β-Nitro-5,10,15-tritolylcorroles

Abstract

Functionalization of the β-pyrrolic positions of the corrole macrocycle with -NO(2) 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 (TTCorrH(3)) with the AgNO(2)/NaNO(2) system, to give mono- and dinitrocorrole derivatives when stoichiometry is carefully controlled. Reactions were found to be regioselective, affording the 3-NO(2)TTCorrH(3) and 3,17-(NO(2))(2)TTCorrH(3) 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-NO(2) 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.

Figure 1

The molecular structure of 2-NO₂TTCorrH₃, with 50% ellipsoids.

Figure 2

The molecular structure of 2,3-(NO₂)₂TTCorrCoPPh₃, with 50% ellipsoids and H atoms not shown.

Figure 3

The molecular structure of 3,12-(NO₂)₂TTCorrCoPPh₃, with 30% ellipsoids and H atoms not shown.

Figure 4

Cyclic voltammograms of TTCorrH₃ in (a) CH₂Cl₂, (b) PhCN and (c) pyridine containing 0.1 M TBAP.

Figure 5

Cyclic voltammograms of TTCorrH₃, 3-NO₂TTCorrH₃ and 3,17-(NO₂)₂TTCorrH₃, in pyridine containing 0.1 M TBAP.

Figure 6

UV-visible spectral changes of 3-NO₂TTCorrH₃ in CH₂Cl₂ during (a) the controlled reduction at -0.95 V, (b) successive addition of piperidine (inset shows the Hill plot), (c) the controlled oxidation at 0.70 V and (d) successive addition of TFA (inset shows the Hill plot).

Figure 7

Energy level scheme for TTCorrH₃ and its nitro derivatives. The Gouterman-derived MOs are indicated with red lines.

Figure 8

Plots of the frontier orbitals of TTCorrH₃ and its nitro derivatives.

Figure 9

Comparison between computed (TDDFT/B3LYP) and experimental absorption spectra of TTCorrH₃ and its nitro derivatives in CH₂Cl₂.

Scheme 1

Synthesis of (NO₂)_xTTCorrH₃.

Chart 1

Molecular structures of β-nitrocorroles presented in this work.