Metal-porphyrin orbital interactions in highly saddled low-spin iron(iii) porphyrin complexes

Article abstract of DOI:10.1021/ic700827w

Substituent effects of the meso-aryl (Ar) groups on the ¹H and ¹³C NMR chemical shifts in a series of low-spin highly saddled iron(III) octaethyltetraarylporphyrinates, [Fe(OETArP)L₂] ⁺, where axial ligands (L) are imidazole (Hlm) and tert-butylisocyanide (^tBuNC), have been examined to reveal the nature of the interactions between metal and porphyrin orbitale. As for the bis(Hlm) complexes, the crystal and molecular structures have been determined by X-ray crystallography. These complexes have shown a nearly pure saddled structure in the crystal, which is further confirmed by the normal-coordinate structural decomposition method. The substituent effects on the CH₂ proton as well as meso and CH₂ carbon shifts are fairly small in the bis(Hlm) complexes. Since these complexes adopt the (d_xy)²(d _xz, d_yz)³ ground state as revealed by the electron paramagnetic resonance (EPR) spectra, the unpaired electron in one of the metal d_π orbitals is delocalized to the porphyrin ring by the interactions with the porphyrin 3e_g-like orbitale. A fairly small substituent effect is understandable because the 3e_g-like orbitale have zero coefficients at the meso-carbon atoms. In contrast, a sizable substituent effect is observed when the axial Hlm is replaced by ^tBuNC. The Hammett plots exhibit a large negative slope, -220 ppm, for the meso-carbon signals as compared with the corresponding value, +5.4 ppm, in the bis(Hlm) complexes. Since the bis(^tBuNC) complexes adopt the (d_xz, d_yz)⁴(d_xy)¹ ground state as revealed by the EPR spectra, the result strongly indicates that the half-filled d_xy orbital interacts with the specific porphyrin orbitale that have large coefficients on the meso-carbon atoms. Thus, we have concluded that the major metal-porphyrin orbital interaction in low-spin saddle-shaped complexes with the (d_xz, d_yz) ⁴(dxy)¹ ground state should take place between the d _xy and a_2u-like orbital rather than between the d _xy and a_1u-like orbital, though the latter interaction is symmetry-allowed in saddled D_2d complexes. Fairly weak spin delocalization to the meso-carbon atoms in the complexes with electron-withdrawing groups is then ascribed to the decrease in spin population in the d_xy orbital due to a smaller energy gap between the d _xy and d_π orbitale. In fact, the energy levels of the d_xy and d_π orbitals are completely reversed in the complex carrying a strongly electron-withdrawing substituent, the 3,5-bis(trifluoromethyl)phenyl group, which results in the formation of the low-spin complex with an unprecedented (d_xy)²(d _xz, d_yz)³ ground state despite the coordination of ^tBuNC.

Full text of DOI:10.1021/ic700827w

Inorg. Chem. 2007, 46, 8193−8207
Metal
−Porphyrin Orbital Interactions in Highly Saddled Low-Spin Iron(III)
Porphyrin Complexes
Yoshiki Ohgo,†,‡ Akito Hoshino,§ Tomoya Okamura,† Hidehiro Uekusa,| Daisuke Hashizume,¶
Akira Ikezaki,† and Mikio Nakamura*,†,‡,§
Department of Chemistry, School of Medicine, Toho UniVersity, Ota-ku, Tokyo 143-8540, Japan,
Research Center for Materials with Integrated Properties, Toho UniVersity, Funabashi 274-8510,
Japan, DiVision of Chemistry, Graduate School of Science, Toho UniVersity, Funabashi 274-8510,
Japan, Graduate School of Science and Engineering, Tokyo Institute of Technology,
Tokyo 152-8551, Japan, and Molecular Characterization Team, RIKEN, Wako, 351-0198, Japan
Received April 30, 2007
Substituent effects of the meso-aryl (Ar) groups on the 1H and 13C NMR chemical shifts in a series of low-spin
highly saddled iron(III) octaethyltetraarylporphyrinates, [Fe(OETArP)L2]+, where axial ligands (L) are imidazole (HIm)
and tert-butylisocyanide (tBuNC), have been examined to reveal the nature of the interactions between metal and
porphyrin orbitals. As for the bis(HIm) complexes, the crystal and molecular structures have been determined by
X-ray crystallography. These complexes have shown a nearly pure saddled structure in the crystal, which is further
confirmed by the normal-coordinate structural decomposition method. The substituent effects on the CH2 proton as
well as meso and CH2 carbon shifts are fairly small in the bis(HIm) complexes. Since these complexes adopt the
(dxy)2(dxz, dyz)3 ground state as revealed by the electron paramagnetic resonance (EPR) spectra, the unpaired
electron in one of the metal dπ orbitals is delocalized to the porphyrin ring by the interactions with the porphyrin
3eg-like orbitals. A fairly small substituent effect is understandable because the 3eg-like orbitals have zero coefficients
at the meso-carbon atoms. In contrast, a sizable substituent effect is observed when the axial HIm is replaced by
tBuNC. The Hammett plots exhibit a large negative slope,
with the corresponding value,
−
220 ppm, for the meso-carbon signals as compared
+
5.4 ppm, in the bis(HIm) complexes. Since the bis(tBuNC) complexes adopt the
(dxz, dyz)4(dxy)1 ground state as revealed by the EPR spectra, the result strongly indicates that the half-filled dxy
orbital interacts with the specific porphyrin orbitals that have large coefficients on the meso-carbon atoms. Thus,
we have concluded that the major metal−porphyrin orbital interaction in low-spin saddle-shaped complexes with
the (dxz, dyz)4(dxy)1 ground state should take place between the dxy and a2u-like orbital rather than between the dxy
and a1u-like orbital, though the latter interaction is symmetry-allowed in saddled D2d complexes. Fairly weak spin
delocalization to the meso-carbon atoms in the complexes with electron-withdrawing groups is then ascribed to the
decrease in spin population in the dxy orbital due to a smaller energy gap between the dxy and dπ orbitals. In fact,
the energy levels of the dxy and dπ orbitals are completely reversed in the complex carrying a strongly electron-
withdrawing substituent, the 3,5-bis(trifluoromethyl)phenyl group, which results in the formation of the low-spin
t
complex with an unprecedented (dxy)2(dxz, dyz)3 ground state despite the coordination of BuNC.
Introduction
properties of naturally occurring heme proteins as well as
synthetic model heme complexes.1-8 Among the porphyrin
π orbitals, either an a1u or a2u orbital is considered to be the
highest-occupied molecular orbital (HOMO) in porphyrins
with D4h symmetry.3-5 Because these orbitals are orthogonal
Interactions between metal and porphyrin orbitals are
important factors that can determine the physicochemical
* To whom correspondence should be addressed. E-mail: mnakamu@
med.toho-u.ac.jp.
† School of Medicine, Toho University.
‡ Research Center for Materials with Integrated Properties, Toho
University.
(1) Physical Method in Bioinorganic Chemistry; Que, L., Jr., Ed.;
University Science Books, Sausalito, CA, 2000.
(2) Inorganic Electronic Structure and Spectroscopy; Solomon, E. I.,
Lever, A. B. P., Eds.; John Wiley & Sons, Inc.: New York, 1999;
Vol. 1.
§ Graduate School of Science, Toho University.
| Tokyo Institute of Technology.
¶ RIKEN.
10.1021/ic700827w CCC: $37.00
Published on Web 08/29/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 20, 2007 8193

Ohgo et al.
Chart 1. Symmetry Presentation of Metal and Porphyrin Orbitals in
Chart 2. Frontier Orbitals of Porphyrin
Metal Porphyrinatesa
a Adapted from refs 9 and 10.
to any of the iron d orbitals, no interactions can be expected
between the a1u or a2u orbitals and iron d orbitals. Actually,
however, the metal porphyrin complexes usually have lower
symmetry because of (i) the presence of peripheral substit-
uents, (ii) the orientation of planar axial ligands such as
pyridine or imidazole, and (iii) the deformation of the
normally planar porphyrin ring. Thus, it is possible that the
porphyrin HOMO, which is originally classified as either
a1u or a2u in D4h complexes, is involved in the interactions
with iron d orbital(s) in various metal porphyrinates. Chart
1 lists the symmetry representations of the metal d and
porphyrin frontier orbitals in metal porphyrinates with planar
D4h, ruffled D2d, and saddled D2d structures.9,10 As shown in
Chart 1, the metal dπ (dxz and dyz) orbitals can interact with
one of the porphyrin π orbitals, HOMO-1, regardless of the
deformation mode of the porphyrin ring since these orbitals
have the same symmetry in D4h and D2d complexes. If we
confine the metal porphyrinates to the low-spin iron(III)
complexes, the dxy orbital could interact with the a2u-like
orbital in ruffled complexes and with the a1u-like orbital in
saddled complexes;9,10 we use hereafter the symmetry labels
such as a1u-like or a2u-like in the ruffled and saddled
complexes, although they should be signified as either b1 or
b2 in deformed complexes as shown in Chart 1. Other
symmetry-allowed interaction should take place between the
2
2
dx -y and a1u-like orbitals in ruffled complexes and between
(3) 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.
the dx -y and a2u-like orbitals in saddled complexes.9-12
However, these interactions must be less important because
2
2
(4) Walker, F. A.; Simonis, U. In Biological Magnetic Resonance;
Berrliner, L. J., Reuben, J., Eds.; Plenum Press, New York, 1993;
Vol. 12, pp 133-273.
2
2
the dx -y orbital is vacant and is located far above the
occupied dxy, dxz, and dyz orbitals in six-coordinate low-spin
iron(III) complexes. In Chart 2, the frontier orbitals of D4h
porphyrin are given.
(5) Walker, F. A. Chem. ReV. 2004, 104, 589-616.
(6) Nakamura, M. Coord. Chem. ReView. 2006, 250, 2271-2294.
(7) Goff, H. M. Nuclear Magnetic Resonance of Iron Porphyrins. In Iron
Porphyrins, I; Lever, A. B. P., Gray, H. B., Eds.; Physical Bioinorganic
Chemistry Series 1; Addison-Wesley: Reading, MA, 1983; pp 237-
281.
There are ample examples showing the presence of the
dxy-a2u interaction in low-spin ruffled complexes.3-5,13-21 The
(8) Bertini, I.; Luchinat, C. In NMR of Paramagnetic Substances; Lever,
A. B. P., Ed.; Coordination Chemistry Reviews 150; Elsevier:
Amsterdam, The Netherlands, 1996; pp 29-75.
(11) 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.
(12) Shao, J.; Steene, E.; Hoffman, B. M.; Ghosh, A. Eur. J. Inorg. Chem.
2005, 1609-1615.
(9) Cheng, R.-J.; Chen, P.-Y.; Lovell, T.; Liu, T.; Noodleman, L.; Case,
D. A. J. Am. Chem. Soc. 2003, 125, 6774-6783.
(10) Cjonradie, J.; Ghosh, A. J. Phys. Chem. B 2003, 107, 6486-
6490.
8194 Inorganic Chemistry, Vol. 46, No. 20, 2007

