Molecular binding behavior of water-soluble calix[4]arenes with asymmetric 4,4′-bipyridinium guests in aqueous solution: regioselective recognition or not?

Abstract

The molecular binding behavior of water-soluble calix[4]arenes (p-sulfonatocalix[4]arene (SC4A) and p-sulfonatothiacalix[4]arene (STC4A)) with two asymmetric 4,4′-bipyridinium guests (N-methyl-N′-adamantane carbomethyl-4,4′-bipyridinium dibromide (MVAd²⁺) and N-methyl-N′-(naphthalen-2-ylmethyl)-4,4′-bipyridinium bromide iodide (MVNp²⁺)) was systematically studied using NMR spectroscopy and microcalorimetry in a neutral aqueous solution. Either the methyl group or the adamantane moiety in MVAd²⁺ could enter into the SC4A and STC4A cavities, without regioselectivity. The STC4A cavity can also accommodate MVNp²⁺, either through incorporation of the methyl group or the naphthalene moiety, without regioselectivity. However, we were surprised to find that MVNp²⁺ could only be included within the SC4A cavity through incorporation of the methyl group, with regioselectivity, which is rare for a flexible host. Furthermore, both SC4A and STC4A can form stable inclusion complexes with the two investigated asymmetric 4,4′-bipyridinium guests, driven by very favorable enthalpy changes, and the thermodynamic origins of the host selectivities for MVAd²⁺ and MVNp²⁺ can be well explained through their binding modes. The finding of this novel regioselective recognition is promising for potential applications in the development of more sophisticated biomimetic materials.