Porphyrin iron monoxide, formation

Formation of the porphyrin iron monoxide (of the Compound I type) as an active oxidizing species in the P-450 system is supported by the successful monoxidation of P 450 by an oxidizing reagent such as iodosobenzene (see Figure 5) (16). This species then effectively hydroxylates an organic substrate. 

The product is exclusively carbon monoxide, and good turnover numbers are found in preparative-scale electrolysis. Analysis of the reaction orders in CO2 and AH suggests the mechanism depicted in Scheme 4.6. After generation of the iron(O) complex, the first step in the catalytic reaction is the formation of an adduct with one molecule of CO2. Only one form of the resulting complex is shown in the scheme. Other forms may result from the attack of CO2 on the porphyrin, since all the electronic density is not necessarily concentrated on the iron atom [an iron(I) anion radical and an iron(II) di-anion mesomeric forms may mix to some extent with the form shown in the scheme, in which all the electronic density is located on iron]. Addition of a weak Bronsted acid stabilizes the iron(II) carbene-like structure of the adduct, which then produces the carbon monoxide complex after elimination of a water molecule. The formation of carbon monoxide, which is the only electrolysis product, also appears in the cyclic voltammogram. The anodic peak 2a, corresponding to the reoxidation of iron(II) into iron(III) is indeed shifted toward a more negative value, 2a, as it is when CO is added to the solution. 

Iron(O) tetraphenylporphyrin is also a catalyst of the electrochemical carbon dioxide reduction to carbon monoxide in DMF electrolytes [29-31]. Under CO, the catalyst is, however, rapidly destroyed by either carboxylation or hydrogenation of the porphyrin ring. The mechanism proceeds through coordination of CO2 to Fe(0). At low temperature (-40°C), a second CO2 molecule adds to the first-coordinated one in an acid-base reaction fashion. This is followed by the cleavage of a C—O bond of the first-coordinated CO2 molecule and concomitant formation of the Fe(II)CO complex and a carbonate anion. After reduction of the Fe(ll)CO complex by the Fe(0) complex, CO is... 

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