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Bronsted acids in a confined environment catalyze transfer hydrogenation of aldehydes (CnH2nO, n = 3–6) by kinetically relevant hydride transfer between hydride donors (e.g., cyclohexadiene, tetralin, cyclohexene, or 3-methyl-1-pentene) and protonated aldehydes. The hydride ion affinity difference between the donor and acceptor pair determines the reaction enthalpy, which reflects the thermodynamic tendency, of hydride transfer. For this class of reactions, the hydride ion affinity difference appears to be the empirical kinetic descriptor for predicting the rates of hydride transfer and, in turn, of transfer hydrogenation through the Bronsted–Evans–Polanyi relation.

The acid dissociation constant Ka of a transition-metal hydride complex is a key thermodynamic quantity for evaluating reactivity and stability of the complexes and their conjugate bases in stoichiometric and catalytic reactions. It can be estimated using a simple ligand acidity constant (LAC) empirical equation for a wide range of complexes. Here, we provide the first density functional theory (DFT) study that supports the additive nature of ligand contributions to the pKa of metal hydride complexes. Specifically, the pKa values of iron hydride complexes [FeH(CO)x(PR3)(5–x)]+ in either tetrahydrofuran or dichloromethane solutions are estimated using the LAC method and DFT calculations. There is a linear correlation between these two methods, and both predict a surprisingly linear increase in pKa over a wide range from approximately −15 for x = 5 to approximately 40 for x = 0. The LAC equation predicts that pKaTHF or pKaDCM increases by 9 units with the replacement of each CO ligand with a trialkylphosphi...

Attempts to generate a proposed rhodium hydride catalytic intermediate instead resulted in isolation of (Cp*H)Rh(bpy)Cl (1), a pentamethylcyclopentadiene complex, formed by C-H bond-forming reductive elimination from the fleeting rhodium hydride. The hydride transfer ability of diene 1 was explored through thermochemistry and hydride transfer reactions, including the reduction of NAD(+).

With a view towards replacing sacrificial hydrogen acceptors in alkane dehydrogenation catalysis, electrochemical methods for oxidative activation of a pincer-ligated iridium hydride intermediate were explored. A 1H(+)/2e(-) oxidation process was observed in THF solvent, with net hydride loss leading to a reactive cationic intermediate that can be trapped by chloride. Analogous reactivity was observed with the concerted hydride transfer reagent Ph3C(+), connecting chemical and electrochemical hydride loss pathways.

The problem of activating N2 and its subsequent hydrogenation to form NH3 has been approached from many directions. One of these approaches involves the use of transition metal hydride complexes. Recently, transition metal hydride complexes of Ti and Ta have been shown to activate N2, but without catalytic formation of NH3. Here, we show that at elevated temperatures (400 °C, 5 MPa), solid-state hydride-containing Ti compounds (TiH2 and BaTiO2.5H0.5) form a nitride-hydride surface similar to those observed with titanium clusters, but continuously (∼7 days) form NH3 under H2/N2 flow conditions to achieve a catalytic cycle, with activity (up to 2.8 mmol·g·–1·h–1) almost comparable to conventional supported Ru catalysts such as Cs–Ru/MgO or Ru/BaTiO3 that we have tested. As with the homogeneous analogues, the initial presence of hydride within the catalyst is critical. A rare hydrogen-based Mars van Krevelen mechanism may be at play here. Conventional scaling rules of pure metals predict essentially no activ...

The reaction of molybdenum, tungsten and manganese carbonyls with several thiazole heterocycle ligands yielded a number of coordinated transition metal complexes 1–10. Of these complexes 1–6 are new compounds which have not been reported to date. The structures of new compounds were characterized by FT-IR and 1H-NMR spectroscopy as well as single-crystal X-ray diffraction analysis. Complexes 1–10 are carbon monoxide releasing molecules that show structure-related anti-cancer activity. The cytotoxicity of all compounds on Hela cells was evaluated by MTT assay, and the results show that carbon monoxide releasing molecules containing such Schiff base ligands may have biomedical applications for their anti-tumor effect.

