Synthesis, spectral characterization, structures, and oxidation state distributions in [(corrolato)FeIII(NO)]n (n = 0, +1, -1) complexes

Article abstract of DOI:10.1021/ic402304e

Two novel trans-A₂B-corroles and three [(corrolato){FeNO} ⁶] complexes have been prepared and characterized by various spectroscopic techniques. In the native state, all these [(corrolato){FeNO} ⁶] species are diamagnetic and display "normal" chemical shifts in the ¹H NMR spectra. For two of the structurally characterized [(corrolato){FeNO}⁶] derivatives, the Fe-N-O bond angles are 175.0(4) and 171.70(3) (DFT: 179.94), respectively, and are designated as linear nitrosyls. The Fe-N (NO) bond distances are 1.656(4) A and 1.650(3) A (DFT: 1.597 A), which point toward a significant Fe^III → NO back bonding. The NO bond lengths are 1.159(5) A and 1.162(3) A (DFT: 1.162 A) and depict their elongated character. These structural data are typical for low-spin Fe(III). Electrochemical measurements show the presence of a one-electron oxidation and a one-electron reduction process for all the complexes. The one-electron oxidized species of a representative [(corrolato){FeNO}⁶] complex exhibits ligand to ligand charge transfer (LLCT) transitions (cor(π) → cor(π)) at 399 and 637 nm, and the one-electron reduced species shows metal to ligand charge transfer (MLCT) transition (Fe(dπ) → cor(π)) in the UV region at 330 nm. The shift of the ν_NO stretching frequency of a representative [(corrolato){FeNO}⁶] complex on one-electron oxidation occurs from 1782 cm^-1 to 1820 cm^-1, which corresponds to 38 cm^-1, and on one-electron reduction occurs from 1782 cm ^-1 to 1605 cm^-1, which corresponds to 177 cm^-1. The X-band electron paramagnetic resonance (EPR) spectrum of one-electron oxidation at 295 K in CH₂Cl₂/0.1 M Bu₄NPF ₆ displays an isotropic signal centered at g = 2.005 with a peak-to-peak separation of about 15 G. The in situ generated one-electron reduced species in CH₂Cl₂/0.1 M Bu₄NPF ₆ at 295 K shows an isotropic signal centered at g = 2.029. The 99% contribution of corrole to the HOMO of native species indicates that oxidation occurs from the corrole moiety. The results of the electrochemical and spectroelectrochemical measurements and density functional theory calculations clearly display a preference of the {FeNO}⁶ unit to get reduced during the reduction step and the corrolato unit to get oxidized during the anodic process. Comparisons are presented with the structural, electrochemical, and spectroelectrochemical data of related compounds reported in the literature, with a particular focus on the interpretation of the EPR spectrum of the one-electron oxidized form.

Full text of DOI:10.1021/ic402304e

Article
pubs.acs.org/IC
Synthesis, Spectral Characterization, Structures, and Oxidation State
Distributions in [(corrolato)FeIII(NO)]n (n = 0, +1, −1) Complexes
Woormileela Sinha,† Naina Deibel,‡,§ Hemlata Agarwala,# Antara Garai,† David Schweinfurth,‡
Chandra Shekhar Purohit,† Goutam Kumar Lahiri,*,# Biprajit Sarkar,*,‡ and Sanjib Kar*,†
†School of Chemical Sciences, National Institute of Science Education and Research (NISER), Bhubaneswar, 751005, India
‡Institut fur Chemie und Biochemie, Anorganische Chemie, Freie Universitat Berlin, Fabeckstraße 34-36, D-14195, Berlin, Germany
̈
̈
§Institut fur Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70550, Stuttgart, Germany
̈
̈
#Department of Chemistry, Indian Institute of Technology−Bombay, Powai, Mumbai 400076, India
S
* Supporting Information
ABSTRACT: Two novel trans-A2B-corroles and three
[(corrolato){FeNO}6] complexes have been prepared and
characterized by various spectroscopic techniques. In the
native state, all these [(corrolato){FeNO}6] species are
1
diamagnetic and display “normal” chemical shifts in the H
NMR spectra. For two of the structurally characterized
[(corrolato){FeNO}6] derivatives, the Fe−N−O bond angles
are 175.0(4)° and 171.70(3)° (DFT: 179.94°), respectively,
and are designated as linear nitrosyls. The Fe−N (NO) bond
distances are 1.656(4) Å and 1.650(3) Å (DFT: 1.597 Å),
which point toward a significant FeIII → NO back bonding.
The NO bond lengths are 1.159(5) Å and 1.162(3) Å (DFT:
1.162 Å) and depict their elongated character. These structural data are typical for low-spin Fe(III). Electrochemical
measurements show the presence of a one-electron oxidation and a one-electron reduction process for all the complexes. The
one-electron oxidized species of a representative [(corrolato){FeNO}6] complex exhibits ligand to ligand charge transfer
(LLCT) transitions (cor(π) → cor(π*)) at 399 and 637 nm, and the one-electron reduced species shows metal to ligand charge
transfer (MLCT) transition (Fe(dπ) → cor(π*)) in the UV region at 330 nm. The shift of the νNO stretching frequency of a
representative [(corrolato){FeNO}6] complex on one-electron oxidation occurs from 1782 cm−1 to 1820 cm−1, which
corresponds to 38 cm−1, and on one-electron reduction occurs from 1782 cm−1 to 1605 cm−1, which corresponds to 177 cm−1.
The X-band electron paramagnetic resonance (EPR) spectrum of one-electron oxidation at 295 K in CH2Cl2/0.1 M Bu4NPF6
displays an isotropic signal centered at g = 2.005 with a peak-to-peak separation of about 15 G. The in situ generated one-
electron reduced species in CH2Cl2/0.1 M Bu4NPF6 at 295 K shows an isotropic signal centered at g = 2.029. The 99%
contribution of corrole to the HOMO of native species indicates that oxidation occurs from the corrole moiety. The results of the
electrochemical and spectroelectrochemical measurements and density functional theory calculations clearly display a preference
of the {FeNO}6 unit to get reduced during the reduction step and the corrolato unit to get oxidized during the anodic process.
Comparisons are presented with the structural, electrochemical, and spectroelectrochemical data of related compounds reported
in the literature, with a particular focus on the interpretation of the EPR spectrum of the one-electron oxidized form.
irreversible. Methemoglobin, where iron is in the +3 oxidation
state, binds NO• in a reversible fashion, whereas oxy-
hemoglobin with iron in the +2 oxidation state, binds NO•
in an irreversible fashion.3 The signaling mechanism of NO• is
directly dependent on the reversible binding of NO•. Thus, it is
evident that the mechanism of binding of NO• with
hemoproteins is of considerable importance from a biological
and chemical point of view. This has led to the development of
an intense research interest in elucidating the mechanism of the
interaction of NO• with hemoproteins and their model
INTRODUCTION
■
The free radical NO• acts as a messenger molecule in biological
systems by activating the ferrous heme of the guanylate cyclase
(sGC).1 Implications of this signaling behavior have been
widely studied by many researchers in chemistry, biology, and
medicine. NO•, widely known as reactive nitrogen species
(RNS), is also essential for homeostasis of cellular organism.2
Understanding the mechanism of redox signaling is of great
importance from the perspective of physiological functions.
Inside cellular organisms, NO• targets a variety of metal-
loproteins including hemoglobin and cytochrome c oxidase
(CcOx). However, the binding of NO• with metalloproteins is
case sensitive. The binding can be either reversible or
Received: September 11, 2013
Published: January 16, 2014
© 2014 American Chemical Society
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Scheme 1. Structures of the A3-Corrole, 1A, and trans-A2B-Corrole, 2A, and 3A and the Corresponding (A3-cor) FeIII NO, 1B,
and (trans-A2B-cor) FeIIINO, 2B and 3B
compounds.3 It is also known that certain enzymes use
(macrocycle)FeNO complexes in the oxidized state as transient
intermediates for the conversion of nitrite to ammonia.4 All
these observations necessarily point toward the extreme
importance of understanding the oxidation state distribution
in higher oxidation states of (macrocycle)FeNO systems.
