Crystal structure of [UO2(NH3)5]NO3·NH3

Uranium chemistry in aqueous solution is dominated by the uranyl cation [UO2]2+, with the uranium atom in the hexa­valent oxidation state. The most prominent representatives are the well-known uranyl nitrates and uranyl halides. In contrast to the [UO2]2+ uranyl cation, the uranyl cation [UO2]+ with penta­valent uranium disproportionates in aqueous solution into the [UVIO2]2+ cation and a tetra­valent uranium species. Only under controlled conditions (Kraus et al., 1949[Kraus, K. A., Nelson, F. & Johnson, G. L. (1949). J. Am. Chem. Soc. 71, 2510-2517.]) and in organic solvents (Arnold et al., 2009[Arnold, P. L., Love, J. B. & Patel, D. (2009). Coord. Chem. Rev. 253, 1973-1978.]) are uranyl cations with penta­valent uranium observable. Here we report on the crystal structure of a UV compound, [UO2(NH3)5]NO3·NH3, obtained from the reaction of UIV with UVI species in anhydrous liquid ammonia. The compound is not stable at temperatures above ca 238 K due to the loss of ammonia of a still unknown amount. Despite several efforts, we have not yet been able to reproduce the synthesis of the compound. Obviously, the two uranium compounds used as educts reacted in a comproportionation reaction in order to form the UV compound reported here. It is possible that the redox potentials in liquid ammonia are reversed compared to aqueous solutions, leading to a comproportionation. Such changes of electrochemical potentials are not uncommon and, for example, are known for the system Cu/Cu+/Cu2+ (Woidy et al., 2015a[Woidy, P., Karttunen, A. J., Widenmeyer, M., Niewa, R. & Kraus, F. (2015a). Chem. Eur. J. 21, 3290-3303.]). However, the detailed reaction UVI + UIV → UV is still unclear, and despite some efforts we were not able to elucidate further reaction products which must be present (e.g. fluoride containing ones).