Iron complexes of tris(4-nitrophenyl)corrole, with emphasis on the (nitrosyl)iron complex

Article abstract of DOI:10.1142/S1088424612500605

The iron complexes of 5,10,15-tris(4-nitrophenyl)corrole have been prepared and characterized by various spectroscopic techniques. The (nitrosyl)iron complex is diamagnetic and its X-ray structure reveals an almost perfectly linear FeNO bond. EPR spectros

Full text of DOI:10.1142/S1088424612500605

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2012; 16: 663–673
DOI: 10.1142/S1088424612500605
Published at http://www.worldscinet.com/jpp/
Iron complexes of tris(4-nitrophenyl)corrole, with emphasis
on the (nitrosyl)iron complex
Pinky Singha, Irena Saltsmana◊, Atif Mahammeda◊, Israel Goldbergb◊, Boris
Tumanskiia and Zeev Gross*a◊
a Schulich Faculty of Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
b School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel
Dedicated to Professor Emanuel Vogel in memoriam
Received 20 February 2012
Accepted 8 March 2012
ABSTRACT: The iron complexes of 5,10,15-tris(4-nitrophenyl)corrole have been prepared and
characterized by various spectroscopic techniques. The (nitrosyl)iron complex is diamagnetic and its
X-ray structure reveals an almost perfectly linear Fe–N–O bond. EPR spectroscopy in conjunction with
15N labelling were used to deduce the redox centre of the one-electron reduction and oxidation products
of the (nitrosyl)iron corrole.
KEYWORDS: corroles, synthesis, iron, 15N-, 57Fe-labeled corroles, NMR, electrochemistry, EPR.
complexes of triarylcorroles (ITC) stand out as the
INTRODUCTION
most extensively investigated [10]. It started with the
Emanuel Vogel’s tremendous achievements during
demonstration that ITC are very useful catalyst for oxo-,
his long and very fruitful carrier include also very
carbene-, and nitrene-transfer reactions [11, 12]. The
critical contributions to the chemistry of corroles [1, 2].
comparison with analogous iron porphyrins revealed that
Some of these were outlined in Jonathan Sessler’s book
ITC are superior for the two latter purposes, but less so for
“Expanded, Contracted & Isomeric Porphyrins,” which
oxygenation catalysis. The most unique group-transfer
includes a sub-chapter entitled “Recent work from Vogel:
reaction catalyzed by ITC is the N-tosylation of olefins
A New Interpretation” [3]. The discussion therein was
by Chloramine-T, a feature not shared by any other iron
mainly focused on Emanuel’s proper assignment of
complex [13]. Only the (chloro)iron(IV) corrole, but not
nickel and copper corroles [4], although it also briefly
any iron(III) complex, was able to catalyze that reaction.
mentioned phenyliron(IV) corrole and its one-electron
This chemical reactivity novelty is related to another
oxidation product [5]. On top of these examples,
aspect- the proper assignment of oxidation states in
Emanuel’s research group also prepared and fully
ITC. Intensive debates regarding the (chloro)iron(IV)
characterized the (pyridine)iron(III), (chloro)iron(IV)
corrole vs. (chloro)iron(III) corrole radical formulation
and (nitrosyl)iron complexes of octaalkylcorroles [6].
continued from Emanuel’s time until most recently [14],
At the turn of the last century, the focus changed to the
but this issue has apparently been resolved by now [15].
much more wide-ranging triarylcorroles that became
The abovementioned low activity of metallocorroles as
accessible only as late as 1999 [7, 8]. Of the numerous
catalysts for oxygen atom transfer reactions relative to
metallocorroles that were introduced since [9], the iron
related porphyrin complexes was advantageously used
for introducing corrole-based catalytic antioxidants
[16–18]. Amphipolar and water soluble ITC have been
SPP full member in good standing
shown to be excellent decomposition catalysts of reactive
oxygen/nitrogen species and the medical relevance of
*Correspondence to: Zeev Gross, email: chr10zg@tx.technion.
ac.il, fax: +972 4-829-5703
Copyright © 2012 World Scientific Publishing Company

664
P. SINGH ET AL.
these findings are under extensive investigation by using
quite a large number of in vitro and in vivo models of
various diseases [19]. Another unexpected reactivity of
ITC, which is also shared with iron(III) porphyrins but
no other metal complexes, is the ability to activate the
NH and SH bonds of amines and thiols, respectively,
with regard to their reactions with diazo compounds such
as ethyldiazoacetate (EDA) [20, 21]. In sharp contrast
with almost all other metal-catalyzed reactions of EDA,
significant evidence points towards the non-involvement
of metal-carbene intermediates in the ITC-catalyzed
reactions [22].
