Hydroxo-Bridged Dicopper(II,III) and -(III,III) Complexes: Models for Putative Intermediates in Oxidation Catalysis

Halvagar, M. R.; Solntsev, P. V.; Lim, H.; Hedman, B.; Hodgson, K. O.; Solomon, E. I.; Cramer, C. J.; Tolman, W. B.
J. Am. Chem. Soc. 2014, 9, 7269 (doi:10.1021/ja503629r).

A macrocyclic ligand (L^4–) comprising two pyridine(dicaroxamide) donors was used to target reactive copper species relevant to proposed intermediates in catalytic hydrocarbon oxidations by particulate methane monooxygenase and heterogeneous zeolite systems. Treatment of LH₄ with base and Cu(OAc)₂•H₂O yielded (Me₄N)₂[L₂Cu₄(μ₄-O)] (1) or (Me₄N)[LCu₂(μ-OH)] (2), depending on conditions. Complex 2 was found to undergo two reversible 1-electron oxidations via cyclic voltammetry and low temperature chemical reactions. On the basis of spectroscopy and theory, the oxidation products were identified as novel hydroxo-bridged mixed-valent Cu(II)Cu(III) and symmetric Cu(III)₂ species, respectively, that provide the first precedence for such moieties as oxidation catalysis intermediates.