Octahedral Ni2 Research Articles

Supramolecular assembly, structure, magnetic property, and proton conductivity of a nickel(II) hydrogen-bonded organosulfonate framework

A three-dimensional nickel(II) hydrogen-bonded organic framework (NiHOF), formulated as [Ni(H2O)2(EtOH)2(MeCN)2][nds] (NiHOF-HGNU-2, H2nds = 1,5-Naphthalenedisulfonic acid, HGNU = Huanggang Normal University), was synthesized and magnetically and electrically characterized. Single-crystal X-ray diffraction (SC-XRD) analysis revealed that extensive hydrogen bonding interactions formed between the [Ni(H2O)(EtOH)(MeCN)]2+cations and organodisulfonate anions nds2−, leading to a highly hydrogen-bonded network. Variable-temperature SC-XRD and thermogravimetric analysis both reveal the crystalline sample of NiHOF-HGNU-2 can be thermally stable up to 400 K. Magnetic investigations reveal the uniaxial magnetic anisotropy (D = -9.87 cm−1) of the distorted octahedral Ni2+ ions of NiHOF-HGNU-2 without slow magnetic relaxation. Alternating current impedance spectroscopy indicate NiHOF-HGNU-2 exhibits temperature- and humidity-dependent proton conduction behavior with the highest measured conductivity reaching 1.15 × 10−4 S cm−1 at 85 °C and 98 % relative humidity, suggesting a supramolecular proton conductor of the NiHOF-HGNU-2. The forgoing results not only provide a new Ni-based proton conductor but also an organodisulfonate strategy for building supramolecular proton-conducting materials.

A 3D supramolecular framework-based electrochemical sensor for highly sensitive detection of ascorbic acid

A benzimidazole carboxylate ligand with multiple coordination sites has been designed by the aldimine condensation reaction of 2-pyridinecarboxal-dehyde and 3,4-diaminobenzoic acid, 2-(2′-pyridyl)-benzimidazole-6-carboxylic acid (H2pybimc), which solvothermally assembled with Ni2+ ions in the mixed solvents of H2O and n-propanol to generate a 3D supramolecular framework, {[Ni(Hpybimc)(NO3)(H2O)]}n (CP 1). The chemical composition and structure of CP 1 have been characterized in detail by various tools. Structural analysis demonstrates that the asymmetrical unit of CP 1 consists of one octahedral Ni2+ ion, one Hpybimc−, one NO3− and one coordinated water molecule. Adjacent Ni2+ ions are linked by Hpybimc− to form a 2D 3-connected ‘fes’-type topology, which further generates a 3D supramolecular architecture by hydrogen bonds and π···π stacking interactions. The carbon paste electrode (CPE) modified with CP 1 is fabricated (CPE 1) and applied for the electrochemical sensor of ascorbic acid (AA), which exhibits good linear correlation with AA in the concentration range of 5.0 × 10−4 to 7.3 mmol L−1 with the limit of detection (LOD) of 0.85 μmol L−1. Moreover, it has satisfactory stability, repeatability, and selectivity towards AA, which can be successfully used for the detections in real samples.

Bandgap engineered BaTiO3-based ferroelectric oxides for photovoltaic applications

Ferroelectric oxides have gained research attention in the field of ferroelectric photovoltaics (PV) after the discovery of power conversion efficiency exceeding the Shockley–Queisser limit in BaTiO3 (BTO) crystals. However, advancement in this field is hindered by the wide bandgap (>3 eV) nature of ferroelectric oxides. In this work, a novel lead-free ferroelectric (1 − x)BTO − xBi(Ni2/3Nb1/3)O3 system was proposed and demonstrated to show bandgap reduction without compromising the polarization. Notably, the system displayed a bandgap reduction from 3.1 to 2.4 eV upon varying the composition from x = 0.0 to 0.05. Particularly, the optimal composition x = 0.02 showed enhancement in polarization (Pmax = 16 μC/cm2) and anomalous PV response with an open-circuit voltage of 6 V at 300 K. The origin of the bandgap reduction and polarization retention is explored experimentally by Raman spectroscopic measurements and analyzed theoretically by density functional theory. Our results revealed that the oxygen octahedral distortions and Ni2+ doping favor bandgap lowering, and Bi3+ ions stabilize the ferroelectric polarization. This study provides insight into the origin of bandgap tuning and paves the route for exploring new low-bandgap ferroelectric material with room temperature polarization.

