Synthesis, crystal structure, and magnetic studies of oxo-centered trinuclear chromium(III) complexes: [Cr3(μ3-O) (μ2-PhCOO)6(H2O)3]NO 3·4H2O·2CH3OH, a case of spin-frustrated system, and...

Article abstract of DOI:10.1021/ic700606e

The synthesis, crystal structure, and magnetic properties of two trinuclear oxo-centered carboxylate complexes are reported and discussed: [Cr ₃(μ₃-O)(μ₂-PhCOO)₆(H ₂O)₃]NO₃·4H₂O·2CH ₃OH (1) and [Cr₃(μ₃-O)(μ₂- PhCOO)₂(μ₂-OCH₂CH₃) ₂(bpy)₂(NCS)₃] (2). For both complexes the crystal system is monoclinic, with space group C2/c for 1 and P₁/n for 2. The structure of complex 1 consists of discrete trinuclear cations, associated NO₃^- anions, and lattice methanol and water molecules. The structure of complex 2 is built only by neutral discrete trinuclear entities. The most important feature of 2 is the unusual skeleton of the [Cr₃O] core due to the lack of peripheral bridging ligands along one side of the triangular core, which is unique among the structurally characterized (μ₃-oxo)-trichromium(III) complexes. Magnetic measurements were performed in the 2-300 K temperature range. For complex 1, in the high-temperature region (T > 8 K), experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = -2J ₁₂S₁S₂ - 2J₁₃S₁S ₃ - 2J₂₃S₂S₃ (J₁₂ = J₁₃ = J₂₃) with J_ij = -10.1 cm^-1, g = 1.97, and TIP = 550 × 10^-6 emu mol^-1. The antisymmetric exchange interaction plays an important role in the magnetic behavior of the system, so in order to fit the experimental magnetic data at low temperature, a new magnetic model was used where this kind of interaction was also considered. The resulting fitting parameters are the following: G _zz = 0.25 cm^-1, δ = 2.5 cm^-1, and TIP = 550 × 10^-6 emu mol^-1. For complex 2, the experimental data could be satisfactorily reproduced by using an isotropic exchange model, H = -2J₁(S₁S₂ + S ₁S₃) - 2J₂(S₂S₃) with J₁ = -7.44 cm^-1, J₂ = -51.98 cm^-1, and g = 1.99. The magnetization data allows us to deduce the ground term of S = 1/2, characteristic of equilateral triangular chromium(III) for complex 1 and S = 3/2 for complex 2, which is confirmed by EPR measurements.

Full text of DOI:10.1021/ic700606e

Inorg. Chem. 2007, 46, 11017−11024
Synthesis, Crystal Structure, and Magnetic Studies of Oxo-Centered
Trinuclear Chromium(III) Complexes: [Cr3( 3-O)( 2-PhCOO)6(H2O)3]NO3
4H2O 2CH3OH, a Case of Spin-Frustrated System, and [Cr3( 3-O)-
2-PhCOO)2( 2-OCH2CH3)2(bpy)2(NCS )3], a New Type of [Cr3O] Core
µ
µ
‚
‚
µ
(
µ
µ
Albert Figuerola,† Vassilis Tangoulis,*,‡ Joan Ribas,† Hans Hartl,§ Irene Bru
1
dgam,§
Miguel Maestro, and Carmen Diaz*,†
Departament de Qu´ımica Inorga`nica i Institut de Nanocie`ncia i Nanotecnologia, UniVersitat de
Barcelona, Mart´ı i Franque`s, 1-11, 08028 Barcelona, Spain, Department of Chemistry, UniVersity
of Patras, 26500 Patras, Greece, Department of Chemistry, Institut fu¨r Chemie, Anorganische und
Analytische Chemie, Freie UniVersita¨t Berlin, Berlin, Germany, and Departamento de Qu´ımica
Fundamental, Facultade de Ciencias, UniVersidade da Corun˜a, 15071 A Corun˜a, Spain
Received March 30, 2007
The synthesis, crystal structure, and magnetic properties of two trinuclear oxo-centered carboxylate complexes are
reported and discussed: [Cr3( 3-O)( 2-PhCOO)6(H2O)3]NO3 4H2O 2CH3OH (1) and [Cr3( 3-O)( 2-PhCOO)2(µ2
µ
µ
‚
‚
µ
µ
-
OCH2CH3)2(bpy)2(NCS)3] (2). For both complexes the crystal system is monoclinic, with space group C2/c for 1
-
and P1/n for 2. The structure of complex 1 consists of discrete trinuclear cations, associated NO3 anions, and
lattice methanol and water molecules. The structure of complex 2 is built only by neutral discrete trinuclear entities.
The most important feature of 2 is the unusual skeleton of the [Cr3O] core due to the lack of peripheral bridging
ligands along one side of the triangular core, which is unique among the structurally characterized (
trichromium(III) complexes. Magnetic measurements were performed in the 2 300 K temperature range. For complex
1, in the high-temperature region (T > 8 K), experimental data could be satisfactorily reproduced by using an
µ3-oxo)-
−
isotropic exchange model, H ) −2J12S1S2
− 2J13S1S3 − 2J23S2S3 (J12 ) J13 ) )
J23) with Jij ) −10.1 cm-1, g
6
1.97, and TIP
)
550
×
10- emu mol-1. The antisymmetric exchange interaction plays an important role in the
magnetic behavior of the system, so in order to fit the experimental magnetic data at low temperature, a new
magnetic model was used where this kind of interaction was also considered. The resulting fitting parameters are
1
the following: Gzz
)
0.25 cm-
,
δ ) 2.5 cm-1, and TIP
)
550
×
10-6 emu mol-1. For complex 2, the experimental
data could be satisfactorily reproduced by using an isotropic exchange model, H ) −2J1(S1S2
with J1 ) −7.44 cm-1, J2 ) −51.98 cm-1, and g
1.99. The magnetization data allows us to deduce the ground
term of S
1/2, characteristic of equilateral triangular chromium(III) for complex 1 and S 3/2 for complex 2, which
is confirmed by EPR measurements.
+ S1S3) − 2J2(S2S3)
)
)
)
Introduction
R is an alkyl or aryl group, have been of great interest as
systems to test current theories of magnetic and electronic
coupling between metal atoms. A feature that has attracted
attention is that when the three metal atoms are identical,
and of the same oxidation state, the complex nearly always
fails to adopt the 3-fold symmetry that might be expected.
