Solvent extraction of metal ions from nitric acid solution using N,N′-substituted malonamides. Experimental and crystallographic evidence for two mechanisms of extraction, metal complexation and ion-pair formation

Article abstract of DOI:10.1039/a605577j

The solvent extraction of actinides including Am^III and Cm^III together with some trivalent lanthanides from nitric acid solutions by two newly synthesized malonamides, N,N′-dimethyl-N,N′-diphenyltetradecylmalonamide (dmptdma) and N,N′-dicyclohexyl-N,N′-dimethyltetradecylmalonamide (dcmtdma) has been investigated and : compared with data for the reference malonamide, N,N′-dibutyl-N,N′-dimethyloctadecylmalonamide (dbmocma). The dependence of the extraction on the nitric acid and malonamide concentrations together with the probable molecular structure of the extraction species from nitric acid solution suggests that there are two principal mechanisms of extraction. For low nitric acid concentrations (up to 1 mol dm^-3) a co-ordinative mechanism dominates for the extraction of metal cations, whereas at higher nitric acid concentrations (1-14 mol dm^-3) an ion-pair mechanism involving the mono- or di-protonated malonamide and the metal anions [M(NO₃)₄]^- or [M(NO₃)₅]^2- appears to be more important. Crystal structures show that in the protonated, unalkylated species Hdcmma ⁺ (dcmma = N,N′-dicyclohexyl-N,N′-dimethylmalonamide) and in the chelated complexes [Nd(NO₃)₃(dcmma)₂], [Nd(NO₃)₃(H₂O)2(dmpma)] and [Yb(NO₃)₃(H₂O)(dmpma)] (dmpma = N,N′-dimethyl-N,N′-diphenylmalonamide) the carbonyl oxygens lie cis to each other suggesting that it is the cis form which is involved in extraction. However, crystal structures of the free unalkylated malonamides N,N′-dicyclohexyl-N,N′-diethylmalonamide and N,N′-dicyclohexyl-N,N′-diisopropylmalonamide show that the carbonyl amide groups adopt a trans configuration in which the carbonyl oxygens are at maximum separation. By contrast, in the crystal structure of the diphenyl derivative dmpma the carbonylamide groups adopt a gauche configuration with an O=C ... C=O torsion angle of 57.2°. Conformational analysis confirms that the differences in these structures reflect the differences between the lowest-energy gas-phase conformations and are not caused by packing effects.

Full text of DOI:10.1039/a605577j

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Solvent extraction of metal ions from nitric acid solution using
N,NЈ-substituted malonamides. Experimental and crystallographic
evidence for two mechanisms of extraction, metal complexation and
ion-pair formation†
Gabriel Y. S. Chan,a Michael G. B. Drew,*,a Michael J. Hudson,*,a Peter B. Iveson,a
Jan-Olov Liljenzin,b Mats Skålberg,b Lena Spjuthb and Charles Madicc
a
Department of Chemistry, University of Reading, PO Box 224, Whiteknights,
Berkshire RG6 6AD, UK
b Department of Nuclear Chemistry, Chalmers University of Technology, S-412 96 Göteborg,
Sweden
c Commissariat à l’Energie Atomique, Bat. 399, B.P. 171, 30207 Bagnols-sur-Cèze, Cedex, France
The solvent extraction of actinides including AmIII and CmIII together with some trivalent lanthanides from nitric
acid solutions by two newly synthesized malonamides, N,NЈ-dimethyl-N,NЈ-diphenyltetradecylmalonamide
(dmptdma) and N,NЈ-dicyclohexyl-N,NЈ-dimethyltetradecylmalonamide (dcmtdma) has been investigated and
compared with data for the reference malonamide, N,NЈ-dibutyl-N,NЈ-dimethyloctadecylmalonamide
(dbmocma). The dependence of the extraction on the nitric acid and malonamide concentrations together with
the probable molecular structure of the extraction species from nitric acid solution suggests that there are two
principal mechanisms of extraction. For low nitric acid concentrations (up to 1 mol dmϪ3) a co-ordinative
mechanism dominates for the extraction of metal cations, whereas at higher nitric acid concentrations (1–14 mol
dmϪ3) an ion-pair mechanism involving the mono- or di-protonated malonamide and the metal anions
[M(NO3)4]Ϫ or [M(NO3)5]2Ϫ appears to be more important. Crystal structures show that in the protonated,
unalkylated species Hdcmma + (dcmma = N,NЈ-dicyclohexyl-N,NЈ-dimethylmalonamide) and in the chelated
complexes [Nd(NO3)3(dcmma)2], [Nd(NO3)3(H2O)2(dmpma)] and [Yb(NO3)3(H2O)(dmpma)] (dmpma = N,NЈ-
dimethyl-N,NЈ-diphenylmalonamide) the carbonyl oxygens lie cis to each other suggesting that it is the cis form
which is involved in extraction. However, crystal structures of the free unalkylated malonamides N,NЈ-
dicyclohexyl-N,NЈ-diethylmalonamide and N,NЈ-dicyclohexyl-N,NЈ-diisopropylmalonamide show that the
carbonyl amide groups adopt a trans configuration in which the carbonyl oxygens are at maximum separation. By
contrast, in the crystal structure of the diphenyl derivative dmpma the carbonyl amide groups adopt a gauche
configuration with an O᎐C ؒ ؒ ؒ C᎐O torsion angle of 57.2Њ. Conformational analysis confirms that the differences
᎐
᎐
in these structures reflect the differences between the lowest-energy gas-phase conformations and are not caused
by packing effects.
One of the aims in nuclear reprocessing is the conversion or
transmutation of the long-lived minor actinides such as ameri-
cium into short-lived isotopes by irradiation with neutrons.1,2 In
order to achieve this transmutation it is necessary to separate
the trivalent minor actinides from the trivalent lanthanides by
solvent extraction because otherwise the lanthanides absorb
neutrons effectively and hence prevent neutron capture by the
transmutable actinides.3,4 One option is to coextract the trival-
ent actinides and lanthanides from the remaining aqueous
waste prior to their separation from each other. Malonamides
of the type (R1R2NCO)2CHR3 have been suggested as possible
coextractants,5 instead of the traditional tributyl phosphate,6,7
(BuO)3PO or (N,N-diisobutylcarbamoylmethyl)octyl(phenyl)-
phosphine oxide.8,9 The latter are phosphorus-containing
extractants which ultimately create undesirable waste even after
incineration. Malonamides, on the other hand, contain only the
elements carbon, oxygen, hydrogen and nitrogen and hence are
completely incinerable extractants. These compounds are weak
bases in which the carbonyl amide groups may bind to metal
ions in a chelate complex.10 In strong acidic solutions the car-
bonyl amide groups may be protonated to form intramolecu-
larly hydrogen-bonded species such as [H(R1R2NCO)2CHR3]+
or [H2(R1R2NCO)2CHR3]2+. The chemical properties of the
malonamides are influenced by the nature of the substituents
on the nitrogen and the central methylene carbon atom. The
choice of R1 and R2 groups is important. When R1 is small (e.g.
methyl) this limits the steric hindrance for metal-ion co-
ordination; R2 is large so that the reagents will dissolve in the
organic phase.5 Owing to the high aqueous solubility of
malonamides, enhanced extraction into the organic phase can
be achieved when the methylene carbon is bound to a hydro-
phobic group (R3) with fourteen or more carbon atoms.11,12 The
presence of this hydrophobic group enhances the solubilities of
both the free ligand and the metal complexes or ion pairs in
the organic phase.
