Factors affecting the electronic ground state of low-spin iron(III) porphyrin complexes

Article abstract of DOI:10.1021/ic001412b

To determine the factors affecting the ground-state electron configuration of low-spin Fe(III) porphyrin complexes, we have examined the ¹H NMR, ¹³C NMR, and EPR spectra of a series of low-spin bis-ligated Fe(III) porphyrin complexes [Fe(Por)L₂]^±, in which the positions of porphyrin substituents and the coordination ability of axial ligands are different. The seven porphyrins used in this study are meso-tetraalkylporphyrins (TRP: R ispropyl, cyclopropyl, or isopropyl), meso-tetraphenylporphyrin (TPP), meso-tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin, and 5,10,15,20-tetraphenyl-2,3,7,8,12,13,17,18-octaalkylporphyrins (ORTPP: R is methyl or ethyl). The porphyrin cores of TRP are more or less S₄-ruffled depending on the bulkiness of the alkyl substituents, while those of ORTPP are highly S₄-saddled. Three types of axial ligands are examined which have the following characteristics in ligand field theory: they are (i) strong σ-donating imidazole (HIm), (ii) strong σ-donating and weak π-accepting cyanide (CN^-), and (iii) weak σ-donating and strong π-accepting tert-butyl isocyanide (^tBuNC). In the case of the bis(HIm) complexes, only the isopropyl complex, [Fe(TⁱPrP)(HIm)₂]⁺, has shown the less common (d_xz, d_yz)⁴(d_xy)¹ ground state; the other six complexes have exhibited the common (d_xy)²(d_xz, d_yz)³ ground state. When the axial imidazole is replaced by cyanide, even the propyl and cyclopropyl complexes have shown the (d_xz, d_yz)⁴(d_xy)¹ ground state; the TPP and ORTPP complexes have still maintained the common (d_xy)²(d_xz, d_yz)³ ground state. In the case of the bis(^tBuNC) complexes, all the complexes have shown the (d_xz, d_yz)⁴(d_xy)¹ ground state. However, the contribution of the (d_xz, d_yz)⁴(d_xy)¹ state to the electronic ground state differs from complex to complex; the (d_xz, d_yz)⁴(d_xy)¹ contribution is the largest in [Fe(TⁱPrP)(^tBuNC)₂]⁺ and the smallest in [Fe(OETPPP)(^tBuNC)₂]⁺. We have then examined the electronic ground state of low-spin [Fe(OEP)(^tBuNC)₂]⁺ and [Fe(ProtoIXMe₂)(^tBuNC)₂]⁺; OEP and ProtoIXMe₂ represent 2,3,7,8,12,13,17,18- octaethylporphyrin and protoporphyrin-IX dimethyl ester, respectively. These porphyrins have a_1u HOMO in contrast to the other seven porphyrins that have a_2u HOMO. The ¹³C NMR and EPR studies have revealed that the contribution of the (d_xz, d_yz)⁴(d_xy)¹ state in these complexes is as small as that in [Fe(OETPP)(^tBuNC)₂]⁺. On the basis of these results, we have concluded that the low-spin iron(III) porphyrins that have (i) strong axial ligands, (ii) highly saddle shaped porphyrin rings, (iii) porphyrins with a_1u HOMO, and (iv) electron withdrawing substituents at the meso positions tend to maintain the common (d_xy)²(d_xz, d_yz)³ ground state.

Full text of DOI:10.1021/ic001412b

Inorg. Chem. 2001, 40, 3423-3434
3423
Factors Affecting the Electronic Ground State of Low-Spin Iron(III) Porphyrin Complexes
Takahisa Ikeue,† Yoshiki Ohgo,† Takashi Saitoh,‡ Tatsuya Yamaguchi,† and
Mikio Nakamura*†,‡
Department of Chemistry, Toho University School of Medicine, Omorinishi,
Ota-ku, Tokyo 143-8540, and Division of Biomolecular Science, Graduate School of Science,
Toho University, Funabashi, Chiba 274-8510
ReceiVed December 18, 2000
To determine the factors affecting the ground-state electron configuration of low-spin Fe(III) porphyrin complexes,
we have examined the 1H NMR, 13C NMR, and EPR spectra of a series of low-spin bis-ligated Fe(III) porphyrin
complexes [Fe(Por)L2](, in which the positions of porphyrin substituents and the coordination ability of axial
ligands are different. The seven porphyrins used in this study are meso-tetraalkylporphyrins (TRP: R is propyl,
cyclopropyl, or isopropyl), meso-tetraphenylporphyrin (TPP), meso-tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin,
and 5,10,15,20-tetraphenyl-2,3,7,8,12,13,17,18-octaalkylporphyrins (ORTPP: R is methyl or ethyl). The porphyrin
cores of TRP are more or less S4-ruffled depending on the bulkiness of the alkyl substituents, while those of
ORTPP are highly S4-saddled. Three types of axial ligands are examined which have the following characteristics
in ligand field theory: they are (i) strong σ-donating imidazole (HIm), (ii) strong σ-donating and weak π-accepting
cyanide (CN-), and (iii) weak σ-donating and strong π-accepting tert-butyl isocyanide (tBuNC). In the case of
the bis(HIm) complexes, only the isopropyl complex, [Fe(TiPrP)(HIm)2]+, has shown the less common
(dxz, dyz)4(dxy)1 ground state; the other six complexes have exhibited the common (dxy)2(dxz, dyz)3 ground state.
When the axial imidazole is replaced by cyanide, even the propyl and cyclopropyl complexes have shown the
(dxz, dyz)4(dxy)1 ground state; the TPP and ORTPP complexes have still maintained the common (dxy)2(dxz, dyz)3
ground state. In the case of the bis(tBuNC) complexes, all the complexes have shown the (dxz, dyz)4(dxy)1 ground
state. However, the contribution of the (dxz, dyz)4(dxy)1 state to the electronic ground state differs from complex
to complex; the (dxz, dyz)4(dxy)1 contribution is the largest in [Fe(TiPrP)(tBuNC)2]+ and the smallest in [Fe-
(OETPPP)(tBuNC)2]+. We have then examined the electronic ground state of low-spin [Fe(OEP)(tBuNC)2]+ and
[Fe(ProtoIXMe2)(tBuNC)2]+; OEP and ProtoIXMe2 represent 2,3,7,8,12,13,17,18-octaethylporphyrin and proto-
porphyrin-IX dimethyl ester, respectively. These porphyrins have a1u HOMO in contrast to the other seven
porphyrins that have a2u HOMO. The 13C NMR and EPR studies have revealed that the contribution of the
(dxz, dyz)4(dxy)1 state in these complexes is as small as that in [Fe(OETPP)(tBuNC)2]+. On the basis of these
results, we have concluded that the low-spin iron(III) porphyrins that have (i) strong axial ligands, (ii) highly
saddle shaped porphyrin rings, (iii) porphyrins with a1u HOMO, and (iv) electron withdrawing substituents at the
meso positions tend to maintain the common (dxy)2(dxz, dyz)3 ground state.
Introduction
tert-butyl isocyanide (tBuNC) and 4-cyanopyridine (4-CNPy)1-10
and (ii) the porphyrin rings that are strongly S4-ruffled such as
meso-tetraisopropylporphyrin, meso-tetramethylchiroporphyrin,
Low-spin Fe(III) porphyrin complexes carrying two axial
ligands usually have the (dxy)2(dxz, dyz)3 ground state.1 However,
recent studies have revealed that some low-spin complexes
tend to adopt the less common (dxz, dyz)4(dxy)1 ground state if
one of the following conditions is satisfied: (i) the axial ligands
that are both weak σ-donors and strong π-acceptors such as
(5) Cheesman, M. R.; Walker, F. A. J. Am. Chem. Soc. 1996, 118, 7373-
7380.
(6) 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.
(7) Safo, M. K.; Marlys, J. M.; Walker F. A.; Debrunner, P. G.; Scheidt,
W. R. J. Am. Chem. Soc. 1997, 119, 9438-9448.
* To whom correspondence should be addressed. E-mail: mnakamu@
med.toho-u.ac.jp.
(8) Pilard, M.-A.; Guillemot, M.; Toupet, L.; Jordanov, J.; Simonneaux,
G. Inorg. Chem. 1997, 36, 6307-6314.
† Toho University School of Medicine.
‡ Graduate School of Science, Toho University.
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1992, 195, 73-76.
(1) (a) Walker, F. A. Paramagnetic Molecules. In Biological Magnetic
Resonance; Berliner, L. J., Reuben, J., Eds.; Plenum Press: New York,
1993; Volume 12, pp 133-274. (b) Walker, F. A. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
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(10) Abbreviations. ORTPP (R ) M or E): dianion of 2,3,7,8,12,13,17,18-
octaalkyl-5, 10, 15, 20-tetraphenylporphyrin where R is methyl (M)
or ethyl (E). OEP: dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin.
i
c
THP: dianion of porphine. TRP (R ) Pr, Pr, or nPr): dianion of
meso-tetraalkylporphyrin where R is isopropyl (iPr), cyclopropyl
(cPr), or propyl (nPr). TArP, TPP, F20-TPP, TCHP, and TMCP:
dianions of meso-tetraarylpoprhyrin, meso-tetraphenylporphyrin, meso-
tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrin, meso-tetracyclohexyl-
porphyrin, and meso-tetramethylchiroporphyrin. ProtoIXMe2: dianion
t
of protoporphyrin IX dimethyl ester. BuNC: tert-butyl isocyanide.
Him: imidazole. 4-CNPy: 4-cyanopyridine. 3-PhPy: 3-phenylpyridine.
10.1021/ic001412b CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/08/2001

