Spin-canted antiferromagnetic phase transitions in alternating phenoxo- and carboxylato-bridged MnIII-salen complexes

Article abstract of DOI:10.1002/ejic.201001215

Three new Mn^IIIcomplexes₂(OCn)](ClO ₄)}_n (1), {[Mn₂(salen)₂(OPh)] (ClO₄)}_n (2) and {[Mn₂(salen) ₂(OBz)](ClO₄)}₂ (3) (where salen = N,N′-bis(salicylidene)-1,2-diaminoethane dianion, OCn = cinnamate, OPh = phenylacetate and OBz = benzoate), have been synthesized and characterized structurally and magnetically. The crystal structures reveal that all three structures contain syn-anti carboxylate-bridged dimeric [Mn₂(salen) ₂(OOCR)]⁺ cations (OOCR = bridging carboxylate) that are joined together by weak Mn...O(phenoxo) interactions to form infinite alternating chain structures in 1 and 2, but the relatively long Mn...O(phenoxo) distance [3.621(2) A] in 3 restricts this structure to tetranuclear units. Magnetic studies showed that 1 and 2 exhibited magnetic long-range order at T_N = 4.0 and 4.6 K (T_N = Neel transition temperature), respectively, to give spin-canted antiferromagnetic structures. Antiferromagnetic coupling was also observed in 3 but no peaks were recorded in the field-cooled magnetization (FCM) or zero-field-cooled magnetization (ZFCM) data, indicating that 3 remained paramagnetic down to 2 K. This dominant antiferromagnetic coupling is attributed to the carboxylate bridges. The ferromagnetic coupling expected due to the Mn-O(phenoxo)...Mn bridge plays an auxiliary role in the magnetic chain, but is an essential component of the bulk magnetic properties of the material.

Full text of DOI:10.1002/ejic.201001215

FULL PAPER
DOI: 10.1002/ejic.201001215
Spin-Canted Antiferromagnetic Phase Transitions in Alternating Phenoxo- and
Carboxylato-Bridged MnIII-Salen Complexes
Paramita Kar,[a] Pampa M. Guha,[a] Michael G. B. Drew,[b] Takayuki Ishida,*[c] and
Ashutosh Ghosh*[a]
Keywords: Manganese complexes / Salen / Magnetic properties / Spin canting
Three new MnIII complexes, {[Mn2(salen)2(OCn)](ClO4)}n (1),
{[Mn2(salen)2(OPh)](ClO4)}n (2) and {[Mn2(salen)2(OBz)]-
(ClO4)}2 (3) (where salen = N,NЈ-bis(salicylidene)-1,2-diami-
noethane dianion, OCn = cinnamate, OPh = phenylacetate
and OBz = benzoate), have been synthesized and charac-
terized structurally and magnetically. The crystal structures
reveal that all three structures contain syn-anti carboxylate-
bridged dimeric [Mn2(salen)2(OOCR)]+ cations (OOCR =
bridging carboxylate) that are joined together by weak
Mn···O(phenoxo) interactions to form infinite alternating
1 and 2 exhibited magnetic long-range order at TN = 4.0 and
4.6 K (TN = Néel transition temperature), respectively, to give
spin-canted antiferromagnetic structures. Antiferromagnetic
coupling was also observed in 3 but no peaks were recorded
in the field-cooled magnetization (FCM) or zero-field-cooled
magnetization (ZFCM) data, indicating that 3 remained para-
magnetic down to 2 K. This dominant antiferromagnetic cou-
pling is attributed to the carboxylate bridges. The ferromag-
netic coupling expected due to the Mn–O(phenoxo)···Mn
bridge plays an auxiliary role in the magnetic chain, but is
an essential component of the bulk magnetic properties of
the material.
chain structures in
1 and 2, but the relatively long
Mn···O(phenoxo) distance [3.621(2) Å] in 3 restricts this
structure to tetranuclear units. Magnetic studies showed that
ation reactions.[4] However, recent interest in this type of
compound has increased since Miyasaka et al.[5] reported a
dimeric manganese(III) Schiff base compound, [Mn2(salt-
men)2(ReO4)2] [H2saltmen = N,NЈ-(1,1,2,2-tetramethyleth-
ylene)bis(salicylideneimine)] showing single-molecule mag-
net (SMM) behavior. Studies of these complexes show that
such compounds can be both ferromagnetic and antiferro-
magnetic, and modulation of the MnIII···MnIII magnetic in-
teraction is possible by varying the apical ligands and the
chemical features of the Schiff-base ligand.[6] On the other
hand, carboxylate bridging ligands displaying multiple
bridging modes are able to mediate effectively magnetic
coupling between metal ions, and have often been used to
prepare molecular magnetic materials.[7] The versatility of
carboxylates as ligands is illustrated by the variety of their
coordination modes when acting as a bridge,[8–11] the most
common being the so-called syn-syn, syn-anti, and anti-anti
modes. Numerous magnetic materials exhibiting diverse
bulk magnetic behavior such as antiferromagnetism, ferro-
magnetism, spin crossover, spin canting, and ferrimagne-
tism have been constructed by exploiting the different coor-
dination modes and structural features of carboxyl-
ates.[12–15] Considering these facts, it may be expected that a
system containing both phenoxo- and carboxylate-bridged
MnIII ions would show interesting magnetic behavior.
Herein, we report the synthesis, XRD determined single
crystal structure, and magnetic properties of three new
MnIII Schiff-base complexes consisting of the dinuclear
Introduction
In recent years considerable attention has been focused
on the design and synthesis of molecular magnetic materials
to further our understanding of fundamental magnetic in-
teractions and magneto-structural correlations, as well as
for the development of new molecule-based materials.[1]
Magnetic phase transitions in molecular materials are of
interest from a fundamental point of view, with regards to
investigating novel magnetic behaviors that differ from
those of traditional magnets based on metal or ionic lat-
tices, but also with an aim of eventually synthesizing new
magnetic materials.
