Accelerating water oxidation - a mixed Co/Fe polyoxometalate with improved turnover characteristics.

Water oxidation is a bottleneck reaction for the establishment of solar-to-fuel energy conversion systems. Earth-abundant metal-based polyoxometalates are promising heterogeneous water oxidation catalysts that can operate in a wide pH range. However, detailed structure-reactivity relationships are not yet comprehensively understood, hampering the design and synthesis of more effective polyoxometalate-based oxidation catalysts. Here we report the synthesis of an ordered, mixed-metal cobalt-iron Weakley archetype [Co II 2 (H 2 O) 2 Fe III 2 (Co II W 9 O 34 ) 2 ] 14- (Co 2 Fe 2 -WS), which unexpectedly highlights the strong influence of the central, coordinatively saturated metal ions on the catalytic water oxidation characteristics. The resulting species exhibits catalytic turnover frequencies which are up to 4× higher than those of the corresponding archetype tetracobalt-oxo species [Co II 2 (H 2 O) 2 Co II 2 (PW 9 O 34 ) 2 ] 10- (Co 4 -WS). It is further striking that the system becomes catalytically inactive when one of the central positions is occupied by a W VI ion as demonstrated by [Co II 2 (H 2 O) 2 Co II W VI (Co II W 9 O 34 ) 2 ] 12- (Co 3 W-WS). Importantly, this study demonstrates that coordinatively saturated metal ions in this central position, which at first glance appear insignificant, do not solely have a structural role but also impart a distinctive structural influence on the reactivity of the polyoxometalate. These results provide unique insights into the structure-reactivity relationships of polyoxometalates with improved catalytic performance characteristics.