Dipyrrolyldiketone Difluoroboron Complexes: Novel Anion Sensors With C‐H⋅⋅⋅X − Interactions

1,3‐Dipyrrolyl‐1,3‐propanediones, synthesized from pyrroles and malonyl chloride, form BF 2 complexes, which represent a new class of naked‐eye sensors for halide and oxoanions. The association mode for the interactions of both the pyrrolyl NH and bridging CH protons with anions was confirmed by 1 H NMR chemical shifts in CD 2 Cl 2 and supported by theoretical studies. The binding constants ( K a ) were estimated as 8.1×10 4 , 2.0×10 3 , 3.3×10 2 , 1.3×10 4 , and 80  m −1 for F − , Cl − , Br − , H 2 PO 4 − , and HSO 4 − by UV/Vis absorption spectral changes in CH 2 Cl 2 . Augmentation of K a compared with dipyrrolylquinoxaline for H 2 PO 4 − is much larger than those for other anions. Contrary to other anions, F − quenches the emission almost completely, which was detected by the fluorescence spectrum as well as the naked‐eye. In the case of the chloride anion complex, the formation of Cl − ‐bridged 1D networks, in which anion is associated with two BF 2 complexes, is observed in the solid state.