked into

ladders through bridging sulfonates. The divergent pyrene

sulfonate linked the 1-D chains into a 2-D layer. The hexa-

methylenetetramine linked Ag centers into 2-D layers, where

the sulfonate only served to fill a single coordination site on

the silver center and the b-picoline forced the silver to

crystallize as 0-D dimeric pairs which were capped by

sulfonate groups.82

In a separate report by Li et al., further neutral ligands were

used to form frameworks with three mixed ligand mono-

sulfonates.83 The neutral ligands used were 4,40-bipyridine,

1,10-(1,4-butanediyl)-bis(imidazole) and b-picoline, and the

sulfonates used were 5-sulfosalicylic acid, p-aminobenzene-

sulfonate and p-hydroxybenzenesulfonate. Again the neutral

ligands took on the structure directing role and frameworks

formed were either 1-D chains with the bridging neutral

ligands or 0-D dimers with the b-picoline. The sulfonates

served as either non-coordinating counter anions or capped

a single coordination site on the Ag. Photoluminescence was

investigated on the solids in both of the above reports.83

The steric effects of the neutral ligands were also investi-

gated in silver sulfonate frameworks through the use of alkyl-

substituted pyrazine derivatives.84 In this case, two sulfonate

ligands were used, p-aminobenzenesulfonate and 6-amino-1-

naphthalenesulfonate, and five mono- or dialkyl-substituted

pyrazine derivatives were used. Eleven structures were

reported, with eight having the expected 1-D chain motif from

bridging pyrazine units; one chain structure was linked into

2-D sheets through a bridging p-aminobenzenesulfonate unit,

and one was linked into 2-D sheets through the 6-amino-1-

naphthalenesulfonate. One interesting structure had the 1-D

chains linked through the 6-amino-1-naphthalenesulfonate

ligand, with the pyrazine acting as a monodentate ligand on

the Ag center. In general it was found that the pyrazine again

played the major structure directing role, although the steric

effects of the alkyl groups and bulky sulfonate ligand, as well

as p-interactions between the neutral and sulfonate ligands

could disrupt the coordination of the pyrazine. Photolumines-

cence studies were performed on these materials.84 More

comment on predictive trends in metal sulfonates will follow

later in this review when discussing alkaline-earth complexes.

For solids that are strongly bonded in two dimensions, such

as metal phosphonates, as mentioned, a fundamental physical

observation is that the layers can be separated and other

molecular species intercalated between the layers. The pliancy

of the layer structure of silver sulfonates would seem to

preclude that but, to certain extents, this is not the case. In

one example,76 after treatment of AgOTs with nonylamine and

heating to 70 1C, the indexed PXRD pattern gave a unit cell

with two axes, closely related to AgOTs, while the third axis

increased markedly. These data were consistent with retention

of the inorganic layers, albeit with minor rearrangement, and

the expected swelling of the structure. A more detailed study

on the intercalation in silver sulfonates was reported by Cote

et al.80 In this case, single-crystal X-ray structural data of an

actual intercalate complex was obtained which showed the

ethanol solvate formed a coordinative interaction. A series of

alcohols of varying chain length were examined and, by

PXRD and thermogravimetric analysis (TGA), it was

confirmed that the entire series was structurally related with

the guest alcohols adopting an identical coordination mode to

the layer as with ethanol, i.e. the intercalation was topotactic.

Intercalation of amines, at lower loadings, was also

observed for the compound Ag(4-pyridylsulfonate).85 This

solid also formed a layered network but not of a prototypical

hybrid inorganic organic solid. Here, linear silver-pyridine

units were crosslinked by 1-D columns of silver sulfonate

aggregates. Unlike hybrid-inorganic–organic solids, these

layers incorporated the R group rather than it being pendant

to the layers, however, like these solids, the layers formed

continuous sheets. At loadings up to a 1 : 1 amine : L ratio,

Ag(4-PSA) showed intercalation confirmed by PXRD experi-

ments. At higher loadings, a network rearrangement occurred

that necessitated cleavage of Ag–pyridine bonds as confirmed

by solid-state 109Ag NMR.86 This type of structural rearran-

gement has been observed with some Cd sulfonate networks

where the guest amines coordinate the metal ion and displace

sulfonate ligands.87,88

The metal sulfonate examples above have their functions

associated with the accessibility of their interlayer regions.

There are examples of functional metal sulfonate networks

where layered structures are observed and the interlayer is

largely densely packed. Monge et al. have reported a family of

lanthanide sulfonates which act as oxidation catalysts.89 Eight

lanthanide arene disulfonate structures were reported of three

structural types, five isostructural frameworks using a 1,5-

naphthalenedisulfonate (1,5-NDS) as linker, and three struc-

tures of two types using a 2,6-naphthalenedisulfonate

(2,6-NDS) linker. The three frameworks (as Nd salts) were

tested as oxidation catalysts for the conversion of linalool into

cyclic hydroxy ethers. Further, the 1,5-NDS framework

was tested as La, Pr and Nd salts for comparison between

lanthanide ions. It should be noted that the pores in these

solids were not of sufficient size to accommodate the molecules

and the authors confirm that the reactions were occurring

on external surfaces. Of the five frameworks tested, the

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La(1,5-NDS) showed the best conversion (100% in 22 h) with

