Coordination Modes of Hydroxamates in Neptunium (V) Complexes in Aqueous Solution

A theoretical study to understand molecular structures and coordination modes of neptunium complexes with three different substituent hydroxamates viz., acetohydroxamate (AHA), benzhydroxamate (BHA) and salicylhydroxamate (SHA) was carried out using density functional theory (DFT). The geometries for different complexes reported in literature MxLyHz (x:y:z) viz 1:1:1, 1:2:2 and 1:2:0 were optimized and interaction energies (Delta E) for complexation process were calculated to obtain the most probable structures of complexes. The E value revealed, the complex stability order as NpO2+-SHA > NpO2-AHA > NpO2-BHA which is in agreement with the reported experimental data. Higher negative charge on AHA compared to BHA leads to its higher interaction with NpO2 which in turn reflects in shorter Np-O-eq (ligand) bond distances. In 1:1:1 complex of NpO2-SHA, the binding mode involving ortho hydroxyl group and -NHO- oxygen atom (seven membered ring) shows most negative interaction energy.However, for binding of second ligand in 1:2:2 NpO2-SHA complex, steric effects dominate and the mode involving five membered ring through oxygen atoms of -CONHO- is most stable.