Water Oxidation by Pentapyridyl Base Metal Complexes? A Case Study.

Area of Science:

• Catalysis
• Inorganic Chemistry
• Materials Science

Background:

• Molecular water oxidation catalysts (WOCs) are crucial for artificial photosynthesis and renewable energy.
• Rational design requires understanding water binding, redox potentials, and O-O bond formation.
• Base metal complexes offer sustainable alternatives to precious metal catalysts.

Purpose of the Study:

• To investigate the catalytic activity and properties of base metal (Mn, Fe, Co, Ni) pentapyridyl complexes as WOCs.
• To elucidate the mechanism of water oxidation for these complexes.
• To identify design principles for improved molecular WOCs.

Main Methods:

• Synthesis and characterization of base metal pentapyridyl complexes.
• Electrochemical analysis (cyclic voltammetry).
• Spectroscopic techniques (XAS, UV-vis).
• Computational studies (DFT).
• Light-driven catalytic experiments using [Ru(bpy)3]2+/S2O82- system.

Main Results:

• Only [Fe(Py5OMe)Cl]+ showed apparent catalytic activity (TON=130) at pH 8.0.
• This activity originated from the formation of amorphous iron oxide/hydroxide (FeOOH) upon complex degradation.
• No M(IV) state was detected for any complex, limited by ligand binding and lack of sigma-donating ligands.
• Complexes preferred binding Cl- or acetonitrile over water-derived species.

Conclusions:

• The tested base metal complexes are not effective molecular WOCs due to degradation or inability to reach higher oxidation states.
• Ligand design must favor water binding and stabilize higher oxidation states for efficient WOCs.
• Future WOC design should incorporate sigma-donor ligands and avoid apical coordination sites prone to degradation.