Out of cross-conjugation: the unexpected structure of tetrazinones

Abstract

Detailed structural and computational investigations have been carried out to elucidate the unprecedented structural properties of NHC-derived tetrazinones. The provocative structural contradictions featured by this class of molecules as emerged from single crystal X-ray diffraction studies include the non-coplanar core ring system with an inter-ring C–C single bond indicating aromaticity but a pattern of bond alteration consistent with an overall quinoidal structure. Combination of periodic and gas-phase calculations identified two key factors affecting the overall structure. The first is a strong tendency to avoid cross-conjugation resulting in a coupled aromatic–quinoidal system whereas the second is the steric demand of the substituents determining the conformational behaviour of the rings. The peculiar electron distribution yields remarkably large polarizations and dipole moments for these molecules which can be key in their potential applications.