Activation of the Aromatic Core of 3,3′-(Pyridine-2,6-diylbis(1 H-1,2,3-triazole-4,1-diyl))bis(propan-1-ol) - Effects on Extraction Performance, Stability Constants, and Basicity

Article abstract of DOI:10.1021/acs.inorgchem.9b02325

The "CHON" compatible water-soluble ligand 3,3′-(pyridine-2,6-diylbis(1H-1,2,3-triazole-4,1-diyl))bis(propan-1-ol) (PTD) has shown promise for selectively stripping actinide ions from an organic phase containing both actinide and lanthanide ions, by preferential complexation of the former. Aiming at improving its complexation properties, PTD-OMe was synthesized, bearing a methoxy group on the central pyridine ring, thus increasing its basicity and hence complexation strength. Unfortunately, solvent extraction experiments in the range of 0.1-1 mol/L nitric acid proved PTD-OMe to be less efficient than PTD. This behavior is explained by its greater pK_a value (pK_a = 2.54) compared to PTD (pK_a = 2.1). This counteracts its improved complexation properties for Cm(III) (log β₃(PTD-OMe) = 10.8 ± 0.4 versus log β₃(PTD) = 9.9 ± 0.5).

Full text of DOI:10.1021/acs.inorgchem.9b02325

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Activation of the Aromatic Core of 3,3′-(Pyridine-2,6-
diylbis(1H‑1,2,3-triazole-4,1-diyl))bis(propan-1-ol)Effects on
Extraction Performance, Stability Constants, and Basicity
,
†,‡
†
,§
§
§
Patrik Weßling,*
Michael Trumm, Elena Macerata,* Annalisa Ossola, Eros Mossini,
∥
∥
,∥
§
†
Maria Chiara Gullo, Arturo Arduini, Alessandro Casnati,* Mario Mariani, Christian Adam,
†
†,‡
Andreas Geist, and Petra J. Panak
†
Institute for Nuclear Waste Disposal, Karlsruhe Institute of Technology, P.O. Box 3640, 76021 Karlsruhe, Germany
‡
§
∥
Institut fu
Politecnico di Milano, Department of Energy, Nuclear Engineering Division, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
Dipartimento di Scienze Chimiche, della Vita e della Sostenibilita Ambientale, Universita di Parma, Area delle Scienze 17/a, 43124
̈
r Physikalische Chemie, Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany
̈
́
́
Parma, Italy
*
S Supporting Information
ABSTRACT: The “CHON” compatible water-soluble ligand
,3′-(pyridine-2,6-diylbis(1H-1,2,3-triazole-4,1-diyl))bis-
propan-1-ol) (PTD) has shown promise for selectively
3
(
stripping actinide ions from an organic phase containing
both actinide and lanthanide ions, by preferential complex-
ation of the former. Aiming at improving its complexation
properties, PTD-OMe was synthesized, bearing a methoxy
group on the central pyridine ring, thus increasing its basicity
and hence complexation strength. Unfortunately, solvent
extraction experiments in the range of 0.1−1 mol/L nitric
acid proved PTD-OMe to be less efficient than PTD. This
behavior is explained by its greater pK value (pK = 2.54)
a
a
compared to PTD (pK = 2.1). This counteracts its improved
a
complexation properties for Cm(III) (log β (PTD-OMe) = 10.8 ± 0.4 versus log β (PTD) = 9.9 ± 0.5).
3
3
INTRODUCTION
In contrast to aminopolycarboxylates, PTD remains efficient in
up to 0.5 mol/L nitric acid, which is advantageous with respect
to process applications. Furthermore, PTD (other than
sulfonated N-heterocyclic compounds) is a “CHON” com-
pound (i.e., containing only carbon, hydrogen, oxygen, and
nitrogen, making it fully combustible to gaseous products
■
Hydrophilic N-donor complexing agents are used in several
solvent extraction processes developed to separate actinides
1
−3
from lanthanides. Such separations may play a role in future
advanced nuclear fuel cycles. On the basis of the chemistry of
4
,5
the TALSPEAK process, actinide and lanthanide ions are co-
extracted, followed by selective stripping of actinides using N-
donor complexing agents such as aminopolycarboxylates or
16
without generating solid wastes). The favorable properties of
PTD such as pronounced selectivity for actinides(III) over
1
4,17
6
−13
lanthanides(III), fast kinetics, and good stability
promising candidate for process development.
make it a
sulfonated N-heterocyclic compounds.
For this purpose, 3,3′-(pyridine-2,6-diylbis(1H-1,2,3-tria-
1
8
14
zole-4,1-diyl))bis(propan-1-ol) (PTD, Scheme 1) is cur-
To further improve its complexation properties, a modified
PTD was synthesized, PTD-OMe (Scheme 1). Adding a
methoxy group in the para position of the pyridine ring to
activate the aromatic core, metal ion complexation should be
improved due to the higher electron density in the aromatic
ring. Such modifications had successfully been employed to
tune the properties of lipophilic of 2,6-bis-triazinyl-pyridine
15
rently studied in the European research program GENIORS.
Scheme 1. Molecular Structure of PTD and PTD-OMe
1
9
(
(
BTP)
BTPhen)
and 2,9-bis-triazinyl-1,10-phenanthroline
20,21
compounds.
