DNA intercalative trinuclear Cu(ii) complex with new trans axial nitrato ligation as an efficient catalyst for atmospheric CO2 fixation to epoxides

Abstract

A trinuclear octahedral Cu^II complex [Cu^II₃(L)₂(μ₂-N₃)₂(trans-NO₃)₂(H₂O)₂(CH₃OH)₂] (1) (L = 3-[{2-(2-pyridinyl) ethyl}imino]-2-butanone oximate) was synthesized and structurally characterized by single crystal X-ray diffraction studies and geometry optimization using DFT/B3LYP. The crystal structure analysis of 1 showed that the two Cu atoms and the central Cu atom are linearly connected through two oximato and two azido (EO-N₃⁻) co-ligands, and the angle between three Cu^II atoms was <Cu2–Cu1–Cu2ⁱ = 180°. Further, the central copper atom was axially coordinated by two nitrate counter ions at the trans axial position which is a unique coordination pattern in trinuclear metal complexes in a CSD search. Complex 1 was employed as an efficient catalyst for carbon dioxide fixation reaction in epoxides to synthesize organic cyclic carbonates. The organic cyclic carbonates were produced at room temperature under 1 bar CO₂ pressure and solvent free conditions in the presence of tetrabutylammonium bromide (TBAB). The high conversion of the substrate as well as high selectivity of the desired product yield was obtained from different epoxides through this catalysis. The turnover frequency for the catalysis reaction was in the range of 270–606 (h⁻¹). The plausible mechanism for the formation of organic cyclic carbonates catalyzed by 1 has been framed based on literature data. The interaction of 1 with calf thymus DNA was studied by spectroscopic and calorimetric techniques. The complex was found to bind with DNA through an intercalative binding mode with the binding affinity (6.65 ± 0.03) × 10⁵ M⁻¹. The negative ΔG⁰ and positive ΔS⁰ values obtained from the calorimetric technique confirmed the spontaneity of the binding process.