Corrole, being a special one among the macrocyclic systems,
may be a perfect choice to study the same. In this regard, it
should be kept in mind that the corrole ring is just one meso-
carbon shorter than related porphyrin macrocycle.5 Compared
to porphyrin which has two imino hydrogens, corrole contains
three imino hydrogens. Therefore, it can act as a trianionic
ligand. Because of the smaller cavity size and more anionic
nature of corrole, it tends to stabilize metals in higher oxidation
states.5 In the majority of the cases, in the native state, the
oxidation state of metal is one unit higher in metallo-corrole
than its metallo-porphyrin analogue. Thus, it has been proven
to be easier and more realistic to study the higher oxidation
states of metal in corrole based macrocycle.5 It was also found
that in various catalytic reactions, like carbene-tranfer and
nitrene-transfer reactions, iron-corrole has been proven to be
more superior than its iron-porphyrin analogue.6 Owing to very
limited availability of information, [(corrolato){FeNO}6]
complexes7 could be a perfect choice to study the oxidation
state distribution and to get deeper insight into the peculiar
reactivities of iron-corrole complexes. A detailed literature
survey reveals that there are very few reports on the in situ
generated oxidized and reduced species of [(cor){FeNO}6]
moieties.7a,b cor stands for corrolato and the {FeNO}6
nomenclature is based on the Enemark and Feltham notation,8
where the exponent term six is obtained by using the
summation of five iron d-electrons and one unpaired electron
from NO. The present work describes the synthesis of one A3-
corrole and two novel trans-A2B-corroles, 5,10,15-tris(4-
bromophenyl)corrole, 1A, 10-(4-bromophenyl)-5,15-bis(4-
cyanophenyl)corrole, 2A, and 10-(2,4,5-trimethoxyphenyl)-
5,15-bis(4-cyanophenyl)corrole, 3A (Scheme 1), and the
corresponding one [(A3-cor){FeNO}6] and two novel
[(trans-A2B-cor){FeNO}6] derivatives, namely, [{5,10,15-tris-
(4-bromophenyl)cor}{FeNO}6], 1B, [{10-(4-bromophenyl)-
5,15-bis(4-cyanophenyl)cor}{FeNO}6], 2B, and [{10-(2,4,5-
trimethoxyphenyl)-5,15-bis(4-cyanophenyl)cor}{FeNO}6], 3B.
Compared to A3-corroles, trans-A2B corroles would be more
versatile scaffolds and would be a better modulator of the steric
and electronic factors for synthesizing a diverse range of metal
complexes. A series of electron-withdrawing (like -CN) and
-releasing (like -OMe) groups were chosen as substituents in
the corrole frameworks keeping in view that these modifications
might influence the energies of the molecular orbitals of the
respective complexes, thus affecting the spectral features.
Moreover, cyanide has been adopted as a substituent in these
corroles because of its tendency to get readily converted into a
series of other organic functional groups. Thus, these
complexes can be considered as potential building blocks for
construction of various other metallocorrole-based architec-
tures. Although the syntheses of free base A3-corrole, 1A, and
[(A3-cor){FeNO}6], 1B, have been already reported in the
literature,7d the crystal structure and UV−vis, IR, and electron
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Computational Details. Full geometry optimizations were carried
out using the DFT method at the (U)B3LYP level for 2B+ and 2B−
and (R)B3LYP for 2B.14 All elements except iron were assigned the 6-
31G(d) basis set. The SDD basis set with effective core potential was
employed for the iron atom.15 Harmonic frequency calculations were
performed on the optimized geometries, representing a minimum on
the potential energy surface. All harmonic frequencies were corrected
by a scaling factor of 0.975 except ν(NO), which was scaled by a factor
of 0.92. All calculations were performed with the Gaussian09 program
package.16 Vertical electronic excitations based on B3LYP optimized
geometries were computed for 2B+, 2B, and 2B− using the TD-DFT
formalism17 in dichloromethane using the conductor-like polarizable
continuum model (CPCM).18 Chemissian19 was used to calculate the
fractional contributions of various groups to each molecular orbital. All
the calculated structures were visualized with ChemCraft.20
Synthesis of 4-(Di(1H-pyrrol-2-yl)methyl)benzonitrile, 1. 4-
(Di(1H-pyrrol-2-yl)methyl)benzonitrile 1 was prepared by slight
modification of the direct synthesis method reported by Lee et al.,
involving the pyrrole-aldehyde condensation in the presence of a large
excess of pyrrole.21 Pyrrole served a dual role of reactant and solvent in
the reaction mixture. In a 100 mL two-necked round-bottomed flask, 2
g of 4-cyano benzaldehyde (0.015 mmol) was dissolved in 21 mL of
pyrrole (0.300 mmol) and, after 233 μL (0.003 mmol) of TFA was
added, was stirred for 20 min at room temperature. The brown-
colored crude product was obtained after dilution with CH2Cl2,
washing with dilute NaOH solution, and concentration of the organic
layer. Excess pyrrole was removed by rotary evaporation. The crude
product was then purified by column chromatography through a bed
of silica-gel (100−200 mesh), using the solvent mixture 85% hexane
and 15% ethylacetate as eluent.
paramagnetic resonance (EPR) spectroelectrochemical inves-
tigations of 1B have not been reported so far. The present
investigation further relates to the synthesis of two new trans-
A2B-corroles, 2A and 3A and their corresponding [(cor)-
{FeNO}6] derivatives. In addition to the synthesis and spectral
characterization of 1A, 1B, 2A, 2B, 3A, and 3B, the crystal
structures of 1B and 2B and electrochemistry, UV−vis, IR, and
EPR-spectroelectrochemical investigations of 1B, 2B, and 3B
are also reported here. To garner further support of the
experimentally obtained results, we have also carried out
density functional theory (DFT) and time dependent-DFT
(TD-DFT) calculations.
EXPERIMENTAL SECTION
■
Materials. The precursors pyrrole, p-chloranil, 2,4,5-trimethox-
ybenzaldehyde, and tetrabutyl ammonium perchlorate (TBAP) were
purchased from Aldrich, USA. 4-Cyano benzaldehyde, 4-bromo
benzaldehyde, and FeCl2·4H2O were purchased from Merck, India.
Other chemicals were of reagent grade. Hexane and CH2Cl2 were
distilled from KOH and CaH2 respectively. For spectroscopy and
electrochemical studies, HPLC-grade solvents were used. Symmetric
A3-corrole 1A was obtained by following a literature reported
procedure;9,10 however, two trans-A2B-corroles 2A and 3A have
been prepared for the first time.
Physical Measurements. UV−Vis spectral studies were per-
formed on a Perkin−Elmer LAMBDA-750 spectrophotometer. The
elemental analyses were carried out with a Perkin−Elmer 240C
elemental analyzer. FT−IR spectra were recorded on a Perkin−Elmer
spectrophotometer with samples prepared as KBr pellets. The NMR
measurements were carried out using a Bruker AVANCE 400 NMR
spectrometer. Tetramethylsilane (TMS) was the internal standard.
Electrospray mass spectra were recorded on a Bruker Micro TOF-QII
mass spectrometer. Cyclic voltammetry measurements were carried
out using a CH Instruments model CHI1120A electrochemistry
system. A glassy carbon working electrode, a platinum wire as an
auxiliary electrode, and an Ag-AgCl reference electrode were used in a
three-electrode configuration. Tetrabutyl ammonium perchlorate
(TBAP) was the supporting electrolyte (0.1 M), and the concentration
of the solution was 10−3 M with respect to the complex. The half wave
For 4-(Di(1H-pyrrol-2-yl)methyl)benzonitrile, 1. The com-
pound 4-(di(1H-pyrrol-2-yl)methyl)benzonitrile was characterized by
various spectral techniques, such as CHN analysis, UV−vis, 1H NMR,
13C NMR, and electrospray mass spectrum. These analyses matched
well with the earlier reported values.21
Synthesis of 5,10,15-Tris(4-bromophenyl)corrole, 1A. 1A was
prepared according to available procedures of corrole synthesis.9
A
total of 0.925 g of 4-bromo benzaldehyde (5 mmol) and 697 μL of
pyrrole (10 mmol) were dissolved in 200 mL of methanol, and
subsequently 200 mL of water was added. Then 4.25 mL of HClaq
(36%) was added, and the reaction was stirred for 3 h. The mixture
was extracted with CHCl3, and the organic layer was washed twice
with water, dried by anhydrous Na2SO4, filtered, and diluted to 300
mL with CHCl3. A total of 1.23 g of p-chloranil (5 mmol) was added,
and the mixture was refluxed for 1 h. The solvent was removed by
rotary evaporation, and the green-colored crude product was purified
by column chromatography through silica gel (100−200 mesh) bed
and using 20% DCM and 80% hexane as eluent.
