Proton Conductive Mononuclear Hydrogen-Bonded Cobalt(II) Spin Crossover Complex

Abstract

The development of molecule-based switchable multifunctional materials remains an important challenge in molecular science. In particular, achieving high proton conductivities in spin crossover (SCO) compounds is of significant interest as the spin state can be controlled via proton transfer induced by an electric field, which leads to magnetoelectric effects and molecular spintronics. However, to the best of our knowledge, no studies have reported on proton-conductive SCO complexes. Thus, herein, we demonstrated the high proton conductivity of a mononuclear hydrogen-bonded Co(II) SCO compound, [Co(COOH-terpy)2](ClO4)2·4H2O (1·4H2O; COOH-terpy = 4′-carboxyl-2,2′:6′,2″-terpyridine). The [Co(COOH-terpy)2] 2+ cation assembled in the solid state via hydrogen-bonding between lattice water molecules and formed a 1D hydrogen-bonded chain. Each 1D chain was linked via CH−π interactions and C–C interactions and produced a complicated 3D supramolecular framework. Magnetic susceptibility and alternative current impedance measurements for 1·4H2O revealed incomplete spin transition behaviors and high-proton-conduction behaviors in identical temperature regions. 1·4H2O exhibited a proton conductivity of 1.68 × 10–3 S cm–1 at 353 K and 98% relative humidity. Notably, this is the first example of a proton-conductive mononuclear SCO compound having the hydrogen-bonding networks. Thus, the results of this study can be used to develop new design guidelines for molecular proton conductors, further leading to the development of multifunctional molecular switching materials.