D-electron Configuration Research Articles

π-Bond Character in Metal-Alkyl Compounds for C-H Activation: How, When, and Why?

C-H bond activation via σ-bond metathesis is typically observed with transition-metal alkyl compounds in d0 or d0fn electron configurations, e.g., biscyclopentadienyl metal alkyls. Related C-H activation processes are also observed for transition-metal alkyls with higher d-electron counts, such as W(II), Fe(II), or Ir(III). A σ-bond metathesis mechanism has been proposed in all cases with a preference for an oxidative addition-reductive elimination pathway for Ir(III). Herein we show that, regardless of the exact mechanism, C-H activation with all of these compounds is associated with π-character of the M-C bond, according to a detailed analysis of the 13C NMR chemical shift tensor of the α-carbon. π-Character is also a requirement for olefin insertion, indicating its similarity to σ-bond metathesis. This observation explains the H2 response observed in d0 olefin polymerization catalysts and underlines that σ-bond metathesis, olefin insertion, and olefin metathesis are in fact isolobal reactions.

High-Pressure Synthesis, Crystal Structure, Chemical Bonding, and Ferroelectricity of LiNbO3-Type LiSbO3.

A polar LiNbO3 (LN)-type oxide LiSbO3 was synthesized by a high-temperature heat treatment under a pressure of 7.7 GPa and found to exhibit ferroelectricity. The crystal structural refinement using the data of synchrotron powder X-ray diffraction and neutron diffraction and the electronic structure calculation of LN-type LiSbO3 suggest a covalent-bonding character between Sb and O. When comparing the distortion of BO6 in LN-type ABO3, the distortions of SbO6 in LiSbO3 and SnO6 in ZnSnO3, which included a B cation with a d10 electronic configuration, were smaller than those of BO6 in LN-type oxides having the second-order Jahn-Teller active B cation, e.g., LiNbO3 and ZnTiO3. The temperature dependence of the lattice parameters, second harmonic generation, dielectric permittivity, and differential scanning calorimetry made it clear that a second-order ferroelectric-paraelectric phase transition occurs at a Curie temperature of Tc = 605 ± 10 K in LN-type LiSbO3. Further, first-principles density functional theory calculation suggested that perovskite-type LiSbO3 is less stable than LN-type LiSbO3 under even high pressure, and the ambient phase of LiSbO3 directly transforms to LN-type LiSbO3 under high pressure. The phase stability of LN-type LiSbO3 and the polar and ferroelectric properties are rationalized by the covalent bonding of Sb-O and the relatively weak Coulomb repulsion between Li+ and Sb5+.

Layered lithium niobium (III) oxide—LiNbO2 as a visible-light-driven photocatalyst for H2 evolution

The search for visible-light photocatalysts is of great significance in clean energy and environmental applications to best utilize solar radiation. Some layered semiconductor photocatalysts have been found to exhibit promising performance levels. Their superior activity has been linked to the layering within the crystal structure, which may facilitate separation of carriers and, hence the reduction and oxidation reactions. Previous investigations on layered materials focused on oxides containing early transition metal cations with d0 electronic configurations utilizing ultraviolet radiation. Attempts to synthesize layered materials with visible light response have been less successful and the photoactivity of such materials is not very high. Here, we have investigated a layered semiconducting compound, LiNbO2 with nominal d2 electronic configuration (Nb3+). As a result of a sub-valence band originating from filled d orbitals, LiNbO2 exhibits visible light absorption extending to 650 nm and demonstrates interesting photocatalytic activity for hydrogen production.

