Electronic effects of para-substituents on the electron configuration of dicyano[meso-tetrakis(p-substituted phenyl)porphyrinato]iron(III) complexes

Article abstract of DOI:10.1016/S0020-1693(02)00819-8

There are two types of electron configuration, (d_xy)²(d_xz, d_yz)³ and (d_xz, d_yz)⁴(d_xy)¹, in low spin iron(III) porphyrin complexes. In order to reveal how the electronic effect of substituents affects the electron configuration of low spin iron(III) porphyrin complexes, we have examined the ¹³C NMR, ¹H NMR, and EPR spectra of a series of tetrabutylammonium (dicyano)[meso-tetrakis(p-substituted phenyl)porphyrinato]ferrate(III), [Fe(p-X-TPP)(CN)₂]^-Bu₄N⁺, in both CD₂Cl₂ and CD₃OD solutions. The chemical shifts of the meso carbon signals, which sharply reflect the electron configuration of low spin iron(III), have changed to a great extent depending on the electron-donating or electron-withdrawing ability of the para-substituent. The isotropic shifts of the meso-carbon signals of [Fe(p-X-TPP)(CN)₂]^-Bu₄N⁺ are determined on the basis of the meso-carbon chemical shifts of the corresponding diamagnetic cobalt(III) complexes, [Co(p-X-TPP)(CN)₂]^-Bu₄N⁺. The plots of the isotropic shifts against Hammett σ_p values have yielded good linear lines with the slopes, -22 and -69 ppm, in CD₂Cl₂ and CD₃OD, respectively. On the basis of these results, we have concluded that the electron-withdrawing groups at the phenyl para-positions stabilize the (d_xy)²(d_xz, d_yz)³ state while the electron-donating groups at the same positions stabilize the (d_xz, d_yz)⁴(d_xy)¹ state. The conclusion is further supported by the ¹H NMR and EPR results.

Full text of DOI:10.1016/S0020-1693(02)00819-8

Inorganica Chimica Acta 335 (2002) 91Á99
/
www.elsevier.com/locate/ica
Electronic effects of para-substituents on the electron configuration of
dicyano[meso-tetrakis(p-substituted phenyl)porphyrinato]iron(III)
complexes
Akira Ikezaki a, Takahisa Ikeue a, Mikio Nakamura a,b,
*
a Department of Chemistry, Toho University School of Medicine, Tokyo 143-8540, Japan
b Division of Biomolecular Science, Graduate School of Science, Toho University, Funabashi 274-8510, Japan
Received 25 October 2001; accepted 28 December 2001
Abstract
There are two types of electron configuration, (dxy )2(dxz, dyz )3 and (dxz, dyz )4(dxy )1, in low spin iron(III) porphyrin complexes. In
order to reveal how the electronic effect of substituents affects the electron configuration of low spin iron(III) porphyrin complexes,
we have examined the 13C NMR, 1H NMR, and EPR spectra of a series of tetrabutylammonium (dicyano)[meso-tetrakis(p-
substituted phenyl)porphyrinato]ferrate(III), [Fe(p-XÁ
TPP)(CN)2]ꢀBu4Nꢁ, in both CD2Cl2 and CD3OD solutions. The chemical
shifts of the meso carbon signals, which sharply reflect the electron configuration of low spin iron(III), have changed to a great
/
extent depending on the electron-donating or electron-withdrawing ability of the para-substituent. The isotropic shifts of the meso-
carbon signals of [Fe(p-XÁ
corresponding diamagnetic cobalt(III) complexes, [Co(p-XÁ
Hammett sp values have yielded good linear lines with the slopes, ꢀ
/
TPP)(CN)2]ꢀBu4Nꢁ are determined on the basis of the meso-carbon chemical shifts of the
/
TPP)(CN)2]ꢀBu4Nꢁ. The plots of the isotropic shifts against
/
22 and ꢀ69 ppm, in CD2Cl2 and CD3OD, respectively. On
/
the basis of these results, we have concluded that the electron-withdrawing groups at the phenyl para-positions stabilize the
(dxy )2(dxz, dyz )3 state while the electron-donating groups at the same positions stabilize the (dxz, dyz )4(dxy )1 state. The conclusion is
1
further supported by the H NMR and EPR results. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Iron(III) porphyrins; Low spin complexes; Electron configuration; Electronic effect; 13C NMR; Hammett plot
1. Introduction
deformation of porphyrin rings; while many low spin
iron(III) complexes show the (dxy)2(dxz, dyz)3 ground
state, the low spin complexes that have axial ligands
The electronic configuration of low spin iron(III)
porphyrin complexes is represented either as (dxy)2(dxz,
dyz)3 or as (dxz, dyz)4(dxy)1 as shown in Scheme 1 [1Á
3].
