Rotaxane-like

Scheme 2. Classification of rotaxane dendrimers Type I, II and III rotaxane dendrimers incorporating rotaxane-like features at the core, termini, and branches, respectively...

Concept Rotaxane-like entities are created by the slippage protocol in which the cavities of macrocyclic polyethers dilate", at elevated temperatures, so as to allow their passage over the relatively bulky stoppers of chemical dumbbells (see Cartoon below). Similarly,... 

Several years ago in our research laboratories, a novel route to the synthesis of kinetically stable rotaxane-like19121 spedes was discovered,1131 which relies upon the aforementioned balance between kinetics and thermodynamics. This route is termed slippage inasmuch as the rotaxane-like species are obtained when a preformed macrocycle bears a cav-... 

Kinetically stable rotaxane-like species have been synthesized without the aid of strong noncovalent bonds previously (I. T. Harrison, Preparation of Rotaxanes by the Statistical Method , J. Chem. Soc Perkin Trans. 11974, 301-304). However, the yields reported were veiy low. 

Kinetically stable superarchitectures can be assembled by relying upon both noncovalent bonds and mechanical coercion. Thus, at elevated temperatures, rotaxane-like complexes (a) are generated when macrocycles slip over the stoppers of chemical dumbbells, while hemicarceplexes (b) are created when guests squeeze through the portals of hemicarcerands. 

Chiu SH, Rowan SJ, Cantrill SJ, Glink PT, Garrell RL, Stoddart JF (2000) A rotaxane-like complex with controlled-release characteristics. Org Lett 2 3631-3634... 

These two complementary cyclic voltammetry experiments confirm that in this rotaxane, like in previously studied related systems, the tetracoordinate Cu(I) state is more stable than the pentacoordinate one and the pentacoordinate Cu(II) state is more stable than the tetracoordinate one. 

Figure 35. Schematic representation depicting the self organization of rotaxane-like entities using the slipping approach in the melt (high concentrations of wheel and axle). R Rotax-ane where the hydrogen bonds between wheel and axle have not yet been formed. R Rot-axane with hydrogen bonds between wheel and axle. Entropy loss upon formation of the mechanical bond is not considered in the graph.

Figure 17.15 Proposed molecular-level mechanism for the operation of solid-state electronic devices based on (a) catenanes like 134+ 115-117 and (b) rotaxanes like 164+ and 174+ 118119 (Adapted with permission from V. Balzani et al., ChemPhysChem 2008, 9, 202-220. Copyright Wiley-VCH Verlag GmbH Co. KGaA.)...

In contrast to modem industry, where technology utilizes the mechanical bond far more ubiquitously on the macroscale than on the nanoscale, Nature depends more vitally on mechanical interlocking at the molecular level. To be certain, a few mechanical bonds can be identified in Nature s macroscale designs, such as the rotaxane-like mammalian spine (Fig. 3a) or a turtle s shell - a suitane in the molecular world [33] (Fig. 3b). But, mechanical bonds are being made and broken incessantly within the mesoscopic world of cells. DNA is foremost among the players in biological MIMs. DNA catenanes [34] and knots [35] are intermediate... 

Polyrotaxane networks 163 have been developed as prototype recyclable cross-linked polymers <2004AGE966>. These systems, which comprise a poly(crown ether)polyurethane and a bisammonium disulfide salt, are cross-linked via mechanical bonds ([3]rotaxane-like substructures) and are capable of undergoing reversible assembly and disassembly as they rely on thiol-disulfide reactions. Macroscopically, cross-linking determines gelation while de-cross-linking enables recovery of the starting materials. 

Polythreaded systems contain motifs that are interweaved via rotaxane-like mechanical links and this implies the presence of closed loops as well as of elements that can thread the loops (Fig. 1.3.2). These two types of moieties may belong to the same unit or may be separately supplied by motifs having different structures. The constituent motifs could be, in principle, OD species, ID polymers or arrays of higher dimensionality. The resulting array can show the same or an increased dimensionality with respect to that of the polythreaded units. 

A representative example of the threading reaction (the principle of which is given in Figure 67) allowing one to make a two-porphyrin rotaxane-like compound is indicated in Figure 68. 

Another threaded system, namely a [6]-rotaxane-like compound," has been proposed as a model of the special pair (SP) of bacteriochlorophylls found in the photosynthetic reaction centers of various photosynthetic bac-teria. -- Two copper(II) porphyrins are brought to close proximity in an interesting process, as represented in Figure 73. 

Rotaxanes have also been part of dendrimers, i.e., dendritic molecules containing rotaxane-like bonds to link... 

Interpenetrating networks have an obvious relationship to molecular catenanes indeed, they can be thought of simply as catenated polymers. Similarly, entangled networks with rotaxane-like relationships have also been reported. ... 

The rotaxane-like orientations observed in the solid state for the centro-symmetrically-located guests in the middle of the [BBIPYBIXYCY]" " cavities conjure up a highly suggestive situation. The marriage of the molecular recognition features from these two host-guest systems, combined with further covalent modification, leads... 

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