Experimental and theoretical investigations of four 3d-4f butterfly single-molecule magnets

Article abstract of DOI:10.1039/c5dt03368c

The syntheses, structures, and characterization of four 3d-4f butterfly clusters are described. With different polyhydroxy Schiff-base ligands 2-(((2-hydroxy-3-methoxyphenyl)methylene)amino)-2-(hydroxymethyl)-1,3-propanediol (H₄L₁) a

Full text of DOI:10.1039/c5dt03368c

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Synopsis for Table of Contents
Abstract
The syntheses, structures, and characterization of four 3d−4f defect-dicubane
clusters are described. With different polyhydroxy Schiff-base ligands of
2-(((2-hydroxy-3-methoxyphenyl)methylene)amino)-2-(hydroxymethyl)-1,3-propanediol
(H4L1) and 2-(2,3-dihydroxpropyliminomethyl)-6-methoxyphenol (H3L2), three
self-assembled heterotetranuclear NiII LnIII complexes (NiII DyIII -L1 (1), NiII TbIII -L2
2
2
2
2
2
2
(2), NiII DyIII -L2 (3)) and one heterohexanuclear CoIII DyIII complex (4) were obtained.
2
2
2
4
The three heterotetranuclear NiII LnIII2 complexes display a central planar butterfly motif
2
consisting of NiII LnIII . The heterohexanuclear complex was built from defect-dicubane
2
2
CoIII DyIII clusters and two DyIII ions by the bridging of pivalate. The vertices of the
2
2
common face of the defect-dicubane core are occupied by transition metal ions in all four
complexes. Magnetic analyses indicate that the complexes exhibit typical
single-molecule magnet behaviour with anisotropy barriers of 33.7 cm-1, 60.3 cm-1, 39.6
cm-1, and 18.4 cm-1 for 1–4, respectively. Ab initio calculations were performed on these
complexes, and determined the low lying electronic structure of each LnIII (Ln = Dy, Tb)
ion and the magnetic interactions. It was found that the two Ln ions may have much more
contribution to the total relaxation barrier through the stronger 3d−4f exchange couplings.

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Experimental and Theoretical Investigation of Four 3d−4f
Butterfly SingleꢀMolecule Magnets
HuaꢀHong Zou a, LiangꢀBing Sheng a, FuꢀPei Liang a,c,* ZiꢀLu Chen a, YiꢀQuan Zhang b,*
aKey Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry
of Education of China), School of Chemistry & Pharmacy of Guangxi Normal University, Guilin
541004, P. R. China. Eꢀmail: fliangoffice@yahoo.com
bJiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing
Normal University, Nanjing 210023, P. R. China. Eꢀmail: zhangyiquan@njnu.edu.cn
cGuangxi Key Laboratory of Electrochemical and Magnetochemical Functional Materials,
College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004, China
Abstract
The syntheses, structures, and characterization of four 3d−4f butterfly clusters are
described.
With
different
polyhydroxy
Schiffꢀbase
ligands
of
2ꢀ(((2ꢀhydroxyꢀ3ꢀmethoxyphenyl)methylene)amino)ꢀ2ꢀ(hydroxymethyl)ꢀ1,3ꢀpropane
diol (H4L1) and 2ꢀ(2,3ꢀdihydroxpropyliminomethyl)ꢀ6ꢀmethoxyphenol (H3L2), three
heterotetranuclear NiII LnIII complexes (NiII DyIII2ꢀL1 (1), NiII TbIII2ꢀL2 (2),
2
2
2
2
NiII DyIII2ꢀL2 (3)) and one heterohexanuclear CoIII2DyIII complex (4) were obtained.
2
4
The three heterotetranuclear NiII LnIII2 complexes display a central planar butterfly
2
topology. The heterohexanuclear complex was built from butterfly CoIII2DyIII cluster
2
and two DyIII ions by the bridging of pivalate. The vertices of the body positions of
the butterfly are occupied by transition metal ions in all four complexes. Magnetic
analyses indicate that the complexes exhibit typical singleꢀmolecule magnet
behaviour with anisotropy barriers of 33.7 cmꢀ1, 60.3 cmꢀ1, 39.6 cmꢀ1, and 18.4 cmꢀ1
for 1–4, respectively. Ab initio calculations were performed on these complexes, and
determined the low lying electronic structure of each LnIII (Ln = Dy, Tb) ion and the
magnetic interactions. It was found that the two Ln ions may have much more
contribution to the total relaxation barrier through the stronger 3d−4f exchange

