Contrasting magnetism of [Mn2III] and [Mr 2IIMn2III] squares

Article abstract of DOI:10.1021/ic902052f

Two tetranuclear manganese distorted square-shaped clusters, [Mn ₄^III(L1)₄(μ₂-OMe)₄] · 2.5H₂O (1)and [Mn₂^IIMn ₂^III(L2)₄(H₂O

Full text of DOI:10.1021/ic902052f

368 Inorg. Chem. 2010, 49, 368–370
DOI: 10.1021/ic902052f
Contrasting Magnetism of [MnIII ] and [MnII MnIII ] Squares
4
2
2
Takuto Matsumoto,† Takuya Shiga,† Mao Noguchi,† Tatsuya Onuki,† Graham N. Newton,† Norihisa Hoshino,†
Motohiro Nakano,‡ and Hiroki Oshio*,†
†Graduate School of Pure and Applied Sciences, University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8571,
Japan and ‡Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1
Yamada-Oka, Suita, Osaka 565-0871, Japan
Received October 16, 2009
Two tetranuclear manganese distorted square-shaped clusters,
their potential application as molecular devices, such as
[MnIII (L1)4(μ2-OMe)4] 2.5H2O (1) and [MnII MnIII (L2)4(H2O)2]-
(PF6)2 CHCl3 CH3OH 1.5H2O (2) (H2L1 = 2-[3-(2-hydroxyphe-
quantum cellular automata.5 The controllable arrangement
of metal ions in multinuclear complexes is achieved through
rational ligand design and the bonding affinities of different
metal ions. Planar multidentate bridging ligands can afford n
ꢀ n square gridlike complexes, such as those reported by
Thompson, Lehn, and others.2 These grid complexes were
formed by spontaneous self-assembly due to the directing
geometries of the ligands. Hydrazone- and polypyridyl-type
ligands have been used by many researchers to construct
n ꢀ n grid complexes. However, most metal ions coordinated
in these complexes have octahedral coordination geometries
and the same oxidation states. Thus, heterometallic gridlike
complexes have been rarely reported.6 Against such a back-
ground, the development of asymmetric planar ligands is
important for the generation of novel grid-type complexes
because asymmetric ligands can stabilize heterometallic and
mixed-valence species. Recently, we reported the first grid-
type single-molecule magnet: a [Co9] complex supported by
symmetric polypyridine ligands.2c To develop our research
further, an asymmetric planar multidentate ligand derived
from 2,6-substituted pyridine was designed, and its com-
plexation behavior has been investigated. This paper des-
cribes the syntheses, crystal structures, and magnetic pro-
perties of two tetranuclear manganese gridlike complexes,
4
2
2
3
3
3
3
nyl)-1H-pyrazol-5-yl]-6-pyridinecarboxylic acid methyl ester; H2L2 =
2-[3-(2-hydroxyphenyl)-1H-pyrazol-5-yl]-6-pyridinecarboxylic acid
ethyl ester), exhibit antiferromagnetic and ferromagnetic inter-
actions between neighboring manganese ions, respectively.
Well-designed polynuclear complexes show various physi-
cal properties and functionalities because of the synergy of
the interactions between metal ions.1 Various complexes,
which have regular arrays such as grids,2 wires,3 and rings,4
have been prepared using a range of synthetic methods.
Tetranuclear complexes with square-shaped arrangements
of metal ions are of great interest to the chemist because of
*To whom correspondence should be addressed. E-mail: oshio@chem.
tsukuba.ac.jp.
(1) (a) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives;
VCH: Weinheim, Germany, 1995. (b) Leininger, S.; Olenyuk, B.; Stang, P. J.
Chem. Rev. 2000, 100, 853. (c) Fujita, M. Chem. Soc. Rev. 1998, 27, 417. (d)
Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnets; Oxford Press:
New York, 2006.
(2) (a) Thompson, L. K.; Waldmann, O.; Xu, Z. Coord. Chem. Rev. 2005,
249, 2677. (b) Ruben, M.; Rojo, J.; Romero-Salguero, F. J.; Uppadine, L. H.;
Lehn, J.-M. Angew. Chem., Int. Ed., 2004, 43, 3644 and references cited therein.
