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第34卷 第3期
2014年6月
物 理 学 进 展
PROGRESS IN PHYSICS
Vol.34 No.3
Jun. 2014
Single Molecule Magnets
Ren Min, Zheng Li-Min*
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, P. R. China
Single molecule magnets (SMMs) refer to those molecules that show slow relaxation of magnetization below a characteristic blocking temperature (TB). Since the first report of SMM behavior in
[Mn12 O12 (O2 CCH3 )16 (H2 O)4 ] · 4H2 O · 2CH3 CO2 H (Mn12 ) in 1993, great efforts have been devoted
to the exploration of new SMMs and their potential applications in the information storage, spintronics and quantum computing etc. A number of mononuclear and polynuclear metal clusters have been
discovered to show SMM behaviors. In this review, we focus on the SMMs of oxo-bridged Mn and Fe
clusters, 3d single ion magnets and lanthanide-based SMMs.
Key words: single molecule magnet; manganese; iron; lanthanide
CLC number : O44
Document Code : A
relaxation time can be extremely long, reaching years
at low temperature (below 2 K),[1] which is reminiscent of bulk magnets. Remarkably, SMMs can also
show a stepped magnetic hysteresis due to quantum
tunneling of the magnetization (QTM).[2] Therefore,
the magnetization relaxation of SMMs obeys two processes depending on the temperature. At high temperature region, the relaxation time τ is thermally
activated following the Arrhenius law with an activation energy equal to U . At very low temperatures,
QTM becomes the fastest pathway of relaxation which
is temperature independent. A crossover occurs experimentally between these two regimes called thermally assisted QTM. In this intermediate range of
temperature, the thermal barrier is short-cut by quantum tunneling, and an effective barrier, Ueff , is found
smaller than U . In many SMM systems, this regime
is the only one seen experimentally before that τ becomes temperature independent. Taking advantage of
both the storage capacity and quantum phenomena,
SMMs appear promising for the building of future integrated nanodevices, such as high density information storage,[3] quantum computing[4] and magnetic
refregeration.[5]
CONTENTS
I. Introduction
119
II. Single molecule magnets based on 3d metal ions
A. Mn-O clusters
B. Fe-O clusters
C. Single 3d-metal ions
120
120
123
124
III. Single molecule magnets based on 4f metal ions
A. Lanthanide phthalocyaninates
B. Lanthanide polyoxometalates
C. Lanthanide organometallics
D. Other mononuclear lanthanide SMMs
E. Other polynuclear lanthanide SMMs
125
125
128
128
129
130
IV. Conclusion
131
References
131
I. INTRODUCTION
Single molecule magnets (SMMs) refer to those
molecules that show slow relaxation of magnetization below a characteristic blocking temperature (TB ).
The slow relaxation is caused by a significant energy
barrier to magnetization reversal, determined by the
combined effect of a high-spin ground state ST and
negative uniaxial anisotropy (D < 0) (Figure 1). The
upper limit of the barrier (U ) is S 2 |D| or (S 2 −1/4)|D|
for integer and half-integer spins, respectively. The
intermolecular interactions must be minimal to avoid
the long range magnetic ordering. The magnetization
Since the first report of SMM behavior in compound
[Mn12 O12 (O2 CCH3 )16 (H2 O)4 ]・4H2 O・2CH3 CO2 H
(Mn12 ) in 1993,[6] a great effort has been devoted to
the preparation and study of new systems with SMM
behaviors. These efforts have led to the discovery of
slow magnetic relaxation in a number of polynuclear
metal clusters,[7] as well as mononuclear lanthanide,[8]
actinide[9] and transition metal compounds.[10] Due
to the limitation of space, the current review will
only focus on the SMMs of oxo-bridged Mn and Fe
clusters, 3d single ion magnets and lanthanide-based
SMMs.
Received date: 2013-2-19
*[email protected]
文章编号: 1000-0542(2014)03-0119-17
119
120
Ren Min et al.: Single Molecule Magnets
III
16+
FIG. 1.
Left: The [MnIV
core of
4 Mn8 (µ-O)12 ]
[Mn12 O12 (O2 CCH3 )16 (H2 O)4 ]·4H2 O·2CH3 CO2 H. Color
codes: MnIV purple red, MnIII brick red, O yellow. Right:
Plot of the potential energy versus the magnetization
direction for a SMM with an S = 10 ground state
(adapted from ref. [1])
II. SINGLE MOLECULE MAGNETS BASED
ON 3D METAL IONS
A. Mn-O clusters
A large amount of manganese clusters have been reported which show relatively large spin ground states,
and large negative D values arising from the JahnTeller (JT) distorted MnIII ions.[11] The largest cluster is Mn84 , showing a SMM below 1.5 K with an
energy barrier of 18 K.[12] However, larger cluster
with high spin ground state cannot guarantee the
higher energy barrier of a SMM. For example, a
Mn19 cluster is not a SMM, although it displays a
record spin ground state of S = 83/2.[13] The increase of the magnetic anisotropy is more important in raising the energy barrier. The record energy barrier for the magnetization reversal among
the 3d metal SMMs is 86 K, held by [MnIII
6 O2 (Etsao)6 (O2 CPh(Me)2 )2 (EtOH)6 ].[14] The second highest barrier is observed in the Mn12 O12 family (up to
71 K).
The family of [Mn12 O12 (O2 CR)16 (H2 O)4 ] ([Mn12 ])
is the first and most thoroughly studied SMMs
to date.[7b] Table I. gives a list of the [Mn12 ]n−
complexes, together with their ground state spin,
anisotropic D values and energy barriers. Compound
[Mn12 O12 (O2 CCH3 )16 (H2 O)4 ] (Mn12 Ac) contains
IV
a [MnIII
The cluster has an
8 Mn4 (µ3 -O)12 ] core.
overall D2d symmetry. The eight MnIII ions define
the external octagon, whereas the four MnIV ions
correspond to the internal tetrahedron (Fig. 1). It
has a spin ground state of S = 10, arising from
antiferromagnetic interactions between the S = 3/2
spins of MnIV ions and the S = 2 spins of MnIII ions
with a negative axial zero-field splitting (D = −0.50
cm−1 ). Single crystal ac magnetic susceptibility
shows that the magnetization of the Mn12 Ac is
highly anisotropic with the easy axis parallel to
the tetragonal axis of the cluster, arising from
the near parallel alignment of the JT axes on the
eight MnIII ions. Slow magnetization relaxation is
observed with an effective energy barrier of 61 K for
Mn12 Ac.[6] Stepped hysteresis loop appears at low
temperature, attributing to the quantum tunneling
of magnetization (QTM). For many [Mn12 ] clusters,
two separate out-of-phase ac susceptibility signals are
observed,[15] which can be explained by the presence
of Jahn-Teller isomers within the same crystal.[16,17]
The fast relaxing species can be suppressed when
the symmetry of the cluster is high.
Complexes
[Mn12 O12 (O2 CCH2 Br)16 (H2 O)4 ]·4CH2 Cl2
(space group I41 /a)[19] and [Mn12 O12 (O2 CCH2 But )16
(MeOH)4 ]·MeOH (space group I − 4)[20] are best
examples of [Mn12 ] clusters with high molecular
symmetry. They show only one out-of-phase ac
susceptibility signal and a “cleaner” hysteresis loop,
which is invaluable for detailed QTM and HFEPR
studies.[21] The energy barrier of the former increases
to 74.4 K.[19]
The neutral [Mn12 ] clusters can be reduced
to produce one-electron, two-electron, and even
three-electron reduced species with general formula
[Mn12 O12 (O2 CR)16 (H2 O)x ]n− (n = 0, 1, 2, 3; x =
3, 4), using versatile carboxylate, mixed carboxylate
and mixed carboxylate/non-carboxylate ligands. In
all cases, the [Mn12 O12 ] core remains essentially
the same (Fig. 1). The added electrons go to the
peripheral MnIII atoms converting them to MnII ,
III
II
−
IV
III
II
giving a MnIV
4 Mn7 Mn ([Mn12 ] ), Mn4 Mn6 Mn2
2−
IV
III
II
3−
([Mn12 ] ) and Mn4 Mn5 Mn3 ([Mn12 ] ) oxidation
state description, respectively. The reduction of MnIII
ions leads to the lowing of the molecular anisotropy,
and hence the energy barrier. For example, the
ground states and D values for a series of compounds
(NPrn4 )z [Mn12 O12 (O2 CCHCl2 )16 (H2 O)4 ]z− ,[38]
decreases from 10 and −0.45 cm−1 for the neutral
[Mn12 ], to 19/2 and −0.35 cm−1 for [Mn12 ]− , 10 and
−0.28 cm−1 for [Mn12 ]2− , and 17/2 and −0.25 cm−1
for [Mn12 ]3− , respectively. The energy barriers also
decrease monotonously from 65 K for [Mn12 ], 45 K
for [Mn12 ]− , 40 K for [Mn12 ]2− , to 26 K for [Mn12 ]3− .
