Dinuclear Ln(III) complexes constructed from an 8-hydroxyquinoline Schiff base derivative with different terminal groups show differing slow magnetic relaxation

Article abstract of DOI:10.1039/c6nj04035g

Two series of dinuclear Ln^III complexes, namely, [Ln(tfa)₂(L)]₂ (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5)) and [Ln(tfa)₂(L′)]₂ (Ln = Gd (6), Tb (7), Dy (8), Ho (9), Er (10)) (tfa = trifluoroacetylacetonate, HL = 2-[[(4-methylphenyl)imino]methyl]-8-hydroxyquinoline and HL′ = 2-[[(4-ethylphenyl)imino]methyl]-8-hydroxyquinoline), were synthesized, and structurally and magnetically characterized. Measurements of alternating-current (ac) susceptibility revealed that complexes 3 and 8 display significant zero-field single-molecule magnetic (SMM) behaviors with barrier energies of U_eff/k_B = 9.14 K, τ₀ = 4.76 × 10^-6 s and U_eff/k_B = 17.50 K, τ₀ = 3.96 × 10^-6 s, respectively. The magnetic studies also revealed that 1 and 6 feature a magnetocaloric effect with magnetic entropy change of -ΔS_m values for 1 and 6 of 19.72 J kg^-1 K^-1 and 19.37 J kg^-1 K^-1 at T = 2 K and ΔH = 7 T, respectively.

Full text of DOI:10.1039/c6nj04035g

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Dinuclear Ln(III) complexes constructed from an
8-hydroxyquinoline Schiff base derivative with
different terminal groups show differing slow
magnetic relaxation†
Cite this:DOI: 10.1039/c6nj04035g
a
Yi-Xin Chang,
Xiao-Pu Zhou,
Wen-Min Wang,a Ru-Xia Zhang,a Hai-Yun Shen,a
a
a
ab
Ni-Ni Wang,
Jian-Zhong Cui
*
and Hong-Ling Gao*ab
Two series of dinuclear LnIII complexes, namely, [Ln(tfa)2(L)]2 (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5)) and
[Ln(tfa)2(L0)]2 (Ln = Gd (6), Tb (7), Dy (8), Ho (9), Er (10)) (tfa = trifluoroacetylacetonate, HL = 2-[[(4-methyl-
phenyl)imino]methyl]-8-hydroxyquinoline and HL0 = 2-[[(4-ethylphenyl)imino]methyl]-8-hydroxyquinoline),
were synthesized, and structurally and magnetically characterized. Measurements of alternating-current
(ac) susceptibility revealed that complexes 3 and 8 display significant zero-field single-molecule magnetic
(SMM) behaviors with barrier energies of Ueff/kB = 9.14 K, t0 = 4.76 ꢀ 10ꢁ6 s and Ueff/kB = 17.50 K,
t0 = 3.96 ꢀ 10ꢁ6 s, respectively. The magnetic studies also revealed that 1 and 6 feature a magnetocaloric
effect with magnetic entropy change of ꢁDSm values for 1 and 6 of 19.72 J kgꢁ1 Kꢁ1 and 19.37 J kgꢁ1 Kꢁ1
at T = 2 K and DH = 7 T, respectively.
Received 22nd December 2016,
Accepted 4th May 2017
DOI: 10.1039/c6nj04035g
rsc.li/njc
effective anisotropic barriers versus energy barriers arising from
Introduction
the two interaction spin entity.10,11 If feasible approaches to
Single-molecule magnets (SMMs) exhibiting slow magnetic rationally enhance the energy barrier of magnetic relaxations in
relaxation have attracted recent research efforts because of {Dy2} SMM systems could be established, there is potential for
their potential for use in high-density information storage, construction of larger polynuclear dysprosium complexes with
molecular spintronics, quantum computing devices, and magnetic better SMM properties, via a bottom-up approach. Many factors
refrigeration.1–5 The Kramers ion, dysprosium(III), with its signifi- including single-ion anisotropy, local coordination geometry,
cant magnetic anisotropy and large magnetic moment, combined ligand field effects, the strength of the magnetic interaction
with the fact that its ground states will always be bistable between lanthanide ions etc., can influence the magnetic relaxa-
irrespective of the ligand-field symmetry, is the most attractive tion properties of lanthanide-based SMMs, making under-
and widely used species for construction of SMMs with high standing of the magnetic structure of lanthanides complex.12
energy barriers, which are not easily satisfied in transition metal- Therefore, it is necessary to differentiate the effects of the single
based SMMs.6 Considerable efforts are being devoted towards parameters. There have been many reports that the ligand field
the search for novel dysprosium species and also the promotion perturbation affects the structures of LnIII complexes and their
of the development of polynuclear dysprosium-based SMMs,7,8 magnetic relaxation behaviors.13,14 Hence, the effective barrier of
and some fascinating results have been reported, such as two- an SMM can be further improved by tuning or optimizing the
step relaxation in a linear {Dy4} architecture and a record energy surrounding ligand field of the anisotropic axis.15 Notably, many
barrier of 721 K in a hydroxide-bridged {Dy2} system.9 Among the recent reports illustrate the effect of electron withdrawal groups
various polynuclear dysprosium complexes, the dinuclear SMM on terminally coordinated ligands capable of fine tuning the
systems are an ideal platform on which to study single-ion anisotropic barrier. In this paper, two dinuclear DyIII complexes
were synthesized, which display outstanding and interesting
a Department of Chemistry, Tianjin University, Tianjin, 300072, China.
magnetic behaviors with different effective anisotropy barriers
and exchange coupled interactions.
