Supramolecular Interactions between Reduced Vitamin K1 and Dissolved CO2 and H2o in Aprotic Solvents

Abstract

Vitamin K1 is a naphthoquinone (NQ) that can be electrochemically reduced at negative potentials in two one-electron steps in aprotic organic solvents. The first electron transfer at potential E 1 forms the anion radical (NQ• –) and the second electron transfer process at potential E 2 results in the formation of the dianion (NQ2–). If a hydrogen bond donor (H-D) such as water or an alcohol is progressively added to the aprotic organic solvent, both E 1 and E 2 shift towards more positive potentials. The shifts in potential are much greater for E 2 compared to E 1 as the hydrogen bond donor is added, so that a sufficiently high donor concentration, E 2 merges with E 1 and only one voltammetric wave is observed at potential E 3 due to the transfer of two-electrons. The shifts in potential with added hydrogen bond donors can be rationalized by supramolecular interactions between the strongly electronegative oxygen atoms in the quinone anions (especially the dianions) and the donor hydrogen atoms, and can be modelled according to an electrochemical square-scheme mechanism involving multiple electron transfer and chemical bonding steps. NQ + e– ⇌ NQ• – E 1 NQ• – + e– ⇌ NQ2– E 2 NQ + 2e– + nH-D ⇌ [NQ(H-D)n]2– E 3 NQ + 2e– + nCO2 ⇌ [NQ(CO2)n]2– E 4 In addition to H-bonding interactions, reduced forms of NQ also undergo supramolecular interactions with CO2(g) when it is purged into solution. Similar to the situation seen for H-bonding, when CO2 is purged into solution, the two one-electron processes merge into one two-electron process at less negative potentials (E 4). The mechanism can be considered analogous to the H-bonding case where the electronegative oxygen atoms in the reduced quinones interact with the electropositive carbon atoms in CO2. However, the supramolecular interactions appear to be stronger with CO2 than with water, since substantially less CO2 needs to be added to cause the two one-electron reduction processes of NQ to merge into one two-electron process. Controlled potential electrolysis experiments were conducted to determine the chemical reversibility of the supramolecular interactions. It was found that, (i) fully reducing NQ under a CO2 atmosphere and then applying an oxidizing potential resulted in the 100% conversion back to the starting material and, (ii) reducing NQ under a CO2 atmosphere and then purging the solution with argon resulted in the CO2 being released as CO2(g) or CO2 • – to leave NQ in its neutral form or as NQ2–. Figure 1. Cyclic voltammograms for 1,4-naphthoquinone in acetonitrile containing 0.2 M Bu4NPF6 at a scan rate of 0.1 V s–1 at a 1 mm diameter planar Pt electrode in the presence and absence of CO2. Figure 1