Synthesis and magnetic properties of an amine-templated Fe2+ (S = 2) sulfate with a distorted Kagome structure

Article abstract of DOI:10.1021/ic061457y

An organically templated iron(II) sulfate of the composition [H ₃N(CH₂)₂NH₂(CH₂) ₂(NH₃]₄[Fe^II₉F ₁₈(SO₄)₆]·9H₂O with a distorted Kagome structure has been synthesized under solvothermal conditions in the presence of diethylenetriamine. The distortion of the hexagonal bronze structure comes from the presence of two different types of connectivity between the FeF₄O₂ octahedra and the sulfate tetrahedra. This compound exhibits magnetic properties different from those of an Fe(II) compound with a perfect Kagome structure and is a canted antiferromagnet at low temperatures.

Full text of DOI:10.1021/ic061457y

Inorg. Chem. 2006, 45, 9475−9479
+
Synthesis and Magnetic Properties of an Amine-Templated Fe2 (S
) 2)
Sulfate with a Distorted Kagome Structure
J. N. Behera and C. N. R. Rao*
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for AdVanced Scientific
Research, Jakkur P. O., Bangalore 560064, India, and Solid State and Structural Chemistry Unit,
Indian Institute of Science, Bangalore 560012, India
Received August 3, 2006
An organically templated iron(II) sulfate of the composition [H3N(CH2)2NH2(CH2)2(NH3]4[FeII9F18(SO4)6]
‚9H2O with a
distorted Kagome structure has been synthesized under solvothermal conditions in the presence of diethylenetriamine.
The distortion of the hexagonal bronze structure comes from the presence of two different types of connectivity
between the FeF4O2 octahedra and the sulfate tetrahedra. This compound exhibits magnetic properties different
from those of an Fe(II) compound with a perfect Kagome structure and is a canted antiferromagnet at low
temperatures.
Introduction
are also found in Fe2+ (S ) 2) Kagome compounds.7 A recent
study of a Ni2+ (S ) 1) Kagome compound has shown it to
be a canted antiferromagnet.8 The Fe2+ Kagome compounds
investigated hitherto are [H3N(CH2)6NH3][FeII1.5F3(SO4)]‚
0.5H2O, I,7a and [H3N(CH2)2NH2(CH2)2NH2(CH2)2NH3]-
Transition metal compounds with the Kagome structure
have been investigated extensively because of their interest-
ing magnetic properties. The literature abounds with studies
of Kagome compounds of Fe3+(S ) 5/2), such as the family
of jarosites, all of which exhibit magnetic frustration or low-
temperature antiferromagnetism.1 Most of the Fe3+ jarosites
investigated are not pure because of the presence of site
defects. It is only recently that Nocera et al.2,3 prepared a
pure Fe3+ jarosite by redox-based hydrothermal methods. In
this compound, the Fe3+ Kagome layer shows an antiferro-
magnetic transition at 61.4 K. A pure S ) 1/2 copper
Kagome compound has been found to show spin frustration,4
while a Co2+ (S ) 3/2) compound shows properties similar
to those of the Fe3+ jarosites.5 On the other hand, V3+ (S )
1) Kagome compounds exhibit ferromagnetic coupling within
the triangles of Kagome layer.6 Ferromagnetic interactions
[FeII F6(SO4)2], II.7b I has a perfect Kagome structure with
3
uniform hexagonal bronze layers, and II has a slightly
distorted structure. A significant question in this context
concerns the relationship between the distortion of the
hexagonal layers and the magnetic properties of the Kagome
compounds. We have now prepared a compound of the
formula [H3N(CH2)2NH2(CH2)2NH3]4[FeII F18(SO4)6]‚9H2O,
9
III, which is highly distorted compared to I and II and shows
different magnetic properties. We describe the structure and
properties of III in this article.
Experimental Section
Synthesis and Characterization. In a typical synthesis of
[H3N(CH2)2NH2(CH2)2NH3]4[FeII F18(SO4)6]‚9H2O, III, 0.263 g of
9
* To whom correspondence should be addressed. E-mail:
cnrrao@jncasr.ac.in. Fax: +91-80-2208-2760.
iron(III) citrate was dispersed in an ethanol (EtOH)-water mixture
(5.8 and 1.8 mL, respectively) under constant stirring. To this
mixture, 0.22 mL of sulfuric acid (98%) and 0.38 mL of diethyl-
(1) (a) Ramirez, A. P. Annu. ReV. Mater. Sci. 1994, 24, 453. (b) Greedan,
J. E. J. Mater. Chem. 2001, 11, 37. (c) Nocera, D. G.; Bartlett, B. M.;
Grohol, D.; Papoutsakis, D.; Shores, M. P. Chem.sEur. J. 2004, 10,
3850. (d) Wills, A. S.; Harrison, A.; Ritter, C.; Smith, R. I. Phys.
