A series of manganese-carboxylate coordination polymers exhibiting diverse magnetic properties

Article abstract of DOI:10.1039/b808691e

The hydrothermal reactions of an asymmetrical 4-(4-carboxyphenylamino)-3,5- dinitrobenzoic acid (H₂cpdba), MnCl₂?4H₂O, or together with 2,2′-bipyridine (2,2′-bpy) or 4,4′- bipyridine (4,4′-bpy) afford three novel molecule-based magnetic coordination polymers [Mn(cpdba)]_n (1), {[Mn₂(cpdba) ₂(2,2′-bpy)₂(H₂O)₂] ?H₂O}_n (2) and {[Mn₂(cpdba) ₂(4,4′-bpy)]?2H₂O}_n (3). Compound 1 has a 3D acentric coordination network containing carboxylate-bridged 1D ladder-like manganese chains with spin-canted antiferromagnetism (J = -3.51 cm^-1 for the coupling along the ladder legs, and zJ′ = 0.22 cm^-1 for coupling along the ladder rungs), whereas compounds 2 and 3 crystallize in the centrosymmetric space groups P1 and C2/c, respectively. 2 exhibits a 1D chain structure, which is extended into a 3D supramolecular network by π-π stacking interactions, while 3 features a quite complex 3D network built up from the cpdba^2- and 4,4′-bpy spacers as well as the carboxylate-bridged Mn(ii) chains. Both 2 and 3 show weak antiferromagnetic coupling interactions (J = -0.55 cm^-1 for 2), and a field-induced spin-flop magnetic transition can also be observed in 2 at ca. 3.2 T at 2 K. The Royal Society of Chemistry.

Full text of DOI:10.1039/b808691e

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www.rsc.org/dalton | Dalton Transactions
A series of manganese–carboxylate coordination polymers exhibiting diverse
magnetic properties†
Wei Li, Hong-Peng Jia, Zhan-Feng Ju and Jie Zhang*
Received 22nd May 2008, Accepted 11th July 2008
First published as an Advance Article on the web 21st August 2008
DOI: 10.1039/b808691e
The hydrothermal reactions of an asymmetrical 4-(4-carboxyphenylamino)-3,5-dinitrobenzoic acid
(H2cpdba), MnCl2·4H2O, or together with 2,2¢-bipyridine (2,2¢-bpy) or 4,4¢-bipyridine (4,4¢-bpy)
afford three novel molecule-based magnetic coordination polymers [Mn(cpdba)]n (1),
{[Mn2(cpdba)2(2,2¢-bpy)2(H2O)2]·H2O}n (2) and {[Mn2(cpdba)2(4,4¢-bpy)]·2H2O}n (3). Compound 1
has a 3D acentric coordination network containing carboxylate-bridged 1D ladder-like manganese
chains with spin-canted antiferromagnetism (J = -3.51 cm-1 for the coupling along the ladder legs, and
zJ¢ = 0.22 cm-1 for coupling along the ladder rungs), whereas compounds 2 and 3 crystallize in the
¯
centrosymmetric space groups P1 and C2/c, respectively. 2 exhibits a 1D chain structure, which is
extended into a 3D supramolecular network by p–p stacking interactions, while 3 features a quite
complex 3D network built up from the cpdba2- and 4,4¢-bpy spacers as well as the carboxylate-bridged
Mn(II) chains. Both 2 and 3 show weak antiferromagnetic coupling interactions (J = -0.55 cm-1 for 2),
and a field-induced spin-flop magnetic transition can also be observed in 2 at ca. 3.2 T at 2 K.
be transferred to magnetic entities when they link paramagnetic
Introduction
metal ions, especially the metal ions with a large anisotropy, to
form ordered structures, the probability of spin-canting should
increase. Aiming at optimizing the conditions for the occurrence
of spin-canting based on the asymmetrical transference and con-
sequently finding an effective way to the construction of molecular
materials with spontaneous magnetizations, we synthesized a V-
type asymmetrical 4-(4-carboxyphenylamino)-3,5-dinitrobenzoic
acid (H2cpdba) ligand, in which the presence of two carboxylic
groups not only can supply diverse super-exchange pathways
being attributed to favourable orbital interactions between the
oxygen p and metal d orbitals, but can also be easily com-
bined with other ancillary ligands.11,12 Herein, we present the
syntheses, crystal structures and magnetic behaviour of three
novel metal–organic hybrid networks having the molecular for-
mula [Mn(cpdba)]n (1), {[Mn2(cpdba)2(2,2¢-bpy)2(H2O)2]·H2O}n
(2) and {[Mn2(cpdba)2(4,4¢-bpy)]·2H2O}n (3). 2,2¢-bipyridine (2,2¢-
bpy) or 4,4¢-bipyridine (4,4¢-bpy) are introduced in order to
explore the effect of co-ligands on the structural symmetry.
