Electron configuration of ferric ions in low-spin (dicyano)(meso-tetraarylporphyrinato)iron(III) complexes

Article abstract of DOI:10.1021/ic990328x

The electron configuration of a series of low-spin (dicyano){meso-tetrakis(2,4,6-trialkylphenyl)porphyrinato}iron(III) complexes, [Fe(R-TPP)(CN)₂]^- where R = Me, Et, or ⁱPr, together with the parent [Fe(TPP)(CN)₂]^-, has been examined in dichloromethane-methanol solution by ¹H NMR, ₁₃C NMR, and EPR spectroscopies. While the ferric ion of [Fe(TPP)(CN)₂]^- has shown a common (d_xy)2(d_xzd_yz)³ configuration, the ferric ions of the alkyl-substituted complexes [Fe(R-TPP)(CN)₂]^- have exhibited the preference of a less common (d_xz,d_yz)⁴(d_xy)₁ configuration. Spectroscopic characteristics of the complexes in which ferric ions take the (d_xz,d_yz)⁴(d_xy)¹ configuration are (i) axial type EPR spectra, (ii) downfield shifted pyrrole and meta signals in ¹H NMR spectra, and (iii) downfield shifted meso-caibon signals in ¹³C NMR spectra. Occurrence of the less common (d_xz,d_yz)⁴(d_xy)¹ configuration in [Fe(R-TPP)(CN)₂]^- has been ascribed to the electronic interaction between iron(d_π) and cyanide(p_π*) orbitals. The interaction stabilizes the d_π orbitals and induces (d_xzd_yz)⁴(d_xy)¹ configuration. Since the electron configuration of (dicyano){meso-tetrakis(2,6-dichlorophenyI)porphyrinato}iron(III), [Fe(Cl-TPP)(CN)₂]^-, which carries bulky electronegative chlorine atoms at the ortho positions, is presented as a common (d_xy)₂(d_xzd_yz)₃, the less common (d_xz,d_yz)⁴(d_xy)¹ configuration in [Fe(R-TPP)(CN)₂]^- can be ascribed, at least partially, to the electron-donating ability of the meso-aryl groups.

Full text of DOI:10.1021/ic990328x

Inorg. Chem. 1999, 38, 3857-3862
3857
Electron Configuration of Ferric Ions in Low-Spin
(Dicyano)(meso-tetraarylporphyrinato)iron(III) Complexes
Mikio Nakamura,*,†,‡ Takahisa Ikeue,†,§ Akira Ikezaki,† Yoshiki Ohgo,† and Hiroshi Fujii*,|
Department of Chemistry, Toho University School of Medicine, Omorinishi, Ota-ku,
Tokyo 143-8540, Japan, Division of Biomolecular Science, Graduate School of Science,
Toho University, Funabashi, Chiba 274-8510, Japan, and Institute for Molecular Science,
Okazaki 444-8585, Japan
ReceiVed March 23, 1999
The electron configuration of a series of low-spin (dicyano){meso-tetrakis(2,4,6-trialkylphenyl)porphyrinato}-
i
iron(III) complexes, [Fe(R-TPP)(CN)2]- where R ) Me, Et, or Pr, together with the parent [Fe(TPP)(CN)2]-,
1
has been examined in dichloromethane-methanol solution by H NMR, 13C NMR, and EPR spectroscopies.
While the ferric ion of [Fe(TPP)(CN)2]- has shown a common (dxy)2(dxz,dyz)3 configuration, the ferric ions of the
alkyl-substituted complexes [Fe(R-TPP)(CN)2]- have exhibited the preference of a less common (dxz,dyz)4(dxy)1
configuration. Spectroscopic characteristics of the complexes in which ferric ions take the (dxz,dyz)4(dxy)1
1
configuration are (i) axial type EPR spectra, (ii) downfield shifted pyrrole and meta signals in H NMR spectra,
and (iii) downfield shifted meso-carbon signals in 13C NMR spectra. Occurrence of the less common (dxz,dyz)4-
(dxy)1 configuration in [Fe(R-TPP)(CN)2]- has been ascribed to the electronic interaction between iron(dπ) and
cyanide(pπ*) orbitals. The interaction stabilizes the dπ orbitals and induces (dxz,dyz)4(dxy)1 configuration. Since the
electron configuration of (dicyano){meso-tetrakis(2,6-dichlorophenyl)porphyrinato}iron(III), [Fe(Cl-TPP)(CN)2]-,
which carries bulky electronegative chlorine atoms at the ortho positions, is presented as a common (dxy)2(dxz,dyz)3,
the less common (dxz,dyz)4(dxy)1 configuration in [Fe(R-TPP)(CN)2]- can be ascribed, at least partially, to the
electron-donating ability of the meso-aryl groups.
Introduction
|gx| ) |gy| ) 2.35 and |gz| ) 1.82 was observed in contrast to
a large gmax type or a rhombic type spectrum in typical low-
Studies on the electron configuration of ferric ions in
porphyrin complexes are of great importance to understand the
physicochemical properties of synthetic complexes as well as
naturally occurring heme proteins.1-4 In previous papers, we
have reported that the spectroscopic properties of a series of
low-spin six-coordinated (meso-tetraalkylporphyrinato)iron(III)
complexes such as [Fe(TRP)(CN)2]-, [Fe(TRP)(CN)(L)], and
spin complexes.9-12 The large spectral change was interpreted
in terms of a change in electron configuration of the ferric ion
from a common (dxy)2(dxz,dyz)3 to a less common (dxz,dyz)4(dxy)1
as the meso substituents become bulkier. We have ascribed the
(dxz,dyz)4(dxy)1 configuration to the highly S4 ruffled structure
of the porphyrin ring especially in the iPr complex; the (dxz,dyz)4-
(dxy)1 ground state is stabilized by the decrease in interaction
[Fe(TRP)(L)2]+ (R ) H, Me, Et, or Pr, and L ) 1-MeIm,
i
2-MeIm, or 2-iPrIm)5 significantly change as the bulkiness of
the meso substituent increases.6-8 For example, the pyrrole
signal in [Fe(TiPrP)(CN)2]- was observed extremely downfield,
δ 12.8 ppm at -35 °C, in the 1H NMR spectrum as compared
with that of the unsubstituted complex [Fe(THP)(CN)2]-, -21.7
ppm at the same temperature. The EPR spectrum of [Fe(TiPrP)-
(CN)2]- was also quite unusual; an axial type spectrum with
(5) Abbreviations: R-TPP, dianion of meso-tetrakis(2,4,6-trialkylphenyl)-
porphyrin where R is methyl (Me), ethyl (Et), or isopropyl (iPr); Me-
TPP, dianion of meso-tetramesitylporphyrin, usually abbreviated as
TMP; [Fe(R-TPP)(CN)2]-, (dicyano){meso-tetrakis(2,4,6-trialkylphenyl)-
porphyrinato}iron(III); Cl-TPP, dianion of meso-tetrakis(2,6-dichlo-
rophenyl)porphyrin; OMTPP, dianion of 2,3,7,8,12,13,17,18-octamethyl-
5,10,15,20-tetraphenylporphyrin; 1-MeIm, 1-methylimidazole; 2-MeIm,
2-methylimidazole; 1,2-Me2Im, 1,2-dimethylimidazole; 2-MeBzIm,
2-methylbenzimidazole; 4-CNPy, 4-cyanopyridine; tBuNC, tert-butyl
isocyanide.
