Bis‐Squaramide‐Based [2]Rotaxane Hosts for Anion Recognition

The first examples of bis‐squaramide axle containing [2]rotaxanes linked via rigid aryl and flexible alkyl spacers synthesised using copper(I) catalysed active metal template methodology are reported. The halide and oxoanion binding properties of the [2]rotaxanes in aqueous‐organic solvent media are examined through extensive 1H‐NMR titration experiments to investigate the impact of integrating multiple squaramide motifs on the anion binding capabilities of the interlocked receptors. These studies reveal that the bis‐squaramide rotaxane host systems exhibit enhanced halide anion binding capabilities relative to a mono‐squaramide axle functionalised rotaxane, demonstrating a rare anti‐Hofmeister bias halide anion selectivity trend in aqueous‐organic mixtures and highlighting the efficacy of the potent solvent shielded hydrophobic interlocked binding pocket created upon mechanical bond formation. Notably, employing a rigid aryl linker between the two squaramide motifs in the axle component enables the rotaxane host to exhibit strong and selective binding of tetrahedral oxoanions. Conversely, a flexible alkyl spacer facilitates trigonal oxoanion selective recognition by the bis‐squaramide [2]rotaxane.