REVERSIBLE-REACTION OF O2 (AND CO) WITH A COPPER(I) COMPLEX - X-RAY STRUCTURES OF RELEVANT MONONUCLEAR CU(I) PRECURSOR ADDUCTS AND THE TRANS-(MU-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 (la-c), CO (1d, nu(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)]+ (1a) reacts to give the dioxygen complex [(LCu)2(O2)]2+ (2) which is stable at -80-degrees-C in EtCN or CH2Cl2 solution [lambda(max) = 525 nm (epsilon = 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 PF6- or ClO4- salts, and crystals suitable for X-ray diffraction were obtained. [(LCu)2(O2)]2+ (2) stands as the first copper-dioxygen complex to be structurally characterized, and full details are reported and compared to structurally related peroxodicobalt(III) complexes also possessing a trans-(mu-1,2-peroxo) coordination. Notable features include Cu...Cu = 4.359(l) angstrom, and the O-O bond length is 1.432(6) angstrom, a typical peroxide bond distance. [(LCu)2(O2)]2+ (2) exhibits very normal looking H-1 and C-13 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.