Mononuclear and trinuclear DyIII SMMs with Schiff-base ligands modified by nitro-groups: first triangular complex with a N-N pathway

Article abstract of DOI:10.1039/c9dt02646k

Using two Schiff-base ligands containing an electron-withdrawing group (NO₂), we obtained two mononuclear and two trinuclear complexes with the general formula [Dy(hni)(NO₃)(DMF)₂]·DMF (1·DMF), [Dy(hni)₂(H₂O)₂]·NO₃·EtOH (2·NO₃·EtOH), [Dy₃(hnc)₃(DMF)₆] (3) and [Gd₃(hnc)₃(DMF)₆] (4) (H-hni: 2-(hydroxyl-3-methoxy-5-nitrophenyl)methylene(isonictino)hydrazine and H₃-hnc: 1,5-bis(2-hydroxy-3-methoxy-5-nitrobenzylidene)carbonohydrazide). Four complexes were confirmed by infrared spectroscopy and single-crystal X-ray diffraction. The crystal structure of complex 3 reveals that the modified Schiff-base ligand provides two different tridentate coordination pockets (ONN and ONO) to encapsulate Dy^III with a unique N-N pathway. The magnetic properties of all four complexes have been investigated using dc and ac susceptibility measurements. The frequency-dependent ac susceptibility is indicative of single-molecule magnetic behavior without and/or with an optimum dc field with a relaxation barrier U_eff = 34 K (400 Oe), 19 K (0 Oe) and 80 K (0 Oe) for complexes 1, 2 and 3, respectively.

Full text of DOI:10.1039/c9dt02646k

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Mononuclear and trinuclear DyIII SMMs with
Schiff-base ligands modified by nitro-groups:
first triangular complex with a N–N pathway†
Cite this: Dalton Trans., 2019, 48,
17331
Li-Wei Cheng, Chi-Lung Zhang, Jun-Yu Wei and Po-Heng Lin
*
Using two Schiff-base ligands containing an electron-withdrawing group (NO2), we obtained two mono-
nuclear and two trinuclear complexes with the general formula [Dy(hni)(NO3)(DMF)2]·DMF (1·DMF),
[Dy(hni)2(H2O)2]·NO3·EtOH (2·NO3·EtOH), [Dy3(hnc)3(DMF)6] (3) and [Gd3(hnc)3(DMF)6] (4) (H-hni: 2-
(hydroxyl-3-methoxy-5-nitrophenyl)methylene(isonictino)hydrazine and H3-hnc: 1,5-bis(2-hydroxy-3-
methoxy-5-nitrobenzylidene)carbonohydrazide). Four complexes were confirmed by infrared spec-
troscopy and single-crystal X-ray diffraction. The crystal structure of complex 3 reveals that the modified
Schiff-base ligand provides two different tridentate coordination pockets (ONN and ONO) to encapsulate
DyIII with a unique N–N pathway. The magnetic properties of all four complexes have been investigated
using dc and ac susceptibility measurements. The frequency-dependent ac susceptibility is indicative of
single-molecule magnetic behavior without and/or with an optimum dc field with a relaxation barrier
Ueff = 34 K (400 Oe), 19 K (0 Oe) and 80 K (0 Oe) for complexes 1, 2 and 3, respectively.
