Macrocyclic Polyethers as Ditopic Co-Receptors for Dual-Centred Secondary Dialkylammonium Guests: from Double-Stranded to Single-Stranded Pseudorotaxanes

Abstract

Recently, we have uncovered a new simple self-assembling system [1] based on the mutual recognition between secondary dialkylammonium dications (R2NH2 +) — such as dibenzylammonium and dibutylammonium cations — and differently-sized macrocyclic polyethers — such as bis-para-phenylene-34-crown-10 (BPP34C10) and dibenzo-24crown-8 (DB24C8). These complexes are stabilised predominantly by electrostatic interactions (including hydrogen bonding) between the R2NH2 + cations and the polyether loops within the cavity of the crown ether. When the macrocycle is sufficiently large, the R2NH2 + cation threads itself through the macrocyclic cavity so as to generate a superstructure with a pseudorotaxane-like geometry in the solid state. We have found that only a single strand, incorporating either one or two [1] (—NH2 +—) centres can be accommodated within the cavity of DB24C8 to form the so-called single- and doublyencircled pseudorotaxanes, respectively (e.g. a [2]- and [3]-pseudorotaxane). However, upon utilising the larger crown ether BPP34C10, two R2NH2 +-containing strands [1] are threaded through the centre of the cavity of either one or two macrocycles to generate a double-stranded [3]pseudorotaxane, and a double-stranded doubly-encircled [4] pseudorotaxane, respectively. These encouraging results have led us to employ [2] either larger macrocyclic polyethers (bearing more polyether loops) or cationic guests incorporating multiple (—NH2 +—) centres to self-assemble multiply-stranded and multiply-encircled