13C NMR studies of the electronic structure of low-spin iron(III) tetraphenylchlorin complexes

Article abstract of DOI:10.1021/ic060508o

A series of low-spin six-coordinate (tetraphenylchlorinato)iron(III) complexes [Fe(TPC)(L)₂]^± (L = 1-Melm, CN ^-, 4-CNPy, and ^tBuNC) have been prepared, and their ¹³C NMR spectra have been examined to reveal the electronic structure. These complexes exist as the mixture of the two isomers with the (d_xy)²(d_xz, d_yz)³ and (d_xz, d_yz)⁴(d_xy)¹ ground states. Contribution of the (d_xz, d_yz)⁴(d _xy)¹ isomer has increased as the axial ligand changes from 1-Melm, to CN^- (in CD₂Cl₂ solution), CN ^- (in CD₃OD solution), and 4-CNPy, and then to ^tBuNC as revealed by the meso and pyrroline carbon chemical shifts; the meso carbon signals at 146 and -19 ppm in [Fe(TPC)(1-Melm)₂] ⁺ shifted to 763 and 700 ppm in [Fe(TPC)(^tBuNC) ₂]⁺. In the case of the CN^- complex, the population of the (d_xz, d_yz)⁴(d _xy)¹ isomer has increased to a great extent when the solvent is changed from CD₂Cl₂ to CD₃OD. The result is ascribed to the stabilization of the d_xz and d_yz orbitals of iron(III) caused by the hydrogen bonding between methanol and the coordinated cyanide ligand. Comparison of the ¹³C NMR data of the TPC complexes with those of the TPP, OEP, and OEC complexes has revealed that the populations of the (d_xz, d_yz)⁴(d _xy)¹ isomer in TPC complexes are much larger than those in the corresponding TPP, OEC, and OEP complexes carrying the same axial ligands.

Full text of DOI:10.1021/ic060508o

Inorg. Chem. 2006, 45, 6728−6739
13C NMR Studies of the Electronic Structure of Low-Spin Iron(III)
Tetraphenylchlorin Complexes
Akira Ikezaki,† Mikio Nakamura,*,†,‡,§ Sandrine Juillard,| and Ge´rard Simonneaux*,|
Department of Chemistry, School of Medicine, Toho UniVersity, Ota-ku, Tokyo 143-8540, Japan,
DiVision of Chemistry, Graduate School of Science, Toho UniVersity, Funabashi 274-8510, Japan,
Research Center for Materials with Integrated Properties, Toho UniVersity, Funabashi 274-8510,
Japan, and Laboratoire Sciences Chimiques de Rennes, UMR CNRS 6226, UniVersite´ de Rennes
1, 35042 Rennes Cedex, France
Received March 26, 2006
A series of low-spin six-coordinate (tetraphenylchlorinato)iron(III) complexes [Fe(TPC)(L)2]± (L
)
1-MeIm, CN-,
4-CNPy, and BuNC) have been prepared, and their 13C NMR spectra have been examined to reveal the electronic
structure. These complexes exist as the mixture of the two isomers with the (dxy)2(dxz, dyz)3 and (dxz, dyz)4(dxy)1
ground states. Contribution of the (dxz, dyz)4(dxy)1 isomer has increased as the axial ligand changes from 1-MeIm,
t
t
to CN- (in CD2Cl2 solution), CN- (in CD3OD solution), and 4-CNPy, and then to BuNC as revealed by the meso
and pyrroline carbon chemical shifts; the meso carbon signals at 146 and −
19 ppm in [Fe(TPC)(1-MeIm)2]+ shifted
to 763 and 700 ppm in [Fe(TPC)(tBuNC)2]+. In the case of the CN- complex, the population of the (dxz, dyz)4(dxy)1
isomer has increased to a great extent when the solvent is changed from CD2Cl2 to CD3OD. The result is ascribed
to the stabilization of the dxz and dyz orbitals of iron(III) caused by the hydrogen bonding between methanol and
the coordinated cyanide ligand. Comparison of the 13C NMR data of the TPC complexes with those of the TPP,
OEP, and OEC complexes has revealed that the populations of the (dxz, dyz)4(dxy)1 isomer in TPC complexes are
much larger than those in the corresponding TPP, OEC, and OEP complexes carrying the same axial ligands.
Introduction
The family of green heme proteins is now quite large,
containing heme d1, an iron isobacteriochlorin dione, or heme
d, an iron chlorin, as prosthetic groups.2 Thus, cytochrome
bd oxidase is a bacterial terminal oxidase that contains three
cofactors: a low-spin heme (b558), a high spin heme (b595),
and a chlorin d.3-5 Whereas X-ray structures have been
published for several heme-copper cytochrome c oxidases,6
no crystal structure is available yet for the cytochrome bd
family. The molecular mechanism of the enzyme action has
been studied in much less detail7,8 than the heme-copper
oxidases. Heme d has also been found in catalases, such as
hydroperoxidase II, from Escherichia coli.9 In contrast to
Nature utilizes iron porphyrins as the prosthetic group for
the majority of heme proteins involved in oxygen transport,
oxygen and peroxide activation, and electron transfer. Thus
there is a considerable amount of data for the physical
properties of iron porphyrins to explain their biological
properties. In contrast, the electronic structure of iron chlorins
still represents a recent active and challenging area,1 because
iron chlorins have been identified as the prosthetic groups
of a number of heme proteins only in recent years.2 A chlorin
is an hydroporphyrin with one reduced pyrrole double bond
in the macrocyle ring.
* To whom correspondence should be addressed. E-mail: mnakamu@
med.toho-u.ac.jp (M.N.).
(3) Sun, J.; Kahlow, M. A.; Kaysser, T. M.; Osborne, J. P.; Hill, J. J.;
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35, 2403-2412.
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† School of Medicine, Toho University.
‡ Graduate School of Science, Toho University.
§ Research Center for Materials with Integrated Properties, Toho
University.
| UMR CNRS 6226, Universite´ de Rennes 1.
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6728 Inorganic Chemistry, Vol. 45, No. 17, 2006
10.1021/ic060508o CCC: $33.50
© 2006 American Chemical Society
Published on Web 07/26/2006

