Kooperativer Bibliotheksverbund

Berlin Brandenburg

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  • 1
    Language: English
    In: Journal of the American Chemical Society, 07 September 2011, Vol.133(35), pp.13852-5
    Description: Reaction of NO(2)(-) with the octahedral cluster ((H)L)(2)Fe(6) in the presence of a proton source affords the hexanitrosyl cluster ((H)L)(2)Fe(6)(NO)(6). This species forms via a proton-induced reduction of six nitrite molecules per cluster, utilizing each site available on the polynuclear core. Formation of the hexanitrosyl cluster is accompanied by a near 2-fold expansion of the ((H)L)(2)Fe(6) core volume, where intracore Fe-Fe interactions are overcome by strong π-bonding between Fe centers and NO ligands. A core volume of this magnitude is rare in octahedral metal clusters not supported by interstitial atoms. Moreover, the structural flexibility afforded by the ((H)L)(2)Fe(6) platform highlights the potential for other reaction chemistry involving species with metal-ligand multiple bonds. Carrying out the reaction of the cluster [((H)L)(2)Fe(6)(NCMe)(6)](4+) with nitrite in the absence of a proton source serves to forestall the nitrite reduction and enables clean isolation of the intermediate hexanitro cluster [((H)L)(2)Fe(6)(NO(2))(6)](2-).
    Keywords: Iron -- Chemistry ; Nitrites -- Chemistry ; Nitrogen Oxides -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 2
    Language: English
    In: Journal of the American Chemical Society, June 1, 2011, Vol.133(21), p.8293-8306
    Description: A trinucleating hexaamide ligand platform is used to characterize the all-ferrous hexanuclear cluster [([super H]L).sub.2][Fe.sub.6], obtained from reaction of 3 equiv of [Fe.sub.2][(Mes).sub.4] (Mes = 2,4,6-[Me.sub.3][C.sub.6][H.sub.2]) with 2 equiv of the ligand ([super H]L)[H.sub.6]. The observed metrical changes accompanying oxidation state changes to the [[Fe.sub.6]] core could be rationalized by considering a qualitative, delocalized molecular orbital description, which provides a set of frontier orbitals populated by Fe 3d electrons.
    Keywords: Oxidation-reduction Reactions -- Analysis ; Iron Oxides -- Chemical Properties ; Amides -- Chemical Properties
    ISSN: 0002-7863
    Source: Cengage Learning, Inc.
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  • 3
    Language: English
    In: Journal of the American Chemical Society, 2011, Vol.133(1)
    Description: Reaction of the high-magnetic anisotropy building unit [ReCl4(CN)2]2- with [Cu(MeCN)6]2+ and hydrotris(pyrazol-1-yl)borate (Tp-) affords the zigzag chain compound (Bu4N)[TpCuReCl4(CN)2]. Dc magnetic susceptibility measurements reveal the presence of ferromagnetic exchange coupling between...
    Keywords: Chemical Sciences ; Coordination Chemistry ; Cyan-Bridged Materials ; Magnetic Phase Diagram ; Chemistry
    ISSN: 0002-7863
    E-ISSN: 1520-5126
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  • 4
    Language: English
    In: Journal of the American Chemical Society, 29 June 2016, Vol.138(25), pp.7804-7
    Description: The ability of magnetic exchange coupling to enable observation of paramagnetic chemical exchange saturation transfer (PARACEST) in transition metal ions with long electronic relaxation times (τs) is demonstrated. Metalation of the dinucleating, tetra(carboxamide) ligand HL with Cu(2+) in the presence of pyrophosphate (P2O7)(4-) affords the complex [LCu(II)2(P2O7)](-). Solution-phase variable-temperature magnetic susceptibility data reveal weak ferromagnetic superexchange coupling between the two S = 1/2 Cu(II) centers, with a coupling constant of J = +2.69(5) cm(-1), to give an S = 1 ground state. This coupling results in a sharpened NMR line width relative to a GaCu analogue, indicative of a shortening of τs. Presaturation of the amide protons in the Cu2 complex at 37 °C leads to a 14% intensity decrease in the bulk water (1)H NMR signal through the CEST effect. Conversely, no CEST effect is observed in the GaCu complex. These results provide the first example of a Cu-based PARACEST magnetic resonance contrast agent and demonstrate the potential to expand the metal ion toolbox for PARACEST agents through introduction of magnetic exchange coupling.
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 5
    Language: English
    In: Journal of the American Chemical Society, June 29, 2016, Vol.138(25), pp.7804-7807
    Description: The ability of magnetic exchange coupling to enable observation of paramagnetic chemical exchange saturation transfer (PARACEST) in transition metal ions with long electronic relaxation times (?s) is demonstrated. Metalation of the dinucleating, tetra(carboxamide) ligand HL with Cu.sub.2+ in the presence of pyrophosphate [(P.sub.2)(O.sub.7).sup.4-] affords the complex [LCu.sup.II.sub.2(P.sub.2)(O.sub.7)].sup.-.The results provide the first example of a Cu-based PARACEST magnetic resonance contrast agent and demonstrate the potential to expand the metal ion toolbox for PARACEST agents through introduction of magnetic exchange coupling.
    Keywords: Copper Compounds – Chemical Properties ; Chemical Reactions – Analysis ; Pyrophosphates – Chemical Properties
    ISSN: 0002-7863
    Source: Cengage Learning, Inc.
