Abstract
Pyridine‐2‐carbaldehyde thiosemicarbazone and pyridine‐2‐carbaldehyde 4__N__‐methyl thiosemicarbazone ligands (HL~I~ and HL~I~mm respectively) undergo an oxidative cyclization when treated with bromate or iodate, leading to 2‐amino‐5‐pyridin‐2‐yl‐1,3,4‐oxadiazole and 2‐methylamino‐5‐pyridin‐2‐yl‐1,3,4‐oxadiazole (HL~II~ and HL~II~mm). This reaction occurs either with free ligands or when coordinated to copper(II). Some of the starting ([{Cu(L~I~mm)(NO~3~)}~2~] (3), intermediate Cu(L~I~)(IO~3~)·(H~2~O) (1), [{Cu(L~I~)I}~2~] (2), [{Cu(L~I~mm)Br}~2~] (4), and final compounds {Cu(HL~II~)(H~2~O)~3~}~2~~2~ (5), [{Cu(HL~II~mm)(H~2~O)~2~(SO~4~)}~2~]·2H~2~O (6), (HL~II~) (7), [HL~II~mm]·3H~2~O (8) have been isolated and characterized by elemental analyses, IR, UV‐visible, NMR, and EPR spectroscopy. The structures of 2, 3, 4, 5, 6, and 8, solved by X‐ray diffraction methods, contain dinuclear entities with either square‐pyramidal (2, 3, 4) or octahedral (5, 6) copper(II) ions. Structural and spectroscopic results suggest that ligand‐to‐metal charge transfer occurs in these compounds. The EPR spectra at 120 K exhibit rhombic (1, 3, 4), isotropic (2), and axial (5, 6) signals. Magnetic measurements show antiferromagnetic couplings for 2, 4, 5, and 6. The susceptibility data were fitted by the Bleaney−Bowers equation for copper(II) dimers. The obtained J/k values are −20.20, −8.85, −2.75, and −2.78 K for 2, 4, 5, and 6, respectively. However, ferromagnetic intradimeric interactions are present in 3 (J/k = +9.90 K) together with antiferromagnetic interdimer coupling (z′J′/k = −1.70 K). Magneto‐structural studies show the influence of the non‐thiosemicarbazone coligand in the magnetic behaviour of these complexes. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)