Aluminum, gallium, germanium, copper, and phosphorus complexes of meso -triaryltetrabenzocorrole

Article abstract of DOI:10.1021/ic400162y

5,10,15-Triaryltetrabenzocorrole complexes of aluminum, gallium, germanium, and phosphorus were synthesized by coordination of these metal ions in the preformed triaryltetrabenzocorrole macrocycle, opening a way to the investigation of different metal com

Full text of DOI:10.1021/ic400162y

Article
pubs.acs.org/IC
Aluminum, Gallium, Germanium, Copper, and Phosphorus
Complexes of meso-Triaryltetrabenzocorrole
Giuseppe Pomarico,† Sara Nardis,† Mario L. Naitana,† M. Graca H. Vicente,‡ Karl M. Kadish,*,§
̧
Ping Chen,§ Luca Prodi,⊥ Damiano Genovese,⊥ 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
⊥Department of Chemistry, “G. Ciamician” Universita
̀
di Bologna, 40126 Bologna, Italy
∥CNR IDASC, via Fosso del Cavaliere, 00133 Rome, Italy
S
* Supporting Information
ABSTRACT: 5,10,15-Triaryltetrabenzocorrole complexes of alumi-
num, gallium, germanium, and phosphorus were synthesized by
coordination of these metal ions in the preformed triaryltetrabenzo-
corrole macrocycle, opening a way to the investigation of different
metal complexes. The UV−vis spectra of these derivatives exhibit a red
shift and broadening of all absorption bands because of the π-extended
aromatic system and distortion of the molecular framework. The
electrochemical and photophysical behaviors of the free base and the
metal complexes of meso-triaryltetrabenzocorrole were investigated and
characterized.
Taking into account the unique optical properties of meso-
INTRODUCTION
■
tetraaryltetrabenzoporphyrins,9,10 we recently investigated the
possibility of converting meso-triarylcorroles to the correspond-
ing meso-triaryltetrabenzocorroles, following two different
synthetic methodologies, for the preparation of these macro-
cycles.11 As expected, the resulting corrole derivatives show a
Soret band that is strongly red-shifted with respect to the
parent corrole, making these macrocycles appealing for their
use as photosensitizers in medicine or as organic substrates in
materials chemistry.12,13
Corroles are contracted porphyrins that are characterized by
substitution of a meso-carbon with a direct pyrrole−pyrrole link,
thus making the molecular skeleton similar to that of corrin, the
vitamin B12 nucleus. In the cornucopia of the all porphyrin
analogues, corroles have now fully come into the limelight,
following the publication of efficient synthetic protocols for the
preparation of 5,10,15-triarylcorroles from commercially
available precursors.1,2 The possibility of preparing a wide
range of meso-triarylcorroles has triggered an exploration of the
chemistry of these macrocycles, leading to a better under-
standing of their reactivity,3 coordination chemistry,4,5 and
photophysics;6,7 such a richness of properties has been
accompanied by an extensive exploitation of corroles in a
number of different applications, both in their free-base form
and as corresponding metal complexes.8
Studies of corrole chemistry have led to a better under-
standing of the structure−property relationships, which is a key
step in the design of new dyes targeted for different
applications. Among the various functionalizations of the
porphyrinoid framework, the fusion of benzene or other
aromatic moieties onto the β positions of the corrole
macrocycle has attracted particular interest because this process
results in a significant modification of the compound’s optical
features, due to expansion of the macrocyclic aromatic π
system.
The coordination of suitable metal ions can further induce
modifications in the corrole optical properties, and the
formation of some metal complexes can lead in many cases
to an enhancement of the photophysical features of the
macrocycle, so that it becomes comparable to or even better
than that of the corresponding porphyrin.5 For this reason, we
decided to prepare metal complexes of meso-triaryltetrabenzo-
corroles, with one aim being to investigate the charges resulting
from an extended aromatic system combined with coordination
of the main-group metal ions. In particular, we chose to prepare
aluminum(III),14 gallium(III),15 germanium(IV),16 and
phosphorus(V)17 benzocorrolates, prompted by the exciting
results reported when these ions were coordinated to common
Received: January 21, 2013
Published: March 12, 2013
© 2013 American Chemical Society
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reduced pressure. The preciptate was washed with CH2Cl2 and
purified by chromatography on silica gel, eluting with tetrahydrofuran
(THF)/hexane from 1:2 to 1:1. Fractions containing aluminum
corrole were collected, the solvent was removed under vacuum, and
the residue was crystallized from CH2Cl2/hexane with the addition of
a few drops of py. Yield: 56% (27 mg). Mp: >300 °C. UV−vis
aryl corroles, particularly the intense fluorescence quantum
yields observed for these metal derivatives.
In this paper, we report synthetic details for the preparation
of these main-group metal complexes along with detailed
photophysical, electrochemical, and spectroelectrochemical
characterization of the different metal complexes of meso-
triaryltetrabenzocorrole (Figure 1).
1
(toluene): λmax, nm (log ε, M−1 cm−1): 474 (4.53), 645 (3.91). H
NMR (300.13 MHz, CDCl3): δ 7.96−7.34 (m, 20 H, β-fused +
phenyls), 4.11−3.34 (m, 24 H, −CO2CH3), 1.63 (br s, 27 H, tert-
butyls). Anal. Calcd for C86H76AlN5O16: C, 70.63; H, 5.24; N, 4.79.
