Macrocycles mechanical bonds

Figure 51. Anionic template synthesis of rotaxane 118 with a bis(phenyl ether) axle. The mechanical bond is formed by reaction of the phenolate-macrocycle complex [32-115] (supramolecular nucleophile) with the semiaxle intermediate 117. 

Rotaxanes and catenanes are two of such interlocked systems which can be described simply as a rod threaded in a macrocycle and captured by two stoppers at each end (rotaxane), and two or more rings which can not be separated unless one or more covalent bonds are broken (catenane). Specific non-covalent interactions are not required to make these systems stable. They do not break apart because of the presence of mechanical bonds [4] in these structures. The fact that they are interlocked does not mean that they are frozen . Conversely, the motions within the final supramolecule can be tailored and controlled by external stimuli making them supramolecular machines... 

This [2]catenane is composed of a jt-electron-deficient tetracationic cyclophane interlocked with a Jt-electron-rich macrocyclic polyether. In addition to a mechanical bond, [jt Jt] stacking interactions between the complementary aromatic units, [C-H---0] hydrogen bonds between the a-bipyridinium hydrogen atoms and the poly-ether oxygen atoms, and [C-H---Jt] interactions between the 1,4-dioxybenzene hydrogen atoms and the p-phenylene spacers in the tetracationic cyclophane hold the two macrocyclic components together and control their relative movements in solution. As a result of the asymmetry of the tetracationic cyclophane, two transla-... 

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... 

An important question in biological electron transfer is related to through-space and through-bond processes.74 Whereas through-bond processes were studied with [2]-rotaxanes 102 and 107, [2]-rotaxane 111 of Figure 2.38 was synthesized with the purpose of addressing the through-space question in a novel approach.75,76 In such a rotaxane the donor (Zn porphyrin stoppers) and the acceptor (Au porphyrin appended to the macrocycle) components are maintained in the same molecule by mechanical bonds only. It is therefore... 

Not only does DNA form itself into catenated and knotted structures, it also rotaxanates itself with macrocyclic enzymes. A,-Exonuclease [30, 37] is an enzyme that participates in DNA replication and repair by fully encircling DNA as it sequentially hydrolyzes nucleotides - a biomolecular rotaxane The structure of the enzyme is shown in Fig. 3d. T4 DNA polymerase holoenzyme [38] is an analogous example its protein subunits clamp around a DNA strand to form a toroid in what chemists of the mechanical bond would call a clipping process. It should be noted that chemists have been able to mimic this concept of a topologically linked catalyst on a polymer [39] using traditional organic catalytic reactions. 

Fig. 26 Active template synthesis [185] (i) metal catalyst is bound in the cavity of a macrocycle, ( ) reagents (half-dumbbells in this example) coordinate the metal within the macrocycle, (Hi) a covalent bond is catalytically formed, resulting also in a mechanical bond, and (iv) catalyst is regenerated and the MIM is expelled...

The earliest examples of mechanically bonded molecular switches were two-station rotaxanes and catenanes (a station is a recognition site for the macrocyclic component) that could be switched by these common stimuli. Our group reported a bistable rotaxane in 1994 that could be switched chemically and electrochemi-cally, utilizing a n-n donor/acceptor recognition motif (see Scheme 2 and explanation in corresponding caption) [107]. Many of the recognition motifs in MIMs that... 

Two higher homologues of the [2]-rotaxane are represented the [3]-rotaxane and the [4]-rotaxane. Whereas [3]-rotaxanes are relatively common [56-60], this is not the case for the linear [4]-rotaxane [61] and its branched or dendritic analogue [62]. Very important in the recent developments of rotaxane chemistry is the [2]-rotaxane whose dumbbell contains two separated sites for interaction of the threaded macrocycle. In this molecule, the macrocycle may be moved between two sites of the dumbbell, in a degenerate manner, if the sites are identical [63-65], or in a controlled manner, thanks to an external trigger, if the sites are different [28-31]. A chiral [2]-rotaxane, whose chirality arises from the mechanical bonding, is obtained if the macrocycle is oriented (by an appropriate substitution pattern) and the dumbbell made non-symmetrical, for example by attaching two different stoppers [66],... 

The photophysical properties of 33 +, 34+ and 35 + were investigated in DMF and the results are here summarized [72,75]. In these systems the redox active porphyrins are on different parts of the structure which are held together by the coordinated metal in 33 + and 35 +, whereas in 34+ they are held together only by mechanical bonds and no covalent electron-transfer pathways exist. These systems are characterized by some flexibility of the polyether gold porphyrin appended macrocycle which could cause, upon... 

A natural extension of the work described in Sect. 2.4 was to prepare a Cu(l)-complexed [2]catenane with macrocycles incorporating the Zn and Au porphyrins. [2]catenanes are topologically non-trivial molecules (non-planar molecular graph) in which two rings are interlocked but not linked [23,24]. Therefore, differentiating the rings with Zn- and Au-porphyrins enables us to study photoinduced electron transfer in mechanical bond systems and also to have information on the conformation of the system. 

A bistable porphyrin-containing [2]rotaxane R3 is synthesized with a shuttling benzylic-amide macrocycle mechanically locked onto the thread subunit via hydrogen bonding with two potential stations, as shown in Figure 5. This macrocycle consists of two pyridine groups, which would be easily coordinated with zinc porphyrin. The Zn(n) coordination of porphyrin moiety on the thread... 

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