PHOSPHINE-STABILIZED, PLATINUM-GOLD AND PALLADIUM-GOLD CLUSTER COMPOUNDS AND APPLICATIONS IN CATALYSIS

This is a comprehensive review on the chemistry and spectroscopic prop erties of phosphine-stabilized, M-centered Au cluster compounds where M=platinum or palladium. The nuclearity of the clusters ranges from 3 (e.g. [Pt(NO3)(PPh(3))(2)(AuPPh(3))(2)](+)) to 25 (e.g. Pt2Ag13(AuPPh( 3))(10)Cl-7). The latter consists of two Pt-centered icosahedral units sharing a common vertex, Many of the clusters contain additional meta ls such as Ag, Cu, or Hg in their periphery, Preparative methods are r eviewed and organized by reaction type. Emphasis has been placed on re actions and transformations of preformed clusters so that the reader w ill acquire a useful knowledge of the reaction chemistry, Structural p roperties of the clusters are also described, The reactivity and struc tural properties of the clusters are rationalized in terms of an elect ron counting model and this is used throughout the review, Spectroscop ic properties (e.g. nuclear magnetic resonance (NMR), fast atom bombar dment mass, UV-visible, and X-ray photoelectron spectroscopy) of the c lusters are described primarily for characterization; however, general trends are pointed out. Several applications in catalysis are also re viewed. Many of the clusters are good catalysts for Hz activation and can serve as potential models for bimetallic surfaces, Kinetic data fo r the H-2-D-2 equilibration reaction (H-2+D-2 reversible arrow 2HD) ar e given and discussed in terms of cluster composition and structure, T he results provide some insight into the well-known synergism observed for Pt-Au and Pd-Au heterogeneous catalysts. The mechanism of catalyt ic H-2-D-2 equilibration under homogeneous conditions is also discusse d. The cluster [Pt(AuPPh(3))(8)](2+) is an excellent catalyst for D-2( g)-H2O(1) isotope exchange in pyridine solution. This interesting and important reaction is described and kinetic results are related to oth er isotope exchange systems, A tentative mechanism is proposed based o n NMR analysis of the reaction mixture. Finally, prospects for future studies are given, The recent discovery of the catalytic applications of these clusters provides renewed interest in this challenging field.