>Magister Scientiae - MSc / Hydrogen has been regarded as an ideal energy carrier for future, it can be stored as a liquid in cryogenic tanks, a gas in high pressure cylinders and as solid in metal hydrides. Hydrogen storage in metal hydrides is of research interest because hydrides often have high energy density than gas or liquid hydrogen and are relatively safe. Ti and Ti alloys are promising hydrogen storage material because they have high affinity for hydrogen, light in weight and react reversibly with hydrogen. This work aims to investigate the hydrogen storage capacity of CP- Ti and Ti6Al4V alloy and Pd/Ti6Al4V alloy, where Pd was deposited on Ti6Al4V alloy. Samples were hydrogenated from room temperature to 650 °C at atmospheric pressure in the vacuum furnace under the 15%H/Ar atmosphere. Hydrogenation was carried out for a period of 3 hours for all samples. Sample composition and layer thickness were determined using Rutherford backscattering spectrometry. The microstructure and phase transformation were investigated using optical microscopy and X-ray diffraction technique. Hydrogen storage capacity was determined using elastic recoil detection analysis and gravimetric method. It was found that hydrogenation temperature has an effect on hydrogen absorption, microstructure and phase transformation. Maximum hydrogen concentration was obtained at hydrogenation temperatures of 550 °C for all materials with 45.57 at.% in CP-Ti, 34.77 at.% in Ti6Al4V alloy and 39 at.% H in Pd/Ti6Al4V coated system. In CP-Ti it was found that hydrogen absorption begins at 550 °C and decreases at hydrogenation temperature of 650 °C and that hydrogenation at both temperatures leads to formation of titanium hydrides and needlelike microstructure. At temperatures below 550 °C no hydrides were formed. For Ti6Al4V alloy ERDA results showed that no significant hydrogen absorption occurred at temperatures below 550 °C and at hydrogenation temperature of 650 °C, hydrogen absorption decreased drastically. The δ- titanium hydride was detected in the sample hydrogenated at 550 °C. Fine needle like microstructure was observed in the sample hydrogenated at 550 °C, and at higher temperature (650 °C ) coarse needles were formed. Pd coatings on Ti6Al4V alloy was found to increase the absorption of hydrogen, and allowing hydrogen to be absorbed at low temperatures. / National Research Foundation (NRF)

Radical cyclizations have become a mainstay of synthetic organic chemistry – useful for the construction of C–C bonds in laboratory-scale applications. However, they are seldom used the industrial scale. In large part, this is because of a reliance on Bu3SnH, widely regarded as the best synthetic equivalent to a hydrogen atom. Transition metal hydrides have emerged as promising alternative hydrogen atom sources. Over the last decade, the Norton group has studied three transition metal systems, with an emphasis on quantifying the M–H bond dissociation energies. Over time, the group has shown that, thermodynamically, first-row transition metal hydrides are good hydrogen atom donors; they often have weak M–H bonds. Modest adjustments to the M–H bond strength result in substantial changes to how a hydride processes a given organic substrate. The Norton group has also studied the kinetics of hydrogen atom transfer, and shown that transition metal hydrides are kinetically competent at transferring hydrogen atoms, both to olefinic substrates and to organic radicals. Some of the transition metal complexes are made catalytic under modest pressures of H2, so they can be used for effecting atom-economical radical reactions. I have leveraged the fundamental kinetic and thermodynamic information that has been gathered by the group to develop new radical reactions – ones that cannot be done by Bu3SnH. Herein are described two cases studies: the first is the generation of α-alkoxy radicals by hydrogen atom transfer to enol ethers (Chapter 2). The second is the development of a radical isomerization and cycloisomerization reactions (Chapter 3). Both of these developments have relied upon an understanding of M–H thermochemistry. Discovering new hydrogen atom donors will lead to discovering new radical reactions. In Chapter 4, I revisit two previously reported transition metal hydrides that are likely to transfer hydrogen atoms: (TMS3tren)CrIV–H and [CpV(CO)3H]–. Although the anionic vanadium hydride was reported as a potent hydrogen atom donor nearly forty years ago, my studies suggest that its M–H bond is actually relatively strong. I have therefore reevaluated the reactivity of [CpV(CO)3H]–, and found that although the 18 electron anionic hydride is not a good hydrogen atom donor, the oxidized 17-electron neutral CpV(CO)3H is an extremely potent one. I have made the reactions with [CpV(CO)3H]– catalytic under H2 (now the reactions are done with an added base). The catalytic reactions that use [CpV(CO)3H]– can enact the exact same transformations that tin does, so I have developed a true catalytic replacement for Bu3SnH.

