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

Abstract

1,3-Dipyrrolyl-1,3-propanediones, synthesized from pyrroles and malonyl chloride, form BF₂ 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 ¹H NMR chemical shifts in CD₂Cl₂ and supported by theoretical studies. The binding constants (K_a) were estimated as 8.1×10⁴, 2.0×10³, 3.3×10², 1.3×10⁴, and 80 m⁻¹ for F⁻, Cl⁻, Br⁻, H₂PO₄⁻, and HSO₄⁻ by UV/Vis absorption spectral changes in CH₂Cl₂. Augmentation of K_a compared with dipyrrolylquinoxaline for H₂PO₄⁻ 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 BF₂ complexes, is observed in the solid state.