Temperature- and Solvent-Induced Crystal-Form Transformations of the Pyridine@p-tert-butylcalix[6]arene Host–Guest System

Maura Malinska

doi:10.1021/acs.cgd.0c01422

Abstract

Crystallization conditions for producing crystal structures of p-tert-butylcalix[6]arene (TBC6) with different conformations were screened. Molecular recognition and self-assembly between the TBC6 host and the guest pyridine led to kinetic trapping and crystallization of intermediate exclusion complexes. These complexes were characterized by single-crystal X-ray diffraction, which revealed that the first-formed crystals are 11:2 polymorphic association complexes with winged-cone host conformations. The crystalline kinetic complex undergoes slow spontaneous dissolution and subsequently recrystallizes as thermodynamic inclusion complexes, namely a 1:3 pseudopolymorphic and three 1:1 polymorphic pyridine@TBC6 complexes. The crystal structure of the host–guest system can be tuned by changing the temperature and supersaturation conditions. Interaction energies were used to generate intermolecular interaction topologies during energy framework analysis, which were used to study the hierarchies of various interactional motifs and their topologies in this series of TBC6 complexes.

Highlights

Calixarenes are macrocyclic compounds composed of phenolic units bonded to each other through methylene groups at their 2,5-positions
Calix[6]arene presents a pinched-cone conformation in almost all available crystal structures; the pinched-cone is the lowest energy conformation and forms R66(12) hydrogenbonded graph-set motifs[10,11] in the solid state structures of many of its previously reported solvent clathrates.[12−14] A recent study showed that the macrocyclic effect and the number of chains created by different types of interaction are the reasons behind the high melting points of calixarene crystals.[4]
Our studies showed that only metastable crystal structures exhibit winged-cone conformations

Summary

INTRODUCTION

Calixarenes are macrocyclic compounds composed of phenolic units bonded to each other through methylene groups at their 2,5-positions. Calix[6]arene presents a pinched-cone conformation in almost all available crystal structures; the pinched-cone is the lowest energy conformation and forms R66(12) hydrogenbonded graph-set motifs[10,11] in the solid state structures of many of its previously reported solvent clathrates.[12−14] A recent study showed that the macrocyclic effect and the number of chains created by different types of interaction are the reasons behind the high melting points of calixarene crystals.[4]. Macrocyclic molecules often crystallize with solvent molecules, unless they are crystallized by sublimation,[18] and move toward the thermodynamically stable state as they transform from their solvated to desolvated forms.[19] In 2002, Atwood et al discovered that p-tert-butylcalix[4]arene undergoes singlecrystal-to-single-crystal phase transitions upon guest uptake and release.[20]. The host−guest binding energies in these complexes were determined using model energies and the energy framework module of CrystalExplorer.[21,22]

EXPERIMENTAL SECTION

RESULTS AND DISCUSSION

CONCLUSIONS

■ REFERENCES