Metalloporphyrins Cation radicals

Paramagnetic Metalloporphyrin Cation Radicals and Their Magnetic Properties. .. 172... 

Dimerization of heterocyclic cation radicals was discovered in recent times. The well known thianthrene cation radical perchlorate forms a dimer in propionitrile solution. This association is, in fact, tetrameric (de Sorgo et al., 1972) that is, the anions are included in the aggregrate (Th +C104)2. Phenoxaselenine cation radical has also been found to form an (M +)2 dimer in sulfuric acid and nitro-methane solutions (Cauquis and Maurey-Mey, 1973). It is not surprising that the very stable metalloporphyrin cation radicals should dimerize, but this was demonstrated only recently with zinc (Fuhrhop et al., 1972), and magnesium (Fajer et al., 1970) octaethyl-porphyrin (MOEP). 

The pairing of anion radicals with their counter cations is a wide-spread and now well documented phenomenon (Szwarc, 1972). In contrast, ion-pair phenomena in cation radical systems are not common, but see p. 222. Shifts in -values caused by halide-ion interaction with the tetramethoxybenzene cation radical have been reported (Sullivan, 1973), and halide-ion splittings of metalloporphyrin cation radical esr spectra have been demonstrated (Fajer et al., 1973). 

The metalloporphyrin cation-radical would likely react with the radical derived from the reagent, with substitution occurring at a meso-position, leading to the formation of a resonance-stabilized carbonium ion of the metalloporphyrin. That substitution occurs in the meso-positions is suggested by the observation that one of the major fractions obtained from the reaction of vanadyl octaethylporphine with sulfuryl chloride contained no methine protons, as demonstrated by the nmr spectrum. The site of substitution being meso is further supported by the fact that reactions of metalloporphyrins with peroxides and bromine provide meso-substituted products. Electro-... 

P. Neta, V. Grebel, and H. Levanon, One-Electron Oxidation of Chlorophyll a and (Tetraphenylporphyrinato)cobalt(II) by Various Metalloporphyrin Cation Radicals. Kinetic Spectrophotometric Studies, J. Phys. Chem., 85 (1981) 2117. 

The most common reactions involving nucleophiles and porphyrin systems take place on the metalloporphyrin 77-cation radical (i.e. the one-electron oxidized species) rather than on the metalloporphyrin itself. One-electron oxidation can be accomplished electrochemi-cally (Section 3.07.2.4.6) or by using oxidants such as iodine, bromine, ammoniumyl salts, etc. Once formed, the 77-cation radicals (61) react with a variety of nucleophiles such as nitrite, pyridine, imidazole, cyanide, triphenylphosphine, thiocyanate, acetate, trifluoroace-tate and azide, to give the correspondingly substituted porphyrins (62) after simple acid catalyzed demetallation (79JA5953). The species produced by two-electron oxidations of metalloporphyrins (77-dications) are also potent electrophiles and react with nucleophiles to yield similar products. 

The possibility of photo-oxidizing zinc and magnesium porphyrins in vitreous matrices in the presence of the efficient acceptor of electrons C(N02)4 was first demonstrated by Kholmogorov and Bobrovskii [54]. The authors observed the formation of N02 free radical and the cation radical of metalloporphyrin, MP+, upon illumination of solutions in the MP absorption band. 

From the data of refs. 64 and 65 it follows that, depending on the nature of the reagents, the metalloporphyrin-sensitized electron transfer from donor to acceptor particles by the tunneling mechanism in vitreous matrices can proceed via the formation of both cation radicals and anion radicals of me tal loporphyrins. 

An analogous cation radical chain process has been proposed for cis to trans isomerization of N-methyl-4-(6-stryl)-pyridinium ions via electron-transfer sensitization by Ru(bpy)-j2+ and metalloporphyrins (145). Quantum yields for isomerization are substantially higher in aqueous anionic micelles versus homogeneous solution due to the higher concentration of cis-styrylpyridinium ions. A radical cation chain mechanism may also account for previous reports of selective cis to trans sensitized photoisomerization of stilbene (25,26). 

