C=N Bond Activation and Hydration by an Iron(III) Complex with Asymmetric Sulfur Oxygenation

Abstract

The presence of asymmetric cysteine sulfur‐oxygenation donors, consisting of sulfenate (SO), sulfinate (SO2), and thiolate (S), around a trivalent iron is essential for the catalytic function of Fe‐ or Co‐type nitrile hydratase (NHase). To gain insight into the role of this asymmetric ligation, an octahedral bisthiolate low‐spin FeIII complex and two of its S‐oxygenation derivatives, unprecedented mixed sulfinate/thiolate and O‐bound bissulfinate complexes, were generated. The placement of anionic nitrogen atoms trans to the thiolates in the bisthiolate complex caused the axial imino C=N bonds to be more polarized and thus resembled the activation of bound nitrile in nitrile hydratase. The polarization was found to be even more severe in the monosulfinate species, and hydration of the Fe‐bound C=N bond by HO– was observed. The generation of an O‐bound bissulfinate octahedral complex implied that the equatorial carboxamido and axial thiolate ligation in Fe‐NHase provided greater electron buffering capacity in keeping the electron density of the FeIII ion constant even in the presence of equatorial S‐oxygenations.