Electronic and steric effects of substituents on the coordinating properties of porphyrins

Article abstract of DOI:10.1134/S107036321203019X

Kinetics of the formation of zinc complexes of porphyrin with various substituents was studied in acetic acid and pyridine. The introduction of strong electron-withdrawing substituents (nitro groups) to the meso-positions of the porphyrin macrocycle was shown to change the reaction rate by an order of magnitude or less compared to the phenyl-substituted analogs. However, the introduction of a large number of bulky substituents leads to the deformation of the porphyrin ligand and thus affects much stronger the coordination properties of porphyrins, decreasing or increasing the rate of the complexation reaction by several orders of magnitude. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S107036321203019X

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 3, pp. 476–481. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © S.G. Pukhovskaya, B.A. Efimovich, A.S. Semeikin, E.A. Kolodina, O.A. Golubchikov, 2012, published in Zhurnal Obshchei Khimii,
2
012, Vol. 82, No. 3, pp. 483–488.
Electronic and Steric Effects of Substituents
on the Coordinating Properties of Porphyrins
S. G. Pukhovskaya, B. A. Efimovich, A. S. Semeikin,
E. A. Kolodina, and O. A. Golubchikov
Ivanovo State University of Chemical Technology, Engelsa pr. 7, Ivanovo, 153460 Russia
e-mail: puhovskaya@isuct.ru
Received December 20, 2010
Abstract—Kinetics of the formation of zinc complexes of porphyrin with various substituents was studied in
acetic acid and pyridine. The introduction of strong electron-withdrawing substituents (nitro groups) to the
meso-positions of the porphyrin macrocycle was shown to change the reaction rate by an order of magnitude or
less compared to the phenyl-substituted analogs. However, the introduction of a large number of bulky
substituents leads to the deformation of the porphyrin ligand and thus affects much stronger the coordination
properties of porphyrins, decreasing or increasing the rate of the complexation reaction by several orders of
magnitude.
DOI: 10.1134/S107036321203019X
R4
The deformation of the porphyrin macrocycle due
R2
R2
to the introduction of a large number of bulky
substituents affects greatly the basic properties of the
porphyrin ligand and, depending on the solvent nature,
increases (in basic solvents) or decreases (in the
solvents of acidic nature) the reaction rate of complex
formation by these ligands by several orders of
magnitude [1, 2]. However, the substithuents them-
selves can more or less affect the process rate as the
electron donors or acceptors.
R1
R1
R5
N
N
N
N
H
R3
H
R1
R1
R2
3
R6
R2
4
1
2
5
6
I, R = Et, R = Me, R = R = Ph, R = NO
2
R = H; II,
1
2
3
5
4
6
1
R = Et, R = Me, R = R = Ph, R = R = NO
2
; III, R =
2
3
4
5
6
1
2
In this regard, the purpose of this study was to
determine the ratio of the contributions of the
substituents electronic and steric effects to the kinetic
parameters of the formation of metalloporphyrins.
R = Me, R = R = R = Ph, R = H; IV, R = R = Me,
3
4
5
6
1
2
3
4
5
R = R = R = R = Ph; V, R = R = Ph, R = R = R =
6
1
2
3
4
5
6
R = H; VI, R = R = Me, R = R = R = R = H.
