Thermodynamic study of the complexation between Nd3+ and functionalized diacetamide ligands in solution

Article abstract of DOI:10.1039/c6dt01694d

A series of amine functionalized ligands, including 2,2′-(benzylazanediyl)bis(N,N′-dimethylacetamide) (BnABDMA), 2,2′-azanediylbis(N,N′-dimethylacetamide) (ABDMA), and 2,2′-(methylazanediyl)bis(N,N′-dimethylacetamide) (MABDMA), are synthesized for the the

Full text of DOI:10.1039/c6dt01694d

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Thermodynamic Study of the Complexation between Nd3+ and
Functionalized Diacetamide Ligands in Solution†
Phuong V. Dau,a Zhicheng Zhang,a Phuong D. Dau,a John K. Gibson,a and Linfeng Rao*a
Received 00th January 20xx,
Accepted 00th January 20xx
A series of amine functionlized ligands, including 2,2’-(benzylazanediyl)bis(N,N'-dimethylacetamide) (BnABDMA), 2,2’-
azanediylbis(N,N'-dimethylacetamide) (ABDMA), and 2,2’-(methylazanediyl)bis(N,N'-dimethylacetamide) (MABDMA), are
synthesized for the thermodyamic study of the complexation with Nd3+ ions. The complexation in solution is investigated
using potentiometry, spectrophotometry, calorimetry, and electrospray ionization mass spectrometry. The results suggest
these ligands act as tridentate ligands. Furthermore, direct comparison between ABDMA and an analogous ether-
functionalized ligand, 2,2'-oxybis(N,N'-dimethylacetamide) (TMDGA), showed that the amine functionalized ligand forms
thermodynamically stronger complexes with Nd3+ ions than the ether-functionalized ligand. In addition, the amine
functionalized ligand can allow the fine-tuning of the binding strength with metal ions via the substitution on the central
amine N atom with different functional groups, which is not possible for ether functionalized ligands such as TMDGA.
DOI: 10.1039/x0xx00000x
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potentials to make the separation process more effective and
environmentally benign. In addition, diamide ligands are
completely combustible due to their compositions of only C, H,
Introduction
In the past decades, carbon-based fuel energy has been a
primary energy source to supply the energy demand of the
world. Unfortunately, the use of carbon-based energy relies
heavily on the limited coal resources, and has been also
criticized for the increase of greenhouse gases in the
environment.1,2 It is thus inevitable to find sustainable,
renewable, cleaner, and more efficient energy alternatives to
replace current carbon-based fuel energy. Nuclear energy has
become a promising alternative resource to replace current
carbon-based energy due to its efficacy, reliability and vastly
natural uranium ores supply. However, the treatment of
nuclear wastes still poses many challenges.3,4 The high level
nuclear waste (HLW) generated during the spent nuclear fuel
reprocessing typically contained large quantities of radioactive
elements such as U, Pu, Am, Np, Cm, and fission products
including lanthanide elements. Efficient separation processes
are under development to separate the radioactive elements
from the HLW so that the nuclear wastes can be safely
N, and O atoms,5-26 so that the use of diamide ligands does not
increase the volume of solid wastes.
Recently, our group has studied the thermodynamics and
solid-state structure of complexes between Nd3+ and a ether-
functionlized
ligand
2,2'-oxybis(N,N'-dimethylacetamide)
(another name for tetramethyl-diglycolamide, TMDGA, Figure
1).8 The underlying reason for the study of these ligands is due
to their similar structures in comparison with TRDGA ligands
such as 2,2'-oxybis(N,N'-dioctylacetamide) (another name for
tetraoctyl-diglycolamide, TODGA, Figure 1). Thermodynamic
and structural studies indicate that the ether-functionalized
ligand, TMDGA, coordinates with Nd3+ ions in a tridentate
mode (the central ether oxygen atom and two amide oxygens
atoms) and the complexation was driven by both enthalpy and
entropy.8
Herein, we report the exploration of a series of amine-
functionalized ligands analogous to the ether-functionalized
ligand TMDGA previously studied, including 2,2’-
disposed of in geological repository.
