Indian Journal of Chemistry
Vol. 51A, May 2012, pp 708-713
Hydrothermal synthesis and characterization
of lanthanide oxalates: In situ oxalate
formation from tartaric acid in presence of KI
Muhammad Athar* & Ashfaq Mahmood Qureshi
Department of Chemistry, Bahauddin Zakariya University,
Multan, Pakistan
Email: [email protected]
and
Guanghua Li, Zhan Shi & Shouhua Feng
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, College of Chemistry, Jilin University,
Changchun, PR China
Email: [email protected]
Received 21 December 2011; revised and accepted 18 April 2012
Two
new
three-dimensional
lanthanide
oxalates
[Pr2(C2O4)3(H2O)4]·2H2O (1) and [Nd2(C2O4)3(H2O)4]·2H2O (2)
and a two-dimensional oxalate [Pr2(C2O4)3(H2O)6]·3H2O (3) have
been prepared by hydrothermal process and characterized by
single crystal X-ray analysis. Compound (1) crystallizes in
monoclinic crystal system with P21/c space group whereas (2)
crystallizes in triclinic crystal system with Pī space group.
Although Pr and Nd are twin lanthanides, they have produced
different crystal structures under the same reaction conditions.
Compound (3) is produced by the in situ hydrothermal
degradation of tartaric acid to oxalate ions in presence of KI.
In this reaction, the iodide anion instead of metal cation, acts as
the reducing agent during the degradation of tartaric acid to
oxalate. Thermogravimetric, elemental analysis and IR studies
have also been carried out for these compounds.
in Scheme 1. Most common is formation of two
five-member chelating rings with two metal ions on
opposite sides (Scheme 1(a)). The formation of (b)
one five-member chelating ring with one metal on one
side, (c) oxo bridge and (d) four-member chelating
ring have also been observed1, 4, 8.
A variety of structural features of metal organic
frameworks having the same metal and ligand can be
obtained by variation in the reaction conditions. One
of the important ways is the in situ ligand formation.
In recent years the scope of in situ ligand formation
during the synthesis of coordination polymers has
increased due to its application in the formation of
new structures having characteristic properties, which
under normal conditions are difficult to achieve.
Many researchers have reported the formation of
oxalate ligands from different compounds under
different conditions with a variety of sources such
as CO29-12, 1,4-diazabicyclo[2.2.2]octane13, porotic
acid in presence of 2CoCO3·3Co(OH)2·nH2O14,
methanol15, ascorbic acid16-18, L(+) tartaric acid19,
2-pyrazine carboxylate20, etc. So far two remarkable
Keywords: Hydrothermal synthesis, Lanthanides, Oxalates,
Degradation, Tartaric acid, Potassium iodide
In the recent years lanthanide oxalates have generated
much attention of researchers due to their structures
and applications. Generally, lanthanide oxalates form
a honeycomb layer structure but many threedimensional crystal structures having channels with
four, five and six-member circumference rings are
also known1-8. The pattern of ring formation of the
metal atoms bridged by the oxalate groups is of
particular importance in determining the properties of
these compounds. One of the factors which influences
the shape of the ring, is pattern of bonding of oxalate
to the metal atoms. Oxalate ligands bridge
metals ions in a number of ways as shown
(a)
(b)
(c)
(d)
Tetradentate oxalate bonding forming two five-member
chelating rings
Bidentate oxalate bonding forming one five-member chelating
ring
Formation of oxo bridge
Formation of four-member chelating ring by utilizing two
oxygen atoms (attached to the same carbon) with a metal ion
Scheme 1
NOTES
mechanistic approaches for the formation of oxalate,
have been proposed. One is the reductive coupling of
CO29 and the other is the oxidation of organic
molecules such as methanol, ethanol or ethylene
glycol13, 20. The reductive coupling of CO2 has always
been reported to be carried out in presence of a metal
cation as reducing agent. It is interesting that in most
of the cases the formation of oxalate was accidental.
Also, it has been observed that in one set of
conditions of the oxalate formation, by changing any
of the reactants in that set, the same results could not
be achieved.
In the present study, the role of KI during the
degradation of tartaric acid to oxalate under the
hydrothermal conditions has been investigated. We
report herein the synthesis of three lanthanide oxalates
[Pr2(C2O4)3(H2O)4]·2H2O (1), [Nd2(C2O4)3(H2O)4]·2H2O (2)
and [Pr2(C2O4)3(H2O)6]·3H2O (3). Their structures
have been determined by single crystal X-ray
analysis. Compound (1) crystallizes in monoclinic
crystal system with P21/c space group whereas (2)
crystallizes in triclinic crystal system with Pī space
group, both having three dimensional structures. Here
we have discussed the structural comparison of the
compounds (1) and (2). Although Pr and Nd are so
called twin lanthanides, they show variations in the
structure formation. Apart from the lanthanide
contraction21 the number and the position of the
coordinated solvent molecules and frequency of
occurrence of oxo bridges in the structure have been
discussed as the possible factors to affect the structure
of the lanthanide oxalates. Compound (3) has two
dimensional layered crystal structure and was
prepared by in situ derivation of oxalate from tartaric
acid in presence of KI under hydrothermal conditions.
