Second Sphere Interaction Allows Selective Reduction of CO and CO2 to CH4

Reduction of oxides of carbon (CO and CO2) into fixed forms of carbon is desirable to achieve sustainable and clean energy. CO, which is an intermediate in CO2 reduction, is challenging to reduce which, in turn, limits the efficient reduction (both electrochemical and photochemical) of CO2 to 2e-/2H+. Inclusion of distal pyridine residue in the second coordination sphere of iron porphyrin allows reduction of CO (and CO2) to CH4 as the only C1 product using water as the proton source. In-situ spectro-electrochemistry and theoretical modelling indicate that the pendent pyridine moiety imposes a hydrogen bonding interaction between the bound CO and adjacent water molecule which stabilizes two low-valent CO adducts i.e., Fe(I)-CO and Fe(0)-CO porphyrins, preventing its dissociation as a partially reduced CO2 species, allowing its further reduction, via a Fe(II)-CHO species, to CH4. The ability to activate CO via second sphere hydrogen bonding interaction in a mononuclear iron porphyrin opens up newer pathways to reduce both CO and CO2 to valuable C1 products.