Substituent effects on the redox reactions of tetraphenylporphyrins.
The effect of substituents on pi radical reactions of para-substituted tetraphenylporphyrins was investigated by cyclic voltammetry in methylene chloride. In all cases electron donating substituents produced a more difficult reduction and an easier oxidation. Plots of E1/2 vs. a yielded Hammett linear free energy relationships for cation radical and dication formation and anion radical and dianion formation. An average reaction constant of p=0.07+/-0.01 V was obtained. This was true for tetraphenylporphyrins containing the central metals VO, Mn, Fe, Co, Ni, Cu, and Zn, as well as the free base H2 (p-X)TPP. The value of p appears not to be directly affected by the central metal oxidation state or the overall charge on the complex.
D055598:Chemical Phenomena; D002621:Chemistry; D008665:Metalloporphyrins; D010084:Oxidation-Reduction; D011048:Polarography; D011166:Porphyrins; D013816:Thermodynamics
D008665:Metalloporphyrins; D011166:Porphyrins