Dipyrrolyl-Functionalized Bipyridine-Based Anion Receptors for Emission-Based Selective Detection of Dihydrogen Phosphate

New cationic anion receptors, based on the use of pyrrole‐substituted bipyridine and coordinated to transition metals, are described. Specifically, polypyridine–ruthenium and –rhodium cores have been functionalized to generate an anion binding site. The design was chosen to probe the influence of the pyrrole‐to‐pyrrole separation on anion‐binding affinities and selectivities; this distance is greater in the new systems of this report (receptors 1 and 2) relative to that present in related dipyrrolyl quinoxaline based receptors 3 and 4. Solution‐phase anion‐binding studies, carried out by means of 1H NMR spectroscopic titrations in [D6]DMSO and isothermal titration calorimetry (ITC) in DMSO, reveal that 1 and 2 bind most simple anions with substantially higher affinity than either 3 or 4. In the case of chloride anion, structural studies, carried out by means of single‐crystal X‐ray diffraction analyses, are consistent with the solution‐phase results and reveal that receptors 1 and 2 are both able to stabilize complexes with this halide anion in the solid state. Fishing out the phosphate: Bearing the same number of pyrrole units (i.e., two) as those present in dipyrrolyl quinoxaline derivatives, the new receptors of this report display a larger cavity size than the latter due to the substitution of the quinoxaline moiety with a bipyridine spacer. Dihydrogen phosphate quenches the emission of the RuII derivative, making these systems of potential interest as anion sensors.