A cubic Fe4Mo4 oxo framework and its reversible four-electron redox chemistry

Area of Science:

• Inorganic Chemistry
• Catalysis
• Materials Science

Background:

• Iron molybdates are vital catalysts in the Formox process.
• Research is driven by the need for molecular models of these catalysts.
• Understanding the structure-activity relationship of iron-molybdenum oxo compounds is key.

Purpose of the Study:

• To synthesize and characterize novel iron-molybdenum oxo clusters.
• To investigate the redox properties and structural changes of these clusters.
• To explore their potential as molecular models for Formox catalysts.

Main Methods:

• Synthesis of an iron-molybdenum oxo cluster, Fe4Mo4, from [(Me3TACN)Fe](OTf)2 and (nBu4N)2[MoO4].
• Electrochemical oxidation and isolation of various redox states using TCNQ, DDQ, and ThPF6.
• Structural characterization (X-ray diffraction), SQUID measurements, Mössbauer spectroscopy, and (16)O/(18)O isotopic exchange experiments.

Main Results:

• A distorted cubic Fe4Mo4 oxo cluster was synthesized and characterized.
• Four reversible oxidation waves were observed, corresponding to Fe(II) → Fe(III) transfers.
• Different redox states (Fe(II)2Fe(III)2Mo(VI)4 to Fe(III)4Mo(VI)4) were isolated and studied.
• A unique structural inversion occurred in the fully oxidized state upon storage.
• (16)O/(18)O exchange experiments showed high oxido ligand mobility, similar to Formox catalysts.

Conclusions:

• The synthesized iron-molybdenum oxo clusters serve as valuable molecular models for Formox catalysts.
• The observed charge delocalization and ion mobility are critical for catalytic activity.
• The study provides insights into the fundamental properties of iron molybdate systems.