The vast majority of the research activity on
ITC was performed on the iron complexes of
tris(pentafluorophenyl)corrole, 2-H3, and its b-pyrrole-
substituted derivatives. The underlying reasons for
this choice are: (a) this corrole is particularly stable,
since its electron withdrawing C6F5 substitutents
remove electron-density from the very electron-rich
macrocycle [23]; (b) its synthetic accessibility is very
high [24]; and (c) its structural assignment is assisted
by 19F NMR [25]. The first two of the above factors are
also shared by tris(4-nitrophenyl)corrole, 1-H3, which
has been reported to be accessible in 21% chemical
yield by the cyclocondensation between pyrrole and
4-nitrobenzaldehyde [26]. It is hence quite surprising
that the coordination chemistry of 1-H3 remains quite
unexplored [27, 28], taking into account also the large
difference in the price of the aldehydes required for the
synthesis of 1-H3 and 2-H3, respectively. Based on the
above, we have decided to explore the iron complexes
of 1-H3, with special emphasis on the properties of the
nitrosyl complex regarding the NO stretching frequency
and the redox processes, in comparison with (nitrosyl)-
iron complexes of other corroles and porphyrins.
the electronic spectra. Mass spectroscopy was performed on
a Finnigan TSQ70 instrument with isobutane as carrier gas
and IR spectra were recorded as thin films on KBr pellet on a
FT-IR spectrometer Bruker Vector 22. Cyclic voltammetric
measurements were recorded on a WaveNow USB
potentiostat Galvanostat (Pine Research Instrumentation),
using Pine AfterNath Data Organizer software. A three
electrode system was used, consisting of a platinum wire
working electrode, a platinum wire counter electrode, and
an Ag/AgCl reference electrode. The CV measurements
were performed in CH2Cl2 solutions, 0.1M in tetrabu-
tylammonium perchlorate (TBAP, Fluka, recrystallized
twice from methanol), and 1 mM substrate under N2 atmo-
sphere at ambient temperature. Scan rates of 10–1000 mV/s
were applied. UV-visible spectroelectrochemical experi-
ments were performed by using a commercial thin layer
quartz glass spectroelectrochemical cell kit (Supplier:
Bio-Logic SAS). The cell contains a thin layer quartz
glass cell (light pass length: 0.5 mm), a platinum gauze
working electrode (80 mesh), and a platinum wire counter
electrode. An Ag/AgCl electrode was used as reference
electrode. Potentials were applied with the WaveNow USB
potentiostat/galvanostat. Time-resolved UV-visible spectra
were recorded with a Hewlett-Packard Model 8453 diode
array spectrophotometer. The EPR spectra were recorded
on a Bruker EMX-10/12 X-band (n = 9.3 GHz) digital
EPR spectrometer equipped with a Bruker N2-temperature
controller. EPR spectra processing and simulation were
performed with the Bruker WIN-EPR and SimFonia
software, respectively.
Synthesis
5,10,15-tris(4-nitrophenyl)corrole 1-H3 was prepared
by the reaction between pyrrole and p-nitrobenzaldehyde
in acetic acid as reported earlier [26]. 2-H3 and its
complexes were synthesized as described in previous
publications [24, 10a].
EXPERIMENTAL
Synthesis of 1-Fe(py)2. 1-H3 (66 mg, 0.1 mmol)
was dissolved in pyridine (40 mL) under N2 and ferrous
chloride tetrahydrate (0.25 g, 2 mmol) was added into it.
The reaction mixture was refluxed for 20 minutes. After
completing the iron insertion, the reaction mixture was
cooled to 25 °C and the solvent was evaporated. The
resulting solid material was dissolved in dichloromethane
containing a few drops of pyridine and chromatographed
over acidic alumina with CH2Cl2/hexane/pyridine
(3:2:0.06) as an eluent. Compound 1-Fe(py)2 was
obtained as a brown solid (yield 46 mg, 53%). UV-vis
(benzene:pyridine 49:1): lmax, nm (log e) 428 (4.9),
583(2.2). 1H NMR (200 MHz; pyridine-d5): dH, ppm -1.7
(2H, s, pyrrole-H), -39.9 (2H, s, pyrrole-H), -54.2 (2H, s,
pyrrole-H), -122.1 (2H, s, pyrrole-H), MS (ESI-, CH3CN):
m/z 750.9 (100%) [M–2Py + 2H2O]-, 792.8 (88%) [M–
Py]-, 826.9 (28%) [M–2Py + 2CH3CN]-.
Materials
Ferrous chloride tetrahydrate (Fluka Chemika),
pyridine (Merck), and deuterated solvents (Aldrich and
Cambridge Isotopes products) were used as received.
