Effects of solvents on the electron configurations of the low-spin dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]iron(III) complex: Importance of the C-H···N weak hydrogen bonding

Article abstract of DOI:10.1021/ic0108383

There are two types of electron configurations, (d_xy)²(d_xz, d_yz)³ and (d_xz, d_yz)⁴(d_xy)¹, in low-spin iron(III) porphyrin complexes. To reveal the solvent effects on the ground-state electron configurations, we have examined the ¹³C-and ¹H-NMR spectra of low-spin dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato] ferrate(III) in a variety of solvents, including protic, dipolar aprotic, and nonpolar solvents. On the basis of the NMR study, we have reached the following conclusions: (i) the complex adopts the ground state with the (d_xz, d_yz)⁴(d_xy)¹ electron configuration, the (d_xz, d_yz)⁴(d_xy)¹ ground state, in methanol, because the dorbitals are stabilized due to the O-H···N hydrogen bonding between the coordinated cyanide and methanol; (ii) the complex also exhibits the (d_xz, d_yz)⁴(d_xy)¹ ground state in nonpolar solvents, such as chloroform and dichloromethane, which is ascribed to the stabilization of the dorbitals due to the C-H···N weak hydrogen bonding between the coordinated cyanide and the solvent molecules; (iii) the complex favors the (d_xz, d_yz)⁴(d_xy)¹ ground state in dipolar aprotic solvents, such as DMF, DMSO, and acetone, though the (d_xz, d_yz)⁴(d_xy)¹ character is less than that in chloroform and dichloromethane; (iv) the complex adopts the (d_xy)²(d_xz, d_yz)³ ground state in nonpolar solvents, such as toluene, benzene, and tetrachloromethane, because of the lack of hydrogen bonding in these solvents; (v) acetonitrile behaves like nonpolar solvents, such as toluene, benzene, and tetrachloromethane, though it is classified as a dipolar aprotic solvent. Although the NMR results have been interpreted in terms of the solvent effects on the ordering of the d_xy and d orbitals, they could also be interpreted in terms of the solvent effects on the population ratios of two isomers with different electron configurations. In fact, we have observed the unprecedented EPR spectra at 4.2 K which contain both the axial- and large g_max-type signals in some solvents such as benzene, toluene, and acetonitrile. The observation of the two types of signals has been ascribed to the slow interconversion on the EPR time scale at 4.2 K between the ruffled complex with the (d_xz, d_yz)⁴(d_xy)¹ ground state and, possibly, the planar (or nearly planar) complex with the (d_xy)²(d_xz, d_yz)³ ground state.

Full text of DOI:10.1021/ic0108383

Inorg. Chem. 2002, 41, 2761−2768
Effects of Solvents on the Electron Configurations of the Low-Spin
Dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]iron(III)
Complex: Importance of the C−H‚‚‚N Weak Hydrogen Bonding
,†,‡
Akira Ikezaki† and Mikio Nakamura*
Department of Chemistry, Toho UniVersity School of Medicine, Ota-ku, Tokyo 143-8540, Japan,
and DiVision of Biomolecular Science, Graduate School of Science, Toho UniVersity,
Funabashi 274-8510, Japan
Received August 6, 2001
There are two types of electron configurations, (d )2(d , d )3 and (d , d )4(d )1, in low-spin iron(III) porphyrin
xy
xz
yz
xz
yz
xy
complexes. To reveal the solvent effects on the ground-state electron configurations, we have examined the 13C-
and 1H-NMR spectra of low-spin dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]ferrate(III) in a variety of
solvents, including protic, dipolar aprotic, and nonpolar solvents. On the basis of the NMR study, we have reached
the following conclusions: (i) the complex adopts the ground state with the (d , d )4(d )1 electron configura-
xz
yz
xy
tion, the (d , d )4(d )1 ground state, in methanol, because the dπ orbitals are stabilized due to the O−H‚‚‚N
xz
yz
xy
hydrogen bonding between the coordinated cyanide and methanol; (ii) the complex also exhibits the (d , d )4(d )1
xz yz
xy
ground state in nonpolar solvents, such as chloroform and dichloromethane, which is ascribed to the stabilization
of the dπ orbitals due to the C−H‚‚‚N weak hydrogen bonding between the coordinated cyanide and the solvent
molecules; (iii) the complex favors the (dxz, d )4(d )1 ground state in dipolar aprotic solvents, such as DMF, DMSO,
yz
xy
and acetone, though the (d , d )4(d )1 character is less than that in chloroform and dichloromethane; (iv) the
xz
yz
xy
complex adopts the (d )2(d , d )3 ground state in nonpolar solvents, such as toluene, benzene, and
xy
xz
yz
tetrachloromethane, because of the lack of hydrogen bonding in these solvents; (v) acetonitrile behaves like nonpolar
solvents, such as toluene, benzene, and tetrachloromethane, though it is classified as a dipolar aprotic solvent.
Although the NMR results have been interpreted in terms of the solvent effects on the ordering of the dxy and dπ
orbitals, they could also be interpreted in terms of the solvent effects on the population ratios of two isomers with
different electron configurations. In fact, we have observed the unprecedented EPR spectra at 4.2 K which contain
both the axial- and large gmax-type signals in some solvents such as benzene, toluene, and acetonitrile. The
observation of the two types of signals has been ascribed to the slow interconversion on the EPR time scale at
4.2 K between the ruffled complex with the (d , d )4(d )1 ground state and, possibly, the planar (or nearly planar)
xz yz
xy
complex with the (d )2(d , d )3 ground state.
xy
xz yz
Introduction
porphyrin rings, (iii) the electronic effects at peripheral
substituents, etc.3 Thus, while most of the low-spin com-
plexes show the electronic ground state with the (dxy)2(dxz,
Two types of electron configurations, (dxy)2(dxz, dyz)3 and
(dxz, dyz)4(dxy)1, exist in low-spin iron(III) porphyrin com-
plexes as shown in Scheme 1.1-3 Major factors determining
the ground-state electron configurations are (i) the ligand
field strength of axial ligands, (ii) the deformation mode of
(1) Walker, F. A.; Simonis, U. Proton NMR Spectroscopy of Model
Hemes. In NMR of Paramagnetic Molecules; Berliner, L. J., Reuben,
J., Eds.; Biological Magnetic Resonance; Plenum Press: New York,
1993; Vol. 12, pp 133-274.
(2) Walker, F. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Chapter
36, Vol. 5, pp 81-183.
