Synthesis, structure, and magnetic properties of a new family of tetra-nuclear {Mn2IIILn2}(Ln = Dy, Gd, Tb, Ho) clusters with an arch-type topology: Single-molecule magnetism behavior in the dysprosium and terbium analogues

Article abstract of DOI:10.1021/ic302742u

Sequential reaction of Mn(II) and lanthanide(III) salts with a new multidentate ligand, 2,2′-(2-hydroxy-3-methoxy-5-methylbenzylazanediyl) diethanol (LH₃), containing two flexible ethanolic arms, one phenolic oxygen, and a methoxy group afforded heterometallic tetranuclear complexes [Mn₂Dy₂(LH)₄(μ-OAc)₂](NO ₃)₂·2CH₃OH·3H₂O (1), [Mn₂Gd₂(LH)₄(μ-OAc)₂](NO ₃)₂·2CH₃OH·3H₂O (2), [Mn₂Tb₂(LH)₄(μ-OAc)₂](NO ₃)₂·2H₂O·2CH ₃OH·Et₂O (3), and [Mn₂Ho ₂(LH)₄(μ-OAc)₂]Cl₂· 5CH₃OH (4). All of these dicationic complexes possess an arch-like structural topology containing a central Mn^III-Ln-Ln-Mn^III core. The two central lanthanide ions are connected via two phenolate oxygen atoms. The remaining ligand manifold assists in linking the central lanthanide ions with the peripheral Mn(III) ions. Four doubly deprotonated LH^2- chelating ligands are involved in stabilizing the tetranuclear assembly. A magnetochemical analysis reveals that single-ion effects dominate the observed susceptibility data for all compounds, with comparably weak Ln...Ln and very weak Ln...Mn(III) couplings. The axial, approximately square-antiprismatic coordination environment of the Ln³⁺ ions in 1-4 causes pronounced zero-field splitting for Tb³⁺, Dy³⁺, and Ho³⁺. For 1 and 3, the onset of a slowing down of the magnetic relaxation was observed at temperatures below approximately 5 K (1) and 13 K (3) in frequency-dependent alternating current (AC) susceptibility measurements, yielding effective relaxation energy barriers of ΔE = 16.8 cm^-1 (1) and 33.8 cm^-1 (3).

Full text of DOI:10.1021/ic302742u

Article
pubs.acs.org/IC
Synthesis, Structure, and Magnetic Properties of a New Family of
III
Tetra-nuclear {Mn2 Ln2}(Ln = Dy, Gd, Tb, Ho) Clusters With an Arch-
Type Topology: Single-Molecule Magnetism Behavior in the
Dysprosium and Terbium Analogues
Vadapalli Chandrasekhar,*,†,‡ Prasenjit Bag,† Manfred Speldrich,§ Jan van Leusen,§ and Paul Kogerler*,§
̈
†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
§Institut fur Anorganische Chemie, RWTH Aachen University, D-52074 Aachen, Germany
̈
S
* Supporting Information
ABSTRACT: Sequential reaction of Mn(II) and lanthanide(III) salts
with a new multidentate ligand, 2,2′-(2-hydroxy-3-methoxy-5-
methylbenzylazanediyl)diethanol (LH3), containing two flexible ethanolic
arms, one phenolic oxygen, and a methoxy group afforded heterometallic
t e t r a n u c l e a r c o m p l e x e s [ M n 2 D y 2 ( L H ) 4 ( μ - O A c ) 2 ] -
(NO3 )2 ·2CH3 OH·3H2 O (1), [Mn2 Gd2 (LH)4 (μ-OAc)2 ]-
(NO3 )2 ·2CH3 OH·3H2 O (2), [Mn2 Tb2 (LH)4 (μ-OAc)2 ]-
(NO3)2·2H2O·2CH3OH·Et2O (3), and [Mn2Ho2(LH)4(μ-OAc)2]-
Cl2·5CH3OH (4). All of these dicationic complexes possess an arch-
like structural topology containing a central MnIII−Ln−Ln−MnIII core.
The two central lanthanide ions are connected via two phenolate oxygen
atoms. The remaining ligand manifold assists in linking the central
lanthanide ions with the peripheral Mn(III) ions. Four doubly
deprotonated LH2− chelating ligands are involved in stabilizing the tetranuclear assembly. A magnetochemical analysis reveals
that single-ion effects dominate the observed susceptibility data for all compounds, with comparably weak Ln···Ln and very weak
Ln···Mn(III) couplings. The axial, approximately square-antiprismatic coordination environment of the Ln3+ ions in 1−4 causes
pronounced zero-field splitting for Tb3+, Dy3+, and Ho3+. For 1 and 3, the onset of a slowing down of the magnetic relaxation was
observed at temperatures below approximately 5 K (1) and 13 K (3) in frequency-dependent alternating current (AC)
susceptibility measurements, yielding effective relaxation energy barriers of ΔE = 16.8 cm−1 (1) and 33.8 cm−1 (3).
ation.11From an understanding of these systems thus far, it
appears that they have to feature two important properties to
INTRODUCTION
■
Multimetallic architectures are attracting considerable interest
function as SMMs: First, they should have a large ground-state
for a variety of reasons including their application as models for
the more complex heterogeneous metal surfaces,1as homoge-
spin; second, they also should possess a large uniaxial negative
neous catalysts,2and as molecular materials.3Most important
magnetic anisotropy (corresponding to a large negative zero-
field splitting energy D);12 however, there have been some
among the multiple challenges in this area is the ability to
assemble and modulate the nuclearity of such ensembles with a
examples of SMMs which possess positive D value.13 The
combination of these factors allows such systems to exhibit
reasonable control on the relative disposition of the various
slow relaxation of magnetization below certain temperatures in
metal ions within such architecture. Achieving this goal requires
a ligand design that allows stitching together various metal ions
the presence of certain applied alternating current (AC) fields.
