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((SN4)-N-Me2(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((SN4)-N-Me2(tren)(NO)](+) (2; g = 4.44, 3.54, 1.97), the isotopically sensitive nu(NO)(nu 15NO) stretching frequency (1685(1640) cm(-1)), and the 0.05 A 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 = S/2) Fe(111) 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)degrees). 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 nu(NO) frequency (1721 cm(-1)). In contrast, the nu(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 nu(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.