Modulating magnetic dynamics of three Dy2 complexes through keto-enol tautomerism of the o-vanillin picolinoylhydrazone ligand

Article abstract of DOI:10.1021/ic2014978

Complexation of dysprosium(III) with the heterodonor chelating ligand o-vanillin picolinoylhydrazone (H₂ovph) in the presence of different bases affords three new dinuclear dysprosium(III) coordination compounds, namely, [Dy₂(ovph)₂(NO₃)₂(H ₂O)₂]·2H₂O (1), [Dy₂(Hovph) (ovph)(NO₃)₂(H₂O)₄]·NO ₃·2CH₃OH·3H₂O (2), and Na[Dy ₂(Hovph)₂(μ₂-OH)(OH)(H₂O) ₅]·3Cl·3H₂O (3), where the aroylhydrazone ligand adopts different coordination modes in respective structures depending on the reaction conditions, as revealed by single-crystal X-ray analyses to be due to their tautomeric maneuver. The magnetic properties of 1-3 are drastically distinct. Compounds 1 and 2 show single-molecule-magnet behavior, while no out-of-phase alternating-current signal is noticed for 3. The structural differences induced by the different coordinate fashions of the ligand may influence the strength of the local crystal field, the magnetic interactions between metal centers, and the local tensor of anisotropy on each Dy site and their relative orientations, therefore generating dissimilar dynamic magnetic behavior.

Full text of DOI:10.1021/ic2014978

ARTICLE
pubs.acs.org/IC
Modulating Magnetic Dynamics of Three Dy Complexes through
2
KetoꢀEnol Tautomerism of the o-Vanillin Picolinoylhydrazone Ligand
†
,‡
§
†,‡
,†
Yun-Nan Guo, Xiao-Hua Chen, Shufang Xue, and Jinkui Tang*
†
State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, People's Republic of China
‡
Graduate School of the Chinese Academy of Sciences, Beijing 100039, People's Republic of China
§
College of Chemistry and Materials Science, Fujian Normal University, Fuzhou, 350007, People's Republic of China
S Supporting Information
b
ABSTRACT: Complexation of dysprosium(III) with the hetero-
donor chelating ligand o-vanillin picolinoylhydrazone (H ovph) in
2
the presence of different bases affords three new dinuclear
dysprosium(III) coordination compounds, namely, [Dy (ovph) -
2
2
(
(
(
NO ) (H O) ] 2H O (1), [Dy (Hovph)(ovph)(NO ) -
3 2 2 2 2 2 3 2
3
H O) ] NO 2CH OH 3H O (2), and Na[Dy (Hovph) -
2
4
3
3
3
3
3
2
2
2
μ -OH)(OH)(H O) ] 3Cl 3H O (3), where the aroylhydra-
2
2
5
3
3
2
zone ligand adopts different coordination modes in respective
structures depending on the reaction conditions, as revealed by
single-crystal X-ray analyses to be due to their tautomeric maneuver.
The magnetic properties of 1ꢀ3 are drastically distinct. Compounds 1 and 2 show single-molecule-magnet behavior, while no out-
of-phase alternating-current signal is noticed for 3. The structural differences induced by the different coordinate fashions of the
ligand may influence the strength of the local crystal field, the magnetic interactions between metal centers, and the local tensor of
anisotropy on each Dy site and their relative orientations, therefore generating dissimilar dynamic magnetic behavior.
’
INTRODUCTION
understand how the magnetic anisotropy of 4f ions directs the
SMM properties of multinuclear compounds, it is helpful to focus
1
ꢀ3
Single-molecule magnets (SMMs)
have attracted increas-
33
on lower-nuclearity clusters. Selection of organic ligands formed
by reaction between the o-vanillin and hydrazine derivatives is a
good choice to construct such clusters, as we and others have
ing attention of both physicists and chemists because of their
potential applications for use in high-density magnetic memories,
quantum computing devices, and molecular spintronics.
Whereas attention was initially focused on coordination clusters
containing 3d metal ions, significant attention is now being
paid to incorporating 4f ions into SMMs in either mixed 3d/4f or
pure 4f clusters.
as terbium(III),
erbium(III)
high-barrier SMMs as a result of their significant magnetic anisot-
ropy arising from the large, unquenched orbital angular momen-
tum. It is now well-established that mononuclear lanthanide com-
plexes may behave as SMMs, when their coordination environment
results in highly anisotropic situations. In addition to these
monometallic systems, polymetallic lanthanide clusters can also
exhibit SMM properties, most of them containing dysprosium-
4,5
6
,7
8,9
22ꢀ24
recently demonstrated (Scheme 1),
for the following two
10
reasons: (1) This kind of multidentate ligand has both two or
more oxygen donors and two or more nitrogen donors. These
donors with suitable relative positions in the ligand can coordi-
1
1ꢀ13
4ꢀ16
The use of the heavy lanthanide ions, such
dysprosium(III),
34
1
11,17 18
nate to several metal centers. (2) The o-vanillin group displays
holmium(III), and
1
9ꢀ21
a variety of bonding geometries, such as monodentate, bidentate
is one of the most promising ways to design
35,36
bridging, and chelate bridging.
