Activity of homogeneous chromium(III)-based alkene polymerization catalysts: Lack of importance of the barrier to ethylene insertion

The barrier to ethylene insertion into the chromium-methyl bond in Cp(H2O)C rMe+ (1), Cp(THF)CrMe+ (2), Cp(imidazol-2-ylidene)CrMe+ (3), (H2NC2H4C5H4)C rMe+ (4), and (Me2NC2H4C5H4)CrMe+ (5) has been calculated using gradient-co rrected density functional theory (DFT) and compared to experimentally reco rded activity data for homogeneous Cr(III)-based polymerization catalysts. The lack of correlation between the two data sets shows that the observed d ifferences in activity among these catalysts do not originate from a differ ence in the ability of the various Cp(donor)CrR+ complexes to instigate the bond-breaking and -forming phases of the Cossee-Arlman-type insertion step . Rather, the superior performance of the donor-substituted Cp-Cr catalysts is probably associated with the enforced proximity of the donor group to t he metal, demonstrating the importance of the constant presence of the dono r ligand in the inner coordination sphere of the chromium atom. For 5, inse rtion of a second ethylene molecule has also been investigated, and the res ults reflect that for the Cr(III)-based catalysts the insertion step does n ot represent a serious bottleneck in the propagation cycle, consistent with the observed high activities of the recently developed amino-substituted C p-Cr catalysts.