Electronic structures of six-coordinate ferric porphyrin complexes with weak axial ligands: Usefulness of 13C NMR chemical shifts

Article abstract of DOI:10.1021/ic0488942

¹H NMR, ¹³C NMR, and EPR spectra of six-coordinate ferric porphyrin complexes [Fe(Por)L₂]ClO₄ with different porphyrin structures are presented, where porphyrins (Por) are planar 5,10,15,20-tetraphenylporphyrin (TPP), ruffled 5,10,15,20- tetraisopropylporphyrin (TⁱPrP), and saddled 2,3,7,8,12,13,17,18- octaethyl-5,10,15,20-tetraphenylporphyrin (OETPP), and axial ligands (L) are weak oxygen ligands such as pyridine-N-oxide, substituted pyridine-N-oxide, DMSO, DMF, MeOH, THF, 2-MeTHF, and dioxane. These complexes exhibit the spin states ranging from an essentially pure high-spin (S = 5/2) to an essentially pure intermediate-spin (S = 3/2) state depending on the field strength of the axial ligands and the structure of the porphyrin rings. Reed and Guiset reported that the pyrrole-H chemical shift is a good probe to determine the spin state in the spin admixed S = 5/2,3/2 complexes (Reed, C. A.; Guiset, F. J. Am. Chem. Soc. 1996, 118, 3281-3282). In this paper, we report that the chemical shifts of the α- and β-pyrrole carbons can also be good probes to determine the spin state because they have shown good correlation with those of the pyrrole-H or pyrrole-C_α. By putting the observed or assumed pyrrole-H or pyrrole-C_α chemical shifts of the pure high-spin and pure intermediate-spin complexes into the correlation equations, we have estimated the carbon chemical shits of the corresponding complexes. The orbital interactions between iron(III) and porphyrin have been examined on the basis of these chemical shifts, from which we have found that both the d _xy-a_2u interaction in the ruffled Fe(TⁱPrP) L₂⁺ and d_xy-a_1u interaction in the saddled Fe(OETPP)L₂⁺ are quite weak in the high-spin and probably in the intermediate-spin complexes as well. Close inspection of the correlation lines has suggested that the electron configuration of an essentially pure intermediate-spin Fe(TⁱPrP)L₂⁺ changes from (d_xy, d_yz)³(d_xy) ¹(d_z2)¹ to (d_xy)²(d _xz, d_yz)²(d_z2)¹ as the axial ligand (L) changes from DMF to MeOH, THF, 2-MeTHF, and then to dioxane. Although the DFT calculation has indicated that the highly saddled intermediate-spin Fe(OETPP)-(THF)₂⁺ should adopt (d _xy, d_yz)³(d_xy)¹(d _z2)¹ rather than (d_xy)²(d _xz, d_yz)²(d_z2)¹ because of the strong d_xy-a_iu interaction (Cheng, R.-J.; Wang, Y.-K.; Chen, P.-Y.; Han, Y.-P.; Chang, C.-C. Chem. Commun. 2005, 1312-1314), our ¹³C NMR study again suggests that Fe(OETPP)(THF)₂ ⁺ should be represented as (d_xy)²(d _xz, d_yz)²(d_z2)¹ because of the weak d_xy-a_iu interaction. The contribution of the S = 3/2 state in all types of the spin admixed S = 5/2,3/2 six-coordinate complexes has been determined on the basis of the ¹³C NMR chemical shifts.

Full text of DOI:10.1021/ic0488942

Inorg. Chem. 2005, 44, 7333−7344
Electronic Structures of Six-Coordinate Ferric Porphyrin Complexes
with Weak Axial Ligands: Usefulness of 13C NMR Chemical Shifts§
Akito Hoshino,‡ Yoshiki Ohgo,† and Mikio Nakamura*,†,‡
Department of Chemistry, School of Medicine, Toho UniVersity, Ota-ku, Tokyo 143-8540, Japan,
and DiVision of Biomolecular Science, Graduate School of Science, Toho UniVersity,
Funabashi 274-8510, Japan
Received August 11, 2004
1H NMR, 13C NMR, and EPR spectra of six-coordinate ferric porphyrin complexes [Fe(Por)L2]ClO4 with different
porphyrin structures are presented, where porphyrins (Por) are planar 5,10,15,20-tetraphenylporphyrin (TPP), ruffled
5,10,15,20-tetraisopropylporphyrin (TiPrP), and saddled 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin
(OETPP), and axial ligands (L) are weak oxygen ligands such as pyridine-N-oxide, substituted pyridine-N-oxide,
DMSO, DMF, MeOH, THF, 2-MeTHF, and dioxane. These complexes exhibit the spin states ranging from an
essentially pure high-spin (S
strength of the axial ligands and the structure of the porphyrin rings. Reed and Guiset reported that the pyrrole-H
chemical shift is a good probe to determine the spin state in the spin admixed S 5/2,3/2 complexes (Reed, C.
A.; Guiset, F. J. Am. Chem. Soc. 1996, 118, 3281 3282). In this paper, we report that the chemical shifts of the
- and -pyrrole carbons can also be good probes to determine the spin state because they have shown good
) 5/2) to an essentially pure intermediate-spin (S ) 3/2) state depending on the field
)
−
R
â
correlation with those of the pyrrole-H or pyrrole-CR. By putting the observed or assumed pyrrole-H or pyrrole-CR
chemical shifts of the pure high-spin and pure intermediate-spin complexes into the correlation equations, we have
estimated the carbon chemical shits of the corresponding complexes. The orbital interactions between iron(III) and
porphyrin have been examined on the basis of these chemical shifts, from which we have found that both the
+
+
dxy− −a1u interaction in the saddled Fe(OETPP)L2 are quite weak
a2u interaction in the ruffled Fe(TiPrP)L2 and dxy
in the high-spin and probably in the intermediate-spin complexes as well. Close inspection of the correlation lines
+
has suggested that the electron configuration of an essentially pure intermediate-spin Fe(TiPrP)L2 changes from
3
1
1
2
2
1
2
2
(dxy, dyz) (dxy) (dz ) to (dxy) (dxz, dyz) (dz ) as the axial ligand (L) changes from DMF to MeOH, THF, 2-MeTHF, and
then to dioxane. Although the DFT calculation has indicated that the highly saddled intermediate-spin Fe(OETPP)-
+
3
1
1
2
2
1
2
2
(THF)2 should adopt (dxy, dyz) (dxy) (dz ) rather than (dxy) (dxz, dyz) (dz ) because of the strong dxy-a1u interaction
(Cheng, R.-J.; Wang, Y.-K.; Chen, P.-Y.; Han, Y.-P.; Chang, C.-C. Chem. Commun. 2005, 1312
1314), our 13C
−
1
2
+
2
2
NMR study again suggests that Fe(OETPP)(THF)2 should be represented as (dxy) (dxz, dyz) (dz ) because of the
weak dxy a1u interaction. The contribution of the S 3/2 state in all types of the spin admixed S 5/2,3/2
six-coordinate complexes has been determined on the basis of the 13C NMR chemical shifts.
−
)
)
Introduction
catalytic processes of naturally occurring heme proteins. A
number of techniques have been used to solve this problem,
which include UV-vis, NMR,EPR, resonance Raman, MCD,
Mo¨ssbauer, EXAFS, SQUID, X-ray crystallography, etc.1
Elucidation of the electronic structures of iron porphyrin
complexes is quite important to understand the function and
* To whom correspondence should be addressed. E-mail: mnakamu@
med.toho-u.ac.jp.
1
Among these techniques, H NMR is particularly useful
† Department of Chemistry, School of Medicine, Toho University.
‡ Division of Biomolecular Science, Graduate School of Science, Toho
University.
because it gives detailed information on the electronic
structures of the complexes in solution at various tempera-
tures. This is because the iron d orbital with an unpaired
§ Abbreviations. TPP, TiPrP, TEtPrP, OEP, and OETPP: dianions of
5,10,15,20-tetraphenylporphyrin, 5,10,15,20-tetraisopropylporphyrin, 5,10,-
15,20-tetrakis(1-ethylpropyl)porphyrin, 2,3,7,8,12,13,17,18-octaethylpor-
phyrin, and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin,
respectively. 4-MePyNO, 3,5-Me2PyNO, PyNO, and 4-ClPyNO: 4-meth-
ylpyridine-N-oxide, 3,5-dimethylpyridine-N-oxide, pyridine-N-oxide, and
4-chloropyridine-N-oxide, respectively.
(1) (a) Que, L., Jr., Ed. Physical Method in Bioinorganic Chemistry,
University Science Books, Sausalito, CA. 2000. (b) Solomon, E. I.;
Lever, A. B. P., Eds. Inorganic Electronic Structure and Spectroscopy;
John Wiley and Sons: New York, 1999; Vol 1.
10.1021/ic0488942 CCC: $30.25
Published on Web 09/13/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 21, 2005 7333