Other important families of Mn-O SMMs include Mn2 , Mn3 , Mn4 and Mn6 clusters.
The
first Mn2 -based SMM is [Mn(saltmen)(ReO4 )]2 ,
which contains a N2 O2 Schiff base ligand N,N’(1,1,2,2-tetramethylethylene)bis(salicylideneiminate)
(saltmenH2 ) (Fig. 2, left).[43] Ferromagnetic interactions are propagated between the MnIII centers
(2J/kB = +5.30 K), leading to an ST = 4 spin ground
state with a uniaxial anisotropy (DMn = −4.0 K).
Very small antiferromagnetic coupling of ca. −0.2 K
was found to present between the dimers. This com-
121
Ren Min et al.: Single Molecule Magnets
TABLE I. SMMs based on Mn12 O12 clusters
Mn12 O12
[Mn12 O12 (O2 CMe)16 (H2 O)4 ]·2MeCOOH·4H2 O
[Mn12 O12 (O2 CC6 H4 F-2)16 (H2 O)4 ]
Space ST
group
10
10
I41 /a
I −4
P ca21
P 21 /c
I −4
P 21 /n
9
10
10
10
10
10
10
10
10
10
D
Ueff
(cm−1 ) (K)
-0.50
61
65.2
31.9
n.a
38
n.a
64
-0.44
65.4
-0.33
64.4
-0.38
74.4
-0.46
-0.43
62.6
-0.49
71.2
64.4
-0.42
62.5
τ0
(s)
2.1 × 10−7
2.3 × 10−9
3.0 × 10−10
2.0 × 10−10
7.7 × 10−9
2.4 × 10−9
1.5 × 10−9
3.3 × 10−9
I −4
P 21 /n
P −1
C2/c
P 2/c
P −1
10
10
10
10
19/2
10
-0.65
-0.34
-0.4
-0.29
-0.34
-0.39
1.4 × 10−10
6.3 × 10−11
2.9 × 10−9
3.3 × 10−8
4.9 × 10−9
7.8 × 10−9
3.3 × 10−11
7.6 × 10−9
5.3 × 10−9
6.7 × 10−9
7.7 × 10−9
6.3 × 10−9
2.6 × 10−9
1.6 × 10−10
5.3 × 10−9
6.6 × 10−9
7.4 × 10−9
5.7 × 10−9
6.0 × 10−9
3.8 × 10−9
1.7 × 10−9
25
2.8 × 10−9
34
7.7 × 10−10
1.4 × 10−8
3.2 × 10−11
1.0 × 10−8
3.4 × 10−9
3.0 × 10−10
5.0 × 10−10
n.a.
n.a.
n.a.
n.a.
2.1 × 10−9
3.0 × 10−9
2.3 × 10−9
3.1 × 10−9
1.6 × 10−8
9.0 × 10−9
35
36
36
37
4.7 × 10−9
[Mn12 O12 (O2 CC6 H4 -p-Me)16 (H2 O)4 ]·(HO2 CC6 H4 -p-Me)
[Mn12 O12 (O2 CC6 H4 -p-Me)16 (H2 O)4 ]·3H2 O
[Mn12 O12 (O2 CCHCHCH3 )16 (H2 O)4 ]·H2 O
[Mn12 O12 (O2 CC6 H4 C6 H5 )16 (H2 O)4 ]·2C6 H5 C6 H4 COOH
[Mn12 O12 (O2 CCH2 Br)16 (H2 O)4 ]·4CH2 Cl2
[Mn12 O12 (O2 CCH2 But )16 (CH3 OH)4 ]·CH3 OH
[Mn12 O12 (O2 CCH2 But )16 (But OH)(H2 O)3 ]·2But OH
[Mn12 O12 (O2 CCH2 But )16 (C5 H11 OH)4 ](C5 H11 OH: 1-pentanol)
[Mn12 O12 (O2 CPhSCH3 )16 (H2 O)4 ]·8CHCl3
[Mn12 O12 (O2 CMe)16 (dpp)4 ]·6.1CH2 Cl2 ·0.4H2 O
(dppH: diphenyl phosphate)
[Mn12 O12 (O2 CCF3 )16 (H2 O)4 ]·2CF3 COOH·4H2 O
[Mn12 O12 (O2 CCF3 )16 (H2 O)4 ]·CF3 COOH·7H2 O
[Mn12 O12 (O2 CC6 F5 )16 (H2 O)4 ]·3CH2 Cl2
[NMe4 ]2 [Mn12 O12 (O2 CC6 F5 )16 (H2 O)4 ]·6C7 H8
[NMe4 ][Mn12 O12 (O2 CC6 F5 )16 (H2 O)4 ]·4.5CH2 Cl2 ·1/2H2 O
[Mn12 O12 (O2 CPet )16 (MeOH)4 ]
Pet COOH = 2,2-dimethylbutyric acid
[Mn12 O12 (O2 CC4 H3 S)16 (H2 O)4 ]·6CH2 Cl2 ·2H2 O
[Mn12 O12 (O2 CC4 H3 S)16 (HO2 CC4 H3 S)(H2 O)2 ]·5CH2 Cl2
C2/c
I2/a
Ibca
P −1
P −1
10
10
0.65K
0.61K
[Mn12 O12 (O2 CCHCl2 )8 (O2 CCH2 But )8 (H2 O)3 ]·CH2 Cl2 ·H2 O
[Mn12 O12 (O2 CCHCl2 )8 (O2 CEt)8 (H2 O)3 ]·CH2 Cl2
[Mn12 O12 (O2 CC6 H5 )8 La4 (H2 O)4 ]·8CH2 Cl2 (H2 La : 10-(4acetylsulfanylmethyl-phenyl)-anthracene-1,8-dicarboxylic acid)
[Mn12 O12 (NO3 )4 (O2 CCH2 But )12 (H2 O)4 ]·MeNO2
[Mn12 O12 (O2 CMe)8 (O3 SPh)8 (H2 O)4 ]
[Mn12 O12 (Z)16 (H2 O)4 ][PF6 ]16
[Mn12 O12 (Z)16 (H2 O)4 ][W6 O19 ]8
[Mn12 O12 (Z)16 (H2 O)4 ][PW12 O40 ]16/3
[Mn12 O12 (Z)16 (H2 O)4 ][(H3 O)PW11 O39 Ni]4
[Mn12 O12 (Z)16 (H2 O)4 ][(H3 O)PW11 O39 Co]4
Z = O2 C-Ph-p-CH2 N(CH2 CH2 CH2 CH3 )3
[NBun4 ]2 [Mn12 O12 (OMe)2 (O2 CPh)16 (H2 O)2 ]·2H2 O·4CH2 Cl2
[NBun4 ]2 [Mn12 O12 (OMe)2 (O2 CPh)16 (H2 O)2 ]·2H2 O·CH2 Cl2
(NBun4 )2 [Mn12 O12 (OMe)2 (O2 CPh)16 (H2 O)2 ]·6CH2 Cl2
(PPh4 )[Mn12 O12 (O2 CEt)16 (H2 O)4 ]
(PPh4 )[Mn12 O12 (O2 CPh)16 (H2 O)4 ] · 8CH2 Cl2
(PPh4 )[Mn12 O12 (O2 CPh)16 (H2 O)4 ]
P −1
P −1
I41 /am
d
C2/c
P −1
10
10
10
-0.45
-0.42
10
10
10
10
10
10
10
-0.46
-0.34
-0.44
-0.40
-0.40
52
53
P bca
P −1
10
50.1
19/2
19/2
19/2
19/2
-0.40
-0.62
−0.44
n.a.
