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

Nitric oxide (NO) is frequently used to probe the substrate-binding site of "spectroscopically silent" non-heme Fe²⁺ sites of metalloenzymes, such as superoxide reductase (SOR). Herein we use NO to probe the superoxide binding site of our thiolate-ligated biomimetic SOR model [Fe^II(S^Me2N₄(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(S^Me2N₄(tren)(NO)] ⁺ (2; g = 4.44, 3.54, 1.97), the isotopically sensitive V _NO(V_15NO) stretching frequency (1685(1640) cm^-1), and the 0.05 Å decrease in Fe-S bond length are shown to be consistent with 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 V_NO frequency (1721 cm^-1). In contrast, the V _O-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 V_Fe-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 Fe^II-NO₂^- and {FeNO}⁶ intermediates similar to those implicated in the mechanism of NiR-promoted NO₂^- reduction.