One interesting phenomenon is spin frustration, which occurs
when all the interactions between spin pairs cannot simul-
taneously have their optimal value. The simplest systems
where this could be observed are an odd-member ring of
antiferromagnetic coupled spins. In complexes which have
the triangular µ3-oxo bridge [M3O] core, when all three
Trinuclear oxo-centered metal complexes of the general
type [M3O(O2CR)6L3]0/+,1a where L is water or pyridine and
* To whom correspondence should be addressed. E-mail: carme.diaz@
qi.ub.es (C.D.).
† Universitat de Barcelona.
‡ University of Patras.
§ Freie Universita¨t Berlin.
Universidade da Corun˜a.
(1) (a) Cannon, R. D.; White, R. P. Prog. Inorg. Chem. 1988, 36, 195.
(b) Blake, A. B.; Yavari, A.; Hatfield, W. E.; Sethulekshmi, C. N. J.
Chem. Soc., Dalton Trans. 1985, 2509. (c) Vincent, J. B.; Chang, H.
R.; Folting, K.; Huffman, J. C.; Christou, G.; Hendrickson, D. N. J.
Am. Chem. Soc. 1987, 109, 5703.
10.1021/ic700606e CCC: $37.00
Published on Web 11/16/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 26, 2007 11017

Figuerola et al.
pairwise interactions are antiferromagnetic, there is the
possibility of spin frustration if the three pairwise interactions
are the same; then, only two of the three spin constraints
can be satisfied simultaneously, and the system is geo-
metrically frustrated. The ground state then is of intermediate
spin, intermediate between the lowest and the highest spins
possible for the three metal ions. It is well-established that
the µ3-oxo bridge in [M3O(O2CR)6L3]n+ complexes is
dominant in propagating the exchange interaction.1b,c Oxo-
centered trinuclear complexes of antiferromagnetic spins are
quite common for transition metal ions like Fe3+,2 Mn3+,1c,3
and Cr3+.2k,4 The isotropic Hamiltonian for the interaction
between any two spins can be written as the scalar product
of the spin operators: H ) -ΣijJijSiSj. Therefore, the energy
is minimized for collinear parallel or antiparallel spin
alignments. The available experimental data are in good
agreement with the isotropic HDVV model in the region of
relatively high temperatures. However, there are a number
of serious discrepancies between the predictions of the theory
and the experimental data at low temperatures. In order to
interpret the magnetic properties of these frustrated systems,
it is necessary to go beyond the framework of the above
isotropic exchange model and to introduce an antisymmetric
exchange interaction, GAB. This antisymmetric exchange
interaction of the type
HAS ) ΣijGij[Si × Sj]
where Gij ) -Gij is the antisymmetric vector constant. As
was pointed out by Tsukerblat and co-workers,5 this equation
was introduced phenomenologically by Dzialoshinski to
explain the weak ferromagnetism of R-Fe2O3 and manganese
carbonate crystals.6 The microscopic meaning of the param-
eter G was revealed in the work of Moriya.7 Therefore, the
operator HAS is usually termed the Dzialoshinski-Moriya
antisymmetric (AS) interaction. AS exchange leads to
qualitative and quantitative changes in the temperature and
field dependences of the magnetic susceptibility at low
temperatures. These features are due to the nonlinear
behavior of levels in the magnetic field when AS exchange
is taken into account, and they consist of a reduction of the
magnetic moments.5 Information about Gij values for isolated
complexes is very scarce. The first experimental observation
of the antisymmetric exchange parameter on the electronic
properties of the trinuclear complex [Cu3(µ3-OH)(pyridine-
2-aldoxime)3 (SO4)]‚10.5H2O was made by Tsukerblat and
co-workers.8 Experimental applications of this exchange term
(2) (a) Sowrey, F. E.; Tilford, C.; Wocadlo, S.; Anson, C. E.; Powell, A.
K.; Bennington, S. M.; Montfrooij, W.; Jayasooriya, U. A.; Cannon,
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Koningsbruggen, P. J.; Arif, A. M.; Shum, W. W.; Millar, J. S. Inorg.
Chem. 2003, 42, 5645. (c) Boudalis, A. K.; Sanakis, Y.; Raptopoulou,
C. P.; Terzis, A.; Tuchagues, J. P.; Perlepes, S. Polyhedron 2005, 24,
1540. (d) Zhang, K. L.; Shi, Y. J.; Gao, S.; Dai, Y. D.; Yu, K. B.;
You, X. Z. Inorg. Chem. Commun. 2004, 7, 584. (e) Bing, Y.; Zhi-
Da, C. J. Coord. Chem. 2002, 55, 241. (f) Supriya, S.; Latha, K. S.;
Das, S. K. Eur. J. Inorg. Chem. 2005, 357. (g) Supriya, S.;
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C¸ elenligil-C¸ etin, R.; Staples, R. J.; Stavropoulos, P. Inorg. Chem. 2000,
39, 5838. (j) Franc¸ois, M.; Saleh, M. I.; Rabu, P.; Souhassou, M.;
Malaman, B.; Steinmetz, J. Solid State Sci. 2005, 7, 1236. (k)
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M.; Diamantopoulos, G.; Papavassiliou, G. Eur. J. Inorg. Chem. 2006,
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S. J. J. Am. Chem. Soc. 1987, 109, 4244-4255.
have been reported for various Cu2+ complexes9,10 as well
3
as for Fe3+3, and Cr3+ carboxylates.11,12
3
In this work, we describe the synthesis, X-ray structure,
and magnetic properties of two cyclic trimeric compounds:
[Cr3(µ3-O)(µ2-PhCOO)6(H2O)3]NO3‚4H2O‚2CH3OH (1) and
[Cr3(µ3-O) (µ2-PhCOO)2(µ2-OCH2-CH3)2(bpy)2(NCS)3] (2).
Complex 1 shows a similar cationic structure to those already
reported,4k,m-4r but the skeleton of the [Cr3O] core of complex
2 is unique among the structurally characterized (µ3-oxo)-
trichromium(III) complexes.
