Mechanistic Aspects of Dinitrogen Hydrogenation Catalyzed by the Geometry-Constrained Zirconium and Titanium Complexes:Computational Studies

摘要

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(l) and Me(2)) and Zr (with R=H(3),Me(4),and ~tBu(5)) coordinate the N2 molecule in a side-on-manner and form highly stable (relative to the dissociation limit of 2n+N2) n-(mu_2,eta~2,eta~2-N2)-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 N2 molecule:they possess appropriate frontier orbitals to add an H2 molecule.Calculations show that n-(mu_2,eta~2,eta~2-N2)-n complexes add an H2 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(M2,eta~2,eta~2-N2) (where [P2N2]=PhP(CH2SiMe2NSiMe2CH2)2PPh) and {(C5Me4H)2Zr}2(mu_2,eta~2,eta~2-N2) show that the proposed complexes 1,3,and 4 will exhibit similar or better reactivity toward the H2 molecule than the latter complexes.