Stefan T. Ochsenbein, Mark Murrie, Eduard Rusanov, Helen Stoeckli-Evans, Chihiro Sekine and H. U. Güdel
Synthesis, Structure, and Magnetic Properties of the Single-Molecule Magnet [Ni₂₁(cit)₁₂(OH)₁₀(H₂O)₁₀]^16-
Inorg. Chem. 41, 5133-5140 (2002)                       

Abstract: The preparation of two new compounds containing the cluster [Ni₂₁(cit)₁₂(OH)₁₀(H₂O)₁₀]^16– is presented, together with a detailed magnetic investigation of one of the compounds. We found that this cluster shows an unexpected stability and that it exists as different stereoisomers. Compound 1 contains the achiral cluster with a D-L configuration, and compound 2 contains a pair of enantiomeric clusters with the configurations D-D and L-L, respectively. Magnetic measurements of 1 in the millikelvin range were necessary to determine the spin ground state of S = 3, and they also revealed a magnetic anisotropy within the ground state. A frequency-dependent out-of-phase signal was found in alternating current susceptibility measurements at very low temperatures, which indicates a slow relaxation of the magnetization. Thus, individual molecules are acting as single magnetic units, which is a rare phenomenon for nickel clusters. The energy barrier exhibited by compound 1 has been calculated to be 2.9 K.