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(COOHterpy) 2 ](ClO 4 ) 2 •4H 2 O (1•4H 2 O; 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•4H 2 O revealed incomplete spin transition behaviors and highproton-conduction behaviors in identical temperature regions. 1•4H 2 O 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.

Highly proton-conducting lanthanide metal–organic frameworks featuring highly oxygenated ligands with slow magnetic relaxation or magnetocaloric effect

“…16 In recent years, proton-conducting magnetic materials as novel bifunctional molecular magnetic materials have emerged, such as proton-conducting single-molecule magnets (SMMs) 17–20 and proton-conducting spin-crossover (SCO) materials. 21,22 Despite the development of several such bifunctional Ln-MOFs, the rational design and construction of highly proton-conducting Ln-MOFs with compelling magnetic properties remains a challenge. These magnetic-electrical dual-functional materials provide potential applications in multi-task electronic devices.…”

Light-Controlled Multiconfigurational Conductance Switching in a Single 1D Metal–Organic Wire

“…Note that so far most SCO switches are based on iron complexes . SCO systems based on Co­(II) compounds are also well documented for complexes having an octahedral geometry, − but appear much rarer in the case of square planar complexes . Namely, the control of spin states in square planar polynuclear Co­(II) complexes at specific positions has hitherto been unknown.…”

A Sodium-Based Phosphonates Metal–Organic Framework with Superprotonic Conductivity

“…Therefore, deliberate attempts are being made to develop high-performance and low-cost PEMs for fuel cells. In recent years, a series of crystalline materials, such as metal–organic frameworks (MOFs), − covalent–organic frameworks (COFs), , and hydrogen-bonded organic frameworks (HOFs), − have emerged as potential PEM materials. Such crystals have a designable and tunable structure and are also highly crystalline.…”