a new three-dimensional open-framework iron(iii) phosphate,

International Journal of Inorganic Materials 2 (2000) 217–223
A new three-dimensional open-framework iron(III) phosphate,
[C N H ][Fe (HPO ) ]
2210 2 44
a,b a,
*
Amitava Choudhury , Srinivasan Natarajan
a
Advanced Materials Research Laboratory
,
Chemistry and Physics of Materials Unit
,
Jawaharlal Nehru Centre for Advanced Scientific Research
,
Jakkur P
.
O
.,
Bangalore
560 064,
India
b
Solid State and Structural Chemistry Unit
,
Indian Institute of Science
,
Bangalore
560 012,
India
Accepted 3 January 2000
Abstract

A new open-framework iron(III) phosphate, I, [C N H ][Fe (HPO ) ] has been hydrothermally synthesized in the presence of
2210 2 44
ethylenediamine (en). The structure is built up from the vertex linkages between the FeO octahedra and the PO tetrahedra, strictly
64
alternating, forming the three-dimensional architecture. The linkages between the FeO and PO polyhedra gives rise to ladder-like
64
edge-shared chains, which are connected variously forming two types of channels. The di-protonated en molecules occupy these channels.
3
˚˚
Crystal data for I, [C N H ][Fe P O ]: a59.341(1), b58.892(1), c59.480(1) A,
b
5117.6(1)8, V5698.1(1) A , space group P
2
/n
2 2 10 2 4 16
23
˚
(No. 13), Z52, M5557.7, D 52.65 g cm , MoKa (
l
50.71073 A), R 50.03, wR 50.08 and S51.10. Magnetic susceptibility studies
calc 1 2
indicate a predominantly antiferromagnetic interaction with T 530 K.  2000 Elsevier Science Ltd. All rights reserved.
N
Keywords
:
A. inorganic compounds; magnetic materials; B. chemical synthesis; C. X-ray diffraction
1. Introduction known. A special case is iron fluro-phosphate, exhibiting a
gradual spin crossover behavior from the low- to the
The area of open-framework materials continues to be of high-spin state as a function of temperature [14]. This iron
interest not only because of the variety of interesting phosphate also possessed large voids bound by 24-T atoms

structures but also due to the potential applications in the (T5Fe, P) in addition to having infinite one-dimensional
area of catalysis, sorption and separation processes [1–3]. chains made of Fe–O/F–Fe linkage [14]. Our continued
Of the many open-framework metal phosphate structures effort on the synthesis of iron phosphates enabled us to
that are known, those of the transition metals are interest- discover a new three-dimensional iron phosphate, I,
ing as they provide a variety of coordination environments [C N H ][Fe (HPO ) ], possessing a narrow arrow-head
2210 2 44
as well as showing interesting magnetic behavior. In the type of one-dimensional channel bound by 8-T atoms. The
last couple of years, a large number of open-framework structure-directing agent, ethylenediamine (en), sits in the
iron phosphate materials have been synthesized and char- middle of these channels. In this paper, the synthesis and
acterized with interesting physical properties [4–13]. The structure of I is presented.
iron phosphate structures, in general, comprise a vertex
linkage between the Fe–O polyhedra and PO tetrahedra
4
forming chain, layer and three-dimensional architectures.
2. Experimental
Most of these materials have been synthesized hydrother-
2
mally in the presence of F ions, which also get incorpo-
The title compound was synthesized from a gel con-
rated as part of the framework in many cases. Our efforts
taining ethylenediamine (en) as the structure-directing
on the hydrothermal synthesis of the iron phosphates
agent. In a typical synthesis, 0.464 g of FeCl –6H O and
32
resulted in a variety of solids, some of which were already
0.169 g of MnCl –4H O was dispersed and dissolved in 3
22
ml of water. To this, 0.7 ml of phosphoric acid (aq. 85
wt.%) and 0.34 ml of en was added and stirred vigorously.
*Corresponding author. Fax: 191-80-846-2766.