Hoshino et al.
Scheme 1. Complexes Examined in This Study
electron interacts with the specific π molecular orbitals of
porphyrin and increases the π spin density at the specific
carbon and nitrogen atoms of the complex. Consequently,
the NMR signals of the protons directly attached to or
attached by two bonds to the carbon atoms exhibit an upfield
or downfield shift. In addition, the unpaired electrons in the
2
2
2
dx -y and dz orbitals can be delocalized through σ bonds to
the protons and induce downfield shifts of the proton signals.
Thus, we can determine which d orbital has unpaired
electrons by the analysis of the observed 1H NMR chemical
shifts. A number of good review articles have been published
on the relationship between the electronic structures of iron
porphyrins and the 1H NMR chemical shifts.2-7 The 1H NMR
spectroscopy has, however, an intrinsic deficit in a sense that
the direct comparison of the chemical shifts in a wide variety
of complexes is impossible. While the chemical shift of the
proton directly bonded to the â-pyrrole carbon is a good
probe to determine whether the complex has unpaired
shifts. We will especially emphasize the usefulness of the
13C NMR chemical shifts for the determination of the
electronic structures of the complexes including the spin
states and electron configurations. Scheme 1 shows the
complexes examined in this study where the axial ligands
(L’s) are weak oxygen ligands such as 4-MePyNO, 3,5-Me2-
PyNO, PyNO, 4-ClPyNO, DMSO, DMF, MeOH, THF,
2-MeTHF, and dioxane.
Experimental Section
2
2
electron in the dx -y orbital, the probe is not applicable to
naturally occurring heme proteins because of the absence of
such protons. In contrast, a direct comparison of the chemical
shifts of three types of carbon atoms constituting the
porphyrin ring, i.e., pyrrole-R, pyrrole-â, and meso carbons,
can be made by 13C NMR spectroscopy. Nevertheless, the
systematic studies to reveal the relationship between the
electronic structures and the chemical shifts of ferric por-
phyrin complexes has been hampered due to the low natural
abundance and low sensitivity inherent to the 13C nucleus.6-17
General Procedure. UV-vis spectra were measured on a
SHIMADZU MultiSpec-1500 spectrophotometer at ambient tem-
perature. 1H and 13C NMR spectra were recorded on a JEOL LA300
spectrometer operating at 300.4 MHz for 1H. Chemical shifts were
referenced to the residual peak of dichloromethane (5.32 ppm for
1H and 53.8 ppm for 13C). EPR spectra were measured at 4.2 K
with a Bruker E500 spectrometer operating at X band and equipped
with an Oxford helium cryostat. The concentrations of EPR samples
were 5-8 mM. The g values were estimated by the simulation of
the observed spectra.
Synthesis. Free base porphyrins such as H2(TPP), H2(TiPrP),
and H2(OETPP) were prepared by the literature methods.18,19 Fe-
(TPP)Cl, Fe(TiPrP)Cl, and Fe(OETPP)Cl were prepared by the
literature methods.20-22 The perchlorate complexes Fe(Por)ClO4
were prepared by the reaction between Fe(Por)Cl and AgClO4 in
THF solution. Caution! Perchlorate salts are potentially explosiVe
when heated or shocked. Handle them in milligram quantities with
care. The crude solid was purified by the recrystallization from
dichloromethane and heptane. A series of six-coordinate complexes
were obtained at ambient temperature by the addition of the CD2-
Cl2 solution of each ligand (L) into a CD2Cl2 solution of Fe(Por)-
ClO4 or Fe(Por)(THF)2ClO4 placed in an NMR sample tube.
Addition of the ligand was continued until each signal showed no
appreciable change in chemical shift.
1
In the present work, we have determined both H and 13C
NMR chemical shifts of a number of six-coordinate S )
5/2, 3/2 complexes [Fe(Por)L2]+ ranging from a quite pure
S ) 5/2 to a quite pure S ) 3/2. The purpose of this study
is to elucidate the contribution of the S ) 3/2 spin state at
ambient temperature on the basis of the NMR chemicals
(2) Walker, F. A. In The Porphyrin Handbook, Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol.
5, Chapter 36, pp 81-183.
(3) Walker, F. A. Inorg. Chem. 2003, 42, 4526-4544.
(4) Walker, F. A. Chem. ReV. 2004, 104, 589-616.
(5) Goff, H. M. Nuclear Magnetic Resonance of Iron Porphyrins. In Iron
Porphyrin; Lever, A. B. P., Gray, H. B., Eds.; Physical Bioinorganic
Chemistry Series 1; Addison-Wesley: Reading, MA, 1983; Vol. I,
pp 237-281.
(6) Bertini, I.; Luchinat, C. In NMR of Paramagnetic Substances. Lever,
A. B. P., Ed.; Coordination Chemistry Reviews 150; Elsevier:
Amsterdam, 1996; pp 29-75.
(7) Bertini, I.; Luchinat, C. In NMR of Paramagnetic Molecules in
Biological Systems; Lever, A. B. P., Gray, H. B., Eds.; Physical
Bioinorganic Chemistry Series 3; Benjamin/Bummings: Menlo Park,
CA, 1986; pp 165-229.
(8) Wu¨thrich, K.; Baumann, R. HelV. Chim. Acta 1973, 56, 585-596.
(9) Wu¨thrich, K.; Baumann, R. HelV. Chim. Acta 1974, 57, 336-350.
(10) La Mar, G. N.; Viscio, D. B.; Smith, K. M.; Caughey, W. S.; Smith,
M. L. J. Am. Chem. Soc. 1978, 100, 8085-8092.
(11) Mispelter, J.; Momenteau, M.; Lhoste, J.-M. J. Chem. Soc., Dalton
Trans. 1981, 1729-1734.
(12) Goff, H. M. J. Am. Chem. Soc. 1981, 103, 3714-3722.
(13) Boersma, A. D.; Goff, H. Inorg. Chem. 1982, 21, 581-586.
(14) Goff, H, M.; Shimomura, E. T.; Phillippi, M. A. Inorg. Chem. 1983,
22, 66-71.
(15) Toney, G. E.; terHaar, L. W.; Savrin, J. E.; Gold, A.; Hatlfield, W.
E.; Sangaiah, R. Inorg. Chem. 1984, 23, 2561-2563.
(16) Toney, G. E.; Gold, A.; Savrin, J. E.; terHaar, L. W.; Sangaiah, R.
Inorg. Chem. 1984, 23, 4350-4352.
(17) Mao, J.; Zhang, Y.; Oldfiled, E. J. Am. Chem. Soc. 2002, 124, 13911-
13920.
Results and Discussion
Spectral Characteristics and Signal Assignments. i. 1H
NMR Spectra. Figure 1 shows the 1H NMR spectra of Fe-
(TiPrP)L2+ taken in CD2Cl2 solution at 298 K where the axial
ligands are (a) 3,5-Me2PyNO, (b) DMSO, (c) DMF, (d)
MeOH, and (e) dioxane. The pyrrole signal moved upfield
as the axial ligand was changed from 3,5-Me2PyNO (57.6
(18) Wagner, R. W.; Lawrence, D. S.; Lindsey, J. S. Tetrahedron Lett.
1987, 28, 3069-3070.
(19) 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.
(20) Cheng, R.-J.; Chen, P.-Y.; Gau, P.-R.; Chen, C.-C.; Peng, S.-M. J.
Am. Chem. Soc. 1997, 119, 2563-2569.
(21) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Nakamura, M.; Fujii, H.; Yokoyama,
M. J. Am. Chem. Soc. 2000, 122, 4068-4076.
(22) Sparks L. D.; Medforth, C. J.; Park, M.-S.; Chamberlain, J. R.; Ondrias,
M. R.; Senge, M. O.; Smith, K. M.; Shelnutt, J. A. J. Am. Chem. Soc.
1993, 115, 581-592.
7334 Inorganic Chemistry, Vol. 44, No. 21, 2005

