A series of triple-stranded lanthanide(III) helicates: Syntheses, structures and single molecular magnets

Article abstract of DOI:10.1016/j.poly.2017.01.011

Bis-β-diketones have been proven to be effective for the construction of multiple-stranded helicates, where there commonly are two distinct Ln³⁺centers with subtle geometric difference. In this study, a series of triple-stranded helicates [Ln₂(BTB)₃(DME)₂]·C₆H₁₄have been prepared with the ligand H₂BTB and trivalent metallic ions, respectively [Ln?=?La (1), Ce (2), Pr (3), Eu (4), Dy (5), Ho (6) and Yb (7), H₂BTB?=?4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl)biphenyl and DME?=?dimethoxyethane]. All products have been fully characterized by IR spectroscopy, elemental analysis, thermogravimetric analysis, and single-crystal X-ray diffraction analyses. Structural analysis reveals that 1–7 are isostructurally crystallized in the orthorhombic space group of Pna2₁and each Ln³⁺ion is ligated to six O atoms of three BTB ligands and two O atoms of one DME molecule. Interestingly, DME molecule as a chelator to the Ln³⁺centers has played a unique role in the crystallization of the triple-stranded helicate 1–7. Magnetic measurement shows that 5 displays significant single-molecule magnetic property, exhibiting magnetic slow relaxation. And the large separation between Dy³⁺ions in the same helicates leads to two distinct magnetic relaxation processes.

Full text of DOI:10.1016/j.poly.2017.01.011

Accepted Manuscript
A series of triple-stranded lanthanide(III) helicates: syntheses, structures and
single molecular magnets
Xingrui Gao, Hongfeng Li, Peng Chen, Wenbin Sun, Pengfei Yan
PII:
S0277-5387(17)30016-5
DOI:
http://dx.doi.org/10.1016/j.poly.2017.01.011
POLY 12415
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
19 September 2016
4 January 2017
7 January 2017
Please cite this article as: X. Gao, H. Li, P. Chen, W. Sun, P. Yan, A series of triple-stranded lanthanide(III) helicates:
syntheses, structures and single molecular magnets, Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.
2017.01.011
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A series of triple-stranded lanthanide(III) helicates: syntheses, structures and
single molecular magnets
Xingrui Gaoa, Hongfeng Lia, Peng Chena,b,*, Wenbin Suna and Pengfei Yana,*
a
b
Key Laboratory of Functional Inorganic Material Chemistry (MOE), School of Chemistry and
Materials Science Heilongjiang University Harbin 150080, PR China.
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University,
Changchun 130012, PR China.
Abstract. Bis-β-diketones have been proven to be effective for the construction of
multiple-stranded helicates, where there commonly are two distinct Ln3+ centers with
subtle geometric difference. In this study, a series of triple-stranded helicates
[Ln2(BTB)3(DME)2]·C6H14 have been prepared with the ligand H2BTB and trivalent
metallic ions, respectively [Ln = La (1), Ce (2), Pr (3), Eu (4), Dy (5), Ho(6) and
Yb(7), H2BTB = 4,4'-bis(4,4,4-trifluoro-1,3-dioxobutyl)biphenyl and DME =
dimethoxyethane]. All products have been fully characterized by IR spectroscopy,
elemental analysis, thermogravimetric analysis, and single-crystal X-ray diffraction
analyses. Structural analysis reveals that 1–7 are isostructurally crystallized in the
orthorhombic space group of Pna21 and each Ln3+ ion is ligated to six O atoms of
three BTB ligands and two O atoms of one DME molecule. Interestingly, DME
molecule as a chelator to the Ln3+ centers has played a unique role in the
crystallization of the triple-stranded helicate 1–7. Magnetic measurement shows that 5
displays significant single-molecule magnetic property, exhibiting magnetic slow
relaxation. And the large separation between Dy3+ ions in the same helicates leads to
two distinct magnetic relaxation processes.
Keywords: helicates, lanthanide, triple-stranded, magnetics

