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

## TL;DR

This paper studies how two macrocyclic compounds form stable complexes with solvents, revealing how their structures and interactions influence supramolecular material design.

## Contribution

The study provides new insights into the complexation behavior and stabilization mechanisms of per-hydroxy pillar[5]arene and pillar[5]quinone with solvents.

## Key findings

- P[5]A–OH encapsulates two acetone molecules via hydrogen bonding and CH−π interactions.
- P[5]Q binds two TCE molecules outside its cavity, stabilized by hydrogen bonds.
- IR spectroscopy and ab initio calculations confirm structural and vibrational changes in solvent complexes.

## Abstract

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 1H 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.