Electron Configuration and Spin Distribution in Low-Spin (meso-Tetraalkylporphyrinato)iron(III) Complexes Carrying One or Two Orientationally Fixed Imidazole Ligands

Article abstract of DOI:10.1021/ic9801241

To understand the orientation effect of coordinated imidazole ligands, a series of low-spin (tetraalkylporphyrinato)-iron(III) complexes, [Fe(TRP)(L)₂J⁺ and [Fe(TRP)(L)(CN)], carrying at least one orientationally fixed imidazole (L) have been prepared. The ¹H NMR pyrrole signals of a series of [Fe(TRP)(2-MeIm)₂]⁺ have shown considerable downfield shifts as the meso substituent becomes bulkier, from -30.4 (R = H) to +5.6 ppm (R = ⁱPr) at -71 °C. These complexes have exhibited four pyrrole signals at lower temperature due to the hindered ligand rotation. The spread of the pyrrole signals decreases from 9.4 (Me) to 8.2 (Et) and then to 5.7 (ⁱPr) ppm. The downfield pyrrole signals together with the small spread in [Fe(TⁱPrP)(2-MeIm)₂]⁺ are in sharp contrast to the other low-spin complexes with orientationally fixed imidazole ligands; the chemical shifts and spreads of the pyrrole signals in [tetrakis(2,4.6-trialkylphenyl)porphyrinato]iron(III) complexes [Fe(R-TPP)(2-MeIm)₂]⁺ (R = Me. Et, ⁱPr) are ca. -20 and ca. 9 ppm, respectively, at -71 °C. The EPR spectra of a series of [Fe(TRP)(2-MeIm)₂]⁺ were then taken at 4.2 K. While the R = H, Me, and Et complexes have shown so-called large g_max type spectra as in the case of [Fe(R-TPP)(2-MeIm)₂]⁺, the ⁱPr complex has exhibited an axial type spectrum. The result indicates that the electron configuration of the ferric ion of [Fe(TⁱPrP)(2-MeIm)₂]⁺ is presented by the unusual (d_xz, d_yz)⁴-(d_xy)¹ in contrast to the other low-spin complexes where ferric ions have the (d_xy)²(d_xz, d_yz)³ configuration. When one of the 2-MeIm ligands in [Fe(TRP)(2-MeIm)₂]⁺ is replaced by CN^-, not only the ⁱPr but also the Me and Et complexes have shown the (d_xz, d_yz)⁴(d_xy)¹ configuration as revealed from the EPR spectra. The pyrrole signals of the ⁱPr complex [Fe(TⁱPrP)(2-MeIm)(CN)] have been observed at 12.2, 14.1, 14.8, and 16.2 ppm at -71 °C. Thus, the spread is only 4.0 ppm. The value is quite different from that of the corresponding [Fe(Me-TPP)(2-ⁱPrIm)(CN)] where the spread reaches as much as 11.4 ppm. On the basis of these results, it is concluded that the spin distribution on the pyrrole β-carbons in the complexes with (d_xz, d_yz)⁴(d_xy)¹ is rather homogeneous even if the coordinated imidazole is orientationally fixed. On the contrary, the fixation induces a larger asymmetric spin distribution on these carbons in the complexes with (d_xy)²(d_xz, d_yz)³ configuration.

Full text of DOI:10.1021/ic9801241

Inorg. Chem. 1998, 37, 2405-2414
2405
Electron Configuration and Spin Distribution in Low-Spin
(meso-Tetraalkylporphyrinato)iron(III) Complexes Carrying One or Two Orientationally
Fixed Imidazole Ligands1
Mikio Nakamura,*,†,‡ Takahisa Ikeue,‡ Hiroshi Fujii,*,§, Tetsuhiko Yoshimura,§ and
Kunihiko Tajima|
Department of Chemistry, Toho University School of Medicine, Omorinishi, Ota-ku, Tokyo 143, Japan,
Division of Biomolecular Science, Graduate School of Science, Toho University, Funabashi, Chiba 274,
Japan, Institute for Life Support Technology, Yamagata Technopolis Foundation, Matsuei, Yamagata
990, Japan, and Department of Polymer Science & Technology, Kyoto Institute of Technology,
Matsugasaki, Sakyo-ku, Kyoto 606, Japan
ReceiVed February 2, 1998
To understand the orientation effect of coordinated imidazole ligands, a series of low-spin (tetraalkylporphyrinato)-
iron(III) complexes, [Fe(TRP)(L)2]+ and [Fe(TRP)(L)(CN)], carrying at least one orientationally fixed imidazole
(L) have been prepared. The 1H NMR pyrrole signals of a series of [Fe(TRP)(2-MeIm)2]+ have shown considerable
i
downfield shifts as the meso substituent becomes bulkier, from -30.4 (R ) H) to +5.6 ppm (R ) Pr) at -71
°C. These complexes have exhibited four pyrrole signals at lower temperature due to the hindered ligand rotation.
The spread of the pyrrole signals decreases from 9.4 (Me) to 8.2 (Et) and then to 5.7 (iPr) ppm. The downfield
pyrrole signals together with the small spread in [Fe(TiPrP)(2-MeIm)2]+ are in sharp contrast to the other low-
spin complexes with orientationally fixed imidazole ligands; the chemical shifts and spreads of the pyrrole signals
i
in [tetrakis(2,4,6-trialkylphenyl)porphyrinato]iron(III) complexes [Fe(R-TPP)(2-MeIm)2]+ (R ) Me, Et, Pr) are
ca. -20 and ca. 9 ppm, respectively, at -71 °C. The EPR spectra of a series of [Fe(TRP)(2-MeIm)2]+ were then
taken at 4.2 K. While the R ) H, Me, and Et complexes have shown so-called “large gmax type” spectra as in
the case of [Fe(R-TPP)(2-MeIm)2]+, the iPr complex has exhibited an “axial type” spectrum. The result indicates
that the electron configuration of the ferric ion of [Fe(TiPrP)(2-MeIm)2]+ is presented by the unusual (dxz, dyz)4-
(dxy)1 in contrast to the other low-spin complexes where ferric ions have the (dxy)2(dxz, dyz)3 configuration. When
one of the 2-MeIm ligands in [Fe(TRP)(2-MeIm)2]+ is replaced by CN-, not only the iPr but also the Me and Et
complexes have shown the (dxz, dyz)4(dxy)1 configuration as revealed from the EPR spectra. The pyrrole signals
i
of the Pr complex [Fe(TiPrP)(2-MeIm)(CN)] have been observed at 12.2, 14.1, 14.8, and 16.2 ppm at -71 °C.
Thus, the spread is only 4.0 ppm. The value is quite different from that of the corresponding [Fe(Me-TPP)(2-
iPrIm)(CN)] where the spread reaches as much as 11.4 ppm. On the basis of these results, it is concluded that
the spin distribution on the pyrrole â-carbons in the complexes with (dxz, dyz)4(dxy)1 is rather homogeneous even
if the coordinated imidazole is orientationally fixed. On the contrary, the fixation induces a larger asymmetric
spin distribution on these carbons in the complexes with (dxy)2(dxz, dyz)3 configuration.
Introduction
methyl signals in low-spin hemoproteins as compared with that
of synthetic ferric porphyrin complexes with the same axial
Studies on the orientation effect of the axially coordinated
imidazole ligands on the heme properties have attracted much
attention in these years in connection with the biological system
where the coordinated ligands are tightly fixed in the cavities
of heme proteins.2-9 The importance of the orientation effect
was originally recognized by the large spread of the heme
ligands.10 For example, the spread of the methyl signals in horse
heart cytochrome c, which has histidine and methionine as axial
ligands, is reported to be 30.0 ppm.11 Similarly, the spreads of
the methyl signals in cytochrome b5 and cytochrome c imida-
zole, both of which have two imidazole ligands at the axial
positions, are 20.5 and 13.5 ppm, respectively,12,13 as compared
* Corresponding authors. E-mail: mnakamu@med.toho-u.ac.jp.
† Toho University School of Medicine.
(4) Traylor, T. G.; Berzinis, A. P. J. Am. Chem. Soc. 1980, 102, 2844-
2846.
‡ Division of Biomolecular Science, Graduate School of Science, Toho
University.
(5) Goff, H. J. Am. Chem. Soc. 1980, 102, 3252-3254.
(6) Walker, F. A.; Buehler, J.; West, J. T.; Hinds, J. L. J. Am. Chem.
Soc. 1983, 105, 6923-6929.
§ Institute for Life Support Technology.
Current address: Institute for Molecular Science, Myodaiji, Okazaki
444, Japan.
(7) Scheidt, W. R.; Chipman, D. M. J. Am. Chem. Soc. 1986, 108, 1163-
| Kyoto Institute of Technology.
1167.
(1) This paper is dedicated to Professor Michinori Ohki on the occasion
(8) Walker, F. A.; Huynh, B. H.; Scheidt, W. R.; Osvath, S. R. J. Am.
Chem. Soc. 1986, 108, 5288-5297.
of his 70th birthday.
(2) La Mar, G. N.; Walker, F. A. In The Porphyrins; Dolphin, D., Ed.;
Academic Press: New York, 1979; Vol. IV, pp 61-157.
(3) Bertini, I.; Luchinat, C. In NMR of Paramagnetic Molecules in
Biological Systems; Lever, A. B. P., Gray, H. B., Eds.;
Benjamin/Cummings: Menlo Park, CA, 1986; pp 165-229.
(9) Yamamoto, Y.; Nanai, N.; Chujo, R.; Suzuki, T. FEBS Lett. 1990,
264, 113-116.
(10) Shulman, R. G.; Glarum, S. H.; Karplus, M. J. Mol. Biol. 1971, 57,
93-115.
