Molecular controllable cubane oxo cluster catalysts and their dynamic role on photoanodes for water oxidation
Photocatalytic water splitting by cuboidal {CaMn4O5} center is one of nature’s most fascinating and important reactions.[1] Knowledge transfer from the key features of the {CaMn4O5} center to other oxo cubane-type water oxidation catalysts has attracted enormous attention due to its ability for direct solar energy-to-fuel conversion. Inspired by this concept, we successfully developed molecular {Co4O4} cubane-type catalysts through both ligand engineering and mixed metal centers.[2,3] To predictively access this highly active motif, we achieved an unprecedented level of synthetic control for the self-assembly of {Co4O4}.[4] Typical systems involving Co(OAc)2 precursor and dpy{OH}O ligand were operated with systematically varied inorganic counteranion types and concentrations. After introducing a toolbox of straightforward inorganic counteranions, the self-assembly processes of oxo clusters revealed a surprisingly strong structure-directing effect of inorganic counteranions in solution. The type of {Co4O4} cubane exhibited a clear selectivity with respect to the counteranions used during the assembly process. Therefore, we opened up a strategy to molecular controllable {Co4O4} cubane synthesis for water oxidation. The combination of experimental screening and complemented DFT calculations were efficient used to track their pathways. These results are expected to provide insight into the widely unknown assembly mechanisms of organically ligated oxo clusters.
The dynamic role of {Co4O4} cubanes on photoanodes for water oxidation was investigated by loading molecular (Co4O4-dpk) on hematite photoanode surfaces.[5] Supported by both photoelectrochemical analyses and related rate law analyses, we revealed that the role of the {Co4O4} cubane cocatalysts alternated from predominate hole reservoirs to catalytic centers with the applied potential. Likewise, several other {Co4O4}-type molecules for heterogeneous cocatalysts were observed to display similar properties. Overall, our works provide a controllable and predictive access to highly performing oxo cluster catalysts via ligand engineering and counteranion selection, and further reveal the dynamic role of {Co4O4} cubanes on photoanodes for water splitting.
[2] Fabio Evangelisti, Robin Güttinger, RenéMoré, Sandra Luber, Greta R. Patzke, J. Am. Chem. Soc. 2013, 135, 18734.
[3] Fangyuan Song, RenéMoré, Mauro Schilling, Grigory Smolentsev, Nicolo Azzaroli, Thomas Fox, Sandra Luber, Greta R. Patzke, J. Am. Chem Soc. 2017, 139, 14198.
[4] Fangyuan Song, Karrar Al-Ameed, Mauro Schilling, Thomas Fox, Sandra Luber, Greta R. Patzke, J. Am. Chem Soc. 2019, 141, 8846.
[5] Jingguo Li, Wenchao Wan, Carlos A. Triana, Zbynek Novotny, Jürg Osterwalder, Rolf Erni, Greta R. Patzke, J. Am. Chem Soc. 2019, 141, 12839.