Ion association with tetra-n-alkylammonium cations stabilizes higher-oxidation-state neptunium dioxocations

Shanna L. Estes,Geng Bang Jin,Baofu Qiao

doi:10.1038/s41467-018-07982-5

Shanna L. Estes, Geng Bang Jin + Show 1 more

Open Access

https://doi.org/10.1038/s41467-018-07982-5

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Abstract

Extended-coordination sphere interactions between dissolved metals and other ions, including electrolyte cations, are not known to perturb the electrochemical behavior of metal cations in water. Herein, we report the stabilization of higher-oxidation-state Np dioxocations in aqueous chloride solutions by hydrophobic tetra-n-alkylammonium (TAA+) cations—an effect not exerted by fully hydrated Li+ cations under similar conditions. Experimental and molecular dynamics simulation results indicate that TAA+ cations not only drive enhanced coordination of anionic Cl– ligands to NpV/VI but also associate with the resulting Np complexes via non-covalent interactions, which together decrease the electrode potential of the NpVI/NpV couple by up to 220 mV (ΔΔG = −22.2 kJ mol−1). Understanding the solvation-dependent interplay between electrolyte cations and metal–oxo species opens an avenue for controlling the formation and redox properties of metal complexes in solution. It also provides valuable mechanistic insights into actinide separation processes that widely use quaternary ammonium cations as extractants or in room temperature ionic liquids.

Highlights

Extended-coordination sphere interactions between dissolved metals and other ions, including electrolyte cations, are not known to perturb the electrochemical behavior of metal cations in water
By measuring electrode potentials in organic media and by characterizing solid reaction products, several recent studies have demonstrated that direct second-sphere coordination of strong Lewis acids to oxo groups enhances the redox activity of metal–oxo complexes, including increasing the oxidizing capacity of manganese–oxo clusters in the oxygenevolving complex of photosystem II5,6 and facilitating oxo group functionalization and reduction of the uranyl ion, UO22+ 3,4
On the basis of the experimental and simulation results, we propose that TAA+ cations promote the formation of innersphere NpV–chloride complexes in solution

Summary

Introduction

Extended-coordination sphere interactions between dissolved metals and other ions, including electrolyte cations, are not known to perturb the electrochemical behavior of metal cations in water. We combine electroanalytical chemistry, vibrational and electronic spectroscopies, X-ray crystallography, and molecular dynamics simulations to characterize the interactions that electrolyte cations (A+ = Li+ or tetra-n-alkylammonium cations (TAA+ = [NMe4]+, [NEt4]+)) have with NpVO2+ cations in aqueous systems, and to understand the effect these interactions have on NpV redox behavior, both in solution and during solution evaporation and crystal formation.

Results

Conclusion