Self-assembly of meso-mono-4-pyridyltriphenylporphyrinatoiron(II) in sublimed layers. Interaction with molecular oxygen

Article abstract of DOI:10.1023/A:1023415001180

Low-temperature (T = 80 K) interaction of the sublimed layers of meso-mono-4-pyridyltriphenylporphyrinatoiron(II) (FeMPyTPP) with dioxygen was studied by IR and electronic absorption spectroscopies. Unlike the meso-tetraphenylporphyrinatoiron(II) (FeTPP)

Full text of DOI:10.1023/A:1023415001180

394
Russian Chemical Bulletin, International Edition, Vol. 52, No. 2, pp. 394—397, February, 2003
Selfꢀassembly of mesoꢀmonoꢀ4ꢀpyridyltriphenylporphyrinatoiron(II)
in sublimed layers. Interaction with molecular oxygen
c
c
T. S. Kurtikyan,a,b R. K. Kazaryan, and V. N. Madakyan
ꢀ
aMolecular Structure Research Center, National Academy of Sciences,
26 Azatutyan prosp., 375014 Yerevan, Republic of Armenia
Fax: (374 1) 23 2145. Eꢀmail: tkurt@msrc.am
bResearch Institute of Applied Chemistry,
70 Bagratunyants prosp., 375005, Yerevan, Republic of Armenia
cM. Geratsi Yerevan Medical State University,
5 ul. Koryuna, 375025 Yerevan, Republic of Armenia
Lowꢀtemperature (T = 80 K) interaction of the sublimed layers of mesoꢀmonoꢀ
4ꢀpyridyltriphenylporphyrinatoiron(II) (FeMPyTPP) with dioxygen was studied by IR and
electronic absorption spectroscopies. Unlike the mesoꢀtetraphenylporphyrinatoiron(II) (FeTPP)
layers, coordination of O2 with FeMPyTPP produces extraꢀcomplexes of two types: in the first
complex one of the axial sites is free, while in the second complex an axial site is occupied by
the pyridine group of the adjacent molecule. The results obtained indicate selfꢀassembly of the
FeMPyTPP molecules through coordination bonds between the iron atom and pyridine group
of the adjacent molecule in the layer. The bonding of O2 by the sublimed FeMPyTPP layers
differs substantially from that by the FeTPP layers, which rapidly loose their ability of oxygen
bonding at room temperature.
Key words: mesoꢀmonoꢀ4ꢀpyridyltriphenylporphyrinatoiron(II), sublimed layers, oxygen
mediators, electronic absorption spectroscopy, IR spectroscopy.
Reversible bonding of dioxygen is one of the most
In this work, we synthesized mesoꢀmonoꢀ4ꢀpyridylꢀ
triphenylporphyrinatoiron(II) (FeМPyТРP) and studied
the O2ꢀbonding ability of its sublimed layers prepared
under various conditions.
important properties of hemeꢀcontaining and related comꢀ
pounds, providing them biological and catalytic activꢀ
ity.1,2 Some metal porphyrins, being efficient oxygen meꢀ
diators, are of interest as oxygen absorbers,3 artificial heꢀ
moglobin,4 and membranes with selective oxygen permeꢀ
ability.5
Experimental
Thin porphyrin layers are promising for the use in
photovoltaic cells,6 gas sensors,7 and heterogeneous cataꢀ
lytic systems.8,9
Monopyridyltriphenylporphyrin (Н2МРуТРР) was syntheꢀ
sized using the method of mixed aldehydes.12 Its purity was
confirmed by the data of thinꢀlayer chromatography, NMR,
and UV spectroscopy coincided with the published data.12 Since
the FeII porphyrin complexes are readily oxidized in air to form
µꢀoxo dimers, we used stable in air hexacoordinate dipiperidine
extraꢀcomplex FeMPyТРP(Pip2) (Pip is piperidine) synthesized
from Н2МРуТРР using a described procedure.13
A weighted sample (∼10 mg) of FeМPyТРP(Pip2) was placed
in a Knudsen cell and stored in high vacuum (∼10–5 Torr) at
∼250 °C for ∼3 h. This storage results in the elimination of the
axial ligands (monitored by measurement of the pressure at the
outlet of the cryostat). Then at the same pressure the temperaꢀ
ture of the cell was raised to 300—320 °C, and FeMPyTPP was
sublimed at this temperature. To prepare layers with a thickness
convenient for IR spectroscopic measurements, sublimation was
carried out for 2—3 h, while it took several of minutes to record
electronic absorption spectra (EAS) at the Soret band or several
tens of minutes for recording bands in the visible region. The СsI
and СaF2 plates (for recording IR spectra and EAS, respecꢀ
tively) placed inside an optical cryostat were cooled with liquid
Studies of sublimed layers of mesoꢀtetraphenylporꢀ
phyrinatoiron(II) (FeTPP)10 and structurally similar
mesoꢀtetraꢀ4ꢀpyridylporphyrinatoiron(II) (FeTPyP)11
have shown that these films loose their О2ꢀbonding ability
after storage at room temperature. In the FeTPyP layers
this process is strongly retarded, and the spectral data
indicate that this is caused by the formation hexaꢀ
coordinate saturated complexes in the layer due to the
interaction of the metal ions with the pyridine groups
of the adjacent molecules. It could be expected that
a decrease in the number of pyridine groups in the
mesoꢀpositions of the porphyrin macrocycle (to the ratio
of the number of metal ions to pyridine groups equal to
unity) would diminish the probability of formation of
hexacoordinate structures and improve the oxygenꢀbondꢀ
ing characteristics of the layer.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 377—380, February, 2003.
1066ꢀ5285/03/5202ꢀ394 $25.00 © 2003 Plenum Publishing Corporation

