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 ₂MgCl to furnish (TEMPO)Ti(CH ₂Ph) ₃ (2), and (ii) protonolysis of Ti(CH ₂Ph) ₄ (3) with hydroxylamines to yield (R ₂NO) ₂Ti(CH ₂Ph) ₂ (R = CH ₂Ph (4), Et (5)). ¹H 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 ₆F ₅) ₃ at 20°C forms 1 equiv of toluene and a cationic benzyltitanium complex exhibiting strong η ⁶-PhCH ₂B(C ₆F ₅) 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.