Found: C, 70.69; H, 5.18 N, 4.83.
[5,10,15-Tris(4-tert-butylphenyl)-2:3,7:8,12:13,17:18-tetrabenzo-
2″,3″,7″,8″,12″,13″,17″,18″-octakis(carboxymethyl)corrolato]-
gallium(pyridine) ([TBCorr]Ga(py), 4). To 40 mg of 2 (0.029 mmol)
dissolved in 20 mL of py was added a large excess of Ga(acac)3, and
the mixture was refluxed under nitrogen, monitoring the course of the
reaction by UV−vis spectroscopy. After 2 h, the solvent was removed
by reduced pressure and the residue purified by chromatography on
silica gel, eluting with a gradient THF/hexane from 1:2 to 1:1.
Fractions containing gallium corrole were collected, the solvent was
removed under vacuum, and the residue was crystallized from
CH2Cl2/hexane in the presence of a few drops of py. Yield: 66%
(29 mg). Mp: >300 °C. UV−vis (toluene): λmax, nm (log ε, M−1 cm−1)
478 (4.78), 657 (4.17). 1H NMR (300.13 MHz, CDCl3): δ 8.12−7.34
(m, 20 H, β-fused + phenyls), 4.23−3.31 (m, 24 H, −CO2CH3), 1.59
(br s, 27 H, tert-butyls). MS (TOF-SIMS, m/z): 1426.1 (M+ − Py).
Anal. Calcd for C86H76GaN5O16: C, 68.62; H, 5.09; N, 4.65. Found: C,
68.69; H, 5.05 N, 4.68.
Figure 1. Structure of the investigated free-base triaryltetrabenzo-
corrole.
[5,10,15-Tris(4-tert-butylphenyl)-2:3,7:8,12:13,17:18-tetrabenzo-
2″,3″,7″,8″,12″,13″,17″,18″-octakis(carboxymethyl)corrolato]-
germanium(hydroxyl) ([TBCorr]Ge(OH), 5). To 40 mg of 2 (0.029
mmol) dissolved in 10 mL of anhydrous N,N-dimethylformamide
(DMF) was added 17 μL (0.15 mmol) of GeCl4, and the mixture was
refluxed under nitrogen while the course of the reaction was
monitored by UV−vis spectroscopy. After 1.5 h, the solvent was
removed and the residue dissolved with CH2Cl2 and purified by
chromatography on silica gel, eluting with THF/hexane from 1:2 to
1:1. Fractions containing germanium corrole were collected and
crystallized from CH2Cl2/hexane. Yield: 57% (24 mg). Mp: >300 °C.
UV−vis (toluene): λmax, nm (log ε, M−1 cm−1) 456 (5.00), 640 (4.18).
1H NMR (300.13 MHz, CDCl3): δ 8.38−7.32 (m, 20 H, β-fused +
EXPERIMENTAL SECTION
■
Materials. Silica gel 60 (70−230 mesh, Sigma Aldrich) was used
for column chromatography. Reagents and solvents (Aldrich, Fluka)
were of the highest grade available and were used without further
purification. Room temperature 1H and 31P NMR spectra were
recorded on a Bruker AV300 spectrometer operating at 300.13 MHz
(1H) or 121.48 MHz (31P). Chemical shifts are given in ppm relative
1
to a residual solvent (CHCl3, 7.26 ppm; CH2Cl2, 5.32 ppm) for H
and to a CDCl3 solution of PPh3 (−6.1 ppm) as the external standard
for 31P. Variable-temperature NMR spectra were recorded on a Bruker
AV400 spectrometer operating at 400.13 MHz using CD2Cl2 as the
solvent. Mass spectrometry (MS) spectra (TOF-SIMS) were recorded
using a positive method with a TOF-SIMS V (IONTOF)
spectrometer. UV−vis spectra were measured in CH2Cl2 or toluene
with either a Varian Cary 50 or a Perkin-Elmer Lambda 45
spectrophotometer. Quartz cuvettes with optical path length of 1 cm
were used. The fluorescence spectra were recorded with an Edinburgh
FLS920 equipped Hamamatsu R928P photomultiplier for emission up
to 800 nm and with a germanium detector for the 800−1600 nm
spectral range. The same instrument connected to a PCS900 PC card
was used for the time-correlated single photon counting experiments.
Luminescence quantum yields (uncertainty 15%) were determined
using solutions of the tetraphenylporphyin free base in toluene as a
reference (Φ = 0.11), and fluorescence intensities were corrected for
inner filter effects according to standard methods.18
Syntheses of Metallobenzocorroles. [5,10,15-Tris(4-tert-butyl-
phenyl)-2:3,7:8,12:13,17:18-tetrabenzo-2″,3″,7″,8″,12″,13″,17″,18″-
octakis(carboxymethyl)corrolato]copper ([TBCorr]Cu, 1) was syn-
thesized following a literature method.11 5,10,15-Tris(4-tert-butyl-
phenyl)-2:3,7:8,12:13,17:18-tetrabenzo-2″,3″,7″,8″,12″,13″,17″,18″-
octakis(carboxymethyl)corrole (TBCorrH3, 2) was prepared by
demetalation of 1 following a published method.19
[5,10,15-Tris(4-tert-butylphenyl)-2:3,7:8,12:13,17:18-tetrabenzo-
2″,3″,7″,8″,12″,13″,17″,18″-octakis(carboxymethyl)corrolato]-
aluminum(pyridine) ([TBCorr]Al(py), 3). To 45 mg of 2 (0.033 mmol)
dissolved in 8 mL of toluene was added 90 μL (0.171 mmol) of a 1.9
M solution of AlEt3 in toluene, and the mixture was stirred at room
temperature under nitrogen while the course of the reaction was
monitored by UV−vis spectroscopy. After 10 min, the reaction
mixture was cooled by an ice bath, and 200 μL of water was added to
destroy excess AlEt3. A total of 1 mL of pyridine (py) was added, after
which the reaction mixture was filtered and the solvent removed by
phenyls), 4.44−3.51 (m, 24 H, −CO2CH3), 1.66 (br s, 27 H, tert-
butyls), −3.90 (br s, 1H, axial −OH). MS (TOF-SIMS, m/z): 1446.46
(M+). Anal. Calcd for C81H72GeN4O17: C, 67.28; H, 5.02; N, 3.87.
