A μ1,1- or μ1,3-carboxylate bridge makes the difference in the magnetic properties of dinuclear MnII compounds

Article abstract of DOI:10.1039/c0dt00902d

Six new dinuclear Mn^II compounds with carboxylate bridges have been synthesized and characterized by X-ray diffraction: [{Mn(phen) ₂}₂(μ-RC₆H₄COO) ₂](ClO₄)₂ with R = 2

Full text of DOI:10.1039/c0dt00902d

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www.rsc.org/dalton | Dalton Transactions
A l1,1- or l1,3-carboxylate bridge makes the difference in the magnetic
properties of dinuclear MnII compounds†
Vero´nica Go´mez,*a Montserrat Corbella,a Merce´ Font-Bardiab and Teresa Calvetb
Received 23rd July 2010, Accepted 22nd September 2010
DOI: 10.1039/c0dt00902d
Six new dinuclear MnII compounds with carboxylate bridges have been synthesized and characterized
by X-ray diffraction: [{Mn(phen)2}2(m-RC6H4COO)2](ClO4)2 with R = 2-Cl (1), 2-CH3 (2), 3-Cl (3),
3-CH3 (4), 4-Cl (5) and 4-CH3 (6). Compounds 1 and 2 show two m1,3-carboxylate bridges in a syn–anti
mode while compounds 3–6 present a very uncommon coordination mode of the carboxylate ligand:
the m1,1-bridge. The magnetic properties of these compounds are very sensitive to the bridging mode of
the carboxylate ligands. While compounds 1 and 2 (m1,3-bridge) display antiferromagnetic interactions,
with J values of -1.41 and -1.66 cm-1, respectively, compounds 3–6 (m1,1-bridge) show ferromagnetic
interactions, with J values of 1.01, 0.98, 1.04 and 1.06 cm-1, respectively. It is worth noting that
compounds 3–6 are the first of their class to be magnetically characterized. The EPR spectra at 4 K for
compounds with antiferromagnetic coupling (1 and 2) are more complex than those for compounds
with a ferromagnetic interaction (3–6). Quite good simulations can be obtained with the ZFS
parameters of the MnII ion DMn ~ 0.095 cm-1 and EMn ~ 0.025 cm-1 for compounds 1 and 2 and DMn
0.060 cm-1 and EMn ~ 0.004 cm-1 for compounds 3–6.
~
Introduction
Manganese-based coordination compounds are of great interest
not only due to their magnetic properties1 but also because of
their importance in bioinorganic chemistry, as many manganese
compounds are being synthesized to mimic several metalloproteins
such as catalase2 and the oxygen-evolving complex (OEC) of
photosystem II.3
Carboxylate ligands are excellent groups due to their biological
relevance and their wide variety of coordination modes, such
Fig. 1 Carboxylate binding modes involved in the compounds reported
as monodentate terminal, chelating, bidentate bridging and
monodentate bridging modes. There are diverse dinuclear MnII
complexes reported in the literature showing only carboxylate
bridges, whether one,4 two,5 three6 or even four,7 as well as other
bridging ligands besides carboxylate ligands, like aquo, hydroxo
or phenoxo/alkoxo.
Regarding dinuclear MnII compounds with just two carboxylate
bridges, the most common coordination mode is the syn–anti m1,3-
bridging mode (Fig. 1). Although the number of this type of
compound is relatively large,8–10 not many of them are magnetically
characterized10 and there are even fewer with their EPR spectrum
reported.9k,10d,e
here.
Dinuclear MnII compounds displaying two carboxylate ligands
in a monodentate bridging mode (m1,1) (Fig. 1) are very rare.
To our knowledge, there are very few complexes,11–13 and in
most of them the carboxylate groups form part of polydentate
ligands.11,12 However, this binding mode can be found in some 1D
MnII systems.14 Nevertheless, there are very few compounds with
their magnetic properties studied12,14c–e or characterized by EPR
spectroscopy.13b,c
In this paper, we report the synthesis of six new dinuclear
MnII compounds with two carboxylate bridges with formula
[{Mn(phen)2}2(m-RC6H4COO)2](ClO4)2 with R = 2-Cl (1), 2-CH3
(2), 3-Cl (3), 3-CH3 (4), 4-Cl (5) and 4-CH3 (6). All compounds (1–
6) have been structurally characterized by X-ray diffraction and
magnetically studied, leading to surprising results.
aDepartament de Qu´ımica Inorga`nica, Facultat de Qu´ımica, Universitat
de Barcelona, Mart´ı i Franque`s 1-11, 08028, Barcelona, Spain. E-mail:
veronica.gomez@qi.ub.es; Fax: +34934907725; Tel: +34934039144
bCristal.lografia, Mineralogia i Dipo`sits Minerals, Universitat de Barcelona,
Mart´ı i Franque`s s/n, 08028, Barcelona, Spain
Experimental
† Electronic supplementary information (ESI) available: Fig S1 View of
p-stacking and C–H ◊ ◊ ◊ p interactions between dinuclear units present in
compound 2. Fig. S2 View of p-stacking and C–H ◊ ◊ ◊ C–H interactions
between dinuclear units present in compound 4. Fig. S3 View of interac-
tions between perchlorate anions and dinuclear units present in compound
6. Fig. S4 Mn–Ob–Mn angle vs. Mn ◊ ◊ ◊ Mn distance plot for compounds
with [Mn2(m-O)2] core. Fig. S5 Plot of Mn–Ob vs. Mn ◊ ◊ ◊ Mn distances
for compounds with [Mn2(m-O)2] core. CCDC reference numbers 785947–
785952. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0dt00902d
Synthesis
All manipulations were carried out at room temperature under
aerobic conditions. Reagents and solvents were obtained from
commercial sources and used without further purification. The
Mn(n-RC6H4COO)2·mH2O (n = 2, 3, 4 and R = Cl, CH3) were
synthesized by the reaction of MnCO3 and n-RC6H4COOH in
11664 | Dalton Trans., 2010, 39, 11664–11674
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boiling water. After several hours, the solution was filtered and
concentrated, giving a pale pink precipitate of the desired product.
Yields were calculated from the stoichiometric reaction.
by filtration and dried in air. Yield 0.87 g, 84%. Anal. Calcd. for
C62H40Cl4Mn2N8O12·CH3CH2OH (1386.78) (%): C, 55.43; H, 3.34;
Cl, 10.23; N, 8.08. Found: C, 55.5; H, 3.1; Cl, 10.0; N, 8.1. Yellow
crystals suitable for X-ray diffraction were obtained by mixing a
CH3CN solution of compound 5 with CH2Cl2 and layering this
mixture with hexanes. Selected IR data (KBr pellet, cm-1): 3444
(m), 3083 (w), 3057 (w), 1609 (s), 1592 (m), 1567 (w), 1519 (w),
1429 (s), 1302 (s), 1145 (w), 1090 (vs), 865 (w), 850 (s), 782 (m),
729(s), 623 (m).
