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Addition of NO to a nonheme dithiolate-ligated iron(II) complex, Fe

SCS pincer ligands have an interesting combination of strong-field and weak-field donors, a coordination environment that is also present in the nitrogenase active site. Here, we explore the electronic structures of iron(II) and iron(III) complexes with such a pincer ligand, bearing a monodentate phosphine, thiolate S donor, amide N donor, ammonia, or CO. The ligand scaffold features a proton-responsive thioamide site, and the protonation state of the ligand greatly influences the reduction potential of iron in the phosphine complex. The N–H bond dissociation free energy, derived from a square scheme, is 56 ± 2 kcal/mol. EPR spectroscopy and SQUID magnetometry measurements show that the iron(III) complexes with S and N as the fourth donors have an intermediate spin (S = 3/2) ground state with large zero field splitting, and X-ray absorption spectra show high Fe–S covalency. The Mössbauer spectrum changes drastically with the position of a nearby alkali metal cation in the iron(III) amido complex, and DFT calculations explain this phenomenon through a change between having the doubly-occupied orbital as d(z)2 or d(yz), as the former is more influenced by the nearby positive charge.

The quest for homoleptic Cr TEMPO species has led to the preparation of pseudo-pentagonal planar (η2-TEMPO)2CrX (X = Cl, TEMPO), with perturbations from TEMPO(−) lone pairs at roughly 0°, 90°, 135°, 225° and 270°.

The electronic structures and contrasting reactivity of [Cu(CF3)4]− and [Cu(CF3)3(CH3)]− were probed using coupled cluster and ab initio valence bond calculations.

Conjugated microporous polymers (CMPs) are porous organic materials that display (semi)conducting behavior due to their highly π-conjugated structures. As such, they are promising next-generation m...

A new nonheme iron(II) complex, FeII (Me3 TACN)((OSiPh2 )2 O) (1), is reported. Reaction of 1 with NO(g) gives a stable mononitrosyl complex Fe(NO)(Me3 TACN)((OSiPh2 )2 O) (2), which was characterized by Mossbauer (δ=0.52 mm s-1 , |ΔEQ |=0.80 mm s-1 ), EPR (S=3/2), resonance Raman (RR) and Fe K-edge X-ray absorption spectroscopies. The data show that 2 is an {FeNO}7 complex with an S=3/2 spin ground state. The RR spectrum (λexc =458 nm) of 2 combined with isotopic labeling (15 N, 18 O) reveals ν(N-O)=1680 cm-1 , which is highly activated, and is a nearly identical match to that seen for the reactive mononitrosyl intermediate in the nonheme iron enzyme FDPnor (ν(NO)=1681 cm-1 ). Complex 2 reacts rapidly with H2 O in THF to produce the N-N coupled product N2 O, providing the first example of a mononuclear nonheme iron complex that is capable of converting NO to N2 O in the absence of an exogenous reductant.

All radical S-adenosylmethionine (radical-SAM) enzymes, including the noncanonical radical-SAM enzyme diphthamide biosynthetic enzyme Dph1–Dph2, require at least one [4Fe–4S](Cys)3 cluster for activity. It is well-known in the radical-SAM enzyme community that the [4Fe–4S](Cys)3 cluster is extremely air-sensitive and requires strict anaerobic conditions to reconstitute activity in vitro. Thus, how such enzymes function in vivo in the presence of oxygen in aerobic organisms is an interesting question. Working on yeast Dph1–Dph2, we found that consistent with the known oxygen sensitivity, the [4Fe–4S] cluster is easily degraded into a [3Fe–4S] cluster. Remarkably, the small iron-containing protein Dph3 donates one Fe atom to convert the [3Fe–4S] cluster in Dph1–Dph2 to a functional [4Fe–4S] cluster during the radical-SAM enzyme catalytic cycle. This mechanism to maintain radical-SAM enzyme activity in aerobic environments is likely general, and Dph3-like proteins may exist to keep other radical-SAM enzymes functional in aerobic environments.

The exposure of CrCl

[Image: see text] A high-spin, mononuclear Ti(II) complex, [(Tp(tBu,Me))TiCl] [Tp(tBu,Me–) = hydridotris(3-tert-butyl-5-methylpyrazol-1-yl)borate], confined to a tetrahedral ligand-field environment, has been prepared by reduction of the precursor [(Tp(tBu,Me))TiCl(2)] with KC(8). Complex [(Tp(tBu,Me))TiCl] has a (3)A(2) ground state (assuming C(3v) symmetry based on structural studies), established via a combination of high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy, solution and solid-state magnetic studies, Ti K-edge X-ray absorption spectroscopy (XAS), and both density functional theory and ab initio (complete-active-space self-consistent-field, CASSCF) calculations. The formally and physically defined Ti(II) complex readily binds tetrahydrofuran (THF) to form the paramagnetic adduct [(Tp(tBu,Me))TiCl(THF)], which is impervious to N(2) binding. However, in the absence of THF, the Ti(II) complex captures N(2) to produce the diamagnetic complex [(Tp(tBu,Me))TiCl](2)(η(1),η(1);μ(2)-N(2)), with a linear Ti=N=N=Ti topology, established by single-crystal X-ray diffraction. The N(2) complex was characterized using XAS as well as IR and Raman spectroscopies, thus establishing this complex to possess two Ti(III) centers covalently bridged by an N(2)(2–) unit. A π acid such as CNAd (Ad = 1-adamantyl) coordinates to [(Tp(tBu,Me))TiCl] without inducing spin pairing of the d electrons, thereby forming a unique high-spin and five-coordinate Ti(II) complex, namely, [(Tp(tBu,Me))TiCl(CNAd)]. The reducing power of the coordinatively unsaturated Ti(II)-containing [(Τp(tBu,Me))TiCl] species, quantified by electrochemistry, provides access to a family of mononuclear Ti(IV) complexes of the type [(Tp(tBu,Me))Ti=E(Cl)] (with E(2–) = NSiMe(3), N(2)CPh(2), O, and NH) by virtue of atom- or group-transfer reactions using various small molecules such as N(3)SiMe(3), N(2)CPh(2), N(2)O, and the bicyclic amine 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene.

