Extraction and coordination behavior of diphenyl hydrogen phosphine oxide towards actinides

Article abstract of DOI:10.1080/00958972.2017.1387653

Extraction behavior of some selected actinides like U(VI), Th(IV), and Am(III) was investigated with three different H-phosphine oxides, viz. diphenyl hydrogen phosphine oxide (DPhPO), dihexyl hydrogen phosphine oxide (DHePO) and diphenyl phosphite (DPP).

Full text of DOI:10.1080/00958972.2017.1387653

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Extraction and coordination behavior of diphenyl
hydrogen phosphine oxide towards actinides
Dhrubajyoti Das, Gopinadhanpillai Gopakumar, C.V.S. Brahmmananda Rao,
N. Sivaraman, Akella Sivaramakrishna & Kari Vijayakrishna
To cite this article: Dhrubajyoti Das, Gopinadhanpillai Gopakumar, C.V.S. Brahmmananda Rao,
N. Sivaraman, Akella Sivaramakrishna & Kari Vijayakrishna (2017): Extraction and coordination
behavior of diphenyl hydrogen phosphine oxide towards actinides, Journal of Coordination
Chemistry, DOI: 10.1080/00958972.2017.1387653
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1387653
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Date: 06 October 2017, At: 18:22

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://doi.org/10.1080/00958972.2017.1387653
Extraction and coordination behavior of diphenyl hydrogen phosphine oxide
towards actinides
DHRUBAJYOTI DAS†, GOPINADHANPILLAI GOPAKUMAR‡, C.V.S. BRAHMMANANDA RAO‡,
N. SIVARAMAN‡, AKELLA SIVARAMAKRISHNA† and KARI VIJAYAKRISHNA*†
†Department of Chemistry, School of Advanced Sciences, VIT University, Vellore 632 014, India
‡Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam 603 102, Tamil Nadu, India
Extraction behavior of some selected actinides like U(VI), Th(IV) and Am(III) was investigated
with three different H-phosphine oxides, viz. diphenyl hydrogen phosphine oxide (DPhPO),
dihexyl hydrogen phosphine oxide (DHePO) and diphenyl phosphite (DPP). The H-phosphine
oxides exhibited a dual nature towards the extraction of actinides where the ligand not only
extracts the metals by cation exchange but also by coordination with the phosphoryl group at
lower and higher acidic concentrations, respectively. Among all ligands employed, DPhPO
showed highest extraction with actinides with a substituent dependent trend as follows: DPhPO >
DHePO > DPP. This trend emphasizes the importance of substituents around the phosphine
oxide towards their extraction of actinides. The coordination behavior of DPhPO was studied by
investigating its corresponding complexes with Th(NO3)4 and UO2(NO3)2. The metal complexes
of these actinides were characterized using FT-IR, 1H and 31P NMR spectroscopic techniques.
Density Functional Theory (DFT) calculations were also performed to understand the electronic
and geometric structure of the ligand and the corresponding metal complexes.
Keywords: Diphenyl hydrogen phosphine oxide; Coordination behavior; Solvent extraction;
Uranium; Actinides
1. Introduction
Chemical separations play an important role in nuclear fuel cycle. The reprocessing of the spent
*Corresponding author. Emails: kari@vit.ac.in; vijayakrishnakari@gmail.com
1