Received: August 1, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02325
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Synthesis of PTD-OMe
PTD-OMe was studied using solvent extraction, time-
resolved laser fluorescence spectroscopy (TRLFS), and
Synthesis of 2,6-Diethynyl-4-methoxypyridine (4). One gram
3.32 mmol) of 4-methoxy-2,6-bis((trimethylsilyl)ethynyl) pyridine is
(
dissolved in 70 mL of a mixture of MeOH/Et O 2:1, and then 2.3 g
2
NMR. The pK value, stability constants for the complexation
a
(
1
16.6 mmol) of K CO is added. The reaction mixtures are stirred for
2 3
of Cm(III), and the Cm(III) speciation in solution were
determined. Computational calculations based on density
functional theory (DFT) of actinide and lanthanide PTD-
OMe complexes were performed to support experimental
findings.
h and then quenched with water. The aqueous layer is extracted
three times with AcOEt. The organic phases collected are dried over
anhydrous Na SO , and then the solvent is removed under reduced
2
4
1
pressure. The product is obtained as a brownish solid in 88% yield. H
NMR (400 MHz, CDCl ): δ 7.01 (2H, s, PyH3), 3.88 (3H, s, Py-
3
1
3
OCH ), 3.13 (2H, s, CCH); C NMR (100 MHz, CDCl ): δ 165.8,
3
3
+
1
43.7, 113.7, 82.1, 77.3, 55.6; HR-MS (ESI+) m/z: [M + H] Calcd
EXPERIMENTAL SECTION
■
for C H NO 158.0600; Found 158.0601; mp 140 °C dec.
1
0
8
Chemicals. PTD-OMe was synthesized as described below
Synthesis of 3-Azidopropan-1-ol. Prepared according to literature
22
(
Scheme 2). D O was purchased from Deutero GmbH. All
procedure. The 9.1 g (140 mmol) of NaN are added to 50 mL of
2
3
commercially available chemicals (Sigma-Aldrich) used in this study
were analytical reagent grade and used without further purification.
Synthesis. Melting points were determined on an Electrothermal
water. When the azide is completely dissolved, 6.87 g (68 mmol) of 3-
chloropropan-1-ol are added dropwise. The reaction mixture is stirred
at 80 °C for 24 h. The aqueous solution is extracted three times with
dichloromethane, and then the organic phases collected are dried over
anhydrous Na SO . The solvent is evaporated in vacuo to obtain the
1
13
apparatus in capillaries sealed under nitrogen. H and C NMR
spectra were recorded on Bruker AV300 and AV400 spectrometers. J
coupling constants are given in hertz. Partially deuterated solvents
were used as internal standards. Electrospray ionization mass
spectrometry (ESI-MS) spectra were recorded on a Waters single
quadrupole instrument SQ Detector, while high-resolution mass
spectrometry (HR-MS) spectra were obtained with a Thermo
Scientific Orbitrap LTQ-XL. Thin-layer chromatography (TLC) was
performed on Merck 60 F254 silica gel, and flash column
chromatography was performed on 230−400 mesh Merck 60 silica
gel.
2
4
1
product as a yellowish liquid. Yield: 85% H NMR (400 MHz,
CDCl ): δ 3.76 (2H, t, J = 6.0 Hz, CH N ), 3.46 (2H, t, J = 6.6 Hz,
3
2
3
CH OH), 1.84 (2H, quint, J = 6.3 Hz, CH CH CH ). HR-MS (ESI
2
2
2
2
+
+) m/z: [M + H] Calcd for C H NOSi 302.1391; Found
1
6
24
2
302.1392.
Synthesis of PTD-OMe. The 0.25 g (1.59 mmol) of 2,6-diethynyl-
4-methoxypyridine and 0.52 g (5.2 mmol) of 3-azidopropan-1-ol are
dissolved in 30 mL of a 1:1 mixture of water and ethanol. Then 7.5
mg (0.03 mmol) of CuSO ·5H O and 63.4 mg (0.32 mmol) of
4
2
Synthesis of 2,6-Diiodo-4-methoxypyridine 1-oxide (1). Prepared
sodium ascorbate are added. The reaction mixture is stirred for 3 d
and then quenched by removal of the solvents under reduced
pressure. The crude is purified by flash column chromatography using
14
according to literature procedure from 4-methoxypyridine 1-oxide
1
in 30% yield. H NMR (400 MHz, CD OD): δ 7.75 (2H, s, PyH3),
3
3
.90 (3H, s, Py-OCH3).
Synthesis of 2,6-Diiodo-4-methoxypyridine (2). Prepared accord-
dichloromethane/methanol 92/8 as eluent. The product was obtained
1
as a white solid in 57% yield. H NMR (400 MHz, CD OD): δ 8.57
3
14
ing to literature procedure from 4-methoxy-pyridine 1-oxide in 90%
yield. H NMR (300 MHz, CD OD): δ 7.42 (2H, s, PyH3), 3.86
(2H, s, Triaz-H), 7.53 (2H, s, PyH3), 4.62 (4H, t, J = 7.2 Hz, CH N),
2
1
4.00 (3H, s, Py-OCH ), 3.64 (4H, t, J = 6.0 Hz, CH OH), 2.20 (4H,
3
3
2
1
3
(
3H, s, Py-OCH3).
quint, J = 6.4 Hz, CH CH N). C NMR (100 MHz, CD OD): δ
2
2
3
Synthesis of 4-Methoxy-2,6-bis((trimethylsilyl)ethynyl) pyridine
167.6, 151.3, 147.6, 123.6, 104.7, 57.9, 54.8, 32.6. ESI-MS (+): 382.3
+
+
+
(
3). 2,6-Diiodo-4-methoxypyridine (1.30 g, 3.60 mmol) is dissolved in
[M + Na] , 398.2 [M+K] , 558.9 [M+Na ascorbate] , 741.5 [2M
+
+
a dry mixture of toluene and diisopropylamine 2:1 (150 mL) under
+Na] . HR-MS (ESI+) m/z: [M + H] Calcd for C H N O
16 22 7 3
inert conditions. Then CuI (0.03 g, 0.15 mmol), Pd(PPh ) (0.08 g,
360.1779; Found 360.1786. mp: 118−120 °C.
