Thermally Induced Solid-Phase Quasi-Intramolecular Redox Reactions of [Hexakis(urea-O)iron(III)] Permanganate: An Easy Reaction Route to Prepare Potential (Fe,Mn)Ox Catalysts for CO2 Hydrogenation.

Research on new reaction routes and precursors to prepare catalysts for CO2 hydrogenation has enormous importance. Here, we report on the preparation of the permanganate salt of the urea-coordinated iron(III), [hexakis(urea-O)iron(III)]permanganate ([Fe(urea-O)6](MnO4)3) via an affordable synthesis route and preliminarily demonstrate the catalytic activity of its (Fe,Mn)Ox thermal decomposition products in CO2 hydrogenation. [Fe(urea-O)6](MnO4)3 contains O-coordinated urea ligands in octahedral propeller-like arrangement around the Fe3+ cation. There are extended hydrogen bond interactions between the permanganate ions and the hydrogen atoms of the urea ligands. These hydrogen bonds serve as reaction centers and have unique roles in the solid-phase quasi-intramolecular redox reaction of the urea ligand and the permanganate anion below the temperature of ligand loss of the complex cation. The decomposition mechanism of the urea ligand (ammonia elimination with the formation of isocyanuric acid and biuret) has been clarified. In an inert atmosphere, the final thermal decomposition product was manganese-containing wuestite, (Fe,Mn)O, at 800 °C, whereas in ambient air, two types of bixbyite (Fe,Mn)2O3 as well as jacobsite (Fe,Mn)T-4(Fe,Mn)OC-62O4), with overall Fe to Mn stoichiometry of 1:3, were formed. These final products were obtained regardless of the different atmospheres applied during thermal treatments up to 350 °C. Disordered bixbyite formed first with inhomogeneous Fe and Mn distribution and double-size supercell and then transformed gradually into common bixbyite with regular structure (and with 1:3 Fe to Mn ratio) upon increasing the temperature and heating time. The (Fe,Mn)Ox intermediates formed under various conditions showed catalytic effect in the CO2 hydrogenation reaction with <57.6% CO2 conversions and <39.3% hydrocarbon yields. As a mild solid-phase oxidant, hexakis(urea-O)iron(III) permanganate, was found to be selective in the transformation of (un)substituted benzylic alcohols into benzaldehydes and benzonitriles.

Solid-Phase "Self-Hydrolysis" of [Zn(NH3)4MoO4@2H2O] Involving Enclathrated Water-An Easy Route to a Layered Basic Ammonium Zinc Molybdate Coordination Polymer.

An aerial humidity-induced solid-phase hydrolytic transformation of the [Zn(NH3)4]MoO4@2H2O (compound 1@2H2O) with the formation of [(NH4)xH(1-x)Zn(OH)(MoO4)]n (x = 0.92-0.94) coordination polymer (formally NH4Zn(OH)MoO4, compound 2) is described. Based on the isostructural relationship, the powder XRD indicates that the crystal lattice of compound 1@2H2O contains a hydrogen-bonded network of tetraamminezinc (2+) and molybdate (2-) ions, and there are cavities (O4N4(µ-H12) cube) occupied by the two water molecules, which stabilize the crystal structure. Several observations indicate that the water molecules have no fixed positions in the lattice voids; instead, the cavity provides a neighborhood similar to those in clathrates. The @ symbol in the notation is intended to emphasize that the H2O in this compound is enclathrated rather than being water of crystallization. Yet, signs of temperature-dependent dynamic interactions with the wall of the cages can be detected, and 1@2H2O easily releases its water content even on standing and yields compound 2. Surprisingly, hydrolysis products of 1 were observed even in the absence of aerial humidity, which suggests a unique solid-phase quasi-intramolecular hydrolysis. A mechanism involving successive substitution of the ammonia ligands by water molecules and ammonia release is proposed. An ESR study of the Cu-doped compound 2 (2#dotCu) showed that this complex consists of two different Cu2+(Zn2+) environments in the polymeric structure. Thermal decomposition of compounds 1 and 2 results in ZnMoO4 with similar specific surface area and morphology. The ZnMoO4 samples prepared from compounds 1 and 2 and compound 2 in itself are active photocatalysts in the degradation of Congo Red dye. IR, Raman, and UV studies on compounds 1@2H2O and 2 are discussed in detail.

Solid-Phase Quasi-Intramolecular Redox Reaction of [Ag(NH3)2]MnO4: An Easy Way to Prepare Pure AgMnO2.