High-energy resolution fluorescence-detected Cu K-edge X-ray absorption spectroscopy (XAS) and single-crystal polarized XAS data are presented toward refining the assignments of bands assigned as excitations from Cu 1s to ligand-localized molecular orbitals. These have been previously dubbed "XAS-metal-ligand charge transfer" (XAS-MLCT) bands. Data are presented for a series of [Cu(xbpy)2]n+ complexes (xbpy = 2,2'-bipyridine (1n+), 4,4'-bisamino-2,2'-bipyridine (2n+), and 4,4'-dimethoxy-2,2'-bipyridine (3n+); n = 1 and 2). Dipolar dependencies of these "XAS-MLCT" bands in both Cu1+ and Cu2+ species lead to reassignment of these features as owing their intensities primarily to Cu 1s → Cu 4p excitations. The transition densities are Cu-localized, highlighting that XAS-MLCT features in Cu XAS spectra are not "charge transfer" transitions but rather quasi-atomic transitions. Although scrutiny of the acceptor orbitals supports assignment as Cu 1s → ligand π* transitions, it ultimately appears that while the ligand orbital energetics govern the positions of these bands the intensity is conferred through a small degree of metal 4p mixing into otherwise ligand-dominated acceptor molecular orbitals.

Nickel anions [(MeCN)Ni(CF3)3]− and [Ni(CF3)4]2– were prepared by the formal addition of 3 and 4 equiv, respectively, of AgCF3 to [(dme)NiBr2] in the presence of the [PPh4]+ counterion. Detailed in...

The rates of catalytic oxidation of cyclohexane and CO are 4 and 20 times higher, respectively, with Cu supported on a cerium-based metal–organic framework (MOF) than on the structurally analogous ...

Structural, spectroscopic, and reactivity studies are presented for an electron transfer series of copper hydroxide complexes supported by a tridentate redox-active ligand. Single crystal X-ray crystallography shows that the mononuclear [CuOH](1+) core is stabilized via intramolecular H-bonds between the H-donors of the ligand and the hydroxide anion when the ligand is in its trianionic form. This complex undergoes two reversible oxidation processes that produce two metastable “high-valent” CuOH species, which can be generated by addition of stoichiometric amounts of 1e(−) oxidants. These CuOH species are characterized by an array of spectroscopic techniques including UV–vis absorption, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopies (XAS), which together indicate that all redox couples are ligand-localized. The reactivity of the complexes in their higher oxidation states toward substrates with modest O─H bond dissociation energies (e.g., 4-substitued-2,6-di-tert-butylphenols) indicates that these complexes act as 2H(+)/2e(−) oxidants, differing from the 1H(+)/1e(−) reactivity of well-studied [CuOH](2+) systems.

Cytochrome (cyt) P460 is a c-type monoheme enzyme found in ammonia-oxidizing bacteria (AOB) and methanotrophs; additionally, genes encoding it have been found in some pathogenic bacteria. Cyt P460 is defined by a unique post-translational modification to the heme macrocycle, where a lysine (Lys) residue covalently attaches to the 13' meso carbon of the porphyrin, modifying this heme macrocycle into the enzyme's eponymous P460 cofactor, similar to the cofactor found in the enzyme hydroxylamine oxidoreductase. This cross-link imbues the protein with unique spectroscopic properties, the most obvious of which is the enzyme's green color in solution. Cyt P460 from the AOB Nitrosomonas europaea is a homodimeric redox enzyme that produces nitrous oxide (N2O) from 2 equiv of hydroxylamine. Mutation of the Lys cross-link results in spectroscopic features that are more similar to those of standard cyt c' proteins and renders the enzyme catalytically incompetent for NH2OH oxidation. Recently, the necessity of a second-sphere glutamate (Glu) residue for redox catalysis was established; it plausibly serves as proton relay during the first oxidative half of the catalytic cycle. Herein, we report the first crystal structure of a cross-link deficient cyt P460. This structure shows that the positioning of the catalytically essential Glu changes by approximately 0.8 A when compared to a cross-linked, catalytically competent cyt P460. It appears that the heme-Lys cross-link affects the relative position of the P460 cofactor with respect to the second-sphere Glu residue, therefore dictating the catalytic competency of the enzyme.

Dicopper complexes templated by dinucleating, pacman dipyrrin ligand scaffolds ((Mes)dmx, (tBu)dmx: dimethylxan-thine-bridged, cofacial bis-dipyrrin) were synthesized by deprotonation/metalation with mesitylcopper (CuMes; Mes: mesityl) or by transmetalation with cuprous precursors from the corresponding deprotonated ligand. Neutral imide complexes ((R)dmx)Cu(2)(μ(2)-NAr) (R: Mes, (t)Bu; Ar: 4-MeOC(6)H(4), 3,5-(F(3)C)(2)C(6)H(3)) were synthesized by treatment of the corresponding dicuprous complexes with aryl azides. While one-electron reduction of ((Mes)dmx)Cu(2)(μ(2)-N(C(6)H(4)OMe)) with potassium graphite initiates an intramolecular, benzylic C─H amination at room temperature, chemical reduction of ((tBu)dmx)Cu(2)(μ(2)-NAr) leads to isolable [((tBu)dmx)Cu(2)(μ(2)-NAr)](−) product salts. The electronic structures of the thermally robust [((tBu)dmx)Cu(2)(μ(2)-NAr)](0/−) complexes were assessed by variable-temperature electron paramagnetic resonance spectroscopy, X-ray absorption spectroscopy (Cu L(2,3)/K-edge, N K-edge), optical spectroscopy, and DFT/CASSCF calculations. These data indicate that the formally Class IIIA mixed valence complexes of the type [((R)dmx)Cu(2)(μ(2)-NAr)](−) feature significant NAr-localized spin following reduction from electronic population of the [Cu(2)(μ(2)-NAr)] π* manifold, contrasting previous methods for engendering iminyl character through chemical oxidation. The reactivity of the isolable imido and iminyl complexes are examined for prototypical radical-promoted reactivity (e.g., nitrene transfer and H-atom abstraction), where the divergent reactivity is rationalized by the relative degree of N-radical character afforded from different aryl substituents.

