Praseodymium(III) chloride

Praseodymium(III) chloride (PrCl₃), also known as praseodymium trichloride, is a compound of praseodymium and chlorine. It is a blue-green solid which rapidly absorbs water on exposure to moist air to form a light green heptahydrate.

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Chemical properties

Praseodymium(III) chloride is a moderately strong Lewis acid, which ranks as "hard" according to the HSAB concept. Rapid heating of the hydrate may cause small amounts of hydrolysis.^[1] PrCl₃ forms a stable Lewis acid-base complex K₂PrCl₅ by reaction with potassium chloride; this compound shows interesting optical and magnetic properties.^[2]

Aqueous solutions of praseodymium(III) chloride can be used to prepare insoluble praseodymium(III) compounds, for example praseodymium(III) phosphate or praseodymium(III) fluoride:

PrCl₃(aq) + K₃PO₄(aq) → PrPO₄(s) + 3 KCl(aq)

PrCl₃(aq) + 3 NaF(aq) → PrF₃(s) + 3 NaCl(aq)

Preparation

Praseodymium(III) chloride can be prepared as a green aqueous solution by reaction of either praseodymium metal or praseodymium(III) carbonate and hydrochloric acid. The anhydrous halide may alternatively be prepared from praseodymium metal and hydrogen chloride.^[2][3]

2 Pr(s) + 6 HCl(aq) → 2 PrCl₃(aq) + 3 H₂(g)

Pr₂(CO₃)₃(s) + 6 HCl(aq) → 2 PrCl₃(aq) + 3 CO₂(g) + 3 H₂O(l)

Anhydrous PrCl₃ can be made by dehydration of the hydrate either by slowly heating to 400 °C with 4-6 equivalents of ammonium chloride under high vacuum^[1],[4], or by heating with an excess of thionyl chloride for four hours^[1],[5]. The anhydrous halide may alternatively be prepared from praseodymium metal and hydrogen chloride^[3]. It is usually purified by high temperature sublimation under high vacuum.^[1]

Uses

PrCl₃ has been used to increase the activity of Pr₆O₁₁ catalysts, which can be used for the oxidation of methane to ethene. This process is becoming an important route to ethene for the manufacture of polyethylene (a common plastic). There are no major uses for praseodymium(III) chloride, though it can be used as a starting point for the preparation of other praseodymium salts.

Precautions

Praseodymium compounds are of low to moderate toxicity, although their toxicity has not been investigated in detail. Wear gloves and goggles.

References

1. ^ ^{a b c d} F. T. Edelmann, P. Poremba, in: Synthetic Methods of Organometallic and Inorganic Chemistry, (W. A. Herrmann, ed.), Vol. 6, Georg Thieme Verlag, Stuttgart, 1997.
2. ^ ^{a b} J. Cybinska, J. Sokolnicki, J. Legendziewicz, G. Meyer Journal of Alloys and Compounds 341, 115-123 (2002).
3. ^ ^{a b} L. F. Druding, J. D. Corbett, J. Am. Chem. Soc. 83, 2462 (1961); J. D. Corbett, Rev. Chim. Minerale 10, 239 (1973)^,
4. ^ M. D. Taylor, P. C. Carter, J. Inorg. Nucl. Chem. 24, 387 (1962); J. Kutscher, A. Schneider, Inorg. Nucl. Chem. Lett. 7, 815 (1971).
5. ^ J. H. Freeman, M. L. Smith, J. Inorg. Nucl. Chem. 7, 224 (1958).

Further reading

1. CRC Handbook of Chemistry and Physics (58th edition), CRC Press, West Palm Beach, Florida, 1977.
2. N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, 1984.
3. S. Sugiyama, T. Miyamoto, H. Hayashi, M. Tanaka, J. B. Moffat Journal of Molecular Catalysis A, 118, 129-136 (1997).
4. Druding L. F., Corbett J. D., Ramsey B. N. (1963). "Rare Earth Metal-Metal Halide Systems. VI. Praseodymium Chloride". Inorganic Chemistry 2 (4): 869 - 871. doi:10.1021/ic50008a055.