A Cubic Fe₄Mo₄Oxo Framework and Its Reversible Four-Electron Redox Chemistry

Abstract

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-κ(3)O,O',O″)]]4 (1, Fe4Mo4) with a distorted cubic structure, where the corners are occupied by (Me3TACN)Fe(2+) 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(II)2Fe(III)2Mo(VI)4 constellation, [Fe4Mo4](TCNQ)2 (2), could be isolated after treatment with TCNQ. The Fe(II)Fe(III)3Mo(VI)4 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(III)4Mo(VI)4 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)])-κ(3)O,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. (16)O/(18)O exchange experiments with labeled water performed with 1 revealed an interesting parallel with the Formox catalyst: water-(18)O 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.

Investigation of Cubic Fe4M4 Frameworks for Application in Nonaqueous Energy Storage

“…Characterization of the product by 1 H NMR spectroscopy revealed two paramagnetically shifted resonances located at 102.7 (6 H, Δ ν 1/2 =280 Hz) and 38.2 (15H Δ ν 1/2 =240 Hz) ppm (Figure S7). This spectrum is similar to that reported previously for Fe 4 Mo 4 O 16 , suggesting successful formation of the desired heterometallic structure. ESI‐MS reveals a series of peaks centered around m / z= 1901, matching the predicted spectrum for Fe 4 W 4 O 16 (Figure S8).…”

“…For the fourth oxidation event in Fe 4 Mo 4 O 16 , [Fe II 1 Fe III 3 ] 3+ / [Fe III 4 ] 4+ , the calculated electron‐transfer rates are slower, on the order of 10 −3 cm s −1 . In our original report, attempts to isolate the 4+ charge state of the iron–molybdate cluster revealed a structural change in the form of an inversion of one of the M=O moieties . We hypothesize that this molecular rearrangement results in the reduction in k 0 for this event.…”

“…To quantify this, we determined the diffusion coefficients (D 0 )a nd heterogeneouse lec- ,t he calculated electron-transfer rates are slower,o n the order of 10 À3 cm s À1 .I no ur original report, attempts to isolate the 4 + charge state of the iron-molybdate clusterr evealed as tructuralc hange in the form of an inversion of one of the M = Om oieties. [13] We hypothesize that this molecular rearrangement results in the reduction in k 0 for this event.S till, the kinetics for all four of the charge-transfer events in Fe 4 Mo 4 O 16 are notably fast, apropertywhich has positive implications on both the energy efficiency and powerd ensity of an RFB system. [18] For the tungstate analogue, Fe 4 W 4 O 16 ,t he electron-transfer rates are overall slower than those observed for 16 ,o nt he order of 10 À2À 10 À3 (Table 2).…”

“…In 2014, part of our research teamr eported the synthesis of the Fe 4 Mo 4 O 16 framework. [13] Generated by self-assembly from monometallic precursors, the distorted, cube-shaped cluster exhibits ar emarkable electrochemical profile, with four,r eversible redox events spanning approximately 1.3 V. The original report detailed spectroscopic investigationst hat assigned the redox chemistry of the heterometallic cube to the iron centers, with the [MoO 4 ] 2À moieties serving asr edox-innocent vertices. The comproportionation constants of the Fe 4 Mo 4 O 16 cluster, ranging from 10 4 to 10 11 ,q ualify it as aR obin and Day Class II delocalized system.…”

“…Isomeric shifts are referenced to alpha-iron at room temperature. Materials:M oO 3 ,( NH 4 ) 2 [13] Me 3 TACN, [23] [Fe(MeCN) 2 ](OTf) 2 [24] and [nBu 4 N] 2 [WO 4 ] [25] were synthesized according to literature. Solvents:M eCN and Et 2 O were dried with an MBraun solvent purification system, degassed and stored over activated molecular sieves (3 )p rior to use and degassed prior to use.…”