Six-Coordinate Ferric Porphyrin Complexes
Figure 2. 1H NMR spectra of (a) Fe(OETPP)(THF)2+ and (b) Fe(OETPP)-
(THF)2+-d20 taken in CD2Cl2 solution at 298 K. Curie plots of each signal
are given in the inset of (a).
of the pyrrole shifts, which indicates that the S ) 3/2
contribution increases on going from 4-MePyNO to THF.28
ii. 13C NMR Spectra. In the case of five-coordinate high-
spin Fe(TPP)X, the R signal appears more upfield than the
â signal; the chemical shifts of the R and â signals in Fe-
(TPP)Cl are 1205 and 1328 ppm, respectively.11,14 The
upfield shift of the R relative to the â signal should be
ascribed to the π spin densities at the neighboring meso
Figure 1. 1H NMR spectra of Fe(TiPrP)L2 taken in CD2Cl2 solution at
+
298 K where L’s are (a) 3,5-Me2PyNO, (b) DMSO, (c) DMF, (d) MeOH,
and (e) 1,4-dioxane. The broad signal at δ -3.9 ppm in (a) is ascribed to
the ligand protons.
2
carbon and nitrogen atoms caused by the dz -a2u interac-
ppm) to DMSO (-7.7 ppm), DMF (-21.5 ppm), MeOH
(-28.3 ppm), and then to dioxane (-39.8 ppm). It is
noteworthy that Fe(TiPrP)(dioxane)2+ exhibits the pyrrole-H
signal fairly upfield, -40 ppm, which should be compared
with that of mono-aqua complex Fe(TPP)(H2O)+, -43 ppm
in C6D6 at 300 K, reported by Evans and Reed.23 Signal
assignment of Fe(TPP)L2+ and Fe(TiPrP)L2+ was carried out
on the basis of the relative integral intensities and the signal
tion;29 the a2u orbital has large coefficients at the meso carbon
and pyrrole nitrogen atoms, as shown in Scheme 2.2 In the
+
six-coordinate high-spin Fe(TPP)L2 , however, the spin
density at the meso carbon should decrease because of the
2
less-favorable dz -a2u interaction. In fact, a fairly large
upfield shift of the meso signal was observed in the high-
spin six-coordinate complexes.14,30 We can therefore expect
a large downfield shift of the neighboring R signals. Thus,
the signals observed at the most downfield position in the
+
widths. In the case of Fe(OETPP)L2 , the ortho-H and one
of the diastereotopic CH2 signals, both corresponding to 8H,
appeared rather close to each other. Thus, the H NMR
spectra of the corresponding phenyl deuterated complexes
Fe(OETPP)L2 -d20 were measured for the assignment. Figure
2 shows the H NMR spectra of Fe(OETPP)(THF)2 and
Fe(OETPP)(THF)2 -d20 as a typical example, which enabled
the assignment of all the signals. Correction: The assignment
1
(23) Evans, D. R.; Reed, C. A. J. Am. Chem. Soc. 2000, 122, 4660-4667.
(24) As was pointed out,25 the assignment of the ortho-H and one of the
+
+
diastereotopic CH2 signals in analogous Fe(OETPP)(4-CNPy)2 in
1
+
Table 2 of ref 26 and Table 1 of ref 27 has to be reversed.
(25) Yatsunyk, L. A.; Walker, F. A. Inorg. Chem. 2004, 43, 757-777.
(26) Ikeue, T.; Ohgo, Y.; Yamaguchi, T.; Takahashi, M.; Takeda, M.;
Nakamura, M. Angew Chem., Int. Ed. 2001, 40, 2617-2620.
(27) Ikeue, T,; Ohgo, Y.; Owendi, O.; Vicente, M. Graca. H.; Nakamura,
M. Inorg. Chem. 2003, 42, 5560-5571.
(28) Hoshino, A.; Nakamura, M. Chem Lett. 2004, 33, 1234-1235.
(29) Cheng, R.-J.; Chen, P.-Y.; Lovell, T.; Liu, T.; Noodleman, L.; Case,
D. A. J. Am. Chem. Soc. 2003, 125, 6774-6783.
(30) Nakamura, M.; Hoshino, A.; Ikezaki, A.; Ikeue, T. Chem. Commun.
2003, 1862-1863.
+
of the ortho-H and one of the diastereotopic CH2 signals in
+
Fe(OETPP)(THF)2+ and analogous Fe(OETPP)(4-CNPy)2
in our preVious papers should be reVersed.24-27 Table 1 lists
the chemical shifts of the complexes examined in this study.
In Table 1, the axial ligands are arranged in descending order
Inorganic Chemistry, Vol. 44, No. 21, 2005 7335

Hoshino et al.
Table 1. 1H NMR Chemical Shifts (CD2Cl2, 298 K, ppm)
pyrrole
CHR
meso-Ar
ligand
H
CHâ
o (HR)
m (Hâ)
p
+
Fe(TPP)L2
4-MePyNO
3,5-Me2PyNO
PyNO
70.7
71.0
68.9
67.2
67.2
60.2
48.1
3.1
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
12.7
12.8
13.3
13.2
13.0
13.5
13.8
14.1
9.4
9.4
9.7
9.7
9.5
9.8
4-ClPyNO
DMSO
9.7
9.9
9.6
9.7
DMF
9.9
9.9
MeOH
10.1
10.1
10.0
10.3
THF
Fe(TiPrP)L2
+
4-MePyMO
3,5-Me2PyNO
PyNO
58.5
57.6
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
(10.8)
(10.9)
(9.9)
(10.7)
(8.0)
(7.3)
(7.3)
(5.7)
(6.4)
(5.9)
(6.2)
(6.3)
(6.1)
(5.8)
(4.6)
(4.2)
n.d.
(4.9)
(5.7)
n.d.
-
-
-
-
-
-
-
-
-
-
41.7
4-ClPyNO
DMSO
35.2
-7.7
-21.5
-28.3
-35.5
-37.3
-39.8
DMF
MeOH
THF
2-MeTHF
dioxane
+
Fe(OETPP)L2
4-MePyNO
3,5-Me2PyNO
PyNO
-
-
-
-
-
21.0
44.2
18.6
43.8
20.0
43.8
15.4
39.2
11.6
37.7
11.7
11.1
38.7
2.4
14.7
14.7
15.3
17.2
15.8
8.7
8.7
7.4
7.4
6.1
10.3
10.4
10.6
10.6
11.4
2.4
2.2
DMSO
0.9
DMF
-0.1
MeOH
THF
-
-
0.3
0.3
15.8
15.8
5.9
5.6
11.5
11.5
Figure 3. Spectral change when 2, 4, 8, and 16 equiv of THF was added
to the CD2Cl2 solution of Fe(TPP)ClO4. The R, â, o, and Cm indicate the
pyrrole-R, pyrrole-â, ortho, and meso carbon signals, respectively.
Scheme 2. Frontier Orbitals of Porphyrin
signals appeared rather downfield, as do those in Fe(TPP)-
+
L2 . Thus, the most downfield-shifted signal was assigned
to the R signal. In the complexes with L ) DMSO, DMF,
MeOH, THF, and dioxane, both the R and â signals showed
a large upfield shift. The signal assignment of these com-
plexes was quite easy because the â signal exhibited clear
doublet due to the C-H coupling as shown in Figure 4b and c.
complexes with L ) 4-MePyNO, 3,5-Me2PyNO, PyNO,
DMSO, and DMF, ranging from 1636 to 1474 ppm, were
assigned to the R carbons. Both the R and â signal showed
a large upfield shift in the THF complex due to the decrease
+
Figure 5 shows the 13C NMR spectra of Fe(OETPP)L2
taken under the same conditions where the axial ligands are
(a) 4-MePyNO, (b) DMSO, and (c) THF. The meso signals
were assigned unambiguously by the spectral comparison
2
2
in spin population in the dx -y orbital. These signals were
assigned by the titration method. Figure 3 shows the spectral
change when THF was added to the CD2Cl2 solution of Fe-
(TPP)ClO4. As THF was added, the signals at 302 and 530
ppm in Fe(TPP)ClO4 shifted to the downfield and upfield
positions and finally reached 547 and 518 ppm, respectively.
Since the meso signal exhibited a large upfield shift from
345 to 60 ppm in the titration process, the signal that moved
from 302 to 547 ppm was assigned to the neighboring R
+ 31
with the meso-13C-enriched Fe(OETPP)L2 . As shown in
Figure 5, the line widths were quite different between the R
and â signals. Since the broad signal at 394 ppm in
Fe(OETPP)(THF)2+ was assigned to the R signal on the basis
of the titration method, the broad signals observed at the
+
downfield position in other Fe(OETPP)L2 were also as-
signed to the R signals. Table 2 lists the chemical shifts of
all the complexes examined in this study.32
iii. EPR Spectra. The EPR spectra were taken in frozen
CH2Cl2 solution at 4.2 K to obtain the additional information
on the spin state. The g values were determined by the
+
carbons of Fe(TPP)(THF)2 .
Figure 4 shows the 13C NMR spectra of Fe(TiPrP)L2
+
taken in CD2Cl2 solution at 298 K where the axial ligands
are (a) 4-MePyNO, (b) DMSO, and (c) MeOH. The meso
signals were assigned by the spectral comparison with the
(31) Sakai, T.; Ohgo, Y.; Ikeue, T.; Takahashi, M.; Takeda, M.; Nakamura,
M. J. Am. Chem. Soc. 2003, 125, 13028-13029.
+
(32) Complete formation of Fe(OETPP)(THF)2 requires a large excess
+ 31
structurally analogous meso-13C-enriched Fe(TEtPrP)L2 .
of THF. The meso carbon chemical shift reported previously, -244
In the PyNO and substituted PyNO complexes, the R and â
ppm at 298 K, contains a small amount of Fe(OETPP)ClO4.31
7336 Inorganic Chemistry, Vol. 44, No. 21, 2005

