β-nitro-5,10,15-tritolylcorroles

Article abstract of DOI:10.1021/ic3007926

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.

Full text of DOI:10.1021/ic3007926

Article
pubs.acs.org/IC
β-Nitro-5,10,15-tritolylcorroles
Manuela Stefanelli,† Giuseppe Pomarico,† Luca Tortora,† Sara Nardis,† Frank R. Fronczek,‡
Gregory T. McCandless,‡ Kevin M. Smith,*,‡ Machima Manowong,§ Yuanyuan Fang,§ Ping Chen,§
Karl M. Kadish,*,§ Angela Rosa,*,⊥ Giampaolo Ricciardi,⊥ and Roberto Paolesse*,†
†Dipartimento di Scienze e Tecnologie Chimiche, Universita
̀
di Roma Tor Vergata, 00133 Roma, Italy
‡Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States
§Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States
⊥Dipartimento di Chimica, Universita
̀
della Basilicata, 85100 Potenza, Italy
S
* Supporting Information
ABSTRACT: 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.
activating nucleophilic substitution on the adjacent β-carbons of
the pyrrole rings is well-known. Furthermore, it is also
interesting to evaluate the influence of the strong electron
withdrawing character of the nitro group on the behavior of the
corrole ring, which is characterized by high electron density and
its noninnocent character as ligand.8 In this connection, we
recently reported the first successful example of a vicarious
nucleophilic substitution performed on corrole nitro-derivatives
by reacting copper and germanium β-(NO2) corrolates with 4-
amino-4H-1,2,4-triazole.9 The reaction afforded β-amino-β-
nitro derivatives, which can have great synthetic value for the
elaboration of new chromophores, as well as extended aromatic
β-fused architectures. To date β-nitrocorrole derivatives of
copper,9 germanium,10 gallium,11 silver,12 cobalt,13 and iron14
have been prepared by using a variety of nitrating systems and
reaction conditions, while all attempts to nitrate the corrole
INTRODUCTION
■
The establishment of efficient synthetic methodologies for the
preparation of corrole1−4 has allowed a more detailed
investigation of its chelating properties as a ligand, and of its
unique behavior in several reactions.5 The investigation of
corrole functionalization reactions is fundamental for the
potential exploitation of this macrocyle in practical uses, since
the manipulation of the peripheral positions can deeply affect
the corrole properties, as shown by the first seminal examples
reported in the literature.6
Compared with porphyrins, the peripheral functionalization
of the corrole macrocycle is possible only in a limited number
of cases, but recently some successful substitutions have been
reported in the literature.2,7 In particular we have been
interested in the nitration reaction, because the insertion of
one or more −NO2 groups onto the β-pyrrole positions
represents one of the most valuable modifications for
researchers interested in functionalization of the corrole
skeleton; for example, the potential of the nitro group in
Received: April 17, 2012
© XXXX American Chemical Society
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Chart 1. Molecular Structures of β-Nitrocorroles Presented in This Work
free-base failed, mainly because of the lability of corroles under
these substitution conditions. This was evidenced under a
variety of reaction conditions with the formation of isocorrole
species. The recent development of demetalation protocols for
the effective removal of silver15 and copper16 from the
corresponding corrole complexes has allowed the preparation
of nitrocorrole free bases from the application of the reductive
DBU/THF system to a silver 3-NO2 corrolate (3-
NO2TtBuCorrAg), obtained by using a large excess of
AgNO2.15 Although this approach allowed circumvention of
the instability of corrole under the nitrating systems tested to
date, it opened the way to preparation of otherwise unavailable
3-NO2 corroles. Such preparations of functionalized free bases
in a single step is highly desirable, particularly in the
preparation of polynitrated free bases. Prompted by the recent
results obtained in the case of nitration of copper corrole
complexes9 carried out with an optimized ratio of silver salt and
sodium nitrite mixture as the nitrating system, we decided to
apply the same reaction conditions to corrole free bases,
surmising that minimizing the oxidant stoichiometry could
preserve corrole aromaticity during the reaction, but still allow
results provide informative details on the behavior of these
macrocycles.
EXPERIMENTAL SECTION
■
General. Silica gel 60 (70−230 mesh, Sigma Aldrich) was used for
column chromatography. Reagents and solvents (Aldrich, Merck or
Fluka) were of the highest grade available and were used without
further purification, except for benzonitrile (PhCN), which was
distilled over P2O5 under vacuum prior to use, and tetra-n-
butylammonium perchlorate (TBAP), which was dried under vacuum
1
at 30 °C for at least one week prior to use. H NMR spectra were
recorded on a Bruker AV300 (300 MHz) spectrometer. Chemical
shifts are given in parts per million relative to residual CHCl3 (7.25
ppm). UV−visible spectra were measured on a Cary 50 spectropho-
tometer. Mass spectra (FAB mode) were recorded on a VGQuattro
spectrometer in the positive-ion mode using m-nitrobenzyl alcohol
(Aldrich) as a matrix.
Diffraction data for chloroform solvates of 2-NO2TTCorrH3 and
2,3-(NO2)2TTCorrCoPPh3 and for a dichloromethane solvate of 3,12-
(NO2)2TTCorrCoPPh3 were collected at low temperature on a Bruker
Kappa Apex-II CCD diffractometer with either Cu Kα (λ = 1.54184
Å) or MoKα (λ = 0.71073 Å) radiation and an Oxford Cryosystems
Cryostream chiller. Refinement was by full-matrix least-squares using
SHELXL,17 with H atoms in idealized positions, except for those on N,
for which coordinates were refined. Absorption corrections were
applied with a multiscan method (Bruker SADABS18).
Synthesis of β-Nitrocorroles. Preparation of 3-(NO2)TTCorrH3.
NaNO2 (75 mg, 1.1 mmol) and AgNO2 (19 mg, 0.12 mmol) were
added to a refluxing solution of TTCorrH3 (70 mg, 0.12 mmol) in
DMF (12 mL), and the progress of the reaction was followed by TLC
analysis and UV−visible spectroscopy. The reaction was complete in
10 min, as indicated by the UV−visible spectrum of the reaction
mixture that showed the spectral features of the desired product. The
reaction mixture was cooled, quenched by addition of few drops of
aqueous hydrazine solution, precipitated by adding distilled water, and
then filtered. The crude product was taken up in CH2Cl2 and applied
to a silica gel column for chromatographic purification. Elution with
CH2Cl2/hexane (7:3) afforded traces of the silver complex,
TTCorrAg, as a first reddish band followed by a second fraction
containing a complex mixture of green and brownish compounds that
were not easy to separate using a column. A subsequent main green
fraction was isolated and crystallized from CH2Cl2/MeOH giving the
3-nitroderivative as an emerald green powder (38 mg, 52% yield). The
spectroscopic characterization of 3-NO2TTCorrH3 was fully in
agreement with data in the literature.16
−
nucleophilic attack by the NO2 ion upon the macrocycle.
We report here the first successful example of nitration of the
corrole free base, obtained by reacting 5,10,15-tritolylcorrole
(TTCorrH3) with the AgNO2/NaNO2 system. Variation of the
molar ratio of reagents allowed us to obtain 3-nitro- and 3,17-
dinitro-derivatives of corrole in satisfying yield, together with
traces of other isomers, namely 2-NO2-, 2,3-(NO2)2, and 3,12-
(NO2)2 corroles (Chart 1). While the substitution pattern of 2-
NO2TTCorrH3 was established structurally by X-ray crystallog-
raphy, in the case of 2,3-(NO2)2TTCorrH3 and 3,12-
(NO2)2TTCorrH3 isomers, the substitution pattern was
unambiguously confirmed by preparation of the corresponding
Co triphenylphosphine complexes, which were also structurally
characterized.
The definition of the synthetic protocols for the preparation
of free base nitrocorroles prompted us to study in detail the
influence of the introduction of the nitro group at the corrole β-
positions. This has been accomplished through a detailed UV−
visible, electrochemical, and spectroelectrochemical character-
ization of these functionalized corroles. The changes in the UV-
visible spectra and in the first reduction potential following β-
nitration have been rationalized by density functional theory
(DFT) and time-dependent DFT (TDDFT) calculations of the
ground and excited states of the TTCorrH3, 2-NO2TTCorrH3,
3-NO2TTCorrH3, and 3,17-(NO2)2TTCorrH3 series of free
base corroles. Taken together, the experimental and theoretical
The second fraction was subjected to preparative TLC on silica gel
plates (CH2Cl2/hexane 1:1 as eluant). A green fraction having 0.52 Rf
was isolated and identified as 2-NO2TTCorrH3.
