Novel spin transition between S = 5/2 and S = 3/2 in highly saddled iron(III) porphyrin complexes at extremely low temperatures

Article abstract of DOI:10.1039/b601412g

A novel spin transition between S = 5/2 and S = 3/2 has been observed for the first time in five-coordinate, highly saddled iron(iii) porphyrinates by EPR and SQUID measurements at extremely low temperatures. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b601412g

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Novel spin transition between S =
2
and S =
2
in highly saddled
iron(III) porphyrin complexes at extremely low temperatures{
Yoshiki Ohgo,*ab Yuya Chiba,c Daisuke Hashizume,d Hidehiro Uekusa,e Tomoji Ozekie and
Mikio Nakamura*abf
Received (in Cambridge, UK) 30th January 2006, Accepted 14th March 2006
First published as an Advance Article on the web 28th March 2006
DOI: 10.1039/b601412g
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A novel spin transition between S =
and S =
has been
2
observed for the first time in five-coordinate, highly saddled
iron(III) porphyrinates by EPR and SQUID measurements at
extremely low temperatures.
A wide variety of research has been carried out on the spin
transition in naturally occurring heme proteins because of their
relevance to the important functional tuning process.1 For
example, the ferrihemes in cytochrome P450 adopt the low-spin
(S = K) in the resting state. However, the spin transition to the
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high-spin state (S =
2
) occurs on binding the organic substrate,
which triggers the one-electron reduction to proceed the catalytic
2
oxidation reaction. The spin transition between S = 5 2 and S = K
Scheme 1 Possible spin transitions (ST) in iron(III) porphyrinates.
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has also been studied in model complexes such as [Fe(OEP)(3-
ClPy)2]+.3,4 In contrast to high-spin and low-spin states, the
caused by the spin–orbit coupling between S = 3 and S = 5
states
2
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2
intermediate-spin state (S = 2) is quite rare.5 Reed and Guiset
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(Scheme 1).10 In the course of our studies on the spin transition in
highly saddled complexes, we found that structurally similar
complexes [Fe(OMTArP)(H2O)]ClO4 (1 and 2), shown in
Scheme 2, serve an unprecedented example showing the spin
reported that [Fe(TPP)]X is in an essentially pure intermediate-spin
state if the axial anionic ligand
X is extremely weak,
[Ag(Br6CB11H6)2]2.6 Ourselves and others have also reported that
highly saddled porphyrins such as [Fe(OETPP)L2]ClO4, and core-
contracted porphycenes and corroles such as [Fe(TPrPc)L2]ClO4
and [Fe(TPFC)L2] adopt a quite pure intermediate-spin state if the
field strengths of the axial ligands are fairly weak.7 Furthermore,
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5
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2
transition between S =
2
and S =
spin states.
the saddled complex with a much stronger pyridine (Py) ligand,
+
[Fe(OETPP)(Py)2] , exhibits a novel spin transition between S = 3
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2
and S = K.8 Occurrence of a similar spin transition has recently
been reported by Rivera and Caignan in the heme metabolism
process mediated by human heme oxygenase.9 While the spin
transitions between S = 5 2 and S = K, and between S = 3 2 and S =
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K have been observed both in heme proteins and model heme
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complexes, the spin transition between S = 5 2 and S = 3 2 has never
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2
been observed before. In fact, the mixed S = and S = spin
2
state is considered to be a quantum mechanical spin admixture
Scheme 2 (a) Complexes 1 and 2 examined in this study, and (b) a
aDepartment of Chemistry, School of Medicine, Toho University,
Ota-ku, Tokyo 143-8540, Japan. E-mail: yohgo@med.toho-u.ac.jp;
mnakamu@med.toho-u.ac.jp
schematic drawing of them.
