Ligand-driven redox transformations and catalytic activity of mononuclear copper complexes: structural and spectroscopic insights

Abstract

Eight mononuclear complexes have been synthesized, namely: [(L1)Cu^I]ClO₄ (1), [(L2)Cu^I]ClO₄·CH₃CN (2·CH₃CN), [(L2)Cu^II(DMF)](ClO₄)₂·2DMF·H₂O (3·2DMF·H₂O), [(L1)Cu^II(MeCN)](ClO₄)₂ (4), [(L1)Cu^II(Cl)]PF₆ (5), [(tren)Cu^II(MeCN)](ClO₄)₂ (6), [(tren)Cu^II(Cl)]PF₆ (7) and [(L2ˊ)Cu^II(DMF)](ClO₄)₂·DMF·H₂O (8·DMF·H₂O). In these complexes, copper atoms are coordinated to tetradentate ligands such as tris-(4-(4-(tert-butyl))benzyl-3-aza-3-butenyl)amine (L1), tris(2-aminoethyl)amine (tren), tris-(4-pyren-1-yl-3-aza-3-butenyl)amine (L2) and bis-(4-pyren-1-yl-3-aza-3-butenyl)aminoethylamine (L2’). The complexes were structurally characterized by X-ray crystallography, along with various spectroscopic techniques. Complexes 1 and 2 are Cu(I) complexes, exhibiting trigonal pyramidal geometry, whereas complexes 3 to 8 are Cu(II) complexes with distorted trigonal bipyramidal geometry. Complex 1 shows a quasi-reversible redox response at E_1/2 = 0.567 V (ΔE = 115 mV), while complexes 2 and 3 exhibit quasi-reversible cyclic voltammograms with E_1/2 values of 0.405 V (ΔE = 80 mV) vs. non-aqueous Ag/Ag⁺. The cyclic voltammograms indicate that the formation of a Cu(II) complex from the oxidation of 1 required more anodic potential than from the oxidation of 2, highlighting the influence of ligand environment on redox properties. Complex 2 and 3 are interconvertible and exhibit high superoxide dismutase (SOD) mimetic activity, efficiently dismutating O₂^•⁻ to O₂ and H₂O₂ with an impressive IC₅₀ value of 5.1 × 10^-7 M. Upon electrochemical oxidation, under N₂ atmosphere complex 1 forms complex 4, which in presence of air, converted to complex 6 through complete breaking of imino bonds, resulting in the formation of aldehyde and carboxylic acid in a 2:1 ratio. The generation of carboxylic acid implies a reaction pathway of formation 6 from 4, involving not only hydrolysis of imino bonds but also oxidative degradation of the imino moiety or its aldehyde products. Furthermore, treatment of complex 2 with KO₂ in the presence of protons leads to the formation of complex 8 and 1-pyrenecarboxylic acid. This transformation likely proceeds via Cu(II)-hydroperoxo intermediates, which oxidize 1-pyrenecarboxaldehyde, itself a product of partial imine bond cleavage in complex 3. Density functional theory (DFT) calculations were conducted to rationalize and support the experimental observations and proposed mechanistic pathways.