Reduced Oxidation States Research Articles

Thermodynamic investigation of reduced barium molybdates

Precise identification and thermodynamic assessment of the various phases in spent nuclear fuel are necessary for the accurate dynamic modeling of nuclear fuel. The Thermodynamics of Advanced Fuels – International Database (TAF-ID) compiles high-quality, self-consistent, and reliable thermodynamic data of various phases in chemical systems of interest for the international nuclear industry, such as the Ba-Mo-O ternary system. The thermodynamic properties of compounds in the Ba-Mo-O ternary system, especially those of hexavalent Mo, have been extensively studied and included in computational thermodynamic assessments. In contrast, compounds with Mo in reduced oxidation states, such as BaMo6O10 and Ba3Mo18O28, have been excluded from such assessments due to a lack of experimentally assessed thermodynamic data. In this work, high-quality samples of the aforementioned compounds were synthesized, and the first experimental assessment of their heat capacities in the temperature range 300–600 K were obtained. Furthermore, a reversible phase transition was identified at ∼380 K for BaMo6O10. The collected experimentally-assessed thermodynamic data from this work and initial calculated estimations of standard enthalpies of formation and entropies for these compounds were successfully integrated into a preliminary, updated Ba-Mo-O thermodynamic assessment using the CALPHAD method. Key thermodynamic properties of the newly included compounds (i.e., standard enthalpy of formation and entropy) were calculated using the new Ba-Mo-O assessment.

Spontaneous Reduction of Transition Metal Ions by One Electron in Water Microdroplets and the Atmospheric Implications

Freshman chemistry teaches that Fe3+ and Cu2+ ions are stable in water solutions, but their reduced forms, Fe2+ and Cu+, cannot exist in water as the major oxidation state due to the fast oxidation by O2 and/or disproportionation. Contrary to these well-known facts, significant fractions of dissolved Fe and Cu species exist in their reduced oxidation states in atmospheric water such as deliquesced aerosols, clouds, and fog droplets. Current knowledge attributes these phenomena to the stabilization of the lower oxidation states by the complexation of ligands and the various photochemical or thermal pathways that can reduce the higher oxidation states. In this study, by spraying the water solutions of transition metal ions into microdroplets, we show the results of the spontaneous reduction of ligated Fe(III) and Cu(II) species into Fe(II) and Cu(I) species, presenting a previously unknown source of reduced transition metal ions in atmospheric water. It is the spontaneously generated electrons in water microdroplets that are responsible for the reduction. Control experiments in the atmosphere and in a glove box filled with precisely controlled gaseous contents reveal that O2, CO2, and NO2 are the major competitors for the electrons, forming O2-, HCO2-, and NO2-, respectively. Taking these findings together, we opine that microdroplet chemistry might play significant but previously underestimated roles in atmospheric redox chemistry.

Determinants of substrate specificity in a catalytically diverse family of acyl-ACP thioesterases from plants

BackgroundACYL-LIPID THIOESTERASES (ALTs) are a subclass of plastid-localized, fatty acyl-acyl carrier protein (ACP) thioesterase enzymes from plants. They belong to the single hot dog-fold protein family. ALT enzymes generate medium-chain (C6-C14) and C16 fatty acids, methylketone precursors (β-keto fatty acids), and 3-hydroxy fatty acids when expressed heterologously in E. coli. The diverse substrate chain-length and oxidation state preferences of ALTs set them apart from other plant acyl-ACP thioesterases, and ALTs show promise as metabolic engineering tools to produce high-value medium-chain fatty acids and methylketones in bacterial or plant systems. Here, we used a targeted motif-swapping approach to explore connections between ALT protein sequence and substrate specificity. Guided by comparative motif searches and computational modelling, we exchanged regions of amino acid sequence between ALT-type thioesterases from Arabidopsis thaliana, Medicago truncatula, and Zea mays to create chimeric ALT proteins.ResultsComparing the activity profiles of chimeric ALTs in E. coli to their wild-type counterparts led to the identification of interacting regions within the thioesterase domain that shape substrate specificity and enzyme activity. Notably, the presence of a 31-CQH[G/C]RH-36 motif on the central α-helix was shown to shift chain-length specificity towards 12–14 carbon chains, and to be a core determinant of substrate specificity in ALT-type thioesterases with preference for 12–14 carbon 3-hydroxyacyl- and β-ketoacyl-ACP substrates. For an ALT containing this motif to be functional, an additional 108-KXXA-111 motif and compatible sequence spanning aa77–93 of the surrounding β-sheet must also be present, demonstrating that interactions between residues in these regions of the catalytic domain are critical to thioesterase activity. The behaviour of chimeric enzymes in E. coli also indicated that aa77–93 play a significant role in dictating whether an ALT will prefer ≤10-carbon or ≥ 12-carbon acyl chain-lengths, and aa91–96 influence selectivity for substrates of fully or partially reduced oxidation states. Additionally, aa64–67 on the hot dog-fold β-sheet were shown to be important for enabling an ALT to act on 3-hydroxy fatty acyl-ACP substrates.ConclusionsBy revealing connections between thioesterase sequence and substrate specificity, this study is an advancement towards engineering recombinant ALTs with product profiles suited for specific applications.