Low-Spin Iron(III) Tetraphenylchlorin Complexes
cytochrome bd oxidase, the crystal structure of catalase HP
II from E. coli has been determined showing a heme d
prosthetic group with a cis-hydroxychlorin γ-spirolactone10
and a tyrosine as the proximal ligand.11 A heme d prosthetic
group with the same configuration has also been found in
the crystal structure of Penicillium Vitale catalase.12 Evidence
favoring coordination of a tyrosinate proximal ligand to the
chlorin iron of E. coli Hp ΙΙ catalase was previously proposed
by Dawson et al.13 Sulfmyoglobin 14,15 and sulfhemoglobin16
are also green heme proteins.
Since the reference paper reported by Holm and co-
workers in 1981,17 many investigations of the NMR and EPR
spectra of low-spin iron(III) complexes of reduced porphyrins
have been published very recently,18 by us19-22 and
others.15,23-32 The nature of the electronic ground state is
not always clear, and more information is needed with these
systems. Magnetic circular dichroism spectroscopy has also
been shown to be of great utility in the identification of
proximal and distal axial ligands in chlorin-containing
proteins.33,34 However, only a limited number of iron chlorin
complexes such as high-spin iron(II),35,36 high-spin iron-
(III),30,37 (µ-oxo)bis[(tetraphenylchlorin)iron(III)],38 and low-
spin iron(III) tetraphenylchlorin18 species have been inves-
tigated with X-ray crystallography. We now report 13C NMR
analyses of a series of low-spin six-coordinated iron(III)
tetraphenylchlorin complexes. The purpose of this study is
to extend the coverage of the electronic ground state of iron
chlorin models using 13C as NMR probes.
Experimental Section
Synthesis. Free base TPC,39 Fe(TPC)Cl,40 Fe(TPC)(CF3SO3),19
[Fe(TPC)(1-MeIm)2]Cl,20 and [Fe(TPC)(tBuNC)2](CF3SO3)19 were
prepared as described previously.
[Fe(TPC)(CN)2]NBu4 was prepared by the addition of 550 µL
of CD2Cl2 to the mixture consisting of Fe(TPC)Cl (30 µmol) and
2.5 equiv of tetrabutylammonium cyanide. The solution was taken
1
into an NMR sample tube for the H NMR measurement. The
1
characterization of all the complexes was done by means of H
and 13C NMR spectra as given in Results and Discussion.
[Fe(TPC)(4-CNPy)2](CF3SO3). Titration experiment revealed
that the addition of 10 equiv of 4-CNPy is enough for the com-
plete conversion of Fe(TPC)(CF3SO3) to [Fe(TPC)(4-CNPy)2]-
(CF3SO3). Thus, the sample for the NMR measurement was
prepared by the addition of 550 µL of CD2Cl2 to the mixture
consisting of Fe(TPC)(CF3SO3) (54 µmol) and 10 equiv of 4-CNPy
placed in a reaction vial. The solution was then taken into an NMR
sample tube.
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J. Am. Chem. Soc. 1989, 111, 7046-7050.
meso-13C-Enriched [Fe(TPC)(1-MeIm)2]Cl and [Fe(TPC)-
(CN)2]NBu4. meso-13C-enriched free base TPC was prepared using
13C-enriched benzaldehyde and pyrrole according to the literature.39
It was then converted to Fe(TPC)Cl by the literature method.40
meso-13C-enriched [Fe(TPC)(1-MeIm)2]Cl was prepared by the
addition of 550 µL of CD2Cl2 to the mixture consisting of meso-
13C-enriched Fe(TPC)Cl (14 µmol) and 4 equiv of 1-MeIm placed
in a reaction vial. meso-13C-enriched [Fe(TPC)(CN)2]NBu4 was
similarly prepared from 4 equiv of tetrabutylammonium cyanide
and meso-13C-enriched Fe(TPC)Cl(14 µmol).
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K. S.; Vainshtein, B. K.; Melik-Adamyan, W.; Bravo, J.; Ferra´n, J.
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meso-13C-Enriched Fe(TPC)ClO4. To the mixture of AgClO4
(69 µmol) and Fe(TPC)Cl (57 µmol) was added 10 mL of THF.
The solution was stirred for 20 min at room temperature. After
evaporation of the solvent, dichloromethane (3 mL) was added to
the resultant solid and the suspension was filtered to remove AgCl.
This procedure was repeated twice. The filtrate was evaporated,
and the resultant solid was dried in vacuo for 4 h at 25 °C. The
prechlorate salt, Fe(TPC)ClO4, thus obtained was used without
further purification.
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H.-K.; Morris, I. K. J. Am. Chem. Soc. 1988, 110, 6352-6358.
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Caution! Perchlorate salts are potentially explosive when heated
or shocked. Handle them in milligram quantities with care.
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Inorganic Chemistry, Vol. 45, No. 17, 2006 6729

Ikezaki et al.
meso-13C-Enriched [Fe(TPC)(4-CNPy)2]ClO4 and [Fe(TPC)-
(tBuNC)2]ClO4. meso-13C-enriched [Fe(TPC)(4-CNPy)2]ClO4 was
prepared by the addition of 550 µL of CD2Cl2 to the mixture
consisting of Fe(TPC)ClO4(13 µmol) and 10 equiv of 4-CNPy. The
solution was taken into an NMR sample tube for the NMR
measurement. meso-13C-enriched [Fe(TPC)(tBuNC)2]ClO4 was
t
similarly prepared from 3 equiv of BuNC and Fe(TPC)ClO4 (26
µmol).
[Co(TPC)(CN)2]NBu4. Free base chlorin was treated with 13
equiv of CoCl2‚6H2O in refluxing CHCl3-CH3OH (3:1) solution
for 4 h.41 The reaction mixture was washed with water to remove
the inorganic material. The organic layer was dried over sodium
sulfate for 1 h. The solution was evaporated to dryness, and the
resultant cobalt(II) complex, Co(TPC), was dried in vacuo for 4 h
at 25 °C. The solid still contained some amount of free base. Thus,
the reaction was repeated for the complete conversion of the free
base to the cabalt(II) complex. To a 550 µL volume of CD2Cl2
solution of Co(TPC) (15 µmol) was added a CD2Cl2 solution
containing 2.5 equiv of tetrabutylammonium cyanide. The solu-
tion, after being stirred for 1 h in a vial, gave cobalt(III) complex
1
[Co(TPC)(CN)2]NBu4 in a quantitative yield. H NMR and 13C
NMR chemical shifts determined in CD2Cl2 at 298 K are given in
Results and Discussion.
NMR Spectroscopy. Samples for NMR studies were prepared
by the addition of 550 µL of CD2Cl2 to the reaction vial containing
Fe(TPC)X and excess ligand under argon atmosphere. 1H and 13
C
Figure 1. 1H NMR spectra of [Fe(TPC)(CN)2](NBu4) taken in (a)
CD2Cl2 and in (b) CD3OD solutions at 298 K. Strong signals ascribed to
the butyl protons are observed between 1 and 4.5 ppm.
NMR spectra were recorded on a JEOL LA300 spectrometer
1
operating at 300.4 MHz for H. Chemical shifts were referenced
1
to the residual peak of dichloromethane-d2 (δ ) 5.32 ppm for H
and 53.8 ppm for 13C) and methanol-d4 (δ ) 3.30 ppm for 1H and
49.0 ppm for 13C). UV-vis spectra were recorded on a Shimadzu
MultiSpec-1500 at ambient temperature.
Chart 1. (a) Atom Numbering and (b) Atom Labeling in
[Fe(TPC)L2]( with C2V Point Groupa
Results
Assignments of 1H NMR Signals. The 1H NMR spectra
of [Fe(TPC)L2]+ where the axial ligands are PMe2Ph,18
t
1-MeIm,20 P(OMe)2Ph,21 and BuNC19 have already been
reported in detail. In the following discussion, we will
describe the signal assignments of three new complexes
including diamagnetic [Co(TPC)(CN)2]NBu4.
1
[Fe(TPC)(CN)2](NBu4). Figure 1 shows the H NMR
a R-C: carbon atoms at the 1, 4, 6, 9, 11, 14, 16, and 19 positions. â-C
and â-H: carbon and hydrogen atoms at the 7, 8, 12, 13, 17, and 18
positions, respectively. Pyrroline-C and pyrroline-H: carbon and hydrogen
atoms at the 2 and 3 positions, respectively. meso: carbon atoms at the 5,
10, 15, and 20 positions.
spectra of [Fe(TPC)(CN)2]- taken in (a) CD2Cl2 and (b)
CD3OD solutions at 298 K. Because of the presence of
the pyrroline ring, these complexes belong to the C2V point
group. Thus, the pyrrole protons give three signals signified
as 7,18-H, 8,17-H, and 12,13-H as shown in Chart 1, and
the ortho, meta, and para protons give two signals. These
signals were assigned on the basis of the relative inten-
sity and multiplicity of each signal together with the 2D
COSY experiments. In the case of [Fe(TPC)(CN)2]- taken
in CD2Cl2 solution, the pyrrole signal at -1.9 ppm was
assigned to the 12,13-H since no cross-peak was observed.
In contrast, a cross-peak was observed between the signals
at 3.3 and -26.0 ppm (Supporting Information S-1). Thus,
these two signals were assigned to the pyrrole protons in
the same pyrrole ring; i.e., 7,8-H and 17,18-H. Two sets of
the phenyl signals were assigned on the basis of the
homonuclear selective decoupling; they are (o, m, p) ) (6.38,
7.94, 6.95 ppm) and (6.66, 7.88, 6.88 ppm) (S-2). The
complete signal assignment was achieved by the NOE
difference spectra. Irradiation of the signal at -1.9 ppm
(12,13-H) caused the enhancement of the signal at 6.66 ppm.
Thus the signal at 6.66 ppm was assigned to the ortho-H of
the 10,15-phenyl rings (S-2). Similarly, irradiation of the
signal at -26.0 ppm caused the enhancement of the signal
at 6.66 ppm (S-2). Thus, the signals at -26.0 and 3.3 ppm
were assigned to the 8,17-H and 7,18-H, respectively.
As is clear from Figure 1, the chemical shifts of
[Fe(TPC)(CN)2]- in CD3OD solution are quite different from
those in CD2Cl2 solution. The signals observed in CD3OD
solution were similarly assigned on the basis of the relative
intensity and multiplicity of each signal, selective homo-
nuclear decoupling, NOE difference spectra (S-3), and 2D
(41) Karweik, D. H.; Winograd, N. Inorg. Chem. 1976, 15, 2336-2342.
6730 Inorganic Chemistry, Vol. 45, No. 17, 2006

