Very Large Magnetic Anisotropy of Cage Cobalt(II) Complexes with a Rigid Cholesteryl Substituent from Paramagnetic NMR Spectroscopy

Variable-temperature
NMR spectroscopy has recently emerged as a
new alternative to the magnetometry methods for studying single molecule
magnets. Its use is based on an accurate determination of magnetic
susceptibility tensor anisotropy Δχ, which is not always
achievable due to some contact contribution to NMR chemical shifts
and possible conformational dynamics. Here, we applied this approach
to cholesteryl-substituted cage cobalt­(II) complexes featuring a very
large magnetic anisotropy. Conformational rigidity and large size
of the cholesteryl substituent with many magnetically nonequivalent
nuclei resulted in an excellent convergence of experimental and calculated 1H and 13C chemical shifts, thus allowing for the
determination of Δχ value for all of the synthesized cobalt­(II)
complexes with a very high accuracy and providing a more reliable
zero-field splitting energy for further calculations.