The following porphyrins were synthesized: 10-
nitro-5,15-diphenyl-3,7,13,17-tetramethyl-2,8,12,18-
tetraethylporphin (I) and 10,20-dinitro-5,15-diphenyl-
In this work we studied the kinetics of the reactions
of the zinc complexes formation with porphyrins
containing strong electron-withdrawing substituents in
one or two meso-positions of the macrocycle: 10-nitro-
3
,7,13,17-tetramethyl-2 ,8,12,18-tetraethylporphin (II)
were obtained by nitration of 5,15-diphenyl-3,7,13,17-
tetramethyl-2,8,12,18-tetraethylporphin (VII) with a
mixture of sodium nitrite and trifluoroacetic acid,
according to [3]. Porphyrin VII was synthesized in
5
,15-diphenyl-3,7,13,17-tetramethyl-2,8,12,18-tetra-
ethylporphin (I), 10,20-dinitro-5,15-diphenyl-3,7,13,17-
tetramethyl-2,8,12,18-tetraethylporphin (II), compared
with 5,10,15-triphenyl-2,3,7,8,12,13,17,18-octamethyl-
porphin (III), 5,10,15,20-tetraphenyl-2,3,7,8,12,13,17,18-
octamethylporphin (IV), 2,3,7,8,12,13,17,18-octa-
phenylporphin (V), and 2,3,7,8,12,13,17,18-octamethyl-
porphin (VI), in acetic acid and pyridine.
3
8% yield by the condensation of dipyrrolylmethane
(VIII) with benzaldehyde under the influence of
chloroacetic acid in methylene chloride in an inert
atmosphere, followed by oxidation of the intermediate
porphyrinogen with p-chloranil [3].
4
76

ELECTRONIC AND STERIC EFFECTS OF SUBSTITUENTS
477
R
Me
Me
Me
CHO
Et
Et
Et
Et
Et
Et
NH
NH
NH
N
N
(
1) CH ClCOOH
2
(
1)
+
(
2) p-chloranil
HN
Me
VIII
Me
R1
VII
Me
1
R = R = Ph.
The nitration with sodium nitrite at the equimolar
ratio of the reagents [reaction (2)] resulted in mono-
nitroporphyrin (I) in 56% yield. Further nitration of I
led to dinitroporphyrin (II) in 38% yield.
R
Me
Me
R
Me
R
Me
Me
Me
Et
Et
Et
Et
Et
Et
Et
Et
1
eq NaNO2
1.5 eq NaNO
CF COOH
2
NH
N
N
NH
N
N
NH
N
N
CF COOH
3
3
NO2
Et
O N
NO2 (2)
Et
2
HN
HN
HN
Et
Et
R1
VII
Me
R1
I
R1
II
Me
Me
Me
Me
Me
1
R = R = Ph.
The formation of complexes of porphyrins with
double-charged metal cations in nonaqueous solutions
proceeds in accordance with Eq. (3).
dence of the effective rate constant (k ) of reaction (3)
ef
on the salt concentration (c). In our case, the order of
reaction (3) with respect to the zinc acetate both in
pyridine and in acetic acid equal unity [6]. The value
of k is related to the true rate constant (k ) according
2 2
Н P + [МХ (Solv)n–2] → МР + 2НХ + (n – 2)Solv. (3)
ef
v
Here X is acido ligand, Solv is a solvent molecule, n is
the coordination number of the metal cation. Typically,
reaction (3) obeys the first order kinetic Eq. (4) with
respect to porphyrin [4, 5].
to equation (5).
n
k
ef = k
v
c (MX
2
).
(5)
The kinetic data on the porphyrins I and II
coordination with zinc acetate are listed in Tables 1
and 2, in comparison with previous results.
n
–
dc(H
2
P)/dt = kc(H
2
P)c (MX
2
),
(4)
n
where k is the rate constant, c (MX ) is the salt
In non-aqueous media the reaction of the complex
formation proceeds in one step by a bimolecular
mechanism. The formation of the transition state
includes, firstly, the restructuring and partial destruct-
2
concentration, c(H P) is the concentration of por-
phyrin. The reaction order with respect to the salt is
determined experimentally by measuring the depen-
2
X
N
X
N
Solv
NH
N
Solv
Solv
H
MP
+
M
H
M
HN
N
Solv
Solv
N
N
Solv
X
X
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 3 2012

4
78
PUKHOVSKAYA et al.
Table 1. Kinetic parameters of the porphyrins I–VI coordina-
tion by the zinc acetate in acetic acid
tion of the coordination sphere of the solvated metal,
secondly, the formation of coordination bonds between
the metal cation and tertiary nitrogen atoms of the
porphyrin, and thirdly, extension and polarization of
the ligand N–H bonds [4].
