Unusual stability of dyads during photochemical hydrogen production

Abstract

Dyads for photochemical water splitting often suffer from instability during irradiation with visible light. However, the use of bis(bidentate) phosphines forming a five-membered ring enhances their stability. The coordination of these phosphor based chelates to soft metals like Pd(II) prolongs the photocatalytic activity to 1000 hours. To avoid contribution to hydrogen production by colloidal metal, a small amount of Hg is added to the reaction mixture. In the course of our investigations, it turned out that colloidal palladium was not able to produce hydrogen under our irradiation conditions. As soon as metallic palladium emerged in our reaction vessels, no further hydrogen production was detected. This is confirmed by the observation that the hydrogen production depends on the kind of ancillary ligands present in the dyads. The first dyads of the type [M_I(bpy)₂(dppcb)M_II(bpy)]⁴⁺ are presented (M_I = Os, M_II = Pd (1); M_I = Ru, M_II = Pd (2); M_I = Os, M_II = Pt (3); M_I = Ru, M_II = Pt (4)). In [Os(bpy)₂(dppcb)Pd(dppm)](PF₆)₄ (5) the ancillary ligand is varied. Furthermore, it is also possible to produce hydrogen in an intermolecular way. Using different bidentate diphosphines instead of a bis(bidentate) tetraphosphine leads to this intermolecular approach, where the chromophore and the water reduction catalyst (WRC) belong now to two molecules. In this case the TON is sensitive to the type of diphosphine, which is only possible if intact molecules act as catalysts and no free palladium(0) is formed.