6-Azahemiporphycene: A new member of the porphyrinoid family

Article abstract of DOI:10.1021/ic9014866

The reaction of 5,10,15-triarylcorrole with 4-amino-4H-1,2,4-triazole provides another example of corroie ring expansion to give the corresponding 6-azahemiporphycene, a novel porphyrin analogue. The facile oxidation of the corroie ring is a required step

Full text of DOI:10.1021/ic9014866

10346 Inorg. Chem. 2009, 48, 10346–10357
DOI: 10.1021/ic9014866
6-Azahemiporphycene: A New Member of the Porphyrinoid Family
Federica Mandoj,† Sara Nardis,† Giuseppe Pomarico,† Manuela Stefanelli,† Luca Schiaffino,†
Gianfranco Ercolani,† Luca Prodi,*,‡ Damiano Genovese,‡ Nelsi Zaccheroni,‡ Frank R. Fronczek,§ Kevin M. Smith,*,§
Xiao Xiao, Jing Shen, Karl M. Kadish,*, and Roberto Paolesse*,†
†Department of Chemical Science and Technologies, University of Rome Tor Vergata, Via della Ricerca
Scientifica 1, 00133 Rome, Italy, ‡Department of Chemistry “G. Ciamician”, Bologna University, via Selmi 2,
40126 Bologna, Italy, §Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803,
and Department of Chemistry, University of Houston, Houston, Texas 77204-5003
Received July 27, 2009
The reaction of 5,10,15-triarylcorrole with 4-amino-4H-1,2,4-triazole provides another example of corrole ring
expansion to give the corresponding 6-azahemiporphycene, a novel porphyrin analogue. The facile oxidation of
the corrole ring is a required step for the ring expansion and for this reason the reaction fails in the case of corroles
bearing meso-phenyl groups carrying electron-withdrawing substituents. Steric requirements also limited the scope of
the reaction, which is not successful in the case of 2,6-disubstituted meso-aryl corroles. The occurrence of an initial
oxidation is further supported by formation of the 6-azahemiporphycene derivative when the reaction is carried out
1
under the same conditions, using a 5- or a 10-isocorrole as starting material. H NMR spectra and X-ray crystal
characterization of 6-azahemiporphycene evidenced the presence of an intramolecular N;H N hydrogen bond in
3 3 3
the inner core of the macrocycle, while photophysical characterization confirmed the aromatic character of the novel
macrocycle, showing an intense Soret-like band around 410 nm in the absorption spectrum. The fluorescence
emission is very modest, and 6-azahemiporphycene showed higher photostability than the corresponding corrole
species. Different metal complexes of 6-azahemiporphycene were prepared following synthetic protocols usually
exploited for the preparation of metalloporphyrins, demonstrating good coordination properties for the macrocycle.
Both the free-base and metal derivatives were characterized by cyclic voltammetry and spectroelectrochemistry in
dichloromethane and benzonitrile. To further detail the behavior of this novel macrocycle, density functional theory
(DFT) calculations were carried out on the basic structure of 6-azahemiporphycene with the aim of assessing
aromaticity and tautomerism, as well as calculating its stability with respect to the 5-aza isomer.
Introduction
modifications. In this regard, Nature is a flawless teacher;
iron porphyrins are used for oxygen binding and activation,
while magnesium chlorins, which possess related macro-
cycles, are catalysts in sunlight energy conversion. Starting
from a similar molecular framework, a change of the central
metal ion and/or saturation of a peripheral double bond can
lead to completely different but equally vital functions.
Developing upon this approach, several pyrrolic macro-
cycles other than porphyrins have been prepared and fully
Porphyrins are ubiquitous in Nature where they perform
functions essential for life. This important role has been the
springboard for a huge number of studies carried out on these
macrocycles during the last two centuries.1 Researchers have
focused on one hand on better understanding mechanisms of
the biological systems and on the other in exploiting this
knowledge to create synthetic platforms that are able to
mimic the almost unique chemistry and physics of the natural
systems. Particular attention in these efforts has been devoted
to the close relationships that exist between the structures of
the macrocycles and their inherent properties, with the
consequent possibility of tuning both by subtle synthetic
(2) Drain, C. M.; Hupp, J. T.; Suslick, K. S.; Wasielewski, M. R.; Chen,
X. J. Porphyrins Phthalocyanines 2002, 6, 243–258.
(3) (a) Sessler, J. L.; Weghorn, S. J. Expanded, Contracted and Isomeric
Porphyrins; Pergamon: Oxford, 1997 (b) Jasat, A.; Dolphin, D. Chem. Rev.
1997, 97, 2267–2340. (c) Sessler, J. L.; Gebauer, A.; Vogel, E. In The Porphyrin
Handbook; Kadish, K. M., Smith, K., Guilard, R., Eds.; Academic: San Diego,
CA, 2000; Vol. 2, pp 1-54 (d) Sessler, J. L.; Gebauer, A.; Weghorn, S. J. In The
Porphyrin Handbook; Kadish, K. M., Smith, K., Guilard, R., Eds.; Academic:
San Diego, CA, 2000; Vol. 2, pp 55-124 (e) Sessler, J. L.; Seidel, D. Angew.
Chem., Int. Ed. 2003, 42, 5134–5175. (f) Misra, R.; Chandrashekar, T. K. Acc.
Chem. Res. 2008, 41, 265–279. (g) Callot, H. J.; Rohrer, A.; Tschamber, Th. New
J. Chem. 1995, 19, 155–159.
*To whom correspondence should be addressed. E-mail: roberto.paolesse@
uniroma2.it (R.P.), kkadish@uh.edu (K.M.K.), kmsmith@lsu.edu (K.M.S.),
luca.prodi@unibo.it (L.P.).
(1) (a) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R.
Eds.; Academic Press: San Diego, CA, 2000; Vols. 1-10 (b) The Porphyrin
Handbook; Kadish, K. M.; Smith, K. M.; Guilard, R. Eds.; Academic Press: San
Diego, CA, 2003; Vols. 11-20.
r
pubs.acs.org/IC
Published on Web 10/01/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10347
characterized in recent years, with the aim both to study in
detail structure-property relationships and to tune them for
a particular targeted application.2 This effort has led to
formation of the so-called “porphyrinoid” family, which
comprises all polypyrrolic macrocycles having molecular
frameworks similar to the parent porphyrin macrocycle.3
In this regard, the porphyrin family can be schematically
divided into three groups: (1) contracted porphyrins, (2)
isomeric porphyrins, and (3) expanded porphyrins, the exact
assignment depending upon the structural variation present
in the macrocyclic skeleton.3a
The above three types of macrocycles have been prepared
by a variety of synthetic routes, which in several cases have
been developed by variations of synthetic procedures already
optimized for porphyrins. Progress in the synthetic organic
chemistry of porphyrins has also stimulated extensions of the
porphyrinoid family to give a variety of new macrocycles.4
In a new synthetic approach for the preparation of por-
phyrinoids, it was demonstrated that the macrocyclic skele-
ton is not a rigid block and can be modified even after its
formation.5 It is important to note that this result was not
unprecedented since examples were reported in the literature
many years before, leading to porphyrins from nickel tetra-
dehydrocorrin salts6 or homoporphyrins from a ring expan-
sion of porphyrins.7 Only recently, however, it has become
evident that these interconversions are not occasional,5 but
rather are quite general and can be exploited for the prepara-
tion of macrocycles that may be difficult to prepare by direct
or previously designed routes.
One group of well-characterized porphyrinoids includes
the corroles or contracted porphyrins, which have assumed in
the past few years a protagonist role.8 The behavior of these
macrocycles can be quite different from the related por-
phyrins with the same metal ions and this has made the
corroles especially promising for specific applications.9 For
example, one characteristic of the corroles is that the central
metal ion is often stabilized in a formal oxidation state which
is higher than that of the corresponding porphyrins, and this
property may be related to the fact that some corroles are
superior to porphyrins as catalysts in specific reactions.10
The corrole is also a protagonist in switching between
different macrocycle structures, through ring-opening and/or
ring-closure reaction sequences. For example, it has been
reported that corroles can undergo a ring-expansion to por-
phyrins11 and that they can also be formed by ring-contraction
of a porphyrin.12 However, the versatility of the corrole is not
confined to the direct pyrrole-pyrrole link formation or
expansion demonstrated by the fact that corroles can be
transformed into hemiporphycenes by reaction with CI4 where
the ring-expansion reaction occurs at one meso carbon
bridge.13
Most recently we discovered an additional example of
corrole ring-expansion that leads to the formation of
6-azahemiporphycene upon reaction of the corrole with
4-amino-4H-1,2,4-triazole.14 As a follow up to this initial
communication, we now present a detailed characterization
of 6-azahemiporphycenes with different central metal ions,
together with the electrochemistry of these compounds and a
computational study with the aim of assessing aromaticity
and tautomerism of these new macrocycles. Although the
synthetic chemistry of corroles was greatly improved after
initial reports for the preparation of meso triarylcorroles, the
reactions described in the present paper will open up a facile
route to the preparation of these novel porphyrinoids.
