Alkene Hydrogenation on Rhodium Complexes Attached to Chemically Modified Silica Supports

Abstract

The interest in discrete molecular clusters of transition metals has grown tremendously among catalytic scientists in recent years. The reason for this growth is twofold. First, analogous geometric arrangements and structural units occur in molecular clusters and on metal surfaces. Moreover, certain similarities between the interaction of ligands and metal clusters and the chemisorption on metal surfaces have been encountered. Discrete clusters have therefore been considered as attractive models for chemisorption and catalysis on metal surfaces (1). Secondly, molecular clusters, namely carbonyl clusters of rhodium and iridium, may be potentially useful as homogeneous catalysts in industrial processes such as hydroformylation (2, 3) and the conversion of synthesis gas CO + H₂ (4). However, homogeneous catalytic processes always bring about several technological problems — such as difficult separation of active catalyst from reaction mixture, loss of noble metal, corrosion — besides their undisputed activity and selectivity advantages. Several attempts have therefore been undertaken to attach molecular clusters with retention of cluster integrity onto organic polymeric and inorganic supports. Rh₆,(CO)₁₆, (5–8) and Ir₄, (CO)₁₆ (9) have been claimed to probably maintain their structural integrity when bound onto phos- phinated polystyrene matrizes.