The crystal structure of 2,8,14,20-tetrapropylpentacyclo[19.3.1.1^3,7.1^9,13.11^5,19]octacosa-1(25),3(28),4,6,9(27),10,12,15(26),16,18, 21,23-dodecaene-4,5,6,10,11,12,16,17,18,22,23,24-dodecol – acetone – water (1(3/3))

1 Source of materials

The synthesis of the title compound, 2,8,14,20-tetrapropylpyrogallol[4]arene was reported previously. ⁴ For crystallization, 50 mg (1.39 mmol) of the compound was dissolved in 1 mL of acetone, overlaid onto 1 mL of water, followed by careful addition of 0.5 mL of toluene on top of the acetone layer, yielding yellow rhombohedral crystals.

2 Experimental details

The data were corrected for Lorentz and polarization effects (SAINT), and semiempirical absorption corrections based on equivalent reflections were applied (SADABS). ¹ The structure was solved by Direct Methods and refined on F² (SHELXTL). ² Hydrogen atoms of 2,8,14,20-tetrapropylpyrogallol[4]arene were added to their geometrically ideal positions, and other hydrogen atoms of solvents were found from the difference map. Hydrogen atoms were found from the difference map. C-bound H atoms were refined using a riding model, with U_iso(H) = 1.5 U_eq(C) for methyl H atoms and 1.2 U_eq(C) for all others. C–H distances were constrained to 0.95 Å for aryl H atoms, 0.98 Å for methyl H, and 0.99 Å for methylene H atoms.

3 Comment

Pyrogallol[4]arenes, a subclass of resorcin[4]arenes, have attracted considerable attention due to their ability to form molecular capsules in both solution and the solid state, as well as their guest-inclusion properties. Research in this area was stimulated by Rebek’s pioneering studies on hydrogen-bonded capsules, which established these assemblies as model systems for investigating weak noncovalent interactions, particularly hydrogen bonding. ⁵ Since the initial report by Gerkensmeier and co-workers on hexameric capsule formation of C-alkyl-substituted pyrogallol[4]arenes in acetonitrile, ⁴ subsequent studies by Atwood, Cohen, Rebek, and others have demonstrated a wide range of capsule architectures. These include hydrogen-bonded hexameric and dimeric capsules influenced by substituent variation and solvent effects, ^{6 , 7 , 8 , 9} metal-coordinated dimeric capsules, ^{10 , 11} hexameric capsules, ¹² and inclusion complexes with diverse organic guests. ^{10 , 13 , 14 , 15 , 16 , 17}

The crystal structure of the title compound reveals one pyrogallol[4]arene molecule in the asymmetric unit, accompanied by three acetone and three water molecules. Notably, one acetone molecule occupies the pocket region of the pyrogallol[4]arene. This acetone appears to interact not only with a water and hydroxyl group through hydrogen bonding but also with a pocket through van der Waals forces. Its presence induces a distortion of the macrocyclic framework. Torsional angle analysis indicates that the cis-oriented set of four pyrogallol units adopts a distorted geometry: two opposite pyrogallol moieties (C1–C6 and C21–C26) exhibit narrower orientations, tilted by 29.88° and 30.32° relative to C7–C8 and C27–C28, respectively, whereas the other two (C11–C16 and C31–C36) are more widely splayed, with tilt angles of 50.56° and 46.63°. Comparable crystal structures of C-substituted pyrogallol[4]arenes incorporating methanol, ¹⁶ or 4,4′-bipyridine and acetone ¹⁷ as guests have been reported.