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Betley, Theodore

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Betley

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Theodore

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Betley, Theodore
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Now showing 1 - 10 of 23
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    Maximizing Electron Exchange in a [Fe3] Cluster
    (American Chemical Society (ACS), 2016) Hernández Sánchez, Raúl; Bartholomew, Amymarie; Powers, Tamara Michelle; Ménard, Gabriel; Betley, Theodore
    The one-electron reduction of (tbsL)Fe3(thf)1 furnishes [M][(tbsL)Fe3] ([M]+ = [(18-C-6)K(thf)2]+ (1, 76%) or [(crypt-222)K]+ (2, 54%)). Upon reduction, the ligand tbsL6– rearranges around the triiron core to adopt an almost ideal C3-symmetry. Accompanying the (tbsL) ligand rearrangement, the THF bound to the neutral starting material is expelled, and the Fe–Fe distances within the trinuclear cluster contract by ∼0.13 Å in 1. Variable-temperature magnetic susceptibility data indicates a well-isolated S = 11/2 spin ground state that persists to room temperature. Slow magnetic relaxation is observed at low temperature as evidenced by the out-of-phase (χM″) component of the alternating current (ac) magnetic susceptibility data and by the appearance of hyperfine splitting in the zero-field 57Fe Mössbauer spectra at 4.2 K. Analysis of the ac magnetic susceptibility yields an effective spin reversal barrier (Ueff) of 22.6(2) cm–1, nearly matching the theoretical barrier of 38.7 cm–1 calculated from the axial zero-field splitting parameter (D = −1.29 cm–1) extracted from the reduced magnetization data. A polycrystalline sample of 1 displays three sextets in the Mössbauer spectrum at 4.2 K (Hext = 0) which converge to a single six-line pattern in a frozen 2-MeTHF glass sample, indicating a unique iron environment and thus strong electron delocalization. The spin ground state and ligand rearrangement are discussed within the framework of a fully delocalized cluster exhibiting strong double and direct exchange interactions.
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    Meta-Atom Behavior in Clusters Revealing Large Spin Ground States
    (American Chemical Society (ACS), 2015) Hernández Sánchez, Raúl; Betley, Theodore
    The field of single molecule magnetism remains predicated on super- and double exchange mechanisms to engender large spin ground states. An alternative approach to achieving high-spin architectures involves synthesizing weak-field clusters featuring close M–M interactions to produce a single valence orbital manifold. Population of this orbital manifold in accordance with Hund’s rules could potentially yield thermally persistent high-spin ground states under which the valence electrons remain coupled. We now demonstrate this effect with a reduced hexanuclear iron cluster that achieves an S = 19/2 (χMT ≈ 53 cm3 K/mol) ground state that persists to 300 K, representing the largest spin ground state persistent to room temperature reported to date. The reduced cluster displays single molecule magnet behavior manifest in both variable-temperature zero-field 57Fe Mössbauer and magnetometry with a spin reversal barrier of 42.5(8) cm–1 and a magnetic blocking temperature of 2.9 K (0.059 K/min).
