Investigations on synthesis, coordination behavior and actinide recovery of unexplored dicyclohexylphosphinic acid

Article abstract of DOI:10.1016/j.poly.2016.07.013

Dicyclohexylphosphinic acid (DcyHPA) and some of its metal complexes with lanthanides (La, Ce, Gd, Sm and Nd) and actinides (U and Th) were prepared and subsequently the structural composition was elucidated. The single crystal XRD data and theoretical analysis revealed that the ligand (DcyHPA) exists as a dimer in solid state. DcyHPA showed dual nature for the extraction of U(VI), Th(IV) and Am(III) from nitric acid medium i.e. at lower acidity, the ligand behaves as an acidic extractant and extracted the metal through cation exchange mechanism while solvation mechanism played a major role for extraction at higher acidity. Additionally, density functional theory (DFT) calculations were demonstrated to understand the electronic structure of thorium-DcyHPA metal complex formed during the extraction process.

Full text of DOI:10.1016/j.poly.2016.07.013

Polyhedron 117 (2016) 741–748
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Investigations on synthesis, coordination behavior and actinide recovery
of unexplored dicyclohexylphosphinic acid
a
b
E. Veerashekhar Goud a, Dhrubajyoti Das a, Akella Sivaramakrishna a, , Kari Vijayakrishna , V. Sabareesh ,
⇑
Gopinadhanpillai Gopakumar c, C.V.S. Brahmmananda Rao c, Mohsin Y. Lone d, Prakash C. Jha d
a Department of Chemistry, School of Advanced Sciences, VIT University, Vellore 632 014, Tamil Nadu, India
b Centre for Bioseparation Technology, VIT University, Vellore 632 014, Tamil Nadu, India
c Chemistry Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, Tamil Nadu, India
d School of Chemical Sciences, Central University of Gujarat, Sector-30, Gandhinagar, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Dicyclohexylphosphinic acid (DcyHPA) and some of its metal complexes with lanthanides (La, Ce, Gd, Sm
and Nd) and actinides (U and Th) were prepared and subsequently the structural composition was elu-
cidated. The single crystal XRD data and theoretical analysis revealed that the ligand (DcyHPA) exists
as a dimer in solid state. DcyHPA showed dual nature for the extraction of U(VI), Th(IV) and Am(III) from
nitric acid medium i.e. at lower acidity, the ligand behaves as an acidic extractant and extracted the metal
through cation exchange mechanism while solvation mechanism played a major role for extraction at
higher acidity. Additionally, density functional theory (DFT) calculations were demonstrated to under-
stand the electronic structure of thorium-DcyHPA metal complex formed during the extraction process.
Ó 2016 Elsevier Ltd. All rights reserved.
Received 11 May 2016
Accepted 10 July 2016
Available online 28 July 2016
Keywords:
Dicyclohexylphosphinic acid
Coordination behavior
Single crystal X-ray analysis
Actinides
Lanthanides
Density functional theory
1. Introduction
have been attempted in the recent past to extract actinides from
radioactive nuclear waste [12]. Some of these extraction studies
The quest for the development of an optimum extractant, effec-
tive in separating a particular metal from its mixture, is an active
field of research in both chemistry and chemical engineering. These
extractants find extensive applications in extractive metallurgy (for
the separation of metal from its ore) and in nuclear fuel cycle for the
separation of actinides from other fission products [1]. In the case of
the latter, solvent extraction and ion exchange are two widely used
separation techniques [2]. In solvent extraction, a specific ligand/
extractant is used in the organic phase in conjunction with suitable
diluents. For instance, PUREX [3], TRUEX [4] and DIAMEX [5] are
some of the processes used throughout the world for separation
of actinides and lanthanides. Apart from the traditional ligand sys-
tems, a variety of N-donor ligands including N,N,N0,N0-tetraalkyl-
7-oxabicyclo[2.2.1]heptane-2,3-dicarboxamides [6], 6-(5,6-dipen-
tyl-1,2,4-triazin-3-yl)-2,20-bipyridine [7], diglycolamide-functional-
ized calix[4]arene [8], 2,6-bis(1,2,4-triazin-3-yl)-pyridine [9],
were also evaluated by theoretical calculations [13].
Being hard in nature, actinides prefer oxygen donor ligands for
coordination and complexation [14]. Organophosphorus esters are
thus regarded as preferred class of compounds for the extraction
and separation of actinides. In nuclear reprocessing industry, tri-
n-butyl phosphate (TBP) has been used as an extractant for more
than six decades to recover Pu from irradiated spent fuel in PUREX
process [15]. The main advantage of TBP is that it is a neutral sol-
vent and hence very effective in extracting uranium and plutonium
at higher acidity. Subsequently, the extracted metal can also be
stripped from the organic phase by reducing the acidity of the
aqueous phase. The other members of organophosphorus family
have not been studied extensively for their potential use as alter-
nate extractants. Basicity of the phosphoryl group in ester can be
enhanced by replacing the C–O–P group by a C–P group. This facil-
itates the extraction behavior which varies for neutral organophos-
phorus extractants in the order: phosphates < phosphonates <
phosphinates < phosphine oxides [16]. Families of dialkyl phos-
phonates, H-phosphonates and di-n-alkyl phosphine oxides
(DAPOs) were synthesized and studied for their potential usage
as extractants in our laboratory [17,18]. In general, research is
carried out in various laboratories for better separations and to
N,N0-dioctyl,
a
-hydroxy acetamide [10] and N,N,N0,N0-tetrakis
(2-hydroxy-3-methyl-5-tert-butylbenzyl) diamino alkanes [11]
⇑
Corresponding author at: Organic Chemistry Division, School of Advanced
Sciences, VIT University, Vellore 632 014, Tamil Nadu, India. Fax: +91 416224 3092.
E-mail address: asrkrishna@vit.ac.in (A. Sivaramakrishna).
http://dx.doi.org/10.1016/j.poly.2016.07.013
0277-5387/Ó 2016 Elsevier Ltd. All rights reserved.

