Guest-induced flexibility in crystals

ENGLISH ABSTRACT: The primary goal of the work presented here was to prepare both organic and metal-organic porous crystals (using crystal engineering strategies) in order to study the guest-induced flexibility of such seemingly rigid materials. The first section describes the structural modification of a known oxacalix[4]arene compound and the ability of the novel derivatives to encapsulate guest molecules in the solid state. Although it was not possible to obtain porous guest-free forms of any of the hosts, an apohost phase of a 2,3-naphthalene-derivative was obtained from dimethyl sulfoxide. This host uses the same principle as a molecular tweezer by capturing the guests between its offset eclipsing naphthalene moieties; a series of solvate structures were obtained. The flexible nature of the host molecule, when enclosing guests of different shapes and sizes, has been illustrated by a systematic conformational study. The work dealing with metal-organic compounds was shown to be the more successful of the two parts and forms the largest portion of the work. A series of discrete metallocycles was synthesised and their ability to function as porous materials was investigated. In all instances the metallocyclic hosts included the solvent of crystallisation within the ring. Desolvation of the included solvent did not occur as a single-crystal to single-crystal process and in some instances desolvation coincided with decomposition of the metallocyclic host even though the guests are situated within continuous channels. Although it was not possible to examine the permeability of the empty host, single-crystal to single-crystal guest exchange occurs rapidly when exposing the acetonitrile solvated metallocycle to several solvents. Significant adjustment of the host conformation, as well as the guest-accessible volume, accompanies the uptake of the different guests. Remarkably, this exchange process can also occur upon exposure to small gaseous guests such as I2, CO2, C2H2 and C2H3Cl. The physico-chemical properties of a known seemingly nonporous metallocycle were investigated in order to formulate a mechanism of transport from one discrete cavity to the next. Crystals of the apohost were shown to be permeable to a series of solvents despite the lack of conventional channels in the host structure. Accurate sorption isotherms measured at four different temperatures revealed host:guest ratios that are comparable to the host:guest ratio inferred from the single-crystal structures. The thermodynamic parameters of sorption ΔHad and ΔSad could be derived from the isotherms and revealed essential information on the affinity of the hosts for particular guests. Significant deviation in the centroid-to-centroid distance between the imidazole rings suggest that the transport of molecules is facilitated by “flapping” of the imidazole rings. The extent of the host flexibility was explored by studying the permeation of relatively large volatile solids such as naphthalene and p-dichlorobenzene. Kinetic sorption isotherms in conjunction with precise single-crystal data revealed a possible mechanism of transport, which was confirmed by molecular mechanics calculations.