Synthesis and characterization of functionalized meso - triaryltetrabenzocorroles

Article abstract of DOI:10.1021/ic4010467

5,10,15-Triaryltetrabenzocorroles functionalized with different electron-withdrawing groups on the β,β′-fused rings have been prepared by a cross-coupling Heck procedure between octabrominated copper corrole and a terminal alkene bearing electron-withdraw

Full text of DOI:10.1021/ic4010467

Article
pubs.acs.org/IC
Synthesis and Characterization of Functionalized meso-
Triaryltetrabenzocorroles
Giuseppe Pomarico,† Sara Nardis,† Manuela Stefanelli,† Daniel O. Cicero,† M. Graca H. Vicente,‡
̧
Yuanyuan Fang,§ Ping Chen,§ Karl M. Kadish,*,§ and Roberto Paolesse*,†,⊥
†Dipartimento di Scienze e Tecnologie Chimiche, Universita
̀
di Roma Tor Vergata, via della Ricerca Scientifica, 1, 00133 Rome, 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
⊥CNR IDASC, via Fosso del Cavaliere, 00133 Rome, Italy
S
* Supporting Information
ABSTRACT: 5,10,15-Triaryltetrabenzocorroles functional-
ized with different electron-withdrawing groups on the β,β′-
fused rings have been prepared by a cross-coupling Heck
procedure between octabrominated copper corrole and a
terminal alkene bearing electron-withdrawing moieties. The
spectroscopic characterization of these complexes showed red-
shifted UV−vis absorption bands characterized by a significant
band broadening. The same feature was observed in the case of
NMR spectra, where low-resolution groups of signals were
observed. This behavior derives from a strong tendency of
1
these macrocycles to aggregate in solution, as has been demonstrated by an H NMR study performed on one of these
tetrabenzocorroles. The influence of the substituents on the fused benzene ring on the properties of the tetrabenzocorroles was
investigated by electrochemistry and spectroelectrochemistry, and comparisons were made between properties of the newly
synthesized compounds and those of the tetrabenzocorroles reported earlier in the literature.
research ranging from medicine to sensors to material
chemistry.10−12
INTRODUCTION
■
Corrole is a tetrapyrrolic macrocycle, which differs from
naturally-occurring porphyrin rings in lacking one meso carbon,
which is replaced by a direct pyrrole−pyrrole link. Despite this
difference in the molecular skeleton, corroles retain an 18-
electron π aromatic system similar to that of porphyrins.
Although corroles are not found in biological systems, the
number of investigations of these compounds in the area of
synthesis and applications has increased significantly over the
past decade and has reached a point where the popularity of
studying corroles is in some cases approaching that of the
related porphyrins.
After the first report of corrole preparation by Johnson and
Kay in 1965,1 the chemistry of this porphyrinoid stood largely
unexplored for several decades; this was mainly due to the
difficult multistep procedure originally needed to prepare the
precursors that lead to the formation of β-alkylcorroles. This
situation changed in 1999, when two different synthetic routes
for the preparation of meso-triarylcorroles starting from
commercially available precursors were independently reported
in seminal papers from the groups of Gross2 and Paolesse.3
Further development of new methods for the large-scale
preparation of a wide range of meso-triarylcorroles4−6 allowed
for more accurate investigations of the properties of this
macrocycle7−9 and their concomitant use in various fields of
This progress led to the possibility of tuning the properties of
corroles by peripheral functionalization, following an approach
successfully used in the case of porphyrins, with one end goal
being the expanded use of these macrocycles in different
applications ranging from catalysis to photovoltaic cells. One
interesting modification of the corrole is to expand the aromatic
system by introduction of fused rings at the β,β′-pyrrole
positions of the macrocycle. This modification is generally
known as annulation and can result in significant variations of
the macrocycle photochemical features.
A number of porphyrinoids have been obtained by the
introduction of different aromatic rings onto the macrocycle,
including benzene,13 naphthalene,14 anthracene,15 phenan-
threne,16 and acenaphthalene.17 The most common derivatives
among these π-extended macrocycles are the benzoporphyrins,
which show different optical features compared with those of
the starting materials;18,19 for example, there are substantial red
shifts of the Soret and Q bands, the latter of which often exhibit
increased extinction coefficients. These variations are interest-
Received: April 26, 2013
Revised: June 26, 2013
Accepted: June 26, 2013
Published: July 5, 2013
© 2013 American Chemical Society
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Scheme 1. Synthetic Pathway for the Preparation of Cu−Triaryltetrabenzocorrolates by the Cross-Coupling Methodology
ing for the exploitation of these compounds in several fields of
research.
groups on the fused benzene rings. These modified
tetrabenzocorroles were prepared by Heck cross-coupling
reactions between the octabromocorrole 1 and terminal olefins
bearing different functional groups. We also describe the
electrochemical properties of these new compounds.
