[Bis(3,5‐diisopropylpyrazol‐1‐yl‐κ N 2 )dihydroborato](triphenylphosphane‐κ P )copper(I)

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/102235/1/S0108270113015965.pd

Synthesis, Crystal Structure and Thermal Reactivity of [ZnX 2 (2-chloropyrazine)] (X = Cl, Br, I) Coordination Compounds

Reaction of zinc(II) halides with 2-chloropyrazine in different solvents leads to the formation of five new coordination compounds that contain either only 2-chloropyrazine or additional water molecules as donor ligands. In the ligand-rich 1:2 compound catena[bis(2-chloropyrazine-N)]di-Μ-chlorozinc(II) ( 1 ) the zinc atom is coordinated by two 2-chloropyrazine ligands and four chlorine atoms in an octahedral fashion. The zinc atoms are connected by the chloride atoms forming linear chains. In the isotypic ligand-rich 1:2 compounds bis(2-chloropyrazine-N)dibromozinc(II) ( 2 ) and bis(2-chloropyrazine-N)diiodozinc(II) ( 3 ) discrete complexes are found in which each zinc atom is coordinated by two 2-chloropyrazine ligands and two halide atoms within distorted tetrahedra. The 1:1 compounds aqua-(2-chloropyrazine-N)dibromozinc(II) ( 4 ) and aqua-(2-chloropyrazine-N)diiodozinc(II) ( 5 ) are also isotypic and form discrete complexes in which the zinc atoms are surrounded by two halide atoms, one 2-chloropyrazine ligand and one water molecule. Upon heating, compounds 1 – 5 form ligand-deficient 1:1 and 2:1 compounds of composition [ZnX 2 (2-chloropyrazine)] (X = halide) and [(ZnX 2 ) 2 (2-chloropyrazine)]. X-ray powder diffraction shows that identical ligand-deficient intermediates are obtained on decomposition of either [ZnX 2 L 2 ] (L = 2-chloropyrazine) or the [ZnX 2 L(H 2 O)] complexes. DFT calculations suggest that the formation of the [ZnX 2 L(H 2 O)] complex is energetically favoured for the heavier halide anions. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/57924/1/605_ftp.pd

Isolation and Characterization of Single and Sulfide‐Bridged Double [4Fe–4S] Cubane Clusters with 4‐Pyridinethiolato Ligands

Cuboidal iron–sulfur clusters, [4Fe–4S], are important electron‐transfer (ET) sites in biology. In addition, more complex structures, usually consisting of modified or fused cubane clusters, are used as active sites in many important enzymes. For example, the Fe–Mo cofactor (FeMoco) of nitrogenase contains two fused cubanes. Here, we report the synthesis of three new para ‐pyridylthiolate ligated iron–sulfur cubane clusters, two single clusters (Bu 4 N) 2 [Fe 4 S 4 (SMePy) 4 ] and (Bu 4 N) 2 [Fe 4 S 4 (SPy) 4 ], and the sulfide‐bridged double cubane (Bu 4 N) 4 [{Fe 4 S 4 (SPy) 3 } 2 S] with 4‐pyridinethiolato exogenous ligands. The properties of these clusters were then explored by 1 H NMR, IR, and UV/Vis spectroscopy, cyclic voltammetry (CV), and X‐ray crystallography. Importantly, (Bu 4 N) 4 [{Fe 4 S 4 (SPy) 3 } 2 S] is the first example of a crystallographically characterized sulfide‐bridged double cubane with all‐thiolato exogenous ligands that is not supported by a large encapsulating ligand. This cluster shows a bridging Fe–S–Fe angle of 104°, its other structural parameters are in close agreement with those of the single‐cluster analog (Bu 4 N) 2 [Fe 4 S 4 (SPy) 4 ]. Finally, the one‐electron‐reduced forms of (Bu 4 N) 2 [Fe 4 S 4 (SPy) 4 ] and (Bu 4 N) 4 [{Fe 4 S 4 (SPy) 3 } 2 S] were studied by low‐temperature electron paramagnetic resonance (EPR) spectroscopy. Both clusters exhibit reversible one‐electron reductions at –401 and –528 mV [vs. the normal hydrogen electrode (NHE)], respectively. The one‐electron‐reduced forms of both clusters show S = 1/2 ground states as evident from EPR spectroscopy at liquid‐helium temperature. The temperature‐dependent data for the double cubane further indicate that the extra electron is trapped in one of the clusters of the dimer and that a low‐lying excited state is likely present in this complex, close in energy to the ground state. The syntheses of unique para ‐pyridylthiolate‐ligated cuboidal clusters, including two single [4Fe–4S] clusters and a sulfide‐bridged double [4Fe–4S] cubane, are described. The properties of these clusters are characterized by 1 H NMR, IR and UV/Vis spectroscopy, cyclic voltammetry, and X‐ray crystallography.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/100291/1/ejic_201300802_sm_miscellaneous_information.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/100291/2/5253_ftp.pd

Fischer‐Tropsch‐Chemie bei Raumtemperatur?

