Benzoannelation stabilizes the dxy 1 state of low-spin iron(III) porphyrinates

Article abstract of DOI:10.1021/ic1024873

A series of low-spin, six-coordinate complexes [Fe(TBzTArP)L₂]X (1) and [Fe(TBuTArP)L₂]X (2) (X=Cl^-, BF₄ ^-, or Bu₄N⁺), where the axial ligands (L) are HIm, 1-MeIm, DMAP, 4-MeOPy, 4-MePy, Py, and CN^-, were prepared. The electronic structures of these complexes were examined by ¹H NMR and electron paramagnetic resonance (EPR) spectroscopy as well as density functional theory (DFT) calculations. In spite of the fact that almost all of the bis(HIm), bis(1-MeIm), and bis(DMAP) complexes reported previously (including 2) adopt the (d_xy)²(d_xz, d_yz) ³ ground state, the corresponding complexes of 1 show the (d _xz, d_yz)⁴(d_xy)¹ ground state at ambient temperature. At lower temperature, the electronic ground state of the HIm, 1-MeIm, andDMAP complexes of 1 changes to the common (d _xy)²(d_xz, d_yz)³ ground state. All of the other complexes of 1 and 2 carrying 4-MeOPy, 4-MePy, Py, and CN^- maintain the (d_xz, d_yz)⁴(d _xy)¹ ground state in the NMR temperature range, i.e., 298-173 K. The EPR spectra taken at 4.2 K are fully consistent with the NMRresults because theHImand 1-MeIm complexes of 1 and 2 adopt the (d _xy)²(d_xz, d_yz)³ ground state, as revealed by the rhombic-type spectra. The DMAP complex of 1 exists as a mixture of two electron-configurational isomers. All of the other complexes adopt the (dxz, dyz)4(dxy)1 ground state, as revealed by the axialtype spectra. Among the complexes adopting the (d_xz, d_yz) ⁴(d_xy)¹ ground state, the energy gap between the dxy and dp orbitals in 1 is always larger than that of the corresponding complex of 2. Thus, it is clear that the benzoannelation of the porphyrin ring stabilizes the (d_xz, d_yz)⁴(d_xy) ¹ ground state. The DFT calculation of the bis(Py) complex of analogous iron(III) porphyrinate, [Fe(TPTBzP)(Py)₂]⁺, suggests that the (d_xz, d_yz)⁴(d _xy)¹ state is more stable than the (d_xy) ²(d_xz, d_yz)³ state in both ruffled and saddled conformations. The lowest-energy states in the two conformers are so close in energy that their ordering is reversed depending on the calculation methods applied.On the basis of the spectroscopic and theoretical results, we concluded that 1, having 4-MeOPy, 4-MePy, and Py as axial ligands, exists as an equilibrium mixture of saddled and ruffled isomers both of which adopt the (d_xz, d_yz)⁴(d_xy)₁ ground state. The stability of the (d_xz, d_yz)⁴(d _xy)¹ ground state is ascribed to the strong bonding interaction between the iron d_xy and porphyrin a1u orbitals in the saddled conformer caused by the high energy of the a1u highest occupied molecular orbital in TBzTArP. Similarly, a bonding interaction occurs between the dxy and a2u orbitals in the ruffled conformer. In addition, the bonding interaction of the dp orbitals with the low-lying lowest unoccupied molecular orbital, which is an inherent characteristic of TBzTArP, can also contribute to stabilization of the (dxz, dyz)4(dxy)1 ground state.

Full text of DOI:10.1021/ic1024873

ARTICLE
pubs.acs.org/IC
1
Benzoannelation Stabilizes the dxy State of Low-Spin Iron(III)
Porphyrinates
Takahisa Ikeue,*,† Makoto Handa,† Adam Chamberlin,‡ Abhik Ghosh,*,‡ Owendi Ongayi,§
M. Grac-a H. Vicente,*,§ Akira Ikezaki,^ and Mikio Nakamura*,^,z
†Department of Chemistry, Faculty of Material Science, Shimane University, 1060 Nishikawatsu-cho, Matsue-shi, Shimane 690-8504,
Japan
‡Department of Chemistry and the Center for Theoretical and Computational Chemistry, University of Tromso, Breivika, N-9037
Tromso, Norway
§Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States
^Department of Chemistry, School of Medicine, Toho University, Ota-ku, Tokyo 143-8540, Japan
zDivision of Chemistry, Graduate School of Science, Toho University, Funabashi 274-8510, Japan
S Supporting Information
b
ABSTRACT: A series of low-spin, six-coordinate complexes
[Fe(TBzTArP)L2]X (1) and [Fe(TBuTArP)L2]X (2) (X = Cl-,
BF4-, or Bu4Nþ), where the axial ligands (L) are HIm, 1-MeIm,
DMAP, 4-MeOPy, 4-MePy, Py, and CN-, were prepared. The
electronic structures of these complexes were examined by 1H
NMR and electron paramagnetic resonance (EPR) spectrosco-
py as well as density functional theory (DFT) calculations. In
spite of the fact that almost all of the bis(HIm), bis(1-MeIm),
and bis(DMAP) complexes reported previously (including 2)
adopt the (dxy)2(dxz, dyz)3 ground state, the corresponding
complexes of 1 show the (dxz, dyz)4(dxy)1 ground state at ambient temperature. At lower temperature, the electronic ground state of
the HIm, 1-MeIm, and DMAP complexes of 1 changes to the common (dxy)2(dxz, dyz)3 ground state. All of the other complexes of 1
and 2 carrying 4-MeOPy, 4-MePy, Py, and CN- maintain the (dxz, dyz)4(dxy)1 ground state in the NMR temperature range, i.e.,
298-173 K. The EPR spectra taken at 4.2 K are fully consistent with the NMR results because the HIm and 1-MeIm complexes of 1
and 2 adopt the (dxy)2(dxz, dyz)3 ground state, as revealed by the rhombic-type spectra. The DMAP complex of 1 exists as a mixture
of two electron-configurational isomers. All of the other complexes adopt the (dxz, dyz)4(dxy)1 ground state, as revealed by the axial-
type spectra. Among the complexes adopting the (dxz, dyz)4(dxy)1 ground state, the energy gap between the dxy and dπ orbitals in 1 is
always larger than that of the corresponding complex of 2. Thus, it is clear that the benzoannelation of the porphyrin ring stabilizes
the (dxz, dyz)4(dxy)1 ground state. The DFT calculation of the bis(Py) complex of analogous iron(III) porphyrinate,
[Fe(TPTBzP)(Py)2]þ, suggests that the (dxz, dyz)4(dxy)1 state is more stable than the (dxy)2(dxz, dyz)3 state in both ruffled and
saddled conformations. The lowest-energy states in the two conformers are so close in energy that their ordering is reversed
depending on the calculation methods applied. On the basis of the spectroscopic and theoretical results, we concluded that 1, having
4-MeOPy, 4-MePy, and Py as axial ligands, exists as an equilibrium mixture of saddled and ruffled isomers both of which adopt the
(dxz, dyz)4(dxy)1 ground state. The stability of the (dxz, dyz)4(dxy)1 ground state is ascribed to the strong bonding interaction
between the iron dxy and porphyrin a1u orbitals in the saddled conformer caused by the high energy of the a1u highest occupied
molecular orbital in TBzTArP. Similarly, a bonding interaction occurs between the dxy and a2u orbitals in the ruffled conformer. In
addition, the bonding interaction of the dπ orbitals with the low-lying lowest unoccupied molecular orbital, which is an inherent
characteristic of TBzTArP, can also contribute to stabilization of the (dxz, dyz)4(dxy)1 ground state.
