A Stable Tetraazafulvalene

Abstract

A genuine double bond joins the two imidazol-2-ylidene halves in compound 1, which was characterized by both X-ray crystallography and NMR spectroscopy. The dissociation of 1 to 1′ was not observed, even though the strength of the CC bond is estimated to be only a few kcalmol−1. This is in accordance with theoretical predictions based on the very large singlet–triplet energy difference for the imidazol-2-ylidenes. Dissociation of the unbridged, singly bridged, or larger ring doubly bridged tetraazafulvalenes is therefore an entropy-driven process.