3424 Inorganic Chemistry, Vol. 40, No. 14, 2001
Ikeue et al.
and meso-tetracyclohexylporphyrin.11-17 The weak crystal field
of axial ligands can place the dxz, dyz orbitals at lower energy
than dxy, resulting in the formation of the (dxz, dyz)4(dxy)1 ground
state.7 The S4-ruffling of the porphyrin core weakens the
3eg(porphyrin)-dπ(iron)13 and strengthens the a2u(porphyrin)-
dxy(iron) interactions.4 The former stabilizes the iron dπ orbitals
and the latter destabilizes the iron dxy orbital, resulting in
the stabilization of the (dxz, dyz)4(dxy)1 state relative to the
(dxy)2(dxz, dyz)3 state. As the S4-ruffling of the porphyrin
core increases, the (dxz, dyz)4(dxy)1 state is further stabilized.
Thus, in the case of the isopropyl complexes, [Fe(TiPrP)L2](,
the (dxz, dyz)4(dxy)1 ground state is maintained in the wide range
of axial ligands with different coordination ability, i.e., from
the very weak ligands such as 4-CNPy to the much stronger
ligands such as Py, 3-MePy, 4-NMe2Py, 2-MeIm, and HIm.18
Then, the question arises as to how the electron configuration
is affected by the deformation mode of porphyrin ring; several
deformation modes have been observed including ruffling,
saddling, doming, and waving.19,20 The study on the effect of
deformation mode on the heme properties must be very
important since the recent research done by Shelnutt and co-
workers have revealed that the nonplanar distortions of por-
phyrins are frequently observed in naturally occurring heme
proteins;21,22 the hemes in cytochromes c, for example, are
highly ruffled, while those in peroxidases are mainly saddled.23,24
We have chosen the low-spin Fe(III) complexes of OMTPP and
OETPP as the models for saddle shaped complexes, because
the porphyrin rings in OMTPP and OETPP complexes always
show the strongly S4-saddled structure.25-28 In this paper, we
Scheme 1
of the low-spin Fe(III) complexes with saddle shaped porphyrin
rings such as [Fe(ORTPP)(HIm)2]+, [Fe(ORTPP)(CN)2]-, and
[Fe(ORTPP)(tBuNC)2]+, and compare the spectroscopic results
with those of the ruffle shaped complexes such as [Fe(TRP)-
(HIm)2]+, [Fe(TRP)(CN)2]-, and [Fe(TRP)(tBuNC)2]+ (R ) nPr,
i
cPr, or Pr); although the X-ray crystallographic results of the
low-spin [Fe(TRP)L2]+ (L ) HIm, CN-, or tBuNC) complexes
have not been reported, the porphyrin rings in meso-tetra-
alkylporphyrinates such as [Ni(TiPrP)],29 [Fe(TiPrP)Cl],30
[Fe(TMCP)Cl],31 [Ni(TCHP)],32 [Fe(TnPrP)Cl],33 [Fe(TEtP)-
(THF)2]+,34 and [Fe(TiPrP)(THF)2]+35 are known to be ruffled.
In the case of [Fe(TiPrP)(THF)2]+, the deviations of the meso
carbon atoms, C5, C10, C15, and C20, from the least-squares
plane of the [Fe(C20N4)] core are -0.675(9), +0.623(8),
-0.579(8), and +0.640(8) Å, respectively. We also report
the spectroscopic results of [Fe(OEP)(tBuNC)2]+ and [Fe-
(ProtoIXMe2)(tBuNC)2]+. The porphyrins of these complexes
have a1u HOMO, while those of the other complexes examined
in this study have a2u HOMO.36 On the basis of the spectroscopic
results, we have concluded that the low-spin complexes with
highly saddled porphyrin ring or with a1u porphyrin HOMO
resist to switch the ground-state electron configuration from
(dxy)2(dxz, dyz)3 to (dxz, dyz)4(dxy)1.
1
report the H NMR, 13C NMR, and EPR spectroscopic results
Py: pyridine. [Fe(ORTPP)(CN)2]-: (dicyano)[(2,3,7,8,12,13,17,18-
octaalkyl-5,10,15,20-tetraphenylporphyrinato)iron(III)] anion. [Fe-
(ORTPP)(tBuNC)2]+: bis(tert-butylisocyanide)[(2,3,7,8,12,13,17,18-
octaalkyl-5,10,15,20-tetraphenylporphyrinato)iron(III)] cation.
(11) Nakamura, M.; Nakamura, N. Chem. Lett. 1991, 1885-1888.
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Chem. 1996, 35, 3731-3732.
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Chem. 1998, 37, 2405-2414.
(15) Wojaczynski, J.; Latos-Grazynski, L.; Glowiak, T. Inorg. Chem. 1997,
36, 6299-6306.
Experimental Section
Spectral Measurement. H and 13C NMR spectra were recorded
1
(16) Wolowiec, S.; Latos-Grazynski, L.; Mazzanti, M.; Marchon, J.-C.
Inorg. Chem. 1997, 36, 5761-5771.
on a JEOL LA300 spectrometer operating at 300.4 MHz for 1H.
Chemical shifts were referenced to the residual peak of dichloromethane
(17) Wolowiec, S.; Latos-Grazynski, L.; Toronto, D.; Marchon, J.-C. Inorg.
Chem. 1998, 37, 724-732.
(δ ) 5.32 ppm for H and 53.8 ppm for 13C). Proton homonuclear
1
(18) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Nakamura, M.; Fujii, H.; Yokoyama,
M. J. Am. Chem. Soc. 2000, 122, 4068-4076.
COSY spectra were collected after the measurement of the standard
1D reference spectra. The 2D COSY spectra were collected by use of
1024 points in t2 over the bandwidth of 8.4 kHz with 512 t1 blocks and
128 scans per block in which 4 dummy scans were included. UV-vis
spectra were recorded on a Hitachi 200-10 spectrophotometer at 25 °C
(19) Senge, M. O.; Medforth, C. J.; Forsyth, T. P.; Lee, D. A.; Olmstead,
M. M.; Jentzen, W.; Pandey, R. K.; Shelnutt, J. A.; Smith, K. M.
Inorg. Chem. 1997, 36, 1149-1163.
(20) Jentzen, W.; Simpson, M. C.; Hobbs, J. D.; Song, X.; Ema, T.; Nelson,
N. Y.; Medforth, C. J.; Smith, K. M.; Veyrat, M.; Mazzanti, M.;
Ramasseul, R.; Marchon, J.-C.; Takeuchi, T.; Goddard, III, W. A.;
Shelnutt, J. A. J. Am. Chem. Soc. 1995, 117, 11085-11097.
(21) Shelnutt, J. A.; Song, X.-Z.; Ma, J.-G.; Jia, S.-L.; Jentzen, W.;
Medforth, C. J. J. Chem. Soc. ReV. 1998, 27, 31-41.
(22) Ma, J.-G.; Zhang, J.; Franco, R.; Jia, S.-L.; Moura, I.; Moura, J. G.;
Kroneck, P. M. H.; Shelnutt, J. A. Biochemistry 1998, 37, 12431-
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(29) Ema, T., Senge, M. O., Nelson, N. Y., Ogoshi, H., Smith K. M. Angew.
Chem., Int. Ed. Engl. 1994, 33, 1879-1881.
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Grazynski, L.; Shang, M.; Scheidt, W. R. Inorg. Chem. 1998, 37,
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(23) Bushnell, G. W.; Louie, G. V.; Brayer, G. D. J. Mol. Biol. 1990, 214,
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Y.; Amachi, T. J. Mol. Biol. 1994, 235, 331-344.
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New York, 1979; Vol. IV, pp 198-256.

Low-Spin Iron(III) Porphyrin Complexes
Inorganic Chemistry, Vol. 40, No. 14, 2001 3425
using CH2Cl2 as solvent. EPR spectra were measured at 4.2 K with a
Brucker ESP-300E spectrometer operating at X band and equipped with
an Oxford helium cryostat. The samples for the EPR measurement were
prepared by the addition of 4 to 6 equiv of the ligands into the CH2Cl2
solutions of [Fe(Porphyrin)Cl] or [Fe(Porphyrin)]ClO4. The concentra-
tions of EPR samples were 5∼8 mM. The observed EPR spectra had
enough quality to determine their g values from the spectra except for
some broad signals. In the latter case, the g-values were estimated by
the simulation of the observed spectra using Bruker WIN-EPR Sim
Fonia program.
4.01, 2.00. Mo¨ssbauer (microcrystalline, 80 K): IS ) 0.50 mm s-1
,
QS ) 3.50 mm s-1. SQUID (microcrystalline, µeff): 3.85 ( 0.05 µB in
20∼300 K.40
[Fe(OEP)(THF)2]ClO4.41 1H NMR (CD2Cl2, 25 °C, δ): 10.3(4H,
meso), 34.6(16H, CH2), 6.2(24H, CH3). Coordinated THF ligand
showed very broad signals at 10.3 and 6.0 ppm.
[Fe(ProtoIXMe2)(THF)2]ClO4. 1H NMR (CD2Cl2, 25 °C, δ):
69.3(3H, CH3), 68.4(3H, CH3), 66.6(3H, CH3), 62.5(3H, CH3), 50.5(1H,
vinyl-R), 48.3(1H, vinyl-R), 40.0(2H, R-CH2), 39.5(2H, R-CH2), 6.7(4H,
â-CH2 x 2 ), 3.64(6H, OCH3 x 2), -8.99(1H, vinyl-â), -9.17(1H, vinyl-
â), -10.39(1H, vinyl-â), -10.59(1H, vinyl-â). Broad signals were
observed at 22 and 11 ppm due to the coordinated THF.
[Fe(Porohyrin)(HIm)2]Cl (Porphyrin ) TRP, TPP). These com-
plexes were prepared by the addition of 4 to 6 equiv of imidazole into
a CD2Cl2 solution of [Fe(Porphyrin)Cl] placed in an NMR sample tube
as described in our previous paper.18
n
Synthesis. Free base porphyrins. H2(TRP)(R ) Pr,37 cPr,18 and
iPr29,38), H2(TPP), H2(OMTPP),25 and H2(OETPP),25 were prepared from
the corresponding aldehydes and pyrroles according to the literature.
H2(OEP) and H2(ProtoIXMe2) were purchased from Aldrich. The meso
13C enriched H2(TPP), H2(OMTPP), and H2(OETPP) were prepared
similarly with the use of benzaldehyde-carbonyl-13C (99 atom % 13C)
purchased from Aldrich.
[Fe(F20-TPP)(1-MeIm)2]Cl. This complex was prepared by the
addition of 4 equiv of 1-methylimidazole into a CD2Cl2 solution of
[Fe(F20-TPP)Cl] placed in an NMR sample.1H NMR (CD2Cl2, 25 °C,
δ): -15.9(8H, Py-H), 19.4(6H, CH3), -7.18(2H, ligand).
[Fe(ORTPP)(HIm)2]ClO4 (R ) Me, Et). A large excess of HIm
was necessary for the complete conversion of [Fe(ORTPP)Cl] into [Fe-
(ORTPP)(HIm)2]Cl. Thus, [Fe(ORTPP)-(THF)2]ClO4 was used instead
of [Fe(ORTPP)Cl]. The CD2Cl2 solution of [Fe(ORTPP)-(THF)2]ClO4
was treated with 4 to 6 equiv of HIm in an NMR sample tube.
[Fe(Porphyrin)Cl](Porphyrin ) TRP, TPP, F20-TPP, ORTPP,
OEP, ProtoIXMe2). Insertion of iron was performed using FeCl2‚4H2O
either in refluxing CH3OH-CHCl3 (1:3) for H2(TRP), H2(OEP), and
H2(ProtoIXMe2) or in refluxing DMF for H2(ORTPP).18,28 The Fe(III)
porphyrin complexes thus formed were treated with diluted HCl and
purified by chromatography on silica gel using CH2Cl2-CH3OH as
eluents. Formation of [Fe(TRP)Cl], [Fe(ORTPP)Cl], [Fe(OEP)Cl], and
[Fe(ProtoIXMe2)Cl] was confirmed by the comparison with the UV-
1
vis and H NMR spectral properties reported before.13,18,28,30 [Fe(F20-
1
Formation of [Fe(ORTPP)(HIm)2]ClO4 was confirmed by the H and
13C NMR spectra at 25 °C.
TPP)Cl] was purchased from Aldrich.
[Fe(OMTPP)(HIm)2]ClO4. 1H NMR (CD2Cl2, 25 °C, δ): 21.1(24H,
CH3), 5.42(8H, o), 6.21(8H, m), 6.79(4H, p). The ligand exchange was
[Fe(Porphyrin)(THF)2]ClO4(Porphyrin ) TRP, TPP, F20-TPP,
ORTPP, OEP, ProtoIXMe2). A THF solution (5 mL) of AgClO4 (3.5
× 10-5 mol) was added to a THF solution (20 mL) of [Fe(Por)Cl] (3.5
× 10-5 mol). The solution was stirred for a minute at room temperature
and then evaporated. Caution! Perchlorate salts are potentially
explosiVe when heated or shocked. Handle them in milligram quantities
with care. Dichloromethane (20 mL) was added to the reaction mixture
and the resultant suspension was filtered to remove silver chloride. After
the evaporation of the filtrate, 20 mL of THF was added to dissolve
the solid and then 20 mL of heptane was added. The solution was
allowed to stand overnight. The purple crystal thus formed was collected
by filtration, washed with hexane, and dried in vacuo for 10 min at 25
°C.39
1
fast on the H NMR time scale at 25 °C. As a result, the free and the
coordinated ligands gave broad signals at the average positions. The
ligand exchange became slow below -40 °C to show the coordinated
1
ligand signals separately. H NMR (CD2Cl2, -79 °C, δ): 23.0(24H,
CH3), 1.79(8H, o), 4.39(8H, m), 5.40(4H, p), 20.6(2H, ligand), 13.0(2H,
ligand), -4.6(2H, ligand). 13C NMR (CD2Cl2, 25 °C, δ): 128.4(s, R-Py),
173.0(s, â-Py), -33.4(q, CH3), 28.5(s, meso), 153.4(s, ipso), 116.6(d,
o), 126.4(d, m), 125.9(d, p).
[Fe(OETPP)(HIm)2]ClO4. 1H NMR (CD2Cl2, 25 °C, δ): 4.5(broad,
8H, CH2), ca. 10 (broad, 8H, CH2), 0.86(24H, CH3), 4.96(8H, o),
5.79(8H, m), 6.72(4H, p). The ligand exchange was fast on the 1H NMR
time scale at 25 °C. However, the process was frozen below -20 °C,
1
[Fe(TiPrP)(THF)2]ClO4. Yield: 69%. H NMR (CD2Cl2, 25 °C,
1
δ): -34.8(8H, Py-H), 13.0(4H, meso R-H), 5.0(24H, meso â-H), 4.5(8H,
THF), 9.5(8H, THF). EPR(CH2Cl2, 4.2 K): g ) 3.99, 1.97. Mo¨ssbauer
giving the signals of the coordinated HIm ligand. H NMR (CD2Cl2,
-70 °C, δ): 2.84(8H, CH2), 12.50(8H, CH2), 1.41(24H, CH3), 2.09(8H,
o), 4.31(8H, m), 5.76(4H, p), 22.2(2H, ligand), 17.5(2H, ligand),
14.6(2H, ligand). 13C NMR (CD2Cl2, 25 °C, δ): 162.4(s, R-Py), 167.0(s,
â-Py), -23.5(t, CH2), 86.6(q, CH3), 7.0(s, meso), 164.0(s, ipso); 107.4(d,
o); 124.6(d, m); 125.0(d, p).
(microcrystalline, 76 K): IS ) 0.34 mm s-1, QS ) 3.71 mm s-1
.
SQUID (microcrystalline, µeff): 3.90 ( 0.10 µB in 50 ∼ 300 K.40 The
crystal structure of this complex was reported elswhere.35
1
[Fe(TcPrP)(THF)2]ClO4. Yield: 70%. H NMR (CD2Cl2, 25 °C,
[Fe(Porphyrin)(CN)2]NBu4 (Porphyrin ) TRP, TPP, F20-TPP,
ORTPP). These complexes were prepared by the addition of 4 to 6
equiv of tetrabutylammonium cyanide into a CD2Cl2 solution of [Fe-
(Porphyrin)Cl] placed in an NMR sample tube as described in our
previous papers.13,18 Formation and spectroscopic results of [Fe(TRP)-
(CN)2]NBu4(R ) iPr, cPr, nPr) and [Fe(TPP)(CN)2]NBu4 have already
been reported.13,18, 42,43
δ): -35.3(8H, Py-H), 32.8(4H, meso R-H), 0.89(8H, meso â-H),
2.79(8H, meso â-H), 7.6(8H, THF), 12.5(8H, THF).
1
[Fe(TnPrP)(THF)2]ClO4. Yield: 77%. H NMR (CD2Cl2, 25 °C,
δ): -19.0(8H, Py-H), 15.6(8H, meso R-H), 4.97(8H, meso â-H),
2.86(12H, meso γ-H), 9.5(8H, THF), 14.6(8H, THF).
[Fe(F20-TPP)(THF)2]ClO4. Yield: 65%. 1H NMR (CD2Cl2, 25 °C,
δ): 41.4(8H, Py-H), 7.9(8H, THF), 19.4(8H, THF).
[Fe(F20-TPP)(CN)2]NBu4. 1H NMR (CD2Cl2, 25 °C, δ): -19.4(8H,
1
[Fe(OMTPP)(THF)2]ClO4. Yield: 67%. H NMR (CD2Cl2, -20
Py-H). 13C NMR (CD2Cl2, 25 °C, δ): 48.6(s, R-Py), 92.0(d, â-Py),
°C, δ): 71.5(24H, CH3), 13.8(8H, o), 6.88(8H, m), 10.40(4H, p),
9.4(8H, THF), 13.8(8H, THF). Another signal for the coordinated THF
ligand might be too broad to detect.
2
1
32.9(s, meso), 109.4(t, JC-F ) 18.1 Hz, ipso), 142.8(d, JC-F ) 244
1
1
Hz, o), 134.8(d, JC-F ) 256 Hz, m), 139.7(d, JC-F ) 256 Hz, p).
[Fe(OMTPP)(CN)2]NBu4. 1H NMR (CD2Cl2, 25 °C, δ): 13.36(24H,
CH3), 5.43(8H, o), 7.16 (8H, m), 6.58(4H, p). 13C NMR (CD2Cl2, 25
°C, δ): 85.3(s, R-Py), 127.2(s, â-Py), -23.2 (q, CH3), 136.6(s, meso),
123.2(s, ipso), 166.8(d, o), 131.7(d, m), 127.6(d, p).
[Fe(OETPP)(THF)2]ClO4. Yield: 87%. 1H NMR (CD2Cl2, 25 °C,
δ): 42.7(8H, CH2), 14.2(8H, CH2), 0.67(24H, CH3), 12.9(8H, o),
6.5(8H, m), 10.6(4H, p), 11.4(8H, THF). EPR(CH2Cl2, 4.2 K): g )
[Fe(OETPP)(CN)2]NBu4. 1H NMR (CD2Cl2, 25 °C, δ): 0.78(24H,
(37) Neya, S.; Funasaki, N. J. Heterocycl. Chem. 1997, 34, 689-690.
(38) Wagner, R. W.; Lawrence, D. S.; Lindsey, J. S. Tetrahedron Lett.
1987, 28, 3069-3070.
CH3); 6.87(16H, CH2); 5.39(8H, o); 6.48(8H, m); 6.56(4H, p). 13C NMR
(39) Full report on the formation, characterization, and spectroscopic
properties of [Fe(Porphyrin) (THF)2]ClO4 will be published elsewhere.
[Fe(TiPrP)(THF)2]ClO4 and [Fe(OETPP)(THF2)ClO4 turned out to be
(41) Ogoshi, H.; Sugimoto, H.; Watanabe, E.; Yoshida, Z.; Maeda, Y.;
Sakai, H.; Bull. Chem. Soc. Jpn. 1981, 54, 3414-3419.
(42) La Mar, G. N.; Gaudio, J. D.; Frye, J. S. Biochim. Biophys. Acta 1977,
498, 422-435.
(43) Nakamura, M.; Ikeue, T.; Ikezaki, A.; Ohgo, Y.; Fujii, H. Inorg. Chem.
1999, 38, 3857-3862.
3
very pure intermediate spin (S ) /2) complexes.40
(40) Ikeue, T.; Saitoh, T.; Yamaguchi, T.; Ohgo, Y.; Nakamura, M.;
Takahashi, M.; Takeda, M. Chem. Commun. 2000, 1989-1990.

