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

Abstract

A macrocyclic ligand (L^4–) comprising two pyridine­(dicarboxamide) 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)] (<b>1</b>) or (Me₄N)­[LCu₂(μ-OH)] (<b>2</b>), depending on conditions. Complex <b>2</b> 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