Iron(III) and iron(IV) corroles: Synthesis, spectroscopy, structures, and no indications for corrole radicals

Article abstract of DOI:10.1021/ic020118b

A delicate control of reaction conditions allows the isolation of several distinctively different iron complexes of tris(pentafluorophenyl)- and tris(2,6-dichlorophenyl)corrole. As long as coordinating ligands are present, the iron(III) complexes are stable in solution. Otherwise they are aerobically oxidized to either mononuclear chloroiron(IV) or dinuclear (μ-oxo)iron(IV) complexes, in acidic and basic solutions, respectively (the latter holds only for tris(pentafluorophenyl)corrole). When treated with NaNO₂, the mononuclear chloroiron(IV) corroles are efficiently converted into diamagnetic iron nitrosyl complexes. The low- and intermediate-spin iron(III), iron nitrosyl, and chloroiron-(IV) corroles were fully characterized by a combination of spectroscopic methods and X-ray crystallography. There was no indication for an open-shell corrole in any of the complexes.

Full text of DOI:10.1021/ic020118b

Inorg. Chem. 2002, 41, 5433−5439
Iron(III) and Iron(IV) Corroles: Synthesis, Spectroscopy, Structures, and
No Indications for Corrole Radicals
,‡
,†
Liliya Simkhovich,† Israel Goldberg,* and Zeev Gross*
Department of Chemistry and Institute of Catalysis Science and Technology,
Technion-Israel Institute of Technology, Haifa 32000, Israel, and School of Chemistry,
Tel AViV UniVersity, Tel AViV 69978, Israel
Received February 11, 2002
A delicate control of reaction conditions allows the isolation of several distinctively different iron complexes of
tris(pentafluorophenyl)- and tris(2,6-dichlorophenyl)corrole. As long as coordinating ligands are present, the iron(III)
complexes are stable in solution. Otherwise they are aerobically oxidized to either mononuclear chloroiron(IV) or
dinuclear (µ-oxo)iron(IV) complexes, in acidic and basic solutions, respectively (the latter holds only for
tris(pentafluorophenyl)corrole). When treated with NaNO , the mononuclear chloroiron(IV) corroles are efficiently
2
converted into diamagnetic iron nitrosyl complexes. The low- and intermediate-spin iron(III), iron nitrosyl, and chloroiron-
(IV) corroles were fully characterized by a combination of spectroscopic methods and X-ray crystallography. There
was no indication for an open-shell corrole in any of the complexes.
Introduction
ods and X-ray crystallography. All the acquired information
supported a low-spin iron(IV) oxidation state for Fe(oec)Cl,
and the diamagnetism of (oec)Fe-O-Fe(oec) was analyzed
as resulting from strong antiferromagnetic coupling between
the two d4 irons. Stabilization of the iron(III) oxidation state
was possible in the presence of pyridine, but surprisingly,
only the pentacoordinated Fe(oec)(py) was isolated. This is
unique to corroles: analogous porphyrin complexes cannot
be isolated because of the very small dissociation constants
of hexacoordinated bis(amine)iron(III) porphyrins.5 Still, in
a recent reexamination of Fe(omc)Cl and Fe(dmhec)Cl,
Licoccia et al claimed that the formally iron(IV) corroles
should be formulated as iron(III) corrole radicals.6 Their
conclusion is based on NMR spectroscopy, relying on the
very low field chemical shifts of the meso-protons therein.
The explanation provided by the authors with regard to the
Synthetic iron porphyrins are extensively investigated
because of their relevance to heme-containing proteins and
enzymes.1 In sharp contrast, iron complexes of corroles
(Scheme 1)sthe one carbon atom short analogues of
porphyrins, which are also related to the metal-chelating
macrocycle in Vitamin B12 (a corrin)sare much less known.
In addition, there is no general consensus about the electronic
structures of iron corroles. The early spectroscopic investiga-
tions (mainly by NMR) by Licoccia and Paolesse led the
authors to the conclusion that the product obtained from
insertion of iron into H3(omc) is a simple low-spin iron(III)
complex and that the ligand field of corroles is stronger than
that of porphyrins.2,3 But the studies of Vogel and co-workers
with iron complexes of H3(oec) revealed that the air-stable
oxidation state is actually higher than iron(III).4 These authors
have fully characterized mononuclear Fe(oec)Cl and di-
nuclear (oec)Fe-O-Fe(oec) by various spectroscopic meth-
(2) Abbreviations: oec, omc, dmhec, tpfc, and tdcc, trianions of octa-
ethylcorrole, octamethylcorrole, 7,13-dimethyl-2,3,8, 12,17,18-hexa-
ethylcorrole, 5,10,15-tris(pentafluorophenyl)corrole, and 5,10,15-tris-
(2,6-dichlorophenyl)corrole; tpp, tmp, and oep, dianions of meso-
tetraphenylporphyrin, meso-tetramesitylporphyrin, and octaethylporphyrin.
(3) Licoccia, S.; Paci, M.; Paolesse, R.; Boschi, T. J. Chem. Soc., Dalton
Trans. 1991, 461.
* To whom correspondence should be addressed. E-mail: chr10zg@
tx.technion.ac.il (Z.G.).
