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

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^II₂(H₂O)₂Fe^III₂(Co^IIW₉O₃₄)₂]¹⁴⁻ (Co₂Fe₂-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^II₂(H₂O)₂Co^II₂(PW₉O₃₄)₂]¹⁰⁻ (Co₄-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^II₂(H₂O)₂Co^IIW^VI(Co^IIW₉O₃₄)₂]¹²⁻ (Co₃W-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.