Demetalation of silver(III) corrolates

Article abstract of DOI:10.1021/ic900859a

Several procedures for the demetalation of silver(III) corrolates have been tested. Acidic conditions induce removal of the silver ion but they can also promote concomitant oxidation of the corroie nucleus to an isocorrole species, the degree of which wil

Full text of DOI:10.1021/ic900859a

Inorg. Chem. 2009, 48, 6879–6887 6879
DOI: 10.1021/ic900859a
Demetalation of Silver(III) Corrolates
Manuela Stefanelli,† Jing Shen,‡ Weihua Zhu,‡ Marco Mastroianni,† Federica Mandoj,† Sara Nardis,†
Zhongping Ou,‡ Karl M. Kadish,*,‡ Frank R. Fronczek,§ Kevin M. Smith,*,§ 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, University of Houston, Houston, Texas 77204-5003, and
§Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803
Received May 4, 2009
Several procedures for the demetalation of silver(III) corrolates have been tested. Acidic conditions induce removal of
the silver ion but they can also promote concomitant oxidation of the corrole nucleus to an isocorrole species, the
degree of which will depend upon the specific acidic media. This oxidation cannot be completely avoided by addition of
hydrazine, particularly in the case of 3-NO2 substituted complexes which are quantitatively converted into the
corresponding 3-NO2, 5-hydroxy isocorroles upon silver ion removal. Several β-nitro isocorrole products were isolated,
and one was structurally characterized. Electrochemical and chemical reductive methods for silver(III) corrolates
demetalation were then tested with the aim to avoid the formation of isocorroles. While reaction with sodium
borohydride was shown to be quite effective to demetalate unsubstituted silver corrolates this was not the case for the
β-nitro derivatives where the peripheral nitro group is reduced by borohydride giving the corresponding 3-amino free
base corrole species. For the β-nitro corrole silver complexes, a successful approach was obtained using DBU/THF
solutions which afforded the 3-NO2 corrole free-base compound as a single reaction product in good yield. These
conditions were also effective for unsubstituted corroles although longer reaction times were necessary in this case. To
study in greater detail the corrole demetalation behavior, selected Ag(III) derivatives were characterized by cyclic
voltammetry in pyridine, and the demetalation products spectrally characterized after controlled potential reduction in a
thin-layer spectroelectrochemical cell.
Introduction
Corroles possess excellent chelating properties and numer-
ous metal derivatives have been synthesized and examined as
to their structure-property relationships for both theoretical
and practical reasons.4 In many cases, the metallocorroles
exhibit a coordination chemistry which is different than that
of the analogous porphyrins because of the reduced inner
cavity size and the trianionic ligand character of the corrole
macrocycle, thus resulting in a stabilization of derivatives
having higher formal oxidation states of the central metal
ions than in the case of the porphyrins.5
Early research on corroles was hampered by the need for
time-consuming and laborious preparation, but new syn-
thetic routes for obtaining 5,10,15-triarylcorroles3 have en-
abled a variety of detailed studies to be carried out on these
macrocycles, leading to a large number of publications on
various aspects of these compounds. The increased avail-
ability of corroles in large quantities has also led to these
compounds being examined for possible use in seve-
ral practical applications.6 From this point of view it is
Porphyrin chemistry represents an active field of research
which has implications in several disciplines, resulting in
many versatile applications such as in photodynamic ther-
apy1 or catalysis.2 In recent years increasing attention has
been devoted to the synthesis and characterization of por-
phyrin analogues, compounds deriving from skeletal mod-
ifications of the porphyrin macrocycle. The most intensely
studied of these compounds are the corroles which are core-
contracted porphyrins having a corrinoid carbon skeleton,
one of the meso-carbon bridges being replaced by a direct
pyrrole-pyrrole link.3
*To whom correspondence should be addressed. E-mail: kkadish@uh.edu
(K.M.K.), kmsmith@lsu.edu (K.M.S.), roberto.paolesse@uniroma2.it (R.P.).
(1) (a) Pandey, R. K.; Zheng, G. In The Porphyrin Handbook; Kadish, K.
M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol.
6, p 157. (b) Kessel, D. J. Porphyrins Phthalocyanines 2008, 12, 877.
(2) Aida, T.; Inoue, S. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 6, p 133.
(3) (a) Paolesse, R. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 2, p 201. (b)
Guilard, R.; Barbe, J.-M.; Stern, C.; Kadish, K. M. In The Porphyrin Handbook
Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2003;
Vol. 18, p 303. (c) Gryko, D. T.; Fox, J. P.; Goldberg, D. P. J. Porphyrins
Phthalocyanines 2004, 8, 1091. (d) Nardis, S.; Monti, D.; Paolesse, R. Mini-Rev.
Org. Chem. 2005, 2, 546.
(4) Palmer J. H., Day M. W., Wilson A. D., Henling L. M., Gross Z.,
Gray H. B. J. Am. Chem. Soc. 2008, 130, 7786 and references therein.
(5) Walker, F. A.; Licoccia, S.; Paolesse R. J. Inorg. Biochem. 2006, 100,
810 and references therein.
(6) Aviv, I.; Gross, Z. Chem. Commun. 2007, 1987.
r
2009 American Chemical Society
Published on Web 06/23/2009
pubs.acs.org/IC

6880 Inorganic Chemistry, Vol. 48, No. 14, 2009
Stefanelli et al.
important to develop protocols for peripheral functionaliza-
tion of the corrole macrocycle with an aim toward tuning and
optimizing the properties of the corrole for a particular
application.
conjugated macrocycle. The investigated compounds are
shown in Chart 1. Because a reduction mechanism is pre-
sumably involved in the demetalation process, in analogy
with silver porphyrins,11 we have performed electrochemical
studies on both the silver triarycorrolates and the corre-
sponding nitro derivatives to elucidate the redox processes
involved and have spectrally characterized the corrole deme-
talation product after controlled potential reduction in a
thin-layer cell.
Our laboratories have been involved in functionalization
of corroles for quite some time,7 with recent publications
being devoted to peripheral bromination, hydroformylation,
and, more recently, nitration of the macrocycle.8 The free-
base corrole frequently exhibits a very specific reactivity,
giving unexpected reaction products. While this behavior is
intriguing and challenging in itself, it is not useful for the
preparation of rationally functionalized macrocycles.