* To whom correspondence should be addressed (e-mail: mnakamu@
med.toho-u.ac.jp).
(6) Nakamura, M.; Ikeue, T.; Neya, S.; Funasaki, N.; Nakamura, N. Inorg.
Chem. 1996, 35, 3731-3732.
† Toho University School of Medicine.
‡ Graduate School of Science, Toho University.
§ Present address: Department of Chemistry, Faculty of Science, Chiba
University, Chiba 263-8522, Japan.
(7) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T. J. Am. Chem. Soc.
1997, 119, 6284-6291.
(8) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T.; Tajima, K. Inorg.
Chem. 1998, 37, 2405-2414.
| Institute for Molecular Science.
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D. P.; Debrunner, P. G.; Scheidt, W. R. J. Am. Chem. Soc. 1994, 116,
7760-7770.
(9) Palmer, G. Electron Paramagnetic Resonance of Hemoproteins. In Iron
Porphyrins, Part II; Lever, A. B. P., Gray, H. B., Eds.; Physical
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1983; pp 43-88.
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10.1021/ic990328x CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/07/1999

3858 Inorganic Chemistry, Vol. 38, No. 17, 1999
Nakamura et al.
between iron dπ(dxz,dyz) and porphyrin 3eg orbitals,7,8 as well
as the increase in interaction between iron dxy and porphyrin
a2u orbitals.2 The former stabilizes dπ orbitals, and the latter
destabilizes the dxy orbital, resulting in the stabilization of
(dxz,dyz)4(dxy)1 as compared with the commonly observed (dxy)2-
(dxz,dyz)3 state.
(TPP)(tBuNC)2]+ and [Fe(TPP)(4-CNPy)2]+.2,4,32 The X-ray
crystallographic studies of these complexes revealed that the
porphyrin core is highly S4 ruffled in spite of the apparent
absence of the steric interactions between ligands and the
porphyrin core. The reason for the nonplanarity was thus
ascribed to the electronic interaction. Walker, Scheidt, and co-
workers pointed out the possible interactions of iron(dπ) orbitals
with a low-lying ligand π* orbital.1-4 Simonneaux and co-
workers also ascribed the novel electron configuration to the
π-accepting nature of the axial ligands.33 Because of this
interaction, dπ orbitals are stabilized to a point lower than the
dxy orbital. The complex is further stabilized by the a2u-dxy
interaction which is possible if the porphyrin ring changes from
the planar to the S4-ruffled structure.2 This interaction raises
the energy level of the dxy orbital and contributes to the increase
in a (dxz,dyz)4(dxy)1 state. Latos-Grazynski, Marchon, and co-
workers showed that the electronic ground state of some low-
spin ferric complexes such as quinoxalinotetraphenylporphyrin
is presented as (dxz,dyz)4(dxy)1 even if the axial ligands are strong
σ-donors such as imidazole and cyanide.34-36 Thus, the electron
configuration of low-spin ferric ions is controlled by the steric
and electronic effects of axial ligands as well as peripheral
substituents.
Contrary to the tetraalkylporphyrin complexes in which the
porphyrin ring itself is more or less nonplanar,13-19 tetraarylpor-
phyrin complexes usually have planar porphyrin rings unless
there is some steric repulsion between axial ligand and meso
aryl groups.20-25 This is because the half thickness of a benzene
ring is 1.7 Å, which is smaller than the van der Waals radius of
a methyl group, 2.0 Å.26 Thus, the steric repulsion between the
meso-aryl group and the pyrrole â-hydrogen can be alleviated
by the rotation of the benzene ring about Cmeso-Caryl bonds to
maintain the porphyrin planarity. In fact, the porphyrin rings
of [Fe(TPP)(CN)2]- and [Fe(TPP)(1-MeIm)2]+ were essentially
planar as revealed by the X-ray crystallographic analysis.27,28
1
Accordingly, these complexes showed the H NMR spectra
typical for low-spin ferric porphyrin complexes; pyrrole â-sig-
nals were observed at -16.4 and -16.8 ppm at 25 °C,
respectively.29,30 The EPR spectra of these complexes are also
quite typical; the former showed a large gmax type spectrum in
which one strong signal was observed at g ) 3.70, and the latter
showed a rhombic type spectrum where three signals were
observed at g ) 2.89, 2.29, and 1.55.10,31 Thus, the ground state
electron configuration of both complexes was assigned as (dxy)2-
(dxz,dyz)3; the energy levels of the dxz and dyz are nearly the same
in the former complex while they are different in the latter.9
Although the electronic ground state of [Fe(TPP)(CN)2]- and
[Fe(TPP)(1-MeIm)2]+ is presented as (dxy)2(dxz,dyz)3, some low-
spin complexes with much weaker axial ligands have a novel
(dxz,dyz)4(dxy)1 state.1-4,32,33 Examples are complexes such as [Fe-
In order to find out the general conditions for the formation
of low-spin ferric porphyrin complexes with the (dxz,dyz)4(dxy)1
configuration, we have examined a series of low-spin (dicyano)-
{meso-tetrakis(2,4,6-trialkylphenyl)porphyrinato}iron(III) com-
i
plexes, [Fe(R-TPP)(CN)2]- in which R ) Me, Et, and Pr,
1
together with the parent [Fe(TPP)(CN)2]-, by means of H
NMR, 13C NMR, and EPR spectroscopies. Similarly, the
electron configuration of (dicyano){meso-tetrakis(2,6-dichlo-
rophenyl)porphyrinato}iron(III), [Fe(Cl-TPP)(CN)2]-, has been
examined to find out the effects of a bulky electronegative group
at the ortho positions. We report that the ground state electronic
configuration of low-spin [Fe(R-TPP)(CN)2]- (R ) Me, Et, and
iPr) is presented as (dxz,dyz)4(dxy)1. We also report the factors
controlling the electron configuration of ferric ions in tet-
raarylporphyrin system.
