Hexahalorhenate(iv) salts of protonated ciprofloxacin: antibiotic-based single-ion magnets

Abstract

Two novel Re^IV compounds of formula [H₂cip][Hcip][ReCl₆]Cl·H₂O (1) and [Hcip]₂[ReBr₆] (2) [(H₂cip)²⁺/(Hcip)⁺ = ciprofloxacindiium cation/ciprofloxacinium cation] have been synthesized and studied structurally and magnetically. 1 crystallizes in the monoclinic system with space group P2₁/c, whereas 2 crystallizes in the orthorhombic system with space group Pbca.1 and 2 are hexahalorhenate(IV) salts obtained with the protonated ciprofloxacin antibiotic. In their crystal lattice, the [ReX₆]²⁻ [X = Cl(1) and Br(2)] anions are well separated from each other through the protonated ciprofloxacindiium (1) and ciprofloxacinium (1 and 2) cations, which are arranged without generating intermolecular Re−X⋯X−Re interactions between the paramagnetic centers. In both compounds, intermolecular π⋯π (1) and π⋯F (2) type interactions [the shortest centroid⋯centroid and centroid⋯fluorine distance being approximately 3.49(1) and 2.95(1) Å, respectively], as well as hydrogen bonds (1 and 2), are present. The self-assembly of these protonated organic cations and Re^IV complexes generate novel supramolecular structures based on this paramagnetic 5d metal ion. The study of their magnetic properties through variable-temperature dc magnetic susceptibility measurements reveals a magnetic behaviour typical of noninteracting mononuclear Re^IV complexes (S_Re = 3/2), with magnetically isolated spin quartets and high values of their zero-field splitting (ZFS), for 1 and 2. In addition, AC magnetic susceptibility measurements in the presence of external dc fields show slow relaxation of the magnetization, which is a behavior compatible with the occurrence of the field-induced single-ion magnet (SIM) phenomenon for 1 and 2. This is the first magnetostructural study performed on salts based on protonated ciprofloxacin antibiotic and a paramagnetic 5d ion.