Synthesis and characterization of a metallocycle-based molecular shuttle

Kinetically stable metallocycle-based molecular shuttles of [2]rotaxanes 4a and 4b, along with [3] rotaxanes 5a and 5b, have been prepared using the rhenium(I)-bridged metallocycle 2 and the dumbbell components containing two stations, 3a and 3b. The rotaxanes were self-assembled by hydrogen bonding interactions upon heating a Cl2CHCHCl2 solution containing their components at 70 degreesC. Each rotaxane was isolated in pure form by silica gel chromatography under ordinary laboratory conditions and fully characterized by elemental analysis and various spectroscopic methods. The H-1 NAIR signals for the amide NH and the methylene -(CH2)(4)- of the station were considerably changed when occupied by the metallocycle. In [2]rotaxane 4b, which has a larger naphthyl spacer, the occupied and unoccupied stations gave widely separated signals in the H-1 NMR spectroscopy at room temperature, but averaged signals of two stations were observed in [2]rotaxane 4a, which has a smaller phenyl spacer. This is attributed to the shuttling of the metallocycle between two stations. The coalescence temperature experiment gave a shuttling rate of similar to670 s(-1) at 19 degreesC in CDCl3, corresponding to an activation free energy (DeltaG(+)) of 13.3 kcal/mol. With respect to the relative position of the chloride in the rhenium(l) center, two diastereomers are possible in the [2]rotaxane and three diastereomers are possible in the [3]rotaxane. In fact, the rotaxanes exist as diastereomeric mixtures in nearly equal amounts of all possible diastereomers on the basis of the amide NH signals of the station in the H-1 NMR spectroscopy.