Effect of structural and electronic properties of substituents on the metal porphyrin formation kinetics

Article abstract of DOI:10.1134/S0036023613040141

5,10,15-trinitro-2,3,7,8,12,13,17,18-octaethylporphyrin was synthesized and the kinetics of its coordination to zinc acetate was studied in comparison with previously studied β-octamethylporphyrin, β-octaphenylporphyrin, 5,10,15-triphenyl-2,3,7,8,12,13,17

Full text of DOI:10.1134/S0036023613040141

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2013, Vol. 58, No. 4, pp. 406–410. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © S.G. Pukhovskaya, V.A. Efimovich, O.A. Golubchikov, 2013, published in Zhurnal Neorganicheskoi Khimii, 2013, Vol. 58, No. 4, pp. 467–472.
COORDINATION COMPOUNDS
Effect of Structural and Electronic Properties of Substituents
on the Metal Porphyrin Formation Kinetics
S. G. Pukhovskaya, V. A. Efimovich, and O. A. Golubchikov
Research Institute of Chemistry of Macroheterocyclic Compounds, Ivanovo State University
of Chemical Technology, Ivanovo, Russia
Received November 15, 2011
Abstract—5,10,15ꢀtrinitroꢀ2,3,7,8,12,13,17,18ꢀoctaethylporphyrin was synthesized and the kinetics of its
coordination to zinc acetate was studied in comparison with previously studied ꢀoctamethylporphyrin,
β
β
ꢀoctaphenylporphyrin, 5,10,15ꢀtriphenylꢀ2,3,7,8,12,13,17,18ꢀoctamethylporphyrin, and 5,10,15,20ꢀtetꢀ
raphenylꢀ2,3,7,8,12,13,17,18ꢀoctamethylporphine. The effect of structural and electronic properties of subꢀ
stituents on the kinetics of formation of metal porphyrins is analyzed. It is shown that the properties of
5,10,15ꢀtrinitroꢀ2,3,7,8,12,13,17,18ꢀoctaethylporphyrin are determined by both strong electronꢀwithdrawꢀ
ing influence of three nitro groups and by the distortion of the planar structure of the tetrapyrrole macrocycle.
DOI: 10.1134/S0036023613040141
The concept of nonꢀplanar deformations of tetrapyrrole in comparison with these data for previously prepared
macrocycles, which is invoked to explain various biochemꢀ
ical functions of systems in vivo [1], stimulated the developꢀ
ment of chemical syntheses and investigations of nonꢀplaꢀ thylporphyrin
β
ꢀoctamethylporphyrin
III) [6], 5,10,15ꢀtriphenylꢀ2,3,7,8,12, 13,17,18ꢀoctameꢀ
IV), and 5,10,15,20ꢀtetraphenylꢀ
(
II),
β
ꢀoctaphenylporphyrin
(
(
nar porphyrins [2, 4] where the deviations from the planar 2,3,7,8,12,13,17,18ꢀoctamethylporphine (V).
structure are caused by the presence of bulky substituents at
R1
R1
R2
the macrocycle periphery. Study of the coordination propꢀ
erties of these porphyrins demonstrated that by macrocycle
deformation it is possible to change the rate of coordination
by more than four orders of magnitude in either direction
with the corresponding change in the activation parameters
[5]. However, the substituents themselves, being either elecꢀ
tron donors or acceptors, can also somewhat influence the
rate of formation of metal porphyrins.
I
: R1 = Et, R2 = Н, R3 = NO2
R1
R3
R1
R3
R1
II: R1 = Me, R2 = R3 = H
III: R1 = Ph, R2 = R3 = H
IV: R1 = Me, R2 = Н, R3 = Рh
1 V: R1 = Me, R2 = R3 = Рh
VI: R1 = Et, R2 = R3 = H
N
N
N
H
H
N
R
R1
R1
R3
Hence, the goal of this investigation was to deterꢀ
mine the ratio of contributions of the electronic and
steric effects of substituents to the kinetic parameters
of formation of metal porphyrins.
