Mendeleev Commun., 2019, 29, 463–465
pyridine core with various substituents at phosphorus atom such
as primary, secondary and tertiary alkyl as well as phenyl and
alkylphenyl groups (Scheme 1).
Table 1 Distribution coefficients D and separation factors SF for the
simultaneous extraction of Amiii and Euiii by ligands 2a–i.a
3 m HNO3
3 m NH4NO3
The yields of compounds 2a–i were not affected by the structure
of substituents at phosphorus atom, thus phosphine sulfides with
cyclohexyl and tert-butyl groups as well as less sterically hindered
aryl-substituted analogues were successfully synthesized. Note
that only one instance of bipyridine-based structure, namely
compound 2a, was synthesized due to its poor extraction efficiency
(see further).
Ligand
DAm
DEu
SFAm/Eu
DAm
DEu
SFAm/Eu
2a
2b
2c
2d
2e
2f
2g
2h
2i
0.003
0.017
0.421
0.431
283
0.003
0.009
0.221
0.246
504
1.0
1.9
1.9
1.8
0.6
2.0
1.7
0.7
1.0
0.023
1.2
0.029
2.1
0.8
0.6
0.7
0.6
0.8
0.6
1.0
0.7
0.7
0.364
0.241
88
0.545
0.405
112
Single crystal of ligand 2a, obtained by slow evaporation of
its acetonitrile solution at room temperature, was investigated by
X-ray diffraction (Figure 1).† According to the data obtained,
two pyridine rings of the 2,2'-bipyridine core are in the same
plane and adopt anti-conformation, this arrangement was reported
for the phosphine oxide counterpart 1a.16 The lengths of P–Ph
bonds in compound 2a (1.811 and 1.815 Å) are similar to those
for the reported analogue,16 while the P–HetAr bond is somewhat
longer, namely 1.840 Å. These bond lengths are also consistent
with the known data for pyridine-2,6-diylbis(diphenylphosphine
sulfide) 2b17 (1.806, 1.810 and 1.823 Å, respectively). The P–S
bonds in ligand 2a have a length of 1.942 Å, which is very close to
the published value (1.943 Å) for analogue 2b, and are distorted
by 16.91° from the plane of the bipyridine core, whereas for
disulfide 2b the distortion is unsymmetrical and reveals larger
values of 21.26° and 27.99°. The C–P–C angles are in the range
of 103.80°–107.21° for compound 2a, while those for disulfide
2b are 101.31°–105.52°. The P–C–N angles for ligand 2a are
wider than those for analogue 2b, namely 117.10° both vs.
113.12° and 114.30°, respectively.
All the synthesized ligands were tested for the separation of
Am and Eu pair by liquid–liquid extraction. Note that despite
the nature of nitric acid as strong oxidant, no decomposition
of the ligands was observed under the extraction conditions, as
confirmed by NMR spectroscopy. The resulting distribution coeffi-
cients D and separation factors SF are given in Table 1.
Phosphine sulfides 2a,b, containing only phenyl groups as
substituents at the phosphorus atom, demonstrated SF values
of 1.0 and 1.9 as well as low distribution coefficients. Methyl
substituents in phenyl groups of compounds 2c,d did not affect
0.014
657
0.007
385
0.228
165
0.390
159
375
565
140
204
0.004
0.004
0.585
0.856
a 0.01 m solutions of ligands 2a–i in m-nitrobenzotrifluoride. An aqueous
phase initially contained trace amounts of 241Am and 152Eu.
the selectivity of the separation, however, about two orders of
magnitude increase in the distribution coefficients was achieved.
With n-octyl groups as substituents in ligands 2e, 2g and 2h,
the solubility of extractants improved greatly, resulting in four
orders of magnitude increase in the distribution coefficients,
namely up to 657. Unexpectedly, two of the three ligands with
n-octyl groups demonstrated preferential extraction of europium
over americium, i.e., inverse selectivity in relation to the desired
preferential binding of actinide, despite the similarity in the
ligand structure. We also evaluated compounds with secondary
and tertiary alkyl groups, namely 2f and 2i. The introduction of
sterically hindered cyclohexyl and tert-butyl substituents resulted
in low distribution coefficients, similarly to ligand 2b; moreover,
the extraction selectivity completely disappeared for compound
2i. An additional investigation was carried out in NH4NO3 solution
as less acidic medium compared with HNO3. This replacement
resulted in an inverse selectivity for all the ligands, while n-octyl-
containing ones had again the largest distribution coefficients.
The range of SF values for all compounds was smaller than that
for nitric acid solution, all D values were somewhat lower as
well. The reason for the inverse selectivity is not evident, because
the consideration of published data is complicated by the dif-
ference in experimental conditions. To the best of our knowledge,
there are no examples of such behavior for ligands with similar
structure.
C(2i)
Next, we investigated the extraction of rare earth elements
by ligands 2b and 2d from 3 m NH4NO3, as well as additionally
from 3 m HNO3 for ligand 2d (Figure 2). Higher D values for
heavier lanthanides have been observed, though according to the
N(1)
C(7)
N(1i)
P(1)
C(2)
C(13)
C(6)
S(1)
0.10
2d from 3 M HNO3
2d from 3 M NH4NO3
0.09
Figure 1 An ORTEP diagram for compound 2a. Thermal ellipsoids are
given at the 50% probability level. Hydrogen atoms are omitted for clarity.
2b from 3 M NH4NO3
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
†
Crystal data for 2a. C34H34N2P2S2, M = 596.72, monoclinic, space
group P21/n, a = 8.6803(2), b = 12.8014(4) and c = 13.2818(3) Å, b =
= 95.389(2)°, V = 1469.35(7) Å3, Z = 2, dcalc = 1.330 g cm–3, m = 2.875 mm–1
,
F(000) = 612. Total of 12376 reflections were measured and 2462
independent reflections (Rint = 0.074) were used in a further refinement,
which converged to wR2 = 0.0986 for all independent reflections [R1 =
= 0.0311 was calculated against F for 2299 observed reflections with
I > 2s(I)]. The data were collected on a STOE diffractometer with a
Pilatus 100K detector, focusing mirror collimation with l(CuKa) =
= 1.54086 Å in rotation method mode at 293(2) K. The structure was
solved and refined with SHELX program. The non-hydrogen atoms were
refined using the anisotropic full matrix least-square procedure.
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Element
CCDC 1887405 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cambridge
Figure 2 Distribution coefficients for extraction of different rare earth
elements by ligands 2b and 2d. m-Nitrobenzotrifluoride is an organic phase,
3 m HNO3 or 3 m NH4NO3 solution is an aqueous phase.
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