During this process iodide ions were oxidized to I2.
The electrons released from iodide ions may have
been involved in the reductive coupling of CO2
generated from thermal decomposition of tartaric
acid. Such a role of iodide anion as reducing agent to
convert tartaric acid to oxalate and forming I2 has not
been reported yet.
Experimental
All the chemicals and solvents were of reagent
grade obtained from commercial sources and were
used without further purification.
Synthesis of compound (1) was afforded by taking
0.4 mmol Pr(NO3)3 dissolved in 10.0 mL water in a
23 mL Teflon lined stainless steel autoclave. Then
709
1.2 mmol oxalic acid was added to it. This reaction
mixture was placed in the oven at 150 ºC for three
days. The resulting crystals were filtered, washed
initially with distilled water and then with acetone and
dried in air at room temperature. The compound
(2) was prepared by the same method using
NdCl3·xH2O as the starting reagent.
In the synthesis of compound (3) oxalic acid was
not used at all. Pr(NO3)3 (0.4 mmol) was dissolved
in 10.0 mL water and 1.2 mmol KI was added to it
and the mixture was stirred for 6 hours. Tartaric acid
(0.4 mmol) was added to it and transferred to a 23 mL
Teflon lined autoclave. The mixture was placed
in oven at 120 ºC for three days. The crystals were
filtered, washed and dried in air.
The elemental analysis was carried out on a
Perkin-Elmer 2400 elemental analyzer and
inductively coupled plasma (ICP) analysis was
performed on a Perkin-Elmer Optima 3300DV ICP
instrument. These results are in good agreement with
the theoretical values. Anal. (%): Found (Calc.)
(1): Pr 43.64 (43.09); C 11.21 (11.02); H 1.89(1.85);
(2): Nd 43.68 (43.66); C 10.99 (10.91); H 1.80 (1.83);
(3): Pr 40.01 (39.8); C 10.24 (10.18); H 2.49 (2.56).
Differential thermal analysis (DTA) and
thermogravimetric analysis (TGA) were carried
out on a Perkin-Elmer DTA 1700 differential thermal
analyzer and a Perkin-Elmer TGA 7 thermogravimetric
analyzer in air with a heating rate of 10 ºC per minute.
The infrared (IR) spectra were recorded on a
Bruker IFS 66v/S spectrometer using KBr pellets.
The crystal structures were determined by single
crystal X-ray diffraction method on a Siemens
SMART CCD diffractometer with graphite
monochromated Mo-Kα (λ = 0.71073 Å) radiation at a
temperature of 293 ± 2K. A hemisphere of data was
collected using a narrow frame method with a scan
width of 0.30° and an exposure time of 20 s/frame.
SAINT processing program was used to process the
data22. The SHELXTL crystallographic software
package was used to solve the crystal structures23.
Details of some structural parameters are given
in Table 1.
Results and discussion
Compound (1) crystallizes in monoclinic crystal
system with P21/c space group in three-dimensional
crystal structure. This compound is similar in
structure to bismuth oxalate hexahydrate24
[Bi2(C2O4)3(H2O)4]·2H2O. In each asymmetric
INDIAN J CHEM, SEC A, MAY 2012
710
unit (Fig. 1a), Pr atom is coordinated to nine
oxygen atoms; two of water and remaining seven
of oxalate ions. There are two non-coordinated
water molecules thus establishing the formula
[Pr2(C2O4)3(H2O)4]·2H2O. Each Pr atom is surrounded
by four oxalate ions three of which lie in the layer
along the bc-plane while the fourth oxalate of each
Pr atom is coordinated perpendicular to the bc-plane,
Table 1— Crystal data for compounds (1) – (3)
Empirical formula
Formula mass
Space group
a (Å)
b (Å)
c (Å)
α (deg.)
β (deg.)
γ (deg.)
