The Build-Up of Bimetallic Transition Clusters By Paul R. Raithby Department of , University of‘ Cambridge, England

The synthesis and reaction chemistry of high nuclearity transition clusters is briejly reviewed, and new synthetic strategies leading to the “rational” synthesis of bimetallic clusters containing metal cores of over 1 nm in dimension are described. The solid state structures of a number of usmiuml , osmiumlgold and rutheniumlcopper bimetallic clusters are discussed with regard to the nature of their formation, und of their bonding and properties. Suggestions are made as to how the synthetic strategies can be adapted to prepare bimetallic clusters of industrially useful combinations of . Recent work showing that bimetallic nunuparticles prepared frum clusters are catalytically active when anchored inside mesoporous silica is also discussed.

Transition metal carbonyl chemistry lic properties as the nuclearity increases, has been an important and developing topic although different sizes of cluster exhibit dif- of research in for the ferent types of metal-like properties under last three decades (1). One of the main appeals different conditions (2). of clusters is that they lie at the interface between One of the main thrusts of cluster chemistry ‘‘conventional” organometallic chemistry and at Cambridge has been to prepare ever larger the chemistry of colloids and of the bulk metal. clusters and to investigate their Figure 1 illustrates the progression in particle physical and chemical properties. A range of size from a single through clusters, with clusters containing more than ten metal metal core sizes of around 1 nm; , has now been prepared and crystallographically with sizes up to 100 nm; leading into the col- characterised (3) and examples in which the loid regime; and then on to the bulk metal. metal atoms “condense” to form structures cor- Indeed, at what size (number of metal atoms) responding to the hexagonal, cubic and body- does a metal cluster stop behaving like an centred cubic packings found in bulk metal have organometallic complex, with bonding proper- been observed, as well as other clusters, such as ties that can be described in terms of discrete [Pt19(C0)22]4-(4), which exhibit five-fold sym- molecular orbitals, and take on metallic prop- metry packing. erties, where the bonding can be described in The diameters of the metal cores in the largest terms of band structure? There is no immedi- of these clusters, such as [Ni,,Pt,(CO)4aH,,]”~ ate answer to this question, but there is a clear (n = 5, 4) are of the order of 2 nm (5). Even progression towards the clusters taking on metal- larger clusters containing copper and selenium

*. .. - ;.;:. .. - .*. 4

Single Cluster Colloid Bulk metal metal Fig. 1 The progression in particle atom size from a single metal atom to Particle diameter roi -lo2i 403i )lo% the bulk metal

Platinum Metals Rev., 1998, 42, (4), 146-157 146 have been prepared, and the largest of these to have been crystallographically characterised is [CU~~~S~~~(PP~~)~~]in which the selenium atoms exhibit “ABA” stacking and the copper atoms occupy interstitial sites (6). There are also reports of transition metal clusters, for instance those containing Auss(7) and Rhss(8), Pt,09(9) and Pd,,, (10) units, and a series of palladium clusters containing up to 2000 metal atoms (I I), which have not yet been crystallographically characterised, but which must have dimensions of the order of 4 nm. Several research groups have proposed that clusters can act as good building blocks in nanoscale architecture and thus will find appli- cation in the fabrication of single electron devices (12). Small metal particles and other transition metal clusters have also been clearly shown to Fig. 2 The metal core structure of form densely packed monolayers on electron [Os,(CO),]*- showing the four triangular microscope grids when they are ligated by faces of the Osmtetrahedron organic surfactant (13). From the viewpoint of , metal clus- ters can be considered as fragments of a metal under catalytic conditions is rather more surface surrounded by a layer of “adsorbed” lig- challenging. However, for the majority of clus- and molecules. Even the largest clus- ter models, for example [Os,,(CO),,]’- (14), only ter carbonyl so far characterised, [Os,,(CO),]’~, “surface” atoms are present, and since the where the metal framework is approximately sub-layers of the bulk metal influence the chem- 0.9 x 0.9 x 0.9 nm in size, contains only surface istry of the surface atoms on a catalytic surface, atoms, with each osmium atom being bonded some aspects of the model are not valid, so the to at least one carbonyl , see Figure 2, “analogy” should be treated with caution. (14). Therefore, the simple answer to the question However, can clusters be regarded as good of whether clusters are good models for models of metal surfaces in heterogeneous cat- heterogeneous catalysts would be “no”. alytic reactions? The “cluster/surface” analogy was pointed out quite early in the development “Rational” Synthesis of High of cluster chemistry (1 5), and ever since then Nuclearity Mixed-Metal Clusters clusters have been used as models for catalytic With a view to preparing precursor materials systems (1 6). that could have applications in catalysis and Certainly, organic molecules bond to catalyt- nanoparticle technology, the “cl~~ter”group in ically active metal surfaces in the same way as Cambridge has been developing strategies for to metal clusters, and analysis of cluster systems synthesising high nuclearity mixed-metal clus- is easier because they can be subjected to the ters containing ten or more metal atoms in their full range of spectroscopic techniques, cluster cores. Even the smallest of these clus- such as IR and multinuclear NMR spectro- ters should have a core diameter in excess of 0.5 scopies, and, in the solid nm and have the advantage, unlike bimetallic state, single crystal X-ray , particles prepared by other routes, that the exact whereas analysing the bonding modes of ratio of the two metallic elements is known. co-ordinated molecules on a metal surface As much of the early cluster synthesis work

