Hydride Chemistry Research Articles

Low valent lead hydride chemistry: hydroplumbylation of phenylacetylene and 1,1-dimethylallene.

Hydroplumbylation reactions of the low valent organolead hydride [(Ar*PbH)2] with phenylacetylene and 1,1-dimethylallene are presented. A vinyl plumbylene was isolated in the case of the alkyne reaction exhibiting a down field NMR signal for the CH unit attached to the low valent heavy atom. 1,1-Dimethylallene affords formation of an allyl plumbylene.

Low-valent group 14 element hydride chemistry: towards catalysis.

The chemistry of group 14 element(ii) hydride complexes has rapidly expanded since the first stable example of such a compound was reported in 2000. Since that time it has become apparent that these systems display remarkable reactivity patterns, in some cases mimicking those of late transition-metal (TM) hydride compounds. This is especially so for the hydroelementation of unsaturated organic substrates. Recently, this aspect of their reactivity has been extended to the use of group 14 element(ii) hydrides as efficient, "TM-like" catalysts in organic synthesis. This review will detail how the chemistry of these hydride compounds has advanced since their early development. Throughout, there is a focus on the importance of ligand effects in these systems, and how ligand design can greatly modify a coordinated complex's electronic structure, reactivity, and catalytic efficiency.

Molecular Magnesium Hydrides.

Solid magnesium hydride [MgH2 ]∞ has been pursued as a potential hydrogen-storage material. Organic chemists were rather interested in soluble magnesium hydride reagents from mid-20th century. It was only in the last two decades that molecular magnesium hydride chemistry received a major boost from organometallic chemists with a series of structurally well-characterized examples that continues to build a whole new class of compounds. More than 40 such species have been isolated, ranging from mononuclear terminal hydrides to large hydride clusters with more than 10 magnesium atoms. They provide not only insights into the structure and bonding of Mg-H motifs, but also serve as models for hydrogen-storage materials. Some of them are also recognized to participate in catalytic transformations, such as hydroelementation. Herein, an overview of these molecular magnesium hydrides is given, focusing on their synthesis and structural characterization.

Synthesis and reductive chemistry of bimetallic and trimetallic rare-earth metallocene hydrides with (C5H4SiMe3)1− ligands

The reductive chemistry of [Cp'2Ln(μ–H)(THF)x]y [Ln = Y, Dy, Tb; Cp' = (C5H4SiMe3)1−; x = 2, 0 and y = 2, 3] was examined to determine if these hydrides would be viable precursors for 4fn5d1 Ln2+ ions that could form 5d1-5d1 metal–metal bonded complexes. The hydrides were prepared by reaction of the chlorides, [Cp'2Ln(μ–Cl)]2, 1-Ln, with allylmagnesium chloride to form the allyl complexes, [Cp'2Y(η3–C3H5)(THF)], 2-Ln, which were hydrogenolyzed. The solvent-free reaction of solid 2-Ln with 60 psi of H2 gas in a Fischer-Porter apparatus produced, in the Y case, the trimetallic species, [Cp'2Y(μ–H)]3, 3-Y, and in the Dy and Tb cases, the bimetallic complexes [Cp'2Ln(μ–H)(THF)]2, 4-Ln (Ln = Dy, Tb). The latter complexes could be converted to 3-Dy and 3-Tb by heating under vacuum. Isopiestic data indicate that 3-Y solvates to 4-Y in THF. Reductions of 4-Y, 4-Dy, and 4-Tb with KC8 in the presence of a chelate such as 2.2.2-cryptand or 18-crown-6 all gave reaction products with intense dark colors characteristic of Ln2+ ions. In the yttrium case, with either chelating agent, the dark green product gives a rhombic EPR spectrum (g1 = 2.01, g2 = 1.99, g3 = 1.98, A = 24.1 G) at 77 K. However, the only crystallographically-characterizable products obtainable from these solutions were Ln3+ polyhydride anion complexes of composition, [K(chelate)]{[Cp'2Ln(μ–H)]3(μ–H)}. Reduction of 1-Y with KC8 in the presence of 2.2.2-cryptand also yields an intensely colored product with an axial EPR spectrum (gx = gy = 2.05, Ax = Ay = 35.5 G; gz = 2.07, Az = 34.5) similar to that of (Cp'3Y)1− ion, but crystals were not obtained from this system.

