Polymorphism and structural diversity in calix[n]arenes (n=4, 6 or 8)

Abstract

This paper reports six crystal structures of five calix[n]arenes (n=4, 6, or 8), highlighting the remarkable ability of this molecular class to self-assemble into diverse architectures and conformations, and rationalizing which structural features predispose calixarenes to act as supramolecular hosts. The structures of p-sulfonatocalix[4]arene and 1,3-bis(carboxymethyl)calix[4]arene reveal open-crown conformations accessible to guest molecules, both in the calcium salt and DMSO solvate forms, respectively. Closed-crown conformations were identified in two conformational polymorphs of hexakis(carboxymethyl)calix[6]arene methyl ester and in a lower-rim n-butyl-functionalized p-tert-butylcalix[8] arene derivative, in which their first crystal structures are reported here. Meanwhile, the p-tert-butylcalix[8]arene without lower-rim functionalization crystallizes with two structurally similar pleated-loop conformers that display distinct packing motifs. Hirshfeld surface analyses and DFT help us also to understand the interactions patterns and conformerism, such as the finding of an atypical high-energy 1,2,3-alternate conformer in which the closed crown is kept by π···π interactions between phenyl rings in one of the two polymorphs of hexakis(carboxymethyl)calix[6]arene methyl ester.