Characterization of the Initial Intermediate Formed during Photoinduced Oxygenation of the Ruthenium(II) Bis(bipyridyl)flavonolate Complex

A ruthenium­(II) flavonolate complex, [Ru^II(bpy)₂fla]­[BF₄], was synthesized to model the reactivity of the flavonol dioxygenases. The treatment of dry CH₃CN solutions of [Ru^II(bpy)₂fla]­[BF₄] with dioxygen under light leads to the oxidative O-heterocyclic ring opening of the coordinated substrate flavonolate, resulting in the formation of [Ru^II(bpy)₂(carboxylate)]­[BF₄] (carboxylate = <i>O</i>-benzoylsalicylate or benzoate) species, as determined by electrospray ionization mass spectrometry. Moderation of the excitation and temperature allowed isolation and characterization of an intermediate, [Ru^II(bpy)₂bpg]­[BF₄] (bpg = 2-benzoyloxyphenylglyoxylate), generated by the 1,2-addition of dioxygen to the central flavonolate ring

Characterization of the Initial Intermediate Formed during Photoinduced Oxygenation of the Ruthenium(II) Bis(bipyridyl)flavonolate Complex

A ruthenium­(II) flavonolate complex, [Ru^II(bpy)₂fla]­[BF₄], was synthesized to model the reactivity of the flavonol dioxygenases. The treatment of dry CH₃CN solutions of [Ru^II(bpy)₂fla]­[BF₄] with dioxygen under light leads to the oxidative O-heterocyclic ring opening of the coordinated substrate flavonolate, resulting in the formation of [Ru^II(bpy)₂(carboxylate)]­[BF₄] (carboxylate = <i>O</i>-benzoylsalicylate or benzoate) species, as determined by electrospray ionization mass spectrometry. Moderation of the excitation and temperature allowed isolation and characterization of an intermediate, [Ru^II(bpy)₂bpg]­[BF₄] (bpg = 2-benzoyloxyphenylglyoxylate), generated by the 1,2-addition of dioxygen to the central flavonolate ring

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