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

When the macrocycles <i>per-</i>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 ¹H NMR and Infrared (IR) spectroscopy, interpreted with <i>ab initio</i> calculations. The <i>ab initio</i> 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.