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

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.