Binuclear phenoxo-bridged manganese(III) compounds
containing salen-type Schiff bases are well established, and
are of interest to chemists because of their important roles
in biological systems e.g. in many metalloenzymes, redox
and nonredox proteins,[2,3] and as catalysts in olefin epoxid-
[a] Department of Chemistry, University College of Science,
University of Calcutta,
92, A.P.C. Road, Kolkata 700009, India
E-mail: ghosh_59@yahoo.com
[b] School of Chemistry, The University of Reading,
P. O. Box 224, Whiteknights, Reading RG 66AD, UK
[c] Department of Engineering Science, The University of Electro-
Communications,
Chofu, Tokyo 182-8585, Japan
E-mail: ishi@pc.uec.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001215.
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units [Mn2(salen)2(OOCR)](ClO4), where
R
=
C6H5- another species, which was shown by elemental analysis to
CH=CH- (1), C6H5CH2- (2) and C6H5- (3). In complexes 1 have the probable molecular formula of [Mn(salen)-
and 2 the dinuclear cationic units are joined together by (RCOO)(H2O)], along with the desired complexes 1–3.
weak Mn···O (phenoxo) interactions to form an alternating
phenoxo- and carboxylato-bridged 1D chain. Complex 3 is
Description of the Structures
best considered to be a tetramer that is formed by the phen-
oxo bridge between two dimers. Interestingly, complexes 1
and 2 show antiferromagnetic phase transitions that can be
explained on the basis of spin canting, whereas complex 3
behaves as a paramagnet with appreciable antiferromag-
netic coupling. Long-range ordering was found to be pres-
ent in 1 and 2 due to the existence of interchain magnetic
interactions. To the best of our knowledge, this is the first
report of the magnetic behavior of such alternating phen-
oxo-carboxylato-bridged MnIII Schiff-base complexes.
All three structures contain [Mn2(salen)2(OOCR)]+ cat-
ions (OOCR = bridging carboxylate) together with a dis-
crete perchlorate anion in the asymmetric unit. In all three
structures, the dimeric cations are involved in weak interac-
tions with adjacent molecules to form polymeric chains in
1 and 2 and tetramers in 3; however the perchlorate anions
are not involved in any close interactions with the metal
atoms.
{[Mn2(salen)2(OCn)](ClO4)}n (1). The asymmetric unit of
1 consists of a dimeric structure that is shown in Figure 1
together with the atomic numbering scheme for the metal
coordination spheres. Selected bond lengths and angles are
summarized in Table 1. Within the asymmetric unit, both
metal ions can be considered as five-coordinate, with square
pyramidal (4 + 1) coordination spheres, when one takes into
account only the five strong bonds in which the metal ions
participate. However, both manganese ions are weakly co-
Results and Discussion
Syntheses
Compounds 1–3 were obtained as dark brown micro
crystals by slow evaporation of methanol solutions of
Mn(ClO4)2·6H2O, H2salen, the corresponding carboxylic
acid and triethylamine in 2:2:1:1 ratios. The MnII ions were
oxidized to MnIII by the oxygen in the atmosphere under
these reaction conditions, and the deprotonated Schiff-base
ligand (salen) molecules occupy the equatorial planes of the
MnIII sites, as is usual for salen-type Schiff-base complexes.
Such a [Mn(salen)]+ species can take up neutral or anionic
ligands in one or both axial positions resulting in various
complexes. In the present investigation, we maintained the
molar ratios of MnII and carboxylate as 2:1, so that a carb-
oxylate ions will bridge two manganese(III)-salen species by
coordinating at one of the axial positions while keeping the
other axial site vacant and thus available for interaction
with phenoxo oxygen atoms of neighboring dinuclear units
to generate polymeric structures (Scheme 1). Maintaining
the ratio of MnII and carboxylate as 2:1 is important be-
cause a higher proportion of Mn(ClO4)2·6H2O in the reac-
tion mixture led to the formation of [Mn(salen)(H2O)]ClO4
as an impurity in all three cases; on the other hand, a higher
Figure 1. The dimeric structure of 1 with ellipsoids drawn at the
proportion of carboxylic acid resulted in the formation of 30% probability level.
Scheme 1. Formation of complexes 1–3.
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Antiferromagnetic Phase Transitions in MnIII-Salen Complexes
ordinated at their second axial positions, which make their by 0.144(1) and 0.194(1) Å, respectively, and do so in the
effective coordination number six with tetragonal coordina- direction of the bridging axial carboxylate oxygen atom.
tion geometry and allow the formation of polymeric 1-D The planes intersect at an angle of 54.9(9)°.
chains. The two metal ions Mn(1) and Mn(2) are coordi-
In addition to these five strong bonds, the metal ions
nated by the four donor atoms of the deprotonated tetra- show weak interactions, at the other axial positions, with
dentate Schiff-base ligand (salen) at the equatorial sites. The the phenoxo oxygen atoms of salen ligands on neighboring
bond lengths are as expected with Mn–O(ligand) length dinuclear units, with Mn(1)–O(30)a 2.543(2) Å (a: 1 – x, 1 –
within the range 1.871(2)–1.879(2) Å, and with Mn–N
y, –z) and Mn(2)–O(50)b 2.743(3) Å (b: –x, –y, 1 – z) thus
lengths within the 1.966(3)–1.982(3) Å range. The two oxy- forming centrosymmetric bridging arrangements as shown
gen atoms O(81) and O(83) of the cinnamate ligand coordi- in Figure 2. These two alternating bridging systems, i.e.
nate at the axial positions of Mn(1) and Mn(2), with corre- phenoxo- and syn-anti carboxylate, constitute the one-di-
sponding bond distances of 2.075(2) and 2.137(3) Å, respec- mensional chain structure of this compound. The two asso-
tively, to form a syn-anti carboxylate bridge between the ciated (phenoxo-bridged) Mn···Mn distances are 3.361(11)
two Mn ions. These axial bonds are significantly longer and 3.497(12) Å, and the distance between the carboxylate-
than the equatorial bonds, which is a result of Jahn–Teller bridged Mn ions [Mn(1)···Mn(2)] is 5.749(3) Å.
distortion due to the Mn ions being in the +3 oxidation
state.