In order to rationalise some of the above observations, two
new malonamides were investigated and compared with the
malonamide,
N,N Ј-dibutyl-N,NЈ-dimethyloctadecylmalon-
amide (dbmodma), which may be regarded as a reference mole-
cule. The malonamides studied all have an N-methyl group
plus one other group, either cyclohexyl or phenyl, attached to
the nitrogen atoms. For both dbmodma and the N,N Ј-
dicyclohexyl-N,N Ј-dimethyltetradecylmalonamide (dcmtdma)
derivatives, the N-alkyl groups are rather bulky and non-planar,
whereas in N,N Ј-dimethyl-N,N Ј-diphenyltetradecylmalon-
amide (dmdptdma) the planar phenyl group is less bulky and
could lie close to the long hydrophobic alkyl chain. The bulky
N-butyl and N-cyclohexyl groups possibly sterically hinder the
chelation of the malonamide to the metal or the formation of
† Non-SI units employed: eV ≈ 1.60 × 10Ϫ19 J, hartree ≈ 4.36 × 10Ϫ18 J,
cal = 4.184 J.
J. Chem. Soc., Dalton Trans., 1997, Pages 649–660
649

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ion pairs. Alternatively, these malonamides can change their
O᎐C ؒ ؒ ؒ C᎐O torsion angle and the O ؒ ؒ ؒ O distance between
O
O
O
O
᎐
᎐
the carbonyl oxygen atoms; these conformational changes can
N
N
N
N
be related to the extraction properties of the malonamides.
C2H5
dcema
C2H5
CH(CH3)2
dcima
CH(CH3)2
N,N Ј-Dicyclohexyl-N,N Ј-dimethylmalonamide
(dcmma),
N,N Ј-dimethyl-N,N Ј-diphenylmalonamide (dmpma), N,N Ј-
dicyclohexyl-N,N Ј-diethylmalonamide (dcema) and N,N Ј-
dicyclohexyl-N,N Ј-diisopropylmalonamide (dcima) were syn-
thesized and all shown (by NMR spectroscopy) to be con-
formationally flexible. In order for chelative complexation
with metal ions to occur there must be a reorientation of the
carbonyl groups from the trans to cis conformer. A similar
rearrangement is also required once the malonamide is proto-
nated, such that the proton is shared between the carbonyl
groups of the same molecule. In order to establish the possible
structures of species involved in the separation processes, the
crystal structures of the free malonamides dcema, dcima and
dmpma together with that of the protonated form Hdcmma+
and also lanthanide complexes of the formulae [Nd(NO3)3(dc-
mma)2], [Nd(NO3)3(dmpma)]ؒ2H2O and [Yb(NO3)3(dmp-
ma)]ؒH2O were determined.
HN
CH(CH3)2
NH
C2H5
O
O
CH3O
OCH3
NH
NH
CH3
CH3
O
O
O
O
N
N
N
N
H3C
CH3
H3C
CH3
Experimental
dmpma
dcmma
All starting chemicals for the syntheses were obtained from
Aldrich and used without further purification. Microanalyses
were carried out by Medac Ltd., Brunel University. Infrared
spectra (4000–350 cmϪ1) were recorded on a Perkin-Elmer
1720-X FT-IR spectrometer with an IRDM data-management
system, 1H (400 MHz) NMR spectra on a JEOL JNM-EX 400
spectrometer using either CD3OD (unalkylated malonamides)
or CDCl3 (alkylated malonamides) as solvent.
Scheme 1 Synthesis of the tetrasubstituted malonamides
refluxed until the pH became neutral (ca. 3 d). Toluene was
removed under vacuum and a yellow solid was formed. This
was dried in an oven, giving pure dcmtdma (60 g, 95%). The
compound dmptdma was prepared in the same way. Again the
characterisation details are summarised in Tables 1 and 2 and
the structural formulae are shown in Scheme 2.
Syntheses
N,NЈ-Tetrasubstituted malonamides. There are a number of
routes for the synthesis of malonamides. These methods
include the reaction of secondary amines with acid esters (I),
with acid anhydrides (II) and with acyl chlorides (III).13 In this
study, we adopted route (I) for the synthesis of N,N Ј-
tetrasubstituted malonamides, as this was found to give the
highest yields.
N,NЈ-Dicyclohexyl-N,NЈ-dimethylmalonamide (dcmma). This
synthesis is given as an example of the method used. N-
Methylcyclohexylamine (300 g, 2.65 mol) was initially mixed
with dimethyl malonate (162 g, 1.23 mol) then heated at 75 ЊC
for 4 h. Throughout this time the extent of reaction was moni-
tored by Fourier-transform IR spectroscopy: the absorption at
1750 cmϪ1 due to the ester carbonyl was replaced by a band at
about 1640 cmϪ1 assigned to an amide structure.13 A white solid
formed on cooling. This was washed with diethyl ether (2 × 100
cm3) and recrystallised from ethyl acetate to give pure dcmma
(216 g, 60 %). The other three N,N Ј-tetrasubstituted malon-
amides were prepared in the same way. The NMR data are
shown in Table 2 and the elemental analyses and melting points
in Table 1. The structural formula of each malonamide is
shown in Scheme 1.
Solvent extraction procedure
Nitric acid solutions were pre-equilibrated with the tracer solu-
tion before distribution ratio determination. The pentaalkyl-
malonamides were vigorously shaken with the aqueous phase
for 5 min in order to enable equilibrium to be reached. After
phase disengagement by centrifugation, aliquots from each
phase were removed for radiometric analysis. The γ energies at
59.6, 122, 185 and 879 keV for 241Am, 152Eu, 235U and 160Tb
respectively were measured using a solid-state HPGe detector.
For the α and β emitters (i.e. 244Cm and 234Th) an LKB Wallac
1219 Rackbeta liquid scintillation counter was used. All
experiments were performed with tert-butylbenzene as diluent.
The distributions of nitric acid between the aqueous and
organic phases were determined by potentiometric titration
using a Mettler DL20 Compact Titrator. The titrations were
performed in ethanolic media to facilitate mixing of the
solutions.
Preparation of crystals
Crystals of dcema were prepared by dissolving the compound
(2 g) in hot ethanol (50 cm3) and allowing the solution to cool
slowly to room temperature (yield 70–75%). Those of dcima
were prepared in a similar way, except that the compound (2 g)
was initially dissolved in ethanol–diethyl ether (1:1, 75 cm3)
(yield 80%). A suitable crystal of dmpma was obtained directly
on slow cooling of the reaction mixture. Crystals of Hdcmma +
were prepared by dissolving dcmma (0.5 g, 1.7 × 10Ϫ3 mol) in
5 mol dmϪ3 HCl (25 cm3). Cobalt() chloride hexahydrate
(0.54 g, 2.27 × 10Ϫ3 mol) was added to the acidic solution. The
blue solution formed was then left in a thermostatted bath at
28 ЊC. Blue crystals were formed after 24 h (yield 70%) (Found:
C, 51.3; H, 7.7; N, 7.3. C34H62Cl4CoN4O4 requires C, 51.6; H,
7.9; N, 7.1%). The neodymium complex of dcmma was pre-
pared as follows: a solution of dcmma (0.5 g, 1.7 × 10Ϫ3 mol) in
Pentaalkylmalonamides. N,N Ј-Dibutyl-N,N Ј-dimethylocta-
decylmalonamide (dbmodma) was supplied by Synthelec AB,
Sweden while the other pentaalkylmalonamides were syn-
thesized at the University of Reading, UK.
