The complex relationship between guest-free polymorphic products and desolvation of p-tert-butylcalix[4]arene inclusion compounds

Abstract

Depending on the details of the technique employed, desolvation of tBC inclusion compounds leads to either a dense, guest-free polymorph, or a guest-free low density polymorph, the latter also having distinct high and low temperature forms.

Pseudopolymorphism of Aliphatic Amine/4‐tert‐Butylcalix[4]arene Inclusion Compounds: Supramolecular Stabilization as a Route to Polar Clusters and Layers

“…This is quite similar to the spectra previously reported for the low-density guest free forms of 4tBC4A (b 0 and b 0 0 ) as obtained through sublimation [43] or desolvation of 4tBC4A:toluene inclusion compounds. [24] The PXRD pattern for this apo form is consistent with that of the b 0 forms, [43] but too few peaks are available to index the cell reliably.…”

“…The majority of these resonances are readily assigned based on previous studies of 4tBC4A inclusion compounds, the information derived from dipolar dephasing experiments to suppress the signals of CH and CH 2 carbons experiencing significant dipolar coupling, and the solution 13 C spectrum of n-butyl-A C H T U N G T R E N N U N G amine. [23,24,29] The resonances attributed to C1 (d 154-150 ppm) and C4 (d 143-140 ppm) in the aromatic ring are of particular diagnostic value. Both carbons show a four-fold splitting, similar C CP/MAS NMR spectra of a) clathrate 1; b) clathrate 2; and c) clathrate 3.…”

“…Previous studies of simple 4tBC4A inclusion compounds have clearly demonstrated that by heating such inclusion compounds (as with toluene inclusions) or introduction of an alternative guest (as with nitrobenzene inclusions), the balance between these forces is altered, allowing for control of the structural motif. [24,29,36] The inclusion of amines in different environments due to the competition of specific and non-specific interactions introduces an intriguing complication when compared with these previous studies. As discussed above, the competition between these forces gives rise to differing degrees of guest stabilization by the host.…”

“…The 13 C CP/MAS spectrum clearly shows an increase in splitting in both the aromatic and aliphatic regions consistent with the densely packed guest free form of 4tBC4A (a), while the PXRD pattern is indexible based on the available single crystal data for this form. [24] Given the lack of endo guests in the structure of clathrate 3, this suggests that the guest free form obtained by desolvation at low temperatures is actually guided by the initial inclusion motif observed, altogether avoiding the balancing of temperature and heating rate dictated by the balance of forces in the toluene inclusion system. [24] Judicious choice of guest size allows one not only to dictate the structure of the inclusion compounds that result, but also that of the empty guest form obtained after desolvation.…”

“…A family of interrelated polymorphs and pseudopolymorphs of 4tBC4A and its simple inclusion compounds have been characterized, including a low-density form which has shown potential as a gas adsorbent. [11,12,[24][25][26] In light of some of the elaborate covalently modified calixarene receptors previously reported, [16,27,28] self-assembly of 4tBC4A frameworks with guest molecules…”