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.