3
298
2
≠
c[Zn(OAc)
2
]×10 ,
k
×10 ,
ΔE,
ΔS ,
Porphyrin
–
1
–1
–1 –1
M
l mol s
kJ mol J mol K
I
0.268
0.268
0.268
0.268
0.52
2.29±0.09 56±2
0.75±0.04 84±2
–96±9
–10±6
The substituents in the porphyrin macrocycle,
depending on their electronic effect (±C and ±I),
determine the distribution of the π-electron density and
hence the degree of solvation in the ground state, as
well as lead to a different protonation of the N–H
bonds. Some bulky substituents force the porphyrin
molecule to adopt the most energetically favorable
conformation, even when it destroys the planar
structure of the macrocycle.
II
III
IV
V
The reaction is very slow
The complex is not formed
481±15
67±1
67±3
39±1
29±9
а
VI
0.50
–
163±3
a
Data of [12].
Theoretical calculations [7–9] and our experimental
studies [1] showed that the spectral changes of
porphyrins with a sterically non-planar structures are
very characteristic, so that the red shift of all bands in
electron absorption spectra (EAS) of a porphyrin can
be attributed to the deformation of its tetrapyrrole
macrocycle with a high confidence.
Table 2. Kinetic parameters of the reaction of porphyrins I–
VI coordination by the zinc acetate in pyridine
2
98
2
≠
c[Zn(OAc)
2
]×
k
×10 ,
ΔE,
ΔS ,
Porphyrin
Spectral characteristics of the studied porphyrins
3
–1
–1
–1 –1
1
0 , M
2.53
2.53
5.00
5.00
2.68
5.00
l mol s
13.4±0.5
43.5±0.5
88±2
kJ mol
J mol K
(
Table 3) showed that replacement of methyl groups in
the β-positions of the pyrrole fragments of macrocycle
porphyrin VI) by the phenyl (V) leads to ~15 nm red
I
91.0±1.0
93.6±1.5
54.6±0.4
47.0±0.9
30.5±3.5
103±1
36±3
54±5
(
II
shift of the absorption bands. At the accumulation of
substituents in the meso-positions of the macrocycle
the red shift of the first absorption band in the EAS
increases and reaches 70 nm in the spectrum of
porphyrin II compared with VII.
а
III
–97±1
–118±3
–208±5
9±3
а
IV
150±3
V
2.08±0.05
0.632±0.005
а
VI
Introduction of strong electron-withdrawing sub-
stituents to the meso-positions promote blue shift of
the bands in the EAS. According to the four-orbital
Simpson–Platt–Gouterman model [10], the blue shift is
due to a decrease in the energy of the highest occupied
molecular orbital (HOMO) owing to the greater
electron density on the methine carbon atoms. Despite
this, all absorption bands suffer the red shift. It is
obvious that at the application of two opposite effects
a
Data of [8].
Table 3. Electron absorption spectra of porphyrins I, II, V–
VII
Absorption bands maxima λmax, nm (log ε)
(
deformation of the porphyrin framework and the –I,
I
II
III
IV
Соре
–C effects of nitrogroups) the structural factor
dominates.
I
665 (3.41) 595 (3.78)
696 (3.78) 607 (3.91)
sh
sh
524 (4.04) 418 (5.04)
541 (4.06) 447 (4.99)
The electron-withdrawing substituents are known to
increase the protonation of the N–H bond and,
therefore, should increase the rate of reaction (3). At
the same time, the acceptor groups reduce the electron
density on the tertiary nitrogen atoms thus impeding
the strengthening of N–M bonds in the transition state
II
V
635 (3.47) 583 (3.95) 550 (4.04) 518 (4.23) 423 (5.13)
620 (3.65) 568 (3.68) 535 (4.60) 500 (4.60) 400 (5.61)
VI
VII
6
25 (3.30) 574 (3.85) 542 (3.70) 508 (4.23) 409 (5.34)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 3 2012

ELECTRONIC AND STERIC EFFECTS OF SUBSTITUENTS
479
and therefore reducing the reaction rate of the metallo-
porphyrin formation.
porphyrins I and II at the interaction with zinc acetate
is significantly lower in acetic acid than in pyridine,
and with the porphyrins III and IV the reaction in
the same conditions proceeds much slower.
