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56. A TMEDA–Iron Adduct Reaction Manifold in Iron‐Catalyzed C(sp2)−C(sp3) Cross‐Coupling Reactions.

Author

Bakas, Nikki J., Sears, Jeffrey D., Brennessel, William W., and Neidig, Michael L.

Subjects
GRIGNARD reagents, X-ray crystallography, IRON, HOMOGENEOUS catalysis, COUPLING reactions (Chemistry)
Abstract

Herein, we expand the current molecular‐level understanding of one of the most important and effective additives in iron‐catalyzed cross‐coupling reactions, N,N,N′,N′‐tetramethylethylenediamine (TMEDA). Focusing on relevant phenyl and ethyl Grignard reagents and slow nucleophile addition protocols commonly used in effective catalytic systems, TMEDA‐iron(II)‐aryl intermediates are identified via in situ spectroscopy, X‐ray crystallography, and detailed reaction studies to be a part of an iron(II)/(III)/(I) reaction cycle where radical recombination with FePhBr(TMEDA) (2Ph) results in selective product formation in high yield. These results differ from prior studies with mesityl Grignard reagent, where poor product selectivity and low catalytic performance can be attributed to homoleptic iron–ate species. Overall, this study represents a critical advance in how amine additives such as TMEDA can modulate selectivity and reactivity of organoiron species in cross‐coupling. [ABSTRACT FROM AUTHOR]

Published
2022
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64. Magnetic circular dichroism studies of iron(ii) binding to human calprotectin† †Electronic supplementary information (ESI) available: Sample details and supplemental magnetic circular dichroism spectra. See DOI: 10.1039/c6sc03487j Click here for additional data file

Author

Baker, Tessa M., Nakashige, Toshiki G., Nolan, Elizabeth M., and Neidig, Michael L.

Subjects
Chemistry
Abstract

Magnetic circular dichroism studies of Fe(ii) binding to human calprotectin demonstrate the nature of Fe(ii) coordination at two different Fe(ii)-binding sites and provide insight into how Ca(ii) modulates Fe(ii) coordination., Calprotectin (CP) is an abundant metal-chelating protein involved in host defense, and the ability of human CP to bind Fe(ii) in a calcium-dependent manner was recently discovered. In the present study, near-infrared magnetic circular dichroism spectroscopy is employed to investigate the nature of Fe(ii) coordination at the two transition-metal-binding sites of CP that are a His3Asp motif (site 1) and a His6 motif (site 2). Upon the addition of sub-stoichiometric Fe(ii), a six-coordinate (6C) Fe(ii) center associated with site 2 is preferentially formed in the presence of excess Ca(ii). This site exhibits an exceptionally large ligand field (10D q = 11 045 cm–1) for a non-heme Fe(ii) protein. Analysis of CP variants lacking residues of the His6 motif supports that CP coordinates Fe(ii) at site 2 by employing six His ligands. In the presence of greater than one equiv. of Fe(ii) or upon mutation of the His6 motif, the metal ion also binds at site 1 of CP to form a five-coordinate (5C) Fe(ii)–His3Asp motif that was previously unidentified in this system. Notably, the introduction of His-to-Ala mutations at the His6 motif results in a mixture of 6C (site 2) and 5C (site 1) signals in the presence of sub-stoichiometric Fe(ii). These results are consistent with a reduced Fe(ii)-binding affinity of site 2 as more weakly coordinating water-derived ligands complete the 6C site. In the absence of Ca(ii), both sites 1 and 2 are occupied upon addition of sub-stoichiometric Fe(ii), and a stronger ligand field is observed for the 5C site. These spectroscopic studies provide further evaluation of a unique non-heme Fe(ii)–His6 site for metalloproteins and support the notion that Ca(ii) ions influence the Fe(ii)-binding properties of CP.