(13) Kutzler, F. W.; Swepston, P. N.; Berkovitch-Yellin, Z.; Ellis, D. E.;
Ibers, J. A. J. Am. Chem. Soc. 1983, 105, 2996-3004.
(14) Ema, T.; Senge, M. O.; Nelson, N. Y.; Ogoshi, H.; Smith, K. M.
Angew. Chem., Int. Ed. Engl. 1994, 33, 1879-1881.
(15) Senge, M. O.; Ema, T.; Smith, K. M. J. Chem. Soc., Chem. Commun.
1995, 733-734.
(16) 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.
(17) Veyrat, M.; Ramasseul, R.; Marchon, J.-C.; Turowska-Tyrk, I.; Scheidt,
W. R. New J. Chem. 1995, 19, 1199-1202.
(18) Mazzanti, M.; Veyrat, M.; Ramasseul, R.; Marchon, J.-C.; Turowska-
Tyrk, I.; Shang, M.; Scheidt, W. R. Inorg. Chem. 1996, 35, 3733-
3734.
(19) Ikeue, T.; Ohgo, Y.; Uchida, A.; Nakamura, M.; Fujii, H.; Yokoyama,
M. Inorg. Chem. 1999, 38, 1276-1281.
(20) Scheidt, W. R.; Lee, Y. J. Struct. Bonding 1987, 64, 1-70.
(21) Scheidt, W. R.; Kirner, J. F.; Hoard, J. L.; Reed, C. A. J. Am. Chem.
Soc. 1987, 109, 1963-1968.
(22) Munro, O. Q.; Marques, H. M.; Debrunner, P. G.; Mohanrao, K.;
Scheidt, W. R. J. Am. Chem. Soc. 1995, 117, 935-954.
(23) Nakamura, M. Bull. Chem. Soc. Jpn. 1995, 68, 197-203.
(24) Nakamura, M.; Ikezaki, A. Chem. Lett. 1995, 733-734.
(25) Nakamura, M.; Kawasaki, Y. Chem. Lett. 1996, 805-806.
(26) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, NY, 1960.
Experimental Section
1
Spectral Measurement. H and 13C NMR spectra were recorded
on a JEOL LA300 spectrometer operating at 300.4 MHz for proton.
Chemical shifts were referenced to the residual peak of the deuterated
solvents such as dichloromethane (δ ) 5.32 ppm for 1H and 53.1 ppm
for 13C) and chloroform (δ ) 7.22 ppm for 1H and 77.2 ppm for 13C).
EPR spectra were measured at 4.2 K in frozen CH2Cl2-CH3OH
solutions with a Brucker ESP-300E spectrometer operating at X band
and equipped with an Oxford helium cryostat.
i
Synthesis. (i) [Fe(R-TPP)(CN)2]-; R ) Me, Et, or Pr. A series
of meso-tetrakis(2,4,6-trialkylphenyl)porphyrins, (R-TPP)H2 where R
i
) Me, Et, and Pr, were prepared according to the Lindsey’s method
in CHCl3 solution using BF3‚ether.30,37 Their spectral and analytical
data have already been reported in our previous paper.30 The 13C NMR
chemical shifts for the free base porphyrins are listed in Table S1 of
the Supporting Information. The high-spin complexes [Fe(R-TPP)Cl],
prepared by refluxing a DMF solution of (R-TPP)H2 with FeCl2‚4H2O,
were converted into the corresponding low-spin dicyano complexes
(27) Scheidt, W. R.; Haller, K. J.; Hatano, K. J. Am. Chem. Soc. 1980,
102, 3017-3021.
(28) Scheidt, W. R.; Osvath, S. R.; Lee, Y. J. J. Am. Chem. Soc. 1987,
109, 1958-1963.
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G. Inorg. Chem. 1997, 36, 6307-6314.
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498, 422-435.
(34) Wojaczynski, J.; Latos-Grazynski, L.; Glowiak, T. Inorg. Chem. 1997,
36, 6299-6306.
(30) Nakamura, M.; Tajima, K.; Tada, K.; Ishizu, K.; Nakamura, N. Inorg.
Chim. Acta 1994, 224, 113-124.
(35) Wolowiec, S.; Latos-Grazynski, L.; Mazzanti, M.; Marchon, J.-C.
Inorg. Chem. 1997, 36, 5761-5771.
(31) Inniss, D.; Soltis, S. M.; Strouse, C. E. J. Am. Chem. Soc. 1988, 110,
5644-5650.
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Chem. 1998, 37, 724-732.
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28, 823-825.
(37) Wagner, R. W.; Lawrence, D. S.; Lindsey, J. S. Tetrahedron Lett.
1987, 28, 3069-3070.

Electron Configuration of Ferric Ions
Inorganic Chemistry, Vol. 38, No. 17, 1999 3859
[Fe(R-TPP)(CN)2]- in an NMR sample tube by the following method.
To a CD2Cl2 solution containing 1.5-2.0 mg of [Fe(R-TPP)]Cl was
added a CD2Cl2 solution (4.0 molar equiv) of tetrabutylammonium
cyanide (Bu4N+CN-) to form [Fe(R-TPP)(CN)2]NBu4. The CD2Cl2-
CD3OD (4:1) solution was prepared by the addition of CD3OD to a
CD2Cl2 solution of [Fe(R-TPP)(CN)2]NBu4.
(ii) [Fe(Cl-TPP)(CN)2]-. This complex was prepared from {meso-
tetrakis(2,6-dichlorophenyl)porphyrinato}iron(III) chloride, [Fe(Cl-TP-
P)]Cl,38 and Bu4N+CN-.
