Diphenylmorpholine CMPO: Synthesis, coordination behavior and extraction studies of actinides

Article abstract of DOI:10.1016/j.poly.2017.11.036

Carbamoylmethyl phosphine oxide (CMPO) derivatives are well known ligands in the separation and extraction of trivalent lanthanides and actinides from nuclear waste. The substituents of CMPO play major role in their selectivity and extractability. We report the synthesis and characterization of diphenylmorpholine carbamoylmethyl phosphine oxide (DPMCMPO) (L₁) and diphenyl-N,N-diethyl carbamoylmethyl phosphine oxide (DPDECMPO) (L₂) using various spectroscopic techniques, such as FT-IR, ¹H, ¹³C, and ³¹P NMR. The molecular structure of DPMCMPO is confirmed by single crystal XRD analysis. The present study aims to understand the influence of substituents on ‘N’ atom of L₁ (morpholine based DPMCMPO) and L₂ (diethyl substituted DPDECMPO) ligands for the extraction of some selected actinide ions such as Th(IV), U(VI) and Am(III). The geometry and electronic structure of these ligands and their respective complexes with Th(NO₃)₄ and UO₂(NO₃)₂ are further explored using density functional theory (DFT) calculations. The employed ligands (L₁ and L₂) show greater distribution values for Th(IV) over U(VI), due to strong “ligand-Th” complexation ability as suggested by DFT calculations.

Full text of DOI:10.1016/j.poly.2017.11.036

Polyhedron 141 (2018) 215–222
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Diphenylmorpholine CMPO: Synthesis, coordination behavior and
extraction studies of actinides
a
a
b
b
Dhrubajyoti Das , Akella Sivaramakrishna , Gopinadhanpillai Gopakumar , C.V.S. Brahmmananda Rao ,
b
a,
⇑
N. Sivaraman , Kari Vijayakrishna
a
Department of Chemistry, School of Advanced Sciences, VIT University, Vellore 632 014, Tamil Nadu, India
Indira Gandhi Centre for Atomic Research, HBNI, Kalpakkam 603 102, Tamil Nadu, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbamoylmethyl phosphine oxide (CMPO) derivatives are well known ligands in the separation and
extraction of trivalent lanthanides and actinides from nuclear waste. The substituents of CMPO play
major role in their selectivity and extractability. We report the synthesis and characterization of
Received 12 September 2017
Accepted 26 November 2017
Available online 6 December 2017
diphenylmorpholine carbamoylmethyl phosphine oxide (DPMCMPO) (L
bamoylmethyl phosphine oxide (DPDECMPO) (L ) using various spectroscopic techniques, such as FT-IR,
H, C, and P NMR. The molecular structure of DPMCMPO is confirmed by single crystal XRD analysis.
The present study aims to understand the influence of substituents on ‘N’ atom of L (morpholine based
DPMCMPO) and L (diethyl substituted DPDECMPO) ligands for the extraction of some selected actinide
ions such as Th(IV), U(VI) and Am(III). The geometry and electronic structure of these ligands and their
1
) and diphenyl-N,N-diethyl car-
2
Keywords:
Carbamoylmethyl phosphine oxides
CMPO
Actinide extraction
Thorium
1
13
31
1
2
respective complexes with Th(NO
)
3 4
and UO
2
(NO are further explored using density functional theory
3
)
2
Uranium
(
(
DFT) calculations. The employed ligands (L
VI), due to strong ‘‘ligand-Th” complexation ability as suggested by DFT calculations.
Ó 2017 Elsevier Ltd. All rights reserved.
1
and L ) show greater distribution values for Th(IV) over U
2
1
. Introduction
Separation of actinides from other fission products plays a key
of actinides in bulk scale is using solvent extraction, where a speci-
fic ligand/extractant is used with suitable diluents. Neutral
organophosphorus ligands like (N,N-diisobutylcarbamoylmethyl)
octylphenyl phosphine oxide [11] are used in current practice in
the TRUEX process. Several other CMPO based ligands were well
studied in the separation of actinides e.g. Am(III) from lanthanides
[12,13]. During the last few decades, the general structure of CMPO
(I, Fig. 1) was modified in several ways and the relationship
between the structure and the extraction ability of these reagents
was extensively studied. Most of the structural modifications are
role in nuclear fuel cycle. In this regard, various methodologies like
solvent extraction or liquid–liquid extraction, ion exchange and
precipitation processes have been proposed [1,2]. Among them,
solvent extraction is the most convenient method of separation
on a bulk scale for actinides. Extractants like TBP (tri-n-butyl phos-
phate) is used in PUREX process [3] for separation of plutonium
and uranium in industrial scale. While octyl, phenyl-N,N-diisobutyl
carbamoylmethyl phosphine oxide used in TRUEX process [4] and
dimethyl dibutyl tetradecyl malonamide (DMDBTDMA) used in
DIAMEX process [5,6] are used for the co-extraction separation of
actinides at pilot plant scale. In the past years different types of
extractants having S, O and N donor atoms for actinide separation
were developed and are well-documented in recent reviews [7–9].
