Platinum Metals Rev., 1994, 38, (2), 60
Polymer-Immobilised Clusters of the Platinum Group Metals
Potential use for Platinum Metals
In this review major developments associated with the synthesis, properties, structure and applications of polymer-immobilised clusters of platinum, palladium, ridium, rhodium, osmium and ruthenium, are presented. Special attention is paid to polymer analogous reactions with metal clusters and new directions involving the polymerisation and copolymerisation of cluster-containing monomers. Some specific features of fixing heterometallic clusters on polymers are examined and the more interesting application of PCNM in catalysis, and future developments in this direction, are discussed.
Polymer-immobilised clusters of the noble metals (PCNM) are of great interest for at least two reasons; first, they are a new type of catalyst which can be used in many organic syntheses* and second they can be widely used as a basis for the production of different polymeric materials with unusual properties. In addition polymer-immobilised clusters can act as a convenient model of metal catalyst surfaces (3). Studying immobilised clusters can provide information about the structure of the catalyst surface, such as metal-substrate bond energies, the stereochemistry of the active centre and structural transformations occurring during catalytic reactions, and also about the mechanism of the catalytic reaction.
At least three fields of chemistry are linked by PCNM: cluster co-ordination chemistry, polymer chemistry and catalysis. There are three major methods for the preparation of PCNM; these are the physical insertion of small metal particles into a polymer, chemical insertion and (co)polymerisation of cluster-containing monomers, see Table I.
Preparation of PCNM Clusters of Unidentified Structure
The methods of preparing platinum metals clusters immobilised on polymers having unidentified structure are based on forming small metal particles from fine powders, salts and mononuclear complexes in the presence of polymers, which act as protective covers. The specific features of these methods are the direct route and the relative simplicity of preparation of the products. PCNM prepared in this way have a wide size distribution, typically 10 to 700 Å, which can change during use. By comparison with typical mechanical mixtures of polymer and large metal particles of micron size, the polymer immobilised clusters additionally have a relatively high dispersion, a uniform size distribution of metal particles within the bulk of the polymer, and a substantial irreversible sorption of macromolecules on the surface of the metallic particles. Such PCNM have a tendency to encompass metal particles within the natural hollows of the polymer matrix, and their formation is often accompanied by chemical reactions. However, the PCNM produced by these methods may be of single-phase metallopoly-meric compositions due to the high dispersion of the metal particles. Various methods for preparing polymer immobilised noble metal clusters are shown in Table I.
Another method for preparing clusters with unidentified structures is by the reduction of mononuclear complexes. This method uses carbon chain polymers or polymers produced by polycondensation as the organic medium for the process. For example, the reduction of palladium chloride, absorbed on polyheteroarylenes, by NaBH4 or hydrogen leads to the formation of Pd0 clusters of size 1 to 3.5 nm (4). Such small particles interact so strongly with the polymer matrix that positive charge, Pd+, appears on the palladium atoms as a result of electron transfer. The p-electron system of the polymer chain promotes this process and subsequently transforms into an ion-radical state.
Similar interactions between formed clusters and the polymer matrix are characteristic of many other metallopolymeric systems (5, 6), including clusters produced by spraying solvated metal atoms in vacuo at 77 K (7). The polymer matrix in such cases acts to stabilise the highly dispersed metal particles and prevents the subsequent enlargement of the clusters.
Metal particle growth prevention by the polymer matrix can be shown by examples from the formation of cluster-, or colloidal particles of palladium, ruthenium, rhodium, osmium, iridium, silver or gold, in protective polymer coverings (8, 9).
The following transformation sequence occurs on boiling RhCl3 with polyvinyl alcohol (PVA) or polyvinylpyrrolidone solutions: in the first stage RhCl3 is co-ordinated by the polymer, then oxoniene RhCl3 complex is formed followed by fixing the hydride complexes as the alkoxide form; alkoxide groups being precursors of hydride forms. This sequence is necessary to form homogeneous colloids and is accompanied by consecutive growth of the particles, see Scheme I.
The clusters formed with thirteen rhodium nuclei have a face centred cubic lattice, and the co-ordination number of rhodium is 12. They are attached to the protective colloid by electrostatic attraction or physical absorption and possibly by co-ordination bonds. The dimensions of the cluster particles may be changed by using a different polymer as well as by the reaction conditions.
