Photochemical generation of nickel(I) complexes and their reaction with hydrogen: nickel hydride catalysed hydrogenation of 1,5-cyclo-octadiene

Abstract

In benzene under an atmosphere of hydrogen, triplet excited-state xanthone or acetone sensitizes the photoreduction of bis(acetylacetonato)nickel(II), Ni(acac)₂, to transient nickel(I) complexes which can be detected by ESR spectroscopy in the presence of stabilizing ligands. The kinetics of these ESR signal decays prove that these nickel(I) complexes are derived from moderately co-ordinating ligands, such as tetrahydrofuran and 1,5-cyclo-octadiene (1,5-COD), and react with hydrogen in the dark to form nickel hydride complexes. An excess of 1,5-COD in the photolysate was sequentially and catalytically isomerized and hydrogenated under the irradiation conditions; i.e., 1,5-COD→1,4-COD→1,3-COD→COE→COA. The nickel hydride complexes are proposed as the key species in the initiation of the catalytic cycle in which steps involving the addition and elimination of nickel hydride bonds and the hydrogenolysis of nickel complexes are assumed. As these alkene reactions cease in the dark and are quenched by a high concentration of 1,3-COD, it can be concluded that one or more steps in the catalytic cycle requires sensitized photoexcitation.