An Ag(i)-linked bis-calix[4]pyrrole molecular capsule and its selective recognition of fluoride anions

Abstract

A well-defined “four-wall” capsule (3) was constructed based on the coordination-driven self-assembly of a tetratopic pyridine-functionalized calix[4]pyrrole cavitand with four stabilizing pyridine⋯Ag⁺⋯pyridine coordination bonds in N,N-dimethylformamide (DMF). Capsule 3 was characterized by means of various techniques, including ¹H nuclear magnetic resonance (NMR) spectroscopy, diffusion ordered NMR spectroscopy (DOSY), electrospray ionization mass spectrometry (ESI-MS), and single crystal X-ray diffraction analyses. The tetra-α-meso-pyridyl extended calix[4]pyrrole ligand 2 and the corresponding dimeric capsule 3 were found to selectively recognize the fluoride anion over other halide anions (chloride, bromide, and iodide anions) in DMF. Cavitand 2 was found to bind and release Ag⁺via a fast equilibrium exchange process on the NMR timescale, whereas the fluoride binding of both 2 and capsule 3 found to be slow on the NMR timescale. The transformation among multiple molecular states, viz. monomeric form 2, capsule 3, and their corresponding fluoride-bound complexes F⁻⊂2 and 2F⁻⊂3, could be modulated by the addition of silver, fluoride, and chloride ions.