(3) (a) Li, J.; Yang, S.; Zhang, F.; Tang, Z.; Ma, S.; Shi, Q.; Wu, Q.;
Huang, Z. Inorg. Chim. Acta 1999, 294, 109. (b) Can˜ada-Vilalta, C.;
Huffman, J. C.; Streib, W. E.; Davidson, E. R.; Christou, G.
Polyhedron 2001, 20, 1375. (c) Wu, R.; Poyraz, M.; Sowrey, F. E.;
Anson, C. E.; Wocadlo, S.; Powell, A. K.; Jayasooriya, U. A.; Cannon,
R. D.; Nakamoto, T.; Katada, M.; Sano, H. Inorg. Chem. 1998, 37,
1913.
(4) (a) Figgis, B. N.; Robertson, G. B. Nature 1965, 694. (b) Schenk, K.
J.; Gu¨del, H. U. Inorg. Chem. 1982, 21, 2253. (c) Gonzalez-Vergara,
E.; Hegenauer, J.; Saltman, P.; Sabat, M.; Ibers, J. A. Inorg. Chim.
Acta 1982, 66, 115. (d) Bradshaw, J. E.; Grossie, D. A.; Mullica, D.
F.; Pennington, D. E. Inorg. Chim. Acta 1988, 141, 41. (e) Harton,
A.; Nagi, M. K.; Glass, M. M.; Junk, P. C.; Atwood, J. L.; Voncent,
J. B. Inorg. Chim. Acta 1994, 217, 171. (f) Anson, C. E.; Bourke, J.
P.; Cannon, R. D.; Jayasooriya, U. A.; Molinier, M.; Powell, A. K.
Inorg. Chem. 1997, 36, 1265. (g) Cannon, R. D.; Jayasooriya, U. A.;
Sowrey, F. E.; Tilford, C.; Little, A.; Bouke, J. P.; Rogers, R. D.;
Vicent, J. B.; Kearley, G. J. Inorg. Chem. 1998, 37, 5675. (h) Fujihara,
T.; Aonahata, J.; Kumakura, S.; Nagasawa, A.; Murakami, K.; Ito, T.
Inorg. Chem. 1998, 37, 3779. (i) Lieberman, R. L.; Bino, A.; Mirsky,
N.; Summers, D. A.; Thompson, R. C. Inorg. Chim. Acta 2000, 297,
1. (j) Palii, S. P.; Richardson, D. E.; Hansen, M. L.; Iversen, B. B.;
Larsen, F. K.; Singerean, L.; Timco, G. A.; Gerbeleu, N. V.; Jennings,
K. R.; Eyler, J. R. Inorg. Chim. Acta 2001, 319, 23. (k) Vlachos, A.;
Psycharis, V.; Raptopoulou, C. P.; Lalioti, N.; Sanakis, Y.; Diaman-
topoulos, G.; Fardis, M.; Karayanni, M.; Papavassiliou, G.; Terzis,
A. Inorg. Chim. Acta 2004, 357, 3162. (l) Baranwal, B. P.; Fatma, T.
J. Mol. Struct. 2005, 750, 72. (m) Winpenny, R.; Smith, A.; Coxall,
B.; Parsons, S.; Messenger, D. Private communication, 2005. (n)
Parsons, S.; Murrie, M.; Teat, S.; Winpenny, R.; Wood, P. Private
communication, 2004. (o) Winpenny, R.; Smith, A.; Parsons, S.;
Messenger, D. Private communication, 2005. (p) Winpenny, R.; Smith,
A.; Parsons, S.; Messenger, D. Private communication, 2005. (q)
Batsanov, A. S.; Struchkov, Y. T.; Gerbeleu, N. V.; Timko, G. A.;
Manole, O. S. Koord. Khim. 1994, 20, 833. (r) Coxall, R. A.; Parkin,
A.; Parsons, S.; Smith, A. A.; Timco, G. A.; Winpenny, R. J. Solid
State Chem. 2001, 159, 321.
The magnetic behavior of complex 1, which can be
regarded as a geometrically frustrated system, is studied by
(5) Tsukerblat, B. S.; Belinskii, M. I.; Fainzil’berg, V. E. SoV. Sci. ReV.
B Chem. 1987, 9, 337.
(6) Dzialoshinski, I. Phys. Chem. Solids 1958, 4, 241.
(7) (a) Moriya, T. Phys ReV. 1960, 117, 635. (b) Moriya, T. Phys. ReV.
1960, 120, 91.
(8) Tsukerblat, B. S.; Kuyavskaya, B. Y.; Belinskii, M. I.; Ablov, A. V.
Theor. Chim. Acta 1975, 38, 131.
