Utilizing Redox Transformations of Quinones to Modulate Weak Donor and Acceptor Bonding Interactions with H2o and CO2 in Organic Solvents

Abstract

Redox chemical transformations can be used to modulate supramolecular interactions between reduced quinones and acceptor molecules, with the application of an electric potential enabling the interactions to be switched on and off. The supramolecular interactions targeted include weak solution phase processes involving H-bonding with water molecules, and interactions between dissolved molecular gases such as CO2 and solution phase electroactive species. The advantage of this approach is that it enables the supramolecular complexing abilities of one molecule to be improved and tuned simply by changing its redox state. For instance, in one redox state the compound may undergo no supramolecular interactions, but once it is oxidized or reduced it undergoes very strong interactions. This enables the development of potential sensing devices as well as the ability to gain fundamental information regarding interactions that are difficult to quantify, including H-bonding.In aprotic organic solvents, neutral quinones (Q) undergo an EE reduction mechanism (E signifies an electron transfer) to first give a radical anion or semiquinone (Q•–) at potential E Q1, followed by an aromatic dianion (Q2–) at a more negative potential, E Q2. When water is progressively added to the solvent, the second reduction wave (E Q2) progressively moves towards more positive potentials until it eventually merges with the first reduction wave (E Q1), so the two electrons transfer at close to one potential. The reason for the shift in potential is because the quinone anions, especially the dianions, undergo very strong H-bonding interactions through the highly electronegative oxygen atoms in the reduced quinones with the hydrogen atoms from the water molecules. In order to be able to quantitatively study the reactions with water, it is essential that the exact water content of the solvent is known, and the water content can be precisely controlled. By performing careful calibration experiments using Karl Fischer coulometric titrations, it was possible to establish working curves of water content versus E Q2 – E Q1 (= ΔE) that enabled the accurate calculation of the water content of an organic solvent simply by recording one voltammogram in the presence of a quinone. Therefore, using this procedure, a voltammetric method for quickly estimating water contents of organic solvents down to the low ppm levels was established.In addition to H-bonding interactions, recent experiments have demonstrated that reduced quinones undergo interactions with CO2 and this has been proposed as a method for removing CO2 from industrial gas streams.The technique involves purging the gas through an ionic liquid containing a quinone and applying a reducing potential to produce the quinone dianion which interacts strongly with CO2 in a similar supramolecular mechanism as for H2O [Figure 1(a)]. Once the CO2 is “trapped”, the [quinone–(CO2)2]2– molecules can be separated, and an oxidizing potential applied which results in the [quinone–(CO2)2]2– complex being oxidized back to the starting material and the CO2 being released.Figure 1(b) shows the results from cyclic voltammetry and electrolysis experiments for the reduction of a quinone in the presence and absence of CO2. Under an argon atmosphere, two one-electron reduction processes are evident due to the reduction to the anion radical then at more negative potentials the dianion. However, when the solution is purged with CO2 gas, the voltametric waves immediately merge into one two-electron process due to the formation of the [quinone–(CO2)2]2– supramolecular complex This species is stable under electrolysis conditions and can exist in the bulk solution for long periods (hours). The mechanism can be varied by purging the solution containing the [quinone–(CO2)2]2– complex with an inert gas such as Ar or N2, resulting in two major pathways.In one pathway, the [quinone–(CO2)2]2– complex can be made to release the CO2 •– and leave the neutral quinone, and in the other pathway the charge remains on the quinone and the neutral CO2 is released as shown in Figure 1(c). Figure 1. (a) Reduction mechanism of 1,4-napthoquinone in the presence of CO2. The counterion for the anionic species is the electrolyte cation, n-Bu4N+. (b) Cyclic voltammograms for 1,4-napthoquinone in the presence and absence of CO2. (c) Proposed pathways for the reaction of NQ(CO2)2 2 – with excess argon gas. Figure 1