Syntheses, crystal structures and magnetic properties of one-, two- and three-dimensional 2,2′-bipyrimidine-containing copper(II) complexes

Abstract

Three new copper(II) complexes of formula [Cu(bipym)(H₂O)₂][NO₃]₂·H₂O 1, [Cu(bipym)(Cr₂O₇)] 2 and [Cu(bipym)(SO₄)]·H₂O 3 (bipym = 2,2′-bipyrimidine) have been synthesized and characterized by single-crystal X-ray diffraction. Compound 1 loses water very easily transforming into [Cu(bipym)][NO₃]₂ 1′. The structure of 1 consists of zigzag chains of bipym-bridged copper(II) ions with unco-ordinated nitrate anions and co-ordinated and crystallization water molecules, whereas the structures of 2 and 3 are made up of bipym-bridged copper(II) chains which are connected through bis(monodentate)-dichromate (2) and -sulfate (3) groups to yield sheet-like (2) and three-dimensional (3) polymers. The copper atom in compounds 1–3 exhibits a distorted elongated-octahedral co-ordination. In 1, it is linked to two bis(chelating) bipym groups and two water molecules in cis positions. The equatorial positions of the octahedron are occupied by three bipym-nitrogen atoms and a water-oxygen, while the axial ones are filled by the remaining bipym-nitrogen and the second aqua ligand. The equatorial mean planes of two adjacent copper(II) ions within the chains are mutually perpendicular. In 2, two different bis(chelating) bipym groups alternate regularly within the chain. The four nitrogen atoms of one of them build the equatorial positions of two neighbouring octahedra and the two remaining equatorial sites around each metal atom are filled by an oxygen atom of the dichromate group and a nitrogen atom of the second bipym. The two trans axial positions are occupied by another nitrogen atom of the second bipym and an oxygen atom of the dichromate anion. The two adjacent equatorial copper(II) mean planes through this second bridging bipym in 2 are mutually parallel. Two adjacent crystallographically independent metal atoms alternate regularly within the bipym-bridged copper(II) chain in 3. Their equatorial plane is built by three nitrogen atoms from bipym and one oxygen atom from a sulfate anion, whereas the axial positions are filled by the remaining bipym nitrogen atom and an oxygen atom from another sulfate anion. The equatorial mean planes of adjacent copper(II) ions within the bipym-bridged chain of 3 are mutually perpendicular as in 1. The metal–metal separations across bridging bipym are 5.646(1) Å in 1, 5.486(2) and 5.765(2) Å in 2 and 5.648(1) and 5.715(1) Å in 3, values which are close to that through bridging dichromate [5.774(1) Å in 2] but shorter than those through bridging sulfate [6.464(2) and 6.417(2) Å in 3]. Variable-temperature magnetic susceptibility measurements reveal that bridging bipym is able to mediate relatively strong (1′ and 2) and weak (1 and 3) antiferromagnetic couplings. The magnitude of the exchange coupling in this series is analyzed and discussed in the light of the complexes structural patterns.