(iii) [Fe(R-TPP)(CN)2]-(pyrrole-d8). A series of pyrrole-deuterated
porphyrins (R-TPP)H2(pyrrole-d8) (R ) Me, Et, iPr) were prepared by
the reaction of pyrrole-d5 with the corresponding aldehydes, and they
were converted to the high-spin complexes [Fe(R-TPP)Cl](pyrrole-d8).
Low-spin dicyano complexes [Fe(R-TPP)(CN)2]-(pyrrole-d8) were
prepared in an NMR sample tube as described above.
(iv) [Fe(Me-TPP)(CN)2]-(meso-13C). Mesitoic acid(13COOH) was
prepared in 43% yield by the treatment of mesitylmagnesium bromide,
obtained from 2-bromomesitylene (0.10 mol) and magnesium (0.11
mol), with 0.045 mol of 13CO2 (99.2 atom % of 13C, Isotech Inc.) in
THF solution followed by the conventional workup procedure; mp
151-153 °C, 13C NMR (CDCl3, 25 °C) 175.5 ppm. The carboxylic
acid (1.00 g, 6.1 mmol) was reduced by BH3-THF (20 mmol) in THF
solution to form 2-(hydroxymethyl)mesitylene in 92% yield;39 mp 83-
85 °C, 13C NMR (CDCl3, 25 °C) 59.2 ppm. The alcohol (1.0 g, 6.6
mmol) was dissolved in 50 mL of CH2Cl2 and was treated with 2.88 g
(33 mmol) of activated MnO2 at 25 °C.40 Formation of mesitaldehyde-
(13CHO) was monitored by thin layer chromatography. The yield was
58%. 13C NMR (CDCl3, 25 °C): 192.4 ppm. Condensation reaction of
mesitaldehyde and pyrrole gave (Me-TPP)H2(meso-13C); 13C NMR
(CDCl3, 25 °C) 117.6 ppm. Insertion of iron was performed by the
conventional method using DMF and FeCl2‚4H2O to form high-spin
[Fe(Me-TPP)Cl]; 13C NMR (CDCl3, 25 °C) 524 ppm. The dicyano
complex [Fe(Me-TPP)(CN)2]-(meso-13C) was prepared similarly in an
NMR sample tube by the addition of Bu4N+CN- into a CD2Cl2 solution
of [Fe(Me-TPP)Cl](meso-13C); 13C NMR (CD2Cl2, 25 °C) 125.1 ppm.
Figure 1. EPR spectra of (a) [Fe(TPP)(CN)2]-, (b) [Fe(Me-TPP)-
(CN)2]-, (c) Fe(Et-TPP)(CN)2]-, and (d) [Fe(iPr-TPP)(CN)2]- taken
in frozen CH2Cl2-CH3OH solution at 4.2 K.
Table 1. EPR g Values of [Fe(TPP)(CN)2]- and
i
[Fe(R-TPP)(CN)2]- (R ) Me, Et, Pr) Taken in Frozen
CH2Cl2-CH3OH Solution at 4.2 K
complexes
gx
gy
gz
[Fe(TPP)(CN)2]-
3.56
1.5
1.5
[Fe(Me-TPP)(CN)2]-
[Fe(Et-TPP)(CN)2]-
[Fe(iPr-TPP)(CN)2]-
2.47
2.45
2.45
2.47
2.45
2.45
1.5
Table 2. 1H NMR Chemical Shifts Measured at -71 °C in
CD2Cl2-CD3ODa
Results
(i) EPR Spectra. EPR spectra of [Fe(TPP)(CN)2]-, [Fe(R-
TPP)(CN)2]- (R ) Me, Et, iPr), and [Fe(Cl-TPP)(CN)2]- were
taken at 4.2 K in frozen CH2Cl2-CH3OH (4:1) solutions as
shown in Figure 1. The g values of these complexes are listed
in Table 1.
complexes
o
m
p
py-H
[Fe(TPP)(CN)2]-
4.01
8.72
10.88
12.89
5.46
-11.68
-5.74
4.39
[Fe(Me-TPP)(CN)2]-
[Fe(Et-TPP)(CN)2]-
(2.35)
(2.49R)
(0.49â)
(1.57R)
(1.27â)
(2.79)
(3.52R)
(1.56â)
(3.61R)
(1.71â)
5.70
[Fe(iPr-TPP)(CN)2]-
[Fe(Cl-TPP)(CN)2]-
12.92
5.97
1
(ii) H NMR. Chemical shifts at -71 °C in CD2Cl2-CD3-
OD (4:1) are listed in Table 2. The assignment of the pyrrole
protons has been done by spectral comparison with the pyrrole-
deuterated complexes. The Curie plots of the pyrrole and meta
proton signals are given in Figure 2, panels a and b, respectively.
(iii) 13C NMR Spectra. Chemical shifts of the meso-13C
signals of [Fe(TPP)(CN)2]- and[Fe(Me-TPP)(CN)2]- were
determined over a wide temperature range by using meso-13C
enriched complexes.41,42 In the case of [Fe(Et-TPP)(CN)2]- and
[Fe(Cl-TPP)(CN)2]-, the chemical shifts were determined by
using the samples without 13C enrichment. Curie plots of these
signals are given in Figure 3.
6.30
-27.04
a Numbers in parentheses are the chemical shifts of the R and â
protons of the substituents.
porphyrin complexes can be determined by EPR spectroscopy.
Three types of EPR spectra have been reported for the low-
spin ferric porphyrin complexes. The first one is a rhombic
spectrum where three signals appear at g ) 2.8-2.9, 2.2-2.4,
and 1.5-1.6.9,10 This type of spectrum is generally observed in
[Fe(TPP)(L)2]+ where L’s are unhindered imidazoles such as
imidazole and 1-methylimidazole. In these complexes two
imidazole ligands align in a parallel fashion above and below
the porphyrin ring. Thus, the energy level of dxz differs from
that of dyz due to the different degree of interaction with the
axially coordinated imidazole ligands, giving rhombic type
Discussion
Electron Configuration of [Fe(R-TPP)(CN)2]-. (i) EPR
Spectra. The electron configuration of low-spin ferric ions in
spectra.12 The second type is sometimes called a “large gmax
”
or “strong gmax” spectrum in which one resolved signal appears
at g > 3.3.11,42 This type of spectrum is observed in the
complexes having two bulky imidazole ligands such as [Fe-
(TPP)(2-MeIm)2]+ and [Fe(TMP)(2-MeIm)2]+.12,30 X-ray crys-
tallographic analyses of these complexes have revealed that the
imidazole ligands are perpendicularly aligned along the diagonal
Cmeso-Fe-Cmeso axes.21,22 In such a situation, the dxz and dyz
orbitals are destabilized to a similar extent by the interaction
(38) Traylor, P. S.; Dolphin, D.; Traylor, T. G. J. Chem. Soc., Chem.
