This article was published as part of the

Hybrid materials themed issue

Guest editors Clément Sanchez, Kenneth J. Shea and Susumu Kitagawa

Please take a look at the issue 2 2011 table of contents to

access other reviews in this themed issue

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This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 575–582 575

Cluster-based inorganic–organic hybrid materialsw

Ulrich Schubert*

Received 14th June 2010

DOI: 10.1039/c0cs00009d

Clusters as building blocks have been used for two types of inorganic–organic hybrid materials.

The first are hybrid polymers, with polymer-like properties and structures, where the cluster units

crosslink the polymer chains. They are prepared by co-polymerization of organic monomers with

functional ligands attached to the clusters. The second type is crystalline metal–organic

framework structures which are obtained by coordination chemistry approaches, i.e. by

coordinating multifunctional organic ligands to cluster units. This tutorial review shows that both

types of cluster-based materials are limiting cases with many options for varying both the cluster

units as well as the connecting organic entities.

Introduction

Inorganic–organic hybrid materials are obtained by combination

of molecular or nanoscale inorganic and organic building

blocks. Their properties not only depend on the kind and

proportion, but also on their dispersion of the individual

constituents. Various types of hybrid materials have been

prepared by various preparative routes, with different

structural characteristics. Any graduation of the dimension

and dimensionality (particles, layers, etc.) of the organic or

inorganic constituents is possible. For example, a topical area

of research is the use of nanoparticles for inorganic–organic

hybrid materials.

This article focuses on to use molecular inorganic clusters1

as inorganic building blocks. Clusters are intermediate

between mono- or binuclear molecular compounds and

nanoparticles and may have special intrinsic physical

properties (e.g., molecular magnets, special optical properties,

redox properties, etc.). Clusters have several advantages

compared to (nano-)particles:

� Clusters are big molecules, i.e. they can be dissolved in

organic solvents, purified, etc., and can be characterized by the

analytical tools of molecular chemistry.

� Surface modification can be monitored by standard

spectroscopic methods.

� Each cluster in a macroscopic sample has the same

composition, size and shape. Nevertheless, clusters of varying

composition, size and shape can be prepared with the same

core elements (e.g. different Ti/O- or Cd/S-based clusters).

This article focuses on a sub-class of so-called ‘‘class II’’

hybrid materials in which the clusters, as the inorganic

constituents, and the organic entities are linked to each other

by strong chemical bonds and are thus part of extended

network structures. ‘‘Class I’’ hybrid materials, where clusters

are just embedded in a polymer or an inorganic matrix phase,

are not subject of this article.

The building block approach for the bottom-up synthesis of

complex structures is also called ‘‘molecular Legos’’, and

there are indeed similarities between the characteristics of

Legos bricks and chemical building blocks:

� The individual building blocks have specific shapes; con-

necting species of different shapes with each other results in

geometrically different supra-structures.

� The building blocks fulfil certain (chemical or physical)

functions in the final system.

� The building blocks are equipped with reactive sites

that allow connecting the building blocks with each other

(connector sites).

� Number and geometrical arrangement of the connector

sites is of utmost importance for the formed structures when

the clusters are connected by organic groups.

Two major classes of cluster-based inorganic–organic

hybrid materials will be discussed and compared in this article,

viz. cluster-crosslinked organic polymers2–5 and 3D coordination

polymers6 with clusters as connector units. 1D coordination

polymers are known since decades, but only the highly porous,

crystalline solids with regular 3D structures, the so-called

Institute of Materials Chemistry, Vienna University of Technology,Vienna, Austria. E-mail: Ulrich.Schubert@tuwien.ac.atw Part of the themed issue on hybrid materials.

Ulrich Schubert

Ulrich Schubert received hisDiploma (1972) and doctoraldegree (1974) from TUMunchen. After postdoctoralwork at Stanford University,he finished his Habilitation atTU Munchen in 1980. From1982 to 1994 he was Professorof Inorganic Chemistry at theUniversity of Wurzburg,Germany. Since 1989 he alsoserved at the FraunhoferInstitute of Silicate Researchin Wurzburg. In 1994 he wasappointed as the Chair ofInorganic Chemistry at Vienna

University of Technology. He is a member of the AustrianAcademy of Sciences and the German Academy Leopoldina,and Fellow of the Royal Society of Chemistry. His group isworking on applied fundamental research on various types ofinorganic–organic hybrid materials and nanocomposites.

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

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576 Chem. Soc. Rev., 2011, 40, 575–582 This journal is c The Royal Society of Chemistry 2011

metal–organic frameworks (MOF),7–9 have gained major

interest as new materials. Cluster-crosslinked polymers and

MOF differ mainly in two ways:

� Cluster-crosslinked polymers are amorphous and have

typical polymer-like properties, while MOFs are crystalline

solids formed under thermodynamic control.

� Cluster-crosslinked polymers are prepared by polymerization

of ligands attached to the cluster while MOFs are obtained by

coordination chemistry approaches, i.e. by coordinating multi-

functional organic ligands (the linkers) to isolated metal

complex moieties or metal clusters (the connectors).

The basic notions and options of both materials classes will

be discussed for selected examples in the following.

Cluster-crosslinked polymers

The basic idea behind this class of hybrid materials is to

employ clusters with polymerizable ligands in organic

polymerization reactions, together with organic co-monomers.