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couplings compared to weak LnꢀLn interactions.
Keywords: 3d−4f heterometallic clusters; butterfly; magnetic properties; Ab initio
calculations; singleꢀmolecule magnet
Introduction
In the past two decades, singleꢀmolecule magnets (SMMs) have attracted great
interest because of their underlying physical phenomena and potential applications
such as quantum computing, highꢀdensity memory storage devices, and molecular
spintronics.1ꢀ4 The advantage of using lanthanide ions in the synthesis of new SMMs
is that they will provide both large spin and considerable single ion anisotropy.
However, the magnetic exchanges coupling interaction between 4f ions are weak and
generally dipolar in nature.5 A combination of two different spin carriers within the
same molecular entity is a current strategy for obtaining new SMMs. The introduction
of 3d ions into pure 4f systems has proven successfully in providing strong exchange
interactions.6 Two such examples have been reported by Murray et al. CoIII2DyIII2 and
CrIII2DyIII complexes that are bridged by benzoic acid show differently
2
exchangedꢀcouple, and result in Ueff values of 88.8 and 77 K, respectively.7 The
barriers of two analogous are similar, however, the obvious magnetic hysteresis with
large coercive magnetic fields was observed in the latter compound, which arises
from the modification of the environment of DyIII when diamagnetic CoIII ions are
replaced by paramagnetic CrIII ions. Recently, FeII DyIII, a compound with a record
2
anisotropy barrier (459 K) for all d−f complexes, was reported by Tong et al.8 This
complex possesses two asymmetric and distorted FeII ions and one quasiꢀD5h DyIII ion.
The ferromagnetically coupled [FeIIꢀDyIIIꢀFeII] unit together with the quasiꢀD5h
symmetry of the DyIII ion are favorable for this system to show singleꢀmolecule
magnets behavior. These results highlight the importance of the exchange coupling by
introducing of 3d ions in lanthanideꢀbased singleꢀmolecule magnets. Up to now, many
different 3d−4f heterometal SMMs with different topological structures have been
reported, such as NiIIꢀLnIII,9 CoIIIꢀLnIII,10 CuIIꢀLnIII,11 MnIII, IVꢀLnIII,12 FeIIIꢀLnIII,13

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ZnIIꢀLnIII,14 and CrIIIꢀLnIII.15
The butterfly topology is one of the most familiar structural motifs in
magnetoꢀchemistry, and their magnetoꢀstructural correlations are often well
discussed.16 Among the SMMs with butterfly magnetic core,17ꢀ19 the 3d−4f
compounds are very interesting from the viewpoint of the magnetoꢀstructural
correlation study. They are able to show two different structure types, that is, the 4f
ions occupying the vertices of the body positions of the butterfly, and that of the
opposite situation,20 which are favorable for the investigation of the mechanism of the
magnetic behaviour of 3d−4f heterometal SMM systems. In contrast, the reported
3d−4f SMMs with 3d ions occupying the vertices of the body positions of the
butterfly
are
rare.21
Recently,
with
two
ligands
of
2ꢀ(((2ꢀhydroxyꢀ3ꢀmethoxyphenyl)methylene)amino)ꢀ2ꢀ(hydroxymethyl)ꢀ1,3ꢀpropane
diol (H4L1, Scheme S1) and 2ꢀ(2,3ꢀdihydroxpropyliminomethyl)ꢀ6ꢀmethoxyphenol
(H3L2), we synthesized four butterfly 3d−4f heterometallic clusters with the vertices
of
the
body
positions
being
occupied
by
3d
ions:
[Dy2Ni2(H2L1)2(µ3ꢀOMe)2(CH3CN)2(NO3)4]·4H2O
(1),
[Ln2Ni2(HL2)2(µ3ꢀOMe)2(CH3CN)2(NO3)4]·4H2O [Ln = Tb (2), Dy (3)], and
[Dy4Co2(HL2)2(ꢁ3ꢀOH)2(piv)10(OH2)2]·2Hpiv·2CH3OH·5H2O [Ln = Dy (4), piv =
pivalate]. The magnetic investigations indicated that all the compounds display the
SMMs properties. Herein, we wish to report the detailed syntheses, structures, and the
experimental and theoretical investigation on the magnetic properties of these four
compounds.
Results and Discussion
Description of the structure of [Dy2Ni2(H2L1)2(µ3ꢀMeO)2(CH3CN)2(NO3)4]·4H2O
(1)
Complex 1 crystallizes in the monoclinic space group P21/n with Z = 2. The
molecule has centrosymmetric site symmetry in the crystal (Figure 1 (left)), that is 1
lies on
a
crystallographic inversion center and consists of
a

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II
[Dy2IIINi2 (ꢁ2ꢀO)4(µ3ꢀMeO)2]4+ butterfly core with NiII ions occupying the vertices of
the body positions. The Ni ions are sixꢀcoordinated adopting distorted octahedral
geometries with N2O4 environments. The equatorial positions of Ni1 (Ni1A) is
occupied by NO3 donor atoms from one η1:η2:η1:η2:η1:ꢁ3 ligand, one µ3ꢀMeOꢀ and
one coordinated acetonitrile molecule. The axial positions are occupied by one
oxygen atom from one coordinated methanol molecule and one nitrogen atom from
2ꢀ
the H2L1 ligand. The values of the Ni–O and Ni–N bond lengths cover
2.034(4)–2.095(4) and 2.012(5)–2.129(6) Å, respectively, which are comparable to
those reported values.22 Dy1 and its symmetry equivalent have nine coordinated
oxygen atoms originating from two η1:η2:η1:η2:η1:ꢁ3 ligands, two nitrate anions, and
one µ3ꢀMeOꢀ anion. This results in nine coordinate DyIII ions with distorted capped
square antiprism geometries. The Dy–O bond distances are in the range of
2.278(4)–2.566(5) Å, and the average value is 2.437 Å. It is connected to Ni1 by one
µ3ꢀMeOꢀ anion bridge, one phenol oxygen atom and one alkoxyl oxygen atom. The
two central Ni ions are linked by two µ3ꢀMeOꢀ anions. It is worth noting that two
µ3ꢀMeOꢀ oxygen atoms, two Dy atoms and two Ni atoms form a butterfly topology.
Two Ni atoms (Ni1, Ni1A) and one Dy atom (Dy1 or Dy1A) form two isosceles
triangle with the Ni–Ni distances of 3.071(1) Å, and DyꢀNi distances of 3.420(2) Å,
respectively. Charge balance consideration reveals that H4L1 ligand coordinates in a
dianionic form with the phenol and one of the alkoxyl groups deprotonated and the
other two left neutral. In addition, two methanol molecules are deprotonated and
coordinated in a µ3ꢀbridging form, and four nitrate anions. The uncoordinated
hydroxyl group of the H4L1 in 1 is disordered over two positions with 0.5/0.5
occupancy. These uncoordinated hydroxyl groups provide rich hydrogen bonding
donors, while the coordinated nitrate anions act as acceptors, these hydrogen bonds
connect the adjacent clusters along the aꢀaxis. Meanwhile, the lattice water molecules
link two adjacent clusters along the cꢀaxis via the formation of hydrogen bonds with
one coordinated oxygen atom and one uncoordinated oxygen atom of L1 ligand. The
aboveꢀmentioned hydrogen bonds lead to the construction of a 2D layer structure
(Figure S1, Table S1). The shortest intermolecular MꢂꢂꢂM separation is 8.756(3) Å,

