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Flavodiiron nitric oxide reductases (FNORs) protect microbes from nitrosative stress under anaerobic conditions by mediating the reduction of nitric oxide (NO) to nitrous oxide (N2O). The proposed mechanism for the catalytic reduction of NO by FNORs involves a dinitrosyldiiron intermediate with a [hs-{FeNO}7]2formulation, which produces N2O and a diferric species. Moreover, both NO and hydrogen sulfide (H2S) have been implicated in several similar physiological functions in biology and are also known to cross paths in cell signaling. Here we report the synthesis, spectroscopic and theoretical characterization, and N2O production activity of an unprecedented monohydrosulfidodinitrosyldiiron compound, with a [(HS)hs-{FeNO}7/hs-{FeNO}7] formulation, that models the key dinitrosyl intermediate of FNORs. The generation of N2O from this unique compound follows a semireduced pathway, where one-electron reduction generates a reactive hs-{FeNO}8center via the occupation of an Fe–NO antibonding orbital. In contrast to the well-known reactivity of H2S and NO, the coordinated hydrosulfide remains unreactive toward NO and acts only as a spectator ligand during the NO reduction process.

We previously reported the synthesis and preliminary characterization of a unique series of low-spin (ls) {FeNO}8–10complexes supported by an ambiphilic trisphosphineborane ligand, [Fe(TPB)(NO)]+/0/−. Herein, we use advanced spectroscopic techniques and density functional theory (DFT) calculations to extract detailed information as to how the bonding changes across the redox series. We find that, in spite of the highly reduced nature of these complexes, they feature an NO+ligand throughout with strong Fe−NO π-backbonding and essentially closed-shell electronic structures of their FeNO units. This is enabled by an Fe−B interaction that is present throughout the series. In particular, the most reduced [Fe(TPB)(NO)]−complex, an example of a ls-{FeNO}10species, features a true reverse dative Fe → B bond where the Fe center acts as a strong Lewis-base. Hence, this complex is in fact electronically similar to the ls-{FeNO}8system, with two additional electrons “stored” on site in an Fe−B single bond. The outlier in this series is the ls-{FeNO}9complex, due to spin polarization (quantified by pulse EPR spectroscopy), which weakens the Fe−NO bond. These data are further contextualized by comparison with a related N2complex, [Fe(TPB)(N2)]−, which is a key intermediate in Fe(TPB)-catalyzed N2fixation. Our present study finds that the Fe → B interaction is key for storing the electrons needed to achieve a highly reduced state in these systems, and highlights the pitfalls associated with using geometric parameters to try to evaluate reverse dative interactions, a finding with broader implications to the study of transition metal complexes with boratrane and related ligands.

Cytochrome (Cyt) P450s are an important class of enzymes with numerous functions in nature. The unique reactivity of these enzymes relates to their heme bactive sites with an axially bound, deprotonated cysteine (a “cysteinate”) ligand (chemically speaking a thiolate). The heme-thiolate active sites further contain a number of conserved hydrogen-bonds (H-bonds) to the bound cysteinate ligand, which have been proposed to tune and stabilize the Fe–S bond. In this work, we present the low-temperature preparation of five ferric heme-thiolate nitric oxide (NO) model complexes that contain one tunablehydrogen-bond to the bound thiolate ligand. We show that the presence of a H-bond has a dramatic effect in stabilizing the thiolate ligand against direct reaction with NO. This observation reinforces the important protective role of H-bonds in Cyt P450s. We further demonstrate that H-bond strength tunes thiolate donor strength, which, in turn, controls the N–O and Fe–NO stretching frequencies and hence, bond strengths. We observe a direct correlation between the Fe–NO and N–O stretching frequencies, indicative of a thiolate σ-transeffect (interaction). Here, very small changes in H-bond strength lead to a surprisingly large effect on the FeNO unit. This result implies that subtle changes in the Cys-pocket of a Cyt P450 can strongly affect reactivity. Importantly, using the Fe–NO/N–O correlation established here, the thiolate donor strength in heme-thiolate enzyme active sites and model complexes can be quantifiedin a straightforward way, using NO as a probe. This spectroscopic correlation provides a quantitative measure of the thiolate’s “push” effect, which is important in O2-activation (Compound I formation) in Cyt P450s in general.

