Octanuclear {Ln(III)8}(Ln = Gd, Tb, Dy, Ho) macrocyclic complexes in a cyclooctadiene-like conformation: Manifestation of slow relaxation of magnetization in the Dy(III) derivative

Article abstract of DOI:10.1021/ic400091j

The synthesis of a series of macrocyclic, isostructural octanuclear lanthanide complexes [Gd₈ (LH₂)₄ (μ-Piv)₄ (η²-Piv)₄ (μ-OMe) ₄]·6CH₃OH·2H₂O (1), [Tb ₈ (LH₂)₄ (μ-Piv)₄ (η²-Piv)₄ (μ-OMe)₄]₄CH ₃OH·4H₂O (2), [Dy₈(LH₂) ₄ (μ-Piv)₄ (η²-Piv)₄ (μ-OMe)₄]·8CH₃OH (3), and [Ho ₈(LH₂)₄(μ-Piv)₄ (η²-Piv)₄ (μ-OMe)₄]·CH ₃OH·4H₂O (4) have been achieved, using Ln(III) nitrate salts, pivalic acid, and a new multidentate chelating ligand (2E,N′E)-N′-(3-((bis(2- hydroxyethyl)amino)methyl)-2-hydroxy-5- methylbenzylidene)-2-(hydroxyimino) propane hydrazide (LH₅), containing two unsymmetrically disposed arms; one side of the phenol unit is decorated with a diethanolamine group while the other side is a hydrazone that has been built by the condensation reaction involving 2- hydroxyiminopropanehydrazide. All the compounds, 1-4, are neutral and are held by the four [LH₂]^3- triply deprotonated chelating ligands. In these complexes all the lanthanide ions are doubly or triply bridged via phenolate, alkoxy, and pivalate oxygens. The metal centers are distributed over the 8 vertices of an octagon, resembling a cyclooctadiene ring core. The details of magnetochemical analysis for complexes 1-4 shows that they exhibit antiferromagnetic interactions between the Ln³⁺ ions through the phenoxo, alkoxo, and pivalato bridging groups. None of the compounds exhibits slow relaxation of the magnetization at zero applied direct current (dc) magnetic field, which could be due to the existence of a fast quantum tunneling relaxation of the magnetization (QTM). In the case of 3, the application of a small dc field is enough as to fully or partly suppress the fast and efficient zero-field QTM allowing the observation of slow relaxation above 2 K.

Full text of DOI:10.1021/ic400091j

Article
pubs.acs.org/IC
Octanuclear {Ln(III)8}(Ln = Gd, Tb, Dy, Ho) Macrocyclic Complexes in a
Cyclooctadiene-like Conformation: Manifestation of Slow Relaxation
of Magnetization in the Dy(III) Derivative
Vadapalli Chandrasekhar,*,†,‡ Prasenjit Bag,† and Enrique Colacio§,⊥
†Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur-208016, India
‡Tata Institute of Fundamental Research, Centre for Interdisciplinary Sciences, 21 Brundavan Colony, Narsingi, Hyderabad-500075,
India
§Departamento de Química Inorgan
́
ica, Facultad de Ciencias, Universidad de Granada, Avenida de Fuentenueva s/n, 18071 Granada,
Spain
S
* Supporting Information
ABSTRACT: The synthesis of a series of macrocyclic, isostructural octanuclear lanthanide complexes [Gd8 (LH2)4 (μ-Piv)4 (η2-
Piv)4 (μ-OMe)4]·6CH3OH·2H2O (1), [Tb8 (LH2)4 (μ-Piv)4 (η2-Piv)4 (μ-OMe)4]4CH3OH·4H2O (2), [Dy8(LH2)4 (μ-Piv)4 (η2-
Piv)4 (μ-OMe)4]·8CH3OH (3), and [Ho8(LH2)4(μ-Piv)4 (η2-Piv)4 (μ-OMe)4]·CH3OH·4H2O (4) have been achieved, using
Ln(III) nitrate salts, pivalic acid, and a new multidentate chelating ligand (2E,N′E)-N′-(3-((bis(2- hydroxyethyl)amino)methyl)-
2-hydroxy-5-methylbenzylidene)-2-(hydroxyimino) propane hydrazide (LH5), containing two unsymmetrically disposed arms;
one side of the phenol unit is decorated with a diethanolamine group while the other side is a hydrazone that has been built by
the condensation reaction involving 2-hydroxyiminopropanehydrazide. All the compounds, 1−4, are neutral and are held by the
four [LH2]3− triply deprotonated chelating ligands. In these complexes all the lanthanide ions are doubly or triply bridged via
phenolate, alkoxy, and pivalate oxygens. The metal centers are distributed over the 8 vertices of an octagon, resembling a
cyclooctadiene ring core. The details of magnetochemical analysis for complexes 1−4 shows that they exhibit antiferromagnetic
interactions between the Ln3+ ions through the phenoxo, alkoxo, and pivalato bridging groups. None of the compounds exhibits
slow relaxation of the magnetization at zero applied direct current (dc) magnetic field, which could be due to the existence of a
fast quantum tunneling relaxation of the magnetization (QTM). In the case of 3, the application of a small dc field is enough as to
fully or partly suppress the fast and efficient zero-field QTM allowing the observation of slow relaxation above 2 K.
