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.     

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.     

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.     

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.     

Transformation of the {Fe(NO)2}9 dinitrosyl iron complexes (DNICs) into S-nitrosothiols (RSNOs) triggered by acid-base pairs

S‐Nitrosation of the coordinated thiolate of dinitrosyl iron complexes (DNICs) to generate S‐nitrosothiols (RSNOs) was demonstrated. Transformation of [{(NO)₂Fe(μ‐StBu)}₂] ( 1‐tBuS ) into the {Fe(NO)₂}⁹ DNIC [(NO)₂Fe(StBu)(MeIm)] ( 2‐MeIm ) occurs under addition of 20 equiv of 1‐methylimidazole (MeIm) into a solution of 1‐tBuS in THF. The dynamic interconversion between {Fe(NO)₂}⁹ [(NO)₂Fe(S‐NAP)(dmso)] ( 2‐dmso ) (NAP=N‐acetyl‐D ‐penicillamine) and [{(NO)₂Fe(μ‐S‐NAP)}₂] ( 1‐NAP ) was also observed in a solution of complex 1‐NAP in DMSO. In contrast to the reaction of complex 2‐MeIm and bis(dimethylthiocarbamoyl) disulfide ((DTC)₂) to yield {Fe(NO)}⁷ [(NO)Fe(DTC)₂] ( 3 ) (DTC=S₂CNMe₂) accompanied by (tBuS)₂ and NO_(g), transformation of {Fe(NO)₂}⁹ 2‐MeIm ( 2‐dmso ) into RSNOs (RS=tBuS, NAP‐S) along with complex 3 induced by the Brønsted acid solution of (DTC)₂ demonstrated that Brønsted acid may play a critical role in triggering S‐nitrosation of the coordinated thiolate of DNICs 2‐MeIm (or 2‐dmso ) to produce RSNOs. That is, DNIC‐mediated S‐nitrosation requires a Brønsted acid–Lewis base pair to produce RSNO. Transformation of DNICs into RSNOs may only occur on the one‐thiolate‐containing {Fe(NO)₂}⁹ DNICs, in contrast to protonation of the two‐thiolate‐containing DNICs [(NO)₂Fe(SR)₂]^? by Brønsted acid to yield [{(NO)₂Fe(μ‐SR)}₂]. These results might rationalize that the known protein‐Cys‐SNO sites derived from DNICs were located adjacent to acid and base motifs, and no protein‐bound SNO characterized to date has been directly derived from [protein–(cysteine)₂Fe(NO)₂] in 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.     

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.     

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.     

{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.     

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.     

Nonheme Iron Mediated Oxidation of Light Alkanes with Oxone: Characterization of Reactive Oxoiron(IV) Ligand Cation Radical Intermediates by Spectroscopic Studies and DFT Calculations

The oxidation of light alkanes that is catalyzed by heme and nonheme iron enzymes is widely proposed to involve highly reactive {Fe^V?O} species or {Fe^IV?O} ligand cation radicals. The identification of these high‐valent iron species and the development of an iron‐catalyzed oxidation of light alkanes under mild conditions are of vital importance. Herein, a combination of tridentate and bidentate ligands was used for the generation of highly reactive nonheme {Fe?O} species. A method that employs [Fe^III(Me₃tacn)(Cl‐acac)Cl]⁺ as a catalyst in the presence of oxone was developed for the oxidation of hydrocarbons, including cyclohexane, propane, and ethane (Me₃tacn=1,4,7‐trimethyl‐1,4,7‐triazacyclononane; Cl‐acac=3‐chloro‐acetylacetonate). The complex [Fe^III(Tp)₂]⁺ and oxone enabled stoichiometric oxidation of propane and ethane. ESI‐MS, EPR and UV/Vis spectroscopy, ¹⁸O labeling experiments, and DFT studies point to [Fe^IV(Me₃tacn)({Cl‐acac}^.+)(O)]²⁺ as the catalytically active species.     

Molecular and crystal structure of a cationic dinitrosyl iron complex with 1,3-dimethylthiourea

A new dinitrosyl iron complex of the composition [Fe(SC(NHCH₃)₂)₂(NO)₂]Cl (I) is obtained by direct nitrosylation of ferrous sulfate and a hydrochloric acid solution of 1,3-dimethylthiourea. The characteristic features of the molecular and crystal structure of complex I is determined by single crystal X-ray diffraction analysis.     