E-mail: cuijianzhong@tju.edu.cn, ghl@tju.edu.cn
b State Key Laboratory of Medicinal Chemical Biology (NanKai University), Tianjin,
300071, China
LnIII complexes not only show unique superiority in SMMs
but also in magnetocaloric effect (MCE). The MCE is based on
the change of magnetic entropy upon application of a magnetic
field and is of great technological importance as it can be used
for cooling applications according to a process known as
† Electronic supplementary information (ESI) available. CCDC 1481074 (1),
1481071 (2), 1481070 (3), 1481072 (4), 1481073 (5), 1491554 (6), 1491556 (7),
1491552 (8), 1491553 (9) and 1491555 (10). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c6nj04035g
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according to a previously reported method,32 as were the
8-hydroxyquinoline Schiff base ligands HL and HL0.33
Elemental analyses for C, H and N were carried out on a
PerkinElmer 240 CHN elemental analyzer. Infrared spectra (IR)
of the complexes in KBr pellets were obtained using a Bruker
Tensor 27 IR spectrometer in the range of 4000–400 cmꢁ1
region. UV-vis spectra were performed on a TU-1901 spectro-
photometer at room temperature. Fluorescence spectra were
taken on a Cary Eclipse fluorescence spectrophotometer sup-
plied by Varian (USA) at room temperature. Powder X-ray
diffraction (PXRD) data were performed on a Rigaka D/max
2500v/pc X-ray powder diffractometer with Cu-Ka radiation
(l = 1.540598 Å), with a scan speed of 51 minꢁ1 in the range
2y = 5–501. Thermal gravimetric analyses (TGA) were carried out
on a NETZSCH TG 209 instrument under air atmosphere from
30 to 800 1C, with a heating rate of 10 1C minꢁ1. The magnetic
measurements were carried out using a Quantum Design
MPMS-XL7 and a PPMS-9 ACMS magnetometer. The diamagnetic
corrections for the complexes were estimated using Pascal’s
constants and magnetic data were corrected for diamagnetic
contributions of the sample holder.34
Scheme 1 Structures of 2-[[(4-methylphenyl)imino]methyl]-8-hydroxy-
quinoline (HL) (R = CH3) and 2-[[(4-ethylphenyl)imino]methyl]-8-hydroxy-
quinoline (HL0) (R = C2H5).
adiabatic demagnetization.16–18 Magnetic refrigeration con-
stitutes one of the potential applications envisioned for poly-
metallic molecules.19,20 To construct molecular clusters with
high MCE, GdIII with seven unpaired 4f electrons was preferentially
selected as the intrinsic nature of GdIII fulfils the requirements of
improving MCEs well, such as having a large spin ground state S,
negligible magnetic anisotropy, and low-lying excited spin
states.21,24 There is further support from reports of Gd-containing
clusters;25–31 however, investigations of MCE of pure Gd-clusters21,28
are rare. Additionally, various structures of discrete Gd-clusters have
been observed, such as [{Gd(OAc)3–(H2O)2}2]ꢂ4H2O,31a {Gd48},31c
[Gd(OH)CO3]n,22 GdF3,23 Gd60,31d and Gd10431e discrete Gd-clusters.
Among them, magnetic studies have revealed an extremely large
DSm for GdF3 up to 528 mJ cmꢁ3 Kꢁ1 for DmoH = 9 T.23 After just
Synthesis of 2-[[(4-methylphenyl)imino]methyl]-8-hydroxy-
quinoline (HL). The HL was synthesized by a simple aldimine
condensation reaction involving 8-hydroxyquinoline-2-carbox-
aldehyde (20.0 mmol) and 4-methylaniline (20.0 mmol) in
methanol. After about 4 h, the crude product was obtained as
a light yellow powder. Then, the resulting precipitate was
washed with methanol and dried in vacuo to give the HL as a
yellow solid. Purification by silica gel chromatography (dichloro-
methane/petroleum ether = 1 : 3) afforded the desired product
HL. The yield of the yellow block product was 3.4 g, 62%.
Elemental analysis (%) calcd for C17H14N2O (262): C 78.1, H
5.2, N 10.6. Found: C 77.9, H 5.3, N 10.7. IR (KBr, cmꢁ1): 3345
(w), 1562 (s), 1511 (s), 1433 (m), 1392 (m), 1374 (m), 1246 (s),
half a year, Gd60 as a good sample has large MCE with ꢁDSmmax
=
66.5 J kgꢁ1 Kꢁ1 31d There are no reports of Gd-cluster complexes
.
with higher nuclearity than Gd104 because of severe challenges in
synthesizing this type of cluster. Inspired by this, the present
study aims to explore the influences of MCE using two GdIII
complexes.
Therefore, to deeply understand the relationship between
the structures and magnetic properties of SMMs and MCE,13 we
decided to use two 8-hydroxyquinoline Schiff bases with different
alkyl (–CH3 and –C2H5) substituents (Scheme 1) to construct
LnIII-based SMMs. Herein, 10 dinuclear LnIII complexes,
[Ln(tfa)2(L)]2 (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5)) and
[Ln(tfa)2(L0)]2 (Ln = Gd (6), Tb (7), Dy (8), Ho (9), Er (10)) (tfa =
1
1076 (s), 950 (m), 819 (s), 664 (m), 542 (w). H NMR data for
HL (400 MHz, DMSO-d6): d 8.79 (s, 1H), d 8.25–8.43 (dd, 2H),
d 7.46–7.53 (dd, 2H), d 7.28–7.34 (dd, 4H), d 7.16–7.18 (d, 1H),
d 2.36 (s, 3H).
trifluoroacetylacetonate, HL
=
2-[[(4-methylphenyl)imino]-
methyl]-8-hydroxy-quinoline and HL0 = 2-[[(4-ethylphenyl)imino]-
methyl]-8-hydroxyquinoline) were synthesized, then structurally
and magnetically characterized. Magnetic studies revealed that
complexes 3 and 8 exhibit different magnetic relaxation beha-
viors, with anisotropic barriers of 9.14 K (3) and 17.50 K (8),
respectively. The different SMM behaviors of the two Dy2 com-
plexes can be ascribed to the different electron-donating effects
induced by the two alkyl groups (–CH3 and –C2H5) of the
8-hydroxyquinoline Schiff base ligands, which further influence
the chemical environment of DyIII ions.
