Regeneration of terminal oxidants by molecular oxygen
in metal-catalyzed
oxidations of organic substrates has the advantage of avoiding the
use of stoichiometric amounts of hazardous and/or expensive reagents
to meet (some of) the green chemistry requirements. In the present
study, photosensitized singlet oxygen oxidation of iodate to periodate
has been used to regenerate the oxidant in polyoxometalate (POM)-catalyzed
oxidation of sulfides to sulfoxides with periodate in water. To the
best of our knowledge, it is the first report on singlet oxygen oxidation
of iodate to periodate. In order to determine the contribution of
photooxidation and oxidation pathways in the formation of sulfoxide,
the oxidation of diphenyl sulfide with a very low reactivity toward
aerobic photooxidation was studied; a sevenfold increase in the conversion
of the sulfide to the diphenyl sulfoxide was observed for the reaction
conducted in the presence of H2TMPyP-PW12O40/IO3
–/O2/hν
compared to that in the presence of H2TMPyP-PW12O40/O2/hν. Also, under the same conditions,
a ca. 1.5-fold increase was observed in the case of methyl phenyl
sulfide, which shows high reactivity toward both the oxidation and
photooxidation reactions. A porphyrin-POM nanocomposite formed by
the electrostatic immobilization of meso-tetra(N-methylpyridinium-4-yl)porphyrin (H2TMPyP) on
PW12O40 was employed for the one-pot oxidation
and photooxidation reactions. Field-emission scanning electron microscopy
(FESEM), X-ray diffraction (XRD), diffuse-reflectance UV–vis
spectroscopy, thermal gravimetric analysis, and Fourier transform
infrared were used to characterize the formation of the hybrid compound.
An average particle size of 42 nm was estimated for H2TMPyP-PW12O40 from XRD peak broadening using the Scherrer
equation. Also, FESEM images showed the formation of nearly spherical
nanoparticles with a size of ca. 200 nm. The redshift of the Soret
band of H2TMPyP upon immobilization on POM was attributed
to strong N–H···O hydrogen-bond interactions
between POM and porphyrin.
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