E-mail address
:
(S. Natarajan) To this solution, 0.2 ml of HF was added and the mixture
1466-6049/00/$ – see front matter  2000 Elsevier Science Ltd. All rights reserved.
PII: S1466-6049(00)00007-6
218 A
.
Choudhury
,
S
.
Natarajan / International Journal of Inorganic Materials
2 (2000) 217
–
223
was stirred until homogeneous. The final mixture, a thick solved by direct-methods using SHELXS-86 [15], which
white gel, with a composition, 2FeCl –6H O–MnCl – readily established the heavy atom (Fe, P) sites and most
32 2
4H O–12H PO –6en–3HF–200H O, was transferred of the light atom (O, N and C) positions. All the hydrogen
234 2
onto a 7 ml PTFE-lined acid digestion bomb and heated at positions were located from difference Fourier maps and
1808C for 64 h. The fill factor was |50%. The resultant for the final refinement the hydrogens were placed
product contained only colorless rods, suitable for single- geometrically and held in the riding mode. The relevant
crystal X-ray diffraction, and was filtered and washed details of the structure determination are presented in
2
thoroughly with deionized distilled water. The pH of the Table 2. Full-matrix least-squares refinement on uF u
mixture did not show any appreciable change during the (atomic coordinates, anisotropic thermal parameters for the
hydrothermal reaction and remained at 2.0. The initial non-hydrogen atoms of the framework, water and amine
characterization was carried out using powder X-ray molecule) were carried out using the program SHELXTL-
diffraction (XRD) and thermogravimetric analysis (TGA). PLUS [16]. The final atomic coordinates along with the

The powder XRD pattern indicated that the product is a thermal parameters for I is presented in Table 3 and the
new material; the pattern is entirely consistent with the bond distances and angles in Tables 4 and 5.
structure determined using the single-crystal X-ray diffrac-
tion. A least-squares fit of the powder XRD (CuKa) lines,
using the hkl indices generated from single-crystal X-ray 3. Results and discussion
data, gave the following cell: a59.333(1), b58.845(1),
˚
c59.447(1) A,
b
5117.6(2)8, which is in excellent agree- The iron phosphate I, [C N H ][Fe (HPO ) ], was
2210 2 44
ment with that determined using the single-crystal XRD. synthesized by hydrothermal methods in the presence of
Powder data for I, [C N H ][Fe (HPO ) ], are listed in structure-directing agent, en, and the structure determined
2210 2 44
Table 1. EDAX analysis indicated that the final product did using the single-crystal X-ray diffraction method. The
not contain any Mn and the Fe–P ratio was 1:2, consistent synthesis, predominantly kinetically controlled, does not
with the stoichiometry determined using the single-crystal show any correlation between the starting composition and
studies. the final solid product. The absence of Mn in the final
A suitable single crystal of I was carefully selected product indicates that there are subtle forces that control
under a polarizing microscope. Data collection was per- the crystallization during the synthesis. The role of Mn in
formed on a Siemens SMART diffractometer with a CCD the synthesis is not yet clear and our efforts to form the
detector in the
u
range 2.29–23.288. The structure was title compound in the absence of Mn did not come to
2
fruition. The role of F ions in the formation open-
framework solids is well documented in the literature [17],
Table 1
2
and we believe, in the present case, F ions act as a

X-ray powder data for I, [C N H ][Fe (HPO ) ]
2210 2 44
mineralizer and facilitate the formation of I, as the final
abc
hkl 2
u
D(2
u
) d D(d) I
2
obs calc rel
product did not contain any fluorine. F ions acting as a
0 1 1 14.561 0.015 6.089 20.006 32.5
mineralizer in the synthesis of aluminum phosphates has
1121 14.913 0.025 5.950 20.001 60.5
been known in the literature [18,19].
2 0 0 21.531 0.012 4.129 20.002 34.4
[C N H ][Fe (HPO ) ] is a new three-dimensional
2210 2 44
2122 24.318 0.000 3.660 0.000 3.4
open-framework network structure made from vertex-
1 2 1 27.228 20.018 3.273 0.002 5.8
1222 27.615 0.011 3.231 20.001 100
linked FeO octahedra and PO tetrahedra incorporating
64
1023 28.628 0.093 3.128 20.001 7.2
di-protonated en molecules within its pores. The asymmet-
0 2 2 29.280 20.052 3.045 0.005 4.3
ric unit of I, consisting of 14 non-hydrogen atoms, is
2 1 1 29.921 20.031 2.983 0.003 29.7