only a minor selectivity for the furan over the pyran rings

(3 : 2 ratio in all cases). The catalytic activity was dependant

on the coordination number around the Ln centers, with the

3-D 2,6-NDS frameworks with nine-coordinate Ln having the

least catalytic activity (17% in 48 h). These authors have also

reported Co2+ and Ni2+ phenanthroline/1,5-naphthalene-

disulfonate coordination polymers which they examined

as oxidation catalysts for the conversion of thioethers into

sulfones and sulfoxides.90

Kennedy et al. have examined the effect of packing on the

properties of sulfonated dyes.91 In these solids, the crystal

structure affects stability to heat, light and solvents, as well as

the crystal morphology. The morphology in turn affects

binding strength, stability, flow and dispersion properties of

the dyes as a crystalline colorant. In one work, they examined

the Na+ and Ca2+ salts of three sulfonated diazobenzene dyes

where the remote phenyl ring was further substituted with

amine or hydroxyl groups. This work was a nice example of

the flexibility of the SO3 group and how coordination was

influenced by small changes in ligand structure. The SO3

adopted Z1, Z2 or Z3 coordination modes and either linked

the structures into 1-D chains, or left the metal ions as discrete

units. Since the metal coordination spheres were pliant alkali

or alkaline-earth ions, the salts crystallized in frameworks that

maximized intermolecular forces between ligands, while still

filling the coordination sphere of the metal to satisfy ionic

interactions. These subtleties were relevant as the lmax for the

salts was affected by small changes in packing.

The same group reported a more encompassing article92

categorizing a range of sulfonated azo dyes with a range of

s-block metals to generalize the observed structures. Li, Na,

Mg, Ca, K, Rb and Ba were crystallized (Fig. 11) with eight

different m- and p-sulfonated azo dyes for the investigation to

form 43 structures. A subset of 12 of these structures was

discussed in the article in order to form general postulates as to

structure directing factors. It was found that all of the

structures formed alternating layers of organic and inorganic

regions. The harder Mg2+ formed solvent separated ion pairs

in all cases. Ca2+ did as well with some of the m-sulfonated

azo dyes. Ca, Ba and Li typically formed 0-D or 1-D

coordination networks where sulfonates bridged metal centers

in the case of the 1-D networks. Na, K and Rb were found to

form a wider range of networks (0-D, 1-D and 2-D) where

both sulfonate and bridging water molecules served to increase

the dimensionality of the networks. The authors rationalized

the trends through electronegativities of the metal ions,

where the more electronegative metals tended to form less

ionic interactions, hence favoring water coordination.

Conversely, the less electronegative metals tended to form

more ionic interactions thereby disfavoring water and favoring

sulfonate coordination. Previous work by Cote et al. related

similar observations to hard-soft acid base theory.93 Although

the metal plays a significant role in the type of structure

formed, the types and positions of substituents on the ligand

also had a key role in the type of structure formed and affected

the coordination of the SO3 group. These authors followed

this study with related work on alkali and alkaline-earth

complexes of the disulfonated azo dye, Orange G.94

The coordinative tendencies of the sulfonate group have

also been investigated for copper(II) and cadmium(II) ions by

Cai et al. In one study Cu(II) was investigated, where 4,40-

biphenyldisulfonate, and 1,5- and 2,6-naphthalenedisulfonate

were used as the anions.74 Bidentate ethylenediamine deriva-

tives as well as cyclam were used in order to coordinate to the

equatorial positions on the octahedral Cu center, and the

sulfonate groups acted as either uncoordinated counter anions

or as bridging ligands coordinated to the axial positions on the

Cu, forming 1-D chains. It was found that increased steric

bulk on either the amine or the sulfonate would preclude the

formation of a Cu–sulfonate bond, and water would fill the

axial position. As well, decreasing steric bulk on the amine or

sulfonate allowed for favorable hydrogen bond interactions in

the network, which allowed for direct coordination of the

sulfonate to the Cu.

In another study by Cai et al., seven cadmium(II) sulfonates

were reported, with the same sulfonate anions as the copper

study above, as well as 4,40-phenyletherdisulfonate.95 For this

study, 2,20-bipyridine, isonicotinamide and cyclam were used

as neutral ligands. In all cases the Cd center was octahedral

and had at least one coordinated sulfonate group. The neutral

ligands again played a major structure directing role, with the

larger 2,20-bipy ligands forcing 0-D molecular units to form,

and the smaller cyclam and isonicotinamide units causing the

formation of 1-D chains where Cd centers were bridged by

sulfonate units. The absence of neutral ligands with 1,5-

naphthalenedisulfonate allowed for the formation of a 2-D

layered solid in which the sulfonate groups bridged Cd centers

into 1-D chains and the chains were bridged through the

naphthyl groups into a layer. The effects of N-methyl- and

N,N0-dimethylethylenediamine on Cd sulfonates was also

investigated in a separate report, with the same sulfonate

anions as above.96 Again, the dominant structure-directing

ligand was the amine, where equatorially coordinated, octa-

hedral [CdN4]2+ centers were bridged into 1-D chains through

axial disulfonate ligands. Significant hydrogen-bonding inter-

actions between sulfonates and amines on adjacent chains

served to link the chains into 2-D networks.

Fig. 11 Classic observation of close packing R groups as well as non-

coordinating H-bonding sulfonate groups with (a) [Mg(H2O)6]+ and

bridging sulfonate groups with (b) Ca2+ in studies of sulfonated azo

dyes.