Specifically, the
(benzylazanediyl)bis(N,N'-dimethylacetamide)
2,2’-azanediylbis(N,N'-dimethylacetamide) (ABDMA), and 2,2’-
(methylazanediyl)bis(N,N'-dimethylacetamide) (MABDMA,
(BnABDMA),
separation between trivalent actinide and lanthanide ions is
extremely difficult due to the similarity in their chemical
behaviors. Diamide ligands such as the family of alkyl-
substituted diglycolamide (TRDGA, Figure 1) ligands3 have
recently become an attractive new class of organic ligands for
the application in actinide separation because of their
Figure 1) in the complexation with Nd3+ ions. The direct
comparison between ABDMA and TMDGA can provide an
insight of the difference between amine versus ether
functionalized ligands in coordinating with Nd3+ ions. In
addition, the structure of ABDMA allows the substitution with
several functional groups such as methyl and benzyl (Figure 1),
which cannot be otherwise achieved with the ether-
functionalized ligands such as TMDGA. The substitution with
different groups modifies the electron density on the central
a.Chemical Science Division, Lawrence Berkeley National Laboratory, One
Cyclotron Rd., Berkeley, CA, USA 94720, Email: lrao@lbl.gov, Phone: 1-510-486-
5427, Fax: 1-510-486-5596.
†Electronic Supplementary Information (ESI) available: [Titration Conditions,
thermograms, and mass spectrophotometry]. See DOI: 10.1039/x0xx00000x
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amine N atom so that the binding ability of the ligands can be
fine-tuned. The stability constants of the 1:1, 1:2, and 1:3
complexes between these ligands and Nd3+ ions were
determined via potentiometry and spectrophotometry.
Calorimetry was also utilized to experimentally determine the
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DOI: 10.1039/C6DT01694D
enthalpy of complexation.
Furthermore, electrospray
ionization mass spectrometry experiments substantiated the
formation of these complexes in solution to further support
the results from other methods.
Scheme 1. Synthesis of BnABDMA, ABDMA, and MABDMA.
Synthesis of 2,2'-(benzylazanediyl)bis(N,N'-dimethylacetamide)
(BnABDMA, Scheme 1). 2-Chloro-N,N'-dimethylacetamide
(2.12 mL, 20.50 mmol), benzylamine (1.21 mL, 10.25 mmol),
K2CO3 (5.50 g, 40.00 mmol), and KI (2.00 g, 12.05 mmol) were
added to CH3CN (~100 mL). The mixture was heated to reflux
for 72 h. The mixture was cooled down to room temperature.
K2CO3 and KI were filtered off. CH3CN was removed under
vacuum conditions to yield a mixture of light yellow liquid and
white solid. The mixture was dissolved in CHCl3. Any
undissolved solid was then filtered off. CHCl3 was removed
under vacuum conditions to yield a light yellow solid as
Figure 1. Structures of different ether-functionalized diamide ligands, and
amine-functionalized diamide ligands.
Experimental
Starting materials and solvents were purchased and used
without further purification from commercial suppliers (Sigma-
Aldrich, Alfa Aesar, EMD, TCI, Cambridge Isotope Laboratories,
Inc., and others). Proton nuclear magnetic resonance spectra
(1H NMR) were recorded by a Bruker FT-NMR spectrometer
(300 MHz). Chemical shifts were quoted in parts per million
(ppm) referenced to the appropriate solvent peak or 0 ppm for
tetramethylsilane (TMS). The following abbreviations were
used to describe the peak patterns when appropriate: br =
broad, s = singlet, d = doublet, dd = doublet of doublet, t =
triplet, q = quartet, and m = multiplet. Coupling constants, J,
were reported in Hertz unit (Hz). Mass spectrophotometry
(MS) experiments were performed using an Agilent 6340
quadrupole ion trap mass spectrophotometer.