Carbamoylmethyl phosphine oxide (CMPO) is a very well-known
ligand for the extraction and separation of actinides from high level
waste (HLW) [10]. One of the most convenient modes of separation
0
00
made on numerous residues at nitrogen (R and R ) and phospho-
1
2
rus (R and R ) atoms in various combinations, such as butyl, hexyl,
octyl, isobutyl, phenyl etc [14,15]. It is well known that the sub-
stituents at nitrogen atom in CMPO molecules affect the distribu-
tion of actinides [16]. Given the background, we set out to
investigate the role of a morpholine group at N atom of CMPO to
understand the influence of the cyclic structures on the extraction
behavior of actinides. Additionally, we have also compared the dis-
1
tribution ratios of morpholine (closed system, L ) and diethyl (open
system, L ) substitutions at CMPO nitrogen atom.
2
⇑
Corresponding author. Fax: +91 416 224 3092.
E-mail addresses: kari@vit.ac.in, vijayakrishnakari@gmail.com (K. Vijayakrish-
na).
https://doi.org/10.1016/j.poly.2017.11.036
277-5387/Ó 2017 Elsevier Ltd. All rights reserved.
0

2
16
D. Das et al. / Polyhedron 141 (2018) 215–222
added drop wise at ice cooled condition under nitrogen atmo-
sphere. The reaction mixture was stirred for 2 h. The reaction mix-
ture was filtered to remove the white precipitate of morpholine
hydrochloride salt and then diluted with 100 mL of ice cold water.
The organic layer was separated and the solvent was removed
under reduced pressure to afford a viscous yellow liquid. The com-
pound was further dried under high vacuum and subjected to var-
ious spectroscopic analyses. The characteristic peaks were
compared with the literature values. The isolated yield of the com-
ꢀ1
1
pound was about 87%. IR (cm ): 1633.71 (C@O). H NMR (400
MHz, CDCl ): d = 3.46–3.48 (t, N-CH , 4H), 3.62 (m, O-CH2, 4H),
2H) ppm. CNMR (d, 100 MHz, CDCl ): 40.55,
2.5, 46.73, 66.6, 165.4. Mass (EI, m/z): 163.17 (observed) (calcu-
Fig. 1. General structure of Carbamoylmethyl phosphine oxide (CMPO).
3
2
1
3
4
4
.013 (s, C–CH
2
-Cl
,
3
2
. Experimental part
lated: 163.6). A similar procedure was followed for the synthesis
of 2-chloro-N,N-diethylacetamide (see Scheme 2).
2.1. Materials and instrumentation
2.3. Preparation of 2-chloro-N,N-diethylacetamide
3
Diphenyl phosphine oxide, Aliquat 336 and CDCl were pro-
cured from Sigma–Aldrich and used as received. Morpholine,
chloroacetyl chloride and other solvents were procured from SD
fine chemicals. The solvents were distilled prior to usage.
Ten g (0.137 mol) of diethyl amine and 7.72 g (0.0684 mol) of
chloroacetyl chloride were used for the reaction. Yield: 92%, IR
ꢀ1
1
(
1
(
cm ): 1643.35 (C@O). H NMR (400 MHz, CDCl
.123 (t, N-CH -CH3, 3H), 1.185–1.221 (t, N-CH -CH3, 3H), 3.31
m, N-CH2, 4H), 4.039 (s, C–CH
3
): d = 1.087–
1
13
31
H, C, and P NMR spectra were recorded with BRUKER DMX-
2
2
13
1
4
00. H chemical shifts were reported relative to the residual pro-
2
-Cl, 2H) ppm. CNMR (d, 100
3
): 12.6, 14.34, 40.69, 41.13, 42.54, 165.98. Mass (EI,
3 3 4
ton resonance in deuterated solvents (all at 298 K, CDCl ). H PO
MHz, CDCl
was used as an external standard for 3 P NMR. FT-IR spectra were
recorded on SHIMADZU Affinity 1 FT-IR spectrometer using KBr
pellet. UV–Vis absorption spectra were recorded with a UNICAM
UV4-100 type double-beam spectrophotometer (ATI UNICAM,
Cambridge, UK). Finnpipette F3 variable volume pipette (100–
1
m/z):149.17 (observed) (calculated: 149.62)
2
.4. Preparation of diphenylmorpholine carbamoylmethyl
phosphineoxide (DPMCMPO) (L
1
)
1
000 and 20–200 ll) (Sigma Aldrich) was used for pipetting ali-
Diphenylmorpholine
DPMCMPO) (L ) was prepared by conventional Michaelis–Becker
reaction [20]. Diphenyl phosphineoxide, 5 g (0.0247 mol) and
.84 g (0.029 mol) of 2-chloro-1-morpholinoethanone were dis-
carbamoylmethyl
phosphineoxide
quots. Single crystal X-ray diffraction measurements for
DPMCMPO was performed on a Bruker AXS Smart APEX II single