The same method was used to prepare palladium particles of diameter 1.8 nm within the protective colloid, polyvinylpyrrolidone. Natural polymers, such as β-cyclodextrine (10), different functionalised cellulose derivatives, oligo- and polysacharides (11), indian silk (12), chitin and chitosane (13), may also be used as stabilising agents for colloidal particles of rhodium, platinum and palladium of diameter 1 to 100 nm.
Synthesis by Polymer Analogous Reactions
The development of new methods is needed to create materials composed of individual clusters or metal atom assemblies of diameters 1.5 to 5 nm with a narrow size distribution. In one method the assembly of polynuclear complexes from mononuclear complexes often occurs during the immobilisation or mononuclear complexes, by functionalised polymers, from highly concentrated solutions, as well as by fixation by urifunctionalised polymers. However, published data show that the nuclearity of the initial complexes can be preserved by fixing binuclear or trinuclear ruthenium complexes on ion-exchange resins, such as Amberlite ER-20 or Dianion CR-10, which contain irninodiacetate groups (14).
More elaborate methods of producing PCNM are based on fixing individual clusters of known structure by polymer macroligands. Macrolig-ands, such as "popcorn" polymers (phosphorilated copolymers of styrene and divinylbenzene Ⓟ-PPh2, or sometimes variations (triple block-copolymer styrene-divinylbenzene-vinyldiphenyl-phosphine) in the form of thin membranes or grains with size dimensions of 200 to 400 mesh are usually employed. Fixing the clusters can be performed by ligand- or ion exchange, oxidative addition, decarbonylation, ligand addition, and so on. Some of the more interesting PCNM prepared by these methods, grouped according to similarities in their preparations, are listed in Scheme II (15–20).
Polymer-immobilised clusters with a one-centre bonded cluster, for example iridium, are formed at low concentrations of phosphine groups, where less than two per cent of the benzene rings are functionalised, or at a statistical distribution of PPh2-groups. A convenient method of showing the structure of immobilised clusters is by comparison of the infrared spectra of the initial substances and the products formed. However, for macroligands with rather high concentrations of PPhrgroups, where more then three per cent of the benzene rings are functionalised, a mixture of clusters connected to the polymer by one and two PPh2 groups is formed. Clusters connected to the polymer by two PPh2 groups are produced as the sole product when the block-copolymer styrene-divinyl-benzene-divinylphenylphosphine, containing 8 to 15 per cent of PPh2-groups, is used. Clusters connected to three PPh2-groups, are prepared in this way (for instance Ⓟ-(PPh2)3RuCl2, see Scheme II).
It is important that immobilised complexes of Ir4 are synthesised by assembly from Ir(CO)2Cl-(p-toluidine) or from Ir2Cl2(C8H12)2 in situ in the presence of zinc and carbon monoxide. Attempts to fix Ir4(CO)l2, dissolved in toluene, permenently with Ⓟ-PPh2 were unsuccessful; a mix ture of mono-, bi- and trisubstituted forms was obtained.
More complicated cluster transformations have been observed; for example, when Rh4(CO)12 was reacted with Ⓟ-PPh2 in hexane at 50°C binuclear clusters were formed (19). However, when this procedure was carried out with Rh6(CO)16 in benzene, a precipitation of black spots of metallic rhodium was observed on the walls of the reactor (20). The black spots were of size 25 to 40 Å, and each spot contained about 100 rhodium atoms. The same effect has also been observed for anionic ruthenium clusters. At higher temperatures of 100 to 150°C the immobilised clusters dissociate to form metal crystallites in the polymers.
Cluster fragments OfIr4(CO)10 can be fixed by other phosphorus containing macroligands on polymers which have been modified with the optically active groups 2,3-(o -isopropylidene-2,3-dioxi)-1,4-bis (diphenylphoshino)-butane (21).
The processes for immobilising noble metal clusters by other polymers have been studied less. In particular, the immobilisation of cluster Os3(CO)12 onpoly(4-vinylpyridine) (P4VPy) in dimethylformamide at 110°C in an atmosphere of carbon monoxide occurs by means of two pyridine rings, one of which is chelated (22).
