Chromium(II)_chloride

Chromium(II) chloride

Chromium(II) chloride

Chemical compound

Chromium(II) chloride describes inorganic compounds with the formula CrCl2(H2O)n. The anhydrous solid is white when pure, however commercial samples are often grey or green; it is hygroscopic and readily dissolves in water to give bright blue air-sensitive solutions of the tetrahydrate Cr(H2O)4Cl2. Chromium(II) chloride has no commercial uses but is used on a laboratory-scale for the synthesis of other chromium complexes.

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Synthesis

CrCl2 is produced by reducing chromium(III) chloride either with hydrogen at 500 °C:

2 CrCl3 + H2 → 2 CrCl2 + 2 HCl

or by electrolysis.

On the laboratory scale, LiAlH4, zinc, and related reductants produce chromous chloride from chromium(III) precursors:

4 CrCl3 + LiAlH4 → 4 CrCl2 + LiCl + AlCl3 + 2 H2
2 CrCl3 + Zn → 2 CrCl2 + ZnCl2

CrCl2 can also be prepared by treating a solution of chromium(II) acetate with hydrogen chloride:[6]

Cr2(OAc)4 + 4 HCl → 2 CrCl2 + 4 AcOH

Treatment of chromium powder with concentrated hydrochloric acid gives a blue hydrated chromium(II) chloride, which can be converted to a related acetonitrile complex.[7]

Cr + n H2O + 2 HCl → CrCl2(H2O)n + H2

Structure and properties

Anhydrous CrCl2 is white[6] however commercial samples are often grey or green. It crystallizes in the Pnnm space group, which is an orthorhombically distorted variant of the rutile structure; making it isostructural to calcium chloride. The Cr centres are octahedral, being distorted by the Jahn-Teller Effect.[8]

Ball-and-stick model of chromium(II) chloride tetrahydrate.

The hydrated derivative, CrCl2(H2O)4, forms monoclinic crystals with the P21/c space group. The molecular geometry is approximately octahedral consisting of four short Cr—O bonds (2.078 Å) arranged in a square planar configuration and two longer Cr—Cl bonds (2.758 Å) in a trans configuration.[3]

Reactions

The reduction potential for Cr3+ + e− ⇄ Cr2+ is −0.41. Since the reduction potential of H+ to H2 in acidic conditions is +0.00, the chromous ion has sufficient potential to reduce acids to hydrogen, although this reaction does not occur without a catalyst.

Organic chemistry

Chromium(II) chloride is used as precursor to other inorganic and organometallic chromium complexes. Alkyl halides and nitroaromatics are reduced by CrCl2. The moderate electronegativity of chromium and the range of substrates that CrCl2 can accommodate make organochromium reagents very synthetically versatile.[9] It is a reagent in the Nozaki-Hiyama-Kishi reaction, a useful method for preparing medium-size rings.[10] It is also used in the Takai olefination to form vinyl iodides from aldehydes in the presence of iodoform.[11]

References

  1. [1]
    Lide, David R., ed. (2009). CRC Handbook of Chemistry and Physics (90th ed.). Boca Raton, Florida: CRC Press. ISBN 978-1-4200-9084-0.
  2. [2]
    Tracy, Joseph W.; Gregory, N.W.; Lingafelter, E.C.; Dunitz, J.D.; Mez, H.-C.; Rundle, R.E.; Scheringer, Christian; Yakel, H.L.; Wilkinson, M.K. (1961). "The crystal structure of chromium(II) chloride". Acta Crystallographica. 4 (9): 927–929. doi:10.1107/S0365110X61002710.
  3. [3]
    Schnering, H.G. von; Brand, B.-H. (1973). "Struktur und Eigenschaften des blauen Chrom(II)-chlorid-tetrahydrats CrCl2.4H2O". Zeitschrift für anorganische und allgemeine Chemie. 402 (2): 159–168. doi:10.1002/zaac.19734020206.
  4. [4]
    Sigma-Aldrich Co., Chromium(II) chloride. Retrieved on 2014-07-04.
  5. [5]
    "MSDS of Chromium(II) chloride". fishersci.ca. Fisher Scientific. Retrieved 2014-07-04.
  6. [6]
    Riley, edited by Georg Brauer ; translated by Scripta Technica, Inc. Translation editor Reed F. (1963). Handbook of Preparative Inorganic Chemistry. Volume 1 (2nd ed.). New York, N.Y.: Academic Press. p. 1337. ISBN 978-0121266011. {{cite book}}: |first= has generic name (help)CS1 maint: multiple names: authors list (link)
  7. [7]
    Holah, David G.; Fackler, John P. (1967). "Chromium(II) Salts and Complexes". Inorganic Syntheses. Inorganic Syntheses. Vol. 10. pp. 26–35. doi:10.1002/9780470132418.ch4. ISBN 9780470132418.
  8. [8]
    Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 1021–1022. ISBN 978-0-08-037941-8.
  9. [9]
    (a) Kazuhiko Takai, K.; Loh, T.-P. "Chromium(II) Chloride" in Encyclopedia of Reagents for Organic Synthesis John Wiley & Sons: New York; 2005. doi:10.1002/047084289X.rc166. (b) Alois Fürstner, "Carbon−Carbon Bond Formations Involving Organochromium(III) Reagents" Chemical Reviews, 1999, 99 (4), 991–1046 doi:10.1021/cr9703360
  10. [10]
    (a) MacMillan, D. W. C.; Overman, Larry E. "Enantioselective Total Synthesis of (−)-7-Deacetoxyalcyonin Acetate. First Synthesis of a Eunicellin Diterpene" J. Am. Chem. Soc. 1995, 117 (41), 10391–10392. doi:10.1021/ja00146a028. (b) Lotesta, S. D.; Liu, J.; Yates, E. V.; Krieger, I.; Sacchettini, J. C.; Freundlich, J. S.; Sorensen, E. J. "Expanding the pleuromutilin class of antibiotics by de novo chemical synthesis" Chem. Sci. 2011, 2, 1258-1261. doi:10.1039/C1SC00116G.
  11. [11]
    Simple and selective method for aldehydes (RCHO) -> (E)-haloalkenes (RCH:CHX) conversion by means of a haloform-chromous chloride system K. Takai, K. Nitta, K. Utimoto J. Am. Chem. Soc.; 1986; 108(23); 7408-7410. doi:10.1021/ja00283a046.
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