In some instances, such compounds also show hysteresis loops
by appropriate bridging groups. The latter is of importance in
similar to those observed in classical magnets.14 The
molecular magnetism, as the bridging ligands mediate the
importance of both S and D cannot be overemphasized; mere
presence of large spin does not ensure SMM behavior. Thus, a
[Mn19]15 complex with an S = 83/2 ground state multiplet
failed to show any SMM behavior because it does not have any
magnetic superexchange and also define the relation between
the magnetic anisotropies of the spin centers.4 Interest in
single-molecule magnets (SMMs)5 and single-chain magnets6
continues to grow, motivated by phenomena such as quantum
tunneling7 and quantum phase interference,8 as well as by
potential applications ranging from quantum computing9and
high-density memory storage devices10 to magnetic refriger-
Received: December 13, 2012
Published: April 24, 2013
© 2013 American Chemical Society
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Table 1. Details of the Data Collection and Refinement Parameters for Compounds 1−4
1
2
3
4
formula
C58H96Mn2Dy2N6O31
1808.29
C58H96Mn2Gd2N6O32
1813.79
C62H99Mn2Tb2N6O31
1852.19
C61H105Cl2Ho2Mn2N4O25
1802.11
M/g mol−1
crystal system
space group
wavelength (MoKα)/Å
unit cell dimensions
a/Å
triclinic
triclinic
triclinic
monoclinic
P21/c
P1
̅
0.71069
P1
̅
0.71069
P1
̅
0.71069
0.71069
15.243(5)
15.268(5)
15.246(5)
15.665(5)
b/Å
15.813(5)
15.808(5)
15.802(5)
32.486(5)
c/Å
17.963(5)
17.965(5)
17.911(5)
15.691(5)
α/deg
66.685(5)
66.994(5)
66.852(5)
90.000(5)
β/deg
82.697(5)
82.487(5)
82.714(5)
108.600(5)
90.000(5)
γ/deg
V/Å3
70.871(5)
70.931(5)
71.074(5)
3757(2)
3772(2)
3753(2)
7568(4)
Z
2
2
2
4
ρc/ g cm−3
μ/mm−1
1.599
1.615
1.639
1.584
2.379
2.149
2.278
2.540
F(000)
1832
1840
1882
3668
crystal dimension/mm3
θ range/deg
limiting indices
0.14 × 0.11 × 0.085
1.70−25.50
−12 ≤ h ≤ 18
−18 ≤ k ≤ 19
−21 ≤ l ≤ 20
20246
0.13 × 0.11 × 0.08
1.88−26.00
−18 ≤ h ≤ 18
−17 ≤ k ≤ 19
−22 ≤ l ≤ 22
21153
0.125 × 0.10 × 0.075
2.25−25.50
−18 ≤ h ≤ 18
−18 ≤ k ≤ 19
−14 ≤ l ≤ 21
19908
0.13 × 0.11 × 0.09
1.86−25.50
−16 ≤ h ≤ 18
−39 ≤ k ≤ 38
−17 ≤ l ≤ 19
40300
reflns collected
independent reflns
completeness to θ
refinement method
data/restraints/params
goodness-of-fit on F2
final R indices
13663 (Rint = 0.0325)
97.6%
full-matrix least-squares on F2
13663/39/915
1.052
14478 (Rint = 0.0348)
97.7%
full-matrix least-squares on F2
14478/47/937
1.045
13614 (Rint = 0.0529)
97.3%
full-matrix least-squares on F2
13609/49/959
1.073
14027 (Rint = 0.0765)
99.7%
full-matrix least-squares on F2
14027/13/897
1.039
R1 = 0.0659,
wR2 = 0.1694
R1 = 0.0823,
wR2 = 0.1888
3.190 and −1.589
R1 = 0.0693
wR2 = 0.1852
R1 = 0.0897,
wR2 = 0.2119
3.800 and −2.054
R1 = 0.0925
wR2 = 0.2285
R1 = 0.1202,
wR2 = 0.2483
3.999 and −1.934
R1 = 0.0579
wR2 = 0.1543
R1 = 0.0840,
wR2 = 0.1771
3.028 and −1.163
[I > 2θ(I)]
R indices (all data)
ρmax/min (e·Å−3
)
appreciable D value. On the other hand, replacing the central
Mn2+ ion with a Dy3+ion gave a Mn9−Dy−Mn9 complex16
which exhibited SMM behavior because of its significant
molecular magnetic anisotropy. Thus, complexes containing 3d
and 4f metal ions are of interest as well as those where single-
ion anisotropy can be realized; note however that the zero-field
splitting of the MJ states in 4f complexes exhibiting slow
relaxation of the magnetization usually do not result in a simple
parabola (as seen for 3d-based SMMs) between the energeti-
cally lowest MJ states but in more complex patterns with
multiple maxima and minima. In this context, many 3d/4f
systems have been investigated; particularly those that contain
Cu2+//Ln3+and Ni2+/Ln3+ systems.17,18 Among the Mn3+/Ln3+
systems several high-nuclearity clusters are known: Mn21Dy,19a
Mn12Gd,19b Mn11Gd2,19c Mn6Dy6,19d Mn5Ln4,19e Mn9Dy8,19f
Mn11Dy4,19g Mn4Ln4,20 Mn2Ln2,21 Mn5Ln6,22a Mn2Ln3,22b
Mn6Ln2,22c Mn12Dy6,22d Mn4Ln2.23 Interestingly, low-nuclearity
aggregates are quite rare.24 We have recently reported a series
of Mn2Ln compounds using a phosphorus-supported tris-
hydrazone ligand.25Among Mn2-Ln2 complexes, only three
examples are thus far known.21 Recognizing that assembling a
tetranuclear aggregate containing two Mn3+ and two Ln3+ ions
requires the design of a chelating, flexible, and sterically
unencumbered ligand system, we have designed a new
multidentate ligand, 2,2′-(2-hydroxy-3-methoxy-5-
methylbenzylazanediyl)diethanol (LH3), which contains two
flexible ethanolic arms, one phenolic oxygen, and a methoxy
group at the neighboring carbon of the phenolic oxygen. This
ligand design is based on a modification of a ligand type that
has been reported in literature earlier for the preparation of a
divanadium(III) complex.26 Based on this ligand system, we
herein report the synthesis, characterization, and detailed
magnetic properties of a series of isostructural heterometallic
tetranuclear {MnIII2Ln2} complexes [Ln = Dy (1), Gd (2), Tb
(3), Ho (4)]. Among the known tetranuclear {MnIII2Ln2}
complexes, compounds 1−4 represent a new structural type.