Introduction
The design and synthesis of highly organized structures have achieved considerable
attention in the supramolecular chemistry and material chemistry [1]. Helicates as the
simplest structural motifs have received much interests since their formation process
is the basis for the supramolecular architectures and device [2]. Various ligands with
distinct geometries and coordination modes have been designed [3] and the
bis-β-diketones have been proven to be qualified to construct the multiple-stranded
helicates as well [4,5]. The control on the shape, flexibility and functionality of the
ligands highlights the diversified structures of the helicates, while the substitution of
the organic spacers provides an impetus for further investigation on their
supramolecular architectures. The bis-β-diketones was initially utilized to construct
transition metal-based complexes for their rigidity as excellent building units [6].
Thereafter, the bis-β-diketones have recently found their importance in the formation
of multiple-stranded lanthanide-based helicates, while the enhanced emission with an
unexpectedly high quantum yield is observed for the multiple-stranded helicates [4a].
Notably, the β-diketonate-Dy strategy has proven successful for the preparation of
the single molecular magnets (SMM) with larger energy barrier [7], ever since Gao et
al reported the structure and magnetic behavior of first β-diketonate-Dy SMM [8].
Commonly, the Dy3+ ions are approximately located in a D4d symmetry, and it
provides a good sample for the theoretical calculation on the easy axis of SMM [9]. It
is well-known that the local magnetic anisotropy of the Dy3+ ions are sensitive to the
subtle changes of the ligand field and local geometry. However, it is difficult to
predict the potential magnetic behavior of the complexes in respect to the complex
coordination environment of Dy3+ centers. A great deal of researches have been done
2

to extend the understanding on this subject and establish a proper synthetic strategy
for the application [10].
We have been devoted to the design and syntheses of bis-β-diketone ligands with
the aim of enforcing triple-stranded and quadruple-stranded topological patterns and
introducing functionality aspects to the resulting complexes [5,11-13]. A variety of
bis-β-diketones have been synthesized with diversified rigid and flexible spacers,
which are used to ligate to the Ln3+ ions, producing multiple-stranded dinuclear
helicates. In addition, triple-stranded and/or quadruple-stranded helicates could be
produced, depending on the geometry of the ligands and the auxiliary moieties [5d].
The multiple-stranded dinuclear systems, featured by the similar coordination
environments of the Dy3+ centers, could provide a way to tune the anisotropy axes to
align the anisotropic axis with higher energy barrier for the spin reversal [10c].
For this reason, we decide to explore and prepare SMM by using different ligands
furnished by the organic spacers. The ligand H2BTBa with 3,3’-substituted biphenyl
as one of the simplest rigid spacers has accomplished a deeper insight into the
crystallization of polymorphs and supramolecular isomers [5a,5e]. The efforts on the
alternative substitution position of the biphenyl groups are still unknown, while
4,4’-substituted biphenyl in more rigid and lengthy geometry would result in a ligand
in a distinct geometry in respect to H2BTBa. And it would lead to different local
environments of the Dy3+ centers. Taking into account of our previous reports, we
present the syntheses, structures and magnetic behavior of a series of triple-stranded
helicates [Ln2(BTB)3(DME)2]·C6H14 [Ln = La (1), Ce (2), Pr (3), Eu (4), Dy (5), Ho(6)
and Yb(7)], which are assembled about
a
bis-β-diketone ligand
4,4'-bis(4,4,4-trifluoro-1,3-dioxobutyl)biphenyl (H2BTB). DME serves as a chelating
3

moiety to the lanthanide centers, while it has played a decisive role in the
crystallization of this series of helicates. The result iterates the concerns on the crystal
engineering for the crystallization of multiple-stranded helicates [5d]. Magnetic
measurement shows that 5 displays significant single-molecule magnetic property,
and the large separation between Dy3+ ions in the same helicates leads to two distinct
magnetic relaxation processes.
Experimental Section
Materials and instrumentation
Elemental analyses were performed on an Elementar Vario EL cube analyzer.
FT-IR spectra were obtained on a Perkin-Elmer Spectrum One spectrophotometer by
using KBr disks in the range of 4000–370 cm–1. MS detection was performed on an
Agilent 6520 Q/TOF mass spectrometer with an ESI source and an AgilentG1607A
1
coaxial sprayer (all from Agilent). The H NMR spectra were recorded on a Bruker
Avance III 400 MHz spectrometer in a CDCl3 solution. The magnetic susceptibility
measurements were obtained using a Quantum Design SQUID magnetometer
−7 to 7
MPMS-3 operating between 1.8 and 300 K for dc-applied fields ranging from
T, and ac susceptibility measurements were carried out under an oscillating ac field of
2 Oe and ac frequencies ranging from 1 to 1000 Hz. A diamagnetic correction was
applied for the sample holder. The sample was analyzed by X-ray photoelectron
spectroscopy (XPS, Kratos, ULTRA AXIS DLD) with monochrome Al Kα (hν =
1486.6 eV) radiation. All binding energies were calibrated by referencing to C 1s
(284.6 eV). Xray powder diffraction data were collected on a Bruker D5005 X-ray
diffractometer with Cu Kα radiation (λ = 1.5406 Å). Thermogravimetric analyses
were obtained on an SDT Q600 thermogravimetric analyzer at a heating rate of 20
4