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Open-framework metal sulfonates

With the observed versatility of the coordination mode of the

sulfonate to layers of metal ions, an easy approach to open

the structure would seem to be to simply employ pillars for the

layers that would disfavor dense packing. To form a layered

sulfonate with an open pillared-layered structure, Cote et al.

examined the tironate anion, 1,3-disulfo-4,6-dihydroxybenzene.97

The SO3 groups on this ligand do not orient at 1801 and so

it was anticipated that some interlayer porosity would be

created. Indeed, the bent conformation of the pillar resulted

in micropores in the interlayer (Fig. 12). The stability of this

structure was augmented by chelation of the hydroxyl groups

to the Ba centers in addition to the SO3 ligation. The pores

were occupied by water molecules, which was to be expected.

Unexpectedly, the hydrogen atoms of the intrachannel water

molecules were readily observed in the single-crystal X-ray

structure (Fig. 12). This observation led to experiments with

the structurally related guest molecule, H2S, that showed that

upon activation, for every water molecule removed,

0.93 equivalents of H2S were taken up. This reversible H2S

uptake constituted the first illustration of functional porosity

in a layered metal sulfonate. More generally, it showed that

permanent pores could exist in a metal sulfonate material. In

this solid, the structural integrity (TGA showed stability

to 4400 1C) was augmented by a chelating catechol moiety

in the tironate ligand. A later example illustrates that a

homoleptic barium sulfonate could be sufficiently robust to

allow for interlayer chemistry.

The pillaring group, 1,3,5-tris(sulfomethyl)benzene could be

envisioned to open channels between the layers of a metal

sulfonate. Similar structures were obtained with this ligand

and both Ag+ and Ba2+�X�. The silver complex98 was formed

as part of a larger study, akin to opening pores in layered

metal phosphonates, where larger organic cores were screened

for their ability to form open channels but in a metal sulfonate.

The silver(I) complex of 1,3,5-tris(sulfonomethyl)benzene did,

in fact, form a pillared layered solid. The overall structure was

that of sheets of SO3-bridged Ag ions separated by mesitylene

units. Each ligand had one sulfonate group coordinating to

one layer and two sulfonate groups ligating to an adjacent

layer. Notably, the pillars were not densely packed which

resulted in the creation of interlayer void space in which water

molecules were included. The channels had dimensions of

8.50(5) � 4.25(5) A and were occupied by two crystallo-

graphically unique water molecules. Gas sorption was not

reported.

In addition to the mesitylene core already mentioned,

silver(I) networks were reported with a,a0,a00,a0 0 0-durenetetra-sulfonate, to form {[Ag4(L)(H2O)2]}N, and 1,3,5,7-tetra-

(4-sulfonophenyl)adamantane, to form {[Ag4(L)(H2O)2]�1.3H2O}N.98 The sulfonated durene complex formed a

three-dimensional structure but not that of a pillared layered

type. With the durene core, the layers were disrupted and 1-D

inorganic columns resulted. Pores defined in this solid were

small as the columns were realistically separated by only a

Ag-coordinated water molecule. Going to the much larger

tetrasulfonate ligand based on the tetraphenyladamantane

core gave much larger pores. The structure of the silver(I)

complex of 1,3,5,7-tetra(4-sulfonophenyl)adamantane

(Fig. 13(a)) clearly showed that the structure was composed

of 1-D columns of Ag-SO3 aggregates crosslinked in two

dimensions by the ligand. Two types of channel were defined

by the adamantane cores of the ligand. The first was primarily

occupied by coordinated water molecules with approximate

dimensions 7.4 � 5.9 A. The second was occupied by dis-

ordered water molecules and had dimensions of 8.3 � 6.0 A.

The distances between adjacent columns were 11.46(1) and

12.49(1) A. In this structure, guest and coordinated water

molecules could be removed and resorbed but the structure did

not persist in the absence of the guest molecules.

To extract some design principles from the structures of the

durene and tetraphenyladamantane analogues,98,99 the

approach of defining an ideal reference structure was again

employed. As with the monosulfonate study, Ag benzene-

sulfonate was employed as a reference and treated as an ideal

2-D silver sulfonate. The most pertinent parameter for struc-

tural comparison was determined to be the distance between

sulfonate S atoms in the structures as they represented the

anchor points to the layers/aggregates and did not vary greatly

with the identity or orientation of the organic moiety to which

they were linked. This distance in Ag benzenesulfonate

wasB5.2 A. It should be noted that this sulfur–sulfur distance

was somewhat flexible. For example, Ag 2-naphthalene-

sulfonate, maintained a layered motif by the Ag sulfonate

layer rearranging to 4.53(1) and 6.01(1) A to accommodate the

broader pendant group. However, these two values were still

centered roughly on the 5.2 A value. The durene analogue was

viewed as having two sets of meta-xylyl spacers constraining

the structure. Only half of each ligand was crystallographically

unique and so there were only two independent sulfonate

groups. The meta sulfur–sulfur distance in this complex was

Fig. 12 Structure of Ba tironate showing the pillared layer motif. The

H atoms of free intra-channel water molecules are clearly visible

(located in the structure) and indicated a pore well-suited for H2S

inclusion.

