Rational design of ion-pair receptors for selective binding of hazardous salts
Final Report Abstract
Ion-pair receptors are molecular scaffolds that contain specific binding sites for both cations and anions, which allow for coordination of electroneutral ion-pairs. While there has been much progress in the molecular design of such receptors and the quantification of ions interacting with these receptors, their functioning has not been fully established: The question whether such receptors can induce ion-pair formation or if they just stabilize already existing ion-pairs was open at the beginning of this project. In this project we elucidated this mode of action of the receptors. Using dielectric spectroscopy, which can directly detect the rotation of dipolar ion-pairs, we investigated ion-pairing in solution in the presence and the absence of receptor molecules. Our results show that these receptor molecules can indeed lead to the formation of ion-pair from initially dissociated ions in solution. Yet, this ability strongly depends on the molecular structure of the receptor: A receptor with electron deficient aromatic rings for anion recognition and a single crown-ether moiety for binding of the cation can only moderately increase the number of ion-pairs in solution; the largest fraction of ion-pairs is already formed in the absence of the receptor. Yet, this receptor allows for solubilization of otherwise sparingly soluble salts in solution. Optimization of the molecular design using two oligo-ether rings to coordinate the cation provided high selectivity for monovalent sodium cations. Our spectroscopic results demonstrated that this design provides an efficient receptor geometry, which can nearly quantitatively induce ion-pair formation from dissociated ions in solution. Together, our results highlight the importance of considering the intrinsic ion-pair formation tendency in the absence of the receptor, in order to quantify the ion-pair receptor efficiency. We demonstrate this for two receptor designs, for which the efficiency to induce ion-pair formation is completely different. Our findings imply that such in-depth evaluation of the receptor efficiencies is pivotal for optimizing their performance via chemical engineering.
Publications
- Tritopic ion-pair receptors based on anion–π interactions for selective CaX2 binding, Dalton Trans. 47, 7883-7887 (2018)
J. Luo, Y.-F. Ao, C. Malm, J. Hunger, Q.-Q. Wang, D.-X Wang
(See online at https://doi.org/10.1039/c8dt01727a) - Enhancement of Ion Pairing of Sr(II) and Ba(II) Salts by a Tritopic Ion-Pair Receptor in Solution, ChemPhysChem 21, 1957-1965 (2020)
B. Kutus, J. Zhu, J. Luo, Q.-Q. Wang, A. Lupan, A. A. A. Attia, D.-X. Wang, J. Hunger
(See online at https://doi.org/10.1002/cphc.202000507) - Dielectric response of light, heavy and heavyoxygen water: isotope effects on the hydrogen-bonding network's collective relaxation dynamics, Phys. Chem. Chem. Phys., 23, 5467-5473 (2021)
B. Kutus, A. Shalit, P. Hamm, J. Hunger
(See online at https://doi.org/10.1039/d0cp06460b)
