Catalysis of the Electrochemical Reduction of Carbon Dioxide by Iron(0) Porphyrins:  Synergystic Effect of Weak Brönsted Acids

Addition of weak Brönsted acids such as 1-propanol, 2-pyrrolidone, and CF3CH2OH triggers a considerable improvement of the catalysis of CO2 reduction by iron(0) tetraphenylporphyrins. Both the catalytic currents and the life time of the catalyst increase without significant formation of hydrogen. Unprecedented values of the turnover numbers per hour can thus be reached. Carbon monoxide is the main product, while formic acid is formed to a lesser extent. The yield of formic acid counterintuitively decreases as the acidity of the acid synergist increases, becoming negligible with CF3CH2OH. Analysis of the reaction kinetics suggests that the action of the acid synergist is to stabilize the initial FeIICO22- carbenoid complex by hydrogen bonding. The formation of a doubly hydrogen-bonded complex opens the route to the cleavage of one of the two C−O bonds resulting in the formation of CO within the iron coordination sphere. The formation of formic acid involves a reaction pathway where the iron−CO2 interactions are weaker. The effect of the acid synergist is an example of electrophilic assistance in a two-electron push−pull mechanism where pulling the electron pair out of the substrate by means of the synergist is as important as pushing electrons from the catalyst into the substrate. With CF3CH2OH, the production of CO is so fast that it commences to inhibit the catalytic reaction. This self-inhibition phenomenon can be satisfactorily modeled under the assumption that product adsorption on the electrode surface obeys a Langmuir equilibrium and that the covered portions of the surface are totally inactive toward reduction of the catalyst.