Low-Spin Iron(III) Tetraphenylchlorin Complexes
the 1H NMR spectrum of [Fe(TPC)(4-CNPy)2]+ taken at 253
K where the ligand exchange is slow enough to give clearly
resolved signals. These signals were assigned similarly by
COSY spectra as in the case of [Fe(TPC)(CN)2]- (S-6, S-7).
The Curie plots of the pyrroline and pyrrole-H signals are
given in Figure 3c. The slopes of the Curie lines are -1.3
× 104 and +0.41 × 104 ppm‚K, respectively, which are quite
close to the corresponding values of [Fe(TPC)(CN)2]- in
CD3OD. Thus, the electronic ground state of [Fe(TPC)-
(4-CNPy)2]+ is considered to be similar to that of [Fe(TPC)-
(CN)2]- in CD3OD.
1
[Co(TPC)(CN)2]NBu4. Determination of the H NMR
chemical shifts of diamagnetic [Co(TPC)(CN)2]NBu4 is
necessary to know the isotropic shifts of paramagnetic
[Fe(TPC)L2](. The signal assignment was carried out by
means of 1D homonuclear decoupling and NOE differ-
ence spectroscopy (S-8) as well as 2D COSY spectroscopy
(S-9). The chemical shifts of the ortho and meta protons in
one phenyl ring were determined to be 7.81 and 7.56 ppm,
respectively, while those of the other phenyl ring were
determined to be 8.00 and 7.63 ppm, respectively, by the
COSY spectra (S-9). The para signals were almost overlap-
ping with the meta signals in each case; they appeared at
7.58 and 7.61 ppm. The NOE difference spectrum showed
the increase in intensity of the ortho signals at 7.81 ppm
when the pyrroline signal at 4.00 ppm was irradiated (S-8).
Thus, the signals at 7.81 and 8.00 ppm were ascribed to the
ortho protons of the 5,20- and 10,15-phenyl groups, respec-
tively. The signals at downfield regions, 8.08 (d) and 8.36
(d) ppm, were assigned to the pyrrole 7,8,17,18-H, while
the singlet at 8.28 ppm was assigned to the 12,13-H. The
NOE difference spectrum showed the increase in intensity
of the pyrrole signals at 8.08 ppm when the ortho signals at
7.81 ppm was irradiated. Similarly, the intensity of the
pyrrole signals at 8.28 and 8.36 ppm increased when the
ortho signal at 8.00 was irradiated. Thus, the doublets at 8.08
and 8.36 ppm were assigned to the 7,18-H and 8,17-H,
respectively.
Figure 2. Change in (a) 1H NMR and (b) 13C NMR chemical shifts when
CD3OD is added to the CD2Cl2 solution of [Fe(TPC)(CN)2]- at 298 K.
Symbols in (a): b, pyrroline-H; O, 7,18-H; 4, 12,13-H; 0, 8,17-H. Symbols
in (b): b, pyrroline-C; O, 7,18-C; 4, 12,13-C; 0, 8,17-C; red O, meso-C.
COSY spectra (S-4, S-5). For example, the signal at 8.7 ppm
showed a correlation peak with that at -10.8 ppm (S-5).
Thus, these signals were assigned to the 7,8,17,18-H. The
complete signal assignment was achieved by the NOE
difference spectra. Thus, the signals at 8.7 and -10.8 ppm
were assigned to the 7,18-H and 8,17-H, respectively. Figure
2a shows the change in chemical shift of the pyrrole and
pyrroline-H signals when CD3OD is added to the CD2Cl2
solution of [Fe(TPC)(CN)2]- at 298 K. When the volume %
of the added CD3OD reached 15.4%, the pyrrole signals at
3.3, -1.9, and -26.0 ppm shifted to 7.1, 0.9, and -16.4
ppm, respectively. Thus, the signals at 8.7, 2.3, and -10.8
ppm observed in CD3OD solution correspond to the signals
at 3.3, -1.9, and -26.0 ppm observed in CD2Cl2 solution,
which in turn indicate that they are assigned to 7,18-H,
12,13-H, and 8,17-H, respectively. It should be noted that
the spread among the pyrrole signals decreased from 29.3
to 19.5 ppm as CD2Cl2 is replaced by CD3OD. Figure 3a,b
shows the Curie plots of the pyrroline and pyrrole-H signals
of [Fe(TPC)(CN)2]- taken in CD2Cl2 and CD3OD solutions,
respectively. In both cases, the chemical shifts vary linearly
with 1/T, but the extrapolated lines do not pass through the
diamagnetic value at 1/T ) 0. Especially, the pyrroline and
8,17-H signals showed a large positive and negative slope
in CD2Cl2 solution, +2.7 × 104 and -2.5 × 104 ppm‚K,
respectively. The intercepts at 1/T ) 0 for these signals
reached as much as -67 and 59 ppm, respectively; the
chemical shifts of the corresponding signals in diamagnetic
[Co(TPC)(CN)2]- are 4.0 and 8.4 ppm. In CD3OD solution,
the signs of the Curie slopes of the pyrroline and 8,17-H
signals were reversed and their absolute values were
diminished to a great extent; they were -1.1 × 104 and
+0.37 × 104 ppm‚K, respectively. The intercepts at 1/T )
0 for the pyrroline and 8,17-H decreased to 42.4 and -23.5
ppm, respectively. Thus, it is clear that the electronic ground
state in CD2Cl2 solution is quite different from that in
CD3OD, which will be discussed later.
The chemical shifts of these complexes are given in Table
1. The axial ligands are arranged in the descending order of
the pyrroline shifts, which corresponds to the ascending order
of the 8,17-H shifts.
Assignments of 13C NMR Signals. Full analysis of the
13C NMR spectra of low-spin [Fe(TPC)L2]+ have not been
reported before. In the following discussion, we will describe
the signal assignments of the complexes carrying 1-MeIm,
CN- in CD2Cl2 solution, CN- in CD3OD solution, 4-CNPy,
t
and BuNC as axial ligands.
[Fe(TPC)(CN)2](NBu4). Figure 5a shows the proton-
decoupled 13C NMR spectrum of [Fe(TPC)(CN)2]- taken in
CD2Cl2 solution at 298 K. The R- and â-C give four and
three signals, respectively, and the meso, ipso, o-, m-, and
p-C give two signals as revealed from Chart 1. Signal
assignment of the carbons with directly bonded protons such
as pyrroline-â, pyrrole-â, o, m, and p has been done on the
basis of the relative intensity and multiplicity of the signal
[Fe(TPC)(4-CNPy)2](ClO4). At ambient temperature, the
ligand dissociation in [Fe(TPC)(4-CNPy)2]+ is taking place
on the 1H NMR time scale to give broad signals ascribed to
the free and coordinated ligand molecules. Figure 4 shows
1
in the proton coupled spectra together with the H{13C}
HMQC experiments. For example, the 13C signal at 48 ppm
Inorganic Chemistry, Vol. 45, No. 17, 2006 6731

