Synthesis, crystal structure and magnetic properties of quasi-linear tetranuclear copper(II) Schiff base complexes formed by covalent linkage of asymmetrically dibridged dicopper(II) units

Alkoxo-phenoxo bridged tetranuclear copper(II) complexes [Cu4L2(O2CC6H4-p-OH)2] (1) and [Cu4L2(O2CC6H4-o-OH)2] (2) containing pentadentate Schiff base ligand N,N′-(2-hydroxypropane-1,3-diyl)bis(salicylaldimine) (H3L) are prepared and structurally characterized. Crystal structures of the complexes show the covalent linkage between two {Cu2L(O2CR)}(R = C6H4-p-OH, C6H4-o-OH) units through the phenoxo atoms of the Schiff base ligand showing axial/equatorial bonding modes. The Cu(1)–O(2)–Cu(2) alkoxo bridge angle is 131° in 1 and 2. The pendant ortho- and para- OH groups of the three-atom bridging carboxylate ligands show no apparent bonding interactions with the metal or other group(s). The complexes show a d–d band near 635 nm in CH2Cl2. Variable temperature magnetic susceptibility measurements in the temperature range 300–18 K show antiferromagnetically coupled spin system. A theoretical fit of the magnetic data using exchange parameters J1 and J2 for the intradimer and interdimer units of the quasi-linear tetrameric core gave values as: J1=−132,J2=−72 cm−1 for 1 and J1=−167,J2=−67 cm−1 for 2.