Stefan T. Ochsenbein, Floriana Tuna, Marzio Rancan, Rachel S. G. Davies, Christopher A. Muryn, Oliver Waldmann, Roland Bircher, Andreas Sieber, Graham Carver, Hannu Mutka, Felix Fernandez-Alonso, Andrew Podlesnyak, Larry P. Engelhardt, Grigore A. Timco, Hans U. Güdel and Richard E. P. Winpenny
Studies of Finite Molecular Chains: Synthesis, Structural, Magnetic and Inelastic Neutron Scattering Studies of Hexa- and Heptanuclear Chromium Horseshoes
Chem. Eur. J. 14, 5144-5158 (2008)                                   

Abstract: 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(O₂CCMe₃)₁₂][HO₂CCMe₃]₂}₂] (4), [{[Me₂NH₂]₃[Cr₆F₁₁(O₂CCMe₃)₁₀]·2.5 H₂O}₄] (5) and [{[iPr₂NH₂]₃[Cr₇F₁₂(O₂CCMe₃)₁₂]}₂] (6). The structures all contain horseshoes of chromium centres, with each Cr···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 tetra-horseshoe 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 J_B 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=-1.18 meV. The single-ion anisotropy parameters for Cr^III were also derived for the two compounds as: for 1, D_Cr=-0.028 meV and |E_Cr|=0.005 meV; for 6, D_Cr=-0.031 meV. Magnetic-field-dependent inelastic neutron scattering experiments on 1 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 +0.12 meV. 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.

Cover Picture: Molecular horseshoe magnets pack in crystals to give beautiful supramolecular arrays. They can also be regarded as finite chains of paramagnetic ions, and, therefore, can also be used to test models for magnetic behavior, such as spin-wave theory, as H. U. Güdel, R. E. P. Winpenny et al. show on page 5144 ff., in their Full Paper.