Solid-state isolation of a unique, small-molecule, supra-heterodimer of large hexameric assemblies of C-methylcalix?4]resorcinarene

31 May 2017

The well-known host molecule C-methylcalix?4]resorcinarene is shown to exist in two unique hexameric arrangements within the same crystal. These two hexameric assemblies are linked via a water molecule into a supra-heterodimer. Nature is able to produce large, supramolecular assemblies of macromolecules with intricate complexities such as found in viruses, cellular membranes and ribosomes.1?3 These complicated structures are held together by non-covalent interactions which are ultimately crucial in the functioning of these complex biological systems. Small-molecule supramolecular chemists are inspired by these complex supramolecular systems in nature, however, large, synthetic, multicomponent (n > 3) supramolecular assemblies which enclose chemical space are relatively rare phenomena in the field of small-molecule, supramolecular chemistry. Atwood and MacGillivray reported the first example of such an assembly by showing that the bowl-shaped host molecule C-methylcalix?4]resorcinarene 1a (Scheme 1) can spontaneously assemble in a nitrobenzene solution to form a large, chiral, supramolecular assembly consisting of 6 molecules of 1a and 8 water molecules, the latter ?stitching? molecules of 1a into a hexameric assembly, 1a6?(H2O)8 via O?H?O hydrogen bonds.4 Despite the approximately 125 structures reported since this discovery containing 1a co-crystallised with various guest and/or solvent molecules,5 only one similar hexameric assembly of 1a was reported by Holman et al. where 6 of the 8 water molecules were replaced by 2-ethylhexanol (2-EH) molecules.6 However, large hexameric supramolecular assemblies have been reported for the structurally related pyrogallolarenes (such as 2a in Fig. 1), which have three hydroxyl groups per aromatic ring, the additional hydroxyl group negating the need for a stitching solvent molecule.7