Redetermination of CaMn2O4

The title compound, calcium dimanganese tetraoxide, CaMn₂O₄, was synthesized hydro­thermally and is made up of edge‐sharing Mn³⁺ distorted octahedral layers and eight‐coordinate Ca²⁺ centered polyhedral layers.     

Structural, magnetic coupling and oxidation state trends in models of the CaMn4 cluster in photosystem II

Density functional theory calculations are reported on a set of isomeric structures I, II and III that share the structural formula [CaMn4C9H10N2O16]q+.(H2O)3 (q= -1, 0, 1, 2, 3). Species I has a skeletal structure, which has been previously identified as a close match to the ligated CaMn4 cluster in Photosystem II, as characterized in the most recent 3.0 angstroms crystal structure. Structures II and III are rearrangements of I, which largely retain that model's bridging ligand framework, but feature metal atom positions broadly consistent with, respectively, the earlier 3.5 and 3.2 angstroms crystal structures for the Photosystem II water-oxidising complex (WOC). Our study explores the influence of the cluster charge state (and hence S state) on several important properties of the model structures; including the relative energies of the three models, their interconversion, trends in the individual Mn oxidation states, preferred hydration sites and favoured modes of magnetic coupling between the manganese atoms. We find that, for several of the explored cluster charge states, modest differences in the bridging-ligand geometry exert a powerful influence over the individual manganese oxidation states, but throughout these states the robustness of the tetrahedron formed by the Ca and three of the Mn atoms remains a significant feature and contrasts with the positional flexibility of the fourth Mn atom. Although structure I is lowest in energy for most S states, the energy differences between structures for a given S state are not large. Overall, structure II provides a better match for the EXAFS derived metal-metal distance parameters for the earlier S states (S0 to S2), but not for S3 in which a significant structural change is observed experimentally. In this S state structure III provides a closer fit. The implications of these results, for the possible action of the WOC, are discussed.     

Magnetic properties of the mixed-valence compounds CaMn3O6 and CaMn4O8

The preparation of two new mixed-valence compounds, CaMn₃O₆ and CaMn₄O₈, are described and their magntic and EPR behavior investigated. The Mn moments in both compounds have nearly spinonly values. CaMn₃O₆ and CaMn₄O₈ order ferrimagnetically near 3 and 89 K, respectively. The broad, Lorentzian EPR lines indicate a significant exchange interaction between Mn³⁺ and Mn⁴⁺ ions. The magnetic and EPR results suggest a strong ferromagnetic interaction between Mn³⁺ and Mn⁴⁺ ions and a comparable antiferromagnetic Mn³⁺Mn³⁺ and/or Mn⁺Mn⁴⁺ interaction.     

Tuning the magnetic properties of Li(x)CrTi0.25Se2 (0.03 < or = x < or = 0.7) by directed deintercalation of lithium

X-ray diffraction (XRD), in situ energy-dispersive X-ray diffraction (EDXRD), X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and magnetic measurements were applied to investigate the effects of lithium deintercalation on pseudolayered Li(0.70)CrTi(0.25)Se(2). A detailed picture of structural changes during the deintercalation process was obtained by combining the results of EDXRD and EXAFS. Removal of Li from the host-guest complex leads to anisotropic contraction of the unit cell with stronger impact on the c axis, which is the stacking axis of the layers. The EDXRD experiments evidence that the shrinkage of the lattice parameters with decreasing x(Li) in Li(x)CrTi(0.25)Se(2) is nonlinear in the beginning and then becomes linear. Analysis of the EXAFS spectra clearly shows that the Cr/Ti-Se distances are affected in a different manner by Li removal. The Cr-Se bond lengths decrease, whereas the Ti-Se bonds lengthen when the Li content is reduced, which is consistent with XRD data. Magnetic measurements reveal a change from predominantly antiferromagnetic exchange (theta(p) = -300 K) interactions for the pristine material to ferromagnetic exchange interactions (theta = 25 K) for the fully intercalated material. Thus, the magnetic properties can be altered under ambient conditions by directed adjustment of the dominant magnetic exchange. The unusual magnetic behavior can be explained on the basis of the variation of the metal-metal distances and the Cr-Se-Cr angles with x, which were determined by Rietveld refinements. Owing to competing ferromagnetic and antiferromagnetic exchange interactions and disorder, the magnetic ground state of the intercalated materials is characterized by spin-glass or spin-glass-like behavior.     

Cu(2)O nanowires in an alumina template: electrochemical conditions for the synthesis and photoluminescence characteristics.

Cu2O nanowires, mainly consisting of (100) and (200) polycrystalline structures with a length of 4 mum are prepared by electrochemical deposition using a porous alumina template. It is found that the optimized electrochemical conditions to prepare Cu2O nanowires are different from those for the formation of a bulk thin Cu2O layer since different pH values are found between the tip of the pores and the bulk, due to diffusion limits in porous alumina with an extremely high aspect ratio of 300. We point out that Cu2O (200), Cu2O (111), Cu, and co-deposited alloys can be obtained under specific electrochemical conditions. In addition, the optical band gap of the prepared Cu2O nanowires with a length of 4 microm and a diameter of 200 nm is estimated to be 2.17 eV from photoluminescence measurements.     

