Synthetic protocols for the nitration of corroles

Article abstract of DOI:10.1142/S1088424611004038

The modification of peripheral positions of corroles by introduction of nitro groups is an important functionalization of this macrocycle. The nitro substituent strongly influences the corrole behavior leading to the preparation of macrocycles with differ

Full text of DOI:10.1142/S1088424611004038

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2011; 15: 1085–1092
DOI: 10.1142/S1088424611004038
Published at http://www.worldscinet.com/jpp/
Synthetic protocols for the nitration of corroles
Giuseppe Pomaricoa, Frank R. Fronczekb, Sara Nardisa, Kevin M. Smithb and
Roberto Paolesse*a
a Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma Tor Vergata, 00133 Roma, Italy
b Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA
Dedicated to Professor Karl M. Kadish on the occasion of his 65th birthday
Received 23 June 2011
Accepted 15 July 2011
ABSTRACT: The modification of peripheral positions of corroles by introduction of nitro groups is
an important functionalization of this macrocycle. The nitro substituent strongly influences the corrole
behavior leading to the preparation of macrocycles with different properties, which can be of interest
for their exploitation as catalysts, sensing layers in chemical sensors or in the field of supramolecular
chemistry. In the last few years we have developed different routes for the β-nitration of the corrole
ring, and we report here novel synthetic protocols which can allow the formation of tri- and tetra-
nitro derivatives, as demonstrated by X-ray analysis. In all of the methodologies used, the presence
of isocorrole species as reaction intermediates was established, which regenerated the corresponding
corrole by metal insertion.
KEYWORDS: corrole, nitration, isocorrole.
Furthermore, the functionalization of corrole is compli-
INTRODUCTION
cated by its lower symmetry compared with porphyrin,
Since the seminal papers on the preparation of aryl-
which could lead to the formation of different substituted
corroles [1, 2], in the last decade the number of publica-
regioisomers. However, pyrroles A and D of corrole
tions related to corrole significantly expanded, allowing
(Fig. 1) are endowed with enhanced reactivity than are
a more detailed study of the chemistry of the macrocycle.
pyrroles B and C, thereby allowing the formation of a
The synthetic routes for the preparation of corrole are
few or even a single product among the huge number of
now significantly improved [3, 4], allowing also studies
potential regioisomers.
of functionalization of the corrole [5], a fundamental step
The most studied functionalization reactions reported
for exploitation of such a macrocycle in practical applica-
for corroles are the chlorosulfonation [7, 8], formylation
tions. The first examples of the potentialities of corrole
[8, 9], nitration [8, 10–12], and bromination [13–17].
derivatives in different fields have been reported in the
In the case of chlorosulfonation, the 2,17- and 3,17-
literature [6].
isomers have been isolated, while in the case of nitra-
The peripheral functionalization of corroles obviously
tion the 3-nitro, and the 3,17-dinitro derivatives could
took advantage of the routes already developed for por-
be obtained, together with the 2,3,17-trinitro analog,
phyrins, although the reactivity of the contracted macro-
depending on the nitrating system chosen. Also the
cycle could be very unexpected. For this reason, it is not
formylation of corrole mainly led to the insertion of the
surprising to isolate novel compounds when corroles are
carbonyl moiety at positions-3 and -17, together with a
reacted under the same conditions used for porphyrins.
different compound, produced by attack of the Vilsmeier
reagent to the macrocycle inner core [9].
SPP full member in good standing
A different behavior occurs in the case of bromina-
tion, because this reaction is more difficult to control than
those mentioned above, and the fully brominated corrole
*Correspondence to: Roberto Paolesse, email: roberto.
paolesse@uniroma2.it, fax: + 39-06-72594328
Copyright © 2011 World Scientific Publishing Company

1086 G. POmarICO et al.
is the most usual product, because otherwise a complex
and impossible to separate mixture of regioisomers is
obtained. Only recently the first examples of partially
brominated Ge-corroles [15], and a tetrabrominated free-
base corrole have been reported in the literature [16, 17].
