Synthesis and properties of complexes of Np(V) and Pu(V) benzoates with phenanthroline

Article abstract of DOI:10.1134/S1066362209030035

Complexes of 1,10-phenanthroline (phen) with Np(V) and Pu(V) benzoates of the compositions NpO₂(phen)(OOCC₆H₅) and PuO₂(phen)(OOCC₆H₅) were synthesized. A powder X-ray diffraction study showed that these compounds, depending on the preparation conditions, exist in the form of two phases having essentially the same composition but different powder patterns. The phases isolated from hot (70-100°C) solutions are isostructural with the previously described complexes AnO₂(bipy)(OOCC₆H₅) (An = Np and Pu, bipy = 2,2′-bipyridine), i.e., in their structure there are dimeric (AnO₂) ₂ ²⁺ cations formed by mutual coordination of two AnO ₂ ⁺ ions via "yl" oxygen atoms. The compounds AnO₂(phen)(OOCC₆H₅) prepared by slow crystallization in the cold or at weak (up to 45°C) heating are isostructural with each other but appreciably differ in the structure from the high-temperature phases. The electronic absorption spectra of the compounds and their thermal behavior were examined.

Full text of DOI:10.1134/S1066362209030035

ISSN 1066-3622, Radiochemistry, 2009, Vol. 51, No. 3, pp. 231–236. © Pleiades Publishing, Inc., 2009.
Original Russian Text © A.A. Bessonov, N.N. Krot, M.S. Grigor’ev, V.I. Makarenkov, 2009, published in Radiokhimiya, 2009, Vol. 51, No. 3, pp. 202–206.
Synthesis and Properties of Complexes of Np(V) and Pu(V)
Benzoates with Phenanthroline
A. A. Bessonov, N. N. Krot, M. S. Grigor’ev, and V. I. Makarenkov
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia
Received July 17, 2008
Abstract—Complexes of 1,10-phenanthroline (phen) with Np(V) and Pu(V) benzoates of the compositions
NpO2(phen)(OOCC6H5) and PuO2(phen)(OOCC6H5) were synthesized. A powder X-ray diffraction study
showed that these compounds, depending on the preparation conditions, exist in the form of two phases having
essentially the same composition but different powder patterns. The phases isolated from hot (70–100°С) solu-
tions are isostructural with the previously described complexes AnO2(bipy)(OOCC6H5) (An = Np and Pu,
bipy = 2,2'-bipyridine), i.e., in their structure there are dimeric (AnO2)22+ cations formed by mutual coordination
of two AnO2+ ions via “yl” oxygen atoms. The compounds AnO2(phen)(OOCC6H5) prepared by slow crystalli-
zation in the cold or at weak (up to 45°С) heating are isostructural with each other but appreciably differ in the
structure from the high-temperature phases. The electronic absorption spectra of the compounds and their ther-
mal behavior were examined.
PACS numbers: 61.10.Nz, 78.40.Ha
DOI: 10.1134/S1066362209030035
Previously [1] we reported on the synthesis and
properties of complexes of Np(V) and Pu(V) with bipy
of the composition AnO2(bipy)(OOCC6H5). The elec-
tronic absorption spectra of the solid compounds sug-
gested the mutual coordination of the AnO+2 ions, i.e.,
formation of cation–cation (CC) bonds in their
structure. This conclusion was confirmed by a single
crystal X-ray diffraction study of AnO2(bipy)·
(OOCC6H5) [2]. In particular, it was shown that the
formulas of the compounds should be more adequately
presented as [AnO2(bipy)(OOCC6H5)]2, with the
dimeric cations (AnO2)22+ formed by CC bonds acting
as coordination centers. The only previously known
compound with CC interaction of this type, along
with AnO2(phen)(OOCC6H5), was double mellitate
of Np(V) and sodium of the composition
Na4(NpO2)2C12O12·8H2O [3]. Therefore, it was inter-
esting to synthesize other Np(V) and Pu(V) com-
pounds similar to [AnO2(bipy)(OOCC6H5)]2 in the
composition and ligand nature and to study their struc-
ture and properties. Here we report on the synthesis
and properties of complexes of Np(V) and Pu(V) ben-
zoates with phenanthroline.
scribed previously [4]. 0.5–1 M ammonium and potas-
sium benzoate solutions were prepared by the same
procedure as in [1]. A working 0.5 M phen solution
was prepared by dissolving a weighed portion of pure
grade phenanthroline in the measured volume of etha-
nol. Pu(VI) was reduced with a 0.5 M N2H4 solution
whose concentration was intermittently checked by
iodometric titration. The devices and experimental pro-
cedures were similar to those in [4].
Preliminary experiments showed that complexes of
phen with Np(V) and Pu(V) benzoates fairly com-
pletely precipitate at room temperature even from very
dilute solutions in the form of voluminous amorphous
precipitates which within a day or more gradually
transform into finely crystalline weakly colored prod-
ucts. The recrystallization considerably accelerates on
heating to 35–45°С. Heating of the amorphous nep-
tunyl(V) compound on a boiling water bath leads to its
transformation into a dense pale green crystalline pre-
cipitate already within 20–50 min. The powder pattern
of the latter precipitate appreciably differs from those
of the Np(V) and Pu(V) compounds prepared by slow
crystallization in the cold. With Pu(V), the precipitates
should be heated with caution to avoid disproportiona-
tion. Nevertheless, with plutonyl(V) benzoate it is also
possible to prepare two phenanthroline complexes by
crystallizing the initially obtained amorphous precipi-
EXPERIMENTAL
The initial 0.145 M Np(V) nitrate and 0.15 M
Pu(VI) perchlorate solutions were prepared as de-
231