Found: C, 67.24; H, 5.06 N, 3.84.
[5,10,15-Tris(4-tert-butylphenyl)-2:3,7:8,12:13,17:18-tetrabenzo-
2″,3″,7″,8″,12″,13″,17″,18″-octakis(carboxymethyl)corrolato]-
phosphorus(dihydroxyl) ([TBCorr]P(OH)2, 6). To 45 mg of 2 (0.033
mmol) dissolved in 15 mL of py was added 261 μL (2.81 mmol) of
POCl3, and the mixture was refluxed under nitrogen while the course
of the reaction was monitored by UV−vis spectroscopy. After 1.5 h,
the solvent was removed and the residue dissolved with CH2Cl2 and
purified by chromatography on silica gel, eluting with ethyl acetate/
hexane from 1:1 to 2:1. Fractions containing phosphorus corrole were
collected, the solvent was removed under reduced pressure, and the
residue was crystallized from CH2Cl2/hexane. Yield: 36% (17 mg).
Mp: >300 °C. UV−vis (toluene): λmax, nm (log ε, M−1 cm−1) 474
(5.03), 645 (4.36). 1H NMR (300.13 MHz, CDCl3): δ 8.36−7.47 (m,
20 H, β-fused + phenyls), 4.29−3.34 (m, 24 H, −CO2CH3), 1.60 (br s,
27 H, tert-butyls), −3.26 (br s, 2H, axial −OH). 31P NMR (121.48
MHz, CDCl3): δ −186. MS (TOF-SIMS, m/z): 1422.95 (M+). Anal.
Calcd for C81H73N4O18P: C, 68.44; H, 5.18; N, 3.94. Found: C, 68.49;
H, 5.24; N, 3.91.
Electrochemical and Spectroelectrochemical Measure-
ments. Absolute dichloromethane (CH2Cl2; 99.8%, EMD Chemicals
Inc.) and pyridine (py; 99.8%, Sigma-Aldrich Chemical Co.) were used
for electrochemistry without further purification. Benzonitrile (PhCN)
was purchased from Aldrich Chemical Co. and distilled over P2O5
under vacuum prior to use. Tetra-n-butylammonium perchlorate
(TBAP), used as the supporting electrolyte, was purchased from Sigma
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Scheme 1. Synthetic Pathway for the Preparation of meso-Triaryltetrabenzocorrole Metal Complexes
Figure 2. UV−vis absorption spectra of 3 (solid line), 4 (dash-dotted line), 5 (dotted line), and 6 (dashed line).
Chemical Co. or Fluka Chemika Co., recrystallized from ethyl alcohol,
and dried under vacuum at 40 °C for at least 1 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 and consisted of a platinum button or 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 that
contained the solvent/supporting electrolyte mixture. All potentials are
referenced to the SCE.
UV−vis spectroelectrochemical experiments were performed with
an optically transparent platinum thin-layer electrode of the type
described in the literature.20 Potentials were applied with an EG&G
model 173 potentiostat/galvanostat. Time-resolved UV−vis spectra
were recorded with a Hewlett-Packard model 8453 diode-array rapid-
scanning spectrophotometer.
under Heck conditions with a 100-fold excess of methyl
acrylate; the reaction intermediate was not isolated but was
further reacted through 6π electrocyclization with aromatiza-
tion of the fused cyclohexane rings, thus allowing formation of
the β-fused benzene rings. This procedure, developed for the
preparation of opp-dibenzoporphyrins,21 leads to the formation
of meso-triaryltetrabenzocorroles in good yield.
Because of the harsh conditions needed for bromination at
the macrocycle peripheral positions, the insertion of a copper
ion is necessary to protect the macrocycle, limiting its
decomposition, which is triggered by an excess of bromine.
This procedure can also be used for the preparation of different
metal complexes, replacing copper with other metals, thus
allowing for formation of the corresponding metallobenzocor-
roles (Scheme 1).
Among the four investigated metal derivatives, only
aluminum22 and gallium23 corrolates could undergo octabromi-
nation; in the case of germanium corrole, only partially
brominated derivatives could be obtained16 because of a
competing ring-opening reaction that occurred when complete
bromination was attempted. This particular behavior can be
attributed to the GeIV oxidation state, with the higher positive
charge acting to withdraw the electron density from the
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The procedure reported
for the preparation of meso-triaryltetrabenzocorrole involves a
three-step reaction,11 where the starting material is meso-
triarylcorrole, which is first metalated with copper and then
fully brominated at the β-pyrrole positions of the macrocycle.
