Structure and magnetic properties of polynuclear chloro- and hydroxo-bridged copper(II) complexes formed by a tetramacrocyclic derivative of 1,4,7-triazacyclononane

Abstract

Two new copper(II) complexes of the ligand 1,2,4,5-tetrakis(1,4,7-triazacyclononan-1-ylmethyl)benzene (Ldur) have been synthesized and characterized by single crystal X-ray studies. The first, [Cu4Ldur(岭OH)4]Cl2(PF6)2 砸H2O (1), was isolated from a solution of Ldur and Cu2+ at pH 9. Under acidic conditions (pH 3), a polymeric complex, {[Cu4Ldur(岭Cl)6](PF6)2 砱0H2O}n (2), crystallized from solution. In both complexes, each of the four triazacyclononane (tacn) rings of the Ldur ligand facially coordinate to separate metal centres. Pairs of Cu(II) centres are then doubly-bridged by hydroxo groups in 1, leading to tetranuclear complex cation units featuring pairs of isolated copper(II) dimers with Cu2(岭OH)2 cores folded at the Ocdots, three dots, centeredO lines. Two forms of the tetranuclear units, featuring slightly different Cu2(岭OH)2 core geometries, are present in equal amounts within the crystal lattice. In complex 2, chloro bridging ligands link pairs of Cu(II) centres from neighbouring tetranuclear units, forming a 1D helical polymeric structure. Variable-temperature magnetic susceptibility measurements suggest that the hydroxo-bridged copper(II) centres within one of the tetranuclear units in 1 are weakly antiferromagnetically coupled (J = -27 cm-1), whilst those in the other interact ferromagnetically (J = +19 cm-1). Similar measurements indicate weak ferromagnetic coupling (J = +16 cm-1) for the chloro-bridged copper(II) centres in 2.