N,NЈ-Dicyclohexyl-N,NЈ-dimethyltetradecylmalonamide
(dcmtdma). This synthesis is given as an example of the method
used. Sodium hydride (5.23 g of a 60% dispersion in mineral oil,
0.13 mol) was washed with light petroleum (b.p. 60–80 ЊC) and
suspended in toluene (100 cm3). The compound dcmma (38.22
g, 0.13 mol) was added and the mixture refluxed until hydrogen
evolution had ceased (ca. 2 h). 1-Bromotetradecane (36.33 g,
0.13 mol) in toluene (80 cm3) was added and the mixture
650
J. Chem. Soc., Dalton Trans., 1997, Pages 649–660

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O
O
O
O
N
N
N
N
H3C
CH3
H3C
CH3
dcmma
NaH, CH3(CH2)13Br
~
Toluene, 135°C, 3d
~
dcmtdma
O
O
O
O
O
O
N
N
N
N
C4H9
H3C
C4H9
CH3
H3C
CH3
H3C
CH3
N
N
dmpma
dmptdma
dbmodma
Scheme 2 Synthesis of dcmtdma and dmptdma. The structure of dbmodma is also shown
EtOH (10 cm3) was added slowly to a stirred solution of
Nd(NO3)3ؒ6H2O (1.12 g, 2.6 × 10Ϫ3 mol) in EtOH (10 cm3).
The slightly purple solution was allowed slowly to evaporate
and crystals were deposited after several days at room tempera-
ture (yield 80%). The corresponding complex of dmpma was
prepared by adding Nd(NO3)3ؒ 6H2O (0.039 g, 0.09 mmol) dis-
solved in propan-2-ol (2 cm3) to a stirred solution containing
dmpma (0.05 g, 0.18 mmol) in absolute ethanol (1 cm3) and
propan-2-ol (2 cm3). The final solution was allowed slowly to
evaporate at room temperature and crystals were deposited
after 2 d. These were filtered off, washed with a small amount
of propan-2-ol and then dried in an oven at 90 ЊC for 5 h
(Found: C, 30.4, H, 3.4; N, 10.7. C17H24N5NdO14 requires C,
30.6; H, 3.6; N, 10.5%). The corresponding ytterbium complex
was prepared by slow addition of a propan-2-ol solution
(5 cm3) containing Yb(NO3)3ؒ5H2O (0.04 g, 0.09 mmol) to a
rapidly stirred propan-2-ol solution (5 cm3) of dmpma (0.05 g,
0.18 mmol). Crystals of [Yb(NO3)3(dmpma)]ؒH2O suitable for
X-ray analysis were obtained after leaving the solution to stand
for 1 d at room temperature (yield 40%).
details. Data were collected at 293(2) K with Mo-Kα radiation
(λ 0.710 73 Å) using the MARresearch image-plate system.
Each crystal was positioned at 75 mm from the plate. Ninety-five
frames were measured at 2Њ intervals with a counting time of 2
min. Data analysis was carried out with the XDS program.14
The structures of the four malonamides were solved using dir-
ect methods with the SHELXS 86 program,15 those of the three
complexes by heavy-atom methods. In all seven structures the
non-hydrogen atoms were refined with anisotropic thermal
parameters. In all except Hdcmma+ all hydrogen atoms were
included in geometric positions. In Hdcmma+ all hydrogen
atoms bonded to carbon were included in geometric positions,
but the two hydrogen atoms bonded to O(1A) and O(1B) were
located in a Fourier-difference map and included with O᎐H
constraints set at 0.95 Å. Also the [CoCl4]2Ϫ anion suffered from
severe anisotropy but a suitable disordered model could not be
found.* In [Nd(NO3)3(dmpma)]ؒ2H2O there were two molecules
in the asymmetric unit. All structures were then refined using
* We also carried out a structure determination of the isomorphous
copper complex: a = 13.650(8), b = 23.736(12), c = 14.562(11) Å,
β = 112.42(1)Њ, U = 4361.4 Å, monoclinic, space group P21/a. It is not of
sufficient quality to report here, but contains protonated malonamide
cations and [CuCl4]2Ϫ anions.
Crystallography
Crystal data are given in Table 8, together with refinement
J. Chem. Soc., Dalton Trans., 1997, Pages 649–660
651

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Table 1 Melting points and elemental analysis data for malonamides
Analysis (%)
Calculated
C
Malonamide
M.p./ ЊC
C
H
N
H
N
dcema
dcima
dcmma
dmpma
dcmtdma
dmptdma
121–122
147–148
117–118
105–106
53–54
70.7
71.9
69.4
72.2
75.8
77.8
10.6
10.9
10.3
6.5
11.9
9.5
8.7
7.8
9.5
9.9
5.6
5.7
70.8
71.9
69.4
72.3
75.9
77.8
10.6
10.9
10.3
6.4
11.9
9.7
8.7
7.9
9.5
9.9
5.7
5.8
48–49
Table 2 Proton NMR assignments for malonamide
either dbmodma or dcmtdma, both of which reach maxima at
ca. 7 mol dmϪ3 HNO3. It is believed that when the nitric acid
concentrations are below 1 mol dmϪ3 the malonamide species
(L) exists in the unprotonated form. Between acid concentra-
tions of 1 and 6 mol dmϪ3 the dominant species is the mono-
protonated HL+NO3Ϫ, while the diprotonated form H2L2+
Malonamide Structural formula
1H NMR (δ)
dcema
1.12–1.18 (6 H, t, a)
1.3–1.9 (20 H, m, b)
3.26–3.38 (4 H, q, c)
3.44–3.52 (2 H, s, d)
4.22–4.35 (2 H, q, e)
b
b
O
O
O
e
e
N
N
c
d
a
a
c
Ϫ
2NO3 is then thought to predominate above 7 mol dmϪ3
CH2CH3
CH2CH3
HNO3.17 The extraction of nitric acid supports this observation
(Fig. 2). The extraction data in Fig. 1 indicate a point of inflec-
tion at ca. 1 mol dmϪ3 HNO3, suggesting a change in extraction
mechanism, which may be related to the malonamide species.