fuel and individual separation of metal ions from spent nuclear waste and high-level liquid waste
(HLLW) has attracted significant global attention towards the development of better, cheaper,
and radiation-resistant ligands. Various methods such as precipitation, ion exchange, and liquid-
liquid extraction (solvent extraction) have been developed and successfully employed for the
separation of actinides and other metal ions in nuclear waste management, amongst which
solvent extraction has been the most convenient method for the separation of actinides on a bulk
scale [1-3]. On the other hand, tributyl phosphate (TBP) is widely employed not only for the
extraction and separation of actinides from the dissolver solution in PUREX [4] and TRUEX
process (based on octyl, phenyl-N, N-diisobutyl carbamoylmethyl phosphine oxide) [5] but also
for the co-extraction of lanthanides and actinides from the HLLW. Apart from the above
mentioned processes, TRPO [6] (trialkyl phosphine oxides), DIDPA [7] (diisodecyl phosphoric
acid) and DIAMEX [8, 9] (diamide extraction) are also used for the co-extraction of actinides.
Numerous ligands with varying substituents and donor atoms like S, O, and N have been
reported towards the separation of actinides from lanthanides and comprehensively discussed in
some excellent reviews [10-13]. Additionally, phosphonate, phosphonothioate and amide-based
ligands are also reported for the extraction of actinides and lanthanides [14]. The extraction
behavior of actinides is governed by the basicity on phosphoryl oxygen [15] which is as follows:
phosphates < phosphonates < phosphinates < phosphine oxides [16, 17]. Consequently, the
extraction ability of organophosphorus extractant depends on the substituents attached to the
phosphorus atom. Higher analogs of TBP and H-phosphonates were also synthesized and studied
previously [18-21]. The extraction behavior of dialkyl phosphine oxides (DAPOs) with various
alkyl substituents from hexyl to decyl side chains and also dicyclohexyl phosphinic acid towards
U(VI) and Am(III) was investigated to understand the influence of alkyl substituents on the
distribution values [22, 23].
To the best of our knowledge, the extraction behavior of phosphine oxides with aromatic
substituents for actinides has not been investigated. In this contribution, we attempt to
demonstrate a comparative study on the extraction behavior of phosphine oxides tagged with
aromatic substituents and alkyl and alkoxy groups towards actinides. In this regard, we present
the extraction studies of actinides with diphenyl hydrogen phosphine oxide (DPhPO) and
compared the same with that of diphenyl phosphite (DPP) and dihexyl hydrogen phosphine
oxide (DHePO). The present study involves synthesis, characterization and extraction behavior
2

of diphenyl hydrogen phosphine oxide (DPhPO) towards actinides like U(VI), Th(IV) and
Am(III) and its comparison on extraction behavior of same metals with diphenyl phosphite
(DPP, also known as diphenyl hydrogen phosphonate) and dihexyl hydrogen phosphine oxide
(DHePO). The coordination behavior of DPhPO was also evaluated towards uranium and
thorium by synthesizing the corresponding metal complexes. Additionally, we have performed
Density Functional Theory (DFT) calculations to probe the electronic structure and geometries of
the ligands and the corresponding metal complexes employed in this study.
2. Experimental
2.1. Materials and Instrumentation
Chlorodiphenyl phosphine, diphenyl phosphite (DPP), diethyl phosphite and CDCl3 (Sigma-
Aldrich) were used as received. Hexyl bromide, magnesium turnings and solvents like diethyl
ether, dichloromethane (DCM) (SD Fine chemicals) were distilled prior to use. Th(NO3)4∙5H2O
(Sigma-Aldrich) was used as received. UO2(NO3)2∙6H2O metal salt and 233U tracers were used
from laboratory stock and their radiochemical purity was checked prior to use.
1H, 13C and 31P NMR spectra were recorded by a BRUKER DMX-400 and 1H chemical
shifts were reported relative to the residual proton resonance in deuterated solvents (all at 298 K,
CDCl3). H3PO4 was used as an external standard for 31P NMR. FT-IR spectra were recorded on a
SHIMADZU Affinity 1 FT-IR spectrometer using KBr pellets. UV-vis absorption spectra were
recorded with a UNICAM UV4-100 type double-beam spectrophotometer (ATI UNICAM,
Cambridge, UK).
2.2. General procedure for preparation of dihexyl hydrogen phosphine oxide (DHePO)
Dihexyl hydrogen phosphine oxide was synthesized by a modified literature procedure [24] as
shown in equation 2. 9.5 g (397.05 mmol) of activated magnesium turnings in 200 mL of dry
THF were taken in a two-necked, round-bottomed flask and to it 65.5 g (397.05 mmol) of
1-bromohexane diluted with 50 mL of dry THF were added dropwise with continuous stirring at
65 °C for 6 h. When the reaction mixture turned dark grey, 18 g (132.35 mmol) of diethyl
phosphite were added dropwise at ice-cooled temperature and the mixture was allowed to stir at
room temperature for 1 h. Excess amount of Grignard reagent was quenched with 1 N HCl
solution. The reaction mixture was subjected under reduced pressure to remove the volatile
3