ReV. B 2000, 61, 6156. (e) Wills, A. S.; Harrison, A. J. Chem. Soc.,
Faraday Trans. 1996, 92, 2161.
(6) (a) Grohol, D.; Papoutsakis, D.; Nocera, D. G. Angew. Chem., Int.
Ed. 2001, 40, 1519. (b) Grohol, D.; Huang, Q.; Toby, B. H.; Lynn, J.
W.; Lee, Y. S.; Nocera, D. G. Phys. ReV. B 2003, 68, 094404. (c)
Papoutsakis, D.; Grohol, D.; Nocera, D. G. J. Am. Chem. Soc. 2002,
124, 2647.
(2) Bartlett, B. M.; Nocera, D. G. J. Am. Chem. Soc. 2005, 127, 8985.
(3) Grohol, D.; Nocera, D. G.; Papoutsakis, D. Phys. ReV. B 2003, 67,
064401.
(4) Shores, M. P.; Nytko, E. A.; Bartlett, B. M.; Nocera, D. G. J. Am.
Chem. Soc. 2005, 127, 13462.
(5) Behera, J. N.; Paul, G.; Choudhury, A.; Rao, C. N. R. Chem. Commun.
2004, 456.
(7) (a) Rao, C. N. R.; Sampathkumaran, E. V.; Nagarajan, R.; Paul, G.;
Behera, J. N.; Choudhury, A. Chem. Mater. 2004, 16, 1441. (b) Paul,
G.; Choudhury, A.; Rao, C. N. R. Chem. Commun. 2002, 1904.
(8) Behera, J. N.; Rao, C. N. R. J. Am. Chem. Soc. 2006, 128, 9334.
10.1021/ic061457y CCC: $33.50
Published on Web 10/13/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 23, 2006 9475

Behera and Rao
Table 1. Crystal Data and Structure Refinement Parameters for III
enetriamine (DETA, 99.98%) were added, followed by the addition
of 0.36 mL of HF (40%). The resultant mixture with the molar
composition of iron(III) citrate/4H2SO4/3.5DETA/100EtOH/100H2O/
8HF had an initial pH of 4 after it had been stirred for 2 h. The
mixture was taken in a 23 mL PTFE-lined acid-digestion bomb
and heated at 180 °C for 4 days. After it was cooled to room
temperature, the product containing thin plate-shaped crystals of
III (yield 30% with respect to Fe) was filtered and washed with
water and then with ethanol.
empirical formula
formula mass
cryst syst
space group
a (Å)
C16H82F18N12O33S6FeII9
2008.01
triclinic
P1h(2)
11.0438(2)
b (Å)
15.8352(2)
c (Å)
19.5662(4)
R (deg)
77.4810(10)
74.2310(10)
71.1710(10)
3085.31(10)
â (deg)
γ (deg)
The initial characterization of III was carried out by powder
X-ray diffraction (PXRD), energy-dispersive analysis of X-rays
(EDAX), thermogravimetric analysis (TGA), and IR spectroscopy.
Magnetic measurements on powdered samples were performed at
temperatures between 2 and 300 K, in a vibrating sample magne-
tometer using a physical property measurement system (quantum
design). PXRD patterns indicated the products to be new materials
and monophasic, the patterns being consistent with those generated
from single crystal X-ray diffraction. EDAX gave the expected
metal/sulfate ratio of 3:2. The fluoride test was performed quali-
tatively, and quantitative analysis was performed by field emission
scanning electron microscopy (FE-SEM). Bond valence sum
calculations and the absence of electron density near fluorine in
the difference Fourier map also provide evidence for the presence
of fluorine. The water content of III was established by thermo-
gravimetric analysis (TGA) to be close to the value given by the
formula.
