Crystal and electronic structure of a mixed-valent Np(IV)-Np(V) compound: [BuMeIm]5[Np(NpO2)3(H2O)6Cl12]

A new mixed valent neptunium compound [BuMeIm](5)[Np(NpO(2))(3)(H(2)O)(6)Cl(12)] was synthesized by evaporation of an ethyl acetate-acetone mixture where the salt [BuMeIm](2)[NpCl(6)] was dissolved. The crystal structure was determined using single crystal X-ray diffraction; it consists of four crystallographically unique Np centers with two different coordination environments in two different oxidation states. In addition, five crystallographically independent dialkylimidazolium cations stabilize the crystal through C-H...Cl hydrogen bonds. [Np(NpO(2))(3)(H(2)O)(6)Cl(12)] unit presents a highly symmetric structure with the formation of three Np(IV)-Np(V)O(2)(+) bonds. According to the crystal structure and density functional theory (DFT) calculations, the stability of the three Np(4+)-NpO(2)(+) interactions is due to the presence of chloride ions around NpO(2)(+) which brings electronic charge into the system and also to the presence of water molecules which create hydrogen bond with chloride ions.