Synthesis, crystal structure and magnetic properties of the cyclic tetranuclear compound [Cu4(pz)4(hppa)2(H2O)4] [pz = pyrazolate; hppa = R,S-2-hydroxo-2-phenyl-2-(1-pyrazolyl)acetate]

The synthesis, X-ray structure and magnetic properties of the neutral tetranuclear copper(II) complex of formula [Cu4(pz)4(hppa)2(H2O)4] (1) [Hpz = pyrazole and hppa = R,S-2-hydroxo-2-phenyl-2-(1-pyrazolyl)acetate] are reported. Remarkably, the structure of 1 reveals the presence of the S- and R-forms of the new hppa ligand which is formed in situ in the complex reaction between copper(II), pyrazole and phenylmalonate in water:methanol solvent mixture under ambient conditions. The two crystallographically independent copper(II) ions [Cu(1)/Cu(2)] are five-coordinate in square pyramidal surroundings. Three nitrogen atoms, from two pz groups and one hppa ligand and one oxygen atom of the same hppa dianion form the basal plane and a water molecule occupies the apical position at Cu(1) whereas two nitrogen atoms from two pz groups plus one carboxylate- and one hydroxo-oxygen atoms from a hppa ligand build the basal plane and a water molecule fills the apical site at Cu(2). Two types of bridging pathways alternate in the edges of the centrosymmetric cyclic tetracopper(II) unit: a single bis-monodentate pz in the longest edge [3.5850(5) A] and a double bridge consisting of a hppa ligand adopting the κ2O:κN:κO″ coordination mode and another bis-monodentate pz group in the shortest edge [3.3228(5) A]. Magnetic susceptibility measurements in the temperature range 50–300 K for 1 show the occurrence of strong antiferromagnetic interactions between the copper(II) ions through the two exchange pathways: J1 = 650(5) cm−1 (across the pz/hppa bridge) and J2 = −200(9) cm−1 (through the single pz linker). These values of the magnetic coupling are analysed and discussed in the light of the literature data of copper(II) complexes where related bridges occur.