Despite many exploratory studies over the past several decades, the presently known transition metals that form homoleptic transition-metal hydride complexes are limited to the Groups 7-12. Here we present evidence for the formation of Mg3 CrH8 , containing the first Group 6 hydride complex [CrH7 ](5-) . Our theoretical calculations reveal that pentagonal-bipyramidal H coordination allows the formation of σ-bonds between H and Cr. The results are strongly supported by neutron diffraction and IR spectroscopic measurements. Given that the Group 3-5 elements favor ionic/metallic bonding with H, along with the current results, the true boundary for the formation of homoleptic transition-metal hydride complexes should be between Group 5 and 6. As the H coordination number generally tends to increase with decreasing atomic number of transition metals, the revised boundary suggests high potential for further discovery of hydrogen-rich materials that are of both technological and fundamental interest.

In this study, the surface properties of type-B silica have been compared with an unmodified silica hydride phase, a diamond hydride phase and silica hydride phases modified with bidentate anchored octyl (BDC8), bidentate anchored octadecyl (BDC18), phenyl and cholesteryl groups. Atomic distributions of the surface elemental composition of each type of stationary phase were determined using energy-dispersive X-ray spectroscopy. For the type-B silica, unmodified silica hydride, diamond hydride as well as BDC18 and cholesteryl silica hydride phases, the increase in carbon contents correlated with more negative surface ζ potential values (R(2) = 0.92). The origin of these more negative ζ potentials has been evaluated with mobile phases up to 100% (v/v) methanol content, with this property attributed to either an increase in the amount of adsorbed hydroxide ions or a decrease in the amount of adsorbed protons on the surfaces modified silica hydride phases of higher carbon content. This property of chemically modified silica hydride phases is in accordance with the unique propensity for hydroxide ions to be preferentially adsorbed onto hydrophobic surfaces of low permittivity and effects due to the specific accumulated water molecules associated with the electrical interfacial double layer of the adsorbent.

In this work, the influence of carbon dioxide concentration in the hydrogen on metal hydride has been investigated. LmNi4.73Mn0.12Al0.15 was employed as a sample material for its durability against inorganic compound and operating temperature and pressure. Mixture of hydrogen and carbon dioxide is supplied to the sample in a double-ended cylinder at a constant flow rate and a part of the supplied gas is released to avoid condensing of impurity at a smaller constant flow rate than inlet. The temperature is maintained at 30 °C. The experimental results show the sample is able to absorb hydrogen selectively at carbon dioxide concentrations less than 30 vol% and the mixture of the carbon dioxide concentration of 20 vol% is purified to less than 0.1%.

Complex hydride systems for hydrogen (H2) generation for supplying fuel cells are being reviewed. In the first group, the hydride systems that are capable of generating H2 through a mechanical dehydrogenation phenomenon at the ambient temperature are discussed. There are few quite diverse systems in this group such as lithium alanate (LiAlH4) with the following additives: nanoiron (n-Fe), lithium amide (LiNH2) (a hydride/hydride system) and manganese chloride MnCl2 (a hydride/halide system). Another hydride/hydride system consists of lithium amide (LiNH2) and magnesium hydride (MgH2), and finally, there is a LiBH4-FeCl2 (hydride/halide) system. These hydride systems are capable of releasing from ~4 to 7 wt.% H2 at the ambient temperature during a reasonably short duration of ball milling. The second group encompasses systems that generate H2 at slightly elevated temperature (up to 100 °C). In this group lithium alanate (LiAlH4) ball milled with the nano-Fe and nano-TiN/TiC/ZrC additives is a prominent system that can relatively quickly generate up to 7 wt.% H2 at 100 °C. The other hydride is manganese borohydride (Mn(BH4)2) obtained by mechano-chemical activation synthesis (MCAS). In a ball milled (2LiBH4 + MnCl2) nanocomposite, Mn(BH4)2 co-existing with LiCl can desorb ~4.5 wt.% H2 at 100 °C within a reasonable duration of dehydrogenation. Practical application aspects of hydride systems for H2 generation/storage are also briefly discussed.