The halogenated complex undergoes a cross-coupling reaction
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macrocycle, thus deactivating the peripheral positions and
hindering formation of fully brominated germanium corrole. A
similar behavior was also expected to occur in the case of
phosphorus corrole because of the positive charge of the
phosphorus(V)-coordinated species.
The limited scope of the complete bromination reaction of
metallocorroles for the subsequent preparation of correspond-
ing tetrabenzocorroles could, however, be easily circumvented
by using a different approach, as shown in Scheme 1. Following
the preparation of copper tetrabenzocorrole 1, the metal ion
was removed by a procedure reported for demetalation of
corrole complexes,19 leading to isolation of 2. The free-base
compound was then further reacted to prepare the desired
metallocorrole derivatives, following a successful approach
earlier reported for the preparation of cobalt and tin
tetrabenzocorroles.11
groups of expected signals for 5 (aromatic, 8.38−7.32 ppm;
carboxymethyl, 4.44−3.51 ppm; tert-butyl, broad singlet at 1.66
ppm), a broad signal at −3.9 ppm was observed due to the
hydroxyl group, which is axially bound to germanium (Figure
S3 in the SI). Narrower absorption bands are seen in the UV−
vis spectrum and the Soret and Q bands appear at 456 and at
640 nm, respectively, with a blue-shifted shoulder for the Q
band (Figure 2). The high values measured in both the
absorption and emission spectra (see the later sections) seem
to indicate a less distorted geometry for 5 than those of
complexes 3 and 4.
The series of meso-triaryltetrabenzocorrole complexes was
completed by the insertion of phosphorus. Free base 2 was
dissolved in py, and an 85-fold excess of POCl3 was added. The
mixture was stirred under reflux and a N2 atmosphere for 90
min. The reaction workup yielded a phosphorus complex with
two axially bound hydroxyl units.
The spectroscopic properties of 6 are similar to those of 5.
The UV−vis spectrum exhibits a Soret band at 474 nm and a Q
band at 645 nm, with a blue-shifted shoulder (Figure 2). Large
absorbance and emission values are obtained (see the
photophysical characterization in a later section), which
suggests a deviation from planarity of the macrocycle similar
to that of 5 but less pronounced with respect to the aluminum
and gallium complexes.
Compound 3 was prepared by reacting 2, dissolved in
toluene, with a 1.9 M AlEt3/toluene solution at room
temperature, under an inert atmosphere. The reaction proceeds
quickly, and complete variation of the UV−vis spectrum was
observed after 10 min. The reaction workup yielded 3, as the
corresponding pyridino derivative.
The number of py moieties coordinated to aluminum(III)
arylcorrolates has recently been clarified.25 This metal may
form a penta- or hexacoordinated species, depending upon the
solvent medium; the pentacoordinated form is observed when
the complex is dissolved in a noncoordinating solvent, while the
addition of different volumes of py to the solvent generates an
equilibrium between the penta- and hexacoordinated species,
with the latter obtained in neat py. The UV−vis spectrum of 3
recorded in a noncoordinating solvent, such as toluene and
CH2Cl2 (Figure 2), showed a Soret band at 474 nm and a Q
band at 645 nm, which is 43 nm red-shifted compared with that
of 1; in the presence of excess py, the spectrum became broad
in the Q-band region as the hexacoordinated species is formed.
Compound 4 was obtained by refluxing the free-base
benzocorrole in py, containing an excess of Ga(acac)3; after 2
h, the UV−vis spectrum of the reaction mixture showed
complete disappearance of the starting material. The reaction
workup yielded the final complex with py as an axial ligand; the
UV−vis spectrum of this compound is very similar to that of 3,
with a Soret band at 478 nm and a weaker absorbance at 657
nm (Figure 2); a further change in the spectrum due to
formation of the hexacoordinated complex was observed when
4 was dissolved in neat py, with a broadening of the Q band,
while a similar variation in the spectrum of 3 occurs in a 1:1
mixture of CH2Cl2/py.
The 1H NMR spectrum of phosphorus corrole is very similar
to that of other metal complexes for the aromatic (8.36−7.47
ppm), carboxymethyl (4.29−3.34 ppm), and tert-butyl (broad
singlet at 1.60 ppm) protons, while the inner −OH groups
appear as a broad signal at −3.26 ppm (Figure S4 in the SI).
The behavior of the coordinated phosphorus ion toward axial
ligands was investigated by attempting to replace OH− with
OCH3− by stirring the phosphorus complex in methanol for 72
h. While no variations were seen in the UV−vis spectrum, the
1H NMR spectrum showed the presence of two doublets of
different intensities, with J = 24 Hz (because of the P−OCH3
coupling) at −1.52 and −1.87 ppm (Figure S5 in the SI). These
doublets did not completely replace the broad signal of the
inner OH, thus confirming the large affinity toward the
hydroxyl groups (in the case of phosphorus triarylcorrole, the
complete conversion of hydroxy into a methoxy derivative
occurred after the addition of 125 equiv of methanol to the
NMR tube17).
Another peculiarity reported for phosphorus corrole17 is the
presence of an equilibrium between the penta- and
hexacoordinated species. To identify the predominant form of
6 in solution, we recorded the 31P NMR spectrum. The
chemical shifts of the phosphorus nucleus in 31P NMR
spectroscopy are sensitive to the coordination number of the
phosphorus center.26 When the phosphorus nucleus is
coordinated to a porphyrin, phthalocyanine, or a related
macrocycle, it shows signals in the −180/−200 ppm region if it
exists in the hexacoordinated form, while evidence of
pentacoordination is given by a signal in the range −90 to
−110 ppm.