In this context, we decided to explore the possibility of
constructing new magnetically interesting molecules using o-
vanillin picolinoylhydrazone (H ovph; Scheme 1d) as such a
2
1
7
versatile ligand. We report herein the synthesis, structures, and
magneticproperties ofthree dinucleardysprosium(III) complexes,
[
Dy (ovph) (NO ) (H O) ] 2H O (1), [Dy (Hovph)(ovph)-
2 2 3 2 2 2 2 2
3
(
NO ) (H O) ] NO 2CH OH 3H O (2), and Na[Dy -
3
2
2
4
3
3
3
3
3
2
2
(III) with different topologies. Such compounds exhibiting large
22ꢀ27
(Hovph) (μ ꢀOH)(OH)(H O) ] 3Cl 3H O (3), assembled
2
2
2
5
3
3
2
observed relaxation barriers
SMMs, potentially bringing the goals of molecule-based informa-
tion storage and processing closer to reality. Other dysprosium-
advance the prospects of
from dysprosium salts and the H ovph ligand. Of particular
2
28
interest in the present study is the clear realization, as revealed by
crystallographical and IR spectral investigations, of the tautomeric
(III) complexes are interesting for their very fascinating physics,
29ꢀ31 24,28,32
such as spin chirality
and multiple relaxation pathways.
Ligand design is one of the key components for achieving
pure dysprosium-based systems with tailor-made properties. To
Received: July 14, 2011
Published: September 08, 2011
r 2011 American Chemical Society
9705
dx.doi.org/10.1021/ic2014978
|Inorg. Chem. 2011, 50, 9705–9713

Inorganic Chemistry
ARTICLE
maneuver of the aroylhydrazone ligand, producing three novel
complexes with different coordination modes (Scheme 2), de-
pending on the reaction conditions. The structural differences
induced by the different coordinate fashions of the ligand are
mostly responsible for the distinct relaxation dynamics observed;
namely, compounds 1 and 2 show SMM behavior, while no out-
of-phase alternating-current (ac) signal is noticed for 3.
Samples were prepared as KBr disks. All magnetization data were
recorded on a Quantum Design MPMS-XL7 SQUID magnetometer.
The variable-temperature magnetization was measured with an external
magnetic field of 1000 Oe in the temperature range of 1.9ꢀ300 K.
Samples were restrained in eicosane to prevent torquing. The experi-
mental magnetic susceptibility data are corrected for the diamagnetism
estimated from Pascal’s tables and sample holder calibration.
X-ray Crystallography. Single-crystal X-ray diffraction measure-
ments of the title complexes were carried out on a SMART CCD area
detector equipped with a graphite crystal monochromator situated in the
incident beam for data collection at 191(2) K. The structures were solved by
direct methods and refined with full-matrix least-squares techniques
’
EXPERIMENTAL SECTION
General Procedures. All chemicals were used as commercially ob-
tained without further purification. Elemental analyses for carbon, hydro-
gen, and nitrogen were carried out on a Perkin-Elmer 2400 analyzer. Fourier
transform IR (FTIR) spectra were recorded with a Perkin-Elmer FTIR
37
using SHELXS-97 and SHELXL-97 programs. Anisotropic thermal
parameters were assigned to all non-hydrogen atoms. The hydrogen
atoms were introduced in calculated positions and refined with a fixed
geometry with respect to their carrier atoms. CCDC 830671 (1),
ꢀ1
spectrophotometer using the reflectance technique (4000ꢀ300 cm ).
830672 (2), and 830673 (3) contain the supplementary crystallographic
Scheme 1. Title Ligand H ovph, the Related Ligands
data for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
2
2
3 1 2 2 2 3
4
23
22
(
H L , H L , and H L ), and Their Coordinating Modes
in Dysprosium(III)-Based SMMs
Synthesis of H ovph. The Schiff-base ligand H
2
ovph is synthe-
2
sized by condensation of picolinoyl hydrazide and o-vanillin in a 1:1 ratio
3
8
in methanol using the reported procedure.
Synthesis of [Dy (ovph) (NO ) (H O) ] 2H O (1). A solution
2
2
3 2
2
2
3
2
of Dy(NO
4.26 mg) in a mixed solvent of methanol (5 mL) and acetonitrile
10 mL) was stirred for 5 min. Then Et N (0.2 mmol) was added, and
3 3 2 2
) 6H O (0.2 mmol, 91.32 mg) and H ovph (0.2 mmol,
3
5
(
3
the mixture was stirred for 3 h. The resultant yellow solution was left
unperturbed to allow for slow evaporation of the solvent. Yellow single
crystals, suitable for X-ray diffraction analysis, were formed after 1 month.
Yield: 38 mg (36%, based on the metal salt). Elem anal. Calcd for
28 30 8 2
C H N O16Dy : C, 31.71; H, 2.83; N, 10.57. Found: C, 31.60; H,
ꢀ
1
2
1
1
1
8
.62; N, 10.39. IR (KBr, cm ): 3392(br), 2993(w), 2842(w), 1599(s),
557(s), 1541(w), 1517(m), 1479(s), 1465(w), 1440(m), 1383(s),
340(s), 1303(m), 1281(m), 1248(m), 1215(m), 1170(w), 1155(s),
083(m), 1055(w), 1021(w), 966(w), 922(w), 859(w), 813(w),
02(w), 749(m), 715(w), 691(m), 639(m), 580(w).
