2,6-Bis(5-(2,2-dimethylpropyl)-1H-pyrazol-3-yl)pyridine as a ligand for efficient actinide(III)/lanthanide(III) separation

Article abstract of DOI:10.1021/ic3000526

The N-donor complexing ligand 2,6-bis(5-(2,2-dimethylpropyl)-1H-pyrazol-3- yl)pyridine (C5-BPP) was synthesized and screened as an extracting agent selective for trivalent actinide cations over lanthanides. C5-BPP extracts Am(III) from up to 1 mol/L HNO₃ with a separation factor over Eu(III) of approximately 100. Due to its good performance as an extracting agent, the complexation of trivalent actinides and lanthanides with C5-BPP was studied. The solid-state compounds [Ln(C5-BPP)(NO₃)₃(DMF)] (Ln = Sm(III), Eu(III)) were synthesized, fully characterized, and compared to the solution structure of the Am(III) 1:1 complex [Am(C5-BPP)(NO₃) ₃]. The high stability constant of log β₃ = 14.8 ± 0.4 determined for the Cm(III) 1:3 complex is in line with C5-BPP's high distribution ratios for Am(III) observed in extraction experiments.

Full text of DOI:10.1021/ic3000526

Article
pubs.acs.org/IC
2,6-Bis(5-(2,2-dimethylpropyl)-1H-pyrazol-3-yl)pyridine as a Ligand
for Efficient Actinide(III)/Lanthanide(III) Separation
†,‡
†
†
Antje Bremer, Christian M. Ruff, †,‡ Denise Girnt,§ Udo Mullich, Jorg Rothe, Peter W. Roesky,*,§
̈
̈
Petra J. Panak,*, †,‡ Alexei Karpov,∥,⊥ Thomas J. J. Muller,∥ Melissa A. Denecke, and Andreas Geist*, †
†
̈
†Institut fur Nukleare Entsorgung, Karlsruher Institut fur Technologie (KIT), P.O. Box 3640, 76021 Karlsruhe, Germany
̈
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‡Institut fur Physikalische Chemie, Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany
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§Institut fur Anorganische Chemie, Karlsruher Institut fur Technologie (KIT), Engesserstraße 15, 76131 Karlsruhe, Germany
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∥Institut fur Organische Chemie und Makromolekulare Chemie, Heinrich-Heine-Universitat Dusseldorf, Universitatsstraße 1, 40225
̈
̈
̈
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Dusseldorf, Germany
̈
S
* Supporting Information
ABSTRACT: The N-donor complexing ligand 2,6-bis(5-(2,2-dimethylpropyl)-1H-
pyrazol-3-yl)pyridine (C5-BPP) was synthesized and screened as an extracting
agent selective for trivalent actinide cations over lanthanides. C5-BPP extracts
Am(III) from up to 1 mol/L HNO3 with a separation factor over Eu(III) of
approximately 100. Due to its good performance as an extracting agent, the
complexation of trivalent actinides and lanthanides with C5-BPP was studied. The
solid-state compounds [Ln(C5-BPP)(NO3)3(DMF)] (Ln = Sm(III), Eu(III)) were
synthesized, fully characterized, and compared to the solution structure of the
Am(III) 1:1 complex [Am(C5-BPP)(NO3)3]. The high stability constant of log β3
= 14.8 0.4 determined for the Cm(III) 1:3 complex is in line with C5-BPP’s high
distribution ratios for Am(III) observed in extraction experiments.
extraction of trivalent actinides but also at studying them
from a more fundamental point of view; this is true for BTP
and BTBP in particular.13−29 The goal of such fundamental
studies is understanding their selectivity on a molecular level,
which in turn could have a positive impact on the development
of new compounds with improved properties.
Following the approach of slightly modifying the BTP
skeleton, we synthesized 2,6-bis(5-(2,2-dimethylpropyl)-1H-
pyrazol-3-yl)pyridine (C5-BPP; Scheme 2).30−33 The structure
of C5-BPP can be viewed as intermediate between the BTPs
and 6-(3,5-dimethyl-1H-pyrazol-1-yl)-2,2′-bipyridine (dmpbi-
py), a ligand with weak complexing power for the trivalent
actinides studied earlier.34
In the following, results from liquid−liquid extraction studies
for Am(III) and Eu(III) are presented and the solid-state
structures of C5-BPP complexes with Sm(III) and Eu(III) are
described. These structures are compared to the solution
structure of an Am(III) complex characterized by extended X-
ray absorption fine structure (EXAFS). Stability constants of
the Cm(III) 1:1, 1:2, and 1:3 complexes are determined by
time-resolved laser fluorescence spectroscopy (TRLFS) and
compared to those for the Cm(III)-BTP26 and Cm(III)-
dmpbipy34 complexes.
INTRODUCTION
■
Chemical liquid−liquid extraction is a widely and successfully
applied technique for separating ionic solutes. However,
customary extracting agents which coordinate metal ions via
oxygen donor atoms are not useful for separating trivalent
actinide and lanthanide ions, due to the similar chemistry of
these elements. Their separation is an essential part of
innovative nuclear fuel cycles, which are under development
in many countries.1−3
Nitrogen heterocycle compounds such as terpyridine
derivatives form stronger complexes with trivalent actinides
than with lanthanides, initiating the search for new N-donor
extracting agents.4,5 Unfortunately, most of these compounds
do not work in solutions of relatively high nitric acid
concentrations (0.5 mol/L and greater), which is a process
requirement. Alkylated 2,6-ditriazinylpyridines (BTP),6−8 alky-
lated 6,6′-ditriazinylbipyridines (BTBP),9−11 and their phenan-
throline derivates (BTPhen)12 have been developed (Scheme
1) and are able to directly extract trivalent actinides from nitric
acid solutions with high selectivity over the lanthanides.
Typically, separation factors for Am(III) over Eu(III) (as
representatives of trivalent actinides and lanthanides) of 100−
300 are achieved.