Manganese porphyrins are used as catalysts in the oxidation of olefins and nonactivated hydrocarbons. Key to these reactions are high-valent Mn–(di)oxo species, for which [Mn(Porph)(X)] serve as precursors. To elucidate their properties, it is crucial to understand the interaction of the Mn center with the porphyrin ligand. Our study focuses on simple high-spin [MnIII(TPP)X] (X = F, Cl, I, Br) complexes with emphasis on the spectroscopic properties of [MnIII(TPP)Cl], using variable-temperature variable-field magnetic circular dichroism spectroscopy and time-dependent density functional theory to help with band assignments. The optical properties of [MnIII(TPP)Cl] are complicated and unusual, with a Soret band showing a high-intensity feature at 21050 cm–1and a broad band that spans 23200–31700 cm–1. The 15000–18500 cm–1region shows the Cl(px/y) → dπ(CT(Cl,π)), Q band, and overlap-forbidden Cl(px/y)_dπ→ dx2–y2transitions that gain intensity from the strongly allowed π → π*(0)transition. The 20000–21000 cm–1region displays the prominent pseudo A-type signal of the Soret band. The strongly absorbing features at 22500–28000 cm–1exhibit A1u⟨79⟩/A2u⟨81⟩ → dπ, CT(Cl,π/σ), and symmetry-forbidden CT character, mixed with the π → π*(0)transition. The strong dx2–y2_B1g⟨80⟩ orbital interaction drives the ground-state MO mixing. Importantly, the splitting of the Soret band is explained by strong mixing of the porphyrin A2u(π)⟨81⟩ and the Cl(pz)_dz2orbitals. Through this direct orbital pathway, the π → π*(0)transition acquires intrinsic metal-d → porphyrin CT character, where the π → π*(0)intensity is then transferred into the high-energy CT region of the optical spectrum. The heavier halide complexes support this conclusion and show enhanced orbital mixing and drastically increased Soret band splittings, where the 21050 cm–1band shifts to lower energy and the high-energy features in the 23200–31700 cm–1range increase further in intensity, compared to the chloro complex.

Despite utilizing a common cofactor binding motif, hemoproteins bearing a cysteine-derived thiolate ligand (heme-thiolate proteins) are involved in a diverse array of biological processes ranging from drug metabolism to transcriptional regulation. Though the origin of heme-thiolate functional divergence is not well understood, growing evidence suggests that the hydrogen bonding (H-bonding) environment surrounding the Fe-coordinating thiolate influences protein function. Outside of X-ray crystallography, few methods exist to characterize these critical H-bonding interactions. Electron paramagnetic resonance (EPR) spectra of heme-thiolate proteins bearing a six-coordinate, Fe(III) heme exhibit uniquely narrow low-spin (S= 1/2), rhombic signals, which are sensitive to changes in the heme-thiolate H-bonding environment. To establish a well-defined relationship between the magnitude of g-value dispersion in this unique EPR signal and the strength of the heme-thiolate H-bonding environment, we synthesized and characterized of a series of six-coordinate, aryl-thiolate-ligated Fe(III) porphyrin complexes bearing a tunable intramolecular H-bond. Spectroscopic investigation of these complexes revealed a direct correlation between H-bond strength and g-value dispersion in the rhombic EPR signal. Using density functional theory (DFT), we elucidated the electronic origins of the narrow, rhombic EPR signal in heme-thiolates, which arises from an Fe–S pπ–dπbonding interaction. Computational analysis of the intramolecularly H-bonded heme-thiolate models revealed that H-bond donation to the coordinating thiolate reduces thiolate donor strength and weakens this Fe–S interaction, giving rise to larger g-value dispersion. By defining the relationship between heme-thiolate electronic structure and rhombic EPR signal, it is possible to compare thiolate donor strengths among heme-thiolate proteins through analysis of low-spin, Fe(III) EPR spectra. Thus, this study establishes EPR spectroscopy as a valuable tool for exploring how second coordination sphere effects influence heme-thiolate protein function.