Features of the electronic structure of adducts of transition metal hydride complexes (Cp*M(dppe)H, dppe is the 1,2-(diphenylphosphino)ethane, M = Fe, Ru, Os; CpM(CO)3H, M = Mo, W) with acids and bases were analyzed with the ADF2014 program using energy decomposition analysis (EDA) by the Ziegler-Rauk method combined with the natural orbitals for chemical valence theory (ETS-NOCV). The nature of orbital interactions in the complex determines the reaction pathway: σMH → σ*OH interaction leads to the proton transfer to hydride ligand, nM → σ*OH leads to the metal atom protonation, nN → σ*MH implies the metal hydride deprotonation, and σMH → n*B corresponds to the hydride transfer to Lewis acid. It was shown that M-H bond polarization change has the similar character upon the formation of complexes with Bronsted and Lewis acids. The ease of polarization of M-H bonds in complexes CpM(CO)3H determines their reactivity as proton and hydride ion donors.

Iron-sulfide complexes with hydride ligands provide an experimental precedent for spectroscopically detected hydride species on the iron-sulfur FeMoco of nitrogenase. In this contribution, we expand upon our recent synthesis of the first iron sulfide hydride complex from an iron hydride and a sodium thiolate (Arnet, N. A.; Dugan, T. R.; Menges, F. S.; Mercado, B. Q.; Brennessel, W. W.; Bill, E.; Johnson, M. A.; Holland, P. L., J. Am. Chem. Soc. 2015, 137, 13220–13223). First, we describe the isolation of an analogous iron sulfide hydride with a smaller diketiminate supporting ligand, which benefits from easier preparation of the hydride precursor and easier isolation of the product. Second, we describe mechanistic studies on the C-S bond cleavage through which the iron sulfide hydride product is formed. In a key experiment, use of cyclopropylmethanethiolate as the sulfur precursor leads to products from cyclopropane ring opening, implicating an alkyl radical as an intermediate. Combined with the results of isotopic labeling studies, the data are consistent with a mechanism in which homolytic C-S bond cleavage is followed by rebound of the alkyl radical to abstract a hydrogen atom from the iron to give the observed alkane and iron-sulfide products.

[Fe]-hydrogenase is a key enzyme involved in methanogenesis and facilitates reversible hydride transfer from H2 to N5,N10-methenyltetrahydromethanopterin (CH-H4MPT+). In this study, a reaction system was developed to model the enzymatic function of [Fe]-hydrogenase by using N,N′-diphenylimidazolinium cation (1+) as a structurally related alternative to CH-H4MPT+. In connection with the enzymatic mechanism via heterolytic cleavage of H2 at the single metal active site, several transition metal complex catalysts capable of such activation were utilized in the model system. Reduction of 1[BF4] to N,N′-diphenylimidazolidine (2) was achieved under 1 atm H2 at ambient temperature in the presence of an equimolar amount of NEt3 as a proton acceptor. The proposed catalytic pathways involved the generation of active hydride complexes and subsequent intermolecular hydride transfer to 1+. The reverse reaction was accomplished by treatment of 2 with HNMe2Ph+ as the proton source, where [(η5-C5Me5)Ir{(p-MeC6H4SO2)NCHPh...

Ninefold coordination of hydrogen is very rare, and has been observed in two different hydride complexes comprising rhenium and technetium. Herein, based on a theoretical/experimental approach, we present evidence for the formation of ninefold H- coordination hydride complexes of molybdenum ([MoH9]3−), tungsten ([WH9]3−), niobium ([NbH9]4−) and tantalum ([TaH9]4−) in novel complex transition-metal hydrides, Li5MoH11, Li5WH11, Li6NbH11 and Li6TaH11, respectively. All of the synthesized materials are insulated with band gaps of approximately 4 eV, but contain a sufficient amount of hydrogen to cause the H 1s-derived states to reach the Fermi level. Such hydrogen-rich materials might be of interest for high-critical-temperature superconductivity if the gaps close under compression. Furthermore, the hydride complexes exhibit significant rotational motions associated with anharmonic librations at room temperature, which are often discussed in relation to the translational diffusion of cations in alkali-metal dodecahydro-closo-dodecaborates and strongly point to the emergence of a fast lithium conduction even at room temperature.

