Saddle-shaped six-coordinate iron(III) porphyrin complexes showing a novel spin crossover between S = 1/2 and S = 3/2 spin states

Article abstract of DOI:10.1002/1521-3773(20010716)40:14<2617::AID-ANIE2617>3.0.CO;2-B

The field strength of the axial ligands determines the spin state of saddled iron(III) porphyrin complexes (see picture). Strong axial ligands (L), such as imidazole and 4-dimethylaminopyridine, lead to the formation of complexes with a pure S = 1/2 state, while weak ligands, such as THF, give complexes with a pure S = 3/2 state. Intermediate strength ligands, such as pyridine and 4-cyanopyridine, give complexes that show a novel spin crossover between the S = 1/2 and S = 3/2 states.

Full text of DOI:10.1002/1521-3773(20010716)40:14<2617::AID-ANIE2617>3.0.CO;2-B

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Commun. 2000, 271, 334 ± 336; b) J. von Lintig, K. Vogt, J. Biol. Chem.
2000, 275, 11915 ± 11920; c) A. Wyss, G. Wirtz, W.-D. Woggon, R.
Brugger, M. Wyss, A. Friedlein, H. Bachmann, W. Hunziker, Biochem.
J. 2001, 354, 521 ± 529.
We and others recently reported that highly nonplanar
(porphyrinato)iron(iii) complexes with weak axial ligands
show a quite pure intermediate spin state.[3, 4] The results were
ascribed to the short Fe Npor bonds of the nonplanar
porphyrin rings and the weak coordination ability of the axial
ligands.[5] We therefore expected that the spin state of
nonplanar [FeIII(oetpp)L2]ClO4 (1 ± 5) could change from
[11] G. M. Wirtz, C. Bornemann, A. Giger, R. K. Müller, H. Schneider, G.
Schlotterbeck, G. Schiefer, W.-D. Woggon, Helv. Chim. Acta 2001, in
press.
Â
[12] J. Retey, A. Umani-Ronchi, J. Seibel, D. Arigoni, Experientia 1966, 22,
502 ± 503.
[13] J. Devery, B. V. Milborrow, Br. J. Nutr. 1994, 72, 397 ± 414.
[14] a) R. R. French, P. Holzer, M. G. Leuenberger, W.-D. Woggon,
Angew. Chem. 2000, 112, 1321 ± 1323; Angew. Chem. Int. Ed. 2000,
39, 1267 ± 1269; b) W.-D. Woggon, Chimia 2000, 57; c) R. R. French, J.
Wirz, W.-D. Woggon, Helv. Chim. Acta 1998, 81, 1521 ± 1527.
[15] C. Engeloch-Jarret, M. G. Leuenberger, W.-D. Woggon, unpublished
results.
+
L
Et
Et
1: L = HIm
2: L = dmap
3: L = py
Ph
Ph
Ph
Ph
–
ClO4
N
Fe
N
Et
Et
4: L = 4-CNpy
5: L = thf
Et
Et
N
L
Et
Et
[16] G. J. Handelman, M. J. Haskell, A. D. Jones, A. J. Clifford, Anal.
Chem. 1993, 65, 2024 ± 2028.
[Fe(Oetpp)L2]ClO4
the pure S ˆ 1/2 to the pure S ˆ 3/2 state as the axial ligand
changes from strong HIm to weak THF; the order of the
coordination ability is HIm > dmap > py > 4-CNpy > thf.[6] Of
particular interest are the spin states of 2 ± 4 because the axial
ligands of these complexes are ranked between HIm and THF.
Table 1 lists the Mössbauer parameters, isomer shift (IS;
relative to a-iron foil at 290 K), and quadrupole splitting (QS)
measured at ambient and liquid nitrogen temperatures. The
QS values for 1 and 2 at ambient temperature were within the
range of low-spin complexes.[7] The IS and QS values for 4
Saddle-Shaped Six-Coordinate Iron(iii)
Porphyrin Complexes Showing a Novel Spin
Crossover between S ˆ 1/2 and S ˆ 3/2 Spin
States**
Takahisa Ikeue, Yoshiki Ohgo, Tatsuya Yamaguchi,
Masashi Takahashi,* Masuo Takeda, and
Mikio Nakamura*
Table 1. Mössbauer parameters and spin state (S) of 1 ± 5.