Highly Saddled Low-Spin Iron(III) Porphyrin Complexes
Chart 3
relevance of this interaction to the biological process was
recently pointed out by Rivera and co-workers in the
metabolism of heme oxygenase.22,23 In contrast, the dxy-a1u
interaction in low-spin saddled complexes is rarely reported.
Quite recently, Cheng and co-workers suggested on the basis
of the DFT calculation that the saddle-shaped intermediate-
spin complex, [Fe(OETPP)(THF)2]+, should adopt the (dxz,
dyz)3(dxy)1(dz2)1 electron configuration.24 This is because the
dxy-a1u interaction is strong enough to lift the dxy orbital
above the dxz and dyz orbitals. Since this conclusion is against
our original proposal that [Fe(OETPP)(THF)2]+ should adopt
the (dxy)2(dxz, dyz)2(dz2)1 electron configuration,25,26 we have
reexamined how the dxy-a1u interaction affects the electronic
structure of saddle-shaped iron(III) porphyrinates. To begin,
the effect of the dxy-a1u interaction on the electronic structure
of low-spin saddled complexes has been examined on the
L2]+ (1a,b-5a,b) (OETArP ) octaethyltetraarylporphyrinates),
where Ar represents various substituted phenyl groups and
the axial ligand (L) is either imidazole (HIm, a) or tert-
butylisocyanide (tBuNC, b) as shown in Chart 3. We have
also prepared the corresponding diamagnetic Zn(OETArP)
complexes (1Zn-5Zn) to estimate the isotropic shifts of some
1H and 13C signals of 1a,b-5a,b. The molecular structures of
some of the bis(imidazole) complexes, 1a-4a, have been
determined by X-ray crystallography.
Results
Structures of [Fe(OETArP)(HIm)2]+. 1. Crystal Pack-
ing. Crystal packing diagrams of 1a-4a are given in Figures
S1-S4 of the Supporting Information, respectively. The
crystals of 1a and 3a are isomorphic; the crystal system is
monoclinic, and the space group is P21 in both complexes,
as listed in Table 1. Each molecule in 1a and 3a is connected
with the neighboring molecules by the hydrogen bonding
between HIm and ClO4-. As a result, chiral molecular helices
represented by (N7)HIm-Fe-HIm(N8)‚‚‚(O3)ClO4(O2)‚‚‚
(N7*)HIm*-Fe*-HIm*(N8*)‚‚‚(O3*)ClO4(O2*) develop
along the b axis, where * means the symmetry transformation
along the b axis (symmetry code: x, 1 + y, z). It should be
noted that chiral crystals are formed due to the chiral
arrangement of the molecules, though the molecules of 1a
and 3a are essentially achiral. Thus, the recrystallization batch
contains the same amount of chiral helices with opposite
helicities, but in a certain single crystal either one of opposite
helices should exist, exclusively.
1
basis of the H and 13C NMR chemical shifts as well as
electron paramagnetic resonance (EPR) g values. Thus, we
have prepared a series of low-spin complexes [Fe(OETArP)-
(13) 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.
(14) Nakamura, M.; Ikeue, T.; Fujii, H.; Yoshimura, T. J. Am. Chem. Soc.
1997, 119, 6284-6291.
(15) Wolowiec, S.; Latos-Grazynski, L.; Mazzanti, M.; Marchon, J.-C.
Inorg. Chem. 1997, 36, 5761-5771.
(16) Wojaczynski, J.; Latos-Grazynski, L.; Glowiak, T. Inorg. Chem. 1997,
36, 6299-6306.
(17) Pilard, M.-A.; Guillemot, M.; Toupet, L.; Jordanov, J.; Simonneaux,
G. Inorg. Chem. 1997, 36, 6307-6314.
(18) Wolowiec, S.; Latos-Grazynski, L.; Toronto, D.; Marchon, J.-C. Inorg.
Chem. 1998, 37, 724-732.
(19) Mazzanti, M.; Marchon, J.-C.; Wojaczynski, J.; Wolowiec, S.; Latos-
Grazynski, L.; Shang, M.; Scheidt, W. R. Inorg. Chem. 1998, 37,
2476-2481.
The crystal of 2a contains eight chloroform molecules and
one cyclohexane molecule. The cyclohexane molecule is on
the center of symmetry in the asymmetric unit. Two
chloroform molecules are highly disordered. One of the
imidazole ligands is connected to the Cl anion by hydrogen
bonding, and the same anion is further connected with the
imidazole ligand of the neighboring complex by hydrogen
bonding. Thus, the one-dimensional hydrogen-bonding net-
work represented by (N6)HIm-Fe-HIm(N8)‚‚‚Cl‚‚‚(N6*)-
HIm*-Fe*-HIm*(N8*)‚‚‚Cl* develops along the a axis,
where * means the symmetry transformation along the a axis
(symmetry code: 1 + x, y, z). It should be noted that the Cl
(20) Veyrat, M.; Ramasseul, R.; Turowska-Tyrk, I.; Scheidt, W. R.; Autret,
M.; Kadish, K. M.; Marchon, J.-C. Inorg. Chem. 1999, 38, 1772-
1779.
(21) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Nakamura, M.; Fujii, H.; Yokoyama,
M. J. Am. Chem. Soc. 2000, 122, 4068-4076.
(22) Caignan, G. A.; Deshmukh, R.; Zeng, Y.; Wilks, A.; Bunce, R. A.;
Rivera, M. J. Am. Chem. Soc. 2003, 125, 11842-11852.
(23) Rivera, M.; Caignan, G. A.; Astashkin, A. V.; Raitsimring, A. M.;
Shokhireva, T. K.; Walker, F. A. J. Am. Chem. Soc. 2002, 124, 6077-
6089.
(24) Cheng, R.-J.; Wang, Y.-K.; Chen, P.-Y.; Han Y.-P.; Chang, C.-C.
Chem. Commun. 2005, 1312-1314.
(25) Sakai, T.; Ohgo, Y.; Ikeue, T.; Takahashi, M.; Takeda, M.; Nakamura,
M. J. Am. Chem. Soc. 2003, 125, 13028-13029.
(26) Hoshino, A.; Ohgo, Y.; Nakamura, M. Inorg. Chem. 2005, 44, 7333-
7344.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8195

Ohgo et al.
Table 1. Crystal Data of a Series of Imidzole Complexes
[Fe(4-OMe-OETPP-
(HIm)2]C1O4
1a
[Fe(OETPP)-
(HIm)2]C1
2a
[Fe(4-CF3-OETPP-
(HIm)2]C1O4
3a
[Fe(2,6-C12-OETPP)-
(HIm)2]C1O4
4a
chemical formula
C70H76N8O8FeCl‚
(CH2Cl2)
0.51 × 0.40 × 0.12
purple
monoclinic
P21 (No. 4)
1333.61
C66H68N8FeC1‚
(C6H12)0.5‚(CHC13)4
0.2 × 0.1 × 0.1
purple
triclinic
P1h (No. 2)
1584.13
C
701H64N8O4F12-
FeCl‚(CH2Cl2)
C66H60N8O4FeCl9‚
(C6H6)0.5‚(CH2Cl2)
0.2 × 0.2 × 0.1
purple
triclinic
P1h (No. 2)
1528.10
cryst dimens (mm)
cryst color
cryst syst
space group
Mr
0.2 × 0.2 × 0.1
purple
monoclinic
P21 (No. 4)
1485.52
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
13.082(1)
18.000(2)
15.531(2)
90
112.151(1)
90
3387.3(6)
2
1.308
12.412(1)
13.784(1)
24.500(1)
74.439(3)
82.914(1)
81.725(3)
3980.0(5)
2
12.918(2)
18.784(3)
15.434(2)
90
110.509(6)
90
3507.7(9)
2
1.406
11.816(4)
14.594(5)
22.483(7)
76.480(15)
83.346(14)
71.053(12)
3562(2)
Z
2
Dx (Mg m-3
T (K)
)
1.322
298
1.425
298
90
298
diffractometer
abs corr
Rigaku VariMax
numerical
(Tmin ) 0.865,
Tmax ) 0.964)
65.1
-18 < h < 18
-25 < k < 20
-16 < l < 21
17 672
Rigaku RAXIS-RAPID
multiscan
(Tmin ) 0.865,
Tmax ) 0.934)
55.0
-16 < h < 16
-17 < k < 17
-31 < l < 31
17 686
Rigaku RAXIS-RAPID
multiscan
(Tmin ) 0.918,
Tmax ) 0.959)
55.0
-16 < h < 16
-22 < k < 24
-20 < l < 20
15 115
Rigaku RAXIS-RAPID
multiscan
(Tmin ) 0.880,
Tmax ) 0.941)
55.0
-15 < h < 15
-18 < k < 18
-28 < l < 29
15 816
2θmax (deg)
limiting indices
no. of indep reflns
no. of reflns with >2σ(I)
no. of params
13 714
859
10 384
993
13 216
926
13 129
863
Rint
0.0366
0.0650
0.0456
0.0351
R1
0.0538
0.0959
0.0607
0.0756
wR2
S
0.1300
1.016
0.2640
1.010
0.1685
1.097
0.2169
1.087
(∆/σ)max
<0.001
<0.001
<0.001
<0.001
anion, which bridges two iron(III) porphyrinates, is further
involved in the hydrogen bonding with the three chloroform
molecules.
In the crystal of 4a, the perchlorate oxygen atoms bind
two molecules by hydrogen bonding to form a one-
dimensional hydrogen-bonding network along the b axis,
which is represented as (N8)HIm-Fe-HIm(N6)‚‚‚(O3)-
ClO4(O1)‚‚‚N(8*)HIm*-Fe*-HIm*(N7*)‚‚‚(O3*)ClO4-
(O1*), where * means the symmetry transformation along
the b axis (symmetry code: x, 1 + y, z).
2. Molecular Structures. The molecular structures of 1a-
4a are given in Figure 1 and Figures S5-S7 of the Supporting
Information, respectively. The displacement of each atom
of the porphyrin core from the 24-atom mean plane is shown
in Figure 2. Table 2 lists the structural parameters of these
complexes together with those of some analogous complexes
reported previously.27-30 Various structural parameters listed
in Table 2 are defined as follows: φ is the orientation angle
of the axial ligand to the nearest NP-Fe-NP axis, θ is the
dihedral angle between two axial ligands, τ is the dihedral
angle between the average N4 plane and each pyrrole
ring, ψ is the dihedral angle between two diagonal pyrrole
rings, η is the twist angle between two diagonal pyrrole
rings, ω is the dihedral angle between phenyl group and the
CR-Cmeso-CR′ plane, and |∆Cmeso| and |∆Câ| are the average
deviation of the meso- and â-pyrrole carbon atoms from the
mean porphyrin plane, respectively.29 Table 3 lists the results
of the normal-coordinate structural decomposition (NSD)
method, which reveals the distortion modes of the complexes
more quantitatively.31-33 Figure 2 and the data in Table 3
clearly indicate that all the bis(Him) complexes examined
in this study commonly exhibit highly saddled structures,
though some structural differences have been observed
among complexes. The average deviations of the â-pyrrole
carbon atoms from the mean porphyrin plane are 1.15-
1.26 Å. These values are close to the corresponding values
in analogous [Fe(OETPP)(DMAP)2]+ and [Fe(OETPP)-
(Py)2]+.27-29 The average deviations of the meso-carbon
atoms are 0.04-0.10 Å. Thus, each complex includes a small
ruffling in a highly saddled porphyrin core. The NSD result
given in Table 3 shows the deformation mode of these
complexes more quantitatively.31-33 The B2u (saddle) coef-
ficient ranges from 3.4809 to 3.7849 and is tremendously
greater than any other coefficients. In fact, the ratio of the
(27) Ogura, H.; Yatsunyk, L.; Medforth, C. J.; Smith, K. M.; Barkigia, K.
M.; Renner, M. W.; Melamed, D.; Walker, F. A. J. Am. Chem. Soc.
2001, 123, 6564-6578.
(28) Ohgo, Y.; Ikeue, T.; Nakamura, M. Inorg. Chem. 2002, 41, 1698-
1700.
(29) Ohgo, Y.; Ikeue, T.; Takahashi, M.; Takeda, M.; Nakamura, M. Eur.
J. Inorg. Chem. 2004, 798-809.
(30) Yatsunyk, L. A.; Dawson, A.; Carducci, M. D.; Nichol, G. S.; Walker,
F. A. Inorg. Chem. 2006, 45, 5417-5428.
(31) Shelnutt, J. A.; Song, X.-Z.; Ma, J.-G.; Jia, S.-L.; Jentzen, W.;
Medforth, C. J. Chem. Soc. ReV. 1998, 27, 31-41.
(32) Jentzen, W.; Song, X.-Z.; Shelnutt, J. A. J. Phys. Chem. B 1997, 101,
1684-1699.
(33) Jentzen, W.; Ma, J.-G.; Shelnutt, J. A. Biophys. J. 1998, 74, 753-
763.
8196 Inorganic Chemistry, Vol. 46, No. 20, 2007