The mechanism for bonding Rh4(CO)12 with poly(4-vinylpyridine), as well as the structures of the products formed by the interaction of Rh6(CO)16, Rh4(CO)12 and Rh2(CO)4Cl2 with NH2-groups of the polymer (aminated polystyrene), requires further study (23). This also applies to the immobilisation Of Ru3(CO)12 on cross-linking macroporous chelating polymers, functionalised with the following: bipyridine, 2-aminopyridine, 2-aminophenol, 2-iminopy-ridine and sodium anthranilate (24), for example; and to products formed by the interaction Of Rh2(CO)4Cl2 with copolymers of styrene and divinylbenzene, modified by N,O-chelating nodes (25).
The interaction of Rh6(CO) 16 (the first cluster to be immobilised (16)) with resin Amberlyst A-21 proceeds in a more complicated manner and leads to the formation of immobilised rhodium clusters of different nuclearity, see Scheme III (26).
The formation Of HOs3(CO)11N+Et3CH2P from Os3(CO)12 has been identified (27). Sulphur-and oxygen-containing matrixes are rarely used to immobilise clusters; polymeric alcohols and acids are the more often used macroligands. Thus, the addition of triosmium clusters to polymeric alcohols proceeds, as in the case of inorganic oxide, by oxidative addition, see Scheme IV (28).
Very stable compounds, for example the bland trinuclear complexes RuIII-O-RuIV-ORuIII, can be fairly easily fixed on the ion-exchange resin Diaion CR-IO (14). It is important to note that the preservation or dissociation of their polynuclear structure depends on the nature of the ligands. Thus, during the heterogenisation of the binuclear acetate complex Rh4(OOCCH3)4 on a polymer which contains ligands of 3(5)-methylpyrazole or imidazole groups (29), the binuclear structure and the degree of oxidation of the rhodium (+2) remain on the polymer. In contrast, for rhodium binuclear complexes of sulphate, acetonitrile and hexafluoroacetylacetonate the rhodium-rhodium bond breaks and the degree of oxidation on the central atom increases. An analogous result has been obtained during fixing the cluster anion Ru5Cl12-2 on Ⓟ-PPh2 (30): the cluster anion can not penetrate into the polymeric matrix and so, for steric reasons, dissociates forming fixed mononuclear and dimer complexes.
There is no data available at present about bonding noble metal clusters by polymer-polymer compositions, unlike mononuclear complexes for which data which is available.
Polymer-Immobilised Bimetallic Clusters
A heterometallic polynuclear centre is preferable to a mononuclear one for many catalytic processes. Such systems can be considered as models of bimetallic catalysts, for example as the contact crystallites on the surface of catalysts used for refining oil distillates industrially. However, PCNM, in contrast to fused catalyst are of uniform structure. As a rule the same methods that are used to immobilise mononuclear complexes are used both to immobilise heterometallic clusters and to identify the structure of the products. The most widely used method is the assembly synthesis of clusters from monometallic complexes. A co-operative dissociation of H2PtCl6-Fe(NO3)2 and/or Rh4(CO)12-CoCl2.6H2O in polymeric matrixes has been carried out.
Bimetallic clusters can be formed in a colloidal dispersion of platinum and palladium stabilised by polyvinylpyrrolidone, if the ratio of palladium:platinum is 41; the dispersion being prepared by the combined reduction Of PdCl2 and H2PtCl6 in a water-ethanol solution in the presence of polyvinylpyrrolidone (10). Clusters of different metals, for example, a mixture of the clusters Co4(CO)12 and Rh4(CO)12 in molar ratios from 1:1 to 3:1, can be fixed on amino-containing ion-exchange resins to use as a model of a bimetallic catalyst (31).
Heterometallic clusters of known structure are of greater use in the same way that immobilised monometallic clusters of known structure are. Phosphorilated polystyrenes are polymer matrixes of optimal structure, and ligand exchange or sometimes ionic exchange is widely used to immobilise clusters on them. The structure of the products is always determined by analysis of the infrared spectra of the carbonyl group clusters and by a comparison of the spectra of individual molecular compounds. Typical examples of immobilised heterometallic clusters are summarised in Scheme V (3,17,32-34). Other examples show the immobilisation of H2FeOs3(CO)13 on cross-linking macroporous chelating polymers (24). There is also data about the immobilisation of Co2Rh2(CO)12 on the ion-exchange resin Dawex-1 (35), as well as for H2FeRu3(CO)13, [FeRh4(CO)15] [NMe4]2 and Rh4-xCox(CO)12, where x = 2 or 3, on macroligands. The mechanism of these processes has not yet been studied, and it is very possible that there are no examples of immobilisation of trimetallic type clusters.
