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

Abstract

Reaction of titanium complex [(L)TiX 2 (THF)] (L = { N, N ′-[(5-R-3- tert -Bu-2-O − -C 6 H 2 )CH 2 ] 2 (C 3 H 2 N 2 )}, R = tert -Bu, L a, R = Me, L b; X = Cl, 1a, 1b; X = Br, L a, 2 ) with 2.0 equiv of PhCH 2 MgCl or MeLi in diethyl ether gave dimethyl complexes [(L)Ti(CH 2 Ph) 2 ] ( 3a, 3b ) and [(L)Ti(CH 3 ) 2 ]( 4a, 4b ) by salt metathesis. Dibenzyl titanium complex [(L a )Ti(CH 2 Ph) 2 ] ( 3a ) absorbs dioxygen gas to afford the oxygen-insertion product [(L a )Ti(OCH 2 Ph) 2 ] ( 5 ) in 57% yield. The reduction of [(L a )MBr 2 (THF)] ( 2 ) with 1 equiv of LiBEt 3 H in toluene gave the titanium(III) species [(L a )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 13 C 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.