To functionalize related porphyrins, the macrocycle is
protected and/or activated toward a particular reaction by
coordination with a given metal ion. In this context, a
reversibility of the metalation and demetalation reactions
involving the macrocycle and the central metal ion is needed
to obtain a functionalized free-base macrocycle at the end of
the reaction. This approach has been hindered for corroles
by a lack of demetalation protocols, and this is due in large
part to the inherent instability of the corrole macrocycle
in acid media where structurally modified corroles and/or
decomposition products are readily formed. Nonetheless,
this situation has changed in recent years, and several
efficient methods have been reported for removal of metal-
locorrole central metal ions under controlled conditions.9
Among the examined compounds are the silver(III) corroles
which were the first examples reported in the literature for a
reversible metalation/demetalation process.9b Demetalation
in the initial report was accomplished by treatment of the
silver corrole with concentrated HCl, but a detailed proce-
dure was not reported.
In our laboratory we have recently developed a regiose-
lective β-nitration method for preparing meso-triarylcorroles
using silver nitrite as a nitrating system.7 This simple proce-
dure affords in moderate yields a 3-nitrosubstituted Ag(III)
corrolate as the reaction product. This compound is particu-
larly appealing because of the well-known potentialities of
the nitro group for efficient incorporation of functionality
into the β-pyrrole positions of the macrocycle, thus allowing
for further manipulation of the corrole platform.10 For this
reason, the availability of a simple procedure for Ag(III)
corrolate demetalation is of fundamental interest to further
exploit this synthetic route, as well as for the preparation of
more elaborate corrole derivatives.
Experimental Section
Chemicals. Pyridine (py) was obtained from Sigma-Aldrich
Co. and used as received for electrochemistry and spectroelec-
trochemistry experiments. Tetra-n-butylammonium perchlo-
rate (TBAP) was purchased from Fluka Chemical Co. and
used without further purification. Silica gel 60 (70-230 mesh,
Merck) was used for column chromatography. Reagents and
solvents (Aldrich, Merck, or Fluka) were of the highest grade
available and were used without further purification.
Instrumentation. Reactions were monitored by thin layer
chromatography and UV-vis spectrophotometry. 1H NMR
spectra were recorded with a Bruker Avance 300 (300 MHz)
instrument, using CDCl3 as solvent: chemical shifts are ex-
pressed in ppm relative to residual CHCl3 (7.25 ppm). UV-vis
spectra were measured with a Varian Cary 50 Spectrophoto-
meter. Mass spectra were obtained using a VG quattro
Spectrometer (FAB). Synthesis of 5,10,15-triarylcorroles
and the corresponding silver complexes were prepared as
previously reported in literature,8a,12 while the preparation
of (TF5PCor)Ag is reported for the first time in the present
work.
Cyclic voltammetry was carried out at 298 K by using an
EG&G Princeton Applied Research (PAR) 173 potentiostat/
galvanostat. A homemade three-electrode cell was used for
cyclic voltammetric measurements and consisted of a glassy
carbon working electrode, a platinum counter electrode, and a
homemade 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.
Thin-layer UV-visible spectroelectrochemical experiments
were performed with a home-built thin-layer cell which has a
light transparent platinum net working electrode.13 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 spectrophot-
ometer. High purity N2 from Trigas was used to deoxygenate the
solution and kept over the solution during each electrochemical
and spectroelectrochemical experiment.
The need for well-defined methods for removal of metal
ions from the corrole macrocycle led us to examine in detail
new routes for demetalation. The results of our studies
are described in the present paper which examines seven
triaryl silver(III) corroles, three of which are nitrated at the
The crystal structure of [(3-NO2)(5-OH)TTisoCor]H2 was
determined at low temperature using data collected with Cu
KR radiation on a Bruker Kappa Apex II diffractometer.
Crystal data: C40H31N5O3, dark green, triclinic space
˚
group P1, a=7.7596(5), b=11.0531(7), c=18.6697(10) A, R=
3
˚
96.019(5), β=98.975(5), γ=96.284(5)°, V=1559.98(16) A , Z=
2, Dcalc=1.341 g cm-3, μ(Cu KR)=0.69 mm-1, T=90 K, 11984
reflections having θmax = 58.9° measured, 4281 independent
reflections (Rint =0.037) were used in the calculations. All H
atoms were located by difference maps. Those on C were placed
in idealized positions, while coordinates for those on N and
(7) Paolesse, R. Synlett 2008, 15, 2215.
(8) (a) Stefanelli, M.; Mastroianni, M.; Nardis, S.; Licoccia, S.; Fronczek,
F. R.; Smith, K. M.; Zhu, W.; Ou, Z.; Kadish, K. M.; Paolesse, R. Inorg.
Chem. 2007, 46, 10791. (b) Mastroianni, M.; Zhu, W.; Stefanelli, M.; Nardis,
S.; Fronczek, F. R.; Smith, K. M.; Ou, Z.; Kadish, K. M.; Paolesse, R Inorg.
Chem. 2008, 47, 11680.
::
::
(9) (a) Broring, M; Hell, C. Chem. Commun. 2001, 2336. (b) Bruckner, C.;
~
Barta, C. A.; Brinas, R. P.; Krause Baue, J. A. Inorg. Chem. 2003, 42, 1673.
(11) Kadish, K. M.; Lin, Q. X.; Ding, J. Q.; Wu, Y. T.; Araullo, C. Inorg.
Chem. 1986, 25, 3236.
(12) (a) Paolesse, R.; Nardis, S.; Sagone, F.; Khoury, R. G. J. Org. Chem.
2001, 66, 550. (b) Paolesse, R.; Marini, A.; Nardis, S.; Froiio, A.; Mandoj, F.;
Nurco, D. J.; Prodi, L.; Montalti, M.; Smith, K. M. J. Porphyrins Phtha-
locyanines 2003, 7, 25.
(c) Mandoj, F.; Nardis, S.; Pomarico, G.; Paolesse, R. J. Porphyrins
Phthalocyanines 2008, 12, 19. (d) Capar, C.; Kolle, E. T.; Ghosh, A. J.
