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

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 carbonyl amide groups adopt a gauche; configuration with an O=C ... C=O torsion angle of 57.2 degrees. 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.