crystal X-ray diffractometer with 1000 CCD detector. Data was col-
lected in room temperature (297 K). The unit cell parameters were
determined from 36 frames measured from different crystallo-
graphic zones. The structure was elucidated by direct methods
and refined employing full-matrix least-squares with the program
(
1
4
solved in 30 mL of dichloromethane (DCM), followed by drop wise
addition of 5 mL of 50% aqueous NaOH solution. Aliquat 336 (200
mg) of was added as phase transfer catalyst (PTC). The reaction
mixture was refluxed for 12 h. The organic layer was separated
and washed three times with 30 mL of water. The volatile impurity
and solvent was removed under reduced pressure and the residue
was purified by recrystallization using chloroform to give colorless
crystals in 7.23 g. Yield: 89% (isolated yield), M.P: 177–179 °C, IR
2
SHELXL-97 [17] refining on F . Packing diagrams were produced
using program PovRay and graphic interface X-seed [18]. ORTEP-PLA-
TON program was used to create the image. It is to be noted that the
crystals of C18
H20NO P
3 1
falls under monoclinic, space group P21/C,
a) 8.6311(3) Å, (b) 14.8698(4) Å, (c) 13.2673(4) Å. 90°, b 90.787
2)°, 90°. V = 16892.78(9). Crystallographic data for DPMCMPO is
ꢀ1
1
(
(
a
(cm ): 1114.86 (P@O), 1627.92 (C@O). H NMR (400 MHz, CDCl ):
3
c
d = 7.8–7.85 (m, Ar-O, 4H), 7.45–7.54 (m, Ar-m and p, 6H), 3.567 (d,
1
deposited to the Cambridge Crystallographic Data Centre (CCDC
499035) and can be obtained free of charge from www.ccdc.-
cam.ac.uk/structures.
JP-H: 14.8 Hz, P-CH -CO, 2H), 3.51–3.62 (m, N-CH -CH -O, 8H)
2
2
2
3
1
13
1
ppm. P NMR (162 MHz, CDCl ): d = 27.34 ppm. CNMR (d, 100
3
MHz, CDCl ): 38 (d), 42.45, 47.42, 66.68, 128.71 (d), 131.51 (d),
3
1
32.27 (d), 132.04 (d, JC-P: 103 Hz), 163.76. Mass (EI, m/z): 329.2
(
observed) (calculated: 329.33).
2.2. Preparation of 2-chloro-1-morpholinoethanone
2
.5. Preparation of diphenyl-N,N-diethyl carbamoylmethyl
The precursor, 2-chloro-1-morpholinoethanone was prepared
phosphineoxide (DPDECMPO) (L
2
)
by modified literature report [19] (see Scheme 1). 12.35 g (0.143
mol) of morpholine, diluted with 60 mL of diethyl ether was taken
into two necked round bottomed flask. To that 8 g (0.072 mol) of
chloroacetyl chloride diluted with 20 mL of diethyl ether was
Diphenyl-N,N-diethyl
carbamoylmethyl
phosphineoxide
(
DPDECMPO) (L ) was synthesized, similar to that of procedure
2
Scheme 1. Synthesis of 2-chloro-1-morpholinoethanone.
Scheme 2. Synthesis of 2-chloro-N,N-diethylacetamide.

D. Das et al. / Polyhedron 141 (2018) 215–222
217
employed for DPMCMPO (L
phineoxide and 3.07 g (0.029 mol) 2-chloro-N,N-diethyl acetamide
were used for the reaction. Product: 7.15 g. Yield: 91% (isolated
1
): 5 g (0.0247 mol) of diphenyl phos-
phases. Liquid scintillation counter (LSC) (Hidex, Finland) is used
2
33
to measure the
Am(III) concentrations in both organic and aqueous phases were
determined by measuring the activities of the 60 keV photons
of the respective phases using NaI(TI) detector. For each measure-
ments triplicate runs were carried out. The distribution ratio (D
a
-activity of
U using dioxane based cocktail.
ꢀ1
yield), M.P: 170–172 °C, IR (cm ): 1201.65 (P@O), 1624.06
c
1
(
7
1
C@O). H NMR (400 MHz, CDCl
3
): d = 7.83–7.88 (m, Ar-O, 4H),
.25–7.51 (m, Ar-m and p, 6H), 0.88–0.91 (t, N-CH -CH3, 3H), 1.1–
-CH3, 3H), 3.23–3.51 (q, N-CH2, 2H), 3.39–3.41 (q,
2
M
)
.13 (t, N-CH
2
was calculated as the ratio of metal ion concentrations in organic
phase to that of aqueous phase (Eq. (1))
1
31
N-CH
(
1
2
2H), 3.408 (d,
JP-H: 15.6 Hz, P-CH
2
-CO, 2H) ppm. P NMR
):
2.76, 40.53, 43.09, 128.51 (d), 131.25 (d), 132.03 (d), 132.27 (d,
13
3 3
162 MHz, CDCl ): d = 28.38 ppm. CNMR (d, 100 MHz, CDCl
½
Mꢃorg
D
M
¼
ð1Þ
J
C-P: 102 Hz), 164.33. Mass (EI, m/z): 315.17 (observed) (calculated:
½Mꢃaq
3
2
2
15.35).
where [M]org and [M]aq are the concentrations of metal ion in
organic and aqueous phases, respectively.