orbital angular momentum that result in magnetic anisotropy,
along with a large number of unpaired 4f electrons, resulting in
high spin, lanthanide complexes are being vigorously
investigated as molecular magnetic materials.9 Some envisaged
applications of these novel magnetic materials include magnetic
refrigeration,10 ultra high-density data storage,11 as well as
quantum computation.12 The first major breakthrough in this
area was the discovery that the double-decker mononuclear
sandwich-type lanthanide [LnPc2] (Ln = Tb (III), Dy(III),
INTRODUCTION
■
For the past couple of years there has been notable interest in
the preparation and structural elucidation of polynuclear
lanthanide complexes.1 This interest is the result of several
factors. Many lanthanide(III) complexes are being investigated
in view of their interesting catalytic,2 photophysical3 and
magnetic properties.4 Thus, lately, lanthanide compounds that
function as single molecule magnets (SMMs),5 single chain
magnets,6 and single ion magnets7 have been studied. SMM
behavior has been correlated to the presence of a large ground-
state spin and a significant magnetic anisotropy within the
molecule.8 Since many lanthanide ions possess unquenched
Received: January 13, 2013
Published: March 28, 2013
© 2013 American Chemical Society
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Article
reaction mixture was allowed to come to room temperature and the
solvent removed in vacuo affording a solid which was washed
dichloromethane (3 × 10 mL) to give LH5 as an off-white solid. Yield:
3.10 g, 87.8%. Mp: 122 °C. FT-IR (KBr) cm−1: 3269 (b), 2920 (bm),
2826 (m), 1670 (s), 1613 (m) 1530 (s), 1470 (s) 1362 (s), 1234 (s),
1171 (s), 1034 (s), 958 (m). 1H NMR (CD3OD, δ, ppm): 8.48 (s, 1H,
imino), 7.41 (s, 1H, Ar−H), 7.06(s, 1H, Ar−H), 3.79 (s, 2H, ArCH2),
3.64 (t, 4H, CH2O), 3.78 (s, 3H, OCH3), 2.69 (t, 4H, NCH2), 2.24 (s,
3H, ArCH3), 2.03 (s, 3H, imino CH3) ESI-MS (m/z): 353.176 (M
+H). Anal. Calcd for C16H24N4O5: C, 54.53; H, 6.86; N, 15.90. Found:
C, 54.40; H, 6.70; N, 15.73.
General Synthetic Procedure for the Preparation of the
Complexes 1−4. All the metal complexes (1−4) were synthesized
according to the following procedure. LH5 (0.071 g, 0.20 mmol) was
dissolved in methanol (10 mL) and subsequently Ln(NO3)3·nH2O
(0.40 mmol) was added to this solution. This reaction mixture was
stirred for 0.5 h. At this stage, pivalic acid (0.041 g, 0.40 mmol) and
triethylamine (0.1 mL, 0.70 mmol) were added, and the mixture was
stirred for a further period of 4 h at room temperature affording a
yellow precipitate. This was filtered, washed with cold methanol (2 × 5
mL), dried and dissolved in 1:1 v/v mixture of dichloromethane and
methanol. X-ray quality crystals of 1−4 were obtained within a week
after slow evaporation of the solvent mixture. The characterization
data for these complexes are given below.
Ho(III) H2Pc = phthalocyanine) complexes showed SMM
behavior based on the single ion anisotropy of the constituent
lanthanide ions.13 Extending this idea, for the past couple of
years, several multinuclear lanthanide aggregates have been
reported, utilizing some multisite coordinating ligands.5 Among
these are the pentanuclear square-pyramid-shaped Dy(III)
aggregate that showed the highest energy barrier (Ueff = 530 K)
to magnetization reversal14 and a {N2}3− radical-bridged
Tb(III) compound that showed a hysteresis up to 14 K.15 In
spite of these reports, polynuclear lanthanide complexes are still
quite sparse. In this regard, from the point of view of chemical
synthesis, there is a challenge to design appropriate ligands
whose coordination action can lead to the generation of
multinuclear lanthanide complexes. An examination of the
literature revealed that the hydrazide-Schiff base ligands are
very much useful for construction of lanthanide clus-
ters.5h,i,23,24c Keeping this in mind we have designed a new
chelating, flexible, and sterically unencumbered multisite
coordinating ligand, (2E,N′E)-N′-(3-((bis(2-hydroxyethyl)-
amino)methyl)-2-hydroxy-5-methylbenzylidene)-2-(hydroxy-
imino) propane hydrazide (LH5). Using this ligand, we have
been able to assemble neutral octanuclear macrocyclic {Ln-
(III)}8 complexes (Ln(III = Gd(III), Tb(III), Dy(III), and
Ho(III)). These macrocycles possess a novel cyclooctadiene-
type conformation. One of these, the Dy(III) analogue shows
slow relaxation of magnetization above 2 K. These results are
discussed herein.
[Gd8 (LH2)4 (μ-Piv)4 (η2-Piv)4 (μ-OMe)4]6CH3OH·2H2O (1). Yield:
0.025 g, 26.2% (based on Gd). Mp: >220 °C. IR (KBr) cm−1: 3362
(b), 2959 (s), 2925 (s), 2865 (m), 1621 (s), 1573 (s), 1541 (m) 1484
(s), 1420 (m), 1357 (s), 1305 (s), 1262 (s), 1223 (s), 1199 (s), 1135
(m), 1087 (s), 1046(s). ESI-MS m/z, ion: 1711.2, [Gd8 (LH2)4 (Piv)6
(OCH3)4 (OH2)2]2+; 1683.1, [Gd8 (LH2)4 (Piv)5 (OCH3)5 (HOCH3)
(OH2)]2+. Anal. Calcd C114H198N16 O48Gd8 (3818.88): C, 35.85; H,
5.23; N, 5.87. Found: C, 35.38; H, 5.01; N, 5.75.
