Titanium Complexes Bearing Bisaryloxy-N-heterocyclic Carbenes: Synthesis, Reactivity, and Ethylene Polymerization Study

Reaction of titanium complex [(L)TiX2(THF)] (L = {N,N′-[(5-R-3-tert-Bu-2-O−-C6H2)CH2]2 (C3H2N2)}, R = tert-Bu, La, R = Me, Lb; X = Cl, 1a, 1b; X = Br, La, 2) with 2.0 equiv of PhCH2MgCl or MeLi in diethyl ether gave dimethyl complexes [(L)Ti(CH2Ph)2] (3a, 3b) and [(L)Ti(CH3)2](4a, 4b) by salt metathesis. Dibenzyl titanium complex [(La)Ti(CH2Ph)2] (3a) absorbs dioxygen gas to afford the oxygen-insertion product [(La)Ti(OCH2Ph)2] (5) in 57% yield. The reduction of [(La)MBr2(THF)] (2) with 1 equiv of LiBEt3H in toluene gave the titanium(III) species [(La)TiBr(THF)2] (6). The molecular structures of 3b, 4b, 5, and 6 have been confirmed by X-ray single-crystal analysis. The solid state structures of these compounds reveal that these hybrid carbene ligands adopt a transoid conformation to form a pseudotrigonal-bipyramidal (for 3b, 4a, 4b, and 5) or octahedral (for 6) coordination geometry around metal centers. These titanium complexes (1, 2, and 6) showed high activities up to ca. 97 kg PE/(mol Ti·h·atm) for ethylene polymerization in the presence of MAO as coactalyst. The 13C NMR analysis revealed that linear polyethylene with low molecular weight was formed by these NHC titanium complexes. No methyl or other long-chain branch could be observed.