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, [Cu-2(L-1)(2)(H2O)(2)] (ClO4)(2) (1, HL1 = 3,5-dipyridyl-4-(2-keto-yl)pyrazole) and [Cu-2(L-2)(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 center dot 2H(2)O establishes the formation of a pyrazole ring from three different carbon centers through C-C bondforming reactions, mediated by cop-per(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 C-C bond-forming reactions is proposed and supported by theoretical calculations.