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

An activated side-on-bound ansa-zirconocene dinitrogen complex, [Me2Si(eta5-C5Me4)(eta5-C5H3-3-tBu)Zr]2(mu2,eta2,eta2-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 N2 ligand to yield one diastereomer of the hydrido zirconocene diazenido complex. Kinetic measurements have yielded the barrier for H2 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) A 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(eta5-C5Me4)(eta5-C5H3-3-tBu)ZrH]2(mu2,eta2,eta2-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.