Unusual C–C Bond Cleavage in the Formation of Amine-Bis(phenoxy) Group 4 Benzyl Complexes: Mechanism of Formation and Application to Stereospecific Polymerization

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(CH2)­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 >NCH2–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(C6F5)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(NC5H9))­CH2NCH­(2-adamantyl-4-MeC6H2O)]­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 PβMMBL 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-tBu2C6H2(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.