bResearch Center for Materials with Integrated Properties, Toho
University, Funabashi 274-8510, Japan
Complexes 1 and 2 were prepared by the condensation reactions
between 3,4-dimethylpyrrole and 3,5-dimethylbenzaldehyde/3,5-di-
tert-butylbenzaldehyde, followed by the insertion of iron using
FeCl2?4H2O in DMF, and then by the addition of a benzene
solution of AgClO4.11 1H NMR (CD2Cl2, 298 K): 1 d = 60.1
(pyrrole-CH3), 3.78 (meta-CH3), 11.7 (ortho-H) and 9.17 (para-H);
2 d = 60.4 (pyrrole-CH3), 11.7 (ortho-H) and 9.12 (para-H). The
large downfield shift of the pyrrole-CH3, as well as of the ortho-
and para-H signals, suggests that these complexes are in the
intermediate-spin state at ambient temperature.12
cDivision of Biomolecular Science, Graduate School of Science, Toho
University, Funabashi 274-8510, Japan
dMolecular Characterization Team, RIKEN, Wako, Saitama 351-0198,
Japan
eDepartment of Chemistry and Materials Science, Tokyo Institute of
Technology, Tokyo 152-8551, Japan
fDivision of Chemistry, Graduate School of Science, Toho University,
Funabashi 274-8510, Japan
{ Electronic Supplementary Information (ESI) available: Temperature
dependent EPR spectra, crystal structure discussion and crystal packing
diagrams. See DOI: 10.1039/b601412g
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Chem. Commun., 2006, 1935–1937 | 1935

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Table 1 Comparison of various geometric parameters in five-
coordinated saddle-shaped iron(III) porphyrin complexes
Fe–Np/ Fe–Xax
sa sb
/
DFe/ N4 area/
sc s2d
Complex
Se Ref.
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1
2
1.946(4) 2.084(4) 0.285(2) 7.45
1.960(2) 2.056(2) 0.232(1) 7.59
This
work
This
work
15
2
3
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2
5
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Fe(OMTPP)Cl
Fe(OETPP)Cl
Fe(OETPP)ClO4 1.963
2.034
2.030
2.247
2.242
2.059
0.464
0.467
0.262
7.87
7.83
7.59
2
2
2
5
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15
7b
3
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a
b
Bond length between the iron and the pyrrole nitrogen. Bond
c
length between the iron and the axial ligand.
Out-of-plane
d
deviation of the iron(III) ion from the N4 plane. The area of the
plane consisting of the four nitrogen atoms of the pyrrole rings.
Spin state at ambient temperature.
e
intermediate-spin species above 70 K. This process turns out to
be reversible because the original spectra were observed when the
temperature was lowered back to 4 K. The temperature
dependence of the EPR spectra could best be explained in terms
Fig. 1 Molecular structures of (a) 1 and (b) 2. Ellipsoids are colored in
grey (carbon), blue (nitrogen), red (oxygen), green (chlorine), cyan
(hydrogen) and magenta (iron). Hydrogen bonds are shown as dotted
lines.
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of the spin transition between S =
2
and S =
. It should be
noted that, while 2 shows an almost complete spin transition
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between S = and S = 2, 1 exists mainly as the intermediate-
2
spin species throughout the temperature range examined. The
population of each species has been determined by computer
simulation of the observed spectra. The van’t Hoff plots yield
DHu and DSu values, corresponding to the equilibrium given in
(eqn. 1), of 60 J mol21 and 48 J mol21 K21 for 1, and 100 J mol21
and 19 J mol21 K21 for 2, respectively.
At first, we expected that the complexes obtained should be
[Fe(OMTArP)]ClO4. However, the X-ray crystallographic analysis
shown in Fig. 1 has revealed that both 1 and 2 are highly saddled
five-coordinated mono(aqua) complexes.13,14 Fig. 2 shows the
perpendicular displacement of the peripheral carbon atoms. The
maximum deviations from the least-squares plane are 1.32–1.35 s,
which are seen in the pyrrole b-carbons for both complexes. Thus,
these complexes can be classified into the group of those that have
a severely saddled porphyrin ring.16 Table 1 lists some geometric
parameters of 1 and 2, together with those of some analogous
complexes.
(1)
Fig. 4 shows the correlation of magnetic susceptibility (x) with 1/T.
The curved line of 1 almost coincides with that of intermediate-
spin Fe(EtioPc)I at higher temperature when 1/T is less than 0.05
(or T is higher than 20 K).18 As 1/T becomes larger than 0.05 (or T
is lower than 20 K), it starts to deviate from the curved line of
Fe(EtioPc)I and approaches that of high-spin Fe(OEP)I. Complex
2 exhibits a similar behavior. These results suggest that the spin
Temperature dependent EPR spectra of 1 and 2 have been
acquired both in frozen CH2Cl2 solution and in the solid state.
Parts of them are given in Fig. 3. Each complex shows a similar
spectral change in both solution and the solid state. The EPR
spectra at 4 K exhibit two types of signals characteristic of high-
spin and intermediate-spin species.17 The g values, determined by
computer simulations of the observed spectra, are as follows: 1,
3
5
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2
transition takes place from the S =
2
to the S =
state as the
temperature is lowered below 20 K.