The 1H NMR spectra of 3 and 4, recorded at room
temperature, show three main groups of unresolved signals
related to the aromatic protons of the meso-phenyl and β-fused
benzene rings, (7.96−7.34 ppm for 3 and 8.12−7.34 ppm for
4); the second group of signals (4.11−3.33 ppm for 3 and
4.23−3.31 ppm for 4) belong to the carboxymethyl
substituents, while the tert-butyl groups appear as a broad
singlet centered at 1.63 ppm for 3 and at 1.51 ppm for 4 (see
Figures S1 and S2 in the Supporting Information, SI).
For the preparation of 5, the free base 2 was dissolved in
anhydrous DMF under an inert atmosphere, after which GeCl4
was added and the resulting mixture stirred under reflux and
under N2 for 2 h. A chloro complex was initially obtained, but
this quickly hydrolyzed, leading to formation of the Ge−OH
derivative. The lability of the initially formed Ge−Cl adduct
The 31P NMR spectrum of the investigated compound shows
a signal at −186 ppm, but no resonances are seen in the range
of −90 to −110 ppm; this result indicates that the
hexacoordinated form is very stable and is not prone to
dissociation to form the pentacoordinated complex. There is
also overlap of several signals instead of just a single resonance,
as occurs for phosphorus triarylcorroles,25 thus suggesting the
presence of different conformers. A second signal at −170 ppm
1
was confirmed by H NMR analysis; in addition to the three
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Figure 3. Emission spectra of 3 (solid line), 4 (dash-dotted line), 5 (dotted line), and 6 (dashed line) benzocorrolates.
Table 1. Photophysical Properties of Metallotetrabenzocorrolates in Toluene at Room Temperature
λabsx, nm
ε, M−1 cm−1
λem, nm
Φ
t, ns
kr, s−1
2.1 × 107
knr, s−1
a
b
3
4
5
6
474
645
478
657
456
640
474
645
34100
8200
670
0.025
1.2
8.1 × 108
3.40 × 108
1.1 × 109
2.2 × 108
61000
14800
101000
15000
108500
22800
679
652
665
0.012
0.042
0.31
2.9
0.9
3.1
4.1 × 106
4.7 × 107
1.0 × 108
a
b
Dual emission; see the text. Upon excitation in the Soret band.
in the 31P NMR spectrum was obtained when ligand exchange
was attempted, again demonstrating the incomplete ligand
substitution (Figure S6 in the SI).
not observed in the aromatic region of the spectrum, which has
the lowest resolution. The only exception was detected with 4;
at −50 °C, the broad signal for the tert-butyl substituents was
replaced by several signals in the region 1.77−1.51 ppm,
suggesting once again the presence of different conformations
(Figure S8 in the SI).
These spectroscopic data corroborate the hypothesis of
considerable distortion of these complexes: the weak
absorbances shown in the UV−vis spectra and the impossibility
to observe signals of coordinated py in the 1H NMR spectra are
in agreement with a strong deviation from planarity and with
the presence of dynamic processes involving these compounds,
which makes ligand exchange very fast and therefore not
observable within the NMR time scale. The affinity of metals
coordinated to tetrapyrrolic macrocycles toward axial ligands
decreases as distortion of the tetrapyrrolic ring increases.
Photophysical Characterization. In general, from the
point of view of their photophysical properties, the metal-
locorroles reported in this manuscript can be divided into two
subgroups.
One general feature characteristic of the 1H NMR spectra for
these benzocorrole derivatives is the low resolution of the
spectral signals, especially in the aromatic region. This is similar
to what was previously reported for the free base and copper
complex.11
Studies carried out on porphyrins bearing substituents at
either the meso- or β-pyrrole positions suggested the presence
of conformers originating from the hampered rotation of
phenyls due to crowding of the macrocycle periphery and an
inversion of the saddle-shaped porphyrin macrocycle.27−29
Unfortunately, we were not able to obtain crystals suitable
for X-ray analysis, which would have provided additional
information about the geometry of tetrabenzocorroles.
However, we can reasonably assume the presence of dynamic
processes due to macrocyclic distortions; from this point of
view, we tried to further characterize these compounds by
investigating their NMR behavior at variable temperature
1
(Figure S7−S9 in the SI). H NMR spectra were recorded at
As far as the absorption spectrum is concerned, 5 and 6 show
a very intense Soret-like band centered at 456 and 474 nm,
respectively, in both dichloromethane and toluene solutions (ε
= 101000 and 108500 M−1 cm−1). They also show a Q-like set
of broader, less intense bands with maxima at 640 and 645 nm,
respectively (ε = 15000 and 22800 M−1 cm−1; see Figure 2).
273, 248, and 223 K with the aim of obtaining better resolved
spectra. We chose the 2, 4, and 5 derivatives for these studies,
with the last two compounds being selected as models for
complexes with a neutral or an anionic ligand.
Despite lowering of the temperature, no significant variation
of the spectra was observed and an increase of resolution was
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Figure 4. Near-IR emission spectra of 5 (dashed line) and 6 (solid line).
The absorption spectra of 3 and 4 show similar bands,
although they are much broader and less intense (εmax = 34100
and 61000 M−1 cm−1). The fluorescence spectra show even
larger differences. In particular, 5 and 6 show at room
temperature an unstructured band in the red region, with a
rather short excited-state lifetime (see Figure 3 and Table 1).