2-NO2TTCorrH3. Mp > 300 °C. UV−visible (CHCl3): λmax, nm
(log ε) 408 (4.52), 460 (4.53), 588 (4.33), 715 (4.31). 1H NMR (300
MHz, CDCl3): 9.13 (d, 1H, J = 4.20 Hz β-pyrrole), 9.08 (s, 1H, β-
pyrrole), 8.74 (d, 1H, J = 4.83 Hz β-pyrrole), 8.64 (d, 1H, J = 4.89 Hz
B
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β-pyrrole), 8.34 (d, 1H, J = 4.95 Hz β-pyrrole), 8.28 (m, 3H, β-pyrrole
+ phenyls), 8.03 (d, 2H, J = 7.92 Hz phenyls), 7.95 (d, 2H, J = 7.83 Hz
phenyls), 7.67 (d, 2H, J = 7.86 Hz phenyls), 7.59 (m, 6H phenyls),
2.70 (s, 3H, p-CH3), 2.67 (s, 6H, p-CH3). Anal. calcd for C40H31N5O2:
C, 78.2; H, 5.1; N, 11.3%. Found: C, 78.3; H, 5.1; N, 11.4%. MS
(FAB): m/z 613(M+). Crystal data: C40H31N5O2 · 1.47 CHCl3,
71.31; H, 4.33; N, 8.60%. Found: C, 71.27; H, 4.35; N, 8.64%. MS
(FAB): m/z 715 (M+ − PPh3). Crystal data: C58H42CoN6O4P·0.5
CH2Cl2, monoclinic space group P2/c, a = 18.225(3), b = 9.2487(14),
c = 29.294(4) Å, β = 105.212(10)°, V = 4764.8(13) Å3, T = 120.0(5)
K, Z = 4, Dx = 1.421 g cm−3, μ(Cu Kα) = 4.12 mm−1. A total of 19 491
data was collected to θmax = 64.9°, R = 0.059 for 6277 reflections with I
> 2σ(I) of 7795 unique data and 644 refined parameters. Disordered
CH2Cl2 was modeled as a half-populated site near a 2-fold axis, CCDC
869741.
Electrochemical and Spectroelectrochemical Measure-
ments. Cyclic voltammetry was carried out with an EG&G model
173 potentiostat/galvanostat. A homemade three-electrode electro-
chemistry cell was used and consisted of a glassy carbon working
electrode, a platinum wire counter electrode, and a saturated calomel
reference electrode (SCE). The SCE was separated from the bulk of
the solution by a fritted-glass bridge of low porosity which contained
the solvent/supporting electrolyte mixture. All potentials are
referenced to the SCE.
UV−visible spectroelectrochemical experiments were performed
with an optically transparent platinum thin-layer electrode of the type
described in the literature.21 Potentials were applied with an EG&G
Model 173 potentiostat/galvanostat. Time-resolved UV−visible
spectra were recorded with a Hewlett-Packard Model 8453 diode
array rapid-scanning spectrophotometer.
Determination of Equilibrium Constants of Free Base
Corroles. In order to study the protonation or deprotonation process,
microliter quantities of TFA/CH2Cl2 and piperidine/CH2Cl2 were
added gradually to 6.0 mL CH2Cl2 solution of TTCorrH3 in a 1.0 cm
homemade glass cell. Solutions of the corroles were freshly degassed
by N2 flux before each measurement and the spectra recorded after
each addition of titration reagents. The concentration of the corrole
solutions were in the 10−6 M range, in order to avoid aggregation of
the macrocycle.
triclinic space group P1, a = 9.6282(10), b = 14.5927(14), c =
̅
15.2094(15) Å, α = 64.348(5), β = 83.229(5), γ = 87.496(5)°, V =
1912.8(3) Å3, T = 90.0(5) K, Z = 2, ρcalcd = 1.370 g cm−3, μ(Cu Kα) =
3.42 mm−1. A total of 19 520 data was collected at to θ = 68.7°. R =
0.059 for 6350 data with I > 2σ(I) of 6674 unique data and 510 refined
parameters, CCDC 847685.
Preparation of 3,17-(NO2)2TTCorrH3. TTCorrH3 (100 mg, 0.18
mmol) was dissolved in DMF (15 mL), and the solution was heated at
reflux. Then, NaNO2 (99 mg, 1.44 mmol) and AgNO2 (55 mg, 0.36
mmol) were added and the progress of the reaction was followed by
TLC analysis and UV−visible spectroscopy. The reaction was
complete in 30 min. The reaction mixture was cooled, quenched by
addition of a few drops of aqueous hydrazine solution, precipitated by
adding distilled water, and then filtered. The crude product was taken
up in CH2Cl2 and applied to a silica gel column for chromatographic
purification. Elution with CH2Cl2 afforded the isocorrole 3-(NO2)-5-
(OH)TTisoCorH2 (10 mg, 9% yield) as the first fraction, followed by
two green bands corresponding to the dinitroisomers 2,3-
(NO2)2TTCorrH3 (2 mg, 1.7% yield) and 3,12-(NO2)2TTCorrH3
(1 mg, 0.8% yield), respectively. The last brilliant green fraction was
collected and crystallized from CH2Cl2/n-hexane giving the 3,17-
(NO2)2TTCorrH3 as a brilliant green powder (24 mg, 20% yield).
3,17-(NO2)2-TTCorrH3. Mp > 300 °C. UV−visible (CHCl3): λmax
,
1
nm (log ε) 439 (4.54), 483 (4.63), 707 (4.52). H NMR (300 MHz,
CDCl3): 8.94 (s, 2H, β-pyrrole), 8.52 (d, 2H, J = 4.80 Hz β-pyrrole),
8.23 (d, 2H, J = 4.86 Hz β-pyrrole), 7.95 (m, 6H, phenyls), 7.55 (m,
6H, phenyls), 2.68 (s, 3H, p-CH3), 2.63 (s, 6H, p-CH3). Anal. calcd for
C40H30N6O4: C, 72.9; H, 4.6; N, 12.8%. Found: C, 72.8; H, 4.6; N,
12.7%. MS (FAB): m/z 658 (M+).
The free base is protonated in acid and deprotonated in basic
solutions. The spectroscopic data were analyzed by the Hill equation,
which gives the equilibrium constant (K) for the reactions.
Preparation of 2,3-(NO2)2-TTCorrCoPPh3. 2,3-(NO2)2-TTCorrH3
(5 mg, 0.08 mmol) in CH2Cl2 (10 mL) and cobalt(II) acetate (60 mg,
0.24 mmol) and triphenylphosphine (63 mg, 0.24 mmol) in CH3OH
(15 mL) were mixed, and the resulting solution was heated at reflux
for 1 h. The solvent was evaporated and the residue dissolved in
CH2Cl2 and filtered through a silica plug to afford the complex as a
green-bluish fraction. Crystallization from CH2Cl2/CH3OH afforded
the title compound in an almost quantitative yield.
