Abstract
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.