Table 1. Bond lengths [Å] and angles [°] in the metal coordination
spheres of complex 1–3.
1
2
3
Mn(1)–O(11)
Mn(1)–O(30)
Mn(1)–N(19)
Mn(1)–N(22)
Mn(1)–O(81)
Mn(2)–O(31)
Mn(2)–O(50)
Mn(2)–N(39)
1.871(2)
1.899(2)
1.966(3)
1.997(3)
2.075(2)
1.875(2)
1.879(2)
1.981(3)
1.982(3)
2.137(3)
94.4(1)
92.5(1)
166.1(1)
173.1(1)
89.4(1)
82.6(1)
97.9(1)
96.6(1)
94.4(1)
87.4(1)
93.3(1)
92.0(1)
166.5(1)
167.8(1)
90.0(2)
82.4(1)
104.2(1)
96.7(1)
94.0(1)
94.0(1)
2.543(2)
2.743(3)
3.361(11)
3.497(12)
5.749
1.856(4)
1.907(4)
1.977(5)
1.975(4)
2.110(4)
1.867(4)
1.897(4)
1.982(5)
1.980(5)
2.127(4)
94.5(2)
91.8(2)
166.6(2)
173.5(2)
89.6(2)
83.3(2)
96.9(2)
98.6(2)
92.4(2)
87.5(2)
94.0(2)
92.2(2)
165.2(2)
167.9(2)
89.6(2)
81.7(2)
99.8(2)
94.5(2)
97.7(2)
91.5(2)
2.576(4)
2.729(4)
3.387(2)
3.525(2)
5.737
1.865(2)
1.908(2)
1.971(2)
1.993(2)
2.098(2)
1.874(2)
1.870(2)
1.977(2)
1.988(2)
2.089(2)
94.2(1)
92.3(1)
165.5(1)
174.3(1)
89.4(1)
83.2(1)
97.2(1)
95.3(1)
96.7(1)
86.9(1)
90.8(1)
91.5(1)
165.8(1)
154.7(1)
90.6(1)
81.5(1)
104.9(1)
99.9(1)
93.1(1)
99.8(1)
2.493(2)
3.621(2)
3.364(1)
4.087(1)
5.478
Mn(2)–N(42)
Mn(2)–O(83)
O(11)–Mn(1)–O(30)
O(11)–Mn(1)–N(19)
O(30)–Mn(1)–N(19)
O(11)–Mn(1)–N(22)
O(30)–Mn(1)–N(22)
N(19)–Mn(1)–N(22)
O(11)–Mn(1)–O(81)
O(30)–Mn(1)–O(81)
N(19)–Mn(1)–O(81)
N(22)–Mn(1)–O(81)
O(31)–Mn(2)–O(50)
O(31)–Mn(2)–N(39)
O(50)–Mn(2)–N(39)
O(31)–Mn(2)–N(42)
O(50)–Mn(2)–N(42)
N(39)–Mn(2)–N(42)
O(31)–Mn(2)–O(83)
O(50)–Mn(2)–O(83)
N(39)–Mn(2)–O(83)
N(42)–Mn(2)–O(83)
Mn(1)···O(30)a
Figure 2. A view of the polymeric structure of complex 1. Ellipsoids
are drawn at the 30% probability level. The weak Mn(2)–O(50) and
Mn(1)–O(30) interactions are shown as dotted lines.
The crystal packing of complex 1 is further stabilized by
offset or slipped stacking π···π interactions (Figure S2, Sup-
porting Information). The phenyl rings of the carboxylate-
bridged dimeric units are stacked with respect to each other
[ring R1: C(32)–C(33)–C(34)–C(35)–C(36)–C(37) with ring
R2: C(44)–C(45)–C(46)–C(47)–C(48)–C(49)] with
a
centroid–centroid distance of 3.728(2) Å (Figure S2, Sup-
porting Information). The π···π interactions are quite
strong because the dihedral angle between the rings is 14.3°
with a slip angle of 21.3°. Similarly, the phenyl rings R3
[C(24)–C(25)–C(26)–C(27)–C(28)–C(29)] and R4 [C(12)–
C(13)–C(14)–C(15)–C(16)–C(17)] are also stabilized by
π···π interactions with a centroid–centroid distance of
3.611(2) Å and the dihedral angle between the rings is 11.6°
and the slip angle is 17.9°.
Mn(2)···O(50)b
Mn(1)···Mn(1)a
Mn(2)···Mn(2)b
Mn1···Mn2
Symmetry codes, a: 1 – x, 1 – y, –z (for 1 and 3), 1 – x, –y, –z
(for 2); b: –x, –y, 1 – z (for 1), 1 – x, 1 – y, 1 – z (for 2), 1 –x, 2 –
y, 1 – z (for 3).
{[Mn2(salen)2(OPh)](ClO4)}n (2). The structure of 2,
shown in Figure 3, is similar to that of 1. Bond lengths,
The root mean squared (r.m.s.) deviations of the four listed in Table 1, are also similar with the Mn–O(salen)
basal donor atoms from their mean planes around Mn(1) bond lengths within the range 1.856(4)–1.868(4) Å, Mn–N
and Mn(2) are 0.053 and 0.004 Å, respectively. The Mn(1) 1.978(4)–1.982(5) Å, and Mn–O(carboxylate), with the li-
and Mn(2) ions deviate from their respective mean planes gand in the axial position, 2.110(4)–2.127(4) Å. The r.m.s.
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deviations of the four basal donor atoms from their mean the phenyl rings R3 [C(44)–C(45)–C(46)–C(47)–C(48)–
planes around Mn(1) and Mn(2) are 0.059 and 0.015 Å, C(49)] and R4 [C(32)–C(33)–C(34)–C(35)–C(36)–C(37)] are
respectively. The Mn(1) and Mn(2) ions deviate from these also stabilized by π–π interactions with a centroid–centroid
mean planes by 0.136(2) and 0.200(1) Å, respectively, and distance of 3.733(3) Å and with a dihedral angle between
do so in the direction of the bridging axial carboxylate oxy- the rings of 15.0° and a slip angle of 22.6°. In this com-
gen atom. The planes intersect at an angle of 58.7(1)°.
pound there is a short Csp2–H···π (phenyl) contact between
C(86)–H(68) and a phenyl group of the salen molecule (Fig-
ure S3, Supporting Information), where the H(86)···Cg(R4)
distance is 2.71 Å and the C(86)–H(86)···Cg(R4) angle is
141° (Cg: centroid of the phenyl ring).