The inorganic cluster core is retained upon polymerization.

A sub-class of such materials, the so-called POSS-reinforced

polymers, is already known for some time and resulted in

various applications. The employed cage compounds

(‘‘clusters’’) in this particular case are polyhedral oligomeric

silsesquioxanes RnSinO3n/2, or spherosilicates (RO)nSinO3n/2,

mostly the octameric derivatives (n = 8) with a cubic cluster

core of silicon atoms bridged by the oxygen atoms. Depending

on the number of polymerizable groups per cluster, the POSS

units can be pending (Rn�1R0SinO3n/2, one polymerizable

group R0) or crosslinking (Rn�xR0xSinO3n/2, x > 1). Various

types of organic polymers have been reinforced by POSS units.

These materials were extensively reviewed and are therefore

not covered in this article.10

The range of properties and potential applications can be

considerably broadened by employing other types of inorganic

cluster, especially those of the transition metals.

Preparation of clusters with functional organic ligands

A necessary requirement for the preparation of cluster-cross-

linked polymers is the availability of clusters with functional,

i.e., polymerizable, organic ligands. Two principal routes are

the synthesis of the clusters in the presence of the functional

ligands or the post-synthesis exchange of cluster ligands.

Both approaches are also used for capping nanoparticles

with (functional) organic groups. Only one example for each

route is given in the following; details have been discussed

elsewhere:2

� When certain metal alkoxides are reacted with carboxylic

acids, one or more alkoxo ligands are substituted by

carboxylate groups in the first step of the reaction. The thus

liberated alcohol may then undergo ester formation in

which water is produced. This is the source of oxide or

hydroxide groups in the formed clusters of the type

Mx(O/OH/OR)y(carboxylate)z. The presence of the organic

ligands inhibits the growth of the cluster core and prevents

the formation of metal oxide gels. The overall reaction

resulting in the formation of Zr6(OH)4O4(OMc)12 (Fig. 1, left)

from Zr(OPr)4 and methacrylic acid (H–OMc) is given in

eqn 1 as an example.11

6Zr(OPr)4 + 20McOH - Zr6(OH)4O4(OMc)12

+ 8McOPr + 16PrOH (1)

� The heterotungstate cluster [PW11O39]7� was derivatized by

reaction with organotrichlorosilanes RSiX3 with polymerizable

groups R (R = vinyl, allyl, or 1-hexenyl).12 The obtained

anionic clusters have the composition [PW11O34(O5Si2R2)]3�

(Fig. 1, right), i.e., four terminal O2� groups were replaced by

two bridging [O2Si(R)–O–Si(R)O2]4� units. The unsaturated

organic groups are thus attached to the cluster core through

W–O–Si–C linkages.

Two cluster features can be varied, viz. (i) composition, size

and shape of the cluster core and (ii) the organic functionality

of the ligands.

Composition, size and shape of the cluster core. Some metal

oxide clusters were obtained from the corresponding metal

salts by reaction with unsaturated carboxylic acids or carboxylate

salts, such as [Fe3O(m-OOCR)6L3]X (OOCR = acrylate or

2-butenate; X = counter-ion)13 with a triangular M3O core

(see below) or the Cr–Ni wheel [Cr7NiF8(OMc)16]+.14

A greater variety of metal oxide clusters with functional

carboxylate ligands were prepared by reacting metal alkoxides

with the corresponding carboxylic acids (similar to eqn (1)).

This includes monometallic clusters of Ti, Zr, Hf, Nb and Ta

of various shapes as well as mixed-metal Ti/Zr(Hf) or Ti/Y

oxide clusters.2 Clusters with different sizes and shapes and

different degrees of substitution can be obtained by varying

the metal alkoxide/carboxylic acid ratio and the kind of

alkoxo groups of the metal alkoxides. Both parameters

influence the relative reaction rates of the carboxylic acid-

consuming reactions (metal substitution and ester formation)

and thus the overall composition of the cluster.2

An advantage of this method is the slow production of

water in the reacting system. Adding ‘‘external’’ water to

the solution usually does not give reproducible results

because the precise dosage of water is very difficult. Synthesis

of Zr10O6(OH)4(OPr)18(allylacetoacetate)6 by hydrolysis of a

solution of Zr(OPr)4 and 0.6 equivalents of allylacetoacetone,

however, has shown that this method is in principle also

feasible.15

Fig. 1 Left: Zr6O4(OH)4(OMc)12 (reproduced by permission of

Springer from ref. 4); right: [PW11O34(O5Si2R2)]3� (reproduced by

permission of Elsevier from ref. 12).

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This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 575–582 577

Post-synthetic exchange of non-functional ligands against

functional ligands is encumbered by the possibility of

cluster rearrangement or degradation during ligand exchange.

