Early Transition Metal Hydride Research Articles

Synthesis of MAX phase-based ceramics from early transition metal hydride powders

MAX phase ceramics are typically prepared by the reactive sintering of elemental powders that are often coarse, expensive, and prone to oxidation. The temperature-driven dehydrogenation of metal hydride powders offers an alternative synthesis approach, as the hydrides decompose into phase-pure, dimensionally fine elemental powder particles. The increased reactivity of these in situ formed, fine powder particles drastically reduces the formation temperature of the antecedent intermetallic phases, without forming excess binary carbides or facilitating powder oxidation in the Ti-Al-C and Zr-Al-C systems. This work elucidates the effect of metal hydrides on the sequence of formation reactions in MAX phase ceramics. In the Zr-Al-C system, the use of coarse, oxidation-prone elemental Zr powders prevented MAX phase formation, whereas spark plasma sintering of ZrH2 powders at 1500 °C produced ceramics containing 60 wt% Zr3AlC2. Similarly, in the Ti-Al-C system, spark plasma sintering of TiH2 powders at 1200 °C produced phase-pure Ti3AlC2 ceramics.

The impacts of charge transfer, localization, and metallicity on hydrogen retention and transport capacity

Solid state hydrides such as early transition metal hydrides are of inestimable importance for the future of hydrogen energy and are actively being investigated for energy conversion and storage applications such as fuel cells, solid-state batteries and neutron moderators. The retention and transport behavior of hydrogen in these hydrides has a huge role on the extended performance of components. While early transition-metal-based compounds exhibit many peculiar properties due to their unique correlated electronic signatures arising from d-orbital electrons, the fundamental chemistry and transport behavior of hydrogen in such hydrides is not well understood. In the present work, using density functional theory, a highly intricate bonding feature is revealed through the theoretical investigation of the electronic structure of early transition metal hydrides YH2 and ZrH2. In particular, a pronounced charge transfer from the transition element to H, results in localized electron densities at deep energy levels. The interplay between intrinsic charge transfer, charge localization, and metallicity in YH2 and ZrH2 leads to strong chemical bonding between metal and hydrogen atoms and large energy barriers for the migration of hydrogen vacancies. Specifically, hydrogen vacancies are found to be stable in the neutral state due to electron screening effects, accompanied by substantially high migration barriers between 0.8–1.2 eV along different crystallographic directions. In contrast, recent literature shows the migration barrier for charged H vacancies in insulating s-block metal hydrides lie between 0.1–0.4 eV, which is suitable for fast conduction applications. This pivotal electron structure difference exploited between early transition metal hydrides and alkali/alkaline earth metal hydrides determines extended hydrogen retention in these early transition metal hydrides. This work explains fundamental differences between the electronic structure of s-block and d-block metal hydrides, and its impact on the mobility of hydrogen vacancies.

Activation and Transformation of Small Molecules by Multimetallic Early Transition Metal Hydride Clusters

Activation and Transformation of Small Molecules by Multimetallic Early Transition Metal Hydride Clusters

Computational study of H2 binding to MH3 (M = Ti, V, or Cr).

A series of amorphous materials based on hitherto elusive early transition metal hydrides MH3 (M = Ti, V, and Cr) and capable of binding H2via the Kubas interaction has shown great promise for hydrogen storage applications, approaching US DoE system targets in some cases [Phys. Chem. Chem. Phys., 2015, 17, 9480; Chem. Mat., 2013, 25, 4765; J. Phys. Chem. C, 2016, 120, 11407]. We here apply quantum chemical computational techniques to study models of the H2 binding sites in these materials. Starting with monomeric MH3 (M = Ti, V, and Cr) we progress to M2H6 and then pentametallic systems, analyzing the H2 binding geometries, energies, vibrational frequencies and electronic structure, finding clear evidence of significant Kubas binding. Dihydrogen binding energies range from 22 to 53 kJ mol-1. In agreement with experiment, we conclude that while TiH3 binds H2 exclusively through the Kubas interaction, VH3 and CrH3 additionally physisorb dihydrogen, making these more attractive for practical applications.

