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.