3426 Inorganic Chemistry, Vol. 40, No. 14, 2001
Ikeue et al.
(CD2Cl2, 25 °C, δ): 134.9(s, R-Py), 140.6(s, â-Py), -19.9(t, CH2),
74.6 (q, CH3), 71.7(s, meso), 145.6(s, ipso), 136.4(d, o), 126.5(d, m),
125.9(d, p).
[Fe(Porphyrin)(tBuNC)2]ClO4 (Porphyrin
)
TRP, TPP,
F20-TPP, ORTPP, OEP, ProtoIXMe2). Complete formation of
[Fe(Porphyrin)(tBuNC)2]Cl was unsuccessful even by the addition of
large excess tBuNC into a CD2Cl2 solution of [Fe(Porphyrin)Cl]. Thus,
[Fe(Porphyrin)(THF)2]ClO4 was used instead of [Fe(Porphyrin)Cl].
The CD2Cl2 solution of [Fe(Porphyrin)(THF)2]ClO4 was treated
t
with 4 to 6 equiv of BuNC in an NMR sample tube. Formation
1
of [Fe(Porphyrin)(tBuNC) ]ClO4 was confirmed at 25 °C by the H
2
and 13C NMR spectra.
[Fe(TiPrP)(tBuNC)2]ClO4. 1H NMR (CD2Cl2, 25 °C, δ): 11.97(8H,
Py-H), 39.51 (8H, CH), 8.05(24H, CH3), -1.74(18H, CH3 of the
coordinated BuNC). 13C NMR (CD2Cl2, 25 °C, δ): 98.8(d, â-Py),
t
t
18.0(q, CH3 of the coordinated BuNC).
[Fe(TcPrP)(tBuNC)2]ClO4. 1H NMR (CD2Cl2, 25 °C, δ): 13.12(8H,
Py-H), -2.21(8H, CH2), -3.16(8H, CH2), -2.04(18H, CH3 of the
t
coordinated BuNC). The meso CH signal was not observed up to δ
500 ppm. 13C NMR (CD2Cl2, 25 °C, δ): 91.0(d, â-Py), 16.4(q, CH3 of
t
the coordinated BuNC).
[Fe(TnPrP)(tBuNC)2]ClO4. 1H NMR (CD2Cl2, 25 °C, δ): 11.60(8H,
Py-H), 95.20(8H, R-CH2), -2.15(8H, â-CH2), 2.17(12H, γ-CH3),
-1.53(18H, CH3 of the coordinated BuNC). 13C NMR (CD2Cl2, 25
t
°C, δ): 84.7 (d, â-Py), 29.5 (q, γ-CH3), 18.4(q, CH3 of the coordinated
tBuNC).
[Fe(TPP)(tBuNC)2]ClO4. H NMR (CD2Cl2, 25 °C, δ): 0.77(8H,
1
Figure 1. 1H NMR spectra of (a) [Fe(OMTPP)(CN)2]-, (b) [Fe(Ti-
PrP)(tBuNC)2]+, and (c) [Fe(OETPP)(tBuNC)2]+ taken in CD2Cl2 at
25 °C. (d) 1H NMR of [Fe(OETPP)(tBuNC)2]+ taken in CD2Cl2 at -40
o), 14.09(8H, m), 3.12(4H, p), 10.01(8H, Py-H), -1.86(18H, CH3 of
the coordinated tBuNC). 13C NMR (CD2Cl2, 25 °C, δ): -293.6(s, R-Py),
82.8(d, â-Py), -122.5(s, ipso), 457.6(broad-s, o), 145.8(d, m), 154.4(d,
t
°C. Integral intensity of the methyl signal of the coordinated BuNC
t
ligand, δ -2.50 ppm, clearly shows the formation of the bis-adduct.
Signal assignment: o, m, and p, ortho-H, meta-H, and para-H; CH3,
pyrrole-methyl; CH2, pyrrole-methylene; L and F, methyl signal of the
coordinated and free tBuNC ligand, respectively; x, tetrabutylammonium
ion; y, THF; s, dichloromethane.
p), 767.0(s, meso), 19.3(q, CH3 of the coordinated BuNC). The meso
carbon signal was observable only when the meso-13C enriched complex
was used.
[Fe(F20-TPP)(tBuNC)2]ClO4. 1H NMR (CD2Cl2, 25 °C, δ): 4.31(8H,
t
Py-H), -1.86(18H, CH3 of the coordinated BuNC).
[Fe(OMTPP)(tBuNC)2]ClO4. 1H NMR (CD2Cl2, 25 °C, δ): 1.26(24H,
CH3); 3.06(8H, o); 12.83(8H, m); 5.16(4H, p). The tert-butyl protons
of the coordinated ligand gave a broad signal due to the exchange with
the free ligand at 25 °C. The broad signal split into two signals below
0 °C, where the coordinated ligand gave a signal corresponding to 18H.
1H NMR (CD2Cl2, -20 °C, δ): -0.49(24H, CH3), 1.96(8H, o),
13.95(8H, m), 4.70(4H, p), -2.24 (18H, CH3 of the coordinated tBuNC).
13C NMR (CD2Cl2, 25 °C, δ): -205.3(s, R-Py), 112.9(s, â-Py), 12.4
(q, CH3), 701.4(s, meso), -84.4(s, ipso), 430.2(broad s, o), 155.8(d,
m), 142.6(d, p). The meso carbon signal was observable only when
the meso-13C enriched complex was used. The tert-butyl carbons of
the coordinated ligand gave a broad signal at 25.9 ppm due to the fast
exchange with the free ligand at 25 °C. The broad signal split into two
sets of quartets below -20 °C, where coordinated ligand showed signals
at δ 13.7 ppm.
â-CH2 × 2), 4.90(1H, â-vinyl), 4.66(1H, â-vinyl), 4.27(1H, â-vinyl),
4.07(1H, â-vinyl), 3.75(6H, OCH3 × 2), -45.04(1H, meso), -44.67(1H,
meso), -43.98(1H, meso), -1.33 (18H, CH3 of the coordinated tBuNC).
13C NMR (CD2Cl2, 25 °C, δ): -101(broad s, R-Py), -113(broad s,
t
R-Py), 396 (broad s, meso), 26.8 (q, CH3 of the coordinated BuNC).
Results
1
1H NMR Spectra. Table 1 shows the H NMR chemical
shifts of [Fe(TRP)L2](, [Fe(TPP)L2](, [Fe(F20-TPP)L2](, and
t
[Fe(ORTPP)L2]( (L ) HIm, CN-, and BuNC) taken at -50
°C in CD2Cl2 solution together with the labeling of the proton
atoms. The chemical shifts of [Fe(OEP)(tBuNC)2]+ and [Fe-
(ProtoIXMe2)(tBuNC)2]+ are also listed. Signal assignments of
these complexes were unambiguously done by the integral
intensities, coupling patterns, and 2D COSY technique. Figures
1a, 1b, and 1c show the 1H NMR spectra of [Fe(OMTPP)(CN)2]-,
[Fe(TiPrP)(tBuNC)2]+, and [Fe(OETPP)(tBuNC)2]+, respec-
tively, taken at 25 °C as typical examples. In contrast to the
case of [Fe(TiPrP)(tBuNC)2]+ shown in Figure 1b, the ligand
[Fe(OETPP)(tBuNC)2]ClO4. 1H NMR (CD2Cl2, 25 °C, δ): 7.46(16H,
CH2), 1.08(24H, CH3), 5.54(8H, o), 11.05(8H, m), 6.31(4H, p). The
tert-butyl protons of the coordinated ligand showed a broad signal at
25 °C, which split into two below 0 °C. At -20 °C, the coordinated
1
tBuNC gave a signal at -2.10 ppm with integral intensity of 18H. H
NMR (CD2Cl2, -20 °C, δ): 8.35(16H, CH2), 1.41(24H, CH3), 5.57(8H,
t
o), 11.42(8H, m), 6.30(4H, p), -2.10(18H, CH3 of the coordinated -
1
BuNC). 13C NMR (CD2Cl2, 25 °C, δ): -3.7(s, R-Py), 144.2(s, â-Py),
-0.5(t, CH2), 59.1(q, CH3), 417.5(s, meso), 38.0(s, ipso), 309.6(d, o),
145.2(d, m), 138.6(d, p). Although the methyl carbons of the coordinated
ligand did not show the separate signal due to the fast ligand exchange
at 25 °C, a new quartet assigned to these carbons appeared at 22.9
ppm at -50 °C.
exchange process is fast on the H NMR time scale in [Fe-
(OETPP)(tBuNC)2]+; a broad signal corresponding to the methyl
protons of the free and coordinated ligands appeared at ca. 1
ppm. Figure 1d shows the 1H NMR spectrum of [Fe(OETPP)-
(tBuNC)2]+ taken at -40 °C. The ligand exchange process is
frozen at this temperature; the tert-butyl signal of the coordinated
tBuNC is observed at -2.5 ppm with the integral intensity of
18 H. The methylene signal appears at 9.2 ppm as a very broad
singlet due to the ring inversion. This signal split into two peaks
below -60 °C and gave clearly separated signals at 5.68 and
18.40 ppm at -80 °C where the ring inversion process is frozen
on the 1H NMR time scale. The chemical shifts of the pyrrole-H
and pyrrole-CH(R) signals of these complexes are plotted against
[Fe(OEP)(tBuNC)2]ClO4. 1H NMR (CD2Cl2, 25 °C, δ): 7.61(16H,
CH2), 3.19(24H, CH3), -37.7(4H, meso), -0.81(18H, CH3 of the
t
coordinated BuNC). 13C NMR (CD2Cl2, 25 °C, δ): -94.9(s, R-Py),
115.9(s, â-Py), 1.7(t, CH2), 59.1(q, CH3), 371.4 (s, meso), 25.7(q, CH3
t
of the coordinated BuNC).
[Fe(ProtoIXMe2)(tBuNC)2]ClO4. 1H NMR (CD2Cl2, 25 °C, δ):
12.41(1H, R-vinyl), 11.91 (1H, R-vinyl), 11.44(3H, CH3), 11.21(3H,
CH3), 11.18(3H, CH3), 10.76(3H, CH3), 7.19(4H, R-CH2 × 2), 4.03(4H,