For trivalent metal ions, from 1 to 6 mol dmϪ3 nitric acid the
dominating species could be HL+M(NO3)4Ϫ. When the nitric
acid concentration increases to 7 mmol dmϪ3 and above the
extracting species becomes the diprotonated H2L2+. After L is
diprotonated the extracted anion could be in the form of two
b
b
O
dcima
1.15–1.19 (12 H, d, a)
1.35–1.85 (20 H, m, b)
3.39–3.44 (2 H, s, c)
3.72–3.82 (2 H, h, d)
4.24–4.40 (2 H, q, e)
e
e
N
N
c
d
a
d
a
CH(CH3)2 CH(CH3)2
a
a
O
O
O
O
O
dcmma
1.29–1.9 (20 H, m, a)
2.8–2.9 (6 H, s, b)
3.4–3.5 (2 H, s, c)
d
d
N
N
N
N
N
c
b
b
H3C
CH3
4.25–4.45 (2 H, q, d)
Ϫ
2Ϫ
M(NO3)4 anions or one M(NO3)5 dianion. In order to
extract these anions the protonated carbonyl groups must be
available and it would appear that the diphenyl derivative can
be protonated more readily than the other two malonamides.
The different extraction mechanisms for trivalent metal ions by
malonamides can thus be described as in equations (1)–(3)
(L = malonamide).
d
d
c
c
dmpma
d
d
d
d
2.9–3.0 (2 H, s, a)
3.08–3.15 (6 H, s, b)
6.9–7.1 (4 H, d, c)
7.35–7.45 (6 H, t, d)
c
c
a
H3C
CH3
b
b
dcmtdma
O
0.8–0.9 (3 H, t, a)
1.2–1.8 (46 H, m, b)
2.65–2.82 (6 H, s, c)
3.4–3.6 (1 H, t, d)
4.3–4.4 (2 H, q, e)
e
e
d
Co-ordination mechanism
N
c
c
b
H3C
CH3
CH2
b
M3+ + 3NO3Ϫ + nL
M(NO3)3Ln
(р1 mol dmϪ3 HNO3) (1)
(CH2)12
a
CH3
g
g
Ion-pair mechanism
g
g
g
g
dmptdma
f
f
0.86–0.90 (3 H, t, a)
1.00–1.30 (24 H, m, b)
1.66–1.72 (2 H, q, c)
3.18 (6 H, s, d)
O
O
e
Ϫ
Ϫ
HL+ + M(NO3)4
HL+M(NO3)4
N
N
f
f
d
d
c
(1 р [HNO3] р 6 mol dmϪ3
)
(2)
H3C
CH3
CH2
3.23–3.27 (1 H, t, e)
6.70 (4 H, m, f)
7.34 (6 H, m, g)
b
(CH2)12
CH3a
2Ϫ
2Ϫ
H2L2+ + M(NO3)5
H2L2+M(NO3)5
(у6 mol dmϪ3 HNO3) (3)
SHELXL 93.16 All calculations were carried out on a Silicon
Graphics R4000 Workstation at the University of Reading.
Atomic coordinates, thermal parameters and bond lengths
and angles have been deposited at the Cambridge Crystallo-
graphic Data Centre (CCDC). See Instructions for Authors,
J. Chem. Soc., Dalton Trans., 1997, Issue 1. Any request to the
CCDC for this material should quote the full literature citation
and the reference number 186/333.
Determination of the number of malonamide molecules
The number of malonamide molecules in the extracted species
can be determined if the distribution ratio (D) for the metal
extraction at a constant pH is plotted as a function of malon-
amide concentration; the slope is approximately equal to the
number of malonamide molecules. (The distribution ratio is
defined as the concentration of the metal species in the organic
phase divided by its concentration in the aqueous phase.) Table
3 shows that there are between two and four malonamide mole-
cules per metal ion, consistent with a co-ordination mechan-
ism but not with an ion-pair mechanism. The discrepancy
arising from conventional slope analysis often results in non-
integral slopes for amide extractants. This is possibly due to the
non-ideality of the organic solution or the presence of extra
amide molecules in the second co-ordination sphere.5 The result
also reflects the possible aggregation of L/HNO3 solvates in the
organic phase (micelle formation).18
Results and Discussion
Metal extraction
Extraction data for some trivalent metals were determined with
dmptdma and dcmtdma and compared with the data for ura-
nium() and thorium() and the same trivalent metals
extracted by dbmodma, Fig. 1. The extraction of metals at
high nitric acid concentration (>7 mol dmϪ3 HNO3) by dmp-
tdma showed a constant increase which cannot be seen with
652
J. Chem. Soc., Dalton Trans., 1997, Pages 649–660

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Fig. 1 Plots of the distribution coefficient D vs. nitric acid concentration for extractions by dcmtdma (a), dbmodma (b) and dmptdma (c)
Nitric acid extraction
extractants; when the nitric acid concentration reaches 7 mol
dmϪ3 the ratio is between one and two. These values suggested
that, in 5 mol dmϪ3 nitric acid, the dominating species is the
The extent of nitric acid extraction does not show any signifi-
cant difference between the two extractants dmptdma and
dbmodma (Fig. 2). The number of nitric acid molecules
extracted per malonamide increases uniformly with the nitric
acid concentration. At 6 mol dmϪ3 HNO3 the ratio between
HNO3 and the malonamides is approximately one for both
monoprotonated form, HL+NO3Ϫ, but in 7 mol dmϪ3 nitric
2Ϫ
acid the diprotonated form H2L2+(NO3)2
predominates.
These observations support the existence of an ion-pair
mechanism involving either the mono- or di-protonated
forms.
J. Chem. Soc., Dalton Trans., 1997, Pages 649–660
653

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Table 3 Results of slope analysis for Am at 0.2–1 mol dmϪ3 malonamide
Malonamide
[HNO3]/mol dmϪ3
Slope
r
[HNO3]/mol dmϪ3
Slope
r
dbmodma
dmptdma
dcmtdma
0.3
0.5
1
2.5
2.6
3.6
0.9940
0.9835
0.9916
3
2
7.5
2.3
2.7
2.2
0.9937
0.9992
0.9979
Fig. 2 Extraction of nitric acid by 0.1 mol dmϪ3 dbmodma in tert-
butylbenzene (᭡), by 0.1 mol dmϪ3 dmptdma in tert-butylbenzene (᭹)
(both continuous lines, left scale) and the average number of HNO3
molecules extracted per dbmodma molecule (᭛) and per dmptdma
molecule (ᮀ) (both dotted lines, right scale)
Fig. 4 Crystal structure of dcima showing the atomic numbering
scheme
Fig. 3 Crystal structure of dcema showing the atomic numbering
scheme
Fig. 5 Crystal structure of dmpma showing the atomic numbering
scheme
Crystal structures
The structures are shown in Figs. 3–9 together with the atomic
numbering schemes. The malonamide moieties are numbered in
identical fashion in the seven structures.
Ligand conformations. The conformation of the malon-
amides can be described by the torsion angles listed in Table 4.
