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Functionalization of the &bgr;-pyrrolic positions of the corroie macrocycle with --NO2 groups Is limited at present to metallocorrolates due to the instability exhibited by corroie free bases under oxidizing conditions. A careful choice of the oxidant can limit the transformation of corroles into decomposition products or isocorroie species, preserving the corroie aromatidty, and thus allowing the insertion of nitro groups onto the corroie framework Here we report results obtained by reacting 5,10,lS-tritolylcorroIe (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 &bgr;-nitro substituants on the corroie properties is studied in detail by UV--visible, electrochemical, and spectroelectrochemical characterization of these fimctionalized corroies. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations of the ground and excited state properties of these &bgr;-nitrocorrole derivatives also afforded significant information, dosely matching the experimental observations. It is found that the &bgr;-NO2 substituents conjugate with the Ä-aromatic system of the macrocycle, which initiates significant changes in both the spectroscopic and redox properties of the so fimctionalized corroies. 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. [ABSTRACT FROM AUTHOR]

The reaction of 5,10,15-triarylcorrole with 4-amino-4H-1,2,4-titazole provides another example of corrole ring expansion to give the corresponding 6-azahemiporphycene, a novel porphyrin analogue. The facile oxidation of the corrole ring is a required step for the ring expansion and for this reason the reaction fails in the ease of corroles bearing meso-phenyl groups carrying electron-withdrawing substituents. Steric requirements also limited the scope of the reaction, which is not successful in the ease of 2,6-disubstituted meso-aryl corroles. The occurrence of an initial oxidation is further supported by formation of the 6-azahemiporphycene derivative when the reaction is carried out under the same conditions, using a 5- or a lO-isocorrole as starting material. 1H NMR spectra and X-ray crystal characterization of 6-azahemiporphycene evidenced the presence of an intramolecular N—H ⋯ N hydrogen bond in the inner core of the macrocycle, while photophysical characterization confirmed the aromatic character of the novel macrocycle, showing an intense Soret-like band around 410 nm in the absorption spectrum. The fluorescence emission is very modest, and 6-azahemiporphycene showed higher photostability than the corresponding corrole species. Different metal complexes of 6-azahemiporphycene were prepared following synthetic protocols usually exploited for the preparation of metalloporphyrins, demonstrating good coordination properties for the macrocycte. Both the free-base and metal derivatives were characterized by cyclic voltammetry and spectroelectrochemistry in dichioromethane and benzonitrite. To further detail the behavior of this novel macrocycle, density functional theory (DFT) calculations were carried out on the basic structure of 6-azahemiporphycene with the aim of assessing aromaticity and tautomerism, as well as calculating its stability with respect to the 5-aza isomer. [ABSTRACT FROM AUTHOR]

Compares factors that affect the stabilization of open-shell cyclopentadienyl-molybdenum(II) complexes through a density functional study. Bond dissociation energies; Pi donation from ligand lone pairs; Extent of the stabilization; Steric interactions.