Solvent Extraction. Organic phase was 0.2 mol/L N,N,N′,N′-
3
4
0
.07 mmol), and trimethylsilylacetylene (0.82 g, 8.32 mmol) are
added, and the reaction mixture is stirred at room temperature. After
4 h the reaction is quenched with water, and the aqueous phase is
23−27
tetra-n-octyl-3-oxapentanediamide (TODGA)
+ 5 vol % 1-
2
octanol in kerosene. Aqueous phase was 80 mmol/L PTD or PTD-
extracted with ethyl acetate (AcOEt). The organic phases collected
OMe in HNO (varied concentration) spiked with each 2.5 kBq/mL
3
2
41
154
are dried over anhydrous Na SO , and the solvents are evaporated
Am(III) and Eu(III).
2
4
under reduced pressure. The crude is purified by flash column
chromatography using hexane/AcOEt 8.5:1.5 as eluent. Yield: 92%
Each 300 μL of aqueous and organic phases were placed in 2 mL
Eppendorf tubes and shaken on an orbital shaker for 60 min at 1100
rpm and 295 K. Following centrifugation for 10 min at 1000 rpm, 200
μL aliquots of both phases were analyzed on a gamma counter
(Packard Cobra Auto-Gamma 5003).
1
H NMR (400 MHz, CDCl ): δ 6.95 (2H, s, PyH3), 3.87 (3H, s, Py-
3
1
3
OCH ), 0.27 (18H, s, Si(CH ) ); C NMR (100 MHz, CDCl ): δ
3
3
3
3
1
65.7, 144.4, 113.2, 103.2, 94.8, 55.5, 0.3. mp 64−66 °C.
B
DOI: 10.1021/acs.inorgchem.9b02325
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Inorganic Chemistry
Article
TRLFS Sample Preparation. Stock solutions containing 0.5 mol/
RESULTS AND DISCUSSION
Solvent Extraction. The solvent extraction behavior of
PTD-OMe was studied using Am(III) and Eu(III) as
representatives for trivalent actinides and lanthanides. Figure
■
L PTD-OMe were prepared by dissolving 50.3 mg of PTD-OMe in
−
3
2
80 μL of 1 × 10 mol/L HClO or 0.44 mol/L HNO . Solutions
4
3
with lower PTD-OMe concentrations were prepared by dilution with
−
3
1
× 10 mol/L HClO or 0.44 mol/L HNO , respectively.
4
3
1
compares distribution ratios (DM(III) = [M(III)] /[M-
TRLFS samples were prepared by adding 4.7 μL of a Cm(III) stock
org
−
5
solution (2.12 × 10 mol/L Cm(ClO ) in 0.1 mol/L HClO ;
4
3
4
2
2
48
47
246
243
244
245
Cm: 89.7%, Cm: 9.4%, Cm: 0.4%, Cm: 0.3%, Cm: 0.1%,
Cm: 0.1%) to 995.3 μL of 1 × 10 mol/L HClO or 0.44 mol/L
HNO , resulting in an initial Cm(III) concentration of 1 × 10 mol/
−3
4
−
7
3
L. Ligand concentration was adjusted by adding appropriate volumes
of the PTD-OMe solutions. TRLFS spectra were recorded following
an equilibration time of 10 min. Preliminary tests showed this to be
sufficient to attain equilibrium.
Solvent extraction samples for TRLFS measurements were
prepared as described in Solvent Extraction, with the exception that
samples were 500 μL per phase each spiked with 4.7 μL of the
2
41
154
Cm(III) stock solution instead of Am and Eu.
TRLFS Measurements. TRLFS measurements were performed at
2
98 K using a Nd:YAG (Surelite II laser, Continuum) pumped dye
laser system (NarrowScan D-R; Radiant Dyes Laser Accessories
GmbH). A wavelength of 396.6 nm was chosen to excite Cm(III). A
spectrograph (Shamrock 303i, ANDOR) with 300, 1199, and 2400
lines per millimeter gratings was used for spectral decomposition. The
fluorescence emission was detected by an ICCD camera (iStar Gen
III, ANDOR) after a delay time of 1 μs to discriminate short-lived,
organic fluorescence, and light scattering.
Figure 1. Distribution ratios for the extraction of Am(III) and Eu(III)
with TODGA/PTD-OMe (solid symbols) and TODGA/PTD (open
symbols). Organic phase, 0.2 mol/L TODGA in TPH/1-octanol (5
vol %). Aqueous phase, 0.08 mol/L PTD-OMe or PTD in nitric acid.
A/O = 1, T = 295 K.
NMR Sample Preparation. NMR samples for pK determination
a
contained initially 9 × 10− mol/L PTD-OMe in an aqueous formic
3
acid/formate buffer containing 10 vol % of D O. pH was measured
2
with a microelectrode (Orion PerpHecT ROSS, Thermo Fisher
Scientific) and a pH meter (Orion Star, Thermo Fisher Scientific)
before and after NMR measurement. The pH was adjusted with 1,
(
III)] ) and separation factors (SF = DEu(III)/DAm(III)) for the
aq
systems TODGA/PTD and TODGA/PTD-OMe as a function
of nitric acid concentration.