† Technion-Israel Institute of Technology.
‡ Tel Aviv University.
(1) (a) Watanabe, Y. J. Biol. Inorg. Chem. 2001, 6, 846 and references
therein. (b) Dolphin, D.; Xie, L. Y. In Metalloporphyrins Catalysed
Oxidations; Montanari, F., Casella, L., Eds.; Kluwer Academic
Publishers: Dordrecht, The Netherlands, 1994; pp 269-306. (c)
Groves, J. T.; Han, Y.-Z. In Cytochrome P-450: Structure, Mecha-
nism, and Biochemistry, 2nd ed.; Ortiz de Montellano, P. R., Eds.;
Plenum Press: New York, 1995; Chapter 1, pp 3-48. (d) Woggon,
W.-D. Bioorg. Chem. 1997, 184, 39.
(4) (a) Vogel, E.; Will, S.; Tilling, A. S.; Neumann, L.; Lex, J.; Bill, E.;
Trautwein, A. X.; Wieghardt, K. Angew. Chem., Int. Ed. Engl. 1994,
33, 731. (b) Caemelbecke, E. V.; Will, S.; Autret, M.; Adamian, V.
A.; Lex, J.; Gisselberecht, J.-P.; Gross, M.; Vogel, E.; Kadish, K. M.
Inorg. Chem. 1996, 35, 184.
(5) Nesset, M. J. M.; Shokhirev, N. V.; Enemark, P. D.; Jacobson, S. E.;
Walker, F. A. Inorg. Chem. 1996, 35, 5188.
(6) Cai, S.; Walker, F. A.; Licoccia, S. Inorg. Chem. 2000, 39, 3466.
10.1021/ic020118b CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/26/2002
Inorganic Chemistry, Vol. 41, No. 21, 2002 5433

Simkhovich et al.
Scheme 1
of the mono- and dinuclear iron(IV) corroles are described
in these publications, and we have also demonstrated that
the catalytic activity of iron(III) and iron(IV) is very
different.9b We now report a detailed analysis of the
electronic structures of several iron complexes of H3(tpfc)
and H3(tdcc), based on spectroscopic methods and X-ray
crystallography. The results reveal no indications for oxidized
corrole in any of the complexes, with isolated low-spin and
intermediate-spin iron(III) in the bis(pyridine) and bis(ether)
complexes, respectively. Quite strong iron to metal back-
donation is apparent in the Fe(NO) corroles, and the closed-
shell corroles support an iron(IV) oxidation state in the
Fe(cor)Cl complexes.
Experimental Section
1
Physical Methods. The H NMR spectra were recorded on a
Brucker AM 200 and Bruker AM 400, operating at 200 and 400
MHz, respectively. Chemical shifts are reported in ppm relative to
residual hydrogens in the deuterated chloroform: δ ) 7.24. An
HP 8452A diode array spectrophotometer was used to record the
electronic spectra. The EPR spectra were recorded on a Bruker
EMX 220 digital X-band radiospectrometer equipped with a Bruker
ER 4121VT temperature control system operating within the
temperature range of 100-700 K. Spectra processing and parameter
calculations were performed using WIN-EPR software. Mass
spectroscopy was performed on a TSQ 70 Finnigan with isobutane
as carrier gas, and IR spectra were recorded as KBr pellets of a
FT-IR Bruker Vector 22.
Synthetic Methods. The synthetic details for the preparation and
full spectroscopic characterization of H3(tpfc) and its iron complexes
are provided in previous publications.7a,b,9a X-ray-quality crystals
of Fe(tpfc)(NO) were obtained via recrystallization from a mixture
of benzene and n-heptane.
conflicting evidence from Mossbauer spectroscopy and
magnetic susceptibility (S ) 1) in favor of low-spin
iron(IV) is that the complexes are composed of an intermedi-
ate-spin iron(III) ion (S ) 3/2) that is strongly coupled in
an antiferromagnetic fashion with the unpaired electron on
the corrole, i.e., Stotal) 3/2 (Fe) - 1/2 (corrole radical) ) 1.
The NMR spectrum of the bis(pyridine)iron corrole (obtained
by adding excess pyridine to Fe(omc)Cl) was analyzed as
reflecting a low-spin iron(III) coupled to the corrole radical
in a ferromagnetic fashion, despite the fact that both the EPR
spectrum and the magnetic moment were only consistent with
a low-spin iron(III) coordinated by a closed-shell corrole.
Our interest in corrole chemistry started with the facile
synthesis of tris(pentafluorophenyl)corrole, H3(tpfc) in Scheme
1, and the demonstration that its metal complexes are potent
catalysts for a variety of reactions.7 The substitution pattern
in H3(tpfc)sonly electron-withdrawing substituentsssuggests
that is should be much more oxidation resistant than all
previously reported corroles, i.e., much more innocent in
terms of metal vs macrocycle oxidation. In addition, the
presence of â-pyrrole hydrogens in H3(tpfc) is a large
advantage for relatively straightforward interpretation of the
1H NMR spectra of its paramagnetic complexes. Additional
information can be obtained by sensing the meso-C6F5
substituents via 19F NMR spectroscopy. In parallel with many
other metal corroles,8 we have prepared the iron complexes
of H3(tpfc) in various oxidation states and with different axial
ligands.8a,9a These syntheses and the X-ray crystal structures
H3(tdcc). A total of 1.32 g (7.5 mmol) of solid 2,6-dichlorobenz-
aldehyde was put in a 50 mL flask that was heated on a hot plate
(about 75-80 °C) to melt the substrate. To the same flask was
added at once 1.05 mL (15 mmol) of pure pyrrole. After being
stirred for 10 min, the mixture was cooled to RT (room temperature)
and the resulting brown solid was dissolved in CH2Cl2. A 0.75 g
amount of DDQ was added to the solution, and the mixture was
stirred for 1 h at RT. Purification of H3(tdcc) was performed on a
chromatographic column with silica gel (20:5 n-hexane/CH2Cl2).
