Oxidation of Thiosulphate using TCNQF_n (n=0, 2, 4) Derivatives with a Tuneable Driving Force: Electrochemical and Spectrophotometric Detection of a Protonated Intermediate

Abstract

Fluorine substituent effects associated with the TCNQFnormaln0/1- and TCNQFnormaln1-/2- (n=0, 2, 4, TCNQ=7,7,8,8‐tetracyanoquinodimethane) half‐cell reactions provide access to a wide driving force range (∼850 mV) which is available to tune redox reactions; in this case the oxidation of normalS2normalO32- to normalS4normalO62- by TCNQFnormaln0 or TCNQFnormaln1- . The TCNQFnormaln0/1-/2- ‐normalS2normalO32- /normalS4normalO62- redox chemistry was studied by steady‐state and transient cyclic voltammetry and UV‐visible spectrophotometry in acetonitrile containing 5 % water and Bu4NPF6 as the supporting electrolyte. In this mixed solvent system, all the TCNQFnormaln0/1- (n=0, 2, 4)‐normalS2normalO32- /normalS4normalO62- reactions were thermodynamically favourable, as were those of TCNQFnormaln1-/2- (n=2, 4). However, TCNQ1- does not oxidize normalS2normalO32- . These findings lead to a prediction that the reversible potential for the normalS2normalO32- /normalS4normalO62- half‐cell reaction lies between −0.083 and −0.278 V vs SHE. Interestingly, in the investigation of the reaction of TCNQ0 and normalS2normalO32- in a 1 : 2 concentration ratio, a protonated TCNQ1- intermediate was detected. In contrast, when the oxidant was TCNQF20 (2,5‐difluoro‐7,7,8,8‐tetracyanoquinodimethane) or TCNQF40 (2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquino‐dimethane), no evidence for an analogous protonated intermediate was found. However, if equimolar ratios of TCNQFnormaln0 (n=0, 2, 4) and normalS2normalO32- were used, the intermediate was again detected. The structure of the intermediate, described as HTCNQFn for convenience, is unknown, but this derivative of the redox chemistry is kinetically very stable and only very slowly deprotonated to TCNQFnormaln1- or reduced by excessnormalS2normalO32- to give oxidized normalS4normalO62- and deprotonated TCNQFnormaln2- . Mechanisms are provided to explain the formation of the protonated intermediate.