SCHEDULED MAINTENANCE
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
Lena
Spjuth
and
and
Abstract
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(NO
3
)
4
]
-
or
[M(NO
3
)
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(NO
3
)
3
(dcmma)
2
],
[Nd(NO
3
)
3
(H
2
O)
2
(dmpma)] and
[Yb(NO
3
)
3
(H
2
O)(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 carbonyl amide groups adopt a
gauche configuration with an
O![[double bond, length as m-dash]](https://www.rsc.org/images/entities/char_e001.gif)
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.
+
(dcmma = N,N′-dicyclohexyl-N
,N′-dimethylmalonamide) and in the chelated
complexes [Nd(NO
3
)
3
(dcmma)
2
],
[Nd(NO
3
)
3
(H
2
O)
2
(dmpma)] and
[Yb(NO
3
)
3
(H
2
O)(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 carbonyl amide groups adopt a
gauche configuration with an
OC · · · 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.
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′-substituted malonamides.
Experimental and crystallographic evidence for two mechanisms of
extraction, metal complexation and ion-pair formation