Interestingly, compounds 1 and 3 exhibit SMM characteristics.
EXPERIMENTAL SECTION
■
Reagents and General Procedures. Solvents and other general
reagents used in this work were purified according to standard
procedures.27 2-Methoxy-4-methylphenol (Sigma Aldrich, U.S.A.) was
used as purchased. Diethanolamine, paraformaldehyde, and Mn-
(OAc)2·4H2O were obtained from SD Fine Chemicals, Mumbai, India,
and were used as such. LnX3·nH2O (X = NO3 for 1−3; X = Cl for 4)
were obtained from Aldrich Chemical Co. and were used as such.
S y n t h e s e s . 2 , 2 ′ - ( 2 - H y d r o x y - 3 - m e t h o x y - 5 -
methylbenzylazanediyl)diethanol (LH3).26 The synthesis of LH3
was carried out by adapting a literature procedure.26 To a stirred
solution of 2-methoxy-4-methylphenol (6.0 g, 43.0 mmol), diethanol-
amine (4.52 g, 43.0 mmol) in dry methanol (40 mL) and
paraformaldehyde (1.93 g, 64.5 mmol) were added, and the solution
was refluxed under a dry nitrogen atmosphere for 3 days. After this, the
solution was evaporated to dryness under vacuum, and the resulting
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a
Scheme 1. Synthesis of Compounds 1−4
a
Atom numbering applicable to compound 1.
crude oily mixture was purified by column chromatography on silica
gel (5:95 methanol/ethyl acetate) to afford LH3 as a pale yellow solid.
Yield: 9.30 g (84.50%). Mp: 60 °C. FT-IR (KBr), cm−1: 3324 (b),
2930 (m), 2919 (m), 1600 (s), 1499 (s), 1457 (s) 1396 (m), 1367 (s),
1243 (s), 1155 (s), 1089 (s). 1H NMR (CDCl3, δ, ppm): 6.55 (s, 1H,
Ar−H), 6.37 (s, 1H, Ar−H), 3.78 (s, 3H, OCH3), 3.75 (s, 2H,
ArCH2), 3.64 (t, 4H, CH2O), 2.69 (t, 4H, NCH2), 2.17 (s, 3H,
ArCH3). ESI-MS (m/z): 256.15 (M+). C/H/N analysis, calcd.for
C13H21NO4: C, 61.16; H, 8.29; N, 5.49%. Found: C, 61.02; H, 8.11; N,
5.30%.
Preparation of the Tetranuclear Complexes 1−4. A general
protocol was applied for the preparation of all the metal complexes
(1−4) as follows: LH3 (0.102 g, 0.40 mmol) was dissolved in
methanol (20 mL). Ln(NO3)3·nH2O (0.20 mmol) (for the
preparation of 1−3) and triethylamine (0.06 mL, 0.40 mmol) were
added to this solution. The reaction mixture was stirred for 1 h. At this
stage, Mn(OAc)2·4H2O (0.049 g, 0.20 mmol) was added, and the
reaction mixture was stirred for a further period of 1.5 h at 20 °C to
afford a clear solution. A deep green-colored solution was obtained
which was stripped off its solvent in vacuo resulting in a green solid
which was washed with diethyl ether and dried affording pure samples
of 1−3. For the preparation of 4, HoCl3·6H2O was used as the
lanthanide salt. X-ray quality dark green crystals of 1−4 were grown by
vapor diffusion of diethyl ether into the methanolic solution of the
corresponding complex. The characterization data for these complexes
are given below.
analysis, calcd. for C58H96 N6O31Mn2Dy2 (1808.29 g mol−1): C, 38.52;
H, 5.35; N, 4.65%. Found: C, 38.20; H, 5.04; N, 4.48%.
[Mn2Gd2(LH)4(μ-OAc)2](NO3)2·2CH3OH·3H2O (2). Yield: 0.065 g
(35.8% based on Gd). Mp: >220 °C. IR (KBr), cm−1: 3189 (b), 2974
(m), 2915 (m), 2861 (m), 1558 (s), 1488 (s), 1420 (m), 1384 (s),
1254 (s), 1156 (s), 1073 (s). ESI-MS m/z, ion: 778.13, [M]2+. C/H/N
analysis, calcd. for C58H96N6O32Mn2Gd2 (1813.79 g mol−1): C, 38.41;
H, 5.33; N, 4.63%. Found: C, 38.15; H, 5.09; N, 4.41%.
