Condensation of a Nickel Tetranuclear Cubane into a Heptanuclear Single-Molecule Magnet

A tetranuclear complex, [Ni4], with a cubane-like structure synthesized from
hexafluoroacetylacetone
gives, after drying at high temperature and treatment with pyridine,
a heptanuclear nickel­(II) complex, [Ni7]. The crystal structures of both compounds have
been determined by single-crystal X-ray diffraction. Their magnetic
properties have been studied by SQUID and μ-SQUID magnetometry
as well as by high-frequency EPR spectroscopy (HF-EPR). For [Ni4], the temperature
dependence of the magnetic susceptibility can be fitted by taking
into account strong Ni···Ni ferromagnetic interactions
which lead to an S = 4 ground-state spin, in good
agreement with the HF-EPR study. For [Ni7], the temperature dependence of the magnetic
susceptibility shows that the Ni···Ni ferromagnetic
interactions are kept within the metal core. However, it was not possible
to fit this with a clear set of parameters, and the ground-state spin
was undetermined. The field dependence of the magnetization indicates
an S = 7 ground-state spin at high field. In contrast,
the temperature dependence of the magnetic susceptibility indicates
a ground-state spin of S = 6 or even S = 5. These results agree with complicated high-frequency EPR spectra
which have been ascribed to the superposition of signals from the
ground spin multiplet and from an excited spin multiplet very close
in energy, with the excited state having a larger S value than the ground state. Very low temperature studies show that
only the heptanuclear complex behaves as a single-molecule magnet.