The {Bis-2,6-[1-(2-imidazol-4-ylethylimino)ethyl]pyridine}copper(I) cation. A synthetic Cu I oxygen carrier in solution as a potential model for oxyhemocyanin

Abstract

The red copper(I) title compound {[CuI(imep)]+} is apparently the first synthetic copper complex to functionally mimic hemocyanin in its capacity to reversibly bind O2 in solution under ambient conditions (1 atm O2; room temperature) with a reversibility factor of ca. 80% per oxy/deoxy cycle. Manometry has established the O2 : Cu reaction stoicheiometry to be the same (1 : 2) as for the oxygenation of hemocyanin. The green oxy form of [CuI(imep)]+ is proposed to be a µ-dioxygen species with each CuN5O centre of the binuclear unit being six-co-ordinate and having two imidazole, one pyridine, and two imine nitrogen-donor atoms. Like oxyhemocyanin, the oxy form of [CuI(imep)]+ is essentially e.s.r. silent at 100 K, indicating a fully coupled S= 0 ground state if the binuclear centre is formally viewed as [CuII–O2–2–CuII]. However, unlike hemocyanin, the [CuI(imep)]+ cation does not form an adduct with CO under ambient conditions. If the imidazole moieties are replaced by pyridine groups. [CuI(pyep)]+, the copper (I) centre no longer reversibly bind O2. This result is consistent with cyclic voltammetric data showing the CuI state for [CuI(pyep)]+ to be stabilized by 130 mV relative to that of [CuI(imep)]+. Cyclic voltammetry has also been used as a probe for observing and following what is presumed to be the reversible oxygenation process of [CuI(imep)]+ and to demonstrate the lack of reactivity with CO. Finally, the [M(imep)][ClO4]2 and [M(pyep)][ClO4]2(M = CuII or ZnII) compounds have been isolated and characterized for purposes of comparison with their CuI analogue.