Outer-Coordination-Sphere Interaction in a Molecular Iron Catalyst Allows Selective Methane Production from Carbon Monoxide

Reduction of oxides of carbon (CO and CO2) to fixed forms of carbon is desirable to achieve sustainable and clean energy. Carbon monoxide (CO), an intermediate product in CO2 reduction, is challenging to reduce, which in turn jeopardizes the direct reduction of CO2 beyond 2e–/2H+ to products like CH3OH and CH4. Iron porphyrins can efficiently reduce CO2 to CO by 2e–/2H+, but further reduction is halted by the rapid dissociation of CO from the reduced iron center. This work shows that CO can indeed be reduced upon inclusion of a pendent pyridine in the second coordination sphere of an iron porphyrin complex efficiently and selectively to CH4 using water or phenol as the proton source. In situ spectro-electrochemistry supported by theoretical calculations indicates that the pendent pyridine moiety imposes a hydrogen bonding interaction between the bound CO and water, which stabilizes two low-valent CO adducts, i.e., Fe(I)–CO and Fe(0)–CO, of iron porphyrins, allowing its complete reduction, via a Fe(II)–CHO species, to CH4. The ability to activate and reduce CO by ne–/nH+ via a second-sphere hydrogen bonding interaction in a mononuclear iron porphyrin opens newer pathways to valorize both CO and CO2 to valuable C1 products.