Understanding the Surprising Oxidation Chemistry of Au-OH Complexes

Abstract

Au is known to be fairly redox inactive (in catalysis) and bind oxygen adducts only quite weakly. It is thus rather surprising that stable Au-OH complexes can be synthesized and used as oxidants for both one- and two-electron oxidations. A charged Au^III -OH complex has been shown to cleave C-H and O-H bonds homolytically, resulting in a one-electron reduction of the metal center. Contrasting this, a neutral Au^III -OH complex performs oxygen atom transfer to phosphines, resulting in a two-electron reduction of the hydroxide proton to form a Au^III -H rather than causing a change in oxidation state of the metal. We explore the details of these two examples and draw comparisons to the more conventional reactivity exhibited by Au^I -OH. Although the current scope of known Au-OH oxidation chemistry is still in its infancy, the current literature exemplifies the unique properties of Au chemistry and shows promise for future findings in the field.

Keywords: gold hydroxides; mechanisms; oxidation; oxygen atom transfer; proton coupled electron transfer.

Figure 1

Examples of Au−OH complexes for which oxidation reactivity studies have been performed.[ ^9a , ^9c , ^9d , ^9m ]

Scheme 1

Reported reactivity of formal (a) Au(I) and (b) Au(III) complexes with phenol.[ ^10c , ^13a ]

Scheme 2

Curly arrow representations of the electron flow in the HAT and cPCET mechanisms. Adapted from Ref. [13c].

Scheme 3

OAT reactivity of (C^C^C)Au−OH. ^[14a]