Six-Coordinate Ferric Porphyrin Complexes
Figure 4. 13C NMR spectra of Fe(TiPrP)L2 taken in CD2Cl2 at 298 K
+
where L’s are (a) 4-MePyNO, (b) DMSO, and (c) MeOH.
computer simulation of the observed spectra. Figure 6 shows
the EPR spectra of Fe(TiPrP)L2+ where L’s are 4-MePyNO,
PyNO, DMF, and dioxane. The complexes with PyNO and
substituted PyNO exhibited the g signal at 5.85-5.70. In
contrast, the complexes with much weaker axial ligands such
as DMSO, DMF, MeOH, THF, 2-MeTHF, and dioxane
showed the broad g signals at 4.2-4.0 together with some
much sharper peaks at 6.2-5.6 corresponding to the minor
species. The populations of the minor species are less than
10%. Figure 7 also shows the EPR spectra of Fe(OETPP)-
+
Figure 5. 13C NMR spectra of Fe(OETPP)L2 taken in CD2Cl2 solution
at 298 K, where L’s are (a) 4-MePyNO, (b) DMSO, and (c) THF. 13C NMR
spectrum of meso-13C enriched Fe(OETPP)(THF)2+ together with the Curie
plots of the meso-13C signal is given in the inset of (c).
i. [Fe(TPP)L2]ClO4. At first, we have examined the
relationship of the chemical shifts between pyrrole-H and
porphyrin carbons. The black lines in Figure 8a-c show the
correlation of the chemical shifts of the pyrrole-H against
those of the R (O), â (0), and meso (4) carbons. The lines
with the correlation coefficients 0.9989, 0.9982, and 0.8392
were obtained for these carbons, respectively. The results
indicate that the chemical shifts of the R and â carbons can
be used as probes as well as those of the pyrrole-H to
determine the spin state.3-7 In contrast, the meso carbon shifts
are less-suitable to elucidate the S ) 3/2 contribution in the
+
L2 where L’s are 4-MePyNO, PyNO, DMF, and MeOH.
The EPR spectrum of the 4-MePyNO complex exhibited
three signals at g ) 5.5, 4.5, and 2.0 as shown in Figure 7a.
Computer simulation suggested the existence of two com-
ponents. The major species have signals at 5.54 and 2.00,
while the minor species have them at 4.50 and 2.00. The
EPR spectrum of the PyNO complex also exhibited three
signals at g ) 5.73, 4.35, and 2.00. The computer simulation
revealed that these signals are ascribed to the single species.
In the case of the DMSO, DMF, MeOH, and THF com-
plexes, they showed the broad g signals at 4.2-4.0. Table
3 lists the EPR g values of all the complexes examined in
+
Fe(TPP)L2 complexes.
We have then estimated the chemical shifts for the pure
high-spin and intermediate-spin Fe(TPP)L2+ complexes. The
six-coordinate complexes such as Fe(TPP)(4-MePyNO)2
and Fe(TPP)(PyNO)2 are considered to be in a quite pure
high-spin state because the g values of these complexes are
close to 6.0.33 Thus, the chemical shift of the pyrrole-H in
the pure high-spin Fe(TPP)L2+ was assumed to be 71 ppm.
Although there is no example showing a pure intermediate-
+
+
this study. All the EPR spectra of Fe(TiPrP)L2 and Fe-
+
(OETPP)L2+ are given in the Supporting Information together
with some simulated spectra.
Determination of the S ) 3/2 Character by 13C NMR
Chemical Shifts. We have then determined the contribution
of the S ) 3/2 in the spin admixed (S ) 5/2,3/2) complexes
at 298 K on the basis of the 1H and 13C NMR chemical shifts.
+
spin state in the six-coordinate Fe(TPP)L2 , the five-
(33) Palmer, G. In Iron Porphyrin, Part II; Lever, A. B. P., Gray, H. B.,
Eds.; Addison-Wesley: Reading, MA, 1983; pp 43-88.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7337

Hoshino et al.
Table 2. 13C NMR Chemical Shifts of [Fe(Por)L2]ClO4 Taken in CD2Cl2 at 298 K (ppm)
pyrrole
meso-Ar
ligand
R
â
CR
Câ
meso
ip (CR)
o (Câ)
m
p
Int(%)
+
Fe(TPP)L2
4-MePyNO
3,5-Me2PyNO
PyNO
1625
1636
1617
1594
1538
1474
1261
547
1248
1249
1240
1218
1226
1179
1024
518
-
-
26
29
27
25
10
13
37
60
171
170
172
173
168
170
170
181
122
123
123
122
127
129
130
129
129
129
130
129
∼0
∼0
∼0
-
-
-
-
-
-
-
-
-
-
-
-
-
-
130
4-ClPyNO
DMSO
130
130
1.7
4.8
8.3
20
59
DMF
(129-131)
MeOH
134
117
131
130
130
THF
129
Fe(TiPrP)L2
+
4-MePyNO
3,5-Me2PyNO
PyNO
1262
1239
1031
889
1038
1015
865
756
290
138
86
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
97
97
(39)
(3)
(42)
(26)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
22
23
110
114
126
145
142
115
127
118
(ca. 40)
36
4-ClPyNO
DMSO
(42)
(43)
(41)
(42)
(51)
(46)
(49)
(30)
27)
45
87
100
100
100
100
100
226
DMF
24
(27)
(28)
MeOH
-47
-122
-154
-156
THF
22
(14)
b
b
2-MeTHF
dioxane
9
(18)
+
Fe(OETPP)L2
-19
4-MePyNOa
3,5-Me2PyNOa
PyNO
598
504
555
485
419
409
394
601
498
512
398
250
239
215
-2
-14
-10
-24
-43
-45
-55
190
191
191
186
190
199
215
257
257
278
307
338
343
354
66
52
126
126
125
122
121
121
118
127
126
125
122
117
116
116
42
55
-40
-63
38
51
DMSO
-139
-4
66
DMF
-221
-49
-57
-74
87
MeOH
-211
89
THF
-269
100
a The assignment of R and â could be reversed. b Either â or Câ appears at 17 ppm.
+
Figure 6. X-band EPR spectra of Fe(TiPrP)L2 taken in frozen CH2Cl2
solution at 4.2 K where L’s are (a) 4-MePyNO, (b) PyNO, (c) DMF, and
(d) dioxane. Small signals signified by * are ascribed to the minor
components.
Figure 7. X-band EPR spectra of Fe(OETPP)L2+ taken in frozen CH2Cl2
solution at 4.2 K, where L’s are (a) 4-MePyNO, (b) PyNO, (c) DMF, and
(d) MeOH. A small signal signified by * is ascribed to the minor component.
coordinate Fe(TPP)(OH2)+ reported by Evans and Reed is
known to be the complex with high degree of the S ) 3/2
character on the basis of the Mo¨ssbauer parameters and
effective magnetic moment.23 Since the pyrrole-H chemical
shifts of five- and six-coordinate intermediate-spin complexes
are supposed to be similar,34,35 the pyrrole-H chemical shift
7338 Inorganic Chemistry, Vol. 44, No. 21, 2005

