Polymer-Bound Tetraruthenium and Tetrairidium Carbonyl Catalysts for Olefin Hydrogenation

Abstract

Tetraruthenium clusters with unique structures have been attached to solid polymeric supports and used to catalyze ethylene hydrogenation at 1 atm and 50–90°C. Polymer-bound analogs of [H 4 Ru 4 (CO) 12–x (PPh 3 ) x ) are synthesized by ligand exchange between [H 4 Ru 4 (CO) 12 ) and poly(styrene-divinylbenzene) membranes functionalized with phosphine ligands. Rates of ethylene hydrogenation are measured with a flow reactor allowing simultaneous recording of the infrared spectra of the functioning catalyst. Each catalyst is stable, exhibiting undiminished activity after thousands of turnovers and presenting a carbonyl spectrum unchanged during catalysis and indistinguishable from that of the membrane incorporating the originally bound tetraruthenium cluster. Unique tetrairidium carbonyl clusters are also anchored to phosphine-functionalized poly (styrene-divinylbenzene), the metal species being identified by carbonyl infrared spectra as the clusters analogous to [Ir 4 (CO) 12–x (PPh 3 ) x ). Infrared spectra of polymer membranes functioning as catalysts for ethylene and cyclohexene hydrogenation in a flow reactor at 1 atm and 40–80° C indicates that the predominant metal species in each catalyst is the originally prepared tetrairidium cluster. Catalytic kinetics measured at <90°C shows that for each olefin, the rate of hydrogenation decreases with an increasing number of phosphine substituents on the tetrairidium cluster. The results suggest that the metal clusters provide the catalytic sites, possibly formed by reversible cleavage of Ir–Ir or Ir–P bonds- to generate coordinative unsaturation.