Abstract
Dibenzyl‐4,4′‐bipyridinium (BIPY^2+^) bis(hexafluorophosphate) and three of its derivatives – disubstituted at the para positions of the benzyl groups with CO~2~Me, F, and Me in turn – have been shown to form 1:1 complexes that are [2]pseudorotaxanes with dibenzo[24]crown‐8 (DB24C8), benzometaphenylene[25]crown‐8 (BMP25C8), and dipyrido[24]crown‐8 (DP24C8) in CD~3~CN solution by ^1^H NMR spectroscopy and in one case in the solid state by X‐ray crystallography. Binding constants (K~a~) for all of these 1:1 complexes, which were determined both (1) by isothermal titration calorimetry in MeCN solution and (2) by the ^1^H NMR spectroscopic single‐point method in CD~3~CN solution, were found to be, on the average, an order of magnitude less than the K~a~ values obtained for DB24C8 and DP24C8 with dibenzylammonium (DBA^+^) hexafluorophosphate and three of its derivatives, also disubstituted at the para positions of the benzyl groups with CO~2~Me, F and Me. In the case of BMP25C8, however, the K~a~ values with both categories (BIPY^2+^ and DBA^+^) of guests are much of a muchness, being both small and error prone. The equilibrium thermodynamics for these small libraries of [2]pseudorotaxanes indicate that the best bistable [2]rotaxanes incorporating both DBA^+^ and BIPY^2+^ recognition sites are going to involve ester functions in their dumbbell components and will employ DP24C8 or, failing that, DB24C8 as the ring component. The BIPY^2+^threads also directed the templated assembly of [2]rotaxanes incorporating the crown ethers (DB24C8, DP24C8, and BMP25C8) and triphenylphosphonium stoppers using the threading followed by stoppering approach. The rotaxanes were characterized in solution by ^1^H NMR spectroscopy, and in one case, in the solid state by X‐ray crystallography.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)