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 trans-1,4-cyclohexanediamine-based cyclic/acyclic diamides/dicarbamide cross-linkers (L1–L6) to know how the ligand structure impacts precipitation behavior of UO22+ from HNO3(aq). In a similar manner to the γ-lactam analogue (L2) we reported previously, 1D coordination polymers of [UO2(NO3)2(Lx)]n were obtained in the use of any Lx’s having β-/δ-lactam (L1, L3), acetamide (L4), propionamide (L5), and N,N,N′-trimethylcarbamide (L6) terminals. These compounds exhibit the typical structural features of uranyl nitrates. Indeed, [UO2(NO3)2(Lx)]n (x = 2–6) deposited in 71–90% yield from 3 M HNO3(aq). In contrast, no [UO2(NO3)2(L1)]n precipitated under the same condition. The packing efficiency of [UO2(NO3)2(Lx)]n and the electronic nature of Lx seem to control the solubility of [UO2(NO3)2(Lx)]n. Reaction kinetics of precipitation of [UO2(NO3)2(Lx)]n 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 E–Z 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).