Porphyrins homodimers

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... 

It was observed in a 2005 article that Co(II) porphyrin-Co(III) corrole dimers are more effective dioxygen reduction electrocatalysts than analogous Co(III)-Co(III) corrole dimers or monomeric Co(III) corroles [145], The heterodimers operated effectively at lower overpotentials and promote complete reduction to water (the average number of electrons transferred per 02 molecule approaches 4 in the best porphyrin-corrole catalyst). It was suggested that the inferior catalytic performance of the corrole homodimers could be due to a reduction in the basicity of the activated intermediate when two Co(III) moieties are involved, leading to a less favorable 4-electron reduction. Heterobimetallic catalysts containing formally Co (IV) corroles were also examined as potential dioxygen reduction catalysts [146]. 

Double bonded ruthenium and osmium homodimers have been synthesized by Coil-man . These two metals are inserted into the porphyrin free base by using the corresponding metal chlorocarbonyl dimer [MCl2(CO)3]2- Irradiation of the resulting carbonyl metal(II) porphyrin irradiated in pyridine solutions yields the bis(pyridine) metal(II) porphyrin and leads to the expected dimers by heating under vacuum. This is shown in Scheme 16. 

Drain [13] has utilized the self-association of 3,5-diacetamido-4-pyridyl substituted porphyrins as a design stratagem for the formation of a variety of H-bonded oligomeric tapes and squares. A monosubstituted derivative 20 can form the simplest homodimer 21 through a quadruple H-bond between diacetamidopyridine units in the meso (5-) position of each porphyrin a hnear trimeric tape 22 predominates in a 2 1 stoichiometric mixture of... 

McMurry cross-coupling reactions have also been found to be useful for the synthesis of the unsymmetrically substituted bisporphyrin 68 and the porphyrin-chlorin heterodimer 69, which were isolated from mixtures of the corresponding homodimers [55] (Figure 6.14). 

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