Pseudorotaxanes and Rotaxanes Formed by Viologen Derivatives

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)