Six-Coordinate Ferric Porphyrin Complexes
Table 3. EPR g Values (CH2Cl2, 4.2 K)
correlation coefficients for the R, â, and meso carbons were
0.9996, 0.9993, and 0.9710, respectively. We noticed that
the signals of the four complexes mentioned above showed
a deviation from the correlation lines and that the deviation
increased as the axial ligand changes from MeOH to THF,
to 2-MeTHF, and then to dioxane as shown by the filled red
marks.
ligand
g values
Fe(TPP)L2
6.11 5.81 2.00 DMF
ligand
g values
+
4-MePyNO
5.85
5.69
4.59
2.00
2.00
2.00
3,5-Me2PyNO 5.85
2.00 MeOH
2.00 THFa
2.00
PyNO
6.00
5.85
5.85
4-ClPyNO
DMSO
6.03 5.48 2.00
2.01
+
Fe(TiPrP)L2
We have then estimated the intrinsic chemical shifts for
+
the high-spin Fe(TiPrP)L2 complex. Although the 4-Me-
4-MePyNO
3,5-Me2PyNO 5.70
PyNO
5.85
2.00 DMFc
2.00 MeOHc
2.00 THF
4.2
4.1
4.0
2.00
2.00
1.97
2.00
2.00
PyNO complex showed the largest contribution of the S )
5/2 among the complexes examined in this study, it is still
not in the pure high-spin state; the pyrrole-H shift at 298 K
and EPR g at 4.2 K are 58.5 ppm and 5.85, respectively.
We have assumed that the pyrrole-H signal of the pure high-
5.70
5.75
4.2
4-ClPyNO
1.99 2-MeTHFc 4.1
2.00 dioxaned 4.1
DMSOb
+
Fe(OETPP)L2
4-MePyNOe
5.54
2.00 DMSOb
2.00 DMF
5.73 4.35 2.00 MeOH
4.3 2.00 THF
4.2
4.2
4.1
4.0
2.01
2.00
1.97
2.00
3,5-Me2PyNOe 5.69
+
spin Fe(TiPrP)L2 should appear more downfield than that
PyNO
4-ClPyNOb
+
of the pure high-spin Fe(TPP)L2 . This is because the
2
2
a Two components are comparable in population. b A weak signal was
observed at g ) 5.6-5.8. The population of this species is less than 5%.
c Some small signals were observed at 6.2-4.3. d Trace amount of high-
spin species is contaminated. e A weak signal was observed at g ) 4.5.
unpaired electron in the dx -y orbital can be delocalized to
the pyrrole-H position more effectively in Fe(TiPrP)L2+ than
+
in Fe(TPP)L2 due to the shorter Fe-NP bond lengths in
the former complexes; the average Fe-NP lengths in Fe-
+
+
(TPP)(THF)2 and Fe(TiPrP)(THF)2 are 2.016 and 1.967
Å, respectively.36,37 In fact, the pyrrole-H signal in five-
coordinate high-spin Fe(TiPrP)Cl appears at 90.4 ppm,38
which is 10.0 ppm more downfield than that of high-spin
Fe(TPP)Cl. Thus, we estimated the chemical shift of the
of Fe(TPP)(OH2)+, -43 ppm in C6D6 at 300 K, was taken
as the chemical shift for the intermediate-spin Fe(TPP)L2
+
complex with axially coordinating oxygen ligand. By putting
the assumed pyrrole-H shift (-43 ppm) of the pure inter-
mediate-spin complex into the equations, we were able to
estimate the chemical shifts of the R, â, and meso carbons
to be -193, 24, and 84 ppm, respectively. The chemical
shifts of the pure high-spin complex were taken from those
of Fe(TPP)(4-MePyNO)2+ to be 1625 (R), 1248 (â), and 26
(meso) ppm Thus, the contribution of the intermediate-spin
state, Int(%), was determined on the basis of the pyrrole-R
chemical shift by eq 1, where δL is the observed R-carbon
pyrrole-H signal in high-spin Fe(TiPrP)L2 to be 81 ppm,
+
which is 10 ppm more downfield than that of high-spin Fe-
+
(TPP)(4-MePyNO)2 . By putting the pyrrole-H shift (81
ppm) into the equations, we obtained the 13C chemical shifts
+
of the pure high-spin Fe(TiPrP)L2 ; they are 1610, 1290,
and 87 ppm for the R, â, and meso carbons, respectively.
Determination of the intrinsic chemical shifts for the
intermediate-spin complex was hampered because of the
deviation of the chemical shifts from the correlation lines in
the four complexes with L ) MeOH, THF, 2-MeTHF, and
dioxane. All of these complexes are supposed to be in an
essentially pure intermediate-spin state at least at 4.2 K
because the EPR g values are in the range of 4.2-4.0.33 In
+
chemical shift of Fe(TPP)L2 .
Int(%) ) [(1625 - δL)/(1625 - (-193))] × 100 (1)
The Int(%) values for the DMSO, MeOH, and THF
complexes were calculated to be 4.8, 20, and 59%, respec-
tively, as listed in Table 2.
+
fact, one of these complexes, Fe(TiPrP)(THF)2 , has been
[Fe(TiPrP)L2]ClO4. We have examined the S ) 3/2
fully characterized as a quite pure intermediate-spin complex
+
contribution in the ruffled Fe(TiPrP)L2 complexes. The
1
on the basis of the H NMR, 13C NMR, EPR, Mo¨ssbauer,
and SQUID data.31,35,39 Another fully characterized six-
chemical shifts of the Py-H in Table 1 indicate that the S )
3/2 character increases on going from the planar TPP to the
ruffled TiPrP for all the axial ligands examined.28 As in the
case of Fe(TPP)L2 , the chemical shifts of the R (O), â (0),
and meso (4) carbon signals were plotted against those of
+
coordinate intermediate-spin complex is Fe(TMCP)(H2O)2 ,
which shows the pyrrole signals at -33.7 and -36.3 ppm
at 295 K in CDCl3-D2O solution.40 Therefore, we were very
much surprised when we found that the pyrrole-H signal of
+
+
Fe(TiPrP)(dioxane)2 appears at -39.8 ppm, which is 4.3
the pyrrole-H signals as shown in panels a, b, and c of Figure
8, respectively; the red symbols are used for Fe(TiPrP)L2 .
+
+
ppm more upfield than that of Fe(TiPrP)(THF)2 . In the
Quite good linear fits were obtained for these carbons if the
chemical shifts of the complexes with extremely weak axial
ligands such as MeOH, THF, 2-MeTHF, and dioxane are
omitted. They are shown in Figure 8 by the red lines. The
(36) Chen, L.; Yi, G.-B.; Wang, L.-S.; Dharmawardana, U. R.; Dart, A.
C.; Khan, M. A.; Richter-Addo, G. B. Inorg. Chem. 1998, 37, 4677-
4688.
(37) Ohgo, Y.; Saitoh, T.; Nakamura, M. Acta Crystallogr. 2001, C57, 233-
234.
(34) The pyrrole-H chemical shifts of five-coordinate intermediate-spin
complexes such as Fe(TiPrP)ClO4 and Fe(TEtPrP)ClO4 are -31.2 and
-32.8 ppm, respectively, while those of the corresponding six-
coordinate intermediate-spin complexes such as Fe(TiPrP)(THF)2+ and
(38) Ikeue, T.; Ohgo, Y.; Uchida, A.; Nakamura, M.; Fujii, H.; Yokoyama,
M.; Inorg. Chem. 1999, 38, 1276-1281.
(39) Ikeue, T.; Saitoh, T.; Yamaguchi, T.; Ohgo, Y.; Nakamura, M.;
Takahashi, M.; Takeda, M. Chem. Commun. 2000, 1989-1990.
(40) Simonato, J.-P.; Pecaut, J.; Le Pape, L.; Oddou, J.-L.; Jeandey, C.;
Shang, M.; Scheidt, W. R.; Wojaczynski, J.; Wolowiec, S.; Latos-
Grazynski, L.; Marchon, J.-C. Inorg. Chem. 2000, 39, 3978-3987.
Fe(TEtPrP)(THF)2 are -35.5 and -32.3 ppm, respectively.35
+
(35) Sakai, T.; Ohgo, Y.; Hoshino, A.; Ikeue, T.; Saitoh, T.; Takahashi,
M.; Nakamura, M. Inorg. Chem. 2004, 43, 5034-5043.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7339