[{Mn(phen)2}2(l-2-ClC6H4COO)2](ClO4)2
(1). Mn(2-
ClC6H4COO)2·2H2O (0.60 g, 1.50 mmol), NaClO4·H2O (0.21 g,
1.50 mmol) and 1,10-phenanthroline (0.59 g, 3.00 mmol), all
dissolved in absolute ethanol, were mixed (total volume ~ 75 mL)
and stirred for 15 min. The yellow solid formed was isolated by
filtration and dried in air. Yield 0.90 g, 88%. Anal. Calcd. for
C62H40Cl4Mn2N8O12·H2O (1358.73) (%): C, 54.80; H, 3.12; Cl,
10.44; N, 8.25. Found: C, 54.7; H, 3.1; Cl, 10.4; N, 8.0. Yellow
crystals suitable for X-ray diffraction were obtained after several
days by slow evaporation, in the refrigerator, of a solution of
the dinuclear compound in 15 mL CH3CN and a drop of water.
Selected IR data (KBr pellet, cm-1): 3417 (m), 3068 (w), 1601 (vs),
1516 (s), 1426 (s), 1401 (s), 1144 (m), 1089 (vs), 1050 (w), 864 (w),
845 (m), 759 (w), 725 (m), 638 (w), 623 (m).
[{Mn(phen)2}2(l-4-CH3C6H4COO)2](ClO4)2 (6). The same
procedure for the preparation of 2 was followed, using Mn(4-
CH3C6H4COO)2·2H2O (0.54 g, 1.50 mmol) in this case. The pale
yellow solid formed was isolated by filtration and dried in air. Yield
0.80 g, 82%. Anal. Calcd. for C64H46Cl2Mn2N8O12 (1299.87) (%):
C, 59.14; H, 3.57; Cl, 5.45; N, 8.62. Found: C, 59.1; H, 3.6; Cl, 5.3;
N, 8.6. Yellow needle-like crystals suitable for X-ray diffraction
were obtained by mixing a CH3CN solution of compound 6 with
CH2Cl2 and layering this mixture with hexanes. Selected IR data
(KBr pellet, cm-1): 3442 (s), 3081 (w), 3056 (w), 1623 (m), 1606 (vs),
1570 (m), 1519 (m), 1430 (s), 1367 (w), 1308 (vs), 1295 (vs), 1224
(w), 1211 (w), 1176 (w), 1145 (m), 1099 (vs), 1018 (w), 963 (w), 865
(m), 854 (s), 775 (s), 730 (vs), 723 (s), 639 (w), 623 (m), 611 (w).
[{Mn(phen)2}2(l-2-CH3C6H4COO)2](ClO4)2 (2). The same
procedure for the preparation of 1 was followed, using Mn(2-
CH3C6H4COO)2·2H2O (0.54 g, 1.50 mmol) in this case. The yellow
solid formed was isolated by filtration and dried in air. Yield 0.76 g,
78%. Anal. Calcd. for C64H46Cl2Mn2N8O12 (1299.88) (%): C, 59.14;
H, 3.57; Cl,5.45; N, 8.65. Found: C, 59.2; H, 3.6; Cl, 5.4; N, 8.7.
Yellow crystals suitable for X-ray diffraction were obtained after
several days by slow evaporation of the ethanolic mother liquor
in the refrigerator. Selected IR data (KBr pellet, cm-1): 3434 (m),
3067 (w), 2923 (w), 1623 (w), 1604 (m), 1583 (s), 1560 (s), 1516
(m), 1425 (s), 1403 (w), 1144 (w), 1120 (s), 1104 (vs), 864 (w), 848
(m), 751 (w), 728 (m), 637 (w), 623 (m).
Physical measurements
Analyses of C, H, N and Cl were carried out by the “Servei de Mi-
croana`lisi” of the “Consell Superior d’Investigacions Cient´ıfiques
(CSIC)”. Infrared spectra were recorded on KBr pellets, in the
range 4000–400 cm-1, with a Thermo Nicolet Avatar 330 FT -
IR spectrometer. Magnetic susceptibility measurements between
2 and 300 K were carried out in a Quantum Design MPMP
SQUID Magnetometer at the “Unitat de Mesures Magne`tiques
(Universitat de Barcelona)”. Two different magnetic fields were
used for the susceptibility measurements, ~ 200 G (2–5 K)
and 3000 G (2–300 K), with superimposable graphs. Pascal’s
constants were used to estimate the diamagnetic corrections for
the compound. The fit was performed by minimising the function
R = R [(cMT)exp - (cM T)calc]2/R(cMT)exp]2. Solid-state EPR spectra
were recorded at X-band (9.4 GHz) frequency using a Bruker
ESP-300E spectrometer, from room temperature to 4 K at the
“Unitat de Mesures Magne`tiques (Universitat de Barcelona)”.
The computational package easyspin15 was used to simulate the
solid-state EPR spectra of dinuclear compounds, considering all
the spin states and including the magnetic interaction between
the MnII ions (J) and the zero-field splitting parameters for each
manganese ion (DMn and EMn) in the calculations.
[{Mn(phen)2}2(l-3-ClC6H4COO)2](ClO4)2 (3). An analogous
procedure for the preparation of 1 was followed, using Mn(3-
ClC6H4COO)2·2H2O (0.60 g, 1.50 mmol) in this case. A yellow
solid was formed, isolated by filtration and dried in air. Yield
0.88 g, 88%. Anal. Calcd. for C62H40Cl4Mn2N8O12 (1340.71) (%): C,
55.54; H, 3.00; Cl, 10.58; N, 8.36. Found: C, 55.4; H, 3.0; Cl, 10.5;
N, 8.2. Yellow crystals suitable for X-ray diffraction were obtained
after several days by slow evaporation of a CH3CN solution of
compound 3 with a drop of water in the refrigerator. Selected IR
data (KBr pellet, cm-1): 3444 (m), 3061 (w), 1616 (s), 1592 (m),
1566 (m), 1519 (m), 1427 (s), 1310 (vs), 1260 (m), 1147 (m), 1085
(vs), 866 (m), 852 (vs), 779 (w), 766 (m), 730 (s), 640 (w), 623 (s).
[{Mn(phen)2}2(l-3-CH3C6H4COO)2](ClO4)2 (4). The same
procedure was followed as for compound 2, using Mn(3-
CH3C6H4COO)2·2H2O (0.54 g, 1.50 mmol) in this case. A yellow
solid was formed, isolated by filtration and dried in air. Yield
0.75 g, 77%. Anal. Calcd. for C64H46Cl2Mn2N8O12 (1299.87) (%):
C, 59.14; H, 3.57; Cl, 5.45; N, 8.62. Found: C, 59.2; H, 3.6; Cl, 5.3;
N, 8.4. Yellow crystals suitable for X-ray diffraction were obtained
by mixing a CH3CN solution of compound 4 with CH2Cl2 and
layering this mixture with hexanes. Selected IR data (KBr pellet,
cm-1): 3416 (s), 3062 (w), 1616 (s), 1599 (m), 1580 (m), 1518 (m),
1427 (s), 1391 (w), 1314 (m), 1282 (w), 1213 (w), 1146 (w), 1097
(vs), 865 (w), 852 (m), 778 (w), 761 (m), 730 (s), 623 (m).
