The effect of functional substitution on thermal stability of pyridinylporphyrins under argon atmosphere

Article abstract of DOI:10.1134/S1070363216040137

The data on thermal stability under argon atmosphere of a series of porphyrin ligands containing meso- and β-positioned electron-donor (Me, Et, or Ph) groups as well as electron-acceptor pyridinyl substituents differing in the heteroatom location are presented. Thermogravimetric analysis and electron absorption spectroscopy studies have shown that pyridinylporphyrins are thermally stable at heating to 360–500°С depending on the functional substitution in the molecule.

Full text of DOI:10.1134/S1070363216040137

ISSN 1070-3632, Russian Journal of General Chemistry, 2016, Vol. 86, No. 4, pp. 835–839. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © N.M. Berezina, Do Ngoc Minh, Yu.I. Tikhonova, N.N. Tumanova, S.S. Guseinov, M.I. Bazanov, M.B. Berezin, A.V. Glazunov,
A.S. Semeikin, 2016, published in Zhurnal Obshchei Khimii, 2016, Vol. 86, No. 4, pp. 648–652.
The Effect of Functional Substitution on Thermal Stability
of Pyridinylporphyrins under Argon Atmosphere
N. M. Berezinaa, Do Ngoc Minha, Yu. I. Tikhonovaa, N. N. Tumanovaa, S. S. Guseinovb,
M. I. Bazanova, M. B. Berezinb, A. V. Glazunova, and A. S. Semeikina
a Ivanovo State University of Chemistry and Technology, Research Institute of Macroheterocyclic Compounds,
Sheremetevskii pr. 7, Ivanovo, 153000 Russia
e-mail: sky_berezina@rambler.ru
b G.A. Krestov Institute of Solutions Chemistry, Russian Academy of Sciences, Ivanovo, Russia
Received August 13, 2015
Abstract—The data on thermal stability under argon atmosphere of a series of porphyrin ligands containing
meso- and β-positioned electron-donor (Me, Et, or Ph) groups as well as electron-acceptor pyridinyl
substituents differing in the heteroatom location are presented. Thermogravimetric analysis and electron
absorption spectroscopy studies have shown that pyridinylporphyrins are thermally stable at heating to 360–
500°С depending on the functional substitution in the molecule.
Keywords: pyridinylporphyrin, degradation, thermal stability, argon, thermogravimetric analysis, electron
absorption spectroscopy
DOI: 10.1134/S1070363216040137
Porphyrins and their structural analogs have been
applied as catalysts of various chemical, electro-
chemical, and photochemical processes [1–3], as thera-
peutic and diagnostic compounds [4–7], brilliance-
forming additives [7], dyes [8, 9], optical materials
[10], chemical sensors [10–12], etc.
position at linear heating in an inert atmosphere
(Scheme 1).
Introduction of additional electron-acceptor sub-
stituents to the molecule of porphyrin generally
reduces the thermal stability of the macrocycle due to
the decrease in the electron density. This observation
was confirmed in this study (see the table). For
example, mono-, di-, and tetrasubstituted pyridinyl-
porphyrins (except for compound 1a) exhibited the
lower onset of thermal decomposition (tstart) as
compared to the unsubstituted analog.
Pyridinyl-substituted porphyrins and their com-
plexes with d-metals are especially attractive, owing to
the capacity of the pyridine nitrogen atom bearing a
lone-electron pair to form bonds with electron-acceptor
molecules, cations, etc.
The estimation of the pyridinylporphyrins potential
for practical applications requires the information on
their stability under diverse conditions; the relevant
data have been almost absent in the literature so far
[13, 14]. For instance, the technology of production of
oxygen cathodes of fuel cells includes the stage of
thermal treatment of the catalyst, and the information
of thermal stability of potential catalysts is of topical
importance.
In the case of the meso-substituted porphyrins even
the substitution of one of the phenyl groups with the π-
acceptor pyridinyl group (compound 2a) resulted in
the tstart decrease by 15°С by comparison with the
parent compound H2TPhP. Further increase in the
number of the electron-acceptor groups (compounds
2b, 2c, 1b) led to the decrease in tstart by 12–22°C.
The data on the thermal stability of the porphyrins
differing in the spatial arrangement of the pyridinyl
substituents with respect to the porphine macrocycle
demonstrated that the tstart value for the 5,10-isomer
(2c) was slightly higher than in the case of the isomer
with the substituents in positions 5 and 15 (2b).
In order to analyze the effects of the nature and the
number of functional substituents in porphyrin molecules,
we performed thermogravimetric analysis of solid
porphyrins 1–4 under conditions of thermal decom-
835