Despite huge progress being made, all of the known N-donor
extracting agents have some unfavorable properties, and the
search for even better compounds continues.
Recent efforts are directed not solely at synthesizing new N-
donor extracting agents and testing them for selective
Received: January 10, 2012
Published: April 19, 2012
© 2012 American Chemical Society
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Scheme 1. BTP, BTBP, and BTPhen
Syntheses. All manipulations of air-sensitive materials were
performed with rigorous exclusion of oxygen and moisture in flame-
dried Schlenk-type glassware either on a dual-manifold Schlenk line,
interfaced to a high-vacuum (10−3 Torr) line, or in an argon-filled
glovebox (MBraun GmbH). Deuterated solvents (≥99 atom % D)
were obtained from Eurisotop or Aldrich. Ln(NO3)3·6H2O was
obtained from Aldrich.
Scheme 2. C5-BPP
Pyridine-2,6-dicarboxylic Acid Diethyl Ester. A 10.0 g amount
(59.9 mmol) of pyridine-2,6-dicarboxylic acid and 2 mL of
concentrated H2SO4 were refluxed for 3 h in 70 mL of dry ethanol
and stored overnight at ambient temperature. The solvent was
removed under reduced pressure. The colorless, viscous residue was
dissolved in dichloromethane and neutralized with aqueous NaHCO3
solution. The aqueous phase was extracted three times with 40 mL of
dichloromethane. The combined organic phases were washed three
times with 40 mL of water and dried with Na2SO4. The solvent was
removed under reduced pressure, and the product was obtained as
colorless crystals. Yield: 9.4 g (42.2 mmol, 70%). Melting point: 45−
46.5 °C. 1H NMR (400.2 MHz, methanol-d4, 300 K): δ 1.43 (t, 6H, 3J
= 7.2 Hz, CH3), 4.46 (q, 4H, 3J = 7.2 Hz, CH2), 8.14 (dd, 1H, 3J = 8.3
EXPERIMENTAL SECTION
■
Instrumentation and Measurements. NMR spectra were
recorded on a Bruker Avance III 400 MHz or a Bruker Avance II
300 MHz spectrometer. Chemical shifts are referenced to internal
solvent resonances and are reported relative to tetramethylsilane (1H
NMR). IR spectra were obtained on a Bruker Tensor 37 FT-IR
instrument. Mass spectra were recorded at 70 eV on a Varian Mat SM
11 instrument. Elemental analyses were carried out with an Elementar
vario EL or vario MICRO cube apparatus. Melting points were
measured in open glass capillaries in a Stuart SMP30 melting point
apparatus.
TRLFS measurements were performed using a Nd:YAG (Surelite,
Continuum) pumped dye laser system (Narrow Scan G-R, Radiant
Dyes). A wavelength of 396.6 nm was used for excitation of Cm(III).
Emission spectra were recorded in the range of 565−645 nm after a
delay time of 1.0 μs to discriminate the Cm emission from the short-
lived fluorescence of the organic ligand. The fluorescence emission was
detected by a spectrograph (Shamrock 303i, ANDOR) with a 900
lines/mm grating and an ICCD Camera (iStar Gen III, ANDOR). The
cuvette holder is temperature controlled; all measurements were
performed at T = 20 °C.
3
3
Hz, J = 7.3 Hz, Py H), 8.29 (d, 2H, J = 7.9 Hz, Py H).
1,1′-(Pyridine-2,6-diyl)bis(5,5-dimethylhexane-1,3-dione). 4,4-Di-
methylpentane-2-one (16.0 mmol, 2.3 mL) and a solution of sodium
methoxide in methanol (30%, 20.0 mmol, 3.5 mL) were stirred for 30
min at room temperature under an argon atmosphere. A solution of
1.7 g (7.5 mmol) of pyridine-2,6-dicarboxylic acid diethyl ester in 40
mL of freshly distilled diethyl ether was added dropwise, and the
reaction mixture was refluxed for 3 h. Subsequently it was cooled to
room temperature and was neutralized with glacial acetic acid. The
organic phase was washed with water (3 × 30 mL), dried with Na2SO4,
filtered, and concentrated in vacuo. The crude product was obtained as
Am L3 edge X-ray absorption fine structure (XAFS) spectra were
recorded at the INE-Beamline for actinide research at the ANKA
synchrotron light source.35 Higher harmonic radiation in the incident
X-ray beam was suppressed by detuning the double crystal
monochromator (DCM)equipped with a pair of Ge ⟨422⟩
crystalsto 70% of maximum photon flux peak intensity. The
incident intensity was measured by an Ar-filled ionization chamber at
ambient pressure. The Am L3 scans were calibrated against the first
inflection point in the X-ray absorption near-edge structure (XANES)
spectrum of a Nb foil, defined as 18 986 eV. Spectra were measured in
fluorescence yield detection mode, recording Am Lα fluorescence
radiation with a 5 pixel energy-dispersive solid-state Ge detector
(Canberra LEGe). Six scans were collected and averaged. Standard
data reduction and least-squares fit techniques36 using the ATHENA37
and UWXAFS38 program packages were applied for EXAFS data
analysis. The k2-weighted EXAFS function χ(k) was obtained following
pre-edge background subtraction, normalization, μ0 spline function
fitting, and conversion from energy to k space using the energy of the
white line maximum to define the ionization energy (E0). Metric
parameters describing the Am(III) coordination (coordination
numbers (N), interatomic distances (R), mean square radial
displacements or EXAFS Debye−Waller factors (σ2), and relative
shifts in ionization potential (ΔE0)) were obtained by least-squares fits
1
a yellowish solid and was used without further purification. H NMR
(400.2 MHz, methanol-d4, 300 K): δ 1.09 (s, 18H, CH3), 2.39 (s, 4H,
CH2C(CH3)3), 6.90 (s, 4H, CH2), 8.11 (dd, 1H, 3J = 8.3 Hz, 3J = 7.4
3
Hz, Py H), 8.23 (d, 2H, J = 7.6 Hz, Py H).