For 5,10,15-Tris(4-bromophenyl)corrole, 1A. The compound
1A was characterized by various spectral techniques, such as CHN
analysis, UV−vis, 1H NMR, 13C NMR, and electrospray mass
spectrum, which also confirmed the purity and identity of 1A. These
analyses matched well with the earlier reported values.10
Synthesis of 10-(4-Bromophenyl)-5,15-bis(4-cyanophenyl)-
corrole, 2A. 2A was prepared by following a general procedure of
corrole synthesis.9 A total of 0.247 g of 5-(4-cyanophenyl)-
dipyrromethane (1 mmol) and 0.092 g of 4-bromo benzaldehyde
(0.5 mmol) were dissolved in 50 mL of MeOH. The reaction was kept
for stirring for 1 h. The mixture was extracted with CHCl3, and the
organic layer was washed twice with H2O, dried by anhydrous Na2SO4,
filtered, and diluted to 250 mL with CHCl3. Then, 0.369 g (1.5 mmol)
of p-chloranil was added, and the mixture was refluxed for 1 h. The
solvent was removed by rotary evaporation, and the green-colored
crude product was purified by column chromatography through silica
gel (100−200 mesh) column and using 80% DCM and 20% hexane as
eluent.
potential E0 was set equal to 0.5 (Epa + Epc), where Epa and Epc are
298
anodic and cathodic cyclic voltammetric peak potentials, respectively.
The scan rate used was 100 mV s−1. The EPR spectra in the X band
were recorded with a Bruker System EMX. Simulations of EPR spectra
were done using the Simfonia program. UV−vis-NIR absorption
spectra were recorded on an Avantes spectrometer system: Ava Light-
DH-BAL (light source), AvaSpec-ULS2048 (UV−vis-detector), and
AvaSpec-NIR256−2.5TEC (NIR-detector). Spectroelectrochemical
measurements were carried out using an optically transparent thin
layer electrochemical (OTTLE) cell.11
Crystal Structure Determination. Single crystals of 1B were
grown by slow diffusion of a solution of the 1B in dichloromethane
into methanol, followed by slow evaporation under atmospheric
conditions, and those for 2B were grown by slow diffusion of a
solution of the (cor){FeNO}6 in dichloromethane into benzene,
followed by slow evaporation under atmospheric conditions. The
crystal data of 1B and 2B were collected on a Bruker Kappa APEX II
CCD diffractometer at 293 K. Selected data collection parameters and
other crystallographic results are summarized in Table S1, Supporting
Information. All data were corrected for Lorentz polarization and
absorption effects. The program package SHELXTL12 was used for
structure solution and full matrix least-squares refinement on F2.
Hydrogen atoms were included in the refinement using the riding
model. Contributions of H atoms for the water molecules were
included but were not fixed. Disordered solvent molecules were taken
out using the SQUEEZE13 command in PLATON.
For 10-(4-Bromophenyl)-5,15-bis(4-cyanophenyl)corrole,
2A. Yield: 45% (150 mg). Anal. calcd (found) for C39H23BrN6
(2A): C, 71.45 (71.53); H, 3.54 (3.46); N, 12.82 (12.75). λmax/nm
(ε/M−1 cm−1) in dichloromethane: 425 (100488), 582 (16544), 623
CCDC-959930 and CCDC-959931 contain the supplementary
crystallographic data for 1B and 2B. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif.
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Scheme 2. Representative Synthesis of 2B
1
(11766), 648 (9409). H NMR (400 MHz, CDCl3) δ 9.063 (broad
in acetonitrile (see Supporting Information, Figure S4) showed peaks
centered at m/z = 667.21 corresponding to [3A + H] (calculated
molecular mass: 666.23). 3A displayed strong fluorescence at 678 nm
(see Supporting Information, Figure S5).
+
singlet, 2H), 8.854 (broad singlet, 2H), 8.611−8.486 (broad multiplet,
8H), 8.130−8.036 (broad multiplet, 6H), 7.922 (broad singlet, 2H).
The electrospray mass spectrum in acetonitrile (see Supporting
Information, Figure S1) showed peaks centered at m/z = 656.724
Synthesis of {5,10,15-Tris(4-bromophenyl)cor}FeIIINO, 1B. A
mixture of 0.025 g of 5,10,15-tris(4-bromophenyl)corrole (0.03 mmol)
and 0.060 g of ferrous chloride tetrahydrate FeCl2·4H2O (0.3 mmol)
was refluxed in a solution of 15 mL of pyridine/methanol (1:2) for
1-1/2 h under dinitrogen. A 1.0 mL aliquot of saturated aqueous
NaNO2 was then added to the hot solution, and the system was
refluxed for an additional 30 min. The solution was then allowed to
cool to room temperature, and the solvent mixture was removed by
rotary evaporation. The solid product was finally dissolved in
dichloromethane and purified by column chromatography using silica
gel (100−200 mesh). The solvent-mixture used for elution of 1B was
20% DCM and 80% hexane. The final form of the compound was
obtained as orange-red crystalline materials.
+
correspond to [2A + H] (calculated molecular mass: 655.544).
Synthesis of 10-(2,4,5-Trimethoxyphenyl)-5,15-bis(4-
cyanophenyl)corrole, 3A. 3A was prepared by following a general
procedure of corrole synthesis.9 0.247 g of 5-(4-cyanophenyl)-
dipyrromethane (1 mmol) and 0.098 g of 2,4,5-trimethoxy
benzaldehyde (0.5 mmol) were dissolved in 50 mL of MeOH. The
reaction was kept for stirring for 1 h. The mixture was extracted with
CHCl3, and the organic layer was washed twice with H2O, dried by
anhydrous Na2SO4, filtered, and diluted to 250 mL with CHCl3. Then,
0.369 g (1.5 mmol) of p-chloranil was added, and the mixture was
refluxed for 1 h. The solvent was removed by rotary evaporation, and
the green-colored crude product was purified by column chromatog-
raphy through silica gel (100−200 mesh) column and using 80%
DCM and 20% hexane as eluent.
For 10-(2,4,5-Trimethoxyphenyl)-5,15-bis(4-cyanophenyl)-
corrole, 3A. Yield: 42% (140 mg). Anal. Calcd (found) for
C42H30N6O3 (3A): C, 75.66 (75.53); H, 4.54 (4.46); N, 12.60
(12.75). λmax/nm (ε/M−1 cm−1) in dichloromethane: 426 (90875),
582 (17250), 627 (13000), 646 (11250). 1H NMR (400 MHz,
CDCl3) δ 8.91 (s, 2H), 8.79 (s, 2H), 8.62 (s, 2H), 8.47 (s, 2H), 8.40
(d, J = 6.4 Hz, 4H), 8.05 (d, J = 6.7 Hz, 4H), 7.55 (s, 1H), 7.02 (s,
1H), 4.22 (s, 3H), 3.95 (s, 3H), 3.62 (s, 3H), −(1.37−1.43) (broad
multiplet, 3H) (see Supporting Information, Figure S2). 13C NMR
(101 MHz, CDCl3) δ 154.04, 150.28, 144.32, 142.53, 141.43, 139.11,
135.43, 132.12, 131.81, 126.86, 126.47, 122.52, 121.94, 119.43, 119.28,
116.78, 113.35, 111.05, 107.12, 98.17, 57.22, 56.86, 56.55 (see
Supporting Information, Figure S3). The electrospray mass spectrum
For {5,10,15-Tris(4-bromophenyl)cor}FeIIINO, 1B. Anal. Calcd
(found) for C37H20Br3FeN5O (1B): C, 52.52 (52.48); H, 2.38 (2.54);
N, 8.28 (8.35). λmax/nm (ε/M−1 cm−1) in dichloromethane:
1
394(84800), 535(9400). νNO = 1773 cm−1. H NMR (400 MHz,
CDCl3) δ 7.98 (d, J = 4.6 Hz, 2H), 7.76−7.62 (m, 13H), 7.54 − 7.50
(m, 3H), 7.38 (d, J = 4.8 Hz, 2H). 13C NMR (101 MHz, CDCl3) δ
148.12, 147.91, 146.09, 138.46, 138.31, 137.35, 132.21, 132.01, 131.74,
131.69, 131.56, 131.53, 131.32, 130.44, 126.72, 126.02, 125.58, 123.13,
122.95, 118.49. The electrospray mass spectrum in acetonitrile
matched well with the earlier reported data.7d
Synthesis of {10-(4-Bromophenyl)-5,15-bis(4-cyanophenyl)-
cor}FeIIINO, 2B. A mixture of 0.050 g of 10-(4-bromophenyl)-5,15-
bis(4-cyanophenyl)corrole (0.076 mmol) and 0.152 g of ferrous
chloride tetrahydrate FeCl2·4H2O (0.76 mmol) was refluxed in a
solution of 15 mL of pyridine/methanol (1:2) for 1-1/2 h under
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Figure 1. Single-crystal X-ray structure of 1B.
dinitrogen. A 1.0 mL aliquot of saturated aqueous NaNO2 was then
added to the hot solution, and the system was refluxed for an
additional 30 min. The solution was then allowed to cool to room
temperature, and the solvent mixture was removed by rotary
evaporation. The solid product was finally dissolved in dichloro-
methane and purified by column chromatography using silica gel
(100−200 mesh). The solvent-mixture used for elution of 2B was 60%
DCM and 40% hexane. The final form of the compound was obtained
as orange-red crystalline materials.
