Syntheses, crystal structures, and magnetic characterization of five new dimeric manganese(III) tetradentate Schiff base complexes exhibiting single-molecule-magnet behavior

Article abstract of DOI:10.1021/ic051648l

Tetradentate Schiff base ligands H₂L (H₂saltmen, H₂salen, H₂-5-Brsalen, and H₂-3,5-Brsalen), derived from the condensation of the corresponding salicylaldehyde or its derivatives with 1,1,2,2-tetramethylethyldiamine or 1, 2-diaminoethane, reacted with MnIII acetate or perchlorate salts and sodium azide or sodium cyanate to produce five Mn^III dimer complexes, [Mn(saltmen)(O ₂CCH₃)]₂·2CH₃CO₂H (1), [Mn(saltmen)(N₃)]₂ (2), [Mn(salen)(NCO)]₂ (3), [Mn(3,5-Brsalen)(3,5-Brsalicylaldehyde)]₂ (4), and [Mn(5-Brsalen)(CH₃OH)]₂(ClO₄)₂ (5). These new complexes have been characterized by IR, elemental analyses, crystal structural analyses, and magnetic studies. Within these Mn^III dimeric complexes, two Mn^III ions are connected by phenolate oxygen atoms with acetate, azide, cyanate, a 3,5-Brsalicyladehyde anion, and a neutral methanol molecule as the axial ligands for complexes 1-5, respectively. Complexes 1-4 exhibit intradimer ferromagnetic exchange and display frequency dependence of ac magnetic susceptibility, possibly showing single-molecule- magnet (SMM) behavior. In contrast, complex 5 shows an intradimer antiferromagnetic coupling probably originating from the relatively shorter Mn-O* distance, compared to those of complexes 1-4.

Full text of DOI:10.1021/ic051648l

Inorg. Chem. 2006, 45, 3538−3548
Syntheses, Crystal Structures, and Magnetic Characterization of Five
New Dimeric Manganese(III) Tetradentate Schiff Base Complexes
Exhibiting Single-Molecule-Magnet Behavior
Zhengliang Lu †,§,# Mei Yuan,‡,# Feng Pan,‡ Song Gao,*,‡ Deqing Zhang,*,† and Daoben Zhu†
1
,
Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100080, China, State Key Laboratory of Rare
Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering,
Peking UniVersity, Beijing 100875, China, and Graduate School of Chinese Academy of Sciences,
Beijing 100080, China
Received September 25, 2005
Tetradentate Schiff base ligands H2L (H2saltmen, H2salen, H2-5-Brsalen, and H2-3,5-Brsalen), derived from the
condensation of the corresponding salicylaldehyde or its derivatives with 1,1,2,2-tetramethylethyldiamine or 1,
2-diaminoethane, reacted with MnIII acetate or perchlorate salts and sodium azide or sodium cyanate to produce
five MnIII dimer complexes, [Mn(saltmen)(O2CCH3)]2
‚2CH3CO2H (1), [Mn(saltmen)(N3)]2 (2), [Mn(salen)(NCO)]2 (3),
[Mn(3,5-Brsalen)(3,5-Brsalicylaldehyde)]2 (4), and [Mn(5-Brsalen)(CH3OH)]2(ClO4)2 (5). These new complexes have
been characterized by IR, elemental analyses, crystal structural analyses, and magnetic studies. Within these MnIII
dimeric complexes, two MnIII ions are connected by phenolate oxygen atoms with acetate, azide, cyanate, a 3,5-
Brsalicyladehyde anion, and a neutral methanol molecule as the axial ligands for complexes 1
Complexes 1 4 exhibit intradimer ferromagnetic exchange and display frequency dependence of ac magnetic
susceptibility, possibly showing single-molecule-magnet (SMM) behavior. In contrast, complex 5 shows an intradimer
antiferromagnetic coupling probably originating from the relatively shorter Mn O* distance, compared to those of
complexes 1 4.
−5, respectively.
−
−
−
Introduction
netism. SMMs exhibit slow relaxation of their magnetization
induced by the combined effect of a high-spin ground state
ST and uniaxial anisotropy D, which generate an energy
barrier between spin-up and spin-down states. Thus, SMMs
are potentially useful for information storage at the molecular
In recent years, the research on single-molecule magnets
(SMMs)1-5 and single-chain magnets (SCMs)6-10 has re-
ceived significant attention in the area of molecular mag-
* To whom correspondence should be addressed. E-mail: gaosong@
pku.edu.cn (S.G.); dqzhang@iccas.ac.cn (D.-Q.Z.).
(3) Barra, A. L.; Caneschi, A.; Cornia, A.; Fabrizi, F.; de Biani, D.;
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† Institute of Chemistry, Chinese Academy of Sciences.
‡ College of Chemistry and Molecular Engineering, Peking University.
§ Graduate School of Chinese Academy of Sciences.
# Mr. Lu¨ and Ms. Yuan contributed equally to the present work.
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3538 Inorganic Chemistry, Vol. 45, No. 9, 2006
10.1021/ic051648l CCC: $33.50
© 2006 American Chemical Society
Published on Web 04/07/2006

Dimeric Mn(III) Tetradentate Schiff Base Complexes
Scheme 1
level. Moreover, SMMs are considered to be unique systems
for studying quantum spin tunneling and quantum phase
interference, which may lead to the application of SMMs in
molecular electronics.11
Among the reported SMMs, the MnIII complexes play very
important roles. For instance, the first SMM is a Mn12
complex.12 In addition to the Mn12 complexes, several Mn4
complexes were found to be SMMs.13-18 Miyasaka et al.19
further minimized the nuclearity of SMMs based on MnIII
complexes and reported a dimeric manganese(III) tetradenate
Schiff base complex showing SMM behavior. In fact, tens
of dimeric MnIII complexes were described previously,20-30
and some of them were magnetically characterized. Both
ferromagnetic and antiferromagnetic interactions between the
two MnIII ions were reported. Modulation of the MnIII‚‚‚MnIII
magnetic interaction by varying the apical ligands and the
chemical features of the Schiff base ligand through changing
the corresponding diamines seems to be possible when the
structures of the dimeric MnIII complexes are considered.