Magnetically driven orbital-selective insulator\u2013metal transition in double perovskite oxides

Interaction-driven metal–insulator transitions or Mott transitions are widely observed in condensed matter systems. In multi-orbital systems, many-body physics is richer in which an orbital-selective metal–insulator transition is an intriguing and unique phenomenon. Here we use first-principles calculations to show that a magnetic transition (from paramagnetic to long-range magnetically ordered) can simultaneously induce an orbital-selective insulator–metal transition in rock-salt ordered double perovskite oxides A2BB′O6, where B is a non-magnetic ion (Y3+ and Sc3+) and B′ a magnetic ion with a d3 electronic configuration (Ru5+ and Os5+). The orbital-selectivity originates from geometrical frustration of a face-centered-cubic lattice on which the magnetic ions B′ reside. Including realistic structural distortions and spin-orbit interaction do not affect the transition. The predicted orbital-selective transition naturally explains the anomaly observed in the electric resistivity of Sr2YRuO6. Implications of other available experimental data are also discussed. This work shows that by exploiting geometrical frustration on non-bipartite lattices, new electronic/magnetic/orbital-coupled phase transitions can occur in correlated materials that are in the vicinity of metal–insulator phase boundary.

Tailoring manganese oxide with atomic precision to increase surface site availability for oxygen reduction catalysis

Controlling the structure of catalysts at the atomic level provides an opportunity to establish detailed understanding of the catalytic form-to-function and realize new, non-equilibrium catalytic structures. Here, advanced thin-film deposition is used to control the atomic structure of La2/3Sr1/3MnO3, a well-known catalyst for the oxygen reduction reaction. The surface and sub-surface is customized, whereas the overall composition and d-electron configuration of the oxide is kept constant. Although the addition of SrMnO3 benefits the oxygen reduction reaction via electronic structure and conductivity improvements, SrMnO3 can react with ambient air to reduce the surface site availability. Placing SrMnO3 in the sub-surface underneath a LaMnO3 overlayer allows the catalyst to maintain the surface site availability while benefiting from improved electronic effects. The results show the promise of advanced thin-film deposition for realizing atomically precise catalysts, in which the surface and sub-surface structure and stoichiometry are tailored for functionality, over controlling only bulk compositions.

Identification of hydrogen species in alunite-type minerals by multi-nuclear solid-state NMR spectroscopy

The various hydrogen species present in a series of synthetic hydroniumjarosite ((H3O)Fe3(SO4)2(OH)6), and ammonioalunite ((NH4)Al3(SO4)2(OH)6) as well synthetic potassium (Cr3+ and V3+) and hydronium (V3+, Cr3+, and Ga3+) analogues were identified and quantified by 1H and 2H MAS NMR spectroscopy. The results confirm the defect mechanism proposed for alunite Nielsen et al. (Am Miner 92: 587–597, 2007), and allow for identification and quantification of even a few percent structural defects. For the paramagnetic samples, the isotropic shift for G2-OH group (V3+, Cr3+, and Fe3+) span more than 1100 ppm, which is related to different d-electron configuration (d2, d3, and d5). Analysis of the 1H and 27Al MAS NMR spectra shows that the synthetic ammonioalunite contains small amounts (5–10%) of hydronium. Furthermore, the close structural relationship between of hydronium and gallium alunite is reflected by the 27Al and 71Ga quadrupole coupling parameters. Thus, the current work demonstrates the applicability of solid state NMR spectroscopy for identification and quantification of hydrogen species in both dia- and paramagnetic minerals.

Silver Oxide‐Based Semiconductors for Solar Fuels Production and Environmental Remediation: a Solid‐State Chemistry Approach

AbstractA solid‐state perspective is offered for a discussion of silver‐based ternary and quaternary oxide semiconductors for applications related to solar fuels photogeneration and environmental remediation. The rather unique d10 electron configuration in these compounds is shown to impart interesting effects on their optoelectronic properties, and in turn, the photocatalytic activity of these compounds. This Minireview focuses on the structural aspects, electronic band structures, synthetic advances, and finally photoelectrochemical and photocatalytic applications of 32 compounds within the two oxide families, of the generic formula: AgxByOz or AgxByCnOz, where B and C are metals or non‐metals other than silver.

