Mössbauer spectroscopic study of an iron(III) complex with crown porphyrin

A complex of ⁵⁷Fe with 5-{4-[((4′-hydroxybenzo-15-crown-5)-5′-yl)diazo]phenyl}-10,15,20-triphenylporphyrin was studied by Mössbauer spectroscopy. The presence of signals of two types in the spectra (a doublet and an extended absorption band over a wide velocity range) suggests that the Fe atoms occupy two structurally different positions in this complex. The dependences of the doublet asymmetry on temperature and the angle between the normal to the sample plane and the γ-ray beam were studied. The isomer shift δ of the doublet in the temperature range from 360 to 5 K changes from 0.25 to 0.41 mm/s, while the quadrupole splitting remains virtually unchanged (Δ ≈ 0.65 mm/s). The relaxation-type absorption over a wide velocity range, the relative area of which strongly varies with temperature, can be described by a broad singlet with the following parameters: δ = 0.30–0.44 mm/s and Γ = 2.8–3.38 mm/s. According to the δ values, both signals are due to Fe(III) derivatives.     

First example of a Cu(I)-(η2-O,O)nitrite complex derived from Cu(II)-nitrosyl

Copper(II) complex, 1, of the bidentate ligand, L [L = bis(2-ethyl-4-methyl-imidazol-5yl)methane] has been synthesized and structurally characterized. Addition of nitric oxide gas to a degassed acetonitrile solution of 1 yielded the corresponding copper(ii)-nitrosyl complex, 2. In acetonitrile, complex 2 on reaction with water afforded the corresponding copper(I)-nitrite complex, 3. Single crystal structure of complex 3 reveals the bidentate nitrite (η(2)-O,O) bonding. This is the first example of a structurally characterized Cu(I)-(η(2)-O,O)nitrite complex with N-donor ligand. The sequence of the formation of these complexes is just the reverse of the key steps of the postulated nitrite reduction cycle by CuNiRs.     

A saccharinate–iron(II) complex with a free saccharin molecule present, [Fe(phen)3]sac2·sacH·6H2O

The crystal structure of the title compound, tris(1,10-phenanthroline-N,N′)­iron(II) bis­[1,2-benziso­thia­zol-3(2H)-onate 1,1-dioxide] 1,2-benziso­thia­zol-3(2H)-one 1,1-dioxide hexahydrate, [Fe(C₁₂H₈N₂)₃](C₇H₄NO₃S)₂·C₇H₅NO₃S·6H₂O, at 120 K consists of slightly distorted octahedral [Fe(phen)₃]²⁺ cations (phen is 1,10-phenanthroline), two saccharinate anions and a free saccharin mol­ecule, as well as six waters of crystallization. The compound has been confirmed as diamagnetic low-spin iron(II) by magnetic measurements. There is extensive hydrogen bonding leading to a three-dimensional network.     

A theoretical study of an unusual Y-shaped three-coordinate Pt complex: Pt(0) σ-disilane complex or Pt(II) disilyl complex?