presented in Fig. 1. There are two crystallographically
1123 30.400 0.093 2.949 20.009 8.6
independent iron and phosphorus atoms. The Fe atoms
3121 30.843 0.029 2.902 20.003 22.9
0 3 1 32.413 20.032 2.782 0.003 3.8
occupy special positions and have occupancy of 0.5. The
0 1 3 33.556 20.030 2.668 0.003 1.0
two distinct Fe atoms each make three Fe–O–P bonds to P
1 2 2 34.678 0.020 2.588 20.001 5.6
neighbors. The Fe–O–P bonds angles are in the narrow
3222 35.914 0.048 2.504 20.004 4.2
region 137.8–152.48 (Table 5) indicating that both the
2322 37.824 20.014 2.378 0.000 1.9
FeO octahedra and PO tetrahedra are regular. The Fe–O
2024 37.970 20.056 2.366 0.004 3.2
64
˚
2124 39.397 20.020 2.286 20.002 13.4
bond distances are in the range 1.967–2.092 A [(Fe(1)–
2323 42.534 0.032 2.127 0.001 1.0
˚˚
O) 52.019 A; (Fe(2)–O) 52.015 A] and the O–Fe–O
av. av.
1 3 3 49.789 0.059 1.833 20.002 5.7
bond angle is in the range 81.9–176.38, which is in
5121 52.246 20.031 1.750 0.001 6.5
agreement with those observed earlier in many of the
2424 57.014 0.017 1.616 20.001 8.9
open-framework iron phosphate structures [4–13]. The two
2325 57.839 20.083 1.592 0.002 3.3

a
distinct phosphorus atoms in I are tetrahedrally coordi-
2
u
22
u
.
obs calc.
b
nated by oxygens and both of them make three P–O–Fe
d 2d .
obs calc.
c
1003I/I . bonds with the remaining P–O vertex being ‘terminal’.
max
A
.
Choudhury
,
S
.
Natarajan / International Journal of Inorganic Materials
2 (2000) 217
–
223
219
Table 2
Crystal data and structure refinement parameters for I, [C N H ][Fe (HPO ) ]
2210 2 44
Empirical formula Fe P O C N H

2 4 16 2 2 14
Crystal system Monoclinic
Space group P
2
/n (No. 13)
Crystal size (mm) 0.2030.2030.28
˚
a (A) 9.341(1)
˚
b (A) 8.892(1)
˚
c (A) 9.480(1)
b
(8) 117.6(1)
3
˚
Volume (A ) 698.1(1)
Z 2
Formula mass 557.7
23
r
(g cm ) 2.65
calc
˚
l
(MoKa) (A) 0.71073
21
m
(mm ) 2.64
u

range (8) 2.29–23.28
Total data collected 2879
Index ranges 210#h#10, 29#k#8, 210#l#9
Unique data 1010
Observed data [
s
.2
s
(I)] 914
R 0.02
merg.
2
Refinement method Full-matrix least-squares on uF u
a
R indices [I.2
s
(I)] R 50.03; wR 50.08
12
R indices (all data) R 50.03; wR 50.08
12
Goodness of fit (S) 1.10
No. of variables 119
23
˚
Largest difference map peak and hole (e A ) 0.734 and 20.687
a22 2 22
W5 1/[
s
(F ) 1 (0.0466P) 1 1.7021P] where P 5 [F 1 2F ]/3.
OOC

