Characterization of crystalline complexes between heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin and various alkanes or alkenes (5 ≤ C ≤ 10)

Crystallization, at room temperature under normal pressure, of heptakis (2,3,6-tri-O-methyl)-beta -cyclodextrin (TRIMEB) in the presence of various hydrocarbons (n-pentane, n-hexane, cyclohexane, methylcyclohexane, (RS)-alpha -pinene, (S)-alpha -pinene and (R)-alpha -pinene) gave two series of isomorphous crystalline phases: Ua (metastable) and Ub (stable). In this study, the molar guest/host ratio is defined as the variable x. On the one hand, the experiments carried out in solution at high supersaturations led to the Ua phase, with x < 1 for linear alkanes (non-stoichiometric and efflorescent phases) and x = 1 for cyclic guests. On the other hand, by slow evaporation, the Ub phase obtained with cyclic components was stoichiometric (x = 1) and non-efflorescent; whereas the Ub phase with n-pentane and n-hexane as guests displayed efflorescent character and was non-stoichiometric (x <approximate to> 0.15). Temperature-resolved X-ray powder diffraction allowed the irreversible transition from Ua to Ub to be observed. Following this thermal process and whatever the nature of the cyclic guest molecule, Ub was non-stoichiometric (x approximate to 0.7 for methylcyclohexane); thus, x was significantly lower than that of the mother phase Ua with x = 1. This suggests a destructive-reconstructive solid-solid transition. The crystal structure solved at 120K of Ub obtained from solution, with methylcyclohexane, reveals that the guest molecule is totally buried within the cavity. The methylcyclohexane mean plane, defined by the C(2), C(3), C(5) and C(6) atoms, is 45 degrees away from the pseudo seven-fold axis of the macrocycle. The methylcyclohexane molecule is disordered within the cavity, and its possible conformations were twisted Chair and twisted boat. These results differ from the conformations reported by Rontoyianni et al., J. Incl. Phenom. 32, 415-428 (1998) for the structure of the same complex solved at 293K. Molecular simulations of n-alkane (C(5) and C(6)) movements along the a axis showed that the Ut, phase structure can easily undergo a partial release of this linear alkane, due to the presence of channels in this structure. Comparison between solid state conformations observed for the TRIMEB molecule in its complexes does not support the notion of 'induced fit' in the inclusion process.