Low-Spin Iron(III) Porphyrin Complexes
Inorganic Chemistry, Vol. 40, No. 14, 2001 3427
Table 1. 1H NMR Chemical Shifts of [Fe(Porphyrin)L2]( Taken in CD2Cl2 Solution at -50 °C
pyrrole
py-HR
meso
aryl
m
porphyrins
TiPrP
TcPrP
L
py-H
0.11
-18.70
py-Hâ
meso
HR
Hâ
Hγ
o
p
tBu
ref
HIm
HIm
-
-
-
-
-
-
16.14
10.88
3.91
-1.52
-2.26
-
-
-
-
-
-
-
-
-
-
18
18
TnPrP
TPP
F20-TPP
OMTPP
OETPP
HIm
HIm
-21.45
-26.05
-
-
-
-
-
-
-
1.72
-
-1.47 -0.46
-
3.52
-
2.91
2.76
-
5.44
-
5.03
4.68
-
5.78
-
5.84
5.99
-
-
-
-
-
18
-
-
-
-
-
-
-
-
-
-
this worka
this work
this work
this work
1-MeImb -24.42
-
-
-
HIm
HIm
-
-
20.89
3.21
11.81
-
-
-
1.28
-
TiPrP
TcPrP
CN-
CN-
12.26
4.28
-
-
-
-
28.68
91.87
6.67
-
-
-
-
-
-
-
-
-
-
18
18
-
-0.15
-1.83
0.09
-
TnPrP
TPP
CN-
-3.48
-28.73
-32.51
-
-
-
-
30.91
-
0.82
-
2.12
-
3.56
3.90
-
-
-
-
4.93
-
6.12
6.54
-
-
-
-
4.96
-
5.84
6.30
-
-
-
-
-
18
CN-
-
-
-
-
-
-
-
-
-
this workc
this work
this work
this work
F20-TPP
OMTPP
OETPP
TiPrP
CN-
-
-
-
-
CN-
18.35
7.89
-
-
-
-
-
CN-
-
0.81
-
-
-
-
tBuNC
tBuNC
tBuNC
tBuNc
tBuNC
tBuNC
tBuNc
tBuNC
12.93
14.70
12.65
11.72
5.48
-
-
50.80
d
10.03
-4.23
-
-2.36 this work
-2.73 this work
-2.12 this work
TcPrP
-
-
-
-
-
TnPrP
TPP
-
-
-
118.50e -4.16
2.28
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-2.38 16.50
0.88 -2.86 this workf
F20-TPP
OMTPP
OETPP
OEP
-
-
-
-
-
-
-3.03 this work
4.24 -2.70 this work
6.43 -2.84 this work
-2.13
9.77g
7.40
i
-
-
0.99 14.99
5.84 11.54
-
1.36
3.74
i
-
-
-58.2
-70.4
-71.5
-
-
-
-
-
-1.32 this workh
-2.55 this work
ProtolXMe2 tBuNC
-
-
a Reported by Satterlee and La Mar, ref 57. b 1-MeIm was used instead of HIm due to the low solubility of the bis(HIm) complex. c Originally
reported by La Mar et al., ref 42. d Signals were not observed up to δ 500 ppm. e Extrapolated value. This signal disappeared below -20 °C.
f Originally reported by Simonneaux et al., ref 2. g Extrapolated value. Signals are too broad due to the ring inversion at -50 °C. At -80 °C, two
signals appeared at 5.68 and 18.40 ppm. h Originally reported by Walker et al., ref 6. i Ring CH3: 9.89, 9.62, 9.58, 9.41. Vinyl(R): 11.42, 10.96.
Vinyl(â): 6.25, 5.93, 5.39, 5.11. CH2(R): 6.62. CH2(â): 4.57; OCH3; 3.79.
inverse temperature. They are shown in the Supporting Informa-
tion.
enriched complexes are also given in the inset of each figure.
Figures 2a and 2b clearly show that both the R-pyrrole and ipso
carbon signals appear as doublets due to the coupling with the
adjacent meso carbons. A part of the 13C NMR spectrum of
[Fe(OETPP)(tBuNC)2]+ taken at -50 °C is also given in the
inset of Figure 2c, which clearly shows the methyl signal of
13C NMR Spectra. Table 2 shows the 13C NMR chemical
shifts of [Fe(TRP)L2](, [Fe(TPP)L2](, and [Fe(ORTPP)L2]( (L
) HIm, CN-, and tBuNC) taken at -50 °C in CD2Cl2 solution
together with the labeling of the carbon atoms. The chemical
shifts of [Fe(OEP)(tBuNC)2]+ and [Fe(ProtoIXMe2)(tBuNC)2]+
taken at the same temperature are also listed. Signals were
assigned on the basis of the acquisition of the proton-coupled
13C NMR spectra. The meso carbon signals in [Fe(ORTPP)-
L2]( and [Fe(TPP)L2]( were assigned by the use of the meso-
13C enriched complexes. The R-pyrrole and ipso-phenyl carbons
of [Fe(ORTPP)(HIm)2]+ and [Fe(ORTPP)(CN)2]- were con-
veniently assigned on the basis of their coupling with the
adjacent meso-carbons in the 13C NMR spectra of the meso-
13C enriched complexes. In the case of [Fe(TRP)(tBuNC)2]+,
only the â-pyrrole and the ligand methyl signals were observ-
able; other signals corresponding to the meso,meso-CR, meso-
Câ, and R-pyrrole carbons were too broad to detect. Figures
2a, 2b, and 2c show the proton coupled 13C NMR spectra of
[Fe(OETPP)(Him)2]+, [Fe(OMTPP)(CN)2]-, and [Fe(OETPP)-
(tBuNC)2]+, respectively, taken at 25 °C as typical examples.
The proton decoupled 13C NMR spectra of the meso-13C
t
the coordinated BuNC ligand at 22.9 ppm.
EPR Spectra. EPR spectra of [Fe(TRP)L2](, [Fe(TPP)L2](,
and [Fe(ORTPP)L2]( (L ) HIm, CN-, and tBuNC) were taken
at 4.2 K in frozen CH2Cl2 solution. Figure 3 shows the observed
(bottom) and calculated (top) spectra of [Fe(TnPrP)(tBuNC)2]+,
[Fe(OMTPP)(tBuNC)2]+, and [Fe(OETPP)(tBuNC)2]+ as typical
examples. In the case of [Fe(TRP)(tBuNC)2], the calculated
spectra fitted well to the observed ones with the use of
appropriate g and g values. The signal fitting was, however,
||
rather poor in the case of [Fe(OMTPP)(tBuNC)2]+ and [Fe-
+
t
(OETPP)(BuNC) 2] . Since the EPR spectra of these complexes
seemed to be rhombic rather than axial, the calculated spectra
were obtained with the use of three different g values as shown
in Figures 3b and 3c. The g values of these complexes thus
obtained are listed in Table 3 together with those of [Fe(OEP)-
(tBuNC)2]+ and [Fe(ProtoIXMe2)(tBuNC)2]+ taken at the same
temperature.