A 70 mg amount (3.8% yield) of pure H3(tdcc) was obtained. UV-
vis (CH2Cl2, λmax, nm) (relative ꢀ): 415 (1.00), 570 (0.32), 609
1
(0.24). H NMR (CDCl3, 200 MHz, δ in ppm, J in Hz): 8.91 (d,
J ) 4.17, 2H, â-pyrrole H), 8.48 (d, J ) 4.8, 2H, â-pyrrole H),
8.33 (m, 4H, â-pyrrole H), 7.73-7.63 (m, 9H, meso-Ph H).
Insertion of Iron into H3(tdcc), for the Synthesis of Fe(tdcc)-
(OEt2)2, Fe(tdcc)(py)2, Fe(tdcc)Cl, and Fe(tdcc)(NO). All iron
corroles were obtained by the previously published procedure for
(8) (a) Simkhovich, L.; Galili, N.; Saltsman, I.; Goldberg, I.; Gross, Z.
Inorg. Chem. 2000, 39, 2704. (b) Meier-Callahan, A. E.; Gray, H. B.;
Gross, Z. Inorg. Chem. 2000, 39, 3605. (c) Gross, Z.; Golubkov, G.;
Simkhovich, L. Angew. Chem., Int. Ed. 2000, 39, 4045. (d) Bendix,
J.; Golubkov, G.; Gray, H. B.; Gross, Z. J. Chem. Soc., Chem.
Commun. 2000, 1957. (e) Bendix, J.; Dmochowski, I. J.; Gray, H. B.;
Mahammed, A.; Simkhovich, L.; Gross, Z., Angew. Chem., Int. Ed.
2000, 39, 4048. (f) Golubkov, G.; Bendix, J.; Gray, H. B.; Mahammed,
A.; Goldberg, I.; Di Bilio, A. J.; Gross, Z. Angew. Chem., Int. Ed.
2001, 40, 2132.
(7) (a) Gross, Z.; Galili, N.; Saltsman, I. Angew. Chem., Int. Ed. Engl.
1999, 111, 1530. (b) Gross, Z.; Galili, N.; Simkhovich, L.; Saltsman,
I.; Botoshansky, M.; Blaser, D.; Boese, K.; Goldberg, I. Org. Lett.
1999, 1, 599. (c) Gross, Z.; Simkhovich, L.; Galili, N. Chem. Commun.
1999, 599.
(9) (a) Simkhovich, L.; Mahammed, A.; Goldberg, I.; Gross, Z., Chem.
Eur. J. 2001, 7, 1041. (b) Simkhovich, L.; Gross, Z. Tetrahedron Lett.
2001, 42, 8089.
5434 Inorganic Chemistry, Vol. 41, No. 21, 2002

Iron(III) and Iron(IV) Corroles
Table 1. Crystal and Experimental Data
Fe(tpfc)(NO)a
Fe(tdcc)(NO)
a
Fe(tpfc)(py)2
empirical formula
fw
cryst system
space group
a/Å
b/Å
c/Å
2C47H18F15FeN6‚3/2C7H16
2165.34
triclinic
P1h
14.0800(5)
16.6300(5)
20.9740(8)
101.32(2)
103.88(2)
93.24(2)
4647.0(3)
2
C37H8F15FeN5O‚C7H16
979.53
triclinic
P1h
7.0690(1)
20.0240(7)
27.8390(9)
92.089(1)
90.149(2)
93.555(2)
3930.4(2)
4
C37H17Cl6FeN5O‚2C6H6
972.32
monoclinic
C2/c
38.6430(13)
14.5000(3)
15.8420(5)
90.00
100.889(1)
90.00
8716.8(4)
8
R/deg
â/deg
γ/deg
V/Å3
Z
T/K
115
110
115
Dc/g‚cm-3
2θ range/deg
no. of unique reflcns
no. of reflcns with I > 2σ(Ι)
no. of refined params
R1 (I > 2σ(Ι))
R1 (all data)
wR2 (all data)
|∆F|max/e‚Å-3
1.548
4-50
16 319
9766
1369
0.068
0.133
1.655
3.5-55
16 762
10 966
1146
0.067
0.116
1.482
4-51
7696
5630
562
0.063
0.092
0.193
0.64
0.203
0.88
0.174
0.96
a There are two crystallographically independent corrole species in the asymmetric unit of these structures.