Synthesis of 2-[[(4-ethylphenyl)imino]methyl]-8-hydroxyquinoline
(HL0). 2-[[(4-ethylphenyl)imino]methyl]-8-hydroxyquinoline (HL0)
was synthesized by a similar method to synthesis of HL, but
using 4-ethylaniline instead of 4-methylaniline. The precipitate
was obtained after filtration, washed with methanol, and dried
in vacuo to give the HL0 ligand as a yellow solid. Purification by
silica gel chromatography (dichloromethane/petroleum ether =
1 : 3) afforded the desired product HL0. The yield of the yellow
block product was 3.2 g, 55%. Elemental analysis (%) calcd for
C18H16N2O (276): C 78.3, H 6.8, N 10.1. Found: C 77.9, H 6.8,
N 9.9. IR (KBr, cmꢁ1): 2959 (w), 2865 (w), 1565 (s), 1506 (s),
1463 (m), 1432 (m), 1373 (s), 1333 (s), 1272 (m), 1244 (m),
1084 (s), 843 (s), 750 (s), 716 (w), 605 (w), 547 (w). 1H NMR data
for HL0 (400 MHz, DMSO-d6): d 8.80 (s, 1H), d 8.26–8.44 (dd, 2H),
d 7.48–7.54 (dd, 2H), d 7.34–7.37 (dd, 4H), d 7.16–7.18 (d, 1H),
Experimental section
Materials and methods
All the chemicals for synthesis were of analytical grade and d 2.40–2.42 (q, 2H), d 1.04–1.09 (t, 3H).
were used without further purification. The reacting salts
Synthesis of [Ln(tfa)2L]2. (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er
Ln(tfa)3ꢂ2H2O (Ln = Gd, Tb, Dy, Ho and Er) were synthesized (5)). All five of the complexes were synthesized by the same method.
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The synthesis of complex 1 is detailed herein. Gd(tfa)3ꢂ2H2O N 4.08. Found: C 45.01, H 3.08, N 3.75. IR (KBr, cmꢁ1): 2966 (w),
(0.01638 g, 0.025 mmol) was dissolved in 20 mL of dry boiling 1632 (s), 1523 (m), 1462 (s), 1368 (m), 1296 (s), 1186 (s), 1137 (s),
n-heptane. A 5 mL CH2Cl2 solution of HL (0.0065 g, 0.025 mmol) 955 (w), 908 (w), 843 (m), 775 (m), 558 (m), 405 (w).
was added, and the mixture was heated for 30 min at around 70 1C.
[Er(tfa)2L0]2 (10). Yield: 61% (based on Er). Elemental analysis
Red crystals of 1 suitable for X-ray analysis were isolated by cooling (%) calcd for C56H46Er2F12N4O10 (1497.53): C 45.24, H 3.16, N
the solution to room temperature and keeping the filtrate in a 3.91. Found: C 44.87, H 3.07, N 3.74. IR (KBr, cmꢁ1): 2966 (w),
refrigerator at 4 1C for a few days.
1632 (s), 1525 (m), 1462 (s), 1368 (m), 1296 (s), 1186 (s), 1137 (s),
[Gd(tfa)2L]2 (1). Yield: 47% (based on Gd). Elemental analysis 955 (w), 907 (w), 842 (m), 775 (m), 558 (m), 502 (w), 405 (w).
(%) calcd for C54H42Gd2F12N4O10 (1449.42): C, 44.96; H, 2.77; N,
4.21. Found: C, 44.71; H, 2.90; N, 3.86. IR (KBr, cmꢁ1): 2922 (w),
Single-crystal X-ray structure determination
1633 (s), 1590 (m), 1517 (m), 1460 (s), 1369 (m), 1292 (s), 1185
(m), 1135 (s), 908 (w), 843 (m), 753 (m), 557 (m), 406 (w).
Single-crystal X-ray diffraction data of 1–10 were collected at a
temperature of 113 K on a computer-controlled Rigaku Saturn
CCD area detector diffractometer equipped with confocal mono-
chromatized Mo-Ka radiation with a radiation wavelength of
[Tb(tfa)2L]2 (2). Yield: 54% (based on Tb). Elemental analysis
(%) calcd for C54H42Tb2F12N4O10 (1452.76): C 44.82, H 2.73, N
3.76. Found: C 44.60, H 2.89, N 3.85. IR (KBr, cmꢁ1): 3038 (w),
0.71073 Å using the o–j scan technique. The structures were
1632 (s), 1521 (m), 1460 (s), 1367 (m), 1292 (s), 1185 (m), 1136
solved by direct methods using SHELXS-97 and refined by full
(s), 982 (w), 907 (w), 844 (m), 755 (m), 557 (m), 408 (w).
matrix least-squares techniques on F2 with SHELX-97.35 Aniso-
tropic thermal parameters were assigned to all non-hydrogen
atoms. Crystallographic data parameters for 1–10 are given in
Tables 1 and 2. The important bond angles and lengths for 1–10
are given in Table S1 (ESI†). CCDC 1481074 (1), 1481071 (2),
1481070 (3), 1481072 (4), 1481073 (5), 1491554 (6), 1491556 (7),
1491552 (8), 1491553 (9) and 1491555 (10).
