Cuprate

Oxides

One of the simplest oxide-based cuprates is the copper(III) oxide KCuO₂. This species can be viewed as the K+ salt of the polyanion [CuO₂⁻]_n. As such the material is classified as a cuprate. This dark blue diamagnetic solid is produced by heating potassium peroxide and copper(II) oxide in an atmosphere of oxygen:

Interest in cuprates sharply increased in 1986 with the discovery of high-temperature superconductivity in the Non-stoichiometric cuprate lanthanum barium copper oxide La_2−xBa_xCuO₄. The T_c for this material was 35 K, well above the previous record of 23K. Thousands of publications examine the superconductivity in cuprates between 1986 and 2001, and Bednorz and Müller were awarded the Nobel Prize in Physics only a year after their discovery.

From 1986 to 2008, many cuprate superconductors were identified. Most famous is yttrium barium copper oxide (YBa₂Cu₃O₇, "YBCO" or "1-2-3"). Another example is bismuth strontium calcium copper oxide (BSCCO or Bi₂Sr₂Ca_nCu_n+1O_2n+6-d) with T_c = 95–107 K depending on the n value. Thallium barium calcium copper oxide (TBCCO, Tl_mBa₂Ca_n−1Cu_nO_2n+m+2+δ) was the next class of high-T_c cuprate superconductors with T_c = 127 K observed in Tl₂Ba₂Ca₂Cu₃O₁₀ (TBCCO-2223) in 1988. The highest confirmed, ambient-pressure, T_c is 135 K, achieved in 1993 with the layered cuprate HgBa₂Ca₂Cu₃O_8+x. Few months later, another team measured superconductivity above 150K in the same compound under applied pressure (153 K at 150 kbar).

Cuprate superconductors usually feature copper oxides in both the oxidation state 3+ as well as 2+. For example, YBa₂Cu₃O₇ is described as Y³⁺(Ba²⁺)₂(Cu³⁺)(Cu²⁺)₂(O²⁻)₇. All superconducting cuprates are layered materials having a complex structure described as a superlattice of superconducting CuO₂ layers separated by spacer layers where the misfit strain between different layers and dopants in the spacers induce a complex heterogeneity that in the superstripes scenario is intrinsic for high temperature superconductivity.

Applications

BSCCO superconductors already have large-scale applications. For example, tens of kilometers of BSCCO-2223 electrical cables are being used in the Large Hadron Collider – the world's largest and highest-energy particle accelerator.

Coordination complexes

Copper forms many anionic "cuprate" coordination complexes with negatively charged ligands such as cyanide, hydroxide, and halides. Copper(I) derivatives tend to be colorless, copper(II) complexes are often turquoise-blue, and copper(III) and copper(IV) complexes are often orange-red.

One example of a copper(I)-based cuprate is tetracyanocuprate(I), [Cu(CN)₄]³⁻.

Copper(II) anions are most common, especially the chlorocuprates, such as trichlorocuprate(II) [CuCl₃]⁻, tetrachlorocuprate(II) [CuCl₄]²⁻ and pentachlorocuprate(II) [CuCl₅]³⁻. The light blue solid sodium tetrahydroxycuprate is well known, it is prepared by heating cupric hydroxide with concentrated sodium hydroxide.

Dilithium tetrachlorocuprate (Li₂CuCl₄) is an effective catalyst for the couplings of alkyl halides in the Grignard reaction. It is prepared by mixing lithium chloride (LiCl) and copper(II) chloride (CuCl₂) in tetrahydrofuran.

There are also rare copper(III) and copper(IV) complexes such as the hexafluorocuprate(III) [CuF₆]³⁻ and hexafluorocuprate(IV) [CuF₆]²⁻, which are strong oxidizing agents.

Organic cuprates

Cuprates have a role in organic synthesis. Organic cuprates often have the formula [CuR₂]⁻ or [CuR₃]²⁻, where R is an alkyl or aryl. These reagents find use as nucleophilic alkylating reagents. In stark contrast to the oxidic cuprates, the handling of these organocopper requires air-free techniques