Highly Saddled Low-Spin Iron(III) Porphyrin Complexes
Figure 1. ORTEP diagrams of 1a together with the atom labeling: (a) top view; (b) side view. Thermal ellipsoids are drawn at the 50% probability level.
saddled deformation reaches as great a value as 95%. As
expected from the small deviation of the meso-carbons from
the mean porphyrin plane, the ratio of the ruffled deformation
is at most 6%. Thus, we can say that these complexes have
nearly pure saddled structure in the crystal. It should be noted
here that 2a has a relatively large A2u (dome) coefficient.
The result can be ascribed to the unique crystal packing of
this complex; one of the imidazole ligands rotates about the
Fe-N(HIm) bond and deviates from the Fe-N(pyrrole) axis
for the formation of hydrogen bonding with the Cl anion.
As a result, the steric environment around this ligand is more
congested than that of the other ligand. Correspondingly, the
average displacement of the â-carbons at the crowded side
decreases to 1.08 Å as compared with 1.23 Å in the less-
crowded opposite side. It is this curious deviation that
increases the A2u (dome) coefficient.
the θ values have decreased to 73.3° and 46.4° in 4a and 2a,
respectively. As mentioned, the fairly small dihedral angle
in 2a has been ascribed to the deviation of one of the
imidazole ligands from the Fe-N(pyrrole) axis to form the
hydrogen bond with the Cl anion. Since imidazole is a less-
bulky ligand as compared with other aromatic ligands such
as pyridine, the imidazole ligand can easily rotate around
the Fe-N(imidazole) bond and deviate from the Fe-
N(pyrrole) axis to the position that is favorable for effective
packing. This must be one of the reasons why the structural
parameters such as φ and θ are different among a series of
[Fe(OETArP)(HIm)2]+ complexes examined in this study.
The ratios of the average bond lengths, Fe-Naxial/Fe-Np,
in 1a-4a are quite close to unity; they are 0.998, 0.998, 1.004,
and 1.007, respectively. The result indicates that the coor-
dination geometry around the iron center is nearly octahedral.
In contrast, the corresponding ratios in analogous [Fe-
(OETPP)(DMAP)2]+ and [Fe(OETPP)(Py)2]+ are much
Although two axial ligands are perpendicularly aligned in
1a and 3a as revealed from θ ) 88.2° and 87.4°, respectively,
Inorganic Chemistry, Vol. 46, No. 20, 2007 8197

Ohgo et al.
Figure 2. Vertical displacement of the peripheral atoms.
larger, 1.025 and 1.109 at 298 K, respectively. The shorter
Fe-Naxial bonds in 1a-4a as compared with those of the bis-
(DMAP) and bis(Py) complexes are the indication that the
ligand field of HIm is much stronger than that of DMAP
and Py. Neither the Fe-NP nor the Fe-Naxial bond has
exhibited appreciable change depending on the p-substituents.
It should be noted here that the structural parameters of 2a
obtained in this study are somehow different from those
reported by Yatsunyk and co-workers.30 The discrepancies
should be ascribed to the differences in crystal system and
space group between these two crystals; the crystal system
and the space group reported by Yatsunyk and co-workers
are trigonal and P3221, respectively.30
Figure 3. 1H NMR spectra of (a) 1b and (b) 5b taken in CD2Cl2 solution
at 298 K.
directing ethyl groups. This orientation is also a commonly
observed one. Complexes such as [Fe(OETPP)(DMAP)2]+,27
[Fe(OETPP)(Py)2]+,28 [Fe(OETPP)(4-CNPy)2]+,36 and Fe-
(OETPP)ClO438 take the same orientation. In the case of 4a,
the ethyl groups take the ududduud orientation, where any
of the two ethyl groups in the same pyrrole ring take the
opposite orientation probably because of the severe steric
repulsion with the bulky chloro groups.
1H NMR Chemical Shifts. 1H NMR spectra of 1b and 5b
taken in CD2Cl2 solution at 298 K are shown in Figure 3 as
typical examples. Table 4 lists the chemical shifts of a series
of low-spin [Fe(OETArP)(HIm)2]+ (1a-5a) and [Fe(O-
ETArP)(tBuNC)2]+ (1b-5b) determined at 298 K together
with the Hammett σ values. The chemical shifts of some
diamagnetic Zn(OETArP) (1Zn-5Zn) are also listed to
estimate the isotropic shifts.39 The chemical shifts of the CH2
signals of 1a-5a given in Table 4a are extrapolated values
from low temperature since these signals are very broad at
298 K due to the inversion of the saddled porphyrin ring. In
parentheses in Table 4a are given the chemical shifts at 223
K where the CH2 protons give separated signals. The
chemical shifts of the CH2 signals of 1b-5b given in Table
4b are the average values of two diastereotopic CH2 signals.
This is because the CH2 protons of 1b-5b exhibit quite sharp
signals at 298 K; they have given two signals only when
the temperature is lower than 223 K.40 In the parentheses of
Close inspection of the molecular structures given in
Figure 1 and Figures S5-S7 (Supporting Information)
reveals that the relative orientation of the eight ethyl groups
is quite different among the four complexes. Since the
stable conformation of an ethyl group attached to the
â-carbon of the pyrrole ring is the one where the â-CR-Câ
(see Chart 3) plane is nearly perpendicular to the pyrrole
plane, there should be two possible orientations in an ethyl
group, u (or up) and d (or down), depending on the posi-
tions of the ethyl group relative to the average porphyrin
plane. Figures S5 and S6 (Supporting Information) clearly
indicate that the eight ethyl groups of 2a and 3a exhibit the
commonly observed uudduudd orientation, though one of
the ethyl groups in 2a is disordered to take both the u and d
orientations. The same orientation of the peripheral
ethyl groups is observed in [Fe(OETPP)(2-MeIm)2]+,27 [Fe-
(OETPP)(DMAP)2]+,29 [Fe(OETPP)(Ph)]+,34 [Fe(OETPP)-
(1-MeIm)2]+,35 [Fe(OETPP)(4-CNPy)2]+,36 and Fe(OETPP)-
Cl.37 In contrast, the ethyl groups of 1a show a uudduuud
orientation where one of the pyrrole rings has oppositely
(34) Bill, E.; Schunemann, V.; Trautwein, A. X.; Weiss, R.; Fischer, J.;
Tabard, A.; Guilard, R. Inorg. Chem. Acta 2002, 339, 420-426.
(35) Yatsunyk, L. A.; Carducci, M. D.; Walker, F. A. J. Am. Chem. Soc.
2003, 125, 15986-16005.
(36) Yatsunyk, L. A.; Walker, F. A. Inorg. Chem. 2004, 43, 757-777.
(37) Cheng, R.-J.; Chen, P.-Y.; Gau, P.-R.; Chen, C.-C.; Peng, S.-M. J.
Am. Chem. Soc. 1997, 119, 2563-2569.
(38) Barkigia, K. M.; Renner, M. W.; Fajer, F. J. Porphyrins Phthalocya-
nines 2001, 5, 415-418.
(39) Hoshino, A.; Ohgo, Y.; Nakamura, M. Tetrahedron Lett. 2005, 46,
4961-4964.
(40) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Yamaguchi, T.; Nakamura, M. Inorg.
Chem. 2001, 40, 3423-3434.
8198 Inorganic Chemistry, Vol. 46, No. 20, 2007

