Structural and magnetic studies of a zigzag chain copper(II) complex and a dipolar spin-coupled copper(II) dimer with the versatile ligand 5-[(2-carboxyphenyl)azo]-1,3-dimethylbarbituric acid

Abstract

On treating H₂L {5-[(2-carboxyphenyl)azo]-1,3-dimethylbarbituric acid} with Cu(NO₃)₂·3H₂O in ethanol–water (4 : 1) the tetranuclear complex [{CuL(H₂O)}₄]1 is obtained. When 1 is dissolved in pyridine(py)–ethanol (1 : 1) the complex [{CuL(py)·3H₂O}_n]2 is isolated. Both complexes in pyridine solution yield [CuL(py)₂]3. A magnetic study of 1 points to a ferromagnetic ground state (S_T= 2). The structures of 2 and 3 have been determined by X-ray crystallographic methods. The structure of 2 consists of infinite zigzag chains running along the b axis, in which copper ions are sequentially bridged by the barbituric ring. The copper presents a distorted square-pyramidal environment. In the basal plane, four bonds are formed with the ligand and the molecule of pyridine, while the apical position is occupied by an oxygen atom belonging to the barbituric ring of an adjacent fragment; the ligand then acts in a tetradentate fashion. The copper–copper intrachain distance is 6.322(3)Å. The structure of 3 consists of discrete [CuL(py)₂] molecules in which the ligand acts in a tridentate fashion. The copper co-ordination is distorted square-pyramidal. The complex packs in the crystal lattice as symmetry-related pairs of molecules, with a Cu ⋯ Cu separation of 5.410(3)Å. From magnetic susceptibility measurements, complex 2 is found to exhibit ferromagnetic intrachain interactions with an exchange coupling of J/2k= 1.7, whereas a Curie–Weiss law is observed for 3. Even though 3 lacks any close contact between molecules which may be regarded as bonding interactions, the polycrystalline EPR spectrum at 90 K exhibits a half-field signal, which is indicative of a spin-coupling process.