Tridentate pincer and pincer-type frameworks characterize several important classes of redox-active ligands of high impact in synthetic chemistry and catalysis. We have recently demonstrated that the linear oligopyrrolic scaffold of tripyrrin-1,14-dione offers a robust tridentate platform for palladium(II) coordination and ligand-based redox chemistry. The tripyrrindione motif is found in the structure of urinary pigment uroerythrin, which was first isolated in the 1970s; nevertheless, the coordination of transition metals and redox chemistry of this ligand system had not been characterized. We found that tripyrrindione coordinates Pd(II) and other transition metals as a planar tridentate ligand. In the Pd(II) species, the square planar coordination sphere of the metal center is completed by a water molecule, which engages in hydrogen bonding interactions with the carbonyl groups of the tripyrrindione framework. Concurrent with metal coordination, the ligand undergoes one-electron oxidation reactions leading to ligand-based radicals that are stable at room temperature. Both electrochemical and chemical redox transformations allow alteration of the oxidation state of the ligand system. This pincer-type tripyrrole therefore behaves as non-innocent ligand and the electronic structure of the resulting complexes is investigated by spectroscopic and computational methods. For paramagnetic systems, the number and type of electronic spins, as well as the interactions thereof, are studied by electron paramagnetic resonance (EPR) spectroscopy. This investigation is producing a detailed description of the metal-binding properties and redox chemistry of the tripyrrindione ligand, thereby providing a basis for future reactivity pathways in stoichiometric and catalytic transformations.

Radical stabilization and ligand-based redox chemistry on oligopyrrolic fragments

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.