Tuning the Equatorial Crystal-Field in Mononuclear DyIIIComplexes to Improve Single-Molecule Magnetic Properties

Article abstract of DOI:10.1021/acs.inorgchem.0c01716

Two DyIII complexes, [Dy(bbpen-F)X] [X = Cl (1), Br (2); H2bbpen-F = N,N′-bis(2-hydroxybenzyl)-N,N′-bis(5-fluoro-2-methylpyridyl)ethylenediamine], have been synthesized that show remarkable single-molecule-magnet behavior with effective barriers of magnet

Full text of DOI:10.1021/acs.inorgchem.0c01716

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Tuning the Equatorial Crystal-Field in Mononuclear DyIII Complexes
to Improve Single-Molecule Magnetic Properties
Li Zhu, Bing Yin,* Pengtao Ma, and Dongfeng Li*
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ABSTRACT: Two DyIII complexes, [Dy(bbpen-F)X] [X = Cl (1), Br (2); H2bbpen-F = N,N′-bis(2-hydroxybenzyl)-N,N′-bis(5-
fluoro-2-methylpyridyl)ethylenediamine], have been synthesized that show remarkable single-molecule-magnet behavior with
effective barriers of magnetic reversal of 837.7 K for 1 and 1149.7 K for 2 under zero direct-current field and hysteresis loops up to
20 K for 1 and 30 K for 2, as confirmed by magnetic properties and ab initio calculations.
n the preceding years, single-molecule magnets (SMMs)
have had potential applications in the fields of molecular
weakening of the equatorial electrostatic field, which could
also have a remarkable effect on the SMM behavior.22b,24,28b,30
The DyIII sandwich system has shown enormous success in
enhancing the axial electrostatic field and weakening the
equatorial electrostatic field at the same time.7b For the stable
Tong PBP DyIII compound,17a according to our best
knowledge, only one work has been reported to date in
which the axial positions of the ligand (phenol groups) are
modified with the electron-donating methyl groups to give a
new DyIII complex with increased Ueff and TB. No effort has
been reported to modify the equatorial pyridyl groups of the
H2bbpen ligand to improve the SMM behavior in this system.
Here, we tried to introduce electron-withdrawing groups,
e.g., an F atom, to the equatorial pyridyl groups, which will
reduce the amount of negative charge of the coordinating
pyridyl N atoms located on the equatorial positions of the
coordination sphere. We propose that it will produce a weaker
equatorial electrostatic field to give better SMM behavior.
Following up with this idea, we synthesized a new ligand
H2bbpen-F with F atoms at the equatorial pyridyl groups and
obtained two corresponding air-stable PBP DyIII complexes, of
which the structures and magnetic properties were researched.
Upon the reaction of H2bbpen-F with DyX3 and triethyl-
amine using acetonitrile as the solvent, well-shaped block
crystals [Dy(bbpen-F)X] [X = Cl (1), Br (2)] were obtained
under solvothermal conditions. Single-crystal X-ray diffraction
analysis (Tables S2 and S3) indicates that two complexes
crystallize in the orthorhombic crystal system with a Pbcn space
group. The structures of the two complexes contain a DyIII ion,
a bbpen-F2− ligand, and Cl− or Br− (Figures 1 and S1−S3).
The center DyIII locations in complexes 1 and 2 have
distorted PBP coordination geometry (Table S4) formed by
I
spintronics, quantum computing, and high-density information
storage materials1−3 because of their large effective energy
barrier (Ueff) and high blocking temperature (TB),4 while Ln
ions have huge magnetic anisotropy, large magnetic moments,
and strong spin−orbit coupling,5,6 which make them attractive
candidates for the preparation of high-performance SMMs.
Therefore, Ln-SMMs have aroused the interest of many
researchers, and mononuclear Ln-SMMs7−31 have highly
contributed because the interion coupling is usually negligible.
Theoretical studies have shown that, to increase Ueff and TB of
a Dy-SMM, one needs to supply a coordination environment
providing a very strong axial crystal-field component (electro-
static field) and a weak equatorial electrostatic field.12,15,22a,28a
High-order symmetries such as C∞, D5h, and D4d are very
important for the restraint of quantum tunneling of magnet-
ization (QTM) because the splitting is forced to be zero in
these ideal symmetries.17c,d,23,25,29a As reported, many Ln-
SMMs with high symmetry have been synthesized, but only a
very small part of them have Ueff and TB above 1000 and 20
K,7b,11,17a,18a−c,19a,13a,b respectively (see Table S1 for a detailed
list of these complexes).