2,3-(NO2)2-TTCorrCoPPh3. Mp > 300 °C. UV−visible (CHCl3):
λmax, nm (log ε) 370 (3.92), 552 (3.47), 1H NMR (300 MHz, CDCl3):
9.06 (d, 1H, J = 4.50 Hz β-pyrrole), 8.35 (d, 1H, J = 4.89 Hz β-
pyrrole), 8.15 (d, 1H, J = 4.89 Hz β-pyrrole), 8.10 (d, 1H, J = 5.07 Hz
β-pyrrole), 8.02 (d, 1H, J = 5.07 Hz β-pyrrole), 7.98 (d, 1H, J = 4.47
Hz β-pyrrole), 7.58−7.36 (m, 15 H, phenyls), 7.22 (m, 3 H, p-
phosphine), 6.94 (m, 6 H, m-phosphine), 5.43 (m, 6 H, o-phosphine),
2.60 (s, 3H, p-CH3), 2.59 (s, 6H, p-CH3). Anal. calcd for
C58H42CoN6O4P: C, 71.31; H, 4.33; N, 8.60%. Found: C, 71.28; H,
4.36; N, 8.63%. MS (FAB): m/z 715 (M+ − PPh3). Crystal data:
C58H42CoN6O4P·0.81 CHCl3, orthorhombic space group Fdd2, a =
37.190(3), b = 37.996(6), c = 13.8041(10) Å, V = 19,506(4) Å3, T =
90.0(5) K, Z = 16, Dx = 1.463 g cm−3, μ(Mo Kα) = 0.58 mm−1. A total
of 36 267 data was collected to θmax = 30.0°. R = 0.059 for 10 933 data
with I > 2σ(I) of 12 276 unique data and 634 refined parameters,
Flack19 parameter 0.022(12). Disordered solvent contribution was
removed using the SQUEEZE procedure,20 CCDC 869740
Quantum Chemical Calculations. All calculations were
performed with Turbomole V6.3 201122 using both pure (BP86)23
and hybrid (B3LYP)24 functionals in combination with the extensively
polarized basis sets of triple-ζ quality including high angular
momentum polarization functions (def2-TZVPP).25 In the BP86
calculations, the resolution of the identity (density fitting) approach26
was used to save computer time in combination with the def2-
TZVPP/J auxiliary basis set.27 The ground state molecular structures
of TTCorrH3, 2-NO2TTCorrH3, 3-NO2TTCorrH3, 3,17-(NO2)2-
TTCorrH3, and related monoanionic species were fully optimized
(without constraints) in the gas-phase and in dichloromethane
solution. For the optimization of the monoanions, the unrestricted
formalism was used. Solvation effects on the optimized structures were
modeled through a dielectric continuum model, which was chosen to
be the COSMO model.28 The optimized structures were confirmed to
be local minima by calculating the harmonic vibrational frequencies.
The thermodynamic parameters for the one-electron reduction
reaction of the investigated metal-free corroles were computed at
the BP86 level, at the geometries optimized in dichloromethane
solution. The zero-point energies (ZPE), thermal corrections, and
entropy terms for the optimized geometries of the neutral and
monoanionic species were obtained from frequency calculations in
dichloromethane. Vertical absorption energies and oscillator strengths
of the lowest singlet excited states of TTCorrH3 and its nitro
derivatives were computed in dichloromethane solution at TDDFT29
level using the hybrid B3LYP24 functional and the DFT/B3LYP
ground state geometries optimized in dichloromethane. The calculated
excitation energies contain, apart from the altered “solvated” orbitals
(slow term), also the contributions from the “fast” solvent response
term.30
Preparation of 3,12-(NO2)2-TTCorrCoPPh3. 3,12-(NO2)2-TTCorr-
CoPPh3 was prepared as reported above for the 2,3-dinitro isomer.
3,12-(NO2)2-TTCorrCoPPh3. Mp > 300 °C. UV−visible (CHCl3):
λmax, nm (log ε) 450 (4.31), 594 (4.08), 1H NMR (300 MHz, CDCl3):
8.98 (s, 1H, β-pyrrole), 8.67(s, 1H, β-pyrrole), 8.53 (d, 2H, J = 4.65
Hz β-pyrrole), 8.30 (d, 2H, J = 5.04 Hz β-pyrrole), 8.21 (d, 2H, J =
5.13 Hz β-pyrrole), 8.07 (d, 2H, J = 4.53 Hz β-pyrrole), 7.66−7.32 (m,
15 H, phenyls), 7.28 (m, 3 H, p-phosphine), 7.18 (m, 6 H, m-
phosphine), 6.53 (m, 6 H, o-phosphine), 2.62 (s, 3H, p-CH3), 2.57 (s,
3H, p-CH3), 2.54 (s, 3H, p-CH3). Anal. calcd for C58H42CoN6O4P: C,
C
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Scheme 1. Synthesis of (NO2)xTTCorrH3
that a careful choice of the oxidant species could potentially
drive the reaction pathway toward nitrocorroles, inhibiting the
further transformation to isocorroles.
RESULTS AND DISCUSSION
■
Synthesis. In the past few years, we have been particularly
interested in the elaboration of synthetic protocols for the
synthesis of β-nitrocorrole derivatives. By using different
nitrating systems, we have been able to establish diverse
methodologies for the convenient preparation of mono- and
dinitro-metallocorrolates, starting in most cases from metal-
locorroles, such as the Cu, Ge, and Fe derivatives. Our attempts
to introduce nitro groups directly onto the free base, to obtain
the corresponding functionalized corrole failed, since the
required β-substitution took place in combination with other
structural modifications, thereby precluding the isolation of the
pure nitrocorrole free base. For example, using a large excess of
AgNO2, both functionalization and metalation of the macro-
cycle occurred simultaneously, affording the corresponding
Ag(III) 3-nitrocorrole as the reaction product.12 In the case of
TFA/NaNO2 or HCl/NaNO2 systems, the formation of β-
nitroisocorroles was observed, but these were converted in a
second step into the corresponding aromatic corrole complexes
by cobalt insertion.13 Our hypothesis for the nitration reaction
mechanism is that the first step requires an oxidant species for
the generation of the corrole π-cation radical, which is then
attacked by nitrite to afford the final product. However, corrole
free bases demonstrate extreme sensitivity under oxidizing
reaction conditions, leading to a quite facile conversion of the
macrocycle into the corresponding isocorrole spe-
cies.9,13,15,31−35 Therefore, the nitration reaction conditions
made the preparation of aromatic nitro-compounds difficult,
instead favoring the formation of isocorrole derivatives through
nucleophile insertion onto a meso-carbon position, which was
observed for the 3-NO2-5-OH-isocorroles.9,31 Since the
oxidation step is crucial for the reaction outcome, we surmised
We recently reported that when the nitration reaction is
performed on corrole using an equimolar amount of an
oxidizing agent, such as chloranil in combination with an excess
of NaNO2, the 3-NO2-5-OH-isocorrole is rapidly obtained as
the main reaction product, confirming the regioselectivity of
both mononitration and the formation of the single 5-
substituted isocorrole isomer.31 Our attention turned toward
other oxidants, and prompted by the results reported recently
for nitration of copper corrolates, we tested the AgNO2/
NaNO2 system, which allows the preparation of mono- and
dinitro-derivatives in good yields just by varying the reagent
stoichiometry.9
Thus, TTCorrH3 was reacted in refluxing DMF with a
mixture of sodium and silver nitrites using a corrole/AgNO2/
NaNO2 molar ratio of 1:1:9 (Scheme 1) with the intention of
achieving mononitration of the corrole. The reaction proceeded
very fast and in 10 min complete disappearance of the starting
compound was observed by TLC analysis, together with the
formation of a green major product. The UV−visible spectrum
of the mixture changed rapidly even from the beginning of the
reaction and showed at the end the typical spectral features of
3-NO2TTCorrH3,15 indicating it as the main product.
Chromatographic purification afforded traces of the silver
complex of the corrole as the first rose-red band together with a
second fraction containing a mixture of brownish and greenish
products, inseparable on the column. A third green fraction was
the major reaction product and corresponded to 3-
NO2TTCorrH3, obtained in 52% yield. It is worth mentioning
that the formation of the 3-nitro-5-hydroxy-isocorrole was not
D
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observed, showing that the amount of silver nitrite was
correctly chosen to allow the β-functionalization, and to
preserve at the same time the integrity of the nitrocorrole
free base formed in the reaction medium. The second band was
further purified on a preparative silica gel TLC plate, using a
mixture of CH2Cl2/hexane as eluant. A green band (Rf = 0.52)
was isolated in sufficient amount to be characterized using the
usual spectroscopic techniques. This fraction showed an
unusual UV−visible spectrum, characterized by a split Soret
band (of similar intensity) at 408 and 460 nm, a less intense
narrow band centered at 588 nm, and a broad band at about
715 nm. The FAB mass spectrum of this compound afforded a
molecular peak at m/z 613, which is consistent with a
mononitrocorrole. The 1H NMR spectrum and integral
calculations confirmed it as a monosubstituted corrole showing
a proton singlet at 9.08 ppm. It is noteworthy that the pattern
of substitution is clearly different from that of the known 3-
NO2TTCorrH3, showing a pyrrolic doublet more deshielded
compared with the singlet, just as observed for the 2-
bromocorrole isomer obtained by reacting the TTCorrH3
with EtMgBr.33 The definitive identification of position C2 as
the site of substitution for this compound was afforded by X-ray
crystallographic analysis, carried out on a single crystal obtained
by slow diffusion of n-hexane into a dilute dichloromethane
solution. This unambiguously confirmed the new monosub-
stituted corrole as the 2-NO2TTCorrH3 isomer (Figure 1).
