Density functional study of ethylene polymerization catalyzed by a zirconium non-cyclopentadienyl complex, L2ZrCH3+. Effects of ligands and bulky substituents

To elucidate the role of electronic and steric effects on catalytic activit ies and to gain some guidance for the design of catalysts, we applied the h ybrid density functional B3LYP and integrated molecular orbital + molecular mechanics (IMOMM) methods to study the mechanism of the chain initiation/p ropagation reaction of L2ZrMe+-catalyzed ethylene polymerization for severa l bidentate non-cyclopentadienyl ligands [L = -O-CH=CH-CH=NH- (1 (trans), 2 (cis)), HCO2 (3), HC(NH)(2) (4), HC(O)(NH) (5 (cis), 6 (trans)), -O-CH=CH- N(CH2) (7)], Bu-t-4 [L = HC((NBu)-Bu-t)(2)], and Bu-t-5 [L = HCO(NtBu)]. It was shown that for model catalysts 1-7 the barrier-for chain initiation re action is 11-21 kcal/mol. Reactant, pi -complex, and product have either ec lipsed or nearly untwisted ligands L, depending on the size of L-Zr ring, w hereas the insertion transition state has the planes of the two ligands L; twisted with respect to each other. Bulky substituents in catalysts tBu-4 a nd Bu-t-5 lead to;a significant twisting of the ligands L in the reactant a nd pi -complex while retaining the structure of the transition state. As a result, the pi -complex is destabilized, and the insertion barrier is lower ed by several kcal/mol. Among all catalysts studied,Bu-t-5 is expected to b e best. It was also shown that polymerization catalyzed by these catalysts is likely to produce linear polyethylene.