The chemical identity of “[Ag(py)2]MnO4” organic solvent soluble oxidizing agent and new synthetic routes for the preparation of [Ag(py)n]XO4 (X = Mn, Cl, and Re, n = 2–4) complexes

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)O_x 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)O_x 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.

[Fe(urea-O)₆](MnO₄)₃ is a selective solid-phase oxidant of benzylic alcohols into benzaldehydes and precursor in the preparation of (Fe,Mn)O_x catalysts for CO₂ hydrogenation into hydrocarbons. The urea ligands are in octahedral propeller-like arrangement around the Fe³⁺ cation, and there are hydrogen bonds between the permanganate anions and the urea ligands. A solid-phase quasi-intramolecular redox reaction of the urea and the permanganate resulted in (Fe,Mn)O, (Fe,Mn)₂O₃, and (Fe,Mn)^T-4(Fe,Mn)^OC-6₂O₄ with an overall Fe to Mn stoichiometry of 1:3.

Structure and Vibrational Spectra of Pyridine Solvated Solid Bis(Pyridine)silver(I) Perchlorate, [Agpy2ClO4]·0.5py

A hemipyridine solvate of bis(pyridine)silver(I) perchlorate, [Agpy2ClO4]·0.5py (compound 1) was prepared and characterized by single crystal X-ray analysis and vibrational spectroscopy (R and low-temperature Raman). Compound 1 was prepared via the trituration of [Agpy2ClO4] and 4[Agpy2ClO4]·[Agpy4]ClO4 (as the source of the solvate pyridine) in a mixed solvent of acetone:benzene =1:1 (v = v) at room temperature. The monoclinic crystals of compound 1 were found to be isomorphic with the analogous permanganate complex (a = 19.1093(16) Å, b = 7.7016(8) Å, c = 20.6915(19) Å, β = 105.515(7)°; space group: C2/c). Two [Agpy2]+ cations formed a dimeric unit [Agpy2ClO4]2, and each silver ion was connected to two ClO4− anions via oxygen atoms. The Ag∙∙∙Ag distance was 3.3873(5) Å, the perchlorate ions were coordinated to silver ions, and the Ag∙∙∙O distances were 2.840(2) Å and 2.8749(16) Å in the centrosymmetric rectangle of Ag-O-Ag-O. The stoichiometric ratio of the monomer [Agpy2ClO4] and the solvent pyridine was 1:0.5. The guest pyridine occupied 527.2 Å3, which was 18.0% of the volume of the unit cell. There was no additional residual solvent-accessible void in the crystal lattice. The solvate pyridine was connected via its a-CH to one of the O atoms of the perchlorate anion. Correlation analysis, as well as IR and low-temperature Raman studies, were performed to assign all perchlorate and pyridine vibrational modes. The solvate and coordinated pyridine bands in the IR and Raman spectra were not distinguishable. A perchlorate contribution via Ag-O coordination to low-frequency Raman bands was also assigned.

Pyridinesilver Tetraoxometallate Complexes: Overview of the Synthesis, Structure, and Properties of Pyridine Complexed AgXO4 (X = Cl, Mn, Re) Compounds

We reviewed the synthesis, structure, and properties of pyridine complexes of AgXO4 (X = Cl, Mn, and Re) compounds with various compositions ([AgPy2] XO4, [AgPy2XO4]·0.5Py, [AgPy4] XO4, and 4 [AgPy2XO4] [AgPy4] XO4). We also clarified the controversial information about the existence and composition of pyridine complexes of silver permanganate, used widely as mild and selective oxidants in organic chemistry. We discussed in detail the available structural and spectroscopic (IR, Raman, and UV) data and thermal behavior, including the existence and consequence of quasi-intramolecular reactions between the reducing ligand and anions containing oxygen.

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.

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.

Unexpected Sequential NH3/H2O Solid/Gas Phase Ligand Exchange and Quasi-Intramolecular Self-Protonation Yield [NH4Cu(OH)MoO4], a Photocatalyst Misidentified before as (NH4)2Cu(MoO4)2

[NH₄Cu(OH)MoO₄] as active photocatalyst in the decomposition of Congo Red when irradiated by UV or visible light has been prepared in an unusual ammonia/water ligand exchange reaction of [tetraamminecopper(II)] molybdate, [Cu(NH₃)₄]MoO₄. [Cu(NH₃)₄]MoO₄ was subjected to moisture of open air at room temperature. Light blue orthorhombic [Cu(NH₃)(H₂O)₃]MoO₄ was formed in 2 days as a result of an unexpected solid/gas phase ammonia-water ligand exchange reaction. This complex does not lose its last ammonia ligand on further standing in open air; however, a slow quasi-intramolecular (self)-protonation reaction takes place in 2-4 weeks, producing a yellowish-green microcrystalline material, which has been identified as a new compound, [NH₄Cu(OH)MoO₄], ( a = 10,5306 Å, b = 6.0871 Å, c = 8.0148 Å, β = 64,153°, C2, Z = 4). Mechanisms are proposed for both the sequential ligand exchange and the self-protonation reactions supported by ab initio quantum-chemical calculations and deuteration experiments as well. The [Cu(NH₃)(H₂O)₃]MoO₄ intermediate transforms into NH₄Cu(OH)(H₂O)₂MoO₄, which loses two waters and yields [NH₄Cu(OH)MoO₄]. Upon heating, both [Cu(NH₃)₄]MoO₄ and [Cu(NH₃)(H₂O)₃]MoO₄ decompose, losing three NH₃ and three H₂O ligands, respectively, and stable [Cu(NH₃)MoO₄] is formed from both. The latter can partially be hydrated in boiling water into [NH₄Cu(OH)MoO₄. This compound can also be prepared in pure form by boiling the saturated aqueous solution of [Cu(NH₃)₄]MoO₄. All properties of [NH₄Cu(OH)MoO₄] match those of the active photocatalyst described earlier in the literature under the formulas (NH₄)₂[Cu(MoO₄)₂] and (NH₄)₂Cu₄(NH₃)₃Mo₅O₂₀.