Jeffrey R Long

Lawrence Berkeley National Laboratory, Berkeley, California, United States

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Publications (217)1980.56 Total impact

  • Journal of Materials Chemistry A. 01/2015;
  • Ehud Tsivion, Jeffrey R Long, Martin Head-Gordon
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    ABSTRACT: In order for hydrogen gas to be used as a fuel, it must be stored in sufficient quantity on board the vehicle. Efforts are being made to increase the hydrogen storage capabilities of metal-organic frameworks (MOFs) by introducing unsatu-rated metal sites into their linking element(s), as hydrogen adsorption centers. In order to devise successful hydrogen storage strategies there is a need for a fundamental understanding of the weak and elusive hydrogen physisorption in-teraction. Here we report our findings from the investigation of the weak inter-molecular interactions of adsorbed hy-drogen molecules on MOF-linkers by using cluster models. Since physical interactions such as dispersion and polariza-tion have a major contribution to attraction energy, our approach is to analyze the adsorption interaction using Ener-gy Decomposition Analysis (EDA) that distinguishes the contribution of the physical interactions from the charge-transfer (CT) "chemical" interaction. Surprisingly, it is found that CT from the adsorbent to the σ*(H2) orbital is pre-sent in all studied complexes and can contribute up to approximately -2 kJ/mol to the interaction. When metal ions are present, donation from the σ(H2) → metal Rydberg-like orbital, along with the adsorbent → σ*(H2) contribution, can contribute from -2 to -10 kJ/mol, depending on the coordination mode. To reach a sufficient adsorption enthalpy for practical usage, the hydrogen molecule must be substantially polarized. Ultimately, the ability of the metallated linker to polarize the hydrogen molecule is highly dependent on the geometry of the metal ion coordination site where a strong electrostatic dipole or quadrupole moment is required.
    Journal of the American Chemical Society 11/2014; · 11.44 Impact Factor
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    ABSTRACT: Single-molecule magnets represent the ultimate size limit for spin-based information storage and processing; however, such applications require large spin relaxation barriers and blocking temperatures. Ongoing efforts to synthesize single-molecule magnets with higher barriers must take into consideration key physical parameters, such as spin ground state, S, and axial zero-field splitting parameter, D, which are both correlated to the barrier height. A third critical parameter that has received less attention is the exchange coupling constant, J. This constant determines the degree of separation between spin ground state and excited states, which must be sufficiently large in a single-molecule magnet to maintain slow magnetization dynamics at elevated temperatures, and also serves to shut down fast quantum relaxation pathways. Toward this end, one synthetic strategy to engender strong magnetic exchange is the incorporation of radical ligands into metal complexes. Within these complexes, the presence of direct exchange between paramagnetic ligand and metal units can result in exceptionally strong magnetic coupling, much stronger in fact than more common superexchange interactions. This review article provides a survey of radical ligand-containing single-molecule magnets, with a brief overview of other classes of metal-ligand radical complexes that could be exploited in the design of new single-molecule magnets. Furthermore, ligand-field and electronic structure considerations in dictating exchange strength and slow magnetic relaxation are highlighted, with the aim of helping to guide the synthesis of future radical ligand-containing single-molecule magnets with even stronger exchange coupling.
    Coordination Chemistry Reviews 11/2014; · 12.10 Impact Factor
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    ABSTRACT: First-row two-coordinate complexes are attracting much interest. Herein, we report the high-yield isolation of the linear two-coordinate iron(I) complex salt [K(L)][Fe{N(SiMe3)2}2] (L=18-crown-6 or crypt-222) through the reduction of either [Fe{N(SiMe3)2}2] or its three-coordinate phosphine adduct [Fe{N(SiMe3)2}2(PCy3)]. Detailed characterization is gained through X-ray diffraction, variable-temperature NMR spectroscopy, and magnetic susceptibility studies. One- and two-electron oxidation through reaction with I2 is further found to afford the corresponding iodo iron(II) and diiodo iron(III) complexes.
    Angewandte Chemie International Edition 11/2014; · 11.34 Impact Factor
  • Katie R. Meihaus, Jeffrey R. Long
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    ABSTRACT: Actinide single-molecule magnetism has experienced steady growth over the last five years since the first discovery of slow magnetic relaxation in the mononuclear complex U(Ph2BPz2)3. Given their large spin-orbit coupling and the radial extension of the 5f orbitals, the actinides are well-suited for the design of both mononuclear and exchange-coupled molecules, and indeed at least one new system has emerged every year. By some measures, the actinides are already demonstrating promise for one day exceeding the performance characteristics of transition metal and lanthanide complexes. However, much further work is needed to understand the nature of the slow relaxation in mononuclear actinide complexes, as well as the influence of magnetic exchange on slow relaxation in multinuclear species. This perspective seeks to summarize the successes in the field and to address some of the many open questions in this up and coming area of research.
