Americium(III) and europium(III) solvent extraction studies of amide-substituted triazine ligands and complexes formed with ytterbium(III)

Article abstract of DOI:10.1039/b104181a

4-Amino-bis(2,6-(2-pyridyl))-1,3,5-triazine L⁴ and several, amide derivatives with hydrophobic alkyl substituents have been synthesised. Solvent extraction studies carried out on Am(III) and Eu(III) with L⁴ and its amide derivatives

Full text of DOI:10.1039/b104181a

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Americium(III) and europium(III) solvent extraction studies of
amide-substituted triazine ligands and complexes formed with
ytterbium(III)
Nathalie Boubals,b Michael G. B. Drew,*a Clément Hill,b Michael J. Hudson,a Peter B. Iveson,a
Charles Madic,b Mark L. Russella and Tristan G. A. Youngsa
a Department of Chemistry, University of Reading, Whiteknights, Reading, UK RG6 6AD.
E-mail: m.g.b.drew@reading.ac.uk
b Commissariat à l’Energie Atomique, Valrhô, DEN/DRCP/SCPS/LCSE, Bât 399, B.P. 17171,
30207, Bagnols-sur-Cèze, Cedex, France
Received 11th May 2001, Accepted 22nd October 2001
First published as an Advance Article on the web 3rd December 2001
4-Amino-bis(2,6-(2-pyridyl))-1,3,5-triazine L4 and several, amide derivatives with hydrophobic alkyl substituents
have been synthesised. Solvent extraction studies carried out on Am() and Eu() with L4 and its amide derivatives
in synergistic combination with α-bromodecanoic acid, show that these ligands can selectively extract actinides with
respect to lanthanides. The structures of [H2L4]ؒ2Clؒ2.5H2O and two amide derivatives have been determined and
show respectively the trans, trans; cis, cis; and cis, trans conformations of the adjacent aromatic rings. These
observed conformations are in agreement with the results of quantum mechanics calculations on L4 and its
protonated derivatives. The structures of two Yb complexes with amide derivatives are also reported with
stoichiometry [Yb(L)(NO3)(H2O)4]ؒ2NO3ؒ0.5H2O and [Yb(L)(NO3)3(H2O)]ؒ2MeCN and show the metal
in 9 coordinate environments.
Introduction
One possible future scenario in nuclear reprocessing is the con-
version or transmutation of the long-lived minor actinides,
such as americium, into short-lived isotopes by irradiation with
neutrons.1 In order to achieve this transmutation it is necessary
to separate the trivalent minor actinides from the trivalent lan-
thanides by solvent extraction, because the lanthanides absorb
neutrons effectively and hence prevent neutron capture by the
transmutable actinides.
For many years we have been designing and testing ligands
for the co-extraction2,3 of lanthanides and actinides from
nuclear waste and their subsequent separation.4,5 Oligoamines
such as 2,2Ј:6Ј2Љ-terpyridine4–6 (L1) and 2,4,6-tris(2-pyridyl)-
1,3,5-triazine5 (L2) have been shown to selectively extract actin-
ides in preference to the lanthanides from nitric acid solutions
into an organic phase. For extraction it proved necessary to use
these ligands in synergistic combination with α-bromodecanoic
acid. Separation factors for Am() relative to Eu() were
found to be around 7 and 10 for L1 and L2 respectively.5
Although a separation factor of around 12 can be obtained
with a more hydrophobic derivative of L2 (2,4,6-tris(4-tert-
butyl-2-pyridyl)-1,3,5-triazine (L3), the synthesis is difficult and
the ligand can only be prepared on a small scale. The promising
solvent extraction results obtained with L2 and L3 led us to the
synthesis of 4-amino-bis(2,6-(2-pyridyl))-1,3,5-triazine (L4).
This ligand contains the same major binding cavity as L2 but is
much more easily functionalised to prepare the appropriate
hydrophobic derivatives. In this study, several new amide
derivatives of L4 have been prepared containing either 2,2,4-
Fig. 1 Structures of ligands.
trimethyl-1-pentyl (L5), 2,2-dimethyl-1-propyl (L6), cyclohexyl
(L7), or heptyl (L8) as the hydrophobic alkyl substituent (Fig. 1).
The Am()/Eu() separation–extraction performance of L4,
L5, L7 and L8 was measured in combination with α-bromo-
decanoic acid. The structures of a bis hydrochloride salt of L4
and of the free ligands L5 3,5,5-trimethylhexanoylamino-bis(2,6-
(2-pyridyl)-1,3,5-triazine) and L7 (4-cyclohexanoylamino-bis-
(2,6-(2-pyridyl))-1,3,5-triazine) together with the corresponding
Yb() complexes of L6 and L7 were also determined.
DOI: 10.1039/b104181a
J. Chem. Soc., Dalton Trans., 2002, 55–62
This journal is © The Royal Society of Chemistry 2002
55

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[Yb(L6)(NO3)3]ؒ2MeCN. This complex was prepared by the
dropwise addition of Yb(NO3)3ؒ5H2O (0.0129 g, 0.03 mM)
dissolved in 1 cm3 CH3CN to a stirred solution containing
L6 (0.01 g, 0.03 mM) also dissolved in 1cm3 CH3CN. Suitable
crystals were obtained at room temperature after 2 days.
Experimental
Synthesis
L4 was prepared as described previously.7 3,5,5-Trimethylhexan-
oyl chloride, tert-butylacetyl chloride, cyclohexane carbonyl
chloride, octanoyl chloride and ytterbium nitrate pentahydrate
(99.9%) were used as received from Aldrich. Pyridine and
acetonitrile were dried over 4 and 3 Å molecular sieves
respectively.
