The C–H bond activation in 1-ethyl-3-methylimidazolium acetate–copper(ii) acetate–water–air (dioxygen) systems

Abstract

Ionic liquid (1-ethyl-3-methylimidazolium acetate, [C₂C₁im][AcO])–copper(II) diacetate monohydrate–water–air (O₂) systems have been investigated by ¹³C NMR, EPR, spectrophotometry, HPLC, and synthetic chemistry methods at different temperatures. The C–H bond activation of [C₂C₁im]⁺ with the formation of the unusual dication 1,1′-diethyl-3,3′-dimethyl-2,2′-biimidazolium ([(C₂C₁im)₂]²⁺) at 50 °C and 1-ethyl-3-methyl-1H-imidazol-2(3H)-one (C₂C₁imO) at 50–85 °C was revealed. Two new complexes with the above compounds, [(C₂C₁im)₂][Cu(AcO)₄] and Cu₂(AcO)₄(C₂C₁imO)₂, were isolated from the systems and characterized by X-ray structural analysis. Catalytic cycles with the participation of copper(II) acetate and dioxygen and the production of [(C₂C₁im)₂]²⁺ and C₂C₁imO have been proposed. The catalysis presumably includes the formation of the Cu^II(O₂)Cu^II active centre with μ–η²:η²-peroxide bridging in analogy with tyrosinase and catechol oxidase activity.