Insights into the Coordination Behavior of Methyl-Substituted Phosphinic Acids with Actinides.

Abstract

The electronic structure and complexation behavior of methyl-substituted phosphinic acids with U(VI) and Pu(IV) were explored by applying quantum chemical methods. In contrast to Ingold's classification, our results indicate that the methyl group is electron-withdrawing, reducing the phosphoryl group electron density in substituted phosphinic acids. The magnitude of the computed complexation energy values increases along with the series, PA → MPA → DMPA, and MP → MMP → MDMP, implying an increasing complexation tendency upon methyl group substitution for both U(VI) and Pu(IV) complexes. One of the nitrate groups in UO2(NO3)2•2L complexes (L = PA, MPA, and DMPA) is in monodentate coordination mode due to the additional stability gained from O2N-O···H hydrogen bonding interactions with the acidic H atoms of respective ligands. The calculation indicates marginally stronger metal-ligand interactions in Pu(IV) complexes compared to that in U(VI), which is supported by the computed complexation energies, M-OP bond lengths, ν(P═O), the extent of metal-ligand charge transfer, and properties of M-OP bond critical points. The energy landscape of substituted phosphinic acid ligands is further analyzed within the framework of the activation strain model to explain the energetic preference of certain conformers.