(c) Shiga, T.; Matsumoto, T.; Noguchi, M.; Onuki, T.; Hoshino, N.; Newton,
G. N.; Nakano, M.; Oshio, H. Chem. Asian J. 2009, 4, 1660.
[MnIII4(L1)4(μ2-OMe)4] 2.5H2O (1) and [MnII2MnIII2(L2)4-
3
(H2O)2](PF6)2 CHCl3 CH3OH 1.5H2O(2) [Figure 1; H2L1=
3
3
3
2-[3-(2-hydroxyphenyl)-1H-pyrazol-5-yl]-6-pyridinecar-
boxylic acid methyl ester; H2L2 = 2-[3-(2-hydroxyphenyl)-
1H-pyrazol-5-yl]-6-pyridinecarboxylic acid ethyl ester
(Scheme 1)].
(3) (a) Yin, C.; Huang, G.-C.; Kuo, C.-K.; Fu, M.-D.; Lu, H.-C; Ke,
J.-H.; Shih, K.-N.; Huang, Y.-L.; Lee, G.-H.; Yeh, C.-Y.; Chen,
C.-h.; Peng, S.-M. J. Am. Chem. Soc. 2008, 130, 10090. (b) Liu, I. P.-C.;
ꢀ
Benard, M.; Hasanov, H.; Chen, I.-W. P.; Tseng, W.-H.; Fu, M.-D.; Rohmer,
M.-M.; Chen, C.-h.; Lee, G.-H.; Peng, S.-M. Chem.;Eur. J. 2007, 13, 8667. (c)
Ismayilov, R. H.; Wang, W.-X.; Wang, R.-R.; Yeh, C.-Y.; Lee, G.-H.; Peng, S.-M.
Chem. Commun. 2007, 1121.
The asymmetric multidentate ligand H2L2 was synthesized
using the Claisen condensation reaction of 2-hydroxoaceto-
phenone with 2,6-pyridinecarboxylic acid ethyl ester.7 The
(4) (a) Affronte, M.; Carretta, S.; Timco, G. A.; Winpenny, R. E. P.
reaction of Mn(OAc)2 4H2O with H2L2 and Et3N in metha-
ꢀ
Chem. Commun. 2007, 1789. (b) Campos-Fernandez, C. S.; Schottel, B. L.;
3
nol yielded the tetranuclear MnIII4 complex 1, while the same
procedure, in the presence of a stoichiometric quantity of
bis(pentamethylcyclopentadienyl)iron as a reducing agent,
Chifotides, H. T.; Bera, J. K.; Bacsa, J.; Koomen, J. M.; Russell, D. H.; Dunbar,
K. R. J. Am. Chem. Soc. 2005, 127, 12909.
(5) (a) Zhao, Y.; Guo, D.; Liu, Y.; He, C.; Duan, C. Chem. Commun.
2008, 44, 5725. (b) Jiao, J.; Long, G. J.; Rebbouh, L.; Grandjean, F.; Betty, A. M.;
Fehlner, T. P. J. Am. Chem. Soc. 2005, 127, 17819. (c) Lent, C. S.; Isaksan, B.;
Lieberman, M. J. Am. Chem. Soc. 2003, 125, 1056. (d) Qi, H.; Sharma, S.; Li, Z.;
Snider, G. L.; Orlov, A. O.; Lent, C. S.; Fehlner, T. P. J. Am. Chem. Soc. 2003,
125, 15250. (e) Lent, C. S. Science 2000, 288, 1597. (f ) Imre, A.; Csaba, G.; Ji, L.;
Orlov, A. O.; Bernstein, G. H.; Porod, W. Science 2006, 311, 205.
(6) (a) Petitjean, A.; Kyritsakas, N.; Lehn, J.-M. Chem. Commun. 2004,
1168. (b) Parsons, S. R.; Thompson, L. K.; Dey, S. K.; Wilson, C.; Howard,
J. A. K. Inorg. Chem. 2006, 45, 8832.
r
pubs.acs.org/IC
Published on Web 12/14/2009
2009 American Chemical Society

Communication
Inorganic Chemistry, Vol. 49, No. 2, 2010 369
yielded a mixed-valence tetranuclear MnII2MnIII complex,
2
2.8 During the synthesis of 1, the ethyl ester ligand H2L2 was
transformed into the methyl ester H2L1 by solvolysis.