The risk of such reactions occurring is minimized, if the

leaving and entering ligands have the same charge and

occupy the same number of coordination sites. Thus,

reaction of Ti16O16(OEt)32 with 2-hydroxyethyl-methacrylate

or vinylphenol resulted in partial exchange of the

ethoxo groups, and Ti16O16(OEt)24(OCH2CH2OMc)816 or

Ti16O16(OEt)32�x(OC6H4CHQCH2)x (x = 4, 8, 16) was

obtained.17 A manganese oxide cluster with polymerizable

carboxylate ligands was prepared by exchanging the acetate

ligands in Mn12O12(OAc)16 (OAc = acetate) against acrylate18

or methacrylate.19 Another example is already mentioned

above, i.e. the preparation of [PW11O34(O5Si2R2)]3� from

[PW11O39]7�.12 Other polyoxometallates, such as [SiW11O39]

8�,

[SiW10O36]8�, [PW11O39]

7�, or [P2W17O61]10� were functionalized

in the same manner, i.e. by reaction with functional organo-

trichloro- or organotrialkoxosilanes.20

Organic functionality of the ligands. Most cluster-based

hybrid polymers were prepared by free-radical polymerization

of (meth-)acrylate or styryl-substituted metal oxo clusters and

organic co-monomers. It was shown, however, that other

polymerization reactions can be applied as well, provided that

the cluster ligands have suitable functionalities. Thus, clusters

containing strained-ring unsaturated ligands were employed in

ring-opening metathesis polymerizations,21,22 clusters

with 4-pentynoate ligands for alkyne–azide click reactions,23

2-bromo-iso-butyrate-substituted clusters as initiator for

atoms-transfer radical polymerization,24 clusters with

3-mercaptopropionate ligands for thiol-ene reactions,25 and

polyoxometallates modified by aminopropyl silane for the

preparation of cluster-crosslinked polyimides.26 Cluster-based

networks were also prepared by oxidation of the thiophene

units of the clusters W8S8(PR2R0)6, where the groups R

0 were

thiophene oligomers.27

Structural issues of cluster-crosslinked polymers

In most cases reported until present, small proportions of the

functional clusters were co-polymerized with organic co-polymers

by standard protocols. The cluster can be dissolved in an

excess of the organic monomer, but solution polymerization is

also possible if the solubility of the cluster in the organic

monomer is too low. Some clusters can also be polymerized in

the absence of organic monomers.

Most of the cluster-based hybrid polymers are highly cross-

linked, even when small cluster proportions are employed,

because of the large number of reactive ligands in most

clusters. Because of the high crosslinking capability of many

clusters, which may result in a gel effect during polymerization,

a step polymerization protocol can improve the completeness

of free radical polymerizations and thus the polymer

properties.28

The materials properties of cluster-based polymers were

discussed elsewhere.2 They depend not only on the

polymerization conditions and the cluster proportion, but also

on the kind of employed cluster. The appearance of the hybrid

polymers is similar to that of the corresponding cluster-free

polymers for low cluster proportions; resins are obtained for

high cluster proportions. Some properties are distinctly

different to that of the parent polymers and are brought about

by the clusters acting as multifunctional crosslinkers as well as

inorganic (nano-)fillers.3,29 Property changes include:

� Cluster-crosslinked polymers are no longer soluble in

organic solvents, but swell instead, as expected for crosslinked

polymers.

� Thermal stability is higher compared with the native

polymers. A purely inorganic residue is obtained when the

organic moieties of hybrid polymers are completely pyrolyzed

in air.

� Thermomechanical properties are improved.

� Mechanical properties (strength, hardness, brittleness,

scratch resistance, etc.) are changed.

� Dielectric properties may change.

A relatively unexploited option is to combine these property

changes with properties that are inherent to the cluster, such as

special magnetic,14,18,19 redox30 or optical properties.

The most pertinent structural issues for cluster-reinforced

polymers are (i) the integrity of the cluster, (ii) the cluster

dispersion in the polymer, and (iii) the length and polydispersity

of the polymer segments connecting the cluster units.

Integrity of the clusters. Several cluster types are prone to

rearrangement or degradation processes when dissolved or

brought in contact with compounds that could interact with

the surface atoms (mostly compounds with Lewis-basic

properties). A question of practical importance thus is whether

the composition and structure of the inorganic constituents is

Fig. 2 Experimental (solid line) and calculated (dotted line) Fourier transform of the k3w(k) function of crystalline Zr6(OH)4O4(OMc)12 (left) and

Zr6(OH)4O4(OMc)12 co-polymerized with methacrylic acid (right) at the Zr–K edge (reproduction from ref. 31 by permission of Springer Publ. Comp.).

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retained during polymerization, i.e. whether the employed

cluster survive the preparation steps of the hybrid polymer.

In many cases reported in the literature it is assumed that the

obtained polymers contain the same cluster as the one

employed as precursor, but experimental evidence is often

missing. Some cluster types, such as polyoxometallates, are

rather robust, and the assumption is justified. Others, for

example many metal sulfide clusters, have very characteristic

UV-Vis signatures and are thus easy to follow up. 183WNMRwas

employed to confirm the integrity of [SiW10O32(O5Si2R2)]4�

after polymerization.12 In other cases, however, the issue is less

trivial, e.g. for neutral oxo/hydroxo/alkoxo clusters of the

early transition metals. In some cases the starting clusters

were compared with the hybrid polymers after polymerization

of the clusters with organic co-monomers by EXAFS

(Fig. 2).14,25,31 Nearly identical fingerprints of the clusters

before and after polymerization confirmed that (in these cases)

the structure of the cluster core was retained.