Solvent extraction studies
Aqueous solutions (800 µL) of diluted nitric acid (0.02 mol LϪ1
≤ [HNO3] ≤ 0.13 mol LϪ1), spiked with radioisotopes 241Am and
152Eu, were contacted for 30 minutes by means of an automatic
vortex shaker with organic solutions (800 µL), containing
either ligand L4, L5, L7 or L8 ([L]initial = 0.02 mol LϪ1), diluted
in a mixture of hydrogenated tetrapropene (TPH) and α-bromo-
decanoic acid ([αBrC10]initial = 1 mol LϪ1). Aqueous and organic
solutions were mixed in 2 mL Nalgene tubes thermostatted at
22 ЊC. After phase separation by centrifugation, 500 µL samples
of both phases were analysed using a gamma counting spectro-
meter (HPGe detector, Eurisys Mesures). The peaks at 59.54
and 121.78 keV were used for 241Am and 152Eu activity measure-
ments, respectively. The concentration of nitric acid in the
aqueous phase at equilibrium ([HNO3]eq) was determined by
automatic titration with NaOH.
Preparation of ligands
4-(3,5,5-Trimethylhexanoylamino)-bis(2,6-(2-pyridyl))-1,3,5-
triazine (L5). L4 (31.78 g, 0.127 M) was stirred as a suspension in
pyridine (500 cm3) under a nitrogen atmosphere. The solution
was heated to ≈115 ЊC and 3,5,5-trimethylhexanoyl chloride
(66.40 g, 0.376 M) was added in one volume to the hot reaction
mixture. The suspension of L4 gradually disappeared on heat-
ing and the solution was allowed to cool after 2 h. After the
volume of solvent was reduced to 100 cm3 and 100 cm3 of
CH2Cl2 was added, the solution was extracted with a saturated
NaHCO3 solution (2 × 200 cm3) and then twice with distilled
water (2 × 200 cm3). The organic phase was dried with sodium
sulfate and the solvents were removed in vacuo to leave a dark
brown oil which was then stirred vigorously with ethyl acetate
(50 cm3) and hexane (500 cm3) for approximately 1 h. A white
precipitate formed which was filtered, washed with cold hexane
and recrystallised from ethyl acetate. (Yield 34 g, 70%), mp 148
ЊC. NMR measurements for this and all other compounds were
The distribution ratio DM for a metallic cation M is defined
as the ratio of the concentration of the metallic species in
the organic phase at equilibrium over its concentration in the
aqueous phase at equilibrium. The error of the measure of DM
(M = Am() or Eu()) was estimated to be within 5%. The
separation factor SFM /M for two metallic cations M1 and M2
is defined as the ratio of their distribution ratios. The error of
the determination of SFAm/Eu was estimated to be 7%.
1
2
1
carried out in CDCl3. H NMR: δ 0.89 (9H, s), 1.08 (3H, d),
1.24 (1H, dd), 1.42 (1H, dd), 2.31 (1H, m), 2.69 (1H, dd),
2.82 (1H, dd), 7.47 (2H, t), 7.91 (2H, t), 8.73 (2H, d), 8.91
(2H, d). Found: C, 67.64; H, 6.67; N, 21.19. C22H25N6O requires
C, 67.67; H, 6.71; N, 21.52%.
Crystallography
The structures of the salt [H2L4]ؒ2Clؒ2.5H2O, the ligands L5ؒ
2H2O and L7, and the ytterbium complexes with L6 and L7 were
determined. Crystal data and refinement details are provided in
Table 1. Data for all 5 crystals were collected with Mo-Kα radi-
ation using the MAR research Image Plate System. The crystals
were positioned 70 mm from the image plate. 95 frames were
measured at 2Њ intervals with a counting time of 2 min. Data
analysis was carried out with the XDS program.8 Default
refinement details are described here while differences for
specific structures are included below. Structures were solved
using direct methods with the SHELX86 program.9 All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
on the carbon atoms and nitrogen atoms were included in cal-
culated positions and given thermal parameters equivalent to
1.2 times those of the atom to which they were attached.
Hydrogen atoms on water molecules were included when they
could be located in a difference Fourier map and refined with
distance constraints. The assignment of the positions of the
nitrogen atoms in the pyridine rings was made straight-
forwardly on the basis of thermal parameters, dimensions, R
values and hydrogen bond positions. An empirical absorption
correction was made for the two metal complexes using the
DIFABS program.10
4-tert-Butylacetanoylamino-bis(2,6-(2-pyridyl))-1,3,5-triazine
(L6). L6 was prepared in a similar manner to L5. H NMR
1
confirmed the presence of L6 and the ligand was used without
further purification for the preparation of the corresponding
Yb complex. 1H NMR: δ 1.11 (9H, s), 2.64 (2H, s), 7.58 (2H, t),
8.01 (2H, t), 8.61 (2H, d) 8.84 (2H, d), 9.11 (1H, br).
4-Cyclohexanoylamino-bis(2,6-(2-pyridyl))-1,3,5-triazine (L7).
L7 was prepared in a similar manner to L5. Yield 64%, mp
1
138 ЊC, H NMR: δ 1.3 (3H, m), 1.6 (2H, m), 1.7 (1H, d), 1.9
(2H, m), 2.1 (2H, m), 2.8 (1H, t), 7.5 (2H, t), 7.9 (2H, t), 8.7
(2H, d), 8.9 (2H, d). Found: C, 66.61; H, 5.63; N, 23.54.
C20H20N6O requires C, 66.65; H, 5.59; N, 23.32%. Crystals suit-
able for X-ray structural analysis were obtained after further
recrystallisation from ethyl acetate.
