Characterization of a High‐Spin Non‐Heme {FeNO}⁸ Complex: Implications for the Reactivity of Iron Nitroxyl Species in Biology

Abstract

Die chemische und elektrochemische Reduktion eines fünffach koordinierten High‐Spin‐Nichthäm‐{FeNO}7‐Komplexes (siehe Struktur: N blau, Fe orange, und O rot) führte zum ersten stabilen High‐Spin(S=1)‐Nichthäm‐{FeNO}8‐Modellkomplex. Der Befund, dass die Reduktion metallzentriert abläuft und eine Verringerung der Fe‐NO‐Kovalenz bewirkt, kann als ein Indiz gelten, dass Nichthäm‐Fe‐NO‐Einheiten in biologischen Systemen durch eine Reduktion für Folgereaktionen aktiviert werden können.

A Series of Iron Nitrosyl Complexes {Fe–NO}6–9 and a Fleeting {Fe–NO}10 Intermediate en Route to a Metalacyclic Iron Nitrosoalkane

“…15,50 Noteworthy, all observed NO stretching frequencies of 2, 3, and 4 are 40−100 cm −1 higher than most reported Fe−NO complexes with the corresponding number of valence electrons in its unit. 5,30,51,52 The successful reduction of the {Fe−NO} 9 (4) to the nitrosoalkane complex 5 also was confirmed by IR vibrational spectroscopy. The intense NO stretching band νÑ O = 1550 cm −1 of 4 is shifted to 1358 cm −1 upon reduction to 5.…”

“…This spectral range (5.20 bis 2.52) is further dominated by three broad singlets of the three magnetically inequivalent methylene groups of the [Cs(crypt-222)] + cation (3.57, 3.48, and 2.53 ppm; 12 H each). In the aromatic region, six singlets (6.85 to 5,6,30 This πbackbonding, in turn, causes shortening of the Fe−N bond, as determined by XRD analyses. The direct occupation of the π* orbitals through ligand-based reduction would lead to a stronger than observed bathochromic NO band shift of about 300 cm −1 .…”

“…This unusual stability of 3 is remarkable for an {Fe−NO} 8 species. 16,30 Access to even higher reduced forms of the [(TIMEN Mes )Fe(NO)] 1); regardless of the NO oxidation state. This linear Fe−NO coordination mode is typically found for complexes with a NO + (nitrosonium) ligand (see {Fe−NO} 6 complex 1) and often lost upon reduction (and formation of nitroxyl, NO − ).…”

Mechanism of N–N Bond Formation by Transition Metal–Nitrosyl Complexes: Modeling Flavodiiron Nitric Oxide Reductases

“…47,48 Correspondingly, the formally NO − ligand does not chemically behave like a nitroxyl group and is generally not very reactive; that is, it cannot be protonated. 49 On the basis of a recent study on [Fe(TMG 3 tren)(NO)] 2+/1+ , the one-electron reduced nonheme {FeNO} 8 complex is best described as a high-spin Fe(II) with a bound triplet NO − , where the spins are again antiferromagnetically coupled to give an S = 1 ground state. 49,50 This one-electron reduction leads to a distinct decrease in the covalency of the Fe−NO bond and a general activation of the Fe−NO unit for chemical reactivity.…”

“…49 On the basis of a recent study on [Fe(TMG 3 tren)(NO)] 2+/1+ , the one-electron reduced nonheme {FeNO} 8 complex is best described as a high-spin Fe(II) with a bound triplet NO − , where the spins are again antiferromagnetically coupled to give an S = 1 ground state. 49,50 This one-electron reduction leads to a distinct decrease in the covalency of the Fe−NO bond and a general activation of the Fe−NO unit for chemical reactivity. Although it seems counterintuitive that two NO − units, which should electrostatically repel one another, can couple to make a N−N bond, the experimental data on FNORs described above conclude that this is in fact the case (by either the di-{FeNO} 7 or sr mechanism).…”