EXPERIMENTAL
5,10,15ꢀtrinitroꢀ2,3,7,8,12,13,17,18ꢀoctaethylporꢀ
phyrin (I) was prepared by nitration of
2,3,7,8,12,13,17,18ꢀoctaethylporphyrin (VI) with a
mixture of sodium nitrite and trifluoroacetic acid
We synthesized 5,10,15ꢀtrinitroꢀ2,3,7,8,12,13,17, 18ꢀ
octaethylporphyrin (
I) containing strong electronꢀwithꢀ
drawing substituents in mesoꢀpositions of the macrocycle
and studied the kinetics of its coordination by zinc acetate according to a reported procedure [7]:
NO2
Et
Et
Et
Et
Et
Et
Et
Et
Et
N
NH
N
NH
10 equiv NaNO
CF COOH
3
2
NO2
Et
N
HN
N
HN
Et
Et
Et
Et
Et
Et
NO2
406

EFFECT OF STRUCTURAL AND ELECTRONIC PROPERTIES
407
NaNO2 (10 equiv, 149.8 mg) was added to a soluꢀ
558
A
0.8
tion of porphyrin VI (108.9 mg) in trifluoroacetic acid.
The solution was stirred at room temperature for
4 days.
544
After completion of the reaction, the mixture was
poured into 50 mL of water, neutralized with a 25%
solution of ammonia until the color changed from
green to redꢀbrown. The precipitate was filtered off,
washed with water, dried in air at room temperature to
a constant weight, dissolved in 30 mL of benzene, and
586
0.6
0.4
0.2
0
639
650
chromatographed on a column (1.5 25 cm) with silꢀ
×
ica gel 60 (230–400) using benzene as the eluent. The
eluate was concentrated to a minimum volume and
5,10,15ꢀtrinitroꢀ2,3,7,8,12,13,17,18ꢀoctaethylporꢀ
phyrin was precipitated with methanol, filtered off,
washed with methanol, and dried at room temperature
to a constant weight. The individual nature of the
compound was checked by TLC on aluminum plates
with attached 0.5 mmꢀthick silica gel F254 layer
(Merck) with C6H6 as the eluent. Yield 73% (79.5 mg).
500
550
600
700
, nm
λ
Fig. 1. Variation of the UV/Vis spectrum during coordinaꢀ
tion of 10,15,20ꢀtrinitroꢀ2,3,7,8,12,13,17,18ꢀoctaethꢀ
ylporphyrin with zinc acetate in pyridine at 318 K.
UV/Vis (CHCl3, λmax, nm (log )): 641 (3.39); 591
ε
(3.67); 513 (4.05); 414 (4.07).
where А0, А, and А are the initial current, and final
absorbances of the solution, respectively.
∞
1Н NMR (
, ppm): 10.07 (s, 1H, mesoꢀH); 3.97
δ
and 3.57 (q and m, 16H, 2,3,7,8,12,13,17,18ꢀ
СH2CH3); 1.81–1.44 m (24H, 2,3,7,8,12,13,17,18ꢀ
СH2CH3); –3.47 (s, 2H, NH).
The order of reaction (1) with respect to zinc aceꢀ
tate is equal to unity both in pyridine and in acetic acid
IR, cm–1: 1536 (δas NO2), 1383 (δs NO2).
[12,13]; therefore, the true secondꢀrate constants (k )
v
Analytical grade zinc acetate was recrystallized were calculated by the equation
from aqueous acetic acid and dehydrated at 380–
390 K [8].
k
(3)
= kef /cZn OAc
.
v
(
)
2
The solvents used in the work (acetic acid, pyriꢀ
dine, chloroform, benzene) were purified before use
by procedures reported in the literature [9, 10].
The activation energy (
ature range was calculated by the Arrhenius equation:
Δ
Е) for the studied temperꢀ
The IR spectrum was recorded on an Avatar 360
FTꢀIR ESP spectrometer in KBr pellets.
