Triggering the Generation of an Iron(IV)-Oxo Compound and Its Reactivity toward Sulfides by RuII Photocatalysis

摘要

The preparation of Fe~(IV)(O)(MePy_2tacn)~(2+) (2, MePy_2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) by reaction of Fe~(II)(MePy_2tacn)(solvent)~(2+) (1) and PhIO in CH_3CN and its full characterization are described. This compound can also be prepared photochemically from its iron(II) precursor by irradiation at 447 nm in the presence of catalytic amounts of Ru~(II)(bpy)_3~(2+) as photosensitizer and a sacrificial electron acceptor (Na_2S_2O_8). Remarkably, the rate of the reaction of the photochemically prepared compound 2 toward sulfides increases 150-fold under irradiation, and 2 is partially regenerated after the sulfide has been consumed; hence, the process can be repeated several times. The origin of this rate enhancement has been established by studying the reaction of chemically generated compound 2 with sulfides under different conditions, which demonstrated that both light and Ru~(II)(bpy)_3~(2+) are necessary for the observed increase in the reaction rate. A combination of nanosecond timeresolved absorption spectroscopy with laser pulse excitation and other mechanistic studies has led to the conclusion that an electron transfer mechanism is the most plausible explanation for the observed rate enhancement. According to this mechanism, the in-situ-generated Ru~(III)(bpy)_3~(3+) oxidizes the sulfide to form the corresponding radical cation, which is eventually oxidized by 2 to the corresponding sulfoxide.