Major factors determining the electron configuration
with extremely low lying p* orbitals [4Á
/
12], and/or
/
highly S4 ruffled porphyrin core [11,13Á18], exhibit the
/
(dxz, dyz)4(dxy)1 ground state. We have recently reported
that the solvent effect also plays an important role in
determining the ground state. For example, the electro-
nic ground state of low spin (dicyano)[meso-tetra-
kis(2,4,6-triethylphenyl)porphyrinato]ferrate(III)
are the ligand field strength of axial ligands and the
Abbreviations: TPP and p-XÁ
tetraphenylporphyrin and meso-tetrakis(p-substituted phenyl)
porphyrin; [Fe(p-XÁTPP)Cl], chloro[meso-tetrakis(p-substituted
phenyl)porphyrinato]iron(III);
tetrabutylammonium
/
TPP, dianions of meso-
changes drastically depending on the solvent properties;
the complex is in the (dxz, dyz)4(dxy)1 state in methanol
and chloroform while it is in the (dxy)2(dxz, dyz)3 state in
benzene, toluene, and tetrachloromethane[15,19,20].
Although we have revealed various factors affecting
the electronic ground state of low spin iron(III) por-
phyrin complexes, the electronic effects of substituents
are still ambiguous [3]. In the previous papers, we have
/
[Fe(p-XÁ
dicyano[meso-tetrakis(p-substituted
TPP)Cl], chloro[meso-
/
TPP)(CN)2]ꢀBu4Nꢁ
,
phenyl)porphyrinato]ferrate(III); [Co(p-XÁ
/
tetrakis(p-substituted phenyl)porphyrinato]cobalt(III); [Co(p-XÁ
/
TPP)(CN)2]ꢀBu4Nꢁ, tetrabutylammonium dicyano[meso-tetrakis(p-
substituted phenyl)porphyrinato]cobaltate(III).
* Corresponding author. Fax: ꢁ
E-mail address: mnakamu@med.toho-u.ac.jp (M. Nakamura).
/
81-3-5493-5430.
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 0 8 1 9 - 8

92
A. Ikezaki et al. / Inorganica Chimica Acta 335 (2002) 91Á99
/
were purchased from Aldrich and Porphyrin Systems,
respectively. A series of (p-XÁTPP)H2 were prepared
according to the literature using pyrrole and the
/
corresponding para-substituted benzaldehydes [24]. Me-
tal insertion reactions to form [Fe(p-XÁ
XÁTPP)], and [Co(p-XÁTPP)Cl] were carried out
according to the literature [25Á27].
/
TPP)Cl], [Co(p-
/
/
/
2.2. Spectral measurement
1H and 13C NMR spectra were measured on a JEOL
Scheme 1. Two types of electron configuration of low spin iron(III)
porphyrin complexes.
1
LA300 spectrometer operating at 300.4 MHz for H.
Chemical shifts were referenced to the residual peaks of
1
5.32 ppm for H and
1
reported that the contribution of the (dxz, dyz)4(dxy
)
deuterated solvents; CD2Cl2(dꢂ
/
53.8 ppm for 13C) and CD3OD(dꢂ3.30 for 1H and 49.0
/
state in (dicyano)[meso-tetrakis(2,6-dichlorophenyl)por-
phyrinato]ferrate(III) and bis(t-butylisocyanide)[meso-
tetrakis(2,3,4,5,6-pentafluorophenyl)porphyrinato]iron-
(III) is much larger than that in the corresponding
ppm for 13C). EPR spectra were recorded on a Bruker
E500 spectrometer operating at X band and equipped
with an Oxford helium cryostat.
(meso-tetraphenylporphyrinato)iron(III)
complexes
2.3. Sample preparation for NMR measurements
[3,19]. On the basis of these results, we have concluded
that the electron-withdrawing substituents at the meso-
positions stabilize the (dxy)2(dxz, dyz)3 state. In order to
further confirm this conclusion, we have planned to
determine the electronic ground state of a series of
tetrabutylammonium dicyano[meso-tetrakis(p-substi-
Samples for the 13C NMR measurements were pre-
pared as follows. [Fe(p-XÁTPP)Cl](20 mg) was placed
/
in an NMR sample tube, to which a 550-ml CD2Cl2
solution of Bu4NꢁCNꢀ (4.0 equiv.) was added via
microsyringe to form 37 to 52 mM solution of [Fe(p-XÁ
/
tuted
TPP)(CN)2]-Bu4Nꢁ (Scheme 2), in which p-substituents
are not only the electron withdrawing ÁF, ÁCl, Á
COOCH3, ÁCF3 and ÁCN but also the electron-
donating ÁOCH3 and ÁCH3 [21,22]. Since these sub-
stituents are located at the phenyl para-positions, the
direct steric and electronic interactions between the p-
substituents and the porphyrin core need not be
considered [23]. Thus, the conclusion obtained here
could reflect the purely electronic effect of substituents.