These two corroles present rather high emission quantum
yields, with the data for 6 being remarkably high (31%).
Overall, its photophysical properties confer to 6 a very high
brightness, comparable to that shown by most currently used
dyes, including derivatives of BODIPYs, xanthones, and
cyanines.
For these two macrocycles, the excitation spectrum is
proportional to the absorption spectrum, indicating that the
fluorescent excited state is also formed regardless of the
excitation wavelength. Similar results were observed at 77 K.
For 5 and 6, no emission was observed in the 800−1600 nm
region either at room temperature or at 77 K, indicating a
negligible efficiency for phosphorescence under all experimen-
tal conditions. However, an emission peak centered at 1270 nm
at room temperature was observed (Figure 4), indicating singlet
oxygen sensitization. The excitation spectrum of the singlet
oxygen emission matches the absorption bands of the corroles,
providing experimental proof of the sensitization mechanism.
The corroles 3 and 4 showed less intense fluorescence at
room temperature. Analysis of the excited-state kinetics shows
that this is due to both a lower radiation rate constant and a
faster nonradiatiave process compared to 6 (Table 1). The
lower intensities in the absorption spectra are in agreement
with these data, suggesting a greater distortion of the
macrocycle skeleton in the case of the 3 and 4 complexes.
This distortion could also be a reason for the lack of singlet
oxygen sensitization observed for these two complexes, which
would require longer triplet-state lifetimes.
summarized in Table 2, which includes literature data for
related triaryl- and octaethylcorroles under the same solution
conditions. Cyclic voltammograms of 1, 3, and 4 in the three
utilized solvents are illustrated in Figures 5 and 6, while cyclic
voltammograms of 2, 5, and 6 are given in the Supporting
Information (Figures S13−S15).
The most well-defined electrochemistry occurs for the
copper derivative 1, which exhibits two reversible oxidations
and three reversible reductions in PhCN and CH2Cl2,
containing 0.1 M TBAP. A second reversible oxidation cannot
be observed in py because of the limited positive potential
window of this solvent.
Both oxidations of copper corrole are assigned as macro-
cycle-centered because the CuIII central metal ion cannot be
further oxidized and the eight electron-withdrawing ester
groups (CO2Me) on the macrocycle should also not be
oxidized in the investigated range of potentials.
In contrast to copper(III) corrole, multiple oxidation
processes are observed for aluminum(III) corrole 3 and
gallium(III) corrole 4 in CH2Cl2 and PhCN, while only two
oxidations are seen for the same corroles in py, which has a
smaller positive potential window. The py axial ligand in 3 and
−
4 can be replaced by a ClO4 anion from the supporting
electrolyte after oxidation, and PhCN can also axially bind to
the central metal ion before or after oxidation, which might give
different ligated forms of each corrole in solution, each of which
might have a different half-wave potential. However, the fact
that the two oxidations of 3 and 4 seem to be split into four
processes in CH2Cl2 or PhCN suggests that the py ligand is
dissociated in CH2Cl2 and PhCN, inducing the probable
formation of a π−π dimer with two equivalent redox centers. A
similar oxidation behavior is seen for germanium(IV) corrole
(see Table 2 and Figure S14 in the SI) in that the two
oxidations are split into four processes in the nonbinding
solvent CH2Cl2, but only two oxidations are observed in PhCN,
the more strongly coordinating solvent.
Electrochemistry and Spectroelectrochemistry. The
electrochemical and UV−vis properties of compounds 1−6
were examined before and after oxidation/reduction in CH2Cl2,
PhCN, and py. The measured half-wave or peak potentials are
The first reduction of 1 is assigned to the CuIII/CuII process,
as has been reported for a large number of copper(III) corroles
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Table 2. Half-Wave and Peak Potentials (V vs SCE) of the Investigated meso-Triaryltetrabenzocorrole Complexes in CH2Cl2,
PhCN, and py (0.1 M TBAP at a Scan Rate of 0.1 V s−1)
oxidation
second
reduction
side reactions
compd
no.