Highly Saddled Low-Spin Iron(III) Porphyrin Complexes
Table 2. Comparison of Structural Parameters for [Fe(OETPP)L2]+
Fe-Np/ Fe-Naxial
/
|∆Cmeso|/ |∆Câ|/
cavity/
Å3
complexes
T/K
Å
Å
φ/deg
θ/deg
τ/deg
ψ/deg
η/deg
ω/deg
0.01 Å
6
0.01 Å
118
1a
2a
2a
2a
3a
4a
90 1.979(2)
298 1.975(3)
170 1.980(4)
170 1.961(4)
298 1.971(3)
298 1.966(3)
298 1.977(2)
298 1.951
1.975(3)
14.6(3)
16.6(3)
11.6(5)
31.8(4)
11.1(4)
-19.0(2)
12.1(5)
17.7(5)
4.0(4)
0.4(4)
5.4(4)
21.8(4)
10.6(2)
25.0(2)
9
88.2(2)
30.2(8)
119.6(1)
2.5(3) 65.4(2)
0.4(4) 64.9(3)
46.7(2)
14.64
1.972(3)
46.4(3)
30.1(4)
57.2(5)
87.4(2)
73.3(2)
53.2
29.5(1)
121.0(1)
4
20
19
6
115
113
114
126
120
122
122
124
121
26.70
a
a
1.936(9)
1.998(7)
1.962(7)
1.973(9)
1.979(3)
46.1(2)
32.3(1)
30.2(2)
30.8(1)
33e
115.5(1)
119.6(2)
118.4(1)
114e
2.0(4) 62.4(2)
5.2(3) 67.7(3)
4.0(2) 66.7(2)
17.62
22.95
1.980(3)
2.030(3)
2.000
10
8
[Fe(OETPP)-
(DMAP)2]+ b)
[Fe(OETPP)-
(DMAP)2]+ c
70
17e
66
28
3
29
[Fe(OETPP)Py2]+ d 298 1.985
2.201
1.1(4)
3.6(3)
6.3(3)
11.6(3)
82.3
32.4(1)
30.8(1)
115.2(2)
118.5(1)
1.2(3) 40.6(1)
3.0(3) 64.5(3)
28.88 32.08
23.19
[Fe(OETPP)Py2]+ d
80 1.957
1.993
85.1
6
a Yatsunyk, L. A. et al.30 b Ohgo, Y. et al.29 c Ogura, H. et al.27 d Ohgo, Y. et al.28 e These angles are calculated on the basis of the structural data
reported by Ogura et al.27
Table 3. NSD of Distortion Modes of Complexes
complexes
Doop
B2u, saddle
B1u, ruffle
A2u, dome
Eg(x), wave (x)
Eg(y), wave (y)
A1u, propeller
sum
sad/sum
ruf/sum
1a
2a
3a
4a
3.5796
3.4837
3.7921
3.6392
3.5796
3.4809
3.7894
3.6298
0.1175
0.0206
0.0716
0.2432
0.0255
0.1341
0.0427
0.0322
0.0700
0.0174
0.0401
0.0714
0.0446
0.0244
0.1084
0.0285
0.0007
0.0068
0.0184
0.0477
3.8379
3.6842
4.0706
4.0528
0.93
0.95
0.93
0.90
0.03
<0.01
0.02
0.06
Table 4. 1H NMR Chemical Shifts in CD2Cl2 Solutions at 298 K. (a) [Fe(OETArP)(HIm)2]+, (b) [Fe(OETArP)(tBuNC)2]+, and (c) [Zn(OETArP)]
Chemical Shifts in Parentheses in (a) and (b) Are Determined at 223 and 193 K, Respectively
complexes
Σσ
CH2
CH2 (avg)
CH3
ortho
meta
para
tBu
(a) [Fe(OETArP)(HIm)2]+
1a
2a
3a
4a
5a
-0.27
0.00
4.74, 10.98a
(3.69, 12.45)
4.37, 10.34a
(3.22, 11.81)
4.32, 10.69a
(3.06, 12.20)
5.54, 9.71a
(4.76, 10.62)
3.56, 10.91a
(2.09, 12.72)
7.86a
(8.07)
7.34
0.74
(1.39)
0.86
5.64
(3.02)
4.97
(2.76)
4.90
5.49
(4.36)
5.8
(4.68)
6.00
(4.83)
5.42
(4.09)
3.35b
(2.73b)
6.73
(7.52)
7.51
(1.28)
0.92
(5.99)
0.54
(7.40)
7.63
(1.39)
1.18
(2.62)
6.32
(5.45)
1.17
(7.69)
7.24
(1.72)
1.05
0.86
5.23
(3.04)
(7.41)
(1.32)
(6.38)
(b) [Fe(OETArP)(tBuNC)2]+
1b
2b
3b
4b
5b
-0.27
0.00
7.10
(5.1, 18.0)
7.46
(5.7, 18.4)
9.93
(7.4, 24.8)
12.39
7.10
(11.6)
7.46
1.14
(1.67)
1.32
5.10
(6.49)
5.54
(6.68)
7.00
10.55
(10.95)
11.05
(11.40)
10.14
(9.75)
9.35
(10.97)
-1.55a
(-4.57)
-1.53a
(-4.06)
-1.03a
(-3.28)
-1.03a
(-2.15)
-0.95a
(-1.78)
6.31
(6.80)
(11.9)
9.93
(1.63)
1.27
0.54
(16.1)
12.39
(14.8)
10.83
(19.1)
(1.86)
1.73
(2.09)
1.36
(8.43)
7.53
(7.43)
7.96
(8.8, 20.8)
10.83
0.86
7.96
(8.94)
(8.0, 30.2)
(1.95)
(8.47)
(c) Zn(OETArP)
1Zn
2Zn
3Zn
4Zn
5Zn
-0.27
0.00
0.54
2.10, 2.65
2.01, 2.61
1.97, 2.63
2.45
2.38
2.31
2.30
2.45
1.52
0.45
0.46
0.49
0.71
1.26
8.19
8.31
8.52
7.24
7.67
8.03
7.76
4.07b
7.79
7.65
8.36
0.86
1.52
8.87
a Extrapolated values; signals are too broad at ambient temperature. b Methoxy signal.
1
Table 4b are given the chemical shifts determined at 193 K
where the CH2 protons give separated signals. As shown in
Figure 3, the coordinated and free tBuNC protons showed a
broad single line at 298 K due to the fast ligand exchange
on the H NMR time scale. However, the signal started to
split into two signals below 273 K. Thus, the chemical shifts
at 298 K are the extrapolated values from low temperature
where the free and coordinated tBuNC give different signals.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8199

Ohgo et al.
Figure 4. Curie plots of the isotropic shifts of (a) CH2, (b) o-H, and (c) m-H signals of (A) [Fe(OETArP)(HIm)2]+ (1a-3a) and (B) [Fe(OETArP)(t-
BuNC)2]+ (1b-3b) taken in CD2Cl2 solutions. Complexes 1-3 are signified by squares, filled circles, and open circles, respectively.
Figure 4 shows the Curie plots of the CH2, o-H, and m-H
signals of 1a-3a and 1b-3b, where the isotropic shifts (δiso)
are plotted against 1/T. The isotropic shifts of some selected
1H signals were determined by δiso ) δobs - δdia. δobs is the
chemical shift of a 1H signal in paramagnetic [Fe(OETArP)-
L2]+ and δdia is the chemical shift of the corresponding signal
in diamagnetic Zn(OETArP). Curie plots for each proton
signal of 1a-3a given in Figure 4A showed a straight line
in the temperature range examined, though the isotropic shifts
at 1/T ) 0 deviated by at most 5 ppm. While the slopes of
the Curie plots for the CH2 signals were positive, those for
the o-H and m-H signals were negative. In contrast, the Curie
plots for each proton signal of 1b-3b given in Figure 4B
exhibited a considerable curvature in the temperature range
examined.
13C NMR Chemical Shifts. The 13C NMR spectra of
1b and 5b taken in CD2Cl2 solution at 298 K are shown in
Figure 5 as typical examples. Table 5 lists the 13C NMR
chemical shifts of a series of low-spin [Fe(OETArP)(HIm)2]+
(1a-5a) and [Fe(OETArP)(tBuNC)2]+ (1b-5b) complexes
determined at 298 K. The 13C NMR chemical shifts of a
Figure 5. 13C NMR spectra of (a) 1b and (b) 5b taken in CD2Cl2 solution
at 298 K.
series of diamagnetic 1Zn-4Zn complexes are also listed; the
13C NMR chemical shifts of 5Zn were not determined due to
its fairly low solubility. In the case of [Fe(OETArP)(HIm)2]+
and Zn(OETArP), the assignment of the R and â signals was
hampered due to their close proximity. Thus, the chemical
shifts of these signals signified by asterisks in Table 5 could
be reversed.
Hammett Plots of the 1H and 13C NMR Isotropic Shifts.
The isotropic shifts of some selected signals of [Fe-
(OETArP)(HIm)2]+ (1a-3a and 5a) and [Fe(OETArP)-
(tBuNC)2]+ (1b-3b and 5b) were plotted against Hammett σ
values by the red and blue lines, respectively, as shown in
Figure 6. We could not determine the isotropic shifts of the
carbon signals of 5a and 5b, since the 13C NMR chemical
shifts of diamagnetic 5Zn were unavailable due to its low
solubility. Thus, we estimated the isotropic shifts of these
carbon signals in 5a and 5b by assuming that the 13C chemical
shifts of 5Zn are the same as those of 3Zn. Figure 6 clearly
indicates that both the isotropic shifts and the slopes of the
Hammett plots are very different between the bis(HIm) and
8200 Inorganic Chemistry, Vol. 46, No. 20, 2007