’ INTRODUCTION
axial ligands must be of primary importance. For example, axial
ligands with strong σ-donating and weak π-accepting ability such
as imidazole (HIm) and 4-(N,N-dimethylamino)pyridine
(DMAP) stabilize the (dxy)2(dxz, dyz)3 ground state. In contrast,
Six-coordinate, low-spin iron(III) porphyrinates adopt either
the common (dxy)2(dxz, dyz)3 or the less common (dxz, dyz)4(dxy)1
ground state, as shown in Scheme 1, depending on various factors
including the nature and number of axial ligands, electronic
effects of peripheral substituents, deformation of the porphyrin
ring, etc.1-8 Among these factors, the nature and number of the
Received: December 13, 2010
Published: March 16, 2011
r
2011 American Chemical Society
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|

Inorganic Chemistry
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1
axial ligands with strong π-accepting ability such as tert-butyliso-
cyanide (tBuNC) and 4-CNPy stabilize the dπ orbital by dπ-pπ*
interactions and lead to the formation of complexes with the less
common (dxz, dyz)4(dxy)1 ground state.9-13 The latter ground
state is further stabilized14-22 by the ruffled deformation of the
porphyrin ring.14-27 This is because the iron dxy orbital is destabi-
lized relative to the dπ orbitals because of an interaction between the
porphyrin a2u and iron dxy orbitals, which is symmetry-allowed in a
ruffled D2d framework.28,29 As an extension of our long-term
studies of unusual electronic structure of ion porphyrins and
Similarly, the H NMR spectra of series [Fe(TBuTArP)L2](
(2; where TBuTArPP2- = dianion of 2:3,7:8,12:13,17:18-tetra-
butano-5,10,15,20-tetrakis(4-nonylphenyl)porphyrin] are given in
1
porphyrinoids, we examined the H NMR and electron para-
magnetic resonance (EPR) spectra of low-spin, six-coordinate
[Fe(TBzTArP)L2]X (1) and compared the spectroscopic data
with those of the corresponding [Fe(TBuTArP)L2]X (2) and
[Fe(OETPP)L2]X (3; where OETPP2- = dianion of 2,3,7,8,12,
13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin) to reveal the
effect of benzoannelation on the electronic structure of low-spin
iron(III) complexes, where the axial ligands (L) examined are
HIm (a), 1-methylimidazole (1-MeIm; b), DMAP (c), 4-meth-
oxypyridine (4-MeOPy; d), 4-methylpyridine (4-MePy; e),
pyridine (Py; f), 4-CNPy (g), and CN- (h) and the counteran-
ion or cation (X) is Cl-, BF4-, or Bu4Nþ (Scheme 2).
’ RESULTS
1H NMR Spectra. Figure 1 shows the 1H NMR spectra of series
[Fe(TBzTArP)L2]( [1; L = HIm, DMAP, 4-MeOPy, and CN-,
where TBzTArP2- = dianion of 2:3,7:8,12:13,17:18-tetrabenzo-
5,10,15,20-tetrakis(4-nonylphenyl)porphyrin] taken in CD2Cl2
1
solutions at 233 K. The H NMR spectra of other complexes
are given in Figure S1 of the Supporting Information (SI).
Scheme 1. Two Types of Electronic Ground States
Figure 1. 1H NMR spectra of 1a, 1c, 1d, and 1h taken in a CD2Cl2
solution at 233 K. Resonances signified as o, m, p, R, β, C, F, and S
indicate the o-H, m-H, p-CH2(R), R-H, β-H, coordinated ligand H, free
ligand H, and solvent signals, respectively. B indicates the butyl H of
tetrabutylammonium cyanide.
Scheme 2. Complexes 1 and 2 Examined in This Study Together with the Labels for Some Carbon Positionsa
a Axial ligands (L) are as follows: a, HIm; b, 1-MeIm; c, DMAP; d, 4-MeOPy; e, 4-MePy; f, Py; g, 4-CNPy; h, CN-. The countercation or -anion (X) is
Cl-, BF4-, or Bu4Nþ.
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1
Table 1. H NMR Chemical Shifts Determined in a CD2Cl2 Solution at 298 K (Upper) and 213 K (Lower)
chemical shifts (δ, ppm)
L
T (K)
R
β
ortho
meta
p-CH2
liganda
δm - δo
slopeb
[Fe(TBzTArP)L2]( (1)
HIm (a)
298
213
298
213
298
213
298
213
298
213
298
213
298
213
7.77
6.32
5.85
3.89
6.31
5.74
6.94
6.76
8.68
9.97
8.78
10.17
8.93
10.31
8.60
9.86
5.27
4.37
5.18
4.33
4.99
4.26
4.24
3.18
3.91
2.77
3.66
2.32
5.35
4.26
9.85
9.47
3.54
3.23
3.70
3.73
3.73
3.90
4.23
4.61
4.17
4.66
4.22
4.53
4.04
4.62
4.62
5.10
5.14
6.53
5.49
7.05
7.54
10.76
8.29
11.47
8.82
12.16
5.84
8.62
þ f -
þ f -
þ f -
þ
-21.31
-21.85
-29.91
-28.10
-26.30
-24.20
-5.10
-2.82
1-MeIm (b)
DMAP (c)
4-MeOPy (d)
4-MePy (e)
Py (f)
8.15
10.32
10.86
10.48
11.31
11.78
13.94
12.20
14.24
12.48
14.48
11.16
12.88
8.13
-6.02
-10.46 (3.73)
-6.41 (6.87)
-7.11 (0.03)
-7.80 (-3.91)
-7.29
8.79
9.28
9.92
11.63
9.78
þ
11.53
9.76
þ
11.45
10.05
11.54
CN- (h)
þ
[Fe(TBuTArP)L2]( (2)
HIm (a)
298
213
298
213
298
213
298
213
298
213
298
213
298
213
298
213
29.67
28.68
29.84
31.20
24.19
28.05
29.12
24.00
28.59
20.18
28.94c
16.49
68.14c
9.75
0.65
-0.63
0.75
5.77
3.88
5.77
3.99
5.28
3.77
6.25
4.64
6.34
4.63
5.61c
4.34
3.04c
4.18
6.14
5.86
6.60
5.78
6.74
5.81
6.73
6.35
7.76
8.51
8.29
9.47
8.72c
10.53
7.66c
12.80
9.15
10.43
2.35
1.78
2.35
1.83
2.39
1.99
2.85
2.92
3.01
3.15
3.15c
3.54
1.97c
1.99
3.33
3.68
0.83
1.89
0.97
1.82
1.45
2.58
1.51
3.87
1.95
4.84
3.11
6.19
4.62
8.62
3.01
4.77
-
-12.96
-11.56
-16.64
-27.42
-31.06
-5.00
14.96
1-MeIm (b)
DMAP (c)
4-MeOPy (d)
4-MePy (e)
Py (f)
-
-0.38
0.55
(17.31)
8.49(16.68)
-
-0.32
0.90
þ
0.85
1.71
þ
0.85
1.80c
-1.35
-1.30
-1.35
þ
2.76
4-CNPy (g)
CN- (h)
2.10c
3.04
curved
-31.06
9.56
2.04
þ
8.75
2.74
[Fe(OETPP)L2]( (3)e
HIm (a)f,g
DMAP (c)f,g
Py (f)f
298
213
298
213
298
213
298
213
298
213
298
213
298
213
7.34h
4.98h
8.26h
7.93h
21.68h
16.93h
27.75h
39.15h
6.87
0.86
1.34
4.97
2.44
5.80
4.50
5.87
4.65
5.77
5.03
5.32
3.98
6.45
5.59
11.05
11.53
5.74
4.68
(6.73)
(5.87)
(6.88)
(5.78)
(10.41)
(7.78)
(12.11)
(14.01)
(6.58)
(5.86)
(6.31)
(6.51)
(11.48)
(12.97)
0.83
2.06
21.66
16.51
32.80
76.16
16.53
14.21
0.83
5.36
0.51
1.42
2.36
-2.87(19.06)
21.15
2.29
-0.39
0.58
13.01
6.96
-7.24
-1.93
-11.14
-15.90
1.04
4-CNPy (g)f,i
CN- (h)j
tBuNCj,k
-0.66
-0.70
0.78
16.46
19.88
5.41
58.43
7.89
0.77
3.49
2.10
7.46
1.09
5.54
(-1.52)
(-3.17)
5.51
9.78
1.15
6.04
5.49
THFi,j
26.13
0.35
15.79
-10.05
-14.38
32.21
1.15
19.06
a Data in parentheses are the chemical shifts of the methyl signals in the coordinating ligand. b Slope of the Curie plots of the m-H signals. þ f -
indicates that the slope changes from þ to - as the temperature is lowered. c Extrapolated value. d See the text. e Data in parentheses are the chemical
shifts of the p-H signal. f Reference 30. g The low-spin complex that adopts the (dxy)2(dxz, dyz)3 ground state. h Averaged chemical shifts of the CH2 (R)
signals. i The complex that adopts the pure S = 3/2 ground state. j Reference 22. k The low-spin complex that adopts the (dxz, dyz)4(dxy)1 ground state.