{[Mn2(salen)2(OBz)](ClO4)}2 (3). The structure of 3 is
also a dimer, as shown in Figure 5. In this compound the
equatorial planes of the metal complexes are approximately
planar, with the r.m.s. deviations of the four donor atoms
from their mean planes around Mn(1) and Mn(2) being
0.074 Å and 0.098 Å, respectively. The Mn(1) and Mn(2)
ions deviate from these mean planes by 0.141(1) and
0.319(1) Å, respectively, and deviate towards the bridging
axial benzoate oxygen atom. The planes intersect at an an-
gle of 62.0(1)°. It may be noted that the angles between the
planes can be correlated with the steric bulks of the bridg-
ing carboxylate ligands, thus the angle is smallest in 1 with
cinnamate and largest in 3 where the aromatic ring of the
carboxylate is close to the metal coordination spheres. The
bond lengths are as expected for Mn–O (ligand) and are in
the range 1.856(4)–1.868(4) Å, Mn–N 1.978(4)–1.982(5) Å,
and for Mn–O(carboxylate) where the ligand is in the axial
position, the distances are within the range 2.110(4)–
2.127(4) Å.
Figure 3. The dimeric structure of 2 with ellipsoids drawn at the
30% probability level.
The packing of the dimers in 2 to form a 1-dimensional
polymer is also similar to that in 1, as shown in Figure 4.
The weak axial interactions are Mn(1)–O(30)a 2.576(4) Å
(a: 1 – x, –y, –z) and Mn(2)–O(50)b 2.729(4) Å (b: 1 – x,
1 – y, 1 – z) and the two Mn···Mn distances are 3.387(2)
and 3.525(2) Å. The Mn(1)···Mn(2) distance within the di-
nuclear unit is 5.737(3) Å. In addition to the weak phenoxo
bridge, the dinuclear units are also stacked with respect to
each other and stabilized by π···π interactions between the
phenyl rings [ring R1: C(24)–C(25)–C(26)–C(27)–C(28)–
C(29) with ring R2: C(12)–C(13)–C(14)–C(15)–C(16)–
C(17)] with a centroid–centroid distance of 3.663(4) Å (Fig-
ure S3, Supporting Information); the dihedral angle be-
tween the rings is 10.7° and the slip angle is 22.6°. Similarly,
Figure 5. The dimeric structure of 3 with the ellipsoids drawn at
the 30% probability level.
The packing of the dimers in the crystal structure of 3 is
however somewhat different than in 1 and 2, as O(30)a is
2.493(2) Å from the vacant axial position on Mn(1) and
O(50)b is 3.621(2) Å from Mn(2). In no way can this latter
distance be considered even as a weak interaction, and
therefore 3 is best considered to be a tetramer, as shown in
Figure 6. The associated Mn···Mn distances are 3.364(1)
and 4.087(1) Å. The distance between Mn(1) and Mn(2)
within the carboxylate-bridged dimeric unit is 5.478(3) Å.
As in 1 and 2, in addition to phenoxo bridges, two dinuclear
Figure 4. A view of the polymeric structure of complex 2. Ellipsoids
are drawn at the 30% probability level. The weak Mn(2)–O(50) and
Mn(1)–O(30) interactions are shown as dotted lines.
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Antiferromagnetic Phase Transitions in MnIII-Salen Complexes
units are also held together by π···π interactions (Figure S4, to be acetate-bridged dimers. The association of the dimeric
Supporting Information) between phenyl rings R1 and R2 units via the phenoxo bridge of any of these complexes was
of the salen ligands [R1: C(24)–C(25)–C(26)–C(27)–C(28)– not analyzed, although the deposited CIF associated with
C(29), R2: C(12)–C(13)–C(14)–C(15)–C(16)–C(17)] with a ref.[16] (for ref.[17] the atomic coordinates for the reported
centroid–centroid distance of 3.806(19) Å, and a dihedral structures are not deposited) reveals that it is a 1D polymer
angle between the rings of 14.3° and a slip angle of 26.0°.
like 1 and 2 with long Mn–O(ligand) distances, 2.453 and
2.686 Å, involving the vacant axial positions of the metal
coordination sphere. The magnetic properties of these com-
plexes were not explored.
Magnetic Study for Compounds 1, 2 and 3
We measured the magnetic properties of 1–3 on a
SQUID magnetometer with an applied field of 0.5 T. These
compounds consist of dimeric motifs, and the magnetiza-
tion and susceptibility were analyzed based on the Mn2 en-
vironments. The χmT vs. T plot for 1 is shown in Figure 7
(a) (where χm = molar magnetic susceptibility). In the high-
temperature region (Ն20 K), the data obeyed the Curie–
Weiss law, χm = C/(T – θ ), where C is the Curie constant
and θ is the Weiss temperature. The parameters were opti-
Figure 6. A view of the tetrameric structure of complex 3. Ellip-
soids are drawn at the 30% probability level. The weak Mn(1)–
O(30) interactions are shown as dotted lines.
A search of the CCDC database shows that only two mized to give C = 5.93(2) cm3 Kmol–1 and θ = –2.0(2) K.
compounds similar to those reported herein, [Mn2(salen)2- The value of C is close to the spin-only value of
[16]
(CH3COO)]ClO4
and [Mn2(7-Me-salen)2(CH3COO)]- 6.0 cm3 Kmol–1 that is expected for two high-spin d4 (S =
[17]
ClO4 are known. Both of these compounds are reported 2) contributions. The g value of 1 was 1.99. The g value is
Figure 7. (a) The χmT vs. T plot for a randomly oriented polycrystalline specimens of 1– 3. Solid lines are the best fit to Equation 2. See
the text for optimized fitting parameters; magnetization–magnetic field strength (M–H) curves for polycrystalline 1 (b) and 2 (c) measured
at 1.8 and 10 K; (d) M–H curves for polycrystalline 3 measured at 1.8, 4.5, and 10 K. Lines are drawn as a guide.
