Doctoral Degrees (Chemistry and Polymer Science)

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Computational assessment and visualization of guest inclusion in porous crystalline materials.
(Stellenbosch : Stellenbosch University, 2020-11) Van Heerden, Dewald Pepler; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: Host-guest systems are a central feature of supramolecular chemistry and an improved understanding of guest inclusion phenomena (i.e., transport and accommodation) may lead to the development of improved functional porous materials. However, the generally greater degree of thermal motion of guest molecule(s) relative to the host often impedes their accurate modelling in crystal structures. Furthermore, activation, the act of removing the guest to yield the empty apohost phase, often results in a loss of single crystallinity. Physical stimuli such as temperature, light, mechanical pressure, gas and liquid sorption, and electric and magnetic fields can induce structural transformations; however, the scarcity of in situ or in operando single-crystal X-ray diffraction (SCXRD) data encumbers the direct elucidation of underlying mechanisms. The work presented in this thesis contribute to the collective understanding of inclusion and dynamic phenomena in the solid state. Results are presented as two manuscripts, both of which have been submitted for peer review. The first manuscript describes the systematic derivation of a fractional occupancy factor θ that allows for “back-of-the-envelope” estimation of host/guest ratios in crystal structures. A common method of evaluating guest-occupiable space involves in silico sampling of free space by means of a spherical probe in van der Waals contact with the host structure. In addition to discussing caveats pertaining to reporting guest-occupiable space, we propose the terms probe-traversable volume (PTV) and probe-accessible volume (PAV) to refer to the regions mapped out by the center and surface of the probe, respectively. High-throughput analysis of close to 40 000 solvates yielded a weighted mean θ value of 51.1(4)%, where θ is the ratio of the molecular volume of the solvent to the PAV. The utility of our methodology is demonstrated through application to case studies encompassing 0D organic and organometallic, as well as metal-organic framework (MOF) materials. As part of an ongoing investigation in our group, the second manuscript reports on a combined theoretical and in situ SCXRD structural analysis of a flexible MOF subjected to methane, ethane, propane and butane gas pressures. Whereas subnetwork displacement is induced by sorption of ethane, distinctly stepped sorption isotherms for propane and butane are associated only with linker rotation; i.e., true gate opening. A turnstile mechanism is elucidated whereby neighboring pockets are temporarily fused to allow diffusion of guest molecules through the seemingly nonporous system.
Defect engineering of UiO-66 metal-organic framework (MOF) for improved hydrogen storage applications
(Stellenbosch : Stellenbosch University, 2021-03) Ledwaba, Mpho Violet; Barbour, Leonard J.; Ren, Jianwei; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: Metal–organic frameworks (MOFs) as a relatively new class of porous crystalline materia ls have attracted much interest in many applications due to their high porosity, diverse structures, and controllable chemical structures. However, the specific geometrical morphologies, limited functions and unsatisfactory performances of pure MOFs hinder their further applicatio ns . Several modifying strategies for engineering MOF crystals have been developed based on their desired features and applications. In nature there are no "ideal crystals" with an infinite periodic repetition or ordering of the same groups of atoms in space. The structure of "real crystals" often deviates from the ideal arrangement and includes a significant density of structural irregularities or deficiencies. Crystal irregularities may arise from compositio na l inhomogeneities, and this term is often used interchangeably. By using defective technology strategies during their synthesis, crystal imperfections may be introduced into the MOF framework, thereby enhancing their performance in the envisioned applications. Defect engineering is one of the most effective approaches that one can use to change their physical and chemical features such as thermal stability, textural properties, mechanical properties and gas adsorption abilities. In order to achieve the desired changes, it is essential to control the defects, otherwise these defects may have an adverse effect on the MOFs. Therefore, it is vitally important to apply synthetic control over defects; the exact nature and concentration of the defects may be controlled by modifying the synthetic conditions and post-synthet ic modifications. Structurally characterising inherent or engineered defects is very challenging and this challenge has not been addressed substantially. This thesis explores the experimental creation of structural defects via post-synthetic modification, the role of structural defects and their relationship to gas adsorption, with emphasis on hydrogen adsorption. Through a combination of techniques, including powder X- ray diffraction (PXRD), thermogravimetric analysis (TGA), acid-base titration and Brunauer– Emmett–Teller (BET) surface area and pore size measurements, missing linkers and missing cluster defects have been identified and analysed. In Chapter 4, we seek to understand the relationship between some of the major synthetic parameters and the physicochem ic a l characteristics of UiO-66 (Universitetet i Oslo) MOF and discover a "non-defective" sampling technique for this material. The technique produces non-defective UiO-66 MOFs at a specific temperature (493 K), with the linker ratio being greater than that of the salt previously reported by Shearer et al. As described in Chapter 5, by varying the concentrations of modulator and the linker, we demonstrate that the linker vacancies can be systematically tuned, leading to significantly increased surface areas. The defects are caused by partial terephthalic acid eplacement with smaller formate groups from the formic acid modulator. The BET surface areas of the obtained samples range from 1200 to 1600 m2.g-1, and the best sample has a surface area that is about 30% higher than the theoretical value of the surface area of defect-free UiO- 66. Additionally, linker vacancies are proven to have profound effects on the gas adsorption behaviour of UiO-66 by improving the hydrogen uptake from 1.51 wt. % to 2.0 wt. % at 77 K and 1 bar. Chapters 5 and 6 include detailed studies of two conventional methods for generating defects (i.e., de novo defect technology and post-synthetic modification (PSM)) discussed in each chapter respectively. Still in chapter 5, experimental investigations are discussed that show the impact of modulator and linker concentration on H2 adsorption and thermal stability. Chapter 6 provides insight into the impact on thermal stability and adsorption properties brought about by the post-synthetic modification methods. The resultant materials typically have high surface areas, large pore volumes and structures with hierarchical pores, which makes them more practical for hydrogen storage applications.
