Iron
hangman porphyrins with phenol, guanidinium, and sulfonic
acid proton donor groups placed above the Fe porphyrin platform reduce
CO2 to CO with Faradaic efficiencies >93%. Computations
show that the activation of CO2 at the Fe center is enhanced
by the hanging group. Intramolecular hydrogen bonding from the phenol
and guanidinium groups results in a 2.1–6.6 kcal/mol stabilization
of CO2 within the hangman pocket; the hanging sulfonate
group is deprotonated, thus resulting in destabilization of the CO2 adduct due to unfavorable electrostatic interactions. Electrochemical
studies show that Fe hangman porphyrins exhibit canonical S-curve
character; together with computation results, the apparent rate constant
for CO2 reduction is found to be governed by CO2 binding within the hangman cleft.
The reduction of CO 2 by electrogenerated Fe(0)TPP can be directed toward formate with a good selectivity (ca. 70%) upon addition of a tertiary amine in the presence of a weak proton donor. Whereas most systems generating formate from CO 2 involve the initial formation of a hydride, we demonstrate herein that the hydride pathway is avoided by preferential binding of CO 2 that may then be protonated by weak acids. The net result is that hydrogen production via the hydride is disfavored, thus leading to high yields of formate. These results demonstrate an alternative route for the activation of CO 2 to formate.
Hydrogen
evolution from a weak acid, acetic acid, occurs at extremely
high rates for iron tetraphenylporphyrin in the presence tertiary
amine as a cocatalyst. Kinetic analysis of H2 evolution
for a range of amines with varying coordinative and acid–base
properties reveals a dramatic rate enhancement derived from enhanced
proton activity of a heteroconjugated adduct between acetic acid and
the amine. Additionally, nonbulky tertiary amines (quinuclidine and
diazabicyclooctane) result in a further increase in activity resulting
from a trans effect for a TPPFe(II)–H hydride intermediate.
These findings reveal the design principle of a cocatalyst for creating
more efficient molecular systems for hydrogen evolution electrocatalysis.
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