For {10-(4-Bromophenyl)-5,15-bis(4-cyanophenyl)cor]-
FeIIINO, 2B. Yield: 55% (31 mg). Anal. Calcd (found) for
C39H20BrFeN7O (2B): C, 63.44 (63.48); H, 2.73 (2.84); N, 13.28
(13.40). λmax/nm (ε/M−1 cm−1) in dichloromethane: 390(56200), 539
(5000). νNO = 1774 cm−1. 1H NMR (400 MHz, CDCl3) δ 8.04 (d, J
= 4.6 Hz, 2H), 7.96−7.90 (m, 8H), 7.76−7.63 (m, 5H), 7.53−7.48 (m,
3H), 7.44−7.43(d, J = 5.0 Hz, 2H). 13C NMR (101 MHz, CDCl3) δ
148.14, 147.38, 146.09, 143.12, 138.18, 137.98, 132.24, 131.89, 131.68,
131.64, 131.62, 131.58, 131.35, 129.30, 126.37, 126.00, 125.81, 123.13,
119.08, 118.76, 112.68. The electrospray mass spectrum in acetonitrile
(see Supporting Information, Figure S6) showed peaks centered at m/
z = 709.308 corresponding to [M+ − NO] (calculated molecular mass:
708.365).
Synthesis of {10-(2,4,5-Trimethoxyphenyl)-5,15-bis(4-
cyanophenyl)cor}FeIIINO, 3B. A mixture of 0.050 mg of 10-(2,4,5-
trimethoxyphenyl)-5,15-bis(4-cyanophenyl)corrole (0.075 mmol) and
0.149 g of ferrous chloride tetrahydrate FeCl2·4H2O (0.75 mmol) was
refluxed in a solution of 15 mL of pyridine/methanol (1:2) for 11/2 h
under dinitrogen. A 1.0 mL aliquot of saturated aqueous NaNO2 was
then added to the hot solution, and the system was refluxed for an
additional 30 min. The solution was then allowed to cool to room
temperature, and the solvent mixture was removed by rotary
evaporation. The solid product was finally dissolved in dichloro-
methane and purified by column chromatography using silica gel
(100−200 mesh). The solvent used for elution of 3B was 100% DCM.
The final form of the compound was obtained as orange-red crystalline
materials.
149.90, 147.40, 147.38, 146.45, 146.37, 146.34, 142.06, 141.99, 141.69,
137.28, 132.15, 130.76, 130.09, 130.03, 129.43, 129.39, 128.81, 127.11,
126.05, 125.88, 125.83, 125.54, 125.48, 119.07, 118.40, 117.31, 117.23,
114.54, 114.12, 111.35, 98.44, 98.17, 56.38, 56.00, 55.51. The
electrospray mass spectrum in acetonitrile (Figure S7, Supporting
Information) showed peaks centered at m/z = 719.09 corresponding
to [M+ − NO] (calculated molecular mass: 719.14).
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Symmetric A3-corrole
1A was already reported in the literature;9,10 however, two
trans-A2B-corroles 2A and 3A were prepared for the first time.
Reaction of the respective aryl-aldehydes and pyrrole, which
were dissolved in a 1:1 mixture of methanol and water in the
presence of HCl, and the subsequent oxidation by p-chloranil
(Scheme 2; representative synthesis of 2B), resulted in the
formation of the corrole derivatives.
The [(cor){FeNO}6] derivatives were synthesized by
following a reported procedure.7 A mixture of the desired
corrole and ferrous chloride tetrahydrate was refluxed in a
solution of pyridine/methanol (1:2) under a dinitrogen
atmosphere. Upon addition of a saturated aqueous NaNO2
solution, the corresponding [(cor){FeNO}6] derivatives 1B,
2B, and 3B were formed. Purity and identity of the free base
corroles and the corresponding [(cor){FeNO}6] derivatives
were demonstrated by their satisfactory elemental analyses (see
Experimental Section), IR spectroscopy data (Figure S8 and
Table S2, Supporting Information), and the electrospray mass
spectra. The electrospray mass spectrum of 1B in chloroform
matched well with the earlier reported data.7d The electrospray
mass spectrum of 2B in acetonitrile (see Supporting
Information, Figure S6) shows peaks centered at m/z =
709.308 corresponding to [M+ − NO] (calculated molecular
mass: 708.365). The electrospray mass spectrum of 3B in
acetonitrile (Figure S7, Supporting Information) showed peaks
centered at m/z = 719.09 corresponding to [M+ − NO]
(calculated molecular mass: 719.14).
For {10-(2,4,5-Trimethoxyphenyl)-5,15-bis(4-cyanophenyl)-
cor}FeIIINO, 3B. Yield: 51% (29 mg). Anal. Calcd (found) for
C42H27FeN7O4 (3B): C, 67.30 (67.48); H, 3.63 (3.74); N, 13.08
(12.95). λmax/nm (ε/M−1 cm−1) in dichloromethane: 388 (96200),
1
485 (25800). νNO = 1772 cm−1. H NMR (400 MHz, DMSO-d6) δ
8.17−8.03 (m, 10H), 7.68−7.64 (dd, J = 9.6, 4.5 Hz, 2H), 7.42−7.40
(dd, J = 4.6, 3.2 Hz, 2H), 7.33−7.31 (t, J = 5.3 Hz, 2H), 7.09−6.93 (m,
2H), 3.93 (d, J = 2.9 Hz, 3H), 3.74 (s, 2H), 3.66 (d, J = 4.8 Hz, 2H),
3.55 (s, 2H). 13C NMR (101 MHz, DMSO) δ 151.08, 150.92, 149.93,
NMR Spectra. The 1H NMR spectrum of 1B exhibits sharp
peaks in accordance with 20 partially overlapping aromatic
protons in the region δ, 8.0−7.3 ppm (see Supporting
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Figure 2. Single-crystal X-ray structure of 2B.
Information, Figure S9). Out of these signals three doublets (δ
= 7.98, 7.38, 7.53) are easily distinguishable with characteristic
coupling constants J = ∼5.0 Hz. On the basis of earlier
observations,22 these doublets can be assigned to β-pyrrolic
hydrogen atoms. Another doublet is overlapped with the aryl
ring protons. The aryl ring protons are observed at the 7.76−
7.50 ppm region. 13C NMR spectrum shows the presence of
the expected 20 signals (see Supporting Information, Figure
surrounded by five nitrogen atoms. The geometry around the
iron is distorted square pyramidal. The Fe−N distances are
1.9059(4), 1.929(4), 1.921(4), and 1.898(4) Å respectively and
are typical for low-spin Fe (III). The N−Fe−N bond angles are
87.44(17)°, 93.18(18)°, 87.05(17)°, and 79.60(17)°. The Fe−
N−O bond angle, 175.0(4)°, justifies calling it as a linear
nitrosyl. The Fe−N (NO) bond distance, 1.656(4) Å, points
toward a significant FeIII → NO back bonding. The NO bond
length, 1.159(5) Å, depicts its elongated character (Scheme 3
and Table 2). The iron atom is elevated from the peripheral 19-
atom corrole carbon ring by 0.512(9) Å to give rise to a domed
conformation. This leads to a consequent deviation of the
pyrrole ring nitrogen atoms from the 19-atom corrole carbon
ring by distances ranging from 0.018 to 0.140 Å. The meso-
substituted phenyl rings are tilted with respect to the mean
corrole plane (considered as the 19-atom corrole carbon ring)
by dihedral angles ranging from 58.05−61.68°. In 2B, iron is
penta coordinated and surrounded by five nitrogen atoms. The
geometry around the iron is distorted square pyramidal.