Spin states of iron(iii) porphyrins are controlled by the
number and nature of axial ligands.[1] The coordination of
nitrogen bases such as imidazole (HIm) and pyridine results in
the formation of low-spin (S ˆ 1/2) six-coordinate complexes.
In contrast, anionic ligands such as Cl and F lead to the
formation of five-coordinate high-spin (S ˆ 5/2) complexes.
Maltempo discussed a spin-admixed S ˆ 3/2, 5/2 state on the
basis of quantum mechanical calculations, and suggested that
the S ˆ 3/2 state is an important contributor to the spin state of
certain bacterial heme proteins known as cytochromes c'.[2]
T
IS
QS
[mms
G1
G2
S
1
1
1
1
[K] [mms
]
]
[mms
]
[mms ]
1
2
3
4
297 0.18
78 0.26
290 0.19
80 0.26
290 0.32
1.82
1.86
2.21
2.31
2.76
2.29
3.26
3.03
2.70
3.65
3.50
0.24
0.40
0.27
0.55
0.27
0.47
0.32
0.47
0.64
0.32
0.77
0.25
0.62
0.32
0.89
0.29
0.64
0.33
0.47
0.64
0.26
0.49
1/2
1/2
1/2
1/2
3/2 ± 1/2
1/2
3/2
3/2
1/2
80 0.25
site A 295 0.37
site A
site B
80 0.57
80 0.20
290 0.41
80 0.50
5
3/2
3/2
[*] Prof. Dr. M. Nakamura, Dr. T. Ikeue, Dr. Y. Ohgo, T. Yamaguchi
Department of Chemistry
1
Toho University School of Medicine
Tokyo 143-8540 (Japan)
(0.37 and 3.26 mms , respectively) were close to those for 5
1
(0.41 and 3.65 mms ); 5 has been fully characterized as the
Fax : (‡81)3-5493-5430
quite pure intermediate-spin complex.[4] Thus, from the
viewpoint of Mössbauer spectroscopy, 1 and 2 are the low-
spin complexes, while 4 is the intermediate-spin complex at
ambient temperature. Figure 1 shows the Mössbauer spectra
of 3 and 4 taken at ambient temperature and 80 K. The
features change as the temperature is lowered. Complex 4
exhibited a new doublet (site B) below 230 K, and the relative
intensities for this site increased on decreasing the temper-
ature. The values for sites A and B are in the range of
intermediate-spin and low-spin complexes, respectively, and
both spin states co-exist at low temperature. This observation
implies the occurrence of a novel spin-crossover process
[Eq. (1)].[8, 9]
E-mail: mnakamu@med.toho-u.ac.jp
Prof. Dr. M. Takahashi, Prof. Dr. M. Takeda
Department of Chemistry
Faculty of Science, Toho University
Funabashi, 274-8510 (Japan)
Fax : (‡81) 47-475-1855
E-mail: takahasi@chem.sci.toho-u.ac.jp
[**] This work was supported by a Grant in Aid for Scientific Research on
Priority Areas (A; no. 12020257 to M.N.) from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan. T.I. is
grateful to the JSPS for a Research Fellowship for young scientists. We
thank the Research Center for Molecular Materials, the Institute for
Molecular Science (IMS), for support. The authors are grateful to Mr.
Masahiro Sakai of the IMS for the assistance with the SQUID
measurements.
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
low spin (S ˆ 1/2)
>
intermediate spin (S ˆ 3/2)
(1)
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Figure 2. Temperature dependence of the effective magnetic moments of
1 ± 5 taken for microcrystalline samples by SQUID magnetometry. Inset:
Effective magnetic moments determined in CH2Cl2 by the Evans method.
Figure 1. 57Fe Mössbauer spectra of 3 and 4 measured on microcrystalline
samples at ambient temperature and 80 K (as indicated in the parentheses).
