Two Tetranuclear Copper( II) Complexes with Open Cubane- Like Cu4O4 Core Framework and Ferromagnetic Exchange Interactions between Copper( II) Ions: Structure, Magnetic Properties, and Density Functional Study

Two alkoxo-bridged copper(II) tridentate Schiff base complexes [{Cu(H(2)L1)}(4)] (1) [H(2)L1 = N-(2-hydroxyethyl)-3,5-di-tert-butylsalicylaldimine] and {Cu(H(2)L2)}(4) (2) [H(2)L2 = N-(2-hydroxyethyl)-4-methoxysalicylaldimine] have been synthesized and structurally and magnetically characterized. X-ray diffraction studies show that 1 and 2 are tetranuclear alkoxo-bridged copper(II) complexes that contain a rather distorted Cu4O4 cubane core of 4+2 type (four short and two long CuCu distances). The coordination of each copper ion can be described as a distorted square pyramid with one nitrogen and four oxygen atoms from three ligands. Variable-temperature magnetic susceptibility measurements on the two tetranuclear complexes 1 and 2 in the range 2-300 K indicate ferromagnetic exchange coupling between copper(II) centers. The magnetic susceptibility data were analyzed by using a simple two-J model with J and J representing the magnetic exchange couplings through the short and long CuCu exchange pathways, respectively. The J values were as follows: J = +28.7 cm(-1) and J = +7.8 cm(-1) for 1, and J = +39.8 cm(-1) and J = +10.2 cm(-1) for 2. The sign and magnitude of the exchange coupling constants were justified on the basis of the structural geometric factors of the bridging Cu(O)(2)Cu fragments, the overlap of the magnetic orbitals, and DFT calculations.