Data in Tables 1 and 2 show that the rate of the
formation of the β-octaphenylporphyrin zinc complex
V is approximately 3–7 times higher compared with β-
octamethylporphyrin VI. Calculations of the structural
parameters of the porphyrin V [11] showed that the
phenyl fragments are located at the angle of ~75° to
the plane of the macrocycle and, consequently, may
In pyridine, a sharp increase in the rate of the
complex formation when β-octamethylporphyrin
contains aryl meso-substituents in its structure (in
going from the porphyrin VI to IV) cannot be caused
only by the electronic effects of substituents, but is
primarily associated with the changes in the geometry
of the molecule (Table 2). However, the replacement
of one phenyl meso-substituent in the sterically
distorted porphyrins by the nitro group (porphyrins I
and III, II and IV) leads to a decrease in the reaction
rate 3.5 to 6 times. Obviously, this is due to the
stronger electron-acceptor influence of the nitro group.
4
affect the state of the reaction center N only via the
system of σ-bonds, showing –I effect. The eight phenyl
substituents at the porphyrin macrocycle reduce the
negative charge on the central nitrogen atoms, thus
facilitating the N–H bonds breaking under the in-
fluence of polar solvent in the transition state. Methyl
groups in the pyrrole β-positions in the porphyrin
macrocycle VI exhibit the opposite effect (+I), which
apparently reduces the reaction rate of the metallo-
porphyrin formation. However, these factors do not
influence significantly the rate of the complexation
reaction, the rate constant varies within the same order
of magnitude.
EXPERIMENTAL
The electron absorption spectra (EAS) and the rate
of reactions of the porphyrin zinc complexes I–VI
were measured on a Shimadzu UV-1800 and Hitachi
U-2000 spectrophotometers. Measurements were per-
formed in thermally-controlled polished glass cells in
the temperature range from 288 to 348 K. Temperature
fluctuations did not exceed ±0.1 K. In all studied cases,
the spectra of the reacting systems included a clear
isosbestic point. A typical example is shown in the
figure.
When acetic acid is used instead of pyridine, the
rate of reaction of metalloporphyrins with a flat
structure of the macrocycle V, VI, as one would ex-
pect, is reduced due to stabilization of the salt
solvation shell (Tables 1 and 2). In acetic acid, salts of
transition metals form a relatively labile solvate
complexes, easily decomposing at the interaction with
porphyrin. Pyridine has a strong electron-donating
properties, and therefore forms a stable coordination
sphere of the metal cation. Its partial destruction upon
complexation with porphyrin requires considerable
energy expenditure, thus pyridine reduces the rate of
the complexation [12].
1
The H NMR spectra were recorded on a Bruker-
5
00 spectrometer at the operating frequency 500 MHz
in CDCl , internal reference TMS.
3
The solvents used (acetic acid, pyridine) were
purified by the methods described in the literature [15,
1
6].
Another situation occurs in the case of the
porphyrins with nonplanar structure of the tetrapyrrole
macrocycle. In the publications [1, 3, 11, 14] based on
calculations of the geometric parameters and on
spectral data (Table 3) it was shown that porphyrins I–
IV have a nonplanar structure, the degree of the
macrocycle deformation increasing with the number of
substituents in meso-positions. The basicity of the
spatially distorted porphyrins was found to increase
significantly compared with the flat analog [14]. There
is a certain relationship between the coordination and
the acid–base properties: The protonation of porphyrin
ligands is a reaction competing with the coordination.
