Laboratory of Crystallography and Coordination Chemistry

National Centre for Scientific Research "Demokritos", Institute of Materials Science

Athens, Greece

 

 

Synthesis of Iron(II) acetate hydrate (ferrous acetate)

     
  Pure and soluble iron(II) acetate hydrate, which can be useful in synthesis of ferrous complexes, is prepared by oxidation of metallic iron in acetic acid. Anhydrous iron(II) acetate is very insoluble and less useful for synthetic purposes. In addition, when certain types of spectroscopy are envisaged, high-purity iron(II) acetate is required. In particular, ferric impurities will show up in EPR spectra even at trace amounts (iron(III) is a Kramers ion), while iron(II) (a non-Kramers ion) gives much weaker EPR signals (if at all).

This is the method proposed by M. A. Scheurer-Kestner (Bull. Soc. Chim. Fr. 1863, 5, 345), as modified by R. N. Rhoda and A. V. Fraioli (Inorg. Synth. 1953, 4, 159-161).

     
 

Apparatus:

Schlenk line with vacuum pump

 

Materials:

Metallic Iron powder (at least 97%)

Acetic acid

Distilled water

Dry acetone*

 

Glassware:

0.5 L Schlenk flask

0.5 L two-necked round-bottom flask

Schlenk frit

 

     

 

 

 

1.

 

 

 

Assemble the two 0.5 L flask on the two sides of the Schlenk frit as shown in Figure 1.

Use two stirring magnets (the second will be used after the apparatus has been inversed) and protect the frit pores from clogging with unreacted iron powder using glasswool. Stopper the second neck of the bottom flask with a rubber septum.

2.

Add 25 g of metallic iron powder to the bottom flask and purge the whole setup by 4-5 vacuum-nitrogen cycles.

 

 

 

 

3.

 

 

 

Deoxygenate in a separate Schlenk flask 90 ml of acetic acid and decanulate it through the rubber septum. Similarly, decanulate 90 mL of deoxygenated distilled H2O into you setup.

 

4.

A small evolution of H2 will commence as the metallic iron is oxidised by water. Open the top valve to an oil bubbler and maintain a small nitrogen flow to impede air from entering.

Allow ~1 hr for this reaction to proceed at this rate.

 

 

 

 

5.

 

 

 

Immerse the setup in an oil bath (or use a heating mantle if available) and heat to reflux (~115 oC). Make sure to tilt your setup so that the refluxing solvents do not pass over the stopcock valves. Continue the reflux for ~6-8 hr.

 

 

 

 

6.

 

 

 

At the end of this period, most of the iron has reacted, but usually not all. Remove from heating and allow to cool.

When the mixture is still warm inverse the apparatus to filter. Use stopcock (C) to evacuate the apparatus and (A) to pressurize with nitrogen in order to accelerate the filtration. The solution should be a pale green color.

 

 

 

 

7.

 

 

 

When the filtration is complete, stop the inflow of nitrogen and open all stopcocks to vacuum (first, (C), then (B), then (A)). The solvents will start evaporating and condense in the cold trap.

 

 

 

8.

 

 

When this is complete, a white foam will form. Close all stopcocks and while under vacuum, enter the apparatus into the glovebox (if the apparatus is not under vacuum, it may explode inside the antechamber).

 

9.

Scrape the foam off the flask walls and grind it in a mortar. Return to the flask, add dry acetone and agitate. Filter on a Buchner funnel. The first filtrate should be light brown. Repeat the washing with dry acetone 2-3 times. The rest of the filtrates should be pale green or colorless.

 

10.

Vacuum dry and store in tightly sealed vials.

 

     
  The dried material should be white, with only a hint of green.

The material should be checked for its purity. Apart from microanalyses that would determine its degree of hydration (normally ~2 H2O solvates are expected per formula unit), X-band EPR spectroscopy is very useful in determining other impurities. Common impurities are:
-monomeric iron(III) (rhombic g ~ 4.3 signal in the EPR spectrum)
-monomeric Mn(II) impurities (g ~ 2.0 sextet in the EPR spectrum)
These can be detected by X-band EPR spectroscopy to levels below 1%.

EPR spectra of iron(II) acetate prepared from 97% metallic iron, might show trace manganese impurities before washing with dry acetone (depending on the quality of metallic iron). However, these totally disappear after 3 washings. The EPR spectra should also be totally free of ferric impurities if all manipulations were well carried out. In the examples below, after the washings, only the EPR cavity is visible at 300 K.

 

 

 

Before washing

 

 

After washing

 

On the other hand, manganese impurities may not show up at all, even in the "impure" material (prior to washing).

 

*

There are various methods for the preparation of dry acetone. The one that was found to be simple enough, while providing acetone of suitable dryness, was distillation over drierite.

Attention! The dry acetone must first be deoxygenated before being entered into the glovebox for the washings.

     
 
 

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