Sodium-coupled electron transfer reactivity of metal–organic frameworks containing titanium clusters: the importance of cations in redox chemistry

Stoichiometric reduction reactions of two metal–organic frameworks (MOFs) by the solution reagents (M = Cr, Co) are described. The two MOFs contain clusters with Ti₈O₈ rings: Ti₈O₈(OH)₄(bdc)₆; bdc = terephthalate (MIL-125) and Ti₈O₈(OH)₄(bdc-NH₂)₆; bdc-NH₂ = 2-aminoterephthalate (NH₂-MIL-125). The stoichiometry of the redox reactions was probed using solution NMR methods. The extent of reduction is greatly enhanced by the presence of Na⁺, which is incorporated into the bulk of the material. The roughly 1 : 1 stoichiometry of electrons and cations indicates that the storage of e⁻ in the MOF is tightly coupled to a cation within the architecture, for charge balance.