Revisiting La2MMnO6 (M = Co, Ni, Cu, Zn) perovskites in view of 3d-electron configuration

We investigate the impact of 3d-electron configuration on the magnetic and DC conductive properties of La2MMnO6 (M = Co, Ni, Cu, Zn) and offer evidence to an enhanced ionic ordering degree with the increase in the 3d electron number of an M ion. A key role of the 3d electronic configuration that influences the B-site ionic arrangement is revealed by changing both the ionic radii and bond covalency. The position of the valence band edge is shifted away from the Fermi level with the increase in 3d electrons, which reveals a strengthened electron localization behavior. Magnetic characterization indicates that the Co, Ni, and Zn samples present magnetic behaviors with half- and full-filled eg configurations, whereas the Cu sample shows a special Jahn-Teller configuration (t2g6eg3), giving rise to the degenerated eg states and significant change in bond parameters. Consequently, the Goodenough-Kanamori rules are not followed and the ferromagnetic coupling is weakened greatly. Further, DC resistivity measurements of samples M = Co, Ni, and Zn present thermally activated conductive behavior, in good accordance with the position shift of valence band edge. However, due to the Jahn-Teller effect arising from Cu2+ ion, a strong electron-phonon coupling is evidenced. This effect provides a suitable hopping channel for small polarons resulting in a considerable rise of its conductivity. Such changes in magnetization and DC conductivity of La2MMnO6 perovskites by modulating the M elements would inspire us to develop novel spin-electronic devices.

Bimetallic zeolitic imidazolate framework as an active excipient of curcumin under physiological condition

Metal-organic frameworks (MOFs) hold great promises in biotechnological application. MOFs have been shown as efficient carriers of therapeutic drugs. Yet MOFs have not been used for simultaneous drug delivery and efficacy enhancement. In this study, a mixed metal MOF copper (Cu2+)-doped zeolitic imidazolate framework-8 (Cu/ZIF-8) was applied for the first time for simultaneous increment of drug stability (through metalation with zinc (Zn2+)) and functionality (via complexation with Cu2+). Copper was introduced in the tetrahedral sites of ZIF-8 in reactions conducted with 25% and 10% Cu2+ by weight of Zn2+ and the reaction time, temperature and metal to ligand ratios were optimized to ensure minimum distortions of ZIF-8 crystallinity. Further experiments were conducted with ZIF-8 prepared by 25% of Cu2+. The as synthesized (Cu Zn (2-methyl imidazolate)2) crystals were loaded with curcumin following a facile route at room temperature. Spectroscopic study of Cur@Cu/ZIF-8 revealed the nature of interaction to be metal chelation with or without adsorption while picosecond-resolved spectroscopic measurement depicted greater charge separation in Cur@Cu/ZIF-8 than Cu/ZIF-8. Cu/ZIF-8 exhibits reasonably high drug loading capacity (22%) with exceptionally high efficiency (83%). Attachment with Cu/ZIF-8 increased the aqueous stability of curcumin by several orders of magnitude. Reactive oxygen species (ROS) generation was amplified due to combined interactions of Zn2+ and Cu2+ representing d10 and d9 electronic configurations respectively and enhanced bond breaking tendency of peripheral −O−H bond of curcumin molecule. Note worthily, Cur@Cu/ZIF-8 also exhibits greater anti-bacterial and anti-biofilm effects than free curcumin, a combination that is best suited for biomedical application.

(Invited) d3 Electron Configuration: Basic Properties and Main Misconceptions

Transition metal ions with the 3d3 electronic configuration (such as V2+, Cr3+, Mn4+, Fe5+) are widely used in various optical applications. In particular, the Mn4+ ions play a major role in red phosphors used in white light emitting diodes. Increased interest to these systems is justified by growing number of publications focused on the Mn4+ -based phosphors. However, quite often their spectroscopic properties are misinterpreted and physically incorrect statements can be met in the literature. Therefore, there is an urge need in a detailed and thorough review of the basic properties of the 3d3 electronic configuration, which would include the origin of the electronic energy levels, their degeneracy, symmetry, splitting patterns in crystal fields etc. This is the main goal of this presentation: to tutorize the fundamental optical properties of the 3d3 electronic configuration and to clear up some common misconceptions that we have encountered in the archival literature that pertains to the spectroscopic properties of the ions with the same electron configuration. The methods to properly describe the optical spectra of these systems are given. It is hoped that such systematic presentation of spectroscopic properties of the d3 ions and their variation from the free state to the crystalline solids will be useful for many researchers – mainly experimentalists – actively working in the field and will help them avoiding many mistakes when presenting their experimental results.