The active site clusters of nitrogenase enzymes possess the only examples of carbides in biology. These are the only biological FeS clusters that are capable of reducing N2 to NH4+, implicating the central carbon and its interaction with Fe as important in the mechanism of N2 reduction. This biological question motivates study of the influence of carbon donors on the electronic structure and reactivity of unsaturated, high-spin iron centers. Here, we present functional and structural models that test the impacts of carbon donors and sulfide donors in simpler iron compounds. We report the first example of a diiron complex that is bridged by an alkylidene and a sulfide, which serves as a high-fidelity structural and spectroscopic model of a two-iron portion of the active-site cluster (FeMoco) in the resting state of Mo-nitrogenase. The model complexes have antiferromagnetically coupled pairs of high-spin iron centers, and sulfur K-edge X-ray absorption spectroscopy shows comparable covalency of the sulfide for C and S bridged species. The sulfur-bridged compound does not interact with N2 even upon reduction, but upon removal of the sulfide it becomes capable of reducing N2 to NH4+ with the addition of protons and electrons. This provides synthetic support for sulfide extrusion in the activation of nitrogenase cofactors., High-spin diiron alkylidenes give insight into the electronic structure and functional relevance of carbon in the FeMoco active site of nitrogenase.

Nitrous oxide (N2O) contributes significantly to ozone layer depletion and is a potent greenhouse agent, motivating interest in the chemical details of biological N2O fixation by nitrous oxide reductase (N2OR) during bacterial denitrification. In this study, we report a combined experimental/computational study of a synthetic [4Cu:1S] cluster supported by N-donor ligands that can be considered the closest structural and functional mimic of the CuZ catalytic site in N2OR reported to date. Quantitative N2 measurements during synthetic N2O reduction were used to determine reaction stoichiometry, which in turn was used as the basis for density functional theory (DFT) modeling of hypothetical reaction intermediates. The mechanism for N2O reduction emerging from this computational modeling involves cooperative activation of N2O across a Cu/S cluster edge. Direct interaction of the μ4-S ligand with the N2O substrate during coordination and N–O bond cleavage represents an unconventional mechanistic paradigm to be considered for the chemistry of CuZ and related metal–sulfur clusters. Consistent with hypothetical participation of the μ4-S unit in two-electron reduction of N2O, Cu K-edge and S K-edge X-ray absorption spectroscopy (XAS) reveal a high degree of participation by the μ4-S in redox changes, with approximately 21% S 3p contribution to the redox-active molecular orbital in the highly covalent [4Cu:1S] core, compared to approximately 14% Cu 3d contribution per copper. The XAS data included in this study represent the first spectroscopic interrogation of multiple redox levels of a [4Cu:1S] cluster and show high fidelity to the biological CuZ site.

Diruthenium paddlewheel complexes supported by electron-rich anilinopyridinate (Xap) ligands were synthesized in the course of the first in-depth structural and spectroscopic interrogation of monocationic [Ru

Reduction of nitrite anions [NO2]- takes place in a myriad of environments such as in the soil as part of the biogeochemical nitrogen cycle as well as in acidified nuclear waste. Nitrite reduction typically takes place within the coordination sphere of a redox active transition metal. Lewis acid coordination, however, can dramatically modify the reduction potential of this polyoxoanion to allow for reduction under non-aqueous conditions (-0.74 V vs. NHE). This strategy enables the isolation of a borane-capped nitrite dianion [NO2]2- along with its spectroscopic study consistent with reduction to the N(II) oxidation state. Protonation of the nitrite dianion results in facile loss of nitric oxide (NO) while reaction of the nitrite dianion with nitric oxide results in disproportionation to nitrous oxide (N2O) and nitrite, connecting three redox levels in the global nitrogen cycle.

Lithium peroxide is the crucial storage material in lithium–air batteries. Understanding the redox properties of this salt is paramount toward improving the performance of this class of batteries. Lithium peroxide, upon exposure to [Formula: see text] –benzoquinone (p–C(6)H(4)O(2)) vapor, develops a deep blue color. This blue powder can be formally described as [Li(2)O(2)] [Formula: see text] [LiO(2)] [Formula: see text] {Li[p–C(6)H(4)O(2)]} [Formula: see text] , though spectroscopic characterization indicates a more nuanced structural speciation. Infrared, Raman, electron paramagnetic resonance, diffuse-reflectance ultraviolet-visible and X-ray absorption spectroscopy reveal that the lithium salt of the benzoquinone radical anion forms on the surface of the lithium peroxide, indicating the occurrence of electron and lithium ion transfer in the solid state. As a result, obligate lithium superoxide is formed and encapsulated in a shell of Li[p–C(6)H(4)O(2)] with a core of Li(2)O(2). Lithium superoxide has been proposed as a critical intermediate in the charge/discharge cycle of Li–air batteries, but has yet to be isolated, owing to instability. The results reported herein provide a snapshot of lithium peroxide/superoxide chemistry in the solid state with redox mediation.