Published
2016

65. Facile hydrogen atom transfer to iron(iii) imido radical complexes supported by a dianionic pentadentate ligand† †Electronic supplementary information (ESI) available: Synthesis, characterization data, experimental and computational details. Crystallographic data for [B2Pz4LiPyH]2, 1·THF, 1, 2·Ar, 2·Ad and 4. CCDC 1439938 and 1439242–1439246. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6sc01433j

Author

Spasyuk, Denis M., Carpenter, Stephanie H., Kefalidis, Christos E., Piers, Warren E., Neidig, Michael L., and Maron, Laurent

Subjects
Condensed Matter::Quantum Gases, Chemistry, Physics::Atomic and Molecular Clusters, Physics::Accelerator Physics, Physics::Atomic Physics, Physics::Chemical Physics
Abstract

Facile hydrogen atom transfer from toluene., A dianionic tetrapodal pentadentate diborate ligand is introduced. This ligand forms a high spin neutral iron(ii) complex that reacts with a variety of organoazides to yield transient Fe(iii) imido radicals that are extremely potent hydrogen atom abstractors. The nature of these species is supported by full characterization of the Fe(iii) amido products, kinetic studies, density functional computations and Mössbauer spectroscopy on the –C6H4-p-tBu substituted derivative.

Published
2016

75. Magnetic Circular Dichroism Studies of Iron(ii) Binding to Human Calprotectin

Author

Massachusetts Institute of Technology. Department of Chemistry, Nakashige, Toshiki George, Nolan, Elizabeth Marie, Baker, Tessa M., Neidig, Michael L., Massachusetts Institute of Technology. Department of Chemistry, Nakashige, Toshiki George, Nolan, Elizabeth Marie, Baker, Tessa M., and Neidig, Michael L.

Abstract

Calprotectin (CP) is an abundant metal-chelating protein involved in host defense, and the ability of human CP to bind Fe(ii) in a calcium-dependent manner was recently discovered. In the present study, near-infrared magnetic circular dichroism spectroscopy is employed to investigate the nature of Fe(ii) coordination at the two transition-metal-binding sites of CP that are a His₃ Asp motif (site 1) and a His₆ motif (site 2). Upon the addition of sub-stoichiometric Fe(ii), a six-coordinate (6C) Fe(ii) center associated with site 2 is preferentially formed in the presence of excess Ca(ii). This site exhibits an exceptionally large ligand field (10D[subscript q] = 11 045 cm⁻¹) for a non-heme Fe(ii) protein. Analysis of CP variants lacking residues of the His₆ motif supports that CP coordinates Fe(ii) at site 2 by employing six His ligands. In the presence of greater than one equiv. of Fe(ii) or upon mutation of the His₆ motif, the metal ion also binds at site 1 of CP to form a five-coordinate (5C) Fe(ii)-His₃ Asp motif that was previously unidentified in this system. Notably, the introduction of His-to-Ala mutations at the His₆ motif results in a mixture of 6C (site 2) and 5C (site 1) signals in the presence of sub-stoichiometric Fe(ii). These results are consistent with a reduced Fe(ii)-binding affinity of site 2 as more weakly coordinating water-derived ligands complete the 6C site. In the absence of Ca(ii), both sites 1 and 2 are occupied upon addition of sub-stoichiometric Fe(ii), and a stronger ligand field is observed for the 5C site. These spectroscopic studies provide further evaluation of a unique non-heme Fe(ii)-His₆ site for metalloproteins and support the notion that Ca(ii) ions influence the Fe(ii)-binding properties of CP., National Institutes of Health (U.S.) (Grant P30-ES002109)

Published
2018

76. A combined magnetic circular dichroism and density functional theory approach for the elucidation of electronic structure and bonding in three- and four-coordinate iron(ii)–N-heterocyclic carbene complexes† †Electronic supplementary information (ESI) available: Supplemental MCD and Mössbauer data; MO diagrams, TD-DFT, optimized geometry coordinates and X-ray crystallographic details. CCDC 1023546–1023548. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4sc02791d Click here for additional data file. Click here for additional data file

Author

Fillman, Kathlyn L., Przyojski, Jacob A., Al-Afyouni, Malik H., Tonzetich, Zachary J., and Neidig, Michael L.