The unusual Y-shaped structure of the recently reported three-coordinate Pt complex Pt[NHC(Dip)(2)](SiMe(2)Ph)(2) (NHC = N-heterocyclic carbene; Dip = 2,6-diisopropylphenyl) was considered a snapshot of the reductive elimination of disilane. A density functional theory study indicates that this structure arises from the strong trans influence of the extremely σ-donating carbene and silyl ligands. Though this complex can be understood to be a Pt(II) disilyl complex bearing a distorted geometry due to the Jahn-Teller effect, its (195)Pt NMR chemical shift is considerably different from those of Pt(II) complexes but close to those of typical Pt(0) complexes. Its Si···Si bonding interaction is ~50% of the usual energy of a Si-Si single bond. The interaction between the Pt center and the (SiMe(2)Ph)(2) moiety can be understood in terms of donation and back-donation interactions of the Si-Si σ-bonding and σ*-antibonding molecular orbitals with the Pt center. Thus, we conclude that this is likely a Pt(0) σ-disilane complex and thus a snapshot after a considerable amount of the charge transfer from disilane to the Pt center has occurred. Phenyl anion (Ph(-)) and [R-Ar](-) [R-Ar = 2,6-(2,6-iPr(2)C(6)H(3))(2)C(6)H(3)] as well as the divalent carbon(0) ligand C(NHC)(2) also provide similar unusual Y-shaped structures. Three-coordinate digermyl, diboryl, and silyl-boryl complexes of Pt and a disilyl complex of Pd are theoretically predicted to have similar unusual Y-shaped structures when a strongly donating ligand coordinates to the metal center. In a trigonal-bipyramidal Ir disilyl complex [Ir{NHC(Dip)(2)}(PH(3))(2)(SiMe(3))(2)](+), the equatorial plane has a similar unusual Y-shaped structure. These results suggest that various snapshots can be shown for the reductive eliminations of the Ge-Ge, B-B, and B-Si σ-bonds.     

The atomic structural dynamics of γ-Al2O3 supported Ir-Pt nanocluster catalysts prepared from a bimetallic molecular precursor: a study using aberration-corrected electron microscopy and X-ray absorption spectroscopy

This study describes a prototypical, bimetallic heterogeneous catalyst: compositionally well-defined Ir-Pt nanoclusters with sizes in the range of 1-2 nm supported on γ-Al(2)O(3). Deposition of the molecular bimetallic cluster [Ir(3)Pt(3)(μ-CO)(3)(CO)(3)(η-C(5)Me(5))(3)] on γ-Al(2)O(3), and its subsequent reduction with hydrogen, provides highly dispersed supported bimetallic Ir-Pt nanoparticles. Using spherical aberration-corrected scanning transmission electron microscopy (C(s)-STEM) and theoretical modeling of synchrotron-based X-ray absorption spectroscopy (XAS) measurements, our studies provide unambiguous structural assignments for this model catalytic system. The atomic resolution C(s)-STEM images reveal strong and specific lattice-directed strains in the clusters that follow local bonding configurations of the γ-Al(2)O(3) support. Combined nanobeam diffraction (NBD) and high-resolution transmission electron microscopy (HRTEM) data suggest the polycrystalline γ-Al(2)O(3) support material predominantly exposes (001) and (011) surface planes (ones commensurate with the zone axis orientations frequently exhibited by the bimetallic clusters). The data reveal that the supported bimetallic clusters exhibit complex patterns of structural dynamics, ones evidencing perturbations of an underlying oblate/hemispherical cuboctahedral cluster-core geometry with cores that are enriched in Ir (a result consistent with models based on surface energetics, which favor an ambient cluster termination by Pt) due to the dynamical responses of the M-M bonding to the specifics of the adsorbate and metal-support interactions. Taken together, the data demonstrate that strong temperature-dependent charge-transfer effects occur that are likely mediated variably by the cluster-support, cluster-adsorbate, and intermetallic bonding interactions.     

CHF3…H2O complex revisited: a matrix isolation and ab initio study

Structural Chemistry - The structural and spectroscopic features of the CHF3…H2O complex have been investigated using high-level ab initio calculations and IR matrix isolation spectroscopy....     

A sugar's choice: coordination to a mononuclear or a dinuclear copper(II) complex?

We proposed a decisive role of the number of metal ions at the sugar binding site for carbohydrate-coordinating copper(II) complexes. To verify this hypothesis, we studied the binding of the representatively chosen carbohydrates D-ribose (7), D-mannose (8), D-glucose (9), and D-maltose (10) to structurally related mono- and dinuclear copper(II) complexes in alkaline solution. All carbohydrates coordinate to the metal complexes in a 1:1 molar ratio. Coordination of 7 or 8 to the dinuclear copper(II) complex 1 is about 0.5 order of magnitude stronger than the complex formation with related mononuclear complexes. On contrast, 9, which is an epimer of 8, coordinates stronger to either one of the mononuclear copper(II) complexes in alkaline aqueous solution.     

