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 ReI and RuII 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 pKa values of sulfonamide N−H
protons when probes 2 and 3 are compared. Coordination of probe 1 to a ReI center or probe 2 to a RuII center
increases the intrinsic acidity of sulfonamide N−H protons and results in an enhanced sensitivity to anions.