Subjects
Chemistry
Abstract

Studies of electronic structure and bonding in iron(ii)–NHC complexes using a combined magnetic circular dichroism and DFT approach., Iron salts and N-heterocyclic carbene (NHC) ligands is a highly effective combination in catalysis, with observed catalytic activities being highly dependent on the nature of the NHC ligand. Detailed spectroscopic and electronic structure studies have been performed on both three- and four-coordinate iron(ii)–NHC complexes using a combined magnetic circular dichroism (MCD) and density functional theory (DFT) approach that provide detailed insight into the relative ligation properties of NHCs compared to traditional phosphine and amine ligands as well as the effects of NHC backbone structural variations on iron(ii)–NHC bonding. Near-infrared MCD studies indicate that 10Dq(T d) for (NHC)2FeCl2 complexes is intermediate between those for comparable amine and phosphine complexes, demonstrating that such iron(ii)–NHC and iron(ii)–phosphine complexes are not simply analogues of one another. Theoretical studies including charge decomposition analysis indicate that the NHC ligands are slightly stronger donor ligands than phosphines but also result in significant weakening of the Fe–Cl bonds compared to phosphine and amine ligands. The net result is significant differences in the d orbital energies in four-coordinate (NHC)2FeCl2 complexes relative to the comparable phosphine complexes, where such electronic structure differences are likely a significant contributing factor to the differing catalytic performances observed with these ligands. Furthermore, Mössbauer, MCD and DFT studies of the effects of NHC backbone structure variations (i.e. saturated, unsaturated, chlorinated) on iron–NHC bonding and electronic structure in both three- and four-coordinate iron(ii)–NHC complexes indicate only small differences as a function of backbone structure, that are likely amplified at lower oxidation states of iron due to the resulting decrease in the energy separation between the occupied iron d orbitals and the unoccupied NHC π* orbitals.

Published
2014

81. A Pseudotetrahedral Uranium(V) Complex

Author

Tondreau, Aaron M., primary, Duignan, Thomas J., additional, Stein, Benjamin W., additional, Fleischauer, Valerie E., additional, Autschbach, Jochen, additional, Batista, Enrique R., additional, Boncella, James M., additional, Ferrier, Maryline G., additional, Kozimor, Stosh A., additional, Mocko, Veronika, additional, Neidig, Michael L., additional, Cary, Samantha K., additional, and Yang, Ping, additional

Published
2018
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84. The three-His triad in Dke1: comparisons to the classical facial traid

Author

Diebold, Adrienne R., Neidig, Michael L., Moran, Graham R., Straganz, Grit D., and Solomon, Edward I.

Subjects
Carboxylation -- Analysis, Cysteine -- Chemical properties, Density functionals -- Usage, Histidine -- Chemical properties, Iron -- Chemical properties, Oxidases -- Structure, Oxidases -- Chemical properties, Biological sciences, Chemistry
Abstract

The effect of the three-His motif in Dke1 on the geometric and electronic structure of the [Fe.sup.II] centers is analyzed by a combination of absorption, CD, MCD and VTVH MCD spectroscopies and DFT calculations. The studies have helped in understanding the effects of perturbation of the facial triad ligation of the non-heme ferrous enzymes on their geometric and electronic structure and also their contributions to reactivity.

Published
2010

85. CD and MCD of CytC3 and taurine dioxygenase: role of the facial triad in [alpha]-KG-dependent oxygenases

Author

Neidig, Michael L., Brown, Christina D., Light, Kenneth M., Fujimori, Danica Galonic, Nolan, Elizabeth M., Price, John C., Barr, Eric W., Bollinger, J. Martin, Jr., Krebs, Carsten, Walsh, Christopher T., and Solomon, Edward I.

Subjects
Catalysis -- Research, Crystallography -- Research, Cytochromes -- Research, Oxidases -- Research, Chemistry
Abstract

The article describes a new subclass of [alpha]-ketoglutarate ([alpha]-KG)-dependent oxygenases, which do not contain the conventional facial triad. These compounds are found to exhibit novel reactivity and excellent catalytic activity.