69.5
21.7
64
28
53
62
35
67.09
51.81
66.44
72
71
65.2
41.8
72
67
53
51
51
P 2/c
10
10
11
−0.09
−0.14
−0.22
55.1
57
57.5
55
28
50
25
45
40
26
24
54
50
53
51
28
27
18.5
30.3
34.7
P −4
11
−0.31
33.8
P 21 /c
P −1
(m − MPYNN+ )[Mn12 O12 (O2 CPh)16 (H2 O)4 ]−
19/2
(NPrn4 )[Mn12 O12 (O2 CCHCl2 )16 (H2 O)4 ]
(NPrn4 )2 [Mn12 O12 (O2 CCHCl2 )16 (H2 O)4 ]
(NPrn4 )3 [Mn12 O12 (O2 CCHCl2 )16 (H2 O)4 ]
[NMe4 ]3 [Mn12 O12 (O2 CCHCl2 )16 (H2 O)4 ]
[Fe(C5 Me5 )2 ][Mn12 O12 (O2 CC6 F5 )16 (H2 O)4 ] · 2H2 O
[Fe(C5 H5 )2 ][Mn12 O12 (O2 CC6 F5 )16 (H2 O)4 ]
[Co(C5 Me5 )2 ][Mn12 O12 (O2 CC6 F5 )16 (H2 O)4 ] · 2CH2 Cl2 · C6 H14
[Co(C5 H5 )2 ][Mn12 O12 (O2 CC6 F5 )16 (H2 O)4 ]
[Fe(C5 Me5 )2 ]2 [Mn12 O12 (O2 CC6 F5 )16 (H2 O)4 ]
[Fe(C5 H5 )2 ]2 [Mn12 O12 (O2 CC6 F5 )16 (H2 O)4 ]
(PPh4 )2 [Mn12 O12 (O2 CCHCl2 )16 (H2 O)4 ] · 4CH2 Cl2 · H2 O
(PPh4 )2 [Mn12 O12 (O2 CCHCl2 )16 (H2 O)4 ] · 6CH2 Cl2
[Mn12 O12 (bet)16 (EtOH)4 ](PF6 )14 · 4CH3 CN · H2 O
bet : (CH3 )3 N+ − CH2 − CO−
2
[Mn12 O12 (bet)16 (EtOH)3 (H2 O)](PF6 )13 (OH) · 6CH3 CN · EtOH · H2 O
19/2
10
17/2
17/2
21/2
21/2
Aba2
P 21 /c
−0.35
−0.28
−0.25
−0.23
−0.38
21/2
21/2 −0.36
9.1 × 10−9
1.1 × 10−8
1.6 × 10−9
n.a.
ref
6a
16
17
18
19
20
22
23
24
26
27
28
29
30
31
32
33
38
39
40
2.1 × 10−10 41,42
42
122
Ren Min et al.: Single Molecule Magnets
FIG. 2.
The molecular structure of compounds
Mn(saltmen)(ReO4 )]2 (left, adapted from ref. [43]) and
[MnIII
3 O(O2 CMe)3 (mpko)3 ]ClO4 (right, adapted from ref.
[46])
pound shows slow relaxation of magnetization below
4 K with an activation energy Ueff of 16 K. Hysteresis
appears below 2 K (for field sweep rates of 1.4 T/s)
and becomes temperature independent below 0.6 K,
indicating the QTM effect. By varying the apical
ligands and the chemical features of the Schiff base, it
is possible to modulate the MnIII . . . MnIII magnetic
interactions and the overall magnetic behavior of
the MnIII
dimer.
Those showing SMM behav2
iors include [Mn(saltmen)(X)]2 (X− = CH3 COO,
N3 ), [Mn(salen)(NCO)]2 [salen2− = N,N’-ethylenebis(salicylide
neiminate),
[Mn(3,5-Brsalen)(3,5Brsalicylaldehyde)]2 [3,5-Brsalen2− = N,N’-ethylenebis(3,5-dibromosalicylideneiminate)][44] and [Mn2 (5MeOsaltmen)2 (DCNNQI)2 ] (DCNNQI = N,N’dicyano-1,4-naphthoquinonediiminate
radical).[45]
Their energy barriers are in the range of 19.0 ∼ 28.7
K.
For the oxide-centered, triangular, [Mn3 O(O2 CR)6
L3 ]n+ (n=0, 1) complexes, ferromagnetic interactions can be “turned on” by posing relatively
small, ligand-imposed structural distortions, defined by the Mn-N-O-Mn torsion angles (α).
This leads to the first SMM with a triangular
topology [MnIII
3 O(O2 CMe)3 (mpko)3 ]ClO4 ·3CH2 Cl2
(α = 11.2◦ ) (Fig. 2, right).[46] It has an S = 6
ground state and a negative axial anisotropy
(D = −0.34 cm−1 ). The effective relaxation barrier is Ueff = 10.9 K. Below 0.3 K, the relaxation
is temperature-independent, consistent with relaxation by ground-state QTM. A much higher
effective barrier (37.5 K) is achieved for [MnIII
3 O(Mesalox)3 (2,4’-bpy)3 (ClO4 )]·0.5MeCN which shows a
similar triangular topology but possessing a larger
Mn-N-O-Mn torsion angle of 44.2◦ .[47] Brechin et
al. carried out a systematic study by using R-saoH2
ligands to control the puckering or twisting of their
[48]
central cores of the MnIII
Their energy
3 O triangles.
barriers vary from 25.7 K to 57.0 K. A few Mn3 O
SMMs using salox ligands were also reported.[49]
FIG. 3. (a) Core structure of the [MnIV MnIII
3 O3 Cl] cubane;
(b) The structure of the [Mn4 O3 Cl4 (O2 CEt)3 (py)3 ]2
dimer, denoted [Mn4 ]2 . The dashed lines are C-H · · ·
Cl hydrogen bonds and the dotted line is the close Cl
· · · Cl approach. (c) The magnetization hysteresis loops
of [Mn4 ]2 shown at different temperatures. Reproduced
with permission from ref. [53]. Copyright 2002, Nature
Publishing Group.
Complexes
[MnIV MnIII
[L
3 O3 X(O2 CMe)3 L3 ]
= pyridine (py), dibenzoylmethane (dbm)] contain a distorted cubane structure with a central
6+
[MnIV MnIII
core.[50,51] The MnIV
3 (µ3 -O)3 (µ3 -X)]
III
. . . Mn
antiferromagnetic exchange interactions
leads to an S = 9/2 ground state. For compound
[MnIV MnIII
3 O3 Cl(O2 CMe)3 (dbm)3 ], SMM behavior
is observed at low temperature with the effective
relaxation barrier of Ueff = 11.8 K. Although the
ground state is a Kramer doublet, QTM is observed
below 0.9 K, which is explained by internal magnetic
field within the Mn4 complex due to the nuclear
spins (IMn = 5/2, IH = 1/2).[52] A very interesting
compound is [Mn4 O3 Cl4 (O2 CEt)3 (py)3 ]2 ([Mn4 ]2 )
which crystallizes in the hexagonal space group R − 3
with pairs of Mn4 molecules lying ‘head to head’
on a crystallographic S6 symmetry axis.[53] (Fig.
3a). This [Mn4 ]2 supramolecular arrangement is held
together by six C-H · · · Cl hydrogen bonds between
the pyridine rings on one [Mn4 ] and Cl ions on the
other (Fig. 3b), thus a weak antiferromagnetic (AF)
coupling is mediated between the Mn4 units resulting
in an S = 0 ground state. Wernsdorfer et al. found
that this AF coupling is manifested as an exchange
bias of all tunnelling transitions, and the hysteresis
loops consequently display unique features, such as
the absence of a QTM step at zero field (Fig. 3c).
The phenomenon is very important if SMMs are to
be used for information storage.
Another important family of Mn4 SMMs
is mixed valent clusters containing a planar
III
MnII
rhombus core. For example, compound
2 Mn2
II
[ Mn2 MnIII
2 (O2 C Me)2 (H p d m)6 ] (Cl O4 )2 · 2.5 H2 O
(pdmH2 is pyridine-2,6-dimethanol) shows ferromagnetic interactions between MnIII -MnII and
MnIII -MnIII pairs resulting in an S = 9 ground state
with negative axial ZFS (D = −0.31 cm−1 ).[54,55] It
Ren Min et al.: Single Molecule Magnets
exhibits a SMM behavior with the effective energy
barrier of 16.7 K. A seriers of Mn4 (hmp)6 (hmp
= 2-hydroxymethylpyridine) clusters with similar
III
MnII
rhombus core showing SMM behaviors
2 Mn2
are also reported, with the energy barriers ranging
from 9.2 K to 23.3 K.[56∼62] Other related complexes were reported using bridging ligands such as
triethanolamine (teaH3 ) and N-butyldiethanolamine
(bdeaH2 ). The highest energy barrier is found for
III
compound
[MnII
2 Mn2 (bdea)2 (bdeaH)2 (O2 CPh)4 ]
[63]
(Ueff =26.7 K).
Compound [MnIII
6 O2 (sao)6 (O2 CPh)2 (EtOH)4 ]·EtOH
(saoH2 = salicylaldoxime) is among the first SMMs
based on Mn6 family.[64] It has a nonplanar [MnIII
6 (µ3 O)2 (µ2 -OR)2 ]12+ core, made up of two off-set stacked
7+
triangular subunits bridged by two
[MnIII
3 (µ3 -O)]
central oximato oxygen atoms, with the remaining
four sao2− ligands bridging in a near-planar η 1 : η 1 :
7+
η 1 : µ-fashion along the edges of the [MnIII
3 (µ3 -O)]
triangles (Fig. 4). The ferromagnetic interaction
between the antiferromagnetically coupled Mn3 O
triangles leads to an S = 4 ground state. The energy
barrier and τ0 values are 27.9 K and 2.3 × 10−8
s. Motivated by the triangular Mn3 O clusters in
which the structural distortion could switch on
the ferromagnetic interaction within the trimer,
Brechin and co-workers carried out a systematic
work to investigate whether the additional steric
bulk of the derivatized oximates would enforce
structural distortions.[65−69] They found a “magic
area” (30.4◦ ∼ 31.3◦ ) of the torsion angles to predict
pairwise exchange. When α > 31.3◦ , J > 0 (F).
