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The tripyrrin-1,14-dione biopyrrin, which shares the scaffold of several naturally occurring heme metabolites, is a redox-active platform for metal coordination. We report the synthesis of square planar platinum( ii ) tripyrrindiones, in which the biopyrrin binds as a tridentate radical and the fourth coordination position is occupied by either aqua or tert -butyl isocyanide ligands. These complexes are stable through chromatographic purification and exposure to air. Electron paramagnetic resonance (EPR) data and density functional theory (DFT) analysis confirm that the spin density is located predominantly on the tripyrrindione ligand. Pancake bonding in solution between the Pt( ii ) tripyrrindione radicals leads to the formation of diamagnetic π dimers at low temperatures. The identity of the monodentate ligand ( i.e. , aqua vs . isocyanide) affects both the thermodynamic parameters of dimerization and the tripyrrindione-based redox processes in these complexes. Isolation and structural characterization of the oxidized complexes revealed stacking of the diamagnetic tripyrrindiones in the solid state as well as a metallophilic Pt( ii )−Pt( ii ) contact in the case of the aqua complex. Overall, the properties of Pt( ii ) tripyrrindiones, including redox potentials and intermolecular interactions in solution and in the solid state, are modulated through easily accessible changes in the redox state of the biopyrrin ligand or the nature of the monodentate ligand.
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Primary amines as ligands and linkers in complexes of tripyrrindione radicals
Biopyrrin pigments, which result from the degradation of heme in biological settings, feature three or two pyrrole rings and characteristic pyrrolin-2-one termini. These scaffolds serve as redox-active ligands and electron reservoirs in coordination compounds. Tripyrrin-1,14-dione coordinates divalent transition metals as a dianionic ligand hosting a delocalized radical. Herein, we report the synthesis and characterization of palladium(II) and platinum(II) tripyrrindione complexes featuring a primary amine (i.e., aniline, tert-butylamine, 1,2-ethylenediamine) at the fourth coordination site within square planar geometries. Interligand hydrogen-bonding interactions are observed between the coordinated amine and the carbonyl groups on the tripyrrindione scaffold. Notably, 1,2-ethylenediamine is employed to link two Pt(II) tripyrrindione complexes. As revealed by optical absorption and electron paramagnetic resonance (EPR) spectroscopy, all resulting complexes present ligand-based radicals that are stable at room temperature and when exposed to air. Spin pairing through multicenter interactions leads to [Formula: see text]-dimerization of the tripyrrindione radicals and a decrease in the EPR signal at low temperatures. Electrochemical measurements indicate that the ligand system undergoes quasi-reversible one-electron oxidation and reduction, thus confirming the ability of tripyrrindione to form square planar complexes in three different redox states.
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- PAR ID:
- 10434239
- Date Published:
- Journal Name:
- Journal of Porphyrins and Phthalocyanines
- ISSN:
- 1088-4246
- Page Range / eLocation ID:
- A to I
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1142/S1088424623501109
