Guest-Encapsulation Properties of a Self-Assembled Capsule by Dynamic Boronic Ester Bonds

Two molecules of tetrakis(dihydroxyboryl)-cavitand 1a as an aromatic cavity and four molecules of 1,2-bis(3,4-dihydroxyphenyl)ethane 2 as an equatorial linker self-assemble into capsule 3a via the formation of eight dynamic boronic ester bonds in CDCl3 or C6D6. Capsule 3a encapsulates one guest molecule, such as 4,4'-disubstituted-biphenyl and 2,6-disubstituted-anthracene derivatives, in a highly selective recognition event, wherein the guest substituents are oriented to both aromatic cavity ends of 3a, as confirmed by a H-1 NMR study and X-ray crystallographic analysis. Capsule 3a showed a significant solvent effect on guest encapsulation. The association constant (K-a) of 3a with guests in C6D6 was much greater than that in CDCl3 (450-48 000-fold). The encapsulation of guests within 3a in C6D6 was enthalpically driven, whereas that in CDCl3 tended to be both enthalpically and entropically driven. Thermodynamic studies suggest that the small Ka value in CDCl3 arises from the character of CDCl3 as a competitor guest molecule for 3a, and not from the difference in stability of the boronic ester bonds of 3a in both solvents. We propose a linker partial dissociation mechanism for the guest uptake and release into and out of 3a based on the kinetic studies of guest@3a using 2D EXSY analysis, as well as structural analysis of a guest@3b. The rotation behavior of 4,4'-diacetoxy-2,2'-disubstituted-biphenyls within 3a was also investigated, where the elongation of 2,2'-disubstituents of guests put the brakes on guest rotation within 3a.