2,6-Bis-(5-(2,2-dimethylpropyl)-1H-pyrazol-3-yl)-pyridine (C5-
BPP). A 1.5 g amount of crude 1,1′-(pyridine-2,6-diyl)bis(5,5-
dimethylhexane-1,3-dione) and 1.3 mL of hydrazine hydrate (80% in
H2O, 21 mmol) were dissolved in 150 mL of methanol and refluxed
for 2 h. The solvent was removed under reduced pressure. The residue
was washed with water and diethyl ether (3 × 30 mL each) and dried
under reduced pressure. The product was obtained as a colorless
powder. Yield (over two steps): 2.2 g, 6.4 mmol, 40%. Melting point:
1
259−261 °C. H NMR (400.2 MHz, methanol-d4, 300 K): δ 1.00 (s,
18H, Ha), 2.60 (s, 4H, Hc), 6.75 (s, 2H, H4), 7.72 (d, 2H, 3J = 7.7 Hz,
3
1
H3′, H5′), 7.85 (t, 1H, J = 7.7 Hz, H4′) ppm. H NMR (300.1 MHz,
THF-d8, 298 K): δ 0.98 (s, 18H, Ha), 2.55 (s, 4H, Hc), 6.70 (s, 2H,
H4), 7.70 (br s, 3H, H3′, H4′, H5′), 11.92−12.27 (br m, 2H, NH, H1)
ppm. 13C{1H} NMR (75.5 MHz, THF-d8, 298 K): δ 30.0 (Ca), 32.1
(Cb), 42.2 (Cc), 104.5 (C4), 118.5 (C3′, C5′), 141.5 (C5), 143.4 (C3),
151.8 (C2′, C6′) ppm. IR (ν/cm−1): 2957 (w), 1654 (w), 1574 (w),
1460 (m), 1259 (m), 1090 (m), 1012 (m), 796 (s), 753 (w), 737 (w),
661 (w). MS (EI, 70 eV, 170 °C): m/z (%) 351 [M+, (11)], 336 [M+
− CH3, (6)], 294 [M+ − C4H9, (10)], 209 [M+ − C10H22, (6)], 137
[C8H13N2, (26)], 71 [C5H11, (26)], 57 [C4H9, (45)]. HRMS: calcd for
C21H29N5, 351.2479; found, 351.1892.
2
of the data to the EXAFS equation. The amplitude reduction factor S0
was held constant at 1.0. Theoretical scattering phase shift and
backscattering amplitude functions used in the fits were obtained with
the ab initio multiple-scattering code feff (v8.4)39 based on atomic
clusters derived for Eu(NO3)3(C6H14O4)40 and the [Ln(C5-BPP)-
(NO3)3(DMF)] structure 2 (see Figure 2), where Am (Z = 95) was
replaced by Eu.
[Sm(C5-BPP)(NO3)3(DMF)] (1). C5-BPP (216 mg, 0.61 mmol) and
Sm(NO3)3·6H2O (91 mg, 0.21 mmol) were dissolved in 3 mL of
DMF and stirred at 41 °C for 37 h. Then, the subsequent mixture was
filtered. After removal of the solvent, the white precipitate was
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Scheme 3. Synthesis of C5-BPP
recrystallized from DMF/diethyl ether (1:10), yielding colorless
crystals of 1. Yield: 99 mg, 0.13 mmol, 64%. H NMR (300.1 MHz,
Positional parameters, hydrogen atom parameters, thermal param-
eters, and bond distances and angles have been deposited as
Supporting Information.
Liquid−Liquid Extraction. C5-BPP’s performance as an extract-
ing agent was determined by measuring the distribution of 241Am(III)
and 152Eu(III) between an organic phase containing C5-BPP and an
aqueous phase containing nitric acid, using γ counting. 241Am and
152Eu are radionuclides with distinctive γ lines which are easily
detected. No sample preparation is required.
The organic phase was a solution of 10 mmol/L of C5-BPP + 0.5
mol/L of 2-bromohexanoic acid in kerosene prepared by dissolving
weighed amounts of C5-BPP and 2-bromohexanoic acid in the diluent.
The aqueous phase was 241Am(III) + 152Eu(III) in 0.2−2.0 mol/L
HNO3 prepared by adding 30 μL of a stock solution (60 kBq/mL
241Am(III) + 100 kBq/mL 152Eu(III) in 0.1 mol/L HNO3) to 1470 μL
of HNO3.
Equal volumes (1.5 mL each) of organic and aqueous phases were
contacted 45 min at 20 °C using an orbital shaker (500 rpm). It was
established that 45 min was sufficient to attain equilibrium.33 Samples
were centrifuged to separate phases. 241Am and 152Eu activities were
determined in 1.0 mL aliquots of each phase by γ counting (Packard
Cobra Auto Gamma 5003).
1
THF-d8, 298 K): δ 0.97 (s, 18H, Ha), 2.70 (s, 4H, Hc), 2.75 (s, 3H,
CH3, DMF), 2.88 (s, 3H, CH3, DMF), 6.66 (s, 2H, H4), 7.95 (s, 1H,
CH, DMF), 8.03 (d, 3J = 7.7 Hz, 2H, H3′, H5′), 8.25 (t, 3J = 7.7 Hz, 1H,
H4′), 10.98 (br s, 2H, H1) ppm. 13C{1H} NMR (75.5 MHz, THF-d8,
298 K): δ 29.6 (Ca), 31.1 (CH3, DMF), 32.2 (Cb), 36.1 (CH3, DMF),
39.6 (Cc), 103.2 (C4), 118.5 (C3′, C5′), 141.4 (C5), 144.5 (C3), 153.4
(C2′, C6′), 162.6 (CHO, DMF) ppm. IR (ν/cm−1): 2956 (w), 1650
(w), 1575 (w), 1462/1426 (m(sp)), 1240/1228 (m(sp)), 1115/1098
(m(sp)), 1037/1026/1010 (m(sp)), 828/812 (s(sp)), 758 (w), 736
(w), 735 (w), 675 (m). Anal. Calcd for C24H36N9O10Sm·DMF (1,
835.23): C, 38.88; H, 5.20; N, 16.79. Found: C, 39.19; H, 5.25; N,
16.46.