It is interesting to note that a 3D acentric coordination net-
work containing carboxylate-bridged 1D ladder-like manganese
chains (1), which shows spin-canted antiferromagnetism, can be
obtained in the absence of co-ligands. In the presence of co-
ligands, compounds 2 and 3, with the centrosymmetric space
During past decades, a considerable research effort has been
focused on the design and synthesis of molecule-based mag-
netic materials due to their impressive structural diversity and
intriguing physical properties as well as complicated magneto-
structural correlations.1–4 Judicious choice of appropriate molec-
ular building blocks can effectively mediate the magnetic in-
teraction between the paramagnetic metal ions, and provide
a powerful means to create a rich variety of new materi-
als with interesting magnetic properties, such as spin-canting,
metamagnetic transition, spin-flop transition, etc.5,6 Despite the
great progress in the construction of magnetic molecular ma-
terials, it still remains a challenge to obtain extended net-
works with spontaneous magnetizations and predictable magnetic
properties.
One origin for a spontaneous magnetization may arise from
spin-canting, which is usually due to the antisymmetric exchange
and/or the single-ion anisotropy.7a Since the low symmetry of
the crystal structures is beneficial to the occurrence of the
antisymmetric exchange,8 and may afford a significantly increased
anisotropy barrier,9 it is possible to induce spin-canting by
lowering the symmetry of the systems. Within the realm of crystal
engineering, the unsymmetrical organic ligands have been utilized
as pre-chiral or non-centrosymmetric building blocks to construct
acentric materials.10 If the asymmetry of the organic ligands can
¯
groups P1 and C2/c can be obtained, respectively. The former
exhibits a 1D chain structure, which is extended into a 3D
supramolecular network by p–p stacking interactions arising
from 2,2¢-bpy ancillary ligands, while the latter features a quite
complex 3D network built up from cpdba2- and 4,4¢-bpy spacers
as well as the carboxylate-bridged Mn(II) chains. Both 2 and 3
show weak antiferromagnetic coupling interactions, and a field-
induced spin-flop transition can also be observed in 2 at ca. 3.2 T
at 2 K.
State Key Laboratory of Structural Chemistry, Fujian Institute of Research
on the Structure of Matter, and Graduate School of the Chinese Academy
of Sciences, Fuzhou, Fujian, 350002, China. E-mail: zhangjie@fjirsm.ac.cn;
Fax: +86 591 8371 0051
† Electronic supplementary information (ESI) available: IR spectra for 1–3
and temperature-dependent ac susceptibility data for 1. CCDC reference
numbers 679474–679476. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b808691e
5350 | Dalton Trans., 2008, 5350–5357
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Table 1 Crystal data and structure refinement for compounds 1–3
1
2
3
Formula
Mr
C14H7MnN3O8
400.17
Orthorhombic
C48H36Mn2N10O19
1166.75
Triclinic
¯
P1
C38H26Mn2N8O18
992.55
Monoclinic
Crystal system
Space group
Iba2
C2/c
˚
a/A
18.3259(15)
25.300(2)
7.5240(7)
90.00
90.00
90.00
3488.5(5)
8
1.524
0.804
1608
9.4695(14)
15.348(2)
17.220(2)
93.952(5)
100.876(7)
102.557(5)
2383.2(6)
2
1.618
0.623
1180
0.999
26.7150(19)
12.9166(7)
25.9898(19)
90.00
114.048(3)
90.00
8189.8(9)
8
1.603
˚
b/A
˚
c/A
a/◦
b/◦
g ◦
3
˚
V/A
Z
D
calcd/g cm-3
m/mm-1
0.706
4000
1.003
F(000)
GOF
0.994
Collected reflections
Unique reflections (Rint
13311
3860 (0.0535)
3194
18674
10 746 (0.0449)
6376
31179
9378 (0.0339)
8195
)
Observed reflections [I>2s(I)]
Refined parameters
239
719
597
R1a/wR2b[I>2s(I)]
0.0541/0.1890
0.0678/0.2013
0.0747/0.1870
0.1296/0.2324
0.0623/0.1900
0.0699/0.1986
R1a/wR2b(all data)
2
1/2
a R1 = RꢀFo| - |Fcꢀ/R|Fo|. b wR2 = {R[w(Fo - Fc2)2]/R[w(Fo2)2]}
.