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ARTICLE
Table 2. EPR g Values and Tetragonal Splitting (Δ/λ) Valuesa
ligand (L)
g values
Δ/λb
ref
[Fe(TArTBzP)L2]( (1)
HIm (a)
2.80
2.85
2.53
2.77
2.49
2.43
2.40
2.39
2.29
2.38
2.40
1.47
1.44
tw
tw
tw
1-MeIm (b)
DMAP (c)c
2.36
4-MeOPy (d)
4-MePy (e)
Py (f)
CN- (h)
CN- (h0) (CH3OH)
1.71
1.77
1.83
1.81
1.90
-3.5
-4.1
-4.5
-4.5
-6.3
tw
tw
tw
tw
tw
[Fe(TArTBuP)L2]((2)
HIm (a)
2.97
3.03
3.0
2.34
2.34
1.53
1.56
tw
tw
tw
1-MeIm (b)
DMAP (c)d
4-MeOPy (d)
4-MePy (e)
Py (f)
2.58
2.53
2.54
2.47
2.50
2.39
tw
tw
tw
tw
tw
4-CNPy (g)
CN- (h)
CN- (h0) (CH3OH)
1.70
1.73
-3.7
-4.0
Figure 2. EPR spectra of (A) 1 and (B) 2 taken in frozen CH2Cl2
solutions at 4.2 K, where a-d correspond to the complexes carrying
HIm, DMAP, 4-MeOPy, and CN- (in MeOH). The impurity peaks are
indicated by asterisks.
[Fe(OETPP)L2]( (3)
HIm (a)
DMAP (c)
Py (f)
4-CNPy (g)e
tBuNCf
2.72
3.06
3.39
4.28
2.29
4.01
2.37
2.14
2.08
3.80
2.25
1.64
1.42
22
30
30
30
22
22
Figure S2a,b of the SI. The signal assignment will be described in
the Discussion section. Table 1 lists the chemical shifts of 1 and 2
determined at 298 and 213 K.
2.08
1.92
2.00
THFe
EPR Spectra. EPR spectra of 1 and 2 were taken in frozen
CH2Cl2 solutions at 4.2 K. Some of them are given in Figure 2.
EPR spectra of other complexes are given in Figure S3a,b of the
SI. As shown in Figure 2A, each spectrum of 1 contains some
signals around g = 2.08 (indicated by asterisks), which are
presumably ascribed to the decomposed species. The intensities
of the impurity signals are much smaller in the case of 2, as shown
in Figure 2B, although some complexes exhibit a sharp signal at
g = 2.01 caused by the organic free radical. In the case of 2c, the
signals are poorly resolved. The g values of the major signals of 1
and 2 are listed in Table 2 together with those of analogous 3.22,30
The tetragonal splitting (Δ/λ) values, which indicate the energy
difference between the dxy and dπ(dxz and dyz) orbitals, are also
listed in Table 2 in units of the spin-orbit coupling constant (λ).
A negative value of Δ/λ indicates that the dxy orbital is located
above the dxz and dyz orbitals.31-33
a Frozen CH2Cl2 solutions at 4.2 K. b Tetragonal parameters where λ is a
spin-orbit coupling constant. Negative values indicate that the dxy orbital
is higher than the dπ orbital. c Two species coexist. See the text. d Signals
are too broad. e The pure intermediate-spin (S = 3/2) complex. f The low-
spin complex that adopts the pure (dxz, dyz)4(dxy)1 ground state.
different states [Fe(TBzTPP)(Py)2]þ for a number of different
exchange-correlation functionals, including two with dispersion
corrections. The four states examined for this complex are all rather
close in energy, and indeed their relative ordering varies somewhat
depending on the DFT method used; thus, on the basis of these
calculations alone, it would be difficult to make an unequivocal
prediction about the actual experimental structure and conforma-
tion. Spectroscopic data and the calculations together, however, do
point toward a preferred state for this complex.
DFT Calculations. DFT calculations28,29,34-38 were carried out
to examine the relative stabilities of the (dxz, dyz)4(dxy)1 and (dxy)2
(dxz, dyz)3 states of both the ruffled and saddled conformations in the
bis(Py) complex of structurally similar iron(III) porphyrinate, i.e.,
[Fe(TBzTPP)(Py)2]þ (where TPP2- = dianion of 5,10,15,20-
tetraphenylporphyrin and TBzTPP2- = dianion of 2:3,7:8,12:13,
17:18-tetrabenzo-5,10,15,20-tetraphenylporphyrin). Intermediate
ruffled-saddled geometries were also examined; however, these
relaxed to either the pure ruffled or the pure saddled geometry. In
addition, we also optimized both the ruffled and saddled con-
formations of four-coordinate nickel(II), copper(II), and zinc(II)
complexes of TBzTPP, all of which exhibited a clear preference for
the saddled conformation. Table 3 presents the energetics of
As shown in Table 3, regardless of whether one assumes a
ruffled or a saddled conformation, the calculations favor a (dxz,
dyz)4(dxy)1 state over a (dxy)2(dxz, dyz)3 state. However, decid-
ing between a ruffled or a saddled conformation is more
difficult. Thus, whereas OLYP,39,40 BP8641,42 and BLYP41-44
all indicate a saddled (dxz, dyz)4(dxy)1 state as the lowest-energy
state, including Grimme’s45 dispersion correction results in a
reversal of the lowest-energy states; thus, BP86-D indicates a
ruffled (dxz, dyz)4(dxy)1 state as the ground state, whereas
BLYP-D predicts almost exactly equienergetic ruffled and
saddled (dxz, dyz)4(dxy)1 states. Considering how delicately
the different conformational and spin states are balanced, one
must be careful indeed with ascribing the stability of a state to a
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Table 3. Electronic Configurations, Conformations, and OLYP/TZP Energies of S = 1/2 [Fe(TBzTPP)(Py)2]þ
energy (eV)
state
A
iron config.
geometry
all-electron occupations (R//β)
OLYP
0.00
BP86
0.00
BP86-D
0.00
BLYP
0.00
BLYP-D
0.00
(dxz, dyz)4(dxy)1
ruffled (D2d)
a1 50//50
a2 18//18
b1 21//21
b2 48//47
e 130//130
a1 98//98
a2 39//39
b1 65//65
b2 65//64
a1 45//45
a2 23//23
b1 28//27
b2 41//41
e 130//130
a1 86//86
a2 51//51
b1 65//65
b2 65//64
B
(dxz, dyz)3(dxy)2
ruffled (C2v)
0.23
0.24
0.26
0.11
0.22
0.24
0.01
C
(dxz, dyz)4(dxy)1
saddled (D2d)
-0.30
-0.10
-0.23
D
(dxz, dyz)3(dxy)2
saddled (C2v)
-0.14
0.05
0.26
-0.06
0.17
Table 4. OLYP/TZP Mulliken Spin Populations for Different
States of S = 1/2 [Fe(TBzTPP)(Py)2]þ
The optimized geometries of the different electronic states are
relatively unremarkable. We will therefore limit ourselves to
briefly describing the ruffled and saddled (dxz, dyz)4(dxy)1 states
(Figure 6), the two nearly equienergetic contenders for the
ground state. Both states deviate quite strongly from planarity.
