Direct observation of conformers caused by the difference in ligand orientation in low spin iron(III) porphyrin complex

Article abstract of DOI:10.1246/cl.2000.342

EPR spectrum of low spin bis(dimethylaminopyridine)(3,8, 13,18-tetramesityl-2,7,12,17-tetramethylporphyrinato)iron(III) perchlorate taken in frozen CH₂Cl₂ solution at 4.2 K showed both large g_max and rhombic type signals.

Full text of DOI:10.1246/cl.2000.342

342
Chemistry Letters 2000
Direct Observation of Conformers Caused by the Difference in Ligand Orientation in
Low Spin Iron(III) Porphyrin Complex
Takahisa Ikeue, Tatsuya Yamaguchi, Yoshiki Ohgo, and Mikio Nakamura*
Department of Chemistry, Toho University School of Medicine, Omorinishi, Ota-ku, Tokyo 143-8540
(Received December 28, 1999; CL-991105)
EPR spectrum of low spin bis(dimethylaminopyridine)(3,8,
13,18-tetramesityl-2,7,12,17-tetramethylporphyrinato)iron(III)
perchlorate taken in frozen CH2Cl2 solution at 4.2 K showed
both large gmax and rhombic type signals. The result has been
explained in terms of the presence of two conformers where the
dihedral angles between two ligands are different.
Physicochemical properties of low spin iron(III) porphyrin
complexes as well as naturally occurring heme proteins are
controlled by various factors. In the complexes carrying one or
two imidazole ligands at the axial positions, orientation of the
ligands relative to the heme plane is considered to be one of the
factors.1,2 Although imidazole ligands are fixed in the cavities
of heme proteins, they are rapidly rotating in the synthetic
iron(III) porphyrin complexes. Thus, the observed properties
could be the average of the several conformers where the ligand
orientation is different. However, there is no report on the
observation of conformers in solution. Some time ago, we
reported the first example of the complex in which the axially
1
coordinated 2-methylimidazole ligands are fixed on the H
NMR time scale.3 Even in such a case, the complex existed as a
conformationally pure species.4,5 In this paper, we report the
first example of the direct observation of two conformers in
frozen solution.
The complex in question is bis(dimethylaminopyridine)(3,
8,13,18-tetramesityl-2,7,12,17-tetramethylporphyrinato)iron(III)
perchlorate, [Fe(TMTMP)(DMAP)2]ClO4 (1).6-8 1 was pre-
pared by the addition of 4.0 equiv of DMAP into the CD2Cl2
solution of [Fe(TMTMP)]ClO4 in an NMR sample tube.
[Fe(OEP)(1-MeIm)2]Cl.12
In order to determine the orientation of the axially coordi-
nated DMAP ligands, the EPR spectrum of 1 was taken in
frozen CH2Cl2 solution at 4.2 K.13 Figure 2(a) clearly exhibited
both large gmax and rhombic type spectra. The rhombic compo-
nent has gz = 2.86, gy = 2.28, and gx = 1.62, while the large gmax
component has a signal at gz = 3.58. For the comparison, the
EPR spectra of the analogous [Fe(OEP)(DMAP)2]ClO4(2) and
[Fe(OETPP)(DMAP)2]ClO4(3) were examined; 3 was prepared
by the addition of 4.0 equiv of DMAP into the CH2Cl2 solution
of [Fe(OETPP)]ClO4.14-16 Figures 2(b) and 2(c) show the EPR
spectra of 2 and 3, respectively, taken in frozen CH2Cl2 solu-
tion at 4.2 K. The former exhibited the rhombic spectrum with
gz = 2.81, gy = 2.28, and gx = 1.64,17 while the latter showed the
large gmax type spectrum with gz = 3.24.
Walker, Schidt and coworkers have established that the
large gmax and rhombic type spectra are originated from the per-
pendicular and parallel alignment of the ligands, respective-
ly.17,18 In fact, 2 has two parallelly oriented planar ligands as
revealed from the crystallographic result.17 On the contrary, 3 is
expected to have perpendicularly aligned ligands because the
OETPP ring is highly S4-saddled and creates cavities along the
diagonal N-Fe-N axes.15 Since the planar axial ligands are
placed along the cavities, the stereoisomer with perpendicularly
aligned axial ligands could be formed as in the case of the anal-
ogous cobalt(III) complexes.19 The present result strongly sug-
Formation of 1 was confirmed by the 1H NMR spectrum at -20
˚C as shown in Figure 1. The downfield shifted pyrrole methyl
signal, δ 14.7 ppm, and the upfield shifted meso signal, δ -2.7
ppm, strongly suggest that the complex adopts the common
(dxy)2(dzx, dyz)3 ground state.9-11 The (dxy)2(dzx, dyz)3 ground
state was further supported by the 13C NMR spectra; 1 showed
the α-pyrrole signals at 35.9 and 41.0 ppm, β-pyrrole signals at
135.7 and 144.3 ppm, and meso signal at 23.6 ppm at 25 ˚C,
which are quite close to the corresponding signal positions of
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
343
References and Notes
1
W. R. Scheidt and D. M. Chipman, J. Am. Chem. Soc., 108,
1163 (1986).
2
F. A. Walker, B. H. Huynh, W. R. Scheidt, and S. R. Osvath, J.
Am. Chem. Soc., 108, 5288 (1986).
3
4
M. Nakamura and J. T. Groves, Tetrahedron, 44, 3225 (1988).
F. A. Walker and U. Simonis, J. Am. Chem. Soc., 113, 8652
(1991).
5
6
M. Nakamura and N. Nakamura, Chem. Lett., 1991, 1885.
Abbreviations: TMTMP, OEP, OETPP, TPP, T2,6Cl2PP: diaions
of 3,8,13,18-tetramesityl-2,7,12,17-tetramethylporphyrin,
2,3,7,8,12,13,17,18-octaethylporphyrin, 2,3,7,8,12,13,17,18-
octaethyl-5,10,15,20-tetraphenylporphyrin, meso-tetraphenyl-
porphyrin, and meso-tetrakis(2,6-dichloro-phenyl)porphyrin,
respectively. DMAP: 4-dimethylaminopyridine. 1-VinIm: 1-
vinylimidzaole. HIm: imidazole. 1-MeIm: 1-methylimidazole.
N. Ono, H. Kawamura, M. Bougauchi, and K. Maruyama,
Tetrahedron, 46,7483 (1990).
7
8
9
H. Fujii, J. Am. Chem. Soc., 115, 4641 (1993).
F. A. Walker and U. Simonis, “Proton NMR Spectroscopy of
Model Hemes,” in “NMR of Paramagnetic Molecules,” ed by L.
J. Berliner and J. Reuben, Plenum Press, New York, (1993), pp:
133-274.
10 M. Nakamura, T. Ikeue, H. Fujii, and T. Yoshimura, J. Am.
Chem. Soc. 119, 6284 (1997).
11 M. Nakamura, T. Ikeue, A. Ikezaki, Y. Ohgo, and H. Fujii,
Inorg. Chem. 38, 3857 (1999).
12 [Fe(OEP)(1-MeIm)2]Cl: 13C NMR (CDCl3, 26 ˚C, δ ppm) α-
pyrrole, 45.4; β-pyrrole, 146.7; meso, 23.6 ppm. H. M. Goff,
J. Am. Chem. Soc., 103, 3714 (1981).
13 1H NMR spectra of 1 were taken at low temperature. No broad-
ening due to the hindered rotation of the axial ligand was
observed even at -100 ˚C.
14 [Fe(OETPP]ClO4 was prepared by the addition of 1.0 equiv of
AgClO4 into the THF solution of [Fe(OETPP)Cl].15 The crude
product was recrystallized from THF-hexane. 1H NMR
(CD2Cl2, 25 ˚C, δ ppm): 14.2 (8H, CH2); 43.2 (8H, CH2); 0.7
(24H, CH2); 13.0 (8H, o-H); 6.5 (8H, m-H); 9.7 (4H, p-H).
