New field-induced single ion magnets based on prolate Er(III) and Yb(III) ions: tuning the energy barrier Ueff by the choice of counterions within an N3-tridentate Schiff-base scaffold

Lanthanides have relatively recently been recognized as ideal candidates for the construction of advanced magnetic materials that would allow for their future applications in spintronics and high-density data storage. Despite enormous progress that deals with the control of magnetic anisotropy and slow relaxation of magnetization in Single Molecule Magnets (SMMs), further improvements are still indispensable to go beyond the ultra-low temperature regime. We have thus prepared four lanthanide complexes ([ErL2(OTf)(MeOH)2](OTf)2 (1), [YbL2(OTf)2](OTf) (2), [ErL(NO3)3(H2O)](3) and [YbL(NO3)3(MeOH)]·MeCN (4)) with a tridentate Schiff-base ligand L, to unravel magneto-structural correlations in this new family of field-induced Single Ion Magnets (SIMs). Interestingly, as revealed by the single crystal X-ray diffraction, their structures are synthetically tuned by the choice of the applied counterion. The static and dynamic magnetic properties of 1–4 were investigated revealing that all compounds behave as field-induced Single Ion Magnets (SIMs). Their energy barriers Ueff decrease in the sequence: 4, 2, 3, 1, with an order of magnitude difference between the highest and the lowest value. To correlate the observed magnetic properties with spectroscopic data, low-temperature absorption spectroscopy was performed. This has allowed the determination of the energy levels of the Ln(III) ions and the exact composition of the state vectors for the Ln(III) ground multiplets via crystal-field analysis (CFA) and semiempirical superposition model (SPM) approach. Theoretical and magneto-structural correlation studies indicate that one can modulate the heterotopic coordination spheres around the prolate Er(III) and Yb(III) solely with the counterions. This leads to rarely observed high-coordinate SIM species with the LnNxOy first coordination sphere (where Ln – Er or Yb, x = 3 or 6, y = 2, 3 or 6). Their performance can be related to the intricate interactions between the electron density on the Ln ion and the crystal field created by the surroundings.