The kinetic data agree well with the established laws:
The reaction rate of coordination of spatially distorted
5,15-Diphenyl-3,7,13,17-tetramethyl-2,8,12,18-
tetraethylporphyrin (VII). To a solution of 1.2 g
(5.2 mmol) of 3,3'-diethyl-4,4'-dimethylpyrrolylmethane
[3] and 0.6 ml (5.9 mmol) of benzaldehyde in 400 ml
of methylene chloride in an atmosphere of carbon
dioxide was added while stirring a solution of 1.0 g
(10.6 mmol) of monochloroacetic acid in 20 ml of
methylene chloride. The mixture was stirred in the
dark for 4 h, then 1.9 g (7.7 mmol) of p-chloranil was
added, and the mixture was kept for 12 h at room
temperature. The solvent was evaporated, and the
residue was washed with 5% sodium hydroxide
solution, then with water, and dried. The purification
was performed by chromatography on Al O (III
2
3
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 82 No. 3 2012

4
80
PUKHOVSKAYA et al.
a solution of 49.55 mg (0.079 mmol) of the porphyrin
(
a)
А
VII in 5 ml of trifluoroacetic acid. The mixture was
stirred at room temperature for 10 min, then poured
into 50 ml of water and neutralized with 25% solution
of ammonia until its color changed from green to red-
brown. The precipitate was filtered off, washed with
water, and dried to constant weight. Porphyrin was
dissolved in benzene and chromatographed on alumina
of III degree activity by Brockmann eluting with
benzene. Yield 29.83 mg (56.2%).
EAS (chloroform), λmax, nm (log ε): 665 (3.41), 595
1
(
3.78), 524 (4.04), 418 (5.04). H NMR spectrum, δ,
ppm: 9.84 s (1H, meso-H); 8.08 m (4H, o-H ); 7.75 m
Ph
λ, nm
b)
(6H, m,p-H ); 3.8 q (4H , CH CH ); 3.35 q (4H,
Ph
2
3
CH CH ); 2.28 s (6H, CH ); 2.16 s (6H, CH ), 1.59 t
2
3
3
3
(
А
(6H, CH CH ); 1.27 t (6H, CH CH ), 2.69 s (NH)
2 3 2 3
(
CDCl , int. TMS).
3
10,20-Dinitro-5,15-diphenyl-3,7,13,17-tetramethyl-
2
,8,12,18-tetraethylporphyrin (II) was obtained by
nitration of 41.05 mg (0.079 mmol) of porphyrin VII
with two equivalents (10.9 mg, 0.16 mmol) of sodium
nitrite in 5 ml of TFA, during 90 min. The yield after
purification by chromatographic method was 38 mg
(
65.2%).
EAS (chloroform), λmax, nm (log ε) 696 (3.78), 607
1
(
3.91), 541 (4.06), 447 (4.99). H NMR spectrum, δ,
ppm: 8.08 m (4H, o-H ); 7.81 m (6H, m,p-H ); 3.11 q
Ph
Ph
λ, nm
(8H, 2,8,12,18-CH CH ); 1.96 s (12H, 3,7,13,17-CH ),
2 3 3
1
.18 t (12H, 2,8,12,18-CH CH ); (12H, CH ); 1.59 t
2 3 3
Changes in the EAS in the course of coordination at T =
(
6H, CH CH ); –2.41 br.s (NH) (CDCl , int. TMS).
2 3 3
2
98 K: (a) β-octaphenylporphyrin by the zinc acetate in
acetic acid, (b) 10-nitro-5,15-diphenyl-3,7,13,17-tetramethyl-
,8,12,18-tetraethylporphyrin by zinc acetate in pyridine.
Spectral data of porphyrins I, II, and VII did not
2
differ from those described in the literature [3].
ACKNOWLEDGMENTS
degree of activity by Brockmann), eluent chloroform.
Yield 1.25 g (38%).
This work was supported by the Russian Founda-
tion for Basic Research (grant no.10-03-00305-A).
Individuality of the compound was monitored by
TLC on Silufol plates with the layer thickness of
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