impurity including the solvent and the residue material was purified by recrystallization using
n-hexane to give a colorless amorphous solid in 19.6 g (68.8% as isolated yield). M.P: 75-77 °C,
IR (cm-1): (P=O) 1159.22. 1H NMR (δ/ppm, 400 MHz, CDCl3): 0.89-0.92 (t, CH3CH2, 6H),
1.81-1.88 (m, P-CH2, 4H), 1.32-1.79 (m, CH3CH2, 16H), 6.8 (d, 1JP-H: 448.8 HZ, 1H). 31P NMR
(δ/ppm, 162 MHz, CDCl3): 36.14 (d, 1JP-H: 458.46 HZ). 13C NMR (δ/ppm, 100 MHz, CDCl3):
31.30, 30.43, 28.24 (d), 22.38, 21.74 (d), 13.98.
2.3. Synthesis of actinide complexes
2.3.1. [Th(NO3)4(DPhPO)2] (1). All complexes were synthesized using the following procedure
[25, 26]. Solution of DPhPO (0.154 g, 0.7 mmol) in 5 mL of methanol was added dropwise to
the stirred solution of Th(NO3)4∙5H2O (0.2 g, 0.35 mmol) in 10 mL of methanol. The reaction
mixture was allowed to stir overnight and subjected to high vacuum to remove solvent and other
volatile impurities. The residue was washed with diethyl ether (2×5 mL) which gave a yellow
viscous liquid (0.31 g). The purity and the composition of all complexes were confirmed by
analytical and spectroscopic techniques. Yield: 91% (isolated yield), IR (cm-1): 1278.81 (P=O),
31P NMR (δ/ppm, 162 MHz, CDCl3): 30.27 (d, 1JP-H: 571.62 Hz). 1H NMR (δ/ppm, 400 MHz,
CDCl3,): 7.37-7.44 (m, Ar-p, 4H), 7.55-7.60 (m, Ar-o and Ar-m, 6H), 8.67 (s, P-H, 1H). Anal.
Calcd. for C24H22N4O14P2Th: C, 32.59; H, 2.51; N, 6.33. Observed: C, 31.39; H, 2.21; N, 6.05.
2.3.2. [UO2(NO3)2(DPhPO)2] (2). [UO2(NO3)2∙6H2O (0.2 g, 0.39 mmol) and DPhPO (0.175 g,
0.79 mmol)], Product: 0.325 g, Yield: 92% (isolated yield), IR (cm-1): 1149.57 (P=O), 31P NMR
(δ/ppm, 162 MHz, CDCl3): 25.67 (d, 1JP-H: 486 Hz). 1H NMR (δ/ppm, 400 MHz, CDCl3): δ =
7.54-7.87 (m, Ar, 10H), 9.31 (s, P-H, 1H). Anal. Calcd. for C24H22N2O10P2U: C, 36.11; H, 2.78;
N, 3.51. Observed: C, 35.27; H, 2.51; N, 3.14.
2.4. Computational methodologies
We applied DFT calculations to study the electronic structure and geometries of diphenyl
hydrogen phosphine oxide (DPhPO), dihexyl hydrogen phosphine oxide (DHePO) and diphenyl
phosphate (DPP). Geometry optimizations were performed at DFT level employing B3LYP
functional [27, 28] in conjunction with def2-TZVP basis set. The B3LYP functional is expected
to provide better description for organic molecules [29, 30]. The located stationary points are
4