Ikezaki et al.
1
Figure 3. Curie plots of the H NMR signals: (a) [Fe(TPC)(CN)2]- taken in CD2Cl2; (b) [Fe(TPC)(CN)2]- taken in CD3OD; (c) [Fe(TPC)(4-CNPy)2]+
taken in CD2Cl2. Key: b, pyrrol-H; red b, pyrroline-H.
basis of the HMQC method (S-14, S-15, S-16). Figure 5b′
shows the 13C NMR spectrum of meso-13C-enriched complex.
The strong signals at 293 and 262 ppm were assigned to the
meso carbons. The other four signals with short relaxation
times at 43, 31, -19, and -89 ppm were then assigned to
the R carbons. Figure 2b shows the change in 13C NMR
chemical shifts when CD3OD was added to the CD2Cl2 solu-
tion of [Fe(TPC)(CN)2]-. The titration experiment enables
the correlation of the signals in CD2Cl2 solution to the
corresponding signals in CD3OD. As in the case of pyrrole-H
signals, the spread of the pyrrole â signals decreased from
271 to 183 ppm as CD2Cl2 was replaced by CD3OD.
[Fe(TPC)(1-MeIm)2]Cl. Figure 6a shows the proton-
coupled 13C NMR spectra of [Fe(TPC)(1-MeIm)2]+ taken
Figure 4. 1H NMR spectrum of [Fe(TPC)(4-CNPy)2]+ taken in CD2Cl2
at 253 K. Signals signified by L′, â′, o′, m′, and p′ in the spectra indicate
the ligand, pyrrole-H, and meta signals of [Fe(TPP)(4-CNPy)2]+. S and F
are the signals for the solvent and free ligand, respectively.
in CD2Cl2 solution at 298 K. Signal assignment of the
carbons with directly bonded protons such as pyrroline-â,
o, m, and p was carried out on the basis of the relative
intensity and multiplicity of the signal in the proton coupled
spectra together with the heteronuclear selective decoupling
method. Thus, the broad doublets at 288, 113, and -23 ppm
were assigned to the 8,17-C, 12,13-C, and 7,18-C, respec-
tively, by the heteronuclear selective decoupling of the
corresponding proton signals at -35, -10, and -1.0 ppm,
respectively (S-17). As shown in Figure 6b, the ipso signals
were discriminated from the R and meso signals by partially
relaxed NMR spectra because the former signals were
negative while the latter were positive when the spectrum
was taken with the pulse interval 75 ms in the 180°-τ-90°
pulse sequence. When the temperature was lowered to 273
K, the signal at -19 ppm split into two signals, indicating
that they accidentally overlapped at 298 K. Thus, the five
signals at 224, 171, 146, -19, and -91 ppm were assigned
to either the meso or the R signals. We could unambiguously
assign the signals at 146 and -19 ppm to the meso carbons
by using meso-13C-enriched [Fe(TPC)(1-MeIm)2]+ as shown
in Figure 6c. Thus, the other 4 signals at 224, 171, -19,
and -91 ppm were assigned to the R carbons.
in CD2Cl2 solution, which showed a correlation peak with
the proton signal at -1.9 ppm, was assigned to the 12,13-C
(S-10). Similarly, the signals at -17 and 254 ppm were
assigned to the 7,18-C and 8,17-C, respectively, since these
signals have correlation peaks with the proton signals at 3.3
and -26.0 ppm, respectively (S-11, S-12). The pyrrole-â
signals were also assigned by the heteronuclear selective
decoupling (S-13). The assignments of the R, meso, and ipso
signals are much more difficult. In the inset of Figure 5a′ is
given the partially relaxed proton decoupled 13C NMR
spectrum, which is obtained by the 180°-τ-90° pulse
sequence with τ ) 75 ms. Because of the short relaxation
times of the R, â, and meso carbons relative to those of the
o, m, p, and ipso carbons, the former gave positive signals
while the latter gave negative ones. Thus, the six unassigned
signals in a wide frequency range, i.e., 192, 162, 130, 76,
-20, -87 ppm, were assigned either to the meso or to the
R carbons. As shown in Figure 5a′′, we could unambiguously
assign the signals at 192 and 76 ppm to the meso carbons
by using meso-13C enriched [Fe(TPC)(CN)2]-. The other four
signals at 162, 130, -20, and -87 ppm were then assigned
to the R carbons.
[Fe(TPC)(4-CNPy)2]ClO4. Figure 7a shows the proton-
coupled 13C NMR spectra of [Fe(TPC)(4-CNPy)2]+ taken
in CD2Cl2 solution at 253 K. The most downfield shifted
signals, 476 and 486 ppm, were assigned to the meso signals
since the meso-13C-enriched [Fe(TPC)(4-CNPy)2]+ exhibits
two signals at nearly the same positions as shown in Figure
The 13C NMR spectra of [Fe(TPC)(CN)2]- taken in
CD3OD solution at 298 K is given in Figure 5b. The o, m,
p, pyrroline, and â signals were assigned similarly on the
6732 Inorganic Chemistry, Vol. 45, No. 17, 2006

Low-Spin Iron(III) Tetraphenylchlorin Complexes
Table 1. 1H NMR Chemical Shifts of [Fe(TPC)(L)2]( and [Co(TPC)(L)2]( in CD2Cl2 at 298 K
complexes
pyrroline
7,18
8,17
12,13
o
m
p
ref
[Fe(TPC)L2](
6.85
PMe2Ph
1-MeIm
CN-
64.6
39.1
25.8
6.8
0.9
nd
0.7
-1.0
3.3
-57.8
-35.0
-26.0
-10.8
-11.5
-8.9
0.7
-10.0
-1.9
2.3
6.98
6.17
6.66e
6.56e
4.4
7.62
7.22
7.71
7.11
7.88e
9.28e
7.10
7.11
6.95d
6.63d
5.8
7.44
18
20
this work
this work
21
this work
this work
19
6.17
6.91
6.38d
7.94d
9.62d
11.55
11.46f
12.54f
14.7f
6.88e
6.74e
5.65
CN- a
8.7
5.80d
P(OMe)2Ph
4-CNPy
4-CNPyb
tBuNC
10.9
14.4
16.0
7.9c
5.6
4.5
4.4
7.2
3.7
11.2
3.77f
4.60g
3.64g
1.3g
10.87g
11.74g
12.9g
5.65f
5.17f
3.07f
5.60g
5.01g
3.42g
-0.1
-36.0
-6.6
2.76f
5.1c
-0.39f
[Co(TPC)L2](
CN-
4.00
8.08
8.36
8.28
7.81d
8.00e
7.56d
7.63e
7.58d
7.61e
this work
a In CD3OD solution. b Data at 253 K. c The assignment of the signals at 7.9 and 5.1 ppm could be reversed. d Protons at 5,20-phenyl rings. e Protons at
10,15-phenyl rings. f ,gProtons belong to the same phenyl group.
Figure 6. 13C NMR spectrum of [Fe(TPC)(1-MeIm)2]+ taken in CD2Cl2
solution at 298 K: (a) proton-coupled 13C NMR spectrum; (b) partially
relaxed proton-decoupled 13C NMR spectrum; (c) proton-decoupled 13C
NMR spectrum of meso-13C-enriched complex. Signals signified by R′, â′,
and meso′ are ascribed to [Fe(TPP)(1-MeIm)2]+.
Although we could not observe the meso-13C signals by the
conventional single pulse measurement even by using meso-
13C-enriched complex, two signals certainly appeared at 700
and 763 ppm as shown in the inset (a) when some strong
signals were eliminated by the proton decoupled partially
Figure 5. 13C NMR spectra of [Fe(TPC)(CN)2]- taken in (a) CD2Cl2 and
relaxed 13C NMR spectrum. In the inset (b) is given the 13
C
(b) CD3OD solutions at 298 K. Insets: (a′) partially relaxed spectrum; (a′′)
meso-13C-enriched [Fe(TPC)(CN)2]-; (b′) meso-13C-enriched [Fe(TPC)-
(CN)2]+. Signals signified by meso′ are ascribed to the meso signal of
[Fe(TPP)(CN)2]-. The butyl signals of tetrabutylammonium cyanide are
signified by asterisks.
NMR spectrum of a narrow region between 0 and 40 ppm.
The proton-coupled partially relaxed 13C NMR spectrum
shown in the inset (c) revealed that the methyl signals of
the coordinated tBuNC exactly overlapped with the pyrroline
signals. It should be noted that pyrroline-C signal appeared
at +20 ppm while the corresponding signals of all the other
complexes were observed at rather upfield positions, i.e. -25
to -71 ppm.
[Co(TPC)(CN)2]NBu4. Signal assignments of diamagnetic
[Co(TPC)(CN)2]NBu4 were carried out by 1D and 2D NMR
spectroscopy. Signal assignment of the carbons with directly
bonded protons such as pyrroline-â, pyrrole-â, o, m, and p
7b. The R and ipso signals were discriminated by the spectral
comparison of Figure 7c,d; Figure 7d shows the proton-
coupled partially relaxed spectrum where the ipso-C exhibit
negative signals due to the longer relaxation times as
compared with the R-C.
[Fe(TPC)(tBuNC)2]ClO4. Figure 8 shows the proton-
coupled 13C NMR spectrum of [Fe(TPC)(tBuNC)2]+ taken
in CD2Cl2 solution at 298 K. Signals were assigned similarly.
Inorganic Chemistry, Vol. 45, No. 17, 2006 6733

