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Robert H Morris - One of the best experts on this subject based on the ideXlab platform.
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density functional theory calculations support the additive nature of ligand contributions to the pka of Iron Hydride phosphine carbonyl complexes
Inorganic Chemistry, 2016Co-Authors: Molly M H Sung, Robert H MorrisAbstract:The acid dissociation constant Ka of a transition-metal Hydride complex is a key thermodynamic quantity for evaluating reactivity and stability of the complexes and their conjugate bases in stoichiometric and catalytic reactions. It can be estimated using a simple ligand acidity constant (LAC) empirical equation for a wide range of complexes. Here, we provide the first density functional theory (DFT) study that supports the additive nature of ligand contributions to the pKa of metal Hydride complexes. Specifically, the pKa values of Iron Hydride complexes [FeH(CO)x(PR3)(5–x)]+ in either tetrahydrofuran or dichloromethane solutions are estimated using the LAC method and DFT calculations. There is a linear correlation between these two methods, and both predict a surprisingly linear increase in pKa over a wide range from approximately −15 for x = 5 to approximately 40 for x = 0. The LAC equation predicts that pKaTHF or pKaDCM increases by 9 units with the replacement of each CO ligand with a trialkylphosphi...
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Details of the Mechanism of the Asymmetric Transfer Hydrogenation of Acetophenone Using the Amine(imine)diphosphine Iron Precatalyst: The Base Effect and The Enantiodetermining Step
2016Co-Authors: Weiwei Zuo, Demyan E. Prokopchuk, Alan J. Lough, Robert H MorrisAbstract:The excellent ketone asymmetric transfer hydrogenation (ATH) systems using the precatalysts (S,S)-trans-[FeCl(CO)(PPh2CH2CH2NHCHPhCHPhNCHCH2PAr2)]BPh4 (Ar = Ph (1), p-tolyl (2)) have a fascinating dependence of activity on the base concentration, which is investigated here. The reaction of complex 1 or 2 with 1 equiv of the strong base potassium tert-butoxide in THF for 2–7 days produces the neutral amine(ene-amido) complexes [FeCl(CO)(PPh2CH2CH2NHCHPhCHPhNCHCHPAr2)] (8 and 9). These monodeprotonated complexes have been completely characterized by NMR, EA, and FT-IR spectroscopy and mass spectrometry, and the structure of 9 has been further confirmed by single-crystal X-ray diffraction to reveal a structure with the NH and FeCl bonds parallel and the proton and chloride ligands next to each other. The structures of 8 and 9 and their 1H NMR patterns are similar to those of the active catalyst for the ATH of ketones that is postulated to have NH and FeH bonds parallel with the protonic and hydridic hydrogens adjacent. Identical key nuclear Overhauser effect (NOE) correlations in both 8 and the hydrido complex provide further evidence for the postulated structure of the amine Iron Hydride intermediate. The catalyst system is not active for the transfer hydrogenation of acetophenone, unless greater than 2 equiv of base is added to activate the precatalyst. The addition of base (up to 8 equiv per Iron) increases the reaction rate, while a further increase of the base concentration shows a reduction of activity. The loss of activity with less than 2 equiv of base results from a side reaction of the active amido(ene-amido) complexes with 2-propanol to form an inactive neutral bis(amido) Iron complex, which was characterized by NMR spectroscopy. Structural evidence for this was provided by the X-ray crystal structure determination of an analogous bis(amino) Iron(II) complex, generated from the reaction of the amine(ene-amido) Iron complex with methanol in C6D6 in the presence of BF4–. The presence of excess base prevents this side reaction, thereby favoring the reaction which forms the active amine Iron Hydride species that is in the catalytic cycle. The structure of the transition state for the reaction of the amine hydrido Iron catalyst with acetophenone has been successfully modeled using density functional theory (DFT) calculations. The (R) configuration of the product 1-phenylethanol is induced by the position of the phenyl groups on the catalyst and a π–π stabilizing interaction between the aryl ring on the ketone and the ene-amido moiety on the ligand
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amine imine diphosphine Iron catalysts for asymmetric transfer hydrogenation of ketones and imines
Science, 2013Co-Authors: Ann Julie Lough, Young Feng Li, Robert H MorrisAbstract:A rational approach is needed to design hydrogenation catalysts that make use of Earth-abundant elements to replace the rare elements such as ruthenium, rhodium, and palladium that are traditionally used. Here, we validate a prior mechanistic hypothesis that partially saturated amine(imine)diphosphine ligands (P-NH-N-P) activate Iron to catalyze the asymmetric reduction of the polar bonds of ketones and imines to valuable enantiopure alcohols and amines, with isopropanol as the hydrogen donor, at turnover frequencies as high as 200 per second at 28°C. We present a direct synthetic approach to enantiopure ligands of this type that takes advantage of the Iron(lI) ion as a template. The catalytic mechanism is elucidated by the spectroscopic detection of Iron Hydride and amide intermediates.