oC/min under air atmosphere in the temperature range of 25–780 oC. All
measurements were carried out by using fresh crystals. Single crystals of 1−7 were
selected for X-ray diffraction analysis on a Xcalibur, Eos diffractometer using
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The crystals were kept at
room temperature during data collection. The structures were solved by the direct
methods and refined on F2 by full-matrix least-square using the ShelXL2014 program
[14]. The Ln3+ ions were firstly located, and then non-hydrogen atoms (C, O and F)
were placed from the subsequent Fourier-difference maps and refined anisotropically.
The H atoms were introduced in the calculated positions and refined with fixed
geometry with respect to their carrier atoms. In the case of 1−4, 6 and 7, F1−F3,
F10−F12 and F16−F18 atoms have been modelled as disordered with the equivalent
occupancy. In the case of 6, n-hexane molecules have been treated as a diffuse
contribution to the overall scattering without specific atom positions by
squeeze/platon [15], in respects with the cases of 1−5 and 7. The crystallographic
formula has been modified to include the non-located atoms. The relatively larger
Ueq values of the n-hexane molecules in all cases might be owing to their higher
thermal motion and flexible configuration. The experimental details for the structural
−1478972
determination are presented in Table 1. CCDC: 1478966
supplementary crystallographic data for this paper.
contained the
5

Scheme 1 The structure and the synthetic route for the ligand H2BTB.
Synthesis of 4,4’-diacetylbiphenyl
A 100-mL round-bottomed Schlenk flask was charged with anhydrous AlCl3
(1.82 g, 13.6 mmol), dry dichloromethane (50 mL), and acetyl chloride (1.07 g,
13.6 mmol), resulting in a yellow transparent solution. A dry dichloromethane
solution of biphenyl (1.00 g, 5.5 mmol) was added dropwise to the above
o
solution at -20 C. After kept 12 hours at room temperature, the resulting
mixture was poured into 50 mL ice-water and alkalized to pH = 7 using the
aqueous solution of NaOH. The resulting precipitate was then filtered. The
crude product was crystallized with ethanol and dried under vacuum (0.93 g,
64 %). Anal. Calcd. for C16H14O2 (238.10): C, 80.65; H, 5.92 wt%. Found: C,
80.61; H, 5.88 wt%. IR (KBr, cm-1): 3446 (w), 1600 (s), 1590 (m), 1401 (m),
1
o
1363 (s), 1264 (s). H NMR (400 MHz, CDCl3, 25 C, TMS): δ = 8.08 (d, J =
1
8.4 Hz, 4H), 7.74 (d, J = 7.4 Hz, 4H), 2.67 ppm (s, 6H). The H NMR spectra
were shown in Figure S1–2.
Synthesis of H2BTB
H2BTB was synthesized by the Claisen condensation of ethyl trifluoroacetate
and 4,4’-diacetylbiphenyl in DME (dimethoxyethane). A mixture of sodium
6