(3) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Yamaguchi, T.; Nakamura, M. Inorg.
Chem. 2001, 40, 3423-3434.
* Author to whom correspondence should be addressed. E-mail:
mnakamu@med.toho-u.ac.jp.
† Toho University School of Medicine.
‡ Graduate School of Science, Toho University.
10.1021/ic0108383 CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/23/2002
Inorganic Chemistry, Vol. 41, No. 10, 2002 2761

Ikezaki and Nakamura
Scheme 1. Extreme Electron Configurations of Low-Spin Iron(III)
Porphyrin Complexes
Scheme 2. Complex Examined in This Study Together with the
Labeling of Carbon Atoms
in turn, stabilize the iron dπ(dxz, dyz) orbitals through the dπ-
dyz)3 configuration, the low-spin complexes that have axial
ligands with low-lying π* orbitals4-10 and/or a highly S4
ruffled porphyrin core exhibit the less common ground state
with the (dxz, dyz)4(dxy)1 configuration.11-19 Some years ago,
La Mar and co-workers pointed out that the hydrogen
bonding between the coordinated cyanide and solvent
molecules decreases the σ-donating and increases the π-ac-
cepting ability of the cyanide ligand.20,21 As a result, the
energy difference between the dπ(dxz, dyz) and dxy orbitals
decreases, which leads to the decrease in the magnetic
anisotropy of the iron. We have recently found that, in a
series of low-spin dicyano(meso-tetraalkylporphyrinato)-
ferrate(III) complexes, the ground state with the (dxz, dyz)4-
(dxy)1 electron configuration is stabilized by the addition of
methanol into the CD2Cl2 solutions.13 The results have been
explained in terms of the hydrogen bonding between the
coordinated cyanide and methanol; that is, the O-H‚‚‚N
hydrogen bonding stabilizes the cyanide pπ* orbitals which,
pπ* interactions as La Mar and co-workers pointed out.20,21
-
Since the role of protic solvents on the electron configuration
has been clarified, our next purpose is to generalize the sol-
vent effects on the electron configurations by using a wide
range of solvents, including nonpolar solvents. We have
chosen tetrabutylammonium dicyano[meso-tetrakis(2,4,6-tri-
ethylphenyl)porphyrinato]ferrate(III), [Fe(Et-TPP)(CN)2]--
Bu4N+ (Scheme 2), as a model to elucidate the solvent
effects, because our previous study has suggested that the
energy difference between the dπ and dxy orbitals is rather
small.18 This means that the small perturbation to the
coordinated cyanide caused by solvents could change the
energy levels of the dπ and dxy orbitals of low-spin iron(III)
and alter the physicochemical properties of the complex. In
this paper, we report on the ground-state electron configu-
ration of [Fe(Et-TPP)(CN)2]-Bu4N+ by means of the 13C-
1
NMR, H-NMR, and EPR spectra in a variety of solvents,
including protic, dipolar aprotic, and nonpolar solvents. We
also report that the C-H‚‚‚N weak hydrogen bonding
between the coordinated cyanide and nonpolar solvents, such
as chloroform and dichloromethane, plays an important role
in determining the ground-state electron configuration.
(4) Simonneaux, G.; Hindre, F.; Le Plouzennec, M. Inorg. Chem. 1989,
28, 823-825.
(5) Safo, M. K.; Gupta, G. P.; Watson, C. T.; Simonis, U.; Walker, F.
A.; Scheidt, W. R. J. Am. Chem. Soc. 1992, 114, 7066-7075.
(6) 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.
(7) Cheesman, M. R.; Walker, F. A. J. Am. Chem. Soc. 1996, 118, 7373-
7380.
(8) Walker, F. A.; Nasri, H.; Turowska-Tyrk, I.; Mohanrao, K.; Watson,
C. T.; Shokhirev, N. V.; Debrunner, P. G.; Scheidt, W. R. J. Am.
Chem. Soc. 1996, 118, 12109-12118.
Abbreviations: DMAP, 4-(N,N-dimethylamino)pyridine;
DMSO, dimethyl sulfoxide; DMF, N,N-dimethylformamide;
t-BuNC, tert-butyl isocyanide; HIm, imidazole; Bu4N+CN-,
tetrabutylammonium cyanide; Et-TPP, dianion of meso-
tetrakis(2,4,6-triethylphenyl)porphyrin; TMTMP, dianion of
3,8,13,18-tetramesityl-2,7,12,17-tetramethylporphyrin; [Fe-
(Et-TPP)Cl], chloro[meso-tetrakis(2,4,6-triethylphenyl)por-
phyrinato]iron(III); [Fe(Et-TPP)(CN)2]-Bu4N+, tetrabutyl-
ammonium dicyano[meso-tetrakis(2,4,6-triethylphenyl)por-
phyrinato]ferrate(III); [Co(Et-TPP)], [meso-tetrakis(2,4,6-
trietylphenyl)porphyrinato]cobalt(II); [Co(Et-TPP)Cl], chlo-
ro[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]cobalt-
(III);[Co(Et-TPP)(CN)2]-Bu4N+,tetrabutylammoniumdicyano-
[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]cobaltate(III).
(9) Safo, M. K.; Nesset, M. J. M.; Walker, F. A.; Debrunner, P. G.;
Scheidt, W. R. J. Am. Chem. Soc. 1997, 119, 9438-9448.
(10) Pilard, M.-A.; Guillemot, M.; Toupet, L.; Jordanov, J.; Simonneaux,
G. Inorg. Chem. 1997, 36, 6307-6314.
(11) Nakamura, M.; Nakamura, N. Chem. Lett. 1991, 1885-1888.
(12) Nakamura, M.; Ikeue, T.; Neya, S.; Funasaki, N.; Nakamura, N. Inorg.
Chem. 1996, 35, 3731-3732.
(13) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T. J. Am. Chem. Soc.
1997, 119, 6284-6291.
(14) Wołowiec, S.; Latos-Graz˘yn´ski, L.; Mazzanti, M.; Marchon, J.-C.
Inorg. Chem. 1997, 36, 5761-5771.
(15) Wojaczyn´ski, J.; Latos-Graz˘yn´ski, L.; Glowiak, T. Inorg. Chem. 1997,
36, 6299-6306.
(16) Wołowiec, S.; Latos-Graz˘yn´ski, L.; Toronto, D.; Marchon, J.-C. Inorg.
Chem. 1998, 37, 724-732.
(17) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T.; Tajima, K. Inorg.
Chem. 1998, 37, 2405-2414.
