Sarcophaginates and sepulchrates

Since sarcophaginates and sepulchrates are relatively easy to crystallize, a great number of these compounds are studied by X-ray crystallography, which together with molecular geometry calculations makes it possible to establish their three-dimensional structures both in crystal and in solution. The optical activity of such clathrochelates enables one routinely to utilize circular dichroism measurements to investigate their structure. The spatial and electronic structures of sarcophaginates and sepulchrates are much more seldomly determined by alternative spectral techniques compared with clathrochelates of other types. 

Phosphorus-containing tris-diimine d-metal complexes are isolated as ionic associates with a bulky inorganic BF4 anion and readily crystallize. Therefore, monocrystals of these compounds, suitable for X-ray analysis, were obtained, and X-ray crystallography - the major method for determining their geometry - was applied to all complexes of this type. The subtle features of the electronic structure of macrobicyclic phosphorus-containing d-metal tris-diiminates have been examined by a variety of spectral methods and quantum-chemical calculations. 

For macrobicyclic d-metal tris-dioximates, the X-ray crystallography analysis is performed least often. This is because the charge of the central metal atom in the majority of such compounds is balanced by that of the clathrochelate ligand, and crystals of the resulting intramolecular complexes are purely molecular. Alongside the structural peculiarities in the structure of such complexes, this 

The main parameters describing such a coordination polyhedron are the distortion angle (p (0° and 60° correspond to the TP and TAP, respectively), bite angle a (half the chelate angle), distance h between the coordination polyhedron bases, and the metal-donor atom distance a. 

As mentioned above, the three-dimensional structure of a considerable number of these compounds has been established mainly by X-ray crystallography. The main structural characteristics for sepulchrate and sarcophaginate complexes as well as for free ligands are listed in Tables 2 and 3. 

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Symbols of the apical substituents in the capping fragments of sarcophaginates and sepulchrates. 

As mentioned above, a template synthesis is a very efficient approach to the preparation of sarcophaginates and sepulchrates of certain metals. For all other metal ions, this synthetic pathway is inapplicable or gives the desired products in low yields. For instance, the yields of nickel(II) and chromium(III) sepulchrates resulting from template condensation are only ca 1 and 10%, respectively. This problem can largely be overcome if the synthesis of a variety of metal... 

Methods for the synthesis of sarcophaginates and sepulchrates based on redox and photochemical reactions are discussed in Chapter 5. 

Thus, for theoretical calculations of the sarcophaginate and sepulchrate structures, one should take into account inter- and intramolecular nonbonded interactions and electronic effects. The calculations based on geometric and/or repulsion models are obviously justified only for complexes with insufficient or no preference for TAP or TP structure (ALFSE = 0). In this case, the ligand determines the complex geometry. In all other cases, the contribution of the metal ion electronic configuration cannot be neglected [178],... 

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. 

C NMR spectra, like H NMR data, confirmed the existence of a threefold symmetry axis in symmetric sarcophaginate and sepulchrate molecules, as well as the existence of only one diastereomer in solution. It has been pointed out [121] that three diastereomers are... 

The UV-vis spectra of the cobalt(III) sarcophaginates and sepulchrates contain two bands of d-d transitions from the Aig term to the Tig and T2g terms in the UV and visible regions, which are... 

The UV-vis spectra of sarcophaginates and sepulchrates of the platinum subgroup metal ions are much less informative. In most cases, the d-d transition bands have been observed as a shoulder at CO 4000 cm- to more intensive CTB. Only spectra of rhodium(III) complexes with electronic configuration d oi the encapsulated metal ion show the Aig—> Tigand d-d transition bands at 33 000... 

In all cases mentioned, the spectral and X-ray diffraction data on reaction intermediates and by-products have widely been used in considering kinetic and thermodynamic relationships. The identification of intermediates and by-products is still more important in determination of the mechanisms of sarcophaginate and sepulchrate synthesis because in this case kinetic approaches cannot be used. 

The fact that the synthesis of sarcophaginates and sepulchrates proceeds via the formation of an imine complex was also confirmed by isolation of the semiclathrochelate [Co(sen)]- + complex (as a by-... 

The fact that free ligands can be obtained by demetallation of the preliminarily synthesized complexes is a striking feature observed in the chemistry of sarcophaginates and sepulchrates. Studies on the mechanism of the demetallation (extrusion) occurring in acidic media are of great importance. 

In the case of sarcophaginates and sepulchrates, the M-N distances exceed the sum of the nitrogen radius and the corresponding metal ionic radius in the appropriate oxidation state. Thus, a redox process is not limited by the cavity size, which in bonding can either decrease or increase with the change in the radius of the central ion owing to structural rearrangement. The increase in the cavity size in a reduction process occurs on translational rotation... 

When immobilized on polymer surfaces, cage complexes may be utilized for modifications of electrodes. The increase in the electron-transfer rate on such surfaces is governed by two factors a high rate of electron transfer in cobalt clathrochelates and the regular disposition of these complexes on the surface. The properties of immobilized macrobicyclic complexes have been considered in Refs. 94, 410, and 411. Cyclic voltammetry has been used to characterize the incorporation of a range of structurally different d-metal sarcophaginates and sepulchrates into Nafion polymer [412]. 

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