Cyt P450 nitric oxide (NO) reductase (P450nor) is an important enzyme in fungal denitrification, responsible for the large-scale production of the greenhouse gas N2O. In the first step of catalysis, the ferric heme-thiolate active site of P450nor binds NO to produce a ferric heme-nitrosyl or {FeNO}6intermediate (in the Enemark–Feltham notation). In this paper, we present the low-temperature preparation of six new heme-thiolate {FeNO}6model complexes, [Fe(TPP)(SPh*)(NO)], using a unique series of electron-poor thiophenolates (SPh*–), and their detailed spectroscopic characterization. Our data show experimentally, for the first time, that a direct correlation exists between the thiolate donor strength and the Fe–NO and N–O bond strengths, evident from the corresponding stretching frequencies. This is due to a σ-trans effect of the thiolate ligand, which manifests itself in the population of an Fe–N–O σ-antibonding (σ*) orbital. Via control of the thiolate donor strength (using hydrogen bonds), nature is therefore able to exactly control the degree of activation of the FeNO unit in P450nor. Vice versa, NO can be used as a sensitive probe to quantify the donor strength of a thiolate ligand in a model system or protein, by simply measuring the Fe–NO and N–O frequencies of the ferric NO adduct and then projecting those data onto the correlation plot established here. Finally, we are able to show that the σ-trans effect of the thiolate is the electronic origin of the “push” effect, which is proposed to mediate O–O bond cleavage and Compound I formation in Cyt P450 monooxygenase catalysis.

Nickel-containing enzymes such as methyl coenzyme M reductase (MCR) and carbon monoxide dehydrogenase/acetyl coenzyme A synthase (CODH/ACS) play a critical role in global energy conversion reactions, with significant contributions to carbon-centered processes. These enzymes are implied to cycle through a series of nickel-based organometallic intermediates during catalysis, though identification of these intermediates remains challenging. In this work, we have developed and characterized a nickel-containing metalloprotein that models the methyl-bound organometallic intermediates proposed in the native enzymes. Using a nickel(I)-substituted azurin mutant, we demonstrate that alkyl binding occurs via nucleophilic addition of methyl iodide as a methyl donor. The paramagnetic NiIII-CH3species initially generated can be rapidly reduced to a high-spin NiII-CH3species in the presence of exogenous reducing agent, following a reaction sequence analogous to that proposed for ACS. These two distinct bioorganometallic species have been characterized by optical, EPR, XAS, and MCD spectroscopy, and the overall mechanism describing methyl reactivity with nickel azurin has been quantitatively modeled using global kinetic simulations. A comparison between the nickel azurin protein system and existing ACS model compounds is presented. NiIII-CH3Az is only the second example of two-electron addition of methyl iodide to a NiIcenter to give an isolable species and the first to be formed in a biologically relevant system. These results highlight the divergent reactivity of nickel across the two intermediates, with implications for likely reaction mechanisms and catalytically relevant states in the native ACS enzyme.

The iron(II)-nitroxyl complex [Fe(NO)(L3)](1) (with L3–= a hindered hydrotris(pyrazolyl)borate ligand), a high-spin (hs)-{FeNO}8complex in the Enemark–Feltham notation, is surprisingly stable and is the first of its kind that could be structurally characterized. We further studied this compound using a variety of spectroscopic methods. These results indicate a hs iron(II) center with a bound 3NO–ligand where the spins are antiferromagnetic coupled (St= 1). Vibrational data show that this complex has a very strong Fe−NO bond. DFT calculations support this result and link it to very strong π-donation from the 3NO–ligand to the iron(II) center. Furthermore, a very unusual equilibrium between the hs-{FeNO}8complex and a dinitrosyl iron complex (DNIC) of {Fe(NO)2}9type is observed. The O2reactivity of the complex is finally reported.

