Publications by authors named "Eckhard Bill"

290 Publications

Cooperative Co-Activation of Water and Hypochlorite by a Non-Heme Diiron(III) Complex.

J Am Chem Soc 2021 Sep 7;143(37):15400-15412. Epub 2021 Sep 7.

Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej 55, 5320 Odense M, Denmark.

Aqueous solutions of the iron(III) complex of ,,'-tris(2-pyridylmethyl)ethylenediamine-'-acetate (tpena) react with hypochlorite (ClO) to produce the reactive high-valent [Fe(O)(tpena)]. Under catalytic conditions, in bicarbonate-buffered media (pH 8) with a set ionic strength (10 mM NaCl), kinetic analysis shows that two equivalents of [Fe(O)(tpena)] per one ClO are produced, with benign chloride ions the only byproduct. An unprecedented supramolecular activation of ClO by {(HCO)⊂[(tpena)Fe(μ-O)Fe(Htpena)]} is proposed. This mode of activation has great advantage for use in the catalytic oxidation of C-H bonds in water since: (i) the catalyst scaffold is protected from oxidative degradation and (ii) undesirable radical side reactions which produce toxic chlorinated compounds are circumvented by this novel coactivation of water and ClO. The unique activation mechanism by the Fe-tpena system makes possible the destruction of organic contaminants as an add-on technology to water disinfection by chlorination, demonstrated here through (i) the catalytic oxidation of micropollutant metaldehyde, and (ii) mineralization of the model substrate formate. The resting-state speciation at pH 3, 5, 7, and 9, as well as the catalytically active iron speciation are characterized with Mössbauer and EPR spectroscopy and supported by DFT calculations. Our study provides fundamentally new insights into the design and activation mode of iron-based catalysts relevant to applications in water remediation.
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http://dx.doi.org/10.1021/jacs.1c07669DOI Listing
September 2021

Rational Design of a Confacial Pentaoctahedron Anisotropic Exchange in a Linear ZnIIFeIIIFeIIIFeIIIZnII Complex.

Chemistry 2021 Aug 24. Epub 2021 Aug 24.

Bielefeld University, Chemistry Department, Universitätsstr. 24, 33615, Bielefeld, GERMANY.

The first confacial pentaoctahedron comprised of transition metal ions namely Zn II Fe III A Fe III B Fe III A Zn II has been synthesized using a dinucleating nonadentate ligand. The face-sharing bridging mode enforces short Zn II •••Fe III A and Fe III A •••Fe III B distances of 2.83 and 2.72 Å, respectively. Ab-initio CASSCF/NEVPT2 calculations provide significant negative zero-field splittings for Fe III A and Fe III B with \ D A \ > \ D B \ with the main component along the C 3 axis. Hence, a spin-Hamiltonian comprised of anisotropic exchange, zero-field, and Zeeman term was employed. This allowed by following the boundary conditions from the theoretical results the simulation in a theory-guided parameter determination with J xy =+0.37, J z =-0.32, D A =-1.21, E A =-0.24, D B =-0.35, and E B =-0.01 cm -1 supported by simulations of high-field magnetic Mössbauer spectra recorded at 2 K. The weak but ferromagnetic Fe III A Fe III B interaction arises from the small bridging angle of 84.8° being at the switch from anti- to ferromagnetic for the face-sharing bridging mode.
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http://dx.doi.org/10.1002/chem.202102572DOI Listing
August 2021

A [3Cu:2S] cluster provides insight into the assembly and function of the Cu site of nitrous oxide reductase.

Chem Sci 2021 Jan 15;12(9):3239-3244. Epub 2021 Jan 15.

Institut für Biochemie, Albert-Ludwigs-Universität Freiburg Albertstrasse 21 79104 Freiburg im Breisgau Germany

Nitrous oxide reductase (NOR) is the only known enzyme reducing environmentally critical nitrous oxide (NO) to dinitrogen (N) as the final step of bacterial denitrification. The assembly process of its unique catalytic [4Cu:2S] cluster Cu remains scarcely understood. Here we report on a mutagenesis study of all seven histidine ligands coordinating this copper center, followed by spectroscopic and structural characterization and based on an established, functional expression system for NOR in . While no copper ion was found in the Cu binding site of variants H129A, H130A, H178A, H326A, H433A and H494A, the H382A variant carried a catalytically inactive [3Cu:2S] center, in which one sulfur ligand, S, had relocated to form a weak hydrogen bond to the sidechain of the nearby lysine residue K454. This link provides sufficient stability to avoid the loss of the sulfide anion. The UV-vis spectra of this cluster are strikingly similar to those of the active enzyme, implying that the flexibility of S may have been observed before, but not recognized. The sulfide shift changes the metal coordination in Cu and is thus of high mechanistic interest.
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http://dx.doi.org/10.1039/d0sc05204cDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8179356PMC
January 2021

Experimental and Theoretical Evidence for an Unusual Almost Triply Degenerate Electronic Ground State of Ferrous Tetraphenylporphyrin.

Inorg Chem 2021 Apr 19;60(7):4966-4985. Epub 2021 Mar 19.

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mülheim an der Ruhr, Germany.

