A Cubic Fe4Mo4 Oxo Framework and Its Reversible Four-Electron Redox Chemistry

The potential of iron molybdates as catalysts in the Formox process stimulates research on aggregated but molecular iron–molybdenum oxo compounds. In this context, [(Me3TACN)­Fe]­(OTf)2 was reacted with (nBu4N)2[MoO4], which led to an oxo cluster, [[(Me3TACN)­Fe]­[μ-(MoO4-κ3O,O′,O″)]]4 (1, Fe4Mo4) with a distorted cubic structure, where the corners are occupied by (Me3TACN)­Fe2+ and [MoO]4+ units in an alternating fashion, being bridged by oxido ligands. The cyclic voltammogram revealed four reversible oxidation waves that are assigned to four consecutive FeII → FeIII transfers and motivated attempts to isolate compounds containing the respective cations. Indeed, a salt with a FeII2FeIII2MoVI4 constellation, [Fe4Mo4]­(TCNQ)2 (2), could be isolated after treatment with TCNQ. The FeIIFeIII3MoVI4 stage could be reached via oxidation with DDQ or 3 equiv of thianthrenium hexafluorophosphate (ThPF6), giving [Fe4Mo4]­(DDQ)3 (4) or [Fe4Mo4]­(PF6)3 (5), respectively. The fully oxidized FeIII4MoVI4 state was generated through oxidation with 4 equiv of ThPF6, leading to [Fe4Mo4]­(PF6)4, which showed a unique behavior: upon storage, one of the [MoO]4+ corners inverts, so that the terminal oxido ligand is located in the interior of the cage, leading to the formation of [[(Me3TACN)­Fe]4[μ-([MoO4]3[MoO4(MeCN-κN)])-κ3O,O′,O″)]­(PF6)4 (7). In this form, the compound could no longer be employed to enter the cyclic voltammogram recorded for 1, 3, and 5 from the oxidized side; no discrete redox events were observed. Compounds 1–3 and 7 were characterized structurally and 1, 3, and 7 additionally by SQUID measurements and Mössbauer spectroscopy. The data reveal a high degree of charge delocalization. 16O/18O exchange experiments with labeled water performed with 1 revealed an interesting parallel with the Formox catalyst: water−18O exchanges its label with all of the oxido ligands (bridging and terminal). This property relates to the ion mobility being held responsible for the activity of iron molybdate catalysts compared to neat MoO3 or Fe2O3.