Hexanuclear iron(III) salicylaldoximato complexes presenting the [Fe 6(μ3-O)2(μ2-OR) 2]12+ core: Syntheses, crystal structures, and spectroscopic and magnetic characterization

Article abstract of DOI:10.1021/ic051945q

The use of salicylaldehyde oxime (H₂salox) in iron(III) carboxylate chemistry has yielded two new hexanuclear compounds [Fe ₆(μ₃-O)₂(O₂CPh) ₁₀(salox)₂(L)₂]·xMeCN·yH

Full text of DOI:10.1021/ic051945q

Inorg. Chem. 2006, 45, 2317−2326
Hexanuclear Iron(III) Salicylaldoximato Complexes Presenting the
+
[Fe6(µ3-O)2(µ
2-OR)2]12 Core: Syntheses, Crystal Structures, and
Spectroscopic and Magnetic Characterization
Catherine P. Raptopoulou,*,† Athanassios K. Boudalis,† Yiannis Sanakis,† Vassilis Psycharis,†
Juan Modesto Clemente-Juan,‡ Michael Fardis,† George Diamantopoulos,† and George Papavassiliou†
Institute of Materials Science, NCSR “Demokritos”, 15310 Aghia ParaskeVi, Athens, Greece, and
Instituto de Ciencia Molecular, UniVersidad de Valencia, c/ Doctor Moliner 50,
46100 Burjassot, Spain
Received November 10, 2005
The use of salicylaldehyde oxime (H2salox) in iron(III) carboxylate chemistry has yielded two new hexanuclear
compounds [Fe6( 3-O)2(O2CPh)10(salox)2(L)2] xMeCN yH2O [L MeCONH2, x 6, y 0 (1); L H2O, x 2,
3 (2)]. Compound 1 crystallizes in the triclinic space group P1 with (at 25 C) a 13.210(8) Å, b 13.87(1)
17.04(1) Å, R ) 105.79(2) â ) 96.72(2) γ ) 116.69(2) , V 1. Compound
2578.17(2) Å3, and Z
2 crystallizes in the monoclinic space group C2/c with (at 25 C) a 21.81(1) Å, b 17.93(1) Å, c
Å, â ) 111.70(2) , V 4. Complexes 1 and 2 contain the [Fe6(
10070(10) Å3, and Z
3-O)] triangular subunits linked by two
µ
‚
‚
)
)
)
)
)
y
)
h
°
)
)
Å, c
)
°
,
°
,
°
)
)
°
)
)
)
27.72(1)
+
°
)
)
µ
3-O)2(
µ
2-OR)2]12 core
-
and can be considered as two [Fe3(
ligands, which show the less common
µ
µ
µ
2-oximato O atoms of the salox2
:
3 η1
:
η2
:
η
1 coordination mode. The benzoato ligands are coordinated through
2 η1 η1 mode. The terminal MeCONH2 ligand in 1 is the hydrolysis product of the acetonitrile
the usual syn,syn-µ : :
solvent in the presence of the metal ions. Mo¨ssbauer spectra from powdered samples of 2 give rise to two well-
resolved doublets with an average isomer shift consistent with that of high-spin FeIII ions. The two doublets, at an
approximate 1:2 ratio, are characterized by different quadrupole splittings and are assigned to the nonequivalent
FeIII ions of the cluster. Magnetic measurements of 2 in the 2
interactions between the FeIII ions, stabilizing an S
0 ground state. NMR relaxation data have been used to
−300 K temperature range reveal antiferromagnetic
)
investigate the energy separation between the low-lying states, and the results are in agreement with the susceptibility
data.
Introduction
demands, the most productive way so far has been the
systematic exploration of the reaction conditions affecting
the identity of the products. This, in other words, constitutes
a “trial and error” method in which suitable modifications
Polynuclear 3d metal complexes have been extensively
studied over the past years not only as valuable models for
the study of biological systems1 but also as molecular
materials with promising properties (magnetic,2 optical,3
electronic,4 catalytic,5 etc) that are closely related to their
structural characteristics. Thus, to exploit these properties,
it is crucial to understand their origin and to be able to fine-
tune them through synthetic modifications. To satisfy these
(2) For recent examples involving manganese and iron complexes, see:
(a) Tasiopoulos, A. J.; Vinslava, A.; Wernsdorfer, W.; Abboud, K.
A.; Christou, G. Angew. Chem., Int. Ed. 2004, 43, 2117. (b) Soler,
M.; Wernsdorfer, W.; Folting, K.; Pink, M.; Christou, G. J. Am. Chem.
Soc. 2004, 126, 2156. (c) Aliaga-Alcalde, N.; Edwards, R. S.; Hill, S.
O.; Wernsdorfer, W.; Folting, K.; Christou, G. J. Am. Chem. Soc. 2004,
126, 12503. (d) Wang, S.; Zuo, J.-L.; Zhou, H.-C.; Choi, H. J.; Ke,
Y.; Long, J. R.; You, X.-Z. Angew. Chem., Int. Ed. 2004, 43, 5940.
(e) Oshio, H.; Hoshino, N.; Ito, T.; Nakano, M. J. Am. Chem. Soc.
2004, 126, 8805.
(3) (a) Niu, Y.; Song, Y.; Hou, H.; Zhu, Y. Inorg.Chem. 2005, 44, 2553.
(b) Susumu, K.; Duncan, T. V.; Therien, M. J. J. Am. Chem. Soc.
2005, 127, 5186. (c) Meng, X.; Song, Y.; Hou, H.; Fan, Y.; Li, G.;
Zhu, Y. Inorg. Chem. 2003, 42, 1306. (d) Tian, L.; Zhang, W.; Yang,
B.; Lu, P.; Zhang, M.; Lu, D.; Ma, Y.; Shen, J. J. Phys. Chem. B
2005, 109, 6944. (e) Hou, H.; Wei, Y.; Song, Y.; Mi, L.; Tang, M.;
Li, L.; Fan, Y. Angew. Chem., Int. Ed. 2005, 44, 6067.
* To whom correspondence should be addressed. E-mail: craptop@
ims.demokritos.gr. Tel: +3210-6503365. Fax: +3210-6519430.
† NCSR “Demokritos”.
‡ Universidad de Valencia.
(1) (a) Brechin, E. K.; Knapp, M. J.; Huffman, J. C.; Hendrickson, D.
N.; Christou, G. Inorg. Chim. Acta 2000, 297, 389. (b) Lippard, S. J.
Angew. Chem., Int. Ed. Engl. 1988, 27, 344. (c) Lippard, S. J.; Berg,
J. M. Principles of Bioinorganic Chemistry; University Science
Book: Mill Valley, CA, 1994. (d) Mishra, A.; Wernsdorfer, W.;
Abboud, K. A.; Christou, G. Chem. Commun. 2005, 54.