phenyl)porphyrinato]ferrate(III),
[Fe(p-XÁ
/
TPP)(CN)2]ꢀBu4Nꢁ. For the H NMR measurements,
a CD2Cl2 solution of [Fe(p-XÁ
TPP)(CN)2]ꢀBu4Nꢁ
(7.5 mM) was prepared by the similar procedure. The
CD3OD solutions of [Fe(p-XÁ
TPP)(CN)2]ꢀBu4Nꢁ
1
/
/
/
/
/
/
/
/
/
were prepared similarly from [Fe(p-XÁTPP)Cl] (10
/
mg) and Bu4NꢁCNꢀ (4.0 equiv.) and they were used
for both the 1H and 13C NMR measurements. The
CD2Cl2 solutions of [Co(p-XÁ
TPP)(CN)2]ꢀBu4Nꢁ
/
were similarly prepared from [Co(p-XÁTPP)Cl] (10
/
mg) and Bu4NꢁCNꢀ (4 equiv.) and they were used
1
for H and 13C NMR measurements. For signal assign-
ments, 2D HMQC and HMBC spectra of [Fe(p-XÁ
/
2. Experimental
TPP)(CN)2]ꢀBu4Nꢁ and [Co(p-XÁTPP)(CN)2]ꢀBu4-
/
Nꢁ were measured.
2.1. Materials
Deuterated solvents such as dichlorometh-
ane(CD2Cl2) and methanol (CD3OD) were purchased
3. Results
from
Merck.
Tetrabutylammonium
cy-
TPP)Cl]
3.1. 13C NMR Spectra of [Fe(p-XÁ
(CN)2]ꢀBu4Nꢁ and [Co(p-XÁTPP)(CN)2]ꢀBu4Nꢁ
/TPP)-
anide(Bu4NꢁCNꢀ) and [Fe(p-CH3OCOÁ
/
/
3.1.1. CD2Cl2 solution
Fig. 1(a) shows the 13C NMR spectrum of [Fe(p-CNÁ
TPP)(CN)2]ꢀBu4Nꢁ taken in CD2Cl2 at 25 8C. Fig.
/
1(b) shows the 13C NMR spectrum of [Fe(p-OCH3Á
/
TPP)(CN)2]ꢀBu4Nꢁ taken in CD2Cl2 at 25 8C. The
meso, a-Py, and b-Py signals moved from d 74.3, 47.3
and 92.9 ppm to d 94.4, 40.2, and 89.8 ppm, respec-
TPP)(CN)2]ꢀBu4Nꢁ
Scheme 2. [Fe(p-X-TPP)(CN)2]ꢀBu4Nꢁ and the labelling for the 13
NMR signals.
C
tively, on going from [Fe(p-CNÁ
/
to [Fe(p-OCH3Á
/
TPP)(CN)2]ꢀBu4Nꢁ. Table 1 lists the

A. Ikezaki et al. / Inorganica Chimica Acta 335 (2002) 91Á
/
99
93
in Table 1 as italic letters. The data in Table 1 indicate
that the meso-carbon chemical shifts change to a great
extent depending on the p-substituents. The downfield
shift of the meso-signal was the largest in the methoxy
complex and decreased in the order given in Eq. (1):
OCH3 ꢀÁCH3 ꢀÁHꢀÁFꢀÁClꢀÁCOOCH3 ꢀÁCF3
ꢀÁCN
(1)
Fig. 2 shows the temperature dependence of the carbon
chemical shifts of [Fe(p-CNÁ
TPP)(CN)2]ꢀBu4Nꢁ and
[Fe(p-OCH3Á
TPP)(CN)2]ꢀBu4Nꢁ taken in CD2Cl2
/
/
solution. The largest difference in chemical shifts
between these two complexes was observed in the
meso-carbon throughout the temperature range exam-
ined. The observed chemical shifts of the meso and a-
pyrrole carbons are represented as dobs
where ddia and diso indicate a diamagnetic shift and an
isotropic shift, respectively [28Á30]. The isotropic shifts
ꢂ
/
ddia
ꢁ
/
diso,
Fig. 1. 13C NMR spectra of (a) [Fe(p-CNÁ
(b) [Fe(p-OCH3 Á
TPP)(CN)2]ꢀBu4Nꢁ in CD2Cl2 at 25 8C. The
labeling for the signals is given in Scheme 2.
