Rhodium-phosphine complex catalysts tethered on silica-supported heterogeneous metal catalysts: arene hydrogenation under atmospheric pressure

The rhodium-phosphine complex, RhCl(CO)[Ph2P(CH2)(3)Si(OC2H5)(3)](2) (Rh-P), was tethered to silica-supported metal heterogeneous catalysts, Pd-SiO2, Ni-SiO2 and Au-SiO2, to give the tethered complex catalysts Rh-P/Pd-SiO2, Rh-P/Ni-SiO2 and Rh-P/Au-SiO2. An IR (DRIFTS) study shows that the structure of the tethered rhodium-phosphine complex (Rh-P) in all of these combination catalysts is the same as that of the free Rh-P complex. These heterogenized complex catalysts consisting of a tethered complex on a supported metal (TCSM) were used to catalyze the hydrogenation of arenas under the mild conditions of 40 degrees C and 1 arm of H-2. The most active TCSM catalyst Rh-P/Pd-SiO2 is not only much more active than the homogeneous Rh-P complex and Pd-SiO2 but also much more active than the rhodium-phosphine complex tethered on just SiO2 (Rh-P/SiO2). The Rh-P/Pd-SiO2 catalyst gives a maximum turnover frequency (TOF, mol H-2/mol Rh min) of 2.9 and a turnover (TO, mol H-2/mol Rh) of 1940 during a 23 h period in the hydrogenation of toluene. When the tethered rhodium-phosphine complex catalyst stands in air for 2 months or longer, the phosphine ligand is oxidized to the phosphine oxide. This air-aged Rh-P/Pd-SiO2 is much more active than the fresh catalyst for the hydrogenation of toluene, but the air-aged Rh-P/SiO2 becomes inactive for this reaction. These two air-aged catalysts were characterized by solid state P-31 NMR and DRIFTS using CO as a probe. (C) 1999 Elsevier Science B.V. All rights reserved.