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.