X-ray structural analyses of 1 and 2 reveal that both
complexes have 2 ꢀ 2 gridlike tetranuclear manganese cores
bridged bythe pyrazolegroups of the2,6-substitutedpyridine
ligands (L1 and L2).9 1 crystallizes in the orthorhombic space
group Pnnn, where the asymmetric unit is half of the mole-
cule. In 1, each manganese ion has a square-pyramidal N2O3
coordination environment with two pyrazole nitrogen atoms,
two oxygen atoms of μ2-MeO- ions, and one phenoxo
oxygen atom. The pyridyl nitrogen atoms of the ligands
weakly coordinate to the manganese ions, where the average
˚
distance between manganese and nitrogen atoms is 2.618 A,
suggesting axial elongation along the N6-Mn1-O8 and
N3-Mn2-O7 vectors. The coordination bond lengths with
(7) Ligand synthesis. 6-[1,3-Dioxo-3-(2-hydroxyphenyl)propionyl]pyridine-
2-carboxylic acid ethyl ester: A solution of sodium ethoxide (prepared from
sodium (6.99 g, 304 mmol)), acetophenone (22.88 g, 168 mmol), and 2,6-
pyridinedicarboxylic acid ethyl ester (25.0 g, 112 mmol) in 100 mL of diethyl
ether was refluxed for 2 h under a nitrogen atmosphere. The solvent was
removed by filtration, and aqueous acetic acid (15%) was added to the residue.
The resulting yellow solid was filtered, and the recrystallization of the crude
product from methanol yielded yellow needle crystals (16.5 g). Yield: 47%.
Anal. Calcd for C17H15N1O5: C, 65.17; H, 4.83; N, 4.47. Found: C, 64.95; H,
4.92; N, 4.42. 1H NMR (270 MHz, CDCl3): δ 1.52 (t, J=8.1 Hz, 3H, CH3), 4.52
(q, J=8.1 Hz, 2H, CH2), 7.02-6.93(m, 2H, ph), 7.49(t, J=7.0 Hz, 1H, py), 7.69
(s, 1H), 8.19-7.95 (m, 2H, ph), 8.27-8.20 (m, 2H, py), 12.07 (s, 1H, OH), 15.15
(s, 1H, OH). H2L2: Hydrazine monohydrate (3.5 g, 70 mmol) was added to an
ethanolic solution of 6-[1,3-dioxo-3-(2-hydroxyphenyl)propionyl]pyridine-2-
carboxylic acid ethyl ester (20.0 g, 63 mmol), and the mixture was refluxed
for 1 h. The resulting mixture was evaporated and allowed to stand for several
hours at 5 °C to yield colorless needles (11.88 g, 38 mmol). Yield: 61%. Anal.
Calcd for C17H15N3O3 0.5H2O: C, 64.14; H, 5.04; N, 13.20. Found: C, 64.46;
3
H, 5.19; N, 13.28. 1H NMR (270 MHz, CDCl3): δ 1.47 (t, J=7.2 Hz, 3H, CH3),
4.49 (q, J=7.2 Hz, 2H, CH2), 6.94 (t, J=7.4 Hz, 1H, ph), 7.06 (t, J=8.1 Hz, 2H,
ph), 7.27-7.21 (m, 1H, ph), 7.63-7.60 (m, 1H, pz), 8.08-7.84 (m, 3H, py),
12.10 (s, 1H, OH), 10.80 (s, 1H, NH).
Figure 1. ORTEP diagrams of 1 (top) and 2 (bottom). Ellipsoids are at
30% probability, carbon atoms are represented by full ellipsoids, and all
hydrogen atoms and solvent molecules are excluded for clarity.
(8) Synthesis of 1: A solution of Mn(OAc)2 4H2O (49 mg, 0.2 mmol) in
Scheme 1. Ligands H2L1 and H2L2
3
methanol (5 mL) was added to the mixture of H2L2 (62 mg, 0.2 mmol) and
triethylamine (56 μL, 0.4 mmol) in methanol (5 mL). Chloroform (10 cm3)
was added to the resulting solution. After the solution was stirred for several
minutes, NEt4BF4 (22 mg, 0.1 mmol) was added, and the resulting solution
was refluxed for 10 min before hot filtration. The filtrate was allowed to cool
to room temperature and stand for a few days to give brown plate crystals of
1. Anal. Calcd for C68H56Mn4N12O16: C, 52.27; H, 3.94; N, 10.76. Found: C,
50.67; H, 3.76; N, 10.36. Synthesis of 2: A mixed solution of Mn-
(OAc)2 4H2O (24.5 mg, 0.1 mmol) in methanol (3 mL) and bis(pentamethyl-
3
equatorially coordinated atoms are in the range of
cyclopentadienyl)iron (32.6 mg, 0.1mmol) in chloroform (3 mL) was added
to the mixture of H2L2 (62 mg, 0.2 mmol) and triethylamine (112 μL, 0.8
mmol) in methanol (4 cm3). The mixture was stirred for several minutes.