Ben Bradshaw - One of the best experts on this subject based on the ideXlab platform.
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Iron Hydride Radical Reductive Alkylation of Unactivated Alkenes.
Organic letters, 2019Co-Authors: Mar Saladrigas, Josep Bonjoch, Ben BradshawAbstract:Iron-catalyzed hydrogen atom transfer-mediated intermolecular C–C coupling reactions between alkenes and tosylhydrazones, followed by in situ cleavage of the tosylhydrazine intermediates using Et3N, are described. The process involves a new strategic bond disconnection resulting in the reductive alkylation of nonactivated alkenes. The reaction is operationally simple, proceeds under mild conditions, and has a wide substrate scope.
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Hydrogen Atom Transfer (HAT)-Triggered Iron-Catalyzed Intra- and Intermolecular Coupling of Alkenes with Hydrazones: Access to Complex Amines
2018Co-Authors: Mar Saladrigas, Josep Bonjoch, Guillem Loren, Ben BradshawAbstract:A methodology for the coupling of alkenes with aldehyde- or ketone-derived Cbz-hydrazones to form a new C–C bond through a radical process is described. The sequence comprises an initial in situ generation of a putative Iron Hydride followed by a hydrogen atom transfer to an alkene, a coupling with a hydrazone, and a final reduction of the nitrogen-centered radical. Hydrogenation of the obtained hydrazines renders amines, including valuable tert-alkyl amines
Wolfgang Lubitz - One of the best experts on this subject based on the ideXlab platform.
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asymmetry in the ligand coordination sphere of the fefe hydrogenase active site is reflected in the magnetic spin interactions of the aza propanedithiolate ligand
Journal of Physical Chemistry Letters, 2019Co-Authors: Edward J Reijerse, Vladimir Pelmenschikov, Stephen P Cramer, James A Birrell, Casseday P Richers, Martin Kaupp, Thomas B Rauchfuss, Wolfgang LubitzAbstract:[FeFe] hydrogenases are very active enzymes that catalyze the reversible conversion of molecular hydrogen into protons and electrons. Their active site, the H-cluster, contains a unique binuclear Iron complex, [2Fe]H, with CN– and CO ligands as well as an aza-propane-dithiolate (ADT) moiety featuring a central amine functionality that mediates proton transfer during catalysis. We present a pulsed 13C-ENDOR investigation of the H-cluster in which the two methylene carbons of ADT are isotope labeled with 13C. We observed that the corresponding two 13C hyperfine interactions are of opposite sign and corroborated this finding using density functional theory calculations. The spin polarization in the ADT ligand is shown to be linked to the asymmetric coordination of the distal Iron site with its terminal CN– and CO ligands. We propose that this asymmetry is relevant for the enzyme reactivity and is related to the (optimal) stabilization of the Iron-Hydride intermediate in the catalytic cycle.
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a strenuous experimental journey searching for spectroscopic evidence of a bridging nickel Iron Hydride in nife hydrogenase
Journal of Synchrotron Radiation, 2015Co-Authors: Yoshitaka Yoda, Hongxin Wang, Hideaki Ogata, Yoshihito Tanaka, Wolfgang LubitzAbstract:Direct spectroscopic evidence for a Hydride bridge in the Ni–R form of [NiFe] hydrogenase has been obtained using Iron-specific nuclear resonance vibrational spectroscopy (NRVS). The Ni–H–Fe wag mode at 675 cm−1 is the first spectroscopic evidence for a bridging Hydride in Ni–R as well as the first Iron-Hydride-related NRVS feature observed for a biological system. Although density function theory (DFT) calculation assisted the determination of the Ni–R structure, it did not predict the Ni–H–Fe wag mode at ∼675 cm−1 before NRVS. Instead, the observed Ni–H–Fe mode provided a critical reference for the DFT calculations. While the overall science about Ni–R is presented and discussed elsewhere, this article focuses on the long and strenuous experimental journey to search for and experimentally identify the Ni–H–Fe wag mode in a Ni–R sample. As a methodology, the results presented here will go beyond Ni–R and hydrogenase research and will also be of interest to other scientists who use synchrotron radiation for measuring dilute samples or weak spectroscopic features.