Received 24th June 2019,
Accepted 30th October 2019
DOI: 10.1039/c9dt02646k
rsc.li/dalton
metal centers are also arranged in a toroidal shape, and has
attracted much interest because of the unique property of
Introduction
Single-molecule magnets (SMMs) display a slow relaxation of the toroidal response of multiferroic materials.6 Since
magnetization at the molecular level, and the molecular nano- [Dy3(HL)3(μ3-OH)(CH3OH)2(H2O)4]
(HL:
(3-hydroxy)-N′-((8-
magnets are characterized by the ability to display a magnetic hydroxyquinolin-2-yl)methylene)picolinohydrazide) was syn-
hysteresis of molecular origin.1 The first pure lanthanide- thesized, displaying a non-magnetic ground state and a toroi-
based SMM was reported by Ishikawa et al. in 2003, which dal magnetic state,7a the fascinating magnetic properties have
indicated that the lanthanide ions were ideal candidates for ignited new interests in exploring larger dysprosium com-
constructing SMMs with a high relaxation barrier due to the pounds with Dy3 building blocks, which retain the functional-
intrinsic strong spin–orbit coupling and large magnetic an- ity and toroidal arrangement of single-ion magnetic anisotropy
isotropy.2 Many different kinds of multinuclear lanthanide axes.7 Recent efforts have also been engaged in the design of
SMMs ranging from mononuclear to Dy60 cluster have been ligands to further understand the magnetic interactions
synthesized to pursue SMMs with high energy barriers.3 In between lanthanide ions of the dysprosium complexes with tri-
addition to large lanthanide complexes, the cyclic architec- angular topology. For example, Layfield, Chilton et al. investi-
tures of the lanthanide centers have also attracted great atten- gated the properties of arsenic-, selenium- and antimony-
tion since these metal complexes might be one of the promis- ligated Dy3 triangles,7g,h and Dunbar et al. successfully syn-
ing molecular materials for future applications in quantum thesized radical-bridged lanthanide metallacyclic triangles.7j
computing, information storage and as multiferroic materials In stark contrast to the organometallic complexes synthesized
with the magnetoelectric effect.4 This new class of magnetic under an inert atmosphere of N2, the air-stable Dy3 triangular
materials with wheel-shaped topology is called Single- complexes were mainly synthesized by Schiff-base ligands. To
Molecule Toroics (SMTs),5 where the magnetic moments of the our knowledge, all the bridges between the two DyIII ions of
these complexes were achieved through the phenoxide or car-
bonyl oxygen atoms of the Schiff-base ligands.
Our recent success in synthesizing multinuclear lanthanide
complexes with carbohydrazide-type ligands inspired us to
Department of Chemistry, National Chung Hsing University, 250 Kuo Kang Rd.,
Taichung 402, Taiwan, Republic of China. E-mail: poheng@dragon.nchu.edu.tw;
Tel: +886 (4) 28840411 ext. 724
explore this chemistry due to the stereoisomers of the
†Electronic supplementary information (ESI) available. CCDC 1897352, 1897353,
ligands.8 The complex may have different pathways by synthe-
1897393 and 1948085. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/C9DT02646K
sizing using cis–trans stereoisomers of the ligand (Scheme 1).
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1439 (m), 1412 (w), 1286 (br), 1200 (m), 1153 (m), 1098 (m),
1062 (m) 1009 (m), 983 (m), 936 (w), 898 (m), 842 (s), 789 (s),
743 (s), 706 (w), 681 (s), 656 (w), 628 (w).
Synthesis of 1,5-bis(2-hydroxy-3-methoxy-5-nitrobenzylidene)
carbonohydrazide (H3-hnc)
Scheme 1 Coordination modes of 1,5-bis(2-hydroxy-3-methoxybenzy-
lidene)carbonohydrazide (H2-hmc, left) and 1,5-bis(2-hydroxy-3-
methoxy-5-nitrobenzylidene)carbonohydrazide (H3-hnc, right).
An ethanol (50 mL) solution of carbohydrazine (0.01 mole,
0.9 g) was mixed with an ethanol (50 mL) solution of 3-meth-
oxyl-5-nitrobenzaldehyde (0.02 mole, 3.94 g). The resulting
solution was stirred and refluxed overnight, yielding a brown
precipitate. The product was filtered, washed with ethyl ether
and dried in vacuo. Yield = 85%. NMR (DMSO-d6, 400 MHz):
11.23 (s, 1H), 8.48 (s, 2H), 7.75 (s, 1H), 3.95 (s, 3H). IR (KBr,
cm−1): 3628 (m), 3525 (m), 3097 (m), 2980 (br), 1716 (s), 1604
(m), 1549 (m), 1513 (m), 1348 (m), 1268 (br), 1214 (m), 1166
(m), 1102 (s), 1074 (m), 1020 (m), 986 (m), 945 (m), 884 (m),
848 (m), 756 (w), 741 (s), 615 (br), 576 (w), 464 (m), 447 (w).
Scheme 2 Synthetic routes for complexes 1–4.