.6. Synthesis of actinide complexes
Thorium: Similar procedure of uranium extraction described
above was employed for the extraction of Th(IV). The concentra-
tion of thorium was determined spectrophotometrically using
arsenazo-III as the chromogenic agent [23] in the initial aqueous
phase and after equilibrium with appropriate ligand. Equilibrium
aqueous concentration was subtracted from the initial feed con-
centration to estimate the concentration of thorium in the organic
phase. All experiments were carried out at least in triplicate and
certainty in all the measurements were within the standard devi-
ation of 5%. The distribution ratio can be defined as
.6.1. [Th(L
The following procedure (as per literature reports [21,22]) was
adopted for the synthesis of [Th(L (NO ], and other metal com-
plexes. Th(NO O (0.2 g, 0.35 mmol) was added to a stirred
ꢁ5H
solution of L (0.346 g, 1.053 mmol) in dichloromethane. Reaction
1 3 3 4
) (NO ) ] (1)
)
1 3
3 4
)
3
)
4
2
1
mixture was stirred for 24 h to obtain white colored precipitate.
The solvent and volatile impurities were removed completely
under reduced pressure followed by washing with diethyl ether
(
2 ꢂ 5 mL) to afford a colorless solid. The composition and the pur-
ity of all the complexes were confirmed by spectroscopic and ana-
½
MꢃaqðiÞ ꢀ ½MꢃaqðfÞ
ꢀ1
lytical techniques. Yield: 90%, M.P: 151–153 °C, IR (cm ): 1276.88
P@O), 1587.42 (C@O). 31P NMR (d, 162 MHz, CDCl
): 43.11 ppm.
O): 7.66 (m, Ar-O, 4H), 7.82 (m, Ar-m
-CO, 2H), 3.58 (m, N-CH -CH -O, 8H)
DM
¼
ð2Þ
½MꢃaqðfÞ
(
3
1
H NMR (d, 400 MHz, D
and p, 6H), 3.64 (s, P-CH
2
where [M]aq(i) and [M]aq(f) concentration of metal ions before and
after contact with organic phase. All the experiments were carried
out at various nitric acid concentrations ranging from 0.01 M to 6
2
2
2
2
.6.2. [Th(L
The procedure for the preparation of the complex is similar to
that for [Th(L (NO ]. The amounts of the reactants are, Th
0.2 g (0.35 mmol) and DPDECMPO(L 0.332 g
1.053 mmol). Yield: 91%, M.P: 140–142 °C, IR (cm ): 1280.73
2 3 3 4
) (NO ) ] (2)
d
M. The distribution coefficient (K ) was measured as a function of
equilibration time to study the extraction kinetics. The distribution
coefficient was measured in several time intervals and the equilib-
rium condition was optimized as 90 min. Hence all the samples
were equilibrated for 90 min.
)
1 3
3 4
)
(NO
(
P@O), 1587.42 (C@O). 3 P NMR(d, 162 MHz, DMSO-D
(
): 7.76–7.81 (m, Ar-O, 4H), 7.5–7.56
3
4
) ꢁ5H
2
O
2
)
ꢀ
1
1
1
6
): 39.47. H
NMR (d, 400 MHz, DMSO-D
6
1
(
m, Ar-m and p, 6H), 3.7 (d, JP-H: 16 Hz, P-CH
2
-CO, 2H), 3.16–3.38
-CH3, 3H)
2.8. Computational methods
(
m, N-CH2, 4H), 0.86–0.9 (t, CH -CH , 3H), 1.06–1.1 (t, CH
2
3
2
We have applied density functional theory (DFT) calculations to
understand the electronic structure and geometries of diphenyl-
2.6.3. [UO (L )
2 1 2
3 2
(NO ) ] (3)
UO (NO
2
3
)
2
ꢁ6H
2
O 0.1 g (0.199 mmol) and DPMCMPO(L
1
) 0.131 g
morpholine carbamoylmethyl phosphineoxide (DPMCMPO) (L
diphenyl-N,N-diethyl carbamoylmethyl phosphine oxide
(DPDECMPO) (L ), and their respective complexes with Th(NO
and UO (NO . All ligand geometries were optimized employing
1
),
ꢀ1
(
0.398 mmol). Yield: 89%, M.P: 118–120 °C, IR(cm ): 1112.93
(
P@O), 1593.02 (C@O). 31P NMR (d, 162 MHz, DMSO-D
6
): 28.82.
2
3 4
)
1
H NMR (d, 400 MHz, DMSO-D
.54 (m, Ar-m and p, 6H), 3.79 (d,
.32–3.47 (m, N-CH -CH -O, 8H).
6
): 7.75–7.79 (m, Ar-O, 4H), 7.48–
2
3 2
)
1
7
3
J
P-H: 14 Hz, P-CH
2
-CO, 2H),
the most common B3LYP functional [24,25] in conjunction with
the triple-fdef2-TZVP basis sets [26,27]. The located stationary
points were subsequently characterized as energy minima by per-
forming harmonic vibrational frequency calculations at the same
level. In order to speed up the calculations, the resolution-of-iden-
tity (RI) approximation was applied in conjunction with appropri-
ate auxiliary basis sets [28–30]. Effect of dispersion was considered
by incorporating the latest atom-pair-wise dispersion correction
by Grimme et al. with Becke-Johnson damping (D3BJ) [31,32].
