Synthesis and extraction behavior of alkyl and cyclic aminophosphonates towards actinides

Article abstract of DOI:10.1016/j.ica.2018.07.002

Alkyl and cyclic substituted aminophosphonates (AmPs) were synthesized and characterized with various spectroscopic techniques. The molecular structures of diphenyl phenyl aminophosphonate (DPhPhAmP) and diphenyl cyclohexyl aminophosphonate (DPhCyAmP) wer

Full text of DOI:10.1016/j.ica.2018.07.002

Accepted Manuscript
Research paper
Synthesis and Extraction behavior of Alkyl and Cyclic Aminophosphonates to-
wards Actinides
Dhrubajyoti Das, C.V.S. Brahmmananda Rao, N. Sivaraman, Akella
Sivaramakrishna, Kari Vijayakrishna
PII:
S0020-1693(18)30563-2
https://doi.org/10.1016/j.ica.2018.07.002
ICA 18348
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
14 April 2018
1 July 2018
2 July 2018
Please cite this article as: D. Das, C.V.S. Brahmmananda Rao, N. Sivaraman, A. Sivaramakrishna, K. Vijayakrishna,
Synthesis and Extraction behavior of Alkyl and Cyclic Aminophosphonates towards Actinides, Inorganica Chimica
Acta (2018), doi: https://doi.org/10.1016/j.ica.2018.07.002
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Synthesis and Extraction behavior of Alkyl and Cyclic
Aminophosphonates towards Actinides
a
b
b
a
Dhrubajyoti Das , C.V.S. Brahmmananda Rao, N. Sivaraman, Akella Sivaramakrishna, Kari
a,
Vijayakrishna, *
a
Department of Chemistry, School of Advanced Sciences, VIT University, Vellore 632 014, Tamil
Nadu, India
b
Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam 603 102, Tamil Nadu, India
Corresponding Author: Kari Vijayakrishna
E-mail: kari@vit.ac.in and vijayakrishnakari@gmail.com
Abstract
Alkyl and cyclic substituted aminophosphonates (AmPs) were synthesized and characterized
with various spectroscopic techniques. The molecular structures of diphenyl phenyl
aminophosphonate (DPhPhAmP) and diphenyl cyclohexyl aminophosphonate (DPhCyAmP)
were elucidated based on the single crystal XRD analysis. Extraction behavior of these
synthesized AmPs were investigated with some actinides [U(VI), Th(IV), and Am(III)] and also
studied their acid uptake as a function of nitric acid concentration. Here we have compared the
extraction behavior of aminophosphonates (AmPs) towards actinides as a function of alkyl,
cyclic and alicyclic substituents.
Keywords: Aminophosphonates;Solvent extraction; Actinides; Uranium; Thorium
1

1
. Introduction
Nuclear spent fuel contains a large amount of different actinide ions along with many fission
products [1]. Recovery of those actinides from other metal ions is a challenging task and also
plays an important role in nuclear waste management [2]. Over these years various processes
were adopted for the separation of actinides including precipitation, co-precipitation, ion
exchange and solvent extraction methods [3, 4]. Solvent extraction process has been proved to be
the most favorable method for separation and purification of actinides in large scale.
Organophosphorous and amide based neutral and acidic extractants like TBP (tri-n-butyl
phosphate) [5], CMPO (octyl, phenyl- N, N-diisobutyl carbamoylmethyl phosphine oxide) [6],
TRPO (trialkyl phosphine oxides) [7, 8] HD2EHP (di(2-ethylhexyl) phosphoric acid) [9],
DIDPA (di-isodecyl phosphoric acid) [10], DMDBTDMA (N,N,N′,N′- dimethyldibutyl tetradecyl
malonamide) [11], TODGA (N,N,N′,N′- tetraoctyl malonamide) [12] are being used in many
processes like PUREX [13], TRUEX [14] and DIAMEX [15] which are used for the separation
of actinides and lanthanides (not on commercial scale). N-donor extractants like BTP (2,6-
di(1,2,4-triazin-3-yl)pyridine)
and
BTBPs
(6,6′-bis(5,6-dialkyl-1,2,4-triazin-3-yl)-2,2′-
bypiridines) also used for the separation and extraction of Am(III). Recently a tetradentate N-
donor ligand N,N’-diethyl-N,N’-ditolyl-2,9-diamide-1,10-phenanthroline (Et-Tol-DAPhen) was
reported as a promising ligand for the separation and extraction of actinides from nuclear waste
stream [16-19]. TBP has been extensively used in nuclear repossessing industry for the recovery
of uranium and plutonium from irradiated fuel since last six decades [20]. TBP is a mild
extractant and can extract actinides at higher acidity and strip at lower acidity. Being hard acids,
actinides prefer oxygen donor ligands like organophosphorous esters as the preferred class of
2

