Diazene dehydrogenation follows H₂ addition to coordinated dinitrogen in an ansa-zirconocene complex

An activated side-on-bound ansa-zirconocene dinitrogen complex, [Me ₂Si(η⁵-C₅Me₄)(η⁵- C₅H₃-3-^tBu)Zr]₂(μ₂, η²,η²-N₂), has been prepared by sodium amalgam reduction of the corresponding dichloride precursor under an atmosphere of N₂. Both solution spectroscopic and X-ray diffraction data establish diastereoselective formation of the syn homochiral dizirconium dimer. Addition of 1 atm of H₂ resulted in rapid hydrogenation of the N ₂ ligand to yield one diastereomer of the hydrido zirconocene diazenido complex. Kinetic measurements have yielded the barrier for H ₂ addition and in combination with isotopic labeling studies are consistent with a 1,2-addition pathway. In the absence of H₂, the hydrido zirconocene diazenido product undergoes swift diazene dehydrogenation to yield an unusual hydrido zirconocene dinitrogen complex. The N=N bond length of 1.253(5) Å determined by X-ray crystallography indicates that the side-on-bound N₂ ligand is best described as a two-electron reduced [N₂]^2- fragment. Comparing the barrier for deuterium exchange with [Me₂Si(η⁵-C₅Me ₄)(η⁵-C₅H₃-3- ^tBu)ZrH]₂(μ₂,η², η²-N₂H₂) to diazene dehydrogenation is consistent with rapid 1,2-elimination of dihydrogen followed by rate-determining hydride migration to the zirconium. This mechanistic proposal is also corroborated by H₂ inhibition and the observation of a normal, primary kinetic isotope effect for dehydrogenation.