A tetradentate N-ligand binds Cu^II in a geometric and electronic environment that resembles the properties of the Cu_B site of particulate Methane Monooxygenase (pMMO). Its Cu^I counterpart undergoes ligand oxidation during exposure to air, establishing the flexibility of the tetradentate scaffold that allows O₂ activation and subsequent formation of a cupric amidato complex. This reactivity may be relevant for a reevaluation of the active site of pMMO, in which Cu_B has been relegated due to its “saturated” coordination environment.

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- (Cu^II) or bimetallic (2Cu^I) nature of the so-called Cu_B 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 Cu_B 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 Cu_B 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 Cu_B.