compounds for actinides recovery [21]. Extraction of actinides depends on the basicity of the
phosphoryl oxygen, as well as it is related to the substituents of the extractants [22]. The nature
of the substituents also plays an important role on the extractability. Numerous classes of dialkyl
alkyl phosphonates [23-26] and their functionalized resin [27], H-phosphonates [28, 29] and di-
n-alkyl phosphine oxides [30, 31] (DAPOs) were synthesized and their extraction behavior
towards actinides was studied and reported earlier [32-35]. In the literature a variety of
phosphates with various substituents have been employed for the extraction of actinide.
ꢀ
ꢁꢂꢃꢄꢅꢃꢆꢄꢅꢃꢂꢇꢈꢉꢆꢊ ( ꢀꢋꢆꢌ,ꢊ ꢇꢍꢆꢃꢊ
ꢎꢏ ꢂꢃꢐꢂꢊ ꢇꢆꢊ ꢄꢅꢃꢆꢄꢅꢃꢑꢇꢀꢁꢒꢇꢈꢉꢆꢊ ꢇꢑꢉ ꢑꢉꢍꢇꢈꢁꢓꢉꢍꢔꢊ ꢂꢉꢐꢊ ꢕꢍꢇꢆꢆꢊ
ꢃꢖꢊ
ꢎꢍ ꢁꢗꢇꢂꢒꢆꢊꢒꢉꢑꢁꢓꢉꢒꢊꢖꢑꢃꢀꢊꢈꢅꢉꢊꢓꢉꢑꢔ well-known phosphine oxide family and offer fascinating
opportunities. This area has the potential to grow as increasing demand for carefully designed
ligands of this type in the nuclear industry towards the treatment and separation of nuclear waste.
It seemed a constructive exercise to analyze the nature of these ligands in the coordination
spheres of some selected actinides, especially as these complexes possess potentially reactive P–
N,ꢊꢎP–C–N and P–N–C linkages. Eventually, our search for newer ligands has lead us to
investigate the extraction behavior of phosphonates towards the actinide ions having a primary
amine group in the phosphonate moiety. Also we wanted to study the effect of linear, aromatic
and cyclic substitutions in distribution ratio of actinides. In this connection, we have synthesized
a class of alkyl and cyclic substituted aminophosphonates (AmPs) and studied their extraction
behavior towards various actinides.
2
. Experimental
2
.1. Materials and Instrumentation
3