PCNM Formation in the Course of Polymerisation
The formation of polymer-immobilised noble metal complexes during the preparation of the polymeric matrix has advantages both in simplifying the synthesis of the complexes and in determining their structure. The methods for preparing PCNM which were considered earlier are generally accompanied by numerous processes, the most important of which is an increase or decrease in their nuclearity.
The polymerisation of vinyl monomers in the presence of formed small dispersive particles of the platinum metals is not a commonly used method at present. However, in recent years methods for the polymerisation of cluster-containing monomers (as for other types of metal-containing monomers (36)) have aroused great interest. As this area of study is presently undergoing development, we shall demonstrate its possibilities with some examples.
The cluster monomers listed below were synthesised by the interaction of trinuclear clusters M3(CO)12 (where M is osmium or ruthenium), Os3(CO)11(CH3CN), Os3(CO)10(CH3CN)2 and (m -H)Os3(m -OR)(CO)10 (where R is hydrogen or phenyl) with conventional monomers, such as 4-vinylpyridine, (4VPy) acrylic acid and allyl-sulphide, see Scheme VI (37).
It is interesting to note that in such monomers the pyridine ring is chelated as in the case for the product of the interaction of Os3(CO)12 with poly(4-vinylpyridine) (P4VPy) (see above). At the same time the interaction Of Rh6(CO)16 with P4VPy in the presence of (N-oxide of trimethylamine) proceeds under mild conditions and is accompanied by the formation of the basic product, the monosubstituted derivative Rh6(CO)15. (4VPy), and small amounts of di-substituted compounds Rh6(CO).(4VPy)2 (38). These are easy separated by chromatography and may be isolated individually. In contrast to the Os3-derivatives the monomer Rh6(CO)15.(4VPy) is an octahedron cluster with 11-end and 4-μ-bridge carbonyl ligands, and 4-vinylpyridine is connected to the rhodium atom only through the nitrogen atom, and occupies the co-ordination position of the twelfth terminal carbonyl group. The average length of the rhodium-rhodium bond is 2.762 Å and is similar to that of other rhodium clusters.
The composition of the polymers and their molecular weights are controlled by the usual methods, such as the composition of the monomer mixture and the polymerisation conditions. It is important that in such PCNM mutual thermal stabilisation of the polymer and the cluster in its chain, is observed. For example, the temperature of dissociation of clusters in copolymers containing the Os3 complex and 4-vinylpyridine increases from 295 to 450°C, but the temperature for the thermal destruction of the polystyrene framework increases by 50 to 100°C (the cluster units are 0.5 to 1.0 mol per cent of the contents). The effect of the polymer chain is such that it can dissipate the diffusion of energy from the free rotary-oscillatory movement of the cluster into translation in the polymer segments, which therefore increases the thermal stability of the polymer.
It may be that such an approach will soon become wide spread and that new classes of polymer-immobilised noble metal clusters can be prepared with its help. This also applies to polymer-immobilised clusters of noble metals prepared by polycondensation of cluster-containing compounds, which is not as yet known.
PCNM in Catalysis
Although a role for polymer-immobilised clusters of noble metals in catalysis has only just begun to develop, some publications and reviews have appeared (3, 40). The most important applications of PCNM in catalysis, in our opinion, are summarised in Table II.
Such catalysts can be used as powders, beads, balls, thin membranes (of thickness up to 7 nm), swelling gels, and sometimes in solutions. Kinetic parameters have many features in common with catalysis using immobilised mononuclear complexes. For instance, the rate for ethylene hydrogenation using polymer-immobilised noble metal clusters is ten times higher than for cyclohexene hydrogenation, and the ratio between the rates for cyclohexene and benzene hydrogenation is 25 to 50. The rate of reaction decreases with the accumulation of phosphine ligands in the cluster. The M0 particles, where M is the noble metal, formed by reduction of the polymer-immobilised noble metal clusters, for example by hydrogen, differ in their sizes and activities in olefin hydrogenation and isomerisation. This isomerisation is promoted by hydrogen and occurs on the co-ordinated unsaturated active centres. The enlargement of the produced metal particles depends on both the method of preparing the catalysts and the conditions of catalysis.
