Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 230

for the desired minimally-toxic sulfoxide product, CEESO), the
stoichiometry and the evidence for a non-radical chain mechanism are
presented and discussed. This work represents a foray into “functionally
smart” materials, materials that not only sense and adapt to ambient
conditions but also execute important functions when appropriate. In this
instance, the function is catalyzing the facile oxidative degradation of toxic
sulfur compounds if and whenever they are present.
2. DISCOVERY OF
CATALYTIC OXIDATION SYSTEM
By combining heteropolyanions (polyoxometalates; POMs for
convenience) and selected cations in acetonitrile, more than 150
combinations were assayed for their catalytic activity towards selective
CEES oxidation to CEESO by dioxygen under ambient (room temperature
and atmospheric pressure) conditions. The main criteria in choosing POMs
were their ability to undergo reversible redox transformations and to catalyze
homogeneous oxidations either by peroxides or other terminal oxidants. The
cations chosen included redox-active transition metal ions or cations
conventionally used as the counterions in POMs. In control experiments the
chloride, nitrate or perchlorate salts of the same transition metal ions were
also examined. The list of these catalytic systems and some selected results
were recently published. 22
Most of the screened combinations (catalysts) showed little if any
catalytic activity. Only two catalysts exhibited considerable activity, both of
which included The catalysts contained 1 equivalent of
or and five equivalents of A
mixing of with these POMs resulted in a formation of a white
precipitate, which elemental analysis showed to be NaCl. It was
hypothesized that removal of a chloride ligand from produced an
active catalyst. To explore this idea Ag(I) a stronger halide
abstractor in acetonitrile than Na(I), was used to remove a chloride anion
from the Au center. The rate of CEES oxidation by this POM-free system
based on Au and Ag was significant, and by varying the ratios of
and the rate was increased several-fold. Interestingly, the
Au(III)/Ag(I) system appeared to be inactive when was replaced
with Subsequently, by varying the ratios of different Ag(I) salts it
was established that the rate was very sensitive to the Au(III): ratio. A
detailed study revealed that the most active catalyst was formed when
and were combined in a 1 : 1 : 1 ratio, suggesting a
formation of the complex. This catalytic system is
significantly faster than the two most reactive catalysts in the literature for