Two Tetranuclear Copper(II) Complexes with Open Cubane-Like Cu_4O_4 Core Framework and Ferromagnetic Exchange Interactions between Copper(II) Ions: Structure, Magnetic Properties, and Density Functional Study

摘要

Two alkoxo-bridged copper(II) tridentate Schiff base complexes [{Cu(H_2L1)}_4] (1) [H_2L1 = N-(2-hydroxyethyl)-3,5-ditert- butylsalicylaldimine] and {Cu(H_2L_2)}_4 (2) [H_2L_2 = N-(2- hydroxyethyl)-4-methoxysalicylaldimine] have been synthesized and structurally and magnetically characterized. X-ray diffraction studies show that 1 and 2 are tetranuclear alkoxobridged copper(II) complexes that contain a rather distorted Cu_4O_4 cubane core of 4+2 type (four short and two long Cu···Cu 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 tetra- nuclear 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 Cu···Cu 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)_2Cu fragments, the overlap of the magnetic orbitals, and DFT calculations.