Interplay between β-Diimino and β-Diketiminato Ligands in Nickel Complexes Active in the Proton Reduction Reaction

[Image: see text] Two Ni complexes are reported with κ(4)-P(2)N(2) β-diimino (BDI) ligands with the general formula [Ni(XBDI)](BF(4))(2), where BDI is N-(2-(diphenylphosphaneyl)ethyl)-4-((2-(diphenylphosphaneyl)ethyl)imino)pent-2-en-2-amine and X indicates the substituent in the α-carbon intradiimine position, X = H for 1(BF(4))(2) and X = Ph for 2(BF(4))(2). Electrochemical analysis together with UV–vis and NMR spectroscopy in acetonitrile and dimethylformamide (DMF) indicates the conversion of the β-diimino complexes 1(2+) and 2(2+) to the negatively charged β-diketiminato (BDK) analogues (1-H)(+) and (2-H)(+) via deprotonation in DMF. Moreover, further electrochemical and spectroscopy evidence indicates that the one-electron-reduced derivatives 1(+) and 2(+) can also rapidly evolve to the BDK (1-H)(+) and (2-H)(+), respectively, via hydrogen gas evolution through a bimolecular homolytic pathway. Finally, both complexes are demonstrated to be active for the proton reduction reaction in DMF at E(app) = −1.8 V vs Fc(+/0), being the active species the one-electron-reduced derivative 1-H and 2-H.