Conducting Coronene Cation Radical Salt Containing Magnetic Metal Ions

Coronene is the smallest homologue of benzene and is the smallest fragment of graphene among 6-fold symmetric polycyclic aromatic hydrocarbons. In this study, we obtained the first coronene cation radical solid containing magnetic counterions by an electrochemical method. Coronene monocations in the 1:1 salt, (coronene^•+)(FeBr₄^-), assemble in a stacking manner via π-π interactions, which lead to a rather high room-temperature conductivity of 0.6 S cm^-1. The salt shows semiconducting behavior as expected from the calculated band structure, and activation energies were estimated to be 0.25 eV at T ≥ 220 K and 0.18 eV at T ≤ 220 K. The magnetic susceptibility follows the Curie-Weiss law down to about 30 K, with a Curie constant (4.47 emu K mol^-1) expected for S = 5/2 spins of iron(III) ions and a high Weiss temperature (-32.2 K). Upon further cooling, the salt exhibits a susceptibility kink at 16.2 K followed by the loss of a significant fraction of the susceptibility due to long-range antiferromagnetic ordering. Theoretical calculations predicted that the indirect π-d magnetic exchange interaction through C-H···Br hydrogen bonds is equal to J_πd = -3.10 K. Although the absolute value is lower than that of the direct d-d magnetic exchange interaction between the FeBr₄^- anions (J_dd = -13.35 K), it is evident that the π-d interactions play a certain role in determining the magnetic behavior. Considering that an isomorphous salt, (coronene^•+)(GaBr₄^-) involving a nonmagnetic counterpart GaBr₄^-, exhibits singlet-triplet magnetic behavior with a spin gap of 1.44 × 10³ K, it is most likely that in (coronene)(FeBr₄) the nonmagnetic π-electrons serve as mediators of the magnetic ordering of d-spins through the π-d interactions.