Highly Reactive CoIII,IV2(µ-O)2 Diamond Core Complex That Cleaves C-H Bonds.

ABSTRACT

The selective activation of strong sp3 C-H bonds at mild conditions is a key step in many biological and synthetic transformations and an unsolved challenge for synthetic chemists. In nature, soluble methane monooxygenase (sMMO) is one representative example of nonheme dinuclear iron-dependent enzymes that activate strong sp3 C-H bonds by a high-valent diiron(IV) intermediate Q. To date, synthetic model complexes of sMMO-Q have shown limited abilities to oxidize strong C-H bonds. In this work, we generated a high-valent CoIII,IV2(µ-O)2 complex 3 supported by a tetradentate tris(2-pyridylmethyl)amine (TPA) ligand via one-electron oxidation of its CoIII2(µ-O)2 precursor 2. Characterization of 2 and 3 using X-ray absorption spectroscopy and DFT calculations showed that both species possess a diamond core structure with a short Co···Co distance of 2.78 Å. Furthermore, 3 is an EPR active species showing an S = 1/2 signal with clearly observable hyperfine splittings originated from the coupling of the 59Co nuclear spin with the electronic spin. Importantly, 3 is a highly reactive oxidant for sp3 C-H bonds, and an oxygenation reagent. 3 has the highest rate constant (1.5 M-1 s-1 at -60 °C) for oxidizing 9,10-dihydroanthracene (DHA) compared to diamond core complexes of other first-row transition metals including Mn, Fe and Cu reported previously. Specifically, 3 is about 4-5 orders of magnitude more reactive than the diiron analogs FeIII,IV2(µ-O)2 and FeIV2(µ-O)2 supported by TPA and related ligands. These findings shed light on future development of more reactive approaches for C-H bond activation by bioinspired dicobalt complexes.