α-Aminophosphonates, -phosphinates, and -phosphine oxides are a group of organophosphorus compounds that were investigated as extraction agents for rare earth (RE) metals and actinoids for the first time in the 1960s. However, more systematic investigations of their extraction properties towards REs and actinoids were not started until the 2010s. Indeed, recent studies have shown that these α-amino-functionalized compounds can outperform the commercial organophosphorus extraction agents in RE separations. They have also proven to be very efficient extraction and precipitation agents for recovering Th and U from RE concentrates. These actinoids coexist with REs in some of the commercially important RE-containing minerals. The efficient separation and purification of REs is becoming more and more important every year as these elements have a pivotal role in many existing technologies. If one also considers the facile synthesis of α-amino-functionalized organophosphorus extractants and precipitation agents, it is expected that they will be increasingly utilized in the extraction chemistry of REs and actinoids in the future. This review collates α-aminophosphonates, -phosphinates, and -phosphine oxides that have been utilized in the separation chemistry of REs and actinoids, including their most relevant synthetic routes and molecular properties. Their extraction and precipitation properties towards REs and actinoids are also discussed.

The efficient and environmentally sustainable separation process for rare earth elements (REE), especially for adjacent lanthanoids, remains a challenge due to the chemical similarity of REEs. Tetravalent actinoids, thorium, and traces of uranium are also present in concentrates of REEs, making their separation relevant. This study reports six simple water-soluble aminobis(phosphonate) ligands, RN[as precipitating agents for REEs, Th, and U, as well as gives insight into the coordination modes of the utilized ligands with REEs at the molecular level. Aminobis(phosphonates) 4−6 with longer carbon chains were found to separate selectively thorium, uranium, and scandium from REEs with short precipitation time (15 min) and excellent separation factors that generally range from 100 to 2000 in acidic aqueous solution. Ligands 1−6 also improved separation factors for adjacent lanthanoids in comparison to traditional oxalate precipitation agents. Importantly, precipitated metals can be recovered from the ligands with 3 molar HNO 3 with no observed ligand decomposition enabling the possibility of recycling the ligands in the separation process. NMR-monitored pH titrations for 1 showed deprotonation steps at pK a 1.3, 5.55, and >10.5, which indicate that the ligands remain in a deprotonated [L] −1 form in the pH range of 0−4 used in the precipitation studies. 31 P NMR titration studies between 1 and M(NO 3 ) 3 (M = Y, La, Lu) gave satisfactory fits for 1:3, 1:2, and 1:1 metal−ligand stoichiometries for Y, La, and Lu, respectively, according to an F-test. Therefore, aminobis(phosphonate) precipitation agents 1−6 are likely to form metal complexes with fewer ligands than traditional separation agents like DEHPA, which coordinates to REEs in 1:6 metal−ligand ratio.