The intermediacy of metal–NNH2complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N2) fixation. In this context, we have shown that triphosphine-supported Fe(N2) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH2) complexes over an array of charge and oxidation states. Upon exposure to further H+/e–equivalents, these species either continue down a distal-type Chatt pathway to yield a terminal iron(IV) nitride or instead follow a distal-to-alternating pathway resulting in N–H bond formation at the proximal N atom. To understand the origin of this divergent selectivity, herein we synthesize and elucidate the electronic structures of a redox series of Fe(NNMe2) complexes, which serve as spectroscopic models for their reactive protonated congeners. Using a combination of spectroscopies, in concert with density functional theory and correlated ab initio calculations, we evidence one-electron redox noninnocence of the “NNMe2” moiety. Specifically, although two closed-shell configurations of the “NNR2” ligand have been commonly considered in the literature—isodiazene and hydrazido(2−)—we provide evidence suggesting that, in their reduced forms, the present iron complexes are best viewed in terms of an open-shell [NNR2]•–ligand coupled antiferromagnetically to the Fe center. This one-electron redox noninnocence resembles that of the classically noninnocent ligand NO and may have mechanistic implications for selectivity in N2fixation activity.

Ethynylferrocene (FcC≡CH) was utilized as a redox-active metalloligand for the synthesis of polynuclear coinage metal complexes. The reaction of [FcC≡CLi] with tri-tert-butylphosphine metal chlorides [tBu3P-MCl] (M = Au, Ag, Cu) yielded different heteronuclear ferrocene-funtionalized alkyne complexes featuring metallophilic interactions. Furthermore, the redox properties of the ferrocenyl-functionalized tetracopper complex were investigated by cyclic voltammetry and UV–vis–near-IR spectroelectrochemistry. They indicate the compounds’ redox-rich nature and a weak electronic coupling between the redox-active ferrocenyl units over a large distance.

Ruthenium(II) tetraamine nitrosyl complexes with N-heterocyclic ligands are known for their potential as nitric oxide (NO•) donors, capable of releasing NO•through either direct photodissociation or one-electron reduction of the Ru(II)NO+center. This study delivers a novel insight into the one-electron reduction mechanism for the model complex trans-[RuII(NO)(NH3)4(py)]3+(RuNOpy, py = pyridine) in phosphate buffer solution (pH 7.4). In situFT-IR spectroelectrochemistry reveals that the pyridine ligand is readily released upon one-electron reduction of the nitrosyl complex, a finding supported by nuclear magnetic resonance spectroscopy (1H NMR) and electrochemistry coupled to mass spectrometry (EC-MS), which detect free pyridine in solution. However, direct evidence of NO•release from RuNOpy as the primary step following reduction was not observed. Interestingly, FT-IR results indicate that the isomers of the nitrosyl complex, cis-[Ru(NO)(NH3)4(OH)]+and trans-[Ru(NO)(NH3)4(OH)]+, are formed following reduction and pyridine labilization, initiating an outer-sphere electron transfer process that triggers a chain electron transfer reaction. Finally, nitric oxide is liberated as an end product, arising from the reduction of the hydroxyl isomer complexes cis-[Ru(NO)(NH3)4(OH)]2+and trans-[Ru(NO)(NH3)4(OH)]2+. This study provides new insights into the reduction mechanism and transformation pathways of ruthenium nitrosyl complexes, contributing to our understanding of their potential as NO•donors.