Two monoclinic polymorphs of [Ag(NH3)2]MnO4 containing a unique coordination mode of permanganate ions were prepared, and the high-temperature polymorph was used as a precursor to synthesize pure AgMnO2. The hydrogen bonds between the permanganate ions and the hydrogen atoms of ammonia were detected by IR spectroscopy and single-crystal X-ray diffraction. Under thermal decomposition, these hydrogen bonds induced a solid-phase quasi-intramolecular redox reaction between the [Ag(NH3)2]+ cation and MnO4- anion even before losing the ammonia ligand or permanganate oxygen atom. The polymorphs decomposed into finely dispersed elementary silver, amorphous MnOx compounds, and H2O, N2 and NO gases. Annealing the primary decomposition product at 573 K, the metallic silver reacted with the manganese oxides and resulted in the formation of amorphous silver manganese oxides, which started to crystallize only at 773 K and completely transformed into AgMnO2 at 873 K.

An unknown component of a selective and mild oxidant: structure and oxidative ability of a double salt-type complex having κ1O-coordinated permanganate anions and three- and four-fold coordinated silver cations.

Compounds containing redox active permanganate anions and complexed silver cations with reducing pyridine ligands are used not only as selective and mild oxidants in organic chemistry but as precursors for nanocatalyst synthesis in low-temperature solid-phase quasi-intramolecular redox reactions. Here we show a novel compound (4Agpy2MnO4·Agpy4MnO4) that has unique structural features including (1) four coordinated and one non-coordinated permanganate anion, (2) κ1O-permanganate coordinated Ag, (3) chain-like [Ag(py)2]+ units, (4) non-coordinated ionic permanganate ions and an [Ag(py)4]+ tetrahedra as well as (5) unsymmetrical hydrogen bonds between pyridine α-CHs and a permanganate oxygen. As a result of the oxidizing permanganate anion and reducing pyridine ligand, a highly exothermic reaction occurs at 85 °C. If the decomposition heat is absorbed by alumina or oxidation-resistant organic solvents (the solvent absorbs the heat to evaporate), the decomposition reaction proceeds smoothly and safely. During heating of the solid material, pyridine is partly oxidized into carbon dioxide and water; the solid phase decomposition end product contains mainly metallic Ag, Mn3O4 and some encapsulated carbon dioxide. Surprisingly, the enigmatic carbon-dioxide is an intercalated gas instead of the expected chemisorbed carbonate form. The title compound is proved to be a mild and efficient oxidant toward benzyl alcohols with an almost quantitative yield of benzaldehydes.

Polarized IR reflectance spectra of the monoclinic single crystal K2Ni(SO4)2.6H2O: dispersion analysis, dielectric and optical properties.

Polarized IR reflectance spectra of K2Ni(SO4)2.6H2O single crystal (belonging to the group of Tutton salts) were recorded at near-normal incidence. From the dispersion analysis performed on the spectra recorded from the ac crystal plane, mode parameters: transversal frequency, oscillator strength, attenuation constant and the orientation of the transition moment were determined. The polarized spectrum along the b crystallographic axis was also recorded and a dispersion analysis performed. Comparison between the spectroscopically obtained transition moment directions with those obtained from the structure data for various modes is discussed. All dielectric tensor component values were obtained for the whole mid-IR frequency range. Also, the real and the imaginary parts of the complex indexes of refraction for the waves with wave vector in the direction of the b crystallographic axis and in the ac plane (when the direction of the electric vector is oriented along the b axis) were found as functions of frequency.

Infrared reflectance spectra of some optically biaxial crystals: on the origin of isosbestic-like points in the polarized reflectance spectra.

An investigation of the IR polarized reflectance spectra of the orthorhombic K2SO4, KHSO4 and monoclinic CaSO4 x 2H2O, K2Ni(SO4)2 x 6H2O, (NH4)2Ni(SO4)2 x 6H2O was performed in order to explain the appearance of particular intersection points in their spectra. The analogy in the origin of the appearance of intersection points and the well-known isosbestic points in UV-vis and IR spectra was discussed. The reason for such an appearance was identified in the way that the individual reflectances (for radiation polarized along principal dielectric axes) sum up to give the reflectance under arbitrary polarization. This summation may also produce supplementary bands in the reflectance spectra, not predicted by the group theory. It was shown that relatively large LO-TO splitting is needed for the supplementary band(s) to appear, but also the overlapping region must be taken into account.

The IR reflectance spectra of the nu 3(SO4(2-)) and nu 4(SO4(2-)) band regions of some Tutton salts using polarized radiation: testing the model dielectric function.

The investigation of the vibrational bands of the SO(4)(2-) ions (in the nu(3) and nu(4) frequency regions) of six different Tutton salts was performed with specular IR reflectance spectroscopy using polarized radiation, on single crystal samples. The reflectance function under oblique incidence using dielectric model function as parameter (originally derived for optically uniaxial crystals) appeared to be readily applicable for the investigated monoclinic crystals. The frequencies of the transversal and longitudinal phonons were obtained by fitting of spectra recorded from (0 1 0), (0 0 1) and (1 0 0) crystal planes. Further, the symmetry types of all experimentally detected phonons were identified. Some of the results were further confirmed from IR absorption spectra recorded at liquid nitrogen temperature (LNT) of isomorphously isolated SO(4)(2-) ions into the corresponding selenate matrices.