1
product (2.40 g, 8.71 mmol, ~85%). H NMR (CDCl3, 300 MHz):
δ 7.33 (m, 5 H), δ 3.87 (s, 2 H), δ 3.55 (s, 4H), δ 2.94 (s, 6 H), δ
2.90 (s, 6 H). MS (C15H23N3O2): Calc. [MW] = 277.18, Found
[MW+H]+ = 278.2, [MW+Na]+ = 300.2.
Synthesis
of
2,2'-azanediylbis(N,N'-dimethylacetamide)
(ABDMA, Scheme 1). BnABDMA (1.00 g, 3.60 mmol) was
dissolved in ethanol (EtOH, 30 mL) in a scintillation vial. Pd/C
(10 wt%, 1.90 g, 1.80 mmol Pd) was added slowly to the
scintillation vial. The vial was placed in a Parr bomb reactor.
The Parr bomb was sealed and evacuated under vacuum
conditions. The bomb reactor was pressurized with H2 (~5.0
psi). The bomb reactor was evacuated under vacuum
conditions and re-pressurized with H2 (~5.0 psi). The reaction
was stirred at room temperature for 3 days. After 3 days, the
bomb reactor was evacuated under vacuum conditions and re-
pressurized with H2 (~5.0 psi). The reaction was stirred at
room temperature for another 3 days. This process was
repeated again. The bomb reactor was then vented. The vial
was taken out of the bomb reactor. Pd/C was filtered off with
the aid from celite. EtOH was removed under vacuum
conditions to yield a white solid as product (383 mg, 2.0 mmol,
57%). 1H NMR (CDCl3, 300 MHz): δ 3.59 ppm (s, 4H), δ 2.96
ppm (s, 12H), δ 2.17 ppm (br, s, 1H). MS (C8H17N3O2): Calc.
[MW] = 187.13; Found [MW+H]+ = 188.2 [MW+Na]+ = 210.2.
Milli-Q water was used in preparing all solutions. Nd3+
stock solutions were prepared by dissolving Nd(NO3)3xH2O
(Aldrich, 99.9%). NaNO3, HNO3, and NaOH solutions were
prepared from solid NaNO3 (Aldrich, 99.9%), concentrated
HNO3 (16 M, Aldrich), and 1 M NaOH (Aldrich). The
concentrations of Nd3+, H+, OH- in the stock solutions were
determined by ethylenediaminetetraacetic acid (EDTA)
titrations, Gran’s titration,27 and standard acid/base titration
with primary standards (potassium hydrogen phthalate, or
tris(hydroxymethyl)aminomethane)), respectively. Ligand
solutions were prepared directly by weighting the ligands. The
ionic strength of all solutions used in potentiometry,
calorimetry, and spectrophotometry was adjusted to 1.0 M at
25 °C by adding appropriate amounts of NaNO3 as the
background electrolyte.
In this paper, the neutral ligands, protonated ligands, and
the Nd3+ complexes are denoted as L, HL+, and NdLj3+ where j =
1, 2, and 3, respectively.
Synthesis of 2,2'-(methylazanediyl)bis(N,N'-dimethylacetamide)
(MABDMA, Scheme 1).
