Thermodynamic, Spectroscopic, and Computational Studies of f-Element Complexation by N-Hydroxyethyl-diethylenetriamine-N,N′,N,N-tetraacetic Acid

Article abstract of DOI:10.1021/acs.inorgchem.6b02897

Potentiometric and spectroscopic techniques were combined with DFT calculations to probe the coordination environment and determine thermodynamic features of trivalent f-element complexation by N-hydroxyethyl-diethylenetriamine-N,N′,N,N-tetraacetic acid, HEDTTA. Ligand protonation constants and lanthanide stability constants were determined using potentiometry. Five protonation constants were accessible in I = 2.0 M (H⁺/Na⁺)ClO₄. UV-vis spectroscopy was used to determine stability constants for Nd³⁺ and Am³⁺ complexation with HEDTTA. Luminescence spectroscopy indicates two water molecules in the inner coordination sphere of the Eu/HEDTTA complex, suggesting HEDTTA is heptadentate. Luminescence data was supported by DFT calculations, which demonstrate that substitution of the acetate pendant arm by a N-hydroxyethyl group weakens the metal-nitrogen bond. This bond elongation is reflected in HEDTTA’s ability to differentiate trivalent actinides from trivalent lanthanides. The trans-lanthanide Ln/HEDTTA complex stability trend is analogous to Ln/DTPA complexation; however, the loss of one chelate ring resulting from structural substitution weakens the complexation by ～3 orders of magnitude. Successful separation of trivalent americium from trivalent lanthanides was demonstrated when HEDTTA was utilized as aqueous holdback complexant in a liquid-liquid system. Time-dependent extraction studies for HEDTTA were compared to diethylenetriamine-N,N,N′,N,N-pentaacetic acid (DTPA) and N-hydroxyethyl-ethylenediamine-N,N′,N′-triacetic acid (HEDTA). The results indicate substantially enhanced phase-transfer kinetic rates for mixtures containing HEDTTA.

Full text of DOI:10.1021/acs.inorgchem.6b02897

Article
pubs.acs.org/IC
Thermodynamic, Spectroscopic, and Computational Studies of
f‑Element Complexation by N‑Hydroxyethyl-diethylenetriamine-
N,N′,N″,N″‑tetraacetic Acid
,
†
†
‡
‡
Travis S. Grimes,* Colt R. Heathman, Santa Jansone-Popova, Vyacheslav S. Bryantsev,
‡
§
,†
Sriram Goverapet Srinivasan, Masahiko Nakase, and Peter R. Zalupski*
†
Aqueous Separations and Radiochemistry, Idaho National Laboratory, Idaho Falls, Idaho, 83415, United States
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
‡
§
Actinide Chemistry Group, Japan Atomic Energy Agency, Material Science Research Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun,
Hyogo-pref 679-5148 Japan
*
S Supporting Information
ABSTRACT: Potentiometric and spectroscopic techniques were combined with
DFT calculations to probe the coordination environment and determine
thermodynamic features of trivalent f-element complexation by N-hydroxyethyl-
diethylenetriamine-N,N′,N″,N″-tetraacetic acid, HEDTTA. Ligand protonation
constants and lanthanide stability constants were determined using potentiometry.
+
+
Five protonation constants were accessible in I = 2.0 M (H /Na )ClO . UV−vis
4
spectroscopy was used to determine stability constants for Nd3+ and Am
3+
complexation with HEDTTA. Luminescence spectroscopy indicates two water
molecules in the inner coordination sphere of the Eu/HEDTTA complex,
suggesting HEDTTA is heptadentate. Luminescence data was supported by DFT
calculations, which demonstrate that substitution of the acetate pendant arm by a
N-hydroxyethyl group weakens the metal−nitrogen bond. This bond elongation is
reflected in HEDTTA’s ability to differentiate trivalent actinides from trivalent
lanthanides. The trans-lanthanide Ln/HEDTTA complex stability trend is analogous to Ln/DTPA complexation; however, the
loss of one chelate ring resulting from structural substitution weakens the complexation by ∼3 orders of magnitude. Successful
separation of trivalent americium from trivalent lanthanides was demonstrated when HEDTTA was utilized as aqueous holdback
complexant in a liquid−liquid system. Time-dependent extraction studies for HEDTTA were compared to diethylenetriamine-
N,N,N′,N″,N″-pentaacetic acid (DTPA) and N-hydroxyethyl-ethylenediamine-N,N′,N′-triacetic acid (HEDTA). The results
indicate substantially enhanced phase-transfer kinetic rates for mixtures containing HEDTTA.
trivalent lanthanides from Am3+ and Cm . The TALSPEAK
process balances the organic soluble organophosphorus
extractant against an aqueous medium with metal ions, DTPA,
3+
1
. INTRODUCTION
Aminopolycarboxylic acids are well-known Lewis bases that
contain both N and O donor atoms. The nitrogen donor groups
are found in the amine backbone, while the oxygen donor atoms
are available through acetic acid pendant arms functionalized to
the amine groups. These chelators have high affinity for metal
ions, forming strong complexes through polydentate coordina-
3
+
and carboxylic acid buffer, adjusted to pH 3.0−4.0. The An /
Ln differentiation is accomplished due to selective coordina-
tion of An metal ions by DTPA in the aqueous environment,
while a transport of Ln metal ions to the organic phase is
facilitated by a liquid cation exchanger. Carboxylic acid buffer is
needed to maintain aqueous phase acidity as the organo-
3+
3+
3+
1
tion. Since aminopolycarboxylic acid metal complexes are very
stable they are present in a variety of household commodities,
such as foods, shampoos, and detergents, and find many
applications in industrial (photography, water treatment, soil
remediation) and medical (imaging, radioimmunotherapy,
+
phosphorus extractant exchanges H for metal ions during
extraction. Although it efficiently separates trivalent actinides
from trivalent lanthanides, one major drawback to TALSPEAK
chemistry is slow attainment of liquid−liquid distribution
equilibrium. The kinetic limitation has been attributed to
energetics of structural rearrangement necessary for dissociation
2
−10
chelation) sectors.
The aminopolycarboxylate, diethylenetriamine-
N,N,N′,N″,N″-pentaacetic acid (DTPA) ubiquitous in industrial
products, is also a key component for successful 4f−5f group
of [M(DTPA)]2 complexes. This process obstacle has been
−
13
11,12
subdued by the addition of high concentrations of carboxylic acid
separations. Weaver and Kapplemann
developed the TAL-
SPEAK process (Trivalent Actinide Lanthanide Separations
using Phosphorus-reagent Extraction from Aqueous Kom-
plexes), a liquid−liquid extraction system, which separates
Received: November 29, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02897
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 1. Synthetic scheme for the preparation of N-hydroxyethyl-diethylenetriamine-N,N′,N″,N″-tetraacetic acid, HEDTTA (structure 3).
buffer (1.0−2.0 M) at the expense of extraction system “ideality”
trivalent lanthanides was demonstrated when HEDTTA was
14,15
and thermodynamic predictability.
Recently, several new
utilized as aqueous holdback complexant in a liquid−liquid
system. Interestingly, time dependencies for the partitioning of
trivalent f-elements indicate the enhancement of kinetic rates
beyond those expected based solely on progressive reduction of
ligand’s denticity.
1
6−21
processes
have been developed using the TALSPEAK
3
+
3+
framework to improve Ln /An separations, but the kinetic
shortcomings still plague the new systems.
This work, a third contribution in a series by this group,
22,23
continues to evaluate thermodynamic and kinetic impacts of new
aminopolycarboxylic acid holdback reagents designed to
facilitate 4f−5f element separations using a TALSPEAK-type
2
. EXPERIMENTAL SECTION
2
2
2.1. Reagents. 2,2′-((2-((Carboxymethyl)(2-((carboxymethyl)(2-
chemistry platform. In the first study ethylenediamine-
N,N,N′,N′-tetraacetic acid was modified by the substitution of
two acetate pendant arms with two acetylglycine functionalities,
forming hexadentate ethylenediamine-N,N′-di(acetylglycine)-
N,N′-diacetic acid (EDDAG-DA). The induction effects of the
amide groups reduced the overall “softness” of the amine
hydroxyethyl)amino)ethyl)amino)ethyl)azanediyl)diacetic acid
(IUPAC name for HEDTTA) was isolated as triflate or HCl salts
according to synthetic scheme presented in Figure 1. A detailed
description of the synthetic methodology and product characterization
is provided in the Supporting Information. Metal ions investigated using
potentiometric and spectrophotometric methods to determine stability
3
+
3+
3+
3+
backbone, subsequently reducing An selectivity, but promoted
metal complex formation over ligand protonation in lower pH
regimes. The same amide substitution was made on a
constants include Y , the lanthanide series (excluding Pm ), and Am .
