The degradation of heme in mammalian cells leads to the formation of dipyrrolic and tripyrrolic oligopyrroles known as biopyrrins. These lower-order linear fragments of macrocyclic tetrapyrroles, containing two or three pyrrole units, are ubiquitous in nature; nevertheless, their coordination chemistry is not well established. Compared to the macrocyclic systems, linear tripyrrolic and dipyrrolic ligands offer enhanced freedom of coordination geometry for the metal center and concurrently enable access to the metal center for additional ligands. In this study, two classes of bioinspired linear oligopyrroles (propentdyopent and tripyrrindione) were found to form transition metal complexes featuring ligand-based redox reactivity. With late divalent transition metals, the tripyrrindione ligand binds as a dianionic radical with 1:1 stoichiometry, and the resulting square planar complexes are stable at room temperature. Electrochemical analyses confirm the presence of ligand-based redox events. This investigation is producing a detailed description of tripyrrindione and propentdyopent ligands acting as electron reservoirs. In addition, current X-ray diffraction and electron paramagnetic resonance (EPR) studies on tripyrrindione complexes are beginning to reveal intermolecular interactions between planar ligand-based radicals and formation of neutral π-π dimers.

Ligand-based redox chemistry in transition metal complexes of linear oligopyrroles

253rd National Meeting (2017)

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