Living in a Cage Is a Restricted Privilege - Beauty in Chemistry - page 148

5 Bis-Tren Cages: Comfortable Shelters for Metal Ions,
Anions and Both Metal Ions and Anions Together

Tren, the trivial name for tris(2-aminoethyl)amine 16, is a classical tetradentate
ligand of metal coordination chemistry [ 59 ]. Due to its tripodal structure, tren
favours the formation of five-coordinate complexes of trigonal bipyramidal
geometry, in which the four amine nitrogen atoms occupy four sites of the coordi-
nation polyhedron, the fifth one being left available to a solvent molecule or to
an anion. Figure 27a shows the crystal structure of the [Cu II (tren)(H 2 O)] 2+ com-
plex [ 60 ]. The complex exhibits a compressed trigonal bipyramidal geometry,
which means that axial bonds (e.g. Cu II -tertiary amine nitrogen) are shorter than
equatorial bonds (Cu II -secondary amine nitrogen). This implies that the fifth
ligand, whether a solvent molecule or an anion, being axial, profits from especially
strong coordinative interactions. The situation is opposite to that observed with the
twin tetramine ligand trien (trivial name for triethylenetetramine 17), which gives
five-coordinate complexes of square pyramidal geometry with copper(II). In view
of its linear connectivity, trien spans the corners of the square with its donor atoms,
establishing rather strong coordinative interactions. On the other hand, the distance
between Cu II and the apical ligand (a solvent molecule or an anion) is usually
large, which gives evidence for a relative weak bonding interaction. The crystal
structure of the [Cu II (trien)(H 2 O)] 2+ complex is shown in Fig. 27b , for comparative
purposes [ 61 ].
Thus, the flexible ligand tren, in the presence of a metal, gives rise to a nice
cavity. Quite interestingly, such a cavity can be made permanent through the
reactions illustrated in Fig. 28 . In particular, two molecules of tren undergo Schiff
base condensation with three molecules of a chosen dialdehyde, to give a cage-
shaped Schiff base derivative [ 62 ]. Precipitation of the base favours the displace-
ment to the right of the equilibrium. The “reversibility” of the C ¼ N bond (i.e. its

Fig. 27 Crystal and
molecular structures of
(a) [Cu II (tren)(H 2 O)] 2+ ,
displaying a compressed
trigonal bipyramidal
coordination geometry [ 60 ];
and (b) [Cu II (trien)(H 2 O)] 2+ ,
which shows a square
pyramidal geometry [ 61 ]. The
Cu II -OH 2 distance is
distinctly smaller in the tren
complex than in the trien
complex