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Inorg. Chem. 2002, 41, 5821. (c) Ferrer, S.; Haasnot, J. G.; Reedijk,
J.; Mu¨ller, E.; Biagini Cingi, M.; Lanfranchi, M.; Manotti Lanfredi,
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11018 Inorganic Chemistry, Vol. 46, No. 26, 2007

Oxo-Centered Trinuclear Cr(III) Complexes
Table 1. Crystallographic Data for Complexes 1 and 2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1
1
2
Cr1-O1
Cr1-O3
Cr1-O7
Cr1-O5
Cr2-O1
1.904(3)
1.976(2)
1.947(2)
2.031(3)
1.892(3)
Cr2-O2
Cr2-O4
Cr2-O6
Cr2-O8
Cr2-O9
1.957(2)
1.968(2)
1.973(3)
1.974(3)
2.049(2)
empirical formula
formula mass
cryst syst
space group
Z
a (Å)
b (Å)
c (Å)
C44H58Cr3NO2 5
1156.87
monoclinic
C2/c
C41H36Cr3N7O 7S3
990.95
monoclinic
P1/n
4
4
O1-Cr1-O7
O7-Cr1-O7#1
O1-Cr1-O3
O7-Cr1-O3
O7#1-Cr1-O3
O3-Cr1-O3#1
O1-Cr1-O5
O7-Cr1-O5
O3-Cr1-O5
O1-Cr2-O2
O1-Cr2-O4
O2-Cr2-O4
O1-Cr2-O6
O2-Cr2-O6
95.64(7)
168.73(13)
93.44(7)
87.69(11)
91.63(11)
173.12(14)
180.0
84.36(7)
86.56(7)
93.81(8)
94.60(8)
94.48(10)
95.47(9)
88.31(11)
O4-Cr2-O6
O1-Cr2-O8
O2-Cr2-O8
O4-Cr2-O8
O6-Cr2-O8
O1-Cr2-O9
O2-Cr2-O9
O4-Cr2-O9
O6-Cr2-O9
O8-Cr2-O9
Cr2#1-O1-Cr2
Cr2#1-O1-Cr1
Cr2-O1-Cr1
169.34(10)
95.60(9)
10.9596(9)
24.9214(19)
21.9436(17)
90
92.268(2)
90
5988.7(8)
1.276
0.609
0.71073
173(2)
1.86-28.30°
18057
4420
7436
359
R1 ) 0.0532
wR2 ) 0.1622
R1 ) 0.0904
wR2 ) 0.1809
0.991
12.037(3)
16.864(4)
21.384(5)
90
100.210(5)
90
4272.0(18)
1.541
0.955
0.71073
173(2)
1.55-20.81°
22149
3407
4473
644
R1 ) 0.0507
wR2 ) 0.1311
R1 ) 0.0700
wR2 ) 0.1433
1.035
170.42(10)
86.54(11)
89.02(12)
177.97(11)
84.80(10)
84.05(11)
85.97(11)
85.84(11)
120.01(14)
120.00(7)
120.00(7)
R (deg)
â (deg)
γ (deg)
V (Å3)
F (calcd) (g/cm3)
µ
calcd (mm-1
λ(Mo KR) (Å)
)
T (K)
θ range for data collection
total reflns
reflns I > 2σ
reflexions collected
params refined
final R indices [I > 2σ(I)]
The structures were solved by direct methods, using the SHELXS
computer program,13 and refined by full-matrix least-squares method
with the SHELX97 computer program,13 using 7436 reflections for
final R indices [for all data]
2
1 and 4473 reflections for 2. The function minimized was Σw||Fo|
2 2
- |Fc| | , where w ) [σ2(I) + (0.1067P)2]-1 for 1 and w ) [σ2(I)
goodness-of-fit on F2
largest diff peak, hole (e Å-3
)
1.255 -0.471
1.061 -0.472
+ (0.0927P)2 + 4.466P]-1 for 2, and P ) (|Fo| + 2|Fc| )/3, f, f′,
and f′′ were taken from International Tables of X-ray Crystal-
lography.14 For 1, the hydrogen atoms were calculated with the
exception of those corresponding to the water and methanol
molecules that could not be located. For 2, 10 hydrogen atoms were
calculated, and 25 hydrogen atoms were located from a difference
synthesis, while the remaining hydrogen atoms were refined without
constraints. The computed hydrogen atoms were refined using a
riding model with the isotropic temperature factor for H atoms equal
to 1.2 times the equivalent temperature factor of the atom which is
linked.
2
2
considering that the antisymmetric exchange interaction is
operating. The corresponding calculations and results are
reported.
Experimental Section
Materials. All starting materials were purchased from Aldrich
and were used without further purification.
Synthesis of the New Complexes. [Cr3(µ3-O)(µ2-PhCOO)6-
(H2O)3]NO3‚4H2O‚2CH3OH (1). A solution of Cr(NO3)3‚9H2O
(0.80 g, 2 mmol) in methanol (20 mL) was added to an acetonitrile
(20 mL) solution of benzoic acid (1.45 g, 12 mmol) and stirred
under reflux for 1 h. Green crystals, suitable for X-ray structure
analysis, were grown by slow evaporation at room temperature of
the green solution after 1 day (yields 90%). Anal. Calcd for 1
(C44H58Cr3NO25): C, 46.68; N, 1.21; H, 5.05. Found: C, 46.4; N,
1.2; H, 5.2.
Physical Measurements. Magnetic measurements were carried
out in the “Unitat de Messures Magne`tiques (Universitat de
Barcelona)” on polycrystalline samples (20 mg) with a Quantum
Design SQUID MPMS-XL magnetometer working in the 2-300
K range. The magnetic field was 10000G at high temperature and
200 G at low temperature. The field-dependent magnetization was
measured in the applied magnetic field range 0-5 T at 2 K. The
diamagnetic corrections were evaluated from Pascal’s constants.
The X-band EPR spectra were recorded on powder samples with a
Bruker 300E automatic spectrometer, varying the temperature
between 4 and 300 K.
[Cr3(µ3-O)(µ2-PhCOO)2(µ2-OCH2-CH3)2(bpy)2(NCS)3] (2). A
solution of NH4SCN (0.84 g, 10.98 mmol) in DMF (10 mL) and
bpy (0.86 g, 5.49 mmol) were added to a solution of complex 1
(1.86 g, 1.83 mmol) in ethanol (15 mL). The resulting solution
was allowed to stand at 138 °C for 1 day under autogenous pressure
in a Teflon-lined stainless steel autoclave. Slow cooling of the
resulting solution to room temperature afforded brown crystals
suitable for X-ray structure analysis (yields 75%). Anal. Calcd for
2 (C41H36Cr3N7O7S3): C, 49.69; N, 9.89; H, 3.66; S, 9.71. Found:
C, 50.0; N, 9.8; H, 3.7; S, 9.7.
Results and Discussion
Description of the Structure of Complexes [Cr3(µ3-O)-
(µ2-PhCOO)6(H2O)3]NO3‚4H2O‚2CH3OH (1) and [Cr3(µ3-
O)(µ2-PhCOO)2(µ2-OCH2-CH3)2(bpy)2(NCS)3] (2). The
structure of these complexes was determined by X-ray
crystallography. For both complexes, the crystal system is
monoclinic, with space group C2/c for complex 1 and P1/n
for complex 2. Selected bond lengths and angles are listed
in Tables 2 and 3 for complexes 1 and 2, respectively.
The structure of complex 1 is built by trinuclear cations
Crystal Structure Determination. Crystal data and details on
the data collection and refinement are summarized in Table 1.
Suitable single crystals of 1 and 2 were selected and mounted on
a Bruker SMART-CCD area diffractometer with graphite mono-
chromated. Unit-cell parameters were determined from 359 frames
for 1 and 244 for 2 of intensity data covering 0.3° in ω over a
hemisphere of the reciprocal space by combination of three exposure
sets, and refined by least-squares method. Intensities were collected
using ω/2θ. Lorentz-polarization and absorption corrections were
made.