Iron porphyrins exhibit unrivalled catalytic activity for electrochemical CO-to-CO conversion. Despite intensive experimental and computational studies in the last 4 decades, the exact nature of the prototypical square-planar [Fe(TPP)] complex (; TPP = tetraphenylporphyrinate dianion) remained highly debated. Specifically, its intermediate-spin ( = 1) ground state was contradictorily assigned to either a nondegenerate A state with a (d)(d)(d) configuration or a degenerate E state with a (d)(d)(d)/(d)(d)(d) configuration. To address this question, we present herein a comprehensive, spectroscopy-based theoretical and experimental electronic-structure investigation on complex . Highly correlated wave-function-based computations predicted that A and E are well-isolated from other triplet states by ca. 4000 cm, whereas their splitting Δ is on par with the effective spin-orbit coupling (SOC) constant of iron(II) (≈400 cm). Therfore, we invoked an effective Hamiltonian (EH) operating on the nine magnetic sublevels arising from SOC between the A and E states. This approach enabled us to successfully simulate all spectroscopic data of obtained by variable-temperature and variable-field magnetization, applied-field Fe Mössbauer, and terahertz electron paramagnetic resonance measurements. Remarkably, the EH contains only three adjustable parameters, namely, the energy gap without SOC, Δ, an angle θ that describes the mixing of (d)(d)(d) and (d)(d)(d) configurations, and the ⟨⟩ expectation value of the iron d orbitals that is necessary to estimate the Fe magnetic hyperfine coupling tensor. The EH simulations revealed that the triplet ground state of is genuinely multiconfigurational with substantial parentages of both A (<88%) and E (>12%), owing to their accidental near-triple degeneracy with Δ = +950 cm. As a consequence of this peculiar electronic structure, exhibits a huge effective magnetic moment (4.2 μB at 300 K), large temperature-independent paramagnetism, a large and positive axial zero-field splitting, strong easy-plane magnetization ( ≈ 3 and ≈ 1.7) and a large and positive internal field at the Fe nucleus aligned in the plane. Further in-depth analyses suggested that ≫ is a general spectroscopic signature of near-triple orbital degeneracy with more than half-filled pseudodegenerate orbital sets. Implications of the unusual electronic structure of for CO reduction are discussed.
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http://dx.doi.org/10.1021/acs.inorgchem.1c00031DOI Listing
April 2021

Synthetic ferripyrophyllite: preparation, characterization and catalytic application.

Dalton Trans 2021 Jan 12;50(3):850-857. Epub 2021 Jan 12.

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany.

Sheet silicates, also known as phyllosilicates, contain parallel sheets of tetrahedral silicate built up by [SiO] entities connected through intermediate metal-oxygen octahedral layers. The well-known minerals talc and pyrophyllite are belonging to this group based on magnesium and aluminium, respectively. Surprisingly, the ferric analogue rarely occurs in nature and is found in mixtures and conglomerates with other materials only. While partial incorporation of iron into pyrophyllites has been achieved, no synthetic protocol for purely iron-based pyrophyllite has been published yet. Here we report about the first artificial synthesis of ferripyrophyllite under exceptional mild conditions. A similar ultrathin two-dimensional (2D) nanosheet morphology is obtained as in talc or pyrophyllite but with iron(iii) as a central metal. The high surface material exhibits a remarkably high thermostability. It shows some catalytic activity in ammonia synthesis and can serve as catalyst support material for noble metal nanoparticles.
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http://dx.doi.org/10.1039/d0dt03125aDOI Listing
January 2021

Ligand Tailoring Toward an Air-Stable Iron(V) Nitrido Complex.

J Am Chem Soc 2021 Jan 12;143(3):1458-1465. Epub 2021 Jan 12.

Department of Chemistry and Pharmacy, Inorganic Chemistry, Friedrich-Alexander-University Erlangen-Nürnberg (FAU), Egerlandstraße 1, 91058 Erlangen, Germany.

A new supporting ligand, -[2-(3-mesityl-idazol-2-ylidene)ethyl]amie (TIMMN), was developed and utilized to isolate an air-stable iron(V) complex bearing a terminal nitrido ligand, which was synthesized by one-electron oxidation from the iron(IV) precursor. Single-crystal X-ray diffraction analyses of both complexes reveal that the metal-centered oxidation is escorted by iron nitride (Fe≡N) bond elongation, which in turn is accompanied by the accommodation of the high-valence iron center closer to the equatorial plane of a trigonal bipyramid. This contrasts with the previous observation of the only other literature-known Fe(IV)≡N/Fe(V)≡N redox pair, namely, [PhB(Im)FeN]. On the basis of Fe Mössbauer, EPR, and UV/vis electronic absorption spectroscopy as well as quantum chemical calculations, we identified the lesser degree of pyramidalization around the iron atom, the Jahn-Teller distortion, and the resulting nature of the SOMO to be the decisive factors at play.
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http://dx.doi.org/10.1021/jacs.0c11141DOI Listing
January 2021

Histidine-Gated Proton-Coupled Electron Transfer to the Cu Site of Nitrous Oxide Reductase.

J Am Chem Soc 2021 01 30;143(2):830-838. Epub 2020 Dec 30.

Institut für Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstraße 21, 79104 Freiburg im Breisgau, Germany.

Copper-containing nitrous oxide reductase (NOR) is the only known enzyme to catalyze the conversion of the environmentally critical greenhouse gas nitrous oxide (NO) to dinitrogen (N) as the final step of bacterial denitrification. Other than its unique tetranuclear active site Cu, the binuclear electron entry point Cu is also utilized in other enzymes, including cytochrome oxidase. In the Cu site of NOR, a histidine ligand was found to undergo a conformational flip upon binding of the substrate NO between the two copper centers. Here we report on the systematic mutagenesis and spectroscopic and structural characterization of this histidine and surrounding H-bonding residues, based on an established functional expression system for NOR in . A single hydrogen bond from Ser550 is sufficient to stabilize an unbound conformation of His583, as shown in a Asp576Ala variant, while the additional removal of the hydrogen bond in a Asp576Ala/Ser550Ala double variant compelled His583 to stay in a bound conformation as a ligand to Cu. Systematic mutagenesis of His583 to Ala, Asp, Asn, Glu, Gln, Lys, Phe, Tyr, and Trp showed that although both the Cu and Cu sites were present in all the variants, only the ones with a protonable side chain, i.e., His, Asp, and Glu, were able to mediate electron transfer at physiological pH. This observation is in line with a proton-coupled electron transfer mechanism at the Cu site of NOR.
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http://dx.doi.org/10.1021/jacs.0c10057DOI Listing
January 2021

A Pseudotetrahedral Terminal Oxoiron(IV) Complex: Mechanistic Promiscuity in C-H Bond Oxidation Reactions.

