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

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 .