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

Abstract

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. K0.1[Co(bpy)]2.9[Fe(CN)6]2 ([Cobpy–Fe]), K0.2[Co(phen)]2.8[Fe(CN)6]2 ([Cophen–Fe]), {[Co(bpy)2]3[Fe(CN)6]2}[Fe(CN)6]1/3 ([Cobpy2–Fe]), and {[Co(phen)2]3[Fe(CN)6]2}[Fe(CN)6]1/3 Cl0.11 ([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 pentanuclear 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 ∼50 times higher than that of [Co–Fe] (1.8 × 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.