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.

模板导向合成策略为机械互锁分子（MIMs）的制备提供了常规路径，这类分子的机械键可通过多种弱相互作用实现稳定。本文报道了一种制备带有可滑动互锁富电子环的中性[2]索烃的方法：以两种简并供体-受体型[2]索烃为起始原料，该类索烃由四阳离子环双(百草枯-对-亚苯基)环番（CBPQT⁴+）与分别含(i)对苯二酚（HQ）或(ii)1,5-二氧萘（DNP）识别单元的冠醚构成，并通过将环番组分进行四电子还原得到中性形态。冠醚中HQ、DNP单元与CBPQT⁴+环之间的供体-受体相互作用是稳定[2]索烃的核心作用力，当环番组分被还原为自由基阳离子态时，该相互作用会被显著削弱，而在完全还原状态下几乎完全消失。通过紫外-可见光谱、电子顺磁共振谱与核磁共振谱对溶液状态下的样品进行表征，结果显示[2]索烃的氧化还原特性变化可大幅降低互锁环的环旋转与回旋运动能垒，使得中性状态下的互锁环可彼此自由滑动。完全还原态索烃的固态结构表明，互锁环的相对排布发生了显著变化：冠醚的乙二醇链段位于中性CBPQT⁰环的空腔内部。针对索烃四种不同氧化态对应的能级开展的量子力学研究，佐证了上述结论。带有可滑动环的索烃与轮烷有望展现出独特的性能。