EXPERIMENTAL SECTION
■
[Tb8 (LH2)4 (μ-Piv)4 (η2-Piv)4 (μ-OMe)4]4CH3OH·4H2O (2). Yield:
0.032 g, 33.3% (based on Tb). Mp: >220 °C. IR (KBr) cm−1: 3341
(b), 2958 (s), 2901 (s), 2867 (m), 1622 (s), 1574 (s), 1543 (m) 1484
(s), 1421 (m), 1357 (s), 1303 (s), 1271 (s), 1222 (s), 1198 (s), 1135
(m), 1090 (s), 1046(s). ESI-MS m/z, ion: 1587.1, [Tb8 (LH2)4 (Piv)3
(OCH3)6 (OH)]2+; 1615.1, [Tb8 (LH2)4 (Piv)4 (OCH3)4 (OH)2]2+
Anal. Calcd C112H192N16O48Tb8 (3802.26): C, 35.38; H, 5.09; N, 5.89.
Found: C, 35.10; H, 4.95; N, 5.75.
Reagents and General Procedures. Solvents and other general
reagents used in this work were purified according to standard
procedures.16 p-Cresol and paraformaldehyde were obtained from S.D.
Fine Chemicals, Mumbai, India, and anhydrous magnesium chloride
was obtained from Alfa Aesar and were used as such. Ln(NO3)3·nH2O
were obtained from Aldrich Chemical Co. and were used as such. 2-
Hydroxy-5-methylbenzaldehyde (C1),17 3-(chloromethyl)-2-hydroxy-
5-methylbenzaldehyde (C2),18 and (E)-2-(hydroxyimino)-
propanehydrazide (C4)19 were synthesized by adapting the literature
procedure.
[Dy8 (LH2)4 (μ-Piv)4 (η2-Piv)4 (μ-OMe)4]8CH3OH (3). Yield: 0.035 g,
36.1% (based on Dy). Mp: >220 °C. IR (KBr) cm−1: 3349 (b), 2958
(s), 2902 (s), 2869 (m), 1622 (s), 1575 (s), 1546 (m) 1484 (s), 1421
(m), 1357 (s), 1304 (s), 1271 (s), 1222 (s), 1199 (s), 1135 (m), 1089
(s), 1048(s). ESI-MS m/z, ion: 1602.6, [Dy8 (LH2)4 (Piv)3 (OCH3)6
(OH)]2+; 1574.1, [Dy8 (LH2)4 (Piv)2 (OCH3)8]2+. Anal. Calcd
C116H200 N16O48 Dy8 (3886.92): C, 35.84; H, 5.19; N, 5.77. Found: C,
35.50; H, 4.96; N, 5.60.
Syntheses. 3-((Bis(2-hydroxyethyl)amino)methyl)-2-hydroxy-5-
methylbenzaldehyde (C3). 3-(Chloromethyl)-2-hydroxy-5-methyl-
benzaldehyde (C2) (6.00 g, 32.50 mmol) was taken in dry
tetrahydrofuran (40 mL) and added dropwise to a mixture of
diethanolamine (3.52 g, 33.50 mmol) and dry triethylamine (6.82 g,
67.50 mmol) taken in a solvent mixture of dry tetrahydrofuran (40
mL) and methanol (5 mL), under N2 atmosphere. The reaction
mixture was stirred at room temperature for 24 h. After this, the
solution was filtered to remove triethylamine hydrochloride, and the
filtrate was stripped off the solvent in vacuo to afford an oily mass.
This was then dissolved in dichloromethane (40 mL) and washed
twice with water (2 × 25 mL). The organic layer was separated, dried
over anhydrous sodium sulfate, and the solvent from it in vacuo to
afford an oily product. This was purified over a silica gel column
(eluent: v/v: 5:95 methanol/ethyl acetate) to afford C3 as a yellow
liquid. Yield: 6.20 g, 75.3%. FT-IR (KBr) cm−1: 3373 (b), 2870 (s),
1672(s), 1609 (s), 1473 (s) 1399 (m), 1278 (s), 1223 (s), 1150 (m),
[Ho8 (LH2)4 (μ-Piv)4 (η2-Piv)4 (μ-OMe)4]4CH3OH·4H2O (4). Yield:
0.030 g, 31.2% (based on Ho). Mp: >220 °C. IR (KBr) cm−1: 3345
(b), 2958 (s), 2903 (s), 2870 (m), 1624 (s), 1576 (s), 1547 (s) 1484
(s), 1421 (m), 1357 (s), 1304 (s), 1271 (s), 1222 (s), 1199 (s), 1135
(m), 1091 (s), 1048(s). ESI-MS m/z, ion: 1656.1,
[Ho8(LH2)4(Piv)4(OCH3)5(OH2)(OH)]2+; 1684.7, [Ho8 (LH2)4
(Piv)4(OCH3)4 (OH2)5 (OH)2]2+. Anal. Calcd C112H192 N16O48Ho8
(3850.26): C, 34.94; H, 5.03; N, 5.82. Found: C, 34.58; H, 4.81; N,
5.61
Instrumentation. Melting points were measured using a JSGW
melting point apparatus and are uncorrected. IR spectra were recorded
as KBr pellets on a Bruker Vector 22 FT IR spectrophotometer
operating at 400−4000 cm−1. 1H NMR was recorded on a JEOL-JNM
LAMBDA model 400 spectrometer using CDCl3 operating at 400
MHz. Elemental analyses of the compounds were obtained from
Thermoquest CE instruments CHNS-O, EA/110 model. Electrospray
ionization mass spectrometry (ESI-MS) spectra were recorded on a
Micromass Quattro II triple quadrupole mass spectrometer. Electro-
spray ionization (positive ion, full scan mode) was used as was
methanol as solvent for desolvation. Capillary voltage was maintained
at 2 kV, and cone voltage was kept at 31 kV.
