High-yield preparation of new crown ether-based cryptands and improving complexation with paraquat, paraquat derivatives and diquat

Efficient syntheses of two novel cryptands 2a and 2b based on bis(m-phenylene)-32-crown-10 are reported. Unimolecular macrocyclization of 9 through copper(II)-mediated Eglinton coupling generated the diacetylene-containing cryptand 2a in 93% yield. Pd/C-catalyzed hydrogenation of 2a yielded the cryptand 2b (74%). The complexation behavior of a series of cryptand hosts (1a–2b) with dicationic guests (3–6) has been comprehensively investigated by NMR spectroscopy, mass spectrometry and X-ray analysis. It is found that all four cryptands binding with guests form 1:1 complexes both in solution and in the solid state, except the case of cryptand 1a/1b with 5 forming 2:1 complex. Moreover, it is demonstrated that cryptand 2a/2b not only binds guests more strongly than the corresponding BMP32C10, but also has greater interacting affinities than the cryptand 1a/1b. Meanwhile, the association constants of the N-methyl-substituted guests 4 and 5 with cryptands are higher than non-methyl-substituted guest 3. The X-ray analysis of crystals of these complexes revealed that the complexation is stabilized by π–π stacking interactions and CH⋯O hydrogen bonds between the cryptands and the guests. Additionally, the paraquat derivative 3 penetrated symmetrically through the cavity of the two same 29-membered rings of cryptand 2 in the solid state, while the threading of paraquat unit into the cavity of cryptand 1 was unsymmetrical both in crystal structures of 1a3 and 1b3.

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