Copper(II)-Mediated Oxidative Transformation of vic-Dioxime to Furoxan: Evidence for a Copper(II)-Dinitrosoalkene Intermediate

The mononuclear copper(II) complex [Cu(H₂L¹)₂(H₂O)](ClO₄)₂ (1) (where H₂L¹ = 1,10-phenanthroline-5,6-dioxime) reacts with copper(II) perchlorate in acetonitrile at ambient conditions in the presence of triethylamine to afford a copper(II) complex, [Cu(L³)₂(H₂O)](ClO₄)₂ (2a), of 1,10-phenanthroline furoxan. A similar complex [Cu(L³)₂Cl](ClO₄) (2) is isolated from the reaction of H₂L¹ with copper(II) chloride, triethylamine, and sodium perchlorate in acetonitrile. The two-electron oxidation of the vic-dioxime to furoxan is confirmed from the X-ray single crystal structure of 2. An intermediate species, showing an absorption band at 608 nm, is observed at −20 °C during the conversion of 1 to 2a. A similar blue intermediate is formed during the reaction of [Cu(HDMG)₂] (H₂DMG = dimethylglyoxime) with ceric ammonium nitrate, but H₂DMG treated with ceric ammonium nitrate does not form any intermediate. This suggests the involvement of a copper(II) complex in the intermediate step. The intermediate species is also observed during the two-electron oxidation of other vic-dioximes. On the basis of the spectroscopic evidence and the nature of the final products, the intermediate is proposed to be a mononuclear copper(II) complex ligated by a vic-dioxime and a dinitrosoalkene. The dinitrosoalkene is generated upon two-electron oxidation of the dioxime. The transient blue color of the dioxime-copper(II)-dinitrosoalkene complex may be attributed to the ligand-to-ligand charge transfer transition. The intermediate species slowly decays to the corresponding two-electron oxidized form of vic-dioxime, i.e. furoxan and [Cu(CH₃CN)₄](ClO₄). The formation of two isomeric furoxans derived from the reaction of an asymmetric vic-dioxime, hexane-2,3-dioxime, and copper(II) perchlorate supports the involvement of a dinitrosoalkene species in the intermediate step. In addition, the oxidation of 2,9-dimethyl-1,10-phenanthroline-5,6-dioxime (H₂L²) to the corresponding furoxan and subsequent formation of a copper(I) complex [Cu(L⁴)₂](ClO₄) (3) (where L⁴ = 2,9-dimethyl-1,10-phenanthroline furoxan) are discussed.