Selective self-assembly and acid-base controlled de-/rethreading of pseudorotaxanes constructed using multiple recognition motifs

Abstract

The selective self-assembly of a threadlike tetracation, containing two dialkylammonium (NH2/+) centers and a 4,4'-bipyridinium (bpym2+) unit, with an assortment of macrocyclic polyethers has been investigated. Spectroscopic studies, supported by X-ray crystallographic analyses, have demonstrated the formation of pseudorotaxanes between the thread and the macrocyclic polyethers. In particular, the macrocyclic polyethers dibenzo[24]crown-8 (DB24C8) and bisp-phenylene[34]crown-10 (BPP34C10) associate selectively with the thread's NH2/+ and bpym2+ units, respectively, to generate novel multicomponent pseudorotaxane and pseudopolyrotaxane superarchitectures as a consequence of hydrogen-bonding and charge-transfer interactions. BPP34C10's preference for bpym2+ units is shared by several of its dinaphtho-containing crown-10 congeners, which self- assemble with the tetracationic thread to produce [2]pseudorotaxanes in which the bpym2+ moiety is encircled by the macrocyclic polyether. The reversible acid-base controlled de/rethreading of [n]pseudorotaxanes based on the multi functional tetracationic thread has also been studied by 1H NMR, absorption, and fluorescence spectroscopies. Only one DB24C8 macrocycle is ejected from a [3]pseudorotaxane-created from the tetracationic thread and two DB24C8 molecules when both of its DB24C8-encircled NH2/+ centers are deprotonated with base, leading to a [2]pseudorotaxane whose bpym2+ unit is associated with the remaining DB24C8 macroring. The [3]pseudorotaxane is reestablished upon reprotonation of the thread with a stoichiometric amount of acid. Treatment of this species with an excess of acid causes protonation of the DB24C8 macrorings, which are completely dethreaded thereafter. Again, the dethreading process can be reversed upon neutralization of the surplus acid with base. Addition of dicyclohexano[24]crown-8 (DCy24C8) to a solution of the [3]pseudorotaxane induces the dethreading of one of the two DB24C8 macrocycles, so that a [4]pseudorotaxane is produced in which the tetracation's NH2/+ and bpym2+ units are encircled by DCy24C8 and DB24C8 macrorings, respectively. Reversible acid-base de-/rethreading processes have also been observed for [2]pseudorotaxanes bearing the dinaphtho-containing crown-10s.