Flexible Co(II)-and Ni(II)-Based Cationic 2D Metal–Organic Frameworks Based on a Charge-Neutral (O,O)-Donor Bridge

Two new metal–organic frameworks based on highly flexible 1,4-diazabicyclo[2.2.2]octane N,N′-dioxide (odabco) ligands were successfully synthesized and characterized. Their crystallographic formulae are [M(DMF)2(odabco)2](ClO4)2·dioxane, where M2+ = Co2+ (1) and Ni2+ (2), and DMF is N,N-dimethylformamide. The title compounds possess cationic 2D coordination networks filled with perchlorate anions and dioxane solvent molecules in the interlayer space, with 20% solvent accessible volume. Carbon dioxide adsorption measurements for desolvated samples 1a and 2a gave 511 m2/g and 377 m2/g specific surface areas, respectively, revealing the first example of gas adsorption properties in the structure based on a flexible odabco bridge, despite the presence of large counteranions within the positively charged network. Magnetization measurements for 1, 1a, 2 and 2a reveal their paramagnetic nature to be in a reasonable agreement with crystal structures, and almost no solvent dependence of the magnetization characteristics. A decrease in the effective magnetic moment observed at low temperatures is attributed mostly to zero-field level-splitting in the octahedral Ni2+ and Co2+ ions.

Impactive cadmium modifications to enhance the structural-optical relationship in nickel borate glasses

Cadmium-doped borate glass composition was prepared via the melt-quenching technique. The nature of the glass structure was detected by X-ray diffraction (XRD). The physical parameters, such as density (ρ) from Archimedes’ principle, molar volume (Vm), Cd2+ ions concentration (NCd), and inter-atomic distance (Ri) were evaluated, providing structural information on glass samples. Fourier transform infrared (FT-IR) analysis was used to study the fundamental vibrational modes of structural units, confirming the conversion of BO3 to BO4 units, indicating a strongly bonded and high connectivity of the network. Some important optical findings were extracted from the analysis of optical absorption spectra. For example, the mitigation in the optical band gap, the increase in Urbach energy, the deterioration in the metallization criterion, and the enhancement in the electron-phonon interaction were obtained with cadmium oxide addition. The nonlinearity characteristics were enhanced, making these glass samples have viability in the non-linear optics sector. The obtained data from the electron spin resonance (ESR) revealed that the nickel ions exist in a divalent oxidation state with octahedral symmetry. Interestingly, the optical absorption transitions of Ni2+ ions were accurately deconvoluted and assigned to octahedral and tetrahedral positions. The equations of ligand field parameters of octahedral Ni2+ ions were correctly presented. The equation of Racah C parameter was deduced for the first time. Then, the ligand field parameters, such as the ligand field strength (10 Dq), Racah parameters (B and C), and other related parameters, were precisely assessed. The covalent nature between Ni2+ ions and the ligands was detected from the nephelauxetic effect.