Throughout the calculations, we have used increased integration
grids (Grid6 in ORCA convention) and tight SCF convergence crite-
ria. We have employed the hybrid density functional PBE0 [33]
with 25% HF exchange, for the calculation of the metal complexes
2
2
2
2
.6.4. [UO (L
2
)
2
(NO
3 2
) (4)
UO (NO
2
3
)
2
ꢁ6H
2
O 0.1 g (0.199 mmol) and DPDECMPO(L ) 0.125
2
ꢀ
1
g (0.398 mmol). Yield: 93%, M.P: 158–160 °C, IR (cm ): 1273.03
(
P@O), 1602.85 (C@O), 31P NMR (d, 162 MHz, DMSO-D
6
): 31.69.
1
H NMR (d, 400 MHz, DMSO-D
6
): 7.75–7.8 (m, Ar-O, 4H), 7.49-
1
7
3
CH
.56 (m, Ar-m and p, 6H), 3.69 (d,
.16-3.39 (m, N-CH , 4H), 0.85-0.89 (t, CH
-CH , 3H).
J
2
P-H: 13.6 Hz, P-CH -CO, 2H),
2
2
-CH , 3H), 1.05-1.08 (t,
3
2
3
2
.7. Batch studies of actinides by solvent extraction
(
Th and U). For thorium and uranium atoms an ECP basis set was
Uranium and Americium: 2 mL of ligand of interest (0.2 M) in
employed, where the 60 inner-shell core electrons were replaced
by an effective core potential (ECP) generated for the neutral atom
using quasi-relativistic methods, [34–36]. The resulting explicitly
treated electrons were described by the standard def2-TZVP basis
sets and are referred to hereafter as def2-TZVP-ECP. All quantum
chemical calculations were performed with ab initio program pack-
age ORCA version 3.0.3 [37].
chloroform (already pre-equilibrated with appropriate nitric acid
concentrations) and 2 mL of nitric acid were taken in an equilibra-
tion tube and spiked with 2 U/ Am tracer and equilibrated for
33
241
9
0 min in a constant temperature bath at 30 °C. Concentrations
of uranium and americium in both aqueous and organic phase
were measured by sampling suitable aliquots from both the

2
18
D. Das et al. / Polyhedron 141 (2018) 215–222
3
. Results and discussion
.1. Synthesis and characterization of CMPO ligands
Derivatives of CMPO are very good candidates for the extraction
3
Scheme 4. Synthesis of diphenyl-N,N-diethyl carbamoylmethyl phosphineoxide
DPDECMPO) (L ).
of actinides from the spent fuel. Here the substitutions having dif-
ferent alkyl (short and long chain) and aryl groups on P and N atom
play a key role in their selectivity and extractability. It is proposed
to study the influence of cyclic and acyclic substitutions at CMPO
nitrogen atom over their extractability. In this connection, we have
designed two different molecules namely morpholine based CMPO
(
2
L
L
1
which is a cyclic system and diethyl (acyclic) substituted CMPO
. The proposed ligands L and L were synthesized by conven-
2
1
2
tional Michaelis–Becker reaction (Schemes 3 and 4). The purity
and structural confirmation of the compounds were established
3
1
1
13
by FT-IR, P, H and C NMR spectral data (see Supporting Infor-
mation for spectral data). A substantial downfield shift was
3
1
observed in P NMR of ligands [L
when compared to the precursor, diphenyl phosphine oxide [d
1.65 (d)] and this suggest the formation of the compounds L
and L . Further, the molecular structure of compound L is con-
1 2
= d 27.34 (s), L = d 28.38 (s)]
2
1
2
1
firmed by a single-crystal XRD data, whose ORTEP diagram is
shown in Fig. 2. The bond distances of P1–O1 (1.4802(12) Å) and
C14–O2 (1.2246(17) Å) in L were in good agreement with the tra-
1
ditional unsaturated P@O and C@O bonds. The bond angles of O1–
P1–C13 and O2–C14–C13 are found to be 113.63(7)° and 119.05
(
12)° respectively. Further it was found that the positions of C@O
and P@O are not cis to each other. The bond angles and bond
lengths of L is almost identical with the previously reported
Fig. 2. Crystal structure of diphenylmorpholine carbamoymethyl (DPMCMPO) (L
Some selected bond lengths (Å): P1–O1, 1.4802(12); C14–O2, (1.2246(17); C13–P1,
.8162(14); C14–N1, 1.3401(18); and bond angles (°): O1–P1–C13, 113.63(7); O2–
1
).
1
1
C14–C13 119.05(12) and O2–C14–N1, 121.74(14).
diphenyl-N,N-dimenthyl carbamoylmethyl phosphineoxide and
also the positions of C@O and P@O are trans to each other [38].
recorded between 25 and ꢀ50 °C (See Figs. S34 and S35 in Support-
ing). Irrespective the recorded temperature (25 to ꢀ50 °C), in all
conditions the complexes showed clear single peak without signif-
icant change in peak position in P VT NMR studies, this suggest
the formation of single complex.