    Dalton Transactions 10/2014; · 4.10 Impact Factor
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    ABSTRACT: Analysis of the CO2 adsorption properties of a well-known series of metal-organic frameworks M2(dobdc) (dobdc4−= 2,5-dioxido-1,4-benzenedicarboxylate; M = Mg, Mn, Fe, Co, Ni, Cu, and Zn) is carried out in tandem with in-situ structural studies to identify the host-guest interactions that lead to significant differences in isosteric heats of CO2 adsorption. Neutron and X-ray powder diffraction and single crystal X-ray diffraction experiments are used to unveil the site-specific binding properties of CO2 within many of these materials while systematically varying both the amount of CO2 and the temperature. Unlike previous studies, we show that CO2 adsorbed at the metal cations exhibits intramolecular angles with minimal deviations from 180°, a finding that indicates a strongly electrostatic and physisorptive interaction with the framework surface and sheds more light on the ongoing discussion regarding whether CO2adsorbs in a linear or nonlinear geometry. This has important implications for proposals that have been made to utilize these materials for the activation and chemical conversion of CO2. For the weaker CO2 adsorbents, significant elongation of the metal–O(CO2) distances are observed and diffraction experiments additionally reveal that secondary CO2 adsorption sites, while likely stabilized by the population of the primary adsorption sites, significantly contribute to adsorption behavior at ambient temperature. Density functional theory calculations including van der Waals dispersion quantitatively corroborate and rationalize observations regarding intramolecular CO2 angles and trends in relative geometric properties and heats of adsorption in the M2(dobdc)–CO2 adducts.
    Chemical Science 08/2014; · 8.60 Impact Factor
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    ABSTRACT: The syntheses and magnetic properties of six new compounds featuring the radical-bridged dilanthanide complexes [(Cp*2Ln)2(-tppz•)]+ (Ln = Gd (1), Tb, 2; Dy, 3; tppz = 2,3,5,6-tetra(2-pyridyl)pyrazine) and [(Cp*2Ln)2(-tppz•)]– (Ln = Gd, 4; Tb, 5, Dy, 6) are reported. Cyclic voltammograms for compounds 1-3 reveal that the tppz ligand can reversibly undergo multiple redox changes. Hence, in the two sets of compounds isolated, 1-3 and 4-6, the redox-active ligand tppz exists in the monoanionic (tppz•–) and trianionic (tppz•3–) forms, respectively. Substantial LnIII-tppz•– exchange coupling is found for the cationic tppz•– radical-bridged species of 1-3, as suggested by a rise in MT at low temperatures. For the Gd compound 1, fits to the data yielded a coupling constant of J = –6.91(4) cm–1, revealing antiferromagnetic coupling to give an S = 13/2 ground state. Both of the TbIII and DyIII-containing compounds 2 and 3 exhibit single-molecule magnet behavior under zero applied dc field. Importantly, the Dy congener shows a divergence of the field-cooled and zero-field-cooled dc susceptibility data at 2.8 K and magnetic hysteresis below 3.25 K. Interestingly, the coupling constant of J = –6.29(3) cm–1 determined for the trianionic tppz•3– radical-bridged Gd compound 4 is of similar magnitude to that of the tppz•–-bridged analogue 1. However, the anionic tppz•3–-bridged species containing TbIII and DyIII centers, compounds 5 and 6, do not exhibit slow magnetization dynamics under zero and applied dc fields. Computational results indicate a doublet ground state for the bridging tppz•3– unit, with a different distribution for the spin density orientation towards the LnIII centers. These results have important implications for the future design of molecule-based magnets incorporating exchange-coupled lanthanide-radical species.
