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.