Experimental Section
(18) Nakamura, M.; Ikeue, T.; Ikezaki, A.; Ohgo, Y.; Fujii, H. Inorg. Chem.
1999, 38, 3857-3862.
Materials. Bu4N+CN- was purchased from Aldrich. Highly pure
tetrachloromethane (>99.9%) was purchased from Wako. All the
deuterated solvents were purchased from Merck. Deuterated
solvents used in this study were methanol-d4 (CD3OD), dimethyl
sulfoxide-d6 (DMSO-d6), N,N-dimethylformamide-d7 (DMF-d7),
(19) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Nakamura, M.; Fujii, H.; Yokoyama,
M. J. Am. Chem. Soc. 2000, 122, 4068-4076.
(20) Frye, J. S.; La Mar, G. N. J. Am. Chem. Soc., 1975, 97, 3561-3562.
(21) La Mar, G. N.; Gaudio, J. D.; Frye, J. S. Biochim. Biophys. Acta 1977,
498, 422-435.
2762 Inorganic Chemistry, Vol. 41, No. 10, 2002

Effects of SolWents on the Electron Configurations
acetone-d6 ((CD3)2CO), acetonitrile-d3 (CD3CN), chloroform-d
(CDCl3), dichloromethane-d2 (CD2Cl2), benzene-d6 (C6D6), and
toluene-d8 (C6D5CD3). Chloroform-d was washed several times with
concentrated sulfuric acid and then with dilute sodium carbonate
solution and water. It was then dried over potassium carbonate and
distilled in an argon atmosphere shortly before use.22 Other solvents
were dried over an activated molecular sieve (4.0 Å) before use.
equiv) into an NMR sample tube that contained 20 mg of
[Fe(Et-TPP)Cl].
Solutions of [Fe(Et-TPP)(CN)2]-Bu4N+ were similarly prepared
in other deuterated solvents. For the signal assignments 2D HMQC
and HMBC techniques were used as well as the conventional 1D
spectra. 1H- and 13C-NMR spectra were measured at various
temperatures. The temperature range examined in each solvent was
as follows: CD3OD (60 to -100 °C), CDCl3 (60 to -60 °C),
CD2Cl2 (30 to -100 °C), DMSO-d6 (70 to 25 °C), DMF-d7 (70 to
-60 °C), (CD3)2CO (70 to -70 °C), CD3CN (70 to -40 °C), C6D6
(70 to 0 °C), C6D5CD3 (70 to -80 °C), CCl4 (60 to -20 °C).
Titration. A CD2Cl2 solution of [Fe(Et-TPP)(CN)2]-Bu4N+,
which was prepared from [Fe(Et-TPP)Cl] (20 mg) and Bu4N+CN-
Synthesis. (Et-TPP)H2, [Fe(Et-TPP)Cl], [Fe(Et-TPP)OAc], and
[Co(Et-TPP)] were prepared according to the literature methods.23-25
[Fe(Et-TPP)Cl]: 1H-NMR (CD2Cl2, 25 °C, δ) 0.56 (12H, o-CH3),
ca. 3.0 (12H, o-CH3), ca. 3.0 (8H, o-CH2), 5.91 (8H, o-CH2), 2.25
(12H, p-CH3), 4.27 (8H, p-CH2), 14.1 (4H, m-H), 15.7 (4H, m-H),
80.0 (8H, Py-H). [Fe(Et-TPP)OAc]: 1H-NMR (CD2Cl2, 25 °C, δ)
0.36 (12H, o-CH3), 1.94 (12H, o-CH3), 1.94 (12H, p-CH3), 3.68
(8H, p-CH2), 12.69 (4H, m-H), 13.87 (4H, m-H), 78.9 (8H, Py-H),
41.0 (3H, OAc-CH3). The o-methylene signals were too broad to
detect. [Co(Et-TPP)]: UV-vis (CH2Cl2) λmax (log ꢀ) 414 (5.41),
1
(4.0 equiv), was titrated with acetic acid. The H-NMR spectrum
was taken to monitor the change in the pyrrole chemical shift in
each case after the addition. Titration experiments with CD3OD,
CDCl3, phenol, and trifluoroacetic acid were similarly done.
1
531 (4.17); H-NMR (CD2Cl2, 25 °C, δ) 0.82 (24H, o-CH3), 3.92
Results and Discussion
(16H, o-CH2), 2.56 (t, J ) 7.5 Hz, 12H, p-CH3), 4.22 (q, J ) 7.5
Hz, 8H, p-CH2), 9.20 (8H, m-H), 15.0 (8H, Py-H).
General Consideration. As shown in Scheme 1, there
are two types of electron configurations, (dxy)2(dxz, dyz)3 and
(dxz, dyz)4(dxy)1, in low-spin iron(III) porphyrin complexes.1-3
If the dxy orbital is located above the dxz and dyz orbitals (dπ
orbitals) in the energy diagram, then the complex has a
ground state with the (dxz, dyz)4(dxy)1 electron configuration.
In the following discussion, the electronic state whose
ground-state configuration is (dxz, dyz)4(dxy)1 or (dxy)2(dxz, dyz)3
is expressed as a (dxz, dyz)4(dxy)1 or (dxy)2(dxz, dyz)3 ground
state, respectively. Depending on the energy difference
between the dxy and dπ orbitals, the excited-state electron
configurations contribute to the electronic state of the
complex. If the dxy orbital is located far above the dπ orbitals
in the energy diagram, then the complex has a pure (dxz, dyz)4-
(dxy)1 ground state. On the other hand, if the energy difference
is rather small, then the contribution of the excited-state
electron configurations increases. The physicochemical
properties of the complex, therefore, change depending on
the energy difference between the dxy and dπ orbitals.
In principle, a low-spin complex should adopt either the
(dxz, dyz)4(dxy)1 or (dxy)2(dxz, dyz)3 ground state depending on
the d orbital ordering determined by the ligand field strength
of axial ligands, deformation of porphyrin rings, electronic
effects of peripheral substituents, etc. It is possible, however,
that a complex exists as the equilibrium mixture of the two
isomers with the different electronic ground states; one has
the (dxz, dyz)4(dxy)1 ground state, while the other has the (dxy)2-
(dxz, dyz)3 ground state as shown in eq 1.