Defective Metal‐Organic Frameworks with Tunable Porosity and Metal Active Sites for Significantly Improved Performance in Styrene Oxidation

Metal active sites and sufficient porosity in metal-organic frameworks (MOFs) are crucial parameters determining the performances of catalysis, guest molecule adsorption, etc. Herein, through in situ introduction of Ru sites with different levels to Cu-BTC structure together with post-synthetic activation at 180 °C, a series of hierarchically porous CuRu-BTC (HP-CuRu-BTC) MOFs were obtained. Besides, selective thermal decomposition (STD) treatment was carried out at 240 °C to further tune the hierarchical pores and metal sites, yielding rare case of metal nanoparticles (NPs)@HP-CuRu-BTC composites. After full characterization by XRD, N2 physisorption, SEM, ICP and XPS, these HP-CuRu-BTC and NPs@HP-CuRu-BTC samples possess high surface area (682-1199 m2 g-1 ), hierarchical pores and highly distributed metal sites with reduced oxidation states (Cu+ and Ru2+ ), indicating regulation of both metal sites and hierarchical pores. The HP-CuRu-BTC and NPs@HP-CuRu-BTC were further employed as catalysts for the heterogeneous styrene oxidation reaction under mild condition. Compared to microporous Cu-BTC with unary metal component, HP-CuRu-BTC-3 and NPs@HP-CuRu-BTC-3 exhibited more than 2 times higher styrene conversion after 7 hours reaction under same condition.

Enhanced hydrogen adsorption properties of mesoporous nano-TiO2@SnO2

Today, hydrogen as being one of the most advantageous energy supplies for various applications and specifically not with carbon emissions, alluring strategies are made possible for its production and storage. Since the metal oxides (MOs) semiconductors give rise to the development of new technologies related to renewable energy and environmental issues, this work is envisioned to study the hydrogen adsorption properties of TiO2@SnO2 catalyst synthesized via novel approach by integrating sol–gel and thermal decomposition methods. The synthesized TiO2 and its composite have been analyzed by sophisticated instruments to reveal the drastic enhancement of hydrogen adsorption. In this work, hydrogen measurements were accomplished by quartz crystal (QC) microbalance method. The Raman scattering experiment of the composite revealed the lower peak intensity compared with pure TiO2, which demonstrated the surface defects that created oxygen vacancies with reduced oxidation states of the metal centers. Meanwhile, the mesoporous nanostructure of TiO2@SnO2 could be confirmed via High resolution transmission electron (HR-TEM) microscopic analysis. Besides, the intermediate states of the composite were analyzed through Photoluminescence (PL) spectra which demonstrated the delay in recombining charges. The Barrett-Joyner- Halenda (BJH) method illustrated the larger pore size and pore volume with decreased surface area of the composite. The addition of SnO2 into TiO2 has reported 4 times greater the adsorption of pristine TiO2 particles, because of the capacity of SnO2 to hinder pores. Moreover, the titania oxidation states play a predominant role in the procurement of larger H2 adsorption. Also, the Ti4+ and Sn4+ reveal fragile Kubas type of adsorption that facilitated the hydrogen storage.