Commun. 1984, 279-280.
(39) Yoon, N. M.; Pak, C. S.; Brown, H. C.; Krishnamurthy, S.; Stocky,
T. P. J. Org. Chem. 1973, 38, 2786.
(40) Sandler, S. R.; Karo, W. In Organic Functional Group Preparation;
Blomquist, A. T., Ed.; Academic Press: New York, 1968; p 147.
(41) Signal assignment was carried out on the basis of 1D and 2D (FG-
HMQC, FG-HMBC, COSY, and NOESY) NMR techniques.
(42) Orme-Johnson, N. R.; Hansen, R. E.; Beinert, H. Biochem. Biophys.
Res. Commun. 1971, 45, 871-878.

3860 Inorganic Chemistry, Vol. 38, No. 17, 1999
Nakamura et al.
observed in the complexes with much stronger axial ligands
such as cyanide, pyridines, and imidazoles, if the complexes
have highly deformed porphyrin rings.7,8,35,36,44 The axial type
spectra have been ascribed to the less common (dxz,dyz)4(dxy)1
electron configuration as mentioned in the Introduction.
The large gmax type EPR spectrum of [Fe(TPP)(CN)2]- shown
in Figure 1 suggests that the electron configuration of the ferric
ion of this complex is (dxy)2(dxz,dyz)3 in CH2Cl2-CH3OH
solution. In contrast, the EPR spectra of the alkyl-substituted
complexes all showed axial type spectra, indicating that the
electron configuration is presented as (dxz,dyz)4(dxy)1. The dif-
ferences in energy levels between dxy and dπ(dxz,dyz) orbitals in
the alkyl-substituted complexes are calculated to be ca. 2.9λ.45,46
The value is comparable in magnitude to those of [Fe(TPP)(4-
CNPy)2]+ and [Fe(Me-TPP)(4-CNPy)2]+, 1.72λ and 2.92λ,
respectively,2,3 while it is much smaller than the corresponding
values of [Fe(TiPrP)(CN)2]- and [Fe(TPP)(tBuNC)2]+, 4.82λ and
8.33λ, respectively.4,7
(ii) Pyrrole Signals in 1H NMR Spectra. The electron
configuration of ferric ions can also be determined by 1H NMR
spectroscopy. The data in Table 2 demonstrate the existence of
large spin densities on the pyrrole â-carbons in [Fe(TPP)(CN)2]-;
the chemical shift of the pyrrole protons of this complex is
-11.68 ppm at -71 °C. The upfield shift can be explained in
terms of the interaction between porphyrin(3eg) and iron(dπ)
orbitals.47-49 Since 3eg orbitals have large electron densities on
the pyrrole â-carbon atoms, the charge transfer from the
porphyrin 3eg to the singly occupied iron dπ orbitals induces
an upfield shift of the protons directly bonded to these carbons.
Thus, the large upfield shift observed in the pyrrole signal of
[Fe(TPP)(CN)2]- proves the existence of an unpaired electron
in the dxz or dyz orbital and supports the (dxy)2(dxz,dyz)3 config-
uration. In contrast, the pyrrole signals in [Fe(Me-TPP)(CN)2]-,
[Fe(Et-TPP)(CN)2]-, and [Fe(iPr-TPP)(CN)2]- moved downfield
and appeared at -5.74, +4.39, and +5.97 ppm, respectively,
at -71 °C. The result suggests that the spin densities on the
pyrrole â-carbons decreased to a great extent in the alkyl-
substituted complexes. The decrease can be explained if the
contribution of (dxz,dyz)4(dxy)1 toward the electronic ground state
increases in these complexes; the interaction between porphyrin-
(3eg) and singly occupied iron(dxy) orbitals must be very weak
because of the orthogonality of these orbitals in a planar D4h
porphyrin complex. Thus, the downfield pyrrole shifts in the
alkyl-substituted complexes support the (dxz,dyz)4(dxy)1 config-
uration determined by the EPR spectra. However, there seem
to be some discrepancies between the 1H NMR and EPR results.
While the chemical shifts of the pyrrole protons are still quite
different among the three complexes, the EPR g values are
nearly the same, g ) 2.45-2.47 and g| ) 1.5. The discrep-
ancies must be ascribed to the difference in temperature where
these values were obtained; even if the contribution of the (dxy)2-
(dxz,dyz)3 state is not small at 202 K where NMR spectra were
taken, it could be negligibly small at 4.2 K where EPR
measurements were carried out. Rather small differences in the
Figure 2. Curie plots of the pyrrole (a) and meta-proton (b) signals
i
of [Fe(TPP)(CN)2]-, [Fe(R-TPP)(CN)2]- (R ) Me, Et, Pr), and [Fe-
(Cl-TPP)(CN)2]- taken in CD2Cl2-CD3OD solution.
Figure 3. Curie plots of the meso-carbon signals of [Fe(TPP)(CN)2]-,
[Fe(R-TPP)(CN)2]- (R ) Me, Et, iPr), and [Fe(Cl-TPP)(CN)2]- taken
in CD2Cl2-CD3OD solution.
with the imidazole π orbitals. Thus, the orbital degeneracy is
maintained, giving large gmax type spectra. Complexes with
linear cyanide ligand such as [Fe(TPP)(CN)2]- also exhibit large
gmax type spectra for similar reasons.31 Recent studies have
revealed a third type of EPR spectrum, an axial spectrum, in
complexes with very weak ligands such as tert-butyl isocya-
nide,4,32 trifluoroethyl isocyanide,43 4-cyanopyridine,1,2 and
dimethyl phenylphosphonite.33 The axial type spectra were also
(44) Nakamura, M.; Nakamura, N. Chem. Lett. 1991, 1885-1888.
