Browsing by Author "Winpenny, REP"

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    Inelastic neutron scattering and EPR studies of cobalt dimers
    (Australian Institute of Physics, 2011-02-03) Mole, RA; Boeer, AB; Simeoni, GG; Collison, D; McInnes, EJL; Timco, GA; Winpenny, REP
    The phenomenon of single molecule magnetism has been known for over fifteen years and the fundamentals of this behaviour are well understood; the observed hysteresis is due to an energy barrier for spin reversal, the magnitude of this energy barrier is given by the magnitude and anisotropy of the molecular spin. Despite intensive research it has not been possible to improve the performance of the best single molecule magnets to obtain operating temperatures above a few Kelvin. Recent work has suggested that the anisotropy parameter might play a more important role than previously thought. As such clusters consisting of octahedral Co(II) are promising candidates due to the large anisotropy associated with the spin orbit coupling of the 4T1g ground term. There are however no clear cut cobalt single molecule magnets and the magnetic properties of coordination clusters consisting of octahedral cobalt (II) ions are often complex. Here I present an inelastic neutron scattering study of two cobalt dimers Co2(D2O)(L)4(HL)2(C5D5N)2 and Co2(D2O)L4(HL)4 where L = (CD3)3Co2H [1]. These dimers share a common core; though display different bulk magnetic properties. The dimer was chosen as it is the simplest possible exchange coupled unit that can be studied. The use of inelastic scattering for this problem is incredibly powerful, both due to the zero field nature of the technique and the flexible selection rules [2]. The INS data is presented together with the results of multiple frequency EPR and bulk magnetic properties. The complimentary nature of these techniques allows both the energy scale and the ground state to be determined.
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    Metal distribution and disorder in the crystal structure of [NH2Et2][Cr7MF8(tBuCO2)16] wheel molecules for M = Mn, Fe, Co, Ni, Cu, Zn and Cd
    (International Union of Crystallography, 2014-12) Larsen, FK; Overgaard, J; Christensen, M; McIntyre, GJ; Timco, GA; Winpenny, REP
    The homometallic wheel compound [Cr8F8(O2CCMe3)16] formed with fluorine and pivalic acid ligands can be modified by introducing in the synthesis process a divalent cation M capable of octahedral coordination instead of one of the trivalent Cr centres in the ring. Heterometallic mono-anionic species [Cr7MF8(O2CCMe3)16]− can form diethylammonium salts and be crystallized from ethylacetate solution as compounds with the general formula [NH2Et2][Cr7MF8(tBuCO2)16][C4H8O2]0.5 for M = Mn, Fe, Co, Ni, Cu, Zn and Cd. Their structures are isomorphous, belonging to the space group P21/c. The study has determined the degree of order for the individual M heterometal over the possible metal positions of the ring in the crystal structure by modelling based on X-ray diffraction data. The model took into account disorder in tert-butyl groups of the pivalate ligands and in the position and orientation of the ethylacetate solvent molecule. The heterometal turned out to be partly ordered in the crystal structure. © 2014, International Union of Crystallography.
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    Relationships between electron density and magnetic properties in water-bridged dimetal complexes
    (ACS Publications, 2014) Overgaard, J; Walsh, JPS; Hathwar, VR; Jørgensen, MRV; Hoffman, C; Platts, JA; Piltz, RO; Winpenny, REP
    The experimental and theoretical electron density distributions in two structurally similar transition metal dimers (M = Ni, Co; see Figure) were analyzed using the atoms-in-molecules (AIM) approach, and selected properties related to the chemical bonding are compared to measured intramolecular magnetic exchange interaction parameters.The electron densities in two analogous dimetallic transition metal compounds, namely, [M2(μ-OH2)(tBuCOO)4(tBuCOOH)2(C5H5N)2] (M = Co(1), Ni(2)), were determined from combined X-ray and neutron single-crystal diffraction at 100 K. Excellent correspondence between the thermal parameters from X- and N-derived atomic displacement parameters is found, indicating high-quality X-ray data and a successful separation of thermal and electronic effects. Topological analysis of electron densities derived from high-resolution X-ray diffraction, as well as density functional theory calculations, shows no direct metal–metal bonding in either compound, while the total energy density at the bond critical points suggests stronger metal–oxygen interactions for the Ni system, in correspondence with its shorter bond distances. The analysis also allows for estimation of the relative strength of binding of terminal and bridging ligands to the metals, showing that the bridging water molecule is more strongly bound than terminal carboxylic acid, but less so than bridging carboxylates. Recently, modeling of magnetic and spectroscopic data in both of these systems has shown weak ferromagnetic interactions between the metal atoms. Factors related to large zero-field splitting effects complicate the magnetic analysis in both compounds, albeit to a much greater degree in 1. The current results support the conclusion drawn from previous magnetic and spectroscopic measurements that there is no appreciable direct communication between metal centers. © 2014, American Chemical Society.
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    A spectroscopic investigation of magnetic exchange between highly anisotropic spin centers
    (Wiley-Blackwell, 2011-04-18) Boeer, AB; Barra, AL; Chibotaru, LF; Collison, D; McInnes, EJL; Mole, RA; Simeoni, GG; Timco, GA; Ungur, L; Unruh, T; Winpenny, REP
    Structurally similar, magnetically diverse: The exchange and Zeeman parameters of two dimetallic, six-coordinate cobalt(II) compounds—i.e., with orbitally degenerate metal ions—have been determined by a combination of high-resolution EPR and INS spectroscopies, together with ab intio calculations. It is suggested that the correlation between the local crystal field about the CoII centers and the superexchange pathway controls the magnetic interaction. © 2011, Wiley-Blackwell.
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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.