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 (C 5 H 4 SiH 2 NR)M, where M = Ti (with R = H( 1) and Me (2 )) and Zr (with R= H( 3), Me( 4 ), and t Bu( 5 )) coordinate the N 2 molecule in a side-on manner and form highly stable (relative to the dissociation limit of 2 n + N 2 ) n -(μ 2,η 2,η 2 -N 2 )- n dimer (where n = 1 − 5 ), and satisfy the first necessary condition of hydrogenation of the coordinated N 2 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 -(μ 2,η 2,η 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 {[P 2 N 2 ]Zr} 2 (μ 2,η 2,η 2 -N 2 ) (where [P 2 N 2 ] = PhP(CH 2 SiMe 2 NSiMe 2 CH 2 ) 2 PPh) and {(C 5 Me 4 H) 2 Zr} 2 (μ 2,η 2,η 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.