Superconductivity and unexpected chemistry of germanium hydrides under pressure

Following the idea that hydrogen-rich compounds might be high-T$_c$ superconductors at high pressures, and the very recent breakthrough in predicting and synthesizing hydrogen sulfide with record-high T$_c$ = 203 K, ab initio evolutionary algorithm for crystal structure prediction was employed to find stable germanium hydrides. In addition to the earlier structure of germane with space group Ama2, we propose a new C2/m structure, which is energetically more favorable at pressures above 278 GPa (with inclusion of zero point energy). Our calculations indicate metallicity of the new C2/m phase of germane with T$_c$ = 67 K at 280 GPa. Germane is found to exhibit thermodynamic instability to decomposition to hydrogen and the new compound Ge$_3$H$_{11}$ at pressures above 300 GPa. Ge$_3$H$_{11}$ with space group I$\bar{4}$m2 is found to become stable at above 285 GPa with T$_c$ = 43 K. We find that the pressure-induced phase stability of germanium hydrides is distinct from its analogous isoelectronic systems, e.g., Si-hydrides and Sn-hydrides. Superconductivity stems from large electron-phonon coupling associated with the wagging, bending and stretching intermediate-frequency modes derived mainly from hydrogen.

Transition‐Metal Complexes with (C–C)→M Agostic Interactions

The coordination of sigma bonding electron density to a Lewis acid was for some time largely the domain of boron hydride chemistry. Subsequently it was found that C–H bonds could undergo intramolecular coordination to metal centers, forming what are now known as “agostic bonds” and “agostic complexes”. Thereafter, even the H–H bond in molecular hydrogen was found to be capable of coordination, leading to what are commonly referred to as “sigma complexes”. Eventually, coordination by much more shielded C–C bonds was established, and is of particular significance with regard to to the interest in C–C bond activation. This review focuses on the key developments in this field, including the nature of the bonding interactions in these and some related species, as revealed by spectroscopic, structural, and theoretical studies, their involvement in important chemical reactions such as alkene polymerizations and metathesis, and comparisons with the other types of electron‐deficient bridge bonding.

Mer, fac, and Bidentate Coordination of an Alkyl-POP Ligand in the Chemistry of Nonclassical Osmium Hydrides.

Nonclassical and classical osmium polyhydrides containing the diphosphine 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant(PiPr2)2), coordinated in κ3-mer, κ3-fac, and κ2-P,P fashions, have been isolated during the cyclic formation of H2 by means of the sequential addition of H+ and H- or H- and H+ to the classical trihydride OsH3Cl{xant(PiPr2)2} (1). This complex adds H+ to form the compressed dihydride dihydrogen complex [OsCl(H···H)(η2-H2){xant(PiPr2)2}]+ (2). Under argon, cation 2 loses H2 and the resulting unsaturated fragment dimerizes to give [(Os(H···H){xant(PiPr2)2})2(μ-Cl)2]2+ (3). During the transformation the phosphine changes its coordination mode from mer to fac. The benzofuran counterpart of 1, OsH3Cl{dbf(PiPr2)2} (4; dbf(PiPr2)2 = 4,6-bis(diisopropylphosphino)dibenzofuran), also adds H+ to afford the benzofuran counterpart of 2, [OsCl(H···H)(η2-H2){xant(PiPr2)2}]+ (5), which in contrast to the latter is stable and does not dimerize. Acetonitrile breaks the chloride bridge of 3 to form the dihydrogen [OsCl(η2-H2)(CH3CN){xant(PiPr2)2}]+ (6), regenerating the mer coordination of the diphosphine. The hydride ion also breaks the chloride bridge of 3. The addition of KH to 3 leads to 1, closing a cycle for the formation of H2. Complex 1 reacts with a second hydride ion to give OsH4{xant(PiPr2)2} (7) as consequence of the displacement of the chloride. Similarly to the latter, the oxygen atom of the mer-coordinated diphosphine of 7 has a tendency to be displaced by the hydride ion. Thus, the addition of KH to 7 yields [OsH5{xant(PiPr2)2}]- (8), containing a κ2-P,P-diphosphine. Complex 8 is easily protonated to afford OsH6{xant(PiPr2)2} (9), which releases H2 to regenerate 7, closing a second cycle for the formation of molecular hydrogen.

