"Catch and release" of the Cp^N3 ligand using cobalt: dissociation, protonation, and C-H bond thermochemistry

The coordination chemistry of an amine-rich Cp^N3 ligand has been explored with cobalt. We demonstrate that in the presence of NaCo(CO)₄, the cationic precursor [CpN3]+ yields the complex CpN3CoI(CO)2. While 2e^- oxidation generates new Co^III complexes such as [CpN3Co(NCMe)3]2+ and CpN3CoI2(CO), subsequent ligand loss is facile, generating free [CpN3]+ or the protonated dication [CpN3H]2+. We have structurally characterized both these ligand release products via single crystal X-ray diffraction and obtained thermochemical C-H bond strengths via experiment and density functional theory (DFT). Upon reversible 1e^- reduction, the radical cation [CpN3H]˙+ has a weak C-H BDFE of 52 kcal mol^-1 in acetonitrile. Mechanistic analysis shows that [CpN3H]˙+ undergoes radical-radical disproportionation in the absence of exogenous H-atom acceptors, which is supported by deuterium isotope labelling experiments. Structural comparison of these organic molecules shows a high degree of iminium-like electron delocalization over the C-N bonds connected to the central five-membered ring.