10.1021/ic051945q CCC: $33.50
Published on Web 02/09/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 5, 2006 2317

Raptopoulou et al.
of the reaction conditions lead to new molecular structures
with optimized physical properties.6,7 The experience gained
in each step is used to design new synthetic routes, thus
forming a positive feedback loop. However, at this moment
we are still in the realm of serendipitous assembly as far as
3d cluster chemistry is concerned.8 Therefore, more studies
in this direction need to be carried out.
recently reported13 the first two members of a new class of
SMMs containing exclusively MnIII ions with blocking
temperatures greater than 2 K, compounds [Mn6O2(O2CR)2-
(salox)6(EtOH)4] [R ) Me (I); Ph (II)], where salox2- adopts
the rare µ3:η1:η2:η1 coordination mode, which usually leads
to high-nuclearity complexes.14 Expanding our research into
iron(III) carboxylate chemistry, we have reported15 the
synthesis and structural, spectroscopic, and magnetic char-
acterization of two neutral trinuclear oxo-centered ferric
complexes, [Fe3(µ3-O)(O2CPh)5(salox)L1L2] [L1 ) L2 )
MeOH (III); L1 ) EtOH and L2 ) H2O (IV)]. In this case,
the salox2- ligand adopts the common µ2:η1:η2:η1 coordina-
tion mode so the possibility of further use of the oximato O
atom to bridge a second metal and to increase the nuclearity
of the derived complex is present. By working in the weakly
coordinating solvent MeCN, we isolated the hexanuclear
complexes [Fe6(µ3-O)2(O2CPh)10(salox)2(L)2]‚xMeCN‚yH2O
[L ) MeCONH2, x ) 6, y ) 0 (1); L ) H2O, x ) 2, y ) 3
(2)], which are studied structurally. 2 is additionally studied
spectroscopically (57Fe Mo¨ssbauer, solid-state NMR) and
magnetically. Furthermore, in addition to the reaction
pathways for the formation of 1 and 2, we report a new
reaction pathway to III and IV, to better map this reaction
system. In both 1 and 2, the salox2- ligand adopts the rare
µ3:η1:η2:η1 coordination mode, leading to a topology of the
six FeIII ions analogous to that of the hexanuclear manganese-
(III) complexes I and II (other crystallographically estab-
lished coordination modes of Hsalox- and salox2- have been
reported by us elsewhere15).
We have recently been interested in polynuclear metal
clusters that may display single-molecule magnet (SMM)
behavior both because of their potential technological ap-
plications9 and because SMMs exhibit interesting physical
and quantum phenomena previously predicted by theory.10
To function as SMMs, the metal clusters should combine a
large-spin ground state S and an Ising (or easy-axis) type of
magnetization expressed by an axial zero-field splitting tensor
with D < 0.11 While it is easier to satisfy the first requirement
by choosing metal ions with large spin values (e.g., FeIII
S
) 5/2, Mn III S ) 2, Mn IV S ) 3/2, etc.) and suitable bridging
ligands to promote high-spin ground states through suitable
exchange interactions and spin topology, it is very difficult
to satisfy the second requirement in a controllable way.
Hence, a plethora of high-spin molecules do not display
SMM behavior, and further studies need to be carried out
by using new ligands or a combination of ligands in order
to achieve this goal.
Our approach to the field concerns the investigation of
the reaction between 3d metal ions and salicylaldehyde oxime
(H2salox) in the presence of carboxylates. H2salox is a
multifunctional ligand that in its mono- or dianionic form
can adopt various coordination modes, giving rise to a
number of homo- and heterometallic complexes.12 We have
Experimental Section
Materials. All manipulations were performed under aerobic
conditions using materials as received (Aldrich Co.). All chemicals
and solvents were reagent-grade. Basic iron(III) benzoate “[Fe3-
(µ3-O)(O2CPh)6(H2O)2(OH)]” was synthesized as previously de-
scribed.16
(4) (a) Yu, G.; Yin, S.; Liu, Y.; Shuai, Z.; Zhu, D. J. Am. Chem. Soc.
2003, 125, 14816. (b) Bordiga, S.; Lamberti, C.; Ricchiardi, G.; Regli,
L.; Bonino, F.; Damin, A.; Lillerud, K.-P.; Bjorgen, M.; Zecchina, A.
Chem. Commun. 2004, 2300. (c) Shi, J.-M.; Xu, W.; Liu, Q,-V.; Liu,
F.-I.; Huang, Z.-I.; Lei, H.; Yu, W.-T.; Fang, Q. Chem. Commun. 2002,
756. (d) Coronado, E.; Clemente-Leo´n, M.; Gala´n-Mascaro´s, J. R.;
Gime´nez-Saiz, C.; Go´mez-Garc´ıa, C. J.; Mart´ınez-Ferrero, E. J. Chem.
Soc., Dalton Trans. 2000, 3955.
(5) For recent representative examples involving iron complexes, see: (a)
Legros, J.; Bolm, C. Chem.sEur. J. 2005, 11, 1086. (b) Avenier, F.;
Dubois, L.; Dubourdeaux, P.; Latour, J.-M. Chem. Commun. 2005,
480. (c) Be´nisvy, L.; Chottard, J.-C.; Marrot, J.; Li, Y. Eur. J. Inorg.
Chem. 2005, 999.
(6) (a) Brockman, J. T.; Abboud, K. A.; Hendrickson, D. N.; Christou,
G. Polyhedron 2003, 22, 1765. (b) Chakov, N. E.; Wernsdorfer, W.;
Abboud, K. A.; Hendrickson, D. N.; Christou, G. J. Chem. Soc., Dalton
Trans. 2003, 2243. (c) Kuroda-Sowa, T.; Nogami, T.; Konaka, H.;
Maekawa, M.; Munakata, M.; Miyasaka, H.; Yamashita, M. Polyhe-
dron 2003, 22, 1795.
(7) (a) Tsohos, A.; Dionyssopoulou, S.; Raptopoulou, C. P.; Terzis, A.;
Bakalbassis, E. G.; Perlepes, S. P. Angew. Chem., Int. Ed. 1999, 38,
983. (b) Papaefstathiou, G. S.; Perlepes, S. P.; Escuer, A.; Vicente,
R.; Font-Bardia, M.; Solans, X. Angew. Chem., Int. Ed. 2001, 40, 884.
(c) Boudalis, A. K.; Donnadieu, B.; Nastopoulos, V.; Clemente-Juan,
J. M.; Mari, A.; Sanakis, Y.; Tuchagues, J. P.; Perlepes, S. P. Angew.
Chem., Int. Ed. 2004, 43, 2266.
(8) Winpenny, R. E. W. J. Chem. Soc., Dalton Trans 2002, 1.
(9) (a) Leuenberger, M. N.; Loss, D. Nature 2001, 410, 789. (b) Cavallini,
M.; Gomez-Segura, J.; Ruiz-Molina, D.; Massi, M.; Albonetti, C.;
Rovira, C.; Veciana, J.; Biscarini, F. Angew. Chem., Int. Ed. 2005,
44, 888.
(10) (a) Friedman, J. R.; Sarachik, M. P.; Tejada, J.; Ziolo, R. Phys. ReV.
Lett. 1996, 76, 3830. (b) Thomas, L.; Lionti, F.; Ballou, R.; Gatteschi,
D.; Sessoli, R.; Barbara, B. Nature 1996, 383, 145. (c) Gatteschi, D.;
Sessoli, R.; J. Magn. Magn. Mater. 2004, 272-276, 1030.
(11) Christou, G.; Gatteschi, D.; Hendrickson, D. N.; Sessoli, R. MRS Bull.
2000, 25, 1.
Physical Measurements. Elemental analysis for C, H, and N
was performed on a Perkin-Elmer 2400/II automatic analyzer. IR
spectra were recorded as KBr pellets in the range of 4000-400
cm-1 on a Bruker Equinox 55/S FT-IR spectrophotometer. Variable-
temperature magnetic susceptibility measurements were carried out
on polycrystalline samples of 2 in the 2.0-300 K temperature range
using a Quantum Design MPMS SQUID susceptometer under a
magnetic field of 1.5 T. Diamagnetic corrections for the complexes
were estimated from Pascal’s constants. The calculation of the
magnetic susceptibility was accomplished using a modified version
of MAGPACK.17 Minimization was carried out with an adapted
(12) Chaudhuri, P. Coord. Chem. ReV. 2003, 243, 143.
(13) Milios, C. J.; Raptopoulou, C. P.; Terzis, A.; Lloret, F.; Vicente, R.;
Perlepes, S. P.; Escuer, A. Angew. Chem., Int. Ed. 2004, 43, 210.
(14) (a) Thorpe, J. M.; Beddoes, R. L.; Collison, D.; Garner, C. D.;
Helliwell, M.; Holmes, J. M.; Tasker, P. A. Angew. Chem., Int. Ed.