3428 Inorganic Chemistry, Vol. 40, No. 14, 2001
Ikeue et al.
Table 2. 13C NMR Chemical Shifts of [Fe(Porphyrin)L2]( Taken in CD2Cl2 Solution at -50 °C
pyrrole
py(â)
meso
CR(ipso)
aryl
m
porphyrins
L
py(R)
RR
Râ
meso
Câ
Cγ
o
p
ref
TiPrP
HIm
HIm
HIm
HIm
HIm
HIm
-28.3
11.8
0.0
76.5
79.6
73.6
77.1
-
-
-
-
-
-
-
-
-
331.6
97.1
73.1
25.2
-11.5
-55.3
-6.2
172.5
17.5
64.5 12.4
-
-
-
-
-
-
-
-
-
-
-
18
18
18
TcPrP
TnPrP
TPP
14.5
-2.0
81.2
143.5
-186.0
-84.2
-72.7
18.2
81.2
128.8
c
(136.9)
(151.0)
(165.8)
-134.5
-98.9
-56.6
(122.9)
(127.6)
(147.1)
c
-
-
-
-
-
-
122.9 125.5 123.3 this work
108.3 122.1 123.9 this work
97.1 122.3 122.6 this work
OMTPP
OETPP
TiPrP
145.6 -50.3
156.1 -38.8 104.8 -37.2
-
-
CN-
CN-
CN-
CN-
CN-
CN-
tBuNC
tBuNC
tBuNC
54.7
61.2
61.2
86.2
131.6 -43.0
142.4 -34.3
98.8
91.0
84.7
60.0
90.2
-
-
-
-
-
-
639.6
386.7
336.1
65.1
87.4
45.1
c
309.4
91.4
-
-
-
-
-
-
-
-
-
18
18
18
TcPrP
TnPrP
TPP
-
249.7 17.9
-
-
-
-
c
c
c
-
-
-
-
-
-
148.0 122.5 125.0 this worka
151.8 128.9 125.4 this work
132.5 125.7 125.1 this work
OMTPP
OETPP
TiPrPb
TcPrPb
TnPrPb
TPP
-
-
95.7
-
-
-
-
-
-
-
-
-
-
-
-
-
-
this work
this work
this work
c
c
-
c
c
c
c
-
-
-
29.5
-
tBuNC -429.3
tBuNC -376.5d
-
-
997.3d (-223.6)
579.6 149.3 166.5 this work
568.8 169.9 146.3 this work
323.7 146.4 137.4 this work
-
-
OMTPP
OETPP
OEP
18.9
-
979.4d (-189.8)
-
tBuNC
13.3
136.4 -11.0
96.4
e
77.7
58.6
e
416.2
491.1
396b
(48.0)
-
-
-
tBuNC -187.4
1.7
e
-
-
-
-
-
this work
this work
ProtoIXMe2 tBuNC -100.9b
-
-117.8b
a Originally reported by Wu¨thrich and Baumann, ref 47. b Data at 25 °C. c Signals are too broad to detect. d Extrapolated from high temperature.
e Assignment is not clear.
Table 3. EPR g Values of [Fe(Porphyrin)L2]( Taken in Frozen
CH2Cl2 Solution at 4.2 K Together with the Ground State Electron
Configuration
configuration, we have examined the solution structure of the
nonplanar porphyrin complexes. When the porphyrin ring is
highly deformed, the planar ligands such as imidazole and
porphyrins
L
g1
g2
g3
ground statea
ref
pyridine tend to orient themselves along the cavities formed by
the nonplanar porphyrin ring.11,44 In the previous papers, we
reported that the rotation of the coordinated 2-MeIm ligand in
[Fe(TiPrP)(2-MeIm)2]+ is frozen on the 1H NMR time scale at
low temperature to show four pyrrole signals, two isopropyl
methine signals, and four methyl signals.12,18 Correspondingly,
the 13C NMR spectrum of the same complex exhibited four
signals for the â-pyrrole carbon and two signals for the meso
carbon as shown in Figure 4a; the R-pyrrole carbon also showed
four signals between -6 and -90 ppm at -20 °C. These results
suggest that the planar 2-MeIm ligands are fixed perpendicularly
along the C(meso)-Fe-C(meso) axes, which in turn suggests
that the porphyrin core in [Fe(TiPrP)L2]( is S4-ruffled and
creates the cavities along these axes in solution as shown in
Figure 5a.12 The result is consistent with the crystal structures
of [Fe(TiPrP)(THF)2]ClO4, in which porphyrin rings are highly
S4-ruffled.35
TiPrP
HIm
HIm
HIm
HIm
HIm
HIm
CN-
CN-
CN-
CN-
CN-
CN-
2.55 2.55
2.87 2.42
2.90 2.35 (1.45)
2.87 2.29 1.56
2.84 2.31 1.58
2.72 2.37 1.64
2.42 2.42 1.74
-
-
dxy
dπ
dπ
dπ
dπ
18
18
18
56
this work
this work
18
18
18
73
this work
this work
this work
this work
this work
this workb
this work
this work
this work
this workc
this work
TcPrP
TnPrP
TPP
OMTPP
OETPP
TiPrP
dπ
dxy
dxy
dxy
dπ
dπ
dπ
dxy
dxy
dxy
dxy
dxy
dxy
dxy
dxy
dxy
TcPrP
TnPrP
TPP
2.47 2.47
2.49 2.49
-
-
-
-
-
3.70
3.48
3.31
-
-
-
OMTPP
OETPP
TiPrP
tBuNC 2.16 2.16 1.96
tBuNC 2.16 2.16 1.95
tBuNC 2.16 2.16 1.95
tBuNC 2.18 2.18 1.93
tBuNC 2.31 2.31 1.86
tBuNC 2.20 2.17 1.95
tBuNC 2.29 2.25 1.92
tBuNC 2.29 2.29 1.86
TcPrP
TnPrP
TPP
F20-TPP
OMTPP
OETPP
OEP
The porphyrin cores in [Fe(ORTPP)L2]( are supposed to be
S4-saddled in solution, since the crystallographic analysis of the
free bases H2(ORTPP) as well as their metal complexes always
shows the strongly S4-saddled porphyrin core.25-28 In fact,
Medforth and co-workers observed four signals for the methyl
and o-phenyl protons in [Co(OETPP)(3-PhPy)2]+.45 The result
was interpreted in terms of the mutually perpendicular alignment
ProtoIXMe2 tBuNC 2.30 2.30 1.86
a The dxy and dπ stand for the ground state with the (dxz, dyz)4(dxy)1
and (dxy)2(dxz, dyz)3 electron configurations, respectively. b Originally
reported by Walker et al. at 77 K in frozen CH2Cl2; g1 ) g2 ) 2.21, g3
) 1.93, ref 6. c Originally reported by Walker et al. at 77 K in frozen
CH2Cl2; g1 ) g2 ) 2.28, g3 ) 1.83, ref 6.
Discussion
(44) Walker, F. A.; Simonis, U. J. Am. Chem. Soc. 1991, 113, 8652-8657.
(45) Medforth, C. J.; Muzzi, C. M.; Shea, K. M.; Smith, K. M.; Abraham,
R. J.; Jia, S.; Shelnutt, J. A. J. Chem. Soc., Perkin Trans. 2 1997,
833-837.
Solution Structure of [Fe(TRP)L2]( and [Fe(ORTPP)L2](.
Before discussing the factors that affect the ground state electron

Low-Spin Iron(III) Porphyrin Complexes
Inorganic Chemistry, Vol. 40, No. 14, 2001 3429
Figure 4. Temperature dependent 13C NMR spectra of (a) [Fe(T PrP)-
i
(2-MeIm) 2]+ 18 and (b) [Fe(OETPP)(2-MeIm) 2]+ (meso-13C enriched)
taken in CD2Cl2 solution.
Figure 2. 13C NMR spectra of (a) [Fe(OETPP)(HIm)2]+, (b)
[Fe(OMTPP)(CN)2]-, and (c) [Fe(OETPP)(tBuNC)2]+ taken in CD2-
Cl2 at 25 °C without decoupling of the proton region. 13C NMR spectra
of the meso 13C enriched complexes taken under the broad band
decoupling of the proton region are given in the insets of (a), (b), and
(c). A part of the spectrum of [Fe(OETPP)(tBuNC)2]+ taken at -50
°C is also given in the inset of (c). Signal assignment: o, m, p, and ip,
ortho-, meta-, para-, and ipso-carbons; py(R) and py(â), R- and
â-carbons of the pyrrole ring; L and F, methyl signal of the coordinated
and free tBuNC ligand, respectively; s, dichloromethane; x, tetrabutyl-
ammonium ion. Note that both the py(R) and ipso(ip) carbon signals
split into two signals in the meso 13C enriched complexes as shown in
the insets of (a) and (b).
Figure 5. Orientation of the axially coordinated 2-MeIm ligands in
(a) [Fe(TiPrP)(2-MeIm)2]+ and (b) [Fe(OETPP)(2-MeIm) ]+.
2
the porphyrin cores in [Fe(ORTPP)L2]( also maintain the
strongly saddled structure in solution. To determine the por-
phyrin structure in solution, the NMR spectra of [Fe(OETPP)-
(2-MeIm)2]+ have been examined at low temperature. Although
1
the H NMR spectra of this complex showed the broadening
and complicated splitting of the methyl, methylene, and phenyl
signals below -80 °C due to the slow rotation of the coordinated
2-MeIm ligands, it was difficult to assign these signals under
the present conditions (300 MHz NMR, -80 °C). Thus, we
have examined the ligand orientation by means of the 13C NMR
spectra of meso-13C enriched [Fe(OETPP)(2-MeIm)2]+. Figure
4b shows the 13C NMR spectra of the meso carbon region. The
meso signal appeared at δ -79.6 ppm at -20 °C, which
broadened as the temperature was lowered and split into three
signals below -60 °C. At -70 °C, three clearly separated
signals were observed at δ ) -0.6, -27.5, and -57.5 ppm
with the intensity ratios of 1:2:1, respectively. The result
indicates that the axial ligands are fixed along the diagonal
N-Fe-N axes on the 13C NMR time scale as shown in Figure
5b, which in turn suggests that the strongly S4-saddled structure
is maintained in solution.25-28 It should be noted that the
hindered rotation of the 2-MeIm ligand was not observed even
at -100 °C in the case of meso-13C enriched [Fe(OMTPP)(2-
MeIm)2]+. It seems to be strange that the rotational barrier of
the 2-MeIm ligand in [Fe(OETPP)(2-MeIm)2]+ is greatly
different from that in [Fe(OMTPP)(2-MeIm)2]+ despite the
similar degree of nonplanarity expected for both complexes;
the deviations of the py(â) carbon atoms from the average
porphyrin plane in [Fe(OMTPP)Cl] and [Fe(OETPP)Cl] are 1.05
( 0.10 Å and 1.15 ( 0.11 Å, respectivley.28 The results could
be explained if we assume the difference in rigidity of these
complexes. In the ligand rotation process, the conformational
Figure 3. EPR spectra of (a) [Fe(TnPrP)(tBuNC) 2]+, (b) [Fe(OMTPP)-
(BuNC) 2] , and (c) [Fe(OETPP)( BuNC) 2] . Bottom: observed spectra
taken in frozen CH2Cl2 solution. Top: simulated spectra.
+
t
+
t
of the 3-PhPy ligands along the N-Co-N axes above and below
the porphyrin ring. Thus, the porphyrin core in [Co(OETPP)-
(3-PhPy)2]+ maintains the S4-saddled structure in solution and
creates the cavities along these axes. It is then expected that