3
.2. Synthesis, characterization and coordination chemistry of metal
complexes
3
1
[
Th(L
(L
1
)
3
(NO
3
)
4
] (1), [Th(L
] (4) complexes were prepared by reacting the cor-
and L in
2 3 3 4 2 1 2 3 2
) (NO ) ] (2), [UO (L ) (NO ) ] (3) and
[
UO
2
2
)
2
(NO )
3 2
responding salts of Th(IV) and U(VI) with ligands L
1
2
3.3. Distribution studies
dichloromethane solution. All the synthesized metal complexes
were characterized using FT-IR and NMR spectral data. 3 P NMR
spectra of all the metal complexes were significantly deshielded
with respect to the free ligand, which confirm the formation of
metal complexes. The purity of the metal complexes was ascer-
tained by the appearance of a new single peak in their 31P NMR
data. For example, ligand L resonates at 27.34 ppm but the corre-
1
sponding thorium and uranium complexes show signals at 43.17
and 28.32 ppm respectively. Thorium complexes showed a promi-
1
1
The extraction behaviour of the two CMPO based ligands (L and
L ) was studied individually with Th(IV), U(VI) and Am(III) in CHCl
2 3
at 303 K. Chloroform was used as a diluent due to the poor solubil-
ity of these ligands in n-dodecane and xylene. These are neutral
extractants and extract the metal ions through phosphoryl coordi-
nation like other neutral extractants. The expression for the extrac-
tion of metal ions by these compounds can be written as follows.
3
1
zþ
þ zNOꢀ þ nEðorgÞ ꢀ MðNO Þ ꢁ nE
nent shift in P NMR spectra indicating the formation of stronger
thorium complexes compared to uranium (Fig. 3).
M
ð3Þ
in chloroform at 303 K,
ðAqÞ
3ðAgÞ
3
z
ðOrgÞ
Fig. 4a shows the Du(VI) in 0.2 M L
1
and L
2
In Fig. 3, a broad peak for Th(L
observed. To understand the type of complex formation the metal
to ligand ratio for Th(NO with L and L was changed from 1:1 to
:3 and P NMR was recorded. Irrespective of metal to ligand
2 3 3 4 1 3 3 4
) (NO ) and Th(L ) (NO ) was
as a function of equilibrium aqueous nitric acid concentration. D
(VI)
values increase steeply with acid concentration, which is due
u
3
)
4
1
2
3
1
to the increased basicity of the phosphoryl oxygen leading to the
formation of stronger complexes. Distributions values are almost
1
3
1
ration, in P NMR for the corresponding complexes showed single
peak for both L and L at around 43 and 42 ppm respectively (See
Figs. S32 and S33 in Supporting information). In continuation,
same for L
1
and L
2
up to 1 M of HNO
3
concentrations. But from 2
of L is marginally higher
1
2
3
1
M HNO concentration onwards the D
P
3
u(VI)
2
1
than that of L and it maintains the same trend even at higher
acidities.
variable temperature NMR of these corresponding complexes were
D
Th(IV) follows the same trend as that of Du(VI) (Fig. 4b). When
compared with Du(VI) and DTh(IV), the distribution values for Am
(
1 2 3
III) by L and L showed deviation above 4 M HNO concentration
(Fig. 4c). In general, it is observed that the common extractants
show more distribution value for U(VI) when compared with Th
IV). But in the case of L and L , a reverse trend is observed. Distri-
(
1
2
bution values for Th(IV) is almost three fold higher than the Du(VI)
Scheme 3. Synthesis of diphenylmorpholine carbamoylmethyl phosphineoxide
(
DPMCMPO) (L
1
).
indicating selectivity of these ligands towards thorium over ura-

D. Das et al. / Polyhedron 141 (2018) 215–222
219
Fig. 3. Overlay of 31P NMR spectra of free ligands and their Uranium and Thorium complexes.
1 2
Fig. 4. Variation of (a) DU(VI), (b) DTh(IV) and (c) DAm(III) with equilibrium aqueous phase nitric acid concentration for DPMCMPO (L ) and DPDECMPO (L ) at 303 K.
nium. Thus ligands L
thorium over uranium. The distribution values for Th(IV) and U(VI)
for L and L are almost identical. The results suggest that cyclic
substitution on N atom i.e., morpholine based system L and its
analogue of open system L showed identical distribution values.
1 2
and L can be used for selective extraction of
1
2
1
2
3
3.4. Effect of NaNO on extraction of thorium
The effect of ionic strength on the extraction of Th(IV) was stud-
ied by varying concentration of sodium nitrate from 0.0 to 6 M
with 0.01 M HNO with extractants L and L using 0.2 M solution
in chloroform. Fig. 5 shows the variation of the distribution ratio of
Th(IV) with increase in the concentration of NaNO . The initial rise
3
1
2
3
in the D value is less and the rise becomes steep above 2 M nitrate
concentration. This behavior can be explained by the increase in
the thermodynamic activity of the metal and concomitant
decreases in the activity of water. The ionic strength of the solution
increases and hence behaves like salting out agent. The efficiency
of extraction increases with the increase in the concentration of
nitrate anions, because these species are considered as weak com-
plexing anions and thus behave as salting out agents. This supports
the nitrate complexes being extracted because of the higher the
ionic strength of the solution.
