Synthesis of cluster compounds

In initial studies on transition metal cluster chemistry synthetic work was frequently based on chance discoveries. However the explosive development of this branch of chemistry in the last fifteen years has also produced an accelerated evolution of cluster synthetical methods. On going from casual synthesis to tailored cluster synthesis, the systematic preparation of a great variety of homo and heteronuclear cluster compounds is already possible. 

Fundamental consideration of the type of condensation reactions involved in cluster formation shows that the methods most frequently used for metal cluster synthesis may be classified in the following three groups  

Condensation reactions induced by the presence of coordinately unsaturated species, 

Condensation reactions between coordinately saturated species with different oxidation states, and 

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Dinuclear thiolato bridged complexes play a key role in the synthesis of cluster compounds and in biological... 

The synthesis of cluster compounds and sub-valent species in molten salt solution was mainly developed by Corbett and his co-workers. In this section and... 

F.18 Synthesis of cluster compounds by photolysis of azido complexes [heterometallic Au clusters]... 

Synthesis of Cluster Compounds 129 Table 2.23. Cluster synthesis by condensation of metallates with neutral metallic species... 

Table 9.9. Phase transfer synthesis of cluster compounds [49]...

In the process of clustering compounds to try to form series, and examining all of the data for the compound in a cluster, it may become apparent that there is an insufficient number of compounds, or enough range in the data to reach a decision as to whether a cluster of compounds is a series with demonstrated SAR. This problem can be addressed through searching of databases for additional related compounds or limited synthesis to provide the critical compounds. 

In an ever increasing number of compounds tolylidyne ligands triply-bridge a triangle of metal atoms, one of which is tungsten and the other two may or may not involve the same element. Thus two types of core structure are possible fM2W(y3-CR)l (Class A) or [M1M2W(y3-CR)] (Class B). Two methods are available for the synthesis of clusters of Class A. viz... 

The single-bond compounds between Si, Ge, and Sn and chalcogens (S, Se, Te) are reactive and used as synthetic intermediates in the synthesis of organic compounds, organochalcogen compounds, organo-silicon, -germanium and -tin compounds, and transition metal-chalcogen complexes and clusters. 

Ni and Co or of oxophilic metals, for example. Re, is still poorly studied the surface-mediated synthesis of bimetallic carbonyl clusters is limited to a few examples the surface-mediated synthesis of metal compounds without carbonyl ligands has just begun with the silica-mediated synthesis of [RhH2(PMe3)4] by treatment of bis (allyl) rhodium with PMe3 followed by H2 [121] the silica-mediated synthesis of tantalum clusters has been investigated recently but the products were not extracted from the surface-for example, treatment of silica physisorbed Ta(CH2Ph)5 in H2 at 523 K for 20 h led to tri-tantalum clusters, as shown by EXAFS spectroscopy [122]. 

The [Ms(/i3-E)(/t2-E)8Lg] (I), [ (/ - )(/ 2- 2) 8] (II), and [M3E5Lg] (III) types of cluster compounds (Fig. 29) are known, and because of the importance of triangular cluster units as building blocks of higher clusters, much effort has been put into the synthesis of such clusters. 

Cluster complexes can also be used as starting compounds for the synthesis of heterocarbonyl compounds via ligand replacement. Treatment of [Os3(H)2(CO)9(NMe3)] with S = CPh2 afforded two different osmium clusters depending on the reaction conditions. Each contained a 77 -bound thiobenzophenone ligand.117... 

It has been said that noticeably few metal clusters have been prepared by designed or rational synthetic procedures (52, 55). Indeed, most clusters have been prepared by placing together a variety of reagents, allowing them to react, and then examining the reaction mixtures to find out what compounds have been prepared. This is particularly true of mixed-metal clusters, and a real need exists for the development of synthetic procedures that can be used for the designed synthesis of particular compounds. 

The [Bai4CaN6] cluster can be considered as a Ca-centered Bas cube with each face capped by a Ba atom, and the six N atoms are each located inside a Bas tetragonal pyramid, as shown in Fig. 12.6.6. The stmcture can be viewed alternatively as a cluster composed of the fusion of six N-centered BasCa octahedra sharing a common Ca vertex. In the synthesis of this type of cluster compounds, variation of the atomic ratio of the Na/K binary alloy system leads to a series of compounds of variable stoichiometry Bai4CaN6]Na,v withx = 7,8,14,17,21... 

Our group has used a combined metathesis-PKR for the synthesis of tricyclic compounds in one step. The process starts from pure cobalt complexed dienynes 52. The cobalt cluster acts first as a protecting group to avoid undesired enyne metathesis processes. The methodology allows the formation of tricyclic [6.5.5] (53) and [7.5.5] (54) structures including, in some examples, oxygen or nitrogen. Tricycles 53 are obtained in a total stereoselective manner, while compounds 54 are formed as mixtures of two diastereomers (Scheme 17) [110]. 

Considering the fact that the number of polyaromatic compounds known is large, this result should be no surprise. Of course neither the mno rule nor the cluster-fusion rule by itself gives any insight on the synthesis of the compounds. But they do suggest many such fused systems should be possible to make and that is added justification for seeking their syntheses. 

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