Formation of novel transition metal hydride complexes with ninefold hydrogen coordination

Hydrogen Absorption Reactions of Hydrogen Storage Alloy LaNi5 under High Pressure

Hydrogen can be stored in the interstitial sites of the lattices of intermetallic compounds. To date, intermetallic compound LaNi₅ or related LaNi₅-based alloys are known to be practical hydrogen storage materials owing to their higher volumetric hydrogen densities, making them a compact hydrogen storage method and allowing stable reversible hydrogen absorption and desorption reactions to take place at room temperature below 1.0 MPa. By contrast, gravimetric hydrogen density is required for key improvements (e.g., gravimetric hydrogen density of LaNi₅: 1.38 mass%). Although hydrogen storage materials have typically been evaluated for their hydrogen storage properties below 10 MPa, reactions between hydrogen and materials can be facilitated above 1 GPa because the chemical potential of hydrogen dramatically increases at a higher pressure. This indicates that high-pressure experiments above 1 GPa could clarify the latent hydrogen absorption reactions below 10 MPa and potentially explore new hydride phases. In this study, we investigated the hydrogen absorption reaction of LaNi₅ above 1 GPa at room temperature to understand their potential hydrogen storage capacities. The high-pressure experiments on LaNi₅ with and without an internal hydrogen source (BH₃NH₃) were performed using a multi-anvil-type high-pressure apparatus, and the reactions were observed using in situ synchrotron radiation X-ray diffraction with an energy dispersive method. The results showed that 2.07 mass% hydrogen was absorbed by LaNi₅ at 6 GPa. Considering the unit cell volume expansion, the estimated hydrogen storage capacity could be 1.5 times higher than that obtained from hydrogen absorption reaction below 1.0 MPa at 303 K. Thus, 33% of the available interstitial sites in LaNi₅ remained unoccupied by hydrogen atoms under conventional conditions. Although the hydrogen-absorbed LaNi₅H_x (x < 9) was maintained below 573 K at 10 GPa, LaNi₅H_x began decomposing into NiH, and the formation of a new phase was observed at 873 K and 10 GPa. The new phase was indexed to a hexagonal or trigonal unit cell with a ≈ 4.44 Å and c ≈ 8.44 Å. Further, the newly-formed phase was speculated to be a new hydride phase because the Bragg peak positions and unit cell parameters were inconsistent with those reported for the La-Ni intermetallic compounds and La-Ni hydride phases.

Unusual Intramolecular Motion of ReH92- in K2ReH9 Crystal: Circle Dance and Three-Arm Turnstile Mechanisms Revealed by Computational Studies

The nonahydridorhenate dianion ReH₉^2- is a unique rhenium polyhydride complex due to its remarkably high coordination number; however, its detailed polytopal rearrangement process in either solution or crystal is so far unclear. In this work, our quantum chemical calculations have identified two previously unreported fluxional mechanisms for the ReH₉^2- dianion in the K₂ReH₉ crystal: three-arm turnstile rotation and circle dance mechanism. These two polytopal rearrangements in the crystal offer an alternative interpretation to the pulse and wide-line NMR spectra (Farrar et al. J. Chem. Phys. 1969, 51, 3595). The previously postulated hindered rotation of the whole ReH₉^2- dianion in K₂ReH₉ (White et al. J. Chem. Soc., Faraday Trans. 2 1972, 68, 1414) turns out to be a combination of the above-mentioned two elementary fluxional processes. In addition, our calculations have confirmed the Muetterties' D_3h⇌C_4v rearrangement as the intramolecular motion for the ReH₉^2- dianion in solution.

Stabilization and electronic topological transition of hydrogen-rich metal Li5MoH11 under high pressures from first-principles predictions

Regarded as doped binary hydrides, ternary hydrides have recently become the subject of investigation since they are deemed to be metallic under pressure and possibly potentially high-temperature superconductors. Herein, the candidate structure of Li₅MoH₁₁ is predicted by exploiting the evolutionary searching. Its high-pressure phase adopts a hexagonal structure with P6₃/mcm space group. We used first-principles calculations including the zero-point energy to investigate the structures up to 200 GPa and found that the P6₃cm structure transforms into the P6₃/mcm structure at 48 GPa. Phonon calculations confirm that the P6₃/mcm structure is dynamically stable. Its stability is mainly attributed to the isostructural second-order phase transition. Our calculations reveal the electronic topological transition displaying an isostructural second-order phase transition at 160 GPa as well as the topology of its Fermi surfaces. We used the projected crystal orbital Hamilton population (pCOHP) to examine the nature of the chemical bonding and demonstrated that the results obtained from the pCOHP calculation are associated with the electronic band structure and electronic localized function.