Herein we report five new dimeric manganese(III) tetradenate
schiff base complexes: [Mn(saltmen)(O2CCH3)]2‚2CH3CO2H
(1), [Mn(saltmen)(N3)]2 (2), [Mn(salen)(NCO)]2 (3), [Mn-
(3,5-Brsalen)(3,5-Brsalicyladehyde)]2 (4), and [Mn(5-Brsalen)-
(CH3OH)]2(ClO4)2 (5). The corresponding Schiff base ligands
used in this paper were listed in Scheme 1. Within these
new dimeric MnIII complexes, two MnIII ions are connected
by phenolate oxygen atoms with acetate, azide, cyanate, a
3,5-Brsalicyladehyde anion, and a neutral methanol molecule
as the axial ligands for complex 1-5, respectively. Magnetic
studies indicate that complexes 1-4 exhibit obvious ferro-
magnetic coupling between the two MnIII ions and frequency
dependence of ac magnetic susceptibility, one of the most
important characteristic behaviors of SMMs, while the two
MnIII ions are antiferromagnetically coupled in complex 5.
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Experimental Section
Materials and Measurements. All the chemicals used in
syntheses were of reagent grade and were used without further
purification. The tetradentate Schiff base ligands H2L (H2saltmen,
H2salen, H2-5-Brsalen, and H2-3,5-Brsalen) were synthesized by
mixing the corresponding salicylaldehyde or its derivatives and
1,1,2,2-tetramethylethyldiamine or 1, 2-diaminoethane in a 2:1 mole
ratio in ethanol.
Caution: Although we have experienced no problem with
complexes 2-5 reported in this work, perchlorate salts and sodium
azide are potentially explosive and should be handled in small
quantities and very carefully.
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K.; Rheingold, A. L.; Christou, G.; Hendrickson, D. N. Polyhedron
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Infrared spectra were recorded as KBr pellets on a Perkin-Elmer
Paragon 1000 FT-IR spectrometer. Elemental analyses were carried
out on a Perkin-Elmer 240C analyzer. Magnetic measurements were
carried out on an Oxford Maglab 2000 System or a Quantum Design
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1996, 247, 57.
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M. J. Chem. Soc., Dalton Trans. 2002, 1528.
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Sousa, A.; Watkinson, M.; McAuliffe, C. A.; Pritchard, R. G.; Beddoes.
R. L. J. Chem. Soc., Dalton Trans. 1996, 2935.
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Inorganic Chemistry, Vol. 45, No. 9, 2006 3539

Lu1 et al.
Table 1. Crystallographic and Refinement Data for Complexes 1-5
1
2
3
4
5
formula
fw
cryst syst
space group
a (Å)
C48H58Mn2N4O12
992.86
monoclinic
P21/c
11.024(2)
18.905(4)
12.107(2)
90
105.42(3)
90
2432.4(8)
2
1.356
0.583
1040
C40H44Mn2N10O4
838.74
monoclinic
P21/c
8.0512(16)
16.874(3)
14.346(3)
90
103.84(3)
90
1892.5(7)
2
1.472
0.724
872
C34H28Mn2N6O6
726.50
orthorhombic
Pbca
13.131(3)
14.177(3)
16.710(3)
90
C46H26Br12Mn2N4O8
1831.51
monoclinic
P21/c
12.606(3)
15.713(3)
14.343(3)
90
115.67(3)
90
2560.6(9)
2
C34H32Br4Cl2Mn2N4O14
1221.06
monoclinic
P21/n
11.019(2)
13.047(3)
14.914(3)
90
102.78(3)
90
2090.9(7)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
90
90
3110.7(11)
4
1.551
0.869
1488
F
calcd (Mg/m3)
2.375
9.914
1728
1.939
4.624
1200
µ (mm-1
F(000)
)
reflns collected
unique reflns
GOF
final R indices
[I > 2σ(I)]
R indices
23 257
9072
1.080
5737
3753
0.870
R1)0.0812
wR2)0.1488
R1)0.1923
wR2)0.1867
44 409
3548
0.951
R1)0.0346
wR2)0.0603
R1)0.1110
wR2)0.0696
41 120
4752
1.073
R1)0.0703
wR2)0.2118
R1)0.1178
wR2)0.2315
37 369
34 800
0.995
R1)0.0495
wR2)0.1195
R1)0.0909
wR2)0.1333
R1a)0.0604
wR2b)0.1425
R1 ) 0.0792
wR2 ) 0.1537
(all data)
2
2
2
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) ∑[w(Fo - Fc )2]/∑[w(Fo )2]1/2
.
MPMS-XL5 SQUID magnetometer. The diamagnetic corrections
were calculated from Pascal’s constants.
brown crystals were collected by filtration (62% yield based on
Mn). Anal. Calcd for C40H44Mn2N10O4: C, 57.26; H, 5.29; N, 16.71.
Found: C, 56.71; H, 5.22; N, 16.58. IR (KBr, cm-1): 2040, 1604,
1540, 1466, 1441, 1393, 1299, 1144, 751, 626.
X-ray Crystallographic Study. Single-crystal X-ray diffraction
data for complexes 1 and 2 were collected on a Rigaku R-AXIS
RAPID IP diffractometer at 293 K using graphite-monochromated
Mo KR radiation (λ ) 0.71073 Å) and the oscillation scans
technique in the range of 1.85° < θ < 27.48°, while those for
compounds 3-5 were collected using a NONIUS Kappa-CCD
diffractometer with Mo KR radiation (λ ) 0.71073 Å) at 293 K.
The structures were solved by direct methods using the program
SHELXS-9731 and refined by full-matrix least-squares methods on
F2 using SHELXL-97.32 All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms defined by the stereochemistry
were placed at their calculated positions and allowed to ride on
their host carbons in coordinates. A summary of the crystallographic
data and structural determination and refinement details for
complexes 1-5 is provided in Table 1.