ΔЕ
= 19.1[(T1T2)/(T2
–
T
1)]log(k2
/k
1),
(4)
The 1H NMR spectra of porphyrin
I in CDCl3 were
measured on a Brukerꢀ500 spectrometer operating at
500 MHz (internal TMS standard).
UV/Vis spectra were measured on Shimadzu
UVꢀ1800 and Hitachi Uꢀ2000 spectrophotometers.
≠
ΔS
and the transition state formation entropy (
found by the equation
) was
≠
ΔS
= 19.1log
k
+
ΔЕ
/
T
– 253.
(5)
v
The complex formation of porphyrins with zinc
acetate was studied by spectrophotomertry in temperꢀ
atureꢀcontrolled groundꢀstoppered cells in the temꢀ
perature range of 288–348 K. The temperature variaꢀ
tion did not exceed 0.1 K. In all cases, the spectra of
the reacting systems exhibited clearꢀcut isosbestic
points (Fig. 1).
The formation of metal porphyrins follows the first
kinetic order with respect to the ligand [11]. The
kinetic experiments were carried out under a 100ꢀfold
excess of Zn(OAc)2 with respect to porphyrin, and this
The rate constants for the formation of zinc comꢀ
plexes of porphyrins in pyridine and acetic acid
are summarized in the table.
I–V
RESULTS AND DISCUSSION
According to the mechanism of formation of metal
porphyrins proposed by Berezin [14] and confirmed
by subsequent studies [15, 16], the key factors deterꢀ
mining the rate of coordination are, first, the rearꢀ
rangement and the partial destruction of the coordinaꢀ
allowed for calculating the effective rate constants (kef
)
of reaction (1)
H2P + Zn(OAc)2
by the equation
kef = (1/
→
ZnP + 2AcOH
)/( )],
(1)
(2)
tion sphere of the metal solvate complex [MX2Solv4
and, second, stretching and polarization of ligand
H bonds:
]
τ
)ln[(A0
–
A
A – A
N
⎯
∞
∞
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 58 No. 4 2013

408
PUKHOVSKAYA et al.
Kinetic parameters of porphyrin coordination to zinc acetate in pyridine and in acetic acid
≠
ΔS
≠
ΔS ,
J/(mol K)
k298
×
102,
Δ
E
,
,
k298
×
102,
Δ
E
,
cZn(OAc)
103 mol/L
,
×
cZn(OAc)
103 mol/L
,
×
2
2
L mol s–1 kJ/mol J/(mol K)
L mol s–1 kJ/mol
Porphyrin
Pyridine solvent
Acetic acid solvent
β
ꢀOctamethylporꢀ
5.00
2.68
5.00
0.63 0.01 103
1
9
3
0.50
0.52
0.50
67
1
39
67
1
3
–163
29
3
9
phyrin
βꢀOctaphenylporꢀ
2.08 0.05 30.5 3.5 –208 10
481 15
phyrin
5,10,15,15ꢀTripheꢀ
nylꢀ ꢀoctamethylꢀ
porphine
89
3
54.6 0.4 –97
1
3
2
Too low rate to be measured
Too low rate to be measured
β
5,10,15,20ꢀTetꢀ
raphenylꢀ ꢀoctaꢀ
methylporphine
5.00
0.08
1179 13 47.0 0.9 –118
0.50
β
5,10,15ꢀTrinitroꢀ
2053 11
27 0.7 –136
0.056
3907 20
33
3
–113
8
βꢀoctaethylporꢀ
phyrin
X
X
N
H
N
solv
solv
solv
solv
N
NH
+
MP
M
.
H
M
N
HN
solv
N
solv
N
X
X
The substituents of the porphyrin macrocycle, the planar structure of the porphyrin nucleus, thus
which have particular electronic effects ( and ), exerting a characteristic influence on their acid–base
С
I
determine the electron density distribution on the N– and coordination properties.
