Opening the Co(μ-O) Diamond Core by Lewis Bases Leads to Enhanced C-H Bond Cleaving Reactivity.

The high-valent diiron(IV) intermediate is the key oxidant that cleaves strong C-H bonds of methane in the catalytic cycle of soluble methane monooxygenase (sMMO). sMMO- was previously reported as a bis-μ-oxo Fe(μ-O) diamond core but was recently described to have an open core with a long Fe···Fe distance. We recently reported a high-valent Co(μ-O) diamond core complex () that is highly reactive with sp C-H bonds. In this work, we demonstrated that the C-H bond cleaving reactivity of can be further enhanced by introducing a Lewis base X, affording faster kinetic rate constants and the ability to cleave stronger C-H bonds compared to . We proposed that first reacts with X in a fast equilibrium to form an open core species X-Co-O-Co-O (). We were able to characterize using EPR spectroscopy and DFT calculations. exhibited an = 1/2 EPR signal distinct from that of the parent complex . DFT calculations showed that has an open core with the spin density heavily delocalized in the Co-O unit. Moreover, has a more favorable thermodynamic driving force and a smaller activation barrier than to carry out C-H bond activation reactions. Notably, is at least 4 orders of magnitude more reactive than its diiron open core analogues. Our findings indicate that the diamond core isomerization is likely a practical enzymatic strategy to unmask the strong oxidizing power of sMMO- necessary to attack the highly inert C-H bonds of methane.