Synthesis, structure, magnetism and proton conductivity of a cyanide-bridged NiIICoIII framework

A two-dimensional cyanide-bridged NiIICoIII coordination polymer, {[Ni(cyclam)]3[Co(CN)6]2·16H2O}n (1, cyclam = 1,4,8,11-tetraazacyclotetradecane), was constructed from hexacyanidocobaltate(III) anions, cyclam ligands and Ni2+ ions. Single-crystal X-ray structural analysis revealed that the [Ni(cyclam)]2+ species were bridged by [Co(CN)6]3- building units, leading to a 2D honeycomb topological structure. Interestingly, 1D microporous channels within the framework were formed with guest water molecules located. An N2 adsorption experiment supports the porosity of the compound. Magnetic investigations reveal the easy-axis magnetic anisotropy of the distorted octahedral Ni2+ ions in 1. However, no single-ion magnet behavior was observed. Due to the presence of significant hydrogen-bonded water channels, temperature- and humidity-dependent proton conduction behavior was observed in 1. The electrical conductivity reaches 2.9 × 10−4 S cm−1 at 44 °C and 100 % relative humidity. The forgoing results provide a relatively rare porous cyanide-bridged magnetic framework with proton conduction behavior. Additionally, this compound also highlights that cyanide-bridged frameworks are a potential great platform for the development of high-performance proton-conducting materials.

Oxygen vacancies induced by charge compensation tailoring Ni-doped Co3O4 nanoflakes for efficient hydrogen evolution

The electronic structure modulation of the catalysts shows great significance towards hydrogen evolution reaction (HER). A charge compensation was realized by doping octahedral Ni2+ ions into the Co3O4 lattice, where the oxygen vacancies generate to maintain the electric neutrality. The oxygen vacancies lead to the local lattice distortion of Co3O4, trigger the concomitant electron redistribution, as well as optimize the d-band center of Ni-Co3O4. Then, the accelerated charge transfer and matter conversion boost the HER kinetics of Ni-Co3O4. As expected, the obtained Ni-Co3O4 catalyst exhibits the brilliant HER catalytic performance with a low overpotential of only 47 mV at 10 mA cm−2, and a Tafel slope of 46 mV dec−1. The density function theory calculations verify that this charge compensation strategy could efficiently promote the water dissociation and hydrogen transformation kinetics. Our work paves an effective avenue for rationally contriving high-efficiency catalysts towards HER.

Structure and Luminescence Properties of Transparent Germanate Glass-Ceramics Co-Doped with Ni2+/Er3+ for Near-Infrared Optical Fiber Application.

An investigation of the structural and luminescent properties of the transparent germanate glass-ceramics co-doped with Ni2+/Er3+ for near-infrared optical fiber applications was presented. Modification of germanate glasses with 10–20 ZnO (mol.%) was focused to propose the additional heat treatment process controlled at 650 °C to obtain transparent glass-ceramics. The formation of 11 nm ZnGa2O4 nanocrystals was confirmed by the X-ray diffraction (XRD) method. It followed the glass network changes analyzed in detail (MIR—Mid Infrared spectroscopy) with an increasing heating time of precursor glass. The broadband 1000–1650 nm luminescence (λexc = 808 nm) was obtained as a result of Ni2+: 3T2(3F) → 3A2(3F) octahedral Ni2+ ions and Er3+: 4I13/2 → 4I15/2 radiative transitions and energy transfer from Ni2+ to Er3+ with the efficiency of 19%. Elaborated glass–nanocrystalline material is a very promising candidate for use as a core of broadband luminescence optical fibers.

Structural significance of nickel sites in aluminosilicate glasses

We use Ni ions as a structural probe to investigate the influence of modifier cations (Na, K, Mg, Ca, and Ba) on the structure of aluminosilicate glasses. The color of these glasses varies with the nature of the cations present in glass. Structural origin of this change is investigated by optical absorption and X-ray absorption spectroscopies. Our results reveal pronounced changes in Ni coordination. Tetrahedra Ni2+ is present in all these glasses and Ni2+ in 5-fold coordinated site also appears. There is a weak contribution from octahedral Ni2+. The concentration of tetrahedral Ni2+ deduced from the optical absorption spectroscopy increases with decreasing CFS, explaining the variation of color from yellow and brown to purple. This is in line with X-ray absorption spectroscopy data. Cation-oxygen distances and site symmetry indicate a similar structural behavior of Mg2+ and Ni2+, being a good structural probe of Mg.