-
[Cr3(µ3-O)(µ2-PhCOO)6(H2O)3]+, with NO3 ions acting as
(13) Sheldrick, G. M. A computer program for determination of crystal
structure; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(14) International Tables of X-ray Crystallography; Kynoch Press: Bir-
mingham, U.K., 1974; Vol. iv, pp 99-100, 149.
Inorganic Chemistry, Vol. 46, No. 26, 2007 11019

Figuerola et al.
Table 3. Selected Bond Lengths (Å) and Angles [deg] for 2
Cr01-O1
Cr01-O6
Cr01-O7
Cr01-O2
Cr01-O5
Cr01-N1
Cr02-O1
Cr02-O6
Cr02-O3
1.922(4)
1.964(4)
1.968(4)
2.014(4)
1.992(4)
2.013(6)
1.895(4)
1.936(4)
1.982(4)
Cr02-N2
Cr02-N21
Cr02-N22
Cr03-O1
Cr03-O7
Cr03-O4
Cr03-N3
Cr03-N32
Cr03-N31
2.019(6)
2.064(5)
2.076(5)
1.902(4)
1.958(4)
1.988(4)
2.015(6)
2.050(5)
2.064(5)
O1-Cr01-O6
O1-Cr01-O7
O6-Cr01-O7
O1-Cr01-O5
O5-Cr01-O6
O5-Cr01-O7
O1-Cr01-N1
O6-Cr01-N1
O7-Cr01-N1
O5-Cr01-N1
O1-Cr01-O2
O6-Cr01-O2
O7-Cr01-O2
O5-Cr01-O2
N1-Cr01-O2
O1-Cr02-O6
O1-Cr02-O3
O3-Cr02-O6
N2-Cr02-O1
O6-Cr02-N2
O3-Cr02-N2
O6-Cr02-N21
O3-Cr02-N21
N2-Cr02-N21
80.38(16)
82.00(15)
162.14(17)
90.93(16)
87.87(17)
89.55(16)
176.90(19)
101.7(2)
96.03(19)
91.45(19)
89.24(16)
91.12(17)
91.52(16)
178.93(18)
88.42(19)
81.76 (16)
91.51(16)
91.82(17)
176.2(2)
N22-Cr02-O1
O6-Cr02-N22
O3-Cr02-N22
N2-Cr02-N22
N22-Cr02-N21
O1-Cr02-N21
O1-Cr03-O7
O1-Cr03-O4
O4-Cr03-O7
O1-Cr03-N3
O7-Cr03-N3
O4-Cr03-N3
O1-Cr03-N32
O7-Cr03-N32
O4-Cr03-N32
N3-Cr03-N32
O1-Cr03-N31
O7-Cr03-N31
O4-Cr03-N31
N3-Cr03-N31
N31-Cr03-N32
Cr01-O1-Cr03
Cr01-O1-Cr02
Cr03-O1-Cr02
95.21(17)
175.88(19)
91.04(19)
87.9(2)
78.1(2)
91.96(17)
82.77(16)
90.40(16)
93.62(16)
177.70(18)
94.96(18)
90.10(18)
90.11(17)
93.58(17)
172.78(18)
89.68(19)
93.65(17)
171.68(18)
93.91(18)
88.55(19)
78.87(19)
97.11(16)
98.12(16)
164.5(2)
95.0(2)
90.63(18)
99.14(19)
168.9(2)
86.53(19)
counteranion, and solvent methanol and water molecules. The
ORTEP view, with atom labeling scheme and the core of
the trinuclear entity are given in Figure 1a,b, respectively.
Their skeletal structure is very similar to those of other
trinuclear chromium(III) carboxylates.4c,h,j,m-r,15-19 The three
metal ions lie at the corners of a nearly equilateral triangle
of separations: Cr2-Cr2 ) 3.276 Å and Cr1-Cr2 ) 3.287
Å. The center of the triangle is occupied by triply bridging
oxygen not displaced out of the plane. The µ3-oxide atom is
the common vertex of the coordination octahedral around
the three Cr3+ ions. The trigonal geometry at the µ3-O bridge
is reflected by the analogous angles: Cr2-O1-Cr2 )
120.01° and Cr1-O1-Cr2 ) 119.99°, very close to 120°.
The benzoate ligand acts as bidentate and bridges couples
of Cr3+ ions, so that four benzoate oxygen atoms surround
each chromium(III) ion. A water molecule, in trans position
to the shared oxo ligand, acts as the sixth donor. Therefore,
the coordination at each Cr3+ ion includes six Cr-O bonds
in a distorted octahedral environment. An examination of
the angle values in the structure shows that all the µ3-O-
Cr-O (benzoate) angles exceed 90°, whereas all the O(ben-
zoate)-Cr-O(water) ones are smaller (Table 2). The trinu-
clear cations are self-assembled through hydrogen bonds
Figure 1. (a) ORTEP view of complex [Cr3(µ3-O)(µ2-PhCOO)6(H2O)3]-
NO3‚4H2O‚2CH3OH (1) with atom labeling scheme. (b) Core of the
trinuclear entity of 1.
giving rise to a 2-D supramolecular structure in the xy plane
(Figure S1, Supporting Information). The O9, O5 atoms of
the water molecules coordinated to the chromium ions, and
the O13/O13′ and O12 oxygen atoms of the NO3- counter-
anion and the O18 of the methanol molecules of crystalliza-
tion participated in this supramolecular structure. The shortest
inter-trinuclear distance Cu-Cu is 8.15 Å. Short contact
distances are shown in Table S1 (Supporting Information).
The structure of complex 2 is built only by neutral
trinuclear [Cr3(µ3-O)(µ2-PhCOO)2(µ2-OCH2-CH3)2(bpy)2-
(NCS)3] entities. The ORTEP view, with atom labeling
scheme, and the core of the trinuclear entity are given in
Figure 2a,b, respectively. The three metal atoms lie at the
corners of a nearly isosceles triangle of separations Cr(01)‚
‚‚Cr(02) ) 2.883 Å and Cr(01)‚‚‚Cr(03) ) 2.866 Å along
equivalent sides of the isosceles triangle, and Cr(02)‚‚‚Cr-
(03) ) 3.768 Å along the inequivalent one. The center of
the triangle is occupied by a triple-bridging oxygen atom.
