Impact of the proximity effect on uranyl coordination of conformationally variable weakly-bonded cucurbit[6]uril-bipyridinium pseudorotaxane

Abstract

To explore the proximity effect in uranyl coordination of weakly-bonded cucurbit[6]uril(CB[6])-bipyridinium ligands, a new pseudorotaxane precursor C7BPCN3@CB[6] containing 1,1′-(heptyl-1,7-diyl)bis(3-cyanopyridin-1-ium) bromide (C7BPCN3) with elongated alkyl chains and meta-substituted cyano groups, has been synthesized and used to react with uranyl cations. Due to the weak binding affinity between the host and guest components of the supramolecular ligand, five new uranyl-rotaxane coordination polymers URCP1–URCP5 have been prepared, in which the pseudorotaxane linkers have shown great conformation diversity and abundant coordination behaviors. Three different coordination modes are present in the as-synthesized uranyl compounds, one of which is host–guest synergetic coordination due to the proximity effect between the meta-coordination carboxylate groups of C7BPCN3 and the portal carbonyls of CB[6]. Specially, the elongated spacer and weak-bonded feature make both CB[6] host and C7BPCN3 at one end of the pseudorotaxane ligand involved in the synergetic chelating with the uranyl centre. Meanwhile, at the other end, only the C7BPCN3 guest can participate in the first coordination sphere of uranium atom. This is the first time that this unique coordination mode has been observed in uranyl coordinating polyrotaxanes. This work demonstrates the proximity effect on uranyl coordination of weakly-bonded CB[6]-bipyridinium pseudorotaxane linkers, and provides an alternative approach to achieve metal coordination regulation and structure diversity in uranyl coordination polymers involving mechanically interlocked molecules.