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indicating that the molecules are quite well isolated.
Fig. 1. Molecular structure of 1 (left) and 3 (right). The nonꢀcoordinated solvent molecules and the
hydrogen atoms are omitted for clarity. Color code: Ni, purple; Dy, yellow; O, red; N, light blue;
C, green.
Description of the structure of [Ln2Ni2(HL2)2(µ3ꢀOMe)2(CH3CN)2(NO3)4]·4H2O
(Ln = Tb (2), Dy (3))
Single crystal measurements indicated that complexes 2−3 are isostructural. Here,
we will describe the structure of 3 in detail; it crystallizes in the triclinic space group
Pꢀ1 such that the molecule has centrosymmetric in the crystal (Figure 1 (right)).
Complex 3 has a planar butterfly Dy2Ni2 core in which the two DyNi2 triangles are
each bridged by a single µ3ꢀMeOꢀ ligand, one lying above and the other below the
Dy2Ni2 plane. The Dy and Ni atoms are nonacoordinated and hexacoordinated,
respectively. The Dy1 cation is coordinated by one terminal alkoxo and one ꢁ2ꢀalkoxo
2ꢀ
2ꢀ
O atom from one HL2 , one phenol O atom and one methoxy O from another HL2 ,
two chelate nitrate anions, and one µ3ꢀMeOꢀ group with a nearly perfect monocapped
squareꢀantiprismatic geometry. The average Dy–O bond length is of 2.436 Å. The Ni
centers have octahedral environments, coordinated by one phenoxide O atom, an
2ꢀ
imino atom along with a ꢁ2ꢀalkoxo O atom from one HL2 , two µ3ꢀMeOꢀ groups, and
one CH3CN molecule. The Dy and Ni are connected by a bridging alkoxo group and
one µ3ꢀMeOꢀ, resulting in butterfly Dy2Ni2 motif with NiII ions occupying the vertices
of the body positions. Hydrogen bond interactions between the coordinated hydroxyl
group of the HL22ꢀ ligands and the neighbor coordinated nitrate anions ligands, which
holds molecules of 3 in chains running along cꢀdirection (Figure S2, Table S2). The

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closest intermolecular Dy···Dy distance is 6.322(2) Å. Dy···Dy distance along chain
direction is even larger with a value of 8.747(2) Å.
Description
of
the
Structure
of
[Dy4Co2(HL2)2(ꢁ3ꢀOH)2(piv)10(OH2)2]·2Hpiv·2CH3OH·5H2O (4)
Singleꢀcrystal Xꢀray diffraction measurements revealed that complex 4 crystallize
in the triclinic space group Pꢀ1. It is a heterometallic hexanuclear cluster consisting of
two CoIII and four DyIII ions, with the asymmetric unit containing half of the complex
(one CoIII ion and two DyIII ions) which lies upon an inversion center. The core of
complex 4 lies on a crystallographic inversion centre and consists of a coplanar
III
butterfly [Dy2 Co2III(ꢁ2ꢀO)4(ꢁ3ꢀOH)2]6+ core (comprising Dy1, Dy1A, Co1, and
III
Co1A) with two Dy ions (Dy2 and Dy2A) locating above and below the Dy2IIICo2
plane (Figure 2). The CoIII ions occupy the vertices of the body positions of butterfly
III
[Dy2 Co2III(ꢁ2ꢀO)4(ꢁ3ꢀOH)2]6+ core. Co1 centre is in a distorted octahedral
environment. Equatorial sites are occupied by one bridging alkoxo oxygen, one
bridging phenoxo oxygen, one ꢁ3ꢀOHꢀ oxygen and an oxygen atom from 1,3 bridging
pivalate ligands while axial sites are filled by an oxygen atom from ꢁ3ꢀOHꢀ and one
nitrogen from imino. Dy1 and its symmetry equivalent have eight coordinated oxygen
atoms originating from two ŋ2:ŋ2:ŋ1:ŋ2:ŋ1:ꢁ4 ligands, three pivalate anions, and one
ꢁ3ꢀOHꢀ oxygen with distorted square antiprismatic geometry. The Dy1–O bond
distances are in the range of 2.202(1)–2.530(1) Å, and the average value is 2.378 Å. It
is connected to Dy2 through two pivalate anions bridges and one ŋ2:ŋ2:ŋ1:ŋ2:ŋ1:ꢁ4
ligand bridge. Furthermore, it is also connected to Co1A through one pivalate anion
bridge, one ꢁ3ꢀOHꢀ ion and ŋ2:ŋ2:ŋ1:ŋ2:ŋ1:ꢁ4 ligand bridge. The sevenꢀcoordinated
Dy2/Dy2A ion adopts a capped trigonal prismatic geometry, with average Dy–O bond
lengths of 2.323 Å. The seven oxygen atoms from one ŋ2:ŋ2:ŋ1:ŋ2:ŋ1:ꢁ4 ligand, one
coordinated water molecule and four pivalate anions ligand which show a variety of
bonding modes. Two of the four pivalate ligands coordinated to Dy1 and Dy2 with
ꢁ2ꢀbridging mode. The remaining pivalate ligands connected to Dy2 with
monocoordinated
and
chelated
mode,
respectively.
The
butterfly