We have been interested in the study of the effect of aromatic
ring annulation to the corrole macrocycle and reported the first
preparation of a meso-triaryltetrabenzocorrole,20 which was
obtained by two different synthetic routes that were previously
exploited to obtain analogous tetrabenzoporphyrins.21,22 One
method was based on the condensation of an arylaldehyde with
tetrahydroisoindole, a suitable pyrrole bearing a six-membered
ring as precursor of the annulated benzene ring. Higher yields
of the target corroles were achieved with an alternative
synthetic route, shown in Scheme 1, based on a cross-coupling
reaction between a brominated copper corrole precursor and
methyl acrylate under Heck conditions. The tetrabenzocorroles
obtained by this method are characterized by large red shifts of
the absorption bands as a result of the extended aromatic
system.
Following the discovery of an efficient synthetic pathway for
the preparation of triaryltetrabenzocorroles, we investigated the
coordination of different metal ions in the core of the
macrocycle (Al, Ga, Ge, Cu, P) and the associated axial
ligands.23 Noteworthy results were obtained for the phosphorus
complex, which showed the highest fluorescence quantum yield
(0.31 in toluene). The potential applications of this type of
compound as an optical sensor, a fluorophore in bioimaging, or
a singlet oxygen sensitizer in photodynamic therapy, warrant
their further syntheses and investigation.
EXPERIMENTAL SECTION
■
Materials. Silica gel 60 (70−230 mesh, Sigma Aldrich) was used
for column chromatography. Reagents and solvents (Aldrich, Fluka)
were HPLC-grade and were used without further purification. 1H
spectra were recorded on a Bruker AV300 spectrometer operating at
300.13 MHz. Chemical shifts are given in parts per million relative to
residual solvent (7.26 ppm for CHCl3 or 5.32 ppm for CH2Cl2). UV−
vis spectra were measured in CH2Cl2 with a Varian Cary 50
spectrophotometer. IR spectra were recorded in CHCl3 solutions
with a Perkin-Elmer 100 FT-IR spectrometer using KBr cells. Mass
spectra (TOF-SIMS) were recorded using positive method with
TOFSIMS V (IONTOF) spectrometer or (FAB mode) on a
VGQuattro spectrometer in the positive ion mode using m-nitrobenzyl
alcohol (Aldrich) as a matrix.
General Procedure for [Triaryltetrabenzocorrolato]Cu Syn-
thesis. Octabrominated copper corrole 1 (140 mg, 0.101 mmol),
Pd(AcO)2 (23.0 mg, 0.101 mmol, 1 equiv), PPh3 (69.0 mg, 0.264
mmol, 2.62 equiv), K2CO3 (120 mg, 0.870 mmol, 8.57 equiv), and
EWG-alkene (100-fold excess) were dissolved in a mix of anhydrous
toluene/DMF (22 + 15 mL) in a Schlenk tube under nitrogen. The
mixture was then degassed via five freeze−pump−thaw cycles before
the vessel was purged with nitrogen, sealed, and heated at 125 °C for
72 h. The solvent was removed under reduced pressure and the
residue was purified as described below.
In the present study, we report the preparation of new
tetrabenzocorroles containing different electron-withdrawing
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[5,10,15-Tris(4-tert-butylphenyl)-2:3,7:8,12:13,17:18-tetrabenzo-
2″,3″,7″,8″,12″,13″,17″,18″-octacarboxymethylcorrolato]Cu,
[CH3CO2-TBC]Cu, 2. This was prepared as previously reported.20
[5,10,15-Tris(4-tert-butylphenyl)-2:3,7:8,12:13,17:18-tetrabenzo-
2″,3″,7″,8″,12″,13″,17″,18″-octacyanocorrolato]Cu, [CN-TBC]Cu, 3.
The residue was purified by chromatography on silica gel eluted with
2% CH3OH in CH2Cl2, followed by crystallization from CH2Cl2/
hexane, to afford the desired product. Yield: 22% (26 mg). Mp > 300
°C. UV−vis (CH2Cl2) λmax, nm (log ε): 474 (4.72), 605 (3.92), 686
(3.69). IR (CHCl3): 2228 cm−1 (CN). 1H NMR (300.13 MHz,
CDCl3): δ 7.71−7.50 (m, 20 H, β-fused + phenyls), 1.44 (br s, 27 H,
tert-butyls). MS (FAB, m/z): 1154 (M+); 1094 (M+ − Cu). Anal.
Calcd for C73H47CuN12: C, 75.86; H, 4.10; N, 14.54%. Found: C,
75.81; H, 4.13; N, 14.48%.
Electrochemical and Spectroelectrochemical Measure-
ments. Absolute dichloromethane (CH2Cl2, 99.8%, EMD Chemicals
Inc.) and pyridine (py, 99.8%, Sigma-Aldrich) were used for
electrochemistry without further purification. Benzonitrile (PhCN)
was purchased from Sigma-Aldrich and distilled over P2O5 under a
vacuum prior to use. Tetra-n-butylammonium perchlorate (TBAP),
used as supporting electrolyte, was purchased from Sigma-Aldrich or
Fluka, recrystallized from ethyl alcohol, and dried under a vacuum at
40 °C for at least one week prior to use.
Cyclic voltammetry was carried out with an EG&G model 173
potentiostat/galvanostat. A homemade three-electrode electrochemis-
try cell was used consisting of a platinum button or 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 that
contained the solvent/supporting electrolyte mixture. All potentials
were referenced to the SCE.
