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

作者：Falkenhagen Jan P; Braun Beatrice; Bill Eckhard; Sattler Dominik; Limberg Christian^*

来源：Inorganic Chemistry, 2014, 53(14): 7294-7308.

DOI：10.1021/ic500584a

The potential of iron molybdates as catalysts in the Formox process stimulates research on aggregated but molecular iron-molybdenum oxo compounds. In this context, [(Me(3)TACN)Fe](OTf)(2) was reacted with (nBu(4)N)(2)[MoO4], which led to an oxo cluster, [[(Me(3)TACN)Fe][mu-(MoO4-kappa O-3,O%26apos;,O %26apos;%26apos;)]](4) (1, Fe4Mo4) with a distorted cubic structure, where the corners are occupied by (Me(3)TACN)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 -%26gt; Fe-III transfers and motivated attempts to isolate compounds containing the respective cations. Indeed, a salt with a (Fe2Fe2Mo4VI)-Fe-II-Mo-III constellation, [Fe4Mo4](TCNQ)(2) (2), could be isolated after treatment with TCNQ The (FeFe3Mo4VI)-Fe-II-Mo-III 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 (Fe4Mo4VI)-Mo-III 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 [[(Me(3)TACN)Fe](4)[mu([MoO4](3)[MoO4(MeCN-kappa N)])-kappa O-3,O%26apos;,O %26apos;%26apos;)](PF6)(4) (7). In this form, the compound could no longer be employed to enter the cyclic voltarnmogram 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 Mossbauer spectroscopy. The data reveal a high degree of charge delocalization. O-16/O-18 exchange experiments with labeled water performed with 1 revealed an interesting parallel with the Formox catalyst: water-O-18 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.

• 出版日期2014-7-21