Effect of Oxygen-Donor Charge on Adjacent Nitrogen-Donor Interactions in Eu3+Complexes of Mixed N,O-Donor Ligands Demonstrated on a 10-Fold [Eu(TPAMEN)]3+Chelate Complex

Article

Eﬀect of Oxygen-Donor Charge on Adjacent Nitrogen-Donor

Interactions in Eu Complexes of Mixed N,O-Donor Ligands

Demonstrated on a 10-Fold [Eu(TPAMEN)] Chelate Complex

Kathleen Schnaars,* Masashi Kaneko,* and Kiyoshi Fujisawa

Cite This: Inorg. Chem. 2021, 60, 2477 −2491

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sı Supporting Information

ABSTRACT: To reduce high-level radiotoxic waste generated by

nuclear power plants, highly selective separation agents for minor

actinides are mandatory. The mixed N,O-donor ligand N,N,N′,N′-

tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine

(

H TPAEN; 1) has shown good performance as a masking agent in

Am /Eu separation studies. Adjustments on the pyridyl backbone

to raise the hydrophilicity led to a decrease in selectivity and a

decrease in M −N interactions. An enhanced basicity of the

pyridyl N-donors was given as a cause. In this work, we examine

whether a decrease in O-donor basicity can promote the M −N

interactions. Therefore, we replace the deprotonated “charged”

carboxylic acid groups of TPAEN⁴by neutral amide groups and

introduce N,N,N′,N’-tetrakis[(6-N″,N′′-diethylcarbamoylpyridin-2-

−

yl)methyl]ethylenediamine (TPAMEN; 2) as a new ligand. TPAMEN was crystallized with Eu(OTf) and Eu(NO ) ·6H O to

3 3

form positively charged 1:1 [Eu(TPAMEN)] complexes in the solid state. Alterations in the M−O/N bond distances are

−

compared to [Eu(TPAEN)] and investigated by DFT calculations to expose the diﬀerences in charge/energy density distributions

4−

at europium(III) and the donor functionalities of the TPAEN and TPAMEN. On the basis of estimations of the bond orders,

atomic charges spin populations, and density of states in the Eu and potential Am and Cm complexes, the speciﬁc contributions of

−

the donor−metal interaction are analyzed. The prediction of complex formation energy diﬀerences for the [M(TPAEN)] and

[

M(TPAMEN)] (M = Eu , Am ) complexes provide an outlook on the potential performance of TPAMEN in Am /Eu

separation.

INTRODUCTION

example of the tripodal ligands tris[(2-pyridyl)methyl]amine

■

(

TPA) and tris[6-((2-N,N-diethylcarbamoyl)pyridyl)methyl]-

The separation of long-lived radiotoxic minor actinides (MAs:

amine (TPAAM) (Figure 1), where the Ln −N bond

distance for TPAAM was elongated. A comparison with

i.e., Am , Cm ) and lanthanides (Ln ) is crucial for the

success of the partitioning and transmutation strategy to

reduce the high-level radioactive waste inventory of nuclear

α,α′,α′′-nitrilotris(6-methyl-2-pyridinecarboxylic acid)

TPAA) (Figure 1) revealed that the Ln −O and Ln −

power plants in the future. Their very similar physicochem-

ical properties still render the separation of MAs and Ln

challenging. To increase the economics of current separation

processes, the development of highly selective separation

agents is mandatory. Especially, softer N-donor ligands have

bond distances in these ﬂexible systems are similar

independent of the oxygen donor charge. Unfortunately,

structural diﬀerences and variations in the type of associated

anions and solvent molecules, completing the coordination

sphere of the Ln complexes with TPAAM and H TPAA,

proven to have good selectivity toward MAs over Ln due to

the slight presence of a covalent character in their interaction

prevent a direct correlation between the O-donor type (neutral

or negatively charged) on the Ln −N bond distances in this

toward MAs. Unfortunately, pure N-donor ligands hardly

study. Whether the observed tendencies in the bond distances

coordinate the Ln /An (An denotes actinides) ions under

highly acidic concentrations as a consequence of concurrent

Received: November 18, 2020

Published: January 27, 2021

protonation reactions. Mixed N,O-donor ligands overcome

this problem by additional oxygen moieties attached to the

nitrogen scaﬀold to increase the ligand’s basicity.

Less selective harder O-donors counteract the interactions

between the Ln /An ions and the softer N-donors in the

same molecule. Bravard et al. demonstrated this by the

2021 American Chemical Society

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Inorg. Chem. 2021, 60, 2477−2491

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nitrogen in the order H TPAEN (1) < H TPAEN-p-OCH <

H TPAEN-p-C H NO. As a consequence,the An −N

bond distance decreases, whereas the An −O and the

An −N bond distances experience an elongation. The

author explains this trend as a steric eﬀect pushing the N and

O away from the actinide. However, this tendency also

agrees with the concept of a negative charge-saturated cation

with a reduced aﬃnity for further interactions toward other

donor atoms in the same molecule as discussed above.

In this study, we intend to promote the M −N

interactions by reducing the basicity of adjacent donor

functions. To achieve this without tremendously altering the

coordination environment, our idea is to replace the cation-

exchanging O-donor atoms of H TPAEN (1) by neutral O-

donors of amide groups. The amide group is a popular

structural motif in separation agents for f-block elements, such

as NTAamide (Figure 1) and BLPhen (Figure 1), because

of its high acid stability, relatively hard electron donor

Figure 1. Molecular schemes of TPA, TPAA, TPAAM, NTAamide,

BLPhen, TPEN, H TPAEN (1), H TPAEN-p-OCH , H TPAEN-p-

character in comparison to other ketones, and good

C H NO, and TPAMEN (2).

hydrophobic properties due to the possibility of introducing

nonpolar alkyl chains at the amide nitrogen. Herein, we

report on the decadentate amide-substituted TPEN derivative

N,N,N′,N′-tetrakis[(6-N″,N′′-diethylcarbamoylpyridin-2-yl)-

methyl]ethylenediamine (TPAMEN; 2) (Figure 1). We give

two synthetic paths to obtain TPAMEN (2) and investigate its

are the consequence of steric restrictions or are the result of

saturation in charge density at the cation, lowering the metal−

ligand attraction, as has been described in quantum

mechanistic studies by Berny et al., remains unclear.

^c^o^m^p^l^e^x^a^tⁱ^oⁿ^b^e^h^a^vⁱ^o^r^t^o^w^a^r^d^E^u⁽^O^T^f⁾3 ^aⁿ^d^E^u⁽^N^O3⁾3^·

Among the numerous perturbing parameters in the complex

coordination systems of f-block elements, including high

coordination numbers of between 6 and 12, diﬀerent

coordination geometries, and varying amounts of additional

solvent and anion interactions, speciﬁc structure-related

reactivity changes are hard to identify. Therefore, to enable

structure−reactivity correlations, ligands with a high number of

available donor functionalities, capable of completely saturat-

ing the coordination sphere, are of great interest. Chromo-

H O. A comparison of the diﬀerent molecular structures of

both salts serves as an indicator for possible steric inﬂuences of

the packing, aﬀecting the Eu −O/N /N bond distances of

the complexes in the solid state. DFT calculations of

−

[

Eu(TPAMEN)] and [Eu(TPAEN)] estimate how the

alterations in the experimental M −L bond distances aﬀect

the electron-/energy-density properties, the bond order of the

present metal−ligand interactions, and the eﬀect of these on

the atomic charge and spin population at the cation and each

donor functionality. In addition, a prediction of the equivalent

phore Ln receptors for biomedical applications, which need

to be stable under physiological conditions and should avoid

additional water coordination to gain higher quantum yields,

fulﬁll this condition. A prominent example is N,N,N′,N′-

tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine

Am and Cm complexes serves as a further validation of the

potential eﬀect on actinide complexation and evaluates

TPAMEN’s potential for selective Am /Eu separation.

With this exemplary investigation of TPAMEN in relation to

(

H TPAEN; 1) (Figure 1), capable of completely encapsulat-

4−

TPAEN , we want to determine the complex interplay of

donor−metal interactions and their contribution to d- and f-

orbital overlap. Moreover, we want to emphasize the

importance of shifts in the strength of donor−metal

interactions as a result of basicity changing modiﬁcations on

adjacent donor groups and encourage its consideration in the

development of new separation agents in the future.

ing Ln ions by 10-fold coordination. H TPAEN is a

tetrapodal, decadentate ligand with a 6-fold N-donor frame-

work, based on N,N,N′,N′-tetrakis(2-pyridylmethyl)-

ethylenediamine (TPEN) (Figure 1), extended by four

additional carboxylic acid groups. The TPEN framework also

renders this ligand as having promise for selective MA

separation, which has initiated several studies on its Am /

10,11

Eu and Am /Cm selectivity.

Despite its high

separation factor for Am /Eu , H TPAEN’s poor solubility

EXPERIMENTAL SECTION

■

under acidic conditions limits its application in separation

processes and requests further adjustments of the ligand

Materials and Methods. The chemicals used were purchased

from Sigma-Aldrich, Merck, Wako, and TCI. CH Cl , CH CN,

design. The attempts by Gracia et al. to increase the

solubility of the ligand in aqueous media by introducing polar

groups in the para position of the pyridyl ring resulted in the

CH OH, and ethylenediamine were predried by storage over activated

3 Å (CH Cl , CH CN, CH OH) or 5 Å (ethylenediamine) molecular

sieves. If not stated diﬀerently, all chemicals were used without

further puriﬁcation. Deionized water was obtained from a Merck

desired higher solubility but also in a loss of Am /Eu

Milli-Q reference A water puriﬁcation system. For path I, 1 was

selectivity.

A quantum chemical study by Huang et al. on the Am /

synthesized in a ﬁve-step synthesis by starting from commercially

available 2,6-dipicolinic acid according to a protocol by Gracia et al.

Cm selectivity of H TPAEN (1), H TPAEN-p-OCH , and

The number of associated hydrochlorides and hydrates was

determined by elemental analysis. For path II , precursor 3 was

synthesized in a ﬁve-step synthesis (see Scheme S1 in the Supporting

Information) by starting from the commercially available 2,6-

dipicolinic acid (4). The single-step syntheses of compounds,

H TPAEN-p-C H NO (C H NO denotes a morpholine

group) (Figure 1) pointed out that the An −N bond

distances in these ligands correlate with their separation ability

3+ 12

for Am and Cm . An analysis of the electronic structure

also suggested an increased charge density at the pyridyl

including dimethylpyridine-2,6-picolinate (5),

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Table 1. Crystallographic Data for 9 and 10

[

Eu(TPAMEN)][Eu(NO ) ]·2CH CN·0.45H O (9)

[Eu(TPAMEN)]OTf ·1.125H O·0.545CH CH OH[+solvents] (10)

CCDC no.

chemic formula

formula wt

temp (K)

2018225

C H Eu N O

1587.26

178.0

2018224

C₅₀_.₀₉H₆₉_.₂₇EuF N O₁₄_.₆₇S₃

1465.36

178.0

70.9

18 22.45

cryst syst

space group

a (Å)

b (Å)

c (Å)

orthorhombic

Pnna

16.3723(3)

21.5347(4)

18.0522(3)

orthorhombic

Pbca

15.3371(19)

19.232(2)

42.232(5)

α (deg)

β (deg)

γ (deg)

V (Å³)

6364.7(2)

1.656

12456.6(3)

1.563

density (g·cm⁻³)

F(000)

3210

5993

radiation type

μ(Mo Kα) (mm⁻¹)

crystal size (mm)

no. of measd rﬂns

no. of indep rﬂns

R(int)

no. of variables

residuals: R1 (I > 2σ(I))

residuals: R (all rﬂns)

Mo Kα (λ = 0.71073)

2.043

0.213 × 0.083 × 0.047

193036

7309

0.0565

463

0.0477

0.0499

0.0981

1.275

Mo Kα (λ = 0.71073)

1.204

0.126 × 0.078 × 0.040

376698

14260

0.0573

958

0.0678

0.0736

0.1534

1.297

residuals: wR2 (I > 2σ(I))

goodness of ﬁt

R1 = ∑||F | − |F ||/∑|F |; wR2 = [∑(w(F − F ) )/∑w(F ) ]1/2_.

