title: Highly anisotropic rhenium(IV) complexes: new examples of mononuclear single-molecule magnets.
abstract: The rhenium(IV) complex (NBu4)2[ReBr4(ox)] (1) (ox = oxalate and NBu4(+) = tetra-n-butylammonium cation) has been prepared and its crystal structure determined by X-ray diffraction. The structure is made up of discrete [ReBr4(ox)](2-) anions and bulky NBu4(+) cations. Each [ReBr4(ox)](2-) anion is surrounded by six NBu4(+) cations, which preclude any significant intermolecular contact between the anionic entities, the shortest rhenium···rhenium distance being 9.373(1) Å. Variable temperature dc and ac magnetic susceptibility measurements and field-dependent magnetization experiments on polycrystalline samples of 1 reveal the occurrence of highly anisotropic magnetically isolated Re(IV) centers (S(Re) = 3/2), which exhibit slow relaxation of the magnetization at very low temperatures in a dc field. Ac measurements conducted on a polycrystalline sample of the complex (NBu4)2[ReCl4(ox)] (2) [compound isostructural to 1 whose structure and dc magnetic susceptibility study were previously reported in Tomkiewicz, A.; Bartczak, T. J.; Kruszyński, R.; Mroziński, J. J. Mol. Struct. 2001, 595, 225] show a similar behavior, both complexes thus constituting new examples of mononuclear single-molecule magnets. High-frequency and -field electron paramagnetic resonance on polycrystalline samples of 1 and 2 and on single crystals of 2 allowed for the determination for the first time of the negative sign and confirmed a significant magnitude and rhombicity (E/D) of the zero-field splitting tensor of the [ReCl4(ox)](2-) and [ReBr4(ox)](2-) centers, originating from a combination of spin-orbit coupling and low molecular symmetry. D and E values of 1 and 2 were estimated through magnetization measurements and theoretically calculated through complete active space and density functional theory methodologies.
authors: Departament de Química Inorgànica/Instituto de Ciencia Molecular (ICMol), Universitat de València , C/Catedrático José Beltrán 2, 46980 Paterna (València), Spain.
authors: José
authors: J
authors: Martínez-Lillo
authors: Teresa F
authors: TF
authors: Mastropietro
authors: Elsa
authors: E
authors: Lhotel
authors: Carley
authors: C
authors: Paulsen
authors: Joan
authors: J
authors: Cano
authors: Giovanni
authors: G
authors: De Munno
authors: Juan
authors: J
authors: Faus
authors: Francesc
authors: F
authors: Lloret
authors: Miguel
authors: M
authors: Julve
authors: Saritha
authors: S
authors: Nellutla
authors: J
authors: J
authors: Krzystek