Klaus Gieb

Indian Institute of Technology Kanpur, Cawnpore, Uttar Pradesh, India

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Publications (11)52.93 Total impact

  • [show abstract] [hide abstract]
    ABSTRACT: The reaction between Ln(III) chloride and NiCl2·4H2O salts in presence of a multidentate sterically unencumbered ligand, (E)-2,2'-(2-hydroxy-3-((2-hydroxyphenylimino)methyl)-5-methylbenzylazanediyl)diethanol (LH4) leads to the synthesis of four isostructural pentanuclear hetereometallic complexes [Ni2Dy3(LH)4]Cl (1), [Ni2Gd3(LH)4]Cl (2), [Ni2Tb3(LH)3(LH2)]Cl2 (3), [Ni2 Ho3 (LH)3 (LH2)]Cl2 (4) with unprecedented topology. Here the two compounds 1 are 2 are monocationic and crystallize in chiral space group, P212121 whereas compounds 3 and 4 are dicationic and crystallize in achiral space group P21/n. The total metal framework, {Ni2Ln3} unit is held by four triply deprotonated ligands [LH](3-) in 1 and 2 whereas in case of 3 and 4 three triply deprotonated [LH](3-) and one doubly deprotonated [LH2](2-) ligands are involved. In these complexes both the lanthanide ions and the nickel(II) ions are doubly bridged and the bridging is composed of oxygen atoms derived from either phenolate or ethoxide groups. The analysis of SQUID measurements reveal a high magnetic ground state and a slow relaxation of the magnetization with two relaxation regimes for 1. For the thermally activated regime we found an effective energy barrier of Ueff = 85 K. Micro Hall probe loop measurements directly proof the single-molecule magnet (SMM) nature of 1 with a blocking temperature of TB = 3 K and an open hysteresis for sweep rates faster than 50 mT/s.
    Inorganic Chemistry 11/2013; 52(22):13078-86. · 4.59 Impact Factor
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    ABSTRACT: Reaction of nickel(II) chloride hexahydrate with N-n-butyldiethanolamine H(2)L (3) in the presence of LiH in anhydrous THF leads to the formation of the unique octanuclear chloro-bridged nickel(II) double cubane [({Ni(II)(4)(μ(3)-OH)Cl(3)(HL)(3)}μ(2)-Cl)(2)] (4) in 57% yield. According to single crystal X-ray structure analysis, complex 4·4CH(2)Cl(2) possesses a [({Ni(4)(μ(3)-OH)(μ(3)-O)(3)(OH)(3)(N)(3)(Cl)(3)}μ(2)-Cl)(2)] core and crystallizes in the monoclinic space group P2(1)/c with a = 18.292(2), b = 19.8972(5), c = 23.295(2) Å, β = 98.408(6)°, V = 8387.3(8) Å(3), and four molecules in the unit cell. The analysis of the SQUID magnetic susceptibility data identified 4 as a weakly coupled dimer (J(1) = 14.5 K, J(2) = -0.6 K) with a ground state of S = 0, resulting from two S = 4 states of each {Ni(4)} subunits. Although complex 4 does not show an ac out-of-phase signal in a zero dc field at temperatures of 1.8 K and higher, low-temperature magnetization measurements revealed that complex 4 is a single-molecule magnet and shows hysteretic magnetization characteristics with typical temperature and sweep-rate dependencies. The eye-catching feature of complex 4 is the presence of two different blocking temperatures (0.9 K around zero field and 1.3 K at higher fields). The origin of this highly unusual behavior can be assigned to the dimer-nature of the interaction between the two S = 4 units. Furthermore STM and current imaging tunnelling spectroscopy (CITS) were performed on aggregates of 4 drop-coated on highly oriented pyrolytic graphite (HOPG) surfaces. CITS measurements show a strong contrast in the area of the nickel centers and a HOMO-LUMO gap of approximately 0.8 V.
