Energetic, structural, and dynamic aspects of ethylene polymerization mediated by homogeneous single-site "constrained geometry catalysts" in the presence of cocatalyst and solvation:: An investigation at the ab initio quantum chemical level

Mechanistic aspects of the ethylene insertion pathway into the Ti-methyl bond of the [H2Si(C5H4)((BuN)-Bu-t)]TiCH3+H3CB(C6F5)(3)(-) ion pair have been analyzed at the ab initio level, employing a double-zeta quality basis set and second-order perturbative Moller-Plesset (MP2) and by hybrid density functional B3LYP methods, including solvation effects and thermal and pressure corrections to 298 K/1.0 atm. Three basic reaction pathways are identified as viable. Ethylene approach from the side opposite the H3CB(C6F5)(3)(-) counteranion is energetically most favored and occurs in a concerted (intermediateless) fashion. The other two channels involving olefin approach on the same side as the H3CB(C6F5)3- group are energetically similar, and each occurs via two discrete steps: W anion displacement with formation of an intermediate T-ethylene complex, (ii) ethylene insertion into the Ti-C bond (the slow step). These latter pathways are more strongly solvent-assisted because of the larger attendant ion pair separation. Structural and energetic analysis of the [H2Si(C5H4)((BuN)-Bu-t)]-Ti(n-C3H7)+H3CB(C6F5)(3)(-) insertion product shows the existence of several stable conformations. All such structures can be classified into two different types depending on the [H2Si(C5H4)((BuN)-Bu-t)]Ti(n-C3H7)+...H3CB(C6F5)(3)(-) contact distance. Structures with short Ti+...[H3CB(C6F5)(3)](-) contacts involve metal coordinative saturation by the CH3- group or a counteranion aryl F. For structures with longer Ti+...[H3CB(C6F5)(3)](-) contacts, the counter-anion remains out of the metal coordination sphere and agostic interactions occur between the n-propyl chain and Ti. The relative stabilities of these classes of structures are an index of the preferred olefin enchainment mechanism: chain migratory insertion (dissociated ion pair) versus nonmigratory insertion (associated ion pair). Ethylene insertion into the Ti-n-C3H7 bond of the [H2Si(C5H4)((BuN)-Bu-t)]Ti(n-C3H7)+H3CB(C6F5)(3)(-) ion pair occurs with a reduced overall energy barrier versus the corresponding methyl derivative, indicating that propagation should be more rapid than initiation. These results are discussed in the context of the large quantity of emerging experimental data.