742
E.V. Goud et al. / Polyhedron 117 (2016) 741–748
understand the fundamental coordination chemistry of the acti-
nides with various ligands to optimize the separation processes.
There is a scope for the development of new separation technolo-
gies and fundamental discoveries by creating appropriate models
to elucidate the chemistry and environmental behavior of these
important elements [19].
7H2O)] and [LaCl3ꢁ7H2O)] were used as received. The actinides,
241Am and 233U tracers were used from laboratory stock solutions
and their radiochemical purity was checked prior to their use.
2.3. Procedure for preparation of DcyHPA
In this context, we have studied the cyclohexyl derivative of
phosphinic acid (dicyclohexylphosphinic acid (DcyHPA)), which
was not explored before as a potential extractant for actinide sep-
aration and recovery. In the present work, we focus on the synthe-
sis, characterization and extraction abilities of DcyHPA. The
structural compositions of lanthanide/actinide metal complexes
and electronic structure of the ligand was investigated by applying
suitable experimental (single crystal studies, spectroscopic and
analytical techniques) and theoretical (density functional theory
calculations) methodologies.
A
calculated amount of magnesium turnings (2.6001 g,
99.0 mmol) was added to the two necked round bottomed flask
equipped with condenser and dropping funnel in an inert nitrogen
atmosphere. A small amount of dry tetrahydrofuran (THF) was
added as a solvent. This reaction mixture was refluxed for 5 min
and a slow addition of cyclohexyl bromide (14.6005 g, 89.6 mmol)
was performed through a dropping funnel for initiation of the reac-
tion. Once the reaction was initiated, the remaining amount of
alkyl halide was added drop wise and refluxed for 2 h with contin-
uous stirring. The resulting ash-black solution of Grignard was
cooled to 0 °C and added diethylphosphite (7.002 g, 30 mmol) in
THF (50 mL) under stirring for 2 h. The formation of product was
monitored by TLC and to this reaction mixture, dil. HCl (0.1 N)
was added to quench the excess Grignard reagent at ice cold tem-
perature. The upper organic layer was separated and to the residue
2 ꢀ 25 mL dichloromethane was added to extract the remaining
product. The organic extract was dried using anhydrous Na2SO4
and filtered. The volatile components were removed under reduced
pressure. The obtained product was purified with column
chromatography by eluting with a mixture of ethyl acetate and iso-
propyl alcohol (3:1) to get a colorless crystalline solid. The yield
was 9.5 g (76%) and m.p. 141–143 °C. 1H NMR (400 MHz, CDCl3):
d 8.3 (s, 1H, P–OH), 1.8–1.89 (m, 2H, P–CH), 1.69–1.72 (q, 8H,
P–CH–CH2), 1.4–1.42 (m, 8H, P–CH–CH2–CH2), 1.2 (m, 4H,
2. Experimental
2.1. Instrumentation
1H, 13C, and 31P{1H} NMR spectra were recorded on a Bruker
DMX-400 spectrometer and all 1H chemical shifts were reported
relative to the residual proton resonance in deuterated solvents
(all at 298 K, CDCl3). Infrared spectra were recorded on a Shimadzu
Affinity 1 FT-IR Spectrometer UV–vis absorption spectra were
recorded with a UNICAM UV4-100 type double-beam spectropho-
tometer (ATI UNICAM, Cambridge, UK) and Heraeus CHNS rapid
micro analyzer was used for elemental analysis. The mass spectral
data were recorded by means of LC-MS. The liquid chromatography
(LC) was performed on a reverse phase column (Acquity UPLC BEH
P–CH–CH2–CH2–CH2). 13C NMR (CDCl3, 100 MHz):
d
34.9
1
2
3
(d, JCP = 91.0 Hz), 24.6 (d, JCP = 3.3 Hz), 25.9, 26.1 (d, JCP = 13.6 -
Hz). 31P NMR (CDCl3, 162 MHz): d 61.55. The analytical data of
DcyHPA closely agrees with the literature report [22].
C18, 1.7
l
m, 2.1 ꢀ 50 mm) using Acquity UPLC (Waters). Methanol
and water were used as the mobile phase and no additives were
added to the mobile phase. The mass spectrometric data were
acquired on Quattro Premier XE (Micromass MS Technologies),
an electrospray ionization (ESI) triple quadrupole mass spectrom-
eter, which is connected to Acquity UPLC.
2.4. General procedure for the preparation of metal complexes
Single crystal X-ray diffraction measurements for DcyHPA were
performed on a Bruker Smart 1000 CCD diffractometer at 120(2) K.
The structure was solved by direct methods and refined employing
full-matrix least-squares with the program SHELXL-97 [20] refining
on F2. Packing diagrams were produced using the program PovRay
and graphic interface X-seed [21]. The crystals of C12H23O2P1 are
triclinic, space group P1; (a) 6.57960(10) Å, (b) 9.5197(2) Å, (c)
Th(NO3)4ꢁ5H2O (0.1 g, 0.175 mmol) was added to a stirred solu-
tion of DcyHPA (0.08 g, 0.35 mmol) in a solvent mixture containing
CHCl3: HNO3 (3.5 mL, 70%: 1.5 mL, 2% (v/v), 30%). The mixture was
stirred for about 24 h to get the light yellow colored solid. The yield
of the complex was 101.7 mg (97%).
A similar procedure was adopted to prepare other complexes by
taking the appropriate amounts of ligand and metal salts are as fol-
lows: La(DcyHPA)3 [LaCl3ꢁ7H2O (0.1005 g, 0.27 mmol) and DcyHPA
(0.1801 g, 0.81 mmol)], Ce(DcyHPA)3 [CeCl3ꢁ7H2O (0.1001 g,
0.268 mmol) and DcyHPA (0.185 g, 0.805 mmol)], Gd(DcyHPA)3
[Gd(NO3)3ꢁ6H2O (0.0502 g, 0.11 mmol) and DcyHPA (0.0769 g,
0.33 mmol)], Sm(DcyHPA)3[Sm(NO3)3ꢁ6H2O (0.0506 g, 0.11 mmol)
and DcyHPA (0.0771 g, 0.33 mmol)], Nd(DcyHPA)3 [Nd(NO3)3ꢁ6H2-
O (0.0501 g, 0.11 mmol) and DcyHPA (0.0801 g, 0.34 mmol)], Th
(NO3)2(DcyHPA)2 [Th(NO3)4ꢁ5H2O (0.1004 g, 0.17 mmol) and
DcyHPA (0.0802 g, 0.35 mmol)], UO2(DcyHPA)2 [UO2(NO3)2ꢁ6H2O
(0.2003 g, 0.51 mmol) and DcyHPA (0.2303 g, 1.01 mmol)].