1999, 38, 1119. (b) Chaudhuri, P.; Hess, M.; Rentschler, E.; Wey-
hermu¨ller, T.; Flo¨rke, U. New J. Chem. 1998, 553. (c) Zerbib, V.;
Robert, F.; Gouzerh, P. J. Chem. Soc., Chem. Commun. 1994, 2179.
(15) Raptopoulou, C. P.; Sanakis, Y.; Boudalis, A. K.; Psycharis, V.
Polyhedron 2005, 24, 711.
(16) Earnshaw, A.; Figgis, B. N. J. Chem. Soc. A 1966, 1656.
(17) (a) Borra´s-Almenar, J. J.; Clemente-Juan, J. M.; Coronado, E.;
Tsukerblat, B. S. Inorg. Chem. 1999, 38, 6081. (b) Borra´s-Almenar,
J. J.; Clemente-Juan, J. M.; Coronado, E.; Tsukerblat, B. S. J. Comput.
Chem. 2001, 22, 985.
2318 Inorganic Chemistry, Vol. 45, No. 5, 2006

Hexanuclear Iron(III) Salicylaldoximato Complexes
version of MINUIT.18 The error factor R is defined as
Table 1. Crystallographic Data for 1‚6MeCN and 2‚2MeCN‚3H2O
1‚6MeCN
2‚2MeCN‚3H2O
2
(xexp - xcalc
)
R )
formula
fw
space group
T, °C
λ, Å
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
C
100H88Fe6N10O28
2212.90
P1h
C88H76Fe6N4O31
2020.63
C2/c
∑
2
Nxexp
25
25
where N is the number of experimental points. Mo¨ssbauer spectra
were taken with a constant-acceleration spectrometer using a 57Co
(Rh) source at room temperature and a variable-temperature Oxford
cryostat. 1H-pulsed NMR experiments were collected on a Bruker
MSL200 spectrometer operating at 4.7 T (200.1 MHz for 1H NMR).
An Oxford 1200 CF continuous-flow cryostat was employed for
measurements in the range of 5-300 K. The T1 spin-lattice
relaxation time was obtained using the standard saturation tech-
nique.19 The relaxation recovery, M(t), was generally a nonexpo-
nential function, mainly because of the many proton-nonequivalent
nuclei, and the relaxation data were fitted to a modified stretched
exponential function of the form20
Mo KR (0.710 73 Å)
13.210(8)
13.87(1)
17.04(1)
105.79(2)
96.72(2)
116.69(2)
2578.17(2)
1
Mo KR (0.710 73 Å)
21.81(1)
17.93(1)
27.72(1)
111.70(2)
10070(10)
4
F
calcd/Fmeasd, g cm-3
1.425/1.41
0.902
1.333/1.32
0.917
µ(Mo KR), mm-1
R1a
0.0718b
0.1876b
0.0726c
0.1759c
wR2a
a w ) 1/[σ2(Fo ) + (aP)2 + bP] and P ) (max)Fo (0) + 2Fc )/3. R1 )
2
2
2
∑(|Fo| - |Fc|)/∑(|Fo|) and wR2 ) {∑[w(Fo2 - Fc )2]/∑[w(Fo )2]}1/2
.
b For
2
2
n-1
t
T1
t
T1
M(t) ) M0 exp -
1 +
3922 reflections with I > 2σ(I). c For 2607 reflections with I > 2σ(I).
(
)
[
]
Method B. A quantity of III (0.042 g, 0.042 mmol) or IV (0.071
g, 0.071 mmol) was dissolved in MeCN (30 or 20 mL, respectively),
and the resulting deep-brown solutions were refluxed for 5 h. The
brown precipitates that formed were filtered off, washed with cold
MeCN, dried in vacuo, and characterized as 2 by FT-IR spectros-
copy. Characteristic IR data (cm-1, KBr pellet): 1596, ν(CdN);
1552, νas(CO2); 1406, νs(CO2) [∆ ) 146 cm-1]; 1295, ν(N-Oox);
482, νas(Fe3O). Yield: 0.03 g, ∼75%.
In the short time regime, t , T1, this model exhibits the desired
single-exponential behavior and has a finite time derivative at the
origin.
n-1
t
T1
t
t
T1
t ) 0: exp -
1 +
f exp -
(
)
[
]
[
]
T1
In the long time regime, t . T1, the model exhibits the limiting
stretched-exponential behavior:
[Fe3(µ3-O)(O2CPh)5(salox)(MeOH)2] (III). H2salox (0.068 g,
0.50 mmol) was added to a refluxing orange solution of “[Fe3(µ3-
O)(O2CPh)6(H2O)2(OH)]” (0.164 g, 0.17 mmol) in MeOH (20 mL).
The reflux was continued overnight, and small amounts of a black
precipitate were filtered off. The resulting solution was left for slow
evaporation, and after a period of 1 week, deep-brown needles
formed and were filtered off, washed with cold MeOH, and dried
in vacuo (yield: 0.13 g, ∼80%). Unit-cell determination of the
needles identified them as complex III [a ) 19.85(3) Å, b ) 27.17-
(4) Å, c ) 19.88(2) Å, â ) 98.88(3)°, and V ) 10590(10) Å3].15
[Fe3(µ3-O)(O2CPh)5(salox)(EtOH)(H2O)] (IV). H2salox (0.068
g, 0.50 mmol) was added to a refluxing orange solution of “[Fe3-
(µ3-O)(O2CPh)6(H2O)2(OH)]” (0.164 g, 0.17 mmol) in EtOH (20
mL). The reflux was continued for 1 h more, and the deep-brown
solution was left for slow evaporation. After a period of 2 weeks,
deep-brown prismatic crystals formed and were filtered off, washed
with cold MeOH, and dried in vacuo (yield: 0.12 g, ∼70%). Unit-
cell determination of the crystals identified them as complex IV [a
) 15.12(2) Å, b ) 15.63(2) Å, c ) 20.18(3) Å, â ) 95.32(3)°,
and V ) 4750(10) Å3].15
X-ray Crystallography. A red prismatic crystal of 1 (0.08 ×
0.18 × 0.55 mm) and a red prismatic crystal of 2 (0.12 × 0.16 ×
0.75 mm) were mounted in capillaries. Diffraction measurements
were made on a Crystal Logic Dual Goniometer diffractometer
using graphite-monochromated Mo KR radiation. Significant crystal
data and parameters for data collection are reported in Table 1.
Unit-cell dimensions were determined and refined by using the
angular settings of 25 automatically centered reflections in the range
of 11° < 2θ < 23°. Three standard reflections, monitored every
97 reflections, showed less than 3% intensity fluctuation and no
decay. Lorentz and polarization corrections were applied using
Crystal Logic software. The structures were solved by direct
methods using SHELXS-8621 and refined by full-matrix least-squares
techniques on F 2 with SHELXL-97.22 In both structural cases, the
crystals had poor diffraction ability (despite their sufficient size)
n-1
n
t
T1
t
T1
t
T1
exp -
1 +
f exp -
(
)
( )
[
]
[
]
Preparation of the Complexes. [Fe6(µ3-O)2(O2CPh)10(salox)2-
(MeCONH2)2]‚6MeCN (1). FeCl3‚6H2O (0.135 g, 0.50 mmol) was
added to a refluxing solution of H2salox (0.069 g, 0.50 mmol) and
sodium benzoate (0.216 g, 1.50 mmol) in acetonitrile (20 mL). The
color of the solution immediately turned to deep red. After 24 h of
reflux, a small amount of brown precipitate was filtered off, and it
was characterized as 1 by FT-IR spectroscopy. The deep-brown
filtrate was sealed and left undisturbed for about 2 months. By that
time, X-ray-quality red crystals of 1 had formed (yield: 0.02 g,
∼11%). The crystals of 1 were collected by filtration, washed with
cold MeCN, and dried in vacuo. The resulting powder was analyzed
as solvent-free. Anal. Calcd (found) for C88H70Fe6N4O28: C, 53.75
(53.60); H, 3.59 (3.55); N, 2.85 (2.82).
