Crystal Structure of Regularly T_h‐Symmetric [U(NO₃)₆]²⁻ Salts with Hydrogen Bond Polymers of Diamide Building Blocks

Abstract

Hexanitratouranate(IV), [U(NO3)6]2−, has been crystallized with anhydrous H+‐involving hydrogen bond polymers connected by selected diamide building blocks. Thanks to the significant moderation of electrostatic interactions between the anions and cations, the molecular structure of [U(NO3)6]2− in these compounds is regularly Th‐symmetric. The f–f transitions stemming from 5f2 configuration of U4+ are strictly forbidden by the Laporte selection rule in such a centrosymmetric system, so that the obtained compounds are nearly colourless in contrast to other UIV species usually coloured in green.

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

“…As predicted from the U(IV)analogues (1,2), these compounds consist of [Np(NO 3 ) 6 ] 2À together with two L molecules. As shown in Tables S1 and S2 (ESI †), lattice parameters of 3 and 4 are almost identical with those of the corresponding U(IV)-analogues 1 and 2, 16 respectively, indicating that the obtained Np compounds are isomorphic and isostructural to these U(IV)-analogues. These facts are strong evidence that the Np centre in both 3 and 4 still remains tetravalent.…”

“…Due to strong radioactivity of 237 Np (2.63 Â 10 7 Bq g À1 ), further characterization methods like elemental analysis, IR, and powder XRD are unavailable at present. However, taking into account our recent results of well-characterized U(IV)-analogues, (HL) 2 16 the most probable identities of 3 and 4 are crystalline salts of a centrosymmetric hexanitratoneptunate(IV), [Np(NO 3 ) 6 ] 2À .…”

“…However, its strength is still much weaker than that of an actual O-H bond (e.g., 436 kJ mol À1 for methanol). 33 Indeed, the C]O groups (mean 1.26 A for 3, mean 1.27 A for 4) are slightly longer than free L (1.23 A for both L1 and L2), 16 but still exhibit double bonding character on the basis of covalent radii (C]O: 1.24 A, C-O: 1.43 A). 34 This means that there is no direct protonation to make an actual covalent bond between the carbonyl O and H + .…”

“…The selected structural parameters of 3 and 4 are summarized in Table 2 together with those of the U(IV)analogues, 1 and 2. 16 During crystallization process, initial hydration around H + in the reaction mixtures was fully removed. Instead, the dehydrated H + interacts with the neighbouring L molecules to make a [H + /L] n hydrogen bond polymer as shown in Fig.…”

“…The nal cycle of full-matrix least-squares renement on F 2 was based on observed reections and parameters, and converged with unweighted and weighted agreement factors, R and wR, respectively. Crystallographic data of 3 and 4 were summarized in Table 1, and compared with those of U(IV) analogues, (HL) 2 (2)) we reported previously 16 in Tables S1 and S2 (ESI †), respectively.…”