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 Fe^II → Fe^III transfers and motivated attempts to isolate compounds containing the respective cations. Indeed, a salt with a Fe^II₂Fe^III₂Mo^VI₄ constellation, [Fe₄Mo₄]­(TCNQ)₂ (2), could be isolated after treatment with TCNQ. The Fe^IIFe^III₃Mo^VI₄ 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 Fe^III₄Mo^VI₄ 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.