Experimental Section
Reagents and solvents (Sigma-Aldrich, Fluka and Carlo
Erba Reagenti) were of synthetic grade and used without
further purification. Silica gel 60 (70-230 mesh) and neutral
alumina (Brockmann grade III) were used for chromato-
graphy. Chloroform Uvasol was obtained from Merck; triflic
acid and tributylamine were obtained from Aldrich. Benzo-
nitrile (PhCN) was purchased from Aldrich Chemical Co.
and distilled over P2O5 under vacuum prior to use. Tetra-n-
butylammonium perchlorate (TBAP, 99%) was purchased
from Fluka Chemical Co. and used as received.
1H NMR spectra were recorded on a Bruker AV300 (300
MHz) spectrometer. Chemical shifts are given in parts per
million relative to tetramethylsilane (TMS). UV-vis spectra
were measured on a Cary 50 spectrophotometer. Mass spec-
tra were recorded on a VGQuattro spectrometer in the
positive-ion mode, using m-nitrobenzyl alcohol (NBA,
Aldrich) as a matrix (FAB), or on a Voyager DE STR
Biospectrometry workstation in the positive mode, using R-
cyano-4-hydroxycinnamic acid as a matrix (MALDI).
Electronic spectra were measured in chloroform using a
Perkin-Elmer Lambda 40 spectrophotometer. Corrected
emission and excitation spectra (450 W Xe lamp) and excited
state lifetimes were obtained with a modular UV-vis-NIR
Spectrofluorimeter Edinbourg, equipped with a single
photon counting apparatus. Corrections for instrumental
response, inner filter effects, and phototube sensitivity were
2þ
performed.15 Ru(bpy)3 in aerated H2O (Φ =0.028) was
used as a standard for florescence quantum yields. Emission
spectra in a rigid, transparent 2-methylcyclohexane matrix at
77 K were recorded using quartz tubes immersed in a quartz
Dewar filled with liquid N2. A correction for a difference in
the refraction index was introduced when necessary.
Diffraction data were collected on a Nonius KappaCCD
diffractometer equipped with graphite-monochromated Mo
KR radiation (λ =0.71073 A) and an Oxford Cryostream
low-temperature device.
(4) (a) Horn, S.; Dahms, K.; Senge, M. O. J. Porphyrins Phthalocyanines
2008, 12, 1053–1077. (b) Jux, N. Angew. Chem., Int. Ed. 2009, 48, 4284–4286.
(5) (a) Callot, H. Dalton Trans. 2008, 6346–6357. (b) Senge, M. O.;
Sergeeva, N. N. Angew. Chem., Int. Ed. 2006, 45, 7492–7495.
(6) Grigg, R.; Johnson, A. W.; Richardson, K.; Smith, M. J. J. Chem. Soc.
C 1970, 1289–1295.
(7) (a) Callot, H. J.; Tschamber, T. Tetrahedron Lett. 1974, 15, 3155–3158.
(b) Callot, H. J.; Tschamber, T. J. Am. Chem. Soc. 1975, 97, 6175–6178.
(8) (a) Nardis, S.; Monti, D.; Paolesse, R. Mini-Rev. Org. Chem. 2005, 2,
546–564. (b) Paolesse, R. Synlett 2008, 2215–2230. (c) Gryko, D. T.
J. Porphyrins Phthalocyanines 2008, 12, 906–917.
(9) (a) Aviv, I.; Gross, Z. Chem. Commun. 2007, 1987–1999. (b) Aviv-
Harel, I.; Gross, Z. Chem.;Eur. J. 2009, 15, 8382–8394. (c) Flamigni, L.;
Gryko, D. T. Chem. Soc. Rev. 2009, 38, 1635–1646. (d) Lvova, L.; Di Natale, C.;
D'Amico, A.; Paolesse, R. J. Porphyrins Phthalocyanines, 2009, in press.
(10) Simkhovich, L.; Mahammed, A.; Goldberg, I.; Gross, Z. Chem.;
Eur. J. 2001, 7, 1041–1055.
Cyclic voltammetry was carried out with an EG&G model
173 potentiostat-galvanostat. A homemade three-electrode
electrochemistry cell was used and consisted of a glassy
(13) Paolesse, R.; Nardis, S.; Stefanelli, M.; Fronczek, F. R.; Vicente,
M. G. H. Angew. Chem., Int. Ed. 2005, 44, 3047–3050.
(11) Gros, C. P.; Barbe, J.-M.; Espinosa, E.; Guilard, R. Angew. Chem.,
Int. Ed. 2006, 45, 5642–5645.
(12) Kin Tse, M.; Zhang, Z.; Mak, T. C. W.; Shing Chan, K. Chem.
Commun. 1998, 1199–1200.
(14) Mandoj, F.; Stefanelli, M.; Nardis, S.; Mastroianni, M.; Fronczek,
F. R.; Smith, K. M.; Paolesse, R. Chem. Commun. 2009, 1580–1582.
(15) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of
Photochemistry, 3rd ed.; CRC Taylor & Francis: Boca Raton, FL, 2006.

10348 Inorganic Chemistry, Vol. 48, No. 21, 2009
Mandoj et al.
Chart 1
(1.32 g, 8.3 mmol) was degassed by bubbling nitrogen for
10 min, and trifluoroacetic acid (16 μL, 0.2 mmol) was then
added. The mixture was stirred for 15 min at room temperature
(RT) and then diluted with CH2Cl2 (25 mL). The reaction
mixture was left at RT for 1 h; chloranil (1.74 g, 7.01 mmol)
was added, and the mixture was stirred for another 10 min. The
solvent and the residual pyrrole were removed under reduced
pressure, the crude mixture dissolved in CH2Cl2 and then
chromatographed on silica gel using a mixture of CH2Cl2/
CHCl3 (1:1) as eluent. The red-purple fraction was recrystallized
from CH2Cl2/hexane (yield 13%; 254 mg).
Spectroscopic data for 8. Found: C, 72.9; H, 5.5; N, 7.7
C43H38O6N4 requires C, 73.1; H, 5.4; N, 7.9%. UV-vis: λmax
-
(CHCl3), nm 408 (log ε, 4.94), 566 (4.13), 604 (3.97), and 632
1
(3.63); H NMR δ (CDCl3, J [Hz]) 8.80 ppm (2 H, d, J 4.0,
β-pyrrole), 8.51 (2 H, d, J 4.6, β-pyrrole), 8.33 (2 H, d, J 4.7,
β-pyrrole), 8.30 (2 H, d, J 4.1, β-pyrrole), 7.66 (3 H, t, J 8.4,
phenyl), 6.98 (6 H, m, phenyl), 3.57 (12 H, s, -OCH3), 3.54 (6 H,
s, -OCH3); MS (MALDI): m/z 707 (Mþ).
General Procedure for Preparation of 6-Azahemiporphycene
Derivatives. Route A. In a 100 mL round-bottomed flask,
with a stirring bar, the corrole (0.1 mmol) was dissolved in
22 mL of toluene/abs. ethanol mixture (10:1); NaOH
(0.5 mmol) and 4-amino-4H-1,2,4-triazole (1.2 mmol) were
added, and the mixture was refluxed for about 2 h, monitoring
the progress of the reaction by UV-vis spectroscopy and TLC.
After cooling to RT, the solvent was evaporated under
vacuum, and the crude product dissolved in 50 mL of chloro-
form and washed three times with water. The organic phase
was dried over sodium sulfate and the product purified by
chromatography on silica gel, first using CH2Cl2 to elute traces
of unreacted corrole. The other two fractions were then eluted
using a CH2Cl2/CHCl3 (2:1) mixture. The second red eluted
fraction corresponded to 6-azahemiporphycene, while the
third blue-green fraction was characterized as an open-chain
tetrapyrrole.
