Spectral and complex-forming properties of ?-bromo-substituted porphyrins in N,N-dimethylformamide

Article abstract of DOI:10.1134/S107036321207016X

Contributions of structural and electronic effects to the reactivity and chromophore properties of tetrapyrrole macrocycles modified by bromine atoms were revealed on the basis of the kinetic data on the complex formation of 2-bromo-5,10,15,20-tetraphenyl

Full text of DOI:10.1134/S107036321207016X

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 7, pp. 1278–1283. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © O.V. Mal’tseva, N.V. Chizhova, N.Zh. Mamardashvili, 2012, published in Zhurnal Obshchei Khimii, 2012, Vol. 82, No. 7, pp. 1179–
1184.
Spectral and Complex-Forming Properties
of β-Bromo-Substituted Porphyrins in N,N-Dimethylformamide
O. V. Mal’tseva, N. V. Chizhova, and N. Zh. Mamardashvili
Krestov Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia
e-mail: olga_toldina@mail.ru
Received May 26, 2011
Abstract—Contributions of structural and electronic effects to the reactivity and chromophore properties of
tetrapyrrole macrocycles modified by bromine atoms were revealed on the basis of the kinetic data on the
complex formation of 2-bromo-5,10,15,20-tetraphenylporphyrin, 2,3,12,13-tetrabromo-5,10,15,20-tetraphenyl-
porphyrin, and 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin with zinc(II) and copper(II)
acetates in N,N-dimethylformamide, and also by measuring spectral characteristics of porphyrin ligands and
zinc and copper porphyrinates.
DOI: 10.1134/S107036321207016X
Formation and decomposition of metal porphyry-
nates defining their ability to existence in live systems
and to functioning as industrial catalysts, coloring
agents, photo and thermal stabilizers of polymers have
been studied within many decades [1–10].
of spectral and complex-forming properties of tetra-
phenylporphin in the course of sequential chemical
modification of β-pyrrole positions of a macrocycle by
bromine atoms. It was found that changes in the mole-
cular structure of tetraphenylporphyrin Н2TPP (I), for
example, on passing to 2-bromo-5,10,15,20-tetraphenyl-
porphyrin Н2TPP(β-Br) (II), 2,3,12,13-tetrabromo-
5,10,15,20-tetraphenylporphyrin Н2TPP(β-Br)4 (III),
and 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenyl-
porphyrin Н2TPP(β-Br)8 IV, strongly affect the rate
and mechanism of the complex formation:
Salts of two-charge transition metal ions, i.e.
cations of metals with sizes corresponding to a
macrocycle cavity size, react most readily with por-
phyrins. However, the formation of metal porphyry-
nаtes with these salts proceeds extremely slowly. A
full completion of the reactions often requires heating
or a long time. It is connected first of all with the fact
that the limiting stage of the reactions of macrocycles
with cations is the stage of the transition state forma-
tion [1]. In perch-type complexes, where a cation-guest
remains on the apex of a binding cavity of a host, the
destructtion of a cation solvate shell is rather insigni-
ficant, whereas in capsular complexes, such as por-
phyrinаtes, it is maximal. In a transition state of the
complex formation the cation should be desolvated,
this process being energy unprofitable and leading to a
high-energy transition state. As a result the following
rule is valid: the more rigidly a ligand is preorganized
for cation binding, the lower is the process rate. Rigid
aromatic porphyrin molecules react with metal cations
at low rates.
Н2TPP(X)m + [МА2(Solv)m–2] → МTPP(X)m + 2HA.
Here МА2(Solv)m–2 is a metal salt solvate with a
coordination number m in a solvent medium (M =
Zn2+, Cu2+), A is the salt anion, and X is an electron-
donor or electron-acceptor substituent.
The features of the chemical behavior of substituted
porphyrins is caused mainly by a complicated branched
macrocycle, a strong conjugation of a π-electronic system
[1], and a macrocyclic effect, i.e. a spatial macrocycle
distortion and a steric screening of reaction centers by
substituents. β-Substituted por-phyrins are of special
interest, as in this case, accord-ing to the published
data and our own results, the electronic influence of a
substituent on the macrocycle π-electronic system is
maximal as compared to meso-substituents [1–6, 10].
The aim of the present work was to determine the
effects of a macrocycle deformation and electronic and
structural contributions of substituents on the variation
When bromine atoms are introduced in β-positions
of pyrrole fragments, the number of π-electrons in the
1278

