Browsing by Author "Bircher, R"

Now showing 1 - 7 of 7
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    Controlled synthesis of isomorphous coordination polymers via in situ ligand transformation reaction: crystal structure, thermal and magnetic properties
    (American Chemical Society, 2010-09) Nadeem, MA; Bhadbhade, MM; Bircher, R; Stride, JA
    A series of isomorphous [M2(ip)2(L)2]n (M = Co, Ni; ip = isophthalate; L = 4,4′-dipyridyl and pyrazine) coordination polymers was synthesized via in situ ligand transformation reactions, in which 1,2,3-benzene tricarboxylic acid (1,2,3-btcH3) undergoes decarboxylation to isophthalate (ip), which was found to be highly dependent on the reaction temperature. The polymers crystallize in the same crystal triclinic system P. The magnetic properties were investigated by a Quantum Design physical property measurement system (PPMS); all four compounds feature almost isostructural M(II) dimers having ferromagnetic (FM) interactions. The interdimer interactions mediated by two different carboxylate groups of ip are weak, while there is no significant magnetic exchange via the 4,4′-dipyridyl ligands along the chain direction. However, substitution with pyrazine leads to antiferromagnetic exchange of nearest neighbor dimers. © 2010, American Chemical Society
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    Magneto-structural correlations of a three-dimensional Mn based metal-organic framework
    (Royal Society of Chemistry, 2010-05-14) Nadeem, MA; Craig, DJ; Bircher, R; Stride, JA
    A 3D metal–organic framework, [Na{Mn3(Hbtc)2(btc)}·5H2O]n ( 1) (H3btc = 1,3,5-benzene tricarboxylic acid), was synthesized under hydrothermal conditions. The structure of 1 was established by single crystal X-ray diffraction analysis; 1 crystallizes in the monoclinic space group P2/c, a = 9.753(3) Å, b = 12.751(2) Å, c = 14.174(4) Å, = 109.41(1)°. The complex 1 is isostructural to previously reported MIL-45 and consists of one dimensional wave like chains of carboxylate bridged hexa-coordinated Mn(II) ions. Variable temperature magnetic susceptibility measurements revealed dominant antiferromagnetic exchange interactions and the intra-chain exchange constants J1 = -2.4 cm-1 and J2 = -0.6 cm-1 compare well with literature values for similar materials. Inter-chain interactions are expected to be very small in this compound and there is no indication of magnetic ordering phenomena in the temperature range from 300–2 K. © 2010, Royal Society of Chemistry
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    [Mn-6] under pressure: a combined crystallographic and magnetic study
    (Wiley-VCH Verlag Berlin, 2008-03) Prescimone, A; Milios, CJ; Moggach, S; Warren, JE; Lennie, AR; Sanchez-Benitez, J; Kamenev, KV; Bircher, R; Murrie, M; Parsons, S; Brechin, EK
    Folding under pressure: High-pressure crystallography of an Mn6 single-molecule magnet reveals dramatic changes in the intramolecular geometry of the magnetic core. These structural changes effect the magnetic properties of the molecule: the magnitude of the ferromagnetic exchange between the metals is decreased, and under extreme pressure switches to antiferromagnetic. © 2008, Wiley-VCH Verlag Berlin
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    New family of ferric spin clusters incorporating redox-active ortho-dioxolene ligands.
    (American Chemical Society, 2009-08-17) Mulyana, Y; Nafady, A; Mukherjee, A; Bircher, R; Moubaraki, B; Murray, KS; Bond, AM; Abrahams, BF; Boskovic, C
    Seven new di-, tri-, tetra-, and hexanuclear iron complexes that incorporate a polydentate Schiff base and variously substituted catecholate ligands have been synthesized from the trinuclear precursor [Fe3(OAc)3(L)3] (1), where LH2 = 2-[[(2-hydroxyethyl)imino]phenylmethyl]-phenol. These were isolated as the compounds [Fe3(OAc)(Cat)(L)3] (2), [Fe6(OAc)2(Cat)4(L)4] (3), [Fe4(3,5-DBCat)2(L)4] (4), [Bu4N][Fe4(OAc)(3,5-DBCat)4(L)2] (5a, 5- is the complex monoanion [Fe4(OAc)(3,5-DBCat)4(L)2]-), [Fe4(OAc)(3,5-DBCat)3(3,5-DBSQ)(L)2] (6), [Fe2(Cl4Cat)2(L)(LH2)(H2O)] (7), and [Et3NH]2[Fe2(Cl4Cat)2(L)2] (8a, 8²- is the complex dianion [Fe2(Cl4Cat)2(L)2]2-), where CatH2 = catechol; 3,5- DBCatH2 = 3,5-di-tert-butyl-catechol; 3,5-DBSQH = 3,5-di-tert-butyl-semiquinone, and Cl4CatH2 = tetrachlorocatechol. While compounds 2-4, 5a, 7, and 8a were obtained by directly treating 1 with the appropriate catechol, compound 6 was synthesized by chemical oxidation of 5a. These compounds have been characterized by single crystal X-ray diffraction, infrared and UV-visible spectroscopy, voltammetry, UV-visible spectroelectrochemistry, andmagnetic susceptibility and magnetization measurements. An electrochemical study of the three tetranuclear complexes (4, 5-, and 6) reveals multiple reversible redox processes due to the o-dioxolene ligands, in addition to reductive processes corresponding to the reduction of the iron(III) centers to iron(II). A voltammetric study of the progress of the chemical oxidation of compound 5a, together with a spectroelectrochemical study of the analogous electrochemical oxidation, indicates that there are two isomeric forms of the