σ-Resonance Stabilization of Aminomethylene by N-Halogenation

Amino substitution effectively stabilizes carbenes via π-resonance, enabling their isolation as N-heterocyclic carbenes (NHCs). Our group has recently demonstrated that N-halogenation of pyridinylidenes (Hammick intermediates) provides an additional stabilization mode through a rarely utilized σ-resonance. However, this σ-resonance interaction might not yet have reached its full potential due to the constriction of the carbene motif within a cyclic structure. Thus, we have studied the impact of N-halogenation (X = Br, I) on the structure, energetics, and spectroscopic properties of the parent open-chain aminocarbene, aminomethylene, utilizing fully unconstrained CCSD(T) and NEVPT2 geometry optimizations together with a def2-TZVPP basis set. We find that the halogenated carbenes prefer a Z configuration of the C-N bond, which is impossible in a cyclic arrangement. Halogenated aminomethylenes are kinetically protected against unimolecular rearrangement by barriers exceeding 35 kcal mol^-1. Short C-N bond lengths (1.23 Å) and wide carbene angles (~125°) indicate cooperative π- and σ-resonance stabilization, resulting in carbene stabilization energies (CSEs) increased by ~12-13 kcal mol^-1 relative to the parent aminomethylene. The electronic structure is characterized by the σ* orbital of the N-X bond as the lowest unoccupied molecular orbital.