These are the four torsion angles along the backbone from one
acyclic carbon atom to the other [e.g. C(acyclic)᎐N᎐C᎐C,
N᎐C᎐C᎐C, C᎐C᎐C᎐N and C᎐C᎐N᎐C (acyclic)]. In addition, the
O(1)᎐C(2) ؒ ؒ ؒ C(4)᎐O(5) torsion angles and O(1) ؒ ؒ ؒ O(5) dis-
tances are listed. The conformations of dcema and dcima are
very similar with a tggt (t = trans, g = gauche) pattern along the
Fig. 6 Crystal structure of Hdcmma+ showing the atomic numbering
scheme. There are two cations in the asymmetric unit with similar
geometries. Only one cation is shown. The hydrogen bond is shown as a
dotted line
backbone and an overall trans conformation for O᎐C ؒ ؒ ؒ C᎐O
᎐
᎐
with the torsion angle being close to 180Њ. Both molecules
exhibit approximate C2 symmetry with the rotation axis passing
through the central carbon atom C(3). The carbonyl group is cis
to the acyclic groups on the nitrogen and trans to the bulkier
cyclohexane group. The O ؒ ؒ ؒ O distances are 4.44 and 4.48 Å
respectively. By contrast in dmpma the backbone shows a ttgt
pattern with an overall O᎐C ؒ ؒ ؒ C᎐O torsion angle of 57.2Њ and
᎐
᎐
an O ؒ ؒ ؒ O distance of 3.29 Å. Clearly there is a major differ-
ence between the conformations with R = cyclohexyl and
R = phenyl. The compound Hdcmma+ contains two discrete
654
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Fig. 7 Crystal structure of [Nd(NO3)3(dcmma)2] showing the atomic numbering scheme
cationic units together with a [CoCl4]2Ϫ anion. The two cations
(denoted A and B) have very similar geometries. For both cat-
ions it proved possible to locate the hydrogen atom bonded
to O(1) and as is apparent from Fig. 6 this atom forms an
intramolecular hydrogen bond to O(5). The O(1) ؒ ؒ ؒ O(5),
H(1) ؒ ؒ ؒ O(5) and O(1)᎐H(1) ؒ ؒ ؒ O(5) dimensions are 2.39, 1.45
Å, 165Њ in A and 2.40, 1.56 Å, 144Њ in B. For these cations the
formation of the hydrogen bonds leads to a planar arrangement
of the backbone with O(1)᎐C(2) ؒ ؒ ؒ C(4)᎐O(5) torsion angles
of 1.9Њ in A and 2.3Њ in B. The overall backbone pattern how-
ever is cggt and as shown in Fig. 6 the alkyl groups are arranged
differently at the two ends of the malonamide. Thus at one end
the cyclohexyl group is trans to the carbonyl and at the other
the methyl group is trans. It is not clear why this should be but
it is noteworthy that this conformation is found in the two
independent cations. It is also significant that in both cations
it is the carbonyl group trans to the methyl group that is pro-
tonated rather than the other carbonyl which is trans to the
cyclohexyl group.
group to give the dimethyl analogue dcmma. For both struc-
tures many different conformations were obtained but only the
four lowest-energy ones for each are shown in Table 4. Note
that in both cases the lowest-energy conformation is similar to
that found in the crystal structures, thus dmpma has the ttgt
backbone with an O᎐C ؒ ؒ ؒ C᎐O torsion angle of Ϫ70.4Њ and
᎐
᎐
dcmma has the tggt backbone with an O᎐C ؒ ؒ ؒ C᎐O torsion
᎐
᎐
angle of 161.2Њ. It is clear from the molecular mechanics calcu-
lations that this difference is due to a large number of small
energy terms rather than a few large terms. For dmpma the
other three low-energy conformations all show much larger
O ؒ ؒ ؒ O distances. The third lowest-energy conformation has a
similar conformation to that observed in dcmma. By contrast in
dcmma the three lowest-energy conformations are tggt, tggc
and cggc, thus all having similar gauche-gauche arrangements
around the central N᎐C᎐C᎐C and C᎐C᎐C᎐N bonds but differ-
ent arrangements of the methyl and cyclohexyl groups relative
to the backbone. The fourth lowest-energy conformation is ttgt,
equivalent to that found for the lowest energy of dmpma.
To confirm these energy preferences, particularly as the
energy differences are small and the force field imperfect, we
also carried out quantum-mechanics calculations on these eight
lowest-energy conformations. Using the GAUSSIAN 94 pro-
gram,21 the geometries were those obtained from the refinement
following the conformational analysis. The 6-31* basis set was
used. The results confirm the conformational preferences found
by the molecular mechanics calculations and the energies are
listed in Table 4. The GAUSSIAN 94 program was also used to
investigate the structure of the malonamide cation found in
Hdcmma+. Single-point calculations confirm the stability of the
cation with the intramolecular hydrogen bond compared to an
alternative structure with the hydrogen atom pointing away
from the second oxygen atom which could lead to inter-
molecular hydrogen bonding. Respective energies were
Ϫ920.240 and Ϫ920.214 hartree.
A search of the Cambridge Crystallographic Database19
revealed seven structures containing the malonamide moiety
and the O᎐C ؒ ؒ ؒ C᎐O torsion angles all range between 66.2 and
᎐
᎐
115.9Њ with none being close to 180Њ (or less surprisingly 0Њ).
Some of these structures have special features like substitution
on the central carbon atom C(3) or internal hydrogen bonds
which may cause this torsion angle to have a specific value but
this is not true of all of them. While we have established a
significant difference between the cyclohexyl and phenyl struc-
tures that is consistent with the different extraction properties
of the two types of malonamide, it is not obvious why they
should have different structures. We therefore investigated
malonamide conformations using both molecular mechanics
and quantum mechanics methods.
We first used the QUANTA/CHARMm package20 for con-
formational analysis. Molecular dynamics was employed, heat-
ing the molecule up to 3000 K. The time step was 1 fs and after
equilibration 1000 structures were saved at 100 fs intervals.