Among these different functionalizations, we have
been particularly interested in the definition of synthetic
protocols for the β-nitration of corroles. The usefulness
of the nitro group comes from the possibility to further
modify the porphyrinoid macrocycle in a selective way
[18, 19]; indeed the ability of this substituent to drive the
attack of another functional group on corrole has been
reported, making it possible to prepare functionalized
compounds not achievable by straightforward synthetic
pathways; a second interesting capability of –NO2 is its
reduction to the amino group, which could be the first
step toward exploitation of aminocorroles and the con-
struction of covalently linked porphyrinoid moieties.
However, in our previous work the preparation of
β-nitrocorroles was limited to the introduction of one or
two nitro groups, probably because of the deactivating
nature of the nitro group towards the further substitution;
for this reason we have been interested to study the exploi-
tation of more effective nitrating systems able to increase
the level of the nitration above that so far achieved. Fur-
thermore it is also interesting to investigate the possible
concomitant nitration of the meso-phenyl groups, which
has not yet been observed in the case of corrole.
data were collected at low temperature on a Nonius Kap-
paCCD diffractometer equipped with MoKα radiation
(λ = 0.71073 Å) and an Oxford Cryosystems Cryostream
chiller. Refinement was by full-matrix least squares using
SHELXL [20], with H atoms in idealized positions,
guided by difference maps. In 1, one of the four chloro-
form solvent molecules was disordered, and its electron
density was removed using the SQUEEZE [21] proce-
dure. In 2, a disorder exists in which the third NO2 group
is on C17 rather than C3 12.2(5)% of the time. The three
atoms of the minor site were treated as isotropic. Crys-
tallographic data for 1: C55H34CoN8O8P, CHCl3, triclinic
space group P-1, a = 15.9753(15), b = 24.101(3), c =
25.382(3)Å, α = 90.495(4), β = 90.369(7), γ = 90.674(6)˚,
V = 9771.3(19)Å3, T = 90.0(5) K, Z = 8, ρcalcd = 1.556 g cm-3,
µ(MoKα) = 0.62 mm-1. A total of 121,297 data was col-
lected to θ = 25.6˚. R = 0.083 for 19,806 data with Fo2 >
2σ(Fo2) of 31,240 unique data and 2739 refined param-
eters, CCDC 830942. For 2: C55H35CoN7O6P, monoclinic
space group P21/n, a = 10.4692(15), b = 23.662(3), c =
18.703(2) Å, β = 103.077(6)˚, V = 4513.0(10) Å3, T =
100.0(5) K, Z = 4, ρcalcd = 1.442 g cm-3, µ(MoKα) = 0.48
mm-1. A total of 48,573 data was collected to θ = 22.9˚.
R = 0.051 for 4380 data with Fo2 > 2σ(Fo2) of 6276
unique data and 642 refined parameters, CCDC 830941.
Syntheses of β-nitrocorroles
Preparation (method A — compounds 1, 2, 3).
45 mg of TPCorrH3 (0.085 mmol) were dissolved in 6
mL of TFA, and 59 mg (0.85 mmol, 10 equiv.) or 590
mg (8.5 mmol, 100 equiv.) were added. The mixture was
stirred at room temperature for 5 min or 1 h (in the case of
3). After that time the reaction was quenched with 60 mL
of water and the mixture was extracted with CH2Cl2. The
organic phase was washed with aqueous NaHCO3, water
and dried over anhydrous Na2SO4, before the solvent was
removed under reduced pressure.
We report here the results obtained by reacting
5,10,15-triphenylcorrole (TPCorrH3) in TFA/NaNO2
mixture, showing the formation of poly β-nitrated deriv-
atives, obtained as Co triphenylphosphine complexes
upon metalation of the corresponding isocorrole species,
which were formed under the reaction conditions.
EXPERIMENTAL
Preparation (method B — compounds 4, 5). 45 mg
of TPCorrH3 (0.085 mmol) were dissolved in 30 mL of
CH2Cl2, and 189 mg (2.74 mmol, 32 equiv.) dissolved
in 30 mL of HCl 0.9 M were added. The mixture was
stirred at room temperature for 5 min, monitoring the
course of reaction by UV-vis spectrophotometry. After
that time acid was neutralized with Na2CO3. The mix-
ture was extracted with CH2Cl2 and the organic phase
washed twice with water and dried over anhydrous
Na2SO4; the solvent was then removed under reduced
pressure.