Preparation of Complex 1. A solution of manganese(III) acetate
dihydrate (0.27 g, 1.0 mmol) in methanol (10 mL) was added to
H2saltmen (0.32 g, 1.0 mmol) in methanol (20 mL). After the brown
solution was heated to 50 °C and stirred for 30 min, 10 mL of
water was added to the reaction mixture, and it was filtered. The
red-black solution collected was left to stand for one week at room
temperature to form dark brown crystals. The crystals were collected
by suction filtration (yield of 81% based on manganese(III) acetate
dihydrate). Anal. Calcd for C48H58Mn2N4O12: C, 58.07; H, 5.89;
N, 5.65. Found: C, 58.09; H, 5.83; N, 5.68. IR (KBr, cm-1): 1723,
1605, 1543, 1469, 1442, 1398, 1290, 1146, 757, 626.
Preparation of Complex 3. A solution of Mn(ClO4)2‚6H2O (0.36
g, 1.0 mmol) in methanol (20 mL) was added to H2salen (0.32 g,
1.0 mmol) in methanol (20 mL), followed by the addition of Et3N
(0.28 mL, 2.0 mmol). The solution was heated to 50 °C and stirred
for 2 h; then NaOCN (0.07 g, 1.0 mmol) dissolved in a minimum
of water was added. After another 2 h, the filtrate was collected
and left to stand for several weeks at room temperature. The dark
brown crystals were collected by filtration (90% yield based on
Mn). Anal. Calcd for C34H28Mn2N6O6: C, 56.21; H, 3.88; N, 11.57.
Found: C, 56.05; H, 3.86; N, 11.43. IR (KBr, cm-1): 3019, 2948,
2170, 2164, 1623, 1601, 1542, 1468, 1446, 1296, 904, 800, 760.
Preparation of Complex 4. A solution of Mn(ClO4)2‚6H2O (0.09
g, 0.25 mmol) in methanol (5 mL) was added to H2-3, 5-Brsalen
(0.11 g, 0.25 mmol) in methanol (20 mL), followed by the addition
of solid NaOH (0.02 g, 0.50 mmol). The solution was refluxed for
4 h, and then the filtrate was collected and left to stand for several
weeks at room temperature. The dark brown crystals were collected
by filtration (45% yield based on Mn). Anal. Calcd for C46H26-
Br12Mn2N4O8: C, 30.17; H, 1.43; N, 3.06. Found: C, 30.25; H,
1.46; N, 2.92. IR (KBr, cm-1): 3063, 2925, 2854, 1623, 1581, 1510,
1436, 1408, 1375, 1294, 1213, 1167, 747, 722.
Preparation of Complex 5. A solution of Mn(ClO4)2‚6H2O (0.09
g, 0.25 mmol) in methanol (5 mL) was added to H2-5-Brsalen (0.09
g, 0.25 mmol) in methanol (20 mL), followed by the addition of
NaOH (0.02 g, 0.50 mmol). The solution was heated to 50 °C and
stirred for 2 h. The filtrate was collected and left to stand for several
weeks at room temperature. The dark brown crystals were collected
by filtration (62% yield based on Mn). Anal. Calcd for C34H32Br4-
Cl2Mn2N4O14: C, 33.44; H, 2.64; N, 4.59. Found: C, 33.31; H,
2.58; N, 4.50. IR (KBr, cm-1): 3600, 3491, 3319, 1627, 1612, 1528,
1456, 1371, 1286, 1263, 1112, 831, 809, 689, 655.
Preparation of Complex 2. A solution of Mn(ClO4)2‚6H2O (0.36
g, 1.0 mmol) in methanol (20 mL) was added to H2saltmen (0.32
g, 1.0 mmol) in methanol (20 mL), followed by the addition of
Et3N (0.14 mL, 1.0 mmol). The solution was refluxed for 1 h before
solid NaN3 (0.07 g, 1.0 mmol) was added. Hot water (10 mL) was
added to the hot solution in methanol. The filtrate was collected
and left to stand for several weeks at room temperature. The dark
(31) Sheldrick, G. M. SHELXL97, Program for Crystal Structure Refine-
ment; University of Go¨ttingen: Go¨ttingen, Germany, 1998.
(32) Sheldrick, G. M. SHELXS97, Program for Crystal Structure Solution;
University of Go¨ttingen: Go¨ttingen, Germany, 1998; Acta Crystallogr.
1990, A46, 467.
Results and Discussion
Syntheses. Compounds 1-5 were prepared as dark brown
crystals by slow evaporation of the resulting methanol
3540 Inorganic Chemistry, Vol. 45, No. 9, 2006

Dimeric Mn(III) Tetradentate Schiff Base Complexes
Figure 1. ORTEP drawings of the MnIII dimers for complexes 1-5: (a) 1, (b) 2, (c) 3, (d) 4, and (e) 5.
solution with a small amount of water. During the prepara-
tion, heating and stirring were needed to favor the reactions.
To get more examples for the investigation of the magnetic
coupling rule of MnIII dimer complexes, we chose four types
of Schiff base ligands as secondary ligands including H2-
saltmen, H2salen, H2-5-Brsalen, and H2-3,5-Brsalen, as well
as five axial ligands for controlling the resulting compounds.
Complex 1 was synthesized using a literature method with
manganese(III) acetate dihydrate as the starting material, and
the acetate group acts as the axial ligand in the resulting
product. In complexes 2-5, divalent Mn(ClO4)2‚6H2O was
substituted for manganese(III) acetate dihydrate as the
starting material to avoid the coordination of CH3COO-; in
the reaction, the MnII ion may be oxidized by the oxygen in
the atmosphere to MnIII ion in the product in combination
with the basic atmosphere. As a result, complexes 2-5 were
obtained with azide, cyanate, a 3,5-Brsalicyladehyde anion,
and a neutral methanol molecule as the axial ligands,
Inorganic Chemistry, Vol. 45, No. 9, 2006 3541

Lu1 et al.
Figure 2. Crystal packing for complexes 1-5. The green lines display the Jahn-Teller distortions of two symmetry-related MnIII dimers: (a) 1, (b) 2, (c)
3, (d) 4, and (e) 5.
respectively. The 3,5-Brsalicyladehyde anion originated from
the decomposition of the 3,5-Brsalen ligand caused by
refluxing in a strong basic environment.
to the phenolate oxygen atom of the another [Mn(L)(X)] unit.
