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

Lucy E. Darago, Michael L. Aubrey, Chung Jui Yu, Miguel I. Gonzalez, Jeffrey R. Long

A three-dimensional network solid composed of Fe(III) centers and paramagnetic semiquinoid linkers, (NBu4)2Fe(III)2(dhbq)3 (dhbq(2-/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. 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 metal-organic framework. Pursuit of stoichiometric control over the ligand redox states resulted in synthesis of the reduced framework material Na0.9(NBu4)1.8Fe(III)2(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 metal-organic frameworks.