Mechanistic Investigations of Photoinduced Oxygenation of Ru(II) Bis-bipyridyl Flavonolate Complexes

We previously reported that a Ru-bound flavonolate model of flavonol dioxygenases, [Ru^II(bpy)₂(3-hydroxyfla)]­[PF₆], photochemically reacts with dioxygen in two different manners. Broad-band excitation generates mixtures of products characteristic of 1,3-addition of dioxygen across the central pyrone ring, as is observed in enzymatic reactions. However, low temperature excitation at wavelengths longer than 400 nm generates a unique Ru-bound 2-benzoatophenylglyoxylate product resulting from a 1,2-dioxetane intermediate. Herein, we investigate this reactivity in a series of Ru­(II)­bis-bipyridyl flavonolate complexes [Ru^II(bpy)₂(3-hydroxyfla^R)]­[PF₆] (bpy = 2,2′-bipyridine; fla = flavonolate; R = <i>p</i>-OMe (<b>1</b>), <i>p</i>-Me (<b>2</b>), <i>p</i>-H (<b>3</b>), <i>p</i>-Cl (<b>4</b>)), and [Ru^II(bpy)₂(5-hydroxyfla)]­[PF₆] (<b>5</b>). The complexes’ structures, photophysical and electrochemical properties, and photochemical reactivity with oxygen were investigated in detail. Two different reaction product mixtures, from 1,2- and 1,3-additions of dioxygen, are observed by illumination into distinct excitation/emission manifolds. By analogy to previous reports of excited state intramolecular proton transfer, the two manifolds are attributed to tautomeric diradicals that predict the observed reactivity patterns