Eight-Electron Reduction of CO2 to CH4 by a Bioinspired Mo-Fe-S Cubane Catalyst

Technical Summary: Eight-Electron Reduction of CO2 to CH4 by a Bioinspired Mo-Fe-S Cubane Catalyst

Problem Statement
Metal-sulfur (M-S) clusters are ancient biological cofactors capable of catalyzing the reductive activation of small molecules like CO2 and N2. While enzymes such as carbon monoxide dehydrogenase (CODH) and nitrogenases demonstrate that biological M-S clusters can activate CO2 and reduce CO, these functions are typically separated across different enzyme systems. Furthermore, biological nitrogenases that can reduce CO2 directly to hydrocarbons exhibit very low turnover numbers (TONs < 100). Synthetic M-S clusters have historically been limited to the two-electron reduction of CO2 to CO or formate. A significant challenge remains in achieving sustained, high-turnover conversion of CO2 directly to hydrocarbons (specifically methane, CH4) at a single active site. This eight-electron transformation requires not only the activation of CO2 but also the retention of a metal-bound CO intermediate long enough to undergo further six-electron reduction before dissociation.

Methodology
The study utilized a bioinspired [Cp3Mo3S4Fe] cubane cluster (where Cp = C5Me5), designated as 1, as the primary catalyst. The catalytic system operated under ambient conditions using tetrahydrofuran (THF) as the solvent, SmI2 as a single-electron reductant, and H2O as a proton source.

Key experimental approaches included:

• Catalytic Screening: Evaluation of sterically varied precursors ([CpR3Mo3S4FeCl] with CpR = Cp*, CpL, CpXL) to determine the impact of steric accessibility on catalytic activity.
• Control Experiments: Testing individual precursors and common iron complexes (e.g., Fe(TPP)Cl, FeCp2) to rule out decomposition products or simple iron salts as the active species.
• Isotopic Labeling: Using 13CO2 to verify the carbon source of the methane product via Gas Chromatography-Mass Spectrometry (GC-MS).
• Kinetic Analysis: Measuring initial turnover frequencies (TOFs) and reaction orders to establish the molecular nature of the catalysis and distinguish it from nanoparticle formation.
• Intermediate Interception: Attempting to isolate catalytic intermediates under stoichiometric conditions using KC8 as a reductant under CO2 atmosphere, followed by X-ray crystallography and IR spectroscopy.
• Stoichiometric Conversion: Independently synthesizing the neutral Fe-CO cluster (5) and subjecting it to SmI2/H2O to test its direct conversion to CH4.
• Computational Support: Density Functional Theory (DFT) calculations were employed to model binding energies of CO2 and CO, and to propose a catalytic cycle.

Key Results

1. High-Performance Catalysis: The [Cp*3Mo3S4FeCl] cluster (1) catalyzed the direct eight-electron reduction of CO2 to CH4 with turnover numbers (TONs) exceeding 150,000. The reaction showed high selectivity for CH4 (35% of consumed reducing equivalents) with only trace amounts of CO (0.54%) and significant H2 evolution as a competing pathway.
2. Steric Dependence: Catalytic activity was highly dependent on the steric profile of the Cp ligands. The least hindered cluster (1) was the most active, while bulkier analogs (2 and 3) showed negligible activity, suggesting that steric accessibility at the Fe site is critical for CO2 binding and reduction.
3. Mechanistic Evidence:
   • Isotopic Confirmation: GC-MS analysis confirmed that the carbon in the produced CH4 originated exclusively from CO2.
   • Intermediate Identification: Under stoichiometric conditions, the reaction of 1 with KC8 under CO2 yielded an anionic Fe-CO adduct, [Cp*3Mo3S4Fe(CO)]– (4). This species was structurally characterized by X-ray crystallography.
   • Intermediate Competence: The neutral Fe-CO cluster (5) and the anionic species (4) both catalyzed CO2 reduction with activities comparable to or exceeding the chloride precursor. Crucially, stoichiometric treatment of 5 with SmI2 and H2O converted the bound CO to CH4 in near-quantitative yield (0.96 equiv), proving the Fe-CO species is a competent intermediate rather than a dead-end product.
   • Kinetics: The reaction exhibited first-order dependence on the cluster concentration, supporting a molecular catalytic mechanism rather than one involving aggregated nanoparticles.
4. Durability: Large-scale experiments using a massive excess of reductant (4 million equiv. SmI2) demonstrated that the catalyst framework remains intact, with TONs reaching 157,000 for the neutral Fe-CO cluster (5) at 0 °C.

Significance and Claims
The authors claim that this work represents the highest turnover number reported to date for a homogeneous molecular catalyst converting CO2 to CH4, surpassing previous M-S cluster catalysts by approximately four orders of magnitude.

The study establishes that a single molecular metal-sulfur cluster can integrate both CO2 activation and the complete reduction of the resulting CO intermediate. This challenges the notion that such multi-step reductions require separate enzymatic systems or protein scaffolds.

A central finding is the identification of "productive retention" of a metal-bound CO intermediate as a key design principle. Unlike CODH, which releases CO, or nitrogenase, where CO can inhibit catalysis, the [Cp*3Mo3S4Fe] framework maintains the Fe-CO bond with sufficient strength to prevent dissociation but allows for further reduction to CH4. This is evidenced by the unusually low CO stretching frequencies (1690–1857 cm⁻¹) observed, indicating strong Fe→CO π backdonation and substantial activation of the ligand.

The paper concludes that the [Cp*3Mo3S4Fe] system demonstrates the intrinsic ability of metal-sulfur frameworks to coordinate multi-electron/multi-proton transfer using simple reagents (H2O in THF) without the need for structured protein hydrogen-bonding networks. This provides a molecular blueprint for advancing catalytic CO2 reduction beyond the two-electron stage toward hydrocarbon fuels.