Geometric analysis and formability of the cubic A2BX6 vacancy ordered double perovskite structure

A geometric analysis of the cubic A2BX6 structure commonly formed by metal halides is presented. Using the “hard-sphere” approximation, where the ions are represented by spheres of a fixed radius, we derive four limiting models that each constrain the distances between constituent ions in different ways. We compare the lattice parameters predicted by these four models with experimental data from the Inorganic Crystal Structure Database (ICSD). For the fluorides, the maintenance of the AX bond length at the sum of the A and X radii gives the best approximation of the lattice parameter, leading to structures with widely separated BX6 octahedra. For the heavier halides, a balance between forming an A-site cavity of the correct size and maintaining suitable anion–anion distances determines the lattice parameter. It is found that in many A2BX6 compounds of heavier halides, the neighboring octahedra show very significant anion–anion overlap. We use these models to predict a compound with A-site rattling and use density functional theory (DFT) to confirm this prediction. Finally, we use the geometric models to derive formability criteria for vacancy-ordered double perovskites

Room-Temperature Synthesis of Transition Metal Clusters and Main Group Polycations from Ionic Liquids

Main group polycations and transition metal clusters had traditionally been synthesized via high-temperature routes by performing reactions in melts or by CTR, at room-temperature or lower temperature by using so-called superacid solvents, and at room-temperature in benzene–GaX3 media. Considering the major problems associated with higher temperature routes (e.g. long annealing time, risk of product decomposition, and low yield) and taking into account the toxicity of benzene and liquid SO2 in room-temperature or lower temperature synthesis, a soft and sustainable chemical approach has been developed, employing a Lewis-acidic IL [bmim]Cl/AlCl3. This new alternative reaction medium has proven to be an excellent solvent system for the single–step synthesis of main group polycations and transition metal clusters. X-ray diffraction and Raman spectroscopy have been used for the structural characterization of the isolated compounds. Physical properties and quantum chemical calculations of some of the compounds have also been carried out

Mechanistic studies of HPP epoxidase and DXP reductoisomerase: applications to biosynthesis and antibiotic development

textThe focus of this dissertation is the study of two enzymes, DXR and HppE. DXR catalyzes the first committed step in the MEP pathway, which is the pathway most eubacteria, archeabacteria, algae, and the plastids of plants use for the biosynthesis of isoprenoid. Since mammals utilize the mevalonate pathway and isoprenoids are essential for survival, all enzymes in the MEP pathway are excellent antibiotic targets. One antibiotic that has promise in the fight against malaria is the natural product fosmidomycin, whose antibiotic activity is due to its ability to bind and inhibit DXR. With a deeper understanding of DXR's catalyzed reaction, it will be possible to design a more sophisticated and potent antibiotic. To probe the mechanism of DXR, two fluorinated substrate analogues, 3F-DXP and 4F-DXP, and a fluorinated product analogue, FCH₂-MEP were designed and analyzed as possible substrates or inhibitors. To further analyze the mechanism of DXR, a 2° [²H]-KIE study was conducted using the equilibrium perturbation method. The second enzyme this dissertation examines is HppE, which catalyzes the final step in the biosynthesis of the antibiotic, fosfomycin. Fosfomycin is a clinically useful antibiotic for the treatment of limb-threatening diabetic foot infections and urinary tract infections. Chemically speaking, HppE is unique for two reasons. First, HppE's epoxidation differs from Nature's standard method of epoxide formation by alkene oxidation, where the epoxide oxygen is derived from molecular oxygen. For HppE, the epoxide is formed through the dehydrogenation of a secondary alcohol; thus the epoxide oxygen is derived from the substrate. Second, HppE is a unique member of the mononuclear non-heme iron-dependent family of enzymes. HppE differs from all other mononuclear non-heme iron-dependent enzymes by requiring NADH and an external electron mediator for turnover but not requiring [alpha]-KG, pterin, ascorbate, or an internal iron-sulfur cluster. After a study was published on the activity of zinc-reconstituted HppE from Streptomyces wedmorensis, the proposed iron and NADH dependent mechanism of HppE was reevaluated and was reconfirmed. The HppE from Pseudomonas syringae (Ps-HppE) was also purified and was characterized biochemically and spectroscopically. The results of [²H] and [¹⁸O]-KIE studies on Ps-HppE are also reported.Chemistry and BiochemistryBiochemistr

