Mechanistic studies on the in-situ generation of furoxan ring during the formation of Cu(II) coordination compound from dioxime ligand: Theoretical and experimental study

Abstract

The copper(II) coordination compound, [Cu(Lʹ)(NO3)2] (1) where Lʹ = 3,4-di(pyridin-2-yl)-1,2,5-oxadiazole 2-oxide, was synthesized from the reaction of Cu(NO3)2·3H2O and dioxime ligand, (1E,2E)-1,2-di(pyridin-2-yl)ethane-1,2-dione dioxime (L), in methanol under aerial atmosphere. The dark-blue crystals of 1 were obtained by solvent evaporating method and were characterized by elemental analysis, FT-IR spectroscopy and single crystal X-ray studies. Structural studies indicated that in 1 a furoxan ring is generated during the formation of mononuclear Cu(II) coordination compound from the in-situ intramolecular cyclization reaction of dioxime moiety of the initial ligand. The density functional theory (DFT) along with B3LYP functional was employed to propose a rational mechanism for cyclization of dioxime (L) and formation of the furoxan ring in 1. It was found that the isolated ligand (L) is not an acidic compound while its acidity is enhanced dramatically in coordination compound in the presence of NO3− anions assisting its deprotonation. Ring formation is catalyzed by Cu in the second deprotonation step, i.e. Cu(II) accepts the negative charge of L, produced upon deprotonation, to assist formation of the neutral aromatic furoxan ring. In the last step, Cu(I) is oxidized to Cu(II) by solvated oxygen, O2(CH3OH)n, to retrieves its structure and oxidation state.