0
.1, or 0.01 mol/L HCl or NaOH solutions.
NMR Measurements. NMR spectra were recorded at T = 300 K
on a Bruker Avance III 400 spectrometer operating at a resonance
frequency of 400.18 MHz for H nuclei. The spectrometer was
equipped with a z-gradient observe room-temperature probe.
Chemical shifts were referenced internally to tetramethylsilane
Am(III)/Eu(III) separation is achieved for DAm(III) < 1 and
DEu(III) > 1. With PTD, this is the case at 0.15−0.6 mol/L nitric
acid, with a selectivity in the range of 500 > SF > 180. With
PTD-OMe, separation is achieved at 0.1−0.3 mol/L nitric acid,
with a selectivity in the range of 60 > SF > 20. Contrary to
expectation, PTD-OMe is a less efficient ligand compared to
PTD.
1
(
TMS) (δ(TMS) = 0 ppm) by the deuterium lock signal of D O.
2
1
For single scan H spectra, standard 90° pulse sequences were used.
Water suppression was achieved by the WATERGATE
28,29
pulse
sequence. All spectra were recorded with 32k data points and were
zero filled to 64k points. For WATERGATE spectra, eight scans were
acquired per spectrum with a relaxation delay of 2 s. Exponential
window functions with a line broadening factor of 0.05 Hz were
applied for processing.
Determination of the pK Value by NMR. The insertion
a
of an activating −OMe group on the pyridine nucleus of the
ligand will impact the pK value. Therefore, the pK value of
a
a
1
PTD-OMe was determined by evaluating the shifts of its H
NMR signals as a function of the measured pH. Note that the
ionic strength is not constant during titration. Because of the
rather low ionic strength (10 to 10 mol/L), however, no
corrections for the pH were performed. Corresponding H
Theoretical Model. Protonation and the complex formation of
PTD-OMe were investigated. Structures were optimized employing
−
2
−1
3
0
31
density functional theory at the B3-LYP /def2-TZVP level of
1
32
theory as implemented in the TURBOMOLE program package. To
avoid the known problem of spin contamination in Gd(III) complexes
computed by the B3-LYP functional, the BH-LYP functional was
used instead on all metal ion complexes. NMR shielding constants
were calculated for all atoms of unprotonated and protonated PTD-
NMR spectra at different pH values are shown in Figure 2.
33
1
The H NMR signals of PTD-OMe show a downfield shift
1
with decreasing pH due to protonation of the ligand. The H
NMR signals shift in a pH range of 4.02−1.25. No shifts are
observed beyond this range, indicating the existence of only
the protonated or unprotonated ligand. Both species are
characterized as follows (for proton assignment see Figure 3):
34
OMe using the MPSHIFT routine in TURBOMOLE. Binding
energies corrected for basis set superposition error (BSSE) of
3+
3+
[
M(PTD-OMe)3]
and [M(PTD)3]
were determined on the
MP2/def2-TZVP level, which was shown to yield good results when
• Unprotonated PTD-OMe (pH = 4.39): 1H NMR
35
comparing to experimental separation factors (SF). The Cm(III)
36
(400.18 MHz in formic acid/formate buffer +10 vol %
ion was described by a ECP60MWB small-core pseudopotential,
36
D O): δ 8.20 (s, 2H, H-1); 6.93 (s, 2H, H-3); 3.65 (s,
2
and the Gd(III) was described by an ECP28MWB small-core
3
3
H, H-2); 3.58 (t, J
= 6.30 Hz, 4H, H-6); 2.09
pseudopotential. Differences in selectivity have recently been
H−H
3
37
(
quint, J
= 6.68 Hz, 4H, H-5).
connected to atomic polarizabilities of the coordinating atoms. To
investigate the ligands at hand within this approach, atomic charges
and dipole polarizabilities were calculated on the B3LYP/aug-cc-
H−H
1
• Protonated PTD-OMe (pH = 0.96): H NMR (400.18
MHz in formic acid/formate buffer +10 vol % D O): δ
2
38
37,39
pVTZ level using the Hirshfeld method.
8.75 (s, 2H, H-1); 7.65 (s, 2H, H-3); 4.12 (s, 3H, H-2);
C
DOI: 10.1021/acs.inorgchem.9b02325
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Inorganic Chemistry
Article
1
Figure 2. H NMR spectra of PTD-OMe in formic acid/formate (H-fa) buffer with 10 vol % D O at varied pH. The water signal at δ = 4.702 ppm
2
was suppressed using the WATERGATE technique.
1
1
Figure 3. Shift of the H signals in respect to the H shift of the unprotonated PTD-OMe as a function of pH.
3
6
.57 (t, 3JH−H = 6.21 Hz, 4H, H-6); 2.14 (quint, 3JH−H
.55 Hz, 4H, H-5).
=
therefore, only an average of the signals of the two species is
detected. Consequently, the peak shift (Δδ , eq 1) at a given
i
pH relative to the shift of the unprotonated species (δ =
Spectra in the pH range of 1.25−3.87 contain both
protonated and unprotonated species. The presence of only
one set of H signals indicates fast proton exchange, and,
0
δpH=4.39) is used for pK
determination. The relative peak shifts
a
1
for all protons of PTD-OMe are shown in Figure 3.
D
DOI: 10.1021/acs.inorgchem.9b02325
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Inorganic Chemistry
Article
Δδ = δ − δ
(1)
values derived from the shifts of H-1, H-2, and H-3 are
i
i
0
reported in Table 1. The average pK of PTD-OMe has a value
a
With decreasing pH, aromatic protons shift more strongly
downfield than protons farther away from the aromatic center.