The Y3 and lanthanide perchlorate stocks were prepared as previously
+
2
2
3+
described. The Am perchlorate stock solution was obtained from
Idaho National Laboratory stocks and purified using N,N,N′,N′-tetra-n-
octyldiglycolamide (DGA) extraction chromatographic resin (Ei-
chrom). The Am was first adsorbed on a DGA column and then
eluted using dilute HCl. Several evaporation cycles were performed to
23
diethylenetriamine backbone in the second study, forming
octadentate diethylenetriamine-N′,N″-bis(acetylglycine)-
N,N′,N″-triacetic acid (DTTA-DAG) in an attempt to regain
3+
3
+
3+
ligand softness required for efficient An /Ln differentiation.
drive off the HCl, and the final material was dissolved in 0.1 mM HClO .
4
The Am3 concentration was determined using a spectrophotometric
+
The study demonstrated a pathway to enhanced phase-transfer
+
24
kinetics via H -catalyzed dissociation of the aminopolycarbox-
titration method reported by Tian and Shuh. Metal ion solutions
3
+
3+
ylate metal complexes. The bisamide substitution enhanced the
reagent’s ability to complex metal ions through more efficient
competition with the protonation equilibria while maintaining
investigated with solvent extraction techniques include Am and La −
3+
Ho group of lanthanides, prepared from lanthanide nitrate stocks
previously standardized using ICP-MS.
3
+
Sodium perchlorate and sodium nitrate salts (ACS reagent grade)
were purchased from GFS Chemicals. Concentrated background
electrolyte stock solutions were prepared separately (5.0 mol/kg). Salt
crystals were dissolved in 18 MΩ deionized water, filtered through a fine
glass frit filter, and recrystallized by heating to reduce the volume until
the salts crashed out of solution. After cooling, the remaining solution
was decanted thoroughly. The salt crystals were dissolved in a minimal
amount of 18 MΩ deionized water, and each solution was standardized
efficient separation of Am from all lanthanides below pH = 2.0.
In this study a different approach is used to enhance the phase-
transfer kinetics. Danesi and Cianetti studied the interfacial mass
transfer of Eu3 in a liquid−liquid system containing a buffered
aqueous solution of an aminopolycarboxylate and a nonaqueous
mixture of the liquid cation exchanger (bis-2-ethylhexylphos-
+
13
phoric acid, HDEHP) in dodecane. The authors demonstrated
that mass transfer across the liquid−liquid boundary is fully
controlled by the rates of interfacial chemical reactions, with no
diffusional resistance due to migration through the stagnant
+
using ion-exchange chromatography (Dowex 50WX8 beads, H form,
1
00−200 mesh) and potentiometric titrations.
Sodium hydroxide solutions were prepared by adding the desired
1
3
amount of 50% w/w NaOH (Sigma-Aldrich) and NaClO in 18 MΩ
4
interfacial zones. Accordingly, the authors recommended the
use of smaller, easier to unravel (relative to DTPA) complexants
as the only option to speed up the liquid−liquid partitioning
equilibria. This strategy is employed here through a specific
structural modification of DTPA. A single acetate pendant arm of
DTPA was replaced by a hydroxyethyl group, thus reducing the
denticity of the reagent. A thorough thermodynamic and kinetic
characterization of N-hydroxyethyl-diethylenetriamine-
N,N′,N″,N″-tetraacetic acid, HEDTTA, is presented. The
coordination environment of a trivalent f-element/HEDTTA
complex was probed by luminescence spectroscopy, and its
theoretical geometry was described via ab initio molecular
dynamics simulations based on density functional theory (DFT).
Effects of reduction of the f-element coordination environment
on the stability of metal chelate were studied using potentiometry
and spectroscopy. The differentiation of trivalent actinides from
deionized water. The ionic strength was adjusted to match the titrand
solutions to maintain constant ionic strength throughout the titrations.
All NaOH solutions were standardized by titration of primary standard
potassium hydrogen phthalate (KHP).
Organic extractant 2-ethylhexyl phosphonic acid mono-2-ethyhexyl
ester (HEH[EHP]) was purchased from Yick-Vic Chemicals. The
25
HEH[EHP] was purified using the third phase formation method and
determined to be >99% by potentiometric titration. The organic diluent
n-dodecane (Sigma-Aldrich) was used without further purification. The
organic phases were prepared by dissolving weighed amounts of
HEH[EHP] in n-dodecane.
2
.2. Potentiometry. Acid dissociation (K ) and metal complex
a
stability (βmhl) constants were determined by potentiometric titration
using a Mettler Toledo T-50 graphix autotitrator. Titrand solution
temperatures were held constant at 25.0 ± 0.1 °C with a jacketed beaker
and a circulating water bath. All titrations were blanketed with hydrated
nitrogen gas (bubbled through 1.0 M NaOH) to prevent CO2
B
DOI: 10.1021/acs.inorgchem.6b02897
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
absorption. A 1.5 L solution of 5.0 mM HEDTTA (I = 2.0 M adjusted
distribution ratios were calculated using the following equation, where
[M]i,aq corresponds to initial concentration of metal in aqueous mixture
and [M]f,aq the final concentration of metal in aqueous mixture after
equilibration with nonaqueous solution
+
+
with (H /Na )ClO ) was prepared at pC 1.65. This pCH condition
4
H
guaranteed access to the lowest obtainable K by a pH electrode and
a
control of the starting pC for each titration, thus eliminating the need
H
for pCH corrections in the fitting exercises. A Ross Orion semimicro
[
M]i,aq − [M]f,aq
glass electrode was used to measure changes in pC . The electrode
H
DM
=
filling solution was changed from KCl to 5.0 M NaCl to prevent KClO4
precipitation. The high concentration of NaCl was used for rapid
electrode response and to minimize variations in the junction potential.
The electrode was calibrated (I = 2.0 M, T = 25.0 ± 0.1 °C) by strong
acid−strong base titration with standardized reagents. Gran anal-
[M]f,aq
(1)
When radioisotope tracers were utilized, distribution ratios were
determined from the ratio of activity in organic and aqueous phase.
Phase-transfer kinetic experiments (aqueous into organic) were
performed by monitoring time-dependent partitioning of radioisotopes
2
6−28
ysis
of the titration data provided a calibration curve (pC versus
H
+
241
139
154
mV) to determine the H concentration from hydrogen ion activity.
Acid dissociation and metal complex stability titration data were
Am, Ce, and Eu (Eckert and Ziegler) between two immiscible
liquid phases. Nonaqueous phases contained liquid cation exchangers
(2-ethylhexyl)-2-ethylhexyl-phosphonic acid, HEH[EHP], or di(2-
ethylhexyl)phosphoric acid, HDEHP, in n-dodecane. Preliminary
equilibration experiments identified the required content of ion
exchanger to ensure partitioning of the majority (>80%) of metal ions
into the nonaqueous environment at equilibrium. Aqueous phases
contained 20 mM holdback complexant (except for 5 mM DTPA) in 1
29
analyzed using Hyperquad2013 fitting software.