Enhanced Electrical Conduction in Anatase TaON via Soft Chemical Lithium Insertion toward Electronics Application

Metal oxynitride semiconductors with the d0 or d10 electron configuration are promising materials for nontoxic pigments and photocatalysts, but their electrical properties have scarcely been studied. Anatase TaON (δ-TaON) is a metastable polymorph of TaON, and its epitaxial thin films show good semiconducting properties such as a wide tunability of electrical conductivity and a rather high electron mobility comparable to that of anatase TiO2. However, the density of carrier electrons (ne) provided by anion vacancies is limited to ∼1 × 1020 cm–3, so establishing a method for carrier doping of anatase TaON remains a critical issue for its use in electronics applications. In this report, we used soft chemical insertion of Li into interstitial sites of anatase TaON epitaxial thin films by using an n-butyllithium solution, and the resulting material showed a higher ne (3.5 × 1020 cm–3) than anion-deficient anatase TaON films. Additionally, the Li-inserted anatase TaON showed an enhanced Hall mobility (μH) of o...

Determination of d-Orbital Populations in a Cobalt(II) Single-Molecule Magnet Using Single-Crystal X-ray Diffraction.

The tetrahedral cobalt(II) compound (Ph4P)2[Co(SPh)4] was the first mononuclear transition-metal complex shown to exhibit slow relaxation of the magnetization in zero external magnetic field. Because the relative populations of the d orbitals play a vital role in dictating the magnitude of the magnetic anisotropy, the magnetic behavior of this complex is directly related to its electronic structure, yet the exact role of the soft, thiophenolate ligands in influencing the d-electron configuration has previously only been investigated via theoretical methods. To provide detailed experimental insight into the effect of this ligand field, the electron density distribution in this compound was determined from low-temperature, single-crystal X-ray diffraction data and subsequent multipole modeling. Topological analysis of the electron density indicates significant covalent contributions to the cobalt-sulfur bonds. The derived d-orbital populations further reveal a fully occupied d z2 orbital, minor d xz orbital population, and nearly equal population of the d xy, d x2- y2, and d yz orbitals. Notably, we find that an electrostatic interaction between Co(II) and one hydrogen atom from a thiophenolate group in the xz plane increases the energy of the d x2- y2 orbital, leading to the nearly equal population with d xy and strong magnetic anisotropy.

SnI4 ⋅(S8 )2 : A Novel Adduct-Type Infrared Second-Order Nonlinear Optical Crystal.

A simple adduct from tin tetraiodide SnI4 and octasulfur S8 , SnI4 ⋅(S8 )2 (1), is obtained employing a facile reaction. The combination of Sn4+ ions with d10 electron configuration, acentric SnI4 tetrahedra, and lone-pair effects of S8 , makes 1 a phase-matchable infrared NLO crystal with a moderate second-harmonic generation (SHG) response and a very high laser-induced damage threshold (LIDT), which is well confirmed by the DFT calculations.

SnI4⋅(S8)2: A Novel Adduct‐Type Infrared Second‐Order Nonlinear Optical Crystal

AbstractA simple adduct from tin tetraiodide SnI4 and octasulfur S8, SnI4⋅(S8)2 (1), is obtained employing a facile reaction. The combination of Sn4+ ions with d10 electron configuration, acentric SnI4 tetrahedra, and lone‐pair effects of S8, makes 1 a phase‐matchable infrared NLO crystal with a moderate second‐harmonic generation (SHG) response and a very high laser‐induced damage threshold (LIDT), which is well confirmed by the DFT calculations.