Synthesis,structure, and Mössbauer study of [Fe(H2O)(2)(C(9)O(6)H4)].H2O: a two-dimensional iron(II) trimellitate (MIL-67)

The two-dimensional (2D) iron trimellitate [Fe(H(2)O)(2)(C(9)O(6)H(4))].H(2)O, labeled MIL-67, has been obtained under hydrothermal conditions (473 K, 48 h). In the 2D structure of MIL-67, the Fe(2+) ions display two different octahedral environments: [FeO(4)(H(2)O)(2)] and [FeO(2)(H(2)O)(4)]. These octahedra share an apical water molecule to form infinite chains. The chains are linked by partly deprotonated C(9)O(6)H(4)(2-) anions to give hybrid organic-inorganic layers; the remaining acidic-CO(2)H group is dangling in the interlayer space. Below 8(1) K, MIL-67 displays a canted antiferromagnetic behavior, according to analyses via magnetic measurements and M?ssbauer spectroscopy. Crystal data for MIL-67 are as follows: triclinic; space group P1 (No. 2), with a = 6.9671(2) A, b = 7.3089(3) A, c = 12.5097(3) A, alpha = 78.758(1) degrees, beta = 89.542(2) degrees, and gamma = 65.197(1) degrees; volume V = 565.21(3) A(3); and Z = 2.     

Studies on hydrogen ferrite phase in γ-Fe2O3 synthesised from ferrous fumarate half hydrate precursor

Thermal techniques viz., TG/DTA/DSC methods have been employed in understanding the formation and decomposition of hydrogen ferrite phase (H-F) as a metastable intermediate in the conversion of -Fe₂O₃·xH₂O, to -Fe₂O₃. Magnetic hysteresis measurements carried out at liquid N₂ and at room temperature for the vacancy ordered -Fe₂O₃ and its H-F phase also supplement these studies.The author thanks prof. M.R. Udpa, IIT, Madras for supplying TG/DTA data on Stanton Redcroft Instrument and to the Head, Department of Chemistry, S.P. University, Vallabh Vidyanagar for providing the facilities.     

Synthesis of calcium titanate from [Ca(H2O)3]2[Ti2(O2)2O(NC6H6O6)2]·2H2O as a cheap single-source precursor

Calcium titanate (CaTiO₃) was conveniently synthesized by thermal decomposition of a single-source precursor [Ca(H₂O)₃]₂[Ti₂(O₂)₂O(NC₆H₆O₆)₂]·2H₂O at low temperature. This single-source precursor was characterized by elemental analysis, IR spectrum, thermal gravimetric analysis and X-ray single crystal diffraction. The calcined products at different temperature were further characterized by powder X-ray diffractions and IR spectra. The morphology, microstructure, and crystallinity of the resulting CaTiO₃ materials have been characterized by SEM and TEM. The BET measurement revealed that the CaTiO₃ powders had a surface area of 14.0 m²/g. In addition, the microwave dielectric properties of the resulting CaTiO₃ material have been measured.     

Mesoporous α-Fe(2)O(3) thin films synthesized via the sol-gel process for light-driven water oxidation

This work reports a facile and cost-effective method for synthesizing photoactive α-Fe(2)O(3) films as well as their performances when used as photoanodes for water oxidation. Transparent α-Fe(2)O(3) mesoporous films were fabricated by template-directed sol-gel chemistry coupled with the dip-coating approach, followed by annealing at various temperatures from 350 °C to 750 °C in air. α-Fe(2)O(3) films were characterized by X-ray diffraction, XPS, FE-SEM and electrochemical measurements. The photoelectrochemical performance of α-Fe(2)O(3) photoanodes was characterized and optimized through the deposition of Co-based co-catalysts via different methods (impregnation, electro-deposition and photo-electro-deposition). Interestingly, the resulting hematite films heat-treated at relatively low temperature (500 °C), and therefore devoid of any extrinsic dopant, achieve light-driven water oxidation under near-to-neutral (pH = 8) aqueous conditions after decoration with a Co catalyst. The onset potential is 0.75 V vs. the reversible hydrogen electrode (RHE), thus corresponding to 450 mV light-induced underpotential, although modest photocurrent density values (40 μA cm(-2)) are obtained below 1.23 V vs. RHE. These new materials with a very large interfacial area in contact with the electrolyte and allowing for a high loading of water oxidation catalysts open new avenues for the optimization of photo-electrochemical water splitting.     

