Stable Hydrazyls and Push–Pull (Capto-Dative) Aminyl Free Radicals

Electron paramagnetic resonance spectroscopy is at present the preferred method for studying free radicals. Push–pull aminyls show exceptional stability, which is explained satisfactorily by Linnett's double-quartet theory in terms of electronic spin. Stable hydrazyls such as 2,2-diphenyl-1-picrylhydrazyl and 2,2-diphenyl-1-cyanohydrazyl can actually be viewed as push–pull aminyls. A consequence of Linnett's theory is the increased bond order for the NN bond in hydrazyls, which is confirmed experimentally by X-ray crystallography and the high intramolecular rotation barrier. For aminyls and hydrazyls whose nitrogen center is not part of a ring, this survey lists the variety of electron donors and electron acceptors known so far. Keywords: aminyl and hydrazyl stable free radicals; nitrogen-centered persistent free radicals; push–pull stabilization; capto-dative stabilization; merostabilization; electron paramagnetic resonance spectra