Electronic Conductivity, Ferrimagnetic Ordering, and Reductive Insertion Mediated by Organic Mixed-Valence in a Ferric Semiquinoid Metal–Organic Framework

A three-dimensional
network solid composed of FeIII centers
and para­magnetic semi­quinoid linkers, (NBu4)2­FeIII2­(dhbq)3 (dhbq2–/3– = 2,5-dioxido­benzo­quinone/​1,2-dioxido-4,5-semi­quinone),
is shown to exhibit a conductivity of 0.16 ± 0.01 S/cm at 298
K, one of the highest values yet observed for a metal–organic
framework (MOF). The origin of this electronic conductivity is determined
to be ligand mixed-valency, which is characterized using a suite of
spectro­scopic techniques, slow-scan cyclic voltammetry, and
variable-temperature conductivity and magnetic susceptibility measurements.
Importantly, UV–vis–NIR diffuse reflectance measurements
reveal the first observation of Robin–Day Class II/III mixed
valency in a MOF. Pursuit of stoichio­metric control over the
ligand redox states resulted in synthesis of the reduced frame­work
material Na0.9­(NBu4)1.8FeIII2(dhbq)3. Differences in electronic
conductivity and magnetic ordering temperature between the two compounds
are investigated and correlated to the relative ratio of the two different
ligand redox states. Overall, the transition metal–semi­quinoid
system is established as a particularly promising scaffold for achieving
tunable long-range electronic communication in MOFs.