Thin-layer UV−visible spectroelectrochemical experiments were
performed with a home-built thin-layer cell that had a light transparent
platinum net working electrode. Potentials were applied and
monitored with an EG&G PAR Model 173 potentiostat. Time-
resolved UV−visible spectra were recorded with a Hewlett-Packard
Model 8453 diode array spectrophotometer. High-purity N2 from
Trigas was used to deoxygenate the solution and was kept over the
solution during each electrochemical and spectroelectrochemical
experiment.
[5,10,15-Tris(4-tert-butylphenyl)-2:3,7:8,12:13,17:18-tetrabenzo-
2″,3″,7″,8″,12″,13″,17″,18″-octaphenylcorrolato]Cu, [Ph-TBC]Cu, 4.
The residue was purified by chromatography on silica gel eluted with
CH2Cl2/hexane (2:1), followed by crystallization from CH2Cl2/
CH3OH, to afford the desired product. Yield: 21% (33 mg). Mp > 300
1
°C. UV−vis (CH2Cl2) λmax, nm (log ε): 475 (4.92), 606 (4.17). H
NMR (300.13 MHz, CD2Cl2): δ 7.61−6.68 (m, 60 H, β-fused + meso-
phenyls + δ,δ′-phenyls), 1.49−1.33 (m, 27 H, tert-butyls). MS (FAB,
m/z): 1564 (M+). Anal. Calcd for C113H87CuN4: C, 86.75; H, 5.60; N,
3.58%. Found: C, 86.69; H, 5.56; N, 3.53%.
[5,10,15-Tris(4-tert-butylphenyl)-2:3,7:8,12:13,17:18-tetrabenzo-
2 ″ , 3 ″ , 7 ″ , 8 ″ , 1 2 ″ , 1 3 ″ , 1 7 ″ , 1 8 ″ - o c t a k i s ( 2 , 3 , 4 , 5 , 6 -
pentafluorophenylmethyl)corrolato]Cu, [F5PhCH2-TBC]Cu, 5. The
residue was purified by chromatography on silica gel eluted with
CH2Cl2/hexane (1:1), to afford the desired product. Yield: 21% (51
mg). Mp > 300 °C. UV−vis (CH2Cl2) λmax, nm (log ε): 454 (4.55),
RESULTS AND DISCUSSION
■
1
The formation of the triaryltetrabenzocorrole by the cross-
coupling methodology involves reaction of an octabrominated
copper corrolate with terminal alkenes possessing electron-
withdrawing groups, as previous reported for the preparation of
tetrabenzoporphyrin systems.22 The first tetrabenzocorrole
obtained by this procedure was prepared starting from the
halogenated corrole 1 and methyl acrylate, leading to the
formation of an annulated corrole functionalized with eight
carboxymethyl moieties on the fused rings. The presence of
substituents at the periphery of the macrocycle influences the
optical features of this derivative as compared to the spectral
properties of the unsubstituted tetrabenzocorrole.20
608 (4.08). H NMR (300.13 MHz, CDCl3): δ 7.63−7.34 (m, 20 H,
β-fused + phenyls), 3.53−3.47 (m, 16 H, −CH2F5Ph), 1.41 (br s, 27
H, tert-butyls). MS (TOF-SIMS, m/z): 2396.34 (M+). Anal. Calcd for
C121H63CuF40N4: C, 60.65; H, 2.65; N, 2.34%. Found: C, 60.72; H,
2.60; N, 2.28%.
[5,10,15-Tris(4-tert-butylphenyl)-2:3,7:8,12:13,17:18-tetrabenzo-
2″,3″,7″,8″,12″,13″,17″,18″-octacarboxy-(1,1,1,3,3,3,-
hexafluoroisopropyl)corrolato]Cu, [IsoF6-TBC]Cu, 6. The residue was
purified by chromatography on silica gel eluted with CH2Cl2/hexane
(1:1), to afford the desired product. Yield: 24% (61 mg). Mp > 300
1
°C. UV−vis (CH2Cl2) λmax, nm (log ε): 498 (4.85), 620 (4.11). H
NMR (300.13 MHz, CDCl3): δ 7.63−7.33 (br m, 20 H, β-fused +
phenyls), 5.98−5.76 (br m, 8 H, −CO2CH(CF3)2), 1.45−1.39 (m, 27
H, tert-butyls). MS (TOF-SIMS, m/z): 2507.99 (M+). Anal. Calcd for
C97H55CuF48N4O16: C, 46.45; H, 2.21; N, 2.23%. Found: C, 46.39; H,
2.25; N, 2.19%.
