CHEMICAL CATALYSIS OF ELECTROCHEMICAL REACTIONS - HOMOGENEOUS CATALYSIS OF THE ELECTROCHEMICAL REDUCTION OF CARBON-DIOXIDE BY IRON(0) PORPHYRINS - ROLE OF THE ADDITION OF MAGNESIUM CATIONS

Iron("0") porphyrins catalyze the electrochemical reduction of CO2. The main reduction product is CO. In DMF, with tetraalkylammonium salts as supporting electrolyte, the porphyrin is however destroyed by carboxylation and/or hydrogenation of the ring after a few catalytic cycles. The presence of a hard electrophile such as Mg2+ ion dramatically improves the rate of the reaction, the production of CO, and most importantly, the stability of the catalyst. The reaction mechanism involves the introduction of one molecule of CO2 into the iron coordination sphere. The addition of a second molecule of CO2 acts as a Lewis acid and then allows the breaking of one C-O bond of the first CO2 molecule thus leading to CO. This process is accelerated by Mg2+ ions in a way that depends upon the temperature. At low temperatures (-40-degrees-C), the Mg2+ ions facilitate the decomposition of the complex containing two molecules of CO2, whereas, at room temperature, Mg2+ ions triggers the breaking of the bond at the level of the complex containing a single molecule of CO2 in its coordination sphere. The combined action of iron("0") porphyrins and of Mg2+ ions offers a remarkable example of a bimetallic catalysis where an electron-rich center starts the reduction process and an electron-deficient center assists the transformation of the bond system.