cs0c00559_si_001.pdf (3.55 MB)
Harnessing Noninnocent Porphyrin Ligand to Circumvent Fe-Hydride Formation in the Selective Fe-Catalyzed CO2 Reduction in Aqueous Solution
journal contribution
posted on 2020-05-21, 15:55 authored by Ya-Qiong Zhang, Jia-Yi Chen, Per E. M. Siegbahn, Rong-Zhen LiaoThe iron–porphyrin
complex with four positively charged N,N,N-trimethyl-4-ammoniumphenyl
substituents (called WSCAT) is an efficient catalyst for the reduction
of CO2 to CO in aqueous solution with excellent selectivity.
Density functional calculations have been carried out to explore the
reaction mechanism and the origin of selectivity. The porphyrin ligand
was found to be redox noninnocent and accept two electrons and one
proton, while the ferrous ion keeps its oxidation state as +2 during
the reduction. The FeII–porphyrin diradical intermediate
then performs a nucleophilic attack on CO2, coupled with
two electron transfers from the porphyrin ligand to the CO2 moiety. Subsequently, an intramolecular proton transfer takes place
from the porphyrin nitrogen to the carboxylate oxygen, affording an
FeII–COOH intermediate. An alternative pathway to
form the critical FeII–COOH intermediate, involving
the attack on CO2 by an unprotonated two-electron reduced
FeII–porphyrin diradical species followed by protonation,
was found to be possible as well. Finally, proton transfer from the
carbonic acid in the aqueous solution to the hydroxyl moiety results
in the cleavage of the C–O bond and the production of a CO
molecule. The formation of an FeII-hydride species, a critical
intermediate for the production of H2 and formic acid,
was found to be kinetically much less favorable than the protonation
of the porphyrin nitrogen, even though it is thermodynamically more
favorable. The prevention of this metal-hydride formation pathway
explains why this catalyst is highly selective for the reduction of
CO2 in aqueous solution.