Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 247

Based on structural properties of Au(I) complexes, the formation of a
binuclear complex 4 bridged by two Cl-ligands with a linear
moiety is reasonable, eq 30, Scheme 2.
Dioxygen binding to this complex promoted by the presence of two
thioether ligands results in the formation of a mixed Au(III)/Au(I) complex
5 , with a square-planar peroxo-Au(III) and linear Au(I) moieties. Mixed-
valent Au-complexes are precedented in the literature. The vibrational
spectra of some dialkyl sulphide complexes of gold (III) and gold (I) halides
have been reported. 56 A mixed-valence complex of gold with dimethyl
sulfoxide has been reported. 57,58 The unimolecular rearrangement of 5
results in the formation of a complex 6 , eq 33. Transfer of
terminal oxygen atom from the peroxo-group to the sulfur atom of the
thioether, eq 32, can be definitively ruled out since it would result in only
50% incorporation of oxygen-18 from labeled water during the catalytic
reaction. A heterolytic cleavage of O-O bond in 6 , assisted by protons,
would result in the formation of the catalytically active complex 1 , eq 34,
without intermediate formation. Scheme 2, while highly speculative,
is compatible with the structural properties of Au(I) and Au(III) complexes
in the literature and our current experimental data.
Scheme 2 suggests that an active catalyst can be prepared starting from a
Au(I) complex and proper amounts of nitrate and chloride salts in the
presence of a strong acid. We attempted to prepare such a catalyst by
mixing p -toluenesulfonic acid, and CEES in
acetonitrile under oxygen and have found that the rate of catalysis increases
with increasing acid concentration. For example, the reaction occurs very
slowly if at all when no acid is present, but reactivity is clearly visible when
one equivalent of p -toluenesulfonic acid acid is present. When three
equivalents are used, the reactivity is comparable to the

system.

12. EFFECT OF LIGANDS ON REACTIVITY

As described above and in recently published mechanistic studies 22 a ratio
of catalyst components of 1 produced the most reactive
catalyst, indicating that is necessary for high catalytic activity. To
assess this necessity, was systematically replaced with other ligands
and Surprisingly, the
stoichiometric substitution of with produced a catalyst with ~2-
fold higher activity than in the case of (Figure 2). However, this new
system was not thoroughly investigated because is more toxic than
and would be less desirable for many applications. Other ligands led to