Solvents like dichloromethane and hexane were obtained
from S. D. Fine Chemicals Ltd. Tetrabutyl ammonium
perchlorate was obtained from Aldrich and was
recrystallized twice from methanol. Sodium borohydride
and tris-(4-bromophenyl)aminium hexachloroantimonate
were obtained from Aldrich.
Physical measurements
The 1H NMR spectra were recorded on a Bruker Avance
200 spectrometer operating at 200 MHz. Chemical shifts
1
in the H NMR spectra are reported in ppm relative to
Synthesis of 1-FeCl. 1-Fe(py)2 (40 mg) was dissolved
in CH2Cl2, the solution was washed twice with HCl (7%)
the residual hydrogens in the deuterated solvents. An HP
8452A diode array spectrophotometer was used to record
Copyright © 2012 World Scientific Publishing Company
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IRON COMPLEXES OF TRIS(4-NITROPHENYL)CORROLE, WITH EMPHASIS ON THE (NITROSYL)IRON COMPLEX
665
and finally with water. During the acidic washing, the
color of the mixture changed from red-brown to pale
green. The solution was dried over Na2SO4 and the solvent
was evaporated. Compound 1-FeCl was obtained by
recrystallization from CH2Cl2 and hexane as a green solid
(yield 32 mg, 93%). UV-vis (CH2Cl2): lmax, nm (log e)
nitrogen until the measurements. The sample preparation
was done in a glove box.
RESULTS AND DISCUSSION
1
409 (6.3), 396 (4.6). H NMR (200 MHz; CDCl3): dH,
Synthesis and NMR characterization
ppm 5.1 (2H, s, pyrrole-H), -5.7 (2H, s, pyrrole-H), -7.3
(2H, s, pyrrole-H), -38.5 (2H, s, pyrrole-H), 23.5 (2H, s,
ortho-H of meso-Ph), 22.4 (2H, s, ortho-H of meso-Ph),
21.5 (2H, s, ortho-H of meso-Ph), -1.2 (2H, s, meta-H
of meso-Ph), -1.3 (2H, s, meta-H of meso Ph), -2.1 (2H,
s, meta-H of meso Ph). MS (ESI-, CH2Cl2): m/z 748.9
[M–H]-.
Free base corrole 1-H3 was prepared via the
condensation of p-nitrobenzaldehyde with excess pyrrole
in acetic acid medium. Methods for inserting iron into
core of corrole were already fully described in previous
publications [10]. We adopted the same procedure to
insert iron into corrole 1-H3, by refluxing the solution
of 1-H3 in pyridine with excess ferrous chloride under
an inert atmosphere. Column chromatography in the
presence of pyridine in the eluting solvent mixture led to
isolation of the red colored bis-pyridine iron(III) complex
1-Fe(py)2 [10c,d]. The treatment of 1-Fe(py)2 with 7%
HCl at room temperature provided the pale green iron
complex 1-FeCl, due to aerobic oxidation (Scheme 1).
The iron complexes 1-Fe(py)2 and 1-FeCl are
Synthesis of 1-Fe(NO) and 1-Fe(15NO). 1-FeCl
(20 mg, 26.7 µmol) was dissolved in CH2Cl2 (20 mL) and
a saturated solution of NaNO2 (2 mL) was added into it.
After 3 h of stirring at rt, the color of the solution became
reddish orange. The organic solvent was washed with
water, dried over Na2SO4 and evaporated. The compound
was purified by column chromatography over silica
gel using hexane/CH2Cl2 (2:3) as eluent. Compound
1-Fe(NO) was obtained as a red solid (yield 8.5 mg,
43%). UV-vis (CH2Cl2): lmax, nm (log e) 383 (11.22), 540
(1.81). 1H NMR (300 MHz; CDCl3): dH, ppm 7.37 (2H, d,
3J(H,H) = 4.7 Hz, pyrrole-H), 7.54 (2H, d, 3J(H,H) = 4.7
Hz, pyrrole-H), 7.76 (2H, d, 3J(H,H) = 4.7 Hz, pyrrole-H),
7.85 (1H, dd, 3J(H,H) = 8.3 Hz, phenyl-H), 7.98 (1H, dd,
3J(H,H) = 8.3 Hz, phenyl-H), 8.04 (4H, d, 3J(H,H) = 8.8
Hz, phenyl-H), 8.10 (2H, d, 3J(H,H) = 4.7 Hz, pyrrole-H),
8.46 (1H, dd, 3J(H,H) = 8.3 Hz, phenyl-H), 8.50 (1H, dd,
3J(H,H) = 8.3 Hz, phenyl-H), 8.51 (4H, d, 3J(H,H) = 8.6
Hz, phenyl-H). IR (KBr): l, cm-1 1775 (NO stretching).