1017 w, 932 w, 874 w, 851 w, 775 m, 768 m, 737 w, 724 w, 704 w,
673 w, 653 w, 627 w, 548 w, 503 w, 454 w, 414 w.
Experimental
Materials and methods
{[Mn2(cpdba)2(4,4¢-bpy)]·2H2O}n (3). The procedure was the
same as that for 1 except 4,4¢-bpy (0.125 mmol, 0.020 g) was added.
Dark-red block crystals of 3 were obtained in 24% yield based on
manganese. Anal. calcd for C38H26N8O18Mn2 (Mr = 992.55): C
45.99, H 2.64, N 11.29. Found: C 46.37, H 2.98, N 11.47. FT-IR
(KBr, cm-1): 3406 br, 3314 w, 3077 w, 1660 s, 1610 m, 1599 vs,
1562 s, 1509 m, 1484 w, 1438 w, 1408 m, 1377 s, 1344 m, 1277 m,
1218 w, 1182 w, 1101 w, 1064 w, 1040 w, 1013 w, 931 w, 869 w,
834 w, 809 m, 773 m, 757 w, 728 w, 700 w, 683 w, 622 w, 576 w,
510 w.
All chemicals and solvents were of reagent grade and were used
as received. IR spectra were recorded with a Spectrum One FT-
IR spectrophotometer as KBr pellets in the range 4000–400 cm-1.
Elemental analyses were determined with an Elementar Vario EL
III elemental analyzer. Magnetic susceptibility data were mea-
sured using a Quantum Design PPMS-9T system. Diamagnetic
corrections were made for the sample holder and sample. 4-
(4-Carboxyphenylamino)-3,5-dinitrobenzoic acid (H2cpdba) was
synthesized according to the literature.13
Single-crystal structure determination
[Mn(cpdba)]n (1). MnCl2·4H2O (0.25 mmol, 0.049 g), H2cpdba
(0.125 mmol, 0.044 g), and NaOH (0.25 mmol, 0.010 g) were mixed
in 8 mL of distilled water, and the solution was stirred for 1 h and
transferred into a 23 mL Teflon-lined autoclave at 160 ◦C for 5 d
and then slowly cooled to room temperature. Dark-red needle
crystals of 1 were obtained in 20% yield based on manganese.
Anal. calcd for C14H7N3O8Mn (Mr = 400.17): C 42.02, H 1.76, N
10.50. Found: C 42.46, H 2.87, N 10.47. The slightly high hydrogen
content might result from the water absorption of 1 within its
porous structure. FT-IR (KBr, cm-1): 3434 m, 3357 m, 1625 m,
1601 s, 1537 m, 1499 m, 1447, 1390 s, 1384 vs, 1357 m, 1290 m,
1183 m, 1113 w, 1099 w, 1015 w, 866 w, 783 m, 752 w, 722 w, 705 w,
677 w, 618 w, 593 w, 484 w, 444 m, 417 w.
The intensity data were collected on a Rigaku Mercury CCD
diffractometer with graphite-monochromated Mo Ka radiation
14
˚
(l = 0.71073 A) at 293 K. CrystalClear software was used for
data reduction. The structures were solved by direct methods
and refined by full-matrix least-squares procedure on F2 using
the SHELXS-97 and SHELXL-97 programs.15 All non-hydrogen
atoms were refined with anisotropic thermal parameters. H atoms
attached to carbon atoms were geometrically placed and refined
using a riding model. However, the H atoms of the water molecules
in 1–3 have not been included in the final refinement. The
crystallographic data are summarized in Table 1 and selected bond
distances and angles of the three compounds are listed in Table 2,
Table 3 and Table 4, respectively.