The three protonated N atoms in the interior of the molecule
force marked deviations of the corrole system from coplanarity.
The mean deviation of its 23 atoms is 0.159 Å, with a maximum
of 0.415(4) Å. The nitrated “A” pyrrole and the adjacent “B”
pyrrole not directly bonded to it come nearest to being
coplanar, forming a dihedral angle of 10.5(6)°. The main
distortion of the corrole is a twist, in which the other two
pyrroles “C” and “D” tip alternately above and below this plane,
forming dihedral angles of 18.1(3) and 20.6(2)° with it and
23.8(3)° with each other. The N atoms of the two alternately
tipped pyrroles are somewhat pyramidal, with their H atoms
lying 0.14(4) and −0.30(4) Å out of the best planes of their
respective pyrroles. The third N−H hydrogen atom is not
tipped out of plane, but forms an N−H···N hydrogen bond
(N···N 2.623(3) Å; N−H···N 134(3)°). The nitro group lies
essentially in the plane of the pyrrole to which it is bonded,
with a C1−C2−N−O torsion angle 1.2(4)°. The two directly
bonded pyrroles exhibit a N−C1−C19−N twist of 7.6(3)°.
With the aim of driving the reaction toward dinitrocorrole
derivatives, we modified the amounts of nitrating agents by
altering the corrole/AgNO2 molar ratio to 1:2 (Scheme 1). Of
course, in this case, we were aware that the increase of the
oxidant reagent could induce the formation of the isocorrole,
instead of forming dinitrocorroles. The reaction of TTCorrH3
with silver and sodium nitrites in a 1:2:8 molar ratio, afforded in
30 min a mixture of compounds observable from the TLC
analysis as green and bluish-green bands together with a
number of decomposition products.
Chromatographic purification on silica gel using dichloro-
methane as eluant afforded, after elution of small traces of the
silver tritolylcorrolate, a first major fraction consisting of the 3-
NO2-5-OH-TTisoCorrH2, obtained in 9% yield. Afterward, a
small quantity of two green fractions was isolated and subjected
to spectroscopic identification. The band having the greater Rf
value showed a UV−visible spectral profile quite similar to 3-
NO2TTCorrH3, but generally red-shifted, while the second
fraction displayed a strongly red-shifted Soret band at 472 nm
together with two satellite bands at 613 and 666 nm. The mass
spectrometry analysis afforded a molecular peak at m/z 658 for
both compounds, tentatively identifying them as two different
dinitrocorrole isomers. The proton NMR spectra of both
compounds were of poor resolution, hampering definitive
identification of the sites of substitution by the nitro groups.
The last green fraction obtained by column chromatographic
purification was the main reaction product, obtained in 20%
yield, and fully characterized as 3,17-(NO2)2TTCorrH3. The
UV−visible spectrum was unusual, having a split Soret band at
439 and 483 nm and an intense broad absorption at ca. 700 nm.
To allow a more definitive identification of the other
dinitrocorrole isomers, we decided to prepare the correspond-
ing Co complexes, by reaction of these products with
Co(OAc)2 and PPh3; the reaction provided cobalt complexes
1
having well resolved H NMR spectra. In the case of the first
compound, the lack of proton singlets together with the
presence of a downfield shifted doublet above 9 ppm, clearly
separated from the other five pyrrolic resonances at ca. 8−8.5
ppm, led to the conclusion that for this compound the
substitution occurred on the C2 and C3 positions of the corrole
macrocycle. We were able to obtain crystals of this isomer
suitable for X-ray crystallographic analysis, which helped to
elucidate the structure of the corresponding free base as 2,3-
(NO2)2TTCorrH3.
The structure of 2,3-(NO2)2TTCorrCoPPh3 is shown in
Figure 2. The Co atom has square pyramidal coordination with
Co−N distances in the range 1.869(3)−1.886(3) Å (mean
1.880 Å), Co−P distance 2.2321(8) Å, and the Co is
0.2634(13) Å out of the best plane of the four pyrrole N
atoms. The 23-atom C19N4 corrole core deviates only slightly
Figure 1. Molecular structure of 2-NO2TTCorrH3, with 50%
ellipsoids.
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more distorted from planarity than that of 2,3-
(NO2)2TTCorrCoPPh3, having an rms deviation of 0.159 Å
and maximum deviation 0.339(4) Å for C8. The distortion is
best described as a saddle, although all four N atoms lie on the
same side of the 23-atom best plane by a mean distance of
0.118 Å. The two nitro groups are twisted out of the corrole
best plane by similar amounts, forming torsion angles: C4−
C3−N−O, −40.0 (6)° and C11−C12−N−O, 29.2 (5)°.
It is worth mentioning that in all cases the use of the
AgNO2/NaNO2 system allowed the β-functionalization of the
macrocycle, without inducing the concomitant metalation,
which occurs when the reaction is carried out with an excess of
silver nitrite. In this case the AgNO2 acts only as oxidant, and
the proper amount chosen prevented the metalation of the
macrocycle. The transient formation of the silver corrole
complex, then demetalated by the final addition of hydrazine
can be safely excluded, because hydrazine is unable to promote
silver corrole demetalation and because the same products were
again obtained when the nitration reaction was carried out
avoiding the addition of this reagent.
Electrochemistry and Spectroelectrochemistry. The
redox behavior of free base corroles is complicated in
nonaqueous media by the facile gain or loss of protons,
which can give in solution a mixture of the neutral corrole along
with either its protonated [(Corr)H4]+ or deprotonated
[(Corr)H2]− form. To investigate the effect of nitration and
the solvent on both the overall oxidation/reduction mechanism
and the electrochemically initiated protonation/deprotonation
process, a series of three corroles, TTCorrH3, 3-
NO2TTCorrH3, and 3,17-(NO2)2TTCorrH3, were selected
for electrochemical and spectroelectrochemical characterization
in pyridine, PhCN, and CH2Cl2.
Figure 2. Molecular structure of 2,3-(NO2)2TTCorrCoPPh3, with
50% ellipsoids and H atoms not shown.
from planarity, with an rms deviation 0.059 Å and maximum
deviation 0.119(3) Å for C7. The small distortion from
planarity is a slight saddle. One nitro group lies nearly in the
corrole best plane, with C1−C2−N−O torsion angle −6.9(5)°,
while the other is more nearly orthogonal to the corrole, with
C2−C3−N−O torsion angle −73.1(4)°.
The second Co complex showed two proton singlets at 8.98
and 8.67 ppm, respectively, and three different proton signals of
equal intensity at 2.62, 2.57, and 2.54 ppm, indicating it to be
an asymmetric product. Also in this case, the X-ray crystallo-
graphic characterization (Figure 3) led to identification of 3,12-
We earlier reported the electrochemistry and spectroelec-
trochemistry of several meso-substituted corroles in PhCN and
pyridine, including TTCorrH3.36 It was demonstrated that the
electroactive species in PhCN was the neutral corrole CorrH3,
while in pyridine, the monoanion [(Corr)H2]− is the prevalent
species,36 the deprotonation process being almost complete. A
similar solvent effect on deprotonation is seen for the currently
investigated corroles, as seen by the cyclic voltammograms in
Figure 4 for TTCorrH3 in CH2Cl2, PhCN and pyridine.
The first reduction of TTCorrH3 is irreversible in all three
solvents and located at Ep = −1.33 to −1.40 V for a scan rate of
0.1 V/s. Only a residual amount of neutral CorrH3 is present in
the basic pyridine solvent and a small peak current is seen for
reduction of the fully protonated corrole. This contrasts with
CH2Cl2, where CorrH3 is the main electroactive species in
solution and the peak current is much higher for the first
reduction. Sweeping the potential from −0.40 to values beyond
the first reduction in PhCN or pyridine drives to completion
the deprotonation of TTCorrH3, after which the deprotonated
[(TTCorr)H2]− product undergoes a reversible one-electron
ring-reduction, which is located at E1/2 = −1.90 V in PhCN and
−1.88 V in pyridine.
As indicated above, TTCorrH3 undergoes deprotonation in
both CH2Cl2 and PhCN after the first one-electron reduction
and this then generates [(Corr)H2]−. As a result, when
reversing the scan at potential negative of the first reduction
and then scanning in a positive direction, a new redox couple
assigned to [(Corr)H2]−/[(•Corr)H2] can be seen in CH2Cl2
and PhCN. This process is located at E1/2 = −0.09 to −0.08 V
and it is not observed in CH2Cl2 or PhCN, when sweeping the
potential toward more positive values from the initial potential
Figure 3. Molecular structure of 3,12-(NO2)2TTCorrCoPPh3, with
30% ellipsoids and H atoms not shown.