Route B. In a 50 mL round-bottomed flask, with a stirring bar,
5,10,15-tris-(4-methylphenyl)-5-hydroxy-isocorrole (a) [or 5,10,
15-tris-(4-methylphenyl)-10-hydroxy-isocorrole (b)] (30 mg, 0.05
mmol) was dissolved in 11 mL of a toluene/ethanol absolute
mixture (10:1); NaOH (10 mg, 0.25 mmol) and 4-amino-4H-1,2,4-
triazole (50.4 mg, 0.6 mmol) were then added and the mixture
refluxed for about 30 min as the progress of the reaction was
monitored by UV-vis spectroscopy and TLC. The reaction
workup was carried out as described above. Yield: 54%, 16 mg
(a), 60%, 17 mg (b).
6-Aza-5,11,16-triphenylhemiporphycene 9. The red solid was
recrystallized from CH2Cl2/methanol (yield 48%, 26 mg).
Found: C, 82.2; H, 4.8; N, 12.9 C37H25N5 requires C, 82.3; H,
4.7; N, 13.0%. UV-vis: λmax(CHCl3), nm 412 (log ε, 5.12) and
571 (4.20); 1H NMR δ (CDCl3, J [Hz]) 8.99 ppm (1 H, d, J 4.6,
β-pyrrole), 8.86 (1 H, d, J 4.4, β-pyrrole), 8.72 (1 H, d, J 4.8,
β-pyrrole), 8.62 (1 H, d, J 4.3, β-pyrrole), 8.45 (4 H, m, β-pyrrole
þ phenyl), 8.31 (1 H, d, J 4.6, β-pyrrole), 8.16 (2 H, m, phenyl),
8.08 (1 H, m, β-pyrrole), 8.04 (2 H, m, phenyl), 7.73 (9 H, m,
phenyl), 4.06 (1 H, s, NH), 2.94 (1 H, s, NH); MS (MALDI): m/z
540 (Mþ).
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-visible spectroelectrochemical experiments were per-
formed with an optically transparent platinum thin-layer
electrode of the type described in the literature.16 Potentials
were applied with an EG&G Model 173 potentiostat-galvano-
stat. Time-resolved UV-visible spectra were recorded with a
Hewlett-Packard Model 8453 diode array rapid-scanning
spectrophotometer.
6-Aza-5,11,16-tris-(4-methylphenyl)hemiporphycene 10. The
red solid was recrystallized from CH2Cl2/hexane (yield 51%;
30 mg). Found: C, 82.4; H, 5.3; N, 11.9 C40H31N5 requires C,
82.6; H, 5.4; N, 12.0%. UV-vis: λmax(CHCl3), nm 416 (log ε,
5.27) and 571 (4.33); 1H NMR δ (CDCl3, J [Hz]) 8.93 ppm (1 H,
d, J 4.3, β-pyrrole), 8.80 (1 H, d, J 4.5, β-pyrrole), 8.68 (1 H, d,
J 4.1, β-pyrrole), 8.59 (1 H, d, J 4.5, β-pyrrole), 8.37 (4 H, m,
β-pyrrole þ phenyl), 8.28 (1 H, d, J 4.5, β-pyrrole), 8.06 (3 H, m,
β-pyrrole þ phenyl), 7.91 (2 H, d, J 7.8, phenyl), 7.59 (4 H, m,
phenyl), 7.49 (2 H, d, J 7.8, phenyl), 4.24 (1 H, s, NH), 3.13 (1 H,
s, NH), 2.67 (6 H, s, -CH3), 2.62 (3 H, s, -CH3); MS (MALDI):
m/z 582 (Mþ).
Corroles 1-7 (see Chart 1) were prepared following litera-
ture methods.17
5,10,15-Tris-(2,6-dimethoxyphenyl)corrole 8. A solution of
pyrrole (5.6 mL, 80 mmol) and 2,6-dimethoxybenzaldehyde
(16) Lin, X. Q.; Kadish, K. M. Anal. Chem. 1985, 57, 1498–1501.
(17) (a) Paolesse, R.; Nardis, S.; Sagone, F.; Khoury, R. G. J. Org. Chem.
2001, 66, 550–556. (b) Paolesse, R.; Marini, A.; Nardis, S.; Froiio, A.; Mandoj, F.;
Nurco, D. J.; Prodi, L.; Montalti, M.; Smith, K. M. J. Porphyrins Phthalocya-
nines 2003, 7, 25–36.

Article
6-Aza-5,11,16-tris-(4-methoxyphenyl)hemiporphycene 11. The
Inorganic Chemistry, Vol. 48, No. 21, 2009 10349
[6-Aza-5,11,16-tris(4-tert-butylphenyl)hemiporphycenato]Ni-
ckel 16. Purification of the crude reaction mixture was per-
formed on a silica gel column eluting with CH2Cl2. The green
fraction was recrystallized from CH2Cl2/methanol. Yield 74%,
17 mg. Found: C, 77.1; H, 6.3; N, 8.9 C49H49N5Ni requires C,
76.8; H, 6.4; N, 9.1%. UV-vis: λmax(CH2Cl2), nm 412 (log ε,
4.84), 607 (3.85) and 648 (3.93); 1H NMR δ (CDCl3, J [Hz]) 8.85
ppm (1 H, d, J 4.6, β-pyrrole), 8.70 (2 H, dd, β-pyrrole), 8.60
(1 H, d, J 4.7, β-pyrrole), 8.57 (1 H, d, J 4.9, β-pyrrole), 8.53 (1 H,
d, J 4.7, β-pyrrole), 8.49 (1 H, d, J 4.9, β-pyrrole), 8.24 (3 H, m,
β-pyrrole þ phenyl), 7.96 (2 H, d, J 8.1, phenyl), 7.87 (2 H, d,
J 8.1, phenyl), 7.71 (6 H, m, phenyl), 1.58 (9 H, s, tert-butyl), 1.57
(9 H, s, tert-butyl), 1.51 ppm (9 H, s, tert-butyl); MS (MALDI):
m/z 767 (Mþ).
red solid was recrystallized from CH2Cl2/methanol (yield
56%; 35 mg). Found: C, 76.1; H, 5.1; N, 10.9. C40H31N5O3
requires C, 76.3; H, 5.0; N, 11.1%. UV-vis: λmax(CHCl3), nm
424 (log ε, 5.00) and 574 (4.05); 1H NMR δ (CDCl3, J [Hz]) 8.88
ppm (1 H, d, J 4.3, β-pyrrole), 8.76 (1 H, d, J 4.4, β-pyrrole), 8.62
(1 H, d, J 4.3, β-pyrrole), 8.55 (1 H, d, J 4.4, β-pyrrole), 8.44 (2 H,
d, J 8.6, phenyl), 8.35 (1 H, d, J 3.3, β-pyrrole), 8.29 (1 H, d, J 4.5,
β-pyrrole), 8.22 (1 H, d, J 4.5, β-pyrrole), 8.09 (2 H, d, J 8.5,
phenyl), 8.04 (1 H, m, β-pyrrole), 7.93 (2 H, d, J 8.4, phenyl),
7.32 (4 H, d, J 8.3, phenyl), 7.23 (2 H, d, J 8.4, phenyl), 4.53 (1 H,
s, NH), 4.07 (6 H, s, -OCH3), 4.04 (3 H, s, -OCH3), 3.81 (1 H, s,
NH); MS (MALDI): m/z 630 (Mþ).
Crystal data for 11: dark red plate, monoclinic, space group C2/
c, a =37.860(12), b =10.166(2), c =17.081(5) A, β =94.528(6)°,
[6-Aza-5,11,16-tris(4-tert-butylphenyl)hemiporphycenato]Co-
pper 17. Purification of the crude reaction mixture was per-
formed on a neutral alumina column eluting with CH2Cl2/
hexane (2:1). The green fraction was recrystallized from
CH2Cl2/methanol. Yield 65%, 15 mg. Found: C, 76.5; H, 6.6;
N, 8.8 C49H49CuN5 requires C, 76.3; H, 6.4; N, 9.1%. UV-vis:
V =109.082(12) A3, Z =8, Dcalc =1.346 g cm-3, μ =0.087 mm-1
,
T =90.0(5) K, 22317 reflections collected with θmax < 25.5°, 5698
independent reflections (Rint =0.072) which were used in all the
calculations, 3334 data with I >2σ(I). Final residuals (for 442
parameters) were R1 [I >2σ(I)] =0.059, wR2 (all data) =0.140,
CCDC 736599.
