Oxalate, squarate and croconate complexes with bis(2-pyrimidylcarbonyl)amidatecopper(II): synthesis, crystal structures and magnetic properties

The preparation and magnetic properties of three copper(II) compounds of formulae [Cu2(bpcam)2(H2O)2(C2O4)] (1), [Cu2(bpcam)2(H2O)4(C4O4)] · 10 H2O (2) and Cu2(bpcam)2(C5O5)(H2O)3 (3) [bpcam = bis(2-pyrimidyl)amidate, View the MathML sourceC2O42-=dianionofoxalicacid,C4O42-=dianionof3,4-dihydroxycyclobut-3-ene-1,2-dione and View the MathML sourceC5O52-=dianionof4,5-dihydroxycyclopent-4-ene-1,2,3-trione] are reported. The structures of two of them (1 and 2) have been solved by single crystal X-ray diffraction and consists of centrosymmetric discrete copper(II) dinuclear units bridged by bis-bidentate oxalate (1) and bis-monodentate squarate (2), with the bpcam group acting as a terminal tridentate ligand. Each copper atom in 1 exhibits a distorted elongated octahedral coordination geometry. Three bpcam nitrogen atoms and one oxalate oxygen define the basal plane while the other oxalate oxygen and a water molecule take up the axial positions. Each copper atom in 2 is in an elongated octahedral surrounding with three bpcam nitrogen atoms and one squarate oxygen in the equatorial plane and two water molecules in the axial positions. The intramolecular copper–copper separations are 5.677(1) (1) and 7.819(53) Å (2). Magnetic susceptibility measurements for 1–3 in the temperature range 1.9–290 K show the occurrence of weak ferromagnetic interactions through oxalato (J = +0.75 cm−1) and squarato (J = +1.26 cm−1), the Hamiltonian being defined by View the MathML sourceHˆ=-JSˆA·SˆB. These values are analyzed and discussed in the light of the available magneto-structural data for analogous systems. The quasi-Curie law observed in 3 (θ = −1.15 K) contrasts with the significant antiferromagnetic interaction through bis-chelating croconate in other structurally characterized croconate-bridged copper(II) complexes and rules out the presence of bridging croconate in this compound.