Time-Resolved Far-IR Spectroscopic Analysis of Mechanochemical Prussian Blue Synthesis

Abstract

Ferric ferrocyanide (Fe4[Fe(CN)6]3), more commonly known as Prussian Blue, has a crystal lattice composed of iron atoms in alternating oxidation states connected by cyanide bridges. Although Prussian Blue can be synthesized rapidly by mixing aqueous solutions of ferric chloride (FeCl3) and potassium ferrocyanide (K4[Fe(CN)6]), this method provides little insight into the solid-state kinetics or possible intermediates of the reaction. For pigments like Prussian Blue, the more gradual process of mechanochemical synthesis allows changes in reaction concentrations to be observed in real time, whether qualitatively through color change or quantitatively through spectroscopy. This study investigates the mechanochemical formation of Prussian Blue using time-resolved mid- and far-infrared spectroscopy. While the mid-IR region (400 - 4000 cm-1) contains the characteristic cyanide triple bond stretching band (~2080 cm-1), analysis in the far-IR region (10 - 700 cm-1) provided new insights by allowing the examination of the iron-to-ligand peaks that break and form during the reaction. The mid-IR cyanide peak intensity increased by 10% relative to the baseline during each 10-minute interval up to 30 minutes, regressed by 5% at 40 minutes, and increased again by 10% at the completion of the reaction. Additionally, a peak in the far-IR region (~490 cm-1) emerged early in the reaction and exhibited a similar growth-regression-growth trend. These time-resolved spectral changes suggest the solid-state synthesis of Prussian Blue to be a multi-step mechanochemical pathway involving transient intermediates and demonstrate the value of far-IR spectroscopy for probing coordination complex formation.