inserting iron into H3(tpfc),9a i.e., heating of dry DMF or pyridine
solution of H3(tdcc) under N2 in the presence of about 20 equiv of
FeCl2. The products that were obtained directly after metal insertion
are Fe(tdcc)(OEt2)2 (after purification of the complex via chroma-
tography on silica gel with diethyl ether) or Fe(tdcc)(py)2 (via
recrystallization of Fe(tdcc)(OEt2)2 from a mixture of benzene and
n-heptane in the presence of pyridine). To obtain Fe(tdcc)Cl, the
red-brown Fe(tdcc)(OEt2)2 was dissolved in CH2Cl2 and washed
extensively by aqueous HCl. Pure Fe(tdcc)Cl (brown color) was
obtained by recrystallization from CH2Cl2/n-hexane. The yields of
all the reactions described herein were in the range of 90-95%
for pure crystalline materials. The last Fe(III) complex, Fe(tdcc)-
(NO), was obtained from the iron(IV) chloride as discussed in our
previous publication.9a Final purification by recrystallization from
benzene and n-heptane afforded highly crystalline Fe(tdcc)(NO)
in about 50% yield. X-ray-quality crystals of Fe(tdcc)(NO) were
obtained via recrystallization from a mixture of benzene and
n-heptane.
814.9 (30) [MH]+, 784.6 (100) [MH+ - (NO)]. MS (-DCI) [m/z
(%)]: 815.1 (10) [M]-, 785.1 (100) [M- - (NO)]. IR (KBr): ν )
1783 cm-1 (NO).
Crystallography. The diffraction measurements were carried out
on a Nonius KappaCCD diffractometer, using graphite-monochro-
mated Mo KR radiation (λ ) 0.7107 Å). The crystalline samples
of the analyzed compounds were covered with a thin layer of light
oil and cooled to 110-115 K to minimize solvent escape, structural
disorder, and thermal motion effects and increase the precision of
the results. The crystal and experimental data for Fe(tpfc)(py)2,
Fe(tpfc)(NO), and Fe(tdcc)(NO) are summarized in Table 1. These
structures were solved by direct methods (SIR-92)10 and refined
by full-matrix least-squares on F2 (SHELXL-97).11 All non-
hydrogen atoms were refined anisotropically. The hydrogens were
located in idealized positions (the methyls being treated as rigid
groups) and were refined using a riding model with fixed thermal
parameters [Uij ) 1.2Uij(equiv) for the atom to which they are
bonded]. The three compounds crystallized with uncoordinated
solvent, which is disordered in the crystal lattice even at the low
temperature. Partial rotational disorder characterizes also some of
the pentafluorophenyl and dichlorophenyl rings of the corrole
molecules (as it is demonstrated in particular by excessively large
thermal displacement parameters of the corresponding halogen and
carbon atoms), affecting to some extent the quality of the crystals
(high mosaicity) and diffraction data (high percentage of weak
reflections) and, thus, the precision of the crystallographic deter-
mination.
Fe(tdcc)(OEt2)2. UV-vis (diethyl ether; λmax, nm) (relative ꢀ):
408 (1.00), 550 (0.26), 730 (0.05). 1H NMR (CDCl3, 200 MHz, δ
in ppm): 47.7, 32.7, -61.8, -135.4 (8H, â-pyrrole H), 10.49 and
9.23 (9H, meso-Ph H).
Fe(tdcc)(py)2. UV-vis (C6H6, λmax, nm) (relative ꢀ): 414 (1.00),
550 (0.27), 752 (0.07). 1H NMR (pyridine-d5, 200 MHz, δ in
ppm): 3.5, -34.7, -49.3, -121.3 (8H, â-pyrrole H), 8.9 and 9.23
(9H, meso-Ph H). MS (-DCI) [m/z (%)]: 785.1 (100) [M- - 2py].
Fe(tdcc)Cl. UV-vis (CH2Cl2, λmax, nm) (relative ꢀ): 368 (0.92),
394 (1.00). 1H NMR (CDCl3, 200 MHz, δ in ppm): 10.0 (s, para-H
of meso-Ph, 2H), 9.7 (s, para-H of meso-Ph, 1H), 6.6 (s, 2H,
â-pyrrole), -2.9 (s, 2H, meta-H of meso-Ph), -3.3 (s, 2H, meta-H
of meso-Ph), -3.8 (s, 2H, â-pyrrole), -4.3 (s, 1H, meta-H of meso-
Ph), -4.8 (s, 1H, meta-H of meso-Ph), -8.9 (s, 2H, â-pyrrole),
-37.5 (s, 2H, â-pyrrole). MS (+DCI) [m/z (%)]: 821.9 (5) [MH]+,
784.9 (100) [MH+ - Cl]. MS (-DCI) [m/z (%)]: 820.0 (20) [M]-,
785.1 (100) [M- - Cl].
Results and Discussion
Synthesis. The procedures for insertion of iron into
H3(tpfc) and the electrochemistry of the complexes were fully
described in one of our previous publication.9a In general,
metalation of H3(tpfc) proceeds readily in hot DMF with
FeCl2 as the metal source. The oxidation state of the isolated
Fe(tdcc)(NO). UV-vis (CH2Cl2, λmax, nm) (10-4ꢀ): 378 (7.82),
(10) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giacovazzo,
C.; Guagliardi, A.; Polidori, G. SIR-92. J. Appl. Crystallogr. 1994,
27, 435.