Ikezaki et al.
Figure 7. 13C NMR spectra of [Fe(TPC)(4-CNPy)2]+ taken in CD2Cl2 solution at 253 K: (a) proton-coupled spectrum; (b) proton-decoupled spectrum of
meso-13C-enriched complex; (c) proton-coupled spectrum of -200 to 50 ppm region; (d) proton-decoupled partially relaxed spectrum of -200 to 50 ppm
region.
Figure 8. 13C NMR spectra of [Fe(TPC)(tBuNC)2]+ taken in CD2Cl2 solution at 298 K. Insets: (a) downfield region of the meso-13C-enriched complex;
(b) proton-coupled 13C NMR spectrum between 0 and 40 ppm; (c) proton-decoupled partially relaxed 13C NMR spectrum of the same region as (b).
were carried out on the basis of the 1H{13C} HMQC
experiments. Thus, the signals at 128.2, 133.8, and 129.6
ppm were assigned to the 7,18-C, 8,17-C, and 12,13-C,
respectively. Similarly, the signal at 34.8 ppm was assigned
to the pyrroline-C. Signal assignment of the carbons without
directly bonded protons such as meso, ipso, and R-C was
done by the HMBC experiment. Thus, the signal at 109.9
ppm was assigned to the 5,20-C since it showed correlation
peaks with the 1H signals at 4.0 ppm (pyrroline-H) and 7.81
ppm (ortho-H) in the HMBC spectra (S-18). Similarly, the
signal at 123.0 ppm was assigned to the 10,15-C since it
1
showed a correlation peak with the H signals at 8.00 ppm
6734 Inorganic Chemistry, Vol. 45, No. 17, 2006

Low-Spin Iron(III) Tetraphenylchlorin Complexes
Table 2. 13C NMR Chemical Shifts of [Fe(TPC)L2]( and [Co(TPC)(CN)2]- in CD2Cl2 at 298 Ka
(a) Chlorin Ring
complexes
pyrroline
meso
R
7,18
8,17
12,13
[Fe(TPC)L2](
1-MeIm
CN-
-70
-71
-45
-25
-34
20
146 (56)
192 (89)
293 (184)
413 (243)
486 (289)
763 (767)
-19
76
262
396
476
700
224 (42)
162 (41)
43 (27)
11 (46)
-44 (-17)
-179 (-294)
171
130
31
-19
-67
-204
-19
-20
-91
-87
-23 (99)
-17 (90)
16 (89)
20 (150)
12 (113)
85 (83)
288
254
199
222
213
70
113 (99)
48 (90)
54 (89)
77 (150)
57 (113)
86 (83)
CN- b
-19
-89
4-CNPy
4-CNPyc
tBuNC
-35
-100
-297
-129
-172
-342
[Co(TPC)L2](
155 (143) 147
CN-
35
110 (119)
123
139
142
128 (133)
134
130 (133)
(b) Phenyl Group
complexes
ipso
o
m
p
[Fe(TPC)L2](
147 (131)
173e (149)
229e (195)
288 (225)
326 (259)
432 (458)
1-MeIm
CN-
139
118d
58d
-15
-57
-104
129 (137)
136
177d
258d
331
385
552
127
129d
140d
144
148
159
128 (125)
127
129d
134
140
143
153
128 (126)
115e (127)
84e (104)
22 (74)
132e (125)
135e (131)
137 (132)
138 (133)
145 (146)
130e (127)f
134 (132)f
145 (137)
145 (139)
153 (155)g
CN- b
4-CNPy
4-CNPyc
tBuNC
-7 (46)
-104 (-123)
[Co(TPC)L2](
133.9 (135)
CN-
142.7
131.8 (127)
132.8
127.9
127.5 (127)
127.2
127.2 (128)f
a Data in parentheses are the chemical shifts of the corresponding signals of [Fe(TPP)L2]( and [Co(TPP)(CN)2]-. b In CD3OD. c Data at 253 K. d Carbons
at 5,20-phenyl rings. e Carbons at 10,15-phenyl ring. f Reference 42. g Reference 45.
Scheme 1. Two Types of Electronic Ground States
(ortho-H). The signals at 142.7 and 141.8 ppm were assigned
to the ipso-C since they showed the correlation peaks with
the meta-H signals at 7.56 and 7.63 ppm, respectively. The
signals at 154.5 and 141.7 ppm were assigned to the 1,4-C
and 11,14-C, respectively, due to the presence of the
correlation peaks at 4.00 (pyrroline-H) and 8.28 ppm
(12,13-H), respectively. The other two signals, 147.1 and
138.8 ppm, were assigned to the 6,19-C and 9,16-C though
the complete assignment is not successful at this point.
Table 2 shows the 13C NMR chemical shifts of [Fe(TPC)-
L2]( and [Co(TPC)(CN)2]-, where the axial ligands of
[Fe(TPC)L2]( are arranged in the same order as those in
Table 1. The data in Table 2 clearly indicate that the pyrroline
state, the complexes usually exhibit a nearly planar porphyrin
and meso carbon signals continuously move downfield as
ring. Both the pyrrole-H and meso-C signals appear at rather
the axial ligand changes from 1-MeIm to tBuNC. The chem-
upfield positions due to the dπ-3eg interactions. This is
ical shifts of the meso, R, â, and phenyl carbon signals of
because the 3eg orbital has large coefficients at the â pyrrole
the corresponding porphyrin complexes, [Fe(TPP)L2]( and
and zero coefficient at the meso carbon atoms.43 In the case
[Co(TPP)(CN)2]-, taken under the same conditions are also
listed in the parentheses of Table 2.42
(44) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Nakamura, M.; Fujii, H.; Yokoyama,
M. J. Am. Chem. Soc. 2000, 122, 4068-4076.
(45) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Yamaguchi, T.; Nakamura, M. Inorg.
Chem. 2001, 40, 3423-3434.
(46) Ikeue, T,; Ohgo, Y.; Ongayi, O.; Vicente, M. G. H.; Nakamura, M.
Discussion
Electron Configurations of the Iron(III) Chlorinates.
Inorg. Chem. 2003, 42, 5560-5571.
General Consideration. As shown in Scheme 1, there are
two types of electronic ground states, (dxy)2(dxz, dyz)3 and (dxz,
dyz)4(dxy)1, in low-spin iron(III) porphyrinates.43 Extensive
studies using NMR spectroscopy have revealed that not only
the pyrrole-H but also the meso-C chemical shifts are
powerful probes to elucidate the electronic ground state.43-53
In the low-spin [Fe(TPP)L2]( with the (dxy)2(dxz, dyz)3 ground
(47) Rivera, M.; Caignan, G. A. Anal. Bioanal. Chem. 2004, 378, 1464-
1483.
(48) Hoshino, A.; Nakamura, M. Chem. Commun. 2005, 915-917.
(49) Hoshino, A.; Ohgo, Y.; Nakamura, M. Inorg. Chem. 2005, 44, 7333-
7344.
(50) Safo, M. K.; Walker F. A.; Raitsimring, A. M.; Walters, W. P.; Dolata,
D. P.; Debrunner, P. G.; Scheidt, W. R. J. Am. Chem. Soc. 1994, 116,
7760-7770.
(51) Walker, F. A.; Nasri, H.; Turowska-Tyrk, I.; Mohanrao, K.; Watson,
C. T.; Shokhirev, N. V.; Debrunner, P. G.; Scheidt, W. R. J. Am.
Chem. Soc. 1996, 118, 12109-12118.
(42) Ikezaki, A.; Ikeue, T.; Nakamura, M. Inorg. Chim. Acta 2002, 335,
91-99.
(43) 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.
(52) Pilard, M.-A.; Guillemot, M.; Toupet, L.; Jordanov, J.; Simonneaux,
G. Inorg. Chem. 1997, 36, 6307-6314.
(53) Simonneaux, G.; Schu¨nemann, V.; Morice, C.; Carel, L.; Toupet, L.;
Winkler, H.; Trautwein, A. X.; Walker, F. A. J. Am. Chem. Soc. 2000,
122, 4366-4377.
Inorganic Chemistry, Vol. 45, No. 17, 2006 6735

