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Magnetic interactions between d1 [MOR4]- units of molybdenum and tungsten

AutorGarcía-Monforte, M. Ángeles ; Alonso, Pablo J.; Arauzo, Ana B.; Baya, Miguel ; Luis, Fernando; Martín, Antonio ; Menjón, Babil ; Rillo, Conrado
Fecha de publicación2016
CitaciónSCTE (2016)
ResumenFive-coordinate oxo compounds with formula [MOX4]- and open-shell electron configuration (d1) are known for all Group 6 metals. This kind of model compounds have attracted a great deal of attention over the years mainly because of two reasons: 1) they have played a key role in the proper understanding of the chemical bond between the oxo ligand and the metal center, and 2) because of their relevance to various Mo-containing enzymatic systems operating as oxotransferases and hydroxylases. In any case, the X ligands under consideration have always been typical (σ+π)-donors, such as halogens or thiolates, which should have a distinct effect on the precise ordering of the molecular orbitals. The properties of homologous compounds with the MO3+ unit being located in a coordination framework of s-only ligands are virtually unknown. Here we report on the synthesis and characterization of open-shell, five-coordinate [MOR4]- oxo-derivatives for the heavier Group 6 metals (M = Mo, W), with R being the bulky perfluorophenyl group C6F5, which acts mainly as a σ-donor ligand. The metal centers in the [NBu4][MO(C6F5)4] salts are located in regular square-pyramidal coordination environments as established by single-crystal X-ray diffraction. Thus, the paramagnetic (d1) MO3+ units are placed within an organic framework of near tetragonal symmetry defined by the four basal C-donor atoms. The C6F5 groups are hard ligands of mainly σ-donor character. This fact greatly simplifies the detailed description of the electronic structure of the [MOX4]− unit, which we consider best portrayed as containing a M≡O triple bond. Crystals of the [NBu4][MO(C6F5)4] salts of the heavier Group 6 metals Mo and W are isomorphic and isostructural with those of the lighter metal, Cr. In all cases, the crystal lattice is formed by alternating layers of cations and anions. The square-pyramidal [MO(C6F5)4]- anions show weak intermolecular contacts between meta- and para-C atoms of neighboring and roughly parallel perfluorophenyl rings (interplanar angle ~10°), as shown in Figure 1 for M = W. We consider these weak π-π interactions as the most likely path to explain the interactions between the individual magnetic centers that become noticeable at very low temperatures. The magnetic properties of these open-shell derivatives have been studied by a combination of spectroscopic (EPR), calorimetric and magnetochemical techniques. At higher temperatures, a simple paramagnetic behavior is observed both in solution and in the solid state, corresponding to separate single-spin (d1) magnetic entities.
DescripciónResumen del trabajo presentado a la 20th International Conference on Solid Compounds of Transition Elements, celebrada del 11 al 15 de abril de 2016 en Zaragoza (España).
URIhttp://hdl.handle.net/10261/148866
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