Bronsted Base-Induced Rearrangement and Nucleophilic Addition of O/N-Functionalized NHCs and Relative Group 4 Metal Complexes for Ethylene Polymerization Catalysis

The O/N-functionalized NHC precursors 1-[RNHC(O)CH2]-3-[2-OH-3,5-Bu-t(2)-(C6H2)CH2]-imidazolium bromide, [H-3(1a-f)]Br (a: R = 2,6-Pr-i(2)-(C6H3); b: R = 2,4,6-Me-3-(C6H2); c: R = 2,6-Me-2-(C6H3); d: R = 4-Me-(C6H4); e: R = 4-Cl-(C6H4.); f: R = Bu-t), have been synthesized and characterized. Reactions between the aryloxy/amido-NHC precursors [H-3(1a-e)]Br and Ag2O resulted in the NHC rearrangement to compounds [2-OH-3,5-(Bu-t)(2)-(C6H2)CH2] [R]NC(O)-CH2-(C3N2H3) (4a-e) in 68-89% yield. The preliminary study suggested that, after deprotonation, the o-quinone methide (o-QM) intermediate was in situ generated by the C-N cleavage and proton transfer of the resultant aryloxybenzyl NHCs, followed by the nucleophilic attack of o-QM to the nitrogen atom of the amido group. Treatment of salicylaldimine-functionalized NHC precursor 1-Pr-i-[2-HO-C6H2-3,5-Bu-t(2)-C(H)=N-CH2CH2]-imidazolium bromide, [H-2(2)]Br, with sodium hydride and group 4 metal sources MCl4(THF)(2) (M = Ti, Zr) step by step afforded rare zwitterionic complexes [M(13)Cl-4] (M = Ti, 10; Zr, 11) in 50-53% yield. The formation of phenolate-amine 13 was attributed to the nucleophilic addition of the formed NHC to the imine carbon after deprotonation. The corresponding products 4a and {[Ti(13)Cl-4](2)(mu-O)} (12) from the controlled hydrolysis of 10 have been confirmed by X-ray single-crystal analysis. Two novel NHC precursors, {H-2(5)}Br and [H-3(7)]Br, together with silver complex [Ag-2(5)(2)] (6) were conveniently derived. Several relative group 4 metal complexes, [MX2(k(2)-N,O-OC6H2-3,5-Bu-t(2)-C(H)=N-CH2CH2-Im)(2)] [Br](2) (M = Zr, X = Cl, 8; M = Hf, X = OSiMe3, 9) and [Ti(7)Cl-2] (14), have been prepared and tested for ethylene polymerization with MAO as cocatalyst. Complex 14 showed the highest catalytic activity of up to ca. 114 kg PE/(mol Ti.h.atm) to produce linear polymer.