Rhodium phosphine complexes immobilized on silica as active catalysts for 1-hexene hydroformylation and arene hydrogenation

Abstract

Abstract In immobilizing the rhodium complexes [Rh(acac)(CO)(P)] ( 1 ) and [Rh(acac)(P) 2 ] ( 2 ) (P = Ph 2 PCH 2 CH 2 Si(OMe) 3 ) onto SiO 2 , acetylacetone is found to be released through protonation of the acac ligand by the acidic silica-OH groups. The resulting complexes [Rh(O-{SiO 2 }(HO-{SiO 2 })(CO)(P-{SiO 2 })] ( 1a ) and [Rh(O-{SiO 2 })(HO-{SiO 2 })(P-{SiO 2 }) 2 ] ( 2a ) were successfully tested with respect to their catalytic action on 1-hexene hydroformylation as well as benzene and toluene hydrogenation. The reaction outcome, viz. the formation of aldehydes versus isomerization, depends strongly on the presence and concentration of a phosphine co-catalyst. Thus, while 1a gave only a 17% yield of aldehyde in the absence of phosphines, the yield is increased to 54% in the presence of phosphinated silica P-{SiO 2 } or even 94% if PPh 3 is added to the solution. Without extra added phosphine, both 1a and 2a effect mainly the isomerization of 1-hexene to 2-hexene. Pre-catalyst 1a catalyzes also the hydrogenation of benzene at 10.5 atm H 2 and 90 °C to give cyclohexane with a TOF of 608 h −1 .