Base-Metal Nanoparticle-Catalyzed Hydrogen Release from Ammine Yttrium and Lanthanum Borohydrides

Solid ammine metal borohydrides [M­(BH₄)_<i>m</i>(NH₃)_<i>n</i>, AMBs] are promising materials for low temperature, high capacity hydrogen generation. Retention of metal halide co-products, arising from typical mechanochemical synthetic methods, is shown to have negative impacts on dehydrogenation properties of yttrium AMB. Halide-free yttrium and lanthanum AMBs, M­(BH₄)₃(NH₃)₄, have been synthesized directly by treatment of MCl₃ with 3 equiv of NaBH₄ in thf followed by filtration, cooling, and exposure to liquid ammonia. The peak dehydrogenation temperature of the Y analog decreased from previously reported 179 to 160 °C while the ammonia peak temperature increased from 86 to 165 °C. To enhance the dehydrogenation properties and increase the selectivity of gas formation from these AMBs, base-metal nanoparticle catalysts, M′NPs; M′ = Fe, Co, Ni, and Cu) were employed. Preparation of the M′NPs from M′Cl₂ and liquid hexylamine–borane allowed for separation of the B–Cl byproducts by subsequent solvent washing. Sonification of the M′NPs in toluene followed by addition of the solid AMB afforded composite AMB–M′NP–BN solids. Thermolysis data indicated a threefold reduction in ammonia release from the Y–Co and fourfold for the La–Fe composite. The purity of the released hydrogen was estimated to be 97.9 mol % for Y–Co and 98.9 mol % for La–Fe

A Pressure Induced Structural Dichotomy in Isostructural Bis-1,2,3-thiaselenazolyl Radical Dimers

The pressure dependence of the crystal and molecular structure of the bis-1,2,3-thiaselenazolyl radical dimer [<b>1b</b>]₂ has been investigated over the range 0–11 GPa by powder diffraction methods using synchrotron radiation and diamond anvil cell techniques. At ambient pressure, the dimer consists of a pair of radicals linked by a hypervalent 4-center 6-electron S---Se–Se---S σ-bond in an essentially coplanar arrangement. The dimers are packed in cross-braced slipped π-stack arrays running along the <i>x</i>-direction of the monoclinic (space group <i>P</i>2₁/<i>c</i>) unit cell. Pressurization to 11 GPa causes the unit cell dimensions <i>a</i> and <i>c</i> to undergo a slow but uniform compression, while the <i>b</i>-axis is slightly elongated. There is virtually no change in the molecular structure or in the slipped π-stack crystal architecture. This behavior is in marked contrast to that of the isostructural radical dimer [<b>1a</b>]₂, where the basal fluorine is replaced by hydrogen. Pressurization of this latter material induces a phase change near 4–5 GPa, characterized by a sharp contraction in <i>a</i> and <i>c</i>, and a correspondingly large increase in <i>b</i>. At the molecular level, the transition is associated with a buckling of the σ-bonded dimer to a more conventional π-bonded arrangement. Geometry optimized DFT band structure calculations on [<b>1b</b>]₂ replicate the observed structural changes and indicate that compression widens both the valence and conduction bands but does not induce band gap closure until >13 GPa. This result is consistent with the measured thermal activation energy for conduction <i>E</i>_act, which indicates that a metallic state requires pressures > 10 GPa

A Pressure Induced Structural Dichotomy in Isostructural Bis-1,2,3-thiaselenazolyl Radical Dimers

