Living in a Cage Is a Restricted Privilege - Beauty in Chemistry - page 135

pH 3, in the presence of oxalate, it promotes the formation of H 2 and CO 2 ,

according to the reaction:

¼

HC 2 O 4 þ H þ þ hv ! 2CO 2 þ H 2

with a turnover number 700 [ 22 ]. This unique reactivity results from the combi-

nation of two properties: (1) the relatively high-rate of the electron transfer, derived
from the strained arrangement of the cage framework; and (2) the stability of the
Co II complex in a moderately acidic solution (whereas non-caged Co II complexes
decompose). At this stage, it could seem convenient to verify whether other metals,
encapsulated by the sepulchrand, display unusual and unprecedented properties.
However, any attempt to demetallate the complex, in order to obtain the free
sepulchrand, failed. In particular, in the absence of an encapsulated metal, the
sepulchrand decomposes, due to intrinsic instability of methylenediamine groups.
For instance, reduction of [Co III (sep)] 3+ with Zn in 1 M HCl induces decomposition
to Co 2+ (aq),

>

+ H 3 NCH 2 CH 2 NH 3 +

and NH 4 + , practically reversing the process

illustrated in Fig. 8 .
In conclusion, a metal centre has built up around itself a superb container, of
which it remains prisoner. The only, but unhappy, way to escape is to crash the
beautiful vase. This tale is reminiscent of a short story by Luigi Pirandello
(1867-1936) entitled The jar [ 23 ]. A Sicilian landowner bought an enormous
earthenware jar, large enough to contain the great amount of olive oil he expects
to produce. To his disappointment, he realises that the jar is broken in two distinct
pieces. Therefore he calls a renowned craftsman to repair the huge amphora. The
repairman decides to fix the jar with glue and iron wire and asks that the two pieces
of the jar are closed around him, because he wants to make the repair from inside.
At the end, the jar is perfectly fixed, but the craftsman remains prisoner. Then,
a long contest begins involving the repairman, who wants to break the jar, and the
landowner, who insists that the jar must not be broken, in the presence of many
partaking wits. Finally, the frustrated landowner kicks the jar, which rolls down a
bank to crash into a tree, freeing the craftsman.
Fortunately, things at the molecular level can go better than in the real
(or literary) world. In fact, a very stable and indestructible version of polyamine
cages of the sepulchrand type was obtained by Sargeson by simply replacing NH 3 ,
in the template reaction illustrated in Fig. 8 , with the stronger triprotic acid
CH 3 NO 2 , to give the [Co III (NO 2 ) 2 sar] 3+ complex [ 24 ]. Notice first that the
substituents on the caps can be modified and finally eliminated according to the
sequence of reactions illustrated in Fig. 10 . To these hexamines, the general trivial
name of “sarcophagines” (abbreviated to sar) was given, in keeping with the current
necrophilic nomenclature. The sar framework is robust and resistant to the most
drastic conditions, which allows demetallation and isolation of the metal-free
hexamine, whether (NH 2 ) 2 sar or sar, in good yield [ 25 ]. Demetallation was
carried out on the Co II cage complex, to avoid the intrinsic inertness of the Co III
centre, according to two main procedures. In one process, a suspension of the
[Co II (NH 3 ) 2 sar]Cl 4 complex salt in a deoxygenated 48% HBr solution was heated
at reflux for 3 h under dinitrogen, to give a clear solution of the blue [Co II Br 4 ] 2