.3. Spectrophotometric Titrations. Absorption spectra for
2
−
[
Nd(HEDTTA)] complexation (550−610 nm, 0.1 nm interval)
were collected using an Agilent Cary-6000i UV−vis−NIR spectropho-
tometer equipped with a 100 cm waveguide capillary flow cell (World
Precision Instruments) at ambient temperature (20 ± 1 °C). Solution A
3+
3+
+
+
(
[Nd ] = 0.533 mM, I = 2.0 M, pC = 2.04) and solution B ([Nd ] =
M (H /Na )NaNO and were adjusted to pC = 3.0 using NaOH. To
H
3
H
0
.538 mM, [HEDTTA] = 14.482 mM, I = 2.0 M, pC = 9.02) were
slow down the phase-transfer equilibrium and enhance reagent
differences on a kinetic scale measurable by solvent extraction, the
buffers were omitted. Phase-transfer kinetic studies for HEDTTA were
compared to conventional aminopolycarboxylate reagents HEDTA and
DTPA. Organic solutions containing either HEH[EHP] or HDEHP
H
mixed in appropriate amounts to produce an HEDTTA dependence at
3+
constant volume (2.5 mL), [Nd ], and I = 2.0 M. The solutions were
mixed to produce 20 spectra for the titration. Absorption spectra for
−
[
Am(HEDTTA)] complexation (400−600 nm, 0.37 nm interval) were
+
+
collected in a 1 cm quartz cuvette (Starna) using a Flame-S-VIS-NIR-ES
Ocean Optics device coupled with to an Ocean Optics DH-2000-BAL
light source through a 2 m (200 μm) fiber optic cable at ambient
temperature (20 ± 1 °C). Titrand solution A ([Am ] = 0.796 mM, I =
.0 M, pC = 1.92, 0.8 mL) was titrated with 0.8 mL of solution B
were pre-equilibrated using a 1 M (H /Na )NaNO , pC = 3.0 solution
3 H
prior to contact with the aqueous complexant mixtures containing
radioisotopes. Two-phase experiments were vortexed on a Glass-Col
multitube shaker (motor speed setting 50) and centrifuged, and the
liquid phases were sampled for radiometric measurements. Liquid−
3+
2
(
H
3+
3+
[Am ] = 0.788 mM, [HEDTTA] = 16.721 mM, I = 2.0 M, pC = 4.58)
liquid distribution of Eu only was monitored by measuring its activity
H
using titrant additions that varied from 5 to 150 μL. Separate titrations
(15−300 keV energy window) using the Packard D5003 Cobra Gamma
counter. Partitioning of mixed radioisotope samples was measured on an
ORTEC GEM50P4 coaxial HPGe detector equipped with a DSPEC
gamma spectrometer (59.54 keV peak for Am-241, 123.07 keV peak for
Eu-154, and 165.86 keV peak for Ce-139).
were performed to measure pCH after each ligand addition. Stability
−
constants of the [M(HEDTTA)] complexes were resolved using the
30
HypSpec spectral data fitting program.
.4. Fluorescence Measurements. Luminescence lifetimes were
2
collected using a HORIBA Jobin Yvon IBH FluoroLog-3 fluorometer
adapted for time-resolved measurements. A submicrosecond xenon flash
lamp (Jobin Yvon, 5000XeF) coupled to a double-grating excitation
monchromator for spectral selection was used as the light source. The
detector consists of a single-photon detection module (HORIBA Jobin
Yvon IBH, TBX-04-D) coupled with a fast rise time photomultiplier
tube, a wide bandwidth preamplifier, and a picosecond constant fraction
discriminator. Data were collected using an IBH Data Station Hub and
analyzed using the DAS 6 decay analysis software package from
HORIBA Jobin Yvon IBH. Lifetime data were modeled using single- and
3
. RESULTS
.1. Potentiometric Determination of Acid Dissocia-
3
tion Constants and Ln/HEDTTA Stability Constants.
Experimental and theoretical titration curves combined with
HEDTTA speciation are shown in Figure 2. Best fits were
obtained when using a five-dissociable proton model to represent
the experimental data. The acid dissociation reactions for
HEDTTA are summarized by cumulative equilibria expression
as described by eq 2. Table 1 lists the acid dissociation constants
for HEDTTA resolved in this study, together with pK values
previously determined at I = 2.0 M for DTPA and DTTA-PA for
comparison.
(pK ≈ 16) is not readily dissociable in the pC
potentiometric titration; therefore, it was treated as an inert
2
double-exponential decay curves. The χ values ranged between 1.03
and 1.05 for all curve-fitting iterations. Samples were analyzed in 1.0 cm
quartz cuvettes (Starna); the temperature was controlled at 25.0 ± 0.1
a
°
C using a water-jacketed cuvette holder and a circulating water bath.
.5. Density Functional Theory Calculations. Theoretical
calculations were performed to simulate the participation of
CH CH OH in the coordination of europium metal ion. Density
35,36
The proton associated with the ethyl alcohol
2
range typical of
a
H
−
2
2
2
+
3+
functional theory (DFT)-based molecular dynamics (MD) simulations
pendant arm for modeling purposes. The H6R and H7R
species have weak proton association, and the acidic pK values
were not accessible using potentiometric titration methods due
to the limitations of the glass electrode.
were carried out in the Born−Oppenheimer approximation using the
a
31−34
VASP software.
A detailed description of calculations is provided in
the Supporting Information.
.6. Solvent Extraction. Solvent extraction experiments were
conducted to test the selectivity of HEDTTA toward actinides. Aqueous
2
+4−n
+3−n
+ H+K
n
H
L
⇄ H7−n
L
3
+
8−n
a
phase (1.0 mM Ln , 20 mM HEDTTA, 0.5 M malonate buffer, pC 3.0,
H
+
+
+
+3−n
I = 2.0 M adjusted (H /Na )NaNO ) was pre-equilibrated using a
[H ]·[H7
L
]
3
−n
4−n
Glass-Col multitube vortexer with neat n-dodecane for 30 min and then
centrifuged for 5 min. Organic phase (0.08 M HEH[EHP]/n-dodecane)
was pre-equilibrated with metal ion free aqueous phase for 30 min and
then centrifuged for 5 min. Equal phase ratios (0.5 mL each) of pre-
equilibrated aqueous and organic phases were contacted for 30 min and
then centrifuged for 5 min. Aqueous phases were sampled before and
after equilibrium; the samples were diluted with 2% nitric acid and
analyzed using ICP-MS to determine lanthanide concentration. The
=
, n = 1...7
+
[H
L
]
(2)
8−n
−
Stability constants for [Ln(HEDTTA)] complexation were
determined by titrating 1:1 metal/HEDTTA mixtures. The
results of a typical Ln /HEDTTA titration, calculated best fit
curve, and Nd3+ complex speciation are shown in Figure 3. The
large inflection point at ∼7.75 equiv of NaOH is typical of 1:1
3+
C
DOI: 10.1021/acs.inorgchem.6b02897
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
+
Figure 3. Potentiometric titration of HEDTTA in the presence of Nd ,
Figure 2. Potentiometric titration of HEDTTA to determine
3+
superimposed on the distribution of Nd species. Titrant: 0.328 M
protonation constants overlaid on distribution of ligand species. Titrant:
+
+
+
+
NaOH, I = 2.0 M (Na /H )ClO . Titrand: C
= 4.68 mM
0
.330 M NaOH, I = 2.0 M (Na /H )ClO . Titrand: C
= 4.68 mM
4
HEDTTA
4
HEDTTA
+
+
HEDTTA, C = 0.022 M, CNd3+ = 4.95 mM, Vint = 25.068 mL, I = 2.0 M
HEDTTA, C + = 0.022 M, V = 25.098 mL, I = 2.0 M (Na /H )ClO ,
H+
H
int
4
+
+
(
Na /H )ClO , T = 25.0 ± 0.1 °C. Experimental pC (○), calculated
4 H
3+
T = 25.0 ± 0.1 °C. Experimental pC (□), calculated pC (solid orange
H
H
+
−
pCH (solid orange line), Nd (green dashed line), NdHL(aq) (red
line), H L (dashed black line), H L (red dashed line), H L (green
dashed line), H L (blue dashed line), HL (cyan dashed line), and
5
4
3
−
2−
3−
dashed line), NdL (blue dashed line).
2
4
−
L
(magenta dashed line).
[
MHL(aq)]
−
+
HEDTTA
ML + H ⇄ MHL
K
= [
(aq)
111
−
+
ML ]·[H ]
(5)
metal/ligand complex formation. In previous f-element/DTPA
−
2−
complexation studies, both [M(DTPAH)] and [M(DTPA)]
3.2. Spectrophotometric Determination of Stability
Constants. Absorbance spectra of free metal and metal ligand
complexes are shown in Figure 4A and 4B. Typically, Nd (ionic
radius =1.109 Å) is used as a nonradioactive metal ion surrogate
37,38
complexes have been reported in acidic aqueous media.