characterized as energy minima by evaluating real harmonic vibrational frequencies at the same
level. The resolution-of-identity (RI) approximation was applied in conjunction with the
appropriate auxiliary basis sets to speed up the computations [31-33]. In order to account for
dispersion effects, empirical Grimme-type dispersion corrections were incorporated during this
step using the latest atom-pairwise dispersion correction with Becke-Johnson damping (D3BJ)
[34, 35]. Increased integration grids (Grid 6 in ORCA convention) and tight SCF convergence
criteria were used throughout the calculations. For the calculation of metal complexes, the hybrid
density functional PBE0 [36], with 20% HF exchange, was employed and tight SCF convergence
criteria was used throughout the calculations. For thorium atom, 60 inner-shell core electrons
were replaced by an effective core potential (ECP) generated for the neutral atom using quasi-
relativistic methods [37-39] and explicitly treated electrons were described by the standard def2-
TZVP basis sets. This basis set combination referred to hereafter as def2-TZVP-ECP was
adopted for previous theoretical calculations of actinide metal complexes [40, 41]. All
calculations were performed with ORCA version 3.0.3 program package [42].
2.5. Batch studies of uranium and thorium by solvent extraction
Uranium and americium: All extractants were pre-equilibrated with appropriate nitric acid
concentration prior to performing the extraction studies. Equal amounts of H-phosphine oxide
solutions in xylene (0.01 M for U(VI) and 0.3 M for Am(III)) and appropriate nitric acid were
taken in an equilibration tube, spiked with 233U/241Am tracer and equilibrated in a constant
temperature bath at 303 K for 2 h. After attainment of equilibrium, suitable aliquots from both
phases were taken for radiometric assay of actinides. 233U -activity was obtained from the
liquid scintillation counting using dioxane based cocktail and from that the equilibrium
concentrations of U(VI) in both 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,
[M]org
DM =
(1)
[M]aq
5

where [M]org and [M]aq are the concentrations of metal ion in organic and aqueous phases,
respectively.
Thorium: The extraction of Th(IV) was performed by following the similar experimental
procedure mentioned for uranium using 0.01 M solutions of extractant in xylene. Thorium
amount was calculated spectrophotometrically using arsenazo-III as the chromogenic agent [43]
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:
[M]aq(i) - [M]aq(f)
DM =
(2)
[M]aq(f)
where [M]aq(i) and [M]aq(f) concentration of metal ions before and after contact with organic
phase. All experiments were carried out at least in triplicate. Batch sorption studies were
performed in nitric acid media ranging from 0.01 M - 6 M.
3. Results and discussion
3.1. Synthesis and characterization of DHePO
The effect on distribution values of some selected actinides like U(VI), Th(IV) and Am(III) by
di-n-alkyl phosphine oxides and H-phosphonates as a function of alkyl chain length were studied
previously [20, 21]. But the effect on distribution values of actinides with the change in
substitution from acyclic to cyclic groups in H-phosphine oxides will be interesting to
investigate. On that note, we have chosen three different H-phosphine oxides, viz. diphenyl
hydrogen phosphine oxide (DPhPO), dihexyl hydrogen phosphine oxide (DHePO) and diphenyl
hydrogen phosphonate or diphenyl phosphite (DPP), having a different substitution on phosphine
oxide moiety. DHePO was prepared by the reported literature (chart 1). Structural confirmation
and the purity of DHePO were established by different spectral characterization techniques like
FT-IR, 31P, 1H and 13C NMR spectroscopy (see Supporting Information for spectral data).
DHePO showed strong infrared absorptions at 1159.22 cm-1 for the presence of the phosphoryl
group. Both compounds showed a substantial downfield shift in 31P NMR chemical shifts
(DHePO: δ = 36.14 {d} ppm) compared to the precursor, diethyl phosphite (δ = 7.45 {d} ppm),
6