(45) Taylor, C. P. S. Biochim. Biophys. Acta 1977, 491, 137-149.
(46) Bohan, T. L. J. Magn. Reson. 1977, 26, 109-118.
(47) La Mar, G. N.; Walker, F. A. In The Porphyrins; Dolphin, D., Ed.;
Academic Press: New York, 1979; Vol. IV, pp 61-157.
(48) Goff, H. Nuclear Magnetic Resonance of Iron Porphyrins. In Iron
Porphyrin, I; Lever, A. B. P., Gray, H. B., Eds.; Physical Bioinorganic
Chemistry Series 1; Addison-Wesley: Reading, MA, 1983; pp 237-
281.
(49) 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.
(43) Geze, C.; Legrand, N.; Bondon, A.; Simonneaux, G. Inorg. Chim.
Acta 1992, 195, 73-76.

Electron Configuration of Ferric Ions
Inorganic Chemistry, Vol. 38, No. 17, 1999 3861
pyrrole shifts among the unsubstituted and alkyl-substituted
complexes at 25 °C can also be explained in terms of the
increased contribution of the (dxy)2(dxz,dyz)3 state at higher
temperature.
ground state is ordered as follows: [Fe(Me-TPP)(CN)2]- < [Fe-
(Et-TPP)(CN)2]- e [Fe(iPr-TPP)(CN)2]-.
Electron Configuration of [Fe(Cl-TPP)(CN)2]-. In order
to find out the effect of bulky electronegative groups at the ortho
positions, the 1H NMR spectra of [Fe(Cl-TPP)(CN)2]- have been
examined. The Curie plots of the pyrrole and meta signals are
given in Figure 2, panels a and b, respectively, together with
those of [Fe(TPP)(CN)2]- and [Fe(R-TPP)(CN)2]-. The Curie
slope of the pyrrole signal of [Fe(Cl-TPP)(CN)2]- showed a
much larger negative value than that of [Fe(TPP)(CN)2]- as is
clear from Figure 2a. Thus, the pyrrole signal of [Fe(Cl-TPP)-
(CN)2]- moved to -27.04 ppm at -71 °C as compared with
-11.68 ppm in [Fe(TPP)(CN)2]-. The difference was also
observed in the Curie plots of the meta signal; while the other
four complexes showed positive slopes, [Fe(Cl-TPP)(CN)2]-
showed a small but negative slope. These results suggest that
the contribution of the (dxz,dyz)4(dxy)1 configuration to the
electronic ground state is much larger in [Fe(TPP)(CN)2]- than
in [Fe(Cl-TPP)(CN)2]-. This is because the existence of the
electronegative chlorine atoms at the ortho positions in [Fe(Cl-
TPP)(CN)2]- weakens the coordination ability of the porphyrin
toward low-spin ferric ion. In such a situation, metal to axial
ligand π back-bonding is also expected to be weakened as
compared with that in [Fe(TPP)(CN)2]-. As a result, the energy
level of the dπ orbitals is far above that or the dxy orbital in
[Fe(Cl-TPP)(CN)2]-, giving a much purer (dxy)2(dxz,dyz)3 con-
figuration. The 13C NMR results also support the (dxy)2(dxz,dyz)3
configuration in [Fe(Cl-TPP)(CN)2]-. As shown in Figure 3,
the meso-carbon signal appeared at 79.1 ppm at 25 °C and 76.2
ppm at -50 °C, suggesting a small spin density on meso-carbon
atoms.
The difference in electron configuration can also be demon-
strated by the Curie slopes of the pyrrole signals as shown in
Figure 2a. While the Curie slope of [Fe(TPP)(CN)2]- showed
a small negative value, those of the Me, Et, and iPr complexes
were positive. The negative Curie slope of the pyrrole signal is
commonly observed in the low-spin ferric porphyrin complexes
with the (dxy)2(dxz,dyz)3 configuration.30 These results suggest
that the ferric ions in the alkyl-substituted complexes are in the
(dxz,dyz)4(dxy)1 state, while that of the unsubstituted complex is
in the (dxy)2(dxz,dyz)3 state. A curvature observed in the Curie
plots suggests that the ground state electron configuration is
the admixture of (dxz,dyz)4(dxy)1 and (dxy)2(dxz,dyz)3.50
(iii) Meta Signals in 1H NMR Spectra. A similar tendency
was observed in the chemical shifts of meta signals. The data
in Table 2 indicate that the meta signal moves downfield from
i
8.72 (R ) H) to 12.92 ppm (R ) Pr). The chemical shift of
the iPr complex should be compared with those of [Fe(Me-TPP)-
(4-CNPy)2]+ (δ 14.59 at -80 °C) and [Fe(TPP)(iBuNC)2]+ (δ
ca. 18 ppm at -80 °C based on the Curie plots in Figure 3 of
ref 32), both of which have ferric ions with the (dxz,dyz)4(dxy)1
electron configuration.1,32 The downfield shifts of meta signals
are usually observed in high-spin ferric porphyrin complexes
such as [Fe(TPP)]Cl and [Fe(Me-TPP)]Cl, which have been
ascribed to the large spin densities on the meso-carbons; the
π-spins on the meso-carbons delocalize to the aryl groups
directly bonded to them and induce a downfield shift of the
meta protons.47-49 Thus, the 1H NMR results suggest the
existence of large spin densities on the meso-carbons especially
On the basis of the 1H and 13C NMR results of [Fe(Cl-TPP)-
(CN)2]-, it is concluded that the presence of the electron-
withdrawing chlorine atoms at the meso-aryl groups stabilizes
the common (dxy)2(dxz,dyz)3 configuration.
i
in the Pr complex. The result is an indication that the spin
transfer from the low-spin ferric ion to the meso carbons takes
place through the iron(dxy) and porphyrin(a2u) interaction; the
a2u orbital has large spin densities on the meso-carbons.