λ
max(CH2Cl2), nm 431 (log ε, 5.01) and 615 (4.16); MS
(MALDI): m/z 771 (Mþ).
6-Aza-5,11,16-tris(4-tert-butylphenyl)hemiporphycene 12. The
spectroscopic data are reported in a previous paper.14
[6-Aza-5,11,16-tris(4-tert-butylphenyl)hemiporphycenato]Zi-
nc 18. Purification of the crude reaction mixture was performed
on a silica gel column eluting first with CH2Cl2 and finally with
CHCl3. The green fraction was recrystallized from CH2Cl2/
methanol. Yield 86%, 20 mg. Found: C, 75.9; H, 6.5; N, 9.3
General Procedure for Preparation of 6-Aza-5,11,16-tris(4-
tert-butylphenyl)hemiporphycene Metal Complexes. Mn(III)
and Fe(III) Complexes. In a 50 mL round-bottomed flask,
with a stirring bar, 6-aza-5,11,16-tris(4-tert-butylphenyl)hemipor-
phycene (20 mg, 0.03 mmol) was dissolved in 10 mL of DMF and a
3-fold excess of MnCl2 or FeCl2 salt was added. The mixture was
heated to reflux and the progress of the reaction followed by
UV-vis spectroscopy and TLC. After disappearance of absorption
peaks of the starting material, the solvent was evaporated under
vacuum and the crude mixture purified by chromatography on
neutral alumina using CHCl3/hexane (1:1) as eluent.
C49H49N5Zn requires C, 76.1; H, 6.4; N, 9.1%. UV-vis: λmax
-
(CH2Cl2), nm 436 (log ε, 4.99), 535 (3.69), 570 (3.78), and 618
1
(4.24); H NMR δ (CDCl3, J [Hz]) 8.99 ppm (1 H, d, J 3.8,
β-pyrrole), 8.91 (1 H, d, J 4.0, β-pyrrole), 8.68 (2 H, m,
β-pyrrole), 8.54 (1 H, d, J 4.2, β-pyrrole), 8.47 (1 H, m, β-
pyrrole), 8.36 (1 H, d, J 4.3, β-pyrrole), 8.11 (4 H, d, J 7.6,
phenyl), 7.80 (4 H, d, J 7.5, phenyl), 7.08 (5 H, br, β-pyrrole þ
phenyl), 1.71 (9 H, s, tert-butyl), 1.62 (9 H, s, tert-butyl), 1.30
ppm (9 H, s, tert-butyl); MS (MALDI): m/z 773 (Mþ).
M(II) Complexes where M =Co, Ni, Cu, and Zn. In a 50 mL
round-bottomed flask, with a stirring bar, 6-aza-5,11,16-tris
(4-tert-butylphenyl)hemiporphycene (20 mg, 0.03 mmol) was
dissolved in 10 mL of CHCl3, and a saturated solution of
M(OAc)2 salt in methanol was added. The mixture was heated
to reflux, and the progress of the reaction followed by UV-vis
spectroscopy and TLC. After disappearance of absorption
peaks for the starting material, the solvent was evaporated
under vacuum, and the crude mixture purified by column
chromatography.
Spectroscopic data of compounds 19-22 are reported in
Table 1. Crystal data for 20: dark red parallelepiped, monoclinic,
space group C2/c, a =28.711(2), b =13.3259(10), c =18.9309(15)
A, β =90.199(5)°, V =7242.9(9) A 3, Z =8, Dcalc =1.321 g cm-3
,
μ =2.621 mm-1, T =90.0(5) K, 49650 reflections collected with
max < 69.0°, 6591 independent reflections (Rint =0.041) which
θ
were used in all the calculations, 5561 data with I >2σ( I). Final
residuals(for 604 parameters) wereR1 [I >2σ(I)]=0.035, wR2 (all
data) =0.093. Two of the three MePh groups are disordered, each
into two conformations. The chloroform solvent molecule is also
disordered into two orientations, sharing a single Cl atom.
Chloro[6-Aza-5,11,16-tris(4-tert-butylphenyl)hemiporphycen-
ato]Manganese 13. Purification of the crude reaction mixture
was performed on a neutral alumina column, eluting first with
CH2Cl2 and then with CHCl3. The green fraction was recrys-
tallized from CH2Cl2/hexane. Yield 73%, 18 mg. Found: C,
73.4; H, 6.1; N, 8.7 C49H49ClMnN5 requires C, 73.7; H, 6.2; N,
8.8%. UV-vis: λmax(CH2Cl2), nm 440 sh, 473 (log ε, 4.53), 542
(3.77) and 636 (3.84). MS (MALDI): m/z 798 (Mþ).
Results and Discussion
Synthesis. We previously described how 3-NO2-6-aza-
hemiporphycene was serendipitously obtained during our
studies involving the peripheral functionalization of
5,10,15-triarylcorroles.14 Our intention was to prepare
the 2-amino-3-nitro substituted corrole by reacting the
Ag(III) 3-nitrocorrole complex with 4-amino-4H-1,2,4-
triazole through vicarious substitution directed by the
nitro group.18 However under the basic reaction condi-
tions utilized, the corrole was readily demetalated,19 and
the resulting free-base corrole macrocycle reacted with
the triazole leading to 6-azahemiporphycene.14 The nitro
substituents had no effect in either promoting or orienting
the reaction, and we obtained the same product by
Chloro[6-Aza-5,11,16-tris(4-tert-butylphenyl)hemiporphycen-
ato]Iron 14. Purification of the crude reaction mixture was
performed on a neutral alumina column eluting with CH2Cl2/
hexane (1:1). The green fraction was washed with 1 M HCl and
recrystallized from methanol. Yield 61%, 15 mg. Found: C,
73.9; H, 6.1; N, 8.7 C49H49ClFeN5 requires C, 73.6; H, 6.2; N,
8.8%. UV-vis: λmax(CH2Cl2), nm 412 (log ε, 4.68) and 577
(3.63). MS (MALDI): m/z 799 (Mþ).
[6-Aza-5,11,16-tris(4-tert-butylphenyl)hemiporphycenato]Co-
balt 15. Purification of the crude reaction mixture was per-
formed on a neutral alumina column eluting with CH2Cl2/
hexane (1:1). The green fraction was recrystallized from
CH2Cl2/hexane. Yield 68%, 16 mg. Found: C, 76.9; H, 6.2; N,
9.2 C49H49CoN5 requires C, 76.7; H, 6.4; N, 9.1%. UV-vis:
(18) Richeter, S.; Hadj-Aıssa, A.; Lee, A.; Leclercq, D. Chem. Commun.
¨
2007, 2148–2150.
(19) Stefanelli, M.; Shen, J.; Zhu, W.; Mastroianni, M.; Mandoj, F.;
Nardis, S.; Ou, Z.; Kadish, K. M.; Fronczek, F. R.; Smith, K. M.; Paolesse,
R. Inorg. Chem. 2009, 48, 6879–6887.
λ
max(CH2Cl2), nm 413 (log ε, 4.97) and 584 (3.83); MS
(MALDI): m/z 767 (Mþ).

10350 Inorganic Chemistry, Vol. 48, No. 21, 2009
Mandoj et al.
