Synthesis of dinuclear and trinuclear Ni(Ⅱ) complexes based on double-armed Salamo-type ligand containing N2O2 donors: Crystal structures, theoretical calculations, and catalytic oxidase activity

Abstract

Two polynuclear Ni(II) complexes, [Ni₂(L)(C₅H₅ N)₂]⋅CH₂Cl₂ (1) and [Ni₃(L)(μ_1,3OAc)₂] (2), were synthesized through the reaction of a double-armed salamo-type ligand H₄ L with Ni(NO₃)₂·6H₂O (using pyridine as an auxiliary ligand) and Ni(OAc)₂·4H₂O, respectively. Both complexes were characterized by X-ray crystallography, PXRD, and a range of spectroscopic techniques. Crystal structure analysis revealed that complex 1 has a dinuclear linear structure with coordinated pyridine, while complex 2 exhibits a trinuclear structure with central symmetry, where two phenoxides and two acetates bridge one of the units. The acetate bridges adopt the μ₂-η¹: η¹ coordination mode. Theoretical calculations, including molecular electrostatic potentials (MEPs) and density functional theory (DFT), were employed to predict reaction sites and assess the overall reactivity of the complexes. Both complexes 1 and 2 demonstrated catalytic activity, oxidizing 3,5-Ditert‑butyl catechol (3,5-DTBC) to the corresponding o-quinone (3,5-DTBQ), as monitored by UV-visible spectroscopy under ambient, aerobic conditions. Kinetic analysis using the Michaelis-Menten equation indicated turnover numbers (K_cat) of 599 h⁻¹ and 443 h⁻¹ for complexes 1 and 2, respectively. The mechanistic pathway of the catalytic reaction was further elucidated through ESI-mass spectrometry and cyclic voltammetry.