1
1083 (s). H NMR (CDCl3, δ, ppm): 9.88 (s, 1H, −CHO), 7.26 (s,
1H, Ar−H), 7.18(s, 1H, Ar−H), 3.58 (t, 4H, CH2O), 3.67 (s, 2H,
ArCH2), 2.63 (t, 4H, NCH2), 2.24 (s, 3H, ArCH3). ESI-MS (m/z):
254.139 (M+H). Anal. Calcd for C13H19NO4: C, 61.64; H, 7.56; N,
5.53. Found: C, 61.37; H, 7.39; N, 5.37.
(2E,N′E)-N′-(3-((bis(2-hydroxyethyl)amino)methyl)-2-hydroxy-5-
methylbenzylidene)-2-(hydroxyimino)propanehydrazide (LH5). To a
stirred solution of C4 (2.54 g, 10.0 mmol) in dry methanol (20 mL),
C3 (1.18 g, 10.0 mmol) also dissolved in dry methanol (20 mL) was
added dropwise over a period of 20 min, and the resulting reaction
mixture was stirred for 12 h under N2 atmosphere at 60 °C. The
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Table 1. Details of the Data Collection and Refinement Parameters for Compounds 1−4
1
2
3
4
formula
C114 H198 N16 O48 Gd8
3818.88
C112 H192 N16 O48Tb8
3802.26
C116 H200 N16 O48 Dy8
3886.92
C112 H192 N16 O48 Ho8
3850.26
M/g
crystal system
space group
wavelength (MoKα
triclinic
tetragonal
I4(1)/a
tetragonal
tetragonal
I4(1)/a
P1
̅
0.71069
I4(1)/a
)
0.71069
0.71069
0.71069
unit cell dimensions (Å, deg)
a = 17.936(5)
b = 18.337(5)
c = 26.005(5)
α = 72.163(5)
β = 73.427(5)
γ = 65.490(5)
7284(3)
a = 18.261(3)
b = 18.261(3)
c = 44.911(9)
α = 90
a = 18.228(3)
b = 18.228(3)
c = 44.831(9)
α = 90
a = 18.183(3)
b = 18.183(3)
c = 44.626(9)
α = 90
̀
β = 90
β = 90
γ = 90
β = 90
γ = 90
γ = 90
V/Å3
14975(4)
14896(4)
14755(4)
Z
2
4
4
4
ρc/g cm−3
μ/mm−1
1.741
1.686
1.733
1.730
3.672
3.806
4.041
4.318
F(000)
3780
7520
7680
7584
cryst size (mm3)
θ range (deg)
limiting indices
0.125 × 0.105 × 0.085
2.22 to 26.00°.
−22 ≤ h ≤ 18,
−22 ≤ k ≤ 22,
−32 ≤ l ≤ 32
52005
0.14 × 0.115 × 0.09
2.23 to 25.50
−22 ≤ h ≤ 22,
−22 ≤ k ≤ 22,
−38 ≤ l ≤ 54
40483
0.13 × 0.11 × 0.085
1.21 to 25.49
−21 ≤ h ≤ 22,
−22 ≤ k ≤ 21,
−54 ≤ l ≤ 34
40419
0.14 × 0.11 × 0.095
2.24 to 25.99
−17 ≤ h ≤ 22,
−22 ≤ k ≤ 22,
−50 ≤ l ≤ 54
41387
reflns collected
ind reflns
28460 [R(int) = 0.0333]
6960 [R(int) = 0.0549]
99.8
6918 [R(int) = 0.0907]
99.8
7223 [R(int) = 0.0592]
99.7
completeness to θ (%)
refinement method
data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
99.4
full-matrix least-squares on F2 full-matrix least-squares on F2 full-matrix least-squares on F2 full-matrix least-squares on F2
28460/56/1721
1.026
6960/15/433
1.051
6918/56/443
1.053
7231/42/409
1.050
R1 = 0.0425,
wR2 = 0.0971
R1 = 0.0607,
wR2 = 0.1065
3.175 and −1.224
R1 = 0.0475,
wR2 = 0.1242
R1 = 0.0585,
wR2 = 0.1333
2.595 and −1.257
R1 = 0.0548,
wR2 = 0.1439
R1 = 0.0820,
wR2 = 0.1685
2.467 and −1.674
R1 = 0.0492,
wR2 = 0.1222
R1 = 0.0647,
wR2 = 0.1327
2.211 and −1.456
R indices (all data)
largest diff. peak and hole(e·Å−3
)
Scheme 1. Synthesis of Ligand LH5
X-ray Crystallography. The crystal data and the cell parameters for
1−4 are given in Table 1. The crystal data for 1−4 have been collected
on a Bruker SMART CCD diffractometer using a Mo Kα sealed tube.
The program SMART20a was used for collecting frames of data,
indexing reflections, and determining lattice parameters, SAINT20a for
integration of the intensity of reflections and scaling, SADABS20b for
absorption correction, and SHELXTL20c,d for space group and
structure determination and least-squares refinements on F2. All the
structures were solved by direct methods using the programs
SHELXS-9720e and refined by full-matrix least-squares methods
against F2 with SHELXL-97.20e Hydrogen atoms were fixed at
calculated positions, and their positions were refined by a riding
model. All non-hydrogen atoms were refined with anisotropic
displacement parameters. The crystallographic figures used in this
manuscript have been generated using Diamond 3.1e software.20f In
the case of 3 (Supporting Information, Figure S2), one carbon atom of
the tertiary butyl group of chelating pivalic acid group is doubly
disordered (C34A and C34B).