[Mn2Tb2(LH)4(μ-OAc)2](NO3)2·2H2O·2CH3OH·Et2O (3). Yield: 0.076
g (41.03% based on Tb). Mp: >220 °C. IR (KBr), cm−1: 3173 (b),
2977 (m), 2920 (m), 2852 (m), 1557 (s), 1488 (s), 1420 (m), 1384
(s), 1254 (s), 1155 (s), 1064 (s). ESI-MS m/z, ion: 779.12, [M]2+. C/
H/N analysis, calcd. for C62H99N6O31Mn2Tb2(1852.19 g mol−1): C,
40.20; H, 5.39; N, 4.54%. Found: C, 39.85; H, 5.11; N, 4.36%.
[Mn2Ho2(LH)4(μ-OAc)2]Cl2·5CH3OH (4). Yield: 0.079 g (42.5%
based on Ho). Mp: >220 °C. IR (KBr), cm−1: 3392 (b), 2971 (w)
2914 (m), 2862 (m), 1560 (s), 1489 (s), 1422 (m), 1360 (s), 1255
(s), 1157 (s), 1069 (s). ESI-MS m/z, ion: 785.12, [M]2+. C/H/N
analysis, calcd. for C61H105N4O25Cl2Mn2Ho2 (1852.19 g mol−1): C,
40.59; H, 5.86; N, 3.10%. Found: C, 40.12; H, 5.53; N, 2.90%.
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
1
operating at 400−4000 cm−1. H NMR spectra were 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. Methanol was used as the solvent for the electrospray
[Mn2Dy2(LH)4(μ-OAc)2](NO3)2·2CH3OH·3H2O (1). Yield: 0.080 g
(44.4% based on Dy). Mp: >220 °C. IR (KBr), cm−1: 3132 (b), 2978
(m), 2919 (m), 2853 (m), 1558 (s), 1489 (s), 1420 (m), 1384 (s),
1253 (s), 1156 (s), 1064 (s). ESI-MS m/z, ion: 783.13, [M]2+. C/H/N
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Figure 1. Some earlier-known examples of Mn2(III)Ln2 compounds.21b,c The left example contains pivalate ligands which bind the two central
lanthanide ions along with a terminal manganese ion. The example on the right is a carboxylate-free system and contains μ3-OH ligands that bridge
the central manganese ions along with a terminal lanthanide ion.
ionization (positive ion, full scan mode). Capillary voltage was
maintained at 2 kV, and cone voltage was kept at 31 kV.
Heisenberg−Dirac−van Vleck approach assuming only nearest-
neighbor interactions (i.e., a Mn−Ln−Ln−Mn connectivity), Hex
=
̂
3+
̂
3+
̂
3+
̂
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 AXS SMART APEX CCD X-ray diffractometer using a
Mo sealed-tube X-ray source. The program SMART28a was used for
collecting frames of data, indexing reflections, and determining lattice
parameters, SAINT28a for integration of the intensity of reflections and
−2(J1(SLn ·SLn ) + J2(SLn ·SMn3+)).
RESULTS AND DISCUSSION
■
Synthetic Aspects. The multisite coordinating ligand, LH3,
was prepared by a one-pot synthesis involving the reaction of 2-
methoxy-4-methylphenol, diethanolamine, and paraformalde-
hyde. The ligand LH3, is based on a basic phenol-framework
and contains two unsymmetrically disposed substituents.
Adjacent to the phenol unit on one side is a diethanolamine
group and on the other side a methoxy group (Scheme 1).
Thus, a total of five coordination sites are available: one
nitrogen, one phenolate oxygen, two alkoxy (or -CH2OH units)
and one methoxy oxygen atoms. Among these, the weakly
coordinating methoxy group can specifically bind to the
lanthanide ions. The flexible diethanolamine arms of the ligand
containing three coordination sites, along with the phenolate
oxygen atom, can effectively bridge different metal centers. In
accordance with this expectation, LH3 reacts with Mn-
(OAc)2·4H2O and LnX3·nH2O (in a 2:1:1 stoichiometric
ratio, in the presence of triethylamine as the base) in methanol
affording the heterometallic tetranuclear dicationic complex
salts [(LH)4(AcO)2Mn2Ln2]X2 (1−4) in good yields (Scheme
1; see the Experimental Section for details of syntheses). The
formation of these complexes is depicted in eq 1.We believe
that aerial oxygen is responsible for the oxidation of Mn(II) to
Mn(III) under the basic reaction conditions provided by Et3N.
The molecular structures of all four complexes were
determined by single-crystal X-ray crystallography (vide
infra). Compounds 1−4 retain their molecular integrity in
solution as evidenced by the detection of their molecular ion
peaks in their ESI-MS spectra (see Experimental Section; see
also Supporting Information, Figures S2−S5). It is interesting
to note that the preparation of two of the previously known
Mn2Ln2 complexes used either (2-hydroxymethyl)pyridine or
scaling, SADABS28b for absorption correction, and SHELXTL27 for
c,d
space group and structure determination and least-squares refinements
on F2. All structures were solved by direct methods using the programs
SHELXS-9728e and refined by full-matrix least-squares methods
against F2 with SHELXL-97.28e 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.28f The
high R1 = 0.0925 for compound 3 is likely due to the small crystal size,
disordered water molecules, and a Q′ peak value of 3.9 around Tb1;
despite several tries, we were not able to obtain better crystals.
