Rhodium amine complexes tethered on silica-supported metal catalysts. Highly active catalysts for the hydrogenation of arenes

摘要

Rhodium amine complexes, RhCl(CO)(2)[Et2N(CH2)(3)Si(OCH3)(3)] (Rh-NEt2), RhCl(CO)(2)[H2N(CH2)(3)Si(OC2H5)(3)] (Rh-NH2) and RhCl(COD)[H2NCH2CH2NH(CH2)(3)Si(OCH3)(3)] (Rh(COD)(N-N)), were tethered to the silica-supported metal heterogeneous catalysts, M-SiO2 (M = Pd, Ni, Au), to give the TCSM (tethered complex on supported metal) catalysts, Rh-NEt2/M-SiO2 (M = Pd, Ni, Au), Rh-NH2/Pd-SiO2 and Rh(COD)(N-N)/Pd-SiO2. These TCSM catalysts exhibit activities, at 40 degrees C and 1 arm of H-2 pressure, for the hydrogenation of arenes that are higher than those of the separate homogeneous rhodium amine complexes, the separate silica-supported metal heterogeneous catalysts or the rhodium complex catalysts tethered on just SiO2. The activities of the TCSM catalysts are strongly affected by both the tethered rhodium amine complex and the SiO2-supported metal. Among these TCSM catalysts, Rh-NEt2/Pd-SiO2 exhibits the highest activity for the hydrogenation of toluene; its maximum TOF is 7.2 mol H-2, (mol Rh min)(-1) while its TO is 1919 mol H-2 (mol Rh)(-1) during a 5 h period. IR(DRIFT) spectral studies of the TCSM catalysts before and after being used for the hydrogenation of toluene show that during the hydrogenation, the two CO ligands of Rh-NEt2/M-SiO2 (M = Pd, Ni, Au) are lost from the rhodium center. After standing in air for one month, Rh-NEt2/Pd-SiO2 becomes more active for the hydrogenation of toluene, but the activity of the air-aged Rh(COD)(N-N)/Pd-SiO2 is lower than that of the fresh catalyst. [References: 54]