Structures of potassium calixarene crown ether inclusion complexes and application in polymerization of rac-lactide
Reaction of 1,3-dipropoxy-p-tert-butyl-calixarene (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 calixarene-crown ligands of 1,3-dihydroxy-p-tert-butyl-calixarene-crown-5 (L2H2) and 1,2-dihydroxy-p-tert-butyl-calixarene-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 Pm = 0.67 and Pm = 0.73, respectively.