˚
The P–O distances are in the range 1.508–1.586 A [(P(1)– in agreement with those reported earlier [4–13]. Bond
˚˚
O) 51.537 A; (P(2)–O) 51.534 A] and the average valence sum calculations [21] indicated that the valence
av. av.
O–P–O bond angles are 109.58 and 109.48, respectively, states of the Fe, P and O are 13, 15 and –2, respectively,
for P(1) and P(2). Of the eight O atoms, six are bonded in agreement with the framework formula.
with two Fe atoms and one P atom. The remaining two The three-dimensional framework of I,
˚
oxygens with distances P(1)–O(7)51.568 A and P(2)– [C N H ][Fe (HPO ) ], is built up from FeO octahedra
2210 2 44 6
˚
O(8)51.586 A are formally terminal –OH groups. Termi- and PO tetrahedra sharing vertices. The vertex linking
4
nal hydroxyl groups, for example, in H PO –0.5H O and polyhedra form 4-membered rings made up of [Fe P O ]
34 2 22 4
a-zirconium phosphate have distances of 1.551 and 1.558 units and are connected edge-wise forming ladder-like
˚
A, respectively [20]. The above geometrical parameters are chains. The ladder-like chains are connected together by
Table 3
4
Atomic coordinates [310 ] and equivalent isotropic displacement param- Table 4
2
3 a
˚
eters [A 310 ] for I, [C N H ][Fe (HPO ) ] Selected bond lengths in I, [C N H ][Fe (HPO ) ]
2210 2 44 2210 2 44
a
˚˚
Atom xy zU(eq) Moiety Distance (A) Moiety Distance (A)

Fe(1) 2500 8818(1) 2500 7(1) Fe(1)–O(1) 2.016(3) Fe(2)–O(4) 1.987(3)
Fe(2) 2500 3958(1) 7500 7(1) Fe(1)–O(2) 1.995(2) Fe(2)–O(5) 1.967(3)
P(1) 3370(1) 1003(1) 5669(1) 7(1) Fe(1)–O(3) 2.048(2) Fe(2)–O(6) 2.092(3)
[1 [2
P(2) 1376(1) 6586(1) 4676(1) 7(1) Fe(1)–O(1) 2.016(3) Fe(2)–O(4) 1.987(3)
[1 [2
O(1) 97(3) 8546(3) 1181(3) 13(1) Fe(1)–O(2) 1.995(2) Fe(2)–O(5) 1.967(3)
[1 [2
O(2) 2422(3) 7326(3) 4034(3) 11(1) Fe(1)–O(3) 2.048(2) Fe(2)–O(6) 2.092(3)
[3
O(3) 2519(3) 10557(3) 1092(3) 11(1) P(1)–O(1) 1.508(3) P(2)–O(2) 1.519(3)
[4
O(4) 2445(3) 5485(3) 5945(3) 14(1) P(1)–O(3) 1.533(3) P(2)–O(4) 1.514(3)
[5
O(5) 141(3) 4124(3) 6613(3) 12(1) P(1)–O(6) 1.538(3) P(2)–O(5) 1.516(3)
O(6) 2375(3) 2252(3) 5927(3) 12(1) P(1)–O(7) 1.568(3) P(2)–O(8) 1.586(3)
O(7) 3462(3) 2390(3) 6722(3) 14(1)
O(8) 801(3) 7833(3) 5499(3) 18(1) Organic moiety
[1
N(1) 4442(4) 2983(4) 3073(4) 16(1) N(1)–C(1) 1.496(5) C(1)–C(1) 1.519(8)
C(1) 3331(5) 4114(4) 3218(3) 13(1)
a
Symmetry transformations used to generate atoms: [1, 2 x 1 1/2, y,
a
U(eq) is defined as one-third of the trace of the orthogonalized U 2 z 1 1/2;[2, 2 x 1 1/2, y, 2 z 1 3/2;[3, x 1 1/2, 2 y 1 1, z 1 1/2;
I
,
j
tensor. [4, 2 x 1 1/2, y 2 1, 2 z 1 1/2; [5, 2 x, 2 y 1 1, 2 z 1 1.
220 A