methoxide (1.40 g, 20.0 mmol) and ethyltrifluoroacetate (2.90 g, 20.0 mmol) in
40 mL dry DME was stirred for 10 min, followed by the addition of
4,4’-diacetylbiphenyl (2.00 g, 8.4 mmol). Then, it was further stirred at room
temperature for 24 h (Scheme 1). The mixture was poured into 100 mL
ice-water and acidified to pH = 2–3 using hydrochloric acid (2.0 M), and the
white precipitate was filtered and dried under vacuum. Recrystallization from
acetone gave white flake crystals (2.5 g, 68 %). Anal. Calcd. for C20H12F6O4
(430.06): C, 55.83; H, 2.81. Found: C, 55.89; H, 2.84. IR (KBr, cm-1): 3118,
1
1602, 1445, 1270, 1208, 1187, 1106, 1059, 792, 723, 709, 627, 577. H NMR
(400 MHz, CDCl3, 25 oC, TMS): δ = 8.09 (d, J = 8.53 Hz, 4H), 7.80 (d, J = 8.53
Hz, 4H), 6.64 ppm (s, 4H). ESI-MS m/z 430.0708 (M+H+).
Syntheses of Ln2(BTB)3(DME)2·C6H14
The
synthetic
methods
for
the
triple-stranded
helicates
Ln2(BTB)3(DME)2·C6H14 were similar and the preparation of
5
was given as an
example. 0.29 g DyCl3·6H2O, 0.50 g H2BTB and 0.24 g triethylamine were
refluxed in MeOH (20 mL) for 3h, and the solution was kept stirred for extra 24
hours at room temperature. The addition of deionized water led to the precursor
of white precipitates, which was filtered, collected and dried under vacuum.
Single crystals of
diffusion of -hexane into its DME/CHCl3 (1/3, v/v) solution in 7 days.
Interestingly, it is found the crystallization of is affected by the radius of
5 suitable for X-ray analysis were obtained by the slow
n
1−7
lanthanide ions, the discrepancy of which is well-known arisen from the
lanthanide contraction. Crystals in the larger size, better quality and higher
7

yields are obtained for the lighter lanthanides with larger radius than the heavier
under the similar condition.
La2(BTB)3(DME)2·C6H14 (1)
Yield: 76 %. Anal. Calcd. for C74H64La2F18O16 (1829.08): C, 48.59; H, 3.53
wt%. Found: C, 48.53; H, 3.49 wt%. IR (KBr, cm–1): 2952, 1648, 1618, 1579,
1367, 1289, 1262, 1193, 1084, 1058, 1011, 785.
Ce2(BTB)3(DME)2·C6H14 (2)
Yield: 75 %. Anal. Calcd. for C74H64Ce2F18O16 (1831.49): C, 48.53; H, 3.52
wt%. Found: C, 48.50; H, 3.49 wt%. IR (KBr, cm–1): 2952, 1648, 1603, 1578,
1369, 1294, 1262, 1191, 1082, 1058, 1010, 785.
Pr2(BTB)3(DME)2·C6H14 (3)
Yield: 70 %. Anal. Calcd. for C74H64Pr2F18O16(1833.08): C, 48.49; H, 3.52 wt%.
Found: C, 48.44; H, 3.48 wt%. IR (KBr, cm–1): 2953, 1643, 1606, 1579, 1366,
1294, 1261, 1191,1079, 1058, 1008, 786.
Eu2(BTB)3(DME)2·C6H14 (4)
Yield: 62 %. Anal. Calcd. for C74H64Eu2F18O16 (1855.17): C, 47.91; H, 3.48
wt%. Found: C, 47.85; H, 3.41 wt%. IR (KBr, cm–1): 2956, 1647, 1610, 1581,
1369, 1298, 1263, 1192, 1082, 1058, 1010, 785.
Dy2(BTB)3(DME)2·C6H14 (5)
Yield: 56 %. Anal. Calcd. for C74H64Dy2F18O16 (1876.25): C, 47.37; H, 3.44
wt%. Found: C, 47.31; H, 3.41 wt%. IR (KBr, cm–1): 2953, 1646, 1614, 1579,
1369, 1291, 1262, 1193, 1081, 1057, 1009, 786.
Ho2(BTB)3(DME)2·C6H14 (6)
8

