Acidity and Hydrogen Exchange Dynamics of Iron(II)‐Bound Nitroxyl in Aqueous Solution

Abstract

Nitroxyl‐iron(II) (HNO‐FeII) complexes are often unstable in aqueous solution, thus making them very difficult to study. Consequently, many fundamental chemical properties of FeII‐bound HNO have remained unknown. Using a comprehensive multinuclear (1H, 15N, 17O) NMR approach, the acidity of the FeII‐bound HNO in [Fe(CN)5(HNO)]3− was investigated and its pKa value was determined to be greater than 11. Additionally, HNO undergoes rapid hydrogen exchange with water in aqueous solution and this exchange process is catalyzed by both acid and base. The hydrogen exchange dynamics for the FeII‐bound HNO have been characterized and the obtained benchmark values, when combined with the literature data on proteins, reveal that the rate of hydrogen exchange for the FeII‐bound HNO in the interior of globin proteins is reduced by a factor of 106.