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 (BMP25CB), and dipyrido[24]crown-8 (DP24C8) in CD3CN solution by H-1 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 H-1 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 DB24CB 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. 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 H-1 NMR spectroscopy, and in one case, in the solid state by X-ray crystallography.