[Dy(tfa)2L]2 (3). Yield: 56% (based on Dy). Elemental analysis
(%) calcd for C54H42Dy2F12N4O10 (1459.92): C 44.71, H 3.15, N
3.63. Found: C 44.39, H 2.88, N 3.84. IR (KBr, cmꢁ1): 2922 (w),
1633 (s), 1590 (m), 1517 (m), 1369 (m), 1292 (s), 1185 (m), 1135
(s), 1100 (m), 908 (w), 843 (m), 753 (m), 557 (m), 406 (w).
[Ho(tfa)2L]2 (4). Yield: 53% (based on Ho). Elemental analysis
(%) calcd for C54H42Ho2F12N4O10 (1464.78): C, 43.84; H, 3.23; N,
3.54. Found: C, 44.24; H, 2.87; N, 3.82; IR (KBr, cmꢁ1): 2924 (w),
1633 (s), 1590 (m), 1520 (m), 1461 (s), 1367 (m), 1294 (s), 1185
Results and discussion
(m), 1136 (s), 980 (w), 909 (w), 844 (m), 754 (m), 558 (m), 426 (w).
Crystal structures of 1–5
[Er(tfa)2L]2 (5). Yield: 58% (based on Er). Elemental analysis
Single-crystal X-ray diffraction, IR analysis indicates that com-
plexes 1–5 are essentially isomorphous and crystallize in a
triclinic crystal system in the space group P1. Taking the
dysprosium complex 3 as an example to describe the structure,
(%) calcd for C54H42Er2F12N4O10 (1469.44): C 43.96, H 3.15, N
3.97. Found: C 44.10, H 2.86, N 3.81. IR (KBr, cmꢁ1): 2926 (w),
%
1634 (s), 1528 (s), 1403 (s), 1367 (m), 1292 (s), 1184 (s), 1136 (s),
953 (w), 909 (w), 845 (m), 777 (m), 751 (m), 684 (m), 591 (w), 559
(m), 407 (w).
the partially labeled crystal structure is shown in Fig. 1a. The
Synthesis of [Ln(tfa)2L0]2. (Ln = Gd (6), Tb (7), Dy (8), Ho (9),
Er (10)). The syntheses of complexes 6–9 are very similar to
those of 1–5, with the only difference being the use of HL0
(0.025 mmol, 0.0069 g) instead of HL (0.025 mmol, 0.0065 g).
After about 3 days, block-shaped deeply red-colored crystals
were obtained.
dinuclear dysprosium complex is composed of two [Dy(tfa)2L]
units bridged by two phenoxo groups. Each Lꢁ ligand serves as
a tridentate ligand and chelates the Dy atom through one
phenoxide oxygen atom (O1), one pyridyl ring nitrogen atom
(N1) and one imine nitrogen atom (N2), and four oxygen atoms
(O2, O3, O4 and O5) from two tfaꢁ ligands. The DyIII centers
adopt a distorted dodecahedron geometry (Fig. 1b). Dy1 and
Dy1a are bridged by two phenoxido oxygen atoms (O1 and O1a)
of two ligands, leading to a Dy2O2 core with a Dy–O–Dy angle of
109.26(8)1 and a Dy–Dy distance of 3.8304(5) Å. The lengths of
the Dy–O bonds are in the range of 2.3209(19)–2.331(2) Å and
the Dy–N bond distances are 2.4520(2) and 2.685(3) Å. These
bond distances are comparable with those reported in other
phenoxo-bridged lanthanide complexes.36 Moreover, for 3, a
dihedral angle of 30.16(8)1 was observed between the planes of
the methylphenyl ring and the quinoline ring.
[Gd(tfa)2L0]2 (6). Yield: 51% (based on Gd). Elemental analysis
(%) calcd for C56H46Gd2F12N4O10 (1467.39): C 46.01, H 2.37, N 4.04.
Found: C 45.79, H 2.45, N 3.81. IR (KBr, cmꢁ1): 2966 (w), 1631 (s),
1523 (m), 1461 (s), 368 (m), 1294 (s), 1186 (s), 1138 (s), 1099 (m), 953
(w), 906 (w), 843 (m), 777 (m), 748 (m), 557 (m), 401 (w).
[Tb(tfa)2L0]2 (7). Yield: 57% (based on Tb). Elemental analy-
sis (%) calcd for C56H46Tb2F12N4O10 (1480.81): C 45.67, H 3.31,
N 3.42. Found: C 45.38, H 3.11, N 3.78. IR (KBr, cmꢁ1): 2966 (w),
1631 (s), 1592 (m), 1461 (s), 1368 (m), 1295 (s), 1186 (s), 1138 (s),
955 (w), 907 (w), 843 (m), 776 (m), 558 (m), 405 (w).
[Dy(tfa)2L0]2 (8). Yield: 59% (based on Dy). Elemental analy-
sis (%) calcd for C56H46Dy2F12N4O10 (1487.97): C 45.33, H 2.89,
Crystal structures of 6–10
N 4.07. Found: C 45.35, H 2.44, N 3.90. IR (KBr, cmꢁ1): 2966 (w), Complexes 6–10 are isomorphous and crystallize in the mono-
1632 (s), 1592 (w), 1461 (s), 1368 (m), 1295 (s), 1186 (s), 1137 (s), clinic space group P21/c. Therefore, the molecular structure of 8
955 (w), 907 (w), 843 (m), 776 (m), 558 (m), 404 (w).