The strongest shifts are observed for the pyridine protons (H-
of 2.54 ± 0.08. The attachment of a methoxy group, as
expected for an electron-donating moiety, to the pyridine core
results in a slightly increased basicity compared to PTD (pK =
a
3
; Δδmax = 0.725 ppm), followed by the triazole protons (H-1;
14
2
.1).
Δδ = 0.552 ppm) and the methoxy protons (H-2; Δδ
=
max
max
Complexation of Cm(III) with PTD-OMe at pH = 3.
0
.470 ppm). Shifts of the protons H-5 and H-6 of the propanol
TRLFS was utilized to study the complexation of Cm(III) with
PTD-OMe. Stability constants and the speciation in solution
were determined. Because of its favorable spectroscopic
properties, Cm(III) was used to represent Am(III). This is
valid due to the similar chemical properties of both elements.
The evolution of the Cm(III) fluorescence spectra resulting
units are negligible. The shift of the proton H-4 cannot be
evaluated due to the WATERGATE method. The strong shifts
of the protons in the aromatic region indicate protonation in
the aromatic region and most probably on the pyridine
nucleus.
To determine the pK value, proton signals H1, H2, and H3
a
6
8
from the D′ → S′ transition is shown in Figure 4 as a
7
/2
7/2
were evaluated individually. With eq 2, relative concentrations
were calculated. The species distribution is given in the
Supporting Information, Figure S1.
function of the PTD-OMe concentration.
Without addition of PTD-OMe the emission band of the
40
Cm(III) aqua ion is observed at 593.8 nm. With addition of
PTD-OMe new emission bands evolve at 600.1, 605.6, and
608.8 nm. The bathochromic shift is explained by the
Δδi
χi =
Δδmax
(2)
6
increased splitting of the D′ state due to the complexation
7
/2
The Henderson−Hasselbalch eq 3 was used to determine the
of Cm(III) with PTD-OMe. The emission bands are in
pK value of PTD-OMe.
a
excellent agreement with those of the Cm(PTD) complexes
n
−
3
17
n+
(n = 1−3) in 1 × 10 mol/L HClO4, indicating that the
observed emission bands correspond to the Cm(PTD-OMe)
i [LH ] y
j
n
z
j
z
logj
z = −n × pH + pK
j
j
z
z
a
n
[
L]
(3)
k
{
complexes (n = 1−3).
With single-component spectra of the Cm(PTD-OMe)n
Slope analyses of H-1, H-2, and H-3 shifts as a function of pH
are shown in the Supporting Information, Figure S2. Slopes
see Table 1) of −1 confirm that one proton is transferred. pK
complexes obtained by peak deconvolution (Figure 5) and
13
the respective fluorescence intensity factors (FI = 1, FI =
(
1
2
a
1
.1, and FI = 1.1), the species distribution of the [Cm(PTD-
3
3
+
OMe)n] complexes (n = 1−3) as a function of the free (i.e.,
unprotonated and uncomplexed) PTD-OMe concentration
was derived (see Figure 6). The free ligand concentration was
Table 1. Slopes and pK Values for the Protonation of PTD-
a
OMe
proton
slope
pKa
calculated using eq 4 with [L] being the initial ligand
0
+
H-1
H-2
H-3
−1.06 ± 0.03
−1.06 ± 0.03
−1.03 ± 0.03
2.55 ± 0.05
2.51 ± 0.03
2.57 ± 0.06
concentration, [H ] the initial proton concentration, K the
0
a
ligand protonation constant, and χ the relative fraction of the
i
Cm(III) complex species present at a given ligand concen-
tration.
Figure 4. Normalized Cm(III) emission spectra in 1 × 10− mol/L HClO with increasing PTD-OMe concentrations. [Cm] = 1 × 10−7 mol/L.
3
4
ini
E
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Inorganic Chemistry
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To verify the stepwise complexation according to eq 5 (L =
PTD-OMe)
3
+
3+
[
Cm(L)n−1] + L ⇋ [Cm(L) ] (n = 1 − 3)
(5)
n
slope analyses were performed using eq 6, with the results
shown in the Supporting Information, Figure S4.
3
+
i
y
j [Cm(L)n]
z
j
z
logj
z = 1 × log([L]free) + log Kn′
j
j
3+ z
z
[
Cm(L)
]
(
6)
k
n−1
{
Slopes of m = 1.13 ± 0.06, m = 0.98 ± 0.15, and m = 1.19 ±
.07 confirm the stepwise complexation according to eq 5.
With eq 7, conditional stability constants were determined:
1
2
3
0
log β′ = 3.4 ± 0.3, log β′ = 7.0 ± 0.4, and log β′ = 10.8 ±
1
2
3
0
.4. Calculated and experimental relative fractions deviate
−
4
−4
slightly in the range from 1 × 10 to 4 × 10 mol/L. This is
due to the lower signal-to-noise ratio of the single-component
spectra of the 1:1 and 1:2 complexes used for peak
deconvolution.
Figure 5. Normalized single-component spectra of the [Cm(PTD-
OMe)n]3+ (n = 0−3) complexes in 1 × 10 mol/L HClO4.