This chapter describes the activation of dinitrogen by various transition metal hydride complexes. A number of mononuclear transition metal hydride complexes can incorporate dinitrogen, but they are usually difficult to induce N–N bond cleavage. In contrast, multimetallic hydride complexes can split and hydrogenate dinitrogen through cooperation of the multiple metal hydrides. In this transformation, the hydride ligands serve as the source of both electron and proton, thus enabling the cleavage and hydrogenation of dinitrogen without extra reducing agents and proton sources. Generally, the reactivity of the metal hydride complexes is significantly influenced by their composition (nuclearity) and metal/ligand combination.

In analogy to the previously reported tetranuclear magnesium hydride cluster with a bridged dianionic bis-beta-diketiminate ligand, a related zinc hydride cluster has been prepared. The crystal structures of these magnesium and zinc hydride complexes are similar: the metal atoms are situated at the corners of a tetrahedron in which the vertices are bridged either by dianionic bis-b-diketiminate ligands or hydride ions. Both structures are retained in solution and show examples of H-center dot center dot center dot H- NMR coupling (Mg: 8.5 Hz; Zn: 16.0 Hz). The zinc hydride cluster [NN-(ZnH)(2)](2) thermally decomposes at 90 degrees C and releases 1.8 equivalents of H-2. In contrast to magnesium hydride clusters, there is no apparent relationship between cluster size and thermal decomposition temperature for the zinc hydrides. DFT calculations reproduced the structure of the zinc hydride cluster reasonably well and charge density analysis showed no bond paths between the hydride ions. This contrasts with calculations on the analogous magnesium hydride cluster in which a counter-intuitive H-center dot center dot center dot H- bond path was observed. Forcing a reduced H-center dot center dot center dot H- distance in the zinc hydride cluster, however, gave rise to a H-center dot center dot center dot H- bond path. Such weak interactions could play a role in H-2 desorption. The presumed molecular product after H-2 release, a Zn(I) cluster, could not be characterized experimentally but DFT calculations predicted a cluster with two localized Zn-Zn bonds.

Several five-membered heterocyclic molecules were studied theoretically as organic hydride donors. The density functional theory and ab initio methods are employed to study the direct one-step or multistep sequence suggested for the hydride transfer from the selected molecules: H atom/electron, electron/proton/electron or electron/H atom. Out of the three multistep mechanisms, electron/H atom seems to be a probable pathway in the presence of suitable catalyst/photoreaction that can cause ionization. In the lack of such catalyst/photoreaction, the direct hydride transfer seems to be most probable with the presence of suitable hydride acceptor. A detailed mechanism of the hydride transfer from the five-membered heterocylic compounds is important in understanding chemical and biological reactions and required for scientifically designing and synthesizing new desired five-membered heterocyclic compounds as organic hydride donor. Copyright © 2014 John Wiley & Sons, Ltd.

Density functional theory (DFT) was used to investigate ruthenium hydride-catalyzed addition reactions of benzaldehydes to isoprenes. Calculation indicated that two π bonds of isoprene had different reactivity, and the less substituted one had better reactivity. The role of the ligands hydride, carbonyl, chloride, and triphenylphosphine of RuHCl(CO)(PPh3)3 were studied in our present work. The hydride was active, and promoted this reaction. The carbonyl or chloride was not used as a carrier of hydrogen migration reaction, so they could not change the reaction channels. The only role of them was the transformation of the electron and geometry structures of those intermediates and transition states. The triphenylphosphine decreased generally the free energies of the intermediates and transition states. The di-ruthenium complexes were not an efficient catalyst.

