Electron-transfer reactions metalloporphyrins

Photoinduced electron transfer reactions in supramolecularmodel systems based on metalloporphyrins 97YGK557. 

The data presented in this chapter indicate that many photochemical and dark electron transfer reactions with the participation of metalloporphyrin molecules and their ion radicals can proceed by the tunneling mechanism. Also, among the processes of electron phototransfer involving metallopor-phyrins, there occur not only stoichiometric but also photoctalytic reactions. In the latter case, the metalloporphyrins themselves are not consumed, but they provide the phototransfer of electrons between donor and acceptor par-ticels. 

Light-Induced Electron Transfer Reactions of Metalloporphyrins and Polypyridyl Ruthenium Complexes in Organized Assemblies... 

Although such a large k value is rather unusual in electron-transfer reactions of metalloporphyrins [1], the electron transfer kinetics for generation of iron(IV) por-... 

Metalloporphyrins can also act efficient catalysts in a variety of chemical and biological redox reactions. This area has been extensively studied and the catalytic functions of metalloporphyrins via electron transfer reactions have been reviewed Volume II, Part 2, Chapter 8 [270]. 

One of the most prominent properties of metalloporphyrins is the accessibility of several oxidation states, which is crucial in electron-transfer reactions, e.g. oxygenation catalysis or oxygen transport. In most cases a larger number of these oxidation states are stable on the timescale of cyclovoltammetric experiments or even longer, an example of such multiredox behaviour can be seen in Figure 4.1. 

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)]. 

D.M. Guldi, P. Neta, and K.-D. Asmus, Electron Transfer Reactions Between and Radical Ions of Metalloporphyrins and Arenes, J. Phys. Chem., 98 (1994) 4617. 

The nature of the porphyrin ring may influence reactions which are photoactivated or sensitized by visible light. Although electron transfer from metalloporphyrins to other substrates has been widely studied, examples for activation of the ligand bound to the metal upon irradiation have been very limited. The reactivity of o-bonded aluminum and indium porphyrins towards carbon dioxide has been studied. 

The long-range electron transfer reactions in ruthenium-modified myoglobin, in which the labile heme unit has been replaced by various metalloporphyrins, have been reviewed. The reductions of the [Ru(NH3)5] moiety, attached at His-48, by Pd- and Pt-substituted hemes in myoglobin proceed at rates of 9 1 x 1(P and 1.2x lO s", respectively. The difference in rates for electron transfer between Fe (heme) and Mg or Zn(porphyrin) centers in [a(Fe(II)P),j3(M T)] hemoglobin hybrids indicates a direct process as opposed to the involvement of a conformational gate. Using [Co(NH3)5Cl] to quench the Zn state, a rate constant of 2.4 x 10 s has been measured for back electron transfer within [a(Zn- -P)i8(Fe(III)CN)]. ... 

In this chapter, we will concentrate on the subjects of synthetic strategies for (1) porphyrins and the capability of the products for molecular recognition of small organic substrates via intermolecular and noncovalent interactions (2) effective electron-transfer reaction regulated by intermolecular interaction " and (3) self-assembled porphyrin to construct supramolecular system for the guest. The Structures of porphyrins and their metalloporphyrin derivatives have attracted chemists due to their suitability as a host framework for organic guest molecules. 

Porphyrin-nitrosyl complexes with six other metal ions are also known, and all but one of which has been electrochemically investigated. These are Ru [69, 73, 94-96], Os [5], Rh[97], Cr [98], Mo [99] and Mn [100]. Some nitrosyl metalloporphyrins can be reversibly reduced or oxidized by one or two electrons without loss of the NO ligand and this generally occurs when the electrode reactions involve the 7T-conjugated macrocycle in the case of a metal-centered reduction or oxidation, however, the electron-transfer reactions will most often be accompanied by a loss of the NO ligand, resulting in an irreversible oxidation as shown in Fig. 7 for the case of (TPP)Cr(NO) and (TPP)Mn(NO) in CH2CI2. 

Carbon dioxide radical anions, C02 , are commonly used in aqueous chemistry as a reducing agent for metalloporphyrins or as intermediate in the formation of superoxide anion. COf has been reported to undergo efficient electron transfer reactions with methyl violo-gen, quinones, alkyl halides, fumarates, nitro and nitrosobenzenes and chlorinated benzaldehydes. With nitrobenzenes and chlorinated benzaldehydes, electron attachment occurs on the nitro and aldehyde groups, respectively. CO2 radicals have also been reported to add to some unsaturated compounds such as acrylamide and pyridin-3-ol. Efficient hydrogen abstraction from mercaptobenzenes have also been reported. 

Electron transfer reactions from metalloporphyrin dimers have been studied using molecules that have a quinone moiety chemically attached to the porphyrin ring. These metalloporphyrin complexes are designed such that charge separation can be accomplished between the porphyrin and the quinone, and that these transients can then be used as oxidants. 

Electrochemistry of metalloporphyrins containing Group 13 metals which are a-bonded to different aryl and alkyl groups has been reported by Radish and Guilard . All of the complexes can be reversibly reduced by one or two single electron transfer reactions. 

Secondly, Fig. 5 shows that the polymeric rate constants parallel values of heterogeneous rate constants that have been observed for the electrochemical reactions of solutions of the corresponding dissolved porphyrin monomers. (The slope of the line is 0.5). This re-emphasizes what was said above, that measurements of electron hopping in polymers can give rate constants that are meaningful in the context of the metalloporphyrin s intrinsic electron transfer chemistry. 

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