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very slightly smaller than 2, which is often observed for χmT vs. T plot of 3 exhibited no anomaly (Figure 7, a). The
high-spin Mn3+ compounds.[18] On cooling, the χmT value χmT value for this sample monotonically decreased upon
decreased, but a small hump was observed in the data at cooling. The Curie–Weiss analysis of the 20–300 K data af-
ca. 4.5 K. This behavior was well reproduced, without any forded C = 5.90(2) cm3 Kmol–1, g = 1.98, and θ =
thermal hysteresis, after heating and cooling of the sample. –5.5(1) K. The magnetization curves imply the presence of
As Figure 7 (b) shows, the magnetization (M) of 1 mea- appreciable antiferromagnetic coupling in 3 (Figure 7, d).
sured at 10 K displayed almost linear field dependence. On From a close look at the magnetization curve recorded at
the other hand, significant excess magnetization at ca. 1 T 1.8 K, we see an indication of inflated magnetization in ad-
was recorded at 1.8 K. A χmT–T anomaly is related to a dition to a Brillouin-type curve. This phenomenon was also
M–H curve anomaly, and this excess magnetization is re- observed in the data for 1 and 2, but because the tempera-
sponsible for the cusp in the χmT–T curve (Figure 7, a). At ture at which the anomaly can be observed is much lower
higher applied fields, the magnetization behavior became for 3 the magnitude of the interaction in this specimen is
identical to that recorded at 10 K, implying that the speci- smaller than for 1 and 2.
men violated the Curie–Weiss law. The excess magnetization
To investigate the presence of a magnetic phase transition
was almost constant over the 1.8–3 K range (see Figure S1, we measured FCM, remnant magnetization (RM), and
Supporting Information). The constant dM/dH slope gives zero-field-cooled magnetization (ZFCM) data for 1, 2, and
rise to a χmT value proportional to T, which is in agreement 3 (Figure 8, a). In the data for 1 a clear field-cooled magne-
with the final drop below 4.5 K in the χmT vs. T plot. The tization (FCM) peak was found at 4.0 K. No RM was ob-
magnetization at 9 T (3.2ϫ104 ergOe–1 mol–1) was con- served. The ZFCM and FCM curves for 1 coincide with
siderably smaller than the theoretical saturation magnetiza- each other. Accordingly, we can confirm that the data for
tion (4.5ϫ104 ergOe–1 mol–1). The very small slope implies this specimen showed no hysteresis. The data for compound
the presence of dominant antiferromagnetic interactions in 2 also showed no hysteresis, and a clear peak was found at
this system. No magnetic hysteresis was observed.
4.6 K in the FCM and ZFCM curves. These findings sug-
The magnetic behavior of 2 is similar to that of 1. The gest that the specimens undergo antiferromagnetic phase
χmT vs. T plot for 2 showed Curie–Weiss behavior in the transitions at the temperatures at which the peaks were ob-
high temperature region [C = 5.77(3) cm3 Kmol–1, g = 1.96, served in their data. On the other hand, compound 3
and θ = –8.6(4) K for T Ն20 K]. A χmT peak was found at showed no peak in its FCM or ZFCM data, and no RM
ca. 4.5 K. The M–H curves for 2 were also similar to those was recorded, indicating that 3 is a paramagnet down to
of 1 (Figure 7, c); almost linear field dependence was ob- 1.8 K.
served for the data recorded at 10 K, whereas an additional
Alternating current (ac) magnetometry is a versatile tool
magnetization was superposed at ca. 1 T on the data curve for determining a magnetic phase transition temperature,
recorded at 1.8 K. In contrast to the results of 1 and 2, the and for investigating single-molecule and single-chain mag-
Figure 8. (a) The FCM, RM, and ZFCM results for polycrystalline 1–3. FCM was measured at 5 Oe upon cooling, RM at zero field
upon heating after the FCM measurements, and ZFCM at 5 Oe upon heating. The FCM and ZFCM data almost coincide; (b) The ac
magnetic susceptibility curves, χacЈ (in-phase) and χacЈЈ (out-of-phase), for 1–3. The ac field was applied with an amplitude of 5 Oe and
frequencies of 10–10000 Hz. Solid and dotted lines are guides. Inset shows an expansion of the low temperate region of the χacЈЈ curve
for 3.
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2075–2085

Antiferromagnetic Phase Transitions in MnIII-Salen Complexes
nets that show slow magnetization reversal.[19] The ac mag- stroying the centrosymmetry between neighboring spins has
netic susceptibilities of 1–3 were measured (Figure 8, b), been noted[24] e.g. the bridges in low-dimensional magnets
and we confirmed that 1 and 2 showed antiferromagnetic such as carboxylate ligands[25] (like in the present study),
phase transitions (TN) at 4.0 and 4.6 K, respectively, azide,[24,26] and pyrimidine.[27,24]
whereas 3 did not. The in-phase portion (χacЈ) of the curves
There are two reasons for spin canting, one is the DM
for 1–3 follow practically the same profiles as the curves interaction and the other is single-ion anisotropy.[25,28]
recorded in the FCM and ZFCM measurements. The peak Manganese(III) compounds often exhibit pronounced mag-
positions did not alter regardless of frequency, indicating netic anisotropy defined by a negative D value (where D is
that both 1 and 2 are not single-chain/molecule magnets, the zero-field splitting parameter) in Equation (1) due to
super-paramagnets, or spin-glasses. When a fast ac fre- Jahn–Teller distortion of the metal coordination sphere.
quency was applied a small, but meaningful, peak was ob- The z-axis is defined as the magnetic easy axis at each ion.
served at 3.5 K in the out-of-phase portion (χacЈЈ) of the
data for 1. Similar behavior was found at 3.2 K in the data
for 2, but the peak is much smaller even at 10000 Hz. The
temperature at which the data for 1 and 2 become fre-
quency-dependent is lower than TN. This may be due to the
(1)
There are two crystallographically independent manga-
fact that there is possibly an activation energy associated nese ions in the unit cells of 1 and 2, and the axial direction
with the movement of the magnetic domain boundary.[20,21] on the ions is canted in a zigzag manner along the chain.
