38 Discussion 



number, and their basicity now more correctly established by Phillips' interesting 

 method. It is also interesting to note that evidence for a monocation had previously 

 been obtained by Neuberger and Scott for deuteroporphyrin disulphonic ester, 

 somewhat resembling anionic detergents, although Walter had not been able to 

 confirm this. 



With regard to the spectra of porphyrin dications, it is of interest that on closer 

 observation four, not two, absorption bands can be observed in the visible part of 

 the spectrum. This leads me to doubt the correlations assumed by Piatt for the neutral 

 and acid spectra of porphyrins. In this connexion I should like to point out that the 

 two bands I and III of neutral porphyrin spectra are not at all influenced in a similar 

 manner by the substitution of an electron-attracting group on the porphin nucleus. 

 Thus all formyl and ketonyl-substituted porphyrins have their band III greatly 

 increased, but their band I greatly diminished as the table on p. 37 shows. 



While it is true that band I is the most variable and band III the second in variability, 

 there is, in fact, little difference between the variability of ^niM of bands II, III and IV 

 in a variety of different porphyrins. 

 Phillips: Walter's failure {J. Amer. chem. Soc. 75, 3860, 1953) to detect the monocationic 

 species observed by Neuberger and Scott {Proc. Roy. Soc. A. 213, 307, 1952) was due 

 to an unfortunate choice of experimental conditions. This aspect has been discussed 

 by Scott (/. Amer. chem. Soc. 77, 325, 1955). 



The Reactions between Metal Ions and Porphyrins 



By J. H. Wang and E. B. Fleischer (Yale) 



Wang : Phillips and his co-workers suggested that the combination of porphyrin esters 

 with Zn++ to form Zn++-porphyrin derivatives takes place through a displacement 

 rather than a dissociation type of mechanism. I would like to report some work 

 which not only confirms his suggestion but also gives a more detailed understanding 

 of this displacement mechanism. 



We found that the rates of the successive steps in the combination of a metal ion 

 and a porphyrin derivative can be markedly affected by varying the solvent composi- 

 tion, presumably due to the change in solvation of the metal ion. In acetone solutions 

 some metal ions, Cu++, Bi+++, Hg++, Cd++, etc., react readily with the dimethyl ester 

 of protoporphyrin even at room temperature to form the corresponding metallo- 

 porphyrin, whereas other metal ions, Fe++, Fe+++, Cr+++, Pt+++"'", Sn++, Zn++, etc., 

 form a new type of complex with absorption spectra markedly different from that of 

 the corresponding metallo-porphyrins; only upon heating do their spectra change 

 to those of the metallo-protoporphyrins. 



The spectra of some protoporphyrin derivatives in chloroform solution are shown 

 in Fig. 1 . The spectra are respectively (A) protoporphyrin dimethyl ester, (B) haemin 

 dimethyl ester, (C) the new type of complex formed between ferric chloride and proto- 

 porphyrin dimethyl ester, and (D) the dihydrochloride of protoporphyrin dimethyl 

 ester. 



If alcohol or pyridine is added to the chloroform solution of this new complex 

 formed between ferric chloride and protoporphyrin dimethyl ester, the spectrum 

 changes immediately to that of protoporphyrin dimethyl ester, (A) in Fig. 1 . This 

 shows that the binding between Fe"''++ and protoporphyrin in the new complex is 

 quite weak, since the protoporphyrin can readily be displaced by other ligands. We 

 suggest that this new complex has a sitting-atop type of structure as illustrated in 

 Fig. 2. It is also of interest to note that the spectrum of the sitting-atop complex of 

 Fe+++ bears striking resemblance to that of protoporphyrin dication, PH4++, as 

 shown by diagram (D) in Fig. 1. This observation suggests that the observed absorp- 

 tion in (C) is probably due to electronic transitions in the protoporphyrin rather than 

 the metal part of the complex. 



Similarly, if acetone solutions of the sitting-atop complexes of Fe++, Fe+++, Cr+++, 

 Pt++++, Sn++, Zn++ respectively are diluted with water the spectrum immediately changes 



