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

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journal contribution
posted on 16.12.2019 by Yan Li, Suhashini Handunneththige, Erik R. Farquhar, Yisong Guo, Marat R. Talipov, Feifei Li, Dong Wang
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.