Porphyrins Phthalocyanines 2008, 12, 964. (e) Ngo, T. H.; Van Rossom, W.;
Dehaen, W.; Maes, W. Org. Biomol. Chem. 2009, 7, 439.
(10) Jaquinod, L. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vol. 1, p 201.
(13) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1498–1501.

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6881
Chart 1. Molecular Structures of the Corroles Studied
O were refined. Final residuals (for 446 parameters) were
R1 [I > 2σ(I)]=0.039, wR2 (all data)=0.104, CCDC 722641.
General Procedures for Demetalation. (a). CHCl3/H2SO4
Method. Concentrated H2SO4 (1 mL) was added to a chloro-
form solution (10 mL) of (TtBuTPCor)Ag (0.2 mmol). The
progress of the demetalation reaction was monitored by UV-
vis spectroscopy, by diluting an aliquot of the reaction mixture
in CH3OH. The immediate formation of the corrole cation was
observed and, after 5 min, distilled water (50 mL) was added,
and the organic phase extracted by adding chloroform (50 mL).
The organic mixture was washed twice with distilled water,
then neutralized twice with aqueous NaHCO3 and dried over
Na2SO4. Purification on silica gel (eluant: dichloromethane/
hexane 1:1) and crystallization from CH2Cl2/hexane afforded
the demetalated corrole (TtBuTPCor)H3 (0.019 mmol, 18%
yield), identical to an authentic specimen prepared according
to literature methods.8a
vacuum, and the residue dissolved in CH2Cl2. The organic phase
was washed twice with distilled water and then dried over
Na2SO4. Chromatography on silica gel (eluant:CH2Cl2/hexane
3:1), followed by crystallization from CH2Cl2-CH3OH af-
forded the corresponding corrole free base (yield 38%).
(d). DBU/THF Method. To a stirred solution of silver corrole
complex (0.053 mmol) dissolved in tetrahydrofuran (THF,
10 mL) 154 μL of DBU (1.05 mmol) were added. The progress
of the reaction was monitored by UV-vis spectroscopy and
TLC (silica gel, eluant: CH2Cl2/hexane 3:1). When the silver
complex was fully consumed, the solvent was removed under
vacuum, the residue dissolved in CH2Cl2, and the organic
mixture washed twice with distilled water (50 mL). The organic
solution was dried over Na2SO4 and then purified on silica gel
(eluant: CH2Cl2/hexane 1:1). All fractions containing the cor-
role free base were then collected, and the solvent was evapo-
rated. The residue was dissolved in CHCl3 (10 mL), and an
aliquot of an aqueous solution of hydrazine (1 mL) was added.
The organic mixture was washed twice with distilled water, dried
over Na2SO4, and then crystallized from CH2Cl2-CH3OH,
affording the demetalated corrole [(TtBuTPCor)H3: 62% yield,
(NO2TtBuPCor)H3: 45% yield].
(b). NaBH4 Method. (TtBuTPCor)Ag (28 mg, 0.035 mmol)
was dissolved in 30 mL of a CH2Cl2/CH3OH (4:1) solvent
mixture, and the solution degassed with Ar for 5 min. NaBH4
(66 mg, 1.75 mmol) was added, and the reaction was monitored
by thin-layer chromatography (TLC, silica gel, dichlorometh-
ane elution) and UV-vis spectroscopy. The demetalation was
complete after 1 h. After this time, the solution was filtered
through a Celite plug, the solvent removed under vacuum, and
the residue was purified by column chromatography on silica
gel (eluant: CH2Cl2/hexane 4:1). The first green fraction
was isolated and crystallized from CH2Cl2/CH3OH, affording
13 mg (0.019 mmol, 54% yield) of the corrole free base
(TtBuTPCor)H3.
Preparation of (TF5PCor)Ag. (TF5PCor)H3 (80 mg, 0.1
mmol) was dissolved in acetonitrile (10 mL), and silver(I) acetate
(55 mg, 0.33 mmol) was added. Stirring of the solution was
continued at room temperature (RT), and the progress of the
reaction was monitored by TLC and UV-vis spectroscopy.
After 18 h, the reaction mixture was filtered through a Celite
plug, the solvent removed under vacuum, and the residue
purified by column chromatography on silica gel. Hexane was
initially used as eluant, and increasing amounts of chloroform
(up to 20%) afforded the desired product as the first fraction and
the unreacted free-base-corrole as the second. The first red-
violet fraction was isolated and crystallized from acetone/H2O,
affording 32 mg (0.036 mmol, 36%) of (TF5PCor)Ag as purple
crystals. UV-vis: λmax (CH2Cl2, log ε): 421 (5.01), 493 (3.76),
530 (3.87), 564 (4.32) nm. 1H NMR (300 MHz, CDCl3): δ 9.38
(c). NaOH/EtOH-Toluene Method. (TtBuTPCor)Ag (0.03
mmol) was dissolved in a mixture of EtOH/toluene (0.5 mL/
10 mL) containing 1.5 mmol of NaOH, and the solution was
refluxed for about 1.5 h. The progress of the demetalation
reaction was monitored by UV-vis spectroscopy and TLC
(silica gel, CH2Cl2-petroleum ether (3:1)). After the starting
complex was fully consumed, the solvent was removed under

6882 Inorganic Chemistry, Vol. 48, No. 14, 2009
Stefanelli et al.
(d, 1J = 4.2 Hz, 2H, β-pyrrolic), 9.00 (d, 1J = 4.2 Hz, 2H,
being H2SO4, halogenated acids, or organic acids like TFA,
the exact choice depending upon the stability by the complex.
However, the exploitation of such harsh acid protocols to
metallocorroles has been problematic because of the in-
creased reactivity of the corrole macrocycle in acid media,
which can easily lead to peripheral substitution and/or
decomposition products.14
β-pyrrolic), 8.82 (d, 1J=4.0 Hz, 4H, β-pyrrolic)
Spectroscopic Characterization of Compounds. [(3-NO2)-
(5-OH)tBuTPisoCor]H2. Anal. found for C49H49N5O3: C,
77.8: H, 6.6; N, 9.4. Calcd: C, 77.9; H, 6.7; N, 9.3%. UV-
vis: λmax (CH2Cl2, log ε)=398 (4.79), 622 (4.09), 674 (4.08) nm.
