Impacts of Oxo Interactions on Np(V) Crown Ether Complexes.

Abstract

Intermolecular interactions between the oxo group of an actinyl cation and other metal cations (i.e., cation-cation interactions) are dependent on the strength of the actinyl bond. These cation-cation interactions are prominently observed for the neptunyl cation [Np(V)O2]+ and are sufficiently stable enough to explore using a variety of chemical techniques. Herein, we investigate these intermolecular interactions in the neptunyl 18-crown-6 system, because this macrocyclic ligand provides both stable coordination and the proper sterics to engage the oxo group in bonding with both low-valent metal cations and neighboring neptunyl units. We report the structural and spectroscopic characterization of five neptunyl, [Np(V,VI)O2]+,2+, compounds: Np1a ([NpO2(18-crown-6)]ClO4), Np1b ([NpO2(18-crown-6)]AuCl4), Na-Np ([Np(V)O2(18-crown-6)(Na(H2O)(18-crown-6)][Np(VI)O2Cl4], Np-Np ([NpO2(18-crown-6)](NpO2Cl2NO3)], and Np-Cl (NpO2Cl(H2O)1.75). Each of these compounds were prepared from the ambient reactions of Np(V) in HX (where X = Cl, NO3) with the 18-crown-6 ether molecule. Structural information obtained from single-crystal X-ray diffraction data was paired with solid-state and solution Raman spectroscopy to provide information on the interaction of the neptunyl oxo atom with neighboring cations. Neptunyl (Np═O) bond lengths are not perturbed upon interaction with the Na+ cation (Na-Np), but elongation is observed upon formation of a neptunyl-neptunyl interaction (Np-Np). This is also the first structurally characterized isolated, molecular complex that contains a simple T-shaped neptunyl-neptunyl interaction. Raman spectroscopy indicates little perturbation to the neptunyl bond until the formation of the neptunyl-neptunyl motif, which also results in activation of the ν3 asymmetric stretch. Additional spectroscopic studies indicated that the neptunyl 18-crown-6 inclusion complexes form in solution and persist in the presence of other low-valence cations.