For complexes 1-5, the local coordination geometry around
the MnIII center can be described as a distorted octahedron.
As expected for octahedral MnIII complexes, the Jahn-Teller
distortion leads to elongated axial bond lengths, in particular
for complexes 1-4. The ORTEP drawings of complexes 1-5
are shown in Figure 1, and the corresponding molecular
arrangements in the crystal lattices are displayed in Figure
2. In the crystal packing of complexes 1-5, two symmetrical
arrangements of MnIII dimers are found, the Jahn-Teller
distortions of which are also displayed in Figure 2. Their
selected bond lengths and angles are summarized in Table
2, while Table 3 lists the “key ” bond lengths and angles of
complexes 1-5 for the discussion of the correlation between
the structural features and the MnIII-MnIII magnetic interac-
tions.
FTIR Spectra. The IR spectra of compounds 1-5 exhibit
strong absorption at 1600-1630 cm-1, assignable to the
ν(CdN or CdO) absorption. Also, the characteristic absorp-
tion of CH or NH groups is detected in a range of 2900-
3350 cm-1. For compounds 2-3, the strong absorption at
about 2000-2200 cm-1 is attributed to the asymmetric
stretching vibration of azide or cyanate groups. The strong
-
absorption peak for vibration of ClO4 is also observed at
about 1100 cm-1 in compound 5.
Crystal Structures. The structures of all five MnIII
complexes are similar to those of dimeric MnIII complexes
reported previously.20-30 Each complex can be described as
a dimer of [Mn(L)(X)], where L and X are the tetradentate
Schiff base ligand (L ) saltmen2-, salen2-, 5-Brsalen2-, and
3,5-Brsalen2-) and the corresponding axial ligand (X )
Complex 1 with Acetate as the Outer Axial ligand. The
MnIII center is coordinated by two nitrogen (N(1) and N(2))
atoms, two phenolate oxygen atoms from saltmen2- (O(1)
and O(2)), and two apical oxygen atoms from acetate (O(3))
and the phenolate group (O(2A)) of saltmen2- of the
-
CH3COO-, N3 , NCO-, 3,5-Brsalicyladehyde anion, and
CH3OH), respectively. Two units of [Mn(L)(X)] are con-
nected through MnIII-O* bonding, where the O* atom refers
3542 Inorganic Chemistry, Vol. 45, No. 9, 2006

Dimeric Mn(III) Tetradentate Schiff Base Complexes
Table 2. Selected Bond Lengths (Å) and Angles (deg) of Complexes 1-5
1
2
3
Mn(1)-O(1)
Mn(1)-O(2)
Mn(1)-N(1)
Mn(1)-N(2)
Mn(1)-O(3)
1.8675(14)
1.8906(13)
1.9778(14)
1.9871(15)
2.1130(14)
Mn(1)-O(1)
Mn(1)-O(2)
Mn(1)-N(1)
Mn(1)N(2)
1.879(4)
1.863(4)
1.991(4)
1.998(5)
2.103(6)
Mn(1)-O(1)
Mn(1)-O(2)
Mn(1)-N(1)
Mn(1)-N(2)
Mn(1)-N(3)
1.8667(15)
1.8983(14)
1.9677(17)
1.9877(17)
2.105(2)
Mn(1)-N(3)
O(1)-Mn(1)-O(2)
O(1)-Mn(1)-N(1)
O(2)-Mn(1)-N(1)
O(1)-Mn(1)-N(2)
O(2)-Mn(1)-N(2)
N(1)-Mn(1)-N(2)
O(1)-Mn(1)-O(3)
O(2)-Mn(1)-O(3)
N(1)-Mn(1)-O(3)
N(2)-Mn(1)-O(3)
91.61(6)
93.11(6)
164.83(6)
170.36(6)
91.77(6)
81.43(6)
97.24(6)
95.32(6)
98.36(6)
91.43(6)
O(2)-Mn(1)-O(1)
O(2)-Mn(1)-N(1)
O(1)-Mn(1)-N(1)
O(2)-Mn(1)-N(2)
O(1)-Mn(1)-N(2)
N(1)-Mn(1)-N(2)
O(2)-Mn(1)-N(3)
O(1)-Mn(1)-N(3)
N(1)-Mn(1)-N(3)
N(2)-Mn(1)-N(3)
90.31(16)
166.00(19)
93.20(17)
91.88(17)
163.00(19)
80.91(18)
95.8(2)
98.06(19)
97.1(2)
98.5(2)
O(1)-Mn(1)-O(2)
O(1)-Mn(1)-N(1)
O(2)-Mn(1)-N(1)
O(1)-Mn(1)-N(2)
O(2)-Mn(1)-N(2)
N(1)-Mn(1)-N(2)
O(1)-Mn(1)-N(3)
O(2)-Mn(1)-N(3)
N(1)-Mn(1)-N(3)
N(2)-Mn(1)-N(3)
93.91(6)
91.59(7)
163.59(7)
167.24(7)
89.29(7)
82.16(8)
97.69(7)
100.54(7)
94.01(7)
93.86(7)
4
5
Mn(1)-O(1)
Mn(1)-O(2)
Mn(1)-N(1)
Mn(1)-N(2)
Mn(1)-O(3)
1.875(7)
1.918(7)
1.995(8)
2.000(9)
2.014(8)
Mn(1)-O(1)
1.908(3)
Mn(1)-O(2)
Mn(1)-N(1)
Mn(1)-N(2)
Mn(1)-O(3)
1.857(3)
1.979(4)
1.981(3)
2.222(3)
O(1)-Mn(1)-O(2)
O(1)-Mn(1)-N(1)
O(2)-Mn(1)-N(1)
O(1)-Mn(1)-N(2)
O(2)-Mn(1)-N(2)
N(1)-Mn(1)-N(2)
O(1)-Mn(1)-O(3)
O(2)-Mn(1)-O(3)
N(1)-Mn(1)-O(3)
N(2)-Mn(1)-O(3)
94.9(3)
90.9(3)
165.9(4)
165.2(3)
89.2(3)
82.1(3)
98.7(3)
97.1(3)
94.7(3)
94.8(4)
O(2)-Mn(1)-O(1)
O(2)-Mn(1)-N(1)
O(1)-Mn(1)-N(1)
O(2)-Mn(1)-N(2)
O(1)-Mn(1)-N(2)
N(1)-Mn(1)-N(2)
O(2)-Mn(1)-O(3)
O(1)-Mn(1)-O(3)
N(1)-Mn(1)-O(3)
N(2)-Mn(1)-O(3)
95.99(12)
173.95(13)
89.58(13)
92.01(13)
169.11(13)
82.17(14)
91.42(13)
92.00(12)
90.74(14)
95.19(14)
Table 3. Pertinent Bond Distances (Å) and Angles (deg) for the Dimeric Cores and Magnetic Parameters of Complexes 1-5, [Mn(L)(H2O)]2(ClO4)2,
[Mn2(5-Brsalen)(H2O)2]2(ClO4)2, and [Mn(salen)(H2O)]2(ClO4)2H2O.
