On the origin of dinitrogen hydrogenation promoted by [(η5-C5Me4H)2Zr]2(μ2,η2,η2-N2)

The origin of the hydrogenation of the dinitrogen ligand in [(η5-C5Me4H)2Zr]2(μ2,η2,η2-N2) has been investigated by a combined computational and experimental study. Density functional theory calculations on the zirconocene dinitrogen complex demonstrate significant imido character in the zirconium nitrogen bonds, arising from effective π-back-bonding from the low-valent zirconium and the side-on bound N2 ligand. The twisted ground-state structure of the N2 complex is a key requirement for nitrogen hydrogenation, as calculations on the model complex [(η5-C5H5)2Zr]2(μ2,η2,η2-N2) reveal reduced overlap as the dihedral angle between the zirconocene wedges approaches 0°. Experimentally, isotopic labeling studies on the microscopic reverse are consistent with a 1,2-addition mechanism for nitrogen hydrogenation. Copyright © 2004 American Chemical Society.