Copper(II) complexes with 2,5-bis(2-pyridyl)pyrazine and oxalate and croconate: Synthesis, crystal structure and magnetic properties

Two new copper(II) complexes of formula [Cu2(2,5-dpp)(C2O4)2(H2O)4] (1) and [Cu2(2,5-dpp)(C5O5)2(H2O)4]·3H2O (2) [2,5-dpp = 2,5-bis(2-pyridyl)pyrazine, C2O42− = oxalate and C5O52− = croconate (dianion of 4,5-dihydroxycyclopent-4-ene-1,2,3-trione)] have been prepared and their structures determined by X-ray crystallographic methods. Compounds 1 and 2 are dinuclear complexes where the 2,5-dpp molecule acts as a bis-bidentate bridge between the two copper(II) ions, the electroneutrality being achieved by the presence of bidentate oxalate (1) and croconate (2) ligands. Each copper(II) ion exhibits an elongated octahedral CuN2O4 environment with two nitrogen atoms from 2,5-dpp and two oxalate (1)/croconate (2)-oxygens occupying the equatorial positions and two water molecules (1 and 2) in the axial sites. The values of the copper–copper separation across bridging 2,5-dpp are 6.773(2) (1) and 6.728(2) Å (2). Variable-temperature magnetic susceptibility measurements of 1 and 2 in the temperature range 1.9–300 K reveal the occurrence of weak intramolecular antiferromagnetic interactions. The poor ability of the 2,5-dpp molecule to mediate significant magnetic interactions in its copper(II) complexes when adopting the bis-bidentate coordination agrees with that exhibited by the related polypyridine ligand 2,3-bis(2-pyridyl)pyrazine (2,3-dpp) but it contrasts with the better efficiency of the pyrazine (pyz) and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) bridging ligands.