Recognition bipyridinium

Reaction (Scheme 2) of salt 5-2PF6 with an excess of 6, in the presence of the macrocydic polyether BPP34C10, provided1181 [BPP34C10-7] [PF6]4 via a threading-followed-by-stoppering approach.1231 This rotaxane incorporates a free 4,4-bipyridinium recognition site within its dumbbell-shaped compo-... 

Particularly interesting pseudorotaxane species can be constructed by using components which incorporate different recognition functions, such as a thread (23 ) containing ammonium and bipyridinium functions (Figure 9a) [12], or species... 

Figure 12. Shuttling of a ring between two equivalent stations in a [2]rotaxane. (a) The bipyr-idinium-based cyclophane shuttles from one 1,4-dioxybenzene recognition site to the other at a rate of c. 2000 s in (CD3)2CO at ambient temperature [39a,bj. (b) The 1,4-dioxybenzene-based macrocyclic polyether shuttles from one bipyridinium recognition site to the other one at a rate of c. 3 X 10 s under the same conditions [39c. ...

In a recent full paper [32], a molecular switch 116+, which can be driven either photochemically or electrochemically, has been described. Synthesized in a 59% yield from the corresponding dumbbell compound using the slippage approach, this [2]rotaxane contains (Fig. 16) two chemically distinct recognition sites around which the BPP34C10 macrocycle can reside. Of the two sites, the unsubstituted bipyridinium unit is the better electron acceptor, and so it is around this site that the macrocycle encircles itself preferentially. However, upon reduction of the bipyridinium site to its radical cation - either... 

Fig. 16. A photo- and electrochemically controllable molecular shuttle. The unperturbed rotaxane 116+ exists preferentially in the translational isomer in which the BPP34C10 crown ether resides around the bipyridinium unit, a Photochemical excitation of the Ru(bipy)3 unit results in PET to the bipyridinium site, and consequent translation of the crown ether to the 3,3dimethylbipyridinium unit, which is a less efficient recognition site for the cyclophane CBPQT4+ than a bipyridinium system. This process occurs only in the presence of a sacrificial reductant which reduces the Ru(III) center back to its Ru(II) state in order to prevent charge recombination, b Conversely, upon electrochemical reduction of the bipyridinium unit, the crown ether takes up residency around the 3,3 -dimethylbipyridi-nium site. This process is reversed through electrochemical oxidation of the bipyridinium radical cation back to the dication...

These happy coincidences meant that, if this unsymmetrical tetracationic cyclophane is concatenated with a symmetrical crown ether, there is every chance that the bipyridinium unit of the cyclophane will reside preferentially inside the cavity of the 71-electron-rich macrocyclic polyether. With a lower reduction potential than its extended counterpart, the bipyridinium unit will be reduced first in a voltammetric experiment, and hence it is expected that the cyclophane will circumrotate in order that the vinylogous bipyridinium recognition site - now a much better 7t-electron-receptor system than the radical cation of the bipyridinium unit - resides within the cavity of the crown ether. 

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