X-ray diffraction data collection and refinement
X-ray diffraction measurements and resolution of compounds 1–
5 were carried out at the “Unidade de Raios X (Universidade de
Santiago de Compostela)” while compound 6 was elucidated at
the “Unitat de Difraccio´ de Raigs X (Serveis Cientificote`cnics,
Universitat de Barcelona)”. Crystals of compounds 1–5, obtained
as described in the experimental section, were mounted onto
a sealed tube and X-ray crystallographic data were collected
at 100 K. All measurements were made on a Bruker Apex-
II CCD diffractometer with graphite monochromated Mo-Ka
[{Mn(phen)2}2(l-4-ClC6H4COO)2](ClO4)2 (5). An analogous
procedure was followed as for 1, using Mn(4-ClC6H4COO)2·2H2O
(0.60 g, 1.50 mmol) in this case. A yellow solid was formed, isolated
˚
radiation (l = 0.7107 A). The structures were solved using the
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Table 1 Crystallographic data for compounds 1–6
1·6CH3CN
2·2CH3CH2OH
3
4
5
6
Chemical formula
C74H58Cl4Mn2-
N14O12
C68H58Cl2Mn2-
N8O14
C62H40Cl4Mn2-
N8O12
C64H46Cl2Mn2-
N8O12
C62H40Cl4Mn2-
N8O12
C64H46Cl2Mn2-
N8O12
Formula weight
Crystal colour, habit
T/K
1587.02
yellow, prism
100(2)
1392.00
yellow, prism
100(2)
0.7107
triclinic
¯
P1
1340.70
yellow, prism
100(2)
0.7107
triclinic
¯
P1
1299.87
colourless, prism
100(2)
0.7107
triclinic
¯
P1
1340.70
colourless, prism
100(2)
0.7107
monoclinic
1299.87
yellow, prism
248(2)
0.7107
monoclinic
˚
l (Mo-Ka)/A
0.7107
Crystal system
Space group
triclinic
¯
P1
C2/m
C2/m
Crystal size/mm
0.44 ¥ 0.29 ¥ 0.14 0.46 ¥ 0.32 ¥ 0.18 0.34 ¥ 0.30 ¥ 0.05 0.33 ¥ 0.17 ¥ 0.17 0.18 ¥ 0.09 ¥ 0.09 0.2 ¥ 0.1 ¥ 0.1
˚
a/A
12.6230(6)
13.0026(3)
13.3200(4)
116.6890(10)
106.457(2)
94.124(2)
1821.98(10)
1
1.446
0.57
814
2.4 to 26.4
h = -15 → 14
k = -16 → 14
l = 0 → 16
9.4995(3)
13.1136(4)
14.1160(4)
111.089(2)
95.032(2)
102.951(2)
1571.31(9)
1
1.471
0.56
718
2.7 to 24.4
h = -11 → 11
k = -15 → 14
l = 0 → 17
5947/4/436
1.09
9.5307(2)
12.0676(2)
13.4362(3)
65.6420(10)
88.2400(10)
84.0110(10)
1399.96(5)
1
1.590
0.72
682
2.7 to 30.5
h = -13 → 13
k = -15 → 17
l = 0 → 19
8521/0/455
1.05
9.5435(4)
12.0805(5)
13.3549(5)
66.425(2)
88.325(2)
84.093(2)
1403.57(10)
1
1.538
0.62
666
2.7 to 25.8
h = -11 → 11
k = -15 → 15
l = -16 → 16
5596/189/462
1.05
14.585(2)
19.584(3)
9.5953(13)
90
96.055(9)
90
2725.4(7)
2
1.634
14.975(8)
20.112(6)
9.582(5)
90
96.48(3)
90
2867(2)
2
1.506
0.61
˚
b/A
˚
c/A
a /◦
b /◦
g /◦
3
˚
V/A
Z
rc/g cm-3
m/mm-1
0.74
1364
F(000)
1332
q range/◦
Index ranges
2.6 to 25.5
h = = -18 → 18
k = 0 → 24
l = 0 → 11
2878/0/237
1.28
2.6 to 32.3
h = -22 → 22
k = -30 → 30
l = -12 → 13
4730/2/233
1.10
Data/restraints/parameters 7427/0/481
Goodness-of-fit on F2
1.01
0.0352, 0.0971
0.0468, 0.1004
b
R1a, wR2 [I > 2s(I)]
0.033, 0.0788
0.0396, 0.0817
0.0336, 0.086
0.0437, 0.091
0.0374, 0.0838
0.0588, 0.0920
0.086, 0.2048
0.1063, 0.2106
0.0625, 0.1512
0.1129, 0.1709
b
R1a, wR2 (all data)
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = ꢀFo| - |Fcꢀ/ |Fo|. b wR2 = { [w(Fo - Fc )2]/ [w(Fo2)2]}1/2, w = 1/[s (Fo2) + (aP)2 + bP], where P = [max(Fo2,0) + 2Fc ]/3.
2
2
2
2
SIR97 program16 and refined by full-matrix least-squares method
using the SHELXL97 program.17 Hydrogen atoms were treated
by a mixture of independent and constrained refinement. A
crystal of compound 6, obtained as described in the experimental
section, was mounted on a MAR345 diffractometer with an image
plate detector and graphite monochromated Mo-Ka radiation
substituent in the ortho position show a m1,3-coordination mode
while compounds with R in the meta or para position show
a m1,1-coordination mode. This different coordination mode of
the carboxylate ligand can be observed in the IR spectra. These
compounds show different bands in the range 1620–1300 cm-1
due to the carboxylate groups. For compounds 1 and 2, two strong
bands at ~ 1603 and ~ 1403 cm-1 can be assigned to the asymmetric
and symmetric vibrations from the carboxylate groups, with values
of D = na(COO) - ns(COO) of ~ 200 cm-1, which are indicative of
carboxylate ligands coordinated in a bidentante bridging mode
(m1,3).20 However, the IR spectra of compounds 3–6 show these
two strong bands at ~ 1612 and ~ 1309 cm-1, with a D value of
~ 303 cm-1, indicating that the carboxylate ligands are bridged in
a monatomic mode (m1,1).20 On the other hand, broad bands at ~
1100 cm-1 and a moderate intensity band at 623 cm-1 are assigned
to the perchlorate ions. The phen ligand shows characteristic bands
at 1518, 1427, 865, 850 and 723 cm-1.
˚
(l = 0.7107 A). The structure was solved using the SHELXS
program18 and refined by a full-matrix least-squares method with
the SHELX97 program.17 Hydrogen atoms were computed and
refined using a riding model with an isotropic temperature factor
equal to 1.2 times the equivalent temperature factor of the atom
which is linked. Crystal data collection and refinement parameters
for all compounds (1–6) are given in Table 1.
Results and discussion
Synthesis
The formation of the dinuclear complex could be explained
by the assembly of two mononuclear fragments, both with a
monodentate carboxylate. This assembly can be through the O2
atom, leading to a m1,3-bridging mode, or through the O1 atom,
which leads to a m1,1-bridge (Scheme 1). Two factors can contribute
to this different behavior: the steric and the electronic effects.