These structures were then minimised and analysed. The crystal
structures of dmpma and dcema were used as starting models,
although in the latter the ethyl group was replaced by a methyl
Metal complexes. The structures of the three complexes all
contain malonamide acting as a bidentate ligand to a lanthanide
metal. Bond lengths in the metal co-ordination spheres are list-
ed in Tables 5–7. Torsion angles illustrating the malonamide
J. Chem. Soc., Dalton Trans., 1997, Pages 649–660
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656
J. Chem. Soc., Dalton Trans., 1997, Pages 649–660

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Table 5 Geometry (distances in Å, angles in Њ) of the metal co-
ordination sphere in [Nd(NO3)3(dcmma)2]
Nd᎐O(11)
Nd᎐O(21)
Nd᎐O(15)
Nd᎐O(25)
Nd᎐O(301)
2.462(10)
2.476(9)
2.490(9)
2.512(10)
2.547(13)
Nd᎐O(202)
Nd᎐O(201)
Nd᎐O(102)
Nd᎐O(103)
Nd᎐O(302)
2.573(13)
2.630(12)
2.641(13)
2.685(13)
2.82(2)
O(11)᎐Nd᎐O(21)
O(11)᎐Nd᎐O(15)
O(21)᎐Nd᎐O(15)
O(11)᎐Nd᎐O(25)
O(21)᎐Nd᎐O(25)
O(15)᎐Nd᎐O(25)
O(11)᎐Nd᎐O(301)
O(21)᎐Nd᎐O(301)
O(15)᎐Nd᎐O(301)
O(25)᎐Nd᎐O(301)
O(11)᎐Nd᎐O(202)
O(21)᎐Nd᎐O(202)
O(15)᎐Nd᎐O(202)
O(25)᎐Nd᎐O(202)
O(301)᎐Nd᎐O(202)
O(11)᎐Nd᎐O(201)
O(21)᎐Nd᎐O(201)
O(15)᎐Nd᎐O(201)
O(25)᎐Nd᎐O(201)
O(301)᎐Nd᎐O(201)
O(202)᎐Nd᎐O(201)
O(11)᎐Nd᎐O(102)
O(21)᎐Nd᎐O(102)
147.9(3)
69.7(3)
141.6(3)
81.8(3)
68.4(3)
149.7(3)
83.0(4)
75.0(4)
117.3(4)
67.1(4)
139.8(4)
71.6(4)
73.8(4)
128.6(4)
129.8(5)
130.7(4)
70.5(4)
74.6(4)
134.5(4)
84.4(5)
49.7(5)
69.9(4)
118.0(4)
O(15)᎐Nd᎐O(102)
O(25)᎐Nd᎐O(102)
O(301)᎐Nd᎐O(102)
O(202)᎐Nd᎐O(102)
O(201)᎐Nd᎐O(102)
O(11)᎐Nd᎐O(103)
O(21)᎐Nd᎐O(103)
O(15)᎐Nd᎐O(103)
O(25)᎐Nd᎐O(103)
O(301)᎐Nd᎐O(103)
O(202)᎐Nd᎐O(103)
O(201)᎐Nd᎐O(103)
O(102)᎐Nd᎐O(103)
O(11)᎐Nd᎐O(302)
O(21)᎐Nd᎐O(302)
O(15)᎐Nd᎐O(302)
O(25)᎐Nd᎐O(302)
O(301)᎐Nd᎐O(302)
O(202)᎐Nd᎐O(302)
O(201)᎐Nd᎐O(302)
O(102)᎐Nd᎐O(302)
O(103)᎐Nd᎐O(302)
74.1(4)
87.1(4)
145.1(5)
84.4(5)
130.0(5)
109.1(4)
71.3(4)
110.9(4)
68.6(4)
131.5(4)
69.0(5)
114.8(5)
46.8(4)
69.6(4)
108.9(4)
70.3(4)
109.5(4)
47.2(4)
113.0(5)
67.0(5)
133.0(4)
178.0(5)
Fig. 8 Crystal structure of [Nd(NO3)3(H2O)2(dmpma)] showing the
atomic numbering scheme. There are two molecules in the asymmetric
unit with similar geometries. Molecule A is shown
bone, one being cis and the other trans. This asymmetric con-
formation was also found in the cation Hdcmma+. However, in
the other ligand the cttc arrangement is found.
Structures [Nd(NO3)3(H2O)2(dmpma)] and [Yb(NO3)3-
(H2O)(dmpma)] are shown in Figs. 8 and 9. Both contain only
one dmpma ligand rather than two. In the case of the neo-
dymium complex, several attempts were made to prepare com-
plexes, each time using different metal:ligand ratios, but with
ratios of 3:1, 2:1, 1:1 and 1:2 the 1:1 complex precipitated in
each case (yields ca. 50%). A Fourier-transform IR study of
extracted lanthanide malonamide species in benzene suggested
that the solutions consisted of a mixture containing complexes
with both one and two malonamide ligands. The nature of
these spectra suggested, however, that both complexes were
anhydrous.24 It seems likely that if both complexes are formed in
solution the solubility of each in the chosen solvent determines
which precipitates. The complex [Nd(NO3)3(H2O)2(dmpma)]
contains ten-co-ordinated neodymium bonded to three nitrates,
the malonamide and two water molecules, while [Yb(NO3)3-
(H2O)(dmpma)] contains nine-co-ordinated ytterbium, three
nitrates, the malonamide, but only one water molecule. The dif-
ference in stoichiometry is probably due to the differing sizes of
the two metals, Nd being an early lanthanide with a larger
radius and Yb a late lanthanide with a smaller radius. There
are two molecules in the asymmetric unit for the neodymium
complex, but both have similar dimensions. In both cases the
bond lengths are approximately in the sequence Nd᎐O
(malonamide) < Nd᎐O (water) < Nd᎐O (nitrate) with distances
in the ranges 2.40(1)–2.49(1), 2.54(1)–2.59(1) and 2.58(1)–
2.69(1) Å respectively. For the ytterbium complex the same
order pertains but with much shorter distances, Yb᎐O (malon-
amide)2.27(1)–2.30(1), Yb᎐O (water)2.35(1)and Yb᎐O (nitrate)
2.40(1)–2.48(1) Å. It is interesting that, for these structures con-
taining just one malonamide ligand, the conformation is more
Fig. 9 Crystal structure of [Yb(NO3)3(H2O)(dmpma)] showing the
atomic numbering scheme
conformation are given in Table 4. The structure of [Nd-
(NO3)3(dcmma)2] is shown in Fig. 7. The metal atom is ten-co-
ordinate being bonded in a bidentate fashion to three nitrates
and two malonamides. As is apparent from the figure, the
malonamides are on opposite sides of the metal, with the
nitrate ions fitted in an equatorial girdle. The bond lengths to
the malonamide oxygen atoms at 2.46(1)–2.51(1) Å are shorter
than those to the nitrate oxygen atoms [2.55(2)–2.82(2) Å].
