David M. Low, Gopalan Rajaraman, Madeleine Helliwell, Grigore Timco, Joris van Slageren, Roberta Sessoli, Stefan T. Ochsenbein, Roland Bircher, Christopher Dobe, Oliver Waldmann, Hans-Ulrich Güdel, Mark A. Adams, Eliseo Ruiz, Santiago Alvarez and Eric J. L. McInnes
A Family of Ferro- and Antiferromagnetically Coupled Decametallic Chromium(III) Wheels
Chem. Eur. J. 12, 1385-1396 (2006)                 

Abstract: The synthesis and crystal structures of a family of decametallic Cr^III "molecular wheels" are reported, namely [Cr₁₀(OR)₂₀(O₂CR’)₁₀] [R’=Me, R=Me (1), Et (2); R’=Et, R=Me (3), Et (4); R’=CMe₃, R=Me (5), Et (6)]. Magnetic studies on 1-6 reveal a remarkable dependence of the magnetic behaviour on the nature of R. In each pair of complexes with a common carboxylate (R’) the nearest neighbour Cr···Cr magnetic exchange coupling is more antiferromagnetic for the ethoxide-bridged (R=Et) cluster than for the methoxide analogue. In complexes 2, 4 and 6 the overall coupling is weakly antiferromagnetic resulting in diamagnetic (S=0) ground states for the cluster, whilst in 1 and 5 it is weakly ferromagnetic thus resulting in very high-spin ground states. This ground state has been probed directly in the perdeuterated version of 1 ([D]1) by inelastic neutron scattering experiments, and these support the S=15 ground state expected for ferromagnetic coupling of ten Cr^III ions, and they also indicate that a single J-value model is inadequate. The ground state of 5 is large but not well defined. The trends in J on changing R are further supported by density functional calculations on 1-6, which are in excellent agreement with experiment. The very large changes in the nature of the ground state between 1 and 2, and 5 and 6 are the result of relatively small changes in J that happen to cross J=0, hence changing the sign of J.