SCS pincer ligands have an interesting combination of strong-field and weak-field donors that is also present in the nitrogenase active site. Here, we explore the electronic structures of iron(II) and iron(III) complexes with such a pincer ligand, bearing a monodentate phosphine, thiolate S donor, amide N donor, ammonia, or CO. The ligand scaffold features a protonresponsive thioamide site, and the protonation state of the ligand greatly influences the reduction potential of iron in the phosphine complex. The N–H bond dissociation free energy can be quantitated as 56 ± 2 kcal/mol. EPR spectroscopy and SQUID magnetometry measurements show that the iron(III) complexes with S and N as the fourth donors have an intermediate spin (S = 3/2) ground state with large zero field splitting, and X-ray absorption spectra show high Fe–S covalency. The Mössbauer spectrum changes drastically with the position of a nearby alkali metal cation in the iron(III) amido complex, and DFT calculations explain this phenomenon through a change between having the doubly-occupied orbital as dz2 or dyz, as the former is more influenced by the nearby positive charge.

A new nonheme iron(II) complex, Fe(II)(Me(3)TACN)((OSi(Ph2))(2)O) (1), is reported. Reaction of 1 with NO((g)) gives a stable mononitrosyl complex Fe(NO)(Me(3)TACN)((OSi(Ph2))(2)O) (2), which was characterized by Mössbauer (δ = 0.52 mm s(−1), |ΔE(Q)| = 0.80 mm s(−1)), EPR (S = 3/2), resonance Raman (RR) and Fe K-edge X-ray absorption spectroscopies. The data show that 2 is an {FeNO}(7) complex with an S = 3/2 spin ground state. The RR spectrum (λ(exc) = 458 nm) of 2 combined with isotopic labeling ((15)N, (18)O) reveals ν(N-O) = 1680 cm(−1), which is highly activated, and is a nearly identical match to that seen for the reactive mononitrosyl intermediate in the nonheme iron enzyme FDPnor (ν(NO) = 1681 cm(−1)). Complex 2 reacts rapidly with H(2)O in THF to produce the N-N coupled product N(2)O, providing the first example of a mononuclear nonheme iron complex that is capable of converting NO to N(2)O in the absence of an exogenous reductant.

Reduction of nitrite anions (NO

Metalation of the deprotonated dipyrrin (AdFL)Li with NiCl2(py)2 afforded the divalent Ni product (AdFL)NiCl(py)2 (1) (AdFL: 1,9-di(1-adamantyl)-5-perfluorophenyldipyrrin; py: pyridine). To generate a reactive synthon on which to explore oxidative group transfer, we used potassium graphite to reduce 1, affording the monovalent Ni synthon (AdFL)Ni(py) (2) and concomitant production of a stoichiometric equivalent of KCl and pyridine. Slow addition of mesityl- or 1-adamantylazide in benzene to 2 afforded the oxidized Ni complexes (AdFL)Ni(NMes) (3) and (AdFL)Ni(NAd) (4), respectively. Both 3 and 4 were characterized by multinuclear NMR, EPR, magnetometry, single-crystal X-ray crystallography, theoretical calculations, and X-ray absorption spectroscopies to provide a detailed electronic structure picture of the nitrenoid adducts. X-ray absorption near edge spectroscopy (XANES) on the Ni reveals higher energy Ni 1s → 3d transitions (3: 8333.2 eV; 4: 8333.4 eV) than NiI or unambiguous NiII analogues. N K-edge X-ray absorption spectroscopy performed on 3 and 4 reveals a common low-energy absorption present only for 3 and 4 (395.4 eV) that was assigned via TDDFT as an N 1s promotion into a predominantly N-localized, singly occupied orbital, akin to metal-supported iminyl complexes reported for iron. On the continuum of imido (i.e., NR2−) to iminyl (i.e., 2NR−) formulations, the complexes are best described as NiII-bound iminyl species given the N K-edge and TDDFT results. Given the open-shell configuration (S = 1/2) of the iminyl adducts, we then examined their propensity to undergo nitrenoid-group transfer to organic substrates. The adamantyl complex 4 readily consumes 1,4-cyclohexadiene (CHD) via H-atom abstraction to afford the amide (AdFL)Ni(NHAd) (5), whereas no reaction was observed upon treatment of the mesityl variant 3 with excess amount of CHD over 3 hours. Toluene can be functionalized by 4 at room temperature, exclusively affording the N-1-adamantyl-benzylidene (6). Slow addition of the organoazide substrate (4-azidobutyl)benzene (7) with 2 exclusively forms 4-phenylbutanenitrile (8) as opposed to an intramolecular cyclized pyrrolidine, resulting from facile β-H elimination outcompeting H-atom abstraction from the benzylic position, followed by rapid H2-elimination from the intermediate Ni hydride ketimide intermediate., Nickel-supported nitrenoids exhibit iminyl character, as determined by multi-edge XAS and TDDFT analysis, demonstrate efficacy for C–H activation and nitrene transfer chemistry.

Seventeen Cu complexes with formal oxidation states ranging from CuI to CuIII are investigated through the use of multiedge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) c...

