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

Abstract

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. The magneto-structural properties of two new alkoxo-phenoxo bridged tetranuclear copper(II) complexes of stoichiometry [Cu₄L₂(O₂CR)₂] (R = C₆H₄-p-OH 1, C₆H₄-o-OH 2) are described. The complexes display axial/equatorial covalent bonding between two {Cu₂L(O₂CR)} units through the phenoxo oxygen atoms and 1 shows intermolecular hydrogen bonding with a lattice trapped isopropanol. The magnetic studies show antiferromagnetically coupled metal centers with J₁=-132,J₂=-72 cm^-1 for 1 and J₁=-167,J₂=-67 cm^-1 for 2.