Selfꢀassembly of iron(II) mesopyridylporphyrin
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 2, February, 2003
395
nitrogen and used as supports. Thoroughly dried above Р2О5
and precooled oxygen was slowly supplied to the cryostat from a
receiver equipped with a manometer, which measured the equiꢀ
librium pressure in the cryostat. Further the sublimed layer was
stored for ∼30 min, and the temperature was changed in the
following sequence: 80 → 120 → 80 K. Then oxygen was reꢀ
moved, and EAS or IR spectra were recorded.
IR spectra were recorded on a Specord Mꢀ80 spectrophoꢀ
tometer. The spectral gap width was 4 cm–1. Electronic absorpꢀ
tion spectra were obtained on a Specord Мꢀ40 spectrophotomꢀ
eter. NMR spectra were recorded using a Varian Mercury 300
spectrometer in a CDCl3 solution.
Two absorption bands of stretching vibrations of the
coordinated О2 and a change in their intensity after temꢀ
perature training of the sample are similar to those obꢀ
served previously for the sublimed СоMPyTPP samples.14
However, for the latter the difference in the ν(O2) freꢀ
quencies of two new bands is 80 cm–1, and the presence
of two types of bound О2 is manifested in the IR spectra to
a greater extent.
The EAS (Fig. 2) undergo remarkable changes in the
presence of O2 also upon temperature training of the
sample. The Soret band in the lowꢀtemperature sublimed
FeМРуТРР sample exhibits a longꢀwave shoulder at
435 nm. In the presence of О2, the maximum of the band
undergoes a bathochromic shift (dotted line), which inꢀ
creases after sample training in the 80 → 200 → 80 K cycle
(see Fig. 2, dotted line). The band in the visible region
behaves similarly. For the sample heated to room temꢀ
perature under dynamic vacuum (see Fig. 2, dotted line)
where the oxygen complex decomposes, the maximum of
the Soret band shifts toward the wavelength region in
which the longꢀwave shoulder was earlier observed. The
band in the visible region structurizes and manifests a
pronounced shoulder at 600 nm. An analogous EAS is
observed for the sublimed samples prepared by deposition
on the surface at room temperature. Unlike the IR specꢀ
tra, manifestations of dioxygen coordination in the EAS
of FeМРуТРР are much stronger than those for the preꢀ
viously studied14 СоMPyTPP complexes.
Results and Discussion
When О2 is let on the thin (to 5 µm) FeМРуТРР
layers prepared by sublimation on a surface cooled by
liquid nitrogen, two new bands at 1150—1200 cm–1 (see
Fig. 1, b) appear in the IR spectrum. They are localized in
the immediate vicinity of the absorption band of the metal
porphyrin at 1178 cm–1 (Fig. 1, а) and can distinctly be
seen only in the difference spectra (Fig. 1, c) with maxima
at 1184 and 1173 cm–1. An increase in the temperature of
the layer to 200 K followed by cooling results in an increase
in the intensity of the lowꢀfrequency band (Fig. 1, d).
When oxygen is let on the layer prepared by sublimation
of the porphyrin on the surface at room temperature,
mainly the lowꢀfrequency band with a maximum at
1173 cm–1 appears in the difference spectrum.
These results can be interpreted as follows. The abꢀ
sorption of О2 by the lowꢀtemperature sublimed FeТРР
samples with microporosity and ability to coordinate poꢀ
tential ligands over the whole layer thickness has previꢀ
ously10 been found. The band at 1187 cm–1 was assigned10
to ν(O2) in the pentacoordinate O2•FeТРР adduct found
T
a
previously15 in an argon matrix with ν(O2) = 1195 cm–1
.
In similar experiments11 with mesoꢀtetraꢀ4ꢀpyridylporꢀ
b
c
A
1.4
1.2
× 5
1.0
0.8
0.6
0.4
0.2
d
10%
1200 1180 1160 ν/cm–1
400
450
500
550 600
λ/nm
Fig. 1. IR spectrum of the sublimed FeМРуТРР layer at 80 K in
the ν(O2) region of coordinated dioxygen (a), after inlet of O2
(28 mm Hg) into a cryostat and annealing in the 80 → 120 → 80 K
cycle (b), difference spectrum b—a (c), and difference spectrum
after the 80 → 200 → 80 K cycle (d).
Fig. 2. Electronic absorption spectrum of the sublimed
FeМРуТРР layer at 80 K before (solid line) and after inlet of O2
(30 mmHg) into a cryostat and annealing in the 80 → 120 → 80 K
(dotted line) and 80 → 200 → 80 K (dashed line) cycles, and after
heating of the layer in vacuo to room temperature (dashed line).