The dynamics of new self-assembled porous materials
(Stellenbosch : Stellenbosch University, 2015-12) Nikolayenko, Varvara I.; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept of Chemistry and Polymer Science.
ENGLISH ABSTRACT: principal goal of this study was to prepare a wide selection of crystalline second- and third-generation metal-organic materials in order to investigate guest sequestration and storage capabilities, as well as guest exchange and sorption-induced dynamics in the solid state. Chapters I and II contain a review of the relevant literature, as well as a description of the experimental techniques employed in this work. Chapter III describes the diversity of metal-organic frameworks (MOFs) obtained from a small selection of pyridyl-functionalised ligands (varying in length and rigidity) combined with a series of carboxylic acids (also varying in length and rigidity) in the presence of one of four transition metals. Twenty one different MOFs varying in metal coordination mode, degree of interpenetration, solvent-accessible space and guest were obtained. Four of these MOFs undergo single-crystal to single-crystal (SC-SC) transformations, including activation, gas sorption and solvent exchange. Chapter IV focuses on two isostructural MOFs. The monochromic zinc-containing framework undergoes SC-SC guest exchange, indicating a mild preference for para-xylene, while gas sorption experiments revealed a clear preference for carbon dioxide. The pleochroic crystals of the cobalt analogue showed no xylene selectivity, but gas sorption experiments revealed preferential selectivity toward methane carbon dioxide and ethane. Crystallisation from solutions containing both metals (and the same ligands) resulted in the formation of trichroic solid solutions, the colour of which could be fine-tuned by varying the ratio of the two metals. To complete the study, a homoepitaxial crystal was grown. Chapter V focuses on a modified version of the ligand described in Chapter IV, with electron-withdrawing fluorine atoms located on the central phenyl ring. Use of this ligand resulted in charaterisation of a twofold interpenetrated porous framework, differing from that described in the previous chapter. Activation, as well as guest exchange, elicited “crank-handle” conformational changes in the ligand. Sorption experiments showed the material to be selective for carbon dioxide over several linear alkanes. Reaction using the hydrochloride salt of this ligand yielded a novel interdigitated MOF. Substitution of cadmium for zinc in the reaction gave a third non-interpenetrated framework where the ligand has undergone [2 + 2] cycloaddition and is present in the anti conformation. SC-SC activation as well as guest exchange experiments revealed this framework to be less flexible than its zinc counterpart. Exposure of the activated material to carbon dioxide indicated substantial uptake of carbon dioxide. A mixture of the DMF and DMSO produced two additional isoskeletal structures differing in paddlewheel construction, ligand conformation and, as a result, in the solvent-accessible void space. Chapter VI describes the response of five MOFs based on diarylethene ligands to light. The structural changes underlying the colour change differ in each case. The first framework is a porous twofold interpenetrated MOF that undergoes change in colour and shape upon UV irradiation. When the same crystal was exposed to white light, the parameters revert back to those of the original form. The activated form of a second fourfold interpenetrated highly porous MOF showed an affinity for carbon dioxide, as well as a change in colour upon irradiation. Incorporation of a more flexible ligand gave a twofold interpenetrated porous material that showed low temperature activation but could not be evacuated without loss of crystallinity. UV irradiation induced a colour change but no structural change. The fourth framework is a twofold interpenetrated structure that collapses to a seemingly non-porous form upon removal of the solvent. UV irradiation of this material results in ring-closure of the diarylethene ligand, along with a colour change. Remarkably, all these processes occur in a SC-SC fashion. This is the first example of photo-induced ring closure of a diarylethene molecule incorporated into a MOF. The fifth framework exhibits a high degree of interpenetration (fourfold) but unlike all previous examples, irradiation with UV light induced no visible colour change and no structural change.