Insertion of cobalt. The compound was dissolved
in 30 mL of CH2Cl2 and 10 mL of CH3OH, and a three
fold excess (with respect to the starting TPCorrH3) of
Co(AcO)2 and PPh3 were added. The mixture was stirred
under reflux for 1 h, and then the solvent was removed
under reduced pressure and residue purification by col-
umn chromatography then followed. The purification
details are described for each case as follows.
General
Silica gel 60 (70–230 mesh, Sigma Aldrich) or neutral
alumina oxide (Grade III, Merck) were used for column
chromatography. Reagents and solvents (Aldrich, Merck
or Fluka) were of the highest grade available and were
1
used without further purification. H NMR spectra were
recorded on a Bruker AV300 (300 MHz) spectrometer.
Chemical shifts are given in ppm relative to residual
CHCl3 (7.25 ppm). UV-vis spectra were measured on a
Cary 50 spectrophotometer. Mass spectra (FAB mode)
were recorded on a VGQuattro spectrometer in the pos-
itive-ion mode using m-nitrobenzyl alcohol (Aldrich) as
a matrix.
X-ray crystallographic data
Crystals 1 and 2 for X-ray crystallographic analysis
were grown from methanol/CDCl3 solutions. Diffraction
Copyright © 2011 World Scientific Publishing Company
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SynthetIC PrOtOCOlS fOr the nItratIOn Of COrrOleS 1087
[2,3,12,18-(NO2)4-TPCorr]Co(PPh3) (1). This frac-
tion was collected from silica gel chromatography eluting
with CH2Cl2; it was crystallized from CH2Cl2/CH3OH.
9.09 (s, 1H, β-pyrrole), 8.59 (d, 1H, J = 4.61 Hz,
β-pyrrole), 8.35 (d, 1H, J = 4.61 Hz, β-pyrrole), 8.31 (d,
1H, J = 4.96 Hz, β-pyrrole), 8.14 (d, 1H, J = 4.96 Hz,
Yield 8% (7 mg), mp > 300 °C. UV-vis (CH2Cl2): λmax
,
β-pyrrole), 8.42 (s, 1H, β-pyrrole), 8.27 (q, 2H, J = 5.71
Hz, β-pyrrole), 7.75 (m, 9H, phenyl), 8.08 (d, 1H, J =
5.06 Hz, β-pyrrole), 8.00 (d, 2H, J = 5.09 Hz, β-pyrrole),
7.66–7.50 (m, 13H, phenyl), 7.30 (m, 2H, phenyl), 7.15
(m, 3H, p-phosphine), 6.80 (m, 6H, m-phosphine), 5.83
(m, 6H, o-phosphine). Anal. calcd. for C55H37CoN5O2P:
C, 74.24; H, 4.19; N, 7.87%. Found: C, 74.31; H, 4.26;
N, 7.82.
1
nm (log ε) 382 (4.20), 528 (3.76), 601 (3.81). H NMR
(300 MHz, CDCl3): δ, ppm 8.19 (d, 2H, J = 5.07 Hz,
β-pyrrole), 8.06 (d, 2H, J = 5.22 Hz, β-pyrrole), 7.74–
7.53 (m, 15H, phenyl), 7.30 (m, 3H, p-phosphine), 7.00
(m, 6H, m-phosphine), 5.21 (m, 6H, o-phosphine). Anal.
calcd. for C55H34CoN8O8P: C, 64.46; H, 3.34; N, 10.93%.
Found: C, 64.48; H, 3.38; N, 10.89.
[2,3,18-(NO2)3-TPCorr]Co(PPh3) (2). This fraction
was collected from silica gel chromatography eluting with
diethyl ether and was crystallized from CH2Cl2/CH3OH.
Nitration on iron-corrolates (method B, compounds
6–8). 66 mg of [TPCorr]Fe(Cl) (0.108 mmol) were dis-
solved in 40 mL of CH2Cl2, and 238 mg (3.45 mmol, 32
equiv.) dissolved in 40 mL of HCl 0.9 M were added.