4-Octanoylamino-bis(2,6-(2-pyridyl))-1,3,5-triazine (L8). L8
was prepared in a similar manner to L5. Yield 62%, H NMR:
1
δ 0.88 (3H, t), 1.24–1.38 (6H, m), 1.44 (2H, q), 1.80 (2H, q), 2.90
(2H, t), 7.5 (2H, t), 7.9 (2H, t), 8.7 (2H, d), 8.9 (2H, d). Found:
C, 67.02; H, 6.46; N, 22.33. C21H24N6O requires C, 67.00; H,
6.43; N, 22.32%.
In the structure of the salt [H2L4]ؒ2Clؒ2.5H2O, there are two
formula units in the asymmetric unit. All the hydrogen atoms
on the water molecules were located in a difference Fourier map
and included in the refinement with distance constraints. The
structure of L7 contained no solvent. L5 contained two water
molecules in the asymmetric unit but the hydrogen atoms on
these solvent molecules were not located. For [Yb(L7)(NO3)-
(H2O)4]ؒ2NO3ؒ0.5H2O the hydrogen atoms on the water mole-
cules were not located. The data were of poor quality and only
the metal atom was refined anisotropically. For [Yb(L6)(NO3)3-
(H2O)]ؒ2MeCN the hydrogen atoms on the water molecules
were located and refined with distance constraints. All struc-
tures were refined on F 2 till convergence using SHELXL.11
CCDC reference numbers 172878–172882.
[H2L4]ؒ2Clؒ2.5H2O. L4 was dissolved in 2 M HCl and after
complete evaporation of the solution, suitable crystals were
obtained after a few weeks.
Preparation of metal complexes
[Yb(L7)(NO3)(H2O)4]ؒ2NO3ؒ0.5H2O. This complex was pre-
pared by stirring Yb(NO3)3ؒ5H2O (0.010 g, 0.02 mM) and
L7 (0.008 g, 0.02 mM) in 20 cm3 CH3CN until complete
dissolution had occurred. The solution was allowed slowly to
evaporate at room temperature. Suitable crystals were formed
after 2 days.
56
J. Chem. Soc., Dalton Trans., 2002, 55–62

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Table 1 Crystal data and structure refinement details
Compound
[H2L4]ؒ
2Clؒ2.5H2O
C13H17Cl2N6O2.5 C20H20N6O
368.23
293(2)
Triclinic, P1
10.158(12)
13.890(15)
14.249(15)
69.20(1)
67.61(1)
71.60(1)
1699
L7
L5ؒ2H2O
[Yb(L7)(NO3)(H2O)4]ؒ [Yb(L6)(NO3)3(H2O)]ؒ
2NO3ؒ 0.5H2O
C20H29N9O14.5Yb
800.56
2MeCN
C24H28N11O10Yb
803.61
Empirical formula
Formula weight
Temperature/K
Crystal system, space group
a/Å
b/Å
c/Å
α/Њ
C22H29N6O3
425.51
293(2)
360.42
293(2)
293(2)
293(2)
¯
Monoclinic, P21/c Monoclinic, P21/a Monoclinic, C2/c
Monoclinic, C2/c
8.935(14)
12.887(14)
15.004(17)
71.69(1)
87.14(1)
78.96(1)
1610
9.946(12)
17.15(2)
11.849(14)
(90)
109.12(1)
(90)
1910
4, 1.254
0.082
12.458(14)
14.997(17)
12.515(14)
(90)
93.54(1)
(90)
2334
4, 1.211
0.083
24.35(3)
16.760(18)
14.812(17)
(90)
103.49(1)
(90)
5878
4, 1.809
3.266
β/Њ
γ/Њ
Volume/Å3
Z, Calculated density/Mg mϪ3
Absorption coefficient/mmϪ1
F(000)
2, 1.439
0.404
764
2, 1.658
2.974
760
908
3184
798
Reflections collected
Unique reflections/Rint
Data/restraints/parameters
Final R indices [I > 2σ(I )] R1
5890
5890
5890/15/457
0.0479
6387
7430
6980
5198
5198
4198/2/428
0.0296
0.0712
0.0367
0.0755
0.997, Ϫ1.596
3383/0.0310
3383/0/245
0.0844
0.2336
0.1544
0.2846
0.360, Ϫ0.233
4416/0.0761
4416/4/297
0.0920
0.2439
0.2214
0.3061
0.697, Ϫ0.277
4420/0.0762
4420/0/190
0.1119
0.2809
0.2547
0.3363
3.540, Ϫ1.922
wR2 0.1284
R1 0.0722
wR2 0.1419
Largest diff. peak and hole/e ÅϪ3
0.260, Ϫ0.272
R indices (all data)
Table 2 Extraction of Am() and Eu() by different synergistic
See http://www.rsc.org/suppdata/dt/b1/b104181a/ for crystal-
lographic data in CIF or other electronic format.
systemsa
Ligand
L1
[HNO3]eq
DEu()
DAm()
SFAm/Eu
Theoretical calculations
There are three possible conformations for L4 which can be
characterised by the N–C–C–N torsion angles as tt (trans,
trans), ct (cis, trans) and cc(cis, cis). We have analysed these con-
formations for L4, [HL4]ϩ and [H2L4]2ϩ using the Gaussian94
program.12 Starting models were built using the CERIUS2
software13 and the three rings were made approximately
coplanar but no symmetry was imposed. Structures were then
optimised using the 6-31G** basis set.