[Eu(C5-BPP)(NO3)3(DMF)] (2). C5-BPP (180 mg, 0.51 mmol) and
Eu(NO3)3·6H2O (76 mg, 0.17 mmol) were dissolved in 3 mL of DMF
and stirred at 48 °C for 120 h. Then, the subsequent mixture was
filtered. After removal of the solvent, the white precipitate was
recrystallized from DMF/diethyl ether (1:10) to yield colorless
crystals of 2. Yield: 116 mg, 0.15 mmol, 90%. IR (ν/cm−1): 2957 (w),
1654 (w), 1574 (w), 1460 (w), 1259 (s), 1090 (s), 1012 (s), 796 (vs),
753 (w), 736 (w), 661 (w). Anal. Calcd for C24H36N9O10Eu·3DMF (2;
981.84): C, 40.37; H, 5.85; N, 17.12. Found: C, 40.87; H, 5.95; N,
16.88.
Am(III) and Eu(III) distribution ratios DAm(III) and DEu(III) are
DM(III) = [M(III)org]/[M(III)aq]. They are simply calculated from the
organic phase and aqueous phase count rates of the respective
X-ray Crystallographic Studies of 1 and 2. Crystals of 1 and 2
were obtained from DMF/diethyl ether (1:10). A suitable crystal of
compound 1 or 2 was covered in mineral oil (Aldrich) and mounted
onto a glass fiber. The crystal was transferred directly to the −73 or
−123 °C N2 cold stream of a Stoe IPDS 2 diffractometer.
radionuclide. The separation factor is SFAm(III)/Eu(III) = DAm(III)/DEu(III)
.
Distribution ratios (which were 10−3 < D < 103, see Results and
Discussion) typically are reproducible within 20%.42
TRLFS Titration. The TRLFS titration experiments were
performed using a stock solution of 6.67 × 10−6 mol/L Cm(III)
(89.7% 248Cm, 0.1% 247Cm, 9.4% 246Cm, 0.1% 245Cm, 0.3% 244Cm,
and 0.4% 243Cm) in 0.03 mol/L of HClO4. A 15 μL portion of the Cm
stock solution was dissolved in 985 μL of methanol. The initial
Cm(III) concentration was 1.0 × 10−7 mol/L. Ligand solutions were
prepared by dissolving 3.51 mg of C5-BPP in 985 μL of methanol and
subsequent dilution with 15 μL of distilled water (to maintain a
constant water content during titration), resulting in a 10−2 mol/L
stock solution. For the titration experiments C5-BPP solutions with
concentrations between 10−6 and 10−2 mol/L were prepared and
successively added. The resulting solutions were allowed to equilibrate
for 10 min before measurement to ensure equilibration. The C5-BPP
concentration ranged from 0 to 4.68 × 10−3 mol/L.
EXAFS Sample Preparation. The Am(III)-C5-BPP complex was
measured in solution. A 35 μL portion of an Am(III) stock solution
(aqueous 0.5 M HNO3 solution containing 30 MBq/mL of 243Am and
17 MBq/mL of 241Am, corresponding to 17 mmol/L of Am) was
gently heated in a glass vial to near dryness and then 300 μL of 10
mmol/L C5-BPP in 2-propanol added (2-propanol had to be used
instead of methanol because of limited solubility; however, the
complexation properties of methanol and 2-propanol are quite
similar). A 250 μL amount of the resulting solution (2 mmol/L of
Am), sample 3, was transferred to a polypropylene-capped vial, the cap
All structures were solved by the Patterson method (SHELXS-
97).41 The remaining non-hydrogen atoms were located from
successive difference Fourier map calculations. The refinements were
carried out by using full-matrix least-squares techniques on F2,
minimizing the function (Fo − Fc)2, where the weight is defined as
4Fo /2(Fo ) and Fo and Fc are the observed and calculated structure
factor amplitudes using the program SHELXL-97.41 The hydrogen
atom contributions of compounds 1 and 2 were calculated but not
refined. The residual electron densities in each case were of no
chemical significance.
2
2
Crystal Data for 1: C24H36N9O10Sm·3C3H7NO, Mr = 979.75,
triclinic, a = 9.9870(6) Å, b = 16.2280(10) Å, c = 16.2640(11) Å, α =
115.420(5)°, β = 100.090(5)°, γ = 98.440(5)°, V = 2269.7(2) Å3, T =
150(2) K, space group P1, Z = 2, μ(Mo Kα) = 1.363 mm−1, 42 592
̅
reflections measured, 9644 independent reflections (Rint = 0.1684).
The final R1 value was 0.0476 (I > 2σ(I)). The final Rw(F2) value was
0.1194 (all data). The goodness of fit on F2 was 0.968.
Crystal Data for 2: C24H36N9O10Eu·3C3H7NO, Mr = 981.87,
triclinic, a = 10.074(2) Å, b = 15.930(3) Å, c = 16.520(3) Å, α =
112.71(3)°, β = 101.94(3)°, γ = 98.08(3)°, V = 2320.2(8) Å3, T =
200(2) K, space group P1, Z = 2, μ(Mo Kα) = 1.420 mm−1, 18 504
̅
reflections measured, 9545 independent reflections (Rint = 0.0709).
The final R1 value was 0.0492 (I > 2σ(I)). The final Rw(F2) value was
0.0977 (all data). The goodness of fit on F2 was 0.808.