The mixture was stirred at room temperature for 5 min,
monitoring the course of reaction by UV-vis spectropho-
tometry. After that time the acid was neutralized with
Na2CO3. The mixture was extracted with CH2Cl2 and the
organic phase was washed twice with water and dried
over anhydrous Na2SO4; then the solvent was removed
under reduced pressure. The residue was purified by col-
umn chromatography on silica gel eluting with CH2Cl2/
hexane 80/20%.
Yield 14% (12 mg), mp > 300 °C. UV-vis (CH2Cl2): λmax
,
1
nm (log ε) 380 (3.97), 535 (3.52), 606 (3.66). H NMR
(300 MHz, CDCl3): δ, ppm 8.36 (d, 1H, J = 5.07 Hz,
β-pyrrole), 8.29 (s, 1H, β-pyrrole ), 8.23 (d, 2H, J = 5.19
Hz, β-pyrrole), 8.08 (d, 4H, J = 5.04 Hz, β-pyrrole), 7.75–
7.65 (m, 9H, phenyl), 7.62–7.49 (m, 7H, phenyl), 7.30
(m, 3H, p-phosphine), 6.93 (m, 6H, m-phosphine), 5.13
(m, 6H, o-phosphine). Anal. calcd. for C55H35CoN7O6P:
C, 67.42; H, 3.60; N, 10.00%. Found: C, 67.46; H, 3.58;
N, 10.05.
[2,3,17,18-(NO2)4-TPCorr]Co(PPh3) (3). This frac-
tion was collected from silica gel chromatography eluting
with CH2Cl2/diethyl ether 60/40% and was crystallized
from CH2Cl2/CH3OH. Yield 5% (4 mg), mp > 300 °C.
UV-vis (CH2Cl2): λmax, nm (log ε) 401 (4.47), 452 (4.36),
[(3-NO2)-TPCorr]Fe(NO) (6). Yield 8% (6 mg).
Spectroscopic data are in agreement with those reported
in the literature [26].
[(3,17-NO2)2-TPCorr]Fe(NO) (8). Yield 14% (11 mg).
Spectroscopic data are in agreement with those reported
in the literature [26].
1
532 (3.99), 615 (4.21). H NMR (300 MHz, CDCl3): δ,
ppm 8.69 (s, 1H, β-pyrrole), 8.42 (s, 1H, β-pyrrole), 8.27
(q, 2H, J = 5.71 Hz, β-pyrrole), 7.75 (m, 9H, phenyl),
7.41 (m, 6H, phenyl), 7.73 (m, 3H, p-phosphine), 6.97
(m, 6H, m-phosphine), 5.23 (m, 6H, o-phosphine). Anal.
calcd. for C55H34CoN8O8P: C, 64.46; H, 3.34; N, 10.93%.
Found: C, 64.42; H, 3.37; N, 10.90.
RESULTS AND DISCUSSION
Our previous investigations on the nitration of meso-
arylcorrole, either free-base or metal complexes, allowed
us to optimize the procedure for the preparation of mono-
and dinitro-derivatives; to investigate the possibility to
introduce additional nitro groups on the peripheral posi-
tions of the corrole ring we decided to test the effective-
ness of different nitrating systems.
Our attention was attracted by the exploitation of the
TFA/NaNO2 system; it is interesting to note that under
these conditions the nitration of porphyrin meso-phenyl
rings has been reported [22]; in this case TFA is not only
involved in the formation of the nitrating agent, but it also
drives the nitration on the phenyl rings, deactivating the
pyrrolic positions toward substitution through protona-
tion of the macrocycle inner core. However, differently
from porphyrin, corrole has demonstrated to be reactive
also after protonation, as in the case of Vilsmeier formy-
lation; corrole free-base, in fact, can be directly formy-
lated, while in the case of porphyrin it is necessary to
carry out the reaction on appropriate metal complexes to
avoid the deactivation due to the formation of the macro-
cycle dication.
[(3-NO2)(5-OH)TPIsoCorr] (4). This fraction was
collected from silica gel chromatography eluting with
CH2Cl2 (second fraction eluted) and was crystallized
from CH2Cl2/CH3OH. Yield 16% (8 mg), mp > 300 °C.