Dispersion of the clusters in the polymers. A very suitable

method for studying the cluster distribution in the polymer is

small-angle X-ray scattering (SAXS), because the electron

density difference between the organic and inorganic entities

is large. For a random dispersion of the clusters, a single

maximum is expected which shifts to lower scattering vectors

(larger average cluster–cluster distance) when the cluster

proportion is decreased. A random dispersion was observed

in several cases.18,21,32 In other cases, however, the SAXS data

indicated some clustering of the metal oxide clusters, either

because two maxima were observed or because a single maximum

did not shift upon varying the cluster proportion.16,32

We investigated the latter case in some detail for poly-

styrene (PS) crosslinked by Zr6O4(OH)4(OMc)12, where

only one maximum in the SAXS curves was observed at about

q= 3.7 nm�1.33 The maximum did not shift significantly when

the cluster proportion in the polymer was varied between

0.24 and 0.87 mol% (Fig. 3). The cluster–cluster distance

calculated from the maximum was 1.7–1.8 nm, which was

significantly less than the calculated average distance assuming

that the clusters are randomly distributed in the polymer.

Interpretation of the SAXS data resulted in a structural model

of randomly distributed cluster aggregates of different sizes

(‘‘clusters of clusters’’), where only the packing density of the

aggregates increased with increasing cluster proportion. The

size of the aggregates is in the lower nanometre range. Bigger

aggregates can be excluded, inter alia because the samples are

macroscopically transparent.

The polymer segments connecting the cluster units. Owing to

the high degree of crosslinking, the polymers are insoluble and

molecular mass determinations are not feasible. In one case we

succeeded to destroy the crosslinking cluster and then to

determine the molecular mass distribution of the remaining

organic segments. This allowed some conclusions on the

lengths of the polymer chains connecting the clusters in the

hybrid polymer. When Zr4O2(OMc)12 is treated with acetyl-

acetone, the mono-nuclear complex Zr(acac)2(OMc)2 is formed.

When the experiment was repeated with PS or poly(methyl-

methacrylate) (PMMA) crosslinked with Zr4O2(OMc)12, all

Zr could be extracted from the polymer after degradation of

the clusters. Analysis of the resulting Zr-free polymer gave

Mn of 7.1 � 105 for PMMA and 1.0 � 105 for PS. The poly-

dispersities, however, were very high, viz. 8.8 and 4.4,

respectively.34 This shows that the clusters are connected by

organic cords of strongly varying lengths.

Metal–organic framework structures with clusters as

connectors

One-dimensional coordination polymers are obtained when

bidentate ligands of the type X–Y–X tether two metal

complexes and two groups X coordinate to each metal centre

� � �X–MLn–X–Y–X–MLn–X–Y–X–� � �

(MLn is a metal complex fragment containing n co-ligands

L not integrated in the polymer backbone; X is a coordinating

group which provides a stable link to the MLn fragment; and

Y is an organic or inorganic spacer). This can be extended to

2-D or 3-D network polymers by coordinating more than two

groups X to one metal centre and, optionally, by means of

tri- or tetradentate ligands. In the 3-D metal–organic frameworks

(MOFs), metal ions (the connectors) are bridged in three

dimensions with organic linkers. Linkers (also called ‘‘spacer’’)

and connectors (also called ‘‘node’’) assemble through Lewis

acid–Lewis base interactions (coordinative bonds) or ionic

interactions, where the metals are the Lewis acidic centres or

cations, and the organic linkers the Lewis basic centres or

anions.

MOFs are usually obtained as microcrystalline solids, i.e.,

ordered networks are formed under thermodynamic control.

Apart from the small crystallite sizes, disorder and poly-

morphism may additionally render structure analyses by

diffraction methods difficult. For this reason computer

simulation methods were developed to aid both preparative

chemistry and interpretation of X-ray diffraction data.8,35

A special feature of MOF-type materials is their high porosity

and surface areas.MOFs thus have high potential for applications

where high surface areas are required, such as sorption,

Fig. 3 Experimental SAXS profile of PS crosslinked with

0.24–0.87 mol% of Zr6O4(OH)4(OMc)12 (Zr6). The SAXS profile of

cluster-free PS is included for comparison (lower curve). The solid lines

are fits. Reproduced by permission of Wiley-Interscience from ref. 33.

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separation, catalysis, etc. Complete characterization of MOFs

must therefore include quantification of the porosity,

investigations on the framework stability, analysis of guests

that might be entrapped or adsorbed in the pores, etc.

Topologies of MOF frameworks are determined by several

factors:

� The Legos bricks characteristics of the connector ion or

cluster, as outlined above, especially the number and spatial

orientation of the coordination sites.

� The number and geometrical arrangement of the

coordinating groups of the linker, as well as the rigidity of

the spacer group (Y).

� The nature of the donor atoms of the linkers.

� Auxiliary ligands which may block coordination sites at

the metal ion or cluster.

� Additional non-covalent or non-ionic interactions

(hydrogen bonds, p–p stacking, hydrophobic interactions, etc.).

� The presence of organic guest molecules acting as

templates, or non-coordinated counter-ions of the employed

metal salts.

The linkers are mostly compounds with two or more amino,

carboxylate or phosphonate groups (taking a liberal view on

whether phosphonates are organic ligands). Coordination

compounds can also act as linkers, but are not considered

here because the obtained structures are not strictly

inorganic–organic hybrid materials. The same applies to

cyanide- or oxalate-bridged structures.

