Catalytic CO2 fixation into cyclic carbonates by copper(II) complexes of N4 ligands under mild conditions

Abstract

A series of novel copper(II) complexes [Cu(L)NO3]NO3 (1-5) of linear N4 ligands were synthesized, fully characterized, and evaluated as catalysts for CO2 fixation. Single-crystal X-ray studies of complexes 1, 3, and 4 revealed nearly ideal square-pyramidal coordination geometries (τ = 0.01-0.28). The Cu(II)/Cu(I) redox couples are in the range -0.145 to -0.368 V (vs. Ag/AgCl) in aqueous solution, where 1 and 2 showed nearly reversible redox process (E, 63-72 mV) and however redox process is far from reversibility for imidazole and benzoimidazol containing complexes 3 - 5 (E, 102-236 mV). Electronic absorption spectra exhibited characteristic d-d transitions at 640-689 nm, while axial EPR parameters (g∥ = 2.26-2.32; A∥ = 110-178 × 10⁻4 cm⁻¹) confirmed five coordination geometries in solution. Bonding parameters (α² = 0.663-0.857, β² = 0.964-1.253, γ² = 0.704-1.067, K∥ = 0.761-0.855, and K⊥ = 0.554-0.723) derived from EPR and spectral data indicate significant out-of-plane π-bonding (K∥ > K⊥) in all complexes. All complexes efficiently catalyze the fixation of CO₂ on epoxides to form cyclic carbonates under solvent-free conditions at room temperature and 1 atm CO₂ pressure in the presence of Bu₄NBr. Complex 1 exhibited the highest catalytic activity, achieving a yield of up to 84% and a turnover number (TON) of 1680. It is one of the most efficient copper-based catalysts under these mild conditions. Furthermore, complex 1 demonstrated broad substrate scope, converting eight different epoxides into the corresponding cyclic carbonates with excellent selectivity (>99%) and yields ranging from 56–84%. This catalytic performance is strongly influenced by the electronic nature of the ligands. This sustainable, cost-effective catalytic protocol offers a promising approach to the synthesis of industrially relevant cyclic carbonates from CO₂.