Chemical fixation of atmospheric CO2 in tricopper(ii)-carbonato complexes with tetradentate N-donor ligands: reactive intermediates, probable mechanisms, and catalytic and magneto-structural studies

Abstract

In the present era, the fixation of atmospheric CO₂ is of significant importance and plays a crucial role in maintaining the balance of carbon and energy flow within ecosystems. Generally, CO₂ fixation is carried out by autotrophic organisms; however, the scientific community has paid substantial attention to execute this process in laboratory. In this report, we synthesized two carbonato-bridged trinuclear copper(II) complexes, [Cu₃(L¹)₃(μ₃-CO₃)](ClO₄)₃ (1) and [Cu₃(L²)₃(μ₃-CO₃)](ClO₄)₃ (2) via atmospheric fixation of CO₂ starting with Cu(ClO₄)₂·6H₂O and easily accessible pyridine/pyrazine-based N₄ donor Schiff base ligands L¹ and L², respectively. Under very similar reaction conditions, the ligand framework embedded with the phenolate moiety (HL³) fails to do so because of the reduction of the Lewis acidity of the metal center, inhibiting the formation of a reactive hydroxide bound copper(II) species, which is required for the fixation of atmospheric CO₂. X-ray crystal structures display that carbonate-oxygen atoms bridge three copper(II) centers in μ₃ syn–anti disposition in 1 and 2, whereas [Cu(HL³)(ClO₄)] (3) is a mononuclear complex. Interestingly, we also isolated an important intermediate of atmospheric CO₂ fixation and structurally characterized it as an anti–anti μ₂ carbonato-bridged dinuclear copper(II) complex, [Cu₂(L²)₂(μ₂-CO₃)](ClO₄)₂·MeOH (2-I), providing an in-depth understanding of CO₂ fixation in these systems. Variable temperature magnetic susceptibility measurement suggests ferromagnetic interactions between the metal centers in both 1 and 2, and the results have been further supported by DFT calculations. The catalytic efficiency of our synthesized complexes 1–3 was checked by means of catechol oxidase and phenoxazinone synthase-like activities. While complexes 1 and 2 showed oxidase-like activity for aerobic oxidation of o-aminophenol and 3,5-di-tert-butylcatechol, complex 3 was found to be feebly active. ESI mass spectrometry revealed that the oxidation reaction proceeds through the formation of complex–substrate intermediations and was further substantiated by DFT calculations. Moreover, active catalysts 1 and 2 were effectively utilized for the base-free oxidation of benzylic alcohols in the presence of air as a green and sustainable oxidant and catalytic amount of TEMPO in acetonitrile. Various substituted benzylic alcohols smoothly converted to their corresponding aldehydes under very mild conditions and ambient temperature. The present catalytic protocol showcases its environmental sustainability by producing minimal waste.