Mn‚‚‚Mn*
(intradimer)
Mn‚‚‚Mn*
(interdimer)
Mn-O
Mn-O*
O-Mn-O*
g
D (cm-1
)
J (cm-1
)
ref
1
2
3
4
5
a
b
c
1.8906(13)
1.863(4)
1.8983(14)
1.918(7)
1.908(3)
1.906(6)
1.912(3)
1.891(3)
2.813(5)
3.190(2)
2.793(0)
2.728(2)
2.395(3)
2.419(7)
2.305(2)
2.490(3)
99.57(5)
98.39(16)
97.82(0)
100.00(1)
99.85(11)
3.641(5)
3.922(5)
3.584(1)
3.597(3)
3.307(4)
3.350(3)
3.181(1)
3.361(2)
8.666(1)
8.051(2)
7.522(1)
9.336(4)
8.920(1)
F
F
F
F
AF
AF
AF
AF
1.98
1.98
2.03
2.00
1.96
-1.9
-1.0
-0.3
-4.1
-1.0
1.35
0.6
0.73
0.55
-0.45
this work
this work
this work
this work
this work
24
76.6(1)
80.7(1)
25
27
a [Mn(L)(H2O)]2(ClO4)2(L)N-(acetylacetonylidene)-N′-(R-methylsalicylidene)-ethylenediamine). b [Mn2(5-Brsalen)(H2O)2]2(ClO4)2. c [Mn(salen)(H2O)]2(ClO4)2H2O.
neighboring [Mn(L)(X)] unit. Among them, O(1), N(1), N(2),
and O(2) define the basal plane, from which Mn(1) (Mn-
(1A)) deviates by 0.189 Å. As expected for octahedral MnIII
ions, the Jahn-Teller distortion leads to elongated axial bond
lengths of Mn(1)-O(3) and Mn(1)-O(2A) (2.1130(14) and
2.813(5) Å, respectively) Within the dimer, the two MnIII
ions are linked through the phenolate oxygen atoms (O(1)
and O(1A)); the angle of Mn(1)-O(2A)-Mn(1A) is 99.6°,
and the two MnIII ions are separated by 3.641(5) Å, which
is slightly longer than that of [Mn2(saltmen)2(ReO4)2]19 and
similar to that of [Mn(3-MeO-salmen)Cl]2.22
The crystal lattice also contains molecules of acetic acid
in addition to the MnIII dimers. There are short interatomic
contacts among the atoms of acetic acid and those of the
acetate (apical) and the Schiff base ligands: O(4)‚‚‚H(6B)
) 1.772 Å, O(5)‚‚‚H(19A) ) 2.523 Å, O(5)‚‚‚H(20A) )
2.507 Å, and O(6)‚‚‚H(22A) ) 2.534 Å, as shown in Figure
2a. The shortest interdimer distance of Mn‚‚‚Mn is 8.666(1)
Å, indicating that the magnetic interaction between dimers
may be very weak.
Complex 2 with Azido as the Outer Ligand. The basal
plane of the MnIII center of complex 2 is again composed of
O(1), N(1), N(2), and O(2) from saltmen2-. Mn(1) (Mn(1A))
lies above the basal plane by 0.247 Å. The apical positions
-
are occupied by one nitrogen atom of N3 with a Mn(1)-
N(3) bond length of 2.103(6) Å and the oxygen atom from
phenolate group (O(2A)) of saltmen2- of the neighboring
[Mn(L)(X)] unit. The bond length of Mn(1)-O(2A) is 3.190-
(2) Å, which is longer than that of the corresponding bond
in complex 1 (2.813 (5) Å). The two MnIII centers within
one dimer are connected through the oxygen atoms from
phenolate group with a Mn(1)-O(2)-Mn(1A) angle of
98.4°, and they are separated by 3.922(5) Å, which is about
0.3 Å longer than that of complex 1.
Inorganic Chemistry, Vol. 45, No. 9, 2006 3543

Lu1 et al.
Figure 3. ømT vs T plots for complexes 1-5; the red lines represent a best simulation using an SMn ) 2 dimer model described in the text: (a) 1, (b) 2,
(c) 3, (d) 4, and (e) 5.
The MnIII dimers in the crystal lattice are connected
through short interatomic contacts (N(5)‚‚‚H(20A) ) 2.607
Å, N(5)‚‚‚H(13A) ) 2.602 Å) among the nitrogen atoms of
The bond length of Mn(1)-O(2A) is 2.793(0) Å, which is
shorter than that of the corresponding bond in complexes 1
and 2. The distance between the two MnIII centers within
one dimer is 3.584(1) Å, which is comparable to that of
complex 1 and about 0.3 Å shorter than that of complex 2.
Two symmetrical arrangements of MnIII dimers are found
in the crystal lattice, and the Jahn-Teller distortions of these
two units lie in the ab plane, as shown in Figure 2c. No
short interatomic contacts are observed among the MnIII
dimers, and the nearest Mn‚‚‚Mn separation between the
dimers is 7.522(1) Å, which is slightly shorter than those of
complexes 1 and 2.
-
the N3 apical ligand and the hydrogen atoms of the Schiff
base ligand to generate a 2D layer framework (Figure 2b).
The Jahn-Teller axes of the MnIII dimers lie in the bc plane.
The nearest Mn‚‚‚Mn separation between the dimers is 8.051-
(2) Å.
