SOLVENT-EXTRACTION OF METAL-IONS FROM NITRIC-ACID SOLUTION USING N,N'-SUBSTITUTED MALONAMIDES - EXPERIMENTAL AND CRYSTALLOGRAPHIC EVIDENCE FOR 2 MECHANISMS OF EXTRACTION, METAL COMPLEXATION AND ION-PAIR FORMATION

The solvent extraction of actinides including Am-III and Cm-III togeth er with some trivalent lanthanides from nitric acid solutions by two n ewly synthesized malonamides, N,N'-dimethyl-N,N'-diphenyltetradecylmal onamide (dmptdma) and ,N'-dicyclohexyl-N,N'-dimethyltetradecylmalonami de (dcmtdma) has been investigated and compared with data for the refe rence malonamide, N,N'-dibutyl-N,N'-dimethyloctadecylmalonamide (dbmoc ma). The dependence of the extraction on the nitric acid and malonamid e 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 concentra tions(up to 1 mol dm(-3)) a co-ordinative mechanism dominates for the extraction of metal cations, whereas at higher nitric acid concentrati ons (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 t he protonated, unalkylated species Hdcmma(+)(dcmma = N,N'-dicyclohexyl -N,N'-dimethylmalonamide) and in the chelated complexes [Nd(NO3)(3)(dc mma)(2)], [Nd(NO3)(3)(H2O)(2)(dmpma)] and [Yb(NO3)(3)(H2O)(dmpma)] (dm pma = 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 mal onamides N,N'-dicyclohexyl-N,N'-diethylmalonamide and N,N'-dicyclohexy l-N,N'-diisopropylmalonamide show that the carbonyl amide groups adopt a trans configuration in which the carbonyl oxygens are at maximum se paration. By contrast, in the crystal structure of the diphenyl deriva tive dmpma the carbonyl amide groups adopt a gauche; configuration wit h an O=C ... C=O torsion angle of 57.2 degrees. Conformational analysi s confirms that the differences in these structures reflect the differ ences between the lowest-energy gas-phase conformations and are not ca used by packing effects.