6.10, 5.90, 2.00 (S = 5
2
), and 4.20, 3.80, 2.10 (S = 3
); 2, 6.10, 5.90,
). To our great surprise,
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2
2.00 (S = 5
2
), and 4.37, 3.77, 1.99 (S = 3
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2
the signal intensities for the intermediate-spin species increased
while those for the high-spin species decreased as the temperature
was raised. Thus, both 1 and 2 exist exclusively as the
Fig. 2 Perpendicular displacement of 24 peripheral atoms from the N4
mean plane: #(red), 1; %(black), 2.
Fig. 3 EPR spectra of (a) 1 taken in frozen CH2Cl2 solution and (b) 2
taken in the solid state at various temperatures.
1936 | Chem. Commun., 2006, 1935–1937
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2 T. L. Poulos, in The Porphyrin Handbook, ed. K. M. Kadish, K. M.
Smith and R. Guilard, Academic Press, New York, 2000, vol. 4,
pp. 189–218.
3 Abbreviations: OEP, TPP, OETPP, TPrPc, TPFC and EtioPc: dianions
of 2,3,7,8,12,13,17,18-octaethylporphyrin, 5,10,15,20-tetraphenylpor-
phyrin, 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin,
2,7,12,17-tetrapropylporphycene, 5,10,15-tris(pentafluorophenyl)corrole
and 3,6,13,16-tetraethyl-2,7,12,17-tetramethylporphycene, respectively.
3-ClPy: 3-chloropyridine.
4 (a) R. W. Scheidt, D. K. Geiger and K. J. Haller, J. Am. Chem. Soc.,
1982, 104, 495; (b) S. Neya, M. Tsubaki, H. Hori, T. Yonetani and
N. Funasaki, Inorg. Chem., 2001, 40, 1220.
5 (a) D. H. Dolphin, J. R. Sams and T. B. Tsin, Inorg. Chem., 1977, 16,
711; (b) H. Masuda, T. Taga, K. Osaki, H. Sugimoto, Z. Yoshida and
H. Ogoshi, Inorg. Chem., 1980, 19, 950.
6 C. A. Reed and F. Guiset, J. Am. Chem. Soc., 1996, 118, 3281.
7 (a) T. Ikeue, T. Saitoh, T. Yamaguchi, Y. Ohgo, M. Nakamura,
M. Takahashi and M. Takeda, Chem. Commun., 2000, 1989; (b)
K. M. Barkigia, M. W. Renner and J. Fajer, J. Porphyrins
Phthalocyanines, 2001, 5, 415; (c) M. Nakamura, T. Ikeue, Y. Ohgo,
M. Takahashi and M. Takeda, Chem. Commun., 2002, 1198; (d)
A. Hoshino, Y. Ohgo and M. Nakamura, Inorg. Chem., 2005, 44, 7333;
(e) T. Ikeue, Y. Ohgo, M. Takahashi, M. Takeda, S. Neya, N. Funasaki
and M. Nakamura, Inorg. Chem., 2001, 40, 3650; (f) K. Rachlewcz,
Fig. 4 Correlations between magnetic susceptibility (x) and inverse
temperature (1/T). #, 1; %, 2; $, Fe(OEP)I; &, Fe(EtioPc)I.
´
L. Latos-Graz˙ynski, E. Vogel, Z. Ciunik and L. B. Jerzykiewicz, Inorg.
The novel spin transition described above should be ascribable
to the unique molecular and crystal structures of 1 and 2. The data
in Table 1 indicate that the out-of-plane deviations of the iron(III)
ion, which are signified as DFe, are quite different from other
saddle-shaped complexes. While the DFe values of high-spin
complexes are 0.46–0.47 s, they are only 0.23–0.29 s in the
intermediate-spin complexes. As the temperature is lowered, the
unit cell volume is expected to shrink.19 As 1 and 2 have highly
saddled structures with bulky substituents at the porphyrin
periphery, as illustrated in Scheme 2, the shrinkage of the unit
cell could directly affect each molecule from the lateral side. As a
result, the narrow N4 cavities of 1 and 2 could further decrease at
extremely low temperature, which could extrude the iron(III) ion
away from the N4 plane. The large out-of-plane displacement of
the iron(III) ion could stabilize the dx2 2 y2 orbital and induce spin
Chem., 2002, 41, 1979; (g) L. Simkhovich, I. Goldberg and Z. Gross,
Inorg. Chem., 2002, 41, 5433.