confirming the complete formation of the compounds. The structural confirmation was further
carried out by 1H and 13C NMR. The -CH2 groups of alkyl chains from DHePO resonate between
δ = 1.2-1.9 ppm, confirming the presence of alkyl and the aromatic substitutions in the respective
compounds.
3.2. Synthesis, characterization and coordination chemistry of metal complexes
To understand the coordination behavior of the DPhPO for actinides solid state complexes of
DPhPO with UO2(NO3)2 and Th(NO3)4 were synthesized and characterized by various
spectroscopic techniques. DPhPO complex with UO2(NO3)2 and Th(NO3)4 was synthesized by
taking stoichiometric amounts of the ligand and the corresponding metal salt (2:1 ratio) using
methanol as solvent. The volatile compounds including solvent were removed under high
vacuum. The final product was washed with 2×5 mL (diethyl ether (3:1)) to remove slight excess
of ligand, if any. The obtained product was dried under reduced pressure and characterized by
FT-IR, 1H, 31P NMR spectra and elemental analysis. The IR spectra of [UO2(NO3)2(DPhPO)2]
and [Th(NO3)4(DPhPO)2] show that there is a large shift in P=O stretching towards near- and far-
IR regions, respectively, in comparison with the free ligand, indicating coordination to the metal
center through the phosphoryl oxygen (figure 1). The proton coupled 31P NMR spectra of
complexes 1 and 2 were significantly deshielded with respect to the free ligands, which further
confirmed the complete formation of complexes. Ligand shows a doublet at 21.65 ppm but the
corresponding Th(NO3)4 and UO2(NO3)2 complexes resonate at 30.27 {d} ppm and 25.67 {d}
ppm, respectively (figure 2). The purity of the synthesized complexes was also confirmed by the
elemental analysis. This further confirms the metal-to-ligand ratio as 1:2 in uranium and thorium
complexes (Supporting Information, table S1).
3.3. Distribution studies
Substitutions on the phosphine oxide moiety play a very important role in distribution of
actinides. The extraction efficiency of a ligand is dependent on the nature of substituents, which
also greatly influence the performance of the extractants. Previously, authors have studied the
effect of alkyl chain length in distribution values of actinides in various di-n-alkyl phosphine
oxides or H-phosphine oxides. Subsequently, we wanted to study the effect of aromatic
substitutions on H-phosphine oxides. In that connection, we have planned to study and compare
7

the extraction behavior of three different H-phosphine oxides, viz. DPhPO, DHePO and DPP
with some selected actinides, like U(VI), Th(IV) and Am(III).
3.3.1. Extraction mechanism in H-phosphine oxides. Phosphine oxide-based neutral ligands
extract the actinides by solvation mechanism which is through phosphoryl group coordination.
The tautomeric form diesters of H-phosphonic acid is well known in literature [44]. Equation 3
shows the tautomeric form of the H-phosphine oxide where phosphorus atom is trivalent (II) in
nature and exists in equilibrium with its tetracoordinate pentavalent form of phosphorus (I).
O
R P R
H
R P R
OH
(3)
II
I
The expression for cation-exchange reaction is shown in equation 3.
nR2P-OHorg Mn
R2P-OMorg nHaq
aq
(4)
where R2P-OH is the trivalent form II (equation 3) in which H-phosphine oxide exist in the OH
form and where in H+ is exchangeable there by making it acidic extractant in nature.
Giving the equilibrium constant Keq for the reaction
Keq = [R2P-OM]org [H ]n/ [R2P-OH][M]n
aq
(5)
Rearranging equation 5,
D = Keq [R2P-OH]/[H+]n
(6)
The above equation shows that distribution ratio D is inversely proportional to [H+]
concentration.
3.3.2. Extraction of actinides. DU(VI) values for 0.01 M DPhPO, DPP and DHePO in xylene as a
function of nitric acid are given in figure 3 where it was observed that at lower acidities the
DU(VI) values of H-phosphine oxides are higher and a prominent decrease in D value was
8