(iv) Meso Signals in 13C NMR Spectra. The increase in
spin densities on the meso-carbons due to the interaction between
porphyrin a2u and iron dxy can be proved directly by the 13C
NMR measurement. As shown in Figure 3, the meso-carbon
signal in [Fe(TPP)(CN)2]- appeared at 164.2 ppm at 24 °C and
shifted downfield as the temperature was lowered. Similar
downfield shifts were observed in [Fe(Me-TPP)(CN)2]- and [Fe-
(Et-TPP)(CN)2]- though the slopes were much steeper. Thus,
the chemical shifts of the meso-carbons at -50 °C are greatly
different among three complexes; 204.3, 306.3, and 367.0 ppm
for the R ) H, Me, and Et complexes, respectively. The results
clearly indicate that the spin densities on the meso-carbons in
the R ) Me and Et complexes are much larger than that of the
unsubstituted complex, being consistent with the conclusion that
the ferric ions of the alkyl-substituted complexes are in (dxy,dyz)4-
Factors Controlling the Electron Configuration of Low-
Spin (Tetraarylporphyrinato)iron(III) Systems. (i) Steric
Effects To Induce S4 Ruffled Porphyrin Ring. In the previous
papers, we have pointed out three factors stabilizing the less
common (dxz,dyz)4(dxy)1 configuration in bis(cyanide) and bis-
(imidazole) complexes of (meso-tetraalkylporphyrinato)iron-
(III).7,8 They are (a) bulky meso substituents, (b) bulky axial
ligands, and (c) axial ligands with a low-lying π* orbital.
Conditions a and b are steric factors necessary for the S4-ruffled
porphyrin ring. We have reported that the electron configuration
of complexes such as [Fe(TiPrP)(CN)2]-, [Fe(TiPrP)(1-MeIm)2]-,
[Fe(TiPrP)(2-MeIm)2]-, and [Fe(TiPrP)(Py)2]+ are presented as
(dxz,dyz)4(dxy)1 on the basis of the NMR and EPR results.7,8 These
complexes are supposed to have highly S4 ruffled porphyrin
rings regardless of the axial ligands due to the steric repulsion
between bulky meso-isopropyl groups and the pyrrole â-hy-
drogens. Similar deformation is expected in [Fe(Me-TPP)(2-
MeBzIm)2]+ caused by the severe steric repulsion between bulky
axial ligand and meso-mesityl groups. In fact, this complex
i
(dxy)1 state. Since the Pr complex is supposed to have a ferric
ion with the purest (dxz,dyz)4(dxy)1 configuration among the
complexes examined in this study, determination of the meso-
13C chemical shift of this complex must be quite interesting.
The observation of the signal has hampered, however, due to
the broadening and bad signal-to-noise ratio.
1
showed the H NMR, 13C NMR, and EPR spectra typical for
the (dxz,dyz)4(dxy)1 configuration.44 Bulkiness of the axially
coordinated imidazole ligands is quite important in [Fe(Me-
TPP)(L)2]+ since the complex with less bulky 2-MeIm, [Fe-
(Me-TPP)(2-MeIm)2]+, showed a large gmax type signal in EPR
1
On the basis of the H NMR, 13C NMR, and EPR spectro-
scopic results mentioned above, it is concluded that the electron
configuration of the ferric ions in [Fe(R-TPP)(CN)2]- (R ) Me,
Et, iPr) is (dxz,dyz)4(dxy)1, while that of [Fe(TPP)(CN)2]- is (dxy)2-
(dxz,dyz)3. The contribution of (dxz,dyz)4(dxy)1 to the electronic
1
together with the upfield-shifted pyrrole signals in H NMR;
2-MeIm is not bulky enough to change the electron configuration
of ferric ion from (dxy)2(dxz,dyz)3 to (dxz,dyz)4(dxy)1 in the
tetramesitylporphyrin system.8,30 The deformation mode of the
porphyrin ring is also an important factor to determine the
(50) Shokhirev, N. V.; Walker, F. A. J. Phys. Chem. 1995, 99, 17795-
17804.

3862 Inorganic Chemistry, Vol. 38, No. 17, 1999
Nakamura et al.
electron configuration, because our preliminary results have
shown that [Fe(OMTPP)(CN)2]-, which is supposed to have a
highly S4 saddled porphyrin ring,51-53 maintains the common
(dxy)2(dxz,dyz)3 configuration.54 Thus, S4-ruffled deformation of
the porphyrin ring is necessary for the less common (dxz,dyz)4-
(dxy)1 configuration at least in the case of cyanide.
that of imidazoles. In fact, we have shown in our previous paper
that cyanide is a much more suitable ligand than imidazoles to
obtain the (dxz,dyz)4(dxy)1 configuration; the chemical shifts of
the pyrrole protons [Fe(TiPrP)(1-MeIm)2]+ and [Fe(TiPrP)(CN)2]-
are 3.0 and 12.9 ppm, respectively, at -71 °C.7,8 In addition,
the results shown in this paper were obtained in the presence
of methanol, which can lower the pπ* orbitals of cyanide ligand
by making the hydrogen bonding.8,35,57 Thus, the iron dπ orbitals
could be further stabilized by the cyanide pπ* orbitals due to
the metal to ligand π back-bonding.58
(ii) Electronic Effects To Strengthen Metal to Axial Ligand
π Back-Donation. While conditions a and b are the steric
requirements to obtain the complex with the (dxz,dyz)4(dxy)1
configuration, condition c is the electronic requirement to obtain
it. The low-lying π* orbital can stabilize iron dπ orbitals relative
to a dxy orbital.1-4 Such ligands are tert-butyl isocyanide,32 2,2,2-
trifluoroethyl isocyanide,43 4-cyanopyridine,2 and dimethyl
phenylphosphonite.33 In the low-spin ferric porphyrin complexes
carrying these ligands at the axial positions, the energy levels
of dπ orbitals drop to a point lower than or nearly equal to that
of the dxy orbital. Thus, the ground state electron configuration
of iron becomes an admixture of (dxz,dyz)4(dxy)1 and (dxy)2-
(dxz,dyz)3. The contribution of (dxz,dyz)4(dxy)1 could be increased
by the S4-ruffled deformation of the porphyrin ring, since the
deformation of this mode enables the porphyrin(a2u)-iron(dxy)
interaction and raises the energy level of the dxy orbital.2-4 In
contrast, the a2u-dxy interaction is impossible in complexes with
an S4-saddled deformation; the a2u orbital can interact with the
The question is left as to why the electron configuration is
affected by the alkyl groups at the ortho and para positions;
while the electron configuration of ferric ion in the unsubstituted
complex is presented as (dxy)2(dxz,dyz)3, that of the alkyl-
substituted complexes is proved to be (dxz,dyz)4(dxy)1. As
mentioned, the (dxy)2(dxz,dyz)3 configuration of [Fe(Cl-TPP)-
(CN)2]- has been ascribed to the electron-withdrawing nature
of the chlorine atoms, which weakens iron(dπ)-cyanide(pπ*)
interactions. Thus, the electron-donating ability of the alkyl
substituents facilitates the porphyrin(3eg) to iron(dπ) charge
transfer, which in turn strengthens the iron(dπ) to cyanide(pπ*)
back-donation, lowering the energy level of the dπ orbitals. It
is not clear, however, why the contribution of the (dxz,dyz)4(dxy)1
i
configuration is larger in the Pr and Et complexes than in the
dx -y orbital in the saddle-shaped complexes.53 The interaction
causes, however, little effect on the electron configuration of
the low-spin complexes because of the unoccupancy of an
electron spin in this orbital. This is the reason why all of the
complexes with the (dxz,dyz)4(dxy)1 configuration reported so far
have an S4-ruffled porphyrin ring, even if there is no appreciable
steric repulsion that could deform the porphyrin ring. In other
words, if the energy levels of dxy and dyz are close to that of dxy
by some electronic interactions between iron and axial ligands,
or between iron and porphyrin, the porphyrin ring deforms in
an S4-ruffled mode and gains some stabilization due to the
porphyrin(a2u)-iron(dxy) interactions.
