Fluoride-assisted stabilization of manganese(III) in aqueous medium. A general approach to the synthesis of mixed-ligand fluoro complexes of manganese(III)

Abstract

The importance of fluoride as a stabilizing ligand for manganese(III), both in aqueous solution as well as in solid complexes, has been emphasized, and a host of new mixed-ligand fluoro complexes have been synthesized from aqueous solutions. While K₃[MnF₂(C₂0₄)₂].3H₂0 has been synthesized from a redox reaction of KMn0₄ with H₂C₂0₄.2H₂0 in the presence of KF at ca. 0 °C, the other mixed-ligand fluoromanganates(II1). with EDTA, HP0₄^2-, and glycine (glyH) being the coligands, have been synthesized from the reactions of MnO(0H) in 48% HF with the corresponding ligands in water in the presence of alkali-metal fluoride (for EDTA and HPO₄^2-) and alkali-metal carbonate (for glyH). Three molecular complexes, [MnF₃(H₂O)(phen)].2H₂0, [MnF₃(H₂0)(bpy)].2H₂0, and [MnF₃(urea)₂].3H₂O been prepared also from the reactions of MnO(0H) in 40% HF with the respective ligand solutions. All the compounds, except K₃[MnF₂(C₂0₄)₂].3H₂0 are stable in the absence of moisture. Their characterization and structural assessment are based on the results of elemental analyzes, chemical determination of oxidation state, IR and electronic spectroscopic studies, magnetic susceptibility measurements, and in a few cases pyrolysis studies. Whereas the room-temperature magnetic moment of [MnF₃(urea)₂].3H₂0 is 4.3 μ_B, those of the others lie in the range 4.78-5.0 μ_B. A magnetostructural correlationship has been explored by comparing the present empirical magnetic moments with those of the binary fluoromanganates(II1). Pyrolysis of [MnF₃(urea)₂].3H₂O ca . 500 °C affords MnF₃, a compound of acknowledged importance.