β-Nitro-5,10,15-tritolylcorroles

Functionalization of the β-pyrrolic positions of
the corrole
macrocycle with −NO2 groups is limited at present
to metallocorrolates due to the instability exhibited by corrole free
bases under oxidizing conditions. A careful choice of the oxidant
can limit the transformation of corroles into decomposition products
or isocorrole species, preserving the corrole aromaticity, and thus
allowing the insertion of nitro groups onto the corrole framework.
Here we report results obtained by reacting 5,10,15-tritolylcorrole
(TTCorrH3) with the AgNO2/NaNO2 system,
to give mono- and dinitrocorrole derivatives when stoichiometry is
carefully controlled. Reactions were found to be regioselective, affording
the 3-NO2TTCorrH3 and 3,17-(NO2)2TTCorrH3 isomers as the main products in the case
of mono- and disubstitution, in 53 and 20% yields, respectively. In
both cases, traces of other mono- and disubstituted isomers were detected,
which were structurally characterized by X-ray crystallography. The
influence of the β-nitro substituents on the corrole properties
is studied in detail by UV–visible, electrochemical, and spectroelectrochemical
characterization of these functionalized corroles. Density functional
theory (DFT) and time-dependent DFT (TDDFT) calculations of the ground
and excited state properties of these β-nitrocorrole derivatives
also afforded significant information, closely matching the experimental
observations. It is found that the β-NO2 substituents
conjugate with the π-aromatic system of the macrocycle, which
initiates significant changes in both the spectroscopic and redox
properties of the so functionalized corroles. This effect is more
pronounced when the nitro group is introduced at the 2-position, because
in this case the conjugation is, for steric reasons, more efficient
than in the 3-nitro isomer.