{[Mn2(cpdba)2(2,2¢-bpy)2(H2O)2]·H2O}n (2). The procedure
was the same as that for 1 except 2,2¢-bpy (0.125 mmol, 0.020 g)
was added. Orange sheet crystals of 2 were obtained in 34% yield
based on manganese. Anal. calcd for C48H36N10O19Mn2 (Mr =
1166.75): C 49.41, H 3.11, N 12.01. Found: C 49.29, H 3.23, N
11.92. FT-IR (KBr, cm-1): 3429 br, 3333 m, 1622 s, 1602 s, 1532 s,
1474 w, 1441 m, 1393 vs, 1373 s, 1349 m, 1285 m, 1180 w, 1101 w,
Results and discussions
Structure of 1
The single-crystal X-ray diffraction study reveals that compound
1 has a 3D acentric coordination network. The asymmetric unit
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◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for 1
Mn1–O5A
Mn1–O6
Mn1–O7C
2.055(4)
2.094(4)
2.141(4)
Mn1–O8D
Mn1–O7B
Mn1–O8B
2.166(4)
2.207(4)
2.693(3)
O5A–Mn1–O6
O5A–Mn1–O7C
O5A–Mn1–O7B
O5A–Mn1–O8D
O6–Mn1–O8D
104.9(2)
O6–Mn1–O7C
O6–Mn1–O7B
O7C–Mn1–O7B
O7C–Mn1–O8D
O8D–Mn1–O7B
86.95(17)
82.73(17)
151.71(13)
83.74(15)
97.81(15)
112.72(17)
95.40(17)
94.05(18)
160.94(19)
Symmetry codes: (A) x, 1 - y, 0.5 + z; (B) 1.5 - x, 0.5 - y, 0.5 + z; (C) 1.5 -
x, 0.5 + y, z; (D) 0.5 + x, 0.5 + y, 0.5 + z.
◦
˚
Table 3 Selected bond lengths (A) and angles ( ) for 2
Fig. 1 ORTEP drawing of the molecular structure of 1 (50% probability
level). Solvent water molecules and all hydrogen atoms have been omitted
for clarity. Symmetry code: (A) x, 1 - y, 0.5 + z; (B) 1.5 - x, 0.5 - y, 0.5 +
z; (C) 1.5 - x, 0.5 + y, z; (D) 0.5 + x, 0.5 + y, 0.5 + z.
Mn1–N7
Mn1–O2
Mn1–O15A
Mn2–N9
Mn2–O7B
Mn2–O9
N7–Mn1–O1w
N8–Mn1–O1w
N10–Mn2–N9
O2–Mn1–N7
O2–Mn1–O1w
O7B–Mn2–N10
O9–Mn2–N9
O9–Mn2–O7B
O9–Mn2–O2w
O10–Mn1–N8
O10–Mn1–O15A
O15A–Mn1–N7
O15A–Mn1–O2
2.256(4)
2.137(3)
2.113(4)
2.245(4)
2.230(3)
2.112(4)
93.8(2)
Mn1–N8
Mn1–O10
Mn1–O1w
Mn2–N10
Mn2–O8B
Mn2–O2w
N8–Mn1–N7
N9–Mn2–O8B
N10–Mn2–O8B
O2–Mn1–N8
O7B–Mn2–N9
O7B–Mn2–O8B
O9–Mn2–N10
O9–Mn2–O8B
O10–Mn1–N7
O10–Mn1–O2
O10–Mn1–O1w
O15A–Mn1–N8
O15A–Mn1–O1w
2.255(4)
2.107(4)
2.410(7)
2.233(4)
2.288(4)
2.145(4)
72.69(17)
93.84(15)
97.66(14)
110.23(16)
88.13(14)
58.17(13)
92.22(15)
169.12(14)
162.48(15)
96.21(14)
73.0(2)
76.0(2)
73.00(15)
97.76(15)
168.1(2)
148.76(14)
93.40(15)
114.02(14)
91.76(14)
92.57(15)
101.19(16)
88.81(17)
92.18(16)
152.32(16)
85.1(2)
Symmetry codes: (A) -x, -y, 1 - z; (B) x, y, -1 + z.
Scheme 1 Coordination modes of the cpdba2- ligand in 1 (a), 2 (b and c),
3 (a and d).
◦
˚
Table 4 Selected bond lengths (A) and angles ( ) for 3
Mn1–O1
Mn1–O10B
Mn1–O16
2.146(2)
2.110(3)
2.350(2)
2.147(2)
78.16(9)
84.04(10)
53.42(9)
Mn2–O15
Mn2–O16
Mn3–N7
Mn3–O15
O10B–Mn1–O16
O2–Mn2–O16
N7–Mn3–O15
2.591(2)
2.210(2)
2.253(3)
2.328(2)
82.74(10)
87.43(10)
93.60(10)
axis) as shown in Fig. 2, (left). These 1D ladder-like chains acting
as secondary building units (SBUs), are further interconnected
through (2,6-dinitrophenyl)-phenyl-amino moieties of the cpdba2-
molecules to generate an extended 3D open framework in which
Mn2–O2
O1–Mn1–O16
O2–Mn2–O15
O15–Mn2–O16
2
˚
there are 1D elliptic channels of 5.719 ¥ 8.983 A (measured
between opposite hydrogen atoms) without the solvents residing
along c axis, as shown in Fig. 2, (right). It is worth mentioning that
no 3D polycarboxylate–Mn coordination polymers containing 1D
[Mn(CO2)2]n ladders have been reported so far, although there are
a lot of coordination polymers in which 1D chains act as SBUs.17a,18
Symmetry codes: (B) 0.5 + x, -0.5 + y, z.