When α < 30.4◦ , J < 0 (AF). Thus the ground state
of the Mn6 cluster can vary from 4 to 12 simply by
controlling the structural distortion. A record energy
barrier of 86.4 K among SMMs based on 3d transition
metal clusters is observed for compound [MnIII
6 O2 (Etsao)6 (O2 CPh(Me)2 )2 (EtOH)6 ] (average torsion angle
of 39.1◦ ) which shows blocking temperature (TB ) of
ca. 4.5 K.[14]
B. Fe-O clusters
Since high spin FeIII has an 6 S ground state, large
anisotropy cannot be realized for the single FeIII
species. Very few FeIII complexes have been reported to show SMMs behaviors. The first one is
[Fe8 O2 (OH)12 (tacn)6 ]Br8 ·9H2 O (Fe8 , tacn = 1,4,7triazacyclononane) (Fig. 5, left).[70] It also has an
S = 10 ground state but with an Ising type magnetic
anisotropy of about 1/3 that of Mn12 Ac. The low
symmetry in Fe8 results in a sizeable transverse magnetic anisotropy.[71] Although the ac blocking temperature is only 3 K,[72] the Fe8 complex is a good candidate for the study of quantum effects on the magneti-
123
FIG. 4. The core (left) and molecular structure (right)
of [MnIII
6 O2 (sao)6 (O2 CPh)2 (EtOH)6 ] (adapted from ref.
[14]).
FIG. 5. The structures of [Fe8 O2 (OH)12 (tacn)6 ]Br8 ·9H2 O
(left, adapted from ref. [70]) and [FeIII
4 (OMe)6 (dpm)6 ]
(right, adapted from ref. [74]).
zation dynamics.[73]
The propeller-like FeIII
clusters are among
4
the simplest inorganic systems showing SMM
behavior. The first one of this family has a molecular formula of [FeIII
4 (OMe)6 (dpm)6 ] (Hdpm =
dipivaloylmethane).[74] The molecule has 2-fold
symmetry. The four Fe atoms lie exactly on a plane,
the inner Fe atom being in the center of the triangle
(Fig. 5, right). Antiferromagnetic interactions are
found between the central and peripheral Fe ions
(J = −21.1 cm−1 ), while that between the neighboring peripheral ones is ferromagnetic (J 0 = 1.1
cm−1 ). This leads to a spin ground state of S = 5.
The compound shows a uniaxial magnetic anisotropy
with D = −0.20 cm−1 and E = 0.01 cm−1 . The
energy barrier is 3.5 K, which is significantly smaller
than the expected value (U = 7.1 K), attributed
to the quantum tunneling due to the transverse
anisotropies. Through site-specific ligand replacement of the methoxide bridges with a tripodal ligand
R-C(CH2 OH)3 , complexes [FeIII
4 (L)2 (dpm)6 ] [H3 L=
R-C(CH2 OH)3 ] was obtained.[75] The magnetic
anisotropy and energy barriers of the FeIII
SMMs
4
may be tuned by changing the organic groups in
124
Ren Min et al.: Single Molecule Magnets
R-C(CH2 OH)3 .[76∼81] The new derivatives exhibit in
general enhanced magnetic properties with respect
to the parent cluster (Ueff = 11.1∼17.0 K). Their
static and dynamic magnetic parameters correlate
strongly with the helical pitch (γ) of the Fe(O2 Fe)3
core. The axial anisotropy |D| (evaluated from
EPR spectra) and the effective anisotropy barrier
Ueff (extracted from relaxation measurements) both
increase with increasing helical pitch. A related
III[82]
[83]
FeIII
and a chiral FeIII
compounds were
3 Cr
4
also reported with the energy barriers of 7.0 K and 4.1
K, respectively. The FeII -O clusters showing SMM
behaviors are rare, including [FeII
4 (sae)4 (MeOH)4 ],
[FeII
[FeII
4 (3,5-Cl2 4 (5-Br-sae)4 (MeOH)4 ]·MeOH,
[84]
II
sae)4 (MeOH)4 ]
and
[Fe9 (N3 )2 (O2 CMe)8 {(2py)2 CO2 }4 ].[85] Their energy barriers are 28.4 K, 30.
5K, 26.2 K and 41.7 K, respectively.
C. Single 3d-metal ions
Although much attention has been paid to metal
clusters in searching for new SMMs with large barriers, mononuclear transition metal complexes become
attractive very recently. But the number is still rather
few so far. For example, iron(II) complexes of coordination numbers 4 and 3 have been characterized
as having the magnetic signatures of orbital angular
momentum.[86] Long et al. reported that the axial
and transverse zero-field splitting (ZFS) parameter
for the trigonal pyramidal complex K[(tpaMes )Fe] are
D = −40 cm−1 and E = −0.4 cm−1 , respectively.[10a]
The large magnitude of D stems from the presence of
three electrons residing in the 1e orbital set (Fig. 6).
The negative sign of D would indicate a significant
intrinsic spin-reversal barrier of U = S 2 |D| = 227 K.
Slow magnetization relaxation was observed under a
dc field, giving an effective barrier of Ueff = 60.4 K
(τ0 = 2 × 10−9 s). The absence of slow relaxation under zero applied field is attributed to QTM through
spin-reversal barrier. The trigonal pyramidal iron(II)
complexes [(tpaR )Fe]− can show the ability to systematically enhance the magnetic anisotropy of the S = 2
center via increasing the electron donating abilities
of the tris(pyrrolyl-α-methyl)amine ligand.[10b] In the
case of R = tert-butyl, the axial ZFS parameter becomes D = −48 cm−1 , and the effective energy barrier
increases to 93.5 K.
In complex [FeII (N(TMS)2 )2 (PCy3 )], the central
FeII ion is coordinated by one PCy3 and two N(TMS)2
ligands in a trigonal planar arrangement. This leads
to a negative ZFS (D = −7.6 cm−1 ). Slow magnetization relaxation is observed under an external dc field
of 600 Oe, with the energy barrier of 42 K (τ0 = 6 ×
10−7 s). In contrast, complex [FeII (N(TMS)2 )2 (depe)]
in which the FeII ion has a distorted tetrahedral ge-
FIG. 6. Structure of the trigonal pyramidal complex
[(tpaR )Fe]− , R = tert-butyl (a); mesityl (b); phenyl (c);
2,6-difluorophenyl (d); and the splitting of the 3d orbital
energies for a high-spin FeII center in a trigonal pyramidal
ligand field (e) (adapted from ref. [10b]).
ometry does not show SMM behavior.[87]
In complex [5 CpFe(C6 H3 i Pr3 -2,6)] (5 Cp = C5 i Pr5 ),
the Fe-C bond almost coincides with the C5 axis of
the ring ligand. It shows a large negative axial ZFS
(D = −51.4 cm−1 ). The Arrhenius fitting of the ac
susceptibility data leads to effective energy barriers of
40.3 K (τ0 = 6 × 10−6 s) for the process probed at 750
Oe and 143.4 K (τ0 = 7.8 × 10−9 s) for the one probed
at 2500 Oe.[88]
Large magnetic anisotropies can also be achieved
in two-coordinate complexes with a linear L-M-L geometry. Five compounds, Fe[N(SiMe3 )(Dipp)]2 (1)
Fe[C(SiMe3 )3 ]2 (2), Fe[N(H)Ar’]2 (3), Fe[N(H)Ar*]2
(4), and Fe(OAr’)2 (5) feature a linear geometry at the
FeII center, while the sixth compound, Fe[N(H)Ar# ]2
(6), is bent with an N-Fe-N angle of 140.9(2)◦ (Dipp
= C6 H3 -2,6-Pri2 ; Ar’ = C6 H3 -2,6-(C6 H3 -2,6-Pri2 )2 ;
Ar* = C6 H3 -2,6-(C6 H2 -2,4,6-Pri2 )2 ; Ar# = C6 H3 -2,6(C6 H2 -2,4,6-Me3 )2 ). Ac magnetic susceptibility data
for all compounds revealed slow magnetic relaxation
under an applied dc field, with the magnetic relaxation time following a general trend of 1 > 2 > 3 >
4 > 5 >> 6. Arrhenius plots were fit by employing a sum of tunneling, direct, Raman, and Orbach
relaxation processes, resulting in spin reversal barriers of Ueff = 260, 210, 157, 150, and 62 K for 1∼5,
respectively.[89]
The mononuclear Co(II) complexes were
also explored as SMMs.
The first examples
are
[{ArNdCMe}2 (NPh)]Co(NCS)2
and
[{ArNdCPh}2 (NPh)]Co(NCS)2 .