The activation of dioxygen by Fe(II)(Me(3)TACN)(S(2)SiMe(2)) (1) is reported. Reaction of 1 with O(2) at −135 °C in 2-MeTHF generates a thiolate-ligated (peroxo)diiron complex Fe(III)(2)(O(2))(Me(3)TACN)(2)(S(2)SiMe(2))(2) (2) that was characterized by UV–vis (λ(max) = 300, 390, 530, 723 nm), Mössbauer (δ = 0.53, |ΔE(Q)| = 0.76 mm s(−1)), resonance Raman (RR) (ν(O–O) = 849 cm(−1)), and X-ray absorption (XAS) spectroscopies. Complex 2 is distinct from the outer-sphere oxidation product 1(ox) (UV–vis (λ(max) = 435, 520, 600 nm), Mössbauer (δ = 0.45, ΔE(Q) = 3.6 mm s(−1)), and EPR (S = 5/2, g = [6.38, 5.53, 1.99])), obtained by one-electron oxidation of 1. Cleavage of the peroxo O–O bond can be initiated either photochemically or thermally to produce a new species assigned as an Fe(IV)(O) complex, Fe(IV)(O)(Me(3)TACN)(S(2)SiMe(2)) (3), which was identified by UV–vis (λ(max) = 385, 460, 890 nm), Mössbauer (δ = 0.21, |ΔE(Q)| = 1.57 mm s(−1)), RR (ν(Fe(IV)=O) = 735 cm(−1)), and X-ray absorption spectroscopies, as well as reactivity patterns. Reaction of 3 at low temperature with H atom donors gives a new species, Fe(III)(OH)(Me(3)TACN)(S(2)SiMe(2)) (4). Complex 4 was independently synthesized from 1 by the stoichiometric addition of a one-electron oxidant and a hydroxide source. This work provides a rare example of dioxygen activation at a mononuclear nonheme iron(II) complex that produces both Fe(III)–O–O–Fe(III) and Fe(IV)(O) species in the same reaction with O(2). It also demonstrates the feasibility of forming Fe/O(2) intermediates with strongly donating sulfur ligands while avoiding immediate sulfur oxidation.

We report the first Fe─CPh(3) complex, and show that the long Fe─C bond can be disrupted by neutral π-acceptor ligands (benzophenone and phenylacetylene) to release the triphenylmethyl radical. The products are formally iron(I) complexes, but X-ray absorption spectroscopy coupled with density functional and multireference ab initio calculations indicates that the best description of all the complexes is iron(II). In the formally iron(I) complexes, this does not imply that the π-acceptor ligand has radical character, because the iron(II) description arises from doubly-occupied frontier molecular orbitals that are shared equitably by the iron and the π-acceptor ligand, and the unpaired electrons lie on the metal. Despite the lack of substantial radical character on the ligands, alkyne and ketone fragments can couple to form a high-spin iron(III) complex with a cyclized metalladihydrofuran core.

The synthesis and reactivity of a new trimetallic complex Ti(NP)4Ni2 (NP = 2-diphenylphosphinopyrrolide) (3) is reported. Single-crystal X-ray diffraction and X-ray absorption studies point to a un...

Reaction of the mononuclear nonheme complex [Fe(II)(CH(3)CN)(N3PyS)]BF(4) (1) with an HNO donor, Piloty’s acid (PhSO(2)NHOH, P.A.), at low temperature affords a high-spin (S = 2) Fe(II)-P.A. intermediate (2), characterized by (57)Fe 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 (PhSO(2)NOH(−)) to [Fe(II)(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)]BF(4) (4), as evidenced by (1)H NMR, ATR-IR, and Mössbauer spectroscopies. Isotope labeling experiments with (15)N-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 Fe(II) complexes in the assisted degradation of HNO donor molecules.

Previously, we reported the synthesis of Ti[N(o-(NCH2P(iPr)2)C6H4)3] and the Fe–Ti complex, FeTi[N(o-(NCH2P(iPr)2)C6H4)3], abbreviated as TiL (1), and FeTiL (2), respectively. Herein, we describe the synthesis and characterization of the complete redox families of the monometallic Ti and Fe–Ti compounds. Cyclic voltammetry studies on FeTiL reveal both reduction and oxidation processes at −2.16 and −1.36 V (versus Fc/Fc+), respectively. Two isostructural redox members, [FeTiL]+ and [FeTiL]− (2ox and 2red, respectively) were synthesized and characterized, along with BrFeTiL (2-Br) and the monometallic [TiL]+ complex (1ox). The solid-state structures of the [FeTiL]+/0/– series feature short metal–metal bonds, ranging from 1.94–2.38 Å, which are all shorter than the sum of the Ti and Fe single-bond metallic radii (cf. 2.49 Å). To elucidate the bonding and electronic structures, the complexes were characterized with a host of spectroscopic methods, including NMR, EPR, and 57Fe Mössbauer, as well as Ti and Fe K-edge X-ray absorption spectroscopy (XAS). These studies, along with hybrid density functional theory (DFT) and time-dependent DFT calculations, suggest that the redox processes in the isostructural [FeTiL]+,0,– series are primarily Fe-based and that the polarized Fe–Ti π-bonds play a role in delocalizing some of the additional electron density from Fe to Ti (net 13%)., An isostructural redox series of Fe≡Ti complexes was investigated using a combination of spectroscopic methods and density functional theory to elucidate their electronic structures and to understand their polarized metal−metal bonding. Overall, the results support that the redox changes occur primarily at the Fe site though some electron density is delocalized to Ti. Hence, the Ti plays an important role in enhancing the redox flexibility of the single Fe site.

We report the first examples of the carbide ligand in (Cy3P)2Cl2Ru≡C (RuC) developing into a μ3 ligand toward metal centers. Conventionally, sterics exclude this coordination mode, but Fe2(CO)9 and Co2(CO)8 expel bridging CO ligands upon reaction with RuC to form trimetallic (Cy3P)2Cl2Ru═CFe2(CO)8 (RuCFe2) and (Cy3P)2Cl2Ru═CCo2(CO)7 (RuCCo2) complexes. Thus, the proximity offered by metal–metal associations in bimetallic carbonyl complexes allows the formation of trinuclear carbide complexes as verified by NMR, Mossbauer, and X-ray spectroscopic techniques.

