Behavior of Np(V) malonates under hydrothermal conditions

Article abstract of DOI:10.1023/B:RACH.0000024932.79471.0d

Two new Np(V) malonates, LiNpO₂L·2H₂O and KNpO₂L, where L = CH₂(COO)₂, were synthesized, and their characteristics were studied. Similar to their chemical analogs with Na⁺, NH₄⁺, and Cs⁺ ions in the outer sphere, these compounds transform into well crystallized neptunyl(V) compounds of the MNpO₂CO₃· nH₂O type on heating in sealed ampules beneath the layer of the corresponding alkali metal or ammonium malonate solution at a temperature higher than 140°C. Under hydrothermal conditions (NpO₂)₂L·4H₂O is not converted to lower hydrates, and above 135°C it decomposes to form a dark gray amorphous product.

Full text of DOI:10.1023/B:RACH.0000024932.79471.0d

Radiochemistry, Vol. 46, No. 2, 2004, pp. 107 110. Translated from Radiokhimiya, Vol. 46, No. 2, 2004, pp. 97 99.
Original Russian Text Copyright 2004 by Krot, Grigor’ev, Charushnikova.
Behavior of Np(V) Malonates under Hydrothermal Conditions
N. N. Krot, M. S. Grigor’ev, and I. A. Charushnikova
Institute of Physical Chemistry, Russian Academy of Sciences, Moscow, Russia
Received February 20, 2003
Abstract Two new Np(V) malonates, LiNpO2L 2H2O and KNpO2L, where L = CH2(COO)2, were synthe-
sized, and their characteristics were studied. Similar to their chemical analogs with Na+, NH4+, and Cs+ ions in
the outer sphere, these compounds transform into well crystallized neptunyl(V) compounds of the MNpO2CO3
nH2O type on heating in sealed ampules beneath the layer of the corresponding alkali metal or ammonium
malonate solution at a temperature higher than 140 C. Under hydrothermal conditions (NpO2)2L 4H2O is not
converted to lower hydrates, and above 135 C it decomposes to form a dark gray amorphous product.
We reported previously [1] on the synthesis and
with stoichiometric amounts of the corresponding
some characteristics of Np(V) malonates (NpO ) L
alkali or NH OH. The final solutions had pH 6 7,
and their concentrations were in the range 0.2 0.9 M.
All working solutions were prepared in distilled water.
2 2
4
nH O (n = 4, 3, 1), MNpO L (M = NH and Cs), and
2
2
4
NaNpO L 2H O, where L = CH (COO) . Single
2
2
2
2
crystals of tetra- and trihydrate of simple neptunyl(V)
malonate were prepared, and their structures were
studied by X-ray diffraction [2, 3]. In both cases, in-
The spectrophotometric measurements were carried
out with a Shimadzu UV-3100 spectrometer (Japan).
The electronic absorption spectra (EAS) of crystalline
compounds were recorded from pellets pressed with
calcined NaCl as described in [4]. The X-ray powder
patterns were registered in a focusing Guinier camera
+
teresting systems of mutual coordination of the NpO
2
ions through the yl oxygen atoms (cation cation
bonds) were found. The remaining Np(V) malonates
were synthesized only in the finely crystalline state,
and their structure was not elucidated.
(CuK radiation, internal reference Si). The samples
1
were weighed with a Sartorius microbalance (sensitiv-
ity 1 g).
All neptunium(V) malonates were synthesized at
room temperature or under heating on a boiling water
bath. The aim of this work was to study the behavior
of the above compounds under hydrothermal condi-
tions with the aim to prepare some of these in the
crystalline form suitable for X-ray studies. In parallel,
two new compounds of the MNpO L nH O type with
RESULTS AND DISCUSSION
The hydrothermal behavior of simple neptunyl(V)
malonates was studied by an example of (NpO ) L
2 2
2
2
H O prepared as described in [1], i.e., by crystalliza-
2
M = Li and K were synthesized and studied.
tion from an NpO OH solution in a small excess
2
of malonic acid at room temperature. Compounds
EXPERIMENTAL
MNpO L nH O were synthesized considering our
2
2
previous results [1]. For this purpose, to samples of
0.08 0.15 M NpO NO , the corresponding alkali
We used practically neutral (pH 5) working solu-
tions with the Np(V) nitrate concentration of 0.08
2
3
metal or ammonium malonate was added to the con-
centration excess of 0.1 0.2 M, and the mixture was
heated on a boiling water bath for 1 3 h. Then, after
cooling to room temperature, the light bluish crystal-
line precipitates were filtered off on a glass filter
in a vacuum, thoroughly washed with water, and dried
in an air flow for 2 3 h. The final products with
0.15 M, which were prepared by dissolving NpO OH
2
in the stoichiometric amount of dilute HNO . Nep-
3
tunium was purified by anion exchange and precipita-
tion of Np(C O ) 6H O. The solution concentration
2
4 2
2
was determined gravimetrically after evaporation of
the samples to wet salts and calcination of the dry
residues to NpO . The solutions of analytically pure
M = Cs, NH , and Na were identified by X-ray phase
2
4
grade malonic acid were analyzed by titration with
a reference NaOH solution. Alkali metal and ammoni-
um malonates were prepared by neutralization of ac-
curately measured volumes of malonic acid solutions
analysis as CsNpO L, NH NpO L, and NaNpO L
2
4
2
2
2H O, respectively. The composition of the new com-
2
pounds with M = Li and K, corresponding to the for-
mulas LiNpO L 2H O and KNpO L, was determined
2
2
2
1066-3622/04/4602-0107 2004 MAIK Nauka/Interperiodica

