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 MCp*(2) (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.