Angew Chem Int Ed Engl 2021 03 15;60(12):6752-6756. Epub 2021 Feb 15.

Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, 12489, Berlin, Germany.

S=2 oxoiron(IV) species act as reactive intermediates in the catalytic cycle of nonheme iron oxygenases. The few available synthetic S=2 Fe =O complexes known to date are often limited to trigonal bipyramidal and very rarely to octahedral geometries. Herein we describe the generation and characterization of an S=2 pseudotetrahedral Fe =O complex 2 supported by the sterically demanding 1,4,7-tri-tert-butyl-1,4,7-triazacyclononane ligand. Complex 2 is a very potent oxidant in hydrogen atom abstraction (HAA) reactions with large non-classical deuterium kinetic isotope effects, suggesting hydrogen tunneling contributions. For sterically encumbered substrates, direct HAA is impeded and an alternative oxidative asynchronous proton-coupled electron transfer mechanism prevails, which is unique within the nonheme oxoiron community. The high reactivity and the similar spectroscopic parameters make 2 one of the best electronic and functional models for a biological oxoiron(IV) intermediate of taurine dioxygenase (TauD-J).
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http://dx.doi.org/10.1002/anie.202015896DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7985879PMC
March 2021

Biogenesis of a Designed Iron-Sulfur Protein.

ACS Synth Biol 2020 12 13;9(12):3400-3407. Epub 2020 Nov 13.

Center for Advanced Biotechnology and Medicine and the Department of Biochemistry and Molecular Biology, Robert Wood Johnson Medical School, Rutgers University, Piscataway, New Jersey 08854 United States.

expression of metalloproteins requires specific metal trafficking and incorporation machinery inside the cell. Synthetic designed metalloproteins are typically purified without the target metal, which is subsequently introduced through reconstitution. The extra step complicates protein optimization by high-throughput library screening or laboratory evolution. We demonstrate that a designed coiled-coil iron-sulfur protein (CCIS) assembles robustly with [4Fe-4S] clusters . While reconstitution produces a mixture of oligomers that depends on solution conditions, production generates a stable homotrimer coordinating a single, diamagnetic [4Fe-4S] cluster. The multinuclear cluster of assembled CCIS is more resistant to degradation by molecular oxygen. Only one of the two metal coordinating half-sites is required indicating specificity of molecular recognition in recruitment of the metal cluster. CCIS, unbiased by evolution, is a unique platform to examine iron-sulfur protein biogenesis and develop synthetic multinuclear oxidoreductases.
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http://dx.doi.org/10.1021/acssynbio.0c00514DOI Listing
December 2020

Ambiphilicity of a mononuclear cobalt(III) superoxo complex.

Chem Commun (Camb) 2020 Nov;56(94):14821-14824

Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan. and Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan.

Addition of HOTf to a mixture of CoIII(BDPP)(O2˙) (1, H2BDPP = 2,6-bis((2-(S)-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Cp*2Fe produced H2O2 in high yield implying formation of CoIII(BDPP)(OOH) (3), and reaction of Sc(OTf)3 with the same mixture gave a peroxo-bridged CoIII/ScIII5. These findings demonstrate the ambiphilic property of CoIII-superoxo 1.
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http://dx.doi.org/10.1039/d0cc05337fDOI Listing
November 2020

Bioinspired Nickel Complexes Supported by an Iron Metalloligand.

Inorg Chem 2020 Oct 20;59(19):14251-14262. Epub 2020 Sep 20.

Max-Planck-Institut für Chemische Energiekonversion, Stiftstraße 34-36, 45470 Mülheim an der Ruhr, Germany.

Nature utilizes multimetallic sites in metalloenzymes to enable multielectron chemical transformations at ambient conditions and low overpotentials. One such example of multimetallic cooperativity can be found in the C-cluster of Ni-carbon monoxide dehydrogenase (CODH), which interconverts CO and CO. Toward a potential functional model of the C-cluster, a family of Ni-Fe bimetallic complexes was synthesized that contain direct metal-metal bonding interactions. The complexes were characterized by X-ray crystallography, various spectroscopies (NMR, EPR, UV-vis, Mössbauer), and theoretical calculations. The Ni-Fe bimetallic system has a reversible Fe(III)/Fe(II) redox couple at -2.10 V (vs Fc/Fc). The Fe-based "redox switch" can turn on CO reactivity at the Ni(0) center by leveraging the Ni→Fe dative interaction to attenuate the Ni(0) electron density. The reduced Ni(0)Fe(II) species mediated the formal two-electron reduction of CO to CO, providing a Ni-CO adduct and CO as products. During the reaction, an intermediate was observed that is proposed to be a Ni-CO species.
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http://dx.doi.org/10.1021/acs.inorgchem.0c02041DOI Listing
October 2020

Isolation of a Homoleptic Non-oxo Mo(V) Alkoxide Complex: Synthesis, Structure, and Electronic Properties of Penta--Butoxymolybdenum.

J Am Chem Soc 2020 Sep 10;142(38):16392-16402. Epub 2020 Sep 10.

Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr, Germany.

