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

Research on new reaction routes and precursors to prepare catalysts for CO hydrogenation has enormous importance. Here, we report on the preparation of the permanganate salt of the urea-coordinated iron(III), [hexakis(urea-)iron(III)]permanganate (Fe(urea-O)) via an affordable synthesis route and preliminarily demonstrate the catalytic activity of its (Fe,Mn)O thermal decomposition products in CO hydrogenation. Fe(urea-O) contains O-coordinated urea ligands in octahedral propeller-like arrangement around the Fe 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)O as well as jacobsite (Fe,Mn)(Fe,Mn)O), 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 intermediates formed under various conditions showed catalytic effect in the CO hydrogenation reaction with <57.6% CO conversions and <39.3% hydrocarbon yields. As a mild solid-phase oxidant, hexakis(urea-)iron(III) permanganate, was found to be selective in the transformation of (un)substituted benzylic alcohols into benzaldehydes and benzonitriles.