Table 1. Spectroscopic Data of Open Chain Tetrapyrroles
cpd
1H NMRa
UV-visb
MSc
elem. analysis (%)
19
10.87 (2H, br s, NH), 10.01 (1H, br s, NH), 7.80 (2H, d,
J 7.2, ph), 7.46 (13H, m, ph), 7.09 (2H, m, β-py),
6.80 (1H, m, β-py), 6.69 (1H, m, β-py), 6.45 (2H, d, J
4.3, β-py), 6.37 (1H, d, J 4.2, β-py), 6.33 (1H, d, J
5.6, β-py)
386 (4.55), 588 (4.28)
559 [M]þ
found: C, 79.9; H, 4.5; N,
C37H26O2N4
9.9;
requires C, 79.6; H,
4.7; N, 10.0
20
21
11.21 (2H, br s, NH), 10.36 (1H, br s, NH), 7.65 (2H, d,
J 7.9, ph), 7.37 (2H, d, J 7.9, ph), 7.28 (2H, d, J 7.5,
ph), 7.16 (8H, m, β-py þ ph), 6.73 (1H, m, β-py),
6.64 (3H, m, β-py), 6.34 (2H, m, β-py), 2.47 (3H, s,
-CH3), 2.42 (3H, s, -CH3), 2.40 (3H, s, -CH3)
392 (4.67), 592 (4.39)
409 (4.43), 595 (4.12)
601 [M]þ
found: C, 79.9; H, 5.5; N,
9.1;
C40H32O2N4
requires C, 80.0; H,
5.4; N, 9.3
10.76 (2H, br s, NH), 9.90 (1H, br s, NH), 7.81 (2H, d,
J 8.5, ph), 7.41 (2H, d, J 8.5, ph), 7.32 (2H, m, ph),
7.10 (2H, m, ph), 6.98 (4H, m, β-py þ ph), 6.88 (2H,
d, J 8.7, ph), 6.78 (1H, m, β-py), 6.69 (3H, m, β-py),
6.41 (1H, d, J 4.2, β-py), 6.31 (1H, d, J 5.4, β-py),
3.92 (3H, s, -OCH3), 3.87 (3H, s, -OCH3), 3.83 (3H,
s, -OCH3)
649 [M]þ
found: C, 74.3; H, 5.2; N,
C40H32O5N4
8.4;
requires C, 74.1; H,
5.0; N, 8.6
22
10.93 (2H, br s, NH), 10.15 (1H, br s, NH), 7.72 (2H, d,
J 8.0, ph), 7.41 (10H, m, ph), 7.15 (2H, d, J 5.5, β-
py), 6.79 (1H, m, β-py), 6.74 (2H, d, J 4.0, β-py),
6.69 (1H, m, β-py), 6.41 (1H, d, J 4.3, β-py), 6.30
(1H, d, J 5.4, β-py), 1.40 (9H, s, - tert-butyl), 1.35
(18H, s, tert-butyl)
394 (4.62), 598 (4.33)
727 [M]þ
found: C, 80.9; H, 6.7; N,
7.8;
C49H50O2N4
requires C, 81.0; H,
6.9; N, 7.7
a CDCl3, 300 MHz, δ, ppm. b CH2Cl2 λmax, nm (log ε). c FAB (m/z)
reacting the unsubstituted free-base corrole under the
same conditions. This result led us to investigate the scope
of the reaction, utilizing in this case, under the same
reaction conditions, the free base 5,10,15-triarylcorroles
shown in Chart 1.
The reaction was successful in the case of corroles 1-4,
to give 6-azahemiporphycenes 9-12 with yields varying
in a range of 42-58%. The reaction is regioselective and
only the 6-azahemiporphycene isomer was obtained as
the ring-expansion product. Spectral characterization
of the 6-azahemiporphycenes confirmed the aromatic
character of these macrocycles.
The lower symmetry of 6-azahemiporphycene com-
pared with porphyrin and corrole is reflected in the H
1
NMR spectrum by the β-pyrrole hydrogen resonances
that are located in the range of 9.0-8.3 ppm. These
resonances indicate the existence of a diatropic ring
current effect, typical of an aromatic macrocycle,
although the inner-core hydrogen atoms are present as
broad singlets in the range of 4.5-3.0 ppm, respectively;
these are significantly shifted downfield from those of
porphyrins, corroles, and hemiporphycenes.3g A similar
feature has been observed in the case of porphycenes3c
Figure 1. Molecular structure of 11.
which has corresponding values of 2.573(3) A and
129(2)°. The new CdN bond is somewhat localized to
the phenyl-carrying C atom, having a length 1.310(3) A,
while the other C-N bonds are longer, in the range
1.332(3)-1.399(3) A. The 24-atom 6-azahemiporphycene
core is fairly planar, exhibiting a mean deviation of only
0.047 A from its mean plane. The five N atoms are even
more closely coplanar, with a mean deviation of only
0.021 A from their best plane. This is in contrast to the
t-butyl analogue 12, which has a less planar core, with the
additional N atom lying 0.160 A from the best plane of its
four normal corrole N atoms.14
It is interesting to note that the expansion reaction
was not successful in the case of corroles 5-8. In the case
of 5 and 6, the failure of ring-expansion supports our
initial hypothesis that oxidation of the starting corrole
ring is a required step for ring-expansion to give the
azahemiporphycene macrocycle. When the corrole bears
and can be confidently attributed to N-H N intramo-
3 3 3
lecular hydrogen bonding and not to a reduced ring
current effect due to a lower aromatic character of the
macrocycle.
This hypothesis is corroborated by the X-ray crystal
structure of 11, shown in Figure 1. The N-H hydrogen
atoms are localized on the N atoms shown, rather than
being disordered over the four pyrroles. Insertion of the
additional N atom into the ring system leads to a seven-
membered ring intramolecular hydrogen bond which has
both a longer N N distance, 2.843(3) A, and a larger
3 3 3
N;H N angle, 146(2)°, as compared with the “nor-
3 3 3
mal” six-membered ring on the other side of the molecule

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10351
Figure 3. Molecular structures of 23 and 24 isocorroles.
linked pyrroles (C-C range 1.385(2)-1.406(2) A; C-N
range 1.353(2)-1.380(2) A) than in the other two (C-C
range 1.342(2)-1.468(2) A; C-N range 1.334(2)-1.400-
(2) A).
The formation of 20 confirms our previous finding21
that in the case of 5,10,15-triarylcorroles the site where
the ring-opening occurs is the 5-position and not the
direct pyrrole-pyrrole link, as observed in the case of
10-phenyl-β-alkylcorroles.22
It is worth mentioning that 20 is not an intermediate of
the reaction, because any attempt to convert the open
chain tetrapyrrole into the corresponding 6-azahemipor-
phycene species failed; the oxidative ring-opening is
competitive with the corrole ring expansion to 6-aza-
hemiporphycene and the total yield of the reaction de-
pends upon the balance among these competing reaction
pathways. It is interesting to note that in the case of
corroles 5-8, which cannot afford 6-azahemiporphy-
cene, this linear tetrapyrrole species is not observed,
further confirming that ring oxidation is the necessary
step to initiate the corrole conversion.
To further support the hypothesis that oxidation is a
necessary preliminary step for ring expansion, we decided
to react the isocorroles 23 and 24 (Figure 3), obtained by
DDQ oxidation,20 under the same reaction conditions. It
is interesting to note that in both cases the reaction
product was the corresponding 6-azahemiporphycene
species 10.
According to these experimental results we can hypo-
thesize a plausible reaction pathway for the 6-azahemi-
porphycene formation, reported in Scheme 1. The oxida-
tion of corrole to the corresponding radical species is the
first step of the reaction: this derivative can be then
attacked by triazole to give an isocorrole-like species to
give the final 6-azahemiporphycene, or it can be further
oxidized leading to the open chain tetrapyrrole. The facile
conversion of isocorrole species to corrole has been
already reported in the literature,17a and it can explain
the results obtained when isocorrole (like 23 or 24) is the
starting compound; the conversion to the corrole radical
Figure 2. Molecular structure of 20.
electron-withdrawing substituents at the meso positions,
as in the case of 5 and 6, the oxidation is prevented and
this leads to a failure of the reaction with the starting
material being recovered unaltered. In the case of corroles
7 and 8, the steric hindrance of the 2,6-substituents
prevents attack of the triazole to initiate the ring expan-
sion, assuming that the meso-position is the site of the
nucleophilic attack as observed when isocorroles are
formed by DDQ oxidation of meso-triarylcorroles.20 This
hypothesis is confirmed by the failure of the same corroles
5-8 to give isocorrole derivatives by this route.
It should be noted that the formation of 6-azahemipor-
phycene is always accompanied by a polar blue-green
byproduct. Spectroscopic characterization indicated that
this product is an open-chain tetrapyrrole formed upon
oxidative ring-opening of the starting corrole at the 5
position. We were able to grow single crystals of 20
suitable for X-ray characterization, which allowed the
unambiguous characterization of this open chain species
(Figure 2).
