CH···Anion Interaction in BF₂ Complexes of C₃-Bridged Oligopyrroles

Alkyl-substituted derivatives of 1,3-dipyrrolyl-1,3-propanedione BF2 complexes, the efficient receptors for halide and oxoanions with use of bridging CH as well as pyrrole NH, are reported. BF2 complexes with only one pyrrole NH interaction site (2d,e), which exhibit smaller affinities than the basic structure (2b), bind anions tightly, which is inferred by UV/vis absorption spectral changes, compared to the derivatives with an alkyl group at the bridging carbon (2f) or two pyrrole nitrogen sites (2c). With use of 1H NMR and theoretical studies for anion complexes of 2d and 2e, bridging CH (and one β-CH in 2d) as well as pyrrole NH is found to interact with anions.