Complex 3 with Cyanate as the Outer Ligand. The basal
plane of the MnIII center of complex 3 comprises O(1), N(1),
N(2), and O(2) from salen2-. Mn(1) (Mn(1A)) lies above
the basal plane by 0.223 Å. The apical positions are occupied
by one nitrogen atom of NCO- with the Mn(1)-N(3) bond
length of 2.105(2) Å, and the oxygen atom from phenolate
group (O(2A)) of salen2- of the neighboring [Mn(L)(X)] unit.
Complex 4 with the 3,5-Brsalicyladehyde Anion as the
Outer Ligand. The basal plane of MnIII center of complex
4 is composed of O(1), N(1), N(2), and O(2) from 3,5-
3544 Inorganic Chemistry, Vol. 45, No. 9, 2006

Dimeric Mn(III) Tetradentate Schiff Base Complexes
Figure 4. Plots of the magnetization, M, vs the ratio of the external field, H, to temperature (T ) 2-10 K) for complexes 1-5; the red lines represent the
best fit of the data: (a) 1, (b) 2, (c) 3, and (d) 4.
Brsalen2-. Mn(1) (Mn(1A)) lies below the basal plane by
0.218 Å. The apical positions are occupied by one oxygen
atom of the 3,5-Brsalicyladehyde anion, generated from the
decomposition of 3,5-Brsalen, with an Mn(1)-O(3) bond
length of 2.014(8) Å and the oxygen atom from phenolate
group (O(2A)) of 3,5-Brsalen2- of the neighboring [Mn(L)-
(X)] unit. The bond length of Mn(1)-O(2A) is 2.728(2) Å.
Again, the two MnIII ions within one dimer are linked by
the oxygen atoms from phenolate group, and the angle of
Mn(1)-O(2)-Mn(1A) is 100°. The two MnIII ions of one
dimer are separated by 3.597(3) Å, comparable to that of
complex 3.
As shown in Figure 2d, there are short interatomic contacts
(2.46-2.97 Å) between the bromine and oxygen atoms of
the 3,5-Brsalicyladehyde anion (apical ligand) and the
hydrogen atoms of the Schiff base ligand. The nearest
Mn‚‚‚Mn separation between the dimers is 9.366(4) Å,
obviously longer than those of complexes 1-3.
bond length of 2.222(3) Å and the oxygen atom from
phenolate group (O(2A)) of 5-Brsalen2- of the neighboring
[Mn(L)(X)] unit, leading to [Mn(L)(X)]+. The bond length
of Mn(1)-O(2A) is 2.395(3) Å, which is much shorter than
those of the corresponding bonds in complexes 1-4. The
distance between the two MnIII centers, which are connected
by the oxygen atom from the phenolate group with a Mn-
(1)-O(2)-Mn(1A) angle of 99.8°, is 3.307(4) Å within one
dimer, which is about 0.3 Å shorter than that of complex 1.
In addition to the MnIII dimer, complex 5 contains a
perchlorate ion as counterion, the oxygen atoms of which
show short contacts (2.50-2.95 Å) with the hydrogen atoms
of the Schiff base ligand, as shown in Figure 2e. The nearest
Mn‚‚‚Mn separation between the dimers is 8.920(1) Å.
Magnetic Properties of Complexes 1-5. The temperature
dependence of dc magnetic susceptibility of complexes 1-5
was measured on polycrystalline samples in the temperature
range of 1.9-300 K, as shown in Figure 3. ømT values for
these complexes at room temperature are in the range of
5.80-6.23 cm3 mol-1 K (per MnIII2 unit), in good agreement
with the 6.0 cm3 mol-1 K value of two noninteracting high-
spin MnIII ions (S ) 2). For complexes 1-4, the ømT value
increases gradually upon lowering of the temperature to reach
a maximum, indicating a typical ferromagnetic coupling
Complex 5 with Methanol as the Outer Ligand. The
basal plane of the MnIII center of complex 5 consists of O(1),
N(1), N(2), and O(2) from 5-Brsalen2-. In comparison with
complexes 1-4, the MnIII ions are almost lie on the basal
planes, and the apical positions are occupied by one oxygen
atom of neutral methanol molecule with an Mn(1)-O(3)
Inorganic Chemistry, Vol. 45, No. 9, 2006 3545

Lu1 et al.
between MnIII ions. When the temperature is lowered further,
a sudden decrease of the ømT value occurs for complexes 1,
3, and 4, which could be mainly regarded as contribution of
the zero-field-splitting (ZFS) from MnIII ion. But, the
contribution of weak antiferromagnetic interactions between
dimers cannot be ruled out. In contrast, for complex 5, the
ømT value keeps decreasing while the temperature decreases
from 300 to 1.9 K, in agreement with an antiferromagnetic
interaction between MnIII ions. The field dependence of
magnetization of complexes 1-4 (Figure S1a) supports an
overall ferromagnetic interaction for each of them, and the
data for complex 5 (Figure S1b) suggest an antiferromagnetic
interaction in 5, since for compounds 1-4, the M value
quickly increases with the increasing H value to above 4.0
Nâ mol-1 at 10 kOe, while the M value of compound 5 only
gets to below 1.0 Nâ mol-1 at 10 kOe. The intradimer
antiferromagnetic coupling observed for 5 probably origi-
nated from the relatively shorter Mn-O* distance in 5 than
those of complex 1-4.
