New Opportunities in Metal Cluster Catalysis by Using Organic Ligands

Rate and selectivity of supported-metal clusters can be tuned by changing the cluster ligands. Much research has focused on investigating ligand-support interactions. This study, however, emphasizes the effects of organic ligands on catalysis by homogenous and by heterogeneous metal-cluster catalysts. A comparison of calixarene-bound Ir₄-based carbonyl clusters consisting of Ir₄(CO)₉L₃ and Ir₄(CO)₁₁L (L = t-butyl-calixarene[4](OPr)₃(OCH₂PPh₂)) both in homogeneous solution and when immobilized on the surface of silica is performed.  Catalytic studies were performed by subjecting both materials to a reactive mixture of gases for ethylene hydrogenation.  Both clusters successfully catalyze the hydrogenation reaction at room temperature and appear to be stable after a brief approach to steady-state.  At 50 ^oC, the catalyst containing three calixarenes per active site, Ir₄(CO)₉L₃, retains its stability features, showing a rapid approach to steady-state activity.  In contrast, however, catalyst Ir₄(CO)₁₁L consisting of only a single calixarene per active site shows a lack of stability.  We also describe a pretreatment of the calixarene-based clusters, which increases their catalytic activity for ethylene hydrogenation by nearly 50-fold.