A Triad of Highly Reduced,Linear Iron Nitrosyl Complexes: {FeNO}8–10

Given the importance of Fe–NO complexes in both human biology and the global nitrogen cycle, there has been interest in understanding their diverse electronic structures. Herein a redox series of isolable iron nitrosyl complexes stabilized by a tris(phosphine)borane (TPB) ligand is described. These structurally characterized iron nitrosyl complexes reside in the following highly reduced Enemark–Feltham numbers: {FeNO}⁸, {FeNO}⁹, and {FeNO}¹⁰. These {FeNO}^8–10 compounds are each low‐spin, and feature linear yet strongly activated nitric oxide ligands. Use of Mössbauer, EPR, NMR, UV/Vis, and IR spectroscopy, in conjunction with DFT calculations, provides insight into the electronic structures of this uncommon redox series of iron nitrosyl complexes. In particular, the data collectively suggest that {TPBFeNO}^8–10 are all remarkably covalent. This covalency is likely responsible for the stability of this system across three highly reduced redox states that correlate with unusually high Enemark–Feltham numbers.     

Stable Salts of Heteroleptic Iron Carbonyl/Nitrosyl Cations

The oxidation of Fe(CO)₅ with the [NO]⁺ salt of the weakly coordinating perfluoroalkoxyaluminate anion [F‐{Al(OR^F)₃}₂]^? (R^F=C(CF₃)₃) leads to stable salts of the 18 valence electron (VE) species [Fe(CO)₄(NO)]⁺ and [Fe(CO)(NO)₃]⁺ with the Enemark–Feltham numbers of {FeNO}⁸ and {FeNO}¹⁰. This finally concludes the triad of heteroleptic iron carbonyl/nitrosyl complexes, since the first discovery of the anionic ([Fe(CO)₃(NO)]^?) and neutral ([Fe(CO)₂(NO)₂]) species over 80 years ago. Both complexes were fully characterized (IR, Raman, NMR, UV/Vis, scXRD, pXRD) and are stable at room temperature under inert conditions over months and may serve as useful starting materials for further investigations.     

Structural and Spectroscopic Characterization of a High‐Spin {FeNO}6 Complex with an Iron(IV)−NO− Electronic Structure

Although the interaction of low‐spin ferric complexes with nitric oxide has been well studied, examples of stable high‐spin ferric nitrosyls (such as those that could be expected to form at typical non‐heme iron sites in biology) are extremely rare. Using the TMG₃tren co‐ligand, we have prepared a high‐spin ferric NO adduct ({FeNO}⁶ complex) via electrochemical or chemical oxidation of the corresponding high‐spin ferrous NO {FeNO}⁷ complex. The {FeNO}⁶ compound is characterized by UV/Visible and IR spectroelectrochemistry, Mössbauer and NMR spectroscopy, X‐ray crystallography, and DFT calculations. The data show that its electronic structure is best described as a high‐spin iron(IV) center bound to a triplet NO^? ligand with a very covalent iron?NO bond. This finding demonstrates that this high‐spin iron nitrosyl compound undergoes iron‐centered redox chemistry, leading to fundamentally different properties than corresponding low‐spin compounds, which undergo NO‐centered redox transformations.     

A Nonheme Sulfur‐Ligated {FeNO}6 Complex and Comparison with Redox‐Interconvertible {FeNO}7 and {FeNO}8 Analogues

A nonheme {FeNO}⁶ complex, [Fe(NO)(N3PyS)]²⁺, was synthesized by reversible, one‐electron oxidation of an {FeNO}⁷ analogue. This complex completes the first known series of sulfur‐ligated {FeNO}^6–8 complexes. All three {FeNO}^6–8 complexes are readily interconverted by one‐electron oxidation/reduction. A comparison of spectroscopic data (UV/Vis, NMR, IR, Mössbauer, X‐ray absorption) provides a complete picture of the electronic and structural changes that occur upon {FeNO}⁶–{FeNO}⁸ interconversion. Dissociation of NO from the new {FeNO}⁶ complex is shown to be controlled by solvent, temperature, and photolysis, which is rare for a sulfur‐ligated {FeNO}⁶ species.     