Cyclohexyl amine, aniline, phenylphosphinic dichloride, diphenylphosphoryl chloride and
CDCl (Sigma-Aldrich) were used as received. Butyl, hexyl and octyl amine were purchased
3
from Sigma Aldrich and used without any further purification. All the alcohols and solvents
were purchased from SD fine chemicals. Solvents were distilled prior to use. Radiotracers were
used from laboratory stock and their radiochemical purity was checked prior to their use.
1
13
31
1
H, C, and P NMR spectra were recorded by BRUKER DMX-400 and H chemical
shifts were reported relative to the residual proton resonance in deuterated solvents (all at 298 K,
3
1
CDCl ). H PO was used as an external standard for P NMR. FT-IR spectra were recorded on
3
3
4
SHIMADZU Affinity 1 FT-IR spectrometer using KBr pellet. The spectra were recorded at
ambient temperature by making pressed pellets of the compound. UV–vis absorption spectra
were recorded with a UNICAM UV4-100 type double-beam spectrophotometer (ATI UNICAM,
2
33
Cambridge, UK). Radiometric assay of U was carried out using a liquid scintillation counting
technique using an LSC system (Hidex, Finland) coupled to a multi-channel analyzer using a
dioxane-based scintillator cocktail.
2
.2. Single crystal X-ray analysis
Single crystal X-ray diffraction analysis was performed by Bruker AXS SMART APE XII single
crystal X-ray diffractometer equipped with graphite monochromatic MoKαꢊ(λ = 0.71073 Å)
radiation and 62 mm square CCD detector. Intensity data was collected at room temperature
(293K). The unit cell parameters were determined from 36 frames measured (0.5˚ꢊꢄꢅꢁ-scan) from
different crystallographic zones using the method of difference vectors. The intensity data were
collected with an average fourfold redundancy per reflection and optimum resolution (0.80 Å).
The intensity data collection, frames integration, Lorentz and polarization corrections and decay
4

correction were carried out using SAINT-NT (version 7.06a) software. An empirical absorption
correction (multi-scan) was performed using the SADABS program. The crystal structure was
solved by direct methods using SHELXS-97 and refined by full-matrix least-squares using
SHELXL-97 [35]. Molecular geometry was calculated using PARST. All non-hydrogen atoms
were refined using anisotropic thermal parameters. The hydrogen atoms were included in the
structure factor calculation at idealized positions by using a riding model, but not refined. Images
were created with the ORTEP-PLATON program. The CCDC deposition number for diphenyl
phenyl aminophosphonate (DPhPhAmP) and diphenyl cyclohexyl aminophosphonate
(DPhCyAmP) are CCDC 1820500 and CCDC 1820498 respectively.
2
.3. Synthesis of Dibutyl butyl aminophosphonates (DBBAmP)
Dibutyl butyl aminophosphonate was synthesized by Atherton–Todd reaction [37] (Scheme 1).
Ten grams (51.54 mmol) of dibutyl hydrogen phosphonate (DBP) was taken in a two neck RB
containing 25 g of CCl in 50 mL of dry THF. The mixture was stirred for 2 h in an ice bath. To
4
this mixture of 4.46g (61.23 mmol) of butylamine and 15.45 g (154.63 mmol) of triethylamine
(dissolving in 100 mL of THF) was added drop wise over a period of 45-60 minutes. Reaction
mixture was allowed to stir for overnight. Hydrochloride salt of TEA was filtered. The organic
layer was washed with water (3X50 mL), separated and dried over anhydrous Na SO . Solvent
2
4
and other volatile impurities were removed using rotary evaporator and then further dried the
compound through high vacuum pump for 2 h to obtain viscous yellow liquid in 12.56 g (92%
isolated yield). Similar procedure was followed to synthesize DHHAmP, DOOAmP and
-1
1
DCyCyAmP. IR (cm ): 1022.37 (P=O), 3111.85 (N-H). H-NMRꢊ(δ,ꢊ400ꢊMHz,ꢊCDCꢍ): 0.91-
3
5