.
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Natarajan / International Journal of Inorganic Materials
2 (2000) 217
–
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Table 5
a
Selected bond angles in I, [C N H ][Fe (HPO ) ]
2210 2 44
Moiety Angle (8) Moiety Angle (8)
[1
O(1)–Fe(1)–O(1) 166.2(2) O(5)–Fe(2)–O(4) 89.55(11)
[2
O(2)–Fe(1)–O(1) 88.08(11) O(5)–Fe(2)–O(5) 171.4(2)
[1 [2
O(2) –Fe(1)–O(2) 96.63(14) O(5) –Fe(2)–O(4) 84.55(11)
[1 [2
O(2)–Fe(1)–O(1) 82.75(10) O(5)–Fe(2)–O(4) 84.55(11)
[1 [2 [2
O(2) –Fe(1)–O(1) 82.75(10) O(5) –Fe(2)–O(4) 89.54(11)
[1 [1 [2
O(2) –Fe(1)–O(1) 88.08(11) O(4)–Fe(2)–O(4) 93.8(2)
[2
O(1)–Fe(1)–O(3) 91.68(10) O(5)–Fe(2)–O(6) 92.35(10)
[1 [2 [2
O(1) –Fe(1)–O(3) 98.73(11) O(5) –Fe(2)–O(6) 93.93(10)

[2
O(2)–Fe(1)–O(3) 172.54(10) O(4)–Fe(2)–O(6) 176.26(10)
[1
O(2) –Fe(1)–O(3) 90.73(10) O(5)–Fe(2)–O(6) 93.91(10)
[1
O(2)–Fe(1)–O(3) 90.73(10) O(4)–Fe(2)–O(6) 89.59(10)
[1 [1 [2 [2
O(2) –Fe(1)–O(3) 172.54(10) O(4) –Fe(2)–O(6) 89.59(10)
[1 [2
O(1)–Fe(1)–O(3) 98.74(11) O(5) –Fe(2)–O(6) 92.35(10)
[1 [1 [2
O(1) –Fe(1)–O(3) 91.68(10) O(4) –Fe(2)–O(6) 176.26(10)
[1 [2
O(3)–Fe(1)–O(3) 81.94(14) O(6) –Fe(2)–O(6) 87.07(14)
[3 [4 [5
O(1) –P(1)–O(3) 112.3(2) O(4)–P(2)–O(5) 113.4(2)
[3
O(1) –P(1)–O(6) 113.1(2) O(4)–P(2)–O(2) 106.8(2)
[4 [5
O(3) –P(1)–O(6) 107.30(14) O(5) –P(2)–O(2) 113.1(2)
[3
O(1) –P(1)–O(7) 105.6(2) O(4)–P(2)–O(8) 108.1(2)
[4 [5
O(3) –P(1)–O(7) 109.36(14) O(5) –P(2)–O(8) 106.5(2)
O(6)–P(1)–O(7) 109.16(14) O(2)–P(2)–O(8) 108.7(2)
[6
P(1) –O(1)–Fe(1) 152.4(2) P(2)–O(2)–Fe(1) 145.8(2)
[7
P(1) –O(3)–Fe(1) 137.8(2) P(2)–O(4)–Fe(2) 144.4(2)
[5

P(1)–O(6)–Fe(2) 143.0(2) P(2) –O(5)–Fe(2) 139.3(2)
Organic moiety
[1
N(1)–C(1)–C(1) 112.5(3)
a
Symmetry transformations used to generate atoms: [1, 2 x 1 1/2, y, 2 z 1 1/2; [2, 2 x 1 1/2, y, 2 z 1 3/2; [3, x 1 1/2, 2 y 1 1, z 1 1/2; [4,
2 x 1 1/2, y 2 1, 2 z 1 1/2; [5, 2 x, 2 y 1 1, 2 z 1 1; [6, x 2 1/2, 2 y 1 1, z 2 1/2; [7, 2 x 1 1/2, y 1 1, 2 z 1 1/2.
another edge-shared ladder in a direction perpendicular to
the plane of the chains as shown in Fig. 2. It is to be noted
that 4-membered rings have been hypothesized as the
fundamental building units in aluminum and other metal
phosphates [22,23]. The observation of such 4-membered
Fig. 2. The basic building unit in I, [C N H ][Fe (HPO ) ], showing
2210 2 44
the ladder-like edge-shared chains and the connectivity between them.
Fig. 1. ORTEP plot of I, [C N H ][Fe (HPO ) ]. Thermal ellipsoids are Note that the ladders are connected by another edge-shared ladder in a
2210 2 44
given at 50% probability. horizontal direction.
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Natarajan / International Journal of Inorganic Materials
2 (2000) 217
–
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221
Fig. 3. Structure of I, [C N H ][Fe (HPO ) ], along the [1 0 1] direction. Note that the amine molecules sit in one and the –OH group protrude into the