(11) Smith, M.; McLendon, G. J. Am. Chem. Soc. 1981, 103, 4912-4921.
S0020-1669(98)00124-4 CCC: $15.00 © 1998 American Chemical Society
Published on Web 04/21/1998

2406 Inorganic Chemistry, Vol. 37, No. 10, 1998
Nakamura et al.
with 3.2 ppm in protohemin bis(imidazole).2 The large spreads
of the methyl signals have also been observed in metmyoglobin
cyanide,14-16 cytochrome c cyanide,11,17 lignin peroxidase
cyanide,18 and HRP cyanide,18 all of which carry histidyl
imidazole and cyanide as axial ligands; the spreads of the methyl
signals are 22.2, 11.1, 29.3, and 25.9 ppm, respectively, as
compared with 5.4 ppm in the protohemin with imidazole and
cyanide.2 These results suggest that naturally occurring hemo-
proteins have a large asymmetric spin distribution on the
peripheral carbon atoms, although the degree of spread is
different from protein to protein. In contrast, the synthetic
complexes generally have homogeneous spin densities on the
peripheral carbon atoms. The large spread of the methyl signals
in hemoproteins has been reproduced even in the synthetic
model complexes if they carry imidazole ligands with fixed
geometry. Thus, the spread of the methyl signals in the
imidazole chelated heme reaches as much as 17.1 ppm.4
Similarly, the pyrrole signals in the imidazole-appended heme
spread over 12 ppm.5,6 On the basis of these studies, it is
generally accepted that the fixation of the coordinated imidazole
ligand induces large asymmetric spin distribution on the
peripheral carbons.
Some years ago, we have reported the first example of the
hindered rotation of axially coordinated imidazole ligand in bis-
(2-methylimidazole){meso-tetrakis(2,4,6-trimethylphenyl)-
porphyrinato}iron(III) chloride, [Fe(Me-TPP)(2-MeIm)2]Cl.19,20
Unlike the other complexes with fixed imidazole ligands, this
complex is quite unique in a sense that the nonequivalence of
the pyrrole protons is observed only when the rotation of the
coordinated imidazole ligand is hindered. In fact, the complex
showed a single pyrrole signal at a very high field, δ -10.8
ppm at 25 °C, which split into four signals, δ -14.7, -19.0,
-21.0, and -23.3 ppm, at -56 °C. The spread of the pyrrole
signals, 8.6 ppm at -56 °C, suggests an asymmetric spin
distribution on the peripheral carbon atoms due to the fixation
of the coordinated imidazole ligands. The frozen conformation
in solution was determined to be the one where the ligands are
placed perpendicularly along the diagonal Cmeso-Fe-Cmeso
axes.21,22 The structure was further supported by the X-ray
crystallographic analysis of the analogous [Fe(Me-TPP)(1,2-
Me2Im)2]ClO4.23 The porphyrin ring of this complex showed
a highly S4-ruffled structure where the average deviation of the
four meso carbons from the mean porphyrin plane reaches as
much as 0.72 Å. This indicates that the complex has two
cavities developed along the diagonal Cmeso-Fe-Cmeso axes and
that the coordinated imidazole ligands are placed in the cavities
perpendicularly to each other. Thus, the solid structure is
maintained even in solution.
Splitting of the pyrrole signal was commonly observed in
the low-spin [tetrakis(2,4,6-trialkylphenyl)porphyrinato]iron(III)
i
complexes [Fe(R-TPP)(L)2]+ (R ) Me, Et, Pr) carrying a
sterically hindered imidazole (L) such as 2-MeIm, 2-EtIm, 2-
iPrIm, 1,2-Me2Im, 1-Me-2-iPrIm, and BzIm.24 The pyrrole
signals of these complexes were observed at -8 to -27 ppm
with the spread of 8-12 ppm at -56 °C. The EPR spectra
generally showed a so-called “large gmax type” signal, a signal
with gmax > 3.0 as the sole observable spectral feature.8 The
“large gmax type” EPR spectra together with the extremely
upfield shifted pyrrole protons in the NMR spectra clearly
indicate that the ground-state electron configuration of iron is
presented by the usual (dxy)2(dxz, dyz)3 in which the dxz and dyz
orbitals are nearly degenerate.2,25,26 The reason for the upfield
shift of the pyrrole protons is ascribed to the charge transfer
from the porphyrin 3eg(x) and 3eg(y) to the iron dxz and dyz
orbitals, respectively, since the 3eg orbitals have large electron
densities on the pyrrole â-carbons.27 Thus, the fixation of
unsymmetrical 2-MeIm ligands would lower the porphyrin
symmetry from D4h to C2, resulting in the asymmetric spin
distribution on the porphyrin peripheral carbons. This must be
one of the reasons for the spread of the four pyrrole signals,
8-12 ppm at -56 °C, in [Fe(R-TPP)(L)2]+.24
A much larger asymmetric spin distribution has been expected
in the complexes with parallelly fixed imidazole ligands, since
this alignment of the axial ligands can lift the degeneracy of
the dπ orbitals; the unpaired electron of iron resides mainly in
one of the dπ(dxz and dyz) orbitals which is transferred into one
of the 3eg orbitals of porphyrin, resulting in the large asymmetric
spin distribution. However, there have been no reports so far
on the fixation of two planar ligands in a parallel fashion on
the NMR time scale. The crystallographic result mentioned
above suggests that the parallel conformation must be very
unstable since the second planar ligand has to coordinate to the
ferric iron perpendicularly to the cavity, resulting in the large
steric repulsion between the ligand and the porphyrin core. In
a previous paper, we have prepared mixed-imidazole complexes
such as [Fe(Et-TPP)(2-iPrIm)(1-MeIm)]+ in which one of the
axial ligands, 2-iPrIm, is fixed and the other is rapidly rotating
on the NMR time scale.28 Since the orientation effect of the
rapidly rotating ligand is canceled out, one can expect that these
complexes might be good substitutes for the complex having
two ligands aligned in parallel. The pyrrole signals of [Fe(Et-
TPP)(2-iPrIm)(1-MeIm)]+ appeared at δ -13.8, -18.1, -27.9,
and -33.2 ppm at -56 °C. Thus, the spread of the signals
increased to 19.4 ppm, which is almost twice as much as that
of other complexes carrying two hindered ligands. One problem
in this complex is that the 1-MeIm ligand, although rotating
rapidly on the NMR time scale, still tends to take a perpendicular
conformation in each moment of rotation. Thus, the better
(12) Keller, R. M.; Wuthrich, K. Biochim. Biophys. Acta 1980, 621, 204-
217.
(13) Shao, W.; Sun, H.; Yao, Y.; Tang, W. Inorg. Chem. 1995, 34, 680-
687.
(14) Mayer, A.; Ogawa, S.; Schulman, R. G.; Yamane, T.; Cavaleiro, L.
A. S.; Rocha Gonsalves, A. M. d’A.; Kenner, G. W.; Smith, K. M. J.
Mol. Biol. 1974, 86, 749-756.
(15) Emerson, S. D.; La Mar, G. N. Biochemistry 1990, 29, 1556-1566.
(16) Banci, L.; Bertini, I.; Pierattelli, R.; Vila, A. J. Inorg. Chem. 1994,
33, 4338-4343.
(17) Bren, K. L.; Gray, H. B.; Banci, L.; Bertini, I.; Turano, P. J. Am.
Chem. Soc. 1995, 117, 8067-8073.
(18) Banci, L.; Bertini, I.; Pierattelli, R.; Tien, M.; Vila, A. J. J. Am. Chem.
Soc. 1995, 117, 8659-8667.
(19) Nakamura, M.; Groves, J. T. Tetrahedron 1988, 44, 3225-3230.
(20) Abbreviations: H-TPP, dianion of tetraphenylporphyrin; Me-TPP,
dianion of tetrakis(2,4,6-trimethylphenyl)porphyrin or tetramesitylpor-
phyrin; R-TPP, dianions of meso-tetrakis(2,4,6-trialkylphenyl)porphy-
rin; TRP and THP, dianions of meso-tetraalkylporphyrin and -porphine,
respectively; Im, imidazole; 1-MeIm, 1-methylimidazole; 2-MeIm,
2-methylimidazole; 2-EtIm, 2-ethylimidazole; 2-iPrIm, 2-isopropy-
limidazole; 1,2-Me2Im, 1,2-dimethylimidazole; 1-Me-2-iPrIm, 1-meth-
yl-2-isopropylimidazole; BzIm, benzimidazole; 2-MeBzIm, 2-meth-
ylbenzimidzaole; Py, pyridine.
(24) Nakamura, M.; Tajima, K.; Tada, K.; Ishizu, K.; Nakamura, N. Inorg.
Chim. Acta 1994, 224, 113-124.
(25) Hatano, K.; Safo, M. K.; Walker, F. A.; Scheidt, W. R. Inorg. Chem.
1991, 30, 1643-1650.
(26) Safo, M. K.; Gupta, G. P.; Watson, C. T.; Simonis, U.; Walker, F.
A.; Scheidt, W. R. J. Am. Chem. Soc. 1992, 114, 7066-7075.
(27) Goff, H. Iron Porphyrins Part One; Lever, A. B. P., Gray, H. B.,
Eds.; The Addison-Wesley Publishing Co.: Reading, MA, 1983; pp
237-281.
(21) Walker, F. A.; Simonis, U. J. Am. Chem. Soc. 1991, 113, 8652-8657.
(22) Nakamura, M.; Nakamura, N. Chem. Lett. 1991, 1885-1888.
(23) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.;
Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935-954.
(28) Nakamura, M. Chem. Lett. 1992, 2423-2426.