Electronic structure and FeNO conformation of nonheme iron-thiolate-NO complexes: an experimental and DFT study

Reactions of NO and CO with Fe(II) complexes of the tripodal trithiolate ligands NS3 and PS3* yield trigonal-bipyramidal (TBP) complexes with varying redox states and reactivity patterns with respect to dissociation of the diatomic ligand. The previously reported four-coordinate [Fe(II)(NS3)](-) complex reacts irreversibly with NO gas to yield the S = 3/2 {FeNO}(7) [Fe(NS3)(NO)](-) anion, isolated as the Me(4)N(+) salt. In contrast, the reaction of NO with the species generated by the reaction of FeCl(2) with Li(3)PS3* gives a high yield of the neutral, TBP, S = 1 complex, [Fe(PS3*)(NO)], the first example of a paramagnetic {FeNO}(6) complex. X-ray crystallographic analyses show that both [Fe(NS3)(NO)](-) and [Fe(PS3*)(NO)] feature short Fe-N(NO) distances, 1.756(6) and 1.676(3) A, respectively. However, whereas [Fe(NS3)(NO)]- exhibits a distinctly bent FeNO angle and a chiral pinwheel conformation of the NS3 ligand, [Fe(PS3*)(NO)] has nearly C(3v) local symmetry and a linear FeNO unit. The S = 1 [Fe(II)(PS3)L] complexes, where L = 1-MeIm, CN(-), CO, and NO(+), exhibit a pronounced lengthening of the Fe-P distances along the series, the values being 2.101(2), 2.142(1), 2.165(7), and 2.240(1) A, respectively. This order correlates with the pi-backbonding ability of the fifth ligand L. The cyclic voltammogram of the [Fe(NS3)(NO)](-) anion shows an irreversible oxidation at +0.394 V (vs SCE), apparently with loss of NO, when scanned anodically in DMF. In contrast, [Fe(PS3*)(NO)] exhibits a reversible {FeNO}(6)/{FeNO}(7) couple at a low potential of -0.127 V. Qualitatively consistent with these electrochemical findings, DFT (PW91/STO-TZP) calculations predict a substantially lower gas-phase adiabatic ionization potential for the [Fe(PS3)(NO)](-) anion (2.06 eV) than for [Fe(NS3)(NO)](-) (2.55 eV). The greater instability of the {FeNO}(7) state with the PS3* ligand results from a stronger antibonding interaction involving the metal d(z(2)) orbital and the phosphine lone pair than the analogous orbital interaction in the NS3 case. The antibonding interaction involving the NS3 amine lone pair affords a relatively "stereochemically active" dz2 electron, the z direction being roughly along the Fe-N(NO) vector. As a result, the {FeNO}(7) unit is substantially bent. By contrast, the lack of a trans ligand in [Fe(S(t)Bu)3(NO)](-), a rare example of a tetrahedral {FeNO}(7) complex, results in a "stereochemically inactive" d(z(2)) orbital and an essentially linear FeNO unit.     

Dinitrosyl iron complexes (DNICs) containing S/N/O ligation: transformation of Roussin's red ester into the neutral {Fe(NO)2}10 DNICs

Addition of the Lewis base [OPh]- to the THF solution of Roussin's red ester [Fe(mu-SC6H4-o-NHCOPh)(NO)2]2 (1) and [Fe(mu-SC6H4-o-COOH)(NO)2]2 (2), respectively, yielded the EPR-active, anionic {Fe(NO)2}9, [(SC6H4-o-NCOPh)Fe(NO)2]- (3) with the anionic [SC6H4-o-NCOPh]2- ligand bound to the {Fe(NO)2} core in a bidentate manner (S,N-bonded) and [(SC6H4-o-COO)Fe(NO)2]- (4) with the anionic [SC6H4-o-COO]2- ligand bound to the {Fe(NO)2} core in a bidentate manner (S,O-bonded), characterized by IR, UV-vis, EPR, and single-crystal X-ray diffraction. In contrast to the bridged-thiolate cleavage yielding the neutral {Fe(NO)2}9, [(SC6H4-o-NHCOPh)(Im)Fe(NO)2] (Im=imidazole), by addition of 2 equiv of imidazole to complex 1 observed in the previous study, the addition of the stronger sigma-donating and pi-accepting PPh3 ligand triggered the reductive elimination of bridged thiolates of complex 1 to yield the neutral {Fe(NO)2}10, [(PPh3)2Fe(NO)2]. These results unambiguously illustrate one aspect of how the nucleophile L (L=imidazole, PPh3, [OPh]-) functions to control the reaction pathways (bridged-thiolate cleavage, reductive elimination, and deprotonation) upon the reaction of complex 1 and the nucleophile L. The EPR-active, dimeric {Fe(NO)2}9 dinitrosyl iron complex (DNIC) [Fe(mu-SC7H4SN)(NO)2]2 (6), with S and N atoms of the anionic [-SC7H4SN-]- (2-benzothiozolyl thiolate) ligands bound to two separate {Fe(NO)2}9 cores, was also synthesized from reaction of bis(2-benzothiozolyl) disulfide and [(NO)2Fe(PPh3)2]. A straightforward reaction of complex 6 and 4 equiv of [N3]- conducted in THF led to the anionic {Fe(NO)2}9, [(N3)2Fe(NO)2]- (7). Conclusively, the EPR-active, {Fe(NO)2}9 DNICs can be classified into the anionic {Fe(NO)2}9 DNICs with S/N/O ligation, the neutral {Fe(NO)2}9 DNIC with one thiolate and one neutral imidazole ligation, and the cationic {Fe(NO)2}9 DNICs with the neutral N-/P-containing coordinated ligands.     