Treatment of [MoCl(THF)] with MOBu (M = Na, Li) does not result in simple metathetic ligand exchange but entails disproportionation with formation of the well-known dinuclear complex [(BuO)Mo≡Mo(OBu)] and a new paramagnetic compound, [Mo(OBu)]. This particular five-coordinate species is the first monomeric, homoleptic, all-oxygen-ligated but non-oxo 4d Mo(V) complex known to date; as such, it proves that the dominance of the Mo═O group over (high-valent) molybdenum chemistry can be challenged. [Mo(OBu)] was characterized in detail by a combined experimental/computational approach using X-ray diffraction; UV/vis, MCD, IR, EPR, and NMR spectroscopy; and quantum chemistry. The recorded data confirm a Jahn-Teller distortion of the structure, as befitting a d species, and show that the complex undergoes Berry pseudorotation. The alkoxide ligands render the disproportionation reaction, leading the formation of [Mo(OBu)] to be particularly facile, even though the parent complex [MoCl(THF)] itself was also found to be intrinsically unstable; remarkably, this substrate converts into a crystalline material, in which the newly formed Mo(III) and Mo(V) products cohabitate the same unit cell.
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http://dx.doi.org/10.1021/jacs.0c07073DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7517713PMC
September 2020

Mitochondrial [4Fe-4S] protein assembly involves reductive [2Fe-2S] cluster fusion on ISCA1-ISCA2 by electron flow from ferredoxin FDX2.

Proc Natl Acad Sci U S A 2020 08 12;117(34):20555-20565. Epub 2020 Aug 12.

Institut für Zytobiologie und Zytopathologie, Philipps-Universität Marburg, 35032 Marburg, Germany;

The essential process of iron-sulfur (Fe/S) cluster assembly (ISC) in mitochondria occurs in three major phases. First, [2Fe-2S] clusters are synthesized on the scaffold protein ISCU2; second, these clusters are transferred to the monothiol glutaredoxin GLRX5 by an Hsp70 system followed by insertion into [2Fe-2S] apoproteins; third, [4Fe-4S] clusters are formed involving the ISC proteins ISCA1-ISCA2-IBA57 followed by target-specific apoprotein insertion. The third phase is poorly characterized biochemically, because previous in vitro assembly reactions involved artificial reductants and lacked at least one of the in vivo-identified ISC components. Here, we reconstituted the maturation of mitochondrial [4Fe-4S] aconitase without artificial reductants and verified the [2Fe-2S]-containing GLRX5 as cluster donor. The process required all components known from in vivo studies (i.e., ISCA1-ISCA2-IBA57), yet surprisingly also depended on mitochondrial ferredoxin FDX2 and its NADPH-coupled reductase FDXR. Electrons from FDX2 catalyze the reductive [2Fe-2S] cluster fusion on ISCA1-ISCA2 in an IBA57-dependent fashion. This previously unidentified electron transfer was occluded during previous in vivo studies due to the earlier FDX2 requirement for [2Fe-2S] cluster synthesis on ISCU2. The FDX2 function is specific, because neither FDX1, a mitochondrial ferredoxin involved in steroid production, nor other cellular reducing systems, supported maturation. In contrast to ISC factor-assisted [4Fe-4S] protein assembly, [2Fe-2S] cluster transfer from GLRX5 to [2Fe-2S] apoproteins occurred spontaneously within seconds, clearly distinguishing the mechanisms of [2Fe-2S] and [4Fe-4S] protein maturation. Our study defines the physiologically relevant mechanistic action of late-acting ISC factors in mitochondrial [4Fe-4S] cluster synthesis, trafficking, and apoprotein insertion.
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http://dx.doi.org/10.1073/pnas.2003982117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7456137PMC
August 2020

Bimetallic iron-tin catalyst for N to NH and a silyldiazenido model intermediate.

Chem Commun (Camb) 2020 Sep;56(75):11030-11033

Department of Chemistry, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431, USA.

A tin-supported iron catalyst produces 5.9 turnovers of NH3 from N2, using [Ph2NH2]OTf as the acid and CoCp2* as the reductant. Two redox states of the Fe(N2) adduct and an Fe silyldiazenido complex were characterized using X-ray crystallography along with NMR and Mössbauer spectroscopies. Density functional theory calculations reveal that the charge on the Sn center correlates strongly with both the polarization of the N2 moiety and the charge on the distal N atom.
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http://dx.doi.org/10.1039/d0cc04563bDOI Listing
September 2020

A metastable Ru azido complex with metallo-Staudinger reactivity.

Chem Commun (Camb) 2020 Sep;56(73):10738-10741

Department of Chemistry, University of Wisconsin - Madison, 1101 University Avenue, Madison, Wisconsin 53706, USA.

The metastable purple [(Py5Me2)RuIII(N3)]2+ ion reacts with PPh3 at room temperature to form the phosphinimine complex [(Py5Me2)RuII(N(H)PPh3)]2+ and free [H2NPPh3]+ in a combined 23% conversion. Mechanistic studies suggest that this is the first metallo-Staudinger reaction of a late transition metal that bypasses the nitrido mechanism and instead utilizes a Ru-N[double bond, length as m-dash]N[double bond, length as m-dash]N-PPh3 phosphazide intermediate.
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http://dx.doi.org/10.1039/d0cc04426aDOI Listing
September 2020

A Manganese(IV)-Hydroperoxo Intermediate Generated by Protonation of the Corresponding Manganese(III)-Superoxo Complex.

J Am Chem Soc 2020 06 2;142(23):10255-10260. Epub 2020 Jun 2.

Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan.

Earlier work revealed that metal-superoxo species primarily function as radicals and/or electrophiles. Herein, we present ambiphilicity of a Mn-superoxo complex revealed by its proton- and metal-coupled electron-transfer processes. Specifically, a Mn-hydroperoxo intermediate, [Mn(BDPP)(OOH)] (, HBDPP = 2,6-bis((2-()-di(4-bromo)phenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) was generated by treatment of a Mn-superoxo complex, Mn(BDPP)(O) () with trifluoroacetic acid at -120 °C. Detailed insights into the electronic structure of are obtained using resonance Raman and multi-frequency electron paramagnetic resonance spectroscopies coupled with density functional theory calculations. Similarly, the reaction of with scandium(III) triflate was shown to give a Mn(IV)/Sc(III) bridging peroxo species, [Mn(BDPP)(OO)Sc(OTf)] (). Furthermore, it is found that deprotonation of quantitatively regenerates , and that one-electron oxidation of the corresponding Mn-hydroperoxo species, Mn(BDPP)(OOH) (), also yields .
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http://dx.doi.org/10.1021/jacs.0c02756DOI Listing
June 2020

Stoichiometric Formation of an Oxoiron(IV) Complex by a Soluble Methane Monooxygenase Type Activation of O at an Iron(II)-Cyclam Center.