The synthesis and characterization of a Co(II) dithiolato complex Co(M(e3)TACN)(S(2)SiMe(2)) (1) is reported. Reaction of 1 with O(2) generates a rare thiolate-ligated cobalt-superoxo species Co(O(2))(M(e3)TACN)(S(2)SiMe(2)) (2) that was characterized spectroscopically and structurally by resonance Raman, EPR, and X-ray absorption spectroscopies as well as density functional theory (DFT). Metal-superoxo species are proposed to S-oxygenate metal-bound thiolate donors in nonheme thiol dioxygenases, but 2 does not lead to S-oxygenation of the intramolecular thiolate donors, and does not react with exogenous sulfur donors. However, complex 2 is capable of oxidizing the O-H bonds of 2,2,6,6-tetramethylpiperidin-1-ol (TEMPOH) derivatives via H-atom abstraction. Complementary proton-coupled electron-transfer (PCET) reactivity is seen for 2 with separated proton/reductant pairs. The reactivity studies indicate that 2 can abstract H-atoms from weak X-H bonds with BDFE ≤ 70 kcal mol-1. DFT calculations predict that the putative Co(OOH) product has an O-H bond dissociation free energy (BDFE) = 67 kcal mol(−1), which matches the observed pattern of reactivity seen for 2. These data provide new information regarding the selectivity of S-oxygenation versus H-atom abstraction in thiolate-ligated nonheme metalloenzymes that react with O(2).

The reductive coupling of a chelated diimine ligand, Me2C(CH NCH2(2-py))2 (dmp(PM)2), by (TMEDA)TiCl2 afforded a Ti(IV) species containing a cyclopropane ring, Me2cPr(cis-2,3-NCH2(2-py))2TiCl2 (1). Dimeric [{Me2C(CH2N CH(2-py)(CH2(µ-N)CH2(2-py)}Mg]2 (3) was generated in the magnesium reduction of Me2C(CH2N CH(2-py))2 (dmp(PI)2) as its reduction incurred HAT. A di-azaallyl precursor was used to generate 7-membered ring chelates [{DBM(PI− )(PI2−)}CrIII]2 (4) and [{DBM(PI)(PI′)}Fe]2 (5). Structural studies of 1, 3 and 4 are included, as are electronic assessments of the reductive coupling to afford 1, and the paramagnetism of 3.

Ammonia oxidizing bacteria (AOB) use the cytotoxic, energetic molecule hydroxylamine (NH2OH) as a source of reducing equivalents for cellular respiration. Despite disproportionation or violent decomposition being typical outcomes of reactions of NH2OH with iron, AOB and anammox heme P460 proteins including cytochrome (cyt) P460 and hydroxylamine oxidoreductase (HAO) effect controlled, stepwise oxidation of NH2OH to nitric oxide (NO). Curiously, a recently characterized cyt P460 variant from the AOB Nitrosomonas sp. AL212 is able to form all intermediates of cyt P460 catalysis, but is nevertheless incompetent for NH2OH oxidation. We now show via site-directed mutagenesis, activity assays, spectroscopy, and structural biology that this lack of activity is attributable to the absence of a critical basic glutamate residue in the distal pocket above the heme P460 cofactor. This substitution is the only distinguishing characteristic of a protein that is otherwise effectively structurally and spectroscopically identical to an active variant. This highlights and reinforces a fundamental principal of metalloenzymology: metallocofactor inner-sphere geometric and electronic structures are in many cases insufficient for imbuing reactivity; a precisely defined outer coordination sphere contributed by the polypeptide matrix can be the key differentiator between a metalloenzyme and an unreactive metalloprotein.

Fixation and chemical reduction of CO2 are important for utilization of this abundant resource, and understanding the detailed mechanism of C–O cleavage is needed for rational development of CO2 reduction methods. Here, we describe a detailed analysis of the mechanism of the reaction of a masked two-coordinate cobalt(I) complex, LtBuCo (where LtBu = 2,2,6,6-tetramethyl-3,5-bis[(2,6-diisopropylphenyl)imino]hept-4-yl), with CO2, which yields two products of C–O cleavage, the cobalt(I) monocarbonyl complex LtBuCo(CO) and the dicobalt(II) carbonate complex (LtBuCo)2(μ-CO3). Kinetic studies and computations show that the κN,η6-arene isomer of LtBuCo rearranges to the κ2N,N′ binding mode prior to binding of CO2, which contrasts with the mechanism of binding of other substrates to LtBuCo. Density functional theory (DFT) studies show that the only low-energy pathways for cleavage of CO2 proceed through bimetallic mechanisms, and DFT and highly correlated domain-based local pair natural orbital coupled cluster (DLPNO-CCSD(T)) calculations reveal the cooperative effects of the two metal centers during facile C–O bond rupture. A plausible intermediate in the reaction of CO2 with LtBuCo is the oxodicobalt(II) complex LtBuCoOCoLtBu, which has been independently synthesized through the reaction of LtBuCo with N2O. The rapid reaction of LtBuCoOCoLtBu with CO2 to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO2 cleavage by LtBuCo. LtBuCoOCoLtBu is a novel example of a thoroughly characterized molecular cobalt–oxo complex where the cobalt ions are clearly in the +2 oxidation state. Its nucleophilic reactivity is a consequence of high charge localization on the μ-oxo ligand between two antiferromagnetically coupled high-spin cobalt(II) centers, as characterized by DFT and multireference complete active space self-consistent field (CASSCF) calculations.

