Outer-Coordination-Sphere Interaction in a Molecular Iron Catalyst Allows Selective Methane Production from Carbon Monoxide
Suman Patra, Sarmistha Bhunia, Soumili Ghosh, Abhishek Dey
Abstract
Reduction of oxides of carbon (CO and CO 2 ) to fixed forms of carbon is desirable to achieve sustainable and clean energy. Carbon monoxide (CO), an intermediate product in CO 2 reduction, is challenging to reduce, which in turn jeopardizes the direct reduction of CO 2 beyond 2e – /2H + to products like CH 3 OH and CH 4 . Iron porphyrins can efficiently reduce CO 2 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 CH 4 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 CH 4 . The ability to activate and reduce CO by n e – / n H + via a second-sphere hydrogen bonding interaction in a mononuclear iron porphyrin opens newer pathways to valorize both CO and CO 2 to valuable C 1 products.