The number and spatial orientation of the binding sites of

connectors and linkers determines the geometry of the resulting

3-D network. As a structural consequence of the open frame-

work structures, large cages or cavities are formed, which are

the origin of the high porosity of MOFs. As in zeolites, the

cage size and geometry depends on the topology of the

network, and the topology on the connectivity of the building

blocks. For a given structure type, the pore size can be

modulated by the length of the linker and the size of the

connector.

Cluster-based networks are a sub-class of metal–organic

framework (MOF) structures where clusters are ‘‘expanded’’

connectors or ‘‘secondary building blocks’’ (SBU).6 A good

example is MOF-5 (Zn4O(1,4-benzenedicarboxylate)3), one of

the earliest and most prominent MOFs, with [Zn4O]6+ cluster

units as connectors.36 The six edges of the tetrahedral Zn4O

core are bridged by six 1,4-benzenedicarboxylate units, i.e., the

connector unit is octahedrally coordinated (Fig. 4, left). When

the length of the dicarboxylate is varied, the framework

structure is retained, but the cavity size can be varied in a

rather wide range.37 The [Zn4O]6+ cluster unit can be regarded

as a big (expanded) pseudo-atom with octahedral coordination

geometry. The following comparison shows that MOF structures

often are analogues to common inorganic structures. In the

crystal structure of ReO3, the metal atoms are octahedrally

coordinated by six oxygen atoms, and each oxygen atom

bridges two rhenium atoms. This is a very common structure

type for inorganic MX3 compounds. If the bridging O2� in the

ReO3 structure are replaced by bridging �OOC–R–COO�

(‘‘expanded linkers’’), and Re(VI) by the [Zn4O]6+ cluster unit

(‘‘expanded connector’’), the MOF-5 structure is obtained,

i.e., the MOF-5 structure is a metal–organic, and highly

porous, analogue of the ReO3 structure.

Different network symmetry is obtained when the same

linker (benzene-1,4-dicarboxylate) is coordinated to a cluster

unit with the same charge, but different spatial orientation of

the carboxylate ligands. Six edges of the [Cr3O(H2O)2F]6+

core (MIL-101)38 are coordinated by carboxylate ligands in a

trigonal prismatic geometry (Fig. 4, right; the H2O and F

ligands occupy the coordination sites in the Cr3O plane).

Early MOF syntheses involved techniques known to grow

high-quality crystals of simple inorganic salts by reducing the

crystal nucleation rate such as slow evaporation of solvents,

layering of different solvents or slow diffusion of one component

into the solution of another. MOFs are now most commonly

prepared by solvothermal, especially hydrothermal methods.

To this end, the precursors are typically combined in dilute

Fig. 4 Left: octahedral linkage of Zn4O(OOCR)6 clusters with 1,4-benzenedicarboxylate (MOF-5); right: the trigonal prismatic linkage of

Cr3O(OOCR)6(H2O)2F with 1,4-benzenedicarboxylate (MIL-101). Reproduced by permission of the Royal Society of Chemistry from ref. 8.

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580 Chem. Soc. Rev., 2011, 40, 575–582 This journal is c The Royal Society of Chemistry 2011

solutions of polar solvents, or solvent mixtures, and heated in

sealed vessels. The different solubility of the inorganic and

organic components is thus overcome. Furthermore, the

reduced viscosity of water under hydrothermal conditions

enhances diffusion processes and thus crystal growth.

Solvothermal conditions facilitate equilibration of the system,

i.e. formation of ordered structures. This is enabled by the

weaker coordinative bonds (compared with covalent bonds)

which allow detachment of incoherently assembled monomers

or building blocks and re-attachment of the ligands to the

metal centre(s) in thermodynamically more favourable

positions. There is an inverse relationship between the

metal–ligand bond strength (i.e., the reversibility of the bond

formation process) and the robustness of the formed

frameworks. This requires a very deliberate consideration of

the synthesis protocol.

Microwave synthesis has been recently successfully applied

to MOFs as an alternative to conventional heating. Since

MOFs can be prepared by this method in considerably shorter

reaction times, with better yields and purities, a continuous

flow of products appears to be feasible.8

The two preparative strategies for cluster-based MOF

structures differ by the way how the clusters are generated.

The first relies on the self-assembly of MOx building blocks,

i.e., on in situ formation of the cluster units, while the second

exploits discrete clusters as pre-formed SBUs. Reference

should be made, at this point, to the first part of this article.

As has been discussed there, pre-formed clusters are employed

for the preparation of cluster-crosslinked polymers. In situ

formation of the cluster units, however, is also conceivable.

For example, hybrid materials are known which were obtained

from metal alkoxides and carboxylate-substituted polymers,

such as poly(methacrylic acid). Although the nanoscale

structure of such hybrid materials has not been investigated,

it is very likely that metal oxide clusters could have been

formed (similar to eqn (1)) which are capped by the

carboxylate groups of the polymer. Since there is no thermo-

dynamic control (as in MOF syntheses), some cluster/particle

size distribution must be expected.

In situ formation of the cluster connectors

The most common approach for the synthesis of MOFs is

reacting metal salts and organic links in a single step. This

implies that the synthesis conditions must be compatible with

the formation and preservation of the cluster units being

formed and linked in situ.