(meso-Tetraalkylporphyrinato)iron(III)
Inorganic Chemistry, Vol. 37, No. 10, 1998 2407
carried out in propionic acid using FeCl2‚6H2O. The high-spin ferric
porphyrin complexes [Fe(TRP)]Cl thus obtained were purified by
chromatography on silica gel using CH2Cl2-CH3OH as the eluent and
recrystallized from CH2Cl2-hexane. Bis(imidazole) complexes [Fe-
candidate for the study on the orientation effect of axial ligands
must be the complex carrying a planar imidazole ligand (L)
and a linear cyanide ligand (CN-) as presented by [Fe(R-TPP)-
(L)(CN)]. If rotation of the imidazole ligand is hindered, the
complex would give similar information that could be obtained
from the complexes with parallel fixed imidazole ligands.
Recently, we have reported another iron porphyrin system
in which rotation of the coordinated 2-MeIm is hindered on
the NMR time scale. They are a series of bis(2-methylimida-
zole)(meso-tetraalkylporphyrinato)iron(III) complexes [Fe(TRP)-
(2-MeIm)2]+ (R ) Me, Et, and iPr).29 In the case of the methyl
complex, the pyrrole signals started to split below -51 °C and
showed four signals at δ -6.3, -11.3, -14.4, and -15.7 ppm
at -71 °C. Thus, the spread of the pyrrole signals was 9.4
ppm, which is quite close to that of [Fe(Me-TPP)(2-MeIm)2]+,
8.9 ppm, at the same temperature.24 In the isopropyl complex,
on the contrary, the spread decreased to 3.5 ppm at -35 °C.
These results suggest that the fixation of the imidazole ligand
does not always induce a large asymmetric spin distribution on
the peripheral carbons. Acquiring the basic knowledge of the
factors which control the spin distribution on the peripheral
carbons in low-spin ferric porphyrin complexes is quite impor-
tant to understanding the physicochemical properties of the
naturally occurring hemoproteins. Here, we report (i) the
synthesis and characterization of the low-spin (meso-tetraalkyl-
porphyrinato)iron(III) carrying orientationally fixed imidazole
(L) ligands [Fe(TRP)(L)2]+, (ii) the synthesis and characteriza-
tion of the first examples of the mixed-ligand complexes
carrying cyanide (CN) and orientationally fixed imidazole (L)
ligands in both (meso-tetraalkylporphyrinato)iron(III) [Fe(TRP)-
(L)(CN)] and (meso-tetramesitylporphyrinato)iron(III) [Fe(Me-
TPP)(L)(CN)] complexes, (iii) the observation of the unusually
small spread of the pyrrole signals in the mixed-ligand isopropyl
complexes [Fe(TiPrP)(L)(CN)] as compared wtih the other
complexes such as [Fe(Me-TPP)(L)2]+ and [Fe(Me-TPP)(L)-
(CN)], and (iv) the reasons for the homogeneous spin distribu-
tion in the former complexes as compared with the latter ones.
i
(TRP)(L)2]+ (R ) Me, Et, Pr; L ) 1-MeIm, 2-MeIm, 2-iPrIm) were
synthesized by the addition of 2.5 mol equiv of imidazole (L) into CD2-
Cl2 solutions of the high-spin [Fe(TRP)]Cl.29
(ii) [Fe(TRP)(L)(CN)]. To a CD2Cl2 solution of low-spin bis-
(imidazole) complex [Fe(TRP)(L)2]+ placed in an NMR sample tube
was added KCN in CD3OD or tetrabutylammonium cyanide (Bu4N+CN-)
in CD2Cl2 at -78 °C to form [Fe(TRP)(L)(CN)]. Temperature is very
important to obtain the pure complex; addition of KCN at the ambient
temperature resulted in the formation of both [Fe(TRP)(L)(CN)] and
[Fe(TRP)(CN)2]-. In the following experimental procedure to prepare
the mixed-ligand complexes, only [Fe(THP)(1-MeIm)(CN)] and [Fe-
(TMeP)(1-MeIm)(CN)] are described in detail. All of the other
complexes were prepared by the similar procedure as [Fe(TMeP)(1-
MeIm)(CN)].
[Fe(THP)(1-MeIm)(CN)]. To a 300 µL CD2Cl2-CD3OD (9:1)
solution of [Fe(THP)]Cl (1.2 mg, 3.0 × 10-6 mol) and KCN (1.5 mol
equiv) was added a CD2Cl2 solution of 1-MeIm at -78 °C. The
reaction was monitored by 1H NMR. The mixed-ligand complex was
formed when 30 µL (3.0 mol equiv) of 1-MeIm was added as a CD2-
Cl2 solution. Addition of a KCN solution into [Fe(THP)(1-MeIm)2]+
was unsuccessful since [Fe(THP)(1-MeIm)2]+ was highly insoluble in
CD2Cl2. 1H NMR (CD2Cl2-CD3OD, -35 °C): -24.9 (8H, Py H),
-0.1 (4H, meso-H), 20.0 (3H, Im-CH3).
[Fe(TMeP)(1-MeIm)(CN)]. To a 250 µL CD2Cl2 solution of [Fe-
(TMeP)]Cl (1.5 mg, 3.3 × 10-6 mol) placed in an NMR sample tube
was added a 50 µL CD2Cl2 solution of 1-MeIm (3.0 mol equiv). The
cooled solution (-78 °C) of the low-spin [Fe(TMeP)(1-MeIm)2]+ thus
formed was titrated with a CD3OD solution of KCN. The reaction
1
was monitored by the H NMR spectrum. After the total addition of
1
30 µL (2.0 mol equiv to the complex) of the solution, the H NMR
spectrum showed a complete formation of the mixed-ligand complex
[Fe(TMeP)(1-MeIm)(CN)]. 1H NMR (CD2Cl2-CD3OD, -35 °C):
-6.7 (8H, Py H), 42.2 (12H, meso-CH3), 7.5 (3H, Im-CH3).
(iii) Fe(Me-TPP)(L)2]+. Synthesis and 1H NMR spectra of a series
of bis(imidazole) complexes of (tetramesitylporphyrinato)iron(III) [Fe-
(Me-TPP)(L)2]+ have already been reported in our previous paper.24
(iv) [Fe(Me-TPP)(L)(CN)]. To a CD2Cl2 solution containing [Fe-
(Me-TPP)(L)2]+ was added 2.0 mol equiv of KCN at -78 °C. In the
following experimental procedure to prepare the mixed-ligand com-
plexes, only [Fe(Me-TPP)(1-MeIm)(CN)] is described in detail as a
typical example. Other complexes were prepared by the similar
procedure.
[Fe(Me-TPP)(1-MeIm)(CN)]. To a 250 µL CD2Cl2 solution of
high-spin [Fe(Me-TPP)]Cl (2.1 mg, 2.4 × 10-6 mol) was added a 50
µL CD2Cl2 solution of 1-MeIm (3.0 mol equiv) to form the low-spin
complex. To the low-spin complex thus formed was added a 30 µL
of a CD3OD solution of KCN (2.0 mol equiv) at -78 °C. Complete
formation of the mixed-ligand complex was confirmed by the 1H NMR
spectral change. 1H NMR (CD2Cl2-CD3OD, 25 °C): -14.9 (8H, py
H), 0.93 (12H, o-CH3), 1.08 (12H, o-CH3), 1.76 (12H, p-CH3), 6.33
(4H, m-H), 6.41 (4H, m-H), 11.8 (3H, Im-CH3).
Experimental Section
Spectral Measurement. 1H NMR spectra were recorded on a Jeol
LA300 operating at 300.4 MHz. 13C NMR spectra were recorded either
on a Jeol LA300 operating at 75.5 MHz or on a Jeol JNM620 operating
at 155.9 MHz for carbon. Chemical shifts were referenced to residual
CHDCl2 (δ ) 5.32 ppm). EPR spectra were measured at 77 and 4.2
K in frozen CH2Cl2-CH3OH solution with a Brucker ESP-300
spectrometer operating at X band equipped with an Oxford helium
cryostat.
Results
1H NMR Spectra. (i) [Fe(TRP)(L)2]+. The 1H NMR
spectra of [Fe(TRP)(1-MeIm)2]+, [Fe(TRP)(2-MeIm)2]+, and
[Fe(TRP)(2-iPrIm)2]+ (R ) H, Me, Et, iPr) were taken in CD2-
Cl2 at various temperatures, and their chemical shifts at -35
and -71 °C are listed in Table 1a. The pyrrole protons of the
complexes having both meso-alkyl groups and bulky imidazole
ligands showed signal splitting at low temperature. While the
meso-unsubstituted complexes [Fe(THP)(L)2]+ gave pyrrole
Synthesis. (i) [Fe(TRP)(L)2]+. A series of tetraalkylporphyrins
were prepared according to the literature.30,31 Insertion of iron was
i
(29) Nakamura, M.; Ikeue, T.; Neya, S.; Funasaki, N.; Nakamura, N. Inorg.
Chem. 1996, 35, 3731-3732.
(30) Neya, S.; Yodo, Y.; Funasaki, N. J. Heterocycl. Chem. 1993, 30, 549-
550.
signals at extremely high magnetic field, the Pr complexes
showed them in a so-called “diamagnetic region” regardless of
the axial ligands. The pyrrole signals of the Me and Et
complexes appeared in the middle. In general, bis(2-MeIm)
(31) Neya, S.; Funasaki, N. J. Heterocycl. Chem. 1997, 34, 689-690.

2408 Inorganic Chemistry, Vol. 37, No. 10, 1998
Nakamura et al.