Synthesis of [Dy (Hovph)(ovph)(NO ) (H O) ] NO 2CH -
OH 3H O (2). A procedure similar to that for 1 was followed except that
2
3 2
2
4
3
3
3
3
3
2
Et
based on the metal salt). Elem anal. Calcd for C30
H, 3.63; N, 10.16. Found: C, 28.59; H, 3.54; N, 10.02. IR (KBr, cm ):
3
N was replaced by pyridine (1.0 mmol, 0.08 mL). Yield: 42 mg (34%,
45 9 2
H N O24Dy
: C, 29.01;
ꢀ
1
3349(br), 3235(w), 2839(w), 1624(w), 1610(s), 1591(w), 1561(s),
1546(w), 1477(w), 1465(m), 1436(m), 1333(w), 1307(m), 1287(m),
1241(m), 1219(s), 1173(w), 1161(w), 1090(w), 1054(w), 1021(w),
Scheme 2. Reversible Deprotonation and Base-Assisted KetoꢀEnol Tautomerism of H ovph and the Corresponding Coordi-
2
nation Modes in Complexes 1ꢀ3
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Inorganic Chemistry
ARTICLE
9
6
65(w), 921(w), 863(w), 812(w), 789(w), 745(m), 713(w), 697(m),
37(m), 584(w).
’ RESULTS AND DISCUSSION
Crystal Structure of 1. The reaction of Dy(NO ) 6H O
Synthesis of Na[Dy (Hovph) (μ -OH)(OH)(H O) ] 3Cl 3H O
3 3
3
2
2
2
2
2
5
3
3
2
(
3). DyCl3 6H O (0.2 mmol, 75.4 mg) and H ovph (0.2 mmol, 54.26
with H
2
ovph in methanol/acetonitrile in the presence of Et
3
N
3
2
2
mg) in a mixed solvent of methanol (5 mL) and acetonitrile (10 mL) was
stirred for 5 min and gradually formed a red solution. Then 0.3 mL of
produces yellow crystals of 1 after 1 month. Single-crystal X-ray
studies revealed that 1 crystallizes in the triclinic space group P1.
A perspective view of the molecular structure of 1 is represented
in Figure 1. Details for the structure solution and refinement are
summarized in Table 1, and selected bond distances and angles
are listed in Tables 2 and 3. A structural study of 1 showed it to be
methanolic solution of NaN (0.1 mmol) was added to the reaction
3
mixture, and the mixture was stirred for 3 h. The resultant yellow
solution was left unperturbed to allow for slow evaporation of the
solvent. Yellow single crystals, suitable for X-ray diffraction analysis,
were formed after 1 month. Yield: 49 mg (41%, based on the metal salt).
2ꢀ
a dinuclear trans-dysprosium(III) complex. Two ovph ligands
coordinate two dysprosium centers in an antiparallel or “head-to-
tail” fashion with the tridentate unit (O1, N3, and O2) and the
bidentate picolinoyl group (O1 and N1) (Scheme 2). Carbonyl
oxygen atoms (O1 and O1A) of the ligands bind in their
3 6 2
Elem anal. Calcd for C28H44Cl N O17Dy Na: C, 28.21; H, 3.69; N,
ꢀ
1
7
1
1
7
.05. Found: C, 27.97; H, 3.51; N, 6.95. IR (KBr, cm ): 3205(br),
618(s), 1588(s), 1566(s), 1458(s), 1438(w), 1279(m), 1245(s), 1220(s),
167(w), 1113(w), 1090(m), 1066(w), 961(w), 915(w), 815(w),
86(w), 739(m), 713(w), 698(m), 643(m), 622(w), 527(w), 482(w).
ꢀ
conjugate deprotonated enol form (O ) with a C6ꢀO1 distance
of 1.306(7) Å (Table 2) and bridge the two metal centers, giving
rise to a four-membered Dy O rhomb, which exhibits a center of
2
2
symmetry with a Dy Dy distance of 3.8258(6) Å and two
3 3
3
Table 2. Selected Bond Distances (Å) for H ovph in Com-
2
plexes 1ꢀ3
1
2
3
C6ꢀN2
C6ꢀO1
C20ꢀN5
C20ꢀO4
1.308(8)
1.306(7)
1.315(10)
1.297(10)
1.323(11)
1.240(10)
1.325(9)
1.254(8)
1.329(8)
1.232(8)
Figure 1. Molecular structure of complex 1. Organic hydrogen atoms
and lattice water are omitted for clarity.
