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

Two neptunium(III) mellitates, Np(mell)(HO)·1.5HO () and Np(mell)(HO)·2HO (), have been synthesized from NpCl(dme) by reduction with KC and subsequent reaction with an aqueous solution of mellitic acid (Hmell). 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, is indefinitely stable in air, while 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 the energy is low enough to result in spontaneous oxidation.