Sol–gel synthesis of Zn-thiourea-SiO2 thin films from (EtO)3Si(CH2)3NHC(S)NHPh as molecular precursor

In order to study new and convenient sol–gel syntheses to homogeneous nanocomposites systems based on zinc sulphide clusters embedded in films of silica glass, a thiourea-functionalized silane ((EtO)₃Si(CH₂)₃NHC(S)NHPh, SilTu) was investigated as starting precursor. Electrospray ionization mass spectrometry (ESI-MS), FT-IR and NMR spectroscopies clearly evidence that SilTu coordinates Zn²⁺ ions in solution through the thiourea function. Both zinc acetate (Zn(CH₃COO)₂) and zinc chloride (ZnCl₂) were used as zinc source. The SilTuZn²⁺ complex is also stable under sol–gel conditions. Alcoholic solutions of SilTu, zinc acetate and tetraethoxysilane Si(OEt)₄ (TEOS) were used to prepare the thin films by dip-coating. Transparent, homogeneous and crack-free layers were obtained under annealing up to 600 °C. The samples were investigated by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). Beside ZnS formation, the presence of oxygenated zinc species was observed inside the silica matrix.     

Theoretical analysis of spin crossover in iron(II) [2×2] molecular grids

We use quantum-chemical density functional theory calculations to elucidate the origin of spin-crossover pathways in two iron(II) [2×2] molecular grids with carbohydrazide-based bridging ligands. The complexes are characterized energetically and structurally in five available spin states. Special attention is paid to analysis of the structural distortion induced on each iron center by spin transition on any of its neighbors. The evolution of coordination polyhedra is monitored using the Continuous Shape Measures. It is demonstrated that a succession of spin transitions on different centers depends on the character of the induced distortion, either approaching or getting them away from a more regular low-spin geometry. These effects, resulting from the elasticity of bridging ligands, can be modulated by weak perturbations such as a change of the positions of the hydrogen atoms.     

Kinetics of oxygen evolution at α-Fe2O3 photoanodes: a study by photoelectrochemical impedance spectroscopy

Photoelectrochemical Impedance Spectroscopy (PEIS) has been used to characterize the kinetics of electron transfer and recombination taking place during oxygen evolution at illuminated polycrystalline α-Fe(2)O(3) electrodes prepared by aerosol-assisted chemical vapour deposition from a ferrocene precursor. The PEIS results were analysed using a phenomenological approach since the mechanism of the oxygen evolution reaction is not known a priori. The results indicate that the photocurrent onset potential is strongly affected by Fermi level pinning since the rate constant for surface recombination is almost constant in this potential region. The phenomenological rate constant for electron transfer was found to increase with potential, suggesting that the potential drop in the Helmholtz layer influences the activation energy for the oxygen evolution process. The PEIS analysis also shows that the limiting factor determining the performance of the α-Fe(2)O(3) photoanode is electron-hole recombination in the bulk of the oxide.     

Thermodynamic parameters characterizing adsorption of organic compounds from aqueous solutions on iron oxide α-Fe2O3

Adsorption of a series of organic compounds on the surface of -Fe₂O₃ is studied. The experimental data are analyzed using the Frumkin and Langmuir isotherms. The standard Gibbs energies of adsorption G _A ⁰ and G _ads ⁰ corresponding to different standard states are estimated.Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 9, 2004, pp. 1446–1451.Original Russian Text Copyright © 2004 by Afanasev, Aleksandrova, Akulova.