Table 1. Crystallographic Data and Structure Refinement for Complexes 1ꢀ3
1
2
3
formula
C
28
H
30Dy N
2 8
O
16
C
30
H45Dy
2
9
N O
24
C
28
H
3 2 6
44Cl Dy N NaO17
M
r
1059.60
1240.75
1191.03
cryst size [mm]
color
0.22 ꢁ 19 ꢁ 0.17
yellow blocks
triclinic
0.25 ꢁ 0.22 ꢁ 0.20
yellow blocks
0.25 ꢁ 0.21 ꢁ 0.18
yellow blocks
triclinic
cryst syst
space group
T [K]
orthorhombic
P1
P2
1
2
1
2
1
P1
191(2)
191(2)
10.335(2)
16.368(3)
27.235(5)
90
191(2)
a [Å]
9.3234(8)
9.4773(9)
11.4313(10)
69.1140(10)
83.773(2)
66.6130(10)
865.53(13)
1
8.5593(4)
15.5490(8)
15.5897(8)
85.7660(10)
83.7050(10)
82.6390(10)
2041.54(18)
2
b [Å]
c [Å]
α [deg]
β [deg]
γ [deg]
90
90
3
V [Å ]
4607.3(16)
4
Z
ꢀ3
D
calcd [g cm
]
2.033
1.789
1.938
ꢀ1
μ(Mo Kα) [mm
F(000)
]
0.71073
514
0.71073
2448
0.71073
1168
reflns collected
unique reflns
4825
25 708
9016
11 450
3393
7989
R
int
0.0243
0.0620
557/2
1.011
0.0228
param/restraints
GOF
256/6
514/0
1.076
1.052
R1 [I > 2σ(I)]
wR2 (all data)
0.0380
0.0443
0.1038
1.233/ꢀ0.819
0.0404
0.0917
0.1079
ꢀ3
largest diff peak/hole [e Å
]
1.782/ꢀ0.842
3.351/ꢀ1.445
9
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Inorganic Chemistry
ARTICLE
DyꢀOꢀDy angles of 110.12(16)°. In addition, one nitrate (O4
and O5) and one water molecule (O7) are coordinating to each
dysprosium ion, leading to a coordination number of 8, which
exhibits a hula-hoop-like geometry whose cyclic ring (hula hoop)
ligands (O1, N3, O2, O1A, and N1A, as shown in Figures 1
39,40
and S1 in the Supporting Information).
The coordinated water molecule (O7) binds to the neighbor-
a
ing nitrate via an intramolecular hydrogen bond O4
H7BꢀO7
3
3 3
2
ꢀ
is shaped by the stereochemical preferences of two ovph
(symmetry code: a, ꢀx + 1, ꢀy + 1, ꢀz + 1; Figure 1). Every
Dy molecule connects to six lattice water molecules, as shown
2
in Figure 2, left. Meanwhile, each lattice water (O1w) acts as
Table 3. Selected Bond Distances (Å) and Angles (deg) in
Complexes 1ꢀ3
a
a hydrogen-bonding connector, joining three Dy units via
2
a
b
strong O1wꢀH1wA N2 , O7ꢀH7C O1w , and O1wꢀ
3
3 3 3 3 3
c
H1wB O2 bonds (symmetry codes: b, ꢀx + 1, ꢀy + 2, ꢀz +
3 3
3
Compound 1
1
; c, x + 1, y, z; see Table S1 in the Supporting Information for
Dy1ꢀO1
2.319(4)
2.179(4)
2.451(5)
2.477(5)
3.8258(6)
Dy1ꢀO1A
Dy1ꢀO4
2.348(4)
2.462(5)
2.363(5)
2.453(5)
110.12(16)
details and the red circle in Figure 2, right), and, thus, a 2D
supramolecular motif in 1 is afforded, running along the ab plane
Dy1ꢀO2
Dy1ꢀO5
Dy1ꢀO7
(
Figure 2, right).
Dy1ꢀN1A
Dy1 Dy1A
Dy1ꢀN3
Dy1ꢀO1ꢀDy1A
Crystal Structure of 2. A procedure similar to that for 1 except
that Et N was replaced by pyridine yields yellow crystals of 2 after
3
3 3
3
3
weeks. The single-crystal X-ray structure of the dinuclear 2 is
Compound 2
depicted in Figure 3. Compound 2 crystallizes in the space group
Dy1ꢀO1
Dy1ꢀO5
Dy1ꢀO7
Dy1ꢀO10
Dy1ꢀN3
Dy2ꢀO4
Dy2ꢀO12
Dy2ꢀO15
Dy2ꢀN1
Dy1 Dy2
2.320(5)
2.425(5)
2.513(6)
2.405(6)
2.495(7)
2.322(6)
2.473(6)
2.407(6)
2.532(7)
3.8926(8)
113.5(2)
Dy1ꢀO2
2.189(6)
2.509(5)
2.581(6)
2.402(6)
2.334(5)
2.295(6)
2.563(6)
2.391(6)
2.539(8)
111.1(2)
P2 2 2 , with Z = 4. The molecule occupies a general site, in
which two ligands coordinate two dysprosium centers in a
1 1 1
Dy1ꢀO6
parallel fashion, leading to a noncentrosymmetric Dy (μ-O)
Dy1ꢀO8
2
2
core with a Dy Dy distance of 3.8926(8) Å and two DyꢀOꢀDy
Dy1ꢀO11
Dy2ꢀO1
3 3 3
angles of 113.5(2)° and 111.1(2)°. Close inspection of the bond
distances reveals that one of the ligands has undergone ketoꢀ
enol tautomerism (Scheme 1 and Table 2). A comparison of the
