An EPR study of electron transfer reactions involving arenes and mercury(II) or thallium(III) salts

Abstract

EPR Spectroscopic evidence has been found for a number of new examples of the recently-recognised alternative mechanism of arene mercuriation in which collapse of the ArH^˙+ Hg(TFA)₂^˙– radical ion pair leads to the formation of the arylmercury trifluoroacetate ArHg(TFA)^˙+. The mercury is always introduced at the position where the EPR hyperfine coupling a(H) in the parent ArH^˙+ is largest, and the ratio between the coupling constants of the mercury which has been introduced and the hydrogen which has been displaced, a(¹¹⁹Hg–TFA)/a(¹H) is normally about 20.6.

The magnitude of a(¹¹⁹Hg–TFA) in ArHg(TFA)^˙+ is interpreted in terms of the state of hybridisation of the mercury, where the amount of s character in the Ar–Hg bond, which is determined by the number and nature of the other ligands about the mercury, affects the transfer of electron spin by the spin polarisation mechanism.

When the mercury is flanked by oxygen in dioxolane or dioxane rings, the value of a(¹¹⁹Hg)/a(¹H) is low, and it is suggested that this is caused by an increase in the coordination state of the mercury.

Addition of fluorosulfonic acid to these trifluoroacetates ArHg(TFA)^˙+ often gives the corresponding fluorosulfonates ArHgOSO₂F^˙+, in which the ratio a(¹¹⁹HgOSO₂F)/a(¹¹⁹HgTFA) is about 1.15. This increase in a(¹¹⁹Hg) is thought to result from rehybridisation of the mercury induced by the more electronegative fluorosulfonate ligand, putting more s character into the Ar–Hg bond.

Oxidation of a number of arenes with thallium(III) tris(trifluoroacetate) has provided one further example of the formation of aryl trifluoroacetate radical cations Ar(TFA)^˙+, and it is suggested that these are formed by the reductive elimination reaction of thalliated intermediates, ArTl(TFA)₂.