The formed metal complexes were washed with a solvent mix-
ture of dichloromethane and petroleum ether (3:1) to remove any
traces of unreacted starting materials. The metal complexes were
dried in vacuum over anhydrous calcium chloride. The complete
list of isolated metal complexes is given in Table 1.
10.9205(2) Å.
a 84.5380(10)°, b 79.9420(10)°, c 78.8040(10)°.
V = 659.35(2), Goodness-of-fit on F2 1.063. A total of 4202 reflec-
tions were collected, and 3802 independent reflections were used
in further refinement. The structure of DcyHPA was solved by
direct methods and refined by full-matrix technique against F2 in
the anisotropic approximation. Complete details of the structure
are given in the Supporting Information. The hydrogen atoms for
this molecule are not localized. Crystallographic data for DcyHPA
have been deposited with the Cambridge Crystallographic Data
Centre and assigned the number CCDC 1443194.
2.2. Materials and methods
All synthetic procedures were carried out under nitrogen atmo-
sphere using Schlenk line techniques. The commercially available
solvents were distilled from Na/benzophenone before the usage.
The reagents including magnesium turnings, HCl, diethyl phos-
phite, isopropyl alcohol, ethyl acetate and cyclohexyl bromide
(Merck, India) were used as received. Hydrated metal salts (Loba
and Aldrich, India), [Th(NO3)4ꢁ5H2O)], [Nd(NO3)3ꢁ6H2O)], [Sm
(NO3)3ꢁ6H2O)], [Gd(NO3)3ꢁ6H2O)], [UO2(NO3)2ꢁ6H2O)], [CeCl3ꢁ
La(DcyHPA)3: 1H NMR (400 MHz, DMSO-d6): d 1.52 (m, 2H,
P–CH), 1.10 (m, 10H, P–CH–CH2), 1.64 (m, 10H, P–CH–CH2–CH2).
3
13C NMR (100 MHz): d 34.27, 33.33, 25.20 (d, JCP = 13 Hz), 24.11.
31P NMR (162 MHz): d 63.66.
Ce(DcyHPA)3: 1H NMR (400 MHz, DMSO-d6): d 1.58 (m, 2H,
P–CH), 1.11 (m, 10H, P–CH–CH2), 1.69 (m, 10H, P–CH–CH2–CH2).

E.V. Goud et al. / Polyhedron 117 (2016) 741–748
743
Table 1
equilibrium aqueous concentration from the initial feed
concentration.
Spectral data for metal complexes derived from DcyHPA.
S. No.
Compound
Yield (%)
IR (cmꢂ1
)
m.p. (°C)
1.
2.
3.
4.
5.
6.
7.
8.
DcyHPA
76.5
98.2
97.3
96.4
98.4
97.7
96.9
98.2
1159
1155
1157
1149
1141
1103
1026
1056
141–143
128–130
118–120
>270 (dec)
>270 (dec)
>270 (dec)
>270 (dec)
>270 (dec)
2.5.3. Americium
La(DcyHPA)3
Ce(DcyHPA)3
Gd(DcyHPA)3
Sm(DcyHPA)3
Nd(DcyHPA)3
Th(NO3)2(DcyHPA)2
UO2(DcyHPA)2
After pre-equilibration with required acid, 2 mL of the extrac-
tant (0.1 M xylene solution) was further equilibrated with2 mL of
nitric acid, spiked with 241Am tracer in a constant temperature
bath at 303 K for 90 min. The equilibrium concentration of Am
(III) in the aqueous and organic phases was determined by measur-
ing the activities of the 60 keVc photons of the respective phases
using NaI (TI) counter. The experiment was carried out at various
nitric acid concentrations.
3
13C NMR (100 MHz): d 34.41, 33.52, 25.35 (d, JCP = 14 Hz), 24.27
4
(d, JCP = 3 Hz). 31P NMR (162 MHz): d 57.40.
UO2(DcyHPA)2: 1H NMR (400 MHz, DMSO-d6): d 1.57 (m, 2H,
P–CH), 1.10 (m, 10H, P–CH–CH2), 1.69 (m, 10H, P–CH–CH2–CH2).
3. Computational methodologies
3
13C NMR (100 MHz): d 34.32, 33.43, 25.31 (d, JCP = 14 Hz), 24.23
DFT simulations were carried out employing GAUSSIAN 09 pro-
gramme package [25]. The geometries were optimized at Becke’s
three parameter hybrid density functional B3LYP [26,27] level in
conjunction with valence double zeta 6-31G (d) basis sets. Hessian
matrix was exercised at the same level of approximation to ascer-
tain that the given structure is at its energy minima. 31P-NMR was
computed at DFT level by using the gauge independent atomic
orbitals (GIAO) [28] method in conjunction with Dunning’s triple
zeta correlation consistent basis set cc-pVTZ. Natural bond orbital
(NBO) analysis was carried out at B3LYP/6-31G (d) using NBO 3.1
version implemented in GAUSSIAN 09 [25].
The ground state geometry of thorium metal complex in gas
phase was explored using the hybrid functional, PBE0 [29] which
is having 25% HF exchange. In the case of Thorium, 60 inner-shell
core electrons were replaced by an effective core potential
(DKH-ECP) generated for the neutral atom using quasi-relativistic
methods [30], and all other atoms were described by the standard
def2-TZVP basis sets. The resolution-of-identity (RI) approximation
was applied in conjunction with the appropriate auxiliary basis
sets to speed up the calculations.