bond distances in complex 2 shows considerable differences
between the two tautomeric forms of the ligand (Table 2). The
C20ꢀO4 bonds in the keto form [1.240(10) Å] are shorter
than the corresponding bonds C6ꢀO1 in the enolate form
[1.297(10) Å], congruent with the chemical structures of both
Dy2ꢀO5
Dy2ꢀO13
Dy2ꢀO16
Dy2ꢀN6
Dy1ꢀO5ꢀDy2
3
3 3
Dy1ꢀO1ꢀDy2
2
ꢀ
forms (Scheme 1). Therefore, in the dideprotonated ovph
ꢀ
Compound 3
ligand, O1 as an alkoxido atom (O ) bridges the two metal
Dy1ꢀO1
Dy1ꢀO5
Dy1ꢀO7
Dy1ꢀO9
Dy1ꢀN3
Dy2ꢀO3
Dy2ꢀO5
Dy2ꢀO11
Dy1ꢀO14
2.357(4)
2.401(4)
2.393(5)
2.417(5)
2.543(6)
2.504(4)
2.314(4)
2.417(4)
2.490(4)
3.6145(4)
100.08(15)
Dy1ꢀO2
2.321(4)
2.482(4)
2.396(5)
2.528(4)
2.406(4)
2.330(4)
2.422(4)
2.467(5)
2.516(5)
99.72(16)
92.14(15)
centers, similar to that in complex 1. However, the monodepro-
tonated Hovph ligand coordinates two dysprosium centers with
the tridentate unit (O4, N6, and O5) and the bidentate o-vanillin
ꢀ
Dy1ꢀO6
Dy1ꢀO8
group (O5 and O6), in which the aroylhydrazone part keeps the
Dy1ꢀO14
Dy2ꢀO2
41
original keto state. The coordination spheres of Dy1 and Dy2
are each completed by two water molecules and one nitrate
ligand, making them nine-coordinate with a monocapped square-
antiprismatic geometry (Figure S1 in the Supporting Information).
Crystal Structure of 3. The reaction of DyCl 6H O with
Dy2ꢀO4
Dy2ꢀO10
Dy2ꢀO12
Dy2ꢀN6
3
3
2
H ovph in methanol/acetonitrile, in the presence of NaN , pro-
2
3
Dy1 Dy2
Dy1ꢀO2ꢀDy2
Dy1ꢀO14ꢀDy2
3
3 3
duces yellow crystals of 3, whose molecular structure determined
by single-crystal X-ray diffraction is depicted in Figure 4. Com-
pound 3 crystallizes in the space group P1, with Z = 2. Evidently,
Dy1ꢀO5ꢀDy2
a
Symmetry code for 1: A, ꢀx + 1, ꢀy + 1, ꢀz + 1.
Figure 2. (left) Hydrogen bonding between the Dy unit and six peripheral lattice water molecules. (right) 2D supramolecular plane of 1. The red circle
2
2
highlights where a lattice water joins three Dy units.
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Inorganic Chemistry
ARTICLE
Figure 3. Molecular structure of complex 2. Hydrogen atoms and
lattice solvent molecules are omitted for clarity.
Figure 5. IR spectra of a crystalline sample of H
2
ovph and complexes
1
ꢀ3 with ν(CdN) modes (blue vertical bar) and ν(CdO) modes (red
vertical bar).
Figure 4. Dinuclear unit of 3. Hydrogen atoms are omitted for clarity.
ꢀ
in the presence of a weak base (N3 ), both of the aroylhydrazone
ligands remained in the keto form (Scheme 2 and Table 2). Two
ꢀ
Hovph ligands coordinate Dy1 and Dy2 centers in an antipar-
allel fashion with their tridentate unit and bidentate o-vanillin
Figure 6. Temperature dependence of the χ
1ꢀ3.
M
T products at 1 kOe for
ꢀ1
group. Different from 1 and 2, complex 3 exhibits a Dy (μ-O)3
2
core, which is created by the phenoxido groups (O2 and O5) of
ꢀ
two Hovph ligands and one additional μ -hydroxido (O14)
2
The ν(CdN) vibration modes at 1590 and 1608 cm in the
spectrum of the free hydrazone shift to 1591 and 1610 cm in
the spectrum of complex 2 and to 1588 cm in the spectrum of
complex 3, indicating coordination of the azomethine nitrogen.
bridge with a Dy Dy distance of 3.6145(4) Å and three
3
3 3
ꢀ1
DyꢀOꢀDy angles of 99.72 (16)°, 100.08(15)°, and 92.14(15)°.
ꢀ
1
The coordination spheres of Dy1 and Dy2 are completed by five
4
3
ꢀ
water molecules (O8ꢀO12) and one OH ligand (O7), making
ꢀ1
In complex 1, the broad and strong band was found at 1599 cm ,
them nine-coordinate with a distorted monocapped square-anti-
prismatic geometry (Figure S1 in the Supporting Information).
