Thermodynamic Profiles of Salt Effects on a Host–Guest System: New Insight into the Hofmeister Effect

Abstract

Isothermal titration calorimetry was used to probe how salts influence the thermodynamics of binding of guests to cavitand 1. Studies involved six Hofmeister salts covering the range of salting-in to strongly salting-out. The latter were found to reduce affinity. The cause of this was competitive binding of the weakly solvated anion to the hydrophobic pocket of the host. At the other extreme of the Hofmeister series, salts increased guest affinity. Two factors for this were evident. At low concentrations the data fitted a previously reported model that accounts for cation condensation to the outer carboxylates of the host (Carnagie, R.; Gibb, C. L. D.; Gibb, B. C., Angew. Chem., Int. Ed. 2014, 53 (43), 11498-11500). At higher concentrations, an as of yet unidentified contribution was observed that was noted to be guest dependent. Midcontinuum salts such as NaClO3 were found to enhance affinity at low concentrations, but weaken it at high concentrations; a nonmonotonic trend attributed to the aforementioned competing phenomena. In combination with previous work, the data presented here reveal that the Hofmeister effect evident in this system can be mostly attributed to solute-salt interactions.

Binding Hydrated Anions with Hydrophobic Pockets

“…15 Furthermore, ClO 4 − binding is able to induce Hofmeister effects in the binding of amphiphilic guests to 1 . 16 Here we demonstrate that anion binding to the concavity of 1 is general, use ITC to identify that these complexation events are strongly exothermic and entropically penalized, and show with a combination of quantum calculations and molecular dynamics simulations that partial solvation of the guest is key to binding.…”

“…Furthermore, as noted above, acetate demonstrated no affinity for the host. 16a The strong association of Cl 3 CCO 2 − may in part be attributed to the formation of C– H…X hydrogen bonds between the inward pointing benzal protons of the host and the halogen of the guest; 24 a conclusion supported by NMR signal shifts upon complexation (SI). However, the much lower affinity of Cl 2 CHCO 2 − is unusual.…”

The Chaotropic Effect as an Assembly Motif in Chemistry

“…[25] Thechaotropic effect also accounts for the recently reported binding of chaotropic anions to biotinuril, [98] bambusuril, [45,46,97,145] hemicucurbituril, [146] at ricarbazolo triazolophane (not studied in water), [147] and the so-called octaacid (see Figure 3f or structures). [82,83,[148][149][150][151] Thes tudied chaotropic anions included not only those in Table 1but also N 3 À ,C NO À ,S eCN À ,I O 4 À ,R eO 4 À ,a nd SbF 6 À .N oteworthy, the thermodynamic fingerprint (enthalpically driven processes with an egative entropic contribution) is identical in all cases,r egardless of which type of superchaotropic anion and which macrocycle are being used. [10,12,22,23,26,97,98] Finally,t he inclusion preference of metallo-macrocyclic cage compounds towards chaotropic anions stands out as well but has not been mechanistically rationalized in comparable detail.…”

A Potent Halogen‐Bonding Donor Motif for Anion Recognition and Anion Template Mechanical Bond Synthesis

“…[27][28][29] In contrast, the halide anion recognition strength of the hydrogen bonding rotaxane analogue 14·PF 6 ,w here iodotriazole groups are substituted with C À Hp rototriazole motifs is dramatically diminished, demonstrating only very weak affinity to I À and no binding for Cl À and Br À ( Table 3, Entry 2). [27][28][29] In contrast, the halide anion recognition strength of the hydrogen bonding rotaxane analogue 14·PF 6 ,w here iodotriazole groups are substituted with C À Hp rototriazole motifs is dramatically diminished, demonstrating only very weak affinity to I À and no binding for Cl À and Br À ( Table 3, Entry 2).…”

“…Zuschriften erned by the Hofmeister series bias. [27][28][29] In contrast, the halide anion recognition strength of the hydrogen bonding rotaxane analogue 14·PF 6 ,w here iodotriazole groups are substituted with C À Hp rototriazole motifs is dramatically diminished, demonstrating only very weak affinity to I À and no binding for Cl À and Br À ( Table 3, Entry 2). Theremarkable enhancement of halide complexation by XB 1·PF 6 rotaxane by at the very least three orders of magnitude in comparison to HB rotaxane 14·PF 6 illustrates again the superiority of halogen bonding over hydrogen bonding in aqueous anion recognition.…”