Magnetochemistry. Both direct current (DC) and alternating
current (AC) susceptibility data were recorded using a Quantum
Design MPMS-5XL SQUID magnetometer. Microcrystalline samples
were pressure-compacted and immobilized into cylindrical PTFE
sample holders. DC susceptibility data was acquired as a function of
temperature (2.0−290 K) at 0.1 T and, for compound 2, as a function
of the applied magnetic field (0.1−5.0 T) at 2.0 T. AC susceptibility
data were determined in the 0.1−1500 Hz frequency range (T = 1.9−
20 K, Hac = 3 G), in the absence of a static bias field. Only compounds
1 and 3 exhibited significant out-of-phase components above 2.0 K; a
small DC bias field (10 G) did not significantly affect the AC
susceptibility for these compounds. Experimental magnetic data were
corrected for diamagnetic contributions calculated from tabulated
values. Note that all quantities are given in SI units. The magnetic data
was processed using the computational framework CONDON,29
which allows to reproduce both intramolecular exchange coupling and
single ion effects: interelectronic repulsion (Hee), spin−orbit coupling
(Hso), ligand-field effect (Hlf), and the Zeeman effect of an applied
field (Hmag), see Supporting Information for details. Intramolecular
exchange between magnetic centers was modeled using the
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(2-hydroxyethyl)pyridine as the ligands; one of these was
carboxylate-free while the other also contained pivalate
ligands21b,c (Figure 1). The third previously known Mn2Ln2
complex was prepared using 2-ethyl-2-(hydroxymethyl)-
propane-1,3-diol.21a
2Mn(OAc)2 + 2Ln(NO3)3 + 4LH3 + 4Et3N + 0.5O2
→ [MnIII2Ln2(LH)4(μ‐OAc)2](NO3)2 + 2CH3CO2H
+ 4Et3NHNO3 + H2O
(1)
Figure 4. (a) Distorted square antiprismatic coordination geometry of
the dysprosium(III) ions in 1. (b) Distorted octahedral environment
around the manganese(III) ions in 1.
X-ray Crystallography. Single-crystal X-ray study reveals
that the compounds 1−3 crystallize in the space group P1 while
4 crystallizes in P21/c. All the compounds are dicationic in
nature and have the same structural topology with an arch-like
central tetrametallic core where the Mn(III) ions are present in
the periphery and the lanthanide ions in the center (Figures 2
̅
compounds are given in the Supporting Information, Figures
S6−S8, Tables S2−S4.
Of the four ligands involved in the assembly of 1, two are
present as terminal ligands and two others in the center. Each
of the two centrosymmetrically related terminal ligands binds
to a Mn(III) ion through the phenolate oxygen (O18), the
amino nitrogen (N4), a deprotonated [CH2−CH2−O]− arm
(O19), and the alcoholic [CH2−CH2−OH] group (O20). The
protonation state of the alkoxy oxygen was determined through
a bond valence sum (BVS) calculation of O atoms (Supporting
Information, Table S2).30 The BVS value of about 1.1 suggests
that O4, O10, O14, and O20 are monoprotonated, while the
corresponding value of about 2.0 for O3, O9, O13 and O19
confirms their deprotonated state. The oxygen atom of the
deprotonated arm, O19, also bridges the terminal manganese
ion (Mn2) and the central dysprosium ion (Dy2). The
coordination environment around Mn2 is completed by an
oxygen atom of the bridging acetate group (which binds Dy2
and Mn2) and O13 which also bridges Mn2 with Dy2. The two
central ligands are involved, principally, in holding the two
dysprosium ions (Dy1 and Dy2) together, apart from bridging
one of the terminal manganese ions. Thus, each of the two
central ligands, also doubly deprotonated like the terminal
ligands, bridge Dy1 and Dy 2 through the phenolate oxygen
atom (O8). While the methoxy arm of the ligand (O11) binds
to Dy1, the amino nitrogen binds to Dy2. One of the flexible
−CH2CH2OH arms (O14) binds to a dysprosium (Dy2) while
the other, the deprotonated counterpart, [−CH2−CH2−O]−
(O13) bridges Dy2 with Mn2. Finally, the acetate ligand, which
is present as an η1-η1 bridging ligand (O15 and O16), bridges
Dy2 with Mn2. Thus, while Dy(III) and Mn(III) are bridged by
two oxygen atoms emanating from two flexible arms of two
different LH2− ligands (CH2CH2O and CH2CH2OH) and one
μ-OAc group, the two central Dy(III) ions are bridged by only
two phenolate oxygen atoms. This cumulative coordination
action results in three contiguous four-membered rings: two
terminal heterometallic MnDyO2 rings and one central Dy2O2
ring. The two dysprosium ions, Dy1 and Dy2 form the
spirocyclic centers of two four-membered rings each. The
overall topology of this interconnected multi-ring system is
arch-shaped (Figure 3) which is quite distinct and different
from the tetranuclear Mn2Ln2 derivatives known in literature,21
thus far.
Figure 2. Molecular structure of 1; hydrogen atoms, counteranions,
and solvent molecules were omitted for clarity.
and 3). In view of the similarity of the molecular structures of
1−4, we describe the structure of [(LH)4Mn2Dy2]2+ (1) as a
representative example. The structural features of this
compound are detailed in Figures 2−4. Selected bond
parameters of 1 are summarized in Table 2. The molecular
structures and selected bond parameters of the other
Figure 3. View of the arch-type topology of central Mn2Dy2 core.