2 2

(

methoxycarbonyl)picolinic acid (6), 6-(N,N-diethylcarbamoyl)-

gradually dissolved and a yellowish solid spontaneously precipitated.

The reaction was stopped, and the excess thionyl chloride was

evaporated under reduced pressure. The yellowish solid was

redissolved in dry dichloromethane and cooled to 0 °C. Then

diethylamine (28.442 mmol, 3 mL) was added dropwise and the

reaction mixture was stirred for 2 h at 40 °C. To stop the reaction, an

pyridine-2-carboxylic acid (7), N,N-diethyl(6-(hydroxylmethyl)-

pyridine-2-carboxamide (8), and N,N-diethyl(6-chloromethyl)-

pyridine-2-carboxamide (3), were performed as described in the

literature.

FT-IR spectra were measured on a JASCO FT/IR-4600

spectrometer at 2 cm spectral resolution. For the measurement,

−

NH Cl solution was added. The dichloromethane phase was washed 2

the sample was diluted in CH CN, transferred to a 3M Type 61

times with NH Cl solution and 2 times with deionized water. The

polyethylene 19 mm Aperture IR card, and measured after the

evaporation of the solvent. The intensities are reported relative to the

peak with the weakest transmission, using the following abbreviations:

vw = very weak, w = weak, m = medium, s = strong, vs = very strong.

collected organic phases were dried over Na SO and ﬁltered, and the

dichloromethane was evaporated, resulting in a yellowish oil. The

crude product was puriﬁed by column chromatography (neutral

Al O , 3/2 n-hexane/CHCl and MeOH gradient from 0% to 10%),

The H NMR were measured at 300.0 K and 500.13 MHz on a

yielding 0.699 g of the target compound (L·1.5H O·HCl: 66%).

−1

Bruker Advance III 500 spectrometer. The C{ H} NMR and the

Dept-135-NMR spectra were measured at 300.0 K and 100.61 MHz

on a Bruker Advance III 400 spectrometer. The chemical shifts (δ)

are reported in parts per million (ppm) relative to the residual solvent

IR (FT/IR, 298 K, cm ): 3547 (vw), 3503 (vw), 3063 (w), 2973

(s), 2935 (s), 2873 (m), 2849 (m), 2821 (m), 1633 (vs), 1587 (vs),

1571 (vs), 1513 (w), 1483 (vs), 1433 (vs), 1416 (s), 1379 (s), 1362

(s), 1348 (m), 1316 (s), 1298 (s), 1269 (m), 1219 (m), 1204 (m),

1154 (m), 1116 (s), 1100 (m), 1084 (s), 1049 (w), 1015 (w), 1011

(w), 994 (m), 978 (w), 945 (w), 906 (vw), 821 (m), 787 (m), 760

shifts of CHCl in CDCl ( H NMR, 7.26 ppm; C NMR, 77.16

ppm. Multiplicities are described with s for singlets, d for doublets, t

for triplets, q for quadruplets, and m for multiplets. The signals of the

(s), 695 (vw), 639 (w), 587 (vw), 505 (vw). H NMR (500 MHz,

C{ H} spectra were assigned, supported by Dept-135-NMR spectra.

CDCl , ppm): 1.11 (t, J = 6.9 Hz, 12H, H1), 1.24 (t, J = 7.0 Hz,

Mass spectrometry was carried out on a Shimadzu LCMS 8030 liquid

chromatograph mass spectrometer using electrospray ionization

12H, H1), 1.96 (s, 2H O), 2.73 (s, 4H, H10), 3.27 (q, J = 6.9 Hz,

8H, H2), 3.52 (q, J = 7.0 Hz, 8H, 2), 3.76 (s, 8H, H9), 7.39 (d, J =

7.6 Hz, 4H, H5/H7), 7.44 (d, J = 7.8 Hz, 4H, H5/H7), 7.66 (t, J =

7.7 Hz, 4H, H6). C{ H} NMR (125 MHz, CDCl , ppm): 13.0 (4C,

(

ESI). CH CN served as the mobile phase (LC-MS grade). The

m/z range was set from 100 to 1000. The samples were directly

injected, without any puriﬁcation over an LC column. The CHN

analysis was performed on an Elementar Analysatorsysteme Vario

MICRO cube Elemental Analyzer in CHN mode.

C1), 14.4 (4C, C1), 40.3 (4C, C2), 43.4 (4C, C2), 52.7 (2C, C10),

60.7 (4C, C9), 121.4 (4C, C ), 123.1 (4C, C ), 137.4 (4C, C ),

154.5 (4C, C8/C4), 158.7 (4C, C4/C8), 168.7 (4C, C3). LC-MS

Synthesis of N,N,N′,N′-Tetrakis[(6-N″,N″-diethylcarbamoyl-

pyridin-2-yl)methyl]ethylenediamine (TPAMEN; 2) via Path I.

Thionyl chloride (11.3 mL) and a catalytic amount of DMF (1 drop)

were added to N,N,N′,N′-tetrakis[(6-carboxypyridin-2-yl)methyl]-

(m/z): 821.9 [M + H] , 843.85 [M + Na] Anal. Calcd for

C H N O [L + 0.5H O]: C, 66.56; H, 7.89; N, 16.87. Found: C,

65 10 4.5

66.59; H, 7.63; N, 16.46.

Synthesis of N,N,N′,N′-Tetrakis[(6-N″,N″-diethylcarbamoyl-

pyridin-2-yl)methyl]ethylenediamine (TPAMEN; 2) via Path II.

6-(Chloromethyl)-N,N-diethyl-2-pyridinecarboxamide (1.404 g, 6.192

mmol) and K CO (0.888 g, 6.422 mmol) were diluted in dry

ethylenediamine trihydrochloride (H TPAEN; 1) (0.849 g, 1.195

mmol) at 0 °C under an argon atmosphere. The reaction mixture was

heated to 60 °C and stirred for 1.5 h. During this period the solid

479

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acetonitrile (20 mL) under an Ar atmosphere before dry ethylenedi-

amine (103 μL, 1.548 mmol) was added. The reaction mixture was

reﬂuxed for 11 h and stirred for an additional 84 h at RT. After the

reaction was complete, acetonitrile was evaporated and the crude

product was dissolved again in dichloromethane (50 mL). After the

dichloromethane phase was washed with deionized water (3 × 50

mL), the dichloromethane phase was dried over Na SO , ﬁltered, and

may therefore vary from the reality. Due to the high inaccuracy of our

model concerning the triﬂate anions and solvent molecules, the

discussion will only focus on global packing properties and selected

interatomic metal−ligand distances/angles of the [Eu(TPAMEN)]

fragment, to which no constraints and restraints had been applied. To

enter the responses, corresponding to the alerts in the checkcif ﬁles,

into the cif ﬁles, enCIFer 2020.1 was used.

Continuous Shape Measure Calculations (CShM).

evaporated under reduced pressure to yield the target compound as a

pale yellow oil (0.963 g, 1.172 mmol, 70% yield) of the crude product

determine the shape of the coordination polyhedra of the [Eu-

3−

(92% purity). The spectral data are in accordance with those reported

(TPAMEN)] and [Eu(NO ) ] the program SHAPE (2.1) was

3 6

for path I.

employed. To create the imput ﬁle, the corresponding xyz values were

Synthesis of [Eu(TPAMEN)][Eu(NO ) ]·2CH CN·0.45H O (9).

A 20.7 mg portion of 2 (92%), dissolved in 224 μL of CH CN, was

extracted from the cif ﬁles by using OLEX2-1.3-alpha and

2−36

Mercury2020.1.

The closer the value to zero, the higher the

added to a solution of 6.7 mg of Eu(NO ) ·6H O in 249 μL of

agreement to an ideal polyhedral shape.

DFT Calculations. We carried out the geometry optimization,

CH CN. The vial containing the reaction mixture, placed into a larger

vial, containing a 1/1 diethyl ether/n-hexane mixture, and left at room

temperature for slow vapor diﬀusion of the diethyl ether/n-hexane

mixture. After several weeks, blocklike single crystals slowly grew from

the reaction mixture. The isolation of the bulk material was not

attempted, and therefore, the yield was not determined.

determined the vibrational frequency modes, and carried out single-

point energy calculations by using density functional theory (DFT)

calculations, followed by electron population analyses, of [M-

−

(TPAEN)] and [M(TPAMEN)] (M = Eu , Am , Cm ) to

compare their coordination bond properties, such as bond lengths and

bond orders. The starting coordinates for geometry optimization

calculations were referenced to the corresponding single-crystal X-ray

diﬀraction data of [{Eu(TPAEN)}K(H O) ]·4H O (CSD code:

Synthesis of [Eu(TPAMEN)]OTf ·1.125H O·0.545CH CH OH-

[

+solvents] (10). A 168 μL portion of a 0.1 M solution of 2 (92%, 30

mg, 0.033 mmol, in 336 μL EtOH) in EtOH was added to 168 μL of

a 0.1 M solution of Eu(OTf) (10.3 mg, 0.017 mmol, in 168 μL H O)

TAZKEL ) and [Eu(TPAMEN)][Eu(NO ) ]·2CH CN·0.45H O

(9) (this study) for [M(TPAEN)] and [M(TPAMEN)] ,

−

in H O. The mixture was frozen by external cooling with liquid

nitrogen before a layer of diethyl ether was added. The two-layer

system was warmed to RT, and after slow diﬀusion of the diethyl

respectively. We employed a scalar-relativistic zeroth-order regular

38,39

approximation (ZORA)

with segmented all-electron relativisti-

ether into the EtOH/H O solution, crystalline plates of 10, suitable

cally contracted (SARC) basis sets to consider the scalar-relativistic

for X-ray crystallography, formed at the phase border. The isolation of

bulk material was not attempted, and therefore, the yield was not

determined.

eﬀects of the heavy-metal atoms. The SARC basis sets for ZORA were

assigned as {61 /51 /41 /311} for the Eu atom and (91 /81 /

71 /61 ) to Am and Cm atoms for all DFT calculations and the

X-ray Diﬀraction Reﬁnements. The crystal data and reﬁnement

parameters for [Eu(TPAMEN)][Eu(NO ) ]·2CH CN·0.45H O (9)

other atoms as split-valence plus one polarization (SVP) for

geometry optimization and vibrational frequency mode calculations

and triple-ζ valence plus one polarization (TZVP) for single-point

and [Eu(TPAMEN)]OTf ·1.125H O·0.545CH CH OH[+solvents]

(

10) are summarized in Table 1. Suitable single crystals were coated

in Paratone-N oil and mounted on a Dual-Thickness MicroLoop LD

200 μM) purchased from MiTeGEn and placed in a N gas stream.