[Fe6(µ3-O)2(O2CPh)10(salox)2(H2O)2]‚2MeCN‚3H2O (2). Method
A. Solid H2salox (0.034 g, 0.25 mmol) was added to a refluxing
solution of “[Fe3(µ3-O)(O2CPh)6(H2O)2(OH)]” (0.082 g, 0.085
mmol) in MeCN (30 mL). The color of the solution immediately
turned from orange to deep brown. The reflux was stopped, and
the solution was placed in a closed vessel. After 3 days, X-ray-
quality red crystals had formed (yield: 0.064 g, ∼80%). The crystals
were collected by filtration, washed with cold MeCN, and dried in
vacuo. The resulting powder was analyzed as solvent-free. Anal.
Calcd (found) for C84H64Fe6N2O28: C, 53.54 (53.40); H, 3.42 (3.30);
N, 1.49 (1.25).
(18) James, F.; Roos, M. MINUIT Program, a System for Function
Minimization and Analysis of the Parameters Errors and Correlations.
Comput. Phys. Commun. 1975, 10, 345.
(19) Fukushima, E.; Roeder, S. B. W. Experimental Pulse NMR; Addison-
Wesley: Reading, MA, 1981.
(20) Peyron, M.; Pierens, G. K.; Lucas, A.; Hall, L. D.; Stewart, R. C. J.
Magn. Reson. A 1996, 118, 214.
Inorganic Chemistry, Vol. 45, No. 5, 2006 2319

Raptopoulou et al.
According to the first approach, the reaction of FeIII/
PhCO2 /H2salox in a 1:3:1 respective molar ratio in alcoholic
Scheme 1. Reaction Scheme for the Synthesis of Compounds 1, 2,
III, and IV
-
media has afforded the trinuclear oxo-centered compounds
[Fe3(µ3-O)(O2CPh)5(salox)L1L2] [L1 ) L2 ) MeOH (III);
L1 ) EtOH, L2 ) H2O (IV)].15 In both complexes, the
salox2- ligand is coordinated through the common µ2:η1:η1:
η1 mode, providing the possibility of further use of the
oximato O atom to bind to a second metal and to increase
the nuclearity of the derived complexes. By using the same
reaction “blend” in the weakly coordinating acetonitrile, this
possibility has been confirmed and the hexanuclear complex
1 has been isolated according to eq 1. In this case, it is
assumed that the source of the O2- ion is the H2O from the
starting material and/or the solvent.
6FeCl3‚6H2O + 10PhCOONa + 2H2salox MeCN8
[Fe O (PhCO ) (salox) (MeCONH ) ]
+ 10NaCl +
6
2
2 10
2
2 2
1
8HCl + 34H2O (1)
The terminal MeCONH2 ligand is the hydrolysis product of
the acetonitrile solvent, which is favored in the presence of
the metal ion.23 The above reaction afforded compound 1 in
very small quantities, making the study of its spectroscopic
and magnetic properties impossible.
By following the “building block” synthetic approach, the
1:1 reaction between basic iron benzoate “[Fe3(µ3-O)(O2-
CPh)6(H2O)2(OH)]” and H2salox in MeCN has afforded the
hexanuclear complex 2 in good yield, according to eq 2.
and the data were collected in increasing 2θ shells, and in each
case, the data collection was terminated when about 50% of the
collected shell data were unobserved. Nevertheless, the quality of
the collected data was adequate to establish the structure of the
complexes. Further experimental crystallographic details for 1:
2θmax ) 45°; scan speed, 3.0°/min; scan range, 1.6 + R1R2
separation; reflections collected/unique/used, 5494/5212 [Rint
)
0.0348]/5212; 613 parameters refined; (∆/σ)max ) 0.003; (∆F)max
/
(∆F)min ) 0.657/-0.643 e/Å3; R/Rw (for all data), 0.0987/0.2161.
H atoms were introduced at calculated positions as riding on bonded
atoms. All non-H atoms were refined anisotropically, except those
of the MeCN solvents, which were refined isotropically. The
assignment of the terminal ligand MeCONH2 instead of MeCO2H
(which can be the final hydrolysis product of MeCN in the presence
of metal ions) was based on H-bonding interactions between amide
H atoms and the acetonitrile solvent molecules. Further experimental
crystallographic details for 2: 2θmax ) 40°, scan speed, 1.5°/min;
scan range, 1.6 + R1R2 separation; reflections collected/unique/
used, 4667/4549 [Rint ) 0.0265]/4549; 571 parameters refined; (∆/
σ)max ) 0.001; (∆F)max/(∆F)min ) 0.558/-0.286 e/Å3; R/Rw (for
all data), 0.1451/0.2175. H atoms were introduced at calculated
positions as riding on bonded atoms. All non-H atoms were refined
anisotropically, except those of the MeCN and H2O solvents, which
were refined isotropically.
2[Fe3O(O2CPh)6(H2O)2(OH)] + 2H2salox MeCN8
[Fe O (O CPh) (salox) (H2O) ]
+ 2HO2CPh + 2H2O (2)
6
2
2
10
2
2
2
The same reaction mixture in alcoholic media has afforded
the trinuclear compounds III and IV, whose identities have
been proved by unit-cell determination.
From the molecular structures of compounds III and IV,
it is evident that they might be used as “building blocks”
themselves by using the oximato O atom of the salox2-
ligand to bind to a second metal, thus doubling the nuclearity
of the complex. This possibility has been explored by
dissolving compounds III and/or IV in acetonitrile. The
brown precipitate formed was identified as compound 2 by
FT-IR spectroscopy. Thus, it seems that, in the weakly
coordinating MeCN, the apical coordination positions previ-
ously occupied by ROH in III and IV are now available for
coordination by salox2- and subsequent dimerization, thus
increasing the nuclearity of the complex.
Results and Discussion
Synthesis. Synthetic efforts to prepare polynuclear com-
plexes in a more or less rational way have mainly led to two
synthetic approaches. The first one concerns the in situ
reaction of metal salts with suitable ligands, in ratios that
favor the formation of polynuclear instead of mononuclear
complexes, whereas the second one involves the use of small-
nuclearity complexes, called “building blocks”, to produce
higher-nuclearity aggregates. In this work, we have employed
both synthetic routes, which are shown in Scheme 1.
MeCN8
2[Fe3O(O2CPh)5(salox)(EtOH)(H2O)]
IV
[Fe O (O CPh) (salox) (H2O) ]
+ 2EtOH (3)
6
2
2
10
2
2
2
In the IR spectrum of 1, medium-intensity bands at 3063
and 2924 cm-1 are assigned to the ν(CH)aromatic and ν(CH3)
stretching vibrations, respectively. A strong-intensity band
at 1596 cm-1 and a medium-intensity band at 1292 cm-1
(21) Sheldrick, G. M. SHELXS-86: Structure SolVing Program; University
of Go¨ttingen: Go¨ttingen, Germany, 1986.
(22) Sheldrick, G. M. SHELXL-97: Crystal Structure Refinement; Uni-
versity of Go¨ttingen: Go¨ttingen, Germany, 1997.
2320 Inorganic Chemistry, Vol. 45, No. 5, 2006

Hexanuclear Iron(III) Salicylaldoximato Complexes
Figure 2. Partially labeled ORTEP plot of 2 with ellipsoids drawn at the
30% probability level. For clarity, H atoms and phenyl rings have been
omitted. Primed atoms are generated by symmetry (′ ) 0.5 - x, 1.5 - y,
1 - z).
Figure 1. Partially labeled ORTEP plot of 1 with ellipsoids drawn at the
30% probability level. For clarity, H atoms and phenyl rings have been
omitted. Primed atoms are generated by symmetry (′ ) -x + 1, -y + 1,
-z).