benzo-
a
compd
copper series
solvent
ring
first ring CuIII/CuII
CO2Me
first ring ref
[TBCorr]Cu
1
CH2Cl2
PhCN
py
1.38
0.82
0.80
0.71
1.13
0.70
0.78
0.57,
0.07
0.06
−1.38
−1.42
−1.33
−1.59
−1.62
−1.55
−1.62
tw
1.39
tw
tw
30
30
30
31
0.11
b
[Br8TTCorr]Cu
[TTCorr]Cu
[TPCorr]Cu
[OECorr]Cu
CH2Cl2
1.64
1.35
1.37
1.14
0.08
−0.23
−0.19
−0.34
0.43
c
d
d
free base
[TBCorr]H3
2
CH2Cl2
PhCN
0.68,
0.12
−0.72
−0.58
−1.32
−1.30
−1.30
−1.56
−1.50
tw
tw
b
b
0.57
c
0.83,
0.58
0.18
d
d
py
0.70
0.56
0.21
0.09
−0.59
−1.33
−1.51
−1.88
−1.55
tw
32
tw
b
[TTCorr]H3
py
aluminum and gallium [TBCorr]Al(py)
series
3
4
CH2Cl2
0.95, 0.73 0.43,
0.29
−1.35
−1.33
PhCN
1.02, 0.75 0.47,
0.28
−1.56
tw
py
0.82
0.38
1.25, 0.92 0.57,
0.16
1.27, 0.93 0.58,
0.14
0.95, 0.73 0.43,
0.29
−1.27
−1.25
−1.52
−1.48
tw
tw
[TBCorr]Ga(py)
CH2Cl2
PhCN
py
−1.28
−1.25
−1.52
−1.50
tw
tw
15
tw
[T(C6F5)Corr]
Ga(py)
0.75
e
e
germanium series
phosphorus series
[TBCorr]Ge(OH)
5
6
CH2Cl2
1.23, 1.13 0.92,
−0.71, −1.10
−1.35
−1.55
0.78
e
e
e
b
PhCN
py
1.27
0.90
−0.69, −1.10
−1.40
−1.48
−1.58
−1.63
tw
tw
33
e
b
0.78
1.13
−0.65, −1.10
[T(C6F5)Corr]
Ge(OH)
CH2Cl2
e
e
e
e
e
[TBCorr]P(OH)2
CH2Cl2
PhCN
py
1.35, 1.18 0.74
1.34, 1.17 0.75
0.76
0.11, −0.65, −0.81,
−1.33
−1.35
−1.29
−1.54
−1.56
−1.50
tw
tw
tw
e
−1.05
e
e
0.07, −0.66, −0.85,
e
−1.08
e
e
0.20, −0.55, −0.78,
e
−1.02
[TPCorr]P(OH)2
CH3CN 1.22
0.72
0.80
−1.41
−1.58
17
34
b
[OECorr])P(O)
PhCN
1.17
a
b
c
tw = this work. Peak potential. This redox couple can only be seen on the second positive potential sweep after scanning through the first
d
e
reduction. Irreversible deprotonation process. Unknown process with minor current, and most are irreversible.
in nonaqueous media.30,31,35 Examples of UV−vis spectral
changes obtained during this first reduction in a thin-layer cell
are shown Figure 7a and are consistent with this assign-
ment.30,35
TBAP. As mentioned above, the spectral changes of 1 upon
reduction at −0.20 V in the thin-layer cell are consistent with a
metal-centered redox process (see the top set of spectral
changes in Figure 7a). Only minimal changes are observed for
copper(II) corrole after the potential is switched from −0.20 to
−1.55 V (the second reduction), and these changes are
illustrated in Figure 7a. This electron addition is proposed to
occur at one of the fused benzene rings of the compound
containing the electron-withdrawing ester groups (CO2Me),
and this fused benzene ring does not participate in the
conjugated system of the macrocycle. Ester groups attached to a
benzene ring can undergo one-electron or multielectron
reductions,36 and similar “extra” reductions have also been
observed for some ring-expanded porphyrins with CO2R
groups.37
Copper(II) corrole also exhibits two closely spaced reversible
reductions after the first metal-centered process. Only one ring-
centered reduction is generally observed in this potential region
for most corroles without a redox-active central metal ion and
also without an extended π-ring system (see Table 2), but the
other investigated benzocorrole complexes also exhibit two
reductions in the three utilized solvents. On the basis of
spectral changes obtained in each controlled reduction of 1 (see
Figure 7a), the second reduction is assigned to occur at the
electron-withdrawing ester groups (CO2Me), while the third
reduction is assigned as being macrocycle-centered.
Figure 7 compares the spectral changes of 1 and 3 during
controlled potential reduction in PhCN containing 0.2 M
Finally, switching the applied potential from −1.55 to −1.75
V leads to a decrease in the intensity of the Soret (500 nm) and
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octabromo-meso-tritolylcorrole [(Br8TTCorr)Cu; see Table 2]
both have multiple electron-withdrawing groups on the
macrocycle, and the substituent effect of these groups on the
redox potentials should be considered as similar. The first
metal-centered and first ring-centered reductions of compound
1 and (Br8TTCorr)Cu are similar to each other, but the
potentials for oxidation of the two corroles differ by 260−310
mV. Compound 1, with a ring-expanded macrocycle, is easier
to oxidize.
Electrochemical and spectroelectrochemical data of the ring-
expanded free-base corrole 2 suggest that the electrochemical
behavior of 2 is similar to that of the “regular” free-base corrole
(see Table 2 and Figures S13 and S16 in the SI). First of all,
free-base corroles can be easily protonated and/or deproto-
nated in nonaqueous media to give a mixture of the neutral
corrole along with either its protonated, [(Corr)H4]+, or
deprotonated, [(Corr)H2]−, form in solution.32,38,39 Protona-
tion and/or deprotonation of free-base corroles is solvent-
dependent. As shown in Table 2 and Figure S13 in the SI, the
first reduction of 2 is irreversible in all three solvents and
located in a range of potentials between −0.59 and −0.72 V for
a scan rate of 0.1 V s−1. This reduction is coupled with a loss of
one proton from the central nitrogen atoms, giving [(Corr)-
H2]−. A similar deprotonation occurs for free-base tritolylcor-
role (TTCorrH3)32 in py, with the difference of the redox
potentials between the two compounds being due to the
different substituents on the macrocycle. A spectrum assigned
to [(Corr)H2]− is obtained by Spectroelectrochemisty in the
case of the reduced ring-expanded free-base corrole 2, which is
characterized by an intense Soret band at 496−500 nm and two
visible bands at 656−717 nm, with the exact value depending
on the solvent.
