Living in a Cage Is a Restricted Privilege - Beauty in Chemistry - page 133

Fig. 9 (a) Structure of the [Co III (en) 3 ] 3+ complex [ 16 ] and (b) structure of the [Co III (sep)] 3+
complex [ 17 ]; for both complex salts, chloride counterions and hydrogen atoms have been omitted
for clarity. (c) Top view of the triangles obtained by joining the nitrogen atoms of the primary
amine groups of each coordinated en molecule. (d) Top view of the triangles obtained by joining
the nitrogen atoms of the secondary amine groups linked to each one of the two caps of
sepulchrand. (e, f) Top view of two opposite triangular faces of an octahedron and of a trigonal
prism; the two different geometrical arrangements are defined by the torsion angle y . Values of y

are: 53.58 for [Co III (en) 3 ] 3+ (c), and 54.5 0.3 for [Co III (sep)] 3+ (d)

complexes “sepulchrates”. The term is nevertheless appropriate considering that the
metal centre in the [Co III (sep)] 3+ complex is insensitive to any exotic ligand and inert
to the demetallation, as if it were dead and buried in the octamine framework.
In Fig. 8 , for convenience, the structural formulae of [Co III (en) 3 ] 3+ and
[Co III (sep)] 3+ complexes have been represented according to a trigonal prismatic
coordination geometry. However, structural studies on crystalline trichloride salts
have shown that both complexes exhibit a slightly distorted octahedral geometry, as
illustrated in Fig. 9a ,b[ 16 , 17 ]. Notice that in a perfectly regular octahedron, two
opposite triangular faces should be staggered with a torsion angle 60 ,as

illustrated in Fig. 9e . Conversely, a value of y ¼ 0 (eclipsed triangles, Fig. 9f )
defines the other possible six-coordinating geometrical arrangement: the trigonal
prism. The observed torsion angles are very similar for the two complexes,
[Co III (en) 3 ] 3+

y ¼

53.58 [ 16 ] and [Co III (sep)] 3+ 0.3, and are very

close to the value for a regular octahedron [ 17 ]. Co III -N bond distances are those
expected for Co III

y ¼ y ¼ 54.5

0.01 ˚ ;

low-spin amine complexes ([Co III (en) 3 ] 3+

1.96

0.01 ˚ ) and N-Co III -N bond angles are very similar for
both complexes. Thus, being coordinated by three distinct en molecules or by
three ethylenediamine fragments incorporated in a cage framework does not
seem to change very much the binding tendencies of the Co III centre, which in
any case imposes its bonding and geometrical preferences. However, deep
differences are observed in the reactivity of the [Co III (en) 3 ] 3+ and of the
[Co III (sep)] 3+ complex, in particular as far as the redox behaviour is concerned.
Cobalt(III) hexamine complexes undergo a one-electron reduction process to the
corresponding Co II species at a moderately negative potential: for instance, for the

[Co III (sep)] 3+ 1.99