Characterization of crystalline complexes between heptakis(2,3,6-tri-O-methyl)-beta-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 hy drocarbons (n-pentane, n-hexane, cyclohexane, methylcyclohexane, (RS)-alpha -pinene, (S)-alpha -pinene and (R)-alpha -pinene) gave two series of isomo rphous 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, t he experiments carried out in solution at high supersaturations led to the Ua phase, with x < 1 for linear alkanes (non-stoichiometric and efflorescen t phases) and x = 1 for cyclic guests. On the other hand, by slow evaporati on, 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 <appr oximate to> 0.15). Temperature-resolved X-ray powder diffraction allowed th e irreversible transition from Ua to Ub to be observed. Following this ther mal process and whatever the nature of the cyclic guest molecule, Ub was no n-stoichiometric (x approximate to 0.7 for methylcyclohexane); thus, x was significantly lower than that of the mother phase Ua with x = 1. This sugge sts a destructive-reconstructive solid-solid transition. The crystal struct ure solved at 120K of Ub obtained from solution, with methylcyclohexane, re veals that the guest molecule is totally buried within the cavity. The meth ylcyclohexane mean plane, defined by the C(2), C(3), C(5) and C(6) atoms, i s 45 degrees away from the pseudo seven-fold axis of the macrocycle. The me thylcyclohexane molecule is disordered within the cavity, and its possible conformations were twisted Chair and twisted boat. These results differ fro m the conformations reported by Rontoyianni et al., J. Incl. Phenom. 32, 41 5-428 (1998) for the structure of the same complex solved at 293K. Molecula r 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. Compariso n between solid state conformations observed for the TRIMEB molecule in its complexes does not support the notion of 'induced fit' in the inclusion pr ocess.