2 (the corresponding table, Table S1, for compound 1 is given
in the Supporting Information). Both compounds contain the
[Fe6(µ3-O)2(µ2-OR)2]12+ core, whose topology consists of six
FeIII ions arranged as two centrosymmetrically related [Fe3-
(µ3-O)] subunits bridged by two oximato O atoms (O1 and
O1′). The structure of each triangular [Fe3(µ3-O)] subunit is
quite similar to that of the precursor trinuclear compounds
III and IV. There are two pairs, Fe1/Fe2 and Fe2/Fe3, which
are bridged through one µ2-oxide and two µ2-benzoato
ligands, while the pair Fe1/Fe3 is bridged by one µ2-oxide,
one µ2-benzoato, and the oximato ligand. The sixth coordina-
tion position on Fe3, occupied by a terminal monodentate
ligand in the trinuclear III and IV, is in the case of 1 and 2
occupied by a µ2-oximato O atom belonging to the cen-
trosymmetrically related [Fe3(µ3-O)] subunit, thus increasing
the nuclearity of the derived complex. The structure of
complex 2 will be discussed in detail because compounds 1
and 2 have almost identical structures. The three FeIII ions
in each [Fe3(µ3-O)] subunit form an almost isosceles triangle
(see Table 2), and the closest Fe‚‚‚Fe distance between the
two [Fe3(µ3-O)] subunits is 3.274(5) Å. The Fe-Oox bond
distances are in the range of 1.884-1.928 Å, and the [Fe3-
(µ3-O)] subunit is planar. Ions Fe2 and Fe3 have an
octahedral O-rich coordination environment, while Fe1 has
a distorted octahedral O5N coordination geometry. The two
salicylaldoximato ligands adopt the rare µ3:η1:η2:η1 coordina-
tion mode, which has not only been observed in I and II13
and in [MIII6(µ3-O)2(salox)6(µ2-O2CR)2(OH2)2(RCN)2] (M )
are assigned to the ν(C‚‚‚N) and ν(N-Oox) vibrations of the
salicylaldoximato ligand, respectively.24 The frequencies of
the νas(CO2) and νs(CO2) bands of PhCO2- at 1555 and 1408
cm-1, respectively, differ by 147 cm-1, less than that of
NaO2CPh (184 cm-1), as expected for the bridging modes
of benzoate ligation.25 The two bands at 3365 and 3186 cm-1
are assigned to the NH stretching modes of the H-bonded
NH2 group of the terminal MeCONH2, while the carbonyl
absorption (amide I band) is observed at 1659 cm-1.26
A
medium-intensity band at 480 cm-1 in the spectrum of 1 is
characteristic of the presence of the [Fe3O] moiety in the
structure. In the IR spectrum of 2, medium-intensity bands
at 3409 and 3063 cm-1 are attributed to the ν(OH)H O and
2
ν(CH)aromatic vibrations, respectively. A strong-intensity band
at 1597 cm-1 and a medium-intensity band at 1295 cm-1
are assigned to the ν(C‚‚‚N) and ν(N-Oox) vibrations of the
salicylaldoximato ligand. The frequencies of the νas(CO2) and
-
νs(CO2) bands of PhCO2 are observed at 1552 and 1407
cm-1 (∆ ) 145 cm-1), respectively. Finally, a medium-
intensity band at 482 cm-1 is assigned to the Vas(Fe3O)
vibration. In the IR spectra of both 1 and 2, it is characteristic
that the ν(N-Oox) vibration of the salicylaldoximato ligand
is observed at higher frequency with respect to III and IV,
as a result of the µ3:η1:η2:η1 coordination mode in the
hexanuclear complexes versus the µ2:η1:η1:η1 coordination
mode in the trinuclear ones.
Description of the Structures. Compound 1 crystallizes
in the triclinic space group P1h, and compound 2 crystallizes
in the monoclinic space group C2/c. The molecular structures
of 1 and 2 are given in Figures 1 and 2, respectively, and
selected bond distances and angles for 2 are listed in Table
-
-
-
VIII, CrIII, MnIII, FeIII, RCO2 ) Ph3CCO2 , Me3CCO2 ,
-
-
- 12
Ph2C(OH)CO2 , PhCO2 , C2H5CO2 ) but also in trinuclear
and tetranuclear complexes.14 The Fe-Ooximato bond distances
are 2.025 and 2.099 Å for the chelating and bridging modes,
respectively. The Fe-Ophenoxy and Fe-Noximato bond lengths
are 1.914 and 2.156 Å, respectively. The whole salicylal-
doximato ligand is planar and forms an angle of 44.2° with
the [Fe3(µ3-O)] plane. The benzoato ligands are coordinated
in the common syn,syn-µ2:η1:η1 mode, and the Fe-Ocarboxylato
bond distances are in the range of 1.991-2.057 Å. Although
(23) Constable, E. C. Metals and Ligand ReactiVity; VCH Publishers: New
York, 1996; pp 65-72.
(24) Alexiou, M.; Dendrinou-Samara, C.; Raptopoulou, C. P.; Terzis, A.;
Kessissoglou, D. P. Inorg. Chem. 2002, 41, 4732.
(25) Deacon, G. B.; Phillips, R. J. Coord. Chem. ReV. 1980, 33, 227.
(26) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Chapman
and Hall: London, 1975.
Inorganic Chemistry, Vol. 45, No. 5, 2006 2321

Raptopoulou et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) in 2‚2MeCN‚3H2Oa
Distances
Fe1-O2
1.914(9)
1.928(8)
2.040(9)
2.023(9)
2.057(10)
2.156(10)
3.301(5)
3.274(5)
Fe2-O71
Fe2-O51
Fe2-O41
Fe2-O12
Fe2-O22
Fe2-O61
Fe1‚‚‚Fe3
1.867(8)
2.017(10)
2.044(10)
1.991(9)
2.018(10)
2.116(9)
3.274(5)
Fe3-O71
Fe3-O42
Fe3-O32
Fe3-O52
Fe3-O1
Fe3-O1′
Fe2‚‚‚Fe3
1.884(7)
2.004(9)
2.013(9)
1.999(9)
2.025(7)
2.099(7)
3.260(5)
Fe1-O71
Fe1-O31
Fe1-O11
Fe1-O21
Fe1-N1
Fe1‚‚‚Fe2
Fe3‚‚‚Fe3′
Angles
O2-Fe1-O71
O2-Fe1-O31
O71-Fe1-O31
O2-Fe1-O11
O71-Fe1-O11
O31-Fe1-O11
O2-Fe1-O21
O71-Fe1-O21
O31-Fe1-O21
O11-Fe1-O21
O2-Fe1-N1
O71-Fe1-N1
O31-Fe1-N1
O11-Fe1-N1
O21-Fe1-N1
Fe1-O71-Fe2
Fe3-O1-Fe3′
169.7(4)
93.8(4)
89.0(3)
92.3(4)
97.8(3)
86.8(4)
87.3(4)
90.8(4)
174.5(4)
87.8(4)
85.0(4)
85.0(4)
92.9(4)
177.2(4)
92.6(4)
120.9(4)
105.1(3)
O71-Fe2-O51
O71-Fe2-O41
O51-Fe2-O41
O71-Fe2-O12
O51-Fe2-O12
O41-Fe2-O(12)
O71-Fe2-O22
O51-Fe2-O22
O41-Fe2-O22
O12-Fe2-O22
O71-Fe2-O61
O51-Fe2-O61
O41-Fe2-O61
O12-Fe2-O61
O22-Fe2-O61
Fe1-O71-Fe3
95.2(4)
95.6(4)
91.4(4)
95.4(4)
169.4(4)
86.9(4)
94.8(4)
89.8(4)
169.4(4)
90.0(4)
179.1(4)
85.0(4)
83.6(4)
84.4(4)
86.1(4)
118.4(4)
O71-Fe3-O42
O71-Fe3-O32
O42-Fe3-O32
O71-Fe3-O52
O42-Fe3-O52
O32-Fe3-O52
O71-Fe3-O1
O42-Fe3-O1
O32-Fe3-O1
O52-Fe3-O1
O71-Fe3-O1′
O42-Fe3-O1′
O32-Fe3-O1′
O52-Fe3-O1′
O1-Fe3-O1′
Fe2-O71-Fe3
96.2(4)
92.2(4)
84.7(4)
95.6(4)
88.4(4)
170.1(4)
90.3(3)
173.0(4)
92.4(3)
93.7(3)
165.2(3)
98.5(3)
87.5(3)
86.6(4)
74.9(3)
120.7(4)
a Symmetry transformations used to generate equivalent atoms: ′ ) 0.5 - x, 1.5 - y, 1 - z.