While 4 exhibited a new doublet at low temperature, 3
maintained a set of doublets throughout the temperature
range measured. The IS and QS values of 3, however, showed
an unexpected decrease on lowering the temperature and
were almost the same as those of 2 below 150 K. This result
suggests that 3 exists as the low-spin state below 150 K. Since,
however, the IS and QS values of 3 at ambient temperature
were close to those of intermediate-spin complexes, it is also
considered to be a spin-crossover complex. The observation of
only one set of doublets in 3 could be explained in terms of the
constant, 2.1 Æ 0.1 and 4.2 Æ 0.1 mB, respectively, which are
close to the spin-only values expected for the S ˆ 1/2 and S ˆ
3/2 complexes, respectively. As in the case of the solid sample,
the meff value of 3 in solution decreased from 3.5 mB (298 K) to
2.6 mB (193 K), which suggests the spin-crossover process also
occurs in solution. Table 2 shows the 1H NMR chemical shifts
Table 2. 1H NMR chemical shifts recorded in CD2Cl2 at 223 K.
L
meso Phenyl
Pyrrole
Axial ligand
ring protons
o
m
p
CH2
CH3
1.3
1.3
0.3
1.7
1
fast spin transition on the Mössbauer time scale (107 s ) over
1
2
3
4
HIm
dmap
py
2.8
3.2
8.5
4.7
4.8
5.1
4.2
6.0
6.0
8.5
3.2, 11.8
150 ± 290 K when two iron sites are present.
3.4, 12.5
8.2, 27.9
19.3, 55.5
15.8, 2.7
35.9, 29.6
113.7, 72.5
Figure 2 shows the effective magnetic moments (meff) of
microcrystalline samples measured with a SQUID magneto-
meter over 2 ± 300 K. The results confirm that 1 and 2 are in
the S ˆ 1/2 state. A major part of 3 is in the S ˆ 1/2 state below
150 K, but the population of the S ˆ 3/2 state considerably
increases above this temperature. Similarly, 4 is a mixture of
the S ˆ 1/2 and S ˆ 3/2 states below 200 K, although it exists
almost exclusively as the S ˆ 3/2 complex above 200 K. These
results support the occurrence of the spin-crossover process
[Eq. (1)] in 3 and 4, as suggested by the Mössbauer spectros-
copy results. As mentioned, 5 is a quite pure intermediate-spin
complex as shown by the meff value being maintained at 3.80 Æ
0.09 mB in the range 5 ± 300 K.[4] The EPR spectra taken for the
solid samples at 4.2 K are consistent with the conclusions
obtained from the Mössbauer spectroscopy and SQUID
magnetometry measurements.[10]
4-CNpy
14.4
13.7
at 223 K. The large downfield shifts of the CH2 and ligand (L)
signals on going from 2 to 4 suggest that the spin densities at
the b-pyrrole and the ligand carbon atoms increase from 2 !4.
The result can be explained in terms of the increase in
population of the S ˆ 3/2 state relative to that of the S ˆ 1/2
state.[4] Table 3 shows the 13C NMR chemical shifts at
223 K.[11] The most characteristic feature is the large upfield
shift of the meso carbon atom (C Ph) on going from 2 (d ˆ
28) to 4 (d ˆ 451); the chemical shift of the meso carbon
atom at d ˆ 451 (d ˆ 235 at 298 K) is unprecedented in
13C NMR spectroscopy of iron(iii) porphyrins, and suggests
that the major part of 4 is in the S ˆ 3/2 state.[12±14] Thus, the
observation of the extremely upfield-shifted signal of the
meso carbon atom by 13C NMR spectroscopy could be a good
proof for the intermediate-spin complex.[15] In fact, the pure
intermediate-spin complex 5 also showed the signal corre-
The spin states of these complexes in solution have been
studied. The solution magnetic moments (inset, Figure 2)
were determined by the Evans method over the temperature
range 193 ± 298 K. The meff values of 2 and 4 were almost
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Table 3. 13C NMR chemical shifts recorded in CD2Cl2 at 223 K.