Table 1. 1H NMR Chemical Shifts of a Series of Low-Spin (Tetraalkylporphyrinato)iron(III) Complexes, [Fe(TRP)(L)2]+,a [Fe(TRP)(CN)2]-,b
and [Fe(TRP)(L)(CN)]a
(a) [Fe(TRP)(L)2]+ and [Fe(TRP)(CN)2]
Py H of L
meso-R-H of L
Im-CH3 of L
R
1-MeIm
2-MeIm
2-iPrIm
CN- 1-MeIm
2-MeIm
2-iPrIm CN- 1-MeIm 2-MeIm 2-iPrIm
Hc
Hd
-24.0 -22.5
-30.9 -30.4
e
e
-23.6
-29.2
24.4
29.9
64.3 19.0
14.7
17.5
3.5 -5.1
-1.6 -2.4, -12.4
2.8 -4.8
2.2 -1.7, -10.2
-4.5 -4.0, -9.1
-6.8 -4.6, -10.2
e
e
Mec -19.6 -8.2
-7.4
0.0
1.1
8.7 38.7
8.9 39.0, 49.6
2.4 16.9
39.4
Med -24.3 -6.3, -11.3, -14.4, -15.7 -4.9, -10.5, -12.8, -15.6
Etc -19.7 -9.4
-7.8
38.8, 50.3 78.9 21.2
17.8 27.8 19.0
0.6 15.4, 16.4, 17.8 17.8, 20.5 30.3 22.9
-3.0
Etd -25.9 -9.8, -14.1, -17.1, -18.0 -5.9, -10.2, -13.0, -14.3 -5.0
iPrc
iPrd
2.8 2.6, 3.8, 6.9, 7.3
3.0 2.5, 3.9, 7.8, 8.2
2.3, 5.3, 5.3, 6.6
2.5, 6.3, 6.7, 7.9
12.1 18.3 17.9, 21.3
12.9 21.5 20.4, 25.2
15.8, 18.8 27.8
17.6, 21.4 30.9
1.9
0.9
(b) [Fe(TRP)(L)(CN)]
meso-R-H of L
2-MeIm
Py H of L
2-MeIm
Im-CH3 of L
1-MeIm 2-MeIm 2-iPrIm
R
1-MeIm
2-iPrIm
1-MeIm
2-iPrIm
Hc
-24.9 -21.0
-27.1 -26.8
-6.7 1.8
e
e
2.9
4.0
1.9
f
(-0.1) (-0.2)
(-3.8) (-2.9)
42.2 66.2
52.6 78.3
17.7 33.4
20.2 38.6
25.7 27.5 (broad)
30.1 32.4 (2H), 33.1 (1H), 33.7 (1H) 32.2 (broad)
e
e
71.6
89.3
33.9
40.4
20.0
26.6
7.5
7.4
7.7
21.4
27.3
e
e
Hd
Mec
Med
Etc
-0.8 -5.0
-2.0 -6.7
-0.3 -4.8
-0.6 -6.2
-6.0 -4.8
-7.3 -5.8
-7.1 2.7
-7.7 1.8
-9.7 1.5
Etd
iPrc
iPrd
8.8
11.7 10.8, 12.3, 12.6, 14.0 11.0, 11.8, 12.2, 14.1
13.3 12.2, 14.1, 14.8, 16.2 12.2, 13.6, 14.2, 16.3
26.8 (2H), 27.2 (2H) -2.4
-3.6
a This work. b Reference 32. c Chemical shifts at -35 °C. d Chemical shifts at -71 °C. e Solubility is too low. f Signals are too broad.
and bis(2-iPrIm) complexes exhibited the pyrrole signals at lower
magnetic field than the corresponding bis(1-MeIm) complexes.
In Figure 1a are given the Curie plots of the pyrrole signals in
[Fe(TRP)(1-MeIm)2]+ and [Fe(TRP)(2-MeIm)2]+. Although the
pyrrole protons of [Fe(TRP)(2-MeIm)2]+ showed four signals
at low temperature, the average positions were used in the Curie
plot. Except for the iPr complexes, the slopes of the Curie plots
were negative; both [Fe(TiPrP)(1-MeIm)2]+ and [Fe(TiPrP)(2-
MeIm)2]+ showed small but positive slopes. For comparison,
the chemical shifts of the bis(cyanide) complexes [Fe(TRP)-
(CN)2]- are also listed in Table 1a.32
(ii) [Fe(TRP)(L)(CN)]. The 1H NMR spectra of [Fe(TRP)-
(1-MeIm)(CN)], [Fe(TRP)(2-MeIm)(CN)], and [Fe(TRP)(2-
iPrIm)(CN)] were taken in CD2Cl2-CD3OD over a wide
temperature range, -14 to -82 °C. The chemical shifts at -35
and -71 °C are given in Table 1b. The Curie plots of the
pyrrole signals of [Fe(TRP)(1-MeIm)(CN)] and [Fe(TRP)(2-
MeIm)(CN)] are given in Figure 1b. In a series of [Fe(TRP)-
(1-MeIm)(CN)] complexes, the unsubstituted complex (R ) H)
showed a pyrrole signal at δ -24.9 ppm at -35 °C, which is
quite typical as a low-spin ferric porphyrin complex. The Me
and Et complexes exhibited them at lower field, δ -6.7 and
i
-7.7 ppm at -35 °C, respectively. In the case of the Pr
complex, the pyrrole signal appeared at fairly low field, +11.7
ppm. Neither pyrrole nor meso-alkyl signals of these complexes
showed appreciable broadening even at -82 °C. While the
pyrrole signals of the unsubstituted and Et complexes moved
to higher magnetic field as the temperature was lowered, that
of the iPr complex moved to the opposite direction. The pyrrole
signal of the Me complex showed little dependence on tem-
1
perature. The H NMR signals of both the pyrrole and meso
R-protons in [Fe(TRP)(2-MeIm)(CN)] appeared at lower mag-
netic field as compared with those of the corresponding [Fe-
(TRP)(1-MeIm)(CN)]. Although the signals in the Me and Et
complexes showed considerable broadening at lower tempera-
ture, they did not split even at -71 °C. The clear splitting of
Figure 1. Curie plots of the pyrrole signals.
the signals was observed, however, in the Pr complex [Fe(Ti-
PrP)(2-MeIm)(CN)]; the pyrrole protons gave four signals with
i
equal integral intensities at δ 12.2, 14.1, 14.8, and 16.2 ppm
and the meso-CH showed three signals with 2:1:1 intensity ratios
(32) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T. J. Am. Chem. Soc.
1997, 119, 6284-6291.

(meso-Tetraalkylporphyrinato)iron(III)
Inorganic Chemistry, Vol. 37, No. 10, 1998 2409
at 25 °C. When the temperature was lowered, carbon signals
of the porphyrin core spread into 2 or 4 peaks even in the Me
complex. Because of the low solubility and low signal-to-noise
ratios, the signal assignment was not completed at low tem-
perature. Thus, only the chemical shifts of the meso carbons
at 25 and -60 °C are listed in Table 3.
Spread of the Signals. The spread of the pyrrole â- and
meso R-proton signals is defined by the maximum difference
in chemical shifts at low temperature where the rotation of axial
ligands is slowed on the NMR time scale. These values are
extracted from Tables 1 and 2 and are summarized in Table
4a-c.
(i) Pyrrole Proton Signals. The spread of the pyrrole signals
in [Fe(TRP)(L)2]+ decreased as the bulkiness of the meso
substituents increased. In the case of [Fe(TRP)(2-MeIm)2]+,
the spreads were 9.4, 8.2, and 5.7 ppm at -71 °C for the Me,
i
Et, and Pr complexes, respectively. Similarly, the spread
decreased from 10.7 (R ) Me) to 8.4 (Et) and then to 5.4 (iPr)
ppm in [Fe(TRP)(2-iPrIm)2]+. A much smaller spread was
observed in the mixed-ligand complexes [Fe(TRP)(L)(CN)],
i
although the signal splitting was observed only in the Pr
complexes. The spreads were 4.0 and 4.1 ppm for [Fe(TiPrP)-
(2-MeIm)(CN)] and [Fe(TiPrP)(2-iPrIm)(CN)], respectively. In
the case of the tetraarylporphyrin complexes, [Fe(R-TPP)(L)2]+,
the spreads were 9.0-11.3 ppm. In contrast to the tetraalkyl-
porphyrin system, the spread further increased when one of the
axial ligands was replaced by CN-; the spread of [Fe(Me-
TPP)(2-iPrIm)(CN)] was 11.4 ppm as compared wtih 10.9 ppm
in [Fe(Me-TPP)(2-iPrIm)2]+.
Figure 2. 1H NMR spectra of (a) [Fe(TiPrP)(2-MeIm)(CN)] and (b)
[Fe(Me-TPP)(2-MeIm)(CN)] taken at -71 °C in CD2Cl2. The inset
shows the 1H NMR spectrum of the corresponding pyrrole-deuterated
complex taken at the same temperature.
at 32.4, 33.1, and 33.7 ppm at -71 °C as shown in Figure 2a.
In the case of [Fe(TRP)(2-iPrIm)(CN)], both the pyrrole H and
meso R-H signals of the isopropyl complex [Fe(TiPrP)(2-iPrIm)-
(CN)] showed splitting even at -10 °C; the pyrrole protons
showed four signals while the meso-R protons gave two signals
with equal intensities at -35 °C. The result contrasts with the
case in [Fe(TiPrP)(2-MeIm)(CN)], where the meso-R protons
gave three signals with 2:1:1 intensity ratio. Further lowering
of the temperature caused broadening of the meso-R signals and
gave a single line at -71 °C.