2-Chloro-N,N'-dimethylacetamide
(2.96 mL, 30.00 mmol), methylamine hydrochloride (1.00 g,
14.80 mmol), K2CO3 (6.00 g, ~45 mmol), KI (1.00 g, 6.00 mmol),
and 6.0 M NaOH (~1 mL) were added to CH3CN (~100 mL). The
Ligands Synthesis
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mixture was heat to reflux for 72 h. The mixture was cooled An isothermal microcalorimeter (ITC 4200, Calorimetry Science
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DOI: 10.1039/C6DT01694D
down to room temperature. K2CO3 and KI were filtered off. Corp.) was used to determine the enthalpies of the
CH3CN was removed under vacuum conditions to yield a protonation of ligands and the complexation of Nd3+ ion with
mixture of light yellow liquid and white solid. The mixture was ligands. Detailed information about this microcalorimeter and
dissolved in CHCl3. Any undissolved solid was then filtered off the calibration process were reported previously.8 Multiple
via vacuum filtration. CHCl3 was removed under vacuum titrations using different concentrations of ligands, Nd3+, and
conditions to yield a light yellow sold as product (2.65 g, 13.17 H+ in the initial solution were performed. The enthalpies of
1
mmol, ~89%). H NMR (CDCl3, 300 MHz): δ 3.33 (s, 4 H), δ 2.98 protonation and complexation were calculated by analysing
(s, 6H), δ 2.86 (s, 6H), δ 2.36 (s, 3H). MS (C9H19N3O2): Calc. the calorimetric data using the HypDeltaH program.29 The
[MW] = 201.15, Found [MW+H]+ = 202.2, [MW+Na]+ = 224.2.
enthalpy of the formation of NdNO32+, 1.5 kJ mol-1,8 was used
as constants in the fitting model.
Potentiometry
ESI-Mass Spectrometry
The protonation constants of the ligands and the stability
constants of Nd3+ complexes with ligands were determined by An Agilent 6340 quadrupole ion trap mass spectrometer with a
potentiometric titrations at 25 °C. Titration set up has been micro electrospray ionization (ESI) source was used to identify
reported in detail previously.8 The titrations were carried out Nd3+ complexes. Water-ethanol (<5% water) spray solutions
using an automatic titration system consisting of a glass were used in the experiments. In high resolution mode, the
titration cell, a MetroOhm pH meter (model 713), a pH instrument has a detection range of 50 – 2200 m/z and a
electrode, a MetroOhm dosimat (model 765), and a computer. resolution of ~0.25 m/z. The intensity distribution of ions in
The temperature of the titration cell was kept at (25 ± 0.1) °C the mass spectra was highly dependent on instrument
using an external circulating water bath. The titration parameters, which are different when tuning the optimum
solutions were kept free of CO2 via the use of Ar gas. The inner signals for the tri-cation versus mono-cation complexes.
solution of the electrode was replaced with 1M NaCl to reduce Instrument parameters used to obtain the MS spectra of these
the electrode junction potential. Before each titration, the complexes were similar to those employed in previous
electrode was calibrated using an acid/base titration with reports.30,31
standardized HNO3 and NaOH solutions so that the emf
readings could be converted to the concentrations of H+ in the
subsequent titration. For each system, at least 3 independent
titrations with different concentrations of ligands, Nd3+ ion, or
Results and Discussion
Protonation of amine-functionalized ligands
acidity were carried out. The protonation and complexation
Unlike the ether-functionalized ligands such as TMDGA, the
stability constants were determined by fitting the titration
amine functionalized ligands, BnABDMA, ABDMA, and
MABDMA, can be protonated in aqueous solution. Figure 2
shows a representative protonation titration of BnABDMA.
Representative protonation titrations of ABDMA and
MABDMA are shown in Figure S1 of Supporting Information.
From multiple titrations, the protonation constants of
2+
data using the program Hyperquad 2008.28 The NdNO3
complex was included in the fitting model using the stability
constant (log
= -0.19).8
Spectrophotometry
BnABDMA, ABDMA, and MABDMA (logK, where
K =
Spectrophotometric titrations were carried out to confirm the
stability constants of Nd3+ complexes with ligands using a Cary
5G UV-Vis-NIR spectrophotometer with a 10 mm cuvette (1
nm spectral bandwidth). The spectrophotometric titrations
were monitored at 620 nm to 560 nm with 0.2 nm step size.
For a typical titration experiment, appropriate amount of
ligand solution was added into a cuvette containing 2.2 mL of
Nd3+ solution. The solution in the cuvette was mixed
thoroughly for 1-2 minutes before the spectrum was collected.