L
meso
ipso
o
m
p
Pyrrole(a)
Pyrrole(b)
CH2
CH3
1
2
3
4
HIm
dmap
py
37
166
159
249
458
97
102
31
122
122
124
130
123
123
119
105
143
132
284
590
156
164
223
304
38
40
53
80
105
102
162
264
28
135
451
4-CNpy
160
sponding to the meso carbon atom to be fairly upfield shifted
(d ˆ 265 at 223 K).[16]
saddle-shaped 4 for steric reasons, which further increases the
energy gap.[17] Consequently, [FeIII(TiPrP)(4-CNPy)2]ClO4
maintains the low-spin state. If 4-CNpy is replaced by thf,
both saddle-shaped 5 and ruffled [FeIII(TiPrP)(thf)2]ClO4
show quite pure intermediate-spin characteristics.[4]
Figure 3 shows the temperature dependence of the 13C
chemical shifts of the meso carbon atoms. While the chemical
In conclusion, we have shown that the saddle-shaped
complexes [FeIII(oetpp)L2]ClO4 exhibit a novel spin-crossover
process between the S ˆ 1/2 and S ˆ 3/2 states, and that the
S ˆ 3/2 population increases as the coordination ability of the
axial ligand is weakened. It should be emphasized here that
the novel spin-crossover process in six-coordinate iron(iii)
porphyrins is realized by the tremendous deformation of the
porphyrin rings. The large destabilization of the dx2 orbital
y2
caused by the short Fe Npor bonds insulates the S ˆ 5/2 state
from the other electronic ground states. The spin state of the
iron(iii) ions can hence be finely tuned by the field strength of
the axial ligands, thus leading to the formation of the complex
with the pure S ˆ 1/2 state (1), complexes showing the novel
S ˆ 1/2 > S ˆ 3/2 process (2 ± 4), and the complex with the
pure S ˆ 3/2 state (5).
Figure 3. Temperature dependence of the chemical shifts of the meso 13C
atoms of 1 ± 4 taken in CD2Cl2.
Experimental Section
[FeIII(oetpp)Cl] and meso-13C-enriched [FeIII(oetpp)Cl] were prepared
shift of the meso carbon atom of 3 was quite similar to that of
4 at 298 K, the value approached that of 2 at 200 K. Since 2
and 4 are considered to be the pure S ˆ 1/2 and S ˆ 3/2
complexes, respectively, in this temperature range, the spin-
crossover constant Ksc of 3 is calculated according to
Equation(2).
according to the literature.[18, 19]
5: A solution (15 mL) of AgClO4 (6.1 mg, 2.9 Â 10 5 mol) in THF was
added to a dry solution of [FeIII(oetpp)Cl] (26.1 mg, 2.8 Â 10 5 mol) in THF
(20 mL). After the removal of AgCl by filtration, heptane (20 mL) was
added, and the solution was allowed to stand overnight. The purple crystals
were collected by filtration, washed with hexane, and dried in vacuo for
10 min at 258C to give 25 mg (87%) of 5, which was characterized as the
quite pure intermediate-spin complex.[4] 1 ± 4 were prepared by the addition
of an excess amount of ligand to a solution of 5 in CH2Cl2 followed by
recrystallization from CH2Cl2/heptane. The 1H and 13C NMR data are
shown in Tables 1 and 2, respectively.
Ksc
ˆ
(ddmap dpy)/(dpy d4-CNpy
)
(2)
The plots of log(Ksc) versus 1/T showed a good linear
The 1H and 13C NMR spectra were recorded on
a JEOL LA300
relationship, from which the thermodynamic parameters were
obtained as DH ˆ 16.9 kJmol and DS ˆ 66.6 Jmol 1 K .
Although the spin-crossover process between the S ˆ 1/2
and S ˆ 3/2 states is not unprecedented in iron(iii) com-
plexes,[8, 9] this is, to the best of our knowledge, the first
example in iron(iii) porphyrin complexes.
spectrometer operating at 300.4 MHz for proton spectroscopy. EPR
spectra were recorded on a Bruker E500 spectrometer operating at the
X band and equipped with an Oxford helium cryostat. The solid-state
magnetic susceptibilities were measured between 2 and 300 K under a
magnetic field of 0.5 T with a SQUID magnetometer (Quantum Design
MPMS-7). The measured data were corrected for diamagnetic contribu-
tions. Iron-57 Mössbauer spectra were measured on a Wissel Mössbauer
spectrometer system. The samples were kept in a gas-flow cryostat and the
57Co(Rh) source was kept at room temperature. Isomer shifts (d) are given
relative to a-iron foil at room temperature.