Compound 3 exhibited a decrease in χacЈ and a concomitant Such conditions will bring about a spin-canted structure as
increase in χacЈЈ at ca. 2 K when the ac frequency was ele- a result of a combination of antiferromagnetic coupling and
vated. Although this frequency dependence is small, this strong anisotropy.[24–28] The carboxylate bridge causes the
finding seems to indicate that 3 shows slow magnetization neighboring coordination basal planes to intersect at angles
reversal that is characteristic of single-molecule/single-chain of 54.9(1) and 58.7(1)° for 1 and 2, respectively, which im-
magnets (Figure 8, b, inset).
plies a theoretical maximum cant angle of 27–29°. The pres-
All the compounds showed a distinct χmT decrease below ence of antiferromagnetic coupling across the carboxylate
ca. 30 K, which indicates the presence of dominantly anti- groups is responsible for the small cant angles. In the case
ferromagnetic couplings in all of them, but residual mag- where single-ion magnetic anisotropy is operative, we have
netic moments of 1 and 2 were present below the TN, as to pay attention to the fact that the DM interaction is al-
indicated by the small hump in the magnetization curves ways operative at the same time because the lack of centro-
for these complexes. This observation can be explained symmetry is favorable for DM interactions. However, only
plausibly in terms of spin canting (Figure 9). The canting the large magnetic anisotropy can explain the spin dynam-
angles for these complexes can be estimated from the ics observed in 1 and 2 (Figure 8, b).
residual moments by the extrapolation of the magneti-
In a small applied field the canted antiferromagnetic
zation curves as H Ǟ 0. The residual moment of phases of 1 and 2 are in the ground state, but on applying a
5.7ϫ103 ergOe–1 mol–1 for 1 afforded a cant angle of 7.3°, field of Ͼ 3 Tgenuine antiferromangetic character becomes
and for 2 the residual moment of 3.6ϫ103 ergOe–1 mol–1 strong in these specimens. This feature is different from the
gave a cant angle of 4.6°.
behavior of ordinary canted antiferromagnets, since con-
ventional canted antiferromagnets usually exhibit linear
field dependence with increasing applied field.[22,23,28]
Furthermore, we find another broad hump around 6–7 T
in parts b and c of Figure 7; a simple spin-canting model
cannot explain these additional increases or decreases in
magnetization. One possible reason for these humps in the
data is the competing interaction between the zero-field
splittings and the magnetic couplings that have different
amplitudes.[29] These features seem to be characteristic of
the hierarchical interactions that are due to the low dimen-
Figure 9. Schematic drawing of spin canting.
The magnetic exchange coupling leads only to a parallel sionality of the structures.
or antiparallel spin alignment, and therefore the origin of
This semi-quantitative analysis of the exchange param-
the driving force for the spin canting is worthy of dis- eters and magneto-structural relationship requires further
cussion. The Dzyaloshinskii–Moriya (DM) interaction[22,23] comment. It is well known that carboxylate anions play a
is expressed as DDM·(SiϫSj) where the constant vector DDM role as an antiferromagnetic coupler for various transition
is approximately proportional to (Δg/g)J (where J = ex- metal ions including Mn3+ [30,31]
This bridging structures
.
change interaction constant), and Δg is the deviation of g have been classified based on syn and anti configurations,[31]
from 2.[23] In general, spin-orbit coupling affects Δg and and syn-anti bridges, as found in 1, 2 and 3, are reported
DDM, and Mn3+ coordination compounds may exhibit ap- to transmit antiferromagnetic coupling.[30] The largest cou-
preciable DM antisymmetric exchange despite g being ne- pling (J1) may be attributed to the caroxylato linkage, de-
arly equal to 2. The importance of bridging ligands in de- spite the Mn···Mn distance being longer than that of the
Eur. J. Inorg. Chem. 2011, 2075–2085
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2081

P. Kar, P. M. Guha, M. G. B. Drew, T. Ishida, A. Ghosh
FULL PAPER
(2)
phenoxo linkage. The double Mn–O(phenoxo)–Mn* link-
Compound 3 showed no peak in its FCM or χacЈ data.
ages possess centrosymmetry that is incompatible with the The magnetization and the ac in-phase susceptibility re-
canted spin structure. mained constant down to 1.8 K and finally exhibited fre-
A numerical analyses of the high temperature regions of quency dependence around 2 K, as shown in Figure 8 (b,
the χmT vs. T plots for 1–3 can give information pertaining inset). Therefore, the coupling J1 (canted antiferro.) is oper-
to the largest J parameter for these complexes. Such an ative in Mn(1)–O–C–O–Mn(2) resulting in a residual mo-
approximation is needed because there are too many pa- ment per dinuclear unit. On the other hand, J2 is negligible
rameters to enable a complete analysis to be performed. A or absent in Mn–O–Mn*. Each dinuclear unit is magneti-
simple antiferromagnetic model can be use to estimate the cally isolated; that is, the residual moments form approxi-
dominant antiferromagnetic interaction in canted antiferro- mately ideal paramagnets in the units. The observed Mn–
magnets. The values of J1 were estimated for 1–3 from the O(phenoxo) distance in 3 [3.621(2) Å], which is relatively
χmT vs. T data for T Ն 10 K on the basis of a nearest-neigh- long compared with those in 1 and 2, is consistent with
boring S = 2 dinuclear model with the Heisenberg spin- this argument. In summary, the present study of series 1–3
Hamiltonian H = –2J1 S1·S2. By using Equation (2) we ob- provides a typical discussion of the magneto-structure rela-
tained J1/kB = –0.41(4), –1.58(3), and –1.11(1) K for 1, 2, tionship; the Mn–O–Mn* distance is an important and sen-
and 3, respectively.