Platinum Metals Rev., 1998, 42, (4) 147 Fig. 3 The formation of the “spiked”-triangular cluster [Os,H,(CO),,] from the reaction of the I activated cluster [OS~(CO)~~(M~CN)~]with [OsHZ(CO)d] involved pyrolysis or thermolysis techniques and cluster with a neutral monometal complex. resulted in a range of products, all in low yields, The second route involves the ionic coupling fiom a single reaction, it has been necessary to reaction between a carbonyl cluster anion and develop synthetic strategies where one target a monometal cationic species, again to increase cluster can be obtained in good yield the cluster nuclearity by one. In Figure 4, the (17). tetranuclear osmium cluster [Os,H,(CO) ,,I is In order to achieve this, two fairly straight- initially reduced with WPh,CO to form the forward synthetic routes are available, given that dianion [Os,H,(CO) ,,I ‘-, and then treated the starting materials for the production of high immediately with the labile cation [M(q6- nuclearity clusters are usually low nuclearity C6H6)(MeCN),I2+(M = Ru, 0s) to form the carbonyl clusters. pentanuclear, neutral cluster [Os&W,(CO),,(q6- The first route, illustrated in Figure 3 by the C6H6)](1 9). By choosing appropriate cation formation of [OS~H~(CO)~~],involves the acti- and anion charges and ratios, the cluster nuclear- vation of the binary carbonyl [Os,(CO),,] with ity can be increased by two units, as in the Me,NO, in the presence of MeCN and results reaction of [Os,(CO),,]’~with two equivalents in the oxidation of carbonyl to carbon of [Ru(q5-CsH5)(MeCN),]’to give the pen- dioxide and the occupation of the vacant co- tanuclear cluster [Os3Ru,(CO),,(q’-C,H,),] (20). ordination sites with labile MeCN ligands. The This latter ionic coupling route has been par- subsequent addition of the neutral mononuclear ticularly successful, and has been extensively complex [OSH,(CO)~]displaces the MeCN exploited to prepare a wide range of higher groups and affords the “spiked”-triangular clus- nuclearity clusters containing carbocyclic ter [OS,H~(CO),~](18). In this method the ligand groups (21). cluster nuclearity is increased by one, by the The method has also been used by a number reaction of a neutral, activated, low nuclearity of research teams for synthesising mixed-metal

Insertion M = Ru, 05 NCMe (Oc)2 ..!.a \ [H,Os,CCO),,]

(C0)3 H ,,OS,,M(CO)~I(C~H~)

Fig. 4 The synthesis of [OsdMH,(CO),,(~6-C6H6)](M = Ru, 0s) from the coupling of [OS~H~(CO)~~]~~with [M(t16-C6H6)(MeCN),]2+ (M = Ru, 0s)