MS (ESI-, CH2Cl2): m/z 743.9 [M–H]-.
1
paramagnetic, which is demonstrated by their H NMR
spectra (Figs 1a and 1b, respectively). Their oxidation and
spin states were assigned based on the paramagnetic shifts
of the b-pyrrole CH resonances of iron corroles, in relation
topreviouspublications[10].In1-Fe(py)2,alltheb-pyrrole
CH resonances appear at high field region (-1.7, -39.9,
-54.2 and -122.1 ppm), thus pointing towards low spin
iron(III). On the other hand, the spectrum of 1-FeCl is
similar to that of the analogous [(tpfc)FeCl, 2-FeCl], with
less shifted high field resonances for the b-pyrrole CH’s
(5.1, -5.7, -7.3 and -38.5 ppm). In this case, additional
resonances attributable to the p-nitrophenyl groups are
also seen. All the low field chemical shifts (~20 ppm) are
confidently assigned to ortho-H protons, because they are
absent in chloroiron complexes of tris(ortho-substituted-
phenyl)corroles [10d]. The most straightforward 1H
NMR assignment of the iron corroles may be obtained by
converting the paramagnetic complexes to diamagnetic
(nitrosyl)iron corrole [10b,c]. This was achieved by the
treatment of 1-FeCl with a saturated solution of sodium
nitrite at room temperature, leading to the orange-colored
(nitrosyl)iron corrole 1-Fe(NO), a {FeNO}6 complex
by the Enemark and Feltham nomenclature [29]. Using
Na15NO2 instead of sodium nitrite led to the 15N-labeled
complex 1-Fe(15NO). The nitrosyl(iron) complexes
2-Fe(NO) and 2-Fe(15NO) of the most extensively studied
trispentafluorophenylcorrole were also prepared by the
same methodology. In addition, the isotopically pure 57Fe
complexes 2-57Fe(NO) and 2-57Fe(15NO) were prepared
from 2-57FeCl, which was obtained via metallation of
2-H3 by 57Fe in acetic acid.
The 15N substituted analog of compound 1-Fe(NO),
1-Fe(15NO)] was synthesized as described above, by
using 15N-labelled NaNO2.
Crystal data: 2(C H FeN O ) 5(CHCl ), Mr =
·ꢀ
37 20
8
7
3
2085.76, monoclinic, space group Cc, a = 25.643(2),
b = 16.4041(10), c = 21.3666(17) Å, b = 111.874(3)°,
V = 8340.8(11) Å3, Z = 4, T/K = 110(2), Dc/g.cm-3
=
1.661. Final R1 = 0.123 for 11666 reflections with F >
4s (F), R1 = 0.142 and wR2 = 0.30 for all 13989 data.
Preparation of [1-Fe(NO)]-. 1-Fe(NO) (10 mg,
13.4 µmol) and sodium borohydride (1 mg, 26.9 µmol)
were dissolved in CH2Cl2 (5 mL) and ethanol (5 mL),
respectively. An equal volume was taken from each
solution and mixed within the quartz ESR cuvette. The
ESR tube was sealed and kept frozen with liquid nitrogen
until the measurements. The sample preparation was
done in a glove box.
Preparation of [1-Fe(NO)]+. 1-Fe(NO) (5.7 mg, 7.7
µmol) and tris(4-bromophenyl)aminium hexachloro-
antimonate (1.5 mg, 1.9 µmol) were dissolved in CH2Cl2
(5 mL) in separate vials. An equal volume was taken
from each vial and mixed within the quartz ESR cuvette.
The ESR tube was sealed and kept frozen with liquid
The 1H NMR spectrum of 1-Fe(NO) (Fig. 1c)
