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 (Ka) 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 Ka 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 Ka 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.