Ikezaki et al.
of the (dxz, dyz)4(dxy)1 type complexes with D4h symmetry,
the dxy orbital is orthogonal to any of the porphyrin frontier
orbitals. Actually however, the low-spin complexes adopting
the (dxz, dyz)4(dxy)1 ground state almost always exhibit the
ruffled structure.50-53 Thus, the iron dxy orbital can interact
with the porphyrin a2u orbital because both orbitals are
signified as b2 in the ruffled complexes with D2d sym-
metry.43,54,55 The interaction between these two orbitals
should induce a large downfield shift of the meso-C signal
since the a2u orbital has a large coefficient at the meso carbon
atoms. By contrast, the pyrrole-H signal appears close to the
diamagnetic position despite the a2u-dxy interaction, because
the a2u orbital has zero coefficient at the â pyrrole carbon
atoms.56 We have recently found the direct EPR evidence
showing that low-spin [Fe(TArP)(CN)2]- (Ar ) 2,4,6-
triethylphenyl) exists as an equilibrium mixture of the two
isomers with different electronic ground states.57 Because
the energy gap between these two isomers is expected to be
quite small, they are rapidly interconverting in solution as
shown in eq 1.57,58 Thus, the observed chemical shift of the
nucleus i, signified by δi(obs), is expressed by eq 2, where
δi(dπ) and δi(dxy) are the chemical shifts and p(dπ) and p(dxy)
are the population of the isomers with the (dxy)2(dxz, dyz)3
and (dxz, dyz)4(dxy)1 ground state, respectively.57 However, this
aspect may be more complicated if we consider that excited
states can be thermally populated, as it has been previously
reported in some cases with low-spin iron(III) heme proteins
and iron porphyrins.32,59-62
Chart 2. Hu¨ckel Frontier Orbitals of the Chlorin Ring for Low-Spin
Fe(III) Complexes with (dxy)2(dxz, dyz)3 Ground State Reported by Cai,
Lichtenberger, and Walker32 a
a
The highest (singly) occupied orbital is dyz-A-2. A-1 and S-1 are
analogues of the 1a1u and 3a2u orbitals of the porphyrin ring, respectively.
(dxy)2(dxz, dyz)3 {K
\
} (dxz, dyz)4 (dxy)1
heavy mixing among metal d and chlorin π orbitals. In
addition to these interactions, the dxy orbital can interact with
the S-1 orbital if the chlorin ring is ruffled as in the case of
the porphyrin complexes.32 The NMR chemical shifts listed
in Tables 1 and 2 should be the results of orbital interactions
mentioned above. Thus, we have determined the electronic
δi(obs) ) p(dπ)δi(dπ) + p(dxy)δi(dxy)
(2)
The molecular orbitals of the chlorin ring are different from
those of the porphyrin because of the reduction of one of
the pyrrole rings. Recent theoretical work on the electronic
structure of low-spin (octaethylchlorinato)iron(III) complexes
with the (dxy)2(dxz, dyz)3 ground state has shown that the A-1
and S-1 orbitals correspond to the 1a1u(π) and 3a2u(π)
orbitals, respectively, in the D4h porphyrin complexes.32
Similarly, the A-2 and S-2 orbitals correspond to the 3eg(y)
and 3eg(x) orbitals, respectively. These orbitals are shown
in Chart 2. The important consequence from the calculation
is that the dyz orbital appears to interact almost equally with
the two antisymmetric orbitals, A-2 and A-1, because of the
1
structures of a series of complexes on the basis of the H
and 13C NMR chemical shifts.
[Fe(TPC)L2]+ (L ) PMe2Ph, 1-MeIm). These complexes
have been reported to adopt the (dxy)2(dxz, dyz)3 ground state
1
on the basis of the H NMR and EPR spectroscopy as well
as X-ray crystallography.18,20 The 13C NMR chemical shifts
listed in Table 2 also support the (dxy)2(dxz, dyz)3 ground state.
The average chemical shift of the meso carbon signals of
[Fe(TPC)(1-MeIm)2]+ is 64 ppm, which is ca. 53 ppm more
upfield than that of diamagnetic [Co(TPC)(CN)2]-. Among
the six â-C, 8,17-C have the largest spin densities as is
revealed from their largest downfield shift, 288 ppm, together
with the largest upfield shift of the 8,17-H, -35.0 ppm,
respectively. Thus, the major interactions that affect the 1H
and 13C NMR chemical shifts in these complexes are the
dyz-A-2 and to a minor extent the dyz-A-1 and dxz-S-2
interactions.
(54) Ghosh, A.; Gonzalez, E.; Vangberg, T. J. Phys. Chem. B 1999, 103,
1363-1367.
(55) Cheng, R.-J.; Chen, P.-Y.; Lovell, T.; Liu, T.; Noodleman, L.; Case,
D. A. J. Am. Chem. Soc. 2003, 125, 6774-6783.
(56) Simonneaux, G.; Hindre, F.; Le Plouzennec, M. Inorg. Chem. 1989,
28, 823-825.
(57) Ikezaki, A.; Nakamura, M. Inorg. Chem. 2002, 41, 2761-2768.
(58) Rivera, M.; Caignan, G. A.; Astashkin, A. V.; Raitsimring, A. M.;
Shokhireva, T.K.; Walker, F. A. J. Am. Chem. Soc. 2002, 124, 6077-
6089.
[Fe(TPC)(CN)2]-. The 1H and 13C NMR spectra of
[Fe(TPC)(CN)2]- shown in Figures 1 and 5 are quite sim-
ilar to those of [Fe(TPC)(1-MeIm)2]+ if they are taken in
CD2Cl2 solution. The large upfield shift of the 8,17-H, -26
ppm, together with the large downfield shift of the pyrroline-
(59) Kurland, R. J.; McGarvey, B. R. J. Magn. Reson. 1970, 2, 286-301.
(60) Shokhirev, N. V.; Walker, F. A. J. Phys. Chem. 1995, 99, 17795-
17804.
(61) Banci, L.; Bertini, I.; Luchinat, C.; Pierattelli, R.; Shokhirev, N. V.;
Walker, F. A. J. Am. Chem. Soc. 1998, 120, 8472-8479.
(62) Bertini, I.; Luchinat, C.; Giacomo, P. Eur. J. Inorg. Chem. 2000, 2473-
2480.
6736 Inorganic Chemistry, Vol. 45, No. 17, 2006