Synthesis of [Dy(hni)(NO3)2(DMF)2]·DMF (1·DMF)
To a solution of Dy(NO3)3·5H2O (0.125 mmol, 0.055 g) in ethyl
acetate (15 mL), a solution of H-hni (0.125 mmol, 0.040 g) in
ethyl acetate (10 mL) and DMF (5 mL) was added. The result-
ing pale dark orange solution yielded dark orange crystals of
the mononuclear complex, [Dy(hni)(NO3)2(DMF)2]·DMF (1) in
6% (0.0061 g) yield after three days. IR (KBr, cm−1): 3054 (br),
1648 (w), 1299 (w), 1106 (s), 1070 (m), 1032 (m), 993 (m), 908
(m), 884 (s), 819 (m), 778 (m), 744 (w), 682 (s), 536 (m), 502
(m). Elem Anal. Calcd for [Dy(hni)(NO3)2(DMF)2]·DMF
(1·DMF): C, 33.89; H, 3.76; N, 15.11. Found: C, 33.65; H, 3.93;
N, 15.35. CCDC 1897352.†
Herein, we introduced an electron-withdrawing group, NO2,
on the para-position of o-vanillin and further reacted with iso-
nicotinohydrazide and carbohydrazine to synthesize 2-(hydroxyl-
3-methoxy-5-nitrophenyl)methylene(isonictino)hydrazine (H-hni)
and 1,5-bis(2-hydroxy-3-methoxy-5-nitrobenzylidene)carbono-
hydrazide (H3-hnc), respectively. The synthesis, X-ray structure
and magnetic properties of the two mononuclear and two tri-
nuclear complexes, [Dy(hni)(NO3)2(DMF)2]·DMF (1·DMF), [Dy
(hni)2(H2O)2]·NO3·EtOH (2·NO3·EtOH), [Dy3(hnc)3(DMF)6] (3)
and [Gd3(hnc)3(DMF)6] (4), were reported. Complex 3 is the
first triangular example in which each DyIII ion was linked by
an unusual N–N bridge, which acted as potential magnetic
pathways (Scheme 2).
Synthesis of [Dy(hni)2(H2O)2]·NO3·EtOH (2·NO3·EtOH)
To a solution of Dy(NO3)3·5H2O (0.125 mmol, 0.055 g) in
ethanol (15 ml), a solution of H-hni (0.125 mmol 0.040 g) and
pyridine (0.020 mL, 0.25 mmol) in ethanol (15 mL) was added.
The resulting pale dark orange solution yielded dark orange
crystals of the mononuclear complex, [Dy(hni)2(H2O)2]·
NO3·EtOH (2·NO3·EtOH) in 16% (0.0193 g) yield after three
days. IR (KBr, cm−1): 3393 (br), 1596 (m), 1493 (m), 1389 (m),
1326 (m), 1104 (s), 1031 (w), 986 (m), 906 (w), 854 (w), 776 (w),
746 (s), 719 (m), 538 (w). Elem Anal. Calcd for [Dy
(hni)2(H2O)2]·NO3·EtOH (2·NO3·EtOH): C, 37.20; H, 3.24; N,
12.90. Found: C, 37.18; H, 3.33; N, 13.01. CCDC 1897353.†
Syntheses and structural
determination
Experimental section
All chemicals except 3-methoxy-5-nitrosalicylaldehyde, which
was prepared as described elsewhere,9 were purchased from
Strem Chemicals or Alfa Aesar and used without further
purification.
Synthesis of 2-(hydroxyl-3-methoxy-5-nitrophenyl)methylene
(isonictino)hydrazine (H-hni)
Synthesis of [Dy3(hnc)3(DMF)6] (3)
To a solution of Dy(NO3)3·5H2O (0.125 mmol, 0.055 g) in MeCN
A
methanol (50 mL) solution of isonicotinohydrazide (15 mL), a solution of H3-hnc (0.125 mmol, 0.056 g) and NEt3
(0.01 mole, 1.37 g) was mixed with a methanol (50 mL) solu- (0.070 mL, 0.5 mmol) in MeCN (5 mL) and DMF (10 mL) was
tion of 3-methoxyl-5-nitrobenzaldehyde (0.01 mole, 1.97 g). added. The resulting pale dark orange solution yielded dark
The resulting solution was stirred and refluxed overnight, orange crystals of the trinuclear complex, [Dy3(hnc)3
yielding a brown precipitate. The product was filtered, washed (DMF)6] (3) in 5.3% (0.015 g) yield after two days. IR (KBr, cm−1):
with ethyl ether and dried in vacuo. Yield = 85%. NMR (DMSO- 3305 (br), 2936 (m), 1653 (s), 1565 (s), 1507 (s), 1296 (br), 1107
d6, 400 MHz): 12.40 (s, 1H), 11.81 (s, 1H), 8.78 (m, 3H), 8.29 (s), 985 (w), 890 (w), 842 (m), 781 (m), 764(m), 675 (w), 595 (m).
