A Mononuclear, Nonheme FeII-Piloty's Acid (PhSO2NHOH) Adduct: An Intermediate in the Production of {FeNO}7/8 Complexes from Piloty's Acid

Reaction of the mononuclear nonheme complex [Fe-II(CH3CN)(N3PyS)]BF4 (1) with an HNO donor, Piloty's acid (PhSO2NHOH, P.A.), at low temperature affords a high-spin (S = 2) Fe-II-P.A. intermediate (2), characterized by Fe-57 Mossbauer and Fe K-edge X-ray absorption (XAS) spectroscopies, with interpretation of both supported by DFT calculations. The combined methods indicate that P.A. anion binds as the N-deprotonated tautomer (PhSO2NOH-) to [Fe-II(N3PyS)](+), leading to 2. Complex 2 is the first spectroscopically characterized example, to our knowledge, of P.A. anion bound to a redox-active metal center. Warming of 2 above -60 degrees C yields the stable {FeNO}(7) complex [Fe(NO)(N3PyS)]BF4 (4), as evidenced by H-1 NMR, ATR-IR, and Mossbauer spectroscopies. Isotope labeling experiments with N-15-labeled P.A. confirm that the nitrosyl ligand in 4 derives from PA. In contrast, addition of a second equivalent of a strong base leads to S-N cleavage and production of an {FeNO}(8) species, the deprotonated analog of an Fe-HNO complex. This work has implications for the targeted delivery of HNO/NO-/NO center dot to nonheme Fe centers in biological and synthetic applications, and suggests a new role for nonheme Fe-II complexes in the assisted degradation of HNO donor molecules.