Influence of Thiolate Ligands on Reductive N-O Bond Activation. Probing the O2 - Binding Site of a Biomimetic Superoxide Reductase Analogue and Examining the Proton-Dependent Reduction of Nitrite

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      Publication Details (including relevant citation information):

      Gloria Villar-Acevedo, Elaine Nam, Sarah Fitch, Jason Benedict, John Freudenthal, Werner Kaminsky, and Julie A. Kovacs*

     
      Department of Chemistry, University of Washington, Seattle,   Washington 98195, United States

      J. Am. Chem. Soc. 2011, 133,   1419–1427

      dx.doi.org/10.1021/ja107551u

      

      Abstract:

      Nitric oxide (NO) is frequently used to probe the   substrate-binding site of “spectroscopically silent” non-heme   Fe2+
      sites ofmetalloenzymes, such as superoxide reductase (SOR).   Herein we use NO to probe the superoxide binding site of our   thiolateligated biomimetic SOR model [FeII(SMe2N4(tren))]+ (1).   Like NO-bound trans-cysteinate-ligated SOR (SOR-NO), the rhombic   S = 3/2 EPR signal of NO-bound cis-thiolate-ligated   [Fe(SMe2N4-(tren)(NO)]+ (2; g = 4.44, 3.54, 1.97), the   isotopically sensitive νNO(ν15NO) stretching frequency   (1685(1640) cm-1), and the 0.05 Å decrease in Fe-S bond length   are shown to be consistentwith the oxidative addition of NO to   Fe(II) to afford an Fe(III)-NO- {FeNO}7 species containing   high-spin (S = 5/2) Fe(III) antiferromagnetically coupled to NO-   (S = 1). The cis versus trans positioning of the thiolate does   not appear to influence these properties. Although it has yet to   be crystallographically characterized, SOR-NO is presumed to   possess a bent Fe-NO similar to that of 2 (Fe-N-O = 151.7(4)).   The N-O bond is shown to be more activated in 2 relative to N-   and O-ligated {FeNO}7 complexes, and this is attributed to the   electron-donating properties of the thiolate ligand.   Hydrogen-bonding to the cysteinate sulfur attenuates N-O bond   activation in SOR, as shown by its higher νNO frequency (1721   cm-1). In contrast, the νO-O frequency of the SOR peroxo   intermediate and its analogues is not affected by H-bonds to the   cysteinate sulfur or other factors influencing the Fe-SR bond   strength; these only influence the νFe-O frequency. Reactions   between 1 and NO2- are shown to result in the proton-dependent   heterolytic cleavage of an N-O bond. The mechanism of this   reaction is proposed to involve both FeII-NO2- and {FeNO}6   intermediates similar to those implicated in the mechanism of   NiR-promoted NO2- reduction.

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