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

Abstract

We previously reported that a Ru-bound flavonolate model of flavonol dioxygenases, [RuII(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 [RuII(bpy)2(3-hydroxyflaR)][PF6] (bpy = 2,2'-bipyridine; fla = flavonolate; R = p-OMe (1), p-Me (2), p-H (3), p-Cl (4)), and [RuII(bpy)2(5-hydroxyfla)][PF6] (5). 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.