    Dalton Transactions 03/2012; 41(12):3553-61. · 3.81 Impact Factor
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    ABSTRACT: We have synthesized the triplesalen-based single-molecule magnet (SMM) [Mn-III Mn-6(III)](3+) as a variation of our SMM [Mn-III Cr-6(III)](BPh4)(3). The use of the rod-shaped anion lactate (lac) was intended to enforce a rod packing and resulted in the crystallization of [ MnIII 6MnIII](lac) 3 in the highly symmetric space group R3. This entails a crystallographic S-6 symmetry of the [Mn-III Mn-6(III)](3+) molecules, which in addition are all aligned with the crystallographic c axis. Moreover, the molecular environment of each [Mn-III Mn-6(III)](3+) molecule is highly symmetric. Single-crystals of [Mn-III Mn-6(III)](lac) (3) exhibit a double hysteresis at 0.3 K with a hysteretic opening not only for the spin ground state up to 1.8 T, but also for an excited state becoming the ground state at approximate to 3.4 T with a hysteretic opening up to 10 T. Ab initio calculations including spin-orbit coupling establish a nonmagnetic behavior of the central MnIII low-spin (l.s.) ion at low temperatures, demonstrating that predictions from ligand-field theory are corroborated in the case of MnIII l.s. by ab intio calculations. Simulations of the field-and temperature-dependent magnetization data indicate that [Mn-III Mn-6(III)](3+) is in the limit of weak exchange (J << D) with antiferromagnetic interactions in the trinuclear Mn-3(III) triplesalen subunits resulting in intermediate S* = 2 spins. Slight ferromagnetic interactions between the two trinuclear Mn-3(III) subunits lead to a ground state in zero-field that is approximately described by a total spin quantum number S = 4. This ground state exhibits only a very small anisotropy barrier due to the misalignment of the local zero-field splitting tensors. At higher magnetic fields of approximate to 3.4 T, the spin configuration changes to an all-up orientation of the local MnIII spins, with the main part of the Zeeman energy needed for the spin-flip being required to overcome the local MnIII anisotropy barriers, while only minor contributions of the Zeeman energy are needed to overcome the antiferromagnetic interactions. These combined theoretical analyses provide a clear picture of the double-hysteretic behavior of the [Mn-III Mn-6(III)](3+) single-molecule magnet with hysteretic openings up to 10 T.
    Chemical Science 01/2012; 3(9):2868-2882. · 8.31 Impact Factor
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    ABSTRACT: We report on high field and low temperature magnetization measurements of a novel MnMn6(CH3H5O3)3 complex. A home-made micro-Hall-probe magnetometer was used to perform the characterization at mK temperatures and fields up to 17,. Most 3d-ion based single-molecule magnets, known up to now, have a spin ground state well separated from the first excited state, leading to the formation of giant spin at low temperatures. In contrast to this situation, the ground state (S=6) of the present complex can already be exited at moderate magnetic fields. Surprisingly, magnetic hysteresis was observed for both the ground state and the first exited state leading to a double hysteresis in the low temperature magnetization measurements. The blocking temperature was found to be TB 1.3,. Origin and possible consequences of this unusual behavior will be discussed.
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    ABSTRACT: First members of a new family of heterometallic Mn/Ni complexes [Mn(2)Ni(3)X(2)L(4)(LH)(2)(H(2)O)(2)] (X = Cl: 1; X = Br: 2) with the new ligand 2-{3-(2-hydroxyphenyl)-1H-pyrazol-1-yl}ethanol (H(2)L) have been synthesized, and single crystals obtained from CH(2)Cl(2) solutions have been characterized crystallographically. The molecular structures feature a quasi-linear Mn(III)-Ni(II)-Ni(II)-Ni(II)-Mn(III) core with six-coordinate metal ions, where elongated axes of all the distorted octahedral coordination polyhedra are aligned parallel and are fixed with respect to each other by intramolecular hydrogen bonds. 1 and 2 exhibit quite strong ferromagnetic exchange interactions throughout (J(Mn-Ni) ≈ 40 K (1) or 42 K (2); J(Ni-Ni) ≈ 22 K (1) or 18 K (2)) that lead to an S(tot) = 7 ground state, and a sizable uniaxial magnetoanisotropy with D(mol) values -0.55 K (1) and -0.45 K (2). These values are directly derived also from frequency- and temperature-dependent high-field EPR spectra. Slow relaxation of the magnetization at low temperatures and single-molecule magnet (SMM) behavior are evident from frequency-dependent peaks in the out-of-phase ac susceptibilities and magnetization versus dc field measurements, with significant energy barriers to spin reversal U(eff) = 27 K (1) and 22 K (2). Pronounced quantum tunnelling steps are observed in the hysteresis loops of the temperature- and scan rate-dependent magnetization data, but with the first relaxation step shifted above (1) or below (2) the zero crossing of the magnetic field, despite the very similar molecular structures. The different behavior of 1 and 2 is interpreted in terms of antiferromagnetic (1) or ferromagnetic (2) intermolecular interactions, which are discussed in view of the subtle differences of intermolecular contacts within the crystal lattice.