    Chemical Science 08/2014; · 8.60 Impact Factor
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    ABSTRACT: The well-known frameworks of the type M2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) have numerous potential applications in gas storage and separations, owing to their exceptionally high concentration of coordinatively unsaturated metal surface sites, which can interact strongly with small gas molecules such as H2. Employing a related meta-functionalized linker that is readily obtained from resorcinol, we now report a family of structural isomers of this framework, M2(m-dobdc) (M = Mg, Mn, Fe, Co, Ni; m-dobdc(4-) = 4,6-dioxido-1,3-benzenedicarboxylate), featuring exposed M(2+) cation sites with a higher apparent charge density. The regioisomeric linker alters the symmetry of the ligand field at the metal sites, leading to increases of 0.4-1.5 kJ/mol in the H2 binding enthalpies relative to M2(dobdc). A variety of techniques, including powder X-ray and neutron diffraction, inelastic neutron scattering, infrared spectroscopy, and first-principles electronic structure calculations, are applied in elucidating how these subtle structural and electronic differences give rise to such increases. Importantly, similar enhancements can be anticipated for the gas storage and separation properties of this new family of robust and potentially inexpensive metal-organic frameworks.
    Journal of the American Chemical Society 08/2014; · 11.44 Impact Factor
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    ABSTRACT: The ubiquity of vanadium oxo complexes in the V+ and IV+ oxidation states has contributed to a comprehensive understanding of their electronic structure and reactivity. However, despite being predicted to be stable by ligand-field theory, the isolation and characterization of a well-defined terminal mononuclear vanadium(III) oxo complex has remained elusive. We present the synthesis and characterization of a unique terminal mononuclear vanadium(III) oxo species supported by the pentadentate polypyridyl ligand 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine (PY5Me2). Exposure of [V(II)(NCCH3)(PY5Me2)](2+) (1) to either dioxygen or selected O-atom-transfer reagents yields [V(IV)(O)(PY5Me2)](2+) (2). The metal-centered one-electron reduction of this vanadium(IV) oxo complex furnishes a stable, diamagnetic [V(III)(O)(PY5Me2)](+) (3) species. The vanadium(III) oxo species is unreactive toward H- and O-atom transfer but readily reacts with protons to form a putative vanadium hydroxo complex. Computational results predict that further one-electron reduction of the vanadium(III) oxo species will result in ligand-based reduction, even though pyridine is generally considered to be a poor π-accepting ligand. These results have implications for future efforts toward low-valent vanadyl chemistry, particularly with regard to the isolation and study of formal vanadium(II) oxo species.
    Inorganic Chemistry 08/2014; · 4.79 Impact Factor
  • Source
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    ABSTRACT: Six metal-organic frameworks of the M2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) structure type are demonstrated to bind carbon monoxide reversibly and at high capacity, as required for its efficient separation from gas mixtures. Infrared spectra indicate that, upon coordination of CO to the divalent metal cations lining the pores within these frameworks, the C-O stretching frequency is blue-shifted, consistent with nonclassical metal-CO interactions involving little or no π back-donation. Structure determinations from powder neutron diffraction data reveal M-CO distances ranging from 2.09(2) Å for M = Ni to 2.49(1) Å for M = Zn and M-C-O angles ranging from 161.2(7)° for M = Mg to 176.9(6)° for M = Fe. Significantly, electronic structure calculations employing density functional theory (DFT) resulted in good agreement with the trends apparent in the infrared spectra and crystal structures only upon allowing the extended framework structure to relax in response to CO binding. These results represent the first crystallographically characterized magnesium and zinc carbonyl compounds and the first high-spin manganese(II), iron(II), cobalt(II), and nickel(II) carbonyl species. Adsorption isotherms collected at 25, 35, and 45 °C indicate reversible adsorption, with capacities for the Fe, Co, and Ni frameworks approaching one CO per metal cation site at 1 bar, corresponding to loadings as high as 6.0 mmol/g and 157 cm(3)/cm(3). Consistent with CO binding at the metal cation sites, the six frameworks display (negative) isosteric heats of CO adsorption ranging from 52.7 to 27.2 kJ/mol along the series Ni > Co > Fe > Mg > Mn > Zn, following the Irving-Williams stability order and in good agreement with the results of DFT calculations. The reversible CO binding at high capacity and moderate energy suggests that these frameworks may be of utility for the separation of CO from various industrial gas mixtures, including CO/H2 and CO/N2. Selectivities determined from gas adsorption isotherm data using ideal adsorbed solution theory (IAST) over a range of gas composi-tions at 1 bar and 298 K indicate that all six M2(dobdc) frameworks could potentially be used as solid adsorbents to replace current cryogenic distillation technologies, with the choice of M dictating adsorbent regeneration energy and the level of purity of the resulting gases.
