Vanadium complexes derived from oxacalix[6]arenes: structural studies and use in the ring opening homo-/co-polymerization of ε-caprolactone/δ-valerolactone and ethylene polymerization

Abstract

Reaction of Na[VO(tBuO)₄] (generated in situ from VOCl₃ and NaOtBu) with p-tert-butyltetrahomodioxacalix[6]areneH₆ (L¹H₆) afforded, after work-up (in MeCN), the mixed-metal complex [(VO)₂(μ-O)Na₂(L¹)(MeCN)₄]·5(MeCN) (1·5MeCN), whilst the oxo complex {[VO]₄L¹} (2·6MeCN) was isolated via the use of [VO(OnPr)₃]. Reaction of L¹H₆ with [V(Np-CH₃C₆H₄)(OtBu)₃] afforded the complex {[V(Np-CH₃C₆H₄)]₂L¹} (3·7MeCN·0.5CH₂Cl₂). Use of similar methodology afforded the imido complexes {[V(Np-RC₆H₄)]₂L¹} (R = OMe (4); CF₃ (5); Cl (6); F (7)); on one occasion, reaction of [V(Np-CH₃C₆H₄)(OEt)₃] with L¹H₆ afforded the product [VO(L²)]₂·4MeCN (8·4MeCN) (L² = 2-(p-CH₃-C₆H₄NCH)-4-tBu-C₆H₂O-6-CH₂)-4-tBuC₆H₂OH) in which L¹ has been cleaved. For comparative catalytic ring opening polymerization (ROP) studies, the known complexes [VOL³] (L³ = oxacalix[3]arene) (I), [V(Np-CH₃-C₆H₄)L³]₂ (II), [Li(MeCN)₄][V₂(O)₂Li(MeCN)(L⁶H₂)₂] (L⁶H₆ = p-tert-butylcalix[6]areneH₆) (III) and [(VO)₂L⁸H] (L⁸H₈ = p-tert-butylcalix[8]areneH₈) (IV) have also been prepared. ROP studies, with or without external alcohol present, indicated that complexes 1 to 8 exhibited moderate to good conversions for ε-Cl, δ-VL and the co-polymerization thereof. Within the imido series, a positive influence was observed when electron withdrawing substituents were present. These systems afforded relatively low molecular weight products and were also inactive toward the ROP of rac-lactide. In the case of ethylene polymerization, complexes 3, 5 and 7 exhibited highest activity when screened in the presence of dimethylaluminium chloride/ethyltrichloroacetate; the activity of 4 was much lower. The products were highly linear polyethylene with M_w in the range 74–120 × 10³ Da.