Yield: 47 %. Anal. Calcd. for C74H64Ho2F18O16 (1881.11): C, 47.25; H,3.43
wt%. Found: C, 47.21; H, 3.41 wt%. IR (KBr, cm–1): 2952, 1645, 1610, 1579,
1369, 1298, 1263, 1192, 1080, 1060, 1009, 786.
Yb2(BTB)3(DME)2·C6H14 (7)
Yield: 36 %. Anal. Calcd. for C74H64Yb2F18O16 (1897.33): C, 46.84; H,3.40
wt%. Found: C, 46.77; H, 3.38 wt%. IR (KBr, cm–1): 2948, 1648, 1614, 1578,
1369, 1301, 1263, 1192, 1083, 1059, 1010, 786.
FT–IR spectra
The infrared spectra of 1–7 are similar, and 5 is selected as an example for further
discussion. As shown in Figure S3, the vibration bands at 1646–1579 cm–1 are
attributed to the C═O stretching vibration of the ligands. In 5, the observation of the
band shift by 20 cm–1 compared with the free ligand reveals the coordination of Dy3+
ions to the O atoms of the ligand (Figure S4). The absence of the band at 3114 cm–1 in
5 confirms the deprotonation of the bis-β-diketonate groups in H2BTB as well. Due to
the keto-enol tautomerization in the β-diketones, the C–O stretching vibrations could
be observed at 1187 cm–1 in the free ligand, while the corresponding intensity at 1193
cm–1 is strengthened in respect to the incorporation of DME molecules. The C═C
stretching vibrations of biphenyl are observed at 1579 cm–1 and the vibration bands at
1262 cm–1 are assigned to the C–C stretching vibrations of the diketonate moieties.
The multiple bands observed around 2953 cm–1 are attributed to the stretching
vibration of the C–H groups of DME molecules, whose existence confirms the
coordination of DME to the Dy3+ ions in 5. Apart from the high frequency stretching
vibrations of C–H, the in- and out-of-plane bending vibrations of C–H can also be
9

observed at 1369 and 1009 cm–1. The peaks at 1081 and 1057 cm–1 are attributed
to the C–F stretching vibration.
Thermal behavior
The thermogravimetric analysis on the fresh crystals of
1
−
7
are conducted.
o
For each compound, the first weight loss is clearly visible between 100 C and
o
210 C. It is attributed to the removal of
n
-hexane molecule, which fill out the
o
o
intermolecular spaces. The second weight loss appears from 250 C to 290 C
and it is assigned to the decomposition of two DME molecules attached to the
Ln3+ centers. The following two steps are owing to the removal of BTB ligands
and the final residue corresponds to the lanthanide oxide Ln2O3 for
1 and 4–7,
based on the observed and calculated values. However, it is well known that the
oxide CeO2 is formed upon the thermal decomposition of the cerium-based
compound, and the final weight value is 18.74 wt% for CeO2 (calcd. 18.79
wt%) in 2. Pr6O11 is well-known formed at high temperature as well and the
residue weight value agrees well with the theoretical values [16]. The detailed
discussion on the thermogravimetric analyses are presented in the ESI (Figure
S5).
Results and Discussion
Single crystal X-ray diffraction analysis reveals that 1–7 are isostructurally
crystallized in the orthorhombic space group of Pna21 (Figure S6). Therefore, a full
description of 5 is given as a representative example. As shown in Figure 1, 5 is a
triple-stranded dinuclear helicate. In the asymmetric unit of 5, there are two Dy3+
centers, three BTB ligands, two DME and one n-hexane molecule. Each
crystallographically distinct Dy3+ ion is eight-coordinated to six O atoms of three
10

BTB ligands and two O atoms of one DME molecule in the square antiprism
geometry (Figure 1) [17]. It gives rise to the formation of the triple-stranded helicate,
where three deprotonated BTB are wrapped about two Dy3+ ions. The structural
analysis reveals that each dinuclear complex has homochiral Dy3+ centers in either
left-handed Λ–Λ or right-handed ∆–∆ helix. The Dy–O distances are in the range of
2.271(9)–2.562(10) Å, which are in agreement with the reported values (Table S2)
[5a]. The geometry of each Dy3+ center is slightly different and detailed bond lengths
are listed in Table S1. The dihedral angles between the two phenyl groups in each
BTB are in the range of 13.9–29.7°, while the phenyl groups of BTBa twist more with
the dihedral angles in the range of 40.3–65.3° [5a]. The Ln···Ln distance in the same
helicate is 14.401(1) Å for 1 down to 14.210(1) Å for 7, in accordance with the
lanthanide contraction effect (Table 2). The results are larger than the reported values
[5a]. As far as we know, no cavity could be constituted up by three ligands to
accommodate the guest species in the triple-stranded system. The n-hexane molecules
are found among the helicates, filling up the intermolecular space [5a,5e]. In the
−H…F and F…F interactions have
structures of 1–7, plenty of weak intermolecular C
been observed among the helicates and the guest molecules, which are of helps to the
stability of the whole structures (Figure 2, Table S2) [18]. And it iterates the
importance of the termini –CF3 groups in the crystallization of the helicates, and
guides the design and synthesis for the potential ligand [5].
11