Published
2007

88. A Combined Probe-Molecule, Mössbauer, Nuclear Resonance Vibrational Spectroscopy, and Density Functional Theory Approach for Evaluation of Potential Iron Active Sites in an Oxygen Reduction Reaction Catalyst

Author

Kneebone, Jared L., primary, Daifuku, Stephanie L., additional, Kehl, Jeffrey A., additional, Wu, Gang, additional, Chung, Hoon T., additional, Hu, Michael Y., additional, Alp, E. Ercan, additional, More, Karren L., additional, Zelenay, Piotr, additional, Holby, Edward F., additional, and Neidig, Michael L., additional

Published
2017
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89. Polyoxovanadate–Alkoxide Clusters as a Redox Reservoir for Iron

Author

Li, Feng, primary, Carpenter, Stephanie H., additional, Higgins, Robert F., additional, Hitt, Mark G., additional, Brennessel, William W., additional, Ferrier, Maryline G., additional, Cary, Samantha K., additional, Lezama-Pacheco, Juan S., additional, Wright, Joshua T., additional, Stein, Benjamin W., additional, Shores, Matthew P., additional, Neidig, Michael L., additional, Kozimor, Stosh A., additional, and Matson, Ellen M., additional

Published
2017
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95. CD and MCD studies of the non-heme ferrous active site in (4-hydroxyphenyl)pyruvate dioxygenase: correlation between oxygen activation in the extradiol and alpha-KG-dependent dioxygenases

Author

Neidig, Michael L., Kavana, Michael, Moran, Graham R., and Solomon, Edward I.

Subjects
Chemical compounds -- Research, Oxides -- Research, Chemistry
Abstract

A study was conducted, with circular dichroism (CD) and magnetic circular dichroism (MCD), to examine the resting Fe site of (4-hydroxyphenyl)pyruvate dioxygenase (HPPD) and its interaction with (4-hydroxyphenyl)pyrvate (HPP) substrate. The result provides the insight into the mechanism of oxygen binding and activation by HPPD, and its active-site structural relationship to both alpha-KG-dependent dioxygenases and extradiol class of mononuclear non-heme iron enzymes.

Published
2004

96. Crystal structure of bromidopentakis(tetrahydrofuran-κO)magnesium bis[1,2-bis(diphenylphosphanyl)benzene-κ²P,P']cobaltate(-1) tetrahydrofuran disolvate.

Author

Girigiri, Patience B., Carpenter, Stephanie H., Brennessel, William W., and Neidig, Michael L.

Subjects
TETRAHYDROFURAN, PHOSPHINES, CRYSTAL structure
Abstract

Structural characterization of the ionic title complex, [MgBr(THF)5]-[Co(dpbz)2]·2THF [THF is tetrahydrofuran, C4H8O; dpbz is 1,2-bis(diphenylphosphanyl) benzene, C30H24P2], revealed a well-separated cation and anion cocrystallized with two THF solvent molecules that interact with the cation via weak C--H...O contacts. The geometry about the cobalt center is pseudotetrahedral, as is expected for a d10 metal center, only deviating from an ideal tetrahedral geometry because of the restrictive bite angles of the bidentate phosphane ligands. Three THF ligands of the cation and one co-crystallized THF solvent molecule are each disordered over two orientations. In the extended structure, the cations and THF solvent molecules are arranged in (100) sheets that alternate with layers of anions, the latter of which show various π-interactions, which may explain the particular packing arrangement. [ABSTRACT FROM AUTHOR]

Published
2019
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98. Activation and Functionalization of the Uranyl Ion by Electrochemical Reduction

Author

Golwankar, Riddhi R., Makoś, Małgorzata Z., Cajiao, Nathalia, Neidig, Michael L., Oliver, Allen G., Day, Cynthia S., Day, Victor W., Glezakou, Vassiliki-Alexandra, and Blakemore, James D.

Abstract

Interconversion of the oxidation states of uranium enables separations and reactivity schemes involving this element and contributes to technologies for recycling of spent nuclear fuels. The redox behaviors of uranium species impact these processes, but use of electrochemical methods to drive reactions of molecular uranium complexes and to obtain molecular insights into the outcomes of electrode-driven reactions has received far less attention than it deserves. Here, we show that electro-reduction of the uranyl ion (UO22+) can be used to promote stepwise functionalization of the typically unreactive oxo groups with exogenous triphenylborane (BPh3) serving as a moderate electrophile, avoiding the conventional requirement for a chemical reductant. Parallel electroanalytical, spectrochemical, and chemical reactivity studies, supported by spectroscopic findings and structural data from X-ray diffraction analysis on key reduced and borylated products, demonstrate that our electrochemical approach largely avoids undesired cross reactions and disproportionation pathways; these usually impact the multicomponent systems needed for uranyl functionalization chemistry. Joint computational studies have been used to map the changes associated with U–O activation and to quantify the free energy differences related to key reactions. Taken together, the results suggest that electrochemical methods can be used for selective interconversion of molecular actinide species, reminiscent of methods commonly employed in transition metal redox catalysis.

