Efficient Preparation of Separable Pseudo[n]rotaxanes by Selective Threading of Oligoalkylammonium Salts with Cucurbit[7]uril

Abstract

We describe the efficient preparation of well-defined, homogeneous pseudo[n]rotaxanes (n = 2, 3, 4, 5) by the selective threading of oligoalkylammonium salts with cucurbit[7]uril (CB[7]) and the subsequent simple separation of the pure pseudo[n]rotaxanes from the complicated system. A series of well-defined oligoalkylammonium salts were prepared by stepwise synthesis and their molecular recognition and self-assembly with a solution of CB[7] in CF(3)CO(2)D/D(2)O (1:1 v/v) were investigated. As a result of the high affinity of CB[7] to the p-xylene diammonium units (-NH(2)(+)CH(2)C(6)H(4)CH(2)NH(2)(+)-) in the oligoalkylammonium salts, the CB[7] rings threaded onto these cationic threads to form pseudorotaxane structures. Interestingly, due to the repulsive dipole-dipole electrostatic interactions between the CB[7] units and steric hindrance, the threading process exhibited a high selectivity and a unique self-assembling mode was discovered in which two CB[7] units cannot bind the same ammonium site. The as-formed well-defined pseudo[n]rotaxanes can be easily separated as pure compounds from the mixture by simple counterion exchange with NH(4)PF(6) and the pure pseudo[n]rotaxanes are stable and partially soluble in normal organic solvents. Such facile preparation and separation are mainly ascribed to the high binding constant between CB[7] and the oligoalkylammonium salts and the unique solubility of CB[7] in protonic acid. As most pseudorotaxanes reported in the literature exist as a dynamic mixture and their separation is usually difficult, our research represents one good example in which pure pseudo[n]rotaxanes (n = 2, 3, 4, 5) can be obtained by a simple "threading-followed-by-precipitation" method.