energy calculations. M06-L was used as the exchange-correlation

functional for the geometry optimization calculations as well as for

thermodynamic analysis in the complex formation reaction. We chose

M06-L because it showed good reproduction of the metal−ligand

(

The diﬀraction data were measured on a Rigaku XtaLAB P200

diﬀractometer using Mo Kα (λ = 0.71073 Å) radiation at 178 K for 9

and 10. Empirical absorption corrections were performed with

−

bond lengths of [M(TPAEN)] (M = Eu , Ce , Am ) by

benchmarking of density functionals within an optimized time of pure

CrysAlisPRO. All structures were solved by direct methods

DFT calculations according to a study by Shi et al. Furthermore, the

(

SIR2008) using CrystalStructure, and reﬁnement was performed

M06-L functional was employed for single-point energy calculations

for electron population analyses to enable a comparison of the results

in OLEX2-1.3-alpha with SHELXL by least-squares minimization

against F . During the reﬁnement, ﬁrst isotropic and then anisotropic

−

of diﬀerent [M(TPAEN)] derivatives by Shi et al. The solvent

eﬀect of water was also implicitly considered for all self-consistent-

ﬁeld (SCF) calculations using the conductor-like solvation model

thermal parameters for all non-hydrogen atoms were used. Hydrogen

atoms were calculated and placed in idealized positions. Images of the

4,26

molecular structures were created with OLEX2-1.3-alpha

and

(COSMO) method for both geometry optimization and single-

further modiﬁed with Gimp.2.10.14. In 9 two of the NEt groups

point energy calculations. The dielectric constant and refractive index

were set to 80.4 and 1.33, respectively, and the COSMO radii were

assigned to Eu, Am, Cm, O, N, C, and H atoms as 1.90, 1.99, 1.95,

1.72, 1.83, 2.00, and 1.30 Å, respectively, to consider water solvation

eﬀects in the COSMO calculations. We obtained the electronic

have a disorder of approximately 50%, in which the ethyl residues are

ﬂipped around the CH group to open space for disordered water

molecules in the packing. Structure 10 shows a high degree of solvent

and anion disorder. According to their positioning in the packing the

triﬂate anions are disordered over two and three positions. To ﬁt the

triﬂate molecules on the related residual electron density peaks, the

−

ground states of [M(TPAEN)] and [M(TPAMEN)] , which were

set to a spin septet for M = Eu , Am and a spin octet for M =

28,29

Fragment DB

extensions of OLEX2-1.3-alpha were used. The

Cm , by using an unrestricted Kohn−Sham treatment to conﬁrm the

equilibrium structures to be at a local minimum by vibrational

frequency mode calculations. We used the resolution of the identity

(RI) approximation for all SCF calculations with the same criteria in

the convergence and grid number as in our previous DFT studies.

high degree of disorder hindered a direct assignment of all solvent

molecules in the structure, and a solvent mask was applied. However,

the multiple overlaps and vicinities of the residual electron density

peaks to the triﬂate anions led to diﬃculties with the electron count of

the solvent mask. To solve this problem, in a second approach, two

triﬂate anions were squeezed to enable an electron count in the

solvent-relevant void. A concise summary of the reﬁnement

parameters, the applied constraints and restraints, and a report on

the di ﬀ erent solvent mask attempts can be found in the Supporting

Information . The amount of crystalline solvent was modeled in

accordance with the residual electron density peaks. If possible, the

occupancy was optimized as a free parameter by the reﬁnement

program. In cases where the residual electron density was very low,

the occupancy was adjusted by trial under observation of the ORTEP

ellipsoids. The amount of solvent is purely deﬁned by the model and

All SCF calculations were performed under a convergence condition,

in which the threshold value of total energy change during the

−8

iteration was set as 10 hartree. Grid point parameters were set to a

Lebedev194 angular grid with an integral accuracy of 4.34 in geometry

optimization and numerical vibrational frequency mode calculations

and a Lebedev302 angular grid with an integral accuracy of 4.67

followed by a Lebedev434 ﬁnal angular grid with an integral accuracy

of 5.01 in single-point energy calculations, in which special grids were

assigned to Eu with an integral accuracy of 14. All DFT calculations

and natural population analyses were performed by using ORCA ver.

3.0 and NBO ver. 6.0 programs, respectively.

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Inorganic Chemistry

Article

Scheme 1. Synthesis of TPAMEN (2) by Paths I and II

Reagents and conditions: (a) (1) SOCl , DMF (cat.), 0−60 °C, (2) Et NH, CH Cl , 0−40 °C (66%); (b) K CO , CH CN, reﬂux, 11 h (70%).

RESULTS AND DISCUSSION

assume that in the ﬁrst trial by serendipity a crystal seed

formed from the oily residue and gave this one square blocklike

crystal, whereas in the other trials this was not achieved. There

seems to be a thin window of parameters depending on

external temperature and the ratio of acetonitrile/diethyl ether

and n-hexane where the complex stays soluble and does not

form a third layer. We suspect that temperature ﬂuctuations in

our laboratory may have hindered the crystal formation in the

■

Ligand Synthesis. For the synthesis of TPAMEN (2) two

diﬀerent reaction paths were considered (see Scheme 1): (I) a

direct amide conversion of the carboxyl groups in H TPAEN

and (II) the construction of the TPEN framework on the basis

of a 6-(chloromethyl)-2-pyridine carboxamide derivative,

already containing the amide moiety. The starting material

H TPAEN (1) for reaction I was synthesized by a procedure

further trials. The reaction with Eu(OTf) readily formed a

reported by Gracia et al. The amide conversion was achieved

crystalline solid upon the slow addition of diethyl ether to a

by chlorination of the carboxylic acid groups in 1 with an

solution of TPAMEN and Eu(OTf) in methanol, acetonitrile,

excess of SOCl , followed by a subsequent reaction of the acid

or ethanol/water. Only the last solvent provided suitable single

chloride with diethylamine to give the product 2. The reaction

conditions were chosen on the basis of a procedure by Mariani,

Casnati, et al. for the amide conversion of monomethyl

dipicolinate (see also Scheme S1c in the Supporting

c r y s t a l s o f [ E u ( T P A M E N ) ] O T f · 1 . 1 2 5 H O ·

.545CH CH OH[+solvents] (10) for X-ray crystallographic

3 2

studies.

Crystal data of 9 and 10 are given in Table 1. Selected bond

Information). Depending on the prior puriﬁcation proce-

distances of the Eu coordination environment for [Eu-

dure, H TPAEN can be obtained with varying amounts of

(

TPAMEN)] are summarized in Table 2 and for [Eu-

hydrochlorides and hydrates. To ensure complete conversion

of all four carboxylic acid groups independent of the number of

associated hydrates and hydrochloride, a ≥5 times excess of

3−

NO ) ] in Table 3. A selection of angles and torsion angles

of the [Eu(TPAMEN)] fragments in 9 and 10 is summarized

in Table S3 in the Supporting Information. Both complexes

crystallize in primitive orthorhombic space groups: 9 in Pnna

diethylamine based on the molar mass of pure H TPAEN was

employed. The reaction gave the isolated TPAMEN (2) in

fairly good yield (66%).

Table 2. Selected Bond Distances (Å) for [Eu(TPAMEN)]³⁺

In reaction II, precursor 3 was reacted with ethylenediamine

in the presence of K CO to assemble the TPEN framework.

in [Eu(TPAMEN)][Eu(NO ) ]·2CH CN·0.45H O (9) and

[

Eu(TPAMEN)]OTf ·1.125H O·

3 2

.545CH CH OH[+solvents] (10)

Also, the second reaction gave 2 in good yield (70%). The

starting compound 3 was obtained from 2,6-pyridinedicarbox-

ylic acid in a ﬁve-step synthesis in accordance with diﬀerent

literature protocols (see Scheme S1 in the Supporting

,18,19

Information).

Structural Characterization of Eu -TPAMEN Com-

plexes in the Solid State. TPAMEN (2) was reacted with

Eu(OTf) and Eu(NO ) ·6H O to obtain single crystals for

Eu −O

Eu1−O1

Eu1−O2

2.437(3)

2.397(3)

Eu1−O1

Eu1−O2

Eu1−O3

Eu1−O4

2.416(4)

2.457(4)

2.418(4)

2.436(4)

3 3

structural investigations of the Eu -TPAMEN complexation.

The attempts to obtain crystals from the reaction of TPAMEN

with Eu(NO ) ·6H O resulted primarily in the precipitation of

Eu −N

Eu1−N1

2.786(4)

Eu1−N1

Eu1−N6

2.782(5)

2.784(5)

an oily residue containing the complex species. Only once was

it possible to grow a blocklike single crystal of [Eu-

Eu −N

(

TPAMEN)][Eu(NO ) ]·2CH CN·0.45H O (9) by vapor

3 6 3 2

Eu1−N2

Eu1−N4

2.734(4)

2.599(4)

Eu1−N2

Eu1−N4

Eu1−N7

Eu1−N9

2.605(5)

2.710(5)

2.600(5)

2.699(5)

diﬀusion of n-hexane/diethyl ether into a saturated solution

of the reactants in acetonitrile. Several attempts to reproduce

the complexation under the same as well as slightly altered

stoichiometric, temperature, and solvent conditions failed. We

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Inorganic Chemistry

Article

−

Table 3. Selected Bond Distances (Å) and Angles (deg) for [Eu(NO ) ] in [Eu(TPAMEN)][Eu(NO ) ]·2CH CN·0.45H O

3 6

(

NO3

Eu −O

Eu2−O3

Eu2−O4

2.573(4)

2.577(4)

Eu2−O6

Eu2−O7

2.562(4)

2.538(4)

Eu2−O9

Eu2−O10

2.577(4)

2.603(4)

_ONO3−Eu −O

NO3

O3−Eu2−O4

O6−Eu2−O7

O9−Eu2−O10

49.63(12)

49.88(13)

48.86(14)

O3−Eu2−O9′_a

O4−Eu2−O4′

O4−Eu2−O10′

176.45(14)

159.26(16)

124.07(13)

O6−Eu2−O6′

O7−Eu2−O10

O9−Eu2−O4′

177.75(19)

163.59(13)

132.39(13)

NO3

···Eu ···N

N6···Eu2···N6′

N6···Eu2···N7′

111.77(17)

89.77(13)

N7···Eu2···N7′_a

N6···Eu2···N8′

151.50(19)

148.43(16)

N8···Eu2···N8′_a

N7···Eu2···N8′

72.70(20)

87.08(15)

x + 1/2, y , z .

Figure 2. Stereoscopic view along the b (left) and c (right) axes of the crystal packing in [Eu(TPAMEN)][Eu(NO ) ]·2CH CN·0.45H O (9).

Molecules are displayed as ball and stick diagrams, Eu atoms are emphasized with larger spheres, [Eu(TPAMEN)] molecules are colored in

accordance with their enantiomeric form (blue, Λ; magenta, Δ), and hydrogen atoms have been omitted for clarity.

(

P2/n2 /n2/a) and 10 in the higher symmetric space group

parallel (1/4 oﬀset) to the vertical (a,c) diagonal passing

through the 0 point. Furthermore, a view along c [001] (Figure

Pbca (P2 /b2 /c2 /a).

The asymmetric unit of 9 consists of two crystallographic

2, right) shows that [Eu(TPAMEN)] enantiomers (diago-

nally) aligned at the same (a,c) glide plane have the also same

orientation (e.g., all ethylene bridges point downward). Their

orientation is reversed at the adjacent glide plane. Two layers

independent half-molecules of [Eu(TPAMEN)] and [Eu-

3−

(

NO ) ] , one uncoordinated H O molecule with 0.225

3 6 2

occupation, and one free acetonitrile. The Eu ions in

−

3−

[

Eu(NO ) ] and [Eu(TPAMEN)] occupy special posi-

of (a,c) glide planes sandwich a layer of [Eu(NO ) ]

3 6

tions of symmetry elements in the unit cell. The Eu ion in

(hexanitrato) anions, leading to a dense packing of the

[Eu(TPAMEN)] sits on a 2-fold rotational axis along the

diﬀerent complex molecules in the unit cell. With a multiplicity

−

direction (2x, 1/4, 1/4), which generates the second half of the

molecule (x, y + 1/2, z + 1/2). It also lies on the diagonal (a,c)

glide plane n ((1/2, 0, 1/2)x, 1/4, z), which aﬀords the

generation of the opposite enantiomer (the asymmetric unit

displays the Λ isomer, and the Δ isomer is generated by this

symmetry operation). The enantiomeric forms result from the

helical arrangement of the picolyl amide arms of TPAMEN

of 8, the unit cell contains 4 [Eu(NO ) ] , [Eu-

3 6

(TPAMEN)] 8 0.225 occupied water, and 8 acetonitrile

molecules.

