Porphyrin radical dimers

Figure 5.21 The oxidized porphyrin radicals form very stable dimers, in which the unpaired electrons reversibly form a bond.

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.)...

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. 

As previously, porphyrin radical cations can be obtained electrochemically instead of using chemical oxidants. In this context, Osuka and coworkers showed that similar direct meso-meso couplings can be performed by electrochemical oxidation in place of chemical pathway. Indeed, dimers and longer oligomers were obtained by electrolysis at a potential corresponding to the first ring oxidation of the macrocycle [168]. 

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... 

Reduction of nitrostyrene with aqueous TiCl3 gives a 3,4-diarypyrrole directly in moderate yield (Eq. 10.46).52 The reaction proceeds via dimerization of anion radicals of nitrostyrene and reduction of the nitro function in the dimer to imines. Reduction of dinitrile with diisobutylalu-minum hydride (DIBAL) gives a-free pyrroles (Eq. 10.47) 53 both reactions may proceed in a similar mechanism. These pyrroles are useful intermediates for functionalized porphyrins. 

Specifically Netzel et al. ( - ), in studies of face-to-face , covalently-linked MgP-P dimers, found evidence for the formation within 6 psecs of a low-lying, relatively long-lived intramolecular CT state of the type MgP -P" in polarizable or highly polar solvents and in solvents where chloride ion coordinates with the magnesium ion of the MgP-macrocycle. These workers also observed the formation of benzoquinone anion radicals as stable photoproducts of the CT formation process when the experiments were carried out in the presence of benzoquinone ( ). This approach provides a more direct test for the formation of an intramolecular CT state, and the results are in sharp contrast to those typically observed when porphyrin Ktt, ) states are quenched in the presence of benzoquinone (23). 

Reactions of monomeric and dimeric rhodium(II) porphyrins with carbon monoxide - As already reported in Sect. 3.3, a carbonylrhodium(II) porphyrin behaves as an acyl radical. Hence, if possible, dimerization or coupling reactions occur. Evidence for the formation of isomeric 2 1 Rh(P) CO adducts, namely a monoadduct of the dimer and a metallo ketone complex, and a dimeric 1 1 adduct in the reaction of [Rh(OEP)]2 with carbon monoxide according to sequences (38) and (39) has been presented [340,341] solution equilibria and structures have been studied essentially by lHNMR, 13CNMR, and IR spectroscopy. The first half of sequence (38) and reaction (39) occurred in parallel at CO pressures up to 12 atm at 297 K. At higher pressures, or at lower temperatures, the double-insertion of CO shown in the last step of (38) was observed. 

The quenching of the radical formed along step (a) in sequence (42) cannot occur by uptake of a second M(P) radical, but rather by its dimerization according to step (b). If the extremely hindered porphyrin ligand in monomeric Rh(TTiPP) was involved in sequence (42), the coupling shown in step (b) was considerably slowed down. 

---