/
TPP)(CN)2]ꢀBu4Nꢁ and
/
(25 8C) of the meso and a-pyrrole carbons were
determined by taking the chemical shifts of the corre-
/
sponding carbons in [Co(p-XÁ
TPP)(CN)2]ꢀBu4Nꢁ as
/
13C NMR chemical shifts of a series of [Fe(p-XÁ
TPP)(CN)2]ꢀBu4Nꢁ taken at 25 and ꢀ
50 8C. The
chemical shifts of the corresponding diamagnetic [Co(p-
XÁ
TPP)(CN)2]ꢀBu4Nꢁ taken at 25 8C are also listed
/
the diamagnetic shifts(ddia). The plots of the isotropic
shifts (25 8C) of the meso and a-pyrrole carbons against
Hammett sp values are given in Fig. 3(a) and (b),
/
/
respectively [31]. Good linear lines with the slopes of ꢀ
/
Table 1
13C NMR chemcial shifts of [Fe(p-XÁTPP)(CN)2]ꢀBu4Nꢁ in CD2C12
/
b
X
sp
meso
a-Py
b-Py
ipso
o
m
p
p-X
a
(a) 25 8C
Á
/
OCH3
CH3
H
ꢀ0.28
ꢀ0.14
0.00
94.4
118.7
93.5
40.2
143.7
40.3
89.8
133.3
89.9
118.0
134.8
123.1
139.5
127.2
142.4
124.1
138.3
127.3
140.8
134.2
134.1
146.1
135.8
146.9
152.1
135.7
150.4
134.7
148.7
134.8
148.5
136.0
146.9
135.8
144.7
143.4
134.9
142.8
135.2
110.7
112.3
125.9
127.5
125.3
126.8
111.8
113.6
125.0
127.0
126.0
121.8
123.7
128.6
130.7
158.8
159.6
136.8
137.4
126.7
127.6
161.9
163.0
132.9
134.0
128.5
128.3
129.7
110.4
111.9
54.9
55.8
19.5
21.5
Á/
119.0
89.0
143.5
41.3
133.3
90.3
Á/
119.1
84.2
143.4
42.4
133.4
90.6
Á/
F
0.15
118.0
81.1
143.5
42.9
133.4
90.9
Á/
Cl
0.24
117.9
79.2
143.3
43.6
133.5
91.4
c
Á/
Á/
COOCH3
0.44
0.53
51.7, 165.5
123.2
CF3
74.8
43.9
91.6
117.9
74.3
117.8
143.1
47.3
142.9
133.6
92.9
133.7
125.1
Á/
CN
0.70
117.2
119.5
(b) ꢀ50 8C
Á
Á/
Á/
Á/
Á/
Á/
Á/
Á/
/
OCH3
CH3
H
ꢀ0.28
ꢀ0.14
0.00
47.0
46.0
41.0
35.0
33.8
32.4
28.3
26.1
10.7
10.7
11.5
12.3
12.6
12.8
12.8
13.7
87.2
87.1
87.6
88.4
88.8
89.3
89.7
90.3
116.4
121.3
125.2
122.2
125.0
131.8
131.3
132.8
140.1
138.7
136.7
136.2
135.4
133.9
132.3
132.1
107.2
122.8
121.9
108.5
121.7
122.8
118.6
125.6
155.6
134.0
123.8
158.6
129.7
125.2
125.0
107.2
53.5
18.3
F
0.15
0.24
Cl
COOCH3
CF3
CN
0.44
0.53
51.0, 164.1
121.6
116.1
0.70
a
Chemical shifts of the corresponding diamagnetic [Co(p-XÁ
TPP)(CN)2]ꢀBu4Nꢁ are written in italic letters.
/
b
c
Hammett sp values [31].
13C NMR measurement is not done for [Co(p-COOCH3 Á
/
TPP)(CN)2]ꢀBu4Nꢁ
.

94
A. Ikezaki et al. / Inorganica Chimica Acta 335 (2002) 91Á99
/
Fig. 2. Temperature dependence of the 13C NMR chemical shifts of
the meso, a-Py and b-Py carbon atoms of [Fe(p-CNÁ
TPP)(CN)2]ꢀBu4Nꢁ and [Fe(p-OCH3 ÁTPP)(CN)2]ꢀBu4Nꢁ in
CD2Cl2. The meso, a-Py and b-Py carbons of [Fe(p-CNÁ
TPP)(CN)2]ꢀBu4Nꢁ are signified as k, I and ^, respectively, while
those of [Fe(p-OCH3 Á
TPP)(CN)2]ꢀBu4Nꢁ are given by m, j and m.
/
/
/
/
21.8 and ꢁ7.5 ppm were obtained for the meso and a-
/
pyrrole carbons, respectively.
Fig. 3. Plots of the carbon isotropic shifts of [Fe(p-XÁ
/
TPP)(CN)2]ꢀBu4Nꢁ against Hammett sp values in CD2Cl2 and
CD3OD at 25 8C: (a) meso and (b) a-Py carbons. The symbols, k
and I, are isotropic shifts obtained in CD2Cl2 and CD3OD solutions,
respectively. (a) meso and (b) a-Py carbons.
3.1.2. CD3OD solution
Table 2 lists the 13C NMR chemical shifts of a series
of [Fe(p-XÁ
TPP)(CN)2]ꢀBu4Nꢁ taken in CD3OD at
/
3.2. 1H NMR Spectra of [Fe(p-XÁ
/
TPP)(CN)2]ꢀ
25 8C. The change in chemical shift depending on the p-
substituents was much larger in CD3OD than that in
CD2Cl2 solution. For example, the meso-signal moved
Bu4Nꢁ and [Co(p-XÁTPP)(CN)2]ꢀBu4Nꢁ
/
from d 136.5 to 203.9 ppm on going from [Fe(p-CNÁ
/
3.2.1. CD2Cl2 solution
Table 3 shows the 1H NMR chemical shifts of a series
of [Fe(p-XÁ
TPP)(CN)2]ꢀBu4Nꢁ taken in CD2Cl2 at 25
and ꢀ50 8C. The pyrrole signal moved from d ꢀ17.62
to ꢀ16.09 ppm at 25 8C, when CN groups were
replaced by OCH3 groups. Although the change in
chemical shifts of the pyrrole signals was rather small,
the magnitude of downfield shift followed the order
given in Eq. (1). The chemical shifts of the correspond-
TPP)(CN)2]ꢀBu4Nꢁ to [Fe(p-OCH3Á
/
TPP)(CN)2]ꢀ
Bu4Nꢁ. The downfield shift of the meso signal was
the largest in the methoxy complex and decreased in the
order given in Eq. (1). The plots of isotropic shifts
(25 8C) of the meso and a-pyrrole carbons against
Hammett sp values in CD3OD solution are demon-
strated in Fig. 3(a) and (b), respectively. Good linear
/
/
/
/
lines with the slopes of ꢀ
/
69.5 and ꢁ22.6 ppm were
/
obtained for the meso and a-pyrrole carbons, respec-
tively. Although much larger change in chemical shift
ing
signals
of
diamagnetic
[Co(p-XÁ
/
TPP)(CN)2]ꢀBu4Nꢁ taken at 25 8C are also listed in
Table 3 as italic letters. Fig. 4 shows the Curie plots of
was expected for each carbon atom at ꢀ50 8C, the low
/
solubility of the samples hampered the 13C NMR
measurements.