Distance of Fe (III) from mean corrole plane is 0.516 (5) Å.
The Fe−N distances are 1.919(3) (DFT: 1.927 Å), 1.895(2)
(DFT: 1.902 Å), 1.889(3) (DFT: 1.901 Å), and 1.939(3)
(DFT: 1.932 Å) Å (Table 1) and are typical for low-spin
Fe(III). The N−Fe−N bond angles are 87.91(11)° (DFT:
87.72°), 79.06(12)° (DFT: 79.45°), 87.82(12)° (DFT:
87.59°), and 92.32(11)° (DFT: 93.10°) (Table 1). The Fe−
N−O bond angle, 171.70(3)° (DFT: 179.94°) (Table 1),
justifies calling it as a linear nitrosyl. The Fe−N(NO) bond
distance, 1.650(3) Å (DFT: 1.597 Å) (Table 1), points toward
a significant FeIII → NO back bonding. The N−O bond length,
1.162(3) Å (DFT: 1.162 Å) (Table 1), depicts its elongated
character.
1
S10). The H NMR spectrum of 2B exhibits the expected
number of 20 partially overlapping aromatic protons in the
region δ = 8.04−7.4 ppm (Figure S11, Supporting
Information). Out of these signals, two doublets (δ = 8.04,
7.44) are easily distinguishable with characteristic coupling
constants J = ∼5.0 Hz. These doublets are assigned as β-
pyrrolic hydrogen atoms. Two more doublets are overlapped
with the aryl ring protons. The aryl ring protons are observed at
the 7.96−7.48 ppm region. 13C NMR spectrum (see
Supporting Information, Figure S12) shows the presence of
1
the expected 21 signals. The H NMR spectrum of 3B shows
sharp resonances corresponding to 18 partially overlapping
aromatic protons in the region δ, 8.18−6.9 ppm (see
Supporting Information, Figure S13). Nine methoxy protons
appear in the region of 3.95−3.55 ppm. 13C NMR spectrum
(see Supporting Information, Figure S14) shows the presence
of expected 38 signals due to complete asymmetric nature of
the molecule. Out of those 38 signals, three methoxy group
signals (δ = 56.38, 56.00, and 55.51) can be easily distinguished
by their characteristic chemical shifts. All the signals in the
[(cor){FeNO}6] derivatives show an upfield shift compared to
free base corroles. The 1H and 13C NMR data thus
unequivocally establish the diamagnetic nature of the iron
complexes 1B−3B.
Crystal Structures. The crystal structures of 1B and 2B are
shown in Figures 1 and 2, respectively. The crystal system is
triclinic for 1B and monoclinic for 2B. The unit cell of 1B
contains two molecules and that of 2B contains eight
molecules. Important crystallographic parameters are presented
in Table S1, Supporting Information. The DFT calculated bond
distances and angles match fairly well with the experimental
values for 2B (Table 1). In 1B, iron is penta-coordinated and
Because of the square-pyramidal geometry around the central
metal atom, the nitrogen atoms of the pyrrole rings in the
macrocycle 2B are alternately tilted up and down the mean
corrole plane by a range of 0.0367−0.0873 Å. This prominent
saddling effect in the corrole ring of 2B is also consistent with
the alternate deviation of the dihedral angles between the
pyrrole rings with the mean corrole plane, which lie between
1.906−4.925°. The substituted phenyl rings at the meso-
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two iron atoms is 8.60(7) Å. The crystal structure analyses of
both the compounds 1B (see Supporting Information, Figures
S15 and S17) and 2B (see Supporting Information, Figures S16
and S18) show several C−H···N interactions, C−H···C
interactions, C−H···O interactions, and C−H···π interactions.
1B molecules are stacked together and form layer-like
arrangements throughout the crystal lattice. Similarly, 2B
molecules are stacked together and form cylindrical tubelike
arrangements throughout the crystal lattice. These tubes are
filled with benzene molecules. Inside the tube, benzene
molecules are interconnected through π−π stacking inter-
actions {see Supporting Information, Figure S16b}. The closest
distance between two neighboring [(cor){FeNO}6] molecules
responsible for π−π stacking interactions is ∼3.18 Å for 1B and
∼3.32 Å for 2B. This clearly indicates that there is a reasonably
strong adjacent parallel plane π−π stacking interaction between
the [(cor){FeNO}6] molecules. The shortest intermolecular
distance between two [(cor){FeNO}6] molecules responsible
for C−H···O interactions, C−H···C interactions, C−H···N
interactions, and C−H···π interactions are close to ∼2.71 Å,
∼2.85 Å, ∼3.01 Å, and 2.75−4.04 Å respectively for 1B and
∼2.69 Å, ∼2.89 Å, ∼2.85 Å, and 2.89−3.47 Å respectively for
2B. This is quite clearly indicative of the fact that fairly strong
intermolecular C−H···π interactions, C−H···O, C−H···N, and
C−H···C interactions are also present between adjacent
[(cor){FeNO}6] molecules. It has been assumed that the
solid state structures are stable due to intermolecular parallel-
displaced π−π stacking interactions, C−H···π interactions, C−
H···O, C−H···N, and C−H···C interactions.23
Redox Properties of the 1B, 2B, and 3B. The redox
properties of the 1B, 2B, and 3B were investigated in CH2Cl2/
0.1 M TBAP (TBAP = tetra-n-butyl ammonium perchlorate)
by using cyclic voltammetric techniques and differential pulse
voltammetric techniques (Figure 3 and Table 3). The oxidation
processes at the positive side of Ag-AgCl reference electrode
were recorded by using a platinum working electrode; a glassy−
carbon working electrode was used for recording the reduction
processes. All the three iron complexes 1B, 2B, and 3B
exhibited one reversible oxidative couple E0298, V (ΔEp, mV):
+0.54 (70) (1B), +0.58 (70) (2B), and +0.56(70) (3B) versus
ferrocene/ferrocenium. They also showed one reversible
reductive couple E0298, V (ΔEp, mV): −0.67(70) (1B),
−0.61(70) (2B), and −0.63(70) (3B) versus ferrocene/
ferrocenium. The effect of the substituents in the corrolato
ring on the redox potentials of the metal complexes is marginal.
UV−vis, IR, and EPR spectroelectrochemical investigations
were done on both the oxidative couple and reductive couple to
gain insights into the electronic structures of the various states.
UV−vis, IR, and EPR Spectroelectrochemistry. Elec-
tronic spectral data of the compounds in CH2Cl2 (see
Supporting Information, Figure S19) are shown in Table S2
(see Supporting Information). All the three complexes 1B, 2B,
and 3B exhibit Soret bands in the range of 385−395 nm.
Another weak band in the range of 485−540 nm is observed in
the visible region.7 The molar absorption coefficient of the
Soret bands is in the range of (5.6−9.6) × 104 M−1 cm−1, and
the molar absorption coefficient of the bands in the visible
region is in the range of (5.0−26.0) × 103 M−1 cm−1.
