Catalysis very mixed-metal clusters

Research into cluster catalysis has been driven by both intrinsic interest and utilitarian potential. Catalysis involving "very mixed -metal clusters is of particular interest as many established heterogeneously catalyzed processes couple mid and late transition metals (e.g., hydrodesulfurization and petroleum reforming). Attempts to model catalytic transformations arc summarized in Section II.F.I., while the use of "very mixed -metal clusters as homogeneous and heterogeneous catalysis precursors are discussed in Sections I1.F.2. and I1.F.3., respectively. The general area of mixed-metal cluster catalysis has been summarized in excellent reviews by Braunstein and Rose while the tabulated results are intended to be comprehensive in scope, the discussion below focuses on the more recent results. 

Homogeneous catalysis by transition metal clusters has been reviewed from the perspective of the specific transformations.Examples of very mixed-metal clusters catalyzing processes homogeneously are collected in Table IX. As is generally the case with homogeneous catalysis, the catalytic precursor is well defined, but the nature of the active catalyst is unclear. 

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. 

A review of very-mixed -metal carbonyl clusters, covering complexes containing transition metals separated by at least three d-block groups has been published, and Adams has briefly reviewed the reactivity of segregated bimetallic clusters and their role in catalysis. ... 

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