Crystal structure and electrical conductivity of Ba<i>R</i><sub>2</sub>ZnO<sub>5</sub> (<i>R</i> = Sm, Gd, Dy, Ho, and Er)—a new structure family of oxide-ion conductors

Metal oxides containing the divalent zinc cation (Zn2+, d10 electronic configuration) as an essential element are promising candidates for oxide-ion conductors, because there are several good oxide-ion conductors containing other d10 cations, such as Ga3+ and Ge4+. In the present study, we have found a new structure family of oxide-ion conductors, BaY2CuO5-type (BaCuY2O5-type) BaR2ZnO5 compounds (R = rare earths). BaR2ZnO5 compounds with a BaY2CuO5-type structure were found to be good candidates for oxide-ion conductors by the bond-valence method. BaR2ZnO5 compounds (R = Sm, Gd, Dy, Er, and Ho) were synthesized by solid-state reactions, and their electrical conductivities and crystal structures were investigated. Rietveld analyses of BaR2ZnO5 (R = Sm, Gd, Dy, Er, and Ho) using synchrotron X-ray powder diffraction data gave good fitting for the BaY2CuO5-type structure. BaHo2ZnO5 had the highest total electrical conductivity in air among BaR2ZnO5 compounds (R = Sm, Gd, Dy, Er, Ho). Oxide-ion conduction was confirmed for BaHo2ZnO5 by electrical conductivity measurements conducted under various oxygen partial pressures. Neutron powder diffraction data of BaHo2ZnO5 were analyzed by the Rietveld method, and the oxide-ion conduction paths were investigated by the bond-valence method.

A Series of Rare-Earth Borates K7MRE2B15O30 (M = Zn, Cd, Pb; RE = Sc, Y, Gd, Lu) with Large Second Harmonic Generation Responses

A series of new rare-earth borate crystals K7MRE2B15O30 (M = Zn, Cd, Pb; RE = Sc, Y, Gd, Lu) were synthesized by solid-state reaction method. All of the title compounds crystallize in the noncentrosymmetric trigonal space group R32, and their structures consist of the B5O10 groups and the KO8, KO6, MO6, and REO6 polyhedra. Each isolated B5O10 group is connected by the REO6 (Sc3+, Y3+, Gd3+, Lu3+) groups forming an intricate three-dimensional network, and the K+ and M2+ (Zn2+, Cd2+, Pb2+) cations fill into the void space. Because of the introduction of different nonlinear optical (NLO)-active structural units (cations with d10 electronic configuration and stereoactive lone pairs) into rare-earth metal borates, these compounds exhibit second harmonic generation (SHG) responses ranging from 1.5 to 2.1 times that of KH2PO4 (KDP), and are phase-matchable. Interestingly, this work indicates that the NLO properties can be artificially adjusted by systematically replacing the bivalent M2+ and trivalent RE3+ catio...

Transition-Metal-Incorporated Aluminum Oxide Thin Films: Toward Electronic Structure Design in Amorphous Mixed-Metal Oxides

Amorphous metal oxides have numerous applications spanning from electronics to catalysis. The ability to rationally design the electronic structure of such materials would thus have significant impact. Previous work has shown that the amorphous structure allows for uniformly incorporating a large number of different metal cations into ultrasmooth thin films. Here we investigate how the atomic structure, electronic structure, and optical absorption properties of amorphous aluminum oxide (a-Al2O3) thin films are modulated when metal cations of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, or Ga are added to form MyAl1–yOx where y varies from 0.1 to 0.7. We use X-ray absorption spectroscopy (XAS) to analyze oxidation state and local bonding around the incorporated cations (e.g., coordination number), valence-band X-ray photoelectron spectroscopy (VB-XPS) to assess the impact of the cations on the valence band electronic structure, and optical absorption spectroscopy to assess the presence and energies of new unoccupied electronic states. Cations with partially filled d-orbitals (e.g., Fe3+, Cr3+, Co2+, Ni2+, Cu2+, etc.) add filled 3d electronic states at the valence band edge of a-Al2O3 as well as unfilled 3d electronic states within the a-Al2O3 band gap. Cations with the d10 electronic configuration, such as Zn2+ and Ga3+, introduce filled 3d electronic states at energies well below the valence band edge in addition to new unfilled electronic states in the a-Al2O3 band gap. Cations with unfilled d-orbitals (e.g., those with a d0 configuration such as V5+ and Cr6+) do not modify the valence band of a-Al2O3, but affect the optical properties by inserting unfilled 3d electronic states within the band gap. These results provide guidance for the rational design of the electronic structure of amorphous mixed-metal oxides.