The pressure dependence of the crystal and molecular structure of the bis-1,2,3-thiaselenazolyl radical dimer [<b>1b</b>]₂ has been investigated over the range 0–11 GPa by powder diffraction methods using synchrotron radiation and diamond anvil cell techniques. At ambient pressure, the dimer consists of a pair of radicals linked by a hypervalent 4-center 6-electron S---Se–Se---S σ-bond in an essentially coplanar arrangement. The dimers are packed in cross-braced slipped π-stack arrays running along the <i>x</i>-direction of the monoclinic (space group <i>P</i>2₁/<i>c</i>) unit cell. Pressurization to 11 GPa causes the unit cell dimensions <i>a</i> and <i>c</i> to undergo a slow but uniform compression, while the <i>b</i>-axis is slightly elongated. There is virtually no change in the molecular structure or in the slipped π-stack crystal architecture. This behavior is in marked contrast to that of the isostructural radical dimer [<b>1a</b>]₂, where the basal fluorine is replaced by hydrogen. Pressurization of this latter material induces a phase change near 4–5 GPa, characterized by a sharp contraction in <i>a</i> and <i>c</i>, and a correspondingly large increase in <i>b</i>. At the molecular level, the transition is associated with a buckling of the σ-bonded dimer to a more conventional π-bonded arrangement. Geometry optimized DFT band structure calculations on [<b>1b</b>]₂ replicate the observed structural changes and indicate that compression widens both the valence and conduction bands but does not induce band gap closure until >13 GPa. This result is consistent with the measured thermal activation energy for conduction <i>E</i>_act, which indicates that a metallic state requires pressures > 10 GPa

A Pressure Induced Structural Dichotomy in Isostructural Bis-1,2,3-thiaselenazolyl Radical Dimers

The pressure dependence of the crystal and molecular structure of the bis-1,2,3-thiaselenazolyl radical dimer [<b>1b</b>]₂ has been investigated over the range 0–11 GPa by powder diffraction methods using synchrotron radiation and diamond anvil cell techniques. At ambient pressure, the dimer consists of a pair of radicals linked by a hypervalent 4-center 6-electron S---Se–Se---S σ-bond in an essentially coplanar arrangement. The dimers are packed in cross-braced slipped π-stack arrays running along the <i>x</i>-direction of the monoclinic (space group <i>P</i>2₁/<i>c</i>) unit cell. Pressurization to 11 GPa causes the unit cell dimensions <i>a</i> and <i>c</i> to undergo a slow but uniform compression, while the <i>b</i>-axis is slightly elongated. There is virtually no change in the molecular structure or in the slipped π-stack crystal architecture. This behavior is in marked contrast to that of the isostructural radical dimer [<b>1a</b>]₂, where the basal fluorine is replaced by hydrogen. Pressurization of this latter material induces a phase change near 4–5 GPa, characterized by a sharp contraction in <i>a</i> and <i>c</i>, and a correspondingly large increase in <i>b</i>. At the molecular level, the transition is associated with a buckling of the σ-bonded dimer to a more conventional π-bonded arrangement. Geometry optimized DFT band structure calculations on [<b>1b</b>]₂ replicate the observed structural changes and indicate that compression widens both the valence and conduction bands but does not induce band gap closure until >13 GPa. This result is consistent with the measured thermal activation energy for conduction <i>E</i>_act, which indicates that a metallic state requires pressures > 10 GPa

A Pressure Induced Structural Dichotomy in Isostructural Bis-1,2,3-thiaselenazolyl Radical Dimers

The pressure dependence of the crystal and molecular structure of the bis-1,2,3-thiaselenazolyl radical dimer [<b>1b</b>]₂ has been investigated over the range 0–11 GPa by powder diffraction methods using synchrotron radiation and diamond anvil cell techniques. At ambient pressure, the dimer consists of a pair of radicals linked by a hypervalent 4-center 6-electron S---Se–Se---S σ-bond in an essentially coplanar arrangement. The dimers are packed in cross-braced slipped π-stack arrays running along the <i>x</i>-direction of the monoclinic (space group <i>P</i>2₁/<i>c</i>) unit cell. Pressurization to 11 GPa causes the unit cell dimensions <i>a</i> and <i>c</i> to undergo a slow but uniform compression, while the <i>b</i>-axis is slightly elongated. There is virtually no change in the molecular structure or in the slipped π-stack crystal architecture. This behavior is in marked contrast to that of the isostructural radical dimer [<b>1a</b>]₂, where the basal fluorine is replaced by hydrogen. Pressurization of this latter material induces a phase change near 4–5 GPa, characterized by a sharp contraction in <i>a</i> and <i>c</i>, and a correspondingly large increase in <i>b</i>. At the molecular level, the transition is associated with a buckling of the σ-bonded dimer to a more conventional π-bonded arrangement. Geometry optimized DFT band structure calculations on [<b>1b</b>]₂ replicate the observed structural changes and indicate that compression widens both the valence and conduction bands but does not induce band gap closure until >13 GPa. This result is consistent with the measured thermal activation energy for conduction <i>E</i>_act, which indicates that a metallic state requires pressures > 10 GPa