Highly Saddled Low-Spin Iron(III) Porphyrin Complexes
Table 5. 13C NMR Chemical Shifts of (a) [Fe(OETArP)(HIm)2]+, (b)
[Fe(OETArP)(tBuNC)2]+, and (c) Zn(OETArP) Taken in CD2Cl2
Solutions at 298 K
complexes meso
R
â
ortho meta para ipso
CR
Câ
(a) [Fe(OETArP)(HIm)2]+
1a
2a
3a
4a
5a
-2 182a 179a 102 111 156 163 -23
91
7
5
162a 167* 107 124 125 162 -24
157a 166a 108 122
166 -24
87
86
92
85
-7 149a 161a 113 128 128 154 -27
6
144a 164a 109 128 119 161 -25
(b) [Fe(OETArP)(tBuNC)2]+
1b
2b
3b
4b
5b
430 -11 143 317 126 171 n.d.
0
58
59
75
93
87
417
280
166
194
-5 143 310 144 138
37 -2
79 152 244 135 n.d.
136 157 205 133 133 125 -21
117 161 207 139 126 128 -18
98 -10
(c) Zn(OETArP)
1Zn
2Zn
3Zn
4Zn
5Zn
119 148a 144a 135 113 160 135
119 147a 144a 136 127 128 142
118 147a 144a 136 124 131 145
113 147a 144a 141 129 131 139
too broad to detect
19.9 17.5
19.8 17.5
20.0 17.4
20.0 16.1
a The assignments of the R and â signals could be reversed.
Figure 7. EPR spectra of bis(HIm) complexes (1a and 3a-5a) and bis(t-
BuNC) complexes (1b and 3b-5b) taken in frozen CH2CH2 solution at 4.2-
20 K.
Table 6. EPR g Values of [Fe(OETArP)(HIm)2]+ and
[Fe(OETArP)(tBuNC)2]+ Taken in Frozen CH2Cl2 Solution at 4.2-20 K
1
Figure 6. Hammett plots of the H and 13C NMR isotropic shifts of bis-
complexes
g1
g2
g3
complexes
g1
g2
g3
(HIm) complexes (1a-3a and 5a, red), and bis(tBuNC) complexes (1b-3b
and 5b, blue): (a) CH2-H, (b) CR, and (c) meso-C.
1a
2a
3a
4a
5a
2.73 2.36 1.65
2.72 2.37 1.64
2.71 2.36 1.66
3.45
1b
2b
3b
4b
5b
2.216
2.29
2.250
2.370
1.925
1.92
1.908
1.680
2.25
bis(tBuNC) complexes. The slopes of the CH2, CR, and
meso-C lines in [Fe(OETArP)(HIm)2]+ are 0.27, -1.4, and
+5.4 ppm, respectively, while those in [Fe(OETArP)(t-
BuNC)2]+ are +4.2, -16, and -220 ppm, respectively.
2.73 2.34 1.69
2.524 2.186 1.859
values of the energy levels of the three d orbital are fixed at
a constant value.
EPR g Values. The EPR spectra taken in frozen CH2Cl2
or CH2Cl2/toluene solution at 4-20 K are given in Figure
7. The EPR spectra of both 2a and 2b are omitted since they
have already been reported in our previous paper.40 Figures
S8(A)-S8(D) of the Supporting Information show the
simulated spectra. The g values thus obtained are listed in
Table 6. In the case of [Fe(OETArP)(HIm)2]+, all the
complexes except 4a exhibited rhombic-type spectra; 4a
showed a so-called large gmax-type spectrum. In the case of
[Fe(OETArP)(tBuNC)2]+, all the complexes except 5b showed
axial-type spectra. It is quite unusual that 5b exhibited the
Molecular Orbital Calculation. The energy levels of
some important molecular orbitals in a series of Zn(OETArP)
complexes (1Zn-4Zn) have been determined by DFT calcula-
tion. As the p-substituent changes from OMe (1Zn) to H (2Zn),
and then to CF3 (3Zn), both the HOMO and lowest unoc-
cupied molecular orbital (LUMO) are gradually stabilized,
as shown in Figure S9 of the Supporting Information. In the
case of 4Zn (2,6-Cl2), both the HOMO and LUMO are located
between those of 2Zn and 3Zn despite the presence of two
electron-withdrawing chloro substituents at the 2,6-positions.
t
Discussion
rhombic-type spectrum despite the coordination of BuNC.
Figure 8 demonstrates the energy gap between the dxy and
dπ orbitals of [Fe(OETArP)(tBuNC)2]+ determined on the
basis of the EPR g values.41-44 In each complex, the average
General Considerations about the Effects of meso
Substituents on the Heme Electronic Structure. The
electronic ground state of low-spin iron(III) porphyrinates
can most directly be determined by EPR spectroscopy.
Complexes that adopt the (dxy)2(dxz, dyz)3 ground state exhibit
either the rhombic or large gmax-type spectra depending on
the orientation of planar axial ligands; the complexes with
parallel aligned axial ligands show the rhombic-type spectra
while those with perpendicularly aligned axial ligands show
(41) Walker, F. A. Coord. Chem. ReV. 1999, 185-186, 471-534.
(42) 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.
(43) Bohan, T. L. J. Magn. Reson. 1977, 22, 109-118.
(44) Taylor, C. P. S. Biochim. Biophys. Acta 1977, 491, 137-149.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8201

Ohgo et al.
highly ruffled structure.47-49 This is because the complex
can be stabilized by the interaction between the half-filled
dxy orbital and the filled a2u orbital by ruffling the porphyrin
core. Since the a2u orbital has a large coefficient at the meso
position, the interaction induces not only a large downfield
shift but also a large substituent effect to the meso-carbon
signals.
Another symmetry-allowed interaction in low-spin iron-
(III) complexes should be the dxy-a1u interaction in the
saddled D2d complexes, as recently pointed out by Cheng
and co-workers.24 As revealed by the EPR and NMR studies,
t
the low-spin iron(III) porphyrinates having BuNC as axial
ligand always exhibit the (dxz, dyz)4(dxy)1 ground state regard-
less of the structure of the porphyrin cores. This is because
t
the low-lying π* orbitals of BuNC stabilize the dπ orbitals
of iron(III) to the point that is lower than the dxy orbital.3-6
In fact, highly saddled [Fe(OETPP)(tBuNC)2]+ (2b) is well
characterized to adopt the (dxz, dyz)4(dxy)1 ground state on the
basis of the EPR, 1H NMR, and 13C NMR spectroscopy.40,50
It is then expected that the 1H and 13C NMR chemical shifts
of a series of saddled complexes could reveal which
porphyrin orbital interacts with the half-occupied dxy orbital.
If the dxy-a1u interaction is the major interaction in these
complexes, the effect of the meso-aryl substituent on the 1H
and 13C chemical shifts must be quite small. Furthermore,
the meso-carbon signal should appear at an extremely upfield
position. This is because the a1u orbital has large coefficient
at the R and zero coefficient at the meso-carbon atoms as
shown in Chart 2. If, on the contrary, the dxy-a2u interaction
is the major interaction because of some contribution of the
ruffled deformation in solution, the meso-carbon signal
should appear at an extremely downfield position. In addition,
the substituent effects on the meso-carbon and meso-aryl
proton shifts must be much larger because the meso-carbons
have large coefficients in the a2u orbital, as shown in Chart
2. Thus, the substituent effect on the chemical shifts can tell
the nature of the metal-porphyrin orbital interaction.
Orbital Interactions in Low-Spin [Fe(OETArP)(HIm)2]+
Complexes. Low-spin bis(imidazole) complexes usually
adopt the (dxy)2(dxz, dyz)3 ground state. In fact, we have already
reported that highly saddled [Fe(OETPP)(HIm)2]+ adopts the
(dxy)2(dxz, dyz)3 ground state on the basis of the EPR and NMR
results.40 The EPR spectra given in Figure 7 clearly indicate
that all the complexes examined in this study adopt the (dxy)2-
(dxz, dyz)3 ground state at least at extremely low temperatures
because these complexes exhibited either rhombic (1a-3a
and 5a) or large gmax-type (4a) spectra. However, the EPR
spectral types observed here were against our expectation.
As proposed by Walker and co-workers, EPR spectral type
is closely connected with dihedral angles, θ, between two
Figure 8. Relative energy levels of the dxy, dxz, and dyz orbitals of bis(t-
BuNC) complexes (1b-5b) obtained on the basis of the EPR g values in
frozen CH2Cl2 solution at 4.2-20 K where λ is a spin-orbit coupling
constant. In each complex, the average value of the energy levels of three
d orbitals is fixed at a constant value.
the large gmax-type spectra.30,35,45,46 In contrast, the complexes
that adopt the (dxz, dyz)4(dxy)1 ground state always exhibit the
axial-type spectra.3,40
While the EPR method is used for the determination of
the heme electronic ground state at extremely low temper-
atures, the NMR method is available at much higher
1
temperatures.1-8 The H and 13C NMR chemical shifts of
low-spin iron(III) porphyrinates are composed of the contact
and dipolar terms. In the case of the planar D4h complexes
with the (dxy)2(dxz, dyz)3 ground state, the dπ(dxz or dyz) orbital
carrying an unpaired electron can interact with the filled 3eg
orbitals and to a lesser extent with the vacant 4eg* orbitals
of porphyrin.3-6 Thus, the unpaired electron in the dπ orbital
delocalizes to the porphyrin ring, especially to the pyrrole â
carbons since the 3eg orbital has relatively large coefficient
at these positions. As mentioned, the dπ-3eg type interaction
is possible not only in the planar D4h but also in the six-
coordinate ruffled and saddled D2d complexes because both
the dπ and 3eg orbitals are represented as e symmetry, as
shown in Chart 1. Thus, the [Fe(OETArP)(L)2]( complexes
with the (dxy)2(dxz, dyz)3 ground state should induce the
downfield shift of the CH2 signals and upfield shift of the
1
CR signals.26 Furthermore, the H and 13C chemical shifts
must be insensitive to the meso-phenyl substituents since the
3eg orbital has a zero coefficient at the meso-carbon atoms,
as shown in Chart 2.
In the complexes with the (dxz, dyz)4(dxy)1 ground state, the
dxy orbital carrying an unpaired electron is orthogonal to any
of the porphyrin π orbitals in the D4h complexes. Thus, there
should be no spin delocalization to the peripheral carbon
atoms. Actually however, the low-spin iron(III) porphyrinates
with the (dxz, dyz)4(dxy)1 ground state commonly exhibit the
(47) 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.
(48) 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.
(45) Walker, F. A.; Reis, D.; Balke, V. L. J. Am. Chem. Soc. 1984, 106,
6888-6898.
(46) Walker, F. A.; Huynh, B. H.; Scheidt, W. R.; Osvath, S. R. J. Am.
Chem. Soc. 1986, 108, 5288-5297.
(49) 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.
(50) Yatsunyk, L. A.; Walker, F. A. Inorg. Chem. 2004, 43, 4341-4352.
8202 Inorganic Chemistry, Vol. 46, No. 20, 2007