0.03
0.05
0.08
0.11
2.8
0.3
0.09
0.03
27
9.5
10
9
3.5
0.8
0.3
9
L2
L4
L5
0.03
0.05
0.08
0.11
8.7
0.9
0.2
0.08
124
12
2.1
0.8
14
14
11
9
0.03
0.05
0.08
0.11
4.6
0.5
0.08
0.02
45
9.5
10
9.5
5.5
0.7
0.2
Results and discussion
11.5
Synthesis
Hydrophobic amide derivatives of 4-amino-bis(2,6-(2-pyridyl))-
1,3,5-triazine L4 were synthesised by reaction of L4 with the
appropriate acid chloride in refluxing pyridine. This method
has been used previously for the acylation of 2,4-diamino-s-
triazines.14 There is a wide choice of hydrophobic substituents
because of the large number of commercially available acid
chlorides. The two ligands containing 2,2-dimethyl-1-propyl
and cyclohexyl alkyl groups (L6 and L7), were prepared in order
to facilitate crystallisation of the corresponding lanthanide
complexes. The more organo-soluble L5 and L8 were prepared
for the solvent extraction experiments. The 2,2,4-trimethyl-1-
pentyl alkyl chain in L5 was chosen because it bears a closer
resemblance to the TPH solvent used as the organic phase in
the extraction experiments. TPH (hydrogenated tetrapropene)
is an industrial aliphatic diluent containing highly branched
alkanes.
0.02
0.04
0.06
0.09
0.13
2.2
0.5
0.12
0.04
0.02
14
4.8
1.1
0.39
0.21
6.5
9.5
9
9
9
L7
L8
0.03
0.05
0.10
2.2
0.24
0.04
24
2.8
0.4
11
12
10
0.03
0.06
0.10
2.4
0.3
0.03
19
3.5
0.4
8
12
12
a Organic solution: [αBrC10]initial = 1 M, [L]initial = 0.02 mol LϪ1, TPH.
Aqueous solution: [HNO3]initial = variable, T = 22 ЊC.
the extraction of [ML]3ϩ complexes in TPH. The results of
these experiments are summarised in Table 2 together with the
results related to ligands L2 and L3 and are also shown in Fig. 2
for ligands L4 and L5, and Fig. 3 for ligands L7 and L8,
respectively.
Extraction studies
The synergistic extraction of trivalent actinides and lanthanides
observed when combining tridentate planar ligands such as L2
with α-bromodecanoic acid (αBrC10) in TPH can be described
by the following equilibrium:5
Although not as efficient as ligand L2, ligands L4, L5, L7 and
L8 appeared to be as efficient as ligand L1 when used in a syn-
ergistic mixture with α-bromodecanoic acid for the extraction
of Am() and its separation from Eu(). For the three rather
hydrophobic substituted ligands L5, L7 and L8, the log–log plots
of the experimental DM values vs. [HNO3]eq fitted well with a
linear regression. The slope of which was close to Ϫ3, in agree-
ment with the above proposed extraction equilibrium. However,
for the non-substituted ligand L4, the log–log plots of the
M3ϩ ϩ 3αBrC10 ϩ L
ML(αBrC10)3 ϩ 3Hϩ
We have studied the synergistic extraction of Am() and
Eu() by combining ligands L4, L5, L7 or L8 with α-bromo-
decanoic acid, used here as a cationic exchanger. The replace-
ment of nitrate anions by α-bromodecanoate anions enhances
J. Chem. Soc., Dalton Trans., 2002, 55–62
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Fig. 2 Am() and Eu() extraction by L4 and L5 in a synergistic
mixture with α-bromodecanoic acid. Organic solution: [αBrC10]initial
=
=
1 M, [L]initial = 0.02 mol LϪ1, TPH. Aqueous solution: [HNO3]initial
variable, T = 22 ЊC. The symbols denote each measurement of DM. The
approximately horizontal lines represent calculated SF values.
Fig. 4 The structure of the [H2L4]2ϩ cation A showing the hydrogen
bond pattern (dotted lines). Ellipsoids at 30% probability.
Fig. 3 Am() and Eu() extraction by L7 and L8 in a synergistic
mixture with α-bromodecanoic acid. Organic solution: [αBrC10]initial
=
=
1 M, [L]initial = 0.02 mol LϪ1, TPH. Aqueous solution: [HNO3]initial
variable, T = 22 ЊC. The symbols denote each measurement of DM. The
approximately horizontal lines represent calculated SF values.
Fig. 5 The structure of the [H2L4]2ϩ cation B showing the hydrogen
bond pattern (dotted lines). Ellipsoids at 30% probability.
experimental DM values vs. [HNO3]eq were better fitted with a
linear regression, the slope of which is close to Ϫ4, possibly
because of the redistribution of this rather hydrophilic ligand in
the aqueous acidic phase after protonation (i.e.: [HNO3]eq
0.08 M).
≥
hydrogen bonds between the water molecules and the chloride
anions. It is likely that the short intramolecular contacts in the
ligands between N(11) and N(25), and between N(31) and
N(23) of ca. 2.70 Å represent weak hydrogen bonds.
Structural studies
Ligands. L4 is sparingly soluble in most solvents and we were
unable to prepare suitable crystals of the free ligand. We did,
however, manage to crystallise the corresponding dihydrochlor-
ide salt on evaporation of a 2 M HCl solution containing the
ligand. The structure of [H2L4]ؒ2Clؒ2.5H2O shows two discrete
cations, together with anions and solvent water molecules in the
asymmetric unit. Both independent cations are protonated in
the same way with the hydrogen atoms situated on the nitrogen
atoms in the pyridine rings. The three nitrogen atoms N(11),
N(21), N(31) are arranged in a trans, trans (tt) formation. It is
possible that this conformation and protonation pattern is
facilitated by the presence of the water molecules and chloride
anions and this is considered in the theoretical section below.