Guest effects on the solid-state dynamics of selected inclusion compounds
(Stellenbosh : Stellenbosch University, 2016-12) Engel, Emile R.; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
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.
Guest-induced flexibility in crystals
(Stellenbosch : Stellenbosch University, 2012-03) Heyns, Anneli; Barbour, Leonard J.; Stellenbosch University. Faculty Sciences. Dept. of Chemistry and Polymer Science.
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.
Host-guest dynamics in copper-based metal-organic frameworks
(Stellenbosch : Stellenbosch University, 2017-12) Bezuidenhout, Charl X.; Barbour, Leonard J.; Esterhuysen, Catharine; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: A vast number of metal-organic framework materials (MOFs) and structures have been reported in the literature. These materials are of interest as they exhibit a wide range of remarkable properties such as porosity, magnetism, chromism, thermal expansion and dynamic phase-change behaviour. However, many of the reports do not include investigations of all these possible properties, which provides an opportunity to revisit such materials. This study reports three sets of related MOFs that were studied in order to gain insight into their properties and dynamic host-guest interactions. In the first section three isoreticular copper(II)glutarate based 3D MOFs, whose pore dimensions vary according to the length of the 4,4´-bipyridyl linkers, allows exploration of the effect of increasing pore dimensions on the sorption behaviour of the frameworks. These MOFs capture CO2 under supercritical conditions and retain the gas under ambient conditions. The retention of the CO2 makes it possible to model the positions of the molecules within the channels of the frameworks using single-crystal X-ray diffraction analysis (SCXRD). Comparison of the CO2 gas sorption isotherms with the trend in thermal evolution of the CO2 adducts provided insight into the dynamics of the host-guest interactions within the MOFs. Theoretical models constructed from crystallographic data were used to calculate framework electrostatic potential maps through DFT and semi-empirical methods. The maps yielded a detailed picture of the electrostatic interactions prevailing at the sorption sites, which shows an electrostatic complementarity between the sorption sites and the CO2 molecules for two of the MOFs. The δ+ carbon of the CO2 molecules interacts with the δ– carboxylate moieties of the framework. Furthermore, both δ– oxygen atoms of each of the CO2 molecules interact with several δ+ hydrogen atoms of the framework. This electrostatic complementarity between CO2 and the sorption site results in a strong interaction, which stabilises the gas within these MOFs. The second set of MOFs studied comprises two 3D Cu(II)-glutarate-based MOFs with flexible linkers, [Cu2(glu)2(bpa)] and [Cu2(glu)2(bpp)], that undergo spontaneous phase changes upon solvent loss at room temperature. These MOFs are an extension of the isoreticular series of the previous section. Using SCXRD, we show that the phase changes result in new narrow-channel phases, with a large reduction in solvent accessible volume as compared with the original wide-channel phases. Moreover, the [Cu2(glu)2(bpa)] MOF displays a stepped sorption isotherm upon CO2 sorption at RT. This is indicative of the framework reverting to the wide-channel phase. The positions of the CO2 molecules in the channels of the frameworks were determined using SCXRD analysis of crystals exposed to supercritical CO2. Finally, a scan of the potential energy surface using molecular mechanics was conducted to elucidate the mechanism by which the phase change occurs. This appears to be a direct enantiomeric conversion of the glutarate ligands as a result of structural constraints. The third section describes a one-dimensional porous coordination polymer (PCP) [Cu2(acetate)2(dptz)]n•2CHCl3, that possesses pleochroic properties, i.e. the crystals appear to have different colours depending on their orientation. Solvent exchange with acetonitrile and nitromethane reveals that crystals of this PCP are also solvatochromic. This allows the solvent exchange process to be monitored using optical microscopy with polarised light. The experiment revealed an unexpected double V-shaped pattern in the crystals, which could be explained using an overlay of the measured crystal facets with the crystal structure, along with modelling of crystal surface adsorption at the different crystal facets. Furthermore, we identified that the crystals must be growth twins subject to the double re-entrant corner effect.