(d, 1H), 7.85 (m, 2H), 7.78 (d, 1H), 3.96 (s, 3H). IR (KBr, cm−1): Elem Anal. Calcd for [Dy3(hnc)3(DMF)6]·(3H2O): C, 35.54; H, 3.74;
3393 (br), 3006 (w), 1668 (s), 1606 (w), 1557 (m), 1513 (br), N, 14.21. Found: C, 35.78; H, 3.79; N, 14.51. CCDC 1897393.†
17332 | Dalton Trans., 2019, 48, 17331–17339
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Synthesis of [Gd3(hnc)3(DMF)6] (4)
widely applied for the syntheses of multinuclear lanthanide
complexes.12 Herein, 3-methoxy-5-nitrosalicylaldehyde was pre-
pared from o-vanillin following literature procedures9 and
further reacted with isonicotinic acid hydrazide to synthesize
the ligand, H-hni, without changing the chelating sites.
However, only the mononuclear complexes were obtained
using the H-hni ligand with different reaction conditions. For
this reason, we changed isonicotinohydrazide into carbohydra-
zide for the condensation reaction in order to modify the
Schiff-base ligand with more potential coordination pockets,
and H3-hnc was obtained for further metal complex syntheses
(Scheme 3).
To a solution of Gd(NO3)3·5H2O (0.125 mmol, 0.0533 g) in
MeCN (15 mL), a solution of H3-hnc (0.125 mmol, 0.056 g) and
NEt3 (0.070 mL, 0.5 mmol) in MeCN (5 mL) and DMF (10 mL)
was added. The resulting pale dark orange solution yielded
dark orange crystals of the trinuclear complex, [Gd3(hnc)3(DMF)6]
(3) in 12% (0.032 g) yield after two days. IR (KBr, cm−1): 3305
(br), 2931 (m), 1651 (s), 1562 (s), 1485 (s), 1275 (br), 1103 (s),
984 (w), 887 (w), 838 (m), 779 (m), 744 (m), 668 (w), 590 (m).
Elem Anal. Calcd for [Gd3(hnc)3(DMF)6]·(5H2O): C, 35.47; H, 3.93;
N, 14.39. Found: C, 35.14; H, 3.53; N, 14.23. CCDC 1948085.†
X-ray crystallographic studies
Single crystals of complexes 1–4 suitable for X-ray diffraction
measurements were mounted on the Bruker D8 VENTURE.
The unit cell was determined using the Bruker SMART APEX 3
software suite to employ graphite-monochromated Mo-Kα radi-
ation (λ = 0.71073 Å), and intensity data were collected with ω
scans. The data collection and reduction were performed with
the CrysAlisPro software,10 and the absorptions were corrected
by the SCALE3 ABSPACK multiscan method.11 The space group
determination is based on the symmetry and the systematic
absences displayed in the diffraction pattern. The structure
was solved and refined with the Olex2 1.2-ac21 package.
Anisotropic thermal parameters were used for all non-H
atoms, and fixed isotropic parameters were used for H atoms.
Structural description of complexes 1–3
The obtained [Dy(hni)(NO3)2(DMF)2]·DMF (1·DMF) crystallizes
in the monoclinic P21/n space group. A partially labelled X-ray
structure of 1 is shown in Fig. 1 and the X-ray information
(including cell parameters) is given in Table S1.† Selected
bond distances and angles are given in Table S2.† The DyIII
ion of the mononuclear complex adopts a nine-coordinate
spherical tricapped trigonal prism as determined by SHAPE
2.1,13 where one ligand, two nitrate molecules and two co-
ordinated DMF solvent molecules occupy the coordination
sites (Fig. S1a and Table S5†). The polydentate Schiff-base
ligand coordinates to the Dy center via two O atoms (O1 and
O2) and one N atom (N3). Charge balance considerations for
the molecule indicate that the ligand must have one negative
charge resulting from one deprotonated O2 atom and one
Magnetic measurement
Direct current (dc) magnetic susceptibility measurements were per-
formed on a Quantum Design MPMS7 magnetometer equipped
with a 7.0 T magnet, operating in the range of 2.0–300.0 K.
Alternating current (ac) susceptibility measurements were carried
out on a Quantum Design PPMS-9 magnetometer equipped with a
9.0 T magnet and operating in the range of 1.8–300 K. Ac frequen-
cies ranged from 100 to 10 000 Hz and temperatures ranged from
1.8 to 30 K. Diamagnetic corrections were estimated from Pascal’s
constant and subtracted from the experimental susceptibility data to
determine the molar paramagnetic susceptibility of the compound.