Figure 3 shows the asymmetric unit of 10. It consists of one

independent [Eu(TPAMEN)] molecule, three noncoordi-

nated triﬂate anions, one of which shows a disorder (S2A/

S2B) over two positions and one has a more severe disorder

over three positions (S3A/S3B/S3C), a 1.0 (O14) and a 0.125

(O16) occupied crystal water and one 0.55 occupied ethanol

molecule (O15). There are further electron density peaks in

the vicinity of the triﬂate anions (see Q1 and Q2 Figure 3) S1

encapsulating the Eu³ion. Likewise, the [Eu(NO ) ]

3−

molecule is completed by a rotation around the 2-fold

rotational axis passing through the Eu³⁺center along the c

direction (2 1/4, 0, z). A view along the b direction [010] (see

−

Figure 2, left) reveals that [Eu(NO ) ]

and [Eu-

and S2A/S2B and [Eu(TPAMEN)] , which could not be

(

TPAMEN)] form spatially separated and enantiomerically

assigned to certain solvent molecules (further information is

summarized in the Supporting Information). Unlike the case

pure strings around the 2-fold screw axis in the b direction.

Thereby the opposite enantiomers alternate diagonally along

the (a,c) plane. The crystal water takes the cavity around

for 9, the Eu of [Eu(TPAMEN)] in 10 is not placed on a

symmetry operation. Only the “highly disordered triﬂate”

[Eu(TPAMEN)] close to the ethyl amide groups and to

(S3A/S3B/S3C) overlaps with the 2 screw axis along a, and

according its presence (occupancy 0.225), the ethyl group

the hydrogen-bonded water molecules lie on the axial (a,b)

glide plane (in the a direction). The large ORTEP ellipsoids

indicate the broad thermal deﬂection of the atoms. In addition,

takes two diﬀerent orientations. The acetonitrile molecules

3−

align around [Eu(NO ) ] along the (a,c) room diagonal and

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Inorganic Chemistry

Article

where the ethylene bridge is deﬁned as the head and the

diethylamide groups as the tails) [Eu(TPAMEN)] molecules

lead to an elongated c axis (42.232(5) Å), as is shown in Figure

. The [Eu(TPAMEN)] molecules have ordered layers

parallel to the axial glide planes (a,b) in the direction of a

view [010]) and the 2-fold screw axis along b. The space

(

between those layers is occupied by alternating layers of triﬂate

S3, ﬁlling the space between the tail to tail oriented

[

Eu(TPAMEN)] molecules, and H O molecules, occupying

the cavities between the head to head oriented [Eu-

(

TPAMEN)] molecules. We suspect that due to the large

cavity between the tail to tail oriented [Eu(TPAMEN)]

molecules the degree of freedom in the orientation of triﬂate

ion (S3) is increased, causing its severe degree of disorder. The

two residual triﬂate anions align in the same layer as the

[

Eu(TPAMEN)] molecules.

Despite the diﬀerences in counterions and in packing, the

Figure 3. ORTEP diagram of the asymmetric unit of [Eu-

[Eu(TPAMEN)] cations in both complexes are very similar

(

TPAMEN)]OTf ·1.125H O·0.545CH CH OH[+solvents] (10)

(see Figure 3 and Figure 5 , left). Eu is 10-fold coordinated by

with numeration of selected heteroatoms. thermal ellipsoids are

shown at the 50% probability level, and hydrogen atoms attached to

carbon have been omitted for clarity.

0 displays a racemic mixture of Δ and Λ isomers of

[Eu(TPAMEN)] (see Figure 4). The opposite isomer is

obtained through reﬂection at a glide plane.

With the same multiplicity of 8, but the absence of a

symmetry element on the [Eu(TPAMEN)]³molecule as in

0, the unit cell of 9 contains 8 times [Eu(TPAMEN)]OTf₃·

.125H O·0.545CH CH OH[+solvents] molecules and there-

fore, has about twice the dimension of the unit cell in 9. The

four layers of alternately oriented (head to head/tail to tail,

Figure 5. ORTEP diagram of the C -symmetric [Eu(TPAMEN)]

fragment in [Eu(TPAMEN)][Eu(NO ) ]·2CH CN·0.45H O (9)

(

left) (thermal ellipsoids are at the 50% probability level and

hydrogens have been omitted for clarity) and complex polyhedron

right).

(

the N O donor set of TPAMEN, including the two apical

amine nitrogens N , the four pyridyl nitrogens N , and the

four amide oxygens O. Continuous shape measure (CShM)

calculations show that the formed complex polyhedron most

resembles a tetradecahedron (TD-10: 2.83 for 9 and 3.02 for

0) but is also very similar to a staggered dodecahedron

SDD-10: 2.89 for 9 and 3.52 for 10). The second polyhedron

shows a higher agreement with the coordination polyhedron of

(

−

[

Eu(TPAEN)] (TD-10, 2.85; SDD, 2.48). There are some

variations in the orientation of the ethyl substituents at the

amide groups of TPAMEN in both complexes. The angles in

the [Eu(TPAMEN)] molecules are largely congruent (see

Table S3 ). Only the N −Eu −O angles and the N −C−C−

O dihedral angles diﬀer by up to 5 and 9°, respectively.

However, the mean Eu −donor bond distances (Eu −O ,

ave

.42(3) Å for 9 and 2.43(2) Å for 10; Eu −N

2.67(10) Å

ave

for 9 and 2.65(6) Å for 10; Eu −N _a_v_e, 2.79(1) Å for 9 and

.78(1) Å for 10), in accordance with Table 2, are consistent

in both complexes. Therefore, we suspect that the diﬀerent

packing environments and the varying angles of the complexes

discussed herein have a minor eﬀect on the bond distances in

Figure 4. Stereoscopic view of the crystal packing in [Eu-

[

Eu(TPAMEN)] molecules. The comparably large esds for

(

TPAMEN)]OTf ·1.125H O·0.545CH CH OH[+solvents] (10).

3 2 3 2

the Eu −N

result from the unequal alignment of the

Molecules are displayed as ball and stick diagrams, Eu atoms are

ave

picolyl amide arms, where two picolyl amide arms in line with

the ethylenediamine bridge have longer bond distances

(2.73(1) Å for 9; 2.70(1) Å for 10) and the two vertically

emphasized with larger spheres, [Eu(TPAMEN)] molecules are

colored in accordance with their enantiomeric form (blue, Λ;

magenta, Δ), and hydrogen atoms have been omitted for clarity.

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aligned picolyl amide arms shorter Eu −N bond distances

Article

coordinated by six bidentate nitrate ligands, giving an

icosahedral coordination polyhedron (CShM IC-12: 1.81)

of the oxygen donors. According to CShM calculation (OCT,

7.08; TP, 3.35) the arrangement of the six nitrate N

atoms is closest to a slightly distorted trigonal prism, as

indicated by the green plains in Figure 6. The Eu −O

bond distances vary between 2.537(4) and 2.603(4) Å (see

(

2.60(1) Å for 9; 2.60(1) Å for 10).

−

In comparison to [Eu(TPAEN)] (Eu −O 2.42(1) Å,

NO3

Eu −N

2.65(4) Å, Eu −N

2.91(1) Å), the mean

ave

Eu −O and Eu −N bond distances are on the order of the

NO3

standard deviation (3σ), whereas the Eu −N

bond

ave

distances in the [Eu(TPAMEN)]³fragments are signiﬁcantly

NO3

shorter by 0.13 Å. The similar Eu −O bond distances in

Table 3), and the mean inner O −Eu −O

angle of the

−

[

Eu(TPAEN)] and [Eu(TPAMEN)] show that the diﬀer-

bidentate nitrate coordination of 49(1)° is consistent with

ence in charge has only a minor eﬀect on the strength of the

those observed for other hexakis(nitrato)europate(III) spe-

49,50

Eu −O interactions. Similar observations have been reported

cies.

for Ln −O bond distances of the tripodal analogues H TPAA

Molecular Geometry Optimization. Figure 7 displays

the optimized geometries of [Eu(TPAEN)] and [Eu-

(TPAMEN)] and the corresponding Am and Cm

complexes in aqueous solution using M06-L. Estimated bond

−

and TPAAM, varying between 0 and 5 Å (La > Nd

Lu ). It is also conceivable that the additional coordination

of K in in [{Eu(TPAEN)}K(H O) ] weakens the Eu −O

interaction. However, there is no signiﬁcant diﬀerence in the

Eu −O bond distances of the oxygen atoms involved in the

K coordination and the that solely bound to Eu , opposing

this consideration. The mean value of the Eu −N bond

distances at 2.78(1) Å is notably shortened by 0.13 Å in

comparison to that of [Eu(TPAEN)] , pointing to an

lengths between Eu , Am , and Cm and the donor atoms of

4−

TPAEN and TPAMEN are summarized in Table 4. We

added the Am /Cm geometry optimizations to the

discussion to provide an outlook on how the introduction of

neutral oxygen donors in TPAMEN changes its reactivity

toward actinides in relation to TPAEN .

All six optimized geometries (see Figure 7) show the metal

ions coordinated by the N O donor set of the ligands. The

−

enhanced M −N interaction. Furthermore, the approx-

imately 3 and 11° enhanced mean N −Eu −N angles of the

opposite lying picolyl amide arms at 179(1)° (almost 180°)

calculated geometries reproduce the shorter Eu −N bond

length of [Eu(TPAMEN)] in comparison to that of

−

and 139(1)° in comparison to [Eu(TPAEN)] (176 and

−

[

(

Eu(TPAEN)] . Accordingly, the Eu

TPAMEN)] moves closer to the center of the cavity by

ion of [Eu-

28°), respectively, indicate that the Eu ion moves closer to

the center of the cavity by the 10 donor atoms of TPAMEN. A

the donor atoms, as displayed in Figure 7. The diﬀerence in the

comparison of the dihedral angles discloses a signiﬁcant

Eu −N bond lengths of 0.16 Å is consistent with the

3+ py

widening of the N −C−C−O angles (up to |32°|), showing

experimental diﬀerence. The calculated Eu −N bond

lengths are overestimated by ∼0.1 Å in comparison to the

that the amide carbonyl in contrast to the carboxylate group

does not have a coplanar alignment to the pyridyl ring. Only

−

_t_h_e_c_a_r_b_o_x_y_l_a_t_e_s_u_b_s_t_i_t_u_e_n_t_n_o_t_c_o_o_r_d_i_n_a_t_e_d_t_o_K+ in

experimental values for both [Eu(TPAEN)] and [Eu-

(

TPAMEN] . This overestimation is also observed in the

[

{Eu(TPAEN)}K(H O) ] shows a higher deviation from the

2 3

DFT benchmarking on the geometry optimization for

planar alignment by 15°. Therefore, the K coordination may

make a notable contribution to the orientation of the

carboxylate group.

−

[

Eu(TPAEN)] by 0.05−0.11 Å.