2
2
Me complex even if their electron-donating ability is almost
the same. Whatever the reasons are, it is now clear that the
electron configuration of ferric ions, which greatly affects the
spin distribution on the peripheral carbon atoms, is controlled
by a very subtle change in electronic as well as steric factors of
porphyrin substituents. Further systematic work is currently in
progress in this laboratory to clarify the relationship between
substituents and electron configuration in low-spin (meso-
tetraarylporphyrinato)iron(III) systems.
Conclusion
The electron configuration of a series of low-spin (dicyano)-
In the present system, steric reasons seem to be an unlikely
explanation for the S4-ruffled structure of the porphyrin rings.
First of all, steric repulsion of the meso-aryl groups with pyrrole
â-hydrogens or with linear cyanide ligands is expected to be
negligibly small as is deduced from the crystal structures of
{meso-tetrakis(2,4,6-trialkylphenyl)porphyrinato}iron(III) com-
i
plexes, [Fe(R-TPP)(CN)2]- (R ) Me, Et, Pr), together with
[Fe(Cl-TPP)(CN)2]- and [Fe(TPP)(CN)2]-, has been examined
by 1H NMR, 13C NMR, and EPR spectroscopies. The electron
configuration of the ferric ions is greatly affected by the
substituents at the meso-phenyl groups. While the electron-
withdrawing ortho chlorine atoms stabilize a common (dxy)2-
(dxz,dyz)3 configuration, the electron-donating alkyl groups such
as methyl, ethyl, and isopropyl stabilize a less common (dxz,dyz)4-
(dxy)1 configuration. The results have been interpreted in terms
of the stronger π back-donation from iron(dπ) to cyanide(pπ*)
in [Fe(R-TPP)(CN)2]- than in [Fe(Cl-TPP)(CN)2]-; the dπ-
pπ* interaction lowers the iron dπ(dxz,dyz) orbitals relative to the
[FeIII(TMP)(1-MeIm)2]+ and [FeII(TMP)(CN)2]2- 55,56
Thus, the
.
unusual electron configuration should be ascribed to electronic
reasons rather than steric ones. Although the π-accepting ability
of cyanide is supposed to be much weaker than that of tert-
butyl isocyanide and 4-cyanopyridine, it is much stronger than
(51) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner, M.
W.; Smith, K. M. J. Am. Chem. Soc. 1990, 112, 8851-8857.
(52) Cheng, R.-J.; Chen, P.-Y.; Gau, P.-R.; Chen, C.-C.; Peng, S.-M. J.
Am. Chem. Soc. 1997, 119, 2563-2569.
d
xy orbital, inducing the (dxz,dyz)4(dxy)1 configuration. Thus, the
less common (dxz,dyz)4(dxy)1 state in [Fe(R-TPP)(CN)2]- is
caused by the electronic interactions rather than the steric ones.
The results are in contrast to those obtained from meso-
tetraalkylporphyrin complexes [Fe(TRP)(CN)2]- in which por-
phyrin deformation caused by the bulky meso-alkyl groups is
the major factor to determine the electron configuration.
(53) Renner, M. W.; Barkigia, K. M.; Zhang, Y.; Medforth, C. J.; Smith,
K. M.; Fajer, J. J. Am. Chem. Soc. 1994, 116, 8582-8592.
(54) The 1H NMR chemical shifts of [Fe(OMTPP)(CN)2]- are 5.44 (ortho),
7.15 (meta), 6.57 (para), and 13.44 (Me) at 25 °C. The chemical shifts
of the phenyl protons are quite close to those of [Fe(TPP)(CN)2]-.
The EPR spectrum at 4.2 K showed a signal at g ) 3.48, which is
also close to g ) 3.70 in [Fe(TPP)(CN)2]-. These results strongly
suggest that the electron configuration of the ferric ion in highly saddle
shaped [Fe(OMTPP)(CN)2]- should be presented as (dxy)2(dxz,dyz)3.
Nakamura, M.; Ikeue, T.; Yamaguchi, T.; Ohgo, Y.; Fujii, H. To be
published.
Acknowledgment. This work was supported by a Grant in
Aid for Scientific Research (No. 10640551, to M.N.) from
Ministry of Education, Science, Culture and Sports.
(55) Safo, M. K.; Gupta, G. P.; Walker, F. A.; Scheidt, W. R. J. Am. Chem.
Soc. 1991, 113, 5497-5510.
Supporting Information Available: 13C NMR chemical shifts of
a series of (R-TPP)H2 taken in CDCl3 at 25 °C. This material is available
free of charge via the Internet at http://pubs.acs.org.
(56) Bartczak, T. J.; Wolowiec, S.; Latos-Grazynski, L. Inorg. Chim. Acta
1998, 277, 242-246.
(57) La Mar, G. N.; Del Gaudio, J.; Frye, J. S. Biochim. Biophys. Acta
1977, 498, 422-435.
(58) Lukas, B.; Silver, J. Inorg. Chim. Acta 1986, 124, 97-100.
IC990328X