Quaternary Cocrystals Based on Halide-Binding Foldamers through Both Hydrogen and Halogen Bonding

A series of quaternary cocrystals (R4N)2[L(X)2]·Sol (1-6) have been obtained through a combination of halogen bond (XB) and hydrogen bond (HB) interactions between a dihalide foldamer (formed by a tetrakis(urea) ligand L and halide ions Cl-, Br-, or I-), tetraalkylammonium cation, and a halogenated alkane. Using halide anions as the halogen- and hydrogen-bond acceptor is favorable because they are spherical, symmetric and electronegative. The HB donors include the tetrakis(urea) L, which has strong binding affinity toward anions, and tetraalkylammonium cation, while the halo-alkane is the XB donor. It has been found that the tetraalkylammonium cation and halogenated solvent can significantly affect the XB formation and lead to different binding modes in the quaternary cocrystals. In particular, while the tetraethylammonium ion (TEA+) prefers to bridge two [L(X)2]2- foldamers through C-H···X- HBs (in 1, 2, 4-6), the S-α-methylcholine cation does not serve as bridges, and two chloroform molecules connect two [L(I)2]2- foldamers through C-Cl···I- XBs (in 3).