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 activat ion 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 presen ce of the organo-Lewis acid cocatalyst B(C6F5)(3) and various solvation med ia. 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 gam ma C-H/C-C agostic interactions in selectively stabilizing various conforma tions of the TiR "' group as well as the diminished charge on Ti with the i ntroduction of electron-donatings ligand substituents. The calculated ion p air 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 interp lay of ligand electronic and steric characteristics which weaken the precur sor Ti-CH3 bond and stabilize the cationic product. The ion pair Ti . . .H3 CB interaction is predominantly electrostatic in character and describable by a rather flat potential energy surface for elongation, and the energetic s of heterolysis are strongly influenced by the capacity of the other Ti li gands and solvation to stabilize the separated charges.