232
BESSONOV et al.
Table 1. Powder X-ray diffraction patterns of I and III (the
line intensities were estimated visually)
in solutions heated to 35–40°С, which was followed by
keeping the suspensions at room temperature for 1–
3 days.
NpO2(phen)OOCC6H5 (I,
PuO2(phen)OOCC6H5 (III,
low-temperature phase)
low-temperature phase)
To determine the composition of the synthesized
products, we initially performed a qualitative reaction
with sodium tetraphenylborate to make sure that
the compound contained no potassium cations. There-
fore, taking into account the synthesis procedure,
the compound formulas could be presented as NpO2·
(phen)х(OOCC6H5)·nH2O. The composition was re-
fined by gravimetric analysis for neptunium
(calcination of accurately weighed portions at 800°С to
NpO2). The following results were obtained. For the
product prepared in the cold: sample weight, mg:
3.453. Found NpO2, %: 46.37. Calculated NpO2 in
NpO2(phen)(OOCC6H5) (I), %: 47.19. For the com-
pound crystallizing on heating: sample weight, mg:
23.595. Found NpO2, %: 46.73. Calculated NpO2 in
NpO2(phen)(OOCC6H5) (II), %: 47.19.
dexp, Å
9.146
9.012
8.716
8.410
8.300
7.846
7.728
7.432
6.964
6.723
6.680
6.651
6.391
6.274
5.319
5.238
4.840
4.611
4.072
4.552
3.959
3.806
3.581
3.445
Irel
100
60
70
70
50
40
20
15
15
15
15
10
10
10
10
15
10
50
40
20
10
10
10
10
dexp, Å
9.162
9.055
8.699
8.451
8.317
7.814
7.706
7.395
6.970
6.723
6.651
6.400
6.296
5.337
5.230
4.852
4.602
4.542
4.075
3.976
3.796
3.582
3.448
3.390
Irel
90
100
70
100
100
80
25
40
20
20
20
20
10
20
10
15
20
10
20
15
10
7
Thus, phen complexes with neptunyl(V) benzoate
prepared in the cold and on heating have the composi-
tions expressed by the same formula NpO2(phen)·
(OOCC6H5).
Procedures for preparing phen complexes of Pu(V)
benzoates, based on the results of the preliminary ex-
periments, consisted of the following steps. Aliquots of
0.15 M PuO2(ClO4)2 solution with pH ~4 were diluted
with water to a concentration of (3–5) × 10–3 M, and a
0.5 M ammonium benzoate solution (to a concentra-
tion of ~0.01 M) and an equimolar amount of N2H4
(relative to plutonyl) were added. After mixing for
~1 min, required to convert Pu(VI) to the pentavalent
state, a 0.5 M phen solution was added to a concentra-
tion of ~0.01 M, and the voluminous precipitates
formed were coagulated for 1–2 days in the cold or for
7–9 h on heating to 65–70°С. At room temperature, we
obtained a finely crystalline yellowish-gray precipitate,
and from heated solutions, a denser and darker prod-
uct. The compounds were separated from the mother
liquor on a glass frit and dried as described above for
the synthesis of the corresponding Np compounds I
and II. After checking the compounds for the absence
of NH4+ ions and measuring their powder X-ray pat-
terns (Tables 1, 2; Figs. 1, 2), we ascribed to them the
common formula PuO2(phen)(OOCC6H5). To confirm
the composition of the compounds, we analyzed them
for plutonium by calcination at 800°С to PuO2. We
obtained the following results. For the product pre-
pared at room temperature: sample weight, mg:
11.842. Found PuO2, %: 46.88. Calculated PuO2 in
5
5
tates by either prolonged coagulation in the cold or
heating to 70°С. Taking into account the results of pre-
liminary experiments, synthesis of the neptunyl(V)
compounds was performed by the following proce-
dure.
A sample of practically neutral 0.145 M NpO2NO3
solution was diluted with water by a factor of 15–40,
and a 1.5–2-fold excess (over the equimolar ratio) of
0.5 M potassium benzoate and 0.5 M phen solutions
(the latter contained 0.1 M HNO3 to decrease pH) was
added. After stirring, the precipitate was coagulated on
a boiling water bath for 1–2 h, filtered off in a vacuum
on a glass frit, thoroughly washed with water, and
dried on the filter in an air flow for no less than 3 h. In
the other experiments, the compound was precipitated
similarly, but crystallization was performed for 2–3 h
RADIOCHEMISTRY Vol. 51 No. 3 2009

