Opening the CoIII,IV2(μ-O)2 Diamond Core by Lewis Bases Leads to Enhanced C-H Bond Cleaving Reactivity

The high-valent diiron(IV) intermediate Q is the key oxidant that cleaves strong C–H bonds of methane in the catalytic cycle of soluble methane monooxygenase (sMMO). sMMO- Q was previously reported as a bis-μ-oxo Fe IV 2 (μ-O) 2 diamond core but recently described to have an open core with a long Fe•••Fe distance. We recently reported a high-valent Co III,IV 2 (μ-O) 2 diamond core complex ( 1 ) that is highly reactive with sp 3 C–H bonds. In this work, we demonstrated that the C–H bond cleaving reactivity of 1 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 1 . We proposed that 1 first reacts with X in a fast equilibrium to form an open core species X–Co III –O–Co IV –O ( 1-X ). We were able to characterize 1-X using EPR spectroscopy and DFT calculations. 1-X exhibited an S = 1/2 EPR signal distinct from that of the parent complex 1 . DFT calculations showed that 1-X has an open core with the spin density heavily delocalized in the Co IV –O unit. Moreover, 1-X has a more favorable thermodynamic driving force and a smaller activation barrier than 1 to carry out C–H bond activation reactions. Notably, 1-X is at least four orders of magnitude more reactive than its diiron open core analogs. Our findings indicate that the diamond core isomerization is likely a practical enzymatic strategy to unmask the strong oxidizing power of sMMO- Q necessary to attack the highly inert C–H bonds of methane.