Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 248

completely inactive catalytic systems. Thus, or are definitely

necessary for high catalytic activity.
Above it was noted that the replacement of with increases the
initial reaction rate. This increase in rate is consistent with the involvement
of a ligand transfer reaction in the rate limiting step, eq 9 in Scheme 1, in the
proposed mechanism (for detailed discussion see section 8 and 9). However,
there is another factor, which considerably affects the overall reaction rate.
In acetonitrile at [CEES] > 0.1 M a considerable part of total Au(III) is in the
form of the inactive complex 2 , which results in a saturation of the initial
rate with increasing CEES concentration (Figure 4). Eq 20 perfectly
describes this complex experimental dependence (solid line on Fig 3).
Because a ligand binds more strongly than to Au(III) 58 , (eq 8,
Scheme 1) is lower for the Br-Au complex than the Cl-Au complex. Nitrite
is a softer ligand than nitrate, and subsequently it also binds stronger to
Au(III) (which prefers to complex with soft ligands) than nitrate. 59 Several
gold (III) nitrite complexes have been identified 60-62 , but none have been
isolated. Thus, both and evidently drive eq 8 to the left thereby

decreasing
Changing the value of has a dramatic effect on the reaction rate
according to eq 20. The magnitude of this effect also depends on CEES and
concentrations and is demonstrated by dashed and dotted lines in
Figures 3 and 4. These theoretical dependencies are computed using
different values for It is clearly seen that a decrease in the equilibrium
constant, results in a considerable increase in the reaction rate. Thus,
higher activity for and is at least partly explained by a decrease of

for these ligands.
A more thorough investigation shows that this ligand replacement also
reduced the induction period and decreased the inhibition of the overall
reaction by CEESO product (Figures 2). As a consequence, a significantly
higher conversion of CEES to CEESO can be achieved in the system with a
ligand. Inhibition of the reaction by CEESO product arises from
formation of the inactive complex 2' with the sulfur ligands, analogous to 2 ,
but with one of the CEES ligands replaced by a CEESO 22 ( see Scheme 3
and section 13 below). The analogous complex with two sulfoxide ligands,
2” , is also inactive. A replacement of bromide or nitrite with CEESO in a
complex similar to 1 can be described by equilibrium constants and
which are analogous to Because a replacement of bromide or nitrite by
CEESO is less favourable is lower for bromide or nitrite than for
chloride and nitrate), product inhibition is less pronounced.
An induction period in CEES oxidation is likely to be the result of slow
catalyst formation during the reoxidation of Au(I) by dioxygen at the
beginning of the reaction. 22 However, since reoxidation is not a rate limiting
step during the main process, a kinetic evaluation of the ligand effect on the
rate of Au(I) reoxidation is not possible.