Synthesis and chemical characterization of CuII, NiII and ZnII complexes of3,5-bis(20-pyridyl)-1,2,4-oxadiazole and 3-(20-pyridyl)5-(phenyl)-1,2,4-oxadiazoleligands

The synthesis and structural characterization of NiII, CuII and ZnII complexes of two chelating 1,2,4-oxadiazole ligands, namely 3,5-bis(20-pyridyl)-1,2,4-oxadiazole (bipyOXA) and 3-(20-pyridyl)5-(phenyl)-1,2,4-oxadiazole (pyOXA), is here reported. The formed hexacoordinated metal complexes are[M(bipyOXA)2(H2O)2](ClO4)2 and [M(pyOXA)2(ClO4)2], respectively (M = Ni, Cu, Zn). X-ray crystallography,1H and 13C NMR spectroscopy and C, N, H elemental analysis data concord in attributing them anoctahedral coordination geometry. The two coordinated pyOXA ligands assume a trans coplanar disposition,while the two bipyOXA ligands are not. The latter result is a possible consequence of the formationof H-bonds between the coordinated water molecules and the nitrogen atom of the pyridine in position 5of the oxadiazole ring. The expected splitting of the d metal orbitals in an octahedral ligand field explainsthe observed paramagnetism of the d8 and d9 electron configuration of the nickel(II) and copper(II)complexes, respectively, as determined by the broadening of their NMR spectra.