Reactions of Titanium- and Zirconium-Based 1-Metallacyclobuta-2,3-diene Complexes with Diazenes: N═N Bond Cleavage versus N-C Bond Formation and Dearomatization

The reactions of two [rac-(ebthi)Ti]- and [rac-(ebi)Zr]-based 1-metallacyclobuta-2,3-diene complexes with diazenes are described. With azobenzene, cleavage of the N═N bond is observed in the case of Zr, leading to the formation of dinuclear Zr(IV) imido complex 1. In contrast, the insertion of azobenzene into the Ti-C bond of the titanacycle produces six-membered aza-metallacycle 6, which could only be characterized by nuclear magnetic resonance spectroscopy. With geometrically restricted benzo[c]cinnoline, activation and dearomatization of the phenyl group of the substrate occur, leading to the formation of unusual mononuclear [M = Zr (3)] and dinuclear complexes [M = Ti (7)]. Reaction mechanisms leading to the formation of these unusual complexes are proposed on the basis of control experiments.