Chapter 12
The Azido Ligand: A Useful Bridge for Designing High-Dimensional Magnetic Systems 1
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Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: October 24, 1996 | doi: 10.1021/bk-1996-0644.ch012
R. Cortés , L. Lezama , F. A. Mautner , and T. Rojo 1
Departamento de Quimica Inorgánica, Facultad de Farmacia, Universidad del Pais Vasco, Apartado 450, E-01080 Vitoria-Gasteiz, Spain Departamento de Quimica Inorgánica, Facultad de Ciencias, Universidad del Pais Vasco, Apartado 644, E-48080 Bilbao, Spain Institut für Physikalische und Theoretische Chemie, Technische Universität Graz, A-8010 Graz, Austria 2
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Designing new high-dimensional magnetic molecular systems built from coordination compounds has recently been a point of attention for inorganic chemists. The variety of coordination chemistry provides the synthesizers with a useful tool to build magnetic molecular architectures interesting for their properties, which arise from the interaction among their subunits. Strategies based on the reaction of appropriate terminal and bridging ligands with paramagnetic metal ions allow the preparation of oligomeric species whose nuclearity and magnetic properties may, in any sense, be tailored (1). By using good superexchange bridging groups such as oxalate or cyanide, extended lattices of antiferromagnetic or ferrimagnetic systems which show magnetic order at low temperatures have been achieved (2,3). In this way, the azido ligand represents a good choice for the design of new magnetic systems. This ligand is able to give ferromagnetic exchange interactions (through its end-on (EO) coordination mode) and antiferromagnetic ones (through the end-to-end (EE) mode). It is also important to note the great versatility in their coordination modes, giving rise to high nuclearity systems. The Scheme shows the different bridging modes actually observed for this ligand. Former studies carried out for the azido ligand led to dinuclear entities, generally doubly bridged in the end-to-end form (4,5). More unusual end-on mode was obtained in some copper(II) dinuclear systems (6). Our preliminary work in this field was done with the aim of obtaining compounds exhibiting the unusual end-on bridging mode. From those previous studies, we have been searching for a continuous increase in the dimensionality of the prepared compounds in order to study their magneto-structural correlations. In this chapter we report the most significant structural and magnetic results obtained by our group for dinuclear, one-, two-, and three-dimensional systems.
0097-6156/96/0644-O187S15.00/0 © 19% American Chemical Society
In Molecule-Based Magnetic Materials; Turnbull, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
MOLECULE-BASED MAGNETIC MATERIALS
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Downloaded by STANFORD UNIV GREEN LIBR on October 8, 2012 | http://pubs.acs.org Publication Date: October 24, 1996 | doi: 10.1021/bk-1996-0644.ch012
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Figure 1. Crystal structure of [M(N )2(terpy)]-H 0 (M= Mn, Ni). 3
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In Molecule-Based Magnetic Materials; Turnbull, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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12. CORTES ET AL.
189
The Azido Ligand and nD Magnetic Systems
Dinuclear Compounds. Work centered on copper(II)-halide dinuclear systems led us to observe that the use of rigid tridentate aromatic amines favors a ci5-coordination of the bridging groups around the metal ion. Using the halide compounds as precursors, we have developed a synthetic strategy to obtain cw-pseudohalide-bridged dinuclear compounds of general formula [M(pseudohalide) (L )]-H 0 [M= N i , M n ; L = 2,2':6 ,2"-terpyridine (terpy), N'-(2-pyridin-2-ylethyl)-pyridine-2-carbaldimine (pepci)] (7-9). As a result, several nickel(II) (7,8) and manganese(H) (9) dinuclear azido compounds, always exhibiting the EO bridging mode, have been obtained. A molecular structure of this kind of dinuclear system is shown in Figure 1. The general structure for all these complexes consists of centrosymmetric dinuclear units where the metal ions are doubly bridged by E O - N ligands, with the tridentate ligand occupying three of the four equatorial sites of the coordination polyhedron of each metal and a terminal azido group to complete the hexacoordination. In all cases, the values of the bridging angles are lower than 105°. Ferromagnetic interactions between the metal centers are favored with this kind of bridging (6-9). In this way, the magnetic results for the nickel(II) and manganese(II) compounds are illustrated in Figure 2 as an example. As can be observed, the x T value increases upon cooling, reaching a maximum after which it rapidly decreases in the case of the nickel compound, as a consequence of the zero-field splitting, while for manganese the % T value remains stable at lower temperatures. This clearly indicates intradimer ferromagnetic interactions in both complexes. In fact, ferromagnetic exchange constants in the range 5-40 c m , which are proportional to the value of the bridging angle, have been obtained for the nickel compounds. However, the only manganese(II) compound known, with a bridging angle of 101°, exhibits a J value of 2.5 cm . Ferromagnetic behavior associated with this kind of bridging has usually been explained by using the spin polarization theory (10), which predicts ferromagnetism for the range of bridging angle values. However, in the case of copper dimers it has recently been demonstrated (77) that ferromagnetic behavior prevails at small bridging angles (
