Single-Molecule Magnetism, Enhanced Magnetocaloric Effect, and Toroidal Magnetic Moments in a Family of Ln4 Squares

Article abstract of DOI:10.1002/chem.201502720

Three cationic [Ln₄] squares (Ln=lanthanide) were isolated as single crystals and their structures solved as [Dy₄(μ₄-OH)(HL)(H₂L)₃(H₂O)₄]Cl₂·(CH₃OH)₄

Full text of DOI:10.1002/chem.201502720

DOI: 10.1002/chem.201502720
Full Paper
&
Single-Molecule Magnets
Single-Molecule Magnetism, Enhanced Magnetocaloric Effect, and
Toroidal Magnetic Moments in a Family of Ln Squares
4
[a]
[a]
[a]
[c]
[b]
Chinmoy Das, Shefali Vaidya, Tulika Gupta, Jamie M. Frost, Mattia Righi,
[
c]
[b]
[a]
Euan K. Brechin, Marco Affronte,* Gopalan Rajaraman,* and
[a]
Maheswaran Shanmugam*
À1
Abstract: Three cationic [Ln ] squares (Ln=lanthanide) were
isolated as single crystals and their structures solved as
J = +0.01 and J =À0.01 cm for 1 as the two distinct ex-
4
1
2
III
change interactions between the Dy ions. Similar parame-
[
Dy (m -OH)(HL)(H L) (H O) ]Cl ·(CH OH) ·(H O) (1), [Tb (m -
ters are also obtained for complex 1 (and 2) from specific
4
4
2
3
2
4
2
3
4
2
8
4
4
OH)(HL)(H L) (MeOH) ]Cl ·(CH OH) ·(H O) (2) and [Gd (m -
heat measurements. A very weak antiferromagnetic super-
2
3
4
2
3
4
2
4
4
4
À1
OH)(HL)(H L) (H O) (MeOH) ]Br ·(CH OH) ·(H O)
(3).
The
exchange interaction (J =À0.043 cm and g=1.99) is ob-
2
3
2
2
2
2
3
4
2
3
1
structures are described as hydroxo-centered squares of lan-
thanide ions, with each edge of the square bridged by
served between the metal centers in 3. The magnetocaloric
effect (MCE) was estimated by using field-dependent mag-
netization and temperature-dependent heat-capacity meas-
urements. An excellent agreement is found for the ÀDSm
values extracted from these two measurements for all three
2À
a doubly deprotonated H2L ligand. Alternating current
magnetic susceptibility measurements show frequency-de-
pendent out-of-phase signals with two different thermally
assisted relaxation processes for 1, whereas no maxima in
cM“ appears above 2.0 K for complex 2. For 1, the estimated
effective energy barrier for these two relaxation processes is
complexes. As expected, 3 shows the largest ÀDS variation
m
À1 À1
(23 JKg K ) among the three complexes. The negligible
magnetic anisotropy of Gd indeed ensures near degeneracy
in the (2S+1) ground state microstates, and the weak
super-exchange interaction facilitates dense population of
low-lying excited states, all of which are likely to contribute
to the MCE, making complex 3 an attractive candidate for
cryogenic refrigeration.
2
9 and 100 K. Detailed ab initio studies reveal that complex
possesses a toroidal magnetic moment. The ab initio cal-
1
culated anisotropies of the metal ions in complex 1 were
employed to simulate the magnetic susceptibility by using
the Lines model (POLY_ANISO) and this procedure yields
Introduction
single-molecule-magnet (SMM) behavior in a mononuclear
[
6]
terbium phthalocyanine complex. Following this, several lan-
thanide-based SMMs, in particular dysprosium-based clusters,
ranging from monomers to tetracosanuclear species, have
The large magnetic moments and unquenched first-order orbi-
tal angular momenta associated with certain lanthanide ions
make their coordination complexes potentially useful in vari-
[
1h,7]
been reported.
Among them, multinuclear clusters often
[
1]
[2]
ous applications, such as information storage, spintronics,
show unusual magnetic properties. Fascinating recent exam-
[
3]
[4]
[5]
molecular switches, spin valves, and qubits. The academic
investigation of the magnetic behavior of lanthanide-based
molecular cages has been reinvigorated since the discovery of
ples include Dy triangles in which the SMM behavior origi-
3
[
8]
nates from the excited states, Dy and Dy K clusters exhibit-
5
4 2
[
1b]
ing enormous effective-energy-barrier (U ) values, and the
eff
presence of magnetic bistability in symmetric, oligomeric, lan-
thanide complexes as a result of a toroidal arrangement of ani-
[
a] C. Das, S. Vaidya, T. Gupta, Prof. Dr. G. Rajaraman, Prof. Dr. M. Shanmugam
Department of Chemistry, Indian Institute of Technology Bombay
Powai, Mumbai, Maharashtra, 400076 (India)
E-mail: eswar@chem.iitb.ac.in
[
7b]
sotropic axes.
Although various promising synthetic approaches have been
reported in efforts to increase the Ueff value, the blocking tem-
rajaraman@chem.iitb.ac.in
perature (T ) associated with the onset of hysteresis in molecu-
B
[
b] M. Righi, Prof. Dr. M. Affronte
[
9]
lar systems still sits below T=14 K, hampering any realistic
development of molecular devices for practical applications.
This is partly due to the lack of understanding of the com-
bined roles of orbital (L) and spin (S) angular momenta to the
resultant magnetic moment of the molecule. Moreover, further
complications may arise due to the varying contributions of L
CNR Institute of Nanosciences S3 and Dipartimento di Scienze Fiscihe
Infomatiche e Matematiche, Università di Modena e Reggio Emilia
via G. Campi213 A, 41125 Modena (Italy)
E-mail: marco.affronte@unimore.it
[
c] J. M. Frost, Prof. Dr. E. K. Brechin
EaStCHEM School of Chemistry, The University of Edinburgh
David Brewster Road, Edinburgh, EH9 3FJ (UK)
[
10]
and S with different geometries around the lanthanide ions.
Molecular complexes constructed of metal ions with negligible
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502720.
Chem. Eur. J. 2015, 21, 15639 – 15650
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Full Paper
III
magnetic anisotropy, such as Gd , have been proposed for var-
ious applications, including contrast agents in magnetic reso-
[
11]
[12]
nance imaging,
spin-labeling,
dynamic nuclear polariza-
[
13]
tion, and cryogenic refrigeration, which takes advantage of
[
14]
the intrinsic magnetocaloric effect (MCE) of these materials.