The electronic effect of the R group (Me or Cl) is very different,
however the coordination mode of the carboxylate depends on the
position of the substituent (ortho or meta and para) in the phenyl
ring. So, the steric effect appears to be the most important factor
in the type of bridge formed in the dinuclear complex. Moreover,
it is interesting to note that analogous compounds to 1, 3 and 5
with 2,2¢-bipyridine (bpy) instead of phen show a m1,3 coordination
In previous papers we reported that the reaction between
manganese(II) carboxylates and bidentate nitrogenated ligands
(NN) leads to different types of compounds depending on the
-
MnII/NN ratio and the presence or absence of ClO4 .10d,e,19
In the present work, we carried out the reaction between
Mn(RC6H4COO)2·mH2O (R = 2-Cl, 2-CH3, 3-Cl, 3-CH3, 4-
Cl, 4-CH3), 1,10-phenanthroline (phen) and NaClO4 in a
1 : 2 : 1 ratio, obtaining ionic dinuclear compounds with formula
[{Mn(phen)2}2(m-RC6H4COO)2](ClO4)2 (1–6) in all cases. How-
ever, although the procedure followed to synthesize these dinuclear
compounds was the same, two different modes of coordination
of the carboxylate ligand are found. Compounds with the R
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Scheme 1 The two coordination modes of the carboxylate bridge found
for compounds reported here.
mode (deducible from their IR spectra),9j,10e so the phen ligand also
seems to play an important role in the different behavior observed.
Description of structures
[{Mn(phen)2}2(l-2-ClC6H4COO)2](ClO4)2·6CH3CN (1·6CH3-
CN) and [{Mn(phen)2}2(l-2-CH3C6H4COO)2](ClO4)2·2CH3CH2-
OH (2·2CH3CH2OH). The cationic structures of compounds 1
and 2 are depicted in Fig. 2. Selected bond lengths and angles are
listed in Table 2.
Fig. 2 Crystal structures of the cationic complexes of compounds 1 (top)
and 2 (bottom), showing the atom labelling scheme and ellipsoids at 50%
probability. Hydrogen atoms are omitted for clarity. Symmetry codes: (a)
-x, -y+1, -z+1, (b) -x+1, -y+1, -z+2.
Both compounds show an analogous structure, crystallizing in
II
¯
the triclinic space group P1. The Mn ions are bridged by two
m1,3-carboxylate ligands in a syn–anti mode, with a Mn ◊ ◊ ◊ Mn
˚
distance of ~ 4.72 A. The hexacoordination of each manganese ion
is completed by two phen ligands, leading to a distorted octahedral
environmentaroundMnII ions, withMn–Odistances much shorter
˚
than Mn–N distances (av. 2.136 and 2.266 A for compound 1 and
˚
2.140 and 2.259 A for compound 2, respectively). All distances
agree with those reported for analogous compounds.8–10
Compound 1 shows p-stacking between phen ligands of neigh-
bouring molecules, leading to chains (Fig. 3). These chains are
interconnected through ClO4 anions and CH3CN molecules via
Fig. 3 p-stacking between phen ligands of dinuclear units of compound
1 generating chains.
-
2 displays structural parameters very similar to compound 1,
the interactions between molecules are different. Compound 2
also shows p-stacking between phen ligands of neighbouring
complexes to generate chains, but these interactions are off-
set in this case (Fig. S1, ESI†). Moreover, there are C–H ◊ ◊ ◊ p
interactions between phen ligands of neighbouring molecules,
leading to chains in another direction. The presence of ClO4
anions (with two disordered oxygen atoms) and CH3CH2OH
molecules form the 3D system.
hydrogen bonds, generating a 3D system. Although compound
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) with standard deviations
in parentheses for compounds 1 and 2
1·6CH3CN
2·2CH3CH2OH
-
Mn(1) ◊ ◊ ◊ Mn(1a)
Mn(1)–N(1)
Mn(1)–N(12)
Mn(1)–N(15)
Mn(1)–N(26)
Mn(1)–O(31a)
Mn(1)–O(29)
4.726(5)
Mn(1) ◊ ◊ ◊ Mn(1b)
Mn(1)–N(4)
Mn(1)–N(3)
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–O(2b)
Mn(1)–O(1)
4.712(2)
2.2420(16)
2.2898(16)
2.2734(15)
2.2602(16)
2.1381(13)
2.1342(12)
2.2473(14)
2.2822(14)
2.2527(14)
2.2527(15)
2.1441(12)
2.1364(12)
[{Mn(phen)2}2(l-3-ClC6H4COO)2](ClO4)2 (3), [{Mn(phen)2}2-
(l-3-CH3C6H4COO)2](ClO4)2 (4), [{Mn(phen)2}2(l-4-ClC6H4-
COO)2](ClO4)2 (5) and [{Mn(phen)2}2(l-4-CH3C6H4COO)2]-
(ClO4)2 (6). The cationic complexes of 3 and 4 are shown in
Fig. 4 and those of 5 and 6 in Fig. 5. Selected bond lengths
and angles are given in Table 3 (compounds 3 and 4) and Table
4 (compounds 5 and 6). Compounds 3 and 4 crystallize in the
O(31a)–Mn(1)–N(15) 167.64(5)
O(2b)–Mn(1)–N(1) 168.38(5)
O(29)–Mn(1)–N(12)
N(1)–Mn(1)–N(26)
162.23(6)
163.09(6)
O(1)–Mn(1)–N(3)
N(4)–Mn(1)–N(2)
163.59(5)
168.99(6)
Symmetry codes: (a) -x, -y+1, -z+1, (b) -x+1, -y+1, -z+2.
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◦
˚
Table 3 Selected bond lengths (A) and angles ( ) with standard deviations
in parentheses for compounds 3 and 4
3
4
Mn(1) ◊ ◊ ◊ Mn(1a)
Mn(1)–N(1)
Mn(1)–N(12)
Mn(1)–N(15)
Mn(1)–N(26)
Mn(1)–O(29)
Mn(1)–O(29a)
O(29)–Mn(1)–N(12)
O(29a)–Mn(1)–N(15)
N(1)–Mn(1)–N(26)
O(29)–Mn(1)–O(29a)
Mn(1)–O(29)–Mn(1a)
3.469(1)
3.454(2)
2.256(2)
2.263(2)
2.244(2)
2.266(2)
2.2010(16)
2.1637(16)
160.26(7)
157.63(7)
159.44(7)
75.37(7)
104.63(7)
2.2563(11)
2.2596(11)
2.2412(11)
2.2655(11)
2.1667(9)
2.2065(10)
160.75(4)
157.56(4)
159.90(4)
75.03(4)
104.97(4)
Fig. 4 Crystal structures of the cationic complexes of compounds 3 (left)
and 4 (right), showing the atom labelling scheme and ellipsoids at 50%
probability. Hydrogen atoms are omitted for clarity. Symmetry codes: (a)
1-x, -y,1-z.
Symmetry codes: (a) 1-x, -y,1-z.