followed with increasing nitric acid concentration i.e. 0.01 to 0.1 M. The D values decrease as
the aqueous phase acidity was enhanced to 2 M nitric acid and a slight increase at 4 M.
H-phosphine oxides behave like acidic extractants. Tautomeric equilibrium occurs between P=O
and P-OH (equation 3) at lower acidities and hence the extraction of the metals is through cation-
exchange mechanism. With the increase of the acidity, the distribution ratios fall gradually. The
equilibrium between P=O and P-OH drives towards the left (equation 3) at higher nitric acid
concentration, reducing the extraction of metal. At higher acidity the extraction is through the
P=O group which is through solvation mechanism (equation 7). At higher concentration of nitric
acid, the uranyl ion forms a series of complex species: [UO2NO3]+, [UO2(NO3)2], [UO2(NO3)3]-.
The extraction behavior of H-phosphine oxides is similar to neutral extractants at higher nitric
acid concentration and hence extracts neutral actinide species. Above 4 M nitric acid
concentration, a copious amount of the anionic nitrate complex [UO2(NO3)3]- is formed which is
not extracted by H-phosphine oxides and hence there is a reduction in D values.
++
-
UO2(NO3)2 . 2H-PO(org)
(7)
UO2 (aq) + 2NO3 (aq) + 2H-PO(org)
Figure 4 gives the variation of D values for Th(IV) using 0.01 M solutions of H-phosphine
oxides in xylene (diluent) where Th(IV) shows a similar trend as that of U(VI) which confirms
the dual nature of extraction of H-phosphine oxides, i.e. at lower acidity it behaves as an acidic
extractant while at higher acidity as a neutral extractant.
The lower distribution ratios observed for U(VI) with DPhPO, DPP and DHePO
compared to phosphate system are due to the use of lower extractant concentration employed in
the present study, 0.01 M extractant/xylene, whereas in the case of phosphate-based systems e.g.
TBP, typically 1.1 M solutions are used, where distribution ratio as high as 28 [19] was observed
at 4 M HNO3 medium. The extractants (DPhPO, DPP, DHePO) are stronger compared to
phosphate systems and hence the amount of these molecules used is in the range of 0.01 M in
order to get appreciable amount of metal in the aqueous phase which can be analyzed.
Distribution of americium by 0.3 M solutions in xylene of H-phosphine oxides is given in
figure 5. The distribution trend of Am(III) is also similar to Th(IV) and U(VI) distribution for all
three ligands and suggests that the mechanism of extraction is identical in both. The higher
distribution ratio for Am(III) at lower acidity compared to U(VI) and Th(VI) is due to the use of
9

higher concentration of the extractant. The higher concentration of the extractant was employed
to obtain sufficient activity of americium in the organic phase as the distribution ratio is expected
to be lower in this system. Distributions of Am(III) at higher acidities are very low as P=O group
is not strong enough to extract Am(III).
Among these three H-phosphine oxides, DPhPO shows a better extraction towards the
actinides in comparison with DPP and DHePO. The extraction behavior of these H-phosphine
oxides towards U(VI) and Th(IV) is in the following order: DPhPO > DHePO > DPP. This
phosphoryl group in phosphine oxide is more basic than phosphonate and hence in H-phosphine
oxides have higher distribution ratios than DPP. This is reflected in the higher D where the
phosphoryl group of the extractant participates through solvation in the higher nitric acid region.
Among the H-phosphine oxides, the substitution of phenyl groups around phosphoryl molecule
show higher distribution ratios than linear hexyl groups. One possible reason for higher D in the
phenyl system is the compact nature of the molecule, thereby making the metal complex less
bulky and hence requiring less energy to transfer the complex from the aqueous phase to organic
phase as compared to linear alkyl hexyl group. The explanation of the reason for the differences
in the extraction behavior at lower acidity by cation-exchange mechanism is not clear. The
cation-exchange mechanism depends on the number of hydrogen atoms replaced for each metal
ion depending on the charge of the metal. In the case of uranium extraction, each uranyl ion
requires two molecules of the extractants to satisfy the charge and in the case of thorium and
americium the requirement is four and three molecule of ligands, respectively. The spatial
requirement dictates the ease of formation of the metal complex. The trend observed is not in
line with the number of molecule required to satisfy the charge of the metal. If we take DPhPO at
0.01 M nitric acid concentration, the expected trend would have been U>Am>Th, whereas the
observed trend is Am>U>Th. So there are more than one factor that is operational at lower
acidity and thereby deciding the value of D.
3.4. DFT Studies
In an attempt to understand the electronic structure and geometries of the ligands, a systematic
geometry optimization procedure was adopted. Ten starting geometric configurations were
considered for each ligand. Each of these structures was optimized at B3LYP/def2-TZVP level
and the energetically lowest-lying conformer was considered for further studies. These stationary
10

