Mechanochemistry, an Easy Technique to Boost the Synthesis of CuI Pyrazine Coordination Polymers

Depending on the experimental conditions, the solid-state reactions of CuI with pyrazine (pyz) yield three distinct coordination polymers (CPs): a double chain polymer [Cu2I2(pyz)]n (yellow powder) and a single strand [CuI(pyz)]n (red powder) and its new isomeric compound [(CuI)2(pyz)2]n (orange powder), which present dimers of CuI bridged by the pyrazine ligands. The conversions among the three CPs were studied: by heating to 110 °C, [CuI(pyz)]n or [(CuI)2(pyz)2]n convert into [Cu2I2(pyz)]n, which reverts to the starting compounds upon kneading or grinding in the presence of pyrazine. The orange isomer [(CuI)2(pyz)2]n is obtained only when the solid-state reaction is performed with neat grinding or by direct melting of the pyrazine; it is unstable in the presence of solvent or vapor, and it readily transforms into the red isomer. The structure of [(CuI)2(pyz)2]n was determined by X-ray powder diffraction. [Cu2I2(pyz)]n reacts also with 4-cyanopyrazine to yield the mixed ligand compound [(CuI)(4CN-py)2 py]n, which, when heated, decomposes into [Cu2I2(pyz)]n and [(CuI)4(4CN-py)5]n.