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5.835(2) A, considerably above the ideal 5.2 A distance. This

resulted in the observed one-dimensional structure rather than

a two-dimensional network. As a comparison, with the

mesitylene analogue only one side of the ligand was

constrained by the meta-orientation of the sulfonate groups

and so the third SO3 group could effectively function as a

‘‘filler’ to permit the formation of continuous layers. With the

tetraphenyladamantane derivative, sulfonate sulfur atoms

were situated distance of 12.65 A apart, well beyond that

required to preclude a simple layered solid. The design

principles which evolved from this work were that for silver

sulfonate networks, any spacer that would rigidly position

sulfonate groups farther than 5.2 A apart should disfavor a

layered solid and likely form 1-D columns. Going a step

further, one could say that a ligand which imposed such the

same constraint in two dimensions could dictate the formation

of 0-D clusters. This potential was illustrated by the Ba2+

complex of 1,3,5,7-tetra(4-sulfonophenyl)adamantane.99 With

each Ba ion carrying double the charge of a silver ion, half the

metal centers were available to crosslink the ligands resulting

in the first observation of 0-D clusters in metal sulfonate

chemistry (Fig. 13(b)).

As mentioned above, the Ba2+ complex100 of 1,3,5-tris-

(sulfonomethyl)benzene (Fig. 14) had a very similar structure

to that of the Ag+ complex. This was a result of the channels

in the network including a chloride ion. This solid demons-

trated the first example of ion exchange in a metal sulfonate.

In {Ba2[(L)(H2O)5]Cl}, the dimensions of the channels were

approximately 9.1 � 7.9 A and the Cl� ions were supported

only by long H-bonds to coordinated water molecules. Given

the ‘‘free’’ nature of the chloride ion, anion exchange was

examined. As a general comment, it is not typical that metal

sulfonate structures form as bulk solids with mixed anions.

Without exception, the few which do possess a very stable

three-dimensional cationic metal sulfonate skeleton which

requires additional charge compensation. Screening the

BaX2 family with 1,3,5-tris(sulfonomethyl)benzene gave inter-

esting results. With BaF2, the only solid isolable was BaF2

itself. With BaBr2, an isomorphous material to the chloride

complex was obtained. Iodide was too large for the pores and

a different crystalline, yet undetermined, phase was obtained.

Interestingly, when the triacid form of the ligand was

complexed with Ba(OH)2 so that no secondary ion was

present, the mixture remained soluble inferring that an

extended network was not forming. Attempts to exchange

Br� for Cl� in {Ba2[(L)(H2O)5]Cl}, were unsuccessful,

however, F� could be efficiently exchanged. This selectivity

is contrary to that typically observed for ion-exchange

materials.101 In the presence of one equivalent of fluoride

ion, a 75–80% exchange of the chloride ions was observed in

3 h and complete exchange was obtainable in 3 days. This was

confirmed by AgCl gravimetric analysis, PXRD, 19F NMR

spectroscopy, and elemental analysis. In this publication, it

was also proposed that exchange occurred via two-way

passage of halide ions providing intrachannel solvents were

mobile, a condition that was confirmed by NMR experiments

in D2O. A final noteworthy comment concerning the F� ion

exchange is that whereas the F� adduct could be prepared by

ion exchange, it could not be prepared directly from BaF2.

This is perhaps the strongest evidence for a heterogeneous

exchange mechanism with retention of framework integrity.

Fig. 14 Structure of the Ba2+ complex of 1,3,5-tris(sulfonomethyl)-

benzene. Exchangeable Cl� ions are included in the channels. If, in this

depiction, the Cl� ions were replaced with free water molecules, the

image would be an accurate representation of the Ag+ complex of the

same ligand.

Fig. 13 Two complexes of 1,3,5,7-tetra(4-sulfonophenyl)adamantane:

(a) the Ag+ structure showing the 1-D columns (into the page) and

(b) the Ba2+ structure, with ligated dioxane) showing 0-D clusters.

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In keeping with the theme of using larger organic cores to

disperse crosslinking sulfonate groups, Chandler et al.

published two works concerning the use of sulfonated metal

complexes as building blocks for MOFs.102,103 A goal of this

research was to link luminescent metal complexes into a

porous, guest-accessible structure. Both papers reported a

series of lanthanide ions complexed to disulfonated bipyridine

N-oxide ligands. One paper dealt with a series of 3:1

ligand : Ln complexes that were then crosslinked by Ba2+

ions102 and the other dealt with a series of 4 : 1 ligand : Ln

complexes that were bridged by [Na4Cl]3+ aggregates.103 For

the 3 : 1 Ba complexes, all networks were isomorphous and

contained open channels which readily absorbed and desorbed

water accompanied by a sponge-like shrinkage and expansion

of the host. CO2 sorption measurements confirmed micro-

porosity giving a DR surface area of 718 m2 g�1 and an

average pore size of 6.4 A. The luminescent properties of the

lanthanide building blocks were retained in the porous

network solid. From the luminescence data, it was possible

to attribute the sponge-like properties of the network to the

Ba2+ coordination sphere rather than the Ln3+ center.

For the NaCl bridged 4 : 1 complexes,103 the cross-linking

by [Na4Cl]3+ clusters, which resembled ideal faces of the halite

structure, gave a three-dimensional network with two-

dimensional channels. Both coordinated and uncoordinated

water molecules, up to B80%, could be reversibly removed

leaving a rigid and porous framework with a DR surface area

of 426 m2 g�1 as determined using CO2 adsorption. The

permanent microporosity of the framework was also

supported by the energy level splitting in the luminescence

spectra which was maintained in both the hydrated and mostly

dehydrated frameworks indicating minimal variation in the Ln

coordination spheres.