Prior studies have shown that the complexation reaction was
fast and the absorbance became stable within 30 seconds of
mixing. The stability constants for Nd3+ complexes determined
by potentiometry were inputted to check the goodness of the
fit for the spectrophotometric data via HypSpec.29 The
[HL+]/([H+][L]) are determined to be (6.36 ± 0.03), (7.12 ± 0.03),
and (7.64 ± 0.03), respectively, and listed in Table 1. The trend
in the protonation constants of the three ligands, BnABDMA <
ABDMA < MABDMA, reflects the electron density on the
central amine N atom: the electron-withdrawing group such as
benzyl makes BnABDMA the more acidic ligand than ABDMA,
while the electron-donating methyl group makes MABDMA
the more basic ligand than ABDMA.
2+
NdNO3 complex was also included in the fitting model using
the stability constant (log
= -0.19).8
Calorimetry
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the majority of systems where successive complexes form. In
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contrast, the potentiometric data shoDwOI:t1h0a.1t03t9h/eC6DstTe0p16w9i4sDe
stability constants of the Nd3+/MABDMA complexes are 4.40
(NdL3+), 3.12 (NdL23+), and 4.63 (NdL33+). Obviously, the third
stepwise stability constant from potentiometry is
unreasonable high. We do not have satisfactory explanation
for this discrepancy, but the calorimetric data described in a
3+
following section indicate that, in the case of Nd(MABDMA)3
,
the constant from spectrophotometry (10.50 ± 0.50) provides
better fit for the calorimetric titration data.
Figure 2. Representative potentiometric titration for the protonation of
BnABDMA . I = 1.0 M NaNO3, T = 25 °C. Detailed titration conditions are in Table
S1. Symbols:
line – L.
 - observed pCH, blue line 
calculated pCH; red line – HL+, black
Stability constants of Nd3+ complexes determined by
potentiometry
Figure 3 shows representative potentiometric titrations for the
complexation of Nd3+ ions with the three ligands. Analysis of
the experimental data indicates that the data are best
described by the formation of three successive Nd3+
complexes, as shown by reaction (1):
Nd3+
+
nL
=
[NdLn]3+
(1)
whereas n = 1, 2, and 3. The stability constants of the 1:1, 1:2,
and 1:3 metal to ligand complexes between Nd3+ ions and the
BnABDMA are (2.92 ± 0.03), (5.08 ± 0.03), and (7.13 ± 0.03),
respectively. These stability constants of Nd3+ complexes with
ABDMA are slightly higher (4.08 ± 0.03, 6.93 ± 0.03, and 10.02
± 0.03), indicating ABDMA binds stronger to Nd3+ ions in
comparison to BnABDMA. Similarly, the stability constants of
Nd3+ complexes with MABDMA are (4.40 ± 0.03), (7.52 ± 0.12),
and (12.15 ± 0.03) for the 1:1, 1:2, and 1:3 metal to ligand
complexes, respectively. The binding strength of the ligands
follow the order: MABDMA > ABDMA > BnABDMA. The
stability constants determined by potentiometry are
summarized in Table 1.
Stability constants of Nd3+ complexes confirmed by
spectrophotometry
As shown in Figure 4, the absorption bands of Nd3+ are
generally red-shifted with additions of ligands, indicating the
interactions between the ligands and Nd3+ ions.8 Using the
stability constants determined by potentiometry, the spectra
3+
were very well fitted for all the complexes except the NdL3
complex with MABDMA. However, in the case of
[Nd(MABDMA)3]3+, spectrophotometric titration suggests a
much lower stability constant (10.50 ± 0.50) compared to that
from the potentiometric data (12.15 ± 0.03). The data from
spectrophotometry show that the stepwise stability constants
of the Nd3+/MABDMA complexes are 4.40 (NdL3+), 3.12
(NdL23+), and 2.98 (NdL33+), a trend consistent with those for
Figure 3. Representative potentiometry titrations and speciation diagrams for the
complexation of Nd3+ with BnABDMA (top), ABDMA (middle), and MABDMA (bottom)
at 25 °C. Detailed titration conditions are in Table S2. Different lines represent Nd3+
2+
3+
3+
(black), NdNO3 (red), NdL3+ (green), NdL2 (cyan), and NdL3 (magenta). Emptied
circle represents observed pCH, and blue line represents the calculated pCH.
