Macrobicyclic ligand

Recently considerable attention has been directed at anion binding ligands. Macrobicyclic 27 29) and macrotricyclic amines 30,31) were topologically designed to host anions such as spherical Cl-, linear Nf 32). These anion substrates are incorporated into macrocyclic cavities lined with appropriate anion-binding sites capable of forming hydrogen bonds like those of protonated amines (see /, below). 

Another group of very effective ligands that have recently been employed to coordinate alkali metal cations are the macrobicyclic polydentate ligands that J.-M. Lehn has termed... 

The remarkable physical properties exhibited by the divalent macrobicyclic cage complex [Co(sep)]2+ (29) are unparalleled in Co chemistry.219 The complex, characterized structurally, is inert to ligand substitution in its optically pure form and resists racemization in stark contrast to its [Co(en)3]2+ parent. The encapsulating nature of the sep ligand ensures outer sphere electron transfer in all redox reactions. For example, unlike most divalent Co amines, the aerial oxidation of (29) does not involve a peroxo-bound intermediate. 

Smaller macrobicycles and macrotricycles bearing less that six N-donors are also known. These ligands comprise tertiary or secondary amines and are derived from insertion of one or more straps between pairs of potentially bridgehead N-donors contained within a small ring macrocycle, and in this sense they may be considered analogs of the well-known azacryptands. The inability to offer six N-donors, and the presence of tertiary amines in these ligands leads to the preference for divalent over trivalent Co. 

The spectrum of the octahedral Ni11 complex (580) with a hexaaza macrobicyclic ligand is similar to that of [Ni(en)3]2+, but the ligand field strength of the complex is increased slightly by the capping of the [Ni(en)3]2+ moiety with two tris(methylene)amino groups.1461... 

Sargeson and co-workers have reported the use of [Pt(en)3]4+ in template reactions to produce the platinum(IV) complexes of the macrobicyclic ligands sep and (N02)2sar (170).477 These reactions are analogous to those that occur around cobalt(III). However, in contrast to the [Co((N02)2sar)]3+ system, reduction of the pendant dinitro groups did not yield amines, but hydroxyamine groups.478... 

An attractive feature of this route is that the macrobicyclic intermediate 40a can be readily N-alkylated without affecting the masked thiolate functions (209). Thus, reductive methylation of 40a with formaldehyde and formic acid under Eschweiler-Clarke conditions, followed by deprotection of 40b with Na/NH3 provides the otherwise inaccessible N6S2 ligand H2L19 in nearly quantitative yield (Scheme 1). Several other aza-thioethers 40c-p and their corresponding thiophenolate ligands were prepared by this route (see Fig. 30 and Table I) and some of them will be discussed further in later sections (210-215). 

The deprotection of the polyamine-thioethers 40a,b can also be accomplished in similar good yields with Li/naphthalene in THF (214). Cleavage of the thioether functions also commenced with [PdCl2(NCMe)2] in the case of 40b (216), and it is likely that the other macrobicycles will also undergo this metal-ion mediated cleavage reaction as well (Scheme 2). Note that this reaction cleaves only one of the two thioether bonds forming a macrocyclic ligand with one thiophenolate and one vinyl-thioether moiety, as... 

Figure 18 Macrobicyclic ligand 1,15-diaza-3,4 12,13-dibenzo-5,8>11-trioxybicyclo[13,3,1]nonadecane (C22H28N2O3).

Lehn and coworkers52 reported the synthesis, crystal structure and dinuclear copper(I) complexes of tris-carotenoid macrobicyclic ligands. The macrobicycles 89 and 90 were obtained in good yields in a one-step macrobicyclisation condensation between the tripode N(CH2CH2NH2)3 and the polyolefinic dialdehydes 93 and 94. 

In comparison, both the free ligand and the dinuclear Cu(I) cryptate of an analogous macrobicyclic structure possessing a diphenylmethane group as a central unit display only two resonances for the CH2CH2 fragment, as is the case here only for the complexes 91 and 92. This points to the special conformation features of the free macrobicycles 89 and 90. 

We believe that these large changes in rate constant are primarily attributable to restrictions imposed upon the inner-coordination sphere by the macrocyclic ligands. Presumably, the use of a suitable macrobicyclic ligand, such as you have used... 

The tripodal tris-catechol ligand trencam, (258), is an enterobactin analogue and is very specific for Fe compared with Fe +, as is reflected in the reduction potential of —1.04V (vs. NHE for Fe/enterobactin —0.99V). The stability constant for [Fe(trencam)] is The macrobicyclic... 

Fig. 3. Synthetic strategies towards macrobicyclic ligands of type G...

Table 6. Approximate cavity diameters in macrocyclic, macrobicyclic and macrotricyclic ligands...

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