Inclusion of Methylamines with the Crystal of p-tert-Butylthiacalix[4]arene: Inclusion Selectivity and Its Switching by Solvent Polarity

Abstract

The crystal of p-tert-butylthiacalix[4]arene (2) includes mono-, di-, and trimethylamines from their respective aqueous solutions to give 1:1 (host/guest) inclusion crystals. In competitive inclusion of these three amines, dimethylamine is selectively included. X-ray crystallographic analysis of the inclusion crystals of dimethylamine and trimethylamine reveals that the amines are included by forming salts with the calixarene (H 4 L). In the inclusion crystal of dimethylamine, two dimethylammonium ions and two water molecules form an aggregate, which is interposed between two calix anions (H 3 L − ) arranged in a tail-to-tail manner to construct an exo complex. In the inclusion crystal of trimethylamine, however, a trimethylammonium ion is included in the cavity of the calix anion to form an endo complex. In competitive inclusion of dimethylamine and trimethylamine, the guest selectivity can be switched by changing the polarity of solvent; dimethylamine and trimethylamine are selectively included from low-(32.4 ≤ ε ≤ 65.9) and high-permittivity solvents (88.9 ≤ ε ≤ 132.6), respectively. Mechanistic studies reveal that the inclusions of dimethylamine and trimethylamine are favored under kinetic and thermodynamic control, respectively.

Inclusion of Amine Isomers with Open-Chain Hosts Having a Partial Structure of p-tert-Butylthiacalixarene

“…As a result, the initial selectivity toward 4-methylpyridine is maintained in hexane. We previously reported a similar observation that low-polarity solvents inhibit the guest exchange of dimethylamine with trimethylamine from inclusion crystals of compound 1 with dimethylamine …”

“…On the contrary, the accumulation of host/guest complexes produces a difference in the thermodynamic stabilities of the resulting inclusion crystals with different guest compounds to realize thermodynamic selectivity. We succeeded in the inclusion of alcohols, carboxylic acids, and amines with high guest selectivities using crystals of compound 1 under kinetic or thermodynamic control. Switching of the guest selectivity by changing the solvent polarity or inclusion temperature was also achieved in some cases.…”

“…We succeeded in the inclusion of alcohols, carboxylic acids, and amines with high guest selectivities using crystals of compound 1 under kinetic or thermodynamic control. Switching of the guest selectivity by changing the solvent polarity or inclusion temperature was also achieved in some cases. However, the guest scope of the inclusion with the crystals of compound 1 is limited by the narrow calix cavity and the packing structure of the resulting inclusion crystals.…”

“…We previously reported a similar observation that low-polarity solvents inhibit the guest exchange of dimethylamine with trimethylamine from inclusion crystals of compound 1 with dimethylamine. 10 Regioisomers of Methylquinoline. Competitive inclusion among 2-, 6-, and 8-methylquinoline was conducted using diacid 2 and 10 molar equivalents each of the amine isomers under conditions similar to those used for the competitive inclusion among the methylpyridine isomers (Table 2).…”

Selective inclusion of p-xylene by bis-hydrazone compounds

“…14 Despite these advances, organic host molecules have emerged as a potential candidate for the separation of molecular isomers using their inclusion property. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] Designing host molecules for selective inclusion is extremely challenging due to the unpredictable packing of molecules in the crystal structure. Nevertheless, the separation of xylene isomers by the selective inclusion property of certain organic host compounds has been studied by several research groups.…”