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

Abstract

The preparation and magnetic properties of three copper(II) compounds of formulae [Cu 2(bpcam) 2(H 2O) 2(C 2O 4)] ( 1), [Cu 2(bpcam) 2(H 2O) 4(C 4O 4)] · 10 H 2O ( 2) and Cu 2(bpcam) 2(C 5O 5)(H 2O) 3 ( 3) [bpcam = bis(2-pyrimidyl)amidate, C 2 O 4 2 - = dianion of oxalic acid , C 4 O 4 2 - = dianion of 3 , 4 - dihydroxycyclobut - 3 - ene - 1 , 2 - dione and C 5 O 5 2 - = dianion of 4 , 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 H ˆ = - J S ˆ A · S ˆ 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 ( 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.