Design and synthesis of functionalized coordination polymers as recyclable heterogeneous photocatalysts

The functionalized ligand 9,10-anthraquinone-1,4-dicarboxylate acid (H₂AQDC) was designed and synthesized in order to develop metal-organic coordination polymers as heterogeneous catalysts with a photosensitizing feature. Two major considerations of the ligand design are anthraquinone moieties for photosensitizing to harvest light and carboxylate groups for polymeric coordination toward less solubility. A series of transition metal complexes based on this ligand were synthesized subsequently, namely {Co(AQDC)(H₂O)₃·2H₂O}_n (Co-AQDC), {Ni(AQDC)(H₂O)₃·2H₂O}_n (Ni-AQDC), {[Cu(AQDC)(H₂O)₃][Cu(AQDC)(H₂O)₂(DMF)]·(H₂O)₄}_n (Cu-AQDC), {Zn_1.5(AQDC)(OH)(H₂O)₂·H₂O}_n (Zn-AQDC), {Ag₂(AQDC)(CH₃OH)}_n (Ag-AQDC). Both the ligand and its transition metal complexes are able to catalyze the visible-light driven oxidation reactions of alkynes into 1,2-diketones in air under mild conditions, in which compound Ni-AQDC demonstrates the best activity. This catalyst can be easily isolated from the reaction mixture by filtration with a trace amount of loss in solution and is ready for recycled use after simple washing and drying without any need for regeneration. Remarkably, the catalyst shows no loss of activity after five catalytic cycles and X-ray powder diffraction proves no change in the structure after five runs. This designed metal-organic coordination polymer represents an environmentally friendly, economical and recyclable photocatalyst, constituting a good candidate for photocatalytic organic syntheses in terms of green chemistry.