108
KROT et al.
Table 1. Interplanar spacings (dexp
,
) and relative intensities* (I) of lines in the X-ray powder patternes of double Np(V)
malonates
LiNpO2L 2H2O
dexp
dcalc
hkl
I
dexp
dcalc
hkl
I
7.59
6.10
6.01
5.43
7.60
6.08
6.04
5.43
100
101
001
110
011
020
211
111
120
210
102
021
002
012
121
220
211
300
302
130
310
vs
vs
vs
vs
m
s
2.383
2.366
2.246
2.178
2.139
2.122
2.090
2.060
1.9701
1.9429
1.8993
1.8783
1.8478
1.8212
1.8135
1.7954
1.7861
1.7516
1.7323
1.7059
2.383
2.365
2.246
2.179
2.139
2.122
2.092
2.059
1.9702
1.9410
1.8982
1.8807
1.8490
1.8187
1.8109
1.7959
1.7870
1.7494
1.7314
1.7073
022
112
221
231
230
320
122
301
232
040
123
140
141
321
330
323
023
241
113
420
w
w
vw
m
m
w
4.76
4.77
3.876
3.587
3.547
3.458
3.406
3.294
3.263
3.023
2.811
2.792
2.717
2.597
2.534
2.507
2.449
2.404
3.882
3.583
3.559
3.458
3.415
3.302
3.266
3.019
2.814
2.787
2.716
2.595
2.535
2.504
2.450
2.210
s
s
w
vw
vw
w
vs
vs
m
s
vw
vw
w
vw
m
w
vw
s
m
m
w
w
m
w
w
w
w
m
KNpO2L**
dexp
I
dexp
I
dexp
I
dexp
I
9.44
7.60
6.45
5.67
5.09
5.02
4.79
4.39
s
3.316
3.249
3.197
3.020
2.973
2.859
2.800
2.711
2.673
2.548
2.524
2.456
2.432
2.376
s
m
w
vw
m
vw
w
m
vs
s
2.337
2.277
2.221
2.206
2.194
2.180
2.157
2.123
2.096
2.076
2.057
2.039
2.027
1.9903
m
m
vw
s
1.9608
1.9462
1.9373
1.8490
1.8157
1.7967
1.7787
1.7204
1.6487
1.6085
1.5870
1.5555
1.5300
1.5086
m
m
m
w
m
w
m
s
m
m
vw
m
w
w
w
m
vs
m
w
vs
w
vs
w
vs
m
w
m
s
vw
vw
w
vw
m
w
m
w
m
4.03
3.960
3.826
3.663
3.613
3.553
m
vs
s
s
* (vs) very strong, (s) strong, (m) medium, (w) weak, and (vw) very weak.
** Plus 14 lines down to d = 1.1533
.
exp
by chemical analysis for neptunium and malonate ion
by procedures described in [1]. For LiNpO L 2H O,
The X-ray powder patterns of LiNpO L 2H O and
2 2
KNpO L are given in Table 1. The X-ray powder pat-
2
2
2
2
found, %: Np 57.2, 57.6; CH (COO) 24.9, 24.3.
tern of LiNpO L 2H O was indexed in monoclinic
2
2
2
2
2
2
2
Calculated, %: Np 57.25; CH (COO) 24.64.
system with the unit cell parameters a = 8.3167(8),
b = 7.7641(5), c = 6.6045(6) = 113.89(1) , V =
, space group P2 or P2 /m.
,
For KNpO L, found, %: Np 58.1, 57.9; CH
2
2
3
2
390(1)
1
1
(COO)
24.3, 25.1. Calculated, %: Np 57.81;
2
2
CH (COO) 24.88.
Comparison of data listed in Table 1 with published
2
2
RADIOCHEMISTRY Vol. 46 No. 2 2004