Fig. 5. Variation of distribution of Th(IV) with varying nitrate concentration at fixed
0.01 M HNO
3
.

2
20
D. Das et al. / Polyhedron 141 (2018) 215–222
Another experiment has been performed to evaluate the effect
of the acid protons (H ion) on extraction of Th(IV) with L and L
1 2
tant metal complex differ under different loading conditions. It is
experimentally established that under low loading conditions,
the ligand is primarily monodentate and extracts the metal ion
through the coordination of the phosphoryl group. Taken together,
in the present study, we have considered only those structures
where ligands are in monodentate coordination mode. All resulting
starting geometries were subjected to geometry optimizations so
as to derive the energetically lowest-lying structure at PBE0/
def2-TZVP-ECP level. Both uranium and thorium complexes are
in singlet ground state and the resulting lowest-energy structure
+
at 0.2 M solution in chloroform by varying the nitric acid to sodium
nitrate concentration where in the total nitrate concentration was
fixed at 3 M. Fig. 6 shows that the DTh(IV) value increased with the
decrease in acid protons. This observation can be attributed to the
salting out effect of nitrate ions and decrease in competition
between acid and metal ions for the extraction.
3
.5. Density functional theory studies
for Th(NO
3
)
4
ꢁ3L
1
, Th(NO
3
)
4
ꢁ3L
2
, UO
2
(NO
3
)
2
ꢁ2L
1
and UO
2
3
(NO )
2
ꢁ2L
2
are represented in Fig. 8a–d, respectively. In both complexes the
nitrate groups are distributed unsymmetrically around the metal
We have adopted a systematic geometric search procedure to
understand the electronic structure of the ligands, diphenylmor-
pholine carbamoylmethyl phosphine oxide (L ) and diphenyl-N,
N-diethyl carbamoylmethyl phosphine oxide (L ). Five and nine
starting geometric configurations were considered for the ligands
and L , respectively. The resulting structures were subjected to
1 2
atom and are bidentate in nature. Both L and L ligands are coor-
1
dinating to the metal atom through phosphoryl oxygen atoms.
Previous theoretical studies on the extraction behaviour of TBP
indicated that the effectiveness of extraction by a particular ligand
can be directly correlated to the computed extraction energy [40].
In this regard, we have evaluated the extraction energy for the
respective complexes as the energy balance of the following
equations.
2
L
1
2
geometry optimization at B3LYP/def2-TZVP level and for each
ligand we have identified a set of energetically lower-lying con-
formers. The lowest energy conformers for L
in Fig. 7a and b, respectively. The optimized geometry of L
1
and L
2
are illustrated
is very
1
ThðNO
3
Þ4ðaqÞ þ 3L1ðchloroformÞꢀ! ThðNO
3
Þ4 ꢁ 3L1ðchloroformÞ
ð4Þ
ð5Þ
ð6Þ
much similar to the geometry derived from the X-ray crystallo-
graphic data. It is to be noted that, for both the ligands, the car-
bonyl and phosphoryl oxygen atoms are negatively charged, with
the large value derived for the latter. The calculated Mulliken
charges for carbonyl and phosphoryl oxygen atoms are ꢀ0.39
and ꢀ0.59 electrons respectively. The corresponding phosphorus
and carbon atoms are positively charged (average charges on P
and C are 0.65 and 0.25, respectively) with a large charge separa-
tion estimated for the P@O unit. The geometric orientation of the
ThðNO
3
Þ4ðaqÞ þ 3L2ðchloroformÞꢀ! ThðNO
3
Þ4 ꢁ 3L2ðchloroformÞ
UO ðNO Þ
2
3
þ 2L1ðchloroformÞꢀ!UO ðNO Þ ꢁ 2L
2
3
1ðchloroformÞ
2ðaqÞ
2
UO
2
ðNO
3
Þ2ðaqÞ þ 2L2ðchloroformÞꢀ!UO
2
ðNO
3
Þ2 ꢁ 2L2ðchloroformÞ
ð7Þ
In the beginning of the extraction, Th(NO
3
)
4
and UO
2
3
(NO )
2
spe-
P@O and C@O groups are similar in both L
After establishing the electronic structure of the ligands, we
were interested in their respective complexes with Th(NO and
UO (NO . We have considered seven starting geometric configu-
rations for the complex, by distributing four nitrate groups and
ligand units around thorium and uranium metal atoms. In the case
of thorium complex, three ligand units were distributed around the
metal nitrate, whereas for uranium system only two ligands were
considered. The choice on the number of ligands is based on the
experimental information on stoichiometry. Both monodentate
and bidentate coordination mode of the ligands were explored in
the past [39] and studies suggested that the structure of the extrac-
1
2
and L .
cies are in the aqueous medium whereas the ligands are in chloro-
form environment. Upon extraction, the respective metal solvates
formed at the aqueous organic interface and are extracted to the
chloroform medium. Due to this fact the actual solvent environ-
ment of the metal complexes is chloroform. The influence of the
solvent environment was considered by performing single point
calculations on the optimized geometries with the conductor-like
screening model (COSMO) [41] (dielectric constant of aqueous
3 4
)
2
3 2
)
phase,
ECP level. The calculated extraction energies are ꢀ97.9 and
ꢀ87.2 kcal/mol for Th(NO ꢁ3L and Th(NO ꢁ3L , respectively.