Solubility of uranium oxide in ternary aluminosilicate glass melts

Uranium solubility was measured in melts belonging to the CaO-Al2O3-SiO2 (CAS) and MgO-Al2O3-SiO2 (MAS) systems using the Pt wire loop technique, enabling independent control of the temperature (1400 °C), glass composition, and oxygen fugacity (-16.1<log(fO2)<-0.7). The low sample masses allowed the equilibrium state to be reached quickly and rapid quenching of the glasses was performed in order to immobilize each system as close as possible to the molten state. The compositions of the different quenched glasses were analyzed by EDS. Uranium solubility decreased with decreasing oxygen fugacity, highlighting the lower solubility of uranium at reduced oxidation states. For each system, uranium solubility was constant from log(fO2)<-9.7. Moreover, different uranium behavior was evidenced between the two ternary systems. Modification of the Al content affected only uranium solubility in the CAS compositions, while uranium volatilization for oxidizing conditions was noted in the MAS system. This difference in behavior may be attributed to structural changes and probably to the variable proportions of [5]Al in each glass system.

Toward an energy-efficient synthesis method to improve persistent luminescence of Sr2MgSi2O7:Eu2+,Dy3+ materials

The synthesis of persistent luminescent materials usually requires a multi-step long time annealing at high temperatures (>1200°C) in a resistive oven, causing a huge energy consumption. Also, to achieve reduced oxidation states of emitter ions (e.g., Eu3+ → Eu2+ ), the H2(g) atmosphere is often used, which can be dangerous and increase the costs of the process. Therefore, the development of a quick and new single-step green strategy, using in-situ low-risk atmosphere (e.g., CO(g)) and a microwave-assisted solid-state (MASS) method has been encouraged. In this work, we present a single-step method to synthesize the compound Sr2MgSi2O7:Eu2+,Dy3+ using the MASS method and the results were compared with those prepared by a conventional ceramic method. The luminescent material was prepared in 25 min of synthesis using carbon as a microwave susceptor and CO(g) atmosphere source at the same time. A higher concentration of Eu2+ emitter was identified by XANES in the MASS method product, which has a significant effect on the luminescence efficiency, as well as an improvement in the optical properties, leading to an emission 100 times more intense. Furthermore, to understand the Eu3+ reduction process under CO(g) atmosphere, we present here the innovative results of in-situ XANES analysis for the Sr2MgSi2O7:Eu2+,Dy3+ material. Finally, the MASS method makes it possible to prepare the materials with less than 5% of the ceramic method's duration in time. The energy-saving and better-quality persistent luminescent properties obtained in the MASS method provide viable applications on anti-counterfeiting markers, solar cell sensitizers, and other luminescent technologies.

Facile synthesis of novel, known, and low-valent transition metal phosphates via reductive phosphatization

Novel and known low valent transition metal phosphates (TMPs) are accessible via a novel and facile pathway denoted as reductive phosphatization. The method allows syntheses of TMPs especially with reduced oxidation states.

Sources of parasitic features in the visible range of oxide transparent ceramics absorption spectra

AbstractThis research focused on the identification of parasitic signal sources introduced by sintering under reductive atmospheres of oxide transparent ceramics. The examination encompassed a wide set of materials, like Mg‐spinel, magnesia, alumina. YAG, yttria, zirconia and titania, so as to allow one to detect general rules if operating. The main finding is the dependence between the ability of such ceramics to generate, under reductive conditions, intrinsic parasitic light‐absorbing centers and their composition and structure. Thus it was determined that oxides based on transition element cations have ,in this sense, abilities not present in the case of oxides of elements from the main periodic table groups. The reason is the possibility of non‐absorbing transition cations to be reduced to oxidation states that absorb light. Drive toward reduced oxidation states is motivated by the stabilization energy associated with the ligand fields acting upon the such cations. It has been also found out that YAG lattice distortion, by dopants like tetravalent silicon or zirconium facilitates reduction of Y 3+ to Y 2 + (a light absorbing, d1, cation).Oxygen vacancies do not significantly influence visible light absorption. Carbon atoms penetrate inside ceramics, even dense ones and cause achromatic tints formation.