The molecular structure shows that oxidative ring-
opening introduces a benzoyl group and an oxygen
atom at the terminal R-pyrrolic positions, producing a
biliverdin-like species. Compound 20 adopts a twisted
conformation in which the central two pyrrole rings form
a porphyrin-like intramolecular hydrogen bond with
N
N distance 2.685(2) A and are approximately copla-
3 3 3
nar, forming a dihedral angle of 7.7(3)°. The pyrrole
carrying the carbonyl oxygen also forms an intramole-
cular hydrogen bond to the same acceptor, N
N
3 3 3
2.840(2) A, and is twisted out of the plane of the two
central pyrroles, forming a dihedral angle of 33.32(7)°
with them. Both the directly linked pyrroles carry N-H
hydrogen atoms, and are in an antiperiplanar conforma-
tion, with N-C-C-N torsion angle 177.78(13)°. Bond-
ing appears to be more delocalized in the two directly
(21) Nardis, S.; Mandoj, F.; Paolesse, R.; Fronczek, F. R.; Smith, K. M.;
Prodi, L.; Montalti, M.; Battistini, G. Eur. J. Inorg. Chem. 2007, 2345–2354.
(22) (a) Tardieux, C.; Gros, C.; Guilard, R. J. Heterocycl. Chem. 1998, 35,
965–970. (b) Paolesse, R.; Froiio, A.; Nardis, S.; Mastroianni, M.; Russo, M.;
Nurco, D. J.; Smith, K. M. J. Porphyrins Phthalocyanines 2003, 7, 585–592.
(20) Nardis, S.; Pomarico, G.; Fronczek, F. R.; Vicente, M. G. H.;
Paolesse, R. Tetrahedron Lett. 2007, 48, 8643–8646.

10352 Inorganic Chemistry, Vol. 48, No. 21, 2009
Mandoj et al.
Scheme 1. Reaction Pathway for the 6-Azahemiporphycene Formation
species in the reaction conditions can give the reaction
final products. This hypothesis can explain the regio-
selectivity of the reaction and the formation of 6-aza-
hemiporphycene as product even when 24 is used as
starting reagent.
Photophysical Characterization. This study was carried
out using 12 and the Zn complex 18 as model compounds.
The absorption spectrum of 12 in chloroform shows a
very intense Soret-like band centered at 416 nm (ε =
71000 M-1 cm-1) and a Q-like set of broader, less intense
bands with maximum at 571 nm (ε=9900 M-1 cm-1) and
two shoulders at 533 and 605 nm (Figure 4). The spectrum
of 12 is thus less intense, and the energy of the lowest
absorption is at higher energies than that of 1 or tetra-
phenylporphyrin, (TPP)H2, indicating a lower degree of
aromaticity for 12, as also suggested by computational
results.
shape. The excitation spectrum is in this case propor-
tional to the absorption spectrum, indicating that the S1
state is also formed with unitary efficiency when S2 and
other excited states are initially populated.
Similar results were observed at 77 K. No emissions
were observed in the 800-1600 nm region, indicating a
negligible efficiency both for phosphorescence (at RT and
at 77 K) and for singlet oxygen sensitization (RT) in these
experimental conditions. It should be noted that 12 is
rather stable under irradiation with visible light while 1, in
particular, undergoes rapid degradation upon excitation
at 420 nm with a medium pressure Hg lamp. The spec-
trum of 12 is only slightly affected after 8 h of continuous
irradiation.
Interestingly, the addition of 1 equiv of triflic acid led to
a small red shift in the spectrum, accompanied by a
lowering of the absorption coefficient of the Soret-like
band, while the Q-bands underwent noticeable changes
The fluorescence spectrum at RT shows a moderately
weak, broad, unstructured band at 642 nm (Φ =1.2 ꢀ
10-3), presenting an excited state lifetime of 4.4 ns
(Figure 4). It should be noted that the radiative rate
constant, kr, calculated according to eq 1,14
(Figure 4). At the same time, a structured band at λmax
=
602 nm appears in the emission spectrum, presenting a
higher intensity (Φ =2.0 ꢀ 10-3), a smaller Stokes-shift,
and a lower excited state lifetime (<50 ps) (Figure 5). The
limit value of kr estimated for the monoprotonated
structure is more than 10-fold faster than for the unpro-
tonated compound. All of these results suggest in this case
a less distorted structure for the excited state. It is worth
noting that the protonation process is perfectly reversed
by adding a stoichiometric amount of base such as
tributylamine.
kr ¼ Φ=τ
ð1Þ
is for 12 much slower (3 ꢀ 105 s-1) than that observed for
(TPP)H2 and 1 (8.5 ꢀ 106 s-1 and 1.6 ꢀ 107 s-1
,
respectively), suggesting a distorted/non-rigid structure
for the excited state, in agreement with the observed band

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10353
Figure 4. UV-vis spectral variations of 12 upon addition of 1 equiv of triflic acid (solid line). Addition of 1 equiv of tributylamine restores the original
spectrum (dashed line).
Figure 6. UV-vis absorption (solid line) and emission (dashed line)
spectrum of Zn complex 18.
Figure 5. Emission spectrum of 12 before (solid line) and after (dashed
line) addition of 1 equiv of triflic acid.
Computational Results. Density functional theory
(DFT) calculations were carried with the aim of assessing
aromaticity and tautomerism of the basic structure of 6-
As far as the Zn complex 18 is concerned, it shows
generally a more intense spectrum compared with 12,
azahemiporphycene 25, Figure 7 (pyrrole rings A, B, C, D
with the Soret-like band centered at 436 nm (ε =98000
M
-1 cm-1, Figure 6). The fluorescence band is in this case
and positions 1 and 6 are indicated in the structure), as
structured with a maximum at 626 nm, and with a
relatively low quantum yield (3.8 ꢀ 10-4) and a short
lifetime (0.23 ns). From these data, a kr value of 1.5 ꢀ 106
s-1 can be calculated, higher than for 12 (but lower than
for the free-base corrole), suggesting for 18 a lower
distortion, as also supported by the higher absorption
coefficient. As observed also for free-base 6-azahemipor-
phycene, the excitation spectrum is proportional to
the absorption spectrum. In this case, the band typical
of singlet oxygen in the NIR region can be observed,
although of very low intensity, indicating a higher
intersystem crossing efficiency as expected because of
the presence of the Zn ion. Also the Zn complex 18 is
much more stable than 1 under irradiation with visible
light.
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E. Robb,;
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr., T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson,
B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision
E.01; Gaussian, Inc.: Wallingford, CT, 2004.

10354 Inorganic Chemistry, Vol. 48, No. 21, 2009
Mandoj et al.
Table 2. NICS(0) and NICS(1), in ppm, at the B3LYP/6-311þG(d) Level of
Theory for 6-Azahemiporphycene, Porphyrin, Corrole, and Hemiporphycene
ring
NICS(0)
NICS(1)
6-azahemiporphycene
porphyrin
corrole
-12.0
-15.0
-13.3
-12.7
-11.0
-13.6
-11.9, -12.7a
-11.7
hemiporphycene
a Owing to the significant deviation from planarity of the corrole ring,
NICS(1) and NICS(-1) values are significantly different, thus both have
been reported.
Figure 7. Basic structure of 6-azahemiporphycene 25.
well as its relative stability with respect to the 5-aza
isomer. All calculations were carried out with the pro-
gram Gaussian 03.23 The various issues are discussed
below under separate headings.
Table 3. Relative Electronic Energies (in kcal mol-1) of the Six Tautomers of
6-Azahemiporphycene
Aromaticity. The Nucleus Independent Chemical Shift
(NICS) is presently the most popular index to probe
aromaticity because of its simplicity and efficiency.24 It
is defined as the negative of the magnetic shielding of a
virtual uncharged nucleus placed in selected points in the
vicinity of the molecule, typically at ring center, NICS(0),
or 1 A above the ring center, NICS(1). Significantly
negative NICS values in interior positions of rings indi-
cate the presence of induced diatropic ring currents or
“aromaticity”, whereas positive values at each point
denote paratropic ring currents and “antiaromaticity”.
The NICS index has been routinely utilized to compare
and quantify the aromaticity of porphyrins and porphyr-
inoids.25 Following the general usage,24,25 we have opti-
mized the geometry of 6-azahemiporphycene, porphyrin,
corrole, and hemiporphycene, using the B3LYP/6-
311þG(d) method, and computed the NICS(0) and
NICS(1) at the same level of theory. The results are shown
in Table 2.