Ghosh and co-workers studied the influence of peripheral
substituents on the optical spectra of copper triarylcorroles25
and reported that a significant variation of the Soret band
maxima could occur, depending upon the specific substituents
present on the meso-phenyl rings. This result was attributed to a
ligand-to-metal charge transfer transition, which was assigned as
one signature of the “noninnocent” character of the corrole
ligand. The assignment is also supported by the saddle structure
of copper corroles, even in the absence of peripheral crowding,
and results from the deviation from planarity necessary to allow
a metal (dx2−y2)−corrole π orbital interaction. It should be
noted, however, that peripheral crowding can enhance the red
shifts of the Soret bands; in the case of tetrabenzocorroles, we
were able to determine the structure of the copper complex of
the unsubstituted 5,10,15-triphenyltetrabenzocorrole,20 which
is saddled, but the distortion was not significantly different from
what is observed for other copper corrole derivatives. On the
other hand, we hypothesize that the molecular structure should
be significantly different in the case of tetrabenzocorroles
substituted on the fused benzene rings; thus, we conducted the
present study to investigate how the electronic character of the
substituents influence the redox and chemical properties of the
corresponding complexes.
NMR Measurements. NMR experiments were performed at 300
K on a Bruker Avance 600 MHz with a 5 mm inverse broad-band
probe equipped with z-axis gradients. All data were processed with
TopSpin. A stock solution was prepared at an initial concentration of
5.6 μmol/L with toluene-d8 as the solvent. Dilutions of 1:10, 1:100,
1:1000, and 1:5000 were prepared starting from the stock solution.
1H NMR diffusion experiments were performed using the LED
sequence with bipolar gradients24 on the stock solution. The
attenuation measured with this sequence is given by
I/I0 = −exp[D(γ δG2/π)2(Δ − δ/3 − τ/2)]
(1)
H
where I/I0 is the normalized signal intensity, D is the diffusion
coefficient, δ is the duration of the gradient pulse, γH is the
gyromagnetic ratio of 1H, G is the gradient strength, Δ is the diffusion
time, and τ is eddy current delay. Typical acquisition parameters:
recycle delay time between diffusion experiments, 3 s; Δ, 80 ms; δ, 4
ms; and τ, 5 ms.
Hydrodynamic radius (Rh) of an equivalent spherical particle was
calculated using the Stokes−Einstein equation:
kBT
6πηRh
D =
(2)
In the present study, we have prepared and characterized five
copper triaryltetrabenzocorrolates that differ in the electron-
where η is the macroscopic viscosity value of the solvent, T is the
absolute temperature, and kB is the Boltzmann constant.
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Figure 1. UV−vis spectra of the investigated tetrabenzocorroles, recorded at the following concentrations: 2 = 10.1 μM; 3 = 15.2 μM; 4 = 11.7 μM;
5 = 24.6 μM; 6 = 11.6 μM.
withdrawing groups (EWG) on the fused benzene rings. The
alkene-EWG substrates chosen were cyanoacrylate, vinyl-
benzene, allylpentafluorobenzene, and 1,1,1,3,3,3-hexafluoroi-
sopropylacrylate (Scheme 1). The presence of EWG is
fundamental for the synthesis of benzo derivatives by the
Heck cross-coupling methodology. Similar reaction conditions
leading to 220 have been applied to the preparation of
benzocorrole derivatives 3−6. Specifically, a 100-fold excess of
alkene versus 1 was used in order to ensure complete bromine
substitution on the corrole at 125 °C for 72 h in the presence of
in situ-formed Pd(0) catalyst. Because the Heck-based route is
sensitive both to the reaction time and to the temperature, as
we observed during the preparation of tetrabenzocorroles
bearing the −CO2CH3 moieties20 and as reported for the case
of tetrabenzoporphyrins synthesis,22 we decided to investigate
the influence of these parameters on the reaction yields. When
we tried to reduce the reaction time, we noted the presence of
starting material (1 h) or a complex mixture of products (1
day), indicating an incomplete conversion of the reaction
intermediates to the desired tetrabenzocorrole. Reducing the
temperature to 110 °C and carrying out the reaction for three
days afforded a reaction mixture with spectroscopical features
different from those of the tetrabenzocorrole. Taking into
account the failure of these attempts, we decided to perform
the preparation of 3−6 using the same procedure adopted for
2. Although a certain amount of decomposition resulted from
these reaction conditions, we still observed formation of the
desired products in comparable yields, indicating that the
different EWGs do not have a significant effect on the reactivity
of the substrates when the cross-coupling reaction takes place.
The overall yields for the products were always higher than
20%, a value that can be considered satisfying if we consider
that the preparation of the triaryltetrabenzocorroles is a
sequence of three consecutive steps. The novel benzocorroles
were purified by column chromatography.
each other, whereas that of 5 and 6 are very different in terms
of the position of the Soret band in CH2Cl2. The UV−vis
spectra of 2 and 3 both have a Soret band maximum at around
474 nm, which is surprising considering the fact that the
compounds display quite different electrochemistry as dis-
cussed in a later section of the manuscript.
The parent peak in the mass spectrum of 3 is assigned to the
demetalated corrole, indicating a certain lability of the central
metal ion in this compound (Supporting Information Figure
S1); IR analysis shows a characteristic signal of the C−N triple
bond at 2228 cm−1. The 1H NMR spectrum exhibits two main
groups of signals: a broad singlet at 1.44 ppm for the tert-butyl
groups (27 protons) and an unresolved multiplet at 7.50−7.71
ppm for the 20 aromatic protons (Supporting Information
Figure S2).
Compound 4 was prepared by reacting 1 with vinylbenzene.
