Anchored complexes of rhodium and iridium for the hydrogenation of alkynes and olefins with parahydrogen

Abstract

Several catalysts synthesized by covalently binding Rh and Ir complexes to NH₂– and PPh₂– functional groups of the linker chains on the functionalized silica gel surface were compared in gas-phase hydrogenation of propyne and propene. Their stability, activity and product selectivity were evaluated. Furthermore, the use of parahydrogen in the reaction provided useful mechanistic information by delivering nuclear spin hyperpolarization of the reaction products with the associated enhancement of their NMR signals. In contrast to Rh-based catalysts which showed a pronounced tendency to reduction, for Ir-based catalysts an XPS analysis did not reveal any indication of the formation of a metal phase under reducing conditions upon hydrogenation of propyne or propene at 120 °C. Catalysts with a π-acceptor ligand PPh₂– demonstrated greater activity in the gas-phase hydrogenation of propyne/propene in comparison with σ-donor ligand NH₂–. Furthermore, NMR signal enhancements of up to ca. 700- (Rh) and 800-fold (Ir) were achieved. Remarkably, the Ir metal complex bound via the phosphine moiety demonstrated high stability in gas-phase hydrogenation of propyne along with high catalytic activity and pairwise H₂ addition selectivity, yielding substantial quantities of strongly hyperpolarized propene.