MOFs based on [Zn4O]6+ cluster units as connectors are

obtained by heating a solution of Zn(NO3)2 hydrate and the

dicarboxylic acid in an organic solvent in a closed vessel.36

During this process, the cluster units form and assemble

through the dicarboxylate linkers. MIL-101 (Fig. 4) is

similarly obtained by hydrothermal reaction of Cr(NO3)3,benzene-1,4-dicarboxylic acid and aqueous hydrofluoric

acid.38 Several mono- and polynuclear species were discovered

in the Zn2+/1,4-benzenedicarboxylic acid/solvent system for

the synthesis of MOF-5, which depend, inter alia, on the

counter-ion, the metal/ligand ratio, the solvent, etc.36,39

This observation emphasizes the self-assembling nature of

the process: the metal-containing species most suitable for

stable network structures are ‘‘extracted’’ from the reaction

solution.

Main-group clusters can also be created in situ. MIL-110,

where [Al8(OH)15(H2O)3]9+ cluster units are connected

through 1,3,5-benzenetricarboxylate linkers, was synthesized

by hydrothermal treatment of a mixture of Al(NO3)3, 1,3,5-

benzenetricarboxylic acid and aqueous nitric acid.40

The highly acidic conditions (pH E 0) are required for the

formation of the particular Al/O cluster.

When the tetrahedral quadridentate linker tetrakis(4-pyridyl-

oxymethylene)methane (TPOM; C[CH2O–C5NH4]4) was

reacted with Cd(SPh)2 and thiourea under solvothermal

conditions in acetonitrile/methanol, ([Cd8S(SPh)14]2TPOM)

(MCOF-9) was formed.41 Each of the four nitrogen atoms of

TPOM is used for crosslinking. Only two corners of the clusters,

however, are occupied by N donors from organic linkers (the

remaining two corners are occupied by SPh� groups). Although

this resulted in a belt-like structure, this example shows that the

synthesis method is not restricted to metal oxo clusters and that

other cluster connectors can be obtained as well.

Networks from pre-formed clusters

Using pre-formed clusters is the most rational and deliberate

way of MOF synthesis, and the term ‘‘secondary building

unit’’ (SBU) gets a practical meaning. In this approach,

clusters with monofunctional ligands bonded to the surface

atoms are employed, which are then exchanged by the bi- or

multifunctional linkers to connect the cluster units with each

other. Therefore, no functional cluster-ligands are required, in

contrast to the preparation of cluster-based polymers

discussed in the first part of this article. This ligand exchange

reaction, however, must also proceed without loss of the

integrity of the cluster.

An illustrative example is the reaction of the so-called

‘‘basic iron(III) acetate’’ with various dicarboxylic acids

HOOC–R–COOH.42 Basic iron(III) acetate is [Fe3O(OAc)6L3]X,

where X is a negatively charged counter-ion and L neutral

ligands, such as water, alcohols, amines, etc. The structure of

[Fe3O(OAc)6L3]+ is the same as that of Cr3O(OOCR)6(H2O)2F

described before (Fig. 4). When the iron cluster is reacted with

the dicarboxylic acids, acetic acid is liberated and the MOF

structure is produced with retention of the trinuclear cluster SBU

(eqn (2)). The topology of MIL-88 (R = C2H2, X = OAc,

L = CH3OH) is essentially the same as that of MIL-101

discussed above (since the structure of the cluster connector is

the same). Analogous MOFs were produced with M3O7+ units

of other trivalent metals.

[Fe3O(OAc)6L3]X + 3HOOC–R–COOH

- [Fe3O(OOC–R–COO)3L3]X + 6AcOH (2)

In this example, a mono-carboxylate ligand was exchanged by

a bis-carboxylate ligand. Exchange of a certain ligand against

a ligand of another type is feasible as well. The clusters

M5(benzotriazolate)6(NO3)4(H2O) have a tetrahedral arrangement

of four six-coordinate M2+ ions centered on the fifth one.

Each of the outer metal ions is substituted by a chelating

nitrate ligand. Exchange of the latter against either

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This journal is c The Royal Society of Chemistry 2011 Chem. Soc. Rev., 2011, 40, 575–582 581

dicarboxylates43 or the TCNQ radical anion (7,7,8,8-tetracyano-

p-quinodimethane)44 also resulted in MOF structures.

An example which connects the two topics of this article

with each other, i.e., MOFs with cluster connectors and

cluster-based polymers, is the reaction of the previously

discussed cluster Zr6O4(OH)4(OMc)12 (Fig. 1) with dicarboxylic

acids. Exchange of the methacrylate ligands against muconate

or terephthalate ligands resulted in the formation of a porous

network with retention of the cluster units.45 As shown in

Fig. 5, two cages were formed, a tetrahedral with a diameter of

120 pm and an octahedral with a diameter of 75 pm.

Yaghi et al. recently published a comprehensive study of

transition-metal carboxylate clusters which may serve as

building blocks for MOFs.46 Their compilation of the

geometries of 131 such building blocks, with different

compositions, structures and connectivities, shows impressively

the great potential of this approach for the construction of

new cluster-based framework structures.

Conclusions

Two types of inorganic–organic hybrid materials were discussed,

where clusters constitute the inorganic components. In fact,

cluster-crosslinked hybrid polymers and metal–organic frame-

works with clusters as connectors are limiting cases. In the

MOF structures, the clusters are connected by relatively short

and rigid organic groups in a ratio which is determined by the

connectivity of the building blocks. This results in network

topologies which are analogues of the crystal structures of

inorganic solids (although the structures are expanded). In

contrast, relatively small cluster proportions are employed for

the synthesis of cluster-crosslinked polymers. This means in turn

that extended organic segments, with broad size distribution,

connect the cluster units. Such materials are amorphous and have

polymer-like properties. It is apparent that in between these

extremes there is plenty of room for other materials developments.