is described in detail. In comparison with 3, complex 8 is
[Ho(tfa)2L0]2 (9). Yield: 55% (based on Ho). Elemental analysis composed of two [Dy(tfa)2L0] units bridged by two phenoxo
(%) calcd for C56H46Ho2F12N4O10 (1492.83): C 44.85, H 3.36, groups (Fig. 1c). The structures of two Dy2 complexes are very
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Table 1 Crystal data and structure refinement for complexes 1–5
Complex
1
2
3
4
5
Formula
C52H42F12N4O10Gd2 C52H42F12N4O10Tb2 C52H42F12N4O10Dy2 C52H42F12N4O10Ho2 C52H42F12N4O10Er2
Mr (g molꢁ1
)
1449.41
113(2)
1452.75
113(2)
1459.91
113(2)
1464.77
113(2)
1469.44
113(2)
Temperature (K)
Crystal system
Space group
Triclinic
P1
Triclinic
P1
Triclinic
P1
Triclinic
P1
Triclinic
P1
%
%
%
%
%
a (Å)
b (Å)
c (Å)
a (deg)
11.485(3)
11.837(2)
12.232(3)
94.787 (1)
117.172(4)
108.494(2)
1350.6(5)
1
11.162(4)
11.852(2)
12.135(3)
61.644(19)
89.40(3)
75.41(3)
1362.1(7)
1
11.1652(12)
11.8584(8)
12.0735(11)
62.342(5)
87.199(8)
74.965(7)
1362.6(2)
1
11.174(3)
11.8895(13)
12.008(3)
62.77(2)
87.11(3)
74.71(3)
1363.7(6)
1
11.500(4)
11.839(3)
12.251(4)
94.770(3)
117.275(5)
108.496(4)
1353.3(7)
1
b (deg)
g (deg)
Volume (Å3)
Z
Calculated density (Mg mꢁ3
)
1.782
2.537
1.771
2.677
1.779
2.823
1.784
2.982
1.803
3.183
Absorption coefficient (mmꢁ1
)
F(000)
710
712
714
716
718
5.01 to 55.22
17 404
2y range for data collection (deg) 6.016 to 55.076
Reflections collected
6.002 to 50.038
14 488
6.044 to 55.058
17 468
6.064 to 55.074
17 056
17 520
Independent reflection
Data/restraints/parameters
Goodness-of-fit on F2
6105 [R(int) = 0.0295] 4715 [R(int) = 0.0926] 6167 [R(int) = 0.0313] 6128 [R(int) = 0.0559] 6097 [R(int) = 0.0339]
6105/42/402
1.060
4715/12/427
1.062
R1 = 0.0804,
wR2 = 0.1963
R1 = 0.0881,
wR2 = 0.2032
6167/12/427
1.008
R1 = 0.0293,
wR2 = 0.0754
R1 = 0.0320,
wR2 = 0.0762
6128/12/427
1.115
R1 = 0.0550,
wR2 = 0.1531
R1 = 0.0585,
wR2 = 0.1550
6097/0/402
1.080
R1 = 0.0279,
wR2 = 0.0788
R1 = 0.0316,
wR2 = 0.0802
Final R indices [I 4 2s(I)]
R1 = 0.0189,
wR2 = 0.0507
R1 = 0.0204,
wR2 = 0.0512
R indices (all data)
Table 2 Crystallographic data and structure refinements for 6–10
Complex
6
7
8
9
10
Formula
C
54H46F12N4O10Gd2 C54H46F12N4O10Tb2 C54H46F12N4O10Dy2 C54H46F12N4O10Ho2 C54H46F12N4O10Er2
Mr (g molꢁ1
)
1477.47
113(2)
1480.81
113(2)
1487.97
113(2)
1492.83
113(2)
1497.49
113(2)
Temperature (K)
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/c
a (Å)
b (Å)
c (Å)
a (deg)
12.3847(11)
22.1376(17)
11.3751(10)
90.00
12.3438(11)
22.1120(18)
11.3848(10)
90.00
12.287(3)
22.050(4)
11.404(2)
90.00
12.448(9)
22.405(12)
11.522(8)
90.00
12.2757(11)
22.0146(18)
11.4203(11)
90.00
b (deg)
114.766(2)
90.00
2831.8(4)
2
1.733
2.422
114.7210(10)
90.00
2822.7(4)
2
1.742
2.586
114.696(4)
90.00
2807.1(10)
2
1.760
2.743
114.509(12)
90.00
2924(3)
2
1.696
2.784
114.753(2)
90.00
2802.7(4)
2
1.774
3.075
g (deg)
Volume (Å3)
Z
Calculated density (Mg mꢁ3
)
Absorption coefficient (mmꢁ1
)
F(000)
1452
1456
6.616 to 55.076
28 407
1460
6.638 to 55.274
35 296
1464
6.534 to 54.262
35 277
1468
6.648 to 55.064
28 004
2y range for data collection (deg) 6.604 to 54.992
Reflections collected
28 422
Independent reflection
Data/restraints/parameters
Goodness-of-fit on F2
6452 [R(int) = 0.0230] 6387 [R(int) = 0.0283] 6430 [R(int) = 0.0310] 6444 [R(int) = 0.0283] 6362 [R(int) = 0.0267]
6452/170/498
1.465
6387/170/498
1.482
R1 = 0.0699,
wR2 = 0.1572
R1 = 0.0724,
wR2 = 0.1580
6430/176/498
1.475
R1 = 0.0871,
wR2 = 0.2012
R1 = 0.0888,
wR2 = 0.2017
6444/170/498
1.464
R1 = 0.0736,
wR2 = 0.1589
R1 = 0.0762,
wR2 = 0.1598
6362/170/498
1.441
R1 = 0.0494,
wR2 = 0.1032
R1 = 0.0559,
wR2 = 0.1048
Final R indices [I 4 2s(I)]
R = 0.0685,
wR2 = 0.1517
R1 = 0.0713,
wR2 = 0.1525
R indices (all data)
similar, but the subtle structural difference is the different alkyl complex 8. The Dyꢂ ꢂ ꢂDy distance and Dy–O–Dy angle are
substitution of the two ligands. The center DyIII ions all have a 3.8099(12) Å and 108.8(3)1 for 8, respectively, which are smaller
similar distorteꢁd dodecahedron DyO6N2 coordination sphere than those of 3. Furthermore, the dihedral angle between the
formed by a L0 ligand (O1, O1a, N1 and N2) and two tfaꢁ planes of the ethyphenyl ring and quinoline ring is 37.19(5)1 in 8,
ligands (O2, O3, O4 and O5), with little difference in the bond which is larger than that of 3. These interesting features are
lengths and angles (Fig. 1d). Different from complex 3, the induced by the two alkyl groups (–CH3 and –C2H5) of the
lengths of the Dy–O bonds are in the range of 2.294(8)–2.370(9) Å 8-hydroxyquinoline Schiff base ligands, where the subtle differ-
and the Dy–N bond distances are 2.443(10) and 2.700(10) Å for ences result from different electron-donating effects.