New evidences in CO oxidation and selective chemisorption of carbon oxides on different alkaline ferrite crystal phases (NaFeO2 and LiFeO2)

Different thermogravimetric and CO oxidation analyses were performed on sodium and lithium ferrites (NaFeO2 and LiFeO2) to study CO-CO2 sorption selectivity and mechanism, in presence and absence of oxygen. In both phases, CO is chemisorbed producing the corresponding alkaline carbonate and secondary iron phases with reduced oxidation states. In all the cases, NaFeO2 showed better sorption and oxidation properties than the Li-based ferrite. The sorption and catalytic behavior observed between the alkaline ferrites studied are associated to the crystal structures and alkali composition. Although Li and Na chemical affinity to these gases is similar, their availability changes based on the ferrite crystal structure. Additionally, on CO oxidation properties, the ferrite with tetrahedrally coordinated Fe3+ ions presented better efficiency against CO oxidation than the ferrite with octahedrally coordinated Fe3+ ions.

Speciation of arsenic and antimony in basaltic magmas

This study applies X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structures (EXAFS) spectroscopy at the K-edge to determine the speciation of arsenic and antimony in a suite of basaltic glasses synthesized over a range of oxygen fugacity (fO2) at 1200 °C and 0.1 MPa. Experiments were executed in evacuated fused silica ampoules using a variety of solid metal metal-oxide buffers to achieve fO2’s ranging from FMQ −3.3 to FMQ +5.7 (where FMQ is the fayalite magnetite quartz buffer). The oxidation state was calculated using linear combination fitting (LCF) to spectral reference material with known oxidation states using the XANES spectra. Speciation results were corrected for the quench effect of iron. Trivalent arsenic and antimony were determined to be the dominant oxidation state in the samples, with pentavalent arsenic contributing to less than 10% of the budget of arsenic unless the fO2 is greater than FMQ +5.3 ± 0.9, while pentavalent antimony was not observed. Additionally, no reduced oxidation states of arsenic or antimony were found in the glasses even at the lowest fO2 investigated. Structural parameters such as the coordination number and bond length were determined by fitting theoretical electron scattering paths to the EXAFS spectra. Arsenic is coordinated by three oxygens at 1.78 ± 0.01 Å forming AsIIIO3E (where E is the lone pair of electrons) trigonal pyramids. Antimony is coordinated by three oxygens at 1.98 ± 0.01 Å, interpreted to be in a trigonal pyramid structure similar to As3+. As both metalloids are primarily present in the trivalent state over the range of terrestrial fO2 (FMQ −3 to FMQ +5) it is expected that each will behave incompatibly during basaltic melting or crystallization, owing to (1) the tendency for these elements to form oxyanions, and (2) their poor match in ionic radius and charge for the major cation sites in mafic minerals.

An Infrared Spectroscopic Study Toward the Formation of Alkylphosphonic Acids and Their Precursors in Extraterrestrial Environments.

The only known phosphorus-containing organic compounds of extraterrestrial origin, alkylphosphonic acids, were discovered in the Murchison meteorite and have accelerated the hypothesis that reduced oxidation states of phosphorus were delivered to early Earth and served as a prebiotic source of phosphorus. While previous studies looking into the formation of these alkylphosphonic acids have focused on the iron-nickel phosphide mineral schreibersite and phosphorous acid as a source of phosphorus, this work utilizes phosphine (PH3), which has been discovered in the circumstellar envelope of IRC +10216, in the atmosphere of Jupiter and Saturn, and believed to be the phosphorus carrier in comet 67P/Churyumov-Gerasimenko. Phosphine ices prepared with interstellar molecules such as carbon dioxide, water, and methane were subjected to electron irradiation, which simulates the secondary electrons produced from galactic cosmic rays penetrating the ice, and probed using infrared spectroscopy to understand the possible formation of alkylphosphonic acids and their precursors on interstellar icy grains that could become incorporated into meteorites such as Murchison. We present the first study and results on the possible synthesis of alkylphosphonic acids produced from phosphine-mixed ices under interstellar conditions. All functional groups of alkylphosphonic acids were detected through infrared spectroscopically, suggesting that this class of molecules can be formed in interstellar ices.

A forming-free bipolar resistive switching behavior based on ITO/V2O5/ITO structure

Forming-free bipolar resistive switching behavior in an ITO/V2O5/ITO structure is observed. While the bottom ITO layer functions as a common ground electrode, the top ITO layer is an active element and used as an oxygen reservoir, with an additional metal electrode patterned on its top for making contact. In contrast to typical metal/transition metal oxide/metal based resistive memories, our device exhibits a low resistance state in its virgin state and is switched to a high resistance state when a forward bias of ∼+2.5 V is applied. The device can be reset to its original state at a reverse bias of ∼–1.5 V. A noticeable decrease in switching voltage with a reduced top contact area is observed, indicating a strong electric field enhanced switching mechanism. Different from the widely seen conductive filament mechanism in bipolar switching, we explain the switching behavior by the migration of oxygen ions at the top ITO/V2O5 interface. When oxygen ions are extracted to the ITO side, an interfacial layer with reduced oxidation states is formed and acts as a Schottky barrier that suppresses the current through the whole device. The results suggest future applications in low power, high speed integrated non-volatile memories.