(NO2)2TTCorrCoPPh3, confirming the corresponding corrole
free base as the 3,12-(NO2)2TTCorrH3 isomer. It is interesting
to note that this isomer is, to the best of our knowledge, the
first example of a disubstituted corrole where pyrroles A and C
are functionalized, instead the usual A and D pattern.
The structure of 3,12-(NO2)2TTCorrCoPPh3 is illustrated in
Figure 3 and is quite similar to that of 2,3-
(NO2)2TTCorrCoPPh3, with a somewhat more nonplanar
corrole core. The Co atom has square pyramidal coordination
with Co−N distances in the range 1.873(3)−1.892(3) Å (mean
1.878 Å), Co−P distance 2.2113 (11) Å, and the Co is
0.2734(15) Å out of the best plane of the four pyrrole N atoms.
The 23-atom C19N4 corrole core is approximately three times
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solvents is given in Table 1, while typical cyclic voltammograms
are shown in Figure 5 for the three corroles in pyridine,
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.
of −0.4 V. This oxidation of [(Corr)H2]− is however seen in
pyridine on all scans, in a positive or negative direction (E1/2
=
0.09 V).
To investigate the effect of nitration on both the overall
oxidation/reduction mechanism and the protonation/deproto-
nation processes, 3-NO2 TTCorrH3 and 3,17-
(NO2)2TTCorrH3 were also characterized electrochemically
and spectroelectrochemically in pyridine, PhCN, and CH2Cl2.
The addition of one or two nitro substituents to the β-
pyrrole positions of the macrocycle leads to a positive shift in
containing 0.1 M TBAP. As shown in this figure, the addition of
one or two nitro substituents to the β-pyrrole positions of the
macrocycle leads to similar positive shifts in E1/2 for the first
two oxidations. A positive shift in E1/2 of 130−160 mV for each
additional nitro group is at the 3 and 17 positions of the
macrocycle. For example, the [(Corr)H2]−/[(•Corr)H2]
reaction in pyridine is located at E1/2 = 0.09 V for TTCorrH3
(with no NO2 group), while 3-NO2TTCorrH3 (with one NO2
group) is oxidized at E1/2 = 0.25 V and 3,17-(NO2)2TTCorrH3
(with two NO2 groups) is oxidized at E1/2 = 0.41 V. It is also
worth mentioning that a much larger substituent effect of the
nitro group is observed for the reductions than for the
oxidations. For example, a 540 mV shift in E1/2 for the second
reduction is seen upon going from TTCorrH3 to 3-
NO2TTCorrH3. Although the magnitude of the shift in E1/2
for the second and third reductions is reduced to 290 and 310
mV respectively, upon going from 3-NO2TTCorrH3 to 3,17-
E1/2 for each oxidation or reduction of the nitro-substituted free
base corroles, as compared to measured half wave potentials for
the oxidation or reduction of TTCorrH3. This is as expected,
due to the strong electron-withdrawing character of the NO2
substituents, with the magnitude of the positive shift in E1/2
varying as a function of both the number of the nitro groups on
the macrocycle (0, 1, or 2), as well as the specific electrode
reaction.
A summary of redox potentials for of TTCorrH3, 3-
NO2TTCorrH3, and 3,17-(NO2)2TTCorrH3 in the different
Table 1. Half-Wave or Peak Potentials (V vs SCE) for the Different Forms of (NO2)xTTCorrH3 in Solvents Containing 0.1 M
TBAP
ox
red
solvent
cpd
TTCorrH3
second
first
first
second
third
a
a
a
a
a
a
a
a
a
a
pyridine
0.56
0.09
0.25
0.41
0.33
−1.33
−0.87
−0.75
−1.40
−0.84
−0.62
−1.40
−0.84
−0.62
−1.88
−1.34
−1.05
a
3-NO2TTCorrH3
3,17-(NO2)2TTCorrH3
TTCorrH3
0.70
−1.71
−1.40
a
0.83
b
b
b
b
b
b
CH2Cl2
PhCN
−0.09
0.16
a
a
a
a
3-NO2TTCorrH3
3,17-(NO2)2TTCorrH3
TTCorrH3
0.50
0.34
0.94
−1.42
−1.60
−1.39
0.33
−1.06
−1.90
−1.39
−1.08
−0.08
0.14
3-NO2TTCorrH3
−1.76
−1.38
3,17-(NO2)2TTCorrH3
0.35
a
b
Peak potential at scan rate of 0.1 V/s. This redox couple can only be seen on the second positive potential sweep after scanning through the first
reduction.
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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).
(NO2)2TTCorrH3, the effect of NO2 on these processes is still
much larger than that observed for the first oxidation. This
trend in substituent effects was also observed for β-NO2
substituted Fe(III), Cu(III), and Ge(IV) corroles having similar
structures9,10,37 and fits with the major effect of the nitro group
being to lower the energy of the lowest unoccupied molecular
orbital (LUMO), vide infra.
deprotonated species in the electrochemical reaction, where
[(Corr)H2]− is characterized by absorption bands at 396, 463,
and 666 nm. The diagnostic log−log plot of the titration data is
shown in the inset of Figure 6b and has a slope of 1.0 with a
log K = 3.5 for dissociation of one proton under the given
solution condition.
As shown by the cyclic voltammogram insert in Figure 6c, 3-
NO2TTCorrH3 undergoes an irreversible to quasi-reversible
one-electron oxidation at Ep = 0.50 V in CH2Cl2 at scan rate of
0.1 V/s. Similar irreversible oxidations have been observed for
some free-base porphyrins, where the product of the first
electron abstraction involves a protonation process,38 although
in the case of free-base corroles the oxidation mechanism seems
to be more complicated. Our previous studies36b suggest that
the one-electron oxidation of triarylcorroles initially gives the
corrole radical cation, [(Corr)H3]+, which then rapidly loses a
proton after reaction with another molecule of the neutral
corrole in solution to generate a mixture of the free-base neutral
corrole radical, (Corr)H2, and the corrole monocation,
[(Corr)H4]+. In Figure 6c and d, it is possible to compare
the spectral changes observed for 3-NO2TTCorrH3, when a
controlled potential oxidation at 0.70 V was carried out (see
Figure 6c), and when the same macrocycle was titrated with
successive additions of TFA in CH2Cl2 solution. The final UV−
As mentioned above, the first reduction of the investigated
corroles is irreversible in pyridine, as well as in CH2Cl2 and
PhCN (see Table 1). In all three solvents, the reduction leads
to formation of the deprotonated [(Corr)H2]− species as the
final product. An example of the thin-layer UV−visible spectral
changes observed during the first one-electron reduction of 3-
NO2TTCorrH3 in CH2Cl2 are shown in Figure 6a. As seen in
this figure, the initial spectrum of 3-NO2TTCorrH3 is
characterized by three separated Soret bands at 396, 438, and
464 nm and two visible Q-like bands at 589 and 667 nm.
Controlled potential reduction at −0.95 V in the thin-layer cell
leads to a decreased intensity of the 396 and 464 nm bands, as
well as to a complete disappearance of the 438 nm band, while
the two initially separated visible bands become merged into a
single peak at 630−667 nm (Figure 6a). These spectral changes
match perfectly spectral changes observed in the same solvent
during a titration of 3-NO2TTCorrH3 with piperidine (Figure
6b) and gives further evidence for formation of the
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visible spectra under these two conditions are quite similar to
each other.
structural analysis was performed in the gas-phase and in
dichloromethane solution, using both BP86 and B3LYP
functionals. The optimized structures showed very little
sensitivity to the used functional and solvation effects. Table
3 collects the relevant structural parameters computed at the
B3LYP level, in the gas-phase, for TTCorrH3, 2-NO2-
TTCorrH3, 3-NO2TTCorrH3, and 3,17-(NO2)2TTCorrH3.
For 2-NO2TTCorrH3, comparison is made between the
theoretical and experimental data.
