Synthesis, Crystal Structures, and Magnetic Properties of Dinuclear and Hexanuclear Copper(II) Complexes with Cyclam-Based Macrocyclic Ligands Having Four Schiff-Base Pendant Arms

Reaction of a series of dodecadentate ligands (H4L), 1,4,8,11-tetrakis(salicylideneaminoethyl)-1,4,8,11-tetraaza-cyclotetradecane and its substituted derivatives, with copper(II) salts afforded dinuclear copper(II) complexes, [Cu2L], which were characterized by IR and UV-vis-NIR spectroscopy, and temperature dependence of magnetic susceptibilities (4.5-300 K). Single-crystal X-ray crystallography of these complexes revealed that each copper(II) ion is bound by the chelating of the two Schiff-base pendant arms outside the central tetraazacyclotetradecane ring forming a distorted square plane with an intramolecular Cu-Cu distance of 8.692(2)-8.949(2) angstrom. In accordance with the crystal structures, the magnetic interaction between the two copper(II) atoms is very weak. In the case of 1,4,8,11-tetrakis(3-methoxysalicylideneaminoethyl)-1,4,8,11-tetraazacyclotetradecane (H(4)tmsaec), a hexanuclear complex with a crystallographic inversion center, [Cu-6(O2CCH3)(8)(tmsaec)], was isolated. In the asymmetric unit, one copper atom is bonded to one pendant arm and two copper atoms are bound to another pendant arm with the methoxy group of the Schiff-base moiety with the Cu-Cu distance of 3.096(1) angstrom, giving a novel system containing monodentate, bidentate, and bridging acetate ions in the same molecule. Magnetic susceptibility data shows that a weak antiferromagnetic interaction is operating between the closest two copper atoms.