Along with coordination on Dy1, O7 and O9 also bridge Na1
in accordance with the presence of a ν(NdC) vibration, due to
the formation of a new NdC bond upon coordination with
deprotonation of the hydrazone ligand. Similar variations were
observed for dysprosium(III) complexes of the H L ligand
ions below and above the Dy molecule (Figure S2 in the Sup-
2
2
3
porting Information). The band distances of Dy1ꢀO7[2.393(5) Å]
22
(
Scheme 1).
ꢀ
and Dy1ꢀO9 [2.417(5) Å] are corresponding to one OH
Magnetic Properties. Direct-current (dc) magnetic suscept-
ligand and one water molecule, respectively.
ibility studies of 1ꢀ3 have been carried out in an applied magnetic
field of 1000 Oe in the temperature range 300ꢀ2 K. The plot of
χ T versus T, where χ is the molar magnetic susceptibility, is
IR Spectra. The IR spectrum of the ligand exhibits a band at
273 cm due to ν(NH) stretches, indicative of its ketonic
ꢀ
1
3
M
M
nature in the solid state, which was also observed in complexes 2
shown in Figure 6. The observed χ T values at 300 K are 27.3,
M
ꢀ
1
ꢀ1
3
ꢀ1
(
3235 cm ) and 3 (3205 cm ). The absence of the ν(NH)
2
8.1, and 27.4 cm K mol for 1ꢀ3, respectively, which are in
3
ꢀ1
band in the IR spectrum of complex 1 is consistent with
good agreement with the expected value of 28.34 cm K mol
3
8
5
6
enolization.
for two uncoupled dysprosium(III) ions (S = / , L = 5, H15/2,
2
The ν(CdO) absorption at 1650ꢀ1700 cmꢀ1 in the spec-
4
and g = / ). The temperature dependence of the magnetic
3
ꢀ1
trum of the free hydrazone is observed at 1618 and 1624 cm in
susceptibilities for all compounds shows different thermal evolu-
complexes 2 and 3, respectively, in accordance with coordination
tion. For 3, the χ T product begins a very slow decrease at higher
M
42
through the carbonyl oxygen (Figure 5). In complex 1, no
absorption attributable to ν(CdO) was observed, further in-
temperatures, with the rate of decrease becoming steadily larger
below 90 K and then further decreasing sharply to reach a
3
ꢀ1
dicating full deprotonation of H ovph in 1.
minimum of 15.6 cm K mol at 2 K. The Stark sublevels of
2
9
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Inorganic Chemistry
ARTICLE
0
Figure 7. Temperature dependence of the in-phase (χ ) and out-
00
of-phase (χ ) ac susceptibility for 3 under zero dc field at 1000 Hz.
Figure 8. Temperature (left) and frequency (right) dependence of the
ac susceptibility for 1 as a function of the temperature below 25 K (left)
and the ac frequency between 10 and 1500 Hz (right) under a zero
dc field.
the anisotropic dysprosium(III) ions are thermally depopulated
when the temperature is lowered, resulting in a decrease of the
4
4,45
χMT value.
It is thus likely that this thermal behavior is
associated with the thermal depopulation of the dysprosium(III)
excited states and that Dy Dy interactions are insignificant by
3
3 3
3
3
comparison in 3. However, the χ T product for 1 gradually
M
3
decreases with the temperature to reach a minimum of 25.3 cm
ꢀ1
K mol at about 26 K and then increases sharply to a maximum
3
ꢀ1
value of 33.9 cm K mol at 2 K, while for complex 2, χ T
M
3
ꢀ1
increases only slightly to 23.7 cm K mol at 2 K after reaching a
3
ꢀ1
minimum value of 22.5 cm K mol at 6 K. These behaviors
account for the competition between thermal depopulation of
the dysprosium(III) excited states and ferromagnetic interaction
within complexes 1 and 2.
Magnetization (M) data for 1 and 2 were collected in the
0
ꢀ70 kOe field range below 5 K. The nonsuperimposition of the
M versus H/T data on a single mastercurve (Figures S3 and S4 in
the Supporting Information) suggests the presence of magnetic
anisotropy and/or the lack of a well-defined ground state, where
the low-lying excited states might be populated when a field is
ꢀ1
Figure 9. Magnetization relaxation time, τ, versus T plot for 1 under a
zero dc field. The solid line is fitted with the Arrhenius law. Inset:
ColeꢀCole plots measured at 1.9ꢀ15 K in a zero dc field. The solid lines
are the best fits to the experimental data, obtained with the generalized
Debye model with α parameters below 0.2.
4
6
applied. The magnetization eventually reaches the value of 13.7
μ for 1 (10.5 μ for 2) at 2 K and 70 kOe. This value is much
B
B
lower than the theoretical value for two noninteracting
4
15
dysprosium(III) ions [g ꢁ J = / ꢁ /2 = 10 μB per
J
3
between 1.9 and 16 K (Figure 9). Above 11 K, the relaxation fol-
dysprosium(III)], most likely due to the crystal-field effect at
lows a thermally activated mechanism, affording an energy barrier of
the dysprosium(III) ion, which eliminates the 16-fold degener-
ꢀ
7
6
47
6
9 K with a preexponential factor (τ ) of 5.3 ꢁ 10 s based on
acy of the H
ground state. Especially, a residual slope is
0
1
5/2
the Arrhenius law [τ = τ exp(U /kT)], which is consistent with
the expected τ
that log τ becomes weakly dependent on T (more specifically
1/τ µ T) as the temperature is decreased. This behavior char-
acterizes a crossover from a thermally activated Orbach mechanism
that is predominant at high temperature to a direct or phonon-
observed for complex 2 at high field (>60 kOe), indicating failure
of the magnetization to saturate and some anisotropy in the
system (Figure S5 in the Supporting Information).