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/86803/1/8133_ftp.pd

Role of structural dynamics in selectivity and mechanism of non-heme Fe(II) and 2-Oxoglutarate-dependent Oxygenases involved in DNA repair

AlkB and its human homologue AlkBH2 are Fe(II)- and 2-oxoglutarate (2OG)-dependent oxygenases that repair alkylated DNA bases occurring as a consequence of reactions with mutagenic agents. We used molecular dynamics (MD) and combined quantum mechanics/molecular mechanics (QM/MM) methods to investigate how structural dynamics influences the selectivity and mechanisms of the AlkB- and AlkBH2-catalyzed demethylation of 3-methylcytosine (m3C) in single (ssDNA) and double (dsDNA) stranded DNA. Dynamics studies reveal the importance of the flexibility in both the protein and DNA components in determining the preferences of AlkB for ssDNA and of AlkBH2 for dsDNA. Correlated motions, including of a hydrophobic β-hairpin, are involved in substrate binding in AlkBH2–dsDNA. The calculations reveal that 2OG rearrangement prior to binding of dioxygen to the active site Fe is preferred over a ferryl rearrangement to form a catalytically productive Fe(IV)═O intermediate. Hydrogen atom transfer proceeds via a σ-channel in AlkBH2–dsDNA and AlkB–dsDNA; in AlkB–ssDNA, there is a competition between σ- and π-channels, implying that the nature of the complexed DNA has potential to alter molecular orbital interactions during the substrate oxidation. Our results reveal the importance of the overall protein–DNA complex in determining selectivity and how the nature of the substrate impacts the mechanism

Heme-protein vibrational couplings in cytochrome c provide a dynamic link that connects the heme-iron and the protein surface

The active site of cytochrome c (Cyt c) consists of a heme covalently linked to a pentapeptide segment (Cys-X-X-Cys-His), which provides a link between the heme and the protein surface, where the redox partners of Cyt c bind. To elucidate the vibrational properties of heme c, nuclear resonance vibrational spectroscopy (NRVS) measurements were performed on 57Fe-labeled ferric Hydrogenobacter thermophilus cytochrome c 552, including 13C8-heme-, 13C 515N-Met-, and 13C15N-polypeptide (pp)-labeled samples, revealing heme-based vibrational modes in the 200- to 450-cm-1 spectral region. Simulations of the NRVS spectra of H. thermophilus cytochrome c552 allowed for a complete assignment of the Fe vibrational spectrum of the protein-bound heme, as well as the quantitative determination of the amount of mixing between local heme vibrations and pp modes from the Cys-X-XCys-His motif. These results provide the basis to propose that heme-pp vibrational dynamic couplings play a role in electron transfer (ET) by coupling vibrations of the heme directly to vibrations of the pp at the protein - protein interface. This could allow for the direct transduction of the thermal (vibrational) energy from the protein surface to the heme that is released on protein/protein complex formation, or it could modulate the heme vibrations in the protein/protein complex to minimize reorganization energy. Both mechanisms lower energy barriers for ET. Notably, the conformation of the distal Met side chain is fine-tuned in the protein to localize heme-pp mixed vibrations within the 250-to 400-cm-1 spectral region. These findings point to a particular orientation of the distal Met that maximizes ET

Exploring the Limits of Dative Boratrane Bonding: Iron as a Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl Complexes

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

Exploring the Limits of Dative Boratrane Bonding: Iron as a Strong Lewis Base in Low-Valent Non-Heme Iron-Nitrosyl Complexes

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

Electronic Structures of an [Fe(NNR_2)]^(+/0/–) Redox Series: Ligand Noninnocence and Implications for Catalytic Nitrogen Fixation

The intermediacy of metal–NNH_2 complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N_2) fixation. In this context, we have shown that triphosphine-supported Fe(N_2) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH_2) complexes over an array of charge and oxidation states. Upon exposure to further H^+/e^– equivalents, these species either continue down a distal-type Chatt pathway to yield a terminal iron(IV) nitride or instead follow a distal-to-alternating pathway resulting in N–H bond formation at the proximal N atom. To understand the origin of this divergent selectivity, herein we synthesize and elucidate the electronic structures of a redox series of Fe(NNMe_2) complexes, which serve as spectroscopic models for their reactive protonated congeners. Using a combination of spectroscopies, in concert with density functional theory and correlated ab initio calculations, we evidence one-electron redox noninnocence of the “NNMe_2” moiety. Specifically, although two closed-shell configurations of the “NNR_2” ligand have been commonly considered in the literature—isodiazene and hydrazido(2−)—we provide evidence suggesting that, in their reduced forms, the present iron complexes are best viewed in terms of an open-shell [NNR_2]^•–ligand coupled antiferromagnetically to the Fe center. This one-electron redox noninnocence resembles that of the classically noninnocent ligand NO and may have mechanistic implications for selectivity in N_2 fixation activity

Electronic Structures of an [Fe(NNR_2)]^(+/0/–) Redox Series: Ligand Noninnocence and Implications for Catalytic Nitrogen Fixation

The intermediacy of metal–NNH_2 complexes has been implicated in the catalytic cycles of several examples of transition-metal-mediated nitrogen (N_2) fixation. In this context, we have shown that triphosphine-supported Fe(N_2) complexes can be reduced and protonated at the distal N atom to yield Fe(NNH_2) complexes over an array of charge and oxidation states. Upon exposure to further H^+/e^– equivalents, these species either continue down a distal-type Chatt pathway to yield a terminal iron(IV) nitride or instead follow a distal-to-alternating pathway resulting in N–H bond formation at the proximal N atom. To understand the origin of this divergent selectivity, herein we synthesize and elucidate the electronic structures of a redox series of Fe(NNMe_2) complexes, which serve as spectroscopic models for their reactive protonated congeners. Using a combination of spectroscopies, in concert with density functional theory and correlated ab initio calculations, we evidence one-electron redox noninnocence of the “NNMe_2” moiety. Specifically, although two closed-shell configurations of the “NNR_2” ligand have been commonly considered in the literature—isodiazene and hydrazido(2−)—we provide evidence suggesting that, in their reduced forms, the present iron complexes are best viewed in terms of an open-shell [NNR_2]^•–ligand coupled antiferromagnetically to the Fe center. This one-electron redox noninnocence resembles that of the classically noninnocent ligand NO and may have mechanistic implications for selectivity in N_2 fixation activity