(ii) Meso r-Proton Signals. The spread of the meso
R-signals decreased on going from the bis(imidazole) complex
to the corresponding mixed-ligand complex. Thus, in the
2-MeIm series, it changed from 4.8 ppm in [Fe(TiPrP)(2-
MeIm)2]+ to 1.3 ppm in [Fe(TiPrP)(2-MeIm)(CN)]+. Similar
decrease was observed in the 2-iPrIm series, from 3.8 ppm in
[Fe(TiPrP)(2-iPrIm)2]+ to ca. 0.4 ppm in [Fe(TiPrP)(2-iPrIm)-
(CN)].
EPR Spectra. (i) [Fe(TRP)(2-MeIm)2]+. Although bis-
(imidazole) complexes were EPR silent at 77 K, they gave clear
signals at 4.2 K as shown in Figure 3. The unsubstituted
complex [Fe(THP)(2-MeIm)2]+, though too insoluble to obtain
a spectrum with good signal-to-noise ratio, showed a large gmax
type signal at g ) 3.45. The Me and Et complexes exhibited
broad signals at g ) 2.9 and 3.0, respectively. In contrast, the
iPr complex showed an axial type spectrum where a sharp signal
appeared at g ) 2.58. The EPR g values of these complexes
are listed in Table 5.
(iii) [Fe(Me-TPP)(L)2]+. The 1H NMR spectra of [Fe(Me-
TPP)(L)2]+ have already been reported in our previous paper.24
For comparison with other complexes, the chemical shifts are
listed in Table 2a.
(iv) [Fe(Me-TPP)(L)(CN)]. The 1H NMR spectra of [Fe(Me-
TPP)(L)(CN)] (L ) 1-MeIm, 2-MeIm, 2-iPrIm) were measured
in CD2Cl2-CD3OD over a wide temperature range. The
chemical shifts of these complexes at 25 and -71 °C are listed
in Table 2b. In [Fe(Me-TPP)(1-MeIm)(CN)], the pyrrole signal
appeared at -14.9 ppm at 25 °C and moved to the higher
magnetic field as the temperature was lowered. The sharpness
of the signal was maintained even at -71 °C. In contrast, [Fe-
(Me-TPP)(2-MeIm)(CN)] showed a pyrrole signal at -6.3 ppm
at 25 °C, which broadened considerably at lower temperature
and started to split below -91 °C. Much clearer splitting of
the signal was observed in [Fe(Me-TPP)(2-iPrIm)(CN)], where
the pyrrole signals appeared at -12.7, -5.8, -1.4, and -0.9
ppm at -85 °C as shown in Figure 2b. The assignment of these
signals was confirmed by the spectral comparison with the
corresponding pyrrole deuterated complex. The ortho-methyl
protons also showed complicated signals at this temperature.
13C NMR Spectra. The 13C NMR spectra of unlabeled [Fe-
(TMeP)(2-MeIm)2]+, [Fe(TEtP)(2-MeIm)2]+, and [Fe(TiPrP)-
(2-MeIm)2]+ were taken at various temperatures. In these
complexes, meso-carbon signals appeared at the lowest magnetic
field. While the Me and Et complexes showed the meso signals
(ii) [Fe(TRP)(2-MeIm)(CN)]. The mixed-ligand complexes
were obtained by the addition of 2.0 mol equiv of Bu4N+CN-
as a CH3OH solution into a CH2Cl2 solution of [Fe(TRP)(2-
MeIm)2]+. The EPR spectra of these complexes taken at 4.2
K are given in Figure 4. Although the unsubstituted complex
showed a large gmax type spectrum, the Me and Et complexes
showed sharp axial type spectra. Close inspection of the figure
revealed that the sample was contaminated with small amounts
of bis(cyanide) and bis(imidazole) complexes. Further addition
of cyanide into the solution increased the intensity of the peak
ascribed to bis(cyanide). Thus, by the addition of various
amounts of cyanide into [Fe(TRP)(2-MeIm)2]+, we were able
to assign the signals derived from the mixed-ligand complex.
The g values of these complexes are listed in Table 5 together
with those of the corresponding [Fe(TRP)(CN)2]-.32
i
at δ 195 and 200 ppm, respectively, the Pr complex showed
two signals at much lower magnetic field, δ 323 and 394 ppm

2410 Inorganic Chemistry, Vol. 37, No. 10, 1998
Nakamura et al.
Table 2. 1H NMR Chemcial Shifts of a Series of Low-Spin [Fe(Me-TPP)(L)2]+ a and [Fe(Me-TPP)(L)(CN)]b
L
Py-H
o-Me
m-H
p-Me
Im-Me
(a) [Fe(Me-TPP)(L)2]+
1-MeImc
1-MeImd
2-MeImc
2-MeImd
2-iPrImc
2-iPrImd
-17.2
-30.9
-10.8
0.8
0.0
1.5
6.0
4.9
7.2
1.7
17.0
1.0
25.7
2.0
5.3
5.0
-2.3
-10.9, 0.0
-24.4, -22.0, -20.0, -15.4
-10.8
-7.6, -3.3, 4.8, 10.8
5.5, 5.5, 6.8, 9.8
7.6
7.2, 7.2, 7.6, 10.6
0.8, 2.2
2.9
1.8
-24.3, -21.2, -19.4, -13.4
-7.1, -2.7, 6.2, 12.5
1.1, 2.7
(b) [Fe(Me-TPP)(L)(CN)]
0.9, 1.1
1-MeImc
1-MeImd
2-MeImc
2-MeImd
2-iPrImc
2-iPrImd
-14.9
6.3, 6.4
1.76
11.8
16.3
3.2
-23.4
0.6, 0.7
6.4, 6.5
1.46
-6.3
1.5, 1.7
8.1, 8.2
2.34
-6.1 (broad)
1.5 (broad)
9.7, 9.8
2.58
3.1
-6.1
1.8, 1.9
8.20, 8.23
2.10
-2.4
-6.2
-12.9, -6.7, -1.9, -1.5
-6.6, -1.8, 3.5, 4.1, 5.7
8.2-10.5e
2.7, 2.9
a Data from ref 24. b This work. c Chemical shifts at 25 °C. d Chemical shifts at -71 °C. e Several signals for the m-protons were observed at
-85 °C at δ 9.8, 10.0, 10.5, 10.9, 11.1, and 11.5 ppm.
Table 3. 13C NMR Chemical Shifts of the Meso Carbons in
[Fe(TRP)(2-MeIm)2]Cl Taken in CDCl3
R
Me
Et
25 °C
194.6
200.0
-60 °C
∆δa
117
b
148
265
b
322.5
393.8
397
507
iPr
110
a Difference in chemical shifts at -60 °C. b Signals were not
observed due to the low solubility.
Table 4. Spread of the Pyrrole and Meso R-Proton Signals at -71
°C
(a) Pyrrole Signals (Tetraalkyl Complexes)a
[Fe(TRP)(L)2]+
[Fe(TRP)(L)(CN)]
2-MeIm
2-iPrIm
R
2-MeIm
2-iPrIm
Me1
Et
9.4
8.2
5.7
10.7
8.4
5.4
b
b
4.0
b
b
4.1
iPr
(b) Pyrrole Signals (Tetraaryl Complexes)c
[Fe(R-TPP)(L)2]+
[Fe(R-TPP)(L)(CN)]
2-MeIm
2-iPrIm
9.0 10.9
Figure 3. EPR spectra of a series of [Fe(TRP)(2-MeIm)2]+ taken at
4.2 K in frozen CH2Cl2 solution. From the top to the bottom, R ) iPr,
Et, Me, and H.
R
2-MeIm
2-iPrIm
Me
Et
b
d
d
11.4
d
Table 5. EPR Parameters of [Fe(TRP)(2-MeIm)2]+ (A),a
[Fe(TRP)(2-MeIm)(CN-)] (B),a and [Fe(TRP)(CN)2]- (C)b Taken at
4.2 K in CH2Cl2-CH3OH Solution
9.5
8.9
11.3
10.4
iPr
d
(c) Meso R-Signals (Tetraalkyl Complexes)a
[Fe(TRP)(L)2]+
[Fe(TRP)(L)(CN)]
A
B
C
R
|gx|
|gy|
|gz|
|gx|
|gy|
|gz|
|gx|
|gy|
|gz|
R
2-MeIm
2-iPrIm
2-MeIm
2-iPrIm
H
Me
Et
c
c
3.45
2.9
c
c
3.3
c
c
3.5
c
c
2.1
1.9
2.5
2.5
2.5
2.5
1.6d 2.43 2.43 1.69
1.6d 2.47 2.47 1.61
Me
Et
10.6
2.4
4.8
11.5
2.7
3.8
b
b
1.3
b
b
3.0
iPr 2.58 2.58 1.45d 2.45 2.45 1.67 2.35 2.35 1.82
iPr
ca. 0.4
a This work. b Signal splitting was not observed even at -71 °C.
a This work. b Data from ref 32. c Difficult to determine. d Calculated
value.
c Data in ref 24 were extrapolated to -71 °C. d Not examined.
for the meso-R protons. Thus, the stable conformation in this
complex is consistent with the one given in Figure 5b.
However, the chemical shift of one of the meso R-protons could
coincide with the other. This is possible because the meso
R-signals in [Fe(TiPrP)(2-MeIm)(CN)] appeared in quite a
narrow region, 32.4 (2H), 33.1 (1H), and 33.7 (1H) ppm at -71
°C. Thus, we cannot rule out the conformation given in Figure
5a. Considering the fact that the porphyrin ring in the Ni(Ti-
PrP) is highly deformed and has two cavities along the diagonal
Cmeso-Fe-Cmeso axes,33 it might be much reasonable to consider
that the 2-iPrIm ligand is placed along the cavity as in the case
of [Fe(TiPrP)(2-MeIm)(CN)].
