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

Colourless crystalline compounds of centrosymmetric [Np(NO)] were yielded from 3 M HNO 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(NO)], 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(NO)], the Np centre is twelve-coordinated with 6 bidentate NO , and therefore, present in an icosahedral geometry bearing inversion centre. In such a centrosymmetric system, any f-f transitions stemming from the 5f electronic configuration of Np are electric-dipole forbidden. This is the reason why the compounds currently obtained were colourless unlike ordinary Np(iv) species, which are olive-green.