BEHAVIOR OF Np(V) MALONATES UNDER HYDROTHERMAL CONDITIONS
data [1] showed that LiNpO L 2H O is not isostruc-
109
2
2
tural with NaNpO L 2H O. The same is true for the
2
2
pair NH NpO L and KNpO L. At the same time, it
4
2
2
should be noted that LiNpO L 2H O and KNpO L
2
2
2
are similar to the previously studied compounds of the
MNpO L nH O type in their color, EAS, and features
2
2
of crystallization from a solution. In all the cases, the
precipitation from a solution proceeds fairly slowly,
which is typical for compounds with a polymeric
structure. The rate of crystallization grows with in-
creasing Np(V) concentration and temperature and
somewhat decreases with increasing concentration of
malonate ions. The latter fact is apparently caused by
formation of the fairly stable Np(V) bimalonate com-
plex in the solution [5, 6].
Electronic absorption spectra of crystalline (1) LiNpO L
2
2H O and (2) KNpO L.
2
2
The experiments showed that heating of (NpO ) L
2 2
4H O in a hermetically sealed ampule to 125 C under
2
a layer of 0.02 M H L does not result in decomposi-
2
tion of the compound. Heating to 135 C results in
slow transformation of the compound into a dark gray
flocculent product. The similar pattern is observed in
It is seen from the figure that, in the EAS of
LiNpO L 2H O and KNpO L, similar to the spectra
the case of addition of (NH ) L to the liquid phase
4 2
2
2
2
up to the concentration of 0.01 0.05 M, though the
rate of decomposition of the compound somewhat de-
creases. Thermal instability of simple Np(V) malo-
nates under hydrothermal conditions excludes the pos-
sibility of preparation by this procedure of single crys-
tals of (NpO ) L 2H O or anhydrous salt, formed
of related compounds [1], the absorption band at
approximately 980 nm, characteristic of the hydrated
+
NpO ions, is considerably weakened. This fact sug-
2
gests centrosymmetrical oxygen surrounding of the
neptunium atom in these salts [7]. The most probable
coordination polyhedron (CP) of the central atom is a
hexagonal bipyramid, apparently somewhat distorted.
Unfortunately, with this symmetry of CP we cannot
judge from EAS the presence or absence of cation
cation bonding in the structure of the compounds.
2 2
2
under heating of the tetra- and trihydrate to 140 and
200 C, respectively [1].
The behavior of double malonates under hydro-
thermal conditions strongly depends on the kind of the
2
outer-sphere cation, excess of L , and the presence of
KNpO L and especially LiNpO L 2H O are ap-
2
2
2
2
malonic acid. At [L ] < 0.3 M, in the absence of
preciably soluble in water. For example, after stirring
for 6 h excess LiNpO L 2H O with 0.06 M Li L the
H L, above 135 C the compounds MNpO L 2H O
2
2
2
2
2
2
3
with M = Li and Na transform into dark gray prod-
Np concentration in the liquid phase was 8.5 10
M
ucts, as in the case of simple malonates. When H L is
2
(15 C). After precipitation of KNpO L by the above
2
3
added to the liquid phase to a concentration of 0.05
0.1 M, LiNpO L 2H O under the layer of 0.25 M
procedure, 1.3 10 M Np(V) was found in the
2
2
2
mother liquor with [L ] = 0.14 M.
Li L at a temperature of approximately 150 C slowly
2
After keeping at 115 C for 1 h and cooling in a
desiccator over granulated KOH, the weight loss of
LiNpO L 2H O corresponded to one H O molecule.
transforms into a dense bluish precipitate consisting of
hexagonal plate-like prisms. At 165 C, this process is
complete in 3 h. Further heating to 180 C does not
affect the precipitate. In the case of NaNpO 2H O,
2
2
2
However, the monohydrate formed gradually absorbs
water at storage in air and within approximately
4 days transforms into the initial compound.
2
2
Table 2. Parameters of hexagonal unit cells of anhydrous
double Np(V) carbonates
To study the hydrothermal behavior of Np(V)
malonates, glass ampules (D = 6 mm) were charged
with weighed portions of the compounds and a small
volume (0.3 1 ml) of water or solution of the corre-
sponding M L with addition of H L to a concentra-
tion of approximately 0.02 M with the aim to decrease
pH of the liquid phase and suppress hydrolysis of
neptunyl(V). The ampules were hermetically sealed
and kept in an oven with automatic temperature con-
trol for several hours or days. The temperature varia-
tion during heating was in the limits of 2 C.
Refer-
ences
3
Compound
a,
c,
V,
2
2
KNpO2CO3
KNpO2CO3
5.120
9.971
226.4
[9, 10]
5.104(1) 10.104(1) 227.95(7) This
work
NH4PuO2CO3 5.090
10.390
233.1
[11]
CsNpO2CO3 5.118(1) 12.079(1) 274.01(8) This
work
RADIOCHEMISTRY Vol. 46 No. 2 2004

