Backbone Dehydrogenation in Pyrrole-Based Pincer Ligands

Area of Science:

• Organometallic Chemistry
• Coordination Chemistry
• Ligand Design

Background:

• The development of novel ligands is crucial for tuning the reactivity and electronic properties of metal complexes.
• Pyrrole-based pincer ligands offer unique coordination environments but their electronic properties can be further modulated.

Purpose of the Study:

• To synthesize and characterize metal complexes featuring a dehydrogenated pyrrole pincer ligand (dPNP).
• To investigate the impact of ligand dehydrogenation on the redox potentials and reactivity of cobalt and nickel complexes.
• To explore the electronic structure and properties of the resulting dPNP metal complexes.

Main Methods:

• Treatment of metal chlorides with benzoquinone to achieve ligand dehydrogenation.
• Electrochemical measurements (cyclic voltammetry) to determine redox potentials.
• X-ray crystallography for structural determination of key complexes.
• Density Functional Theory (DFT) calculations to probe electronic structure.
• Electron Paramagnetic Resonance (EPR) spectroscopy to characterize radical species.

Main Results:

• Successful synthesis of cobalt and nickel complexes with the dehydrogenated ^tBudPNP ligand.
• Significant shifts in redox potentials for both M(II/III) and M(II/I) couples upon ligand dehydrogenation.
• Isolation of challenging hydride and ethyl cobalt complexes with the dPNP ligand.
• Formation of a rare square-planar Co(III) species and a Co(I) complex with coordinated N₂.
• Observation of a pincer-based π-radical anion upon further reduction of the Co(I) complex, characterized by EPR.

Conclusions:

• Dehydrogenation of the PNP ligand to dPNP alters the electronic properties of the metal center, making the ligand a more reducing donor.
• The dPNP ligand enables the isolation of previously inaccessible metal complexes.
• The study provides insights into the electronic structure of pincer complexes and the role of ligand design in redox tuning.