Alkali metal cuprates(III)

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In inorganic chemistry, alkali metal cuprates(III) refers to a family of copper salts of the form MCuO2, where M is an alkali metal. They are cuprate salts and coordination polymers. Their primary interest is as simple models of the polymeric cuprate(III) sheet anion, notionally CuO−2, which also appears in cuprate superconductors.

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General properties

Cuprates(III) are very strong oxidants. They are unstable in water, typically oxidizing the medium. [1]

Potassium cuprate

Potassium cuprate Potassium cuperate.png
Potassium cuprate

One of the simplest oxide-based cuprates is potassium cuprate(III) KCuO2. This species can be viewed as the K+ salt of the polyanion [CuO−2]n. As such the material is classified as an oxide cuprate. This dark blue diamagnetic solid is produced by heating potassium peroxide and copper(II) oxide in an atmosphere of oxygen: [2]

K2O2 + 2 CuO → 2 KCuO2

KCuO2 was discovered first in 1952 by V. K. Wahl and W. Klemm, they synthesized this compound by heating copper(II) oxide and potassium superoxide in an atmosphere of oxygen. [3]

2 KO2 + 2 CuO → 2 KCuO2 + O2

It can also be synthesized by heating potassium superoxide and copper powder: [4]

KO2 + Cu → KCuO2

KCuO2 reacts with the air fairly slowly. It starts to decompose at 760 K (487 °C; 908 °F) and its color changes from blue to pale green at 975 K (702 °C; 1,295 °F). Its melting point is 1,025 K (752 °C; 1,385 °F). [3] [4]

In fact, KCuO2 is a non-stoichiometric compound, so the more exact formula is KCuOx and x is very close to 2. This causes the formation of defects in the crystal structure, and this leads to the tendency of this compound to be reduced. [4]

Sodium cuprate

Sodium cuprate(III) NaCuO2 can be produced by using hypochlorites or hypobromites to oxidize copper hydroxide under alkaline and low temperature conditions. [1]

2 NaOH + CuSO4 → Cu(OH)2↓
2 Cu(OH)2 + 2 NaOH + NaClO → 2 NaCuO2 + NaCl + 3 H2O

Sodium cuprate(III) is reddish-brown, but turns black gradually as it decomposes to copper(II) oxide. [1] In order to prevent decomposition, it must be prepared at low temperature in the absence of light.[ citation needed ]

Other cuprates

Other cuprates(III) of alkali metals are known; the structures of KCuO2 (potassium cuprate(III)), RbCuO2 (rubidium cuprate(III)) and CsCuO2 (caesium cuprate(III)) have been determined as well. [5]

RbCuO2 (blue-black) and CsCuO2 (black) can be prepared by reaction of rubidium oxide and caesium oxide with copper(II) oxide powders, at 675 K (402 °C; 755 °F) and 655 K (382 °C; 719 °F) in oxygen atmosphere, respectively. They both reacts with air rapidly, unlike KCuO2. [4]

4 CuO−2 + 2 H2O → 4 CuO + O2 + 4 OH−

References

  1. 1 2 3 Magee, J. S.; Wood, R. H. (1965). "Studies of Sodium Cuprate(III) Stability". Canadian Journal of Chemistry. 43 (5): 1234–1237. doi:10.1139/v65-164.
  2. ↑ G. Brauer, ed. (1963). "Potassium Cuprate (III)". Handbook of Preparative Inorganic Chemistry. Vol. 2 (2nd ed.). NY: Academic Press. p. 1015.
  3. 1 2 Wahl, Von Kurt; Klemm, Wilhelm (1952). "Über Kaliumcuprat(III)". Zeitschrift für Anorganische und Allgemeine Chemie. 270 (1–4): 69–75. Bibcode:1952ZAACh.270...69W. doi:10.1002/zaac.19522700109.
  4. 1 2 3 4 Costa, Giorgio A.; Kaiser, Elena (1995). "Structural and thermal properties of the alkaline cuprate KCuO2". Thermochimica Acta. 269–270: 591–598. doi:10.1016/0040-6031(95)02575-8.
  5. ↑ Hestermann, Klaus; Hoppe, Rudolf (1969). "Die Kristallstruktur von KCuO2, RbCuO2 und CsCuO2". Zeitschrift für Anorganische und Allgemeine Chemie (in German). 270 (1–4): 69–75. Bibcode:1969ZAACh.367..249H. doi:10.1002/zaac.19693670506.
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