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···rhenium distance being 9.373(1) A. 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 (SRe = 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.; Bartc...