Both have a
distorted square-pyramidal geometry with the CoII
centers lying above the basal plane. This leads to
significant spin-orbit coupling for the d7 CoII ions and
consequently to slow relaxation of the magnetization
under a static dc field that is characteristic of SMM
behavior. The effective energy barriers are 16 K
(τ0 = 3.6 × 10−6 s) and 24 K (τ0 = 5.1 × 10−7 s),
respectively.[90]
Field-induced
slow
magnetization
relaxation is also observed in compound cis[CoII (dmphen)2 (NCS)2 ]·0.25EtOH.
The
highly
rhombically distorted octahedral environment is
Ren Min et al.: Single Molecule Magnets
FIG. 7. Left: Structure of the tetrahedral [Co(SPh)4 ]2−
complex. Right: Electronic configuration and d-orbital energy level splitting for the molecule, with energies derived
using the angular overlap model (adapted from ref. [92]).
responsible for the strong axial and rhombic magnetic
anisotropy of the high-spin CoII ion (D = +98 cm−1 ,
E = +8.4 cm−1 ). The activation energy resulting
from the Arrhenius fitting of the ac data is 24.2∼26.0
K [τ0 = (3.0 − 4.4) × 10−7 s].[91]
Noting that all the above mentioned mononuclear
transition metal-based SMMs require application of
a dc field to disrupt fast QTM process. Compound
(Ph4 P)2 [Co(SPh)4 ] remains to be an exception (Fig.
7). In this complex, the CoII ion is tetrahedrally coordinated with an axial ZFS of D = −70 cm−1 . It
displays SMM behavior in the absence of an applied
magnetic field. The effective energy barrier is Ueff =
30 K (τ0 = 1.0 × 10−7 s).[92]
III. SINGLE MOLECULE MAGNETS BASED
ON 4f METAL IONS
Lanthanides are widely used in magnet technology.
The interest in f-element SMMs was boosted by the
report of Ishikawa et al. in 2003 that the mononuclear
bis-phthalocyanine compounds [Pc2 Ln] can show slow
magnetic relaxation.[93] The origin of the magnetism
is from both orbital and spin angular momentums of
a single lanthanide ion, which is placed in a ligand
field, giving the lowest sublevels a large |Jz | value and
energy gaps from the rest of the sublevels. Due to the
single ion features of them, these complexes are also
called single-ion magnets. Table II. lists the pure 4f
monomers of clusters that showing SMM behaviors.
A. Lanthanide phthalocyaninates
In TBA+ [Pc2 Ln]− [Ln = Tb, Dy, Ho, Er, Tm, Yb;
Pc = dianion of phthalocyanine; TBA+ = N(C4 H9 )+
4 ],
the trivalent lanthanide ion is placed in an eight coor-
125
dinate square antiprism ligand field made by two Pc
ligands with approximate D4d symmetry (Fig. 8a).
The high-order axial coordination field results in a
strong axial anisotropy along the C4 axis with potential to exhibit slow relaxation of magnetization. However, the slow magnetization relaxation is observed
only in two of the complexes with the thermally activated barriers of 331 and 40 K for Tb and Dy species,
respectively.[93] The energy barrier of Tb complex is
significantly larger than those found in the 3d-metal
based SMMs. Such a behavior is strongly related to
the sublevel structures of the ground state multiplets
of the complexes. The lowest substates in the Tb complex is Jz = ±6, which are the maximum and minimum values in the J = 6 ground state. The energy
separation from the rest of the substates is more than
576 K (Fig. 9a). For the Dy complex, the lowest
substates are characterized as Jz = 13/2 and −13/2,
which are the second largest in the J = 15/2 ground
state. The magnetic hysteresis measurements in the
subkelvin temperature range show clear evidence of
QTM for Tb, Dy, Ho compounds.[93,94] The quantum
process is a result of the resonant quantum tunneling between entangled states of electron and nuclear
spin systems.[93,94] For the Dy complex with half integer ground state, no tunneling should occur because
of the Kramers theorem of spin parity. But the coupling of J = 15/2 with nuclear spin I = 5/2 lead to
an integer total spin. Nevertheless, the step structure
at H 6= 0 T is not clear because of the significantly
reduced tunnel splitting gap.[95]
The splitting of the crystal field levels is, however, very sensitive to the environment. For example, Ruben and co-workers employed solid state 1 H
NMR to analyze the spin dynamics of the [Pc2 Tb]−
complex in a series of different diamagnetically diluted preparations, TBA+ [Pc2 Tb]− ×9[TBA]Br and
TBA+ [Pc2 Tb]− ×143[TBA]Br, using excess tetrabutylamonium bromide as matrix complement.[96]
The activation energy increases from 840 K in the
undiluted sample TBA+ [Pc2 Tb] to 922 K in the diamagnetically diluted samples. The observation emphasize that even the diamagnetic [TBA]Br matrix
arrangement around the [Pc2 Tb]− complexes can alter the splitting of the crystal field levels, and hence
the TbIII spin dynamics. The fact that small differences in molecular surrounding could trigger substantial modifications of the SMM property is of utmost
importance for ongoing research on surface-deposited
bis-phthalocyaninato terbium (III) molecules targeting the realization of single molecular data storage.[96]
The double-decker phthalocyaninates can exist not
only in the anionic form [Pc2 Ln]− , but also in the
neutral form [Pc2 Ln]0 or cationic form [Pc2 Ln]+ . The
[Pc2 Ln]0 molecule has two spin systems, i.e. an un-
126
Ren Min et al.: Single Molecule Magnets
TABLE II. SMMs based on lanthanide and actinide ions and clusters
Space group Ueff (K)
[(C4 H9 )4 N][Pc2 Tb]
331
[(C4 H9 )4 N][Pc2 Dy]
40.3
[(C4 H9 )4 N][Pc2 Ho]
n.a.
(n − Bu4 N)+ [{Pc(Oet)8 }2 Dy]−
79.1
[(C4 H9 )4 N][Tb(Pc − R)2 ](R = −C15 H31 )
640
[(C4 H9 )4 N][Tb(Pc − R)2 ](R = −C3 H7 )
616
[(C4 H9 )4 N][Tb(Pc − R)2 ](R = −CH(CH3 )Ph)
666
[{Pc(OR)8 }2 Tb](R = CH3 (CH2 )11 OCH(CH3 )CH2 −)
690(cr)
liquid-crystalline
607(dis)
[(Pc)2 TbIII ]0
P 21 21 21
590
Dy(obPc)2
P 21 /n
63
[{Pc(OEt)8 }2 TbIII ]+ (SbCl6 )− [Pc(OEt)8 = dianion
791
of 2,3,9,10,16,17,23,24- octaethoxyphthalocyanine]
[Pc(OEt)8 2 Dy]+ (SbCl6 )−
38.8
Tb2 (obPc)3
P −1
331
Dy2 (obPc)3
[Tb(obPc)2 ]Cd[Tb(obPc)2 ]
211
[Dy(obPc)2 ]Cd[Dy(obPc)2 ]
[Pc(OC4 H9 )8 ]Dy[Pc(OC4 H9 )8 ]Cd[Pc(OC4 H9 )8 ]Dy[Pc(OC4 H9 )8 ] P 2/c
22.4
[Pc(OC4 H9 )8 ]Tb[Pc(OC4 H9 )8 ]Cd[Pc(OC4 H9 )8 ]Tb[Pc(OC4 H9 )8 ] n.a.
312.7
[Pc(OC4 H9 )8 ]Tb[Pc(OC4 H9 )8 ]Cd(Pc)Tb(Pc)
n.a.
291.5
[Pc(OC4 H9 )8 ]Tb[Pc(OC4 H9 )8 ]Cd(Pc)Y(Pc)
n.a.
228.9
Na9 [ErW10 O36 ] · 35H2 O
P −1
55.2
Na9 [Ho(W5 O18 )2 ]·x H2 O
P −1
n.a.
K12 DyP5 W30 O110 ·nH2 O
24
K12 HoP5 W30 O110 ·nH2 O
0.8
(Cp∗ )Er(COT)
P nma
323.2
197.4
[DyIII (COT” )2 Li(THF)(DME)]
P 21 /c
18(0 Oe)
24
23 (100 Oe)
30
43 (200 Oe)
43 (600 Oe)
[Dy(FTA)3 L1 ]
P 32
54.4
[Dy(NTA)3 L2 ]
C2/c
30.4
[Dy(acac)3 (H2 O)2 ] · H2 O · C2 H5 OH
P 21 /n
66.1
Dy(acac)3 (phen)
P 21 /c
63.8
[Dy(acac)3 (dpq)]
P −1
136
[Dy(acac)3 (dppz)]
P −1
187
[Dy(TTA)3 (bpy)]
P 21 /n
58.0
[Dy(TTA)3 (phen)]
P −1
85.0
Na[Dy(DOTA)(H2 O)] · 4H2 O
P −1
61
Na[Er(DOTA)(H2 O)] · 4H2 O
P −1
39
Na[Yb(DOTA)(H2 O)] · 4H2 O
P −1
29
[Dy(9Accm)2 (NO3 )(dmf)2 ]
Pn
23
Zn(L)Dy(sal)(NO3 )Br
P 21 /n
336
94
[Zn3 Dy(LPr )(NO3 )3 (MeOH)3 ] · 4H2 O
P −1
25.8
[ErIII ZnII 3(L1 )(Oac)(NO3 )2 (H2 O)1.5 (MeOH)0.5 ]
P 21 21 21
3.7
24.6
[Dy(dbm)3 LR] · 2H2 O
P 21 21 21
46.9
L = 2,5-bis(4,5-pinene-2-pyridyl)pyrazine
[Dy2 (dbm)6 LR ] · 2H2 O
P 21
89.1
[Dy2III (valdien)2 (NO3 )2 ]
P −1
76
[Dy2 (ovph)2 (NO3 )2 (H2 O)2 ] · 2H2 O
P −1
69
198
τ0 (s) )
6.3× 10−8
6.3× 10−6
n.a.
