Synthesis,Recognition And Transport Studies Of Strapped Calix[4] Pyrrole For Sulfate

Sulfate plays an extremely important role in life activities,and the design and synthesis of artificial receptors capable of recognizing and transporting sulfate have received widespread attention from chemists.However,sulfate has a very high hydration energy（–1080 k J/mol）,making strong and selective binding of this anion by neutral receptors a very difficult task.On the other hand,the transport of sulphate across membranes is a major challenge in supramolecular chemistry.As another supramolecular main compound after calixarene,calixpyrrole has attracted much attention in the field of molecular recognition.Among the many functional calix[4]pyrroles,the strapped calix[4]pyrrole has a unique three-dimensional cavity structure,which could better match complicated anions（such as sulfate ions）.At the same time,the strapped unit could cooperate with calix[4]pyrrole moiety to complex anions,and the synergistic effect of both greatly promotesd the selectivity and binding ability of the receptor towards anions.The strapped calix[4]pyrrole is increasingly favored by researchers in complicated anion recognition.In this study,the groups with hydrogen bond donor site were introduced into the strapped position of calix[4]pyrrole,and the anion receptors with multiple binding sites were designed and synthesized.The recognition ability of sulfate was studied by UV-vis spectroscopy and ¹H NMR,and the recognition mechanism between host and guest was further studied by ¹H NMR and X-ray single crystal diffraction.Finally,U-tube experiments were used to explore their sulfate transport ability.The main research contents are as follows:1.Pyridinebisthiazolamine unit contains two basic thiazole groups.Under acidic conditions,the two thiazole groups containing basic nitrogen atoms would be protonated.The protonated thiazole groups not only promoted the bonding ability of the NH due to its lack of electrons,but also generates ionic hydrogen bonding sites that can coordinate with the calix[4]pyrrole group to complexe the sulfate.So,pyridinebisthiazolamine strapped calix[4]pyrrole I-1 was designed and synthesized,and was confirmed by NMR spectroscopy,mass spectrometry,etc.The anion(SO₄^2-,H₂PO₄^-,Ac O^-,Cl^-,Br^-,I^-)properties of I-1 under acidic conditions in solution were studied by UV-vis spectroscopy and ¹H NMR spectroscopy.The results showed that in DMSO,calix[4]pyrrole I-1 displayed high selectivity and strong binding ability for SO₄^2-(Ka=3.45×10⁵ M^-1)in the presence of 2 equiv.of nitric acid followed by Cl^->>H₂PO₄^-,Ac O^-,Br^-,and I^-(these anions have almost no interaction with pyridinebisthiazolamine strapped calix[4]pyrrole I-1.The host-guest interactions between pyridinebisthiazolamine strapped calix[4]pyrrole I-1 and sulfate were further analyzed by ¹H NMR spectroscopy and single crystal X-ray diffraction.The results showed that the pyrrole NH and the thiazole-linked NH underwent significant downfield shifts due to the formation of hydrogen bonds with sulfate.Beyond these changes,a completely new singlet appeared at 14.56 ppm,which was assigned to the two protonated N⁺H groups on the thiazole rings.Such a large chemical shift value also implied that the two protonated N⁺H groups on the thiazole rings formed strong hydrogen bonds with sulfate,which is consistent with what the single crystal structure of the sulfate complex showd.Based on the above studies,pyridinebisthiazolaminepyridyl strapped calix[4]pyrrole was used to transport sulfate ions.It was found that the compound I-1 was able to transport sulfate ions in the form of sulfuric acid from the supply phase to the receiving phase in a U-tube transport assay.2.Our recent discovery suggested that the unique three-dimensional cavity structure of benzenebistriazole-strapped calix[4]pyrrole could well match the sulfate,and its strapped unit could cooperate with calix[4]pyrrole moiety to complex sulfate.In this study,it was used as a model molecule,and the hydrogen bond donor pyridinyl bistriazole group was introduced into the strapped position of calix[4]pyrrole,to design and synthesize pyridinyl bistriazole strapped calix[4]pyrrole II-1,and was confirmed by NMR spectroscopy,mass spectrometry,etc.The anion(SO₄^2-,H₂PO₄^-,Ac O^-,Cl^-,Br^-,I^-,NO₃^-,Cl O₄^-)properties of II-1 was studied by UV-vis spectroscopy and ¹H NMR spectroscopy,anion receptor shows high sulfate binding affinity and selectivity in DMSO-0.5%H₂O.The affinities for the anions were found to decrease according to the following sequence:SO₄^2-(2.97×10⁵ M^-1)>H₂PO₄^->Ac O^->Cl^->Br^->I^-,NO₃^-,Cl O₄^-（these anions have almost no interaction with pyridinylbistriazole strapped calix[4]pyrrole II-1）,which is consistent with the order of the association constants obtained by ¹H NMR.The host-guest interactions between pyridinylbistriazole strapped calix[4]pyrrole II-1 and sulfate were further investigated using ¹H NMR spectroscopy,and the results showed that the pyrrole NH and the triazole CH underwent significant downfield shifts due to the formation of hydrogen bonds with sulfate.When II-1 was exposed to sulfate,two sets of distinguishable resonances were seen for the proton signals in the ¹H NMR spectrum before saturation was achieved.Accompanied by a corresponding disappearance of the original signals,completely new peaks appeared with the addition of 1 equiv.of sulfate anions.These changes in the chemical shifts show that the formation of a strong 1:1 complex and a rate of binding/release equilibrium between II-1 and the sulfate anion that is slow on the NMR time scale.Finally,we have used U-tube experiments to explore the sulfate transport ability of II-1.Unfortunately,receptor II-1 could not transport the sulfate ion from the supply phase to the receiving phase,which may be led by the hydrophobicity of II-1.