vol (Å3)
Z
2
T (°C)
F
20
2.161
0.71073
2.403
calcd (g cm-3
)
λ(Mo KR) (Å)
µ (mm-1
)
θ range (deg)
total data collected
Rint
R [I > 2σ(I)]
R (all data)
GOF (S)
1.09-23.24
8682
0.0475
R1 ) 0.0642a, wR2 ) 0.1766b
R1 ) 0.112, wR2 ) 0.2129
0.906
2
2
2
a R1 ) ∑|F0| - |Fc|/∑|F0|. b wR2 ) {[w(F0 - Fc )2] /[w(F0 )2]}1/2, w
) 1/[σ2(F0)2 + (aP)2 + bP], P ) [F02 + 2Fc ]/3, where a ) 0.1302 and b
2
) 0.
final refinements for III are listed in Table 1. The positions of the
fluorine atoms in III were located primarily by examination of their
thermal parameters. Their assignment as oxygen instead of fluorine
invariably leads to nonpositive definite values when they were
refined with anisotropic displacement parameters. The powder X-ray
diffraction pattern of III was in good agreement with the simulated
pattern based on the single-crystal data, indicative of phase purity.
The infrared spectrum of III showed characteristic bands in the
980-1010 cm-1 region from ν1 and in the 1090-1140 cm-1 region
2-
from ν3 of SO42-. The bending mode of SO4 was in the 450-
600 cm-1 region. The stretching and bending modes of the NH2/
+
NH3 groups and H2O were also in the expected ranges.9
Results and Discussion
Single-Crystal Structure Determination. A suitable single
crystal of compound III was carefully selected under a polarizing
microscope and mounted at the tip of the thin glass fiber using
cyanoacrylate adhesive. The single-crystal structure determination
by X-ray diffraction was performed on a Siemens SMART-CCD
diffractometer equipped with a normal focus, 2.4 kW sealed-tube
X-ray source (Mo KR radiation, λ ) 0.71073 Å) operating at 40
kV and 40 mA. The structure was solved by direct methods using
SHELXS-97,10 which readily revealed all the heavy-atom positions
(Fe and S) and allowed us to locate the other non-hydrogen (C, N,
O, and F) positions from the difference Fourier maps. An empirical
absorption correction based on symmetry-equivalent reflections was
applied using SADABS.11 All the hydrogen positions were found
in the difference Fourier maps. For the final refinement, the
hydrogen atoms of the amine were placed geometrically and held
in the riding mode. The last cycles of refinement included atomic
positions, anisotropic thermal parameters for all the non-hydrogen
atoms, and isotropic thermal parameters for hydrogen atoms of the
amine. The hydrogen positions for water molecules were excluded
from the final refinement. Full -matrix least-squares structure
refinement against |F2| was carried out using the SHELXL-9712
package of programs. Details of the structure determination and
[H3N(CH2)2NH2(CH2)2NH3]4[FeII F18(SO4)6]‚9H2O, III, has
9
an asymmetric unit with 85 non-hydrogen atoms, of which
58 belong to the inorganic framework and 37 belong to the
extraframework including nine water molecules (Figure 1a).
There are 10 crystallographically distinct Fe atoms and six
S atoms with all the Fe atoms in octahedral geometry. The
Fe atoms have fluorine and oxygen neighbors to form FeF4O2
octahedra. There are two types of octahedral arrangements
around the metal ion. In one, four F atoms are in equatorial
positions, and two O are in the axial position as in jarosites;
the other type has three F and one O in equatorial positions
and one F and one O in axial positions (Figure 1b). Anionic
layers of vertex-sharing FeIIF4O2 octahedra and SO4 tetra-
hedra are linked by Fe-F-Fe and Fe-O-S bonds. Note
that in the perfect Kagome lattice of I, the six coordination
of the metal ion is satisfied by the presence of four F atoms
in equatorial positions, with the two axial positions occupied
by the oxygen atoms of the sulfate (Figure 1c). As a result
of the two types of Fe octahedra, the hexagonal structure of
III gets distorted from that of a perfect Kagome lattice.
Because 33% of the sulfate tetrahedra share equitorial oxygen
with the iron octahedra, they do not lie perfectly on the
triangular lattice and, instead, get tilted. The bridging
fluorines connect Fe(II) ions with an Fe-F-Fe angle of
124.6-132.7° to form a triangular µ-fluoro trimer, which is
capped by the sulfate anion. Because of the presence of F
and O in both axial and equatorial positions of the octahedra,
two types of triangular lattices are created, and the structure
(9) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds; Wiley-Interscience, New York, 1978.
(10) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure Deter-
mination; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (b)
Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.
