BF2Complex of Fluorinated Dipyrrolyldiketone: A New Class of Efficient Receptor for Acetate Anions

Abstract

The β-fluorinated derivative (2b) of the 1,3-dipyrrolyl-1,3-propanedione BF₂ complex has been prepared from 3,4-difluoropyrrole and malonyl chloride, followed by treatment with BF₃·OEt₂. Despite the simple, acyclic, and neutral structure, 2b exhibits efficient 1:1 binding for anions in CH₂Cl₂ using the bridging CH and pyrrole NH as interaction sites. The binding constant (K_a) of 2b for acetate (CH₃CO₂^-), associating more effectively than anions such as F^-, Cl^-, Br^-, H₂PO₄^-, and HSO₄^-, is estimated to be 9.6 × 10⁵ M^-1, ∼9 times larger than that of the β-H derivative 2a (1.1 × 10⁵ M^-1). The UV−vis and fluorescence spectral changes of 2b elucidate the effective recognition of an amino acid, such as phenylalanine, in the anionic form; this is also supported by CD spectral changes with mirror images by l- and d-isomers. Furthermore, in the solid state, BF₂ complex 2b provides Cl^--bridged supramolecular networks and, in sharp contrast, deprotonated “anionic” self-assembled structures by F^- binding.