Advanced 19F-NMR studies shed new light on encapsulation of isosteric guests in the hexameric capsules of resorcin[4]arenes and pyrogallol[4]arenes

Abstract

The hexameric capsules of resorcin[4]arenes and pyrogallol[4]arenes are fascinating, catalytically active, and highly accessible structures having large cavities. Despite the apparent similarity between these two types of hexamers, the hexameric capsules of C₁₁-resorcin[4]arenes (1) are much more efficient nanoreactors than the C₁₁-pyrogallol[4]arene (2) capsules. In this study, we investigated the encapsulation of two bulky and structurally related isosteric guests namely adamantane-1-carboxylic acid (3) and 3,5,7-tri-fluoro adamantane-1-carboxylic acid (4) into these hexamers in a competitive (chloroform) and a non-competitive (benzene) solvent. Through the application of NMR spectroscopy, diffusion NMR, and ¹⁹F guest exchange saturation transfer (GEST) applied for the first time on such hexameric capsules, we show that the two apparently similar hexamers behave differently towards these two isosteric guests. We found that in C₆D₆ the hexamers of 1 preferentially encapsulate the non-fluorinated guest 3 over guest 4, while the hexamers of 2 preferentially encapsulate the fluorinated isosteric guest 4. For the hexameric capsule of 2 encapsulating guest 4, ¹⁹F-NMR shows that the disruption of the hexameric capsule by methanol is a more complex process than one would have anticipated revealing, for the first time, three populations of 4 having different exchange rates. The combination of ¹H/¹⁹F diffusion and ¹⁹F-GEST NMR provides new insights into these important and catalytically active capsular systems demonstrating the advantages of using this combination of NMR methods to explore such supramolecular systems in solution.