Easily‐prepared Hydroxy‐containing Receptors Recognize Anions in Aqueous Media

Abstract

Despite their ready availability, O−H groups have received relatively little attention as anion recognition motifs. Here, we report two simple hydroxy‐containing anion receptors that are prepared in two facile steps followed by anion exchange, without the need for chromatographic purification at any stage. These receptors contain a pyridinium bis(amide) motif as well as hydroxyphenyl groups, and bind mono‐ and divalent anions in 9:1 CD3CN:D2O, showing a selectivity preference for sulfate. Notably, a “model” receptor that does not contain hydroxy groups shows only very weak sulfate binding in this competitive solvent mixture. In the solid state, X‐ray crystallographic studies show that the receptors tend to form extended assemblies with anions; however, 1H and DOSY NMR studies as well as molecular dynamics simulations show that only 1:1 complexes are present in solution. Molecular dynamics simulations suggest that one of the receptors suffers from competing intramolecular hydrogen bonding, while another binds partially‐hydrated anions, with the receptor's O−H groups forming hydrogen bonds to water molecules within the anion's coordination sphere.

Chain Entropy Beats Hydrogen Bonds to Unfold and Thread Dialcohol Phosphates inside Cyanostar Macrocycles To Form [3]Pseudorotaxanes

“…The role of terminal hydroxyl (-OH) groups on recognition properties therefore remain untested. Hydroxyl groups known to be strong hydrogen bond donors 75 capable of anion binding, [76][77][78] and while interactions with phosphates are expected, 79 their role has not been studied. Similar interactions have been seen but between NH hydrogen bond donors and phosphates in receptors.…”

Hydroxy Groups Enhance [2]Rotaxane Anion Binding Selectivity

“…The use of OÀ H hydrogen bond donors for anion recognition has lagged behind that of NÀ H and even CÀ H donors, [20,21] which is perhaps surprising given their facile synthesis and potent hydrogen bond donor characteristics (e.g. pK a s are typically approximately 10 for aromatic [22] and 15 for aliphatic [23] OÀ H groups, respectively).…”

“…We recently studied the simple cationic pyridinium receptors 2 • BF 4 and 3 • BF 4 , which contain phenolic OÀ H groups (Figure 1b). [20] Both acyclic hosts bound a variety of anions in aqueous acetonitrile (90 : 10 CD 3 CN : D 2 O), with a preference being shown for sulfate. We also observed that 3 + displayed significantly stronger anion binding than an analogous pyridinium receptor without hydroxy groups.…”

Hydroxy groups enhance [2]rotaxane anion binding selectivity

“…The use of O-H hydrogen bond donors for anion recognition has lagged behind that of N-H and even C-H donors, 20,21 which is perhaps surprising given their facile synthesis and potent hydrogen bond donor characteristics (e.g. pKas are typically in the range of 10-15 for aromatic 22 and aliphatic 23 O-H groups, respectively).…”

“…We recently studied the simple cationic pyridinium receptors 2•BF4 and 3•BF4, which contain phenolic O-H groups (Figure 1b). 20 Both acyclic hosts bound a variety of anions in aqueous acetonitrile (90:10 CD3CN:D2O), with a preference being shown for sulfate. We also observed that 3 + displayed significantly stronger anion binding than an analogous pyridinium receptor without hydroxy groups.…”