Recent developments in the chemistry of hydride and dihydrogen complexes of iron, cobalt, and nickel are summarized. Applications in homogeneous catalysis are emphasized.

This study highlights that Fe additives offer better catalytic properties than carbon, Fe-C (iron carbide/carbon composites), and Fe-Mg (Mg2 FeH6 ) additives for the low-temperature dehydrogenation of magnesium hydride. The in situ X-ray diffraction measurements prove the formation of a Mg2 FeH6 phase in iron additive loaded MgH2 . Nonetheless, differential scanning calorimetry data suggest that this Mg2 FeH6 phase does not have any influence on dehydrogenation properties of MgH2 . On the other hand, the composite system Mg2 FeH6 /MgH2 shows significantly improved dehydrogenation properties even in absence of further additives. It is suggested that the improved system performance of Fe loaded MgH2 is attributed to restrictions on crystal growth of MgH2 and the catalytic behavior of Fe nanoparticles, rather than any intrinsic catalytic properties offered by the formed mixed metal phase Mg2 FeH6 .

We present the synthesis and structural characterisation of a transition metal oxide hydride, LaSrCoO_3H_{0.7}, which adopts an unprecedented structure in which oxide chains are bridged by hydride anions to form a two- dimensional extended network. The metal centers are strongly coupled by their bonding with both oxide and hydride ligands to produce magnetic ordering up to at least 350 K. The synthetic route is sufficiently general to allow the prediction of a new class of transition metal-containing electronic and magnetic materials., Comment: 6 pages, including 3 figures

Transition metal hydride complexes are usually amphoteric, not only acting as hydride donors, but also as Brønsted-Lowry acids. A simple additive ligand acidity constant equation (LAC for short) allows the estimation of the acid dissociation constant Ka(LAC) of diamagnetic transition metal hydride and dihydrogen complexes. It is remarkably successful in systematizing diverse reports of over 450 reactions of acids with metal complexes and bases with metal hydrides and dihydrogen complexes, including catalytic cycles where these reactions are proposed or observed. There are links between pKa(LAC) and pKa(THF), pKa(DCM), pKa(MeCN) for neutral and cationic acids. For the groups from chromium to nickel, tables are provided that order the acidity of metal hydride and dihydrogen complexes from most acidic (pKa(LAC) -18) to least acidic (pKa(LAC) 50). Figures are constructed showing metal acids above the solvent pKa scales and organic acids below to summarize a large amount of information. Acid-base features are analyzed for catalysts from chromium to gold for ionic hydrogenations, bifunctional catalysts for hydrogen oxidation and evolution electrocatalysis, H/D exchange, olefin hydrogenation and isomerization, hydrogenation of ketones, aldehydes, imines, and carbon dioxide, hydrogenases and their model complexes, and palladium catalysts with hydride intermediates.

Atomic hydrogen on the surface of a metal with high hydrogen solubility is of particular interest for the hydrogenation of carbon dioxide. In a mixture of hydrogen and carbon dioxide, methane was markedly formed on the metal hydride ZrCoHx in the course of the hydrogen desorption and not on the pristine intermetallic. The surface analysis was performed by means of time-of-flight secondary ion mass spectroscopy and near-ambient pressure X-ray photoelectron spectroscopy, for the in situ analysis. The aim was to elucidate the origin of the catalytic activity of the metal hydride. Since at the initial stage the dissociation of impinging hydrogen molecules is hindered by a high activation barrier of the oxidised surface, the atomic hydrogen flux from the metal hydride is crucial for the reduction of carbon dioxide and surface oxides at interfacial sites.

