Schiff-Base ligand based Ni(III) and Ni(II) complexes for catalytic application in CO2 fixation reactions

Abstract

A Ni(III) complex with a Ni(II) counterpart, formulated as 2[Ni(L)(HL)(SCN)].0.5[Ni(Cl)(SCN)₃].0.5(SCN) (1), and a Ni(II) complex, [Ni(L)(NCS)] (2), were synthesized using the Schiff base ligand (E)-4‑bromo-2-(((2-(dimethylamino)ethyl)imino)methyl)-6-methoxyphenol [HL]. Both complexes were structurally characterized by single-crystal X-ray diffraction. The high oxidation state of the metal in 1 was confirmed by single-crystal X-ray diffraction, X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and EPR analysis study, which, as far as our survey shows, represents the first report of a Ni(III) complex comprising Schiff base ligands with a Ni(II) counterpart. The presence of the Lewis acidic nickel center, as well as the Lewis basic groups (-NMe₂ or -OMe) in both complexes, makes them suitable as catalysts for carbon dioxide fixation into epoxides for the production of organic cyclic carbonates. The use of greenhouse gas CO₂ (at 1 bar) and solvent-free conditions marks the catalytic protocol as environment friendly. Various aliphatic, aromatic, and endocyclic epoxides smoothly undergo the fixation reaction through this catalytic process, producing the respective carbonate products in moderate to high yields (51–100 %) at 90 °C. The conditions of these catalytic reactions were optimized, and high turnover numbers (TON) of 1224–2400, along with turnover frequencies of 66–200 h^–1, were achieved. Density functional theory (DFT) calculations were used to determine the relative energy of transition states, intermediates, and products involved in the catalytic cycles and to describe the mechanistic pathway in detail.