A straightforward synthetic route towards tetranuclear copper(II) complexes: reactions between binuclear complexes and exo-bidentate or exo-bis(bidentate) ligands

The reactions between binuclear copper(II) complexes of the type [L(μ-OH)Cu2](ClO4)2 with exo-dentate ligands lead to tetranuclear complexes with both short and long distances between the metallic centers (L=the compartmental end-off ligand, 2,6-bis[N-2-pyridylethyl)formimidoyl]-phenolato). Three new copper(II) complexes have been obtained by using the following spacers: bis(4-pyridyl)ethane (1), the dianion of the acetylenedicarboxylic acid (2) and 4,4′-bipyridine-N,N′-dioxide (3). The crystal structures of the three compounds have been solved. Compound 1 contains cationic [Cu4] rectangles, with the short edges constructed from the preexisting μ-OH and μ-phenoxo groups, and the long ones constructed from bis(4-pyridyl)ethane molecules. The crystal 2 contains centrosymmetric tetranuclear cationic species, which result by connecting two binuclear units with one dicarboxylato linker. Each carboxylato group forms a third bridge between the copper(II) ions within the binuclear moiety (syn–syn bridging more). In compound 3, one 4,4′-bipyridine-N,N′-dioxide molecule (4,4′-bpno) connects two copper(II) ions arising from two binuclear cationic entities. The two binuclear moieties are almost parallelly disposed above and below the 4,4′-bpno bridge. The Cu(II) ions bridged by 4,4′-bpno show a square-pyramidal stereochemistry, with the oxygen atom arising from 4,4′-bpno in the apical position. The other two copper(II) ions, which are not bridged by 4,4′-bpno, exhibit a square-planar coordination geometry.