Isolation of a Bimetallic Cobalt(III) Nitride and Examination of Its Hydrogen Atom Abstraction Chemistry and Reactivity toward H ₂

Abstract

Room temperature photolysis of the bis(azide)cobaltate(II) complex [Na(THF)_<i>x</i>][(^ketguan)Co(N₃)₂] (^ketguan = [(<i>t</i>Bu₂CN)C(NDipp)₂]^-, Dipp = 2,6-diisopropylphenyl) (<b>3a</b>) in THF cleanly forms the binuclear cobalt nitride Na(THF)₄{[(^ketguan)Co(N₃)]₂(μ-N)} (<b>1</b>). Compound <b>1</b> 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 Co^III═N═Co^III 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 <b>1</b>. Indeed, <b>1</b> is unreactive toward weak C-H donors and even cocrystallizes with a molecule of cyclohexadiene (CHD) in its crystallographic unit cell to give <b>1</b>·CHD as a room temperature stable product. Notably, addition of pyridine to <b>1</b> or photolyzed solutions of [(^ketguan)Co(N₃)(py)]₂ (<b>4a</b>) 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) (<b>5</b>) formed from intermolecular hydrogen atom abstraction (HAA) of strong C-H bonds (BDE ∼ 100 kcal/mol). Kinetic rate analysis of the formation of <b>5</b> 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 <b>4a</b> under H₂ and D₂ atmospheres (150 psi), which leads to the exclusive formation of the bis(imido) complexes [(^ketguan)Co(μ-NH)]₂ (<b>6</b>) and [(^ketguan)Co(μ-ND)]₂ (<b>6-D</b>), 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.