1H NMR (300 MHz, CDCl3): δ=16.37 (s, 1H, NH), 14.97 (s,
1H, NH), 7.53 (s, br, 5H, phenyl), 7.50 (s, br, 1H, phenyl), 7.40
(m, 5H, phenyl þ β-pyrrolic), 7.15 (d, 1J=3.63 Hz, 2H, phenyl),
6.92 (m, 2H, β-pyrrolic), 6.73 (d, 1J=4.71 Hz, 1H, β-pyrrolic),
6.55 (m, 1H, β-pyrrolic), 6.36 (s, 1H, β-pyrrolic), 6.30 (m, 1H,
β-pyrrolic), 1.42 (s, 9H, -C(CH3)3), 1.39 (s, 9H, -C(CH3)3), 1.27
ppm (s, 9H, -C(CH3)3). LRMS (FAB): m/z 755 (Mþ).
Considering this crucial aspect, we initially used the bi-
::
phasic method reported by Bruckner et al.,9b to determine the
most accurate experimental protocol. For this purpose, we
prepared a solution of (TtBuPCor)Ag in chloroform and
added the same volume of 1 M HCl. The progress of the
reaction was monitored by UV-vis spectroscopy after acid
addition and no changes in the spectral features were ob-
served under these conditions. Increasing the acid concen-
tration to 4 M led to formation of the corrole cation in 1 h,
but the reaction workup afforded almost quantitatively
the starting complex together with traces of an isocorrole
species,7,15 as recently reported in the case of copper corrole
demetalation using different acidic mixtures.9b Similar results
were obtained after adding a few drops of concentrated HCl
to a chloroform mixture containing (TtBuPCor)Ag.
Whenthesameprocedurewasappliedto(NO2TtBuPCor)Ag,
the UV-vis spectrum revealed a complete transforma-
tion of the starting complex into a different compound in
about 12 h. TLC analysis showed the almost quantitative
formation of a green band having a lower Rf value than the
starting macrocycle. Subsequent purification of the crude
mixture afforded an analytically pure compound that was
spectroscopically characterized as an isocorrole. The pre-
sence of a blue-shifted Soret band below 400 nm and two
bands above 600 nm in the visible spectrum combined with
NMR data which showed resonances at about 15 ppm for the
inner NH and the β-pyrrolic resonances shifted below 7 ppm,
supported formation of [(3-NO2)(5-OH)tBuTPisoCor]H2.
This is analogous to results from our previous experiments
on the demetalation of Cu(III) corrolates.9b Attempts to
obtain crystals suitable for X-ray analysis were unsuccessful,
but the same demetalation protocol carried out on (NO2TT-
Cor)Ag afforded a single compound with similar spectro-
scopic features, for which single crystals were obtained by
slow diffusion of methanol into a diluted dichloromethane
solution. An X-ray crystal structure determination of this
compound unambiguously confirmed it to be [(3-NO2)-
(5-OH)TTisoCor]H2 (Figure 1). Although the oxidative for-
mation of isocorroles generally gives a statistical mixture of
5- and 10-substituted compounds,15 the demetalation of
3-NO2-triarylcorroles affords exclusively the 5-hydroxy sub-
stituted isocorrole, whose formation is clearly favored by the
intramolecular hydrogen bond established between the
hydroxyl and nitro groups on the compound (as evidenced
in Figure 1).
[(3-NO2)(5-OH)TTisoCor]H2. Anal. found for C40H31N5O3:
C, 76.4: H, 4.9; N, 11.2. Calcd: C, 76.3; H, 5.0; N, 11.1%. UV-
vis: λmax (CH2Cl2, log ε)=397 (4.75), 616 (4.06), 672 (4.01) nm.
1H NMR (300 MHz, CDCl3): δ=16.35 (s, 1H, NH), 14.95 (s,
1H, NH), 7.53 (s, br, 4H, phenyl), 7.50 (s, br, 1H, phenyl), 7.40
(m, 5H, phenyl þ β-pyrrolic), 7.15 (d, 1J=3.63 Hz, 2H, phenyl),
6.92 (m, 2H, β-pyrrolic), 6.73 (d, 1J=4.71 Hz, 1H, β-pyrrolic),
6.55 (m, 1H, β-pyrrolic), 6.36 (s, 1H, β-pyrrolic), 6.30 (m, 1H,
β-pyrrolic), 1.42 (s, 9H, -C(CH3)3), 1.39 (s, 9H, -C(CH3)3), 1.27
ppm (s, 9H, -C(CH3)3). LRMS (FAB): m/z 628 (Mþ).
[(10-OH)tBuTPisoCor]H2. Anal. found for C49H50N4O: C,
82.9: H, 6.9; N, 7.7. Calcd: C, 82.8; H, 7.1; N, 7.8%. UV-vis:
λmax (CH2Cl2, log ε) = 360 (4.47), 433 (4.68), 670 (3.83), 725
(3.86) nm. 1H NMR (300 MHz, CDCl3): δ=15.62 (s, 2H, NH),
1
7.59 (d, 1J = 8.4 Hz, 2H, phenyl), 7.48 (d, J = 8.1 Hz, 4H,
phenyl), 7.26 (d, 1J=8.1 Hz, 4H, phenyl), 7.06 (d, 1J=7.8 Hz,
2H, phenyl), 6.69 (m, 4H, β-pyrrolic), 6.64 (d, 1J=4.5 Hz, 2H,
β-pyrrolic), 6.40 (d, 1J = 4.2 Hz, 2H, β-pyrrolic), 1.61 ppm
(s, 27H, -C(CH3)3). LRMS (FAB): m/z 710 (Mþ).
