INSIGHTS into the structures adopted by titanocalix[6 and 8]arenes and their use in the ring opening polymerization of cyclic esters

Abstract

Interaction of p-tert-butylcalix[6]areneH₆, L¹H₆, with [TiCl₄] afforded the complex [Ti₂Cl₃(MeCN)₂(OH₂)(L¹H)][Ti₂Cl₃(MeCN)₃(L¹H)]·4.5MeCN (1·4.5MeCN), in which two pseudo-octahedral titanium centres are bound to one calix[6]arene. A similar reaction but employing THF resulted in the THF ring-opened product [Ti₄Cl₂(μ₃-O)₂(NCMe)₂(L)₂(O(CH₂)₄Cl)₂]·4MeCN (2·4MeCN), where LH₄ = p-tert-butylcalix[4]areneH₄. Interaction of L¹H₆ with [TiF₄] (3 equiv.) led, after work-up, to the complex [(TiF)₂(μ-F)L¹H]₂·6.5MeCN (3·6.5MeCN). Treatment of p-tert-butylcalix[8]areneH₈, L²H₈, with [TiCl₄] led to the isolation of the complex [(TiCl)₂(TiClNCMe)₂(μ₃-O)₂(L²)]·1.5MeCN (4·1.5MeCN). From a similar reaction, a co-crystallized complex [Ti₄O₂Cl₄(MeCN)₂(L²)][Ti₃Cl₆(MeCN)₅(OH₂)(L²H₂)]·H₂O·11MeCN (5·H₂O 11MeCN) was isolated. Extension of the L²H₈ chemistry to [TiBr₄] afforded, depending on the stoichiometry, the complexes [(TiBr)₂(TiBrNCMe)₂(μ₃-O)₂(L²)]·6MeCN (6·6MeCN) or [[Ti(NCMe)₂Br]₂[Ti(O)Br₂(NCMe)](L²)]·7.5MeCN (7·7.5MeCN), whilst use of [TiF₄] afforded complexes containing Ca²⁺ and Na⁺, thought to originate from drying agents, namely [Ti₈CaF₂₀(OH₂)Na₂(MeCN)₄(L²)₂]·14MeCN (8·14MeCN), [Na(MeCN)₂][Ti₈CaF₂₀NaO₁₆(L²)₂]·7MeCN (9·7MeCN) or [Na]₆[Ti₈F₂₀Na(MeCN)₂(L²)][Ti₈F₂₀Na(MeCN)_0.5(L²)]·15.5(C₂H₃N) (10·15.5MeCN). In the case of [TiI₄], the ladder [(TiI)₂(TiINCMe)₂(μ₃-O)₂(L²)]·7.25CH₂Cl₂ (11·7.25CH₂Cl₂) was isolated. These complexes have been screened for their potential to act as catalysts in the ring opening polymerization (ROP) of ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and rac-lactide (r-LA), both in air and N₂. For ε-CL and δ-VL, moderate activity at 130 °C over 24 h was observed for 1, 9 and 11; for r-LA, only 1 exhibited reasonable activity. In the case of the co-polymerization of ε-CL with δ-VL, the complexes 1 and 11 afforded reasonable conversions and low molecular weight polymers, whilst 4, 6, and 9 were less effective. None of the complexes proved to be active in the co-polymerization of ε-CL and r-LA under the conditions employed herein.