Multi-Centered Solid-Phase Quasi-Intramolecular Redox Reactions of [(Chlorido)Pentaamminecobalt(III)] Permanganate-An Easy Route to Prepare Phase Pure CoMn2O4 Spinel

We synthesized and structurally characterized the previously unknown [Co(NH3)(5)Cl](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 horizontal ellipsis O-Mn hydrogen bonds act as redox centers to initiate a solid-phase quasi-intramolecular redox reaction even at 120 degrees C involving the Co(III) centers. The product is an amorphous material, which transforms into [Co(NH3)(5)Cl]Cl-2, NH4NO3, and a todorokite-like solid Co-Mn oxide on treatment with water. The insoluble residue may contain {(Mn4Mn2O12)-Mn-III-O-IV}(n)(4n-), {(Mn5MnO12)-Mn-III-O-IV}(n)(5n-) or {(Mn6O12)-O-III}(n)(6n-) frameworks, which can embed 2 x n (Co-II and/or Co-III) cations in their tunnels, respectively, and 4 x 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 (CoM2O4)-M-II-O-III spinel with an average size of 16.8 nm, and gaseous N-2, N2O and Cl-2. 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 degrees C (in air) or 420 degrees C (under N-2), respectively.