A
previous structure determination of [La(NO3)3(tema)2]
(tema = N,N,N,NЈ-tetraethylmalonamide) had been carried out
with R1 = R2 = Et.22 While not isomorphous, the structures are
very similar. There are two other known structures containing
lanthanides and malonamides, viz. [M(tema)4][PF6]3 (M = Sm
or Er). Here the lanthanides are eight-co-ordinated being
bonded to four bidentate ligands.23
The conformation of the malonamide ligand is somewhat
surprisingly significantly non-planar with O᎐C ؒ ؒ ؒ C᎐O tor-
᎐
᎐
sion angles of Ϫ52.1 and 51.4Њ in the two ligands. The back-
bones are tttc and cttc, but with one angle of the central two
angles differing significantly from the ideal at 136.4 and
Ϫ132.0Њ in the two ligands. It is interesting that the ligands are
also not planar in [La(NO3)3(tema)2] with torsion angles of 48.0
and Ϫ54.2Њ. Even in the tetramalonamide structures23 two of the
ligands are significantly non-planar (Ϫ32.0, Ϫ32.0Њ) while the
other two are approximately planar at 1.6 and 1.6Њ. One
unexpected feature of [Nd(NO3)3(dcmma)2] is that in one ligand
the methyl groups are in different positions relative to its back-
closely planar with O᎐C ؒ ؒ ؒ C᎐O torsion angles of (Ϫ20.5,
᎐
᎐
Ϫ20.2Њ) and 14.0Њ respectively. In both cases, however, the
arrangement of the ligands is similar in that the carbonyl
J. Chem. Soc., Dalton Trans., 1997, Pages 649–660
657

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Table 6 Bond lengths (Å) and angles (Њ) for [Nd(NO3)3(H2O)2-
(dmpma)]
Table 7 Distances (Å) and angles (Њ) in the metal co-ordination sphere
of [Yb(NO3)3(H2O)(dmpma)]
Molecule A
Nd(1)᎐O(5)
Nd(1)᎐O(1)
Nd(1)᎐O(w1)
Nd(1)᎐O(w2)
Nd(1)᎐O(71)
Nd(1)᎐O(52)
Nd(1)᎐O(51)
Nd(1)᎐O(62)
Nd(1)᎐O(72)
Nd(1)᎐O(61)
Molecule B
Yb᎐O(1)
Yb᎐O(5)
Yb᎐O(300)
Yb᎐O(62)
Yb᎐O(73)
2.274(6)
2.296(8)
2.350(8)
2.396(8)
2.447(8)
Yb᎐O(51)
Yb᎐O(61)
Yb᎐O(53)
Yb᎐O(71)
2.460(8)
2.474(9)
2.473(7)
2.481(8)
2.428(8)
2.477(8)
2.541(9)
2.580(9)
2.595(8)
2.600(9)
2.610(9)
2.617(9)
2.630(9)
2.669(9)
Nd(2)᎐O(5)
Nd(2)᎐O(1)
Nd(2)᎐O(61)
Nd(2)᎐O(w1)
Nd(2)᎐O(w2)
Nd(2)᎐O(72)
Nd(2)᎐O(51)
Nd(2)᎐O(53)
Nd(2)᎐O(62)
Nd(2)᎐O(71)
2.398(7)
2.490(8)
2.575(8)
2.581(9)
2.594(9)
2.617(9)
2.618(9)
2.637(9)
2.672(10)
2.688(8)
O(1)᎐Yb᎐O(5)
75.8(3)
77.7(3)
84.3(3)
91.4(3)
125.9(3)
144.7(3)
153.1(3)
89.7(3)
123.9(3)
78.8(3)
126.6(2)
152.7(3)
85.9(3)
73.8(3)
74.9(3)
81.7(3)
72.6(3)
152.2(3)
O(62)᎐Yb᎐O(61)
O(73)᎐Yb᎐O(61)
O(51)᎐Yb᎐O(61)
O(1)᎐Yb᎐O(53)
O(5)᎐Yb᎐O(53)
O(300)᎐Yb᎐O(53)
O(62)᎐Yb᎐O(53)
O(73)᎐Yb᎐O(53)
O(51)᎐Yb᎐O(53)
O(61)᎐Yb᎐O(53)
O(1)᎐Yb᎐O(71)
O(5)᎐Yb᎐O(71)
O(300)᎐Yb᎐O(71)
O(62)᎐Yb᎐O(71)
O(73)᎐Yb᎐O(71)
O(51)᎐Yb᎐O(71)
O(61)᎐Yb᎐O(71)
O(53)᎐Yb᎐O(71)
53.5(3)
72.3(3)
121.6(3)
73.8(3)
144.8(2)
72.3(3)
72.4(3)
125.1(3)
52.8(2)
119.4(3)
140.9(2)
77.2(3)
71.8(2)
127.5(3)
52.5(3)
75.5(3)
116.1(3)
117.7(2)
O(1)᎐Yb᎐O(300)
O(5)᎐Yb᎐O(300)
O(1)᎐Yb᎐O(62)
O(5)᎐Yb᎐O(62)
O(300)᎐Yb᎐O(62)
O(1)᎐Yb᎐O(73)
O(5)᎐Yb᎐O(73)
O(300)᎐Yb᎐O(73)
O(62)᎐Yb᎐O(73)
O(1)᎐Yb᎐O(51)
O(5)᎐Yb᎐O(51)
O(300)᎐Yb᎐O(51)
O(62)᎐Yb᎐O(51)
O(73)᎐Yb᎐O(51)
O(51)᎐Yb᎐O(61)
O(5)᎐Yb᎐O(61)
O(300)᎐Yb᎐O(61)
O(5)᎐Nd(1)᎐O(1)
70.4(3)
76.5(3)
74.9(3)
80.9(3)
72.4(3)
144.9(5)
77.2(3)
131.1(3)
131.2(2)
67.1(3)
139.1(3)
80.4(3)
68.5(3)
117.2(3)
142.9(3)
138.2(3)
73.9(3)
114.4(3)
67.9(3)
112.7(3)
50.3(3)
120.9(3)
147.1(3)
78.2(4)
136.9(4)
81.3(3)
72.3(4)
100.9(3)
125.1(3)
134.4(3)
145.6(4)
69.4(4)
49.2(3)
95.8(3)
69.4(3)
67.7(3)
72.7(3)
130.4(3)
65.0(3)
132.0(3)
68.3(3)
108.9(3)
149.1(3)
48.2(3)
94.1(3)
O(5)᎐Nd(2)᎐O(1)
70.0(3)
77.6(3)
131.4(3)
80.1(4)
70.2(4)
69.3(3)
75.9(3)
74.6(4)
130.9(3)
142.5(6)
119.1(3)
148.0(3)
79.7(3)
139.2(4)
78.2(4)
139.7(3)
81.7(3)
141.5(3)
117.1(3)
69.1(3)
73.0(4)
139.1(3)
74.9(3)
113.0(3)
68.7(3)
114.1(2)
101.8(3)
49.7(3)
126.1(3)
135.2(3)
49.7(3)
72.4(4)
145.0(4)
67.3(3)
94.2(3)
69.1(3)
70.5(3)
128.8(3)
67.3(3)
131.6(3)
65.1(3)
48.5(3)
110.1(3)
150.3(3)
94.7(3)
O(5)᎐Nd(1)᎐O(w1)
O(1)᎐Nd(1)᎐O(w1)
O(5)᎐Nd(1)᎐O(w2)
O(1)᎐Nd(1)᎐O(w2)
O(w1)᎐Nd(1)᎐O(w2)
O(5)᎐Nd(1)᎐O(71)
O(1)᎐Nd(1)᎐O(71)
O(w1)᎐Nd(1)᎐O(71)
O(w2)᎐Nd(1)᎐O(71)
O(5)᎐Nd(1)᎐O(52)
O(1)᎐Nd(1)᎐O(52)
O(w1)᎐Nd(1)᎐O(52)
O(w2)᎐Nd(1)᎐O(52)
O(31)᎐Nd(1)᎐O(52)
O(5)᎐Nd(1)᎐O(51)
O(1)᎐Nd(1)᎐O(51)
O(w1)᎐Nd(1)᎐O(51)
O(w2)᎐Nd(1)᎐O(51)
O(71)᎐Nd(1)᎐O(51)
O(52)᎐Nd(1)᎐O(51)
O(5)᎐Nd(1)᎐O(62)
O(1)᎐Nd(1)᎐O(62)
O(w1)᎐Nd(1)᎐O(62)
O(w2)᎐Nd(1)᎐O(62)
O(71)᎐Nd(1)᎐O(62)
O(52)᎐Nd(1)᎐O(62)
O(51)᎐Nd(1)᎐O(62)
O(5)᎐Nd(1)᎐O(72)
O(1)᎐Nd(1)᎐O(72)
O(w1)᎐Nd(1)᎐O(72)
O(w2)᎐Nd(1)᎐O(72)
O(71)᎐Nd(1)᎐O(72)
O(52)᎐Nd(1)᎐O(72)
O(51)᎐Nd(1)᎐O(72)
O(62)᎐Nd(1)᎐O(72)
O(5)᎐Nd(1)᎐O(61)
O(1)᎐Nd(1)᎐O(61)
O(w1)᎐Nd(1)᎐O(61)
O(w2)᎐Nd(1)᎐O(61)
O(71)᎐Nd(1)᎐O(61)
O(52)᎐Nd(1)᎐O(61)
O(51)᎐Nd(1)᎐O(61)
O(62)᎐Nd(1)᎐O(61)
O(72)᎐Nd(1)᎐O(61)
O(5)᎐Nd(2)᎐O(61)
O(1)᎐Nd(2)᎐O(61)
O(5)᎐Nd(2)᎐O(w1)
O(1)᎐Nd(2)᎐O(w1)
O(61)᎐Nd(2)᎐O(w1)
O(5)᎐Nd(2)᎐O(w2)
O(1)᎐Nd(2)᎐O(w2)
O(61)᎐Nd(2)᎐O(w2)
O(w1)᎐Nd(2)᎐O(w2)
O(5)᎐Nd(2)᎐O(72)
O(1)᎐Nd(2)᎐O(72)
O(61)᎐Nd(2)᎐O(72)
O(w1)᎐Nd(2)᎐O(72)
O(w2)᎐Nd(2)᎐O(72)
O(5)᎐Nd(2)᎐O(51)
O(1)᎐Nd(2)᎐O(51)
O(61)᎐Nd(2)᎐O(51)
O(w1)᎐Nd(2)᎐O(51)
O(w2)᎐Nd(2)᎐O(51)
O(72)᎐Nd(2)᎐O(51)
O(5)᎐Nd(2)᎐O(53)
O(1)᎐Nd(2)᎐O(53)
O(61)᎐Nd(2)᎐O(53)
O(w1)᎐Nd(2)᎐O(53)
O(w2)᎐Nd(2)᎐O(53)
O(72)᎐Nd(2)᎐O(53)
O(51)᎐Nd(2)᎐O(53)
O(5)᎐Nd(2)᎐O(62)