3
1
0
1
.94(t, CH CH , 9 H), 3.94-4.01 (m, P-O-CH 4 H), 1.32-1.68 (m, CH CH 14 H), P-NMR (δ,ꢊ
3 2 2, 3 2,
62 MHz, CDCl ): 9.27
3
-1
2
.4. Dihexyl hexyl aminophosphonates (DHHAmP): IR (cm ): 1065.43 (P=O), 3140.11(N-H).
1
H-NMRꢊ(δ,ꢊ400ꢊMHz,ꢊCDCl ): 0.86-0.88 (t, CH CH , 9 H), 2.83-2.89 (m, P-O-CH 4 H), 1.28-
3
3
2
2,
3
1
1
.67 (m, CH CH 24 H), 3.93-3.99 (m, NHCH 2 H). P-NMR (δ,ꢊ162ꢊMHz,ꢊCDCꢍ): 9.27.
3 2, 2, 3
-1
2
.5. Dioctyl octyl aminophosphonates (DOOAmP): IR (cm ): 1036.81 (P=O), 3221.12 (N-H).
1
H-NMRꢊ(δ,ꢊ400 MHz, CDCl ): 0.86-0.89 (t, CH CH , 9 H), 2.84-2.89 (m, P-O-CH 4 H), 1.27-
3
3
2
2,
3
1
1
.67 (m, CH CH 36 H), 3.93-3.99 (m, NHCH 2 H), P-NMR (δ,ꢊ162ꢊMHz,ꢊCDCꢍ): 9.43.
3 2, 2, 3
-1
2
.6. Dicyclohexylcyclohxyl aminophosphonate (DCyCyAmP): IR (cm ): 1220.94 (P=O), 3226.91
1
(
N-H). H-NMR (δ,ꢊ400ꢊMHz,ꢊCDCꢍ): 4.281(m, P-O-CH 2 H), 2.95(m, P-NH-CH 1 H), 1.1-
3
,
,
3
1
1
.93 (m, CH CH CH 30 H). P-NMR (δ,ꢊ162ꢊMHz,ꢊCDCꢍ): 6.58.
2 2 2, 3
2
.7. Synthesis of Diphenylcyclohexyl aminophosphonate (DPhCyAmP)
In an two neck RB 10g (37.22 mmol) of diphenyl phosphoryl chloride dissolved in 80 mL dry
THF was taken and to that mixture of 4.54g (45.73 mmol) of cyclohexylamine and 11.21g
(111.3 mmol) of triethylamine (dissolving in 100 ml of THF) was added drop wise over a period
of 45-60 min in an ice bath. Reaction was carried out in nitrogen atmosphere. The reaction
mixture allowed to reach at room temperature and kept for overnight stirring. White precipitate
of hydrochloride salt of TEA was filtered. Organic layer was washed with water (3X50 mL),
separated and dried over anhydrous Na SO and the volatile solvent and other impurities was
2
4
removed using rotary evaporator. Required compound was obtained as colorless solid (isolated
yield 87.5%). Same procedure was used for the synthesis of diphenylphenyl aminophosphonate
6

-
(DPhPhAmP) and monophenyldicyclohexyl aminophosphonate (MPhDcyAmP) ligands. IR (cm
1
1
)
: 1190.08 (P=O), 3207.62 (N-H). H-NMR (δ, 400 MHz, CDCl ): 7.27 (d, Ar-o, 4 H), 7.33 (t,
3
Ar-m, 4 H), 7.16 (t, Ar-p, 2 H), 3.26 (m, P-NH-CH 1 H), 1.14-1.195 (m, CH CH CH 24 H).
,
2
2
2,
3
1
13
P-NMR (δ,ꢊ162MHz, CDCl ): -1.68. C-NMR (δ, 100 MHz, CDCl ): 24.98, 25.32, 35.39,
3
3
5
1.26, 120.23, 124.76, 129.62, 151.03.
2
.8. Diphenylphenyl aminophosphonate (DPhPhAmP): Ten grams (37.22 mmol) of diphenyl
phosphoryl chloride in 80 mL dry THF, was made to reacted with 4.25g of aniline (45.73 mmol)
in presence of 11.21g (111.3 mmol) of triethylamine (dissolving in 100 mL of THF) in ice
cooled condition and the reaction was allowed to stir overnight. White precipitate of
hydrochloride salt of TEA was filtered from the reaction mixture. The organic layer was washed
with water (3X50 mL), separated, dried over anhydrous Na SO and the volatile solvent was
2
4
removed using rotary evaporator. Required compound was obtained as colorless solid (isolated
-1
1
yield 84.29%). IR (cm ): 1176.58 (P=O), 3170.97 (N-H). H-NMR (δ, 400 MHz, CDCl ): 7.03
3
3
1
13
(
t, NH-Ar-p, 1 H), 7.03 (m, Ar-o, m, p 14 H). P-NMR (δ,ꢊ162MHz, CDCl ): -6.81. C-NMR
3
(
δ, 100 MHz, CDCl ): 118.22, 120.39 122.25, 125.31, 129.30, 129.75, 139.12, 150.33.
3
2
.9. Monophenyl dicyclohexyl aminophosphonate (MPhDCyAmP): Ten grams (47.39 mmol) of
phenyl dichlorophosphate in 80 mL of dry THF was made to react with two equivalent of
cyclohexyl amine (9.4g, 94.78 mmol) in presence of excess of triethylamine (17. 65 g, 189.56
mmol) (dissolving in 150 mL of THF) at ice cooled condition. The reaction was allowed to stir
overnight. After filtering the white precipitate of hydrochloride salt of TEA the organic layer was
washed with water (3X50 mL), separated and dried over anhydrous Na SO The volatile solvent
2
4.
was removed using rotary evaporator. Required compound was obtained as colorless solid
7