2210 2 44
other creating hydrophobic and hydrophilic channels. Hydrogens of the amine molecule are not given for clarity.
Fig. 4. Structure of I, [C N H ][Fe (HPO ) ], along the a axis. The amine molecules are not given for clarity.
2210 2 44
222 A
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Choudhury
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Natarajan / International Journal of Inorganic Materials
2 (2000) 217
–
223
directing amine molecules sits in the middle of one of the
channels with the terminal –OH groups of the phosphates
protrude into the another as shown in Fig. 3. The formation
of two types of channels, one hydrophobic and another
hydrophilic, is noteworthy. Along the [1 0 0] direction, the
connectivity gives rise to narrow arrowhead-type one-
dimensional channels, as shown in Fig. 4. Thus, I, possess
two distinct channels.
Thermogravimetric analysis of I was carried out in N
2
atmosphere from room temperature to 7008C using a
21
heating rate of 108C min , as shown in Fig. 5. The result
shows two mass losses, a sharp one and a second rather
broad one. The first mass loss of about 12% occurring in
the region 350–3808C corresponds to the loss of the amine

and some adsorbed water (calc. 11.1%) and the second
mass loss of 8% in the region 400–7008C corresponds to
the loss of the –OH group (calc. 6.5%). The loss of the
Fig. 5. Thermogravimetric analysis of I, [C N H ][Fe (HPO ) ].
2210 2 44
amine and the –OH group resulted in the collapse of the
framework, leading to the formation of an amorphous
rings, formed between the FeO octahedra and PO material (XRD).
64
tetrahedra (Fig. 2), is crucial as they constitute the majority The structure of I possesses strong hydrogen bond
of the building blocks in I. The ladder-like chains are interactions between the amine and the framework. The
connected in such a way forming 8-membered one-dimen- presence of two terminal –OH groups also ensured that
sional channels along the [1 0 1] direction. The structure- there are intra-framework hydrogen bond interactions in I.
Fig. 6. The variation of magnetic susceptibility as a function of temperature in I, [C N H ][Fe (HPO ) ]. Inset shows the variation of inverse
2210 2 44
susceptibility with temperature.
A
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2 (2000) 217
–
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223
Table 6
interest, help and encouragement. One of us (AC) thanks
Important hydrogen bond interaction in I, [C N H ][Fe (HPO ) ]

2210 2 44
the Council of Scientific and Industrial Research (CSIR),
˚
Moiety Distance (A) Moiety Angle (8)
Government of India, for the award of a research fellow-
ship.
O(3)–H(1) 2.157(1) O(3)–H(1)–N(1) 137.7(1)
O(8)–H(3) 2.560(1) O(8)–H(3)–N(1) 142.6(1)
O(2)–H(3) 2.080(1) O(2)–H(3)–N(1) 169.2(1)
aa
O(7)–H(10) 1.978(3) O(7)–H(10)–O(8) 148.6(1)
N(1)–H(20) 2.181(2) N(1)–H(20)–O(7) 162.4(1)
References
O(4)–H(4) 2.548(2) O(4)–H(4)–C(1) 133.5(1)
a
Intra-framework.
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¨
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[3] Cheetham AK, Loiseau T, Ferey G. Angew Chem Int Ed
˚
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˚
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N
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284.
p
[10] DeBord JRD, Reiff WM, Haushalter RC, Zubieta J. J Solid State
The
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2
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The authors thank Prof. C.N.R. Rao FRS, for his keen