Na3FeH7 and Na3CoH6: Hydrogen-Rich First-Row Transition Metal Hydrides from High Pressure Synthesis

The formation of ternary hydrogen-rich hydrides involving the first-row transition metals TM = Fe and Co in high oxidation states is demonstrated from in situ synchrotron diffraction studies of reaction mixtures NaH–TM–H₂ at p ≈ 10 GPa. Na₃FeH₇ and Na₃CoH₆ feature pentagonal bipyramidal FeH₇^3– and octahedral CoH₆^3– 18-electron complexes, respectively. At high pressure, high temperature (300 < T ≤ 470 °C) conditions, metal atoms are arranged as in the face-centered cubic Heusler structure, and ab initio molecular dynamics simulations suggest that the complexes undergo reorientational dynamics. Upon cooling, subtle changes in the diffraction patterns evidence reversible and rapid phase transitions associated with ordering of the complexes. During decompression, Na₃FeH₇ and Na₃CoH₆ transform to tetragonal and orthorhombic low pressure forms, respectively, which can be retained at ambient pressure. The discovery of Na₃FeH₇ and Na₃CoH₆ establishes a consecutive series of homoleptic hydrogen-rich complexes for first-row transition metals from Cr to Ni.

In situ synchrotron diffraction studies of reaction mixtures NaH−TM−H₂ (TM = Fe, Co) at p ≈ 10 GPa revealed the formation of ternary hydrides Na₃FeH₇ and Na₃CoH₆ featuring pentagonal bipyramidal Fe(IV)H₇³⁻ and octahedral Co(III)H₆³⁻ complexes, respectively. The discovery of Na₃FeH₇ and Na₃CoH₆ establishes a consecutive series of homoleptic hydrogen-rich complexes for first-row transition metals from Cr to Ni.

First principles investigation of electron correlation and Lifshitz transition within iron polynitrides

Metal poly-nitrogen compounds are gaining great interests as potential high energy density materials. Several iron polynitrides have been recently synthesized and investigated under high pressure (2018Nature Communications92756). In this work the electron correlations within these iron poly-nitrogen compounds were self-consistently determined, benchmarked with those obtained from linear response approach. Along with the increase of the concentration of nitrogen, the Coulomb interaction strengths show a monotonic decrease, where FeN and FeN₂are antiferromagnetic and the others are ferromagnetic. Then the electron correlation is studied along with the pressure, where the electrons are more delocalized as pressure becomes higher. One electronic topological transition was found for FeN₂, owing to a breaking of symmetry of spin and a transition of magnetism induced by a structural change. The band structure, densities of states, Fermi surface and absorption spectra were calculated and discussed.

Na-Ni-H Phase Formation at High Pressures and High Temperatures: Hydrido Complexes [NiH5]3- Versus the Perovskite NaNiH3

The Na-Ni-H system was investigated by in situ synchrotron diffraction studies of reaction mixtures NaH-Ni-H₂ at around 5, 10, and 12 GPa. The existence of ternary hydrogen-rich hydrides with compositions Na₃NiH₅ and NaNiH₃, where Ni attains the oxidation state II, is demonstrated. Upon heating at ∼5 GPa, face-centered cubic (fcc) Na₃NiH₅ forms above 430 °C. Upon cooling, it undergoes a rapid and reversible phase transition at 330 °C to an orthorhombic (Cmcm) form. Upon pressure release, Na₃NiH₅ further transforms into its recoverable Pnma form whose structure was elucidated from synchrotron powder diffraction data, aided by first-principles density functional theory (DFT) calculations. Na₃NiH₅ features previously unknown square pyramidal 18-electron complexes NiH₅ ^3-. In the high temperature fcc form, metal atoms are arranged as in the Heusler structure, and ab initio molecular dynamics simulations suggest that the complexes are dynamically disordered. The Heusler-type metal partial structure is essentially maintained in the low temperature Cmcm form, in which NiH₅ ^3- complexes are ordered. It is considerably rearranged in the low pressure Pnma form. Experiments at 10 GPa showed an initial formation of fcc Na₃NiH₅ followed by the addition of the perovskite hydride NaNiH₃, in which Ni(II) attains an octahedral environment by H atoms. NaNiH₃ is recoverable at ambient pressures and represents the sole product of 12 GPa experiments. DFT calculations show that the decomposition of Na₃NiH₅ = NaNiH₃ + 2 NaH is enthalpically favored at all pressures, suggesting that Na₃NiH₅ is metastable and its formation is kinetically favored. Ni-H bonding in metallic NaNiH₃ is considered covalent, as in electron precise Na₃NiH₅, but delocalized in the polyanion [NiH₃]^-.