Ethylene Oligomerization over Nickel Supported Silica-Alumina Catalysts with High Selectivity for C10+ Products

The nickel (II) loading silica-alumina under various treatments in terms of aging temperature, Si/Al ratio and activation temperature were investigated by XRD, N2 adsorption-desorption, TEM, UV-Vis, NH3-TPD and XRF and then applied to catalyze the ethylene oligomerization. High aging temperature, low Si/Al ratio and high activation temperature were beneficial to high selectivity for C10+ products because of a reasonable match between Ni active sites and acid sites, high Ni loading content and less octahedral coordination Ni2+ species, respectively. Ni loading content was more important than the number of acid sites for high yield of C10+ products, and less octahedral coordination Ni2+ species favored less by-products produced at high reaction temperature. In addition, other experimental conditions, such as reaction temperature, weight hourly space velocity (WHSV) and nickel precursor were discussed in the paper.

Dielectric dispersion and impedance spectroscopy of NiO doped Li2SO4–MgO–P2O5 glass system

Li2SO4–MgO–P2O5 glasses mixed with a varied quantity of NiO were prepared. Detailed spectroscopic investigations (viz., OA, IR and Raman) spectra were carried out. Optical absorption spectral results indicated that Ni2+ ions in the glass network reside in octahedral (Oh) as well as tetrahedral (Td) sites; these results have also pointed out the growing concentration of Ni2+ ions in Oh sites as NiO concentration is increased up to 0.8 mol%. Results of infrared and Raman spectral studies indicated increased internal chaos in the sulpho-phosphate glass network with an increase of NiO concentration up to 0.8 mol%. Values of dielectric permittivity (both real and imaginary components), a.c. conductivity, σac, are observed to be gradually increasing with NiO content in the concentration range 0–0.8 mol%. Electric moduli spectra have exhibited dipolar relaxation effects and the dipolar relaxation time τ estimated from these spectra exhibited a decreasing trend with the raise of NiO quantity up to 0.8 mol%. The complexes of octahedral Ni2+ ions with oxygen ions are predicted to be the possible dipoles responsible for the dipolar effects. Variation of σac with NiO content exhibited an increasing trend up to 0.8 mol%. Quantitative analysis of this observation indicated ionic contribution to σac is dominant in the glasses containing small quantities of NiO (< 0.8 mol%), whereas in the glasses containing higher quantities of NiO (> 0.8 mol%) polaronic contribution to the conductivity seems to be present. Overall, the results of dielectric properties of studied glass system indicted that the glass with low quantities of NiO (< 0.8 mol%) can be considered as useful material as electrolytes in the solid-state batteries.

Broadband NIR emission from transparent fluorosilicate glass-ceramics containing Rb2SiF6:Ni2+ nanocrystals

Ni2+-doped fluorosilicate glass-ceramics with broadband emission have been prepared. The structural characterizations show that Rb2SiF6 nanocrystals separated from the glassy phase in the sample with a composition of RbF-ZnF2-SiO2:Ni2+. Moreover, the obtained Rb2SiF6 nanocrystals with the size of around 30 nm rely on stringent control of the heat-treatment temperatures. The experimental data reveal that Ni2+ selectively incorporates into the cubic Rb2SiF6 nanocrystals and locates at a typical 6F-coordinated octahedra. Meanwhile, a broadband emission in the important NIR range from 1200 to 2000 nm appears with the precipitation of Rb2SiF6 nanocrystals. The emission can be ascribed to the 3T2g(3F) → 3A2g(3F) transition of octahedral Ni2+ incorporated in the Rb2SiF6 crystalline phase. The Rb2SiF6:Ni2+ containing glass-ceramics provide an innovative choice for the development of photonic glasses with broadband emission.

