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, νCO = 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 [λmax = 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry