Iron arsenate frameworks

Article abstract of DOI:10.1039/b702847d

Six new iron arsenate framework structures, Fe₂As ₂O₇·2H₂O, [Fe₆As ₈O₃₂H₄]^2-(1,4-butanediamininium ²⁺)·2H₂O, [Fe₄As₆

Full text of DOI:10.1039/b702847d

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www.rsc.org/dalton | Dalton Transactions
Iron arsenate frameworks†
a
a
b
a
Seth B. Wiggin, Robert W. Hughes, Daniel J. Price and Mark T. Weller*
Received 5th March 2007, Accepted 24th April 2007
First published as an Advance Article on the web 10th May 2007
DOI: 10.1039/b702847d
2
−
Six new iron arsenate framework structures, Fe
2
As
2
O
7
·2H
2
O, [Fe
As
6
As
8
O
32
H
2−
4
] (1,4-butane-
2
+
2−
2+
2+
diamininium )·2H
2
O, [Fe
4
As
6
O
22
H
2
] (piperazinium ), [Fe
5
5
O
24
H
4
] (piperazinium )·2H
2
O,
2
−
2+
[
Fe As
6
7
O
31
H
5
] (dabco ) and LiFeAsO OH have been synthesised under hydrothermal conditions.
4
Incorporation of the amine cation templates leads to more open framework geometries and, in contrast
to iron phosphates which have topologies based on PO tetrahedra, the iron arsenate structures
typically contain protonated As(O,OH) units. The magnetic properties of the iron arsenates studied
show Curie–Weiss behaviours with maxima in the v(T) vs. T plots in the range 10–50 K.
4
4
Introduction
A number of iron arsenates have been reported in the
11
literature. [Fe
3
(AsO
4
)
2
12
(HAsO
4
)
2
](C
4
N
2
H
12
)
and [Fe
3
(HAsO ) ]-
4
6
Materials that adopt open-framework structures built from linked
oxo-polyhedral units, MO
(
C
2
N
2
H
10)·NH
3
·NH
4
are both examples of amine cation
n
, have been an area of intense research
templated 3D structures. Fluorine can also be incorporated
1
over the past few years. The interest in this class of compounds
comes from a wide variety of potential applications, though these
have mainly resulted from catalytic and adsorptive properties
seen in compounds such as the zeolites (aluminosilicates) and
aluminophosphates. Incorporation of transition metals into the
framework often endows the materials with additional properties
such as optical, electronic and magnetic effects. Transition metal
containing frameworks based on phosphate groups have received
most attention, although other tetrahedral moieties such as
13
into the structure forming frameworks such as Fe
2
(AsO
)F ](C
4
)F,
11,14
[
H
Fe
3
F
3
(AsO
4
)(HAsO
4
)
2
](C
4
N
2
H
12
)
1.5
and [Fe
3
(AsO
4
6
3
2
N -
15
12
)
3/4
,
which contains mixed valence iron. Oxalate units
have been incorporated into several hybrid oxalate–arsenate
16
materials, for example [Fe
4
(HAsO
4
)
6
(C
2
O
4
)
2
] (C
4
N
2
H
12
)
2
and
17
[
NH (CH )CH(NH )CH [Fe
3
2
3
3
]
3
6
(AsO
4
)
2
(HAsO
4
)
6
(C
2
O ) ].
4
3
Despite these reports iron arsenate chemistry has remained
relatively unexplored especially in comparison with that of iron
phosphates. In this work we report the synthesis and characterisa-
tion, including preliminary magnetism data for several phases, of
six new iron arsenate materials.
2
3
4
5
borate, arsenate, sulfate and selenate have also been successfully
3
−
incorporated. Hence the combination of PO
first row transition metal elements (Fe, Co, Ni) and with zinc
has led to numerous framework structures, see for example the
4
with the late
6
Experimental
zeolite gismondine topology of ZnPO
4
(H
3
N(CH
2
)
3
NH
3
)
0.5. With
iron, several phosphate framework structures have gained impor-
tance with properties and applications associated with redox/ion
Synthesis
7
8
(
de)intercalation, e.g. LiFePO
4
, molluscicides (FePO
4
·2H
2
O),
For all materials, the reagents were sealed in a 23 ml Parr autoclave.
The autoclave was heated in an oven, held at temperature and then
oven cooled to room temperature. Solid products were recovered
by filtration and washed with water, then ethanol before drying at
9
catalysis (Fe
glass frameworks).
Of the other oxotetrahedral units, TO
is arsenate frameworks that show, perhaps, the greatest potential
for forming framework structures in combination with the first
row transition metals. Note that the larger size of the arsenate,
2
(PO
3
OH)P
2
O
7
)
and magnetism (iron phosphate
10
n−
4
, T = B, As, S, Se, it
◦
80 C.
Fe
0.4500 g, 2.5 mmol), 5 M arsenic acid solution ((synthesised from
As (Aldrich, 99%)), 1.5 ml, 7.5 mmol), tetramethylammonium
2
As
2
O
7
.2H O (1). Iron(II) oxalate dihydrate (Aldrich, 99%,
2
3
−
3−
AsO
4
, anion compared to phosphate, PO
4
, can lead to quite
2
O
3
different framework topologies for a particular transition metal
species and, hence, dissimilar properties. Furthermore frameworks
incorporating arsenate into frameworks, using the hydrothermal
conditions under which they normally form, often integrate the
hydroxide 25 wt% soln (Aldrich, 0.9 ml, 2.5 mmol) and water
◦
(7 ml). Heated to 150 C and held for 120 h. Product crystallised
as colourless plates.
protonated units As(O
the similar acidity of arsenic acid (pK
3
OH) and/or As(O
2
(OH)
values; 2.25 (K
; 2.15, 7.2, 12.7).
2
); this is despite
2
−
2+
[
Fe
6
As
8
O
32
H
4
] (1,4-butanediamininium )·2H
2
O (2). Iron(II)
a
1
) 6.8 (K )
2
oxalate dihydrate (0.4500 g, 2.5 mmol), 5 M arsenic acid solution
1.5 ml, 7.5 mmol), 1 M 1,4-diaminobutane solution (Aldrich,
9%, 2.5 ml, 2.5 mmol) and water (6 ml). Heated to 150 C and
11.6 (K
3
)) and of phosphoric acid (pK
a
(
9
◦
a
held for 120 h. Product occurred as colourless rod like crystals.
School of Chemistry, University of Southampton, Southampton, UK SO17
1
BJ. E-mail: mtw@soton.ac.uk
2
−
2+
b
[Fe
4
As
6
O
22
H ] (piperazinium ) (3). Iron(II) oxalate dihydrate
2
Department of Chemistry, University of Glasgow, Glasgow, UK G12 8QQ
(0.4500 g, 2.5 mmol), 5 M arsenic acid solution (1.5 ml, 7.5 mmol),
†
Electronic supplementary information (ESI) available: Table S1 - key
hydrogen bond distances. See DOI: 10.1039/b702847d
1 M piperazine solution (Aldrich, 99%, 2.5 ml, 2.5 mmol) and
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◦
water (6 ml). Heated to 160 C and held for 72 h. Product
crystallised as blue needles.
2
−
2+
[Fe
5
As
5
O
24
H
4
] (piperazinium )·2H
2
O (4). Iron (II) oxalate
dihydrate (0.4500 g, 2.5 mmol), 5M arsenic acid solution (0.5 ml,
2
2
.5 mmol), 1M piperazine solution (Aldrich, 99%, 2.5 ml,
.5 mmol) and water (6 ml). Heated to 160 C and held for 72 h.
◦
Product occurred as green fragments.
2
−
2+
[Fe
6
As
7
O
31
H ] (dabco ) (5). Iron(II) oxalate dihydrate
5
(
0.4500 g, 2.5 mmol), 5 M arsenic acid solution (1.5 mL,
7
9
.5 mmol), 1 M 1,4-diazabicyclo[2.2.2]octane solution (Aldrich,
8%, 2.5 ml, 2.5 mmol) and water (6 ml). Heated to 150 C and
◦
held for 120 h. Product precipitated as green plate crystals.
LiFeAsO OH (6). Iron sulfate heptahydrate (Aldrich, 99%,
4
0
9
.6951 g, 2.5 mmol), ammonium dihydrogen arsenate (Alfa Aesar,
8%, 0.3975 g, 2.5 mmol), 1 M lithium hydroxide solution
(
2
Aldrich, 98%, 1.5 ml, 1.5 mmol) and water (5 ml). Heated to
20 C and held for 24 h. Product occurred as green needle-shaped
◦
crystals.
Crystallographic determination
Suitable crystals of each material were selected for data collection
on a Nonius Kappa CCD area detector diffractometer (Mo Ka
radiation) at 120(2) K. All calculations were carried out using the
1
8
19
WINGX system (Version 1.70.01) and SHELX-97. Absorption
20
corrections were applied using SADABS. Key data collection and
refinement details are given in Table 1.
CCDC reference numbers 639058–639063.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b702847d
Thermal analysis
Samples of (1) and (5) were analysed using a Polymer Laboratories
STA1500. In a typical experiment, 10–15 mg of compound was
◦
heated in flowing air from room temperature up to 850 C at
◦
−1
1
0 C min . Mass loss and heat flow were recorded as a function
of temperature.
Variable temperature powder X-ray diffraction
A sample of (1) was analysed using a Bruker D8 Advance
diffractometer with Anton Parr HTK1200 furnace stage. The
˚
instrument was operated using Cu Ka
1
radiation (k = 1.54056 A)
and a SOL-X energy discriminating detector. Data were collected
◦
from 10–110 in 2h at 30, 150, 250, 325, 350, 375, 400, 425, 550
◦
and 600 C. The initial and final data sets were collected over 9 h
and the intermediate sets were collected for 5 h.
Magnetic susceptibility measurements
These measurements were performed using a Quantum Design
magnetic properties measuring system (MPMS) in an external
magnetic field of 5 T.
2
936 | Dalton Trans., 2007, 2935–2941
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◦
◦
Results and discussion
370 C, and then on to 600 C at a slower rate. The mass then
remained constant to maximum temperature. The total mass
loss of around 8% was close to the calculated loss of 9% for
Fe
2
As
2
O
7
·2H
2
O (1)
complete dehydration. A further sample of Fe
then analysed by VTXRD in order to identify the dehydration
products. The powder X-ray diffraction pattern collected, in situ,
from a sample heated to 325 C showed the formation of FeAsO
indicating that the dehydration/oxidation has proceeded via the
reaction:
2
As
2
O
7
·2H
2
O was
The formation of (1) required the presence of tetramethylammo-
nium hydroxide in the synthesis medium, where it increases the
solution pH, but the organic reagent is not incorporated into
the final structure. The structure of Fe
isostructural with the reported phosphate Fe
two distinct edge sharing FeO (H
◦
4
2
As
2
O
7
·2H
2
O, which is
21
2
P
2
O
7
·2H
2
O, has
5
2
O) octahedra that alternate
to form an infinite zigzag chain. Separate chains are linked
Fe
2
As
2
O
7
·2H
2
O + 1/2O
2
→ 2FeAsO
4
+ 2H
2
O
by As
2
O
7
units to form a dense 3D framework with channels
˚
approximately 3 × 10 A in the a direction. Fig. 1 shows the
This is in contrast to the known magnesium compositional ana-
22
view down the a axis detailing the channel like structure, and
logue, Mg As O ·2H O, which dehydrates to form Mg As O .
2
2
7
2
2
2
7
◦
2+
6
a representation rotated by 90 in the c axis to show the packing
The material contains chains of Fe ions, making it a d
system. A v(T) plot (Fig. 2) shows a maximum at about
of arsenate groups and iron chains. As can be seen from Table 1 the
structure has a relatively high calculated density compared with the
templated frameworks obtained in this work, vide infra, showing
that other than the channels along the a axis the framework has
little free space.
3
−1
−1
13(1) K with vmax at 0.160 cm mol . A plot of v (T) shows
that Curie–Weiss behaviour is followed above ∼100 K and
The hydrogen atoms were located during the refinement process
and were fixed at standard O–H distances. There was no significant
difference in Fe–O bond lengths between the terminal bonds
to water and other bridging oxygen atoms. IR spectroscopy
confirmed the presence of hydroxyl groups with a strong peak
−
1
centred at 3470 cm .
The water molecules in the structure are responsible for six intra-
framework hydrogen bonds of varying strength, detailed in Table
S1 (see ESI).† The water molecule orientations (approximately
◦
9
0 apart) presumably maximise the hydrogen bonding contacts
across the channels in the structure where the water molecules are
in close proximity.
