Monocopper model of CuB site of pMMO in N4-environment oxidizes C−H bonds

Abstract

Discrepancies regarding the coordination environment, donor atoms, nuclearity, and oxidation state of the active site of particulate methane monooxygenase (pMMO), a copper-dependent enzyme capable of activating the strong C−H bond of methane, persist despite numerous structural and spectroscopic studies. To address the proposed mono- (CuII) or bimetallic (2CuI) nature of the so-called CuB site, we report the bis(benzimidazole)-based NMe−N,N’-(1-Me-2-CH2C7H4N2)2C6H4 ligand (N4) and its copper complexes. In the solid state [Cu(N4)(ClO4)]ClO4 features tetragonal geometry defined by the chelating ligand and an axial perchlorate; geometric and EPR parameters are very close to those reported for the CuB site. Attempts to obtain a dicopper(I) analog resulted in [Cu(N4)][CuCl2], based on spectroscopic, electrochemical, and ESI-MS data. Although these results support the assignment of CuB as a monometallic site, air exposure of [Cu(N4)][CuCl2] leads to ligand oxidation in the structurally characterized [Cu(N4=O)Cl], raising the possibility of distorted tetragonal Cu(I) centers activating O2 and oxidizing substrates, in C−H activation chemistry that may take place at CuB.