Electronic structure and complexation behavior of di-n-alkyl phosphine oxides

In nuclear fuel cycle, organophosphorus compounds are considered as preferred class of extractants for the extraction of actinides from other metal ions. In these compounds the extraction ability is directly correlated to the basicity of phosphoryl oxygen atom and can be enhanced by replacing the C-O-P group with a C-P group. For neutral organophosphorus extractant series, this trend will vary in the order: phosphate < phosphonates < phosphinates < phosphine oxides. In the present study, we have applied density functional theory calculations (DFT) to understand the electronic structures and extraction behavior of four members of di-n-alkyl phosphine oxide (DAPO) family (ie. di-n-hexyl phosphine oxide (DHePO), di-n-heptyl phosphine oxide (DHpPO), di-n-octyl phosphine oxide (DOPO) and di-n-decyl phosphine oxide (DDPO)). The electronic similarity of various phosphine oxide ligands implies similarity in their extraction behaviour. The in-house experimental studies indicated that these extractants behave like acidic extractants under lower acidity because of the tautomeric equilibrium between P=O and P-OH. In this connection, we have applied DFT calculations to investigate the mechanism of tautomerism reaction in DHePO, where tetra coordinate pentavalent and trivalent phosphorus are in equilibrium. Our calculations supported a bimolecular mechanism (energy barrier of ∼27.3 kcal/mol). In order to understand the coordination behaviour of ligands during extraction, we have also derived the lowest energy structures for Th(NO₃)₄.2DHePO and UO₂(NO₃)_2.2DHePO at PBE0/def2-TZVP level. (author)