Influence of counter anions on the structures and magnetic properties of trinuclear Cu(II) complexes containing a μ 3-OH core and Schiff base ligands

Abstract

Four trinuclear Cu(II) complexes, [(CuL 1) 3(μ 3-OH)](NO 3) 2 ( 1), [(CuL 2) 3(μ 3-OH)](I) 2·H 2O ( 2), [(CuL 3) 3(μ 3-OH)](I) 2 ( 3) and [(CuL 1) 3(μ 3-OH)][Cu II 3] ( 4), where HL 1 (8-amino-4-methyl-5-azaoct-3-en-2-one), HL 2 [7-amino-4-methyl-5-azaoct-3-en-2-one] and HL 3 [7-amino-4-methyl-5-azahept-3-en-2-one] are the three tridentate Schiff bases, have been synthesized and structurally characterized by X-ray crystallography. All four complexes contain a partial cubane core, [(CuL) 3(μ 3-OH)] 2+ in which the three [CuL] subunits are interconnected through two types of oxygen bridges afforded by the oxygen atoms of the ligands and the central OH − group. The copper(II) ions are in a distorted square-pyramidal environment. The equatorial plane consists of the bridging oxygen of the central OH − group together with three atoms (N, N, O) from the Schiff base. The oxygen atom of the Schiff base also coordinates to the axial position of Cu(II) of another subunit to form the cyclic trimer. Magnetic susceptibilities have been determined for these complexes over the temperature range of 2–300 K. The isotropic Hamiltonian, H = − J 12 S 1 S 2 − J 13 S 1 S 3 − J 23 S 2 S 3 has been used to interpret the magnetic data. The best fit parameters obtained are: J = −54.98 cm −1, g = 2.24 for 1; J = −56.66 cm −1, g = 2.19 for 2; J = −44.39 cm −1, g = 2.16 for 3; J = −89.92 cm −1, g = 2.25 for 4. The EPR data at low temperature indicate that the phenomenon of spin frustration occurs for complexes 1– 3.