The term cluster has had a multitude of meanings and was once considered a description of a complex containing at least three metal atoms bound together by M-M interactions. As of late it has now been considered true to name any aggregate a 'cluster'.1 The carbonyl clusters of osmium are of huge importance to metal cluster chemistry largely due to its ability to form clusters with different nuclearities, ranging from one to eight metal atoms.2, 3 This is unusual in that the highest nuclearity of neutral binary carbonyl clusters that are well characterized is three (Fe and Co).5 The chemistry of tetra nuclear clusters has had a seemingly slow start in comparison to those of higher nuclearity with the first having not been reported until 1987.3
The main interest in the tetranuclear clusters of osmium has stemmed from their stability, the easy introduction of new ligands without disruption of the metal core and the ease of analysis with X rays since suitable crystals may be grown without much difficulty.5 Rather than forming chains, the molecule agglomerates hence maximising the M-M bonds. Carbonyl ligands in particular are capable of stabilising cluster compounds encouraging the favourable overlap of metal atomic orbitals of Osmium, giving birth to a vast array of reactive properties.1 The structure, Reactivity and importance in catalysis of H4Os4(CO)12 will be discussed her along with its derivatives.
1.1 Structure and bonding
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The structure of a transition metal cluster is not always obvious from inspection of the molecular formula and must be broken down into its constituent parts to examine the polyhedral metal core. There are however disadvantages to this approach as assumptions must be made of the M-M, M-L and M-L interactions, either that they have no effect on each other or that they are sufficiently different in magnitude that there is no apparent effect on one another.6 However, the metal geometry of transition metal clusters is easily and reliably predicted by electron counting and the cluster structure rationalized by the Effective Atomic Number rule (EAN).7
EAN count = 18x - 2y
Where x is the nuclearity of the cluster and y the number of metal-metal bonds. Osmium forms clusters with Ï€ -acceptor ligands such as Carbonyls which have the ability to withdraw electron density from the cluster thus depopulating the skeletal molecular orbitals, the metal itself is usually present in a low oxidation state and all the bonding interactions are kinetically inert to bond dissociation for the prevention of colloid formation or defragmentation over the formation of a cluster.CATALYSIS REF. For clusters with more than five metals the polyhedral skeletal electron pair theory (PSEPT) is utilized instead.
REFClusters obeying the EAN rule are known as "electron precise" as the metal-metal interactions have two-center, 2 electron bonds. Tetra nuclear clusters of osmium typically have cluster valence electron counts of 60 electrons and a tetrahedral metal structure.
The x-ray structure of H4Os4(CO)12 as determined by Johnson et. Al depicts four osmium atoms within a distorted tetrahedron, with four long (0.296nm) and two short (0.282 nm) metal-metal bonds with a near D2h symmetry whereby the longer metal bonds are presumably bridged by hydrogen.8
There are predominantly two types of method used to synthesise tetra osmium clusters: from lower nuclearity clusters and from clusters with a higher nuclearity. However the most widely used is that of the lightly stabilized derivatives of Os3(CO)12.
1.2.1 Synthesis from lower nuclearity species.
The tetranuclear hydrido cluster of osmium has been synthesized from the hydrogenation of Os3(CO)12 in high yields9:
4[Os3(CO)12] + 6H2 3[Os4(Âµ-H)4(CO)12] + 12CO
Synthesis in this manner, by the hydrogenation of the parent carbonyls, where Os3(CO)12 is physisorbed on hydroxylated silica also gives the tetra nuclear cluster in high yield, the surface mediated approach provides a more controlled and more reactive route to the synthesis of organometallic compounds. 10
Os3(CO)12 was dissolved in degassed Dichloromethane and mixed with silica under an inert atmosphere, after removal of the solvent and subjection to a steady flow of H2 gas (700 Torr) a significant sublimation occurred at a temperature of 150Â°C, with IR analysis it was shown that a mixture of the tri and tetra osmium compounds were present. A five day reaction at a lower temperature of 100Â°C yielded H4Os4(CO)12 in its entirety. Extraction of the product gives relatively high yields of 68%.