Pressure-Induced Changes on The Electronic Structure and Electron Topology in the Direct FCC → SH Transformation of Silicon

X-ray diffraction experiments at 80 K show that when silicon is compressed under hydrostatic conditions the intermediate high-pressure phases are bypassed leading to a direct transformation to the simple hexagonal structure at 17 GPa. A maximum entropy analysis of the diffraction patterns reveals dramatic alterations in the valence electron distribution from tetrahedral covalent bonding to localization in the interstitial sites and along the one-dimensional silicon atom chain running along adjacent hexagonal layers. Changes in the orbital character of the unoccupied states are confirmed using X-ray Raman scattering spectroscopy and theoretical Bethe-Salpeter equation calculations. This is the first direct observation indicating that the silicon valence electrons in 3s and 3p orbitals are transferred to the 3d orbitals at high density which proves that electrons of compressed elemental solids migrate from their native bonding configuration to interstitial regions

Benzoquinone-Bridged Heterocyclic Zwitterions as Building Blocks for Molecular Semiconductors and Metals

In pursuit of closed-shell building blocks for single-component organic semiconductors and metals, we have prepared benzoquino-bis-1,2,3-thiaselenazole QS, a heterocyclic selenium-based zwitterion with a small gap (λ_max = 729 nm) between its highest occupied and lowest unoccupied molecular orbitals. In the solid state, QS exists in two crystalline phases and one nanocrystalline phase. The structures of the crystalline phases (space groups <i>R</i>3<i>c</i> and <i>P</i>2₁/<i>c</i>) have been determined by high-resolution powder X-ray diffraction methods at ambient and elevated pressures (0–15 GPa), and their crystal packing patterns have been compared with that of the related all-sulfur zwitterion benzoquino-bis-1,2,3-dithiazole QT (space group <i>Cmc</i>2₁). Structural differences between the S- and Se-based materials are interpreted in terms of local intermolecular S/Se···N′/O′ secondary bonding interactions, the strength of which varies with the nature of the chalcogen (S vs Se). While the perfectly two-dimensional “brick-wall” packing pattern associated with the <i>Cmc</i>2₁ phase of QT is not found for QS, all three phases of QS are nonetheless small band gap semiconductors, with σ_RT ranging from 10^–5 S cm^–1 for the <i>P</i>2₁/<i>c</i> phase to 10^–3 S cm^–1 for the <i>R</i>3<i>c</i> phase. The bandwidths of the valence and conduction bands increase with applied pressure, leading to an increase in conductivity and a decrease in thermal activation energy <i>E</i>_act. For the <i>R</i>3<i>c</i> phase, band gap closure to yield an organic molecular metal with a σ_RT of ∼10² S cm^–1 occurs at 6 GPa. Band gaps estimated from density functional theory band structure calculations on the ambient- and high-pressure crystal structures of QT and QS correlate well with those obtained experimentally

The Power of Packing: Metallization of an Organic Semiconductor

Benzoquino-bis-1,2,3-dithiazole <b>5</b> is a closed shell, antiaromatic 16π-electron zwitterion with a small HOMO–LUMO gap. Its crystal structure consists of planar ribbon-like molecular arrays packed into offset layers to generate a “brick-wall” motif with strong 2D interlayer electronic interactions. The spread of the valence and conduction bands, coupled with the narrow HOMO–LUMO gap, affords a small band gap semiconductor with σ_RT = 1 × 10^–3 S cm^–1 and <i>E</i>_act = 0.14 eV for transport within the brick-wall arrays. Closure of the band gap to form an all-organic molecular metal with σ_RT > 10¹ S cm^–1 can be achieved by the application of pressure to 8 GPa

Heat, Pressure and Light-Induced Interconversion of Bisdithiazolyl Radicals and Dimers

