Large structural changes upon protonation of Fe4S4 clusters: the consequences for reactivity

Area of Science:

• Inorganic Chemistry
• Bioinorganic Chemistry
• Computational Chemistry

Background:

• Iron-sulfur clusters ([Fe4S4X4](2-)) are crucial in biological electron transfer.
• Understanding their reactivity, especially acid-catalyzed ligand substitution, is vital for synthetic and natural systems.
• Previous mechanisms for substitution reactions lacked comprehensive structural and kinetic explanations.

Purpose of the Study:

• To elucidate the structural changes upon protonation of [Fe4S4X4](2-) clusters.
• To revise and provide a consistent mechanistic interpretation of acid-catalyzed ligand substitution reactions.
• To support proposed mechanisms with computational data and kinetic analysis.

Main Methods:

• Density functional theory (DFT) calculations to model cluster structures and intermediates.
• Kinetic studies of acid-catalyzed ligand substitution reactions with varying X ligands.
• Simulations of reaction steps to validate proposed mechanisms.

Main Results:

• Protonation of a μ3-S ligand in [Fe4S4X4](2-) clusters leads to S-Fe bond cleavage and formation of a unique three-coordinate Fe atom.
• This structural change facilitates indirect ligand substitution pathways involving solvent and incoming ligands.
• Kinetic data reveal distinct rate-determining steps (ligand dissociation vs. nucleophilic attack) depending on the nature of the X ligand (halide, thiolate, phenoxide).

Conclusions:

• The study presents a revised and consistent mechanism for acid-catalyzed ligand substitution reactions in [Fe4S4X4](2-) clusters, based on DFT calculations and kinetic data.
• The unique three-coordinate Fe site plays a pivotal role in the substitution process.
• These findings have significant implications for understanding the reactivity of both natural and synthetic iron-sulfur clusters.