Is it homogeneous Pt(II) or heterogeneous Pt(0)n catalysis?: Evidence that Pt(1,5-COD)Cl2 and Pt(1.5-COD)(CH3)2 plus H2 form heterogeneous, nanocluster plus bulk-metal Pt(0) hydrogenation catalysts

An investigation of the question of Is it homogeneous or heterogeneous catalysis? is reported when using Pt-II(1,5-COD)X-2 (X = halogen, alkyl) precatalysts for the hydrogenation of olefins. Using product studies, kinetic evidence, and Hg-0 poisoning experiments, it is shown that Pt-II(1,5-COD)Cl-2 is a precatalyst and must be reduced to Pt-0 nanoclusters and bulk metal as the true hydrogenation catalyst. An investigation of the related complex Pt-II(1,5-COD)(CH3)(2) reveals that this complex does not form a hydrogenation catalyst by itself under H-2, in agreement with the literature. Kinetic and Hg-0 poisoning evidence confirms that Pt-II(1,5-COD)(CH3)(2), too, forms a Pt-0 heterogeneous catalyst if other metals (Ir-0, NO) are used as seeds to initiate the reduction of Pt-II. A short review of the use of Pt-II(1,5-COD)Cl-2 in hydrosilylation reactions is given, illustrating the continued controversy surrounding the nature of the true catalyst in that literature system. (c) 2005 Elsevier B.V. All rights reserved.