J Am Chem Soc 2020 04 18;142(13):5924-5928. Epub 2020 Mar 18.

Institut für Chemie, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany.

In soluble methane monooxygenase enzymes (MMO), dioxygen (O) is activated at a diiron(II) center to form an oxodiiron(IV) intermediate that performs the challenging oxidation of methane to methanol. An analogous mechanism of O activation at mono- or dinuclear iron centers is rare in the synthetic chemistry. Herein, we report a mononuclear non-heme iron(II)-cyclam complex, -, that activates O to form the corresponding iron(IV)-oxo complex, -, via a mechanism reminiscent of the O activation process in MMO. The conversion of - to - proceeds via the intermediate formation of an iron(III)-superoxide species , which could be trapped and spectroscopically characterized at -50 °C. Surprisingly, is a stronger oxygen atom transfer (OAT) agent than -; performs OAT to - or PPh to yield - quantitatively. Furthermore, - oxidizes the aromatic C-H bonds of 2,6-di--butylphenol, which, together with the strong OAT ability of , represents new domains of oxoiron(IV) and superoxoiron(III) reactivities.
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http://dx.doi.org/10.1021/jacs.9b13756DOI Listing
April 2020

Bis(imino)pyrazine-Supported Iron Complexes: Ligand-Based Redox Chemistry, Dearomatization, and Reversible C-C Bond Formation.

Inorg Chem 2020 Feb 28;59(4):2604-2612. Epub 2020 Jan 28.

Anorganisch-Chemisches Institut , Universität Heidelberg , Im Neuenheimer Feld 276 , 69120 Heidelberg , Germany.

Iron complexes supported by novel π-acidic bis(imino)pyrazine (PDI) ligands can be functionalized at the nonligated nitrogen atom, and this has a marked effect on the redox properties of the resulting complexes. Dearomatization is observed in the presence of cobaltocene, which reversibly reduces the pyrazine core and not the imine functionality, as observed in the case of the pyridinediimine-ligated iron analogues. The resulting ligand-based radical is prone to dimerization through the formation of a long carbon-carbon bond, which can be subsequently cleaved under mild oxidative conditions.
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http://dx.doi.org/10.1021/acs.inorgchem.9b03665DOI Listing
February 2020

Dispersion Forces Drive the Formation of Uranium-Alkane Adducts.

J Am Chem Soc 2020 Jan 13;142(4):1864-1870. Epub 2020 Jan 13.

Department of Molecular Theory and Spectroscopy , Max-Planck Institute for Kohlenforschung , Kaiser Wilhelm-Platz-1 , 45470 Mülheim-an-der-Ruhr , Germany.

Single-crystal cryogenic X-ray diffraction at 6 K, electron paramagnetic resonance spectroscopy, and correlated electronic structure calculations are combined to shed light on the nature of the metal-tris(aryloxide) and η-H, C metal-alkane interactions in the [((ArO)tacn)U(cy-C6)]·(cy-C6) adduct. An analysis of the ligand field experienced by the uranium center using ab initio ligand field theory in combination with the angular overlap model yields rather unusual U-O and U-N bonding parameters for the metal-tris(aryloxide) interaction. These parameters are incompatible with the concept of σ and π metal-ligand overlap. For that reason, it is deduced that metal-ligand bonding in the [((ArO)tacn)U] moiety is predominantly ionic. The bonding interaction within the [((ArO)tacn)U] moiety is shown to be dispersive in nature and essentially supported by the upper-rim Bu groups of the (ArO)tacn ligand. Our findings indicate that the axial alkane molecule is held in place by the guest-host effect rather than direct metal-alkane ionic or covalent interactions.
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http://dx.doi.org/10.1021/jacs.9b10620DOI Listing
January 2020

Unusual Magneto-Structural Features of the Halo-Substituted Materials [Fe (5-X-salMeen) ]Y: a Cooperative [HS-HS]↔[HS-LS] Spin Transition.

Chemistry 2020 Apr 24;26(21):4766-4779. Epub 2020 Mar 24.

Department of Chemistry, College of Science, Sultan Qaboos University, Private Bag 36, Al-Khod 123, Muscat, Sultanate of Oman.

X-ray structures of the halo-substituted complexes [Fe (5-X-salMeen) ]ClO (X=F, Cl, Br, I) [salMeen=N-methyl-N-(2-aminoethyl)salicylaldiminate]at RT have revealed the presence of two discrete HS complex cations in the crystallographic asymmetric unit with two perchlorate counter ions linking them by N-H ⋅⋅⋅O interactions. At 90 K, the two complex cations are distinctly HS and LS, a rare crystallographic observation of this coexistence in the Fe -salRen (R=alkyl) spin-crossover (SCO) system. At both temperatures, crystal packing shows dimerization through C-H ⋅⋅⋅O interactions, a key feature for SCO cooperativity. Moreover, there are noncovalent contacts between the complex cations through type-II halogen-halogen bonds, which are novel in this system. The magnetic profiles and Mössbauer spectra concur with the structural analyses and reveal 50 % SCO of the type [HS-HS]↔[HS-LS] with a broad plateau. In contrast, [Fe (5-Cl-salMeen) ]BPh ⋅2MeOH is LS and exhibits a temperature-dependent crystallographic phase transition, exemplifying the influence of lattice solvents and counter ions on SCO.
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http://dx.doi.org/10.1002/chem.201904744DOI Listing
April 2020

Planar three-coordinate iron sulfide in a synthetic [4Fe-3S] cluster with biomimetic reactivity.

Nat Chem 2019 11 14;11(11):1019-1025. Epub 2019 Oct 14.

