Pseudorotaxanes and rotaxanes formed by viologen derivatives

Dibenzyl-4,4′-bipyridinium (BIPY²⁺) bis(hexafluorophosphate) and three of its derivatives - disubstituted at the para positions of the benzyl groups with CO₂Me, 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 CD₃CN solution by ¹H 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 ¹H NMR spectroscopic single-point method in CD₃CN 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 CO₂Me, F and Me. In the case of BMP25C8, however, the K_a values with both categories (BIPY²⁺ 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 BIPY²⁺ 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 BIPY²⁺ 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 ¹H NMR spectroscopy, and in one case, in the solid state by X-ray crystallography.