Crystallization of colourless hexanitratoneptunate(iv) with anhydrous H⁺ countercations trapped in a hydrogen bonded...

Colourless crystalline compounds of centrosymmetric [Np(NO₃)₆]^2- were yielded from 3 M HNO₃ aq in the presence of double-headed 2-pyrrolidone derivatives (L). In the obtained crystal structures, H⁺ was also involved as a countercation to compensate for the negative charge of [Np(NO₃)₆]^2-, where the initial hydration around H⁺ was fully removed during crystallization despite it having the strongest hydration enthalpy. Instead, this anhydrous H⁺ was captured by L to form a [H⁺⋯L] hydrogen bonded polymer. In [Np(NO₃)₆]^2-, the Np⁴⁺ centre is twelve-coordinated with 6 bidentate NO₃ ^-, and therefore, present in an icosahedral geometry bearing inversion centre. In such a centrosymmetric system, any f-f transitions stemming from the 5f³ electronic configuration of Np⁴⁺ are electric-dipole forbidden. This is the reason why the compounds currently obtained were colourless unlike ordinary Np(iv) species, which are olive-green.