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

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, [(CyPBP)­CoH]2 (1), supported by bis­(phosphino)­boryl and bis­(phosphino)­hydridoborane
ligands. This complex reacts reversibly with a second equivalent of
H2 (1 atm) and serves as an olefin hydrogenation catalyst
under mild conditions (room temperature, 1 atm H2). A bimetallic
Co species is invoked in the rate-determining step of the catalysis
according to kinetic studies. A structurally related NiINiI dimer, [(PhPBP)­Ni]2 (3), has also
been prepared. Like Co catalyst 1, Ni complex 3 displays reversible reactivity toward H2, affording the
bimetallic complex [(PhPBHP)­NiH]2 (4). This
reversible behavior is unprecedented for NiI species and
is attributed to the presence of a boryl–Ni bond. Lastly, a
series of monomeric (tBuPBP)­NiX complexes (X = Cl (5), OTf (6), H (7), OC­(H)O (8)) have been prepared.
The complex (tBuPBP)­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.