The equivalent Cr(01)-Cr(02) and Cr(01)-Cr(03) sides of
the triangle are each bridged by the µ2-oxygen atom of the
ethanolate ligand and by a bidentate benzoate group; no
bridges were present along the inequivalent Cr(02)-Cr(03)
(15) Anson, C. E.; Chai-Sa’aed, N.; Bourke, J. P.; Cannon, R. D.;
Jayasooriya, U. A.; Powell, A. K. Inorg. Chem. 1993, 32, 1502.
(16) Glowiak, T.; Kozlowsk, H.; Erre, L. S.; Micera, G. Inorg. Chim. Acta
1996, 248, 99.
(17) Mullica, D. F.; Pennington, D. E.; Bradshaw, J. E.; Sappenfield, E.
L. Inorg. Chim. Acta 1992, 191, 3.
(18) Kato, H.; Nakata, K.; Nagasawa, A.; Yamaguchi, T.; Sasaki, Y.; Ito,
T. Bull. Chem. Soc. Jpn. 1991, 64, 3463.
(19) Chang, S. C.; Jeffrey, G. A. Acta Crystallogr. 1970, B26, 673.
11020 Inorganic Chemistry, Vol. 46, No. 26, 2007

Oxo-Centered Trinuclear Cr(III) Complexes
Figure 3. Magnetic susceptibility data in øMT form of 1. Solid lines above
the experimental data are theoretical curves derived from Hamiltonian 1.
In the inset is shown the low-temperature susceptibility data along with
the theoretical curve.
5.628 emu mol-1 K expected for three independent Cr3+ ions,
to the value of 0.43 emu mol-1 K at 10 K. Below that
temperature, a plateaulike feature appears until 2 K (0.35
emu mol-1 K) which is clear evidence that the total spin
1
(ST) for the ground state is ST ) /2.
The isotropic Heisenberg Dirac van Vleck (HDvV) Hamil-
tonian (eq 1) is the main term used to interpret the magnetic
data.
H ) -2J12S1S2 - 2J13S1S3 - 2J23S2S3
(1)
Superexchange has been studied in a large number of oxo-
centered trinuclear complexes, and it is known that the
coupling is antiferromagnetic (Jij < 0). Since the three
chromium atoms of the [Cr3O] unit define a quasiequilateral
triangle, the three Cr3+ ions can be considered equivalent,
and so the three Jij constants are equal. If we assume three
equal Jij’s, the data are reasonably reproduced with Jij )
-10.1 cm-1, g ) 1.97(1), and TIP ) 550 × 10-6 emu mol-1
(TIP ) temperature-independent paramagnetism). The qual-
ity of the magnetic susceptibility data does not vary
Figure 2. (a) ORTEP view of complex [Cr3(µ3-O)(µ2-PhCOO)2(µ2-OCH2-
CH3)2(bpy)2(NCS)3] (2) with atom labeling scheme. (b) Core of the
trinuclear entity of 2.
side. This lack of peripheral bridging ligands between Cr2-
Cr3 is the most important structural feature which is unique
among the structurally characterized (µ3-oxo)trichromium-
(III) complexes, and this characteristic is responsible for the
anomalous Cr-O1-Cr: 98.12° (Cr(01)-O1-Cr(02)), 97.11°
(Cr(01)-O1-Cr(03)), and 164.52° (Cr(02)-O-Cr(03). This
indicates that the hybrids of the oxygen central atoms are
farther than regular sp2 hybridation. Chromium atoms Cr-
(02) and Cr(03) are also coordinated to bipyridine nitrogen
atoms, and all three chromium atoms have a terminal
thiocyanate ligand linked by the nitrogen atom. The Cr-
O-Cr angles from the µ2-ethanolate are 95.30° (Cr(02)-
O6-Cr(01)) and 93.79° (Cr(01)-O7-Cr(O3)). The neutral
trinuclear entities are isolated; no hydrogen bonds or π-π
stacking are operating in the structure, so we can conclude
that only van der Waals forces are responsible for the packing
in the global structure.
significantly if we adopt an isosceles triangle (J12 ) J13
*
J23). The resolution of our data in the high-temperature range,
however, does not allow us to make an accurate determina-
tion of the difference |J12 - J23|. In the inset of Figure 3 is
shown the low-temperature magnetic data where an obvious
discrepancy exists between the magnetic model and the
experimental points. In order to clarify the low-temperature
behavior, a systematic measurement of the susceptibility data
was carried out at different magnetic fields and is shown in
Figure 4. It is well documented that the antisymmetric
exchange interaction plays an important role in the magnetic
behavior of the system, and so a new magnetic model was
used where this kind of interaction was also considered:
Magnetic Studies.[Cr3(µ3-O)(µ2-PhCOO)6(H2O)3]NO3‚
4H2O‚2CH3OH (1). The temperature dependence of the
magnetic susceptibility data in øMT form (øM being the
magnetic susceptibility per [Cr3] unit) is shown in Figure 3.
The øMT data decrease smoothly from a value around 4.73
emu mol-1 K at room temperature, smaller than the value
H ) -2J0(S1‚S2 + S2‚S3) - 2J1(S1‚S3) +
G([S1 × S2] + [S2 × S3] + [S3 × S1]) +
(gz cos θ + gxy sin θ)âH(S1 + S2 + S3) (2)
Inorganic Chemistry, Vol. 46, No. 26, 2007 11021

Figuerola et al.
Figure 4. Low-temperature susceptibility data, at different magnetic fields,
in the form of øMT vs T of 1. The solid lines represent the theoretical curves
according to Hamiltonian 2. See text for details.