Titanium Hydride Complex BaCa2Ti2H14 with 9-Fold Coordination

We present the high-pressure synthesis and crystal structure of a novel titanium hydride complex, BaCa₂Ti₂H₁₄, with 9-fold coordination. It comprises a unique dinuclear [Ti₂H₁₄]^6- complex that consists of a pair of Ti⁴⁺ ions each coordinated by nine hydrides in the monocapped square antiprism, distinguished from the known 9-fold coordination in the mononuclear tricapped trigonal prism of [MH₉]^x-. The dinuclear hydride complex is stabilized by three-center two-electron bonding at the four bridging Ti-H-Ti bonds to compensate for the lack of valence electrons in the Ti⁴⁺ ions. Optical measurements show that BaCa₂Ti₂H₁₄ is a band insulator with a wide band gap of 2.25 eV. Density functional theory calculations reveal that the top of the valence band is dominated by H-1s-derived states, as expected from the 9-fold coordination, which would present a playground for electronic properties such as high-T_c superconductivity when doped with hole carriers or under high pressure.

Unexpected Molecular Structure of a Putative Rhenium-Dioxo-Benzocarbaporphyrin Complex. Implications for the Highest Transition Metal Valence in a Porphyrin-Type Ligand Environment

A combination of quantum chemical calculations and synthetic studies was used to address the possibility of very high (>6) valence states of transition metals in porphyrin-type complexes. With corrole as a supporting ligand, DFT calculations ruled out Re(VII) and Ir(VII) dioxo complexes as stable species. Attempted rhenium insertion into benzocarbaporphyrin (BCP) ligands on the other hand led to two products with different stoichiometries - Re[BCP]O and Re[BCP]O2. To our surprise, single-crystal structure determination of one of the complexes of the latter type indicated an ReVO center with a second oxygen bridging the Re-C bond. In other words, although the monooxo complexes Re[BCP]O are oxophilic, the BCP ligand cannot sustain a trans-ReVII(O)2 center. The search for metal valence states >6 in porphyrin-type ligand environments must therefore continue.

Exploring the Mg-Cr-H System at High Pressure and Temperature via in Situ Synchrotron Diffraction

The complex transition metal hydride Mg₃CrH₈ has been previously synthesized using high pressure conditions. It contains the first group 6 homoleptic hydrido complex, [Cr(II)H₇]^5-. Here, we investigated the formation of Mg₃CrH₈ by in situ studies of reaction mixtures of 3MgH₂-Cr-H₂ at 5 GPa. The formation of the known orthorhombic form (o-Mg₃CrH₈) was noticed at temperatures above 635 °C, albeit at a relatively slow rate. At temperatures around 750 °C a high temperature phase formed rapidly, which upon slow cooling converted into o-Mg₃CrH₈. The phase transition at high pressures occurred reversibly at ∼735 °C upon heating and at ∼675 °C upon slow cooling. Upon rapid cooling, a monoclinic polymorph (m-Mg₃CrH₈) was afforded which could be subsequently recovered and analyzed at ambient pressure. m-Mg₃CrH₈ was found to crystallize in P2₁/n space group (a = 5.128 Å, b = 16.482 Å, c = 4.805 Å, β = 90.27°). Its structure elucidation from high resolution synchrotron powder diffraction data was aided by first-principles DFT calculations. Like the orthorhombic polymorph, m-Mg₃CrH₈ contains pentagonal bipyramidal complexes [CrH₇]^5- and interstitial H^-. The arrangement of metal atoms and interstitial H^- resembles closely that of the high pressure orthorhombic form of Mg₃MnH₇. This suggests similar principles of formation and stabilization of hydrido complexes at high pressure and temperature conditions in the Mg-Cr-H and Mg-Mn-H systems. Calculated enthalpy versus pressure relations predict o-Mg₃CrH₈ being more stable than m-Mg₃CrH₈ by 6.5 kJ/mol at ambient pressure and by 13 kJ/mol at 5 GPa. The electronic structure of m-Mg₃CrH₈ is very similar to that of o-Mg₃CrH₈. The stable 18-electron complex [CrH₇]^5- is mirrored in the occupied states, and calculated band gaps are around 1.5 eV.