sitive variable, not only for intrachain magnetic coupling,
We attempted to estimate J1 more precisely by introduc- but also for bulk magnetism and single-molecule/single-
ing the parameter D into the analyses.[33] However, the opti- chain magnet behavior.
mized parameters were unrealistic when compared with the
typical values for dinuclear Mn3+ complexes,[33c] presum-
Conclusions
ably because of possible over-parameterization of the small
We have synthesized, by varying the carboxylate group,
and monotonic decrease of the χmT vs. T profiles down to
three very rare alternating carboxylate- and phenoxo-
10 K.
bridged MnIII complexes of salen. The crystal structures re-
The axial Mn–O(phenoxo)–Mn* linkage can aid ferro-
veal that in all three complexes the bridging mode of the
magnetic coupling (defined by the coupling parameter J2).
carboxylate remains the same (syn-anti), but the angle of
If the ferromagnetic interaction across the phenoxo bridge
intersection between the two equatorial planes of the carb-
was dominant in these complexes, the χmT value would
oxylate-bridged MnIII ions varies systematically. This varia-
show an increase upon cooling of the compounds. There-
tion seems to have appreciable effect on the association of
fore, for 1–3 J2 is semi-quantitatively determined to be
the dinuclear molecules through axial Mn···O(phenoxo) in-
smaller in absolute magnitude than J1.
teractions to form the 1D chain structure; in 3, in which
Several oxo-bridged dinuclear Mn3+ systems are known
the intersection angle is the largest, propagation of chain is
to possess ground high-spin states.[32] Furthermore, a
terminated after each tetranuclear entity. From the mag-
number of dinuclear Mn3+ compounds with salen-based li-
netic point of view, the present systems have been shown to
gands have been characterized as ground high-spin mole-
possess two types of magnetic ground states as a result of
cules.[5,6] The first reported single-chain magnet,
the balance between antiferromagnetic and ferromagnetic
[Mn3+2Ni2+], also involves ferromagnetically related salen-
couplings in the alternating S = 2 chains, which in turn
based Mn2 building blocks.[34] These reports support our
depend upon the geometries of the Mn–O(phenoxo)–Mn
hypothesis that ferromagnetic Mn–O–Mn* coupling is pres-
and Mn–O–C–O–Mn bridges. The assignment of magnetic
ent in compounds 1–3. Note that isolated dinuclear Mn3+
couplings to the geometrical structures has been proven by
building blocks involving salen-based bridges are known to
taking into consideration a spin-canted arrangement in the
be potential single-molecule magnets.[6a] It can be con-
structures. Compounds 1 and 2 exhibit long-range order,
cluded that the relatively large inter-block interactions (J1)
and the origin of the magnetic moments is due to the spin
in 1 and 2 are unfavorable for the development of single-
canting. Unfortunately, the compounds 1–3 do not show
molecule magnets. The canted spin structure forms before
hysteresis above 2 K as would be expected for a single-mole-
they can behave as single-molecule magnets.
cule/single-chain magnet. The Mn2 building block is versa-
Compounds 1 and 2 must have an interchain magnetic
tile, and the present study informs us that attention must
interaction because they exhibited long-range magnetic or-
be paid to the competition between intra- and inter-block
der. The ground states of 1 and 2 are antiferromagnetic,
interactions. The latter is unexpectedly larger than the for-
and accordingly the interchain interaction should be anti-
mer in the case of the materials reported herein.
ferromagnetic for the magnetic chains that have relatively
small moments due to spin canting. Therefore, the order of
the exchange couplings can be concluded to be as follows:
Experimental Section
|J1 (canted antiferro.)| Ͼ |J2 (ferro.)| Ͼ |Jinterchain (anti-
Starting Materials: Salicylaldehyde, 1,2-ethanediamine, benzoic
ferro.)|.
acid (HOBz), phenyl acetic acid (HOPh) and cinnamic acid
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2075–2085

Antiferromagnetic Phase Transitions in MnIII-Salen Complexes
(HOCn) were purchased from commercial sources and used as re-
ceived. All other solvents were of reagent grade and were used with-
out further purification.
Complex 3: Yield 1.61 g; 75%. C39H33ClMn2N4O10 (863.02): calcd.
C 54.28, H 3.85, N 6.49; found C 54.37, H 3.97, N 6.56. IR (KBr
pellet): ν = 1618 [ν(C=N)], 1534 [ν (C=O)], 1440 [ν (C=O),
˜
as
s
ν(ClO4 )], 1098 cm–1. λmax (nm) and εmax (dm3 mol–1) (CH3CN
–
Caution! Perchlorate salts of metal complexes with organic ligands
are potentially explosive. Only small amounts of material should be
prepared and handled with great care.
solution): 391 and 965.
Physical Measurements: Elemental analyses (C, H and N) were per-
formed with a Perkin–Elmer 240C elemental analyzer. IR spectra
in KBr (4500–500 cm–1) were recorded with a Perkin–Elmer RXI
FT-IR spectrophotometer. The electronic absorption spectra
(1000–200 nm) of the complexes were recorded in CH3CN with a
Hitachi U-3501 spectrophotometer.
Schiff-base
Ligand
N,NЈ-Bis(salicylidene)-1,2-diaminoethane
(H2salen): The tetradentate Schiff-base ligand was prepared by the
condensation of salicylaldehyde (1.05 mL, 10 mmol) and 1,2-eth-
anediamine (0.31 mL, 5 mmol) in methanol (10 mL).