Chart 1. Schematic Diagram of Structures of I and II
Table 3. Structural Characteristics of the Triangular [M3(µ3-O)] Unit in
Compounds 2 and I-IV
2a
Ib
IIb,c
IIIa,c
IVa
Distances (Å)
M1‚‚‚M3
M1‚‚‚M2
M2‚‚‚M3
3.274
3.301
3.260
3.163
3.258
3.245
3.157
3.253
3.262
3.253
3.222
3.267
3.199
3.298
3.269
Angles (deg)
115.0
121.1
M1-O-M3
M1-O-M2
M2-O-M3
118.4
120.9
120.7
115.3
120.5
120.5
116.0
121.9
121.7
117.2
119.3
123.2
120.0
a M ) FeIII. b M ) MnIII. c Average values of the two crystallographically
independent molecules in the asymmetric unit.
similar, compounds 1 and 2 present different lattice structures
because of H-bonding interactions. In 1, the amide H atoms
of the MeCONH2 ligands are H-bonded to the acetonitrile
solvent molecules, leading to the formation of 1D polymeric
chains (Figure S1 in the Supporting Information). On the
other hand, no intermolecular H-bonding interactions are
observed between the hexanuclear complexes in 2, leading
to isolated species in the crystal lattice.
A comparison of the structural characteristics of the
triangular [Fe3(µ3-O)] subunit in complex 2 with those of
the precursor trinuclear complexes III and IV (for a graphical
representation, see Scheme 1) shows equivalent trends for
the interatomic distances and angles between the FeIII ions
in terms of the types of bridging interactions (Table 3). In
all three complexes, in the Fe1/Fe3 pair, bridged by one µ2-
oxide, one µ2-benzoato, and one oximato bridge, a ∼3.26-Å
separation is observed between the ferric ions, while the Fe-
O-Fe angle of ∼117° is the smallest one in the triangular
unit. On the other hand, in the Fe1/Fe2 and Fe2/Fe3 pairs,
bridged by one µ2-oxide and two µ2-benzoato ligands, the
Fe‚‚‚Fe distances are differentiated (Fe1‚‚‚Fe2 ∼3.32 Å; Fe2‚
‚‚Fe3 ∼ 3.27 Å) despite the similar Fe-O-Fe angles
(∼121°). Moreover, the approach of a second triangular [Fe3-
(µ3-O)] subunit in the hexanuclear complex 2 at ca. 3.3 Å
does not seem to affect the structural characteristics of the
[Fe3(µ3-O)] triangle itself.
A comparison of the structural characteristics of the
triangular [M3(µ3-O)] subunit in the hexairon complex 2 with
those in I and II is also very interesting (for a graphical
representation of I and II, see Chart 1). The metal ion
topology in the three compounds is exactly the same, despite
the different bridging modes observed in the triangular
subunits. In the cases of I and II, there are two pairs, Mn1/
Mn2 and Mn2/Mn3, bridged by one µ2-oxide and one
oximato group, and one pair, Mn1/Mn3, which is bridged
by one µ2-oxide, one µ2-carboxylato, and one oximato group.
In the latter pair, the MnIII ions are separated by ∼3.16 Å
and the Mn-O-Mn angle is ∼115°, values which are the
smallest ones within the triangular subunit. The same trends,
i.e., the smallest metal-metal interatomic distance and the
largest deviation of the M-O-M angle from the ideal 120°
value, are also observed in the ferric complex 2. In the two
former pairs, the Mn‚‚‚Mn separations are almost identical
(∼3.25 Å) and the Mn-O-Mn angles are ∼120°. The same
2322 Inorganic Chemistry, Vol. 45, No. 5, 2006

Hexanuclear Iron(III) Salicylaldoximato Complexes
Figure 4. Exchange-interaction pattern for complex 2.
Figure 3. Mo¨ssbauer spectrum from a powdered sample of 2 at 295 K.
Solid lines represent theoretical simulations assuming two doublets, A and
B, with the parameters quoted in Table 4.
Table 4. Mo¨ssbauer Parameters for Complex 2
a
iron site
T (K)
δ (mm s-1
)
∆EQ (mm s-1
)
Γ
1/2 (mm s-1
)
A (FeO6)
B (FeO5N)
A (FeO6)
B (FeO5N)
A (FeO6)
B (FeO5N)
295
295
77
77
4.2
4.2
0.43(1)
0.43(1)
0.53(1)
0.53(1)
0.53(1)
0.54(1)
0.55(1)
1.36(1)
0.62(1)
1.23(1)
0.64(1)
1.27(1)
0.13/0.14
0.16/0.22
0.15/0.17
0.17/0.22
0.16/0.17
0.18/0.19
Figure 5. øM vs T and øMT vs T experimental data for complex 2 and the
theoretical curve based on the Hamiltonian of eq 4 (solution a).
a Γ1/2 values for the left and right lines. The doublets are given as ΓL/
ΓR.
geneous O6 donor atom sets. On the other hand, the bond
lengths span the ranges 1.914-2.156 Å for Fe1, 1.867-2.116
Å for Fe2, and 1.884-2.099 Å for Fe3. Thus, Fe1 and Fe2
are characterized by a less symmetric environment than Fe3.
These two competing factors should affect the electron-field
gradient around the iron nuclei and, therefore, the quadrupole
splittings. From the relative area of the two sites, the most
reasonable assignment is that the minority doublet B is due
to Fe1 whereas doublet A, representing the majority of the
species, is assignable to both Fe2 and Fe3. The conclusion,
therefore, is that ligand inhomogeneity is more influential
to the quadrupole splitting than geometrical distortion. The
same conclusion was drawn for complexes III and IV
previously studied.15 We note here the similarity of the
spectra between complexes 2, III, and IV, which all exhibit
two doublets at a 2:1 ratio. Small albeit discernible differ-
ences for the ∆EQ values of sites A and B in the three
complexes can be identified.
trend is true for the Fe-O-Fe angles in 2, but the Fe‚‚‚Fe
distances are substantially differentiated (∼3.27 Å). Overall,
the triangular subunits in 2, I, and II are best described as
isosceles triangles, where in 2 the base defined by Fe1/Fe2
is directed toward the outside of the hexanuclear core while
in I and II the corresponding base defined by Mn1/Mn3 is
directed toward the inside of the hexanuclear core.
Mo1ssbauer Spectroscopy. Mo¨ssbauer spectra from pow-
dered samples of 2 were collected between 4.2 and 295 K.
These consist of two well-resolved quadrupole-split doublets
(sites A and B) with Mo¨ssbauer parameters typical of high-
spin FeIII in octahedral O/N environments. A characteristic
Mo¨ssbauer spectrum at 295 K is shown in Figure 3. Fairly
narrow lines are observed, indicating a large degree of
structural homogeneity. The asymmetry is attributed to
alignment of the crystallites in the sample holder. The
doublets exhibit similar isomer shifts but quite different
quadrupole splittings (∆EQA < ∆EQB), which is indicative
of significant differences in the homogeneity and/or geometry
of the coordination spheres.