836
BEREZINA et al.
Scheme 1.
N
N
N
N
N
H
N
H
N
N
N
N
H
N
H
N
N
N
N
N
1а
1b
N
N
N
N
H
N
H
N
H
N
N
N
N
N
N
H
N
H
N
H
N
N
N
2c
2b
2а
N
N
N
CH3
CH3
CH3
CH3
CH3
CH3
H3C
H3C
CH3
H3C
CH3
H3C
H3C
CH3
CH3
NH
N
N
NH
N
N
NH
N
HN
HN
HN
N
CH3
CH3
H3C
C2H5
CH3
C2H5
CH3
C2H5
C2H5
CH3
C2H5
C2H5
CH3
3а
N
3c
3b
N
N
CH3
CH3
H
5C2
C2H5
C2H5
H5C2
C2H5
C2H5
C2H5
C2H5
H5C2
NH
N
N
NH
N
N
NH
N
HN
HN
N
HN
N
H5C2
C
H5
2
H
5C2
CH3
CH3
CH3
CH3
CH3
CH3
N
N
4а
4c
4b
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 4 2016

THE EFFECT OF FUNCTIONAL SUBSTITUTION ON THERMAL STABILITY
837
Thermogravimetric analysis data on decomposition of pyridinylporphyrins 1–4 and tetraphenylporphyrin (H2TPhP) under
argon atmospherea
Compound
H2TPhP [16]
1а
mo, mg
5.631
3.883
tstart, °C
–
tmax, °C
–
tend, °C
–
ma1exp, %
tstart, °C
470.2
tmax, °C
tend, °C
498.5
531.2
ma2exp, %
51.54
–
486.1
304.15
328.96
345.64
3.43
501.0
501.31
525.25
49.62
1b
2а
2b
2c
3а
2.187
3.140
1.920
4.458
1.465
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1.39
458.5
455.0
448.2
452.5
416.7
467.3
467.7
456.7
460.4
472.5
478.1
465.5
468.2
445
41.51
44.95
40.04
40.46
67.45
–
–
–
–
411.1
437.1
3b
3c
4а
4b
4c
3.135
2.730
2.416
2.492
2.256
183.6
236.8
–
249.7
287.2
–
276
297.8
–
6.07
5.91
–
400.8
387.5
365.1
363.6
360.9
412.8
402.1
375.9
373.5
369.5
421.2
411.0
395.6
388.5
389.1
59.03
55.40
55.24
51.46
52.56
210.7
232.2
259.3
257.6
267.6
275.8
2.07
2.13
a
tstart, temperature of the onset of the process; tmax, temperature of the maximum endoeffect; tend, temperature of the end of the process;
ma1exp and ma2ex, experimental value of weight loss.
It was found that thermal stability of the studied
unchanged electron absorption spectra of the specimen
after heating (Fig. 2).
compounds decreased with the nitrogen atom of the
pyridinyl fragment located closer to the macrocycle
(within each of the compounds groups 3a–3c and 4a–
4c, see the table). For example, the onset of the
thermal decomposition of the 3N-isomer of the mono-
pyridinylporphyrin 3b was almost by 17°C lower than
that of the 4N-isomer 3a, and that of the 2N-isomer 3c
was by further 13°C lower than in the case of the 3N-
isomer 3b. That was evidently due to the redistribution
of the electron density over the macrocycle. The same
trend was observed for compounds 4a–4c.
In summary, the obtained data on the thermal
stability of the pyridinyl-substituted porphyrins in the
solid state under argon atmosphere revealed the
correlation between the molecular structure of the
compound and its thermochemical properties within
the isomers group, but not for the whole set of the
DTG
The molecule symmetry strongly affected the onset
of the thermal decomposition as well. In particular, in
the cases of symmetrical porphyrins 1a and 4a the
decomposition started at higher temperature as com-
pared to the unsymmetrical analogs 1b, 4c, or 3a–3c.
Thermal decomposition of the ligands studied in
this work occurred in a single stage and was
accompanied by complete decomposition of the
molecule (Fig. 1). Several of the studied compounds
exhibited slight mass loss at relatively low
temperature. This process was not connected with the
substance decomposition, as confirmed by the
TG
T, °C
Fig. 1. Thermogram of porphyrin 1a under argon atmosphere.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 4 2016