Selected bond distances (Å) and bond angles (deg): Mn(1)−O(9) =
1.918(8), Mn(1)−O(3) = 1.902(7), Dy(1)−O(8) = 2.315(7),
Dy(1)−O(12) = 2.326(7), Dy(1)−O(9) = 2.304(7), Dy(1)−O(3)
= 2.359(7), Dy(1)−O(8) = 2.315(7), Dy(1)−O(12) = 2.326(7),
Dy(2)−O(19) = 2.343(7), Dy(2)−O(13) = 2.300(7), Mn(2)−O(13)
= 1.923(7), Mn(2)−O(19) = 1.900(7), Dy(1)−Mn(1) = 3.301(2),
Dy(1)−Dy(2) = 3.712(1) Dy(2)−Mn(2) = 3.297(2), Mn(1)−O(3)−
Dy(1) = 101.0(3), Mn(1)−O(9)−Dy(1) = 102.5(3), Mn(2)−
O(19)−Dy(2) = 101.3(3), Mn(2)−O(13)−Dy(2) = 102.1(3),
Dy(2)−O(12)−Dy(1) = 106.2(3), Dy(2)−O(8)−Dy(1) = 106.8(3),
Mn(1)−Dy(1)−Dy(2) = 133.88(4), Mn(2)−Dy(2)−Dy(1) =
133.71(4).
The two manganese ions, Mn1 and Mn2 are in the +III
oxidation state, as evidenced from their characteristically axially
elongated octahedral MnO5N coordination geometries with the
Jahn−Teller axes along O4−Mn1−O5 and O15−Mn2−O20,
respectively, with axial bond lengths of Mn1−O5, 2.204(8) and
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Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for Compound 1
bond lengths around
manganese(1)
Mn(1)−N(1)
Mn(1)−O(2)
Mn(1)−O(3)
Mn(1)−O(9)
Mn(1)−O(4)
Mn(1)−O(5)
manganese(2)
Mn(2)−N(4)
Mn(2)−O(18)
Mn(2)−O(13)
Mn(2)−O(19)
Mn(2)−O(15)
Mn(2)−O(20)
2.076(9)
1.860(8)
1.902(7)
1.918(8)
2.298(8)
2.204(8)
2.057(9)
1.872(7)
1.923(7)
1.900(7)
2.202(8)
2.287(9)
Mn(1)−O(3)−Dy(1)
Mn(1)−O(9)−Dy(1)
Dy(2)−O(12)−Dy(1)
Dy(2)−O(8)−Dy(1)
Mn(2)−O(19)−Dy(2)
Mn(2)−O(13)−Dy(2)
Mn(1)−Dy(1)−Dy(2)
Mn(2)−Dy(2)−Dy(1)
101.0(3)
102.5(3)
106.2(3)
106.8(3)
101.3(3)
102.1(3)
133.88(4)
133.71(4)
bond lengths around
dysprosium(1)
dysprosium(2)
Dy(1)−N(2)
2.574(9)
2.359(7)
2.299(8)
2.315(7)
2.304(7)
2.369(8)
2.488(7)
2.326(7)
Dy(2)−O(16)
2.286(8)
Dy(1)−O(3)
Dy(1)−O(6)
Dy(1)−O(8)
Dy(1)−O(9)
Dy(1)−O(10)
Dy(1)−O(11)
Dy(1)−O(12)
Dy(2)−O(7)
Dy(2)−O(8)
Dy(2)−O(12)
Dy(2)−O(13)
Dy(2)−O(14)
Dy(2)−N(3)
Dy(2)−O(19)
2.485(7)
2.309(7)
2.314(7)
2.300(7)
2.380(8)
2.600(8)
2.343(7)
Mn1−O4, 2.298(8) Å that are much longer than the equatorial
bond lengths, Mn1−O2, 1.860(8), Mn1−O3, 1.902(7), and
Mn1−O9, 1.918(8) Å (Figure 4). The BVS calculations
(Supporting Information, Table S1) also support the assign-
ment of an oxidation state of +III to the manganese ions.30 The
two central dysprosium ions reside in distorted square-
antiprismatic DyO7N environments (Figure 4). The Dy−O
bond distances are, as anticipated, longer than the Mn−O bond
distances. Unlike the Mn−O bond distances, the Dy−O bond
distances are nearly similar (average: 2.351(8) Å) with the
shortest distance of 2.299(8) Å for the Dy1−O6 bond
involving the acetate oxygen, and the longest distance of
2.488(7) Å involving the methoxy oxygen (Dy1−O11). The
intramolecular intermetallic distances involved are Mn···Dy,
3.300 Å, and Dy···Dy, 3.712 Å (Table 2). The crystal structure
of 1 reveals the formation of a supramolecular polymeric one-
dimensional (1D) chain through intermolecular O−H·····O
interactions (Supporting Information). The hydrogen bond
parameters involved in these interactions are tabulated in
Supporting Information, Table S5.
Figure 5. Temperature dependence of μeff at 0.1 T for compounds 1−
4 as well as simulated curves for single Mn3+ and Ln3+ ions in their
idealized coordination environments. Solid symbols: experimental data
(green circles: 1; orange pentagons: 2; red squares: 3; blue triangles:
4); solid graphs: least-squares fits (black or gray) and simulated single-
ion contributions (individually labeled).