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III
[Dy2 Co2III(ꢁ2ꢀO)4(ꢁ3ꢀOH)2]6+ cores are held together primarily via two ꢁ3ꢀOHꢀ and
ꢁ2ꢀbridging pivalate ligands. Obviously, these small ligands are able to stabilise the
hexametallic core by coating inside and outside the metals. The two lattice water
molecules O2W and O3W connect the adjacent clusters along the crystallographic
bꢀaxis (Fig S3, Table S4). The coordinated water molecule O4W connect two pivalate
anions through O−H···O interactions along the crystallographic aꢀaxis (Fig S4, Table
S4). These result in 2D hydrogenꢀbonded sheet (Fig S5, Table S4). The closest
intermolecular Dy···Dy distance is 5.871(2) Å. In particular, complexes 2−4 are the
first
example
of
3d−4f
complexes
with
2ꢀ(2,3ꢀdihydroxpropyliminomethyl)ꢀ6ꢀmethoxyphenol as ligand.
Fig. 2. Molecular structures of the coordination cluster in complex 4. The nonꢀcoordinated solvent
molecules and the hydrogen atoms are omitted for clarity. Color code: Ni, purple; Dy, yellow; O,
red; N, light blue; C, green.
Magnetic Properties
The temperature dependence of the magnetic susceptibilities of complexes 1−4
were each measured on powdered samples over the temperature range 2ꢀ300 K under
an applied direct current magnetic field of 1000 Oe (Figure 3). For complex 1, the DC
studies reveal a room temperature χMT value of 29.89 cm3 K mol−1, which is lower
than that expected value of 30.34 cm3 K mol−1 for the noninteracting ions. The χMT
products of 1 increase slightly with decreasing temperature, reaching maximum
values of 63.93 cm3 K mol−1 at 2 K. The χMT value of 26.57 cm3 K mol−1 at 300 K for
the complex 2 is slightly higher than the value of 25.84 cm3 K mol−1 expected for two

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NiII (S = 1) and two TbIII (4f8, J = 6, S = 3, L = 3, 7F6) magnetically isolated ions. On
lowering the temperature, the χMT value increases gradually to reach a maximum
value of 105.28 cm3 K mol−1 at 3 K and then decreases abruptly. For 3, the χMT value
of 30.42 cm3 K mol−1 at 300 K for the NiII−DyIII complex is close to the expected
value of 30.34 cm3 K mol−1 for two NiII and two DyIII (4f9, J = 15/2, S = 5/2, L = 5,
6H15/2) magnetically isolated ions, and the χMT value increases gradually to reach a
maximum value of 57.78 cm3 K mol−1 at 10 K and then decreases to 48.21 cm3 K
mol−1 at 2 K. In complexes 1−3, the Ni–O–Ni angles range from 94.45 to 95.46o, and
NiII···NiII distances range from 3.067 to 3.071 Å, respectively. Evidently,
ferromagnetic coupling occurs between the two NiII ions. The shape of the χMT vs T
plots for 1−3 also indicates intramolecular ferromagnetic interactions between the
paramagnetic centers within the butterfly units.23 For 4, the product χMT at 300 K is
58.52 cm3 K mol−1, which is in agreement with the expected value for nonꢀinteracting
spins of four DyIII and two diamagnetic CoIII ions.24 χMT gradually decreases with the
lowering of temperature from 300 to 180 K and then further gradually increases to
reach ca. 63.72 cm3 K mol−1 at 14 K. On lowering the temperature, the χMT value
abruptly decreases, finally reach the minimum value of 45.35 cm3 K mol−1 at 2 K.
This thermal evolution may suggest the presence of ferrimagnetic interactions
between the spin carriers. However, it is difficult to comment on the interactions of
Dy–Dy in 4, with the DyIII ions having their intrinsic complicated magnetic
characteristics which include the presence of spin–orbit coupling and magnetic
anisotropy.25 The slight decrease in χMT on decreasing the temperature from 300 to
180 K can be ascribed to the thermal depopulation of Stark sublevels of the DyIII
ions.26 Between 180 and 14 K, the χMT product increases to reach 63.72 cm3 K mol−1
before plummeting to reach a value of 45.35 cm3 K mol−1 at 2 K. If the latter variation
is probably associated with the presence of magnetic anisotropy or weak
antiferromagnetic interaction between hexanuclear complexes, the increase of the χMT
product between 180 and 14 K suggests the presence of a highꢀspin groundꢀstate for
this complex that could be the result of either a ferriꢀ or ferromagnetic arrangement of
the Dy(III)ꢀCo(III)ꢀDy(III) spins.