As expected, the introduction of one or two nitro groups at
the β-positions of the corrole macrocycle influences the
corresponding acid−base properties of the molecule. Spectral
changes of the type reported in Figure 6b and d for 3-
NO2TTCorrH3 were exploited to obtain equilibrium constants
for deprotonation (log Ka) and protonation (log Kb) processes
of the three investigated free-base corroles in CH2Cl2, and the
corresponding protonation−deprotonation constants are re-
ported in Table 2. The equilibrium constant for proton
As observed in other metal-free triarylcorroles,39c,40 in
TTCorrH3 the macrocycle exhibits a significant distortion
from planarity to minimize the steric hindrance between the
inner protons. The macrocycle adopts a puckered conformation
in which the pyrrole rings turn either slightly up or down.
According to the computed twist angle between the pyrrole
mean planes, θ1, the pyrrolenine ring A and the adjacent pyrrole
not directly bonded to it, B, are close to be coplanar, mostly
due to the presence of an intramolecular hydrogen bond
between the N2−H group and the iminic N1 atom (N1−N2 =
2.621 Å, ∠N2−H2···N1 = 132.2°). The D and C rings are tipped
up and down with respect to the 23 atom mean plane forming a
dihedral angle (θ3) of 22.2° with each other. The two directly
bonded pyrroles, A and D, show a N−C−C−N twist angle
(φ1) of 10.3°. Comparison of the structural parameters of 2-
NO2TTCorrH3 with those of TTCorrH3 shows that the
introduction of a nitro group at the C2 position has very little
impact on the geometry of the corrole macrocycle. As indicated
by the C1−C2−N5−O1 angle (φ2), the NO2 group is
substantially coplanar with the pyrrolenine ring A. This,
together with a relatively short C2−N5 bond length (1.430
Å), suggests conjugation between the corrole and the nitro
substituent. The geometrical data listed in Table 3 for 2-
NO2TTCorrH3 show agreement between theory and experi-
ment, thus confirming that the puckering of the corrole
macrocycle is dictated by intrinsic electronic factors. Because of
steric hindrance with the adjacent phenyl ring, the nitro group
in 3-NO2TTCorrH3 is no longer coplanar with the pyrrolenine
ring A, the C4−C3−N5−O1 angle amounting to 31.4°. This
results in decreased conjugation between the nitro group and
the corrole macrocycle, as confirmed by the elongation of the
C−N bond. In turn, the phenyl ring on the 5 position tilts
toward the bipyrrolic block by 50.2°. This is reflected in a slight
increase of macrocycle puckering. In 3,17-(NO2)2TTCorrH3,
the puckering of the corrole macrocycle is further enhanced
relative to the mononitrated corroles, due to the cooperative
effects of the nitro substituents. It is worth noting, however,
Table 2. Equilibrium Constants for Deprotonation (log Ka)
and Protonation (log Kb) of (NO2)xTTCorrH3 in CH2Cl2
log Ka
log Kb
reacting with
piperidine
reacting with trifluoroacetic
cpd
TTCorrH3
acid
1.1
3.5
4.9
4.5
3.9
3.3
3-NO2TTCorrH3
3,17-(NO2)2TTCorrH3
dissociation increased upon going from TTCorrH3 (log Ka =
1.1) to 3-NO2TTCorrH3 (log Ka = 3.5) and then to 3,17-
(NO2)2TTCorrH3 (log Ka = 4.9), while for log Kb, a reverse
trend is obtained (see Table 2), indicating that the acidity of
free-base corroles increases, while the basicity decreases upon
increasing the number of peripheral nitro substituents. It is
interesting to note that the effect of the nitro groups is higher
for the increase of the acidity constants than that observed for
the corresponding Kb.
Theoretical Studies. To rationalize the effects of
introducing nitro groups at the periphery of TTCorrH3, the
structural, electronic, optical, and electrochemical properties of
TTCorrH3 and its nitro derivatives were theoretically
investigated using DFT and TDDFT methods.
(a) Molecular Structures. In metal-free corroles the inner
hydrogen atoms can be distributed in different ways among the
four nitrogen atoms,39 which results in the formation of
tautomers. In this study only one tautomeric form of
TTCorrH3, 2-NO2TTCorrH3, 3-NO2TTCorrH3, and 3,17-
(NO2)2TTCorrH3 was examined theoretically. In the choice of
the tautomers of TTCorrH3 and its nitro derivatives, we have
been guided by the X-ray structural data available for
TPCorrH3 (TPCorrH3 = 5,10,15-triphenylcorrole)39c and 2-
NO2TTCorrH3, respectively. In both compounds the inner
hydrogen atoms are located on B, C, and D pyrrole rings. The
Table 3. Selected Bond Lengths (Å), Bond Angles (deg), and Metrical Parameters Calculated for TTCorrH3 and its Nitro
Derivatives, 2-NO2TTCorrH3, 3-NO2TTCorrH3, and 3,17-(NO2)2TTCorrH3
a
TTCorrH3
2-NO2TTCorrH3
3-NO2TTCorrH3
3,17-(NO2)2TTCorrH3
b
C−N(nitro)
N1−N2
1.430/1.421(3)
2.639/2.623(3)
131.5/134(3)
9.4/10.5(6)
1.445
2.623
130.4
12.4
1.447/1.441
2.627
2.621
132.2
10.2
N2−H2···N1
129.8
c
θ1
13.7
d
θ3
22.2
22.4/23.8(3)
11.0/7.3(3)
23.6
27.9
e
φ1
10.3
12.1
14.4
f
φ2
2.8/1.2(4)
31.4
32.9/20.0
a
b
Experimental values (this work) in italics. C2−N5, C3−N5, and C3−N5/C17−N6 bond lengths for 2-NO2TTCorrH3, 3-NO2TTCorrH3, and 3,17-
c
d
(NO2)2TTCorrH3, respectively. Dihedral angle between the B and A pyrrole mean planes. Dihedral angle between the C and D pyrrole mean
planes. N1−C5−C19−N4 torsional angle. C1−C2−N5−O1, C4−C3−N5−O1, and C4−C3−N5−O1/C16−C17−N6−O3 torsional angles for 2-
NO2TTCorrH3, 3-NO2TTCorrH3, and 3,17-(NO2)2TTCorrH3, respectively.
e
f
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that while the twisting of the nitro group on the 3 position is
very much the same as in 3-NO2TTCorrH3 (32.9° vs 31.4°),
that of the nitro group on the 17 position is only 20.0°, leading
to a more effective conjugation with the pertinent pyrrole.
(b) Ground-State Electronic Structure and Optical Spectra.
To provide an interpretation of the UV/vis spectral changes
accompanying the nitration of TTCorrH3, TDDFT calculations
of the electronic absorption spectra in dichloromethane
solution were performed for 2-NO2TTCorrH3, 3-
NO2TTCorrH3, 3,17-(NO2)2TTCorrH3, and the parent
TTCorrH3. Before dealing with the excited states, we briefly
discuss the ground-state electronic structure of the investigated
metal-free corroles. To highlight the electronic effects of the
nitro groups, the electronic structure of the unsubstituted free-
base is taken as a reference. An energy level scheme of the
highest occupied and lowest unoccupied Kohn−Sham orbitals
of TTCorrH3 and its nitro derivatives is shown in Figure 7.
lying set of π-phenyl orbitals. According to the level scheme of
Figure 7, the outstanding effect of introducing nitro groups into
TTCorrH3 is the stabilization of the LUMO and, to a lesser
extent, of the HOMO−1. As a result, the LUMO/LUMO+1
splitting increases dramatically on going from TTCorrH3 to the
nitro derivatives. The large stabilization of the LUMO moving
from TTCorrH3 to the nitro derivatives is due to the C−N(nitro)
π-bonding character of this orbital well visible in the plots
displayed in Figure 8. In 3-NO2TTCorrH3 the LUMO is
slightly less stabilized than in 2-NO2TTCorrH3 because the
nitro group is rotated by 31.4° with respect to the pertinent
pyrrole plane and hence the overlap between the C3 and N5
2pz orbitals is less effective than in 2-NO2TTCorrH3 where the
nitro group is almost coplanar with the pertinent pyrrole. In the
dinitro derivative both the LUMO and LUMO+1 experience a
significant downward shift as compared to the parent
TTCorrH3. This is because both these MOs have C−N(nitro)
π-bonding character. Due to the presence of two C−N(nitro) π-
bonding interactions, the stabilization of the LUMO and
LUMO+1 is quite large in 3,17-(NO2)2-TTCorrH3. However,
it should be noted that, for overlap reasons, the C−N(nitro) π-
bonding interaction involving the nitro group on the 3 position
is somewhat less effective than that involving the nitro group on
the 17 position. As pointed out earlier, the nitro groups on the
3 and 17 positions are rotated with respect to the plane of the
pertinent pyrrole by 32.9° and 20.0°, respectively.
