Second sphere control of CO2 reduction selectivity by iron porphyrins: The role of spin state

Iron porphyrins are reported to reduce CO₂ selectively to CO. Hydrogen bonding interactions are reported to facilitate this 2e⁻/2H⁺ reduction process. A series of iron porphyrins are reported here where hydrogen bonding distal super structure is introduced systematically. The results show that the selectivity for 2e⁻/2H⁺ CO₂ reduction changes dramatically from CO to HCOOH as hydrogen bonding interactions are introduced. The resonance Raman data of the Fe(II)-COOH intermediate, trapped at −80^oC, show that the spin state of this species changes from low spin to high spin as hydrogen bonding is introduced. The spin state of the Fe(II)-COOH intermediate seems to determine the selectivity of 2e⁻/2H⁺ CO₂ reduction in iron porphyrins.