Results and discussion
Ligand design
Polydentate Schiff-base ligands derived from o-vanillin, which
Fig. 1 Molecular structure of complex 1. Color code: Dy (yellow), O
(red), N (blue) and C (gray). H atoms were omitted for clarity.
provided O,N,O,O-based multi-chelating sites, have been
Scheme 3 Synthesis of two ligands for complex formation.
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double bond between C6 and O1. In comparison with the The coordination modes of the three ligands between any two
other reported mononuclear complex, [Dy(hmb)(NO3)2(DMF)2] DyIII ions are all the same, where the two DyIII ions were held
(Hhmb:
(N′-(2-hydroxy-3-methoxybenzylidene)benzohydra- together by μ-κ3:κ3-hnc3−. In contrast to the literature com-
zide),14 it is notable that the nitro-functional group at the para plexes with similar ligands,8b the carbohydrazine moieties
position of phenolate in hni− affected the coordination were all shown in trans–trans conformations, which were
environments of DyIII. The two nitrate anions are perpendicu- switched to cis–trans conformations in complex 3, leading to
lar in complex 1, but vertical in the complex reported in the lit- the N–N pathway (Fig. 3b) (N2–N3, N2a–N3a and N2b–N3b
erature (Fig. S2†).
between Dy1–Dy1a, Dy1a–Dy1b and Dy1b–Dy1, respectively).
In order to obtain various structures with the same ligand, For this reason, the three intramolecular Dy⋯Dy distances
different reaction conditions have been applied for the synth- are 5.862 Å (Fig. 3c), which are longer than other literature
eses. A suspension of Dy(NO3)3·5H2O (0.5 equiv.) and H-hni triangular complexes with Schiff-base ligands7a–e with
(0.5 equiv.) in EtOH (30 mL) was treated with 1 equiv. of pyri- Dy⋯Dy⋯Dy angles close to 60.0°, giving a nearly perfect equi-
dine. The resulting yellow solution yielded rectangular, light lateral triangle. Moreover, N3, N3a and N3b are also settled on
yellow crystals of another mononuclear complex [Dy the triangular plane, where the three O, N, N coordination
(hni)2(H2O)2]·NO3·EtOH (2·NO3·EtOH) in 16% yield after one pockets of the three ligands are all above the plane (Fig. 3d).
week. No crystals were obtained without EtOH since the EtOH
Magnetic properties
solvent molecule was crystallized in the structure as a co-
solvent. Complex 2 crystallized in the monoclinic P2/C space Direct current (dc) magnetic measurements were performed
group. The structure is composed of an eight coordinated between 1.8 and 300 K, with an applied dc field of 1000 Oe.
spherical triangular dodecahedron DyIII ion, where two ligands The temperature dependence of the χT product can be
and two coordinated H2O solvent molecules (O6 and O6A) observed in Fig. 4. The room temperature values of the χT pro-
occupy the coordination sites (Fig. S1b and Table S5† shape). ducts for complexes 1, 2, 3 and 4 were 13.96, 14.69, 40.23 and
The ligands coordinated with DyIII in the same coordination 21.89 cm3 K mol−1, respectively. The free ion approximation
modes as complex 1 as the keto-form of hni−. Furthermore, for a single DyIII ion is 14.17 cm3 K mol−1 (6H15/2, S = 5/2, L =
−
owing to the necessity for charge balance, the NO3 ion was 5, g = 4/3), and it is 7.88 cm3 K mol−1 (8S7/2, S = 7/2, L = 0, g =
also present within the lattice structure (Fig. 2).