In summary, comparison between the one-electron levels of
the nitro corroles and those of the parent free-base reveals that
the most striking differences are the increased LUMO/LUMO
+1 splitting and the diminished HOMO/LUMO energy gap,
which are mainly due to the downshift of the LUMO in the
nitro derivatives.
We see now how the previously discussed electronic
structure changes accompanying the nitration of the corrole
macrocycle reflect on the UV−visible spectroscopic properties
of the investigated nitrocorroles. The excitation energies and
oscillator strengths calculated for the lowest singlet excited
states of TTCorrH3 and its nitro derivatives are reported in
Tables 4−7 together with the major one-electron transitions
contributing to the excited-state solution vectors.
In Figure 9, the computed and experimental absorption
spectra of these compounds in CH2Cl2 are compared.
Considering first TTCorrH3 as point of reference for the
nitro derivatives, only two excited states are computed in the
energy regime of the four Q bands, the 11A and 21A. As found
in TPPH2,42 these states originate from out-of-phase
combination of the Gouterman transitions. However, due to
the lifting of the LUMO/LUMO+1 degeneracy induced by the
appreciable distortion of the macrocycle in TTCorrH3, the
mixing coefficients of the Gouterman transitions change
significantly relative to TPPH2. As a matter of fact, the 11A
excited state in TTCorrH3 is mostly described by the HOMO
→ LUMO transition, the transition out of the HOMO−1 into
the LUMO+1 entering in this state with only a minor (15%)
weight. The 21A is instead a nearly 50:50 mixture of the
HOMO → LUMO+1 and HOMO−1 → LUMO transitions.
Due to a less effective mixing of the contributing transitions
and, hence, to a less effective cancellation of their large dipole
moments, the oscillator strength computed for the 11A state is
significantly larger than that of the 21A. By analogy with the
assignment previously proposed for TPPH2,42 the Q bands
observed in the CH2Cl2 absorption spectrum of TTCorrH3 at
652 and 574 nm are assigned to the 11A and 21A states,
Figure 7. Energy level scheme for TTCorrH3 and its nitro derivatives.
The Gouterman-derived MOs are indicated with red lines.
The plots of the two highest occupied and of the two lowest
unoccupied MOs of these compounds are displayed in Figure 8.
The HOMO−1 and HOMO of TTCorrH3 resemble the
HOMO−1(au) and HOMO(b1u) of TPPH2 (D2h symmetry).41
Less pronounced is the similarity between the LUMO and
LUMO+1 of TTCorrH3 and the LUMO(b2g) and LUMO
+1(b3g) of TPPH2 (D2h symmetry).41 In the nitro derivatives
and in the parent TTCorrH3, a quite large energy gap separates
the HOMO−1 from the lower lying MOs, which are largely
localized on the aryl groups and on the pyrrolenine ring. In the
nitro derivatives, one or two MOs having some amplitude on
the nitro groups interpose between the LUMO+1 and the high-
J
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Figure 8. Plots of the frontier orbitals of TTCorrH3 and its nitro derivatives.
Table 4. Composition, Vertical Excitation Energies, E (eV/
nm), and Oscillator Strengths, f, for the Lowest Optically
Allowed Excited States of TTCorrH3 in CH2Cl2
distinct peak at 588 nm in the UV−visible spectrum of 2-
NO2TTCorrH3 and as a broad feature at around 600 nm in the
spectra of the other nitro derivatives. The calculations also
account for the observed red shift of this band on going from
TTCorrH3 to the nitro derivatives. Distinct from TTCorrH3,
the nitro corroles exhibit a complex B band system
characterized by several overlapping features. The increased
complexity of the B band system in nitrocorroles is related to
the involvement of non-Gouterman transitions in the excited
states accounting for this spectral region. The calculations
indicate that the Gouterman-type transitions, which account for
the nearly degenerate B bands of TTCorrH3, mix to a variable
extent with transitions out of the HOMO−2 (a MO largely
localized on the aryl groups and on the pyrrolenine ring) into
the LUMO, or with transitions out of the HOMO into the
LUMO+2 (a MO with sizable amplitude on the nitro groups).
As for 2-NO2TTCorrH3, the two prominent B bands are well
accounted for by the 41A and 51A excited states computed at
449 and 392 nm and with oscillator strength of 0.8068 and
0.7931, respectively. According to their composition (see Table
5), these states are largely composed of the Gouterman-type
transitions with the above-mentioned non-Gouterman tran-
sitions entering with a minor, although not negligible, weight.
Four intense excited states are computed in the energy regime
of the three B band system of 3-NO2TTCorrH3. On the basis
of their energy, the 31A and 41A are assigned to the lowest
energy features at 465 and 438 nm, respectively, and the nearly
degenerate 51A and 61A states to the most prominent feature at
397 nm.
state
11A
composition (%)
E
f
150a → 151a (78)
149a → 152a (15)
150a → 152a (47)
149a → 151a (46)
149a → 151a (48)
150a → 152a (47)
149a → 152a (80)
150a → 151a (14)
2.12/585
0.2905
21A
31A
41A
2.30/539
2.96/418
3.05/406
0.03580
1.122
1.439
respectively, although the computed absorption wavelengths
are somewhat blue-shifted with respect to the experimental
values. According to the TDDFT calculations, the Q bands at
616 and 465 nm are identified as vibrational in origin. The B
band profile of TTCorrH3 is nicely accounted for by the nearly
degenerate 31A and 41A excited states. These originate from in-
phase combinations of the Gouterman type transitions and,
hence, have large oscillator strength (see Table 4).
Coming to the nitrocorroles, in the energy regime of the Q
bands, TDDFT calculations predict, just as for the parent
TTCorrH3, only two excited states, the 11A and 21A. As can be
inferred from the composition of these states in Tables 4−7 for
all three nitro derivatives, the 11A is a nearly pure (>90%)
HOMO → LUMO state and the 21A is a mixture of the
HOMO → LUMO+1 and HOMO−1 → LUMO transitions.
The lowest excited state nicely accounts for the energy and
intensity of the longest wavelength Q-band. The 21A excited
state is responsible for the higher energy Q-band appearing as a
The B band system of the dinitrocorrole is characterized by
an intense, sharp feature at 483 nm followed to the blue by a
broad, less intense feature. The energy and oscillator strength
K
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Table 5. Composition, Vertical Excitation Energies, E (eV/
nm), and Oscillator Strengths, f, for the Lowest Optically
Allowed Excited States of 2-NO2TTCorrH3 in CH2Cl2
state
composition (%)
E
f
11A
21A
161a → 162a (94)
160a → 162a (72)
161a → 163a (25)
161a → 164a (82)
160a → 163a (16)
161a → 163a (59)
160a → 162a (20)
160a → 164a (16)
160a → 163a (49)
159a → 162a (31)
1.74/711
2.21/560
0.2655
0.1042
31A
41A
2.61/474
2.76/449
0.07460
0.8068
51A
3.16/392
0.7931
Table 6. Composition, Vertical Excitation Energies, E (eV/
nm), and Oscillator Strengths, f, for the Lowest Optically
Allowed Excited States of 3-NO2TTCorrH3 in CH2Cl2
state
composition (%)
E
f
11A
21A
161a → 162a (91)
160a → 162a (69)
161a → 163a (27)
161a → 164a (38)
161a → 163a (37)
160a → 162a (14)
160a → 163a (32)
161a → 164a (27)
161a → 163a (23)
159a → 162a (54)
161a → 164a (17)
160a → 163a (14)
159a → 162a (36)
160a → 163a (33)
161a → 164a (14)
1.86/667
2.19/565
0.3338
0.1304
31A
41A
51A
61A
2.65/468
2.73/455
3.07/403
3.13/396
0.3584
0.3085
0.4981
0.5669
In the energy regime of the broad B feature, the calculations
predict four excited states, two of which, the nearly degenerate
61A and 71A, account for most of the intensity of this band.