Vibrational states of tetrahedral molecular species in electric fields. A theoretical model for quantitative description of the latent symmetry effects in vibrational spectroscopy of matrix isolated tetrahedral species.

A theoretical model that enables a quantitative description of latent symmetry effects in the case of matrix isolated tetrahedral species is developed. Several particular cases are considered, that are of special interest from vibrational spectroscopic viewpoint. It is shown that the observed appearances of the IR spectra of various XY4n- anions isomorphously isolated in solid matrices may be systematically explained in terms of the orientation of the effective local crystalline field vector. The model also principally enables calculation of the magnitude of the effective local homogeneous field vector as well as its direction with respect to the symmetry elements of the dopant species, and it can be successively corrected accounting for the field non-homogeneity. Also, a solid theoretical basis is for the first time presented for the significantly smaller site group splitting of the nu4 modes of dopant XY4n- species in comparison to the splitting of nu3 ones, which is often observed in the IR spectra. However, the presented approach has potentially a much wider applicability, as it in facts considers the vibrational states of tetrahedral molecular species in electric fields, which may be of a different origin, for example, electrodes in electrochemical systems, external fields in Stark-type spectroscopies, etc.

The dielectric model function. An evidence for the nu3(SO4-) phonon mixing in gypsum using polarized IR specular reflectance spectroscopy.

A complete investigation of the phonon mixing in the nu3(SO4(2-)) frequency region was made, under experimental geometry where the reflecting crystal plane and the plane of incidence contain two principal dielectric axes. It was shown that although it is monoclinic, in the frequency region under inspection, gypsum could be treated as an orthorhombic crystal. More sophisticated model dielectric functions were introduced for the explanation of the Evans holes in the reflectance spectra. Using the model dielectric function, an appearance of the shoulder at the higher frequency part of the reflectance band was explained in terms of longitudinal and quasilongitudinal phonons. A possibility for Evans type interaction was encountered between the nu3(SO4(2-)) Au symmetry type mode and probably some of the combination modes.

Infrared spectra of permanganate anions in potassium perchlorate matrices: vibrational anharmonicity, effective symmetry and vibrational mode mixing effects.

Fourier transform infrared spectra of MnO4- anions isomorphously isolated in potassium perchlorate matrices were recorded at room and low temperature (LT, approximately 100 K). On the basis of the detected second-order vibrational transitions involving the dopant species nu3 mode components, anharmonicity constants and harmonic eigenvalues for these modes were calculated. Despite the fact that, rigorously speaking, the appearance of the spectra of dopant permanganate anions may be explained in terms of a Cs site symmetry, the nu3 stretching region resembles an approximate A1+E splitting (characteristic for a local C3nu or even higher symmetry), which is not expected, even within the latent symmetry approach. We explain such spectral patterns on the basis of vibrational mode mixing (a 'Fermi-like' resonance) of the MnO4- nu1 mode with the nu3, site-group component. With the results of degenerate case stationary perturbation theory, we show that in the present case the Fermi-like resonance is predominantly responsible for the observed spectral features. The appearance of the region of second-order vibrational transitions in the spectra of dopant permanganate anions may be better explained in terms of the (rigorous) crystallographic Cs site group (corresponding to the crystallographic Pnma space group), instead of the 'latent' (effective) symmetry site group C2nu (corresponding to the latent symmetry space group Imma).

Dicesium trans-Tetraaquadichlorochromium(III) Chloride: Redetermination of the Crystal Structure and Infrared Study of the Water Spectrum.

Infrared spectral studies of the solid led us to believe that the published crystal structure of dicesium trans-tetraaquadichlorochromium(III) chloride,trans-Cs(2)[CrCl(2)(H(2)O)(4)]Cl(3) might be incorrect. Crystal data: Cs(2)CrCl(5). 4H(2)O, a = 17.484(9) Å, b = 6.099(3) Å, c = 6.928(3) Å, beta = 106.06(5) degrees, monoclinic, C2/m, Z = 2 molecules per cell. The redetermination has revealed disorder in the positions of the water molecules. Instead of one type of H(2)O molecule being present as found in the original study, two sets of such molecules with four nonequivalent O.Cl contacts were found. The presence, in the O-D stretching region of the spectra of samples with low deuterium content, of three bands with intensities close to 2:1:1 (rather than the expected four) is believed to be a consequence of different degrees of nonlinearity of the two hydrogen bonds formed by the water molecules of one of the two existing types.