Published: October 21, 2011 r2011 American Chemical Society 11375 dx.doi.org/10.1021/ic200979q | Inorg. Chem. 2011, 50, 11375–11383 ARTICLE pubs.acs.org/IC Copper(II)-Mediated Oxidative Transformation of vic-Dioxime to Furoxan: Evidence for a Copper(II)-Dinitrosoalkene Intermediate Oindrila Das, † Sayantan Paria, † Ennio Zangrando, ‡ and Tapan Kanti Paine †, * † Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata-700032, India ‡ Dipartimento di Scienze Chimiche e Farmaceutiche, University of Trieste, Via Licio Giorgieri 1, 34127 Trieste, Italy b S Supporting Information ’ INTRODUCTION Furoxans (1,2,5-oxadiazole-2-oxides) and benzofuroxans are important heterocycles for their potential NO donor 1 activity with wide application in biological, medicinal, and synthetic chemistry. 110 The five-membered heterocyclic rings are formed by two-electron oxidative cyclization of vic-dioximes. A large number of oxidizing agents like hypohalite, ferricyanide, ceric ions, nitrogen oxides, manganese dioxide, and lead tetraacetate have been used for oxidative transformation of vic-dioximes. 11,12 Copper(II) perchlorate is known as a strong oxidizing agent with the redox potential of Cu 2+ /Cu + in acetonitrile being 0.952 V versus SCE. 13 The strong oxidizing nature of copper(II) per- chlorate in acetonitrile has recently been documented in the oxidation of aromatic amines to generate amine radical cations. 1416 We have reported on the oxidation of vic-dioximes to the corresponding furoxans in high yield at ambient conditions mediated by copper(II) ions. 17 As a natural progression of our earlier work we have started investigating the mechanism of two-electron oxidation of vic- dioximes to furoxans. There are few examples where the involve- ment of iminoxyl radical species has been suggested during the one-electron oxidation of monoximes. 1820 Low-temperature EPR study was reported for iminoxyl radical generated upon chemical oxidation of monooxime. 2124 In the electrochemical oxidation of vic-dioximes, a two-electron oxidized species, for example oxoimmonium cation via iminoxyl radical has been proposed that lead to the cyclized product. 25 Dinitrosoalkenes have been proposed as short-lived intermediates for the isomer- ization of unsymmetrical furoxans. 2628 The existence of an equilibrium between a furoxan ring and the corresponding dinitrosoalkene species has been established by trapping the latter with diene in a DielsAlder reaction 29 and also by theoretical calculations. 30,31 Direct evidence for the formation of dinitroso intermediate is available from IR and UV spectro- scopic studies of photolysis product of either benzofuroxan or o- nitrophenyl azide in Ar matrices at 1214 K. 26,29,32,33 In course of investigating the mechanism of copper(II)-mediated oxidation reaction of dioxime to furoxan, we were interested to see if such Received: May 10, 2011 ABSTRACT: The mononuclear copper(II) complex [Cu(H 2 L 1 ) 2 - (H 2 O)](ClO 4 ) 2 (1) (where H 2 L 1 = 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(L 3 ) 2 (H 2 O)](ClO 4 ) 2 (2a), of 1,10- phenanthroline furoxan. A similar complex [Cu(L 3 ) 2 Cl](ClO 4 ) (2) is isolated from the reaction of H 2 L 1 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 ] (H 2 DMG = dimethylglyoxime) with ceric ammonium nitrate, but H 2 DMG 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(CH 3 CN) 4 ](ClO 4 ). 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 (H 2 L 2 ) to the corresponding furoxan and subsequent formation of a copper(I) complex [Cu(L 4 ) 2 ](ClO 4 ) (3) (where L 4 = 2,9-dimethyl-1,10-phenanthroline furoxan) are discussed.