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  • 6
    Language: English
    In: Journal of the American Chemical Society, 02 May 2012, Vol.134(17), pp.7521-9
    Description: The model compounds (NBu(4))(2)[ReCl(4)(CN)(2)] (1), (DMF)(4)ZnReCl(4)(CN)(2) (2), and [(PY5Me(2))(2)Mn(2)ReCl(4)(CN)(2)](PF(6))(2) (3) have been synthesized to probe the origin of the magnetic anisotropy barrier in the one-dimensional coordination solid (DMF)(4)MnReCl(4)(CN)(2) (4). High-field electron paramagnetic resonance spectroscopy reveals the presence of an easy-plane anisotropy (D 〉 0) with a significant transverse component, E, in compounds 1-3. These findings indicate that the onset of one-dimensional spin correlations within the chain compound 4 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior. Within this picture, it is the transverse E term associated with the Re(IV) centers that determines the easy axis and the anisotropy energy scale associated with the relaxation barrier. The results demonstrate for the first time that slow magnetic relaxation can be achieved through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy, thus providing a new direction in the design of single-molecule and single-chain magnets.
    Keywords: Chemistry;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 7
    Language: English
    In: Journal of the American Chemical Society, 23 March 2011, Vol.133(11), pp.3824-7
    Description: The four-coordinate iron(II) phosphoraniminato complex PhB(MesIm)(3)Fe-N═PPh(3) undergoes an S = 0 to S = 2 spin transition with T(C) = 81 K, as determined by variable-temperature magnetic measurements and Mössbauer spectroscopy. Variable-temperature single-crystal X-ray diffraction revealed that the S = 0 to S = 2 transition is associated with an increase in the Fe-C and Fe-N bond distances and a decrease in the N-P bond distance. These structural changes have been interpreted in terms of electronic structure theory.
    Keywords: Chemistry;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 8
    Language: English
    In: Journal of the American Chemical Society, 13 November 2013, Vol.135(45), pp.16845-8
    Description: One-electron reduction of the complex [(TPyA)2Fe(II)2((NPh)L(2-))](2+) (TPyA = tris(2-pyridylmethyl)amine, (NPh)LH2 = azophenine = N,N',N",N'''-tetraphenyl-2,5-diamino-1,4-diiminobenzoquinone) affords the complex [(TPyA)2Fe(II)2((NPh)L(3-•))](+). X-ray diffraction and Mössbauer spectroscopy confirm that the reduction occurs on (NPh)L(2-) to give an S = 1/2 radical bridging ligand. Dc magnetic susceptibility measurements demonstrate the presence of extremely strong direct antiferromagnetic exchange between S = 2 Fe(II) centers and (NPh)L(3-•) in the reduced complex, giving an S = 7/2 ground state with an estimated coupling constant magnitude of |J| ≥ 900 cm(-1). Mössbauer spectroscopy and ac magnetic susceptibility reveal that this complex behaves as a single-molecule magnet with a spin relaxation barrier of U(eff) = 50(1) cm(-1). To our knowledge, this complex exhibits by far the strongest magnetic exchange coupling ever to be observed in a single-molecule magnet.
    Keywords: Chemistry;
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 9
    Language: English
    In: Journal of the American Chemical Society, 26 November 2014, Vol.136(47), pp.16489-92
    Description: The porphyrinic metal-organic framework (MOF) PCN-224 is metalated with Fe(II) to yield a 4-coordinate ferrous heme-containing compound. The heme center binds O2 at -78 °C to give a 5-coordinate heme-O2 complex. For the first time, this elusive species is structurally characterized, revealing an Fe(III) center coordinated to superoxide via an end-on, η(1) linkage. Mössbauer spectroscopy supports the structural observations and indicates the presence of a low-spin electronic configuration for Fe(III). Finally, variable-temperature O2 adsorption data enable quantification of the Fe-O2 interaction, providing a binding enthalpy of -34(4) kJ/mol. This value is nearly half of that observed for comparable 6-coordinate, imidazole-bound heme-O2 complexes, a difference that further illustrates the importance of axial ligands in biological heme-mediated O2 transport and storage. These results demonstrate the ability of a MOF, by virtue of its rigid solid-state structure, to enable isolation and thorough characterization of a species that can only be observed transiently in molecular form.
    Keywords: Ferrous Compounds -- Chemistry ; Oxygen -- Chemistry
    ISSN: 00027863
    E-ISSN: 1520-5126
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  • 10
    Language: English
    In: Journal of the American Chemical Society, March 24, 2010, Vol.132(11), p.3980-3988
    Description: The reaction of ([Bu.sub.4]N)CN with Re[Cl.sub.4][(THF).sub.2] has helped in the synthesis of a high-anisotropy building unit, trans-[[Re[Cl.sub.4][(CN).sub.2]].sup.2-], which has represented the paramagnetic complex with a mixture of cyanide and halide ligands. The field-dependent magnetization of the iron congener has displayed a major hysteresis effect at low temperature, thus showing magnetlike behavior in this one-dimensional system.
    Keywords: Manganese Compounds -- Chemical Properties ; Manganese Compounds -- Electric Properties ; Manganese Compounds -- Magnetic Properties ; Chemical Reactions -- Analysis ; Cyanides -- Chemical Properties ; Cyanides -- Electric Properties ; Cyanides -- Magnetic Properties
    ISSN: 0002-7863
    Source: Cengage Learning, Inc.
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