Molecular binding behaviors between tetrasulfonated bis(m-phenylene)-26-crown-8 and bispyridinium guests in aqueous...

A highly water-soluble crown ether bearing four sulfonate groups (1) was synthesized by sulfonation of neutral bis(m-phenylene)-26-crown-8. The complexation behavior of 1 with two bispyridinium-based guests, methyl viologen (MV(2+)) and 1,2-bis(pyridinium)ethane (BPE(2+)), was systematically investigated in both aqueous solution and the solid state by (1)H NMR spectroscopy, crystallography, and microcalorimetry. The crystallographic studies of superstructures MV(2+)⊂1 and BPE(2+)⊂1 show that MV(2+) forms an interpenetrated complex with 1, and conversely, BPE(2+) is bound outside the cavity of 1 as an electrostatic complex. Furthermore, microcalorimetric titration reveals the thermodynamic origins of this different binding process; that is, MV(2+) threads through the cavity of 1 with a significant enthalpy change, while BPE(2+) mainly associates with the sulfonate groups of 1, accompanied by a dominant entropy change. The obtained results demonstrate a structure-dependent binding process in complexes MV(2+)⊂1 and BPE(2+)⊂1, depending on the relative locations of positive charges on nitrogen atoms and the bridge linkers between two pyridinium moieties in the guest molecules.