Tetraphenolate niobium and tantalum complexes for the ring opening polymerization of ε-caprolactone

Abstract

Reaction of the pro-ligand α,α,α′,α′-tetra(3,5-di-tert-butyl-2-hydroxyphenyl-p-)xylene-para-tetraphenol (p-L¹H₄) with two equivalents of [NbCl₅] in refluxing toluene afforded, after work-up, the complex {[NbCl₃(NCMe)]₂(μ-p-L¹)}·6MeCN (1·6MeCN). When the reaction was conducted in the presence of excess ethanol, the orange complex {[NbCl₂(OEt)(NCMe)]₂(μ-p-L¹)}·3½MeCN·0.614toluene (2·3½MeCN·0.614toluene) was formed. A similar reaction using [TaCl₅] afforded the yellow complex {[TaCl₂(OEt)(NCMe)]₂(μ-p-L¹)}·5MeCN (3·5MeCN). In the case of the meta pro-ligand, namely α,α,α′,α′tetra(3,5-di-tert-butyl-2-hydroxyphenyl-m-)xylene-meta-tetraphenol (m-L²H₄) only the use of [Nb(O)Cl₃(NCMe)₂] led to the isolation of crystalline material, namely the orange bis-chelate complex {[Nb(NCMe)Cl(m-L²H₂)₂]}·3½MeCN (4·3½MeCN) or {[Nb(NCMe)Cl(m-L²H₂)₂]}·5MeCN (4·5MeCN). The molecular structures of 1–4 and the tetraphenols L¹H₄ and m-L²H₄·2MeCN have been determined. Complexes 1–4 have been screened as pre-catalysts for the ring opening polymerization of ε-caprolactone, both with and without benzyl alcohol or solvent present, and at various temperatures; conversion rates were mostly excellent (>96%) with good control either at >100 °C over 20 h (in toluene) or 1 h (neat).