On
3
+
the basis of the analogous structure of HEDTTA the same metal
3
+
complexation equilibria were used to model the Ln /HEDTTA
3+
titration data.
of Am (ionic radius = 1.09 Å) due to similar ionic radii. Optical
In this study, the conventional nomenclature βmhl and Kmhl are
used where subscripts define the complex stoichiometry with
respect to metal ion (m), hydrogen ion (h), and ligand (l) for the
overall reactions and stepwise reactions, respectively. The
stability constants for M /HEDTTA complexation are shown
in Table 2. Metal ion complexation equilibria for HEDTTA are
expressed by eqs 3, 4 and 5.
absorbances due to f−f transitions at λ = 575 nm for trivalent
max
4
4
neodymium ( I → G ) and λ = 503 nm for trivalent
9
/2
5/2
max
7
5
americium ( F → L ) were monitored. Both transitions are
sensitive to metal/ligand coordination.
0
6
3
9,40
In both titrations, the
3
+
intensities of both peaks corresponding to free metal decreased
as HEDTTA concentration increased. The optical absorbance
3+
3+
features were red shifted (585 nm for Nd , 507 nm for Am ),
resulting from the coordination of trivalent f-elements by
HEDTTA.
−
[
ML ]
M3+ + L4− ⇄ ML−
HEDTTA
β1
= [
01
3+
4−
M ]·[L ]
(3)
The spectrophotometric titration data was analyzed using
30
HypSpec software. The best theoretical representation of
spectral information was afforded when three absorbing species
M3 + H + L ⇄ MHL(aq)
+
+
4−
3+
−
(M , ML , and MHL ) were present in the model. The
(
aq)
3+
3+
calculated molar absorptivities for Nd and Am species are
shown in Figure 4C and 4D. The formation equilibria of two
absorbing metal complexes ML and MHL are described by
[
MHL(aq)]
HEDTTA
β1
= [
11
3+
+
4−
M ]·[H ]·[L ]
(4)
−
(
aq)
a
Table 1. Acid Dissociation Constants for HEDTTA at 25.0 ± 1 °C
b
−log10 Kam
−log10 Kam
HEDTTA
HL3
n
−log10 Kan
DTTA-PA
HL4−
m
DTPA
HL4−
−
7
6
5
4
3
2
1
9.49 ± 0.02
8.19 ± 0.01
4.31 ± 0.02
2.56 ± 0.02
2.15 ± 0.02
8
7
6
5
4
3
2
1
9.64
8.86
4.52
3.54
2.79
9.50
8.31
4.38
2.53
2.41
H L2
−
H2L
3−
H2L
3−
2
H3L−
H4L(aq)
H5L+
H6L2+
H7L3+
H3L
2−
H3L
2−
−
−
H4L
H4L
H5L(aq)
H6L+
H7L2+
H8L3+
H5L(aq)
H6L+
2+
H7L
3+
H8L
a
+
+
35,36
b
+
+
I = 2.0 M (H ,Na )ClO . Data for DTTA-PA and DTPA displayed for comparison.
I = 0.1 (H ,K )NO
4
3
D
DOI: 10.1021/acs.inorgchem.6b02897
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
+
Table 2. Cumulative Stability Constants log β and log β for M /HEDTTA Complexes Determined Using Potentiometry
1
0
101
10 111
+
+
a
(
25.0 ± 0.1 °C) and Spectroscopy (20 ± 1 °C) in 2.0 M (Na /H )ClO
4
HEDTTA
DTPA
metal
log10 β101
log10 β111
log10 K111
log10 β101
log10 K111
Y3+
La3
17.18 ± 0.01
15.38 ± 0.01
16.35 ± 0.01
17.00 ± 0.01
17.28 ± 0.01
18.15 ± 0.01
18.10 ± 0.01
18.09 ± 0.01
18.18 ± 0.01
18.12 ± 0.01
18.05 ± 0.01
17.92 ± 0.01
17.79 ± 0.01
17.72 ± 0.02
17.68 ± 0.02
17.50 ± 0.06
18.34 ± 0.08
19.83 ± 0.01
18.31 ± 0.03
19.15 ± 0.02
19.35 ± 0.02
19.42 ± 0.02
20.22 ± 0.03
20.42 ± 0.01
20.27 ± 0.01
20.34 ± 0.01
20.43 ± 0.01
20.43 ± 0.01
20.47 ± 0.01
20.63 ± 0.01
20.77 ± 0.02
20.69 ± 0.02
20.41 ± 0.02
20.67 ± 0.04
2.65 ± 0.01
2.93 ± 0.02
2.81 ± 0.01
2.36 ± 0.01
2.14 ± 0.02
2.07 ± 0.01
2.32 ± 0.01
2.18 ± 0.01
2.15 ± 0.01
2.31 ± 0.01
2.38 ± 0.01
2.56 ± 0.01
2.83 ± 0.01
3.05 ± 0.01
3.01 ± 0.01
2.91 ± 0.05
20.13
18.02
19.06
19.64
20.23
20.79
21.03
21.15
21.15
21.23
21.43
21.41
21.10
20.96
21.14
1.74
2.36
1.72
1.71
1.34
1.59
1.54
1.23
1.61
1.58
1.33
1.25
1.35
1.57
1.07
+
3+
Ce
3
+
Pr
3
+
+
Nd
Sm
3
3
+
Eu
3
+
Gd
3
+
+
Tb
Dy
3
3
+
Ho
Er3
+
Tm3
+
3
+
Yb
3
+
Lu
3
+
b
Nd
Am3+
b
2.33 ± 0.07
a
+
+
35
Stability constants are compared to Ln/DTPA complexes determined in 2.0 M (Na /H )ClO using potentiometry. Stepwise log K constants
4
10 111
3+
3+
b
for the protonation of Ln /HEDTTA and Ln /DTPA complexes are tabulated for comparison. Denotes stability constants determined using
spectrophotometry.
Figure 4. Spectrophotometric titrations of (A) Nd(ClO ) and (C) Am(ClO ) with HEDTTA accompanied by calculated molar absorptivities for (B)
4
3
4 3
3
+
3+
+
+
Nd and (D) Am absorbing species. (A) Titrant: C
= 0.3536 mM, CHEDTTA = 14.482 mM, pC = 9.02, I = 2.0 M (Na /H )ClO . Titrand: 0.533
Nd3+
H
4
+
+
mM Nd(ClO ) , pC = 2.04, I = 2.0 M (Na /H )ClO , T = 20.0 ± 1.0 °C. (C) Titrant: C
= 0.788 mM, CHEDTTA = 16.721 mM, pC = 4.58, I = 2.0 M
H
4
3
H
4
Am3+
+
+
+
+
(
Na /H )ClO . Titrand: C
= 0.5796 mM, pC = 1.92, I = 2.0 M (Na /H )ClO , T = 20.0 ± 1.0 °C.
Am3+ H 4
4
3+
3+
eqs 3 and 4, respectively. The stability constants for Nd /
HEDTTA complexes determined using spectroscopy were
comparable to potentiometric results given differences in
experimental conditions (Table 2).
3.3. Luminescence Lifetimes of Eu /HEDTTA Solu-
tions. Luminescence lifetime measurements were collected for
trivalent europium in the presence of HEDTTA in aqueous
environment adjusted to pCH = 1.0, 2.4, and 5.0. The
experimental data and calculated best fit lines are shown in
E
DOI: 10.1021/acs.inorgchem.6b02897
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Inorganic Chemistry
Article
Figure 5. Trivalent europium luminescence is produced by
Table 3. Luminescence Lifetimes and Waters of Hydration for
5
a
relaxation of electronic transitions from D (lowest excited
Eu/HEDTTA Complexes at Various pC Conditions
0
H
%
species
pCH
τ (μs)
kobs (s−1)
nH2O (±0.5)
Eu3+
EuL−
EuHL(aq)
1
2
2
5
.0
.4
.4
.0
109
120
458
483
9196
8354
2185
9.2
8.3
1.9
1.7
100
31.6
0.0
0.0
0.0
0.0
0.0
37.5
99.8
30.9
0.2
2071
0.0
a
3+
Aqueous phase: [Eu ] = 0.001 M, [HEDTTA]total = 0.010 M, I =
total
+
+
2
.0 M (H /Na )ClO .
4
ion and significantly dehydrated metal due to formation of
−
EuHL(aq) and EuL complexes.
3
.4. Liquid Liquid Distribution Measurements. Figure 6
3
+
shows the distribution of trivalent lanthanides and Am between
Figure 5. Luminescence lifetime data for aqueous mixtures containing
3
+
Eu ion and HEDTTA, collected at different pC conditions. For all
H
+
+
solutions CEu = 1.0 mM, CHEDTTA = 10.0 mM, I = 2.0 M (Na /H )ClO ,
4
T = 25.0 ± 0.1 °C. Solid red lines represent best fits calculated using
exponential functions.
