Lanthanide complexes based on an anthraquinone derivative ligand and applications as photocatalysts for visible-light driving photooxidation reactions

Four isostructural lanthanide coordination complexes based on 3,7-diamino-9,10-anthraquinone-2,6-disulfonate (dianionic, L) have been synthesized by hydrothermal method, namely [Er(L)(H₂O)₆]•[Er(H₂O)₈]•2L•8H₂O (Er-L), [Tm(L)(H₂O)₆]•[Tm(H₂O)₈]•2L•8.5H₂O (Tm-L), [Yb(L)(H₂O)₆]•[Yb(H₂O)₈]•2L•9H₂O (Yb-L), [Lu(L)(H₂O)₆]•[Lu(H₂O)₈]•2L•9H₂O (Lu-L). Single-crystal X-ray analysis reveals the existence of both coordinated and free ligand L in the crystal structure. Versatile sulfonate groups on these distinct L ligands, together with very rich coordinated and lattice water molecules, form a lot of hydrogen-bonding motifs that contribute to the stabilization of the crystal packing. It is interesting that the ligands stack into columns through strong π-π interactions and the centroid-centroid distances are between 3.281 and 3.331 Å. These ligands are stacked in an alternate off-set mode to avoid the steric hindrance between the bulky sulfonate groups, generating a repeated structural unit involving six stacked ligands. These lanthanide complexes proved to be good heterogeneous photocatalyst for promoting the visible-light driving photooxidation reactions of diarylacetylenes and thioethers. The Er-L complex exhibited the best catalytic activity and showed good catalytic efficiency over a wide range of substrates for both reaction systems. The Er-L photocatalyst can be easily isolated by simple filtration as crystalline material upon completion of the photooxidation reaction without structure change, and can be recycled for at least five catalytic cycles with persistent catalytic efficiency without any need of activation or regeneration. This family of lanthanide complexes represent a category of promising heterogeneous photocatalysts in terms of green chemistry, with the potential of promoting organic transformations highly efficiently under the irradiation of visible light.