Hydrogen Oxidation and Production Using Nickel-Based Molecular Catalysts with Positioned Proton Relays

Highly efficient electrocatalysts for both hydrogen evolution and hydrogen oxidation have been
designed, synthesized, and characterized. The catalysts in their resting states are air-stable, mononuclear
nickel(II) complexes containing cyclic diphosphine ligands with nitrogen bases incorporated into the ligand
backbone. X-ray diffraction studies have established that the cation of [Ni(PPh2NPh2)2(CH3CN)](BF4)2, 6a,
(where PPh2NPh2 is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane) is a trigonal bipyramid with bonds
to four phosphorus atoms of the two bidentate diphosphine ligands and the nitrogen atom of an acetonitrile
molecule. Two of the six-membered rings formed by the diphosphine ligands and Ni have boat conformations
with an average Ni- - -N distance to the two pendant bases of 3.4 Å. The cation of [Ni(PCy2NBz2)2](BF4)2,
6b, (where Cy = cyclohexyl and Bz = benzyl) is a distorted square planar complex. For 6b, all four six-membered rings formed upon coordination of the diphosphine ligands to the metal are in the boat form. In
this case, the average Ni- - -N distance to the pendant base is 3.3 Å. Complex 6a is an electrocatalyst for
hydrogen production in acidic acetonitrile solutions, and compound 6b is an electrocatalyst for hydrogen
oxidation in basic acetonitrile solutions. It is demonstrated that the high catalytic rates observed with these
complexes are a result of the positioning of the nitrogen base so that it plays an important role in the
formation and cleavage of the H−H bond.