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 }[Fe(CN) 6 ] 1/3 ( [Cobpy2-Fe] ), and {[Co(phen) 2 ] 3 [Fe(CN) 6 ] 2 }[Fe(CN) 6 ] 1/3 Cl 0.11 ( [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), K 0.1 Co 2.9 [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.