Tetranuclear Uranyl Polyrotaxanes: Preferred Selectivity toward Uranyl Tetramer for Stabilizing a Flexible Polyrotaxane Chain Exhibiting Weakened Supramolecular Inclusion.

Introduction of mechanically interlocked components into actinide-based metal-organic materials such as polyrotaxanes will generate an entirely new type of inorganic-organic hybrid materials showing more supramolecular encapsulation-based dynamics. In this work, tetranuclear uranyl-directed polyrotaxanes (UO2 )4 O2 -C5A3-CB6 (1) and (UO2 )4 O2 -C6A3-CB6 (2), which are the first actinide pseudorotaxanes with high-nuclearity uranium centers, were obtained through systematic extension of the string spacer in pseudorotaxane ligands from 1,4-butylene (C4) to 1,5-pentylene (C5) and 1,6-hexylene (C6). Both of the as-synthesized tetranuclear uranyl polyrotaxanes were structurally characterized and analyzed. Considering the structure of UO2 -C4A3-CB6 and the 1,4-butylene string spacer, the preference for the uranyl tetramer may be related to the configurational inversion of the pseudorotaxane ligands from trans mode to cis mode on coordination to the uranyl center. Detailed structural analysis suggests that the length of the stretched string molecules for CB6-encapsulated pseudorotaxanes has remarkable effect on the supramolecular inclusion interactions and the configurations of pseudorotaxanes, and should be responsible for the configurational inversion of pseudorotaxane spacers and subsequent distinct changes of the uranyl building units and geometric structures.