porphyrins - macrocycles - aromaticity
Significance
<P>Reaction of cobalt porphyrin
1 with diiodoacetylene
in the presence of FeCl
3 followed by treatment with K
2CO
3 affords
compound
2, which is reduced with SmI
2 to
form porphyrin
3. Characterization of this
neutral C=C porphyrin reveals alternating single and double
C-C bonds, owing to the antiaromatic properties of the
compound. Further, cyclic voltammetry measurements indicate an easy
oxidation of
3 to its dicationic, 18π-electron
aromatic form. Chemical oxidation of
3 in
the presence of AgOTf affords ionic species
4. NMR
measurements and DFT calculations confirm the aromatic character
of
4, that does not possess alternating
single and double C-C bonds.</P>
Comment
<P>Although the typical aromatic character of porphyrins is preserved
in compound
4, its electronic structure
significantly differs from that of classical metalloporphyrins.
In particular, the UV/Vis absorbance spectrum of
4 features several intense bands where
the single Soret band is usually observed for porphyrins.
Such properties make this new type of porphyrins of potential interest
for light-harvesting systems.</P>
