Halogeno-coordinated iron corroles

Article abstract of DOI:10.1021/ic0491332

The first full assignment of ¹H NMR chemical shifts for iron corroles and the first synthesis of a series of (halogeno)iron corroies reveal very large effects of the axial ligands on the corresponding spectra, which apparently reflect differences in the relative importance of metal-to-corrole and corrole-to-metal π-donation. These findings pave the way for a thorough analysis of the electronic structures of such complexes.

Full text of DOI:10.1021/ic0491332

Inorg. Chem. 2004, 43, 6136−6138
Halogeno-Coordinated Iron Corroles
Liliya Simkhovich and Zeev Gross*
Department of Chemistry and Institute of Catalysis Science and Technology,
Technion - Israel Institute of Technology, Haifa 32000, Israel
Received July 2, 2004
1
The first full assignment of H NMR chemical shifts for iron corroles
and the first synthesis of a series of (halogeno)iron corroles reveal
very large effects of the axial ligands on the corresponding spectra,
which apparently reflect differences in the relative importance of
Chart 1. Formal Drawing of Previously Reported Five-Coordinate
Iron Corroles and the Two Electronic Configurations that Are Consistent
with Experimental Magnetic Dataa
metal-to-corrole and corrole-to-metal
π-donation. These findings
pave the way for a thorough analysis of the electronic structures
of such complexes.
The involvement of high-valent metalloporphyrins as
reaction intermediates in the various processes that are
catalyzed by heme enzymes continues to serve as a main
inspiration for research into synthetic porphyrins and por-
1
phyrin analogues. Most issues regarding structural and
electronic factors that affect spin and oxidation states in iron
2
porphyrins are well resolved to date, but this is not the
situation for iron corroles, which were recently found to
catalyze quite a variety of reactions.2 The main shortcom-
ings that hamper the latter research are the limited number
of well-characterized complexes and the vague assignments
of H NMR resonances therein. The quite extensive debate
about the proper definition of the electronic state [iron(IV)
corrole vs iron(III) corrole radical] in complexes with the
general formula of Fe(cor)(Cl) clearly suffers from the fact
that the four magnetically different â-pyrrole substituents (R
,3
a
M in b is another iron corrole, i.e., a (cor)Fe-O-Fe(cor) complex).
1
4
We now report the preparation of a series of five-
coordinated (halogeno)iron complexes of 5,10,15-tris(penta-
fluorophenyl)corrole and 5,10,15-tris(2,6-dichlorophenyl)-
corrole, H (tpfc) and H
3 3
(tdcc), respectively.6 The two
particular corroles were chosen for three reasons: (a) they
are much more stable than all other corroles, (b) they are
the only ones whose metal complexes were used as catalysts,
and (c) they complement each other in terms of spectroscopic
sensing of the meso-aryl groups by F and H NMR
7
)
H or alkyl in Chart 1) were never assigned by NMR
analysis, as well as the lack of any information about
complexes with other halogeno ligands.2-5
1
9
1
1
spectroscopies, respectively. Full assignment of all H NMR
*
To whom correspondence should be addressed. E-mail: chr10zg@
techunix.technion.ac.il.
resonances was achieved via selective deuteration of H
3
(tpfc)
(
1) (a) Watanabe, Y. Curr. Opin. Chem. Biol. 2002, 6, 208. (b) Fujii, H.
and H (tdcc). The results revealed two very large effects of
3
Coord. Chem. ReV. 2002, 226, 51.
1
(2) Walker, F. A. Inorg. Chem. 2003, 42, 4526.
axial ligands on the H NMR spectra of the iron complexes:
variations in chemical shift that were most pronounced for
â-pyrrole protons and differences in broadness of resonances
that were most distinctive for meso-aryl protons. These
phenomena seem more consistent with iron(IV) corrole
(
3) (a) Gross, Z.; Simkhovich, L.; Galili, N. Chem. Commun. 1999, 599.
(
b) Gross, Z.; Golubkov, G.; Simkhovich, L. Angew. Chem., Int. Ed.
000, 39, 4045. (c) Golubkov, G.; Bendix, J.; Gray, H. B.; Mahammed,
A.; Goldberg, I.; DiBilio, A. J.; Gross, Z. Angew. Chem., Int. Ed. 2001,
2
4
0, 2132. (d) Simkhovich, L.; Gross, Z. Tetrahedron Lett. 2001, 42,
8089.
(
4) (a) Simkhovich, L.; Goldberg, I.; Gross, Z. Inorg. Chem. 2002, 41,
433. (b) Vogel, E.; Will, S.; Schulze Tilling, A.; Neumann, L.; Lex,
5
(6) (a) Gross, Z.; Galili, N.; Saltsman, I. Angew. Chem., Int. Ed. 1999,
38, 1427. (b) Gryko, D. T.; Koszarna, B. Org. Biomol. Chem. 2003,
1, 350.
J.; Bill, E.; Trautwein, A. X.; Wieghardt, K. Angew. Chem., Int. Ed.
Engl. 1994, 33, 731.
(
5) Steene, E.; Wondimagegn, T.; Ghosh, A. J. Phys. Chem. B 2001, 105,
(7) Geier, G. R., III; Chick, J. F. B.; Callinan, J. B.; Reid, C. G.;
Auguscinski, W. P. J. Org. Chem. 2004, 69, 4159.
11406; J. Phys. Chem. B 2002, 106, 5312.
6136 Inorganic Chemistry, Vol. 43, No. 20, 2004
10.1021/ic0491332 CCC: $27.50
© 2004 American Chemical Society
Published on Web 08/31/2004

