Rotaxanes transition metal complexes

Electrochemically Driven Molecular Machines Based on Transition-metal Complexed Catenanes and Rotaxanes... 

The [2]-rotaxane 81 of Figure 2.28 owes its existence to transition metal-complex fragments as stoppers.60 The thread 77 designed for its synthesis... 

Another area of NEMS that is receiving tremendous attention is the mimicry of biological systems, aptly referred to as biomimetics. For instance, in the development of linear molecular muscles that undergo contraction and extension movements. Initial work in this field utilized transition metal complexes containing rotaxanes and catenanes, due to the nondestructive redox processes occurring on the metal centers.Though these complexes were actuated by a chemical reaction, the movement was in a noncoherent manner. In order to better mimic skeletal muscle movement, one has to look at the mode of motion within the most efficient molecular machines - in our human bodies. 

Transition metal-complexed catenanes and rotaxanes as light-driven molecular machines prototypes 05CL742. 

Fig. 3 The macrocycle A incorporating a coordinating fragment thick line) interacts with a metal center black circle) and an asymmetrical open chain chelate B bearing one porphyrin and a precursor function X which is small enough to pass through the ring after the threaded intermediate C is formed, the additional porphyrin ring is constructed, affording the transition metal-complexed rotaxanes D and E. Demetalation leads to the free-ligand rotaxanes F and G from D and E, respectively...

The situation is not significantly different when the polymer films are modified by catalytic centers, such as clusters of transition metals [329,330,332,334,338, 344,345], polyoxometallates [327,335,371], porphyrins, phthalocyanines and their analogs [307,335], other transition metal complexes [341,342], biomolecules [372], arenas and rotaxane [328,336], etc. see [7,15, 312-314,331, 340,343,373] for reviews. 

The situation could have been markedly different when the precursors contain coordinating moieties or transition metals, which could perturb the course of the reaction by interacting with the copper(I) catalyst or act as competitors, respectively. Interestingly, the presence of various transition metal-complexed fragments in the precursors is not particularly detrimental and does not seem to lower the preparative yields. Click chemistry has been successfully used with various transition metal complexes as precursors (Cu(I) or Fe(II)). The iron(II)-templated synthesis of a [3]rotaxane is remarkably efficient since the yield of a 4-fold stoppering reaction is close to quantitative. An impressive example... 

Figure 4 The first rolaxane (4) bearing transition metal complex fragments as stoppers. It is also the first cyclodextrin-based rotaxane the a-cyclodextrin ring is threaded onto an alkyl chain ended by amino groups, which bind cobalt(III) centres coordinated to cthylcncdiaminc chelates [4a].

In this chapter, we will focus on transition metal-based catenanes and rotaxanes. We will restrict ourselves to compounds that are set in motion by an electrochemical signal. Indeed, the electrochemical techniques represent privileged methods for piloting these machines since they contain electroactive transition metal centers or complexes. In addition to triggering the motions, electrochemistry allows to investigate the dynamic properties of the compounds. 

The precatenate intermediate can be regarded as a threaded complex, which is thermodynamically stabilized by coordination bonds, from this simple consideration, a transition metal-templated synthesis of rotaxanes was devised, the principle of which is shown in Figure 28 [117]. The threading step (i) is a complexation reaction. 

Figure 29 Assembling a dumbbell chelate (B) and an open chelate (A) around a transition metal cation with a kinetically labile coordination sphere will provide a mixture of homoleptic ((D) and (E)) and heteroleptic (C) complexes. Only the latter will be productive for the formation of a [2]-rotaxane.

Figure 43 Principle of transition metal-templated construction of a [2]-rotaxane with two different porphyrinic stoppers (same conventions as in Figure 42). (i) Macrocycle (A) is threaded onto chelate (B), end-blocked by a porphyrin at one extremity and fiaictionalized with a reactive group X, which is a precursor to the second porphyrin stopper, affording prerotaxane (C) (ii) construction of the second porphyrin, leading to metal-complexed [2]-rotaxane (D).

Figure 51 Principle of transition metal-templated synthesis of a [3]-rotaxane, from two chelating macrocycles (B) and a bis-chelate-containing molecular thread (A) functionalized with reactive end groups X (same conventions as in Figure 43). (ii) Threading step, affording prerotaxane (C) construction of the porphyrin stoppers providing copper(I)-complexed [3]-rotaxane (D).

Fig. 11 Principle of transition metal-templated synthesis of a [2]rotaxane. A thick line represents a dpp chelate, a black dot represents a metal cation, a hatched diamond represents a Au(III) porphyrin and an empty diamond represents a Zn(II) porphyrin. The transition metal controls the threading of Au(III) porphyrin-pendant macrocycle (A) onto chelate (B), to form prerotaxane (C). Construction of the porphyrin stoppers at the X functions leads to the metal complex [2]rotaxane (D). Removal of the template cation forms the free rotaxane (E)...

The precursor C to the rotaxane E, called prerotaxane, is obtained in one step from a gold porphyrin macrocycle A and a difunctionalized thread B, thanks to the gathering properties of the transition metal (black dot). Construction of the porphyrin blocking groups is the kineticaUy templated key step leading to the metal-complexed rotaxane structure D. The desired rotaxane E is obtained after removd of the metal template from D. 

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