Mechanistic aspects of dinitrogen hydrogenation catalyzed by the geometry-constrained zirconium and titanium complexes: Computational studies

We applied the density functional theory to propose a new class of compounds affording hydrogenation of the coordinated dinitrogen molecule under mild conditions. These calculations have demonstrated that geometry-constrained complexes with a formula Of (C5H4SiH2NR)M, where M = Ti (with R = H(1) and Me(2)) and Zr (with R= H(3), Me(4), and Bu-t(5)) coordinate the N-2 molecule in a side-on manner and form highly stable (relative to the dissociation limit of 2n + N-2) n-(mu(2),eta(2),eta(2)-N-2)-n dimer (where n = 1-5), and satisfy the first necessary condition of hydrogenation of the coordinated N2 molecule. These complexes also satisfy the second necessary condition for the successful hydrogenation of the side-on coordinated N-2 molecule: they possess appropriate frontier orbitals to add an H-2 molecule. Calculations show that n-(mu(2),eta(2),eta(2)-N-2)-n complexes add an H-2 molecule with a reasonable barrier (ca. 20, 14-16, and 15-18 kcal/mol for n = 1, 3, and 4, respectively); Zr complex 3, is expected to be more reactive than its Ti analogue 1. Replacement of R = H in 3 with R= Me (i.e., going from n = 3 to n 4) has no significant effect on the calculated H-H addition barriers. The comparison of the calculated H-H addition barriers of these species with those (18-20 and 24 kcal/mol barriers) for experimentally and computationally well-studied dizirconium complexes {[P2N2]Zr}(2)(mu(2),eta(2),eta(2)-N-2) (where [P2N2] = PhP(CH2SiMe2NSiMe2CH2)(2)PPh) and {(C5Me4H)(2)Zr}(2)(mu(2),eta(2),eta(2)-N-2) show that the proposed complexes 1, 3, and 4 will exhibit similar or better reactivity toward the H-2 molecule than the latter complexes.