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Fig. 1 (a) Molecular structure for 3 (all hydrogen atoms are omitted for clarity); (b) coordination polyhedrons for the DyIII ions in complex 3; (c) molecular
structure for 8 (all hydrogen atoms are omitted for clarity); (d) coordination polyhedrons for the DyIII ions in complex 8.
Powder X-ray diffraction (PXRD) and thermogravimetric
analysis (TGA)
the unquenched first-order orbital momentum of lanthanide
ions (except for GdIII) under the ligand-field effect.37,38 As is
known, the 4fn configuration of LnIII is split into 2S+1LJ states by
interelectronic repulsion and spin–orbit coupling, and can be
further split into Stark components (2J + 1 if n is even, J + 1/2 if
n is odd; n: the number of electrons) under the crystal-field
perturbation. With cooling temperature, depopulation of the
Stark sublevels will lead to a decline in the value of wMT, which
may shield the weak ferromagnetic/antiferromagnetic coupling
nature between adjacent lanthanide ions. Therefore, one cannot
determine the magnetic interaction accurately based simply on
the trend of wMT.
Variable-temperature magnetic susceptibility measurements
were carried out for samples 1–5 and 6–10 in the range of
2–300 K at an applied field of 1000 Oe; the wMT versus T plots are
shown in Fig. 2. At room temperature, the wMT values of complexes
1–5 are 15.67, 23.28, 28.45, 27.90, and 22.76 cm3 K molꢁ1, respec-
tively. These values are in good agreement with the expected
theoretical values (1: 15.76; 2: 23.63; 3: 28.34; 4: 28.16; 5:22.96)
for two non-interacting lanthanide ions: GdIII (8S7/2, S = 7/2), TbIII
(7F6, S = 3), DyIII (6H15/2, S = 5/2), HoIII (5I8, S = 2), ErIII (4I15/2, S = 3/2).
As the temperature decreases, the wMT value of complex 1 stays
almost constant in the temperature range 300–20 K and then drops
The purity of crystalline powders of 1–10 was confirmed by
powder X-ray diffraction (PXRD); results are shown in Fig. S1
and S2 (ESI†). These results are in good agreement with the
XRD patterns simulated from the single-crystal data, indicating
high purity of the obtained samples.
To identify the thermal stabilities of these complexes, ther-
mal gravimetric analyses of complexes 1–10 were carried out
under air atmosphere with a heating rate of 10 1C minꢁ1 in the
temperature range of 30 to 800 1C. TGA curves of 1–10 (Fig. S3,
ESI†) have similar profiles, exhibiting two main weight loss
steps up to decomposition of the framework. Therefore, as a
representative, only the TGA curve of 1 is discussed in detail.
From 30 1C to about 280 1C, no weight loss occurs in 1, implying
that no solvent molecules are present in the crystal lattice. The
TGA curve of 1 indicates thermal stability up to 280 1C, and then
weight loss in the range of 280 to 300 1C, after which the
framework decomposes gradually. Finally, the residue is expected
to be the corresponding lanthanide oxide.
Magnetic properties
Direct current (dc) magnetism. Investigating the magnetic rapidly to a minimum value of 7.60 cm3 K molꢁ1 at 2 K, which
properties of lanthanide compounds is complicated because of indicates the presence of a weak antiferromagnetic interaction
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Fig. 2 (a) Temperature dependence of the wMT products at 1 kOe for 1–5; (b) temperature dependence of the wMT products at 1 kOe for 6–10.
between the GdIII ions. For complexes 2–5, the wMT value for 1 and ꢁ0.76 K for 6 (Fig. S6, ESI†). According to the formula
decreases slowly with cooling temperature from 300 to 50 K of (2)–(4), we fit the wMT versus T data.
and more rapidly below 50 K, reaching minimum values of
wM = C/(T ꢁ y)
wM = (2Ng2b2/kT)(A/B)
(1)
4.99 cm3 K molꢁ1 for 2, 8.42 cm3 K molꢁ1 for 3, 6.25 cm3 K molꢁ1
for 4, and 13.94 cm3 K molꢁ1 for 5. The wMT vs. T plots
of complexes 6–10 are very similar to those of complexes 1–5.
