Abstract
Group 4 tetrabenzyl compounds MBn4 (M = Zr, Ti), upon protonolysis with an equimolar amount of the tetradentate amine-tris(phenol) ligand N-[(2,4-(Bu2C6H2)-Bu-t(CH2)OH](3) in toluene from -30 to 25 degrees C, unexpectedly lead to amine-bis(phenoxy) dibenzyl complexes, BnCH2N[(2,4-(Bu2C6H2)-Bu-t(CH2)O](2)MBn2 (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)center dot THF in CD2Cl2 at room temperature generates the corresponding benzyl cations {BnCH2N[(2,4-'Bu2C6H2(CH2)O](2)MBn(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-(Pr2C6H3)-Pr-i)N=C(3,5-(Bu2C6H2)-Bu-t)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 beta-methyl-alpha-methylene-gamma-butyrolactone (beta MMBL) at room temperature into the highly stereoregular polymer P beta MMBL with an isotacticity up to 9996 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-(Bu2C6H2)-Bu-t(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.