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

Abstract

Research on new reaction routes and precursors to prepare catalysts for CO 2 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 ](MnO 4 ) 3 ) via an affordable synthesis route and preliminarily demonstrate the catalytic activity of its (Fe,Mn)O x thermal decomposition products in CO 2 hydrogenation. [Fe(urea-O) 6 ](MnO 4 ) 3 contains O-coordinated urea ligands in octahedral propeller-like arrangement around the Fe 3+ 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) 2 O 3 as well as jacobsite (Fe,Mn) T-4 (Fe,Mn) OC-6 2 O 4 ), 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 CO 2 hydrogenation reaction with <57.6% CO 2 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.

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

“…The shortest Fe−Fe distances varied between 6.433 and 6.976 Å. 1,17,22,34 The hexakis(urea)iron compounds [Fe(Urea) 6 ](I 3 ) 3 (compound 12) 34 and [Fe(Urea) 6 ]I 3 (compound 13) 22 formed in the Fe II /Fe III −I − /I 2 /I 3 − −urea system were studied by Kuz'mina et al (Table 2). The shiny blackish crystals of compound 12 belonged to the hexagonal crystal system.…”

“…Generally, the concentrated aqueous solution of compound 1 or 2 was mixed with the concentrated aqueous solution of the highly water-soluble (NH 4 + or Na + ) salt of the desired anions in a 1:3 (permanganate) or 1:1.5 (dichromate, persulfate) molar ratio. 1,11,12,17 To obtain [Fe(urea) 6 ](MnO 4 ) 3 (compound 5), [Fe-(urea) 6 ] 2 (Cr 2 O 7 ) 3 (compound 6), and [Fe(urea) 6 ] 2 (S 2 O 8 ) 3 (compound 7), a concentrated aqueous solution of compound 1 and NaMnO 4 , Na 2 Cr 2 O 7 , and (NH 4 ) 2 S 2 O 8 were mixed, respectively. 11,12 Compounds 5 and 7 precipitated immediately upon mixing the solutions, while compound 6 could be isolated after concentrating the solution by evaporation.…”

“…The cryo-DSC studies on these compounds showed that no phase transformations occurred between −140 K and their decomposition temperatures. 1,17 The dark purple permanganate compound (compound 5) shows unique structural features. Its structure shows notable pseudosymmetry since the complex cation and anions fit R3̅ c space group symmetry.…”

“…Mossbauer Parameters of Hexakis(urea)iron(III) complexes with Different Anions (X) a Tami et al studied the K edge 1s → 3d pre-edge features of the perchlorate compound (compound 4) with X-ray absorption spectroscopy (XAS) and extended X-ray absorption fine structure (EXAFS) methods. Since the cation of compound 4 has O h geometry (thus it is centrosymmetric), the only the transition allowed is the electric quadrupole transition, (t 2g ) 3 (e g ) 2 ( 5 A 1g ) → (t 2g ) 2 (e g ) 2 ( 5 T 2g ) or (t 2g ) 3 (e g ) 2 ( 5 A 1g ) → (t 2g ) 3 (e g ) 1 ( 5 E g ). These two peaks appear at 7113 and 7114 eV on the XAS spectra of compound 4.…”

“…calculations, respectively, to prove the correctness of their evaluation, which showed good agreement between the experimental and calculated parameters. 1,17,53 Russo et al evaluated the Mossbauer spectra with a single Lorentizan line. However, the curve did not fit well on the two sides of the spectra.…”