110
KROT et al.
on heating for 3 h in 0.3 M Na L + 0.05 M
was indexed in the monoclinic system: a = 12.540(1),
b = 6.242(1), c = 6.248(1) = 91.05(1) , V =
. Based on the unit cell volume, the
compound was identified as NH NpO CO 0.5H O
(Z = 4). The results of studying carbonates will be
described in more detail in a separate paper.
2
H L, druses of crystals in the form of grayish-green
,
2
3
elongated tetrahedral prisms are formed. An increase
488.98(12)
in the Na L concentration to 0.25 M results in forma-
2
4
2
3
2
tion of a mixture of the above crystals and a gray
amorphous decomposition product.
The compounds MNpO L with M = NH , K, and
2
4
REFERENCES
Cs gradually transform into dense bluish crystalline
2
products even at [L ] = 0.03 M in the presence of
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3. Grigor’ev, M.S., Charushnikova, I.A., Krot, N.N.,
0.05 M H L at a temperature of approximately 140 C.
2
This pattern is retained with increasing concentration
of malonate ions to 0.3 M and temperature to 180 C.
In all cases, the solutions over precipitate in ampules
are transparent and colorless.
The crystalline products formed from MNpO L
2
et al., Radiokhimiya, 1993, vol. 35, no. 4, pp. 31 37.
nH O under hydrothermal conditions were studied by
2
4. Krot, N.N., Bessonov, A.A., and Perminov, V.P.,
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X-ray phase analysis. The X-ray powder patterns
showed that, under these conditions, double carbo-
nates of the general formula MNpO CO nH O were
2
3
2
formed, with the K and Cs salts being anhydrous.
Their X-ray powder patterns were indexed [7] and
refined [8] in the hexagonal system (Table 2). They
are similar to those published for isostructural mono-
carbonates KNpO CO and NH PuO CO [9 11].
2
3
4
2
3
The analysis of X-ray powder patterns of double
Np(V) carbonates with Li and Na showed that, under
hydrothermal conditions, mixtures of products consist-
ing of anhydrous salts and hydrates were formed. The
single crystals of LiNpO CO 2H O were obtained,
2
3
2
and its crystal structure was solved. The monoclinic
unit cell parameters are a = 8.775(1), b = 5.058(1),
10. Keenan, T.K. and Kruse, F.H., Inorg. Chem., 1964,
vol. 3, no. 9, pp. 1231 1232.
11. Ellinger, F.H. and Zachariasen, W.H., J. Phys. Chem.,
3
c = 6.643(1)
group Cm.
,
= 90.04(1) , V = 294.8
, space
The X-ray powder pattern of the ammonium salt
1954, vol. 58, no. 5, pp. 405 408.
RADIOCHEMISTRY Vol. 46 No. 2 2004