displays four doublets (7.37, 7.54, 7.76, 8.1 ppm) with
characteristic coupling constants of J ~ 4.7 Hz [25], which
correspond to the eight b-pyrrole hydrogen atoms. The
Copyright © 2012 World Scientific Publishing Company
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666
P. SINGH ET AL.
NO2
NO2
L
N
N
N
N
HN
FeCl2, Py
reflux, N2
Fe
NO2
NO2
O2N
O2N
N
NH HN
L
L = Py
1-H3
1-Fe(py)2
7% HCl, rt, air
NO2
NO2
Cl
NO
N
N
N
N
N
N
N
N
NaNO2 ,rt
Fe
Fe
NO2
O2N
NO2
O2N
Na15NO2
1-FeCl
1-Fe(NO)
1-Fe(15NO)
Scheme 1. Synthetic scheme for the iron complexes of corrole 1-H3
1
ο
Fig. 1. H NMR spectra of different iron complexes of corrole 1-H3 at 25 C: (a) 1-Fe(py)2 in pyridine-d5; (b) 1-FeCl in CDCl3;
(c) 1-Fe(NO) in CDCl3
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 666–673

IRON COMPLEXES OF TRIS(4-NITROPHENYL)CORROLE, WITH EMPHASIS ON THE (NITROSYL)IRON COMPLEX
667
penta-coordinated 1-Fe(NO) is of Cs symmetry, leading
to magnetically identical C5 and C15 aryls and a unique
C10 aryl. This is reflected in a 2:1 ratio of ortho-H protons
at 8.04 ppm (4H of the C5 and C15 aryls) and 8.50 ppm
(2H of the C10 aryl) characterized by J ~ 8.8 Hz coupling
constants. Surprisingly, the meta-H resonances are more
separated, as seen by the three double doublets at 7.85,
7.98 and 8.46 ppm (the three other signals overlap with
the ortho-H protons).
crystals that were obtained by the slow evaporation of a
saturated solution of 1-Fe(NO) in chloroform.
Crystallography
The diffraction measurements were carried out on
a Nonius Kappa CCD diffractometer, using graphite
monochromated Mo Ka radiation (l = 0.7107 Å). The
structure was solved by direct method (SIR-97) and
refined by full-matrix least squares on F2 (SHELX-97).
All non-hydrogen atoms were refined anisotropically.
The compound 1-Fe(NO) crystallized with high content
of uncoordinated solvent molecules, which are partly
disordered in the crystal lattice, thus affecting the crystal
quality and lowering the precision of the otherwise
unequivocal structure determination.
The spectroscopy-based assignment of 1-Fe(NO) was
confirmed by X-ray crystallography, based on single
The asymmetric unit of the 1-Fe(NO) crystals
contains two crystallographically independent (nitrosyl)-
iron corrole molecules (Fig. 2), along with five molecules
of the chloroform solvent. The most important structural
parameters are tabulated in Table 1.
The iron ion in the square pyramidal coordination
sphere is displaced out of the basal plane of the four
pyrrole N-atoms by 0.468(5) and 0.464(5) Å towards
the axial ligand. As discussed earlier [28], the neutral
(nitrosyl)iron corroles are similar to the cationic (nitrosyl)
iron porphyrins by means of a linear Fe–N–O moiety,
consistent with {FeNO}6. The structure of 1-Fe(NO)
reveals Fe–N–O angles of 173(1)°, similar to previously
reported iron corroles (Table 2). This feature is also
consistent with the earlier mentioned diamagnetism.
A diagnostic tool of the coordination mode of
nitrosyl ligands is IR, because of the large differences
between the n(N–O) stretching frequencies of linear
and bent Metal–N–O moieties. The FT-IR spectra of
the (nitrosyl)iron corrole complexes were obtained
as thin films on KBr (Fig. 3), revealing that the NO
stretching frequency in both 1-Fe(NO) and 2-Fe(NO)
are well within the range of linear nitrosyl(iron)
complexes expected for {FeNO}6 [30]. This assignment
was reinforced by the isotope sensitivity of n(14N–O)
Fig. 2. Structure (ORTEP view) of the two crystallographically
independent molecule of 1-Fe(NO), showing 50% probability
thermal ellipsoids. Hydrogen atoms are omitted for clarity
Table 1. Selected bond lengths (Å) of the two molecules of
1-Fe(NO) in the asymmetric unit
Bond length
Molecule 1
Molecule 2
Fe–N21
Fe–N22
Fe–N23
Fe–N24
Fe–N(NO)
1.914(10)
1.906(9)
1.897(9)
1.967(10)
1.966(9)
1.910(9)
1.648(11)
1.888(10)
1.886(10)
1.675(14)
N–O
Fe–plane
1.144(15)
0.468(5)
1.155(13)
0.464(5)
Table 2. Structural parameters and the NO stretching frequency of 1-Fe(NO), in comparison with previously characterized (nitrosyl)-
iron corroles (oec = octaethylcorrole, tdcc = tris(o-dichlorophenyl)corrole, ttc = tris(p-methylphenyl)corrole, tmopc = tris(p-
methoxyphenyl)corrole)
Complex
FeN(corrole)(Å) Fe–NO ang (deg) Fe out-of-plane(Å)
N–O(Å)
NO stret. cm-1
1767
Ref.