Nitrogen K-edge X-ray absorption spectra (XAS) were obtained for 19 transition metal complexes bearing bipyridine, ethylenediamine, ammine, and nitride ligands. Time-dependent density functional theory (TDDFT) and DFT/restricted open configuration interaction singles (DFT/ROCIS) calculations were found to predict relative N K-edge XAS peak energies with good fidelity to experiment. The average difference (|ΔE|) between experimental and linear corrected calculated energies were found to be 0.55 ± 0.05 eV and 0.46 ± 0.04 eV, respectively, using the B3LYP hybrid density functional and scalar relativistically recontracted ZORA-def2-TZVP(-f) basis set. Deconvolution of these global correlations into individual N-donor ligand classes gave improved agreement between experiment and theory with |ΔE| less than 0.4 eV for all ligand classes in the case of DFT/ROCIS. In addition, calibration method-dependent values for the N 1s → 2p radial dipole integral of 25.4 ± 1.7 and 26.8 ± 1.9 are obtained, affording means to estimate the nitrogen 2p character in unfilled frontier molecular orbitals. For the complexes studied, nitrogen covalency values correlate well to those calculated by hybrid DFT with an R2 = 0.92 ± 0.01. Additionally, as a test case, a well-characterized PNP ligand framework (PNP = N[2-P(CHMe2)2-4-methylphenyl]21−) coordinated to NiII is investigated for its ability to act as a redox non-innocent ligand. Upon oxidation of (PNP)NiCl with [FeCp2](OTf) to its radical cation, [(PNP)NiCl](OTf) (OTf = triflate), a new low-energy feature emerges in the N K-edge XAS spectra. This feature is assigned as N 1s to a PNP-localized acceptor orbital exhibiting 27 ± 2% N 2p aminyl radical character, obtained using the aforementioned nitrogen covalency calibration. Combined, these data showcase a direct spectroscopic means of identifying redox-active N-donor ligands and also estimating nitrogen 2p covalency of frontier molecular orbitals in transition metal complexes.

International audience

Ammonia-oxidizing bacteria (AOB) convert ammonia (NH(3)) to nitrite (NO(2)(–)) as their primary metabolism, and thus provide a blueprint for the use of NH(3) as a chemical fuel. The first energy-producing step involves the homotrimeric enzyme hydroxylamine oxidoreductase (HAO), which was originally reported to oxidize hydroxylamine (NH(2)OH) to NO(2)(–). HAO uses the heme P460 cofactor as the site of catalysis. This heme is supported by seven other c hemes in each monomer that mediate electron transfer. Heme P460 cofactors are c-heme-based cofactors that have atypical protein cross-links between the peptide backbone and the porphyrin macrocycle. This cofactor has been observed in both the HAO and cytochrome (cyt) P460 protein families. However, there are differences; specifically, HAO uses a single tyrosine (Tyr) residue to form two covalent attachments to the macrocycle whereas cyt P460 uses a lysine (Lys) residue to form one. In Nitrosomonas europaea, which expresses both HAO and cyt P460, these enzymes achieve the oxidation of NH(2)OH and were both originally reported to produce NO(2)(–). Each can inspire means to effect controlled release of chemical energy. Spectroscopically studying the P460 cofactors of HAO is complicated by the 21 non-P460 heme cofactors which obscure the active site. However, mono-heme cyt P460 is more approachable biochemically and spectroscopically. Thus, we have used cyt P460 to study biological NH(2)OH oxidation. In aerobic conditions, substoichiometric production of NO(2)(–) was observed along with production of nitrous oxide (N(2)O). Under anaerobic conditions, however, N(2)O was the exclusive product of NH(2)OH oxidation. We have advanced understanding of the mechanism of this enzyme and have showed that a key intermediate is a ferric nitrosyl that can dissociate the bound nitric oxide (NO) molecule and react with O(2), thus producing NO(2)(–) abiotically. Because N(2)O was the true product of one P460 cofactor-containing enzyme, this prompted us to reinvestigate whether NO(2)(–) is enzymatically generated from HAO catalysis. Like cyt P460, we showed that HAO does not produce NO(2)(–) enzymatically, but unlike cyt P460, its final product is NO, establishing it as an intermediate of nitrification. More broadly, NO can be recognized as molecule common to the primary metabolisms of all organisms involved in nitrogen “defixation.” Delving deeper into cyt P460 yielded insights broadly applicable to controlled biochemical redox processes. Studies of an inactive cyt P460 from Nitrosomonas sp. AL212 showed that this enzyme was unable to oxidize NH(2)OH because it lacked a glutamate residue in its secondary coordination sphere that was present in the active N. europaea cyt P460 variant. Restoring the Glu residue imbued activity, revealing that a second-sphere base is Nature’s key to controlled oxidation of NH(2)OH. A key lesson of bioinorganic chemistry is reinforced: the polypeptide matrix is an essential part of dictating function. Our work also exposed some key functional contributions of non-canonical heme-protein cross-links. The heme-Lys cross-link of cyt P460 enforces the relative position of the cofactor and secondary-sphere residues. Moreover, the cross-link prevents the loss of the axial histidine residue, which stops catalysis, emphasizing the importance of this unique post-translational modification.

Nickel anions [(MeCN)Ni(CF

The nickel anions [(MeCN)Ni(CF3)3]1– and [Ni(CF3)4]2– were prepared by formal addition of three and four equivalents of [AgCF3] to [(dme)NiBr2] in the presence of supporting [PPh4] counter-ion. Detailed insights into the electronic properties of these new compounds were obtained through the use of density functional theory (DFT) calculations, spectroscopy-oriented configuration interaction (SORCI) calculations, X-ray absorption spectroscopy, and cyclic voltammetry. The data collectively show that trifluoromethyl complexes of nickel, even in the most common oxidation state of nickel(II), are highly covalent systems whereby a hole is distributed on the trifluoromethyl ligands and surprisingly rendering the metal to a physically more reduced state. In the cases of [(MeCN)Ni(CF3)3]1– and [Ni(CF3)4]2–, these complexes are better described as physically d9 metal complexes. [(MeCN)Ni(CF3)3]1– is electrophilic and reacts with other nucleophiles like phenoxide to yield the unsupported [(PhO)Ni(CF3)3]2– salt, revealing the broader potential of [(MeCN)Ni(CF3)3]1– in the development of ligandless trifluoromethylations at nickel. Proof-in-principle experiments show that reaction of [(MeCN)Ni(CF3)3]1– with an aryl iodonium salt yields trifluoromethylated arene, presumably via a high valent, unsupported, and formally organonickel(IV) intermediate. Evidence for the feasibility of such intermediates is provided with the structurally characterized [Ni(CF3)4(SO4)][PPh4]2, which was derived through the two electron oxidation of [Ni(CF3)4]2–.