Synthesis and structural characterisation of metal hydrides in two important systems are presented. The first system presented is low valent cobalt and nickel complex hydrides with the compositions BaMg5Co2H10, RbMg5CoNiH10, SrMg2CoH7and Sr4Mg4Co3H19 featuring nickel with oxidation state of 0 and cobalt with oxidation state +I and -I. The second system presented is polyanionic gallium complex hydrides with the compositions RbGaH2, RbxK(1−x)GaH2 (0.5≤x≤1), CsxRb(8−x)Ga5H15 (0≤x≤8) and Cs10Ga9H25 featuring novel hydrogenous polyanionic gallium hydride clusters mimicking common hydrocarbons. The syntheses of the compounds were performed at elevated temperatures and at moderate hydrogen pressures (50-100 bar). The structural investigations were mainly done by X-ray powder diffraction (XRPD) and neutron powder diffraction (NPD). The metal-hydrogen bond was investigated by vibrational spectroscopy using Fourier Transform IR-spectroscopy (FTIR) and Inelastic Neutron Scattering (INS).By subtle changes in the compositions of the hydrides it was possible to induce major changes in band gaps, oxidation states and structures. / <p>At the time for the doctoral defence the following papers were unpublished and had a status as follows: Paper 1: Manuscript; Paper 2: Accepted; Paper 5: Manuscript</p>

In addition to their potential use at low to moderate temperatures in mobile fuel cell technologies, metal hydrides may also find application as high temperature tritium getterers in the U.S. DOE Next Generation Nuclear Plant (NGNP). We use Density Functional Theory to identify metal hydrides capable of sequestering tritium at temperatures in excess of 1000 K. First we establish the minimum level of theory required to accurately capture the thermodynamics of highly stable metal hydrides and determine that isotope effects can be neglected for material screening. Binary hydride thermodynamics are largely well established, and ternary and higher hydrides typically either do not form or decompose at lower temperatures. In this thesis we investigate anomalous systems with enhanced stability in order to identify candidates for the NGNP application beyond the binary hydrides. Methods implemented in this work are particularly useful for deriving finite temperature phase stability behavior in condensed systems. We use grand potential minimization methods to predict the interstitial Th−Zr−H phase diagram and apply high throughput, semi-automated screening methodologies to identify candidate complex transition metal hydrides (CTMHs) from a diverse library of all known, simulation ready ternary and quaternary CTMHs (102 materials) and 149 hypothetical ternary CTMHs based on existing prototype structures. Our calculations significantly expand both the thermodynamic data available for known CTMHs and the potential composition space over which previously unobserved CTMHs may be thermodynamically stable. Initial calculations indicate that the overall economic viability of the tritium sequestration system for the NGNP will largely depend on the amount of protium rather than tritium in the metal hydride gettering bed feed stream.

Thesis (Ph. D.)--University of Washington, 2007. / Vita. Includes bibliographical references (leaves 119-130).

No description available.