UV-vis (CH2Cl2): λmax, nm (log ε) 390 (4.21), 614 (3.56),
1
666 (3.53). H NMR (300 MHz, CDCl3): δ, ppm 16.36
(s, 1H, NH), 14.81 (s, 1H, NH), 7.64–7.52 (m, 8H, phe-
nyl), 7.45–7-41 (m, 5H, phenyl), 7.31 (m, 2H, phenyl),
7.18 (d, 1H, J = 2.82 Hz, phenyl), 7.10 (d, 1H, J = 4.61
Hz, β-pyrrole), 6.95 (d, 1H, J = 4.61 Hz, β-pyrrole),
6.89 (d, 1H, J = 4.74 Hz, β-pyrrole), 6.67 (d, 1H, J =
4.72 Hz, β-pyrrole), 6.50 (m, 1H, β-pyrrole), 6.31 (s,
1H, β-pyrrole), 6.25 (m, 1H, β-pyrrole). Anal. calcd. for
C37H25N5O3: C, 75.62; H, 4.29; N, 11.92%. Found: C,
75.72; H, 4.25 N, 11.98.
[(3-NO2)-TPCorr]Co(PPh3) (5). This fraction was
collected from silica gel chromatography eluting with
CH2Cl2 (third fraction eluted) and crystallized from
CH2Cl2/CH3OH. Yield 14% (11 mg), mp > 300 °C. UV-
vis (CH2Cl2): λmax, nm (log ε) 369 (4.21), 408, shoulder
(4.14), 588 (3.81). 1H NMR (300 MHz, CDCl3): δ, ppm
Furthermore, in the case of corrole it would be inter-
esting to investigate the possibility of nitration at the
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1088 G. POmarICO et al.
meso-phenyls rings, by analogy with the case of porphy-
rins. We decide to test this reaction on corrole free-base,
using TPCorrH3 as starting material, in order to compare
the reactivity of pyrrole versus phenyl groups.
The 1H NMR spectrum of the first green band showed
two singlets (1H each) at 15.5 and 14.4 ppm for the inner
hydrogens; this chemical shift is a signature of an isocor-
role species, as already observed in the case of similar
compounds [23]; furthermore, the presence of two sig-
nals suggested the formation of an isocorrole derivative
where the interruption of the aromatic system takes place
at position 5. The absence of the ring current led to a
high field shift of the β-pyrrolic hydrogens; from 6.35 to
7.00 ppm two doublets and two multiplets (total integral
4H) pointed to the formation of a poly-nitrated deriva-
tive; moreover, a fifth signal for pyrrolic protons was
detected at lower field (7.70 ppm), among the unresolved
multiplets related to the resonance of the meso-phenyl
protons. This chemical shift is a typical signature for
β-nitroisocorrole derivatives, as already observed in the
case of demetalation of the Ag mononitro corrole com-
plex [24], which afforded the (3-NO2)(5-OH)TTiso
CorrH2, and it can be attributed to the electron withdrawing
influence of the –NO2 group. All these data are consis-
tent with the formation of a trinitroisocorrole; taking in
TPCorrH3 was dissolved in a few mL of TFA and 10
equivalents of NaNO2 were added (Scheme 1); the initial
greensolution,correspondingtothecationic[TPCorrH4]+,
became brownish, and the resulting UV-vis spectrum
completely changed. The reaction was quenched with
water and TLC analysis of the mixture showed two green
bands, together with a consistent amount of starting mate-
rial decomposition (non-mobile red-brown spot).
The purification of the reaction products was quite
difficult: column chromatography separation using polar
eluants was not effective, while reduction of the polar-
ity of the eluting solvents caused the compounds to stick
on the column, further decreasing the reaction yields,
as indicated by the small amount of products collected.
However, subsequent purification by preparative thin-
layer chromatography allowed us to readily isolate the
products.
PPh3
NO2
NaNO2: 10 eq
Reaction time: 5 min
O2N
O2N
N
N
N
Co
N
1
O2N
PPh3
N
O2N
O2N
NaNO2: 10 eq
Reaction time: 5 min
HN
N
NH
N
(1) TFA/NaNO2
Co
N
N
(2) Co(AcO)2/PPh3
HN
2
O2N
O2N
PPh3
N
NaNO2: 100 eq
Reaction time: 1 h
O2N
N
N
Co
N
O2N
O2N
3
Scheme 1. Products of nitration with TFA/NaNO2
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SynthetIC PrOtOCOlS fOr the nItratIOn Of COrrOleS 1089
account the larger reactivity of the directly linked pyr-
role subunits, we hypothesized this compound to be the
2,3,18-(NO2)3(5-OH)TPisoCorrH2.
