Photoinitiated dioxygenase-type reactivity of open-shell 3d divalent metal flavonolato complexes

Irradiation of 3-hydroxyflavonolato (3-Hfl) complexes of Mn^II, Co^II, Ni^II and Cu^II (1-4) at 300 nm under aerobic conditions results in dioxygenase-type reactivity and the formation of the corresponding divalent metal O-benzoylsalicylato (O-bs) complexes 8-11 and CO. The latter were characterized by using multiple methods, including elemental analysis, X-ray crystallography, NMR and/or EPR spectroscopy, mass spectrometry and IR spectroscopy. Compounds 1-4 serve as catalysts for the photoinduced reactivity of 3-hydroxyflavonol (3-HflH) to produce O-benzoylsalicylic acid as the major product. Spectroscopic studies (UV/Vis and ¹H NMR) show that each O-benzoylsalicylato complex 8-11 reacts with one equiv. of 3-hydroxyflavonol to regenerate 1-4 and enable turnover reactivity. Unlike what is observed for free 3-HflH, photoinduced reactions involving 1-4 and excess flavonol show only minor amounts of flavonol isomerization reactivity. These results indicate that the presence of a metal ion, whether under stoichiometric or catalytic conditions, facilitates the photoinduced degradation of 3-HflH to produce a carboxylic acid and CO as products.