Ti3+-, V2+/3+-, Cr2+/3+-, Mn2+-, and Fe2+-Substituted MOF-5 and Redox Reactivity in Cr- and Fe-MOF-5

The metal nodes in metal-organic frameworks (MOFs) are known to act as Lewis acid catalysts, but few reports have explored their ability to mediate reactions that require electron transfer. The unique chemical environments at the nodes should facilitate unusual redox chemistry, but the difficulty in synthesizing MOFs with metal ions in reduced oxidation states has precluded such studies. Herein, we demonstrate that MZn3O(O2C-)6 clusters from Zn4O(1,4-benzenedicarboxylate)3 (MOF-5) serve as hosts for V(2+) and Ti(3+) ions and enable the synthesis of the first MOFs containing these reduced early metal ions, which can be accessed from MOF-5 by postsynthetic ion metathesis (PSIM). Additional MOF-5 analogues featuring Cr(2+), Cr(3+), Mn(2+), and Fe(2+) at the metal nodes can be obtained by similar postsynthetic methods and are reported here for the first time. The inserted metal ions are coordinated within an unusual all-oxygen trigonal ligand field and are accessible to both inner- and outer-sphere oxidants: Cr(2+)- converts into Cr(3+)-substituted MOF-5, while Fe(2+)-MOF-5 activates NO to produce an unusual Fe-nitrosyl complex.

Surface, structural and catalytic properties of heteropolymolybdate supported on vanadia dispersed alumina catalysts

12-Molybdophosphoric acid (MPA) supported on V2O5 dispersed γ-Al2O3 catalysts with different loadings were prepared and characterized by BET surface area, X-ray diffraction, FT-infrared, laser Raman, X-ray photoelectron spectroscopy and temperature programmed reduction techniques. The catalytic activities were evaluated for the aerobic oxidation of benzyl alcohol. The conversion of benzyl alcohol increased with the amount of MPA content and the catalyst with 15 wt% of MPA showed highest activity. The synergistic effect of V2O5 and MPA was observed for the oxidation of benzyl alcohol compared to MPA on alumina without V2O5. The XPS results suggest the participation of both Mo and V in the reaction as the used catalysts showed the reduced oxidation states of both Mo and V. The high selectivity of the catalysts is due to the presence of V2O5, which induces the redox nature to the catalyst and also preventing the decomposition of MPA on Al2O3.

Novel Pyridazine Based Scorpionate Ligands in Cobalt and Nickel Boratrane Compounds

Heating of 6-methylpyridazine-3-thione (HPn(Me)) and 6-tert-butylpyridazine-3-thione (HPn(tBu)) with potassium borohydride in diphenylmethane in a 3:1 ratio gave two new scorpionate ligands K[HB(Pn(Me))(3)] and K[HB(Pn(tBu))(3)]. Single crystal X-ray diffraction analysis of the methyl derivative K[HB(Pn(Me))(3)] revealed a dimeric species with one potassium atom coordinated by six sulfur atoms of two scorpionate ligands and a second potassium atom coordinated by three nitrogen atoms of one of the two ligands as well as by three water molecules. The reaction of K[HB(Pn(tBu))(3)] with nickel(II) chloride or cobalt(II) chloride in CH(2)Cl(2) led to the new boratrane compounds [M{B(Pn(tBu))(3)}Cl] (M = Ni 1, Co 3) where a formal reduction of the metal ions to Ni(I) and Co(I), respectively, and activation of the B-H bond occurred. Similar reactivity was observed by employing K[HB(Pn(R))(3)] (R = Me, tBu) and nickel(II) chloride in water. Reaction with cobalt(II) chloride in water also gave boratrane compounds [Co{B(Pn(R))(3)}(Pn(R))] (R = tBu 4, Ph 5), but instead of a chloride a bidentate pyridazinethionate ligand from a defragmentated scorpionate is found in the molecules. The molecular structures of all nickel and cobalt compounds were determined by single crystal X-ray diffraction analyses confirming the formation of boratranes in compounds 1-5. Magnetic measurements confirm the reduced oxidation states and the paramagnetic character of the Ni(I) and Co(I) complexes. Supportive DFT studies were carried out for a better understanding of the electronic nature of the metal-boron bond of the boratrane complexes.