Magnetic Susceptibility Studies. The øMT product of 2
at 300 K is 7.27 cm3 mol-1 K, significantly lower than the
5
theoretically expected value for six noninteracting S ) /2
ions (26.28 cm3 mol-1 K), indicating antiferromagnetic
interactions (Figure 5). This is corroborated by the constant
drop of the øMT product upon cooling, reaching a value of
0.63 cm3 mol-1 K at 2.2 K. The magnetic susceptibility, øM,
shows a constant increase upon cooling, which may be
associated with a number of factors such as a magnetic
ground state, low-lying magnetic excited states, or paramag-
netic impurities. This will be analyzed in detail below.
Consideration of the molecular geometry (Figure 4) would
suggest that five exchange parameters have to be taken into
The spectra can be simulated by assuming a 2:1 ratio for
doublets A and B, respectively. The derived Mo¨ssbauer
parameters are presented in Table 4. An increase of the
isomer shift upon cooling is attributed to a second-order
Doppler effect.27
Examination of the structure reveals that Fe1 exhibits an
O5N donor atom set, whereas Fe2 and Fe3 exhibit homo-
(27) Greenwood, N. N.; Gibb, T. C. Mo¨ssbauer Spectroscopy; Chapman
and Hall: London, 1971; pp 148-164.
Inorganic Chemistry, Vol. 45, No. 5, 2006 2323

Raptopoulou et al.
account in order to describe the magnetic behavior of 2: JA
) J33′, JB ) J23 ) J2′3′, JC ) J12 ) J1′2′, JD ) J13 ) J1′3′ and
JE ) J13′ ) J1′3. However, a closer inspection of the molecular
structure allows for simplifications by considering the
following: (i) the Fe1/Fe2 and Fe2/Fe3 pairs are bridged
by one µ2-oxide and two µ2-benzoates; the Fe1/Fe3 pair is
bridged by one µ2-oxide, one µ2-benzoato, and one oximato
bridge; (ii) the Fe-O-Fe angles for the Fe1/Fe2, Fe2/Fe3,
and Fe1/Fe3 pairs, which define the main superexchange
pathways between ferric ions, are 120.95, 120.70, and
118.35°, respectively; (iii) atoms Fe1 and Fe3 of the Fe1/
Fe2/Fe3 triangle both participate in interactions with atoms
outside the triangle (Fe3′ and Fe1′/Fe3′, respectively),
whereas atom Fe2 is the only one that does not; Fe2 can be
considered the apex of an isosceles triangle with side Fe1/
Fe3 as its base. Because of i-iii, interactions JB and JC are
grouped together.
Figure 6. Surface error plot of J2 vs J3 (with g ) 2.00) revealing the
minima a and b for |J3| > |J2| and |J3| < |J2|, respectively.
Thus, we can consider JA ) J1, JB ) JC ) J2, JD ) J3,
and JE ) J4. The corresponding Hamiltonian is
therefore, wondered if there could be a second solution with
different relative strengths of J2 and J3, within the isosceles
triangle Fe1/Fe2/Fe3. For that reason, the parameter space
of J2 and J3 was explored by keeping J1 and J4 fixed to their
previously defined values, and an error contour plot of J2 vs
J3 was drawn (Figure 6). This verified the minimum
previously mentioned (a) and revealed a second one (b) at
J2 ∼ -30 cm-1 and J3 ∼ -20 cm-1. Using those as initial
conditions and by the release of J1 and J4, a second solution
was obtained, with parameters J1 ) -15.0 cm-1, J2 ) -38.2
cm-1, J3 ) -19.9 cm-1, J4 ) -0.2 cm-1, g ) 2.08, with R
) 2.8 × 10-3 (solution b′). We observe that the relative
strengths have now been inversed, like in basic iron
carboxylates, with J2 being stronger. However, the quality
of this fit is inferior; thus, this solution is ruled out. Another
observation is that no minimum is found for J2 ) J3, which
justifies our fitting strategy by which this constraint was
abolished.
Hˆ ) -2[J1Sˆ3Sˆ3′ + J2(Sˆ1Sˆ2 + Sˆ2Sˆ3 + Sˆ1′Sˆ2′ + Sˆ2′Sˆ3′) + J3
(Sˆ1Sˆ3 + Sˆ1′Sˆ3′) + J4(Sˆ1Sˆ3′ + Sˆ1′Sˆ3)] (4)
Because of the relatively large number of fitting param-
eters, initial fitting attempts were carried out by considering
certain simplifications to verify that the problem is not
overparametrized. Exchange interaction J4 is expected to be
the weakest one because of the large Fe1‚‚‚Fe3′ separation
and because the magnetic interaction is mediated by a
diatomic oximato bridge. In addition, J2 and J3 are expected,
on the basis of structural data, to be close in strength. Initial
fitting attempts were carried out by considering J4 ) 0 and
J2 ) J3; however, these failed to yield an acceptable fit.
Releasing only one of these two constraints resulted in
marginal improvements. It was, therefore, decided that all
four exchange interactions are necessary for the interpretation
of the magnetic susceptibility data, with J4 * 0 and J2 * J3.
With those considerations, the best-fit parameters were J1
) -19.7 cm-1, J2 ) -35.8 cm-1, J3 ) -63.8 cm-1, J4 )
-4.99 cm-1, g ) 2.07, with R ) 1.3 × 10-4. One can deduce
from these results that J4 is indeed very weak, as expected,
and that g is close to 2.0, which is reasonable for high-spin
ferric ions. The ground state is characterized by S ) 0. The
first excited states are only 1.0 cm-1 (S ) 1) and 10.0 cm-1
(S ) 2) higher than the ground state, thus contributing to
the magnetic susceptibility, because of their thermal popula-
tion even at 2 K (Figure S2 in the Supporting Information).
A point that we thought of as worth exploring concerns
the structural relationship of the complex with triangular
basic iron carboxylates (see Description of the Structures).
The magnetic properties of those are normally interpreted
through a 2J model, considering an equilateral triangle, in
which interactions along the sides of the triangle are
associated with a parameter J and those along its base with
a parameter J′. It is common for those28 to present two best-
fit solutions, one with |J| > |J′| and one with |J| < |J′|. We,
As a general conclusion, therefore, we may say that the
complex retains some of the magnetic properties of simple
basic iron carboxylates, which are, however, severely
distorted because of the interaction of two triangular units.
Specifically, although there are solutions for two pairs of
J2/J3 values, only one is satisfactory, unlike basic iron
carboxylates in which both are close in quality. In addition,
although the overall coupling is antiferromagnetic in both
types of complexes, the odd number of spins in Fe3 clusters
leads to half-integer spin systems (usually S ) 1/2), whereas
the even number of spins in 2 leads to an integer spin system
with S ) 0.
These results can be compared to a series of studies carried
out on hexanuclear ferric complexes containing two magneti-
cally coupled [Fe3(µ3-O)] triangular cores. It has been shown
that the spin of the ground state can greatly vary depending
on the coupling scheme and the relative strengths of the
coupling constants. In one such case,29 complexes [Fe6O2-
(29) Christmas, C. A.; Tsai, H.-L.; Pardi, L.; Kesselman, J. M.; Gantzel,
P. K.; Chadha, R. K.; Gatteschi, D.; Harvey, D. F.; Hendrickson, D.
N. J. Am. Chem. Soc. 1993, 115, 12483.
(28) Boudalis, A. K.; Sanakis, Y.; Raptopoulou, C. P.; Terzis, A.;
Tuchagues, J.-P.; Perlepes, S. P. Polyhderon 2005, 24, 1540.