The bond lengths of the optimized actinide complexes tend

to be shorter in comparison to the Eu complexes, except for

Figure 6 displays the anionic hexakis(nitrato)europate(III)

3−

the M −O bond lengths, which increase in the order Eu

ave

species [Eu(NO ) ] present in 9. Hexakis(nitrato) anions

Cm < Am . Apart from the M −N

bond lengths of

ave

commonly cocrystallize from reactions of neutral ligands, for

−

49,50

[Eu(TPAEN)] and [Am(TPAEN)] no signiﬁcant variations

example, multidentate macrocycles or TEDGA,

and the

in the bond lengths are observed. The bond lengths of the

corresponding nitrate salt of the lighter up to medium

_l _a _n _t _h _a _n _i _d _e _s _._T _h _e _E _u3 ion in [Eu(NO ) ] is 12-fold

3−

Am and Cm complexes with the same ligand are close to

equal. On comparison with the experimental data for the

−

[Am(TPAEN)] complex, less of a pronounced overestima-

tion for M −N

and an underestimation for the M −

ave

are displayed. In contrast to the theoretical data the

ave

M −N

distances of the experimental data show no

signiﬁcant diﬀerence between the Eu and Am₃complexes.

Comparing both ligands, we observe shorter M −N

and

ave

longer M −O

distances for the complexes [M-

ave

(

TPAMEN)] (M = Am , Cm ). The plain diﬀerences

between the bond lengths in the Eu and Am complexes

suggest a higher eﬀect on the TPAEN complexation and,

except for the Eu complex, the bond distances are shorter for

[

M(TPAMEN)] and also the shorter bond distances for

−

M(TPAEN)] (M = Am , Cm ) point to a more stable

complex formation with TPAEN.

Bond Critical Point (BCP) Analysis. More insights into

the electron, energy density, and interaction character of the

formed bonds are given by a bond critical point (BCP)

^aⁿ^a^l^y^sⁱ^s^.^T^h^e^e^l^e^c^t^r^oⁿ^d^eⁿ^sⁱ^t^y^ρBCP (>0.2, open-shell “covalent”

−

Figure 6. ORTEP diagram of C -symmetric [Eu(NO ) ] in

3 6

[

Eu(TPAMEN)][Eu(NO ) ]·2CH CN·0.45H O (9) with integrated

complex polyhedra (thermal ellipsoids are shown at the 50%

probability level).

3 6 3 2

interaction; <0.1, closed-shell interaction referring to ionic, van

der Waals, or hydrogen bonding) and the Laplacian ∇ ρ

BCP

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Figure 7. Ball and stick diagrams of the DFT optimized structures of [M(TPAEN)] and [M(TPAMEN)]³⁺(M = Eu, Am, Cm) structures using

−

M06-L.

Table 4. Bond Analysis of the Complexes [M(TPAEN)] and [M(TPAMEN)]³⁺(M³⁺= Eu , Am , Cm )

−

bond length (Å)

Eu³⁺

Am³⁺

Cm³⁺

exptl

calcd

2.951

2.76(2)

2.413(6)

exptl

M(TPAEN)]⁻

calcd

exptl

calcd

[

M −N

2.91(1)

2.65(4)

2.42(1)

2.90(2)

2.69(2)

2.48(1)

2.841(4)

2.72(2)

2.46(2)

2.841(2)

2.73(5)

2.451(7)

ave

M −N

ave

M −O

ave

[

M(TPAMEN)]³

M −N

2.78(1)

2.66(6)

2.42(2)

2.789

2.76(4)

2.44(2)

2.736(4)

2.75(4)

2.50(2)

2.743(5)

2.75(5)

2.48(2)

ave

M −N

ave

M −O

ave

Reference 9. Average of all corresponding M −O/N bond distances determined for 9 and 10 in this work. Reference 51. Values are based on

EXAFS.

Table 5. Bond Critical Point (BCP) Analyses of [M(TPAEN)] and [M(TPAMEN)]³⁺(M³⁺= Eu , Am , Cm )

−

^ρBCP (e a₀⁻³)

Am³⁺

∇ ρ

(e a₀⁻⁵)

10⁻³H_B_C_P

Am³⁺

BCP

Eu³⁺

Cm³⁺

Eu³⁺

M(TPAEN)]⁻

Am³⁺

Cm³⁺

Eu³⁺

Cm³⁺

[

M −N

0.0190

0.0264

0.0480

0.0275

0.0328

0.0502

0.0274

0.0327

0.0502

0.0635

0.0935

0.2096

M(TPAMEN)]³

0.0872

0.0881

0.1114

0.2092

0.0870

0.1113

0.2137

1.26

1.20

0.25

−0.31

−0.35

−1.62

−0.26

−0.40

−1.41

ave

M −N

ave

M −O

ave

[

M −N

0.0270

0.0271

0.0433

0.0348

0.0320

0.0443

0.0340

0.0317

0.0455

0.1067

0.1064

0.1901

0.1060

0.1047

0.1976

0.68

1.04

1.31

−1.12

−0.46

−0.31

−1.12

−0.48

−0.34

ave

M −N

0.0948

0.1957

ave

M −O

ave

(

<0, covalent IA/electron density is concentrated in the bond;

0, electron density is diluted from the bond) are values that

are necessary to characterize the interaction type between two

exceeds the kinetic energy part (H_B_C_P< 0), the interaction

between the two atoms is deﬁned as covalent; otherwise, a

closed-shell interaction is present.

atoms. In some cases, e.g. electron-rich atoms, where an

excessive charge accumulation in the bond area is restricted by

the exclusion principle, the aforementioned conditions may

not be suﬃcient to identify a covalent bond. The energy

density H is a suﬃcient condition to describe the covalency

of a bond. It is deﬁned by a positive kinetic and a negative

potential energy share. When the potential energy part

The results of the BCP analysis are summarized in Table 5.

For ρ_B_C_Pand ∇ ρ

all values are <0.1 and >0, respectively,

BCP

pointing to closed-shell interactions. The energy densities

H_B_C_P, however, show a diﬀerent tendency and point to closed-

shell interactions of the Eu complexes and open-shell

BCP

interactions in the case of the Am and Cm complexes.

The H_B_C_Pvalues are very small; therefore for the bonding in

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Article

the actinide complexes we suggest slightly covalent inter-

and the metal cations. The smallest bond orders (0.06−0.11)

actions. The increase in ρ_B_C_Pand ∇ ρ

is related to the

are displayed for the M −N

interactions. The diﬀerence in

BCP

ave

decrease in bond distances (see Table 4). In contrast to the

oxygen donor charge is reﬂected by a drop in the M −O

−

ave

values reported in the literature, the H values of the M −

bond orders from ∼0.33 for [M(TPAEN)] to ∼0.22 for

BCP

O_a_v_einteractions in the Am³and Cm complexes both are

[M(TPAMEN)] . Aﬃliated with changes in bond length, the

negative and point to covalent interactions. Also, for

M −N ave interactions show an opposite trend with bond

−

[

EuTPAEN)] H_B_C_Pof M −O displays the least ionic

orders of ∼0.21 and ∼0.34 for the respective complexes with

ave

4−

character, which seems to be unusual in a comparison of

nitrogen and oxygen donor interactions with lanthanides and

may imply back-bonding from the metal. We veriﬁed our

calculations by reproducing H_B _C _P values of a given optimized

structure in the literature (see Table S4 in the Supporting

Information), and we suspect that slight diﬀerences in the

optimized structures are the origin of the present diﬀerences.

TPAEN and TPAMEN. The diﬀerences among the three

metal complexes of each ligand are rather subtle (<0.05). The

actinide complexes show very similar values. Only the higher

−

bond orders for M −Oave in [M(TPAEN)] and for M −

ave in [M(TPAMEN)] point to slightly stronger

interactions with the Am ion.

A closer inspection of the Eu and Am³⁺complex MBOs

implies that the donor−metal interactions are not straightfor-

wardly related to the observed bond lengths. For example, in

Furthermore, the values obtained for H_B_C_Pare very small and

may be more prone to ﬂuctuations. The ρ_B_C_Pand ∇ ρ

BCP

interactions in [M(TPAEN)]⁻

the case of the M −N

values are very close and in general show similar tendencies.

Except for this, as expected, interactions with shorter bond

distances show higher electron densities ρ_B_C_Pbut also a

stronger dilution of charge from the bond due to a higher

concentration on the bond. There is also a tendency toward

smaller H_B_C_Pvalues for interactions with shorter interatomic

distances, pointing to less ionic and more open-shell character

of these interactions. Although there is clear enhancement in

the nitrogen donor−metal interactions in all [M-

ave

(

M = Eu , Am ), a higher population of binding orbitals is

−

detected for [Eu(TPAEN)] . It is also conspicuous that,

despite the similar or even slightly larger M −N

bond

ave

lengths in comparison to M −N ave^,^t^h^e^b^oⁿ^d^o^r^d^e^r^s

resulting from the N coordination show higher values in

general. Accordingly, we suspect a favorable orbital overlap of

binding orbitals and thus their enhanced population in case of

the N donor coordination. Moreover, this suggests a greater

eﬀect of the N donor function on the metal−ligand

(

TPAMEN)] complexes, the summation of all donor−ligand

interaction, which in the case of [Am(TPAMEN)] with a

interactions suggests, as a consequence of the signiﬁcantly

−

bond order of 0.362 even exceeds the strongest carboxylate O

donor interactions observed for TPAEN complexes.

smaller H

(

values for all M −O interactions in [M-

BCP

ave

4−

−

TPAEN)] , that the TPAEN complexes are less ionic and

Mulliken Population Analysis (MPA)/Natural Popula-

more covalent in character in the case of the actinide

tion Analysis (NPA). Table 7 summarizes the atomic charges,

complexes. On comparison of the M −O interactions, the

ave

spin populations, and electronic conﬁgurations of Eu , Am

results suggest that the negatively charged carboxylate oxygen

−

and Cm in [M(TPAEN)] and [M(TPAMEN)] obtained

56 57

provides less of an electrostatic interaction for Eu and more

_o_f_a_c_o_v_a_l_e_n_t_i_n_t_e_r_a_c_t_i_o_n_f_o_r_A_m3 and Cm . The neutral

by MPA and NPA. The alterations of charge and spin

population at each single donor function are rather subtle (see

Table S5 in the Supporting Information).

amide oxygen in TPAMEN, however, seems to provide

interactions with stronger electrostatic character. As a

The atomic charge q and the excess of the spin populations

consequence of this, mainly the nature of the M −N

ave

ρ_s_p_i_nby MPA increase and decrease, respectively, from Eu

interactions experiences a shift from more electrostatic

character to less electrostatic and more covalent character,

which even exceeds that of the M −O amide interaction.

Mayer Bond Order (MBO). The Mayer bond order

analysis provides an overview of all contributions to the

bonds. Higher bond orders point to an increased electron

population of binding orbitals as well as a reduction or absence

over Am to Cm for both TPAEN and TPAMEN.

Regardless of the metal type the electronic conﬁgurations at

the d-orbitals remain largely similar and only vary between

ave

−

∼0.71 in the [M(TPAEN)] complexes and ∼0.46 in the

[

M(TPAMEN)] complexes. This variation can likely be

4−

attributed to the negative carboxylate oxygens in TPAEN ,

providing higher electronic charge to the d-orbitals. However,

this variation is not reﬂected by a drastic diﬀerence in the q or

4,55

of electron population in antibinding orbitals.

Our results

−

of the MBO analysis of the [M(TPAEN)] and [M-

TPAMEN)] (M = Eu , Am , Cm ) complexes are

−

ρ_s_p_i_nvalues for [M(TPAEN)] and [M(TPAMEN)] of the

same cation. The excess of the f-orbital electron conﬁguration,

however, decreases from Eu over Am to Cm . On

comparison of the complexes [M(TPAEN)] and [M-

TPAMEN)] , the f-orbital population in the TPAMEN

(

summarized in Table 6. With values between 0 and 0.4 we

4−

observe weak interactions between the TPAEN /TPAMEN

−

(

−

Table 6. Mayer Bond Order (MBO) of [M(TPAEN)] and

complexes is slightly enhanced, which is also reﬂected by a

[

M(TPAMEN)] (M = Eu , Am , Cm )

somewhat enhanced spin population of the [M(TPAMEN)]

complexes.

