Interlocked molecules rotaxanes

Ceo-TTF molecules, interlocked molecules (catenanes, rotaxanes), donor-acceptor macrocycles, cage molecules, etc. (Jeppesen et al, 2004). It is beyond the scope of this book to review such developments and I appeal to the curiosity of researchers really not familiar with such macromolecules to see how big the molecules can become. 

Interlocked molecules such as rotaxanes initially gained interest due to their interesting topology and associated synthetic challenge, but recent efforts have showed that they can be used in many important applications that will be discussed in this chapter (Scheme 6.1). 

Scheme 6.1 Schematic representation of interlocked molecules (a) pseudorotaxane. (b) rotaxane and (c) catenane.

The choice of one of these four routes for the preparation of a rotaxane depends mainly on the chemical nature of the different components and the chemistry required to establish the interlocked molecule. An interaction between the two individual components is very often the driving force in the synthesis of rotaxanes. 

Fortunately, more efficient methods for the complexation of macrocyclic hosts with acyclic guest molecules have become available with the advent of supramo-lecular chemistry, resulting in higher yields in rotaxane and catenane synthesis. In the following sections, the preparation of different types of interlocked molecules, with the use of host-guest recognition, is discussed. It should be noted that these template-directed methods differ significantly from the above-mentioned stochastic approach [28]. 

The synthesis of interlocked molecules has become commonplace over the past 25 years with the gradual development of a number of highly facile template methods for their construction. What were once laboratory curiosities have now taken a prominent place in the broad field of supramolecular chemistry, especially regarding their uses and further potential as molecular switches and machines [1], We present here an overview of the main synthetic approaches to these molecules, with a focus on methods in which macrocyclization reactions result in interlocked products. The analysis is by no means meant to be comprehensive or exhaustive in detail, but rather to convey the variety and utility of the selected synthetic strategies in generating abiotic rotaxane and catenane superstructures. 

Interlocked molecules are those assemblies of two or more molecules which are linked by a so-called mechanical bond [2]. The individual molecules are not connected covalently in any way but are linked via their spatial relationship to one another. The nature of this phenomenon is such that it necessarily involves a macrocyclic component as one or more of the molecules which compose the assembly. The simplest forms of these assemblies are represented by a [2]rotaxane and a [2]catenane (Figure 10.1), where the bracketed numeral preceding the name indicates the number of individual molecules comprising the interlocked product. The synthesis of a rotaxane may be executed by a number of different routes, only one of which involves the formation of a macrocyclic component in the final... 

Fig. 2 Graphical representations of the structures of mechanically interlocked molecules (MIMs) a catenane (left) and a rotaxane (right)...

Catenanes, as the name of Latin origin suggests (catena = chain), are chain-like molecules composed of at least two cyclic compounds (i.e., a [2]catenane) that are not covalently linked to one another but nevertheless cannot be separated unless covalent bond breakage occurs. Catenanes thus belong, together with rotaxanes (Section 14.12.4.1), to the wider family of interlocked molecules. 

A special issue devoted to molecular machines appeared in Accounts of Chemical Research in 2001. It reflects the current interest for this field in which ruthenium complexes act as important tools. Molecular machines are characterized by a mobile part and a stationary part. Photochemical and electrochemical inputs can make a machine work, offering the advantage of being switched on and off easily and rapidly. Mechanically interlocked molecules, such as rotaxanes and catenanes, are suitable candidates. Crown ethers, cyclophanes, and calixarenes are representative families of the cyclic... 

Before going on to discuss molecular electronic machines, it will be useful to describe their structural foundation at a molecular level, namely those based on interlocked molecules. Interlocked molecules can take on a variety of forms, the most common being catenanes, rotaxanes, knots [16], and carceplexes [17]. For the purpose of this review, only catenanes, rotaxanes and their geometrically related complexes - pseudorotaxanes [18] - will be discussed. When conferred with the ability to undergo some mechanical motion as a result of an applied stimulus - be it chemical, electrochemical, or photochemical - these interlocked molecular and interpenetrated supramo-lecular systems often take on the characteristics of molecular machines [19]. 

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