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 [CoII2(H2O)2FeIII2(CoIIW9O34)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 [CoII2(H2O)2CoII2(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 WVI ion as demonstrated by [CoII2(H2O)2CoIIWVI(CoIIW9O34)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.

Light-harvesting, 3rd generation RuII/CoII MOF with a large, tubular channel aperture.

A photoactive, hetero-metallic CoII/RuII-based metal-organic framework (MOF) with a large channel aperture, ca. 21 Å, is reported. The photophysical properties of the MOF are derived from the RuII nodes giving rise to emission centred at ca. 620 nm and relatively long triplet 3MLCT lifetimes. In addition to the optical attributes, the 1H-imidazo [4,5-f][1,10]-phenanthroline ligand imparts structural functionality to the MOF which is composed of alternating CoII- and RuII-based nodes of Δ and Λ helicity. The framework maintains its integrity upon activation and shows gas sorption behaviour that is characteristic of mesoporous materials promoting high CO2 sorption capacities and selectivities over N2.