Discussion
Stable Conformation. Low-temperature 1H NMR spectrum
of [Fe(TiPrP)(2-MeIm)(CN)] showed four signals for the pyrrole
protons and three signals for the meso-R protons. The intensity
ratio of the latter signals was 2:1:1 as shown in Figure 2a. On
the basis of the splitting pattern of the H NMR signals, the
frozen conformation of this complex was determined to be the
1
one where the coordinated 2-MeIm is aligned along the Cmeso
-
Fe-Cmeso axis as shown in Figure 5a. The low-temperature
1H NMR spectra of [Fe(TiPrP)(2-iPrIm)(CN)] showed four
signals for the pyrrole and two signals with equal intensities

(meso-Tetraalkylporphyrinato)iron(III)
Inorganic Chemistry, Vol. 37, No. 10, 1998 2411
[Fe(H-TPP)(2-MeIm)2]+.34 Thus, the electron configuration of
[Fe(THP)(2-MeIm)2]+ should be presented as (dxy)2(dxz, dyz)3.
In the case of [Fe(TMeP)(2-MeIm)2]+ and [Fe(TEtP)(2-
MeIm)2]+, the EPR spectra were quite unusual in a sense that
they gave single unsymmetrical signal. Although the g values
of these complexes were quite small, 2.9 and 3.0 for the Me
and Et complexes, respectively, their spectra resemble large gmax
type signal in shape. Similar EPR spectra were reported by
Safo et al. in bis(3-ethylpyridine)- and bis(3-chloropyridine)-
{meso-tetramesitylporphyrinato}iron(III) complexes, which
showed single-feature large gmax type spectra but with unusually
low g values, 2.89 and 3.07, respectively.26 Thus, the present
EPR results together with the upfield-shifted pyrrole signals,
-8.2 (R ) Me) and -9.4 ppm (R ) Et) at -35 °C, suggest
that the ground-state electron configuration of the ferric ions in
[Fe(TMeP)(2-MeIm)2]+ and [Fe(TEtP)(2-MeIm)2]+ is (dxy)2(dxz,
dyz)3 although the energy differences between dxy and dπ(dxz,
dyz) orbitals are expected to be much closer than that of [Fe-
(THP)(2-MeIm)2]+. In the case of [Fe(TiPrP)(2-MeIm)2]+, a
sharp axial type EPR signal and an extremely downfield-shifted
pyrrole proton signals were observed, which clearly indicate
that the electron configuration of the iron is best presented by
the unusual (dxz, dyz)4(dxy)1.
Figure 4. EPR spectra of a series of [Fe(TRP)(2-MeIm)(CN)] taken
at 4.2 K in frozen CH2Cl2-CH3OH solution. From the top to the bottom,
i
R ) Pr, Et, Me, and H.
(ii) [Fe(TRP)(L)(CN)]. The data in Table 1b show that the
pyrrole protons of [Fe(TRP)(L)(CN)] appear at lower magnetic
field than those of the corresponding [Fe(TRP)(L)2]+. In the
complexes with L ) 2-MeIm, for example, the pyrrole chemical
shifts of [Fe(TRP)(2-MeIm)(CN)] were -21.0, +1.8, +1.8, and
+12.4 ppm at -35 °C for R ) H, Me, Et, and iPr, respectively,
as compared with -22.5, -8.2, -9.4, and +5.2 ppm in [Fe-
(TRP)(2-MeIm)2]+. As for the unsubstituted complex, [Fe-
(THP)(2-MeIm)(CN)], the upfield shifted pyrrole signal together
with the large gmax type EPR signal at g ) 3.3 shown in Figure
4 suggests that the electron configuration should be presented
by (dxy)2(dxz, dyz)3. In contrast, the pyrrole signals in the Me
and Et complexes appeared in a so-called diamagnetic region,
which might be the indication that the electron configuration
of these complexes changed from (dxy)2(dxz, dyz)3 to (dxz, dyz)4-
(dxy)1 by the replacement of one of the imidazole ligands with
CN-. In fact, these complexes showed axial type EPR spectra
as shown in Figure 4. It is, therefore, concluded that the electron
configuration of the ferric ions in [Fe(TRP)(2-MeIm)(CN)] is
given by (dxz, dyz)4(dxy)1 if the meso-carbons carry alkyl
substituents.
(iii) [Fe(Me-TPP)(L)2]+ and [Fe(Me-TPP)(L)(CN)]. As
shown in Table 2a,b, all the pyrrole proton signals in [Fe(Me-
TPP)(L)2]+ and [Fe(Me-TPP)(L)(CN)] appeared in the high-
field region typical to the low-spin complexes, -30 to -6 ppm
at -71 °C. Thus, the electron configuration of the ferric ions
should be presented by the usual (dxy)2 (dxz, dyz)3, which is
supported by the EPR spectra of [Fe(Me-TPP)(L)2]+ reported
previously; [Fe(Me-TPP)(2-MeIm)2]+ showed a large gmax type
spectrum with g ) 3.17, and [Fe(Me-TPP)(HIm)2]+ gave
rhombic spectrum with gz ) 2.92, gy ) 2.29, and gx ) 1.57.24,26
The EPR spectrum of the analogous [Fe(H-TPP)(Py)(CN)] was
also reported to show a large gmax type signal at g ) 3.31.35
(iv) Correlation between Pyrrole and Imidazole Methyl
Shifts. The chemical shifts of the coordinated imidazole protons
can be affected by the electron configuration of the ferric ion.
If the electron configuration of the ferric ion is the usual (dxy)2-
Figure 5. Conformation of the mixed-ligand complexes. The imidazole
ligand is aligned along the diagonal (a) Cmeso-Fe-Cmeso axis and (b)
N-Fe-N axis.
The conformation of the coordinated imidazole ligand in [Fe-
(Me-TPP)(2-MeIm)(CN)] is determined to be similar to that in
[Fe(TiPrP)(2-MeIm)(CN)] on the basis of the 1H NMR splitting
pattern. Especially suggestive is the splitting pattern of the meta
protons shown in Figure 2b. These protons showed at least
five signals in the region between 9.8 and 11.5 ppm, which is
only explainable by the conformation given in Figure 5a.
Electron Configuration of Iron. (i) [Fe(TRP)(L)2]+. The
1H NMR data in Table 1a show that the pyrrole signal in [Fe-
(TRP)(L)2]+ moves to the lower magnetic field as the bulkiness
of the meso substituent increases. In a series of [Fe(TRP)(2-
MeIm)2]+ complexes, for example, the pyrrole chemical shift
changed from -22.5 ppm in [Fe(THP)(2-MeIm)2]+ to +5.2 ppm
(average of the four signals) in [Fe(TiPrP)(2-MeIm)2]+ at -35
°C. In a previous paper, we have observed a similar phenom-
enon in the low-spin [Fe(TRP)(CN)2]- and ascribed it to the
change in electron configuration of the ferric ion from the usual
(dxy)2(dxz, dyz)3 in the R ) H complex to the unusual (dxz,
dyz)4(dxy)1 in the R ) iPr complex.32 The EPR results given in
Figure 3 and Table 5 clearly indicate that the change in electron
configuration also takes place in this system. In the case of
[Fe(THP)(2-MeIm)2]+, the large gmax type signal centered at g
) 3.45 was observed. This value is quite close to g ) 3.40 of
(33) 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, W. A., III;
Shelnutt, J. A. J. Am. Chem. Soc. 1995, 117, 11085-11097.
(34) Walker, F. A.; Reis, D.; Balke, V. L. J. Am. Chem. Soc. 1984, 106,
6888-6898.
(35) Inniss, D.; Soltis, S. M.; Strouse, C. E. J. Am. Chem. Soc. 1988, 110,
5644-5650.

2412 Inorganic Chemistry, Vol. 37, No. 10, 1998
Nakamura et al.
interaction between iron and porphyrin orbitals: (i) the decrease
in the interaction between iron dπ(dxz and dyz) and porphyrin
3eg orbitals due to the less effective overlap of the orbitals32
and (ii) the increase in the interaction between iron dxy and
porphyrin a2u orbitals as Walker and co-workers pointed out.38,39
The weakened dπ and 3eg interaction would stabilize the dπ
orbitals and contribute to the formation of the unusual (dxz, dyz)4-
(dxy)1 electron configuration. The latter interaction, the interac-
tion between iron dxy and porphyrin a2u orbitals becomes possible
in the complexes with S4 ruffled structure since the a2u orbital,
which is orthogonal to the dxy orbital in the porphyrin complex
with a D4h symmetry, has xy components and thus can interact
with the dxy orbital. The strong a2u-dxy interaction would
destabilize the dxy orbital relative to the dyz and dxz orbitals and
contribute to the change in electron configuration from (dxy)2-
(dxz, dyz)3 to (dxz, dyz)4(dxy)1. Because of these reasons, the ferric
ions in the complexes with highly S4-deformed porphyrin ring
exhibit (dxz, dyz)4(dxy)1. Recent studies by Latos-Grazynski and
co-workers using highly deformed chiroporphyrins have shown
the same results.40 One of the experimental methods to ascertain
Figure 6. Correlation of the chemical shifts at -35 °C between pyrrole
and imidazole methyl proton signals in a series of complexes [Fe(TRP)-
(2-MeIm)2]+ (1, R ) H; 2, R ) Me; 3, R ) Et; 4, R ) iPr), [Fe(TRP)-
(2-MeIm)(CN)] (5, R ) H; 6, R ) Me; 7, R ) Et; 8, R ) iPr), [Fe(Me-
TPP))(2-MeIm)2]+ (9), and [Fe(Me-TPP)(2-MeIm)(CN)] (10).