2324 Inorganic Chemistry, Vol. 45, No. 5, 2006

Hexanuclear Iron(III) Salicylaldoximato Complexes
At lower temperatures and particularly when the temper-
ature is comparable to the magnetic exchange interaction J
(kBT ≈ J), there is a growing of strong correlations in the
fluctuations of the magnetic moments of the molecule. These
correlations manifest themselves as an enhancement of the
relaxation rate 1/T1 at a temperature on the order of the
magnetic exchange constant J/kB. This behavior is similar
to the critical enhancement of relaxation in 3D magnetic
systems at the transition temperature to long-range magnetic
order. In molecular clusters, because of the finite size of the
spin system, the low-lying energy states are well separated
among themselves, and in this temperature range, the spin
dynamics is governed by magnetic excitations between the
ground state and the next excited state separated by an energy
difference ∆. The appearance of the NMR 1/T1 peak can be
explained with the use of a semiphenomenological model
used in previous NMR studies in molecular rings.31b,33 In
particular, in this approach, the nuclear relaxation rate 1/T1
has contributions proportional to the probability of occupation
of the different energy levels. The first contribution comes
from the first excited state exp(-∆/kBT) weighted by the
sum of all of the energy states (the partition function), and
the second contribution comes from the remaining states
approximated by a continuum. Indeed, because the first
excited states are 1.0 cm-1 (S ) 1) and 10.0 cm-1 (S ) 2)
higher than the ground state, as obtained from the suscep-
tibility data, it is expected that the nuclear relaxation rate
will probe the energy difference ∆ ) 10 cm-1 ) 14.4 K
(with the energy difference of 1 cm-1 being too small to be
detected by NMR), and this is clearly demonstrated by the
experimental NMR peak at ∼15 K (Figure 7). According to
this simplified model, the expression for the NMR relaxation
can be written as
Figure 7. 1H T1 spin-lattice relaxation time of complex 2 as a function
of temperature (the open circles represent the experimental data, the dashed
line shows the temperature dependence of øMT in arbitrary units, and the
solid line represents the fit to the experimental data according to eq 5).
(OH)2(O2CMe)10(hep)2] [hep- ) anion of 2-(2-hydroxyeth-
yl)pyridine] and [Fe6O2(OH)2(O2CMe)10L2] [L ) 2-(Ν-
methylimidazol-2-yl)-2-hydroxypropane] with identical cou-
pling schemes proved to exhibit S ) 5 and 0 ground states,
respectively, because of differences in the relative coupling
strengths. In the case of a prismatic Fe6 cluster,30 an S ) 1
ground state was found to be stabilized, whereas examples
of other FeIII complexes with S ) 5 ground states are
6
mentioned in ref 28. In this respect, complex 2 is a new
example of a spin-coupling scheme, which leads to an S )
0 ground state because of its particular coupling features.
Solid-State NMR Studies. T1 spin-lattice relaxation in
paramagnetic and magnetic materials is a powerful tool to
investigate the electron-spin dynamics of the system because
the nuclei under investigation probe the fluctuating local
magnetic fields induced at the nuclear sites through the
hyperfine interaction.
The 1H T1 spin-lattice relaxation time of complex 2 was
measured at a frequency of 200.145 MHz between room and
liquid-helium temperatures. The nuclear relaxation rate 1/T1
as a function of temperature is shown in Figure 7. At high
temperatures, 1/T1 exhibits a weak temperature dependence,
while at low temperatures, a peak appears at ∼15 K. The
appearance of a peak in 1/T1 at a temperature on the order
of the magnetic exchange constant J/kB has been frequently
encountered in previous 1H NMR studies of molecular
clusters and rings.31
In particular, at high temperatures (kBT . J), the magnetic
moments in the cluster are weakly correlated and the system
behaves like a paramagnet at high temperatures. In this limit,
the spin dynamics is dominated by the behavior of the static
response function, and it can be shown32 that in this limit
1/T1 ∼ øΜT, where øΜ is the uniform static susceptibility.
This proportionality is shown in Figure 7, where the
dashed line shows the temperature dependence of øΜT in
arbitrary units.
1
T1
∆
kBT
) C exp -
Z-1
(5)
(
)
0
where Z ) ∫∞D(E) exp(-E/kBT) dE is the partition func-
tion, D(E) is the distribution of energy levels, and C is a
constant.
The experimental 1/T1 data have been fitted to eq 5 using
∆ ) 15 K and assuming a Gaussian distribution for the
continuum of the remaining states with a mean energy of
1200 K and a width of 220 K. The fit is shown as a solid
curve in Figure 7, and it can be seen that it describes
adequately the experimental data.
Therefore, NMR relaxation data have been used to probe
the nature of the low-lying states and in particular the energy
separation between them, and the results are in accordance
with the susceptibility data.
Conclusions
Further investigation of the FeIII/PhCO2 /salox2- reaction
-
system has provided conclusions on certain aspects of its
chemistry. Specifically, the use of weakly coordinating
MeCN as the solvent led to two hexanuclear complexes [Fe6-
(30) Shweky, I.; Pence, L. E.; Papaefthymiou, G. C.; Sessoli, R.; Bino, J.
A.; Lippard, S. J. J. Am. Chem. Soc. 1997, 119, 1037.
(31) (a) Lascialfari, A.; Jang, Z. H.; Borsa, F.; Gatteschi, D.; Cornia, A. J.
Appl. Phys. 1998, 83, 6946. (b) Lascialfari, A.; Gatteschi, D.; Borsa,
F.; Cornia, A. Phys. ReV. B 1997, 55, 14341.
(33) Fardis, M.; Diamantopoulos, G.; Karayianni, M.; Papavassiliou, G.;
(32) Moriya, T. Prog. Theor. Phys. 1962, 28, 371.
Tangoulis, V.; Konsta, A. S. Phys. ReV. B 2001, 65, 14412.
Inorganic Chemistry, Vol. 45, No. 5, 2006 2325

Raptopoulou et al.
(µ3-O)2(O2CPh)10(salox)2(L)2]‚xMeCN‚yH2O [L ) MeCONH2,
x ) 6, y ) 0 (1); L ) H2O, x ) 2, y ) 3 (2)], which can be
regarded as dimers of trimers. Their synthesis has been
different exchange coupling constants was required, leading
to an S ) 0 ground state. The first excited states were found
to be only 1.0 cm-1 (S ) 1) and 10.0 cm-1 (S ) 2) higher
than the ground state, a fact that has been confirmed by the
presence of a significant enhancement of the 1/T1 NMR
relaxation rate at ∼15 K attributed to the energy difference
between the ground and the S ) 2 energy states.
-
achieved (i) by the FeIII/PhCO2 /H2salox reaction mixture
in MeCN, (ii) by the reaction between “basic iron benzoate”
and H2salox in MeCN, and (iii) by recrystallization of the
previously characterized trinuclear precursor complexes III
and IV in MeCN, thus exploring the parameter space of this
reaction system. On the other hand, the use of strongly
coordinating alcoholic solvents stabilizes the respective
trinuclear complexes III and IV. Both 1 and 2 contain the
[Fe6(µ3-O)2(µ2-OR)2]12+ core, whose topology consists of six
FeIII ions arranged as two centrosymmetrically related [Fe3-
(µ3-O)] subunits bridged by two oximato O atoms (O1 and
O1′). Mo¨ssbauer spectra of 2 revealed that, as in the case of
the trinuclear N/O-coordinated complexes III and IV, the
primary factor influencing the quadrupole splittings is the
donor atom homogeneity of the coordination sphere. It was
thus possible to clearly distinguish between sites possessing
O6 and O5N coordination spheres. For the interpretation of
the magnetic susceptibility data, a model involving four
Acknowledgment. We thank the Greek General Secre-
tariat of Research and Technology for support within the
frame of the Competitiveness EPAN 2000-2006, Centers
of Excellence #25. A.K.B. thanks the Greek State Scholarship
Foundation (IKY) for support through a postdoctoral grant.
Supporting Information Available: X-ray crystal crystal-
lographic files, in CIF format, for 1 and 2, as well as a figure of
the polymeric chains of 1 formed by H-bonding interactions, a table
with selected bond distances and angles in compound 1, and the
energy level plot for 2. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC051945Q
2326 Inorganic Chemistry, Vol. 45, No. 5, 2006