Reversible Reaction of O₂ (and CO) with a Copper(I) Complex. X-ray Structures of Relevant Mononuclear Cu(I) Precursor Adducts and the trans-(µ-1,2-Peroxo)dicopper(II) Product

The study of copper-dioxygen interactions and reactivity is of interest in chemical systems which involve redox activity or oxidative transformations and in biological systems where copper proteins are known to effect a variety of functions including dioxygen transport, substrate oxygenation, and O₂ reduction. Here, we describe reactions of O₂ (and CO) with a mononuclear copper(I) complex containing a tripodal tetradentate ligand L (tris(2-pyridylmethyl)- amine). Cationic copper(I) complexes [LCu(D)]⁺ (D = RCN (1a–c), CO (1d, ν_CO = 2075 cm⁻¹), or PPh₃ (1e)) can be isolated as stable compounds, and the X-ray structures of [LCu(PPh3)]⁺ (1e) and [L′Cu(CH₃CN)]⁺ (1′, L′ = bis(2-pyridylmethyl)-(5-carbomethoxy-2-pyridylmethyl)amine) are reported. Compound 1e exhibits a pseudotetrahedral tetracoordination with ligation from two pyridines plus alkylamine and phosphine donors; the third pyridine donor is uncoordinated. Complex 1′ possesses a distorted pentacoordinate structure, resembling more closely that found for typical Cu(II) complexes with L, i.e., with a trigonal bipyramidal geometry. [LCu(MeCN)]⁺ (la) reacts to give the dioxygen complex [(LCu)₂(O₂)]²⁺ (2) which is stable at −80 °C in EtCN or CH₂Cl₂ solution [λ_max = 525 nm (ϵ = 11 500 M⁻¹ cm⁻¹)]. The binding of O₂ and CO to 1a is reversible, and UV-vis spectroscopy is used to demonstrate cycling between la and 2 (vacuum cycling). Thermally unstable solids [(LCu)₂(O₂)]²⁺ (2) were isolated as or salts, and crystals suitable for X-ray diffraction were obtained. [(LCu)₂(O₂)]²⁺ (2) stands as the first copperdioxygen complex to be structurally characterized, and full details are reported and compared to structurally related peroxodicobalt(III) complexes also possessing a trans-(µ.-1,2-peroxo) coordination. Notable features include Cu⋯Cu = 4.359(1) Å, and the O-O bond length is 1.432(6) Å, a typical peroxide bond distance. [(LCu)₂(O₂)]²⁺ (2) exhibits very normal looking ¹H and ¹³C NMR spectra, indicative of strong magnetic coupling between Cu(II) ions. This and other properties are compared to those of oxyhemocyanin (O₂ carrier) and other Cu₂O₂ complexes.