Molecular and supported Ti(iii)-alkyls: efficient ethylene polymerization driven by the π-character of metal-carbon bonds and back donation from a singly occupied molecular orbital.

While Ti(iii) alkyl species are the proposed active sites in Ziegler-Natta ethylene polymerization catalysts, the corresponding well-defined homogeneous catalysts are not known. We report that well-defined neutral β-diiminato Ti(iii) alkyl species, namely [Ti(nacnac)(CH Bu)] and its alumina-grafted derivative [(AlO)Ti(nacnac)(CH Bu)], are active towards ethylene polymerization at moderate pressures and temperatures and possess an electron configuration well-adapted to insertion of ethylene. Advanced EPR spectroscopy showed that ethylene insertion into a Ti(iii)-C bond takes place during polymerization from Ti(nacnac)(CH Bu). A combination of pulsed EPR spectroscopy and DFT calculations, based on a crystal structure of [Ti(nacnac)(CH Bu)], enabled us to reveal details about the structure and electronic configurations of both molecular and surface-grafted species. For both compounds, the α-agostic C-H interaction, which involves the singly occupied molecular orbital, indicates a π character of the metal-carbon bond; this π character is enhanced upon ethylene coordination, leading to a nearly barrier-less CH insertion into Ti(iii)-C bonds after this first step. During coordination, back donation from the SOMO to the π*(CH) occurs, leading to stabilization of π-ethylene complexes and to a significant lowering of the overall energy of the CH insertion transition state. In d alkyl complexes, ethylene insertion follows an original "augmented" Cossee-Arlman mechanism that involves the delocalization of unpaired electrons between the SOMO, π*(CH) and σ*(Ti-C) in the transition state, which further favors ethylene insertion. All these factors facilitate ethylene polymerization on Ti(iii) neutral alkyl species and make d alkyl complexes potentially more effective polymerization catalysts than their d analogues.