Synthesis, Photophysical, and Anion-Sensing Properties of Quinoxalinebis(sulfonamide) Functionalized Receptors and Their Metal Complexes

We report the synthesis, characterization, and photophysical properties of a series of organic receptors and their corresponding Re^I and Ru^II metal complexes as anion probes featuring bis(sulfonamide) interacting sites incorporating highly chromophoric π-conjugated quinoxaline moieties. The interactions with various anions were extensively investigated. These probe molecules are capable of recognizing F^-, OAc^-, CN^-, and H₂PO₄^- with different sensitivities. The probe−anion interactions can be easily visualized via naked-eye colorimetric or luminescent responses. Probe 1 has the weakest acidic sulfonamide N−H protons and therefore simply forms hydrogen-bonding complexes with F^-, OAc^-, CN^-, and H₂PO₄^-. Probe 2 undergoes a stepwise process with the addition of F^- and OAc^-:  formation of a hydrogen-bound complex followed by sulfonamide N−H deprotonation. Direct sulfonamide N−H deprotonation occurs upon the addition of CN^-, while only a hydrogen-bound complex forms with the H₂PO₄^- ion for probe 2 in a dimethyl sulfoxide (DMSO) solution. Similar probe−anion interactions occur in probe 3 with the addition of F^-, CN^-, or H₂PO₄^-. However, only a genuine hydrogen-bound complex forms in the presence of the OAc^- ion in a DMSO solution of probe 3 because of the subtle difference in the pK_a values of sulfonamide N−H protons when probes 2 and 3 are compared. Coordination of probe 1 to a Re^I center or probe 2 to a Ru^II center increases the intrinsic acidity of sulfonamide N−H protons and results in an enhanced sensitivity to anions.