Thus, adjacent pyrrole rings in the saddled conformation are
tilted by just over 30ꢀ relative to each other, whereas antipodal
pyrrole rings in the ruffled conformation are twisted by over 50ꢀ
relative to each other. As expected, the Fe-NPor distances are
significantly shorter in the ruffled conformation (1.937 Å) than in
the saddled conformation (1.977 Å). Overall, this is a remarkable
and indeed unique example of a porphyrin derivative whose
ruffled and saddled conformations are almost identical in energy.
atom
A
B
C
D
Fe
0.7402
0.0416
0.9389
0.3307
-0.0293
0.0900
0.9266
NPor
CR
-0.0062
0.0128
-0.0176, 0.0116
0.0034, 0.0060
-0.0030, 0.0203
-0.0078
-0.0216
-0.0001
0.0878
Cβ
0.0044
-0.0078
-0.0344
0.0040
Cmeso
Ci-Ph
Co-Ph
Cm-Ph
Cp-Ph
NPy
-0.0001, -0.0052
0.0005
-0.0054
0.0020
0.0008
-0.0001
0.0002
-0.0014
0.0004
-0.0005
-0.0003
0.0002
0.0001
-0.0001
-0.0252
-0.0024
-0.0009
-0.0059
-0.0019
-0.0002
-0.0098
-0.0043
-0.0219
’ DISCUSSION
CR-Py
-0.0012
1
Signal Assignments in H NMR Spectra. The signals were
specific metal-ligand orbital interaction, although we will
attempt to do so.
assigned by the 2D COSY spectra, which are given in Figure S4a-dof
the SI. In the case of 1e, the signal at 2.1 ppm showed the correlation
with the two signals at 1.8 and 4.5 ppm. Thus, the signals at 2.1, 1.8,
and 4.5 ppm were assigned to p-CH2(β), p-CH2(γ), and p-CH2(R),
respectively. The downfield shift of the p-CH2(R) signal is a direct
indication that the meso-C atoms have a sizable amount of spin density,
which should induce the upfield and downfield shifts to the o-H and
m-H signals, respectively. Thus, the two signals at 13.7 and 3.1 ppm,
which exhibited the correlation peak, were assigned to the m-H and o-
H signals, respectively. Another correlation was observed between the
two signals at 11.1 and 9.8 ppm. Thus, they belong to the fused
benzene ring. The assignment of R-H and β-H is quite difficult. We
tentatively assigned the broader and more downfield shifted signal to
R-H because these complexes adopt the (dxz, dyz)4(dxy)1 ground state
as mentioned in the following section; the low-spin complexes with
the (dxz, dyz)4(dxy)1 ground state induces a positive dipolar shift to the
protons on the porphyrin ring.
As shown in Table 4, the ruffled and (dxz, dyz)4(dxy)1 states
exhibit significant positive spin populations on the meso-C
atoms, attributable to the Fe(dxy)-porphyrin(a2u) orbital
interaction, which is shown in Figure 3. Interestingly, the
saddled (dxz, dyz)4(dxy)1 state also has significant meso spin
populations; these, however, are negative spin populations.
This state features an unusually strong Fe(dxy)-porphyrin-
(a1u) orbital interaction, resulting in an exceedingly small Fe
spin population and large quantities of positive spin on the
TBzTPP ligand; this is shown in Table 4 and in Figures 4 and 5.
The strength of this orbital interaction is a result of the high
energy of the TBzTPP a1u highest occupied molecular orbital
(HOMO);46 thus, for closed-shell derivatives such as a free base
or ZnIITBzTPP, the a1u HOMO is far higher in energy (i.e.,
corresponding to a lower ionization potential) than the a2u
HOMO, with the latter being similar in energy to that in an
analogous TPP complex.
Methodology To Determine the Electronic Ground
State. 1. NMR Method. Electronic ground states can be determined
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Figure 3. Selected frontier MOs of the ruffled (dxz, dyz)4(dxy)1 state of [Fe(TBzTPP)]þ (i.e., state C). The spin-unrestricted MOs that best correspond
to a singly occupied MO are indicated by a blue legend.
1
by the H and 13C NMR spectra.2-8 In the complexes with the
(dxy)2(dxz, dyz)3 ground state, the unpaired electron in the dπ orbitals
can be delocalized to the porphyrin ring especially on the pyrrole β-C
atoms and induces the upfield shift of the pyrrole β-H signals. In the
complexes with the (dxz, dyz)4(dxy)1 ground state, the unpaired
electron in the dxy orbital can be delocalized to the ruffled porphyrin
ring especially on the meso-C atoms by interaction with the porphyrin
a2u orbital;2-8 the a2u orbital has a large coefficient at the meso-C
atoms. Thus, we can expect that the o-H and p-H signals appear
upfield, while the m-H signal appears downfield in the complexes
having phenyl groups at the meso positions.
We chose the m-H signal as a probe to reveal the electronic
structure of series 1. The theoretical background is given
below.47-50 The observed chemical shift (δobs) of the m-H
signal can be given by eq 1, where δiso is the isotropic shift and
δdia is the chemical shift of the m-H signal in a suitable
diamagnetic complex. The δiso value, which is the sum of the
contact shift (δcon) and dipolar shift (δdip), can be expressed as
eq 2 in the case of complexes with axial symmetry, where Kcon
and Kdip are positive constants, FC is a spin density at the m-C
atom, and (3 cos2 θ - 1)/r3 is a geometric factor of the m-H atom.
δobs ¼ δiso þ δdia
δiso ¼ δcon þ δdip
ð1Þ
Our previous papers showed that even the highly saddled
[Fe(OETPP)(tBuNC)2]þ with the (dxz, dyz)4(dxy)1 ground state
exhibits the meso-C and m-H signals fairly downfield, 416 and 11.1
ppm, respectively, at 298 K.22 The reason for the downfield shift of
the meso-C and m-H signals in highly saddled (dxz, dyz)4(dxy)1-
type complexes is not clear at this point. Presumably, the saddled
porphyrin ring in the solid state is ruffled to some extent in
solution, which will be discussed later.
¼ ½ - KconFC þ Kdipðg 2 - g^ Þð3 cos2 θ - 1Þ=r3ꢀ=T ð2Þ
2
In the low-spin complexes with the (dxy)2(dxz, dyz)3 ground state,
the spin density at the meso-C atom is supposed to be zero
because the major interaction takes place between the half-filled
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Figure 4. Selected frontier MOs of the saddled (dxz, dyz)4(dxy)1 state of [Fe(TBzTPP)]þ (i.e., state A). The spin-unrestricted MOs that best
correspond to a singly occupied MO are indicated by a blue legend.
dπ and filled porphyrin 3eg orbitals; the 3eg orbitals have a
negligibly small coefficient at the meso-C atoms. Thus, the slope
of the Curie plots for the m-H signal should be negative because
In the (dxz, dyz)4(dxy)1-type ruffled complexes, the meso-C
atom has a sizable amount of spin density because of interaction
between the half-occupied dxy and filled a2u orbitals. The positive
spin at the meso-C atoms is delocalized to the attached phenyl
groups and induces the negative π spin density at the m-C atoms.
Thus, the slope for the contact shift, -KconFC, should be positive.
2
FC at the m-C atom is nearly zero and (g 2 - g^ )(3 cos2θ - 1)/r3
isnegative; notethatg 2 - g^2 is positive, while (3 cos2θ - 1)/r3 is
negative for m-H.
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Figure 5. Spin densities (0.0025 e/Å3) for states A-D for [Fe(TBzTPP)]þ. Majority and minority spin densities are indicated in blue and
red, respectively.
