PhenTAA: A Redox-Active N₄-Macrocyclic Ligand Featuring Donor and Acceptor Moieties

Area of Science:

• Coordination Chemistry
• Supramolecular Chemistry
• Electrochemistry

Background:

• Macrocyclic ligands are crucial in coordination chemistry, influencing metal center properties and reactivity.
• Ligand-centered redox activity in metal complexes is key for applications in catalysis, sensing, and molecular electronics.
• Understanding the interplay between macrocycle structure and redox behavior is essential for designing functional materials.

Purpose of the Study:

• To synthesize and characterize a novel PhenTAA macrocycle and its nickel complexes.
• To investigate the ligand-centered redox activity and stability of these complexes.
• To explore the influence of substituents on the electrochemical properties and redox states.

Main Methods:

• Synthesis of the PhenTAA macrocycle and subsequent formation of [Ni(R2PhenTAA)]n complexes.
• Electrochemical techniques, including cyclic voltammetry (CV), to determine redox potentials.
• Spectroscopic methods such as UV/Vis-SEC and X-band Electron Paramagnetic Resonance (EPR) for characterization.
• Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations for theoretical insights.

Main Results:

• The novel PhenTAA macrocycle forms stable nickel complexes with extensive ligand-centered redox activity.
• Accessible ligand charges range from -2 to +2, with distinct donor (o-phenylenediamide) and acceptor (imine) sites.
• Substituent effects (R=H, Me, Ph) influence complex stability and redox accessibility, with R=Me/Ph enabling a stable [Ni(R2PhenTAA)]2- species.
• Electrochemical stability window observed from -2.4 to +1.8 V (vs Fc/Fc+) for Me/Ph substituted complexes.

Conclusions:

• The [Ni(R2PhenTAA)]n complexes exhibit remarkable electrochemical versatility with up to five distinct ligand-based oxidation states.
• The ligand framework facilitates tunable redox properties, controlled by substituents and accessible over a wide potential range.
• These findings open avenues for designing novel redox-active molecular materials with tailored electrochemical characteristics.