H bonds and affect the degree of solvation in the
Figure 2 shows the stereo view of the conformers of
ground and transition states. The introduction of elecꢀ
tronꢀwithdrawing substituents increases protonization
of the N–H bonds and hence, increases the rate of
complexation.
molecule
I differing by the orientation of the ethyl
groups relative to the tetrapyrrole ring mean plane.
The structure of the conformers was optimized by the
semiempirical quantum chemical PM3 method using
Analysis of published data [2–5, 17] demonstrated the HyperChem program (version 6). It can be seen
that the introduction of bulky substituents into mesoꢀ that due to steric repulsion from ethyl substituents, the
and βꢀpyrrole positions of the porphyrin ring disrupts nitro groups are virtually perpendicular to the mean
Fig. 2. Stereo view of the mesoꢀtrinitroꢀ
β
ꢀoctaethylporphyrin conformers.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 58 No. 4 2013

EFFECT OF STRUCTURAL AND ELECTRONIC PROPERTIES
409
plane of the macrocycle, which is highly distorted and
А
has a saddle structure. Correspondingly, it should be
expected that the properties of this porphyrin are
determined by both the strong electronꢀwithdrawing
1
2
–I effect of three nitro groups, and the distortion facꢀ
tor of the planar structure of the tetrapyrrole ring.
The data of the Table indicate that for porphyrins II
and III with a planar macrocycle structure, the rate of
formation of zinc complexes is somewhat (3–8ꢀfold)
increases when methyl groups (+
I
substituent effect)
effect).
3
5
are replaced by phenyl groups (–
I
4
As expected, on going from acetic acid to pyridine,
the rate of formation of metal porphyrins with the plaꢀ
nar macrocycle structure decreases upon stabilization
of the solvation shell of the salt [18] (table).
6
The situation changes considerably on going to
porphyrins with nonplanar structure of the tetrapyrꢀ
role ring. According to Xꢀray diffraction for the solid
phase and calculation data, porphyrins IV and
V are
nonplanar, the degree of ring deformation increasing
with increase in the number of phenyl substituents [5,
19, 20].
500
550
600
650
λ
, nm
The basicity of spatially distorted phenyl derivaꢀ
tives of octaalkylporphyrins is higher compared with
Fig. 3. UV/Vis spectra of porphyrins: (
1) 5,10,15ꢀtripheꢀ
nylꢀ2,3,7,8,12,13,17,18ꢀoctaethylporphyrin in benzene
and (
(3)
2
) the same in benzene containing 1.5% acetic acid;
5,10,15ꢀtrinitroꢀ2,3,7,8,12,13,17,18ꢀoctaethylporꢀ
planar analogs [5]. The data of Fig. 3 (curves
clearly demonstrate this. The addition of even 1.5%
acetic acid to a benzene solution of porphyrin drasꢀ
1 and 2)
phyrin in benzene and (
4) the same in acetic acid;
V
(
5) 2,3,7,8,12,13,17,18ꢀoctaethylporphyrin in benzene
tically changes the UV/Vis spectral pattern. The visiꢀ
ble region of the spectrum shows two bands instead of
four bands and the intensity of the first, longꢀwaveꢀ
length, band sharply increases, which attests to vigorꢀ
ous acid–base interaction of the porphyrin tertiary
nitrogen atoms with acetic acid. In the case of porphyrins
with planar macrocycle, similar spectral changes were
observed only when the content of acetic acid in its mixꢀ
and (
ture.
6) the same in a 1 : 9 (v/v) benzene–acetic acid mixꢀ
from II to III increases the reaction rate not more than
eightfold.
The change of the planar conformation of the macꢀ
rocycle to the saddle conformation on going from II to
IV and
ture with benzene is 90% (Fig. 3, curves 5 and 6).
V in the transition state of coordination, first,
There is a certain relationship between the coordiꢀ ensures more effective solvation of the leaving protons
nation and acid–base properties: protonation of the
porphyrin ligands competes with coordination reacꢀ
tion. The obtained kinetic data are in good agreement
with the established regularities: due to the protonaꢀ
tion of the coordination center (perhaps, due to the
formation of strong hydrogen bonds), the rate of coorꢀ
of porphyrin and, second, in this case, the transition
state is attained at much less pronounced destruction
of the coordination sphere of the pyridinate solvate
complexes of zinc acetate. These factors not only
result in a sharp increase in the reaction rate in pyriꢀ
dine but also considerably lower the activation energy
and entropy.
dination of spatially distorted porphyrins IV and
zinc acetate in acetic acid is too low to be measured.