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sealed shut with epoxy glue, and the sealed vial placed in the INE-
Beamline standard sample containment chamber for measurement.
mol/L nitric acid under the experimental conditions used. The
selectivity for Am(III) over Eu(III) is very good, in the range of
SFAm(III)/Eu(III) ≈ 100, which lies in the range of separation
factors found for BTP and BTBP.7,8,11 Am(III) back-extraction
(i.e., DAm(III) < 1) requires a nitric acid concentration of >1
mol/L. Unfortunately, a precipitate occurs for ≥1.3 mol/L
nitric acid. Nevertheless, a more complete solvent extraction
study33 shows that the precipitation issue is solved by
modifying the diluent. The C5-BPP ligand also shows good
chemical stability toward high nitric acid concentrations and
fast extraction kinetics. This study also reveals that slope
analysis is not suitable to determine the composition of the
extracted complexes, due to strong ligand aggregation and
ligand−ligand interaction.
The slopes for log DAm(III) and log DEu(III) vs log [HNO3] are
−3.4 and −3.6, respectively. This is in agreement with a cation-
exchange mechanism, where a slope of 3 is expected for log
DAm(III) or log DEu(III) vs pH. The steeper slopes in Figure 1 are
likely due to the fact that distribution ratios are simply plotted
vs the nitric acid concentration without taking into account
activity coefficients.
RESULTS AND DISCUSSION
■
Ligand Synthesis. C5-BPP was synthesized in three steps
(Scheme 3) starting from pyridine-2,6-dicarboxylic acid, which
was converted to the corresponding diethyl ester in the first
step. In a Claisen condensation the diethyl ester was treated
with 4,4-dimethylpentan-2-one in the presence of sodium
methoxide to give the corresponding β-diketone, which was
reacted without further purification with hydrazinium hydrate
in CH2Cl2 to give the desired product C5-BPP in good yield.
1
The purity of the desired product was confirmed by H and
13C{1H} NMR spectroscopy.
Liquid−Liquid Extraction. Prior screening tests have
shown that, in contrast to BTP7,8 and BTBP,11 C5-BPP does
not extract Am(III) and Eu(III) nitrates from nitric acid by
solvation according to
M3+ + 3NO3− + nC5‐BPP
aq
org
aq
→ M(NO3)3(C5‐BPP)n org
To our knowledge, these results show that C5-BPP is able to
extract Am(III) from more acidic solutions with better
selectivity over Eu(III) than any other N-donor extracting
agent in combination with 2-bromocarboxylic acid.4
However, C5-BPP does extract Am(III) with high selectivity
over Eu(III) in the presence of a lipophilic anion source such as
a 2-bromocarboxylic acid by a cation-exchange mechanism:32,33
Sm(III) and Eu(III) Complex Structures. Stirring a 3:1
mixture of C5-BPP with Ln(NO3)3·6H2O (Ln = Sm(III),
Eu(III)) in DMF at elevated temperature for a prolonged time
did not result in the formation of complexes with the central
metal ion coordinated by three ligands. Instead, the
monoligated compounds [Ln(C5-BPP)(NO3)3(DMF)] (Ln =
Sm(III) (1), Eu(III) (2)) were obtained as colorless crystals
after recrystallization from DMF/diethyl ether (1:10) (Scheme
4). These results are in contrast with those for the related
ligand 2,6-bis(1H-pyrazol-3-yl)pyridine with LnCl3·6H2O (Ln
= Eu(III), Gd(III), Tb(III), Ho(III)) in methanol, which upon
treatment with KPF6 yields complexes with three ligands
coordinating to the lanthanide atom.43 Obviously, the 2,6-
bis(1H-pyrazol-3-yl)pyridine ligand substitutes water molecules
M3+ + nC5‐BPP + 3HAorg
⇌ MA3(C5‐BPP)n org + 3H+
aq
org
aq
The dependence of Am(III) and Eu(III) distribution ratios
and the separation factor on nitric acid concentration is shown
in Figure 1. Am(III) is extracted (i.e., DAm(III) > 1) from <1
−
of the lanthanides atoms but not the NO3 anions (see below
for the solid-state data). Similar observations were recently
made using 6-(3,5-dimethyl-1H-pyrazol-1-yl)-2,2′-bipyridine
(dmpbipy) as ligand;34 in this case NO3 anions were also
−
not replaced. This observation is significant, since the
separation of actinides from lanthanides, e.g., in the SANEX
process, is usually performed in HNO3 solutions.
The new complexes were characterized by standard analytical
and spectroscopic techniques, and the solid-state structure was
established by single-crystal X-ray diffraction. As result of the
paramagnetic metals only NMR data of compound 1 could be
obtained. As expected, the peaks are broadened. Most
characteristic are the signals of the tBu group of the alkyl
Figure 1. Extraction of Am(III) and Eu(III): dependence of Am(III)
and Eu(III) distribution ratios and separation factor on initial nitric
acid concentration. Aqueous phase: 241Am(III) + 152Eu(III) in HNO3.
T = 293 K.
Scheme 4. Synthesis of 1 and 2
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chain, which are observed at δ(1H) 0.78 ppm and δ(13C{1H})
32.5 ppm, only slightly shifted from the values observed for
noncoordinated C5-BPP. In the aromatic region two typical
signals for the pyridine ring and one signal for the pyrazolyl ring
were observed.
Compounds 1 and 2 are isostructural, both crystallizing in
the triclinic space group P1 and having two molecules of the
̅
complex in the unit cell (Figure 2). The central metals are 10-
Figure 3. Normalized fluorescence spectra of Cm(III) in methanol
with increasing C5-BPP concentration. [Cm(III)]ini = 1 × 10−7 mol/L;
[C5-BPP] = (0−4.68) × 10−3 mol/L.
603.7, 607.7, and 611.6 nm. The emission band of the
[Cm(solv)]3+ species decreases simultaneously. The spectro-
scopic results show that three different Cm-C5-BPP species are
formed in the ligand concentration range of 9.9 × 10−8−4.7 ×
10−3 mol/L. These are attributed to the 1:1, 1:2, and 1:3
[Cm(C5-BPP)n]3+ complexes with n = 1−3 (see below).
The fluorescence spectra are analyzed by peak deconvolution
using the spectra of the four single components ([Cm(solv)]3+
and [Cm(C5-BPP)n]3+; n = 1−3) displayed in Figure 4. The
resulting speciation diagram is shown in Figure 5.
Figure 2. Solid-state structure of 2 showing the atom-labeling scheme,
omitting hydrogen atoms. Selected bond lengths (Å) and angles (deg)
(also given for the isostructural compound 1): Eu−O1 = 2.478(4),
Eu−O2 = 2.551(4), Eu−O4 = 2.503(4), Eu−O5 = 2.520(4), Eu−O7
= 2.549(5), Eu−O8 = 2.550(4), Eu−O10 = 2.388(4), Eu−N2 =
2.545(4), Eu−N3 = 2.622(4), Eu−N4 = 2.541(5); N2−Eu−N3 =
62.38(14), N2−Eu−N4 = 124.70(14), N3−Eu−N4 = 62.41(14);
Sm−O1 = 2.518(3), Sm−O2 = 2.525(4), Sm−O4 = 2.594(3), Sm−
O5 = 2.545(4), Sm−O7 = 2.604(4), Sm−O8 = 2.497(3), Sm−O10 =
2.348(3), Sm−N2 = 2.548(4), Sm−N3 = 2.628(3), Sm−N4 =
2.583(4); N2−Sm−N3 = 62.65(11), N2−Sm−N4 = 124.81(11), N3−
Sm−N4 = 62.45(11).
fold coordinated by three κ2O,O′-NO3 groups, the tridentate
−
C5-BPP ligand, and one DMF molecule, forming a distorted
hexadecahedron (see Figure S1 in the Supporting Informa-
tion).44 The Ln−O bond distances of the nitro groups are
within the expected ranges of 2.518(3)−2.604(4) Å (1) and
2.478(4)−2.549(5) (2) Å.34 The bite angles of the C5-BPP
ligand are N2−Ln−N3 = 62.65(11)° (1) and 62.38(14)° (2),
N2−Ln−N4 = 124.81(11)° (1) and 124.70(14)° (2), and N3−
Ln−N4 = 62.45(11)° (1) and 62.41(14)° (2). The Ln−N
bond length of the pyridine ring (Ln−N3 = 2.628(3) Å (1),
2.622(4) Å (2)) is slightly longer than the Ln−N bond
distances of the five-membered pyrazolyl ring (Ln−N2 =
2.548(4) Å (1) and 2.545(4) Å (2), Ln−N4 = 2.584(4) Å (1)
and 2.541(5) Å (2)). A comparison of compound 2 with the 9-
fold coordinated tris(2,6-bis(1H-pyrazol-3-yl)pyridine)euro-
pium cation43 does not reveal any significant differences in
their Eu−N bond distances.
Figure 4. Normalized fluorescence spectra of the pure components
[Cm(solv)]3+ and [Cm(C5-BPP)n]3+ (n = 1−3).
TRLFS. The evolution of the Cm(III) fluorescence emission
At low ligand concentrations [Cm(solv)]3+ is the dominant
species. With increasing C5-BPP concentration the 1:1 Cm-C5-
BPP complex is formed and dominates in the concentration
range 2 × 10−7−3 × 10−5 mol/L. At concentrations greater
than 10−4 mol/L the 1:3 Cm-C5-BPP complex is the dominant
species. In the concentration range 10−6−10−3 mol/L the 1:2
Cm-C5-BPP complex is also present, with a relative fraction of
approximately 40% at 10−4 mol/L. Figure 5 also includes the
relative species distribution calculated with the log Kn (n = 1−
3) values determined below. Comparison with the experimental
data shows an excellent agreement in the concentration range
10−7−2 × 10−4 mol/L. At the highest ligand concentrations
studied, the experimental values differ significantly from the
calculated curve, which is due to colloid formation observed for
[C5-BPP] ≥ 10−4 mol/L.
resulting from the 6D′7/2
→
8S′7/2 transition in methanol
(containing 3.3 mol % of water from the Cm(III) stock
solution) and in the presence of increasing amounts of C5-BPP
is shown in Figure 3. The spectra are normalized to the same
peak area for better comparison. At zero ligand concentration a
broad emission band at 599.3 nm with a shoulder at 603.7 nm
is observed, which originate from different [Cm(solv)]3+
species with varying numbers of methanol and/or water
molecules in the inner coordination sphere. According to ref
45, the spectrum of [Cm(solv)]3+ results from two different
species with nine methanol and eight methanol/one water
molecules in the inner coordination sphere, respectively. With
increasing C5-BPP concentration the emission band shifts to
higher wavelength, displaying distinct emission maxima at
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from the initial ligand concentration in due consideration of the
fraction of the complexed species. Slopes of 0.96 0.05, 0.93
0.04, and 0.94 0.05 are derived by linear regression for the
stepwise formation of the [Cm(C5-BPP)n]3+ complexes (n =
1−3). These results are in excellent agreement with the model
described by eq 1 and confirm the unambiguous assignment of
the three observed species with emission maxima at 603.7,
607.7, and 611.6 nm to be the 1:1, 1:2, and 1:3 Cm-C5-BPP
complexes, respectively.
The conditional stability constants log Kn (n = 1−3) for the
stepwise complexation of Cm(III) with C5-BPP are calculated
according to eq 3.
[Cm(C5‐BPP)n]3+
Kn =
[Cm(C5‐BPP)n−1]3+[C5‐BPP]
(3)
Figure 5. Relative Cm(III) species concentrations in methanol as a
function of the total C5-BPP concentration. Symbols give
experimental data from peak deconvolution. Lines designate relative
species calculated with log K1 = 6.9, log K2 = 4.3, and log K3 = 3.6.