Extending Reaction Mechanism Generator to Silicon Hydride Chemistry

Understanding the gas-phase chemistry of silicon hydrides is the first step to building a realistic kinetic model for chemical vapor deposition (CVD). Functionality for thermodynamic and kinetic data estimation of silicon hydrides was added to the open-source software Reaction Mechanism Generator (RMG). Using the updated RMG, a detailed kinetic model was built for SiH4 thermal decomposition. The generated model was used to perform reactor simulations at various process conditions for comparison to prior SiH4 decomposition experiments in a flow tube. Results show that the RMG-generated model can reasonably replicate experimental results for SiH4 concentration profiles at different temperatures and residence times. While the effect of changing initial SiH4 concentration is not captured, a first pass sensitivity analysis reveals that reasonable errors in reaction rates could contribute to the discrepancy.

ChemInform Abstract: Coinage Metal Hydrides: Synthesis, Characterization, and Reactivity

AbstractReview: 262 refs.

Muonium Chemistry at Diiron Subsite Analogues of [FeFe]-Hydrogenase.

The chemistry of metal hydrides is implicated in a range of catalytic processes at metal centers. Gaining insight into the formation of such sites by protonation and/or electronation is therefore of significant value in fully exploiting the potential of such systems. Here, we show that the muonium radical (Mu.), used as a low isotopic mass analogue of hydrogen, can be exploited to probe the early stages of hydride formation at metal centers. Mu. undergoes the same chemical reactions as H. and can be directly observed due to its short lifetime (in the microseconds) and unique breakdown signature. By implanting Mu. into three models of the [FeFe]‐hydrogenase active site we have been able to detect key muoniated intermediates of direct relevance to the hydride chemistry of these systems.

Muonium Chemistry at Diiron Subsite Analogues of [FeFe]‐Hydrogenase

AbstractThe chemistry of metal hydrides is implicated in a range of catalytic processes at metal centers. Gaining insight into the formation of such sites by protonation and/or electronation is therefore of significant value in fully exploiting the potential of such systems. Here, we show that the muonium radical (Mu.), used as a low isotopic mass analogue of hydrogen, can be exploited to probe the early stages of hydride formation at metal centers. Mu. undergoes the same chemical reactions as H. and can be directly observed due to its short lifetime (in the microseconds) and unique breakdown signature. By implanting Mu. into three models of the [FeFe]‐hydrogenase active site we have been able to detect key muoniated intermediates of direct relevance to the hydride chemistry of these systems.

ON THE CHEMISTRY OF HYDRIDES OF N ATOMS AND O+ IONS

ABSTRACT Previous work by various authors has suggested that the detection by Herschel/HIFI of nitrogen hydrides along the low-density lines of sight toward G10.6-0.4 (W31C) cannot be accounted for by gas-phase chemical models. In this paper we investigate the role of surface reactions on dust grains in diffuse regions, and we find that formation of the hydrides by surface reactions on dust grains with efficiency comparable to that for H2 formation reconciles models with observations of nitrogen hydrides. However, similar surface reactions do not contribute significantly to the hydrides of O+ ions detected by Herschel/HIFI that are present along many sight lines in the Galaxy. The O+ hydrides can be accounted for by conventional gas-phase chemistry either in diffuse clouds of very low density with normal cosmic-ray fluxes or in somewhat denser diffuse clouds with high cosmic-ray fluxes. Hydride chemistry in dense dark clouds appears to be dominated by gas-phase ion–molecule reactions.