The Fe2(NO)2 Diamond Core: A Unique Structural Motif In Non‐Heme Iron–NO Chemistry

Non‐heme high‐spin (hs) {FeNO}⁸ complexes have been proposed as important intermediates towards N₂O formation in flavodiiron NO reductases (FNORs). Many hs‐{FeNO}⁸ complexes disproportionate by forming dinitrosyl iron complexes (DNICs), but the mechanism of this reaction is not understood. While investigating this process, we isolated a new type of non‐heme iron nitrosyl complex that is stabilized by an unexpected spin‐state change. Upon reduction of the hs‐{FeNO}⁷ complex, [Fe(TPA)(NO)(OTf)](OTf) ( 1 ), the N‐O stretching band vanishes, but no sign of DNIC or N₂O formation is observed. Instead, the dimer, [Fe₂(TPA)₂(NO)₂](OTf)₂ ( 2 ) could be isolated and structurally characterized. We propose that 2 is formed from dimerization of the hs‐{FeNO}⁸ intermediate, followed by a spin state change of the iron centers to low‐spin (ls), and speculate that 2 models intermediates in hs‐{FeNO}⁸ complexes that precede the disproportionation reaction.     

{FeNO}7-Type Halogenido Nitrosyl Ferrates: Syntheses,Bonding, and Photoinduced Linkage Isomerism

Mononitrosyl–iron compounds (MNICs) of the Enemark–Feltham {FeNO}⁷ type can be divided into a doublet (S=1/2) and a quartet (S=3/2) spin variant. The latter relies on weak-field co-ligands such as amine carboxylates. Aqua-only co-ligation appears to exist in the long-known “brown-ring” [Fe(H₂O)₅(NO)]²⁺ cation, which was prepared originally from ferrous salts and NO in sulfuric acid. A chloride variant of this species, the green [FeCl₃(NO)]⁻ ion, was first prepared analoguosly by using hydrochloric instead of sulfuric acid. As a tetrahedral species, it is the simple prototype of sulfur-bonded {FeNO}⁷ (S=3/2) MNICs of biological significance. Although it has been investigated for more than a century, neither clean preparative routes nor reliable structural parameters were available for the [FeCl₃(NO)]⁻ ion and related species such as the [FeCl₂(NO)₂]⁻ ion, a prototypical dinitrosyliron species (a “DNIC”). In this work, both issues have been resolved. In addition, we report on a computational study on the ground- and excited-state properties including an assignment of the chromophoric transitions. Photoinduced metastable isomers were characterised in a combined experimental and computational approach that resulted in the confirmation of a single photoinduced linkage isomer of the paramagnetic nitrosyl–metal coordination entity.     

Nitrous‐Acid‐Mediated Synthesis of Iron–Nitrosyl–Porphyrin: pH‐Dependent Release of Nitric Oxide

Two iron–nitrosyl–porphyrins, nitrosyl[meso‐tetrakis(3,4,5‐trimethoxyphenylporphyrin]iron(II) acetic acid solvate ( 3 ) and nitrosyl[meso‐tetrakis(4‐methoxyphenylporphyrin]iron(II) CH₂Cl₂ solvate ( 4 ), were synthesized in quantitative yield by using a modified procedure with nitrous acid, followed by oxygen‐atom abstraction by triphenylphosphine under an argon atmosphere. These nitrosyl porphyrins are in the {FeNO}⁷ class. Under an argon atmosphere, these compounds are relatively stable over a broad range of pH values (4–8) but, under aerobic conditions, they release nitric oxide faster at high pH values than that at low pH values. The generated nitric‐oxide‐free iron(III)–porphyrin can be re‐nitrosylated by using nitrous acid and triphenylphosphine. The rapid release of NO from these Fe^II complexes at high pH values seems to be similar to that in nitrophorin, a nitric‐oxide‐transport protein, which formally possesses Fe^III. However, because the release of NO occurs from ferrous–nitrosyl–porphyrin under aerobic conditions, these compounds are more closely related to nitrobindin, a recently discovered heme protein.     

Stepwise nitrosylation of the nonheme iron site in an engineered azurin and a molecular basis for nitric oxide signaling mediated by nonheme iron proteins

Mononitrosyl and dinitrosyl iron species, such as {FeNO}⁷, {FeNO}⁸ and {Fe(NO)₂}⁹, have been proposed to play pivotal roles in the nitrosylation processes of nonheme iron centers in biological systems. Despite their importance, it has been difficult to capture and characterize them in the same scaffold of either native enzymes or their synthetic analogs due to the distinct structural requirements of the three species, using redox reagents compatible with biomolecules under physiological conditions. Here, we report the realization of stepwise nitrosylation of a mononuclear nonheme iron site in an engineered azurin under such conditions. Through tuning the number of nitric oxide equivalents and reaction time, controlled formation of {FeNO}⁷ and {Fe(NO)₂}⁹ species was achieved, and the elusive {FeNO}⁸ species was inferred by EPR spectroscopy and observed by Mössbauer spectroscopy, with complemental evidence for the conversion of {FeNO}⁷ to {Fe(NO)₂}⁹ species by UV-Vis, resonance Raman and FT-IR spectroscopies. The entire pathway of the nitrosylation process, Fe(ii) → {FeNO}⁷ → {FeNO}⁸ → {Fe(NO)₂}⁹, has been elucidated within the same protein scaffold based on spectroscopic characterization and DFT calculations. These results not only enhance the understanding of the dinitrosyl iron complex formation process, but also shed light on the physiological roles of nitric oxide signaling mediated by nonheme iron proteins.

