Modular Crystalline Hosts Derived from Mandelic Acid. Host Synthesis, Inclusion Formation, and Xray Crystal Structures of Two Free Hosts and One Inclusion Complex with Methanol

Abstract

Six new host compounds 2–5 (a, b) in optically resolved and racemic forms derived from mandelic acid and having particular lateral substituents were synthesized. Their properties in crystalline inclusion formation were studied and discussed relative to the unsubstituted parent molecules 1 (a, b). Crystal structures of two optically resolved free host compounds (2a, 3a) and of a respective methanol inclusion complex [2a · MeOH (1:1)] have been determined by Xray analysis [2a · a = 6.1641(2), b = 19.932(1), c = 14.3469(7) Å, orthorhombic, P2₁2₁2₁, D _c = 1.200 g·cm⁻³, Z = 4, R = 0.051 for 1650 observed reflexions; 3a: a = 6.2828(3), b = 24.686(4), c = 15.000(1) Å, orthorhombic, P2₁2₁2₁, D _c = 1.149 g · cm⁻³, Z = 4, R = 0.048 for 2154 observed reflexions; 2a · MeOH (1:1): a = 13.7316(7), b = 5.8722(2), c = 12.7330(6) Å, β = 99.278(4)°, monoclinic, P2₁, D _c = 1.149 g · cm⁻³, Z = 2, R = 0.046 for 1823 observed reflexions]. In all compounds, the molecules are arranged in columns via OH…O hydrogen bonds. In the free hosts only one hydroxyl group acts as donor and the other one is involved in OH…phenyl interactions. However, in the complex, both hydroxyl groups of the host are connected through the methanol molecule by hydrogen bonds being responsible for the formation of the chains. Organic structures with R < 0.030 retrieved from the Cambridge Structural Database revealed the absence of correlation between the C—O bond length and the strength of the hydrogen bond (length of the O…O distance) in the C(sp³)OH·O(sp³) fragments.