Photochemical Activation of Ruthenium(Ⅱ)-Pyridylamine Complexes Having a Pyridine-N-Oxide Pendant toward Oxygenation of Organic Substrates

摘要

Ruthenium(Ⅱ)-acetonitrile complexes having η~3 -tris(2-pyridylmethyl)arnine (TPA) with an uncoordinated pyridine ring and diimine such as 2,2'-bipyridine (bpy) and 2,2'-bipyrimidine (bpm), [Ru~Ⅱ(η~3-TPA)(diimine)(CH_3CN)]~(2+), reacted with m-chloroperbenzoic acid to afford corresponding Ru(Ⅱ)-acetonitrile complexes having an uncoordinated pyr-idine-N-oxide arm, [Ru~Ⅱ(η~3-TPA-O)(diimine)(CH_3CN)]~(2+), with retention of the coordination environment. Photoirradia-tion of the acetonitrile complexes having diimine and the η~3 -TPA with the uncoordinated pyridine-N-oxide arm afforded a mixture of [Ru~Ⅱ (TPA)(diimine)]~2+, intermediate-spin (S =1) Ru(Ⅳ)-oxo complex with uncoordinated pyridine arm, and intermediate-spin Ru(Ⅳ)- oxo complex with uncoordinated pyridine-N-oxide arm. A Ru(Ⅱ) complex bearing an oxygen-bound pyridine-N-oxide as a ligand and bpm as a diimine ligand was also obtained, and its crystal structure was determined by X-ray crystallography. Femtosecond laser flash photolysis of the isolated O-coordinated Ru(Ⅱ)- pyridine-N-oxide complex has been investigated to reveal the photodynamics. The Ru(Ⅳ)-oxo complex with an uncoordinated pyridine moiety was alternatively prepared by reaction of the corresponding acetonitrile complex with 2,6-dichloropyridine-N-oxide (Cl_2py-O) to identify the Ru(Ⅳ)-oxo species. The formation of Ru(Ⅳ)-oxo complexes was concluded to proceed via intermolecular oxygen atom transfer from the uncoordinated pyridine-N-oxide to a Ru(Ⅱ) center on the basis of the results of the reaction with Cl_2py-O and the concentration dependence of the consumption of the starting Ru(Ⅱ) complexes having the uncoordinated pyridine-N-oxide moiety. Oxygenation reactions of organic substrates by [Ru~Ⅱ(η~3-TPA-O)(diimine)(CH_3CN)]~(2+) were examined under irradiation (at 420 ± 5 nm) and showed selective allylic oxygenation of cyclohexene to give cyclohexen-1-ol and cyclohexen-1-one and cumene oxygenation to afford cumyl alcohol and acetophenone.