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, [Cu4Ldur(mu(2)-OH)(4)]Cl2PF6)(2) center dot 8H(2)O (1), was isolated from a solution of L-dur and Cu2+ at pH 9. Under acidic conditions (pH 3), a polymeric complex, {(Cu4Ldur([mu(2)-Cl)(6)](PF6)(2) center dot 10H(2)O}(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) dinners with Cu-2(mu(2)-OH)(2) cores folded at the O center dot center dot center dot O lines. Two forms of the tetranuclear units, featuring slightly different Cu-2(mu(2)-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 I D helical polymeric structure. Variable-temperature magnetic susceptibility measurements suggest that the hydroxo-bridged copper(II) centres within one of the tetranuclear units in I 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. (c) 2005 Elsevier B.V. All rights reserved.