Platinum Metals Rev., 1998,42, (4) 148 has a core diameter in the surface plane of approximately 1.2 nm (12 A). %P This coupling process, more correctly called redox , resulting in cluster build- up, can also be achieved between the carbido- stabilised, decanuclear cluster anion [OsloC(CO)z4]z~and [Hg(O,SCF,),] to afford the anion [ { OsloC(CO)z4}zHg]2,Figure 6. Here the mercury atom links the two decanuclear clusters by bridging an edge of each Oslounit, forming a cluster containing 2 1 metal atoms (24). In this reaction, the appropriate choice of the mercury(I1) salt, [Hg(O,SCF,),], is the key b to the formation of the higher nuclearity mixed- Fig. 5 The structure of the “raft” cluster metal cluster. This is partly because the deca- [{Os,(CO)rlHg}s] showing the linking ofthree osmium dianion precursor has low nucleo- Os, triangles to the central Hg3 triangle philicity (the negative charge being delocalised over the ten metal centres) and because there is an increased propensity for the reaction to clusters containing the coinage metals by the follow alternative pathways involving partial reaction of carbonyl cluster anions with cationic degradation or rearrangement of the metal copper, silver and gold complexes (22).

Osmium-Mercury Clusters So far, our most extensive series of studies into the formation of mixed-metal clusters contain- ing ten or more metal atoms have also involved the reaction of a range of ruthenium and osmium cluster carbonyl anions with late transition metal cations, such as [Ha]’ (X = C1, CF,), [AuPR,]’ (R = alkyl, aryl) and [CU(NCM~)~]’.The first indication that cluster build-up could occur to produce higher nuclearity clusters came from the metathesis reaction of [OS,H(CO)~~]~with mercury(I1) salts. The product of the reaction was the extremely photolabile, dodecanuclear ‘‘rafl” cluster [ { Os,(CO) IHg},] shown in Figure 5 (23). It is significant that the three mercury atoms form a central triangle with Hg-Hg distances in the range 3.08-3.12 A. The “OS,(CO)~~”fragments bridge this central tri- angle, with each “Os(CO),” group forming bonds to two mercury atoms (0s-Hg in the range 2.71-2.76 A); the co-ordinated “Os(CO),” b groups each form one bond to a mercury atom L (0s-Hg in the range 2.98-3.05 A). This clus- Fig. 6 The molecular structure of the mercury ter can be viewed as a model for a bimetallic linked cluster dianion [{Os,,C(CO),,},Hg]*~ surface, and while being only one layer thick

Platinum Metals Rev., 1998,42, (4) 149 The Hg-Hg bonds in the Hg, dianion are an average 2.927 A long, but in the HgZdianion 0 0 0 [Os,,Hg2C,(CO)42]2~,which has also been crys- 4 PLP4 tallographically characterised and found to have a pair of mercury atoms linking the two Os, units, the Hg-Hg bond is significantly shorter at 2.745 A long.

Osmium-Gold Clusters A wide range of reactions which form high nuclearity osmium-gold clusters has been car- ried out (27), but this discussion will be restricted to two key syntheses which yield important information about the build-up processes in high nuclearity clusters. The first reaction is that between [OS,~C(CO),,]~~and the polygold cation [(AuPR,),O]+(PR, = PCy,, PPh,, PMeJ’h) which affords the fourteen atom cluster [OsloC(CO) ,,Au(AuPR,) ,] (28). The structure of the PCy, derivative has been char- acterised crystallographically and is shown in Fig. 7 The structure of the dianion Figure 8. The tetracapped octahedral geome- [MisHgX~(Co)u]’- try of the parent Os,, dianion is retained, and the four gold atoms form a tetrahedral cluster which is linked to the osmium core via one gold framework. Again, the metal core in this cluster atom that bridges an 0s-0s edge of one of the is somewhat asymmetric but has a maximum dimension of approximately 1.6 nm.