Mechanism and kinetics of early transition metal hydrides, oxides, and chlorides to enhance hydrogen release and uptake properties of MgH2

Selected hydrides (TiH2, ZrH2), chlorides (VCl3, ScCl3) or oxides (V2O5) utilized as additives facilitating hydrogen release and uptake for magnesium hydride were investigated using mechano-chemical treatment and in-situ synchrotron radiation powder X-ray diffraction studies. The fastest hydrogen desorption and absorption kinetics for MgH2 was observed for a sample with 5 mol% V2O5 at 320 °C. Additional activation of the system (2 cycles, vacuum/p(H2) ~150 bar, 450 °C) leads to significant improvement of the kinetics even at lower temperatures, 270 °C. The observed prolific effect is achieved through the full reduction of vanadium oxides and formation of an efficient vanadium catalyst as nanoparticles and possibly interfacial effects in the MgO/Mg/MgH2/V system introduced during cycling hydrogen release and uptake in hydrogen/dynamic vacuum at 450 °C. Nanostructuring as well as hydrogen permeability via vanadium nanoparticles may improve kinetics and reduce the apparent activation energy for hydrogen release. Thus, the enhancement of hydrogen release/uptake in the MgH2 owe to “in situ” formation of vanadium nanoparticles by reduction of V2O5.

Fragmentation reactions of realgar caused by early transition metal hydrides

The reaction of realgar with [Cp′2MH3] in boiling toluene gave [Cp′2M2As2S6] (Cp′=t-BuC5H4; M=Nb: 1; Ta: 2), while [Cp2WH2] (Cp=C5H5) reacted with As4S4 to give [Cp2W(H)(η1-As5S2)] (6). The structures were determined by X-ray crystallography. Compounds 1 and 2 are dimers in which two Cp′M units are symmetrically bridged by a μ,η2:2-AsS3 ligand, a sulfide ligand and a heteroallylic μ,η2:2-AsS2 ligand. The structure of 6 belongs to the type of bent metallocenes in which tungsten is surrounded tetrahedrally by two Cp ligands, a crystallographically found hydride atom and the new As5S2 cage. The reaction of 1 with W(CO)5THF gave [Cp′2Nb2As2S6·W(CO)5] (3) and [Cp′2Nb2As2S6W(CO)3·W(CO)5] (4). Compound 3 is a monoadduct of 1 bearing the W(CO)5 fragment at the As atom of the AsS3 ligand. The structure of 4 contains a distorted cubane-like Nb2WS4As cluster with an integrated W(CO)3 unit. A similar cluster 5 containing a CuI fragment instead of W(CO)3 was prepared from 1 and CuI. The structure is completed by an attached As4S3 cage, the As3 basis of which is oriented towards iodide. Electrochemical investigations of 1, 3, and 4 shows for each compound quasi-reversible one-electron reductions with E1/2=−0.73V (1), −0.55V (3) and −0.67V (4), respectively.

Niobaziridine Hydrides

Presented herein are synthetic, structural, and reactivity studies delineating the characteristics of the niobaziridine hydride functional group as it pertains to the stabilization of trisanilide niobium complexes of the type Nb(N[R]Ar)3 (1R, Ar = 3,5-Me2C6H3). Utilization of the N-isopropyl anilide ligand, N[i-Pr]Ar, results in the niobaziridine hydride dimer [Nb(H)(η2-Me2C═NAr)(N[i-Pr]Ar)2]2 ([2i-Pr-H]2). Dimer [2i-Pr-H]2 is thermally unstable at room temperature and decomposes via ortho-metalation and i-Pr radical ejection to a species containing a Nb−Nb bond. The ligand variant N[Np]Ar (Np = neopentyl) provides the room-temperature-stable niobaziridine hydride monomer Nb(H)(η2-t-Bu(H)C═NAr)(N[Np]Ar)2 (2Np-H). Thermal decomposition of 2Np-H at elevated temperature (75 °C) provides the neopentyl imido complex Nb(NNp)(Ar)(N[Np]Ar)2 (5Np). H/D isotopic labeling studies provide evidence for reversible β-H elimination interconverting 2Np-H and its trisanilide tautomer [Nb(N[Np]Ar)3] (1Np), with the latter thereby implicated as an intermediate during the 2Np-H → 5Np conversion. Reactivity studies between 2Np-H and certain small-molecule substrates confirm that the niobaziridine hydride group can effectively mask a reactive d2 Nb(III) trisanilide center. However, 2Np-H exhibits insertion chemistry when treated with a variety of unsaturated organic substrates, thus demonstrating a pronounced tendency to additionally function as a Lewis acidic, early transition metal hydride species. A general mechanism accounting for the divergent reactivity of 2Np-H is proposed. Niobaziridine hydride complexes derived from the amido ligands N[CH2Ad]Ar, N[Cy]Ar, and NCy2 (Ad = 1-adamantyl, Cy = cyclohexyl) are also presented, and their thermal behavior and reaction chemistry are compared with those of 2Np-H. In addition, the radical anion of 2Np-H is reported and compared with the neutral d1 molybdaziridine hydride complex Mo(H)(η2-Me2C═NAr)(N[i-Pr]Ar)2 (3i-Pr-H), to which it is isoelectronic.