Ruthenium hydride was synthesized at a hydrogen pressure of about 14 GPa in a diamond-anvil cell. Energy-dispersive x-ray diffraction was used to monitor the ruthenium crystal structure as a function of hydrogen pressure up to 30 GPa. The hydride formation was accompanied by phase transition from the original hcp structure of the pristine metal to the fcc structure. Our results confirmed the theoretical prediction of ruthenium hydride formation under hydrogen pressure. The standard Gibbs free energy of the ruthenium hydride formation reaction was calculated assuming the pressure of decomposition as the equilibrium pressure.

Ruthenium will fix it: CO(2) undergoes reduction to HCO(2)(-) when placed over a solution of a ruthenium complex bearing an NADH model ligand 1 (black in right structural formula). The organic hydride transfer is triggered by the addition of benzoate anion, which rapidly forms a complex with 1, a complex that is a stronger reductant than 1. A photocatalytic variant of the reaction using triethanolamine as a sacrificial reagent has also been developed.

Dehydrogenation of NaAlH 4 can be greatly facilitated by activated carbon catalysts. The catalytic function can be further enhanced by decorating the carbon with Co, Ni, or Cu nanoparticles. The decomposition temperature was lowered by as much as 100 °C using a 3 wt.% Co or Ni-decorated activated carbon, comparable to a Ti-based catalyst, which were the most effective among the metals tested. The catalytic effect is likely due to a combination of hydrogen spillover effect, high contact area between carbon and the hydride, and confinement of the hydride as nano-sized domains in the pores of the carbon matrix. The catalysts were also effective in facilitating rehydrogenation of NaAlH 4 under moderate pressure (75.8 bar H 2 ) and low temperature (120 °C), when no rehydrogenation would occur without the catalyst. The fact that this new catalyst system is not specific to any hydride offers many potential applications.

The new compound Ir(COD)(CO)2GePh3, 1, was obtained when a solution containing [Ir(COD)Cl]2 and HGePh3 was treated with a solution of BuLi under a purge with CO. The tin homologue Ir(COD)(CO)2SnPh3...

Two kinds of metal hydride alloys as the bi-functional catalyst concept for hydrogen generation and oxidation in hydrogen-diffusion electrodes were investigated. The AB5-type hydride electrode shows much higher catalytic activities than the Zr-based AB2-type hydride electrode. However, the activity of Zr-based hydride electrodes can be improved only after removal of zirconium oxides on surface by a 1.0 M HF solution. The experiments demonstrated that the both metal-hydride hydrogen-diffusion electrodes for cycles of hydrogen generation (12 h) and oxidation (12 h) had good stability under the current densities of 100 and 50 mA/cm2, respectively. The results also showed that small amounts of oxygen below 500 ppm and moisture up to 145,000 ppm in the hydrogen gas have little effect on the activity. It indicated that the hydride alloys as the non-noble-metal bi-functional catalysts in a reversible MH-air fuel cell have potential applications. Keywords: Metal hydride, Catalyst, Hydrogen, Gas-diffusion electrode, Air-MH fuel cell

This study examines the use of transition-metal hydride complexes that can be generated by the heterolytic cleavage of H(2) gas to form B-H bonds. Specifically, these studies are focused on providing a reliable and quantitative method for determining when hydride transfer from transition-metal hydrides to three-coordinate BX(3) (X = OR, SPh, F, H; R = Ph, p-C(6)H(4)OMe, C(6)F(5), (t)Bu, Si(Me)(3)) compounds will be favorable. This involves both experimental and theoretical determinations of hydride transfer abilities. Thermodynamic hydride donor abilities (DeltaG(o)(H(-))) were determined for HRh(dmpe)(2) and HRh(depe)(2), where dmpe = 1,2-bis(dimethylphosphinoethane) and depe = 1,2-bis(diethylphosphinoethane), on a previously established scale in acetonitrile. This hydride donor ability was used to determine the hydride donor ability of [HBEt(3)](-) on this scale. Isodesmic reactions between [HBEt(3)](-) and selected BX(3) compounds to form BEt(3) and [HBX(3)](-) were examined computationally to determine their relative hydride affinities. The use of these scales of hydride donor abilities and hydride affinities for transition-metal hydrides and BX(3) compounds is illustrated with a few selected reactions relevant to the regeneration of ammonia borane. Our findings indicate that it is possible to form B-H bonds from B-X bonds, and the extent to which BX(3) compounds are reduced by transition-metal hydride complexes forming species containing multiple B-H bonds depends on the heterolytic B-X bond energy. An example is the reduction of B(SPh)(3) using HRh(dmpe)(2) in the presence of triethylamine to form Et(3)N-BH(3) in high yields.

