A Mononuclear, Nonheme Fe^II–Piloty’s Acid (PhSO₂NHOH) Adduct: An Intermediate in the Production of {FeNO}^7/8 Complexes from Piloty’s Acid

Abstract

Reaction of the mononuclear nonheme complex [FeII(CH3CN)(N3PyS)]BF4 (1) with an HNO donor, Piloty’s acid (PhSO2NHOH, P.A.), at low temperature affords a high-spin (S = 2) FeII-P.A. intermediate (2), characterized by 57Fe Mössbauer 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 [FeII(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 °C yields the stable {FeNO}7 complex [Fe(NO)(N3PyS)]BF4 (4), as evidenced by 1H NMR, ATR-IR, and Mössbauer spectroscopies. Isotope labeling experiments with 15N-labeled P.A. confirm that the nitrosyl ligand in 4 derives from P.A. 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‧ to nonheme Fe centers in biological and synthetic applications, and suggests a new role for nonheme FeII complexes in the assisted degradation of HNO donor molecules.

Photoinduced NO and HNO Production from Mononuclear {FeNO}6 Complex Bearing a Pendant Thiol

“…The infrared (IR) spectrum of 1 (KBr pellet, Figure S2) shows a ν NO band at 1829 cm −1 and a broad S−H stretching band (ν SH ) at ∼2440 cm −1 , which are consistent with the presence of these functional groups determined by the X-ray diffraction analysis of 1. The ν NO band at 1829 cm −1 of 1 shifts to 1792 cm −1 for 15 NO-labeled 1; the shift of 37 cm −1 upon the substitution of 14 NO with 15 NO is close to the calculated value based on Hooke's law (Figure S2). A comparison of the ν SH stretching frequency for 1 and the free ligand TMS PS2H 2 (2478 cm −1 ) 73 2), as confirmed by various spectroscopies and single-crystal X-ray diffraction.…”

“…After the first 20 s of irradiation, subsequent photolysis results in intensity loss of the ν NO at 1823 cm −1 with the concurrent growth of ν NN of N 2 O at 2219 cm −1 (in toluene) and a proposed {FeNO} 7 species with ν NO at 1774 cm −1 (from blue to red traces in Figure 2a). Photoirradiation of 15 NO-labeled 1 also exhibits similar reactions. A shift of the NO peak from 1796 to 1786 cm −1 in toluene occurs first, followed by decay of the new peak at 1786 cm −1 in the subsequent photolysis, along with the concomitant formation of 15 N 2 O at 2150 cm −1 (Figure S9).…”

“…Photoirradiation of 15 NO-labeled 1 also exhibits similar reactions. A shift of the NO peak from 1796 to 1786 cm −1 in toluene occurs first, followed by decay of the new peak at 1786 cm −1 in the subsequent photolysis, along with the concomitant formation of 15 N 2 O at 2150 cm −1 (Figure S9). The GC analysis of the reaction headspace exhibits a signal (t r = 6.52 min) that is assigned to N 2 O (∼15.0% yield, Figure S10).…”

“…The lower energy ν NO band of 5 − , which is shifted by 75 cm −1 in the CH 2 Cl 2 solution from that of 6, is comparable to that of [Mn(NO)-(L) 2 ] 2− (a {MnNO} 6 species; LH 2 = 1,2-benzenedithiol and toluene-3,4-dithiol) and [Mn(NO)(L) 2 ] − (a {MnNO} 5 species). 99 The photoirradiation of 15 NO-labeled 1 in the presence of [Mn III ( TMS PS3)(DABCO)] in CH 2 Cl 2 solution at room temperature displays similar reactivity in which the NO peak is shifted from 1703 to 1670 cm −1 for 15 NO-labeled 5 − and from 1778 to 1744 cm −1 for 15 NO-labeled 6, respectively (Figure S25). It is noted that no IR bands assigned to N 2 O are observed in the range of 2200−2300 cm −1 upon photolysis of the mixture containing 1 and [Mn III ( TMS PS3)(DABCO)] under the same conditions.…”

“…Nitric oxide (NO) and its one-electron reduced derivative nitroxyl (HNO/NO – ) play significant roles in many physiological processes. − The target receptors of NO and HNO/NO – often contain iron centers (heme or nonheme) or sulfur ligands (thiol or thiolate), leading to the formation of NO adducts responsible for biological processes. , For example, the formation of cyclic guanosine monophosphate (cGMP) triggered by binding of NO to the iron-heme center in soluble guanylate cyclase (sGC) represents a key step for neuromodulation or vasodilation. , HNO also induces a vasorelaxation response and displays a more vasodilative effect than NO, thereby showing promise as a positive cardiac inotrope for heart failure treatment. , Inspired by the importance of NO and HNO in biological systems or medical applications, many efforts have been made to prepare NO- or HNO-releasing agents based on metal–NO compounds. − The release of NO from exogenous metal–nitrosyl sources can be stimulated by changes in pH, solvation, heat, and light. ,− In contrast with the NO-releasing chemistry from metal–nitrosyl sources, the generation of HNO from metal–nitroxyl species (M­(HNO/NO – )) remains relatively unexplored. It is generally proposed that the reduction of the iron–NO complexes, {FeNO} x ( x = 6 or 7; the Enemark–Felthan notation), to electron-rich {FeNO} 8 species is an essential step in HNO-releasing chemistry. − Subsequent protonation of the Fe-bound NO species carrying a more likely NO – character would yield the formation of HNO. − However, protonation of the {FeNO} 8 species by adventitious proton sources frequently results in the formation of corresponding {FeNO} 7 species and H 2 . − …”