3430 Inorganic Chemistry, Vol. 40, No. 14, 2001
Ikeue et al.
change of the nonplanar porphyrin ring should take place
concomitantly to minimize the ligand-porphyrin repulsion.46
Thus, the high barrier to ligand rotation in the OETPP complex
relative to that in the OMTPP complex strongly suggests that
the former is much more rigid than the latter. As discussed later,
the rigidity of porphyrin ring plays an important role to
determine the electron configuration.
information on the spin distribution of any porphyrin carbons
including meso, py(R), and py(â) regardless of the substituent
positions at the porphyrin periphery. In fact, we have established
that not only the meso carbon but also the py(R) carbon signals
appear at the characteristic positions in the (dxz, dyz)4(dxy)1 type
complexes; they appear at extremely downfield and upfield
positions, respectively.11,14,18 In the case of [Fe(TiPrP)(4-
CNPy)2]+, for example, the meso and py(R) carbon signals were
observed at 918 and -295 ppm at -50 °C, respectively.18 The
large upfield shift of the py(R) carbon signal can be explained
in terms of the spin polarization effect from the meso carbon
and pyrrole nitrogen atoms; these atoms have the largest spin
densities in the (dxz, dyz)4(dxy)1 type complexes. More impor-
tantly, we can compare the contribution of the (dxz, dyz)4(dxy)1
state to the electronic ground state in a wide variety of
complexes such as TRP, TPP, ORTPP, OEP, and ProtoIXMe2
on the basis of the chemical shifts of these carbon atoms; the
comparison of the 1H NMR chemical shifts is sometimes
hampered because some complexes have no protons at the meso
and/or â-pyrrole positions.
Spectroscopic Methods to Determine the Electronic Ground
State. We have already reported that, while [Fe(THP)(CN)2]-
and [Fe(TPP)(CN) 2]- have the common (dxy)2(dxz, dyz)3 ground
i
c
n
state,13,43 [Fe(TRP)(CN)2]- (R ) Pr, Pr, and Pr) adopt the
less common (dxz, dyz)4(dxy)1 ground state.13,18 The difference
in the electronic ground states between [Fe(THP)(CN)2]- and
[Fe(TRP)(CN)2]- is mainly ascribed to the presence of the
S4-ruffled porphyrin cores in the latter complexes;13 the S4-
ruffling makes the interaction between the iron dxy and porphyrin
a2u orbitals possible and raises the energy level of the dxy orbital
as Walker, Debrunner, Scheidt, and co-workers pointed out.4
This mode of porphyrin deformation, therefore, stabilizes the
(dxz, dyz)4(dxy)1 state relative to the (dxy)2(dxz, dyz)3 state. The
difference in orbital interactions in the (dxy)2(dxz, dyz)3 and
(dxz, dyz)4(dxy)1 type complexes could induce a large change in
spin densities at the porphyrin carbon and nitrogen atoms. In
fact, our recent studies using [Fe(TRP)L2]( have revealed that
the major spin densities in the (dxz, dyz)4(dxy)1 type complexes
are at the meso carbon and pyrrole nitrogen atoms; these atoms
have 0.045 and 0.057 electrons, respectively, in the case of
[Fe(TiPrP)(4-CNPy)2]+.18 The large spin densities at the
meso carbon atoms is one of the characteristic features of the
(dxz, dyz)4(dxy)1 type complexes; the recent spectroscopic studies
have revealed that not only the meso-substituted complexes such
as [Fe(TRP)(4-CNPy)2]+ and [Fe(TArP)(CN)2]-,18,43 but also
the meso-unsubstituted [Fe(OEP)(tBuNC)2]+, have large spin
densities at the meso carbon atoms.6 Thus, the spin distribution
in the (dxz, dyz)4(dxy)1 type complexes differs from that in the
(dxy)2(dxz, dyz)3 type complexes where the major spin densities
are at the â-pyrrole carbon atoms.47-52
The ground-state electron configuration determined on the
basis of the 13C NMR and 1H NMR spectra was further
confirmed by the types of the EPR spectra; the axial type EPR
spectra correspond to the complexes with the (dxz, dyz)4(dxy)1
ground state while the large gmax or rhombic type spectra to
those with the (dxy)2(dxz, dyz)3 ground state.53,54
Ground-State Electron Configuration. As mentioned in the
Introduction, the axial ligands examined in this study are typical
bases to form the low-spin Fe(III) porphyrin complexes. The
electronic effect of these ligands on the Fe(III) ion is, however,
quite different. Although imidazole and cyanide are strong
σ-donors, the π-accepting abilities of these ligands are different;
cyanide is a much stronger π-acceptor than imidazole.55,56 In
contrast, tert-butyl isocyanide is classified as a weak σ-donor
1
and a strong π-acceptor.2,6 We have measured the H NMR,
13C NMR, and EPR spectra of the complexes having these
ligands at the axial positions and examined how the ground state
of the low-spin complexes changes depending upon the struc-
tural and electronic differences of the porphyrin ring.
The difference in spin distribution results in the extremely
1
different NMR spectra. The H NMR characteristics of the
(dxz, dyz)4(dxy)1 type complexes as compared with those of the
(dxy)2(dxz, dyz)3 type complexes have been extensively studied.
They are summarized as follows:1-4,6,8,9,11-18,43 (i) the down-
field shift of the meso-CH and meta-H signals in TRP and
TArP complexes, respectively, (ii) the upfield shift of the
meso-H in OEP complexes, and (iii) the downfield shifts of
the py-H signals. While the characteristics (i) and (ii) are
originated from the large spin densities at the meso carbons,
(iii) is ascribed to the smaller spin densities at the py(â) carbons
in the (dxz, dyz)4(dxy)1 type complexes. The sign reversal in
dipolar shift also contributes to the downfield shift of the py-H
signals.1,18
(i) Bis(imidazole) Complexes. (a) [Fe(TRP)(HIm)2]+ (R )
c
n
iPr, Pr, Pr). The electronic ground state of these complexes
has been reported in our previous paper on the basis of the NMR
and EPR spectra.18 Only the isopropyl complex [Fe(TiPrP)-
c
(HIm)2]+ adopts the (dxz, dyz)4(dxy)1 ground state; the Pr and
nPr complexes showed the common (dxy)2(dxz, dyz)3 ground state.
It should be noted that the electronic effects of the propyl,
cyclopropyl, and isopropyl groups are considered to be quite
similar. Thus, the difference in the electron configuration among
three complexes should be ascribed to the degree of nonplanarity
of the porphyrin rings. That is, the S4-ruffling of the porphyrin
structure affects the electron configuration and stabilizes the
(dxz, dyz)4(dxy)1 state.
In principle, 13C NMR chemical shifts can be a better probe
to determine the electronic ground state, because they give the
(b) [Fe(TPP)(HIm)2]+. This complex shows the (dxy)2(dxz, dyz)3
ground state as revealed from the large upfield shift of the
pyrrole proton signal as well as the rhombic EPR spec-
trum.53,57,58
(46) Saitoh, T.; Ikeu, T.; Ohgo, Y.; Nakamura, M. Tetrahedron 1997, 53,
12487-12496.
(47) Wu¨thrich, K.; Baumann, R. HelV. Chim. Acta 1973, 56, 585-596.
(48) Wu¨thrich, K.; Baumann, R. HelV. Chim. Acta 1974, 57, 336-350.
(49) La Mar, G. N.; Viscio, D. B.; Smith, K. M.; Caughey, W. S.; Smith,
M. L. J. Am. Chem. Soc. 1978, 100, 8085-8092.
(50) Goff, H. M. J. Am. Chem. Soc. 1981, 103, 3714-3722.
(51) Goff, H. M. Nuclear Magnetic Resonance of Iron Porphyrins. In Iron
Porphyrin, I; Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley:
Reading, MA, 1983; Physical Bioinorganic Chemistry Series 1, pp
237-281.
(52) Bertini, I.; Luchinat, C. NMR of Paramagnetic Substances. In
Coordination Chemistry ReViews 150; Lever, A. B. P., Ed.; Elsevier:
Amsterdam, 1996; pp 29-75.
(53) Walker, F. A.; Reis, D.; Balke, V. L. J. Am. Chem. Soc. 1984, 106,
6888-6898.
(54) Walker, F. A.; Reis, D.; Balke, V. L. J. Am. Chem. Soc. 1986, 108,
5288-5297.
(55) Shriver, D. F.; Atkins, P. W.; Langford, C. H. In Inorganic Chemistry,
2nd ed.; Oxford University Press: Oxford, 1994; pp 246-261.
(56) Frye, J. S.; La Mar, G. N. J. Am. Chem. Soc. 1975, 97, 3561-3562.
(57) Satterlee, J. D.; La Mar, G. N. J. Am. Chem. Soc. 1976, 98, 2804-
2808.

Low-Spin Iron(III) Porphyrin Complexes
Inorganic Chemistry, Vol. 40, No. 14, 2001 3431
(c) [Fe(ORTPP)(HIm)2]+ (R ) Me, Et). Table 1 shows the
existence of the downfield shifted methyl and methylene signals
in [Fe(OMTPP)(HIm)2]+ and [Fe(OETPP)(HIm)2]+, respec-
tively. The results suggest that the â-pyrrole carbons have a
considerable amount of spin densities, which is one of the char-
acteristic features of the complexes having the (dxy)2(dxz, dyz)3
ground state.50 Similarity of the chemical shifts of the phenyl
protons in [Fe(ORTPP)(HIm)2]+ and [Fe(TPP)(HIm)2]+ also
supports the (dxy)2(dxz, dyz)3 ground state. The assignment was
further confirmed by the EPR spectra; both complexes exhibited
the rhombic spectra as in the case of [Fe(TPP)(HIm)2]+.53,58
However, the 13C NMR chemical shifts in Table 2 show some
deviations from those of [Fe(TPP)(HIm)2]+. The major differ-
ence lies in the chemical shifts of the py(R) and py(â) carbon
atoms. While the chemical shifts of the py(R) and py(â) of
[Fe(TPP)(HIm)2]+ were -2.0 and 77.1 ppm at -50 °C,
respectively, those of the corresponding carbons in [Fe(OETPP)-
(HIm)2]+ were 143.5 and 156.1 ppm. The chemical shifts of
the meso carbon atoms also showed some difference; they were
25.2 and -37.2 ppm for [Fe(TPP)(HIm)2]+ and [Fe(OETPP)-
(HIm)2]+, respectively. The results might be the indication that
the iron(dπ) and porphyrin(pπ) interactions are perturbed by the
S4-saddled deformation of the porphyrin ring, which will be
discussed in the latter part of this paper.
downfield on going from [Fe(TRP)(HIm)2]+ to [Fe(TRP)(CN)2]-
and then to [Fe(TRP)(tBuNC)2]+. In the case of the nPr
complexes, the chemical shift of the meso R-proton changed
from 1.72 to 30.91 and then to 118.5 ppm at -50 °C. The results
suggest that the spin densities at the meso carbons have increased
t
to a great extent as the axial HIm is replaced by BuNC. As
mentioned, the 13C NMR spectra of a series of [Fe(TRP)-
(tBuNC)2]+ showed neither the meso carbon nor the py(R)
carbon signals even at 25 °C. The results indicate that both the
meso and py(R) carbons have very short relaxation times, which
must be ascribed to the fairly large spin densities at the meso
carbon and pyrrole nitrogen atoms.18 The EPR spectra of these
complexes showed clear axial type signals. Figure 3a demon-
strates the EPR spectrum of [Fe(TnPrP)(tBuNC)2]+ as a typical
example. The g values were determined to be g ) 2.16 and g
||
1
) 1.95. On the basis of the H NMR, 13C NMR, and EPR
n
c
results, it is clear that the [Fe(TRP)(tBuNC)2]+ (R ) Pr, Pr,
or iPr) complexes have a very pure (dxz, dyz)4(dxy)1 ground state.
(b) [Fe(TPP)(tBuNC)2]+. The ground-state electron con-
figuration of this complex was already determined as
(dxz, dyz)4(dxy)1.2,6 The coordination of the weak axial ligand
stabilizes the iron dπ(dxz, dyz) orbitals to the point lower than
or equal to the dxy orbital.6,7 In this situation, if the normally
planar TPP ring deforms in a S4-ruffled fashion, further
stabilization can be obtained by the iron dxy and porphyrin a2u
interaction, inducing the less common (dxz, dyz)4(dxy)1 config-
uration. In fact, the porphyrin core of this complex is reported
to be strongly S4-ruffled as revealed by the X-ray crystal-
lographic analysis.6 The 13C NMR data listed in Table 2 also
support this conclusion; the chemical shifts of the meso and
R-pyrrole carbons were estimated to be 997 and -429 ppm,
respectively, at -50 °C.
(c) [Fe(ORTPP)(tBuNC)2]+ (R ) Me, Et). The similarity
of the phenyl proton chemical shifts in [Fe(OMTPP)(tBuNC)2]+
and [Fe(TPP)(tBuNC)2]+ suggests that the ground-state electron
configuration of the former complex is also represented as
(dxz, dyz)4(dxy)1. The conclusion is further supported by the
presence of the extremely downfield and upfield shifted meso
and py(R) carbon signals, respectively; these signals appeared
at 979 and -377 ppm at -50 °C. The 13C NMR spectra of
[Fe(OETPP)(tBuNC)2]+ also showed the downfield and upfield
shifted meso and py(R) signals, respectively. The chemical shifts
of these signals are, however, markedly different from those
of [Fe(OMTPP)(tBuNC)2]+; the meso and the py(R) signals
were observed at 416 and 13.3 ppm, respectively, at -50 °C.
These results suggest that the spin densities at the meso carbon
atoms in [Fe(OETPP)(tBuNC)2]+ are much smaller than those
of the analogous [Fe(OMTPP)(tBuNC)2]+. On the basis of the
NMR results, the following two points have become clear for
[Fe(OMTPP)(tBuNC)2]+ and [Fe(OETPP)(tBuNC)2]+: (i) the
ground states of both complexes are represented mainly as the
(dxz, dyz)4(dxy)1 state and (ii) the contribution of the (dxz, dyz)4(dxy)1
state to the electronic ground state is much larger in [Fe-
(OMTPP)(tBuNC)2]+ than in [Fe(OETPP)(tBuNC)2]+. The as-
signment of the ground-state electron configuration of all the
complexes examined in this study is listed in Table 3, where
the dxy and dπ stand for the (dxz, dyz)4(dxy)1 and (dxy)2(dxz, dyz)3
ground states, respectively.
(ii) Bis(cyanide) Complexes. (a) [Fe(TRP)(CN)2]- (R )
c
iPr, Pr, nPr). Ground-state electron configuration of these
complexes has already been determined as (dxz, dyz)4(dxy)1 in
our previous papers.13,18 The chemical shifts of the pyrrole
proton and the meso carbon atoms in these complexes clearly
indicate that the contribution of the (dxz, dyz)4(dxy)1 state to
the electronic ground state increases in the following order:
[Fe(TnPrP)(CN)2]- e [Fe(TcPrP)(CN)2]- < [Fe(TiPrP)(CN)2]-.
Since the electronic effects of the alkyl groups on the porphyrin
ring are considered to be quite similar, the increase in the
(dxz, dyz)4(dxy)1 contribution should again be ascribed to the
degree of S4-ruffling of the porphyrin ring.
(b) [Fe(TPP)(CN)2]-. Electron configuration of this complex
1
was determined to be (dxy)2(dxz, dyz)3 on the basis of the H
NMR, 13C NMR, and EPR spectroscopic data.42,43
(c) [Fe(ORTPP)(CN)2]- (R ) Me, Et). The 1H NMR
chemicals shifts in Table 1 indicate that both [Fe(OMTPP)(CN)2]-
and [Fe(OETPP)(CN)2]- adopt the common (dxy)2(dxz, dyz)3
ground state. The large downfield shifts of the methyl and
methylene signals in these complexes correspond to the large
upfield shift of the pyrrole protons in [Fe(TPP)(CN)2]-, sug-
gesting that the major spin densities are at the â-pyrrole carbon
atoms. The 13C NMR chemical shifts in Table 2 also support
this configuration because the meso carbon signals appeared
rather upfield, 87.4 and 45.1 ppm, respectively. The large gmax
-
type EPR spectra observed for these complexes are the direct
evidence for the (dxy)2(dxz, dyz)3 configuration; the g values are
3.48 and 3.31 as listed in Table 3. The downfield shifts of the
R- and â-pyrrole carbon signals were again observed in
[Fe(OETPP)(CN)2]-; the chemical shifts were 128.8 and 142.4
ppm, respectively, as compared with 18.2 and 86.2 ppm in
[Fe(TPP)(CN)2]-.
(iii) Bis(tert-butyl isocyanide) Complexes. (a) [Fe(TRP)-
(tBuNC)2]+ (R ) iPr, cPr, nPr). The low-spin Fe(III) complexes
consisting of the S4-ruffled porphyrin ring and the weak axial
ligands with low lying pπ* orbitals are expected to have a very
pure (dxz, dyz)4(dxy)1 ground state. In fact, the data in Table 1
indicate that both the pyrrole and meso R-proton signals moved
Factors Affecting the Electronic Ground State. (i) Axial
Ligands. Effects of axial ligands on the ground state electron
configuration of the low-spin Fe(III) porphyrin complexes have
been studied extensively in [Fe(TPP)L2]+ and [Fe(TRP)L2]+.3,18
The present results are consistent with the previous ones in the
sense that the contribution of the (dxz, dyz)4(dxy)1 state increases
(58) La Mar, G. N.; Walker, F. A. J. Am. Chem. Soc. 1973, 95, 1782-
1790.

