Macrobicyclic framework

The essential information on the spatial and electronic structure of diamagnetic sarcophaginates and sepulchrates in solution has been obtained from their and NMR spectra, NMR is a more precise and more reliable instrument for investigations of such complexes due to a wide chemical shift range. The carbon atoms of the macrobicyclic framework are numbered as shown in Scheme 101. 

The i/c=N values in IR spectra of the di- and hexachloride clathrochelate precursors are the lowest of all those known so far for clathrochelate iron(II) tris-dioximates. The vc=n bands of both types of dioximate fragments distinctly appeared in the case of partially substituted Cs-nonsymmetric complexes. Alongside the vm-o and vb-o bands of the macrobicyclic framework, the IR spectra of ribbed-functionalized clathrochelates also contained characteristic lines of the substituents in dioximate fragments [65, 68]. 

Some data on the redox reactions of the substituents at the macrobicyclic framework are represented in Table 51. A comparison of the data from Tables 42 and 51 indicates that the encapsulated cobalt(III) ion and the substituents reduce at substantially different potential values. The nitro group reduction depends slightly on the... 

Hydrogen bonds between the water molecule and the oxygens of the macrobicyclic framework could stabilize such species. 

Macrobicyclic ligands incorporating heterocyclic subunits. Vogtle (58) and Newkome (59) have successfully prepared cryptands containing respectively one (58) and three (59) pyridine subunits in the macrobicyclic framework (Figure 2.18). The ligand (58) forms complexes with many metal cations, and binuclear cobalt and copper complexes of (59) were isolated. 

A hexaamide covalent capsule 148 with an extended macrobicyclic framework has been described in [111] as a first polynucleating amide caging ligand with two of three pyridine-containing ribbed fragments having the same orientation and the third one looking at the... 

Severin and coworkers reported (146) the reaction of tris(2-aminoethyl)amine and 4-formylphenylboronic acid with penta-erythritol to give, via multicomponent assembly, the boronic acid based macrobicyclic cage 35 (Fig. 25). The cage has the form of an ellipsoid with a diameter of 20.5 A and binds two Cud) ions in a fashion similar to the smaller tren-based cryptands. The reversible formation of boronic esters has also been employed to build other hollow structures such as nanotubes (147) and porous covalent organic frameworks (148,149). 

It is undoubtedly of interest that the substituents in the clathrochelate framework and in apical groups affect the structure and properties of macrobicyclic complexes. In particular, it was noted that N-methylation of the complexes must stabilize the lowest oxidation states of the encapsulated metal ion. In this case, one should take into account the steric effects of substituents whose introduction influences the dissociation kinetics of the sarcophaginates [112]. 

The direct X-ray diffraction study is undoubtedly the most optimal and informative structural method for all compounds. Unfortunately, no one has obtained crystals of the binuclear iron(II) azineoximates, which are suitable for X-ray crystallography [193], The structure of these complexes was deduced from EXAFS data, which allow one to obtain the function of the radial atom distribution (RAD) and hence to determine coordination numbers and the distances from the encapsulated iron ions to the clathrochelate framework atoms, and from Fe Mdssbauer spectra, which make it possible to predict the distortion angle (p (see above). Macrobicyclic iron(II) tris-dioximates... 

The quasi-reversible character of these redox reactions indicates that some geometrical changes are taking place without changing the clathrochelate framework. As in the case of trinuclear cobalt complexes, macrobicyclic nickel dioximates are characterized by a high oxidation state stabilization in comparison with other nickel complexes. For example, for Ni(HDm)2 bis-dioximate, two processes (oxidation and reduction) were observed for nickel(II) ion [80],... 

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