Stepwise nitrosylation from Fe(ii) to {FeNO}⁷, {FeNO}⁸ and then to {Fe(NO)₂}⁹ is reported for the first time in the same protein scaffold, providing deeper understanding of the detailed mechanism of dinitrosyl iron complex formation.     

X‐Ray Emission Spectroscopy: A Spectroscopic Measure for the Determination of NO Oxidation States in Fe–NO Complexes

Extensive study of the electronic structure of Fe‐NO complexes using a variety of spectroscopic methods was attempted to understand how iron controls the binding and release of nitric oxide. The comparable energy levels of NO π* orbitals and Fe 3d orbitals complicate the bonding interaction within Fe? NO complexes and puzzle the quantitative assignment of NO oxidation state. Enemark–Feltham notation, {Fe(NO)_x}ⁿ, was devised to circumvent this puzzle. This 40‐year puzzle is revisited using valence‐to‐core X‐ray emission spectroscopy (V2C XES) in combination with computational study. DFT calculation establishes a linear relationship between ΔE_σ2s*‐σ2p of NO and its oxidation state. V2C Fe XES study of Fe? NO complexes reveals the ΔE_σ2s*‐σ2p of NO derived from NO σ_2s*/σ_2p→Fe_1s transitions and determines NO oxidation state in Fe? NO complexes. Quantitative assignment of NO oxidation state will correlate the feasible redox process of nitric oxide and Fe‐nitrosylation biology.     

One Electron Makes Differences: From Heme {FeNO}7 to {FeNO}8

The first X‐ray single‐crystal structure of a {FeNO}⁸ porphyrin complex [Co(Cp)₂][Fe(TFPPBr₈)(NO)], and the structure of the {FeNO}⁷ precursor [Fe(TFPPBr₈)(NO)] are determined at 100 K. The two complexes are also characterized by FTIR and UV/Vis spectroscopy. [Fe(TFPPBr₈)(NO)]^? shows distinct structural features in contrast to a nitrosyl iron(II) porphyrinate on the Fe? N? O^? moiety, which include a much more bent Fe? N? O^? angle (122.4(3)°), considerably longer Fe? NO^? (1.814(4)) and N? O^? (1.194(5) Å) bond distances. These and the about 180 cm^?1 downshift ν_N‐O stretch (1540 cm^?1) can be understood by the covalently bonding nature between the iron(II) and the NO^? ligand which possesses a two‐electron‐occupied π* orbital as a result of the reduction. The overall structural features of [Fe(TFPPBr₈)(NO)]^? and [Fe(TFPPBr₈)(NO)] suggest a low‐spin state of the iron(II) atom at 100 K.     

Spectroscopic and Computational Study of a Nonheme Iron Nitrosyl Center in a Biosynthetic Model of Nitric Oxide Reductase

A major barrier to understanding the mechanism of nitric oxide reductases (NORs) is the lack of a selective probe of NO binding to the nonheme Fe_B center. By replacing the heme in a biosynthetic model of NORs, which structurally and functionally mimics NORs, with isostructural ZnPP, the electronic structure and functional properties of the Fe_B nitrosyl complex was probed. This approach allowed observation of the first S=3/2 nonheme {FeNO}⁷ complex in a protein‐based model system of NOR. Detailed spectroscopic and computational studies show that the electronic state of the {FeNO}⁷ complex is best described as a high spin ferrous iron (S=2) antiferromagnetically coupled to an NO radical (S= 1/2) [Fe²⁺‐NO^.]. The radical nature of the Fe_B‐bound NO would facilitate N? N bond formation by radical coupling with the heme‐bound NO. This finding, therefore, supports the proposed trans mechanism of NO reduction by NORs.     