[(5-OH)tBuTPisoCor]H2. Anal. found for C49H50N4O: C,
82.6: H, 7.0; N, 7.8. Calcd: C, 82.4; H, 7.1; N, 7.7%. UV-vis:
λ
max (CH2Cl2, log ε)=407 (4.69), 680 (3.86), 738 (3.83) nm. 1H
NMR (300 MHz, CDCl3): δ=16.21 (s, 1H, NH), 15.84 (s, 1H,
NH), 7.59 (d, 1J=8.1 Hz, 2H, phenyl), 7.42 (d, 1J=7.8 Hz, 2H,
phenyl), 7.27 (m, 6H, phenyl), 7.07 (d, 1J=8.1 Hz, 2H, phenyl),
1
1
6.95 (d, J=4.5 Hz, 1H, β-pyrrolic), 6.84 (d, J=4.5 Hz, 1H,
1
β-pyrrolic), 6.56 (m, 2H, β-pyrrolic), 6.51 (d, J=4.5 Hz, 1H,
β-pyrrolic), 6.30 (m, 1H, β-pyrrolic), 6.14 (m, 1H, β-pyrrolic),
6.02 (m, 1H, β-pyrrolic), 1.61 (s, 18H, -C(CH3)3), 1.59 (s, 9H, -C
(CH3)3). LRMS (FAB): m/z 710 (Mþ).
(NO2TtBuPCor)H3. Anal. found for C49H49N5O2: C, 79.3:
H, 6.9; N, 9.4. Calcd: C, 79.5; H, 6.7; N, 9.5%. UV-vis: λmax
(CH2Cl2, log ε)=397 (4.64), 438 (4.68), 465 (4.69), 664 (4.29) nm.
1H NMR (300 MHz, CDCl3): δ=9.07 (s, 1H, β-pyrrolic), 8.66 (s,
br, 3H, phenyl), 8.52 (s, 1H, β-pyrrolic), 8.37 (s, 1H, β-pyrrolic),
8.28 (s, 1H, β-pyrrolic), 8.20 (s, br, 2H, phenyl), 8.04 (m, 4H,
phenyl), 7.80 (m, 6H, phenyl), 1.60 ppm (s, 27H, -C(CH3)3).
LRMS (FAB): m/z 740 (Mþ) .
(NH2TTCor)H3. Anal. found for C40H33N5: C, 85.0: H, 4.8;
N, 10.2. Calcd: C, 85.1; H, 4.8; N, 10.1%. UV-vis: λmax
(CH2Cl2, log ε) = 418 (4.71), 578 (4.06), 635 (3.88) nm. 1H
NMR (300 MHz, CDCl3): δ = 8.86 (d, 1J = 4.7 Hz, 1H,
1
1
β-pyrrolic), 8.81 (d, J=3.8 Hz, 1H, β-pyrrolic), 8.55 (d, J=
4.7 Hz, 1H, β-pyrrolic), 8.51 (d, 1J=4.3 Hz, 1H, β-pyrrolic), 8.45
1
(m, 2H, β-pyrrolic), 8.29 (d, J=7.6 Hz, 2H, phenyl), 8.06 (d,
The intramolecular OH O hydrogen bond to the nitro
3 3 3
1J=7.4 Hz, 4H, phenyl), 7.98 (s, 1H, β-pyrrolic), 7.63 (m, 4H,
phenyl), 7.55 (d, J = 7.4 Hz, 2H, phenyl), 2.69 ppm (s, 9H,
group is normal and approximately linear, having O
O
3 3 3
1
˚
distance 2.714(2) A and a 163(2)° angle about H. The four
corrole N atoms are essentially coplanar, exhibiting a mean
deviation of 0.010(1) A. NH hydrogen atoms appear ordered
-CH3). LRMS (FAB): m/z 691 (Mþ).
˚
Results and Discussion
and have a direction preference toward one adjacent N atom
Studies of demetalation were carried out with (TtBuPCor)
Ag and the corresponding 3-(NO2) derivative. We started
with the most general routes previously used in the case of
porphyrins. These involve the removal of the central metal
ion in aqueous solutions containing different acids, examples
(14) Ou, Z.; Sun, H.; Zhu, W.; Da, Z.; Kadish, K. M. J. Porphyrins
Phthalocyanines 2008, 12, 1.
(15) Nardis, S.; Pomarico, G.; Fronczek, F. R.; Vicente, M. G. H.;
Paolesse, R. Tetrahedron Lett. 2007, 48, 8643.

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6883
This protocol reduced decomposition of the starting complex
and remarkably prevented oxidation of the corrole ring,
which leads in general to the formation of isocorroles.
Considering these favorable aspects, we applied the above
procedure to both silver complexes, but the results were not
as satisfying as in the case of the copper corrolates.
Applying the demetalation procedure to (NO2TtBuPCor)Ag
again led to a nitro isocorrole as the main reaction product,
while better results were obtained with (TtBuPCor)Ag.
Monitoring the progress of the reaction by UV-vis spec-
troscopy showed the starting complex to be totally consumed
in 5 min with formation of a product ascribed to a π-radical
species of the corrole macrocycle, as recently reported in the
literature.17 Treatment of this reaction product in the mixture
with aqueous NaHCO3 and purification on a column
afforded three fractions. The first isolated brown band
corresponded to the above-mentioned corrole radical that
could be easily converted into its aromatic form by treatment
with a few drops of aqueous hydrazine. The second green
fraction was identified as the expected (TtBuPCor)H3 while a
more polar third green band corresponded to an isocorrole,
which was the main reaction product. Although reduction of
the compound in the first band by hydrazine increased the
amount of the demetalated corrole, the overall reaction yield
was still very low as compared with the analogous reaction
performed on copper corrolates.
Using acidic mixtures of different strengths has been
demonstrated to be quite ineffective for demetalation of
silver corroles, and for this reason we turned our attention
to reductive methods used for the removal of silver ion from
porphyrins and carbaporphyrins.11 Collman et al. reported in
1983 the use of NaBH4 as an efficient demetalation reagent
for silver porphyrins,18 and its reducing mechanism was
elucidated by Cowan and Sanders19 a few years later.
Following a slightly modified protocol, we demetalated
(TtBuPCor)Ag by treatment of the complex with excess
NaBH4 in a CH2Cl2/CH3OH solution at room temperature.
In this case, the color of the reaction mixture rapidly turned
from claret to brilliant green, and in about 1 h the starting
complex was consumed and converted into the expected free-
base corrole, (TtBuPCor)H3 in good yield, with no evidence
of an isocorrole.
It should be mentioned that reduction of the silver ion in
this experiment proceeded to its Ag(0) form, as determined
from the typical silver mirror layered on the glass surface.