(11) Sheldrick, G. M. SADABS: Siemens Area Detector Absorption
Correction Program; University of Go¨ttingen: Go¨ttingen, Germany,
1994.
(12) Sheldrick, G. M. SHELXTL-PLUS, Program for Crystal Structure
Solution and Refinement; University of Go¨ttingen: Go¨ttingen, Ger-
many, 1997.
9476 Inorganic Chemistry, Vol. 45, No. 23, 2006

Fe2+ Sulfate with a Distorted Kagome Structure
Figure 2. View down the c axis showing the stacking of the layers in III.
DETA molecules are not shown in the interlayer space for purpose of clarity.
(Figure 2). The layers in III are stacked along the b axis in
an ABAB fashion and are held together by hydrogen bonding
interactions with the triprotonated amine and water mol-
ecules.
In Figure 3, we compare the structure of III with that of
the perfect Kagome structure of I and the slightly distorted
structure of II. Note that I contains only vertex-shared
octahedra and, hence, symmetrical hexagons characteristic
of the tungsten bronze layer (Figure 3a).14 In II, the distorted
structure results from the edge sharing of octahedra with two
types of triangular units (Figure 3b). We realized that the
distortion in III (Figure 3c) is far greater than that in II.
The TGA curve of III (N2 atmosphere, range of 30-900
°C, heating rate 5 °C/min, given as Supporting Information)
showed weight losses in three steps. The first weight loss
corresponds to the loss of guest water molecules in the 60-
200 °C range [obsd ) 7.8%, calcd ) 8%]; the major weight
loss in the 200-400 °C range corresponds to the loss of
amine and HF [obsd ) 30.6%, calcd ) 31.8%]. The third
weight loss in the region of 400-800 °C corresponds to the
removal of fluorine and the decompositions of sulfate [obsd
) 27%, calcd ) 25%]. The sample heated at 900 °C diffracts
weakly, and the PXRD corresponds mixture of Fe2O3 and
FeO (JCPDS files 00-003-0800 and 00-002-1186, respec-
tively).
The magnetic properties of the jarosites are known to be
sample dependent.1d,1e It is expected that any small perturba-
tion would have a strong effect on the ground-state manifold.
In Figure 4, we present the magnetic data on III. From the
high-temperature inverse-susceptibility data of III, recorded
at 1000 Oe (see inset a of Figure 4), we estimate a Weiss
temperature of -105 K (θp), suggesting dominant antifer-
romagnetic exchange interactions. The effective magnetic
moment per iron atom is 5.02 µB, which is close to the spin-
only (S ) 2) value of 4.9 µB for Fe2+ and is comparable to
the values reported for similar compounds.15 Furthermore,
Figure 1. (a) ORTEP plot of [C16N12H64][FeII9F18(SO4)6]0‚9H2O, III.
Thermal ellipsoids are given at 50% probability. The connectivity of the
iron octahedra and the sulfate tetrahedra in III and in the perfect Kagome
structure of I are shown in panels b and c, respectively.
gets distorted from the normal Kagome lattice. One type of
three-ring trio is formed by the vertex-sharing of the FeF4O2
octahedra creating a perfect three-membered ring which is
capped by the sulfate tetrahedron as in jarosites. On the other
hand, the three-ring trio formed by the sharing of an
equatorial corner between two octahedra and the axial corners
shared with two equatorial positions a third octahedron gives
rise to a distorted triangular unit. The presence of both perfect
and distorted triangular units gives rise to symmetric and
unsymmetric hexagons in III.
The Fe-O bond distances in III are in the range of
2.127(5)-2.176(5) Å with (Fe-O)av ) 2.158 Å. The Fe-F
bond distances are in the range of 2.008(4)-2.119(4) Å with
(Fe-F)av ) 2.068 Å. Bond valence calculations13 using
r0(Fe-F) ) 1.65 Å and r0(Fe-O) ) 1.734 Å (Fe(1) ) 1.96,
Fe(2) ) 1.97, Fe(3) ) 1.98, Fe(4) ) 1.89, Fe(5) ) 1.98,
Fe(6) ) 1.90, Fe(7) ) 1.85, Fe(8) ) 1.96, Fe(9) ) 1.91,
and Fe(10) ) 1.93) and the average bond distance value
indicate that the valence state of all the iron atoms is +2.
