Copper(II) acetate

Copper(II) acetate, also referred to as cupric acetate, is the chemical compound with the formula Cu2(OAc)4 where AcO- is acetate (CH3CO2-). The hydrated derivative, which contains one molecule of water for each Cu atom, is available commercially. Cu2(OAc)4 is a dark green crystalline solid, whereas Cu2(OAc)4(H2O)2 is more bluish-green. Since ancient times, copper acetates of some form have been used as fungicides and green pigments. Today, Cu2(OAc)4 is used as a source of copper(II) in inorganic synthesis and as a catalyst or an oxidizing agent in organic synthesis. Copper acetate, like all copper compounds, emits a blue-green glow in a flame.

History
Copper(II) acetate is the primary component of verdigris, the blue-green substance that forms on copper during long exposures to atmosphere. It was historically prepared in vineyards, since acetic acid is a byproduct of fermentation. Copper sheets were alternately layered with fermented grape skins and dregs left over from wine production and exposed to air. This would leave a blue substance on the outside of the sheet. This was then scraped off and dissolved in water. The resulting solid was used as a pigment, or combined with arsenic trioxide to form copper acetoarsenite, a powerful insecticide and fungicide called Paris or Schweinfurt green.

Uses in chemical synthesis
The uses for copper(II) acetate are more plentiful as a catalyst or oxidizing agent in organic syntheses. For example, Cu2(OAc)4 is used to couple two terminal alkynes to make a 1,3-diyne:
 * Cu2(OAc)4 +  2 RC≡CH  →  2 CuOAc  +  RC≡C-C≡CR  +  2 HOAc

The reaction proceeds via the intermediacy of copper(I) acetylides, which are then oxidized by the copper(II) acetate, releasing the acetylide radical. A related reaction involving copper acetylides is the synthesis of ynamines, terminal alkynes with amine groups using Cu2(OAc)4.[2]

Structure
Cu2(OAc)4(H2O)2 adopts the "Chinese lantern" structure seen also for related Rh(II) and Cr(II) tetraacetates. One oxygen atom on each acetate is bound to one copper at 1.97 Å (197 pm. Completing the coordination sphere are two water ligands, with Cu-O distances of 2.20 Å (220 pm).  The two five-coordinate copper atoms are separated by only 2.65 Å (265 pm), which is close to the Cu--Cu separation in metallic copper.[5].  The two copper centers interact resulting in a diminishing of the magnetic moment such that near 90 K, Cu2(OAc)4(H2O)2 is essentially diamagnetic due to cancellation of the two opposing spins.  Cu2(OAc)4(H2O)2 was a critical step in the development of modern theories for antiferromagnetic coupling.

Synthesis
Copper(II) acetate has been synthesized for centuries by the method described in the history section. This method, however, leads to an impure copper(II) acetate. In a laboratory, a much purer form can be synthesized in a simple three-step procedure. The overall reaction is as follows:[6]
 * 2 CuSO4.5H2O +  4 NH3  +  4 CH3COOH  →  Cu2(OAc)4(H2O)2  +  2 [NH4]2[SO4] + 8 H2O

The hydrate form can be dehydrated by heating at 100 °C in a vacuum:
 * Cu2(OAc)4(H2O)2 →  Cu2(OAc)4  +  2 H2O

Heating a mixture of anhydrous Cu2(OAc)4 and copper metal affords colorless, volatile cuprous acetate:
 * 2 Cu +  Cu2(OAc)4  →  4 CuOAc