The UV−vis spectrum of this corrole shows the Soret band at
475 nm and a single Q-band at 606 nm. The 1H NMR
spectrum shows resonances for the tert-butyl groups as a
multiplet at 1.33−1.49 ppm. There is also a multiplet at 6.68−
7.61 ppm for the aromatic protons (Supporting Information
Figure S3). The presence of several signals for the alkyl
substituents and the origin of the high-field aromatic protons of
4 with respect to 2 is likely due to aggregation phenomena. A
similar behavior was recently reported for undecaarylcorroles26
where π−π interaction occurs between the meso-phenyl group
and the two adjacent β-phenyl pyrrolic substituents or by the β-
phenyl rings on the same pyrrole. The formation of the desired
product is also confirmed by mass analysis, with a peak at 1568
m/z, corresponding to the molecular weight of the desired
product (Supporting Information Figure S4). We also observe
three other signals differing by approximately 100, 200, and 300
units from the value of the molecular peak of 4. These signals
correspond to fragmentation of the fused benzene ring followed
by loss of the −C−CH−Ph unit linked to the pyrrole.
The UV−vis spectra for compounds 3−6 are shown in
Figure 1 along with the spectrum of 2, which was characterized
in an earlier study.23 With the exception of compound 5, a large
red shift of the Soret band is observed (in the range 40−80 nm
with respect to the unsubstituted Cu triarylcorrole), whereas
weak absorbances in the Q-band region of the spectra remain
relatively unchanged. The spectra of 2−4 are quite similar to
The benzocorrole 5, obtained by reacting 1 with
allylpentafluorobenzene, exhibits the most severe broadening
of the absorbance bands among all the compounds investigated.
The 1H NMR spectrum of 5 shows three groups of signals: (1)
a broad singlet at 1.41 ppm for the tert-butyl groups, (2)
multiplets (16 H) in the range 3.47−3.53 that belong to the
methylene bridge between the pentafluorophenyl groups and
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1
1
Figure 2. (A) H NMR spectrum of 2 in toluene-d8. (B) H−13C HSQC spectrum of 2 in toluene-d8.
the macrocycle, and (3) the meso-phenyl and β,β′-fused
benzene protons (Supporting Information Figure S5). The
series of novel tetrabenzocorroles was completed with the
preparation of 6, obtained from 1 and 1,1,1,3,3,3-haxafluor-
oisopropyl acrylate. Among all the compounds synthesized, this
benzocorrole shows the largest red shift of the Soret band,
benzocorroles obtained by the cross-coupling methodol-
ogy,20,23 and it has been confirmed for the tetrabenzocorroles
3−6. We decided to study this peculiarity of tetrabenzocorroles
in more detail using 2 as case study to obtain more information
on the behavior of these macrocycles. The choice of 2 is due to
the presence of the peripheral carboxymethyl susbstituent,
which afford a simplified 1H NMR spectrum. In fact, compound
2 shows signals in the three expected regions of the spectrum
according to its chemical structure (Figure 2A): aromatic
protons show broad signals at δ around 7.5 ppm, O−CH3
groups at 3.5 ppm, and tert-butyl CH3 protons at 1.4 ppm. The
integral values for the three regions are those expected for the
chemical structure of 2. However, the number of signals,
particularly in the 3.5 and 1.4 ppm regions, are larger than those
expected for a single molecule (4 for the O−CH3 and 2 for the
1
which is located at 498 nm. The H NMR spectrum of this
benzocorrole is characterized by its low resolution, with a
multiplet at 1.39−1.45 ppm for the tert-butyl moieties, a
multiplet at 5.76−5.98 for the hydrogen atom on the
hexafluoroisopropyl substituents, and multiplets for the
aromatic protons in the region 7.33−7.63 ppm (Supporting
Information Figure S7).
NMR Characterization of 2. The drawback of the low-
resolution NMR spectrum was earlier reported for all of the
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tert-butyl CH3). The large number of signals can be better
To further characterize the mixture present in solution at the
highest concentration, we have measured the self-diffusion
coefficients by gradient-pulsed NMR experiments using
different signals in the 3.5 and 1.4 ppm regions. The values
are reported in Table 1. These values are used in the Stokes−
1
visualized in the H−13C HSQC spectrum (Figure 2B). The
chemical shift of the carbon nuclei are very similar for the
whole set of signals belonging to each region, indicating that
1
the H signals of the 3.5 and 1.4 ppm regions belong to O−
CH3 and tert-butyl CH3 groups, respectively. A second feature
1
of the H NMR spectrum of 2 is the differential line shape
Table 1. Self-Diffusion Coefficients (D) Measured at 300 K
in Toluene for Compound 2 and the Calculated
Hydrodynamic Radius for Different Signals in the 3.5 and
1.4 ppm Region
observed for aromatics proton, which are much broader than
the methyl protons. This fact clearly points to a dynamic
equilibrium between different forms of 2 in solution. Although
the large number of signals observed in the 3.5 and 1.4 regions,
together with the low number of conformational freedom
degrees expected for this molecule, safely excludes a conforma-
tional equilibrium, we ruled out this possibility by running
variable temperature 1H NMR experiments, registering the
spectra using CD2Cl2 for low temperatures and toluene-d8 for
high temperature. No variations of the spectral features were
observed in the temperature range compatible with solvents
used. This fact leaves molecular association as the only possible
explanation for the heterogeneity of signals observed.
