Formazan as a Noninnocent Ligand: Catalytic Activity of [Ni(3-cyano-1,5-mesitylformazanate)((2-mercaptopyridinate)-κN,S)] in Electrochemical Hydrogen Production

Formazanates are the mono-deprotonated form of formazans, which exhibit a notable property of noninnocent behavior. Here, a new nickel–formazanate complex, [Ni(3-cyano-1,5-mesitylformazanate)((2-mercaptopyridinate)-κN,S)], was synthesized and characterized via NMR spectroscopy, crystallography, and elemental analysis. Cyclic voltammetry using water as the proton source revealed the catalytic activity of the complex for electrochemical hydrogen production. Further study with acetic acid as a proton source estimated an overpotential of 0.73–0.79 V, current efficiency of 82.1%, and turnover number of 12.2 (for 2 h). The catalytic mechanism is discussed based on electrochemical data, spectral changes, and quantum calculations, concluding that the noninnocent ligand plays an important role in catalytic behavior. A side-product complex and a dinuclear derivative complex were also characterized crystallographically.