Combined Homogeneous and Heterogeneous Catalysts. Rhodium and Platinum Isocyanide Complexes Tethered on Silica-Supported Metal Heterogeneous Catalysts: Arene and Cyclohexanone Hydrogenation

Abstract

Rhodium and platinum isocyanide complexes RhCl(CO)[CN(CH2)3Si(OC2H5)3]2 (Rh−CNR2), RhCl[CN(CH2)3Si(OC2H5)3]3 (Rh−CNR3), and PtCl2[CN(CH2)3Si(OC2H5)3]2 (Pt−CNR2) were tethered to the silica-supported metal heterogeneous catalysts M−SiO2 (M = Pd, Pt, Ru) to give the TCSM (tethered complex on supported metal) catalysts Rh−CNR2/Pd−SiO2, Rh−CNR3/M−SiO2 (M = Pd, Pt, Ru), and Pt−CNR2/Pd−SiO2. These TCSM catalysts were used to catalyze the hydrogenation of arenes (Rh−CNR2/Pd−SiO2 and Rh−CNR3/M−SiO2) and cyclohexanone (Pt−CNR2/Pd−SiO2) under the mild conditions of 40 °C and 1 atm. They exhibit activities that are higher than those of the separate homogeneous rhodium (or platinum) isocyanide complex, the separate silica-supported metal heterogeneous catalyst, or the rhodium (or platinum) complex catalyst tethered on just SiO2. The activities of the TCSM catalysts are strongly affected by the nature and loading of the supported metal in the catalyst. Among the three silica-supported metal M−SiO2 (M = Pd, Pt, Ru) catalysts, the rhodium complex Rh−CNR3 tethered on Pd−SiO2 exhibits the highest activity for the hydrogenation of toluene (TOF = 5.5 mol H2/(mol Rh min) and TO = 2420 mol H2/mol Rh during 8.5 h). The Rh−CNR3/Pd−SiO2 catalyst with 10 wt % Pd is more active than its counterparts with higher or lower palladium loadings. IR (DRIFT) spectral studies of the TCSM catalysts before and after being used for toluene hydrogenation show that the isocyanide ligands remain coordinated to the rhodium (or platinum) center even after extended use. Atomic emission spectroscopic analysis of hydrogenation solutions shows that there is no rhodium (or platinum) leaching into the solutions.