COMMUNICATION
Scheme 1. Synthetic Pathways for Preparation of the New
Five-Coordinate Iron Complexes of H3(tdcc)
formulation than the alternative iron(III) corrole radical
description.
The syntheses of the new five-coordinated iron corroles
relied on the previously described complexes shown in
Scheme 1. This includes all synthetic details for preparation
and full spectroscopic characterization of H
3
(tpfc) and
6
H
3
(tdcc) and their six-coordinated iron(III) and the (chloro)-
8
iron(IV) complexes. The chloro- and bromo-coordinated
complexes 3-Cl, 3-Br, 4-Cl, and 4-Br were prepared
from the corresponding (bis-ether)iron(III) corroles via
treatment with the appropriate mineral acid (HCl, HBr) in
aerobic solutions. To obtain Fe(tdcc)I, the red-brown
Fe(tdcc)(OEt
extensively with aqueous HI. The last (halogeno)iron com-
plex of H (tdcc)sFe(tdcc)Fswas obtained by heating of a
2 2
) was dissolved in benzene and washed
3
dry and acid-free dichloromethane solution of Fe(tdcc)Cl
with an excess of AgF. The yields of all of the reactions
described herein were in the range of 90-95% for pure
crystalline or powder materials. All halogeno-coordinated
Figure 1. 1H NMR spectra of (a) natural-abundance H3(tdcc), (b) partially
deuterated H3(tdcc), (c) natural-abundance Fe(tdcc)Cl, and (d) partially
deuterated Fe(tdcc)Cl prepared from the sample whose spectrum is shown
in b. Measurements were performed in (a,b) CDCl3 and (c,d) C6D6 at
293 K.
iron complexes of H
stable, but the fluoro- and iodo-coordinated iron complexes
of H (tpfc) (3-F and 3-I) that were obtained in analogy to
3
(tdcc) (4-F, 4-Cl, 4-Br, 4-I) were
Similar procedures were carried out for the other com-
plexes, allowing for a complete assignment of the resonances
for all complexes. Whenever the assignment was ambiguous,
the procedures were repeated with batches of corroles that
differed in the extent of deuteration: the shorter the reaction
time of the corroles with trifluoroacetic acid-d, the more the
initial selectivity of deuteration of C3 g C2 > C8 > C7 is
3
4-F and 4-I decomposed quite rapidly in solution.
The first step of the investigations was to assign observed
1
chemical shifts in the H NMR spectra of the paramagnetic
complexes to specific hydrogen atoms. This task became
feasible as a result of the recent demonstration of selective
deuteration of corroles and the full NMR assignment of
9
a
reflected in the outcome. Finally, the assignment of aryl
resonances relies on the 2-fold symmetry that enforces one
unique and two identical aryls, reflected in 1:2 sets of
9
triarylcorroles and their diamagnetic complexes. The ap-
1
proach is illustrated in Figure 1, which shows the H NMR
1
19
spectra of natural-abundance and partially deuterated corrole,
together with those of the corresponding natural-abundance
and partially deuterated (chloro)iron(IV) complexes. The
analysis of Figure 1b revealed that about 50% of H3, 50%
of H2, and 20% of H8 of the free base corrole were replaced
by deuterium in this particular case, a comparison between
the spectra in Figure 1c and d exposed a decrease in the
intensities of the resonances at 3.4, -8.6, and -34.4 ppm,
which was confirmed by complementary information ob-
H and F NMR resonances for para-H and meta-H in
Fe(tdcc)X and for ortho-F, meta-F, and para-F in the
Fe(tpfc)X complexes.
The chemical shifts and assignments that are shown in
Figure 2 and summarized in Figure 3 reveal very large effects
of the coordinated halide ions on the spectra of the (halo-
geno)iron(IV) complexes. Another aspect that is also apparent
from Figure 2 is the large effect of the axial ligands on the
line widths, from very broad resonances in Fe(tdcc)F to very
sharp in Fe(tdcc)I. This is further illustrated in Figure 4,
which serves to demonstrate that the resonances of Fe(tdcc)I
are narrow enough that the small J coupling constants of its
aryl hydrogen atoms can be resolved.
2
tained from the H NMR spectra of the same complex.
(
8) (a) Simkhovich, L.; Galili, N.; Saltsman, I.; Goldberg, I.; Gross, Z.
Inorg. Chem. 2000, 39, 2704. (b) Simkhovich, L.; Mahammed, A.;
Goldberg, I.; Gross, Z. Chem. Eur. J. 2001, 1041.
(
9) (a) Gross, Z.; Mahammed, A. J. Porphyrins Phthalocyanines 2002,
Importantly, both phenomena are reminiscent of similar
effects of axial ligands on the spectra of manganese(III) and
6
, 553. (b) Balazs, Y. S.; Saltsman, I.; Mahammed, A.; Tkachenko,
E.; Golubkov, G.; Levine, J.; Gross, Z. Magn. Reson. Chem. 2004,
2, 624-635.
10
4
iron(III) porphyrins, clearly indicating that there is no need
Inorganic Chemistry, Vol. 43, No. 20, 2004 6137