4
(d, JCP = 3 Hz). 31P NMR (162 MHz): d 57.77.
Th(NO3)2(DcyHPA)2: 1H NMR (400 MHz, DMSO-d6): d 1.51
(m, 2H, P–CH), 1.64 (m, 10H, P–CH-CH2), 1.07 (m, 10H, P-CH-
CH2-CH2). 13C NMR (100 MHz): d 34.06, 33.18, 25.18 (d, JCP = 14
3
Hz), 24.08. 31P NMR (162 MHz): d 60.32.
Sm(DcyHPA)3: 1H NMR (400 MHz, DMSO-d6):
(m, 2H, P–CH), 1.57 (m, 10H, P–CH–CH2), 0.83 (m, 10H,
d
1.45
P–CH–CH2–CH2). 13C NMR (100 MHz):
d 33.95, 33.07, 25.04
(d, JCP = 13 Hz), 23.96. 31P NMR (162 MHz): d 60.77.
3
Nd(DcyHPA)3: 1H NMR (400 MHz, DMSO-d6):
d
1.51
(m, 2H, P–CH), 1.63 (m, 10H, P–CH–CH2), 1.08 (m, 10H,
P–CH–CH2–CH2). 13C NMR (100 MHz):
d 34.17, 33.29, 25.24
(d, JCP = 15 Hz), 24.10. 31P NMR (162 MHz): d 59.20.
3
Gd(DcyHPA)3: 1H NMR (400 MHz, DMSO-d6): d 1.51 (m, 2H,
P–CH), 1.63 (m, 10H, P–CH–CH2), 1.08 (m, 10H, P–CH–CH2–CH2).
13C NMR (100 MHz): d 24.39, 25.44. 31P NMR (162 MHz): d 56.40.
2.5. Batch studies of actinides by solvent extraction
Empirical Grimme-type dispersion corrections were incorpo-
rated during this step using the latest atom-pairwise dispersion
correction with Becke–Johnson damping (D3BJ) [31]. Increased
integration grids (GridX9 in ORCA convention) and tight SCF con-
vergence criteria were used throughout the calculations. The DFT
calculations were performed with ORCA version 3.0.3 program
package [32].
In general, 1.1 M concentration of extractant in n-dodecane was
used in reprocessing industry for the separation of uranium and
plutonium from the dissolver fuel solution [23]. DcyHPA has a lim-
ited solubility in n-dodecane because of its polar nature and hence
all the studies here were carried out by using xylene as a diluent.
The maximum solubility of DcyHPA in xylene was 0.3 M. The con-
centration of the extractant used in the present study was 0.01 M
in xylene for uranium and thorium while in the case of americium,
0.1 M concentration of the extractant was used.
4. Results and discussions
2.5.1. Uranium
4.1. Synthesis and characterization of DcyHPA
The distribution studies of uranium were carried out by using
233U tracer. The extractant was pre-equilibrated with appropriate
nitric acid concentration. 2 mL of the extractant (0.01 M) in xylene
and 2 mL of nitric acid were taken in an equilibration tube and
spiked with 233U tracer. The samples were equilibrated in a con-
stant temperature bath at 303 K for 90 min.
Initially we have attempted to synthesize dicyclohexylphos-
phine oxide (H-phosphine oxide, DcyHPO) by reacting diethyl
phosphate and cyclohexyl magnesium bromide (cf. Scheme 1).
But the isolated product showed a single peak in 31P-NMR. It is
well-known that H-phosphine oxides and H-phosphonates exhibit
a doublet in their 31P-NMR spectra, this clearly suggests that the
obtained compound does not have P–H bond and rules out the
existence of DcyHPO. The molecular structure of the obtained com-
pound was further confirmed by single crystal X-ray analysis data
as DcyHPA (Fig. 1). The instability of dicyclohexylphosphine oxide
(DcyHPO) was attributed due to the presence of bulky cyclohexyl
groups. This closely agrees with the literature report on the influ-
ence of steric bulk of cycloalkyl substituents in rearrangement
reactions [33]. Interestingly, the oxidized molecule DcyHPA con-
tains ‘AP@O(OH)’ group, which could be a potential chelating
ligand for the f-metal coordination complexes. This prompted us
to explore the coordination and extraction behavior of DcyHPA
The equilibrium concentrations of U(VI) in both the phases
were measured by sampling a known quantity of the aqueous
and organic phases and the a
-activity of 233U was obtained from
the liquid scintillation counting using dioxane based cocktail.
2.5.2. Thorium
The experimental procedure used for the extraction of Th(IV) by
the DcyHPA was similar to that of uranium extraction described as
above. The thorium concentrations in the initial aqueous phase and
at equilibrium were determined spectrophotometrically using
arsenazo-III as the chromogenic agent [24]. The concentration of
thorium in the organic samples was estimated by subtracting the

744
E.V. Goud et al. / Polyhedron 117 (2016) 741–748
O
O P O
H
O
P
H
MgBr
N2, 2 h stirr, RT
dil. HCl
unstable product
DcyHPO
dry.THF, N2
Reflux, 2 h
Br
Mg
O
P
OH
DcyHPA
Scheme 1. Route for synthesis of dicyclohexylphosphinic acid from
cyclohexylmagnesium bromide.
Fig. 2. Crystal packing diagram of DcyHPA.
experimental results. It is obvious from the Table 2 that there is
a good agreement between theoretical and experimental results;
the marginal deviation could be attributed due to crystal close
packing and hydrogen bonding.
It is prudent to determine 31P chemical shift for DcyHPA
because of its extreme sensitivity towards electronic structure.
The range of which is within 1000 ppm and therefore a feeble
change in structure will be strongly reflected in its chemical shifts
[36]. The isotropic chemical shift obtained for DcyHPA with refer-
ence to H3PO4 at B3LYP/cc-pvtz is 60.23 ppm (experimental chem-
ical shift is 61.55 ppm) reflecting the finest structural conformity.
