Non-negligible Axial Ligand Effect on Electrocatalytic CO₂ Reduction with Iron Porphyrin Complexes

Iron(III) porphyrin complexes have been demonstrated as one of the efficient molecular catalysts for the electrochemical reduction of CO₂. However, the role of axial ligands coordinated with a metal center in the complex on the electrochemical CO₂ reduction activity has not been fully explored yet. Herein, iron(III) tetraphenylporphyrin thiocyanate (FeTPP-SCN) is synthesized from a commercially available catalyst of FeTPP-Cl by a counteranion exchanging reaction. Cyclic voltammetry measurements showed that the catalytic activity of FeTPP-SCN is noticeably suppressed in the DMF solutions. The structural dynamics of the axial ligand in FeTPP-SCN are further examined by the FTIR and ultrafast IR spectroscopies, where the SCN ligand is employed as the local vibrational probe. Vibrational relaxation measurements showed that the reorientational dynamics of SCN ligands was strongly restricted in DMF solution, suggesting that the subtle electrostatic interaction between the ligands and metal center in the complex can have a non-negligible effect on its catalytic activity.