7
state) to the ground state F (where j = 0, 1, 2, 3...).
j
Luminescence yields are quenched by nonradiative vibrational
decay through OH oscillators (typically coordinated H O
2
molecules). The degree of quenching is directly proportional
to the number of H O molecules found in the inner coordination
2
41
sphere. Horrocks and Sudnick established the correlation
between luminescent lifetime and inner coordination sphere
H O molecules. Waters of hydration were calculated using the
2
42
empirical equation developed by Kimura and Kato, where kobs
is the decay constant for the luminescent lifetime and is
−
1
expressed in s .
3
+
3+
3+
Figure 6. Distribution of (blue squares) La −Ho (except Pm ),
−
3
Eu
obs
NH2O = 1.05 × 10 k − 0.44
(6)
139
3+
154
3+
241
3+
(green triangles) Ce and Eu , and (red circles) Am between
a nonaqueous solution of ion exchanger and an aqueous mixture of
The aqueous acidities were chosen to maximize the
3+
HEDTTA complexant. Aqueous phase: 1.0 mM total Ln , 0.5 M
3
+
−
concentrations of free Eu , EuHL , and EuL species in the
+
+
(aq)
Malonate, 20.0 mM HEDTTA, 1.0 M (H /Na )NO , pC = 3.0.
3
H
3+
Eu /HEDTTA mixtures. As the pC was increased lumines-
H
Organic phase: 0.1 M HEH[EHP] in n-dodecane.
cence lifetimes became longer. Single-exponential functions were
used to obtain luminescence lifetimes (τ) for pC 1.0 and 5.0
systems (best fit with χ = 1.03 and 1.05, respectively), indicating
H
2
nonaqueous environment containing HEH[EHP] in n-dodecane
the presence of a single luminescent species. A single-exponential
and aqueous electrolyte mixture containing HEDTTA holdback
1
39
3+
function was used to model the pC 2.4 system which produced a
reagent. The figure also shows the distribution ratios for Ce
H
2
154
3+
best fit of χ = 2.1 and suggested two waters of hydration.
and Eu determined using radiometric techniques, illustrating
Therefore, based on the poor fit using a single-exponential
function and the known speciation from stability constant
determinations, a double-exponential function was used to
good agreement with the ICP-MS data. The extraction efficiency
3
+
increases throughout the investigated lanthanide group (La −
3+
Ho ). Three tetrads are evident in the extraction data. The
increased lanthanide distribution across the series observed in
the HEDTTA system closely resembles TALSPEAK chemistry
model the pC 2.4 system. The fitting iterations indicated two
H
distinguishable luminescent components and produced a best fit
2
3+
3+
with χ = 1.05. Luminescence decay rates, hydration numbers
with the exception of La . Here the distribution ratio of La is
∼1, compared to ∼10 in the original TALSPEAK formulation.
The distribution data shows that effective Ln/An differentiation
3
+
calculated using eq 6, and Eu /HEDTTA speciation are shown
in Table 3. The short 109 μs lifetime (τ), measured at a pC of
H
3
+
1
.0, is consistent with full quenching from nonradiative
is facilitated by HEDTTA suppressing the partitioning of Am
vibrations through OH oscillators, indicating the presence of a
to the organic phase. The minimum group separation factor
3
+
Ln
fully hydrated (n = 9) Eu metal ion in solution. At pC 5.0 the
(SF = D /D ) was driven by lanthanum (D /D = 8) for 20
H
An
Ln
An
La
Am
longer quenching (483 μs) yielded 1.7 coordinated waters
mM HEDTTA, pC = 3.0 aqueous complexing environment.
H
according to eq 6, suggesting the presence of 2 waters of
Figure 7a compares the time-dependent partitioning trends
collected when ion exchangers facilitated the forward extraction
3
+
hydration in the inner coordination sphere of Eu complex. At
3
+
−
3+
pC 2.4 a mixture of free Eu , EuHL , and EuL species is
of Eu from aqueous mixtures of HEDTTA, HEDTA, and
H
(aq)
present in solution. The best fit representation of the quenching
data confirms the presence of a fully hydrated europium metal
DTPA. The distribution of europium increases steadily for each
system until the phase-transfer equilibrium is manifested by a
F
DOI: 10.1021/acs.inorgchem.6b02897
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Inorganic Chemistry
Article
HEDTTA may be inferred from the luminescence results
(
nH2O = 1.7) assuming seven inner-sphere waters of hydration are
expelled from aqua metal ion due to inner-sphere coordination
by HEDTTA. This heptadentate coordination matches other
literature accounts of f-element complexation by DTTA-type
reagents, where a single acetate group of DTPA has been
43,44
replaced by coordination-inert moieties.
The comparison of
luminescence quenching at pCH 1.0 and 5.0 suggests the
hydroxyethyl functionality is inert to metal binding, likely
extending out into bulk solvent. This is consistent with a report
45
by Moeller and Horowitz that suggested the hydroxyethyl
functionality of HEDTA does not participate in the ligand
complex. The presence of a protonated complex MHL(aq) at pCH
2
.4 cannot be distinguished from the influence of the ML−
complex on luminescence lifetime decay signatures. The double-
exponential fit resolves two main components contributing
different luminescence quenching properties. The resolved n
H2O
=
1.9 value for the coordinated Eu3+ species represents the
−
average hydration number for both EuL and EuHL(aq)
complexes, suggesting heptadentate coordination by HEDTTA
4
6
in both cases. Tian and Rao argued that octadentate
coordination of metal ion remains the same for the [Eu-
(
DTPA)]2 complex and its protonated form. The authors
−
combined the DFT calculations and luminescence lifetime data
to suggest the protonation of metal complex likely occurs on the
oxygen of one acetate group, which undergoes a rotation to
orient carbonyl oxygen toward the metal to retain the
octadentate coordination. The heptadentate coordination of
−
europium ion by HEDTTA in EuL and EuHL( complexes
aq)
Figure 7. (a) Time-dependent liquid−liquid distribution of 154Eu
between organic solutions of ion exchanger in n-dodecane and aqueous
reiterates the inert nature of the alcohol moiety to metal ion
coordination.
The density functional theory calculations were performed to
gain further understanding of alcohol’s influence of structural
organization of the metal complex. Several plausible complex
geometries for two different alcohol binding modes (A,
+
mixtures containing HEDTTA, HEDTA, or DTPA in 1 M (H /
+
Na )NO , pC = 3.0. T = 20.0 ± 1.0 °C: (red open diamonds) 0.065 M
3
H
HEH[EHP]/20.0 mM HEDTTA; (blue open circles) 0.065 M
HEH[EHP]/20.0 mM HEDTA; (black open squares) 0.04 M
241
HDEHP/5.0 mM DTPA. (b) Time-dependent distribution of Am
39
154
3+
(
black open squares), 1 Ce (blue open circles), and Eu (red open
coordinating; B, noncoordinating) were optimized for Eu −
HEDTTA interaction. As shown in Figure 8, Eu is coordinated
3+
diamonds) between 0.065 M HEH[EHP] in n-dodecane and 20 mM
HEDTTA, 1 M NaNO , pC = 3.0.
3
H
plateau. The kinetic trends follow the HEDTTA > HEDTA >
DTPA order as translated from time required to reach the
constant liquid−liquid distribution of metal ion (i.e., plateau).
The equilibrium is reached within ∼30 min for HEDTTA and
∼
90 min for HEDTA and is not attained until ∼5 h of phase
contact for DTPA mixtures. Time-dependent forward extraction
241
139
154
of Am, Ce, and Eu is presented in Figure 7b for a liquid−
liquid system containing 0.065 M HEH[EHP] in nonaqueous n-
dodecane phase and 20 mM HEDTTA adjusted to pC 3.0 in the
H
aqueous phase. Efficient separation of trivalent lanthanides from
3
+
Figure 8. Optimized structures of Eu −HEDTTA complexes in water.
(a) Complex A with the −CH CH OH group directly coordinated to
trivalent actinides is on display here, yielding good separation
Ce
Am
Eu
Am
2
2
factors SF ≈ 50 and SF ≈ 150 at equilibrium.
the metal ion. (b) Complex B with the −CH CH OH group not
coordinated to the metal ion. Eu atom is pink, O atoms are red, N atoms
are blue, C atoms are cyan, and H atoms are white.
