Researchers in this field have investigated photochemical and electrochemical CO 2 reduction to CO or formate, even to methanol, using transition metal electrodes, metal complexes, semiconductors, and also organic molecules, and the details of these achievements can be found in many reviews recently published. 14−31 While recent progress in this field is quite remarkable in photochemical CO 2 reduction using semiconductors or heterogeneous systems, most experiments have not been confirmed using labeled CO 2 (i.e., 13 CO 2 ) and H 2 O (i.e., H 2 18Figure 1. Z-Scheme for photocatalytic CO 2 reduction coupled to methanol oxidation. Reproduced with permission from ref 46.

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(P(Ph)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2), 6a, (where P(Ph)(2)N(Ph)(2) 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 A. The cation of [Ni(P(Cy)(2)N(Bz)(2))(2)](BF(4))(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 A. 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.

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