the pyrrole signals in [Fe(p-CNÁ
and [Fe(p-OCH3Á
/
TPP)(CN)2]ꢀBu4Nꢁ
/
TPP)(CN)2]ꢀBu4Nꢁ. Good linear

A. Ikezaki et al. / Inorganica Chimica Acta 335 (2002) 91Á
/
99
95
Table 2
13C NMR chemcial shifts of [Fe(p-XÁ
/
TPP)(CN)2]ꢀBu4Nꢁ in CD3OD at 25 8C
X
meso
a-Py
Â21
b-Py
ipso
o
m
p
p-X
56.1
Á
Á/
Á/
/
OCH3
CH3
H
203.9
195.8
183.8
173.2
163.7
152.8
145.8
136.5
88.3
88.6
89.4
90.2
90.8
91.7
92.4
93.3
89.5
203.9
199.5
194.7
191.8
186.9
180.0
177.8
173.8
116.1
131.4
131.3
116.9
129.8
130.6
126.5
133.1
162.8
142.5
131.7
167.0
138.3
132.8
132.8
114.5
22.4
96.6
103.8
102.7
107.9
117.8
119.0
122.5
19.1
26.6
30.7
33.3
36.5
38.5
41.3
Á/F
Á/Cl
Á
Á/
Á/
/
COOCH3
CF3
CN
52.5, 166.4
122.4
117.1
Table 3
1H NMR chemcial shifts of [Fe(p-XÁ
/
TPP)(CN)]ꢀBu4Nꢁ in CD2C12
X
Py
o
m
p
p-X
a
(a) 25 8C
Á
/
OCH3
CH3
H
ꢀ16.09
8.89
4.48
8.08
4.47
8.05
4.55
8.18
4.50
8.14
4.44
8.12
4.54
4.56
8.39
4.51
8.31
5.97
7.24
6.26
7.53
6.36
7.73
6.00
7.40
6.27
7.70
6.89
6.45
8.08
6.46
8.01
3.25
4.06
1.94
2.68
Á/
ꢀ16.12
8.88
Á/
ꢀ16.55
8.88
6.08
7.71
Á/
F
ꢀ16.82
8.85
Á/
Cl
ꢀ16.97
8.88
Á/
Á/
COOCH3
ꢀ17.24
ꢀ17.62
8.85
3.45
CF3
Á/
CN
ꢀ17.62
8.83
Fig. 4. Temperature dependence of the 1H NMR chemical shifts of the
pyrrole protons in [Fe(p-CNÁ
TPP)(CN)2]ꢀBu4Nꢁ (k) and [Fe(p-
OCH3 Á
TPP)(CN)2]ꢀBu4Nꢁ (m) in CD2Cl2.
(b) ꢀ50 8C
/
Á
Á/
Á/
/
OCH3
CH3
H
ꢀ29.77
ꢀ29.98
ꢀ30.44
ꢀ30.76
ꢀ30.89
ꢀ30.85
ꢀ31.44
ꢀ31.63
1.84
1.86
4.49
4.76
4.86
4.51
4.75
5.41
4.95
4.98
2.47
0.90
/
Â1.9
1.80
4.93
Á/F
ing signal taken in CD2Cl2 solution. The pyrrole signals
showed particularly large downfield shifts; the pyrrole
Á/Cl
1.73
1.82
1.85
1.73
Á
Á/
Á/
/
COOCH3
CF3
CN
2.89
signal of [Fe(p-OCH3Á
from ꢀ29.8 ppm in CD2Cl2 to ꢀ
50 8C. The magnitude of the downfield shift again
/
TPP)(CN)2]ꢀBu4Nꢁ shifted
7.0 ppm in CDCl3 at
/
/
ꢀ
/
a
Chemical shifts of the corresponding diamagnetic [Co(p-XÁ
/
TPP)(CN)2]ꢀBu4Nꢁ are written in italic letters.
obeyed the order given in Eq. (1). The Hammett plots of
the pyrrole signals given in Fig. 5 showed a good
104 and ꢀ
/
1.18ꢃ
/
104
linearity with the slope of the line was ꢀ5.0 ppm.