Hoshino et al.
+
+
+
Figure 8. Chemical shift correlation. (a) Py-H vs Py-R in Fe(TPP)L2 (black) and Fe(TiPrP)L2 (red), (b) Py-H vs Py-â in Fe(TPP)L2 (black) and
Fe(TiPrP)L2+ (red), (c) Py-H vs meso in Fe(TPP)L2+ (black) and Fe(TiPrP)L2+ (red), and (d) Py-CR vs Py-R (O), Py-â (0), and meso (4) in Fe(OETPP)L2
.
+
+
The filled red marks in (a)-(c) correspond to the signals of Fe(TiPrP)L2 where L’s are MeOH, THF, 2-MeTHF, and dioxane.
previous paper, we reported that there are two types of
intermediate-spin complexes with different electron configu-
rations, S ) 3/2(dxy) and S ) 3/2(dπ), and that the highly
from the S ) 3/2(dxy) to the S ) 3/2(dπ). Since the meso
+
signal of Fe(TiPrP)(DMF)2 appears at the most downfield
position, 145 ppm, we assumed that this complex adopts the
purest S ) 3/2(dxy) among the complexes examined in this
study; a minor contribution from the S ) 5/2 and S ) 3/2(dπ)
could not be neglected. Thus, we estimated the 13C chemical
shifts of the R, â, and meso carbons of the pure S ) 3/2(dxy)
complex to be equal to those of the DMF complex; they are
24 (R), 138 (â), and 145 (meso) ppm. The Int(%) is then
+
ruffled Fe(TEtPrP)(THF)2 should adopt the S ) 3/2(dxy)
electron configuration;31 the S ) 3/2(dxy) and S ) 3/2(dπ)
indicate the electron configurations of the intermediate-spin
3
1
1
2
2
complexes represented as (dxz, dyz) (dxy) (dz ) and (dxy) (dxz,
2
1
2
dyz) (dz ) , respectively. It is then expected that the difference
in the pyrrole-H chemical shifts among the intermediate-
spin complexes with ruffled porphyrin core should be
ascribed to the difference in the S ) 3/2(dxy) contribution.
Thus, the upfield shift of the pyrrole-H shift observed when
the axial ligand changes from DMF (-21.5 ppm), to MeOH
(-28.3 ppm), to THF (-35.5 ppm), to 2-MeTHF (-37.3
ppm), and then to dioxane (-39.8 ppm) suggests that the
contribution of the S ) 3/2(dxy) decreases by this order. Close
inspection of the data in Table 2 reveals that the meso signal
continuously moves downfield from 97 to 145 ppm as the
axial ligand changes from 4-MePyNO to DMF and reaches
the maximum value in the DMF complex. When the axial
ligand changes from DMF to dioxane, the meso signal shifts
upfield from 145 to 117 ppm. The curious behavior of meso
signal again supports for the change in electron configuration
+
expressed by eq 2 in the ruffled Fe(TiPrP)L2 complexes
with the S ) 5/2, 3/2(dxy) spin system, on the basis of the
pyrrole R chemical shifts, where δL is the observed R-carbon
chemical shift of Fe(TiPrP)L2 .
+
Int(%) ) [(1610 - δL)/(1610 - 24)] × 100
(2)
Thus, the Int(%) values for the 4-MePyNO, PyNO, DMSO,
and DMF complexes were calculated to be 22, 37, 87, and
100%, respectively, as listed in Table 2. Similarly, if we
+
assume that Fe(TiPrP)(dioxane)2 showing the pyrrole-H
signal at the most upfield position (-39.8 ppm) is the pure
S ) 3/2(dπ) complex, then the 13C chemical shifts of the R,
â, and meso signals are -156, 9, and 117 ppm, respectively.
7340 Inorganic Chemistry, Vol. 44, No. 21, 2005

Six-Coordinate Ferric Porphyrin Complexes
1
+
Table 4. Observed or Estimated 13C and H NMR Chemical Shifts of Pure High-Spin and Pure Intermediate-Spin Fe(Por)L2 Complexes with three
Different Porphyrin Structuresa
spin state
porphyrin
R-C
â-C
Py-CR
meso-C
pyrrole-H
examples
+
S ) 5/2
TPP
1625
1610
719
-193
-156
24
1248
1290
868
24
9
138
215
-
-
26
87
168
84
117
145
-269
71
81
-
-43
-40
-22
-
Fe(TPP)(4-MePyNO)2
c
TiPrP
c
c
OETPP
TPP(dπ)b
TiPrP(dπ)b
TiPrP(dxy)b
OETPP
36
-
S ) 3/2
+
-
-
-55
Fe(TiPrP)(Dioxane)2
Fe(TiPrP)(DMF)2
+
+
394
Fe(OETPP)(THF)2
a Bold entries are the observed chemical shifts. b The dπ and dxy indicate the (dxy) (dxz,dyz) (dz ) and (dxy, dyz) (dxy) (dz ) electron configurations, respectively.
c The chemical shifts of the pyrrole-H and Py-CR signals were estimated on the basis of the chemical shifts of the analogous complexes. Other chemical
shifts were obtained from the correlation equations.
2
2
1
3
1
1
2
2
The contributions of the S ) 3/2(dxy) in the intermediate-
spin Fe(TiPrP)(MeOH)2+ and Fe(TiPrP)(THF)2+ are roughly
estimated to be 60 and 20%, respectively.
It is difficult, however, to determine the Py-CR chemical shift
of the pure high-spin complex because even Fe(OETPP)(4-
MePyNO)2 exhibits the admixed intermediate-spin state;
+
the g value of this complex was determined to be 5.54, as
listed in Table 3. We tried to estimate the chemical shifts of
the pure high-spin six-coordinate Fe(OETPP)L2+ on the basis
of the reported chemical shifts of the analogous complexes.
The chemical shifts of the Py-CR in high-spin Fe(OEP)Cl
and Fe(OEP)(PyNO)2+ are 79.2 and 57.5 ppm, respectively,
indicating that the Py-CR signal moves upfield by 21.7 ppm
on going from high-spin five-coordinate to six-coordinate
complex.42 Since the average chemical shift of the Py-CR
signals in Fe(OETPP)Cl is 57.8 ppm, the Py-CR shift in
hypothetical high-spin Fe(OETPP)L2+ is expected to be 36.1
ppm, which is 21.7 ppm more upfield than that of Fe-
(OETPP)Cl. In the present discussion, we have approximated
Fe(OETPP)Cl as a high-spin complex at 298 K, though the
EPR data at 4.2 K suggest ca. 5% contribution of the S )
3/2 spin state.43 The chemical shifts of the Py-CR in
Fe(OETPP)L2+ were then correlated with those of the R, â,
and meso signals. The result is shown in Figure 8d. Quite
good linearity was observed for the R, â, and meso carbon
shifts; the correlation coefficients were 0.9755, 0.9906, and
0.9869, respectively. By putting the assumed Py-CR shift (36
ppm) for the pure high-spin complex into the equations, we
obtained the 13C chemical shifts of the R, â, and meso
carbons to be 719 (R), 868 (â), and 168 (meso) ppm. The
chemical shifts of the pure intermediate-spin complex are
iii. [Fe(OETPP)L2]ClO4. Determination of the Int(%)
value from the H NMR chemical shifts is much more
1
difficult in Fe(OETPP)L2+ because the chemical shifts of the
Py-CH2 protons cannot be good probes to differentiate S )
3/2 from S ) 5/2. For example, the average Py-CH2 shifts
in Fe(OETPP)L2+ are not much different; they are 31.2, 31.9,
27.3, 24.4, and 24.9 ppm for L ) 3,5-Me2PyNO, PyNO,
DMSO, MeOH, and THF, respectively; the axial ligands (L)
are arranged in descending order of the field-strength.28
Similarly, the average CH2 shifts in Fe(OETPP)X are 35.7,
34.0, 32.7, and 30.2 ppm for X ) F-, Cl-, Br-, and I-,
although the former two complexes are in the high-spin and
the latter two are in the quite pure intermediate-spin state.41
Relatively small ligand dependence of the CH2 shifts as
compared with that of the pyrrole-H shifts in the spin
admixed complexes can be explained in terms of the effect
of spin delocalization through σ bonds. While the unpaired
2
2
electron in the dx -y orbital effectively delocalizes to the
pyrrole-H positions through σ bonds, the delocalization to
the Py-CH2 positions must be greatly attenuated due to the
presence of an additional σ bond. As a result, the upfield
shift of the Py-CH2 signals caused by the depopulation of
2
2
the dx -y orbital becomes much smaller than that of the
pyrrole-H signals. In addition, the contact shift of the CH2
protons must be affected by the conformation of the ethyl
group.6 Since the conformation should change from complex
to complex due to the steric interactions between ligand and
the porphyrin core, the correlation between the CH2 chemical
shifts and Fe(III) spin state is expected to be worse.
+
taken from those of Fe(OETPP)(THF)2 . The Int(%) values
can then be estimated on the basis of the R, â, meso, or
Py-CR chemical shifts. Equation 3 shows the Int(%) obtained
on the basis of the Py-CR chemical shifts:
We have therefore examined the 13C NMR data to seek a
better probe that can reflect the spin state of Fe(OETPP)-
L2 . The Py-CR seems to be the most plausible candidate
Int(%) ) [(36 - δL)/(36 - (-55))] × 100
(3)
+
Thus, the Int(%) values for the PyNO, DMSO, MeOH, and
THF complexes were calculated to be 51, 66, 89, and 100%,
respectively, as listed in Table 2.
because it is directly bonded to the â pyrrole carbon as in
the case of the pyrrole-H. In addition, the conformational
effect of the ethyl groups on the Py-CR chemical shifts is
supposed to be quite small as compared with that on the
13C NMR Chemical Shifts of Pure High-Spin and
Intermediate-Spin Complexes of Six-Coordinate Fe(Por)-
+
Py-CH2. Since Fe(OETPP)(THF)2 is known to be in an
+
L2 . In Table 4, the chemical shifts of the pure high-spin
essentially pure intermediate-spin state, the chemical shift
and pure intermediate-spin six-coordinated Fe(Por)L2+ with
+
of the Py-CR in Fe(OETPP)(THF)2 , -55.0 ppm, was taken
as the chemical shift of the pure intermediate-spin complex.39
(42) Hoshino, A.; et al. unpublished results.
(43) Schu¨nemann, V.; Gerdan, M.; Trautwein, A. X.; Haoudi, N.; Mandon,
D.; Fischer, J.; Weiss, R. Tabard, A.; Guilard, R. Angew. Chem., Int.
Ed. 1999, 38, 3181.
(41) Nakamura, M.; Ikeue, T.; Ohgo, Y.; Takahashi, M.; Takeda, M. Chem.
Commun. 2002, 1198-1199.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7341

