Living in a Cage Is a Restricted Privilege - Beauty in Chemistry - page 131

Fig. 6 Template synthesis of [Ni II (cyclam)] 2+ from linear 3.2.3-tet 3 and its demetallation to give
the uncomplexed cyclam ligand. Steps i - iv are described in the text

suitable for Schiff base condensation with one molecule of glyoxal, to form the
diene macrocycle. The reversibility of the formation of the C N double bond

favours the attainment of the thermodynamically most stable species (the macrocy-
clic complex) with respect to non-cyclic oligomers. Then, NaBH 4 is added in the
same vessel in order to hydrogenate the two C

¼

N bonds, thus giving weaker, but
kinetically stable C-N single bonds. The [Ni II (cyclam)] 2+ complex can be
precipitated as a yellow-orange perchlorate salt by addition of concentrated
perchloric acid. If wished, boiling an aqueous solution of [Ni II (cyclam)](ClO 4 ) 2 ,
in the presence of excess CN (and OH ), demetallates the complex and the cyclam
ligand can be isolated as a white solid. The Ni II template synthesis of cyclam was
reported in 1972 by Barefield [ 12 , 13 ]. This was the natural evolution of the
pioneering work on the template synthesis of tetra-aza macrocycles carried out by
Busch and Curtis during the preceding decade [ 14 , 15 ].
“Template” is a widely used term in chemistry and in biology. Looking at its
etymology, “template” is the phonetic English version of the French word templet ,
diminutive of temple, which hints at the pediment of a Greek-Latin temple (see
Fig. 7 ): in fact, the shape of the pediment provides the most simple pattern to be
used as a guide to the form of a piece being made: the triangle. Triangular shapes
are not common in coordination chemistry, where the most simple template
remains the square, which can be conveniently provided by 3d metals, Ni II and
Cu II in primis .
However, transition metals can also act as templates to make shapes in three
dimensions. In 1977, Alan Sargeson reported the astonishing one-pot synthesis
of a cage-shaped polyamine, templated by Co III , which offered an octahedral
pattern [ 16 ]. Mechanistic details of the synthesis, taking place in water, in open
air, at room temperature, are illustrated in Fig. 8 .
The labile Co 2+ ionwasmade to react with threemolecules of ethylenediamine (en),
to give the octahedral [Co II (en) 3 ] 2+ complex, which was then oxidised by air to
the inert [Co III (en) 3 ] 3+

¼

(Fig. 8 , step i). Then, Schiff base condensation with six