Inclusion Studies of Metal-Organic Hosts
(Stellenbosch : Stellenbosch University, 2012-03) Potts, Storm Victoria; Barbour, Leonard J.; Haynes, Delia A.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: The primary objective of the work presented in this thesis was to prepare and investigate the properties of novel inclusion complexes of metal-organic host materials. Two types of host materials were studied, namely 0D metallocycles and 3D metal-organic frameworks (MOFs). The first section details the synthesis and characterisation of six ditopic imidazole derived ligands. These conformationally flexible ligands were reacted with a variety of transition metal salts in a systematic manner with a view to synthesising 0D metallocycles. The structures of 29 novel coordination complexes were elucidated by single-crystal X-ray diffraction and five of these were 0D metallocycles. The thermal stability of the five metallocyclic inclusion complexes was investigated and it was found that all five of the metallocycles have remarkably high thermal stability with respect to the loss of their included volatile solvents. Indeed, in some cases the solvent is only lost when the host decomposes. This is a relatively rare phenomenon, with only a few examples of such systems reported in the literature. A computational study was undertaken in an attempt to rationalise this high thermal stability in terms of guest-guest and host-guest interaction energies. Furthermore, although the single crystals do not survive the desolvation process, all five of these systems exhibit Class II porosity (i.e. the included solvent could readily be exchanged for a different solvent guest without significant disruption to the host packing arrangement). This exchange process was monitored as a single-crystal to single-crystal transformation. The final section reports the attempted gas-phase diffusion of two organic radicals into four porous crystalline materials. The radicals diffused into only one of the four possible host systems, thereby indicating a selective inclusion process. The structures of the two new inclusion complexes were elucidated from powder X-ray diffraction data and it was demonstrated that the properties of both the organic radical as well as the host framework are modified in the new material. Furthermore, the host framework acts as a “vessel” in which the organic radicals, which are normally highly reactive species, can be stored under ambient conditions within the channels of the host framework for a period of three months without degradation. The host framework can thus be used for controlled release of the reactive species by exposure to a suitable solvent.
Investigating the solid-state dynamics of 1D coordination polymers
(Stellenbosch : Stellenbosch University, 2021-03) Van Wyk, Lisa Mercene; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: The aims of this study were twofold: (i) to produce a series of 1D materials exhibiting novel solid-state packing, and (ii), to study these materials with a view to gaining insight into their structure-property relationships. Studies of the ability of the materials to undergo several single-crystal to single-crystal transformations were prioritised. Both dynamic behaviours and transformations were observed in the series produced. The first study describes the mechanochemical separation of two solvates that crystallise concomitantly under solvothermal conditions. Variation of the solvent water composition, and the introduction of aging periods before and after grinding provided various methods of preparing either solvate. A third solvate was prepared in a separate solvent system from the first two. Finally, solvent exchange allowed relatively facile interconversion between all three forms The second study focused on the transient porosity of two 1D coordination polymers. These materials show extreme contraction upon guest release equating to a reduction of up to 39% of the unit-cell volume. Importantly, these transformations progress in a single-crystal to single-crystal fashion and represent some of the largest reported lithotropic contractions. Consequently, the contortion of the conceptually linear ligand in one material far exceeds previous reports. These materials both resorb liquid guests, and one material sorbs vapours. Furthermore, greater ease of structural reopening occurs with each consecutive vapour sorption cycle. In Chapter 5, the low-temperature phase-change behaviour and structural dynamics in terms of thermal expansion of a series of materials, where only the metal centre is varied, are studied. This yielded a series of materials with analogous packing motifs. In nearly all cases, colossal linear and volumetric thermal expansion were recorded in the temperature range 100–270 K. In some cases, supercolossal linear thermal expansion was exhibited, in addition to extreme anisotropy. The results are comparable to the largest linear and most anisotropic thermal expansion reported, but far exceed that reported for 1D coordination polymers. The largest anisotropic thermal expansion occurred in solid solutions, indicating that this behaviour is tuneable. Studies of structure-property relationships and solid-state dynamics require firstly single-crystal to single crystal transformations and secondly bulk phase purity. In 1D coordination polymers both these criteria are often problematic. This work addresses both these concerns, providing methods to overcome or circumvent these hindrances. Furthermore, several of the outcomes in this work exceed or are comparable to the limits of similar solid-state dynamics reported in the literature
Self-assembly of new porous materials
(Stellenbosch : University of Stellenbosch, 2009-03) Jacobs, Tia; Barbour, Leonard J.; Bredenkamp, M. W.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: The primary objective of the work was to prepare and investigate new porous materials using the principles of crystal engineering. Both organic and metal-organic systems were studied and the work can best be divided into two separate sections: 1. The crystal engineering of Dianin’s Compound, a well-known organic host. 2. The design and synthesis of a series of related porous coordination compounds consisting of discrete, dinuclear metallocycles. The first section discusses the synthetic modification of Dianin’s compound in order to engineer a new clathrate host with an altered aperture size. Although this study ultimately failed to isolate the host material in its porous guest-free form, the work led to the discovery of a chiral host framework that aligns guest molecules in a polar fashion, and consequently displays non-linear optical properties. These findings are unprecedented in the long history of crystal engineering of Dianin’s compound and its analogues. This section also describes desorption studies of the new inclusion compound, as well as the known thiol analogue of Dianin’s compound. Systematic characterisation of these desorbed phases has raised interesting fundamental questions about desolvation processes in general. The second section constitutes the major portion of the work. A series of related isostructural coordination metallocycles were synthesised and their structure-property relationships were investigated using a variety of complementary techniques. These metallocyclic compounds all crystallise as solvates in their as-synthesised forms, and different results are obtained upon desolvation of the materials. In each case, desolvation occurs as a single-crystal to single-crystal transformation and three new “seemingly nonporous” porous materials were obtained. A single-crystal diffraction study under various pressures of acetylene and carbon dioxide was conducted for one of the porous metallocycles. This enabled the systematic study of the host deformation with increasing equilibrium pressure (i.e. with increasing guest occupancy). The observed differences in the sorption behaviour for acetylene and carbon dioxide are discussed and rationalised. Gravimetric gas sorption isotherms were also recorded for the three different porous materials and the diffusion of bulkier molecules through the host was also investigated structurally. Finally, a possible gas transport mechanism is postulated for this type of porous material (i.e. seemingly nonporous), and this is supported by thermodynamic and kinetic studies, as well as molecular mechanics and statistical mechanics simulations.