δ (ppm)
D/10−10 (m2/s)
Rh (nm)
1.159
1.217
1.247
1.290
1.311
1.329
1.346
1.377
3.039
3.291
3.343
3.354
3.374
3.517
3.616
3.663
4.45
4.69
4.71
4.85
4.82
4.97
4.84
4.80
4.38
6.94
6.08
5.66
5.63
4.97
4.72
4.75
0.91
0.86
0.86
0.84
0.84
0.82
0.84
0.85
0.93
0.58
0.67
0.72
0.72
0.82
0.86
0.85
1
In order to further explore this possibility, H NMR spectra
were acquired after successive dilutions of the sample (Figure
3). As can be observed for the 3.5 ppm region, there are
changes in the chemical shift of the signals and the number of
signals detected with concentration.
Einstein equation (eq 2) to extract the size of the solute
molecules treated as Brownian particles of spherical shape.27
The calculated hydrodynamic radius (Rh) corresponds to an
equivalent spherical particle showing the same hydrodynamic
behavior as that of the solute molecule. The calculated Rh
values are also indicated in Table 1.
A comparison between the extracted value of the equivalent
hydrodynamic radius and the actual molecular size of 2 can be
performed if the molecular shape of this type of molecules is
used. On the basis of several crystallographic structures of
corrole compounds, a single molecule of 2 can be described as a
disk with a height (l) of 0.3 nm and a diameter (d) of 1.6 nm.
Equation 3 can be used to define the relationship between Rh
and the height-to-diameter radio, p = l/d:28
Rh
l
1
2
=
p−2/3[a1 + a2(ln p) + a3(ln p)2 + a4(ln p)3]
(3)
with 0.1 ≤ p ≤ 20, and
a1 = 1.155; a2 = 1.597 × 10−2;
a3 = 9.020 × 10−2; a4 = 6.914 × 10−3
(4)
Associations of two, three, or four molecules form disks with
different heights but approximately equal diameters. When the
interaction of two molecules in the asymmetric cell of crystal
structures of corrole molecules is analyzed, one can assume that
each molecule added on the top will increase the height by 0.4
nm. Equation 3, when applied to these different molecular
associations, yields Rh values ranging from 0.6 to 0.9 nm (Table
2). Comparison of the calculated Rh values from the NMR
signals in Table 1 with those expected for different associations
between one and four molecules of 2 produces a picture that
explains the high number of signals observed in the proton
1
Figure 3. H NMR spectra of 2 in toluene-d8 at different dilutions
(−OCH3 groups region).
In the most dilute condition, a single signal is emerging,
probably belonging to the monomeric molecule. Similar
behavior was observed in the 1.4 ppm region (data not
shown). On the other hand, aromatic signals were not
detectable at low concentrations, probably due to a residual
broadening.
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chemistry of the ring-expanded corrole 2, was earlier described
in the literature,23 and the redox properties of four newly
synthesized ring-expanded corroles that have different electron-
withdrawing substituents on the fused benzene rings of the
tetrabenzocorrole are examined in the present study.
Cyclic voltammograms illustrating the oxidations and
reductions of compounds 3−6 in CH2Cl2, 0.2 M TBAP, and
PhCN are shown in Figure 4, whereas cyclic voltammograms in
pyridine and 0.2 M TBAP are given in Supporting Information
Figure S9. The measured half-wave or peak potentials are
summarized in Table 3, which includes previously published
data for compound 2.23 The proposed site of electron transfer
for each redox reaction (conjugated ring system, metal center,
or fused benzene ring) is also given in the table, with these
assignments being based on the measured spectral changes,
measured E1/2 values, and previous assignments of the electron
transfer site reported in the literature for related com-
pounds.23,34,35
Table 2. Equivalent Hydrodynamic Radius (Rh) for Different
Molecular Associations of Compound 2, Considered As a
Disk-Like Particle with Diameter d = 1.6 nm
number of associated molecules
l (nm)
p
Rh (nm)
1
2
3
4
0.3
0.7
1.1
1.5
0.19
0.44
0.69
0.94
0.62
0.73
0.82
0.90
NMR spectrum (Figure 1) and the signal simplification that
correlates with sample dilution (Figure 2). Compound 2 exists
at high concentrations as a complex mixture of different
molecules associated in equilibrium. In the case of three and
four associated molecules, it is likely that there is more than one
type of molecular orientation within the complex, leading to
1
different chemical shifts for the H nuclei. The value of the
exchange kinetic constant is of the same order of magnitude as
the chemical shift difference for aromatic protons, leading to
severe broadening. It is possible that the difference for the
chemical shift of methyl protons is even larger, leading to a slow
exchange scenario in which separated signals are observed.
