Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 251

At a higher conversion of CEES, more inactive Au(III)-complexes are
formed. As a result the reaction slows down faster than expected accounting
only for CEES consumption. In other words, the reaction is self-inhibited by the
reaction product, CEESO. This self-inhibition is clear in Figure 1, a line with
(+) symbols represent the kinetics of CEES consumption assuming that no self-
inhibition takes place (exponential kinetics). The extent of self-inhibition is
evidently dependent on the binding properties between sulfoxides and Au(III),
represented by the equilibrium constant (eq 42). A stronger binding of
the sulfoxide to Au(III) (higher results in a stronger self-inhibition.
Dashed and dotted lines in Figure 1 represent the theoretical curves describing
CEES consumption using different values of The best fit is obtained

when (a solid line in Figure 1).
The formation of inactive complexes is accompanied by a complexation of a
nitrate anion and a large Au(III) cation, 2' , 2” or 2' . The formation of these
charged species should be more favourable in a polar solvent such as
This explains why self-inhibition is more pronounced in than in
(for additional discussion see section 15). Au(III)-Br complexes are
larger than similar Au(III)-Cl complexes because the ionic radius of in
crystals is higher than that of 1.96A and 1.81, respectively. The enthalpy of
solvation also decreases in the order implying that the
formation of the analogous bromide complexes, 2' and 2” is less favourable.
Therefore, as mentioned in section 12, product inhibition is less severe when a
Cl-ligand is replaced with Br.

14. CO-CATALYSIS BY TRANSITION METAL IONS

Since the reduction of a Au(III) complex by CEES is the rate limiting step, it
was speculated that transition metal ions in high oxidation states might catalyze
the reduction of Au(III). Both Fe(III) and Cu(II) were screened for their activity
in acetonitrile with and ligands. Both these metals eliminate induction
period (Figure 5), but only Cu(II) gave a rate enhancement (i.e. co-catalyst
activity), particularly in the presence of (see Figure 2 in recent
article). 23 For example, the initial oxidation rate at ~3.0 M CEES is
approximately 2 times faster for than for while
is 8 times faster than It is important to
note that addition of Cu(II) salts to the system eliminates the
inhibition effect of the sulfoxide product, allowing for almost the complete
oxidation of CEES to CEESO.

Co-catalysis by a copper salt is even more