The second band, olive green in color, showed similar
1
behavior upon H NMR analysis; a signal at 14.92 ppm
and the β-pyrrolic protons upfielded in the region
6.7–7.2 ppm indicated the formation of a different nitro-
isocorrole. The presence of a single resonance for the
inner protons (2H) supported the formation of the 10-
substituted isocorrole. Since this fraction showed one
singlet (1H) and two doublets (2H each), we assumed it
was the second isocorrole isomer of the trinitro deriva-
tive: 2,3,18-(NO2)3(10-OH)-TPisoCorrH2. A more accu-
rate identification of the structures was not possible,
since we did not obtain crystals good enough for X-ray
diffraction analysis.
Fig. 1. X-ray structure of [2,3,12,18-(NO2)4-TPCorr]Co(PPh3)
(1) with 40% ellipsoids. Only one of the four independent mol-
ecules is illustrated, and H atoms and solvent are not shown
Having in mind to improve the purification and the
characterization of the products, we repeated the reac-
tion, and after the work up the metalation of the mixture
with Co(AcO)2 and PPh3 was carried out. We choose this
metal because its coordination to the macrocycle drives
the rearomatization of isocorrole to corrole [25], and
because of the diamagnetic behavior of the pentacoor-
dinated cobalt complex. The crude reaction mixture was
purified by silica gel column chromatography, first elut-
ing with CH2Cl2, then with CH2Cl2-diethyl ether 1:1 and
finally with diethyl ether.
the β-pyrrolic positions retain high reactivity, driving the
nitro moieties to attach to the macrocyclic ring instead of
the meso-aryl substituents.
A second fraction, blue-green in color was isolated.
1H NMR spectroscopy suggested the formation of a tri-
nitrocorrole, as indicated by the presence of one singlet
corresponding to the proton of the monosubstituted
pyrrole. X-ray analysis, carried out on a single crystal
obtained by a slow diffusion of methanol into a CDCl3
solution of the corrole derivatives, allowed us to iden-
tify the position of the nitro substitution; this compound
was identified as [2,3,18-(NO2)3-TPCorr]Co(PPh3) (2,
Fig. 2).
Also in this case we again observed the anomalous
behavior of corrole, where the C18 undergoes the attack of
the nitrating agent easier than does C17, as we expected.
In the crystal structure, a disorder exists in which C17 is
nitrated with population about 12%. While it is possible
that this indicates the presence of a small amount of the
tetranitro compound 3, it appears that C3 is not nitrated
when C17 is. Thus the disorder more likely involves the
overlap of two enantiomers of trinitro compound 2.
The crystal of compound 1 contains four independent
Co complex molecules. In all, the Co atom is in a square
1
The first green band was characterized by H NMR;
the formation of the Co complex was confirmed by the
disappearance of the signals around 14–15 ppm, while
three new groups of signals belonging to the ortho, meta
and para protons of the coordinated phosphine con-
firmed complex formation and, more interesting for the
our objectives, we observed that the pyrrolic protons
were shifted over 8 ppm; this is tangible evidence of the
restored aromaticity. Two singlet (1H each) and a multip-
let (2H) showed the presence of a polysubstituted corrole
in an asymmetric way; all these data are consistent with
the formation of a tetranitro derivative.
No similar compound was isolated during the previ-
ous experiment, probably because of a lower stability of
the free-base of this compound, which mainly decom-
posed during the purification step. Crystals obtained by
slow diffusion of methanol into a CDCl3 solution of this
compound allowed us to unambiguously identify the
compound as [2,3,12,18-(NO2)4-TPCorr]Co(PPh3) (1,
Fig. 1).
This result was quite surprising due to the insertion of
–NO2 at C12; we expected for a tetranitro derivative that
the susbtitution would take place on pyrroles A and D,
since they should be endowed with a larger reactivity; in
the compound we identified, the fully functionalization
of one pyrrole was accomplished by the insertion of a
–NO2 on positions 18 and 12, instead of the presumed
more reactive 17.
