Synthesis and Characterization of Solvent-Complexes of per -Hydroxy Pillar[5]arene and Pillar[5]quinone: Experimental and Computational Insights

High Resolution Image Download MS PowerPoint Slide When the macrocycles per- hydroxy pillar[5]arene (P[5]A–OH) and pillar[5]quinone (P[5]Q) are recrystallized from suitable solvents like acetone and 1,1,2,2-tetrachloroethane (TCE), respectively, the solvent molecules form host–guest complexes in a 1:2 stoichiometric ratio. These complexes survive prolonged exposure to vacuum at room temperature. Herein, we report improved yields for the preparation of 1,4-dimethoxypillar[5]arene (DMP[5]A) and its derivatives P[5]A–OH and P[5]Q, including detailed recrystallization procedures. The complexation behavior of these solvent molecules was investigated by 1 H NMR and Infrared (IR) spectroscopy, interpreted with ab initio calculations. The ab initio calculations demonstrate that P[5]A–OH encapsulates two acetone molecules within its cavity, whereas P[5]Q binds with two TCE molecules outside the cavity. These complexes are stabilized by a combination of hydrogen bonding and CH−π interactions. IR spectroscopy confirmed the absorption of acetone by P[5]A–OH and displayed a blue-shifted C═O stretch (1779 vs 1727 cm –1 in free acetone) and red-shifted O–H modes (3384/3622 cm –1 ). Our charge transfer and structural analysis indicate that the C═O bond of acetone is contracted (1.24 Å→1.22 Å) due to enhanced polarization. For P[5]Q, TCE absorption is driven by hydrogen bonds, evidenced by the shift of C–H IR peaks near 3000 cm –1 and red-shifted C–Cl/C═O vibrations. Our study highlights the critical role of functional groups and noncovalent interactions in guest orientation, providing key insights for designing supramolecular materials.