Coinage Metal Hydrides: Synthesis, Characterization, and Reactivity.

Hydride complexes of copper, silver, and gold encompass a broad array of structures, and their distinctive reactivity has enabled dramatic recent advances in synthesis and catalysis. This Review summarizes the synthesis, characterization, and key stoichiometric reactions of isolable or observable coinage metal hydrides. It discusses catalytic processes in which coinage metal hydrides are known or probable intermediates, and presents mechanistic studies of selected catalytic reactions. The purpose of this Review is to convey how developments in coinage metal hydride chemistry have led to new organic transformations, and how developments in catalysis have in turn inspired the synthesis of reactive new complexes.

Tuning the Oxidation State, Nuclearity, and Chemistry of Uranium Hydrides with Phenylsilane and Temperature: The Case of the Classic Uranium(III) Hydride Complex [(C5Me5)2U(μ-H)]2

This work demonstrates that the oxidation state and chemistry of uranium hydrides can be tuned with temperature and the stoichiometry of phenylsilane. The trivalent uranium hydride [(C5Me5)2U–H]x (5) was found to be comprised of an equilibrium mixture of U(III) hydrides in solution at ambient temperature. A single U(III) species can be selectively prepared by treating (C5Me5)2UMe2 (4) with 2 equiv of phenylsilane at 50 °C. The U(III) system is a potent reducing agent and displayed chemistry distinct from the U(IV) system [(C5Me5)2U(H)(μ-H)]2 (2), which was harnessed to prepare a variety of organometallic complexes, including (C5Me5)2U(dmpe)(H) (6), and the novel uranium(IV) metallacyclopentadiene complex (C5Me5)2U(C4Me4) (11).

ChemInform Abstract: Synthesis of (+)‐Harmicine.

A facile convergent access to the important indole alkaloid (+)-harmicine is described, starting from tryptamine and (R)-acetoxysuccinic anhydride via the corresponding acetoxysuccinimide in very good overall yield. Regioselective reduction of an unsymmetrical imide carbonyl group and acid-catalyzed stereoselective intramolecular cyclization were the key features involved. The directing group to induce asymmetry was finally detached via the corresponding iodide by using tributyltin hydride chemistry.

Hydride compounds of zinc

The chemistry of zinc hydrides, including the main approaches to the synthesis, structure and physicochemical properties of compounds containing zinc-hydrogen bond, is systematized the review.

Epitaxial thin film growth of LiH using a liquid-Li atomic template

We report on the synthesis of lithium hydride (LiH) epitaxial thin films through the hydrogenation of a Li melt, forming abrupt LiH/MgO interface. Experimental and first-principles molecular dynamics studies reveal a comprehensive microscopic picture of the crystallization processes, which sheds light on the fundamental atomistic growth processes that have remained unknown in the vapor-liquid-solid method. We found that the periodic structure that formed, because of the liquid-Li atoms at the film/MgO-substrate interface, serves as an atomic template for the epitaxial growth of LiH crystals. In contrast, films grown on the Al2O3 substrates indicated polycrystalline films with a LiAlO2 secondary phase. These results and the proposed growth process provide insights into the preparation of other alkaline metal hydride thin films on oxides. Further, our investigations open the way to explore fundamental physics and chemistry of metal hydrides including possible phenomena that emerge at the heterointerfaces of metal hydrides.

Synthesis of (+)-Harmicine

A facile convergent access to the important indole alkaloid (+)-harmicine is described, starting from tryptamine and (R)-acetoxysuccinic anhydride via the corresponding acetoxysuccinimide in very good overall yield. Regioselective reduction of an unsymmetrical imide carbonyl group and acid-catalyzed stereoselective intramolecular cyclization were the key features involved. The directing group to induce asymmetry was finally detached via the corresponding iodide by using tributyltin hydride chemistry.