Magnetic Measurements. Field dependence of the magnet-
ization at different temperatures and variable temperature (2−300 K)
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magnetic susceptibility measurements on polycrystalline samples were
carried out with a Quantum Design SQUID MPMS XL-5 device
operating at different magnetic fields. Alternating current (ac)
susceptibility measurements were performed using an oscillating ac
field of 3 Oe and ac frequencies ranging from 1 to 1500 Hz. The
experimental susceptibilities were corrected for the sample holder and
diamagnetism of the constituent atoms by using Pascal’s tables.
Scheme 2. Synthesis of Compounds 1−4
RESULTS AND DISCUSSION
■
Synthetic Aspects. The multisite coordinating Schiff base
ligand LH5 was prepared by a three-step synthetic protocol
involving the conversion of the precursor C1 to C3 and its
subsequent condensation with C4 (Scheme 1). Thus, LH5 is
based on a basic phenol-framework and contains two
unsymmetrically disposed arms; one side of the phenol unit
is decorated with a diethanolamine group while the other side is
a hydrazone that has been built by the condensation reaction
involving 2-hydroxyiminopropanehydrazide (C4). Potentially
LH5 has 9 coordination sites although because of steric
restrictions some of them would not be expected to participate
in coordination. Further, the choice of the coordinating groups
has been based on the literature precedent reports that they
would favor binding to lanthanide ions. Thus, −CH2CH2OH
group from diethanolamine part has been shown in its both
deprotonated and protonated forms to be effective for
assembling 3d/4f and 4f compounds.21 Similarly ligands built
from vanillin and hydrazine-based motifs have found use in
preparing mixed-valent manganese-containing polynuclear
compounds.22 Also, in situ generated ligands obtained by the
condensation of hydrazine-based ligands with vanillin have
been used to prepare 4f-containing compounds.23 Another
aspect that was considered in the design of LH5 was that the
ethanolamine side arm is sufficiently flexible and at the same
time can provide a dual mode of binding. Thus, while the free
form can function as a terminal ligand, the deprotonated form
can act in a bridging mode. In accordance with all the above
expectations, LH5 reacts with Ln(NO3)3·nH2O and pivalic acid
(PivH) (in a 1:2:2 stoichiometric ratio, in the presence of
triethylamine as the base) in methanol affording the neutral
macrocyclic octanuclear lanthanide(III) complexes, [Ln8(LH2)4
(μ-Piv)4 (η2-Piv)4 (μ-OMe)4] 1−4 in moderate yields (Scheme
2; see the experimental section for details of syntheses). The
molecular structure of all the four complexes (1−4) was
determined by single-crystal X-ray crystallography (vide infra).
ESI-MS of 1−4 was carried out to check if the structural
integrity of the macrocycles was maintained in solution. While
the parent ion peaks could not be detected, we were able to
find peaks that correspond to [Gd8 (LH2)4 (Piv)5 (OCH3)5
(HOCH3) (OH2)]2+ (m/e: 1683.1), [Gd8 (LH2)4 (Piv)6
(OCH3)4 (OH2)2]2+ (m/e: 1711.2); [Tb8 (LH2)4 (Piv)3
(OCH3)6 (OH)]2+ (m/e: 1587.1), [Tb8 (LH2)4 (Piv)4
( O C H 3 ) 4 ( O H ) 2 ] 2 + ( m / e : 1 6 1 5 . 1 ) ; [ D y 8
(LH2)4(Piv)2(OCH3)8]2+ (m/e: 1574.1), [Dy8 (LH2)4 (Piv)3
(OCH3)6 (OH)]2+ (m/e: 1602.6); [Ho8 (LH2)4 (Piv)4
(OCH3)5 (OH2) (OH)]2+ (m/e: 1656.1), [Ho8 (LH2)4
(Piv)4 (OCH3)4 (OH2)5 (OH)2]2+ (m/e: 1684.7) (see Figure
1 for a representative ESI-MS; see also Supporting
Information).
Figure 1. ESI-MS of 2 (inset picture shows isotopic distribution
pattern of the two octanuclear fragments).
sisting of one triply deprotonated ligand [LH2]3−, two
lanthanide ions, two pivalate ions, and one methoxide ion
(Figure 3). On the other hand the asymmetric unit of 1
contains the full molecule. However, all of the compounds
possess similar structural features. In view of this, in the
following we describe the molecular structure of [Tb8(LH2)4(μ-
Piv)4(η2-Piv)4(μ-OMe)4] (2) as a representative example. The
molecular structure of 2 is given in Figure 2. The structural
X-ray Crystallography. Single-crystal X-ray study revealed
that the compounds 2−4 crystallize in the space group I4(1)/a
whereas compound 1 crystallizes in triclinic space (P1) group.
̅
The asymmetric unit of 2−4 contains one-fourth of the total
molecule, namely, [Ln2(LH2)(μ-Piv)(η2-Piv)(μ-OMe)] con-
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details of all other compounds are given in the Supporting
Information.
