In recent decades, the design and synthesis of magnetic porous coordination polymers have attracted considerable attention, because magnetic porous coordination polymers show multiple functionalities, for example, in multiferroic materials and chiral magnets. In particular, the switching of magnetism by external stimuli (e.g. light, temperature, pressure, and guest molecules ) has been actively investigated.
Guest-induced transformation of magnetism using a neutral guest offers perspectives on the correlation of the electronic structure and magnetic properties of porous coordination polymers. However, the electrochemical insertion of ionic guests, that is, a pair of an ion and electron, into a porous coordination polymer directly manipulates the electronic structure, and thus the magnetic properties. Therefore, this approach is expected to open new avenues for the design of porous coordination polymers with multifunctionalities. Furthermore, the electrochemical insertion/extraction of ions into/from porous coordination polymers offers the possibility to establish fabrication techniques for novel multifunctional materials, which cannot be obtained by the conventional synthetic methods.
Here, we describe the ion-induced reversible transformation of magnetism in a Prussian blue analogue (PBA). PBA is one of the most studied magnetic coordination polymers and exhibits various magnetic properties because of its strong intermetallic interactions through the bridging CN ligands. Important electrochemical reactions of PBA are the insertion/extraction of various cations (e.g. K + , Na + , and Li + ) into/from the 3D connected porous structure. Here, we used a bimetallic Fe III -CN-Cu II PBA (CuFe-PBA) as a porous magnetic host, because CuFe-PBA undergoes a ferromagnetic transition at 20 K with a simple spin configuration (S Fe = 1/2 and S Cu = 1/2). As ionic guests Li ions were used to be inserted/extracted electrochemically.
CuFe-PBA was synthesized by using a precipitation method. The resultant composition of the compound was determined to be K 0.14 Cu II 1.43 [Fe III (CN) 6 ]β’5 H 2 O by inductively coupled plasma mass spectroscopy for K, Cu, and Fe and a standard microanalytical method for C, H, and N. The powder X-ray diffraction (XRD) pattern (Figure in the Supporting Information) indicated a single cubic phase without impurities, and the calculated lattice parameters (a = 10.1337( ) , V = 1040.7(2) 3 ) were consistent with the previously reported values for a bimetallic CuFe-PBA. The mean particle size was estimated to be 40 nm using TEM (Figure in the Supporting Information). The Raman spectrum showed a broad n(CN) peak centered at 2145 cm Γ1 , which confirmed the valence state of Fe III -CN-Cu II . Electrochemical insertion and extraction of Li ions were performed by using a three-electrode glass cell, in which the Li metal was employed as counter and reference electrodes. The cutoff voltages were 2.0 V (vs. Li/Li + ) for insertion of Li ions and 4.3 V for extraction of Li ions to prevent the decomposition of CuFe-PBA and the organic solvent.
We used 1m ethylene carbonate/diethyl carbonate solutions of LiClO 4 as electrolytes. For our approach, one of the most important requirements is to achieve a homogeneous electronic structure with a uniform concentration of Li ions in the PBA particles. If the Li ion is inserted rapidly, slow diffusion of the Li ions in the solid-solution particles and slow movement at the phase boundary in the phase-separated particles could result in inhomogeneous compounds. Thus, we carried out the electrochemical insertion/extraction of Li ions by repeated application of a low-density current (18 mA g Γ1 ) for 10 min, followed by an interruption of 20 min to allow the PBA to equilibrate, that is, we used the galvanostatic intermittent titration technique (GITT). The electrochemical reaction (Scheme 1) is described as given in Equation (1).
x Li