of 1 contains a cpdba2- ligand and a Mn2+ ion as illustrated
in Fig. 1. The Mn2+ ion is located in a distorted octahedral
environment surrounded by a chelating carboxylate group and
four monodentate carboxylate groups from different cpdba2-
ligands, respectively. Five Mn–O bond lengths are in the range
˚
˚
of 2.055(4)–2.207(4) A and the sixth one is 2.693(3) A. Although
it is rather longer than the other five bonds, similar Mn–O bond
lengths have been reported previously.16 The cpdba2- ligand in 1
shows a m5-coordination mode. The carboxylate group near the
nitryl groups adopts a bidentate bridging mode to link two Mn2+
ions, while another carboxylate group binds to three Mn2+ ions
2
in a m3-h bridging mode (Scheme 1a). Such a m5-binding mode
is still very scarce despite the fact that numerous polycarboxylate
complexes have been reported.17 Each Mn2+ ion is bridged by
the carboxylate groups of the cpdba2- ligands to form a 1D
[Mn(CO2)2]n ladder-like chain along the c axis with Mn ◊ ◊ ◊ Mn
Fig. 2 1D Mn(CO2)2 ladder chain (left) and the 3D open framework with
1D channels extending along the c axis (right) in 1.
˚
˚
distances of ca. 3.74 A (along the a axis) and 3.86 A (along the c
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Structure of 2. As shown in Fig. 3, the asymmetric unit of 2
contains two Mn2+ ions, two 2,2¢-bpy ligands, two cpdba2- ligands,
two coordinated water molecules and one lattice water molecule.
Mn1 has a distorted octahedral coordination environment with a
chelating 2,2¢-bpy ligand, three carboxylate groups from different
cpdba2- ligands in a monodentate coordination mode, and one
water molecule. Similarly, Mn2 also has a distorted octahedral
geometry with two nitrogen atoms from a 2,2¢-bpy, a chelating
and a monodentate carboxylate group from two different cpdba2-
ligands, and one water molecule. The cpdba2- ligand exhibits two
kinds of bridging modes in the structure of 2. One cpdba2- ligand
connects three manganese ions with two carboxylate groups in
monodentate and bridging modes, respectively (Scheme 1b), while
another binds to two Mn2+ ions by two carboxylate groups in
monodentate and chelating coordination modes (Scheme 1c).
The Mn1 and Mn2 ions are linked through one carboxylate
group in syn–anti mode, giving rise to a dinuclear subunit.
These dinuclear units are connected by a (2,6-dinitrophenyl)-
phenyl-amino spacer to form a 1D polymeric chain (Fig. 4, top).
Interestingly, there are inter-chain p–p stacking interactions, which
arise from 2,2¢-bpy ligands. The 2,2¢-bpy ligand coordinated to the
Mn1 atoms employs its single-pyridyl ring to form p–p stacking
˚
with a centroid-to-centroid distance of 3.782 A and an inter-planar
˚
pyridine distance of 3.56 A, whereas another 2,2¢-bpy ligand
coordinated to Mn2 atoms employs its double-pyridyl ring to form
a p–p interaction with a centroid-to-centroid distance of 3.645 A
and an inter-planar distance of 3.51 A. Finally, these 1D chains
interweave with each other to form a 3D supramolecular network
as shown in Fig. 4, bottom.