3432 Inorganic Chemistry, Vol. 40, No. 14, 2001
Ikeue et al.
on going from [Fe(Porphyrin)(Him)2]+ to [Fe(Porphyrin)(CN)2]-
and then to [Fe(Porphyrin)(tBuNC)2]+. In other words, the
(dxz, dyz)4(dxy)1 character increases as the σ-donating ability of
ligands is weakened and π-accepting ability is strengthened.
(ii) Deformation Mode of Porphyrins. We have reported
that the electron configuration of [Fe(TRP)(CN)2]- changes from
(dxy)2(dxz, dyz)3 to (dxz, dyz)4(dxy)1 as the bulkiness of alkyl
substituents increases. Since the electronic effects of these
groups on the porphyrin ring are considered to be quite similar,
the result has been ascribed to the increase in magnitude of the
S4-ruffled structure.13
we conclude that the low-spin iron(III) complexes with the
saddle shaped structure resist to switch the electronic ground
state from the common (dxy)2(dxz, dyz)3 to the less common
(dxz, dyz)4(dxy)1.60,61
The conclusion mentioned above can be explained as follows.
In the S4 saddled complexes, the a2u-dxy interaction is weakened
as compared with that in the S4 ruffled ones. In contrast, the
3eg-dπ interaction is expected to be strengthened due to the
short metal-nitrogen bond lengths27,28 as well as the efficient
overlap between the 3eg and dπ orbitals. The large downfield
shifts of the R- and â-pyrrole signals observed in the saddled
complexes could be ascribed to this interaction since the 3eg
orbitals have electron densities at the R- and â-pyrrole carbon
atoms. Thus, the (dxy)2(dxz, dyz)3 state is stabilized relative to
the (dxz, dyz)4(dxy)1 state as the porphyrin ring changes from
the planar to the saddled structure.
It is much more difficult to elucidate the effect of S4-saddled
structure on the electronic ground state. This is because both
geometry and electronic structure of porphyrin ring change on
going from the planar TPP to the S4-saddled ORTPP ligand.
Thus, we have to find a suitable model complex in which
porphyrin ring is S4-saddled, while the electronic structure of
porphyrin is similar to that of the planar complex. Although it
is very difficult to find such model complexes, we can extract
some information from the 13C NMR chemical shifts of [Fe-
(TPP)(tBuNC)2]+, [Fe(OMTPP)(tBuNC)2]+, and [Fe(OETPP)-
(tBuNC)2]+ listed in Table 2. The chemical shifts of the py(R),
py(â), and meso carbons showed only a small change on going
from [Fe(TPP)(tBuNC)2]+ to [Fe(OMTPP)(tBuNC)2]+; they are
-429, 60, 997 ppm in the former and -377, 90, 979 ppm in
the latter complex, respectively. The introduction of the eight
methyl groups at the â-pyrrole carbons apparently gives only a
little influence on the chemical shifts of the porphyrin carbon
atoms. The similarity of the 13C NMR chemical shifts between
[Fe(TPP)(tBuNC)2]+ and [Fe(OMTPP)(tBuNC)2]+ should be
considered, however, to be the result of the two different
factors: (i) change in porphyrin structure caused by the steric
effects of the eight methyl groups and (ii) change in electronic
structure caused by the electron donation of the eight methyl
groups. Surprisingly, the introduction of the eight ethyl groups
changed the chemical shifts of the py(R), py(â), and meso
carbons to a great extent; they are 13, 136, and 416 ppm,
respectively, in [Fe(OETPP)(tBuNC)2]+. Since the electron
donating ability of ethyl group is similar to that of methyl group,
the large difference in chemical shifts observed between [Fe-
(OMTPP)(tBuNC)2]+ and [Fe(OETPP)(tBuNC)2]+ is caused by
the difference in steric effects of these groups. We ascribe the
results to the rigidity of porphyrin ring in [Fe(OETPP)-
(tBuNC)2]+ as compared with that in [Fe(OMTPP)(tBuNC)2]+.
In both complexes, the (dxz, dyz)4(dxy)1 state is stabilized by the
a2u-dxy interaction if the porphyrin ring could deform in a S4
ruffled fashion. However, the OETPP core is much more rigid
than the OMTPP core as revealed from the high barrier to
rotation of the coordinated 2-MeIm ligand; the meso carbons
of [Fe(OETPP)(2-MeIm)2]+ showed three signals below -50
°C as shown in Figure 4b, while those of [Fe(OMTPP)(2-
MeIm)2]+ maintained singlet even at -100 °C. The rigidity of
the OETPP core is also reflected on the barrier to inversion of
the deformed porphyrin ring; the inversion barrier of [Fe-
(OETPP)Cl] is estimated to be 15.7-16.0 kcal mol-1 as
compared with 10.1-10.5 kcal mol-1 in [Fe(OMTPP)Cl].28,59
Thus, the contribution of the (dxz, dyz)4(dxy)1 state on the
electronic ground state is smaller in [Fe(OETPP)(tBuNC)2]+ than
in [Fe(OMTPP)(tBuNC)2]+ since much more energy is required
for the ruffling of the OETPP core. Consequently, the isotropic
shifts of the py(R) and meso carbon signals are smaller than
those of [Fe(OMTPP)(tBuNC)2]+. On the basis of these results,
t
As is well-known, BuNC is the ligand to stabilize the
(dxz, dyz)4(dxy)1 ground state.2,6 In fact, all the complexes
examined in this study showed the (dxz, dyz)4(dxy)1 ground
state. On the basis of the EPR g values listed in Table 3, the
energy parameters ∆ and V were calculated. In the case of [Fe-
(TnPrP)(tBuNC)2]+, the ∆ and V values were -10.7 λ and 0.0
λ, respectively, in units of the spin-orbit coupling constant
λ.62-64 The results indicate that the dxy orbital is located 10.7 λ
above the degenerated dxz and dyz orbitals. In the case of
[Fe(TiPrP)(tBuNC)2]+, the |∆| value reached as much as 11.3
λ, which is one of the largest values ever reported for the low-
spin Fe(III) porphyrin complexes.65-67 Figure 6 shows the
relative energy differences among three d orbitals. Thus, the
contribution of the (dxz, dyz)4(dxy)1 state increases in the
following order at 4.2 K in the case of the bis(tBuNC)
complexes:
OETPP < OMTPP, TPP < TnPrP, TcPrP, TiPrP
Similar order is established on the basis of the 13C NMR meso
shifts. The 13C NMR data reported in this and in our previous
papers indicate that the complexes showing the meso-carbon
signals below 300 ppm, δ > 300, exhibit the axial type EPR
spectra though the temperatures examined are greatly different
between two methods: 4.2 K in EPR vs 223 K in 13C NMR.18,43
The result could be the indication that, when the (dxz, dyz)4(dxy)1
contribution exceeds a threshold value, e.g., δ 300 ppm for
(60) One of the reviewers pointed out that saddling is invariably ac-
companied by ruffling of the porphyrin core as in the case of [Fe-
(OMTPP)Cl] and [Fe(OETPP)Cl],28 and that it is not possible to
separate the effects of saddling and ruffling. However, our preliminary
result on the X-ray crystallographic analysis of [Fe(OETPP)Py2]ClO4
has revealed that the OETPP ring is essentially saddle shaped; the
rotation of the pyrrole planes with respect to the mean porphyrin plane
are at most 3°.61 Obviously, an extensive crystallographic study
including the structural comparison of [Fe(OMTPP)L2]ClO4 and [Fe-
(OETPP)L2]ClO4 for various axial ligands (L’s) is necessary for the
better understanding of the effect of the deformation mode of porphyrin
ring on the electronic ground state, which is now in progress in this
laboratory.
(61) Ohgo, Y.; Ikeue, T.; Nakamura, M. To be published.
(62) Taylor, C. P. S. Biochim. Biophys. Acta 1977, 491, 137-149.
(63) Bohan, T. L. J. Magn. Reson. 1977, 26, 109-118.
(64) Palmer, G. Electron Paramagnetic Resonance of Hemoproteins. In Iron
Porphyrins, Part II.; Lever, A. B. P., Gray, H. B., Eds.; Addison-
Wesley: Reading, MA, 1983; Physical Bioinorganic Chemistry Series
2, pp 43-88.
(65) A couple of papers have appeared recently describing the formation
of the complex with very pure (dxz, dyz)4(dxy)1 configuration.66,67
(66) Moore, K. T.; Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc. 1999,
121, 5196-5209.
(67) Simonneaux, G.; Schu¨nemann, V.; Morice, C.; Carel, L.; Toupet, L.;
Winler, H.; Trautwein, A. X.; Walker, F. A. J. Am. Chem. Soc. 2000,
122, 4366-4377.
(59) Nakamura, M.; Yamaguchi, T.; Ohgo, Y. Inorg. Chem. 1999, 38,
3857-3862.