Moreover, zinc complexes of porphyrins IV and disꢀ
V to
The introduction of three nitro groups into the
V
mesoꢀpositions of
stantially the spectral, acid–base, and coordination
properties of
βꢀoctaethylporphyrin changes subꢀ
sociate upon dissolution in acetic acid to give the iniꢀ
tial porphyrins in the protonated form.
I.
The sharp (by a factor of hundreds or thousands)
increase in the complexation rate in pyridine upon the
introduction of mesoꢀphenyl substituents into the
All absorption bands of the electronic spectrum of
shift bathochromically by 15–20 nm as compared
I
with the UV/Vis octaethylporphyrin. These changes
are typical of porphyrins with nonꢀplanar aromatic
βꢀoctamethylporphyrin on going from II to IV and V
(table) cannot be attributed to the electronic effects of
substituents. In our opinion, this is due, first of all, to
the disruption of planar structure of the tetrapyrrole
nucleus of the molecule. Recall that the replacement
nucleus. In the 1H NMR spectrum of
I
, the NH proꢀ
ton signal is shifted downfield with respect to that in
ꢀoctaethylporphyrin, –3.47 and –3.75 ppm, respecꢀ
β
of eight methyl groups by eight phenyl groups on going tively. This is caused by the combined influence of the
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 58 No. 4 2013

410
PUKHOVSKAYA et al.
2. O. A. Golubchikov, S. G. Pukhovskaya, and E. M. Kuvꢀ
porphyrin deformation and the electronꢀwithdrawing
effect of the nitro groups.
shinova, Usp. Khim. 74, 263 (2005).
3. O. A. Golubchikov, S. G. Pukhovskaya, and E. M. Kuvꢀ
shinova, Progress in the Chemistry of Porphyrins, Ed. by
O. A. Golubchikov (Izd. NII khimii SPbGU, St. Peꢀ
tersburg, 2004), Vol. 4 [in Russian].
4. E. I. Zen’kevich, E. I. Sagun, V. N. Knyukshto, et al.,
Progress in the Chemistry of Porphyrins, Ed. by O. A.
Golubchikov (Izd. NII khimii SPbGU, St. Petersburg,
2004), Vol. 4 [in Russian].
5. S. G. Pukhovskaya, Extended Abstracts of Doctoral
Dissertation in Chemistry (Inst. of Chemistry, RAS,
Ivanovo, 2009).
6. S. G. Pukhovskaya, V. A. Efimovich, A. S. Semeikin,
and O. A. Golubchikov, Russ. J. Inorg. Chem. 55 (9),
1494 (2010).
7. E. A. Kolodina, Candidate’s Dissertation in Chemistry
(Inst. Of Chemistry, RAS, Ivanovo, 2010).
8. Yu. V. Karyakin and I. I. Angelov, Pure Chemical
Reagents (Khimiya, Moscow, 1974) [in Russian].
9. A. Weissberger, E. S. Proskauer, J. A. Riddick, and
E. E. Toops, Jr., Organic Solvents: Physical Properties
and Methods of Application (Interscience, New York,
1955; Inostrannaya Literatura, Moscow, 1958).
The data of Fig. 3 indicate unambiguously that the
basicity of
influence of the nitro groups. The UV/Vis spectrum of
in acetic acid barely differs from the spectrum in benꢀ
zene (curves and ).