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Key Hydride Vibrational Modes in [NiFe] Hydrogenase Model Compounds Studied by Resonance Raman Spectroscopy and Density Functional Calculations
2012Co-Authors: Hannah S Shafaat, Frank Neese, Katharina Weber, Taras Petrenko, Wolfgang LubitzAbstract:Hydrogenase proteins catalyze the reversible conversion of molecular hydrogen to protons and electrons. While many enzymatic states of the [NiFe] hydrogenase have been studied extensively, there are multiple catalytically relevant EPR-silent states that remain poorly characterized. Analysis of model compounds using new spectroscopic techniques can provide a framework for the study of these elusive states within the protein. We obtained optical absorption and resonance Raman (RR) spectra of (dppe)Ni(μ-pdt)Fe(CO)3 and [(dppe)Ni(μ-pdt)(μ-H)Fe(CO)3][BF4], which are structural and functional model compounds for the EPR-silent Ni–SI and Ni–R states of the [NiFe] hydrogenase active site. The studies presented here use RR spectroscopy to probe vibrational modes of the active site, including metal–Hydride stretching vibrations along with bridging ligand–metal and Fe–CO bending vibrations, with isotopic substitution used to identify key metal–Hydride modes. The metal–Hydride vibrations are essentially uncoupled and represent isolated, localized stretching modes; the Iron–Hydride vibration occurs at 1530 cm–1, while the nickel–Hydride vibration is observed at 945 cm–1. The significant discrepancy between the metal–Hydride vibrational frequencies reflects the slight asymmetry in the metal–Hydride bond lengths. Additionally, time-dependent density functional theory (TD-DFT) calculations were carried out to obtain theoretical RR spectra of these compounds. On the basis of the detailed comparison of theory and experiment, the dominant electronic transitions and significant normal modes probed in the RR experiments were assigned; the primary transitions in the visible wavelengths represent metal-to-metal and metal-to-ligand charge transfer bands. Inherent properties of metal–Hydride vibrational modes in resonance Raman spectra and DFT calculations are discussed together with the prospects of observing such vibrational modes in metal–Hydride-containing proteins. Such a combined theoretical and experimental approach may be valuable for characterization of analogous redox states in the [NiFe] hydrogenases
Hao Jiang - One of the best experts on this subject based on the ideXlab platform.
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generation of carbon radical from Iron Hydride alkene exchange enhanced reactivity selects the reactive spin state
ACS Catalysis, 2019Co-Authors: Wenzhen Lai, Hao Jiang, Hui ChenAbstract:Iron-Hydride and alkene constitute a promising pair of reactants to access alkyl carbon radicals. However, the fundamental mechanistic scenario remains unclear. High-level coupled cluster calculations reported in this work demonstrate that for Iron(III)-Hydrides, hydrogen atom transfer (HAT) is favored over hydrometalation. The oxidation state of Iron makes a remarkable difference in the reactivity of the Iron-Hydride such that ferric FeIII–H is much more reactive in HAT than ferrous FeII–H. Akin to Iron(IV)-oxo, exchange-enhanced reactivity (EER) dictates the intrinsic spin HAT reactivity of Iron(III)-Hydride. However, it is not two-state reactivity (TSR) but single-state reactivity (SSR) that operates in HAT with Iron(III)-Hydride. The reactivity insights gained herein for alkyl liberation from Iron-alkyls may have profound mechanistic implications on the Iron-dependent bioorganometallic radical chemistry in radical SAM enzymes.
Patrick L Holland - One of the best experts on this subject based on the ideXlab platform.