This is similar to what was reported during the demetalation
of silver porphyrinates.11 Application of the same protocol to
the silver(III) β-nitro corrolates could open the way to the
corresponding free-base corrole, an appealing compound
never reported in literature. When we carried out the reaction
on (NO2TTCor)Ag we observed a considerable change in the
visible spectrum consistent with formation of a free-base
corrole after only 10 min. TLC analysis revealed the complete
disappearance of the starting complex and formation of a
green band having a lower Rf value than the silver nitrocor-
role as the main product. The solution was filtered on Celite
to remove the generated Ag(0) and then purified on a
chromatographic column. Unfortunately this compound
Figure 1. Molecular structure of [(3-NO2)(5-OH)TTisoCor]H2.
(N-H N angles 135(2)°) over the other (N-H N <
3 3 3
3 3 3
110°). The 23-atom corrole core has a flattened saddle
conformation, with the nitrated pyrrole forming a dihedral
angle of 9.0(3)° with the pyrrole ring opposite it, while the
other two opposite pyrroles are more steeply inclined, form-
ing a dihedral angle of 29.7(1)°. Overall, the 23 C and N
˚
atoms have a mean deviation of 0.18 A from coplanarity,
with a maximum of 0.536(2) A for a C atom on the periphery
˚
a pyrrole ring.
This result led us to turn our attention to other demetala-
tion systems exploiting acidic conditions; we decided to use
trifluoroacetic acid (TFA) in CH2Cl2, a system recently
reported for the reductive demetalation of 3-Aryl- and
3-arylethynyl NCP silver(III) complexes.16 For this purpose,
(TtBuPCor)Ag was dissolved in the CH2Cl2/TFA mixture,
and the course of the reaction followed by UV-vis spectros-
copy. After5 min, theUV-vis. spectrumindicated formation
of the corrole cation, but subsequent treatment with an
aliquot of TEA showed an overlapping of the optical features
for both the starting complex and an isocorrole. After 45 min
the complete disappearance of the original silver corrolate
was observed, but the UV-vis spectrum indicated exclusive
formation of an isocorrole. Neutralization with aqueous
NaHCO3 and TLC of the crude reaction product confirmed
the absence of the expected free-base corrole among the
reaction products. After treatment by column chromatogra-
phy, the hydroxyisocorrole was recovered. Application of the
same demetalation procedure to (NO2TtBuPCor)Ag led
to an almost immediate conversion to the nitroisocorrole
which possesses identical spectroscopic characteristics as that
obtained using the biphasic method described above.
The failure of the above-described demetalation protocols,
which leave unchanged the starting complex or completely
transform it into the corresponding isocorrole, led us to
investigate as the acidic mixture CHCl3/H2SO4 which was
recently used for the efficient removal of copper ions from
meso-triaryl-, β-octaalkyl- and fully substituted corroles.9b
(17) Shen, J.; Shao, J.; Ou, Z.; E, W.; Koszarna, B.; Gryko, D. T.; Kadish,
K. M. Inorg. Chem. 2006, 45, 2251.
(18) Collman, J. P.; Bencosme, S. C.; Durand, R. R.Jr.; Kreh, R. P.;
(16) Ishizuka, T.; Yamasaki, H.; Osukab, A.; Furuta, H. Tetrahedron
2007, 63, 5137.
Anson, F. C. J. Am. Chem. Soc. 1983, 105, 2699.
(19) Cowan, J. A.; Sanders, J. R. M. Tetrahedron Lett. 1986, 27, 1201.

6884 Inorganic Chemistry, Vol. 48, No. 14, 2009
Stefanelli et al.
Figure 2. 1H NMR spectrum of (NH2TTCor)H3.
was quite labile on silica and mostly decomposed during
elution. However traces of the product could be collected and
were used for spectroscopic characterization. The proton
NMR spectrum is reported in Figure 2.
with potential optoelectronic features. Such a compound
could be obtained through application of the Barton-Zard
procedure, which similarly affords β-fused pyrroloporphy-
rins23 by reaction of some (2-nitro-5,10,15,20-tetraphenyl-
porphyrinato) metal complexes with R-isocyanoacetic esters
in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU).
For this purpose we reacted (NO2TtBuPCor)Ag with ethyl
isocyanoacetate in a dry refluxing THF/iPr-OH solvent
mixture, using excess of DBU as base. After3 h, TLC analysis
showed the starting material to be fully consumed, and a
main green compound was formed. The reaction product was
purified by column chromatography after which the first
green eluted fraction was isolated and spectral characteriza-
tion indicated it to be (NO2TtBuPCor)H3.
The UV-vis spectrum of this compound is quite uncom-
mon among tetrapyrrolic macrocycles, having an unusual
Soret band which is split into three bands of similar intensity
at 397, 438, and 465 nm. There is also a broadband at about
600 nm (Supporting Information, Figure S1). FAB mass
spectrometry showed the compound to have a molecular
peak at m/z 740, and the 1H NMR spectrum showed a singlet
at 9.08 ppm, corresponding to the resonance of the proton on
C2, which is shifted upfield with respect to the starting
complex. To confirm our assignment, we dissolved the
compound in pyridine and added excess silver acetate; in a
few minutes silver insertion was shown to be successful by
both UV-vis and TLC monitoring, and (NO2TtBuPCor)Ag
was recovered.
The identification of this product as (NH2TTCor)H3 was
unexpected, since sodium borohydride is able to reduce
aromatic nitro compounds only in catalytic processes that
employ metal catalysts such as Pt, Pd, or Ru supported on
active carbon, polymers, and metal oxides, or Raney-Ni
catalysts.20 A plausible explanation for this result derives
from the recent development of novel catalytic systems based
on silver nanoparticles, which have been used for the selec-
tive hydrogenation of a range of chloronitrobenzenes21 and
4-nitroaniline.22 Considering these catalytic processes, we
propose a mechanism for the demetalation/reduction of the
silver nitrocorrole to the aminocorrole where NaBH4 first
reduces Ag(III) to Ag(I) and then is dissociated from the
macrocycle and reduced to silver metal in the solution. This
process leads to the formation of Ag(0) nanoparticles that
constitute the catalytic system responsible for reduction of
the nitro group to an amino group.