/
lines with the slopes of ꢀ
/
1.21ꢃ
/
ppm K were obtained for these complexes, respectively.
The Curie lines of all the other complexes (not shown)
were located between these two lines. Fig. 5 shows the
isotropic shifts of the pyrrole protons in a series of
3.3. EPR spectra of [Fe(p-XÁ
/
TPP)(CN)2]ꢀBu4Nꢁ
[Fe(p-XÁ
/
TPP)(CN)2]ꢀBu4Nꢁ plotted against Ham-
EPR spectra of
a
series of
[Fe(p-XÁ
/
TPP)(CN)2]ꢀBu4Nꢁ were taken in frozen solutions of
CH2Cl2 and CH3OH at 4.2 K. In CH2Cl2 solution, each
mett sp values. A good linear line with the slope of ꢀ
/
1.7 ppm was obtained.
spectrum commonly showed a strong signal at gꢂ
together with some small signals ranging from gꢂ2.6 to
1.8. In CH3OH solution, a strong signal was observed at
gꢂ2.5 in all the complexes examined. EPR spectra of
[Fe(p-CH3Á
TPP)(CN)2]ꢀBu4Nꢁ taken in these sol-
vents are given in Fig. 6 as typical examples.
/
3.6
/
3.2.2. CD3OD solution
Table 4 lists the 1H NMR chemical shifts of a series of
TPP)(CN)2]ꢀBu4Nꢁ determined in CD3OD
/
[Fe(p-XÁ
at 25 and ꢀ
large downfield shift as compared with the correspond-
/
/
/
50 8C, respectively. Each signal showed a

96
A. Ikezaki et al. / Inorganica Chimica Acta 335 (2002) 91Á99
/
Fig. 5. Plots of the isotropic shifts of the pyrrole-H of [Fe(p-XÁ
/
Fig. 6. EPR spectra of [Fe(p-CH3 Á
TPP)(CN)2]ꢀBu4Nꢁ taken in
frozen solutions of (a) CH2Cl2 and (b) CH3OH at 4.2 K.
/
TPP)(CN)2]ꢀBu4Nꢁ against Hammett sp values in CD2Cl2 and
CD3OD at 25 8C. The symbols, k and I, are isotropic shifts
obtained in CD2Cl2 and CD3OD solutions, respectively.
Table 4
1H NMR chemcial shifts of [Fe(p-XÁTPP)(CN)2]ꢀBu4Nꢁ in CD3OD
[3,11,19,20,32]. In general, the meso-carbon signal
moves downfield as the (dxz, dyz)4(dxy)1 contribution
increases [3,11]. This is because the spin densities at the
meso-carbon atoms greatly increase as the (dxz,
dyz)4(dxy)1 contribution increases. As Fig. 2 shows, the
largest change in chemical shifts has been observed in
the meso carbon when the p-CN groups are replaced by
the p-OCH3 groups. In the previous paper, we have
pointed out that the complexes with the (dxy)2(dxz, dyz)3
ground state have negative diso values for meso carbons,
while those with the (dxz, dyz)4(dxy)1 ground state have
positive ones. Thus, we have determined the isotropic
shift (diso) by the method described in the Result section.
The isotropic shifts of the meso carbon obtained in
CD2Cl2 solution at 25 8C are plotted against the
Hammett sp values, which yields a good linear line
/
X
Py
o
m
p
p-X
(a) 25 8C
Á
Á/
Á/
Á/
Á/
Á/
Á/
Á/
/
OCH3
CH3
H
ꢀ6.31
ꢀ6.78
ꢀ7.69
ꢀ8.32
ꢀ8.98
ꢀ9.81
ꢀ10.46
ꢀ11.11
5.47
5.46
5.61
5.65
5.57
5.68
5.73
5.66
7.80
8.06
8.04
7.62
7.82
8.36
7.86
7.80
3.87
3.20
6.44
F
Cl
COOCH3
CF3
CN
3.80
(b) ꢀ50 8C
Á
Á/
Á/
Á/
Á/
Á/
Á/
Á/
/
OCH3
CH3
H
ꢀ7.03
ꢀ7.98
4.55
4.56
4.71
4.76
4.65
4.68
4.68
4.49
8.59
8.81
8.60
8.03
8.18
8.58
7.94
7.76
4.01
3.90
ꢀ10.02
ꢀ11.21
ꢀ12.42
ꢀ14.01
ꢀ15.44
ꢀ16.73
5.97
F
Cl
with the slope of ꢀ21.8 ppm as shown in Fig. 3(a). The
/
COOCH3
CF3
CN
3.74
negative slope indicates that the meso-carbon moves
downfield as the electron-donating ability of the p-
substituent increases. The result indicates that the
electron-donating substituent stabilizes the (dxz,
dyz)4(dxy)1 state while the electron-withdrawing substi-
tuent stabilizes the (dxy)2(dxz, dyz)3 state. A good
linearity suggests that the electron configuration of the
4. Discussion
4.1. Effects of para-substituents on the electron
configuration
iron(III) ions in [Fe(p-XÁ
TPP)(CN)2]ꢀBu4Nꢁ is deter-
/
mined by the single factor, that is, the electronic effect of
the p-substituents. Since the diso values of all the
complexes examined are in the negative region, these
complexes adopt the ground state with the (dxy)2(dxz,
dyz)3 electron configuration in CD2Cl2 solution.