Eu³⁺

Am³⁺

Cm³⁺

For the NPA the excess in ρ population also decreases in

spin

_M₍_T_P_A_E_N₎_]−

[

the order Eu > Am > Cm . Herein the actinide complexes

exhibit a negative excess (=deﬁcient). This deﬁciency of spin

population may indicate a contribution by the metal ion. The

atomic charge populations q of the complexes [M-

M −N

0.222

0.108

0.327

0.202

0.094

0.333

0.208

0.093

0.323

ave

M −N

ave

M −O

ave

[

M(TPAMEN)]³⁺

0.315

(

TPAMEN)] increase from Eu over Am to Cm .

M −N

0.362

0.080

0.213

0.335

0.066

0.214

−

ave

However, for the complexes [M(TPAEN)] a diﬀerent

M −N

0.103

0.239

ave

order, Am < Eu < Cm , is observed. The electronic

conﬁguration at the d-orbital is at ∼0.73 for the [M-

M −O

ave

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Inorg. Chem. 2021, 60, 2477−2491

Inorganic Chemistry

Article

Table 7. Atomic Charge Population, Spin Population, and Electronic Conﬁguration after Mulliken (MPA) and Natural

−

Population Analysis (NPA) of [M(TPAEN)] and [M(TPAMEN)] (M = Eu , Am , Cm )

MPA

NPA

ρ_s_p_i_n

orbital population

ρ_s_p_i_n

electronic conﬁguration

[

M(TPAEN)]⁻

p⁰^.⁰⁶

_E_u3⁺

Am³⁺

Cm³⁺

6.40

6.31

7.24

0.73 0.09

[Xe]4f⁶^.⁴⁴5d⁰^.⁶⁹6s⁰^.¹⁷6p⁰^.⁰¹

1.714

1.862

1.872

6.333

6.225

7.062

1.503

1.416

1.748

6.016

5.818

6.708

0.70 0.16 −0.03

6.43 0.76 0.20 0.01

[Rn]5f 6d 7s 7p

0.71 0.18 0.00

7.17 0.75 0.21 0.01

[Rn]5f 6d 7s 7p

M(TPAMEN)]³

[

_E_u3⁺

Am³⁺

Cm³⁺

6.46

6.35

7.28

0.47 0.10 0.13

[Xe]4f⁶^.⁴⁷5d⁰^.⁶³6s⁰^.¹⁷6p⁰^.⁰¹

1.838

1.994

2.058

6.374

6.273

7.092

1.450

1.519

1.842

6.031

5.827

6.710

0.45 0.11 0.09

6.43 0.65 0.20 0.01

[Rn]5f 6d 7s 7p

0.45 0.13 0.09

7.16 0.65 0.21 0.01

[Rn]5f 6d 7s 7p

Mulliken/Natural atomic charge. Mulliken/Natural atomic spin population. Without core orbital population.

−

(

TPAEN)] complexes similar to the results from an MPA. At

∼

0.64 the d-orbital electronic conﬁguration in the complexes

M(TPAMEN)] are slightly larger in comparison to the

[

MPA. The general trend toward smaller d-orbital population as

a consequence of coordination with the neutral oxygen donors

in TPAMEN is also reﬂected by the NPA. The oxygen donor

charge seems to have a minor eﬀect on the excess of the f-

orbital electronic conﬁguration, which remains consistent for

the equivalent complexes of both ligands. A minor enhance-

ment in the f-orbital population (0.03) can be observed for the

complex [Eu(TPAMEN)] . On comparison of all three metal

ions, the f-orbital population excess of Cm³is at ∼0.17

signiﬁcantly lower than that of the other two metal cations

(∼0.44). A look at the electron conﬁguration distribution at

the f- and d-orbitals in combination may oﬀer an explanation

for the diﬀerent trends observed for the atomic charge

population q. In [Am(TPAEN)]⁻the increased d-orbital

population and almost equal f-orbital population may be the

reason for the smaller atomic charge population. However, for

the [M(TPAMEN)]³complexes with similar d-orbital

populations mainly the variation of the f-orbital electron

population leads to the present order.

Density of States (DOS) Analysis. Figure 8 shows the

−

density of states (DOS) diagrams of [M(TPAEN)] and

[

M(TPAMEN)] . Partial densities of states (PDOSs) of the

metal f-orbital electrons are shown as solid lines and overlap

population densities of states (OPDOSs) between the metal f-

orbitals are shown as dashed lines. The curves are convoluted

with a half-width of 0.5 eV. Red and blue dashed lines denote

Figure 8. Diagrams of the density of states (DOS) for bonds between

metal d- or f-orbitals and donor atoms. Partial DOS curves are

presented as black solid lines, and overlap population DOS curves for

M −O and M −N bonds are displayed as red and blue dashed

lines, respectively.

OPDOS curves of M −O

and M −N

bonds,

total

respectively. The OPDOS curves in the d-orbital DOS diagram

display regardless of the metal type exclusively positive

distributions for both M (d)−O

and M (d)−N

total

bonds, representing bonding-type overlaps. In contrast with

that, the f-orbital DOS diagram shows positive OPDOS

distributions as well as negative OPDOS distributions,

indicating bonding and antibonding-type overlaps. The integral

values of OPDOS from the HOMO-29 to the HOMO for the

α-spin orbitals in 10⁻²electron units are also shown in Figure

TPAMEN. The DOS values are available inTable S6 in the

Supporting Information.

Complex Formation Energies. Gibbs energies were

calculated on the basis of the single-point energies and normal

vibrational frequencies at the same level as for the geometrical

optimization. We estimated the diﬀerence in complex

. The integral values increase for the M −N

bonds and

total

−

decrease for the M −O

bonds from [M(TPAEN)] to

formation energies between Eu and Am on the basis of

total

[

M(TPAMEN)] , reﬂecting the shift toward stronger N −

eq 1 to investigate whether or not Am /Eu selectivity

4−

metal and weaker amide oxygen−metal interactions also

observed in the BCP and MBO analyses. The diﬀerences

increases from TPAEN to TPAMEN. A negative Gibbs

energy diﬀerence (ΔG) (see eq 2), on the basis of eq 1,

suggests that AmL is more stable than EuL in aqueous solution

and also that AmL has a higher complex formation stability. As

starting compounds the hydration complexes [M(H O) ]

and [M(NO )(H O) ] were considered. Unfortunately, the

3 2 7

between Eu and Am for all donor−ligand interactions of

both ligands suggests that through the dominant increase of

the OPDOS of the Am −N

bonds a greater distinction

total

between Eu and Am is obtained in the complexation by

487

Inorg. Chem. 2021, 60, 2477−2491

Inorganic Chemistry

Article

Table 8. Thermodynamic Results for Complexation Reactions Based on Eq 1

ΔG (kJ mol⁻¹)

−

ΔΔG (kJ mol⁻¹

−7.2

ΔΔE (kJ mol⁻¹

tot

reaction

n = 0

n = 1

.1 M HNO₃

L = TPAEN⁴

L = TPAMEN

)

−3.8

−25.0

−11.0

−32.2

−8.0

−12.3 [−10.0(10) , −12.0(22) ]

−19.5

−

Experimental value based on a microcalorimetry measurement of [Eu(TPAEN)] in ref 10. Experimental value based on time-resolved laser

ﬂuorescence spectroscopy of [Eu(TPAEN)] in ref 10.

−

optimization by M06-L failed to converge; therefore, the

geometries optimized by the BP86 functional had to be used.

The thermal correction to Gibbs energy (G^c^o^r^r) under the

standard conditions (see eq 3) was calculated by using quasi-

harmonic oscillator and rigid rotator approximations. The

calculation details and numerical thermodynamic data are

available in Table S7 in the Supporting Information.

lowering the energy of the valence orbitals, we raise the

possibility for an interaction with the energetically lower lying

f-orbitals. That results likely in a shift in population of bonding

and antibonding orbitals, which then in relation to orbital

overlap strengthens or weakens the M−D bonding interactions

of each donor function, individually. The charge properties of

each single donor atom designate the corresponding size and

energy level of their valence orbitals and thus their reactivity

[

EuL] + [Am(NO ) (H O)9−2n^]

toward the valence orbitals of the metal cation. The resulting

overlap population (OPDOS, MPA, NPA) and the subsequent

ratio of bonding and antibonding interactions, which can be

described with the bond order (e.g., MBO, Wiberg bond

indices) or electron delocalization (δ), then deﬁne the bond

strengths of each single M−D interaction. All further bond

→

[AmL] + [Eu(NO ) (H O)₉₋₂_n]

(1)

ΔG(L) = {G(AmL) + G(Eu(NO ) (H O)9−2n⁾^}

−

{G(EuL) + G(Am(NO ) (H O)₉₋₂_n)} (2)

parameters such as equilibrium bond length, charge density

tot

corr

BCP, Laplacian ∇ ρBCP, and energy density HBCP render

G = E + G

Table 8 summarizes the thermodynamic data based on eq 1.

(3)

descriptive parameters of the chemical bond and arise from the

bond strength and in the bonding involved for donor atom

types or functional groups as well as steric parameters such as

the coordination number.

4−

For both TPAEN and TPAMEN the ΔG values are negative.

The experimental ΔG value of TPAEN is reported to be

−

10(1) kJ mol in 0.1 M HNO aqueous solution. The

The alteration of charge (basicity) properties at speciﬁc

positions aﬀects the energetic level of the ligand molecule

ratio between [M(H O) ] and [M(NO )(H O) ] in 0.1 M

HNO aqueous solution (C < 10 M) at zero ionic strength

is estimated to be 53:47 for M = Eu and 65:35 for M

−

orbitals as a whole. Depending on the extent of charge

alteration and its distribution in the molecule, this will be more

Am by using stability constants corrected with the Davies

or less pronounced and may result in a switch in order

equation: log β = 0.95 and 0.74 for M = Eu , Am ,

respectively. The ΔG value of TPAEN is calculated as a

weighted sum by a 60:40 ratio for the sake of simplicity to be

concerning the highest occupied molecular orbital (HOMO).

Accordingly, this eﬀect is very sensitive to the composition of

the individual ligand system and thus to the energetic

compatibility of the donor functions valence orbitals with

that of the target metal cation. Therefore, an estimation of this

eﬀect without further quantum chemical investigation of the

explicit ligand system will likely have only limited precision.

Utilizing this interplay in bonding and antibonding interactions

by adjusting the HOMO properties of the ligand may be the

key to speciﬁcally address diﬀerent unoccupied energetically

low lying valence orbitals of certain metal cations to further

adjust the selectivity properties. However, this is probably only

possible in a very restricted range of energy levels and requires

precisely adjusted ligand properties.

−

12.3 kJ mol , being consistent with the experimental value.

For TPAMEN the weighted sum of the ΔG values is −19.5 kJ

−

−1

mol , which is −7.2 kJ mol (ΔΔG) lower than that of

−

TPAEN . We also computed the ΔΔG values on the basis of

the B2PLYP functional, which is known to reproduce Am /

46,62

Eu selectivity with various ligands,

diﬀerence of −4.3 kJ mol . This value suggests that TPAMEN

also favors Am over Eu in the aqueous complex formation

reaction and indicates that the stability of its Am complex

relative to that of its Eu complex is higher than that of

TPAEN. ΔΔE also shows the almost same values of ΔΔG as

and obtained a

−

tot

shown in Table 8, indicating that the lower ΔG value of

TPAMEN in comparison to that of TPAEN can be attributed

to a diﬀerence in inner energy, including ionic and covalent

interactions between the metal and L.