The protonated cations A and B are shown in Figs. 4 and 5
together with their hydrogen bond patterns, details of which are
listed in Table 3. Surprisingly, the patterns are different for A
and B. Thus the intramolecular hydrogen bonds show that, in
cation A, the amine nitrogen atom N(27) is hydrogen bonded to
a water molecule and a chloride anion, whilst in B it is bonded
to two chloride anions. Similarly N(11A) is bound to a chloride
anion and N(31A) to a water molecule, while both N(11B) and
N(31B) are bonded to water molecules. In addition, there are
The structure of L7 is shown in Fig. 6. There is one strong
hydrogen bond between N(41) and N(11) (x, 0.5 Ϫ y, 0.5 ϩ z)
with an N ؒ ؒ ؒ N distance of 3.00 Å. The arrangement of the
three nitrogen atoms is cis, cis (cc). On the other hand, the
structure of L5ؒ2H2O exhibits the ligand in the ct conformation
as shown in Fig. 7. A water molecule O(200) forms donor
hydrogen bonds to both N(31) at 2.84(1) and O(43) at 2.79(1)
Å. The second water molecule in the asymmetric unit (not
shown in the Figure) forms two hydrogen bonds to O(200) but
not to L5. In addition N(11) forms an intermolecular hydrogen
bond to N(41) (5 ϩ x, 0.5 Ϫ y, z) at 2.94 Å.
The fact that L5 is in the ct conformation and L7 is in the cc
conformation (and indeed that [H2L4]2ϩ is in the tt conform-
ation) is particularly interesting. Our theoretical calculations on
the parent amine (see below) show that there are only small
energy differences between these three conformations, much
lower than is found for ligands (e.g. 2,2Ј:6Ј2Љ-terpyridine (L1))6
with a central pyridine ring. The energy barrier between con-
formations is also likely to be much lower in these ligands con-
taining a central triazine ring than in those containing a central
pyridine ring where the ortho-hydrogen atoms on adjacent rings
restrict rotation.
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J. Chem. Soc., Dalton Trans., 2002, 55–62

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Fig. 6 The structure of L7 with ellipsoids at 30% probability.
Table 3 Hydrogen bond distances (Å) in the structures
[H2L4]ؒ2Clؒ2.5H2O
Table
4
Dimensions in [Yb(L7)(NO3)(H2O)4]ؒ2NO3ؒ0.5H2O and
[Yb(L6)(NO3)3(H2O)]ؒ2MeCN
[Yb(L7)(NO3)(H2O)4]ؒ2NO3ؒ0.5H2O
O(100) ؒ ؒ ؒ Cl(1)
O(100) ؒ ؒ ؒ Cl(2)$2
O(101) ؒ ؒ ؒ Cl(3)
O(101) ؒ ؒ ؒ Cl(4)$4
O(102) ؒ ؒ ؒ Cl(1)
O(103) ؒ ؒ ؒ Cl(3)
O(103) ؒ ؒ ؒ Cl(4)
O(104) ؒ ؒ ؒ Cl(4)
O(104) ؒ ؒ ؒ Cl(2)$1
N(11A) ؒ ؒ ؒ N(25A)
N(11A) ؒ ؒ ؒ Cl(2)
3.15
3.11
3.17
3.14
3.07
3.21
3.21
3.13
3.11
2.71
3.09
N(11B) ؒ ؒ ؒ O(101)$3
N(11B) ؒ ؒ ؒ N(25B)
N(27A) ؒ ؒ ؒ O(100)$2
N(27A) ؒ ؒ ؒ Cl(1)
N(27B) ؒ ؒ ؒ Cl(3)$3
N(27B) ؒ ؒ ؒ Cl(3)
N(31A) ؒ ؒ ؒ O(102)
N(31A) ؒ ؒ ؒ N(23A)
N(31B) ؒ ؒ ؒ O(103)
N(31B) ؒ ؒ ؒ N(23B)
2.69
2.70
2.84
3.26
3.33
3.34
2.68
2.72
2.69
2.69
Yb(1)–O(102)
Yb(1)–O(101)
Yb(1)–O(103)
Yb(1)–N(21)
Yb(1)–O(100)
2.290(16)
2.313(15)
2.385(18)
2.39(2)
Yb(1)–O(42)
Yb(1)–N(31)
Yb(1)–O(41)
Yb(1)–N(11)
2.437(19)
2.52(2)
2.548(18)
2.57(2)
2.398(16)
[Yb(L6)(NO3)3(H2O)]ؒ2MeCN
Yb(1)–O(41)
Yb(1)–O(100)
Yb(1)–O(51)
Yb(1)–O(61)
Yb(1)–O(62)
2.277(4)
2.298(4)
2.362(4)
2.400(4)
2.404(4)
Yb(1)–N(21)
Yb(1)–O(52)
Yb(1)–N(11)
Yb(1)–N(31)
2.419(4)
2.470(4)
2.489(5)
2.519(5)
L7
N(41) ؒ ؒ ؒ N(11)$1
3.00
the lanthanide complexes formed with L4 in which we obtained
suitable crystals with the majority of the lanthanides.15 The ease
of crystallisation using L4 was probably due to the stabilising
effect in the crystal of intermolecular hydrogen bond formation
through the 4-amino group.