NH4PF6 (163 mg, 1.0 mmol) was added to the mixture, and the resulting
solution was filtered. The filtrate was allowed to stand for a few days at room
temperature to give brown rhombic plate crystals of 2. Anal. Calcd for
C68H59F12Mn4N12O16.5P2: C, 44.91; H, 3.27; N, 9.25. Found: C, 45.66; H,
3.49; N, 9.61.
˚
1.858-2.018 A. The bond lengths, observed Jahn-Teller
distortions, and bond valence sum (BVS) calculations10
suggest that all manganese ions are trivalent. 2 crystallizes
in the triclinic space group P1. Although there are four
crystallographically independent manganese ions in the com-
plex, chemically they can be grouped into two types. The
Mn1 and Mn3 ions have a square-pyramidal N2O3 coordina-
tion environment, coordinated by two pyrazole nitrogen
atoms, two phenoxo oxygen atoms, and one oxygen atom
of a water molecule, with basal coordination bond lengths in
(9) Crystal data for 1: brown plate crystals (0.30 ꢀ 0.20 ꢀ 0.05 mm3),
˚
C68H61Mn4N12O18.5, M=1562.03, orthorhombic, Pnnn, a=20.135(2) A, b=
3
˚
˚
˚
20.145(2) A, c=17.2055(18) A, V=6978.8(12) A , Z=4, dcalcd=1.459 mg
m
-3, μ(Mo KR)=0.784 mm-1, T=100 K. A total of 35 094 were collected
(4.74° < θ < 48.58°) of which 6328 unique reflections (Rint=0.0487) were
˚
measured. R1=0.0713, wR2=0.2068 (I > 2σ), and S=1.055. Crystal data for
the range of 1.848-1.989 A and axial water molecule bond
2: brown rhombic plates (0.42 ꢀ 0.25 ꢀ 0.04 mm3), C70H64Cl3F12Mn4N12
-
˚
lengths of 2.145(8) and 2.197(6) A. The Mn2 and Mn4 ions
˚
˚
O16.5P2, M=1949.34, triclinic, P1, a=16.063(3) A, b=16.722(3) A, c=
have a highly distorted and elongated octahedral N4O2
coordination geometry with two pyridyl nitrogen atoms
and two pyrazole nitrogen atoms with bond lengths in the
3
˚
˚
17.021(3) A, R=76.977(3)°, β=73.151(3)°, γ=70.571(3)°, V=4084.3(13) A ,
Z=2, dcalcd=1.563 mg m-3, μ(Mo KR)=0.839 mm-1, T=200 K. A total of
16 607 were collected (4.16° < θ < 46.56°) of which 11 393 unique reflections
(Rint=0.0265) were measured. R1=0.0641, wR2=0.1653 (I > 2σ), and S=
0.926. Both data sets were treated with the SQUEEZE program to deal with
disordered solvent molecules, The crystallographic formulas have been
amended to include the number of solvent molecules suggested by
SQUEEZE (see the CIF files).
˚
ranges of 2.239-2.251 and 2.168-2.209 A, respectively.
The coordination environment is completed by two weakly
(10) BVS of 1: Mn1 for þ3, 3.001; Mn2 for þ3, 3.001.

370 Inorganic Chemistry, Vol. 49, No. 2, 2010
Matsumoto et al.
neighboring manganese ions are treated as being identical
for all four bridges in 1 and 2, respectively, because the same
bridging modes are active between all metal centers in each
cluster. Fitting of the experimental data gave g=1.94 and J=
-1.29 K for 1 and g=1.97, J=þ0.84 K, Dfixed=-0.22 K,
and zJ0 = -1.74 K for 2, where J represents the exchange
coupling constant between neighboring manganese ions
(Figure 2, inset).12 The observed J value for 1 is in a range
similar to that of previously reported MnIII-NN-MnIII/
MnIII-O-MnIII interaction pathways;13 however, there are
no known examples of MnIII-NN-MnII mixed-valence
interaction pathways to allow an effective comparison with 2.
