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 3 ) 6 ] 2- were yielded from 3 M HNO 3 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 3 ) 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] n hydrogen bonded polymer. In [Np(NO 3 ) 6 ] 2- , the Np 4+ centre is twelve-coordinated with 6 bidentate NO 3 - , and therefore, present in an icosahedral geometry bearing inversion centre. In such a centrosymmetric system, any f-f transitions stemming from the 5f 3 electronic configuration of Np 4+ are electric-dipole forbidden. This is the reason why the compounds currently obtained were colourless unlike ordinary Np(iv) species, which are olive-green.