Structures of potassium calix[4]arene crown ether inclusion complexes and application in polymerization of rac-lactide

Abstract

Reaction of 1,3-dipropoxy-p-tert-butyl-calix[4]arene (L¹H₂) with KN(SiMe₃)₂ afforded a one-dimensional (1D) chain complex [K₂L¹]_n (1). Upon reaction with 1 equivalent 18-crown-6, complex 1 can convert to complex [K₂(18-crown-6)L¹] (2) which possesses a sandwich structure. Treatment of two calix[4]arene-crown ligands of 1,3-dihydroxy-p-tert-butyl-calix[4]arene-crown-5 (L²H₂) and 1,2-dihydroxy-p-tert-butyl-calix[4]arene-crown-5 (L³H₂) with KN(SiMe₃)₂ gave the dinuclear complex [K₂L²] (3) and the mononuclear complex [K(THF)L³H] (4), respectively. Complexes 1–4 were all characterized by single-crystal X-ray diffraction techniques. The variable temperature ¹H NMR spectrum indicates there is a quick rotation equilibrium of the two phenoxy groups in complex 3. In addition, complexes 1–4 have been tested for the ring-opening polymerization (ROP) of rac-lactide and the results showed that complexes 2 and 3 are highly active for the ROP of rac-lactide. The obtained polymers displayed low dispersity values (Đ) and the molecular weights are close to the calculated ones. Furthermore, complexes 2 and 3 show moderate isoselectivities of P_m = 0.67 and P_m = 0.73, respectively.