Low-Spin Iron(III) Tetraphenylchlorin Complexes
H, 25.8 ppm, clearly indicates that the complex also adopts
the (dxy)2(dxz, dyz)3 ground state. In other words, the isomer
with the (dxy)2(dxz, dyz)3 ground state predominantly exists in
the equilibrium state. It should be noted that the population
of the (dxz, dyz)4(dxy)1 isomer in [Fe(TPC)(CN)2]- is much
larger than that in [Fe(TPC)(1-MeIm)2]+ since the meso-C
signals in the former complex appeared more downfield than
those of the latter one.
[Fe(TPC)(tBuNC)2]+. This complex is well characterized
to adopt the (dxz, dyz)4(dxy)1 ground state on the basis of the
1H NMR, EPR, and X-ray crystallography.19 The large
upfield shift of the pyrroline-H signal, -36.0 ppm, suggests
that the pyrroline R-C has a sizable amount of negative spin.
Corresponding to a large upfield shift of the pyrroline-H
signal, this complex exhibited the pyrroline-C signal at fairly
downfield position, 20 ppm. The result again indicates that
the π spin density at the pyrroline R-C has changed from a
large positive to a negative value as the axial ligand changes
Addition of CD3OD to the CD2Cl2 solution of [Fe(TPC)-
(CN)2]- greatly affects the chemical shifts of the pyrroline-H
and meso-C signals as shown in Figure 2a,b; the pyrroline-H
signal shifted upfield from 25.8 to 6.8 ppm while the meso-C
signals shifted to the opposite direction from 192 and 76
ppm to 293 and 262 ppm. The large downfield shift of the
meso-C signals suggests the increase in spin densities at the
meso carbons caused by the interaction between S-1 and the
half-filled dxy orbital in a probably ruffled chlorin core. These
results clearly indicate that the isomer with the (dxz, dyz)4-
(dxy)1 ground-state predominates in CD3OD solution. This
is because the hydrogen bonding between CD3OD and
coordinating CN- lowers the energy level of the π*(CN-)
orbital and stabilizes the dxz and dyz orbitals relative to the
dxy orbital. The same phenomenon was observed in the
porphyrin complexes.63-67 Figure 3a,b shows the Curie plots
of the 1H signals in [Fe(TPC)(CN)2]- determined in CD2Cl2
and CD3OD solutions, respectively. The large positive slope
of the pyrroline-H signal in CD2Cl2 solution, +2.7 × 104
ppm‚K, changed to the negative one, -1.1 × 104 ppm‚K, in
CD3OD solution, which is consistent with the large decrease
in π spin densities at the pyrroline R-C. The fact that the
8,17-H and pyrroline-C signals still appear rather upfield
positions, -10.8 and -45 ppm, respectively, suggests that
a considerable amount of the (dxy)2(dxz, dyz)3 isomer exists in
the equilibrium state.
t
from 1-MeIm to BuNC. We expected, however, that the
pyrroline-C signal of [Fe(TPC)(tBuNC)2]+ should appear
much more downfield than the corresponding signal of
diamagnetic [Co(TPC)(CN)2]-, 34.8 ppm, because of the
presence of negative spin at the neighboring R-C’s. The
smaller downfield shift could be explained in terms of the
cancellation of the contact contribution to the pyrroline-C
signal due to the negative spin at the two adjacent R-C’s.
That is, the downfield shift of the pyrroline-C caused by the
negative spin at the directly bonded R-C is canceled by the
upfield shift caused by the negative spin at the other R carbon
atom.
As shown in the data in Tables 1 and 2, the chemical shifts
of all the pyrrole-H and â-C in [Fe(TPC)(tBuNC)2]+ are
much closer to their diamagnetic positions than those of any
other complexes examined in this study. The results suggest
that the spin densities at the â-C are quite small in [Fe(TPC)-
(tBuNC)2]+. The most characteristic feature in the 13C NMR
spectrum is the large downfield shifts of the meso-C signals,
763 and 700 ppm, which are ascribed to the large amount
of spin densities at these carbons caused by the strong
interaction between the S-1 and half-filled dxy orbital. In
contrast, all the neighboring carbons that are directly attached
to the meso-C exhibited the signals at significantly upfield
positions; the R- and ipso-C signals appeared at -179 to
-342 ppm and at -104 ppm, respectively. This is because
the large π spin densities at the meso carbon atoms polarize
the adjacent Cmeso-CR and Cmeso-Cipso bonds.68,69
1
[Fe(TPC)(4-CNPy)2]+. The H NMR data in Table 1
suggest that the chemical shifts of [Fe(TPC)(4-CNPy)2]+ are
quite close to those of [Fe(TPC){P(OMe)2Ph}2]+ adopting
the (dxz, dyz)4(dxy)1 ground state.21 The large downfield shifts
of the meso-C signals, which reached 486 and 476 ppm,
suggest the increase in positive spin at these carbon atoms
caused by the interaction between S-1 and half-filled dxy
orbital. Correspondingly, the negative slope of the pyrro-
line-H Curie plots shown in Figure 3c increased to -1.3 ×
104 ppm‚K. The results indicate that the replacement of the
axial cyanide ligand by 4-CNPy greatly stabilized the (dxz,
dyz)4(dxy)1 isomer. It should be noted, however, that the
8,17-H and pyrroline-C signals appeared rather upfield
positions, -8.9 and -25 ppm, respectively. The results
suggest that the population of the (dxy)2(dxz, dyz)3 isomer is
still not small.
Curie Plots of the Carbon Signals. Figure 9A shows the
Curie plots of the meso-, R-, â-, and pyrroline-C signals of
a series of [Fe(TPC)L2](, where L’s are (a) 1-MeIm, (b) CN-
(in CD2Cl2), (c) CN- (in CD3OD), and (d) 4-CNPy. Curie
t
plots of the BuNC complexes were not available because
the signals were too broad to detect at lower temperatures.
In Figure 9A, the Curie plots of the meso signals are
expressed by the red filled circle. They exhibited a good
linearity though the intercepts at 1/T ) 0 deviated from the
corresponding diamagnetic values. The average slopes and
the intercepts for these complexes are as follows: (a) -3.3
× 104 ppm‚K, 175 ppm; (b) -6.0 × 104 ppm‚K, 338
ppm; (c) 10 × 104 ppm‚K, -62.5 ppm; (d) 13 × 104
ppm‚K, -16.1 ppm. The slopes of the Curie plots clearly
(63) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T. J. Am. Chem. Soc.
1997, 119, 6284-6291.
(64) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T.; Tajima, K. Inorg.
Chem. 1998, 37, 2405-2414.
(68) Goff, H. M. Nuclear Magnetic Resonance of Iron Porphyrins. In Iron
Porphyrins, I; Lever, A. B. P., Gray, H. B., Eds.; Physical Bioinorganic
Chemistry Series 1; Addison-Wesley: Reading, MA, 1983; pp 237-
281.
(69) Bertini, I.; Luchinat, C. In NMR of Paramagnetic Substances; Lever,
A. B. P., Ed.; Coordination Chemistry Reviews 150; Elsevier:
Amsterdam, 1996; pp 29-75.
(65) Wołowiec, S.; Latos-Graz˘yn´ski, L.; Mazzanti, M.; Marchon, J.-C.
Inorg. Chem. 1997, 36, 5761-5771.
(66) Wołowiec, S.; Latos-Grz˘yn´ski, L.; Toronto, D.; Marchon, J.-C. Inorg.
Chem. 1998, 37, 724-732.
(67) La Mar, G. N.; Gaudia, J. D.; Frye, J. S. Biochim. Biophys. Acta 1977,
498, 422-435.
Inorganic Chemistry, Vol. 45, No. 17, 2006 6737

Ikezaki et al.
dyz)4(dxy)1 isomer at higher temperature. Similarly, the nega-
tive deviation in the CN- (in CD3OD) and 4-CNPy com-
plexes should be ascribed to the increase in population of
the (dxy)2(dxz, dyz)3 isomer at higher temperature. Another
notable feature in Figure 9A is the difference in spread of
the pyrrole â chemical shifts given by black open circles.
While the maximum differences in the chemical shifts of
the â signals were 461 and 458 ppm at 223 K for the 1-MeIm
and CN- (in CD2Cl2) complexes, respectively, they were 183
and 197 ppm at the same temperature for the CN-
(CD3OD) and 4-CNPy complexes, respectively. The result
suggests that the spin densities at the peripheral â-C are quite
different in the former complexes adopting the (dxy)2(dxz, dyz)3
ground state while they are not much different in the latter
complexes with the (dxz, dyz)4(dxy)1 ground state. Similarly,
while the differences in the chemical shifts of the meso
signals were 230 and 221 ppm at 223 K for the 1-MeIm and
CN- (in CD2Cl2) complexes, they were only 8 and 34 ppm
for the CN- (CD3OD) and 4-CNPy complexes, respectively.
The differences in chemical shifts among the nonequivalent
â-C and meso-C signals clearly indicate that the A-1 and
A-2 orbitals in Chart 2 interact with the singly occupied dyz
orbital in [Fe(TPC)(1-MeIm2]+ and [Fe(TPC)(CN)2]- (in
CD2Cl2) while the S-1 orbital mainly involves in the inter-
action with the singly occupied dxy orbital in [Fe(TPC)(CN)2]-
(in CD3OD) and [Fe(TPC)(4-CNPy)2]+.
Comparison with the Analogous Complexes. First, we
have compared the electronic structures of the TPC com-
plexes with those of the TPP complexes. Table 2 shows the
13C NMR chemical shifts of a series of [Fe(TPC)L2]( and
[Fe(TPP)L2](. As mentioned, the presence of the unpaired
electron in the dxy orbital induces the large downfield shift
of the meso signal together with the large increase in the
Curie slopes. Thus, the electronic structures of [Fe(TPC)-
L2]( were compared with those of [Fe(TPP)L2]( in terms of
the meso carbon chemical shifts and the Curie slopes. Figure
9B shows the Curie plots of the meso, R, and â carbon signals
of a series of [Fe(TPP)L2](, where L’s are (a) 1-MeIm, (b)
CN- (in CD2Cl2), (c) CN- (in CD3OD), and (d) 4-CNPy.
As in the case of the chlorin complexes, the meso signal of
the 1-MeIm and the CN- (in CD2Cl2) complexes showed a
positive deviation, while that of the CN- (in CD3OD)
complex showed a negative deviation. In the case of the
4-CNPy complex, the Curie plots showed a considerable
curvature. We observed a similar temperature dependence
of the meso signals in some highly saddled complexes with
weak nitrogen bases such as [Fe(OETPP)(Py)2]+ and
[Fe(OMTPP)(4-CNPy)2]+.46,70 The phenomenon was ascribed
to the spin transition between S ) 3/2 and S ) 1/2. Thus,
the curvature of the Curie plots in [Fe(TPP)(4-CNPy)2]+
could be explained in terms of the equilibrium among (dxz,
dyz)4(dxy)1, (dxy)2(dxz, dyz)3, and S ) 3/2. The slopes and
intercepts of [Fe(TPP)L2]( are (a) -2.5 × 104 ppm‚K, 138
ppm, (b) -4.0 × 104 ppm‚K, 222 ppm, (c) 3.8 × 104
ppm‚K, 57 ppm, and (d) 3.2 × 104 ppm‚K, 170 ppm; the
curved plots in (d) was treated as a linear line. Thus, the
Figure 9. Curie plots of the carbon signals in a series of (A) [Fe(TPC)-
L2]( and (B) [Fe(TPP)L2](, where red-, blue-, and black-filled circles
represent the meso, R, and pyrroline signals, respectively, and the open
square indicates the â signals.
indicate that the 1-MeIm and CN- (in CD2Cl2) complexes
exist mainly as the (dxy)2(dxz, dyz)3 isomer while the CN- (in
CD3OD) and 4-CNPy complexes exist mainly as the (dxz,
dyz)4(dxy)1 isomer. One of the reasons for the deviation from
the diamagnetic positions in these complexes should be
ascribed to the temperature dependence of the equilibrium
constant given in eq 1. Thus, the positive deviation of the
meso signals in the 1-MeIm and CN- (in CD2Cl2) complexes
should be ascribed to the increase in population of the (dxz,
(70) Ikeue, T.; Ohgo, Y.; Yamaguchi, T.; Takahashi, M.; Takeda, M.;
Nakamura, M. Angew. Chem., Int. Ed. 2001, 40, 2617-2620.
6738 Inorganic Chemistry, Vol. 45, No. 17, 2006