V (Å3)
Z
Dcalcd (mg/m3)
μ (mm-1)
λ (Å)
T (ºC)
R1a, [I > 2σ(I)]
wR2b, [I > 2σ(I)]
(1)
(2)
(3)
C6H12O18Pr2
653.98
P21/c
9.8917(3)
8.2883(3)
10.1887(4)
90
99.040(2)
90
824.95(5)
2
2.633
5.930
0.71073
20(2)
0.0315
0.0782
C6H12O18Nd2
660.64
Pī
6.0482(12)
7.5937(15)
8.9091(18)
98.38(3)
99.77(3)
96.80(3)
394.62(14)
1
2.780
6.604
0.71073
20(2)
0.0141
0.0424
C6H18O21Pr2
708.02
P21/c
11.239(2)
9.6154(19)
10.321(2)
90
114.25(3)
90
1016.9(4)
2
2.312
4.831
0.71073
20(2)
0.0190
0.0525
R1 = Σ⎜⎜Fo⎜−⎜Fc⎜⎜/Σ⎜Fo⎜. b)
wR2 = {Σ[w(Fo2−Fc2)2]/Σ[w(Fo2)]2}1/2.
a
thus extending the network three dimensionally
in the ab-plane. A channel having a 4-member
circumference ring comprising four Pr atoms and four
oxalate groups can be viewed along a-axis. The four
Pr atoms of this circumference ring are not exactly
planer. This is because one oxygen atom of each of
the three oxalates in bc-plane is coordinated to two
Pr atoms, which results in the formation of an oxo
bridge between two Pr atoms (Fig. 2a). The empty
space of the channels is occupied by the coordinated
and non-coordinated water molecules. If we view the
structure along c-axis, we see oblong shaped channels
having 5-member circumference rings comprising
five Pr atoms and four oxalate groups.
Compound (2) crystallizes in triclinic crystal
system with Pī space group having three-dimensional
crystal structure. This compound shows similarity
in structure with bismuth oxalate octahydrate24
[Bi2(C2O4)3(H2O)4]·4H2O but with the difference in
the number of non-coordinating water molecules.
In the asymmetric unit as shown in Fig. 1b each
Nd atom is coordinated to nine oxygen atoms; two
of water and remaining seven of oxalate ions. There
are two non-coordinated water molecules thus
establishing the formula [Nd2(C2O4)3(H2O)4]·2H2O. In
the layer along bc-plane we see the channels having
6-member circumference ring comprising six Nd
atoms and four oxalate groups. This channel is
rectangular shaped instead of the usual hexagonal due
to the unequal number of Nd atoms and oxalate
Fig. 1—ORTEP drawing of the polymer unit of (a) (1) and (b) (2) showing the coordination environment. Thermal ellipsoids are
shown at 50 % probability.
NOTES
711
Fig. 2—Three dimensional structure of (a) (1) viewed from b-axis showing the oxo bridges and (b) (2) viewed from a-axis showing the
oxo bridges. The oxygens of oxo bridge come from oxalates attached to the metal along a-axis.
groups. In the channel circumference, linking of Nd
atoms is observed in two different patterns. In one
pattern two Nd atoms are linked through a single
oxalate groups forming usual 5-member chelating
rings, whereas in the other pattern two Nd atoms are
linked through two oxalate groups forming oxo bridge
as shown in Fig. 2b. In (2), out of the four oxalate
groups attached to each Nd atom, two oxalates are
involved in the formation of oxo bridges. Coordinated
and non-coordinated water molecules occupy the
empty spaces of the channels. The three-dimensional
network is formed by the extension of Nd linkage
through oxalate groups along the a-axis. In the
layer along the ac-plane, 4-member circumference
rings are formed comprising four Nd atoms and four
oxalate groups.
Although Pr and Nd both are the neighboring
lanthanides, usually called the twins lanthanides,
the oxalates of the both have been crystallized in
different space groups and different three-dimensional
networks. This feature might have developed
due to the difference in occurrence of oxo bridges
(3 in (1) and 2 in (2)) as well as due to the bonding
positions of the coordinated water molecules. In (1)
the two coordinated water molecules are positioned
on the opposite directions of the Pr atom having
O(w1)-Pr-O(w2) bond angle of 144.65°, whereas in
(2) the two coordinated water molecules are in the
same direction having O(w1)-Nd-O(w2) bond angle
of 70.23°.
Compound (3) has been obtained by the
hydrothermal treatment of a mixture of tartaric acid
and Pr(NO3)3 in presence of KI. The crystal structure
of (3) has already been reported25. Single crystal
X-ray characterization showed that the compound
crystallizes in monoclinic crystal system having P21/c
space group with two-dimensional network. A single
unit consists of two Pr atoms, three oxalate ions,
six coordinated water molecules and three
non-coordinated water molecules, thus establishing
the formula [Pr2(C2O4)3(H2O)6]·3H2O. Each Pr atom
is coordinated to three oxalate ions in a triangular
planer fashion utilizing its six coordination site.
The remaining three sites are occupied by three
water molecules, which are out of the plane of oxalate
ions. In this way a hexagonal planer ring is formed
resulting in the two-dimensional crystal lattice.
In compound (3) the oxalate groups do not
show formation of oxo bridges as has been seen in
(1) and (2).
