Inner and Outer Beauty - Beauty in Chemistry - page 11

Fig. 14 Calculated 1 H NMR
shifts as a consequence of
location within the inner
space of the M 4 L 6 assembly
[ 24 ]
Fig. 15 Three-dimensional view of Nitschke's tetrahedral assembly for the protective encapsula-
tion of P 4 . Iron atoms are drawn in purple , carbon atoms gray , nitrogen atoms blue , and phospho-
rous atoms are orange . The sulfonate groups, which help solubilize the assembly in water, are
yellow and red [ 24 ] . Reprinted with permission from AAAS
the interior of the cluster is strongly de-shielding, while the spaces close to the walls
and apertures instead shield guests. The result is that encapsulated species have
diagnostic shifts, which provide information about the guest orientation within the
cavity of the M 4 L 6 assembly [ 23 ] (Fig. 14 ).
Another example, which combines both the self-assembly type of cluster and the
intention illustrated by Cram of stabilizing otherwise unstable guests, is the work by
Nitschke in which the tetrahedral and highly elemental form of phosphorous, P 4 ,is
stabilized [ 24 ]. This structure is shown in Fig. 15 . The importance of this was
highlighted in a Nature Commentary [ 25 ]:
White phosphorus reacts with oxygen to produce an oxide (P 2 O 5 ). This oxide then reacts
with any water that is around to form phosphoric acid. The phosphorus-phosphorus bonds
of P 4 are weak compared with the stronger phosphorus-oxygen bonds of P 2 O 5 ; in other
words, the oxide is thermodynamically much more stable than white phosphorus, and this
drives the reaction to such an extent that white phosphorus spontaneously combusts in air.
One might therefore assume that Mal and colleagues' nanoflasks simply isolate P 4
molecules from oxygen. But this isn't the case: oxygen molecules are smaller than P 4
molecules, and must therefore also be able to gain access to the flasks' interiors. Instead, the
tight confinement of P 4 molecules prevents the formation of phosphorus-oxygen bonds
during the first steps of phosphorus oxidation - there simply isn't room for the reaction
intermediates to form. This is the first time that a reactive species has been stabilized by
such an effect, and represents a fundamental advance for the field.