Thermogravimetric analysis was carried out on an 11.420 mg
◦
sample of Fe
2
As
2
O
7
·2H
2
O. The sample was heated to 850
C
Fig. 2 Left: Plot of magnetic susceptibility (black circles) and inverse
◦
−1
at 10 C min . Mass was lost in two stages, from 200 to
susceptibility (white circles) versus temperature for Fe
2
As
2
O
7
·2H
2
O.
Fig. 1 Left: Structure of Fe
tetrahedral units black; hydrogen positions are shown as grey shaded thermal ellipsoids.
2
As
2
O
7
·2H
2
6 4
O looking down the a direction. Right: View down the c axis. FeO octahedra are shaded grey and AsO
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−
3
−1
aCurie–WeissfitgivesC =3.57(1)cm molK andh=−4.3(2) K.
seen from the list of such bonds in Table S1 (see ESI),† the length,
and therefore strength, of the hydrogen bonds is quite varied. The
amine molecule only forms two bonds to the framework. One of
these, (O11 · · · N1) is presumably quite weak due to the large bond
A plot of leff(T) shows a decreasing moment on cooling and
a room-temperature moment of 5.29 l
tibility of the isostructural cobalt sample has been investigated
previously, and reveals a similar behaviour. In this cobalt material
the susceptibility reaches a maximum at ca. 10 K before a sharp
drop, indicative of antiferromagnetic behaviour. A Curie–Weiss fit
gave C = 6.41 cm mol K and h = 8.29 K.
B
. The magnetic suscep-
23
˚
length, 3.456(4) A. The water molecule appears to be more strongly
held with three bonds to the framework, indicating this may be
harder to remove during thermal decomposition, i.e. requiring
higher temperatures. The atoms within the structural framework
of the material all have relatively small thermal displacement
parameter ellipsoids. However, the oxygen atom of the water
molecule, O1W, has significantly larger thermal motion consistent
with a greater degree of mobility of this non-framework moiety.
This structure is the first 1,4-butanediamininium cation tem-
plated iron arsenate material reported; though n-alkyl diamine
cations have been used for similar framework structures such
−
3
−1
2
−
2+
[Fe
6
As
8
O
32
H
4
] (1,4-butanediamininium )·2H
2
O (2)
In (2) the protonated amine acts as a template and is incorporated
into the final product. The structure consists of iron arsenate
chains running along the a axis with approximate dimensions
× 8 A. The chains (shown in Fig. 3) have three edge sharing
˚
5
iron octahedra separated by a layer of corner sharing arsenate
tetrahedra, so that the layer iron octahedra are linked by 8-rings
with neighbouring layers. The chains form into a 3D structure
by linking to four other chains via corner sharing oxygen atoms
from both the arsenate tetrahedra and iron octahedra. This corner
sharing between chains leaves large channels in the structure,
of similar size to the iron arsenate chains. The water and 1,4-
butanediamininium cations are situated in these large channels,
the long dimension of the amine cation running parallel to the
iron arsenate chains, as shown in Fig. 3.
as (NH
3
(CH
2
)
3
NH
3
)
0.5[Fe(OH)AsO ], a 2D layered iron arsenate
4
24
reported by Liao et al. There are two reported iron phos-
phate materials with a 1,4-butanediamininium cation template,
though both these have a markedly different framework motif
to the iron arsenate material reported herein. The hexagonal
25
material Fe
has isolated FeO
Fe (C
adopts a 2D layered structure.
8
P
14
O
62
H
24(C
4
H
14
N
2
)
3
reported by Korzenski et al.
6
octahedra. The iron fluorophosphate material
26
5
P
6
F
3
O
27
H
6
4
H
14
N
2
)
3
·2H
2
O reported by Riou-Cavellec et al.
2
−
2+
[
[
Fe
Fe
4
As
As
6
5
O
O
22
24
H
H
2
4
] (piperazinium ) (3) and
2
−
2+
5
] (piperazinium )·2H
2
O (4)
Structures (3) and (4) demonstrate the effect that varying molar
ratios can have on product formation even with the same template.
The same reagents and reaction conditions were used, but (3) has
an As : Fe ratio of 1.5 : 1, and was isolated from an arsenic rich
solution with an As : Fe molar ratio of 3 : 1, while (4) has As : Fe
of 1 : 1 and is derived from a solution with an equimolar As : Fe
ratio.