6
Fe(NO)(oec)
2-Fe(NO)a
1.909(3)
176.9(3)
0.470(1)
1.171(4)
1.910(4),
1.910(4)
177.3,
178.0(4)
1.164,
1.166(4)
0.465, 0.464(2)
1790
10a
Fe(NO)(tdcc)
Fe(NO)(ttc)
1.910(4)
1.913(2)
1.898(4)
172.3(4)
177.1(2)
172.0(4)
0.452(2)
0.455(1)
0.449(2)
1.169(5)
1.162(2)
1.076(4)
1783
1761
1767
10a
28
Fe(NO)(tmopc)
28
1.888(10),
1.935(9)
172.8(10),
173.3(9)
0.468(5),
0.464(5)
1.144(15),
1.154(13)
1-Fe(NO)a
1775
this work
a There are two crystallographically independent corrole species in the asymmetric unit of these structures.
Copyright © 2012 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2012; 16: 667–673

668
P. SINGH ET AL.
Fig. 3. FT-IR spectra (films on KBr pellets) of (nitrosyl)iron corroles: (a) 1-Fe(NO) (gray) and 1-Fe(15NO) (black); (b) 2-Fe(NO)
(gray), 2-57Fe(NO) (black), 2-57Fe(15NO) (dotted), and 2-Fe(15NO) (long dashed line)
vs. n(15N–O), shifting from 1775 cm-1 in 1-Fe(NO) to
Table 3. FT-IR band stretching frequencies of the investigated
1733 cm-1 in the 15N-labeled complex 1-Fe(15NO) (Dexp.
=
(nitrosyl)iron corroles and of some analogous complexes
reported in the literature
42 cm-1). A practically identical shift of 41 cm-1 was
identified for the tpfc-ligated complexes, from 1797
cm-1 in 2-Fe(NO) to 1756 cm-1 in 2-Fe(15NO). As may
have been expected, the effect of 57Fe substitution was
marginal: 1797 in 2-Fe(NO) vs. 1799 in 2-57Fe(NO) and
1756 cm-1 in 2-Fe(15NO) vs. 1759 cm-1 in 2-57Fe(15NO).
The main motivation for the preparation of the
57Fe-labeled complexes was not for that purpose, but
rather for possible determination of the Fe–NO modes.
The spectral area where such bands may be suspected
(600–400 cm-1), based on investigation of natural and
synthetic (nitrosyl)iron complexes by Resonance Raman
spectroscopy [31], was examined. Unfortunately, no
isotope-sensitive bands could be identified with large
enough confidence by IR spectroscopy.
a
Complex
Isotope N–O stret. (cm-1) Dexp. Ref.
Fe
N
O
1-Fe(NO)
1-Fe(15NO)
2-Fe(NO)
2-Fe(15NO)
56 14 16
56 14 16
56 14 16
56 15 16
1775
1733
1797
1756
b
b
b
b
+42
+41
-2
2-57Fe(NO)
57 14 16
57 15 16
56 14 16
56 14 16
1799
1759
1767
1859
1837
b
b
2-57Fe(15NO)
Fe(NO)(oec)
[Fe(NO)(tpfpp)]+
+38
6
32
[Fe(15NO)(tpfpp)]+ 56 15 16
+22 32
a Relative to the 56Fe–14N–16O complex. b This work.
Additional insight was obtained from the data in
Table 3, which lists the NO stretching frequencies of
the corroles under study together two other selected
{FeNO}6 (nitrosyl)iron complexes [32]. One interesting
aspect is the quite large effect of the chelating
macrocycle in these isoelectronic complexes on the
n(NO) frequency: 1859, 1797, 1775, and 1767 cm-1 for
[Fe(NO)(tpfpp)]+, 2-Fe(NO), 1-Fe(NO), and Fe(NO)-
(oec), respectively (tpfpp = the dianionic form of
5,10,15,20-tetrakispentafluorophenylporphyrin; oec = the
trianionic form of 2,3,6,7,11,12,16,17-octaethylcorrole).
This pattern clearly reflects the electron-richness: much
larger for the trianionic corrolates than for the dianionic
porphyrinates; and within the former series, largest for
the b-pyrrole-alkyl-subsituted Fe(NO)(oec), followed
by meso-nitrophenyl-substituted 1-Fe(NO), and least so
for the meso-pentafluorophenyl-substituted 2-Fe(NO).
We also note that the experimental isotope shifts in the
(nitrosyl)iron triarylcorrole complexes are 38–42 cm-1,
much larger than the expected 32 cm-1 for a simple
diatomic model, but perfectly in the range of linear
nitrosyl-metal complexes [33].