Following the idea proposed by Shull and co-workers of em-
[15]
ploying nanoparticles for magnetic refrigeration, a plethora
of 0–3D molecule-based materials have been reported as
promising candidates, particularly by Evangelisti and co-work-
ers, who recently described certain molecular requirements, in-
cluding the importance of a large magnetic density (i.e., the
use of lightweight ligands), in achieving improved MCE effi-
[
14e,16]
ciency, together with large S, weak J, and negligible D.
In this line of interest and to reveal a new generation of lan-
thanide based clusters with both isotropic and anisotropic lan-
thanide metal ions, we have employed the multidentate Schiff
base ligand (H L, Scheme 1), for which the coordination poten-
4
[10a,17]
tial has been previously investigated.
Scheme 1. Synthetic scheme followed to make H
Section for full details.
4
L. See the Experimental
Figure 1. A) A representative crystal structure of the cationic complexes 1–3.
B) The general metal core found in complexes 1–3. The same labelling
scheme has been followed for all three complexes.
III
In this article, we report three analogous Ln squares of
4
III
general formula [Ln (m -OH)(HL)(H L) (Y) ]X cluster (Ln=Dy ,
Table 1. Crystallographic parameters for complexes 1–3.
4
4
2
3
4
2
III
III
Tb , Gd ; Y=solvent, X=halide). The structures were deter-
mined by single-crystal X-ray diffraction. The magnetothermal
properties were investigated by variable-temperature, variable-
field, susceptibility, magnetization and specific heat measure-
ments. To further support our experimental findings, we per-
formed CASSCF+RASSI-SO calculations. This detailed experi-
mental and theoretical investigation reveals that complex 1 ex-
hibits a rare toroidal magnetic moment, which is evident from
the magnetization measurements, and complex 3 shows large
1
2
3
formula
size [mm]
system
space group
a [Š]
b [Š]
c [Š]
a[8]
b [8]
C
48
H
68
N
4
O
25Cl
2
Dy
4
C
52
H
72
N
4
O
25Cl
2
Tb
4
C
52 72 4 2 4
H N O25Br Gd
0.17”0.12”0.09 0.16”0.11x 0.08 0.16”0.13”0.09
triclinic
P1¯
triclinic
P1¯
triclinic
P1¯
14.000(3)
15.650(3)
17.790(4)
101.07(3)
91.48(3)
106.25(3)
3659.3(13)
2
13.914(5)
15.625(6)
18.201(7)
102.620(4)
92.951(5)
105.233(5)
3701(2)
2
14.029(7)
15.692(8)
18.363(9)
103.411(7)
92.689(5)
105.499(7)
3764(3)
2
variation in ÀDS , making it a promising candidate for cryo-
g[8]
V [Š ]
m
3
genic refrigeration.
Z
1
À3
calcd [gcm
]
1.654
1.669
1.714
2
q
max
47.64
MoKa
0.71073
100
27407
58.26
MoKa
0.71075
100
65208
50.64
MoKa
0.71073
100
50807
Results and Discussion
radiation
l [Š]
T [K]
The reaction between the Schiff base ligand H L and a lantha-
4
nide halide (DyCl ·6H O, TbCl ·6H O or GdBr ·6H O) in the pres-
3
2
3
2
3
2
reflns
ence of NaOMe in methanol results in the formation of pale
yellow single crystals of complexes 1–3 after one week.
X-ray diffraction reveals the structure to be [Ln (m -
ind. reflns
reflns with
I>2s(I)
R1
11213
6798
19250
15596
13500
11560
4
4
0.0660
0.1562
0.0689
0.1825
0.0806
0.2266
III
OH)(HL)(H L) (Y) ]X (where Ln=Dy , Y=H O, X=Cl (1); Ln=
2
3
4
2
2
wR2
III
III
Tb , Y=MeOH, X=Cl (2); and Ln=Gd , Y=MeOH, X=Br (3)).
The molecular structures of all three complexes are isomor-
phous, a representative structure is given in Figure 1, with indi-
vidual structures in Figure S1 in the Supporting Information.
The corresponding crystallographic data is listed in Table 1.
The lanthanide ions are in their usual trivalent oxidation state
and occupy the corners of a square, at the center of which lies
À
a m -OH ion, linking all four lanthanide ions. The oxidation
4
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state of this m -bridging O atom was confirmed by bond va-
intermolecular hydrogen bonding (O13···O73=2.721(5);
4
lence sum calculations (Table S1 in the Supporting Informa-
O33···O53=2.611(8);
C12···O73=2.636(6);
C17···O73=
[
18]
tion). The Ln ions are not coplanar; there is a small twist of
2.657(4) Š).
1
98 (for 1) and 18.88 (for 2 and 3) in the dihedral angle within
Careful analysis of the bond lengths of the noncoordinated
hydromethyl groups of all the Schiff base ligands in 1 reveals
that C19–O13 (1.332(17) Š) has a significantly shorter bond
length than the other three (C39–O33 (1.401(18)), C59–O53
À
the Ln plane (Figure 2D). The m -OH ion possesses a distorted
4
4
(
1.425(20), C79–O73 (1.410(15) Š)). This suggests that only the
C19–O13 arm is deprotonated. A similar trend is followed for
complexes 2 and 3. Each lanthanide ion in 1–3 is surrounded
by nine donor atoms (LnO N ) and exists in a distorted, mono-
8
1
capped, square, anti-prismatic geometry, as confirmed by con-
[
19]
tinuous measurements with the SHAPE software. The square
Ln -m -OH] core is rather rare, limited to only three complexes,
[
4
4
based on a recent search (June 2015) of the Cambridge Struc-
[
20]
tural Database (CSD).
For example, Wong and co-workers reported the complex
[
Ln (m -O)L (NO ) (MeOH) ] (Ln=Gd and Tb; L=1,3-bis(2-hy-
4
4
2
3 4
2
droxy-3-methoxybenzylamino)propan-2-ol) in 2001, in which
[
20c]
the Ln ions are eleven coordinate,
and Thompson et al. re-
ported a series of [2x2] square grids of formula [Ln (L1) (m -
4
4
4
O)(m -1,1-N ) ] (Ln=Dy and Tb) with carbohydrozone (L1) li-
2
3 4
20b,21]
[
gands.
In the latter each metal ion is surrounded by nine
donor atoms in a capped, square, anti-prismatic geometry.