-1
1
(
isolated yield 86%). IR (cm ): 1199.72 (P=O), 3228.84 (N-H). H NMRꢊ(δ,ꢊ400ꢊMHz,ꢊCDCꢍ):
3
7
.16 (d, Ar-o, 2 H), 7.24 (t, Ar-m, 2 H), 7.03 (t, Ar-p, 1 H), 3.034 (m, P-NH-CH, 2 H), 1.05-1.86
3
1
13
(
m, CH CH CH 20 H). P NMR (δ,ꢊ162ꢊMHz,ꢊCDCꢍ): 9.34. C NMR (δ,ꢊ100ꢊMHz,ꢊCDCꢍ):
2 2 2, 3 3
2
5.1, 25.44, 35.87, 50.6, 120.26, 123.97, 129.45, 151.54.
2
.10. Batch studies of actinides by solvent extraction
Uranium and Americium: The extractant was pre-equilibrated with appropriate nitric acid
concentration prior to the extraction studies. 2 mL of ligand solution (0.01 molar) in xylene and 2
mL of corresponding nitric acid solutions were taken in an equilibration tube, spiked with
2
33 241
U/ Am tracer and equilibrated in a constant temperature bath at 303K for 2 h. After
attainment of equilibrium, suitable aliquots from both the phases were taken for radiometric
2
33
assay of actinides.
U -activity was obtained from the liquid scintillation counting using
dioxane based cocktail and from that the equilibrium concentrations of U(VI) in both the phases
were measured. The equilibrium concentration of Am (III) in the aqueous and organic phases
was computed by measuring the activities of the 60 keV  photons of the respective phases using
NaI(Tl) counter. The precision in the counting by detectors is ±5%. The distribution ratio (DM)
was calculated as the ratio of concentration of the metal ion in organic phase to that in aqueous
phase
(1)
8

Where [M]org. and [M]aq. are the concentrations of metal ion in organic and aqueous phases
respectively. All the experiment was carried out at various nitric acid concentrations ranging
from 0.01M to 6 M.
Thorium: The extraction of Th(IV) was performed by following the similar experimental
procedure mentioned for uranium using 0.01M solutions of extractant in xylene. Thorium
amount was calculated spectrophotometrically using arsenazo-III as the chromogenic agent [38]
in the initial and at equilibrium aqueous phase, subsequently the organic samples were estimated
by subtracting the equilibrium aqueous concentration from the initial feed concentration. The
distribution ratio can be defined as
(2)
Where [M]aq(i) and [M]aq(f) concentration of metal ions before and after contact with organic
phase.
9