4.1.1. 13C NMR spectroscopy
We have established that the meso-carbon chemical
shift is a fairly good probe to determine the electronic
ground state of iron(III) porphyrin complexes

A. Ikezaki et al. / Inorganica Chimica Acta 335 (2002) 91Á
/
99
97
Electronic effects of substituents are also examined in
CD3OD solution. Methanol is known to involve in the
hydrogen bonding with the coordinated cyanide ligand
in low spin dicyano(porphyrinato)ferrate(III) com-
plexes. The hydrogen bonding weakens the s-donating
ability and strengthens the p-accepting ability of the
cyanide [21,22]. Thus, the energy levels of the dp orbitals
decrease, and consequently the contribution of the (dxz,
the (dxz, dyz)4(dxy)1 state increases [3,11]. In an extreme
case where the low spin iron(III) is in the nearly pure
1
(dxz, dyz)4(dxy
)
state as in the case of [Fe(TiPrP)(t-
BuNC)2]ClO4, the pyrrole signal is observed at fairly
downfield, 12.0 ppm, at 25 8C.
The large upfield shifts of the pyrrole signals together
with the similarity of the chemical shifts as shown in
Table 3 and Fig. 4 indicate that the (dxz, dyz)4(dxy
1
)
dyz)4(dxy)1 state increases. In fact, several examples have
characters of these complexes are rather small and that
they are not much different depending upon the para-
substituents in CD2Cl2 solution. Rather small change in
pyrrole chemical shifts is also reported in a series of
bis(1-methylimidazole) complexes of phenyl para-sub-
stituted porphyrin complexes[33]. Fig. 5 shows the
correlation between the Hammett sp values and the
isotropic shifts of the pyrrole protons obtained in
CD2Cl2 and CD3OD solutions at 25 8C. Good linear
lines are obtained in both solutions, though the slopes
1
been reported on the increase in the (dxz, dyz)4(dxy
)
contribution by the addition of methanol [15,20]. The
plots of the meso-carbon isotropic shifts against Ham-
mett sp values have yielded a good linear line as shown
in Fig. 3(a). The slope of ꢀ
/
69.5 ppm is more than three
times as much as that in CD2Cl2 solution. This means
1
that the contribution of the (dxz, dyz)4(dxy
)
state
increases steeply in CD3OD solution as the electron-
donating ability of substituents increases. Furthermore,
the isotropic shifts are positive, 19 Â
complexes examined. These results strongly indicate that
TPP)(CN)2]ꢀBu4Nꢁ adopt the
(dxz, dyz)4(dxy)1 ground state in CD3OD solution.
The isotropic shifts of the a-Py carbons obtained in
CD2Cl2 and CD3OD at 25 8C are also plotted against
the Hammett sp values and they are shown in Fig. 3(b).
Although the Hammett plots have yielded good linear
lines in both CD2Cl2 and CD3OD, the slopes are rather
/
85 ppm, in all the
are very much different; ꢀ
5.0 ppm in CD3OD. The larger slope of the Curie line in
CD3OD solution as compared with that in CD2Cl2
/
1.7 ppm in CD2Cl2 versus ꢀ
/
a series of [Fe(p-XÁ
/
solution again suggests that the contribution of the (dxz,
)
1
dyz)4(dxy
CD2Cl2 solution as the electron-donating ability of
state increases steeply in CD3OD than in
1
substituents increases. Thus, the H NMR results are
totally consistent with the conclusion obtained by the
13C NMR results.
small, ꢁ
/
7.5 (CD2Cl2) and ꢁ22.6 (CD3OD) ppm,
/
compared with those of the meso-carbon. Thus, the
isotropic shifts of the a-Py carbons are less sensitive to
the p-substituents than those of the meso-carbons. As
the contribution of the (dxy)2(dxz, dyz)3 state increases,
the spin density at the meso-carbon decreases and those
at the a-Py carbon increases. Thus, the positive slopes in
Hammett plots indicate that the electron-withdrawing
substituents stabilize the (dxy)2(dxz, dyz)3 state.
