Squaramide based ion pair receptors possessing ferrocene as a signaling unit

We synthesized ion pair receptors1and2consisting of a benzocrown ether cation binding site and aferrocene-supported squaramide anion binding domain and compared their binding ability with anionreceptor3, lacking a crown ether unit, using spectroscopic, spectrophotometric, and electrochemicalmeasurements in acetonitrile solution. All receptors were obtained in a modular fashion by sequentialamidation of dimethyl squarate with corresponding amines, which allows for the simple introduction of aferrocene unit and enables them to act as sensors. We found that ion pair receptors1and2recognizedthe tested anions more strongly in the presence of cations, whereas the homotopic anion receptor3wasunable to bind sodium or potassium salts more strongly than tetrabutylammonium salts. This enhance-ment in anion binding was attributed to a cation complexation induced alteration in the nature of thephenyl ring substituents, directly linked to the squaramide anion binding site, changing from electron-donating to more withdrawing. Solid state X-ray measurements support this design principle of the recep-tors and reveal strong coordination of anions and cations to the corresponding binding domains. In both1·NaCl and2·KCl systems, organic moieties linked by the cations form 1-D polymeric structures in thecrystal lattice. However, in the case of the2·NaCl complex the polymers’shape and orientation result inthe formation of porous crystals with approximately 12% of the space unoccupied. Electrochemicalmeasurements showed when the ion pair receptors (but not the anion receptor) were pretreated withsodium or potassium cations, the addition of anions resulted in greater changes in oxidation andreduction potentials compared to the addition of anions to the same receptors in the absence of cations.Overall, the study demonstrates that squaramides offer a convenient platform for constructing ion pairsensors in modular fashion by varying the cation binding site and reporters. This opens up the possibilityof recognizing salts in real life scenarios where simultaneous binding of anions and cations is needed.