When a similar reaction occurs between Q I) [M,,C(CO),,]’~(M = Os, Ru) and the mercury \8P salt [Hg(O,CCF,),], a different type of product, [M18Hg3CZ(CO)IZ]2~,is obtained (24, 25), although the metal core still contains 21 metal atoms. This also contains a central Hg, trian- gle, Figure 7, linking two “M9C(CO)ZI”units, derived from the framework of the tetracapped octahedral starting materials [MI&( CO)zr] 2- by the loss of an “Os(CO),” vertex. As in [{OS,(CO),~H~},I it is of interest that the mer- cury atoms have linked together between two osmiumhthenium cores forming, in metal- lurgical terms, a mercury domain. Another fascinating feature of the cluster dianion [OS,,H~,C~(CO)~~]~-is that it undergoes reversible photochemical and redox extrusion of mercury atoms to give a complete series of Fig. 8 The molecular structure of high nuclearity clusters with general formula [OS~~C(CO),~U(A~PC~~):~] [Os,sHg,Cz(CO),2]”~(n = 1-3, m = 1-4) (26).

Platinum Metals Rev., 1998, 42, (4) 150 Fig. 9 (left) The molecular structure of the osmium-gold complex [Osla(CO),,(AuPPhzMe),] Fig. 10 (right) The structure of the [OS,~(CO)~,(AUPP~~M~),]complex when it is viewed from one end of the cluster. The novel tubular nature of the metal cnre composed of osmium atoms is clearly visible

tetrahedral caps. The mean Au-Au distance to indicate a bonding interaction, while the truns within the Au, tetrahedron is only 2.7 1 A, which axial osmium atoms [Os(2)... Os(3a) and suggests that the Au-Au bonds are relatively Os(3)... Os(2a)l have moved closer together than strong. As also observed for the osmium-mer- would be expected in an octahedron, to an aver- cury clusters, the gold atoms have a tendency age distance of 3.3 A. The pairs of gold atoms to “cluster” together to form a domain, and are separated by 4.43 A, and the length of the do not become incorporated into the osmium tube including the gold phosphine groups atom framework. exceeds 1 nm. In order to support this type of In the second key reaction the non-carbido geometry the metal bonding must be delocalised decaosmium cluster dianion [0~10(c0)26]’~is in character. further reduced with KPh,CO, presumably to give a tetra-anion, that is treated in situ with Ruthenium-Copper Clusters [AuPPh,Me]+ to give a new type of high Of all the cations discussed above, nuclearity cluster [Oslo(CO),,(AuPPh2Me),],[Cu(NCMe),]’ is the most versatile for use in see Figure 9 (29). cluster build-up reactions. It has been used in At first sight, from the structure of this four- combination with a number of ruthenium clus- teen atom cluster, it appears that the four ter anions to produce a range of novel, high [AuPPh,Me]+cations merely cap the four end nuclearity, mixed copper-ruthenium clusters. faces of a bioctahedral osmium core, but the For example, in dichloromethane, the view looking from one end of the cluster to the reaction of the octahedral ruthenium anion other, Figure 10, shows that the octahedra are [RU~(~~-H)(CO),~]~with an excess of distorted and that a novel tubular structure has [Cu(NCMe),]’ affords the dianionic cluster formed. The 0s-0s equatorial edges [Os(4)- [{Ru,H(C0)12)2Cu7C1,]2~,see Figure 11 (30). Os(5a), Os(1)-Os(la), Os(5)-0s(4a)] have The fifteen-atom cluster core contains two expanded to lie in the range 3.29-3.32 A, a dis- Ru, tetrahedra linked through a Cu7unit which tance that is significantly longer than is judged may be described as two fused square-based

Platinum Metals Rev., 1998, 42, (4) 151 Fig. 11 The structure of the [{RulH(CO)lr}rCu,Cld]2~dianion showing the central Cu: unit

-vCIO)

pyramids sharing a common triangular face. tion of the original Ru, octahedron has not Three chloro ligands each symmetrically bridge occurred, and the two octahedra are linked pairs of copper atoms, while the remaining cop- through two Cu, tetrahedra which share a com- per atom, Cu(5), forms no bonds with ligands mon edge, generating two butterfly arrange- but has eight metal contacts. Thus, the imme- ments which bond to the ruthenium units. diate environment around Cu(5) is similar to Overall, the eighteen-atom cluster can be viewed that in metallic copper and, overall, again it is as a linear condensation of four octahedra. Two seen that the element with the formal d'O elec- edges of the Cu, unit are symmetrically bridged tron configuration has formed the central by chloride . domain, and the transition metal units are fused For both reactions, the presence of chloride to its periphery. ions is apparently necessary, even if, in the first The product of this reaction is very sensitive case, they are abstracted from the solvent. to the nature of the solvent used. In the However, by simply altering the solvent, from presence of [(Ph,P),N]Cl in MeCN, when dichloromethane to [(Ph,P),N] C1 in MeCN, [Ru,H(CO),J is treated with a large excess good yields of high nuclearity clusters with ruthe- of [Cu(NCMe),]+, a different product, nium:copper ratios of 8:7 (approximately 1: 1) [{RU,H(CO),,),CU~C~,]~,is obtained in good and 2:1, respectively, are obtained from a room yield, see Figure 12 (30). In this case degrada- temperature reaction.