To understand the stability and reactivity of molecules DcyHPO
and DcyHPA, frontier molecular orbital (FMO) analysis was carried
out [37]. The splitting of FMOs can be directly correlated to the
kinetic stability of molecules as the separation between these orbi-
tals is the result of intramolecular charge transfer from donor to
acceptor molecular orbitals. From the calculated energy gap values
it is clear that the molecule DcyHPA (8.01 eV) possesses little
higher kinetic stability than DcyHPO (7.99 eV). To gain further
insight regarding the stability the molecules of interest (DcyHPO
and DcyHPA), delocalization second-order perturbation theory
analysis has been taken into account. For each donor (i) and accep-
tor (j) the stabilization energy, E(2), associated with the delocaliza-
tion i ? j is determined as given in Eq. (1).
Fig. 1. Molecular structure of DcyHPA (ORTEP diagram with 40% probability
ellipsoids). Some selected bond lengths (Å): O1–P2 1.546(4), O1–H1 0.8200, O2–P2
1.496(3), O3–P1 1.522(3), O4–P1 1.552(3), O4–H4 0.8200; bond angles (°):
P2–O1–H1 109.4, P1–O4–H4 109.5, O3–P1–O4 114.53(19), O3–P1–C1 108.9(2),
O4–P1–C1 104.5(2), O3–P1–C7 109.7(2), O4–P1–C7 107.8(2), C1–P1–C7 111.4(2),
O2–P2–O1 113.63(19), O2–P2–C13 109.3(2), O1–P2–C13 108.5(2), O2–P2–C19
110.4(2), O1–P2–C19 104.8(2), C13–P2–C19 110.2(2)°.
with various f-metal salts. Single crystal X-ray analysis suggests a
dimeric nature of the ligand in solid state (cf. Figs. 1 and 2).
The single crystal X-ray data of DcyHPA showed two distinct
phosphorus-oxygen bonds i.e. the bond lengths for P–O were
observed as 1.546(4) and 1.552(3) whereas 1.496(3) and 1.522(3)
for P@O. The hydrogen bonding puts this compound in dimeric
nature in solid state. Dimerization is a common phenomenon in
acidic type of compounds like R2PO2H in benzene [34], earlier
reports also confirm this for acidic extractants in nonpolar solvents
[35]. The bond angles of O3–P1–O4 and O2–P2–O1 were shown as
114.53(19)° and 113.63(19)° respectively.
2
F
ðijÞ
Eð2Þ ¼
D
Eijqi
ð1Þ
ðEj ꢂ EiÞ
Larger magnitude of E(2) indicates intensive interaction
between donor–acceptor molecular orbitals. Out of possible
intramolecular interactions accountable for delocalization of
Table 2
Structural parameters (bond lengths in Å and bond angles in degree) computed at
B3LYP/6-31G(d) level.
Molecule
DcyHPA
Bond length/bond angle
Theoretical
Experimental
4.2. Theoretical calculations on the stability of DcyHPO and DcyHPA
C–P
1.84
1.80
P–O
1.49
1.49
DcyHPA
C–P–C
C–P–O
109.20
109.26
111.36
114.24
The structural parameters obtained as a result of geometry
optimization has been taken into account for comparison with

E.V. Goud et al. / Polyhedron 117 (2016) 741–748
745
Table 3
Stabilization energy calculated via second-order perturbation theory analysis. Values
that are above the threshold of 20 kcal/mol reported below.
Molecule
Donor–acceptor orbitals
E(2) (kcal/mol)
DcyHPO
DcyHPA
LP O ? BD⁄ P–H
21.22
76.05
56.21
22.15
21.41
21.69
20.66
BD⁄ P–O ? BD⁄ P–OH
BD P–OH ? BD⁄ P–O
BD C–P ? BD⁄ P–O
BD C–P ? BD⁄ P–OH
BD C–P⁄ ? BD⁄ P–O
BD P–O ? BD⁄ P–OH
LP is lone pair of electrons, BD and BD⁄ are bonding and antibonding molecular
orbitals, respectively.
charge, those having large contribution are mentioned in the
Table 3. Apparently, molecule DcyHPA displays larger number of
such interactions and hence accounts for greater stability.
Fig. 3. FT-IR overlay of DcyHPA and its metal complexes.
4.3. Synthesis of metal complexes
protonated Th(IV) complex ([M++5H+], m/z 819.46) with a metal-
to-ligand ratio of 1:3 was observed. Moreover, a new MS peak at
m/z 588.32, attributed to the [Th(DcyHPA)(NO3)2] complex, occurs.
Two trace species at m/z 692.59 and 523.32 are attributed to [Th
(DcyHPA)2] and [Th(DcyHPA)(NO3)], respectively. The MS peak of
the ligand (DcyHPA) was not observed. Other peaks at m/z
798.29 and 803.46 (most abundant) were assigned to [M–O]+ and
[M–O + 5H+] respectively, which could be due to the reduction of
one of the P@O bonds. Further, the peak observed at m/z 391.29
could be due to the formation of [Th(OH)2(NO3)2]+.
The observed higher melting points indicate greater thermal
stability of these complexes (Table 1). Interestingly the Ce and La
complexes decompose at lower temperatures than the ligand and
all other complexes decompose at temperatures higher than
270 °C.
Attempts were made to understand the electronic structures of
metal complexes of DcyHPA and here Th(IV) complex of DcyHPA is
taken as an example. DFT calculations were carried out to investi-
gate the geometric and electronic structure of the thorium metal
complex formed. Four starting geometric configurations were con-
sidered by distributing two nitrate and two dicyclohexylphospho-
nic acid groups around the metal atoms. These configurations were
then subjected to geometry optimization at PBE0/def2-TZVP-ECP
level. The ground state of the complex is in singlet spin state and
the lowest energy structure is illustrated in Fig. 5.
