Carbamoylmethyl phosphonates

This paper describes a comparison of the extraction behavior of selected. actinide(III), (IV), and (VI) ions by the dihexyl-N, N-diethyl analogs of carbamoylmethyl-phosphonate and phosphinate and octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide. 

Although a variety of extractants have been studied and tested for their suitability in analytical separations of actinides, perhaps the most recent milestone in the process use of actinide extractants is the application of the bidentate carbamoylmethyl-phosphonates (17,18,19). The pioneering work on these compounds was done 15 years ago by Siddall (20,21). Their principal advantage is their ability to extract the trivalent, tetravalent, and hexavalent actinides from considerably more concentrated nitric acid media than other extractants. 

Bidentate phosphate compounds. The organophosphorus bidentate extractants are neutral species extractants that undergo no keto-enolization and have no exchangeable hydrogens (as is the case in extraction by the bidentate diketones). They contain either two P=0 groups or one P=0 and one C=0 group. The carbamoylmethyl-phosphonates (CMPs) and carbamoylphosphonates (CPs) are examples of the latter, while the tetraalkyldiphosphonates and tetraalkyl-diphosphinedioxides [or bis-(disubstituted phosphinyl)-alkanes] are examples of the former. 

Several bifunctional organophosphorus ligands have been found to be efficient liquid-liquid extractants for lanthanide and actinide ions present in highly acidic aqueous nuclear waste solutions. The (carbamoylmethyl)phosphonates, (RO)2P(0) CHzCCOlNRj, and in particular dihexyl [(iV,iV-diethylcarbamoyl)methyl] phos-phonate (DHDECMP) have attracted the most attention because of the low aqueous-phase solubilities and superior radiolytic stabilities of the ligands. 

Syntheses for (carbamoylmethyl)phosphonates based upon the Aibusov and Michaelis reactions have been described briefly by Siddall." DHDECMP ( 60% purity) is available from a commercial source [Wateree Chemical]. The ligands are obtained typically with acidic phosphonate impurities, which have a deleterious effect on extraction selectivity. In order to overcome this problem, Mclsaac and coworkers designed a puriflcation procedure for DHDECMP that provides samples with 98-99% purity. ... 

The increasing interest in (carbamoylmethyl)phosphonates, the fact that only one of these ligands is commercially available, the lack of adequate detail for their syntheses, and the relative inaccessibility of the puriflcation scheme require descriptions of general procedures for the syntheses and puriflcation of the ligands. The procedures are outlined here for R = R = Et. However, the same preparations have been used for R = Me, i-Pr, Bu, Hex with R = Et. 

Although the (carbamoylmethyl)phosphonates are not particularly air-sensitive, it is found that the purities of the products are enhanced by performing the following reactions under dry nitrogen. 

CMP, CMPO system. Horwitz and Kalina (1984), Horwitz et al. (1986) developed neutral bifunctional extractants CMP (RiR2-N,N-di(R ) carbamoylmethyl phosphonate) and CMPO (RiR2-N,N,- di(R )-carbamoylmethyl phosphine oxide, see Fig. 18.20) for the separation of trivalent actinide ions. With thiocyanate as a counter ion in the CMPO system, americium is preferentially extracted and the separation factor S.E (Am /Eu ) is about seven (Muscatello etal. 1982). 

As extractants of polyvalent metal ions from acidic solutions, these reagents have one additional limitation they are also moderately efficient extractants for mineral acids. As the neutral organophosphorus compound extracts the free acid, the free-extractant concentration in the counter-phase is reduced, and metal-ion extraction efficiency suffers. To overcome this limitation, considerable effort has been expended in recent years to evaluate the ability of carbamoylphosphonates (CPs,(RO)2 PO(CO)N(R )2), carbamoylmethyl phosphonates (CMPs (RO)2POCH2(CO)N(R )2), and carbamoylmethylphosphine oxides (CMPOs, R2POCH2(CO)N(R )2) to extract lanthanide and actinide metal ions. These extractants are referred to collectively as bifunctional extractants. The first report of these ligands were made by Siddall (1963,1964). 

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