Molecular and supported ruthenium complexes as photoredox oxidation catalysts in water

Abstract

A new bidentate pyridylpyrazole ligand functionalized with the pyrene group, 3-(2-pyridyl)-1-(pyrazolyl)methylpyrene (pypz–pyr), was synthesized, together with a successful homogeneous photoredox catalyst containing the above ligand and the tridentate terpyridine (trpy), trans-[Ru^II(trpy)(pypz–pyr)H₂O](PF₆)₂trans-3. This last compound was obtained from the corresponding chlorido complex trans-[Ru^IICl(trpy)(pypz–pyr)]PF₆trans-2. All compounds were characterized chemical and electrochemically. The molecular complexes were successfully heterogenized on reduced graphene oxide (rGO) through non-covalent interactions, thanks to the pyrene group. The heterogeneous rGO@trans-3 system was characterized by UV-vis, SEM, and DPV techniques. The homogeneous trans-3 and heterogeneous rGO@trans-3, acting both as the photosensitizer and the catalyst, via PCET processes were tested in the photooxidation of alcohols and trans-β-methylstyrene as substrates, in an aqueous medium under neutral conditions and using visible light. The photoredox trans-3 displays moderate yields and good selectivity values. The supported system keeps intact its homogeneous catalytic properties, but shows an enhancement of yields, provided by its anchoring to the graphene support. The recyclability of the photocatalytic system was demonstrated by performing five runs of the rGO@trans-3, each one without loss of selectivity and yield. To date, to the best of our knowledge, no reports of well-defined molecular aqua ruthenium catalysts on rGO supports have been described in the literature for the photoredox oxidation of alcohols in water, without the presence of an additional photosensitizer.