Metal oxide photoelectrodes for solar water splitting

textEfficient solar water splitting – using sunlight to produce hydrogen from water – has been an ambitious goal of the scientific community for over 40 years. At its heart this is a materials problem, with the photoelectrodes used in a photoelectrochemical cell having to satisfy all the constraints of a photovoltaic material (light absorption, charge transport) as well as being stable in water and having appropriately positioned band edges. Of the metal oxide systems studied for this purpose, we identified iron oxide (hematite, α-Fe2O3), tungsten trioxide (WO3) and an emerging (at the time) material, bismuth vanadate (BiVO4) as the most promising. In this dissertation we sought to understand and address the shortcomings of these materials, namely, carrier transport in BiVO4 and α-Fe2O3 and light absorption in WO3. We synthesized high quality single crystals of undoped and Mo and W-doped BiVO4 using the floating zone technique and carried out fundamental transport measurements. Electrons were shown to form small polarons and the Hall effect mobility was low, ~10-1 cm2 V-1 s-1 at 300 K. Critically, the mobility measured by the Hall effect may be vastly different from the drift mobility. Small-polaron hopping was found to be in the adiabatic regime and anisotropic conductivity was related to the structural arrangement of vanadium ions. Electrons are also thought to form small polarons in α-Fe2O3, but a thorough analysis had not been performed. We grew single crystals of Ti:α-Fe2O3 and characterized their electron transport to evaluate this model and probe the large anisotropy thought to occur between the basal planes. These revealed that the adiabatic small-polaron model was appropriate. Interestingly, electron transport in Ti:α-Fe2O3 was shown to be near-isotropic, contradicting the common view in the literature. Finally, we studied the effects of sulfur or iodine incorporation in WO3 with the aim to improve its visible light harvesting ability. Both of these impurities did increase visible light absorption, but performance was degraded in all cases except for very low concentrations of sulfur doping. These impurities likely form inter-gap defect bands which allow the absorption of longer wavelength light, but also degrade transport properties if present in large amounts.Chemical Engineerin

