Secondary Phosphine Oxide Functionalized Gold Clusters and their Application in Photoelectrocatalytic Hydrogenation Reactions

Ligands in ligand-protected metal clusters play a crucial role, not only because of their interaction with the metal core, but also because of the functionality they provide to the cluster. Here, we report the utilization of secondary phosphine oxide (SPO), as a new family of functional ligands, for the preparation of an undecagold cluster Au11-SPO. Different from the common used phosphine ligand (i.e. triphenylphosphine, TPP), the SPOs in Au11-SPO work as electron-withdrawing anionic ligands. While coordinating to gold via the phosphorus atom, the SPO ligand keeps its O atom available to act as a nucleophile. Upon photoexcitation, the clusters are found to inject holes into p-type semiconductors (here bismuth oxide is used as a model), sensitizing the p-type semiconductor in a different way compared to the photosensitization of a n-type semiconductor. Furthermore, the Au11-SPO/Bi2O3 photocathode exhibits a much higher activity towards the hydrogenation of benzaldehyde than a TPP-protected Au 11 sensitized Bi2O3 photocathode. Control experiments and
density functional theory studies point to the crucial role of the cooperation between gold and the SPO ligands on the selectivity towards the hydrogenation of the C=O group in benzaldehyde.