Tiara[5]arenes: Synthesis, Solid-State Conformational Studies, Host-Guest Properties, and Application as Nonporous Adaptive Crystals

Abstract

Tiara[5]arenes (T[5]s), a new class of five-fold symmetric oligophenolic macrocycles that are not accessible from the addition of formaldehyde to phenol, were synthesized for the first time. These pillar[5]arene-derived structures display both unique conformational freedom, differing from that of pillararenes, with a rich blend of solid-state conformations and excellent host-guest interactions in solution. Finally we show how this novel macrocyclic scaffold can be functionalized in a variety of ways and used as functional crystalline materials to distinguish uniquely between benzene and cyclohexane.

Keywords: five-fold symmetry; host-guest chemistry; nonporous adaptive crystals; polymorphism; tiara[5]arene.

Figure 1

Tiara[5]arene, a para‐bridged oligophenolic macrocycle, which is not attainable by condensing phenol and formaldehyde, can be synthesized by taking a detour through rim‐differentiated pillar[5]arene.

Scheme 1

The metamorphosis of rim‐differentiated pillar[5]arenes (RD‐P[5]s) into tiara[5]arenes (T[5]s).

Figure 2

X‐ray crystal structures of a) T[5]‐(OMe)₅, b) CH₃CN⊂T[5]‐(OMe)₅, and c) H₂O⊂T[5], showing assorted solid‐state conformations of the tiara[5]arene scaffold. Packing modes in (a), (b), and (c) are viewed from [1$\bar{1}$ 0], [110], and [111] directions, respectively. Most hydrogen atoms and solvent molecules are omitted for clarity. C silver/gold for different enantiomeric conformers, O red, N blue, H white.

Figure 3

Host–guest interactions between tiara[5]arene and cationic species. a) Partial ¹H NMR spectra (400 MHz, MeOD‐d ₄, 298 K) of T[5], chloride salts of methylpyridinium and methyl viologen, and their 1:1 mixtures at 5 mm concentrations. b),c) The association constants of b) [Py⊂T[5]]⋅Cl and c) [MV⊂T[5]]⋅2Cl 1:1 complexes in MeOD‐d ₄ are determined by NMR titration method.

Scheme 2

Versatile derivatizations of the tiara[5]arene scaffold at the ortho, para, and phenolic positions.

Figure 4

Outline of the benzene/cyclohexane fractionation process involving tiara[5]arene‐based nonporous adaptive crystals. a) First, crystals of T[5]‐(OMe)₅ obtained from CH₃CN were subjected to heat/vacuum for solvent removal. b) The activated T[5]‐(OMe)₅ crystalline material was exposed to a 1:1 benzene/cyclohexane mixture vapor (50:50 v/v) under ambient conditions. c) Subsequent PXRD and X‐ray crystallography analyses confirmed the phase change of T[5]‐(OMe)₅ after vapor adsorption, while d) ¹H‐NMR time‐dependent vapor‐solid sorption plot and gas chromatography analysis revealed the preferential uptake of benzene over cyclohexane.