Complex Cascades

Hancock, R. D. and Martell, A. E., Ligand design for selective complexation of metal-ions in aqueous-solution , Chem. Rev. 1989,89,1875-1914. 

One of the earliest Schiff base macrocycles to exhibit a haemocyanine-like structure was the copper(II) perchlorate complex of 5.5 which binds readily to azide or hydroxide. The azide complex exhibits two square pyramidal copper binding domains with the basal plane occupied by one pyridyl nitrogen atom and two imine functionalities as well as a terminal azide ligand. The apices of the two pyramidal coordination polyhedra are linked by a single bridging azide anion. Continuing the biomimetic theme, manganese(II) cascade complexes of the unsymmetrical 5.6 have 

Macrobicyclic cascade cryptands have also been prepared such as the series of bis(tren) derived (tren = tris(2-aminoethyl)amine, see Section 4.4.3) compounds 5.9. The di-nickel(II) and di-copper(II) complexes of the cages with various spacers all bind a metal cation into each tren unit. These metal 

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Fig. 20. (a) Schematic illustration of the formation of a cascade complex (b) a heterodinuclear cation complex of a diloop crown receptor and (c) a typical... 

The binuclear Co(n) cryptate (monohydroxo bridged) was also found to combine reversibly with dioxygen yielding a doubly-bridged cascade complex containing both hydroxo and peroxo bridges within the cryptate structure. 

An example of the above mentioned cascade complexation of carboxylates by macrocyclic receptors containing metal ionic centers is the inclusion of oxalate by the dien dicobalt complex 9 (Martell, Mitsokaitis) [12]. Similarly, the -cyclodextrin (jS-CD) 10, modified with a zinc cation bound by a triamine side chain, encapsulates anions like 1-adamantylcarboxylate in its cavity, fixing them by combined electrostatic and hydrophobic interactions [13], Zinc s group achieved the enantioselective transport of the potassium salts of N-protected amino acids and dipeptides by making use of the cation affinity of... 

A cascade complex is one in which an additional substrate may be included between metal cations (or eventually anions) in a ditopic or polytopic ligand. 

Numerous dinucleating macrocyclic and macrobicyclic ligands have been synthesized, in particular by the versatile amine + carbonyl — imine reaction they form dinuclear metal complexes as well as cascade complexes with bridging groups [2.58-2.63, 3.24-3.27, 4.1-4.4], for instance in dicobalt complexes that are oxygen carriers [3.26]. 

Faced with the challenge of ion pair binding, there are three basic ion pair receptor designs contact ion pair receptors (including cascade complexes) in which the anion and cation are bound as a contact ion pair, ditopic receptors with individual, well-separated anion and cation binding sites, and zwitterion receptors, Figure 5.2. We will discuss some examples of each type of complex in the following sections. 

Figure 10. Crystal structure of the fluoride cascade complex of 6 showing die side view (A) and view down the pseudo-threefold axis (B).

Many artificial systems have been designed recently to imitate the function and behaviour of native enzymes - biomimetic chemistry [27]. Among them, calixarene-based receptors bearing one, two or three Zn(II) complexes on the upper rim were prepared as a model for phosphoesterases [28-31]. Dinuclear receptor 25 was reported to enhance the rate of transesterification of the RNA model substrate 2-hydroxypropyl-p-nitrophenyl phosphate more than 20,000 times compared with the non-catalysed reaction. The complexation mode for the phosphate anion can be described as cascade complexation where the anion is coordinated within the cavity formed by two zinc cations. 

Two chiral [2.2.1]cryptands, one incorporating the methyl 4,6-O-benzylidene-a-D-mannopyranoside unit (70), the other the methyl 4,6-0-[(S)-phenylethylidene]-a-D-mannopyranoside unit (77), may exhibit the same two different modes of complexa-tion as before binding of primary alkylammonium cations via hydrogen bonds between the NH-f group and the heteroatoms of the ligand or cascade complexation of ion pairs. Chart 2 shows the constitutions of the two cryptands 70 and 77 and is also indicative that the approach of guest molecules may occur from the more sterically hindered side. 

The second class of dinuclear Cu —Cu complexes is represented by several cascade complexes, in which a pair of cations is encapsulated within a cryptand host (XVII-XIX), with a bridging CN ligand spanning the space between the cations. In these cases, the dinuclear complex is stabilized due to the encapsulated cations rather than by the cyanide bridge. Lu et al., (78) reported a product of this type, [Cu°2(XVII)(CN)](C104)3 (Fig. 9), as a result of the unexpected C C bond cleavage of acetonitrile in the presence of [Cu 2(XVII)]( 104)4. These authors established that the reaction rate exhibits a first-order dependence... 

Chiral di mination studies with the tetra-amine 54 were also performed with respect to racemic molecular anions via the formation of cascade complexes. Regulation of the chiral discrimination may be achieved to some extent by the nature of the cation initially complexed. Efficient ion pairing with molecular anions is favoured in solvents of low polarity. In this respect, aqueous solutions of alkali metal salts of ( )-mandelic acid or (+)-a-hydroxy-l-naphthaleneacetic acid were extracted into a CDCI3 phase where the ligand 54 is dissolved. In case of the mandelate anion, the anion ligand ratios in the CDCI3 layer were as follows Na (0.6 1), (1 1),... 

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