Mononitrosyl tris(thiolate) iron complex [Fe(NO)(SPh)3]- and dinitrosyl iron complex [(EtS)2Fe(NO)2]-: formation pathway of dinitrosyl iron complexes (DNICs) from nitrosylation of biomimetic rubredoxin [Fe(SR)4]2-/1- (R = Ph, Et)

Nitrosylation of the biomimetic reduced- and oxidized-form rubredoxin [Fe(SR)4]2-/1- (R = Ph, Et) in a 1:1 stoichiometry led to the formation of the extremely air- and light-sensitive mononitrosyl tris(thiolate) iron complexes (MNICs) [Fe(NO)(SR)3]- along with byproducts [SR]- or (RS)2. Transformation of [Fe(NO)(SR)3]- into dinitrosyl iron complexes (DNICs) [(RS)2Fe(NO)2]- and Roussin's red ester [Fe2(mu-SR)2(NO)4] occurs rapidly under addition of 1 equiv of NO(g) and [NO]+, respectively. Obviously, the mononitrosyl tris(thiolate) complex [Fe(NO)(SR)3]- acts as an intermediate when the biomimetic oxidized- and reduced-form rubredoxin [Fe(SR)4]2-/1- exposed to NO(g) were modified to form dinitrosyl iron complexes [(RS)2Fe(NO)2]-. Presumably, NO binding to the electron-deficient [Fe(III)(SR)4]- and [Fe(III)(NO)(SR)3]- complexes triggers reductive elimination of dialkyl/diphenyl disulfide, while binding of NO radical to the reduced-form [Fe(II)(SR)4]2- induces the thiolate-ligand elimination. Protonation of [Fe(NO)(SEt)3]- yielding [Fe(NO)(SPh)3]- by adding 3 equiv of thiophenol and transformation of [Fe(NO)(SPh)3]- to [Fe(NO)(SEt)3]- in the presence of 3 equiv of [SEt]-, respectively, demonstrated that complexes [Fe(NO)(SPh)3]- and [Fe(NO)(SEt)3]- are chemically interconvertible. Mononitrosyl tris(thiolate) iron complex [Fe(NO)(SPh)3]- and dinitrosyl iron complex [(EtS)2Fe(NO)2]- were isolated and characterized by X-ray diffraction. The mean NO bond distances of 1.181(7) A (or 1.191(7) A) in complex [(EtS)2Fe(NO)2]- are nearly at the upper end of the 1.178(3)-1.160(6) A for the anionic {Fe(NO)2}9 DNICs, while the mean FeN(O) distances of 1.674(6) A (or 1.679(6) A) exactly fall in the range of 1.695(3)-1.661(4) A for the anionic {Fe(NO)2}9 DNICs.     

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.     

Interaction of nitric oxide with tetrathiolato iron(II) complexes: relevance to the reaction pathways of iron nitrosyls in sulfur-rich biological coordination environments

The mechanism of formation of dinitrosyl iron complexes (DNICs) coordinated by cysteine residues at iron-sulfur protein sites has received little attention in the chemical literature. As a logical first step toward elucidating this mechanism and characterizing new iron-nitrosyl intermediates, we investigated the interaction of NO (g) and NO+ with iron-sulfur complexes chosen to mimic sulfur-rich iron sites in biology. The reaction of NO (g) with [Fe(StBu)4]2- cleanly affords the mononitrosyl complex, [Fe(StBu)3(NO)]- (1), a previously unknown species evoked in this chemistry. Reaction of [Fe(StBu)4]2- with NO derivatives, such as NO+, yields the corresponding dinitrosyl S-bridged Roussin red ester [Fe2(mu-StBu)2(NO)4] (2). The nitrosyl complexes 1 and 2 can chemically convert to the DNIC, [Fe(StBu)2(NO)2]- (3). The results should aid in the spectroscopic identification and elucidation of reaction pathways for the nitrosylation of iron in biologically related sulfur-rich coordination environments.     