◦
˚
Table 4 Selected bond lengths (A) and angles ( ) with standard deviations
in parentheses for compounds 5 and 6
5
6
Mn(1) ◊ ◊ ◊ Mn(1b)
Mn(1)–O(23)
Mn(1)–N(1)
3.415(2)
2.189(4)
2.262(5)
2.237(6)
161.68(15)
161.55(21)
77.46(0)
102.54(1)
3.460(1)
2.1991(16)
2.280(2)
Mn(1)–N(12)
2.267(2)
N(12)–Mn(1)–O(23a)
N(1)–Mn(1)–N(1a)
O(23)–Mn(1)–O(23a)
Mn(1)–O(23)–Mn(1b)
160.44(8)
160.72(11)
76.24(10)
103.76(10)
Symmetry codes: (a) -x, y, -z, (b) -x,1-y, -z.
dinuclear units interact by p-stacking between phen ligands and
phenyl rings of neighbouring molecules and C–H ◊ ◊ ◊ p interactions
-
leading to layers (Fig. 6), which are interconnected via ClO4
anions generating the 3D system. Compound 4 displays 50%
disorder in the 3-methylbenzoate rings. This is not a thermal
disorder but structural, as both possibilities can not coexist due to
steric hindrance. This compound also shows p-stacking between
phen ligands of neighbouring dinuclear units, generating chains.
Fig. 5 Crystal structures of the cationic complexes of compounds 5 (left)
and 6 (right), showing the atom labelling scheme and ellipsoids at 50%
probability. Hydrogen atoms are omitted for clarity. Symmetry codes: (a)
-x, y, -z, (b) -x,1-y, -z, (c) x,1-y, z.
-
The presence of ClO4 anions lead to the 3D system via hydrogen
bonds (Fig. S2, ESI†). Compound 5 displays disorder in the
-
ClO4 anions. The dinuclear complexes form chains through C–
H ◊ ◊ ◊ Cl interactions of 4-chlorobenzoate ligands of neighbouring
molecules (Fig. 7a). These chains are interconnected via p-stacking
between phen ligands of neighbouring chains (Fig. 7b). The
3D system is obtained by C–H ◊ ◊ ◊ O interactions (Fig. 7c) and
¯
triclinic space group P1 while compounds 5 and 6 crystallize
in the monoclinic space group C2/m. All compounds show an
analogous structure. Each MnII ion coordinates two phen and
two carboxylate ligands, which bridge both MnII ions in a very
uncommon coordination mode: the m1,1. The Mn ◊ ◊ ◊ Mn distance
is ~ 3.45 A, much shorter than those for compounds 1 and 2
(~ 4.72 A), in agreement with the different bridging mode. As
has been said, the carboxylate ligands act as monatomic bridges,
with Mn–Obridge distances of ~ 2.19 A, slightly larger than those
for compounds 1 and 2 (~ 2.14 A). The Mn2O2 core is a planar
-
interactions between the dinuclear units and the ClO4 anions.
-
Compound 6 also displays disorder in the ClO4 anions, but just
˚
in the oxygen atoms. The dinuclear complexes can form chains in
all three directions through hydrogen bonds between phen ligands
and perchlorate anions, leading to a 3D system (Fig. S3, ESI†).
The m1,1-bridging mode is found in several trinuclear MnII com-
plexes with a [Mn3(RCOO)6] core.19 However in that case, there is
usually at least a weak interaction between the non-coordinated
oxygen atom and the terminal MnII ions as they do not complete
their hexacoordination. A study made by Lippard considered the
distance between the non-coordinated or dangling oxygen and
the MnII ions as a distinctive parameter of the monodentate
˚
˚
˚
rhombus, with Mn–Ob–Mn angles of ~ 105◦ for compounds
3 and 4 and slightly smaller for compounds 5 and 6 (~ 103◦).
The torsion angles between the phenyl rings and the carboxylate
groups are very small (even 0◦ for compounds 5 and 6), meaning
that the benzoate rings are perpendicular to the Mn2O2 plane.
Compound 3 displays disorder in the 3-chlorobenzoate rings,
with occupancies of 0.5205 (A atoms) and 0.4795 (B atoms). The
bridging.21 This distance is ~ 3.84 A for compounds 3 and 4 and
˚
˚
~ 3.77 A for compounds 5 and 6, too large for an interaction,
11668 | Dalton Trans., 2010, 39, 11664–11674
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Fig. 6 p-stacking and C–H ◊ ◊ ◊ p interactions between dinuclear units of
compound 3.
Fig. 8 cM T vs. T and cM vs. T (inset) plots for compounds 1 (ꢀ) and 2
(᭺). The solid line is the best fit to the experimental data.
8.75 cm3mol-1K assuming g = 2). As the temperature decreases,
the cM T values fall until reaching 0.92 (1) and 1.05 cm3 mol-1
K (2) at 2 K, indicative of an antiferromagnetic coupling. This
behavior is also observed in cM vs. T plots (inset Fig. 8). At 300 K,
c
M values are 0.03 cm3 mol-1 for both compounds, increasing with
temperature until reaching a maximum of 0.50 cm3 mol-1 at ~ 5.5 K
for compound 1 and 0.37 cm3mol-1 at ~ 6.0 K for compound 2. The
experimental data were fitted by using the spin Hamiltonian H =
-JS1·S2 and the corresponding susceptibility expression for two
high spin MnII ions.22 For compound 1, the best fit for cM T (and
cM ) data was obtained with J = -1.41 (-1.30) cm-1, g = 2.05 (2.01)
and R = 1.07 ¥ 10-4 (1.41 ¥ 10-4). For compound 2, the best fit for
cM T (and cM ) data was obtained with J = -1.66 (-1.71) cm-1, g =
2.00 (1.98) and R = 6.47 ¥ 10-4 (9.25 ¥ 10-4). These J values agree
with the range ~ 0 to -1.94 cm-1 found in the literature for other
MnII compounds with two syn–anti carboxylate bridges.9j,10b–e
There are two kinds of compounds reported in the literature
where the MnII ions are bridged only through two carboxylate
ligands coordinated in a syn–anti mode: dinuclear complexes and
one-dimensional systems. Table 5 summarize the most important
structural data for these compounds and their magnetic coupling
constants.
All compounds show a weak (or negligible) antiferromagnetic
coupling, with J values between 0 and -2 cm-1. The difference
between these compounds is in the RCOO bridge and/or the
capping ligand (phen, bpy or tpa). Therefore, there are two
factors that could affect the magnetic properties: the electronic
effects due to the different carboxylate bridge and the structural
parameters. It is interesting to note that compounds 1 and D,
with the same 2-ClC6H4COO bridging ligand, show different
degrees of magnetic coupling, the compound with bpy being a
little more antiferromagnetic. These compounds show appreciable
differences in their structural parameters. On the other hand, the
one-dimensional systems G and H, with the same 3-ClC6H4COO
bridging ligand, show similar J values, in agreement with the
similarity of their structural parameters. The difference between
the one-dimensional systems and the dinuclear complexes is the
presence of one (in 1D systems) or two (in the dinuclear complexes)
bidentate capping ligands coordinated to the MnII ions. The
coordination of two phen ligands, more rigid than bpy ligands,
Fig. 7 C–H ◊ ◊ ◊ Cl interactions (a), p-stacking (b) and C–H ◊ ◊ ◊ O interac-
tions (c) between dinuclear units of compound 5. Only hydrogen atoms
involved in contacts are not omitted.
which indicate that these four compounds reported here represent
dinuclear MnII complexes containing pure monodentate bridging
carboxylates. Moreover, in these compounds, the MnII ions are
already hexacoordinated.