In addition, the slope for the dipolar shift, (g 2 - g^ )(3 cos2θ
- 1)/r3, should also be positive because the g 2 - g^2 value is
negative in the (dxz, dyz)4(dxy)1-type complexes. Consequently,
the slope of the Curie plots for the m-H signal is positive.
Therefore, the chemical shift and slope of the Curie plots of
the m-H signal can tell the electronic ground state of the low-spin
complexes.
of the planar axial ligands.2 If planar axial ligands such as
imidazole and pyridine adopt a mutually parallel alignment above
and below the porphyrin ring, the complexes exhibit the rhom-
bic-type spectra. If, on the contrary, they adopt a mutually
perpendicular alignment, the complexes exhibit single-feature
large gmax-type spectra where a strong signal is observed at g > 3.2.
Walker and co-workers revealed that the borderline between the
parallel and perpendicular alignments is ca. 57ꢀ.51 In contrast,
complexes with the less common (dxz, dyz)4(dxy)1 ground state
exhibit the axial-type spectra where g^ values are usually smaller
than 2.5 and g values are 1.7-2.0.2 If the energy level of the dxy
orbital is extensively higher than the energy levels of the dπ(dxz,
dyz) orbitals, the complex adopts a quite pure (dxz, dyz)4(dxy)1
ground state, where both g^ and g values are close to 2.0.2-8
Electronic Ground State of 1. Figure 7 shows the Curie plots
of the o-, m-, R-, β-, and p-CH2(R) signals of series 1. Each signal
in 1a-1c exhibits a considerable curvature. The results indicate
that the (dxz, dyz)4(dxy)1 ground state is destabilized at lower
temperature. Thus, in the case of 1a, the m-H signal (b, red)
shows a slight downfield shift as the temperature is lowered from
313 to 273 K because 1a adopts the (dxz, dyz)4(dxy)1 ground state
at ambient temperature. As the temperature is further lowered
from 273 to 183 K, this signal is shifted to the opposite direction.
The results are indicative of a gradual transition of the electronic
ground state from (dxz, dyz)4(dxy)1 to (dxy)2(dxz, dyz)3. The m-H
2
Walker and co-workers proposed that the electronic ground
state of low-spin iron(III) porphyrinates having meso-aryl groups
such as [Fe(TPP)L2](, [Fe(OETPP)L2](, and [Fe(TPC)L2](
(where TPC2- = dianion of 5,10,15,20-tetraphenylchlorin) can
be determined by the δm - δp and δm - δo values; δo, δm, and
δp are the chemical shifts of the o-, m-, and p-phenyl signals,
respectively.2,3 If δm - δp and δm - δo are both large and
positive, then the complex has large amounts of positive spin at
the meso-C atoms, which, in turn, indicates that the complex
adopts the (dxz, dyz)4(dxy)1 ground state. In contrast, if δm - δp
and δm - δo are small with δm - δp positive and δm - δo usually
negative, then the complex adopts the (dxy)2(dxz, dyz)3 ground
state because the meso-C atoms have little or no spin density.
2. EPR Method. The electronic ground state of low-spin
complexes can most clearly be determined by EPR spectroscopy,
which is usually taken at extremely low temperatures. Complexes
with the common (dxy)2(dxz, dyz)3 ground state exhibit either
rhombic or large gmax-type spectra depending on the orientation
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Table 1 lists the δm - δo values for 1-3. All of the low-spin
complexes in 1 exhibit δm - δo values from 4.6 to 8.8 ppm at 298 K
and from 5.1 to 12.2 ppm at 213 K. Close inspection of the data in
Table 1 further indicates that the δm - δo values increase on going
from 1a-1c to 1d-1f. The results suggest that the spin densities
at the meso positions in 1d-1f are larger than those in 1a-1c. It
should be noted that the δm - δo value of the DMAP complex
(1c) is quite close to that of [Fe(OETPP)(tBuNC)2]þ adopting
the (dxz, dyz)4(dxy)1 ground state; they are 5.49 and 5.51 ppm for
1c and [Fe(OETPP)(tBuNC)2]þ, respectively, at 298 K.22
These results strongly suggest that the dxy orbital is located
above the dπ orbitals even in the case of 1c. Thus, it is clear that
the TBzTArP core stabilizes the (dxz, dyz)4(dxy)1 ground state as
compared with the OETPP core.
The EPR spectra given in Figure 2 and g values listed in Table 2
are totally consistent with the NMR data. While 1a and 1b exhibit
rhombic-type spectra characteristic of the (dxy)2(dxz, dyz)3 ground
state, 1d-1f and 1h show axial-type spectra characteristic of the
(dxz, dyz)4(dxy)1 ground state. The DMAP complex (1c) is the
borderline case; the complex exhibits both the axial- and rhom-
bic-type signals, showing the coexistence of electron configura-
tional isomers even at 4.2 K.
DFT calculations of [Fe(TBzTPP)(Py)2]þ suggest that the
(dxz, dyz)4(dxy)1 state is more stable than the (dxy)2(dxz, dyz)3 state
for both the ruffled and saddled conformations. The dispersion-
corrected calculations indicate that the ruffled conformation with
the (dxz, dyz)4(dxy)1 state is the most stable state, followed by the
saddled conformation with the same state, as shown in Table 3.
Because the energy difference between these states is quite small, it
is reasonable to consider an equilibrium between these two isomers,
as indicated by eq 4, where dxy indicates the (dxz, dyz)4(dxy)1
1
ground state.
1
1
saddledðdxy Þ a ruffledðdxy
Þ
ð4Þ
Figure 6. Structural highlights (Å, deg) of the ruffled and saddled (dxz,
dyz)4(dxy)1 states: Fe-N distances (in blue), displacements of selected
atoms from the mean porphyrin plane (in magenta), and selected
dihedrals (in red).
The observed chemical shift (δobs) for the m-H signal can be
expressed by eq 5, where p is the population ratio of the saddled
isomer and δ(saddled) and δ(ruffled) are the m-H chemical
shifts of each isomer. The first term in eq 5 should be negative
(upfield shift) because the Mulliken spin population of m-C in
the saddled complex (state C in Table 4) is slightly positive.
However, the second term should be positive (downfield shift)
because the corresponding Mulliken spin-population value in the
ruffled complex (state A in Table 4) is slightly negative.
signals in 1b and 1c show a similar temperature dependence. In
contrast, the Curie plots of the m-H signals in 1d exhibit a good
straight line with a positive slope, i.e., 1450 ppm K. Similarly, 1e,
3
1f, and 1h show a good straight line, as given in the Figure S5a of
the SI; the slopes are 1500, 1520, and 1290 ppm K, respectively.
3
Furthermore, these lines intercept the axis of the ordinate at 7.17
(1d), 7.23 (1e), 7.39 (1f), and 6.83 (1h) ppm, which are close to
the m-H chemical shift, 7.64 ppm, of the corresponding free-base
porphyrin. Thus, 1d-1f and 1h maintain the (dxz, dyz)4(dxy)1
ground state in the temperature range where the NMR spectra
are taken. The signs of the Curie plots are also listed in Table 1.
Figure 8 demonstrates comparisons of the Curie plots of (a)
m-H and (b) R-H signals in 1a-1f and 1h. Signals of 1a-1c are
signified by filled circles with red, blue, and green color,
respectively. The Curie plots of all of the other complexes are
given by the black symbols: 1d (O), 1e (0), 1f (]), and 1h (4).
In both the m-H and R-H signals, the curved lines change
gradually to straight lines with positive slope as the axial ligand
is changed from HIm (a) to 1-MeIm (b) and then to DMAP (c).
Concomitantly, the curved line moves upward. The Curie plots
of 1d-1f and 1h show almost overlapping straight lines although
the Curie plots of the m-H signal of 1h are located below those of
1d-1f by 1-2 ppm.