Fig. 2. X-ray structure of [2,3,18-(NO2)3-TPCorr]Co(PPh3) (2)
with 40% ellipsoids. H atoms and the partially-occupied NO2
group at C17 are not shown
It is noteworthy that no modification of phenyl rings
occurred; despite the protonation of the inner nitrogen,
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1090 G. POmarICO et al.
pyramidal coordination geometry with PPh3 in the api-
cal position. The Co–P distances range from 2.2242(17)–
2.2432(17) Å, with a mean value of 2.2344 Å. The Co
atom lies 0.269 Å (mean of four) out of the N4 basal
plane, and Co–N distances lie in the range 1.865(5)–
1.896(5) Å, with a mean value of 1.883 Å. In compound
2, the Co coordination is nearly identical to that in 1,
with Co–P distance 2.2259(12) Å, Co–N distances in the
range 1.871(3)–1.881(3) Å with mean 1.876 Å, and the
Co atom lying 0.273(1) Å out of the N4 basal plane.
Even though the products were isolated in low yield,
for the first time we were able to prepare different cor-
roles having more than two nitro groups; for this rea-
son we continued to investigate the potential of this
reaction by varying some experimental parameters, as
follows. The amount of NaNO2 was increased to 100
equivalents; by TLC analysis greater decomposition was
observed together with the formation of two green frac-
tions with UV-vis spectra very similar to those reported
in the case of the reaction carried out with 10 equivalents
of sodium nitrite. The insertion of Co led to the isola-
tion of a main product, corresponding to 2. The reaction
time was then extended up to one hour; by the purifi-
cation of Co derivatives two fractions were collected;
[2,3,12,18-(NO2)3-TPCorr]Co(PPh3) (1) and the symmet-
ric [2,3,17,18-(NO2)3-TPCorr]Co(PPh3) (3).
Unfortunately we did not obtain good crystals of
1
the symmetric nitro-corrole, but the H NMR spectra
showed a pattern of signals consistent with our identifi-
cation of the products, due to two singlets (1H each) and
a broad doublet (2H) for 1, and only two doublets (2H
each) for 3.
The products obtained showed how the nitro group
strongly influenced the reactivity of corrole, affording
products unexpected in terms of regioselectivity usually
observed, inducing also the formation of 1 wherein the
further functionalization is directed to the 12-position.
However it is also interesting to note that the substitution
occurred always at the β-positions of the macrocycle, not
involving the meso-phenyl groups in contrast to the case
of porphyrins.
The large decomposition we had in the different exper-
iments represents a serious drawback of this method.
So we turned our attention to a milder nitrating system,
already exploited for the nitration of N-confused porphy-
rins [26, 27]. The protocol is based on a heterogeneous
system, where the porphyrinoid is dissolved in CH2Cl2
and NaNO2 is dissolved in dilute HCl (Scheme 2).
Corrole free-base was reacted with 32 equivalents of
NaNO2 for 5 min; when no more variations of the UV-vis
spectrum were observed, the acid was neutralized with
1
subsequent characterization by H NMR indicated that
when the reaction takes place for longer times, the trini-
tro corrole continues to react, affording the asymmetric
O2N HO
N
NH
N
HN
4
HN
NH
N
(1) HCl/NaNO2
(2) Co(AcO)2/PPh3
HN
O2N
PPh3
N
N
N
Co
N
5
Scheme 2. Products of nitration with HCl/NaNO2
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 1090–1092

SynthetIC PrOtOCOlS fOr the nItratIOn Of COrrOleS 1091
Na2CO3 and the mixture was extracted with CH2Cl2. TLC
analysis of the reaction mixture showed the presence of
new products together with some decomposed material.