Department of Chemistry, Yale University, New Haven, CT, USA.

Iron-sulfur clusters are emerging as reactive sites for the reduction of small-molecule substrates. However, the four-coordinate iron sites of typical iron-sulfur clusters rarely react with substrates, implicating three-coordinate iron. This idea is untested because fully sulfide-coordinated three-coordinate iron is unprecedented. Here we report a new type of [4Fe-3S] cluster that features an iron centre with three bonds to sulfides, and characterize examples of the cluster in three oxidation levels using crystallography, spectroscopy, and ab initio calculations. Although a high-spin electronic configuration is characteristic of other iron-sulfur clusters, the three-coordinate iron centre in these clusters has a surprising low-spin electronic configuration due to the planar geometry and short Fe-S bonds. In a demonstration of biomimetic reactivity, the [4Fe-3S] cluster reduces hydrazine, a natural substrate of nitrogenase. The product is the first example of NH bound to an iron-sulfur cluster. Our results demonstrate that three-coordinate iron supported by sulfide donors is a plausible precursor to reactivity in iron-sulfur clusters like the FeMoco of nitrogenase.
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http://dx.doi.org/10.1038/s41557-019-0341-7DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6858550PMC
November 2019

A Series of Iron Nitrosyl Complexes {Fe-NO} and a Fleeting {Fe-NO} Intermediate en Route to a Metalacyclic Iron Nitrosoalkane.

J Am Chem Soc 2019 10 17;141(43):17217-17235. Epub 2019 Oct 17.

Department of Chemistry and Pharmacy, Inorganic Chemistry , Friedrich-Alexander-University Erlangen-Nürnberg (FAU) , Egerlandstrasse 1 , D-91058 Erlangen , Germany.

Iron-nitrosyls have fascinated chemists for a long time due to the noninnocent nature of the NO ligand that can exist in up to five different oxidation and spin states. Coordination to an open-shell iron center leads to complex electronic structures, which is the reason Enemark-Feltham introduced the {Fe-NO} notation. In this work, we succeeded in characterizing a series of {Fe-NO} complexes, including a reactive {Fe-NO} intermediate. All complexes were synthesized with the tris--heterocyclic carbene ligand tris[2-(3-mesitylimidazol-2-ylidene)ethyl]amine (TIMEN), which is known to support iron in high and low oxidation states. Reaction of NOBF with [(TIMEN)Fe] resulted in formation of the {Fe-NO} compound [(TIMEN)Fe(NO)(CHCN)](BF) (). Stepwise chemical reduction with Zn, Mg, and Na/Hg leads to the isostructural series of high-spin iron nitrosyl complexes {Fe-NO} (-). Reduction of {Fe-NO} with Cs electride finally yields the highly reduced {Fe-NO} intermediate, key to formation of [Cs(crypt-222)][(TIMEN)Fe(NO)], () featuring a metalacyclic [Fe-(NO-NHC)] nitrosoalkane unit. All complexes were characterized by single-crystal XRD analyses, temperature and field-dependent SQUID magnetization methods, as well as Fe Mössbauer, IR, UV/vis, multinuclear NMR, and dual-mode EPR spectroscopy. Spectroscopy-based DFT analyses provide insight into the electronic structures of all compounds and allowed assignments of oxidation states to iron and NO ligands. An alternative synthesis to the {Fe-NO} complex was found via oxygenation of the nitride complex [(TIMEN)Fe(N)](BF). Surprisingly, the resulting {Fe-NO} species is electronically and structural similar to the [(TIMEN)Fe(N)] precursor. Based on the structural and electronic similarities between this nitrosyl/nitride complex couple, we adopted the strategy, developed by Wieghardt et al., of extending the Enemark-Feltham nomenclature to nitrido complexes, rendering [(TIMEN)Fe(N)] as a {Fe-N} species.
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http://dx.doi.org/10.1021/jacs.9b08053DOI Listing
October 2019

Conversion of a Fleeting Open-Shell Iron Nitride into an Iron Nitrosyl.

Angew Chem Int Ed Engl 2019 Dec 22;58(49):17589-17593. Epub 2019 Oct 22.

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470, Mülheim an der Ruhr, Germany.

Terminal metal nitrides have been proposed as key intermediates in a series of pivotal chemical transformations. However, exploring the chemical activity of transient tetragonal iron(V) nitrides is largely impeded by their facile dimerization in fluid solutions. Herein, in situ EPR and Mössbauer investigations are presented of unprecedented oxygenation of a paramagnetic iron(V) nitrido intermediate, [Fe N(cyclam-ac)] (2, cyclam-ac =1,4,8,11-tetraazacyclotetradecane-1-acetate anion), yielding an iron nitrosyl complex, [Fe(NO)(cyclam-ac)] (3). Further theoretical studies suggest that during the reaction a closed-shell singlet O atom is transferred to 2. Consequently, the N-O bond formation does not follow a radical coupling mechanism proposed for the N-N bond formation but is accomplished by three mutual electron-transfer pathways between 2 and the O atom donor, thanks to the ambiphilic nature of 2.
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http://dx.doi.org/10.1002/anie.201908689DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6899486PMC
December 2019

Electronic Structure, Vibrational Spectra, and Spin-Crossover Properties of Vacuum-Evaporable Iron(II) Bis(dihydrobis(pyrazolyl)borate) Complexes with Diimine Coligands. Origin of Giant Raman Features.

Inorg Chem 2019 Oct 17;58(19):12873-12887. Epub 2019 Sep 17.

Institute of Inorganic Chemistry , Christian-Albrechts-University Kiel , Max-Eyth-Strasse 2 , 24118 Kiel , Germany.

