Copper(I) bromide

Copper(I) bromide is the chemical compound with the formula CuBr. This diamagnetic solid adopts a polymeric structure akin to that for zinc sulfide. The compound is widely used in the synthesis of organic compounds.

Preparation, basic properties, structure
The pure solid is colourless, although samples are often colored due to the presence of copper(II) impurities (see picture). It is commonly prepared by the reduction of cupric salts with sulfite in the presence of bromide:
 * 2 CuBr2 +  H2O  +  SO32-  →  2 CuBr  +  SO42-  +  2 HBr

Alternatively, and analogous to the synthesis of copper(I) chloride, it can be generated by dissolving copper powder in hydrobromic acid:
 * 2 HBr +  2 Cu   →   2 CuBr  +  H2

CuBr is insoluble in most solvents due to its polymeric structure, which features four-coordinated, tetrahedral Cu centers interconnected by bromide ligands (ZnS structure). Upon treatment with Lewis bases, CuBr converts to molecular adducts. For example with dimethyl sulfide, the colorless complex is formed:
 * CuBr +  S(CH3)2  →  CuBr(S(CH3)2)

In this coordination complex, the copper is two-coordinate, with a linear geometry. Other soft ligands afford related complexes. For example triphenylphosphine gives CuBr(P(C6H5)3), although this species has a more complex structure.

Applications in organic chemistry
In the Sandmeyer reaction, CuBr is employed to convert anilines into the corresponding aryl bromides:
 * ArN2+ +  CuBr  →  ArBr  +  N2  +  Cu+

The aforementioned complex CuBr(S(CH3)2) is widely used to generate organocopper reagents. Related CuBr complexes are catalysts for Atom Transfer Radical Polymerization and Cu-catalyzed Cross-Dehydrogenative Couplings (CDC).