Highly Saddled Low-Spin Iron(III) Porphyrin Complexes
ligand, which could prohibit the flexibility of the ligand
molecules and maintain the dihedral angle in the solid even
in solution.
To reveal the electronic ground state at ambient temper-
1
ature, we have examined the H and 13C NMR chemical
1
shifts. The H NMR data listed in Table 4 indicate that the
CH2 signals appear at δ 7.2-7.9 ppm as compared with 1.5-
2.5 ppm in diamagnetic [Zn(OETArP)]. The 13C NMR data
listed in Table 5 indicate that the CR signals appear at δ -23
to -27 ppm, which are by 43-47 ppm more upfield than
those of the corresponding diamagnetic [Zn(OETArP)]. The
meso-C signals are also observed upfield, at δ -7 to +7
ppm, as compared with δ 113-119 ppm in diamagnetic [Zn-
(OETArP)]. The downfield shift of the CH2 signals and the
upfield shift of the CR signals are the direct indication that
the pyrrole â carbons have sizable amounts of positive spin,
which in turn indicates that all of these complexes adopt the
(dxy)2(dxz, dyz)3 ground state at 298-198 K; the unpaired
electron in the dπ orbital delocalizes to the porphyrin ring
especially on the â-carbon atoms due to the interactions
between iron dπ and porphyrin 3eg-like orbitals. The large
upfield shift of the meso signal also support this electronic
structure since the 3eg orbital has a zero coefficient at the
meso-carbon positions; the meso-C signal shifts upfield if
the spin density of the meso-carbons is negligibly small and
that of the neighboring R-carbons is fairly large.6,51
1
The isotropic shifts of some selected H and 13C signals
of [Fe(OETArP)(HIm)2]+ (1a-3a and 5a) were plotted
against Hammett σ values, as shown by the red lines in
Figure 6. The slope of the Hammett plots of CH2 signals in
Figure 6a is +0.27 ppm. The results indicate that the
substituent effects on the CH2 chemical shifts are fairly small.
The Hammett plots of the CR and meso-C signals are given
in parts b and c of Figure 6, respectively. The slopes of these
lines are again fairly small; they are -1.4 and +5.4 ppm,
respectively. The results indicate that all the low-spin [Fe-
(OETArP)(HIm)2]+ complexes examined in this study adopt
the (dxy)2(dxz, dyz)3 ground state even at ambient temperatures
and that the major interaction affecting the chemical shifts
occurs between the half-filled iron dπ and filled porphyrin
3eg-like orbitals.
Orbital Interactions in Low-Spin [Fe(OETArP)(t-
BuNC)2]+ Complexes. As mentioned, low-spin bis(tBuNC)
iron(III) porphyrinates always adopt the (dxz, dyz)4(dxy)1
ground state regardless of the type of porphyrins.40 In fact,
the EPR spectra of all the complexes except 5b are axial,
which clearly indicate that they adopt the (dxz, dyz)4(dxy)1
ground state at extremely low temperatures. We happened
to find that 5b is actually the first bis(tBuNC) complex of
iron(III) porphyrinate that shows the (dxy)2(dxz, dyz)3 ground
state. If we expand the range from porphyrins to porphy-
rinoids, diazaporphyrin iron(III) complex [Fe(DAzP)(t-
ByNC)2]+ is the first example that shows the (dxy)2(dxz, dyz)3
ground state.52
Figure 9. EPR spectra of bis(HIm) complexes (1a, 3a, and 4a) taken in
the solid at 4.2 K.
planar axial ligands such as imidazole and pyridine.30,35,45,46
Thus, EPR spectrum changes from rhombic to large gmax
-
type if θ becomes larger than ca. 57°.30 In fact, 2a with a
dihedral angle of 46.4° showed a rhombic spectrum and 4a
with a dihedral angle of 73.2° showed a large gmax-type
spectrum. In the case of 1a and 3a, however, the observed
EPR spectra are rhombic, though the dihedral angles of these
complexes are larger than 57°; they are 88.2° and 87.4°,
respectively. The discrepancies should be ascribed to the
difference in dihedral angles between solution and solid; the
dihedral angles in 1a and 3a in solution could be much
smaller than 57°. To ascertain if this is the case, we have
measured the EPR spectra of 1a, 3a, and 4a at 4.2 K in the
solid and given them in Figure 9. Although 1a and 4a showed
a signal at g ) 4.3 probably due to the contamination of the
monoadduct, all of these complexes commonly exhibited
strong large gmax-type signals at g ) 3.3-3.5 together with
several minor peaks classified as rhombic-type signals. Thus,
the results obtained from the solid samples are consistent
with the general rule proposed by Walker and co-workers.30
As mentioned, only 4a exhibits the large gmax-type spectrum
both in solution and in the solid. The results should be
ascribed to the presence of bulky 2,6-chloro groups near the
(51) Rivera, M.; Caignan, G. A. Anal. Bioanal. Chem. 2004, 378, 1464-
1483.
(52) Ohgo, Y.; Neya, S.; Uekusa, H.; Nakamura, M. Chem. Commun. 2006,
4590-4592.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8203

Ohgo et al.
In contrast to the case of [Fe(OETArP)(HIm)2]+, most of
the 1H and 13C signals in [Fe(OETArP)(tBuNC)2]+ exhibited
considerable substituent dependence, as revealed from the
data in Tables 4 and 5. The Hammett plots of some 1H and
13C signals of 1b-3b and 5b are also given in Figure 6 by
the blue lines for comparison with those of [Fe(OETArP)-
(HIm)2]+. For each signal, the substituent effect on the
chemical shifts is considerably much larger in [Fe(OETArP)-
(tBuNC)2]+ than in [Fe(OETArP)(HIm)2]+. The isotropic shift
of the CH2 signal has increased from +4.72 to +9.31 ppm
with the increase in Hammett σ value as shown in Figure
6a. Correspondingly, the isotropic shift of the CR signal has
decreased from -20 to -38 ppm as shown in Figure 6b.
The isotropic shift of the meso-C signal has also decreased
from 311 to 76 ppm as shown in Figure 6c on going from
1b to 5b. Thus, the Hammett plot shows a large negative
slope, -220 ppm, as compared with +5.4 ppm in [Fe-
(OETArP)(HIm)2]+. The results suggest that the spin density
of the â-carbons gradually increases while that of the meso-
carbons drastically decreases with the increase in Hammett
σ value of the meso-phenyl substituents. If the dxy-a1u-type
interaction is predominant in these complexes, we can expect
that the isotropic shifts of the meso-carbons should exhibit
little substituent dependence since the a1u orbital has a zero
coefficient at the meso-carbon positions. Furthermore, the
dxy-a1u-type interaction should induce a large upfield shift
of the meso signal relative to that of the corresponding
diamagnetic complex. However, even the complex with
electron-withdrawing groups such as 3b has exhibited the
meso signal at 280 ppm, which is by ca. 160 ppm more
downfield than that of the diamagnetic complex. Thus, we
have concluded that the major interaction in [Fe(OETArP)-
(tBuNC)2]+ occurs between the half-filled dxy and filled a2u-
like orbitals, not between the dxy and a1u-like orbitals.
It is noteworthy that the isotropic shift of the CR signal
changes from -20 to -40 ppm on going from 1b to 5b, which
clearly indicates that the spin density at the â-pyrrole
positions gradually increases with an increase in the Hammett
σ value. This is because the electron-withdrawing substituent
at the meso-phenyl group lowers the energy level of the a2u-
like orbital, weakens the dxy-a2u interaction, and conse-
quently stabilizes the dxy orbital relative to the dπ orbitals.
This is most explicitly demonstrated in the energy gap
diagram in Figure 8 calculated on the basis of the EPR g
values. The tetragonality parameter, |∆/λ|, tends to decrease
on going from 1b to 4b; these paramaters are 8.05, 6.93, 7.00,
and 2.53, respectively. In the case of 5b, the energy levels
of the dxy and dπ orbitals have been reversed, as revealed by
the rhombic EPR spectrum. Small energy gap between the
dxy and dπ orbitals can also be seen from the curvatures in
the Curie plots of these complexes as shown in Figure 4B,
which should be ascribed to the contribution from the
thermally accessible excited state.3 As shown in Figure S9
(Supporting Information), DFT calculation of a series of
analogous Zn(OETArP) complexes (1Zn-4Zn) has also
indicated the stabilization of the a2u-like orbital due to the
electron-withdrawing groups at the phenyl ring; the energy
levels of the a2u-like HOMO are -4.05, -4.23, and -4.68
eV for 1Zn-3Zn, respectively. Obviously, there are some
discrepancies in the relationship between the |∆/λ| values
and the HOMO energy levels in 3Zn and 4Zn. While the |∆/
λ| value in 4b is much smaller than that in 3b, the HOMO
energy level of 4Zn is higher than that of 3Zn.
As we have pointed out in a previous paper, some low-
spin complexes such as [Fe(TArP)(CN)2]- exhibit two types
of EPR signals, i.e., axial and large gmax-type signals, in
solution at extremely low temperatures.5,53 We have ex-
plained the result in terms of the existence of two isomeric
complexes adopting different electron configurations; one
is a ruffled complex with the (dxz, dyz)4(dxy)1 electron
configuration and the other is a planar complex with the
(dxy)2(dxz, dyz)3 electron configuration as shown in eq 1.5,53
The interconversion of the two isomers is fast on the NMR
(dxz, dyz)4(dxy)1 a (dxy)2(dxz, dyz)3
δobs ) pxyδxy + pπδπ
(1)
(2)
time scale above 173 K where the NMR spectra are taken,
while it is slow on the EPR time scale at 4.2 K where the
EPR spectra are taken. During the preparation of this paper,
a paper written by Scheidt et al. has appeared, expressing a
different explanation for the presence of two types of EPR
signals.54,55 On going from 1b to 4b, the population of the
isomer with the (dxy)2(dxz, dyz)3 ground state should increase.
1
The observed chemical shifts of the H and 13C signals in
low-spin complexes can then be expressed by eq 2 where
pxy and pπ are the populations and δxy and δπ are the chemical
shifts of the isomers with the (dxz, dyz)4(dxy)1 and (dxy)2(dxz,
dyz)3 ground state, respectively. Since the CR and meso signals
of the (dxz, dyz)4(dxy)1 isomer appear more downfield than
those of the (dxy)2(dxz, dyz)3 isomer, the increase in population
of the (dxy)2(dxz, dyz)3 isomer results in the upfield shift of
these signals.
Interestingly, the isotropic shift of CR in 4b is rather close
to that of 4a, though the electronic ground states of these
complexes are different; δiso ) -41 and -47 ppm for 4b
and 4a, respectively. This does not necessarily mean that the
population of the (dxy)2(dxz, dyz)3 isomer in 4b is close to that
of 4a. The result should rather be explained in terms of the
effective spin delocalization in the (dxy)2(dxz, dyz)3 isomer of
the bis(tBuNC) complex as compared with that of the bis-
(HIm) complex. The interaction between the dπ and 4eg*-
like orbitals could also be considered since the 4eg*-like
orbital is stabilized due to the presence of electron-withdraw-
ing aryl groups at the meso positions as deduced from the
energy-level diagrams of analogous Zn(OETArP). As men-
tioned, we have recently reported that [Fe(DAzP)(tBuNC)2]+,
(53) Ikezaki, A.; Nakamura, M. Inorg. Chem. 2002, 41, 2761-2768.
(54) Regarding eq 1 in our previous paper,53 Scheidt et al. described that
the interconversion of two conformers on the EPR timescale is not
necessary and even unlikely, given the low temperature of the
measurement.55 In the previous paper, we have described that the
mutual exchange between the two species in eq 1 is slow on the EPR
timescale, which indicates that the rate constant for interconversion
is much smaller than 109 s-1; it could be 10-5 s-1 or less.
(55) Li, J.; Noll, B. C.; Schulz, C. E.; Scheidt, W. R. Inorg. Chem. 2007,
46, 2286-2298.
8204 Inorganic Chemistry, Vol. 46, No. 20, 2007