Hoshino et al.
three different porphyrin structures are listed. As mentioned
by 921 and 392 ppm, respectively, while the meso signal
moved downfield by 147 ppm. In contrast, the dxy-a1u
interaction is supposed to be quite small in the high-spin
saddled complexes.
1
in the previous section, the 13C and H chemical shifts in
Table 4 were obtained either from the observed spectra or
from the correlation lines shown in Figure 8. We will discuss
the chemical shifts in Table 4 from the viewpoint of the
iron-porphyrin orbital interactions.
We have then examined the orbital interactions in the
intermediate-spin complexes. In the intermediate-spin TPP
complex, the dπ-3eg interaction is considered to be the major
interaction.2 Since the 3eg orbital has relatively large coef-
ficients at the pyrrole nitrogen and â carbon and zero
coefficient at the meso carbon atoms, as shown in Scheme
2, we can expect the upfield shift of the R signal. The
expectation is confirmed by the data in Table 4; the chemical
shift of the R signal is estimated to be -193 ppm. The large
upfield shift of the pyrrole-H signal indicates that the
complex adopts the S ) 3/2(dπ) electron configuration, which
is further confirmed by the fact that the isotropic pyrrole-H
In the high-spin TPP complex, the large downfield shifts
of the R-C, â-C, and Py-H signals can be explained in terms
of the strong σ-donation of nitrogen atoms to the half filled
dx -y orbital.2-7 If the planar TPP is replaced by the ruffled
2
2
TiPrP, we can expect at least three factors that affect the 13
C
chemical shifts: (i) the contraction of the Fe-N bond,44,45
(ii) the interaction between dxy and a2u orbitals,2,46 and (iii)
the interaction between dx -y and a1u orbitals.47,48 Factor (i)
induces further downfield shift to the R, â, and pyrrole-H
signals, factor (ii) causes the downfield shift of the meso
and upfield shift of the R signal,21,49,50 and factor (iii) would
shift the R and â signals to the downfield and the meso signal
to the upfield positions; note that the a1u orbital has large
coefficients at the R and â carbons and zero coefficient at
the nitrogen and meso carbon atoms, as shown in Scheme
2. The data in Table 4 indicate the upfield shift of the R and
the downfield shift of the â, meso, and pyrrole-H signals,
supporting the major contribution of factors (i) and (ii).
It should be noted here that the downfield shift of the meso
signal due to the dxy-a2u interaction in the high-spin ruf-
fled complexes is rather small, ca. 60 ppm, as compared
with ca. 700 ppm in the corresponding low-spin com-
plexes.49
2
2
+
shift of intermediate-spin Fe(TPP)L2 (-52 ppm) is twice as
+
much as that of low-spin Fe(TPP)(HIm)2 (-26 ppm); the
latter complex is known to adopt the (dxy)2(dxz, dyz)3 electron
configuration.
If the planar TPP is replaced by the ruffled TiPrP in the S
) 3/2(dπ) complexes, we can expect that the difference in
the 13C NMR shifts should be rather small. In fact, the
chemical shifts of these two complexes are quite similar, as
shown in Table 4. If the electronic structure changes from
the S ) 3/2(dπ) to the S ) 3/2(dxy) in the ruffled complexes,
one of the dπ-3eg interactions diminishes while the dxy-a2u
interaction occurs. It is expected that the loss of the dπ-3eg
interaction should shift the R signal downfield, while the
occurrence of the dxy-a2u interaction should shift the meso
signal downfield. In fact, the data in Table 4 indicate that
both the R and meso signals showed downfield shifts by 173
and 28 ppm, respectively. Rather small downfield shift of
the meso signal again suggests that the dxy-a2u interaction
is quite weak in the intermediate-spin complexes as in the
case of the high-spin complexes. If the planar TPP is replaced
by the saddled OETPP, the interaction between the filled
If the planar TPP is replaced by the saddled OETPP, we
can also expect at least three factors affecting the 13C NMR
chemical shifts: (i) the less-effective nitrogen donation to
2
2
the half filled dx -y orbital, (ii) the interaction between the
dx -y and a2u orbitals,51,52 and (iii) the interaction between
the dxy and a1u orbitals.47,48 Factor (i) induces the upfield shift
to the R and â signals, factor (ii) causes the downfield shift
of the meso and an upfield shift of the R signal, and factor
(iii) would shift the R and â signals to the downfield and
the meso signal to the upfield positions. The data in Table
2 suggest that factors (i) and (ii) mainly determine the 13C
NMR chemical shifts; the R and â signals shifted upfield
2
2
2
2
a
2u and empty dx -y orbital becomes possible in addition to
the dπ-3eg interaction. As proposed by Cheng and co-
workers, the former interaction could induce the negative
spin to the a2u orbital by the spin polarization mechanism.48
As a result, the meso and R signals move upfield and
downfield, respectively.
(44) Scheidt, W. R. In The Porphyrin Handbook, Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol.
3, Chapter 16, pp 49-112.
(45) Senge, M. O.; Bischoff, I.; Nelson, N. Y.; Smith, K. M. J. Porphyrins
Phthalocyanines 1999, 3, 99-116.
Electron Configurations of the Intermediate-Spin Com-
plexes. We have then examined the electron configuration
+
of the intermediate-spin Fe(OETPP)L2 . In the previous
(46) Safo, M. K.; Walker F. A.; Raitsimring, A. M.; Walters, W. P.; Dolata,
D. P.; Debrunner, P. G.; Scheidt, W. R. 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.
(47) Ghosh, A.; Vangberg, T.; Gonzalez, E.; Taylor, P. J. J. Porphyrins
Phthalocyanines 2001, 5, 345-356.
communication, we reported that the intermediate-spin Fe-
(OETPP)(THF)2+ should adopt the S ) 3/2(dπ).31 We reached
this conclusion on the basis of the unique 13C NMR chemical
+
shifts of the low-spin Fe(OETPP)L2 . An example is the
chemical shifts of Fe(OETPP)(HIm)2+ with the (dxy)2(dxz, dyz)3
electron configuration. This complex shows the R, â, and
meso signals at 163, 167, and 7.0 ppm, respectively.49 In
(48) Cheng, R.-J.; Wang, Y.-K.; Chen, P.-Y.; Han Y.-P.; Chang, C.-C.
Chem. Commun. 2005, 1312-1314.
(49) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Yamaguchi, T.; Nakamura, M. Inorg.
Chem. 2001, 40, 3423-3434.
+
(50) Ikeue, T.; Ohgo, Y.; Ongayi, O.; Vicente, G. H.; Nakamura, M. Inorg.
Chem. 2003, 42, 5560-5571.
contrast, the chemical shifts in Fe(TPP)(HIm)2 are very
much different despite the same electron configuration; they
are 39, 94, and 55 ppm for the R, â, and meso signals,
respectively. Therefore, the large downfield shifts of the R
and â signals observed in Fe(OETPP)(THF)2+ are consistent
(51) Renner, M. W.; Barkigia, K. M.; Zhang, Y.; Medforth, C. J.; Smith,
K. M.; Fajer, J. J. Am. Chem. Soc. 1994, 116, 8582-8592.
(52) Ghosh, A.; Harvorsen, I.; Nilsen, H. J.; Steene, E.; Wondimagegn,
T.; Lie, R.; Caemelbecke, E. van; Guuo, N.; Ou, Z.; Kadish, K. M. J.
Phys. Chem. B 2001, 105, 8120-8124.
7342 Inorganic Chemistry, Vol. 44, No. 21, 2005

