Recently, reductive conversion of CO₂ to useful carbon resources draws broad attentions from environmental and catalytic to biological sciences. Though many metal complexes as well as pure metals including mainly mononuclear rare metals such as Ru or Re have been proposed to date, they show rather high overpotentials and/or poor product selectivities. Thus, new catalyst design for efficient and robust CO₂reduction is strongly demanded.

   In biological systems, anaerobic bacteria and archaea use oxygen-sensitive Ni-/Fe-containing carbon monoxide dehydrogenases (CODHases), which realize efficient mutual conversion between CO₂ and CO at extremely low overpotentials (< 100 mV) (Fig. 1A).^[1]  The dinuclear active site motif with base metal ions is attractive as chemical models. According to this strategy, we designed iron porphyrin dimers linked by 1,2-phenylene group (Fig. 1B) as the catalysts and applied them for electrochemical CO₂ reduction.  The catalysts show extremely high catalytic TOF and high CO selectivity at relatively low overpotentials (η).  When electron-withdrawing meso-aryl groups are used, η becomes as low as 0.4 V.^[2]  From the detailed CV analysis, Fe porphyrin monomer, e.g., FeTPP, shows the dischage current at its Fe⁰ state in CO₂ atmosphere and Fe^II/III and Fe^I/II couples appear at the same potential as observed in Ar.  The FeTPP dimer, however, shows a considerable positive shift of its Fe^I/II couple in CO₂. This suggests that the Fe^II state of the dimer will make strong association with CO₂ molecule and the resultant complex shows its reduction peak at more positve potential than its original Fe^I/II couple.  Since Fe^II porphyrin monomesr are generally intact with CO₂, this is the first example of the formation of the CO₂-binding to FeII porphyrin by its dimerization. This is the reason of the considerable decrease of the CO₂ reduction overpotential.  This result also supports the two nucleus motif as observed in CODHase active site promote the CO₂ reduction at low η.

 When two Fe ions are separated in a larger distance as observed in 1,3-phenylene linked Fe dimer, this cooperative effect of the two Fe^II ions is not work any longer and the m-dimer just work as two monomers.

The present Fe porphyrin dimmr catalysts are quite robust and any current drop was not observed after 12 h at a constant potential electrolysis. High CO selectivity of these Fe catalysts is also advantage for their practical use on the aspect of easy product separation.

Regerences

[1] J.-H. Jeoung and H. Dobbek, Science, 318, 1461 (2007); J. Fesseler, J.-H. Jeoung, and H. Dobbek, Angew. Chem. Int. Ed., 54, 8560 (2015).

[2] E. Mohamed, Z. Zaharan, and Y. Naruta, Chem. Commun, 51, 16900 (2015). (Cover article)