Homogeneous Hydrogenation by Defined Metal Clusters

Abstract

The catalytic potential of well-defined transition-metal clusters in homogeneous reactions has attracted a great deal of attention over the years, as they represent a natural bridge between mononuclear complexes, metal nanoparticles, and metal-oxide, -sulfide and related surfaces used in heterogeneous catalysis. The molecular nature of metal clusters, together with their solubility properties, provides the advantages of classical mononuclear homogeneous catalysts (high activity, high selectivity, moderate operating conditions, possibility of catalyst design and modification), while the polynuclear framework can offer the possibility of multi-metallic cooperative effects often identified as a key element in the desirable properties of solid heterogeneous catalysts. Therefore, metal clusters can be expected, in principle, to combine the positive aspects of homogeneous and heterogeneous catalytic reactions and, perhaps more importantly, they may react through unique pathways associated with the cluster structures and thereby catalyze reactions not accessible by mononuclear or heterogeneous catalysts. Nevertheless, despite the impressive amount of work that has been devoted to develop these concepts over several decades, the great expectations first advanced during the mid-1970s have not yet been fully accomplished [1][2][3][4][5][6].From a different perspective, well-defined metal clusters have served as useful models for discerning the complex mechanisms of heterogeneous catalytic systems. The structural trends for metal clusters are now well understood, and their reactions in solution can be studied in detail by relatively simple chemical and spectroscopic methods, thereby producing important information at the molecular level -something that is very difficult to achieve on solid catalysts. The knowledge thus gained from studying metal cluster chemistry can be extrapolated, with adequate caution, to heterogeneous reactions [1][2][3][4][5][6].Another trend that has received considerable recent attention is the decomposition of metal clusters under controlled conditions on solid supports or on liquid suspensions, which generates small metallic particles of specific size, struc- 199The Handbook of Homogeneous Hydrogenation. Edited by J. G. de Vries and C. J. Elsevier