Synthesis, structural characterization, and computational simulations of heteroleptic cobalt(ii) complexes: toward efficient catalysis in environmental remediation and cross-coupling chemistry

Abstract

This work aims to study the catalytic chemistry of cobalt-based heteroleptic heterogeneous photocatalysts, including photocatalytic degradation of organic pollutants and cross-coupling reactions. Two novel heteroleptic cobalt(II) complexes (designated as 1 and 2) were synthesized under ambient conditions using a Schiff base ligand (HL₁) in conjunction with 4-hydroxyphenylphenol (L₂) ligand and subsequently characterized through a suite of spectroscopic and electrochemical techniques. The photocatalytic efficacy of these complexes was systematically evaluated towards the degradation of representative organic dyes, viz. methylene blue (MB), malachite green (MG), and methyl orange (MO), under ultraviolet irradiation from a 9 W mercury vapor lamp. Complex 2 achieved maximum degradation efficiencies of 94.12% (MG), 91.50% (MO), and 90.68% (MB), at a catalyst loading of only 6 mg. The degradation process conformed to the Langmuir–Hinshelwood (pseudo-first-order) kinetic model, implicating a physisorption-driven surface reaction mechanism. DFT calculations corroborated the experimental findings, revealing a narrower HOMO–LUMO gap for complex 2, accounting for its enhanced photo-reactivity. Quantum yield (ϕ) and figure of merit (FOM) values demonstrated efficient photon utilization and favourable reaction kinetics, while the remarkably low electrical energy per order (EE/O) value of 45.33 kWh for MG underscores the superior energy efficiency of these systems relative to previously reported photocatalysts operating under comparable or more intense irradiation conditions. Through an investigation into the catalytic properties, efficacy of the complexes was assessed in Suzuki–Miyaura and Buchwald–Hartwig cross-coupling reactions. These reactions, performed under visible light irradiation, yielded the corresponding biaryl compounds in excellent yields.