At 300 K, the wMT values of complexes 6–10 are 15.52, 23.15,
27.84, 27.15 and 23.69 cm3 K molꢁ1, respectively. These values
are close to the calculated values (6: 15.76; 7: 23.63; 8: 28.34; 9:
28.16; 10: 22.96 cm3 K molꢁ1) for two noninteracting lanthanide
ions: GdIII (8S7/2, S = 7/2), TbIII (7F6, S = 3), DyIII (6H15/2, S = 5/2),
HoIII (5I8, S = 2), and ErIII (4I15/2, S = 3/2). For 6, the wMT vs. T curve
is similar to that of 1; upon cooling, the wMT value is almost
constant over the temperature range 300–20 K and then it drops
sharply to a minimum value of 7.79 cm3 K molꢁ1 at 2.0 K. The
decreasing trend indicates the presence of a weak antiferro-
magnetic interaction between the GdIII ions. For 7–10, on cool-
ing, the wMT values decrease gradually in the temperature range
300–50 K, and then the wMT values rapidly reach minima of
4.52 cm3 K molꢁ1 for 7, 8.65 cm3 K molꢁ1 for 8, 5.22 cm3 K molꢁ1
for 9, and 9.18 cm3 K molꢁ1 for 10 at 2 K, respectively.
(2)
A = ex + 5e3x + 14e6x + 30e10x + 55e15x + 91e21x + 140e28x
(3)
B = 1 + 3ex + 5e3x + 7e6x + 9e10x + 11e15x + 13e21x + 15e28x
(4)
which are derived from the isotropic spin Hamiltonian
ˆ ˆ
ˆ
H = ꢁJSAꢂSB, where J is the exchange coupling parameter,
ˆ
ˆ
x = J/kT, and SA and SB are the spin operators of the local spins
(SA = SB = 7/2). The best-fitting results give: J = ꢁ0.057 cmꢁ1
,
ˆ
ˆ
g = 2.02 for 1 and J = ꢁ0.063 cmꢁ1, g = 2.00 for 6 (Fig. 3). The
negative parameters of y and J values further corroborate that a
weak antiferromagnetic coupling between GdIII ions existed in
1 and 6. For 2–5 and 7–10, a sudden decrease is observed in the
wMT. These features can be generally attributed to one or a
combination of the following two phenomena: (i) antiferro-
magnetic interactions between the lanthanide centers; (ii) the
thermal depopulation of the Stark sublevels.39,40
In addition, the variable temperature susceptibility data
obey the Curie–Weiss law (eqn (1)) with constant y = ꢁ0.71 K
Fig. 3 The experimental wMT vs. T for complexes 1 (a) and 6 (b). Red solid line is the best fit obtained.
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The field dependencies of magnetization measurements Alternating current (ac) magnetism
Paper
were carried out in the range of 0–8 T at 2–10 K for 1 and 6
(Fig. 4(a and b)), with M values rapidly increasing at low
magnetic fields. The magnetization of 1 and 6 increases stea-
dily with the applied field and reaches 13.43 Nb for 1 and
13.28 Nb for 6 at 2 K and 8 T, which is lower than the calculated
value of 14 Nb for two GdIII (g = 2, S = 7/2). Then we explored
DSm in evaluating MCE on 1 and 6, considering the very weak
GdIIIꢂ ꢂ ꢂGdIII magnetic interaction. The MCE can be calculated
To investigate the presence of slow relaxation of the magnetiza-
tion, which may originate from an SMM behavior, ac measure-
ments were performed on complexes
3 and 8 in the
temperature range 2–20 K with zero dc field and a 3 Oe ac field
at frequencies between 111 and 2311 Hz (Fig. 5). For complexes
3 and 8, both the in-phase (w0) and out-of-phase (w00) signals of
ac susceptibilities exhibit strong frequency dependence and
good peaks are observed, implying that the two Dy2 complexes
possess slow magnetic relaxation, typical of SMM behavior. For
3, w0 shows a maximum value which starts to decrease in the
2–4 K range, while the peaks of w00 can only be found at
frequencies higher than 1111 Hz. Upon cooling, increases of
w00 are clearly observed in the 111–1111 Hz range, indicating
that blocking is not complete and that thermally activated spin
reversal is gradually replaced by a quantum tunneling mecha-
nism at zero dc field.44 For 8, w0 shows a maximum value which
starts to decrease in the 2–10.6 K range, while w00 can be found
at maximum at frequencies higher than 711 Hz. Compared with
3, remarkable peak shapes were observed in a 0 Oe dc field and
the peaks were also shifted to higher temperature. In addition,
the different and interesting magnetic relaxation behaviors of 3
and 8 might result from electronic effects induced by the
different alkyl substitution of two 8-hydroxyquinoline Schiff
base ligands.
by employing the Maxwell equation as follows: DSmðTÞDH
¼
Ð
½@MðT; HÞ=@TꢃHdH.41 According to the equation, the ꢁDSm
values of 1 and 6 can be obtained; the plots of ꢁDSm versus T
are shown in Fig. 4(c and d). For 1, the maximum value of ꢁDSm
is 19.72 J kgꢁ1 Kꢁ1 (calculated as ꢁDSm = 2R ln(2S + 1)/Mw,
expected maximum ꢁDSm is 23.85 J kgꢁ1 Kꢁ1) for a field change
DH = 7 T at 3.0 K. The gap between the experimental data and
the theoretical value mainly originates from the antiferro-
magnetic interactions in 1. The expected maximum ꢁDSm of
6 is 23.73 J Kꢁ1 kgꢁ1, calculated as 2R ln(2S + 1)/Mw for two
uncoupled GdIII ions, which is larger than the maximum value
of ꢁDSm (19.37 J kgꢁ1 Kꢁ1 for T = 2 K and DH = 7 T). The
observed ꢁDSm values of 1 and 6 are smaller than those of
reported GdIII-based molecular systems;42 however, they are
significantly larger than those for the antiferromagnetic {Gd2}
complex (17.25 J kgꢁ1 ꢁ1, DH = 7 T at 3 K).43
K
Fig. 4 M versus H plots for 1 (a) and 6 (b) at T = 2.0–10.0 K and H = 0–8 T. Temperature dependence of magnetic entropy change (ꢁDSm) as calculated
from the magnetization data of 1 (c) and 6 (d) at T = 2–10 K and H = 0–7 T.
