Proton-Conducting Hydrogen-Bonded Framework of a Cobalt(II) Single-Ion Magnet Sulfonate

Diamagnetic metal sulfonates have been widely reported, while paramagnetic species are very rare, especially those that exhibit interesting magnetic and/or proton conduction properties. Herein, we report the synthesis, structure, magnetic, and proton-conducting properties of a hydrogen-bonded cobalt(II) organosulfonate complex. The coordination self-assembly of Co^II salts and 8-quinolinesulfonic acid ligands affords a mononuclear Co^II sulfonate featuring both coordinated and noncoordinated sulfonic acid O atoms and axial coordinated water molecules. Notably, the Co^II units are further connected by short S-O···H-O hydrogen-bonding interactions between SO₃^- and coordinated H₂O, leading to a three-dimensional (3D) hydrogen-bonded network. This hydrogen-bonded sulfonate exhibits superior thermal stability, as proved by variable-temperature single-crystal and powder X-ray diffraction and thermogravimetric analysis (TGA) analysis. Variable-temperature and variable humidity ac impedance spectroscopy indicated this cobalt sulfonate is a good superionic proton conductor with the highest measured conductivity of 1.5 × 10^-3 S cm^-1 at 90 °C under 97% relative humility, originating from 1D zigzag hydrogen-bonded chains. In addition, field-induced slow magnetic relaxation was observed via dynamic ac magnetic susceptibility measurements. These results show not only the first proton-conducting Co(II) single-ion magnet sulfonate but also a ″magnetic anisotropic metal ion-organosulfonate-coordinated water″ approach for the design and preparation of bifunctional metalo-hydrogen-bonded organic framework (MHOF) materials.