Molecular Binding Behaviors between Tetrasulfonated Bis(m‑phenylene)-26-crown‑8 and Bispyridinium Guests in Aqueous Solution

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 (MV2+) and 1,2-bis­(pyridinium)­ethane (BPE2+), was systematically investigated in both aqueous solution and the solid state by 1H NMR spectroscopy, crystallography, and microcalorimetry. The crystallographic studies of superstructures MV2+⊂1 and BPE2+⊂1 show that MV2+ forms an interpenetrated complex with 1, and conversely, BPE2+ 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, MV2+ threads through the cavity of 1 with a significant enthalpy change, while BPE2+ 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 MV2+⊂1 and BPE2+⊂1, depending on the relative locations of positive charges on nitrogen atoms and the bridge linkers between two pyridinium moieties in the guest molecules.