Synthesis of [Co(Et-TPP)Cl]. The chloroform solution (30 mL)
of [Co(Et-TPP)] (150 mg) was added into an aqueous solution
containing saturated sodium chloride and 1.5 g of FeCl3. Bubbling
of this solution with air at ambient temperature for 10 h gave [Co-
(Et-TPP)Cl]. The reaction mixture was washed three times with a
1 M HCl (30 mL) solution and once with water. The pure material
was obtained by the chromatography on alumina. [Co(Et-TPP)Cl]:
yield, 124 mg (80%); UV-vis (CH2Cl2) λmax (log ꢀ) 411 (4.55),
551 (4.10); 1H-NMR (CD2Cl2 δ) 0.63 (t, J ) 7.5 Hz, 12H, o-CH3),
0.89 (t, J ) 7.5 Hz, 12H, o-CH3), 1.48 (t, J ) 7.6 Hz, 12H, p-CH3),
1.98 (q, J ) 7.5 Hz, 8H, o-CH2), 2.46 (q, J ) 7.5 Hz, 8H, o-CH2),
2.92 (q, J ) 7.6 Hz, 8H, p-CH2), 7.23 (s, 4H, m-H), 7.31 (s, 4H,
m-H), 8.49 (s, 8H, Py-H); 13C-NMR (CD2Cl2 δ) 15.3 (4C, o-CH3),
15.6 (4C, o-CH3), 15.9 (4C, p-CH3), 27.3 (4C, o-CH2), 27.7 (4C,
o-CH2), 29.4 (4C, p-CH2), 125.1 (4C, m), 125.5 (4C, m), 126.4
(4C, meso), 133.5 (4C, â-Py), 138.1 (4C, ipso), 142.4 (4C, o), 143.1
(4C, o), 145.2 (4C, p), 150.9 (8C, R-Py).
Synthesis of [Co(Et-TPP)(CN)2]-Bu4N+. A CDCl3 solution of
Bu4N+CN- (4.0 equiv) was added to [Co(Et-TPP)Cl] and placed
in an NMR sample tube with a microsyringe to give [Co(Et-TPP)-
1
(CN)2]-Bu4N+ quantitatively. H-NMR (CD2Cl2 δ) 0.76 (t, J )
7.5 Hz, 24H, o-CH3), 1.46 (t, J ) 7.5 Hz, 12H, p-CH3), 2.21 (q, J
) 7.5 Hz, 16H, o-CH2), 2.89 (q, J ) 7.5 Hz, 8H, p-CH2), 7.24 (s,
8H, m-H), 8.47 (s, 8H, Py-H); 13C-NMR (CD2Cl2, δ) 15.5 (p-CH3),
15.9 (o-CH3), 27.8 (p-CH2), 29.4 (o-CH2), 115.1 (meso), 124.3 (m),
132.5 (â-Py), 137.8 (ipso), 143.1 (R-Py), 144.0 (p), 145.6 (o).
1
Spectral Measurement. H- and 13C-NMR spectra were mea-
sured on a JEOL LA300 spectrometer operating at 300.4 MHz for
1H. Chemical shifts were referenced to the residual peaks of
1
deuterated solvents. H-NMR chemical shifts in CCl4 were refer-
(dxy)2(dxz, dyz)3 a (dxz, dyz)4(dxy)1
(1)
enced to the residual peak of CDCl3 placed in a capillary. EPR
spectra were recorded on a Bruker E500 spectrometer operating at
X band and equipped with an Oxford helium cryostat.
We have already reported that the 13C-NMR chemical
shifts of meso carbons are a powerful probe for determining
the electronic ground state.3,19 For example, the meso signal
shows a large downfield shift when axially coordinated HIm
is replaced by much weaker 4-CNPy in [Fe(TiPrP)L2]+; the
chemical shifts are 332 and 918 ppm at -50 °C for [Fe-
(TiPrP)(HIm)2]+ and [Fe(TiPrP)(4-CNPy)2]+, respectively.19
The result indicates that the (dxz, dyz)4(dxy)1 character increases
on going from [Fe(TiPrP)(HIm)2]+ to [Fe(TiPrP)(4-CNPy)2]+,
Sample Preparation for NMR Measurements. A CDCl3
solution of [Fe(Et-TPP)(CN)2]-Bu4N+ (35 mM) was obtained by
the addition of a 550 µL CDCl3 solution of Bu4N+CN- (4.0
(22) Riddick, J. A.; Bunger, W. B. Organic SolVent, Techniques of
Chemistry; Wiley-Interscience: New York, 1970; Vol. II.
(23) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828-836.
(24) Nakamura, M.; Tajima, K.; Tada, K.; Ishizu, K.; Nakamura, N. Inorg.
Chim. Acta 1994, 224, 113-124.
(25) Nakamura, M. Bull. Chem. Soc. Jpn. 1995, 68, 197-203.
Inorganic Chemistry, Vol. 41, No. 10, 2002 2763

Ikezaki and Nakamura
Table 1. 13C-NMR Chemical Shifts of [Fe(Et-TPP)(CN)2]-Bu4N+
Determined in Various Solvents at -20 °C
solvent
meso
R-Py
â-Py
ipso
o
m
p
CD3OD
CDCl3
CD2Cl2
DMF-d7
DMSO-d6
275.4 -17.0 76.6
68.8 236.5 130.0 146.5
91.8 202.0 125.6 142.5
97.0 193.2 125.3 143.1
191.4
172.3
134.9
130.8
123.3
100.3
96.1
9.3 76.2
10.4 78.5
18.6 78.0 106.9 178.4 125.0 143.1
16.2 78.5 107.0 176.4 124.5 142.4
20.7 77.2 109.9 174.2 124.4 142.6
20.4 81.1 113.4 165.1 123.4 142.6
25.2 80.3 116.7 164.9 123.9 142.2
23.8 79.6 116.1 164.5 123.7 142.1
25.6 82.2 119.1 158.2 122.6 140.8
a
(CD3)2CO
CD3CN
C6D5CD3
a
C6D6
95.5
79.6
CCl4
a The value is extrapolated from high temperature.
which could be explained by either one or both of the
following two ways on the basis of the discussion given
above. (a) The energy gap between the dxy and dπ orbitals,
∆E ) E(dxy) - E(dπ), increases on going from [Fe(TiPrP)-
(HIm)2]+ to [Fe(TiPrP)(4-CNPy)2]+. (b) The population of
the isomer with the (dxz, dyz)4(dxy)1 ground state is much larger
in [Fe(TiPrP)(4-CNPy)2]+ than in [Fe(TiPrP)(HIm)2]+. In this
case, the chemical shifts of the meso carbon signal should
be given by the weighted average, as shown in eq 2, under
the assumption that the interconversion is very fast on the
13C-NMR time scale.
