The Hydrogen-Bonded Dianion of Vitamin K<sub>1</sub> Produced in Aqueous–Organic Solutions Exists in Equilibrium with Its Hydrogen-Bonded Semiquinone Anion Radical

When the quinone, vitamin K<sub>1</sub> (VK<sub>1</sub>), is electrochemically reduced in aqueous-acetonitrile solutions (CH<sub>3</sub>CN with 7.22 M H<sub>2</sub>O), it undergoes a two-electron reduction to form the dianion that is hydrogen-bonded with water [VK<sub>1</sub>(H<sub>2</sub>O)<sub><i>y</i></sub><sup>2–</sup>]. EPR and voltammetry experiments have shown that the persistent existence of the semiquinone anion radical (also hydrogen-bonded with water) [VK<sub>1</sub>(H<sub>2</sub>O)<sub><i>x</i></sub><sup>–•</sup>] in aqueous or organic–aqueous solutions is a result of VK<sub>1</sub>(H<sub>2</sub>O)<sub><i>y</i></sub><sup>2–</sup> undergoing a net homogeneous electron transfer reaction (comproportionation) with VK<sub>1</sub>, and not via direct one-electron reduction of VK<sub>1</sub>. When 1 mM solutions of VK<sub>1</sub> were electrochemically reduced by two electrons in aqueous-acetonitrile solutions, quantitative EPR experiments indicated that the amount of VK<sub>1</sub>(H<sub>2</sub>O)<sub><i>x</i></sub><sup>–•</sup> produced was up to approximately 35% of all the reduced species. <i>In situ</i> electrochemical ATR-FTIR experiments on sequentially one- and two-electron bulk reduced solutions of VK<sub>1</sub> (showing strong absorbances at 1664, 1598, and 1298 cm<sup>–1</sup>) in CH<sub>3</sub>CN containing <0.05 M H<sub>2</sub>O led to the detection of VK<sub>1</sub><sup>–•</sup> with strong absorbances at 1710, 1703, 1593, 1559, 1492, and 1466 cm<sup>–1</sup> and VK<sub>1</sub>(H<sub>2</sub>O)<sub><i>y</i></sub><sup>2–</sup> with strong absorbances at 1372 and 1342 cm<sup>–1</sup>.