1
1
It is interesting to compare the spin state of saddle-shaped 4
with that of ruffled [FeIII(TiPrP)(4-CNPy)2]ClO4.[6] Although
both complexes are supposed to have short Fe Npor bonds as a
result of the deformation of the porphyrin ring, [FeIII-
(TiPrP)(4-CNPy)2]ClO4 is in the low-spin state with a (dxz,
dyz)4(dxy)1 electron configuration.[14] The result is ascribed to
the difference in the energy gap between the dz2 and dp
orbitals; the energy gap is larger in [FeIII(TiPrP)(4-CNPy)2]-
ClO4 than in 4 since the dp orbitals of the former complex is
located below the dxy orbital. In addition, the coordination of
4-CNPy is much stronger in the ruffled complex than in
Received: February 5, 2001
Revised: May 7, 2001 [Z16551]
[1] W. R. Scheidt in The Porphyrin Handbook, Vol. 3 (Eds.: K. M.
Kadishi, K. M. Smith, R. Guilard), Academic Press, San Diego,
2000, pp. 49 ± 112.
[2] M. M. Maltempo, J. Chem. Phys. 1974, 61, 2540 ± 2547.
Angew. Chem. Int. Ed. 2001, 40, No. 14
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[3] J.-P. Simonato, J. Pecaut, L. L. Pape, J.-L. Oddou, C. Jeandey, M.
Shang, W. R. Scheidt, J. Wojacynski, S. Wolowiec, L. Latos-Grazynski,
J.-C. Marchon, Inorg. Chem. 2000, 39, 3978 ± 3987.
[4] T. Ikeue, T. Saitoh, T. Yamaguchi, Y. Ohgo, M. Nakamura, M.
Takahashi, M. Takeda, Chem. Commun. 2000, 1989 ± 1990.
Molecular Paneling by Coordination: An M15L6
Hexahedral Molecular Capsule having Clefts
for Reversible Guest Inclusion
Kazuhiko Umemoto, Hitoshi Tsukui,
Takahiro Kusukawa, Kumar Biradha, and
Makoto Fujita*
[5] D. R. Evans, C. A. Reed, J. Am. Chem. Soc. 2000, 122, 4660 ± 4667.
[6] Abbreviations: oetpp, TiPrP, and TEtP are dianions of
2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin, meso-
tetraisopropylporphyrin, and meso-tetraethylporphyrin, respectively.
dmap: 4-dimethylaminopyridine; py: pyridine; 4-CNpy: 4-cyanopyr-
idine; HIm: imidazole; 2-MeIm: 2-methylimidazole.
[7] J. Sams, T. B. Tsin in The Porphyrins, Vol. IV (Ed.: D. Dolphin),
Academic Press, New York, 1979, pp. 425 ± 478.
[8] K. D. Hodges, R. G. Wollmann, S. L. Kessel, D. N. Hendrickson, D. G.
Van Derveer, E. K. Barefield, J. Am. Chem. Soc. 1979, 101, 906 ± 917.
[9] W. O. Koch, V. Schünemann, M. Gerdan, A. X. Trautwein, H.-J.
Krüger, Chem. Eur. J. 1998, 4, 686 ± 691.
The construction of three-dimensional (3D) molecular
structures by linking two-dimensional (2D), planar organic
components through metal coordination provides a new
concept that is termed molecular paneling.[1] A family of 2D
components are coordinated by transition metals to give rise
to various hollow 3D polyhedral structures.[2±8] The triangular
exo-hexadentate ligand 2 has been recently shown to give
M18L6 hexahedral coordination capsule 4 by linking together
with PdII building block 1.[4b] This coordination capsule has a
[10] EPR measurements: 1: g ˆ 2.79, 2.35, 1.63 (S ˆ 1/2); g ˆ 3.25 (S ˆ 1/2).