Figure 1. Ligands used in the present study
. Results and discussions
3
3
.1. Synthesis of aminophosphonates
All aminophosphonates were synthesized by modified literature report [37]. Purity and structural
conformations of all synthesized AmPs were established by various spectroscopic techniques
3
1
1
13
such as P, H and C NMR (See supporting information for spectral data). Three alkyl
aminophosphonates viz. DBBAmP, DHHAmP, DOOAmP and one cyclic aminophosphonate,
DCyCyAmP were synthesized from their corresponding H-phosphonates by reacting with
3
1
appropriate amines in presence of CCl and TEA as shown in Scheme 1. In P-NMR, a
4
substantial downfield shift was observed in the AmPs compared to their precursor the H-
phosphonate. Also a singlet was observed in the final aminophosphonate whereas the H-
1
0

3
1
phosphonate appears as a doublet in P NMR suggesting the formation of the compound. For
example, DCyCyAmP appears as a singlet at δ6.58 where the starting material dicyclohexyl H-
phosphonate resonates as doublet at δ4.52 which confirms the complete conversion (Figure 2).
Scheme 1. Synthesis of alkyl aminophosphonates (AmPs)
DOOAmP
(c)
(b)
DHHAmP
(a)
DBBAmP
3
1
Figure 2. P NMR overlay of alkyl aminophosphonates
3
1
31
Similarly DPhCyAmP ( P = δ-1.68) and DPhPhAmP ( P = δ-6.81) were synthesized from
3
1
diphenyl phosphoryl chloride ( P = δ 6.1) reacting cyclohexyl amine and phenyl amine
31
respectively (Scheme 2), which has shown a significant up-field shift in P NMR compared to
the starting material. Two equivalents of cyclohexyl amine was reacted with one equivalent of
phenylphosphinic dichloride to form MPhDCyAmP (Scheme 3) and the product derived from
this reaction mixture gives a singlet at δ9.34 which is much more deshielded than the precursor
1
1

3
1
resonating at δ-1.8 in P NMR (Figure 3). All these AmPs showed strong infrared absorptions
-1
in the range of 1100-1200 cm due to phosphoryl groups. The molecular structures of
DPhCyAmP (Figure 4) and DPhPhAmP (Figure 5) were further confirmed by single crystal X-
ray analysis. Both the molecules showed significantly short P=O bond distances as 1.4565(16)
and 1.4663(11) Å respectively. The P-O bond lengths were observed in the range of 1.5785(11)
to 1.5924(17) Å. In DPhPhAmP molecule, the P-N bond length was slightly longer (1.6249(13)
Å) when compared to DPhCyAmP (1.6062(18) Å). Both the structures did not exhibit any
considerable changes in the bond angles.
Scheme 2. Preparation of diphenylcyclohexyl aminophosphonate (DPhCyAmP) and diphenyl
phenyl aminophosphonate (DPhPhAmP)
Scheme 3. Preparation of monophenyl dicyclohexyl aminophosphonate (MPhDCyAmP)
1
2

(
d)
c)
MPhDCyAmP
(
DPhPhAmP
(
b)
a)
(
DPhCyAmP
DCyCyAmP
3
1
Figure 3. P NMR overlay of cyclic aminophosphonates
Figure 4. Molecular structure (ORTEP) of DPhCyAmP obtained by single crystal X-Ray
analysis at 40 % ellipsoid levels. Some selected bond lengths (Å) are given here: P1 O2
1
.4565(16), P1 O3 1.5912(15), P1 O1 1.5924(17) and P1 N1 1.6062(18); selected bond angles
°):O3 P1 O2 114.21(9), O3 P1 O1 116.64(10), O2 P1 O1 93.18(9), O3 P1 N1 113.15(10), O2
P1 N1 109.37(9) and O1 P1 N1 108.51(9).
(
1
3