δobs ¼ pðsaddledÞ δðsaddledÞ þ pðruffledÞ δðruffledÞ
ð5Þ
3
3
The observed chemical shift for the m-H signal of structurally
analogous [Fe(TBzTArP)(Py)2]þ (1f) is 12.48 ppm at 298 K,
which can be explained if the second term in eq 5 is predominant.
The straight line in the Curie plots suggests that the equilibrium
constant corresponding to eq 4 is invariant throughout the NMR
measurement.
On the basis of these results, we concluded that all of the
complexes of 1 adopt the (dxz, dyz)4(dxy)1 ground state at least at
ambient temperature regardless of the kinds of axial ligands. As the
temperature is lowered, the electronic ground state gradually
changes from (dxz, dyz)4(dxy)1 to (dxy)2(dxz, dyz)3 in the cases of
1a-1c. At an extremely low temperature, both 1a and 1b adopt
the (dxy)2(dxz, dyz)3 ground state, while 1c exists as an equilib-
rium mixture of two electron configurational isomers, as revealed
from the EPR spectra. In contrast, all the other complexes, 1d-1f
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Figure 7. Curie plots of some signals in 1a-1d, where the symbols of blue b, red b, O, 0, and Δ show the chemical shifts of o-, m-, R-, β-, and
p-CH2, respectively.
and 1h, maintain the (dxz, dyz)4(dxy)1 ground state from 298 to
4.2 K. Therefore, the stability of the (dxz, dyz)4(dxy)1 ground state
increases as the axial ligand changes from HIm to
Py in the order given as follows:
ruffled conformer, the dxy orbital should also be destabilized
relative to the dπ orbitals because of interaction with the
porphyrin a2u orbital, as revealed from the large Mulliken spin
population of the meso-C atoms, i.e., 0.0878. In addition, the dπ
orbitals of both conformers can be stabilized by bonding inter-
action with the low-lying LUMO of the TArTBzP ring, which is
further verified by the bathochromic shifts of the absorption
bands in TBzTArP, as compared with the corresponding TBu-
TArP shown in Figure S6a,b of the SI.
HIm < 1-MeIm < DMAP < 4-MeOPy < 4-MePy < Py, CN-
It should be noted that the (dxz, dyz)4(dxy)1 ground state
observed in the HIm complex (1a) at ambient temperature is
quite unusual because almost all of the previously reported HIm
complexes exhibit the (dxy)2(dxz, dyz)3 ground state. The result
suggests that the stability of the (dxz, dyz)4(dxy)1 ground state is
an inherent characteristic of the TArTBzP core.
Electronic Ground State of 2. The Curie plots of series 2
given in Figures 9 and 10 are instructive. While the Curie plots of
the m-H signals of 1a-1c exhibit a curvature as shown in
Figures 7, 8, and S5b of the SI, those of 2a-2c show straight
lines with negative slopes; they are -480, -530, and -1030
The reason for the stability of the less common (dxz, dyz)4(dxy)1
state in 1 can be expressed as follows. As suggested by DFT
calculations, [Fe(TBzTPP)(Py)2]þ appears to exist as an equi-
librium mixture of two conformers with the (dxz, dyz)4(dxy)1
ground state. In the saddled conformation, there is a strong
interaction between the iron dxy and porphyrin a1u orbitals, as
revealed from the large Mulliken spin population of the CR
atoms, i.e., 0.0900, together with the exceedingly small spin
population of the Fe atom, i.e., 0.3307. The interaction destabi-
lizes the dxy orbital and places it above the dπ orbitals. In the
ppm K, respectively. Negative slopes are also observed in the
3
Curie plots of the m-H signals in other typical low-spin com-
plexes with the (dxy)2(dxz, dyz)3 ground state such as
[Fe(OETPP)(HIm)2]þ and [Fe(TPP)(HIm)2]þ; they are -
1200 and -490 ppm K, respectively.22,52 By contrast, the Curie
3
plots of the m-H signals in 2d-2f and 2h exhibit reasonably good
straight lines with positive slopes; they are þ550, þ890, þ1340,
and þ900 ppm K, respectively. These values are only 37-88%
3
of the corresponding values of 1d-1f and 1h. Furthermore, the
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K. Similarly, 2h shows a reasonably good linearity throughout the
temperature range examined, suggesting that the complex main-
tains the (dxz, dyz)4(dxy)1 ground state. In contrast to these
complexes, the curvature of the R-CH2 signals increases gradu-
ally on going from 2d to 2f, suggesting that the population of S =
3/2 increases at higher temperature in this order. It should be
noted, however, that the S = 3/2 populations in 2d-2f are still not
predominant even at 298 K because the chemical shifts of the R-
CH2 signals of these complexes are ca. 30 ppm compared with the
corresponding chemical shift, 88.6 ppm, of the pure S =3/2 complex,
i.e., [Fe(TBTXP)(THF)2]þ (where THF = tetrahydrofuran).53
The curvature observed in the R-CH2 signals of 2d-2f, although
it is not observed in the m-H signals of the same complexes, can
be explained in terms of the large differences in the chemical shifts
3
between the (dxz, dyz)4(dxy)1 and S = /2 complexes. Thus, the
R-CH2 signal is a sensitive probe of the detailed electronic structure
of 2. The spin transition is most explicitly observed in 2g, which
exhibits the R-CH2 signal at an extremely downfield position,
49.8 ppm, at 273 K. The result indicates that 2g exists mainly as
3
S = /2 at ambient temperature, like [Fe(OETPP)(4-CNPy)2]þ.
This signal moves upfield upon a decrease in the temperature and
approaches that of 2h at 213 K, as shown in Figure 10b, suggesting
that the complex is S = 1/2 (dxy) at this temperature.
EPR spectra shown in Figure 2B are totally consistent with
the conclusion obtained by the NMR spectra. The complexes
2a and 2b exhibit rhombic-type spectra, while 2d-2h exhibit
axial-type spectra. Thus, the former complexes maintain the
(dxy)2(dxz, dyz)3 ground state, while the latter complexes
maintain the (dxz, dyz)4(dxy)1 ground state in the temperature
range 298-4.2 K. In the case of 2c, very broad unresolved
signals are observed at g = 3.0-2.3. Presumably, the two low-
field signals in the rhombic-type spectrum overlap because of
poor resolution. The data in Table 2 indicate that the g^ values
of 2d-2h are larger than those of the corresponding 1d-1h.
The results suggest that the energy gap between the dxy and dπ
orbitals for 1d-1h is larger than that for the corresponding
complexes 2d-2h. A direct comparison of the tetragonal
splitting is only possible for 1 h0 and 2h0. As expected, the
former is -6.3λ, while the latter is -4.0λ.
On the basis of these results, we concluded that, while 2a-2c
and 2h adopt the (dxy)2(dxz, dyz)3 and (dxz, dyz)4(dxy)1 ground
states, respectively, throughout the temperature range examined by
NMR and EPR spectroscopy, 2d-2f exhibit spin transition from the
Figure 8. Curie plots of the m-H (a) and R-H (b) signals of 1a-1f
and 1h.
3
1
mixed S = /2, /2 state to the mixed (dxy)2(dxz, dyz)3 and (dxz,
dyz)4(dxy)1 state as the temperature is lowered from 298 to 173 K.
