Chiral polyhedron

The zwitterionic A5S7-fluorosilicates 4-22 were isolated as crystalline solids. Compounds 4-8, 13, and 17-22 were structurally characterized in the solid state by single-crystal X-ray diffraction. In contrast to the achiral zwitterions 4-16, the zwitterions 17-22 are chiral, the respective crystals consisting of pairs of enantiomers [(A)- and (C)-enantiomers]. In all cases, the /-coordination polyhedron was found to be a somewhat distorted trigonal bipyramid, with fluorine atoms in the two axial sites. This is illustrated for 6 and 19 in Fig. 1. Selected geometric parameters for compounds 4-8, 13, and 17-22 are listed in Table I. As can be seen from these data, the axial Si-F distances [1.647(2)-1.743(1) A] are significantly longer than the equatorial ones [1.589(2)-1.638(1) A]. The Si-Cl distances amount... 

Attempts to produce descriptors similar to cis and trims for stereochemicidly more complicated coordination entities have tailed to achieve generality, and labels such as foe and mer are no longer recommended. Nevertheless, a diastereoisomeric structure may be indicated for any polyhedron using a configuration index as an affix to the name or formula. Finally, the chiralities of enantiomeric structures can be indicated using chirality symbols. 

The inversion of the N-atoms in the diethylenetriamine backbone is precluded. The central N-atom is always chiral, since the ethylene bridges around it have different conformations (<5 and A). Furthermore, the two other N-atoms are chiral as well in the bisamides and chirality of the metal ion should be taken into account. DTPA derivatives with three chiral N-atoms can be wrapped around the Ln3 + ion in 16 different ways (8 diastereomeric pairs, see Fig. 1). In crystal structures of these compounds the Ln3+ coordination polyhedron can generally be best described as a tricapped trigonal prism (TTP). For [Ln(DTPA)]2-A1=A2=A3 = B1=B2, and, consequently, only two enantiomers remain (1/1 ). Analogously, it can be seen that the DTPA-bisamides can have 4 diastereomeric pairs. 

What is chirality The term chirality denotes the property of a molecule to be nonidentical to its mirror image. All coordination polyhedrons with identical ligands are achiral. When the ligands are different, the chiral compounds may be formed. Thus, a tetrahedron with four different ligands is chiral. A trigonal bipyramid and tetragonal pyramid may be (but not necessarily will be) chiral if three (or more) ligands are of different types, e.g. 

Figure 3.31 The structures of (a) [Er2(Ala)4(H20)g] + and (b) [Er(Pro)2(H20)s] "+ [RE, black (large balls) O, grey N, black (small balls) C, white H, omitted)]. (Redrawn from the CIF files of T. Glowiak et al., Ligand chirality effect on the structure and its spectroscopic consequences in [Lu2(Ala)4(H20)8] (C104)6 crystals, Polyhedron, 15 (17), 2939-2947, 1996 [127] and A.Z. Ma et al, Structure of an erbium coordination compound with L-proline, ([Er(Pro)2(H20)5]Cl3 , Acta Crystallographica Section C, 49 (5), 865-867, 1993 [128].)...

This polyhedron is chiral and can be drawn as either enantiomer by appropriate choice of the 60 vertices, either red or blue, in Figure 2.24. All the lower orbit structures, O12, O20 and O30 shown in the second column of projections in Figure 2.24 are achiral and identical to those found by coalescing local sets of 10, 6 and 4 vertices in full Ih point symmetry. 

Figure 26.14 Supramolecular chirality provided by the tilt of the elements, (a) An enantiomorphic pair of polyhedrons,...

The examples presented in this section illustrate an elegant approach by Stang and coworkers to the design of chiral supramolecular polygons (squares, cages) and also chiral three-dimensional polyhedrons. The combination of tailored ligand connectors with the metal corners derived from the BINAP framework should allow the preparation of a variety of different supramolecular chiral objects. 

Ligand topology and geometry Extra stability (chelate, macrocyclic, and cryptate effects) Topicity Conformational stereochemistry (coordination polyhedron) Geometric stereochemistry (cis—Irans and fac-mer isomerism) Chirality... 

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