Hydrogen-bonded metal sulfonate complexes

Sulfonates have been observed to participate extensively in

regular hydrogen bonding schemes primarily to metal-bound

water and ammonia. While typically, a H-bonded network

would not fall under the umbrella of MOF chemistry, a few

examples will be presented here as: (1) some contain metal ions

which form coordinate bonds to the sulfonates in addition to

H-bonds; (2) some of these networks demonstrate guest

exchange and one example even presents permanent porosity

as confirmed by reversible gas sorption.

Sulfonate anions are highly complementary with ligated

water molecules in a cis orientation on a metal center.104 We

had shown that the mesitylenetrisulfonate ligand could

perfectly cap the triangular face of a metal octahedron

forming six charge-assisted H-bonds.105,106 Excluding inter-

molecular H-bonds, the divalent and trivalent metal

complexes of this ligand did not form extended structures.

The ligand, hexakis(sulfonomethyl)benzene, could be viewed

as two such mesitylene ligands placed back to back.

With divalent metal ions, H-bonded solids where each

mesitylene-like face capped the triangular face of a metal ion

were observed. However, with Al3+, an unusual network was

formed resulting from each sulfonate group forming both

primary and secondary sphere interactions with the Al centers

(Fig. 15).107 Each SO3 group formed two H-bonds to water

molecules coordinated to an Al center, as expected, however,

the third oxygen formed a bond directly to another Al ion. The

result was the formation of a 3-D open structure sustained by

a combination of primary and secondary sulfonate ligation.

Porosity by gas sorption analysis was not confirmed in

this work.

Much of the inspiration for the work on H-bonded

sulfonate inclusion complexes came from the work on guani-

dinium sulfonates.108,109 These H-bonded solids could be

perceived as expanded versions of metal phosphonates where

the metal ion is replaced by a larger guanidinium cation and

M–O bonds are replaced by longer H-bonds. The net result is

that whereas pillars in metal phosphonates are densely packed,

the guanidinium sulfonates are the most extensive host–guest

inclusion family reported. Based on this work, it was

postulated that a metal hexaamine complex could mimic two

guanidinium cations and form layered networks with inter-

layer void space.110 Trivalent hexaamine cations offer more

inert building blocks but not an ideal charge stoichiometry to

mimic two monovalent cations. Despite that, layered solids

with the ability to reversibly uptake guest molecules have been

reported. The p-xylenedisulfonate complex of [Co(NH3)6]3+

formed both as a hydrate and as p-aniline inclusion

complex.111 A series of PXRD studies confirmed that the

aniline inclusion was reversible provided a trace amount of

water (vapor) was present. The inclusion was shape selective as

1,4-diaminobenzene was also sorbed but not broader diamine

guest molecules.

Further design advances from the previous work led to the

first observation of a permanently porous H-bonded solid.112

The first design advance concerned the stabilization of a

divalent ‘‘hexaamine’’, the ideal charge complement, using

the tripodal ligand tris(aminomethyl)ethane (tame) complexed

to Ni2+. The second involved removing steric congestion

around the methylene linkages of the tame ligand by using

Fig. 15 The channels permeating the [Al(H2O)3]3+ complex of

hexakis(sulfonomethyl)benzene. Each Al center forms both primary

and secondary sphere interactions with the sulfonates.

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sulfoethynyl substituted aryl pillars rather than using an aryl

sulfonate. The net result of these modifications was a highly

insoluble product that, when refluxed with a benzene template,

transformed to the desired network. From PXRD data and

the single-crystal structure of the monosulfonated analogue,

the structure of {[Ni(tame)](4,4 0-bis(sulfoethynyl)biphenyl)}

was determined (Fig. 16). N2 sorption isotherms gave surface

areas of B560 m2 g�1 for Langmuir and DR equations. This

complex made two key illustrations. First, the weaker inter-

actions allowed the network solid to optimize its structure in

the presence of the aromatic guests. Secondly, the solid that

resulted was the first example of a permanently porous solid

sustained exclusively by charge-assisted hydrogen bonds.

In separate reports, Sevov et al. have investigated the effects

of both the guest molecules and the charge of the metal center

in similar hydrogen bonded frameworks. In one report, frame-

works were formed from 4,40-biphenyldisulfonate and

[Co(NH3)6]3+ with a variety of guest solvents; all of the

frameworks also include water as a secondary guest.113 Six

structures were reported, where all exhibited a 3-D structure of

alternating H-bonded [Co(NH3)6]-sulfonate layers bridged

through the organic biphenyl unit. In all of the structures,

the metal : ligand ratio was 2 : 3 and the octahedral metal

centers had their C4 axes oriented perpendicular to the plane

of the layers. The sulfonate pillars adjusted position slightly

depending on the guest in order to accommodate the steric

demands of the guest and maximize hydrogen bonding inter-

actions. In the second report, the charge of the metal center

was adjusted by changing the primary coordination sphere of

Co3+ to include an oxalate anion and ethylenediamine, form-

ing a [Co(en)2(ox)]+ complex.114 The purpose of adjusting the

charge was to give a metal : ligand ratio of 2 : 1, thereby

increasing the space allowed for guest molecules as fewer

pillars were required. Six structures were reported, in all

structures the typical bilayer motif was again observed as

above, although in this case there were H-bond interactions

between metal complexes as well, from the presence of the

oxalate anion. These frameworks were able to accommodate

larger aromatic guest molecules due to the increased space

between organic pillars. An example was also given where two

protonated guests were included and the overall charge of the

framework was adjusted to a 4 : 3 metal : ligand ratio.

