Linker Redox Mediated Control of Morphology and Properties in Semiconducting Iron-Semiquinoid Coordination Polymers

The emergence of conductive 2D, and less commonly 3D, coordination polymers (CPs) and metal–organic frameworks (MOFs) promises novel applications in chemical sensing, energy storage, optoelectronics, thermoelectrics, and spintronics. While classic CPs and MOFs now have relatively sophisticated synthetic parameters to control morphology, crystallinity, and phase purity, similar parameters are not thoroughly understood for electronically more complex materials. In particular, many linkers used in conducting CPs have multiple accessible redox states and the relationship between starting linker oxidation state and final material structure and properties is not well understood. Here we report a new 3D semiconducting coordination polymer, Fe5(C6O6)3, which is composed of hexagonal Fe2(C6O6)3 layers which are bridged by additional Fe ions. This material, which is a fusion of 2D Fe-semiquinoid materials and recently reported 3D cubic Fex(C6O6)y materials, is obtained by using a different initial redox-state of the C6O6 linker. The material displays high electrical conductivity (0.02 S cm–1), broad electronic transitions in the visible to middle-infrared region, promising thermoelectric behavior (S2σ = 4.2×10–9 W m–1 K–2), and strong antiferromagnetic interactions even at room temperature. The unique structure and properties of this material illustrates that controlling the oxidation states of redox-active components in conducting CPs can be a “presynthetic” strategy to carefully tune material topologies, properties, and functionalities in contrast to more commonly encountered post-synthetic modifications.