Magnetic susceptibility
Temperature-dependent direct current (dc) magnetic suscepti-
bility measurements on polycrystalline samples of 1–3 were
carried out between T=300 and 2.0 K in an external magnetic
field of H=0.1 T (Figure 3). The observed room temperature
Figure 2. Average bond lengths and bond angles for 1–3 (A–C). D) A view to
highlight the distortion in the Ln square, with a Ln···Ln···Ln···Ln dihedral
4
angle of 198.
3
À1
c T values of 55.60 (for 1), 47.20 (for 2), and 30.98 cm Kmol
M
square planar geometry in all three complexes (Figure 2). The
average diagonal bond angles found in these complexes are
(for 3) are slightly lower than the expected values of 56.67 (g=
6
7
4/3; S=5/2; L=5; H
for 1), 47.25 (g=3/2; S=3; L=3; F
6
15/2
3
À1
8
]
Ln1O1Ln3=166.28 and ]Ln2O1Ln4=166.68.
for 2), and 31.5 cm Kmol (g=2.0; S=7/2; L=0, S for 3)
7/2
III
III
III
III
The bond lengths between the Ln ions and the central hy-
for magnetically dilute Dy , Tb , and Gd ions, respectively.
droxide ion range from 2.506 to 2.529 Š (for 1), 2.494 to
.553 Š (for 2), and from 2.506 to 2.586 Š (for 3). The average
Ln–Ln distance is 3.577 (for 1), 3.592 (for 2), and 3.623 Š (for
). Each edge of the square is bridged by one phenoxo
The c T values for 1 and 2 gradually decrease from room tem-
perature to T=45 K, likely due to the depopulation of mJ
M
2
levels, with the depopulation effects being more evident in
3
1 than in 2. The c T value drops rapidly below 45 K, and reach-
M
2À
3
À1
oxygen (from the o-vanillin moiety) from one H2L and an
es a value of 25.4 (for 1) and 26.99 cm Kmol (for 2) at 5.0
2À
alkoxo O atom from the hydroxymethyl arm of a second H2L
and 2.5 K, respectively. On the contrary, for 3, the c T value re-
M
ligand. The remaining coordination sites of the Ln ions are
filled by methoxy O atoms of o-vanillin, azomethine N atoms,
the O atom from the second arm of the hydromethyl group of
mains constant from room temperature to T=100 K, below
3
À1
which it steadily decreases to a value of 12.95 cm Kmol at
2.31 K. The decrease in c T values for all three complexes at
M
2
À
H2L (the third arm of the latter remains protonated and non-
bonded), and terminally bonded water and methanol mole-
cules.
lower temperatures is due to the combined effects of some, or
all, of the following phenomena: magnetic anisotropy (1, 2), in-
tramolecular antiferromagnetic exchange interactions, intermo-
lecular dipolar exchange interactions, and thermal depopula-
tion effects.
There is a significant difference in the observed LnÀO bond
lengths between the bridging alkoxide (2.262–2.320(8) (1),
2
.278–2.322(7) (2), and 2.297–2.362(8) Š (3)) and the terminally
Isothermal magnetization M(H) measurements performed on
polycrystalline samples of 1–3 are summarized in Figure 3,
with full details reported in the Supporting Information (Figur-
es S2–S4). From Figure 3 it is evident that complexes 1 and 2
bonded alcohol (2.453–2.514(15) (1), 2.457–2.492 (10) (2), and
2
.479–2.511(8) Š (3)), suggesting the latter to be protonated.
The charge-balance requirements further support the as-
sumption of two halide counter ions present in the crystal lat-
show saturation at approximately 19.3 and 15.3 Nm at 2.0 and
B
tice, balancing the 2+ cationic charge of the metal cluster. The
2.2 K, respectively. These values are approximately half the ex-
pected value for four noninteracting or very weakly coupled
2À
nonbonded arm of the hydromethyl group of H2L facilitates
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H ¼ À2ðJ1SGd1SGd2 þ J1SGd2SGd3 þ J1SGd3SGd4 þ J1SGd1SGd4
À2ðJ2SGd1SGd3 þ J2SGd2SGd4Þ þ m gm H
Þ
s
B
To probe the magnetization relaxation dynamics of both
1
and 2, ac magnetic susceptibility measurements were per-
formed on polycrystalline samples of 1 (Figure 4) and 2 (Fig-
ure S6–S7 in the Supporting Information) at various frequen-
cies. For 2, the maxima in c “ is well below the instrument
M
temperature limit in the presence and absence of an applied
external dc magnetic field (Figure S7). On the contrary, for
complex 1 there are two thermally assisted c ” signals with
M
maxima clearly evident corresponding to fast and slow mag-
netization relaxation processes at low (T=2.5–5.2 K) and high
(
T=3.0–15 K) temperatures, respectively (Figure 4). Similar be-
havior was recently reported in related tetrameric clusters by
[
23]
Layfield and co-workers. However, the origin of two magnet-
ization relaxation processes in the latter complex are distinctly
different from 1 (see below). Lanthanide complexes are prone
M
Figure 3. A) Variable-temperature c T plots for complexes 1–3, measured
with an applied field of 0.1 T. The line denotes the best fit obtained for com-
plex 3 with the parameters given in the text. B) Isothermal field-dependent
magnetization measurements for 1–3 measured from H=0–7 T at T=2.2 K.
lanthanide ions, indicating the presence of significant magnet-
[
1h]
ic anisotropy, and consistent with the non-superimposable
reduced magnetization curves (Figure S5 in the Supporting In-
formation). On the contrary, the magnetization of complex 3 is
III
as expected for four weakly coupled, isotropic Gd ions, with
M close to saturating at a value of 28 NmB.
To extract the spin Hamiltonian parameters for the isotropic
cluster 3, magnetic susceptibility data were fitted by matrix di-
[22]
agonalization with the PHI software. Based on the crystal
structure parameters of 3, we initially employed two different J
values: J corresponds to nearest-neighbor interactions round
1
the ‘outside’ of the square where the super-exchange interac-
tion is mediated between the metal ions through phenoxo,
alkoxo, and m -hydroxide bridges, and J describes the next-
4
2
nearest-neighbor (diagonal) interactions (Gd1O1Gd3 and
Gd2O1Gd4) mediated exclusively by the central m -hydroxide.
4
The Heisenberg Hamiltonian used for fitting the data is shown
in the equation below. The experimental data are very well re-
produced with the following parameters J =À0.043, J =
1
2
À1
À0.043 cm with g=2.0. Because J =J , we repeated the fit
1
2
À1
with just one J value; which afforded J =À0.043 cm and g=
1
1
.99 (Figure 3). The fitting parameters clearly suggest that the
exchange is very weak and the cluster can essentially be re-
garded as a paramagnet. Such weak exchange interactions be-
tween the metal centers are an essential ingredient for observ-
ing an enhanced MCE (see below).
