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 a distorted square pyramidal geometry for 1 and square planar geometries for 2 and 3 around the NiP₂S₂ core. Electrochemical studies revealed a small variation in redox potentials (ΔE_1/2 ≈ 60 mV), suggesting a weak electronic effect of ligands from dppm (1) to dppp (3). However, a significant difference in catalytic half-wave potentials (ΔE_cat/2 ≈ 200 mV in 4 mM CH₃COOH) indicates a strong influence of the P–Ni–P bite angles. The catalytic activity of the complexes is significantly influenced by the chelate ring size and their P–Ni–P bite angles, which follow the trend 92.07(4)° (3) > 86.84(6)° (2) > 75.24(3)° (1). The enhanced electrocatalytic performance of 3 with a low overpotential (∼600 mV), a high turnover frequency (∼706 s⁻¹), and faradaic efficiency (88%) is attributed to the conformational flexibility of the six-membered chelate ring in 3 due to the dppp ligand. Both experimental data and DFT calculations support an ECEC mechanism for the HER catalysed by heteroleptic Ni(II) complexes with the formation of Ni(III)–H intermediate species.