Boryl−Metal Bonds Facilitate Cobalt/Nickel-Catalyzed Olefin Hydrogenation

Abstract

New approaches toward the generation of late first-row metal catalysts that efficiently facilitate two-electron reductive transformations (e.g., hydrogenation) more typical of noble-metal catalysts is an important goal. Herein we describe the synthesis of a structurally unusual S = 1 bimetallic Co complex, [(^(Cy)PBP)CoH]_2 (1), supported by bis(phosphino)boryl and bis(phosphino)hydridoborane ligands. This complex reacts reversibly with a second equivalent of H_2 (1 atm) and serves as an olefin hydrogenation catalyst under mild conditions (room temperature, 1 atm H_2). A bimetallic Co species is invoked in the rate-determining step of the catalysis according to kinetic studies. A structurally related Ni^INi^I dimer, [(^(Ph)PBP)Ni]_2 (3), has also been prepared. Like Co catalyst 1, Ni complex 3 displays reversible reactivity toward H_2, affording the bimetallic complex [(^(Ph)PBHP)NiH]_2 (4). This reversible behavior is unprecedented for Ni^I species and is attributed to the presence of a boryl–Ni bond. Lastly, a series of monomeric (^(tBu)PBP)NiX complexes (X = Cl (5), OTf (6), H (7), OC(H)O (8)) have been prepared. The complex (^(tBu)PBP)NiH (7) shows enhanced catalytic olefin hydrogenation activity when directly compared with its isoelectronic/isostructural analogues where the boryl unit is substituted by a phenyl or amine donor, a phenomenon that we posit is related to the strong trans influence exerted by the boryl ligand.