4.5×10−6
6.35×10−11
1.34 ×10−10
2.22×10−11
1.5 ×10−9
1.6×10−5
6.9×10−9
8.3 ×10−6
1.1×10−10
4.7 ×10−8
3.6 × 10−7
5.5 × 10−9
8.4 × 10−9
5.9 × 10−8
1.6 × 10−8
n.a.
8.17 × 10−11
3.13 × 10−9
6 × 10−6
3 × 1016
3 × 10−5
6 × 10−6
3 × 10−7
3 × 10−7
8.7 × 10−6
4.5 × 10−6
8.1 × 10−7
5.7 × 10−6
3.1 × 10−8
1.3 × 10−8
3.4 × 10−7
3.8 × 10−7
7 × 10−11
2.5 × 10−8
4 × 10−7
1.3 × 10−6
1.1 × 10−5
1.1 × 10−9
1.2 × 10−6
5.3 × 10−7
1.4 × 10−7
5.9 × 10−8
6.0 × 10−7
5.3 × 10−7
7.3 × 10−9
ref
93
93
94
97
98
98
98
99
99
100,101
102
103
97
105
106,107
106
106
108
109
109
109
110
111
113
113
114
114
116
116
116
116
116
116
117
118
119
120
121
121
122
122
123
125
125
126
127
127
128
129
129
130
130
130
131
132
132
127
Ren Min et al.: Single Molecule Magnets
Space group
C2/m
[Dy0.87 Yb1.13 (H2 cht)2 Cl4 (H2 O)(MeCN)] · MeCN
[Dy2 (H2 cht)2 Cl4 (H2 O)(MeCN)] · MeCN
{[(Me3 Si)2 N]2 (THF)Dy}2 (µ − η 2 : η 2 − N2 )
{[(Me3 Si)2 N]2 (THF)Tb}2 (µ − η 2 : η 2 − N2 )−
[Dy3 (µ3 − OH)2 L3 Cl(H2 O)5 ]Cl3 · 4H2 O · 2MeOH · 0.7MeCN
[Dy3 (HSA)5 (SA)2 (phen)3 ](H2 SA = salicylicacid)
[Dy3 (HL)(H2 L)(NO3 )4 ](H4 L = N, N, N0 ,
N’-tetrakis(2-hydroxyethyl)-ethylene-diamine)
[Ln4 (OH)2 L2 (acac)6 ] · 2H2 L · 2CH3 CN(H2 L = N, N0
-bis(salicylidene)-1,2-cyclohexanediamine)
[Dy6 (µ3 − OH)4 L4 L02 (H2 O)9 Cl]Cl5 · 15H2 O
[Dy5 O(Oi Pr)13 ]
C2/m
P −1
P −1
C2/c
P na21
P −1
P −1
P bca
[Ho5 O(Oi Pr)13 ]
[Dy6 (ovph)4 (Hpvph)2 Cl4 (H2 O)2 (CO3 )2 ] · CH3 OH · H2 O · CH3 CN
n.a.=not available
paired π electron on the Pc ligand with S = 1/2 spin
and a LnIII ion with 4f electrons. Compound [Pc2 Ln]0
crystallizes in orthorhombic space group P 21 21 21 .
The twisted angle between the two Pc rings is 41.4◦ ,
causing a pseudo 4-fold axis perpendicular to the Pc
rings and a distorted antiprismatic coordination envi00
ronment. Peaks of the out-of-phase ( χM ) component
of the π-radical [Pc2 Ln]0 have been observed at 50,
43 and 36 K with ac magnetic fields of 103, 102 and
10 Hz, respectively, which are more than 10 K higher
than the corresponding values of the anionic complex
[Pc2 Tb]− with a closed-shell π-system. The energy
barrier is increased to 590 K, about two times that of
[Pc2 Tb]− compound.[100,101]
The oxidation of neutral species [{Pc(OEt)8 }2 Tb]0
can result in compound [{Pc(OEt)8 }2 Tb]+ (SbCl6 )− ,
where Pc(OEt)8 is a dianion of 2,3,9,10,16,17,23,24octaethoxyphthalocyanine. The magnetization reversal barrier for [{Pc(OEt)8 }2 Tb]+ (SbCl6 )− increases to 791 K, 8% greater than that of
[{Pc(OEt)8 }2 Tb]0 .[103] Such a significant increase is
attributed to the increased multiplet splitting by the
strengthened ligand field, resulting from the longitudinal contraction of the coordination space of the Tb3+
ion induced by the ligand oxidation.
The lanthanide phthalocyanine can also form
“triple-deckers” composed of three Pc ligands and two
lanthanide ions. The lanthanide ions are placed along
the fourfold symmetry axis with a separation of about
3.6 Å. In this case, the f-f or dipole-dipole interactions should be considered. By studying the magnetic
properties of PcTbPcTbPc* ([Tb,Tb]), PcYPcTbPc*
([Y,Tb]) and PcTbPcYPc* ([Tb,Y]) (Pc* = dianion of
2, 3, 9, 10, 16, 17, 23, 24-octabutoxy-phthalocyanine),
Ishikawa et al found that the effect of QTM, which
governs the relaxation process of the mono-Tb com-
C2
P 21 /n
Ueff (K)
100
29
124
177
326
61.7
65
42.6
90.9
22
200
528
46.6
400
76
τ0 (s) )
8 × 10−9
8.2 × 10−9
2.2 × 10−8
1.5 × 10−5
1.0 × 106
5.8 × 10−7
3.7 × 10−6
(1.5 kOe)
1.5 × 10−9
4.7 × 10−10
3.8 × 10−6
1.5 × 10−9
1.2 × 10−6
ref
133
133
133
134
135
136
141
142
142
143
143
144
145
145
146
147
plex in near zero magnetic fields, is removed in the
[Tb, Tb] by presence of f-f interaction.[104]
Yamashita and co-workers reported a related tripledecker compound [Tb2 (obPc)3 ] (obPc = dianion of
2,3,9,10,16,17,23,24-octabutoxyphthalocyanine) with
an energy barrier of 331 K.[105] Interestingly this
compound shows dual magnetic relaxation processes
in the low temperature region in the presence
of a dc magnetic field.
To further investigate
the influence of dipole-dipole (f-f) interactions
on magnetic relaxation, a family of multipledecker phthalocyaninato dinuclear lanthanide (III)
single-molecule magnets has been reported.[106,107]
The results show the quadruple-decker terbium
compound
{[Tb(obPc)2 ]Cd[Tb(obPc)2 ]},
and
the
quintuple-decker
compound
{[Tb(obPc)2 ]Cd(obPc)Cd[Tb(obPc)2 ]} show clearly
dual relaxation processes in ascent of dc field below
10 K, but the dysprosium compounds show single
relaxation process.
Jiang and co-workers reported the quadruple-decker
complex { [ Pc (OC4 H 9 )8 ] Dy [ Pc (OC4 H 9 )8 ] Cd [ Pc
(OC4 H 9 )8 ] Dy [ Pc (OC4 H 9 )8 ] } which shows SMM
behaviors with the energy barrier of 22.4 K.[108] They
further studied the magnetic properties of four related
Tb complexes {[Pc(OC4 H9 )8 ]Tb[Pc(OC4 H9 )8 ] Cd [Pc
(OC4 H9 )8 ]Tb[Pc(OC4 H9 )8 ]} and {[Pc(OC4 H9 )8 ]M1 }
[Pc(OC4 H9 )8 ]Cd(Pc)M2 (Pc)} (M1 -Cd-M2 = Tb-CdTb, Tb-Cd-Y, Y-Cd-Tb).[109] The energy barriers
of the Tb-Cd-Tb systems are found to be larger
than that of the monoterbium analogue, indicating
again the effect of the long-distance intramolecular
f-f interactions in the axial direction on the magnetic
properties of sandwich-type quadruple-decker complexes. Besides, the QTM can be suppressed by the
long-distance intramolecular f-f interaction.