4.1.3. EPR spectroscopy
As mentioned in Section 3, the EPR spectra of these
complexes are quite simple. At 4.2 K in CH2Cl2
solution, they commonly show the large gmax type
signals at gꢂ3.6. In contrast, the axial type signals
/
are observed at gꢂ2.6 in CH3OH solution. It is well
/
established that, while the complexes with the (dxy)2(dxz,
dyz)3 ground state exhibit either the large gmax type or
4.1.2. 1H NMR spectroscopy
the rhombic type spectra, those with the (dxz, dyz)4(dxy
)
1
The electron configuration can also be determined by
the 1H NMR chemical shifts and the temperature
dependence of the pyrrole signals. In the complexes
with the (dxy)2(dxz, dyz)3 ground state, the unpaired
electron is transferred to the b-pyrrole carbon atoms via
ground state show the axial type spectra. The results
indicate that all the complexes examined are in the
(dxy)2(dxz, dyz)3 state in CH2Cl2 solution while they are
in the (dxz, dyz)4(dxy)1 state in CH3OH solution. Thus,
the EPR results are consistent with the 13C and 1H
NMR results mentioned above. It should be noted that
the g values are almost the same both in CH2Cl2 and
CH3OH solutions regardless of the change in p-sub-
stituents. In the previous paper, we have reported that
the chemical shifts of the meso carbons in
[Fe(TiPrP)(HIm)2]ꢁ and [Fe(TiPrP)(4-CNPy)2]ꢁ are
very much different, 332 and 918 ppm, respectively, at
the 3egÁdp orbital interactions. Thus, the proton atoms
/
directly bonded to these carbons show significantly
upfield shifted signals as compared with those of the
corresponding diamagnetic complexes. The metal cen-
tered dipolar shift d
MC
dip
also contributes to the upfield
shift of the pyrrole signal [26]. In contrast, the upfield
shift of the pyrrole signal is reduced in the complexes
with the (dxz, dyz)4(dxy)1 ground state because of the
ꢀ50 8C. Yet, the gÞ values of these complexes are
/
decrease in spin densities at the b-pyrrole carbon atoms.
rather small, 2.55 and 2.41 at 4.2 K. Since the difference
in chemical shifts of the meso-carbons in CD3OD
solution is at most 67 ppm at 25 8C, it would not be
surprising if the similar g values are obtained in a series
MC
The sign reversal of the d
dip
values in the (dxz,
dyz)4(dxy
) type complexes also contributes to the
downfield shift of the pyrrole signal. Consequently, the
pyrrole signal moves downfield as the contribution of
1
of [Fe(p-XÁ
TPP)(CN)2]ꢀBu4Nꢁ.
/

98
A. Ikezaki et al. / Inorganica Chimica Acta 335 (2002) 91Á99
/
Scheme 3. Schematic presentation of the change in energy levels in iron d orbitals by the introduction of substituents at the phenyl para-positions.
1
The electron-donating substituents stabilize the (dxz , dyz )4(dxy
)
state as shown in (a), while the electron-withdrawing substituents stabilize the
3
(dxy )2(dxz , dyz
)
state as shown in (b).
4.2. Reasons for the substituent effect on the electron
configuration
the axial ligands have no low lying p* orbitals. Thus, it is
difficult to extract the electronic effects of alkyl groups.
In the present study, we have used a series of [Fe(p-XÁ
/
As mentioned, the electronic configuration of low
spin iron(III) porphyrin complexes is expressed either as
(dxy)2(dxz, dyz)3 or as (dxz, dyz)4(dxy)1. The presence of
the electron donating groups at the p-positions of the
meso-phenyl groups destabilizes the porphyrin a2u
orbital, because the a2u orbital has the largest electron
densities at the meso-carbon atoms. In the low spin
iron(III) porphyrin complexes with the D4h symmetry,
however, the electronic effect of p-substituents should
have little influence on the isotropic shifts of any carbon
and hydrogen atoms, since the a2u orbital is orthogonal
to any of the iron d orbitals. In order for the a2u orbital
to interact, the deformation of the porphyrin core is
necessary. The a2u orbital can interact with the iron dxy
orbital if the porphyrin ring is ruffled [2,3,11,15]. In
other words, the low spin complexes are destabilized by
TPP)(CN)2]ꢀBu4Nꢁ where there is no direct interac-
tion between the p-substituents and the porphyrin ring.
1
Thus, the change in contribution of the (dxz, dyz)4(dxy
)
state observed in the present study should be ascribed
solely to the electronic effect of the p-sbustituents.
In conclusion, we have revealed that the (dxy)2(dxz,
dyz)3 state is stabilized by the electron-withdrawing
substituents at the meso-position while the (dxz,
dyz)4(dxy)1 state is stabilized by the electron-donating
substituents at the same position.
Acknowledgements
This work was supported by the grant-in-aid for
scientific research on priority areas (A) (no. 12020257 to
M.N.) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan, and by the Nukada
Fund for the Advancement of Science (to A.I.) of Toho
University. Thanks are due to the Research Center for
Molecular Materials, the Institute for Molecular Science
(IMS) for low temperature EPR measurements.
the ruffling while they are stabilized by the a2uÁdxy
/
interaction caused by the ruffling. The porphyrin
structure and the electron configuration of iron(III)
could be determined by the two opposing factors. The
introduction of the electron donating substituents de-
stabilizes the a2u orbital, which decrease the energy gap
between the a2u and dxy orbitals and strengthen the a2u
Á
/
dxy interaction by the ruffling of the core. Thus, the
electron-donating substituent deforms the porphyrin
ring in a S4 ruffled fashion and stabilizes the (dxz,
dyz)4(dxy)1 state. In contrast, when the electron-with-
drawing substituents are introduced at the para-posi-
tions, the a2u orbital is stabilized and consequently the
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