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    Testing the Polynuclear Hypothesis: Multielectron Reduction of Small Molecules by Triiron Reaction Sites
    (American Chemical Society (ACS), 2013) Powers, Tamara Michelle; Betley, Theodore
    High-spin trinuclear iron complex (tbsL)Fe3(thf) ([tbsL]6– = [1,3,5-C6H9(NC6H4-o-NSitBuMe2)3]6–) (S = 6) facilitates 2 and 4e– reduction of NxHy type substrates to yield imido and nitrido products. Reaction of hydrazine or phenylhydrazine with (tbsL)Fe3(thf) yields triiron μ3-imido cluster (tbsL)Fe3(μ3-NH) and ammonia or aniline, respectively. (tbsL)Fe3(μ3-NH) has a similar zero-field 57Fe Mössbauer spectrum compared to previously reported [(tbsL)Fe3(μ3-N)]NBu4, and can be directly synthesized by protonation of the anionic triiron nitrido with lutidinium tetraphenylborate. Deprotonation of the triiron parent imido (tbsL)Fe3(μ3-NH) with lithium bis(trimethylsilyl)amide results in regeneration of the triiron nitrido complex capped with a thf-solvated Li cation [(tbsL)Fe3(μ3-N)]Li(thf)3. The lithium capped nitrido, structurally similar to the pseudo C3-symmetric triiron nitride with a tetrabutylammonium countercation, is rigorously C3-symmetric featuring intracore distances of Fe–Fe 2.4802(5) Å, Fe–N(nitride) 1.877(2) Å, and N(nitride)–Li 1.990(6) Å. A similar 2e– reduction of 1,2-diphenylhydrazine by (tbsL)Fe3(thf) affords (tbsL)Fe3(μ3-NPh) and aniline. The solid state structure of (tbsL)Fe3(μ3-NPh) is similar to the series of μ3-nitrido and -imido triiron complexes synthesized in this work with average Fe–Nimido and Fe–Fe bond lengths of 1.941(6) and 2.530(1) Å, respectively. Reductive N═N bond cleavage of azobenzene is also achieved in the presence of (tbsL)Fe3(thf) to yield triiron bis-imido complex (tbsL)Fe3(μ3-NPh)(μ2-NPh), which has been structurally characterized. Ligand redox participation has been ruled out, and therefore, charge balance indicates that the bis-imido cluster has undergone a 4e– metal based oxidation resulting in an (FeIV)(FeIII)2 formulation. Cyclic voltammograms of the series of triiron clusters presented herein demonstrate that oxidation states up to (FeIV)(FeIII)2 (in the case of [(tbsL)Fe3(μ3-N)]NBu4) are electrochemically accessible. These results highlight the efficacy of high-spin, polynuclear reaction sites to cooperatively mediate small molecule activation.
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    Metal Atom Lability in Polynuclear Complexes
    (American Chemical Society (ACS), 2013) Eames, Emily V.; Hernández Sánchez, Raúl; Betley, Theodore
    The asymmetric oxidation product [(PhL)Fe3(μ-Cl)]2 [PhLH6 = MeC(CH2NHPh-o-NHPh)3], where each trinuclear core is comprised of an oxidized diiron unit [Fe2]5+ and an isolated trigonal pyramidal ferrous site, reacts with MCl2 salts to afford heptanuclear bridged structures of the type (PhL)2Fe6M(μ-Cl)4(thf)2, where M = Fe or Co. Zero-field, 57Fe Mössbauer analysis revealed the Co resides within the trinuclear core subunits, not at the octahedral, halide-bridged MCl4(thf)2 position indicating Co migration into the trinuclear subunits has occurred. Reaction of [(PhL)Fe3(μ-Cl)]2 with CoCl2 (2 or 5 equivalents) followed by precipitation via addition of acetonitrile afforded trinuclear products where one or two irons, respectively, can be substituted within the trinuclear core. Metal atom substitution was verified by 1H NMR, 57Fe Mossbauer, single crystal X-ray diffraction, X-ray fluorescence, and magnetometry analysis. Spectroscopic analysis revealed that the Co atom(s) substitute(s) into the oxidized dimetal unit ([M2]5+), while the M2+ site remains iron-substituted. Magnetic data acquired for the series are consistent with this analysis revealing the oxidized dimetal unit comprises a strongly coupled S = 1 unit ([FeCo]5+) or S = 1/2 ([Co2]5+) that is weakly antiferromagnetically coupled to the high spin (S = 2) ferrous site. The kinetic pathway for metal substitution was probed via reaction of [(PhL)Fe3(μ-Cl)]2 with isotopically enriched 57FeCl2(thf)2, the results of which suggest rapid equilibration of 57Fe into both the M2+ site and oxidized diiron site, achieving a 1:1 mixture.
