Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 232

system for oxidation shares all the above features but has additional
components and therefore is more mechanistically complex.

4. IN SITU CATALYST PREPARATION

As is the case with both Ag salt precursors, and
by themselves are completely inactive as catalysts. Moreover, the addition
of a soluble Ag(I) salt (e.g.

or to does not result in

precipitation of AgCl, as expected via eq 2:

However, the subsequent addition of CEES to this Au(III)/Ag(I) solution
leads to the immediate disappearance of the yellow chromophore of Au(III),
and solid AgCl forms (confirmed by elemental analysis after isolation). The
stoichiometry for this catalyst preparation reaction was determined to be eq
3:

If solid AgCl is filtered off and the solvent is removed, the remaining
solid contains Au, CEES and/or CEESO moieties but no Ag. Attempted
recrystallization of this material was unsuccessful, and Au(III) was reduced
to a catalytically inactive Au(0) colloid. It is observed that once Au(0)
forms in the system, it is not reoxidized, leading to an irreversible
inactivation of the catalyst. However, use of the correct ratios of precursors
in the presence of or even ambient air, produces soluble and stable Au-
based catalysts, some of which retain activity for at least one week. Most
Au(I) complexes, with the exception of disproportionate in
and several disproportionation equilibria have been measured. 31-34 Although
disproportionation is less favourable kinetically in solution, it is
possible that the catalyst inactivates by hydrolysis of the Au(I) intermediate
followed by its reduction to form Au(0). 35
In acetonitrile, catalytic thioether oxidation may exhibit a short induction
phase which is followed by the main reaction (Figure 1). This induction
period depends on the concentrations of CEES, Au, and At higher
concentrations the induction period is shorter (Table 1 shows the
dependence of the induction period). During the induction period the system
remains colorless, neither CEES nor are consumed, and no CEESO is
formed. At the end of the induction period, the solution rapidly becomes
yellow. The electronic absorption spectrum of this yellow species is fully