This structure is a stable minimum, as the calculated harmonic
vibrational frequencies predicted real values at the same level. The
bidentate DcyHPA ligands are in cis-orientation with average Th–O
distance amounting to 2.37 Å. This is relatively shorter than the
average Th–O distance (2.49 Å), resulting from nitrate group
All lanthanide complexes (i.e. La, Ce, Gd, Sm and Nd) were syn-
thesized by taking stoichiometric amounts of dicyclohexylphos-
phinic acid (DcyHPA) and the corresponding metal salt (3:1 ratio)
in a Schlenk flask followed by addition of chloroform. This mixture
was then refluxed under reduced pressure for about 12 h. For acti-
nide complexes (i.e. Th and U metal salts) the ligand to metal ratio
was maintained at 2:1. In all cases, colorless to pale yellow crys-
talline solids as products were obtained in quantitative yields.
The traces of solvent were removed under reduced pressure and
the obtained product was washed with 2 ꢀ 5 mL (dichloromethane
and petroleum ether (3:1)) to remove slight excess of ligand, if any.
The desired product was dried by applying high vacuum and char-
acterized by 1H, 31P, 13C NMR and FT-IR spectral data. The purity of
all the metal complexes was confirmed by elemental analyses. This
further confirms the metal to ligand ratio as 1:3 in lanthanide com-
plexes and 1:2 in actinide complexes (Chart 1). The FT-IR data for
metal complexes with DcyHPA are given in Table 3. The compar-
ison of P@O and PAOH stretches between DcyHPAand the corre-
sponding lanthanide followed by actinide complexes reveals the
strong coordination behavior of the free ligand. The IR stretch of
DcyHPA is 1159 cmꢂ1, whereas for lanthanide and actinide com-
plexes it varies from 1103 to 1157 cmꢂ1 and 1026–1056 cmꢂ1
respectively (Fig. 3 and Table 1). It was observed that all the puri-
fied metal complexes except [Th(NO3)2(DcyHPA)2] did not show
the presence of Cl and NO3 groups due to the establishment of
,
new metal–oxygen
in Chart 1.
r-bonds resulting from the ligand as depicted
Mass spectrum of the Th(IV) complex [Th(NO3)2(DcyHPA)2] was
recorded in DMSO. As shown in Fig. 4, the MS peak of the
O
N
P
O
P
O
O
O
P
O
O
O
O
La
P
O
O
O
Th
O
O
P
O
N
O
Chart 1. Proposed structures of La(III) and Th(IV) metal complexes of DcyHPA.

746
E.V. Goud et al. / Polyhedron 117 (2016) 741–748
Fig. 4. ESI Mass spectrum of [Th(NO3)2(DcyHPA)2].
HX represents the ligand, and OH form is represented as H. A close
inspection of equation DcyHPO reveals that at lower acidity, the
equilibrium will be shifted towards the product, which results in
the extraction through ion exchange mechanism. On the other
hand, at higher acidity, the higher concentration of H+ drives the
reaction to the reactant side (LeChatelier’s principle), with no
metal complex formation. The phosphoryl group is thus solely
responsible for the extraction at higher acidity.
Aczþ þ zHXðorgÞ ꢀ AcXzorg þ zHþ
ð2Þ
ðaqÞ
aq
The variation of distribution ratio for U(VI) using 0.01 M
DcyHPA in xylene under various nitric acid concentrations was
investigated and the results are plotted in Fig. 6(a). It was observed
that at lower acidity (i.e. 0.01 M HNO3), DU(VI) value for DcyHPA is
considerably large and it decreases gradually with the increase of
acid concentration. After 0.5 M nitric acid concentration, the D
value increases again and reaches a maximum at 1.5 M.
It shows a steady decrease thereafter and remains almost con-
stant from 3 to 6 M nitric acid concentration. The increase in the
D value after 0.5 M nitric acid concentration is a clear indication
that the extraction is occurring via P@O group coordination (cf.
Eq. (3)).
Fig. 5. Optimized geometry of the Thorium complex of DcyHPA at PBE0/def2-TZVP-
ECP level. Colour code: Light blue ball is thorium, orange balls are phosphorus, grey
balls are carbon, white balls are hydrogen, red colour represents oxygen and dark
blue colour is nitrogen atoms. (Color online.)
UOþ2 þ
þ 2NOꢂ3 þ 2HAðorgÞ ꢀ UO2ðNO3Þ ꢁ 2HAðorgÞ
ð3Þ
2
ðaqÞ
aq
coordination. Mulliken population analysis reveals more electron
population on the phosphoryl oxygen (ꢂ0.58 e) compared to that
derived for coordinated oxygen atoms of nitrate groups (ꢂ0.39 e).
The sterically less crowded configuration, with bidentate DcyHPA
ligands in trans-orientation, is energetically lying 4.3 kcal/mol
above the lowest energy structure. The stability of the cis-isomer
can be attributed primarily due to intramolecular dispersion
interactions.
Fig. 6(b) illustrates the variation of distribution ratios for Th(IV)
using 0.01 M solutions of DcyHPA in xylene. The D value is maxi-
mum at this acid concentration, followed by a gradual decrease
with increase in nitric acid concentration. This trend is similar to
that observed in U(VI) and suggests that the mechanism of extrac-
tion at higher acidities proceeds via solvation mechanism (i.e. coor-
dination through P@O group) [18a]. Even though the absolute
values of distribution ratios are considerably low for Am(III) (cf.
Fig. 6(c)), the trend for extraction remains the same as in the case
of U(VI) and Th(IV). These studies indicated a dual nature of extrac-
tion behavior i.e. at lower acidity, the DcyHPA behaves as an acidic
extractant to extract metal ions through cation exchange mecha-
nism while at higher acidity solvation mechanism was observed.
The results also indicate that it is feasible to extract Th(IV)
selectively in a mixture of U(VI), Th(IV) and Am(III) at lower acid
concentrations. Our studies suggested promising extraction
abilities for dicyclohexylphosphinic acid towards the extraction
4.4. Distribution studies of actinides
As DcyHPA is having two coordination sites, viz. P@O and PAOH
groups, this was further experimentally investigated as a ligand by
studying the extraction of metal ions such as, U(VI), Th(IV) and Am
(III) from nitric acid medium. At lower acidity, it follows an ion
exchange mechanism for the extraction of actinides (cf. Eq. (2)).
