Tripyrrindione as a redox-active ligand: Palladium(II) coordination in three redox states

The tripyrrin-1,14-dione scaffold of urinary pigment uroerythrin coordinates divalent palladium as a planar tridentate ligand. Spectroscopic, structural and computational investigations reveal that the tripyrrindione ligand binds as a dianionic radical, and the resulting complex is stable at room temperature. One-electron oxidation and reduction reactions do not alter the planar coordination sphere of palladium(II) and lead to the isolation of two additional complexes presenting different redox states of the ligand framework. Unaffected by stability problems common to tripyrrolic fragments, the tripyrrindione ligand offers a robust platform for ligand-based redox chemistry. The positive, the negative, and the radical: The tripyrrin-1,14-dione scaffold of urinary pigment uroerythrin coordinates divalent palladium as a planar tridentate ligand. The resulting complex presents a ligand-based radical and is stable at room temperature. One-electron oxidation and reduction reactions lead to the characterization of a series of three Pd^II complexes featuring this ligand framework in three redox states.