Bonding topology

Hawthorne, F. C. (1992b). Bond topology, bond valence and structural stability. In G. D. Price and N. L. Ross (eds). The Stability of Minerals. London Chapman and Hall, pp. 25-87. 

Numerous examples of catenanes 1, rotaxanes 2, and trefoil knots 3 (Scheme 1) have been previously reported in the literature and are still attracting considerable attention (see Chapters 4 and 6-8) [1-5]. These aesthetically appealing molecules have in common that the topological bonds occurring in catenanes 1 and trefoil knots 2 and the mechanical bonds connecting the component parts of rotaxanes 3 are defined at a molecular scale without ambiguity [1, 2, 4]. 

A totally new situation arises from the presence of defined topological bonds in polymer systems. The last documented example is given by polyrotaxanes 7 in which defined topological bonds occur between the macrocycles and the polymer chain (considered as infinite). The polyrotaxanes are composed of a polymer chain on to which a certain number of macrocycles is threaded. For short polymer chains, the end-capping by stoppers prevents the macrocycles unthreading from the chain [27, 28] (Scheme 3). Multicatenanes 8 are structurally related to polyrotaxanes 7 and can be viewed as cyclic analogs of polyrotaxanes [29]. 

The interest in macromolecular systems containing defined topological bonds, such as polyrotaxanes 7, multicatenanes 8, polycatenanes 9, poly[2]catenanes 10, and polymeric catenanes 11, is dual. First, these macromolecules represent daunting synthetic and characterization challenges which deserve attention in their own... 

The interest in and fascination with macromolecular architectures containing defined topological bonds arise, on the one hand, from the synthesis of well-defined polymers of ever increasing structural complexity and, on the other hand, from the prospect of discovering new physical behavior resulting directly from the presence of topological bonds. 

Beside synthetic features, the investigation of unprecedented physical behavior resulting directly from the presence of defined topological bonds is particularly exciting. The most directly accessible macromolecular architecture for physical measurements is the poly[2]catenane 10, because poly[2]catenanes containing... 

Scheme 29. Unreported macromolecular architectures containing defined topological bonds polycatenane 9, linear poly[3]catenane 74, poly[2]catenane network 75, multicatenane network 76, rigid polymeric catenane 77, polymeric trefoil knot 78, and polyknot 79.

All this reflects excitement by today s knowledge and, even more fascinating, tomorrow s potential developments conjoindy in the synthetic chemistry and experimental physics of macromolecular architectures containing defined topological bonds. 

FIGURE 14. CC bond orders n and bond ellipticities i of cyclopropyl homoconjugated molecules (a) norcaradiene, (b) bicyclo[2.1.0]pentene, (c) bicyclo[3.1. OJhexenyl cation. On the right, the preferred mode of electron delocalization is indicated by dashed lines. Also given is the number of delocalized electrons as calculated from topological bond orders. See text... 

In catenanes the mechanical bond that holds the components together is also a topological bond (the interlocked rings cannot be separated without cleavage of one ring), whereas in rotaxanes only deformations of one of the components are required to dissociate the system. Therefore rotaxanes owe their existence to the larger size of the stoppers as compared to the ring diameter, that is, to steric factors. 

The Zintl anion has an electronic structure which is somewhat in between a completely singly bonded (q = -14) and a carbonate related system (q = -12). On the basis of the available topological bond vectors and the actual electron count one would expect 2/3 of a 7t-bond in the trigonal part. A metallic conductivity is anticipated, e.g., overlap of unfilled 7t-type orbitals along the ecliptically stacked Si moieties. 

The non-trivial character and the structural peculiarities of compounds with topological bonds stimulates the elaboration of equally non-trivial approaches to the solution of synthetic problems in this field. In this respect it seems... 

The synthesis of the models shown above acquired additional meaning from the absolute novelty of the stereochemistry problems which became available to experimental studies. While it is premature to make definite suggestions about the peculiarities of the chemistry of compounds having topological bonds, the uniqueness of their structure seems to guarantee that a number of non-trivial phenomena, such as unusual chelating properties, regulated catalytic activity, etc., will be discovered in this field. ... 

Hosoya, H., and Murakami, M., Topological index as applied to p-electronic systems. II. Topological bond order. Bull. Chem, Soc. J, 48(12), 3512-3517 (1975),... 

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