15 R.-J. Cheng, P.-Y. Chen, P.-R. Gau, C.-C. Chen, and S.-M.
Peng, J. Am. Chem. Soc. 119, 2563 (1997).
gests that 1 has two conformers in which the dihedral angles
between two axial ligands are different.20 Observation of the
two types of EPR signals in a single complex is not unprece-
dented.18,21,22 For example, [Fe(TPP)(HIm)2]Cl shows two
overlapping rhombic spectra,21 and [Fe(T2,6Cl2PP)(1-Vin-
Im)2]ClO4 displays both large gmax and rhombic spectra.18 In
these cases, however, the EPR spectra were taken in the solid
state. In the frozen solution where the solid-state packing
effects are absent, both complexes exhibit a single rhombic
spectrum. To our knowledge, 1 is the first example showing
the presence of conformers in frozen solution. If we assume
that the equilibrium constant between two isomers is ca. 10 in
favor of the perpendicular isomer, the ∆G˚ is calculated to be
80 J/mol at 4.2 K. Thus, this phenomenon is observable when
the difference in thermodynamic stability of the isomers is fair-
ly small.
The results presented here indicate the importance of EPR
spectroscopy to elucidate the presence of conformers and their
relative ratios. EPR measurement at various temperatures
could yield the thermodynamic parameters, ∆H˚ and ∆S˚, which
would help understand the properties of these biologically rele-
vant complexes.
The authors thank Dr. Hiroshi Fujii of the Institute for
Molecular Science for his generous supply of (TMTMP)H2.
This work was supported by a Grant-in-Aid for Scientific
Research (No 10640551) from Ministry of Education, Science,
Culture and Sports of Japan.
16 [Fe(OETPP)(DMAP)2]ClO4: 1H NMR (CD2Cl2, -50 ˚C, δ ppm):
3.4 (8H, CH2); 12.5 (8H, CH2); 1.3 (24H, CH3); 2.7 (8H, o-H);
4.8 (8H, m-H); 6.0 (4H, p-H); 15.8 (4H, ligand); -2.7 (4H, lig-
and); 18.0 (12H, NCH3). 13C NMR (CD2Cl2, -50 ˚C, δ ppm):
132 (Py-α); 164 (Py-β); -28 (meso); 159 (ipso); 102 (o); 122
(m); 123 (p); -40 (CH2); 102 (CH3).
17 The g values of 2 taken in frozen CH2Cl2 solution at 25 K were
reported to be 2.818, 2.278, and 1.642. M. K. Safo, G. P.
Gupta, F. A. Walker, and W. R. Scheidt, J. Am.Chem. Soc., 113,
5497 (1991).
18 K. Hatano, M. K. Safo, F. A. Walker, and W. R. Scheidt, Inorg.
Chem., 30, 1643 (1991).
19 C. J. Medforth, C. M. Muzzi, K. M. Shea, K. M. Smith, R. J.
Abraham, S. Jia, and J. A. Shelnutt, J. Chem. Soc. Perkin Trans.
2, 1997, 833.
20 In the case of 2, the two DMAP ligands are coplanar. The ori-
entation of the axial ligands with the closest Fe-N vector is
36˚.17 Introduction of the bulky mesityl groups at 3,8,13, and
18 positions would destabilizes this conformation. As a result,
the energy difference between the parallel and perpendicular
conformers is supposed to be much smaller, making the obser-
vation of both conformers possible.
21 W. R. Scheidt and S. R. Osvath, Y. J. Lee, J. Am. Chem. Soc.,
109, 1958 (1987).
22 R. Quinn, J. S. Valentine, M. P. Byrn, and C. E. Strouse, J. Am.
Chem. Soc., 109, 3301 (1987).