The traditional reagent for H• transfer in organic chemistry is 𝓃-Bu₃SnH, which has a Sn–H bond dissociation energy (BDE) of 78.5 kcal/mol. There are, however, many disadvantages of employing 𝓃-Bu₃SnH in radical reactions. The transfer of H• from tin is necessarily stoichiometric, with 𝓃-Bu₃Sn–X being the eventual product. Overall, the tin reactions have poor atom economy; n-Bu3SnH cannot be regenerated from 𝓃-Bu₃Sn• or 𝓃-Bu₃Sn–X with hydrogen, and no general methods of regenerating the tin hydride with other hydride sources have been reported. Standard purification methods leave unacceptable levels of residual tin in the products of n-Bu3SnH reactions. Alternatives are clearly needed. Transition metal hydrides represent a class of promising reagents to replace 𝓃-Bu₃H. Due to their typically weaker M-H bonds, transition metal hydrides are often able to transfer H• to C=C and generate radicals — a reaction that 𝓃-Bu₃SnH cannot do. Furthermore, many transition-metal hydrides can be regenerated from hydrogen gas, an event that requires that the M–H BDE be over 56 kcal/mol. By combining this reaction with the H• transfer, metalloradicals can often catalyze the formation of radicals from H₂. Over the years, the Norton group has studied several transition metal hydride systems and demonstrated their applications in different scenarios. The kinetics and thermodynamics of these systems have been studies in detail, and they are shown be competent hydrogen atom donors to unsaturated organic substrates and to organic radicals. Some of these metal hydrides can be made catalytic under hydrogen pressure, thus providing an atom-economical way to effect radical reactions. Specifically, the thermodynamic properties of the chromium hydride HCpCr(CO)₃ have been carefully studied. Based on this information, I developed a Ti/Cr cooperative catalytic system featuring multiple interactions between the two metal systems. Herein are described three applications of this Ti/Cr catalytic system: anti-Markovnikov hydrogenation of epoxides (Chapter 2), reductive cyclization of epoxy enones under H₂ (Chapter 3), and aziridine isomerization to allyl amines (Chapter 4). I have also explored new hydrogen atom acceptors. I was able to catalyze hydrodefluorination of CF₃-substituted olefins with a nickel hydride (Chapter 5). The reaction was demonstrated to be initiated by a hydrogen atom transfer from the Ni(II)-H to the olefin substrates. This also expands our toolbox of metal hydrides for transferring hydrogen atom to olefin substrates. With a different cobaloxime catalyst, I was able to catalyze the cycloisomerization of CF₃-substituted dienes (Chapter 6). In Chapter 7, I developed a method to achieve a broad range of hydrofunctionalizations of olefins with hydrogen atom transfer from metal hydrides in situ. Hydrogen atom transfer to olefins was followed by TEMPO trapping to form TEMPO adducts. A subsequent photocatalytic substitution on those TEMPO adducts with different nucleophiles affords various hydrofunctionalized products.

The complex, IrH₂(PMe₃)₃Cl (1), was previously found to be, not only unexpectedly water-soluble but also an effective homogeneous catatyst for the hydrogenation of unsaturates in water. The results of extensive ³¹P NMR studies on the aqueous system of (1) indicate that (1) is in equilibrium with the iridium(III) dihydrido “aquo” complex, [IrH₂(PMe₃)₃(H₂O)]⁺, and not the μ-chloro bridged complex, { [IrH₂(PMe₃)₃]₂Cl}⁺ (2), as previously reported. The calculated K_eq value for the aqueous equilibrium is (0.0037 ± 0.0003) M. Thermodynamic data (ΔH = 30.8 kJ/mol, ΔS = 56.0 J/(Kmol), and ΔG = 14.1 kJ/mol) obtained from variable temperature ³¹P NMR studies are consistent with the proposed equilibrium system. The complexes IrH₂(PMe₃)₃X (X = O₂CPh (3), I (4), and Br (6) were synthesized and examined. The complexes IrH₂(PMe₃)₃X (X = H₂O and F) could not be isolated. (3) was determined to dissociate completely in water to form the iridium(III) dihydrido “‘aquo” complex, [IrH₂(PMe₃)₃(H₂O)]⁺, seemingly explaining the greater catalytic activity of (3). Solubility of the halo complexes decreased from moderately soluble (1), to slightly soluble (6), to very slightly soluble (4). The solubilities of (4) and (6) were too low to allow quantification of their equilibria. Finally it was observed that linear relationships exist between the electronegativity of the ligand, X, and the ¹H and ³¹P NMR chemical shifts of the hydrides and the phosphines for the complexes, IrH₂(PMe₃)₃X. These relationships are consistent with the findings of Birnbaum. / Ph. D.