(11) Sheldrick, G. M. SHELXL-97. Program for the Refinement of Crystal
Structures from Diffraction Data; University of Goettingen: Goet-
tingen, Germany, 1997.
1
538 (1.16). H NMR (CDCl3, 400 MHz, δ in ppm, J in Hz): δ )
7.98 (d, J ) 4.65, 2H, â-pyrrole H), 7.57 (d, J ) 4.88, 2H, â-pyrrole
H), 7.58-7.42 (m, 9H, meso-Ph H), 7.26 (d, J ) 4.87, 2H, â-pyrrole
H), 7.20 (d, J ) 4.86, 2H, â-pyrrole H). MS (+DCI) [m/z (%)]:
Inorganic Chemistry, Vol. 41, No. 21, 2002 5435

Simkhovich et al.
iron corrole is determined by the workup procedure. Elution
of the crude reaction mixture with solutions that contain
either diethyl ether or pyridine results in the red iron(III)
complexes, Fe(tpfc)(OEt2)2 and Fe(tpfc)(py)2, respectively.
When no coordinating ligands are added (the DMF is also
completely removed), an extensive extraction with either
aqueous HCl or NaOH solutions allows the quantitative
isolation of the iron(IV) complexes, Fe(tpfc)Cl and
[Fe(tpfc)]2O, respectively. An additional formally iron(III)
complex of H3(tpfc) was Fe(tpfc)(NO), which was obtained
by treating Fe(tpfc)Cl with NaNO2. Some of the complexes
can be converted into each other by simple means, as
illustrated in Scheme 1. In addition to the above methodolo-
gies, [Fe(tpfc)]2O can also be obtained via the crystallization
of Fe(tpfc)(OEt2)2 from aerobic mixtures of benzene and
n-heptane in the absence of diethyl ether. In the presence of
excess pyridine, Fe(tpfc)Cl was not stable and was auto-
reduced to Fe(tpfc)(py)2. Except for Fe(tpfc)(OEt2)2, X-ray-
quality crystals were obtained for all complexes. In our first
publication about corroles, we also reported the novel tris-
(2,6-dichlorophenyl)corrole, H3(tdcc),7a whose large ortho-
phenyl substituents may serve as to avoid the formation of
µ-oxo bis metallic corroles. Insertion of iron into H3(tdcc)
was performed exactly by the same methodology as in the
case of H3(tpfc). The new Fe(tdcc)(OEt2)2, Fe(tdcc)(py)2, Fe-
(tdcc)Cl, and Fe(tdcc)(NO) complexes were prepared and
characterized by spectroscopy, as well as by X-ray crystal-
lography for the nitrosyl complex. As anticipated, a complex
analogous to [Fe(tpfc)]2O is not formed with this corrole.
Crystallography. The structures of the iron(IV) com-
plexes, Fe(tpfc)Cl and [Fe(tpfc)]2O, were described in two
separate publications.8a,9a The previously reported structure
of Fe(tpfc)(py)2 was obtained with a large R factor, and we
have now succeeded in growing better crystals from a
different crystallization environment.9a In addition, we have
obtained X-ray-quality crystals of the two nitrosyl complexes,
Fe(tpfc)(NO) and Fe(tdcc)(NO). The three new structures
are depicted in Figure 1, and selected structural parameters
of all iron triarylcorrole complexes that were isolated so far
are summarized in Table 2. The structure of Fe(tpfc)(py)2
consists of a quite planar macrocycle with the metal ion
located perfectly in the center. The Fe-N(pyrrole) bond
lengths (1.862-1.906 Å) are comparable to that in Co(tpfc)-
(py)2 (1.873-1.900 Å) and significantly shorter than in
Cr(tpfc)(py)2 (1.926-1.952 Å).12 As there cannot be any
doubt that Co(tpfc)(py)2 contains a closed-shell corrole (it
is a perfectly diamagnetic low-spin cobalt(III) complex);12a
the same conclusion is most likely for Fe(tpfc)(py)2. The
significantly longer Fe-N(pyridine) bond lengths in the iron
corrole (2.022-2.042 Å) relative to the analogous cobalt
complex (1.994 Å) is reasonable, considering the different
ligand field stabilization energies of low-spin d5 and d6 metal
ions. Another expectation for low-spin iron(III) complexes
that is fulfilled in Fe(tpfc)(py)2 is the mutual arrangement
Figure 1. X-ray structures of (a) Fe(tpfc)(NO), (b) Fe(tdcc)(NO), and (c)
Fe(tpfc)(py)2. In (a) and (b), for clarity only, the mean Cl positions of the
rotationally disordered central aryl ring are displayed.
of the coordinated pyridine molecules. The twist angles in
the two independent species are only 2.3 and 4.2°; i.e., they
are almost perfectly parallel to each other as expected for
an electronic configuration of (dxy)2(dxz,dyz)3.13 The corrole
macrocycle is essentially flat. The largest deviation from the
mean plane defined by all the corrole atoms is only 0.14 Å,
and the twist angles between adjacent pyrrole rings are within
2-9°.