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Figure 4. Spectrophotometric titrations of Nd3+ complexation with BnABDMA, ABDMA, and MABDMA (top), and molar absorptivity of species (bottom). Detailed
titration conditions are in Table S3. The molar absorptivity of NdNO32+ was omitted for clarity.
Verification the formation of Nd3+ complexes by ESI-MS
1:1, 1:2, and 1:3 Nd3+/L complexes for all three ligands were
identified by ESI-MS experiments (Figure 5, Figures S4, S5, and
S6). The 1:1 and 1:2 Nd3+/L complexes were observed as singly
-
or doubly charged species with NO3 as counter anions. In the
case of 1:3 Nd3+/L complexes, only a triply charged species is
observed. The intensity of the triply charged species is weak
but consistent with the natural isotopic abundances of Nd.
The weak intensity is due to the lower stability of relatively
small complexes with high electric charge, which are
susceptible to charge-separation during the ESI process. Since
the two spectra in Figure 5 were acquired using different
instrumental parameters, the observed results herein cannot
Figure 5. ESI-MS characterization of Nd3+ complexes with BnABDMA. In the top
be used to quantify the abundance of species,32 but only to
spectrum, triply and doubly charged Nd3+ complexes were observed, including
verify the formation of the complexes in a qualitative way. It
3+
Nd(ABDMA)3 and Nd(BnABDMA)2(NO3)2+
.
An inset figure shows a peak to peak
should be noted that In addition, singly charged NdO+ specie
was also observed in the ESI-MS experiments, potentially due
to the reactions between Nd3+ and anodic oxidation products
of water as reported elsewhere (Figures S4, S5, and S6).32
3+
separation of ∆pp = 0.3 m/z for Nd(ABDMA)3
charged complexes of Nd(BnABDMA)(NO3)2+ and Nd(BnABDMA)2(NO3)2
.
The bottom spectrum shows singly
+
.
Enthalpies of protonation of the ligands and enthalpies of
Nd3+ complexation by calorimetry
At least three independent calorimetric titrations were carried
out to determine ΔH of protonation for each ligand (Figure S2).
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The values of ΔH of protonation are –(31.20 ± 0.30) kJ mol-1, - Both potentiometric and spectrophotometric experiments
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(37.2 ± 2.1) kJ mol-1, and –(33.5 ± 0.60) kJ mol-1, for BnABDMA, indicate that the binding strength of the three ligands follows
DOI: 10.1039/C6DT01694D
ABDMA, and MABDMA, respectively. Similarly, multiple the trend of the increase in the basicity of the ligands,
titrations were performed in order to determine the ΔH of BnABDMA < ABDMA < MABDMA. This trend can be explained
complexation for the Nd3+ complexes. A representative by the electronic effect of the substitution groups in the
thermalgram of the titration is shown in Figure 6. The ligands. In comparison with ABDMA, the electron-withdrawing
experimental heat, calculated heat, and the speciation of Nd3+ ability of the phenyl group in BnABDMA reduces the electron
species in the titrations are shown in Figure 7. The enthalpies density on the central N atom and makes BnABDMA a less
of complexation were calculated in conjunction with the basic ligand. Therefore, the Nd3+/BnABDMA complexes are
stability constants obtained by potentiometry and weaker than the Nd3+/ABDMA complexes. On the other hand,
spectrophotometry. It was found that, in the case of the electron-donating ability of the methyl group in MABDMA
[Nd(MABDMA)3]3+ where significantly different values of increases the electron density on the central N atom and
stability constants were obtained by potentiometry and makes MABDMA a more basic ligand. Therefore, the
spectrophotometry, using the stability constant determined by Nd3+/MABDMA complexes are stronger than the Nd3+/ABDMA
spectrophotometry allows a better fit than using that complexes.
