Comparison of Cobalt and Nickel Complexes with Sterically Demanding Cyclic Diphosphine Ligands: Electrocatalytic H2 Production by [Co(PtBu2NPh2)(CH3CN)3](BF4)2

The cyclic diphosphine ligands (P2N2ph)-N-tBu and (P2N2Bz)-N-tBu have been synthesized and used to prepare new complexes of Co(II) and Ni(II) with the formula [M((P2N2R)-N-tBu)(CH3CNn](BF4)(2) (n = 2,3). The products have been characterized by variable-temperature NM R data, X-ray diffraction studies, and cyclic voltammetry, and properties of the new complexes have been compared with those of previously studied complexes containing (P2N2R)-N-ph ligands. The variation of either phosphorus or nitrogen substituents in these ligands can result in significant differences in the structure, electrochemistry, and reactivity of the metal complexes. [Co((P2N2Ph)-N-tBu)(CH3CN](3)](BF4)(2) is found to be an effective electrocatalyst for the formation of hydrogen using bromoanilinium tetrafluoroborate as the acid, with a turnover frequency of 160 s(-1) an overpotential of 160 mV. These cobalt derivatives are a promising class of catalysts for further study and optimization.