Chelate‐Thiolate‐Coordinate Ligands Modulating the Configuration and Electrochemical Property of Dinitrosyliron Complexes (DNICs)

As opposed to the reversible redox reaction ({Fe(NO)₂}¹⁰ reduced‐form DNIC [(NO)₂Fe(S(CH₂)₃S)]^2? ( 1 )?{Fe(NO)₂}⁹ oxidized‐form [(NO)₂Fe(S(CH₂)₃S)]^?), the chemical oxidation of the {Fe(NO)₂}¹⁰ DNIC [(NO)₂Fe(S(CH₂)₂S)]^2? ( 2 ) generates the dinuclear {Fe(NO)₂}⁹–{Fe(NO)₂}⁹ complex [(NO)₂Fe(μ‐SC₂H₄S)₂Fe(NO)₂]^2? ( 3 ) bridged by two terminal [SC₂H₄S]^2? ligands. On the basis of the Fe K‐edge pre‐edge energy and S K‐edge XAS, the oxidation of complex 1 yielding [(NO)₂Fe(S(CH₂)₃S)]^? is predominantly a metal‐based oxidation. The smaller S1‐Fe1‐S2 bond angle of 94.1(1)° observed in complex 1 (S1‐Fe1‐S2 88.6(1)° in complex 2 ), compared to the bigger bond angle of 100.9(1)° in the {Fe(NO)₂}⁹ DNIC [(NO)₂Fe(S(CH₂)₃S)]^?, may be ascribed to the electron‐rich {Fe(NO)₂}¹⁰ DNIC preferring a restricted bite angle to alleviate the electronic donation of the chelating thiolate to the electron‐rich {Fe(NO)₂}¹⁰ core. The extended transition state and natural orbitals for chemical valence (ETS‐NOCV) analysis on the edt‐/pdt‐chelated {Fe(NO)₂}⁹ and {Fe(NO)₂}¹⁰ DNICs demonstrates how two key bonding interactions, that is, a Fe?S covalent σ bond and thiolate to the Fe d charge donation, between the chelating thiolate ligand and the {Fe(NO)₂}^9/10 core could be modulated by the backbone lengths of the chelating thiolate ligands to tune the electrochemical redox potential (E_1/2=?1.64 V for complex 1 and E_1/2=?1.33 V for complex 2 ) and to dictate structural rearrangement/chemical transformations (S‐Fe‐S bite angle and monomeric vs. dimeric DNICs).     

Dinitrosyl iron complexes: From molecular electrocatalysts to electrodeposited‐film electrodes for hydrogen evolution reaction

Clean and large‐scale production of hydrogen via water splitting triggered by active, robust, and low‐cost electrocatalysts is a promising and sustainable strategy for energy conversion and storage. In this study, a series of four‐coordinated chelating amine‐bound {Fe(NO)₂}¹⁰ dinitrosyl iron complexes (DNICs) [(L)Fe(NO)₂] were synthesized to investigate how the electronic structure of [Fe(NO)₂] unit of DNICs was tailored to promote the electrocatalytic hydrogen evolution reaction (HER) triggered by the homogeneous DNICs' molecular catalysts and the heterogeneous DNIC‐derived electrodeposited‐film electrodes. The electrochemical studies demonstrate that HER onset potentials of those DNICs in neutral sodium sulfate aqueous solution are dependent on their IR ν_(NO) stretching frequencies, indicating that the electron‐rich [Fe(NO)₂] core modulated by the synergistic cooperation of the electron‐donating ability and steric effect of methyl‐/hydrogen‐substituted diamine‐coordinated ligands, presumably, benefits the formation of metal‐hydride intermediate to reduce the required onset potential. In contrast with homogeneous catalyst retaining its molecular integrity during the catalytic HER process, it is noticed that DNICs [(L)Fe(NO)₂] act as the precursor of the active heterogeneous HER catalyst during the electrocatalytic HER process. It is presumed that the intermolecular hydrogen‐bonding interactions among DNICs [(L)Fe(NO)₂] may control the particle sizes of DNIC‐derived electrodeposited film to modulate HER efficiency.     

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.