Figure 5. View of the of central Tb8 core. Selected bond distances (Å)
and bond angles (deg) are as follows Tb(1)−O(2) = 2.377(6),
Tb(1)−O(4) = 2.312(6), Tb(1)−O(6) = 2.269(6), Tb(1)−O(3) =
2.369(6), Tb(2)−O(2) = 2.353(6), Tb(2)−O(4) = 2.292(6), Tb(2)−
O(3)* = 2.361(6),Tb(2)−O(6)* = 2.266(6), Tb(1)−Tb(2) = 3.854
(8), Tb(2)−Tb(1)* = 3.733 (7). Tb(2)−O(2)−Tb(1) = 109.2(2),
Tb(2)−O(4)−Tb(1) = 113.7(2), Tb(2)*−O(6)−Tb(1) = 110.8(2),
Tb(2)*−O(3)−Tb(1) = 104.2(2), Tb(2)*−Tb(1)−Tb(2) =
121.54(2), Tb(1)*−Tb(2)−Tb(1) = 122.83 (2). * Atoms are
generated by the symmetry operation −y+1/4, x+1/4, −z+1/4 and
y−1/4, −x+1/4, −z+1/4.
Figure 2. Molecular structure of 2 (hydrogen atoms and solvent
molecules were omitted for clarity).
2 is a neutral 16-membered macrocycle (considering the
shortest path involving Tb−O−Tb linkages) and possesses a
wheel type structural topology (Figure 5). The macrocyclic
core is made up of interconnected four-membered Tb2O2 rings
with the terbium ions serving as the spirocyclic nodes. The
assembly of the octanuclear macrocycle is made possible by the
involvement of four triply deprotonated [LH2]3− ligands which
are arranged on the outer surface of the lanthanide wheel. This
structural feature, presumably, is responsible for the favorable
solubility of 2 in chlorinated organic solvents such as
dichloromethane and chloroform. Among the 9 potential
coordination sites on the ligand the hydrazine nitrogen
(adjacent to the carbonyl carbon) and the hydroxylamine
oxygen, do not participate in binding to the metal ions.
Interestingly the CO unit, presumably in its enolate form,
functions as a bridging ligand binding two Tb(III) ions
together. While one of the ethanolamine arms is deprotonated
and acts as a briding ligand, the other, in its free form functions
as part of chelating ligand along the nitrogen atom. Expectedly,
the phenolate oxygen is involved in a bridging coordination
mode. Both the imino nitrogen atoms are monodentate and
form part of the chelating rings around the terbium ions
(Scheme 2, Figure 3). Thus, overall, each LH23− simultaneously
binds to four terbium ions (Figure 4) in a μ4- η1: η2: η1: η2: η1:
η1: η2 fashion. In addition to LH23−, 8 pivalate ions are involved
in coordination; four of these are chelating while four others are
bridging. Finally, four methoxide ions are also involved in a
bridging coordination. Thus, in the octanuclear core two
adjacent Tb(III) ions such as Tb1 and Tb2 are bridged by the
amido oxygen (O2) and one of the deprotonated ethanolic
arms (O4); the immediate neighboring four-membered ring
formed contains Tb1 and Tb2* that are bridged by the
phenolate oxygen (O3) and the methoxide oxygen (O6). The
inter-Tb(III) distance in the four-membered rings ranges from
3.855(4) Å and 3.733(4) with Tb−O−Tb angles of 104.2(2)−
113.7(2)°. Overall, in the macrocycle, there are two types of
Tb(III) ions. Both of these have a coordination number of 8;
however, while Tb1 has a distorted square-antiprism geometry
(7O, 1N), Tb2 has a distorted trigonal dodecahedral geometry
Figure 3. Asymmetric unit of 2 (hydrogen atoms and solvent
molecules are omitted for clarity).
Figure 4. Binding mode of the ligand ([LH2]3−) with Terbium(III)
ions.
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(6O, 2N) (Figure 7). The Tb−O bond distances are nearly
similar (average: 2.369 (6) Å) with the shortest distance of
Interestingly, all the 8 terbium ions present in the macrocycle
are distributed over the 8 vertices of an octagon in a puckered
configuration which resembles the cyclooctadiene ring core
(Figure 6). The two planes, plane1(Tb1*, Tb2*, Tb1**,
Tb2**) and plane2 (Tb1, Tb2, Tb1***, Tb2***) are almost
parallel, and the mean plane deviation of the terbium atoms
from the corresponding planes is given in the Supporting
Information, Figure S4, Table S4−S5. Although some
octanuclear lanthanide complexes are previously known
(Table 2),24 the current family has a different and unique
topology.
Figure 6. Arrangement of Tb8 unit in cyclooctadiene-like
conformation in 2.
The crystal structure of 2 reveals the presence of
intermolecular C−H·····O interactions which assist in the
formation of a supramolecular polymeric association along the
crystallographic a-axis (Supporting Information).
Magnetic Properties. The temperature dependences of
χMT for complexes 1−4 (χM is the molar magnetic
susceptibility per Ln8 unit) in the range 300−2 K were
measured in an applied magnetic field of 0.1 T and are
displayed in Figure 8. At room temperature, the χMT value for 1
Figure 7. (a) Distorted square antiprism geometry around the Tb1 (b)
Distorted trigonal-dodecahedron environment around the Tb2 ion in
2. Selected bond distances (Å) are as follows Tb(1)−O(6) = 2.269(6),
Tb(1)−O(4) = 2.312(6), Tb(1)−O(3) = 2.369(6), Tb(1)−O(9) =
2.369(6), Tb(1)−O(2) = 2.377(6), Tb(1)−O(7) = 2.459(7), Tb(1)−
O(8) = 2.514(7), Tb(1)−N(3) = 2.528(7), Tb(2)−O(6)* =
2.266(6), Tb(2)−O(4) = 2.292(6), Tb(2)−O(2) = 2.353(6),
Tb(2)−O(3)* = 2.361(6), Tb(2)−O(10)* = 2.394(6), Tb(2)−
O(5) = 2.504(6), Tb(2)−N(1) = 2.542(7), Tb(2)−N(4) = 2.577(8).