Electronic Structure Modulation in an Exceptionally Stable Non‐Heme Nitrosyl Iron(II) Spin‐Crossover Complex

The highly stable nitrosyl iron(II) mononuclear complex [Fe(bztpen)(NO)](PF₆)₂ (bztpen=N‐benzyl‐N,N′,N′‐tris(2‐pyridylmethyl)ethylenediamine) displays an S=1/2?S=3/2 spin crossover (SCO) behavior (T_1/2=370 K, ΔH=12.48 kJ mol^?1, ΔS=33 J K^?1 mol^?1) stemming from strong magnetic coupling between the NO radical (S=1/2) and thermally interconverted (S=0?S=2) ferrous spin states. The crystal structure of this robust complex has been investigated in the temperature range 120–420 K affording a detailed picture of how the electronic distribution of the t_2g–e_g orbitals modulates the structure of the {FeNO}⁷ bond, providing valuable magneto–structural and spectroscopic correlations and DFT analysis.     

Electronic structure of dinuclear iron nitrosyl complexes with different ligands at two iron centers

The electronic structures of three dinuclear iron complexes were determined with the DFT method. The complexes contain a {Fe(NO)₂}⁹ unit and thiolate, nitrosyl, carbonyl and amine ligands at the second iron atom. The two iron atoms are bridged by thiolate ligands. In the lowest energy states of these complexes, the iron atoms possess spin S = 1, 3/2 or 5/2, depending on the coordinated ligands and their mutual arrangement. Nitrosyl is coordinated as NO⁻ antiferromagnetically coupled to iron, and the two iron units are antiferromagnetically coupled to each other.     

EPR-Untersuchungen an Nitrosyl-Eisen-Chelaten des Typs [FeNO(S,S)(P,P)]

EPR Investigations on Nitrosyl Iron Chelates of the Type [FeNO(S, S)(P, P)] The EPR powder spectra of [FeNO(mnt)(diphos)] and {FeNO(mnt)[P(C₆H₅)₃]₂} (mnt = maleonitriledithiolate, diphos = tetraphenylethylenediphosphine), diamagnetically diluted in the corresponding cobalt chelates, are reported. The nitrosyl iron chelate {FeNO(mnt)[P(C₆H₅)₃]₂} could, However, be only isolated in the host chelate. According to the EPR parameters obtained for [FeNO(mnt)(diphos)] the MO of the unpaired electron consists of the metal d_z2 and the corresponding ligand orbitals. The g tensor components and the ligand hyperfine structure coupling constants found for {FeNO(mnt)[P(C₆H₅)₃]₂} are more in agreement with an MO for the unpaired electron consisting of the metal d_yz and the corresponding ligand orbitals. Large structural deviations are responsible for this difference.     

Electronic structure and bonding of {Fe(PhNO2)}6 complexes: a density functional theory study

Reduction of nitro-aromatic compounds (NACs) proceeds through intermediates with a partial electron transfer into the nitro group from a reducing agent. To estimate the extent of such a transfer and, therefore, the activity of various model ferrous-containing reductants toward NAC degradation, the unrestricted density functional theory (DFT) in the basis of paired L?wdin-Amos-Hall orbitals has been applied to complexes of nitrobenzene (NB) and model Fe(II) hydroxides including cationic [FeOH]+, then neutral Fe(OH)2, and finally anionic [Fe(OH)3]-. Electron transfer is considered to be a process of unpairing electrons (without the change of total spin projection Sz) that reveals itself in a substantial spin contamination of the unrestricted solution. The unrestricted orbitals are transformed into localized paired orbitals to determine the orbital channels for a particular electron-transfer state and the weights of idealized charge-transfer and covalent electron structures. This approach allows insight into the electronic structure and bonding of the {Fe(PhNO2)}6 unit (according to Enemark and Feltham notation) to be gained using model nitrobenzene complexes. The electronic structure of this unit can be expressed in terms of pi-type covalent bonding [Fe+2(d6, S = 2) - PhNO2(S = 0)] or charge-transfer configuration [Fe+3(d5, S = 5/2) - {PhNO2}- ((pi*)1, S = 1/2)].     

Heterobimetallic Mn/Co hybrid complexes composed of proximate organometallic and classical coordination sites

A series of highly unsymmetric heterobinuclear Mn/Co complexes is reported, in which an organometallic CpMn(CO)₂ fragment and a classical Werner-type cobalt(II) subunit are arranged in close proximity by means of a bridging pyrazolate. Two ligand scaffolds are employed that differ by the chelate size of the tripodal tetradentate {N4} binding site for cobalt. Molecular structures of three complexes with either nitrate or acetate coligands have been characterized by X-ray crystallography. IR and UV-Vis-spectroelectrochemistry reveals that oxidation of the heterobimetallic systems is highly localized at the organometallic manganese site, while electrochemical reduction occurs at cobalt. Structural and spectroscopic features as well as trends for the redox potentials of the Mn^I/Mn^II couple suggest that changes at the cobalt(II) Werner-type subunit have only minor effects on the properties of the organometallic site.     