{[Mn2(salen)2(OCn)](ClO4)}n (1), {[Mn2(salen)2(OPh)](ClO4)}n (2)
Static (direct current) magnetic susceptibilities on polycrystalline
samples of 1–3 were measured on a Quantum Design SQUID mag-
netometer (MPMS-7) with applied magnetic fields of 5000 and
500 Oe over a temperature range of 1.8–300 K. Typical sample
masses were 20–50 mg. The magnetic response was corrected with
diamagnetic blank data recorded for the empty sample holder. The
diamagnetic contribution of the sample itself was estimated from
Pascal’s constant. The magnetization curves were recorded from
–70 to +70 kOe. The alternating current magnetic susceptibilities
were obtained on a Quantum Design PPMS ac/dc magnetometer
with polycrystalline samples. The ac frequency was varied from 10
to 10000 Hz with an amplitude of 5 Oe. Hysteresis curves at 9 T
were recorded on the PPMS magnetometer. The magnetization was
calibrated with the SQUID results.
and {[Mn2(salen)2(OBz)](ClO4)}2 (3):
A methanolic solution
(10 mL) of the ligand H2salen (5 mmol) was added to a methanolic
solution (10 mL) of Mn(ClO4)2·6H2O (1.805 g, 5 mmol) with con-
stant stirring. After ca. 15 min methanolic solutions (5 mL) of cin-
namic acid (for 1) (0.370 g, 2.5 mmol), phenyl acetic acid (for 2)
(0.340 g, 2.5 mmol) and benzoic acid (for 3) (0.305 g, 2.5 mmol)
were added to the salen mixtures. Triethylamine (0.35 mL,
2.5 mmol) was added drop wise to each of the solutions with con-
stant stirring. The color of all the solutions turned to dark brown
immediately. Slow evaporation of the resulting brown solutions
gave dark brown microcrystalline 1–3. Once the volume of the solu-
tions were reduced to ca. 10 mL, the solids were filtered off and
washed with diethyl ether, and then redissolved in CH3CN for 1
and in CH3OH for 2 and 3. Layering these brown solutions with
Et2O gave well formed X-ray quality single crystals.
Crystallographic Studies: Crystal data for the three crystals are
given in Table 2. For 1, 2 and 3, 10199, 10076, 9809 independent
data were collected, respectively, with Mo-Kα radiation at 150 K
with an Oxford Diffraction X-Calibur CCD System. The crystals
were positioned 50 mm from the CCD detector. A total of 321
frames were measured for each crystal with a counting time of 10 s
per frame. Data analyses were carried out with the CrysAlis pro-
gram.[35] The structures were solved by direct methods with the
SHELXS97 program.[36] The non-hydrogen atoms were refined
with anisotropic thermal parameters. The hydrogen atoms bonded
Complex 1: Yield 1.39 g; 63%. C41H35ClMn2N4O10 (889.06): calcd.
C 55.39, H 3.97, N 6.30; found C 55.48, H 3.89, N 6.38. IR (KBr
pellet): ν = 1621 [ν(C=N)], 1534 [ν (C=O)], 1441 [ν (C=O),
˜
as
s
–
ν(ClO4 )], 1098 cm–1. λmax (nm) and εmax (dm3 mol–1) (CH3CN
solution): 398 and 917.
Complex 2: Yield 1.51 g; 69%. C40H35ClMn2N4O10 (877.05): calcd.
C 54.78, H 4.02, N 6.39; found C 54.84, H 4.10, N 6.45. IR (KBr
pellet): ν = 1619 [ν(C=N)], 1543 [ν (C=O)], 1438 [ν (C=O),
˜
as
s
–
ν(ClO4 )], 1094 cm–1. λmax (nm) and εmax (dm3 mol–1) (CH3CN to C atoms were included in the refinement model at geometric
solution): 395 and 930.
positions and given thermal parameters equivalent to 1.2 times
Table 2. Crystal data and structure refinement details for complexes 1–3.
1
2
3
Formula
Formula weight
Space group
C41H35ClMn2N4O10
889.06
P1
C40H35ClMn2N4O10
877.05
P1
C39H33ClMn2N4O10
829.65
P1
¯
¯
¯
Crystal system
a [Å]
b [Å]
c [Å]
α [°]
β [°]
triclinic
triclinic
triclinic
10.3388(7)
13.6814(10)
14.5854(9)
72.555(6)
69.831(6)
82.744(6)
1846.9(2)
2
1.599
0.825 (Mo-Kα)
912
0.02ϫ0.17ϫ0.18
2.37–30.00
0.048
10.207(3)
13.581(3)
14.380(4)
100.70(2)
108.22(3)
99.90(2)
1803.1(8)
2
1.615
0.843 (Mo-Kα)
900
0.02ϫ0.19ϫ0.22
2.17–30.00
0.070
10.090(3)
13.318(2)
14.441(3)
100.803(14)
107.32(2)
98.770(17)
1774.1(6)
2
1.616
0.856 (Mo-Kα)
884
0.04ϫ0.22ϫ0.22
2.65–30.0
0.042
γ [°]
V [Å3]
Z
Calcd. density (Dcalc ) [gcm–3]
Absorption coeff. (μ) [mm–1]
F(000)
Crystal size [mm3]
θ range [°]
R(int)
Unique data
10199
4865
0.0617, 0.1236
0.811
10076
3322
0.0743, 0.1851
0.736
9809
5814
0.0508, 0.1176
0.866
Data with IϾ2σ(I)
R1, wR2
GOF on F2
Eur. J. Inorg. Chem. 2011, 2075–2085
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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P. Kar, P. M. Guha, M. G. B. Drew, T. Ishida, A. Ghosh
FULL PAPER
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[12]
CCDC-788085 (for 1), -788086 (for 2) and -788087 (for 3) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
cle): M–H curves for polycrystalline 1 and 2; supramolecular chain
structure of complexes 1 and 2 and tetramer of complex 3 showing
phenoxo bridges, π···π and CH···π interactions; crystal packing dia-
grams for complexes 1, 2 and 3; experimental and simulated pow-
der X-ray diffraction patterns of complexes 1, 2 and 3.
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Acknowledgments
Paramita Kar is thankful to the Council of Scientific and Industrial
Research (CSIR), India, for research fellowship [sanction number
09/028(0733)/2008-EMR-I]. We thank Engineering and Physical
Sciences Research Council (EPSRC) and the University of Reading
for funds for the X-Calibur system. We also are thankful for finan-
cial support by the Ministry of Education, Culture, Sports, Science
and Technology, Japan (grant numbers 21110513 and 22350059).
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Received: November 19, 2010
Published Online: March 29, 2011
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2085