A series of cobalt bis(benzenedithiolate) complexes with varying benzenedithiolate (general abbreviation: bdt2–) ring substitutions (S2C6X42–) were prepared and adsorbed on inexpensive electrodes composed of (a) reduced graphene oxide (RGO) electrodeposited on fluorine-doped tin oxide (FTO) and (b) highly ordered pyrolytic graphite (HOPG). The catalyst-adsorbed electrodes are characterized by X-ray photoelectron spectroscopy. Catalyst loading across the ligand series improved notably with increasing halide substitution [from 2.7 × 10–11mol cm–2for TBA[Co(S2C6H4)2] (1) to 6.22 × 10–10mol cm–2for TBA[Co(S2C6Cl4)2] (3)] and increasing ring size of the benzenedithiolate ligand [up to 3.10 × 10–9mol cm–2for TBA[Co(S2C10H6)2] (6)]. Electrocatalytic analysis of the complexes immobilized on HOPG elicits a reductive current response indicative of dihydrogen generation in the presence of mildly acidic aqueous solutions (pH 2–4) of trifluoroacetic acid, with overpotentials of around 0.5 V versus SHE (measured vs platinum). Rate constant (kobs) estimates resulting from cyclic voltammetry analysis range from 24 to 230 s–1with the maximum kobsfor TBA[Co(S2C6H2Cl2)2] (2) at an overpotential of 0.59 V versus platinum. Controlled-potential electrolysis studies performed in 0.5 M H2SO4at −0.5 V versus SHE show impressive initial rate constants of over 500 s–1under bulk electrolysis conditions; however, steady catalyst deactivation over an 8 h period is observed, with turnover numbers reaching 9.1 × 106. Electrolysis studies reveal that halide substitution is a central factor in improving the turnover stability, whereas the ring size is less of a factor in optimizing the long-term stability of the heterogeneous catalyst manifolds. Catalyst deactivation is likely caused by catalyst desorption from the electrode surfaces.

Fungal denitrification plays a crucial role in the nitrogen cycle and contributes to the total N2O emission from agricultural soils. Here, cytochrome P450 NO reductase (P450nor) reduces two NO to N2O using a single heme site. Despite much research, the exact nature of the critical “Intermediate I” responsible for the key N–N coupling step in P450nor is unknown. This species likely corresponds to a Fe-NHOH-type intermediate with an unknown electronic structure. Here we report a new strategy to generate a model system for this intermediate, starting from the iron(III) methylhydroxylamide complex [Fe(3,5-Me-BAFP)(NHOMe)] (1), which was fully characterized by 1H NMR, UV–vis, electron paramagnetic resonance, and vibrational spectroscopy (rRaman and NRVS). Our data show that 1is a high-spin ferric complex with an N-bound hydroxylamide ligand that is strongly coordinated (Fe–N distance, 1.918 Å; Fe-NHOMe stretch, 558 cm–1). Simple one-electron oxidation of 1at −80 °C then cleanly generates the first model system for Intermediate I, [Fe(3,5-Me-BAFP)(NHOMe)]+(1+). UV–vis, resonance Raman, and Mössbauer spectroscopies, in comparison to the chloro analogue [Fe(3,5-Me-BAFP)(Cl)]+, demonstrate that 1+is best described as an FeIII-(NHOMe)•complex with a bound NHOMe radical. Further reactivity studies show that 1+is highly reactive toward NO, a reaction that likely proceeds via N–N bond formation, following a radical–radical-type coupling mechanism. Our results therefore provide experimental evidence, for the first time, that an FeIII-(NHOMe)•electronic structure is indeed a reasonable electronic description for Intermediate I and that this electronic structure is advantageous for P450nor catalysis because it can greatly facilitate N–N bond formation and, ultimately, N2O generation.