points were characterized as energy minima by evaluating real harmonic vibrational frequencies
at the same level. The lowest-energy conformer for DHePO is already reported [21]. The
energetically lowest-lying conformers for DPhPO, DHePO and DPP are illustrated in figure 6. In
all three structures, the phosphorus atom is positively charged with considerable amount of
electron density localized on the phosphoryl oxygen atom. Mulliken charges for oxygen and
phosphorus atoms are -0.55 e and 0.59 e for DHePO, -0.55 e and 0.58 e for DPhPO, and -0.49 e
and 0.72 e for DPP, respectively. The comparatively large positive charge on phosphorus atom in
DPP is due to the presence of two additional oxygen atoms that are directly coordinated to
phosphorus atom.
After establishing the geometries of the ligands, we turned our interest towards the
corresponding metal complexes with thorium atom. For the calculations, we have considered 1:2
stoichiometry as the same stoichiometric amounts were considered for the synthesis of
corresponding metal complexes. Ten starting geometric configurations were generated for each
metal complex by distributing four nitrate groups and two ligands around thorium atom. These
starting structures were subjected to geometry optimization at PBE0/def2-TZVP-ECP level. For
Th(NO3)4·2DHePO, the lowest-energy structure is reported already and therefore is taken from
reference 21. The optimized geometries of the three thorium metal complexes are shown in
figure 7. The coordination geometry of all three metal complexes is similar. In all complexes, the
ligands are oriented in the opposite phases of the metal nitrate unit and all the nitrate groups are
bidentate in nature.
4. Conclusion
Extraction behavior of U(VI), Th(IV) and Am(III) was studied by three homologues of
H-phosphine oxide having three different substituents around the phosphoryl group. Studies
reveal that DPhPO shows better extractability than DHePO and DPP for these three actinides.
Distribution values for actinides of these H-phosphine oxides are in the following order: DPhPO
> DHePO > DPP. These extractants also showed a dual nature of extraction behavior, i.e. at
lower acidity they behave as acidic extractants and extract the metal ions through cation-
exchange mechanism, while at higher acidity the mode of extraction is through solvation
mechanism. Based on the DFT calculations we were able to establish lowest energy structures
11

for DHePO, DPhPO, DPP and their corresponding metal complexes, Th(NO3)4·2DHePO,
Th(NO3)4·2DPhPO and Th(NO3)4·2DPP. The Mullikan population analysis shows considerable
amount of positive charge on phosphorus atom and charge concentration on phosphoryl oxygen.
Acknowledgement
Kari thanks UGC-DAE, IGCAR-Kalpakkam, [CSR-KN/CRS-53/2013-14/652] for the financial
support. Authors also thank DST-VIT-FIST for NMR, VIT-SIF for FT-IR and other
instrumentation facilities. Authors gratefully acknowledge computing time on NEHA, the High-
Performance Computing Facility at IGCAR, India.
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14

O
O
O
C6H13 P C6H13
H
O P O
H
P
H
a
b
c
Chart 1. Ligands used in present study a. Diphenyl Phosphite (DPP), b. Diphenyl hydrogen
phosphine oxide (DPhPO), c. Dihexyl hydrogen phosphine oxide (DHePO).
15