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high frequency fe h vibrations in a bridging Hydride complex characterized by nrvs and dft
Angewandte Chemie, 2018Co-Authors: Vladimir Pelmenschikov, Hongxin Wang, Stephen P Cramer, Cory K Macleod, Sean F Mcwilliams, Kazimer L Skubi, Patrick L HollandAbstract:: High-spin Iron species with bridging Hydrides have been detected in species trapped during nitrogenase catalysis, but there are few general methods of evaluating Fe-H bonds in high-spin multinuclear Iron systems. An 57 Fe nuclear resonance vibrational spectroscopy (NRVS) study on an Fe(μ-H)2 Fe model complex reveals Fe-H stretching vibrations for bridging Hydrides at frequencies greater than 1200 cm-1 . These isotope-sensitive vibrational bands are not evident in infrared (IR) spectra, showing the power of NRVS for identifying Hydrides in this high-spin Iron system. Complementary density functional theory (DFT) calculations elucidate the normal modes of the rhomboidal Iron Hydride core.
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synthesis and mechanism of formation of Hydride sulfide complexes of Iron
Inorganic Chemistry, 2017Co-Authors: Nicholas A Arnet, Sean F Mcwilliams, Brandon Q Mercado, Daniel E Derosha, Patrick L HollandAbstract:Iron–sulfide complexes with Hydride ligands provide an experimental precedent for spectroscopically detected Hydride species on the Iron–sulfur MoFe7S9C cofactor of nitrogenase. In this contribution, we expand upon our recent synthesis of the first Iron sulfide Hydride complex from an Iron Hydride and a sodium thiolate (Arnet, N. A.; Dugan, T. R.; Menges, F. S.; Mercado, B. Q.; Brennessel, W. W.; Bill, E.; Johnson, M. A.; Holland, P. L., J. Am. Chem. Soc. 2015, 137, 13220−13223). First, we describe the isolation of an analogous Iron sulfide Hydride with a smaller diketiminate supporting ligand, which benefits from easier preparation of the Hydride precursor and easier isolation of the product. Second, we describe mechanistic studies on the C–S bond cleavage through which the Iron sulfide Hydride product is formed. In a key experiment, use of cyclopropylmethanethiolate as the sulfur precursor leads to products from cyclopropane ring opening, implicating an alkyl radical as an intermediate. Combined with th...
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synthesis spectroscopy and hydrogen deuterium exchange in high spin Iron ii Hydride complexes
Inorganic Chemistry, 2014Co-Authors: Thomas R Dugan, William W. Brennessel, Cory K Macleod, Eckhard Bill, Patrick L HollandAbstract:Very few Hydride complexes are known in which the metals have a high-spin electronic configuration. We describe the characterization of several high-spin Iron(II) Hydride/deuteride isotopologues and their exchange reactions with one another and with H2/D2. Though the Hydride/deuteride signal is not observable in NMR spectra, the choice of isotope has an influence on the chemical shifts of distant protons in the dimers through the paramagnetic isotope effect on chemical shift. This provides the first way to monitor the exchange of H and D in the bridging positions of these Hydride complexes. The rate of exchange depends on the size of the supporting ligand, and this is consistent with the idea that H2/D2 exchange into the Hydrides occurs through the dimeric complexes rather than through a transient monomer. The understanding of H/D exchange mechanisms in these high-spin Iron Hydride complexes may be relevant to postulated nitrogenase mechanisms.
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mechanistic insight into n n cleavage by a low coordinate Iron ii Hydride complex
Journal of the American Chemical Society, 2007Co-Authors: Azwana R Sadique, William W. Brennessel, Elizabeth A Gregory, Patrick L HollandAbstract:The reaction pathways of high-spin Iron Hydride complexes are relevant to the mechanism of N2 reduction by nitrogenase, which has been postulated to involve paramagnetic Iron-Hydride species. However, almost all known Iron Hydrides are low-spin, diamagnetic Fe(II) compounds. We have demonstrated that the first high-spin Iron Hydride complex, LtBuFeH (LtBu = bulky beta-diketiminate), reacts with PhN=NPh to completely cleave the N-N double bond, giving LtBuFeNHPh. Here, we disclose a series of experiments that elucidate the mechanism of this reaction. Crossover and kinetic experiments rule out common nonradical mechanisms, and support a radical chain mechanism mediated by Iron(I) species including a rare eta2-azobenzene complex. Therefore, this high-spin Iron(II) Hydride can break N-N bonds through both nonradical and radical insertion mechanisms, a special feature that enables novel reactivity.