M M
Figure 4. Frequency-dependent A) in-phase (c ’) and B) out-of-phase (c “) ac
magnetic susceptibility for 1 at the indicated frequencies. C) Arrhenius plot
constructed for both slow and fast relaxation processes.
Chem. Eur. J. 2015, 21, 15639 – 15650
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[
1b]
[28]
to exhibit fast quantum tunneling of magnetization (QTM),
gram to extract the exchange-coupled energy levels and the
exchange parameters. We begin our discussion with the calcu-
lation of the single-ion anisotropy parameters, followed by
a detailed analysis of the exchange spectrum.
likely to be operable in the low-temperature region, however
the thermally assisted Orbach process appears to be the domi-
nant relaxation process in complex 1. Observation of frequency
dependent out-of-phase susceptibility (c “) signals (for 1) is
M
a characteristic signature of SMM behavior under zero applied
external magnetic field. The Arrhenius plot constructed for
these high and low T regimes are shown in Figure 4C. The Ueff
value for the reorientation of magnetization for the fast and
III
Single-ion anisotropy of Dy
The energy spectrum and g-tensors for the Kramers doublets
6
III
(
KDs) of the ground H1 multiplet for the four Dy ions in
5/2
slow relaxation processes are 29 and 100 K with t =1.324”
0
compound 1 are shown in the Supporting Information (Ta-
bles S3–S6), with the excited states lying at 3000 cm . In 1,
the ground state (GS) KD (Figure 6) shows an almost Ising type
À7
À8
1
0
and 1.179”10 s, respectively. The deviation from lineari-
À1
ty for the slow relaxation process below 7.0 K suggests that
other relaxation processes, such as Raman and direct process-
anisotropy for all four metal sites (Table 2) that is, g is close to
zz
[10e,24]
es, are also likely to be operable.
The presence of two relaxations processes in ac data is clear-
ly visible in the Cole–Cole plot (Figure 5). Attempts to fit the
observed two relaxation processes with the generalized Debye
Figure 5. Cole–Cole plot of complex 1 measured in the absence of external
magnetic field (Hdc =0).
Figure 6. Ab initio computed ground KD state anisotropy axis along the z di-
rection in 1. The arrows exhibit coupled-state orientations (ferromagnetic in-
teraction between neighbors); N, C, and H atoms have been removed for
clarity. The gzz orientations in all the Dy sites differ due to the underlying
structural distortions at the individual metal sites.
III
model failed; hence we fitted only the major relaxation process
(
Table S2 in the Supporting Information). The a-value ranges
from 0.39 to 0.51. Further relaxation data have been measured
for 1 in the presence of an external magnetic field of H=0.2 T
between T=2.0–25 K. Figure S8 in the Supporting Information
shows that the slow relaxation process observed at higher
temperature is more prominent than the fast relaxation pro-
cess observed at lower temperature.
Table 2. Calculated energy spectrum, g tensors and tilt angles (q) of the
principal anisotropy axes of first excited state (ES) on Dy1, Dy2, Dy3, and
Dy4 sites with respect to the GS for complex 1.
À1
Sites
Dy1
gxx
gyy
gzz
Energy [cm
]
q [8]
GS KD
0.049
0.976
0.043
1.192
0.091
1.799
0.090
0.168
0.103
1.982
0.069
3.127
0.199
5.654
0.023
0.253
19.794
15.819
19.768
14.998
19.554
12.629
19.868
16.968
0.000
134.6
0.000
147.8
0.000
145.0
0.000
190.0
–
The Arrhenius plot constructed for the high temperature re-
1st ES KD
GS KD
14.6
–
10.5
–
5.5
–
À9
laxation process yields U =194 K with t =6.38”10 s. This
Dy2
Dy3
Dy4
eff
0
1
st ES KD
GS KD
st ES KD
experiment unambiguously confirms that QTM is one of the
major relaxation processes in the ac relaxation dynamics in the
absence of an external magnetic field. This could be the likely
reason why the estimated energy barrier is significantly lower
1
GS KD
1st ES KD
168.9
[
25]
than the computed effective energy barrier (see below).
To understand the electronic structures of anisotropic com-
[26]
plex 1, detailed ab initio calculations were performed (see
the Supporting Information for computational details and Fig-
ure S9). As complex 1 exhibits SMM characteristics, calculations
were performed only on this structure. Two sets of calculations
were performed on complex 1. Primarily, using the SINGLE A-
20, with a small transverse anisotropy. The axiality of the g-ten-
sors gradually decreases up to fourth KD (Tables S3–S6 in the
Supporting Information), thereafter increasing and reaching ax-
iality for the 8th KD, which is comparable to the value of
lowest KD (close to 20). The observation of such mirror sym-
metry in the magnetic properties in the KDs is in sharp con-
trast to the trend in perfectly axial systems. The relative ener-
[
27]
NISO program, we calculated the magnetic anisotropy of the
III
individual Dy ions. We then employed the POLY_ANISO pro-
Chem. Eur. J. 2015, 21, 15639 – 15650
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gies of the eight lowest lying KDs along with the computed
III
anisotropy for all four Dy ions in complex 1 are given in Ta-
bles S3–S6 in the Supporting Information. The ground state is
found to possess zero magnetic moment in the xy plane and is
entirely oriented along the z axis (L ). The two lowest KDs,
z
shown in Figure 6, are characterized by definite projection of
the total angular moment of the anisotropy axis. At the Dy4
site the ground- and first-excited-state anisotropic axes are in
opposite directions, whereas in the other three sites they are
co-parallel.
To probe the mechanism of single-ion relaxation, data
beyond the ground state KDs need to be analyzed. Magnetic
relaxation of lanthanide species occurs due to three main fac-
[
10e]
tors in the absence of intermolecular interactions:
) through QTM between the ground state KDs as a conse-
quence of large transverse anisotropy in the ground state KDs,
1
[29]
2
) through Orbach/Raman processes which account for the
relaxation via the excited states and occurs due to the non-co-
incidence of the principle anisotropic axes, and 3) through
thermally assisted QTM (TA-QTM), which accounts for relaxa-
tion via the excited states due to the non-Ising nature of the
excited KDs. A qualitative mechanism of the relaxation for 1 ob-
tained from ab initio calculations is shown in Figure 7. Here
the states are arranged according to the values of the magnet-
ic moments. The number at each arrow connecting any two
states is the mean absolute value of the matrix elements of
the transition magnetic moments between the corresponding
states. For all the Dy sites (Dy1, Dy2, Dy3, and Dy4) the trans-
verse anisotropy observed is small, and QTM between the
ground state KDs is expected to be weak.
