Syntheses, crystal structures and magnetic properties of chromato-, sulfato-, and oxalato-bridged dinuclear copper(II) complexes

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Four dinuclear copper(II) complexes of formula [Cu2(bpca)2(H2O)3(CrO4)]·H2O (1), [Cu2(bpca)2(H2O)3(SO4)]·H2O (2), [Cu2(bpca)2(H2O)2(C2O4)]·2H2O (3), and [Cu2(bpca)2(C2O4)] (4) [bpca=bis(2-pyridylcarbonyl)amide anion] have been synthesized and their magnetic behavior has been investigated as a function of temperature. The structures of 1–3 have been determined by single-crystal X-ray diffraction, whereas the structure of 4 was already known. The structures of this family of complexes are made up of neutral chromate-O1,O1′ (1), sulfate-O1,O1′ (2) and oxalate-O1,O2:O1′,O2′-bridged (3 and 4) dinuclear copper(II) units. The two copper atoms within the dinuclear unit of the isomorphous compounds 1 and 2 show different surroundings: they exhibit distorted square pyramidal (Cu(2)) and octahedral (Cu(1)) surroundings with the three bpca-nitrogen atoms and either a chromate (1)/sulfate (2)-oxygen atom (Cu(2)) or a water-oxygen atom (Cu(1)) defining the equatorial positions, whereas the axial sites are occupied by a water molecule (Cu(2) and Cu(1)) and a chromate (1)/sulfate (2)-oxygen atom (Cu(1)). Each copper atom of the centrosymmetric compound 3 is six-coordinated with the three bpca-nitrogen atoms and an oxalate-oxygen forming the equatorial plane, whereas the axial positions are occupied by the other oxalate-oxygen and a water molecule. Complex 4 is also centrosymmetric, each copper atom exhibiting a distorted square pyramidal surrounding. The equatorial plane is the same as in 3, and an oxalate-oxygen occupies the axial position. The intramolecular copper-copper distances are 3.660(1) Å (1), 3.747(1) Å (2) and 5.631(1) Å (3) (5.442(1) Å in 4). The magnetic study of 1–4 reveals the occurrence of weak intramolecular antiferro- (1 and 2) and ferromagnetic (3 and 4) interactions. The magnitude and nature of the magnetic coupling through these extended bridges are analyzed and discussed in the light of the available structural data.