L5ؒ2H2O
N(11) ؒ ؒ ؒ N(41)$3
O(43) ؒ ؒ ؒ O(200)
O(200) ؒ ؒ ؒ N(31)
2.94
2.79
2.84
O(100) ؒ ؒ ؒ O(200)$7
O(100) ؒ ؒ ؒ O(200)$6
2.91
2.96
The structure of the [Yb(L7)(NO3)(H2O)4]2ϩ cation is shown
in Fig. 8. There are two nitrate anions together with a dis-
ordered water molecule in the asymmetric unit. The structure
of the cation shows the Yb to be in a 9-coordinate environment
with the metal bond lengths shown in Table 4, together with the
atomic numbering scheme. The water molecules show the
shortest bonds although there is significant variation [2.29(2)–
2.39(2) Å] but in addition, each of them forms two strong
hydrogen bonds with adjacent non-coordinated nitrate anions.
This formation of a doubly charged ionic complex containing
a lanthanide with only one nitrate ion is unique and is not
found in any of the 100 or so structures that we have deter-
mined of lanthanide nitrate complexes with terdentate nitrogen
ligands.4,6,15 The bond to the central triazine nitrogen atom
Yb(1)–N(21) is at 2.39(2) Å much shorter than the bonds to the
outer pyridine nitrogen atoms (Yb(1)–N(31) 2.52(2), Yb(1)–
N(11) 2.57(2) Å). The amide N–H group is hydrogen bonded to
a nitrate oxygen atom (which is not bonded to the metal) from
an adjacent molecule, viz (N(41) ؒ ؒ ؒ O(54) (0.5 Ϫ x, 0.5 Ϫ y,
1 Ϫ z), 2.84 Å. There are many hydrogen bonds (Table 3)
[Yb(L7)(NO3)(H2O)4]ؒ2NO3ؒ0.5H2O
O(100) ؒ ؒ ؒ O(61)$4
O(100) ؒ ؒ ؒ O(62)$4
O(101) ؒ ؒ ؒ O(44)$6
O(101) ؒ ؒ ؒ O(61)$5
2.79
2.96
2.97
3.07
O(102) ؒ ؒ ؒ O(64)$5
O(102) ؒ ؒ ؒ O(51)$1
N(41) ؒ ؒ ؒ O(54)$2
2.67
2.86
2.82
[Yb(L6)(NO3)3]ؒ2MeCN
O(100) ؒ ؒ ؒ N(500)$5
2.76
N(41) ؒ ؒ ؒ N(400)$4
3.14
Symmetry elements. For [H2L4]ؒ2Clؒ2.5H2O: $1 x, y, 1 ϩ z; $2 1 Ϫ x, 1
Ϫ y, Ϫz; $3 Ϫx Ϫ 1, 2 Ϫ y, Ϫz; $4 Ϫx Ϫ 1, 2 Ϫ y, 1 Ϫ z. For L7: $1 x, 0.5
Ϫ y, 0.5 ϩ z. For L5ؒ2H2O: $3 0.5 ϩ x, 0.5 Ϫ y, z; $6 2.5 Ϫ x, 0.5 ϩ y, 1
Ϫ z; $7 x ϩ 0.5, 0.5 Ϫ y, z Ϫ 1. For [Yb(L7)(NO3)(H2O)4]ؒ2NO3ؒ
0.5H2O: $1 x, 1 Ϫ y, z Ϫ .5; $2 0.5 Ϫ x, 0.5 Ϫ y, 1 Ϫ z; $4 1 Ϫ x, y, 0.5 Ϫ
z; $5 1 Ϫ x, 1 Ϫ y, 1 Ϫ z; $6 x, 1 Ϫ y, 0.5 ϩ z. For [Yb(L6)(NO3)3]ؒ
2MeCN: $4 1 Ϫ x, 1 Ϫ y, Ϫz; $5 x ϩ 1, y, z.
Ytterbium complexes. We were able to prepare suitable crys-
tals of two Yb() complexes formed with L6 and L7 but were
unable to crystallise the corresponding complexes formed with
the larger lanthanides. This is in direct contrast to our study on
J. Chem. Soc., Dalton Trans., 2002, 55–62
59

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Fig. 7 The structure of L5ؒ2H2O with ellipsoids at 30% probability. One water molecule, present in the asymmetric unit, is not shown.
Fig. 8 The structure of the [Yb(L7)(NO3)(H2O)4]2ϩ cation with the atomic numbering scheme. Ellipsoids at 30% probability. Hydrogen atoms on the
water molecules could not be located and are not shown.
between the coordinated water molecules and nitrates in
adjacent molecules.
than the bonds to the other nitrogen atoms (2.489(5), 2.519(5)
Å). The amide nitrogen atom in this case is hydrogen bonded to
a solvent acetonitrile molecule (N(41) ؒ ؒ ؒ N(400) (1 Ϫ x, 1 Ϫ y,
Ϫz) 3.14 Å).