There have also been reports of numerous hydrogen-bonded

networks formed from p-sulfonatocalix[n]arenes, systems

containing sulfonated phenoxy groups linked into rings

through methylene bridges. The chemistry of both calixarenes

and sulfonated calixarenes was initially driven by molecular

inclusion phenomena but these molecules have shown

interesting network properties as well. Solid-state structures

of sulfonated calix[n]arene units with a variety of metals,

particularly the lanthanides, have been reported. The first

crystal structure report on a metal p-sulfonatocalix[4]arene

appeared in 1988.115 As well this ligand was used in order to

demonstrate the first aromatic–p hydrogen bonds to water

crystallographically.116 A large amount of work on this subject

was performed in the 1990s, and since then two reviews were

published; the first review highlighted ‘‘Russian-doll’’

complexes formed with p-sulfonatocalix[4,5]arenes and

trivalent cations, and the second reviewed work on

p-sulfonatocalix[4,5]arene metal complexes up to 2001.117,118

The present discussion will briefly cover work done beyond

2001 on p-sulfonatocalix[n]arenes, where n = 4, 5, 6 and 8.

The p-sulfonatocalix[n]arenes where n = 4 or 5 are typically

locked into a bowl-like conformation due to the lack of

flexibility in these systems. These smaller ring systems tend

to encapsulate chelated cationic centers into so-called

‘‘Russian-doll’’ complexes, where each side of the capsule is

connected through a second-sphere coordination to a

hydrated metal cation. The p-sulfonatocalix[4]arenes have also

been characterized to form ‘‘Ferris wheel’’ complexes where a

single sulfonate group will coordinate to a crown-chelated

metal cation. For the most part these structures are dense,

zero-dimensional hydrogen bonded networks with alternating

hydrophilic and hydrophobic layers, formed with di-120,122,123

or trivalent119,121 cations. In some cases where the trivalent

metal cation is large enough, direct coordination of sulfonate

groups is possible and 2-D coordination polymers result.119,121

Larger ringed p-sulfonatocalix[n]arenes where n = 6 and 8

have also been used; in the larger ring systems there is larger

conformational flexibility in the calix[n]arene ring and the

bowl shape present in the calix[4,5]arenes is typically

not observed. Again, the structures containing the larger

p-sulfonatocalix[6]arenes tend to be zero-dimensional hydrogen-

bonded networks.124–126 One-dimensional coordination poly-

mers have also been observed with p-sulfonatocalix[6]arene,

4,40-bipyridine N-oxide and Eu3+, where the Eu3+ centers are

bridged by both the N-oxide and calix[6]arene.126 One struc-

ture has also been reported using p-sulfonatocalix[8]arenes,

where Eu3+ centers are linked by 4,40-bipyridine N-oxide into

a 2-D wavy-brick coordination polymer and the coordination

polymer is extended into three dimensions through coordina-

tion by the calix[8]arene.127 This 3-D structure appeared to be

dense. There is one report of an unusual 2-D coordination

Fig. 16 Carbon dioxide sorption isotherm of hydrogen bonded

{Ni(tame)(4,40-bis(sulfoethynyl)biphenyl)} showing permanent porosity.

Top left inset: Representation of hydrogen bonds (dashed lines)

between sulfonate groups and Ni(tame) clusters. Bottom right inset:

Representation of a model from PXRD of the open pore structure of

Ni(tame). Color code: Ni-cyan; S-yellow; O-red; N-blue; C-grey;

H-white.

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polymer formed with p-sulfonatocalix[4]arene, hexamethylene-

tetramine (hmt) and Ag+, shown in Fig. 17. Here, Ag

coordination by bridging hmt units created layers. One of five

unique Ag centers was coordinated to a sulfonated calixarene.

The layers combined through hydrogen-bonding interactions

creating large channels along one axis of the network that were

filled with solvent water and acetonitrile. Porosity was claimed

but gas sorption data were not given.

A recent report was given by Geremia et al. on a supra-

molecular framework formed from a modified sodium

p-sulfonatocalix[4]arene and a tetracationic porphyrin.128 In

this case the hydroxyl groups on the calixarene have been

modified to have additional carboxylic acid groups attached

through an ether linkage, and meso-tetrakis(4-N-methyl-

pyridyl)porphyrin was used as the porphyrin (Fig. 18). In this

case the calix[4]arene : porphyrin ratio is 4 : 3 with sodium ions

satisfying the remaining charge imbalance. The porphyrin

units occur as staggered trimeric stacks with the middle

porphyrin’s pyridinium groups fitting in the bowls of four

calix[4]arenes. The sodium ions serve to bridge two calix[4]-

arene units through the carboxylate groups, forming a 2-D

layered solid in the xy plane which interpenetrates with the

same 2-D layer in the xz plane, leaving large interconnected

voids in the framework filled with solvent molecules. The

uptake of ZnCl2 and NiCl2 was demonstrated for this frame-

work, with resulting single-crystal to single-crystal trans-

formations observed. Interestingly, due to the stacked nature

of the porphyrins, the divalent metals served only to displace

Na+ from the carboxylate groups capping the calix[4]arene

cones, rather than being coordinated by the porphyrins.