After purification on a silica gel column eluted with
dichloromethane, two green fractions were collected and
characterized by 1H NMR; also in this case the products
isolated corresponded to isocorrole species, as suggested
by the presence of one (2H) or two singlets (1H each) in
the region 15–16 ppm, and the upfielded signals of the
pyrrolic protons. To facilitate better characterization we
repeated the reaction, and the mixture was reacted by
metalation with Co ion; the purification afforded three
products. The first red band was unreacted Co-corrole as
confirmed by comparison of this product with a pure spec-
imen of [TPCorr]Co(PPh3); the second corresponded to
the [(3-NO2)(5-OH)TPIsoCorr], while the last band was
identified as the [(3-NO2)TPCorr]Co(PPh3). To explain
the partial failure in the aromatization of the isocorroles,
we should consider that the presence of a nitro group on
position 3 deeply affects the stability of isocorroles, mak-
ing isomer-5 more stable than isomer-10. This is due to
the H bond between the OH and NO2 groups, which ham-
pers the rearomatization of the macrocycle. Since this
stabilizing effect is not present on the 10-OH-isocorrole,
this compound rearranges to form the corresponding
Co-complex of nitro-corrole.
X1
X2
NO
N
N
Fe
N
N
X3
6: X1 = NO2; X2 = X3 = X4 = H
7: X1 = X4 = NO2; X2 = X3 = H
8: X1 = X3 = H; X2 = X4 = NO2
X4
Scheme 3. Products of the nitration on iron-corrolates
those in the literature [28]. The presence of the axially
bonded NO group conferred a diamagnetic character to
these compounds, which allowed their characterization
by H NMR spectroscopy. Between the mono- and the
dinitro-derivatives, a small quantity of a reddish com-
pound was isolated.
Traces of a different compound were eluted between
the mono-nitro and the dinitro-Fe-corroles. The presence
of two singlets at 8.09 and 8.59 ppm at 1H NMR analysis
is compatible with the formation of an asymmetric disub-
stituted corrole 8.
1
1
This hypothesis was supported by H NMR spectra,
where the second band (isocorrole-like, emerald green in
color) showed the presence of two signals for the inner
protons at 16.36 and 14.81 ppm, several signals (doublets
and multiplets) of the β-pyrroles resonating in the region
6.24–7.18 ppm and a singlet (1H) for the protons close
to the nitro group at 6.31 ppm. For the second dark green
band, no signals were observed in the region 14–16 ppm,
and the resonance for the β-pyrroles were shifted to
lower field than those of the other nitro-derivative, with
the singlet located in this case at 9.09 ppm.
CONCLUSION
The results obtained demonstrate that stable corroles
bearing up to four peripheral nitro substituents can be
successfully synthesized using our new approach. These
species can be of great interest since, on one hand nitro
groups greatly influence the behavior of the macrocycle
and on the other they can be used for further functional-
ization of the corrole ring.
When the amount of nitrating agent was increased to
132 equivalent, mainly decomposition of starting mate-
rial was observed.
Acknowledgements
This research was supported by the Italian MiUR
(PRIN project 2007C8RW53), United States National
Institutes of Health (K.M.S. grant CA 132861). The pur-
chase of the diffractometer was made possible by Grant
No. LEQSF(1999–2000)-ENH-TR-13, administered by
the Louisiana Board of Regents.
Since we investigated the nitration on metallo-
corroles, to find out the effect of metal ion on corrole
chemistry, we decide to test the last nitrating system on
corrole complexes (Scheme 3) in order to compare the
results with what we have recently reported for nitration
on iron corroles [26]. [TPCorr]FeCl was dissolved in
dichloromethane, and the mixture reacted with NaNO2 in
dilute HCl; subsequent work-up afforded three main frac-
tions. The first result we observed was that only traces of
[Corr]Fe(NO) were obtained, while this compound was
the major product when the reaction was carried out with
the published procedure [26]. Among the other collected
compounds, the first red band and the third green band
were identified as the [(3-NO2)TPCorr]Fe(NO) (6) and
the [(3,17-NO2)2TPCorr]Fe(NO) (7), respectively, as
confirmed by comparison of the spectroscopic data with
Supporting information
Crystallographic data have been deposited at the Cam-
bridge Crystallographic Data Center (CCDC) under num-
bers CCDC 830941 and 830942. Copies can be obtained
on request, free of charge, via www.ccdc.cam.ac.uk/
conts/retrieving.html or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK (fax: +44 1223-336-033 or email: deposit@
ccdc.cam.ac.uk).
Copyright © 2011 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2011; 15: 1091–1092

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J. Porphyrins Phthalocyanines 2011; 15: 1092–1092

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