Average values of log K1 = 6.9 0.2, log K2 = 4.3 0.1, and
log K3 = 3.6 0.1 are determined from the spectroscopic data,
resulting in stability constants of log β2 = 11.2 0.3 and log β3
Stepwise formation of the 1:1, 1:2, and 1:3 Cm-C5-BPP
complexes is described by eq 1. To ascertain the stoichiometry
= 14.8
0.4 for the 1:2 and the 1:3 Cm(III)-C5-BPP
complexes. In a previous study34 using a similar ligand,
dmpbipy, only the 1:1 Cm(III)-dmpbipy complex formed in
1-octanol solution, having the conditional stability constant log
K1 = 2.80 0.02. This difference mirrors dmpbipy’s inferior
behavior as an extracting agent.
+C5‐BPP
[Cm(solv)]3+ HooooooooI [Cm(C5‐BPP)]3+
−C5‐BPP
+C5‐BPP
+C5‐BPP
HooooooooI [Cm(C5‐BPP)2]3+ HooooooooI [Cm(C5‐BPP)3]3+
−C5‐BPP
−C5‐BPP
A stability constant of log β3 = 14.4 0.1 was found for the
Cm(III)-BTP 1:3 complex.26 This value is similar to that
reported here for C5-BPP but was determined using a different
diluent to dissolve the ligand (water/methanol 1:1 instead of
methanol for C5-BPP). The weaker coordination of methanol
compared to that of water is expected to cause an increase in
the value of the stability constant for C5-BPP. Nevertheless, the
Cm(III) TRLFS results indicate that BTP and C5-BPP show
similar affinity for Cm(III).
(1)
of the [Cm(C5-BPP)n]3+ (n = 1−3) complexes, the logarithm
of the concentration ratio [Cm(C5-BPP)n]3+/[Cm(C5-
3+
BPP)n−1
]
(n = 1−3) is plotted as a function of log [C5-
BPP] according to eq 2.
[Cm(C5‐BPP)n]3+
[Cm(C5‐BPP)
⎛
⎜
⎝
⎞
⎟
⎠
log
= log K + n log[C5‐BPP]
3+
]
Similar results have been observed for Eu(III)-C5-BPP
complexes in solution. The Eu(III) fluorescence spectra
n−1
(n = 1 − 3)
(2)
5
7
5
7
resulting from the D0 → F1 and D0 → F2 transitions in
the presence of increasing amounts of C5-BPP are displayed in
Figure S5 (Supporting Information). The spectroscopic results
show that four different Eu(III) species are formed in the ligand
concentration range of (0−3.5) × 10−3 mol/L. In accordance
with the Cm(III) results, these species are attributed to the
[Eu(solv)]3+ and the 1:1, 1:2, and 1:3 [Eu(C5-BPP)n]3+
complexes with n = 1−3. Due to the small shifts in the Eu(III)
fluorescence spectra, a quantitative determination of the four
individual species was not possible. Nevertheless, the 1:3
[Eu(C5-BPP)3]3+ complex which forms at a ligand concen-
tration of >1 × 10−3 mol/L was verified by fluorescence lifetime
measurements. In the course of the titration the fluorescence
lifetime increases from 182 μs ([Eu(solv)]3+ complex) to 2262
μs. According to ref 46, the fluorescence lifetime corresponds
to a 9-fold coordination of Eu(III), confirming the formation of
the [Eu(C5-BPP)3]3+ complex. In comparison to the case for
Cm(III), considerably higher ligand concentrations are required
for a quantitative formation of the 1:3 complex. This is in good
agreement with the results of the extraction experiments
displaying the separation factors SFAm(III)/Eu(III) ≈ 100.
The double-logarithmic plot of the concentration ratios is
shown in Figure 6. The free ligand concentration is determined
EXAFS Analysis. The Am L3-EXAFS of sample 3 ([Am] =
2 mmol/L, [C5-BPP] = 10 mmol/L in 2-propanol) is depicted
in Figure 7. The transformation range and metric parameters
obtained from fits are summarized in Table 1.
Figure 6. Double-logarithmic plots of [Cm(C5-BPP)n]3+/[Cm(C5-
3+
BPP)n−1
]
(n = 1−3) concentration ratios versus free C5-BPP
concentration.
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Figure 7. Am L3-EXAFS for Am(III)-C5-BPP, sample 3: (a) experimental Fourier transform (FT) magnitude (circles) and real part (triangles), with
solid lines giving theoretical curves from fit results; (b) experimental k2-weighted data (thin solid line, vertically shifted for clarity), back-transformed
data (circles), and theoretical fit curve (solid line).
a
Table 1. EXAFS Fit Parameters: ΔE0 Global Parameter (g) for All Shells, with S02 Fixed at 1.0
k range (Å−1) fit range (Å)
shell
N
R (Å)
ΔE0 (eV)
−6.88 (g)
σ2 (Å2)
r factor (%)
2.45−14.95
1.45−4.75
O(N)
N(C)
C(N)
Odist
9.5
8.5
2.54(1)
2.78(2)
3.77(3)
4.16(3)
0.0084
0.0107
0.0069
0.0053
0.4
12.9
9.3
a
Errors in coordination numbers are estimated to be 10% for the first two shells and 20−40% for higher shells.
Three peaks are discernible from the FT magnitudeat ∼2,
The following conclusions can be drawn from attempts to fit
the data using these models: neither models 1 and 3 nor the
exclusive nitrate coordination according to model 2 satisfac-
torily reproduces the experimental data over the full k range.