Dynamic behavior in metal sulfonates

In comparison to phosphonates (and even carboxylates), the

trend with sulfonate frameworks is that, with a few exceptions,

they are generally less robust. That said, the spherical and

weaker ligating nature of the SO3 group predisposes the

network to certain degrees of flexibility. For sulfonate

frameworks, this dynamic behavior is more the rule than the

exception. A larger discussion of the types of framework

callisthenics possible with coordination frameworks will

undoubtedly be presented in this special issue and so here,

we will highlight a few examples that show the role sulfonate

ligation can play in solid state dynamics. Our experience has

been that solid-state dynamics are enhanced by pairing

sulfonates with equally pliant metal cations such as d10 centers

or alkaline earth ions.

A silver sulfonate has been reported that showed selective

guest uptake through a structural rearrangement.129 The

network [Ag(3-pyridylsulfonate)(MeCN)0.5], formed 24-

membered rings with exclusively pyridine and sulfonate ligated

silver centers. The rings packed in an eclipsed fashion to give

channels containing the MeCN guests. Heating to remove the

MeCN brought about conversion to a denser still crystalline

phase. Treatment of this phase with MeCN liquid or vapor

brought about a reversion to the nascent structure. This work

presented X-ray structures of the dry network and the MeCN

solvate but, most importantly, offered a mechanism of inter-

conversion based upon this data (Fig. 18). The macroscopic

analogy of the mechanism was that of a cardboard box folding

sideways and this was confirmed by site occupancies and

changes in coordination spheres of the Ag ions. Fundamental

to the conversion was the ability of the SO3 group to offer

simultaneous multidirectional ligation and the d10 silver(I) ion

to accommodate the structure shift in its coordination sphere.

Notably, no Ag–py bonds needed to be cleaved, only the

weaker Ag–O (sulfonate) bonds. This type of behavior led to

the analogy of the ligating ability of a sulfonate group with a

‘‘Ball of Velcro.’’ Beyond the reversible MeCN sorption

observed for this system, selectivity with other structurally

similar molecules was examined; propionitrile, MeOH, EtOH

or THF were not taken up.

Fig. 17 Open-framework structure made using hexamethylenetetramine,

p-sulfonatocalix[4]arene and Ag+. Two layers are represented,

with all non-coordinating solvent molecules removed. Color code:

Ag-pink; S-yellow; O-red; N-blue; C-grey.

Fig. 18 Crystal structures of the solvated (left) and dry (right) forms

of Ag(3-pyridylsulfonate); MeCN has been removed from the channels

on the left to show the proposed rearrangement that occurs in the solid

state to covert to the dry phase in a crystal-to-crystal transformation.

All metal ions are Ag+ and the different colors represent crystallo-

graphically unique centers.

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Chandler et al. reported the Ba2+ salt of 1,3,5-trisulfono-

benzene which is a solid that undergoes multiple single-crystal

to single-crystal transformations while trapping and releasing

gas molecules (Fig. 19).130 An initial structure of a highly

hydrated phase (17.5% water) was presented which had 1-D

channels permeating it. Single-crystal structures were also

presented which showed the pores collapsing at 12% hydra-

tion and then a dense phase at 3% hydration. All water

molecules in the 3% structure were Ba-ligated. Further heating

to remove ligated water molecules gave a structure that

revealed that the sulfonate groups had rotated in order to

maintain oxygen atom ligation to the Ba centers; the Ba

centers were eight-coordinate in all four structures. This

rearrangement necessitated shifting of components and the

formation of ‘‘Ba-belted’’ closed pores between ligand

molecules. This anhydrous phase was observed to release

bubbles of gas upon rehydration to reverse the structural

transformation. The bubbling observed from the network

was an exceptionally odd phenomenon and merited closer

examination. Varying the atmosphere to CO2, O2, N2, CH4,

Ar, or NH3 during heating to the anhydrous phase yielded

solids which released gas on rehydration. To confirm that the

bubbles of gas were in fact originating from the external

atmosphere a colorimetric experiment was performed.

Crystals of the hydrated Ba sulfonate were fully dehydrated

in NH3(g) then further heated under vacuum to liberate

externally adsorbed NH3. The dried crystals were then placed

in an aqueous Co(NO3)2 solution. The Co2+ ions readily

adsorbed to the crystal’s surface to give a pink color. This

coincided with release of gas from the crystal as water

rehydrated the Ba ions and opened the closed pores.

In B2 min, the crystals changed color from pink to a deep

green characteristic of hexaammine Co(II). As any externally

sorbed gases would have been removed in the desorption cycle,

the only source for NH3(g) was the interior of the solid,

confirming that the external atmosphere was trapped during

dehydration. With this material, any gas, with the exceptions

of H2 and He, composing the external atmosphere during the

dehydration was captured and stored. The main illustrations

from this particular work were that a sulfonate solid can be

truly adaptable leading to facile transformations between

different structures. The second, more practical illustration

concerns the gas storage. The vast majority of molecular

materials for storing gases rely on either physisorption or

chemisorption of gases as their means of storage. A dichotomy

in the storage problem is that materials which store gases

efficiently typically do not release them rapidly, and conversely,

facile release is often correlated with ineffective storage at

elevated temperatures. This network presented a solid able to

absorb the ambient atmosphere and store it at 150 1C under

vacuum while providing instantaneous release of the gas at

room temperature simply by addition of water to the solid.