determined by potentiometry. Therefore, we elect to
The influence of the functional groups on the stability
designate the value of log3 (10.50 0.50) for constants implies two aspects of the complexation: (1) the
=

[Nd(MABDMA)3]3+, and neglect the value of (12.15 ± 0.03) interaction of amine-functionalized ligands with Nd3+ is
from potentiometry. The enthalpies of protonation and predominantly electrostatic in nature; (2) the central amine
complexation are summarized in Table 1, together with the nitrogen atoms of these ligands definitely participate in the
entropies of complexation accordingly calculated from the bonding with the Nd3+ ions in solution. It is very likely that,
enthalpies and stability constants.
similar to the ether oxygen-functionalized TMDGA, all the
three amine-functionalized ligands (BnABDMA, ABDMA, and
MABDMA) coordinate to Nd3+ ions in a tridentate mode.
Thermodynamic trends in Nd3+ complexes with BnABDMA,
ABDMA, and MABDMA
Ligand
TMDGA
Methoda
Logβ
ΔH (kJ mol-1)
ΔS (J K-1 mol-1)
Ref
8
Nd3+ + L = NdL3+
sp, cal
sp, cal
sp, cal
(3.53 ± 0.10)
(5.84 ± 0.19)
(6.80 ± 0.19)
-(10.9 ± 0.9)
-(15.6 ± 1.5)
-(19.3 ± 2.2)
(26 ± 1)
(39 ± 2)
(59 ± 7)
Nd3+ + 2L = NdL2
Nd3+ + 3L = NdL3
BnABDMA
3+
3+
this work
this work
this work
H + L = HL+
pot,cal
(6.36 ± 0.09)
(2.92 ± 0.09)
(5.08 ± 0.09)
(7.13 ± 0.09)
-(31.2 ± 0.3)
-(13.3 ± 0.6)
-(22.6 ± 1.2)
-(30.9 ± 0.9)
(17+1)
(11 ± 1)
(21 ± 3)
(33 ± 2)
Nd3+ + L = NdL3+
pot, sp, cal
pot, sp, cal
pot, sp, cal
Nd3+ + 2L = NdL2
3+
Nd3+ + 3L = NdL3
3+
ABDMA
H + L = HL+
pot, cal
(7.12 ± 0.09)
(4.08 ± 0.09)
(6.93 ± 0.09)
(10.02 ± 0.9)
-(37.2 ± 2.1)
-(13.5 ± 0.6)
-(20.5 ± 2.1)
-(39.4 ± 1.5)
(11+6)
(32± 2)
(64 ± 6)
(60 ± 4)
Nd3+ + L = NdL3+
pot, sp, cal
pot, sp, cal
pot, sp, cal
Nd3+ + 2L = NdL2
3+
Nd3+ + 3L = NdL3
3+
MABDMA
H + L = HL+
pot, cal
pot, sp, cal
pot, sp, cal
sp, cal
(7.64 ± 0.09)
(4.40 ± 0.09)
(7.52 ± 0.36)
(10.50 ± 0.50)
-(33.5 ± 0.6)
-(11.4 ± 0.3)
-(23.4 ± 1.2)
-(34.3 ± 1.5)
(34+2)
(46 ± 1)
(65 ± 3)
(86 ± 4)
Nd3+ + L = NdL3+
Nd3+ + 2L = NdL2
3+
Nd3+ + 3L = NdL3
3+
a
Table 1. Overall equilibrium constants, ΔH, and ΔS for the protonation and complexation of BnABDMA, ABDMA, and MABDMA, with Nd3+ at 25 °C and I = 1.0 NaNO3. Pot:
potentiometry, sp: spectrophotometry, cal: calorimetry.