Solid-state dynamics of porous materials
(Stellenbosch : Stellenbosch University, 2013-12) Grobler, Ilne; Barbour, Leonard J.; Esterhuysen, Catharine; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: A number of porous metal-organic and organic crystalline materials were prepared with a view to study the solid-state dynamics involved in sorption and guest-exchange processes. The main goal was to gain an understanding of the mechanisms involved in structural changes induced by external stimuli. To this end, single-crystal X-ray diffraction (SCD) experiments were carried out under variable temperature and pressure conditions, supplemented by thermal analysis and sorption experiments.
The structural analysis of imidazole-functionalised metallocycles
(Stellenbosch : Stellenbosch University, 2012-12) Loots, Leigh-Anne; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: The primary objective of this study was to synthesise novel metallocyclic compounds and analyse their crystal structures. To this end seven novel imidazole-functionalised ligands were synthesised and reacted with a variety of transition-metal halide salts. In addition to this study a new, simple, yet robust methodology for the analysis of π···π packing motifs in aromatic molecules is described. Seventeen homeotypic metallocyclic compounds were obtained with a ligand containing a dimethoxyphenyl spacer between imidazole functionalities. These compounds form reliably from a number of solvent systems involving acetonitrile and they all include acetonitrile molecules as part of their host assembly. In each case a second guest molecule is enclosed within the walls of the metallocycles. These compounds are compared by means of thermal analysis, calculated powder X-ray diffraction patterns as well as crystal packing similarity calculations. Metallocycles prepared from a phenanthrene-based ligand form an unprecedented infinite π···π stack, which induces the assembly of an infinite catenane. This catenane forms concomitantly with its topological isomer, which consists of unlinked metallocycles. The intermolecular interactions responsible for the two topologically unique structures were investigated. Finally, a further twelve metallocycles were prepared from four novel imidazolefunctionalised ligands and their structures were analysed for any similarities and/or differences. A few of these crystals showed the release of solvent guest molecules as singlecrystal to single-crystal transformations.
Structure-property relationships in stimulus-responsive cadmium- and zinc-based metal-organic framework materials
(Stellenbosch : Stellenbosch University, 2020-03) Claassens, Isabella; Haynes, Delia A.; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: The study of flexible MOFs or soft porous crystals is an exciting and fast-expanding area of MOF research. These materials respond dynamically to external stimuli, including temperature, mechanical pressure, guest sorption or light. Flexible MOFs are prime candidates for gas separation and storage, catalysis and sensing applications. Thus far the focus has largely been on the creation of new MOFs. However, focusing on fine-tuning and controlling the properties of these materials can produce advanced materials with potential applications in several areas of industry. This study investigates the structure-property relationships of three related flexible MOFs to explore ways in which to tune and control their response to specific stimuli. The effects of guest inclusion, temperature and light on the structure of the material are studied. The first two studies place special focus on photochemical cycloaddition and exploring means with which to control the outcome of the reaction. We describe the use of guest exchange as a means to control the position of photochemical [2+2] cycloaddition between bpeb ligands in a new Cd (II) MOF. The selective synthesis of one of two different isomeric products is demonstrated by exchanging one solvent in the channels of the MOF for another. The nature of the guest controls the conformation of the organic linker (bpeb) within the framework, which has a direct effect on the regioselectivity of the cycloaddition reaction. The study demonstrates how a response to different guests in a flexible MOF can be utilised as a synthetic tool and is the first example of the synthesis of both isomers of the cyclized bpeb dimer in the same framework. In the second study temperature change is employed as a stimulus control regioselectivity of the photochemical cycloaddition reaction in the same Cd(II) MOF. The isomeric product formed depends on the temperature at which irradiation is carried out. A rare temperature-induced phase transition alters the conformation of the bpeb ligand in the MOF, and thereby the position at which cycloaddition occurs. The phase transition is fully described and characterised using variable temperature X-ray diffraction techniques. This work also highlights the multistimulus responsive nature of this Cd(II) MOF, which responds to temperature as well as solvent exchange and light. The final section describes a new highly flexible four-fold interpenetrated fluorinated MOF that undergoes breathing and subnetwork displacement modes of flexibility to give interesting gas sorption behaviour. The effect of fluorinated substituents on the gas sorption behaviour is investigated by comparing it to the non-fluorinated isoreticular analogue. Sorption analysis reveals significant changes in gate-opening pressure as well as the degree of hysteresis observed between the two frameworks. In situ SCXRD techniques and computational analysis are employed to describe and identify the interactions responsible for the change in behaviour. This study shows that fluorination of pillar linkers is a viable pathway to developing frameworks that exhibit favourable sorption profiles for future studies. These studies improve the understanding of how to control or fine-tune MOFs that change their properties in response to particular stimuli.