The vibrational properties of spin-crossover complexes [Fe(HB(pz))(L)] (pz = pyrazole) containing L = 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) ligands are investigated by temperature-dependent infrared and Raman spectroscopy. For comparison, the analogous cobalt(II) complexes [Co(HB(pz))(L)] (L = bipy and phen) and iron(II) compounds with L = 4,4'-dimethyl-2,2'-bipyridine and 4,7-dimethyl-1,10-phenanthroline coligands are studied. Highly intense, structured bands (giant Raman features, GRFs) are observed in the resonance Raman spectra of all Fe(II) complexes between 400 and 500 cm at low temperatures in the HS state which, for the SCO complexes, is excited by the Raman laser. On the basis of magnetic field Mössbauer and saturation magnetization data electronic Raman effects are excluded to account for these features. Furthermore, detailed vibrational analysis also allows excluding a vibrational resonance Raman effect involving one of the modes of the individual complexes as a possible origin of the GRFs. Consequently, these features are attributed to coherent two-phonon excitation of metal-ligand stretching vibrations in molecular dimers coupled by π-π stacking interactions.
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http://dx.doi.org/10.1021/acs.inorgchem.9b01813DOI Listing
October 2019

Spectroscopic Description of the E State of Mo Nitrogenase Based on Mo and Fe X-ray Absorption and Mössbauer Studies.

Inorg Chem 2019 Sep 23;58(18):12365-12376. Epub 2019 Aug 23.

Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , 45470 Mülheim an der Ruhr , Germany.

Mo nitrogenase (N2ase) utilizes a two-component protein system, the catalytic MoFe and its electron-transfer partner FeP, to reduce atmospheric dinitrogen (N) to ammonia (NH). The FeMo cofactor contained in the MoFe protein serves as the catalytic center for this reaction and has long inspired model chemistry oriented toward activating N. This field of chemistry has relied heavily on the detailed characterization of how Mo N2ase accomplishes this feat. Understanding the reaction mechanism of Mo N2ase itself has presented one of the most challenging problems in bioinorganic chemistry because of the ephemeral nature of its catalytic intermediates, which are difficult, if not impossible, to singly isolate. This is further exacerbated by the near necessity of FeP to reduce native MoFe, rendering most traditional means of selective reduction inept. We have now investigated the first fundamental intermediate of the MoFe catalytic cycle, E, as prepared both by low-flux turnover and radiolytic cryoreduction, using a combination of Mo Kα high-energy-resolution fluorescence detection and Fe K-edge partial-fluorescence-yield X-ray absorption spectroscopy techniques. The results demonstrate that the formation of this state is the result of an Fe-centered reduction and that Mo remains redox-innocent. Furthermore, using Fe X-ray absorption and Fe Mössbauer spectroscopies, we correlate a previously reported unique species formed under cryoreducing conditions to the natively formed E state through annealing, demonstrating the viability of cryoreduction in studying the catalytic intermediates of MoFe.
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http://dx.doi.org/10.1021/acs.inorgchem.9b01951DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6751781PMC
September 2019

Mononuclear Manganese(III) Superoxo Complexes: Synthesis, Characterization, and Reactivity.

Inorg Chem 2019 Aug 22;58(15):9756-9765. Epub 2019 Jul 22.

Max-Planck-Institut für Chemische Energiekonversion , Mülheim an der Ruhr D-45470 , Germany.

Metal-superoxo species are typically proposed as key intermediates in the catalytic cycle of dioxygen activation by metalloenzymes involving different transition metal cofactors. In this regard, while a series of Fe-, Co-, and Ni-superoxo complexes have been reported to date, well-defined Mn-superoxo complexes remain rather rare. Herein, we report two mononuclear Mn-superoxo species, Mn(BDPP)(O) (, HBDPP = 2,6-bis((2-()-diphenylhydroxylmethyl-1-pyrrolidinyl)methyl)pyridine) and Mn(BDPP)(O) (, HBDPP = 2,6-bis((2-()-di(4-bromo)phenylhydroxyl-methyl-1-pyrrolidinyl)methyl)pyridine), synthesized by bubbling O into solutions of their Mn precursors, Mn(BDPP) () and Mn(BDPP) (), at -80 °C. A combined spectroscopic (resonance Raman and electron paramagnetic resonance (EPR) spectroscopy) and computational study evidence that both complexes contain a high-spin Mn center ( = 2) antiferromagnetically coupled to a superoxo radical ligand ( = 1/2), yielding an overall = 3/2 ground state. Complexes and were shown to be capable of abstracting a H atom from 2,2,6,6-tetramethyl-1-hydroxypiperidine (TEMPO-H) to form Mn-hydroperoxo species, Mn(BDPP)(OOH) () and Mn(BDPP)(OOH) (). Complexes and can be independently prepared by the reactions of the isolated Mn-aqua complexes, [Mn(BDPP)(HO)]OTf () and [Mn(BDPP)(HO)]OTf (), with HO in the presence of NEt. The parallel-mode EPR measurements established a high-spin = 2 ground state for and .
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http://dx.doi.org/10.1021/acs.inorgchem.9b00767DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6685055PMC
August 2019

A Two-Coordinate Iron(II) Imido Complex with NHC Ligation: Synthesis, Characterization, and Its Diversified Reactivity of Nitrene Transfer and C-H Bond Activation.

Inorg Chem 2019 Jun 14;58(11):7634-7644. Epub 2019 May 14.

State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis , Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences , 345 Lingling Road , Shanghai 200032 , People's Republic of China.