Gold-Tellurium Clusters and Tetrachloridoaurates with Divalent Cations

The solid state reaction of AuX3, Te and a Lewis-acidic metal halide results in a series of compounds containing the new cationic gold-tellurium cluster (Au6Te18Xn)(6-n)+. AuTe2X can be used instead of AuX3, but depending on the kind of Lewis-acidic metal halide the product can be different. The Lewis acids AlX3, GaCl3, ZrCl4, MoOCl3, BiCl3, NbCl5 and TaCl5 have all been used to successfully obtain (Au6Te18)[ZrCl6]3, (Au6Te18)[AlX4]6, (Au6Te18X)[MCl4]5, (Au6Te18Cl)[MoOCl4]5, (Au6Te18Cl2)[Mo2O2Cl8]2, (Au6Te18Cl2)[Bi4Cl16] and (Au6Te18Cl2)[MCl6]4. With two exceptions the crystal structures of all target compounds were solved, even though the x-ray diffraction data for some compounds give ambiguous results, because of disordered sites within the cation. The structure of the cation itself can be derived from a cube. Taking a 3x3x3 cube, analogous to the Rubik's cube, all 27 blocks represent a possible atom position. The central block of each face is occupied by a gold atom. The cube's centre and two corner positions are unoccupied and the remaining 18 positions are occupied by tellurium. For compounds with halide atoms X attached to the cation, these atoms occupy one or both of the empty corner positions. Further investigation towards the physical properties of this new class of compounds reveals a high diamagnetic moment for the cation itself (χmol = -8.2 * 10-4 cm3/mol). Both compounds containing isolated MoV atoms show the expected paramagnetism corresponding to their d1 configuration (1.67 µB and 1.83 µB). Semi-conducting behaviour is observed for three of the compounds and is expected for the untested compounds, too, because all samples are of black-metallic appearance. Additionally, the electronic structure of three representatives is investigated by theoretical calculations, which reveal band gaps between 0.4 eV to 2.0 eV. These calculations also provide valuable help for the interpretation of the ambiguous x-ray diffraction data sets. As a second central result achieved in this work, the first salts of divalent metal cations with [AuCl4]- anions are reported. The compounds Cd[AuCl4]2 and Zn[AuCl4]2*(AuCl3)1.115 are obtained from reactions of MCl2 and elemental gold in liquid chlorine and subsequent annealing. The crystal structures of both compounds are reported herein. The latter compound possesses an incommensurately modulated composite crystal structure. The first subsystem contains polymeric chains of zinc(II) tetrachloroaurate(III), while the second subsystem consists of Au2Cl6 dimers, which are located in channels built up by the first subsystem. The structural parameters of these dimers show only small deviations from neat AuCl3. It is revealed from a detailed analysis of the crystal structure that the 4+2 coordination sphere, usually found for Au3+, is only achieved for both subsystems through interaction with each other.Gold-Tellur-Cluster und Tetrachloridoaurate mit zweiwertigen Kationen Die Festkörperreaktion zwischen AuX3, Te und einem Lewis-saurem Metallhalogenid ergibt eine Reihe von Verbindungen, die den neuen kationischen Gold-Tellur-Cluster (Au6Te18Xn)(6-n)+ enthalten. An Stelle von AuX3 kann auch AuTe2X eingesetzt werden, allerdings kann dies je nach eingesetztem Metallhalogenid zu unterschiedlichen Produkten führen. Aus Umsetzungen mit den Lewis-Säuren AlX3, GaCl3, ZrCl4, MoOCl3, BiCl3, NbCl5 und TaCl5 wurden erfolgreich die Verbindungen (Au6Te18)[ZrCl6]3, (Au6Te18)[AlX4]6, (Au6Te18X)[MCl4]5, (Au6Te18Cl)[MoOCl4]5, (Au6Te18Cl2)[Mo2O2Cl8]2, (Au6Te18Cl2)[Bi4Cl16] und (Au6Te18Cl2)[MCl6]4 synthetisiert. Mit Ausnahme zweier Verbindungen konnten die Einkristallstrukturen von allen Zielverbindungen gelöst und verfeinert werden, obwohl die Beugungsdaten für einige Verbindungen aufgrund von fehlgeordneten Lagen innerhalb des Kations nicht eindeutig sind. Die Struktur des Kations kann von einem Würfel abgeleitet werden. Ausgehend von einem 3x3x3 Würfel, etwa einem Zauberwürfel, können alle 27 Blöcke als mögliche Atompositionen aufgefasst werden. Der mittlere Block jeder der sechs Flächen ist mit einem Gold-Atom besetzt. Die zentrale Position im Inneren des Würfels und zwei diagonal gegenüberliegende Ecken bleiben frei. Die übrigen 18 Positionen werden mit Tellur-Atomen besetzt. In Verbindungen, in denen am Kation zusätzlich ein oder zwei Halogenatome X gebunden sind, besetzen diese eine oder beide der bisher unbesetzten Ecken. Die Untersuchung der physikalischen Eigenschaften dieser neuen Substanzklasse, ergibt ein hohes diamagnetisches Moment (χmol = -8.2 * 10-4 cm3/mol) für das Kation. Die beiden Verbindungen, die isolierte MoV Atome enthalten, zeigen, wie zu erwarten, Paramagnetismus in Übereinstimmung mit dend1 konfigurierten Mo-Atomen (1.67 µB und 1.83 µB). Halbleiter-Eigenschaften werden für drei Verbindungen experimentell belegt und aufgrund des schwarz-metallischen Aussehens für alle synthetisierten Verbindungen vermutet. Zusätzlich ergeben theoretische Rechnungen an drei Vertretern Bandlücken zwischen0.4 eV und 2.0 eV. Mit Hilfe dieser Rechnungen wird ebenfalls die Lösung der Einkristallstrukturen der mehrdeutigen Datensätzen unterstützt. Als zweites wichtiges Ergebnis dieser Arbeit, wird über die ersten Salze eines zweiwertigen Metallkations mit [AuCl4]- als Gegenion berichtet. Die Verbindungen Cd[AuCl4]2 und Zn[AuCl4]2*(AuCl3)1.115 können aus der Reaktion von elementarem Gold mitMCl2 in flüssigem Chlor mit anschließendem Tempern erhalten werden. Die Einkristallstrukturen beider Verbindungen wurden bestimmt. Die letztgenannte Verbindung besitzt eine inkommensurabel modulierte Kompositstruktur. Das erste Subsystem besteht aus Ketten von Zink(II)-tetrachloroaurat(III). Das zweite Subsystem enthält Au2Cl6 Dimere, die sich in Kanälen, die vom ersten Subsystem aufgespannt werden, befinden. Die Struktur der Dimere unterscheidet sich kaum von reinem AuCl3. Eine eingehende Analyse der Einkristallstruktur zeigt, dass die 4+2 Koordinationssphäre, wie sie überlicherweise für Au3+ beobachtet wird, wechselseitig durch die beiden Subsysteme vervollständigt wird