The metallobiochemistry underlying the formation of the inorganic N–N-bond-containing molecules nitrous oxide (N(2)O), dinitrogen (N(2)), and hydrazine (N(2)H(4)) is essential to the lifestyles of diverse organisms. Similar reactions hold promise as means to use N-based fuels as alternative, carbon-free energy sources. This review discusses research efforts to understand the mechanisms underlying biological N–N bond formation in primary metabolism and how the associated reactions are tied to energy transduction and organismal survival. These efforts comprise studies of both natural and engineered metalloenzymes, as well as synthetic model complexes.

The introduction of (N2)3–• radicals into multinuclear lanthanide molecular magnets raised hysteresis temperatures by stimulating strong exchange coupling between spin centers. Radical ligands with larger donor atoms could promote more efficient magnetic coupling between lanthanides to provide superior magnetic properties. Here, we show that heavy chalcogens (S, Se, Te) are primed to fulfill these criteria. The moderately reducing Sm(II) complex, [Sm(N††)2], where N†† is the bulky bis(triisopropylsilyl)amide ligand, can be oxidized (i) by diphenyldichalcogenides E2Ph2 (E = S, Se, Te) to form the mononuclear series [Sm(N††)2(EPh)] (E = S, 1-S; Se, 1-Se, Te, 1-Te); (ii) S8 or Se8 to give dinuclear [{Sm(N††)2}2(μ-η2:η2-E2)] (E = S, 2-S2; Se, 2-Se2); or (iii) with Te=PEt3 to yield [{Sm(N††)2}(μ-Te)] (3). These complexes have been characterized by single crystal X-ray diffraction, multinuclear NMR, FTIR, and electronic spectroscopy; the steric bulk of N†† dictates the formation of mononuclear complexes with chalcogenate ligands and dinuclear species with the chalcogenides. The Lα1 fluorescence-detected X-ray absorption spectra at the Sm L3-edge yielded resolved pre-edge and white-line peaks for 1-S and 2-E2, which served to calibrate our computational protocol in the successful reproduction of the spectral features. This method was employed to elucidate the ground state electronic structures for proposed oxidized and reduced variants of 2-E2. Reactivity is ligand-based, forming species with bridging superchalcogenide (E2)−• and subchalcogenide (E2)3–• radical ligands. The extraordinarily large exchange couplings provided by these dichalcogenide radicals reveal their suitability as potential successors to the benchmark (N2)3–• complexes in molecular magnets., A spectroscopically verified computational study reveals the potential for giant exchange couplings using bridging dichalcogenide σ- and π-radicals as the ideal successors to dinitrogen radicals in lanthanide molecular magnets.

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, Mossbauer, 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.

Nitrous oxide (N2O) contributes significantly to ozone layer depletion and is a potent greenhouse agent, motivating interest in the chemical details of biological N2O fixation by nitrous oxide reductase (N2OR) during bacterial denitrification. In this study, we report a combined experimental/computational study of a synthetic [4Cu:1S] cluster supported by N-donor ligands that can be considered the closest structural and functional mimic of the CuZ catalytic site in N2OR reported to date. Quantitative N2 measurements during synthetic N2O reduction were used to determine reaction stoichiometry, which in turn was used as the basis for density functional theory (DFT) modeling of hypothetical reaction intermediates. The mechanism for N2O reduction emerging from this computational modeling involves cooperative activation of N2O across a Cu/S cluster edge. Direct interaction of the μ4-S ligand with the N2O substrate during coordination and N–O bond cleavage represents an unconventional mechanistic paradigm to be considered for the chemistry of CuZ and related metal–sulfur clusters. Consistent with hypothetical participation of the μ4-S unit in two-electron reduction of N2O, Cu K-edge and S K-edge X-ray absorption spectroscopy (XAS) reveal a high degree of participation by the μ4-S in redox changes, with approximately 21% S 3p contribution to the redox-active molecular orbital in the highly covalent [4Cu:1S] core, compared to approximately 14% Cu 3d contribution per copper. The XAS data included in this study represent the first spectroscopic interrogation of multiple redox levels of a [4Cu:1S] cluster and show high fidelity to the biological CuZ site., Experimental data and computational modeling indicates an active role for the bridging sulfide ligand in a synthetic CuZ model.

We report a mild and efficient electrochemical protocol to access a variety of vicinally C–O and C–N difunctionalized compounds from simple alkenes. Detailed mechanistic studies revealed a distinct reaction pathway from those previously reported for TEMPO-mediated reactions. In this mechanism, electrochemically generated oxoammonium ion facilitates the formation of azidyl radical via a charge-transfer complex with azide, TEMPO–N3. DFT calculations together with spectroscopic characterization provided a tentative structural assignment of this charge-transfer complex. Kinetic and kinetic isotopic effect studies revealed that reversible dissociation of TEMPO–N3 into TEMPO• and azidyl precedes the addition of these radicals across the alkene in the rate-determining step. The resulting azidooxygenated product could then be easily manipulated for further synthetic elaborations. The discovery of this new reaction pathway mediated by the TEMPO+/TEMPO• redox couple may expand the scope of aminoxyl radical chemistr...

A quick freeze shows an enzyme's secretsOrganic radicals are chemically useful in enzymatic reactions but are often hard to observe, owing to their short lifetimes. Donget al.used rapid freeze-quench methods to trap two intermediates formed by a noncanonical radical S-adenosylmethionine (SAM) enzyme: a fragmented SAM molecule bound to the iron-sulfur cluster through an iron-carbon bond and a product-like radical. The structure of the SAM-bound enzyme reveals a noncolinear arrangement of carbon, sulfur, and iron atoms. The arrangement of bonds suggests that the organometallic intermediate may be created through a two-electron nucleophilic mechanism. A subsequent radical intermediate is formed on the protein substrate and resolves by oxidation to form the amino acid product diphthamide.Science, this issue p.1247