This structural type, [Yb(L7)(NO3)(H2O)4]2ϩ, is different
from the two types obtained previously for Yb() and L4 as
in [Yb(L4)(NO3)2(H2O)2], [NO3], Yb is coordinated to one tri-
dentate L4 ligand, two bidentate nitrates and two water mole-
cules and in [Yb(L4)(NO3)3(H2O)], Yb is coordinated to one
L4, two bidentate nitrates, one monodentate nitrate and one
water molecule.15 The latter structure is similar to that of
[Yb(L6)(NO3)3(H2O)] which is shown in Fig. 9. This structure
also contains two solvent acetonitrile groups in the asymmetric
unit. This structure is also 9-coordinate as one of the nitrate
anions is monodentate. As is usually the case, the shortest
Yb–O bond is to the monodentate nitrate Yb(1)–O(41) 2.277(4)
Å, followed by the bond to the water molecule Yb(1)–O(100)
2.298(4) Å. The next shortest bonds are those to the bidentate
nitrate although the bond to O(52) at 2.470(4) Å is significantly
longer than the other three (2.362(4), 2.400(4), 2.404(4) Å). As
was found for [Yb(L7)(NO3)(H2O)4]ؒ2NO3ؒ0.5H2O, the bond to
the triazine nitrogen atom is, at 2.419(4) Å, significantly shorter
We ascribe no particular significance to the difference in
stoichiometry between the two metal complexes of Yb with L6
and L7. In previous work we have shown that there can be very
different coordination geometries for specific metals with these
planar terdentate ligands and it seems likely that the replace-
ment of one bidentate and one monodentate nitrate by three
water molecules is not unusual and that both structures
together with others are likely to co-exist in solution. The only
thing in common is that in both cases the metal cations are
9-coordinate as is usually found for ytterbium. It is likely that
with these ligands the smaller lanthanides are 9-coordinate and
have the form [ML(NO3)n(H2O)m]pϩ with n = 1, 2 or 3; m = 6 Ϫ
2n ϩ x, where x is the number of monodentate nitrates. General
structural trends show that the number of monodentate nitrates
will be either 1 or 0, and that at least one bidentate nitrate will
be coordinated so that the charge p on the complex is 0, 1 or 2.
60
J. Chem. Soc., Dalton Trans., 2002, 55–62

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Fig. 9 The structure of the [Yb(L6)(NO3)3(H2O)] complex with ellipsoids at 30% probability. One of the two solvent acetonitrile molecules is shown.
It can also be considered that there are several different entities
for the complexes in solution.
absence of a metal or another coordinating species. While our
calculations have been carried out in the gas phase, it seems
likely that the similar qualitative results will pertain in solution,
that it will be more favourable for terdentate ligands with cen-
tral triazine rings to form the cc conformation necessary for
metal complexation than those with a central pyridine ring.
We then investigated the possible structures of [HL4]ϩ cations.
Given that N(11) and N(31) are equivalent as indeed are N(25)
and N(23) in cc and tt conformations we constructed 14 differ-
ent structures, combinations of the nitrogen to be protonated
and the three possible conformations. The cc conformation
with N(21) protonated was ca. 10 kcal molϪ1 lower in energy
than all the others. The relative stability of this configuration is
probably due to a combination of factors, N(21) is the preferred
nitrogen to be protonated, and also it can form weak hydrogen
bonds to the ortho nitrogen atoms in the terminal pyridine
rings. In addition, there are no steric repulsions owing to
adjacent ortho hydrogen atoms. The next most favourable con-
figuration is the cc conformation with N(11) protonated but
this only forms one weak intramolecular hydrogen bond. The ct
conformation with N(21) protonated is disfavoured owing to
repulsion between ortho hydrogen atoms on N(21) and C(33). It
is interesting that in this case (and in others) the presence of two
mutually ortho-hydrogen atoms leads to a twist in the rings of
ca. 33–46Њ leading to a loss of conjugation. The highest energies
(>44 kcal molϪ1) occur with N(27) protonated even though this
does not lead to increased steric repulsions so that these ener-
gies must be due to the unfavourable nature of protonation at
that atom.
It is interesting to note that 2,6-bis(5,6-dipropyl-1,2,4-triazin-
3-yl)pyridine which provides a separation ratio for An/Ln of
greater than 100,16 when used without a synergist, forms the 1
: 3 complex in the presence of nitrate with the smaller lanth-
anides (Sm–Lu)17 but not for the larger lanthanides (La–Sm).18
This suggests that ligands that always form 1 : 1 complexes are
not likely to give high separation ratios in the absence of a
synergist. While we have only determined crystal structures
with Yb and not the other lanthanides, all indications from
previously obtained structural information2,4,15 is that when Yb
forms 1 : 1 complexes with particular ligands then so do the
other lanthanides and therefore we conclude that these ligands
do not form 1 : 3 complexes with any lanthanide. Another fac-
tor that may lessen the usefulness of L4 and derivatives is that
they always form hydrogen bonds with either solvent molecules
3ϩ
or other metal complexes, a feature not present in the ML3
complex of 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine,
which has a hydrophobic exterior. The propensity for hydrogen
bond formation as indicated by Table 3 is widespread in all the
crystal structures for ligands and metal complexes alike and
the aggregation of molecules may occur in solution to prevent
efficient extraction.
Theoretical analysis of L4, [HL4]؉ and [H2L4]2؉
Results are summarised in Table 5 for L4, [HL4]ϩ and [H2L4]2ϩ.