SYNTHESIS AND PROPERTIES OF COMPLEXES OF Np(V) AND Pu(V)
233
Table 2. Powder X-ray diffraction patterns of II and IV (the line intensities were estimated visually)
[NpO2(phen)(OOCC6H5)]2 (II, high-temperature phase)
[PuO2(phen)(OOCC6H5)]2 (IV, high-temperature phase)
dcalc
d
exp, Å
Irel
100
100
50
dcalc, Å
9.161
8.703
8.373
7.715
6.926
6.770
6.611
6.273
5.633
5.387
5.228
5.146
5.108
4.676
4.628
4.625
4.581
4.530
4.417
4.186
4.035
4.002
3.948
3.632
3.564
3.544
3.531
3.462
3.348
3.320
3.256
3.092
3.079
3.021
3.019
2.9896
2.9404
2.9239
2.9086
2.8807
2.8783
2.8481
2.8364
2.8193
2.7699
2.7519
2.7258
2.7162
2.6772
2.6618
2.6348
2.6302
2.6140
hkl
011
002
101¯
101
110
012
111¯
111
112¯
020
112
021
013
103
120
113¯
022
121¯
121
202¯
014
122
023
212
114
213¯
123
220
221
032
131
124
223¯
301¯
115
204
133¯
025
311¯
214
223
133
312¯
311
034
116¯
232¯
125
312
041
321¯
320
224
dexp, Å
9.171
8.697
8.333
7.672
6.889
6.769
6.583
6.264
5.630
5.387
5.173
5.130
5.088
4.666
4.622
4.595
4.520
4.414
4.184
4.036
4.005
3.943
3.632
3.561
Irel
100
100
40
60
70
10
60
60
10
20
40
15
20
50
50
50
10
40
60
10
20
30
10
40
Å
hkl
011
002
101¯
101
110
012
111¯
111
112¯
020
112
021
013
103
113¯
022
200
121
202¯
014
122
023
212
114
213¯
123
221¯
130
131¯
222¯
131
223¯
115
301¯
204
133¯
214
223
311
230
034
231
232¯
125
312
041
232
320
116
134
140
322
215
9.099
8.652
8.363
7.680
6.921
6.761
6.623
6.266
5.636
5.387
5.223
5.144
5.111
4.678
9.166
8.691
8.353
7.709
6.923
6.768
6.604
6.272
5.624
5.394
5.227
5.152
5.104
4.672
4.617
4.583
4.514
4.420
4.176
4.030
4.004
3.948
3.630
3.562
3.535
3.532
3.444
3.341
3.303
3.302
3.259
3.075
3.017
3.015
2.9880
2.9406
2.8796
2.8783
2.8164
2.8127
2.7705
2.7503
2.7251
2.7155
2.6751
2.6653
2.6295
2.6280
2.6039
2.6035
2.5843
2.5772
2.5576
50
70
10
40
30
20
20
50
20
20
30
4.634
50
4.585
4.542
4.415
4.183
4.038
4.005
3.950
3.638
3.567
60
5
20
15
5
10
20
10
20
3.533
10
3.536
10
3.444
3.336
10
20
3.458
3.353
3.311
3.265
10
15
30
15
3.301
60
3.260
3.069
40
10
3.087
3.025
10
10
3.019
20
2.9913
2.9471
20
20
2.9886
2.9473
2.9311
2.9073
10
10
5
2.8809
2.8177
20
20
5
2.8811
2.8428
10
5
2.7691
2.7561
2.7275
2.7140
2.6776
2.6631
10
10
10
10
5
2.8179
2.7743
2.7599
2.7319
2.7203
2.6819
2.6686
10
10
5
5
5
5
2.6271
2.6025
25
25
5
5
2.6325
2.6105
10
5
2.5804
2.5585
5
10
RADIOCHEMISTRY Vol. 51 No. 3 2009

