Copper(II) complexes of 3,4,5-trisubstituted pyrazolates: in situ formation of pyrazole rings from different carbon centers.

The synthesis and characterization of two pyrazolate-bridged dicopper(II) complexes, [Cu2(L1)2(H2O)2](ClO4)2 (1, HL1=3,5-dipyridyl-4-(2-keto-pyridyl)pyrazole) and [Cu2(L2)2(H2O)2](ClO4)2 (2, HL2=3,5-dipyridyl-4-benzoylpyrazole), are discussed. These copper(II) complexes are formed from the reactions between pyridine-2-aldehyde, 2-acetylpyridine (for compound 1) or acetophenone (for compound 2), and hydrazine hydrate with copper(II) perchlorate hydrate under ambient conditions. The single-crystal X-ray structure of compound 1⋅2 H2O establishes the formation of a pyrazole ring from three different carbon centers through CC bond-forming reactions, mediated by copper(II) ions. The free pyrazoles (HL1 and HL2) are isolated from their corresponding copper(II) complexes and are characterized by using various analytical and spectroscopic techniques. A mechanism for the pyrazole-ring synthesis that proceeds through CC bond-forming reactions is proposed and supported by theoretical calculations.