Cations and Cation Radicals of Porphyrins and Ethyl Chlorophyllide a
Felton, R.H., David Dolphin, Borg, D.C., and Fajer, J.
J. Am. Chem. Soc.
The electrolytic and chemical preparation of cation radicals of various porphyrins and Et chlorophyllide a are described as well as the formation of the dication of Mg octaethylporphyrin (MgOEP). Electrolysis of MgOEP and CH2Cl2 at +0.7 v. (vs. S.C.E.) yielded a blue-green solution and the resulting species was shown by coulometry to be the product of a 1-electron oxidation The E.S.R. measurements on these solutions showed a single line, g = 2.0028, 2.5 G wide. MgOEP underwent a similar 1-electron oxidation when treated with Br in either CH2Cl2 or CHCl3. The product of the chemical oxidation was spectroscopically similar to the cation radical observed via electrolysis and was stable in the solvents. Additional evidence for a simple cation radical, in which an electron was abstracted from the porphyrin ring, is found in the reversible 1-electron oxidation of Mg and Zn tetraphenylporphyrins (MgTPP, ZNTTP). Continued electrolysis of MgOEP+ at +0.9 v. (vs. S.C.E.) resulted in the removal of an additional electron. The 2-electron reduction of this new species generated 95% of the initial MgOEP. Electrolysis of MgTPP+ and ZnTPP+ also proceeded in dication stages. Co(II) octaethylporphyrin [Co(II)OEP] and CHCl3 underwent 2 distinct 1-electron oxidations when treated with Br2. The 1st step brought about the oxidation of divalent to trivalent Co giving [Co(III)OEP]+Br-. Further oxidation of the trivalent complex yielded a green species formulated as [Co(III)OEP]2+2Br-. The 2-electron oxidation stage could also be reached by electrolysis in CH2Cl2. The electrochemical oxididation of Et chlorophyllide a was investigated; 1-electron oxidation. at 0.6 v. vs. S.C.E. yielded a yellow solution. The product exhibited a single E.S.R. signal 9G wide, g = 2.0028. Attempted chemical oxidation by Br2 led to allomerized material.