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    Complex N-Heterocycle Synthesis via Iron-Catalyzed, Direct C-H Bond Amination
    (American Association for the Advancement of Science (AAAS), 2013) Hennessy, Elisabeth; Betley, Theodore
    The manipulation of traditionally unreactive functional groups is of paramount importance in modern chemical synthesis. We have developed an iron-dipyrrinato catalyst that leverages the reactivity of iron-borne metal-ligand multiple bonds to promote the direct amination of aliphatic C–H bonds. Exposure of organic azides to the iron dipyrrinato catalyst furnishes saturated, cyclic amine products (N-heterocycles) bearing complex core-substitution patterns. This study highlights the development of C–H bond functionalization chemistry for the formation of saturated, cyclic amine products and should find broad application in the context of both pharmaceuticals and natural product synthesis.
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    Synthesis of Open-Shell, Bimetallic Mn/Fe Trinuclear Clusters
    (American Chemical Society (ACS), 2013) Powers, Tamara Michelle; Gu, Nina; Fout, Alison R.; Baldwin, Anne M.; Hernández Sánchez, Raúl; Alfonso, Denise Marie; Chen, Yu-Sheng; Zheng, Shao-Liang; Betley, Theodore
    Concomitant deprotonation and metalation of hexadentate ligand platform tbsLH6 (tbsLH6 = 1,3,5-C6H9(NHC6H4-o-NHSiMe2tBu)3) with divalent transition metal starting materials Fe2(Mes)4 (Mes = mesityl) or Mn3(Mes)6 in the presence of tetrahydrofuran (THF) resulted in isolation of homotrinuclear complexes (tbsL)Fe3(THF) and (tbsL)Mn3(THF), respectively. In the absence of coordinating solvent (THF), the deprotonation and metalation exclusively afforded dinuclear complexes of the type (tbsLH2)M2 (M = Fe or Mn). The resulting dinuclear species were utilized as synthons to prepare bimetallic trinuclear clusters. Treatment of (tbsLH2)Fe2 complex with divalent Mn source (Mn2(N(SiMe3)2)4) afforded the bimetallic complex (tbsL)Fe2Mn(THF), which established the ability of hexamine ligand tbsLH6 to support mixed metal clusters. The substitutional homogeneity of (tbsL)Fe2Mn(THF) was determined by 1H NMR, 57Fe Mössbauer, and X-ray fluorescence. Anomalous scattering measurements were critical for the unambiguous assignment of the trinuclear core composition. Heating a solution of (tbsLH2)Mn2 with a stoichiometric amount of Fe2(Mes)4 (0.5 mol equiv) affords a mixture of both (tbsL)Mn2Fe(THF) and (tbsL)Fe2Mn(THF) as a result of the thermodynamic preference for heavier metal substitution within the hexa-anilido ligand framework. These results demonstrate for the first time the assembly of mixed metal cluster synthesis in an unbiased ligand platform.
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    Examination of pigments on Thai manuscripts: the first identification of copper citrate
    (Wiley-Blackwell, 2008) Eremin, Katherine; Stenger, Jens; Huang, Jo-Fan; Aspuru-Guzik, Alan; Betley, Theodore; Vogt, Leslie Ann; Kassal, Ivan; Speakman, Scott; Khandekar, Narayan
    Samples from Thai manuscripts dated to the 18th to 20th century were analyzed by Raman spectroscopy and Fourier-transform infrared spectroscopy (FTIR) to determine the pigments used. This suggested a change in palette from the 18th to 20th century, with use of imported pigments in the later manuscripts. In the 18th century, the main green used was an organic copper salt, which was replaced by emerald green and mixtures of Prussian blue with gamboge, chrome yellow and zinc yellow (zinc potassium chromate). Chrome yellow was used in addition to gamboge in one later 19th century manuscript. Similarly, indigo in the 18th century manuscripts was replaced by Prussian blue and then synthetic ultramarine in the 19th century manuscripts. Lead white was the main white pigment in all but one manuscript, which contained huntite, a magnesium calcium carbonate. Huntite also occurred in mixtures with other pigments in two other manuscripts. In all the works studied, red lead, vermilion and red earth were used for red, orange and pink shades and red earth in brown areas. The organic copper salt used in the 18th century gave good FTIR spectra but could not initially be matched with any published compound. X-ray diffraction (XRD) suggested this was a copper citrate phase, and examination of the literature showed that the FTIR spectra matched those published for a hydrated copper citrate. Raman spectra were obtained from this organic copper salt, which showed close agreement with those obtained from synthetic copper citrate. Copper citrate has not been identified previously as an artist’s material, although its use has been postulated on the basis of historical texts. Minor copper formate and/or copper chloride were also identified by XRD and scanning electron microscopy (SEM) in some green samples containing copper citrate.