Inspection of the data in Table 2 reveals a consistent
order of NICS(0) and NICS(1) values suggesting the
following order of aromaticity: porphyrin > corrole >
hemiporphycene > 6-azahemiporphycene. However, it
must be noted that the differences among NICS values are
small, indicating that all four compounds possess a
comparable aromatic character. The small loss of aroma-
ticity upon passing from hemiporphycene to 6-azahemi-
porphycene is easily understood as due to the small
perturbation in ring current introduced by the more
electronegative nitrogen atom.
tautomer
B3LYP/6-311þþG(d,p)
M05-2X/6-311þþG(d,p)
25AC
25BD
25AB
25AD
25BC
25CD
0.0
0.3
20.2
4.3
4.4
21.0
0.0
1.6
21.4
5.1
5.9
22.3
on adjacent rings can be classified as cis. To evaluate the
relative stability of the above tautomers we carried the
corresponding geometry optimizations at the B3LYP/6-
311þþG(d,p) level of theory. The choice of the basis set,
with polarization and diffuse functions on the hydrogens
beside heavy atoms, was suggested by the role that hydro-
gen bonding inside the cavity may have on the relative
stability of the tautomers. Relative electronic energies are
reported in the first column of Table 3.
The tautomers can be grouped according to their
energies and structural features into three families: the
trans tautomers, 25AC and 25BD; the planar cis tautomers,
25AD and 25BC; and the nonplanar cis tautomers, 25AB
and 25CD. The trans tautomers are the most stable. The
planar cis tautomers, 25AD and 25BC, are about 4.5 kcal
mol-1 less stable than the corresponding trans isomers
probably because of repulsion involving the parallel
dipole moments associated with the two N-H bonds.
The nonplanar cis tautomers, 25AB and 25CD, are more
than 20 kcal mol-1 less stable than the corresponding
trans isomers because of transannular steric repulsions
between the two hydrogens that cause a significant devia-
tion of the corrole core from planarity.
We were somewhat surprised to find that the two trans
isomers have approximately the same energy, especially
because the X-ray structures show the exclusive existence
of the tautomer 25AC in the solid state. Since Zhao and
Truhlar’s recently proposed M05-2X functional has
been demonstrated to outperform the popular B3LYP
functional in the energetic description of organic sys-
tems,26b we carried out geometry optimizations of the
six tautomers also at the M05-2X/6-311þþG(d,p) level
of theory. The results reported in the second column of
Table 3 showed the same pattern of the B3LYP results,
although now the experimentally found tautomer
appears 1.6 kcal mol-1 more stable than the other trans
tautomer. In contrast with the computed planar structure
of the tautomer 25AC, the X-ray crystal structures of the
aryl substituted 6-azahemiporphycenes14 show that the
corrole core is significantly warped in the solid state. We
suspected that the aryl groups could be the cause of such
Tautomerism. The 6-azahemiporphycene basic struc-
ture 25 features two hydrogen atoms inside the ring cavity
on pyrrole rings A and C, respectively. This is the
tautomer that has been detected in the X-ray crystal
structures of 12 and 3-nitro-6-aza-5,11,16-tris(4-tert-
butylphenyl)hemiporphycene.14 For convenience we
indicate this tautomer as 25AC. In addition to 25AC, five
other tautomers can be envisaged, depending upon the
pair of protonated pyrrole rings, namely, 25BD, 25AB
,
25AD, 25BC, and 25CD. Tautomers 25AC and 25BD can be
classified as trans the two hydrogens being on opposite
rings, whereas the other tautomers with the two hydrogens
ꢀ
(24) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; von Rague
Schleyer, P. Chem. Rev. 2005, 105, 3842–3888.
~
(25) (a) Cyranski, M. K.; Krygowski, T. M.; Wisiorowski, M.; van
ꢀ
Eikema Hommes, N. J. R.; von Rague Schleyer, P. Angew. Chem., Int. Ed.
1998, 37, 177–180. (b) Furuta, H.; Maeda, H.; Osuka, A. J. Org. Chem. 2001, 66,
8563–8572. (c) Yoon, Z. S.; Noh, S. B.; Cho, D. G.; Sessler, J. L.; Kim, D. Chem.
ꢀ
ꢀ
Commun. 2007, 2378–2380. (d) Stepien, M.; Latos-Graz_ynski, L. Top. Hetero-
cycl. Chem. 2009, 19, 83–153.

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10355
warp. To put this hypothesis to the test we calculated the
energy of the two trans tautomers of 6-aza-5,11,16-
triphenylhemiporphycene at the M05-2X/6-311þþG-
(d,p)//M05-2X/6-31G(d) level of theory. The lowest
energy conformations of both the tautomers with res-
pect to rotations around the phenyl-corrole axes were
found by a systematic conformational search at
M05-2X/6-31G(d) level. The presence of the phenyl
rings causes significant deviations from planarity in both
the structures. The AC tautomer is now 2.9 kcal mol-1
more stable than the corresponding BD tautomer thus
justifying the fact that only the AC tautomer has been
found in the solid state. The origin of this increased
relative stability can be understood by comparing
the geometries of the phenyl substituted structures with
the corresponding unsubstituted ones: the warping
caused by the phenyl groups increases the distance
between the central hydrogens in the case of the AC
tautomer thus decreasing their steric repulsion, whereas
the opposite occurs in the case of the BD tautomer where
the same distance is reduced.
significantly greater stability of the aryl substituted
6-azahemiporphycene. Inspection of the geometry of
the 5-aza isomer suggests that steric effects between the
phenyl rings and the corrole structure play a major role
in increasing its energy.
Metal Complexes. The metal complexes 13-18 were
prepared to investigate both the coordination abilities of
the macrocycle 12 and the behavior of the resulting metal
derivatives. We adopted synthetic procedures usually
exploited for the preparation of the analogous metallo-
porphyrinates,27 obtaining in all cases good yields of the
metal complexes of 12. These results indicate that the
strong NH-N intramolecular hydrogen bond present in
the macrocyclic inner core does not hinder the coordina-
tion properties of the 6-azahemiporphycene, which is
able to stabilize both divalent metal ions such as Co(II),
Ni(II), Cu(II), and Zn(II) and trivalent metals such as
Mn(III) and Fe(III). These last two complexes are
particularly interesting because the presence of the
chloride axial ligand leads to a mixture of enantiomers
due to the lower symmetry of the 6-azahemiporphycene
structure compared with a porphyrin. Spectroscopic
characterizations of these complexes show features ex-
pected for porphyrinoid metal derivatives. Considering
that in the case of porphycene metal complexes, electro-
chemical behavior showed notable peculiarities, we
decided to study in detail the electrochemistry of the
6-azahemiporphycene metal derivatives. These results
are described below.
Electrochemistry. Compounds 12-18 were examined
as to their electrochemical and spectroelectrochemical
behavior in PhCN and CH2Cl2 containing 0.1 M TBAP.
An example of the cyclic voltammograms obtained in
PhCN are displayed in Figure 8, and a summary of the
half-wave or peak potential for each redox reaction is
given in Table 4.
As shown in Figure 8, the free-base and metal(II)
derivatives of 6-azahemiporphycenes all undergo two
well-defined reversible reductions. Each one-electron
addition occurs on the macrocycle as confirmed by
UV-visible spectroelecrochemistry in a thin-layer cell
that indicates a loss of intensity in the Soret band and
produces a typical π-anion radical type spectrum as the
final electroreduction product (see examples of spectral
changes in Figures 9b and S1-S2). The first reductions of
the MnIIICl and FeIIICl derivatives (13 and 14) are
located at E1/2 =-0.21 V (MnIII/MnII) or Epc =-0.28
V (FeIII/FeII) and are easier than for reduction of the free-
base or metal(II) complexes (12 and 15-18). Further
reversible reductions are obtained at E1/2 =-1.21 and
-1.52 V (compound 13, MnIIICl) or E1/2 = -1.31 V
(compound 14, FeIIICl).
Interestingly, the most stable conformer of the AC
tautomer corresponds to the X-ray structure conforma-
tion; thus computational results are in perfect agreement
with crystallography both for the position of the central
protons and for the dihedral angle around the phenyl-
corrole bonds.
