Porphyrins cation radical dimer

Figure 6.6.1 Molecular structure of zinc porphyrinate cation radical dimers in solution and the UV/vis/NIR spectra of the zinc octaethylporphyrinate cation radical —) and its diamagnetic dimer (-------). (From Fuhrhop et al, 1972 Song et al, 1989.)...

Tn other studies, reaction of [MnX(TPP)] (X = Nf or OCN ) with iodosylbenzene in hydrocarbon or halocarbon solvents yielded products formulated as yi-oxo dimers, [MnlvX(TPP)]20.678 [Mn,vN3(TPP)]20 has been characterized by X-ray diffraction (see Section 41.5.7.1). Infrared evidence supports678,679 the formulation of the dimers as being metal-centred oxidized products rather than containing porphyrin cation radicals the possibility of ligand-centred versus metal-centred oxidation in higher-valent metallo porphyrins has been discussed by several authors.680-682... 

Le Mest Y, L Her M, Hendricks NH, Kim K, Collman JP. 1992. Electrochemical and spectroscopic properties of dimeric cofacial porphyrins with nonelectroactive metal centers. Delocalization processes in the porphyrin rr-cation-radical systems. Inorg Chem 31 835... 

The TT-cation radicals of PFe-O-FeP species, the /u.-oxo dimers of Fe porphyrins, are convenient chemical oxidants of the chloroiron porphyrins, and they are interesting species in their own right. Those in which P = TPP or one of its derivatives have NMR shifts that suggest the same (a2 ) ground state for the porphyrin TT-cation radical, this time delocalized over the two porphyrin rings. ... 

The neutrally charged /x-nitrido dimer of TPPFe, [(TPPFe)2N] and its phthalocyanine " " and mixed-macrocycle counterparts, are isoelectronic with the TT-cation radical of the /x-oxo dimers discussed above, but they have a different electron configuration. " NMR shifts are more consistent with their description as delocalized system having two Fe + rather than a delocalized porphyrin TT-cation radical. This complex readily reacts reversibly with dioxygen to produce an EPR-active species that has... 

The ubiquitous porphyrin homodimers are only of interest in their oxidized form. The zinc octaethylporphyrin cation radical, for example, forms a diamagnetic dimer with an extremely strong charge transfer band in the near infrared (Figure 5.21). In crystalline material, all four pyrroles lie exactly on top of each other with an interplanar distance of 0.42 nm. The tt, Tr -dimer behaves like a... 

While the Co corrole-Fe/Mn porphyrin dyads were active electrocatalysts, the homobimetallic Co porphyrin-corrole dyads operate at lower overpotentials and favor 4-electron reduction to a greater extent. This was attributed to the locus of reduction in each complex apparently, the porphyrin is reduced first in the homometallic dimer, whereas the corrole is the site of the first reduction in the heterometallic dimers. A later article presented EPR and spectroelectrochemical evidence supporting the assignment of a Co(III) Ji-cation electronic configuration for the oxidized derivatives of monomeric Co triarylcorrolates, suggesting that the dimeric five-coordinate Co corrole catalysts are also Co(III) 7i-cation radicals [147]. Further study is needed to elucidate this point. 

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). 

Mn" porphyrins may be electrochemically oxidized in non-aqueous media to yield Mn " cation radicals and dications as the probable products." The redox potentials depend on the nature of the solvent, counter ion present, of any axially-bound ligands as well as on the basicity of the porphyrin ring involved. Water-soluble Mn " porphyrins are oxidized by a range of oxidants in aqueous alkaline solution to the corresponding Mn porphyrins which appear to exist as fx-oxo dimers using hypochlorite as oxidant, a second oxidation step also occurs. In this case the product has been postulate to be a Mn 0x0 porphyrin. The Mn and Mn porphyrins show only limited stability in water and revert to the stable Mn" porphyrin upon standing in the dark for several... 

An Fe oxidation state, apart from those of catalase and peroxidase compounds 1 and II, is attained only in the carbene complexes, RR C—Fe (Por), nitrene complexes,R N—N=Fe (Por), or the dimeric compounds, Fe (Por)=C=Fe (Por), Fe (Por)=N—Fe or) and [Fe (Por)—O—Fe HPor)]". Both carbene and nitrene complexes are diamagnetic, and the former appear to coordinate RNHj, py, Im, ROH, and RS . They lose the axial ligand in the presence of an excess of pyridine, and form Fe"(Por)(py)2. Though the iron oxidation state in these formally Fe (Por) complexes is still ambiguous, their reactivity implies an iron(III) oxidation state. Attempted synthesis of Fe porphyrins by a one-electron oxidation of Fe (TPP)Cl resulted in formation of the corresponding porphyrin n cation radical Fe "(TPP) . The high oxidation state iron porphyrins are of particular interest in relation to the cytochromes P-450, peroxidases and catalases, and Fe" 0(Por)L (L = 1-MeIm, py, Pip) have been spectroscopically characterized (Scheme 18). - "... 

The porphyrin also undergoes aerial oxidation in acid solutions. This time, a ir-cation radical is generated with unpaired electron density delocalised over the macrocycle. This radical is also produced via a conproportionation reaction between the porphyrin and the two-electron oxidised compound. There is some evidence that the Ti-cation radical, as generated from the porphyrin, is in fact a radical dimer. It is interesting to note that the aerial oxidation of the porphyrin affords radicals that can apparently be made to switch their unpaired electron density from different parts of the molecule depending on the acidity or basicity of the surrounding medium. 

Measurements of the oxidation potentials yielded results which reflected the close proximity of the two phthalocyanine planes. The first two oxidation potentials (one-electron oxidations from each phthalocyanine ring) were 100 mV apart, suggesting the delocahzation of the cation radical over the two phthalocyanines jt-electronic framework. This behavior shows some similarity to that of the porphyrin dimers, and is expected to favor energy- and electron-transfer reactions. Another characteristic feature of this phthalocyanine dimer is that its fluorescence quantum yields are almost the same as those of the corresponding monomers (0.45, 0.26, and 0.76 for Zn(OBu), Zn(f-Bu), and Mg(f-Bu) phthalocyanines, respectively). Such a highly fluorescent phthalocyanine dimer has never been reported before. 

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