396
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 2, February, 2003
Kurtikyan et al.
solutions of mesoꢀsubstituted ferroporphyrins in the
pentacoordinate highꢀspin state.20—22 Such a pattern is
observed when a nitrogen base acts as an extraꢀligand. At
the same time, the dioxygen complex exists in the lowꢀ
spin state,22 whereas the ferroporphyrin containing no
axial ligands demonstrates the state with an intermediate
spin.23 Since the energies of the dπꢀorbitals of iron and
еgꢀorbitals of porphyrin are close, the EAS are very sensiꢀ
tive to the spin state of Fe atom in the complex. This is
clearly confirmed by the spectra presented in Fig. 2.
The amounts of bound oxygen in the sublimed FeTPP
layers stored in vacuo at room temperature followed by
cooling and oxygen supply are insufficient for its spectroꢀ
scopic detection. The FeTPP layers prepared by sublimaꢀ
tion on the surface at room temperature behave similarly.
At the same time, in both the first and second cases, the
FeMPyTPP layers remain active toward О2 binding. These
layers retain their activity for much longer time (for weeks)
than the FeTPyP layers, which loose completely their
ability to coordinate О2 for a week. The spectroscopic
measurements show that the FeMPyTPP layers, unꢀ
like FeTPP, are oxidized very slowly (increase in the
Fig. 3. Structures of the pentaꢀ and hexacoordinate dioxygen
complexes in the sublimed FeМРуТРР layer.
νas(Fe—O—Fe) band of the µꢀoxo dimer at 870 cm–1
)
even upon storage in air.
phyrinatoiron(II), the ν(O2) band appeared at 1172 cm–1
and was attributed to a hexacoordinate complex, whose
fifth coordinate is occupied by the pyridine group of
the adjacent molecule in the layer. Thus, it seems reasonꢀ
able to assign the new bands found in our work for
FeМРуТРР to two types of dioxygen complexes scheꢀ
matically shown in Fig. 3. Indeed, when additional elecꢀ
tronꢀdonating ligands, such as nitrogenꢀcontaining bases,
are coordinated to the sixth coordinate, ν(O2) decreases
and ν(М—O2) increases compared to those of the pentaꢀ
coordinate complexes.2 This is related to the additional
transfer of the electron density through the dꢀorbitals of
the metal to the antibonding π∗gꢀorbitals of the oxygen
atom, strengthening the М—О2 bond and weakening
the О—О bond. For example, for the pentacoordinate
FeTPP(О2) complex prepared in the argon matrix, the
ν(O2) band is at 1195 cm–1, while it is observed
at 1157 cm–1 in the spectrum of the hexacoordinate
(Pip)FeTPP(О2) complex.16 We found no IR data on
ν(O2) of the PyFeTPP(О2) complex. However, in the
spectrum of the PyFe(Cap)О2 complex, where Fe(Cap) is
These differences in the oxygenꢀbonding ability of the
sublimed layers of the structurally similar compounds can
be attributed, in our opinion, to their supramolecular
structure. In the mesoꢀtetraarylporphyrin compounds,
whose aryl groups contain no functional substituents caꢀ
pable of specific intermolecular interactions, the molꢀ
ecules are arranged in layers with parallel planes of the
porphyrin cycles.24 When such interactions occur (hydroꢀ
gen bonds, coordination, etc.), the supramolecular orgaꢀ
nization of the system can substantially change.25—28
In the 4ꢀpyridylꢀsubstituted derivatives, the intermoꢀ
lecular coordination compels adjacent molecules to be
arranged perpendicularly to each other. The freshly subꢀ
limed FeМРуТРР layers consist of structures containing
both above types of supramolecular organization. Such a
system is thermodynamically nonequilibrium and can be
transformed in time into a more stable system with preꢀ
vailing of the pentacoordinate FeMPyTPP molecules.
Since the phenyl groups predominate in the mesoꢀposiꢀ
tions, the probability of formation of the hexacoordinate
saturated structures is low. The mutually perpendicular
arrangement of the adjacent molecules favors the layer to
retain porosity28 and ability to link various ligands. It is
likely that this structure also prevents, to a great extent,
µꢀoxodimerization, which proceeds through the formaꢀ
tion of complexes with the bridging dioxygen molecule.
Such compounds were detected in solutions at low temꢀ
peratures by NMR 29 and Raman spectroscopy.30
the "capped" porphyrin,17 ν(O2) is 1175 cm–1 18
which is
,
very close to that observed by us. The influence of the
electronꢀdonating ligand in the sixth coordinate on ν(O2)
of the coordinated dioxygen is much stronger in the coꢀ
balt porphyrins14 than in the iron porphyrins, which is
commonly explained by the multiplet character of the
M—O2 bond in the latter.19
The EAS of the lowꢀtemperature FeMPyTPP layer
heated in vacuo or the layer prepared by sublimation on
the surface at room temperature is close to the spectra of
Analogous conclusions about the structure of the subꢀ
limed FeMPyTPP layers have been made from the data
on their interaction with СО 31 and NO.32

Selfꢀassembly of iron(II) mesopyridylporphyrin
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 2, February, 2003
397
Attention of researchers33 is presently focused on
complexes of mesoꢀpyridylꢀsubstituted porphyrins beꢀ
cause of their ability to selfꢀassembly due to coordination
of the pyridine ligand of one molecule with the metal ion
of the adjacent molecule. Various supramolecular enꢀ
sembles were prepared from Zn, Ru, and Os metal
porphyrins. Similar properties of the Fe and Со porꢀ
phyrinates, which are true biomimetics, remain virtually
unstudied.
14. T. S. Kurtikyan, G. G. Martirosyan, R. K. Kazaryan, and
V. N. Madakyan, Izv. Akad. Nauk, Ser. Khim., 2001, 613
[Russ. Chem. Bull., Int. Ed., 2001, 50, 638].
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J. Am. Chem. Soc., 1982, 104, 3744.
16. K. Nakamoto, I. R. Paeng, T. Kuroi, T. Isobe, and H. Oshio,
J. Mol. Struct., 1988, 189, 293.
17. R. P. Evstigneeva, Porfiriny: struktura, svoistva, sintez [Porꢀ
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1985, 333 pp. (in Russian).
18. R. D. Jones, J. R. Budge, P. E. Ellis, Jr, J. E. Linard, D. A.
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This work was financially supported by the INTAS
(Grant 911).
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20. J. P. Collman, R. Schwenninger, M. Rapta, M. Broring, and
L. Fu, Chem. Commun., 1999, 137.
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Received July 2, 2002