˚
˚
Structure of 3. The asymmetric unit of compound 3 contains
three crystallographically independent Mn2+ ions, two cpdba2-
ligands, one 4,4¢-bpy ligand and two lattice water molecules
(Fig. 5). All Mn2+ ions have distorted octahedral coordination
environments. Mn1 is surrounded by six carboxylate oxygen
atoms from six cpdba2- ligands, Mn2 is coordinated by one
carboxylate group in a chelating mode, three carboxylate groups
in a monodentate mode and one pyridyl nitrogen atom, while
Mn3 is coordinated by four carboxylate oxygen atoms from four
cpdba2- ligands, and two nitrogen atoms from two 4,4¢-bpy ligands
(Fig. 6, top). Each Mn1 atom is bridged to both Mn2 by two
syn–syn carboxylate bridges and one m2-O bridge from the m3-
carboxylate group to give a trinuclear unit. This trinuclear unit
is further connected to two Mn3 atoms through one syn–anti
carboxylate bridge and one m2-O bridge from the m3-carboxylate
group, finally resulting in the formation of 1D [Mn(CO2)2]n chains
with Mn2–Mn1–Mn2–Mn3 repeat units (Fig. 6, top), in which
˚
the Mn1 ◊ ◊ ◊ Mn2 and Mn2 ◊ ◊ ◊ Mn3 distances are 3.58 and 4.15 A,
the Mn1–O16–Mn2 and Mn2–O15–Mn3 bond angles are 103.36
and 114.88◦, respectively. The 4,4¢-bpy ligand coordinates to Mn2
and Mn3 through two terminal N atoms, which may hold the 1D
[Mn(CO2)2]n chains together in a 2D layer as illustrated in Fig. 6,
bottom. The whole structure of 3 can be described as quite a
complex 3D network constructed by cpdba2- and 4,4¢-bpy ligand
(ESI, Fig. S4).† It is interesting to note that the cpdba2- ligand
also adopts two types of bridging modes in 3, one cpdba2- anion
coordinates to four Mn2+ ions in the bidentate bridging mode
(Scheme 1d), while another adopts the rare m5-bridging mode like
that observed in 1.
Fig. 3 ORTEP drawing of the molecular structure of 2 (30% probability
level). All hydrogen atoms and lattice water molecules have been omitted
for clarity. Symmetry codes: (A) -x, -y, 1 - z; (B) x, y, -1 + z.
Fig. 5 ORTEP drawing of the molecular structure of compound 3 (30%
probability level). All solvent water molecules and hydrogen atoms have
been omitted for clarity.
Magnetic properties
The thermal variations of the cm and cmT products obtained
at an applied field of 1 T for 1 are shown in Fig. 7, top. At
room temperature, the cmT value is 3.80 emu K mol-1, which
is lower than that expected for an isolated Mn2+ ion (4.375 emu
K mol-1 for S = 5/2 with g = 2.0), suggesting the presence of an
appropriate antiferromagnetic interaction.19 As the temperature
Fig. 4 1D polymeric chain (top) and 3D network constructed by p–p
stacking interactions between 1D chains (bottom) in 2. All hydrogen atoms
and solvent water molecules have been omitted for clarity. The dashed lines
represent p–p stacking interactions.
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a long distance across the cpdba2- ligand plays a negligible role
in propagating magnetic exchanges, therefore the magnetic ex-
changes within the [Mn(CO2)2]n ladder are dominant. If we neglect
the comparatively weak magnetic coupling interaction mediated
by the anti–anti carboxylate groups20 along the diagonal, there are
two predominant super-exchange pathways within the chain: one
2
connects Mn2+ ions via two carboxylate groups in m3-h bridging
mode (the rungs of the ladder) with the Mn–O–Mn bond angle
of 100.08◦, which lies in the range that ferromagnetic couplings
can be expected,21 and another one links Mn2+ ions through one
2
syn–syn and one m3-h bridging carboxylate groups into uniform
linear chains (the ladder leg) with the Mn–O–Mn bond angle of
125.26◦. Mn–O–Mn bond angles larger than 110◦ usually induces
antiferromagnetic couplings in Mn2+ polynuclear complexes.21b
Furthermore, the triatomic (–OCO–) syn–syn mode may induce
more efficient antiferromagnetic coupling than other triatomic
bridges, with the magnitude comparable to or even greater than the
monatomic (–O–) bridge.22 Since the temperature dependence of
the susceptibility has revealed that antiferromagnetic interaction
dominates the magnetic property of 1, it is reasonable to consider
the coupling along the ladder leg first. Thus, we can estimate
the exchange interaction of this ladder-like chain by a following
simple approach in which the ladder can be treated as the linkage
of two uniform chains with intra-chain and inter-chain exchange
constants. For the intra-chain antiferromagnetic interaction (J),
we used the classical 1D uniform chain theoretical expression
proposed by Fisher.23
Fig. 6 1D [Mn(CO2)2]n chain with Mn2–Mn1–Mn2–Mn3 repeat units
(top). The 4,4¢-bpy ligands connect the 1D [Mn(CO2)2]n chains to form a
2D layer in 3 (bottom). Symmetry codes: (A) -x, y, 0.5 - z; (B) 0.5 + x,
-0.5 + y, z.