8 (a) T. Ikeue, Y. Ohgo, T. Yamaguchi, M. Takahashi, M. Takeda and
M. Nakamura, Angew. Chem., Int. Ed., 2001, 40, 2617; (b) Y. Ohgo,
T. Ikeue and M. Nakamura, Inorg. Chem., 2002, 41, 1698.
9 M. Rivera and G. A. Caignan, Anal. Bioanal. Chem., 2004, 378, 1464.
10 M. M. Maltempo, J. Chem. Phys., 1974, 61, 2540.
11 K. M. Barkigia, M. D. Berber, J. Fajer, C. J. Medforth, M. W. Renner
and K. M. Smith, J. Am. Chem. Soc., 1990, 112, 8851.
12 (a) T. Ikeue, Y. Ohgo, O. Ongayi, M. G. H. Vicente and M. Nakamura,
Inorg. Chem., 2003, 42, 5560; (b) SQUID magnetometry has revealed
that the effective magnetic moments for 1 and 2 at 300 K are 3.71 mB
and 3.82 mB, respectively.
13 Crystal and molecular structural data for 1: C60H62ClFeN4O5, M =
1010.44, orthorhombic, a = 14.4113(3), b = 16.9296(4), c = 21.1176(5) s,
V =5152.2(2) s3, T = 110 K, space group P212121 (no. 19), Z = 4,
m(Mo-Ka) = 0.324 mm21, 53820 reflections measured, 18104 unique
(Rint = 0.0787) which were used in all calculations. R(F) = 0.0631 (I >
2s(I)), 0.0891(all data), wR(F2) = 0.156 (I > 2s(I)), 0.166 (all data),
goodness of fit S = 0.941. CCDC 297040. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b601412g. Diffraction
data were collected on a Rigaku MERCURY CCD system installed on
the NW2 beam line of the Advanced Ring (AR), the Photon Factory
(PF) of the High Energy Accelerator Research Organization (KEK)
using a 18.00 keV (l = 0.68878 s) monochromatized X-ray beam.
14 Crystal and molecular structural data for 2: C85H112Cl3FeN4O5, M =
1415.94, triclinic, a =15.160(4), b = 15.846(4), c = 18.698(5) s, a =
68.533(7), b = 79.192(9), c = 83.227(9)u, V = 4100(2) s3, T = 298 K,
space group P-1 (no. 2), Z = 2, m(Mo-Ka) = 0.331 mm21, 58762
reflections measured, 17675 unique (Rint = 0.16) which were used in all
calculations. R(F) = 0.131 (I > 2s(I)), 0.22 (all data), wR(F2) = 0.33 (I >
2s(I)), 0.38 (all data), goodness of fit S = 1.039. CCDC 297041. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b601412g.
transition from the S = 3 2 to the S = 5 2 state. Needless to say, the
X-ray analysis of these complexes at extremely low temperature is
clearly necessary to prove the above hypothesis. This now in
progress in our laboratory.
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In summary, we have found a spin transition between high-spin
and intermediate-spin states by EPR spectroscopy and SQUID
magnetometry for the first time.
This work was supported by a Research Promotion Grant from
Toho University Graduate School of Medicine (no. 05–21 to
Y. O.), and by a Grant-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan (no. 16550061 to M. N.). Thanks are due to the Research
Center for Molecular-Scale Nanoscience, the Institute for
Molecular Science (IMS). The synchrotron radiation experiment
was performed under the approval of the Photon Factory
Program Advisory Committee (proposal no. 2005G021). Thanks
are due to Professor Shin-ichi Adachi for his help during the
synchrotron radiation experiment.
15 R.-J. Cheng, P.-Y. Chen, P.-R Gau, C.-C. Chen and S.-M. Peng, J. Am.
Chem. Soc., 1997, 119, 2563.
16 M. O. Senge, in The Porphyrin Handbook, ed. K. M. Kadish, K. M.
Smith and R. Guilard, Academic Press, New York, 2000, vol. 1,
pp. 239–347.
17 F. A. Walker, in The Porphyrin Handbook, ed. K. M. Kadish, K. M.
Smith and R. Guilard, Academic Press, New York, 2000, vol. 5, pp. 81–
183.
18 Y. Ohgo, S. Neya, T. Ikeue, M. Takahashi, M. Takeda, N. Funasaki
and M. Nakamura, Inorg. Chem., 2002, 41, 4627.
Notes and references
1 S. J. Lippard and J. M. Berg, in Principles of Bioinorganic Chemistry,
University Science Books, Mill Valley, CA, 1994.
19 Y. Ohgo, T. Ikeue, M. Takahashi, M. Takeda and M. Nakamura, Eur.
J. Inorg. Chem., 2004, 4, 798.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 1935–1937 | 1937