As has been indicated, dinuclear MnII compounds displaying
this type of bridging mode are found in very few cases, the
carboxylate group usually forming part of polydentate ligands,
which are not comparable because they may possibly adopt
this coordination mode due to steric hindrance or preferences
for other coordinating atoms. There are just three dinuclear
compounds reported in the literature similar enough to compare
the structural parameters.13 The compounds reported here (3–6)
˚
show the shortest Mn ◊ ◊ ◊ Mn distance, with a value of ~ 3.45 A,
˚
which is outside the range 3.525–3.589 A. They also display a large
˚
˚
C–Ob distance, ~ 1.31 A compared to the range 1.165–1.281 A,
˚
and a very large Mn–Od distance, ~ 3.80 A in comparison with
˚
the range 3.405–3.538 A. All the other distances and angles agree
with those reported in the literature.
Magnetic properties
Magnetic susceptibility data were recorded for all compounds (1–
6) from room temperature to 2 K. cM T vs. T and cM vs. T plots
of compounds 1 and 2 are displayed in Fig. 8. Both compounds
show analogous magnetic behavior. At 300 K, the cM T values are
9.21 and 8.59 cm3 mol-1 K for compounds 1 and 2 respectively,
close to the typical value for two uncoupled MnII ions (S = 5/2,
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Table 5 Selected structural and magnetic data for MnII systems with a [Mn2(m-RCOO)2]2+ core
b
c
d
e
Compounda
danti
dsyn
a
b
Jf/cm-1
Ref
Dinuclear complexes
[Mn2(ClCH2COO)2(phen)4](ClO4)2
[Mn2(Hphth)2(phen)4](Hphth)2
[Mn2(2-ClC6H4COO)2(phen)4](ClO4)2
[Mn2(2-CH3C6H4COO)2(phen)4](ClO4)2
[Mn2(m-C6H4COO)2(bpy)4](ClO4)2
[Mn2(m-2-ClC6H4COO)2(bpy)4](ClO4)2
[Mn2(CH3COO)(tpa)](TCNQ)2
A
B
1
2.129
2.134
2.134
2.137
2.139
2.199
2.160
2.150
2.118
2.138
2.144
2.118
2.117
2.060
99.9
94.7
94.26
95.8
103.6
96.4
138.2
128.3
123.89
127.4
151.9
130.9
133.5
0
10d
10c
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10e
9j,10e
10b
-0.34
-1.41
-1.66
-1.76
-1.79
-1.94
2
C
D
E
107.6
1D systems
[Mn(ClCH2COO)2(phen)]n
[Mn(bpy)(3-ClC6H4COO)2]n
[Mn(3-ClC6H4COO)2(phen)]n
F
G
H
2.183
2.196
2.200
2.134
2.104
2.091
97.9
100.5
100.4
123.9
134.4
136.1
-0.89
-1.72
-1.80
10d
10e
19
a Compounds: Hphth = monodeprotonated phthalate; tpa = tris(2-pyridylmethyl)-amine; TCNQ = tetracyanoquinodimethane. b Mn–Oanti distance.
c Mn–Osyn distance. d Oanti–Mn–Osyn angle. e Mn–Osyn–C angle. f Magnetic exchange coupling parameter based on the Hamiltonian H = -JS1·S2.
could explain the greater distortion of the octahedral coordination
and, consequently, more differences in the structural parameters
of the carboxylate bridges.
cMT vs. T plots of the remaining dinuclear MnII compounds are
depicted in Fig. 9 (compounds 3 and 4) and Fig. 10 (compounds
5 and 6). All compounds show analogous magnetic behavior. At
300 K, the cM T values are 9.41, 9.55, 8.89 and 9.28 cm3 mol-1 K for
compounds 3–6 respectively, close to the expected value for two
uncoupled high spin MnII ions (S = 5/2, 8.75 cm3mol-1K assuming
g = 2).
Fig. 10 cM T vs. T and M vs. H (inset) plots for compounds 5 (᭢) and 6
(ꢁ). The solid line is the best fit to the experimental data.
Table 6 Parameters obtained from the fit of cM T data of compounds 3–6
Compound
J/cm-1
zJ¢/cm-1
g
R
3
4
5
6
1.01
0.98
1.04
1.06
-0.015
-0.019
-0.026
-0.018
2.03
2.05
1.99
2.02
1.93 ¥ 10-5
5.01 ¥ 10-5
4.65 ¥ 10-5
3.12 ¥ 10-5
Fig. 9 cM T vs. T and M vs. H (inset) plots for compounds 3 (ꢀ) and 4
(♦). The solid line is the best fit to the experimental data.
this case, the experimental data were fitted by using the spin Hamil-
tonian H = -JS1·S2 and taking into account intermolecular inter-
actions (zJ¢);23 the best fit parameters obtained are listed in Table 6.
As has been indicated, there are very few dinuclear compounds
with MnII ions bridged through two m1,1-carboxylate ligands analo-
gous to those reported here, just three, and none of them is magnet-
ically characterized. Regarding other dinuclear complexes with the
same core but where the carboxylate bridges belong to polydentate
ligands, only three of them show hexacoordinated MnII ions,
these compounds displaying weak antiferromagnetic coupling.12a–c
However, as is shown in Table 7, there are four 1D systems which
consist of dinuclear entities with the [Mn2(m1,1-RCOO)2]2+ core
interconnected by long ligands. These compounds show ferro-
magnetic coupling, with J values in the range 0.92–1.80 cm-1.14c–e
As the temperature decreases, the cMT values increase until
reaching a maximum value of 14.00 cm3 mol-1 K at 2.5 K (3), 14.01
cm3 mol-1 K at 2.5 K (4), 13.00 cm3 mol-1 K at 3.0 K (5) and 13.75
cm3 mol-1 K at 2.7 K (6). Below this temperature, cM T values fall
slightly. This behavior is characteristic of ferromagnetic coupling,
which is confirmed by the field dependence of the magnetization at
2 K, showing M/NmB values indicative of ten unpaired electrons
(insets Fig. 9 and Fig. 10). The cM T expected value for a system
with a ground state S = 5 is 15 cm3 mol-1 K; the lower values found
at ~ 2.5 K and the decreasing of cM T below this temperature could
be explained by the existence of intermolecular antiferromagnetic
