Inorganic Synthesis

Potassium ferrioxalate synthesis

Theory:

Oxalate C2O42- is a dianion derived from oxalic acid. The anion has a planar configuration due to the hybridization of carbon atoms. In a free state it acquires D2h symmetry. It is usually used in coordination chemistry for its ability to create strong coordination bond, binding preferably in bidentate manner. Coordination of dianion in this mode results in the creation of highly stable five-membered chelate ring. It can also act as a bridging ligand (e.g. [{Ni(dien)}2(µ-ox){µ-Ni(CN)4}] [1]. The metal centers bridged by oxalate ligand were found to exhibit strong magnetic coupling [2]. The simple oxalate coordination compounds can be prepared by reacting precipitated metal hydroxide with potassium/sodium oxalate at elevated temperatures.

Procedure:

Iron(III) chloride hexahydrate (5 g, 18.5 mmol) was dissolved in 10 mL of distilled water. Potassium hydroxide (4 g, 71.4 mmol) solution was added to the solution of Iron(III) chloride (30% excess). The red precipitate of hydrated iron(III) oxide was filtered off and washed wth distilled water. The precipitate was transfered into the beaker containing potassium oxalate solution (10.22 g, 55.5 mmol) under constant stirring (can be exchanged for sodium oxalate). The reaction temperature was held at 60 °C for one hour. The clear vivid green solution was filtered. After few days the crystals started to form. The crystals were recrystallized from distilled water several times.

picture

Na3[Fe(C2O4)3] after three recrystallizations

doprce

K3[Fe(C2O4)3] after two recrystallizations

Literature:
[1] Iñaki Muga et al. Binuclear Ni(II) complexes based on bridging oxalate and tetracyanometallates: Synthesis, crystal structure and magnetic properties (2002). Vol. 21, Iss. 25–26, p. 2631–2638
[2] Ya-Guang Sun et al. Bis(imidazole) coordination polymers controlled by oxalate as an auxiliary ligand (2015). Vol. 68, Iss. 7, p. 1199-1212