Two novel unusual compounds [Co(NCO) 2 (bpa)] (1) and [Cu(N 3 ) 2 (bpa)] (2) with cyanate and azide pseudohalides are shown in this work. These compounds have been characterized by X-ray diffraction measurements, Fourier transform-infrared (FT-IR) and UV-visible spectroscopies, thermogravimetric studies, electron spin resonance (ESR) spectroscopy, and magnetic measurements. Compound 1 shows a one-dimensional (1D) structure with the cobalt atoms bridged by bpa ligands and with the metal atoms showing an unusual tetrahedral coordination sphere. Compound 2 shows a two-dimensional (2D) structure, where the copper(II) ions are bridged by double 1,3-azido ligands to form chains that are joined through single bpa bridges to give the global 2D structure. Magnetic measurements show predominance of spin-orbit coupling for the cobalt compound (1), while ferromagnetic interactions, unusual for the 1,3-azido bridging, are observed in compound 2. This compound represents the first example of copper-double (μ 1,3 -azido)-bridged chain exhibiting ferromagnetic interactions through this kind of bridging.
' INTRODUCTIONThe design and synthesis of molecular-based magnetic materials with ferromagnetic coupling continue to be a challenge in magnetochemistry. 1 Azide and cyanate pseudohalides have focused the attention in this field due to their ability as magnetic, even ferromagnetic, exchange couplers. As we know, azide as a short bridging ligand that can provide rich coordination modes and efficient pathways of magnetic exchange, and hence can affect the spin ground state of a molecule, has been widely used in magnetochemistry. 2 This ion can act as a bridging ligand either in the end-to-end (μ-1,3-N 3 ) or in the end-on (μ-1,1-N 3 ) mode to give polynuclear complexes from dinuclear to three-dimensional (3D) systems. It is the general norm that the μ-1,3-coordination mode gives rise to antiferromagnetism, while μ-1,1-coordination results in ferromagnetic 3 interactions between the metal centers. From the large number of complexes with azido ligands magnetically characterized in the last decades, 2,4 examples exhibiting ferromagnetic coupling for the end-to-end type of bridging are really scarce. 5 The characterized compounds with the cyanate ligand are very scarce in comparison with those of the azide ligand. 6 This ion shows a lower versatility and clearly prefers the end-on N-cyanate bridging mode, which has been shown to give ferromagnetic type interactions. It is therefore interesting to obtain more examples concerning this bridging ligand.On the other hand, the 4-4 0 -type bpa ligand has been chosen because of its recognized ability to interconnect metal ions and increase the dimensionality of the systems where it participates. 7