Indirect Transformation of Coordination‐Polymer Particles into Magnetic Carbon‐Coated Mn3O4 (Mn3O4@C) Nanowires for Supercapacitor Electrodes with Good Cycling Performance

Carbon‐coated Mn₃O₄ nanowires (Mn₃O₄@C NWs) have been synthesized by the reduction of well‐shaped carbon‐coated bixbyite networks and characterized by TEM, X‐ray diffraction, X‐ray photoelectron spectroscopy, and electrochemical experiments. To assess the properties of 1D carbon‐coated nanowires for their use in supercapacitors, cyclic voltammetry and galvanostatic charging–discharging measurements were performed. Mn₃O₄@C NWs could be charged and discharged faster and had higher capacitance than bare Mn₃O₄ nanostructures and other commercial materials. The capacitance of the Mn₃O₄@C NWs was 92 % retained after 3000 cycles at a charging rate of 5 A g^?1. This improvement can be attributed to the carbon shells, which promote fast Faradaic charging and discharging of the interior Mn₃O₄ core and also act as barriers to protect the inner core. These Mn₃O₄@C NWs could be a promising candidate material for high‐capacity, low‐cost, and environmentally friendly electrodes for supercapacitors. In addition, the magnetic properties of the as‐synthesized samples are also reported to investigate the influence of the carbon coating.     

Studies of a molecular hourglass: synthesis and magnetic characterisation of a cyclic dodecanuclear {Cr10Cu2} complex

The synthesis, structure, EPR, and magnetic studies of two dodecanuclear heterometallic cyclic clusters are reported. The compounds have the general formula [R(2)NH(2)](2)[Cr(10)Cu(2)F(14)(O(2)CCMe(3))(22)] (R=Me, 1 or iPr, 2). Both structures contain an array of metal centers which describe an approximate "hourglass", with an ammonium cation in the center of each half of the figure. The chromium sites are all six-coordinate, with the two copper sites five-coordinate. The majority of metal-metal edges are bridged by a single fluoride and two pivalate ligands, while two Cr--Cu edges are bridged by a single fluoride and a single pivalate. Magnetic studies show that 1 and 2 exhibit similar (but not identical) behavior, which can be attributed to ten antiferromagnetic and two ferromagnetic exchange interactions around the ring which gives an S=0 ground state. Quantum Monte Carlo calculations have been used to quantify the exchange interactions by successfully simulating the susceptibility for the full temperature range and thus clarifying the distinction between 1 and 2. EPR spectroscopy shows signals due to excited states, and a variable-temperature study has provided an estimate of the energy gap between the first excited state (S=1) and second excited state (S=2) for 1 that is consistent with the value obtained using the QMC method.     

Thermal oxide synthesis and characterization of Fe_3O_4 nanorods and Fe_2O_3 nanowires

Fe3O4 nanorods and Fe2O3 nanowires have been synthesized through a simple thermal oxide reaction of Fe with C2H2O4 solution at 200-600℃ for 1 h in the air. The morphology and structure of Fe3O4 nanorods and Fe2O3 nanowires were detected with powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The influence of temperature on the morphology development was experimentally investigated. The results show that the polycrystals Fe3O4 nanorods with cubic structure and the av...     

Nanocomposites NiFe2O4/SiO2 and CoFe2O4/SiO2-Preparation by Sol-Gel Method and Physical Properties

This paper aims to characterise the systems NiFe₂O₄/SiO₂ and CoFe₂O₄/SiO₂ prepared by the sol-gel method. After heat treatment, the various samples have been studied by means of X-ray diffraction, Mössbauer spectroscopy, magnetic measurements and transmission electron microscopy (HR TEM).X-ray diffraction and Mössbauer spectra confirmed the presence of the spinel phase. HR TEM observations revealed the nanocrystals with the size in the range of 2–25 nm. Magnetic measurements showed a superparamagnetic behaviour of the samples heated at lower temperature (800°C) and ferrimagnetic character for the samples heated at higher temperature (900, 1000°C).The final phase composition of the heated samples depends on the preparation conditions. The samples, treated up to 300°C in vacuum and then subsequently heated at 800°C or 900°C, do not contain hematite (the most stable phase at higher temperatures). On the contrary, the samples heated at 1000°C or 1250°C display certain content of hematite.     

