Synthesis, Structure, and Reactivity of Hydroxylaminato Alkyltitanium Complexes

Alkyltitanium complexes bearing hydroxylaminato ligands were synthesized by two methods: (i) reaction of the stable nitroxide radical 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) with TiCl 3 to generate (TEMPO)TiCl 3 (1) followed by alkylation with PhCH 2 MgCl to furnish (TEMPO)Ti(CH 2 Ph) 3 (2), and (ii) protonolysis of Ti(CH 2 Ph) 4 (3) with hydroxylamines to yield (R 2 NO) 2 Ti(CH 2 Ph) 2 (R = CH 2 Ph (4), Et (5)). 1 H NMR studies of these compounds demonstrate that the hydroxylaminato ligands exhibit both η 1 - and η 2 -binding modes with varying degrees of hemilability. X-ray crystallographic analysis of 5 shows that titanium adopts a six-coordinate "propeller" conformation in which the hydroxylamine anions are η 2 -bound. The reaction of the complex 2 with B(C 6 F 5 ) 3 at 20 °C forms 1 equiv of toluene and a cationic benzyltitanium complex exhibiting strong η 6 -PhCH 2 B(C 6 F 5 ) 3 anion coordination, in which one of the TEMPO methyl groups has undergone C-H bond activation as evidenced by 1 H, 1 3 C NMR, gHSQC, gHMBC, and gROESY experiments. Complexes 2-5 exhibit very low activities for propylene polymerization.