It is worth mentioning that Tong’s group reported the first
SMM with pentagonal-bipyramidal (PBP) geometry (D5h) in
2013.17c Later on, they utilized a hexadentate ligand, N,N′-
bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)-
ethylenediamine (H2bbpen), to obtain the first Dy-SMM
possessing Ueff over 1000 K and TB around 20 K with PBP
geometry.17a Although current records of the highest Ueff(2218
K) and TB(80 K) are held by a sandwich DyIII compound,7b
the works of Tong’s group are still highly attractive because of
the fact that the obtained crystals are solvent-free and stable in
the air. It should be reasonable to believe that the solvent-free
and stable complexes can minimize the possibility of giving rise
to complex behaviors because of decomposition and/or
distortion of the structure.17a
Received: June 10, 2020
So far, more efforts have been made to amend the
anisotropy of LnIII ions in enhancing the axial electrostatic
field; however, few studies have been reported on the
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Figure 1. Crystal structure of 2. H atoms are omitted for clarity. Color
code: Dy, yellow; Br, plum; O, red; N, blue; C, gray. A = −x + 2, y, −
3
z + /2.
two O and four N atoms of a bbpen-F2− ligand and one Br
(Cl) atom of DyBr3 (DyCl3). In the axial direction, the two
negatively charged O atoms of phenol are coordinated to DyIII
with the rather short Dy1−O1 bond lengths of 1 and 2,
2.160(2) and 2.155(2) Å, respectively. Also, the O1−Dy1−
O1A bond angles of 1 and 2 are 156.82(13)° and 157.52(11)°,
respectively. In the equatorial direction, the four N atoms and
one Br (Cl) atom formed five coordination sites through an
average Dy−N bond length of 2.581 Å for 2 (2.576 Å for 1)
and a long Dy1−Br1 (Dy1−Cl1) distance of 2.8462(6) Å for 2
[2.6680 (14) Å for 1; Table S3]. The closest intermolecular
distance between two DyIII ions is 8.502 Å for 1 and 8.549 Å
for 2 (Figure S2 and Table S5). The structures are very similar
to Tong’s complexes,17a and the specific bond lengths and
angles of these complexes are shown in Table S6 for
comparison.
Temperature-dependent magnetic susceptibility experiments
were fulfilled on polycrystalline samples of these complexes
under a 1000 Oe external field and the temperature range
300−2 K. The values of χMT are 14.00 and 13.81 cm3·K·mol−1
for 1 and 2 at room temperature, respectively (Figure S4). The
χMT values of complexes 1 and 2 are in accordance with the
theoretical value (J = 15/2, g = 4/3, 14.17 cm3·K mol−1) for one
magnetically separated DyIII ion. Along with the temperature
drops, the χMT value also decreases marginally, and then it
dramatically drops at low temperature and reaches the values
of 6.77 cm3·mol−1·K (for 1) and 4.01 cm3·mol−1·K (for 2) at 2
K, strongly implying the presence of crystal-field splitting with
separated excited-state Kramers doublets. The field dependent
on the magnetization of 1 and 2 was measured from the zero
direct-current (dc) field to 70 kOe at 2.0 K (Figure S5). The
magnetization values achieved are respectively 5.25 and 4.98
NμB for 1 and 2 at 70 kOe, which are lower than the
theoretical saturation value of 10 NμB. Unsaturation of
magnetization was probably a result of the crystal-field effects,
low-lying excited states, and/or existence of significant
magnetic anisotropy.19b,22c,31
Figure 2. Magnetic hysteresis loops of 1 (top) and 2 (bottom) with a
200 Oe·s−1 sweep rate. Insets: Zoomed-in hysteresis loops at 19−20 K
for 1 (29−30 K for 2) with a 20 Oe·s−1 sweep rate.
2 show higher TB than [Dy(bbpen)Br], indicating improve-
ment in the SMM properties of the two complexes. To better
investigate the dynamics of the magnetic properties, alternat-
ing-current (ac) susceptibility calculations were carried out at
temperature dependences in zero dc and 2.5 Oe ac fields for
complexes 1 and 2. Complexes 1 and 2 display obvious
temperature dependencies for out-of-phase susceptibility (χ″),
the peaks of which are in good shape, demonstrating typical
SMM behaviors (Figures S6 and S7). Besides, the frequency
dependencies on the ac susceptibility measurements were
performed under zero dc field to analyze the dynamics of the
magnetic properties for 1 and 2. The data show typical slow
magnetic relaxation behavior (Figures 3 and S8−S11).