Effective Control of the Electron-donating Ability of Phosphines by using Phosphazenyl and Phosphoniumylidyl Substituents

Phosphoniumylidyl and phosphazenyl groups are effective substituents to increase the electron-donating ability of tertiary phosphines. However, the influence of structural variations among those substituents on the electronic properties of the phosphines is little explored. Herein, we show that protonation of the ylidic carbon atom of phosphoniumylidyl phosphines increases the Tolman electronic parameter (TEP) by ΔTEP = 16.0–18.8 cm^–1. Furthermore, phosphazenyl phosphines were synthesized with isopropyl groups (NP{iPr}₃) and tetramethylguanidino groups (NP{tmg}₃) at the phosphonium center. Determination of their TEP values reveals a remarkable low substituent parameter of χ = –18.5 cm^–1 for the NP(tmg)₃ group. In addition, we prepared the corresponding gold(I) complexes and determined their solid-state structures using single-crystal X-ray diffraction studies to analyze the steric profile of the new phosphine ligands.     

Specific Interaction of Nitrogen(II) Oxide with Sublimed Layers of Iron(II) meso-Mono-4-Pyridyltriphenylporphyrinate

IR studies revealed that unlike iron(II) meso-tetraphenylporphyrinate (FeTPP), NO interacts with sublimed layers of iron(II) meso-mono-4-pyridyltriphenylporphyrinate (FeM4PyTPP) to give two types of nitrosyl complexes. The stretching vibration frequency of the coordinated NO in one of them is close to that observed in the nitrosyl FeTPP complex, while in the other complex, it is 28 cm^–1lower. The spectral differences observed upon coordination of NO by two porphyrins with similar structures are explained by the fact that in the sublimed FeM4PyTPP layers, the structures are formed in which the pyridyl group of one molecule is coordinated by the metal ion of the neighboring molecule, while the nitrosyl ligand occupies the sixth coordination site. The thermal stability of the complexes formed, the effect of the extra ligands on the layer structure, and the mechanism of the nitrosyl ligand exchange for its isotopic analog ¹⁵NO are discussed.     

Synthese und Eigenschaften von (Acido)(nitrosyl)phthalocyaninato(2–)ruthenium

Synthesis and Properties of (Acido)(nitrosyl)phthalocyaninato(2–)ruthenium (Acido)(nitrosyl)phthalocyaninato(2–)ruthenium, [Ru(X)(NO)pc^2–] (X = F, Cl, Br, I, CN, NCO, NCS, NCSe, N₃, NO₂) is obtained by acidification of a solution of bis(tetra(n-butyl)ammonium) bis(nitro)phthalocyaninato(2–)ruthenate(II) in tetrahydrofurane with the corresponding conc. mineral acid or aqueous ammonium salt solution. The nitrite-nitrosyl conversion is reversal in basic media. The cyclic and differential pulse voltammograms show mainly three quasi-reversible one-electron processes at 1.05, –0.65 and –1.25 V, ascribed to the first ring oxidation and the stepwise reduction to the complexes of type {RuNO}⁷ and {RuNO}⁸, respectively. The B < Q < N regions in the electronic absorption spectra are still typical for the pc^2– ligand, but are each split into two strong absorptions (14500/16500(B); 28000/30500(Q); 34500/37000 cm^–1(N)), whose relative intensities strongly depend on the nature of the axial ligand X. In the IR spectra is active the N–O stretching vibration between 1827 (X = I) and 1856 cm^–1 (F), the C–N stretching vibration at 2178 (X = NCO), 2072 (NCS), 2066 (NCSe), 2093 cm^–1 (CN), the N–N stretching vibration of the azide ligand at 2045 cm^–1, the fundamentals of the nitrito(O) ligand at 1501, 932, and 804 cm^–1, and the Ru–X stretching vibration at 483 (F), 332 (Cl), 225 (Br), 183 (I), 395 (N₃), 364 (ONO), 403 (CN), 263 (NCS), and 231 cm^–1 (NCSe). In the resonance Raman spectra, excited in coincidence with the B region, the Ru–NO stretching vibration and the very intense Ru–N–O deformation vibration are selectively enhanced between 580 and 618 cm^–1, and between 556 and 585 cm^–1, respectively.