Brønsted 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-tBu2-(C6H2)­CH2]-imidazolium bromide, [H3(1a–f)]Br (a: R = 2,6-iPr2-(C6H3); b: R = 2,4,6-Me3-(C6H2); c: R = 2,6-Me2-(C6H3); d: R = 4-Me-(C6H4); e:
R = 4-Cl-(C6H4); f: R = tBu), have been synthesized and characterized. Reactions
between the aryloxy/amido-NHC precursors [H3(1a–e)]Br and Ag2O resulted in the NHC
rearrangement to compounds [2-OH-3,5-(tBu)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-iPr-3-[2-HO-C6H2-3,5-tBu2-C­(H)N-CH2CH2]-imidazolium bromide, [H2(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)­Cl4] (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)­Cl4]2(μ-O)}
(12) from the controlled hydrolysis of 10 have been confirmed by X-ray single-crystal analysis. Two novel
NHC precursors, {H2(5)}Br and [H3(7)]­Br, together with silver complex [Ag2(5)2] (6) were conveniently
derived. Several relative group 4 metal complexes, [MX2(k2-N,O-OC6H2-3,5-tBu2-C­(H)N–-CH2CH2-Im)2]­[Br]2 (M = Zr, X = Cl, 8; M = Hf,
X = OSiMe3, 9) and [Ti(7)­Cl2] (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.