δobs ) xδ(dxy) + yδ(dπ)
(2)
Here, δobs means the observed chemical shift of the meso
signal, δ(dxy) and δ(dπ) are the chemical shifts of the meso
signals in the isomers with the (dxz, dyz)4(dxy)1 and (dxy)2(dxz,
dyz)3 ground states, respectively, and x and y are the
population ratios. While a single species exists in the case
of (a), two species are present in the case of (b). As
mentioned later, we could actually observe two isomers at
4.2 K in frozen CH2Cl2 solution by EPR spectroscopy; one
has the (dxz, dyz)4(dxy)1 ground state, and the other has the
(dxy)2(dxz, dyz)3 ground state. Even in this case, the isomer
with the (dxz, dyz)4(dxy)1 ground state must have a large energy
gap between the dxy and dπ orbitals in order to explain the
presence of a fairly large downfield shifted meso signal in
[Fe(TiPrP)(4-CNPy)2]+. In other words, even if the isomer
with the (dxz, dyz)4(dxy)1 ground state exists exclusively, the
meso carbon signal does not appear at this downfield region
unless the energy gap between the dxy and dπ orbitals is large.
For this reason, we will discuss the (dxz, dyz)4(dxy)1 character
in terms of the energy gap between the dxy and dπ orbitals
below.
Ground-State Electron Configuration of [Fe(Et-TPP)-
(CN)2]-Bu4N+. (A) 13C-NMR Spectroscopy. We have
established that the electronic ground state of iron(III)
porphyrin complexes particularly affects the meso carbon
chemical shifts.3,18,19,26,27 In general, the more the (dxz, dyz)4-
(dxy)1 ground state is stabilized, the more the spin densities
at the meso carbon increase. Consequently, the meso carbon
signal appears downfield. Table 1 lists the chemical shifts
of the complex obtained in various solvents at -20 °C.
Figure 1 shows the temperature dependence of the meso
carbon shifts, which clearly indicates that the downfield shifts
increase in the order given in eq 3.
Figure 1. Temperature dependence of the 13C-NMR chemical shifts of
the meso carbon of [Fe(Et-TPP)(CN)2]-Bu4N+ (a) in all the solvents exam-
ined, and (b) in nonpolar solvents, such as CCl4, C6D6, and C6D5CD3, as
well as in a dipolar aprotic solvent such as CD3CN. Solvents: CD3OD
(b), CDCl3 (2), CD2Cl2 (3), DMF-d7 ((), DMSO-d6 (4), (CD3)2CO ()),
CD3CN (0), C6D5CD3 (O), C6D6 (×), and CCl4 (9). Chemical shifts of
the meso carbon of [Co(Et-TPP)(CN)2]-Bu4N+ taken in CD2Cl2 solution
(crossed box).
CCl4 < C6D6, C6D5CD3, CD3CN <
(CD3)2CO, DMSO-d6, DMF-d7 <
CD2Cl2 < CDCl3 < CD3OD (3)
Close examination of the data in Table 1 has revealed the
following relationships between the meso carbon chemical
shifts (-20 °C) and the solvents: (i) the meso signal appears
fairly downfield, δ ) 275 ppm, in a protic solvent, such as
methanol; (ii) large differences in chemical shifts are
observed even among the nonpolar solvents; for example,
the chemical shifts in CDCl3 and CD2Cl2 are δ ) 191 and
172 ppm, respectively, as compared with δ ) 80-100 ppm
in C6D5CD3, C6D6, and CCl4; (iii) the meso signals appear
in a quite narrow range of δ ) 129 ( 6 ppm, in dipolar
aprotic solvents, such as DMF-d7, DMSO-d6, and (CD3)2-
CO, which are located between those in the two types of
nonpolar solvents mentioned above; (iv) the meso signal in
2764 Inorganic Chemistry, Vol. 41, No. 10, 2002

Effects of SolWents on the Electron Configurations
Table 2. 1H-NMR Chemical Shifts of [Fe(Et-TPP)(CN)2]-Bu4N+
Determined in Various Solvents at 25 and -20 °C
CD3CN appears rather close to that in C6D5CD3 or C6D6,
ca. 100 ppm, though CD3CN is classified as a dipolar aprotic
solvent.
25 °C
-20 °C
solvent
Py-H
m-H
Py-H
m-H
To obtain valuable information on the electronic ground
state of low-spin iron(III) porphyrin complexes, it is neces-
sary to extract the contact shift (δcon) from the observed
chemical shift (δobs).1,2,19,28-31 The observed shift is repre-
sented as δobs ) δdia + δiso, where δdia and δiso indicate a
diamagnetic and isotropic shift, respectively. Our previous
work has revealed that the signs of the δiso values of meso
carbons are different depending upon the electronic ground
states; the isotropic shifts are positive (downfield shift) in
the complexes with the (dxz, dyz)4(dxy)1 ground state, while
they are negative (upfield shift) in those complexes with the
(dxy)2(dxz, dyz)3 ground state.19 Thus, the electronic ground
state of the complexes could be determined by the signs of
the meso carbon isotropic shifts. In fact, the δiso value of
[Fe(TiPrP)(4-CNPy)2]ClO4, a typical complex with the (dxz,
dyz)4(dxy)1 ground state, was determined to be +584 ppm,19
while that of [Fe(TPP)(1-MeIm)2]Cl, a typical complex with
the (dxy)2(dxz, dyz)3 ground state, was reported to be -73
ppm.32 We have calculated the isotropic shift by using the
meso carbon shift of [Co(Et-TPP)(CN)2]-Bu4N+ in CD2Cl2
solution as δdia; the chemical shift of the meso carbon is
maintained almost constant at 25 °C at 115.5 ( 0.6 ppm in
various solvents ranging from methanol to benzene. Thus,
the linear line drawn from the temperature dependence of
the meso carbon shift of [Co(Et-TPP)(CN)2]-Bu4N+ in
CD2Cl2 solution separates the solvents where the complex
adopts the (dxy)2(dxz, dyz)3 ground state from those where it
has the (dxz, dyz)4(dxy)1 ground state, as shown in Figure 1.