2) for a single GdIII ion. Thus, the free ion approximations are
The single X-Ray crystallography study reveals that com- 14.17, 42.51 and 23.64 cm3 K mol−1 for the mononuclear, tri-
plexes 3 and 4 are isostructural. As an example, the structure nuclear dysprosium complexes and the trinuclear gadolinium
of the dysprosium analogue, complex 3, will be described. The complex, respectively, which are all near the experimentally
reaction of Dy(NO3)3·5H2O with H3-hnc in DMF/MeCN in the observed values. The χT product displays a gradual decrease
presence of NEt3 produces yellow crystals of 3 after 2 days. with temperature for complexes 1 and 2, which could be
Single crystal X-ray studies reveal that 3 crystallized in the related to the inherent magnetic anisotropy from the DyIII
cubic space group P . A perspective view of the molecular ions, the Stark level depopulation and/or the occurrence of
ˉ
a3
structure of 3 is represented in Fig. 3a, and the details for the SMM behavior. A similar thermal behavior is seen in complex
structure solution and refinement are summarized in 3 as the χT value slowly decreases over decreasing temperature
Table S1.† Each DyIII ion of 3 was bonded to two H3-hnc until 100 K, upon which the χT value rapidly decreases to a
ligands and two DMF terminal solvents, leading to an eight co- minimum of 27.73 cm3 K mol−1 at 1.8 K. The decrease in the
ordinated triangular dodecahedron (Fig. S1c and Table S5†). χT product down from room temperature is more likely attribu-
ted to a combination of potential causes (i.e., weak antiferro-
magnetic interactions between the DyIII ions, namely, the
inherent magnetic anisotropy from DyIII ions and/or depopula-
tion of excited states). The magnetic interaction between the
two lanthanide ions can be understood by the gadolinium ana-
logue. Indeed, the GdIII ions present no spin–orbit coupling at
the first order. Thus, the decrease in the χT value to a
minimum of 18.99 cm3 K mol−1 at 1.8 K for complex 4 directly
reveals the presence of a weak antiferromagnetic interaction
between the GdIII ions.
The isotherm magnetization (M) measurements (Fig. S7–
S10† for complexes 1 and 2, respectively) below 8 K reveal a
rapid increase in magnetization at low magnetic fields. Above
20 000 Oe, a more gradual increase is observed for three com-
plexes, reaching near saturation at low temperatures (M = 4.50,
5.44 and 12.96μB at 2 K under 70 000 Oe, respectively). This be-
Fig. 2 Molecular structure of complex 2. Color code: Dy (yellow), O
havior displays the presence of magnetic anisotropy within the
system, which is expected for DyIII. This is further confirmed
(red), N (blue) and C (gray). H atoms and counter ion (NO3) were omitted
for clarity.
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Fig. 3 (a) Partially labelled molecular structure of [Dy3(hnc)3(DMF)6](3). (b) Simplified representation of the structure of [Dy3] emphasizing one of
the hnc3− ligand in black. (c) The central core of complex 3 with Dy–Dy distances. (d) Side view of crystal structures of 3 with the plane through
three DyIII ions. The ligands are depicted in different colors to show the coordination modes. Color code: Dy (yellow), O (red), N (blue) and C (gray).
H atoms were omitted for clarity. DMF molecules were omitted for clarity in (d).
To investigate the magnetic relaxation dynamics of these
three complexes, alternating current (ac) magnetic suscepti-
bility measurements were performed under a zero direct
current (dc) field at various frequencies (100–10 000 Hz)
between 30 and 1.8 K. A frequency dependence tail of a peak is
observed in the out-of-phase signal, χ″, below 11 K for complex
1 (Fig. 5a), indicating potential SMM behavior at very low
temperature. However, it is difficult to quantify the energy
barrier without a full peak with maxima. Such behavior gen-
erally indicates that the slow relaxation of the magnetization
is highly influenced by the quantum tunneling of the magne-
tization (QTM) through the spin reversal barrier, which was
also observed in mononuclear SMMs reported in the litera-
ture with similar coordination environments. For this reason,
ac measurements were also carried out under an optimum dc
field of 400 Oe for complex 1 (Fig. 5b), which revealed a fre-
Fig. 4 Temperature dependence of the χT product of complexes 1
(red), 2 (blue), 3 (black) and 4 (green) at 1000 Oe.
quency-dependent signal with a clear out-of-phase (χ″) peak.
The thermally activated relaxation follows an Arrhenius-like
behavior (τ = τ0 exp(Ueff/kT)), where the anisotropic energy
barrier is calculated to be Ueff = 34 K, τ0 = 3.6 × 10−7
(Fig. S27†).
s
in Fig. S8 and S10,† where the nonsaturation and non-super-
Dynamic magnetic measurements were also carried out on
position of the isotemperature lines were apparent in the complex 2, and a frequency dependence of χ″ without applying
reduced magnetization plots at the indicated temperatures for the dc field was observed in Fig. 5c. The effective energy
both complexes.
barrier and relaxation time were obtained through fitting the
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Fig. 5 Frequency dependence of the out-of-phase (χ’’) susceptibility under zero (left) and applied dc field (right) for complexes 1 (a, b (400 Oe)) and
2 (c, d (800 Oe)).