Ni2Mo3O8: Complex antiferromagnetic order on a honeycomb lattice.

Theoretical studies have predicted the existence of topological magnons in honeycomb compounds with stripy or zigzag antiferromagnetic (AFM) order. Here we report the discovery of AFM order in the layered and noncentrosymmetric honeycomb nickelate Ni2Mo3O8 through a combination of magnetization, specific heat, x-ray and neutron diffraction, and electron paramagnetic resonance measurements. The AFM order is complex, with a mixture of stripy and zigzag character on an integer spin noncentrosymmetric honeycomb lattice (P63 mc). Further, each of the two sublattices of the bipartite honeycomb lattice is comprised of a different crystal field environment, i.e., octahedral and tetrahedral Ni2+, respectively, enabling independent substitution on each. Replacement of Ni by Mg on the octahedral site suppresses the long-range magnetic order and results in a weakly ferromagnetic state. Conversely, substitution of Fe for Ni enhances the strength of the AFM exchange and increases the ordering temperature. Thus, Ni2Mo3O8 provides a platform on which to explore the rich physics of S = 1 on the honeycomb lattice in the presence of competing magnetic interactions with a noncentrosymmetric, formally piezopolar, crystal structure.

Host-Guest Extraction of Heavy Metal Ions with p-t-Butylcalix[8]arene from Ammonia or Amine Solutions.

The capacities of the p-t-butylcalix[8]arene (abbreviated as H8L) host to extract toxic divalent heavy metal ions and silver from aqueous solution phases containing ammonia or ethylene diamine to an organic phase (nitrobenzene, dichloromethane, or chloroform) were carried out. When the metal ions were extracted from an aqueous ammonia solution, the metal ion selectivity for extraction was found to decrease in the order Cd2+> Ni2+> Cu2+> Ag+> Co2+> Zn2+. When the aqueous phase contained ethylene diamine, excellent extraction efficiencies of 97% and 90% were observed for the heavy metal ions Cu2+ and Cd2+, respectively. Under the same conditions the extraction of octahedral type metal ions, namely, Co2+ and Ni2+, was suppressed. The extraction of transition metal cations by H8L in ammonia and/or amine was found to be pH dependent. Detailed analysis of extraction behavior was investigated by slope analysis, the continuous variation method, and by loading tests.

Spectroscopic, dielectric dispersion and dc conductivity studies of Sb2O3doped lithium fluoro borophosphate glasses mixed with small concentrations of NiO

Glasses of the composition(20–x) LiF–10 Sb2O3–10 B2O3–60P2O5: x NiO(0 < x < 0.25) were synthesized by conventional rapid melt quenching method. The non–crystalline nature of the samples was confirmed by XRD analysis and the glass forming abilities were analyzed by DTA studies. The compositional dependence of structural aspects of the glasses was analyzed by FT–IR and Raman studies. The DTA, FT–IR and Raman studies suggested an increasing degree of disorder in the glass network with increasing concentration of NiO up to 0.20 mol %. The reversal trend observed beyond 0.20 mol% is attributed to an increasing polymerization of glass network. The optical absorption (OA) and photoluminescence (PL) studies were also carried on these glasses. The results of OA and PL spectral studies suggested that the gradual increase of octahedral Ni2+ ions with the increase in the concentration of NiO up to 0.20 mol%. Electrical properties viz., dielectric dispersion, DC and AC conductivity of the titled glass samples were also performed on titled glass samples. The observed increase in the values of dielectric constant, dielectric loss and AC conductivity (at any frequency and temperature) with the increase in the concentration of NiO up to 0.20 mol%is ascribed to the increasing space charge polarization in the glass network. The results of conductivity studies indicated that there is a mixed conduction (both ionic and polaronic) and the polaron hopping seems to prevail over ionic conduction in the glasses containing less than 0.20 mol% of NiO.