Electrochemistry
Given the limited information about the oxidized and
reduced states of (nitrosyl)iron corroles, emphasis was
given to their characterization. The cyclic voltammogram
of 1-Fe(NO) reveals reversible oxidation and reduction
waves with E1/2 values of 1.03 V and -0.11 V, respectively
(Fig. 4).
The spectroelectrochemical results for 1-Fe(NO) are
shown in Fig. 5. Holding the applied potential at -0.3 V
reveals that the intensity of the Soret band at 382 nm
decreased and shifted to 395 nm, and that the single band
at 544 nm was replaced by two new bands at 524 nm and
662 nm (Fig. 5a). Identical characteristics were obtained
by the chemical reduction of 1-Fe(NO) with sodium
borohydride, in a CH2Cl2:C2H5OH (1:1) solvent mixture
(Fig. 5b). Formulation of this reduced species by virtue
of the redox centre, i.e. the metal or the macrocycle, was
achieved by EPR spectroscopy (vide infra).
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669
reduction and red shift of the Soret band, as well as
the growth of a new absorbance at about 650 nm. This
phenomenon is different from what has been observed
in nitrosyl(iron) tetraarylporphyrins, where the Soret
band of the {FeNO}7 complexes are slightly blue
shifted and more intense than those of the {FeNO}6
complexes [34].
EPR spectroscopy
The chemically reduced iron complexes [1-Fe(NO)]-
and [1-Fe(15NO)]- with coordinated 14NO and 15NO,
respectively, were examined by EPR. The spectrum of
[1-Fe(NO)]- (Fig. 7a) in CH2Cl2:C2H5OH (1:1) at 290 K
displays an intense triplet due to the interaction of the
unpaired electron with the 14N nuclei, with g = 2.032 and
aN(14N) = 16.0 G. The 15N- labeled complex [1-Fe(15NO)]-
discloses an intense doublet (g=2.032) (Fig. 7b), with
aN(15N) = 22.3 G. The ratio of the hyperfine coupling
constants aN(15N)/aN(14N) is 1.4, perfectly consistent with
the ratio of the nuclear magnetic moments of 15N and
14N; µ(15)/µ(14) =1.4 [35] .
Fig. 4. Cyclic voltammogram of 1-Fe(NO) in CH2Cl2 containing
0.1 M TBAP
The EPR spectrum of [1-Fe(NO)]- in the frozen
CH2Cl2:C2H5OH solvent mixture (130 K) is rhombic
(Fig. 8a), displaying diagonal components of the g-tensor
(g1 = 2.014, g2 = 2.037, and g3 = 2.046). The hyperfine
coupling tensor A with 14N (A1 = 16.1 G, A2 = 17.2 G,
and A3 = 16.0 G) may be deduced from the spectrum as
well, as illustrated by the spectral simulation that is based
on these parameters (Fig. 8b). The spectrum of the 15N-
labeled complex [1-Fe(15NO)]- reveals identical g-factors,
as well as the expected 40% increase in the hyperfine
coupling tensors A of A1 = 22.8 G, A2 = 24.0 G, and
Upon oxidation of 1-Fe(NO) at an applied potential
of 1.2 V, the Soret band intensity decreased and its lmax
shifted to the red by 13 nm. In addition, the 544 nm
band disappeared and a new band appeared at 636 nm
(Fig. 6a). Chemical oxidation of 1-Fe(NO) by tris(4-
bromophenyl)aminium hexachloroantimonate induced
identical changes (Fig. 6b). Comparison of the spectral
changes occurring by either oxidation or reduction of
1-Fe(NO) reveals a surprising similarity: an intensity
Fig. 5. UV-vis spectral changes of 1-Fe(NO) upon (a) controlled reduction potential at -0.3 V in CH2Cl2 containing 0.1 M TBAP,
and (b) chemical reduction by sodium borohydride in CH2Cl2:C2H5OH solvent mixture. Black line for 1-Fe(NO) and gray line for
the reduced species
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670
P. SINGH ET AL.
Fig. 6. UV-vis spectral changes of 1-Fe(NO) via (a) controlled oxidation potential at 1.2 V in CH2Cl2 containing 0.1 M TBAP, and
(b) chemical oxidation by tris(4-bromophenyl)aminium hexachloroantimonate, in CH2Cl2. Black line for 1-Fe(NO) and gray line
for the oxidized species
Fig. 7. EPR spectra of (a) [1-Fe(NO)]- (b) [1-Fe(15NO)]-, in
CH2Cl2:C2H5OH at 290 K
A3 = 22.0 G (Fig. 8d). Taken together, the EPR results
of the reduced (nitrosyl)iron complex are fully consistent
with an FeNO-centered redox process [36]. This
Fig. 8. EPR spectra in frozen CH2Cl2:C2H5OH solution at 130 K
of (a) [1-Fe(NO)]- and (c) [1-Fe(15NO)]-, together with the
conclusion is substantiated via comparison with literature
simulated spectra (b) and (d), respectively)
data on porphyrin-chelated {FeNO}7 complexes (Table
4) by virtue of the similarity of both the g-tensor and the
paramagnetic particles: the intensive isotropic part, due
to the organic radical (tris(4-bromophenyl)aminium
hexachloroantimonate) that was applied as oxidant, and
two features marked by (*), which are attributed to the
hyperfine coupling constants.