Low-Spin Iron(III) Porphyrin Complexes
Inorganic Chemistry, Vol. 40, No. 14, 2001 3433
from the EPR and 1H NMR spectra of the corresponding radical
cations. We have then examined the low-spin complexes where
the porphyrins have a1u HOMO. Such porphyrins are OEP and
ProtoIXMe2.36 Walker, Debrunner, Scheidt, and co-workers have
reported that [Fe(OEP)(tBuNC)2]+ has the (dxz, dyz)4(dxy)1 ground
state on the basis of the 1H NMR, EPR, Mo¨ssbauer, and X-ray
crystallography.6 We have examined the complexes such as [Fe-
(OEP)(tBuNC)2]+ and [Fe(ProtoIXMe2)(tBuNC)2]+, focusing our
attention on how much the (dxz, dyz)4(dxy)1 state contributes to
the electronic ground state. The 1H NMR spectra of [Fe-
(ProtoIXMe2)(tBuNC)2]+ taken at -50 °C showed the meso
signals fairly upfield, -70.4 and -71.6 ppm. These values are
close to -58.2 ppm in [Fe(OEP)(tBuNC)2]+. The results clearly
indicate that [Fe(ProtoIXMe2)(tBuNC)2]+ also adopts the
(dxz, dyz)4(dxy)1 ground state; the meso signals in the (dxy)2(dxz, dyz)3
type complexes appear at δ ca. 5 ppm.51 The 1H NMR method
is difficult, however, to rank the (dxz, dyz)4(dxy)1 contribution in
a variety of complexes, since the porphyrins with a2u HOMO
have no protons at the meso positions. Thus, we examined the
13C NMR spectra of these complexes. The chemical shifts of
the meso carbons were 767, 419, and 396 ppm at 25 °C for the
TPP, OEP, and ProtoIXMe2 complexes, respectively, indicating
that the (dxz, dyz)4(dxy)1 contribution is much larger in the TPP
complex than in the OEP and ProtoIXMe2 complexes. The EPR
g values obtained at 4.2 K are consistent with the NMR chemical
shifts at -50 °C; the |∆| values for the TPP, OEP, and
ProtoIXMe2 complexes are 9.16, 5.73, and 5.63 λ, respectively,
Thus, the (dxz, dyz)4(dxy)1 contribution in these complexes is as
small as that of the OETPP complexes.
Figure 6. Relative energy levels of the three d orbitals in a series of
[Fe(Porphyrin)(tBuNC)2]+ represented in units of spin-orbit coupling
constant λ, where Porphyrins are TiPrP, OMTPP, TPP, and OETPP.
meso carbon at 223 K, the electronic ground-state falls into the
(dxz, dyz)4(dxy)1 type at 4.2 K, giving the axial type EPR spectra.
In the case of the bis(CN-) and bis(HIm) complexes, the
following order has been established on the basis of the 13C
The results mentioned above indicate that the complexes with
a1u HOMO tend to maintain the (dxy)2(dxz, dyz)3 state even for
the axial ligands which prefer the (dxz, dyz)4(dxy)1 state. This is
because, the a1u HOMO has small spin densities at the pyrrole
nitrogens. Thus, the participation of this orbital to the charge-
transfer becomes fairly small. In contrast, the a2u HOMO has
large spin densities at the pyrrole nitrogens and meso carbons
and can directly participate in charge transfer when the porphyrin
is ruffled. Thus, the electron donating alkyl groups at the meso
positions destabilize the a2u orbital and strengthen the a2u- dxy
interactions.
1
and H NMR chemical shifts:
OETPP e OMTPP e TPP < TnPrP, TcPrP < TiPrP
In any case, the (dxz, dyz)4(dxy)1 contribution is the smallest in
the saddled OETPP complexes and the largest in the ruffled
TiPrP complexes. The rhombic EPR spectra observed in [Fe-
(OMTPP)(tBuNC)2]+ and [Fe(OETPP)(tBuNC)2]+ indicate that
the energy levels of the dxz and dyz orbitals are different as shown
in Figure 6. The results suggest that the four Fe-N(porphyrin)
bonds are no longer equivalent in frozen CH2Cl2 solution at
4.2 K;30,68 they are equivalent if porphyrin core is S4-saddled.
This might be the indication that both [Fe(OMTPP)(tBuNC)2]+
and [Fe(OETPP)(tBuNC)2]+ have saddle shaped structure with
some ruffled deformation.20,30 The V values of [Fe(OMTPP)-
(tBuNC)2]+ and [Fe(OETPP)(tBuNC)2]+, 1.32λ and 0.86λ,
respectively, indicate that the magnitude of ruffling is larger in
the former complex than in the latter, suggesting the less rigidity
in the OMTPP ring.
(iv) Electronic Effects of Peripheral Substituents. As we
have just mentioned, the introduction of the electron donating
alkyl substituents to the meso positions increases the
(dxz, dyz)4(dxy)1 contribution. It is then expected that the electron
withdrawing groups at the meso positions stabilizes the a2u
orbital and decreases the (dxz, dyz)4(dxy)1 contribution. To
confirm this hypothesis, we have examined the 1H NMR spectra
t
of [Fe(F20-TPP)L2]( (L ) HIm, CN-, and BuNC) and com-
pared the chemical shifts with those of the parent [Fe(TPP)L2](
(iii) a1u and a2u HOMOs. It is well-known that the porphyrin
HOMO is either a1u or a2u depending on the positions and the
electronic nature of substituents.36,69 All the low-spin complexes
so far discussed have meso-substituted porphyrins with a2u
HOMO, i.e., TRP,36 TPP,36,70-72 and ORTPP,27,59 as revealed
t
(L ) HIm, CN-, and BuNC). The pyrrole signals of [Fe(F20-
TPP)L2]( were observed at -24.4, -32.5, and +5.48 ppm
t
at -50 °C for L ) HIm, CN-, and BuNC, respectively, as
compared with -26.1, -28.7, and +11.7 ppm in [Fe(TPP)-
L2](. Especially important is the difference in chemical shifts
between [Fe(F20-TPP)(tBuNC)2]+ and [Fe(TPP)(tBuNC)2]+;
+5.48 ppm for the former and +11.7 ppm for the latter com-
plex. Since both of these complexes adopt the (dxz, dyz)4(dxy)1
ground state, the pyrrole proton chemical shifts suggest that
the (dxz, dyz)4(dxy)1 contribution is much larger in [Fe(TPP)-
(68) Ochsenbein, P.; Mandon, D.; Fischer, J.; Weiss, R.; Austin, R.; Jayaraj,
K.; Gold, A.; Terner, J.; Bill, E.; Muther, M.; Trautwein, A. X. Angew.
Chem., Int. Ed. Engl. 1993, 32, 1437-1438.
(69) Jayaraj, K.; Terner, J.; Gold, A.; Roberts, D. A.; Austin, R. N.; Mandon,
D.; Weiss, R.; Bill, E.; Mu¨ther, M.; Trautwein, A. X. Inorg. Chem.
1996, 35, 1632-1640.
(70) Goff, H. M.; Phillippi, M. A. J. Am. Chem. Soc. 1983, 105, 7567-
7571.
1
(tBuNC)2]+. The H NMR result is further supported by the
EPR result; the g and g values of [Fe(TPP)(tBuNC)2]+ are
(71) Boersma, A. D.; Goff, H. M. Inorg. Chem. 1984, 23, 1671-1676.
(72) Gans, P.; Buisson, G.; Due´e, H.; Marchon, J.-C.; Erler, B. S.; Scholz,
W. F.; Reed, C. A. J. Am. Chem. Soc. 1986, 108, 1223-1234.
(73) Inniss, D.; Soltis, S. M.; Strouse, C. E. J. Am. Chem. Soc. 1988, 110,
5644-5650.
||
2.18 and 1.93, respectively, while those of [Fe(F20-TPP)-
(tBuNC)2]+ are 2.31 and 1.86. Similar results have already been
reported by our previous paper on the electron configuration of

3434 Inorganic Chemistry, Vol. 40, No. 14, 2001
Ikeue et al.
a series of [Fe(TArP)(CN)2]- complexes; replacement of the
meso-2,4,6-trialkylphenyl groups by the meso-2,6-dichloro-
phenyl groups has changed the electron configuration from
(dxz, dyz)4(dxy)1 to (dxy)2(dxz, dyz).43
We have mentioned in the previous section that the contribu-
tion of the (dxz, dyz)4(dxy)1 state in bis(CN-) and bis(HIm)
complexes increases in the following order:
state differs, however, from complex to complex. On the basis
of the 13C NMR chemical shifts of meso carbons and EPR g
values, it is concluded that the (dxz, dyz)4(dxy)1 character increases
in the following order: OETPP < OMTPP, TPP < TnPrP,
TcPrP, TiPrP. The similar order is established for the bis(CN-)
and bis(HIm) complexes: OETPP e OMTPP e TPP < TnPrP,
TcPrP < TiPrP. The smaller contribution of the (dxz, dyz)4(dxy)1
state in the OETPP complexes than in the OMTPP complexes
is explained in terms of the rigidity of the OETPP core; [Fe-
(OETPP)(tBuNC)2]+ requires much larger energy for ruffling
than [Fe(OMTPP)(tBuNC)2]+ to stabilize the (dxz, dyz)4(dxy)1
ground state. In the complexes with the a1u porphyrin HOMO
such as [Fe(OEP)(tBuNC)2]+ and [Fe(ProtoIXMe2)(tBuNC)2]+,
the contribution of the (dxz, dyz)4(dxy)1 state is as small as that
in [Fe(OETPP)(tBuNC)2]+. Similarly, the complexes with
electron withdrawing substituents at the meso positions such
as [Fe(F20-TPP)(tBuNC)2]( have shown the smaller contribution
of the (dxz, dyz)4(dxy)1 state. On the basis of these results, we
have concluded that the low-spin Fe(III) complexes having (i)
strong σ-donors such as HIm, (ii) highly S4-saddled porphyrin
rings such as OETPP, (iii) porphyrins with a1u HOMO such as
OEP, and (iv) porphyrins with electron withdrawing substituents
at the meso positions such as F20-TPP, resist switching the
electronic ground state from the common (dxy)2(dxz, dyz)3 to the
less common (dxz, dyz)4(dxy)1.
OETPP e OMTPP e TPP < TnPrP, TcPrP < TiPrP
The larger contribution of the (dxz, dyz)4(dxy)1 state in TRP
complexes than in TPP complexes could be ascribed, to some
extent, to the electron donating ability of the alkyl groups.
Therien and co-workers have recently reported that bis(pyridine)
complex of [meso-tetrakis(heptafluoropropyl)porphyrinato]-
iron(III) shows a quite pure (dxz, dyz)4(dxy)1 electron configu-
ration.66 The gx, gy, and gz are 2.07, 2.07, and 1.99, respectively.
Thus, the |∆/λ| reaches as much as 26.4. Heptafluoropropyl
group is bulky and electron withdrawing. Thus, two opposing
factors are operating in this complex. Obviously, much more
examples are necessary for the complete understanding of the
electronic effect of peripheral substituents on the ground-state
electron configuration of low-spin iron(III) porphyrin complexes.
Conclusion
1
Combined analysis of the H and 13C NMR chemical shifts
Acknowledgment. The authors thank Prof. Hiroshi Kihara
of Tokoha University for useful suggestions on the orbital
interactions. This work was supported by the Grant in Aid for
Scientific Research on Priority Areas(A) (No 12020257 to M.N.)
from Ministry of Education, Culture, Sports, Science and
Technology, Japan. T.I. is grateful to the JSPS Research
Fellowship for young scientists. Thanks are due to the Research
Center for Molecular Materials, the Institute for Molecular
Science (IMS).
and EPR g values have revealed that the highly saddle shaped
[Fe(ORTPP)(CN)2]- (R ) Me and Et) has the common
(dxy)2(dxz, dyz)3 ground state. This contrasts to the ruffled
n
c
i
[Fe(TRP)(CN)2]- (R ) Pr, Pr, and Pr) which adopt the less
common (dxz, dyz)4(dxy)1 ground state. The difference in
electronic ground state is explained in terms of the weaker
a2u-dxy and stronger 3eg-dπ(dxz, dyz) interactions in the saddle
shaped [Fe(ORTPP)(CN)2]- than in the ruffled [Fe(TRP)(CN)2]-.
Replacement of CN- by HIm further destabilizes the dπ orbitals,
since the π-accepting ability of imidazole is weaker than that
of cyanide. Thus, even [Fe(TnPrP)(HIm)2]+ and [Fe(TcPrP)-
(HIm)2]+ have shown the common (dxy)2(dxz, dyz)3 ground state.
In contrast, all the bis(tBuNC) complexes examined in this study
have shown the less common (dxz, dyz)4(dxy)1 ground state. The
contribution of the (dxz, dyz)4(dxy)1 state to the electronic ground
Supporting Information Available: Curie plots of the pyrrole
protons and pyrrole-CH(R) protons in [Fe(TRP)L2]( (R ) iPr, cPr, nPr),
[Fe(TPP)L2](, [Fe(F20-TPP)L2](, and [Fe(ORTPP)L2]( (R ) Me, Et).
This material is available free of charge via the Internet at http://
pubs.acs.org.
IC001412B