2
cchain = [Nb g2S(S + 1)/3kT](1 + u)/(1 - u)
(1)
Where u = coth[JS(S + 1)/kT] - kT/JS(S + 1) and S =
5/2. The J parameter is based on the spin Hamiltonian H =
-2JRSi·Si+1 and the S operator is treated as a classical spin
operator. The inter-chain interaction (zJ¢) through the rungs of
the ladder was treated by the molecular field approximation24
2
cladder = cchain/[1 - 2zJ¢cchain/Nb g2]
(2)
Using this approximate model, the susceptibility data were
simulated, giving the best fit in the whole temperature range with
the parameters J = -3.51 cm-1, zJ¢ = +0.22 cm-1 and g = 1.94. The
agreement factor R = R(cobsT - ccalcT)2/R(cobsT)2 is 8.4 ¥ 10-5.
The large negative J value is consistent with the magnetic cou-
pling interactions observed in some Mn2+ carboxylate complexes
2
bridged by the syn–syn and m3-h bridging carboxylate groups with
larger Mn–O–Mn bond angles.22,25 While the inter-chain exchange
pathway zJ¢ originates from two m2-oxygen bridges together with
two syn–anti carboxylate bridges. The syn–anti carboxylate bridge
usually transmits weak antiferromagnetic coupling interactions,22
which counteracts the ferromagnetic interactions via two m2-
oxygen bridges to some extent and leads to a comparatively small
positive zJ¢ value.
Below 4 K and at an applied field of 0.01 T, the cmT product
experiences an upturn, reaching 1.18 emu K mol-1 at 2 K. When
the applied field is larger than 0.1 T, the increasing tendency of
the cmT product is inconspicuous and completely disappears at
higher fields. Upon cooling, the cm value increases smoothly to a
maximum at ca. 24.0 K and then decreases slightly. Below 5.5 K, a
relatively abrupt increase, which is more obvious in lower magnetic
fields, can be observed (Fig. 7, bottom). This low temperature
field-dependent behaviour suggests the occurrence of very weak
Fig. 7 Temperature dependence of the cm and cmT products of com-
pound 1 within the whole temperature range (top), as well as at low
temperature and different applied fields (bottom).
is lowered from 300 K to 75 K, there is a smooth decrease
in cmT, whereas below 75 K, the cmT value decreases sharply
and reaches 0.12 emu K mol-1 at 2 K. The Weiss constant q,
obtained by fitting the reciprocal magnetic susceptibility (1/cm)
to the Curie–Weiss law above 40 K, is -38.6 K. The structural
analysis indicates that the minimum distance between adjacent
˚
1D [Mn(CO2)2]n ladder chains is ca. 13.55 A (Mn ◊ ◊ ◊ Mn). Such
5354 | Dalton Trans., 2008, 5350–5357
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ferromagnetic interactions in 1, which may arise from the presence
of canting between the anti-parallel alignment of the spins.7
The canting behaviour in 1 can be confirmed by the M(H)
curve, which has a fast increase of the magnetization at very
low fields and a linear variation of M(H) at H > 0.065 T. The
curve reaches a value of 0.81Nb at 7 T and is far less than
the saturated value of 5Nb expected for an isolated Mn2+ ion
(Fig. 8). This lack of saturation at high applied field is consistent
with the presence of a spin-canted antiferromagnetic state. The
hysteresis loop measured at 2 K shows values of the coercive field
(Hc) and remnant magnetization (MR) of 48.5 Oe and 0.0094
Nb, respectively (the inset of Fig. 8), being characteristic of a
soft magnet. The canting angle is estimated to be about 0.11◦
from the expression sina = MR/Ms.26 In 1, the in-phase signal
(cm¢) presents a maximum at ca. 3 K, whereas the out-of-phase
signal (cm¢¢) is not observed (ESI, Fig. S5).† The similar ac
susceptibility behaviour has been reported for some Mn2+ and Co2+
complexes.27 In all these magnetic entities the magnetic moment
arising from canting is so small that the loss of energy related
to the out-of-phase signal can be ignored. It is well known that
the occurrence of spin canting is usually caused by either single-
ion magnetic anisotropy or antisymmetric exchange in magnetic
entities.7 Due to the negligible single-ion anisotropy for high-
spin Mn2+,7 the observation of the spin canting in 1 should
arise from the antisymmetric magnetic exchange, which is related
to the antisymmetric nature of the magnetic entities. In 1, the
antisymmetry of the ligand is transferred to the whole 3D structure
to finally lead to a non-centrosymmetric space group Iba2, which
is in the absence of an inversion center between the neighbouring
Mn2+ ions and is low enough to allow the antisymmetric exchange
coupling to operate. Such spin-canting effects arising from the
antisymmetric exchange interaction have been observed in some
isotropic Mn2+ and Fe3+ complexes.28
Fig. 9 Temperature dependence of cmT product at 0.5 T for compound
2. The solid line corresponds to the best fit to a dimer model. The inset
shows the temperature dependence of cm at different external fields in the
range 0.1–4.5 T.