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110
100
90
Complex 1
Complex 2
Complex 3
Complex 4
H = 1000 Oe
80
70
60
50
χ
χ
χ
χ
40
30
0
50 100
200 250 300
T1/50K
Fig. 3. Plots of χMT vs T for 1−4 under applied dc magnetic field of 1000 Oe.
The field dependence of magnetization (0−50 kOe) for complexes 1 and 4 each
show an initial rapid increase up to a field of 10 kOe, followed by almost linear
increase with field, finally reaching values of 17.05 Nβ (1) and 39.31 Nβ (4) at 50 kOe
and 2 K, but without saturation. For complexes 2−3, upon increasing the applied
external magnetic field, the magnetization increases up to 19.75 Nβ and 24.06 Nβ very
close to the expected saturation value of 22 Nβ and 24 Nβ (9 Nβ for TbIII ion, 10 Nβ
for DyIII ion and 2 Nβ for NiII ion) at 5 T, respectively. The sharp increase of M values
at lower dc field indicates the presence of intramolecular ferromagnetic interactions.
Furthermore, the M vs H/T (Supporting Information, Figures S6ꢀS9) plots of 1−4 at
different temperatures are not superposed, indicating the presence of low lying energy
states and/or anisotropy in the system.27
The magnetic properties of complexes 1−3 are governed by three factors: the
thermal population of the Stark components of LnIII, the NiIIꢂꢂꢂNiII interaction, and the
NiIIꢀLnIII interaction. To probe the slow relaxation of the magnetization and quantum
tunneling effects within these systems, variable temperature and frequency ac
magnetic measurements were performed for complexes 1−4, utilizing a 2.5 Oe
oscillating field and a zeroꢀapplied dc magnetic field (Fig 5). A strong
frequencyꢀdependence of the inꢀphase (χ′) (Figure S10) and outꢀofꢀphase (χ′′) (Figure
5) signals was observed for these complexes. The blocking temperatures at 1000 Hz

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for 1−4 could be observed at 4.3, 3.2, 4.5 and 4.8 K, respectively.
Fig. 4. M vs. H plot at various temperatures for complexes 1−4.
Fig. 5. Temperature dependent outꢀofꢀphase ac susceptibility for 1
(Hac = 2.5 Oe).
−4 in the absence of a dc field
Linear Arrhenius fitting (τ vs 1/T) of the data for complexes 1−4 measured under
zero applied magnetic field (Figure 6) results in energy barriers of 33.7 cmꢀ1 (1), 60.3
cmꢀ1 (2), 39.6 cmꢀ1 (3) and 18.4 cmꢀ1 (4) with relaxation times τ0 = 3.6×10ꢀ8 (1),

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2.3×10ꢀ7 (2), 3.3×10ꢀ8 (3) and 8.7×10ꢀ6 (4) s. We summed up the magnetic data of
3dꢀ4f SMMs with the 3d ions occupy the vertices of the body positions of the
butterfly up to now (Table S5). The complexes 1−3 display relatively higher
anisotropy barrier, especially, the complex 2 exhibits the highest anisotropy barrier of
60.3 cmꢀ1 (86.71 K) among the Ni−Tb SMMs reported so far.
Fig. 6. Arrhenius semilog plots of the relaxation time vs 1/T of complexes 1−4. The solid lines
correspond to a linear fit in the thermally activated range of temperatures.
Data for the complexes 1−4 are a clear signature of SMMs behavior. From the
position of the maximum of the imaginary component of the susceptibility, χ″, we
extracted the relaxation time, defined as τ(Tmax) = 1/(2πν). Plotting these data as τ
versus 1/T and fitting them using the Arrhenius law afforded the plot depicted in the
Figure 6. The ColeꢀCole diagram can be used to study the distribution of the
relaxation process, which is frequently characterized and discussed for SMMs. The
data of 1−4 plotted as ColeꢀCole diagrams are shown in Figure S11. The shape of the
ColeꢀCole plot of 1−4 are relatively symmetrical and can be fitted to the generalized
Debye model with a parameter α ranging from 0.12 to 0.08 for 1, 0.15ꢀ0.11 for 2,
0.21ꢀ0.15 for 3, and 0.19ꢀ0.13 for 4, respectivelly, The relatively small α value
indicates that a single relaxation time is mainly involved in the present relaxation
process independently of the temperature.
To confirm the SMM behavior, magnetic hysteresis measurements were