234
BESSONOV et al.
Table 2. (Contd.)
[NpO2(phen)(OOCC6H5)]2 (II, high-temperature phase)
[PuO2(phen)(OOCC6H5)]2 (IV, high-temperature phase)
d
exp, Å
Irel
dcalc, Å
2.5814
2.5808
2.5641
2.5606
2.5013
2.4781
2.4613
2.4591
2.4549
2.4431
hkl
140
322¯
141¯
206¯
313
323¯
107¯
322
135¯
043
dexp, Å
Irel
dcalc
Å
hkl
313
035
107¯
135¯
043
2.4998
2.4995
2.4564
2.4536
2.4452
2.5855
2.5621
10
2.4994
25
10
2.4564
2.4437
10
10
2.5017
2.4856
10
5
2.4612
2.4487
10
10
PuO2(phen)(OOCC6H5) (III), %: 47.38. For the com-
pound isolated from hot solutions: sample weight, mg:
6.784. Found PuO2, %: 46.15. Calculated PuO2 in
PuO2(phen)(OOCC6H5) (IV), %: 47.38.
As seen from Table 2, the powder X-ray diffraction
patterns of II and IV (Fig. 2) are essentially similar to
each other and to that of the previously prepared
[NpO2(bipy)(OOCC6H5)]2 [1]. Therefore, it can be as-
sumed that in the structures of the new phen com-
plexes, as in the structures of the bipy complexes [1,
2], there are dimeric [AnO2]22+ ions formed by two “yl”
cations coordinated to each other in the monodentate
fashion. Hence, the formulas of II and IV should be
more adequately presented as [AnO2(phen)(OOCC6H5)]2.
It is interesting that this type of CC interactions is rela-
tively seldom and was previously observed only in
the structures of Na4[(NpO2)2C6(COO)6]·8H2O [3],
[AnO2(bipy)(OOCC6H4F-o)]2 [6], and [AnO2(bipy)·
(OOCC6H5)]2 [1], An = Np, Pu.
Fig. 1. X-ray diffraction patterns (bar diagrams) of com-
pounds isolated from solutions at room temperature:
(a) NpO2(phen)OOCC6H5 (I) and (b) PuO2(phen)OOCC6H5
(III).
The powder X-ray diffraction patterns of com-
pounds I and III crystallizing at room temperature
(Fig. 1, Table 1) appreciably differ from those of com-
pounds II and IV (Fig. 2, Table 2), despite certain
similar features. Also, the crystallinity of I and III is
somewhat lower: Despite prolonged keeping under a
layer of the mother liquor, the lines in the powder pat-
terns of these compounds are somewhat broadened,
especially at large reflection angles. The powder pat-
terns of the dry salts and wet products were identical,
which indicates that the line broadening cannot be at-
tributed to partial amorphization of the compounds in
the course of drying. The powder patterns of II and IV
were indexed in the monoclinic system, space group
P21/n, with the unit cell patameters given in Table 3.
Fig. 2. X-ray diffraction patterns (bar diagrams) of com-
pounds crystallizing from hot solutions: (a) [NpO2(phen)·
(OOCC6H5)]2 (II) and (b) [PuO2(phen)(OOCC6H5)]2 (IV).
In the electronic absorption spectra of crystalline I
and II, there are narrow strong bands (Fig. 3) with
RADIOCHEMISTRY Vol. 51 No. 3 2009