the importance of the a2u-dxy interaction is to measure the 13
C
NMR spectra of a series of complexes. Since the 13C chemical
shift directly reflects the spin densities on carbons,41 the
complexes with the (dxz, dyz)4(dxy)1 configuration are expected
to show the meso carbon signals at more paramagnetically
shifted positions than those with the (dxy)2(dxz, dyz)3 configura-
tion; the a2u orbital has a large spin density on the meso
carbons.42 The data in Table 3 indicate that, while the Me and
Et complexes gave singlets at δ 194.6 and 200.0 ppm at 24 °C,
(dxz, dyz)3, the imidazole signals are expected to appear at
paramagnetically shifted positions.36 This is because the
imidazole pπ orbitals have correct symmetry to interact with
the singly occupied iron dπ orbitals. In contrast, the paramag-
netic shifts of the imidazole protons must be rather small in the
complexes with the unusual (dxz, dyz)4(dxy)1 configuration, since
the unpaired electron is in the dxy orbital which is orthogonal
to the imidazole pπ orbitals in a D4h porphyrin complex. Thus,
examination of the relationship between pyrrole and imidazole
methyl shifts must be a good test to confirm the electron
configuration of the low-spin ferric porphyrin complexes; the
paramagnetic shift of the imidazole methyl protons is expected
to decrease as the meso substituent becomes bulkier. Figure 6
shows the relationship between the pyrrole and imidazole methyl
shifts in [Fe(TRP)(2-MeIm)2]+ and [Fe(TRP)(2-MeIm)(CN)].
As the pyrrole signals move to the lower magnetic field, in other
words, as the electron configuration of the ferric ion changes
from (dxy)2(dxz, dyz)3 to (dxz, dyz)4(dxy)1, the imidazole methyl
signals shift to the higher magnetic field, from 21.4 ppm in
[Fe(THP)(2-MeIm)(CN)] to -6.0 ppm in [Fe(TiPrP)(2-MeIm)-
(CN)] at -35 °C. Since the chemical shift of the imidazole
methyl protons in the corresponding diamagnetic [Co(TiPrP)-
(2-MeIm)2]+ is -2.3 ppm,37 the paramagnetic shift actually
decreased as the electron configuration changes from (dxy)2(dxz,
dyz)3 to (dxz, dyz)4(dxy)1. It should be noted that the complexes
4 and 6-8 in Figure 6, all of which are located to the right and
bottom of the graph, have the ferric ions with (dxz, dyz)4(dxy)1
configuration as confirmed by the EPR spectra.
i
respectively, the Pr complex showed two signals due to the
hindered rotation of the imidazole ligands at much lower
magnetic field, 322.5 and 393.8 ppm. If we assume the
chemical shift of the analogous diamagnetic [Co(Me-TPP)(2-
MeBzIm)2]+, 119.5 ppm at 25 °C, as a diamagnetic reference,43
i
the isotropic shifts of the R ) Me, Et, and Pr complexes are
calculated to be 75.1, 80.5, and 238.7 ppm (average of two
signals), respectively. Since the electron configuration of the
Me and Et complexes is determined as (dxy)2(dxz, dyz)3 and that
i
of the Pr complex as (dxz, dyz)4(dxy)1 on the basis of the EPR
i
results, the large spin densities on the meso carbons in the Pr
complex are interpreted as the consequence of the unusual
electron configuration caused by the a2u-dxy interactions in this
complex.44
(ii) Bulky Axial Ligands. The unusual electron configuration
would be stabilized by the coordination of bulky imidazole
ligands, since the coordination induces further deformation of
the porphyrin ring.23,37,45-47 Thus, the complexes with bulkier
(38) 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.
(39) Cheesman, M. R.; Walker, F. A. J. Am. Chem. Soc. 1996, 118, 7373-
7380.
(40) Wolowiec, S.; Latos-Grazynski, L.; Mazzanti, M.; Marchon, J.-C.
Inorg. Chem. 1997, 36, 5761-5771.
(41) Goff, H. M. J. Am. Chem. Soc. 1981, 103, 3714-3722.
(42) Fajer, J.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1979; Vol. IV, pp 197-256.
Factors Stabilizing the Unusual Electron Configuration.
(i) Bulky Meso Substituents. In a previous paper, we have
concluded that the low-spin [Fe(TRP)(CN)2]- with a deformed
porphyrin ring tends to have a ferric ion with the unusual
electron configuration.32 According to the recent studies on the
molecular structure of a series of [Fe(TRP)Ni] complexes, the
ruffling dihedral angle, defined by the CRN-NCR for nitrogens
in the diagonal pyrrole rings, increases as the size of the meso
(43) Nakamura, M.; Ikezaki, A. Chem. Lett. 1995, 733-734.
(44) A recent 1H NMR study on the low-spin bis(cyanide) and bis(pyridine)
complexes of (quinoxalino)(tetraphenylporphyrinato)iron(III) have
revealed that the electron configurations of the iron(III) of these
complexes are also presented as (dxz, dyz)4(dxy)1, which was explained
in terms of the π-accepting nature of this porphyrin as compared with
that of TPP: Wojaczynski, J.; Latos-Grazynski, L.; Glowiak, T. Inorg.
Chem. 1997, 36, 6299-6306.
i
substituents increases; the angles in the Me, Et, and Pr
complexes are calculated to be 25.3, 21.0, and 36.6°, respec-
tively.33 The ruffling would cause two major effects on the
(36) Satterlee, J. D.; La Mar, G. N. J. Am. Chem. Soc. 1976, 98, 2804-
2808.
(45) Scheidt, W. R.; Kirner, J. F.; Hoard, J. L.; Reed, C. A. J. Am. Chem.
Soc. 1987, 109, 1963-1968.
(46) Nakamura, M. Bull. Chem. Soc. Jpn. 1995, 68, 197-203.
(37) Saitoh, T.; Ikeue, T.; Ohgo, Y.; Nakamura, M. Tetrahedron 1997, 53,
12487-12496.

(meso-Tetraalkylporphyrinato)iron(III)
Inorganic Chemistry, Vol. 37, No. 10, 1998 2413
Thus, the iron dπ orbital can be stabilized by the cyanide π*
orbitals due to π back-bonding from metal to ligand.52 In the
Me and Et complexes where the porphyrin deformation is
shallow, dπ orbitals are slightly higher than the dxy orbital as
revealed from the EPR spectra of [Fe(TMeP)(2-MeIm)2]+ and
[Fe(TEtP)(2-MeIm)2]+. Coordination of cyanide decreases the
energy level of the dπ orbitals through dπ-pπ* interaction,
resulting in the change in electron configuration from (dxy)2-
i
(dxz, dyz)3 to (dxz, dyz)4(dxy)1. In the Pr complexes where the
porphyrin ring is highly deformed, electronic effect of the axial
ligands would be less important than that in the Me and Et
complexes, since the energy level of the dxy orbital is already
higher than that of the dπ orbitals. This is the reason why the
i
chemical shifts of the pyrrole and meso R-protons in the Pr
complexes are less dependent on the axial ligands than those
of the Me and Et complexes; pyrrole signals in the iPr complexes
appeared in a relatively narrow region, δ 0.2-12.3 ppm, as
compared with those of the Me complexes, δ -19.6 to 2.9 ppm.
Common NMR Features of the Complexes with (dxz, dyz)4-
(dxy)1 Configuration. Figure 7 clearly shows that the com-
plexes with (dxz, dyz)4(dxy)1 configuration have pyrrole signals
at much lower field than those with (dxy)2(dxz, dyz)3. The
borderline of the chemical shifts separating the two classes of
complexes seems to be around 0 ppm at -35 °C in the
complexes examined in this study. In the Me and Et series,
the complexes 8 and 9 satisfy this condition. The electron
configuration of 8 has already been confirmed to be (dxz, dyz)4-
(dxy)1 by the EPR spectra. Although the EPR spectrum of 9 is
not available, the ferric ion of this complex is supposed to have
the (dxz, dyz)4(dxy)1 configuration on the basis of the NMR
chemical shifts. In the case of the iPr series, all the complexes
examined satisfy the condition, indicating that the electron
configuration of these complexes should be presented by (dxz,
dyz)4(dxy)1 regardless of the axial ligands.
Another common feature of the complexes with the unusual
electron configuration appears in the slopes of the Curie plots.
The Curie plots of the pyrrole signals in the low-spin ferric
porphyrin complexes usually give large negative slopes ranging
from -9000 to -5000 ppm‚K in the case of [Fe(R-TPP)-
(L)2]+.24,53 In fact, the Curie slopes of the typical low-spin
complexes examined in this study such as [Fe(THP)(1-MeIm)2]+
and [Fe(THP)(2-MeIm)(CN)] were -8900 and -7700 ppm‚K,
respectively, as shown in Figure 1a,b. As the meso substituent
and imidazole ligand become bulkier, the Curie slope gradually
increased; the slopes in [Fe(TEtP)(2-MeIm)2]+ and [Fe(TEtP)-
(1-MeIm)(CN)] were -5800 and -2700 ppm‚K, respectively.