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performed using a conventional SQUID magnetometer on polycrystalline samples of
1−3 between −5 and 5 T at 2 K. Although were hardly observed, narrow hysteresis
loops could still be seen from Figure S12−S14, which evidenced the behavior of 1−3
as molecular magnets. The hysteretic behavior of complex 4 is different to that of
complexes 1−3, in this case, no hysteresis loop was observed at 2 K using a
conventional SQUID magnetometer (Figure S15). The existence of quantum
tunneling at zero field would explain why the loops observed at 2 K are small.
Ab initio Calculations for 1−4
Completeꢀactiveꢀspace selfꢀconsistent field (CASSCF) calculations on individual
lanthanide (Ln) fragments of complexes 1−4 on the basis of Xꢀray determined
geometry have been carried out with MOLCAS 7.828 and SINGLE_ANISO29 program
package (see Supporting Information for computational details). The other Ln (DyIII
for 1, 3 and 4; TbIII for 2) and transition metal (NiII for 1, 2 and 3; CoIII for 4) ions
were replaced by diamagnetic LuIII and ZnII, respectively. For complexes 1, 2 and 3,
there is only one type of Ln fragment. Thus, we only calculated one Ln fragment for
each of them. For 4, however, there are two types of Dy fragments which were both
calculated by us. The basis sets for all atoms are atomic natural orbitals from the
MOLCAS ANOꢀRCC library: ANOꢀRCCꢀVTZP for DyIII and TbIII ions; VTZ for
close O; VDZ for distant atoms. The calculations employed the second order
DouglasꢀKrollꢀHess Hamiltonian, where scalar relativistic contractions were taken
into account in the basis set and the spinꢀorbit couplings were handled separately in
the restricted active space state interaction (RASSIꢀSO) procedure. The active
electrons in 7 active spaces include all f electrons (CAS(9 in 7) for DyIIIꢀbased
complexes of 1, 3 and 4; CAS (8 in 7) for TbIIIꢀbased complex of 2) in the CASSCF
calculation. To exclude all the doubts we calculated all the roots in the active space.
We have mixed the maximum number of spinꢀfree state which was possible with our
hardware (all from 21 sextets; 128 from 224 quadruplets; 130 from 490 doublets for
the DyIII fragments of complexes 1, 3 and 4). For the TbIII fragment of complex 2, we
mixed the roots coming from the following spinꢀfree states: all from 7 septets; all
from 140 quintets; 68 from 500 triplets.

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The lowest spinꢀorbit energies and the g tensors of DyIII or TbIII ion of four
complexes are shown in Table S6. The main magnetic axes on two DyIII or TbIII ions
are indicated in Figure S16 where the magnetic axes are parallel to each other for each
complex. From Table S6, the energy separations between the ground and the first
excited doublets of 1, 2, and 3 are all larger than 150 cmꢀ1 which indicates that the
DyIIIꢀDyIII, DyIIIꢀNiII and TbIIIꢀNiII exchange interactions will be mainly of the Ising
type.
The magnetic susceptibilities of complexes 1−4 were simulated with the program
30
POLY_ANISO (see Figure S17) using the exchange parameters from Table 1. For
four complexes, the total coupling parameters J (dipolar and exchange) were included
to fit the magnetic susceptibilities. All parameters from Table 1 were calculated with
%
= 1/2 of the DyIII ions. The calculated and experimental
S
respect to the pseudospin
χMT versus T plots of four complexes were shown in Figure S17 in Supporting
Information, where a slight discrepancy of the fit to the experimental data for 1, 2 and
3 because the long relaxation times which make the measured χMT to be lower than
the isothermal susceptibility.7b The computational details of BSꢀDFT on four
complexes can be found in Supporting Information. From Table 1, the calculated
couplings between DyIIIꢀNiII, TbIIIꢀNiII, NiIIIꢀNiII, and DyIIIꢀDyIII of four complexes
using BSꢀDFT and those by fitting using Lines model are all ferromagnetic. However,
the fitting J values are all a little larger than those calculated by BSꢀDFT since
BSꢀDFT methods often underestimate ferromagnetic couplings.30, 31 From Figure S16,
the orientations of the local main magnetic axes of the ground doublets on DyIII or
TbIII ions of 1, 2 and 3 have a little difference, but they have much larger difference
for the two types of DyIII in 4.
Table 1. Calculated exchange coupling constants (J = Jexp + Jdip) (cmꢀ1) using BSꢀDFT and the
fitted exchange coupling parameters (cmꢀ1) within the Lines model of complexes 1
−4.
Complex
BSꢀDFT
7.9
Fitting
11.0
20.3
7.1
1
DyꢀNi
NiꢀNi
17.2
4.7
DyꢀDy

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2
3
4
TbꢀNi
8.5
17.6
5.3
20.4
20.5
6.8
NiꢀNi
TbꢀTb
DyꢀNi
9.5
21.6
20.5
5.7
NiꢀNi
17.8
4.9
DyꢀDy
Dy1ꢀDy2
Dy3ꢀDy4
5.8
13.0
13.0
5.8
The calculated exchange spectrum and the corresponding tunneling gaps and gz
values were given in Table S6. According to a recent proposal by Ungur and
coꢀworkers,32 the relaxation paths can be related to the tunneling gaps. The relaxation
paths of 1, 2, 3 and 4 were shown in Figures 7 and S18, respectively. The blocking
barrier can be defined by the shortest paths, where these quantities are the largest.
Thus, the derived blocking barriers of 1, 2, 3 and 4 are 35.1 cmꢀ1, 61.4 cmꢀ1, 40.3 cmꢀ1,
and 18.8 cmꢀ1, respectively, which are close to the corresponding experimental fitting
values.
Fig. 7. The magnetization blocking barrier in complexes 1 (left) and 2 (right). The arrows show
the connected exchange states. The numbers at each arrow stand for the mean absolute value of
the corresponding matrix element of the transversal magnetic moment.
For 4, the relaxation barrier is only 18.4 cmꢀ1 although the energy separation
between the ground and the first excited Kramers doublets of one DyIII ion is up to
305.2 cmꢀ1 (see Table S7). For 1, 2 and 3 which have stronger 3d−4f interactions,
however, their energy barriers are much larger than that of 4 whose DyIIIꢀDyIII
interactions are relatively much weaker. Especially for 2, the energy barrier is up to

