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 TiCl3 to generate (TEMPO)TiCl3 (1) followed by alkylation with PhCH2MgCl to furnish (TEMPO)Ti(CH2Ph)3 (2), and (ii) protonolysis of Ti(CH2Ph)4 (3) with hydroxylamines to yield (R2NO)2Ti(CH2Ph)2 (R = CH2Ph (4), Et (5)). 1H NMR studies of these compounds demonstrate that the hydroxylaminato ligands exhibit both η1- and η2-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 η2-bound. The reaction of the complex 2 with B(C6F5)3 at 20 °C forms 1 equiv of toluene and a cationic benzyltitanium complex exhibiting strong η6-PhCH2B(C6F5)3 anion coordination, in which one of the TEMPO methyl groups has undergone C−H bond activation as evidenced by 1H, 13C NMR, gHSQC, gHMBC, and gROESY experiments. Complexes 2−5 exhibit very low activities for propylene polymerization.