A redox−active
metal–organic framework, Fe<sub>2</sub>(dobpdc) (dobpdc<sup>4–</sup> = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate),
is shown to undergo a topotactic oxidative insertion reaction with
a variety of weakly coordinating anions, including BF<sub>4</sub><sup>–</sup> and PF<sub>6</sub><sup>–</sup>. The reaction
results in just a minor lattice contraction, and a broad intervalence
charge-transfer band emerges, indicative of charge mobility. Although
both metal- and ligand-based oxidations can be accessed, only the
former were found to be fully reversible and, importantly, proceed
stoichiometrically under both chemical and electrochemical conditions.
Electrochemical measurements probing the effects of nanoconfinement
on the insertion reaction revealed strong anion size and solvent dependences.
Significantly, the anion insertion behavior of Fe<sub>2</sub>(dobpdc)
enabled its use in the construction of a dual-ion battery prototype
incorporating a sodium anode. As a cathode, the material displays
a particularly high initial reduction potential and is further stable
for at least 50 charge/discharge cycles, exhibiting a maximum specific
energy of 316 Wh/kg