Highly Saddled Low-Spin Iron(III) Porphyrin Complexes
which adopts the (dxy)2(dxz, dyz)3 ground state at ambient
temperature despite the coordination of BuNC, shows an
with a negligibly small substituent effect because the a1u
orbital has a zero coefficient at the meso-carbons and a
relatively large coefficient at the pyrrole R-carbons in D4h
iron(III) porphyrinates. Thus, we have concluded that the
half-occupied dxy orbitals in low-spin saddle-shaped bis(t-
BuNC) complexes interact mainly with the a2u-like orbital;
the interaction with the a1u-like orbital should be negligibly
small, though the dxy-a1u interaction is symmetry-allowed
in saddle-shaped complexes. Smaller spin delocalization to
the meso-carbon atoms in the complexes with electron-
withdrawing groups such as 3b and 4b is then ascribed to
the decrease in spin population in the dxy orbital due to a
smaller energy gap between the dxy and dπ orbitals. In fact,
the energy levels of the dxy and dπ orbitals are completely
reversed in 5b, which has strongly electron-withdrawing CF3
groups at the 3- and 5-positions of the meso-phenyl group.
It should be emphasized here that a negligibly small
interaction between half-occupied dxy and porphyrin a1u-like
t
effective spin delocalization to the macrocycle as compared
with [Fe(DAzP)(DMAP)2]+, although the latter complex also
adopts the (dxy)2(dxz, dyz)3 ground state.52 The chemical shifts
of the R-CH2 signals of the butyl groups are -35.4 and
-21.8 ppm for the bis(tBuNC) and bis(DMAP) complexes,
respectively. Correspondingly, the spin densities at the
pyrrole â carbon atoms are determined to be 0.016 and 0.011
in the former complex as compared with 0.010 and 0.0094
in the latter one. Another example showing the effective spin
delocalization in the bis(tBuNC) complex was [Fe(OEC)(t-
BuNC)2]+ reported by Cai et al.56 This complex is quite
unique because the pyrroline-H signal appears at 128.3 ppm
while the meso-H signals resonate at -71.7 and -43.0 ppm.
Extremely large downfield and upfield shifts of the peripheral
proton signals have been interpreted in terms of the equi-
librium between planar complex with the (dxy)2(dxz, dyz)3
ground state and the ruffled complex with the (dxz, dyz)4(dxy)1
ground state; the (dxz, dyz)4(dxy)1 isomer shifts the meso-H
signal to the upfield positions while the (dxy)2(dxz, dyz)3 isomer
induces the downfield shift of the pyrroline-H signal. Since
the chemical shift of the pyrroline-H signal in [Fe(OEC)-
(HIm)2]+, which is a typical complex with the (dxy)2(dxz, dyz)3
ground state, is 12.0 ppm, the large downfield shift in [Fe-
(OEC)(tBuNC)2]+ is a direct indication that the spin delo-
calization from the metal dπ to the porphyrin pπ orbitals is
quite effective.
1
orbitals in saddle-shaped low-spin (S ) /2) iron(III) por-
phyrinates does not necessarily rule out the possibility of
strong dxy-a1u interaction in the corresponding intermediate-
3
spin (S ) /2) [Fe(OETPP)(THF)2]+ complex as proposed
by Cheng and co-workers on the basis of the theoretical
calculation.24 Furthermore, Rivera and co-workers have
reported that a heme-containing enzyme should adopt the S
) 3/2, (dxz)3(dyz)1(dz2)1 electron configuration on the basis of
the 13C NMR data.57 We are now applying a similar
methodology to a series of intermediate-spin [Fe(OETArP)-
(THF)2]+ complexes to reveal the nature of the metal-
porphyrin orbital interactions in the intermediate-spin
complexes.
Conclusion
Hammett plots of the isotropic shifts of the CH2, CR, and
meso-C signals in highly saddled low-spin [Fe(OETArP)-
(HIm)2]+ (1a-5a) and [Fe(OETArP)(tBuNC)2]+(1b-5b), where
Ar represents substituted meso-phenyl groups, have been
examined to reveal if the symmetry-allowed interaction
between iron dxy and porphyrin a1u-like orbitals is strong
enough to affect the chemical shifts of the complexes. EPR
spectra have revealed that all the bis(HIm) complexes
examined in this study adopt the (dxz, dyz)3(dxy)2 ground state.
These complexes have exhibited a fairly small substituent
effect on the chemical shifts because the half-occupied dπ
orbitals carrying unpaired electrons interact with the por-
phyrin 3eg-like orbitals having a zero coefficient at the meso-
carbon positions. In the case of the bis(tBuNC) complexes,
the EPR spectra have revealed that all the complexes except
5b adopt the (dxz, dyz)4(dxy)1 ground state; 5b is actually the
first iron(III) porphyrinate that shows the (dxz, dyz)3(dxy)2
ground state despite the coordination of tBuNC. The chemical
shifts of these complexes have shown a large substituent
dependence as compared with those of bis(HIm) complexes.
The slopes of Hammett plots for the meso-C isotropic shifts
are -220 and +5.4 ppm for bis(tBuNC) and bis(HIm)
complexes, respectively. If the half-occupied dxy orbital in
bis(tBuNC) complexes interacts with the a1u-like orbital, we
can expect a large upfield shift of the meso-C signal together
Experimental Section
Spectral Measurements. UV-vis spectra were measured on a
Shimadzu MultiSpec-1500 spectrophotometer at ambient temper-
ature. Fast atom bombardment mass spectrometry (FAB-MS)
spectra were measured on a JEOL JMS-600H device using
3-nitrobenzyl alcohol as the matrix. 1H and 13C NMR spectra were
recorded on a JEOL LA300 spectrometer operating at 300.4 MHz
1
for H. Chemical shifts were referenced to the residual peak of
dichloromethane (δ ) 5.32 ppm for H and 53.8 ppm for 13C).
1
EPR spectra were measured at 4.2-20 K with a Bruker E500 or
Bruker EMX plus spectrometer operating at the 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. 1. Zn(OETArP). Free base porphyrins, H2(OETArP),
where Ar is substituted phenyls, were prepared by the condensation
reactions between 3,4-diethylpyrrole and substituted benzaldehydes
according to the literature.58,59 The free base porphyrins were
converted to the corresponding zinc complexes Zn(OETArP) for
the characterization. The 1H and 13C NMR chemical shifts of these
complexes are listed in Tables 4 and 6. UV-vis spectra (CH2Cl2,
λ
max nm) and FAB-MS: 1Zn, 457, 588, 646 nm; m/e ) 1020 (M+).
(57) Zeng, Y.; Caignan, G. A.; Bunce, R. A.; Rodriguez, J. C.; Wilks, A.;
Rivera, M. J. Am. Chem. Soc. 2005, 127, 9794-9807.
(58) Evans, B.; Smith, K. M.; Fuhrhop, J.-H. Tetrahedron Lett. 1977, 18,
443-446.
(59) 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.
(56) Cai, S.; Lichtenberger, D. L.; Walker, F. A. Inorg. Chem. 2005, 44,
1890-1903.
Inorganic Chemistry, Vol. 46, No. 20, 2007 8205

