Ligand substituent, anion, and solvation effects on ion pair structure, thermodynamic stability, and structural mobility in "constrained geometry" olefin polymerization catalysts: An ab initio quantum chemical investigation

Ab initio quantum chemical calculations at the MP2 level were performed on the elementary reactions and structural reorganizations involved in activation and ligand binding by the "constrained geometry" olefin polymerization catalyst series R2Si(eta (5)-R'C-4(5))(R "N)Ti(CH3)R "' (R = H, CH3; R' = H, CH3; R " = CH3, t-Bu; R "' = H, CH3, CH2CH2CH3, CH(CH3)(2)) in the presence of the organo-Lewis acid cocatalyst B(C6F5)(3) and various solvation media. Calculated structures of the neutral precursors and resulting ion pairs are in good agreement with the experiment. Analysis of the R2Si(R'C-4(5))(R "N)TiR "'(+) naked cations reveals the importance of alpha, beta, and gamma C-H/C-C agostic interactions in selectively stabilizing various conformations of the TiR "' group as well as the diminished charge on Ti with the introduction of electron-donatings ligand substituents. The calculated ion pair formation enthalpies for the process R2Si(eta (5)-R'C-4(5))(R "N)Ti(CH3)R "' + B(C6F5)(3) -> R2Si(eta (5)-R'C-4(5))(R "N)TiR "'.H3CB(C6F5)(3) are in good agreement with experiment, the magnitudes reflecting a close interplay of ligand electronic and steric characteristics which weaken the precursor Ti-CH3 bond and stabilize the cationic product. The ion pair Ti . . .H3CB interaction is predominantly electrostatic in character and describable by a rather flat potential energy surface for elongation, and the energetics of heterolysis are strongly influenced by the capacity of the other Ti ligands and solvation to stabilize the separated charges.