−3
2
i
3+
y
+
j
[Cm(L) ]
z
[
L]free = 0.5 × ((4 × [L] × K + [H ] + 2 × [H+]
n
z
0
a
0
0
j
z
log β′ = logj
n
j
3+
n
z
z
2
0.5
+
j [Cm ] × [L]
aq
free
(7)
×
−
K + K ) − [H ] − K )
k
{
a
a
0
a
([Cm(III)] × (χ1:1 + 2 × χ1:2 + 3 × χ ))
TRLFS on Solvent Extraction Samples. To study the
1:3
speciation under extraction conditions (0.44 mol/L HNO ),
(
4)
3
TRLFS was performed on both the aqueous and organic
phases of a solvent extraction experiment.
The 1:1 complex starts forming at a PTD-OMe concen-
tration of ≈10 mol/L, with a maximum of 23% at 2.5 × 10−4
−5
The organic phase (Figure 7, top) shows the emission
3
+
17
mol/L. The 1:2 complex has a maximum fraction of 26% at 5.3
spectrum of the [Cm(TODGA)3] complex with its
characteristic emission band at 608.8 nm and a hot band at
595 nm. The species is also verified by its fluorescence lifetime
(see Supporting Information, Figure S5; τ = 403 ± 12 μs;
−
4
×
10 mol/L PTD-OMe. For PTD-OMe concentrations
greater than 3.5 × 10 mol/L the 1:3 complex is the
dominating species. The fluorescence lifetime (τ = 495 ± 20
μs) determined at a ligand concentration of 6.29 × 10 mol/L
see Figure S3) indicates the absence of water molecules in the
first coordination sphere (n(H O) = 0.4 ± 0.5), in
agreement with full coordination by three PTD-OMe
molecules.
−
4
−
3
n(H O) = 0.7 ± 0.5), confirming coordination of three
2
(
TODGA ligands.
4
0
In case of PTD, 80% of the 1:3 complex and 20% of the 1:2
complex are found under the extraction conditions (0.44 mol/
L HNO3). To observe a high selectivity in an extraction
2
17
Figure 6. Cm(III) species distribution in 1 × 10− mol/L HClO as a function of the free PTD-OMe concentration. Symbols, experimental data.
3
4
Lines, calculated with log β′ = 3.4, log β′ = 7.0, and log β′ = 10.8.
1
2
3
F
DOI: 10.1021/acs.inorgchem.9b02325
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
good agreement with the higher conditional stability constants
observed for PTD-OMe compared to PTD. Stability constants
for the lower complex species are less influenced by the
activation of the aromatic core, whereas the conditional
stability constant of the 1:3 complex is almost 1 order of
magnitude higher for PTD-OMe. A linear correlation between
stability constants and pK values has been observed for many
a
41,42
other systems.
The results for PTD-OMe are in good
agreement with this observed trend.
The higher basicity of PTD-OMe explains its inferior
performance in extraction experiments compared to PTD. The
free ligand concentration decreases due to the acidic
conditions used in the extraction experiments (0.44 mol/L
HNO ). Because of the higher pK value of PTD-OMe, the
3
a
concentration of unprotonated ligand is lower compared to
that of PTD. Consequently, Am(III) distribution ratios are less
favorable, that is, higher. Since N-donor ligands such as PTD(-
OMe) have lower affinity for Eu(III) compared to Am(III),
Eu(III) distribution ratios are less affected, and hence
selectivity is lower for PTD-OMe (cf. Figure 1).
As expected, the activation of the aromatic core of PTD
made PTD-OMe a stronger ligand. However, in systems in
which the pH is smaller than the pK value the ligand with the
a
lower pK shows the better extraction performance, as the free
a
ligand concentration is higher.
Quantum-Chemical Calculations. To support the
experimental findings, various computational calculations
were performed using DFT. The effect of the protonation on
NMR shielding was investigated. The NMR spectra (see
Figure 3) reveal the strongest downfield shift for the pyridine
protons (H-3), followed by the triazole (H-1) and methoxy
(
H-2) protons. For the latter, a static model representation is
not able to capture the equivalency of the three methoxy
protons. Hence, average values are reported in the following.
To elucidate the site of protonation, various protonated
structures of PTD-OMe were calculated and compared (Table
Figure 7. Cm(III) emission spectra of the organic (top) and aqueous
(
bottom) phases of a solvent extraction sample. Organic phase, 0.2
3
).
mol/L TODGA in TPH/1-octanol (5 vol %). Aqueous phase, 1 ×
1
−
7
0
mol/L Cm(III) and 0.08 mol/L PTD-OMe in 0.44 mol/L nitric
Table 3. Energy Difference of the Different Optimized
acid.
Protonated PTD-OMe Structures Relative to the Most
a
Stable One
experiment, however, the presence of only the 1:3 complex is
mandatory. To improve the speciation in 0.44 mol/L HNO3
the methoxy group was introduced in PTD-OMe. Yet, multiple
site of protonation
ΔE [kJ/mol]
1
H−N
0
55.8
150.5
311.8
311.2
2
H−N
species are present in 0.44 mol/L HNO in case of PTD-OMe
3
3
H−N
as shown by the emission spectrum of the aqueous phase
H-OMe
H−OH
(
Figure 7, bottom) explaining the observed diminished
selectivity of PTD-OMe.
a
Comparison of PTD-OMe and PTD. An overview of the
Calculated at B3-LYP/def2-TZVP level of theory. For atom
assignment, see Figure 3.
conditional stability constants and pK values of PTD-OMe
a
and PTD is given in Table 2. The pK value of PTD-OMe is
a
≈
0.5 higher than the pK of PTD. This higher basicity is in
The most favorable protonation energy is found for the
a
1
pyridine nitrogen (N ). The protonation of the coordinating
2
Table 2. Conditional Stability Constants of the [Cm(PTD-
OMe)n] and[Cm(PTD)n] Complexes
nitrogen atoms of the triazole rings (N ) is disfavored by 55.8
3
+
3+
3
kJ/mol. The protonation of the other nitrogen atoms (N ) or
the oxygen donors is disfavored even more. Similar trends are
reported for BTP-type ligands, where protonation also takes
place at the pyridine nitrogen, but the protonation of the
coordinating triazine nitrogen atoms is disfavored by only 25.1
1
7
PTD-OMe
PTD
pK = 2.54
a
pK = 2.1
a
n
log β′n
log β′n
19,43,44
kJ/mol.