Ohgo et al.
3
Zn, 459, 592, 648 nm; m/e ) 1172 (M+), 1144 (M+ - 28). 4Zn
,
temperature. A purple prismatic crystal with dimensions of 0.2 ×
0.1 × 0.1 mm thus obtained was used for the diffraction experiment.
Crystal data and experimental and refinement details for crystal
structure determination are summarized in Table 1. The asymmetric
unit contains one porphyrin, one chloride counterion, four chloro-
form molecules as crystal solvent, and a half molecule of cyclo-
hexane on the center of symmetry. The distances and angles in the
cyclohexane molecule were fixed to idealized values. Two of the
four chloroform molecules were highly disordered to have two or
three sites in the structure. The site occupancy for each chloroform
molecule was refined to be 0.65(1)/0.35(1) and refined and fixed
to be 0.44/0.40/0.16. One of the ethyl groups at the Py-â position
was also disordered over two positions with population of 0.57-
(2)/0.43(2). The second-extinction effect was corrected by the
SHELXL method. The extinction coefficient was refined to be
0.016(2).
[Fe(OETArP)(HIm)2]ClO4‚CH2Cl2(3a); Ar ) 4-CF3Ph. The
purple solid of the bis(THF) complex, [Fe(OETArP)(THF)2]ClO4,
Ar ) 4-CF3Ph, was dissolved in a CH2Cl2/heptane solution
containing 4-10 equiv of imidazole, and the solution was allowed
to stand at room temperature. A purple platelet crystal with
dimensions of 0.2 × 0.2 × 0.1 mm thus obtained was used for the
diffraction experiment. The TWIN refinement was applied (Flack
ø parameter ) 0.400). The asymmetric unit contains one porphyrin,
one perchlorate counterion, and one dichloromethane as crystal
solvent. The second-extinction effect was corrected by the SHELXL
method, and the coefficient was refined to be 0.007(1). All non-
hydrogen atoms were refined anisotropically.
468, 601 nm; m/e ) 1172 (M+). 5Zn, 458, 588, 674 nm; m/e )
1416 (M+ - 28).
2. Fe(OETArP)Cl. The insertion of iron to H2(OETArP) was
carried out using FeCl2‚4H2O in refluxing CHCl3-CH3OH (3:1)
solution under argon atmosphere.21,60 The reaction mixture was
treated with diluted HCl solution. The organic layer was separated
and dried over sodium sulfate. Evaporation of the solvents yielded
a dark brown solid, which was purified by chromatography on silica
gel using CH2Cl2-CH3OH as eluents. 1H NMR(CD2Cl2, 298 K, δ
ppm). Ar ) 4-OMePh: 45.3 (4H, CH2); 37.7 (4H, CH2); 34.6 (4H,
CH2); 23.0 (4H, CH2); 3.58 (12H, CH3); 1.41 (12H, CH3); 11.11
(o-H, 4H); 8.54 (o-H, 4H); 12.50 (m-H, 4H); 12.27 (m-H, 4H);
5.52 (p-OCH3, 12H). Ar ) Ph: 46.0 (4H, CH2); 38.3 (4H, CH2);
35.4 (4H, CH2); 23.8 (4H, CH2); 3.65 (12H, CH3); 1.87 (12H, CH3);
10.57 (o-H, 4H); 8.15 (o-H, 4H); 13.00 (m-H, 4H); 12.87 (m-H,
4H); 7.1 (p-H, 4H). Ar ) 4-CF3-Ph: 46.1 (4H, CH2); 41.3 (4H,
CH2); 36.5 (4H, CH2); 26.1 (4H, CH2); 4.23 (12H, CH3); 1.75 (12H,
CH3); 10.27 (o-H, 4H); 7.87 (o-H, 4H); 13.63 (m-H, 4H); 13.54
(m-H, 4H). Ar ) 3,5-(CF3)2-Ph: 45.9 (4H, CH2); 43.4 (4H, CH2);
38.0 (4H, CH2); 28.5 (4H, CH2); 4.69(12H, CH3); 2.27(12H, CH3);
10.21(o-H, 4H); 7.95 (o-H, 4H); 7.19 (p-H, 4H). Ar ) 2,6-Cl2-
Ph: 43.4 (4H, CH2); 39.4 (4H, CH2); 35.3 (4H, CH2); 25.1 (4H,
CH2); 4.18 (12H, CH3); 2.45 (12H, CH2); 13.42 (m-H, 4H); 13.14
(m-H, 4H); 7.70 (p-H, 4H).
3. [Fe(OETArP)(HIm)2]Cl (1a-5a). To a CD2Cl2 solution of
Fe(OETArP)Cl placed in an NMR sample tube was added 4-6
equiv of imidazole. The H NMR spectrum of the sample thus
1
[Fe(OETArP)(HIm)2]ClO4‚(C6H6)0.5(CH2Cl2) (4a); Ar ) 2,6-
Cl2Ph. The purple solid of the bis(THF) complex, [Fe(2,6-Cl2-
OETPP)(THF)2]ClO4, was dissolved in a CH2Cl2/benzene/heptane
solution containing 4-10 equiv of imidazole, and the solution was
allowed to stand at room temperature. A purple platelet crystal with
dimensions of 0.2 × 0.2 × 0.1 mm thus obtained was used for the
diffraction experiment. The asymmetric unit contains one porphyrin,
one perchlorate counterion, one dichloromethane as crystal solvent,
and a half of a benzene molecule on the center of symmetry. The
distances and angles in the benzene molecule were fixed to idealized
values. One of the ethyl groups at the Py-â position is also
disordered over two positions with population of 0.56(1)/0.44(1).
Non-hydrogen atoms except for the benzene molecule were refined
anisotropically.
formed clearly showed the formation of low-spin bis(imidazole)
complexes (1a-5a). The 1H and 13C NMR chemical shifts are listed
in Tables 4 and 6, respectively. Samples for EPR measurements
were similarly prepared by the addition of 6-10 equiv of imidazole
into a CH2Cl2 solution of Fe(OETArP)Cl. The g values are given
in Table 6.
4. [Fe(OETArP)(tBuNC)2]Cl (1b-5b). These complexes were
t
obtained by the addition of 4-6 equiv of BuNC to a CD2Cl2
solution of [Fe(OETArP)(THF)2]ClO4; the latter complexes were
prepared by the addition of AgClO4 to Fe(OETArP)Cl in THF
solution. The 1H and 13C NMR chemical shifts are listed in Tables
4 and 6, respectively. Samples for EPR were similarly prepared by
t
the addition of 6-10 equiv of BuNC into a CH2Cl2 solution of
[Fe(OETArP)(THF)2]ClO4. The g values are given in Table 6.
2. Data Collection. The diffraction data of 1a were collected at
90 K on a Rigaku AFC-8 diffractometer equipped with a Saturn
70 CCD area detector with focused and monochromated Mo KR
radiation (λ ) 0.71073 Å) by a confocal mirror and a rotating anode
generator. The data were collected to a maximum 2θ value of 65.1°.
A total of 1980 images was collected with three different goniometer
settings, and the total oscillation angle of each setting is 330°
corresponding to 660 images, for 1a. The exposure time was 1 s
per degree in ω. The crystal-to-detector distance was 40.09 mm.
The readout was performed in the 0.285714 mm pixel mode. Data
X-ray Crystallographic Analysis. 1. Preparation of Crystals
and Experimental Details. [Fe(OETArP)(HIm)2]ClO4‚CH2Cl2-
(1a); Ar ) 4-OMePh. The purple solid of bis(THF) complex, [Fe-
(OETArP)(THF)2]ClO4, Ar ) 4-OMePh, was dissolved in a
CH2Cl2/heptane solution containing 4-10 equiv of imidazole, and
the solution was allowed to stand at room temperature. A purple
platelet crystal with dimensions of 0.51 × 0.40 × 0.12 mm thus
obtained was used for the diffraction experiment. Crystal data and
experimental and refinement details for crystal structure determi-
nation are summarized in Table 1. The asymmetric unit contains
one porphyrin, one perchlorate counterion, and one dichloro-
methane as crystal solvent. All non-hydrogen atoms were refined
anisotropically.
were processed by the CrystalClear SM program package.53
A
numerical absorption correction was applied.
Diffraction data of 2a, 3a, and 5a were collected at 298 K on a
Rigaku RAXIS-RAPID Imaging Plate diffractometer with graphite
monochromated Mo KR radiation (λ ) 0.71073 Å) and a rotating
anode generator. The indexing was performed from three oscilla-
tions, which were exposed for 0.8 min for 3a and 5a and for 2.5
min for 2a. The data were collected at 298 K to a maximum 2θ
value of 55°. A total of 86, 78, and 192 images, corresponding to
the 388.5°, 390.0°, and 384° oscillation angles, was collected with
three different goniometer settings for 2a, 3a, and 5a, respectively.
[Fe(OETArP)(HIm)2]Cl‚(C6H12)0.5‚(CHCl3)4 (2a); Ar ) Ph.
The purple solid of the chloride complex, Fe(OETPP)Cl, was
dissolved in a CHCl3/cyclohexane solution containing 4-10 equiv
of imidazole, and the solution was allowed to stand at room
(60) 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.
8206 Inorganic Chemistry, Vol. 46, No. 20, 2007

Highly Saddled Low-Spin Iron(III) Porphyrin Complexes
The exposure time was 2.00 min per degree for 2a and 1.00 min
per degree for 3a and 5a. The camera radius was 127.40 mm. The
readout was performed in the 0.100 mm pixel mode. Data were
processed by the PROCESS-AUTO program package. A symmetry-
related absorption correction was carried out by using the program
ABSCOR.62
lography were used for the calculation. All the frontier orbitals
together with the corresponding energy levels were calculated with
the generalized gradient approximation (GGA) of B88-PW9166-69
and a double-ú valence polarization (DZVP) basis set.70
Acknowledgment. This work was supported by Research
Promotion Grants from the Toho University Graduate School
of Medicine (Nos. 05-21 and 06-01 to Y.O.) and by Grant-
in-Aid for Scientific Research from the Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan (No.
18655025 to Y.O. and No. 16550061 to M.N.). This work
was also supported by the Research Center for Materials with
Integrated Properties, Toho University. We also thank the
the Research Center for Molecular-Scale Nanoscience and
the Institute for Molecular Science (IMS).
3. Structure Solution and Refinement. The structures were
solved by direct methods with the program SIR2004.63 The structure
refinement was carried out by the full-matrix least-squares refine-
ment with SHELXL-97.64 All the hydrogen atoms were calculated.
The positional parameters of the H atoms were constrained to have
C-H distances of 0.96 Å for primary, 0.97 Å for secondary, and
0.93 Å for aromatic H atoms. The H-atom U values were con-
strained to have 1.2 times the equivalent isotropic U values of their
attached atoms (1.5 for methyl groups). The atomic scattering fac-
tors were taken from the International Tables for Crystallography.
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge Crys-
tallographic Data Centre as supplementary publication Nos. CCDC-
635316-635319. Copies of the data can be obtained free of charge
on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ,
U.K. (fax: (+44) 1223 336-033; e-mail: deposit@ccdc.cam.ac.uk).
Computational Details. DFT calculations of a series of Zn-
(OETArP) complexes were carried out using the Scigress Explorer
DFT, version 7, program.65 The geometries of the corresponding
[Fe(OETArP)(HIm)2]+ complexes determined by X-ray crystal-
Supporting Information Available: Figures S1-S4, respective
packing diagrams of 1a-4a; Figure S5, ORTEP diagrams of 2a;
Figure S6, ORTEP diagrams of 3a; Figure S7, ORTEP diagrams
of 4a; Figure S8, simulation of the observed EPR spectra; Figure
S9, energy levels of porphyrin π orbitals in Zn(OETArP) (1Zn
-
4Zn) determined by DFT calculation. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC700827W
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