A different comparison is with the nitrosyl complexes,
whose structural parameters are perfectly consistent with
authentic ferric ions coordinated to nitric oxide. The Fe-
NO bonds in both Fe(tpfc)(NO) and Fe(tdcc)(NO) are
practically linear (the three independent Fe-N-O bond
angles are within 172-178°), as expected for {FeNO}6
complexes.14 The five-coordinate metal ion deviates, by
0.45-0.46 Å, from the corrole plane toward the axial ligand,
(13) Walker, F. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol.
5, Chapter 36, pp 81-183.
(14) (a) Richter-Addo, G. B.; Wheeler, R. A.; Hixson, C. A.; Chen, L.;
Khan, M. A.; Ellison, M. K.; Schulz, C. E.; Scheidt, W. R. J. Am.
Chem. Soc. 2001, 123, 6314 and references therein. (b) Scheidt, W.
R. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: San Diego, CA, 2000; Vol. 3, Chapter 16,
pp 83-87. (c) Autret, M.; Will, S.; Caemelbecke, E. V.; Lex, J.;
Gisselbrecht, J.-P.; Gross, M.; Vogel, E.; Kadish, K. M. J. Am. Chem.
Soc. 1994, 116, 9141.
(12) (a) Mahammed, A.; Giladi, I.; Goldberg, I,; Gross, Z. Chem.sEur. J.
2001, 7, 4259. (b) Meier-Callahan, A. E.; Di Bilio, A. J.; Simkhovich,
L.; Mahammed, A.; Goldberg, I.; Gray, H. B.; Gross, Z. Inorg. Chem.
2001, 40, 6788.
5436 Inorganic Chemistry, Vol. 41, No. 21, 2002

Iron(III) and Iron(IV) Corroles
Table 2. Selected Parameters for All Structurally Characterized Iron Complexes of Triarylcorroles
Fe(tpfc)(NO)a
Fe(tdcc)(NO)
a
complex
Fe(tpfc)Cl
[Fe(tpfc)]2O
Fe(tpfc)(py)2
Fe-N(corrole) bond length range/Å
Fe-axial ligand dists/Å
1.880-1.922
2.238(2)
0.367(1)
1.884-1.923, 1.888-1.929
1.709(4), 1.726(4)
0.421(1), 0.426(1)
66.3, 79.0, 78.4, 59.0,
73.1, 68.5
1.862-1.906
1.893-1.926
1.898-1.922
1.641(4)
0.452(2)
2.022, 2.042(4)
0.010, 0.015(1)
1.639, 1.648(4)
0.465, 0.464(1)
64.7, 84.5, 81.3,
78.1, 86.2, 77.3
177.3(4), 178.0(4)
1.164(4), 1.166(4)
1790
Fe out-of-plane displacement/Åb
C6F5-corrole angles/deg
70.8, 64.0, 60.6
79.9, 66.3, 85.6,
80.6, 61.7, 72.0
87.8, 82.3, 83.0
Fe-NO angle/deg
172.3(4)
1.169(5)
1783
NO bond length/Å
NO stretching freq/cm-1
a There are two crystallographically independent corrole species in the asymmetric unit of these structures. b Plane defined by the four inner N atoms.
c The aryl rings in these compounds exhibit in most cases large-amplitude wagging rotational motion, or partial orientational disorder, about the C-C bonds
through which they are bonded to the main corrole framework. The listed dihedral angles between these substituents and the central corrole ring relate to
the average position of the aryl groups.
assuming a square-pyramidal coordination environment and
imparting to the complexes a domed conformation. This
involves also a consistent deviation of the pyrrole N atoms
(toward Fe) from the mean plane of the 19-membered all-
carbon corrole ring, 0.09-0.16 Å in Fe(tpfc)(NO) and 0.04-
0.14 Å in Fe(tdcc)(NO). The latter values reflect also on the
slightly more ruffled corrole structure in Fe(tpfc)(NO) than
in Fe(tdcc)(NO), as may be appreciated by the maximal
deviations of the individual atoms from the mean corrole
plane (0.18 and 0.23 Å vs 0.13 Å) and twist angle range
between adjacent pyrrole rings along the macrocycle
(1-16° vs 3-8°). The new nitrosyl complexes are also quite
ideal {FeNO}6 complexes. As expected for this diamagnetic
electronic state, their NMR spectra are sharp and the
Fe-NO units are linear (Table 2). In addition, the NO
stretching frequencies of Fe(tpfc)(NO), Fe(tdcc)(NO), and
Fe(eoc)(NO) (1790, 1783, and 1767 cm-1, respectively)
display a trend of increasing π-donation ability by these
corroles.14c These values may be compared to that of
corresponding porphyrin complexes, which are in the range
of 1862-1937 cm-1. Another indication for the significantly
stronger π-donation into the Fe(NO) moiety by corroles is
the longer N-O bonds therein, 1.169(5)-1.164(4) Å vs
1.144(3)-1.112(4) Å in porphyrins.14
One feature of tetraarylporphyrin radicals is that the angles
between the aryls and the porphyrin are usually significantly
Figure 2. 1H NMR spectra of (a) Fe(tpfc)(OEt2)2 (25 °C, benzene-d6),
(b) Fe(tpfc)(py)2 (25 °C, benzene-d6), and (c) Fe(tpfc)(py)2 (25 °C, pyridine-
d5). (s and py stand for coordinated pyridine and solvent impurities,
respectively.)
smaller than in closed-shell complexes.15 For example, the
dihedral angles between the phenyl rings and the oxidized
porphyrin in [Fe(tpp•+)Cl]+ are as low as 43.7, 48.8, 39.5,
and 46.2°.15 An inspection of Table 2 reveals that in the
nitrosyl complexes, where the observed data rule out corrole
oxidation, the aryl substituents are aligned in a roughly
perpendicular manner with respect to the corrole macrocycle.