First investigation of non-classical dihydrogen bonding between an early transition-metal hydride and alcohols: IR, NMR, and DFT approach.

The interaction of [NbCp(2)H(3)] with fluorinated alcohols to give dihydrogen-bonded complexes was studied by a combination of IR, NMR and DFT methods. IR spectra were examined in the range from 200-295 K, affording a clear picture of dihydrogen-bond formation when [NbCp(2)H(3)]/HOR(f) mixtures (HOR(f) = hexafluoroisopropanol (HFIP) or perfluoro-tert-butanol (PFTB)) were quickly cooled to 200 K. Through examination of the OH region, the dihydrogen-bond energetics were determined to be 4.5+/-0.3 kcal mol(-1) for TFE (TFE = trifluoroethanol) and 5.7+/-0.3 kcal mol(-1) for HFIP. (1)H NMR studies of solutions of [NbCp(2)H(2)(B)H(A)] and HFIP in [D(8)]toluene revealed high-field shifts of the hydrides H(A) and H(B), characteristic of dihydrogen-bond formation, upon addition of alcohol. The magnitude of signal shifts and T(1) relaxation time measurements show preferential coordination of the alcohol to the central hydride H(A), but are also consistent with a bifurcated character of the dihydrogen bonding. Estimations of hydride-proton distances based on T(1) data are in good accord with the results of DFT calculations. DFT calculations for the interaction of [NbCp(2)H(3)] with a series of non-fluorinated (MeOH, CH(3)COOH) and fluorinated (CF(3)OH, TFE, HFIP, PFTB and CF(3)COOH) proton donors of different strengths showed dihydrogen-bond formation, with binding energies ranging from -5.7 to -12.3 kcal mol(-1), depending on the proton donor strength. Coordination of proton donors occurs both to the central and to the lateral hydrides of [NbCp(2)H(3)], the former interaction being of bifurcated type and energetically slightly more favourable. In the case of the strong acid H(3)O(+), the proton transfer occurs without any barrier, and no dihydrogen-bonded intermediates are found. Proton transfer to [NbCp(2)H(3)] gives bis(dihydrogen) [NbCp(2)(eta(2)-H(2))(2)](+) and dihydride(dihydrogen) complexes [NbCp(2)(H)(2)(eta(2)-H(2))](+) (with lateral hydrides and central dihydrogen), the former product being slightly more stable. When two molecules of TFA were included in the calculations, in addition to the dihydrogen-bonded adduct, an ionic pair formed by the cationic bis(dihydrogen) complex [NbCp(2)(eta(2)-H(2))(2)](+) and the homoconjugated anion pair (CF(3)COO...H...OOCCF(3))(-) was found as a minimum. It is very likely that these ionic pairs may be intermediates in the H/D exchange between the hydride ligands and the OD group observed with the more acidic alcohols in the NMR studies.

