Guest-induced flexibility in crystals

[摘要] ENGLISH ABSTRACT: The primary goal of the work presented here was to prepare both organic and metal-organic porouscrystals (using crystal engineering strategies) in order to study the guest-induced flexibility of suchseemingly rigid materials.The first section describes the structural modification of a known oxacalix[4]arenecompound 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 phaseof a 2,3-naphthalene-derivative was obtained from dimethyl sulfoxide. This host uses the sameprinciple as a molecular tweezer by capturing the guests between its offset eclipsing naphthalenemoieties; 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 systematicconformational study.The work dealing with metal-organic compounds was shown to be the more successful ofthe two parts and forms the largest portion of the work. A series of discrete metallocycles wassynthesised and their ability to function as porous materials was investigated. In all instances themetallocyclic hosts included the solvent of crystallisation within the ring. Desolvation of theincluded solvent did not occur as a single-crystal to single-crystal process and in some instancesdesolvation coincided with decomposition of the metallocyclic host even though the guests aresituated within continuous channels. Although it was not possible to examine the permeability ofthe empty host, single-crystal to single-crystal guest exchange occurs rapidly when exposing theacetonitrile solvated metallocycle to several solvents. Significant adjustment of the hostconformation, as well as the guest-accessible volume, accompanies the uptake of the differentguests. Remarkably, this exchange process can also occur upon exposure to small gaseous guestssuch as I2, CO2, C2H2 and C2H3Cl.The physico-chemical properties of a known seemingly nonporous metallocycle wereinvestigated 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 ofconventional channels in the host structure. Accurate sorption isotherms measured at four differenttemperatures revealed host:guest ratios that are comparable to the host:guest ratio inferred from thesingle-crystal structures. The thermodynamic parameters of sorption ΔHad and ΔSad could bederived from the isotherms and revealed essential information on the affinity of the hosts forparticular 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. Theextent of the host flexibility was explored by studying the permeation of relatively large volatilesolids such as naphthalene and p-dichlorobenzene. Kinetic sorption isotherms in conjunction withprecise single-crystal data revealed a possible mechanism of transport, which was confirmed bymolecular mechanics calculations.

[发布机构] Stellenbosch University