2: g ˆ 3.06, 2.14, 1.42 (S ˆ 1/2). 3: g ˆ 3.39, 2.08 (S ˆ 1/2). 4: g ˆ 4.28,
3.80, 2.08 (S ˆ 3/2).
[11] The signals corresponding to the meso carbon atoms were assigned
unambiguously by the use of meso-13C-enriched complexes. The a-
pyrrole and ipso-phenyl signals were assigned on the basis of their
coupling with the meso carbon atom of the labeled complexes.
[12] High-spin and low-spin complexes show meso signals at d ˆ 500 ± 600
and 50 ± 100, respectively, at room temperature.[13] Some low-spin
complexes with the less common (dxz, dyz)4(dxy)1 electronic config-
uration exhibit the signals further downfield (d ˆ 700 ± 800).[14]
[13] ªPhysical Bioinorganic Chemistry Series 1º: H. Goff in Iron Porphyr-
in, Part
I (Eds.: A. B. P. Lever, H. B. Gray), Addison-Wesley,
Reading, 1983, pp. 237 ± 281.
[14] T. Ikeue, Y. Ohgo, T. Saitoh, M. Nakamura, H. Fujii, M. Yokoyama, J.
Am. Chem. Soc. 2000, 122, 4068 ± 4076.
[15] The large downfield shifts of the a- and b-pyrrole signals are
consistent with the occupancy of unpaired electrons in both the dxz
and dyz orbitals; the unpaired electrons in these orbitals are trans-
ferred to the pyrrole carbon atoms by a dp-3eg interaction. Since the
3eg orbital has zero electron density at the meso carbon atom, the large
upfield shift of the meso signal is ascribed to the spin polarization from
the neighboring a-pyrrole carbon atom.[14]
[16] The extent of the upfield shift of the meso carbon atom is not
necessarily proportional to the S ˆ 3/2 contribution. The large differ-
ence in the shifts of the meso carbon atoms between 4 and 5 (Dd ˆ 186
at 223 K) could be the indication that the dp-3eg interaction in 4 is
much stronger than that in 5 as a result of the difference in the nature
of the axial ligands.
[17] Although the X-ray crystal structures of
4
and [FeIII(TiPrP)(4-
CNpy)2]ClO4 are not available at present, a preliminary result has
shown that saddled 3 has much longer Fe N(axial ligand) bonds than
ruffled [FeIII(TEtP)(2-MeIm)2]Cl; the average bond lengths are
2.203(3) and 2.02(1) Š, respectively. Y. Ohgo, T. Ikeue, M. Nakamura,
unpublished results.
very stable closed-shell structure which makes it difficult to
encapsulate guest molecules. We have designed another
molecular panel 3, which is similar to 2 but misses one
binding site, to prepare a hexahedral capsule with more
[18] K. M. Barigia, M. D. Berber, J. Fajer, C. J. Medforth, M. W. Renner,
K. M. Smith, J. Am. Chem. Soc. 1990, 112, 8851 ± 8857.
[19] R.-J. Cheng, P.-Y. Chen, P.-R. Gau, C.-C. Chen, S.-M. Peng, J. Am.
Chem. Soc. 1997, 119, 2563 ± 2569.
[*] Prof. Dr. M. Fujita,[‡] H. Tsukui, Dr. T. Kusukawa, Dr. K. Biradha
Department of Applied Chemistry
Graduate School of Engineering, Nagoya University
Chikusaku, Nagoya 464-8603 (Japan)
Fax : (‡81)52-789-3199
E-mail: mfujita@apchem.nagoya-u.ac.jp
Dr. K. Umemoto
The Graduate University for Advanced Studies
Myodaiji, Okazaki 444-8585 (Japan)
‡
[ ] Prof. Fujita is responsible for the Core Research for Evolutional
Science and Technology (CREST) project of the Japan Science and
Technology Corporation (JST).
Supporting information for this article is available on the WWW under
http://www.angewandte.com or from the author.
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