Published
2024
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99. Caught in the Act of Substitution: Interadsorbate Effects on an Atomically Precise Fe/Co/Se Nanocluster

Author

Kephart, Jonathan A., Zhou, Daniel Y., Sandwisch, Jason, Cajiao, Nathalia, Krajewski, Sebastian M., Malinowski, Paul, Chu, Jiun-Haw, Neidig, Michael L., Kaminsky, Werner, and Velian, Alexandra

Abstract

Directing groups guide substitution patterns in organic synthetic schemes, but little is known about pathways to control reactivity patterns, such as regioselectivity, in complex inorganic systems such as bioinorganic cofactors or extended surfaces. Interadsorbate effects are known to encode surface reactivity patterns in inorganic materials, modulating the location and binding strength of ligands. However, owing to limited experimental resolution into complex inorganic structures, there is little opportunity to resolve these effects on the atomic scale. Here, we utilize an atomically precise Fe/Co/Se nanocluster platform, [Fe3(L)2Co6Se8L′6]+([1(L)2]+; L = CNtBu, THF; L′ = Ph2PN(−)Tol), in which allosteric interadsorbate effects give rise to pronounced site-differentiation. Using a combination of spectroscopic techniques and single-crystal X-ray diffractometry, we discover that coordination of THF at the ligand-free Fe site in [1(CNtBu)2]+sets off a domino effect wherein allosteric through-cluster interactions promote the regioselective dissociation of CNtBu at a neighboring Fe site. Computational analysis reveals that this active site correlation is a result of delocalized Fe···Se···Co···Se covalent interactions that intertwine edge sites on the same cluster face. This study provides an unprecedented atom-scale glimpse into how interfacial metal–support interactions mediate a collective and regiospecific path for substrate exchange across multiple active sites.

Published
2024
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100. Kinetic and Spectroscopic Studies of N694C Lipoxygenase: A Probe of the Substrate Activation Mechanism of a Nonheme Ferric Enzyme

Author

Neidig, Michael L., Wecksler, Aaron T., Schenk, Gerhard, Holman, Theodore R., Solomon, Edward I., Neidig, Michael L., Wecksler, Aaron T., Schenk, Gerhard, Holman, Theodore R., and Solomon, Edward I.

Abstract

Lipoxygenases (LOs) comprise a class of substrate activating mononuclear nonheme iron enzymes which catalyze the hydroperoxidation of unsaturated fatty acids. A commonly proposed mechanism for LO catalysis involves H-atom abstraction by an FeIII-OH- site, best described as a proton coupled electron transfer (PCET) process, followed by direct reaction of O2 with the resulting substrate radical to yield product. An alternative mechanism that has also been discussed involves the abstraction of a proton from the substrate by the FeIII-OH leading to a ó-organoiron intermediate, where the subsequent ó bond insertion of dioxygen into the C-Fe bond completes the reaction. H-atom abstraction is favored by a high E° of the FeII/FeIII couple and high pKa of water bound to the ferrous state, while an organoiron mechanism would be favored by a low E° (to keep the site oxidized) and a high pKa of water bound to the ferric state (to deprotonate the substrate). A first coordination sphere mutant of soybean LO (N694C) has been prepared and characterized by near-infrared circular dichroism (CD) and variable-temperature, variable-field (VTVH) magnetic circular dichroism (MCD) spectroscopies (FeII site), as well as UV/vis absorption, UV/vis CD, and electron paramagnetic resonance (EPR) spectroscopies (FeIII site). These studies suggest that N694C has a lowered E° of the FeII/FeIII couple and a raised pKa of water bound to the ferric site relative to wild type soybean lipoxygenase-1 (WT sLO-1) which would favor the organoiron mechanism. However, the observation in N694C of a significant deuterium isotope effect, anaerobic reduction of iron by substrate, and a substantial decrease in kcat (3000-fold) support H-atom abstraction as the relevant substrateactivation mechanism in sLO-1.

Published
2007

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