interactions or the zero-field splitting (ZFS) of the ground state. In
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Table 7 Selected structural and magnetic data for MnII compounds with a [Mn2(m-O)2] core
◦
Compounda
Mn–Ob–Mn/◦
Mn ◊ ◊ ◊ Mn/A
Mn–Ob /A
t /
Jd/cm-1
Ref
b
c
˚
˚
3
4
5
6
104.97
104.63
102.54
103.76
3.469
3.454
3.415
3.460
2.187
2.182
2.189
2.199
0
0
0
0
1.01
0.98
1.04
1.06
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Carboxylate in polydentate ligand
[Mn2(Hbida)2(H2O)2
[Mn2(H2O)4L2]
A
B
C
104.92
108.38
105.99
3.446
3.672
3.472
2.173
2.281
2.174
0
0.07
0
-0.94
-0.96
-2.60
12a
12b
12c
[Mn2(H2O)6(HL)2]
1D systems
D
E
F
[Mn(oba)(phen)(H2O)]n
[Mn(HBTC)(H2O)L]n
[Mn(HBTC)(H2O)L]n
[Mn(HBTC)(bpy)(H2O)]n
101.81
104.23
104.39
105.10
3.387
3.489
3.482
3.507
2.182
2.211
2.204
2.209
0
0
0
0
0.92
1.44
1.46
1.80
14c
14d
14d
14e
G
Alkoxo/phenoxo
[Mn2(H2L1)(CH3COO)2]
[Mn2(H2L)2Cl2]
[Mn2(CH3COO)2L]
[Mn(ema)2(H2O)]2
(TMA)2[Mn2L2]
[Mn2(py2ald)2](ClO4)2
[Mn(bpeap)(THF)]2(ClO4)2
[Mn2L1L2](BPh4)2
[Mn(SALPS)]2
H
I
103.66
105.37
102.96
102.67
101.48
104.89
99.90
100.27
101.28
103.68
102.57
3.346
3.443
3.367
3.394
3.332
3.401
3.256
3.280
3.299
3.383
3.518
2.128
2.165
2.152
2.173
2.152
2.145
2.126
2.137
2.134
2.152
2.255
0
0
0
0
> 0e
0.62
0.80
1.00
1.04
1.14
25a
25b
25c
25d
25e
25f
25g
25h
25i
J
K
L
M
N
O
P
Q
R
0.34
16.38
0
11.77
8.13
17.9
25.48
<-0.18f
-1.50
-1.88
-3.20
-4.52
[Mn2L2](ClO4)2
[Mn2L](ClO4)2
25j
25k
N-oxide
S
[Mn(hfac)2(4-cpyNO)]2
[Mn2(dmpo)4(SCN)4(H2O)2]
[Mn(hfac)2(IMHF)]2
[Mn(hfac)2AmPh]2
107.01
109.00
105.88
105.76
107.03
3.567
3.599
3.476
3.465
3.510
2.218
2.211
2.178
2.174
2.183
0
0
0
0
0.23
26a
26b
26c
26d
26e
T
-1.09
-1.72
-1.99
-3.10
U
V
W
[Mn(hfac)2(IMHBithph)]2
0.47
Phosphonate
[Mn2(bbimp)2(H2O)2]
X
102.80
3.358
2.149
0
1.44
27
a A, H3bida = N-(benzimidazol-2-ylmethyl)iminodiacetic acid; B, L = N,N-bis[(1-methylimidazol-2-yl)-methyl]glycinate; C, H3L = 2,6-dioxo-l,2,3,6-
tetrahydropyrimidine-4-carboxylic acid; D, Hoba = 4,4¢-oxybis(benzoic) acid; E, H3BTC = benzene-1,3,5-tricarboxylic acid and L = pyridine-2-
(1-methyl-1H-pyrazol-3-yl); F, L = pyridine-2-(1-methyl-4-bromo-1H-pyrazol-3-yl); H, H2L1 = bis(2,6-bis((2-hydroxypropyl-1,3-di-imino)methyl)-
4-methylphenolate); I, H3L = N-(2-hydroxy-5-nitrobenzyl-iminodiethanol); J, L = 11,23-dimetyl-25,26-dioxo-3,7,15,19-tetra-azatricyclo-hexacosa-
1,2,7,9,11,13,14,19,21-decane; K, Hema = 2-ethyl-3-hydroxy-4-pyrone; L, L = 1,3-bis(5-nitrosalicylideneimino)-2-propanolate; M, py2ald = 6-(bis(pyrid-
2-ylmethyl)aminomethyl)-2-formyl-4-methylphenolato; N, bpeap = bis(o-(bis(2-(1-pyrazolyl)ethyl)amino)phenolate); O, L1 = 1-(2-Pyridylmethylamino)-
2-((2-oxybenzyl)(2-pyridylmethyl)amino)ethane), L2 = (1-(2-pyridylmethyleneamino)-2-((2-oxybenzyl)(2-pyridylmethyl)amino)ethane); P, SALPS =
N,N¢-[1,l¢-dithiobis(phenylene)]bis(salicylideneaminate); Q, L = N,N¢-ethylene-N-(bis(2-pyridylmethyl)amino)-N¢-(eliminatedeneiminate); R, L =
bis((2,6-diformyl-4-chlorophenolate)-N,N¢-(3-((2-pyridylmethyl)aminoethyl)-1,5-diamino3-azapentane))); S, hfac = hexafluoroacetylacetonate and 4-
cpyNO = 4-cyanopyridine-N-oxide; T, dmpo = 3,5-dimethylpyridine-N-oxide; U, IMHF = 1-hydroxo-2-furfural-4,4,5,5-tetramethyl-4.5-dihydro-
1H-imidazole; V, AmPh = 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide; W, IMHBithph = 1-hydroxy-2-bithiophenal-4,4,5,5-tetramethyl-4,5-
dihydro-1H-imidazole and NIT2-bithph = 4,4,5,5-tetramethyl-2-(bithiophenal-2-yl)imidazoline-1-oxyl-3-oxide; X, bbimp = bis(benzimidazol-2-
ylmethyl)imino(methylenphosphonate). b Average values. c Mn–O–Mn–O angle. d Magnetic exchange coupling parameter based on the Hamiltonian
H = -JS1·S2. e Ferromagnetic behavior observed but not fitted. f The authors give this J value based on Tmax is less than 1.5 K.
The analysis of the ferromagnetic chain [MnII(oba)-
(phen)(H2O)]n,14c which shows two monatomic bridges between
MnII ions, reported by Wang et al.24 concluded that angles
~ 103◦ could favour the ferromagnetic coupling for this kind of
compound. This agrees with experimental observations, where
analogous compounds with angles ~ 104◦ show ferromagnetic
coupling (Table 7).
Taking into consideration the m1,1-coordination mode of the
carboxylate, the magnetic interaction in these compounds is
due to the [Mn2(m-O)2]2+ core. Thus, with the aim of finding
magneto-structural correlations, dinuclear complexes with other
monatomic oxo ligands, such as alkoxo/phenoxo,25 N-oxide26
and phosphonate,27 are also included in Table 7. Compounds
with alkoxo/phenoxo bridges show ferro- or antiferromagnetic
behavior, with J values between +1.2 cm-1 and -4.5 cm-1.
Nevertheless, most of these compounds show a non-planar
core, in contrast to compounds with m1,1-RCOO bridges re-
ported here. Compounds with N-oxide as a bridging ligand
also display ferro- or antiferromagnetic interactions (J values
between +0.23 and -3.1 cm-1) and only one them shows a
non-planar [Mn2O2] core. There is also one compound reported
in the literature with two phosphonate bridges, which displays
ferromagnetic coupling between the MnII ions in the planar
core.