In order to explore the relaxation process, the Cole−Cole
curves were studied at variable temperatures for complexes 1
and 2 (Figures S12−S15 and Tables S7 and S8). These curves
obey a generalized Debye model, with the distribution of
relaxation time (α) parameters in the region 0.00−0.02
between 35 and 60 K for 1 and in the region 0.02−0.04
between 30 and 58 K for 2. The small distribution coefficient α
values show that there is a narrow distribution of relaxation
time for the magnetic relaxations of 1 and 2.
To deeply investigate the SMM behavior in 1 and 2, the
blocking of magnetization and field-dependent magnetization
measurements were conducted with an average sweep rate of
200 Oe·s−1 and a field range of −3 to +3 T under different
temperatures (Figure 2). Butterfly-shaped magnetic hysteresis
loops are obviously open at 20 K for 1 and at 30 K for 2 under
zero dc field, which are due to the slow relaxation time for the
complexes. By a comparison with TB in Tong’s work,17a 1 and
The derived ln(τ) versus T−1 plots for complexes 1 and 2
show that the Orbach process is the main process at high
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τ−1 = τQTM−1 + CTn + τ0−1 exp(−U /kBT)
(1)
eff
Compared to 1, the QTM of 2 is apparently weaker, and
thus the fitting of the ln(τ) versus T−1 curves takes into
account only the Orbach and Raman processes, as shown in eq
2.16,26,29b The fitting in the temperature between 11 and 58 K
for complex 2 resulted in τ0 = 1.5 × 10−12 s, Ueff/kB = 1149.7
K, C = 1.8 × 10−4 s−1·K−3.5, and n = 3.5.
τ−1 = CTn + τ0−1 exp(−U /kBT)
(2)
eff
According to ab initio calculations (see the Supporting
Information for the details), the transversal g factor, i.e., gXY,
of the ground-state Kramers doublet of 2 (0.98 × 10−3) is
clearly smaller than that of 1 (0.23 × 10−2). In general, the
QTM rate scales approximately to gXY2. Thus, ab initio
calculations confirmed the fact that the QTM of 2 is apparently
weaker than that of 1. In the aspect of Ueff, on the basis of ab
initio results (Figure S16 and Tables S9 and S10), a recent
theoretical method32 provides the results of 944.0 and 1158.0
K for 1 and 2, respectively. Apparently, theoretical values of
Ueff show good consistency with the results of experimental
fittings. Compared to the previous complexes in Tong’s
work,17a the ab initio results also indicate that the complexes
here possess similar strengths of QTM but larger crystal-field
splittings. Furthermore, the atomic charges (in |e|) and
electrostatic potentials from ab initio calculations for the
complexes (Tables S11 and S12) indicate that the equatorial
pyridyl N atoms of the complexes in this work have obviously
smaller values of the negative charge, which is obviously due to
the electron-withdrawing group on the equatorial plane of the
ligand. Therefore, we tentatively consider that this is the main
influencing factor for the increasing temperature of the open
hysteresis loops of the complexes here.
Figure 3. Frequency dependencies of the out-of-phase ac suscepti-
bility for 1 (top) and 2 (bottom) in zero external field and at the
indicated temperatures.
temperature, in which the fitting of the Arrhenius formula τ =
τ0 exp(Ueff/kBT) gives Ueff/kB = 827.9 K and τ0 = 1.5 × 10−11
s
for 1 and Ueff/kB = 1042.3 K and τ0 = 7.6 × 10−12 s for 2
(Figure 4), while at low temperature, other possible relaxation
processes become competitive. Thus, the ln(τ) versus T−1
curve of 1 is fitted with eq 1,16,26,29b which yields τ0 = 1.4 ×
10−11 s, τQTM = 0.16 s, Ueff/kB = 837.7 K, C = 2.1 × 10−3 s−1·
K−3.3, and n = 3.3.
In summary, we have successfully obtained two mono-
nuclear DyIII complexes 1 and 2, which exhibit relaxation
barriers of up to 837.7 and 1149.7 K and open hysteresis loops
of up to ca. 20 and 30 K, respectively. Compared to Tong’s
complexes, the electron-withdrawing group modified to the
ligand weakens the equatorial electrostatic field of the central
DyIII ion. This modification results into similar strengths of
QTM but larger crystal-field splittings and, eventually, higher
hysteresis temperatures of the systems here. This result may
provide an effective approach to building more high-perform-
ance DyIII-SMMs.
ASSOCIATED CONTENT
■
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01716.
Additional figures, X-ray crystallographic data and
figures, selected bond lengths and angles, magnetic
measurements, and theoretical calculations (PDF)
Accession Codes
CCDC 1999918 and 1999919 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Figure 4. ln(τ) versus T−1 plots at 0 Oe for 1 (top) and 2 (bottom).
The cyan dashed line and pink solid line represent part of the Orbach
process and the best fitting to the multiple relaxation process,
respectively (see the text for the details).
