Uranyl Compounds Involving a Weakly Bonded Pseudorotaxane Linker: Combined Effect of pH and Competing Ligands on Uranyl Coordination and Speciation

Pseudorotaxane-type ligands with tunable structural dynamics offer an opportunity in the exploration of new actinide hybrid materials. In this work, we utilized a weakly bonded pseudorotaxane ligand involving CB[6] and 1, 1'-(heptane-1, 7-diyl)bis(4-(ethoxycarbonyl)pyridin-1-ium) bromides ([C7BPCEt]Br2@CB[6]) to assemble with uranyl ion, and we systematically investigated the effect of different factors including pH and competing ligands on the hydrothermal synthesis of URCPs. Nine uranyl-rotaxane coordination polymers (URCPs) with diversity in coordination mode and topological structure were successfully prepared (two previously reported complexes, URCP1 and URCP2 are also included). The results indicate that sulfate, bromide, CB[6], and C7BPCA (the hydrolyzate of [C7BPCEt]Br2) show a combined influence on the obtained URCPs. At low pH, both CB[6] and C7BPCA can bond with uranyl centers and produce interwoven structures in URCP1, URCP2, and URCP6; at high pH, C7BPCA and competing anions (sulfate and bromide) have priority to coordinate with uranyl ions in URCP3-URCP5 and URCP7-URCP9. Notably, for the first time, bromide anion with lower affinity to uranyl ions is also observed in solid-state uranyl coordination polymer (URCP7-URCP9), which has been demonstrated by both energy dispersive X-ray spectroscopy and single-crystal X-ray structure analysis. In addition, a spontaneously single-crystal-to-single-crystal transformation from URCP3 to URCP4, which is driven by thermodynamics, was observed and explained by computational study. Moreover, it reveals that sulfate with stronger coordination ability can inhibit the hydrolysis of uranyl ion to some extent with only a rarely reported pentanuclear uranyl center found in URCP5 obtained at pH 5.67. These results indicate that the combined effect of competing ligands and pH has great significance in the formation of URCPs in terms of uranyl coordination and speciation and can be an alternative way to design and synthesize uranyl coordination polymers with new topologies.