Fig. 12 The structure of the O(21I [ {Ru,H(CO)17}2C~CI,]2~dianion

Platinum Metals Rev., 1998, 42, (4) 152 Fig. 13 The core geometry in the [ {R~~OHZ(CO)~~}~CU,CI~]~- dianion

This synthetic methodology can be expanded nuclearity cluster, was obtained in quantitative further. The reaction of the decanuclear dian- yield, see Figure 14 (32). The two Ru, octahe- [RU,,H,(CO)~~]~~with excess [CU(NCM~)~]+,dra are linked by a rectangular planar arrange- in dichloromethane, in the presence of chloride ment of four copper atoms, opposite edges of ions, affords the twenty-six-atom cluster which are bridged by chloride ions. Thus, with [ {RuloH,(CO),,}2C~~ClZ]~~with a 70 per cent chloride ions present in the starting material the yield and a ruthenium:copper ratio of 10:3 (31). cluster build-up is not so efficient, resulting in Here, the two Rule units are fused on either side a sixteen-atom cluster with a ruthenium:copper of a Cu, unit which adopts the same geometry ratio of 3: 1, but the yields of the product are as that found in [ { Ru,H(CO) !,} ,CU,CI,]~- improved. (Figure 12). The overall core geometry can be In all of the copper-ruthenium clusters inves- described as six fused octahedra with an addi- tigated, the copper atoms condense to form a tional ruthenium atom at each end capping a central domain and the ruthenium cluster units butterfly face to form a trigonal bipyramid, condense around the periphery to produce nano- Figure 13; this metal framework is over 1.6 sized particles based on the fusion of octahedral nm long. or tetrahedral units, just as is observed in other In order to investigate the role of the high nuclearity clusters and in close packed met- chloride ions in these reactions, the carbido als (3). The advantage of the strategy employed dianion [RU~C(CO)~~]*-was treated with in the synthesis of these copper-ruthenium CuCl, instead of the [Cu(NCMe)J+cation, clusters is that by carefully controlling the reac- and [ {Ru,C(CO),,)2Cu,Cl,]2~,another high tion conditions, nanosized particles with

