“Catch and release” of the CpN3 ligand using cobalt: dissociation, protonation, and C–H bond thermochemistry

Abstract

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 [Cp^N3]⁺ yields the complex Cp^N3Co^I(CO)₂. While 2e⁻ oxidation generates new Co^III complexes such as [Cp^N3Co(NCMe)₃]²⁺ and Cp^N3CoI₂(CO), subsequent ligand loss is facile, generating free [Cp^N3]⁺ or the protonated dication [Cp^N3H]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 [Cp^N3H]˙⁺ has a weak C–H BDFE of 52 kcal mol⁻¹ in acetonitrile. Mechanistic analysis shows that [Cp^N3H]˙⁺ 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.