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

Journal of the American Chemical Society

Volumn 113, Issue 22, 1991, Pages 8455-8466

  Hammouche, Mohamed ^a   Lexa, Doris ^a   Savêant, Jean Michel ^a   Momenteau, Michel ^b  

^a UNIVERSITÉ PARIS DIDEROT   (France)

^b INSTITUT CURIE   (France)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0001580028     PISSN: 00027863     EISSN: 15205126     Source Type: Journal    
DOI: 10.1021/ja00022a038     Document Type: Article

References (168)

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128. The question of the respective weights of iron(0) and iron(I) anion radical resonant forms in the electronic description of iron(“0”) porphyrins is not completely settled for the moment. With superstructured porphyrins bearing secondary amide groups such as those shown in Figure 1, one readily obtains the reversible formation of the iron(“0”) complex in thin-layer spectroelectrochemistry with DMF as the solvent and 0.1 M NEt4ClO4 as supporting electrolyte.20b With TPP (Figure 1), reversibility is only partial, and a spectrum reminiscent of a бr-alkyliron(II) porphyrins19a−c appears superimposed with the same spectrum as that obtained with the superstructured porphyrins. Repeated recrystallization of the supporting electrolyte makes the second spectrum increase at the expense of the first. The spectrum thus assigned to the iron(“0”) porphyrins in DMF in the presence of NEt4+ cations is of the hyperporphyrins type and is clearly different from that reported for the same complex obtained from the reaction with a sodium mirror in THF.20c,d The description of the latter complex as a structure where the main resonant forms would be an iron(I) anion radical20d,c and an iron(II) dianion20e is therefore not suited to the complex we obtain in DMF with NEt4+ as countercation. The latter seems to have more electronic density on the iron center although it is not excluded that some electron density may remain on the ring.
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135. The iron(III) porphyrin has no affinity for CO but has a good affinity for Cl−. In the presence of CO as well as in its absence, wave 1 represents the reduction of Fe111Cl into Fe11Cl−23b,e The fact that wave 1 is insensitive to the presence of CO indicates that CO binds to the iron(II) porphyrin after irreversible expulsion of the Cl− ligand: Fe111Cl + e− s⇉f Fe11Cl−, Fe11Cl− → Fe−(II) + CO→ Fe(II) + CO → Fe11CO (the binding of Fe(II) with CO is faster than its binding with Cl−).
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140. This was confirmed by the cyclic voltammetry of TPPFe111Cl in the presence of CO3(NEt4)2.
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154. There is precedence for transition-metal complexes having two CO2 molecules in their coordination sphere linked in the same way as in the present case.10g In the cited iridium complex, not only the first CO2 molecule is coordinated to the metal through the carbon atom as here but also the second molecule is coordinated to the metal through one of its oxygen atoms unlike what we have represented in Scheme I. In iron porphyrins there are a few examples where some evidence has been provided for structures in which two atoms belonging to the same molecules are coordinated to the iron atom in a cis manner. They concern the binding at iron(lll) porphyrins of doubly reduced dioxygen28g or of the percarbonate resulting from the inclusion of CO2 in this complex.28g Such structures are less likely for iron(II) porphyrins where a carbene-type binding with a Fe-C bond perpendicular to the porphyrin ring seems in order.
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159. The solubility of CO in DMF is known at 20 °C and higher temperatures.24b It appears to decrease when the temperature is lowered and can be estimated to be 2 mM under 1 atm of CO at −40 °C.
160. The Fe1CO−/Fe(l)− ratio may well exceed 20 since the CO concentration in the reaction layer at low scan rate is likely to exceed 2 mM.
161. This equivalence can readily be proven in a rigorous manner at the level of the partial derivative equations, initial and boundary conditions that govern the system.
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