An important aspect of the synthesis of (3) is in situ
formation of oxalate from tartaric acid in the presence
of KI under hydrothermal conditions. During the
process KI was oxidized to I2 changing the color of
the product mixture to orange yellow. The presence of
I2 in the product mixture was confirmed by the simple
starch test. It is important to note that in another series
of experiments the same mixture, which was used
to prepare (3) but without KI, yielded another
three-dimensional crystal structure of Pr tartrate26
[Pr2(H2O)3(C4H4O6)3]·1.5H2O, whereas when Pr(NO3)3
was reacted with oxalic acid and tartaric acid under
the same hydrothermal conditions but in absence
of KI, a new three dimensional compound
INDIAN J CHEM, SEC A, MAY 2012
712
[Pr(C4H4O6)(C2O4)1/2(H2O)]·1.5H2O having both
oxalate and tartrate ions in the structure was obtained.
These results are enough to convince that KI plays
a prominent role in the in situ degradation of
higher dicarboxylic acids into oxalic acid under
hydrothermal conditions.
The logical conversion of tartaric acid to oxalate in
the presence of KI is explained in Scheme 2. The
conversion follows two steps: in step 1, the iodide
anions abstract two protons from the two carboxylic
groups of tartaric acid to generate two HI (acid)
followed by the loss of CO2. The resulting carbanions
at C2 and C3 in a stepwise fashion are protonated by
HI affording ethylene glycol9, 27.
Mechanistic steps for the conversion of tartaric acid to
oxalate in the presence of KI
Scheme 2
The step 2 is the reductive coupling of CO2 to form
oxalate dianion. The carbon in CO2 is electrophilic
and accepts one electron thereby producing CO2−˙,
which is a free radical anion and easily undergoes
radical coupling or dimerization so that a new C-C
bond is formed to make oxalate dianion. The oxalate
anion in turn undergoes complexation with Pr cations.
So far the reductive coupling of CO2 has been
reported in presence of the metal cations. The
important point is that in the present case the reducing
agent is iodide anion. To the best of our knowledge
this has not been reported earlier. The iodide anion
has one extra electron in the last shell farther from the
nucleus therefore the iodide anion relative to other
halides can easily be oxidized to form I2, which can
dissolve in the reaction mixture with a distinct change
in the color. The starting reacting mixture was light
green in color due to Pr(III) ions and after reaction the
color of the mother liquor turned to orange yellow
showing the presence of molecular I2. The presence of
I2 was further testified using starch test.
The in situ ligand formation has great importance
in organic syntheses as well as structure architecture
of the metal-organic frameworks. The pore size in the
compound (3) has been increased to a greater extent
as compared to that in compound (1). Six-member
ring is formed in compound (3) having six Pr atoms
and six oxalate groups, which occupies more
non-coordinated water molecules as compared to
compound (1).
Thermogravimetric analysis of compound (1)
shows that non-coordinating water is lost in the first
step in the temperature range of 115-155 °C and the
coordinated water molecules is lost in the second
step in the temperature range of 180-240 °C. The
dehydrated sample does not show any significant
mass loss up to 400 °C. At 400 °C the sample again
start losing mass and it continues up to about 700 °C
with the total mass loss of 48.6 %, which can be
regarded as the complete conversion of the sample
into metal trioxide. Compound (2) shows the same
thermal decomposition pattern with the difference that
both coordinating and non-coordinating water
molecules are lost in one step in the temperature range
of 165-230 °C.
IR spectra of the three compounds show the bands
which are characteristic of the lanthanide oxalate
compounds. Compound (1) has peaks at 3317, 1619,
1317, 798 cm-1, compound (2) has the peaks at 3346,
1623, 1315, 798 cm-1 and compound (3) has peaks at
3324, 1614, 1315, 798 cm-1.
NOTES
713
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In the present study, two new three-dimensional
lanthanide oxalates [Pr2(C2O4)3(H2O)4]·2H2O and
[Nd2(C2O4)3(H2O)4]·2H2O have been prepared by
hydrothermal process and characterized by single
crystal X-ray analysis. The Pr complex crystallizes in
monoclinic crystal system with P21/c space group
whereas Nd complex crystallizes in triclinic crystal
system with Pī space group. Their structures vary in
the number of oxo bridges and the position of the
coordinated water molecules bonded to the metal
atoms. Another two-dimensional lanthanide oxalate,
[Pr2(C2O4)3(H2O)6]·3H2O, was produced by the in situ
hydrothermal degradation of tartaric acid to oxalate
ions in presence of KI. The role of iodide anion
as reducing agent towards the reductive coupling of
CO2 has not been reported yet.
6
7
8
9
10
11
12
13
14
15
16
Supplementary data
CCDC 682489, 682490 and 682491 contain the
supplementary crystallographic data for compound
(1), (2) and (3). These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html
or from the Cambridge Crystallographic Data Centre,
12 Union road, Cambridge CB21EZ, UK. (Fax:
(+44)1223-336-033; Email: [email protected]).
17
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