Iron terminal imido species are typically implicated as reaction intermediates in iron-catalyzed transformations. While a large body of work has been devoted to mid- and high-valent iron imidos, to date the chemistry of iron(II) imidos has remained largely unexplored due to the difficulty in accessing them. Herein, we present a study on the two-coordinate iron(II) imido complex [(IPr)Fe(NAr)] (3; IPr = 1,3-bis(2',6'-diisopropylphenyl)imidazol-2-ylidene; Ar = 2,6-bis(2',4',6'-triisopropylphenyl)phenyl) prepared from the reaction of an iron(0) complex with the bulky azide ArN. Spectroscopic investigations in combination with DFT calculations established a high-spin S = 2 ground spin state for 3, consistent with its long Fe-N multiple bond of 1.715(2) Å revealed by X-ray diffraction analysis. Complex 3 exhibits unusual activity of nitrene transfer and C-H bond activation in comparison to the reported iron imido complexes. Specifically, the reactions of 3 with CH═CHAr, an electron-deficient alkene, and CO, a strong π acid, readily afford nitrene transfer products, ArCH═CHNHAr and ArNCO, respectively, yet no similar reaction occurs when 3 is treated with electron-rich alkenes and PMe. Moreover, 3 is inert toward the weak C(sp)-H bonds in 1,4-cyclohexadiene, THF, and toluene, whereas it can cleave the stronger C(sp)-H bond in p-trifluoromethylphenylacetylene to form an iron(II) amido alkynyl complex. Interestingly, intramolecular C(sp)-H bond functionalization was observed by adding ( p-Tol)CN to 3. The unique reactivity of 3 is attributed to its low-coordinate nature and the high negative charge population on the imido N atom, which render its iron-imido unit nucleophilic in nature.
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http://dx.doi.org/10.1021/acs.inorgchem.9b01147DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6750749PMC
June 2019

Enhanced Fe-Centered Redox Flexibility in Fe-Ti Heterobimetallic Complexes.

Inorg Chem 2019 May 8;58(9):6199-6214. Epub 2019 Apr 8.

Department of Chemistry , University of Minnesota , 207 Pleasant Street SE , Minneapolis , Minnesota 55455-0431 , United States.

Previously, we reported the synthesis of Ti[N( o-(NCHP( Pr))CH)] and the Fe-Ti complex, FeTi[N( o-(NCHP( Pr))CH)], 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] (2 and 2, respectively) were synthesized and characterized, along with BrFeTiL (2-Br) and the monometallic [TiL] complex (1). The solid-state structures of the [FeTiL] 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 Fe 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] 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%).
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http://dx.doi.org/10.1021/acs.inorgchem.9b00442DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6727590PMC
May 2019

Spectroscopic and Quantum Chemical Investigation of Benzene-1,2-dithiolate-Coordinated Diiron Complexes with Relevance to Dinitrogen Activation.

Inorg Chem 2019 Apr 25;58(8):5111-5125. Epub 2019 Mar 25.

Max Planck Institute for Chemical Energy Conversion , Stiftstrasse 34-36 , D-45470 Mülheim an der Ruhr , Germany.

In this work, a benzene-1,2-dithiolate (bdt) pentamethylcyclopentadienyl di-iron complex [Cp*Fe(μ-η:η-bdt)FeCp*] and its [Cp*Fe(bdt)(X)FeCp*] analogues (where X = NH, NH, H, NH, NHCH, or NO) were investigated through spectroscopic and computational studies. These complexes are of relevance as model systems for dinitrogen activation in nitrogenase and share with its active site the presence of iron, sulfur ligands, and a very flexible electronic structure. On the basis of a combination of X-ray emission spectroscopy (XES), X-ray crystallography, Mössbauer, NMR, and EPR spectroscopy, the geometric and electronic structure of the series has been experimentally elucidated. All iron atoms were found to be in a local low-spin configuration. When no additional X ligand is bound, the bdt ligand is tilted and features a stabilizing π-interaction with one of the iron atoms. The number of lone-pair orbitals provided by the nitrogen-containing species is crucial to the overall electronic structure. When only one lone-pair is present and the iron atoms are bridged by one atom, a three-center bond occurs, and a direct Fe-Fe bond is absent. If the bridging atom provides two lone-pairs, then an Fe-Fe bond is formed. A recurring theme for all ligands is σ-donation into the unoccupied e manifolds of both iron atoms and back-donation from the t manifolds into the ligand π* orbitals. The latter results in a weakening of the double bond of the bound ligand, and in the case of NO, it results in a weakening of all bonds that comprise triple bond. The electron-rich thiolates further amplify this effect and can also serve as bases for proton binding. While the above observations have been made for the studied di-iron complexes, they may be of relevance for the active site in nitrogenase, where a similar N binding mode may occur allowing for the simultaneous weakening of the N σ bond and π bonds.
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http://dx.doi.org/10.1021/acs.inorgchem.9b00177DOI Listing
April 2019

Reduction of CO by a masked two-coordinate cobalt(i) complex and characterization of a proposed oxodicobalt(ii) intermediate.

Chem Sci 2019 Jan 9;10(3):918-929. Epub 2018 Nov 9.

Department of Chemistry , Yale University , New Haven , Connecticut 06520 , USA . Email:

Fixation and chemical reduction of CO are important for utilization of this abundant resource, and understanding the detailed mechanism of C-O cleavage is needed for rational development of CO reduction methods. Here, we describe a detailed analysis of the mechanism of the reaction of a masked two-coordinate cobalt(i) complex, L Co (where L = 2,2,6,6-tetramethyl-3,5-bis[(2,6-diisopropylphenyl)imino]hept-4-yl), with CO, which yields two products of C-O cleavage, the cobalt(i) monocarbonyl complex L Co(CO) and the dicobalt(ii) carbonate complex (L Co)(μ-CO). Kinetic studies and computations show that the κ,η-arene isomer of L Co rearranges to the κ , binding mode prior to binding of CO, which contrasts with the mechanism of binding of other substrates to L Co. Density functional theory (DFT) studies show that the only low-energy pathways for cleavage of CO 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 CO with L Co is the oxodicobalt(ii) complex L CoOCoL , which has been independently synthesized through the reaction of L Co with NO. The rapid reaction of L CoOCoL with CO to form the carbonate product indicates that the oxo species is kinetically competent to be an intermediate during CO cleavage by L Co. L CoOCoL 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.
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http://dx.doi.org/10.1039/c8sc02599aDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6346294PMC
January 2019
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