A unified view of ethylene polymerization by d0 and d0fn transition metals.: Part 2:: Chain propagation

We present a systematic investigation of chain propagation by ethylene insertion into the M-C2H5 bond for a number of d(o) [L]M-C2H5(0,+,2+)-fragments (M = Sc(III), Y(III), La(III), Lu(III), Ti(IV), Zr(IV), Hf(IV), Ce(IV), Th(IV), and V(V); L = NH-(CH)(2)-NH2- [1], N(BH2)-(CH)(2)-(BH2)N2- [2], O-(CH)(3)-O- [3], Cp-2(2-) [4], NH-Si(H-2)-C5H42- [5], [(oxo)(O-(CH)(3)-O)](3-) [6], (NH2)(2)(2-) [7], (OH)(2)(2-) [8], [CH3)(2)(2-) [9], NH-(CH2)(3)-NH2- [10], and O-(CH2)(3)-O2- [11]). For sterically unencumbered systems [L]MC2H5+(C2H4) (L = 7, 8, 9), it is shown that front-side (FS) ethylene insertion barriers follow the order Sc < Y < La and Ti < Zr < Hf. Insertion barriers for group 3 metals are usually lower than those for group 4 metals. The origin of this trend is in the aptitude of the [L]MC2H5n+ framework to occupy trigonal planar arrangement, which previously was shown to follow the trend Sc > Y > La > Ti > Zr > Hf. Backside (BS) insertion barriers, on the other hand, depend little on the identity of the metal center as BS insertion requires little deformation of the metal-ligand framework. For these sterically unencumbered systems, it is found that the insertion reaction proceeds through FS and BS channels in equal parts since FS- and BS-transition states are close in energy. Ligand influence on insertion barriers is such that good pi-donor ligands such as [7] lower the front-side insertion barrier, as they favor trigonal planar over trigonal pyramidal coordination. The activity of different metal centers can be drastically changed by sterically bulky ligands. Steric bulk generally tends to lower insertion barriers, since compression of the active site favors the transition state geometry over the complex geometry.