1
2
3
3.4 ± 0.3
7.0 ± 0.4
10.8 ± 0.4
3.2 ± 0.2
6.8 ± 0.2
9.9 ± 0.5
1H NMR shifts were calculated to further support the site of
protonation. Δδ values (cf. equation 1) between the
i
unprotonated PTD-OMe and all protonated PTD-OMe
G
DOI: 10.1021/acs.inorgchem.9b02325
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
structures were calculated. Table 4 reports Δδ values for the
i
protons of the pyridine (H-3) and the triazole (H-1) ring and
the methoxy group (H-2).
1
Table 4. Differences of the H-NMR Shifts between
Different Protonated PTD-OMe and the Unprotonated
a
PTD-OMe
site of protonation
1
2
3
H−N
H−N
H−N
H-OMe
H−OH
H-1 (Tri)
H-2 (OMe)
H-3 (Py)
0.130
0.326
0.267
0.281
0.157
0.176
0.190
0.166
0.172
0.004
1.106
0.120
−0.021
−0.015
−0.221
1
2
Figure 8. An(III)/Ln(III) selectivity for different N /N polar-
izabilities according to ref 37.
a
Δδ in analogy to equation 1, calculated with the MPSHIFT package
i
[
GdL3] + + [Cm(H O) ]
3
3+
at B3-LYP/def2-TZVP level of theory.
2
9
F [CmL3] + + [Gd(H O) ]
3
3+
2
9
(8)
(9)
Downfield shifts observed experimentally decrease in the
order of Δδ > Δδ > ΔδH‑2. Note that, for the evaluation
3
+
3+
ΔE = BE([CmL ] ) − BE([GdL ] )
g
3
3
H‑3
H‑1
of the calculations, the shifts of H-2 are not considered, as the
methoxy groups rotate very quickly in solution, which leads to
kJ
i
y
j ΔE − 74.1
z
j
g
z
j
mol z
SFCm/Gd expj
=
j−
z
z
larger deviations of the calculations. Therefore, the trend Δδ
H‑3
j
z
j
j
RT
z
z
>
Δδ was evaluated. It was found only for the protonation at
H‑1
(10)
k
{
the pyridine nitrogen or the methoxy oxygen. Since the
protonation of the methoxy oxygen is disfavored by 311.3 kJ/
First, BE of the 1:3 complexes for Gd(III) and Cm(III) were
calculated (Table 5). Although the BE for PTD-OMe
1
mol in all calculations, both protonation energies and H shifts
complexes are higher in general, smaller differences (ΔE , eq
suggest the protonation of PTD-OMe to occur at the pyridine
nitrogen.
Furthermore, the structures of the 1:3 complexes of Cm(III)
and Gd(III) with PTD and PTD-OMe were optimized, and
binding energies (BE) of the complexes and atomic charges
g
9
) in the BE of the 1:3 PTD-OMe complexes of Cm(III) and
Gd(III) were found compared to the corresponding PTD
complexes. Separation factors were calculated using equation
0 taking into account a difference of 74.1 kJ/mol between the
1
35
corresponding aqua ions.
(
q) and dipole polarizabilities (α) of the coordinating nitrogen
1
2
The smaller difference in binding energy (ΔE ) of PTD-
atoms (N and N ) were calculated. It had been shown that
dipole polarizabilities of ligands influence their selectivity.
g
37
OMe leads to a decrease in selectivity by a factor of 5 from a
calculated separation factor (SFCm/Gd) of 31 for PTD to 6 for
PTD-OMe. This is in good agreement with experimental
The calculated values are listed in Table 5.
Introduction of the methoxy group does not affect the
charge density (q) within the pyridine ring in PTD-OMe
compared to PTD but has a significant effect on the dipole
polarizabilities (α) of all nitrogen atoms, both in the pyridine
as well as the triazole ring. This effect is also reflected in the
findings (SFAm/Eu(PTD)exp. = 100−36; SFAm/Eu(PTD-OMe)
exp.
=
12−4). Note that experimentally determined separation
factors reflect the selectivity of both PTD and TODGA, with
6
the Gd(III)/Cm(III) selectivity of TODGA being ∼5.
calculated bond distances between the metal ions and N in
1
the [M(PTD-OMe)3]3 (M = Cm, Gd) complexes, in which
the bond length decreases by 2 pm in both complexes
compared to PTD (cf. Supporting Information, Table S1).
+
CONCLUSION
■
Hoping to improve the extraction and complexation properties
of PTD, a water-soluble complexing agent, PTD-OMe was
synthesized. By placing a methoxy moiety at the 4-position of
the central pyridine the aromatic core was activated. To study
the impact of this activation the pK and the speciation in
aqueous and acidic solutions were investigated.
37
According to theory, An(III)/Ln(III) selectivity with
1
2
respect to the dipole polarizabilities of N versus N for both
PTD and PTD-OMe is plotted in Figure 8 with the optimal
zone for high selectivity highlighted.
a
The dipole polarizabilities of PTD are within the optimal
zone for high selectivity. On the basis of the atomic properties,
a significant decrease in selectivity is expected for PTD-OMe.