Electrochemistry and Spectroelectrochemistry. Elec-
trochemical and spectroscopic studies of triphenylcorroles
(TPC) containing electron-donating or electron-withdrawing
substituents on the meso and/or β-pyrrolic positions of the
macrocycle have been investigated in detail.29−35 The electro-
As shown in Table 3 and Figure 4, the investigated
tetrabenzocorroles can undergo two reversible or quasi-
reversible one-electron oxidations in CH2Cl2 or PhCN, the
first of which is located at E1/2 values between 0.57 and 1.06 V
versus the SCE, and the second of which is between 1.09 and
1.59 V.
The exact number of observed oxidations, as well as the
measured redox potentials, will depend upon the electron-
Figure 4. Cyclic voltammograms of (TBC)Cu derivatives (3−6) in CH2Cl2 and PhCN containing 0.2 M TBAP at scan rate of 0.1 V/s. Some small
reduction peaks, marked by asterisks, are probably due to unstable reduced products.
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the solvent. The splitting of the second oxidation into two
processes can be accounted for by aggregation or dimerization
of the tetrabenzocorrole after the abstraction of one electron.
Three major reductions can be detected for compounds 2, 3,
5, and 6. The first involves a CuIII/CuII process, whereas the
second is assigned to occur at one of the four fused benzene
rings whose reduction is made easier by the presence of the
electron-withdrawing substituent. The fused benzene ring that
is reduced does not participate in the conjugated π system of
the macrocycle; thus, the potentials for this electron addition
should be quite close to E1/2 values for the reduction of a
monosubstituted benzene with the same electron-withdrawing
group.
One example of this is seen in the case of C6H4(CN)2, whose
reduction occurs at about −1.30 V in CH3CN38 as compared to
an E1/2 of −1.30 V for compound 3 in PhCN (see Table 3).
Compound 4, which contains weakly electron-withdrawing
phenyl substituents, does not undergo a reduction at the fused
benzene ring and has behavior typical of what has been
reported for numerous corroles with a “regular” macrocycle.35
The third reduction of 2, 3, 5, and 6 is assigned as occurring at
the π ring system of the tetrabenzocorrole, as is the second
reduction of 4. These ring-centered reductions are all relatively
close to the negative potential limit of the solvent and are quasi-
reversible or irreversible in some cases. Additional ill-defined
reductions with small peak currents are also observed for some
of the compounds (this is especially evident for 3 and 5 in
pyridine). In CH2Cl2 or PhCN, the unknown side reactions are
indicated by an asterisk in the cyclic voltammograms of Figure
4 and are assigned as due to a reduction of trace side products
generated under the given solution conditions.
Table 3. Cyclic Voltammetry of (TBC)Cu Derivatives in
CH2Cl2, PhCN, and Pyridine Containing 0.2 M TBAP at a
Scan Rate of 0.1 V/s
oxidation
reduction
2nd
1st
CuIII/
CuII
2nd
ring
1st
ring
benzo-
a
x
solvent
CH2Cl2
compound
3rd ring
b
Cu 2
1.38
1.58,
1.44
0.82
1.00
0.07
0.18
−1.38
−1.30
−1.59
−1.63
Cu 3
d
Cu 4
Cu 5
Cu 6
1.11
0.57
0.78
1.06
0.80
0.99
−0.28
−0.06
0.21
−1.88
c
−1.55
−1.22
−1.42
−1.30
−1.70
−1.53
−1.62
−1.57
c
1.51
1.39
1.59,
b
PhCN
Cu 2
0.06
Cu 3
0.21
1.47
d
Cu 4
Cu 5
Cu 6
1.09
0.58
0.81
1.04
0.71
0.75
0.54
−0.22
0.01
−1.90
−1.67
b
−1.45
c
c
c
,
,
,
d
d
d
1.54
0.26
−1.22
−1.33
−1.46
−1.49
−1.55
−1.66
−1.86
−1.60
−1.48
b
Pyridine
Cu 2
0.11
d
,e
Cu 3
Cu 4
Cu 5
Cu 6
0.25
−0.20
0.04
d
d
e
0.74
1.05
−1.46
−1.17
d
b
0.23
a
The fused benzene ring with electron-withdrawing substituents does
b
not participate in conjugated system of the macrocycle. Data is taken
c
from ref 23. Some small reduction peaks are seen at Ep = −1.04 to
d
−1.25 V due to trace impurities or side reactions (see Figure 4). Peak
e
potential at a scan rate of 0.1 V/s. Additional reduction observed
between −1.75 and −1.88 V.
The electroreductions and electrooxidation of 3−6 were also
characterized by UV−visible thin-layer spectroelectrochemistry.
Examples of the UV−visible spectral changes that occur during
the first two oxidations of 3 and 6 in PhCN are illustrated in
Figure 5. For both compounds, the spectral transitions between
the neutral and singly or doubly oxidized species are consistent
with macrocycle-centered electron transfer processes of
porphyrin-like molecules.36 The similar final spectrum obtained
after the second split oxidation of 3 and the unsplit oxidation of
6 (see cyclic voltammograms in Figure 4) suggests that the
“extra” oxidation process of 3 at E1/2 = ∼1.59 V in the cyclic
withdrawing properties of the fused phenyl ring substituents
and the specific solvent. In all cases, however, the products of
these two oxidations are a corrole π−cation radical in the first
step and a corrole dication in the second. Only the first
oxidation can be detected in pyridine, due to the limited
positive potential window of this solvent.