Stability of the 5-Aza Isomer. To rationalize the regio-
selectivity observed in the synthesis of aryl substituted
6-azahemiporphycenes14 (i.e., the finding that the nitro-
gen atom is exclusively introduced in position 6), it is
instructive to compare the energy of 5-azahemiporphy-
cene with that of the 6-aza isomer. Of course a careful
theoretical study of the regioselectivity would require a
comparison of the energies of the corresponding transi-
tion states, but owing to the well-known difficulties to
locate and characterize transition states and since tran-
sition states must resemble to some extent the final
products, we thought that a rational for the regioselec-
tivity can be simply gained by comparing the energies of
the final products. Geometry optimizations of unsub-
stituted 5-azahemiporphycene at both the B3LYP/6-
311þþG(d,p) and the M05-2X/6-311þþG(d,p) levels
of theory were carried out. Relative energy of 5-aza-
hemiporphycene with respect to the 6-aza isomer was
0.2 kcal mol-1 with the B3LYP functional and 1.3 kcal
mol-1 with M05-2X functional. Although the relative
energy calculated with the M05-2X functional should
be more reliable,26 it is not large enough to justify the
observed regioselectivity. To investigate a possible role
of the phenyl rings, a comparison between the energies of
the AC tautomer of 5-aza-6,11,16-triphenylhemipor-
phycene and 6-aza-5,11,16-triphenylhemiporphycene
was performed at the M05-2X/6-311þþG(d,p)//
M05-2X/6-31G(d) levels of theory. The most stable
conformation of the 5-aza isomer was found to be
4.7 kcal mol-1 higher in energy than the 6-aza isomer.
Thus, computational results reasonably indicate that
the observed regioselectivity has to be ascribed to a
The ring-centered reductions of each 6-azahemipor-
phycenes are about 250 mV easier than those for the ring-
centered reductions of (TPP)M complexes28 and not so
(27) Buchler, J. W. Porphyrins and Metalloporphyrins; Smith, K. M., Ed.;
Elsevier: Amsterdam, 1975; pp 157-231.
(28) Kadish, K. M.; Royal, G.; Van Caemelbecke, E.; Gueletti L. In The
Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: San Diego, CA, 2003; Vol. 9, pp 1-220.
(26) (a) Wodrich, M. D.; Corminboeuf, C.; Schreiner, P. R.; Fokin, A. A.;
Schleyer, P. v. R. Org. Lett. 2007, 9, 1851–1854. (b) Zhao, Y.; Truhlar, D. G.
Acc. Chem. Res. 2008, 41, 157–167.
(29) Gisselbrecht, J. P.; Gross, M.; Koecher, M.; Lausmann, M.; Vogel,
E. J. Am. Chem. Soc. 1990, 112, 8618–8620.

10356 Inorganic Chemistry, Vol. 48, No. 21, 2009
Mandoj et al.
Table 4. Half-Wave or Peak Potentials (V vs SCE) of 6-Azahemiporphycene
Derivatives in PhCN and CH2Cl2 Containing 0.1 M TBAP
cpda solvent
M
oxidation
1.54b 1.17b
MII/III
reduction
12 PhCN 2H
-0.89 -1.27
13
14
15
16
17
18
MnCl
c
-0.21 -1.21 -1.52
FeCl
Co
Ni
Cu
Zn
1.60b 1.24b 0.95 -0.28b -1.31d
1.63b 1.26b 1.02 0.56b -0.90 -1.43
1.82b 1.48b 1.34b 1.02
-0.93 -1.34
-0.97 -1.37
-1.05 -1.38
1.94b 1.50b 1.30b 1.09b
1.86b 1.35b 1.13b 0.90b
1.75b 1.56b 1.06b
-0.92 -1.26
f
12 CH2Cl2 2H
13
14
15
16
17
18
MnCle
1.38b 1.21 -0.51b -1.18 -1.72
1.68b 1.22b 0.82b -0.26 -0.92 -1.32
1.67b 1.20b 0.98b 0.72b -0.86 -1.43
FeCl
Co
Ni
Cu
Zn
1.86b 1.52b 1.37b 1.00
-0.91 -1.35
-0.96 -1.37
-1.10 -1.39
1.47b 1.08b
1.32b 1.09b 0.85b
a See Chart 1. b Peak potential at a scan rate of 0.1 V/s. c Catalytic
reaction may occur (see Figure 8). d Reactions at Epc = -0.96, -1.04,
and -1.16 V were also observed. e Data were measured at -70 °C.
f Examples reported in Figure S3.
Figure 9. UV-visible spectral changes during the first reduction of (a)
MnIIICl and (b) CuII azahemiporphycenes in PhCN, 0.1 M TBAP.
Epa =0.56 V for a scan rate of 0.1 V/s while most oxidations
of the other derivatives are also irreversible. The current-
voltagecurveforoxidationof theMnIII compound 13 is not
only ill-defined but also has larger than expected currents
for a “simple” electron transfer process in PhCN. However,
at -70 °C in CH2Cl2, a reversible peak is observed at E1/2
=
1.21 V (Supporting Information, Figure S3), and this
suggests a catalytic reaction involving the singly oxidized
species at RT in PhCN. This was not further investigated in
the present study.
UV-visible spectral changes during the first controlled
potential reduction of the MnIIICl 13 and CuII 17 deri-
vatives are shown in Figure 9. Upon applying a potential
of -0.60 V, a new intense Soret band at 448 nm is seen for
13 which indicates a metal-centered reaction (Figure 9a).
This contrasts with what is observed during the first
reduction of 17 (Figure 9b) where the Soret band
decreases in intensity as new near-IR bands grow in at
706 and 751 nm. These latter spectral changes are indi-
cative of a ring-centered reduction. The second reduction
of 13 at -1.21 V leads to [133 ]2- whose spectrum is shown
in Supporting Information, Figure S4.
The first oxidation of 15 involves a Co(II)/Co(III)
process. The electrogenerated Co(III) form of the com-
pound has a Soret-like band at 439 nm after controlled
potential oxidation at 0.70 V (Figure 10a). Controlled
potential reduction of the same compound may generate
either a Co(II) π-anion radical or a Co(I) compound. The
spectrum of the singly reduced product (Figure 10b) lacks
the intense radical “marker bands” seen for the electro-
generated CuII 17 anion radical (Figure 9b), thus suggest-
ing the formation of a Co(I) azahemiporphycene with an
uncharged macrocycle.
Figure 8. Cyclic voltammograms of M-6-aza-5,11,16-tris(4-tert-butyl-
phenyl)hemiporphycene in PhCN containing 0.1 M TBAP where M and
the compound number are indicated in the figure.
different from E1/2 values for tetrapropylporphycenes29
containing the same central metal ion.
The oxidations of compounds 12-18 are not as well-
defined as the reductions and the current-voltage curves
indicate coupled chemical reactions after electron abstra-
ction in each case. A CoII/CoIII process is observed for 15 at

Article
Inorganic Chemistry, Vol. 48, No. 21, 2009 10357
occurring regioselectively at the 6-position. This approach
offers a facile way to prepare a novel member of the
porphyrinoid family, considering the significant develop-
ments recently achieved by the synthetic chemistry of
corrole. Theoretical and spectroscopic characterization
of 6-azahemiporphycene offer important information on
both the formation pathway and behavior of such macro-
cycles. Different metal complexes of 6-azahemiporphycene
have been prepared, and the reductive electrochemistry of
compounds 12-18 resembles that of both porphyrins and
porphycenes with similar metal ions. The singly and
doubly reduced products are stable but this is not the case
for the oxidations where coupled chemical reactions occur
to give unidentified new products. These features, together
with the photophysical behavior showed by 12 and 18,
make these compounds of interest for applications in
catalysis or as chemical sensors, where porphyrinoids have
already shown interesting potential. This research is cur-
rently ongoing in our laboratories.
Acknowledgment. Financial support from Fondazione
Banca del Monte (L.P.), MIUR-PRIN project n.
2007C8RW53 (L.P., R.P.), the U.S. National Institutes
of Health (K.M.S., Grant CA132861), and the Robert A.
Welch Foundation (K.M.K., Grant E-680) are gratefully
acknowledged.
Figure 10. UV-visible spectral changes of CoII azahemiporphycene 15
during (a) the first oxidation at 0.70 V and (b) the first reduction at -1.10
V in PhCN, 0.1 M TBAP.
Supporting Information Available: CIF files of 11 and 20,
optimized geometries, energies of all computed structures, and
UV-visible spectroelectrochemistry. This material is available
free of charge via the Internet at http://pubs.acs.org.
Conclusions
6-Azahemiporphycene represents a further example of
corrole ring “breathing”, with the macrocycle expansion