Effects of Octahedral Ni2+ on Structural and Transport Properties of NixFe3-xO₄ Thin Films

Polycrystalline NixFe<SUB>3-x</SUB>O₄ thin films prepared by using a sol-gel process exhibited phase-pure spinel (Fd3m) structure for the Ni composition up to x = 1.0. X-ray photoemission spectroscopy (XPS) and Raman spectroscopy investigations revealed that Ni ions mostly have valence of +2 and occupy the octahedral sites of the cationic sublattice. The NixFe<SUB>3-x</SUB>O₄ films exhibited higher electrical resistivity than that of Fe₃O₄ below 300 K. The increase in the resistivity is primarily ascribed to decrease in Fe<SUP>2+</SUP>-Fe<SUP>3+</SUP> polaronic hopping rate in the octahedral sites due to the increase in the octahedral Ni<SUP>2+</SUP> population.

Experimental constraints on reconstruction of Archean seawater Ni isotopic composition from banded iron formations

The Ni isotopic systematics in banded iron formations (BIFs) potentially recorded the Ni isotopic composition of ancient seawater over Precambrian geological history. However, the utility of BIFs as proxies requires quantitative knowledge of how Ni isotopes fractionated as dissolved Ni was initially incorporated into iron-rich sediments and how diagenesis may have affected the Ni isotopic systematics. Here we report results of synthesis experiments to investigate the behavior of Ni isotopes during Ni coprecipitation with ferrihydrite and then transformation of ferrihydrite to hematite. Ferrihydrite coprecipitation experiments at neutral pH demonstrated that the dissolved Ni was variably heavier than coprecipitated Ni (likely a mixture of surface-adsorbed and structurally incorporated Ni), with the isotope fractionation becoming larger as the fraction of Ni associated with solid increased (Δ60/58Nisolution-solid=+0.08 to +0.50‰). Further experiments at lower pH (3.7–6.7), in which structurally incorporated Ni likely dominated in solids, documented a decrease in Δ60/58Nisolution-solid from +0.44‰ to −0.18‰ as the pH decreased. The negative value for Δ60/58Nisolution-solid at low pH indicates the enrichment of heavier isotopes in incorporated Ni relative to dissolved and adsorbed Ni, possibly as a result of the presence of a small amount of tetrahedral Ni2+ in addition to octahedral Ni2+ in the ferrihydrite structure. The results of the ferrihydrite experiments thus reflect equilibrium isotope fractionation between three pools of Ni, with δ60/58Ni values in the order of incorporated>dissolved>adsorbed. Hematite was synthesized by transformation of Ni-bearing ferrihydrite in aqueous solution at ∼100°C. A significant amount of Ni (up to 60%) was released (desorbed) from solids into solutions as pH dropped from ∼7 to 4.5–5.5 upon phase transformation. Rinsing of the synthesized hematite in 2M acetic acid released only very small amounts of Ni (<4% of total Ni, presumably surface-adsorbed) that were isotopically heavier (δ60/58Ni=+0.11±0.06‰) than the residues (presumably dominated by incorporated Ni), which had δ60/58Ni of −0.26±0.07‰. The preference of lighter isotopes for the incorporated Ni relative to the surface-adsorbed Ni after phase transformation (most had been released into solution) is probably due to distortion of NiO octahedra in the hematite structure, with weaker NiO bond strengths on average. Hence, the more variable Δ60/58Nisolution-solid values (−0.04 to +0.77‰) observed in hematite experiments most likely reflect thermodynamically driven Rayleigh fractionation, with incorporated Ni unavailable to exchange with dissolved Ni due to continuous reduction in size of the highly reactive surface pool of Ni, through which all solid-solution exchange must occur. Overall, the synthesized hematite was isotopically lighter than the ferrihydrite by ∼0.08‰ in δ60/58Ni, which is however within the current analytical uncertainties (±0.09‰). This implies that earliest diagenesis of BIFs results in very limited change in the isotopic composition of solid-associated Ni. Our experimental results, although conducted in a very simple system that differs from Archean seawater, represent an important step toward reconstruction of the Ni isotopic composition of ancient seawater from Ni isotopic signatures in BIFs.