The structure of (3) has two crystallographically distinct iron
(
III) octahedra that form an infinite zigzag chain through edge
sharing. Arsenate tetrahedra and As units separate the chains;
2
O
7
this creates sheets in the ab plane. The piperazinium molecular ions
occupy space between these sheets. The structure could also be
described as quasi, three-dimensional pore-like, with the arsenate
tetrahedra extending strongly into the interlayer space producing
˚
a closest contact of only 2.46 A between two O10 atoms from
opposite layers.
This is quite different from (4), which possesses a much more
strongly corrugated iron arsenate layer that maintains a regular
spacing between layers. A comparison between the two structures
is shown in Fig. 4. The corrugation arises from the bonding motif
of the iron octahedra. The asymmetric unit contains five iron
atoms that form two parallel chains of edge sharing octahedra.
The chain motif is not a simple zigzag having a repeating wave-like
curve over ten individual octahedra. In contrast to (3) arsenic is
Fig. 3 Upper : Structure of Fe
6
As
8
O
32
H
4
(1,4-diaminobutane)·2H
2
O
looking down the a direction. Lower: Structure of the chains which run
parallel to the a direction. Shading of polyhedra as in Fig. 1. Water oxygen,
carbon and nitrogen are medium grey spheres and hydrogen atoms are
small grey spheres.
present only as AsO
4
tetrahedra with no As
2
7
O units. The arsenate
bridges between the chains of iron octahedra making an infinite
sheet in the ac plane. The iron arsenate framework of (4) has many
hydroxide groups present; evidence for the AsO
2
(OH)
2
tetrahedra
The material forms a number of hydrogen bonds between the
framework and the amine cation and water molecules in the
channel structure, and also intraframework linkages. As can be
and FeO (OH) and two FeO (OH) octahedra present comes from
differences in bond lengths between these and typical As–O and
Fe–O bonds.
4
2
5
2
938 | Dalton Trans., 2007, 2935–2941
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2−
2+
2−
2+
Fig. 4 Comparison of the layer structures of [Fe
as in previous figures.
4
As
6
O
22
H
2
]
(piperazinium ), (left) and [Fe
5
As
5
O
24
H
4
]
(piperazinium )·2H
2
O, (right). Shading scheme
A comparison of the unit cell dimensions of materials (3) and (4)
highlights the difference in bonding motifs. The long b and c axes
of material (4) are due to the large interlayer space and corrugated
structure. The structure of material (4) as a whole exhibits a
complex hydrogen bonding network, details of which are given
in Table S1 (see ESI).† The network is responsible for the location
of the piperazinium molecule and water molecules between the
framework layers. The piperazinium ion forms five separate bonds
to the framework and neighbouring water molecule. By contrast
the amine cation in (3) forms only two hydrogen bonds, implying
the structure may be more likely to possess ion-exchange properties
as the templating cation is less rigidly held in place.
Both structures show relatively small thermal ellipsoids for
the templating amine cation and water molecules. This implies
a low level of disorder and a fixed position for these molecules.
In material (3) hydrogen atoms were given fixed positions for
hydroxyl and amine groups. The amine cation in material (4) has
fixed positions for C–H bonds but refined N–H bond lengths.
The magnetic susceptibility of material (3) has been analysed;
a v(T) plot (Fig. 5) shows a broad maximum at about 30(1) K
Fig. 5 Left: Plot of magnetic susceptibility (black circles) and inverse sus-
ceptibility (white circles) versus temperature for Fe As O22(piperazine ).
4 6
2+
[Fe(OH)(HAsO )(C O )](C N H )·H O, and has a greater struc-
4
2
4
4
2
12
2
tural similarity to (2) than either of these piperazine templated
structures. The same units are also observed in another oxalato-
3
−1
−1
16
with vmax at 0.061 cm mol . A plot of v (T) shows that Curie–
Weiss behaviour is followed above ∼100 K and a Curie–Weiss
iron arsenate piperazine structure, C H As Fe O (C N H ) .
8
12
12
8
64
4
2
12 4
More similar to the structures found in this work for (3)
and (4) is that of the fluoro-iron arsenate piperazinium
As F Fe O H (C N H ) which has a 2D framework with
−
3
−1
fit gives C = 5.6(1) cm mol K and h = 25(1) K. The positive
Weiss constant implies ferromagnetic interactions, although the
overall behaviour of the magnetic susceptibility is characteristic of
antiferromagnetism. A plot of leff(T) shows a decreasing moment
3
5
3
12
2
4
2
12 1.5
14
intercalated piperazine molecules.
on cooling and a room-temperature moment of 5.50 l
B
.