Figure 5. The ORTEP diagram of DPhPhAmP derived from single crystal XRD data drawn at
4
1
1
0 % ellipsoid levels; Some selected bond lengths (Å) are given here: N1 P1 1.6249(13), O1 P1
.5785(11), O2 P1 1.5881(11) and O3 P1 1.4663(11); selected bond angles (°):O3 P1 O1
16.95(7), O3 P1 O2 114.70(7), O1 P1 O2 94.99(6), O3 P1 N1 110.08(6), O1 P1 N1 109.03(7)
and O2 P1 N1 110.18(7).
3
3
.2. Distribution studies of aminophosphonates
.2.1 Effect of alkyl chain length in distribution studies
In the present study, 0.2 M solutions of alkyl AmPs in xylene were used. Figure 6a gives
the variation of distribution coefficient of the for U(VI) by alkyl AmPs at 303K in different nitric
acid concentration ranging from 0.01 –6M. The distribution values increased gradually with the
increase in nitric acid concentration. Alkyl AmPs are neutral ligands and only extracts through
P=O coordination. The alkyl chain length also plays an important role in the distribution
coefficient where we can see distribution value is increasing with the decrease of alkyl chain
length. DBBAmP shows highest distribution among all three followed by DHHAmP and
DOOAmP which suggests that lesser the steric hindrance leads to more distribution.
1
4

(3)
As shown in Figure 6b, DTh(IV) increased with increase in nitric acid concentration and
reached maximum at 2M nitric acid concentration. The decrease in the D value at higher acidity
is due to the inextractable anionic species and considerable extraction of nitric acid. At higher
acid concentration, variation of DTh(IV) is greater than DU(VI) and this may be due to their varied
solvation numbers [25].
(4)
The distribution trend for Am(III) followed the same trend as that of thorium (Figure 6c).
However the distribution of Am(III) is very less for alkyl AmPs as these compounds are not
suitable for extraction of Am(III).
1
5

7
0
3
3
2
2
1
1
5
0
5
0
5
0
5
0
(
a)
60
50
40
30
20
10
0
(b)
0
.2M DBBAMP/Xylene
0.2M DHHAMP/Xylene
.2M DOOAMP/Xylene
0.2 M DBBAMP/Xylene
0.2 M DHHAMP/Xylene
0
0.2 M DOOAMP/Xylene
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
[
HNO ] mol/L
3
[
HNO ] mol/L
3
2
1
1
0
0
.0
.5
.0
.5
.0
0
0
.2 DBBAMP/Xylene
.2 DHHAMP/Xylene
(c)
0.2 DOOAMP/Xylene
0
1
2
3
4
5
6
[
HNO3]
Figure 6. Variation of (a) DU(VI), (b) DTh(IV) and (c) Dam(III) as a function of nitric acid
concentration for alkyl AmPs at 303 K
3
.2.2 Effect of cyclic substitution in distribution studies
Due to poor solubility of the cyclic substituted ligands (cyclic AmPs) in xylene, we have
taken chloroform as diluent. Figure 7 shows the distribution coefficients of uranium and thorium
at 303K as a function of nitric acid concentration for a fixed concentration of cyclic
aminophosphonates (0.2 M) in chloroform. Uranium extraction was observed only in case of
DCyCyAmP and MPhDCyAmP, where as DPhCyAmP and DPhPhAmP showed very less or no
1
6