Here, Ac represents actinide ion, z represents the charge on it,

E.V. Goud et al. / Polyhedron 117 (2016) 741–748
747
0.08
0.06
0.04
0.02
0.00
70
60
50
40
30
20
10
0
0.01M DCPA/Xylene
160
0.1M DcyHPA/Xylene
0.01M DcyHPA/Xylene
140
120
100
80
60
40
20
0
-20
-10
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
[HNO3]
[HNO3]
[HNO3]
(a)
(b)
(c)
Fig. 6. Variation of (a) DU(VI), (b) DTh(IV) and (c) DAm(III) with equilibrium aqueous phase nitric acid concentration for DcyHPA at 303 K.
of various actinides. This extractant can find application in the
removal of uranium and thorium from solution in pH region.
Experimental studies on the stripping behavior are underway to
find a suitable reagent for effective stripping from the organic
phase and these studies will be communicated in due course.
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Sup-
plementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.poly.2016.07.013.
References
[1] W.D. Bond, Thorex process, science and technology of tributyl phosphate, in:
W.W. Schulz, L.L. Burger, J.D. Navratil, K.P. Bender (Eds.), Applications of
Tributyl phosphate in Nuclear Fuel Reprocessing, 3, CRC Press, Florida, 1990,
pp. 225–247.
[2] J. Rydberg, M. Cox, C. Musikas, G.R. Choppin (Eds.), Solvent Extraction
Principle and Practice, second ed., Marcel Dekker, New York, 2004.
[3] (a) D.L. Clark, S.S. Hecker, G.D. Jarvinen, M.P. Neu, The Chemistry of Actinide
and Transactinide Elements L.R. Morss, N.M. Edelstein, J. Fuger, (eds.), third ed.,
Springer, Netherlands, 2006, pp. 813–1279;
5. Conclusions
Dicyclohexylphosphinic acid (DcyHPA) has been synthesized by
the reaction of diethyl phosphite and cyclohexyl magnesium bro-
mide and its possible role as an extractant has been studied in
the present contribution. This molecule was characterized using
a wide range of techniques such as, FT-IR, 1H, 31P{1H} and 13C
NMR, single crystal XRD, and computational methodologies. Single
crystal XRD studies reveal that the molecule exists as a dimer in
solid state, which is also substantiated by theoretical studies. The
metal complexes of DcyHPA with lanthanides (La, Ce, Gd, Sm,
Nd) and actinides (U and Th) were prepared and structural compo-
sition was experimentally determined using spectroscopic and
analytical techniques.
(b) J.M. Joshi, P.N. Pathak, V.K. Manchanda, Solvent Extr. Ion Exch. 23 (2005)
663.
[4] D.G. Kalina, E.P. Horwitz, Solvent Extr. Ion Exch. 3 (1985) 235.
[5] C. Madic, M.J. Hudson, High-level liquid waste portioning by means of
completely in cinerable extractants, EUR 18039 EN, 1998.
[6] S. Sharma, S. Panja, A. Bhattachariya, P.S. Dhami, P.M. Gandhi, S.K. Ghosh,
Dalton Trans. 44 (2015) 12771.
[7] A. Bremer, A. Geist, P.J. Panak, Dalton Trans. 41 (2012) 7582.
[8] P.K. Mohapatra, A. Sengupta, M. Iqbal, J. Huskens, S.V. Godbole, W. Verboom,
Dalton Trans. 42 (2013) 8558.
[9] C.M. Ruff, U. Müllich, A. Geist, P.J. Panak, Dalton Trans. 41 (2012) 14594.
[10] S. Kannan, B.G. Vats, I.C. Pius, D.M. Noronha, P.S. Dhami, P.W. Naik, M. Kumar,
Dalton Trans. 43 (2014) 5252.
[11] A. Riisiö, A. Väisänen, R. Sillanpää, Inorg. Chem. 52 (2013) 8591.
[12] P.J. Panak, A. Geist, Chem. Rev. 113 (2013) 1199.
The role of DcyHPA as a possible extractant was further exper-
imentally investigated by studying the extraction of metal ions
such as, U(VI), Th(IV) and Am(III) from nitric acid medium. These
studies indicated a dual nature of extraction behavior i.e. at lower
acidity, the DcyHPA behaves as an acidic extractant and extracted
metal ions through cation exchange mechanism while at higher
acidity it was achieved via solvation mechanism. Density func-
tional theory (DFT) calculations were also applied to investigate
the geometric and electronic structure of the thorium metal com-
plex formed during extraction.
[13] (a) C.-Z. Wang, J.-H. Lan, Q.-Y. Wu, Y.-L. Zhao, X.-K. Wang, Z.-F. Chai, W.-Q. Shi,
Dalton Trans. 43 (2014) 8713;
(b) A. Sengupta, P.K. Mohapatra, M. Iqbal, J. Huskens, W. Verboom, Dalton
Trans. 41 (2012) 6970;
(c) D. Manna, S. Mula, A. Bhattacharyya, S. Chattopadhyay, T.K. Ghanty, Dalton
Trans. 44 (2015) 1332;
(d) J. Luo, C.-Z. Wang, J.-H. Lan, Q.-Y. Wu, Y.-L. Zhao, Z.-F. Chai, C.-M. Nie, W.-Q.
Shi, Dalton Trans. 44 (2015) 3227.
[14] S. Ahrland, J.O. Lijenzin, R. Rydberg, Comprehensive Inorganic Chemistry, in: J.
C. Bailar Jr., H.J. Emeleus, R. Nhylom, A.F. Trotman-Dickenson (Eds.), vol. 5,
Pergamon Press, Oxford, 1973, p. 337.
[15] D.D. Sood, S.K. Patil, J. Radio Anal. Nucl. Chem. 203 (1996) 547.
[16] (a) L.L. Burger, J. Phys. Chem. 62 (1958) 590;
Acknowledgments
Dr. ASRK is highly thankful to DAE-BRNS (INDIA) (No.