Figure 5. Simulation of the susceptibility equation in the form øMT vs
log T for different values of the G and δ parameters at external field H )
1 T according to eq 2. Solid squares are the experimental points of the
susceptibility data at H ) 1 T of 1.
where J0, J1 are the isotropic exchange interactions (due to
the symmetry lowering) and G is the antisymmetric exchange
vector parameter for two-center interaction. In trigonal
systems, the components of G are assumed to follow the
relation Gz . (Gx,Gy) ≈ 0 (z being the 3-fold axis), which
can be derived from the properties of the G vector.7 This
kind of interaction perturbs only the two lowest Kramers
1
doublets of ST ) /2, and the diagonalization yields the
following energy levels:
2
2
2
2
2 1/2
x
[∆E + |M| ( 2 δ |M| + |M‚G| ]
E ) (
(3)
2
where |M| ) (g2 sin2θ+g2cos2θ) âH is the Zeeman term,
x
xy
z
x
|G| ) 4 3 |Gzz|, assuming |Gxx| ) |Gyy| ∼0 which is valid
x
3
for a weakly axial system with J0 ≈ J1, |M‚G| ) 4
2
2
x
GzzgzâH cos θ, and ∆E ) δ +|G| while the δ factor is
equal to 4|J0 - J1|. It must be pointed out that the sign of
the G is also undefined under these conditions. It has been
3
shown that ST ) /2 is not perturbed by the antisymmetric
exchange term, G, and does not affect the description of the
1
ST ) /2 states.
Figure 6. EPR spectra from a polycrystalline sample of 1 recorded at
different temperatures from 4 K to room temperature. Other EPR condi-
tions: modulation amplitude, 10 Gpp; microwave frequency, 9.40 GHz.
The components of the susceptibility tensor were calcu-
lated according to a generalized susceptibility equation,8 and
it was necessary to consider the parallel and perpendicular
susceptibility components separately, since they are affected
differently by the G vector. Using this approach, it was
possible to fit the low-temperature data, 1.8-8 K, and the
fitting results are shown in Figure 4 as solid lines while the
resulting fitting parameters are, assuming gxy ) gz ) 2.0 as
fixed, |Gzz| ) 0.25 cm-1, δ ) 2.5 cm-1, TIP ) 550 × 10-6
emu mol-1. In order to investigate the influence of G and δ
parameters in the low-temperature behavior of the suscep-
tibility, different simulations were carried out and are shown
in Figure 5. While each of the two paramaters varied, the
other one was fixed to the value obtained from the fitting
procedure. When the δ factor increases, the susceptibility
plateaus are shifted toward higher values (closer to 0.375
emu mol-1 K) while an increase in the G parameter causes
a drastic divergence from the plateau to appear. In Figure
S2 (Supporting Information) is shown a simulation where
only the Zeeman part of eq 1 was considered fixing (G, δ)
) (0, 0). As it is clearly shown, a more anisotropic
Hamiltonian is necessary.
Magnetization data in the field range 0-6.5 T at 2 K are
shown in Figure S3 (Supporting Information) along with the
1
Brillouin function of a ST ) /2 system with g ) 2. This
behavior is in accordance with the susceptibility data and
1
reveals that the ground state is indeed an ST ) /2 one. It
was very difficult to extract information about the previous
parameters from the magnetization data.
EPR Spectroscopy. EPR spectra from a polycrystalline
sample of complex 1 at different temperatures from 4 K to
room temperature are shown in Figure 6. At 4.2 K a strong
absorption peak is observed at geff ) 1.97 accompanied by
a derivative feature at geff ) 1.70. We attribute these signals
11022 Inorganic Chemistry, Vol. 46, No. 26, 2007

Oxo-Centered Trinuclear Cr(III) Complexes
1
to an S ) /2 with axial g-tensors (g , g ) ) (1.98, 1.54).
||
The temperature dependence of these signals indicates that
their origin is from the ground state since after 20 K new
signals appear at lower magnetic fields revealing the popula-
tion of excited states.
For exchange coupled trimers, the large anisotropy in the
1
g-tensor for the S ) /2 ground state with a rather small g
value is usually attributed to the effects of an antisymmetric
exchange interaction according to eq 2.20-22 The effect of
this antisymmetric interaction is to induce an axial anisotropy
in the g-tensor. More explicitly, the g component is not
||
affected whereas a shift toward lower g-values is observed
for the g component. From the EPR findings, compound 1
has isosceles magnetic symmetry or lower (δ * 0); the
antisymmetric exchange is important (G * 0), and ∆E >
hν. The resulting relationship is11
1/2
δ2 - (hν)2
g ) g0
(4)
2
2
[
]
∆E - (hν)
where hν is the microwave energy (≈0.31 cm-1), g0 is the
true g calculated from the magnetic studies (gxy ) gz ) 2.0),
and all the other symbols have the meaning defined in eq 2.
According to the fitting results of the susceptibility data, g /
g0 ) 0.78, which is close to the value obtained from the
EPR spectrum (0.77). This is clear evidence that the
susceptibility results are in accordance with the EPR findings.
[Cr3(µ3-O)(µ2-PhCOO)2(µ2-OCH2-CH3)2(bpy)2-
(NCS)3] (2). The temperature dependence of the magnetic
susceptibility data in the øMT form (øM being the magnetic
susceptibility per [Cr3] entity) is shown in Figure 7, top. At
room temperature, the øMT value is 3.29 emu mol-1 K,
smaller than the value of 5.628 emu mol-1 K expected for
three independent Cr3+ ions, and decreases to the value of
1.88 emu mol-1 K at 50 K. After that temperature, a plateau
appears until 2 K, which is clear evidence that the total spin
Figure 7. Top: Thermal dependence at 10000 G of øMT of 2. Bottom:
Magnetization vs H (2 K) of 2.
3
of a ST ) /2 system with g ) 1.99. This behavior is in
accordance with the susceptibility data.
EPR Spectroscopy. X-band EPR measurements were
carried out in powder samples of 2 and are shown in Figure
S4 (Supporting Information). In the powder spectrum at 4
K, there are well-resolved peaks (shown with asterisks in
Figure 8), which appear to belong to excited states such as
the narrow small peak at about g ) 4.5 as well as the well
defined shoulder at g ) 3.14. Also, for fields greater than
5000 G, it is difficult to observe the contribution of excited
states. All these transitions were excluded from the simula-
tion procedure that was carried out with the following
3
(ST) for the ground state is equal to /2. The fit of the
experimental data, corrected for diamagnetic contributions
and TIP (240 × 10-6/Cr3+),23 was performed according with
the isotropic HDvV Hamiltonian (eq 5)
H ) -2J1(S1S2 + S1S3) - 2J2(S2S3)
(5)
3
Hamiltonian equation for a spin S ) /2:25
where J1 corresponds to the coupling through the equivalent
sides of the triangle and J2 corresponds to the nonequivalent
one (Figure 7, top). The free parameters were J1, J2, and g.