Spectroelectrochemical measurements (UV−vis, FT-IR, and
EPR) of all the nitrosyl derivatives were carried out in
dichloromethane solvent at room temperature. These measure-
ments were carried out to find the characteristic spectroscopic
changes involved in all the accessible redox states of the
Table 1. Selected X-ray and DFT Calculated (B3LYP/6-
31G*/SDD) Bond Distances (Å) and Angles (deg) for 2Bn
(n = 1+, 0, 1−)
2B
2B+
2B−
(DFT)
(DFT)
(DFT)
2B (X-ray)
(S = 0) (S = 1/2) (S = 1/2)
Fe(1)−N(1)
Fe(1)−N(2)
Fe(1)−N(3)
Fe(1)−N(4)
Fe(1)−N(5)
N(5)−O(1)
N(1)−C(2)
N(1)−C(5)
N(2)−C(7)
N(2)−C(10)
N(3)−C(11)
N(3)−C(14)
N(4)−C(16)
N(4)−C(19)
C(1)−C(2)
C(2)−C(3)
C(3)−C(4)
C(4)−C(5)
C(5)−C(6)
C(6)−C(7)
C(7)−C(8)
C(8)−C(9)
C(9)−C(10)
C(10)−C(11)
C(11)−C(12)
C(12)−C(13)
C(13)−C(14)
C(14)−C(15)
C(15)−C(16)
C(16)−C(17)
C(17)−C(18)
C(18)−C(19)
C(19)−C(1)
O(1)−N(5)−Fe(1)
N(1)−Fe(1)−N(2)
N(1)−Fe(1)−N(3)
N(1)−Fe(1)−N(4)
N(1)−Fe(1)−N(5)
N(2)−Fe(1)−N(3)
N(2)−Fe(1)−N(4)
N(2)−Fe(1)−N(5)
N(3)−Fe(1)−N(4)
N(3)−Fe(1)−N(5)
N(4)−Fe(1)−N(5)
1.919(3)
1.895(2)
1.889(3)
1.939(3)
1.650(3)
1.162(3)
1.389(4)
1.385(4)
1.386(4)
1.357(5)
1.357(4)
1.386(4)
1.383(4)
1.379(5)
1.404(4)
1.428(5)
1.354(5)
1.431(5)
1.423(5)
1.383(5)
1.437(5)
1.367(5)
1.428(5)
1.421(5)
1.428(5)
1.372(5)
1.433(5)
1.389(5)
1.414(5)
1.431(5)
1.352(6)
1.433(4)
1.398(5)
171.70(3)
87.91(11)
152.50(12)
92.32(11)
105.45(13)
79.06(12)
150.94(11)
101.25(13)
87.82(12)
100.81(14)
106.66(12)
1.927
1.902
1.901
1.932
1.597
1.162
1.387
1.387
1.380
1.366
1.364
1.377
1.390
1.390
1.407
1.434
1.367
1.436
1.419
1.397
1.438
1.381
1.427
1.415
1.427
1.381
1.438
1.398
1.418
1.436
1.367
1.434
1.405
179.94
87.72
153.99
93.10
101.80
79.45
150.93
105.20
87.59
103.36
103.07
1.931
1.918
1.918
1.970
1.764
1.174
1.380
1.371
1.381
1.344
1.340
1.380
1.369
1.380
1.424
1.430
1.377
1.428
1.457
1.390
1.449
1.378
1.432
1.456
1.433
1.377
1.447
1.393
1.454
1.431
1.376
1.431
1.423
147.89
88.99
158.20
93.73
96.81
79.61
151.96
103.38
88.04
103.84
103.99
1.934
1.911
1.923
1.962
1.837
1.196
1.377
1.386
1.364
1.372
1.369
1.361
1.386
1.376
1.414
1.440
1.369
1.441
1.419
1.418
1.439
1.388
1.422
1.420
1.425
1.386
1.441
1.418
1.419
1.441
1.369
1.440
1.415
135.56
89.12
159.22
93.92
95.89
80.29
154.21
101.01
88.25
103.62
104.12
positions of the corrole rings are placed at angles varying from
65.04−80.54° with respect to the mean corrole plane.
The comparison of all these structural parameters involving
1B and 2B with those of the [(cor){FeNO}6] derivatives
reported by others7 earlier clearly justifies the aforesaid
discussions. In the packing diagram of both 1B and 2B
between two neighboring molecules, NO groups are facing in
opposite directions, and the planes of the rings are
approximately parallel. For 1B the interplanar distance between
two corrole planes is ∼3.53 Å. The distance between two iron
atoms is 7.0949(11) Å. For 2B, the interplanar distance
between two corrole planes is ∼4.63 Å. The distance between
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Scheme 3. Structural Parameters of 1B and 2B
Table 2. Comparison of Structural Parameters and NO Stretching Frequencies of (cor){FeNO}6 Species Having Electronic
Configuration of {FeNO}6 Already Available in the Literature with 1B and 2B
a
compound
Fe−NO bond distance (Å)
N−O bond distance (Å)
Fe−N−O bond angle (deg)
NO stretching freq (cm−1
)
ref
(OEC)Fe(NO)
(TMOPC)Fe(NO)
(TTC)Fe(NO)
(TDCC)Fe(NO)
(TF5PC)Fe(NO)
(TNPC)Fe(NO)
1B
1.631(3)
1.702(4)
1.645(2)
1.641(4)
1.171(4)
176.9(3)
1767
1767
1761
1783
1801
1778
1773
1774
7a
7d
7d
7f
1.076(4)
172.0(4)
1.162(2)
177.1(2)
1.169(5)
172.3(4)
b
1.639(4) , 1.648(4)
1.166(4), 1.171(4)
1.144(15), 1.154(13)
1.159(5)
177.3(4), 178.0(4)
172.8(10), 173.3(9)
175.0(4)
7f
b
1.675(14) 1.648(11)
7d, 7b
1.656(4)
1.650(3)
this work
this work
2B
1.162(3)
171.7(3)
a
OEC = octaethylcorrole, TMOPC = tris(p-methoxyphenyl)corrole, TTC = tris(p-methylphenyl)corrole, TDCC = tris(o-dichlorophenyl)corrole,
b
TF5PC = 5,10,15-tris(pentafluorophenyl)corrole, TNPC = 5,10,15-tris(4-nitrophenyl)corrole. There are two crystallographically independent
molecule exists in the asymmetric unit.
complexes and to establish the site of electron transfer in the
nitrosyl derivatives. Furthermore, these measurements were
also helpful in elucidating the reversibility of the redox
processes. UV−vis, IR, and EPR spectroelectrochemical studies
of all the three nitrosyl derivatives show similar pattern (Figures
4, 5, and 6). Hence, one representative example (complex 3B)
will be discussed here. For rest of the complexes, data are
presented in a tabular form (Tables 4, 5, and 6). Complex 3B
shows its Soret and Q bands at 391, 532, and 631 nm
respectively (Figure 4). On oxidation of 3B to 3B+ in
dichloromethane/0.1 M BuN4PF6 using an optically trans-
parent thin layer (OTTLE) cell,11 the Soret band at 391 nm
loses intensity, and two new bands appear at 365 nm and 397
nm (Figure 4a and Table 4). The initial Q bands lose their
intensity, and new bands appear at 500 and 635 nm. The shift
of the Soret and Q bands to lower energies is consistent with
earlier literature reports.7a,b Literature reports suggest that this
kind of shifting of Soret and Q bands to lower energies is due
to the formation of corrole radical cation or the formation of
Fe(IV) corrole.7a,b On returning back to the initial potential,
the native spectrum of 3B was regained in band position and
intensity to 100%, thus confirming the reversibility of the
oxidation process. The FTIR thin-layer spectroelectrochemical
measurement of one-electron oxidized species 3B•+ in
dichloromethane/0.1 M BuN4PF6 was also performed (Figure
5a and Table 5). The shift of the νNO stretching frequency on
one-electron oxidation occurs from 1782 cm−1 to 1820 cm−1,
which corresponds to 38 cm−1. This small shift is an indication
of the {FeNO}6 unit remaining largely unaffected by the
oxidation step, and the oxidation taking place on the corrolato
ring. In order to get a direct proof of the location of the
unpaired spin in the paramagnetic 3B•+ form, EPR spectros-
copy was performed on the electrogenerated 3B•+ in solution.
The X-band EPR spectrum of 3B•+ at 295 K in CH2Cl2/0.1 M
Bu4NPF6 displays an isotropic signal centered at g = 2.005 with
a peak to peak separation of about 15 G (Figure 6a). The
observation of the signal in fluid solution at 295 K, the
closeness of the g-value to the free electron value of 2.0023, and
the narrow line-width of only 15 G are a clear indication of the
spin being predominantly localized on an organic ligand. The
EPR result, taken together with the relatively small shift of the
NO stretching frequency on moving from 3B to 3B•+ as
observed in IR spectroelectrochemistry, convincingly proves
the generation of a corrole based radical upon one-electron
oxidation of 3B. The one-electron oxidized form is thus best
described as [(cor)2•−{FeNO}6]•+. It should be noted here that
the EPR spectrum of the one-electron oxidized forms of related
(cor)FeNO complexes measured at 120 K have been described
in terms of a low spin Fe(III) signal.7a,b For the present case,
we have convincing data to prove the observation of a corrole
radical signal by EPR spectroscopy. Since the literature reports
also claim oxidation of the cor unit (from UV−vis and IR
spectroscopic experiments), it remains unclear as to how the
final EPR spectrum observed for the oxidized species originates
from a low spin Fe(III) center. In the starting complexes
[(cor)3−{Fe(NO)}6], the {FeNO}6 units are clearly diamag-
netic. An oxidation of the corrole unit will generate a species
[(cor)2•−{Fe(NO)}6]•+ which should logically possess a
corrole centered spin. The only way to obtain a low spin
Fe(III)-type EPR signal for the [(cor)2•−{Fe(NO)}6] form
would be for the NO centered spin and the (cor)2•− spin to
couple antiferromagnetically, leaving a lone spin on Fe(III).