Cl
P
O
P
H
H2O, Diethyl ether
K2CO3
Ice bath, 3h
Scheme 1. Synthesis of diphenyl hydrogen phosphine oxide (DPhPO).
16

Hexyl magnesium
bromide
O
P
O
P
EtO
OEt
H
Dry THF
1h, RT
H
Scheme 2. Synthesis of dihexyl hydrogen phosphine oxide (DHePO).
17

o
Ph
Ph
N
Ph
O
U
P Ph
H
Ph P
o
o
P Ph
H
o
O
Ph P Ph
H
H
o
N
o
Th(NO3)4.5H2O
o
o
o
UO2(NO3)2.6H2O
o
o
Th
o
o
N
H
o
o
o
o
Ph P
Ph
o
N
o
N
o
o
o
N
o
Scheme 3. Formation of uranium and thorium complex with DPhPO.
18

250
200
150
100
50
DPhPO
UO2(NO3)2(DPhPO)2
O=U=O
Th(NO3)4(DPhPO)2
0
1400
1200
1000
cm-1
Figure 1. Overlay of FT-IR spectra of free DPhPO and its U(VI) and Th(IV) complexes.
19

o
N
Ph
o
o
P Ph
o
N
o
o
o
o
H
o
δ 30.27 (d)
1JP-H: 571.6 Hz
Th
H
N
o
o
[Th(NO3)4(DPhPO)2]
Ph P
Ph
o
o
N
Ph
Ph
o
O
U
P Ph
Ph P
δ 25.67(d)
1JP-H: 486 Hz
H
o
o
o
H
o
[UO2(NO3)2(DPhPO)2]
o
o
N
N
o
o
o
O
P
H
21.65 (d)
1JP-H: 481.1 HZ
DPhPO
Ph
Ph
Figure 2. Overlay of 31P NMR spectra of free DPhPO and its U(VI) and Th(IV) complexes.
20

16
14
12
10
8
0.01 M DPhPO/Xylene
0.01 M DPP/Xylene
0.01 M DHePO/Xylene
Cation exchange
at lower acidity
6
4
Coordination through P=O
2
0
0
1
2
3
4
5
6
7
[HNO ] mol/L
3
Figure 3. Variation of DU(VI) with equilibrium aqueous phase nitric acid concentration for
DPhPO, DPP and DHePO at 303 K.
21

8
7
0.01 M DPhPO/Xylene
0.01 M DPP/Xylene
0.01 M DHePO/Xylene
Cation exchange
at lower acidity
6
5
4
3
Coordination through P=O
2
1
0
-1
0
1
2
3
4
5
6
7
[HNO ] mol/L
3
Figure 4. Variation of DTh(IV) with equilibrium aqueous phase nitric acid concentration for
DPhPO, DPP and DHePO at 303 K.
22

100
80
60
40
20
0
0.3 M DPhPO/Xylene
0.3 M DPP/Xylene
0.3 M DHePO/Xylene
Cation exchange
at lower acidity
0
1
2
3
4
5
6
7
[HNO3] mol/L
Figure 5. Variation of DAm(III) with equilibrium aqueous phase nitric acid concentration for
DPhPO, DPP and DHePO at 303 K.
23

A
B
C
Figure 6. Optimized geometries of the ligands (a) DHePO (b) DPhPO and (c) DPP at
B3LYP/def2-TZVP level. (Color code: orange ball is phosphorus, red balls are oxygen, grey
balls are carbon and white balls are hydrogen atoms).
24

A
B
C
Figure 7. Optimized geometries of (a) Th(NO3)4·2DHePO (b) Th(NO3)4·2DPhPO and (c)
Th(NO3)4·2DPP at PBE0/def2-TZVP-ECP level. Bottom: color code for the elements used in
this figure.
25

Graphical abstract
26