These complexes are finally converted to the (dxz, dyz)4(dxy)1 state as
the temperature is further lowered to 4.2 K. In the case of 2g, spin
transition occurs from the nearly pure S = 3/2 to the nearly pure (dxz,
dyz)4(dxy)1state. Similar spin transitions have been observed in
structurally similar [Fe(OETPP)(Py)2]þ (3f), [Fe(OMTPP)L2]þ
(L = 4-CNPy and Py, where OMTPP2- = dianion of 2,3,7,8,
12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphyrin), and
[Fe(TBTXP)L2]þ (L = 4-CNPy and Py).53
δm - δo values are 3.87 (2d), 4.85 (2e), 6.19 (2f), and 4.77 (2h)
ppm at 213 K, which are again much smaller than those of 1d-1f
and 1h; they are 10.8, 11.5, 12.2, and 8.6 ppm, respectively, at the
same temperature. Thus, it is reasonable to conclude that 2a-2c
are in the (dxy)2(dxz, dyz)3 ground state. In contrast, 2d-2f and 2h
are in the (dxz, dyz)4(dxy)1 ground state although the energy gaps
between the dxy and dπ orbitals in these complexes are much
smaller than those in 1d-1f and 1h. The Curie plot of the m-H
signal of the 4-CNPy complex (2g) is quite unique. A considerably
large curvature can be explained in terms of the spin transition
fromS = 3/2 toS = 1/2 withthe (dxz, dyz)4(dxy)1 ground state. Note
that the m-H signal of the pure S = 3/2 complex, [Fe(OETPP)(4-
CNPy)2]þ, appears at 5.27 ppm at 298 K, which is shown in
Figure 10a by the symbol indicated by the arrow.2
Because the six-coordinate, low-spin [Fe(TPP)L2]( and
[Fe(OEP)L2]( complexes adopt the (dxy)2(dxz, dyz)3 ground state
at 4.2 K if the axial ligands are those examined in this study except
for 4-CNPy, it is concluded on the basis of the present work
together with our previous work that stabilization of the (dxz,
dyz)4(dxy)1 ground state in low-spin [Fe(Por)L2]( increases in the
order given below (where TiPrP2- = dianion of 5,10,15,20-
tetraisopropylporphyrin):22
Interestingly, the Curie plots of the R-CH2 signals in all of the
complexes, 2a-2h, exhibit more or less a curvature, as shown in
Figure 10b. Among these complexes, 2a-2c show straight lines
at least below 250 K. Thus, they adopt the (dxy)2(dxz, dyz)3
ground state although the population of S = 3/2 is not zero at 298
OETPP, OEP < OMTPP, TPP < TBuTArP < TBzTArPy < TiPrP
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Figure 9. Curie plots of some signals in 2a, 2b, 2d, and 2h, where the symbols of blue b, red b, 0, and Δ show the chemical shifts of o-, m-, β-, and
p-CH2, respectively.
’ CONCLUSION
almost identical in energy. Thus, the group B complexes are
supposed to exist as an equilibrium mixture of the two con-
formers with the (dxz, dyz)4(dxy)1 ground state. In sharp
contrast to 1, the group A complexes in 2 maintained the
(dxy)2(dxz, dyz)3 ground state throughout the temperature
range examined, i.e., 298-4.2 K. Although the group B com-
plexes in 2 maintained the (dxz, dyz)4(dxy)1 ground state as in
the case of 1, the energy gaps between the dxy and dπ orbitals are
much smaller than those in 1. DFT calculations suggested that
the stability of the (dxz, dyz)4(dxy)1 ground state of 1 may be
ascribed to the strong interaction between the iron dxy and the
high-energy a1u HOMO, which is an inherent characteristic of
the saddled TBzTArP core. In addition, the ruffled conformer
shows a dxy-a2u interaction, which is quite common in low-spin
iron(III) porphyrinates with a ruffled porphyrin core. The
presence of a low-lying LUMO, which is also an inherent
characteristic of the TBzTArP core, should also contribute to
stabilization of the (dxz, dyz)4(dxy)1 ground state via dπ-pπ*
interactions. On the basis of these results, we concluded that
benzoannelation of porphyrin stabilizes the (dxz, dyz)4(dxy)1
ground state in low-spin iron(III) porphyrinates.
The electronic structures of low-spin, six-coordinate 1 and 2,
where the axial ligands (L) are HIm, 1-MeIm, DMAP, 4-MeOPy,
4-MePy, Py, and CN-, were examined by H NMR and EPR
1
spectroscopy as well as DFT calculation. These complexes were
classified into two groups. Group A consists of complexes having
strong σ donors such as HIm, 1-MeIm, and DMAP as axial
ligands, and group B consists of complexes having 4-MeOPy,
4-MePy, Py, and CN- as axial ligands. In the case of 1, the group
A complexes switch the electronic ground state from (dxz,
dyz)4(dxy)1 at ambient temperature to (dxy)2(dxz, dyz)3 at 4.2 K;
the DMAP complex exists as a mixture of two kinds of electron
configurational isomers even at 4.2 K. The group B complexes
maintained the (dxz, dyz)4(dxy)1 ground state in a wide temperature
range, i.e., 298-4.2 K. DFT calculations were carried out on
different conformations and electronic states of [Fe(TBzTPP)
(Py)2]þ. The calculations including Grimme’s dispersion correc-
tions (BP86-D and BLYP-D) indicated that the ruffled conforma-
tion with the (dxz, dyz)4(dxy)1 state is the most stable state, followed
by the saddled conformation with the same state. The complex is
unique in that the highly ruffled and highly saddled conformers are
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added. The resulting solution, after stirring for 1 h at room temperature,
was treated with dichlorodicyanoquinone (DDQ; 1.30 g, 6.60 mmol)
and then was refluxed under argon for 1 h to give a dark-green solution.
The solvent was removed under vacuum, and the residue was purified by
column chromatography on alumina (grade III) using CH2Cl2 as the
eluent. Recrystallization from methanol gave TBuTArPH2 (0.18 g, 56%)
as purple crystals. 1H NMR (300 MHz, CDCl3): δ 8.11-8.08, (d, 8H,
J = 7.7 Hz, o-Ar), 7.52-7.49 (d, 8H, J = 7.8 Hz, m-Ar), 2.97-2.92 (t, 8H,
each, CH2), 2.33-1.29 (m, 40H, CH2), 0.94-0.92 (t, 12H, CH3). UV-
vis [CH2Cl2; λmax, nm (ε, M-1 cm-1)]: 446 (66 084), 542 (5052), 615
(3720), 679 (5698). MS (MALDI): m/z 1335.30 (Mþ). Anal. Calcd for
C96H126N4: C, 84.03; H, 9.56; N, 4.09. Found: C, 84.23; H, 9.35; N,
4.24. 13C NMR (300 MHz, CDCl3): δ 144. 1, 140.7, 136.2, 129.1, 118.5,
37.4, 33.5, 33.1, 31.29, 31.24, 30.97, 30.64, 27.3, 25.1, 24.2, 15.7.
2:3,7:8,12:13,17:18-Tetrabenzo-5,10,15,20-tetrakis(4-nonylphenyl)-
porphyrin, TBzTArPH2. To a toluene (20 mL) solution of TBuTArPH2
(150 mg, 0.12 mmol) was added an excess of copper(II) chloride (162
mg, 1.2 mmol). The mixture was refluxed for 2 h, and then the solvent
was evaporated under vacuum. The resulting residue was dissolved in
CH2Cl2 (150 mL) and washed once with a saturated aqueous NaHCO3
solution and once with water. The organic layer, after drying over
anhydrous Na2SO4, was removed under vacuum. The remaining residue
was purified by column chromatography on alumina using CH2Cl2 as
the eluent. A toluene solution (20 mL) of purple Cu(TBuTArP) (109
mg, 0.78 mmol) was treated with excess DDQ (176 mg, 7.9 mmol), and
the resulting mixture was refluxed for 5 min. The color changed from red
to deep green during the reflux. The reaction mixture was cooled to
room temperature, diluted with CHCl3 (100 mL), and washed once with
a saturated aqueous NaHCO3 solution and once with water. The solvent
was removed under vacuum, and the remaining residue was purified by
column chromatography on alumina using CHCl3 as the eluent.