For the neutral ligand L4, the relative energies of the cc, ct and
tt form were 2.36, 0.00, and 0.49 kcal molϪ1 respectively. This
result contrasts with that found for terpyridine where the rel-
ative energies of the three forms were 12.55, 6.90, and 0.00 kcal
molϪ1 respectively.6 There is a much smaller difference in energy
in L4 for the three conformers compared to terpyridine because
there is no possible clash of ortho hydrogen atoms as the central
ring is a triazine rather than a pyridine. It can be argued that the
differences between the energies of the three conformers in L4
are less than packing effects, which may account for the occur-
rence of the ct and tt forms in the crystal structures of L5 and L7
respectively (Figs. 7 and 6). It is likely that these conformational
preferences in L5 and L7 will be comparable to those of L4. This
is in contrast to terpyridine where the energy differences
between conformers are much more significant such that only
the tt form is likely to be observed. In general it can be noted
from the Cambridge Crystallographic Database that all poly-
pyridine structures exhibit the trans conformation in the
Results for [H2L4]2ϩ are less clear cut and show three con-
figurations with comparable low energies within 5.4 kcal molϪ1,
all with N(11) and N(31) protonated. The lowest energy con-
figuration has the tt conformation in which there are two weak
hydrogen bonds formed between N(11)–H ؒ ؒ ؒ N(25) and
N(31)–H ؒ ؒ ؒ N(23) and no ortho ؒ ؒ ؒ ortho interactions. This is
the structure found in the crystal structure of [H2L4]ؒ2Clؒ
2.5H2O (Figs. 4 and 5) where the configuration is further stabil-
ized by intermolecular hydrogen bonds. The other conform-
ations ct and cc also have low energies. With a cis conformation
the pyridine nitrogen atom N(11) and/or N(31) can form
hydrogen bonds to the central nitrogen atom N(21) but again
there are no undesirable ortho ؒ ؒ ؒ ortho interactions. All other
configurations have an energy of 13.8 kcal molϪ1 greater than
the minimum. There are several features that can be discerned
from the energy distribution. Unlike in [HL4]ϩ protonation of
N(21) is not favourable, because the second proton, wherever
J. Chem. Soc., Dalton Trans., 2002, 55–62
61

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Table 5 Results from quantum mechanics calculations on L4, L2, [HL4]ϩ and [H2L4]2ϩ. Energies are given in kcal molϪ1 relative to the lowest energy
configuration for each type
Conformation
Protonated nitrogen
cc
2.36
ct
0.00
tt
0.49
L4
L2
12.55
9.77
6.90
0.00
[HL4]ϩ
N(11)
N(31)
N(21)
N(25)
N(23)
N(27)
14.67
12.22
10.13
27.07
14.05
46.15
14.55
᎐N(11)
᎐N(11)
᎐
᎐
0.00
22.19
14.55
28.98
᎐N(25)
᎐N(25)
᎐
᎐
51.10
44.38
[H2L4]2ϩ
N(11), N(21)
N(11), N(31)
N(11), N(25)
N(11), N(23)
N(11), N(27)
N(21), N(23)
N(21), N(25)
N(21), N(27)
N(21), N(31)
N(25), N(31)
N(23), N(25)
N(23), N(27)
24.35
5.37
33.99
24.55
41.70
40.53
1.79
34.20
0.00
45.83
13.82
45.70
39.75
14.18
44.15
34.00
50.57
60.49
18.52
26.33
51.03
91.29
᎐N(21), N(25)
᎐N(21), N(25)
᎐
᎐
37.82
50.98
49.71
73.44
᎐N(11), N(21)
᎐N(11), N(21)
᎐
᎐
᎐N(11), N(23)
᎐N(11), N(23)
᎐
᎐
69.04
35.82
᎐N(25), N(27)
᎐
᎐N(25), N(27)
᎐
placed, always leads to ortho ؒ ؒ ؒ ortho intramolecular inter-
actions unless it is situated on N(27) which is an unfavourable
site. Protonation on N(25) {or N(23)} is favourable in the trans
conformation because it gives rise to N(25)–H ؒ ؒ ؒ N(11) or
{N(23)–H ؒ ؒ ؒ N(31)} interactions but not in the cis conform-
ation because of ortho ؒ ؒ ؒ ortho interactions which lead to
rotation of the rings. An intermediate situation arises in con-
figurations such as the cis conformation with N(11) protonated
where there is a weak hydrogen bond between N(11)–H and
N(21). However, a repulsion between N(25)–H and C(13)–H,
which is resolved via an intermediate rotation of 20Њ compared
to the ca. 40Њ, found where there is no such hydrogen bond
formation. We can draw the overall conclusion that the order of
preference for protonation is N(11) > N(21) > N(25) > N(27)
but that this order can be varied according to which other
nitrogen atoms are protonated and the presence of solvent
which can lead to intramolecular hydrogen bonds. The energy
barrier to rotation between cis and trans in L4 with a central
triazine ring is far less than that observed with a central pyridine
ring as in terpy where ortho ؒ ؒ ؒ ortho hydrogen interactions
provide a high energy barrier.
like to thank the EPSRC and the University of Reading for
funding of the image-plate system. The use of the Origin 2000
at the University of Reading High Performance Computer
Centre (HPCC) is gratefully acknowledged.
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Conclusion
The experimental extraction studies indicate that these new
amide ligands have selectivities towards Am() that are com-
parable to that of ligand L2. Furthermore, since they can be
synthesised on a large scale much more easily than ligand L2,
they are promising reagents in industrial solvent extraction for
the separation of An() and Ln(). The solid state studies
show that these ligands form 1 : 1 complexes with the lanth-
anides of a similar type to those formed with the parent L4.
These ligands however show a propensity to form inter-
molecular hydrogen bonds through the amide groups which
may prevent more efficient extraction. These ligands do not
form the 1 : 3 complexes found with 2,6-bis(5,6-dipropyl-1,2,4-
triazin-3-yl)pyridine. The formation of this stoichiometry in a
lanthanide complex might well be indicative of high separation
factors as this ligand has a value for An/Ln of greater than 100.
14 F. H. Beijer, R. P. Sijbesma, J. A. J. M. Vekemans, E. W. Meijer,
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18 M. G. B. Drew, D. Guillaneux, M. J. Hudson, P. B. Iveson and
C. Madic, Inorg. Chem. Commun., 2001, 4, 462.
Acknowledgements
We are grateful for the financial support by the European
Union Nuclear Fission Safety Programme Task 2 (Contracts
F141-CT-96-0010 and F1KW-CT2000-00087). We would also
62
J. Chem. Soc., Dalton Trans., 2002, 55–62