Synthesis, structure, and ethylene/α-olefin polymerization behavior of (cyclopentadienyl)(nitroxide)titanium complexes

Mono-Cp titanium coordination compounds bearing monoanionic ligands derived from stable nitroxyl radicals have been synthesized by two methods: (i) trapping of CpTi(III) species with the stable nitroxyl radical TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) to provide Cp′TiCl₂(TEMPO) (Cp′ = Cp (1) and Cp* (2)) and (ii) salt metathesis of Ti(IV) halides with a nitroxide anion generated by the in situ methylation of tert-butyl-α-phenylnitrone. Alkylation of these complexes with MeLi or MeMgBr furnishes Cp*TiMe₂(TEMPO) (3) and Cp′TiMe₂(ON(^tBu)(CHMePh)) (Cp′ = Cp(4) and Cp* (5)). The molecular structure of 2 has been determined by X-ray crystallography to reveal a monoanionic η¹-TEMPO ligated to titanium. Complexes 3 and 4 activated with ^iprAFPB (2,6,-diisopropyl-N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate) efficiently copolymerize ethylene and 1-hexene to provide copolymers having higher 1-hexene contents and higher productivities than the related Cp*Ti(CH₂Ph)₃ under identical conditions. Comparison of structural and electronic features as well as the ethylene/1-hexene copolymerization behavior of 3 and 4 with the constrained geometry catalyst [MeSi₂(η⁵-Me₄Cp)(η¹-N-tBu)]T iMe₂ (6) provides insights into factors governing high comonomer incorporation by mono-Cp titanium complexes.