6 | J. Name., 2012, 00, 1-3
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Figure 6. Calorimetric titration thermogram of Nd3+ complexation with BnABDMA. Vo
=
0.750 mL, injection volume = 0.05 mL, injection interval = 400 s, T = 25 °C. Detailed
titration conditions are listed in Table S5.
Figure 7. Speciation of Nd3+ and heat in calorimetric titrations for Nd3+ complexation
with BnABDMA (top), ABDMA (middle), and MABDMA (bottom). Detailed titration
conditions are in Table S4. Different lines represent Nd3+ (black), (NdNO3)2+ (red),
(NdL)3+ (green), (NdL2)3+ (cyan), and (NdL3)3+ (magenta). Open circle represents
observed Q, and blue line represents calculated Q.
Comparison of the binding strength between amine-functionalized
ligands and ether oxygen-functionalized TMDGA
In general, the amine-functionalized ligands form stronger
complexes with Nd3+ than the ether oxygen-functionalized
TMDGA (Table 1), except BnABDMA in which the electron-
withdrawing phenyl group reduces its binding strength. The
higher binding affinity of amine ligands probably results from a
combination of a few factors that contribute to the enthalpy of
complexation, including the degree of ligand hydration, steric
effects and modulation of the nitrogen donor ability by the
substitutional groups. This is reflected by the enthalpies of
complexation shown in Table 1. The enthalpies of
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complexation for Nd3+ complexes with the amine ligands are
generally more exothermic (by a few kilojoules per mole to
more than ten kilojoules per mole) than corresponding
complexes with TMDGA. It is also interesting to note that the
stepwise enthalpies and entropies of complexation (Table S6 in
ESI) show different trends for BnABDMA, ABDMA, and
MABDMA. Again, the trends reflect the combination of the few
effects that contribute to the energetics of the complexation,
but it is complicated to assign the origin of these trends to
individual effects.
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As suggested by the trend in the binding strength of
BnABDMA, ABDMA, and MABDMA with Nd3+ complexes, the
amine nitrogen participates in the coordination with Nd3+ and
tridentate complexes form. This is similar to the tridentate
complexes between Nd3+ and TMDGA. However, the binding
strength of the amine functionalized ligands can be fine-tuned
using different substitutional groups on the central nitrogen
atom. Such modification to tune the binding strength is not
feasible for the ether-functionalized ligands such as TMDGA or
the family of TRDGA ligands.
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In summary, thermodynamic parameters of the complexation
between Nd3+ ions and three amine-functionalized ligands,
BnABDMA, ABDMA, and MABDMA, were determined and
compared with those of a previously studied ether oxygen-
functionalized ligand TMDGA. The binding ability of the
amine-functionalized ligands generally form stronger
(19)
Magnusson, D.; Christiansen, B.; Glatz, J.-P.; Malmbeck,
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(20)
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Nave, S.; Modolo, G.; Madic, C.; Testard, F. Solvent
difference in the degree of dehydration of the amine nitrogen
and the ether oxygen. Besides the higher binding strength, the
amine-functionalized ligands offer the advantage of fine-tune
the binding strength by using different substitutional groups
on the amine nitrogen atom. The trend in thermodynamic
parameters within the three amine ligands is consistent with
the electronic effect of the substitutional groups on the
binding strength.
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Sasaki, Y.; Sugo, Y.; Suzuki, S.; Tachimori, S. Solvent
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Acknowledgements
This work was supported by the Director, Office of Science,
Office of Basic Energy Science of the U.S. Department of
Energy (DOE), under Contract No. DE-AC02-05CH11231 at
Lawrence Berkeley National Laboratory (LBNL).
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8 | J. Name., 2012, 00, 1-3
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Three imine-functionalized diamide ligands form tridentate complexes with Nd3+ in aqueous solutions.
The stability constants of the complexes follow the order of the ligand basicity that can be tuned by
different substitutional groups on the imine nitrogen.