A second oxidation of 5 is not observed in CH2Cl2 or PhCN,
implying that the generated radical cation of this compound is
unstable in these two solvents. Moreover, as is seen in Figure 4,
the second oxidation of 3 is split into two processes, one at E1/2
= 1.44−1.47 V and the other at 1.58−1.59 V, depending upon
Figure 5. Thin-layer UV−vis spectral changes of (a) Cu 3 and (b) Cu 6 in PhCN containing 0.2 M TBAP during the indicated oxidations.
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Figure 6. Thin-layer UV−vis spectral changes of CuIII/II reduction for the investigated Cu compounds 3, 5, and 6 in (a) CH2Cl2 and (b) PhCN
containing 0.2 M TBAP.
voltammograms is associated with aggregation or dimerization
and does not effect the final product of the two-electron
abstraction to give a tetrabenzocorrole dication.
compounds 2, 3, 5, and 6 under similar solution conditions.
Consistent with our assigned reduction of a fused phenyl ring
in the second reduction process, the spectral changes recorded
for 3 during the third one-electron reduction resemble what is
expected for a macrocycle-centered electron transfer.
As shown in Figure 6, three different kinds of spectral
changes are seen for the first one-electron reduction of
compounds 3, 5, and 6 in PhCN or CH2Cl2. The first is for
compound 3, which exhibits a 13−20 nm shift in the position
of the Soret band and a moderate intensity shift of the Q-band
at 657−662 nm upon conversion of CuIII to CuII. The second is
for compound 5, which exhibits a decreased intensity of the
Soret band during the metal centered reduction but only small
changes in the wavelength of maximum absorbance. The third
type of spectral change is for compound 6, which exhibits a
substantial shift in the Soret band position along with a large
decrease in intensity of the band after reduction of CuIII to CuII.
The relative intensities of the Soret bands for the CuII form
of the tetrabenzocorrole are quite different from each other,
probably due to a difference in aggregation between the two
compounds. On the other hand, the visible band at 657−704
nm is common to all three compounds. A similar difference in
the types of spectra was observed in our previous study of Cu
β-nitrocorroles35 and was proposed to involve an equilibrium
between a CuII corrole and a CuIII corrole π-anion radical
product in the first reduction process.32−35,37−40 Spectral
changes of 2 and 4 for this process are similar to those of 3.
The spectral changes that occur for compound 3 in PhCN
during the second and third controlled potential reductions are
given in Supporting Information Figure S10. The spectral
changes obtained during the first one-electron addition are
similar to what was reported in our previous study of
compound 2,23 but only minimal changes are observed in the
spectrum of the CuII corrole during the second electron
addition, which we have assigned as the reduction of one
benzene−CN group on the compound. Potentials have been
reported in the literature for electroreduction of benzenes with
one- or two-electron-withdrawing groups (e.g., −CN,41 −NO2,
−Br, and −COOMe42,43), and these values are consistent with
reduction potentials measured in the present study for
The measured E1/2 values of 2−6 shift positively or
negatively with changes in the electron-withdrawing character
of the substituents at the four fused benzene rings of the
macrocycle. The relevant electrochemical linear free energy
relationship is given by the equation ΔE1/2 = 4σρ, where the
substituent constant, σ, provides a measure of the electron-
withdrawing characteristics of the substituent and ρ, the
“reaction constant”, indicates the magnitude of the interactions;
the larger the value of ρ, the larger the substituent effect.44−46
The values of 4σ for 2 (COOMe), 3 (CN), and 4 (C6H5) are
3.28, 4.88, and 0.20, respectively, obtained from ref 41.
Substituent constants are not available for compounds 5 and
6, but correlations between E1/2 for the first oxidation and first
reduction of compounds 2−4 give a linear slope as seen in
Figure 7. The slopes (ρ) of 90−100 mV indicate that these
three compounds follow the same linear free energy relation-
ship and, thus, undergo the same electron transfer mechanism
during the first oxidation and first reduction under the given
experimental conditions. The ΔE1/2 between the first reduction
and first oxidation of compounds 2−4 ranges from 0.75 to 0.85
V, and a similar ΔE1/2 is seen for compounds 5 (0.84 V) and 6
(0.85 V) in CH2Cl2.
CONCLUSIONS
■
The scope of the synthetic route to obtain tetrabenzocorroles
via a Heck cross-coupling reaction is quite general when the
alkene used in the reaction bears an electron-withdrawing
substituent. Using this route, we have successfully prepared
different tetrabenzocorrole Cu complexes bearing electron-
withdrawing groups at the β-fused benzene rings. The
spectroscopic and electrochemical characterization of these
macrocycles gave some insights on the influence of such
substituents on the properties of the corresponding tetraben-
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ACKNOWLEDGMENTS
■
The authors thank the Italian MIUR (R.P., PRIN2009 project
2009Z9ASCA) and the Robert A. Welch Foundation (K.M.K.,
Grant E-680) for financial support.
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Corresponding Author
*E-mail: roberto.paolesse@uniroma2.it.
Notes
■
The authors declare no competing financial interest.
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