Recognition between V- and dumbbell-shaped molecules

A series of 2,6-bis(imino)pyridyl-based V-shaped compounds bearing various para-substituents on the terminal aromatic rings [C₅H ₃N(CHN-C₆H₄R)₂; R = OMe, ⁱPr, Me, H, Cl, F, and CF₃] have been prepared and investigated for their reversible binding with the dumbbell-shaped cations NH₂⁺-{CH₂-C₆H₃(OMe-3,5) ₂}₂ and 9-anthryl-CH₂-NH₂ ⁺-CH₂-C₆H₃(OMe-3,5)₂. Three crystalline V-shaped compounds and a dumbbell hexafluorophosphate were characterised in the solid state by X-ray structural analysis. The binding mode of the 1 : 1 V-shaped molecule·dumbbell complexes was evaluated by ¹H NMR spectroscopy. The binding constants (90-400 M^-1 in dichloromethane) and stoichiometries of the complexes were determined using the Method of Continuous Variations and the Rose-Drago Method based on ¹H NMR spectroscopic data. In a series of V-shaped compounds, the binding strength with both dumbbell cations diminishes with the decreasing electron-donating ability of the R substituents. Specifically, one of the diimine V-shaped compounds shows a stronger binding with the symmetrical dumbbell than with the unsymmetrical anthracene-containing dumbbell. Fluorescence measurements of equimolar mixtures of the V-shaped compounds and the unsymmetrical dumbbell have revealed a reduced anthracene emission which is approximately 50% that of the original intensity. Rapid and complete dissociation (<5 min) of the V-shaped compounds from the dumbbells was realised using an excess of acid or base, whereas only partial dissociation of the complexes was achieved with a large excess of water (<1 h).