Studies of finite molecular chains: Synthesis, structural, magnetic and inelastic neutron scattering studies of hexa- and heptanuclear chromium horseshoes

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.