SYNTHESIS AND PROPERTIES OF COMPLEXES OF Np(V) AND Pu(V)
Table 3. Unit cell parameters of [AnO2(phen)(OOCC6H5)]2, An = Np, Pu
235
Compound
a, Å
b, Å
с, Å
β, deg
V, Å3
[NpO2(phen)(OOCC6H5)]2 (II)
[PuO2(phen)(OOCC6H5)]2 (IV)
9.087(3)
9.0712(16)
10.774(2)
10.7887(15)
17.494(4)
17.466(3)
95.73(2)
95.61(2)
1704.2(8)
1701.1(5)
maxima at about 1000.7 and 1001.4 nm, typical of the
spectra of NpO2+ aqua ions and Np(V) compounds with
five oxygen atoms in the equatorial plane. Hence, the
coordination polyhedron (CP) of the Np atom in I and
II is a pentagonal bipyramid. The bands are considera-
bly shifted to longer wavelengths relative to nep-
tunyl(V) aqua ions. These shifts can be only partially
(to ~4–6 nm) attributed to the presence of benzoate
ions and phen in the equatorial plane of NpO2+, which
was found by measuring the electronic absorption
spectra of dilute neptunyl(V) solutions in the presence
of a large excess of these ligands. The major fraction
of the band shift is due to CC interactions in the struc-
tures of I and II, which is manifested, as noted above,
in formation of dimeric [NpO2]22+ cations described in
[2, 3, 6].
atom is a pentagonal bipyramid [5]. Hence, the equato-
rial coordination number of plutonyl(V) ion in II and
IV is 5.
The derivatogram of II (Fig. 5) confirms the high
thermal stability of the compound. It starts to noticea-
The electronic absorption spectra of III and IV
(Fig. 4) are typical of hydrated PuO+2 ions and plu-
tonyl(V) compounds in which the CP of the central
Fig. 4. Electronic absorption spectra: (a) PuO2(phen)OOC·
C6H5 (III) [4.940 mg/132 mg KBr (melted)] and (b) [PuO2·
(phen)(OOCC6H5)]2 (IV) [5.282 mg/154 mg KBr (melted)].
Fig. 3. Electronic absorption spectra: (a) NpO2(phen)OOC ·
C6H5 (I) [2.077 mg/167 mg KBr (melted)] and (b) [NpO2·
(phen)(OOCC6H5)]2 (II) [1.729 mg/148 mg KBr (melted)].
Fig. 5. Thermal behavior of [NpO2(phen)(OOCC6H5)]2 (II)
in air. Sample weight 23.595 mg, Δmexp = 54.43% (Δmtheor
=
52.97%).
RADIOCHEMISTRY Vol. 51 No. 3 2009

236
BESSONOV et al.
in the structure and certain properties to the previously
studied compounds [AnO2(bipy)OOCC6H5]2 [1, 2].
The similar complexes synthesized in the cold or at
weak heating have somewhat different structure, but
the presence of CC bonds in them is beyond doubt.
Apparently, in I and III the AnO2(phen)(OOCC6H5)
molecules are arranged in the crystal lattice somewhat
differently compared to II and IV.
It should be noted in conclusion that we were able
to prepare single crystals of I by the hydrothermal
method. The details of the structure of I will be de-
scribed in a separate paper.
Fig. 6. Thermal behavior of [PuO2(phen)(OOCC6H5)]2 (IV)
in air. Sample weight 11.842 mg, Δmexp = 53.12% (Δmtheor
52.79%).
=
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bly decompose in air only above 300°С, with an exo-
thermic effect due to combustion of organic products
of ligand degradation. The process ends at about
430°С with the formation of NpO2. The plutonium
complexes are less resistant to heating and even at
220°С start to change their weight (Fig. 6). In this
case, the decomposition processes occur in two steps,
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readily than Np(V). The second step accompanied by
an exothermic effect ends above 450°С with the for-
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Thus, the phenanthroline complexes of Np(V) and
Pu(V) benzoates isolated from hot solutions are similar
RADIOCHEMISTRY Vol. 51 No. 3 2009