838
BEREZINA et al.
porphyrins mixture was extracted with chloroform
A
using a Soxhlet apparatus and purified by chromato-
graphy on alumina (Brockman type II) collecting the
first fraction of 5,10,15,20-tetraphenylporphyrin (yield
1.72 g, 7.7%) and the second fraction of the pyridinyl-
porphyrins. The product was then additionally purified
by chromatography on silica gel 60 (0.063–0.2 mm,
70–230 mesh, ASTM) eluting with chloroform.
5-(Pyridin-3-yl)-10,15,20-triphenylporphyrin (2a).
Yield 1.95 g (8.74%). Electron absorption spectrum
(chloroform), λmax, nm (log ε): 646 (3.56), 590 (3.78),
1
×5
550 (3.87). 515 (4.31), 415 (5.66). Н NMR spectrum
(CDCl3), δ, ppm (J, Hz): 9.53 d (1H, 2'-HPy, J = 1.8),
9. 08 d.d (1H, 6'-HPy, J = 5.0, 1.8), 8. 95 d (2H, 3,7-H,
J = 4.5), 8.92 s (4H, 12,13,17,18-H), 8.85 d (2H, 2,8-
H, J = 4.5), 8.57 d. t (1H, 4'-HPy, J = 5.0, 1.8), 8.28 d
(6H, 2'',6''-HPh, J = 6.4), 7.85–7.77 m (10H, 5'-HPy,
3'',4'',5''-HPh), –2.73 s (2H, NH).
λ, nm
Fig. 2. Electron absorption spectrum (chloroform) of
porphyrin 2a (1) before and (2) after heating to 365оC.
5,15-Bis(pyridin-3-yl)-10,20-diphenylporphyrin
(2b). Yield 0.37 g (1.65%). Electron absorption
spectrum (chloroform), λmax, nm (log ε): 647 (3.60),
studied ligands. Thermal stability of the studied
porphyrins under inert atmosphere was higher than at
their heating in air [15].
1
590 (3.72), 550 (3.83), 515 (4.19), 417 (5.43). Н
NMR spectrum (CDCl3), δ, ppm (J, Hz): 9.55 s (2H,
2'-HPy), 9.11 d (2H, 6'-HPy, J = 4.8), 8.99 d (4H,
3,7,13,17-H, J = 3.1), 8.89 d (4H, 2,8,12,18-H, J =
3.1), 8.59 d (2H, 4'-HPy, J = 4.8), 8.29 d (4H, 2'',6''-
HPh, J = 7.3), 7.86–7.77 m (8H, 5'-HPy, 3'',4'',5''-HPh),
–2.71 s (2H, NH).
EXPERIMENTAL
Thermogravimetric analysis experiments were
performed at the Upper-Volga Regional Center for
Physicochemical Studies using a DSC 204 F1
differential scanning calorimeter equipped with a TG
209 F1 Iris thermobalance (NETZSCH, Germany). A
weighed specimen (2–5 mg) was placed in a platinum
crucible and heated under static argon atmosphere at a
heating rate of 10 K/min over the 298–1223 K range.
Prior to the experiment, the specimens were dried in a
vacuum (<1 mmHg) during 1 day at room temperature.
Electron absorption spectra were recorded using an
SF-56 spectrophotometer. 1Н NMR spectra were
recorded using a Bruker-500 instrument.
5,10-Bis(pyridin-3-yl)-15,20-diphenylporphyrin