Figure 5. Cyclic voltammograms of copper(III) corrole 1 in (a)
CH2Cl2, (b) PhCN, and (c) py containing 0.1 M TBAP.
Q (660 nm) bands of copper corrole, and a new broad band
then appears at 783 nm, consistent with a macrocycle-centered
reaction. Almost identical spectral changes are seen during the
two controlled potential reductions of aluminum(III) corrole at
−1.45 V and then −1.70 V (Figure 7b), and it is therefore
suggested that 1 and 3 both undergo similar reduction
mechanisms.
We previously investigated a series of platinum(II)
porphyrins with different π-expanded macrocycles37 and
demonstrated that ring expansion of these compounds resulted
in a considerable raising of the highest occupied molecular
orbital (HOMO) levels, with only a small effect on the lowest
unoccupied molecular orbital (LUMO) levels. Because the ring
oxidation becomes easier with extended π conjugation and
there is only a small effect on the reduction, the HOMO−
LUMO gap decreased as the size of the π-conjugated system
increased. This is also the case for the ring-expanded corroles.
For example, compound 1 and the copper complex of β-
The free-base corrole 2 also undergoes one or two reversible
(or quasi-reversible) oxidations at potentials between 0.12 and
0.83 V for a scan rate of 0.1 V s−1. For example, the first
oxidation of 2 is located at E1/2 = 0.21 V in py. This process is
only observed in CH2Cl2 and PhCN, after reversal of the initial
negative scan at a potential negative of the first reduction and
then scanning in a positive direction. The oxidation that results
Figure 6. Cyclic voltammograms of (a) 3 and (b) 4 in CH2Cl2, PhCN, and py containing 0.1 M TBAP.
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Figure 7. Thin-layer UV−vis spectral changes of (a) 1 and (b) 3 in PhCN containing 0.2 M TBAP during the indicated reductions.
under these experimental conditions involves the [(Corr)H2]−/
(•Corr)H2) process.
of these tetrabenzocorrole metal complexes demonstrated a
HOMO−LUMO gap decrease, as a consequence of the π-
conjugated system expansion, while the photophysical charac-
terization showed large values measured for the fluorescence
and singlet oxygen sensitization of these complexes, which were
particularly high in the case of the phosphorus derivative.
Because the peripheral methyl ester groups can be hydrolyzed
to make this derivative water-soluble, these features make this
compound particularly promising for PDT and imaging
applications, considering that further optimization of triarylte-
trabenzocorroles may be obtained by the introduction of
suitable substituents on fused benzene rings. These studies are
currently in progress in our laboratories.
The redox-active species is assigned as (Cor)H3 in PhCN or
CH2Cl2 and [(Corr)H2]− in the basic py solvent, where one of
the three central protons has been lost.39 In addition to the
[(Corr)H2]−/(•Corr)H2) reaction of 2, another quasi-rever-
sible oxidation is seen at E1/2 = 0.70 V in py. This oxidation is
separated into two processes in CH2Cl2 and PhCN (at 0.57−
0.83 V for a scan rate of 0.1 V s−1). Similar to that proposed in
earlier studies of other free-base corroles, the oxidation of 2 is
proposed to generate a mixture of the free-base neutral corrole
radical, (•Corr)H2, and the protonated corrole monocation,
[(Corr)H4]+.32,39 The final UV−vis spectrum of 2 obtained
under controlled potential oxidizing conditions is given in
Figure S16 in the SI.
The reductions of [TBCorr]Ge 5 and [TBCorr]P 6 are both
ill-defined (Figures S14 and S15 in the SI), and three to four
unknown side reactions appear in the potential region of +0.20
to −1.10 V, under the given experimental conditions. These
unidentified reactions are currently under continued inves-
tigation. The proposed electron-transfer sites for other redox
reactions are given in Table 2.
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectra of 3−6, MS spectra of compounds 4−6, cyclic
voltammograms of 2, 5, and 6, and UV−vis spectral changes of
2 during the applied potentials. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
CONCLUSIONS
■
*E-mail: kadish@central.uh.edu (K.M.K.), roberto.paolesse@
uniroma2.it (R.P.).
Tetrabenzocorrole represents an interesting example of the
corrole aromatic π-system expansion, operated by the fusion of
benzene rings at the peripheral pyrrole β positions of the
macrocycle. These derivatives can be obtained by a three-step
procedure, which involves the preparation of the copper
complex 2 as the starting tetrabenzocorrole derivative. This
complex can be demetalated to obtain the corresponding free
base, opening the way to the preparation and investigation of
different metal derivatives. The electrochemical characterization
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Italian MIUR (to R.P. and L.P.,
PRIN2009 project 2009Z9ASCA) and the Robert A. Welch
Foundation (to K.M.K., Grant E-680) for financial support.
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E.; Will, S.; Erben, C.; Vogel, E. J. Am. Chem. Soc. 1998, 120, 11986−
11993.
The authors thank Fondazione Roma Dr. Luca Tortora (Tor
Vergata University) for recording the MS spectra.
(32) Stefanelli, M.; Pomarico, G.; Tortora, L.; Nardis, S.; Fronczek, F.
R.; McCandless, G. T.; Smith, K. M.; Manowong, M.; Fang, Y.; Chen,
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2011, 51, 6928−6942.
DEDICATION
■
Dedicated to Professor Maurizio Prato on the occasion of his
60th birthday.
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Chem.Eur. J. 2001, 7, 1041−1055.
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