2012/37C/6/BRNS/623) for the financial assistance. Dr. Kari thanks
UGC-DAE, IGCAR-Kalpakkam, India [CSR-KN/CRS-53/2013-14/652]
for the financial support. Dr. G. Gopakumar acknowledges the
Department of Atomic Energy (DAE), India and IGCAR, Kalpakkam
for the visiting scientist fellowship, and computing time on NEHA,
the high performance computing facility at IGCAR, Kalpakkam,
India. We also thank DST-VIT-FIST for NMR and SIF-VIT University
for other instrumentation facilities.
(b) L.L. Burger, Nucl. Sci. Eng. 16 (1963) 428.
[17] C.V.S. Brahmmananda Rao, T.G. Srinivasan, P.R. Vasudeva Rao, Solvent Extr. Ion
Exch. 28 (2010) 1.
[18] (a) D. Das, E.V. Goud, S. Annam, S. Jayalakshmi, G. Gopakumar, C.V.S.
Brahmmananda Rao, N. Sivaraman, A. Sivaramakrishna, K. Vijayakrishna, RSC
Adv. 5 (2015) 107421;
(b) C.V.S. Brahmmananda Rao, S. Jayalakshmi, S. Subramaniam, N. Sivaraman,
P.R. Vasudeva Rao, Radiochim. Acta 103 (2015) 345;
(c) E.V. Goud, B.B. Pavankumar, Y. Shruthi, A. Paul, A. Sivaramakrishna, K.
Vijayakrishna, C.V.S. Brahmmananda Rao, K.N. Sabharwal, H.S. Clayton, J.
Coord. Chem. 66 (2013) 2647;
(d) B.B. Pavankumar, E.V. Goud, R. Selvakumar, S.K. Ashok Kumar, A.
Sivaramakrishna, K. Vijayakrishna, C.V.S. Brahmamnanda Rao, K.N.
Sabharwal, P.C. Jha, RSC Adv. 5 (2015) 4727–4736;
Appendix A. Supplementary data
(e) E.V. Goud, B.B. Pavankumar, B.H.P. Reddy, K. Vijayakrishna, A.
Sivaramakrishna, K.N. Sabharwal, C.V.S.B. Rao, J. Ind. Chem. Soc. 92 (2015) 760.
[19] A.E.V. Gorden, M.A. DeVore II, B.A. Maynard, Inorg. Chem. 52 (2013) 3445.
[20] (a) G.M. Sheldrick, SHELXL-97 and SHELXS-97, University of Gottingen, Germany,
1997;
CCDC 1443194 contains the supplementary crystallographic
data. These data can be obtained free of charge via http://www.
ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
(b) G.M. Sheldrick, SADABS, Multi-scan Absorption Correction Program for Area
Detector Data, University of Gottingen, Germany, 1996.

748
E.V. Goud et al. / Polyhedron 117 (2016) 741–748
[33] (a) J.A.R. Luft, T. Winkler, F.M. Kessabi, K.N. Houk, J. Org. Chem. 73 (2008)
[21] L.J. Barbour, J. Supramol. Chem. 1 (2001) 189.
[22] T. Cecconie, H. Freiser, Solv. Extr. Ion Exch. 7 (1989) 15.
[23] M. Benedict, T.H. Pigford, H.W. Levi, Nuclear Chemical Engineering, McGraw
Hill Book Company, New York, 1981, pp. 172–174.
8175;
(b) Z. Rappoport, J.F. Liebman, The Chemistry of Cyclobutanes, John Wiley &
Sons Ltd., 2005. Chapter 10.
[24] J.A. Perez-Bustamante, F. Delgado, Analyst 96 (1971) 407.
[25] M.J. Frisch, G.W. Trucos, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, et al., Gaussian 09 Revision
A 02 Gaussian Inc., Wallingford CT, 2009.
[34] (a) C.I. Sainz-Diaz, H. Klocker, R. Marr, H.-J. Bart, Hydrometallurgy 42 (1996) 1;
(b) A. Dumoulin, C. Lalli, P. Retailleau, G. Masson, Chem. Commun. (2015)
5383.
[35] (a) M.I. Baeza, The University of Texas at El Paso. Department of Chemistry,
Synthesis and Characterization of Acetylenic Derivatives of the Actinide
Extractant (aryl)-N, N-di-(alkyl)carbamoylmethyl phosphine Oxide (CMPO),
ProQuest, 2008. 0549989420, 9780549989424;
[26] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.
[27] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B. 37 (1988) 785.
[28] S.H. Vosko, L. Wilk, M. Nusair, Can J. Phys. 58 (1980) 1200.
[29] R. Ditchfied, Mol. Phys. 27 (1974) 789.
[30] C. Adamo, V. Barone, J. Chem. Phys. 110 (1999) 6158.
[31] (a) W. Kuechle, M. Dolg, H. Stoll, H. Preuss, J. Chem. Phys. 100 (1994) 7535;
(b) X. Cao, M. Dolg, H. Stoll, J. Chem. Phys. 118 (2003) 487;
(c) X. Cao, M. Dolg, J. Mole. Struct. (Theochem.) 673 (2004) 203.
[32] (a) S. Grimme, S. Ehrlich, L. Goerigk, J. Comput. Chem. 32 (2011) 1456;
(b) S. Grimme, J. Antony, S. Ehrlich, H. Krieg, J. Chem. Phys. 132 (2010)
154104;
(b) J.F. Malone, D.J. Marrs, M.A. McKervey, P. O’Hagan, N. Thompson, A.
Walker, F. Arnaud-Neu, O. Mauprivez, M.-J. Schwing-Weill, J.-F. Dozol, H.
Rouquette, N. Simon, J. Chem. Soc. Chem. Commun. (1995) 2151;
(c) C.A. Tolman, Chem. Rev. 77 (1977) 313.
[36] J.G. Verkade, L.D. Quin, Methods in Stereochemical Analysis Volumes in the
Series: Phosphorus 31-NMR Spectroscopy in Stereochemical Analysis, VHC
Publishers, Deerfield Beach, Florida, 1987.
[37] Spectrochim. Diwaker, Acta A: Mol. Biomol. Spectr. 128 (2014) 819.
(c) F. Neese, Wiley Interdiscip. Rev. Comput. Mol. Sci. 2 (2012) 73–78.