Multitopic

The tetradentate ligands (340) and (341) form 1 1 metakligand complexes with [IrCl(cod)]2.548 The complexes were tested in the asymmetric hydrogenation of prochiral olefins, providing enantioselectivities up to 36%. The multitopic ligands L, (342) and (343), bind to Ir1 to form [IrL] species which have been characterized by elemental analysis, mass spectrometry, IR and NMR spectroscopy.549 The complexes show enantioselectivities of up to 30% for the hydrogenation of prochiral olefins under mild reaction conditions. 

The huge, multitopic molecule 76, recently synthesized in our laboratories, was conceived as a receptor for trinucleotides, namely ApApA. The compound consists of three adenine pincers connected by two guanidinium linkers. In a 1 1 complex that is currently under investigation in a further cooperation with the MIT group, ApApA is supposed to be bound by interplay of two electrostatic attraction forces, threefold arene-arene stacking, and thirteen hydrogen bonds in all [104]. 

Fedorov, Yu., Fedorova, O., Schepel, N., Gromov, S., Alfimov, M., J. Saltiel (2004) Synthesis and multitopic complex formation of die photochromic bis-crown ether based on benzobis(thiazole), J. Phys. Org. Chem., in press. 

Exodentate multitopic ligands are used to link transition metal ions into building blocks. Some examples of such ligands are 2,4,6-tris(4-pyridyl)-l,3,5-triazine, oligopyridines, and 3- and 4-pyridyl-substituted porphyrins. 

In this chapter, we cover the metal-directed self-assembly of 3D architectures using macrocycles as molecular scaffolds. Particular emphasis will be given to the synthetic strategies leading to coordination cages and to multitopic macrocyclic receptors, as well as to their complexation properties. 

While the design and formation of self-assembled coordination cages described in the previous sections have received much attention, the corresponding metal-directed self-assembly of open multitopic receptors has been almost neglected so far. One reason is related to the need for partially derivatized precursors, which are synthetically more demanding than the corresponding fully derivatized ones. 

Romero-Salguero, F.J., and Lehn, J.-M., Synthesis of multitopic bidentate ligands based on alternating pyridine and pyridazine rings. Tetrahedron Lett., 40, 859, 1999. 

The derivation of quantitative information concerning the potential barrier to internal rotation from such measured frequencies in multitop molecules is a somewhat questionable procedure, which will not be treated in great depth here. For a two-top molecule the potential energy function can be expanded in a Fourier series (omitting sixfold and higher terms) to obtain... 

The rather wide variation in barrier parameters derived from different methods is illustrated in Table 1. The neutron frequencies for liquid propane are used for these barrier comparisons since they are more reliable and, indeed, identical to the gas-phase results within the estimated uncertainty. The same molecular parameters used in the microwave study22,23 are used in each calculation, so that uncertainties in kinetic coupling effects do not affect the relative barrier values. It can be seen (not unexpectedly) that the barrier terms obtained by different methods using identical frequencies differ by as much as 30 percent. It should be noted that, as pointed out by Weiss and Leroi,21 the assumption of a harmonic potential in the Lide and Mann method causes an inherent decrease in the derived V3 terms ( 10 percent) compared with methods retaining the sinusoidal potential. These errors are quite similar for different molecules, however, so that comparisons between barrier results for different molecules by this method can be informative.21 It must obviously be concluded that barrier parameters obtained for multitop molecules on the basis of one or two observed frequencies have limited quantitative significance, except in relative terms, and that one must carefully scrutinize the method of calculation in evaluating such results. 

With these observations in mind, we shall now discuss two other examples of neutron spectra of multitop molecules o-xylene (1,2-dimethyl benzene) and neopentane (tetramethylmethane). o-Xylene is an interesting prototype for internal rotation and top-top coupling in substituted aromatics.29 The results for o-xylene by small k28 and cold neutron18 experiments are shown in Figure 6. The strongest peaks in both these... 

This chapter focuses on supramolecular porphyrin systems that are stable enough to be characterized in solution. Assemblies coordinated only in solids (e.g., crystals) will not be included. In order to maintain these supramolecular structures in solution, strong complementarity and multitopic coordinations are required, rather than a simple monotopic axial coordination. Of these conditions, this discussion will be concerned only with those necessary for the complementary coordinations, and with the presentation of various examples of these coordinations. The issue of multitopic coordination will be dealt with in another forum. Self-complementary coordination can be used to obtain dimers, rings, and one- and two-dimensional arrays. These supramolecular systems should not only influence structure formation, however, but also be closely related to their functions. It is therefore natural to discuss the... 

It is worth remarking that the use of Eq. 3, as well as the Scatchard plot and the Hill plot, must be confined to the case in which a symmetric multitopic donor (or acceptor) interacts with a monotopic acceptor (or donor). Use of these methods to assess cooperativity outside this specific case is improper and has been criticized [31]. 

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