2
2
4. DISCUSSION
4.1. Coordination Environment for Trivalent f-Element
Complexes with HEDTTA. Data in Table 3 shows luminescent
lifetimes, decay rates, number of inner-sphere waters, and
to four carboxylate groups and three amino groups. The
remaining two sites are occupied by (i) one water molecule
and a neutral hydroxyl group of the ligand in complex A and (ii)
two water molecules (the hydroxyl group is unbound) in
complex B. Table 4 compares the calculated average Eu−N and
Eu−O bond lengths for the coordination scenarios A and B with
3
+
percent abundance of Eu species as a function of aqueous
acidity. At pCH 1.0, the protonation reactions of HEDTTA
outcompete the metal ion complexation equilibria leaving the
metal ion fully hydrated. At pC 5.0 the luminescence lifetimes
H
are extended due to displacement of the inner-sphere waters by
−
−2
HEDTTA forming the [Eu(HEDTTA)] complex (99%
those optimized for [Eu(DTPA)] complex based on crystallo-
abundance). The heptadentate coordination of Eu3+ by
graphic data reported by Liu and co-workers. Tian and Rao
47
46
G
DOI: 10.1021/acs.inorgchem.6b02897
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Inorganic Chemistry
Article
a
3+
Table 4. Calculated Eu−N and Eu−O Bond Distances (Angstrsoms) for Eu /HEDTTA Complexes
Eu(DTPA)]−
2
A: [Eu(HEDTTA)]−
B: [Eu(HEDTTA)]−
[
octadentate
octadentate (Figure 8a)
heptadentate (Figure 8b)
Eu−O1
Eu−O2
Eu−O3
Eu−O4
Eu−O5
Eu−Ow1
Eu−Ow2
Eu−OOH
Eu−N1
Eu−N2
Eu−N3
2.37
2.44
2.40
2.51
2.49
2.67
2.39 ± 0.08
2.42 ± 0.08
2.39 ± 0.08
2.44 ± 0.09
2.40 ± 0.08
2.37 ± 0.08
2.39 ± 0.07
2.46 ± 0.09
2.48 ± 0.09
2.51 ± 0.10
2.40 ± 0.07
5.98 ± 0.17
2.64 ± 0.08
2.70 ± 0.09
3.04 ± 0.14
2.53 ± 0.10
2.70 ± 0.09
2.66 ± 0.08
2.84 ± 0.10
2.87
2.76
2.89
a
Averaged for the last 15 ps of trajectory in the canonical ensemble (T = 300 K). Scenario A assumes metal coordination by the −CH CH OH
2
2
3
+
group. Scenario B assumes inertness of the alcohol group in metal binding. Corresponding bond distances for the Eu /DTPA complex are shown
4
4
for comparison.
demonstrated that protonation of [Eu(DTPA)]−2 complex
significantly alters the calculated Eu−O average bond distance
for the affected acetate group. The other Eu−O average bond
lengths were unchanged. Similarly, the calculated Eu−O average
bond lengths for complexes A and B of HEDTTA do not differ
significantly from those observed for DTPA complex, showing
that four acetate groups of HEDTTA are not influenced by
HEDTTA may be compared with the structural modification of
DTTA-PA, where one carboxylate group is moved away from
diethylenetriamine central bridge. Although the equilibrium
constant for the first protonation of DTTA-PA seems unusually
37
low (relative to DTPA at I = 0.1 M), the remaining pK values
a
describing protonation of amine sites show minimal effect of
increased separation between the amine backbone and the
carboxylate group.
46
alcohol substitution. For Eu−N bonds Tian and Rao showed
significant weakening (manifested through longer bond
distances) for the amine group directly connected to acetate
that protonates. The calculated Eu−N3 average bond length for
the coordination scenario A, where participation of alcohol in the
binding of europium is postulated, does not change relative to
The replacement of one acetate group of the DTPA reagent
with an ethanol functionality is strongly manifested in the
reduction of stability constants for the complexation of trivalent
f-elements by HEDTTA (Table 2). The log β values for
10 101
[Ln(HEDTTA)]− complexes are ∼3 orders of magnitude
weaker relative to equivalent 1:1 complexes of DTPA. Such a
decrease in the binding strength is likely due to the somewhat
−2
equivalent Eu−N3 bond of [Eu(DTPA)] complex. This
finding does not adequately translate the thermodynamic
3
+
3+
information as the Eu /HEDTTA stability constant indicates
that interaction between metal ion and ligand weakens
substantially due to acetate/ethanol pendant arm replacement.
This weakening is rather demonstrated by the elongated Eu−N3
bond distance calculated when the noncoordinating alcohol
group is modeled. This theoretical outcome points to alcohol’s
ability to pull away a weakly coordinated amine atom (N3) from
the metal center. The weakened interaction between the metal
ion and one of the amine groups was also manifested in the
thermodynamic characteristics of HEDTTA, as discussed in the
following section.
weakened electrostatic attraction between M and the complex-
ing conjugate base, with a major impact coming from the
elimination of a five-membered chelate ring from the
coordination pocket. The magnitude of weakening follows the
trend expected from conventional aminopolycarboxylates
48
capable of formation of five-membered chelate rings. A simple
transitioning from DTPA (7 rings) to EDTA (5 rings) to
nitrilotriacetic acid, NTA (3 rings) reduces the log β values
1
0 101
2
.5
from ∼21 to ∼16 to ∼11, respectively, for an average 10
49
reduction per chelate ring.
The log K values for the MHL complex are similar to
1
0
111
(aq)
−
4
.2. Thermodynamic Profile of HEDTTA. Table 1
the protonation of H HEDTTA (pK = 2.56), suggesting that
protonation of the ML complex occurs on a carboxylic acid
pendant group. Grimes and Nash showed the electrostatic
interaction between the hydrogen ion and H DTPA is ∼10
times stronger, relative to its isoelectronic [Eu(DTPA)]
3 4
−
compares the determined acid dissociation constants for
35
35
HEDTTA with those reported for DTPA and diethylenetri-
2
−
amine-N,N,N″,N″-tetraacetic-N′-propionic acid, DTTA-PA,
3
3
6
2−
determined by Sawyer and Powell. The HEDTTA pK values
a
+
+
are similar to those reported for DTPA in 2.0 M (H /Na )ClO .
complex. The authors attributed this to a “spherical” compact-
ness of [Eu(DTPA)]2 complex, where carboxylate groups are
shielded from the electrostatic interactions with hydrogen ions.
This relative shift in the protonation equilibrium due to metal ion
coordination is not observed for HEDTTA. The protonation of
4
−
Nearly statistical agreement of all amine protonation sites of
HEDTTA and DTPA indicates that pendant arm substitution
has minimal inductive influence on the diethylenetriamine
backbone. Based solely on such an equivalence of the inductive
effects between HEDTTA and DTPA, the capacity of those
reagents to differentiate trivalent actinides from trivalent
lanthanides should be similar. The inspection of the carboxylic
acid dissociation constants of HEDTTA reiterates the similarity
to DTPA when comparing the least acidic carboxylic protons,
−
−
ML matches the protonation of H HEDTTA , perhaps
3
reiterating the argument of a coordination environment more
susceptible to interactions with bulk solvent.
The trans-lanthanide trend of metal HEDTTA complex
stability constants (Table 2) matches the pattern displayed by
DTPA. The log10β101 values for DTPA range from 18.0 to 21.1
pK . In all, HEDTTA is a weaker acid, relative to DTPA, and its
4
35
operational capacity to delay protonation in favor of the metal
complexation reactions is subdued. The ethanolic substituent of
across the lanthanide series, whereas those for HEDTTA range
3+
3+
from 15.4 (La ) to 17.7 (Lu ). The complex stabilities steadily
H
DOI: 10.1021/acs.inorgchem.6b02897
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Inorganic Chemistry
Article
3
+
3+
increase for La −Sm group, presumably due to increasing
of trivalent f-element complexation strength for HEDTTA
requires the use of phosphonic acid ion exchanger (higher pKa)
to efficiently balance the partitioning equilibrium. The
electrostatic binding. The stability constants reach a plateau
Sm −Ho ), and even slightly decrease for Er −Lu group.
