Selective guest retention in thermal guest-release process in sandwich-type inclusion crystal of cholic acid

Abstract

Ternary inclusion crystals of cholic acid (CA) with two aromatic compounds, m-fluoroaniline (G1) and indene (G2), were investigated from a viewpoint of selective incorporation and release. Recrystallization of CA from an equimolar mixture of G1 and G2 gave the CA crystal in a 47 : 53 molar ratio of G1 to G2. This ternary crystal has a sandwich-type structure with a two-dimensional (2D) host cavity consisting of two non-equivalent sites, A and B. G1 tends to be included in the site A owing to hydrogen bonds, while G2 in the site B. This site affinity for each guest was confirmed in a bulk crystal as well as in a single crystal of the ternary compound. Thermogravimetric analysis revealed that the two guest molecules were released at two different temperature ranges of 50–100 °C and 100–120 °C. Heat treatment of the starting crystal at 90 °C for 10 min yielded the expected intermediate crystal, which had a bilayer structure with a one-dimensional (1D) cavity in a 60 : 40 molar ratio of G1 to G2. Moreover, irrespective of the guest ratios in the starting CA crystals, G1 was preferentially retained in the thermal guest-release process. Such preferential retention of G1 is attributable to the host–guest hydrogen bond between CA and G1 at the site A in the 2D cavity.