ACTIVATION OF MOLECULAR-HYDROGEN IN COBALT-CATALYZED HYDROFORMYLATION

The mechanism of the activation of molecular hydrogen in cobalt-catalyzed hydroformylation of olefins has been studied by high pressure IR spectroscopy using HCo(CO)4 (1) under 100 bar H-2 (or D2) in the absence or presence of CO at room temperature. The treatment of 1 with 100 bar H-2 resulted in the formation of Co2(CO)8 (2) and a small amount of Co4(CO)12 (3), and the transient formation of HCo3(CO)9 (4). In the reaction of 1 with one equivalent of 3,3-dimethyl-butene-1 under 100 bar H-2 both hydrogenation and hydroformylation occur, but the former is much faster. In the presence of large amounts of 1 the predominant path for the hydrogenation of the olefin involves the reaction of two equivalents of 1 with the olefin even under 100 bar of H-2. Under very low partial pressure of CO the stability of 1 is increased and the hydrogenation significantly slowed down. The preferred path of the hydroformylation of the olefin involves the addition of H-2 and CO from gas phase even in the presence of large amount of HCo(CO)4 (1) under 100 bar H-2 and 2.3 bar CO at room temperature. The studies reveal that the mechanism of H-2 activation in the presence of HCo(CO)4 (1) is highly dependent on the reaction conditions. Under 100 bar H-2 and at room temperature the activation of molecular hydrogen starts at a coordinatively unsaturated acyl cobalt carbonyl, yielding an aldehyde and an unknown cobalt species. It is believed that this species is a coordinatively unsaturated hydrido cobalt carbonyl like {HCo(CO)3}, and can activate and catalytically hydroformylate the olefin.