Two Neptunium(III) Mellitate Coordination Polymers: Completing the Series Np-Cf of Trans-Uranic An(III) Mellitates

Two neptunium(III) mellitates, 237Np2(mell)(H2O)9 center dot 1.5H2O (Np-1 alpha) and 237Np2(mell)(H2O)8 center dot 2H2O (Np-1 beta), have been synthesized from 237NpCl4(dme)2 by reduction with KC8 and subsequent reaction with an aqueous solution of mellitic acid (H6mell). Characterization by single-crystal X-ray crystallography and UV-vis-NIR spectroscopy confirms that the neptunium is in its +3 oxidation state and both polymorphs are isostructural to the previously reported plutonium mellitates. Of the two morphologies, Np-1 alpha is indefinitely stable in air, while Np-1 beta slowly oxidizes over several months. This is due to the change in the energy of the metal-ligand charge-transfer absorption exhibited by these compounds attributed to differing numbers of carboxylate bonds to Np(III), where in Np-1 beta the energy is low enough to result in spontaneous oxidation.

Automated summary

Two polymorphs of neptunium(III) mellitate compounds have been synthesized and characterized, confirming their structure and oxidation state. One polymorph is stable in air, while the other slowly oxidizes.