O(1)᎐Nd(2)᎐O(62)
O(61)᎐Nd(2)᎐O(62)
O(w1)᎐Nd(2)᎐O(62)
O(w2)᎐Nd(2)᎐O(62)
O(72)᎐Nd(2)᎐O(62)
O(51)᎐Nd(2)᎐O(62)
O(53)᎐Nd(2)᎐O(62)
O(5)᎐Nd(2)᎐O(71)
O(1)᎐Nd(2)᎐O(71)
O(61)᎐Nd(2)᎐O(71)
O(w1)᎐Nd(2)᎐O(71)
O(w2)᎐Nd(2)᎐O(71)
O(72)᎐Nd(2)᎐O(71)
O(51)᎐Nd(2)᎐O(71)
O(53)᎐Nd(2)᎐O(71)
O(62)᎐Nd(2)᎐O(71)
Conclusion
The use of malonamides for the coextraction of some trivalent
lanthanides and actinides have been investigated. These
malonamides proved to be readily prepared and the method is
suitable for large-scale synthesis. Metal extraction increases as
the nitric acid concentration increases for all three studied
malonamides. The most important feature of the extraction
was that points of inflection were observed for the distribution
coefficients as a function of nitric acid concentration at about 1
and 6 mol dmϪ3. The reasons for this can be related to changes
in the mechanisms of extraction; for example, at low nitric acid
concentrations (р1 mol dmϪ3), a co-ordination mechanism
dominates but as the acidities increase the ion-pair mech-
anism(s) are more important. These proposed mechanisms for
metal extraction were supported by the crystal structures of
[Nd(NO3)3(dcmma)2] for the co-ordination mechanism or
Hdcmma+ for the ion-pair mechanism involving a monoproto-
nated species. The first of these compounds was grown under
neutral conditions which could represent the extracted species
when the malonamide is not protonated. As acidities increase,
malonamides become mono- or di-protonated at the carbonyl
oxygen and the extracted metal species could be in their anionic
form, e.g. M(NO3)4 or M(NO3)52Ϫ. The extent of nitric acid
Ϫ
extraction by dmptdma and dbmodma showed that as the acid-
ities increase above 1 mol dmϪ3 the number of nitric acid mole-
cules extracted per malonamide was one and as the nitric acid
concentration reached 5 mol dmϪ3 there were between one and
two molecules extracted per malonamide. All of this evidence
suggests that the mechanism of extraction of metals by
malonamides depends upon the acidity of the medium: i.e. (1) a
co-ordination mechanism at low acid concentrations and (2) an
ion-pair mechanism at higher acid concentrations.
It was also interesting that the free malonamides dcema and
dcima exist as trans conformers in which the carbonyl groups
are lying mainly trans to each other. In order for the metals to
form chelates with malonamides the carbonyl groups have to
reorientate and lie cis to each other in order to facilitate extrac-
tion via either co-ordination or ion-pair mechanisms.
groups are cis to the methyl groups and trans to the phenyl
rings.
Thus with the dicyclohexyl ligand we can only isolate com-
plexes with two malonamides, while with the diphenyl ligand we
can only isolate complexes with one malonamide. Clearly the
preparation (or not) of crystals is not definite evidence of a
significant difference between the complexation properties of
the two ligands. However, we have also reported here two
further pieces of evidence indicating different behaviour. First
our experimental and computational structural studies have
shown that the cyclohexyl compound is likely to have the trans
O᎐C ؒ ؒ ؒ C᎐O conformation while the phenyl has the gauche.
The diphenyl derivative dmpma on the other hand has a
gauche arrangement of the two carbonyl groups and so the
necessary reorientation is far less. This seems to be consistent
with the fact that dmptdma is able to extract rare earths even at
᎐
᎐
Secondly our extraction studies also show a significant differ-
ence in that the diphenyl compound is far more successful at
extraction than is the dicyclohexyl.
658
J. Chem. Soc., Dalton Trans., 1997, Pages 649–660

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J. Chem. Soc., Dalton Trans., 1997, Pages 649–660
659

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10 T. H. Siddall III, R. L. McDonald and W. E. Stewart, J. Mol. Spec-
the highest acid concentrations compared to dcmtdma and
dbmodma where the extraction reaches a maximum at about 7
mol dmϪ3, decreasing thereafter.
trosc., 1968, 28, 243.
11 C. Musikas, Inorg. Chim. Acta, 1987, 140, 197.
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17 P. Byers and M. J. Hudson, unpublished work.
18 L. Nigond, N. Condamines, P. Y. Cordier, J. Livet, C. Madic,
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Acknowledgements
We are grateful for the financial support by the Swedish
Nuclear and Waste Management Co., SKB and the European
Union Nuclear Fission Safety Programme Task 2 (Contract
F12W-CT91-0112). We are indebted to Pamela D. Chalmers,
Mark R. Feaviour and Philip Turner for their help with the
synthesis and preparation of crystals. We would also like to
thank the EPSRC and The University of Reading for funding
of the image-plate system.
20 QUANTA/CHARMm, Version 3.23, Molecular Simulations Inc.,
Cambridge, UK; Waltham, MA, USA, 1996.
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Received 9th August 1996; Paper 6/05577J
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J. Chem. Soc., Dalton Trans., 1997, Pages 649–660