    Journal of the American Chemical Society 07/2014; · 11.44 Impact Factor
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    ABSTRACT: Enzymatic haem and non-haem high-valent iron-oxo species are known to activate strong C-H bonds, yet duplicating this reactivity in a synthetic system remains a formidable challenge. Although instability of the terminal iron-oxo moiety is perhaps the foremost obstacle, steric and electronic factors also limit the activity of previously reported mononuclear iron(IV)-oxo compounds. In particular, although nature's non-haem iron(IV)-oxo compounds possess high-spin S = 2 ground states, this electronic configuration has proved difficult to achieve in a molecular species. These challenges may be mitigated within metal-organic frameworks that feature site-isolated iron centres in a constrained, weak-field ligand environment. Here, we show that the metal-organic framework Fe2(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) and its magnesium-diluted analogue, Fe0.1Mg1.9(dobdc), are able to activate the C-H bonds of ethane and convert it into ethanol and acetaldehyde using nitrous oxide as the terminal oxidant. Electronic structure calculations indicate that the active oxidant is likely to be a high-spin S = 2 iron(IV)-oxo species.
    Nature Chemistry 07/2014; 6(7):590-5. · 23.30 Impact Factor
  • Selvan Demir, Joseph M. Zadrozny, Jeffrey R. Long
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    ABSTRACT: Single-molecule magnets comprising one spin center represent a fundamental size limit for spin-based information storage. Such an application hinges upon the realization of molecules possessing substantial barriers to spin inversion. Axially symmetric complexes of lanthanides hold the most promise for this due to their inherently high magnetic anisotropies and low tunneling probabilities. Herein, we demonstrate that strikingly large spin reversal barriers of 216 and 331 cm−1 can also be realized in low-symmetry lanthanide tetraphenylborate complexes of the type [Cp*2Ln(BPh4)] (Cp*=pentamethylcyclopentadienyl; Ln=Tb (1) and Dy (2)). The dysprosium congener showed hysteretic magnetization data up to 5.3 K. Further studies of the magnetic relaxation processes of 1 and 2 under applied dc fields and upon dilution within a matrix of [Cp*2Y(BPh4)] revealed considerable suppression of the tunneling pathway, emphasizing the strong influence of dipolar interactions on the low-temperature magnetization dynamics in these systems.
    Chemistry - A European Journal 06/2014; · 5.93 Impact Factor
  • Jarad A. Mason, Mike Veenstra, Jeffrey R. Long
    ChemInform 04/2014; 45(16).
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    ABSTRACT: A photobase generator was used to induce metal–organic framework (MOF) nucleation upon UV irradiation. This method was further developed into a simple, one-step method for depositing patterned MOF films. Furthermore, the ability of our method to coat a single substrate with MOF films having different chemical compositions is illustrated. The method is an important step towards integrating MOF deposition with existing lithographic techniques and the incorporation of these materials into sensors and other electronic devices.
    Angewandte Chemie International Edition 04/2014; · 11.34 Impact Factor
  • Source
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    ABSTRACT: A photobase generator was used to induce metal–organic framework (MOF) nucleation upon UV irradiation. This method was further developed into a simple, one-step method for depositing patterned MOF films. Furthermore, the ability of our method to coat a single substrate with MOF films having different chemical compositions is illustrated. The method is an important step towards integrating MOF deposition with existing lithographic techniques and the incorporation of these materials into sensors and other electronic devices.
    Angewandte Chemie 04/2014;
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    ABSTRACT: Two isostructural series of trigonal prismatic complexes, M(Bp(Me))3 and M(Bc(Me))3 (M = Y, Tb, Dy, Ho, Er, U; [Bp(Me)](-) = dihydrobis(methypyrazolyl)borate; [Bc(Me)](-) = dihydrobis(methylimidazolyl)borate) are synthesized and fully characterized to examine the influence of ligand donor strength on slow magnetic relaxation. Investigation of the dynamic magnetic properties reveals that the oblate electron density distributions of the Tb(3+), Dy(3+), and U(3+) metal ions within the axial ligand field leads to slow relaxation upon application of a small dc magnetic field. Significantly, the magnetization relaxation is orders of magnitude slower for the N-heterocyclic carbene complexes, M(Bc(Me))3, than for the isomeric pyrazolate complexes, M(Bp(Me))3. Further, investigation of magnetically dilute samples containing 11-14 mol% of Tb(3+), Dy(3+), or U(3+) within the corresponding Y(3+) complex matrix reveals thermally-activated relaxation is favored for the M(Bc(Me))3 complexes, even when dipolar interactions are largely absent. Notably, the dilute species U(Bc(Me))3 exhibits Ueff ~33 cm(-1), representing the highest barrier yet observed for a U(3+) molecule demonstrating slow relaxation. Additional analysis through lanthanide XANES, X-band EPR, and (1)H NMR spectroscopies provides evidence that the origin of the slower relaxation derives from the greater magnetic anisotropy enforced within the strongly donating N-heterocyclic carbene coordination sphere. These results show that, like molecular symmetry, ligand donating ability is a variable that can be controlled to the advantage of the synthetic chemist in the design of single-molecule magnets with enhanced relaxation barriers.
