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(μ2-OH)4]Cl2(PF6)2 · 8H2O (1), was isolated from a solution of Ldur and Cu2+ at pH 9. Under acidic conditions (pH 3), a polymeric complex, {[Cu4Ldur(μ2-Cl)6](PF6)2 · 10H2O}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(μ2-OH)2 cores folded at the O⋯O lines. Two forms of the tetranuclear units, featuring slightly different Cu2(μ2-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.