Nitrogenase: A Draft Mechanism

Biological nitrogen fixation, the reduction of N(2) to two NH(3) molecules, supports more than half the human population. The predominant form of the enzyme nitrogenase, which catalyzes this reaction, comprises an electron-delivery Fe protein and a catalytic MoFe protein. Although nitrogenase has been studied extensively, the catalytic mechanism has remained unknown. At a minimum, a mechanism must identify and characterize each intermediate formed during catalysis and embed these intermediates within a kinetic framework that explains their dynamic interconversion. The Lowe-Thorneley (LT) model describes nitrogenase kinetics and provides rate constants for transformations among intermediates (denoted E(n), where n is the number of electrons (and protons), that have accumulated within the MoFe protein). Until recently, however, research on purified nitrogenase had not characterized any E(n) state beyond E(0). In this Account, we summarize the recent characterization of three freeze-trapped intermediate states formed during nitrogenase catalysis and place them within the LT kinetic scheme. First we discuss the key E(4) state, which is primed for N(2) binding and reduction and which we refer to as the "Janus intermediate" because it lies halfway through the reaction cycle. This state has accumulated four reducing equivalents stored as two [Fe-H-Fe] bridging hydrides bound to the active-site iron-molybdenum cofactor ([7Fe-9S-Mo-C-homocitrate]; FeMo-co) at its resting oxidation level. The other two trapped intermediates contain reduced forms of N(2). One, intermediate, designated I, has S = 1/2 FeMo-co. Electron nuclear double resonance/hyperfine sublevel correlation (ENDOR/HYSCORE) measurements indicate that I is the final catalytic state, E(8), with NH(3) product bound to FeMo-co at its resting redox level. The other characterized intermediate, designated H, has integer-spin FeMo-co (non-Kramers; S ≥ 2). Electron spin echo envelope modulation (ESEEM) measurements indicate that H contains the [-NH(2)] fragment bound to FeMo-co and therefore corresponds to E(7). These assignments in the context of previous studies imply a pathway in which (i) N(2) binds at E(4) with liberation of H(2), (ii) N(2) is promptly reduced to N(2)H(2), (iii) the two N's are reduced in two steps to form hydrazine-bound FeMo-co, and (iv) two NH(3) are liberated in two further steps of reduction. This proposal identifies nitrogenase as following a "prompt-alternating (P-A)" reaction pathway and unifies the catalytic pathway with the LT kinetic framework. However, the proposal does not incorporate one of the most puzzling aspects of nitrogenase catalysis: obligatory generation of H(2) upon N(2) binding that apparently "wastes" two reducing equivalents and thus 25% of the total energy supplied by the hydrolysis of ATP. Because E(4) stores its four accumulated reducing equivalents as two bridging hydrides, we propose an answer to this puzzle based on the organometallic chemistry of hydrides and dihydrogen. We propose that H(2) release upon N(2) binding involves reductive elimination of two hydrides to yield N(2) bound to doubly reduced FeMo-co. Delivery of the two available electrons and two activating protons yields cofactor-bound diazene, in agreement with the P-A scheme. This keystone completes a draft mechanism for nitrogenase that both organizes the vast body of data on which it is founded and serves as a basis for future experiments.

Hydrogen–deuterium exchange in hydride chemistry: Dihydrogen bonded complexes as key intermediates

The hydrogen–deuterium exchange mechanism is analyzed at the DFT/B3LYP level for main group element and transition metal hydride compounds (decahydro-closo-decaborate anion, [B10H10]2−, and Cp*Mo(CO)(PMe3)2H, where Cp*=η5-C5Me5) using methanol as deuterium source. The effect of bulk methanol was represented by CPCM calculations combined with the representation of discreet alcohol as the trimer, (CH3OD)3. This model helped to solve the problem of calculation of the reaction Gibbs energy in solution. The dihydrogen bond formation is shown being the first reaction step and determines the selectivity of [1,10-B10H8D2]2− formation. The transition state for the H/D exchange resembles the non-classical (η2-H2) complex and is similar to that for metal hydrides protonation. The evolution of the reaction energy profile with the acid strength is analyzed, showing the gradual transition from the proton-hydride (H/D) exchange to complete proton transfer to hydride ligand. The possibility of η2-H2 complex formation and its stability depend on the properties of both metal atom and proton donor.