The preceding examples show that one-electron reduction of metalloorganic complexes or coordination between a metal and an anion radical ligand may expand an electron shell of the central metal atom. Sometimes, anion radical metallocomplexes contrast in this regard with the cation radical ones. Thus, some metalloporphyrins form cation radicals with charges and unpaired electrons on ligands (Shinomura et al. 1981) and anion radicals with charges and unpaired electrons on metals (Lexa et al. 1989). 

Metalloporphyrins catalyze the autoxidation of olefins, and with cyclohexene at least, the reaction to ketone, alcohol, and epoxide products goes via a hydroperoxide intermediate (129,130). Porphyrins of Fe(II) and Co(II), the known 02 carriers, can be used, but those of Co(III) seem most effective and no induction periods are observed then (130). ESR data suggest an intermediate cation radical of cyclohexene formed via interaction of the olefin with the Co(III) porphyrin this then implies possible catalysis via olefin activation rather than 02 activation. A Mn(II) porphyrin has been shown to complex with tetracyanoethylene with charge transfer to the substrate (131), and we have shown that a Ru(II) porphyrin complexes with ethylene (8). Metalloporphyrins remain as attractive catalysts via such substrate activation, and epoxidation of squalene with no concomitant allylic oxidation has been noted and is thought to proceed via such a mechanism (130). Phthalocyanine complexes also have been used to catalyze autoxidation reactions (69). 

On reaction with a stoichiometric amount of hydroperoxide, catalase and horseradish peroxidase are converted to a green colored intermediate. Compound I (5). The chemical nature of Compound I has been extensively debated since its discovery by Theorell 59). Recently, Dolphin et al. 60) have demonstrated that upon one-equivalent oxidation several metalloporphyrins are converted to stable porphyrin jr-cation radicals, the absorption spectra of which possess the spectral characteristics of Compound I, namely, a decreased Soret w-n transition and an appearance of the 620-670-nm absorption bands. Since Moss et al. 61) proposed the presence of Fe(IV) in Compound I of horseradish peroxidase from Mossbauer spectroscopic measurements, it is attractive to describe Compound I as Fe(IV)-P, where P is a porphyrin w-cation radical. Then, Compound I and Compound ES become isoelectronic. Both contain Fe(IV) and a radical the former as a porphyrin radical (P ) and the latter as a protein radical (R ). Then the reaction cycles of horseradish and cytochrome c peroxidases may be compared as shown in Fig. 4. 

Only a few Fe porphyrin jr-cation radicals have been characterized structurally,i53 i i since they are quite reactive species. They have been characterized mainly by NMR spectroscopy, magnetic susceptibility measurements, and a diagnostic band in the IR spectrum in the region of 1270-1295cm i.15 13 Their main use has been as chemical oxidants of porphyrins, metalloporphyrins, and so on. ... 

The reaction of 6 with TCNE yields only the conventional adduct corresponding to the uncyclized probe and none of the product expected from the cation radical cyclization. That the probe cyclization of the cation radical of 6 would have been observed in the context of an ET mechanism, if it had been involved, was demonstrated by generating the contact ion radical pair of 6+7TCNE via excitation of the charge transfer complex of 6 and TCNE. The cyclobutane cyclization product of the probe reaction was easily detected under these conditions. Consequently, an ET mechanism for this reaction can be confidently excluded. In a similar manner, the epoxidation of 5 and 6 by oxidized metalloporphyrins provides strong evidence against a cation radical mechanism for these reactions [76]. 

The metalloporphyrin V-cation radical spectrum. An extremly broad band covering the whole visible range of the spectrum and often extending into the near infrared is typical of a one-electron oxidation product of the porphyrin ligand. Sometimes strong visible absorption bands around 700 nm are also found with these products, but the broad visible absorptions are always present. The Soret band usually has a low extinction coefficient and is broadened considerably [e.g. Fuhrhop (76)]. 