The corresponding dihedral angles between the mean planes
of the aryl and the corrole residues vary from 65 to 88°. As
the angles in the other complexes (Fe(tpfc)Cl, [Fe(tpfc)]2O,
Fe(tpfc)(py)2) are also within the same range, we conclude
that by crystallographic criteria there is no indication for an
open-shell corrole in any of the complexes.
5/2), at very high field for low-spin iron(III) (S ) 1/2), and
medium-high to medium-low fields for intermediate- and
mixed-spin iron(III) (S ) 3/2 or S ) 5/2 + S ) 1/2).16 The
spectrum of the bis(ether) corrole [Fe(tpfc)(OEt2)2] (Figure
2a) displays two resonances at low field and two at high
field (19.7, 13.4, -60.0, -126.0), consistent with an
intermediate-spin iron(III) complex. In contrast, all four
resonances of the bis(pyridine) corrole [Fe(tpfc)(py)2] (3.2,
-61.1, -65.8, -134.6 ppm) are located at high field as
expected for a low-spin iron(III) complex.17 This is further
Spectroscopy. The most informative 1H NMR resonances
are those of the â-pyrroles, as the extensive work with iron
porphyrins revealed the following rules with regard to their
positioning: at very low field for high-spin iron(III) (S )
(16) Reed, C. A. Inorg. Chim. Acta 1997, 263, 95.
(17) The 1H NMR measurements, performed in benzene only, indicate two
signals at high field and four at low field (Figure 2b), but when
pyridine-d5 was utilized as a solvent, the two resonances at high field
were absent. The same phenomenon was obtained for the iron(III)
complex of H3(tdcc).
(15) Scheidt, W. R.; Lee, Y. J. Struct. Bonding 1987, 64, 1.
Inorganic Chemistry, Vol. 41, No. 21, 2002 5437

Simkhovich et al.
1
Table 3. Paramagnetic H NMR Shifts for High-Valent Iron Corroles and Porphyrins (in ppm)
complex
para-H
meta-H
∆(p-m)
pyrrole H
ref
FeIII(tpp•+)(ClO4)2
FeIII(tpp•+)(Cl)(ClO4)2
FeIII(tpp•+)(Cl)(SbCl6)
FeIII(tpp•+)(imidazole)2
FeIV(tdcc)Cl
-13.2
29.5
35.0
-22.1
10.0, 9.7
35.0
47.2
34.9
37.6
52.5
<15
31.1
66.1
68.8
-40.1
19
20, 21
19
22
this work
9a
-12.4
-12.3
30.4
-2.9, -3.3, -4.3, -4.8
6.6, -3.8, -8.9, -37.5
FeIV(tpfc)Cl
-2.5, -3.1, -12.1, -33.6
FeIV(tmp)(OCH3)2
FeIV(tmp)(O)
7.7
6.2, 6.8
-37.5
5.9
23, 24
25
complex
CH2
CH3
3.16
-0.3, -2.5
meso-H
ref
Fe(oep•+)(Cl)(ClO4)
Fe(oec)Cl
30.5, 29.6
-5.7, -29.9
18
177, 189
20
4a
corroles, without any indications for oxidation of the corrole
macrocycle therein.
As mentioned in the Introduction, the only supporting
evidence for iron(III) corrole radicals comes from the unusual
1H NMR chemical shifts of meso-CH in Fe(omc)Cl. Ac-
cordingly, we have analyzed the spectrum of Fe(tdcc)Cl
(Figure 4) with particular emphasis on para-H and meta-H
of its meso-aryl groups. This allows a comparison with high-
valent iron porphyrins (Table 3), for which a consensus about
the metal vs porphyrin oxidation exists.18 The distinction
between the aryl and â-pyrrole protons was easily achieved,
due to the similarity of the chemical shifts of the latter in
Fe(tdcc)Cl and Fe(tpfc)Cl. The assignment shown in the
figure is based on the following considerations: The aryl
on C10 is different from the magnetically identical groups
on C5 and C15 due to the C2V symmetry of the complex.