The Hamiltonian given in eq 1 has been used to model
the magnetic susceptibility (S1 ) S2 ) SMn, Siz is the z
component of the Si operator)
H ) -2JS1S2 + DMn
S
2 + gµ H
S 2
(1)
∑
∑
i)1,2
iz
B
i
i)1,2
where J is the magnetic coupling constant between two MnIII
ions, DMn is the uniaxial anisotropy of each MnIII ion, g is
the g factor of the Mn ion. The dc susceptibility was
simulated using the general procedure developed by Clem-
ente-Juan and co-workers (MAGPACK program) (the solid
lines in Figure 3).33 Because of the long interdimer separa-
tions of Mn‚‚‚Mn (7.522-9.336 Å) in complexes 1-5, the
interdimer magnetic interactions may be very weak in terms
of the structural view here, so we neglected the influence of
interdimer magnetic coupling in fitting the data.19 The best
set of parameters of g, J, and DMn are given in Table 3. From
these parameters, we can see that the ZFS term, DMn, is
estimated to be in the negative range from -0.3 to -1.9
cm-1, which is consistent with the typical value of -0.38 to
-2.53 cm-1 for those Mn2 dimer analogues,22b,23 except for
the relatively higher value of -4.1 cm-1 for compound 4
for an unknown reason. It is also found that the intradimer
interaction parameters, J, for complexes 1-4 are in the
positive range of 0.55-1.35 cm-1, confirming ferromagnetic
interaction between MnIII ions, and the values are close to
those of reported ferromagnetic MnIII dimers,23 while the
negative J value for complex 5 corresponds to an antifer-
romagnetic interaction between MnIII ions.
Figure 5. Plots of in-phase (øm′, top) and out-of-phase (øm′′, bottom) ac
magnetic susceptibility versus temperature (T) for complex 1: (a) under a
zero dc field and (b) under a dc field of 5 kOe. (c) The Cole-Cole plot for
complex 1; the solid lines represent the best fit of the data.
kB is the Boltzmann constant). The ∆/kB value of compound
3 is the smallest because of its relatively low DMn2 value.
The measurement of the ac magnetic susceptibility at low
temperature was performed on polycrystalline samples of
complexes 1-4 (Figures 5-8). The plots under zero-applied
dc field all show frequency dependence, as expected for
SMMs, although they do not show maxima peaks, except
for compound 2. When the dc field is increased to 5 kOe
for compound 1 and 1 kOe for compounds 2-4, the in-phase
and out-of-phase maxima peaks both shifted to the direction
of higher temperature and could be easily observed above 2
K. This may be caused by the coexistence of the thermal
and tunneling processes for magnetic relaxation. The tun-
neling process shortened the magnetic relaxation time at a
zero dc field. According to the Arrhenius equation (τ ) τ0
exp(Ea/kBT), τ ) 1/(2πf), and T is the temperature at which
Similar to the SMM behavior of [Mn2(saltmen)2(ReO4)2],19
the ground state ST ) 4 was considered to fit well with the
plots of M versus H/T by the least-squares for compounds
1-4 (Figure 4). The best-fit DMn2 values for 1-4 are -1.03,
-0.99, -0.30, and -0.80 cm-1, respectively (solid lines in
Figure 4 and Table S1), leading to a theoretical energy
2
barrier, ∆/kB, of 6.9-23.7 K (∆/kB ) |DMn2|ST /kB, where
(33) Borra´s-Almenar, J. J.; Clemente-Juan, J. M.; Coronado, E.; Tsukerblat,
B. S. J. Comput. Chem. 2001, 22, 985.
3546 Inorganic Chemistry, Vol. 45, No. 9, 2006

Dimeric Mn(III) Tetradentate Schiff Base Complexes
Figure 6. Plots of in-phase (øm′, top) and out-of-phase (øm′′, bottom) ac
magnetic susceptibility versus temperature (T) for complex 2: (a) under a
zero dc field and (b) under a dc field of 1 kOe.
Figure 7. Plots of in-phase (øm′, top) and out-of-phase (øm′′, bottom) ac
magnetic susceptibility versus temperature (T) for complex 3: (a) under a
zero dc field and (b) under a dc field of 1 kOe.
the maximum of the øm′′ occurred under different f (fre-
quency) values from 111 to 9999 Hz), activation energy Ea/
kB, and a pre-exponential factor, τ0, were deduced (Figure
S3). Note that we applied a static dc field to avoid tunneling
at a zero-applied field. On the other hand, the quantum
tunneling of the magnetization at a zero field can reduce the
energy barrier, Ea/kB, as seen in other SMM systems.19,34 In
agreement with this idea, the value of Ea/kB should be
increased with an increase of the applied dc field. To take
compound 2 as an example, from its curve of øm′′ versus T
under a zero dc field, an activation energy, Ea/kB, of 17 K
was deduced following the Arrhenius law. This value of Ea/
kB is lower than the expected value calculated from DMn2
cm3 mol-1, τ ) 4.2 × 10-5 s, and R ) 0.10; the best-fit
results for the øm′′ versus øm′ plot at 3.0 K under a 5 kOe dc
field are øs ) 0.31 cm3 mol-1, øT ) 1.48 cm-1 mol-1, τ )
3.7 × 10-4 s, and R ) 0.10, where øs is the adiabatic
susceptibility, øT is the isothermal susceptibility, τ is the
average relaxation time of magnetization, and the R param-
eter ranges between 0 and 1, quantifying the width of the τ
distribution. As for those fitting parameters, the R parameters
are very similar, whereas the values of øs, øT, and τ have
obvious differences. The applied dc field of 5 kOe increased
the resulting relaxation time (τ) in contrast with the value
under a zero dc field, in agreement with the conclusion
reached from the øm′′ and øm′ versus T plots at different
frequencies.
2
(∆/kB ) |DMn2|ST /kB ) 23 K). When a dc field of 1 kOe
was applied, the Ea/kB value increased to 29 K (Figure S3b).
Strangely, the ac susceptibility of compound 3 measured
under a 1 kOe dc field shows another weak magnetic
relaxation course at lower temperature, shown by the
frequency dependence, in addition to the one at higher
temperature (Figure 7). The Cole-Cole circles measured on
an applied dc field of 1 kOe at temperatures of 1.8-3.5 K
clearly display two semicircles at 1.8 K confirming the
At a fixed temperature and a zero dc field or a certain
applied field, the øm′ and øm′′ values were also measured as
a function of the ac frequency, f, ranging from 10 to 10 000
Hz. From these measurements, Cole-Cole diagrams (øm′′
vs øm′ plots) have been obtained for complexes 1-3 (Figure
5c and Figure S2) and have been simulated using a
generalized Debye model (the solid lines).35 The representa-
tive Cole-Cole plots are depicted in Figure 5c for complex
1. The best-fit parameters for the øm′′ versus øm′ plot at 1.8
K under a zero dc field are øs ) 1.26 cm3 mol-1, øT ) 4.28
(35) (a) Cole, K. S.; Cole, R. H. J. Chem. Phys. 1941, 9, 341. (b) Boettcher,
C. J. F. Theory of Electronic Polarization; Elsevier: Amsterdam, 1952.
(c) Aubin, S. M.; Sun, Z.; Pardi, L.; Krzysteck, J.; Folting, K.; Brunel,
L. J.; Rheingold, A. L.; Christou, G.; Hendrickson, D. N. Inorg. Chem.
1999, 38, 5329. (d) Miyasaka, H.; Cle´rac, Mizushima, K.; Sugiura,
K.; Yamashita, M.; Wernsdorfer, W.; Coulon, C. Inorg. Chem. 2003,
42, 8203.