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    Electronic Perturbations of Iron Dipyrrinato Complexes via Ligand β-Halogenation and meso -Fluoroarylation
    (American Chemical Society (ACS), 2011) Scharf, Austin; Betley, Theodore
    Systematic electronic variations were introduced into the monoanionic dipyrrinato ligand scaffold via halogenation of the pyrrolic β-positions and/or via the use of fluorinated aryl substituents in the ligand bridgehead position in order to synthesize proligands of the type 1,9-dimesityl-β-R4-5-Ar-dipyrrin [R = H, Cl, Br, I; Ar = mesityl, 3,5-(F3C)2C6H3, C6F5 in ligand 5-position; β = 2,3,7,8 ligand substitution; abbreviated (β,ArL)H]. The electronic perturbations were probed using standard electronic absorption and electrochemical techniques on the different ligand variations and their divalent iron complexes. The free-ligand variations cause modest shifts in the electronic absorption maxima (λmax: 464–499 nm) and more pronounced shifts in the electrochemical redox potentials for one-electron proligand reductions (E1/2: −1.25 to −1.99 V) and oxidations (E1/2: +0.52 to +1.14 V vs [Cp2Fe]+/0). Installation of iron into the dipyrrinato scaffolds was effected via deprotonation of the proligands followed by treatment with FeCl2 and excess pyridine in tetrahydrofuran to afford complexes of the type (β,ArL)FeCl(py) (py = pyridine). The electrochemical and spectroscopic behavior of these complexes varies significantly across the series: the redox potential of the fully reversible FeIII/II couple spans more than 400 mV (E1/2: −0.34 to +0.50 V vs [Cp2Fe]+/0); λmax spans more than 40 nm (506–548 nm); and the 57Fe Mössbauer quadrupole splitting (|ΔEQ|) spans nearly 2.0 mm/s while the isomer shift (δ) remains essentially constant (0.86–0.89 mm/s) across the series. These effects demonstrate how peripheral variation of the dipyrrinato ligand scaffold can allow systematic variation of the chemical and physical properties of iron dipyrrinato complexes.
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    Modulation of magnetic behavior via ligand-field effects in the trigonal clusters (PhL)Fe3L*3 (L* = thf, py, PMe2Ph)
    (Royal Society of Chemistry (RSC), 2012) Eames, Emily V.; Harris, T. David; Betley, Theodore
    Utilizing a hexadentate ligand platform, a series of trinuclear iron clusters (PhL)Fe3L*3 (PhLH6 1⁄4 MeC (CH2NPh-o-NPh)3; L* 1⁄4 tetrahydrofuran (1), pyridine (2), PMePh2 (3)) has been prepared. The phenyl substituents on the ligand sterically prohibit strong iron–iron bonding from occurring but maintain a sufficiently close proximity between iron centers to permit direct interactions. Coordination of the weak-field tetrahydrofuran ligand to the iron centers results in a well-isolated, high-spin S 1⁄4 6 or S 1⁄4 5 ground state, as ascertained through variable-temperature dc magnetic susceptibility and low- temperature magnetization measurements. Replacing the tetrahydrofuran ligands with stronger s-donating pyridine or tertiary phosphine ligands reduces the ground state to S 1⁄4 2 and gives rise to temperature-dependent magnetic susceptibility. In these cases, the magnetic susceptibility cannot be explained as arising simply from superexchange interactions between metal centers through the bridging amide ligands. Rather, the experimental data are best modelled by considering a thermally- induced variation in molecular spin state between S 1⁄4 2 and S 1⁄4 4. Fits to these data provide thermodynamic parameters of DH 1⁄4 406 cm 1 and Tc 1⁄4 187 K for 2 and DH 1⁄4 604 cm 1 and Tc 1⁄4 375 K for 3. The difference in these parameters is consistent with ligand field strength differences between pyridine and phosphine ligands. To rationalize the spin state variation across the series of clusters, we first propose a qualitative model of the Fe3 core electronic structure that considers direct Fe–Fe interactions, arising from direct orbital overlap. We then present a scenario, consistent with the observed magnetic behaviour, in which the s orbitals of the electronic structure are perturbed by substitution of the ancillary ligands.
