Three new polynuclear tetracarboxylato-bridged copper(II) complexes: Syntheses, X-ray structure and magnetic properties

The synthesis, crystal structure and magnetic measurements of three new polynuclear tetracarboxylato-bridged copper(II) complexes, i.e. {[Cu-4(phen)(2)(mu-O2CC2H5)(8)] center dot (H2O)}(n) (1), [Cu-2(mu-O2CC6H4OH)(4)(C7H7NO)(2)] center dot 6H(2)O (2) and [Cu-2(mu-O2CCH3)(4)(C7H7NO)(2)] (3) (phen = 1,10-phenanthroline, O2CC6H4OH = 3-hydroxy benzoate, C7H7NO = 4-acetylpyridine) are reported. All compounds consist of Binuclear units, in which two Cu(II) ions are bridged by four syn,syn-eta(1):eta(1):mu carboxylates, showing a paddle-wheel cage type with a square-pyramidal geometry; arranged in different ways. The structure of compound 1 consists of an one-dimensional structure generated by an alternating classical dinuclear paddle-wheel unit and an unusual dinuclear Cu-2(mu-OCOC2H5)(2)(mu-O2CC(2)H(5))(2)(phen)(2) unit, which are connected to each other via a syn,anti-triatomic propionato bridge in an axial-equatorial configuration. The adjacent chains are connected to generate a 2D structure through the face-to-face pi-pi interaction between phen rings. Structures of compounds 2 and 3 both consist of a symmetric Binuclear Cu(II) carboxylate paddle-wheel core and pyridyl nitrogen atoms of 4-acetylpyridine ligand at the apical position, and just differ in the substituents of the equatorial ligands. The magnetic properties have been measured and correlated with the molecular structures. It is found that in the two classical paddle-wheel compounds the Cu(II) ions are strongly antiferromagnetically coupled with J = -278.5 and -287.0 cm(-1) for complexes 2 and 3, respectively. In compound 1 the magnetic susceptibility could be fitted with two different, independent Cu(II) units, one strongly coupled and one weakly coupled; the paddle-wheel dinuclear unit has the strongest anti ferromagetic coupling with a value for J of -299.5 cm(-1), whereas the Cu(II) ions in the propionato-bridged dinuclear unit of 1 display a very weak antiferromagnetic coupling with a value for J = -0.75 cm(-1), due to the orthogonality of the magnetic orbitals. Also the exchange within the chain is therefore very weak. The magneto-structural correlations for complexes 1, 2, and 3 are discussed on the basis of the structural parameters and magnetic data for the complexes. (C) 2008 Elsevier Ltd. All rights reserved.