Highly Anisotropic Rhenium(IV) Complexes: New Examples of Mononuclear Single-Molecule Magnets

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 center dot center dot center dot rhenium distance being 9.373(1) angstrom. 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.; Kruszynski, R.; Mrozinski, 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.