All of these complexes are supposed to have the ferric ions with
the usual (dxy)2(dxz, dyz)3 electron configuration as judged from
their pyrrole shifts and/or EPR spectra. On the contrary, the
Curie slopes were positive in the complexes with (dxz, dyz)4-
(dxy)1 configuration; the slopes of [Fe(TiPrP)(2-MeIm)2]+, [Fe-
(TMeP)(2-MeIm)(CN)], and [Fe(TiPrP)(2-MeIm)(CN)] were
+660, +1100 and +2000 ppm‚K, respectively. The only
exception was [Fe(TEtP)(2-MeIm)(CN)], which showed a very
small negative slope, -370 ppm‚K. Thus, the 1H NMR spectral
features of [Fe(TRP)(L)2]+ and [Fe(TRP)(L)(CN)], where the
ferric ions are presented by (dxz, dyz)4(dxy)1, include a downfield
shift (lower than 0 ppm) and a positiVe Curie slope of the pyrrole
Figure 7. Correlation of the chemical shifts at -35 °C between pyrrole
and meso R-proton signals in a series of [Fe(TRP)(L)2]+ and [Fe(TRP)-
(L)(CN)] complexes. Numbers 1-6 represent the bis(imidazole)
complexes [Fe(TRP)(L)2]+, where L is Im (1), 1-MeIm (2), 2-MeIm
(3), 2-EtIm (4), 2-iPrIm (5), and 1,2-Me2Im (6). Numbers 7-9 represent
the mixed-ligand complexes [Fe(TRP)(L)(CN)], where L is 1-MeIm
(7), 2-MeIm (8), and 2-iPrIm (9).
imidazole ligands are expected to show pyrrole signals at lower
magnetic field than those with less bulky imidazole ligands. It
is also expected that the bulky imidazole ligands shift the meso
R-protons to further lower field due to the stronger a2u-dxy
interaction. To find out if this is the case, the correlation of
the chemical shifts between the pyrrole and meso R-protons in
[Fe(TRP)(L)2]+ and [Fe(TRP)(L)(CN)] was examined at -35
°C. The axial ligands (L) examined are Im, 1-MeIm, 2-MeIm,
2-EtIm, 2-iPrIm, and 1,2-Me2Im. The results given in Figure
7 suggest that the low-field shifts of both the pyrrole and meso
R signals increase as the axial ligand changes in the following
order: Im (1), 1-MeIm (2) < 2-MeIm (3), 2-EtIm (4), 2-iPrIm
(5), 1-MeIm(CN-) (7) < 1,2-Me2Im (6) < 2-MeIm(CN-) (8),
2-iPrIm(CN-) (9). Thus, the complexes with bulkier imidazole
ligands generally shift both the pyrrole and meso R-proton
signals to the lower magnetic field, supporting the hypothesis
described above.
(iii) Axial Ligands with Low-Lying π* Orbitals. Figure 7
shows that cyanide is more effective ligand than imidazole for
the formation of the complexes with unusual electron config-
uration. This is clearly seen in the Me and Et complexes; mixed
ligand complexes 1-MeIm(CN-) (7), 2-MeIm(CN-) (8), and
2-iPrIm(CN-) (9) showed pyrrole and meso R signals at lower
magnetic field than the corresponding bis(imidazole) complexes
1-MeIm (2), 2-MeIm (3), and 2-iPrIm (5). It has been reported
that (dxz, dyz)4(dxy)1 state can be stabilized relative to (dxy)2(dxz,
dyz)3 by the coordination of the axial ligands with weak
σ-donating and strong π-accepting ability.26,38,39,48-51 Although
cyanide ligand is a strong σ donor, it also acts as π acceptor.
(47) Momot, K. I.; Walker, F. A. J. Phys. Chem. A 1997, 101, 2787-
2795.
(48) Simonneaux, G.; Hindre, F.; Le Plouzennec, M. Inorg. Chem. 1989,
28, 823-825.
(51) Pilard, M.-A.; Guillemot, M.; Toupet, L.; Jordanov, J.; Simonneaux,
G. Inorg. Chem. 1997, 36, 6307-6314.
(52) Lukas, B.; Silver, J. Inorg. Chim. Acta 1986, 124, 97-100.
(53) Walker, F. A.; Simonis, U.; Zhang, H.; Walker, J. M.; Ruscitti, T.
M.; Kipp, C.; Amputch, M. A.; Castillo, B. V., III; Cody, S. H.;
Wilson, D. L.; Graul, R. E.; Yong, G. J.; Tobin, K.; West, J. T.;
Barichievich, B. A. New. J. Chem. 1992, 16, 609-620.
(49) Guillemot, M.; Simonneaux, G. J. Chem. Soc., Chem. Commun. 1995,
2093-2094.
(50) 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.

2414 Inorganic Chemistry, Vol. 37, No. 10, 1998
Nakamura et al.
1
signals. The complexes that satisfy these H NMR spectral
that the spin densities on the meso carbons in [Fe(TiPrP)(2-
MeIm)(CN)] are much more homogeneous than those in [Fe-
(TiPrP)(2-MeIm)2]+, although the spin densities themselves are
larger in the former than in the latter.
features are considered to exhibit axial type EPR spectra.
Spin Distribution on the Peripheral Carbons. The data
in Table 4 indicate that the spread of the pyrrole proton signals
decreases as the meso substituent becomes bulkier. In the case
of [Fe(TRP)(2-MeIm)2]+, the spread was 9.4 (Me), 8.2 (Et) and
5.7 (iPr) ppm at -71 °C. In the mixed-ligand complexes such
as [Fe(TiPrP)(2-MeIm)(CN)] and [Fe(TiPrP)(2-iPrIm)(CN)], the
spread further decreased to 4.0 and 4.1 ppm, respectively,
indicating that the spin densities on the pyrrole â-carbons are
quite homogeneous. In contrast, the spread of the pyrrole signals
in tetramesitylporphyrin system was much larger; it was 9.0
ppm in [Fe(Me-TPP)(2-MeIm)2]+, 10.9 ppm in [Fe(Me-TPP)-
(2-iPrIm)2]+, and 11.4 ppm in [Fe(Me-TPP)(2-iPrIm)(CN)] at
-71 °C. Since the complexes with relatively small spread of
the pyrrole signals such as [Fe(TiPrP)(2-MeIm)2]+ and [Fe(Ti-
PrP)(2-MeIm)(CN)] showed axial type EPR spectra, it might
be natural to consider that the homogeneous spin densities on
the pyrrole â-carbons are the consequence of the unusual (dxz,
dyz)4(dxy)1 configuration.
Direct evidence for the spin distribution on the meso carbons
was obtained from the 13C NMR chemical shift. While the meso
carbon signals of the Me complex was observed at 148 and
265 ppm at -60 °C, those of the iPr complex appeared at much
lower field, 397 and 507 ppm at the same temperature. These
data indicate that the spin densities of the meso carbons in the
iPr complex with (dxz, dyz)4(dxy)1 configuration are much larger
than those in the Me complex with (dxy)2(dxz, dyz)3 configuration,
although the difference in spin densities among four meso
carbons is quite similar between these two complexes; the
chemical shift differences at -60 °C were 117 and 110 ppm,
respectively. The results suggest that the spin densities among
i
four meso carbons are much more homogeneous in the Pr
complex than in the Me complex.
Conclusion
The similar trend was observed in the relationship between
the spread of meso R-proton signals and the EPR g values,
though the data for the comparison are quite limited. The
spreads of meso-CH signals and EPR g values in [Fe(TiPrP)-
(2-MeIm)(CN)] were 1.3 ppm (-71 °C) and 2.45 (4.2 K),
respectively, while those in [Fe(TiPrP)(2-MeIm)2]+ were 4.8
ppm and 2.58. Since the g value of the former is smaller than
that of the latter, the ferric ion in the former must have a much
purer (dxz, dyz)4(dxy)1 configuration.26,54-56 In fact, the energy
differences between dxy and dπ orbitals calculated based on the
EPR g values were 3.48λ and 2.14λ for [Fe(TiPrP)(2-MeIm)-
(CN)] and [Fe(TiPrP)(2-MeIm)2]+, respectively. This means
that the ferric ion of the former has 94% (dxz, dyz)4(dxy)1 character
while that of the latter has 84%. It is noteworthy that the meso-
CH signals in [Fe(TiPrP)(2-MeIm)(CN)] appeared at rather low
field, 32.4, 33.1, and 33.7 ppm, as compared with those in [Fe-
(TiPrP)(2-MeIm)2]+, 20.4 and 25.2 ppm. The results indicate
1
Combined H and 13C NMR and EPR studies of a series of
low-spin ferric porphyrin complexes, [Fe(TRP)(L)2]+ and [Fe-
(TRP)(L)(CN)], where R ) H, Me, Et, or iPr and L is 1-MeIm,
2-MeIm, or 2-iPrIm, have revealed that electron configuration
of these complexes changes from (dxy)2(dxz, dyz)3 to (dxz, dyz)4-
(dxy)1 as the bulkiness of meso-alkyl group increases. When
the rotation of axially coordinated 2-MeIm or 2-iPrIm is frozen
at low temperature on the NMR time scale, both the pyrrole â-
and meso R-proton signals as well as the meso-carbon signals
have shown splitting. On the basis of the chemical shifts and
the spreads of these signals, it is concluded that the major spin
densities in the complexes with (dxz, dyz)4(dxy)1 electron con-
figuration are on the meso-carbons and that the spin distributions
on the four meso carbons as well as on the eight pyrrole
â-carbons are quite homogeneous. On the contrary, the major
spin densities in the complexes with (dxy)2(dxz, dyz)3 electron
configuration are on the pyrrole â-carbons and the fixation of
the coordinated imidazole ligands induces relatively large
asymmetric spin distribution both on the four meso- and on the
eight pyrrole â-carbons.
(54) Taylor, C. P. S. Biochim. Biophys. Acta 1977, 491, 137-149.
(55) Bohan, T. L. J. Magn. Reson. 1977, 26, 109-118.
(56) Palmer, G. In Iron Porphyrins Part Two; Lever, A. B. P., Gray, H.
B., Eds.; Addison-Wesley Publishing Co.: Reading, MA, 1983; pp
43-88.
IC9801241