Even for the ‘‘limiting cases’’ discussed in this article, the

preparative possibilities have only been scratched at the

surface. Many additional combinations of clusters and organic

constituents are conceivable. An important issue for both

sub-classes of cluster-based hybrid materials is the availability

of clusters with Legos bricks characteristics. For rational

syntheses of hybrid materials, clusters with a defined chemical

composition and structure are needed which are stable

through all steps of the hybrid material’s syntheses. Although

various kinds of molecular clusters are known, only part of

them fulfil these requirements. A major problem is in many

cases the lack of stability of the cluster core especially during

ligand exchange or reactions involving the ligand shell.

Clusters with functional organic ligands, needed for the

synthesis of cluster-crosslinked polymers, were hardly investigated.

A challenging preparative task is the preparation of clusters

with different kinds of ligands, i.e., functional/non-functional

or exchangeable/non-exchangeable ligands, in a deliberate

ratio and a controlled mutual arrangement.

Cluster-based inorganic–organic hybrid materials have

great potential because clusters are well defined inorganic

building blocks, which can be rationally incorporated in or

connected by organic moieties. The known examples of such

materials are mainly based on rather straightforward clusters

and organic moieties. Interesting new materials are envisioned,

if clusters and/or organic moieties with special properties

would be used, such as clusters with intrinsic magnetic,

electronic, optical or chemical properties.

Acknowledgements

Own work was financially supported by the Austrian Science

Funds (FWF), Wien (projects P12766, P16254 and P19199).

References

1 According to F. A. Cotton (Acc. Chem. Res., 1969, 2, 240), a metalcluster consists of a finite number of metal atoms which are connectedthrough a major part by metal–metal bonds. The term ‘‘cagecompound’’ was proposed for polynuclear compounds connected byligand bridges without metal–metal bonding. In this article the term‘‘cluster’’ is also for the latter compounds in line with current practice.

2 U. Schubert, Chem. Mater., 2001, 13, 3487, and references citedtherein. U. Schubert, in Macromolecules Containing Metal- andMetal-like Elements, ed. A. Abd-El Aziz, C. Carraher, C. Pittmanand M. Zeldin, J. Wiley, New York, 2006, vol. 7, p. 55.

Fig. 5 View of the two cages in the MOF structure formed by reaction of Zr6O4(OH)4(OMc)12 with muconic acid (reproduced by permission of

The Royal Society of Chemistry from ref. 45).

Dow

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ded

by I

ndia

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nive

rsity

- P

urdu

e U

nive

rsity

at I

ndia

napo

lis o

n 19

Aug

ust 2

012

Publ

ishe

d on

28

Sept

embe

r 20

10 o

n ht

tp://

pubs

.rsc

.org

| do

i:10.

1039

/C0C

S000

09D

View Online

582 Chem. Soc. Rev., 2011, 40, 575–582 This journal is c The Royal Society of Chemistry 2011

3 U. Schubert, Macromol. Symp., 2008, 267, 1, and references citedtherein.

4 U. Schubert, J. Sol-Gel Sci. Technol., 2003, 26, 47.5 L. Rozes, N. Stenou, G. Fornasieri and C. Sanchez, Monatsh.Chem., 2006, 137, 501.

6 Selected review articles: R. Robson, Dalton Trans., 2008, 5113;P. J. Hagrman, D. Hagrman and J. Zubieta, Angew. Chem., Int.Ed., 1999, 38, 2638.

7 J. L. C. Rowsell and O. M. Yaghi, Microporous MesoporousMater., 2004, 73, 3.

8 G. Ferey, Dalton Trans., 2009, 4400.9 G. Ferey, Chem. Soc. Rev., 2008, 37, 191; G. Ferey, C.Mellot-Draznieks, C. Serre and F. Millange, Acc. Chem. Res.,2005, 38, 217.

10 Recent review article: K. Pielichowski, J. Njuguna, B. Janowskiand J. Pielichowski, Adv. Polym. Sci., 2006, 201, 225, andreferences cited therein.

11 G. Kickelbick and U. Schubert, Chem. Ber., 1997, 130, 473.12 D. Agustin, J. Dallery, C. Coelho, A. Proust and R. Thouvenot,

J. Organomet. Chem., 2007, 692, 746.13 G. Losada, M. A. Mendiola and M. T. Sevilla, Inorg. Chim. Acta,

1997, 225, 125; K. L. Zhang, Y. J. Shi, X. Z. You and K. B. Yu,J. Mol. Struct., 2005, 743, 73; D. L. Long, P. Kogerler,L. J. Farrugia and L. Cronin, Chem.–Asian J., 2006, 1, 352.

14 V. DiNoto, A. B. Boeer, S. Lavina, C. A. Muryn, M. Bauer,G. A. Timco, E. Negro, M. Rancan, R. E. P. Winpenny andS. Gross, Adv. Funct. Mater., 2009, 19, 3226.