Switching of degree of interpenetration and its effects on porosity of metal-Organic Frameworks (MOFs)
(2015-12) Aggarwal, Himanshu; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science
ENGLISH ABSTRACT: of solvent molecules from the channels. The primary objective of this work was to reinvestigate these systems in order to gain an understanding of the reasons behind loss of porosity in otherwise seemingly highly porous frameworks. The work provides a detailed account of switching of degree of interpenetration and its effects on porosity of MOFs. The first section describes a well-known doubly-interpenetrated framework, [Zn2(ndc)2(bpy)] (ndc = 2,6-naphthalenedicarboxylate and bpy = 4,4′-dipyridyl) which has been studied for loss of porosity upon activation. The zinc-based pillared-layered structure possesses minimal porosity when activated and the framework was thought to collapse upon desolvation, leading to unexpected sorption results. In the present study, it is shown that the structure does not collapse, but converts to its triply-interpenetrated analogue upon desolvation and the transformation occurs in a single-crystal to single-crystal manner under ambient conditions. A mechanism has also been proposed for the conversion and is supported by computational methods. In the second section, the work has been further extended to more robust and entirely different systems. Two known Cd(II) non-interpenetrated doubly-pillared MOFs, [Cd(tp)(4,4′-bpy)] (tp = terphthalate) and [Cd(atp)(4,4′-bpy)] (atp = 2-aminoterephthalate), have been studied for switching of degree of interpenetration. Both of these systems have been reported to form non-interpenetrated as well as doubly-interpenetrated structures. However, the possibility of inter-conversion has not been suggested. In the present study, these MOFs are shown to undergo a change in degree of interpenetration upon loss of solvent molecules from the channels. The transformation in these cases takes place at much higher temperatures as compared to the [Zn2(ndc)2(bpy)] case. In the final section of this thesis, the effect of switching of degree of interpenetration on the porosity of MOFs is demonstrated using a previously reported system, [Co2(ndc)2(bpy)], where an intermediate structure has been successfully isolated by modifying the activation conditions. This framework has also been reported to lose porosity upon desolvation. It has been found that the doubly-interpenetrated structure converts to its triply-interpenetrated form when activated at 120 °C, whereas the same material converts to an intermediate empty doubly-interpenetrated structure when activated under milder conditions. Sorption analysis using the intermediate twofold structure and the converted threefold structure shows a clear difference in the porosities of the two forms. Interestingly, all the transformations occur in single-crystal to single-crystal fashion.