Cupredoxins are copper-dependent electron-transfer proteins that can be categorized as blue, purple, green, and red depending on the spectroscopic properties of the Cu(II) bound forms. Interestingly, despite significantly different first coordination spheres and nuclearity, all cupredoxins share a common Greek Key β-sheet fold. We have previously reported the design of a red copper protein within a completely distinct three-helical bundle protein, α3DChC2.(1) While this design demonstrated that a β-barrel fold was not requisite to recapitulate the properties of a native cupredoxin center, the parent peptide α3D was not sufficiently stable to allow further study through additional mutations. Here we present the design of an elongated protein GRANDα3D (GRα3D) with ΔGu= −11.4 kcal/mol compared to the original design’s −5.1 kcal/mol. Diffraction quality crystals were grown of GRα3D (a first for an α3D peptide) and solved to a resolution of 1.34 Å. Examination of this structure suggested that Glu41 might interact with the Cu in our previously reported red copper protein. The previous bis(histidine)(cysteine) site (GRα3DChC2) was designed into this new scaffold and a series of variant constructs were made to explore this hypothesis. Mutation studies around Glu41 not only prove the proposed interaction, but also enabled tuning of the constructs’ hyperfine coupling constant from 160 to 127 × 10–4cm–1. X-ray absorption spectroscopy analysis is consistent with these hyperfine coupling differences being the result of variant 4p mixing related to coordination geometry changes. These studies not only prove that an Glu41–Cu interaction leads to the α3DChC2 construct’s red copper protein like spectral properties, but also exemplify the exact control one can have in a de novoconstruct to tune the properties of an electron-transfer Cu site.

A diphenylphosphine functionalized benzoic acid was applied for the synthesis of a homoleptic dimolybdenum-based metalloligand, exhibiting four symmetrically placed phosphine donor sites. This allowed subsequent treatment with gold(I), rhodium(I), and iridium(I) precursors to obtain early–late heterometallic complexes as well as Lewis acid–base adducts with BH3. The compounds were in-depth investigated by spectroscopic techniques, single-crystal X-ray diffraction, and femtosecond laser spectroscopy. The coordination of different metal fragments to the dimolybdenum metalloligand leads to a fine-tuning of the system’s optical properties, which correlates well with fluorescence quantum yield measurements. Nevertheless, triplet dynamics still remain the dominating channel in these systems with an intersystem crossing time constant below 1 ps.

Nitric oxide (NO) has a number of important biological functions, including nerve signaling transduction, blood pressure control, and, at higher concentrations, immune defense. A number of pathogenic bacteria have developed methods of degrading this toxic molecule through the use of flavodiiron nitric oxide reductases (FNORs), which utilize a nonheme diiron active site to reduce NO → N2O. The well-characterized diiron model complex [Fe2(BPMP)(OPr)(NO)2]2+(BPMP−= 2,6-bis[(bis(2- pyridylmethyl)amino)methyl]-4-methylphenolate), which mimics both the active site structure and reactivity of these enzymes, offers key insight into the mechanism of FNORs. Presently, we have used computational methods to elucidate a coherent reaction mechanism that shows how one and two-electron reduction of this complex induces N–N bond formation and N2O generation, while the parent complex remains stable. The initial formation of a N–N bond to generate hyponitrite (N2O22–) follows a radical-type coupling mechanism, which requires strong Fe–NO π-interactions to be overcome to effectively oxidize the iron centers. Hyponitrite formation provides the largest activation barrier with ΔG‡= 7–8 kcal/mol (average of several functionals) in the two-electron, super-reduced mechanism. This is followed by the formation of a N2O22–complex with a novel binding mode for nonheme diiron systems, allowing for the facile release of N2O with the assistance of a carboxylate shift. This provides sufficient thermodynamic driving force for the reaction to proceed via N2O formation alone. Surprisingly, the one-electron “semireduced” mechanism is predicted to be competitive with the super-reduced mechanism. This is due to the asymmetry imparted by the BPMP–ligand, allowing a one-electron reduction to overcome one of the primary Fe–NO π-interactions. Generally, mediation of N2O formation by high-spin [{M-NO−}]2cores depends on the ease of oxidizing the M centers and breaking of the M–NO π-bonds to formally generate a “full” 3NO–unit, allowing for the critical step of N–N bond formation to proceed (via a radical-type coupling mechanism).

