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H.-R. Wen et al. / Inorganica Chimica Acta 435 (2015) 274–282
The field dependence of the magnetization was recorded at dif-
curves of the ac magnetic susceptibilities show that both in-phase
and out-of-phase signals are frequency- and temperature-depen-
dent with a series of frequency-dependent peaks for the out-of
phase ac signals (Fig. 7), these are typical features for the field-in-
duced SMM behaviors. The strong anisotropies may result in the
SMM behaviors of 6 and 7 [27]. To obtain the relaxation energy
barrier and relaxation time of 6 and 7, the best fitting based on
ferent temperatures. For 1–8, the magnetizations increase quickly
at very low field, reaching about 0.49, 0.55, 0.03, 0.01, 5.11, 7.55,
5.71 and 0.84 Nb at 10 kOe, respectively (Fig. 5). In the high field
region the increase of magnetizations is slow and linear, which
may be attributed to the anisotropy. At 2 K, the M values reach
to 1.20, 1.31, 0.13, 0.05, 7.05, 8.40, 7.66 and 1.58 Nb at 50 kOe,
respectively, being smaller than the theoretical saturated values
anticipated for one corresponding LnIII ion. This can be explained
by the fact that the depopulation of the Stark levels of the LnIII
2S+1LJ ground state under the ligand-field perturbation produces a
much smaller effective spin [25]. Additionally, the M versus H plots
for 5–7 almost do not show any hysteresis at 2 K (Fig. S3, SI) prob-
ably due to the super-low blocking temperature. The M versus H/T
(Fig. 6a) data of 5 at 2–5 K show nearly overlapping curves, which
indicates there is almost no anisotropy in the complex. While,
there are non-superposition plots of M versus H/T data and a rapid
increase of the magnetizations at low fields for 6 (Fig. 6b), which
eventually reaches the maximum value at 50 kOe without any sign
of saturation. The reason is most likely because of strong aniso-
tropy and important crystal-field effect of the TbIII ions [26].
In order to further elucidate possible SMM behaviors of 6 and 7,
alternating current (ac) susceptibility measurements were per-
formed in the different temperature ranges under Hdc = 0 kOe
and Hac = 2.5 Oe for variable frequencies. No obvious frequency-
and temperature-dependent ac signals were observed for 6 and
7. Therefore, a suitable dc field (2 kOe) was used to suppress the
quantum tunneling effect at low temperatures. For 6 and 7, the
the Arrhénius law 1/Tp = ꢁkB/DE[ln(2
pf) + ln(s0)] [28] gave the
energy barrier of 29.12 K (6) and 18.40 K (7), and the pre-exponen-
tial factor of 3.21 ꢂ 10ꢁ9 s (6) and 7.39 ꢂ 10ꢁ6 s (7) (Fig. 7, inset).
The values are in agreement with the observed s0 and
DE/kB for
DE/kB are higher than those
the LnIII-based SMMs, and the values of
of the similar achiral dinuclear Ni–Ln complexes [17f].
Furthermore, at fixed temperatures of 2 and 3 K with a 2 kOe dc
field for 6, the Cole–Cole plots (Fig. 8) from 1 to 1488 Hz in the
form of v00M versus v0M exhibit multiple relaxation processes and
such
a field-induced multiple relaxation processes was also
observed in the other SMMs/SIMs [29]. The existence of two crys-
tallography independent lanthanide ions probably is the main rea-
son for 6 and 7 to exhibit the multiple relaxation processes [30]. As
aforementioned, complexes 6 and 7 display the field-induced SMM
behaviors. Most of NiII–LnIII complexes behaving as SMMs contain
paramagnetic NiII ions, however, the NiII–LnIII system that exhibits
the highest energy barrier ever found is a {Ni3Dy2} cluster complex
[31] in which the NiII ions, as our compounds, are diamagnetic.
Compared with the similar works using achiral Schiff-base ligands
[23a], we investigate the ac properties of target complexes in
details. Therefore, two field-induced chiral SMMs have been suc-
cessfully constructed, which open a new way to the synthesis of
SMMs in the Ni–Ln system.
4. Conclusion
The synthesis, structural characterization and magnetism of a
new family of NiII–LnIII dinuclear complexes derived from a chiral
Schiff-base ligand have been studied. Complexes 1–8 are neutral
diphenoxo-bridged NiII–LnIII dinuclear compound and crystallize
in the same chiral space group P1. The solid CD spectra confirm
the optical activity and enantiomorphous properties of all these
complexes. The magnetic behaviors of 1–8 attribute to single-ion
properties of the LnIII ions, and crystal-field effects and/or the pos-
sible antiferromagnetic dipole–dipole interaction between the
molecules have a great influence on their magnetic properties.
For 6 and 7, the ac magnetic susceptibilities show that both v0
and v00 are strongly frequency- and temperature-dependent under
a 2 kOe dc field. These results clearly indicate that complexes 6 and
7 are chiral field-induced SMMs.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (21161008 and 91022014) and the Natural
Science Foundation of Jiangxi Province (Grant 20151BAB213003).
Appendix A. Supplementary material
CCDC 1020972–1020975 (complexes 1–3 and 7) and 1043143–
1043146 (complexes 4–6 and 8) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
associated with this article can be found, in the online version, at
Fig. 8. Cole–Cole plots for complex 6 measured at 2 K (a) and 3 K (b) with 2 kOe dc
field.
Wen, He-Rui