For DTPA this leveling off trend in stability constants has been
attributed to decreased entropic stabilization due to lower
hydration of heavier lanthanides, and increased steric repulsion
due to tight coordination pocket for such contracted f-
elements. The similarity of stability constant trends collected
for HEDTTA, relative to DTPA, is somewhat surprising,
considering the lowered constraints imposed by a heptadentate
complexant. Further thermodynamic studies are anticipated to
determine the enthalpic and entropic contributions to Gibbs
energies of [Ln(HEDTTA)] complex formation to explain the
trans-lanthanide pattern in more detail.
The comparison of the stability constants for the formation of
Nd(HEDTTA)] and [Am(HEDTTA)] complexes (deter-
mined using spectrophotometry) show the enhanced affinity for
coordination of trivalent actinides, relative to trivalent
lanthanides. This is partly due to slight differences in charge
density and partly due to small degree of covalency afforded the
An elements. Using ionic radius data produced by Shannon
for metal ions with coordination number 8, the dimensionless
3+
3+
3+
3+
(
3
+
3+
distribution trend for the investigated lanthanides (La −Ho )
shown in Figure 6 steadily increases throughout, with the least
extracted lanthanum limiting the overall differentiation of
trivalent americium from the lanthanides. This partitioning
trend varies appreciably from that observed when DTPA is
4
8
11,12
utilized as the aqueous holdback complexant.
Careful
comparison of trans-lanthanide patterns of stability constants
for HEDTTA and DTPA explains the observed liquid−liquid
distribution differences very well. The prediction of the
equilibrium partitioning of trivalent f-elements in liquid−liquid
mixtures described here follows the combined stoichiometric
−
constant Kex × K
lanthanides to the organic medium, and K
d
. The Kex is the extraction constant of the
d
−
−
[
is the dissociation
constant of the Ln3+ ligand complex in the aqueous mixture.
Accordingly, the strength of the liquid cation exchanger and the
relative ease of Ln3+ ligand complex dissociation balance the
partitioning equilibrium. The trans-lanthanide trends of stability
constants for HEDTTA and DTPA show that light lanthanides
form less stable, more readily dissociable complexes, relative to
heavier 4f elements. Figure 9 illustrates a relative measure of this
3+
50
electrostatic parameter can be calculated using the simple
equation ξ = z /r, where z corresponds to effective ionic charge
2
3+
and r is the ionic radius. The higher ξ for americium ion (Nd =
+
8
.12, Am3 = 8.25) implies a higher charge density, relative to
neodymium ion, resulting in slightly more favorable ionic
3+
3+
binding of Am . The remaining enhancement in the Am
complex stability has been typically attributed to small degree of
covalent interactions made possible by the greater spatial
51
extension of the 5f and 6d orbitals for actinides. The unshielded
orbitals are more available for overlap with the relatively soft
nitrogen donor groups in the HEDTTA backbone. The small
degree of covalency experienced by the actinides is absent for
lanthanide metal ions due to shielding of the 4f orbitals by 5 d
orbitals. The electrons occupying the 4f orbitals are considered
core electrons, and are not available for orbital mixing with
incoming ligands. Accordingly, the lanthanide interactions are
strictly ionic and defined by electrostatic attraction.
3
+
3+
4
.3. Ln /An Separation Using HEDTTA as Holdback
Reagent. The liquid/liquid distribution results shown in Figure
+
3+
8
for Ln3 and Am demonstrate HEDTTA’s ability to
Figure 9. Relative differences in the Gibbs free energies of dissociation
3
+
effectively suppress the distribution ratio of Am and produce
f−5f trivalent element group separation. The separation factor
between Nd (typically the least extracted Ln in TALSPEAK-
−
2−
of (black squares) [Ln(HEDTTA)] and (red circles) [Ln(DTPA)]
complexes as compared to energy required to dissociate equivalent
complexes of Lu .
4
3+
3+
3+
3
+
type chemistries) and Am is slightly lower in the HEDTTA
Nd
Am
Nd
11,12
3+
system, (SF = 26) compared to TALSPEAK (SF = 34).
Am
3
+
The difference in the stability constants for Nd and Am
complexation by HEDTTA is smaller (Δlog β of 0.84) than
that measured for DTPA (Δlog β = 1.3). The diminished
energy balance, showing the net difference in the Gibbs free
energy of complex dissociation as compared to the complex of
lutetium. The plot demonstrates that dissociation of [La-
1
0 101
4
9
1
0 101
3
+
3+
2−
2−
capacity to differentiate Am from Nd may be attributed in
part to the disruption of the octadentate environment of DTPA’s
binding pocket through the introduction of the alcohol. The
elongated Eu−N3 bond predicted by the DFT calculations
supports this experimental finding, suggesting that alcohol
weakens the interaction between the amine and the trivalent
metal. Accordingly, the overall softness of the diethylenetriamine
binding pocket of HEDTTA is reduced, relative to DTPA, as
reflected by differences in the stability constants.
(DTPA)] requires 18 kJ/mol less energy than [Lu(DTPA)] ,
while the energetic difference is 13 kJ/mol for the equivalent pair
of HEDTTA complexes. This consistent difference of ∼5 kJ/mol
is sustained throughout the lanthanide series until holmium. As
such the influence by the phase-transfer reagent on the liquid−
liquid partitioning of lanthanum relative to lutetium (as indicated
by K × K ) will not be as pronounced for HEDTTA as it is for
ex
d
the DTPA-containing mixtures. This is manifested in the liquid−
liquid distribution of lanthanides shown in Figure 6 where the
initially decreasing partitioning trend for light metal ions
observed with DTPA is not reproduced when utilizing
HEDTTA. The lower extractability of light lanthanides, when
coupled with slightly reduced “softness” of HEDTTA due to
The original TALSPEAK recipe balances the use of DTPA
3
+
with phosphoric acid ion exchanger, producing an initial La
>
3+
3+
3+
Ce > Pr > Nd trend of metal partitioning, which is reversed
11,12
for the latter members of the 4f group.
The overall lowering
I
DOI: 10.1021/acs.inorgchem.6b02897
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
alcohol functionalization, yields an overall trivalent 4f−5f
One plausible explanation for HEDTTA’s unexpectedly
enhanced facilitation of kinetics of f-element phase transfer
may reside with the presence of the liquid−liquid interface. A
substitution of the metal-coordinating acetate with an inert
ethanol functionality introduces a less rigid coordination
environment and labels the metal chelate with a hydrophobic
“antennae” directed outward into bulk solvent. Since the
microscopic interfacial layer of water is more compact,
separation factor of 8 in pC 3.0 aqueous environments.
H
4
.4. Kinetics of Liquid−Liquid Distribution Supported
by HEDTTA. Sluggish equilibration of liquid−liquid partitioning
systems has been related to the extraordinary stability of
aminopolycarboxylate metal chelates. The main kinetic
limitation resides with the energetic costs associated with
reorganization of the metal complex during the partitioning
1
3
1
3
61
process. Accordingly, the complexant denticity directly impacts
resembling glycerol-like environment, the diffusion of ions
52
the rates of complex dissociation. The partitioning trends
presented in Figure 7a demonstrate this polydentate kinetic
inhibition for HEDTA and DTPA. The partitioning of Eu3+ is
much slower when octadentate DTPA is present, relative to
pentadentate HEDTA. The observed distribution differences
cannot be attributed to differences in the experimental pH, and
the corresponding enhancement of the rate of complex
through this “stagnant” region to the interface may be altered by
62
the enhanced hydrophobic character of the chelate. The
interfacial structuring is mainly determined by the surface active
molecule present in the nonaqueous environment; however, the
ethanol group of HEDTTA may incorporate some surfactant
character to the metal chelate of this aminopolycarboxylate
6
2
reagent. Further surface tension measurements will be
performed to understand this kinetic enhancement in more
detail. It is important to note that the phase-transfer kinetic trend
52
dissociation due to catalytic influence of hydrogen ion. The
kinetic variance originates from the stability of a complex
containing 8 five-membered chelate rings (DTPA), and nearly
fully dehydrated metal ion, well-shielded from the aqueous
environment. The hydration sphere of a trivalent metal ion is
disrupted to a much lesser extent by HEDTA, and other EDTA-
like reagents, offering easier interaction with bulk water, or
3+
for the partitioning of Eu supported by HEDTTA outperforms
that collected for HEDTA (Figure 7a), which also contains
ethanol functionality.