On the other hand the estimated extraction energies for UO (-
NO and UO (NO ꢁ2L are relatively smaller and amounts
ꢁ2L
e = 80.4 and for chloroform e = 4.81) at PBE0/def2-TZVP-
3
)
4
1
3
)
4
2
2
3
)
2
1
2
3
)
2
2
to ꢀ61.2 and ꢀ59.6 kcal/mol, respectively. The extraction energy
of thorium complexes are considerably larger than that for the ura-
nium complexes. This implies that the ligands form stronger com-
plexes with Th in comparison to U, and therefore can be considered
as potential extractants for selective extraction of Th. This argu-
ment is in agreement with the observation of large D values for
thorium as compared to that of uranium complexes.
7
0
65
0
.2ML /CHCl
1
3
3
60
0.2ML /CHCl
2
55
50
45
4
0
4
. Conclusions
35
3
0
1 2
CMPO based ligands L and L bearing two different substitu-
25
tions on N atom, one with cyclic substitution (i.e., morpholine base
closed system) and other one having acyclic substitution (diethyl
based open system) were synthesized and characterized using
20
15
1
13
31
10
1
FT-IR, H, C, and P NMR spectra. Structure of L was further con-
5
0
firmed by single crystal XRD analysis. No significant changes in dis-
tribution value for Th(IV) and U(VI) were observed when
substitution on N atom of CMPO is changed from closed system
[
3.0+0.0] [2.5+0.5] [2.0+1.0] [1.5+1.5] [1.0+2.0] [0.0+3.0]
(
morpholine based ligand L
ligand L ). The calculated structural parameters for diphenyl mor-
pholine carbamoylmethyl phosphine oxide (L ) agree well with the
X-ray crystallographic data. The geometries of the complexes Th
1
) to open system (diethyl substituted
[
HNO ] +[NaNO ] mol/L
3
3
2
1
Fig. 6. Variation of distribution of Th(IV) with varying nitrate concentration with
respect to HNO
3
.

D. Das et al. / Polyhedron 141 (2018) 215–222
221
1 2
Fig. 7. Optimized geometries of (a) L and (b) L at B3LYP/def2-TZVP level. Color code: orange ball is phosphorus, red balls are oxygen, dark blue ball is nitrogen, grey balls are
carbon, and white balls are hydrogen atoms. (Colour online.)
1 2 3 4 2 3 2
Fig. 8. Optimized geometries of respective complexes of the ligands L and L with Th(NO ) (a and b) and UO (NO ) (c and d), respectively, at PBE0/def2-TZVP-ECP level.
Hydrogen atoms are omitted for clarity. Color code: light blue ball is thorium/uranium, orange ball is phosphorus, red color represents oxygen, dark blue colour indicates
nitrogen, grey balls are carbon, and white balls are hydrogen atoms. (Color online.)
(
NO
3
)
4
ꢁ3L
1
, Th(NO
3
)
4
ꢁ3L
2
, UO
2
(NO
3
)
2
ꢁ2L
1
and UO
2
(NO
3
)
2
ꢁ2L
2
are
tion energies are ꢀ97.9, ꢀ87.2, ꢀ61.2 and ꢀ59.6 kcal/mol, respec-
tively for Th(NO Th(NO ꢁ3L UO (NO ꢁ2L and
ꢁ3L
UO (NO . The computed larger extraction energies of tho-
ꢁ2L
rium complexes in comparison to the uranium systems are in
established using density functional theory calculations. The
ligands are monodentate in nature and coordinates to the metal
atom through the phosphoryl oxygen atom. The computed extrac-
3
)
4
1
,
3
)
4
2
,
2
3
)
2
1
2
3
)
2
2

2
22
D. Das et al. / Polyhedron 141 (2018) 215–222
0
0
agreement with the large D values estimated for thorium com-
plexes. This implies that the ligands form stronger complexes with
Th(IV) and therefore can be considered as potential extractants for
selective extraction of Th. Unusually, a low extraction behavior is
observed with Am ions with the ligands investigated in the present
study compared to the traditional CMPO based ligand systems.
This is an intriguing result that offers interesting opportunities
for the fine tuning of the structure of these ligands. Similarly, com-
puting complexation energies of these species will also be of our
future study.
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Acknowledgements
1
(
Kari thanks UGC-DAE CSR, India [CSR-KN/CRS-53/2013-14/652]
for the financial support and IGCAR, Kalpakkam for instrumenta-
tion facilities. Kari also thanks DST-VIT-FIST for NMR and SIF-VIT
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data for DPMCMPO (ligand L ). These data can be obtained free
1
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
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