Oxidative behavior of TPPZn/anion systems

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The oxidative voltammetry of meso-tetraphenylporphinatozinc(I I), TPPZn, in the presence of coordinating anions has been investigated in order to assess how changes in the metal coordination environment affect the electrochemistry of the porphyrin ligand. In a solution of 0.2 M tetrabutylammonium perchlorate dissolved in 1,2-dichloroethane, TPPZn undergoes two sequential one electron oxidations to form the porphyrin n-cation radical and dication. Evidence is provided that the first oxidation is accompanied by complexation of perchlorate from the supporting electrolyte. This reaction is manifested in a -121 mV shift in the first ring oxidation potential. Addition of coordinating anions to TPPZn-containing solutions causes the entire U.V.-visible spectrum to be red shifted, and the relative intensitie?? of the porphyrin Soret, ??, and a bands to be altered. The magnitude of the red shifts appears to be correlated to the binding strength between the anion and porphyrin. Addition of coordinating anions causes the first ring oxidation potential to be shifted by up to -257 mv ± 1 o mv, the magnitude of which is dependent upon the ratio of binding constants to the cation radical and unoxidized porphyrins. The potential for this redox couple is correlated to the pKa of the anionic ligand, and to the red shifts in the U.V.-visible spectrum. Formation of the dication becomes irreversible in the presence of coordination anions due to nucleophilic attack on the dication by the anion, generating an lsoporphyrin cation. This species is shown to be redox active at more anodic potentia1s, the oxidation apparent1y yie1ding an isoporphyrin dication radica1. A spectroe1ectrochemical ce11, capab1e of performing thin layer and diffusion control1ed vo1tammetries, was designed and constructed for this work.. This ce11 facilitated the performance of in-situ F.T.I.R. spectroelectrochemistry on the TPPZn/anion system for the first time. This technique a11ows the detection of various complexation reactions involving the cation radica1, demonstrates several bands diagnostic for cat ion rad1ca1 format ion, and confirms the gene rat ion of the isoporphyrin cat ion.
Bibliography: p. 99-104.
Pavelich, B. J. (1987). Oxidative behavior of TPPZn/anion systems (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/16418