The EPR spectrum of the one-electron oxidized
derivative of 1-Fe(NO) ([1-Fe(NO)]+, Fig. 9a) in frozen
CH2Cl2 solution is a superposition of signals from two
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IRON COMPLEXES OF TRIS(4-NITROPHENYL)CORROLE, WITH EMPHASIS ON THE (NITROSYL)IRON COMPLEX
671
Table4.gfactorsandcouplingconstantsofvarious(nitrosyl)ironcomplexes(tpp=tetraphenylporphyrin)
Compound
Solvent, T(K)
g factor
A (Gauss)
Ref.
FeII(NO)(tpp)
FeII(NO)(tpp)
[FeII(NO)(oec)]-
[Fe(NO)(oec)]+
Toluene, 120
Toluene, 253
PhCN, 120
CH2Cl2, 77
2.10, 2.06, 2.01
2.054
12.6, 17.2, 17.3
17.4
36
36
6
2.08, 2.04, 2.00
2.02, 2.00, 1.98
6
[1-Fe(14NO)]-
[1-Fe(14NO)]-
[1-Fe(15NO)]-
[1-Fe(15NO)]-
CH2Cl2:C2H5OH, 290
CH2Cl2:C2H5OH, 130
CH2Cl2:C2H5OH, 290
CH2Cl2:C2H5OH, 130
2.032
16
this work
2.046, 2.037, 2.014
2.032
16.1, 17.2, 16.0 this work
22.3
this work
this work
2.046, 2.037, 2.014
22.8, 24, 22
[1-Fe(14NO)]+
CH2Cl2, 130
2.038, 2.004, 1.97
this work
SUMMARY AND CONCLUSIONS
We report an in depth investigation of the (nitrosyl)iron
complex of 5,10,15-tris(4-nitrophenyl)corrole 1-Fe(NO)
and its 15N-labeled analog, by a combination of X-ray
crystallography, IR, NMR, and electrochemistry, as well
as of the one-electron oxidized and reduced complexes
by EPR. The comparison with other (nitrosyl)iron corrole
and porphyrins reveals that 1-Fe(NO) is a genuine
{FeNO}6 complex, with an almost perfectly linear
Fe–N–O bond and large isotope shifts of the nitrosyl
moiety. The reduced complex displays EPR parameters
at both high and low temperatures that imply that it is an
authentic {FeNO}7 complex.
Acknowledgements
This work was supported by the Israel Science
Foundation (ISF) and the United States-Israel Binational
Science Foundation (BSF). Tumanskii B. and Saltsman I.
are grateful for support from the Center of Absorption
in Science, State of Israel, Ministry of Immigrant
Absorption.
Fig. 9. EPR spectrum (frozen CH2Cl2 at 130 K) of [1-Fe(NO)]+,
obtained via oxidation of 1-Fe(NO) by using tris(4-
bromophenyl)aminium hexachloroantimonate as oxidant: (a)
experimental spectrum; (b) simulated spectrum of the oxidant,
(c) spectrum obtained by subtraction (b) from (a)
Supporting information
Crystallographic data for 1-Fe(NO) have been
deposited at the Cambridge Crystallographic Data
Centre (CCDC) under number CCDC-867402. Copies
can be obtained on request, free of charge, via www.
ccdc.cam.ac.uk/data_request/cif or from the Cambridge
Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK (fax: +44 1223-336-033 or
email: deposit@ccdc.cam.ac.uk).
components of the g-tensor of [1-Fe(NO)]+. Apparently,
only partial oxidation was obtained, which is consistent
with the almost identical redox potentials of (tris(4-
bromophenyl)aminium hexachloroantimonate) [9b] and
[1-Fe(NO)]+.
Subtraction of the contribution from tris(4-
bromophenyl)aminium hexachloroantimonate (Fig. 9b)
from the experimental spectrum discloses all components
of the g-tensor of [1-Fe(NO)]+: g1 = 2.038, g2 = 2.004,
g3 = 1.970 (Fig. 9c). The hyperfine coupling tensor A
values could not be obtained with meaningful confidence
because of the applied procedure.
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