Solid-State Electrochemistry of Copper(I) Coordination Polymers Containing Tetrafluoroborate Anions

Host–guest materials based on coordination polymers (CPs) are currently emerging as potential candidates for battery applications. In this context, we describe the preparation of three-dimensional network structures containing BF4 anions and water molecules in the one-dimensional (1D) channels via hydrothermal reactions between Cu­(BF4)2 and 4,4′-bipyridine or 1,2-di-4-pyridylethylene. A systematic characterization of the obtained CPs using single-crystal X-ray diffraction, X-ray absorption fine structure, and an electrochemical test was performed. The results showed that the BF4 anions were electrochemically reduced to BF3 in the cavities of the CPs, with concomitant elimination of a leaving fluoride at room temperature. Using this electrochemical property, a prototype battery, in which the CPs act as the anode and graphite as the cathode, was demonstrated. The cell exhibited a practical discharge potential of ∼1.5 V. This constitutes the first demonstration of CPs showing electrochemical B–F bond activation in the 1D channels and rocking-chair-type fluoride insertion and extraction by changes in the electric potential.