data using the Arrhenius equation, which elicited a value of which is similar to that in other reported DyIII compounds.15
Ueff = 19 K with τ0 = 3.8 × 10−7 s, as can be seen in Fig. S28 This tail in the low temperature region mostly indicates the
(inset),† which is larger than that of complex 1. We also existence of a slower QTM process, where the thermally acti-
carried out ac measurements under various dc fields to deter- vated Orbach relaxation process occurs prominently.16
mine the optimum field (Fig. S19†). The broad peaks were
In order to probe the feasibility of lowering the relaxation
observed under 200 and 400 Oe, which may indicate the mul- probability via the quantum pathway, the ac susceptibility was
tiple exchange interactions with tunneling electrons. measured at 10 K and applied dc magnetic field up to 5000 Oe
Furthermore, the peaks were shifted to the left and a tail of the (Fig. S23†). Since the addition of a static field was found to
peak was observed in the 600–5000 Oe range. In order to truly affect the relaxation dynamics of 3 at 1.8 K, where the
shortcut the QTM, the ac susceptibility measurements were peaks were shifted to the left and a tail of the peak was
also measured under 800 Oe (Fig. 5d). Indeed, the relaxations observed in the 100–10 000 Hz range, the ac susceptibility with
became slower with higher energy barriers in terms of the various static fields was also studied at 10 K and the optimum
same processes at zero dc field, especially the low temperature dc field was found at 2600 Oe (Fig. S23†). An out-of-phase
relaxation with a value of Ueff = 41 K with τ0 = 1.1 × 10−8 s.
signal, with a shifting of the peak maxima was observed for
The dynamic behavior was also investigated without an the resulting data above 7 K (Fig. 6b), and afforded an energy
applied dc field between 30 and 1.8 K for complex 3. The out- barrier of 81 K, with τ0 = 8.0 × 10−8 s (Fig. S31†). The graphical
of-phase (χ″) susceptibilities show significant frequency depen- representation of χ″ vs. χ′ (Cole–Cole plot) in the temperature
dence peaks at a relatively high temperature range above 6 K, range of 6–13 K and 5–15 K with 0 Oe and 2600 Oe, respect-
which clearly indicates that the slow relaxation of magnetiza- ively, further confirms the single relaxation process (Fig. S32
tion arises from the SMM properties. Therefore, the whole and S33†).
temperature dependence could be modelled with the following
Additionally, the energy barrier of complex 3 was compared
equation: τ−1 = τ0−1 exp(−Ueff/kT) + CTn + τ−1QTM. The effective with other DyIII triangular complexes (Table 1), which is higher
energy barrier and relaxation time were obtained through than those of other complexes with the phenoxide bridge
fitting the data, which calculated a value of 80 K with τ0 = 8 × between each DyIII ions (complexes a–g). The energy barrier of
10−8 s. The other parameters can be found in Fig. S30.† complex 3 was close to those of complexes h–j, but much
Remarkably, the increasing of χ″ below 5 K is indicative of the lower than the energy barriers of the SMMs with phosphide-,
other relaxation region at a zero dc field. Indeed, the out-of- arsenide- and antimony-bridges (complexes k–n). Indeed, the
phase peak tails at <5 K could be observed more clearly in the use of ligands with heavier p-block elements could enhance
temperature dependence of the out-of-phase plots (Fig. S26†), the SMM properties because of the influence of the pnictogen
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Fig. 6 Frequency dependence of the out-of-phase (χ’’) susceptibility under zero (a) and 2600 Oe (b) dc field for complex 3.