one-electron oxidized product. A relatively short-lived neutral species (5) that possesses the same ligand arrangement as complex 5- is the kinetic product of both chemical and electrochemical oxidation. After several hours, this species undergoes a significant structural rearrangement to convert to complex 6, which appears to be largely driven by the preference for the 3,5-DBSQ- ligand to bind in a non-bridging mode. Variable temperature magnetic susceptibilitymeasurements for compounds 3, 4, 5a, 6, 7, and8a reveal behavior dominated by pairwise antiferromagnetic exchange interactions, giving rise to a poorly isolated S = 0 ground state spin for compound 3, well-isolated S = 0 ground state spins for complexes 4, 5-, 7 and 8²-, and a well-isolated S = 1/2 ground state spin for complex 6. The ground state spin values were confirmed by low temperature variable field magnetization measurements. The thermal variation of the magnetic susceptibility for compounds 3, 4, 5a, 6, 7, and 8a were fitted and/or simulated using the appropriate Hamiltonians to derive J values that are consistent with magnetostructural correlations that have been reported previously for alkoxobridged ferric complexes. © 2009, American Chemical Society
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    Studies of finite molecular chains: synthesis, structural, magnetic and inelastic neutron scattering studies of hexa- and heptanuclear chromium horseshoes
    (Wiley, 2008-02-05) Ochsenbein, ST; Tuna, F; Rancan, M; Davies, RSG; Muryn, CA; Waldmann, O; Bircher, R; Sieber, A; Carver, G; Mutka, H; Fernandez-Alonso, F; Podlesnyak, A; Engelhardt, LP; Timco, GA; Gudel, HU; Winpenny, REP
    We report the synthesis and structural characterisation of a family of finite molecular chains, specifically [{[R2NH2](3) [Cr6F11 (O2CCMe3)(10)]}(2)] (in which R=nPr 1, Et 2, nBu 3), [{Et2NH}(2) {[Et2NH2](3)[Cr7F12(O2C-CMe3)(12)][HO2CCMe3](2)}(2)] (4), [{[Me2NH2](3)[Cr6F11(O2CCMe3)(10)]center dot 2.5 H2O}(4)] (5) and [I{iPr(2)NH(2)](3)[Cr7F12(O2CCMe3)(12)]}(2)] (6). The structures all contain horseshoes of chromium centres, with each Cr center dot center dot center dot Cr contact within the horseshoe bridged by a fluoride and two pivalates. The horseshoes are linked through hydrogen bonds to the secondary ammonium cations in the structure, leading to di- and tetrahorseshoe structures. Through magnetic measurements and inelastic neutron scattering studies we have determined the exchange coupling constants in 1 and 6. In 1 it is possible to distinguish two exchange interactions, J(A) = -1.1 meV and J(B) = -1.4 meV; J(A) is the exchange interactions at the tips of the horseshoe and JB is the exchange within the body of the horseshoe (1 meV = 8.066 cm(-1)). For 6 only one interaction was needed to model the data: J=-1.18meV The single-ion anisotropy parameters for Cr-III were also derived for the two compounds as: for 1, D-Cr = -0.028 meV and vertical bar ECr vertical bar = 0.005meV; for 6, D-Cr=-0.031meV. Magnetic-field-dependent inelastic neutron scattering experiments on I allowed the Zeeman splitting of the first two excited states and level crossings to be observed. For the tetramer of horseshoes (5), quantum Monte Carlo calculations were used to fit the magnetic susceptibility behaviour, giving two exchange interactions within the horseshoe (-1.32 and -1.65 meV) and a weak inter-horseshoe coupling of +0.12meV. Multi-frequency variable-temperature EPR studies on 1, 2 and 6 have also been performed, allowing further characterisation of the spin Hamiltonian parameters of these chains. © 1999-2020 John Wiley & Sons, Inc.
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    Synthesis, structure and magnetic properties of a novel Tb4 spin cluster and synthesis of a Tb chain
    (Elsevier, 2007-08-06) Bircher, R; Abrahams, BF; Gudel, HU; Boskovic, C
    The syntheses of two new polynuclear TbIII compounds are reported. A tetranuclear complex [Tb4(H2L)2(H4L)2(OAc)8] (1), with H4L=2-((2-hydroxy-benzylidene)-amino)-2-hydroxy-methyl-propane-1,3-diol, has a butterfly-type structure, while [Tb(OAc)3MeOH]∞ (2) is a linear chain. A novel binding mode for H2L2− is observed in 1, with two alkoxides each bridging three TbIII ions. The magnetic properties of 1 are determined by the superposition of the ligand field split 7F6 states of TbIII single-ions located on two crystallographically distinct sites. Weak antiferromagnetic exchange interactions possibly contribute to the lowering of the magnetic moment at the lowest temperatures. © 2007, Elsevier Ltd.
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    Three isolated structural motifs in one crystal: penetration of two 1D chains through large cavities within 2D polymeric sheets
    (Royal Society of Chemistry, 2010-05) Nadeem, MA; Bhadbhade, MM; Bircher, R; Stride, JA
    An unprecedented architecture comprising of three isolated coordination polymers within the same crystalline structure has been obtained, in which two independent infinite one dimensional polymer chains, Cu(bpy)(H2O), fill the voids generated by a third, two dimensional, host framework, Cu3(Hbtc)(btc)(bpy)2. © 2010, Royal Society of Chemistry