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60.3 cmꢀ1. The above results show that the two LnIII ions may have much more
contribution to the total relaxation barrier through the stronger 3d−4f exchange
couplings compared to those with weak LnꢀLn interactions.
Conclusion
In conclusion, we have successfully synthesized four new 3d−4f heterometallic
butterfly SMMs by applying two polyhydroxy Schiffꢀbase ligands. In all four
complexes, the 3d ions occupy the vertices of the body positions of the butterfly.
Experimental and theoretical investigations indicate that there exist stronger magnetic
exchanges in the complexes of 1−3. All four complexes exhibit SMMs properties, and
the complexes 1−3 showing stronger ferromagnetic coupling display relatively higher
anisotropy barrier. Ab initio calculations have been performed. The results suggest
that DyIII/TbIII ions exhibit high axial anisotropy, and the two LnIII ions may have
much more contribution to the total relaxation barrier through the stronger 3d−4f
exchange couplings compared to weak LnꢀLn interactions. These results highlight the
importance of the exchange coupling in 3d−4f SMMs, and the synthesis of 3d−4f
heterometallic systems with enhanced exchange interactions may be an important
avenue in pursuit of SMMs with excellent magnetic performance.
Experimental section
Materials and methods
All chemicals were commercially available and used as received without further
purification.
The
Schiffꢀbase
ligand
2ꢀ(((2ꢀhydroxyꢀ3ꢀmethoxyphenyl)methylene)amino)ꢀ2ꢀ(hydroxymethyl)ꢀ1,3ꢀpropane
diol (H4L1), was prepared by a condensation of tris(hydroxymethyl)aminomethane
and
oꢀvanillin
in
a
1:1
molar
ratio
in
hot
methanol.
2ꢀ(2,3ꢀdihydroxpropyliminomethyl)6ꢀmethoxyphenol (H3L2) was prepared based on
reported procedures.33 IR spectra were recorded in the range of 4000–400 cmꢀ1 on
PerkinꢀElmer Spectrum Two FT/IR spectrometer using a KBr pellet. Elemental

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analysis (C, H, N) was performed on a Elementar Micro cube CHN elemental
analyzer. Magnetic susceptibility measurements were performed in the temperature
range of 2–300 K, using a Quantum Design MPMS SQUIDꢀXLꢀ5 magnetometer
equipped with a 5 T magnet. The diamagnetic corrections for these complexes were
estimated using Pascal’s constants, and magnetic data were corrected for diamagnetic
contributions of the sample holder.34 Alternating current susceptibility measurements
were taken of powdered samples to determine the inꢀphase and outꢀofꢀphase
components of the magnetic susceptibility. The data were collected by increasing
temperature from 2 K to 10 and/or 15 K, with no applied external dc field and a drive
frequency of 2.5 Oe, with frequencies between 1 and 1000 Hz. In the samples where
free movement of crystallites were prevented, silicone grease was employed for the
embedding.
SingleꢀCrystal Xꢀray Crystallography. Diffraction data for these complexes were
collected on a Bruker SMART CCD diffractometer (Mo Kα radiation and λ = 0.71073
Å) in Φ and ω scan modes. The structures were solved by direct methods followed by
difference Fourier syntheses, and then refined by fullꢀmatrix leastꢀsquares techniques
on F2 using SHELXLꢀ97.35 All other nonꢀhydrogen atoms were refined with
anisotropic thermal parameters. Hydrogen atoms were placed in calculated positions
and refined isotropically using a riding model. Xꢀray crystallographic data and
refinement details for the complexes are summarized in Table 2. Full details can be
found in the CIF files provided in the Supporting Information.
Synthesis
of
[Dy2Ni2(H2L1)2(µ3ꢀOMe)2(CH3CN)2(NO3)4]·4H2O
(1).
Dy(NO3)3·6H2O (22.83 mg, 0.05 mmol), Ni(NO3)2·6H2O (14.5 mg, 0.05 mmol), H4L1
(6.38 mg, 0.025 mmol), CH3OH (0.8 mL), CH3CN (0.2 mL) and Et3N (0.03 mL) were
placed in a thick Pyrex tube (ca 20 cm long). The tube was frozen using liquid N2,
evacuated under vacuum and flameꢀsealed. It was allowed to warm to room
o
temperature and heated at 70 C for 72 h, then cooled to room temperature to give
light green block crystals of 1 in 23% yield (based on Dy). Anal. Calc. for

