Sliding-Ring Catenanes

Template-directed protocols provide a routine approach to the synthesis of mechanically interlocked molecules (MIMs), in which the mechanical bonds are stabilized by a wide variety of weak interactions. In this Article, we describe a strategy for the preparation of neutral [2]­catenanes with sliding interlocked electron-rich rings, starting from two degenerate donor–acceptor [2]­catenanes, consisting of a tetracationic cyclobis­(paraquat-p-phenylene) cyclophane (CBPQT⁴⁺) and crown ethers containing either (i) hydroquinone (HQ) or (ii) 1,5-dioxynaphthalene (DNP) recognition units and carrying out four-electron reductions of the cyclophane components to their neutral forms. The donor–acceptor interactions between the CBPQT⁴⁺ ring and both HQ and DNP units present in the crown ethers that stabilize the [2]­catenanes are weakened upon reduction of the cyclophane components to their radical cationic states and are all but absent in their fully reduced states. Characterization in solution performed by UV–vis, EPR, and NMR spectroscopic probes reveals that changes in the redox properties of the [2]­catenanes result in a substantial decrease of the energy barriers for the circumrotation and pirouetting motions of the interlocked rings, which glide freely through one another in the neutral states. The solid-state structures of the fully reduced catenanes reveal profound changes in the relative dispositions of the interlocked rings, with the glycol chains of the crown ethers residing in the cavities of the neutral CBPQT⁰ rings. Quantum mechanical investigations of the energy levels associated with the four different oxidation states of the catenanes support this interpretation. Catenanes and rotaxanes with sliding rings are expected to display unique properties.