C
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AUTHOR INFORMATION
Corresponding Authors
Bing Yin − Key Laboratory of Synthetic and Natural Functional
Molecule of the Ministry of Education, College of Chemistry and
Materials Science, Northwest University, Xi’an 710127, P. R.
China; orcid.org/0000-0001-8668-9504;
■
(5) (a) Gonidec, M.; Biagi, R.; Corradini, V.; Moro, F.; De Renzi, V.;
del Pennino, U.; Summa, D.; Muccioli, L.; Zannoni, C.; Amabilino, D.
B.; Veciana, J. Surface supramolecular organization of a terbium(III)
double-decker complex on graphite and its single molecule magnet
behavior. J. Am. Chem. Soc. 2011, 133, 6603−6612. (b) Zhang, P.;
Guo, Y. N.; Tang, J. K. Recent advances in dysprosium-based single
molecule magnets: structural overview and synthetic strategies. Coord.
Chem. Rev. 2013, 257, 1728−1763. (c) Woodruff, D. N.; Winpenny,
R. E. P.; Layfield, R. A. Lanthanide single-molecule magnets. Chem.
Rev. 2013, 113, 5110−5148. (d) Sessoli, R.; Powell, A. K. Strategies
towards single molecule magnets based on lanthanide ions. Coord.
Chem. Rev. 2009, 253, 2328−2341. (e) Chilton, N. F.; Goodwin, C.
A.; Mills, D. P.; Winpenny, R. E. The first near-linear bis(amide) f-
block complex: a blueprint for a high temperature single molecule
magnet. Chem. Commun. 2015, 51, 101−103.
Email: rayinyin@nwu.edu.cn
Dongfeng Li − Key Laboratory of Pesticide & Chemical Biology,
Ministry of Education, College of Chemistry, Central China
Normal University, Wuhan 430079, P. R. China; orcid.org/
0000-0001-6537-3279; Email: dfli@mail.ccnu.edu.cn
Authors
Li Zhu − Key Laboratory of Pesticide & Chemical Biology,
Ministry of Education, College of Chemistry, Central China
Normal University, Wuhan 430079, P. R. China
Pengtao Ma − Henan Key Laboratory of Polyoxometalate
Chemistry, College of Chemistry and Chemical Engineering,
Henan University, Kaifeng, Henan 475004, P. R. China
(6) (a) Sorace, L.; Benelli, C.; Gatteschi, D. Lanthanides in
molecular magnetism: old tools in a new field. Chem. Soc. Rev. 2011,
40, 3092−3104. (b) Guo, Y. N.; Xu, G. F.; Guo, Y.; Tang, J. K.
Relaxation dynamics of dysprosium(III) single molecule magnets.
Dalton Trans. 2011, 40, 9953−9963. (c) Habib, F.; Murugesu, M.
Lessons learned from dinuclear lanthanide nano-magnets. Chem. Soc.
Rev. 2013, 42, 3278−3288.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01716
Notes
The authors declare no competing financial interest.
(7) (a) Guo, F. S.; Day, B. M.; Chen, Y. C.; Tong, M. L.;
ACKNOWLEDGMENTS
̈
Mansikkamaki, A.; Layfield, R. A. A dysprosium metallocene single-
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molecule magnet functioning at the axial limit. Angew. Chem., Int. Ed.
2017, 56, 11445−11449. (b) Guo, F. S.; Day, B. M.; Chen, Y. C.;
We are thankful for financial support from the National
Natural Science Foundation of China (Grants 21771074 and
21103137) and the computational resources from the chemical
high-performance computing center of Northwest University.
B.Y. is thankful for a fellowship grant under the “Excellent
Young Scholar Plan” (Grant 338050094) from Northwest
University.
̈
Tong, M. L.; Mansikkamaki, A.; Layfield, R. A. Magnetic hysteresis up
to 80 K in a dysprosium metallocene single-molecule magnet. Science
2018, 362, 1400−1403.
(8) Chen, Z. L.; Yu, S.; Wang, R. D.; Li, B.; Yin, B.; Liu, D. C.;
Liang, Y. N.; Yao, D.; Liang, F. P. Three Dy(III) single-ion magnets
bearing the tropolone ligand: structure, magnetic properties and
theoretical elucidation. Dalton Trans. 2019, 48, 6627−6637. (b) Zou,
H. H.; Meng, T.; Chen, Q.; Zhang, Y. Q.; Wang, H. L.; Li, B.; Wang,
K.; Chen, Z. L.; Liang, F. P. Bifunctional Mononuclear Dysprosium
Complexes: Single-Ion Magnet Behaviors and Antitumor Activities.
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