| Using density functional theory, we studied the fundamental steps of olefin polymerization for zwitterionic and cationic Group IV ansa -zirconocenes and a neutral ansa-yttrocene. Complexes [H2E(C5H4)2ZrMe]n (n = 0: E = BH2 (1), BF2 (2), AlH 2 (3); n = +: E = CH2 (4), SiH 2 (5)) and H2Si(C5H4) 2YMe were used as computational models. We investigated the effect of solvent on the reaction energetics for 1 and 5. We found that in benzene the energetics became very similar except that a higher olefin insertion barrier was calculated for 1. The calculated anion affinity of [CH3BF3]− was weaker towards 1 than 5. The calculated olefin binding depended primarily on the charge of the ansa linker, and the olefin insertion barrier was found to decrease steadily in the following order: [H2C(C5H4)2ZrMe] + > [F2B(C-5H4) 2ZrMe] ≈ [H2B(C5H4)2ZrMe] > [H2Si(C5H4)2ZrMe]+ > [H2Al(C5H4)2ZrMe].; We prepared ansa-zirconocene dicarbonyl complexes Me 2ECp2Zr(CO)2 (E = Si, C), and t-butyl substituted complexes (t-BuCp)2Zr(CO)2, Me2E(t-BuCp)2Zr(CO)2 (E = Si, C), (Me2Si)2(t-BuCp)2Zr(CO) 2 as well as analogous zirconocene complexes. Both the reduction potentials and carbonyl stretching frequencies follow the same order: Me2SiCp 2ZrCl2 > Me2CCp2ZrCl2 > Cp2ZrCl2 > (Me2Si)2Cp2ZrCl 2. Additionally, we prepared a series of cationic zirconocene complexes [LZrOCMe3][MeB(C6F5)3] (L = Cp 2, Me2SiCp2, Me2CCp2, (Me 2Si)2Cp2) and studied the kinetics of anion dissociation. We found that the enthalpy of anion dissociation increased from 10.3 kcal·mol −1 to 17.6 kcal·mol−1.; We also prepared series of zirconocene complexes bearing 2,2-dimethyl-2-sila-4-pentenyl with B(C6F5) results in reversible coordination of the tethered olefin to the cationic zirconium center. The kinetics of olefin dissociation have been examined, and the effects of ligand variation has been compared (ΔG‡ for olefin dissociation varies from 12.8 1,2-(SiMe2)2(η5-C5H 3)2Zr(CH3)-(CH2CMe2CH 2CH=CH2) results in rapid β-allyl elimination with loss of isobutene yielding the allyl cation [1,2-(SiMe2)2(η 5-C5H3)2Zr(η3-CH 2CH=CH2)]+. |