Figure 1 The molecular structure of triple-stranded helicate 5 (The C atoms in each
ligand are marked in a different colour. H atoms and guest species have been omitted
for clarity).
−H…F intermolecular
Figure 2 The packing diagram of 5 shows the weak C
interactions, viewed along the b axis. Detailed bond lengths are listed in Table S2.
12

Table 1 Crystal data and structure refinement for 1–7.
Code
1
2
3
4
5
6
7
Empirical formula
Formula weight
Crystal system
space group
a (Å)
C74H64F18La2O16
1829.07
C74H64Ce2F18O16
1831.49
C74H64F18O16Pr2
C74H64Eu2F18O16
1855.17
C74H64Dy2F18O16
1876.25
C74H64F18Ho2O16
1881.11
C74H64F18Yb2O16
1897.33
1833.08
Orthorhombic
Pna21
Orthorhombic
Pna21
Orthorhombic
Pna21
Orthorhombic
Pna21
Orthorhombic
Pna21
Orthorhombic
Pna21
Orthorhombic
Pna21
43.4625(6)
10.1303(2)
17.6022(4)
7750.0(3)
4
43.5356(7)
10.0879(2)
17.4784(4)
7676.2(3)
4
43.4491(13)
10.1166(3)
17.5863(6)
7730.2(4)
4
43.2419(6)
10.04000(10)
17.4211(3)
7563.35(18)
4
43.2398(7)
10.0112(4)
17.3671(5)
7517.9(4)
4
43.2208(8)
10.0045(2)
17.3501(4)
7502.2(3)
4
43.1122(11)
10.0010(4)
17.3151(6)
7465.7(4)
4
b (Å)
c (Å)
Volume (Å3)
Z
Limiting indices
-57≤h≤59
-12≤K≤13
-22≤l≤22
-57≤h≤55
-13≤K≤10
-16≤l≤22
-58≤h≤53
-13≤K≤6
-21≤l≤21
-57≤h≤56
-13≤K≤10
-15≤l≤23
-49≤h≤54
-12≤K≤6
-53≤h≤41
-12≤K≤12
-21≤l≤20
-55≤h≤55
-12≤K≤9
-21≤l≤12
-21≤l≤22
Reflec. collec.
/unique
Rint
73471
18216
0.0502
22575
11149
0.0263
25516
15971
0.0316
75592
15411
0.0322
25992
10723
0.0438
33220
14078
0.0310
25368
13519
0.0306
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
R1 (all data)
wR2 (all data)
GOF of F2
0.0942
0.1908
0.1029
0.1950
1.142
0.0460
0.0954
0.0602
0.1023
1.076
0.0942
0.2028
0.1037
0.2075
1.134
0.0517
0.0908
0.0588
0.0944
1.162
0.0539
0.1040
0.0632
0.1081
1.138
0.0453
0.1118
0.0533
0.1166
1.134
0.0971
0.2584
0.1201
0.2812
1.079