The heterocyclic bisdithiazolyl radical <b>1b</b> (R₁ = Me, R₂ = F) crystallizes in two phases. The α-phase, space group <i>P</i>2₁/<i>n</i>, contains two radicals in the asymmetric unit, both of which adopt slipped π-stack structures. The β-phase, space group <i>P</i>2₁/<i>c</i>, consists of cross-braced π-stacked arrays of dimers in which the radicals are linked laterally by hypervalent 4-center 6-electron S···S–S···S σ-bonds. Variable-temperature magnetic susceptibility measurements on α-<b>1b</b> indicate Curie–Weiss behavior (with Θ = −14.9 K), while the dimer phase β-<b>1b</b> is diamagnetic, showing no indication of thermal dissociation below 400 K. High-pressure crystallographic measurements indicate that the cross-braced π-stacked arrays of dimers undergo a wine-rack compression, but the dimer remains intact up to 8 GPa (at ambient temperature). The resistance of β-<b>1b</b> to dissociate under pressure, also observed in its conductivity versus pressure profile, is in marked contrast to the behavior of the related dimer β-<b>1a</b> (R₁ = Et, R₂ = F), which readily dissociates into a pair of radicals at 0.8 GPa. The different response of the two dimers to pressure has been rationalized in terms of differences in their linear compressibilities occasioned by changes in the degree of cross-bracing of the π-stacks. Dissociation of both dimers can be effected by irradiation with visible (λ = 650 nm) light; the transformation has been monitored by optical spectroscopy, magnetic susceptibility measurements, and single crystal X-ray diffraction. The photoinduced radical pairs persist up to temperatures of 150 K (β-<b>1b</b>) and 242 K (β-<b>1a</b>) before reverting to the dimer state. Variable-temperature optical measurements on β-<b>1b</b> and β-<b>1a</b> have afforded Arrhenius activation energies of 8.3 and 19.6 kcal mol^–1, respectively, for the radical-to-dimer reconversion. DFT and CAS-SCF calculations have been used to probe the ground and excited electronic state structures of the dimer and radical pair. The results support the interpretation that the ground-state interconversion of the dimer and radical forms of β-<b>1a</b> and β-<b>1b</b> is symmetry forbidden, while the photochemical transformation is symmetry allowed

Heat, Pressure and Light-Induced Interconversion of Bisdithiazolyl Radicals and Dimers

The heterocyclic bisdithiazolyl radical <b>1b</b> (R₁ = Me, R₂ = F) crystallizes in two phases. The α-phase, space group <i>P</i>2₁/<i>n</i>, contains two radicals in the asymmetric unit, both of which adopt slipped π-stack structures. The β-phase, space group <i>P</i>2₁/<i>c</i>, consists of cross-braced π-stacked arrays of dimers in which the radicals are linked laterally by hypervalent 4-center 6-electron S···S–S···S σ-bonds. Variable-temperature magnetic susceptibility measurements on α-<b>1b</b> indicate Curie–Weiss behavior (with Θ = −14.9 K), while the dimer phase β-<b>1b</b> is diamagnetic, showing no indication of thermal dissociation below 400 K. High-pressure crystallographic measurements indicate that the cross-braced π-stacked arrays of dimers undergo a wine-rack compression, but the dimer remains intact up to 8 GPa (at ambient temperature). The resistance of β-<b>1b</b> to dissociate under pressure, also observed in its conductivity versus pressure profile, is in marked contrast to the behavior of the related dimer β-<b>1a</b> (R₁ = Et, R₂ = F), which readily dissociates into a pair of radicals at 0.8 GPa. The different response of the two dimers to pressure has been rationalized in terms of differences in their linear compressibilities occasioned by changes in the degree of cross-bracing of the π-stacks. Dissociation of both dimers can be effected by irradiation with visible (λ = 650 nm) light; the transformation has been monitored by optical spectroscopy, magnetic susceptibility measurements, and single crystal X-ray diffraction. The photoinduced radical pairs persist up to temperatures of 150 K (β-<b>1b</b>) and 242 K (β-<b>1a</b>) before reverting to the dimer state. Variable-temperature optical measurements on β-<b>1b</b> and β-<b>1a</b> have afforded Arrhenius activation energies of 8.3 and 19.6 kcal mol^–1, respectively, for the radical-to-dimer reconversion. DFT and CAS-SCF calculations have been used to probe the ground and excited electronic state structures of the dimer and radical pair. The results support the interpretation that the ground-state interconversion of the dimer and radical forms of β-<b>1a</b> and β-<b>1b</b> is symmetry forbidden, while the photochemical transformation is symmetry allowed