Each aryl carries two nonequivalent meta-H protons, syn and
anti to the Fe-Cl bond; the spectral width of para-H
resonances is smaller than of meta-H due to the larger
distance from the paramagnetic metal ion. In Table 3 we
have summarized both the chemical shifts and the differences
between the para and meta protons, ∆(p-m), of the meso-
aryl groups of all relevant complexes. The difference between
Fe(tdcc)Cl and the well-established iron(III) porphyrin cation
radical complexes is highly significant: ∆(p-m) is much
smaller in Fe(tdcc)Cl (<15 ppm vs > 35 ppm), and while
the pattern of low-field para-H and high-field meta-H is
shared by Fe(tdcc)Cl and the pentacoordinated iron(III)
porphyrin cation radical complexes, this is not true for the
â-pyrrole protons (6.6 to -37.5 vs 66 to 69 ppm). In fact,
the trend of both meta-H and pyrrole-H being located at low
field, found in Fe(tdcc)Cl, is only shared by the iron(IV)
porphyrins Fe(tmp)(O) and Fe(tmp)(OCH3)2. Accordingly,
this analysis adds confidence to the conclusion that Fe(tpfc)-
Cl and Fe(tdcc)Cl are not iron(III) corrole radical complexes
but authentic iron(IV) corroles. Finally, we have also put
Figure 3. The EPR spectra of (a) Fe(tpfc)(OEt2)2 (powder, 130 K, insert,
magnetic susceptibility measurements) and (b) Fe(tpfc)(py)2 (1:1:1 CH2-
Cl2/toluene/pyridine; 20 K).
confirmed by the corresponding EPR spectra (Figure 3a,b),
which also follow the well-known general pattern of iron
porphyrins. The spectrum of the bis(ether) complex consists
of a broad feature with g ) 3.8 expected for S ) 3/2, while
the bis(pyridine) complex displays the characteristic gx, gy,
gz pattern for low-spin iron(III) around g ) 2. Finally, the S
) 3/2 spin state of Fe(tpfc)(OEt2)2 was validated via
magnetic susceptibility measurements (Figure 3a, inset),
obtained by SQUID on a powdered sample. We may thus
safely conclude that both complexes are authentic iron(III)
(18) Trautwein, A. X.; Bill, E.; Bominaar, E. L.; Winkler, H. Struct.
Bonding 1991, 78, 1.
(19) Gans, P.; Buisson, G.; Duee, E.; Marchon, J.-C.; Erler, B. S.; Scholz,
W. F.; Reed, C. A. J. Am. Chem. Soc. 1986, 108, 1223.
(20) Phillippi, M. A.; Goff, H. M. J. Am. Chem. Soc. 1982, 104, 6026.
(21) Boersma, A. D.; Goff, H. M. Inorg. Chem. 1984, 23, 1671.
(22) Goff, H. M.; Phillippi, M. A. J. Am. Chem. Soc. 1983, 1054, 7567.
(23) Groves, J. T.; Quinn, R.; McMurry, T. J.; Lang, G.; Boso, B. J. Chem.
Soc., Chem. Commun. 1984, 1455.
(24) Groves, J. T.; Quinn, R.; McMurry, T. J.; Nakamura, M.; Lang, G.;
Boso, B. J. Am. Chem. Soc. 1985, 107, 354.
(25) Groves, J. T.; Gross, Z.; Stern, M. K. Inorg. Chem. 1994, 33, 5065.
5438 Inorganic Chemistry, Vol. 41, No. 21, 2002

Iron(III) and Iron(IV) Corroles
Figure 4. 1H NMR spectrum of Fe(tdcc)Cl (25 °C, CDCl3).
into Table 3 the information for the closely related corrole
and porphyrin complexes Fe(oec)Cl and [Fe(oep+.)Cl](ClO4),
that demonstrates the large differences between the relevant
protons therein. This suggests that Fe(oec)Cl is also an iron-
(IV) corrole rather than an iron(III) corrole radical complex,
as the data of Vogel et al indicate indeed.
iron(III) porphyrin cation radicals but some similarities to
established iron(IV) porphyrins. Accordingly, we conclude
that there is no indication for an open-shell corrole in any
of the complexes; i.e., they are best formulated as authentic
iron(III) and iron(IV) corroles.
Acknowledgment. This research was supported by the
Israel Science foundation under Grant 368/00 and the
Petroleum Research Fund (American Chemical Society).
Partial support by the Technion VPR Fund (New York
Metropolitan Research Fund) and the Fund for promotion
of Research at the Technion is acknowledged as well, and
we also thank Dr. A. J. Di Bilio (Caltech) for the EPR
mesurements on Fe(tpfc)(py)2.
Conclusions
We have isolated several distinctively different iron
complexes of tris(pentafluorophenyl)- and tris(2,6-dichloro-
phenyl)corrole. A combination of spectroscopic methods and
X-ray crystallography allowed the conclusion that the bis-
(ether) and bis(pyridine) complexes are authentic intermedi-
ate-spin and low-spin iron(III) complexes, respectively. The
diamagnetism and the linear Fe-NO bonds in the nitrosyl
complexes were also perfectly consistent with a ferric ion
supported by a closed-shell corrolato ligand. The spectral
comparison of the iron(IV) corroles with high-valent iron
porphyrins reveals highly significant differences relative to
Supporting Information Available: Tables of crystal data,
structure refinement details, atomic coordinates, anisotropic thermal
parameters, bond lengths, and bond angles in CIF format for
Fe(tpfc)(py)2, Fe(tpfc)(NO), and Fe(tdcc)(NO). This material is
available free of charge via the Internet at http://pubs.acs.org.
IC020118B
Inorganic Chemistry, Vol. 41, No. 21, 2002 5439