Enhancement of Ion Pairing of Sr(II) and Ba(II) Salts by a Tritopic Ion‐Pair Receptor in Solution

Abstract

Tritopic ion‐pair receptors can bind bivalent salts in solution; yet, these salts have a tendency to form ion‐pairs even in the absence of receptors. The extent to which such receptors can enhance ion pairing has however remained elusive. Here, we study ion pairing of M2+ (Ba2+, Sr2+) and X− (I−, ClO4−) in acetonitrile with and without a dichlorooxacalix[2]arene[2]triazine‐related receptor containing a pentaethylene‐glycol moiety. We find marked ion association already in receptor‐free solutions. When present, most of the MX+ ion‐pairs are bound to the receptor and the overall degree of ion association is enhanced due to coordinative, hydrogen‐bonding, and anion‐π interactions. The receptor shows higher selectivity for iodides but also stabilizes perchlorates, despite the latter are often considered as weakly coordinating anions. Our results show that ion‐pair binding is strongly correlated to ion pairing in these solutions, thereby highlighting the importance of taking ion association in organic solvents into account.

Recent Advancements in Ion‐Pair Receptors

“…[37] On the other hand, recently Kutus et al has studied that the triazine-based receptor 41 exists in open as well as twisted arrangement, forming the complexes with Ba(ClO 4 ) 2 , Sr(ClO 4 ) 2 , and SrI 2 through ion-pairing in acetonitrile thereby displaying remarkable pairing in free state in the ratio of 1 : 1 upto 50% even at low salt concentrations (Figure 5). [38] The receptor in open form display high selectivity towards iodide as well as stabilizing perchlorate ions, hence enhancing the ion association constants of about 10-25%. However, from the analysis of dielectric relaxation spectroscopy, it was revealed that the higher stability of the complex is due to coordinative hydrogen bonding and anion-π interactions.…”

“…Structural view of dichloro-oxacalix[2]arene[2]triazine receptor a) open and b) twisted conformer of receptor 41, c) optimized structure through B3LYPÀ D3/def2-TZVP level, crystal structure of the ion-pair complex of [41 • Sr(ClO 4 ) 2 ] • H 2 O • CH 3 OH and d) optimized structure of the SrClO 4+ ion-pair bound to the receptor 41. Reprinted with permission from Ref [38]…”