Unexpected Complexity in the Electro-Oxidation of Iodide on Gold in the Ionic Liquid 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide

Abstract

The electro-oxidation of iodide on a gold electrode in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide has been investigated using transient cyclic voltammetry, linear-sweep semi-integral voltammetry, an electrochemical quartz crystal microbalance technique, and coulometry/electrogravimetry. Two oxidation processes are observed, with an electron stoichiometry of 1:1, compared with the well-known 2:1 electron stoichiometry observed on other commonly used electrode materials, such as platinum, glassy carbon, and boron-doped diamond, under identical conditions. Detailed mechanistic information, obtained in situ using an electrochemical quartz crystal microbalance, reveals that this unusual observation can be attributed to the dissolution of the gold electrode in the presence of iodide. Coulometric/electrogravimetric analysis suggests that the oxidation state of the soluble gold species is +1 and that diiodoaurate, [AuI2](-), is the likely intermediate. A proportionally smaller amount of triiodide intermediate is also detected by means of UV-vis spectroscopy. On this basis, it is proposed that iodide oxidation on gold occurs via two parallel pathways: predominantly via a diiodoaurate intermediate 2I(-) + Au ⇌ [AuI2](-) + e(-) and [AuI2](-) ⇌ I2 + Au + e(-) and to a lesser extent via a triiodide intermediate 3I(-) ⇌ I3(-) + 2e(-) and I3(-) ⇌ 3/2I2 + e(-). This proposed mechanism was further supported by voltammetric investigations with an authentic sample of the anionic [AuI2](-) complex.

Electrochemical Behavior and Specific Adsorption of an Iodide-based Ionic Liquid on Au(111)

“…The electrochemical potential window (E pw ), estimated from the difference in the potentials at which the current density started increasing or decreasing rapidly on the cut-off current density of «1.0 mA cm ¹2 , was approximately 2 V. In general, [C n mim] + -based ILs have wide E pw of over 4 V. 4 However, the E pw of [C 4 mim][I] was quite narrow because the dissolution of Au is caused by the formation of gold complexes with iodide. 21 Figure 1b…”

Electrochemistry of Iodide, Iodine, and Iodine Monochloride in Chloride Containing Nonhaloaluminate Ionic Liquids

“…Over the past 2 decades, nonhaloaluminate room temperature ionic liquids (ILs) have attracted significant interest in academia and industry as replacements for volatile molecular solvents in a range of applications. , A promising application of ILs is as the electrolyte in DSSCs, , and for this reason, the electrochemistry of the I – /I 2 couple has been the subject of many investigations in these neoteric solvents. − In a “typical” aprotic IL, such as 1-ethyl-3-methylimidazolium bis­(trifluoromethanesulfonyl)­imide ([C 2 mim]­[NTf 2 ]), the I – /I 2 redox process is analogous to that in acetonitrile, − occurring in two resolved steps (i.e., eqs and ), via an I 3 – intermediate. It should be noted that although the most commonly employed IL anions (e.g., [PF 6 ] − , [BF 4 ] − , [CF 3 SO 3 ] − , and [NTf 2 ] − ) are weak Lewis bases (i.e., “noncoordinating”), ILs containing nucleophilic Cl – or Br – as the constituent anion can be prepared .…”

Towards Understanding the Solvent‐Dynamic Control of the Transport and Heterogeneous Electron‐Transfer Processes in Ionic Liquids

“…[6,16,20] Reports published in this regard have generated immense interesti ne xploring and utilising such interactions and their role in variousp henomena. [21][22][23][24][25][26][27][28][29][30][31][32] In one such report we have demonstrated that these interactions can change the mechanistica spects of elementary steps in ET processes. [24] It has been reported that ILs exhibit both static and dynamic heterogeneity, [19] which makest he continuum description of solvents irrelevant for ILs.…”

“…An interesting feature related to their role as solvents in ET reactions is that the interaction among the IL constituents and the redox probes strongly affects the solvent‐reorganisation energy and hence the nature of potential‐energy surfaces driving the apparent kinetics . Reports published in this regard have generated immense interest in exploring and utilising such interactions and their role in various phenomena . In one such report we have demonstrated that these interactions can change the mechanistic aspects of elementary steps in ET processes …”