distribution values. The distribution ratio of DU(VI) for DCyCyAmP and MPhDCyAmP increased
with increase in nitric acid concentrations. Among four cyclic AmPs, only DCyCyAmP showed
some significant distribution value for thorium (Figure 7b). These cyclic aminophosphonates did
not show any extraction for Am(III).
4
5
0
0
0
0
.2 M DCyCyAmP/CHCl3
2
2
1
1
5
0
5
0
5
0
40
35
(
b)
.2 M DPhCyAmP/CHCl3
.2 M DPhPhAmP/CHCl3
.2 M MPhDCyAmP/CHCl3
3
2
2
1
1
0
5
0
5
0
5
0
5
0
0
0
0
.2 M DCyCyAmP/CHCl
3
.2 M MPhDCyAmP/CHCl
3
.2 M DPhCyAmp/CHCl
3
.2 M DPhPhAmP/CHCl
3
(
a)
-
0
1
2
3
4
5
6
0
5
[
HNO ] mol/L
[HNO ] mol/L
3
3
Figure 7. Variation of (a) DU(VI) and (b) DTh(IV) as a function of nitric acid concentration for
cyclic AmPs at 303 K
Among these alkyl and cyclic substituted aminophosphonates, alkyl aminophosphonates
showed better extraction compared to the cyclic aminophosphonate. In case of alkyl AmPs,
dibutyl butyl aminophosphonate showed a better extraction for all the three actinides viz. U(VI),
Th(IV) and Am(III). The lesser alkyl chain length makes the compound less hindered and this
leads to an efficient coordination with the metal ion present in the solution. The extraction
behavior of these alkyl AmPs towards U(VI), Th(IV) and Am(III) is in the following order:
DBBAmP > DHHAmP > DOOAmP (Figure 8). Among the four cyclic AmPs used,
MPhDCyAmP showed the highest extraction, whereas DPhPhCyAmP and DPhCyAmP did not
show any extraction at all. The lower extractability of these cyclic AmPs (DPhPhCyAmP and
DPhCyAmP) is due to the bulky phenyl groups that makes phosphonate more hindered and less
available for the coordination with metal ions. But the different structural conformations of
cyclohexyl substitution make the phosphonate more active towards the metal centre.
1
7

>
>
DU(VI)
23.62
58.22
10.59
52.75
13.07
DTh(IV)
18.248
*
At 2M HNO concentration
3
Figure 8. Distribution trends for alkyl AmPs towards DU(VI) and DTh(IV) at 2M nitric acid
concentration at 303 K
In another study, we have taken 0.2M TBP solution in xylene where the distribution of
U(VI) was measured in 4M HNO and compared with D
of DBBAmP under identical
U(VI)
3
experimental conditions. A significant difference in distribution was found between the DU(VI) of
TBP and DBBAmP. TBP showed distribution value of 6, whereas DBBAmP showed four times
higher DU(VI) value (than TBP) i,e 28. This suggests that the displacement of one of the butoxy
group (-OC H ) in TBP with butylamine (-NH C H ) can enhance the extractability of uranium.
4
9
4
9
4
. Conclusions
Seven different aminophosphonates with alkyl, cyclohexyl and phenyl substitution were
1
13
synthesized and characterized with various spectroscopic techniques, such as FT-IR, H, C, and
3
1
P NMR. The molecular structure of diphenyl phenyl aminophopshonate (DPhPhAmP) and
diphenyl cyclohexyl aminophosphonate (DPhCyAmP) were elucidated based on the single
crystal XRD analysis. These synthesized AmPs were tested for extraction of U(VI), Th(IV), and
Am(III). Among these AmPs, alkyl substituted ones (alkyl AmPs) showed better distribution
towards the actinides compared to the cyclic substituted aminophosphonates (cyclic AmPs).
Among the three different alkyl substituted aminophosphonates, DBBAmP showed better
1
8

extraction towards the actinides studied. The extraction behavior of these alkyl substituted AmPs
are in the following order: DBBAmP > DHHAmP > DOOAmP. The influence of various
substituents on AmPs was clearly seen on the distribution values, where short alkyl chain length
makes the compound less hindered and more available for the binding with metal ions. In case of
the cyclic substituted AmPs, only MPhDCyAmP showed the significant extraction of uranium
and thorium, among the four. We have demonstrated that DBBAmP shows four fold better
extractability of uranium when compared with commonly used TBP extractant at 4M HNO3
concentration.
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2
2

Highlights
A study on the extraction behavior of aminophosphonates (AmPs) to wards actinides as a function of
alkyl, cyclic and alicyclic substituents. From these studies it is observed that aminopphosphonates
bearing alkyl substation are better for actinide extraction. Its worth mentioning that aromatic
substituted AmPs did not showed any extraction towards the actinides.
2
3

Increases Extractibity
<
<
Lower chain length higher distribution
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
Extraction through P=O coordination
0
1
2
3
4
5
6
7
[
HNO ] mol/L
3
2
4