Figure 7. The ab initio computed magnetization blocking barriers for A) the
Dy1 site (similar energy levels are found for the Dy2 and Dy3 sites) and
B) the Dy4 site. The black lines indicate the KDs as a function of magnetic
moment. The dotted green lines show the possible Orbach processes. The
dotted blue lines show the most probable relaxation pathways for magneti-
zation reversal. The dotted red lines represent the presence of QTM/TA-QTM
between connecting pairs. All excited states for the Dy1/Dy2/Dy3 and Dy4
sites in complex 1 are strong admixtures of different Æm
J
levels. Hence, the
Our calculations confirm this point, as shown in Figure 7; for
both the Dy1 and the Dy4 center the QTM between the
ground state KDs is negligible. The next excited states are lo-
mixing of these wavefunctions has been represented by letters for which
the corresponding wavefunction descriptions have been described in the
computational section of the Supporting Information.
À1
cated at 135, 148, and 145 cm for the Dy1, Dy2, and Dy3
centers, respectively, and these states possess significant trans-
verse anisotropy. The g axis deviates from the direction of the
In accordance with the theoretical prediction, there are two
relaxation processes that are experimentally observed at 20.15
zz
ground state KD by 14.6, 10.47, and 5.478 for the Dy1, Dy2,
and Dy3 centers, respectively (tilt angle, q). This naturally acti-
vates the Orbach/Raman type relaxation via the first excited
state and, as expected, a significant magnetic moment matrix
element is observed for this process (Figure 7A). The trans-
verse anisotropy of the first excited state is large; this leads to
a significant TA-QTM process, which is also reflected in the
computed parameters. The ground and first excited state are
predominantly j Æ15/2>: 0.99j Æ15/2> and j Æ3/2>: 0.91j
Æ13/2>, respectively, for the Dy1, Dy2, and Dy3 sites.
À1
and 69.5 cm (29 and 100 K respectively, Figure 4). Hence, we
can tentatively assign the faster relaxation process, which
occurs at lower temperature, to the Dy1, Dy2, and Dy3 sites,
and the slower magnetization relaxation process, which occurs
at higher temperature, to Dy4. The large deviation in the Ueff
values between the calculated and experimental results can be
attributed to the fact that the ground state QTM and the dipo-
lar/exchange interactions (intra and inter) are not taken into
account in the estimate of the computed barriers. Attempts to
III
III
III
The Dy4 site on the other hand is distinctly different from
the other three sites in complex 1, where the transverse aniso-
tropy is not prominent in the ground state or the first excited
state. However, the tilt angle is large, which leads to relaxation
isolate the diamagnetic analogue (using Y , La , or Lu ) of
complexes 1–3 were not successful. However, we would like to
emphasize that the above-listed parameters significantly influ-
ence the experimental barrier height as shown by us in an un-
III
via the first excited state. Thus for Dy4, the computed effective
related Er complex, for which a tenfold increase in U was
eff
À1
[30]
energy barrier (U ) is 190.0 cm , which is marginally higher
observed with and without dilution (50 and 5 K). We believe
cal
III
than for the other sites. Thus, for the uncoupled Dy sites, two
a similar scenario is also operational for complex 1. The argu-
ment of magnetization relaxation based on single-ion anisotro-
py is valid because the estimate of the exchange coupling be-
relaxation processes are theoretically expected to operate, one
À1
at an average Ucal of 144 cm (Dy1, Dy2 and Dy3) and the
À1
III
other at 190 cm (Dy4).
tween the Dy ions is extremely small (see below).
Chem. Eur. J. 2015, 21, 15639 – 15650
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To offer a rationale on the differences observed between the
four Dy sites, we have examined the structural distortion
[19]
(
using the SHAPE software) at the individual Dy sites. The
deviations are found to be 1.18, 1.22, 1.20, and 1.08 for the
Dy1, Dy2, Dy3, and Dy4 sites, respectively (see Figure S9 in the
Supporting Information), compared with an ideal, monocap-
ped, square anti-prismatic geometry.
Thus the larger distortion of Dy1, Dy2, and Dy3 compared
with Dy4 leads to concomitant smaller barrier heights. To gain
more insight into the mechanism of magnetization relaxation,
we have also computed the crystal-field parameters. If the in-
termolecular and hyperfine interactions are small or negligible,
the probability of a QTM between the ground state KDs is best
[
10h,31]
described by the crystal-field (CF) parameters.
The corre-
q
q
sponding CF Hamiltonian can be expressed as, H =B O ,
CF
k
k
q
q
where Bk represents the crystal field parameter and Ok repre-
Figure 8. The low-lying exchange spectrum and magnetization blocking bar-
rier in complex 1. We have placed all exchange states in compliance with
the value of the magnetic moments (bold black lines).The blue curved
arrows signify tunneling of magnetization within each doublet state, green
curved arrows represent spin-phonon transitions (the numbers are averaged
sents the Stevens operator. Preponderantly larger (see
q
Table S18 in the Supporting Information) nonaxial Bk (where
q¼
6
0 K=2,4,6) terms compared with the corresponding axial
(where q=0 K=2,4,6) terms are found to favor the QTM
q
[34]
B
transition moments in m connecting the corresponding states). For the
k
B
À9
process (Table S7 in the Supporting Information).
transition moment matrix elements, only values below 0.2”10
B
m are
shown (blue arrow); these correspond to the tunnelling between the states.
The observation of a small transverse anisotropy for all the
Dy sites in 1 is also well reproduced in the computed CF pa-
rameters, where the nonaxial terms are larger than the axial
terms (Table S7 in the Supporting Information). We also em-
ployed an electrostatic model to gain information on the ori-
[
32]
entation of the anisotropy axis of the GS in complex 1. This
axis is calculated to be close to the GS ab initio anisotropic
axis, with a deviation of approximately 12.91, 9.28, 14.09, and
1
2.458 for Dy1, Dy2, Dy3, and Dy4, respectively (Figure S9 in
the Supporting Information); this suggests that the orientation
of the anisotropy axis is controlled predominantly by the elec-
trostatic charges of the ligands.