To summarize the effects of solvents on the electronic
ground states from the viewpoint of 13C-NMR spectroscopy,
it might be convenient to classify the solvents into four
types: type A, protic solvents such as CD3OD; type B,
nonpolar solvents such as CDCl3 and CD2Cl2; type C, dipolar
aprotic solvents such as DMF-d7, DMSO-d6, and (CD3)2-
CO; type D, nonpolar solvents such as C6D5CD3, C6D6, and
CCl4. While the complex adopts the (dxz, dyz)4(dxy)1 ground
state in the solvents classified as types A-C, it shows the
(dxy)2(dxz, dyz)3 ground state in the solvents classified as type
D. Figure 1 also indicates that the (dxz, dyz)4(dxy)1 character
is the largest in type A solvents, followed by types B and C.
It should be noted that CD3CN is an exceptional case; the
CD3OD
CDCl3
-6.89
-9.59
8.44
7.54
7.09
6.81
7.24
6.67
6.52
6.49
6.50
6.77
-4.75
-9.97
9.69
8.13
7.74
7.05
7.39a
6.91
6.20
6.63
6.41a
6.58
CD2Cl2
DMF-d7
DMSO-d6
(CD3)2CO
CD3CN
C6D5CD3
C6D6
-12.10
-13.10
-12.50
-13.57
-15.07
-15.23
-15.06
-15.53
-12.28
-15.28
-15.69a
-16.11
-19.76
-18.66
-18.98a
-20.43
CCl4
a The value is extrapolated from high temperature.
complex exhibits the (dxy)2(dxz, dyz)3 ground state in this
solvent, though the solvent is classified as a type C solvent.
1
(B) H-NMR Spectroscopy. Table 2 lists the chemical
shifts of the complex taken in various solvents at 25 and
-20 °C. Figure 2a,b shows the temperature dependence of
the pyrrole and meta proton chemical shifts, respectively.
In the complexes with the (dxy)2(dxz, dyz)3 ground state, the
unpaired electron is transferred to the â-pyrrole carbon atoms
via 3eg(porphyrin)-dπ(iron) interactions. Thus, the protons
directly bonded to these carbons show significantly upfield
shifted signals compared to those of the corresponding
diamagnetic complex. The metal-centered dipolar shift
(δdipMC) also contributes, to some extent, to the upfield shift
of the pyrrole signal. In the complexes with the (dxz, dyz)4-
(dxy)1 ground state, however, the upfield shift of the pyrrole
signal is reduced to a great extent due to the weakening of
the 3eg-dπ interactions. In the complexes with a pure (dxz,
dyz)4(dxy)1 ground state, the pyrrole signal is expected to
appear close to the diamagnetic position (δ ) ca. 8.5)
because of the nearly zero-spin density at the â-pyrrole
MC
carbons.19 Since the δdip term is positive in these com-
plexes, the pyrrole signal could appear even more downfield
than that of the corresponding diamagnetic complex. In fact,
[Fe(TiPrP)(CN)2]-Bu4N+, which has a quite pure (dxz, dyz)4-
(dxy)1 ground state, exhibits the pyrrole signal at δ ) 12.8
ppm at -25 °C.13 The electronic ground state can also be
determined from the meta proton chemical shifts in the case
of meso-tetraarylporphyrin complexes. While the complexes
with the (dxz, dyz)4(dxy)1 ground state show the downfield
shifted meta signal,18 those with the (dxy)2(dxz, dyz)3 ground
state exhibit the upfield shifted meta signal. Figure 3 shows
the correlation of the chemical shifts between the pyrrole
and meta signals in various solvents. Good linearity suggests
that the chemical shifts of these signals are determined
mainly by the electronic ground state of the iron(III) ion.
The following are some characteristics extracted from the
data in Table 2 and Figure 2 on the relationship between
the chemical shifts of the pyrrole signals at -20 °C and the
solvents examined: (i) the pyrrole signal is observed at a
relatively downfield region (δ ) -4.5 ppm) in methanol
(type A) and moves further downfield as the temperature is
lowered; (ii) the pyrrole signals appear at δ ) -10 to -12
ppm in nonpolar solvents such as CDCl3 and CD2Cl2 (type
(26) Ikezaki, A.; Nakamura, M. Chem. Lett. 2000, 994-995.
(27) Ikeue, T.; Ohgo, Y.; Yamaguchi, T.; Takahashi, M.; Takeda, M.;
Nakamura, M. Angew. Chem., Int. Ed. 2001, 40, 2617-2620.
(28) Goff, H. In Iron Porphyrin, Part I; Lever, A. B. P., Gray, H. B., Eds.;
Physical Bioinorganic Chemistry Series 1; Addison-Wesley: Reading,
MA, 1983; pp 237-281.
(29) 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/Cummings: Menlo Park,
CA, 1986; pp 165-229.
(30) Mispelter, J.; Momenteau, M.; Lhoste, J.-M. Heteronuclear Magnetic
Resonance. In NMR of Paramagnetic Molecules; Berliner, L. J.,
Reuben, J., Eds.; Biological Magnetic Resonance; Plenum Press: New
York, 1993; Vol. 12, pp 299-355.
(31) Bertini, I.; Luchinat, C. In NMR of Paramagnetic Substances; Lever,
A. B. P., Ed.; Coordination Chemistry Reviews 150; Elsevier:
Amsterdam, 1996; pp 29-75.
(32) Goff, H. M. J. Am. Chem. Soc. 1981, 103, 3714-3722.
Inorganic Chemistry, Vol. 41, No. 10, 2002 2765

Ikezaki and Nakamura
D) (thus, the difference in chemical shifts between type B
and type D solvents reaches as much as 10 ppm); (iii) the
pyrrole signals are observed at δ ) -15 to -16 ppm in
type C solvents, which are located between those signals
observed in type B and type D solvents. In contrast, the
pyrrole signal in CD3CN appears at δ ) -19.8 ppm, which
is again different from the chemical shifts in other type C
solvents and close to the values in the type D solvents.
The spectral characteristics given above indicate that the
complex adopts the (dxz, dyz)4(dxy)1 ground state in the solvents
classified as types A-C. Figure 2 also indicates that the (dxz,
dyz)4(dxy)1 character is the largest in type A solvents, followed
by type B and type C. In contrast, in type D solvents, the
complex has the (dxy)2(dxz, dyz)3 ground state. Acetonitrile has
exhibited unique character; the complex shows the (dxy)2-
(dxz, dyz)3 ground state in this solvent, though it is classified
as a dipolar aprotic solvent (type C). Thus, the results
obtained by the 1H-NMR spectroscopy are totally consistent
with those obtained by the 13C-NMR spectroscopy.