Synthesis and sorption studies of porous metal-organic hosts
(Stellenbosch : Stellenbosch University, 2013-03) Batisai, Eustina; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: The first part of this study describes the synthesis of new porous materials from basic building blocks. Five structurally related ligands namely: N,N'-bis(3-pyridylmethyl)-naphthalene diimide (L1), N,N'-bis(4-pyridylmethyl)-naphthalene diimide (L2), N,N'-bis(4-pyridylmethyl)- pyromellitic diimide (L3), N,N'-bis(3-pyridylmethyl)-pyromellitic diimide (L4) and 2-(pyridin-4- ylmethyl)-benzene tricarboxylic anhydride (L5) were synthesised. Ligands L1 and L2 were reacted with metal nitrates and carboxylates as co-ligands in a systematic manner with a view to obtaining potentially porous 3–D coordination polymers. Ten structurally diverse coordination polymers were obtained and they were characterised by single-crystal X-ray diffraction, powder X-ray diffraction and thermogravimetric analysis. Four of these compounds absorb moderate amounts of CO2 and, in addition, show sorption selectivity towards CO2 over N2. The reaction of L3 and L4 with transition metal halides yielded two 1–D chains, while the reaction of L5 with transition metal nitrates yielded seven coordination polymers of which four are 2–D and three are 1–D. Of the 2–D structures three are isostructural. The second part of this work describes a variable pressure study of a flexible metal-organic framework [Zn2(BDC)2(BPY)] (BPY = 4,4 -bipyridine and BDC = 1,4-benzene dicarboxylic acid). [Zn2(BDC)2(BPY)] is one of the few examples of a flexible metal-organic framework that undergoes phase transformations in response to gas pressure. The high pressure sorption recorded for this metal-organic framework displays two inflection steps in the pressure range 0 to 30 bar, possibly indicating two phase transformations. The gas-loaded structures for each phase transformation were determined by means of single-crystal X-ray diffraction. High-pressure differential scanning calorimetry was also carried out on the system in order to determine accurate gate-opening pressures, as well as the energies involved with each phase transformation. The results correlate with those obtained from single-crystal X-ray diffraction and high-pressure sorption. The final section reports the mechanochemical synthesis of two Werner complexes [NiCl2(4- PhPy)4] (1), [CoCl2(4-PhPy)4] (2) and their corresponding solid solution [Ni0.5Co0.5Cl2(4-PhPy)4] (3) (PhPy = phenyl pyridine). The solid solution could only be formed by mechanochemical synthesis and not by conventional solution crystallisation methods. The solid solution exhibits sorption properties that differ from those of the pure compounds.
Thermal and mechanical responsiveness of some 4-substituted benzonitriles
(Stellenbosch : Stellenbosch University, 2018-12) Alimi, Lukman Olawale; Barbour, Leonard J.; Smith, Vincent J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: This work is based on three publications. The first manuscript describes the large volumetric thermal expansion of an organic cocrystal over a wide range of temperature (100–300 K). The novel organic multicomponent crystal (cocrystal) (ABN·2DMABN) consisting of 1:2 molar ratio of 4-aminobenzonitrile (ABN) and 4-(dimethylamino)benzonitrile (DMABN) was prepared. It shows linear positive thermal expansion (PTE) along all its three principal axes over the temperature range 100–300 K, which is exclusively dependent on the intermolecular interactions that govern the crystal packing. The associated volumetric thermal expansion coefficient (αv) of 222 MK-1 is the largest reported to date for a cocrystal over such a wide temperature range. The second manuscript explains the mechanical response/behaviour of a rigid small organic molecule, 4-bromobenzonitrile (4-BBN) that has been crystallised using sublimation under dynamic vacuum (0.02 mbar). Interestingly, the crystals show highly flexible plastic behaviour and bends along two orthogonal faces under mechanical stress− a rare phenomenon, resulting in helical twisting, or coiling, which is also rare for crystals of such rigid small organic molecules. The third manuscript discusses the thermal behaviour of the purely organic material 4-aminobenzonitrile (ABN). The single crystals of ABN undergo reversible thermosalience upon cooling from 300 to 100 K and subsequent heating to 300 K. In this study we have demonstrated that the release of accumulated strain in these crystals upon cooling, which results in rapid structural rearrangement, is due to the interplay between the directional and non-directional intermolecular interactions in the system.