The fit made by the irreducible tensor operator formalism
(ITO), using CLUMAG program,24 gave the following
results: J1 ) -7.44 cm-1, J2 ) -51.98 cm-1, g ) 1.99 and
R ) 2.67 × 10-4.
1
3
H ) D Sz2 - S(S + 1) + E(Sx2 - Sy2) + gzµBHzSz +
[
]
gxµBHxSx + gyµBHySy
The simulated spectrum is also shown in Figure 8 as a solid
line.
An isotropic magnetic-field domain line width was used
for the powder spectrum (lw ) 35 G), while the broadness
of the spectrum reveals the g-strain effects. Gaussian
distributions of the g-principal values were used, and the
results are (σgx ) σgy ) 0.1, σgz ) 0.2). The principal
g-values for the powder spectrum are gxy ) 1.96(1), gz )
Magnetization data in the field range 0-5 T at 2 K is
shown in Figure 7 (bottom) along with the Brillouin function
(20) Bencini, A.; Gatteschi, D. In EPR of Exchange Coupled Systems;
Springer-Verlag: New York, 1990.
(21) Gaponenko, V. A.; Eremin, M. V.; Yablokov, Y. V. SoV. Phys. Solid
State 1973, 15, 909.
(22) Honda, M.; Morita, M.; Date, M. J. Phys. Soc. Jpn. 1992, 61, 3773.
(23) Boca, R. Struct. Bonding 2006, 117, 105.
(24) Gatteschi, D.; Pardi, L. Gazz. Chim. Ital. 1993, 123, 231.
(25) Stoll, S. Spectral Simulations in Solid-State EPR. Ph.D. Thesis, ETH
Zurich, 2003.
Inorganic Chemistry, Vol. 46, No. 26, 2007 11023

Figuerola et al.
of basic carboxylates [Cr3O(O2CR)6(H3O)3]+. The hydro-
thermal reaction of complex 1 in ethanol, SCN-, and bpy
produces complex 2 which involves loss of the three
coordinated water molecules and four carboxylic acids and
the coordination of three thiocyanate ions, three ethoxo
bridging ligands, and two bpy molecules. A cluster of higher
nuclearity has not been obtained, but a new skeleton of the
[Cr3O] core was found.
In order to justify the low-temperature magnetic behavior
of complex 1, the antisymmetric exchange interaction term
(G) was added to the HDvV Hamiltonian which represents
the spin-orbit coupling attributed to the orbitally degenerated
ground term. This antisymmetric term splits the two S ) 1/2
Kramers doublets and introduces an axial anisotropy in the
system which can be clearly observed by EPR spectros-
copy: The g and g values are 1.98 and 1.54, respectively,
||
at 2 K. The magnitude of the J value is similar to the value
of the interaction constant observed in analogous [Cr3O]
complexes previously reported.
Complex 2 shows different geometry, and one of the
interactions between two chromium(III) ions (J2) is different
from the other two (J1). The J1 value includes the coupling
through the triple-bridging oxygen atom, the benzoate, and
the ethanolate bridging ligands. The magnitude of J2 is the
highest, since the interaction is mediated by an oxo bridged
with a CrOCr angle of 164.5° for which a high value of
about -60 cm-1 is expected for the exchange coupling
constant by the literature.28 The magnetization data allows
Figure 8. Simulated EPR spectrum (sim line) and temperature dependence
of the powder spectra showing with asterisks transitions belonging to excited
states. See text for details.
1.98(1). The values for the crystal field terms are D ) 1.5-
(5) cm-1 and λ ) E/D ) 0.3, revealing the rhombic character
of the system. Gaussian distributions of the D and E
parameters were also used (σ(D) ) 0.3, σ(E)) 0.03). Several
complexes of Cr3+ have large D-values and were calculated
through simulations of EPR X-band spectra.26 The simulation
showed that the experimental line shape cannot be repro-
1
us to deduce the ground term of S ) /2, characteristic of
equilateral triangular chromium(III), for complex 1 and S )
3/2 for complex 2, which is confirmed by EPR measurements.
Acknowledgment. This work was financially supported
by the Spanish Government (Grant CTQ2006-03949). We
also thank Nu´ria Clos, Unitat de Messures Magne`tiques,
Serveis Cient´ıfics i Te`cnics, Universitat de Barcelona, for
the magnetic measurements.
3
duced by an S ) /2 system with a well-defined value for
E/D, even if one employs angular dependent line shape
distributions of the type W2 ) ∑W 2l 2, where li denotes
x
i
i
Supporting Information Available: Additional tables and
figures. CIF data. CCDC-634867-634868 (1 and 2), which can
be obtained free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. This material
is available free of charge via the Internet at http://pubs.acs.org.
the direction cosines of the applied field. There are well-
resolved peaks which appear to belong to excited states and
influence the line shape dramatically.
Conclusions
IC700606E
The kinetically inert nature of the Cr3+ ion usually requires
reactions to be carried out at high temperature to obtain
polymetallic clusters of high nuclearity:27 one of the most
successful synthetic routes has involved the direct heating
(27) (a) Talbot-Eeckelaers, C. E.; Rajaraman, G.; Cano, J.; Arom´ı, G.; Ruiz,
E.; Brechin, E. K. Eur. J. Inorg. Chem. 2006, 3382. (b) Laye, R. H.;
McInnes, E. J. L. Eur. J. Inorg. Chem. 2004, 2811. (c) McInnes, E. J.
L.; Piligkos, S.; Timco, G. A.; Winpenny, R. E. P. Coord. Chem. ReV.
2005, 249, 2577. (d) Bino, A.; Johnston, D. C.; Goshorn, D. P.;
Halberd, T. R.; Stiefel, E. I. Science 1988, 241, 1479.
(26) Shaham, N.; Cohen, H.; Meyerstein, D.; Bill, E. J. Chem. Soc., Dalton
Trans. 2000, 3082 and references therein.
(28) Wang, C.; Fink, K.; Staemmler, V. Chem. Phys. 1995, 201, 87.
11024 Inorganic Chemistry, Vol. 46, No. 26, 2007