(C) EPR Spectroscopy. Low-spin iron(III) porphyrin
complexes show different EPR spectra depending on the
electronic ground state.2,33 In the case of dicyano complexes,
large gmax-type spectra are observed if the complexes adopt
the (dxy)2(dxz, dyz)3 ground state. In contrast, axial-type spectra
are observed if they form the (dxz, dyz)4(dxy)1 ground state.
Figure 4a-f shows the EPR spectra of the complex taken in
various frozen solvents at 4.2 K. While the complex shows
only the axial-type signals centered at g ) 2.5 in types A-C
solvents, it exhibits both the axial- (g ) 2.5) and large gmax
-
type (g ) 3.7) signals in type D solvents. Thus, the EPR
results indicate that, while the complex adopts the (dxz, dyz)4-
(dxy)1 ground state in types A-C solvents, it exists as a
mixture of two isomers with different ground states in type
D solvents. Again, acetonitrile behaves like type D solvents,
though it is classified as a type C solvent.
Figure 2. Temperature dependence of the 1H-NMR chemical shifts of
[Fe(Et-TPP)(CN)2]-Bu4N+ in various solvents: (a) pyrrole-H and (b) meta-
H. Solvents measured are CD3OD (b), CDCl3 (2), CD2Cl2 (3), DMF-d7
((), DMSO-d6 (4), (CD3)2CO ()), CD3CN (0), C6D5CD3 (O), C6D6 (×),
and CCl4 (9).
Equilibrium between the Complexes with the (dxy)2(dxz,
dyz)3 and (dxz, dyz)4(dxy)1 Ground States. In the previous
paper,wehavereportedthat[Fe(TMTMP)(DMAP)2]+ClO4-shows
two kinds of EPR signals, the large gmax-type and rhombic-
type signals, in frozen dichloromethane solution at 4.2 K.34
The result has been explained in terms of the presence of
two conformers in which only the orientation of the planar
axial ligand is different; one conformer has two axial ligands
oriented in a parallel fashion, while the other has two
perpendicularly oriented ligands. Thus, the observation of
the two types of EPR spectra has been ascribed to the slow
rotation of the axial ligand on the EPR time scale at 4.2 K.
In the present study, we have observed the spectra
containing both the large gmax-type and axial-type signals in
type D solvents as well as in acetonitrile, as shown in Figure
4. The result strongly indicates that two isomers with the
different ground states actually exist in frozen solution at
Figure 3. Correlation of the chemical shifts between the pyrrole and meta
protons in various solvents at -25 °C (correlation coefficient, 0.956).
(33) Palmer, G. Electron Paramagnetic Resonance of Hemoproteins. In Iron
Porphyrins, Part II; Lever, A. B. P., Gray, H. B., Eds.; Physical
Bioinorganic Chemistry Series 2; Addison-Wesley: Reading, MA,
1983; pp 43-88.
(34) Ikeue, T.; Yamaguchi, T.; Ohgo, Y.; Nakamura, M. Chem. Lett. 2000,
342-343.
B), while they appear more upfield at δ ) -19 to -20 ppm
in nonpolar solvents such as C6D5CD3, C6D6, and CCl4 (type
2766 Inorganic Chemistry, Vol. 41, No. 10, 2002

Effects of SolWents on the Electron Configurations
Figure 5. Change in 1H-NMR pyrrole shift against the amount of the
acid added. Each acid was added to a solution containing [Fe(Et-TPP)-
(CN)2]-Bu4N+ and CN- in 1:2 molar ratios at 25 °C: CH3COOH (b),
C6H5OH (4), CD3OD (O), and CDCl3 (0).
Scheme 4. (a) Structural Formula of Alkyl or Aryl Isocyanide and
(b) Resonance Presentation of the Hydrogen Bonding between Proton
Donor and Cyanidea
Figure 4. EPR spectra of [Fe(Et-TPP)(CN)2]-Bu4N+ taken in frozen
solutions at 4.2 K: (a) CD3OD, (b) CDCl3, (c) CD2Cl2, (d) DMF-d7, (e)
DMSO-d6, (f) (CD3)2CO, (g) CD3CN, (h) C6D5CD3, and (i) C6D6.
Scheme 3. Interconversion between the Ruffled Isomer with the (dxz,
dyz)4(dxy)1 Ground State and the Planar (or Nearly Planar) Isomer with
the (dxy)2(dxz, dyz)3 Ground State
aNote that the resonance structure (II) is quite similar to the isocyanide.
After the addition of more than 40 equiv of the acid, [Fe-
(Et-TPP)(CN)2]-Bu4N+ is completely converted to [Fe-
(Et-TPP)Cl] and [Fe(Et-TPP)OAc], as is revealed by the 1H-
NMR spectra.
Similar changes in the pyrrole shift are observed by the
addition of much weaker acids such as phenol and methanol,
though the downfield shift is not as large as in the case of
acetic acid. These results indicate that the hydrogen bonding
to the coordinated cyanide weakens the σ-donating and
strengthens the π-accepting ability of the coordinated cyanide
ligand as La Mar and co-workers pointed out. Thus, the (dxz,
dyz)4(dxy)1 ground state is stabilized in the presence of phenol
and methanol.13,20,21 Scheme 4 shows the resonance structures
of the hydrogen-bonded cyanide ligand. In the presence of
a strong acid, the resonance structure (II) becomes the major
contributor. Since II is supposed to be quite similar to alkyl
or aryl isocyanide (R-NC) in its electronic structure, the
(dxz, dyz)4(dxy)1 ground state is greatly stabilized, as in the
case of bis(R-NC) complexes; all the bis(R-NC) complexes
reported previously show a very pure (dxz, dyz)4(dxy)1 ground
state due to the weak σ-donating and fairly strong π-accept-
ing ability.3,4,8,35,36
4.2 K, and that the mutual exchange between these two
isomers is slow on the EPR time scale at this temperature.
Since the complexes with the (dxz, dyz)4(dxy)1 ground state
always exhibit the ruffled porphyrin ring,2 the observation
of the two isomers should be ascribed to the high barrier to
interconversion between the ruffled complex with the (dxz,
dyz)4(dxy)1 ground state and, possibly, the planar (or nearly
planar) complex with the (dxy)2(dxz, dyz)3 ground state, as
shown in Scheme 3.