Table 2 Interatomic Ln−O and Ln…Ln distance ranges in 1−7
−O (Å)
Ln
−OBTB
−ODME
Ln
Codes
Ln
(Å)
(Å)
Ln…Ln (Å)
−2.637(1)
−2.461(1)
−2.637(1)
1
2
3
4
5
6
7
2.364(1)
2.362(2)−2.662(3)
−2.642(1)
2.364(1)
2.550(9)
14.401(1)
14.410(1)
14.398(1)
14.296(1)
14.266(1)
14.259(1)
14.210(1)
2.362(2)−2.476(2)
−2.452(1)
2.567(2)−2.662(3)
−2.642(1)
2.370(1)
2.370(1)
2.558(8)
2.304(7)−2.597(7)
2.271(1)−2.562(9)
2.304(7)−2.406(6)
2.271(9)−2.379(8)
2.480(5)−2.597(7)
2.446(6)−2.562(9)
−2.561(7)
−2.373(3)
2.262(3)
−2.561(3)
2.262(3)
2.448(2)
2.242(4)−2.558(5)
2.242(4)−2.351(4)
2.418(4)−2.558(5)
As we have previously discussed, the insufficiency in the flexibility of the
4,4’-substituted BTB ligand would result in the formation of triple-stranded helicate
in respect to the 3,3’-substituted BTBa ligand [5a]. Besides of six O atoms from three
BTB ligands, the Ln3+ centers are believed to coordinated to two extra O atoms from
water/methanol, which allow for the possibility to be replaced with proper solvent
molecules. The solvents are well known to have significant impacts on the
crystallization and polymorphs. And it has illustrated the unique role of the diversified
solvents to the crystallization. We have previously prepared three complexes
[Dy2(BTBa)3(CH3OH)4]·3CH3OH,
[Dy2(BTBa)3(DOA)(H2O)2]·4.5DOA
[Dy2(BTBa)3(DME)2]
and
(BTBa
=
3,3'-bis(4,4,4-trifluoro-1,3-
dioxobutyl)biphenyl, DOA =1,4-dioxane) by using various solvents under the similar
condition [5a]. The results have deepened our understandings on the factors
concerned with the crystallization of the multiple-stranded bis-β-diketonate-Ln
helicates, and similar synthetic strategy has been carried out in this work. Various
solvents (methanol, ethanol, DOA and DMF etc) have been mixed with CHCl3 to
14

dissolve the precursor, but we failed to crystallize the helicate, except for DME [19].
The structural analyses on 1−7 reveal that each DME molecule chelates to one Dy3+
center, replacing the water/methanol molecules. It is noted that the auxiliary ligands in
the similar coordination modes (2,2’-bipyridine and 1,10-phenanthroline) are not
helpful for the crystallization. All of the lanthanide elements are commonly known to
have the +3 oxidation state. The relative ease to remove the 4th electron would lead to
the separation of Ce4+ compounds in the basic condition. We areinterested in the
oxidation state of cerium, which has not been included in any multiple-stranded
bis-β-diketonate helicates. Structural analysis indicates that 2 is isostructural to 1 and
3−7, revealing the +3 oxidation state of cerium in 2. X-ray photoelectron
spectroscopy is employed as well to confirm the trivalence of cerium ions in 2, as it is
proposed by the structural analysis (Figure S7). Two strong peaks centered at 885 and
903 eV binding energy of Ce 3d5/2 and Ce 3d3/2 are observed, respectively [20].
Magnetic properties of 5
The direct-current (dc) magnetic measurements are performed in an applied
magnetic field of 1000 Oe (Figure 3) for 5 in the temperature range of 1.8–300 K. At
the room temperature, the value of χT is 28.14, which are close to the expected value
6
for two independent Dy3+ ions (28.34 cm3 K mol–1): Dy3+ (S = 5/2, L = 5, H15/2, g =
4/3, C =14.17 cm3 K mol–1). The χT product remained constant down to 75 K on
lowering the temperature before dropping rapidly down to 24.50 cm3 K mol–1 at 1.8 K.
The decrease of χT is likely due to crystal-field effects (i.e. thermal depopulation of
15

the Ln(III) Stark sublevels) and/or the possible antiferromagnetic dipole–dipole
interaction between the molecules and, even if magnetic anisotropy might also
Figure 3 Plot of χT vs T for 5 in an applied dc field of 1000 Oe in the temperature
range of 1.8–300 K.
Figure 4 Magnetization as a function of H/T for 5.
partially affect low temperature susceptibility. Magnetization (M) data for 5 are
collected in the 0–7 T field range below 5K (Figure 4). The magnetization versus H/T
data at different temperatures show nonsuperposition plots, and a gradual increase of
16