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4A solution of Os3(CO)12 in octane at 120Â°C under an atmospheric pressure of H2 yields H4Os4(CO)12 at a lower 29%. The use of water as the proton source has also been described by the pyrolysis of Os3(CO)12 with water in a Carius tube at 230Â°C, here however a mixture of hydrido carbonyls are formed and the desired product must be recovered lowering the yield further to a mere 15%.10
The silica surface mediated method provides substantial advantages over the other synthesis such as milder reaction conditions, simple product recovery (removal of solvent) and a more direct synthesis route that leads to higher yields and selectivity. Mechanistically there isn't a definite route available yet the theory whereby hydroxyl groups on the surface on the support play a significant role in H4Os4(CO)12 formation has not been rejected.10
1.2.1 Synthesis from higher nuclearity species.
The formation of tetra osmium hydrido clusters is also possible from penta and even hexa osmium clusters by the addition of various reactant species such as carbon monoxide and molecular hydrogen.
1.4 Derivatives: heteronuclear transition metal carbonyl complexes.
It has been stated that metal clusters are somewhat of an intermediate between coordination complexes and bulk metal surfaces or particles. Since the characterisation and mechanistic studies of metal surfaces is difficult the cluster-surface analogy has lead to the prospect of reactions occurring on metal surfaces being explained more thoroughly. Johnson et al has reported that mixed metal clusters with high nuclearities may act as precursors for bimetallic catalysts. A
With this in mind, suggestions have been made that the clusters themselves may act as catalysts and furthermore be able to catalyse reactions that mononuclear species cannot. This exciting prospect leads way for new catalysis and organometallic synthesis. CAT REF + METAL CLUSTERS IN CHEM
It has been found that the structure and reactivity of a cluster is largely effected by the coordination of the ligand to multiple metal centres allowing organic transformations to take place on the ligand which would have been impossible with mononuclear complexes. TM carbonyl clusters have been noted as homogenous catalysts for reactions such as hydroformylation, hydrogenation, C-C and C-H bond activation and the gas-water shift reaction. B
Homonuclear clusters have already been well defined mapping the way for investigation into clusters containing metal - metal bonds between different metals. Mixed metal clusters provide the possibility of new and interesting reactivities and the prospect of enhanced catalytic activity 5, the reasons for which may be attributed to the following:
The selectivity of substrate-cluster interactions may be directed by the polarity of the heterometallic bonds, providing possible multifunctional activation. The adjacent metal centres may react cooperatively.
The cores of these metal clusters resemble a micro alloy and may be used as precursors to heterogenous catalysts.
Molecular clusters as models of metallic catalysts Journal of Molecular Catalysis
Volume 86, Issues 1-3, 3 January 1994, Pages 51-69
Transition Metal Clusters in Homogeneous CatalysisSuss-Fink, G.; Meister, G. Advs. Organomet. Chem. 1993, 35, 41.
1 R. Crabtree, The Organometallic Chemistry of the Transition Metals. 4th edition ed., Editor, John Wiley & Sons, 2005.
2 J. Lewis, J. R. Moss, Journal of Organometallic Chemistry 1993, 444, C51-C52.
3 R. K. Pomeroy, Journal of Organometallic Chemistry 1990, 383, 387-411.
4 B. F. G. Johnson, J. Lewis, Philosophical Transactions of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences 1982, 308, 5-15.
5 R. K. Pomeroy, in Comprehensive Organometallic Chemistry II: A review of literature 1982-1994, ed. by D. F. Shriver, Bruce, M.I., Elsevier Science Ltd, 1995, Vol. 7 Chap. 15, pp. 835-906.
6 B. F. G. Johnson, A. Rodger, Inorganica Chimica Acta 1988, 145, 71-75.
7 T. Eguchi, B. T. Heaton, Journal of the Chemical Society-Dalton Transactions 1999, 3523-3530.
8 B. F. G. Johnson, J. Lewis, P. R. Raithby, C. Zuccaro, Acta Crystallographica Section B-Structural Science 1981, 37, 1728-1731.
9 C. Zuccaro, G. Pamploni, F. Calderazzo, Inorganic Syntheses 1989, 26, 293-295.
10 C. Dossi, R. Psaro, D. Roberto, R. Ugo, G. Zanderighi, Inorganic Chemistry 1990, 29, 4368-4373.