Only the first FT peak at ∼2 Å (R − Δ) is well reproduced
using model 3. A single-path fit of the first shell FT peak yields
9−10 N/O at an average distance of 2.54 Å, in agreement with
the structures of compounds 1 and 2. The rather large Debye−
Waller factor of 0.0085 Å2 reflects structural disorder due to
varying bond distances between Am(III) and coordinating
oxygen atoms from nitrate groups and coordinating N atoms
from pyrazolyl and pyridine rings. Simulations using the feff
code indicated that the second FT peak (∼2.9 Å (R − Δ))
includes intensity from the central N atoms of coordinating
nitrate groups (up to seven, assuming coordination by a single
C5-BPP ligand) and, to a minor extent, from carbon atoms
adjacent to coordinating N atoms in the rings. A single path fit
yields a total of eight to nine N/C backscatterers at an average
distance of 2.78 Å. Again the large σ2 value is indicative of
asymmetric coordination in this shell by the two different types
of ligands. The third FT peak comprises contributions from
further distant C and (noncoordinated) N atoms of C5-BPP
but seems to be dominated by backscattering from the distal
oxygen atoms of coordinating nitrate groups. This signal is
likely enhanced by the focusing effect (three-legged paths) of
the central nitrate N atom in the nearly linear Am(III)−N−
Odist arrangement. Aside from this, multiple scattering paths
2.9, and 4 Å (non-phase-shift-corrected R − Δ values). These
peaks are due to scattering paths between the central Am(III)
cation and ligand atoms, potentially coordinating oxygen atoms
from nitrate groups, or coordinating nitrogen atoms from
pyrazolyl and pyridine rings for the first shell, nitrogen atoms
from coordinating nitrate groups for the second shell, and
either further distant C and N atoms from coordinating C5-
BPP ligands or distal nitrate oxygen atoms comprising the third
shell. Two factors complicate EXAFS fit analysis of the
experimental χ(k) function: N, O, and C are indistinguishable
as backscattering atoms, while the effect of multiple scattering
(mainly triangular) paths in a highly symmetric metal−organic
complex coordination environment, as observed for, e.g.,
Am(III)-BTP3,21 cannot be ruled out.
Several models were tested to reproduce the experimental
EXAFS χ(k) function:
(1) a simulation assuming a symmetric Am(III)(C5-BPP)3
coordination
(2) a cluster containing only coordinated nitrate groups in a
structure derived from ref 40
(3) a 32-atom cluster derived from structure 2 containing
Am(III) as central atom
(4) a peak by peak fit based on single and double scattering
paths calculated for the first and the second model
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seem to play a minor role and were not considered. Any
coordination of solvent 2-propanol molecules cannot be ruled
out, as they are not unambiguously discernible from the EXAFS
data.
carboxylic acid as a lipohilic anion source. Despite its high
complexation strength, C5-BPP also requires a lipohilic anion
source. This implies that differences regarding the extractability
of actinide(III) and lanthanide(III) nitrates do not originate
from differences in ligand strength. Further fundamental studies
on the extractability of trivalent actinide nitrates are strongly
required.
We plan to continue varying the structure of N-donor ligands
systematically to identify trends in complex stability and
extractability. Also, understanding the difference between BTP/
BTBP and BPP (the former extract actinide nitrates, whereas
the latter extract only the more lipophilic actinide 2-
bromocarboxylates) is part of these studies. Future research
and development on new N-donor extracting agents for
separating actinides(III) from lanthanides(III) in the context
of nuclear fuel cycles should focus on finding complexes which
can accommodate nitrate anions required for charge
compensation in a rather hydrophobic environment. In this
context, the results are of major importance for an under-
standing of the extraction mechanism on a molecular level and
are essential for the intelligent design of improved extractants
for future industrial processing of used nuclear fuels.
In summary, the EXAFS analysis of the Am(III)-C5-BPP
complex (sample 3) indicates a [Am(C5-BPP)(NO3)3]
structure similar to structures 1 and 2. Considering the similar
behaviors of Am(III) and Cm(III) and the total C5-BPP
concentration of 10 mmol/L, one would however expect the
formation of the 1:3 complex according to the TRLFS results
(cf. Figure 5). This discrepancy is explained by the presence of
HNO3 in the Am(III) sample (3), which competes with
Am(III) for the C5-BPP ligand. A TRLFS experiment was
performed, adding increasing concentrations of HNO3 to a
sample initially containing exclusively the 1:3 complex (at 611.4
nm). The result clearly demonstrates a transition to the 1:1
complex (at 603.2 nm; Figure 8). The peak at 615.0 nm, not
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic files in CIF format for the structure
determinations of 1 and 2, ORTEP plots, and a figure showing
the coordination polyhedron. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: andreas.geist@kit.edu (A.G.).
■
Figure 8. Normalized fluorescence spectra of Cm(III) in methanol
with increasing HNO3 concentration. [Cm(III)]ini = 1 × 10−7 mol/L;
[C5-BPP]ini = 5 × 10−3 mol/L; [HNO3] = 0−0.1 mol/L.
Present Address
⊥Novartis Institute for BioMedical Research, CH-4002 Basel,
Switzerland.
Notes
The authors declare no competing financial interest.
observed in the spectra shown in Figure 5, may be due to the
formation of a 1:3 complex with a coordinating nitrate anion. A
similar mixed ligand/nitrate species was found for BTP under
specific conditions; this species too has a Cm(III) fluorescence
emission peak which is red-shifted approximately 4 nm from
that of the symmetric 1:3 complex.
ACKNOWLEDGMENTS
■
This work was supported by the German Federal Ministry of
Education and Research (BMBF) under contract numbers
02NUK012A, 02NUK012B, and 02NUK012D, the Baden-
Wurttemberg Stiftung gGmbH, Energie Baden-Wurttemberg
(EnBW-Stiftung), and the Commission of the European
Community (project ACTINET-I3). We thank Dr. Ralf
̈
̈
CONCLUSION
■
C5-BPP initially was synthesized and tested as a selective
extracting agent for trivalent actinides. Due to its good
performance as a selective extracting agent with structural
similarity to BTP, C5-BPP’s complexing properties were
studied from a fundamental point of view.
The solid-state structures of the isostructural lanthanide
complexes [Ln(C5-BPP)(NO3)3(DMF)] (Ln = Sm(III) (1),
Eu(III) (2)) were determined and compared to the solution
structure of the Am(III) 1:1 complex [Am(C5-BPP)(NO3)3]
(3); similar average bond lengths in the first coordination
sphere of 2.54 Å were found. This is in agreement with results
regarding the bond lengths in actinide(III)- and lanthanide-
(III)-BTP complexes.18,21,25
Koppe for performing the DFT studies of the ligand.
̈
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