Recrystallization from methanol afforded Cu(TBzTArP) (65 mg, 60%
yield) as a dark-green solid. Cu(TBzTArP) (60 mg, 0.043 mmol) was
dissolved in concentrated H2SO4 and stirred for 5 min at room
temperature. The acid solution was poured into water and extracted
repeatedly with CHCl3 until the organic extracts were colorless. The
combined organic layers, after drying over anhydrous Na2SO4, were
concentrated under vacuum. Recrystallization from methanol gave
Figure 10. Curie plots of the (a) m-H and (b) R-CH2 signals of 2. The
symbol indicated by the arrow in part a indicates the m-H signal of
[Fe(OETPP)(THF)2]þ with a pure intermediate-spin state.
1
green crystals of TBzTArPH2 (45 mg, 79% yield). H NMR (CDCl3,
298 K): δ 8.22 (d, J = 7.60 Hz, 8H, o-H), 7.64 (d, J = 7.60 Hz, 8H, m-H),
7.38-7.09 (br m, 16H, benzo-H), 3.03 (t, J = 7.60 Hz, 8H, p-CH2),
1.95 (m, 8H, p-CH2), 1.66-1.28 (m, 48H, p-(CH2)6), 0.91 (t, J =
6.8 Hz, 12H, p-CH3), -1.22 (s, 2H, NH). UV-vis [CH2Cl2; λmax, nm
(ε, M-1 cm-1)]: 461 (458 000), 503 (31 900), 589 (18 000), 641 (49 600),
695 (11 300) nm. MS (MALDI-TOF). Calcd for C96H110N4: m/z 1319.9.
Found: m/z 1319.6.
’ EXPERIMENTAL SECTION
General Procedure. NMR samples of six-coordinate complexes,
[Fe(TBzTArP)(L)2]( and [Fe(TBuTArP)(L)2](, were prepared by
the direct addition of an excess (5-10 equiv) of the desired axial ligand
to CD2Cl2 solutions of Fe(TBzTArP)Cl and Fe(TBuTArP)Cl placed in
5 mm NMR sample tubes. 1H NMR spectra were recorded on a JEOL
LA 300 or LA400 spectrometer operating at 300.4 and 400.6 MHz,
respectively, and referenced to the resonance of the residual solvent
protons of CD2Cl2 (δ = 5.32 ppm relative to tetramethylsilane). UV-
vis spectra were recorded in CH2Cl2 solutions at room temperature
using a Shimazu UV-3100 spectrophotometer. EPR spectra were
measured at 4.2-17.0 K on a Bruker E500 spectrometer operating at
X band and equipped with an Oxford helium cryostat. Samples for EPR
measurement were similarly prepared in 5 mm EPR tubes as mentioned
for NMR measurement. The concentrations of the EPR samples were
5-8 mM. The time-of-flight mass spectra were recorded on a Bruker
Daltonics autoflex-T1 mass spectrometer.
Fe(TBzTArP)Cl and Fe(TBuTArP)Cl. Insertion of iron(III) into
TBzTArPH2 and TBuTArPH2 was carried out by the addition of
FeCl2 6H2O into a refluxed N,N-dimethylformamide solution of
3
TBzTArPH2 and TBuTArPH2. The UV-vis spectra of these complexes
are given in Figure S6a of the SI.
[Fe(TBzTArP)(THF)2]BF4 and [Fe(TBuTArP)(THF)2]BF4. These com-
plexes were prepared by the addition of AgBF4 to a THF solution of
Fe(TBzTArP)Cl and Fe(TBuTArP)Cl, respectively, followed by re-
crystallization from CH2Cl2/hexane.
[Fe(TBzTArP)(L)2]Cl and [Fe(TBuTArP)(L)2]Cl (L = HIm, 1-MeIm,
DMAP, 4-MeOPy, 4-MePy, and Py). NMR samples of these complexes
were prepared by the addition of the CD2Cl2 solutions of various axial
ligands (L) mentioned above to the CD2Cl2 solutions of Fe-
(TBzTArP)Cl and Fe(TBuTArP)Cl. As the ligands were added, the
1H NMR spectra showed a drastic change. The ligand solutions were
added until no spectral change was observed.
Synthesis54. 2:3,7:8,12:13,17:18-Tetrabutano-5,10,15,20-tetrakis-
(4-nonylphenyl)porphyrin, TBuTArPH2. 3:4-Butanopyrrole (1.66 g, 1.37
mmol) and 4-nonylbenzaldehyde (0.31 g, 1.37 mmol) were dissolved in
dry, freshly distilled CH2Cl2 (137 mL). The solution was stirred at room
temperature under a slow steady stream of argon for 15 min. The flask
[Fe(TBzTArP)(CN)2](Bu4N) and [Fe(TBuTArP)(CN)2](Bu4N). These
complexes were similarly prepared by the addition of the CD2Cl2
was shielded from light, and BF3 OEt2 (0.02 mL, 0.137 mmol) was
3
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solution of tetrabutylammonium cyanide (Bu4NþCN-) to the CD2Cl2
solutions of Fe(TBzTArP)Cl and Fe(TBuTArP)Cl. The UV-vis
spectra of [Fe(TBzTArP)(CN)2](Bu4N) and [Fe(TBuTArP)(CN)2]
(Bu4N) are given in Figure S6b of the SI.
[Fe(TBuTArP)(4-CNPy)2]BF4. Complete formation of the bis(4-
CNPy) complex was confirmed when a large excess (ca. 40 equiv) of
4-CNPy was added to the CD2Cl2 solution of [Fe(TBuTArP)(THF)2]
BF4. However, the corresponding [Fe(TBzTArP)(4-CNPy)2]BF4 was
not formed even by the addition of 60 equiv of 4-CNPy to the CD2Cl2
solution of [Fe(TBzTArP)(THF)2]BF4.
DFT Calculations. All calculations were carried out with the ADF
2009 program system. An STO-TZP basis set, as well as fine meshes for
numerical integration of matrix elements and tight criteria for geometry
optimization, was used throughout. Calculations were carried out with a
number of functionals including BP86, BLYP, and OLYP. Classic pure
functionals such as BP86 and BLYP often yield somewhat better
geometries for transition-metal complexes, whereas the newer pure
functional OLYP and hybrid functionals such as B3LYP sometimes
overestimate metal-ligand distances, even though the latter generally
yield better energetics.55 In this case, all of the functionals examined
(BP86, BLYP, and OLYP) gave very similar geometries, Mulliken charges,
and spin populations. Including Grimme’s dispersion corrections (such as
BP86-D and BLYP-D) resulted in some modulation of the ruffling and
saddling dihedrals as well as the relative energetics of the different states.
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’ ASSOCIATED CONTENT
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Supporting Information. 1H NMR, COSY, EPR, and
S
b
UV-vis spectra and Curie plots. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
(23) Ema, T.; Senge, M. O.; Nelson, N. Y.; Ogoshi, H.; Smith, K. M.
Angew. Chem., Int. Ed. 1994, 33, 1879–1881.
Corresponding Author
*E-mail: ikeue@riko.shimane-u.ac.jp (T.I.), abhik@chem.uit.no
(A.G.), vicente@lsu.edu (M.G.H.V.), mnakamu@med.toho-u.
ac.jp (M.N.).
(24) Jentzen, W.; Simpson, M. C.; Hobbs, J. D.; Song, X.; Ema, T.;
Nelson, N. Y.; Medforth, C. J.; Smith, K. M.; Veyrat, M.; Ramasseul, R.;
Marchon, J.-C.; Takeuchi, T.; Goddard, W. A.; Shelnutt, J. A. J. Am.
Chem. Soc. 1995, 117, 11085–11097.
(25) Senge, M. O.; Ema, T.; Smith, K. M. J. Chem. Soc., Chem.
Commun. 1995, 733–734.
’ ACKNOWLEDGMENT
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This work was supported by Grants-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan (Grant 22750052 to T.I. and
Grant 22550157 to M.N.). A.G. was supported by the Research
Council of Norway. M.N. thanks the Research Center for
Materials with Integrated Properties, Toho University, for finan-
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Molecular-Scale Nanoscience, the Institute for Molecular
Science.
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