Anthropogenic and climate influences on biogeochemical dynamics and molecular‐level speciation of soil sulfur

The soil environment is a primary component of the global biogeochemical sulfur (S) cycle, acting as a source and sink of various S species and mediating oxidation state changes. However, ecological significance of the various S forms and the impacts of human intervention and climate on the amount and structural composition of these compounds are still poorly understood. We investigated the long-term influences of anthropogenically mediated transitions from natural to managed ecosystems on molecular-level speciation, biogeochemical dynamics, and the apparent temperature sensitivity of S moieties in temperate, subtropical, and tropical environments with mean annual temperature (MAT) ranging from 5 degrees C to 21 degrees C, using elemental analysis and X-ray absorption near-edge structure (XANES) spectroscopy. Land-use and land-cover changes led to the depletion of total soil S in all three ecoregions over a period of up to 103 years. The largest decline occurred from tropical forest agroecosystems (67% Kakamega and 76% Nandi, Kenya), compared to losses from temperate (36% at Lethbridge, Canada, and 40% at Pendleton, USA) and subtropical (48% at South Africa) grassland agroecosystems. The total S losses correlated significantly with MAT. Anthropogenic interventions profoundly altered the molecular-level composition and resulted in an apparent shift in oxidation states of organic S from native ecosystems composed primarily of S moieties in intermediate and highly reduced oxidation states toward managed agroecosystems dominated by organic S rich in strongly oxidized functionalities. The most prominent change occurred in thiols and sulfides, the proportion of which decreased by 46% (Lethbridge) and 57% (Pendleton) in temperate agroecosystems, by 46% in subtropical agroecosystems, and by 79% (Nandi) and 81% (Kakamega) in tropical agroecosystems. The proportion of organic S directly linked to O increased by 81%, 168%, 40%, 92%, and 85%, respectively. Among the various organic S functionalities, thiols and sulfides seem to have higher apparent temperature sensitivity, and thus these organic S moieties may become prone to losses due to land-use changes, even from the cooler regions of the world if MAT of these regions rise in the future.

Effect of oxygen vacancies on ferroelectric behavior of Na 1/2Bi 1/2TiO 3–BaTiO 3 single crystals

One of the important lead-free ferroelectric solid solutions, sodium bismuth titanate–barium titanate Na 1/2Bi 1/2TiO 3–BaTiO 3 (NBT–BT) was grown by the conventional flux technique. In order to study the role of oxygen vacancies on the dielectric/ferroelectric properties, some of the crystal samples oriented along (0 0 1) and (1 0 0) planes were subjected to oxygen and nitrogen annealing processes to create different concentrations of oxygen vacancies in the samples. Dielectric and its loss measurements were carried out to analyze the role of oxygen vacancies and their corresponding dielectric behavior on NBT–BT crystals. Electron energy loss spectrum (EELS) has revealed that increasing oxygen vacancies has reduced oxidation states of Ti. X-ray rocking curve analysis has confirmed the degradation in the structural quality also on increasing the oxygen vacancies.

Evolution of the Structural Chemistry of Vanadium Organodiphosphonate Networks and Frameworks: Structural Consequences of Fluoride Incorporation in the Development of Stable Phases with Void Channels

