Activation and cleavage of the N-N bond in side-on bound [L2M-NN-ML2] (L = NH2, NMe2, (NPr2)-Pr-i, C5H5, C5Me4H) dinitrogen complexes of transition metals from groups 4 through 9

The activation and cleavage of the N-N bond in side-on bound [L2M-NN-ML2] (L = NH2, NMe2, (NPr2)-Pr-i, C5H5, C5Me4H) dinitrogen complexes of transition metals in groups 4 through 9 have been investigated using density functional theory. Emphasis has been placed on Ti, Zr, and Hf (group 4) complexes due to their experimental relevance. Calculations on these species have shown that for cases when the structural configuration corresponds to the terminal [ML2] fragments adopting a perpendicular orientation with respect to the central [N-N] unit, a considerably higher degree of N-N activation is predicted relative to that observed in the experimentally characterized cyclopentadienyl analogues and in related systems involving end-on dinitrogen coordination. An examination of the orbital interactions between the metal-based fragments and the dinitrogen unit shows that both sigma and pi bonding are important in the side-on binding mode, in contrast to the end-on mode where metal-nitrogen p interactions are dominant. This analysis also reveals that the model amide systems possess the orbital properties identied as necessary for successful N-N hydrogenation. A significant result obtained for the amide complexes containing metals from groups 5 (V, Nb, Ta), 6 (Cr, Mo, W), and 7 (Mn, Tc, Re), is the presence of metal-metal bonding in configurations that are considerably distorted from planarity. As a consequence, these complexes exhibit strongly enhanced stability relative to species where metal-metal bonding is absent. In contrast, the d(2) metal-based configurations in the group 4 complexes of Ti, Zr, and Hf are unable to provide the six electrons required for complete reductive cleavage of the dinitrogen unit which is necessary to allow the metal centres to approach one another sufficiently for metal-metal bond formation.