68 



J. E. Falk and D. D. Perrin 



Calvin (1953) have discussed this correlation between electron-withdrawing 

 effect and Eq; potentials of iron complexes rise as the electron-attraction of 

 side-chains increases (Table 3). The differences, although rather small, lie 

 in the expected order. 



In suflEiciently alkaline solutions, complexes of nitrogenous bases with iron- 

 porphyrins give oxidation-reduction potentials which vary Unearly with pH. 

 The reaction can be written: 



Fe+++B2 + OH- ^ Fe++B . OH + B + e 



although the ferric complex may be present mainly as an easily split dimer 

 (Shack and Clark, 1947). Such potentials should therefore be compared at 

 approximately constant base concentration and pH. Around pH 9-6 

 reported potentials are as shown in Table 4, 



Table 4 



Data from Martell and Calvin (1952). 

 * For the reaction Fe+++B2 + OH- ^ Fe++BOH + B + e. 



A different kind of comparison can be made by keeping the porphyrin 

 nucleus constant and varying the base co-ordinating with the iron complexes. 

 Taking the data of Barron (1937) for protoporphyrin-iron we find that at 

 pH 9-2 nicotine, pyridine and a-picoline give oxidation-reduction potentials 

 showing the expected pH-dependence — A^"/ ApH = 0-06. On the other hand, 

 the slope for histidine and pilocarpine is much less, indicating that the 

 complexes Fe+++B2 are also present in significant amounts. To minimize 

 this interference the Eq values listed in Table 5 have been calculated from the 

 data using as low a concentration of bases as possible. The series shows a 

 roughly linear dependence on pK^, with a slope of the order of — AEfApK 

 <^ 0-04 V. This slope is similar in magnitude to that found for 1 : 1 iron 