range with the super-exchange parameters J = -0.55 cm-1, g =
2.04, and R = 1.2 ¥ 10-4. These parameters agree with the
syn–anti carboxylate-bridged dimer with a long Mn ◊ ◊ ◊ Mn dis-
˚
tance of 5.43 A in 2 and are comparable to the observation in the
compounds reported previously.22,29
The comparison of cm vs. T curves at different magnetic fields
from 0.1–4.5 T (the inset of Fig. 9), suggests that there is an obvious
occurrence of a transition to an antiferromagnetic state at the
lower field, with a maximum value at about 4.0 K. On the contrary,
the maximum vanishes above 3 T, indicating that the transition
is suppressed at higher magnetic fields. Furthermore, isothermal
magnetization curves at T = 2 and 3.5 K in the applied field 0–7 T
(Fig. 10) reveal that the magnetization reaches 9.1Nb at 2 K and
8.5Nb at 3.5 K with the highest applied field, which are all less
than the expected saturation value of 10Nb for two isolated Mn2+
ions. The sigmoidal shape of the M(H) dependence as shown in
Fig. 10 and the disappearance of the cm maximum at higher fields
suggest the occurrence of a field-induced magnetic transition.
This field-independent magnetic behaviour is characteristic of a
spin-flop transition, which often occurs in antiferromagnets with
small anisotropy.7 The transition field, determined by a dM/dH
derivative curve, is ca. 3.2 T at 2 K (the inset in Fig. 10).
Fig. 8 Field-dependent magnetization and hysteresis loop for 1 at 2.0 K.
The temperature dependence of the cm and cmT measured at
0.5 T for 2 is shown in Fig. 9 (cm being the magnetic susceptibility
of two Mn2+ ions). The cmT value is about 8.79 emu K mol-1 at
room temperature, close to the expected value for two isolated
octahedral Mn2+ ions (8.75 emu K mol-1). From 300–40 K, there
is a smooth decrease of the cmT value, while from 40–2 K, there
is a sharp decline in cmT, finally reaching 1.19 emu K mol-1. The
cm values show a small maximum at about 4.0 K, suggesting the
occurrence of weak antiferromagnetic interactions between the
two adjacent Mn2+ ions. Taking into account the dimer character
in the structure of 2, the cmT data were fitted by using the
isotropic Heisenberg model H = -2JS1·S2 for two interacting S =
5/2 centers to determine the exchange parameters via a syn–anti
carboxylate group.29 The best fit is given in the whole temperature
Fig. 10 Magnetization curves at 2 and 3.5 K in the applied field range
0–7 T for 2. The inset shows the derivative of the isothermal magnetization
with respect to dc field at 2 and 3.5 K.
-1
Plots of the cmT and cm vs. T for 3 are shown in Fig. 11.
The cmT value of 8.73 emu K mol-1 at room temperature is close
to that expected for two non-interacting high-spin Mn2+ ions. As
the temperature is lowered from 300–50 K, there is a smooth
decrease in the cmT value, whereas below 50 K, the cmT value
decreases sharply and reaches 2.08 emu K mol-1 at 2 K. The
magnetic susceptibility data can be fitted to the Curie–Weiss law
in the whole temperature with q = -9.5 K, indicating dominant
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In conclusion, by utilizing an unsymmetrical carboxylic acid lig-
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have been successfully synthesized and structurally characterized.
It is interesting to note that a novel 3D acentric coordination net-
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chains can be obtained in the absence of co-ligands, whereas the
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Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (No. 20671090/50372069), the Natural Science
Foundation of Fujian Province of China (No. E0220003), and
Key Project from CAS (KJCX2-YW-H01). The authors sincerely
thank Prof. Song Gao and Dr Bing-Wu Wang for their help in
simulating the magnetic data of compound 1.
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