0
eff
ꢀ
6
ꢀ11
1
of 10 ꢀ10 for a SMM. It is worth noticing
0
ac susceptibility measurements were carried out for 1ꢀ3
under a zero dc field to investigate the dynamics of magnetiza-
tion. As shown in Figure 7, complex 3 does not exhibit any out-of-
0
48
phase ac signal. On the contrary, both in-phase (χ ) and out-of-
induced tunneling process taking over at T < 11 K. The data
0
0
phase (χ ) susceptibilities for 1 show the frequency dependence
maximum, signaling the “freezing” of the spins by the anisotropy
barriers, typical features associated with SMM behavior
plotted as ColeꢀCole plots (inset of Figure 9) show a relatively
symmetrical shape and can be fitted to the generalized Debye
49,50
model,
with α parameters below 0.20 (α = 0 for a Debye
(
Figure 8, left). However, the peaks can only be found at
model; Table S2 in the Supporting Information). This result
indicates that a single relaxation time is mainly involved in the
present relaxation process independently of the temperature.
The dynamics of magnetization for 2 were also investigated
from ac susceptibility measurements, in a zero static field at the
0
frequencies higher than 100 Hz. Upon cooling, increases of χ
0
0
and χ are observed below 5 K, indicating that blocking is not
complete and thermally activated spin reversal is gradually
replaced by a quantum tunneling mechanism at zero dc field.
From frequency dependencies of the ac susceptibility (Figure 8,
right), the magnetization relaxation time (τ) has been estimated
0
0
indicated frequencies given in Figure 10. The χ component of
the susceptibility has a strong frequency dependence below 5 K
9
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Inorganic Chemistry
ARTICLE
StructureꢀProperty Relationship. It has been increasingly
identified in dysprosium compounds that the contribution to the
barrier blocking magnetization arises mainly from blockage of the
2
3,52,53
individual lanthanide centers for polynuclear SMMs
be-
cause the magnetic coupling between dysprosium(III) centers is
expected to be very weak owing to the limited radial extension of
54
the 4f orbitals. Thus, for a weakly coupled system, the magnetic
axiality of the ground Kramers doublet of a single ion plays a key
5
5,56
role for the SMM performance of lanthanide complexes.
The
versatility of the coordination of H ovph to dysprosium(III) has
2
been shown by the formation of 1ꢀ3 under different reaction
conditions (Scheme 2). The base strength will influence the
ketoꢀenol tautomerism and deprotonation of the H ovph
2
ligand, resulting in distinct coordination modes to dysprosium-
(
III) centers. Thus, the different magnetic relaxation behaviors
among 1ꢀ3 are probably the result of different coordination
geometries, which are likely to affect the nature or directions of
57
the easy axes through the ligand fields. First, complex 1 has a
local hula-hoop-like geometry close to that reported for Dy
2
22
SMM [Dy (L ) (NO ) (MeOH) ] [H L = (2-hydroxy-3-
2
3 2
3 2
2
2 3
Figure 10. Temperature dependence of the in-phase (top) and out-
of-phase (bottom) ac susceptibility for 2 under a zero dc field.
methoxyphenyl)methylene(isonicotino)hydrazine; Scheme 1],
indicating that such a geometry may be a suitable and robust
ligand field for slow magnetic relaxation on dysprosium(III) ions.
Second, the relative positioning of the different strong oxygen
and nitrogen donor atoms might influence the emergence of
magnetic anisotropy. For example, the phenoxido oxygen atoms
create very short DyꢀO bonds in complexes 1 [Dy1ꢀO2 =
2
.179(4) Å] and 2 [Dy1ꢀO2 = 2.189(6) Å], and it confirmed the
formation of a strong ligand field on the local dysprosium(III)
sites. In complex 3, however, the DyꢀO/N bonds uniformly
distribute between 2.321(4) and 2.543(6) Å for Dy1 and
between 2.314(4) and 2.516(6) Å for Dy2.
In addition, the exchange interaction between the lanthanide
52,53
ions is also expected to contribute to the relaxation.
At low
enough temperature, blockage of magnetization due to exchange
interaction between the metal sites in the weak exchange limit has
0
0
0
Figure 11. Natural logarithm of the ratio of χ over χ versus 1/T of
the data for 2 given in Figure 10. Slope corresponding to energy barrier
30,58,59
been observed in several dysprosium-based SMMs.
It is
worth noting that the magnetic interactions are drastically
different among 1ꢀ3, as shown in Figure 6. These interaction
differences might generate dissimilar anisotropy of lowest exchange
multiplets, therefore affecting the dynamic magnetic behavior.
