Topologically complex molecules, formation

It should be stressed that the coding for the formation of these topologically complex molecules needs to be carefully controlled in order to obtain the desired structures. To illustrate this, consider ligand 7.59, which contains two didentate metal-binding domains. This might be expected to react with octahedral metal ions to give a triple-helical dinuclear complex. Reaction with iron(n) does indeed give a species of stoichiometry [Fe2(7.59)3]4+ however, the crystal structure reveals that an untwisted complex, 7.60, has been formed. 

SUPRAMOLECULAR CHEMISTRY AIDING THE FORMATION OF TOPOLOGICALLY COMPLEX MOLECULES... 

Inclusions of Other Grown Analogues. A variety of crown analogues and hybrid modifications (24—28) with other topological features (lariat ethers (31,32), octopus molecules (33), spherands (eg, (12) (34), torands (35)) including chiral derivatives (36) have been prepared and demonstrated to show particular inclusion properties such as chiroselective inclusion (Fig. 4) (37) or formation of extremely stable complexes (K ">(LR) for (12)... 

Up to this point, we have emphasized the stereochemical properties of molecules as objects, without concern for processes which affect the molecular shape. The term dynamic stereochemistry applies to die topology of processes which effect a structural change. The cases that are most important in organic chemistry are chemical reactions, conformational changes, and noncovalent complex formation. In order to understand the stereochemical aspects of a dynamic process, it is essential not only that the stereochemical relationship between starting and product states be established, but also that the spatial features of proposed intermediates and transition states must account for the observed stereochemical transformations. 

Both molecules, 28 and 30, are topologically chiral the chirality of 30 was proven by H-NMR spectroscopy with the addition of KPFs and Pirkles reagent. This resulted in different chemical shifts for the hydrogen atoms of the clover-leaf knot 30 due to formation of diastereomeric complexes. 

The meaning of a complex formation constant is not as clear-cut as one can wish. The behavior of aqua ions which may be either definite species such as Ni(H60)6" or vague representations of the standard solute in dilute solution has already been discussed. One then must discuss the question When is a ligand bound to a central atom Certainly, this problem does not occur for most complexes of multidentate ligands, but weak complexes are not in an easy position. Prue (89) correctly emphasized that without any specific attraction between two molecules, for purely topological reasons, one would find a complexity constant of 0.2 liter/mole from contact charge-transfer spectra in typical conditions. If one finds a smaller formation constant, the two molecules repel each other... 

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