Aliphatic Carbon−Fluorine Bond Activation Using (C5Me5)2ZrH2

Carbon-fluorine bonds are among the strongest and most inert single bonds found in organic compounds.1 The extraordinary properties of fluorocarbons have led to their increased use commercially and, in turn, to an increased interest in the study of C-F activation. In particular, the development of CFC disposal methods remains a challenging task.2 A variety of early transition metal complexes have been shown to be reactive toward aromatic C-F bonds such as those in perfluorobenzene,2-4 while only a few metal complexes show well characterized reactions with aliphatic fluorocarbons.5 The use of early transition metal hydride reagents in C-F activation remains largely unexplored. In fact, only two examples of C-F activation by early transition metal hydrides have been documented.6 The dihydride complex Cp*2ZrH2 is known to react with a variety of unsaturated small molecules through what are commonly classified as electrophilic concerted addition reactions.7 We have found that Cp*2ZrH2 reacts with many types of fluorinated substrates to give hydrogenated organic products and Cp*2ZrHF. Reaction of Cp*2ZrH2 (1) with 1-fluorohexane in cyclohexaned12 at ambient temperature under 1.3 atm H2 has been found to produce hexane and Cp*2ZrHF (2) in quantitative yield (eq 1).

Chemical Applications of Topology and Group Theory. 33. Symmetry-Forbidden Coordination Polyhedra for Spherical Atomic Orbital Manifolds1

The four chemically significant spherical manifolds of atomic orbitals are the sp3, sd5, sp3d5, and sd5f 7 manifolds of 4, 6, 9, and 13 orbitals found in the chemistry of the main-group elements, the early transition metal homoleptic hydrides and alkyls, most other compounds of the d-block transition metals, and the actinides, respectively. Coordination geometries with an inversion center (e.g., the octahedron) or a unique reflection plane passing through no vertices (e.g., the trigonal prism) are symmetry forbidden for the sd5 manifold thereby accounting for some unusual experimental and computed geometries for six-coordinate early transition metal hydrides and alkyls. The maximum coordination numbers for polyhedra with inversion centers for the nine-orbital sp3d5 and 13-orbital sd5f 7 manifolds are 6 and 12 corresponding to the regular octahedron and regular icosahedron, respectively, for the most symmetrical manifestations of these coordination numbers.

Relative strengths of early transition metal M-H and M-C bonds in substituted niobocenes and tantalocenes. Thermodynamic trends, and electronic factors of olefin insertion into a metal-hydride bond

Principles of M-C and M-H bonding interactions are examined experimentally by high-resolution valence photoelectron spectroscopy. Gas-phase He I and He II photoelectron spectra are reported for the following series of d{sup 0} and d{sup 2} bent metallocenes: Cp{sub 2}MH{sub 3} (M = Nb, Ta), Cp{sub 2}M(CO)L (M = Nb, Ta; L = H and M = Nb; L = CH{sub 3}), and Cp{sub 2}M(C{sub 2}H{sub 4})L (M = Nb, Ta; L = H and M = Ta; L = C{sub 2}H{sub 5}) (Cp = {eta}{sup 5}-C{sub 5}H{sub 5}). The ligand replacements represented by this series of complexes allow stepwise comparison and assignment of each individual low-energy valence ionization band. The He I/He II spectral comparisons show that the lowest ionization energy band of the carbonyl complexes is higher in metal character than the corresponding ionization of the ethylene complexes. This trend indicates extensive electron delocalization in the metal-ethylene bonding that corresponds more accurately to a metallacyclopropane description with relatively strong metal-carbon bonds. The photoelectron data for the early transition metal hydride and alkyl species show that the metal-hydrogen bonds are slightly more stable than the metal-carbon bonds. Correlation of the ionization information for these substituted early metallocenes reveals that themore » stabilization of the metallacyclopropane-hydride structures, M(C{sub 2}H{sub 4})H, relative to the olefin-inserted metal-alkyl structure, M(C{sub 2}H{sub 5}), is partly due to stabilization of the metal electron density by back-bonding to the olefin as indicated by earlier theoretical studies.« less

Hydridic character of early transition metal hydride complexes

The cendency of transition metal hydride complexes to behave as true hydrides, as measured by ability to reduce ketones, appears to depend strongly on the position of the metal in the Periodic Table; the strongest such behavior is exhibited by complexes of metals furthest to the left in the transition series.