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Figure 4. Changes in the UV−vis spectrum of 3B (a) during first
oxidation and (b) during first reduction. Results from OTTLE
spectroelectrochemistry in CH2Cl2/0.1 M Bu4NPF6.
oxidation of similar nitrosyl containing metal complexes of
porphyrin.24
On reduction of 3B to 3B•− in dichloromethane/0.1 M
BuN4PF6 using an optically transparent thin layer electro-
chemical (OTTLE) cell, the Soret band at 391 nm loses its
intensity and is shifted to 417 nm (Figure 4b). The initial Q
bands show a reduced intensity, and three new bands appear at
571, 633, and 761 nm. The shift of the Soret and Q bands to
lower energies is also supported by earlier literature reports.7a,b
The FTIR thin-layer spectroelectrochemical measurement of
one-electron reduced species, 3B•− in dichloromethane/0.1 M
BuN4PF6 was also performed (Figure 5b). The shift of the νNO
stretching frequencies occurs from 1782 cm−1 to 1605 cm−1,
which corresponds to 177 cm−1. As compared to the native
state, the NO band is shifted to lower wavenumbers on
reduction. On oxidation, this band shifts to higher wave-
numbers (see below). This opposite trend is a result of the
different charges induced on the complex on reduction and
oxidation. The 177 cm−1 shift of the NO band to lower energies
on reduction as compared to the native state is a direct
indication that the {FeNO} unit is predominantly affected by
this electron uptake. The IR results thus point to the formation
of a complex [(cor)3−{FeNO}7]•− on one-electron reduction,
which contains a bent {FeNO}7 unit. EPR spectroscopy of the
one-electron reduced form further strengthens this assignment.
Figure 3. (a) Cyclic voltammograms of 1B () in CH2Cl2 (b) cyclic
voltammograms () and differential pulse voltammograms (---) of 2B
in CH2Cl2 and (c) cyclic voltammograms of 3B () in CH2Cl2. The
potentials are versus ferrocene/ferrocenium.
a
Table 3. Electrochemical Data
electrochemical data
compound
oxidation E0, V (ΔEp, mV)
reduction E0, V (ΔEp, mV)
1B
2B
3B
0.54 (70)
0.58 (70)
0.56(70)
−0.67(70)
−0.61(70)
−0.63(70)
a
In dichloromethane. The potentials are versus ferrocene/ferroce-
nium.
Considering the highly covalent nature of the Fe−NO bond,
such a spin situation as stated above is unlikely. There are
reports on the generation of porphyrin centered radicals on the
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Table 4. UV-Vis Data of 1Bn, 2Bn, and 3Bn [n = −1, +1, 0]
a
from Spectroelectrochemistry
complex
λmax [nm] (ε [103 M−1 cm−1])
1B0
1B+
225 (70.3); 272 sh; 396 (85.0); 514 sh; 627 (4.5)
225 (70.3); 283 sh; 361 (58.0); 416 (57.4); 510 sh; 641 (8.0);
882 (3.8)
1B−
225 (69.8); 282 sh; 333 (41.9); 404 (64.6); 565 (16.6); 629 sh;
761 (4.9)
2B0
2B+
2B−
225 (40.1); 262 (24.7); 332 sh; 390 (56.2); 533 (8.5)
226 (40.1); 265 sh; 339 sh; 360 sh; 399 (38.0); 497 sh; 637 (3.7)
224 (42.1); 261 sh; 330 (24.1); 399 sh; 422 (40.7); 570 (12.0);
633 (7.0); 765 (2.1)
3B0
226 (69.0); 260 sh; 296 sh; 333 sh; 391 (96.8); 532 (14.5);
631 (4.6)
3B+
3B−
227 (70.1); 297 (44.2); 365 sh; 397 (58.6); 500 sh; 635 (8.4)
227 (78.0); 293 (44.9); 325 (43.9); 417 (60.9); 571 (19.1); 633 sh;
761 (4.7)
a
Measurements in CH2Cl2/0.1 M Bu4NPF6 (OTTLE spectroelec-
trochemistry).
Table 5. IR Data of 1Bn, 2Bn, and 3Bn [n = −1, +1, 0] from
a
Spectroelectrochemistry
a
complex
IR data νNO (cm−1
)
1B0
1B+
1B−
2B0
2B+
2B−
3B0
3B+
3B−
1779
1820
1606
1785
1825
1604
1782
1820
1605
Figure 5. Changes in the IR spectrum of 3B (a) during first oxidation
and (b) during first reduction. Results from OTTLE spectroelec-
trochemistry in CH2Cl2/0.1 M Bu4NPF6.
a
Measurements in CH2Cl2/0.1 M Bu4NPF6 (OTTLE spectroelec-
trochemistry).
a
b
Table 6. EPR Data of Paramagnetic States
a
complex
EPR data (g)
1B0
1B+
1B−
2B0
2B+
2B−
3B0
3B+
3B−
2.002
2.029; A(14N) = 15.4 G
2.003
2.029 A(14N) = 15.5 G
2.005
2.029 A(14N) = 15.4 G
a
b
g values measured at 295 K. From EPR spectroelectrochemistry in
CH2Cl2/0.1 M Bu4NPF6.
Furthermore, hyperfine coupling to one nitrogen nucleus (I =
1) is observed. The spectrum could be simulated with a a(N)
value of 15.4 G. This signal is thus a direct proof of the spin
being predominantly located in the {FeNO} part, and hence
the EPR spectrum of 3B•− corroborates its assignment as
[(cor)3−{FeNO}7]•− as has been stated above. Previous
literature reports have drawn similar conclusions regarding
the nature of the one-electron reduced form of such
compounds.7a,b
Figure 6. X-band EPR spectrum of (a) 3B•+ and (b) 3B•− with
simulation generated by in situ electrolysis at 295 K in CH2Cl2/0.1 M
Bu4NPF6.
The above observations have been supported by detailed
theoretical investigations. The energy obtained on open shell
singlet optimization is lower than that for a triplet configuration
or closed shell singlet state of 2B, which supports the
The in situ generated one-electron reduced species 3B•− in
CH2Cl2/0.1 M Bu4NPF6 at 295 K shows an isotropic signal
centered at g = 2.029 in its X-band EPR spectrum.
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antiferromagnetic coupling between the unpaired spins on
Fe(III) (d5-low spin, t2g5) and NO•. The 99% contribution of
cor to the HOMO of 2B (Table S3, Supporting Information)
indicates that oxidation occurs from the corrole moiety, which
is further supported by the 95% and 96% contributions of cor
to the α-SOMO and β-LUMO of 2B+ respectively (Table S4,
Supporting Information). This is also reflected in the increase
in Fe−N(cor) bond lengths and shortening of C−N(cor) bond
distances on moving from 2B to 2B+ as seen from their DFT
optimized structures (Table 1, Figure S20, Supporting
Information). A further support arises from the spin density
distribution in 2B+ (Fe: 1.969, NO: −1.020, cor: 0.044; Table
7, Figure 7a) where an appreciable spin of 0.044 arises on the
(DFT)) (Table 8, Figure S21, Supporting Information).
These DFT results showing the occurrence of reduction
predominantly at NO are further proven by the experimentally
obtained triplet EPR showing the hyperfine splitting due to the
I = 1 nuclear spin on 14N, which interacts with the spin of the
unpaired electron on the NO. The experimentally obtained
UV−Vis transitions of native species 2B, one-electron oxidized
species 2B+, and one-electron reduced species 2B− are in good
agreement with the computed values (Table 9). For instance,
2B displays intense ligand-to-ligand charge transfer (LLCT)
transitions in the UV region (cor(π) → cor(π*) at 225 and 262
nm) (Table 9). The visible region transitions at 390 and 533
nm arise due to ligand-to-metal charge transfer (LMCT)
transition (cor(π) → Fe(dπ)) (Table 9). The one-electron
oxidized species 2B+ exhibits LLCT transitions (cor(π) →
cor(π*)) at 399 and 637 nm (Table 9). The one-electron
reduced species 2B− shows metal-to-ligand charge transfer
(MLCT) transition (Fe(dπ) → cor(π*)) in the UV region at
330 nm (Table 9). In the visible region, it displays LLCT bands
at 422 nm (NO(π)/cor(π) → cor(π*)), 422, 570, 633, and 765
nm (cor(π) → cor(π*)) (Table 9).