Exchange-coupled molecule
Due to the axiality of the Dy sites, we simulated the magnetic
interactions between the Dy ions by incorporating contribu-
tions from magnetic dipole–dipole and exchange interactions
within an Ising exchange Hamiltonian. We also calculated the
exchange spectrum (Figure 8) of complex 1 by using the
POLY_ANISO program. An excellent agreement between the si-
mulated and experimental magnetic data (c T (T) and M (H))
M
À1
was observed with the parameters J = +0.01 cm and J =
1
2
À1
À0.01 cm (Figure 9). This is further corroborated by specific
heat measurements that show a well-defined Schottky anom-
aly at liquid-helium temperatures (see below). Our computed
g-tensor for the exchange-coupled system shows zero trans-
verse anisotropy contribution in the GS.
Most importantly, the principal g-tensor (magnetic moment,
Table S8 in the Supporting Information) anisotropy of the
lowest (ground) exchange level of the exchange spectrum is
3
.96. The observation of a conventional magnetic moment (a
deviation from zero) in the GS can be ascribed to the low-sym-
metry nature of the complex. The first excited doublet of the
Figure 9. Poly_ANISO simulated (solid lines) A) magnetic susceptibility and
B) magnetization data with respect to experimental data in complex 1. The
exchange spectrum lies at
a larger energy separation
À1
intermolecular interaction (zJ’) was fixed at À0.14 cmÀ1
(
4.4 cm ) with respect to the GS, compared with a previously
.
Chem. Eur. J. 2015, 21, 15639 – 15650
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[
33]
À1
studied Dy complex
(2.97 cm ), which introduces a very
of 41.58. However, the projected magnetic moment of all four
Dy sites is expected to be on the square plane of the molecule
leading to a toroidal moment. Although analogous square-
4
small low field S shape to the magnetization curve (Figure 9B
inset) in complex 1.
This intramolecular coupling may facilitate preferential infor-
[
20b,c,21]
based complexes have been reported in the literature,
[7b]
mation storage compared with conventional SMMs. An anal-
ysis of the exchange spectrum of 1 (Table S8 in the Supporting
Information) clearly reveals a small magnetic-moment matrix
element for the GS exchange doublet (non-Kramers system),
which subsequently suppresses the QTM completely (Figure 8,
Table S8).
this is the first example to show toroidal magnetic moment in
this family; this is strongly corroborated by ab initio calcula-
tions.
Estimation of magnetocaloric effect (MCE)
The values for all other low-lying states (including excited
states) are found to have low magnitudes, resulting in com-
plete suppression of TA-QTM contributions to the magnetic re-
laxation. These observations lead to the occurrence of relaxa-
tion via a spin-phonon tunnelling mechanism through the ex-
cited states, as indicated by the green arrows in Figure 7.
Significant matrix elements computed between Æ5 and Æ6
To evaluate the MCE efficiency of all three complexes (1–3), de-
tailed magnetization measurements (see the Supporting Infor-
mation, Figures S2–S4) and field-dependent heat-capacity
(Figure 10) measurements were performed on bulk samples.
Change in magnetic entropy (ÀDS) and temperature (DT ) of
ad
the molecular cluster are the two essential thermodynamic pa-
rameters of MCE; these can be extracted from magnetization
measurements by using Maxwell’s thermodynamic relation:
describes a barrier for magnetization reversal of approximately
À1
4
.6 cm (Figure 8, Table S8 in the Supporting Information). An-
other important spin-phonon transition has been observed
from the GS À1 component to the À9 excited state compo-
nents, followed by transition from À9 to À10, À10 to À11,
À11 to À12, À12 to À13, and eventually to the time-reversed
states in the opposite order (green arrows in Figure 8).
This is the sole pathway with substantial magnetic matrix el-
ements connecting GS À1 with other excited states. Addition-
ally, direct tunnelling transitions between higher excited states
(
i.e., between À9 and +9, À10 and +10) are less efficient due
to the extremely small value of magnetic-moment matrix ele-
À10
ments that are of the order of 10 mB. Hence, another barrier
À1
height can be envisioned to occur at approximately 138 cm
(
as shown by the dotted orange line in Figure 8), which is simi-
lar to the slower magnetization relaxation observed from the
ac susceptibility measurements. The observation of such a mul-
tilevel-exchange spectrum corroborates the presence of a slow
magnetization relaxation time in complex 1 due to the signifi-
cantly quenched QTM and TA-QTM processes. As the exchange
interactions are extremely small, at lower temperatures, the in-
dividual Dy sites are expected to have independent orientation
and the small exchange coupling results in a fluctuating mag-
[35]
netic field leading to a very fast relaxation.
The calculated ab initio and electrostatic anisotropy axes lie
at a position that faces the least repulsion from the negatively
charged m -hydroxo ligand. Although not all four gzz axes
4
create a circular pattern, the projected magnetic moment due
to g , which is in the plane of the four lanthanide ions, leads
zz
[7b,36]
to a toroidal magnetic moment.
This is rather clearly visi-
ble in the magnetization data measured at T=2 K, and similar
to the toroidal moments observed in other Dy structures, such
as the Dy6 wheel, Dy3 triangles, and Dy4 butterfly com-
[
8,33,36–37]
plexes.
Analysis of the structural details of complex 1 in-
dicates that it possesses a pseudo C axis of symmetry.
4
The gzz axis of Dy1, Dy2, and Dy3 are in plane with respect
to the pseudo C axis, with tilts of 6.28, 15.98, and 13.28 for
Dy1, Dy2 and Dy3, respectively (Figure 6) and the deviations
4
Figure 10. A–C) Temperature dependence of the heat capacity measured on
polycrystalline samples of 1–3 at the indicated magnetic field (data normal-
ized to the gas constant R). Markers indicate the experimental data and lines
are the calculations described in the text.
are within approximately 108. The Dy4 ion on the other hand
deviates significantly from the pseudo C axis with a tilt angle
4
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Z
Hf
@
MðT; HÞ
DS ðT; HÞ ¼
½
ŠHdH
m
@
T
Hi
where H and H are the initial and final applied magnetic field,
i
f
respectively.
To validate the ÀDSm value extracted from magnetization
measurements for 1–3, heat-capacity measurements were also
performed on polycrystalline samples at various applied mag-
netic fields (Figure 10). A large field-independent contribution
is observed above approximately 5 K; this is attributed to the
lattice. This lattice contribution can be modelled by using an
[38]
a
effective Debye model Clatt =A T , with a=2.4 (for 1, 2 and
) and phenomenological coefficient (A), which was found to
be 0.0095Æ10% (for 1), A=0.011Æ10% (for 2), A=0.010Æ
0% (for 3). This contribution is comparable for the three mol-
3
Figure 11. Entropy variation estimated from both magnetization measure-
ments (points) and specific heat measurements (lines) for 1 for different
magnetic field changes.