128
Ren Min et al.: Single Molecule Magnets
FIG. 8. (a) Schematic diagram of [LnPc2 ]− (Ln = Tb,
Dy, Ho, Er, Tm, or Yb). Reproduced with permission
from ref. 93b. Copyright 2004, American Chemical Society. (b) Structure of the [ErW10 O36 ]9− POM and squareantiprismatic environment of Er (adapted from ref. [110]).
B. Lanthanide polyoxometalates
The family of lanthanide polyoxometalates (POMs)
is formed by POMs encapsulating one or more lanthanide ions in order to give rise to a lanthanide
complex in which the 4f-magnetic ions are subject
to the crystal field created by the POM ligands.
The first two families of POM-based SMMs are reported with the general formula [Ln(W5 O18 )2 ]9− (Ln
= Tb, Dy, Ho and Er) and [Ln(SiW11 O39 )2 ]13− (Ln
= Tb, Dy, Ho Er, Tm and Yb). Among them,
only the [Er(W5 O18 )2 ]9− derivative exhibited slow relaxation of the magnetization associated with SMM
behavior.[110,111] The anisotropy energy barrier was
determined to be 55.2 K.
Compound Na9 [Er(W5 O18 )2 ] is formed by two anionic [W5 O18 ]6− moieties sandwiching the center lanthanide ion. These anionic clusters are surrounded
by sodium cations to balance the charge. Each anionic [W5 O18 ]6− moiety is twisted 44.2◦ with respect
to the other, resulting in a slightly distorted squareantiprismatic environment around the Ln ion (Fig.
8b). The square-antiprism exhibits certain axial compression, in contrast to the axial elongation observed
in “double-decker” [Pc2 Ln]− complexes. In fact, complex [Pc2 Er]− shows no slow relaxation phenomenon,
with the low-lying ground states of MJ = ±1/2.[112]
Interestedly, through the fitting of the susceptibility data of [ErW10 O36 ]9− , a Kramers doublet ground
state with MJ = ±13/2 and two excited states with
MJ = ±1/2 and ±15/2 is present (Fig. 9b). Such a
difference in the energy level scheme seems to be associated with the different axial distortion of the Er
coordination site in these two compounds.
Very recently, another family of lanthanide POMs
with a 5-fold symmetry has been reported by Coronado and co-workers, namely, K12 LnP5 W30 O110 ·nH2 O
(Ln3+ = Tb, Dy, Ho, Er, Tm, and Yb).[113] In
these structures, the lanthanide center can occupy
two equivalent coordination sites, which show a very
unusual 5-fold geometry formed by five phosphate
oxygens and five bridging oxygens between tungsten
atoms, resulting in a pentagonal antiprism coordination site. The shortest Ln-Ln distance is 13.2 Å.
Compared with [ErW10 O36 ]9− , the C5 crystal field
symmetry gives rise to remarkably large off-diagonal
anisotropy parameters A56 , which mix magnetic states
with different MJ values. Consequently, only Dy and
Ho complexes exhibit magnetic hysteresis at low temperature corresponding to SMM behavior. The spin
dynamics, especially at low temperatures, is dominated by fast tunneling processes and strongly affected
by hyperfine interactions and external magnetic fields.
The thermally activated energy barriers turn out to be
very small. However, it can provide attractive candidates for the application as solid-state spin qubits.
C. Lanthanide organometallics
Many organometallic compounds show a doubledecker structure in which the metal center is sandwiched by two aromatic ligands.
The uniaxial
anisotropy is promoted by high symmetry coordination environment and delocalized ligands. The first
organometallic lanthanide SMM was reported by Gao
et al. in 2011,[114] namely (Cp*)Er(COT) (COT =
cyclooctatetraenide, C8 H2−
8 ; Cp* = pentamethylcyclopentadienide, C5 Me−
).
The Er(III) ion is sand5
wiched between the two aromatic rings, being closer
to the COT center (1.66 Å) than to that of the Cp*
ring (2.27 Å) (Fig. 10). Because of the different rings
and the tilting between them, the Er(III) is situated
in an environment of low point group symmetry of Cs .
The COT group is crystallographically disordered in
the temperature range of 10∼120 K. The disorder is
assumed to be static by nature due to the coexistence
of two stable conformers with different COT conformations in the crystal rather than dynamic position
disorder. The local symmetry of Er(III) is approximated to be C∞v . The fine electronic structures are
investigated with ligand field theory, and shows that
the ground state is | ± 15/2i, with the first excited
state | ± 13/2i, lying 273 K above. Ac magnetic measurements show that this compound has two thermally
activated relaxation processes with the energy barriers
of ∆E1 = 323 K (τ01 = 8.17×10−11 s) and ∆E2 = 197
K (τ02 = 3.13 × 10−9 s), attributed to the aforementioned two stable conformers in the crystal. Butterfly
shaped hysteresis loops were recorded at an average
scanning field speed of 550 Oe/min below 5 K. The
isostructural Dy and Ho compounds also show SMM
behaviors, but their energy barriers are much lower
(24.3, 80.6 cm−1 ).[115]
Murugesu and co-workers reported another
organometallic lanthanide SMM with formula
[DyIII (COT” )2 Li(THF)(DME)] (COT” = 1,4bis(trimethylsilyl)cyclooctatetraenyl
dianion).[116]
Ren Min et al.: Single Molecule Magnets
129
FIG. 9. Energy level diagrams of the ground-state multiplets for [LnPc2 ]− (left, reproduced with permission from ref.
93b. Copyright 2004, American Chemical Society) and [Ln(W10 O36 )]9− (right, reproduced with permission from ref.
[111]. Copyright 2009, American Chemical Society).
with the other fields, continues to illustrate that the
low-frequency peak C is the predominant relaxation
pathway. However, close inspection of the peak shape
reveals that the peak signals are broad, suggesting
an overlapping relaxation mechanism (A and C) with
very similar relaxation times. The overlapping peaks
are barely apparent.
FIG. 10. Left: Schematic view of the title compound
(Cp*)Er(COT). Right: Out-of-phase of ac susceptibility
at various temperatures and frequencies in the absent of dc
field. Reproduced with permission from ref. [114]. Copyright 2011, American Chemical Society.
In this case, central DyIII ion is sandwiched by
two COT” ligands. To accommodate the sterically
bulky trimethylsilyl groups, the COT” rings are
arranged in a staggered conformation. The Li ion
interacts with one COT” ring. In zero dc field, the
frequency-dependent ac magnetic susceptibility is
observed. The data in the high temperature region
can be fit by the Arrhenius law giving the relaxation
barrier of Ueff = 18 K and a τ0 value of 6 × 10−6
s (Pathway A). Below 3.75 K, relaxation starts to
become temperature-independent, indicative of a
quantum regime (pathway B). Applying a 100 Oe
field results in a reduction in the quantum tunneling
along with the appearance of the secondary peak,
indicating the coexistence of the new relaxation
pathway C. At 200 Oe, the intensity of the secondary
low frequency peak (C) increases and the intensity
of the primary high-frequency peak (A) decreases.
The data at the optimum field of 600 Oe, consistent
D. Other mononuclear lanthanide SMMs
Li et al. reported the first SMM based on mononuclear lanthanide β-diketone, e.g. [Dy(FTA)3 L] (FTA
= 2-furyltrifluoro-acetonate, L = (S,S)-2,2’-bis(4benzyl-2-oxazoline)), which shows an energy barrier
of Ueff = 54.4 K.[117] In this complex, the DyIII center is eight-coordinated by three β-diketonate anions
and a N,N’-chelating chiral ligand L with a geometry between a bicapped square prism and a square
antiprism. Distinct slow magnetic dynamic behaviors
were found in two polymorphs of [Dy(NTA)3 L’] (NTA
= 2-naphthyltrifluoro-acetonate, L’ = (1R, 2R)-1,2diphenylethane-1,2-diamine),[118] attributed to the
different local environments of DyIII centers in the
crystal. The polymorphic form with a distorted bicapped triangular prismatic coordination geometry in
DyIII ion (C2v symmetry) shows typical features of the
SMM behavior at zero field, while the polymorphic
form with a distorted dodecahedral coordination geometry in DyIII ion (D2d symmetry) shows frequency
0
00
dependent in-phase (χ ) and out-of-phase (χ ) signals
only by applying an external dc field.[118]
Gao and co-workers reported the magnetic behavior
of [Dy(acac)3 (H2 O)2 ] (acac = acetylacetonate),[119]
where the DyIII ion is put on a distorted square-
130
Ren Min et al.: Single Molecule Magnets
antiprismatic coordination geometry with approximately D4d local symmetry, similar to those in
[LnPc2 ]− and [ErW10 O36 ]9− . This complex exhibits
a crossover at 8 K between thermally activated relaxation above 8 K and quantum tunneling relaxation below this temperature.
