Cuprate

Cuprates are a class of compounds that contain copper (Cu) atom(s) in an anion. They can be broadly categorized into two main types:

1. Inorganic cuprates: These compounds have a general formula of XYCu_{m}O_{n}. Some of them are non-stoichiometric. Many of these compounds are known for their superconducting properties. An example of an inorganic cuprate is the tetrachloridocuprate(II) or tetrachlorocuprate(II) ([CuCl4](2-)), an anionic coordination complex that features a copper atom in an oxidation state of +2, surrounded by four chloride ions.

2. Organic cuprates: These are organocopper compounds, some of which having a general formula of [CuR2]−, where copper is in an oxidation state of +1, where at least one of the R groups can be any organic group. These compounds, characterized by copper bonded to organic groups, are frequently used in organic synthesis due to their reactivity. An example of an organic cuprate is dimethylcuprate(I) anion [Cu(CH3)2]-.

One of the most studied cuprates is YBa2Cu3O7, a high-temperature superconducting material. This oxide cuprate has been the subject of extensive research due to its ability to conduct electricity without resistance at relatively high temperatures.

The term 'cuprate' originates from 'cuprum', the Latin word for copper. It is primarily used in the context of oxide materials, anionic coordination complexes, and anionic organocopper compounds, reflecting the diverse roles of copper in chemistry. The term is mainly used in three contexts: oxide materials, anionic coordination complexes, and anionic organocopper compounds.

Oxide cuprates
One of the simplest oxide-based cuprates is potassium cuprate(III) KCuO2. This species can be viewed as the K+ salt of the polyanion [CuO2−]_{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:
 * K2O2 + 2 CuO → 2 KCuO2

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

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.
 * 2 KO2 + 2 CuO → KCuO2 + O2

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


 * KO2 + Cu → KCuO2

KCuO2 reacts with the air fairly slowly. It starts to decompose at 760 K and its color changes from blue to pale green at 975 K. Its melting point is 1025 K.

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 and 655 K in oxygen atmosphere, respectively. Either of them reacts with the air fast, unlike KCuO2.

In fact, KCuO2 is a non-stoichiometric compound, so the more exact formula is KCuO_{x} 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.

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


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

Cuprates(III) are not stable in water, and they can oxidize water as well.


 * 4 CuO2− + 2 H2O → 4 CuO + O2↑ + 4 OH−

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

Coordination complexes
Copper forms many anionic coordination complexes with negatively charged ligands such as cyanide, hydroxide, and halides, as well as alkyls and aryls.

Copper(I)
Cuprates containing copper(I) tend to be colorless, reflecting their d10 configuration. Structures range from linear 2-coordinate, trigonal planar, and tetrahedral molecular geometry. Examples include linear [CuCl2]− and trigonal planar [CuCl3](2−). Cyanide gives analogous complexes but also the trianionic tetracyanocuprate(I), [Cu(CN)4](3−). Dicyanocuprate(I), [Cu(CN)2]−, exists in both molecular or polymeric motifs, depending on the countercation.

Copper(II)
Cuprates containing copper(II) include trichlorocuprate(II), [CuCl3]−, which is dimeric, and square-planar tetrachlorocuprate(II), [CuCl4](2−), and pentachlorocuprate(II), [CuCl5](3−). 3-Coordinate chlorocuprate(II) complexes are rare.

Tetrachlorocuprate(II) complexes tend to adopt flattened tetrahedral geometry with orange colors.

Sodium tetrahydroxycuprate(II) (Na2[Cu(OH)4]) is an example of a homoleptic (all ligands being the same) hydroxide complex.
 * Cu(OH)2 + 2 NaOH → Na2[Cu(OH)4]

Copper(III) and copper(IV)
Hexafluorocuprate(III) [CuF6](3−) and hexafluorocuprate(IV) [CuF6](2−) are rare examples of copper(III) and copper(IV) complexes. They are strong oxidizing agents.

Organic cuprates


Cuprates have a role in organic synthesis. They are invariably Cu(I), although Cu(II) or even Cu(III) intermediates are invoked in some chemical reactions. Organic cuprates often have the idealized formulas [CuR2]− and [CuR3](2−), both of which contain copper in an oxidation state of +1, where R is an alkyl or aryl. These reagents find use as nucleophilic alkylating reagents.