(2c). Yield 0.98 g (4.4%). Electron absorption
spectrum (chloroform), λmax, nm (log ε): 646 (3.71),
1
589 (3.86), 550 (3.98), 514 (4.34), 417 (5.51). Н
NMR spectrum (CDCl3), δ, ppm (J, Hz): 9.54 s (2H,
2'-HPy), 9.10 d (2H, 6'-HPy, J = 5.5), 8.98 d (2H, 3,12-
H, J = 4.4), 8.94 s (2H, 7,8-H), 8.91 s (2H, 17,18-H),
8.87 d (2H, 2,13-H, J = 4.4), 8.57 d (2H, 4'-HPy, J =
5.5), 8.29 d (4H, 2'',6''-HPh, J = 6.0), 7.86–7.78 m (8H,
5'-HPy, 3'',4',5'-HPh), –2.72 s (2H, NH).
Compounds 1, 3, and 4 were synthesized as
described elsewhere [17–20].
5,10,15-Tris(pyridin-3-yl)-20-triphenylporphyrin.
1
Yield 0.56 g (2.5%) Н NMR spectrum (CDCl3), δ,
Compounds 2a–2c. A mixture of 6.9 mL (0.072 mol)
of 3-pyridinylcarbaldehyde, 7.6 mL (0.072 mol) of
benzaldehyde, and 10 mL (0.144 mol) of pyrrole was
gradually added to a mixture of 350 mL of acetic acid,
150 mL of nitrobenzene, and 10.5 mL (0.111 mol) of
acetic anhydride. The reaction mixture was refluxed
during 1.5 h, and nitrobenzene was removed via steam
distillation. The precipitate was filtered off, washed
with water, and dried in air at 70°С. The obtained
ppm (J, Hz): 9.50 s (3H, 2'-HPy), 9.10 d.d (3H, 6'-HPy,
J = 5.5, 1.4), 8.95 d (2H, 8,12-H, J = 4.5), 8.89 s (4H,
3,7,13,17-H), 8.85 d (2H, 2,18-H, J = 4.5), 8.57 d (3H,
4'-HPy, J = 5.5), 8.26 d (2H, 2'',6''-HPh, J = 6.0), 7.86–
7.79 m (6H, 5'-HPy, 3'',4'',5''-HPh), –2.79 s (2H, NH).
5,10,15,20-Tetrakis(pyridin-3-yl)porphyrin. Yield
1
0.21 g (0.95%). Н NMR spectrum (CDCl3), δ, ppm
(J, Hz): 9.50 s (4H, 2'-H), 9.10 d.d (4H, 6'-H, J = 5.6,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 4 2016

THE EFFECT OF FUNCTIONAL SUBSTITUTION ON THERMAL STABILITY
839
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5.6), –2.80 s (2H, NH).
10. Pinto, S.M.A., Lourenco, M.A.O., Calvete, M.J.F.,
Abreu, A.R., Rosado, M.T.S., Burrows, H.D., and
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ACKNOWLEDGMENTS
This work was financially supported by the Russian
Foundation for Basic Research (project no. 14-03-
31232_mol_a, study of the thermal decomposition)
and Ministry of Education and science of Russian
Federation in the scope of the governmental contract
(synthesis of porphyrins).
13. Berezina, N.M., Antina, E.V., Balantseva, E.V.,
Berezin, M.B., Semeikin, A.S., Bazanov, M.I., and
V’yugin, A.I., Izv. Vuzov, Ser. Khim. i Khim. Tekhnol.,
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