The coordination properties of mesoꢀtrinitroꢀ
I decreases under the electronꢀwithdrawing
I
3
4
βꢀ
octaethylporphyrin have some specific features as
compared with nonꢀplanar mesoꢀtriꢀ and tetraphenyl
derivatives of
which are caused by the strong electronꢀwithdrawing
influence of nitro groups. Indeed, porphyrins IV and
do not form zinc complexes in acetic acid, whereas the
complexation of occurs 60 times faster than that of
octamethylporphyrin. Little specific solvation of the
tertiary nitrogen atoms of porphyrin brings to the
βꢀoctamethylꢀ and octaethylporphyrin,
V
I
βꢀ
I
forefront the entropy factor related to the fact that the
transition state of coordination is attained for nonꢀ
planar porphyrin at relatively less pronounced
destruction of the solvation shell of zinc.
Another significant factor is the change in the solꢀ
vation conditions of the NH protons that leave the
reaction area. Upon nonꢀplanar deformation of the
porphyrin macrocycle, the conditions become more
favorable due to the decrease in the van der Waals
repulsion of the solvent molecules that solvate the
leaving protons. In the transition state, the chain of the
10. I. Gyenes, Titration in NonꢀAqueous Media (Akademiai
Kiado, Budapest, 1967; Mir, Moscow, 1871).
11. O. A. Golubchikov, B. D. Berezin, and O. I. Koifman,
Izv. Vyssh. Uchebn. Zaved., Khim. Khim. Tekhnol. 17
,
1265 (1974).
breaking and forming Solv⋅⋅⋅
is anyway not linear [21] but as the macrocycle deforꢀ
mation grows, the Solv⋅⋅⋅ ⋅⋅⋅ increases, which
enhances the Solv interaction.
H⋅⋅⋅N bonds of porphyrin
12. T. A. Ageeva, E. L. Ponomareva, O. A. Golubchikov,
and B. D. Berezin, Koord. Khim. 13, 886 (1987).
13. O. A. Golubchikov, E. M. Kuvshinova, and B. D. Bereꢀ
H
N
H…
zin, Kinet. Katal. 28, 1301 (1987).
Thus, study of the kinetics of formation of zinc
porphyrin complexes in various solvents demonstrated
that strong electronꢀwithdrawing groups in polysubꢀ
stituted porphyrins not only cause the macrocycle
deformation but also affect considerably both the
acid–base interactions with protonꢀdonor solvents
and the kinetic and energy parameters of complexꢀ
ation.
14. B. D. Berezin, Coordination Compounds of Porphyrins
and Phthalocyanines (Nauka, Moscow, 1978) [in Rusꢀ
sian].
15. B. D. Berezin and N. S. Enikolopyan, Metalloporphyꢀ
rins (Nauka, Moscow, 1988) [in Russian].
16. B. D. Berezin, Progress in the Chemistry of Porphyrins
,
Ed. by O. A. Golubchikov (Izd. NII khimii SPbGU, St.
Petersburg, 1997), Vol. 1 [in Russian].
17. M. O. Senge, The Porphyrin Handbook, Ed. by K. M. Kaꢀ
dish, K. M. Smith, and R. Guilard (Academic Press,
San Diego, 2000), Vol. 1, p. 240.
ACKNOWLEDGMENTS
This work was supported by the Russian Foundaꢀ 18. B. D. Berezin and O. A. Golubchikov, Coordination
Chemistry of Solvation Complexes of Transition Metal
Salts (Nauka, Moscow, 1991) [in Russian].
19. K. M. Barkigia, M. D. Berber, J. Fajer, et al., J. Am.
tion for Basic Research, project no. 10ꢀ03ꢀ00305ꢀa,
and Project no. 2.1.1/14169 of the analytical departꢀ
mental target program “Development of scientific
potential of higher school in 2009–2011.”
Chem. Soc. 112, 8851 (1990).
20. M. O. Senge and W. W. Kalisch, Inorg. Chem. 36, 6103
(1997).
REFERENCES
21. O. A. Golubchikov, E. M. Kuvshinova, B. D. Berezin,
1. M. O. Senge, J. Photochem. Photobiol. 16, 3 (1992).
and O. I. Koifman, Kinet. Katal. 27, 1301 (1987).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 58 No. 4 2013