Synthesis of polyoxometalates for detailed solution reactivity studies

PhD ThesisNon-aqueous methodologies provide an opportunity to access a range of polyoxometalates that may not be stable in H2O and enable mechanistic studies into hydrolytic and protonation behaviours, which are fundamental to polyoxometalate chemistry. 17O-enriched (TBA)6[NaPW11O39] was prepared via an efficient non-aqueous route and shown to be a suitable precursor to [(L)MPW11O39]n- (M = Sn2+, Pb2+, Bi3+, Sb3+, Sn4+, Ti4+) for detailed systematic studies. Reactions were monitored by 31P NMR while products were characterised by FT-IR, multinuclear NMR (1H, 13C, 17O, 31P, 119Sn, 183W and 207Pb), solid state NMR, ESI-MS, CHN microanalysis, UV-Vis and/or single crystal XRD. Using this approach, the readily-hydrolysable tin derivatives, (TBA)4[(CH3O)SnPW11O39] and (TBA)8[(μ-O)(SnPW11O39)2] were prepared for the first time and the previously reported (TBA)4[(HO)TiPW11O39] was shown to be stable in DMSO for up to 3 months possibly due to interaction between HO- and DMSO. As a result of the more ionic character of Sn—OCH3 bond compared with Ti—OCH3, (TBA)4[(CH3O)SnPW11O39] was observed to hydrolyse faster than (TBA)4[(CH3O)TiPW11O39] whereas (TBA)4[ClTiPW11O39] with a large excess of H2O hydrolysed more readily than (TBA)4[ClSnPW11O39]. Although (TBA)4[(HO)TiPW11O39] underwent condensation to (TBA)8[(μ-O)(TiPW11O39)2] easily in acetonitrile at room temperature, this reaction only occured for (TBA)4[(HO)SnPW11O39] at elevated temperature (~120oC) in the presence of a water-scavenging agent such as N, N’-dicyclohexylcarbodiimide (DCC). These experimental observations were consistent with DFT calculations on the energetics of the hydrolysis and condensation of (TBA)4[(CH3O)SnPW11O39] and (TBA)4[(CH3O)TiPW11O39]. Protonation studies on the 17O-enriched POMs provided insights into protonation of the MOW sites in (TBA)4[(CH3O)TiPW11O39], (TBA)4[ClMIVPW11O39] (M = Sn, Ti), (TBA)5[MIIPW11O39] (M = Sn, Pb) and (TBA)4[MIIIPW11O39] (M = Sb, Bi) and protonation at both TiOW and TiOTi sites in (TBA)8[(μ-O)(TiPW11O39)2] whilst reactions between (TBA)8[(μ-O)(TiPW11O39)2] and electrophiles indicated possible formation of adducts. Treatment of (TBA)4[(L)SnPW11O39] (L = Cl, HO) with NaBH4 resulted in reduction of the tin heteroatom only whereas reaction between (TBA)5[SnIIPW11O39] and halogens (Br2 and I2) or the molybdate (TBA)3[PMo12O40] showed oxidation of tin (II). Electrochemical studies in acetonitrile revealed no redox processes associated with the heterometals in (TBA)4[(L)SnIVPW11O39] and (TBA)5[PbIIPW11O39] while redox waves assigned to Sn2+/Sn4+ were observed for Abstract ii (TBA)5[SnIIPW11O39] within the potential range studied. Finally, attempts to prepare Lindqvist-type derivatives, [(L)MW5O18]n- (M = Co2+, Mo2+, Sn2+, Pb2+, Fe2+, Cu2+, Cr3+, Sb3+, Bi3+) from a tungstate precursor prepared by hydrolysis of a 3:2 mixture of (TBA)2WO4 and WO(OMe)4 provided evidence that only in certain cases were the required heterometalates formed. Acetonitrile hydrolysis was observed under reaction conditions and the acetamide adduct (TBA)3[{CH3C(O)NH2}CoW5O17(OMe)] was characterised crystallographically. An attempt to prepare [(L)MoIIW5O18]4- produced the crystallographically characterised, one-electron reduced (TBA)3[W6O19].Niger Delta Development Commission (NDDC):, Sokoto State University: COST Action CM1203: The Royal Society of Chemistry