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

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. The ligands include N-(3-dimethylaminopropyl)-N'-pyridin-2-ylmethyl-propane-1,3-diamine (L1), N-(3-dimethylaminopropyl)-N'-quinolin-2-ylmethyl-propane-1,3-diamine (L2), N-(3-dimethylaminopropyl)-N'-(1-methyl-1H-imidazol-2-ylmethyl)-propane-1,3-diamine (L3), N-(3-dimethylaminopropyl)-N'-(1-ethyl-1H-benzoimidazol-2-ylmethyl)-propane-1,3-diamine (L4) and N-(2-dimethylaminobenzyl)-N'-(3-dimethylaminopropyl)-propane-1,3-diamine (L5). 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 exhibited nearly reversible redox processes (ΔE, 63-72 mV) but the redox process was far from reversible for the imidazole- and benzimidazole-containing complexes 3-5 (ΔE, 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 a broad substrate scope, converting eight different epoxides into the corresponding cyclic carbonates with excellent selectivity (>99%) and yields ranging from 56% to 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.