Cationic Heteroleptic Ni(II) Complexes of Dithiocarbamate and Phosphine Ligands: Synthesis, Characterization and Proton Reduction Study

Abstract

Three cationic heteroleptic Ni(II) dithiocarbamate complexes ([Ni(L)(L')n]PF₆, n = 1 or 2) were synthesized and characterized to explore the relationship between diphosphine chelate ring size and their electrocatalytic activity for hydrogen production. These complexes (1-3) feature a common dithiocarbamate ligand (L) and varied diphosphine ligands (L'): dppm (1), dppe (2), and dppp (3). Single-crystal X-ray analysis showed distorted square pyramidal geometry for 1 and square planar geometries for 2 and 3 around NiP₂S₂ core. Electrochemical studies revealed small variation in redox potentials (∆E1/2 ≈ 60 mV) suggesting weak electronic effect of ligands from dppm (1) to dppp (3). However, significant difference in catalytic half-wave potentials (∆Ecat/2 ≈ 200 mV in 4 mM CH3COOH) indicates the strong influence of the P-Ni-P bite angles. The catalytic activity of complexes significantly influenced by chelate ring size and their P-Ni-P bite angles as: 92.07(4)o (3) > 86.84(6)o (2) > 75.24(3)o (1). The enhanced electrocatalytic performance of 3 with low overpotential (~600 mV), high turnover frequency (~706 s⁻¹), and Faradaic efficiency (88%), is attributed to the conformational flexibility of six-membered chelate ring in 3 due to the dppp ligand. Both experimental data and DFT calculations support an ECEC mechanism for HER catalysed by heteroleptic Ni(II) complexes with the formation of Ni(III)–H intermediate species.