One-pot etherification reaction of aromatic and some aliphatic carbonyl compounds with organic halides in the presence of sodium hydride as a reducing reagent proceeded smoothly in dioxane, a polar solvent with higher boiling point, to provide desired ethers in moderate to high yields.

Sodium hydride is a common reagent for substrate activation in nucleophilic substitution reactions. Sodium hydride can behave both as a base and as a source of hydride. This dual ability in the presence of an electrophile such as benzyl bromide results in the formation of byproducts when dimethylformamide or acetonitrile are used as solvents for these reactions. The structural nature of these byproducts is revealed in this report.

Cyclopentadienone-ligated ruthenium complexes, such as Shvo's catalyst, are known to oxidize reversibly alcohols to the corresponding carbonyl compounds. The mechanism of this reaction has been the subject of some controversy, but it is generally believed to proceed through concerted transfer of proton and hydride, respectively, to the cyclopentadienone ligand and the ruthenium center. In this paper we further study the hydride transfer process as an example of a coordinatively directed hydride abstraction by adding quantitative understanding to some features of this mechanism that are not well understood. We find that an oxidant as weak as acetone can be used to re-oxidize the intermediate ruthenium hydride without catalyst re-oxidation becoming rate-limiting. Furthermore, C–H cleavage is a significantly electrophilic event, as demonstrated by a Hammett reaction parameter of ρ = –0.89. We then describe how the application of our mechanistic insights obtained from the study have enabled us to extend the ligand-directed hydride abstraction strategy to include a rare example of an iron(0) oxidation catalyst.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Density functional theory reveals multiple pathways for intramolecular hydride transfer in the cyclopentadienyl and indenyl species (η 5-C 5H 5)Fe(CO) 3H and (η 5-C 9H 7)Fe(CO) 3H. The ability of the indenyl ligand to undergo facile η 5- to η 3-'ring slippage' stabilises the isomer where the hydride is bonded directly to the metal, which opens up a low-energy pathway for hydride transfer from CO to metal. © 2008 Elsevier B.V. All rights reserved.

The chemistry of the hydride complexes of the rare-earth metals has been dominated by complexes with metallocene and half-sandwich ligand frameworks. In contrast to hydride complexes supported by cyclopentadienyl ligands, only relatively few examples of non-metallocene hydride complexes have been reported in the literature. Such complexes are potentially of some interest in view of their chemical reactivity and catalytic applications. This review surveys the chemistry of non-metallocene hydride complexes up to mid-2007, some spectroscopic and structural aspects as well as their reactivity.

Cerium hydride was formed at a moderate temperature (523 K) and atmospheric pressure using a pulse-flow technique. Its adsorption and catalytic properties were probed using deuterium, oxygen, carbon monoxide, carbon dioxide, methanol, and propyne. Deuterium exchange with the hydrogen of the hydride was examined and not all of the hydrogen was found to be exchangeable. It is likely that this was due to the nature of the pulse reaction where the residence time was too short to allow diffusion of hydrogen through from the bulk material. The hydride did not adsorb or react with either carbon monoxide or carbon dioxide. In contrast there was extensive reaction with oxygen resulting in water formation and a significant rise in temperature. The interaction between the hydride and methanol gave rise to adsorption and, at low levels, methane and hydrogen. This unusual reaction was facilitated by the formation of cerium oxide. Propyne hydrogenation was also shown to occur over cerium hydride but a hydrogen source other than the hydride was required.