1
ecules, as expected from the nearly identical structures.
A Schottky anomaly is evident in the low-temperature spe-
cific heat of all three derivatives with a maximum at 1–2 K, aris-
ing from the magnetic-exchange interactions between the
magnetic ions. To get a rough idea of the strength of the inter-
action, we modelled this Schottky anomaly by considering an
effective split between two equally degenerate states separat-
ed by an energy gap T0.
The observed ÀDSm value for 3 is one of the largest known
for an isotropic complex, however this is significantly lower
than for the other isotropic metal complexes reported in the
[
14e,h,16,40]
literature.
Interestingly, the ÀDSm value expressed in
À1 À1
[14c]
Jkg
K
for 3 is larger than that reported for Gd Zn ,
4 8
which contain a similar square Gd core; this is in part due to
4
the lighter mass of our 3 derivative, confirming the strategy of
using a lighter compound to enhance the MCE.
C =R ¼ B ðT =TÞ eðT0 =TÞ=ð1 þ eðT0 =TÞ
2
Þ
2
III
int
0
For four uncoupled Gd ions, the theoretically expected
À1 À1
ÀDSm value is 20.8 Jkg K , however, experimentally we ob-
Data fitting on the three C(T) curves provides these values
for the parameters B=3.7 T and T =7.78 K for 1, B=2.45 T and
T =5.1 K for 2, B=10 T and T =2.35 K for 3. In a first approxi-
serve a value that is higher than the uncoupled one; signifying
that the excited states also participate in polarizing the spins.
Accessibility of possible excited states in the case of 3, even at
low temperatures, is likely due to the weak exchange interac-
tions.
0
0
0
mation the mean value of the exchange interactions JSE be-
2
2
tween four lanthanides leads to an effective gap T =6J g J .
0
J
SE
In this way we are considering six equivalent interactions be-
tween the four magnetic ions, so what is found here is a mean
value for J . We found for 1 J =0.0131 K, for 2 J =0.0105 K
Conclusion
SE
SE
SE
and for 3 J =0.0080 K, comparable to J and J values theoret-
We have reported an unusual family of O-centered Ln (where
SE
1
2
4
ically estimated above. For 2, another term is required to de-
scribe the upturn of the specific heat below 0.7 K: the hyper-
fine coupling can be modelled by using the hyperfine energy
levels of terbium (0.120, 0.149, and 0.178 K) as respective gaps
between the four hyperfine sublevels of the J=6 multiplet
and a multilevel Schottky anomaly expression.
Ln=Dy, Tb or Gd) squares, the structures of which were deter-
mined by single-crystal X-ray diffraction. Dc magnetic suscepti-
bility data on complexes 1–3 show weak antiferromagnetic in-
teractions between the metal ions. Unambiguous evidence for
the existence of antiferromagnetic interactions between the
anisotropic lanthanide ions is confirmed through ab initio cal-
culations and heat-capacity measurements. Measurements of
the ac susceptibility of the anisotropic complexes 1 and 2 in
the 2–35 K temperature range reveal both complexes to show
The magnetic entropy (S ) can also be evaluated from
m
R
S ðT; HÞ ¼ Cm=TdT where Cm is magnetic heat capacity di-
m
rectly obtained from heat-capacity measurements. Notably, the
extracted values are consistent with the values obtained from
magnetization measurements. As expected, complex 3, shows
frequency-dependent out-of-phase signals (c “), although the
M
maxima in c ” for 2 is below the temperature limit of our in-
M
the largest ÀDS among the three complexes (Figure 11). The
strument. On the other hand, complex 1 shows two different
magnetization relaxation processes, one at high temperature
and one at low temperature. The Arrhenius plot constructed
from these two relaxation processes gives energy barriers for
the reversal of the magnetization vector at 29 and 100 K. We
have rationalized the origin of these two relaxations processes
in complex 1 by detailed ab initio calculations, which demon-
strated that the Dy4 site in complex 1 behaves differently com-
pared with the other three sites. Calculations reveal the exis-
tence of a toroidal magnetic moment in complex 1; confirmed
experimentally by the S-shaped magnetization curve at 2.0 K.
m
maximum value obtained for
a
7 T–0 demagnetization
À1 À1
isÀDS =(5.8Æ0.6) R=23 JKg
K
at Tꢀ3 K for 3; ÀDS =
m
m
(
2.2Æ0.4) R at Tꢀ6 K for 1 and ÀDS =(2.0Æ0.4) R at Tꢀ5 K
m
for 2 (Figures S10–S11 in the Supporting Information).
The zero-field splitting due to large spin–orbit coupling as-
sociated with 1 and 2 results in significantly lower degeneracy
of the GS. This leads to a smaller change in the magnetic-en-
tropy value compared with more isotropic clusters. Such sce-
narios have been witnessed already in other lanthanide–metal
[
14h,39]
complexes.
Chem. Eur. J. 2015, 21, 15639 – 15650
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The exchange interaction extracted from the POLY_ANISO pro-
gram is in agreement with experimentally derived parameters.
The MCE of all the three complexes were estimated from two
different experimental techniques, field-dependent magnetiza-
tion and heat-capacity measurements. Complex 3 shows the
6.92 (dd, J=7.88 Hz, 2H), 6.57 (t, J=7.84 Hz, 1H), 4.84 (s, 3H), 3.73
13
(
s, 3H), 3.6 ppm (d, 6H); C NMR d=164.11(C=N), 158.17, 149.53,
1
24.15, 117.12, 114.86, 114.33, 66.43, 61.11, 55.72 ppm; FT-IR (KBr
À1
pellet): n˜ =2919 (s, n(ArÀH)), 1644 cm (s, n(C=N)).
À1 À1
Synthesis of complex 1
largest change in magnetic entropy (23 JKg K ), due to the
near isotropic nature of the GS and weak exchange interac-
tions. The three, analogous, square-based complexes are there-
fore a family of complexes exhibiting a variety of physical
properties, including toroidal magnetic moments, SMM behav-
ior, and enhanced MCE.
