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

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. A tetradentate N‐ligand binds CuII in a geometric and electronic environment that resembles the properties of the CuB site of particulate Methane Monooxygenase (pMMO). Its CuI 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 CuB has been relegated due to its “saturated” coordination environment.