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

Two new copper(II) complexes of the ligand 1,2,4,5-tetrakis(1,4,7-triazacyclononan-1-ylmethyl)benzene (L^dur) have been synthesized and characterized by single crystal X-ray studies. The first, [Cu₄L^dur(μ₂-OH)₄]Cl₂(PF₆)₂ · 8H₂O (1), was isolated from a solution of L^dur and Cu²⁺ at pH 9. Under acidic conditions (pH 3), a polymeric complex, {[Cu₄L^dur(μ₂-Cl)₆](PF₆)₂ · 10H₂O}_n (2), crystallized from solution. In both complexes, each of the four triazacyclononane (tacn) rings of the L^dur 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.