Manipulation of Valence Trapping through Local Order of Fe₃O Units in an Intrinsically Mixed-Valence Coordination...

Mixed-valence coordination polymers (CPs) can exhibit enhanced and tunable conductivity and stimulus-driven phase transitions, making them promising new electronic materials. However, the mechanisms of the electron transfer processes that underlie these properties in the solid state merit further study. Here, we report the synthesis of a new mixed-valence CP, Fe^III₂Fe^IIO(OAc)₆(bpy)₂ (OAc = acetate, bpy = 4,4'-bipyridine), which is obtained from the self-assembly of Fe₃O(OAc)₆ clusters and bpy linkers. Notably, the CP is charge-neutral in its mixed-valence form, making it one of the few established intrinsically mixed-valence CPs. Detailed characterization of the valence and spin states of the constituent Fe ions in the CP via variable-temperature Mössbauer spectroscopy, single-crystal X-ray diffraction, and magnetic susceptibility measurements reveals the occurrence of thermally activated electron transfer (i.e., valence detrapping) and antiferromagnetic exchange between Fe^II and Fe^III ions. Intriguingly, valence-detrapping phenomena are confined to distinct regions of the CP and proceed to varying degrees of completion depending on the solid-state environment of the participating Fe ions. This study presents a convenient strategy for the direct synthesis of intrinsically mixed-valence coordination polymers and highlights the important role of solid-state structural order in manipulating electron transfer mechanisms in Class II Robin-Day mixed-valence materials.