A Charge-Transfer Salt Based on Ferrocene/Ferrocenium Pairs and Keggin-Type Polyoxometalates

Area of Science:

• Inorganic Chemistry
• Organometallic Chemistry
• Materials Science
• Crystallography

Background:

• Polyoxometalates (POMs) are versatile inorganic clusters with tunable properties.
• Ferrocene and its oxidized form, ferrocenium, are foundational organometallic compounds.
• Hybrid materials combining inorganic and organometallic components offer unique functionalities.

Purpose of the Study:

• To synthesize and characterize a novel hybrid inorganic-organometallic salt.
• To investigate the coexistence of ferrocene and ferrocenium within a polyoxometalate framework.
• To explore the structural, electronic, and magnetic properties of the new compound.

Main Methods:

• Single-crystal X-ray diffraction for detailed structural analysis.
• Solid-state diffuse reflectance UV-Vis spectroscopy for charge-transfer studies.
• Mössbauer spectroscopy to confirm the oxidation states of iron centers.
• Topological analysis of ferrocenyl species in the Cambridge Structural Database (CSD).

Main Results:

• Synthesis of [Fe^III(Cp)₂]₄[SiW₁₂O₄₀]·[Fe^II(Cp)₂]·2CH₃OH (1), the first polyoxometalate salt with coexisting ferrocene and ferrocenium.
• Crystal structure reveals four ferrocenium and one ferrocene per Keggin anion, with distinct eclipsed and staggered ferrocenium conformations.
• Organometallic sub-lattice with π-π interactions forms cavities hosting Keggin anions and methanol.
• Spectroscopic studies confirm charge-transfer nature and magnetically isolated Fe^III/Fe^II centers.

Conclusions:

• Compound 1 represents a pioneering example of a hybrid polyoxometalate salt containing both neutral ferrocene and cationic ferrocenium species.
• The study establishes a correlation between Fe···Cp distances and the oxidation state of iron in ferrocenyl compounds.
• Ferrocenyl species in the solid state predominantly adopt either eclipsed or staggered conformations.