A methanolic solution (30 mL) of H L (0.300 g, 1.175 mmol) was de-
protonated by using NaOMe (0.1269 g, 2.350 mmol) and the solu-
4
tion stirred for 30 min before charging DyCl
·6H O (0.3159 g,
2
3
1
.175 mmol) into the reaction flask. The resultant reaction mixture
was stirred for 24 h and then filtered. The filtrate volume was re-
duced to one third of the initial volume and kept for crystallization
without any disturbance. Pale, yellow, block-shaped, single crystals
grew from the filtrate after one week, and were suitable for X-ray
diffraction. Yield: 142 mg (27%, based on Dy). Elemental analysis
(air dried sample) calcd (%): C 29.8, H 4.9, N 2.7; found: C 30.1, H
Experimental Section
All reagents and solvents were purchased from commercially avail-
able sources (Alfa Aesar and Sigma-Aldrich) and were used without
further purification. All reactions were carried out under aerobic
À1
4.6, N 2.6; FT-IR (KBr pellet): n˜ =2921 (s, n(ArÀH)), 1648 cm (s,
n(C=N)).
conditions unless otherwise stated. The ligand H L was synthesized
4
[17o]
as per the literature report,
with a slight modification to im-
Synthesis of complexes 2 and 3
prove the yield of the reaction; see below for details. The chosen
crystal was mounted on a Goniometer with the use of paraffin oil
and the crystal cooled in a stream of liquid nitrogen to 100 K (for
The same procedure was followed for complex 1, by using the cor-
responding equivalence of lanthanide halides (TbCl ·6H O
3
2
1
and 3). For 2, data were collected at room temperature. The data
collection was done on a Rigaku Saturn CCD diffractometer (for
and 3) and an Oxford CCD diffractometer for 2 with a graphite
(0.4388 g, 1.175 mmol for 2 and GdBr
·6H O (0.4665 g, 1.175 mmol)
3 2
for 3) in place of DyCl .6H O. For 2: yield: 137 mg (25%, based on
3
2
1
the Tb). Elemental analysis (air dried sample) calcd (%): C 32.6, H
monochromator (l=0.710738) equipped with an Oxford cryosys-
tems cooler device. The unit cell determination and data reduction
were performed with the Rigaku CrystalClear-SM Expert 2.1 soft-
ware. The structures were solved by direct methods and the re-
4.7, N 2.7; found: C 32.1, H 4.8, N 2.6; FT-IR (KBr pellet): n˜ =2924 (s,
À1
n(ArÀH)), 1644 cm (s, n(C=N)). For 3: yield: 145 mg (26%, based
on the Gd). Elemental analysis (dried sample) calcd (%): C 31.6, H
4
.6, N 2.6; found: C 31.2, H 4.7, N 2.6; FT-IR (KBr pellet): n˜ =2926 (s,
À1
2
n(ArÀH)), 1646 cm (s, n(C=N)).
fined by least-squares procedures on F with SHELXTL package. All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were placed based on the geometry and refined with a riding
model. The solvent molecules were heavily disordered and could
not be model due to diffused electron density in all three com-
plexes. The SQUZEE routine from PLATON resulted in smooth re-
finement of the structures. The loop corresponding to residual
electron-density voids (from PLATON) is appended in the corre-
sponding CIF files.
Crystal structures of all the three complexes and magnetic data
(both dc and ac data) are given in the Supporting Information,
along with full computational details. CCDC 1060622 (1),
1060623 (2), and 1060624 (3) contain the supplementary crystallo-
graphic data for this paper. These data are provided free of charge
by The Cambridge Crystallographic Data Centre.
Acknowledgements
NMR spectra for the H L ligand were recorded on a Bruker Avance
4
III 400 MHz instrument at room temperature. The data were cali-
brated with listed deuterated solvents. Infrared spectra were col-
lected for the solid samples by using KBr pellets on a PerkinElmer
MS likes to thank CSIR (01(2768)/13/EMR-II), DST (SR/S1/IC-32/
2011), DST-Nanomission (SR/NM/NS-1119/2011), and IIT
Bombay for financial support. GR thanks DST-Nanomission (SR/
NM/NS-1119/2011), AISRF. EKB thanks the EPSRC. MS and GR
thank Dr. Ungur and Prof. Chibotaru, Belgium, for their addi-
tional MOLCAS routine and we are indebted to Dr. Ungur for
providing us the POLY_ANISO code for our computational
study.
À1
FT-IR spectrometer in the 400 to 4000 cm range. Variable-temper-
ature dc susceptibility measurements were performed on a quan-
tum design MPMS-SQUID magnetometer. Diamagnetic corrections
were applied for the constituent atoms by using Pascal’s constants.
Heat-capacity measurements were measured on a PPMS-7T system
by using the two-tau method. Microcrystals were pressed into thin
pellets and cut in thin slices of 1–2 mg in weight.
Synthesis of H4L
Keywords: ab initio calculations · clusters · dysprosium ·
magnetism · magnetocaloric effect
In a 250 mL round-bottom flask a methanolic solution of o-vanillin
(
(
5.0002 g, 0.0328 mol, 100 mL MeOH) was added glacial acetic acid
0.3 mL, 5–6 drops) dropwise to activate the carbonyl group in o-
[
1] a) G. Aromí, E. K. Brechin, Struct. Bonding (Berlin) 2006, 122, 1–67;
b) R. J. Blagg, L. Ungur, F. Tuna, J. Speak, P. Comar, D. Collison, W. Werns-
dorfer, E. J. L. McInnes, L. F. Chibotaru, R. E. P. Winpenny, Nat. Chem.
vanillin and stirred for 10 min. Into the reaction flask, solid tris(hy-
droxymethyl)aminomethane (3.981 g, 0.0328 mol) was then added
and the reaction mixture was heated to reflux for 24 h. The light-
yellow precipitate that formed was filtered and washed with n-
hexane several times and dried under vacuum. Yield=7.56 g, 90%.
2013, 5, 673–678; c) D. Gatteschi, A. Caneschi, L. Pardi, R. Sessoli, Sci-
ence 1994, 265, 1054–1058; d) M. Mannini, F. Pineider, C. Danieli, F.
Totti, L. Sorace, P. Sainctavit, M. A. Arrio, E. Otero, L. Joly, J. C. Cezar, A.
Cornia, R. Sessoli, Nature 2010, 468, 417–421; e) M. Mannini, F. Pineider,
P. Sainctavit, C. Danieli, E. Otero, C. Sciancalepore, A. M. Talarico, M.-A.
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The purity of the ligand was confirmed by H and C NMR record-
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Received: July 10, 2015
Published online on September 18, 2015
Chem. Eur. J. 2015, 21, 15639 – 15650
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim