Resonance Raman studies of dioxygen binding to ortho-substituted tetraphenyl- and tetranaphthyl-porphyrinatoiron(II) derivatives with a covalently linked axial imidazole

Article abstract of DOI:10.1039/a802925c

Resonance Raman spectra with 457.9 nm laser excitation have been recorded for the deoxy and oxy complexes of ortho-substituted tetraphenyl- and tetranaphthyl-porphyrinatoiron(II) derivatives with a covalently linked proximal imidazole. The intramolecular imidazole co-ordination gives high-spin iron(II) states in the deoxy forms of these complexes, which are revealed by the appearances of the indicative v₈ bands of the porphyrin ring at 367-375 cm^-1. Their Fe-N (imidazole) stretching modes were directly detected at 201-223 cm^-1, and are discussed in relation to the Fe-N (imidazole) bond strength and the geometry of the imidazole co-ordination. As a result, a significant tilting of the imidazole ring plane from the Fe-N (imidazole) vector is proposed for the double-side encumbered derivatives. The dioxygenation was monitored by the upshift of the porphyrin skeletal band (v₄). The dioxygen adducts of the double-side encumbered porphyrins showed high iron-dioxygen stretching frequencies (579 and 580 cm^-1) relative to those of oxyhemoproteins and other dioxygenated porphyrinatoiron(II) complexes generally observed in the range 568-573 cm^-1. These high frequencies are considered to reflect the decreased Fe-O-O angle induced by the narrow ester-linked cavity. For complexes of 5,10,15,20-tetrakis(o-pivalamidophenyl)porphyrinatoiron(II) with a covalently attached imidazolyl group at the β-pyrrole position the v(Fe-Nε) and v(Fe-O₂) frequencies were found to be fairly similar to those of the same complexes with externally added imidazole ligands. This indicates that the covalent linkage between the imidazole and the porphyrin periphery is flexible and long enough to allow the formation of stable O₂-adduct species.

Full text of DOI:10.1039/a802925c

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Resonance Raman studies of dioxygen binding to ortho-substituted
tetraphenyl- and tetranaphthyl-porphyrinatoiron(II) derivatives with
a covalently linked axial imidazole
Jian Wu, Teruyuki Komatsu and Eishun Tsuchida*,†
Department of Polymer Chemistry, Advanced Research Institute for Science and Engineering,
Waseda University, Tokyo 169-8555, Japan
Resonance Raman spectra with 457.9 nm laser excitation have been recorded for the deoxy and oxy complexes
of ortho-substituted tetraphenyl- and tetranaphthyl-porphyrinatoiron() derivatives with a covalently linked
proximal imidazole. The intramolecular imidazole co-ordination gives high-spin iron() states in the deoxy
forms of these complexes, which are revealed by the appearances of the indicative ν8 bands of the porphyrin ring
at 367–375 cmϪ1. Their Fe᎐N (imidazole) stretching modes were directly detected at 201–223 cmϪ1, and are
discussed in relation to the Fe᎐N (imidazole) bond strength and the geometry of the imidazole co-ordination.
As a result, a significant tilting of the imidazole ring plane from the Fe᎐N (imidazole) vector is proposed for
the double-side encumbered derivatives. The dioxygenation was monitored by the upshift of the porphyrin
skeletal band (ν4). The dioxygen adducts of the double-side encumbered porphyrins showed high iron–dioxygen
stretching frequencies (579 and 580 cmϪ1) relative to those of oxyhemoproteins and other dioxygenated
porphyrinatoiron() complexes generally observed in the range 568–573 cmϪ1. These high frequencies are
considered to reflect the decreased Fe᎐O᎐O angle induced by the narrow ester-linked cavity. For complexes
of 5,10,15,20-tetrakis(o-pivalamidophenyl)porphyrinatoiron() with a covalently attached imidazolyl group at
the β-pyrrole position the ν(Fe᎐Nε) and ν(Fe᎐O2) frequencies were found to be fairly similar to those of the same
complexes with externally added imidazole ligands. This indicates that the covalent linkage between the imidazole
and the porphyrin periphery is flexible and long enough to allow the formation of stable O2-adduct species.
In natural hemoproteins the two axial ligands of the heme
prosthetic group are significantly involved in their biological
Experimental
The corresponding bromides of compounds 1–4, {5-[2-(5-
imidazol-1-ylvaleryloxy)-6-(pivaloyloxy)phenyl]-10,15,20-tris-
[2,6-bis(pivaloyloxy)phenyl]porphyrinato}iron() bromide,
{2-[(8-imidazol-1-yl)octanoyloxymethyl]-5,10,15,20-tetrakis-
functions. The proximal histidine (E7) co-ordinated to the
protoheme in hemoglobin plays a crucial role in regulation of
the O2-binding affinity, and is a trigger for the allosteric
phenomena observed in the dioxygenation process. To get an
insight into the electronic structure of the hemes, several
synthetic porphyrinatoiron() complexes with an intra-
molecular co-ordinated axial base have been prepared.1–5 More
recently, we have synthesized a series of ortho-substituted tetra-
phenyl- and tetranaphthyl-porphyrinatoiron() derivatives 1–4,
in which an N-alkylated imidazole is covalently linked to the
porphyrin periphery.6–8 It has been found that these iron()
complexes can reversibly bind dioxygen in toluene at ambient
temperature. In particular, the double-side encumbered models,
1 and 2, could not form an unfavorable µ-oxo porphyrin
dimer, producing a kinetically stable O2-adduct species. These
four compounds, differing in the structure of substituents on
the porphyrin macrocycle, exhibited varied O2-binding affinities
and kinetics. Consequently, the steric and electronic effects of
the substituent structure on the iron–ligand bonding are of cur-
rent interest to us and resonance Raman (RR) spectroscopy has
been employed as the probe. It is known that Soret excitation of
the synthetic hemes normally gives rich RR spectra consisting
of totally symmetric vibrations of the porphyrin core; some of
them have been identified and found to be sensitive to the axial
ligation.9–11 Especially, the low-frequency (<600 cmϪ1) vibration
associated with motions of the iron atom can be clearly
detected with this technique. We report here the RR spectra of
1–4 and their dioxygen adducts. The observed iron–imidazole
(Fe᎐Nε) and –dioxygen (Fe᎐O2) stretching frequencies are
discussed in terms of the bond strength and geometry.
(2-pivaloyloxynaphthyl)porphyrinato}iron()
bromide,
{2-[(8-imidazol-1-yl)octanoyloxymethyl]-5,10,15,20-tetrakis(o-
pivalamidophenyl)porphyrinato}iron() bromide, and {2-[8-
(2-methylimidazol-1-yl)octanoyloxymethyl]-5,10,15,20-tetra-
kis(o-pivalamidophenyl)porphyrinato}iron() bromide, were
synthesized as previously described.6–8 Dichloromethane
(special grade for fluorescence analysis, Kanto Chemical Co.,
Tokyo) was used without further treatment. The 18O2 (>99%
18O) gas was obtained from Isotec Inc. (USA).
Reduction of the iron() bromides in CH2Cl2 with aqueous
Na2S2O4 under anaerobic conditions afforded the iron() com-
plexes (deoxy form). After the formation of the deoxy complex
had been confirmed by the UV/VIS absorption spectrum,6–8 the
CH2Cl2 layer (ca. 1 mmol dmϪ3) was transferred to a rubber-
septum quartz cell (2 mm thickness) for Raman spectroscopic
measurement. Dioxygenation was then achieved by bubbling
oxygen gas into the solution for 1 min.
Resonance Raman spectra were obtained with excitation of
the 457.9 nm line of a NEC GLG2162 Arϩ ion laser in a back-
scattering geometry at 25 ЊC, using a JASCO NRS-2000 laser
Raman spectrometer equipped with a CCD multichannel
detector. The spectrometer was calibrated with indene.
Results and Discussion
Deoxy complexes
Low-frequency RR spectra of the iron() complexes 1–4 under
argon, namely the deoxy form, are shown in Fig. 1. Complexes
1, 2 and 3 gave a band at 222, 223 and 222 cmϪ1, respectively,
† E-Mail: eishun@mn.waseda.ac.jp
J. Chem. Soc., Dalton Trans., 1998, Pages 2503–2506
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O
O
O
O
N
O
O
O
N
N
O
Fe
N
N
O
O
O
O
O
N
O
O
O
1
O
O
O
O
O
O
O
N
N
O
Fe
N
N
N
O
O
N
Fig. 1 Low-frequency RR spectra of the deoxy complexes 1 (a), 2 (b),
3 (c) and 4 (d) in CH2Cl2 with 457.9 nm excitation. Laser power,
60 mW; accumulation time, 60 s; three scans; sample concentration,
1 mmol dmϪ3; s denotes solvent bands
2
as ≈6 cmϪ1 when replacing the imidazole moiety with methyl-
imidazole. Thus the observed larger shift represents a weakened
Fe᎐N (imidazole) bond in 4 caused by the steric hindrance
of the 2-methyl group on the imidazole ring.
O
O
O
Interestingly, the ν(Fe᎐Nε) frequencies of complexes 1 and 2
are almost the same as that of 3. The double-sided porphyrins
are expected to have a weak Fe᎐N (imidazole) bond due to the
steric hindrance of the rear-side substituents. An infrared spec-
troscopic study of 1 and 2 showed a significant high ν(CO) at
1979 cmϪ1 relative to those of single-face hindered derivatives,
e.g. 1969 cmϪ1 for 5 bonded to mim and CO.6,8 This is presum-
ably caused by the low π-back donation from the central iron to
the bound CO, due to the weak trans-imidazole co-ordination.
The O2- and CO-binding kinetic parameters also supported
this assumption.6,8 Nevertheless, the stretching frequency is
generally determined by both the bond strength and structure
factors including the geometry of the bond. Studies of the
carbon monoxide complexes of hemoproteins, for example,
demonstrated that ν(Fe᎐C) increased according to the inclin-
ation of the Fe᎐C᎐O bond angle.13 This has been attributed to
the reduction of the effective mass of CO for the Fe᎐CO
stretching vibration as the Fe᎐C᎐O angle decreases.14 Results of
molecular simulations‡ of 1 and 2 showed that the imidazole
ring plane is significantly tilted from the Fe᎐N (imidazole) vec-
tor with an angle (ψ) of ca. 150–160Њ, while this angle is almost
180Њ for 3. The inclination of the imidazole ring, normally
resulting in a weakened Fe᎐N (imidazole) bond, may also lead
to a lowering of the effective mass of the imidazole moiety.
Such opposite effects on the stretching frequency probably pro-
duce a higher value than the expected one. Therefore, the
observed ν(Fe᎐Nε) frequencies of 1 and 2 could be interpreted
as corresponding to the tilted geometry of the imidazole ring.
HN
N
NH
O
HN
O
N
N
R
NH
N
Fe
N
O
N
R = H
R = CH3
3
4
whereas 4, an analog of 3 with a C2-methylated imidazole, gave
an overlapped band at 201 cmϪ1. Similar bands were reported
for five-co-ordinated [5,10,15,20-tetrakis(o-pivalamidophenyl)-
porphyrinato]iron() complexes 5 with externally added imid-
azole ligands, which have been assigned to the iron–imidazole
stretching mode.12 Accordingly, the bands at 201–223 cmϪ1
observed here are assigned to the ν(Fe᎐Nε) frequencies for the
intramolecular five-co-ordinated complexes 1–4.
The ν(Fe᎐Nε) frequency of complex 3 at 222 cmϪ1 is close to
that of 5 complexed with 1-methylimidazole (mim) at 225 cmϪ1,
and the same is true for 4 at 201 cmϪ1 and 5 complexed with
1,2-dimethylimidazole (dmim) at 200 cmϪ1. This indicates that
the covalent linkage between the imidazole and the porphyrin
periphery is flexible and long enough to allow intramolecular
iron–imidazole co-ordination. Compared with 3, complex 4
shows an ≈20 cmϪ1 reduced ν(Fe᎐Nε) frequency. Assuming a
diatomic oscillator model and an identical force constant for
the Fe᎐Nε stretching vibration, the frequency shift is calculated
‡ The ESFF forcefield simulation was performed using an Insight II
system (Molecular Simulation Inc.). The structure was generated by
alternative minimizations and annealing dynamic calculations.
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Fig. 2 The RR spectra in the 1200–1400 cmϪ1 region of the deoxy (dotted line) and oxy (solid line) complexes 1 (a), 2 (b), 3 (c) and 4 (d) in CH2Cl2
with 457.9 nm excitation. Measurement conditions for the deoxy complexes as in Fig. 1. Conditions for the oxy complexes: laser power, 5 mW;
accumulation time, 360 s; three scans
Furthermore, several porphyrin skeletal modes have been
identified for hemoproteins and some synthetic hemes, the
frequencies of which are sensitive to the spin state.11 Systematic
studies on tetraphenylporphyrinatoiron() derivatives showed
that the observation of a band at 365–370 or 380–390 cmϪ1 is
indicative of high-spin five-co-ordination or low-spin six-co-
ordination, respectively.15 This band has been assigned to a
deformation mode of the porphyrin ring (ν8).16 The appearance
of a ν8 band at 367–375 cmϪ1 for all the complexes 1–4 under
argon clearly showed the formation of the high-spin five-co-
ordination states (Fig. 1).
Dioxygen adducts
The high-frequency region of the RR spectra of tetraphenyl-
porphyrinatometal derivatives exhibits a strong band at 1340–
1390 cmϪ1, which has been assigned to ν4, a deformed pyrrole-
ring breathing-like mode.17 The ν4 frequency depends on the
electron density in the π* antibonding orbital of the porphyrin
core,18 and hence is sensitive to the axial ligation state of the
central iron. It was found that ν4 bands of complexes 1–4 at
1350–1355 cmϪ1 under argon, which correspond to the five-co-
ordinated high-spin state, shifted immediately to 1369–1375
cmϪ1 after exposure to dioxygen (Fig. 2). This observation
indicates the formation of six-co-ordinated low-spin species,
Fig. 3 Low-frequency RR spectra of 16O2 (a) and 18O2 (b) adducts
of complex 1, and the difference spectrum (c). Laser power, 5 mW;
namely the oxy form. In the spectra of the dioxygenated com-
plexes 1–4, however, the deoxy signals still remain probably due
to photodissociation of the bound O2 during the measurement.
Fig. 3 shows the low-frequency RR spectra of 16O2 and 18O2
adducts of complex 1. A band at 579 cmϪ1 in the spectrum of
the 16O2 adduct shifted to 557 cmϪ1 upon 18O2 substitution. This
is indicated by the difference spectrum in which the strong
porphyrin bands at 385 cmϪ1 are completely cancelled. Con-
sequently, the Fe᎐O2 stretching mode of the oxy complex of
1 is identified at 579 cmϪ1. The low-frequency RR spectra of
the dioxygen adducts of 2, 3 and 4 are shown in Fig. 4, where
the ν(Fe᎐O2) vibrations appear at 580, 566 and 565 cmϪ1,
respectively.
accumulation time, 360 s; three scans; sample concentration, 1 mmol
dmϪ3; s denotes solvent bands
cmϪ1) are nearly identical, while the O2-binding affinity of 3 is
135 times higher than that of 4. Such a lack of correlation is
consistent with previous results for 5 and imidazole-appended
hemes.12,14
The ν(Fe᎐O2) frequencies for the dioxygen adducts of
complexes 3 and 4 are very close to those of 5 with mim and
dmim,12 which were both reported as 568 cmϪ1. Combining the
results of the ν(Fe᎐Nε) and ν(Fe᎐O2) vibrations, it can be con-
cluded that the intramolecular complexes 3 and 4 have similar
iron–ligand bonding characters to those of the corresponding
intermolecular ones, respectively.
It is noteworthy that the ν(Fe᎐O2) frequencies for the oxy
forms of complexes 1 and 2 are appreciably high compared to
those of oxyhemoproteins19 and other dioxygenated porphyrin-
atoiron() complexes11,12,14 generally observed in the range
568–573 cmϪ1. The high frequency probably has its origin in the
Comparison of these frequencies with the O2-binding
₁
affinities [P (O2)] of the iron() complexes 1, 2, 3 and 4 which
₂
have been determined as 13, 18, 0.29 and 38 Torr (Torr ≈ 133
Pa) (25 ЊC, toluene), respectively,6–8 shows that the ν(Fe᎐O2)
₁
frequency lacks a straightforward relation with P (O2). In
₂
particular, the ν(Fe᎐O2) frequencies for 3 (566 cmϪ1) and 4 (565
J. Chem. Soc., Dalton Trans., 1998, Pages 2503–2506
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ably reflects a decreased Fe᎐O᎐O angle which is caused by the
steric hindrance in the ester-linked cavity.
In conclusion, the double-side encumbered complexes 1 and
2 were found to exhibit unique ν(Fe᎐Nε) and ν(Fe᎐O2) vibra-
tions, which are interpreted in terms of the bond geometry.
We are currently attempting to prepare crystals of these
porphyrinato-iron and -zinc complexes to get more detailed
information about the structures.
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compounds. A study on the CO adducts of capped hemes
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Fe᎐CO stretching frequency, which has been ascribed to the
increased Fe→CO back bonding caused by the polar inter-
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Received 20th April 1998; Paper 8/02925C
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J. Chem. Soc., Dalton Trans., 1998, Pages 2503–2506