The above-described demetalation method can be success-
fully employed for the demetalation of silver triarylcorrolates
but it is not suitable for the corresponding nitro derivatives
since it causes peripheral modification of the macrocycle.
Because the free-base NO2-corrole is particularly appealing
for further functionalizations,10 a different demetalation
procedure for such compounds is necessary.
During our studies involving peripheral functionalization
of the corrole macrocyle, we investigated the possibility of
exploiting silver nitrocorrolate as a substrate for preparation
of a β-fused pyrrolocorrole, an appealing starting compound
for construction of more elaborated β-fused architectures
Prompted by these unexpected results, we decided to test
the effectiveness of the demetalation system constituted by
DBU/THF. We dissolved (TtBuPCor)Ag in THF, added a
20-fold excess of DBU, and monitored the course of the
reaction by UV-vis spectroscopy. In about 30 min, the
visible spectrum showed the appearance of a new, distinct
(20) (a) Tafesh, A. M.; Weiguny, J. Chem. Rev. 1996, 96, 2035.
(b) Downing, R. S.; Kunkeler, P. J.; van Bekkum, H. Catal. Today 1997, 121.
(21) Chen, Y.; Wang, C.; Liu, H.; Qiu, J.; Bao, X. Chem. Commun. 2005,
5298.
(22) Zhou, Q.; Qian, G.; Li, Y.; Zhao, G.; Chao, Y.; Zheng, J. Thin Solid
Films 2008, 516, 953–956.
(23) (a) Jaquinod, L.; Gros, C.; Olmstead, M. M.; Antolovich, M.; Smith,
K. M. Chem. Commun. 1996, 1475. (b) Gros, C. P.; Jaquinod, L.; Khoury, R.
G.; Olmstead, M. M.; Smith, K. M. J. Porphyrins Phthalocyanines 1997, 1,
201.

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6885
band at 638 nm which is ascribed to the corrole anion radical.
After 18 h, TLC analysis indicated the complete disappear-
ance of the silver complex, and UV-vis monitoring evi-
denced the typical optical profile of the corrole anion, having
a split Soret band at 421 and 443 nm and a strong Q-band at
638 nm. It is important to note that precipitation of Ag(0)
from the reaction was observed, indicating that a reductive
demetalation mechanism was also occurring in this system.
After workup, TLC analysis showed the free-base corrole,
(TtBuPCor)H3 to be the main product together with a less
polar brown compound which was not easily separated from
the corrole by column chromatograph. However treatment
of the fractions containing these two compounds with
hydrazine converted the brown compound into the corrole,
increasing the final yield. When the same method was applied
to (NO2TtBuPCor)Ag, removal of the silver ion from the
macrocycle was rapid and exhaustive and occurred in about
30 min. Chromatographic purification afforded a single
compound, whose spectroscopic features indicated it to be
the corresponding free-base (NO2TtBuPCor)H3.
Figure 3. Cyclic voltammograms of (TF5PCor)Ag in pyridine contain-
ing 0.1 M TBAP.
To obtain further information on the demetalation
mechanism under basic conditions, we decided to verify if a
strong base, such as NaOH in ethanol/toluene solution, could
represent an effective demetalation system for silver corro-
lates. We dissolved (TtBuPCor)Ag in the alcoholic mixture
and added a large excess of sodium hydroxide. The reaction
was refluxed for about 1.5 h, until TLC analysis indicated
quantitative formation of the red-brown spot, corresponding
to what was observed in the case of the DBU/THF system,
together with traces of the starting compound. After the
necessary workup to neutralize the excess base, purification
on silica gel afforded the compound, which could be con-
verted in moderate yield (38%) to the expected corrole using
hydrazine. An application of the described route to the
corresponding nitro derivative led to rapid formation of a
mixture of isocorrole species and only traces of the expected
demetalated corrole could be detected.
Electrochemistry in Pyridine. Five Ag(III) corroles were
selected for electrochemical characterization in pyridine.
Each compound could be reduced to its Ag(II) and Ag(I)
forms, and both electrode reactions were followed by
demetalation in solution to give a corrole product, iden-
tified as [(Cor)H2]- on the basis of its UV-visible spec-
trum and electrochemistry, previously characterized in
pyridine for a number of derivatives having meso-substi-
tuents similar to the species investigated in the current
study.17
The most stable of the five electrochemically investi-
gated Ag(III) corroles is (TF5PCor)Ag whose cyclic
voltammogram is illustrated in Figure 3. Four redox
processes are observed. Two are assigned to metal-cen-
tered reactions of the initial compound (at E1/2=-0.55 V
and Ep = -1.42 V) and two to the diprotic corrole
demetalation product, in this case [(TF5PCor)H2]-,
which is reversibly reduced to [(TF5PCor)H2]2- at
Figure 4. Cyclic voltammograms of (a) (TtBuPCor)Ag and (b) AgBF4
in pyridine containing 0.1 M TBAP. The electrode reactions involving
dissociated AgI and Ag0 are indicated by *.
when scanning beyond the second reduction, a rapid
demetalation results as indicated by the two reversible
processes at E1/2=-1.62 and 0.38 V on the return sweep.
Two metal-centered reductions followed by demetala-
tion are also seen for the other corroles on the cyclic
voltammetry time scale, as shown in Figures 4 and 5 for
(TTCor)Ag and (TtBuPCor)Ag, respectively, both of
which were also investigated by thin-layer spectroelec-
trochemistry. In both cases, the demetalation product is
reversibly reduced or oxidized at potentials similar to
those of related free-base meso-substituted corroles under
the same solution conditions.17 The half-wave potentials
of the demetalation product are given in Table 1, along
with values of E1/2 or Ep for the metal-centered reactions
of the initial compound.
The voltammetric data in Figures 3-5 are consistent
with the metal-centered reductions and coupled demeta-
lation reactions shown in Scheme 1. The same corrole
demetalation product is generated after the first and
second electron additions and is assigned as [(Cor)H2]-
where Cor represents one of the macrocycles in Chart 1.
Ag(I) and Ag(0) redox reactions are also detected in the
3
E1/2=-1.62 V and reversibly oxidized to [(TF5P Cor)H2]
at E1/2=0.38 V.