Cation distribution in Ni-substituted Mn0.5Zn0.5Fe2O4 nanoparticles: A Raman, Mössbauer, X-ray diffraction and electron spectroscopy study

Abstract In this paper we report the structural, vibration, and electronic-structure parameters (bonding and valence of cations) of single phase cubic mixed spinel nanoparticles of (ZnδMnγFe1−(γ+δ))[NixZn0.5−δMn0.5−γ−xFe1+(γ+δ)]O4 where x = 0.05–0.45 with an aim to determine cation-distribution i.e. δ and γ in these samples. The Raman spectroscopy has established that only Fe and Zn cations occupy tetrahedral interstitial sites in a FCC anion lattice in nearly equal fraction, and Mossbauer spectra have shown that Fe3+ cations are present at both, tetrahedral and octahedral interstitial sites and Ni2+ cations are situated at the octahedral sites in all the substituted samples. The photoelectron spectra also revealed the presence of Fe3+ cations at both the interstitial sites. The best possible cationic distribution in Ni substituted Mn–Zn ferrites has been estimated by reiteratively calculating the intensity ratios of various pairs of X-ray diffraction peaks and matching with the observed intensity ratios.

Cationic structure and charge transport in sol–gel-derived nickel-cobaltite thin films

Conductive nickel-cobaltite (NixCo3−xO4) thin films were synthesized on Al2O3 substrates by using a sol–gel process for varying Ni composition x (≤ 1.0). X-ray diffraction data indicated that the polycrystalline NixCo3−xO4 samples have cubic spinel (Fd $$\bar{3}$$ m) structure with the lattice constant increasing slightly with increasing Ni composition (by 0.5 % for x = 1.0 compared to that of Co3O4). X-ray photoelectron spectroscopy data indicated that the Ni ions in nickel-cobaltite have multi-valence (Ni2+ and Ni3+). The temperature-dependent electrical resistivity (ρ) data of the nickel-cobaltite samples were measured in the 4–300 K range in comparison with Co3O4. At all temperatures (T), the resistivity of NixCo3−xO4 decreased gradually with increasing x down to ρ = 1.9 × 10−2 Ω cm for x = 1.0 at 300 K (reduction by a factor of ~10−3 from that of Co3O4). The decrease in ρ by the Ni doping in Co3O4 can be understood in terms of the polaronic d-electron hopping between octahedral Ni2+ and Ni3+ ions in the spinel lattice. The analysis on ρ(T) for NixCo3−xO4 exhibited a linearity between ln(ρ) and $$T^{ - 1/4}$$ below ~120 K, implying a variable-range hopping.

A heptadentate ligand possessing two phenol groups: Its diverse coordination chemistry and the catalytic behaviors of its transition complexes towards benzene oxidation

Reacting the heptadentate ligand 2,2′-(((pyridine-2,6-diylbis(methylene))bis((pyridin-2-ylmethyl)-azanediyl))bis(methylene))diphenol (H2L) with one equivalent of CuCl2·6H2O, NiCl2·6H2O and FeCl3·6H2O, followed by anion exchange with PF6−, BPh4− and ClO4− formed three mononuclear complexes [Cu(H2L)](Cl)(PF6) (1), [Ni(HL)](BPh4) (2) and [Fe(L)](ClO4) (3), respectively. Their structures in the solid state were determined by X-ray crystallography. The three metal centers in complexes 1–3 adopt distorted tetragonal pyramidal (Cu2+), octahedral (Ni2+) and pentagonal bipyramidal (Fe3+) geometries, respectively. In addition, the electrochemistry and their catalytic activity in the oxidation reaction of benzene to phenol by H2O2 were investigated.