Metalloporphyrin redox potentials

The redox potentials of many metalloporphyrin complexes are sensitive to the nature of the electrolyte in which they are measured. This is because one or both axial positions are coordinated by either solvent or anionic ligands. Such dependencies of E° values have been extensively studied by Kadish and coworkers 29 notably good correlation between E° data and Donor/Acceptor Number properties of solvents have been observed. 

Table 4 The Lifetimes of the Lowest Energy Triplet Excited State and the Redox Potentials of Metalloporphyrins in the Singlet and Triplet States...

Therefore, the changes in the oxidation state at the metalloporphyrin center can also modify the absorption spectra of the peripheral complexes and vice versa. Signihcant shifts of the redox potentials also can be observed. For example, the ring oxidation of the Co P species is shifted to potentials >2.0 V (it is generally found at 1.5 V), as a consequence of the four electron-withdrawing species bond... 

Although this article deals with electron transfers with metalloporphyrins and generally does not cover the chemical reactions of the porphyrin ligand, it is felt that one exception has to be made and that the protonation of the methine bridges of porphyrin n anions should be discussed. This reaction invariably occurs when the reduction of porphyrins is carried out in protic media and is usually fully reversible with a rise in the redox potential of the solution, very much like electron addition and removal [Cfoss (35), Inhoffen (99, 101, 102, 103), Kiselev (113), Mauzerall (129, 130), Peychal-Heiling (147, 148), Seely (156), Shablya (157), Shulga (158), Sidorov (159), Woodward (213, 214)]. 

L. Gueletti, Metalloporphyrins in Nonaqueous Media Database of Redox Potentials, in The Porphyrin Handbook , eds. K. M. Kadish, K. M. Smith, and R. Guilard, Academic Press, San Diego, CA, 2000, Vol. 9, Chap. 59. 

The interest in the properties of metalloporphyrin radical cations in water relates to their potential use as oxidants in solar energy storage schemes. In particular, the oxidation of water to oxygen by highly-oxidized metalloporphyrins has been considered as a model for photosynthesis. The pulse radiolytic studies outlined above provided good understanding of the factors that influence the stability of the radical cations in water. A critical point is the correlation between stability of a porphyrin radical cation and the redox potential for oxidation of the porphyrin. Consequently, porphyrins with a sufficiently high oxidation potential to be useful oxidants of water have short-... 

Catalysis by metal-organic solids may also be applied to redox reactions. An especially intriguing target would be manipulation of hydrocarbon transformations. Many metal-organic networks so far reported in fact contain redox-active transition metals (Cu , Pd", Co" and so on). Metalloporphyrins are potential... 

Fig. 1 Periodic Table of metalloporphyrins. Shaded elements indicate specific elements that have been incorporated into a given porphyrin macrocycle. Extensive tabulations of redox potentials may be found in Ref [21].

The electrochemistry of metalloporphyrins at the start of the 1960s involved, in large part, measurements of standard redox potentials for naturally occurring complexes in aqueous buffered media [14], The choice of an aqueous solvent was often dictated by the biological relevance of the compounds available for study, while the choice of the measurement technique (potentiome-try or polarography at a dropping mercury electrode) was necessitated by the type of available electrochemical instrumentation, virtually all of which was homemade and... 

Finally, there is the question of supporting electrolyte concentration. Most measurements have been made in solutions containing 0.1 M TBAP (the most often utilized salt), but others have utilized solutions with 0.2 M supporting electrolyte, especially in the case of spectroelectrochem-ical measurements. Attention should be paid to this fact since redox potentials measured in solutions of 0.1 M TBAP are not always identical to those measured with TBAP concentrations of 0.01 or 1.0 M.The experimentally obtained differences in potential may amount to several hundred millivolts and will vary as a function of the specific metalloporphyrin and the specific electrode reaction examined [21]. 

In metalloporphyrins with transition metal ions, the redox peaks of central metal ions intervene between the redox peaks of porphyrin ring in many cases as shown in Table 3. Delicate positioning among them are often observed [726]. Comprehensive interpretation of redox potentials of porphyrins has been attempted with the use of ligand parameter El [60]. Axial ligands also affect the redox properties of metalloporphyrins. Detailed studies were reported on Fe, Mn, and Cr porphyrins [56,57] (Fig. 8). 

An impcHtant question in metalloporphyrin redox chemistry is wheAer a given electron transfer occurs at a metal or porphyrin site. The issue frequently is addressed by spectroscopic meAods or by measurement of formal redox potentials. However, Ae rate of electron transfer also can be mAcative of Ae site of charge transfer m a metalloporphyrin [5]. 

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