3
+
3+
The time-dependent partitioning trends for Am , Ce , and
3+
Eu (Figure 7b) demonstrate that HEDTTA operates efficiently
as a holdback complexant of trivalent actinides at pC = 3.0. The
53
secondary coordination reagents. To illustrate, the water
H
3
+
298
6
−1 54
exchange rate of Gd /DTPA chelate (k
= 3.30 × 10 s )
phase-transfer partitioning trends of the americium and cerium
ions are faster relative to europium, as expected from metals of
ex
is approximately 10 times slower relative to chelates formed using
the EDTA-type complexants, such as bisamide-substituted
63
lower charge density. The partitioning studies containing up to
EDTA (k2 = 44 × 10 s ) reported by Platas-Iglesias et al.
98
6
−1
55
3+
1.0 mM Eu in the aqueous environment afforded an efficient,
ex
3
+
The time-dependent trend for the partitioning of Eu
fast option for differentiation of trivalent actinides from trivalent
lanthanides.
facilitated by HEDTTA is surprisingly enhanced relative to
that expected solely from the argument of polydenticity. On the
basis of luminescence studies presented here HEDTTA is
heptadentate in aqueous solutions adjusted to pCH = 3.
Presumably the task of unravelling such a coordination
environment must be of intermediate difficulty relative to
5
. CONCLUSIONS
A single acetate pendant arm of DTPA reagent was replaced by a
N-hydroxyethyl group, thus reducing the denticity of the reagent.
Potentiometric titration of 1:1 Ln/HEDTTA mixtures identified
two species, ML− and MHL(aq), typical of DTPA-type
coordination. Stability constant trends for Ln/HEDTTA
complexation across the lanthanide series were analogous to
previously observed Ln/DTPA complex formation. The
replacement of the acetate pendant arm with the hydroxyethyl
did reduce Ln/HEDTTA complex stability ∼3 orders of
magnitude relative to Ln/DTPA complexes. DFT calculations
help account for this by predicting an elongated Eu−N3 bond
with the hydroxyethyl-functionalized amine. The theoretical
outcome points to alcohol’s ability to pull away a weakly
coordinated amine atom (N3) from the metal center, thus
reducing the “softness” of the reagent. UV−vis spectroscopy was
used to determine stability constants for HEDTTA complexes
with Nd3+ and Am . The results showed consistency between
the two analytical methods as the stability constants for
[Nd(HEDTTA)]− determined by both techniques showed−
good overlap. The stability constant for [Am(HEDTTA)]
3
+
HEDTA and DTPA. The observed Eu partitioning rate
enhancement for HEDTTA must therefore originate with the
specific structural modification of this reagent, where one acetate
pendant arm of DTPA has been replaced by an ethanol group.
Sarka et al. studied kinetic stability and water exchange rates of
3+
Gd complexes of DTPA-type amide-substituted complex-
5
6,57
ants.
The authors indicated that either single or multiple
replacement of an acetate pendant arm of DTPA by an amide
does not appreciably alter the kinetic stability of the chelate. Laus
et al. showed that substitution of an acetate group of DTPA by a
3
+
propionate group increases the water exchange rate for the Gd
2
98
6
−1 58
complex by an order of magnitude (k
= 31 × 10 s ).
ex
Octadentate coordination of the gadolinium ion is maintained
for reagents studied by the Sarka and Laus groups, suggesting
that kinetic stability may be influenced by the flexibility of the
pendant arm functionality. A reduction of polydenticity to
heptadentate coordination was demonstrated by Moriggi et al.,
who did not see significant changes in the kinetic stability of the
metal complex when a single acetate group was replaced by a
3+
−
was ∼1 order of magnitude larger than [Nd(HEDTTA)] ,
demonstrating that HEDTTA maintains selectivity for trivalent
actinides over trivalent lanthanides. Luminescence lifetime
measurements showed the presence of 2 waters inside the
metal’s inner hydration sphere, resulting from displacement of
seven water molecules upon coordination by HEDTTA. Due to
the reduced complex strength (relative to DTPA) TALSPEAK-
type separations were facilitated using a phosphonic acid
extractant HEH[EHP]. As predicted by thermodynamic data
collected in this study, HEDTTA was an effective holdback
reagent for trivalent lanthanide/trivalent actinide differentiation.
59
methyl group relative to bisamide-substituted DTPA reagent.
In contrast, Silverio et al. found a much enhanced water exchange
3+
rate of a Gd complex of DTTA-type compound, where acetate
2
98
6
−1 60
was substituted by a benzyl group (k
= 77 × 10 s ). This
ex
has been attributed to steric strain on the water binding site
imposed by the bulky benzyl group within the metal complex.
60
Clearly, no definitive indication for significant kinetic instability
for heptadentate DTTA-type chelates may be deduced from
these aqueous aminopolycarboxylate systems.
J
DOI: 10.1021/acs.inorgchem.6b02897
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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(
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AUTHOR INFORMATION
■
Corresponding Authors
E-mail: travis.grimes@inl.gov.
E-mail: peter.zalupski@inl.gov.
(13) Danesi, P. R.; Cianetti, C. Kinetics and Mechanism of the
*
Interfacial Mass Transfer of Eu(III) in the System: Bis(2-ethylhexyl)-
phosphoric Acid, n-Dodecane-NaCl Lactic Acid, Polyaminocarboxylic
Acid, Water. Sep. Sci. Technol. 1982, 17 (7), 969−984.
*
ORCID
(14) Nilsson, M.; Nash, K. L. Trans-Lanthanide Extraction Studies in
Travis S. Grimes: 0000-0003-2751-0492
Vyacheslav S. Bryantsev: 0000-0002-6501-6594
Notes
the TALSPEAK System: Investigating the Effect of Acidity and
Temperature. Solvent Extr. Ion Exch. 2009, 27, 354−377.
(15) Nash, K. L. The Chemistry of TALSPEAK: A Review of the
Science. Solvent Extr. Ion Exch. 2015, 33 (1), 1−55.
The authors declare no competing financial interest.
(
16) Braley, J. C.; Grimes, T. S.; Nash, K. L. Alternatives to HDEHP
and DTPA for Simplified TALSPEAK Separations. Ind. Eng. Chem. Res.
012, 51 (2), 629−638.
17) Lumetta, G. J.; Gelis, A. V.; Braley, J. C.; Carter, J. C.; Pittman, J.
ACKNOWLEDGMENTS
■
2
(
The experimental work conducted by T.S.G., C.R.H., M.N., and
P.R.Z. at the Idaho National Laboratory was supported by the
U.S. Department of Energy, Office of Nuclear Energy, DOE
Idaho Operations Office, under contract DE-AC07-05ID14517.
The synthetic work by S.J.-P. and computational studies by
V.S.B. and S.G.S. were supported by the Fuel Cycle Research and
Development Program, Office of Nuclear Energy, U.S. Depart-
ment of Energy. DFT calculations used resources of the National
Energy Research Scientific Computing Center and the Oak
Ridge Leadership Computing Facility at the Oak Ridge National
Laboratory, both of which are supported by the Office of Science
of the U.S. Department of Energy under contract nos. DE-AC02-
W.; Warner, M. G.; Vandegrift, G. F. The TRUSPEAK Concept:
Combining CMPO and HDEHP for Separating Trivalent Lanthanides
from the Transuranic Elements. Solvent Extr. Ion Exch. 2013, 31 (3),
223−236.
(18) Braley, J. C.; Lumetta, G. J.; Carter, J. C. Combining CMPO and
HEH[EHP] for Separating Trivalent Lanthanides from the Transuranic
Elements. Solvent Extr. Ion Exch. 2013, 31 (6), 567−577.
(19) Lumetta, G. J.; Casella, A. J.; Rapko, B. M.; Levitskaia, T. G.;
Pence, N. K.; Carter, J. C.; Niver, C. N.; Smoot, M. R. An Advanced
TALSPEAK Concept Using 2-ethylhexylphosphonic Acid Mono-2-
ethylhexyl Ester as the Extractant. Solvent Extr. Ion Exch. 2015, 33 (4),
2
(
11−223.
0
5CH11231 and DE-AC05-00OR22725, respectively.
20) Lapka, J. L.; Nash, K. L. Advanced TALSPEAK Separations Using
a Malonate Buffer System. Solvent Extr. Ion Exch. 2015, 33 (4), 346−
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