Ribas et al. reported a material with a radical sulfonate

ligand that displayed single-crystal to single-crystal

conversion.131 The ligand, monosulfonated polychlorotriphenyl-

methane, was oxidized by treatment with I2 into the radical

species. Two copper complexes were reported. The first

had the ligand H-bonding via the sulfonate group to

[Cu(py)2(H2O)4]2+ units and the second had the sulfonate

ligand directly coordinating to axial sites of a Jahn–Teller

distorted [Cu(cyclam)]2+ unit. Ethanol molecules were

included in both structures that, upon removal, brought about

single-crystal to single-crystal transformations involving

reorientation of the organic radical. The cyclam system, with

direct ligation of the radical, showed magnetic interaction

between spins however, this was still a relatively weak

magnetic coupling. The weak interaction was attributed to

poor magnetic exchange through the Z1-SO3 group as the

carboxylate analogue showed a markedly stronger magnetic

interaction.

Hu et al. reported the synthesis and adsorption properties of

a linear polymer built from [Cu(2-pyridylsulfonate)2] units

linked via 4,40-bipyridine in the axial sites.132 The chains

packed to form layers while the chains in adjacent parallel

layers were rotated by 901. There were p–p and C–H–pinteractions between adjacent layers but there were still 1-D

pores (5.27 � 4.92 A) present filled with water. The framework

could be desolvated and subsequently resolvated with water,

MeOH or iPrOH vapour all via single-crystal to single-crystal

transformations. The desolvated network could also adsorb

small amounts of benzene and toluene, with selectivity for

smaller guests.

Summary and outlook

The examples presented in this article span a broad range of

structure topologies, inorganic building units, and potential

functions. Here, we will attempt to make some generalizations

from the presented works and from our own experience.

Fig. 19 (Top left) Crystallographic representation of the ‘‘Ba-belted’’

closed pore in Ba 1,3,5-trisulfonobenzene with simulated gas mole-

cules. (Top right) Crystallographic representation of the open-pore Ba

salt with water molecules removed from the channels. (Bottom left)

Photograph of dehydrated single crystals of the Ba salt immediately

after submersion in hydrated paratone oil. (Bottom right) Photograph

of dehydrated single crystals of the Ba salt after readsorption of water

in hydrated paratone oil and the subsequent release of trapped gas.

Color code: Ba-light grey; S-yellow; O-red; N-blue; C-dark grey.

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From the synthetic perspective, for both phosphonates and

sulfonates, the spherical ligating ability of the functional group

makes it easier to extend coordination in three dimensions.

For phosphonates, this is a bit of a double-edged sword as,

while this facilitates the formation of network structures, it

also can lead to insoluble precipitates with low degrees of

order. For phosphonates, especially factoring in their variable

states of protonation, kinetic control of self-assembly,

particularly pH, is crucial. For sulfonates, amorphous

materials are not typically observed. In this case, the possible

variations in ligating mode of the sulfonate are more

manifested as lower degrees of predictability and possibly

the observation of multiple polymorphs.133 That said, there

are general trends one can extract regarding the assembly of

metal phosphonates and sulfonates:

- With the exception of the smallest R groups, for simple

mono- or diphosphonate/sulfonates, layered solids are the

default structure.

- Each layered structure has a certain critical threshold

regarding the breadth of the pendant R group. Organic groups

that exceed the threshold will render the layer structure

impossible and so the assembly will default to the next highest

order of structure, 1-D columns.

- With homoleptic phosphonates and sulfonates, it is

difficult to extrapolate the concept of secondary building units,

as isolable discrete clusters are rarely observed and not regular

in their structures when they are. That said, in some examples

there are clearly infinite SBU motifs which would be expected

to persist with simple modification/extension of the organic

linkers. The fact that new inorganic aggregates are not

uncommon is actually of interest from the perspective of

new electronic/magnetic materials.

- Being sustained more typically by inorganic skeletons with

1-D or 2-D structures, interpenetrated structures are much less

common with phosphonates and sulfonates.

- For sulfonates, hard metal ions remain highly hydrated

and typically zero- or one-dimensional structures results.

Softer metal ions are much better bonding partners.

Regarding the potential applications of phosphonate

and sulfonate MOFs, of course new materials with regular

micropores would be of interest for gas storage or separations.

In contrast to other materials, both phosphonate and

sulfonate MOFs would offer a much greater likelihood of

forming polar pores.134 This would translate to different

selectivity than observed for non-polar pores that are much

more typically observed in MOFs. With sulfonates, their

forte appears to be less in the domain of permanently porous

solids, despite the H-bonded example, and more in the

realm of dynamic and switchable materials. Weaker ligation

coupled with a range of coordination modes is an ideal recipe

for structural dynamics so this is an application where one

would expect sulfonate solids to excel. Another potential

application of materials with polar pores would be ion

conductors. Within the pore size domains of MOFs, one could

envision conduction of protons or Li+ ions. This aspect has

not been extensively studied in MOFs but, given that some

structural mobility in the solid state should augment ion

conduction, a material such as a sulfonate MOF would be

an intriguing candidate.

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