Tuning Energetics of 2 e^-/2H⁺ PCET Properties with Model Ru-bisamido Complexes

Area of Science:

• Coordination Chemistry
• Redox Chemistry
• Supramolecular Chemistry

Background:

• Most bond-breaking/forming redox processes involve two electrons (2e-) and often protons.
• Existing proton-coupled electron transfer (PCET) research predominantly focuses on one-electron, one-proton (1e-/1H+) reactions.
• There is a need for molecular systems that can undergo and be studied for 2e-/2H+ redox transformations.

Purpose of the Study:

• To synthesize and characterize a family of molecular models capable of tunable 2e-/2H+ redox changes.
• To investigate the influence of ligand substitutions on the redox potentials, pKa values, and N-H bond dissociation free energies (BDFEs).
• To explore the phenomenon of potential inversion in these 2e-/2H+ PCET systems.

Main Methods:

• Synthesis of ruthenium complexes with substituted 2,2'-bipyridine (bpy) and 2,3-dimethyl-2,3-butanediamine (en*) ligands.
• Characterization using various spectroscopic techniques (IR, UV-vis, NMR), X-ray diffraction (XRD), electrochemistry, and mass spectrometry.
• Computational analysis including Density Functional Theory (DFT) and Time-Dependent DFT (TD-DFT) calculations.

Main Results:

• Successfully synthesized and characterized ruthenium complexes exhibiting 2e-/2H+ redox behavior.
• Electron-donating and withdrawing groups on the bpy ligand effectively tuned redox potentials, pKa values, and BDFEs of the en* ligands.
• Observed significant potential inversion (ΔpKa > 25 units, ΔE0 > 1.4 V) in these systems, maintained despite electronic effects transmitted through the Ru center.

Conclusions:

• The developed molecular models provide a versatile platform for studying 2e-/2H+ PCET reactions.
• Strong coupling between electrons and protons was confirmed, offering valuable insights into the mechanisms of such transformations.
• The findings contribute to a deeper understanding of redox processes fundamental to various chemical and biological systems.