Using acetate anions to induce translational isomerization in a neutral urea-based molecular switch.

Although the use of cations to switch interlocked machine-like systems between different states is quite common, reports of anion-mediated interlocked switches are rare. To the best of our knowledge, only a few examples of anion-controlled translational isomerism in interlocked molecular switches have been described: 1) the generation of a phenoxide anion on a rotaxane thread to attract Leigh$s macrocycle and 2) exchange of the counteranions of ammonium or pyridinium ions to affect their interactions with complementary crown ether or naphthalene-derived recognition motifs. Because of their ability to act as strong Y-shaped hydrogen-bond donors, urea and its derivatives play key roles in the design of receptors for anions, including both spherical halide anions and Y-shaped carboxylate anions, in solution. Conceptually, a [2]pseudorotaxane complex obtained after threading a urea-derived component through the cavity of a macrocycle should be a suitable system for constructing anion-controllable molecular switches because the recognition of anions by the urea recognition site would encourage relocation of the macrocycle to another recognition site. We are unaware, however, of any such switch having been reported to date. Herein, we report a new ditopic macrocyclic host that is capable of recognizing a diphenylurea-derived thread in a [2]pseudorotaxane fashion in solution. The controllable translational isomerism of its corresponding neutral [2]rotaxane was achieved through the addition and removal of acetate anions. Previously, we reported that the diethylene glycol linkages in bis(para-xylyl)-[26]crown-6 (BPX26C6, Scheme 1) located its two xylene rings at a favorable p-stacking distance and, thus, helped its complexation to (mono)pyridinium and 4,4’bipyridinium ions in solution. Because 2,6-pyridinediamide is also an excellent spacer for locating aromatic rings at a suitable p-stacking distance, we designed the ditopic macrocycle 1, which possesses two xylyl rings linked by both diethylene glycol and 2,6-pyridinediamide spacers, as a host molecule capable of complexing diphenylurea derivatives. We expected that the corresponding [2]pseudorotaxane complexes (Scheme 1) would be stabilized through the cooperative effects of p stacking (of the two xylyl rings about the planar p system of the urea center) and N-H···O hydrogen bonding (between the 2,6-pyridinediamide protons and the urea carbonyl oxygen atom and between the urea amide protons and the ethylene glycol oxygen atoms). We obtained macrocycle 1 in 12% overall yield from the reaction of 4-bromomethylbenzonitrile with diethylene glycol under basic conditions (affording the bis(benzonitrile) 2), the LiAlH4-mediated reduction of 2, and subsequent macrocylization with dimethyl 2,6-pyridinedicarboxylate (Scheme 2). To increase the solubility of diphenylurea in less-polar solvents (i.e., solvents that encourage hydrogen bonding between their solutes), we synthesized a diphenylurea derivScheme 1. Formation of a urea-based pseudorotaxane.