The Metalloporphyrin rc-Dication Spectrum. This is usually very similar to yr-cation radical spectra, but the Soret band has shifted below 350 nm and is even less intense. Often phlorin formation is observed [e.g. Fajer (55)]. 

The various oxidation states and the chemical behavior of the porphyrin ligands in their metal complexes will now be discussed in turn. The jr-cation radicals and the phlorins are considered to be the most important oxidation and reduction products of metalloporphyrins and will therefore be described in more detail. 

Iron(III) porphyrins with imidazole anion(s) as ligands have been synthesized in non-aqueous media"" , and are potential models for heme-containing enzymatic species. Metalloporphyrin vr-cation radicals generated electrochemically (but never from iron porphyrins) are models for compound I or horseradish peroxidase " (see 14.8.5.3.2). 

Several investigations of the redox properties of various free base hydroporphyrins and their metal derivatives have been reported. As is typical of many porphyrins and metalloporphyrins, these hydroporphyrins generally show two oxidations and one or more reductions. The reversibility of these redox reactions depends on the nature of the hydroporphyrin and its stereochemistry. For example, the cyclic voltammograms of ris-H2(OEC) and frans-H2(OEC) were superficially alike, although substantial differences existed in the stability of the cation radicals and dications of the cis and trans isomers [85]. The first oxidation of rrans-H2(OEC) was reversible whereas ds-H2(OEC) was not reversible. However, the notable features observed in the redox chemistry of hydroporphyrins is the shift of both oxidation and reduction potentials of hydroporphyrins towards more negative values compared to porphyrins, i.e., they are more easier to oxidise and difficult to reduce [78]. A significant trend was observed in the electrochemistry of free base octaethyl- [86, 87] and tetraphenyl [88,89] hydroporphyrins (Table 2). The porphyrin and chlorin of each series... 

Goff and coworkers extensively used paramagnetic NMR in identifying the nature of coupling present between the unpaired electron present on the metal and porphyrin ring in metalloporphyrin it-cation radicals [224-228], NMR spectroscopy does probe the environment of protons at the extreme periphery of... 

Anodic oxidation has also been employed to produce stable cation radicals (and sometimes dications) of metalloporphyrins (see, e.g. Stanienda and Biebl, 1967 Fajcr et al., 1970 Wolberg and Manassen, 1970 Lexa and Reix, 1974 and references therein). Since tetraphenylporphin itself produces a stable cation radical in methylene chloride (Tokel et al., 1972), many of these metalloporphyrins (e.g. those with Mg, Zn, and Cu) can be considered as metal stabilized cation radicals, with the unpaired spin density located primarily on the ligand. 

As far as we are aware, reactions of cation radicals with carbonyl compounds were until recently very rare. Cyclization of the 6-/3-ketopropionic ester during oxidation of the metalloporphyrin [52] by iodine is thought by Cox et al., (1969) to occur in the... 

For the insertion of the iron ion into the porphyrin a variety of general procedures have been described and reviewed. In most cases, these methods lead to the formation of Fe " complexes, which are then used to prepare Fe Fe , Fe, and Fe porphyrins. The most commonly employed methods for synthesizing Fe porphyrins are described below. The preparation of the Fe and Fe complexes from the iron (III) porphyrins by chemical or electrochemical means and the oxidized iron porphyrins (Fe TT-cation radicals, Fe, Fe TT-cation radicals, and Fe ) by chemically or electrochemically oxidizing the iron(III) porphyrins is described in more detail in the sections on the corresponding iron porphyrins below. Whereas Fe porphyrins can be photochemically reduced to Fe porphyrins, only a few examples of photooxidations of the iron center are known, which include laser photolysis of the co-condensation products of PFe at 15K to produce PFe = O. Typical photochemical reactions of iron and other metalloporphyrins have been summarized by Suslick and Watson. ... 

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