Metal complexes of tetrapyrrolic macrocycles, such as the porphyrin ligand in heme, are key to the function of numerous natural catalysts. Within the catalytic cycle of several enzymes (e.g., cytochrome P450 monooxygenases, catalases), pyrrole-based ligands engage in redox interplay with coordinated metal centers and host unpaired electrons, thus behaving as redox-active ligands. Linear fragments of macrocyclic tetrapyrroles, containing two or three pyrrole units, are also ubiquitous in nature; nevertheless, their chemistry is less characterized. We explore the coordination chemistry and redox behavior of tripyrrolic and dipyrrolic scaffolds of several natural pigments deriving from bacterial biosynthesis or from oxidative degradation of heme in mammalian physiology. Our electrochemical and electron paramagnetic resonance (EPR) spectroscopic studies reveal that these tridentate and bidentate ligands behave as electron reservoirs in complexes of late transition metals and allow for stabilization of ligand-based radicals at room temperature. In addition, X-ray diffraction data document a wider range of coordination geometries for these ligands when compared to macrocyclic tetrapyrroles. Although the chemistry of linear oligopyrroles has been hampered by their limited stability to redox degradation, this investigation presents new opportunities for the employment of tripyrrolic and dipyrrolic pigments as stable redox-active ligands.

Small pyrrole-based pigments as redox-active ligands

251st National Meeting (2016)

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