This PhD thesis presents investigations of hydrogen incorporation in Zintl phases and transition metal oxides. Hydrogenous Zintl phases can serve as important model systems for fundamental studies of hydrogen-metal interactions, while at the same time hydrogen-induced chemical structure and physical property changes provide exciting prospects for materials science. Hydrogen incorporation in transition metal oxides leads to oxyhydride systems in which O and H together form an anionic substructure. The H species in transition metal oxides may be highly mobile, making these materials interesting precursors toward other mixed anion systems.  Zintl phases consist of an active metal, M (alkali, alkaline earth or rare earth) and a more electronegative p-block metal or semimetal component, E (Al, Ga, Si, Ge, etc.). When Zintl phases react with hydrogen, they can either form polyanionic hydrides or interstitial hydrides, undergo full hydrogenations to complex hydrides, or oxidative decomposition to more E-rich Zintl phases. The Zintl phases investigated here comprised the CaSi2, Eu3Si4, ASi (A= K, Rb) and GdGa systems which were hydrogenated at various temperature, H2 pressure, and dwelling time conditions. For CaSi2, a regular phase transition from the conventional 6R to the rare 3R took place and no hydride formation was observed. In contrast, GdGa and Eu3Si4 were very susceptible to hydrogen uptake. Already at temperatures below 100 ºC the formation of hydrides GdGaH2-x and Eu3Si4H2+x was observed. The magnetic properties of the hydrides (antiferromagnetic) differ radically from that of the Zintl phase precursor (ferromagnetic). Upon hydrogenating ASi at temperatures around 100 oC, silanides ASiH3 formed which contain discrete complex ion units SiH3-. The much complicated β – α order-disorder phase transition in ASiH3 was evaluated with neutron powder diffraction (NPD), 2H NMR and heat capacity measurements.  A systematic study of the hydride reduction of BaTiO3 leading to perovskite oxyhydrides BaTiO3-xHx was done. A broad range of reducing agents including NaH, MgH2, CaH2, LiAlH4 and NaBH4 was employed and temperature and dwelling conditions for hydride reduction examined. Samples were characterized by X-ray powder diffraction (XRPD), thermal gravimetric analysis and 1H NMR. The concentration of H that can be incorporated in BaTiO3-xHx was found to be very low, which is in contrast with earlier reports. Instead hydride reduction leads to a high concentration of O vacancies in the reduced BaTiO3. The highly O-deficient, disordered, phases - BaTiO3-xHy□(x-y) with x up to 0.6 and y in a range 0.05 – 0.2 and (x-y) &gt; y – are cubic and may represent interesting materials with respect to electron and ion transport as well as catalysis. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 5: Manuscript.</p>

Copper hydrides such as [Ph₃PCuH]₆ (Stryker’s Reagent) are textbook reagents in organic chemistry for the selective hydrogenation of α,β-unsaturated carbonyl compounds. Despite their widespread use both stoichiometrically and catalytically, there are many important questions about polynuclear copper hydrides that have not been answered. I have investigated the electron transfer chemistry of [Ph₃PCuH]₆ and related copper hydrides. Copper hydrides (E₁/₂ = –1.0 to –1.2 V vs FcH/FcH⁺) are good one-electron reducing agents. Stopped-flow techniques have allowed the detection of electron transfer intermediates in copper hydride reactions. The fate of the copper containing products after electron transfer or hydride transfer reactions has been investigated. An unusual cationic copper hydride, [(Ph₃P)₇Cu₇H₆]⁺ was found to be the major product of these reactions. Methods of converting this species back to [Ph₃PCuH]₆ have been investigated. The chemistry of this cationic species plays an important role in catalytic use of copper hydrides.