the magnetization at high fields, without a saturation even at 7 T, revealing the
presence of a significant magnetic anisotropy and/or low-lying excited states.
The dynamics of the magnetization for 5 are investigated using alternating current
susceptibility measurements. A temperature dependent increase of the in-phase signal
with the appearance of an out-of-phase signal is observed for 5 (Figure S8). The
increasing of χʹ and χʹʹ below 6 K is indicative of the quantum tunnelling of the
magnetization (QTM) at a zero dc field, which is typical properties for the
lanthanide-based SMMs (Figure S9). To suppress the QTM effect, ac susceptibility
measurements are performed under a static dc field of 2000 Oe for 5 (Figure 5). As
expected, the QTM was suppressed obviously and the full peaks of temperature
dependence of ac susceptibility are observed. The frequency-dependent data in the
temperature range of 2–12 K for 5 display the intensity of the χ’’ increases with
decreasing the temperature and frequency. Moreover, it is worth noting that the two
frequency dependent χ’’ peaks are observed for 5, which is indicative of double
relaxations processes. Multiple relaxation processes have been observed in some
reported f-based SMMs mostly due to the existence of different anisotropic centers or
isomers and conformers in the crystal. In respect to the structures of 5, there are two
crystallographically independent Dy3+ centers, which should be responsible for the
observation of two relaxations processes (Figure S10). The presence of two relaxation
processes is further examined using a graphical representation, χ’’ versus χ’. The
Cole–Cole plots of 5 in the temperature range of 2−10 K exhibits a unique
double-ridge shape and the data can be fitted very well via using the sum of two
17

modified Debye functions [21]. The two distinct peaks of the out-of-phase ac signals
(χ'') at higher frequencies is evident, which reveals the occurrence of a double
relaxation process deriving from two crystallographically independent Dy3+ centers in
5 [22]. The relaxation time is extracted from the frequency-dependent data based on
the Arrhenius law [τ = τ0exp(Ueff/KBT), τ = 1/2πfmax] and the Arrhenius plot obtained
from these data is given in Figure 6. The anisotropic energy barriers are calculated to
be 12.4 K (τ0 = 1.26×10–5 s) and 35.6 K (τ0 = 2.82×10–6 s) for the low temperature
and high temperature domains, respectively. It is noted that the τ0 values are relatively
larger than the expected values for SMM [23], which is probably due to the presence
of quantum tunnelling magnetization.
Figure 5 Frequency dependence of the in-phase (χ', top) and out-of-phase (χ'', bottom)
ac susceptibilities for 5 under 2000 Oe dc field in the temperature range of 2−11 K.
18

Figure 6 The relaxation time is plotted as ln(τ) vs T-1 for 5. The solid lines are fitted
using the Arrhenius law (blue: low temperature domain; red: high temperature
domain).
Conclusions
In summary, a series of triple-stranded helicates [Ln2(BTB)3(DME)2]·C6H14
incorporating various trivalent lanthanide ions (Ln = La3+, Ce3+, Pr3+, Eu3+, Dy3+,
Ho3+ and Yb3+) have been prepared with a rigid bis-β-diketone ligand
4,4'-bis(4,4,4-trifluoro-1,3-dioxobutyl)biphenyl. The synthetic works have revealed
that DME molecules have played unique role in the crystallization of 1−7, while the
weak interaction are responsible to the crystallization as well. The intramolecular
magnetic coupling could be excluded between the Dy3+ centers owing to their large
separation in the same helicates, so the structure of the bis-β-diketonate-Dy SMM can
be divided as two distinct single ion magnets. Therefore, we believed that the
multiple-stranded dinuclear Dy3+ systems, featured by the subtle coordination
environment of the Dy3+ centers, could supply an excellent sample for the chemists
19

and physicists to further understand the magneto-structural correlation. And it
provides a way to tune the anisotropy axes to enhance the whole molecular magnetic
anisotropy with higher energy barrier for the spin reversal.
Supporting Information
1
Electronic Supplementary Information (ESI) available: H NMR spectra, FT–IR
spectra, TG curves, Ce 3d XPS spectrum and the magnetic data are presented as
Figure S1–S10. Selected bond lengths are given as Table S1–S2.
Author Information
Corresponding Author E-mails: jehugu@outlook.com; yanpf@vip.sina.com.
The authors declare no competing financial interest.
Acknowledgements
The authors acknowledge financial support from the NSFC (21572048,
21102039, 21272061 and 51302068).
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26

A series of triple-stranded β-diketonate-Ln helicates have been assembled, where
dimethoxyethane molecules have played a unique role in the crystallization.
27