Impact of Ligand Structures on Precipitation Reactions of Uranyl Nitrate Coordination Polymers with Diamide/Dicarbamide Cross-Linkers

In relevance to advanced nuclear fuel recycling, we investigated the structural chemistry of UO₂(NO₃)₂ coordination polymers bearing <i>trans</i>-1,4-cyclohexanediamine-based cyclic/acyclic diamides/dicarbamide cross-linkers (L1-L6) to know how the ligand structure impacts precipitation behavior of UO₂ ²⁺ from HNO₃(aq). In a similar manner to the γ-lactam analogue (L2) we reported previously, 1D coordination polymers of [UO₂(NO₃)₂(L<i>x</i>)] _<i>n</i> were obtained in the use of any L<i>x</i>'s having β-/δ-lactam (L1, L3), acetamide (L4), propionamide (L5), and <i>N</i>,<i>N</i>,<i>N'</i>-trimethylcarbamide (L6) terminals. These compounds exhibit the typical structural features of uranyl nitrates. Indeed, [UO₂(NO₃)₂(L<i>x</i>)] _<i>n</i> (<i>x</i> = 2-6) deposited in 71-90% yield from 3 M HNO₃(aq). In contrast, no [UO₂(NO₃)₂(L1)] _<i>n</i> precipitated under the same condition. The packing efficiency of [UO₂(NO₃)₂(L<i>x</i>)] _<i>n</i> and the electronic nature of L<i>x</i> seem to control the solubility of [UO₂(NO₃)₂(L<i>x</i>)] _<i>n</i> . Reaction kinetics of precipitation of [UO₂(NO₃)₂(L<i>x</i>)] _<i>n</i> with the cyclic diamides (L2 and L3) was faster than that of the acyclic diamides (L4 and L5), being associated with the rotational isomerism around the amide C-N bonds occurring only in the latter series. Despite the availability of similar <i>E</i>-<i>Z</i> isomerism, the precipitation kinetics in the use of dicarbamide L6 was as rapid as the cyclic amides (L2, L3), being ascribed to the lower energy barrier in its C-N bond rotation compared with those in the acyclic diamides (L4, L5).