Diiminates and diamides as ligands in polymerization catalysts with M(III) (M = Ti, V, Cr) metal centers. A theoretical study

A theoretical study has been carried out on two types of olefin polymerization catalysts based on the same M(III) centers (M = Ti, V, Cr) but different (mono) anionic ligand designs. The first system contains a mono-beta-diiminato ligand that binds through the two imine nitrogens to the M(III) center and has the nitrogen lone-pairs perpendicular to the NMN coordination plane. It is shown that the generic mono-beta-diiminato ligand system without bulky substituents on the nitrogen has barriers (kcal/mol) for insertion (Ti: 10.7; V: 12.2; Cr: 10.4) quite similar to the beta-hydrogen transfer process (BHT) (Ti: 11.2; V: 9.8; Cr: 8.4), which serves as the chain-terminating step. The close proximity of the two barriers is in line with findings for other bidentate nitrogen ligands, where bulky aryl groups on the nitrogens were required in order to increase the BHT barrier relative to insertion and ensure acceptable molecular weights. The second system contains a bridged diamide ligand in which the ligand backbone forces the lone-pairs on the nitrogens into the NMN coordination plane and the bulk of the ligand to be below and above this plane. For this system barriers (kcal/mol) of insertion(Ti: 14.4; V: 9.7; Cr: 15.3) are calculated to be well below the barriers (Ti: 19.0; 18.7; 21.8) for the BTH process, as the BHT transition state is destabilized by the ligand backbone. Especially the V(III) system is predicted to be a good candidate as a high molecular weight polymerization catalyst.