Figure 8. Temperature dependence of the χMT product for 1−4 . The
red solid line shows the best fit for complex 1.
2.269(6) Å being seen for the Tb−O bond involving the
methoxide oxygen (Tb(1)−O(6)). In contrast, the longest
distance, 2.528(7) Å involves the diethanolamine nitrogen
(Tb(1)−N(3). The other Tb−O and Tb−N bond distances are
unexceptional and are summarized in the caption of Figure 7.
(63.16 cm3 mol−1 K) matches well with that expected for 8
noninteracting Gd3+ ions of 63 cm3 mol−1 K (with S = 7/2, g =
2.0). On lowering the temperature, the χMT for 1 remains
almost constant until ∼50 K and then decreases sharply to
reach a value of 23.8 cm3 mol−1 K at 2 K. As the Gd3+ ions
Table 2. Structural and Magnetic Features of Octanuclear Lanthanide Asemblies
compound
core topology
magnetic property
ref.
b
[Dy8(μ4-CO3)4(L′)8-(H2O)8]·10MeOH·2H2O
[Dy8(OH)6(OMe)6(cmnm)10 (ccnm)2(H2O)2(MeOH)2]·3H2O·10MeOH
tub-like conformation
double cubane
SMM Ueff = 74.2 K, τ0 = 2.1 × 10−6
s
5i
a
c
SMM
24a
24b
c
[Dy8(μ -OH)4(ovn)2(mvn)2(p-NO2bz)14(CH3OH)2]·3.09CH3CN·6CH3OH·
planar S shape
SMM
3a
3H2O
a
c
[Dy8(ovph)8(CO3)4(H2O)8]·12CH3CN·6H2O
[Gd8(L″)4(AcO)8(EtOH)4(H2O)4]·8EtOH·4H2O
Nd8(L″)4(AcO)8(MeOH)4(H2O)8]·4MeOH·24H2O
tub-like conformation
wheel-like core
SMM
24c
a
d
non SMM
24d
a
d
wheel-like core
non SMM
24d
d
[Gd8(LH2)4(μ-Piv)4(η2-Piv)4(μ-OMe)4]·6CH3OH·2H2O
[Tb8(LH2)4(μ-Piv)4(η2-Piv)4(μ-OMe)4]·4CH3OH·4H2O
[Dy8(LH2)4(μ-Piv)4(η2-Piv)4(μ-OMe)4]·8CH3OH
[Ho8(LH2)4(μ-Piv)4(η2-Piv)4(μ-OMe)4]·4CH3OH·4H2O
cyclooctadiene
conformation
non SMM
this work
d
cyclooctadiene
conformation
non SMM
this work
this work
this work
c
cyclooctadiene
conformation
SMM
d
cyclooctadiene
conformation
non SMM
a
Abbreviations: H2L′ = (E)-N′-(2-hyborxy-3-methoxybenzylidene)pyrazine-2-carbohydrazide; cmnm = cyano(imino(methoxy)methyl)nitro-
somethanide; ccnm = carbamoyl-cyano nitrosomethanide; ovnH = o-vanillin; mvnH2 = methyl hemiacetal derivative of o-vanillin; p-NO2bz = p-
b
c
nitrobenzoate; H2ovph = o-vanillin picolinoylhydrazone; H4L″ = sulfonylcalix[4]arene. Ferromagnetic ground state. Antiferromagnetic ground
d
state; absence of maxima in the frequency dependent χM″ vs T plot. Antiferromagnetic ground state.
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present no spin−orbit coupling at the first order, the decrease
of the χMT at low temperature points directly to the presence of
a very weak antiferromagnetic interaction between the Gd3+
ions of the wheel-shaped octanuclear Gd8 complex and/or zero-
field splitting (ZFS) effects of the Gd3+ ions. The field
dependence of the magnetization at 2 K for 1 (Figure 9) shows
using the free ion approximation (94.6, 113.4, and 112.6 cm3
mol−1 K for 2, 3, and 4, respectively) for 8 noninteracting
lanthanide ions: Tb3+(7F6, S = 3, L = 3, g = 3/2, C = 11.82 cm3
mol−1 K), Dy3+ (6H15/2, S = 5/2, L = 5, g = 4/3, C = 14. cm3
mol−1 K), and Ho3+ (5I8, S = 2, L = 6, g = 5/4, C = 14.08 cm3
mol−1 K). The χMT product decreases with decreasing
temperature, first slowly down to 100 K and then rapidly to
51.7, 83.5, and 67.0 cm3 mol−1 K at 2.0 K for 2−4, respectively.
The decrease observed in the χMT values is most likely due to a
combination of some very weak Ln3+···Ln3+ antiferromagnetic
interactions and possibly other effects such as magnetic
anisotropy/ligand field and thermal depopulation of the Stark
sublevels of the 2S+1ΓJ ground state of the Ln3+ ion. The
existence of very weak antiferromagnetic interactions in
complexes 2−4 is not unexpected in view of the fact that
isostructural Gd3+, Tb3+, Dy3+, and Ho3+ complexes generally
display magnetic exchange interactions of the same nature.27
The magnetic data on these Ln3+ systems clearly show a
nonzero spin ground state presumably originated from the
closely spaced MJ Stark sublevels.28
Figure 9. M vs H plots for 1 at 2 K. The red solid line represents the
Brillouin function for 8 noninteracting Gd3+ ions.
