Humidity-Responsive Proton Conduction in A Cobalt(ii) Coordination Compound Derived from 2-phenyl-1,3,4-oxadiazole: Structural and Magnetic Investigations

A cobalt (II) coordination compound, diaquabis(2-phenyl-1,3,4-oxadiazole)cobalt(II) chloride, was synthesized, and its structural and magnetic properties and proton conductivity in the presence of moisture were also investigated. The preparation of the ligand, 2-phenyl-1,3,4-oxadiazole, was carried out by the oxidative cyclization of benzaldehyde semicarbazone followed by complexation with cobalt (II) chloride hexahydrate under reflux conditions. The structural formulation of the complex was established by the Rast molecular weight method and complexometric EDTA evaluation. It was confirmed that the metal–ligand stoichiometry was 1:2 with 2 coordinated water molecules and 2 chloride counter ions. Fourier transform infrared (FTIR) spectroscopy showed the formation of the oxadiazole ligand and the cobalt(II) ion coordinates with the N atom of the oxadiazole ring. The change in the ν(C=N) stretching vibration due to coordination and the appearance of the ν(Co–N) and ν(Co–O) bands supported the octahedral structure of the complex. Magnetic susceptibility measurements suggest that the cobalt (II) ion exists in a high-spin octahedral environment containing three unpaired electrons, which agrees with the proposed coordination structure. The proton transport behavior of the complex was analyzed by electrochemical impedance spectroscopy (EIS) at various hydrated conditions and temperatures. The Nyquist impedance plots indicate that as moisture time increases, the resistances of the bulk and grain boundary decreased significantly. Consequently, the proton conductivities enhanced markedly. With increasing hydration, a Warburg diffusion component was observed, signifying diffusion-assisted proton transport through interconnected hydrogen-bonded water networks. According to the temperature-dependent impedance study, it was confirmed that the proton conduction was thermally activated. Enhanced conductivity is mainly due to Grotthuss proton-hopping (assisted by the vehicle mechanism) under highly hydrated conditions. Coordination water molecules, adsorbed atmospheric moisture, and chloride counter ions establish a large hydrogen bond network that can transfer protons throughout the coordination network. The structural characterization, combined with magnetic studies and electrochemical studies, indicates the synthesized cobalt(II) complex has an excellent humidity-responsive proton-conducting behavior and structural and magnetic stability. The results indicate that diaquabis(2-phenyl-1,3,4-oxadiazole)cobalt(II) dichloride may be used as a coordination material for a variety of solid-state proton-conducting electrolytes, proton exchange membrane fuel cells, humidity sensors, electrochemical devices, and low-temperature energy conversion and storage.