To explain the antiferromagnetic interactions of 1, there
are two exchange pathways that must be considered: the
methoxo bridge, which occupies the dz2 orbital of one MnIII
center and the dx2-y2 orbital of the next, and the pyrazole
bridges, which link the dx2-y2 orbitals of neighboring MnIII
centers. Because the dx2-y2 orbitals are unoccupied in MnIII,
the pyrazole bridge leads to negligible magnetic interaction;
however, the methoxo bridge would be expected to lead to
ferromagnetic interactions because of the orthogonality of
the interacting dz2 and dx2-y2 orbitals. The antiferromagnetic
nature of the cluster may be due to the large Mn-O-Mn
angles [118.08(16)-118.09(16)°], which may lead to mixing
of the orbitals and interaction between the dz2 orbital and the
dxy orbital of the neighbor. In the case of 2, the MnIII ions
with square-pyramidal coordination geometry (Mn1 and
Mn3) have their Jahn-Telleraxes filled by coordinatedwater
molecules. Note that each MnIII ion has a vacant dx2-y2
orbital that is oriented toward the bridging pyrazole groups.
These groups constitute the path for the magnetic interaction
between the MnIII and MnII centers, so as a result the
interacting spin of the MnII ions may be partially delocalized
to the vacant dx2-y2 orbital of the MnIII ions, causing the
occurrence of intramolecular ferromagnetic interactions.
In conclusion, two tetranuclear manganese gridlike com-
plexes were synthesized using a rigid multidentate asym-
metric ligand. 1 has four antiferromagnetically coupled
MnIII ions, while 2 consists of two divalent and two tri-
valent manganese ions, all of which are ferromagnetically
coupled.
Figure 2. χmT vs T plots of 1 (4) and 2 (O). Solid lines are theoretical
curves (see the text). Inset: spin states of each complex and magnetic
exchange paths.
coordinated keto oxygen atoms from the ethyl ester groups
˚
with an average Mn-O distance of 2.506 A. The bond
lengths, observed Jahn-Teller distortions, and BVS calcula-
tions11 suggest that Mn1 and Mn3 are trivalent and Mn2 and
Mn4 are divalent. Both complexes have tetranuclear square-
like metal arrangements, but the arrangements of metal ions
in 1 and 2 are square and rhombic, respectively. The manga-
nese centers in 1 are all bridged by pyrazole groups and
methoxo ions, whereas in 2, the manganese ions are only
bridged by pyrazole groups. Both 1 and 2 have gridlike struc-
tures derived from similar components, four manganese ions
and four asymmetric ligands; however, both showed very
different magnetic behavior. Magnetic susceptibility mea-
surements were collected for 1 and 2 in the temperature range
of 1.8-300 K in an applied field of 500 Oe (Figure 2). For 1,
the χmT value of 10.99 emu mol-1 K at 300 K is smaller than
the expected value (12.00 emu mol-1 K) for four magnetically
uncorrelated MnIII ions (S=2; g=2). As the temperature
decreased, the χmT value gradually decreased, reaching a
minimum value of 5.21 emu mol-1 K at 1.8 K. For 2, the χmT
value of 14.87 emu mol-1 K at 300 K is slightly largerthan the
value (14.75 emu mol-1 K) expected for two magnetically
uncorrelated MnIII ions (S = 2; g = 2) and two MnII ions
(S=5/2; g=2). As the temperature decreased, the χmT value
gradually increased, reaching a maximum value of 32.40 emu
mol-1 K at 5.0 K, followed by a sudden decrease. In addition
tothis ferromagnetic behavior, 2 also showed a limiteddegree
of alternating-current susceptibility, suggesting superpara-
magnetic properties (Figure S2 in the Supporting Informa-
tion). These magnetic data were fitted to isotropic exchange
Hamiltonians (see the Supporting Information). It should
be noted that the exchange coupling constants between
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research and for Priority Area “Co-
ordination Programming” (area 2107) from MEXT, Japan.
Supporting Information Available: Additional magnetic data,
isotropic exchange Hamiltonians, and crystallographic data in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
(12) The D value was estimated from the magnetization data at 1.8 K
(Figure S3 in the Supporting Information), and the exchange coupling
constant was then obtained by analyzing the magnetic susceptibility data
with a fixed D value. TIP(1)=400 ꢀ 10-6 emu mol-1, and TIP(2)=200 ꢀ
10-6 emu mol-1
.
(11) BVS of 2: Mn1 and Mn3 for þ3, 3.171 and 3.196, respectively; Mn2
and Mn4 for þ2, 1.843 and 1.817, respectively.
(13) Stoicescu, L.; Jeanson, A.; Duhayon, C.; Tesouro-Vallina, A.;
Boudalis, A. K.; Costes, J.-P.; Tuchagues, J.-P. Inorg. Chem. 2007, 46, 6902.