Abstract
Calixarenes are host molecules that can form complexes with one or more guest molecules, and molecular recognition in calixarenes can be affected by many factors. With a view to establishing molecular recognition rules, the host p-tert-butyl-calix[6]arene (TBC6) was crystallized with different guest molecules (cyclo-hexane, anisole, heptane, toluene, benzene, methyl acetate, ethyl acetate, di-chloro-methane, tetra-hydro-furan and pyridine) and the obtained structures were characterized by X-ray diffraction. With most solvents, 1:1 and/or 1:3 host-guest complexes were formed, although other stoichiometries were also observed with small guest molecules, and crystallization from ethyl acetate produced the unsolvated form. The calculated fill percentage of the TBC6 cavity was ∼55% for apolar guests and significantly lower for polar solvents, indicating that polar molecules can bind to apolar cavities with significantly lower packing coefficients. The most stable crystals were formed by 1:1 host-guest inclusion complexes. The ratio between the apolar surface area and the volume was used to predict the formation of inclusion versus exclusion complexes, with inclusion complexes observed at ratios <40. These findings allow the binding of potential guest molecules to be predicted and a suitable crystal packing for the designed properties to be obtained.
Highlights
Calixarenes, which are vase-like molecules, are widely used in supramolecular chemistry due to the fact that they can be prepared relatively and can be selectively functionalized at different positions to obtain desired shapes and functions (Gutsche, 2008)
Positional disorder was observed in all the crystal structures, and some restraints and constraints were applied during structure refinement to achieve reasonable geometric parameters and anisotropic displacement parameters (ADPs)
Solvated form was only obtained when TBC6 was crystallized from ethyl acetate (11), whereas the remaining solvents formed solvates with 1:3 stoichiometries [benzene (1), pyridine (2) and DCM (3)] or 1:1 stoichiometries [cyclohexane (4), toluene (5), methyl acetate (6), THF (7), pyridine (8), anisole (9) and n-heptane (10)]
Summary
Calixarenes, which are vase-like molecules, are widely used in supramolecular chemistry due to the fact that they can be prepared relatively and can be selectively functionalized at different positions to obtain desired shapes and functions (Gutsche, 2008). The calix[n]arene molecular skeleton consists of a series of phenol rings linked by methylene groups, and structural variants can be obtained by changing the bridging unit or the number of phenolic hydroxyl functionalities Owing to their flexible and geometrically variable interior cavity, which is large enough to accommodate one or more smaller molecules, calix[n]arenes can function as molecular containers or host molecules (Liao et al, 2009; Dalgarno et al, 2006; Lesniewska et al, 2019). The formation of their host–guest complexes is stabilized by intermolecular forces, such as ionic interactions, hydrogen bonding, – interactions, hydrophobic forces and van der Waals forces. Crystal structure analysis is completed by carrying out interaction energy calculations to understand the general trends in host–guest complex formation that centre on the energy rather than the geometric parameters and interaction types
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