Table 7. DFT Calculated (UB3LYP/6-31G*/SDD) Mulliken
Spin Densities for 2Bn (n = 1+, 1−)
complex
Fe
NO
cor
2B+ (S = 1/2)
2B− (S = 1/2)
1.969
2.192
−1.020
−1.202
0.044
0.012
CONCLUSIONS
■
We have presented here the synthesis of two new corrole
ligands. These new, and a literature reported corrole, have been
used to generate a series of [(cor){FeNO}6] derivatives. Purity
and identity of the free base corroles and the corresponding
[(cor){FeNO}6] derivatives have been demonstrated by
various spectroscopic techniques and elemental analysis. The
1H NMR spectra of the [(cor){FeNO}6] derivatives show
sharp resonances, which indicate their diamagnetic character-
istics and the noninnocent nature of the corrole macrocycles.
The Fe−N−O bond angle is 175.0(4)° for 1B and 171.7(3)°
for 2B. This indicates that these are linear nitrosyls of the
{FeNO}6 form. In the packing diagram, 2B molecules are
stacked together and form cylindrical tubelike arrangements
throughout the crystal lattice. These tubes are filled with
benzene molecules, arranged as benzene clusters. All the three
[(cor){FeNO}6] derivatives 1B, 2B, and 3B exhibited one
reversible oxidative couple and one reversible reductive couple
versus ferrocene/ferrocenium. UV−vis, IR, and EPR spectroe-
lectrochemical measurements of one-electron oxidized species
supports the formation of [(cor)2•−{Fe(NO)}6]•+. From earlier
observations, it was established that EPR signals predominantly
originate from a low spin Fe(III) center. However, our
observations clearly prove the predominant location of the
spin on the corrole unit. A rationale also has been provided for
this. Thus, the EPR data point toward a new/alternative
assignment of corrolato-iron-nitrosyl radical cation. UV−vis, IR,
and EPR spectroelectrochemical studies of the reduced form
3B•− indicate that the unpaired electron in that species resides
predominantly on the {FeNO} unit. The above observations
have been supported by DFT and TD-DFT calculations. Thus,
while the investigation of the one-electron oxidized species
delivers the noninnocent character of the corrole ring, the
knowledge of the electron structure of the one-electron reduced
species delivers information about the noninnocent nature of
NO, the oldest known noninnocent ligand. Our results here
reveal that spectroelectrochemistry is a very useful technique
for investigating metal complexes containing several potentially
noninnocent ligands. This combined approach can be used to
establish the electronic structures of such metal complexes in
their various redox states. Information obtained from such
Figure 7. Spin density (UB3LYP/6-31G*/SDD) representations for
(a) 2B+ and (b) 2B−.
corrole moiety unlike the absence of spin on it in 2B. The fact
that oxidation does not occur from NO is established by a
negligible change in the ν(N−O) frequency (Δν ≈ 40 cm−1)
on moving from 2B (1785 cm−1 (exp) and 1778 cm−1 (DFT))
to 2B+ (1825 cm−1 (exp) and 1822 cm−1 (DFT)) (Table 8,
Table 8. IR Spectral Data (Experimental from
a
b
Spectroelectrochemistry and DFT ) for 2Bn (n = 1+, 0, 1−)
a
b
complex
ν(N
−
O)
(cm−1) (exp)
ν(N−
(cm−1) (DFT)
O)
2B0
2B+
2B−
1785
1825
1604
1778
1822
1613
a
Measurements in CH2Cl2/0.1 M Bu4NPF6 (OTTLE spectroelec-
b
trochemistry) B3LYP/6-31G*/SDD/CPCM/CH2Cl2.
Figure S21, Supporting Information). The 40%, 23%, and 38%
contributions of Fe, NO, and cor to the HOMO of 2B (Table
S3) respectively collectively predict the possibility of reduction
on any of the three moieties, although the α-SOMO and β-
HOMO of 2B− having 76% and 96% contributions from cor
respectively (Table S5, Supporting Information) predict the
reduction taking place at the corrole moiety. The spin density
distribution in 2B− of Fe: 2.192, NO: −1.202 and cor: 0.012
shows appreciable amount of spin residing on the Fe and NO
(Table 7, Figure 7b). Moreover, the appreciable lengthening of
the N−O bond from 1.162 Å in 2B to 1.196 Å in 2B− (Table
1) predicts that the reduction occurs at the NO. This is further
supported by the ∼180 cm−1 decrease in the ν(N−O)
frequency on moving from 2B (1785 cm−1 (exp) and 1778
cm−1 (DFT)) to 2B− (1604 cm−1 (exp) and 1613 cm−1
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Table 9. TD-DFT (B3LYP/6-31G*/SDD/CPCM/CH2Cl2) Calculated Electronic Transitions for 2Bn (n = 1+, 0, 1−)
E/eV
λ/nm (expt) (ε/M−1 cm−1
)
λ/nm (DFT) (f)
transitions
character
2B (S = 0)
5.5055
4.7106
3.0014
2.4125
225(40100)
262(24700)
390(56200)
533(8500)
225(0.1098)
263(0.0422)
413(0.4615)
514(0.0012)
HOMO-7 → LUMO+5 (0.48)
HOMO-2 → LUMO+6 (0.49)
HOMO-5 → LUMO (0.50)
HOMO-14 → LUMO (0.38)
cor(π) → cor(π*)
cor(π) → cor(π*)
cor(π) → Fe(dπ)
cor(π) → Fe(dπ)
2B+ (S = 1/2)
2.8725
1.9501
399(38000)
637(3700)
432(0.0084)
636(0.0121)
HOMO-3(α) → LUMO+2(α)(0.46)
HOMO-12(α) → LUMO(α) (0.27)
HOMO(β) → LUMO+1(β) (0.18)
cor(π) → cor(π*)
cor(π) → cor(π*)
cor(π) → cor(π*)
2B− (S = 1/2)
3.8637
2.8912
2.1770
330(24100)
422(40700)
570(12000)
321(0.0539)
429(0.0229)
569(0.0168)
HOMO-8(α) → LUMO+1(α) (0.45)
HOMO-2(β) → LUMO+13(β) (0.48)
HOMO-1(β) → LUMO(β) (0.44)
HOMO-1(β) → LUMO+1(β)(0.42)
HOMO(β) → LUMO+3(β) (0.51)
HOMO-1(β) → LUMO(β) (0.62)
HOMO-1(α) → LUMO(α) (0.60)
Fe(dπ) → cor(π*)
NO(π)/cor(π) → cor(π*)
cor(π) → cor(π*)
cor(π) → cor(π*)
cor(π) → cor(π*)
cor(π) → cor(π*)
cor(π) → cor(π*)
1.9751
1.5899
633(7000)
765(2100)
628(0.0646)
780(0.0022)
K.; Liu, Y.; Feng, D.; Paeng, K.-J.; Ryan, M. D. Inorg. Chem. 1991, 30,
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studies is often useful while investigating the relevance of such
complexes in redox catalysis. Future work from our laboratories
will be focused on those directions.
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ASSOCIATED CONTENT
* Supporting Information
■
S
ESI−MS spectra of 2A, 3A, and 2B. 1H NMR spectrum of 3A,
and 1B. 13C NMR spectra of 1B, 2B, 3B, and 3A. Packing
diagrams of 1B and 2B. Electronic absorption spectra of 1B,
2B, and 3B. UV−vis, IR, and EPR spectroelectrochemical
measurements of 1B and 2B. Electronic absorption and
emission spectra of 3A. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*(G.K.L.) E-mail: lahiri@chem.iitb.ac.in.
*(B.S.) E-mail: Biprajit.Sarkar@fu-berlin.de.
*(S.K.) E-mail: sanjib@niser.ac.in.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support received from the Department of Atomic
Energy (India) is gratefully acknowledged. S.K. thanks DST-
New Delhi for partial funding. Authors thankfully acknowledge
NISER, Bhubaneswar and IOP, Bhubaneswar for providing
infrastructure and instrumental support. Fonds der chemischen
Industrie (FCI) is kindly acknowledged for financial support.
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