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Isolation of a Bimetallic Cobalt(III) Nitride and Examination of Its Hydrogen Atom Abstraction Chemistry and Reactivity toward H2

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posted on 23.04.2020, 15:37 by Debabrata Sengupta, Christian Sandoval-Pauker, Emily Schueller, Angela M. Encerrado-Manriquez, Alejandro Metta-Magaña, Wen-Yee Lee, Ram Seshadri, Balazs Pinter, Skye Fortier
Room temperature photolysis of the bis­(azide)­cobaltate­(II) complex [Na­(THF)x]­[(ketguan)­Co­(N3)2] (ketguan = [(tBu2CN)­C­(NDipp)2], Dipp = 2,6-diisopropylphenyl) (3a) in THF cleanly forms the binuclear cobalt nitride Na­(THF)4{[(ketguan)­Co­(N3)]2(μ-N)} (1). Compound 1 represents the first example of an isolable, bimetallic cobalt nitride complex, and it has been fully characterized by spectroscopic, magnetic, and computational analyses. Density functional theory supports a CoIIINCoIII canonical form with significant π-bonding between the cobalt centers and the nitride atom. Unlike other group 9 bridging nitride complexes, no radical character is detected at the bridging N atom of 1. Indeed, 1 is unreactive toward weak C–H donors and even cocrystallizes with a molecule of cyclohexadiene (CHD) in its crystallographic unit cell to give 1·CHD as a room temperature stable product. Notably, addition of pyridine to 1 or photolyzed solutions of [(ketguan)­Co­(N3)­(py)]2 (4a) leads to destabilization via activation of the nitride unit, resulting in the mixed-valent Co­(II)/Co­(III) bridged imido species [(ketguan)­Co­(py)]­[(ketguan)­Co]­(μ-NH)­(μ-N3) (5) formed from intermolecular hydrogen atom abstraction (HAA) of strong C–H bonds (BDE ∼ 100 kcal/mol). Kinetic rate analysis of the formation of 5 in the presence of C6H12 or C6D12 gives a KIE = 2.5 ± 0.1, supportive of a HAA formation pathway. The reactivity of our system was further probed by photolyzing benzene/pyridine solutions of 4a under H2 and D2 atmospheres (150 psi), which leads to the exclusive formation of the bis­(imido) complexes [(ketguan)­Co­(μ-NH)]2 (6) and [(ketguan)­Co­(μ-ND)]2 (6-D), respectively, as a result of dihydrogen activation. These results provide unique insights into the chemistry and electronic structure of late 3d metal nitrides while providing entryway into C–H activation pathways.