By comparison with homogeneous systems the polymeric carrier substantially suppresses the integration of particles, and the small dimensions, 1 to 2 nm, of the particles produced do not allow them to act as metallic objects. There are probably multinuclear associations, and the dynamics of their formation is very complex. It is important that such processes will be accompanied by saving the co-ordination vacancies after the activation of the catalysts. In many cases the centres, arranged on borders of clusters and stabilised by their electronic systems, are responsible for the catalyst activity. Such co-operative interactions increase the stability and activity of the PCNM and allow them to carry out repeated reactivation and regeneration as well as preventing the precipitation of catalyst from solution, including that on the reactor walls.
The conditions for immobilisation of the clusters and subsequent processes determine the evolutionary transformations of the clusters from being fixed mononuclear complexes to cluster-type structures with a polymeric carrier as macroligand. However it is very difficult to find correlations between the catalytic activity of the PCNM and their nuclearity. We note only that the rate of the water-gas shift reaction increases for complexes immobilised on polymeric amines (23) as: Rh2(CO)4Cl2 < Rh6(CO)16 < Ru4(CO)12 < RhCl(PPh3)3. Additional possibilities for controlling the catalytic properties by changing the nuclearity of the immobilised complex could be created.
There is a series of processes which are catalysed by clusters, including immobilised clusters. For example, polynuclear clusters are the active elements of transport chains in enzyme photo systems (photo systems II of natural photosynthesis) and/or the catalytic centres responsible for redox transformations; natural polymer peptides are their carriers. Immobilised metalloclusters in heterogenised catalysts are used as catalysts for the water-gas shift reaction, see Table II; here the processes proceed under milder conditions.
The immobilisation of metalloclusters is very useful for investigating the mechanism of catalytic reactions, since in many cases it permits the isolation and thus identification of the intermediates. Immobilised Ru3O(OCCH3)6L3+ particles, for example, have been identified during the hydrogenation of cyclooctene by ruthenium complexes fixed on carboxylate matrixes (41). Some researchers (23) think that the two cluster anions Rh14(CO)254- and Rh14H(CO)253- are active forms in the water-gas shift reaction when catalysed by Rh2(CO)4Cl2 on aminated polystyrene.
There are numerous examples of the higher catalytic activity achieved when using polymer-immobilised bimetallic clusters than when using monometallic clusters; in particular the use of cobalt-rhodium and cobalt-ruthenium in hydroformylation reactions. The observed synergism is probably connected with a matrix effect due to the second metal, which isolates the rhodium or ruthenium atoms, respectively, as well as stabilises the intermediates and improves the introduction of carbonyl into the metal-alkyl bond. Other reasons can be linked to a decrease in the charge density on the atom in the active centre of the heterometallic clusters, for example cobalt-rhodium or iron-rhodium, as well as cobalt and iron acting as a ligand for rhodium since the formation of direct bonds rhodium-cobalt-oxygen and possibly rhodium-iron-oxygen prevents caking of the rhodium and of the catalyst (42).
Several examples of the occurrence of segregation of immobilised heterometallic clusters during the catalytic reaction are known. Thus, (μ-H)2RhOs3(CO)10(acac)PPh3 on (Ⓟ-PPh2 fractures during ethylene hydrogenation at 100 °C and butene isomerisation, to form a cluster with an osmium-osmium bond (which explains the isomerisation activity) and a mononuclear rhodium complex, which is transformed into rhodium particles of diameter 1 nm, which is active in the hydrogenation (43).
The examples listed above testify to the intensive development which is going on for PCNM in catalysis; including effective methods for immobilising homo- and heterometallic clusters of the noble metals and identifying their structure. Approaches based on the polymerisation and copolymerisation of cluster-containing monomers are just being developed. However, data on non-carbonyl polymer-immobilised noble metal clusters, such as halides and sulphides are not yet available. This is also true for data on the immobilisation of trimetallic-type clusters and giant clusters.
The quest for effective methods to construct such PCNM is one of the important directions in catalysis. Reviews have recently been published containing data on the immobilisation of noble metal clusters by natural polymers, for example Rh6 (44), Os3 and Ru3 (45) by biopolymers.
In ending it should be noted that the search for methods of improving the properties of polymers may be at the expense of immobilising the metalloclusters. However, increasing the stability of polystyrene, is a possible way to create metallopolymers of uniform structure as new types of construction materials.
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