Clusters, metal heterobimetallic

Heterobimetallic clusters (Figure 58,125 and 126) with solvent-dependent structures were also obtained upon mixing alkali metal tert-butoxides and -trimethylsiloxides in THF, TMEDA, and toluene.184 The common occurrence of heterocubes shows that there is a strong driving force for the formation of heterocubic structures in organozinc alkoxides. Solvent effects are important, however, as demonstrated by the formation of seeo-diheterocubic compounds in TMEDA. 

This reaction was first reported in the early 1970s as an unexpected result in search for the synthesis of new organometallic cobalt complexes [13-18]. Dicobalt octacarbonyl was the only cluster used in its beginnings, although now, many cobalt species and other metal complexes are able to mediate or catalyze this reaction. Thus, the stoichiometric reaction has been performed with Zr, Ni, Fe, Ti, W and Mo derivatives. In addition, heterobimetallic Co - W and Co - Mo complexes (1), are suitable precursors for the PKR and impart a high degree of selectivity in the process giving exclusively endo adducts 2 (Scheme 2) [19,20]. 

Thiolato and S-based ligands have often been also used to bridge two metals with the formation of heterobimetallic complexes. The synthesis and reactivity of cubane-type sulfido clusters containing titanium-late transition metals have been reviewed.1776,1777... 

Curtis has provided some homogeneous work that represents without doubt the closest model available for real Co-Mo-S catalysts. Heterobimetallic clusters, which contain the right combination of metals (Co-Mo) linked through sulfur bridges, were used, thereby beautifully mimicking a Co-Mo-S site [68, 69]. Even though this complex could be considered as just another example of a heterobimetallic cluster, its unique relation to real HDS catalysts seemed to justify its description in some detail in a separate section of its own. 

Interest continues in the synthesis of heterobimetallic carbonyl clusters (see Polynuclear Organowetallic Cluster Complexes and most are prepared by the addition of carbonyl metallates to small- or medium-sized carbonyl clusters. For example, addition of four equivalents of [HFe(CO)4] to [Rh(/x-Cl)(CO)2]2 forms [Fe2Rh(CO)io], which is converted to (99) (L = PPhs) by addition of the phosphine. Similarly, two equivalents of [Rh(CO)4] added to Fe2(C0)g followed by refluxing in acetone forms [Fe2Rh2(CO)io] , which on reaction with [Au(PPhs)]+ gives (100) (L = PPhg). Since [Fe2Rh(CO)io] is a 46-electron cluster, it is coordinatively unsaturated which is reflected by its reactivity (equation 44). Combination of 2 mols of... 

Some interesting clusters are known which combine the features of coinage metal clusters and metal carbonyls. Such heterobimetallic clusters can often be obtained from various reactions of Cu(I) or Ag(I) compounds with the Fe(CO)4 dianion. Thus, a series of clusters is known (Figure 14) having structures constructed from macro-polygonal or macropolyhedral networks with Fe-M -Fe edges (M = Cu, Ag) and typically Fe(CO)4 vertices such as the triangular cluster " Cu3(Fe(CO)4)3 , the but-... 

In summary, we have synthesized various heterobimetallic and heterotrimetallic clusters. The different structural motifs of these clusters are shown in Fig. 7 for general comparison (the ligand on the main group metals M is exclusively Me2Si(NtBu)2). 

This section is concerned with transition metal clusters that, in addition to metal atoms, contain sulfur atoms in the cluster core (rather than just in the peripheral ligands). Metal-metal bonds often supplement sulfur-atom bridges in stabilizing the structures encountered. Generally, such clusters are likely to resemble the HDS active phases of heterogeneous metal-sulfide catalysts to some extent, e.g. in terms of coordination sphere and metallic oxidation states. Because of the large number of molecular metal-sulfide clusters now known, we shall focus on homometallic clusters of Mo(W) and heterobimetallic clusters of Mo(W)-Co(Ni) (next two sections), i.e. molecular clusters containing the elements that are relevant for industrial... 

Klaus M, Stefan B, Robert S, Matthias D. Facile synthesis and structural variation of novel heterobimetallic alkali metal-zinc-alkoxide and-siloxide clusters. J Chem Soc Dalton Trans 2003 3365-9. 

The report of homobimetallic Mg-Mg bonded complexes undoubtedly represents a significant landmark in the development of group 2 chemistry [107], However, the first heterobimetallic group 2 metal-metal bonded complex predates this by more than 30 years [139]. Despite this, they are also few in number compared to body of TM-Metal bonded complexes and their chemistry remains underexplored. While cluster and zintl-phase complexes are known that potentially contain such interactions [140-145], they are not electron precise and will not be considered any further in this section. 

This review is intended to highlight any new reaction types that have been discovered, novel structural features that have been encountered, and chemical patterns and principles that have been observed for this class of compounds. Due to space limitations, only very selective examples of reactions are presented. Heterobimetallic species have been prepared and characterized for most combinations of iron and other transition metals. Their potential in the cooperative activation of small molecules, such as CO and unsaturated hydrocarbons, is an important research focus for this class of compounds. For mixed-metal clusters, enormous efforts have been made to develop rational cluster build-up methods and their potential applications in catalysis. Some important reviews that are pertinent to this type of species have appeared in the literature. 

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