Light-Driven Water Oxidation with Ligand-Engineered Prussian Blue Analogues

Area of Science:

• Materials Science
• Catalysis
• Inorganic Chemistry

Background:

• The coordination environment of catalytic sites is crucial for optimizing catalytic applications.
• Prussian blue analogues (PBAs) are versatile materials with potential catalytic properties.
• Tuning the ligand sphere around metal centers can modulate catalytic activity.

Purpose of the Study:

• To systematically tune the water oxidation activity of catalytic cobalt sites within a Prussian blue framework.
• To investigate the effect of decorating the cobalt coordination sphere with bidentate pyridyl groups (phenanthroline and bipyridine) alongside cyanide ligands.
• To elucidate the role of aqua coordination in the oxygen evolution reaction (OER).

Main Methods:

• Synthesis of novel Prussian blue analogues: [Cobpy-Fe], [Cophen-Fe], [Cobpy2-Fe], and [Cophen2-Fe] by incorporating bidentate pyridyl ligands.
• Structural characterization using techniques to determine molecular and coordination polymer structures.
• Photocatalytic evaluation for water oxidation activity and determination of turnover frequencies (TOFs).
• Computational studies to understand reaction mechanisms and electronic effects.

Main Results:

• Cobalt-cyanide coordination polymers ([Cobpy-Fe] and [Cophen-Fe]) exhibited significantly enhanced water oxidation activity compared to the parent PBA ([Co-Fe]).
• Upper-bound TOFs for [Cobpy-Fe] and [Cophen-Fe] reached 1.3 s⁻¹ and 0.7 s⁻¹, respectively, approximately 50 times higher than [Co-Fe].
• Molecular structures ([Cobpy2-Fe] and [Cophen2-Fe]) were inactive for water oxidation, confirming the necessity of aqua ligands for OER.
• Computational analysis indicated that bidentate pyridyl groups facilitate nucleophilic attack by water on the Co(IV)-oxo intermediate.

Conclusions:

• Decorating cobalt sites in Prussian blue structures with bidentate pyridyl groups and cyanide ligands effectively enhances water oxidation catalysis.
• The presence of aqua ligands is essential for catalytic activity in the oxygen evolution reaction.
• This work presents a new strategy for designing highly active water oxidation catalysts based on Prussian blue coordination polymers.