CONCLUSION

We introduced TPAMEN, an amide-substituted TPEN

derivative, to elucidate the eﬀect of neutral O-donors on the

■

electron distribution around Eu and the consequences for the

ΔΔG = ΔG(TPAMEN) − ΔG(TPAEN)

(4)

Eu −N interactions. Investigations on the TPAMEN

Deduction of Relation between Metal−Donor (M−D)

Bond Parameters and Donor Charge Alterations. By

substituting the carboxylate group with an amide group, we

reduce the charge at the oxygen and basically lower the

complexation behavior toward Eu(OTf) and Eu(NO ) ·

3 3

6H O yielded mononuclear [Eu(TPAMEN)] complexes

−

similar to [Eu(TPAEN)] . TPAMEN completely encapsulates

Eu by a 10-fold coordination of 6 N-donor and 4 O-donor

energies of the valence orbitals of the molecule. This is also

atoms. The consistent values for Eu −O/N /N bond

distances regardless of the structural diﬀerences in both

complexes hint at a negligible eﬀect of the packing on the

emphasized in Figure 8, where the α-spin orbital energies of

−

the complexes [M(TPAEN)] (M = Eu , Am ) are higher

in comparison to that of the complexes [M(TPAMEN)] . By

ligand−metal interactions in the [Eu(TPAMEN)] molecule.

488

Inorg. Chem. 2021, 60, 2477−2491

Inorganic Chemistry

A comparison with [Eu(TPAEN)] shows that, despite the

Complete contact information is available at:

Article

−

Masashi Kaneko − Nuclear Science and Engineering Center,

Japan Atomic Energy Agency, Tokaimura, Ibaraki

319−1195, Japan; orcid.org/0000-0001-5428-2144;

Email: kaneko.masashi@jaea.go.jp

diﬀerence in O-donor charge, the mean Eu −O bond

distances in [Eu(TPAMEN)] remain similar, whereas the

mean Eu −N bond distances are shortened by 0.13 Å.

Accompanying changes in the N −Eu −N angles empha-

Author

size that the Eu ion moves closer into the center of the 10

donor atoms of TPAMEN. According to the BCP, MBO, and

DOS analyses the substitution of the negatively charged

oxygen donor with a neutral oxygen causes a signiﬁcant

decrease in covalency and overlap population of bonding

Kiyoshi Fujisawa − Department of Chemistry, Ibaraki

University, Mito, Ibaraki 310-8512, Japan; orcid.org/

0000-0002-4023-0025

https://pubs.acs.org/10.1021/acs.inorgchem.0c03405

orbital M −O interactions. At the same time the M −

ave

interactions experience a gain in covalency and OPDOS

ave

of the f- and d-orbitals, even surpassing the M −O amide

ave

Author Contributions

interactions. We deduced that the charge alteration leads to a

shift in the energy levels of valence orbitals by the donor

functions, resulting in a redistribution in electron population of

bonding and antibonding orbitals, which then determines the

bond strength of each single M−D bonding interaction. The

eventual eﬀect on TPAMEN separation performance was

estimated on the basis of diﬀerences in complex formation

The manuscript was written through contributions of all

authors. All authors have given approval to the ﬁnal version of

the manuscript. These authors contributed equally.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

energies. The results suggest an improved Am /Eu

separation performance for TPAMEN. The same trend is

■

This work was supported by JSPS KAKENHI grant nos.

JP16H07439 (M.K.) and JP17K14915 (M.K.) and an Ibaraki

University Priority Research Grant (K.F.). We thank Dr. Yuta

Kumagai for his support in the laboratory facilities and with the

LC/MS measurements. We thank the anonymous reviewers

for constructive discussions, adding to the improvement of this

manuscript.

reﬂected by the diﬀerence in integrals of the OPDOSs of the

Eu and Am complexes, where a dominant increase in the

M −N OPDOS integral causes a greater distinction

between the [Eu(TPAMEN)] and [Am(TPAMEN)]

complexes. In addition, this emphasizes once more the

relevance of the M −N interaction for the Am /Eu

selectivity of the ligands. How far these changes in M −N

interactions by TPAMEN aﬀects its actual separation ability

REFERENCES

■

under experimental conditions will be the subject of future

1) Hudson, M. J. Some New Strategies for the Chemical Separation

of Actinides and Lanthanides. Czech. J. Phys. 2003, 53 (S1), A305−

A311.

investigations to determine the Am /Eu separation factors.

By demonstrating how subtle changes in O-donor basicity

−

from [M(TPAEN)] to [M(TPAMEN)] can signiﬁcantly

(2) Dam, H. H.; Reinhoudt, D. N.; Verboom, W. Multicoordinate

alter the M −N interactions, we emphasize the relevance of

basicity modiﬁcations for the strength of adjacent metal−

donor interactions. Taking this under consideration, we hope

to improve ligand design and selectivity estimations for f-block

element receptors in the future.

Ligands for Actinide/Lanthanide Separations. Chem. Soc. Rev. 2007,

36 (2), 367−377.

(3) Kolarik, Z. Complexation and Separation of Lanthanides(III)

and Actinides(III) by Heterocyclic N-Donors in Solutions. Chem. Rev.

Charbonnel, M. C.; Delangle, P. Water-Soluble Tetrapodal N,O

008, 108 (10), 4208−4252.

4) Neidig, M. L.; Clark, D. L.; Martin, R. L. Covalency in F-

Element Complexes. Coord. Chem. Rev. 2013, 257, 394−406.

(5) Heitzmann, M.; Gateau, C.; Chareyre, L.; Miguirditchian, M.;

Ligands Incorporating Soft N-Heterocycles for the Selective

Complexation of Am(III) over Ln(III). New J. Chem. 2010, 34 (1),

ASSOCIATED CONTENT

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03405.

■

08−116.

Details of precursor synthesis, NMR data, X-ray

crystallography, continuous shape measure ﬁles, and

DFT calculations (PDF)

(6) Nash, K. L. A Review of the Basic Chemistry and Recent

Developments in Trivalent F-Elements Separations. Solvent Extr. Ion

Exch. 1993, 11 (4), 729−768.

(

7) Bravard, F.; Bretonnier

e, Y.; Wietzke, R.; Gateau, C.; Mazzanti,

aut, J. Solid-State and Solution Properties of

Accession Codes

M.; Delangle, P.; Pec

Cationic Lanthanide Complexes of a New Neutral Heptadentate

CCDC 2018224 −2018226 contain the supplementary crys-

tallographic data for this paper. These data can be obtained

free of charge via www.ccdc.cam.ac.uk/data_request/cif , or by

emailing data_request@ccdc.cam.ac.uk , or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

N4O3 Tripodal Ligand. Inorg. Chem. 2003, 42 (24), 7978−7989.

8) Berny, F.; Muzet, N.; Troxler, L.; Dedieu, A.; Wipff, G.

Interaction of M Lanthanide Cations with Amide, Pyridine, and

Phosphoryl O = PPh Ligands: A Quantum Mechanics Study. Inorg.

Chem. 1999, 38 (6), 1244−1252.

(9) Chatterton, N.; Bretonniere, Y.; Pecaut, J.; Mazzanti, M. An

̀ ́

Efficient Design for the Rigid Assembly of Four Bidentate

Chromophores in Water-Stable Highly Luminescent Lanthanide

Complexes. Angew. Chem., Int. Ed. 2005, 44 (46), 7595−7598.

AUTHOR INFORMATION

■

Corresponding Authors

Kathleen Schnaars − Nuclear Science and Engineering Center,

Japan Atomic Energy Agency, Tokaimura, Ibaraki

(

10) Borrini, J.; Favre-Reguillon, A.; Lemaire, M.; Gracia, S.;

Arrachart, G.; Bernier, G.; Heres, X.; Hill, C.; Pellet-Rostaing, S.

́ ̀

Water Soluble PDCA Derivatives for Selective Ln(III)/An(III) and

Am(III)/Cm(III) Separation. Solvent Extr. Ion Exch. 2015, 33 (3),

224−235.

19−1195, Japan; orcid.org/0000-0002-6283-9536;

Email: schnaars.kathleen@jaea.go.jp

489

Inorg. Chem. 2021, 60, 2477−2491

Inorganic Chemistry

(28) Kratzert, D.; Holstein, J. J.; Krossing, I. DSR: Enhanced

Article

(

11) Gracia, S.; Arrachart, G.; Marie, C.; Chapron, S.;

Modelling and Refinement of Disordered Structures with SHELXL. J.

Appl. Crystallogr. 2015, 48, 933 −938.

Miguirditchian, M.; Pellet-Rostaing, S. Separation of Am (III) by

Solvent Extraction Using Water-Soluble H4tpaen Derivatives.

Tetrahedron 2015, 71 (33), 5321−5336.

(29) Kratzert, D.; Krossing, I. Recent Improvements in DSR. J. Appl.

Crystallogr. 2018, 51, 928−934.

(12) Huang, P.-W.; Wang, C.-Z.; Wu, Q.-Y.; Lan, J.-H.; Song, G.;

(30) Allen, F. H.; Johnson, O.; Shields, G. P.; Smith, B. R.; Towler,

Chai, Z.-F.; Shi, W.-Q. Understanding Am /Cm Separation with

M. CIF applications. XV. enCIFer: a program for viewing, editing and

H TPAEN and Its Hydrophilic Derivatives: A Quantum Chemical

visualising CIFs. J. Appl. Crystallogr. 2004, 37, 335−338.

Study. Phys. Chem. Chem. Phys. 2018, 20 (20), 14031−14039.

(31) Llunell, M.; Casanova, D.; Cirera, J.; Alemany, P.; Alvarez, S.

(13) Sasaki, Y.; Tsubata, Y.; Kitatsuji, Y.; Sugo, Y.; Shirasu, N.;

SHAPE (2.1); Universitat de Barcelona: Barcelona, Spain, 2013.

Morita, Y.; Kimura, T. Extraction Behavior of Metal Ions by TODGA,

DOODA, MIDOA, and NTAamide Extractants from HNO to n

(32) Macrae, C. F.; Sovago, I.; Cottrell, S. J.; Galek, P. T. A.;

McCabe, P.; Pidcock, E.; Platings, M.; Shields, G. P.; Stevens, J. S.;

Towler, M.; Wood, P. A. Mercury 4.0 : From Visualization to Analysis,

Design and Prediction. J. Appl. Crystallogr. 2020, 53 (1), 226−235.

Dodecane. Solvent Extr. Ion Exch. 2013, 31 (4), 401−415.

14) Jansone-Popova, S.; Ivanov, A. S.; Bryantsev, V. S.; Sloop, F. V.;

Custelcean, R.; Popovs, I.; Dekarske, M. M.; Moyer, B. A. Bis-Lactam-

,10-Phenanthroline (BLPhen), a New Type of Preorganized Mixed

(

(33) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.;

McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de

Streek, J.; Wood, P. A. Mercury CSD 2.0 − New Features for the

Visualization and Investigation of Crystal Structures. J. Appl.

Crystallogr. 2008, 41 (2), 466−470.

N,O-Donor Ligand That Separates Am(III) over Eu(III) with

Exceptionally High Efficiency. Inorg. Chem. 2017, 56 (10), 5911−

917.

15) Siddall, T. H. Effect of Structure of N,N-Disubstituted Amides

on Extraction. J. Phys. Chem. 1960, 64, 1863−1866.