Magnetism. The low-field magnetic DC susceptibility data
for compounds 1−4 exhibit clear signatures of single-ion effects
and weak Ln···Mn as well as very weak Ln···Ln antiferro-
magnetic exchange couplings. To reproduce these data,
common coordination environments are assumed for all Ln3+
sites (square-antiprismatic, see Figure 4a) and all Mn3+
(tetragonally elongated with idealized D4h symmetry, see Figure
4b).
Gd···Gd contact (ca.3.75 Å) is characterized by J1 = −0.07
cm−1, and the two shorter (ca. 3.31 Å) Gd···Mn contacts result
in J2 = −1.14 cm−1, in line with the fact that Ln···Ln (|Jex|≤ 1.0
cm−1) interactions are usually 1 order of magnitude smaller
than Ln···3d (|Jex| ≤ 10 cm−1) interactions. This weak
antiferromagnetic interaction is also well-documented for, for
example, dialkoxide-bridged Mn···Gd23b,24c and diphenoxo-
bridged Gd···Gd31 systems. Note that these J1 and J2 values
correspond to a Weiss temperature of θb = −9.14 K in the high-
temperature expansion limit, in good agreement with the
directly derived value; the small negative residual θa − θb =
−0.26 K likely corresponds to weak antiferromagnetic
intercluster coupling between neighboring {Mn2Gd2} entities,
potentially mediated by both intercluster hydrogen bonds and
dipole−dipole coupling.
Compound 2: The 4f7/3d4 system with the free-ion ground
8
5
terms S7/2 and D0 shows a nearly temperature independent
effective Bohr magneton number of 13.2 (2 × Gd3+ and 2 ×
Mn3+ ions) down to about 150 K, and χ−1; M vs T remains
linear between 50 and 290 K. Taken as a spin-only system, a
least-squares fit to a Curie−Weiss expression yields a Weiss
temperature θa = −9.4 K, that is, a global parameter accounting
for all coupling interactions in 2. The full temperature range is
successfully reproduced (SQ = 0.62%) when both exchange
coupling as well as single-ion effects (Table 3) are taken into
account (Figure 5). Within the tetranuclear complex in 2, the
In contrast to the spin-only (S = 7/2) Gd(III)-based 2, the
lanthanide(III) ions in compounds 1, 3, and 4 in their idealized
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Table 3. Magnetochemical Analysis Details
2
3
1
4
magnetic center
F2(B)/cm−1
F4(C)/cm−1
F6/cm−1
Mn3+(3d4)
Gd3+ (4f7)
Tb3+ (4f8)
97650
68530
52397
1705
Dy3+ (4f9)
Ho3+ (4f10)
101250
71057
91800
94500
64425
66320
49258
50706
54328
ζ/cm−1
1470
1932
2163
μeff (300 K)
13.19
−9.4
2.734
14.62
16.13
15.89
θa/K (50−290 K)
C/10−4 cm3 K mol−1
B2 /cm−1
−4500
14900
7500
−30
−3710
190
370
0
B4 /cm−1
−3940
−4110
0
B4 / cm−1
0
B6 / cm−1
105
195
301
0
J1 /cm−1 (Ln3+−Ln3+)
−0.05
−1.05
0.02
0.08
0.06
J2 /cm−1 (Ln3+−Mn3+)
−0.23
−0.05
−0.11
a
SQ /%
0.7
1.07
1.17
0.9
χdia/ 10−10m3 mol−1
Quality of Fit: SQ={(∑in= 1[(χiobs − χci alc)/χoi bs]2)(1/n)}1/2
−110.16
−110.16
−110.16
−110.16
a
.
D4d-symmetric environments are strongly influenced by ligand-
field effects and spin−orbit coupling. Thus, modeling the
temperature-dependent magnetic moment of 1, 3, and 4
requires accounting for all single-ion effects, whereby the ligand
field parameters determined for 2, as well as standard values for
the spin−orbit coupling constants and the Slater−Condon
parameters F2, F4, and F6, are used as constants in the fitting of
the compounds to avoid overparametrization. In 1, 3, and 4, the
Mn···Ln coupling was found to be weakly antiferromagnetic,
whereas the Ln···Ln coupling is very weakly ferromagnetic
(Table 3).
number of f electrons, going from Tb3+ to Dy3+ to Ho3+, is due
to the increase of the effective nuclear charge of these ions
(Figure 6b). To determine if these splitting patterns lead to an
effective slowing-down of the relaxation of the magnetization
upon an external field change, that is, one of the fundamental
characteristics of single-molecule magnets, AC susceptibility
measurements were performed. Only the Dy3+-based com-
pound 1 and the Tb3+-based compound 3 were found to exhibit
significant slow relaxation above 1.9 K (Supporting Informa-
tion, Figure S10 and S11). For each temperature, the
corresponding AC susceptibility data (the in-phase component
χ′ and the out-of-phase component χ″) can be fitted to a Cole−
Cole equation (Figure 7). The resulting temperature-depend-
ent fit parameters allow the determination of the average
relaxation times of the magnetization, τ, on the basis of an
Arrhenius expression, τ = τ0 exp(ΔE/kBT) (Supporting
Information, Figure S1).32 This results in the effective energy
barriers ΔE = 16.8 ( 0.8) cm−1 (1) and ΔE = 33.8 ( 2.8)
cm−1 (3), as well as the time constants τ0 = 8.30 × 10−9 ( 2.94
× 10−9) s (1) and τ0 = 1.63 × 10−8 ( 1.00 × 10−8) s (3). The
distribution width of τ is quantified by the scalar parameter α in
the generalized Debye model, where a deviation from α = 0
indicates multiple relaxation times due to multiple relaxation
mechanisms. The corresponding values α = 0.1918 ( 0.0693)
(1) and α = 0.1175 ( 0.0669) (3), averaged over all isotherms,
thus show that several relaxation mechanisms exist. We note
that the observed relaxation phenomena are not only related to
the zero-field splitting of the mJ states of the Ln3+ centers, but
are in part also influenced by the magnetic anisotropy of the
Jahn−Teller-distorted Mn3+ centers. The fact that the
anisotropy axes of the individual Ln3+ and the Jahn−Teller
axes of the Mn3+ ions are not oriented in a collinear fashion
because of the arch-type shape of the Mn−Ln−Ln−Mn
backbone reduces the effective energy barriers that are
significantly smaller than the overall ligand field splitting of
the individual Tb3+ and Dy3+ ions in 3 and 1. The arch-type
geometry of these complexes could potentially also cause local
stray fields that increase the transition probabilities of
underlying nonthermal relaxation mechanisms such as quantum
The single-ion effects on the ground state multiplets for Tb3+
(3), Dy3+ (1), and Ho3+ (4) derived from the susceptibility fits
translate into the characteristic zero-field splitting of the J
ground states into the corresponding mJ substates (Figure 6a).