Fig. 14 The structure of the dianion [{hC( C0)w) iC~,Clz]*

Platinum Metals Rev., 1998, 42, (4) 153 the species [OS,,H~,C~(CO)~,]”’(n = 1-3, m = 1-4) (26) have been characterised, and it is not possible to assign realistic oxidation states for the individual mercury atoms in these species. For example, in [OS~,H~,C~(CO),,]~-,in order to balance the formal -4 charge from each “OS,C(CO)~,”fragment, each mercury atom would have to be assigned an unrealistic oxi- dation state of +3. The network of reversible redox transformations shown in Figure 15 con- firms that this series of clusters can act as “elec- tron sinks” (26) with a description of the bond- ing within the metal core best described terms Fig. 15 The network of reversible redox in transformations of [Os,,Hg,,C,(CO),,]”’- of delocalised molecular orbitals. (11 = 1-3, m = M),Fr = ferroeene Clusters as Nanoparticle Precursors For the chemistry outlined above it is clear particular ruthenium:copper ratios can be that it is now possible to prepare a wide vari- obtained in high yields. This control has not ety of bimetallic, nanosized clusters in good yield been evident in previous studies (3). with specific, known ratios of the two metals. In principle, this strategy can be applied with- Some Electrochemical out difficulty to other combinations of transi- Considerations tion metal atoms, such as palladium-rhodium If higher nuclearity bimetallic clusters become and platinum-rhodium, and high nuclearity “metallic” in character they would be expected clusters could be prepared. However, would to undergo extensive redox chemistry, with the these systems have real uses with industrial appli- cluster cores behaving as “electron sinks”. For cations? all of the copper-ruthenium clusters described It has long been established that metal clus- above it is possible to assign a formal oxidation ters can be anchored to oxide surfaces such as state of + 1 to each of the copper atoms in the silica and alumina, and even encapsulated in structures, despite the view that the bonding zeolite cages; after heating the residual metal must be delocalised over the metal core. particles have high catalytic activity (33). It is Similarly, for the tubular osmium-gold clus- also known that bimetallic catalysts often exhibit ter [Osin(CO)?,(AuPPh,Me),](Figure 9) each superior operating stability to monometal sys- gold atom can be assigned an tems, and experiments have shown that rhenium- of + 1 (29). Cyclic voltammetry studies show platinum clusters, supported on alumina, are that this cluster undergoes two reversible one- effective catalysts for naphtha reforming (34). electron reductions, indicating that there is However in these experiments the exact nature no major change in core geometry with the of the cluster precursors and the catalytically uptake of two electrons (27). This is consistent active materials were not known. with the cluster being able to act as an “elec- Recently, Shapley and co-workers have pre- tron sink” since it is able to reversibly take up pared a set of supported bimetallic catalysts from and release a pair of electrons repeatedly, with- two structural isomers of [RejIrC(C0)2,]’ by out it degrading, or without any major struc- deposition onto high surface area alumina and tural change that would cause the redox process activation in dihydrogen at 773 K (35). The spe- to become irreversible. cific activities of the catalysts depend on both The situation for the osmium-mercury sys- the metal framework structure and the coun- tems is more complicated. As noted earlier, all terion present in the precursor (either [NEt,]’

Platinum Metals Rev., 1998, 42, (4) 154 or [N(PPh,)2]+).Interpretation of EXAFS data described above, had been successfully anchored has enabled specific models to be developed for inside mesoporous silica (37). Activation and the catalyst particle nanostructures which cor- anchoring of the adsorbed cluster on the MCM- relate with their catalytic activities. The more 41 silica support was achieved by heating the active catalysts are modelled by a hemisphere sample under dynamic vacuum. EXAFS spec- of close packed metal atoms, with an average troscopy confirmed the presence of a bimetal- diameter of 1 nm, with iridium at the core. In lic particle anchored to the silica atoms a series of related studies, Shapley has also shown through the silver atoms of the cluster. High- that [PtRu,C(CO),,] can be used as a neutral resolution electron microscopy of the heat- cluster precursor for the formation of carbon- treated material shows a uniform distribution supported platinum-ruthenium nanoparticles of the bimetallic nanoparticles aligned along the with exceptionally narrow size and composition zeolite channels. The catalytic performance of distributions (36). The bimetallic particles are the activated, supported bimetallic particles obtained by reduction of the carbido cluster with was tested for hydrogenation of hex-1-ene to . A detailed structural model of the hexane. Initial experiments showed a high selec- nanoparticles was deduced on the basis of in situ tivity (in excess of 99 per cent) and a turnover EXAFS, scanning transmission electron frequency of at least 6300 mol hexane per mol microscopy, microprobe energy-dispersive X- [Ag,RuIo]per hour. ray analysis and electron microdifiaction stud- The success of these studies led to the ies. These experiments show that the nanopar- incorporation of the copper-ruthenium cluster ticles have a Ru:Pt ratio of 5:1, an average anion, [ {RU,C(CO),,}~CU~CI.']~~,described pre- diameter of approximately 1.5 nm and adopt viously (Figure 14) (32), into the mesoporous a face centred cubic close packed structure. This channels of silica (38). Gentle thermolysis of is in contrast to the stable phase of the bulk alloy the anchored clusters gives the bimetallic which is hexagonal close packed. The EXAFS nanoparticles, characterised by X-ray absorp- studies also show that there is a non-statistical tion and FT-IR , and high-reso- distribution of different metal atoms in the lution scanning transmission electron micro- nanoparticles: the platinum atoms exhibit pref- scopy. The copper and ruthenium K-edge X-ray erential migration to the surface of the particles absorption spectra show that these catalytically under an atmosphere of dihydrogen. active particles have diameters of approximately Of particular interest is a recent report by 1.5 nm and display a rosette-shaped structure Johnson, Thomas and colleagues that a clus- with 12 exposed ruthenium atoms that are con- ter anion [A~,Ru,~C~(CO),,C~]~-(Figure 16) nected to a square base composed of relatively with a structure in which a central Ag, triangle concealed copper atoms. In turn, these are links two Ru, square based pyramids, closely anchored by four oxygen bridges to four silicon related to the copper-ruthenium systems atoms of the mesopore. The nanoparticles are