    Journal of the American Chemical Society 03/2014; · 11.44 Impact Factor
  • Zoey R. Herm, Eric D. Bloch, Jeffrey R. Long
    ChemInform 03/2014; 45(10).
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    ABSTRACT: The synthesis and full magnetic characterization of a new series of N2(3-) radical-bridged lanthanide complexes [{(R2N)2(THF)Ln}2(μ3-η(2):η(2):η(2)-N2)K] [1-Ln; Ln = Gd, Tb, Dy; NR2 = N(SiMe3)2] are described for comprehensive comparison with the previously reported series [K(18-crown-6)(THF)2]{[(R2N)2(THF)Ln]2(μ-η(2):η(2)-N2)} (2-Ln; Ln = Gd, Tb, Dy). Structural characterization of 1-Ln crystals grown with the aid of a Nd2Fe13B magnet reveals inner-sphere coordination of the K(+) counterion within 2.9 Å of the N2(3-) bridge, leading to bending of the planar Ln-(N2(3-))-Ln unit present in 2-Ln. Direct current (dc) magnetic susceptibility measurements performed on 1-Gd reveal antiferromagnetic coupling between the Gd(III) centers and the N2(3-) radical bridge, with a strength matching that obtained previously for 2-Gd at J ∼ -27 cm(-1). Unexpectedly, however, a competing antiferromagnetic Gd(III)-Gd(III) exchange interaction with J ∼ -2 cm(-1) also becomes prominent, dramatically changing the magnetic behavior at low temperatures. Alternating current (ac) magnetic susceptibility characterization of 1-Tb and 1-Dy demonstrates these complexes to be single-molecule magnets under zero applied dc field, albeit with relaxation barriers (Ueff = 41.13(4) and 14.95(8) cm(-1), respectively) and blocking temperatures significantly reduced compared to 2-Tb and 2-Dy. These differences are also likely to be a result of the competing antiferromagnetic Ln(III)-Ln(III) exchange interactions of the type quantified in 1-Gd.
    Inorganic Chemistry 02/2014; · 4.79 Impact Factor
  • Energy & Environmental Science 02/2014; · 15.49 Impact Factor
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    ABSTRACT: Two Ni2+-containing metal–organic frameworks, Ni2(dobdc) and Ni2(dobpdc), are shown to be active for the oligomerization of propene in the gas phase. The metal–organic frameworks exhibit activity comparable to Ni2+-exchanged aluminosilicates but maintain high selectivity for linear oligomers. Thus, these frameworks should enable the high yielding synthesis of linear propene oligomers for use in detergent and diesel fuel applications.
    ACS Catalysis 01/2014; 4(3):717–721. · 7.57 Impact Factor

Publication Stats

10k Citations
1,980.56 Total Impact Points

Institutions

  • 2008–2014
    • Lawrence Berkeley National Laboratory
      • • Geochemistry Department
      • • Physics Division
      Berkeley, California, United States
  • 1997–2014
    • University of California, Berkeley
      • Department of Chemistry
      Berkeley, California, United States
  • 2013
    • University of Minnesota Duluth
      • Department of Chemistry and Biochemistry
      Duluth, MN, United States
  • 2012
    • Texas A&M University
      • Department of Chemistry
      College Station, Texas, United States
    • University of Washington Seattle
      • Department of Chemistry
      Seattle, WA, United States
  • 2009–2011
    • CSU Mentor
      Long Beach, California, United States
  • 2010
    • University of Sydney
      • School of Chemistry
      Sydney, New South Wales, Australia
    • Howard Hughes Medical Institute
      Ashburn, Virginia, United States
    • Centre de Recherche Paul Pascal
      Pessac, Aquitaine, France
    • New Mexico State University
      • Department of Chemistry and Biochemistry
      Las Cruces, NM, United States
  • 2007
    • Sun Yat-Sen University
      • Department of Chemical Engineering
      Guangzhou, Guangdong Sheng, China
  • 2005
    • Johns Hopkins University
      Baltimore, Maryland, United States
  • 2004
    • Nanjing University
      • State Key Laboratory of Coordination Chemistry
      Nan-ching, Jiangsu Sheng, China