Indeed, no obvious magnetic coupling between dysprosium-
(III) centers is noticed for compound 3, while a ferromagnetic
interaction was clearly observed within complexes 1 and 2 at low
temperatures (Figure 6). These distinct magnetic behaviors must
be caused by crucial structural differences between the [Dy (μ-O) ]
U
eff = 1.3 K.
down to the lowest measured temperature 1.9 K, indicating the
onset of slow magnetization relaxation and, thus, probable SMM
behavior. Unluckily, the expected maximum due to blocking
could not be observed down to this temperature. To determine
the energy barrier Ueff and τ , another method, recently em-
0
ployed by Bartolom ꢀe et al., is to assume that there is only one
characteristic relaxation process of the Debye type with one
energy barrier and one time constant. With this assumption, one
obtains the following relation (eq ):
2
2
(1 and 2) and [Dy (μ-O) ] (3) cores. As evidenced in Table 3,
2
3
the DyꢀOꢀDy angles in both 1 and 2 are larger than 110°.
However, in compound 3, the resulting DyꢀOꢀDy angles vary
from only 92.14 to 100.08°, which are more than 10° smaller than
those observed in 1 and 2, probably induced by the presence of
0
0
0
lnðχ =χ Þ ¼ lnðωτ0Þ þ Ueff =kT
ꢀ
additional μ-OH bridges. The main, significant disparities
between the three Dy cores are thus found in the DyꢀOꢀDy
0
2
angles. Therefore, different magnetic interactions between the
metal centers induced by the μ-O bridges will be another
important factor for the distinct magnetic dynamics observed
6
0,61
for compounds 1ꢀ3.
It is well-known for other metal ions
linked by μ-O ligands that the MꢀOꢀM angles have a great
62ꢀ66
influence on the magnetic exchange coupling.
One can
rationally expect the same effect for the DyꢀOꢀDy angles.
Indeed, the DyꢀOꢀDy angle will modify the overlap between
the magnetic orbitals of the dysprosium ions and therefore will
9
711
dx.doi.org/10.1021/ic2014978 |Inorg. Chem. 2011, 50, 9705–9713

Inorganic Chemistry
ARTICLE
influence the intradinuclear magnetic interactions (Figure 6),
although such interactions are expected to be very weak. These
features clearly suggest that the strength of the local crystal field
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exchange interactions between them induced by the μ-O bridges,
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(
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ꢀ
In complex 1, the ovph ligand has the capability not only of
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2
003, 302, 1015.
(
(
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6
7
magnetization. Further work to prepare and investigate in-
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(
(
’
CONCLUSION
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(
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2
(
(
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depending on the reaction conditions. The structural differences
induced by the different coordination modes in 1ꢀ3) clearly
2
005, 127, 3650.
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(
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affect the orbital overlaps between the metal centers and ligands,
as well as the local tensor of anisotropy on each dysprosium site
and their relative orientations, therefore generating dissimilar
dynamic magnetic behavior; namely, compounds 1 and 2 show
SMM behavior, while compound 3 does not display slow re-
laxation of magnetization. Theoretical studies are required to
thoroughly analyze the DyꢀOꢀDy angle/magnetic property
relationship. The present results demonstrate that suitable crystal
fields on the dysprosium sites may lead to an efficient blocking of
magnetization. This provides a promising strategy for enhancing
the SMM properties of polynuclear lanthanide-based complexes
via fine-tuning of the local environments of the lanthanide ions.
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(
(
(
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’
ASSOCIATED CONTENT
(
26) Blagg, R. J.; Muryn, C. A.; McInnes, E. J. L.; Tuna, F.;
S
Supporting Information. Tables of hydrogen bonds
Winpenny, R. E. P. Angew. Chem., Int. Ed. 2011, 50, 6530.
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2011, 3, 538.
b
(
Table S1) and relaxation fitting parameters (Table S2) and
figures of crystal structures (Figures S1 and S2) and magnetic
measurements (Figures S3ꢀS7). This material is available free of
charge via the Internet at http://pubs.acs.org.
(
28) Rinehart, J. D.; Meihaus, K. R.; Long, J. R. J. Am. Chem. Soc.
010, 132, 7572.
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2
(
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’
AUTHOR INFORMATION
Corresponding Author
(30) Luzon, J.; Bernot, K.; Hewitt, I. J.; Anson, C. E.; Powell, A. K.;
Sessoli, R. Phys. Rev. Lett. 2008, 100, 247205.
*E-mail: tang@ciac.jl.cn.
(31) Chibotaru, L. F.; Ungur, L.; Soncini, A. Angew. Chem., Int. Ed.
2
008, 47, 4126.
’
ACKNOWLEDGMENT
(32) Car, P.-E.; Perfetti, M.; Mannini, M.; Favre, A.; Caneschi, A.;
Sessoli, R. Chem. Commun. 2011, 47, 3751.
33) Abbas, G.; Lan, Y.; Kostakis, G. E.; Wernsdorfer, W.; Anson,
C. E.; Powell, A. K. Inorg. Chem. 2010, 49, 8067.
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010, 10, 495.
35) Yu, G. M.; Li, Y. H.; Zou, L. F.; Zhu, J. W.; Liu, X. Q. Acta
We thank the National Natural Science Foundation of China
Grants 20871113, 91022009, and 20921002) for its financial
support.
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