Copper manganese oxide spinels and related (multiphase) materials with the formula Cu_xMn_3−xO₄ are the active catalysts in a wide variety of industrially important processes due to their great diversity in their phase relations, metal ion valence/site distribution, and chemical properties. In this review, we summarize the preparation methods and their effects on the composition, properties, and catalytic properties of various Cu_xMn_3−xO₄ catalysts with various Cu/Mn ratios. The main summarized catalytic reactions are the oxidation of carbon monoxide, nitrogen oxide, and hydrogen sulfide and the oxidative removal of organic solvents such as benzene, toluene, and xylene from the air. Some industrially important reactions (steam reforming of methanol or synthesis gas) and the manufacture of organic chemicals (methyl formate, propylene oxide, and benzyl alcohol) catalyzed by Cu_xMn_3−xO₄ spinels are also reviewed.

We synthesized and structurally characterized the previously unknown [Co(NH₃)₅Cl](MnO₄)₂ complex as the precursor of CoMn₂O₄. The complex was also deuterated, and its FT-IR, far-IR, low-temperature Raman and UV-VIS spectra were measured as well. The structure of the complex was solved by single-crystal X-ray diffraction and the 3D-hydrogen bonds were evaluated. The N-H…O-Mn hydrogen bonds act as redox centers to initiate a solid-phase quasi-intramolecular redox reaction even at 120 °C involving the Co(III) centers. The product is an amorphous material, which transforms into [Co(NH₃)₅Cl]Cl₂, NH₄NO₃, and a todorokite-like solid Co-Mn oxide on treatment with water. The insoluble residue may contain {Mn₄^IIIMn^IV₂O₁₂}_n⁴ⁿ⁻, {Mn₅^IIIMn^IVO₁₂}_n⁵ⁿ⁻ or {Mn^III₆O₁₂}_n⁶ⁿ⁻ frameworks, which can embed 2 × n (Co^II and/or Co^III) cations in their tunnels, respectively, and 4 × n ammonia ligands are coordinated to the cobalt cations. The decomposition intermediates decompose on further heating via a series of redox reactions, forming a solid Co^IIM^III₂O₄ spinel with an average size of 16.8 nm, and gaseous N₂, N₂O and Cl₂. The CoMn₂O₄ prepared in this reaction has photocatalytic activity in Congo red degradation with UV light. Its activity strongly depends on the synthesis conditions, e.g., Congo red was degraded 9 and 13 times faster in the presence of CoMn₂O₄ prepared at 550 °C (in air) or 420 °C (under N₂), respectively.

The reaction of ammoniacal AgNO₃ solution (or aq. solution of [Ag(NH₃)₂]NO₃) with aq. NaClO₄ resulted in [Ag(NH₃)₂]ClO₄ (compound 1). Detailed spectroscopic (correlation analysis, IR, Raman, and UV) analyses were performed on [Ag(NH₃)₂]ClO₄. The temperature and enthalpy of phase change for compound 1 were determined to be 225.7 K and 103.04 kJ/mol, respectively. We found the thermal decomposition of [Ag(NH₃)₂]ClO₄ involves a solid-phase quasi-intramolecular redox reaction between the perchlorate anion and ammonia ligand, resulting in lower valence chlorine oxyacid (chlorite, chlorate) components. We did not detect thermal ammonia loss during the formation of AgClO₄. However, a redox reaction between the ammonia and perchlorate ion resulted in intermediates containing chlorate/chlorite, which disproportionated (either in the solid phase or in aqueous solutions after the dissolution of these decomposition intermediates in water) into AgCl and silver perchlorate. We propose that the solid phase AgCl-AgClO₄ mixture eutectically melts, and the resulting AgClO₄ decomposes in this melt into AgCl and O₂. Thus, the final product of decomposition is AgCl, N₂, and H₂O. The intermediate (chlorite, chlorate) phases were identified by IR, XPS, and titrimetric methods.

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.

We synthesized and characterized (IR, Raman, UV, SXRD) hexaamminecobalt(III) dichloride permanganate, [Co(NH₃)₆]Cl₂(MnO₄) (compound 1) as the precursor of Co–Mn–spinel composites with atomic ratios of Co:Mn = 1:1 and 1:3. The 3D−hydrogen bond network includes N–HO–Mn and N–HCl interactions responsible for solid-phase redox reactions between the permanganate anions and ammonia ligands. The temperature-limited thermal decomposition of compound 1 under the temperature of boiling toluene (110 ${}^{\circ }$C) resulted in the formation of (NH₄)₄Co₂Mn₆O_12. which contains a todorokite-like manganese oxide network (Mn^II₄Mn^III₂O₁₂¹⁰⁻). The heat treatment products of compounds 1 and [Co(NH₃)₅Cl](MnO₄)₂ (2) synthesized previously at 500 ${}^{\circ }$C were a cubic and a tetragonal spinel with Co_1.5Mn_1.5O₄ and CoMn₂O₄ composition, respectively. The heating of the decomposition product of compounds 1 and 2 that formed under refluxing toluene (a mixture with an atomic ratio of Co:Mn = 1:1 and 1:2) and after aqueous leaching ((NH₄)₄Co₂Mn₆O₁₂, 1:3 Co:Mn atomic ratio in both cases) at 500 ${}^{\circ }$C resulted in tetragonal Co_0.75Mn_2.25O₄ spinels. The Co_1.5Mn_1.5O₄ prepared from compound 1 at 500 ${}^{\circ }$C during the solid-phase decomposition catalyzes the degradation of Congo red with UV light. The decomposition rate of the dye was found to be nine times faster than in the presence of the tetragonal CoMn₂O₄ spinel prepared in the solid-phase decomposition of compound 2. The todorokite-like intermediate prepared from compound 1 under N₂ at 115 ${}^{\circ }$C resulted in a 54 times faster degradation of Congo red, which is a great deal faster than the same todorokite-like phase that formed from compound 2 under N₂.