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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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Two-Electron Redox Tuning of Cyclopentadienyl Cobalt Complexes Enabled by the Phenylenediamide Ligand
A family of neutral cobalt complexes, [CpR'Co(Ropda)] 1-5, based on the redox-active o-phenylenediamide ligand (Ropda) undergo reversible 2e- oxidation revealed by cyclic voltammetry. This multielectron behavior is observed for all complexes regardless of the substituents on the phenylenediamide ligand, enabling redox tuning over more than 0.5 V. These diamagnetic neutral complexes are best described as delocalized systems with covalent bonding across the cobalt-opda metallocycle, consistent with the closed-shell singlet ground-state predicted by density functional theory (DFT) calculations. Two-electron oxidation using chemical oxidants affords the dicationic species, which are formulated as Co(III)-benzoquinonediimine systems with an additional coordinated acetonitrile ligand. DFT calculations also predict an ECE pathway for the 2e- oxidation, in which the first 1e- step is primarily a ligand-based process with redistribution of electron density to the metal. The associated distortion of the coordination geometry and disruption of the metallocycle bonding enable acetonitrile coordination in the intermediate oxidation state, which is critical for favoring the second electron transfer and accessing the potential inversion.
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- Award ID(s):
- 2117792
- PAR ID:
- 10529808
- Publisher / Repository:
- ChemRxiv
- Date Published:
- Format(s):
- Medium: X
- Institution:
- Rutgers University
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Posted Content:
- https://doi.org/10.26434/chemrxiv-2023-l58hm