Fig. 16 The structure of the metal core of dianion: [A~J~&(CO)UC~]*-

Platinum Metals Rev., 1998, 42, (4) 155 active catalysts for the hydrogenation of hex- ity of the osmiudmercury, rutheniudmercury, 1-ene, diphenylacetylene, phenylacetylene, stil- osmiumigold and copperiruthenium systems bene, cis-cyclooctene and D-limonene, with investigated, the mercury, gold or copper atoms turnover frequencies of 22400, 17610,70, 150 form a central domain and the osmium or ruthe- and 360, respectively, at 373 K and 65 bar of nium cluster units are fused onto the periphery dihydrogen. The catalysts showed no tendency of these central units. In no case did the two to sinter, aggregate of fragment into their metallic components become dispersed through- component metals during these experiments. out the metal core. Lastly, evidence is begin- ning to emerge that nanoparticles derived from Conclusions these and related clusters may prove to be active In this review it has been shown that synthetic catalysts when anchored on silica or alumina strategies to prepare high nuclearity, bimetallic supports. clusters in good yields have been developed. The metal cores of these clusters have dimen- Acknowledgements sions in excess of 1 nm. By careful control of My grateful thanks go to Professor the Lord Lewis reaction conditions it is possible to obtain spe- and Professor Brian F. G. Johnson for their support and encouragement over the years, and for initiating cific target molecules with known ratios of the the research described in this review. I am also indebted two metallic components, and the methodol- to the many research workers in the Department of ogy may be extended further to encompass the Chemistry, at Cambridge, who have carried out the synthetic and structural work described, and to majority of the late transition elements. In the Johnson Matthey for the generous loan of the heavy “condensed” clusters obtained, for the major- transition metal salts.

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Construction of Miniature I1Organo-Rhodium Boxes Many types of molecular cages exist in which C5H5)were used to prepare a series of molec- ions, atoms or molecules can be trapped. These ular “squares”, by reaction with [Cp’RhCl,] or cages are usually held in suspension and are typ- [(cymene)RuCl,] (cymene = 4-isopropyl- ically constructed from bifunctional ligands, with toluene). To assemble the box from the square planar or tetrahedral metal centres at the “squares” the chloride ligands were removed by vertices. Until now there have been no cubic AgPF,. The “molecular boxes” of most inter- shaped organometallic cages. However, if octa- est have the structure [(C5R5)8M8(p-CN),z](M hedral transition metal building blocks could = Rh or Co) and are a subunit of hexa- be constructed, then the assembly of cubic- cyanometalates, of which Prussian blue is one shaped structures should be possible. example. Now, researchers fi-om the University of Illinois The most interestingbox has alternate rhodium have succeeded in constructing a molecular box and cobalt atoms at the vertices, linked by CN from a cubic array of cyano-linked rhodium and groups. Each metal atom can adopt its preferred cobalt octahedra (K. K. Klausmeyer, T. B. octahedral position. The box has edges 5.1 A Rauchfuss and S. R. Wilson, Angew. Chem. long with a volume of - 132 A’, giving enough Znt. Ed., 1998, 37, (12), 1694-1696). space inside to encapsulate a caesium atom. The Tricyanometalates Et4N[Cp’Rh(CN),] and box is also soluble so it could therefore be used K[CpCo(CN),] (where Cp‘ = C5Me5,Cp = for trapping molecules in solution.

Platinum Metals Rev., 1998, 42, (4) 157