The field dependence of the magnetization at 2 K for
compounds 2−4 (Figure 10) shows a slow increase of the
a sigmoidal shape at low field, which supports the
antiferromagnetic interaction and/or ZFS in this complex. As
expected, the experimental magnetization values are well below
the Brillouin function for 8 noninteracting Gd3+ions. At high
field the saturation of the magnetization is almost complete at 5
T, reaching a value of 55.35 NμB, which agrees well with the
theoretical saturation value for 8 Gd3+ ions of 56 NμB.
The magnitude of the antiferromagnetic exchange interaction
in 1 could not be determined by diagonalization matrix
methods because the extremely high dimension of the matrices
to be diagonalized for a Gd8 system. It should be noted that
there are two different types of bridging units in 1 with double
dialkoxo and triple alkoxo/phenoxo/pivalate bridging groups.
As previously pointed by some authors for diphenoxo-bridged
Gd2 complexes,25 it seems that J becomes more negative as the
Gd−O−Gd, and consequently the Gd···Gd, decrease. In view
of this, the larger magnetic coupling (J) would take place
between the Gd3+ ions connected by a triple bridge (short
Gd···Gd distance), whereas the smaller magnetic coupling (J′)
would be assigned to the dialkoxo-bridging pathway (long
Gd···Gd distance). To estimate the value of the magnetic
exchange coupling in 1 we have used a very crude model, in
which each wheel has been considered to be formed by 4 equal
dinuclear Gd2 units with an intradinuclear magnetic interaction
characterized by the J parameter and interdinuclear interactions
that can be calculated by using the molecular field theory.
Taking into account the above considerations, the experimental
data were analyzed with the following Hamiltonian:
Figure 10. Field dependence of the magnetization for compounds 2−
4.
magnetization at low field and a linear increase at high field
without achieving a complete saturation at 5 T. This behavior
suggests the presence of a significant magnetic anisotropy and/
or more likely the presence of low-lying excited states that are
partially [thermally and field-induced] populated. These low-
lying excited states are in agreement with weak magnetic
interactions expected for 4f-4f systems. The magnetization
values for 2−4 at 5 T (40.1, 46, 49.29 NμB, respectively) are
considerably smaller than the expected saturation magnet-
ization value, Ms/NμB = 8gJJ, for 8 Ln3+ ions. The observed
values at 5 T per Ln3+ ion are similar those estimated and
observed for mononuclear Ln3+ complexes where the ligand-
field effects eliminates the J-fold degeneracy of the 2S+1ΓJ
ground state.28b,29
Dynamic ac magnetic susceptibility measurements as a
function of the temperature at different frequencies were
performed on complexes 2−4 under zero-external field but
none of them showed a frequency dependency of the in-phase
(χ′M) and out-of-phase (χ″M) signals. This behavior could be
due to the existence of a fast quantum tunneling relaxation of
the magnetization (QTM) promoted by intermolecular dipolar
interactions and/or hyperfine interactions. When the ac
measurements were performed in the presence of a small
external dc field of 1000 Oe, to fully or partly suppress the
possible fast quantum tunneling relaxation, only compound 3
showed slow relaxation of the magnetization but without
H = −JSGd1 Gd2 − zJ′ Sz Sz
S
The best fitting parameters were J = −0.19 cm−1, zJ′ = −0.1,
and g = 2.018 with R =1.2 × 10−7. Although the obtained values
are in good agreement with the reported coupling constants for
phenoxo-bridged GdIII system complexes, with or without
additional carboxylate bridging groups,25,26 they should be
taken with caution because of (i) the crudeness of the model,
(ii) J and J′ are correlated, (iii) the possible existence of ZFS
splitting of the Gd3+ ions.
The room temperature χMT values of complexes 2−4 are
96.9, 118.9, and 113 cm3 mol−1 K, respectively, which are in
rather good agreement with the expected theoretical values
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Jiang, F.-L.; Hong, M.-C. Inorg. Chem. 2012, 51, 13128. (g) Liao, S.;
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exhibiting any maximum in the temperature dependence of χ″M
above 2 K at frequencies reaching 1400 Hz (see Supporting
Information, Figure S10). This behavior suggests that 3 may
exhibit SMM behavior below 2 K.
CONCLUSION
■
In summary, we have synthesized a series of macrocyclic,
isostructural, neutral octanuclear lanthanide complexes; the
{Ln(III)8} core has a cyclooctadiene-like conformation. Over-
all, the macrocycle contains two types of lanthanides, both with
a coordination number of 8; while the geometry around one of
these is distorted square pyramidal, the other is distorted
trigonal dodecahedron. Compounds 1−4 exhibit antiferromag-
netic interactions between the Ln3+ ions through the phenoxo,
alkoxo, and pivalato bridging groups. None of these compounds
exhibits slow relaxation of the magnetization at zero applied dc
magnetic field which is due to the existence of a fast QTM.
However, in the case of 3, the application of a small dc field is
enough so as to fully or partly suppress the fast and efficient
zero-field quantum tunneling of magnetization allowing the
observation of slow relaxation above 2 K.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data in CIF format. Further details are given in
Figures S1−S10 and Tables S1−S6. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: vc@iitk.ac.in.
■
Notes
The authors declare no competing financial interest.
⊥E.C.: E-mail: ecolacio@ugr.es.
ACKNOWLEDGMENTS
■
V.C. is thankful to the Department of Science and Technology
for a J. C. Bose fellowship. P.B. thanks the Council of Scientific
and Industrial Research, India, for a Senior Research Fellow-
ship. E.C. thanks the Ministerio de Educacion Culturay
́
Deporte (Spain) (Project CTQ2011-24478), the Junta de
́
Andalucia (FQM-195), and the Universidad de Granada for
financial support.
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