titanocene and ethylene

The present study characterizes changes in the electronic structure of reactants during chemical reactions based on the combined charge and energy decomposition scheme, ETS-NOCV (extended transition state-natural orbitals for chemical valence). Decomposition of the activation barrier, ΔE (#), into stabilizing (orbital interaction, ΔE (orb), and electrostatic, ΔE (elstat)) and destabilizing (Pauli repulsion, ΔE (Pauli), and geometry distortion energy, ΔE (dist)) factors is discussed in detail for the following reactions: (I) hydrogen cyanide to hydrogen isocyanide, HCN → CNH isomerization; (II) Diels-Alder cycloaddition of ethene to 1,3-butadiene; and two catalytic processes, i.e., (III) insertion of ethylene into the metal-alkyl bond using half-titanocene with phenyl-phenoxy ligand catalyst; and (IV) B-H bond activation catalyzed by an Ir-containing catalyst. Various reference states for fragments were applied in ETS-NOCV analysis. We found that NOCV-based deformation densities (Δρ (i)) and the corresponding energies ΔE (orb)(i) obtained from the ETS-NOCV scheme provide a very useful picture, both qualitatively and quantitatively, of electronic density reorganization along the considered reaction pathways. Decomposition of the barrier ΔE(#) into stabilizing and destabilizing contributions allowed us to conclude that the main factor responsible for the existence of positive values of ΔE (#) for all processes (I, II, III and IV) is Pauli interaction, which is the origin of steric repulsion. In addition, in the case of reactions II, III and IV, a significant degree of structural deformation of the reactants, as measured by the geometry distortion energy, plays an important role. Depending on the reaction type, stabilization of the transition state (relatively to the reactants) originating either from the orbital interaction term or from electrostatic attraction can be of vital importance. Finally, use of the ETS-NOCV method to describe catalytic reactions allows extraction of information on the role of catalysts in determination of ΔE (#).

Topics: Algorithms; Boranes; Butadienes; Catalysis; Computer Simulation; Cyanates; Electrons; Ethylenes; Hydrogen Cyanide; Isomerism; Models, Chemical; Models, Molecular; Molecular Conformation; Organometallic Compounds; Quantum Theory; Static Electricity; Surface Properties; Thermodynamics

Cp*TiCl(2)(L) [L = C(3)H(3)N(2) (1), 3,5-Me(2)C(3)HN(2) (2) and 3,5-(i)Pr(2)C(3)HN(2) (3)], and Cp*Ti(C(3)H(3)N(2))(3) (4) were prepared in moderate yields by treating Cp*TiCl(3) with the pyrazoles in the presence of Et(3)N or with their corresponding lithium salts in Et(2)O. The structures of 1 and 2 determined by X-ray crystallography indicate that these complexes fold a distorted tetrahedral geometry around Ti, and the pyrazolato ligands coordinate to Ti with eta(2)-N,N'-coordination mode. In contrast, one of the pyrazolato ligand in 4 coordinates to Ti with eta(1)-N-bonding, whereas the other two ligands were bound to Ti with eta(2)-N,N'-fashion. The Cp analogues, CpTiCl(2)(L) [L = C(3)H(3)N(2) (5), 3,5-Me(2)C(3)HN(2) (6), 3,5-(i)Pr(2)C(3)HN(2) (7), and 3,5-Ph(2)C(3)HN(2) (8)], were also prepared by the reaction of CpTiCl(3) with the corresponding lithium salts in Et(2)O or n-hexane. The crystallographic analyses of 5 and 6 revealed that the pyrazolato ligands coordinate to Ti with eta(2)-N,N'-coordination mode. These complexes (1-3, 5-8) exhibited moderate catalytic activities for ethylene polymerization in the presence of methylaluminoxane (MAO), and the activities were highly affected by the substituent on the pyrazolato ligand employed. Complex 1 exhibited the highest activity affording polymer with uniform molecular weight distribution, suggesting that the polymerization proceeded with uniform catalytically active species. An increase in the steric bulk in the pyrazolato ligand led to a slight decrease in the activity by the Cp* analogues, whereas the activity by the Cp analogues increased upon increasing the steric bulk in the pyrazolato ligand employed.

Topics: Catalysis; Crystallography, X-Ray; Ethylenes; Ligands; Models, Molecular; Molecular Conformation; Organometallic Compounds; Pyrazoles; Stereoisomerism