Note that the linear dependency of B2 , B4 , and B6 on the
0
0
0
Figure 6. (a) Zero-field splitting of mJ substates of Ln3+ ions in
compounds 3, 1, and 4, based on ligand field parameters derived from
least-squares fitting (Figure 6, Table 3). (b) Near-linear dependency of
the Wyborne ligand field parameters as a function of the number of 4f
electrons for f8 (Tb3+), f9 (Dy3+), and d10 (Ho3+) based on least-
squares fits for 3, 1, and 4, respectively (lines: visual guides).
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complexes characterized by an arch-type topology. This was
achieved by using an unsymmetrically substituted multisite
coordinating ligand built on a vanillin platform. These
centrosymmetric compounds contain the manganese ions in
the periphery and the lanthanide ions in the center. While the
geometry around the manganese(III) ion is octahedrally
distorted, around the lanthanide ion a distorted elongated
square-antiprismatic geometry is found. DC magnetic analysis
reveals an overall antiferromagnetic interaction between the
metal centers as well as pronounced single-ion effects in all four
compounds. Despite the arched shape of the Mn−Ln−Ln−Mn
backbone of these complexes, which effectively minimizes the
net molecular magnetic anisotropy and possibly increase the
probability of nonthermal relaxation mechanisms, AC magnetic
studies reveal that compounds 1 and 3 retain sufficient
anisotropy to exhibit slow relaxation of magnetization below
1.9 K with energy barriers of ΔE = 16.8 cm−1 (1) and ΔE =
33.8 cm−1 (3), and time constants τ0 = 8.30 × 10−9 s (1) and τ0
= 1.63 × 10−8 s (3). In this context, the significant zero-field
splitting in Dy(III) and Tb(III) ions, in conjunction with the
anisotropy of the Jahn−Teller-distorted Mn(III) ions, appears
to be the dominant factor of the observed SMM behavior of 3
and 4. Given the versatility of this family of {Mn2Ln2}-type
compounds, we will further explore the option to modify the
relative orientation of the ligand fields of both the Ln3+ and the
Mn3+ centers via different chelating ligands to control the
resulting molecular magnetic anisotropy.
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data in CIF format. Further details are given in
Tables S1−S6 and Figures S1−S13. This material is available
free of charge via the Internet at http://pubs.acs.org.
Figure 7. AC susceptibility components for {Mn2Dy2}-type
compound 1 (a) and {Mn2Tb2}-type compound 3 (b). Experimental
data: filled circles, fits to Cole−Cole equation: solid lines.
AUTHOR INFORMATION
Corresponding Author
*E-mail: vc@iitk.ac.in (V.C.), paul.koegerler@ac.rwth-aachen.
de (P.K.).
■
tunneling or Orbach-type relaxation and consequently decrease
the effective relaxation barrier as well. Despite the slow
magnetization relaxation, no hysteresis of the DC magnet-
ization is observed down to 2.0 K.
Notes
The authors declare no competing financial interest.
Analogous to the structural comparison made vide supra, it is
interesting to compare the magnetic behavior of the
{MnIII2LnIII2} family discussed here with related literature
precedents. Among this family, magnetic studies on
MnIII2Ln2(O)(Piv)2(hep)4(NO3)4 (Figure 1a) (hep = 2-(2-
hydroxyethyl)pyridine; Piv = pivalate) revealed that if Ln = Dy
(III), Tb(III), or Ho(III), although slow magnetic relaxation is
observed at low temperatures, maxima of the out-of-phase
signal could not be detected precluding the determination of
the energy barrier and the time constant.21b A similar
o b s e r v a t i o n h a s a l s o b e e n m a d e f o r
[MnIII2Ln2(OH)2(NO3)4(hmp)4(H2O)4](NO3)2 (hmp = 2-
h y d r o x y m e t h y l p y r i d i n e ) . 2 1 c H o w e v e r , f o r
[NMe4]2[MnIII2DyIII2(tmp)2(O2CMe3)4(NO3)4] (tmp = 1,1,1-
tris(hydroxymethyl)propane) the estimated parameters ΔE =
ACKNOWLEDGMENTS
■
V.C. is thankful to the Department of Science and Technology,
for a J. C. Bose fellowship. P.B. thanks Council of Scientific and
Industrial Research, India for Senior Research Fellowship.
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