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Fig. 5 Temperature dependence of the in-phase and out-of-phase components of the ac magnetic susceptibility for 3 (a and b) and 8 (c and d) in zero
dc fields with an oscillation of 3 Oe.
Fig. 6 The ln (t) versus Tꢁ1 plots for complexes 3 (a) and 8 (b) under a zero dc field; the red solid line is best fitted with the Arrhenius law.
To further understand the dynamics of the magnetization of 3 3 and t0 = 3.96 ꢀ 10ꢁ6 s and DE/kB = 17.50 K for 8 (Fig. 6). These
and 8, the frequency dependencies of the alternating-current (ac) parameters are comparable with reported SMMs.45 It is worth
susceptibility were measured under zero dc fields (Fig. S10, ESI†). noting that ln t of 3 and 8 became weakly dependent on T as the
Both the in-phase (w0) and out-of-phase (w00) signals of 3 and 8 temperature decreased, indicating a gradual crossover from a
show temperature dependencies, which also reveal the presence thermally activated Orbach mechanism that is predominant at
of slow relaxation of the magnetization in 3 and 8. The relaxation higher temperatures (red line), indicating that multiple pathways
time t data of complexes 3 and 8 derived from the w00 peaks follow exist for the relaxation of the magnetizations.46
the Arrhenius law [t = t0 exp(DE/kBT)] with the pre-exponential
For complexes 3 and 8, Cole–Cole plots showing the
factor t0 = 4.76 ꢀ 10ꢁ6 s and the effective barrier DE/kB = 9.14 K for in-phase vs. out-of-phase ac susceptibility data reveal an
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Fig. 7 Cole–Cole plots for 3 (a) and 8 (b) measured in zero dc field. The solid lines are the best fit to the experimental data, obtained with the generalized
Debye model with a = 0.11–0.19 for 3 and a = 0.16–0.22 for 8.
asymmetrical semi-circular shape (Fig. 7), which can be fitted In view of the above analysis, the different and interesting
by the generalized Debye model, with parameters a = 0.11–0.19 magnetic relaxation behaviors of 3 and 8 could be attributed to
for 3 and a = 0.16–0.22 for 8, indicating a narrow distribution of the different electron-donating effects induced by the two alkyls
relaxation time.47
(–CH3 and –C2H5) of the 8-hydroxyquinoline Schiff base ligands.
Comparing 3 and 8, these distinctive magnetic behaviors
must be caused by subtle but crucial differences among the
respective structures. Complex 8 displays a slow magnetic
relaxation of magnetization with an energy barrier of 17.50 K,
which is much higher than that of 3 (DE/kB = 9.14 K). The
different ac susceptibility behaviors of 3 and 8 are clearly
related to a subtle perturbation of the different alkyl substitu-
tion of two 8-hydroxyquinoline Schiff base ligands. There were
no obvious deviations in the crystal structure features of 3 and
8. The most pertinent bond distances and angles are within
1–2% deviation. We hypothesized that the immediate cause of
the different ac susceptibility behaviors is geometrical distor-
tion of the coordination spheres. This was proved by the Shape
2.0 analysis (Table S2, ESI†), showing that the Dy coordination
spheres of 3 and 8 are different while belonging to the same
polyhedral shape. Moreover, the crystal packing (complexes 3
and 8 crystallize in different space groups) may influence the
Conclusion
In summary, the structures and magnetic properties were reported
of 10 dinuclear LnIII complexes based on an 8-hydroxyquinoline
Schiff base with different substituents. Complexes 3 and 8 exhibit
different magnetic relaxation barriers of 9.14 K (3) and 17.50 K (8),
respectively. The different magnetic relaxation behaviors of the two
Dy2 complexes originated from the slightly different chemical
environment around the DyIII ions. This demonstrates that the
SMM behavior of lanthanide complexes can be finely tuned by
changing the substituent group of the 8-hydroxyquinoline
Schiff base ligands. These results are important evidence that
the magnetic dynamic behaviors of LnIII-complexes can be
modulated by electronic effects on the ligand set.
magnetic behavior. On further analysis, the main and obvious Acknowledgements
difference between the two Dy2 complexes is the alkyl substi-
This work was supported financially by the National Natural
tuent of the two 8-hydroxyquinoline Schiff base ligands for 3
and 8. It is evident that replacement of the –CH3 group by a
bigger electron-donating group –C2H5 results in a significant
change in magnetic relaxation of 8. The different electron-
donating effects induced by the two alkyls impacts the
8-hydroxyquinoline Schiff base ligands, and consequently may
have an effect on the strength of the chemical bond to the DyIII
ions. This is supported by the slightly different Dy–OPh bond
for 3 (Dy–O(1), 2.3209(19) Å; Dy–O(1a), 2.376(2) Å) compared
with 8 (Dy–O(1), 2.316 (8) Å; Dy–O(1a), 2.370(9) Å). Moreover,
different Dyꢂ ꢂ ꢂDy distances and Dy–O–Dy angles were found for
complexes 3 (3.8304(5) Å and 109.26(8)1) and 8 (3.8099(12) Å
and 108.8(3)1), which may have a significant influence on
magnetic interaction between the DyIII ions. Comparing the bond
lengths and bond angles of 3 and 8 (Table S3, ESI†), could aid
investigation of the influence of different electron-donating groups.
Science Foundation of China (No. 21473121, 21271137 and
21571138) and the State Key Laboratory of Medicinal Chemical
Biology (NanKai University, Opening Foundation No. 201603002).
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