15 C. Sanchez, M. In, P. Toledano and P. Griesmar, Mater. Res. Soc.Symp. Proc., 1992, 271, 669.

16 S. Trabelsi, A. Janke, R. Hassler, N. Zafeiropolous, G. Fornasieri,S. Bocchini, L. Rozes, M. Stamm, J. F. Gerard and C. Sanchez,Macromolecules, 2005, 38, 6068.

17 L. Rozes, G. Fornasieri, S. Trabelsi, C. Creton,N. E. Zafeiropolous, M. Stamm and C. Sanchez, Prog. Solid StateChem., 2006, 33, 127.

18 F. Palacio, P. Oliete, U. Schubert, I. Mijatovic, N. Husing andH. Peterlik, J. Mater. Chem., 2004, 14, 1873.

19 S. Willem, B. Donnadieu, L. Lecren, B. Henner, R. Clerac,C. Guerin, A. Meyer, A. V. Pokrovskii and J. Larionova, New J.Chem., 2004, 28, 919.

20 T. Hasegawa, H. Murakami, K. Shimizu, Y. Kasahara,S. Yoshida, T. Kurashina, H. Seki and K. Nomiya, Inorg. Chim.Acta, 2008, 361, 1385, and references cited therein.

21 Y. Gao, F. R. Kogler, H. Peterlik and U. Schubert, J. Mater.Chem., 2006, 16, 3268.

22 X.Wie, P. J. Carroll and L. G. Sneddon,Chem.Mater., 2006, 18, 1113.23 P. Heinz, M. Puchberger, M. Bendova, S. O. Baumann and

U. Schubert, Dalton Trans., 2010, 39, 7640.24 G. Kickelbick, D. Holzinger, C. Brick, G. Trimmel and E. Moons,

Chem. Mater., 2002, 14, 4382.

25 F. Faccini, H. Fric, U. Schubert, E. Wendel, O. Tsetsgee,K. Muller, H. Bertagnolli, A. Venzo and S. Gross, J. Mater.Chem., 2007, 17, 3297; M. Sangermano, S. Gross, A. Priola,G. Rizza and C. Sada, Macromol. Chem. Phys., 2007, 2560.

26 H. Chen, L. Xie, H. Lu and Y. Yang, J. Mater. Chem., 2007, 17,1258.

27 S. Perruchas, S. Flores, B. Jousselme, E. Lobkovsky, H. Abrunaand F. J. DiSalvo, Inorg. Chem., 2007, 46, 8976.

28 Y. Gao, F. R. Kogler and U. Schubert, J. Polym. Sci., Part A:Polym. Chem., 2005, 43, 6586.

29 F. R. Kogler and U. Schubert, Polymer, 2007, 48, 4990.30 P. Judeinstein, Chem. Mater., 1992, 4, 4; P. Judeinstein, J. Sol-Gel

Sci. Technol., 1994, 2, 147.31 G. Kickelbick, M. P. Feth, H. Bertagnolli, B. Moraru, G. Trimmel

and U. Schubert, Monatsh. Chem., 2002, 133, 919.32 V. Torma, N. Husing, H. Peterlik and U. Schubert, C. R. Chim.,

2004, 7, 495.33 F. R. Kogler, T. Koch, H. Peterlik, S. Seidler and U. Schubert,

J. Polym. Sci., Part B: Polym. Phys., 2007, 45, 2215.34 D. S. Dragan, F. R. Kogler and U. Schubert, Polymer, 2008, 49,

378.35 G. Ferey, C. Serre, C. Mellot-Draznieks, F. Millange, S. Surble,

J. Dutour and I. Margiolaki, Angew. Chem., Int. Ed., 2004, 43,6296.

36 H. Li, M. Eddaoudi, M. O’Keeffe and O. M. Yaghi, Nature, 1999,402, 276.

37 M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter, M. O’Keeffeand O. M. Yaghi, Science, 2002, 295, 469.

38 G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour,S. Surble and I. Margiolaki, Science, 2005, 309, 2040.

39 E. Biemmi, T. Bein and N. Stock, Solid State Sci., 2006, 8,363.

40 C. Volkringer, D. Popov, T. Loiseau, N. Guillou, G. Ferey,M. Haouas, F. Taulelle, C. Mellot-Draznieks, M. Burghammerand C. Riekel, Nat. Mater., 2007, 6, 9724.

41 Q. Zhang, X. Bu, Z. Lin, T. Wu and P. Feng, Inorg. Chem., 2008,47, 760.

42 C. Serre, F. Millange, S. Surble and G. Ferey, Angew. Chem.,Int. Ed., 2004, 43, 6286; S. Surble, C. Serre, C. Mellot-Draznieks,F. Millange and G. Ferey, Chem. Commun., 2006, 284.

43 X. L. Wang, C. Qin, S. X. Wu, K. Z. Shao, Y. Q. Lan, S. Wang,D. X. Zhu, Z. M. Su and E. B. Wang, Angew. Chem., Int. Ed.,2009, 48, 5291.

44 Y.-L. Bai, J. Tao, R.-B. Huang and L.-S. Zheng, Angew. Chem.,Int. Ed., 2008, 47, 5344.

45 V. Guillerm, S. Gross, C. Serre, T. Devic, M. Bauer and G. Ferey,Chem. Commun., 2010, 46, 767.

46 D. J. Tranchemontagne, J. L. Mendoza-Cortes, M. O’Keeffe andO. M. Yaghi, Chem. Soc. Rev., 2009, 38, 1257.

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