Accordingly, lower separation factors SFCm/Gd are expected to
be obtained. To calculate SFCm/Gd values, an exchange reaction
according to equation 8 was considered.
The pK of PTD-OMe was determined using NMR. PTD-
a
OMe (pK = 2.54 ± 0.08) is more prone to protonation than
a
PTD (pK = 2.1). Consequently, greater stability constants
a
41,42
were expected
for the metal ion complexes with PTD-
OMe. TRLFS confirmed the conditional stability constant of
the Cm(III) 1:3 complex to be almost 1 order of magnitude
Table 5. Calculated Atomic Charges (q), Dipole Polarizabilities (α), Binding Energies (BE), and Separation Factors (SF) of
Cm(III) and Gd(III) 1:3 Complexes
N1
N2
q [e]
α [Å3]
q [e]
α [Å3]
BE(Cm) [kJ/mol]
BE(Gd) [kJ/mol]
SFCm/Gd
PTD
PTD-OMe
−0.13
−0.14
0.32
0.42
−0.12
−0.12
0.93
1.10
−3292.3
−3404.7
−3357.9
−3474.3
33
6
H
DOI: 10.1021/acs.inorgchem.9b02325
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
greater for PTD-OMe (log β (PTD-OMe) = 10.8 ± 0.4) than
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for PTD (log β (PTD) = 9.9 ± 0.5).
3
Unfortunately, in solvent extraction experiments (involving
.44 mol/L nitric acid in the aqueous phase) PTD-OMe
0
3
(
559; Oak Ridge National Laboratory: Oak Ridge, TN, 1964.
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NMR experimental data and DFT calculations confirmed
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atom. The lower selectivity of PTD-OMe compared to PTD
was explained by an increased polarizability of the coordinating
nitrogen atoms, actually leaving the small zone of optimum
polarizability.
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Clearly, the positive effect of methoxy substitution observed
Sypula, M.; Magnusson, D.; Mullich, U.; Geist, A.; Bosbach, D.
̈
19−21
for some lipophilic N-heterocyclic extracting agents
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Laboratory-scale counter-current centrifugal contactor demonstration
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overcompensated by increased susceptibility to protonation in
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̈
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ASSOCIATED CONTENT
Supporting Information
■
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*
S
(10) Heathman, C. R.; Grimes, T. S.; Jansone-Popova, S.; Roy, S.;
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02325.
Bryantsev, V. S.; Zalupski, P. R. Influence of a pre-organized N-donor
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Slope analyses, species distribution for the pKa
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11) Heres, X.; Sorel, C.; Miguirditchian, M.; Cames, B.; Hill, C.;
́
̀
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Bisel, I.; Espinoux, D.; Eysseric, C.; Baron, P.; Lorrain, B. Results of
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AUTHOR INFORMATION
Corresponding Authors
E-mail: patrik.wessling@kit.edu. (P.W.)
E-mail: alessandro.casnati@unipr.it. (A.C.)
E-mail: elena.macerata@polimi.it. (E.M.)
■
*
*
*
(
12) Lewis, F. W.; Harwood, L. M.; Hudson, M. J.; Geist, A.;
Kozhevnikov, V. N.; Distler, P.; John, J. Hydrophilic sulfonated bis-
,2,4-triazine ligands are highly effective reagents for separating
1
actinides(III) from lanthanides(III) via selective formation of aqueous
actinide complexes. Chem. Sci. 2015, 6 (8), 4812−4821.
(13) Kaufholz, P.; Modolo, G.; Wilden, A.; Sadowski, F.; Bosbach,
D.; Wagner, C.; Geist, A.; Panak, P. J.; Lewis, F. W.; Harwood, L. M.
Solvent Extraction and Fluorescence Spectroscopic Investigation of
the Selective Am(III) Complexation with TS-BTPhen. Solvent Extr.
Ion Exch. 2016, 34 (2), 126−140.
ORCID
Patrik Weßling: 0000-0002-4410-5344
Christian Adam: 0000-0003-1449-7065
Author Contributions
All authors have given approval to the final version of the
manuscript. All authors contributed equally.
(
14) Macerata, E.; Mossini, E.; Scaravaggi, S.; Mariani, M.; Mele, A.;
Funding
Panzeri, W.; Boubals, N.; Berthon, L.; Charbonnel, M. C.; Sansone,
F.; Arduini, A.; Casnati, A. Hydrophilic Clicked 2,6-Bis-triazolyl-
pyridines Endowed with High Actinide Selectivity and Radiochemical
Stability: Toward a Closed Nuclear Fuel Cycle. J. Am. Chem. Soc.
This work has received funding from the European Research
Council (ERC) under the European Union’s Horizon 2020
research and innovation program (Project GENIORS, Grant
No. 755171). This work has benefited from the equipment and
framework of the COMP-HUB Initiative, funded by the
2
(
016, 138 (23), 7232−7235.
15) Bourg, S.; Geist, A.; Adnet, J.-M.; Rhodes, C.; Hanson, B.
GENIORS, a new European project addressing Gen IV integrated oxide
fuels recycling strategies, Proceedings of GLOBAL 2017, International
Nuclear Fuel Cycle Conference, Seoul, Korea, Sept 24−29, 2017.
“Departments of Excellence” program of the Italian Ministry
for Education, University and Research (MIUR, 2018−2022)
Notes
The authors declare no competing financial interest.
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by means of completely incinerable extractants, EUR 18038; European
Commission: Luxembourg, 1998.
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