2−
2+
[
Fe
6
As
7
O
31
H
5
] (dabco ) (5)
Iron arsenate piperazinium structures have been previously
11
reported. The material, As
4
Fe
3
O
16
H
2
(C
4
N
2
H
12
)
was produced
The material As
7
Fe (C
6
O
31
H
7
4
N H14) (5) crystallises in the triclinic
2
¯
as part of an investigation into an iron arsenate oxalate
system. The material was produced by heating the material
space group, P1. Some difficulties were apparent in modelling
the diffraction data and these stem from structural disorder at
This journal is © The Royal Society of Chemistry 2007
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two sites within the material. The first was the orientation of
the templating dabco unit: this unit was finally modelled by
positioning the molecule over two sites, related by a centre of
inversion so that the four of the six carbon atoms are common to
the two possible dabco orientations. The nitrogen and non-shared
carbon atoms were assigned as half occupancy sites. All hydrogen
atoms were modelled with fixed positions and thermal parameters.
The resultant large and highly elliptical thermal parameters still
found for all atoms within this unit indicate probable further
disorder of the template though the current model replicates the
scattering distribution reasonably well.
out of the plane and forms very close O–O distances (O11–O12 =
˚
1.32(2) A) with those in neighbouring layers. As the sites are half
occupied it is likely that the tetrahedra are randomly distributed
along the surface of the planes but avoiding opposing units.
The hydrogen bonding, shown in Table S1 (see ESI),† reveals
interesting differences between the two possible amine cation
positions. The nitrogen atom N1 makes three hydrogen bond links
with the framework, connecting to O4, O8 and O17. However these
bonds are rather weak with long donor to acceptor distances. By
contrast N2 makes only one hydrogen bond to O4, but this bond is
◦
likely to be much stronger, being both shorter and closer to 180 .
The second site of disorder is associated with the arsenate unit
As4) that is pendent from the main iron arsenate sheets and which
The magnetic behaviour of the sample was analysed using
a SQUID magnetometer in the range 4–300 K. A v(T) plot
(Fig. 7) shows a broad maximum at about 34(1) K with vmax
(
was modelled (together with the associated oxygen atoms O10,
O11 and O12) as a half-occupied site; full occupation of this site
would produce two adjacent arsenate units (attached and facing
towards each other from neighbouring iron arsenate sheets) with
impossibly short oxygen to oxygen close contacts.
3
−1
−1
at 0.0358 cm mol . A plot of v (T) shows that Curie–Weiss
behaviour is followed above 53 K and a Curie–Weiss fit gives C =
−
3
−1
3.83(5) cm mol K and h = −13.3(3) K. A plot of leff(T) shows
a decreasing moment on cooling, however the level has not settled
by room temperature.
A large thermal ellipsoid occurs for the oxygen atom O6. The
reasons for this are unclear, the short As2–O6 bond length of
˚
1
.55(1) A clearly indicates a terminal double bonded oxygen atom
and the reagents used leave few possible alternatives. Evidence for
hydroxyl groups was clearly seen by IR spectroscopy, assigning
the individual –OH groups was performed on the basis of bond
lengths. The terminal oxygen atoms O11, O13 and O16 were
obvious likely candidates, whereas the short bond lengths of O6
and O10 ruled out possible hydroxyl groups. In order to form a
charge neutral material an additional hydroxyl group was required;
investigating the bond lengths further showed the As–O bond
˚
lengths for O3, O5, O7, O8, O11, O14 and O15 were all >1.7 A
and therefore possible hydroxyl groups. Of these, all were three co-
ordinate oxygen atoms, apart from O8 and O11. It was therefore
decided to assign O8 as the final hydroxyl group by adding a
hydrogen atom with fixed bond length and temperature factors.
The structure of (5) is shown in Fig. 6. The iron arsenate layers
along the ab plane are made up from zigzag chains of edge sharing
Fig. 7 Left: Plot of magnetic susceptibility (black circles) and inverse
susceptibility (white circles) versus temperature for Fe As O H (dabco ).