    Journal of the American Chemical Society 02/2011; 133(10):3433-43. · 10.68 Impact Factor
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    ABSTRACT: Reaction of cobalt(II) chloride hexahydrate with N-substituted diethanolamines H(2)L(2-4) (3) in the presence of LiH in anhydrous THF leads under anaerobic conditions to the formation of three isostructural tetranuclear cobalt(II) complexes [Co(II) (4)(Cl)(4)(HL(2-4))(4)] (4) with a [Co(4)(mu(3)-O)(4)](4+) cubane core. According to X-ray structural analyses, the complexes 4 a,c crystallize in the tetragonal space group I4(1)/a, whereas for complex 4 b the tetragonal space group P$\bar 4$ was found. In the solid state the orientation of the cubane cores and the formation of a 3D framework were controlled by the ligand substituents of the cobalt(II) cubanes 4. This also allowed detailed magnetic investigations on single crystals. The analysis of the SQUID magnetic susceptibility data for 4 a gave intramolecular ferromagnetic couplings of the cobalt(II) ions (J(1) approximately 20.4 K, J(2) approximately 7.6 K), resulting in an S=6 ground-state multiplet. The anisotropy was found to be of the easy-axis type (D=-1.55 K) with a resulting anisotropy barrier of Delta approximately 55.8 K. Two-dimensional electron-gas (2DEG) Hall magnetization measurements revealed that complex 4 a is a single-molecule magnet and shows hysteretic magnetization characteristics with typical temperature and sweep-rate dependencies below a blocking temperature of about 4.4 K. The hysteresis loops collapse at zero field owing to fast quantum tunneling of the magnetization (QTM). The structural and electronic properties of cobalt(II) cubane 4 a, deposited on a highly oriented pyrolytic graphite (HOPG) surface, were investigated by means of STM and current imaging tunneling spectroscopy (CITS) at RT and standard atmospheric pressure. In CITS measurements the rather large contrast found at the expected locations of the metal centers of the molecules indicated the presence of a strongly localized LUMO.
    Chemistry 03/2010; 16(16):4784-92. · 5.93 Impact Factor
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    ABSTRACT: We describe a novel method for analyzing the spin states of surface deposited Fe(II) spin crossover (SCO) compounds. The talk focuses on the investigation of [FeII(L)2](BF4)2 (L=2,6-di(1H-pyrazol-1-yl)-4-(thiocyanatomethyl)pyridine)^1 and the comparison to a high spin compound with a similar coordination motif. Single molecules and small clusters were investigated on HOPG. We were able to show a strong current contrast for the different spin states using the CITS technique. Changes of the spin state from high- to low-spin state and vice versa were observed at room temperature. Switching was statistically distributed, indicating a widening of the spin transition compared to the bulk state.^2 ^1 M. Haryono, et al., Eur. J. Inorg. Chem. 2009, 2136. ^2 M.S. Alam, et al., Angew. Chem. (2009) (accepted).
  • Angewandte Chemie International Edition 02/2010; 49(6):1159-63. · 13.73 Impact Factor
  • Angewandte Chemie 12/2009; 122(6):1178 - 1182.
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    Berichte der deutschen chemischen Gesellschaft 11/2009; 2010(2):221 - 232. · 2.94 Impact Factor
  • Berichte der deutschen chemischen Gesellschaft 03/2009; 2009(14):2136 - 2143. · 2.94 Impact Factor