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 [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 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 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.