Apigenines as O,O-bidenate ligands in Oxidorhenium(V) complexes

Apigenin (Hapg) is a naturally occurring flavonoid that is produced and used by nature as yellow pigment in flower petals, but also shows interesting biological and cytotoxic activities. Hence, apigenin is an interesting, renewable resource target for the synthesis of novel metallodrugs for use in anti-cancer therapy. But even more, we were interested in a dual-use approach of these potential metallodrugs as catalysts in classical chemical transformations to even further increase economical and ecological aspects. The coordination properties towards transition metals of apigenin has only been sparsely investigated and was often only poorly characterized. In addition, the extreme insolubility of Apigenin in many solvents complicated research. It could nevertheless be shown that Apigenin does indeed coordinates as an O,O-bidentate ligand.

Within this presentation we will present results on various apigenin derivatives with increased solubility (HapgR) and their resulting complexes of type [ReOCl₃(PPh₃)(apgR)], including a water-soluble version (Re-apg(su)₂). Interestingly, also the coordination chemistry of Re with O,O-bidentate ligands, even with the widely used acac-ligand, is surprisingly small. Obstacles that had to be overcome in the synthesis, including the inability to synthesize bis-ligated complexes of type [ReOCl(apgR)₂], will be discussed. A focus will also be put on water as a solvent and its related challenges of pH-value, hydration, hydrolysis and agglomeration. Finally, our investigations to use these novel complexes as catalysts for olefin epoxidation and perchlorate reduction will be presented and discussed.

The authors gratefully acknowledge financial support from the European Union HORIZON-MSCA through the MET-EFFECT project under grant No. 101086373.