Thermal management of porous materials
(Stellenbosch : Stellenbosch University, 2020-11) Feldmann, Wesley; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: Thermal management is an important consideration for porous materials as adsorbents. Accurate determinations of the thermal effects produced during a sorption process are required to characterise and assess the material for application in gas separation and storage. This work describes the methodologies and insights gained from the calorimetric determination of thermal behaviours related to porous materials. The first manuscript describes the methodology used for the determination of the enthalpy of sorption (ΔH) for a rigid porous material. A modified calorimetric method using the continuous-flow technique is described for the direct determination of ΔH values. We termed the method “pressure-gradient differential scanning calorimetry” (PGDSC). It has the advantage of near infinite resolution operating under quasi-equilibrium conditions to yield reliable results. Method validation was carried out by studying CO2 sorption by CuHKUST over four temperatures. The combination of a heat flow plot and a sorption isotherm enables the determination of ΔH as a function of loading over an extended loading range. The ΔH values compare well with previously reported data determined using isosteric and other calorimetric methods. Additionally, the PGDSC method is less labour-intensive and time-consuming than conventional methods and offers a detailed profile that shows how ΔH changes over a given loading range. The second manuscript describes the extension and use of the abovementioned methodology to analyse the thermal behaviours associated with flexible porous materials. Flexible porous materials display complex thermal behaviours owing to structural transitions under guest loading. Plots of ΔH as a function of pressure were determined using the PGDSC method for CO2 sorption by the flexible metal-organic frameworks MIL-53(Al) and MOF-508b over a range of temperatures. The ΔH plots compare well with literature values and offer high-resolution data leading to additional insights into the thermal effects of sorption processes involving flexible materials. The thermal events are measured and compared for comprehensive thermal assessment. Such assessments are beneficial for identification of materials that exhibit intrinsic thermal management. Materials exhibiting intrinsic thermal management are able to offset endothermic structural transitions against exothermic guest adsorption. MIL-53 shows a gradual and relatively balanced thermal profile, whereas MOF-508b undergoes rapid and large changes in enthalpy leading to potential difficulties when applying external cooling to such a material. However, both materials show reduced total enthalpy when compared to rigid materials, providing evidence for the notion that structural flexibility reduces the overall heat evolved during a sorption process. The final manuscript describes the continuous pressure introduction approach to adsorption calorimetry and the advantages offered in relation to flexible porous materials. These are highlighted through a series of studies using the continuous approach. The key advantage of the approach is the resultant high-resolution data, which effectively increases the limits of detection for thermal effects resulting from materials featuring complex surfaces and structural transitions. The technique offers accurate and comprehensive thermal assessments for structurally flexible materials and the thermal effects derived from sorption-based processes.
What is the possible origin of the 195Pt NMR shielding in the solid state? X-ray diffraction, solid-state NMR studies of deceptively simple C2[PtX6] salts (C = various cations, X = Cl- and Br-)
(Stellenbosch : Stellenbosch University, 2015-03) Odendal, James Arthur; Koch, Klaus R.; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
ENGLISH ABSTRACT: The primary objective of the work presented in this thesis is to investigate the sensitivity and nature of the 195Pt chemical shift in the solid-state, as a function of temperature. We have carried out a series of experimental single crystal X-ray diffraction (SCXRD), solid state nuclear magnetic resonance (SSNMR) and density functional theory (DFT) experiments and calculations on various dication hexachloridoplatinate(IV) salts to paint a picture of the nature of the 195Pt shielding in the solid state. The results obtained from SCXRD analyses performed as a function of temperature on Rb2PtCl6, (NH4)2PtCl6 and [(N(CH3)4]2PtCl6 salts give a detailed account on the changing inter- and intramolecular interactions in dication hexachloridoplatinate(IV) salts upon lattice expansion, as a function of increasing temperature. The 195Pt SSNMR measurements of these salts have a peculiar 195Pt chemical shift sensitivity to changes in temperature. The 195Pt chemical shift trends performed as a function of temperature of the Rb2PtCl6 salt resulted in a linear-like trend with the slope of δ195Pt/T = 0.362 (0.008) ppm/K. The (NH4)2PtCl6 salt trend showed some non-linear characteristics at lower temperatures and tended towards linearity at a higher temperature with the slope δ195Pt/T = 0.228 (0.0126) ppm/K. The 195Pt chemical shift of the [(N(CH3)4]2PtCl6 had quite a different trend due to its well-known structural phase transitions as a function of temperature. The expected sensitivity range of the 195Pt chemical shift, predicted from experimental temperature dependent (100 K and 400 K) SCXRD measurements of Rb2PtCl6 and (NH4)2PtCl6 salts coincide well with that of experimental 195Pt chemical shift sensitivity. The atomic coordinates obtained from SCXRD analyses performed as a function of temperature were used to conduct DFT calculations to extract SSNMR parameters together with electrostatic potentials in order to study the local molecular and electronic structures of Rb2PtCl6 and (NH4)2PtCl6 salts. The computed 195Pt SSNMR parameters gave an account of chemical shift parameters and quadrupolar coupling parameters, which show excellent agreement with experimental 195Pt chemical shift SSNMR measurements. These calculations shed some light on the 35Cl SSNMR parameters, which to date are still unobtainable by means of conventional experimental SSNMR measurements. The computed 35Cl SSNMR shielding parameters showed that the electronic environments of the Cl- ion in dication hexachloridoplatinate(IV) salts are central to the understanding and interpretion of experimental 195Pt chemical shift sensitivity. These computational analyses gave the means to propose a theory whereby the : “polarization of electronic charge density from the PtIV ion towards the Cl- ion as the interaction between the anion and cation are being weakened due to lattice expansion as a function of increasing temperature”. This thesis shows that the combination of these three usually independently used analytical techniques are crucial in understanding the behaviour of experimental 195Pt chemical shift dication hexachloridoplatinate(IV) salts in the solid state.

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Browsing Doctoral Degrees (Chemistry and Polymer Science) by browse.metadata.advisor "Barbour, Leonard J."

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