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A linear relationship between the structural parameters of
compounds reported in Table 7 can be observed: the Mn ◊ ◊ ◊ Mn
distance increases with the Mn–Ob–Mn angle, as well as with the
Mn–Ob distance (Fig. S4–S5, ESI†). A best correlation is found
when only systems with planar core are considered. Nevertheless,
no clear magneto-structural correlation could be established for
these compounds. Complexes with two m1,1-carboxylate bridges,
like 3–6, as well as other compounds with a [Mn2(m-O)2]2+ core,
show similar structural parameters and small J values in all cases,
the magnetic behavior being ferro- or antiferromagnetic.
The [Mn2(m-O)2]n+ core is also found in Mn2 and MnIIIMnIV
IV
compounds, but they show a strong antiferromagnetic behavior.
The dramatic change in their magnetic behavior, compared to
II
the Mn2 systems could be due to structural differences. In these
systems, the Mn–Ob and the Mn ◊ ◊ ◊ Mn distances (~ 1.8 and
˚
~ 2.8 A, respectively) are around 10–20% shorter than those
found for MnII compounds (~2.2 and ~ 3.4 A, respectively).
˚
Moreover, the Mn–Ob–Mn angle is also smaller (~ 96◦ in contrast
with ~ 104◦ for MnII complexes). DFT studies of high-valence
[Mn2(m-O)2]n+ systems, reported by Ruiz–Garcia et al.,28 reveal
that in these compounds the magnetic coupling is dominated
2
2
by the in-plane dx
type superexchange pathway via the oxo
-y
bridges with a small, but non negligible, contribution from direct
2
2
dx
type Mn ◊ ◊ ◊ Mn s-bond. For MnII complexes, with larger
-y
Fig. 11 X-band EPR spectra of powdered samples of the ferromagnetic
dinuclear compounds 3–6 at 4 K. The dashed line is the best simulation
achieved.
Mn ◊ ◊ ◊ Mn distances, the degree of this overlap must be very
small and the dx /dx
2
2
2
2
interaction would be only through the
-y
-y
bridge, decreasing drastically the antiferromagnetic contribution.
Moreover, the presence of two more electrons in each manganese
2
2
ion provides additional pathways (dxy/dx
and dxy/dxy) which
-y
could be ferromagnetic or weak antiferromagnetic contributions
depending on structural parameters.
EPR spectra
EPR spectra of compounds 1–6 were recorded on powdered sam-
ples at different temperatures. At room temperature all complexes
show a broad band centered at g ~ 2 although at low temperatures
the spectra of ferro- and antiferromagnetic compounds are
considerably different. For ferromagnetic compounds (3–6), the
shape of the spectra at room temperature is similar to those at
4 K, only the band width changes a little on cooling and small
features appear at ~ 155 and ~ 250 G (Fig. 11). In contrast,
for the antiferromagnetic compounds (1 and 2) the shape of the
spectra is very sensitive to temperature, showing at 4 K two more
bands at both sides of g ~2 and other features at lower fields
(Fig. 12).
Compounds with ferromagnetic coupling show a spin ground
state S = 5, while for compounds displaying antiferromagnetic
coupling the spin ground state is S = 0. Although at 4 K the latter
compounds should be EPR silent, the small J values allow the
lowest excited states S = 1 and S = 2 to be populated.
The spectra at 4 K of all compounds were simulated with the
easyspin software15 by considering a dinuclear system of two S =
5/2 ions with the magnetic exchange interaction (J) obtained from
the magnetic data and including the ZFS parameters of the single
ion (DMn and EMn). All simulations were performed with g = 2.0.
Fairly good simulations were achieved with D and E values of
0.090 and 0.030 cm-1 (1), 0.100 and 0.020 cm-1 (2), 0.060 and
0.005 cm-1 (3), 0.070 and 0.005 cm-1 (4), 0.045 and 0.002 cm-1 (5)
Fig. 12 X-band EPR spectra of powdered samples of the antiferromag-
netic dinuclear compounds 1 and 2 at 4 K. The dashed line is the best
simulation achieved.
and 0.065 and 0.002 cm-1 (6). These values agree with the small
distortions characteristic of MnII ions.
As can be observed, compounds 1 and 2, with a ground state of
S = 0, show appreciably larger D and E values than compounds
3–6, with a ground state of S = 5, despite displaying quite similar
structural parameters. It is well known that the ZFS has two
contributions: dipolar and anisotropic interactions. For dinuclear
MnII compounds the cofficient for these contributions to the DS
value is very different for S = 5 and S = 1: D5 = (4/9)DMn
+
23
(5/18)Ddip while D1 = (-32/5)DMn + (37/10)Ddip
.
Therefore, the
S = 1 state is more sensitive to both contributions than the S = 5,
and consequently more complex spectra should be obtained for
the antiferromagnetic systems than for the ferromagnetic ones, as
is observed experimentally.
11672 | Dalton Trans., 2010, 39, 11664–11674
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In the simulation of the spectra, a phenomenological D value
has been considered, which does not include the dipole–dipole
interactions. However, it is possible that this contribution is non-
negligible.29 The dipolar interaction is proportional to r-3, r being
the distance between the paramagnetic centers. The Mn ◊ ◊ ◊ Mn
distance in the ferromagnetic compounds is appreciably shorter
than in the antiferromagnetic ones, so the dipolar contribution
should be more important for complexes 3–6 than for complexes
1 and 2.
Summarizing, the EPR spectra for this kind of dinuclear
complex is a tool for differentiating between ferro- and anti-
ferromagnetic compounds. For the antiferromagnetic complexes,
with large Mn ◊ ◊ ◊ Mn distances, the dipolar contribution to the
ZFS should be small, and the D and E values proposed from
the simulation can well describe the system. However, for the
ferromagnetic compounds, the situation is more complex due to
the small contributions of the dipolar and anisotropic interactions.
Moreover, due to the short Mn ◊ ◊ ◊ Mn distance, the dipolar
contribution is not negligible. Therefore, D and E values for the
ferromagnetic compounds must be taken into consideration with
caution.
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Conclusions
Six new dinuclear MnII compounds have been synthesized and
structurally and magnetically characterized. Compounds 1 and 2
show two m1,3-carboxylate bridges in a syn–anti mode while com-
pounds 3–6 present two m1,1-carboxylate bridges. This difference
in the coordination mode of the carboxylate ligands affects the
magnetic behavior in such a way that compounds 1 and 2 display
antiferromagnetic interactions and compounds 3–6 ferromagnetic
interactions. This ferromagnetic behavior, compared to the antifer-
romagnetic interaction found for MnIV compounds with the same
core, is due to the large Mn ◊ ◊ ◊ Mn distance and the Mn–Ob–Mn
angle, which make the JAF interactions diminish. Moreover, the
different magnetic behavior is also reflected in the EPR spectra,
where antiferromagnetic compounds (1 and 2) show much more
complicated EPR spectra at 4 K than ferromagnetic compounds
(3–6).
Acknowledgements
This work was supported by the Ministerio de Ciencia e Inno-
vacio´n of Spain through the project CTQ2009-07264/BQU, the
Comissio´ Interdepartamental de Recerca i Innovacio´ Tecnolo`gica
of la Generalitat de Catalunya (CIRIT) (2009-SGR1454). V.G.
thanks the Ministerio de Ciencia e Innovacio´n for the PhD grant
BES-2007-15668.
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