COMMUNICATION
Figure 3. Chemical shifts of â-pyrrole hydrogen atoms in (halogeno)iron
complexes of H3(tdcc), plotted arbitrarily as a function of the ionic radii of
the halides.
1
Figure 4. Partial H NMR spectrum of Fe(tdcc)I, focusing on the unusually
sharp resonances of the hydrogen atoms of meso-aryl rings.
it is not currently possible to analyze corrole-to-metal
π-donation because there are no reported calculations on the
filled corrole orbitals that are of proper symmetry (lower in
energy than the highest occupied ones that are of improper
symmetry) to interact with the singly occupied d
π
metal
orbitals.
1
Figure 2. Full H NMR spectra of Fe(tdcc)F, Fe(tdcc)Cl, Fe(tdcc)Br, and
The synthesis of a full series of (halogeno)iron corroles
and the assignment of all chemical shifts in the corresponding
H NMR spectra enable an in-depth analysis of the electronic
Fe(tdcc)I (293 K, benzene-d6), also showing the assignment of the â-pyrrole
hydrogen atoms.
1
to invoke oxidized corrole to explain the results. Rather,
ligand-dependent changes in the relative importance of the
direction of π-donation, corrole-to-metal vs metal-to-corrole,
might well be responsible for the quite pronounced differ-
ences. Some evidence for this hypothesis is the increasing
difference between the chemical shifts of H2 and H3 upon
moving from Fe(tdcc)I to Fe(tdcc)F. Metal-to-corrole π-
donation might be expected to be more important in the latter
complex, and published calculations on the π-accepting
LUMO orbitals of closed-shell corroles disclose a large
structures of such complexes. We have already obtained
X-ray-quality crystals of 3-Br, 4-Br, and 4-Cl that,
together with various new spectroscopic data (including about
less electron-poor corroles), will be used for that purpose in
forthcoming publications.
Acknowledgment. This research was supported by the
Israel Science Foundation under Grant 368/00 (Z.G.). A
postdoctoral fellowship for L.S. from the Institute of
Catalysis Science and Technology is acknowledged as well.
11
difference in the coefficients on C2 and C3. Unfortunately,
IC0491332
(
10) (a) La Mar, G. N.; Walker, F. A. J. Am. Chem. Soc. 1975, 97, 5103.
(11) (a) Bendix, J.; Dmochowski, I. J.; Gray, H. B.; Mahammed, A.;
Simkhovich, L.; Gross, Z. Angew. Chem., Int. Ed. 2000, 39,
4048. (b) Ghosh, A.; Taylor, P. R. Curr. Opin. Chem. Biol. 2003, 7,
113.
(
b) Wojaczynski, J.; Latos-Grazynski, L.; Chmielewski, P. J.; Van
Calcar, P.; Balch, A. L. Inorg. Chem. 1999, 38, 3040. (c) Behere, D.
V.; Birdy, R.; Mirta, S. Inorg. Chem. 1982, 21, 386.
6138 Inorganic Chemistry, Vol. 43, No. 20, 2004