Electroactive Anion Receptor with High Affinity for Arsenate

Herein,
we present the synthesis and characterization of a macrocyclic
polyamide cage that incorporates redox-active 1,4-dithiin units. UV/vis
titration experiments with eight anions in acetonitrile revealed high
affinity for H2AsO4– (log β2 = 10.4–0.4+0.4) and HCO3– (log β2 = 8.3–0.4+0.3) over other common anionic guests,
such as Cl– (log K1:1 = 3.20–0.02+0.03), HSO4– (log K1:1 = 3.57–0.03+0.02), and H2PO4– (log K1:1 =
4.24–0.04+0.05), by the selective formation of HG2 complexes. The recognition
of arsenate over phosphate is rare among both proteins and synthetic
receptors, and though the origin of selectivity is not known, exploiting
the difference in the binding stoichiometry represents an underexplored
avenue toward developing receptors that can differentiate between
the two anions. Additional analysis by 1H NMR in 1:3 CD2Cl2/MeCN-d3 found a
strong dependence of anion binding stoichiometry with the solvent
employed. Finally, titration experiments with cyclic voltammetry provided
varying and complex responses for each anion tested, though reaction
between the anion and receptors was observed in most cases. These
results implicate 1,4-dithiins as interesting recognition moieties
in the construction of supramolecular receptors.