Hydrogen-Release Reaction of a Complex Transition Metal Hydride with Covalently Bound Hydrogen and Hydride Ions

The hydrogen-release reaction of a complex transition metal hydride, LaMg₂ NiH₇ , composed of La³⁺ , 2×Mg²⁺ , [NiH₄ ]^4- and 3×H^- , was studied by thermal analyses, powder X-ray, and neutron diffraction and inelastic neutron scattering. Upon heating, LaMg₂ NiH₇ released hydrogen at approximately 567 K and decomposed into LaH_2-3 and Mg₂ Ni. Before the reaction, covalently bound hydrogen (H^c °^v. ) in [NiH₄ ]^4- exhibited a larger atomic displacement than H^- , although a weakening of the chemical bonds around [NiH₄ ]^4- and H^- was observed. These results indicate the precursor phenomenon of a hydrogen-release reaction, wherein there is a large atomic displacement of H^c °^v. that induces the hydrogen-release reaction rather than H^- . As an isothermal reaction, LaMg₂ NiH₇ formed LaMg₂ NiH_2.4 at 503 K in vacuum for 48 h, and LaMg₂ NiH_2.4 reacted with hydrogen to reform LaMg₂ NiH₇ at 473 K under 1 MPa of H₂ gas pressure for 10 h. These results revealed that LaMg₂ NiH₇ exhibited partially reversible hydrogen-release and uptake reactions.

From the Laves Phase CaRh2 to the Perovskite CaRhH3-in Situ Investigation of Hydrogenation Intermediates CaRh2H x

The hydrogenation properties of the cubic Laves phase CaRh₂ and the formation of the perovskite CaRhH₃ were studied by in situ thermal analysis (differential scanning calorimetry), sorption experiments, and in situ neutron powder diffraction. Three Laves phase hydrides are formed successively at room temperature and hydrogen gas pressures up to 5 MPa. Cubic α-CaRh₂H_0.05 is a stuffed cubic Laves phase with statistically distributed hydrogen atoms in tetrahedral [Ca₂Rh₂] voids (ZrCr₂H_3.08 type, Fd3̅ m, a = 7.5308(12) Å). Orthorhombic β-CaRh₂D_3.93(5) (own structure type, Pnma, a = 6.0028(3) Å, b = 5.6065(3) Å, c = 8.1589(5) Å) and γ-CaRh₂D_3.20(10) (β-CaRh₂H_3.9 type, Pnma, a = 5.9601(10) Å, b = 5.4912(2) Å, c = 8.0730(11) Å) are low-symmetry variants thereof with hydrogen occupying distorted tetrahedral [Ca₂Rh₂] and trigonal bipyramidal [Ca₃Rh₂] voids. Hydrogen sorption experiments show the hydrogenation to take place already at 0.1 MPa and to yield β-CaRh₂H_3.8(2). At 560 K and 5 MPa hydrogen pressure the Laves phase hydride decomposes kinetically controlled to nanocrystalline rhodium and CaRhD_2.93(2) (CaTiO₃ type, Pm3̅ m, a = 3.6512(2) Å). The hydrogenation of CaRh₂ provides a synthesis route to otherwise not accessible perovskite-type CaRhH₃.

Unraveling Hidden Mg-Mn-H Phase Relations at High Pressures and Temperatures by in Situ Synchrotron Diffraction

The Mg-Mn-H system was investigated by in situ high pressure studies of reaction mixtures MgH₂-Mn-H₂. The formation conditions of two complex hydrides with composition Mg₃MnH₇ were established. Previously known hexagonal Mg₃MnH₇ (h-Mg₃MnH₇) formed at pressures 1.5-2 GPa and temperatures between 480 and 500 °C, whereas an orthorhombic form (o-Mg₃MnH₇) was obtained at pressures above 5 GPa and temperatures above 600 °C. The crystal structures of the polymorphs feature octahedral [Mn(I)H₆]^5- complexes and interstitial H^-. Interstitial H^- is located in trigonal bipyramidal and square pyramidal interstices formed by Mg²⁺ ions in h- and o-Mg₃MnH₇, respectively. The hexagonal form can be retained at ambient pressure, whereas the orthorhombic form upon decompression undergoes a distortion to monoclinic Mg₃MnH₇ (m-Mg₃MnH₇). The structure elucidation of o- and m-Mg₃MnH₇ was aided by first-principles density functional theory (DFT) calculations. Calculated enthalpy versus pressure relations predict m- and o-Mg₃MnH₇ to be more stable than h-Mg₃MnH₇ above 4.3 GPa. Phonon calculations revealed o-Mg₃MnH₇ to be dynamically unstable at pressures below 5 GPa, which explains its phase transition to m-Mg₃MnH₇ on decompression. The electronic structure of the quenchable polymorphs h- and m-Mg₃MnH₇ is very similar. The stable 18-electron complex [MnH₆]^5- is mirrored in the occupied states, and calculated band gaps are around 1.5 eV. The study underlines the significance of in situ investigations for mapping reaction conditions and understanding phase relations for hydrogen-rich complex transition metal hydrides.