Electrocatalytic Hydrogen Evolution Catalyzed by 3,4-Tolunenedithiolate Nickel Complexes of Bis(diphenylphosphine)amine Ligand Containing An Azahydrophilic Group

Two 3,4-tolunenedithiolate nickel complexes of bis(diphenylphosphine)amine ligand containing an azahydrophilic group, [(CH3)(2)N(CH2)(2)N(PPh2)(2)Ni(tdt)] (1) and [(CH3)(2)N(+)Bz(CH2)(2)N(PPh2)(2)Ni(tdt)] Br- (2) (tdt=3,4-tolunenedithiolate, Bz=PhCH2), have been synthesized and characterized by elemental analysis and spectroscopy (FTIR, UV-vis, H-1, and P-31{H-1} NMR). The electrochemical property of 1 and 2 was investigated in MeCN and MeCN/H2O, respectively. The redox potentials of 1 and 2 are shifted to less-negative potentials as the water contents increase in MeCN/H2O, in which the first reduction potentials for 1 and 2 are shifted positively by 160 and 140 mV in 7 : 3 MeCN/H2O in compassion to neat MeCN. 1 and 2 exhibit the electrocatalysis for hydrogen (H-2) evolution using trifluoroacetic acid (TFA) as the proton source both in MeCN and in MeCN/H2O. Remarkably, 1 and 2 display favorable energetics towards electrocatalytic proton reduction in the presence of water, and the more hydrophilic quaternary ammonium group is beneficial to enhance the catalytic activity for 2. A CEEC (C is a chemical step protonation and E is the electrochemical step reduction) electrocatalytic mechanism for hydrogen production by 1 was proposed in MeCN, where the catalyst 1 firstly undergo rapidly protonation at the coordinating S-1 atom in the tdt ligand to generate intermediate [(NiS1)-S-II-H](+) before undergoing reduction, and the reduction of [(NiS1)-S-II-H](+) yields a key metal hydride species, [Ni-III-H]. Theoretical calculations carried out using density functional theory (DFT) which is consistent with the proposed catalytic reaction mechanism for 1.