(2007). TRANSITION METAL-CATALYZED HYDROSILYLATION OF CARBONYL COMPOUNDS AND IMINES. A REVIEW. Organic Preparations and Procedures International: Vol. 39, No. 6, pp. 523-559.

Catalysis is one of the critical factors in the improvement of hydrogen sorption kinetics in metal hydride systems. Effective catalysts, although usually added in small amounts, enable not only the practical performance of a hydride, but also provide an effective synthesis path for compounds that are otherwise inaccessible. In the search for efficient and inexpensive catalysts for hydrogen sorption reactions, a new type of catalytic compounds was developed. These catalytic complexes demonstrated remarkable potential in a variety of systems, including sodium alanates and magnesium, as well as in hydrogen generation through hydrolysis. These new complexes demonstrated their unique molecular configuration (shown by FT-IR spectroscopy and X-ray diffraction) combined with nano-scale particle size.

An approach is proposed for the quantum-chemical investigation of “hydride ion” transfer based on analysis of the similarity of the order of variation in the ionization potentials, enthalpies, and free energies of affinity to the hydride ion, the hydrogen atom, and the proton in the substrate molecules and also the derivatives of their cations, radicals, and ions to the experimentally established “hydride” series. It was established that the experimental “hydride” mobility series of six chalcogenopyrans based on “semicyclic” 1,5-diketones agrees with the quantum-chemically calculated ionization potentials of the molecules and with the affinity of the respective radicals to the hydrogen atom participating in the transfer. It was found that direct removal of a hydride ion and initial deprotonation of the substrates followed by the removal of two electrons are unlikely. “Hydride” shift mechanisms, in which the first stage is transfer of an electron or hydrogen atom from the chalcogenopyran molecules, are feasible.

Metal hydride is one of key materials for developing fuel-cell vehicles, which is most demanded for reduction of CO2 emission. In this report, our recent work relating to crystal structure and nano-structure of metal hydrides studied by ‘in situ’ X-ray and neutron diffraction for clarifying the mechanism of hydriding reaction of metal hydride will be described.

Synthesis of an anionic hydride from monomeric silsesquioxanes is described. The novel compound, dubbed “silica hydride” is the first of several newly synthesized compounds from an interstitially embedded hydride family. It is a hydride-based compound with H − ions interstitially embedded in a matrix of caged silica. This compound exhibits profoundly different characteristics than other known compounds in hydride family. Unlike saline hydrides, the silica hydride demonstrates no overt or violent reaction with water or air. However it is capable of generating aqueous reductive potential readings of −750 mV for extended time periods. In vitro biological testing demonstrated no cytotoxicity induced by the compound while demonstrating efficacy as an antioxidant. In vivo studies of the compound have shown that it has a significant ability to reduce lactic acid build up in muscles by one-half after exercise. The synthesis of the silica hydride resulted in an approximately 16.8% w/w hydride content, as determined by density changes, proton NMR spectroscopy and ion beam analyses. Scanning and tunneling electron microscopy, Rutherford backscattering spectroscopy (RBS), forward recoil (FReS) ion beam analyses, in addition to Fourier transform infrared spectroscopy, reduction potential and 29 Si CP-MAS solid state NMR were additionally used to characterize the compound.

This review describes the synthesis and reactivity studies of early metal (Sc, Y, Ti, Zr, Hf, V, Nb, Ta) hydride compounds that have appeared over the last 10 years. Every attempt has been made to be comprehensive from 1990 to mid-2001. Complexes reported prior to 1990 have not been discussed. It should also be noted that related subjects that have been recently reviewed including the organic chemistry applications, polyhydride clusters, silica supported Groups 4 and 5 transition metal hydrides and hydride bridged heterobimetallic complexes have been excluded.

There has been a rapid development in the study of metal carbonyl cations, which exhibit distinguishing characteristics in comparison with typical metal carbonyl complexes. This review article gives an overview of the generation, spectroscopic characterization and catalytic application of this new class of metal carbonyls, including the metal carbonyl cluster cations.