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    Catalytic C−H Bond Amination from High-Spin Iron Imido Complexes
    (American Chemical Society (ACS), 2011) King, Evan R.; Hennessy, Elisabeth; Betley, Theodore
    Dipyrromethene ligand scaffolds were synthesized bearing large aryl (2,4,6-Ph3C6H2, abbreviated Ar) or alkyl (tBu, adamantyl) flanking groups to afford three new disubstituted ligands (RL, 1,9-R2-5-mesityldipyrromethene, R = aryl, alkyl). While high-spin (S = 2), four-coordinate iron complexes of the type (RL)FeCl(solv) were obtained with the alkyl-substituted ligand varieties (for R = tBu, Ad and solv = THF, OEt2), use of the sterically encumbered aryl-substituted ligand precluded binding of solvent and cleanly afforded a high-spin (S = 2), three-coordinate complex of the type (ArL)FeCl. Reaction of (AdL)FeCl(OEt2) with alkyl azides resulted in the catalytic amination of C−H bonds or olefin aziridination at room temperature. Using a 5% catalyst loading, 12 turnovers were obtained for the amination of toluene as a substrate, while greater than 85% of alkyl azide was converted to the corresponding aziridine employing styrene as a substrate. A primary kinetic isotope effect of 12.8(5) was obtained for the reaction of (AdL)FeCl(OEt2) with adamantyl azide in an equimolar toluene/toluene-d8 mixture, consistent with the amination proceeding through a hydrogen atom abstraction, radical rebound type mechanism. Reaction of p-tBuC6H4N3 with (ArL)FeCl permitted isolation of a high-spin (S = 2) iron complex featuring a terminal imido ligand, (ArL)FeCl(N(p-tBuC6H4)), as determined by 1H NMR, X-ray crystallography, and 57Fe Mössbauer spectroscopy. The measured Fe−Nimide bond distance (1.768(2) Å) is the longest reported for Fe(imido) complexes in any geometry or spin state, and the disruption of the bond metrics within the imido aryl substituent suggests delocalization of a radical throughout the aryl ring. Zero-field 57Fe Mössbauer parameters obtained for (ArL)FeCl(N(p-tBuC6H4)) suggest a FeIII formulation and are nearly identical with those observed for a structurally similar, high-spin FeIII complex bearing the same dipyrromethene framework. Theoretical analyses of (ArL)FeCl(N(p-tBuC6H4)) suggest a formulation for this reactive species to be a high-spin FeIII center antiferromagnetically coupled to an imido-based radical (J = −673 cm−1). The terminal imido complex was effective for delivering the nitrene moiety to both C−H bond substrates (42% yield) as well as styrene (76% yield). Furthermore, a primary kinetic isotope effect of 24(3) was obtained for the reaction of (ArL)FeCl(N(p-tBuC6H4)) with an equimolar toluene/toluene-d8 mixture, consistent with the values obtained in the catalytic reaction. This commonality suggests the isolated high-spin FeIII imido radical is a viable intermediate in the catalytic reaction pathway. Given the breadth of iron imido complexes spanning several oxidation states (FeII−FeV) and several spin states (S = 0 → 3/2), we propose the unusual electronic structure of the described high-spin iron imido complexes contributes to the observed catalytic reactivity.