Hangman porphyrin complexes

Cobalt macrocychc, or pseudo-macrocyclic, complexes have proven among the most widespread cobalt-based proton reduction catalysts. For example, Fisher and Eisenberg demonstrated in 1980 that some cobalt tetraazamacrocyclic complexes are active in both CO2 and reduction [75]. Similarly, porphyrins have been extensively investigated. Nocera and coworkers showed that cobalt(II) hangman porphyrins can catalyze proton reduction with less overpotential and weaker acids than their standard porphyrin cousins (Fig. 13d) [85, 86]. Both features are thought to be a result of the enhanced proton donation by the carboxylic acid of the hangman substituent. Bren and coworkers showed that the biologically derived cobalt-substituted microperoxidase-11 is stable with a turnover number of 25,000, but the catalytic rate is relatively low at 6.7 s [87]. 

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