Product Control in Visible-Light-Driven CO2 Reduction by Switching Metal Centers of Binuclear Catalysts

Abstract

The paper presents a strategy to control product selectivity in visible-light-driven CO₂ reduction by switching the metal centers in binuclear molecular catalysts. Two catalysts were developed: Co₂(MeL-S)(OAc)₂ (CoCo) and Cu₂(MeL-S)(H₂O)₂·2H₂O (CuCu). CoCo selectively produces CO (96% selectivity, TON = 6188), while CuCu selectively forms HCOOH (98% selectivity, TON = 7540). The distinct selectivities are attributed to structural and electronic differences, particularly the presence of a 3-center-4-electron (3c-4e⁻) Cu···H···Cu bond in CuCu.

Summary

This study demonstrates that switching the metal centers in sulfur-bridged binuclear catalysts enables precise control over CO₂ reduction products under visible light. CoCo favors CO formation, while CuCu favors HCOOH production. The catalysts are stable in aqueous media and work efficiently with non-noble metals, offering a sustainable approach to CO₂ valorization. Mechanistic insights via DFT and spectroscopy validate the influence of metal identity on electron transfer pathways and product outcomes.

Additional Remarks

• Both catalysts function in water-containing (CH₃CN/H₂O, 4/1) systems.
• The study highlights the importance of metal–ligand cooperation and electronic structure in achieving product selectivity.
• This approach mimics biological enzymes like CODH and FDH.

• Photocatalytic efficiency is influenced not only by the metal center but also by the sacrificial electron donor and solvent environment.

Content of the Published Article in Detail

• Introduction: Emphasizes challenges in selective CO₂ photoreduction and the potential of non-noble metal catalysts.
• Catalyst Design: Two bioinspired binuclear complexes (CoCo and CuCu) were synthesized using a sulfur-bridged N₆S-type ligand.
• Photocatalysis: Under visible light, CoCo converts CO₂ to CO with high selectivity, while CuCu yields HCOOH. Product selectivity is influenced by metal type.
• Mechanism: DFT and experimental data show that CuCu promotes HCOOH via a 3c-4e⁻ bond facilitating hydride transfer; CoCo lacks this interaction due to a greater metal–metal distance.

Catalysts tested in this study

• CoCo: Co₂(MeL-S)(OAc)₂
  • Yields CO (TON = 6188, 96% selectivity)

• CuCu: Cu₂(MeL-S)(H₂O)₂·2H₂O
  • Yields HCOOH (TON = 7540, 98% selectivity)

Photosensitizer

[Ru(phen)3][PF6]2

• Ru(phen)₃₂ (Ru-PS)
• In some experiments: [Ru(bpy)₃]Cl₂

• Role: Transfers electrons upon light excitation to the catalyst.

Investigation

• EPR, XPS, CV, DPV, and DFT used for electronic and structural analysis.
• 13CO₂ isotope labeling confirmed CO and HCOOH originate from CO₂.
• Control experiments proved the need for each component (light, PS, catalyst, CO₂, reductant).
• Mechanism: CoCo goes via CO₂ coordination, while CuCu facilitates hydrogenation first, enabled by Cu···Cu proximity and electron delocalization.

Further Information

• Catalysts are stable under reaction conditions for over 10 h.
• Deactivation is primarily due to photosensitizer degradation.
• Reaction can be restarted by adding fresh Ru-PS.

• Linear relationship observed between catalyst concentration and HCOOH production for CuCu.

Sacrificial electron donor

• CoCo: Best performance with TEOA (triethanolamine)
• CuCu: Best performance with TEA (triethylamine)

• Others tested: BINH, VCNa, but with lower efficiency.

Additives

• CH₃COONa: Investigated for its influence on redox behavior.
• Solvent system: CH₃CN/H₂O (4:1 v/v) optimized for activity.

• Hg⁰ poisoning test: Confirmed molecular (not colloidal) catalysis.