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

The elucidation of the ideal coordination environment of a catalytic site has been at the heart of catalytic applications. Herein, we show that the water oxidation activities of catalytic cobalt sites in a Prussian blue (PB) structure could be tuned systematically by decorating its coordination sphere with a combination of cyanide and bidentate pyridyl groups. K-0.1[Co(bpy)](2.9)[Fe(CN)(6)](2) ([Cobpy-Fe]), K-0.2[Co(phen)](2.8)[Fe(CN)(6)](2) ([Cophen-Fe]), {[Co(bpy)(2)](3) [Fe(CN)(6)](2) 1[Fe(CN)(6)](1/3) ([Cobpy2-Fe]), and {[Co(phen)(2)](3)[Fe(CN)(6)](2)}[Fe(CN)(6)](1/3) CI0.11 ([Cophen2 - Fe]) were prepared by introducing bidentate pyridyl groups (phen: 1,10-phenanthroline, bpy: 2,2'-bipyridine) to the common synthetic protocol of Co-Fe Prussian blue analogues. Characterization studies indicate that [Cobpy2-Fe] and [Cophen2-Fe] adopt a pentanu- clear molecular structure, while [Cobpy-Fe] and [Cophen-Fe] could be described as cyanide-based coordination polymers with lower-dimensionality and less crystalline nature compared to the regular Co-Fe Prussian blue analogue (PBA), K0.1Co2.9[Fe(CN)(6)](2)([Co-Fe]). Photocatalytic studies reveal that the activities of [Cobpy-Fe] and [Cophen-Fe] are significantly enhanced compared to those of [Co-Fe], while molecular [Cobpy2-Fe] and [Cophen2-Fe] are inactive toward water oxidation. [Cobpy-Fe] and [Cophen-Fe] exhibit upper-bound turnover frequencies (TOFs) of 1.3 and 0.7 s(-1), respectively, which are similar to 50 times higher than that of [Co-Fe] (1.8 X 10(-2) s(-1)). The complete inactivity of [Cobpy2-Fe] and [Cophen2-Fe] confirms the critical role of aqua coordination to the catalytic cobalt sites for oxygen evolution reaction (OER). Computational studies show that bidentate pyridyl groups enhance the susceptibility of the rate-determining Co(IV)-oxo species to the nucleophilic water attack during the critical O-O bond formation. This study opens a new route toward increasing the intrinsic water oxidation activity of the catalytic sites in PB coordination polymers.

Automated summary

This study demonstrates the tunability of water oxidation activities of catalytic cobalt sites in Prussian blue structure by modifying its coordination sphere, providing a new approach to enhance the intrinsic water oxidation activity of catalytic sites.