6 7 31 7
2+
iron(III) octahedra separated by isolated corner sharing AsO
tetrahedra. The partially occupied As4O tetrahedra is positioned
4
4
The negative Weiss temperature and decreasing moment on
cooling indicate antiferromagnetic behaviour, and this has been
analysed using Fisher’s model of S = 5/2 interacting chains, with
ꢀ
coupling constants J = −1.49(4) K and J = 2.22(4) K.
Relatively few dabco templated frameworks containing iron
are known. There are three phosphates in the literature [C
6
N
2
-
-
H
14
]
2
[Fe
3
(OH)F
3
2
(PO
and [Fe (PO
4
)(HPO
4
)
2
]
2
·H
2
O, [C
(H O)
6
N
]·[C
2
H
14][Fe
2
28
F
2
(HPO )
4
2
7
(
H
2
PO
4
)
2
]·2H
2
O
4
4
)
2
F
2
2
3
6
H
14
2
N ] are all 3D
open framework materials. One iron arsenate–fluoride material
has been reported; [C
2
9
6
H
14
N
2
][Fe
3
(HAsO
4
)
2
(AsO
4
)F
4
]·0.5H
2
O
contains sheets cross-linked by hydrogen–arsenate groups to form
a 3D framework. Structure (5) is the first example of a dabco
templated iron material that does not contain fluorine.
LiFeAsO OH (6)
4
Compound (6) forms under hydrothermal conditions without
the need for an organic template cation; lithium cations act to
balance the negative framework charge. The triclinic structure of
(
6) consists of axially linked chains of corner sharing iron(III)
2+
Fig. 6 Structure of Fe
Shading scheme as per previous figures.
6
As
7
O
31
H
7
(dabco ) looking down the a direction.
octahedra, meaning the iron centres are in a straight line with an
Fe–O–Fe angle of 132 . The chains run perpendicular to the ꢁ111ꢂ
◦
2
940 | Dalton Trans., 2007, 2935–2941
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plane and are bridged by arsenate tetrahedra, which share all four
oxygen atoms with three separate chains of iron octahedra.
The hydroxide group is present on the bridging Fe–O–Fe
oxygen. This bridging oxygen possesses tetrahedral co-ordination
as it is also one of six oxygen atoms that co-ordinates to the
described in this article show only weak cooperative magnetic
interactions between the metal centres, i.e. antiferromagnetic
ordering only at low temperatures, below 50 K.
Acknowledgements
˚
lithium ion within the range 1.97(1)–2.82(5) A. This close packed
−
3
arrangement results in the large calculated density of 3.906 g cm ,
despite the pore like nature of the structure (Fig. 8). The hydrogen
atom H1 forms a bifurcated hydrogen bonding link to oxygen
atoms O2 and O3, cutting across the channels viewed parallel to
the b axis. Details of these bonds are given in Table S1 (see ESI).†
We thank EPSRC for partial studentship funding for SBW and
Dr Mark Light for help with SXD data collection and analysis.
Notes and references
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Fig. 8 Structure of LiFeAsO
scheme as per previous figures; lithium ions are medium grey spheres.
4
·H
2
O looking down the a direction. Shading
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0
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31
32
both the arsenate and phosphate systems, with similar chains
of vertex sharing MO
, M = Mn, octahedra linked in an identical
manner by the TO
1
6
4
, T = P, As, tetrahedra. However in these
manganese analogues the octahedra are strongly distorted due to
the presence of the Jahn–Teller ion Mn(III).
2
2
2
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is mainly as a result of the prevalence of protonated As(O,OH)
4
tetrahedra in these materials; phosphate in polyhedral frameworks
almost invariably links through simple P–O–M bridges, with
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2
T. Rojo, Inorg. Chem., 2001, 40(22), 5691–5694.
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−
2+
966.
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6
As
7
O
31
H
5
] (dabco )
3
3
1 M. Aranda, J. Attfield and S. Bruque, J. Chem. Soc., Chem. Commun.,
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1
991, 604–606.
2
−
2+
in [Fe
6
As
8
O
32
H
4
] (1,4-butanediamininium )·2H
2
O. In common
2 M. Aranda, J. Attfield and S. Bruque, Angew. Chem., Int. Ed. Engl.,
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with many transition metal containing frameworks the materials
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Dalton Trans., 2007, 2935–2941 | 2941