Table 1 The energy barriers for DyIII triangular clusters
Complexes
Formula
Dy–Dy distances (Å)
Energy barrier (applied dc field)
3
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
[Dy3(hnc)3(DMF)6]
5.862, 5.862, 5.862
3.535, 3.530, 3.527
3.499, 3.546, 3.547
3.521, 3.529, 3.530
3.956, 3.957, 3.966
3.943, 3.946, 3.977
3.517, 3.521, 3.546
3.514, 3.526, 3.532
3.476, 3.507, 3.905
7.996, 8.066, 8.096
7.544, 7.555, 7.581
5.717, 5.811, 5.854
5.717, 5.786, 5.829
5.523, 5.443, 5.536
5.312, 5.392, 5.408
5.120, 5.132, 5.157
5.257, 5.274, 5.301
80 K (0 Oe), 81 K (2600 Oe)
37 K (300 Oe)
0.5 K (0 Oe)
[Dy3LRRRRRR(μ3-OH)2(H2O)2(SCN)4] 7b
[Dy3L(μ3-OH)2(NO3)2(H2O)4]7c
[Dy3L(μ3-OH)2(SCN)4(H2O)2]·3MeOH 7c
[Dy3(HL)3(μ3-OH)(CH3OH)3(H2O)2Cl]7a
[Dy3(HL)3(μ3-OH)(CH3OH)2(H2O)4] 7a
[Dy3(μ3-OH)2(Hpovh−)3(NO3)3(CH3OH)2H2O]7d
[Dy3(μ3-OH)2(H2vovh−)3Cl2(CH3OH)(H2O)3]7d
[Dy3(HL)(H2L)(NO3)4]7e
16 K (500 Oe)
34.3 K (1000 Oe)
7.6 K and 45.9 K (0 Oe)
6 K and 53.8 K (1000 Oe)
21.7 K (0 Oe), 82.2 K (1000 Oe)
42.6 K and 90.9 K (0 Oe)
29.6 K and 61.6 K (0 Oe)
60 K (0 Oe)
496.4 K (0 Oe)
391.3 K (0 Oe)
368.3 K (0 Oe)
302.1 K (0 Oe)
{(HAT-μ3)[Dy(tmh)3]3}7f
[Dy3(hfac)6(bptz•−)3]7j
[(η5-Cp′2Dy){μ-Sb(H)Mes}]3
7g
[(η5-Cp′2Dy)3{μ-(SbMes)3Sb}]7g
[(η5-Cp′2Dy){μ-As(H)Mes}]3
7h
7i
[(Cp′2Dy){μ-P(H)Mes}]3
[Li(thf)4]2[(Cp′2Dy)3(μ-PMes)3Li]7i
18.7 K (0 Oe)
33.1 K (0 Oe)
[Li(thf)4]2[(η5-Cp′2Dy)3(μ3-AsMes)3Li]7h
on the DyIII crystal field levels. However, it was notable that the software,17 and the partial charges assigned to the formally
anisotropy axes of complexes h–n were not coplanar with the charged ligands are shown in Fig. S34.† The anisotropy axes of
Dy3 plane, as well as the longer DyIII–DyIII distances, and was complex 3 for the Dy1, Dy1a, and Dy1b ions formed angles
also observed in complex 3 (Fig. 7). We employed electrostatic with the Dy3 plane of 74.66°, 74.49° and 74.53°, respectively,
modelling in order to investigate the orientations of the mag- relative to the Dy3 plane that prevented this triangular
netic anisotropy axes for complex 3 using Magellan magnetic complex from behaving as a SMT.
Fig. 7 Trinuclear core structures of complex 3 showing the direction of the anisotropy axes (green dashed line) for all DyIII ions (side view: left, top
view: right). Color code: Dy (yellow), O (red), N (blue) and C (gray). H atoms were omitted for clarity.
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Conclusions
In conclusion, two mononuclear complexes, [Dy(hni)
(NO3)2(DMF)2]·DMF (1·DMF) and [Dy(hni)2(H2O)2]·NO3·EtOH
(2·NO3·EtOH),
and
two
trinuclear
complexes,
[Dy3(hnc)3(DMF)6] (3) and [Gd3(hnc)3(DMF)6] (4), were pre-
pared from H-hni and H3-hnc ligands. The slow relaxation of
the magnetisation under an optimum field in complexes 1 and
2 confirm the single-ion magnet nature as well as complex 3 is
an SMM with energy barriers in the region of 80 K (0 Oe) and
81 K (2600 Oe), respectively. It is notable that the nitro- func-
tional group affected the coordination environments of the
DyIII ions in both complexes 1 and 2. It is also vital that
complex 3 is the first example of a trinuclear dysprosium
complex with an N–N pathway that is different with the
μ-phenoxide bridges between the lanthanide ions of other tri-
angular coordination complexes. Indeed, the electron-with-
drawing group, NO2, was introduced at the para-position,
which is away from the O,N,O,O-based multi-chelating sites.
However, totally different coordination geometries or ligand
conformations were observed in complexes 1–4 in comparison
with other complexes synthesized by Schiff-base ligands with
the same coordination pockets. This novel idea of ligand
modification can provide rich and versatile coordination
chemistry concepts for future designs of multinuclear lantha-
nide complexes.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by National Chung Hsing University
and the financial support from the Ministry of Science and
Technology, Taiwan (MOST107-2113 M-005-003 and 108-2113
M-005-023). We also thank Prof. Hui-Lien Tsai from National
Cheng Kung University for assistance with magnetic measure-
ments and Prof. Jérôme Long from University of Montpellier
for assistance with the fitting data (including the Raman
process and QTM).
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