The energy barrier for the former is 66.1 K. When the coordination water is replaced by 1,10-phenanthroline (phen),
the energy barrier of compound [Dy(acac)3 (phen)]
is similar.[120] However, if the auxiliary groups are
replaced by large aromatic groups, the ligand field
around the DyIII in compounds [Dy(acac)3 (dpq)] and
[Dy(acac)3 (dppz)] will be enhanced which promotes
the separation of the lowest doubly degerate sublevels
from the rest of the exited states.[121] By utilizing
4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionate (TTA)
and capping ligands such as 2,2’-bipyridine (bipy)
and phen, Tang and coworkers also reported
two SMMs [Dy(TTA)3 (bipy)] (Ueff = 58 K) and
[Dy(TTA)3 (phen)] (Ueff = 85 K).[122]
Sessoli and co-workers reported the magnetic properties of a polycrystalline sample of
Na[Dy(DOTA)(H2 O)]·4H2 O (H4 DOTA = 1,4,7,10tetraazacyclododecane- N, N’, N”, N’”-tetraacetic
acid).[123] This compound behaves like a SMM and,
more interestingly, shows a giant field dependence
on the relaxation time of the magnetization. In zero
static field a temperature-independent underbarrier
mechanism is observed, while the application of a
weak magnetic field induces a thermally activated
regime with an effective barrier of ca. 60 K and
an increase in the relaxation time of six orders of
magnitude at 1.8 K compared to zero static field.
In Na[Dy(DOTA)(H2 O)]·4H2 O, the DyIII ion is in
an idealized square antiprismatic environment with a
twist angle of 39◦ typical of the heaviest lanthanides.
But the tetragonal symmetry is limited to the first
coordination sphere. The magnetic anisotropy of this
compound is then studied by combining measurements of the angular dependence of the single crystal
magnetic susceptibility and luminescence characterization. It is found unprecedently that the easy axis
is not linked to the idealized symmetry axis of the
complex, but is almost perpendicular to it. The results demonstrate that even subtle structural details,
like the position of hydrogen atoms and the consequent orientation of the nonbonding orbitals of the
axial ligand can overcome the symmetry imposed by
the coordination polyhedron.[124]
E. Other polynuclear lanthanide SMMs
In polynuclear lanthanide cluster complexes, the
single-molecule magnet behavior is thought to originate largely from the strong anisotropy of the single
FIG. 11.
Molecular structures of compounds
[DyIII
131)
2 (valdien)2 (NO3 )2 ] (left, adapted from ref.
and [Dy2 (ovph)2 Cl2 (MeOH)3 ] (right, adapted from ref.
[132] ).
lanthanide center, with only weak contributions from
intramolecular exchange coupling. However, compared with the isolated lanthanide ion, the magnetic
behaviors of the polynuclear Ln systems are more
complex due to the effect of the magnetic interactions
and the usual non-collinearity of the main single-ion
anisotropy axes of the different lanthanide ions.
Compound [DyIII
2 (valdien)2 (NO3 )2 ] (H2 valdien =
N1, N3-bis(3-methmethoxysalicylidene) diethylenetriamine) is a symmetrical Dy2 cluster (Fig. 11, left),
exhibits SMM behavior with an anisotropic barrier
Ueff = 76 K.[131] The step-like features in the hysteresis loops observed reveal an antiferromagnetic exchange coupling between the two dysprosium ions.
The ab initio calculations reveal an exchange constant
of JDy − JDy = −0.21 cm−1 . The asymmetrical dinuclear DyIII SMM, [Dy2 (ovph)2 Cl2 (MeOH)3 ]·MeCN
(H2 ovph = pyridine-2-carboxylic acid [(2-hydroxy3-methoxyphenyl)methylene] hydrazide) (Fig. 11,
right), reported by Tang and co-workers, shows a weak
ferromagnetic coupling between the Dy centers, each
with a nearly perfect pentagonal bipyramidal coordination environment.[132] The ab initio calculations
indicate that the quantum tunneling pathways are
strongly suppressed in low-lying exchange multiplets
at low temperatures and the ferromagnetic coupling
comes entirely from a ferromagnetic dipolar interaction (Jdip = 5.36 cm−1 ). Detailed magnetization dynamics studies reveal that the blockage of magnetization in the high-temperature regime occurs at individual Dy sites. Two closely spaced relaxation processes, attributed to the individual Dy ions, can be
well resolved by the sum of two modified Debye functions. Tong et al reported a heterospin dinuclear complex [Dy0.87 Yb1.13 (H2 cht)2 Cl4 (H2 O)(MeCN)]·MeCN
which shows shifts of the relaxation barriers with respect to the barriers observed in homospin Dy2 and
Yb2 isostructural complexes.[133] The difference of activation energies in pure and mixed-metal complexes
is due to small geometrical changes of the environment
of the Ln ions.
Ren Min et al.: Single Molecule Magnets
In order to enhance the magnetic interactions between the 4f ions, Long et al. introduced N3−
2 radical
as bridging ligand to produce dinuclear compounds
{[(Me3 Si)2 N]2 (THF)Ln}2 (µ − η 2 : η 2 −N2 ) (Ln = Gd,
Dy).[134] The fit of dc susceptibility for the GdIII
congener using a spin-only Hamiltonian reveals the
strongest magnetic coupling of J = −27 cm−1 yet observed for this ion. The incorporation of the highly
anisotropic DyIII ion results in a molecule with a
record magnetic blocking temperature of 8.3 K at conventional sweep rates. Further, synergizing the strong
magnetic anisotropy of terbium(III) with the effective
exchange-coupling ability of the N3−
radical create
2
the hardest molecular magnet discovered to date. The
terbium analogue exhibits magnetic hysteresis at 14 K
and a 100 s blocking temperature of 13.9 K.[135] Those
breakthroughs demonstrate that a joint contribution,
combining strong magnetic coupling with single-ion
anisotropy, may ultimately open up a new era in envisioned technological applications for SMMs.
The triangular Dy3 cluster [Dy3 (µ3 − OH)2 L3 ]4+
(HL = o-vanillin) gives a unique example of SMMs
(Fig. 12), showing unusual slow relaxation behavior to 8 K in spite of the almost non-magnetic
ground state.[136] This intriguing behavior originates
from the noncollinearity of the single ion magnetization axes of the DyIII ions, as revealed by single crystal magnetic studies,[137] muon spin lattice
relaxation measurements,[138] as well as ab initio
calculations.[139,140] The peculiar chiral nature of the
ground non-magnetic doublet and the resonant quantum tunneling of the magnetization at the crossings
of the discrete energy levels open new perspectives
in quantum computation and data storage in molecular nanomagnets. This stimulates intensive investigations in utilizing triangular Dy3 units for creating
new SMMs with higher energy barriers.[144]
Considering that much of the fascinating physics of
(TBA)[Tb(Pc)2 ] and other single-ion magnets, such
as Na9 [Er(W5 O18 )2 ], is associated with their fourfold
symmetry, McInnes and Winpenny et al studied the
magnetic properties of an iso-propoxide-bridged dysprosium square-based pyramid [Dy5 O(Oi Pr)13 ] which
has both fourfold symmetry and metal triangles. This
compound shows an energy barrier of 528 K, which is
by far the largest barrier yet observed for any d- or
f -block clusters145
IV. CONCLUSION
In this review, we summarized the progress of single
molecule magnets of metal oxo clusters, single transition metal ions and lanthanide ions or clusters. The
latter is of particular interest due to their high magnetization reversal barriers and high blocking temper-
131
FIG. 12. View of the structure of the triangular Dy3 cluster (adapted from ref. [136]).
atures. Although much need to be understood about
the lanthanide-based SMMs, such as the mechanism
of the relaxation and how to control the magnetic
anisotropy, these molecular materials show promising potential in spintronics and quantum computing. Progresses have been achieved in grafting the
prototype Mn12 , Fe4 and Pc2 Ln systems on different substrates.[148] The XMCD technique was developed to give the information of the oxidation state
of the metal ions in SMM monolayers, and to detect
the magnetic signals arising from layers of a magnetic
molecule.[149] These findings prove that the quantum
spin dynamics can be observed in SMMs chemically
grafted to surfaces, and offer a tool to reveal the organization of matter at the nanoscale.
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Ren Min et al.: Single Molecule Magnets
单分子磁体
任旻、郑丽敏
南京大学化学化工学院，配位化学国家重点实验室
摘 要 : 单分子磁体指的是那些在阻塞温度以下出现磁化强度慢弛豫的分子。 自从 1993 年报道了
第一例单分子磁体 [Mn12 O12 (O2 CCH3 )16 (H2 O)4 ] · 4H2 O · 2CH3 CO2 H (Mn12 ) 以来，人们在探索
新颖的单分子磁体及其在信息存储、自旋电子学和量子计算等方面的潜在应用方面付出了极大的努
力，已经合成得到许多具有单分子磁体性质的单核和多核金属簇合物。 本文将重点介绍基于氧桥联
的锰或铁簇合物、单核过渡金属化合物以及稀土配合物的单分子磁体。
关键 词： 单分子磁体；锰；铁；稀土
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