We have determined a convenient method for the bulk synthesis of high-purity ferric heme-nitrosyl complexes ({FeNO}6in the Enemark–Feltham notation); this method is based on the chemical or electrochemical oxidation of corresponding {FeNO}7precursors. We used this method to obtain the five- and six-coordinate complexes [Fe(TPP)(NO)]+(TPP2–= tetraphenylporphyrin dianion) and [Fe(TPP)(NO)(MI)]+(MI = 1-methylimidazole) and demonstrate that these complexes are stable in solution in the absence of excess NO gas. This is in stark contrast to the often-cited instability of such {FeNO}6model complexes in the literature, which is likely due to the common presence of halide impurities (although other impurities could certainly also play a role). This is avoided in our approach for the synthesis of {FeNO}6complexes via oxidation of pure {FeNO}7precursors. On the basis of these results, {FeNO}6complexes in proteins do not show an increased stability toward NO loss compared to model complexes. We also prepared the halide-coordinated complexes [Fe(TPP)(NO)(X)] (X = Cl–, Br–), which correspond to the elusive, key reactive intermediate in the so-called autoreduction reaction, which is frequently used to prepare {FeNO}7complexes from ferric precursors. All of the complexes were characterized using X-ray crystallography, UV–vis, IR, and nuclear resonance vibrational spectroscopy (NRVS). On the basis of the vibrational data, further insight into the electronic structure of these {FeNO}6complexes, in particular with respect to the role of the axial ligand transto NO, is obtained.

The small, stable heme protein myoglobin (Mb) was modified through cofactor substitution and mutagenesis to develop a new catalyst for carbene transfer reactions. The native heme was removed from wild-type Mb and several Mb His64 mutants (H64D, H64A, H64V), and the resulting apoproteins were reconstituted with ruthenium mesoporphyrin IX (RuMpIX). The reconstituted proteins (RuMb) were characterized by UV–vis and circular dichroism spectroscopy and were used as catalysts for the N–H insertion of aniline derivatives and the cyclopropanation of styrene derivatives. The best catalysts for each reaction were able to achieve turnover numbers (TON) up to 520 for the N–H insertion of aniline, and 350 TON for the cyclopropanation of vinyl anisole. Our results show that RuMb is an effective catalyst for N–H insertion, with the potential to further increase the activity and stereoselectivity of the catalyst in future studies. Compared to native Mb (“FeMb”), RuMb is a more active catalyst for carbene transfer reactions, which leads to both heme and protein modification and degradation and, hence, to an overall much-reduced lifetime of the catalyst. This leads to lower TONs for RuMb compared to the iron-containing analogues. Strategies to overcome this limitation are discussed. Finally, comparison is also made to FeH64DMb and FeH64AMb, which have not been previously investigated for carbene transfer reactions.