Framework fluxionality of organometallic oxides: synthesis, crystal structure, EXAFS, and DFT studies on [[Ru(eta6-arene)]4Mo4O16] complexes

Reactions of the molybdates Na(2)MoO4.2 H2O and (nBu(4)N)2[Mo2O7] with [[Ru(arene)Cl(2)](2)] (arene=C(6)H5CH3, 1,3,5-C6H3(CH3)(3), 1,2,4,5-C6H2(CH3)4) in water or organic solvents led to formation of the triple-cubane organometallic oxides [[Ru(eta(6)-arene)](4)Mo4O16], whose crystal and molecular structures were determined. Refluxing triple cubane [[Ru(eta(6)-C6H5CH3)](4)Mo4O16] in methanol caused partial isomerization to the windmill form. The two isomers of [[Ru(eta(6)-C6H5CH3)](4)Mo4O16] were characterized by Raman and Mo K-edge X-ray absorption spectroscopy (XAS), both in the solid-state and in solution. This triple-cubane isomer was also used as a spectroscopic model to account for isomerization of the p-cymene windmill [[Ru(eta(6)-1,4-CH3C6H4CH(CH3)2)](4)Mo4O16] in solution. Using both Raman and XAS techniques, we were then able to determine the ratio between the windmill and triple-cubane isomers in dichloromethane and in chloroform. Density functional calculations on [[Ru(eta(6)-arene)](4)Mo4O16] (arene=C6H6, C6H5CH3, 1,3,5-C6H3(CH3)3, 1,4-CH3C6H4CH(CH3)2, C6(CH3)6) suggest that the windmill form is intrinsically more stable, provided the complexes are assumed to be isolated. Intramolecular electrostatic interactions and steric bulk induced by substituted arenes were found to modulate but not to reverse the energy difference between the isomers. The stability of the triple-cubane isomers should therefore be accounted for by effects of the surroundings that induce a shift in the energy balance between both forms.     

The First Conducting Spin-Crossover Compound Combining a MnIII Cation Complex with Electroactive TCNQ Demonstrating an Abrupt Spin Transition with a Hysteresis of 50 K

We present herein the synthesis, crystal structure, and electric and magnetic properties of the spin-crossover salt [Mn(5-Cl-sal-N-1,5,8,12)]TCNQ_1.5 ⋅ 2 CH₃CN ( I ), where 5-Cl-sal-N-1,5,8,12=N,N′-bis(3-(2-oxy-5-chlorobenzylideneamino)propyl)-ethylenediamine, containing distinct conductive and magnetic blocks along with acetonitrile solvent molecules. The Mn^III complex with a Schiff-base ligand, [Mn(5-Cl-sal-N-1,5,8,12)]⁺, acts as the magnetic unit, and the π-electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ⁻) is the conducting unit. The title compound ( I ) exhibits semiconducting behavior with room temperature conductivity σ_RT≈1×10⁻⁴ ohm⁻¹ cm⁻¹ and activation energy Δ ≈0.20 eV. In the temperature range 73–123 K, it experiences a hysteretic phase transition accompanied by a crossover between the low-spin S=1 and high-spin S=2 states of Mn^III and changes in bond lengths within the MnN₄O₂ octahedra. The pronounced shrinkage of the basal Mn−N bonds in I at the spin crossover suggests that the d orbital is occupied/deoccupied in this transition. Interestingly, the bromo isomorphic counterpart [Mn(5-Br-sal-N-1,5,8,12)]TCNQ_1.5 ⋅ 2 CH₃CN ( II ) of the title compound evidences no spin-crossover phenomena and remains in the high-spin state in the temperature range 2–300 K. Comparison of the chloro and bromo compounds allows the thermal and spin-crossover contributions to the overall variation in bond lengths to be distinguished. The difference in magnetic behavior of these two salts has been ascribed to intermolecular supramolecular effects on the spin transition. Discrete hydrogen bonding exists between cations and cations and anions in both compounds. However, the hydrogen bonding in the crystals of II is much stronger than in I . The relatively close packing arrangement of the [Mn(5-Br-sal-N-1,5,8,12)]⁺ cations probably precludes their spin transformation.     

Preparation of Pt@Fe2O3 nanowires and their catalysis of selective oxidation of olefins and alcohols

Iron oxide coated platinum nanowires (Pt@Fe(2)O(3)NWs) with a diameter of 2.8 nm have been prepared by the oxygen oxidation of FePt NWs in oleylamine. These "cable"-like NWs were characterised by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and X-ray absorption fine structure analysis. These Pt@Fe(2)O(3) NWs were used as "non-support" heterogeneous catalysts in oxidation of olefins and alcohols. The results revealed that it is an active and highly selective catalyst. Styrene derivatives were tested with molecular oxygen as the sole oxidant, with benzaldehyde successfully obtained from styrene in an absolute yield of 31%, whereas the use of tert-butyl hydroperoxide as the sole oxidant in the oxidation of alcohols led to yields of more than 80% of the corresponding ketone or aldehyde. This unsupported catalyst was found to be more active (TOF=96.5 h(-1)) than other reported Fe(2)O(3) nanoparticle catalysts and could be recycled multiple times without any notable decrease in activity. Our findings will extend the use of such nanomaterial catalysts to new catalytic systems.     

Optical,thermal, and magnetic properties of a microporous fluorinated iron phosphate: (H3O)2[Fe4(H2O)2F4(PO4)2(HPO4)2](H2O)

The non-centrosymmetric microporous fluorinated iron phosphate, (H₃O)₂[Fe₄(H₂O)₂F₄(PO₄)₂(HPO₄)₂](H₂O), is endowed with properties. In fact, the thermogravimetric analysis study shows a mass loss evolution as a temperature function. The optical study was also examined by UV–vis absorption. The magnetic results reveal the appearance of a ferromagnetic behavior at low temperature (Tc = 11.64 K).