Examining the binding mechanism of 3,4-dihydro-3-(2-oxo-2-phenylethylidene) -quinoxalin-2(1H)-one and its fragments to Cu₂₊

The structure and stoichiometries of the complexes that could be formed between Cu₂₊ and 3,4-dihydro-3-(2-oxo-2-phenylethylidene)-quinoxalin- 2(1H)-one (1) were investigated by various spectral techniques such as IR, fluorescence, UV-vis and electron paramagnetic resonance (EPR). The results suggest that initially 3 : 1 and 2 : 1 (1/Cu₂₊) complexes are formed at low Cu₂₊ concentration and upon adding more Cu₂₊, 1 : 1 (preferred) and 1 : 2 complexes are generated. Since 1 possesses two possible binding sites, further exploration was done by testing the binding ability of Cu₂₊ to fragments of 1, namely β-enaminoketone derivatives (2-3) and quinoxaline-2-one (4), and by executing calculations of thermodynamic parameters of the reaction between 1 and Cu₂₊ in ethanol, optimized geometries of the possible complexes, and estimation of stability constants at various stoichiometries. Consequently, a step-by-step binding mechanism is suggested for formation of various complexes between 1 and Cu₂₊.