The bond lengths and angles are in good agreement with
those reported for the Fe2+ in fluorine and oxygen environ-
ments.7 The Mo¨ssbauer spectrum shows the presence of the
characteristic signal from Fe2+. The position of fluorine atoms
is supported by the calculated bond valence sums which lie
in the 0.64-0.72 range. The framework stoichiometry of
[FeII F18(SO4)6] with a -12 charge is balanced by the
9
presence of triprotonated DETA molecules residing in the
crest and trough regions of the corrugated layers and in the
interlayer space where water molecules are also present
(14) Magneli, A. Acta Chem. Scand. 1953, 7, 315.
(15) (a) Paul, G.; Choudhury, A.; Rao, C. N. R. Chem. Mater. 2003, 15,
1174. (b) Fu, A.; Huang, X.; Li, J.; Yuen, T.; Lin, C. L. Chem.sEur.
J. 2002, 8, 2239.
(13) Brown, I. D.; Altermatt, D. Acta Crystallogr., Sect. B 1985, 47, 244.
Inorganic Chemistry, Vol. 45, No. 23, 2006 9477

Behera and Rao
Figure 4. Temperature dependence of the magnetic susceptibility of III
at 1000 Oe. Inset a shows the temperature variation of the inverse
susceptibility at 1000 Oe. Inset b shows magnetic hysteresis at 5 K.
Figure 3. Polyhedral views of the hexagonal Kagome layers in I, II, and
III are shown in panels a, b, and c, respectively. Notice how c has the
most distorted structure and a has the perfect structure.
III shows magnetic hysteresis at low temperatures (inset b
of Figure 4). The magnetization increases with the field
without any evidence for saturation, implying an antiferro-
magnetic component as well, in agreement with the negative
θp. The magnetic behavior of III is quite different from that
of I or II. In Figure 5a and b, we compare the magnetic
susceptibilities of I, II, and III measured at 100 Oe under
field-cooled (FC) and zero-field-cooled (ZFC) conditions.
Particularly important is the fact that magnetic susceptibility
of this distorted compound (III) is very different from the
previous compounds (I and II), mainly in the intermediate
temperature regime, down to very low temperature. The
broad nature of the ZFC susceptibility around T ≈ 10 K
suggests magnetic polarizations caused by some interactions,
the strength of which is in the intermediate energy scale (kT
≈ 10 K). Since this appears in a distorted system, the
magnetic polarizations must have a geometrical origin. It is
by now quite well-known that distortions in a corner shared
Figure 5. Comparison of the magnetic susceptibility data of III at 100
Oe with I and II under (a) field-cooled (FC) and (b) zero-field-cooled (ZFC)
conditions.
Kagome lattice leads to Dzyaloshinsky-Moriya (DM)
interactions, the in-plane components of which can give rise
to magnetic polarizations in the out-of-plane directions with
canted vectors of magnetic moments.16 It is interesting to
(16) Elhajal, M.; Canals, B.; Lacroix, C. Phys. ReV. B 2002, 66, 014422.
9478 Inorganic Chemistry, Vol. 45, No. 23, 2006

Fe2+ Sulfate with a Distorted Kagome Structure
note that the degree of this DM interactions can be tuned by
controlled synthetic methods, as evident from Figure 5. The
fact that the DM interactions can be controlled has a potential
in designing frustrated systems for various magnetic ap-
plications. It is noteworthy that the different degrees of
distortion lead to significant differences in the magnetic
properties of the Fe(II) Kagome compounds, a feature
different from those of the Fe(III) jarosites with a d5 ion. If
such interactions with geometrical origin can be controlled
in both integer spin and half-odd-integer spin Kagome
system17 requires further studies.
ized. The magnetic properties of this compound are distinctly
different from those of other Fe(II) Kagome compounds
possessing perfect or slightly distorted Kagome structures;
the distorted Kagome compound prepared by us shows
canted antiferromagnetism at low temperatures.
Acknowledgment. J.N.B. thanks CSIR, India, for a
research fellowship. The authors thank Dr. S. K. Pati for
helpful discussions.
Supporting Information Available: TGA curve, atomic co-
ordinates, important bond distances and angles, hydrogen bonding
interactions, and X-ray crystallographic information in CIF format
for the structure determination of III. This material is available
free of charge via the Internet at http://pubs.acs.org.
Conclusions
An organically templated iron(II) sulfate with a highly
distorted Kagome lattice has been synthesized and character-
(17) Pati, S. K.; Rao, C. N. R. J. Chem. Phys. 2005, 123, 234703.
IC061457Y
Inorganic Chemistry, Vol. 45, No. 23, 2006 9479