(c) Reduction Potentials. The redox behavior of metal-free
corroles is rather complex owing to the proton transfer
Table 7. Composition, Vertical Excitation Energies, E (eV/
nm), and Oscillator Strengths, f, for the Lowest Optically
Allowed Excited States of 3,17-(NO2)2TTCorrH3 in CH2Cl2
state
composition (%)
E
f
11A
21A
172a → 173a (96)
171a → 173a (59)
172a → 174a (40)
172a → 174a (54)
171a → 173a (35)
171a → 174a (71)
172a → 176a (20)
170a → 173a (68)
172a → 175a (24)
172a → 176a (47)
170a → 173a (16)
171a → 175a (14)
172a → 175a (37)
170a → 173a (20)
171a → 174a (16)
1.72/720
2.04/608
0.4537
0.01098
Figure 9. Comparison between computed (TDDFT/B3LYP) and
experimental absorption spectra of TTCorrH3 and its nitro derivatives
in CH2Cl2.
31A
41A
51A
61A
2.46/503
2.69/462
2.81/442
2.90/427
0.6455
0.01524
0.03678
0.1994
computed for the 31A excited state leaves no doubt about the
assignment of the longer wavelength B band to this state. It is
noteworthy that the 31A state consists of the same Gouterman
transitions as the 21A, HOMO−1 → LUMO and HOMO →
LUMO+1, with approximately reversed weights. The large
transition dipoles of these transitions reinforce in the 31A and
cancel in the 21A state with the result that the former is by far
more intense than the latter.
71A
2.93/422
0.1740
L
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Table 8. Thermodynamic Parameters (eV) for the One-Electron Reduction Reaction1 and One-Electron Reduction Potentials
(V) for TTCorrH3, 2-NO2TTCorrH3, 3-NO2TTCorrH3, and 3,17-(NO2)2TTCorrH3
a
b
ΔHo°x/red
−TΔSo°x/red
ΔEsolv
ΔGo°x/red
Eo°x/red vs SCE
E1/2 vs SCE
TTCorrH3
−2.93
−3.49
−3.44
−3.78
0.02
0.03
0.03
0.04
−1.41
−1.50
−1.47
−1.31
−2.91
−3.52
−3.47
−3.82
−1.46
−0.85
−0.90
−0.55
−1.40
2-(NO2)TTCorrH3
3-(NO2)TTCorrH3
3,17-(NO2)2TTCorrH3
−0.84
−0.62
a
b
ΔEsolv is the difference between the dielectric energy of the reduced species and the dielectric energy of the oxidized species. Experimental values
in dichloromethane solution, this work.
equilibrium reaction CorrH3 ⇌ [CorrH2]−. As seen in the
previous section, the reduction potentials of the three-
protonated species, CorrH3, positively shifts when one or two
nitro groups are introduced at the β-position of the macrocycle.
Also the addition of a second nitro group to the macrocycle
decreases the effect of nitration on the reduction potentials
under the same experimental conditions. To rationalize the
observed trend, we have computed the first reduction potential
o f 2 - N O 2 T T C o r r H 3 , 3 - N O 2 T T C o r r H 3 , 3 , 1 7 -
(NO2)2TTCorrH3, and the parent TTCorrH3 in dichloro-
methane solution, using a DFT/COSMO model.
ΔEsolv values indicate that the energetic contribution of
solvation to the enthalpic term, though relevant, is substantially
constant along the series. Thus, the ΔH°ox/red trend is primarily
determined by the electronic enthalpy that favors energetically
the reduced form over the oxidized one in the order TTCorrH3
≪ 2-NO2TTCorrH3 ≅ 3-NO2TTCorrH3 < 3,17-
(NO2)2TTCorrH3. In nitrocorroles, the one-electron reduced
form is much more stable than the oxidized form as compared
to TTCorrH3, and this is consistent with the added electron
entering a MO with C−N(nitro) π-bonding character in the
former, as witnessed by the significant shortening of the C−
N(nitro) bond. In 2-NO2TTCorrH3 and 3-NO2TTCorrH3, the
C−N(nitro) bond shortens by 0.027 and 0.033 Å, respectively. In
the dinitro derivative, the two C−N(nitro) bonds shorten, on the
average, by 0.025 Å.
The standard reduction potential, E°ox/red, for the reaction
CorrH3(CH2Cl2) + e−(g) → [CorrH3]−(CH2Cl2)
(1)
is related to the standard free energy change relative to the
standard hydrogen electrode (SHE) through the equation
CONCLUSIONS
■
°
°
ΔGox/red − ΔGSHE
°
Eox/red = −
We have reported the first example of efficient insertion of nitro
groups into the β-pyrrole positions of corrole free bases,
effectively overcoming the formation of isocorrole by
controlling the amount and the nature of the oxidant used
for the reaction. The regioselectivity usually observed in the
functionalization of corrole ring is also demonstrated for the
nitration reaction, producing the 3-nitro- and the 3,17-dinitro-
corroles as the main reaction products of mono- and
disubstitution; we have also been able to characterize some
other minor regioisomers, such as the 2-nitro, 2,3-, and the
3,12-dinitro derivatives. It is interesting to note that this last
regioisomer is the first example of antipodal functionalization of
corrole ring.
The introduction of nitro substituents at the β-pyrrole
positions of the corrole ring strongly influence the chemical and
spectroscopic behavior of the macrocycle. The strong electron-
withdrawing character of the nitro group leads to a positive shift
of the E1/2 of the redox processes of corrole and to an increase
of the macrocycle acidity. It is noteworthy that the introduction
of the first nitro group is more effective than the second
substituent in changing the properties of the functionalized
macrocycle. Optical absorption spectra of β-nitrocorroles are
strongly influenced by the peripheral nitro groups, which
increase the number of bands, characterized also by a significant
red shift.
The theoretical results on these β-nitrocorrole derivatives
also afforded significant information, closely matching the
experimental observations. It was 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.
(2)
nF
In eq 2, n is the number of electrons consumed in the
reduction reaction, 1 in the present case, and F is a constant. If
the free energies are expressed in molar units, F is the Faraday
constant (the negative of the charge on one mole of electrons),
but if energies are expressed in electron volts per molecule, as
in the present paper, then F is equal to the unit charge e−.
ΔGo°x/red is the free energy change associated with reaction 1,
whereas ΔG°SHE is the free energy change of the following
reaction
1
2
H+(aq) + e−(g) → H2(g)
(3)
ΔGS°HE has been established in the literature to be −4.28 eV.43
As the experimental half-wave potentials of the investigated
metal-free corroles were measured using SCE as the reference
electrode, the theoretical reduction potentials obtained from eq
2 were converted to SCE referenced potentials by subtracting
0.09 V, which was obtained by taking the literature value of the
reduction potential of aqueous SCE relative to SHE at 298 K
(0.24 V)44 and correcting it to its corresponding value in
dichloromethane solution using the literature value of the liquid
junction potential (ljp) for dichloromethane−water (0.15 V).45
The corrected one-electron reduction potentials of TTCorrH3,
2-NO2 TTCorrH3 , 3-NO2 TTCorrH3 , and 3,17-
(NO2)2TTCorrH3 are compared with the experimental half-
wave potentials in Table 8 where the thermodynamic
parameters computed for the one-electron reduction process
in eq 1 are also reported. The data in Table 8 indicate that there
is an excellent agreement between the theoretically predicted
one-electron reduction potentials and the available experimen-
tal values. The thermodynamic data reveal that the one-electron
reduction process is energetically controlled by enthalpic
factors, the entropic factors playing a negligible role. The
M
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Article
443. (c) Capar, C.; Thomas, K. E.; Ghosh, A. J. Porphyrins
Phthalocyanines 2008, 12, 964−967.
In closing, these nitro-functionalized corroles are also useful
starting materials for further modifications of the corrole ring
and their availability can open new opportunities for the
preparation of functionalized corroles; these studies are now in
progress in our laboratories.
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Karlsruhe and Forschungzentrum Karlsruhe GmbH, 1989−2007,
TURBOMOLE GmbH; available from http://www.turbomole.com.
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files of 2-NO2TTCorrH3, 2,3-(NO2)2TTCorrCoPPh3, and
3,12-(NO2)2TTCorrCoPPh3. This material is available free of
charge via the Internet at http://pubs.acs.org
(b) Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Chem. Phys.
̈
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Lett. 1989, 162, 165.
AUTHOR INFORMATION
Corresponding Author
■
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Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
*E-mail: roberto.paolesse@uniroma2.it (R.P.); kkadish@uh.
edu (K.M.K.); kmsmith@lsu.edu (K.M.S.); angela.rosa@
unibas.it (A.R.).
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the support of MIUR, Italy (PRIN
project 2009Z9ASCA), the US National Institutes of Health
(K.M.S., grant CA132861), and the Robert A. Welch
Foundation (K.M.K., grant E-680).
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