Syntheses, crystal structures and magnetic properties of dinuclear copper(II) complexes with pyrazino[2,3-f][4,7]phenanthroline (pap) as bridging ligand

Abstract

Three dinuclear copper(II) complexes with pyrazino[2,3-f][4,7]phenanthroline (pap) as bridging ligand have been prepared; [Cu₂(pap)(C₂O₄)₂]·5H₂O 1, [Cu₂(pap)(H₂O)₇(SO₄)]SO₄·3H₂O 2 and [Cu₂(pap)(H₂O)₃(NO₃)₃]NO₃3. These are the first metal complexes of pap which have been characterized by X-ray crystallography and magnetic susceptibility measurements. In 1 the dinuclear complex is intercepted by a mirror plane; the bridging pap and the terminal oxalate ligands are bidentate in the equatorial plane of copper. In addition copper has weak axial interactions to oxygen atoms of oxalate in two neighbouring molecules. In 2 the two crystallograpically independent copper atoms are both six-co-ordinated with bridging pap and water molecules in equatorial positions, the axial positions being occupied by two water molecules or one water molecule and one sulfate oxygen atom, respectively. In 3 the co-ordination geometry of one copper is close to square pyramidal with pap, one nitrate oxygen atom and one water in equatorial positions; another water occupies the apical position. The description of the co-ordination sphere of the other copper is complicated by the presence of disorder in one of the co-ordinated nitrate groups. When considering only the major site of the disordered nitrate, the co-ordination geometry may in a first approximation be described as square pyramidal with a significant trigonal bipyramidal distortion. In the square pyramidal description the two pap nitrogen atoms, one water and one nitrate oxygen constitute the equatorial plane and a nitrate oxygen is situated in apical position. All the co-ordinated nitrate ions have a second weak Cu⋯O interaction. The Cu⋯Cu separations across bridging pap are 6.740 (1), 6.834 (2) and 6.808 Å (3). Variable-temperature susceptibility measurements reveal Curie law behaviour with very weak intramolecular antiferromagnetic coupling in all three compounds, the relevant parameters being J = −1.4 cm⁻¹, g = 2.06 for 1, J = −1.5 cm⁻¹, g = 2.10 for 2, and J = −1.3 cm⁻¹, g = 2.08 for 3 (the Hamiltonian being H = −J S_A·S_B).