A Pair of Hydrogen-Bonded Cobalt(II) Complexes Showing the Proton Conduction and Spin Crossover Property

Achieving high proton conductivity in spin-crossover (SCO) compounds is promising for the development of magnetoelectric and spintronics devices. In this work we designed two spin-crossover and proton-conductive bifunctional Co(II) compounds, [Co(Pyrimidine-terpy)₂](BF₄)₂⋅2H₂O (1⋅2H₂O; Pyrimidine-terpy=4′-(5-pyrimidinyl)-2,2′:6′,2“-terpyridine) and [Co(Pyrimidine-terpy)₂](ClO₄)₂⋅2H₂O (2⋅2H₂O). Both compounds undergo the typical spin transitions and have a hydrogen-bonding network consisting of anions with solvent water molecules. At 353 K and under 95 % relative humidity, the proton conductivity of 1⋅2H₂O was 1.9×10⁻⁴ S cm⁻¹ and that of 2⋅2H₂O was 7.5×10⁻⁵ S cm⁻¹. The activation energy analysis indicates that the proton conduction of 1⋅2H₂O follows the Vehicle mechanism in the temperature range of 303–318 K, while the Grotthuss mechanism plays a dominant role in the higher temperature range of 323–353 K. Additionally, 2⋅2H₂O also follows the Grotthuss mechanism in the temperature range of 338–353 K. This study provides new guidelines for the design of novel SCO molecular materials with proton conduction functionality.