Encapsulation of 4,4′-bipyridinium cations by two crown ether molecules: formation and structure of [3]pseudorotaxanes

Abstract

Substituted 4,4′-bipyridinium cations (viologens) typically form 1 : 1 complexes with crown ethers. In this paper the formation of trimolecular [3]pseudorotaxanes whereby each viologen coordinates two dibenzo-24-crown-8, DB24C8, host units have been further studied. Because the viologen cations are tightly encapsulated into the cavity of two coordinated DB24C8 molecules the formation of these [3]pseudorotaxanes is very sensitive to the size of the substituents. Short-chain dialkyl- and diallylviologens furnish stable crystalline [3]pseudorotaxanes but closely related benzylviologens fail to form these complexes. While the apparent association constant for the 1 : 1 [2]pseudorotaxane in acetonitrile is significantly lower than that in acetone, a proton NMR study of dimethylviologen in the presence of various amounts of DB24C8 suggests that in acetonitrile solution the [3]pseudorotaxane stoichiometry is preserved. X-Ray crystal structure characterisation of [3]pseudorotaxanes revealed that the two N-alkylated pyridinium rings of the viologen adopt a twisted conformation and thread two crown ether molecules. The structure is possibly stabilised by hydrogen bonding and ion–dipole interactions.