Hydrogen Bonding between Coordinated Cyanide and
Acids. As shown in Figure 5, the pyrrole proton signal moves
downfield if compounds such as acetic acid, phenol, metha-
nol, and chloroform are added to a [Fe(Et-TPP)(CN)2]Bu4N+-
solution. The result suggests that the (dxz, dyz)4(dxy)1 character
increases as the acid is added due to the stabilization of the
dπ orbitals caused by the hydrogen bonding between the acid
and the coordinated cyanide. In the case of acetic acid, the
signal intensity of [Fe(Et-TPP)(CN)2]-Bu4N+ decreases when
more than 5.0 equiv of the acid is added. Instead, the signals
ascribed to [Fe(Et-TPP)Cl] and [Fe(Et-TPP)OAc] increase.
Weak Hydrogen Bonding between Coordinated Cya-
nide and CDCl3 or CD2Cl2. We have mentioned that the
Inorganic Chemistry, Vol. 41, No. 10, 2002 2767

Ikezaki and Nakamura
chemical shifts of the pyrrole proton and meso carbon signals
are greatly affected by the nature of the solvents; the signals
appear more downfield in CD3OD than in C6D6 and CCl4.
Surprisingly, the chemical shifts are very much different even
among the nonpolar solvents. That is, while the meso carbon
signals appear at 170-190 ppm at -20 °C in type B solvents,
such as CD2Cl2 and CDCl3, they appear at 80-100 ppm in
type D solvents, such as C6D5CD3, C6D6, and CCl4.
Furthermore, the temperature dependence is very much
different between these two types of nonpolar solvents as
shown in Figure 1a,b. Thus, the chemical shift in CD2Cl2
differs from that in C6D5CD3 by as much as 170 ppm at
-70 °C! The difference in chemical shifts is also observed
in the pyrrole protons; the pyrrole signal appears at -7.5
ppm at -70 °C in CD2Cl2 in contrast to the signal at -23.2
ppm in C6D5CD3.
In the previous section, we pointed out that the addition
of acids, such as acetic acid, phenol, and even methanol,
into a CD2Cl2 solution of [Fe(Et-TPP)(CN)2]-Bu4N+ stabi-
lizes the (dxz, dyz)4(dxy)1 ground state via the O-H‚‚‚N
hydrogen bonding with the coordinated cyanide ligand.13,20,21
As shown in Figure 5, the addition of CDCl3 to a CD2Cl2
solution of the complex also induces a downfield shift. The
result suggests that the C-H‚‚‚N weak hydrogen bonding
exists even between the coordinated cyanide and chloroform
molecules.37 The smaller amount of the downfield shift in
chloroform as compared with methanol indicates that chlo-
roform is much weaker than methanol in proton donor ability.
Many examples have been reported on the C-H‚‚‚X type
hydrogen bonding in the crystal.37-39 Chloroform is known
to be a much stronger proton donor than other compounds
having C-H bonds on the basis of the C-H‚‚‚X distances
in the crystal.40-43 Formation of the C-H‚‚‚X hydrogen
bonding is also reported for dichloromethane in the crystal.43
The presence of the C-H‚‚‚X hydrogen bonding has been
reported even in solution between chloroform and pyridine
or between chloroform and DMSO.44 Thus, it must be a
reasonable assumption to ascribe the stabilization of the
(dxz, dyz)4(dxy)1 ground state in CDCl3 and CD2Cl2 to the
C-D‚‚‚N weak hydrogen bonding between the coordinated
cyanide and solvent molecules.
Effects of Dipolar Aprotic Solvents on the Electronic
Ground State. In dipolar aprotic solvents (type C solvents),
the chemical shifts of the meso carbon and pyrrole proton
signals are located between those obtained in the two types
of nonpolar solvents, types B and D. On the basis of the
NMR results, it is concluded that the energy gap between
the dxy and dπ orbitals in type C solvents is less than that in
type B solvents, which should be ascribed to the difference
in proton donor ability among these solvents. As mentioned
in a previous section (General Consideration), it should be
noted here that the results could be explained differently;
that is, the populations of the isomer with the (dxz, dyz)4(dxy)1
ground state are much larger in type B solvents as compared
with those in type C solvents due to the hydrogen bonding.
The proton donor ability is estimated by the C-H‚‚‚X
(X ) N, O, or Cl-) bond distances in the crystal.40-43 In
fact, the X-ray crystallographic studies have revealed that
the C-H‚‚‚X bond distances of acetonitrile, DMSO, and
acetone are longer than those of chloroform and dichlo-
romethane, supporting the hypothesis mentioned above.43
Among the type C solvents examined in this study, CD3CN
differs from the others and resembles the type D solvents.
In fact, the chemical shifts of the meso carbons are 96.5 and
99.0 ppm in CD3CN and C6D5CD3, respectively, at -40 °C.
Similarly, the EPR spectrum in frozen CD3CN shows both
the axial- and large gmax-type signals, as in the case of C6D6
solutions. Thus, CD3CN behaves like type D solvents, which
could be ascribed to the weak proton donor ability.
Acknowledgment. This work was supported by a Grant
in Aid (12020257) for Scientific Research on Priority Areas
(A) (to M.N.) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan, and by the Nukada
Fund for the Advancement of Science of Toho University
(to A.I.). Thanks are due to the Research Center for
Molecular Materials, the Institute for Molecular Science
(IMS).
(35) Simonneaux, G.; Schu¨nemann, V.; Morice, C.; Carel, L.; Toupet, L.;
Winkler, H.; Trautwein, A. X.; Walker, F. A. J. Am. Chem. Soc. 2000,
122, 4366-4377.
(36) Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001,
1587-1592.
Supporting Information Available: 1H-NMR and 13C-NMR
spectra of [Fe(Et-TPP)(CN)2]Bu4N in CD2Cl2 solution. This material
is available free of charge via the Internet at http://pubs.acs.org.
(37) Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond. In Structural
Chemistry and Biology; Oxford University Press: New York, 1999.
(38) Sutor, D. J. J. Chem. Soc. 1963, 1105-1110.
(39) Steiner, T. Chem. Commun. 1997, 727-734.
(40) Desiraju, G. R. J. Chem. Soc., Chem. Commun. 1989, 179-180.
(41) Pedireddi, V. R.; Desiraju, G. R. J. Chem. Soc., Chem. Commun. 1992,
988-990.
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(42) Steiner, T. J. Chem. Soc., Chem. Commun. 1994, 2341-2342.
(43) Steiner, T. New J. Chem. 1998, 1099-1103
(44) Allerhand, A.; Schleyer, P. v. R. J. Am. Chem. Soc. 1963, 85, 1715-
1723.
2768 Inorganic Chemistry, Vol. 41, No. 10, 2002