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

A macrocyclic
ligand (L4–) 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)] (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.