(34) Costes, J. P.; Dahan, F.; Wernsdorfer, W. Inorg. Chem. 2006, 45, 5.
Inorganic Chemistry, Vol. 45, No. 9, 2006 3547

Lu1 et al.
short (2.395(3) Å), and the observed antiferromagnetic
interaction between MnIII ions is probably the result of the
antiferromagnetic interaction due to the MnIII-O* bonds
becoming dominant. The correlation between the MnIII-O*
distances and the magnetic exchange parameters, JF (1.35
cm-1 for 1 and 0.6 cm-1 for 2), for compounds 1 and 2 can
follow with the equation23
JF ) 4.5724 - 1.1868x
(2)
where x ) Mn‚‚‚O* distance (Å) in the range of 2.4-3.7 Å
and the calculated JF values are 1.23 and 0.79 cm-1 for
complexes 1 and 2, respectively (see Figure S4). In all, it
can be concluded that ferromagnetic MnIII dimers may show
SMM behavior, while the MnIII-O* distances may be the
main factor determining if the magnetic coupling is ferro-
magnetic or antiferromagnetic. Also, it is noteworthy that
the anisotropy of a MnIII ion (ZFS term) depends significantly
on the Jahn-Teller distortion that leads to a negative ZFS
parameter in our Mn2 dimer compounds, 1-4.37
Summary
The syntheses, crystal structural characterization, and
magnetic properties of five new MnIII dimers bridged by
phenolate oxygen atoms were described. Complexes 1-4
exhibit intradimer ferromagnetic exchange and display
frequency dependence of ac magnetic susceptibility, probably
showing the characteristic SMM behavior. These results will
enrich the SMM studies. In contrast, complex 5 shows an
intradimer antiferromagnetic coupling probably originating
from the relatively shorter Mn-O* distance. Although it is
still difficult to find a clear correlation between the Mn‚‚‚Mn
magnetic interaction and the structural features of these MnIII
dimeric complexes, it may be concluded that the Mn‚‚‚Mn
magnetic interaction can be modulated by varying the
tetradentate Schiff base and apical ligands. Moreover, these
new MnIII dimer complexes could be used as building blocks
to obtain versatile polymeric architectures by introducing
other magnetic subunits as suggested by Miyasaka et al.38
Figure 8. Plots of in-phase (øm′, top) and out-of-phase (øm′′, bottom) ac
magnetic susceptibility versus temperature (T) for complex 4: (a) under a
zero dc field and (b) under a dc field of 1 kOe.
presence of two magnetic relaxation courses below 1.8 K.
When the temperature was increased to 2.5 K, one of the
two magnetic relaxation courses begins to degenerate, and
it completely disappeared at 3.0 K. This phenomenon may
be caused by the weak intermolecular interaction since the
nearest interdimer Mn-Mn distance is 7.522(1) Å, relatively
shorter than those of compounds 1-3.36
Discussion on Correlation between Structure and
Magnetic Behavior. According to Table 3, the MnIII-O*
distances of the ferromagnetic interacting dimers, 1-4, were
found to be relatively longer than that of 5 and the other
reported complexes, within which the two MnIII ions interact
antiferromagntically.23-25,27 On the basis of the bond lengths
and angles, in particular the unequal bond lengths of two
apical bonds observed for complexes 1-4 as described above
(see Table 3), Jahn-Teller distortion occurs at each MnIII
center. However, as a whole the coordination geometry
around each MnIII center is close to being an octahedron.
Thus the ferromagnetic interaction between MnIII ions for
complexes 1-4 could mainly be the result of the dz2 and dπ
orbitals (dxy, dyz, and dxz orbitals) orthogonality.23 The
antiferromagnetic interaction due to the MnIII-O* bonds with
comparably long bond lengths may be rather weak. For
complex 5, however, the MnIII-O* bond length is relatively
Acknowledgment. The present research was financially
supported by NSFC (20421101, 20372066, 20490210, and
20221101), Chinese Academy of Sciences, and State Key
Basic Research Program. D.-Q.Z. and S.G thank the National
Science Fund for Distinguished Young Scholars. The authors
also thank the anonymous reviewers for their suggestions
and comments which allowed us to largely improve the
manuscript.
Supporting Information Available: CIF files of the crystal
structures of complexes 1-5, M versus H plots for complexes 1-5,
Cole-Cole diagrams for complexes 2 and 3, Arrhenius plots of ln
τ versus 1/T for complexes 1-4, and the plot of the correlation
between the ferromagnetic exchange parameters, J, and the Mn-
O* distances for 1 and 2 and related compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC051648L
(37) Gerritsen, H. J.; Sabinsky, E. S. Phys. ReV. 1963, 132, 1507.
(38) (a) Miyasaka, H.; Matsumoto, N.; Ohkawa, H.; Re, N.; Gallo, E.;
Floriani, C. J. Am. Chem. Soc. 1996, 118, 981. (b) Miyasaka, H.;
Cle´rac, R. Bull. Chem. Soc. Jpn. 2005, 78, 1725.
(36) Boskovic, C.; Bircher, R.; Tregenna-Piggott, P. L. W.; U. Gu¨del, H.;
Paulsen, C.; Wernsdorfer, W.; Barra, A. L.; Khatsko, E.; Neels, A.;
Stoeckli-Evans, H. J. Am. Chem. Soc. 2003, 125, 14046.
3548 Inorganic Chemistry, Vol. 45, No. 9, 2006