Low-Spin Iron(III) Tetraphenylchlorin Complexes
slopes of the CN- (in CD3OD) and 4-CNPy complexes in
TPC are 3-4 times larger than those of the corresponding
TPP complexes. In addition to the Curie slopes, the meso
carbon signals of [Fe(TPC)(CN)2]- (in CD3OD) and
[Fe(TPC)(4-CNPy)2]+ always appeared more downfield than
those of the corresponding TPP complexes. These results
clearly indicate that the TPC core stabilizes the (dxz, dyz)4-
(dxy)1 isomer more effectively than the TPP core. The result
should be ascribed to the flexible nature of the chlorin ring
as compared with the porphyrin ring.71 In contrast to the case
of the CN- and 4-CNPy complexes, the spin densities at
the meso carbons in [Fe(TPC)(tBuNC)2]+ and [Fe(TPP)-
(tBuNC)2]+ are supposed to be quite close because the meso
signals of the TPC and TPP complexes appeared at 732 (on
average) and 767 ppm, respectively. Thus, both [Fe(TPP)-
(tBuNC)2]+ and [Fe(TPC)(tBuNC)2]+ exclusively exist as the
(dxz, dyz)4(dxy)1 isomer.
signal intensity, signal multiplicity, homo- and heteronuclear
selective decoupling, partially relaxed 13C NMR spectra, and
NOE difference spectra together with the 2D COSY and
HMQC measurements. The meso-13C-enriched complexes
have been used successfully for the unambiguous assign-
ments of the meso carbon signals. As in the case of the
corresponding porphyrin complexes [Fe(TPP)L2](, the con-
tribution of the (dxz, dyz)4(dxy)1 ground state has increased as
the axial ligand changes from 1-MeIm, to CN- (in CD2Cl2
solution), CN- (in CD3OD solution), and 4-CNPy, and then
t
to BuNC. In the case of the CN- complex, the population
of the (dxz, dyz)4(dxy)1 isomer has increased to a great extent
when the solvent is changed from CD2Cl2 to CD3OD, which
is ascribed to the hydrogen bonding between the coordinated
cyanide ligand and methanol. While the 1-MeIm and CN-
(in CD2Cl2 solution) complexes exist almost exclusively as
the (dxy)2(dxz, dyz)3 isomer, the CN- (in CD3OD solution) and
the 4-CNPy complexes exist as the mixture of the isomers
with the (dxy)2(dxz, dyz)3 and (dxz, dyz)4(dxy)1 ground state. In
the case of [Fe(TPC)(tBuNC)2]+, the meso-C signals appeare
extremely downfield, 763 and 700 ppm, while all the
pyrrole-H signals are observed quite close to their diamag-
Secondary, we have compared the electronic structures of
the TPC complexes with those of the OEC complexes
reported by Cai and co-workers.32 The largest difference was
observed in the spin densities of the pyrroline R carbon atoms
between [Fe(TPC)(tBuNC)2]+ and [Fe(OEC)(tBuNC)2]+.
While [Fe(TPC)(tBuNC)2]+ showed the pyrroline-H signal
at -36 ppm at 298 K,19 [Fe(OEC)(tBuNC)2]+ exhibited the
corresponding signal at 128 ppm at 213 K; this signal appears
at ca. 140 ppm at 298 K as is revealed from Figure 11 of ref
32. The result indicates that the pyrroline R carbons have
considerable amount of positive spin in [Fe(OEC)(tBuNC)2]+
while they have negative spin in [Fe(TPC)(tBuNC)2]+.
Although the pyrroline R spin densities are quite different,
these complexes commonly have large positive spin at the
meso carbons as is revealed from the downfield shift of
the meso-C and the upfield shift of the meso-H signals. Cai
and co-workers explained the unusual spin distribution in
[Fe(OEC)(tBuNC)2]+ in terms of the rapidly interconverting
equilibrium mixture between planar and ruffled structures;
the unpaired electron occupies predominantly the dyz orbital
in the planar complex and the dxy orbital in the ruffled one.32
This is in sharp contrast to the case of [Fe(TPC)(tBuNC)2]+,
which exists exclusively as the probably ruffled (dxz, dyz)4-
(dxy)1 isomer. The difference should again be ascribed to the
flexible nature of the TPC ring as compared with the OEC
ring. Immediately after we submitted this paper, a paper
written by Cai and co-workers appeared on the Web, which
t
netic positions. Thus, the BuNC complex is considered to
exist exclusively as the (dxz, dyz)4(dxy)1 isomer. Because of
the large positive spin at the meso carbon atoms, the pyrroline
R carbons of this complex have negative spin, which induces
the large upfield shift of the pyrroline-H, -36.0 ppm,
together with the downfield shift of the pyrroline-C signal.
Comparison of the spectral data between [Fe(TPC)L2]+ and
[Fe(TPP)L2]+ has revealed that the population of the (dxz,
dyz)4(dxy)1 isomer is much larger in the TPC than in the TPP
complex if the axial ligand is CN-(in CD3OD solution) or
4-CNPy, which is partly ascribed to the flexible nature of
the chlorin core as compared with the porphyrin core. If
t
the axial ligand is BuNC, both [Fe(TPC)(tBuNC)2]+ and
[Fe(TPP)(tBuNC)2]+ exist exclusively as the (dxz, dyz)4(dxy)1
isomer. Recently, Cai and co-workers have suggested that
[Fe(OEC)(tBuNC)2]+ exists as a mixture of the planar and
ruffled conformations with the unpaired electron predomi-
nantly in the dyz and dxy orbitals, respectively.32 Thus, the
present study reveals that the TPC complexes stabilizes the
(dxz, dyz)4(dxy)1 ground state more effectively than the TPP,
OEC, and OEP complexes carrying the same axial ligands.
Acknowledgment. This work was supported by the
Project Research Grant (No.17-12 to A.I.) of Toho University
School of Medicine and by the Grant in Aid (No. 16550061
to M.N.) for Scientific Research from the Ministry of
Education, Culture, Sports, Science, and Technology of
Japan.
1
discussed the H NMR and EPR spectra of [Fe(TPC)L2]+
where L ) imidazole-d4, 2-methylimidazole, 4-(N,N-di-
methylamino)pyridine, pyridine, and 4-CNPy.72
Conclusions
13C NMR spectra of a series of low-spin chlorin complexes
Supporting Information Available: Listing of homonuclear
and heteronuclear selective decoupling, NOE difference spec-
tra, and 2D COSY, HMQC, and HMBC spectra of a series of
low-spin [Fe(TPC)L2]( and diamagnetic [Co(TPC)(CN)2]-. This
material is available free of charge via the Internet at http://
pubs.acs.org.
t
[Fe(TPC)L2]( (L ) 1-MeIm, CN-, 4-CNPy, and BuNC)
have been examined to reveal the electronic structure. The
signal assignments have been done on the basis of the relative
(71) Stolzenberg, A. M.; Haymond, G. S. Inorg. Chem. 2002, 41, 300-
308.
(72) Cai, S.; Shokhireva, T. K.; Lichtenberger, D. L.; Walker, F. A. Inorg.
Chem. 2006, 45, 3519-3531.
IC060508O
Inorganic Chemistry, Vol. 45, No. 17, 2006 6739