Unusual C-C bond cleavage in the formation of amine-bis(phenoxy) group 4 benzyl complexes: Mechanism of formation and application to stereospecific polymerization

Article abstract of DOI:10.1021/om500661y

Group 4 tetrabenzyl compounds MBn₄ (M = Zr, Ti), upon protonolysis with an equimolar amount of the tetradentate amine-tris(phenol) ligand N[(2,4-^tBu₂C₆H₂(CH ₂)OH]₃ in toluene from -30 to 25 °C, unexpectedly lead to amine-bis(phenoxy) dibenzyl complexes, BnCH₂N[(2,4- ^tBu₂C₆H₂(CH₂)O] ₂MBn₂ (M = Zr (1), Ti (2)) in 80% (1) and 75% (2) yields. This reaction involves an apparent cleavage of the >NCH₂-ArOH bond (loss of the phenol in the ligand) and formation of the >NCH ₂-CH₂Bn bond (gain of the benzyl group in the ligand). Structural characterization of 1 by X-ray diffraction analysis confirms that the complex formed is a bis(benzyl) complex of Zr coordinated by a newly derived tridentate amine-bis(phenoxy) ligand arranged in a mer configuration in the solid state. The abstractive activation of 1 and 2 with B(C₆F ₅)₃·THF in CD₂Cl₂ at room temperature generates the corresponding benzyl cations {BnCH₂N[(2,4- ^tBu₂C₆H₂(CH₂)O] ₂MBn(THF)}⁺[BnB(C₆F₅) ₃]^- (M = Zr (3), Ti, (4)). These cationic complexes, along with their analogues derived from (imino)phenoxy tri- and dibenzyl complexes, [(2,6-ⁱPr₂C₆H₃)N=C(3,5- ^tBu₂C₆H₂)O]ZrBn₃ (5) and [2,4-Br₂C₆H₂(O)(6-CH₂(NC ₅H₉))CH₂N=CH(2-adamantyl-4-MeC ₆H₂O)]ZrBn₂ (6), have been found to effectively polymerize the biomass-derived renewable β-methyl-α-methylene- γ-butyrolactone (_βMMBL) at room temperature into the highly stereoregular polymer P_βMMBL with an isotacticity up to 99% mm. A combined experimental and DFT study has yielded a mechanistic pathway for the observed unusual C-C bond cleavage in the present protonolysis reaction between ZrBn₄ and N[(2,4-^tBu₂C ₆H₂(CH₂)OH]₃ for the formation of complex 1, which involves the benzyl radical and the Zr(III) species, resulting from thermal and photochemical decomposition of ZrBn₄, followed by a series of reaction sequences consisting of protonolysis, tautomerization, H-transfer, oxidation, elimination, and radical coupling.