16) Baaden, M.; Berny, F.; Madic, C.; Schurhammer, R.; Wipff, G.

(34) Macrae, C. F.; Edgington, P. R.; McCabe, P.; Pidcock, E.;

Shields, G. P.; Taylor, R.; Towler, M.; van de Streek, J. Mercury :

Visualization and Analysis of Crystal Structures. J. Appl. Crystallogr.

Theoretical Studies on Lanthanide Cation Extraction by Picolina-

mides: Ligand-Cation Interactions and Interfacial Behavior. Solvent

Extr. Ion Exch. 2003, 21 (2), 199−220.

(

006, 39 (3), 453−457.

35) Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M.; Macrae,

C. F.; McCabe, P.; Pearson, J.; Taylor, R. New Software for Searching

the Cambridge Structural Database and Visualizing Crystal Structures.

Acta Crystallogr., Sect. B: Struct. Sci. 2002 , 58 (3), 389 −397.

17) Williams, D. B. G.; Lawton, M. Drying of Organic Solvents:

Quantitative Evaluation of the Efficiency of Several Desiccants. J. Org.

Chem. 2010, 75 (24), 8351−8354.

(

(36) Taylor, R.; Macrae, C. F. Rules Governing the Crystal Packing

18) Chen, A. Y.; Thomas, P. W.; Stewart, A. C.; Bergstrom, A.;

of Mono-and Dialcohols. Acta Crystallogr., Sect. B: Struct. Sci. 2001, 57

Cheng, Z.; Miller, C.; Bethel, C. R.; Marshall, S. H.; Credille, C. V.;

Riley, C. L.; Page, R. C.; Bonomo, R. A.; Crowder, M. W.; Tierney, D.

L.; Fast, W.; Cohen, S. M. Dipicolinic Acid Derivatives as Inhibitors of

New Delhi Metallo-β -Lactamase-1. J. Med. Chem. 2017, 60 (17),

(

6), 815−827.

(37) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and

Molecules; Oxford University Press: New York, 1989.

(38) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. Relativistic

Regular Two-Component Hamiltonians. J. Chem. Phys. 1993, 99,

4597−4610.

(

267−7283.

19) Macerata, E.; Sansone, F.; Baldini, L.; Ugozzoli, F.; Brisach, F.;

Haddaoui, J.; Hubscher-Bruder, V.; Arnaud-Neu, F.; Mariani, M.;

Ungaro, R.; Casnati, A. Calix[6]Arene-Picolinamide Extractants for

Radioactive Waste Treatment: Effect of Additional Carboxy Binding

Sites in the Pyridine 6-Positions on Complexation, Extraction

Efficiency and An/Ln Separation. Eur. J. Org. Chem. 2010, 2010

(39) van Wullen, C. Molecular Density Functional Calculations in

the Regular Relativistic Approximation: Method, Application to

Coinage Metal Diatomics, Hydrides, Fluorides and Chlorides, and

Comparison with First-Order Relativistic Calculations. J. Chem. Phys.

998, 109 (2), 392−399.

40) Pantazis, D. A.; Neese, F. All-Electron Scalar Relativistic Basis

Sets for the Lanthanides. J. Chem. Theory Comput. 2009, 5 (9), 2229−

238.

41) Pantazis, D. A.; Neese, F. All-electron scalar relativistic basis

sets for the actinides. J. Chem. Theory Comput. 2011, 7 (3), 677−684.

42) Pantazis, D. A.; Chen, X. Y.; Landis, C. R.; Neese, F. All-

Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. NMR

14), 2675−2686.

(

20) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.;

Chemical Shifts of Trace Impurities: Common Laboratory Solvents,

Organics, and Gases in Deuterated Solvents Relevant to the

Organometallic Chemist. Organometallics 2010, 29 (9), 2176−2179.

Electron Scalar Relativistic Basis Sets for Third-Row Transition Metal

Atoms. J. Chem. Theory Comput. 2008 , 4 (6), 908 −919.

(

21) CrysAlisPRO, Ver. 171.36.20; Agilent Technologies: Yarnton,

Oxfordshire, England, 2011.

22) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.;

(43) Zhao, Y.; Truhlar, D. G. A New Local Density Functional for

(

Main-Group Thermochemistry, Transition Metal Bonding, Thermo-

chemical Kinetics, and Noncovalent Interactions. J. Chem. Phys. 2006,

Cascarano, G. L.; De Caro, L.; Giacovazzo, C.; Polidori, G.; Siliqi, D.;

Spagna, R. IL MILIONE: A Suite of Computer Programs for Crystal

Structure Solution of Proteins. J. Appl. Crystallogr. 2007, 40 (3), 609−

25 (19), 194101.

(44) Klamt, A.; Schuurmann, G. COSMO: A New Approach to

(

13.

23) Crystal Structure 4.2.2: Crystal Structure Analysis Package;

Rigaku Corporation: Tokyo 196-8666. Japan.

24) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A.

Dielectric Screening in Solvents with Explicit Expressions for the

Screening Energy and Its Gradient. J. Chem. Soc., Perkin Trans. 2

993, No. 5, 799 −805.

(

(45) Neese, F. An Improvement of the Resolution of the Identity

K.; Puschmann, H. OLEX2: A Complete Structure Solution,

Refinement and Analysis Program. J. Appl. Crystallogr. 2009, 42 (2),

Approximation for the Formation of the Coulomb Matrix. J. Comput.

Chem. 2003, 24 (14), 1740−1747.

(

39−341.

25) Sheldrick, G. M. Crystal Structure Refinement with SHELXL.

Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8.

26) Bourhis, L. J.; Dolomanov, O. V.; Gildea, R. J.; Howard, J. A.

(46) Kaneko, M.; Miyashita, S.; Nakashima, S. Bonding Study on the

Chemical Separation of Am(III) from Eu(III) by S-, N-, and O-Donor

Ligands by Means of All-Electron ZORA-DFT Calculation. Inorg.

Chem. 2015, 54 (14), 7103 −7109.

(

K.; Puschmann, H. The Anatomy of a Comprehensive Constrained,

Restrained Refinement Program for the Modern Computing Environ-

ment - Olex2 Dissected. Acta Crystallogr., Sect. A: Found. Adv. 2015,

(47) Neese, F. The ORCA Program System. Wiley Interdiscip. Rev.:

Comput. Mol. Sci. 2012, 2 (1), 73−78.

(48) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter, J.

E.; Bohmann, J. A.; Morales, C. M.; Landis, C. R.; Weinhold, F. NBO

6.0; Theoretical Chemistry Institute, University of Wisconsin-

Madison: Madison, WI, 2013.

1 (1), 59−75.

27) Spencer Kimball, P. M. and the GDT. GIMP 2.10.14 (GNU

Image Manipulation Program); 2019.

(

490

Inorg. Chem. 2021, 60, 2477−2491

Inorganic Chemistry

(49) Kawasaki, T.; Okumura, S.; Sasaki, Y.; Ikeda, Y. Crystal

Article

Structures of Ln(III) (Ln = La, Pr, Nd, Sm, Eu, and Gd) Complexes

with N,N,N ’,N ’-Tetraethyldiglycolamide Associated with Homoleptic

‑

[

Ln(NO ) )] . Bull. Chem. Soc. Jpn. 2014, 87 (2), 294−300.

50) Nicolo, F.; Plancherel, D.; Bunzli, J. C. G.; Chapuis, G.

Glasslike Structure in Crystalline Macrocyclic Complexes: Synthesis,

X-Ray Diffraction, and Laser-Spectroscopic Investigation of

Neodymium(III) and Europium(III) Complexes with 4,13-Dimeth-

yl-1,7,10,16-Tetraoxa-4,13-Diazacyclooctadecane. Inorg. Chem. 1988,

(

7 (20), 3518−3526.

51) Boubals, N.; Wager, C.; Dumas, T.; Chaneac, L.; Manie, G.;

Kaufholz, P.; Maire, C.; Panak, P. J.; Modolo, G.; Geist, A.; Guilbaud,

P. Complexation of actinide(III) and lanthanide(III) with H TPAEN

for a separation of americium fomr curium and lanthanides. Inorg.

Chem. 2017, 56 (14), 7861−7869.

(52) Cremer, D.; Kraka, E. A Description of the Chemical Bond in

Terms of Local Properties of Electron Density and Energy. Croat.

Chem. Acta 1984 , 57 (6), 1259 −1281.

(53) Mayer, I. Charge, bond order and valence in the AB initio SCF

theory. Chem. Phys. Lett. 1983 , 97 (3), 270 −274.

54) Mayer, I. Bond Order and Valence Indices: A Personal

Account. J. Comput. Chem. 2007, 28 (1), 204−221.

55) Bridgeman, A. J.; Cavigliasso, G.; Ireland, L. R.; Rothery, J. The

Mayer Bond Order as a Tool in Inorganic Chemistry. Dalton Trans.

001, No. 14, 2095−2108.

56) Mulliken, R. S. Electronic Population Analysis on LCAO-MO

Molecular Wave Functions. I. J. Chem. Phys. 1955, 23 (10), 1833−

840.

57) Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural population

analysis. J. Chem. Phys. 1985 , 83 (2), 735 −746.

58) Davies, C. W. The extent of dissociation of salts in water. Part

VIII. An equation for the mean ionic activity coefficient of an

electrolyte in water, and a revision of the dissociation constants of

some sulphates. J. Chem. Soc. 1938, 0, 2093 −2098.

(59) Breen, P. J.; Horrocks, W. W., Jr Europium(III) luminescence

excitation spectroscopy. Inner-sphere complexation of europium(III)

by chloride, thiocyanate, and nitrate ions. Inorg. Chem. 1983, 22 (3),

(

36−540.

60) Zalupski, P. R.; Grimes, T. S.; Heathman, C. R.; Peterman, D.

R. Optical absorption characteristics for F → L and F → F

transitions of trivalent americium ion in aqueous electrolytemixtures.

Appl. Spectrosc. 2017 , 71 (12), 2608 −2615.

(61) Grimme, S. Semiempirical hybrid density functional with

perturbative second-order correlation. J. Chem. Phys. 2006, 124 (3),

34108.

62) Kaneko, M.; Watanabe, M.; Matsumura, T. The Separation

Mechanism of Am(III) from Eu(III) by Diglycolamide and

Nitrilotriacetamide Extraction Reagents Using DFT Calculations.

Dalton Trans. 2016, 45 (43), 17530−17537.

(63) Basch, H.; Robin, M. B.; Kuebler, N. A. Electronic Spectra of

Isoelectronic Amides, Acids, and Acyl Fluorides. J. Chem. Phys. 1968,

9 (11), 5007.

64) da Silva, V. H. M.; Quattrociocchi, D. G. S.; Stoyanov, S. R.;

Carneiro, J. W. M.; da Costa, L. M.; Ferreira, G. B. A DFT Study of

(

the Interaction between [Cd(H O) ] and Monodentate O-, N-, and

S-Donor Ligands: Bond Interaction Analysis. J. Mol. Model. 2018, 24

1), 39.

65) Petit, L.; Daul, C.; Adamo, C.; Maldivi, P. DFT Modeling of

(

the Relative Affinity of Nitrogen Ligands for Trivalent f Elements: An

Energetic Point of View. New J. Chem. 2007, 31 (10), 1738−1745.

(66) Lemonnier, J. F.; Guenee, L.; Beuchat, C.; Wesolowski, T. A.;

́ ́

Mukherjee, P.; Waldeck, D. H.; Gogick, K. A.; Petoud, S.; Piguet, C.

Optimizing Sensitization Processes in Dinuclear Luminescent

Lanthanide Oligomers: Selection of Rigid Aromatic Spacers. J. Am.

Chem. Soc. 2011, 133 (40), 16219−16234.

491