Subject macrocyclic ligands

Cadmium complexes of macrocyclic ligands have not been as widely studied as the corresponding zinc complexes, but the fact that cadmium macrocycles undergo easy metal exchange should make them attractive subjects for future study. The crystal structure of [Cd(TPP)(dioxane)j and its 113CdNMR characteristics have been reported.1144 The formation... 

The thermodynamic origins of the enhanced stabilities of macrocyclic ligands over their acyclic counterparts have been the subject of considerable debate since the term macrocyclic effect was first coined.83 Comparison of thermodynamic data for the several metal ion complexes of the [18]crown-6 and its acyclic counterpart are shown in Table 1. Enthalpy contributions to stabilization appear strongest for the K+ complex, while entropic contributions are stronger for the Na+ complex. Undoubtedly, the factors responsible for the thermodynamics will vary according to ion size, charge, solvation effects and structural preference. Hence, a single definable source of the macrocyclic effect is, in these systems at least, probably nonexistent. 

The energetics of isomer prediction using molecular mechanics is discussed in detail in Chapter 7. One of the results of such a study is the structure of each of the isomers.The archetypal studies in this field relate to the complexes [Co(dien)2]3+ (dien = 3-azapentane-l,5-diamine see Chapter 7). Other important studies include those on macrocyclic ligands (see also Chapter 8). Tetraaza macrocyclic ligands, for example, can adopt a series of configurational isomers, and these have been the subject of numerous molecular mechanics calculations. Consider an equatorially coordinated tetraaza macrocylce. Each of the amine groups can coordinate with the amine proton or substituent disposed above or below the coordination plane. How many isomers result depends on the symmetry of the macrocycle. For example, in the classic case of cyclam (cyclam - 14-ane-N4 = 1,4,8,11-tetraazacyclotetradecane) there are five isomers[12] and these are shown schematically in Fig. 6.3. It is not always possible to prepare or separate all of these isomers and, therefore, in many cases only a minority have been structurally characterized. Thus, the energy-minimized structures represent the best available three-dimensional representations of the other isomers. 

Creative chemistry must begin with synthesis. The chemical synthesis of macrocyclic ligands that can function as selective receptors for ions and neutral molecules is a central theme of modern supramolecular chemistry. Indeed the synthesis of these tailored receptors can be a real challenge, but it is also great fun I hope that this book conveys some of the enthusiasm of the authors for their subject. 

These complexes, all of which contain cobalt within a macrocyclic ligand and an axial cobalt-carbon bond, have been found to be photosensitive and have been the subject of numerous photochemical studies. Space does not permit a detailed summary of all the studies which have been conducted on these classes of compounds, and we present here only a general summary of the various observations and discuss pertinent articles from the most recent literature. The reader is referred to an excellent review of the subject by Koerner von Gustorf et al. 108) which presents a detailed discussion of reports that appeared prior to 1975. 

Macrocyclic ligands with all-nitrogen donor sets are much studied and both tin and, in particular, lead are popular subjects in coordination studies of these ligands. Examples of such ligands used to complex tin include (66) and (67), prepared by Schiff-base condensations. The complex (68) was isolated from an attempted template synthesis of a macrocycle in which the condensation of the component parts of the ligand was incomplete. 

A number of polymer systems containing macrocyclic ligands, such as the crown ethers, have been prepared. This subject has been reviewed a number of times (Blasius and Janzen, 1982 Gramain, 1982 Sahni and Reedijk, 1984 Smid, 1976, 1981a, 1981b Smid et al., 1979 Takagi and Nakamura, 1986 Yokota, 1989). These reviews report on crown ethers or cryptands attached to solid supports or as part of a polymer system. There have been no separate reviews of polymers containing only aza- or peraza-crown macrocycles. 

In such a large subject, this article can only focus on certain aspects, namely those that involve complexation with inorganic substrates. We only consider the synthetic macrocycles, with emphasis on transition metal complexation. Aza, oxa, and, to a lesser extent, thia and phospha macrocycles are also covered. The naturally occurring porphyrins, corrins, corphins, chlorins, and phthalocyanins, as well as the cyclodextrins, are not included. Because of the general complexity of macrocyclic systems and the resulting complicated systematic names, commonly used abbreviations or simplified names will be employed. This review will encompass the synthesis, thermodynamics, structure, and applications of macrocyclic ligands. 

Polymers carrying pendant macrocyclic ligands have been a subject of considerable interest thus poly(crown ether) (9) has been extensively investigated as a complexing agent of metal cations. ""... 

The ligand reaction step may occur either with the template metal still intact or may take place after removal of the metal ion from the ring. As already mentioned, many of the Schiff-base macrocycles are unstable in the absence of a coordinated metal ion. However, for such systems, it has often been possible to hydrogenate the coordinated imine functions directly. The resulting saturated ligands will not be subject to the hydrolytic degradation which occurs for the imine precursors in the absence of their metal ion. 

As with any modern review of the chemical Hterature, the subject discussed in this chapter touches upon topics that are the focus of related books and articles. For example, there is a well recognized tome on the 1,3-dipolar cycloaddition reaction that is an excellent introduction to the many varieties of this transformation [1]. More specific reviews involving the use of rhodium(II) in carbonyl ylide cycloadditions [2] and intramolecular 1,3-dipolar cycloaddition reactions have also appeared [3, 4]. The use of rhodium for the creation and reaction of carbenes as electrophilic species [5, 6], their use in intramolecular carbenoid reactions [7], and the formation of ylides via the reaction with heteroatoms have also been described [8]. Reviews of rhodium(II) ligand-based chemoselectivity [9], rhodium(11)-mediated macrocyclizations [10], and asymmetric rho-dium(II)-carbene transformations [11, 12] detail the multiple aspects of control and applications that make this such a powerful chemical transformation. In addition to these reviews, several books have appeared since around 1998 describing the catalytic reactions of diazo compounds [13], cycloaddition reactions in organic synthesis [14], and synthetic applications of the 1,3-dipolar cycloaddition [15]. 

This section covers the coordination chemistry of aza macrocycles taken as rings of nine or more members, with three or more ring nitrogen atoms, and not included in Chapter 21.1. These ligands have many properties in common with their non-cyclic analogues (Chapter 13) and also have properties in common with other cyclic polydentate ligands (Chapters 21.1 and 21.3). The subject was comprehensively reviewed in 1979,1-6 and in general references included therein will not be listed. 

Such complexes form a precursor to a full discussion of the vast and highly topical field of self-assembly (Chapter 10). We consider them here since they resemble structurally the types of compounds discussed in Section 4.7, but unlike metal-based anion receptors the simple thermodynamic equilibrium between host, anion and complex is not the only process occurring in solution. In fact multiple equilibria are occurring covering all possible combinations of interaction between anions, cations and ligands. These systems have the appeal that the formation of particular metal coordination complexes are thus subject to thermodynamic anion templating (cf. the thermodynamic template effect in macrocycle synthesis, Section 3.9.1) and vice versa. 

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