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

Abstract

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 {Mn4IIIMnIV2O12}n4n-, {Mn5IIIMnIVO12}n5n- or {MnIII6O12}n6n- frameworks, which can embed 2 × n (CoII and/or CoIII) 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 CoIIMIII2O4 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.

[Hexaamminecobalt(III)] Dichloride Permanganate—Structural Features and Heat-Induced Transformations into (CoII,MnII)(CoIII,MnIII)2O4 Spinels

“…In the visible region of spectra, the bands at 510 and 530 nm belong to the permanganate 1 A 1 − 1 T 2 (t 1 −2e) transition, whereas the bands at 490 and 551 nm may belong to the permanganate and cation transitions (ESI Table S7) as well. A similar band system was found in the UV−VIS spectrum of KMnO 4 between 500 and 562 nm [8]. The band at 725 nm is the strongest band and probably consists of the 1 A 1 − 1 T 1 (t 1 −2e) transition of the permanganate ion and a weak component of the 1 A 1 → 5 T 2 transition of the complex cation.…”

“…The band at 725 nm is the strongest band and probably consists of the 1 A1− 1 T1(t1−2e) transition of the permanganate ion and a weak component of the 1 A1→ 5 T2 transition of the complex cation. The 1 A1− 1 T1(t1−2e) transition of the permanganate ion was found at 720 nm for KMnO4 and 710 nm for [Agpy2]MnO4 [8].…”

“…The UV−VIS spectra of solid compound 1 was recorded at room temperature (ESI Figure S10). The spectrum consists of strongly overlapping bands of four possible d-d transitions of the [Co(NH 3 ) 6 ] 3+ cation and CT bands of the permanganate anion [8,46]. As a low-spin complex cation, the ground state of [Co(NH 3 ) 6 ] 3+ is t 2g 6 ( 1 A 1g ).…”

“…solutions of [Co(NH 3 ) 6 ]Cl 3 and also showed by DFT calculations with water around the [Co(NH 3 ) 6 ] 3+ cation [48]. The band observed at 250 cm −1 may be assigned both to the CT band of the complex cation and the 1 A 1 − 1 T 2 (3t 2 −2e) transition of the permanganate ion (it was found at 259 nm for KMnO 4 ), whereas the band at 220 nm may be assigned to the 1 A 1 − 1 T 2 (t 1 −4t 2 ) transition of a permanganate ion (it was found at 227 nm for KMnO 4 ) [8].…”

Hexakis(urea-O)iron Complex Salts as a Versatile Material Family: Overview of Their Properties and Applications

“…The solid phase quasi-intramolecular redox reactions generally occur at low temperatures (between 50–140 °C), and the reactions are exothermic; due to the solid-phase environment and low temperature, these redox reactions result in a disrupted lattice product (amorphous materials), which can crystallize in controlled size with postannealing. ,,− The oxidation power and reducing ability of the ligands in the solid phase are not equal to those parameters found in aqueous solutions. Accordingly, the urea as a reducing ligand will cause special effects in the gaseous environment, and various valence states of the metals may appear in the solid mixed oxides.…”