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. Issue 2 (17th November 2020)

• Record Type: Journal Article
• Authors: Ashuiev, Anton; Allouche, Florian; Wili, Nino; Searles, Keith; Klose, Daniel; Copéret, Christophe; Jeschke, Gunnar
• Is Part Of: Chemical science. Volume 12:Issue 2(2021)
• Page Start: 780
• Page End: 792
• Publication Date: 2020-11-17
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/d0sc04436a
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : Ti(iii ) alkyl species polymerize ethylene via an original mechanism, which involves back donation to the π*(C2 H4 ) and a delocalization of the unpaired electron in the transition state of C2 H4 insertion into the partially alkylidenic Ti(iii )–C bond. Abstract : 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)(CH2 t Bu)2 ] and its alumina-grafted derivative [(Als O)Ti(nacnac)(CH2 t 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)(CH2 t Bu)2 . A combination of pulsed EPR spectroscopy and DFT calculations, based on a crystal structure of [Ti(nacnac)(CH2 t Bu)2 ], 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 C2 H4 insertion into Ti(iii )–C bonds after this first step. During coordination, back donation from the SOMO to the π*(C2 H4 ) occurs, leading to stabilization of π-ethylene complexes and to a significant lowering of the overall energy of the C2 H4 insertion transition state. In d 1 alkyl complexes, ethylene insertion follows an original "augmented" Cossee–Arlman mechanism that involves the delocalization of unpaired electrons between the SOMO, π*(C2 H4 ) 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 1 alkyl complexes potentially more effective polymerization catalysts than their d 0 analogues.