N−C Bond Formation Promoted by a Hafnocene Dinitrogen Complex:  Comparison of Zirconium and Hafnium Congeners

Nitrogen−carbon bond formation from coordinated dinitrogen has been observed upon addition of 2 equiv of PhNCO to the hafnocene dinitrogen complex, [(η⁵-C₅Me₄H)₂Hf]₂(μ₂,η²,η²-N₂). The resulting product most likely arises from initial NC cycloaddition of the first equivalent of heterocumulene, followed by carbonyl insertion of a second equivalent into the newly formed hafnium−nitrogen bond. The resulting product has considerable hafnium imido character, as evidenced by the metrical parameters determined from the solid-state structure as well as reactivity studies, whereby PhNCO, p-tolyl isocyanate, and t-BuC⋮CH each undergo cycloaddition with the hafnium−nitrogen bond. The origin of the nitrogen−carbon-forming chemistry is likely derived from the reluctance of the hafnocene dinitrogen complex to undergo ligand-induced N₂ isomerization, as was observed with the zirconium congener.