Six-Coordinate Ferric Porphyrin Complexes
with the existence of the two-half-filled dπ orbitals; the R,
â, and meso signals appear at 394, 215, and -269 ppm,
respectively. Thus, we assigned the electron configuration
+
of intermediate-spin Fe(OETPP)(THF)2 to be S ) 3/2(dπ)
rather than S ) 3/2(dxy).
Recently, Cheng and co-workers proposed on the basis
of the DFT calculation that the ground state of Fe(OETPP)-
(THF)2+ should be represented as S ) 3/2(dxy) rather than S
) 3/2(dπ).48 This is because the dxy orbital is destabilized
due to the interaction with the a1u orbital which is possible
in the saddled complexes. Since the a1u orbital has relatively
large coefficients at the R and â carbons (especially on the
R carbon atoms) and zero coefficient at the meso carbon,
the strong dxy-a1u interaction could induce the downfield
shifts of the R and â signals and the upfield shift of the meso
signal. However, as we have already mentioned in this paper,
the dxy-a1u interaction in the saddled complexes is quite weak
in the high-spin, and probably in the intermediate-spin
complexes as well because of the long Fe-Np bond lengths.
Furthermore, we noticed that the dxy-a1u interaction in the
saddled complexes is rather weak even in the low-spin
complexes with much shorter Fe-NP bonds, which is
exemplified in the chemical shifts of low-spin Fe(OETPP)-
Figure 9. Curie plots of the pyrrole-H signals of Fe(TiPrP)L2
: b,
+
4-MePyNO; +, 3,5-Me2PyNO; 2, PyNO; 3, 4-ClPyNO; 4, DMSO; !
DMF; O, MeOH; 9, THF.
+
(tBuNC)2 . This complex is known to adopt the (dxz, dyz)4-
weak oxygen ligands and three different porphyrin structures.
We have then considered to extract the factors affecting the
population of the S ) 3/2 in these six-coordinate ferric
porphyrin complexes.
(dxy)1 electron configuration. Therefore, we can expect the
downfield shifts of the R and â signals and the upfield shift
of the meso signal if the dxy-a1u interaction is fairly strong.
Actually however, the R, â, and meso signals are observed
at -5, 143, and 417 ppm, respectively, at 298 K.49
i. Axial Ligands. In the five-coordinate ferric porphyrin
complexes with anionic axial ligands, it is well established
that the population of the S ) 3/2 in the admixed intermedi-
ate-spin complexes increases as the field strength of the
anionic ligand is weakened. Thus, Reed and Guiset ranked
the relative filed strengths of weak anionic ligands on the
basis of the pyrrole-H chemical shifts and called the hierarchy
as magnetochemical series.53 Similar to the five-coordinate
complexes, the determination of the S ) 3/2 character in
the six-coordinate complexes could be a good method to
evaluate the weakness of the axial ligands. In the previous
communication, we ranked the weakness of neutral ligands
with oxygen as a coordinating atom on the basis of the
pyrrole-H chemical shifts; part of the data is included in
Table 1.28 Although the field strengths of PyNO and alkyl-
substituted PyNO’s were difficult to determine in the planar
TPP complexes because of the similarity of the chemical
shifts, they were clearly discriminated in highly ruffled Ti-
PrP complexes. Figure 9 shows the Curie plots of the
The electron configuration of the ruffled intermediate-spin
+
Fe(TiPrP)(THF)2 is also controversial. We reported in the
previous paper that the complex adopts the S ) 3/2(dxy)
ground state.31 Recent theoretical work suggested, however,
that Fe(TiPrP)(THF)2 should adopts he S ) 3/2(dπ) ground
state.48 The major reason is that the S ) 3/2(dxy) complex
should exhibits the meso signal fairly downfield due to the
dxy-a2u interaction in the ruffled complex; the meso carbon
+
chemical shift of Fe(TiPrP)(THF)2 is 115 ppm at 298 K.31
In the present study, we have found that the pyrrole-H signal
in the essentially pure intermediate-spin Fe(TiPrP)L2+ moved
upfield from -22 to -40 ppm as the axial ligand changes
from DMF to dioxane. Similarly, the meso-C signal moved
upfield from 145 to 117 ppm for the same change in the
axial ligand. We have ascribed the upfield shift of both the
pyrrole-H and meso-C signals in a series of intermediate-
+
spin Fe(TiPrP)L2 to the change in electronic state from S
+
pyrrole-H of a series of Fe(TiPrP)L2 . The Curie lines were
) 3/2(dxy) to S ) 3/2(dπ). We have the impression that the
two electronic states, S ) 3/2(dxy) and S ) 3/2(dπ), in ruffled
drawn so that they intercept the ordinate at the diamagnetic
position of the pyrrole-H. The complexes with mainly S )
5/2 showed the positive slope while those with mainly S )
3/2 exhibited the negative slope. Thus, we were able to rank
the field strength of a number of oxygen-containing ligands
by using both planar TPP and ruffled TiPrP complexes. It
should be noted that the order of the shifts is maintained in
a wide range of temperatures, as shown in Figure 9. The
curvature of the Curie lines suggests that the population of
+
Fe(TiPrP)L2 are quite close in energy and that they can
easily be switched by the subtle change of axial ligand.
Obviously, further experimental and theoretical studies are
necessary to fully understand the electron configurations of
the intermediate-spin complexes with nonplanar porphyrin
cores.
Factors Affecting the S ) 3/2 Character in the Six-
Coordinate Spin Admixed Intermediate-Spin Complexes.
As mentioned, we have estimated the population of the S )
+
3/2 in the S ) 5/2,3/2 complexes Fe(Por)L2 with various
(53) Reed, C. A.; Guiset, F. J. Am. Chem. Soc. 1996, 118, 3281-3282.
Inorganic Chemistry, Vol. 44, No. 21, 2005 7343

Hoshino et al.
+
the S ) 3/2 (or S ) 5/2) changes depending upon the
temperature.54 The field strength of these ligands is arranged
in descending order, as shown in eq 4, and they are listed in
Tables 1-3 by the same order.
ate-spin state, Fe(TPP)(THF)2 showed an admixed inter-
mediate-spin state with 68% S ) 3/2 contribution at 298 K.
In the case of the ruffled porphyrin complexes, the paired
electrons of four nitrogen atoms can effectively interact with
2
2
the iron dx -y orbital to destabilize this orbital. In the case
of the saddled porphyrin complexes, the overlap between
the nitrogen lone pair and the iron dx -y orbital could be
less effective due to the saddled core. However, the interac-
4-MePyNO ≈ 3,5-Me2PyNO > PyNO > 4-ClPyNO >
DMSO > DMF > MeOH > THF (4)
2
2
It is clear that the ligands with formally charged oxygen atom
such as PyNO are stronger than those with uncharged oxygen
such as MeOH and THF. A substituent effect is also clearly
seen since 4-MePyNO is stronger than and 4-ClPyNO is
weaker than unsubstituted PyNO. Among eight ligands
examined, THF is the weakest ligands. Thus, bis(THF)
complex always shows the largest population of the S )
2
2
tion between the iron dx -y and porphyrin a2u orbital becomes
symmetry allowed.52 As a result, the dx -y orbital is also
destabilized in the saddled complexes. The data in Tables
1-3 indicate that the Int(%) values actually increased on
going from the planar TPP to the deformed TiPrP and OETPP
cores.20,39-41,55
2
2
+
3/2 state in a series of Fe(Por)L2 .
Acknowledgment. This work was supported by the Grant
in Aid (No 14540521, 16550061) for Scientific Research
from Ministry of Education, Culture, Sports, Science and
Technology, Japan. Thanks are due to the Research Center
for Molecular-Scale Nanoscience, the Institute for Molecular
Science (IMS).
ii. Deformation of the Porphyrin Ring. As mentioned
in our previous papers, the S ) 3/2 spin state is stabilized
in highly deformed porphyrin complexes, which is ascribed
to the destabilization of the dx -y orbital.39,40,53,55 In fact, while
all the bis(THF) complexes with deformed porphyrin ring
such as OETPP and TiPrP exhibited a quite pure intermedi-
2
2
Supporting Information Available: EPR spectra of Fe(TiPrP)-
L2+ and Fe(OETPP)L2+ and simulation of some EPR spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(54) Nesset, M. J. M.; Cai, S.; Shokhireva, T. K.; Shokhirev, N. V.;
Jacobson, S. E.; Jayaraj, K.; Gold, A.; Walker, F. A. Inorg. Chem.
2000, 39, 532-540.
(55) Barkigia, K. M.; Renner, M. W.; Fajer, J. J. Porphyrins Phthalocyanins
2001, 5, 415-418.
IC0488942
7344 Inorganic Chemistry, Vol. 44, No. 21, 2005