Catalysts for selective aerobic oxidation under ambient conditions - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 254

The value of was found to be implying that 26% of the
total Au(III) (at 5 mM) is in the form In contrast,
trifluoroethanol, nitromethane, and 1,2-dichloroethane would coordinate
weakly if at all with the Au complexes. The equilibrium constants are likely
to be very sensitive to the nature of the solvent.
For example, the equilibrium constant for the displacement of
heterocyclic amines (for example pyridine, py) by

from Au(III), eq 44,

changes from 25 in to 0.085 in

The equilibrium (eq 8 in Scheme 1) between a neutral and positively
charged complex, 1 and 2 respectively, depends on the solvent polarity. The
more polar the solvent, the more eq 8 is driven to the right, which increases
and thereby decreases the concentration of the active complex 1 and the
overall reaction rate (see discussion on the effect of in section 12).
However, the solvent effect on the reaction rate is more complex. The
overall rate is controlled not only by the concentration of the reactive
complex 1 , but also by the rate of the reduction of 1 by CEES, eq 9 in
Scheme 1. In this rate limiting step two charged species, chlorosulfonium
and nitrate ions, are formed. Their charge separation should be more
favourable in polar solvents. Thus, predicting the effect of the
solvent on the overall rate could be difficult a priori because and have

dependencies in opposite directions.
The highest oxidation rate is observed in trifluoroethanol, a less polar
solvent than acetonitrile. This higher rate is attributable to a shift of eq 8 to
the left This is also consistent with the weak product
inhibition observed in trifluoroethanol. Formation of the inactive 2'” cation
and nitrate anion, eq 42, is not as favourable in lower polarity solvents
resulting in less self-inhibition. For that reason, CEES oxidation to CEESO
proceeds to almost completion in trifluoroethanol, but significant inhibition
occurs in acetonitrile (Figure 6).
It has also been observed that an induction period is not significant, if it
exists at all, in solvents other than acetonitrile. The length of this induction
period depends on dioxygen concentration and therefore is likely to depend
on the rate of Au(I) oxidation by dioxygen. 22 Since this reaction is not rate
limiting in the catalytic process it is impossible to assess a solvent's effect on
the rate of Au(I) reoxidation.