The initial AgIII/AgII process of (TF5PCor)Ag is rever-
sible at a scan rate of 0.1 V/s, when the negative potential
sweep is terminated prior to the generation of a transient
[(TF5PCor)CorAg(I)]2- species at Ep=-1.42 V. Under
these conditions, there is little to no demetalation on the
cyclic voltammetry time scale (see Figure 3). However,

6886 Inorganic Chemistry, Vol. 48, No. 14, 2009
Stefanelli et al.
Figure 5. (a) Regular and (b) thin-layer cyclic voltammograms of
(TTCor)Ag in pyridine containing 0.1 M TBAP. The electrode reactions
involving free AgI and Ag0 are indicated by *.
Table 1. Half-Wave Potentials (V vs SCE) for Metal-Centered Reductions of
Silver Corroles and Resulting Demetalation Product in Pyridine Containing
0.1 M TBAP (see Scheme 1)
Figure 6. UV-visible spectral changes during the first reduction of
(a) (TF5PCor)Ag, (b) (TtBuPCor)Ag, and (c) (NO2TPCor)Ag in py,
0.1 M TBAP.
initial reductions reactions of demetalated corrole
cpd
AgIII/II AgII/I
oxidation
reduction
-1.62
Scheme 1. Demetalation of Neutral and Reduced Silver Corroles in
Pyridine
(TF5TPCor)Ag
(NO2TPCor)Ag
-0.55 -1.42a
-0.52 -1.14a
0.38
0.23
0.21
-0.03
-0.04
-1.32, -1.78
-1.36, -1.74
-1.92
(NO2tBuPCor)Ag -0.54 -1.17a
(TtBuPCor)Ag
(TTCor)Ag
-0.79 -1.72a
-0.79 -1.70a
-1.90
a Peak potential at a scan rate of 0.10 V/s.
cyclic voltammogram of Figures 4 and 5 and located at
Epc=-0.33 to -0.38 V (reduction) and Epa =-0.08 to
-0.09 V (oxidation). These processes are indicated by an
asterisk in the figures and occur at potentials close to peak
potentials for the reduction and reoxidation of AgBF4 in
pyridine containing 0.1 M TBAP (Figure 4b).
The mechanism shown in Scheme 1 was confirmed by
examining the products of each electrode reaction in a thin-
layer UV-vis cell, and examples of the spectroelectrochemical
data are given in Figures 6 and 7. The first reduction
generates a stable AgII corrole in the case of (TF5PCor)Ag
(Figure 6a) which has bands at 442, 592, and 609 nm,
while (TtBuPCor)Ag and (NO2TPCor)Ag show no evi-
dence of a stable AgII product on the spectroelectrochemical
time scale. For these compounds only the deprotonated
free-base corrole is observed after reduction. Several

Article
Inorganic Chemistry, Vol. 48, No. 14, 2009 6887
Figure 7. UV-visible spectra of the singly and doubled reduced forms of (a) (TF5PCor)Ag and (b) (TF5PCor)H3 in py containing 0.1 M TBAP to give
[(Cor)H2]- and [(Cor)H2]2-
.
Table 2. UV-Visible Spectra of AgIII and AgII Corroles and Demetalation Product in Pyridine (see Scheme 1)
cpd
(Cor)AgIII
[(Cor)AgII]-
demetalation product [(Cor)H2]-
reduced demetalated corrolea [(Cor)H2]2-
(TF5TPCor)Ag 424 566
(NO2TPCor)Ag 436,454 597
442 592 609 436
587
491
451
450
618
440
628
710
861
733
728
b
374,407
641,687
645
644
398,407
431,447
429,449
607,653,674
645
643
b
b
(TtBuPCor)Ag
(TTCor)Ag
428
426
589
563,588
430
428
a Generated by reduction in a thin-layer cell (see Figure 7). b Cannot be obtained on spectroelectrochemistry time scale because of rapid demetalation.
isosbestic points are observed, and the final product has
bands at 430, 451, and 645 nm for [(TtBuPCor)H2]-
(Figure 6b) and 374, 407, 491, 641, and 687 nm for
[(NO2TPCor)H2]- (Figure 6c). These wavelengths are
listed in Table 2.
Further reduction of the deprotonated product at more
negative potentials leads to [(Cor)H2]2- (see Scheme 1) as
evidenced by the spectroelectrochemical data in Figure 7
which compares the spectral reduction product of
(TF5PCor)Ag and (TF5PCor)H3 under the same solution
conditions. As seen in the figure, identical spectra are
obtained after each redox process of the two compounds.
reductant, although the chemical route cannot be used
in the case of 3-NO2 substituted corroles, because of
reduction of the nitro group, with formation of the 3-
NH2 corrole. However, demetalation can be obtained for
the same compound in solution by using a DBU/THF
solvent mixture.
To investigate in greater detail the mechanism for deme-
talation of silver(III) corroles, we have examined the electro-
chemistry of selected Ag(III) corrole complexes and have
elucidated the stepwise Ag(III)/Ag(II) and Ag(II)/Ag(I) re-
actions that occur prior to a rapid demetalation. The use of
electrochemical or chemical demetalation procedures using
DBU/THF can be particularly useful for synthetic purposes
and should remove one of the remaining obstacles that has
slowed down the development of corrole chemistry.
Conclusions
The demetalation of silver(III) corrolates can represent a
useful tool to further develop the synthetic chemistry of other
corrole compounds. However, acidic demetalation condi-
tions inevitably induce an oxidation of the corrole ring, which
is particularly significant in the case of (NO2TPCor)H3. This
oxidation leads to the formation of isocorrole species, one of
which was structurally characterized in the present study to
illustrate that regioselective oxidation occurred at the 5-
position of the corrole ring.
Acknowledgment. The support of Italian MiUR (PRIN
project no. 2007C8RW53) and the Robert A. Welch
Foundation (K.M.K., Grant E-680) are gratefully ac-
knowledged.
Supporting Information Available: CIF file of [(3-NO2)(5-OH)
TTisoCor]H2 and UV-vis spectrum of (NO2TtBuPCor)H3.
This material is available free of charge via the Internet at
http://pubs.acs.org.
The formation of isocorroles can be avoided by using
electrochemical reduction techniques or NaBH4 as a chemical