Ligands sepulchrate

Figure 7-16. A partial mechanism for the stepwise formation of a capping group from the reaction of [Co(en)3]3+ with formaldehyde and ammonia. A similar process leads to the formation of the second cap and the encapsulating sepulchrate ligand.

Selection rules 132 Laporte 132 spin 132 Self-assembly 90 Sepulchrate ligand 56 Sequestering agent 16 Shielding, electrons 20 Soft ligand 78 Soft metal 78 Solid angle factor 85 Solid angle sum 85 Spectrochemical series 112 Spin-only formula 146 Spin selection rule 132 Square planar complexes 108... 

Closely related to the football ligands are the so-called sepulchrate ligands. One can be formed by the condensation of formaldehyde and ammonia onto the nitrogen atoms of tris(ethylenediamine)cobalt(llI). This results in tris(methylene)amino caps on opposite faces of the coordination octahedron. If the synthesis utilizes one of the (A, A)-enantiomers, the chirality of the complex is retained. Furthermore, the complex may be reduced to the corresponding cobalt(II) cation and reoxidized to co-balt(III) without loss of chirality. This is particularly unusual in that, as we shall see in the following chapter, cobalt(ll) complexes are quite labile in contrast to the stability of cobalt(III) complexes. Once again the extra stability of polydentate complexes is demonstrated. 

The sepulchrate ligand, which is hexadentate (the top and bottom nitrogens do not normally coordinate)... 

As for the difference observed for the sepulchrate complex, that may have to do with strain in the ligand. A student of mine has done some calculations considering the fact that the ligand itself may change the preferred distance and change the frequencies. This could account for a large part of the difference between the self-exchange rate constants for the sepulchrate and trisethylenediamine complexes. 

Figure 7-13. A space-filling representation of the cobalt(m) sepulchrate cation showing that the cobalt centre is buried deep within the ligand. The cobalt and nitrogen atoms are shaded. The ligand is oriented such that the two capping nitrogen atoms lie along the x axis.

Some Mechanistic and Stereochemical Aspects of Sepulchrates and Related Encapsulating Ligands... 

Figure 7-17. Views of the two enantiomeric A and A forms of the cobalt(m) sepulchrate cation. The ligand has been oriented such that the capping nitrogen atoms lie along the axis perpendicular to the page. The faces described by the two sets of nitrogen donor atoms in each case have been emphasised. The bold triangle is the face closest to the viewer and the dotted triangle is that furthest away.

A second consequence of this sequential mechanism is even more surprising. The geometry about the metal centre in a sepulchrate is close to octahedral, and this is emphasised in Fig. 7-17. We discussed some of the stereochemical properties of complexes with three didentate chelating ligands in Chapter 2, and in the same way that the [Co(en)3]3+ cation may exist in the two enantiomeric A and A forms, so may the sepulchrate complex. These two enantiomers are shown in Fig. 7-17. The reaction of [Co(en)3]3+ with formaldehyde... 

In addition to the cryptates, which are synthesized apart from metal ions and then used to form complexes, there are other types of multicyclic ligands called encapsulating ligands, which are synthesized around the metal ion and cannot release it. Complexes of this sort are sometimes called sepulchrates. Two of these are (1-XXIII) and (1-XXIV). An encapsulation complex allows studies to be carried out under extremely acidic or basic conditions since the metal ion, though it cannot be removed, can be oxidized or reduced. Such ligands also can enforce unusual coordination geometries in the examples shown the coordination is much closer to trigonal prismatic than to octahedral. 

Other ligands that involve cages with various sized cavities are called sepulchrates one example of which is... 

Sepulchrates (7) are the most noted of the caged macrocyclic ligands and are the nitrogen analogs of the cryptands. The Co-N distances are 1.99 A for Co and 2.16 A for Co from crystallographic data, and do not vary greatly from other cobalt amines. ... 

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