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C30H52Dy2Ni2N8O28: C, 25.46%; H, 3.70%; N, 7.92%; Found: C, 25.51%, H, 3.76%,
N, 7.97%. IR data (KBr pellet, cmꢀ1): 3404 (s), 2934 (w), 1634 (s), 1457 (s), 1384 (s),
1299 (vs), 1221 (s), 1031 (vs), 740 (vs).
Synthesis of [Tb2Ni2(HL2)2(µ3ꢀOMe)2(CH3CN)2(NO3)4]·4H2O (2). Tb(NO3)3·6H2O
(22.65 mg, 0.05 mmol), Ni(NO3)2·6H2O (14.5 mg, 0.05 mmol), H3L2 (0.05mol/L, 1
mL), CH3OH (0.5 mL), CH3CN (0.6 mL) and Et3N (0.02 mL) were placed in a thick
Pyrex tube (ca 20 cm long). The tube was frozen using liquid N2, evacuated under
vacuum and flameꢀsealed. It was allowed to warm to room temperature and heated at
80 oC for 72 h, then cooled to room temperature to give light green block crystals of 6
in 36% yield (based on Tb). Anal. Calc. for Tb2Ni2C32H44N10O22: C, 28.34%; H,
3.27%; N, 10.33%; Found: C, 28.39%, H, 3.35%, N, 10.38%. IR data (KBr pellet,
cmꢀ1): 3331(s), 2950(m), 2833(w), 1647(s), 1471(s), 1381(m), 1305(m), 1222(s),
1116(m), 1036(m), 953(m), 853(m), 784(w), 743(m), 645(w), 515(w).
Synthesis of [Dy2Ni2(HL2)2(µ3ꢀOMe)2(CH3CN)2(NO3)4]·4H2O (3). The preparation
of 3 is similar to that of 2 using Dy(NO3)3·6H2O instead of Tb(NO3)3·6H2O, giving
light green block crystals of 3 in 29% yield (based on Dy). Anal. Calc. for
Dy2Ni2C32H44N10O22: C, 28.20%; H, 3.25%; N, 10.28%; Found: C, 28.26%, H, 3.30%,
N, 10.31%. IR data (KBr pellet, cmꢀ1): 3328(s), 2948(m), 2832(w), 1643(s), 1468(s),
1382(m), 1301(m), 1218(s), 1112(m), 1033(m), 951(m), 858(m), 783(w), 740(m),
645(w), 513(w).
Synthesis
of
the
complex
[Dy4Co2(HL2)2(ꢁ3ꢀOH)2(piv)10(OH2)2]·2
Hpiv·2CH3OH·5H2O (4).
A general synthetic protocol was applied for the preparation of all the complex 4
as follows. H3L2 (2 mL, 0.05 mol/L) was dissolved in methanol (15 mL).
Dy(NO3)3·6H2O (0.5 mmol), pivalic acid (0.30 g, 3.0 mmol) and triethylamine (0.12
mL, 0.90 mmol) were added to this solution. The reaction mixture was stirred for 30
min. At this stage, Co(NO3)2·6H2O (0.1455 g, 0.5 mmol) was added, and the reaction

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mixture was stirred for a further period of 30 min at room temperature to afford a
clear solution. A dark red solution was obtained which was evaporated and redꢀbrown
blockꢀshaped crystals were obtained from the resulting solution after one week. The
characterization data for complex 4 is given below. Yield: 23% (based on Dy). Anal.
Calc. for Dy4Co2C84H156N2O43: C, 38.07%; H, 5.93%; N, 1.06%; Found: C, 38.31%,
H, 6.02%, N, 1.13%. IR data (KBr pellet, cmꢀ1): 3406(s), 2966(m), 2742(w), 2678(m),
2491(w), 1704(m), 1643(s), 1607(w), 1542(s), 1480(s), 1385(s), 1302(m), 1230(s),
1109(w), 1077(m), 1037(m), 963(m), 904(w), 866(m), 738(m), 616(w), 599(w).
Table 2. Details of the Data Collection and Refinement Parameters for Complexes 1−4.
Complex 1
Complex 2
Complex 3
Complex 4
chemical formula
Formula weight
crystal system
space group
a / Å
Dy2Ni2C30H50N8O28
1413.16
monoclinic
P21/n (#14)
11.5441(4)
10.2056(4)
21.1065(7)
90
Tb2Ni2C32H44N10O22
1349.98
Triclinic
Pꢀ1 (No. 2)
8.7774(19)
11.403(3)
12.193(3)
72.702(3)
85.036(4)
83.513(4)
1155.9(5)
1
Dy2Ni2C32H44N10O22
1357.14
Triclinic
Pꢀ1 (No. 2)
8.7471(4)
11.3799(5)
12.1659(6)
72.730(4)
84.913(4)
83.476(4)
1146.99(10)
1
Dy4Co2C84H156N2O43
2650.02
Triclinic
Pꢀ1 (No. 2)
12.5549(17)
14.5434(9)
17.2581(12)
112.329(6)
91.119(8)
107.560(8)
2746.7(5)
1
b / Å
c / Å
α / º
β / º
102.089(3)
90
γ / º
V / Å3
2431.5(2)
2
Z
Dcalcd / g cmꢀ3
ꢁ / mmꢀ1
F(000)
1.930
1.939
1.965
1.554
3.897
3.914
4.119
3.060
1396
662
664
1284
T / K
293(2)
296
293
293
reflections total
22115
10032
23623
20373
Theta
[Deg]
MinꢀMax 3.0, 25.0
1.8, 26.4
2.9, 26.4
2.9, 25.1
ꢀ0.61, 0.77
ꢀ0.55, 0.87
ꢀ0.51, 0.61
ꢀ0.84, 1.62
Min. and Max. Resd.
Dens. [e/Ang^3]
reflections unique
Rint
4268
4670
4676
9758
0.049
0.031
0.046
0.038
GOF on F2
R1 [I>2σ(I)]
wR2 (all reflections)
CCDC
1.06
1.05
1.05
1.07
0.0372
0.0938
1036116
0.0310
0.0822
1036141
0.0247
0.0580
1036145
0.0441
0.1243
1036142

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Acknowledgements
This work was support by National Natural Science Foundation of China (No.
21271050 and 51572050), Guangxi Natural Science Foundation (No.
2015GXNSFDA139007), the State Key Laboratory Cultivation Base for the
Chemistry and Molecular Engineering of Medicinal Resources, Ministry of Science
and Technology of China (CMEMR2014ꢀA07), and the Priority Academic Program
Development of Jiangsu Higher Education Institutions.
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