Hydrothermal reactions of solutions containing a vanadate source, an organodiphosphonate, an organonitrogen component, and HF (V/P/O/F) yield a series of oxyfluorovanadium-diphosphonates with charge-compensation provided by organoammonium cations or hydronium cations. While V/P/O/F networks provide the recurrent structural motif, the linkage between the layers and the details of the polyhedral connectivities within the layers are quite distinct for the five structures of this study. [H2pip][V4F4O2(H2O)2{O3P(CH2)3PO3}2] (1) (pip = piperazine) is a conventional three-dimensional (3D) "pillared" layer structure, whose V/P/O/F networks are buttressed by the propylene chains of the diphosphonate ligands. In contrast, [H2en][V2O2F2(H2O)2{O3P(CH2)4PO3}] (2) and [H2en]2[V6F12(H2O)2{O3P(CH2)5PO3}2 {HO3P(CH2)5PO3H}] (3) are two-dimensional (2D) slablike structures constructed of pairs of V/P/O/F networks sandwiching the pillaring organic tethers of the diphosphonate ligands. Despite the common overall topology, the layer substructures are quite different: isolated {VO5F} octahedra in 2 and chains of corner-sharing {VO(3)F(3)} octahedra in 3. The 3D structure of [H2en]2[V7O6F4(H2O)2{O3P(CH2)2PO3}4].7H2O (4.7H2O) exhibits a layer substructure that contains the ethylene bridges of the diphosphonate ligands and are linked through corner-sharing octahedral {VO6} sites. The connectivity requirements provide large channels that enclose readily removed water of crystallization. The structure of [H3O][V3F2(H2O)2{O3P(CH2)2PO3}2].3.5H2O (5.3.5H2O) is also 3D. Because of the similiarity with 4.7H2O, it exhibits V/P/O/F layers that include the organic tethers of the diphosphonates and are linked through corner-sharing {VO6} octahedra. In contrast to the network substructure of 4.7H2O, which contains binuclear and trinuclear vanadium clusters, the layers of 5.3.5 H2O are constructed from chains of corner-sharing {VO4F2} octahedra. Thermal studies of the open framework materials 4 and 5 reveal that incorporation of fluoride into the inorganic substructures provides robust scaffoldings that retain their crystallinity to 450 degrees C and above. In the case of 4, dehydration does not change the powder X-ray diffraction pattern of the material, which remains substantially unchanged to 450 degrees C. In the case of 5, there are two dehydration steps, that is, the higher temperature process associated with loss of coordinated water. This second dehydration results in structural changes as monitored by powder X-ray diffraction, but this new phase is retained to ca. 450 degrees C. The materials of this study exhibit a range of reduced oxidation states: 1 is mixed valence V(IV)/V(III) while 2 and 4.7H(2)O are exclusively V(IV) and 3 and 5.3.5H2O are exclusively V(III). These oxidation states are reflected in the magnetic properties of the materials. The paramagnetism of 1 arises from the presence of V(III) and V(IV) sites and conforms to the Curie-Weiss law with C = 2.38 em K/(Oe mol) and = -66 K with mu(eff) (300 K) = 4.33 mu(B). Compounds 3-5 exhibit Curie-Weiss law dependence of magnetism on temperature with mu(eff) (300 K) = 5.45 mu(B) for 3 (six V(III) sites), mu(eff) = 4.60 mu(B) for 4 (seven V(IV) sites) and mu(eff) = 4.13 mu(B) for 5 (two V(III) sites). Compound 2 exhibits antiferromagnetic interactions, and the magnetism may be described in terms of the Heisenberg linear antiferromagnetic chain model for V(IV). The effective magnetic moment at 300 K is 2.77 mu(B) (two V(IV) sites).

Theoretical study on the electronic structure and properties of synthetic MoFe3S3 compounds*

AbstractWe examine the electronic structure of a series of MoFe3S3 compounds whose spectroscopic and structural parameters have been recently published (J. Han, K. Beck, N. Ockwig, D. Coucouvanis, J Am Chem Soc 1999, 121, 10448). The synthetic Mo/Fe/S compounds under examination are close structural and functional analogs of essential parts of the FeMo‐cofactor in the enzyme nitrogenase. Using the ZINDO program and the projected unrestricted Hartree–Fock (PUHF) method, we have demonstrated that these compounds exhibit antiferromagnetic coupling between the metal atoms, with calculated triplet and singlet ground states. The metal atoms are found in reduced oxidation states, namely Mo(III), Fe(II), and Fe(I), in agreement with the experimental isomer shifts and the Mössbauer data previously reported. The addition of carbonyl groups to the MoFe3S3 clusters leads to a withdrawal of electron density from the Mo/Fe/S core and thus affects the strength of the metal–metal and metal–sulfur bonds in the compounds. We also show that the removal of pyridine from the coordination of the Mo atom has a notable effect on the electron delocalization in the Mo/Fe/S core and may lead to preferential localization of the electron density on the iron atoms. © 2001 John Wiley &amp; Sons, Inc. Int J Quantum Chem, 2001