Flavo-diiron nitric oxide reductases (FNORs) are a subclass of nonheme diiron proteins in pathogenic bacteria that reductively transform NO to N2O, thereby abrogating the nitrosative stress exerted by macrophages as part of the immune response. Understanding the mechanism and intermediates in the NO detoxification process might be crucial for the development of a more efficient treatment against these bacteria. However, low molecular weight models are still rare, and only in a few cases have their reductive transformations been thoroughly investigated. Here, we report on the development of two complexes, based on a new dinucleating pyrazolate/triazacyclononane hybrid ligand L–, which serve as model systems for nonheme diiron active sites. Their ferrousnitrile precursors [L{Fe(R′CN)}2(μ-OOCR)](X)2(1) can be readily converted into the corresponding nitrosyl adducts ([L{Fe(NO)}2(μ-OOCR)](X)2, 2). Spectroscopic characterization shows close resemblance to nitrosylated nonheme diiron sites in proteins as well as previous low molecular weight analogues. Crystallographic characterization reveals an anti orientation of the two {Fe(NO)}7(Enemark–Feltham notation) units. The nitrosyl adducts 2can be (electro)chemically reduced by one electron, as shown by cyclic voltammetry and UV/vis spectroscopy, but without the formation of N2O. Instead, various spectroscopic techniques including stopped-flow IR spectroscopy indicated the rapid formation, within few seconds, of two well-defined products upon reduction of 2a(R = Me, X = ClO4). As shown by IR and Mössbauer spectroscopy as well as X-ray crystallographic characterization, the reduction products are a diiron tetranitrosyl complex ([L{Fe(NO)2}2](ClO4), 3a′) and a diacetato-bridged ferrouscomplex [LFe2(μ-OAc)2](ClO4) (3a″). Especially 3a′parallels suggested products in the decay of nitrosylated methane monooxygenase hydroxylase (MMOH), for which N2O release is much less efficient than for FNORs.

Dinitrosyl iron complexes (DNICs) are among the most abundant NO-derived cellular species. Monomeric DNICs can exist in the {Fe(NO)2}9or {Fe(NO)2}10oxidation state (in the Enemark–Feltham notation). However, experimental studies of analogous DNICs in both oxidation states are rare, which prevents a thorough understanding of the differences in the electronic structures of these species. Here, the {Fe(NO)2}9DNIC [Fe(dmp)(NO)2](OTf) (1; dmp = 2,9-dimethyl-1,10-phenanthroline) is synthesized from a ferrous precursor via an unusual pathway, involving disproportionation of an {FeNO}7complex to yield the {Fe(NO)2}9DNIC and a ferric species, which is subsequently reduced by NO gas to generate a ferrous complex that re-enters the reaction cycle. In contrast to most {Fe(NO)2}9DNICs with neutral N-donor ligands, 1exhibits high solution stability and can be characterized structurally and spectroscopically. Reduction of 1yields the corresponding {Fe(NO)2}10DNIC [Fe(dmp)(NO)2] (2). The Mössbauer isomer shift of 2is 0.08 mm/s smaller than that of 1, which indicates that the iron center is slightly more oxidized in the reduced complex. The nuclear resonance vibrational spectra (NRVS) of 1and 2are distinct and provide direct experimental insight into differences in bonding in these complexes. In particular, the symmetric out-of-plane Fe–N–O bending mode is shifted to higher energy by 188 cm–1in 2in comparison to 1. Using quantum chemistry centered normal coordinate analysis (QCC-NCA), this is shown to arise from an increase in Fe–NO bond order and a stiffening of the Fe(NO)2unit upon reduction of 1to 2. DFT calculations demonstrate that the changes in bonding arise from an iron-centered reduction which leads to a distinct increase in Fe–NO π-back-bonding in {Fe(NO)2}10DNICs in comparison to the corresponding {Fe(NO)2}9complexes, in agreement with all experimental findings. Finally, the implications of the electronic structure of DNICs for their reactivity are discussed, especially with respect to N–N bond formation in NO reductases.

Two homoleptic Fe(II) complexes in different spin states bearing superbasic terpyridine derivatives as ligands are investigated to determine the relationship between spin state and electrochemical/spectroscopic behavior. Antiferromagnetic coupling between a ligand-centered radical and the high-spin metal center leads to an anodic shift of the first reduction potential and results in a species that shows mixed valency with a moderately intense intervalence-charge-transfer band. The differences afforded by the different spin states extend to the electrochemical reactivity of the complexes: while the low-spin species is a precatalyst for electrocatalytic CO2reduction and leads to the preferential formation of CO with a Faradaic efficiency of 37%, the high-spin species only catalyzes proton reduction at a modest Faradaic efficiency of approximately 20%.