Structure, stacking interactions and magnetism of compounds with oxalate-bridged dinuclear Cu II Cu II and Cu II Nb V units Marijana Juric ´ a , Berislav Peric ´ a , Nevenka Brnic ˇevic ´ a, * , Pavica Planinic ´ a , Damir Pajic ´ b , Kres ˇo Zadro b , Gerald Giester c a Rud - er Bos ˇkovic ´ Institute, Bijenic ˇka cesta 54, 10000 Zagreb, Croatia b Department of Physics, Faculty of Science, University of Zagreb, Bijenic ˇka cesta 32, 10000 Zagreb, Croatia c Institut fu ¨ r Mineralogie und Kristallographie, Universita ¨ t Wien – Geozentrum, Althanstraße 14, 1090 Wien, Austria Received 21 July 2006; accepted 25 August 2006 Available online 3 September 2006 Abstract Two novel compounds of the formulae [{Cu(phen) 2 } 2 (l-C 2 O 4 )][Cu(phen) 2 (l-C 2 O 4 )NbO(C 2 O 4 ) 2 ] 2 Æ 8H 2 O(1) and [{Cu(bpy) 2 } 2 - (l-C 2 O 4 )][Cu(bpy) 2 (l-C 2 O 4 )NbO(C 2 O 4 ) 2 ] 2 Æ 0.5bpy Æ 7H 2 O(2) (phen = 1,10-phenanthroline, bpy = 2,2 0 -bipyridine) have been prepared and characterized by single crystal X-ray diffraction, IR spectroscopy and magnetic susceptibility measurements. The molecular structure of both 1 and 2 consists of a discrete Cu II Cu II oxalate-bridged dinuclear [{Cu(L) 2 } 2 (l-C 2 O 4 )] 2+ cation (A unit) and two Cu II Nb V oxalate- bridged dinuclear [Cu(L) 2 (l-C 2 O 4 )NbO(C 2 O 4 ) 2 ]  anions (B units) (L = phen, bpy). In 1 a crystallographic inversion centre is located in the middle of the oxalate bridge of the A unit, whereas in 2 an analogous inversion centre is missing. In the A units the copper(II) atoms adopt a tetragonally elongated octahedral coordination with the equatorial planes being perpendicular to the mean planes of the oxalate bridge and parallel to each other. In both structures, similar one-dimensional motifs are generated through the ligand stacking interac- tions, with a difference that one free bipyridine molecule, present in 2, intercalates into one of the motifs. It is shown that the phenan- throline ligand, due to its ability of stacking through the central aromatic ring, causes longer intermolecular CuCu distances than the bipyridine ligand. The magnetic susceptibility measurements (1.8–290 K) show the ferromagnetic exchange interaction between the copper(II) atoms in the A units of both compounds, with J = +5.9 cm 1 and +7.9 cm 1 for 1 and 2, respectively (J – the exchange parameter in the isotropic spin Hamiltonian H INT = JS 1 Æ S 2 ). Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Copper–niobium complexes; Dinuclear units; Crystal structures; Oxalate bridges; Ferromagnetic interaction 1. Introduction The use of tris(oxalato)metallates, [M III (C 2 O 4 ) 3 ] 3 (M III = Cr, Fe, Ru, V), in the synthesis of new homo- or heterometallic oxalate-bridged complex systems is con- stantly growing. In the reactions of these anions with organic and inorganic cations different structure types resulted ranging from discrete polynuclear metal species [1–3] to polymeric one- (1D) [4,5], two- (2D) [6–13] or three-dimensional (3D) [14–16] assemblies. In these investi- gations the scientists have been trying to couple two or more paramagnetic metal centres using the oxalate C 2 O 4 2 anion, due to its ability of exhibiting a bis(bidendate) bridging coordination mode and to mediate the magnetic exchange interaction between metal ions. In general, the magnetic properties of these systems follow their structure types. Within the discrete molecules ferro- or antiferromag- netic exchange interactions have been found [1–3]. The 1D structure of [K(18-crown-6)][Mn(H 2 O) 2 Cr(C 2 O 4 ) 3 ] Æ 0.5(18-crown-6) (18-crown-6 = C 12 H 24 O 6 ) shows the ferro- magnetic coupling within the chains as well as between the 0277-5387/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2006.08.024 * Corresponding author. Tel.: +385 1 456 1189; fax: +385 1 468 0098. E-mail address: brnicevi@irb.hr (N. Brnic ˇevic ´). www.elsevier.com/locate/poly Polyhedron 26 (2007) 659–672