Guest effects on the solid-state dynamics of selected inclusion compounds

Engel, Emile R. (2016-12)

Thesis (DPhil)--Stellenbosch University,2016.

Thesis

ENGLISH ABSTRACT: Guest inclusion has always been a central feature of supramolecular chemistry. The present work describes guest effects on particular properties of selected inclusion compounds. Guest replacement is demonstrated as a means of modifying thermal expansion behaviour and porosity. The body of this thesis consists of three manuscripts (two published and one under review). The first manuscript describes a nitromethane solvate of 18-crown-6 that was investigated by means of temperature-resolved single-crystal X-ray diffraction. The compound exhibits exceptionally large positive thermal expansion in two axial directions and exceptionally large negative thermal expansion along the third. The underlying mechanism relies exclusively on weak electrostatic interactions to yield a linear thermal expansion coefficient of −129×10−6 𝐾−1, which is, to the best of our knowledge, the largest negative value yet recorded for an organic inclusion compound. Our discovery of a reported analogue with acetonitrile led us to prepare an isoskeletal series of compounds for further inquiry. The second manuscript demonstrates that guest replacement in a series of isoskeletal organic inclusion compounds can produce remarkable changes in thermal expansion behaviour. The three inclusion compounds have 18-crown-6 as host molecule and nitromethane, acetonitrile or iodomethane as guests. Along principal axis X1 the linear component of thermal expansion is negative for the nitromethane and acetonitrile solvates but zero for the iodomethane solvate. The compounds show varying degrees of large volumetric thermal expansion, with coefficients of 378(22), 226(3) and 256(8)×10−6 𝐾−1 for the nitromethane, acetonitrile and iodomethane solvates, respectively. Crystal structure analysis and computational methods were used to elucidate general features of the underlying mechanism of thermal expansion for the series. Interestingly, the thermosalient effect was observed for the acetonitrile version. To our knowledge this is the first example of thermosalience reported for an inclusion compound. The success with molecular organic crystals prompted similar experimentation with a different class of materials. As inorganic inclusion compounds, MOFs are an obvious choice for experiments involving guest replacement because of their proven capability for guest exchange, and the great interest in MOFs as potential porous sorbents for molecular storage and separation in industry. The final manuscript describes a non-interpenetrated MOF with a paddle-wheel SBU that has been activated by direct thermal evacuation, guest exchange with a volatile solvent, and supercritical CO2 drying. Conventional thermal activation results in a mixture of crystalline phases and some amorphous content. Exchange with a volatile solvent and subsequent vacuum activation produces a pure breathing phase with high sorption capacity, selectivity for CO2 over N2 and CH4, and substantial hysteresis. Supercritical drying can be used to access a guest-free open phase. Pressure-resolved differential scanning calorimetry was used to investigate the systematic loss of sorption capacity by the breathing phase as a function of successive cycles of sorption and desorption.

AFRIKAANSE OPSOMMING: Die insluiting van gas molekules is nog altyd ’n sentrale kenmerk van supramolekulere chemie. Hierdie tesis beskryf die effek van verskillende gas molekules op sekere fisiese eienskappe van uitgesoekte multikomponent kristalle. Dit word bewys dat die vervang van gas molekules ’n manier is om termiese uitsetting en poreusheid te manipuleer. Die resultate is beskryf in drie artikels; twee wat gepubliseer is en een wat nog onder hersiening is. Die eerste artikel beskryf ’n molekulêre kompleks van 18-kroon-6 en nitrometaan wat ondersoek is deur veranderende-temperatuur enkelkristal X-straal diffraksie. Die kompleks ondergaan groot positiewe termiese uitsetting op twee asse en uitsonderlik groot negatiewe termiese uitsetting op die derde as. Die onderligende meganisme van hierdie ongewone termiese uitsetting is eksklusief afhanklik van relatief swak elektrostatiese interaksies wat ’n liniêre termiese uitsettingskoeffisient van −129×10−6 𝐾−1 gee. Sover ons kennis strek is hierdie die grootste negatiewe waarde vir ’n organiese insluitingsverbinding wat nog ooit gepubliseer is. Die tweede artikel wys dat die vervang van gaste in ’n reeks isostrukturele organiese insluitingsverbindings kan lei tot merkwaardige verkille in termiese uitsetting. Die drie insluitingsverbindings het 18-kroon-6 as gasheer en nitrometaan, asetonitriel en jodometaan as gaste. Op die hoof-as X1 is die liniêre komponent van termiese uitsetting negatief vir die nitrometaan en asetonitriel solvate maar nul vir die jodometaan solvaat. Die komplekse het verskillende grade van groot volumetriese termiese uitsetting, met koeffisiente van 378(22), 226(3) en 256(8)×10−6 𝐾−1 vir die nitrometaan, asetonitriel en jodometaan solvate, afsonderlik. Kristalstruktuuranalise en berekeningsmetodes is gebruik om die algemene kenmerke van die onderliggende meganisme van die reeks uit te lig. Dit is interessant dat die “thermosalient” effek opgemerk is vir die asetonitriel solvaat. Sover ons kennis strek is hierdie die eerste voorbeeld van “thermosalience” vir ’n insluitingsverbinding. Die sukses met molekulêre kristalle het ons aangemoedig om soortgelyke experimente uit te voer met ’n ander klas materiale. As anorganiese insluitingsverbindings is metaal-organiese raamwerke ’n natuurlike keuse vir eksperimente wat gasuitruiling behels, omdat kristalle van die klas oor die algemeen gasuitruiling maklik verduur, asook omdat metaal-organiese raamwerke so bekend is vir hul potensiële industriële gebruik vir die stoor en skeiding van verskeie molekules. Die finale artikel beskryf ’n nie-geïnterpenetreerde metaal-organiese raamwerk met ‘n skeprat sekondêre bou-eenheid wat geaktiveer is deur direkte termiese ontruiming, gasuitruiling met ‘’n vlugtige oplosmiddel en superkritiese CO2 aktivering. Gewone termiese aktivering lei tot ’n mengsel van kristallyne fases en ’n amorfiese komponent. Uitruiling met ’n vlugtige oplosmiddel, gevolg deur aktivering teen kamertemperatuur, lei tot ’n “asem haal” fase met ’n hoë kapasiteit vir CO2 adsorpsie bo N2 en CH4 en groot histerese. Differensiële skandeer kalorimetrie met veranderende druk is gebruik om die sistematiese afname in adsorpsiekapasiteit deur die “asem haal” fase as ’n funksie van opeenvolgende siklusse van adsorpsie en desorpsie te ondersoek.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/100059
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