Synthetic porphyrins, dimer formation

Necessary conditions for synthetic models to mimic oxyhemoglobins are formation of a five-coordinate heme precursor with a proximal base (e.g., imidazole, pyridine), prevention of p-peroxo/p-oxo (M-02-M/M-0-M) dimer formation upon oxygenation (i.e., reversible oxygenation), and exclusion of protons and nucleophiles. Model systems should be easily modified and allow probing of distal and proximal influences. Tetraphenyl-porphyrins (TPP) serve as the basis of numerous model complexes, because they are less susceptible to oxidation... 

Rhodium Porphyrins. Chemical syntheses of [CPDRh32 and (P)Rh(R) complexes are well known(4-11). Electrochemical techniques have also been used to synthesize dimeric metal-metal bonded [(TPP)RhJ 2 as well as monomeric metal-carbon a-bonded (TPP)Rh(R) and (0EP)Rh(R)(12-16). The electrosynthetic and chemical synthetic methods are both based on formation of a highly reactive monomeric rhodium(II) species, (P)Rh. This chemically or electrochemically generated monomer rapidly dimerizes in the absence of another reagent as shown in Equation 1. 

Isolated Fe -heme readily reacts with O2. Flowever, under ambient conditions the reaction of Fe -heme or synthetic Fe -porphyrins is not the reversible one analogous to reaction (1), but rather the irreversible reaction (2), resulting in formation of the so-called /u-oxo dimer ... 

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