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 [Cu-2(bpcam)(2)(H2O)(2)(CO4)] (1), [Cu-2(bpCO(2)- cam)(2)(H2O)(4)(C4O4)] center dot 10 H2O (2) and Cu-2(bpcam)(2)(C5O5)(H2O)(3) (3) [bpcam = bis(2-pyrimidyl)amidate, C2O42- = dianion oxalic acid, C4O42- dianion of 3,4-dihydroxycyclobut-3-ene-1, 2-dione and C5O52- dianion of 4,5-dillydroxy cyclopent4-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 I 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) angstrom (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 (H) over cap = -J (S) over cap (A) center dot (S) over cap (B). 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 (theta = - 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. (c) 2005 Elsevier B.V. All rights reserved.