Half metallic ferromagnetism in Ni based half Heusler alloys

The search for emerging materials with ferromagnetic and spin flip properties has attracted widespread interest in material science. Ni based half Heusler alloys have been one of the benchmark half metallic ferromagnetic material owing to their magnetic interaction and promising figure of merit in magnetic memory element. A systematic pathway to design novel Heusler materials is by analyzing structural phase stability, electronic structure, mechanical and magnetic properties. Specifically, we carried out first principles calculations to identify the ferromagnetic and half-metallic properties of Ni based half Heusler alloys XYZ (X = Ni; Y = Cr; Z = Si, Ge, Ga, Al, In, As). The predicted phase stability shows that α-phase is found to be the lowest energy phase compared with β and γ phases. A pressure-induced structural phase transitions from α-phase to γ-phase in NiCrSi, NiCrGe, NiCrGa, NiCrAl, NiCrIn and α-phase to β-phase in NiCrAs are found. Due to the presence of d8 and d5 electronic configuration within Ni2+ and Cr2+ ions respectively, all the Heusler materials show half metallic behavior. Furthermore, the magnetic moments for these half Heusler alloys in all the three different phases (α, β and γ) have been reported. Our work paves the way for designing novel Heusler materials at normal and high pressure.

61Ni synchrotron-radiation-based Mössbauer absorption spectroscopy of Ni nanoparticle composites

We obtained energy-domain 61Ni synchrotron-radiation-based Mossbauer absorption spectra of three materials that relate to nanoparticles: Ni2(C8O6H2) metal-organic frameworks (MOFs), Ni nanoparticles synthesized by complete heat decomposition of the MOFs, and the composites of Ni nanoparticles and the MOFs synthesized by partial decomposition of the MOFs. The 61Ni abundance of all the samples was not enriched but we were successfully able to obtain their spectra in 1 day or less, by using a highly efficient measurement system where the internal conversion electrons from energy standard 61Ni86V14 foil were detected. Although both nanoparticle constituent and MOF constituent in the composites included Ni atoms, the Mossbauer parameters of the Ni nanoparticle constituent could be evaluated; the magnetic hyperfine field of the Ni nanoparticle constituent in the composites was different from that of the Ni nanoparticles obtained by the complete heat decomposition. This difference implied that the 3d and/or 4s electron configuration of the nanoparticle constituent were affected by the MOF constituent in the composites.

Pressure driven spin transition in siderite and magnesiosiderite single crystals

Iron-bearing carbonates are candidate phases for carbon storage in the deep Earth and may play an important role for the Earth’s carbon cycle. To elucidate the properties of carbonates at conditions of the deep Earth, we investigated the pressure driven magnetic high spin to low spin transition of synthetic siderite FeCO3 and magnesiosiderite (Mg0.74Fe0.26)CO3 single crystals for pressures up to 57 GPa using diamond anvil cells and x-ray Raman scattering spectroscopy to directly probe the iron 3d electron configuration. An extremely sharp transition for siderite single crystal occurs at a notably low pressure of 40.4 ± 0.1 GPa with a transition width of 0.7 GPa when using the very soft pressure medium helium. In contrast, we observe a broadening of the transition width to 4.4 GPa for siderite with a surprising additional shift of the transition pressure to 44.3 ± 0.4 GPa when argon is used as pressure medium. The difference is assigned to larger pressure gradients in case of argon. For magnesiosiderite loaded with argon, the transition occurs at 44.8 ± 0.8 GPa showing similar width as siderite. Hence, no compositional effect on the spin transition pressure is observed. The spectra measured within the spin crossover regime indicate coexistence of regions of pure high- and low-spin configuration within the single crystal.