Stable Cobalt Organoazide Complexes and Their Reactivity in Stoichiometric and Catalytic C–H Amination

Catalytic C–H amination using organic azides is attractive for C–N bond formation. Here, Co(HMDS)2 has been investigated as a simple yet appealing catalyst for this reaction. Upon exposure to organoazides, Co(HMDS)2 forms an organoazide adduct that was characterized spectroscopically and crystallographically. With adamantyl-N3 as an organoazide that cannot undergo intramolecular C–H bond amination, gradual interligand C–H bond activation occurred without any spectroscopically detectable intermediate. Photocrystallography uncovered the imido intermediate unambiguously in the solid state. Using (4-azido-4-methylpentyl)-benzene as a substrate induced intramolecular C–H amination and formation of a pyrrolidine adduct. Under catalytic conditions, turnover frequencies (TOFs) up to 300 h–1 were reached, three times higher than that of the fastest iron catalyst known to date. However, turnover numbers (TONs) stalled at a mediocre 35, which was attributed to catalyst decomposition to a tetrazido complex according to spiking experiments. Pyridine as a solvent mitigated catalyst decomposition by competitive bonding to the active site and gave ∼90 TON. DFT calculations suggest that the high TOF originates from a lower barrier of N2 loss compared to iron (29.1 kcal/mol vs 31.7 kcal/mol). Moreover, the catalytically competent species was predicted to be the bis(pyrrolidyl) cobalt complex, generated after two turnovers by H(HMDS) release.