SPECTRAL AND COMPLEX-FORMING PROPERTIES
1279
R8
Ph
R1
modified by bromine atoms from the macrocycle plane
is 4.8°). Benzene rings, as well as in the Н2TPP
molecule, are arranged perpendicularly to the N4H2
planes, the Cβ–Сβ bond lengths decrease [1.33 and
1.34 Å for Н2TPP(β-Br)4 and Н2TPP, respectively],
and the coordination cavity size decreases [4.2 and
4.10 Å for Н2TPP and Н2TPP(β-Br)4, respectively].
R7
Ph
R6
R2
Ph
R3
NH
N
N
HN
Crystallographic data for the octabromo derivative
Н2TPP(β-Br)8[CF3COO]2·0.5CHCl3} are given in [12].
According to these data, all phenyl fragments are
arranged in the same plane, and the macrocycle takes a
saddle-like conformation. Bromine atoms of two
adjacent pyrrole rings in compound IV are lifted above
their planes by 0.170–0.461 Å, and those of two others
are located under planes of pyrrole rings. In compound
III all four bromine atoms are in the same plane. It was
R5
Ph
R4
I, Н2TPP, R1–R8 = H; II, Н2TPP(β-Br), R1 = Br; R2–R8 =
H; R1 = R2 = R5 = R6 = Br; III, Н2TPP(β-Br)4, R3 = R4 =
R7 = R8 = H; IV, Н2TPP(β-Br)8, R1–R8 = Br.
conjugated system of a macrocycle increases, which
results in a greater aromaticity of β-bromo-substituted
tetraphenylporphyrins in comparison with the initial
state. According to [11], Н2TPP(β-Br)4 retains its
planar structure (the deviation of pyrrole rings
1
shown by the Н NMR method [13] that the distorted
structures of compounds III, IV are also retained in
solutions.
II
III
IV
A red shift of the Soret band is observed in the elec-
tronic spectra of porphyrin IV solutions in electron-
donor solvents, and bands in the long wavelength
region become low-intensive and diffuse as compared
with tetraphenylporphin I.
influence of peripheral substituents and also by the
interior chromophore polarization caused by a non-
equivalence of β- and meso-positions [14–16].
Bromine atoms in a macrocycle are under the influence
of two electronic effects: a positive conjugation effect
(+С) and a negative induction effect (–I). The higher
occupied molecular orbital of porphyrin (а2u and а1u) is
located on pyrrole and meso carbon atoms, respec-
It is presumed to originate from two major factors:
special features of the molecule distortion under the
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1280
MAL’TSEVA et al.
Table 1. Position (λmax) and intensity (log ε) of absorption bands (1–5) in electronic spectra of unsubstituted and β-bromo-
substituted tetraarylporphyrins I–IV
Comp.
no.
I
Solvent
1
2
3
4
5
Reference
[19]
Toluene
652(3.81)
645(3.86)
642
592(3.79)
591(3.83)
585
549(3.95)
548(3.97)
542
515(4.28)
513(4.28)
508
415(5.35)
411(5.31)
411
DMF
Chloroform
[21]
Dichloromethane
649(3.79)
653(3.71)
653(3.75)
651(3.76)
678(4.06)
594(3.81)
596(3.70)
597(3.74)
595(3.75)
–
551(3.78)
552(3.64)
553(3.67)
552(3.68)
–
518(4.33)
519(4.23)
519(4.23)
519(4.10)
–
421(5.58)
424(5.41)
424(5.40)
420(5.31)
452(3.75)
[16]
[19]
''
II
Toluene
Pyridine
DMF
''
Acetic acid
Dichloromethane
Chloroform
Toluene
685(4.02)
683(3.91)
680(3.86)
680(3.85)
696(3.53)
711(4.31)
612(3.58)
617(3.58)
614(3.79)
613(3.79)
612(3.63)
–
533(4.30)
535(4.23)
568(3.97)
568(3.97)
536(3.90)
–
–
–
–
–
–
–
436(5.52)
437(5.38)
439(5.36)
442(5.34)
432(4.68)
432(5.10)
[13]
''
III
[19]
''
Pyridine
DMF
Acetic acid
''
Dichloromethane
Toluene
743(3.87)
738(3.85)
760(3.89)
783(3.63)
745(4.17)
626(4.12)
622(4.10)
646(4.08)
627(3.44)
–
569(3.96)
469(5.31)
470(5.25)
477(5.10)
473(4.81)
–
370(4.52)
367(4.41)
371(4.38)
393(4.10)
–
[14]
[19]
''
IV
568(3.95)
Pyridine
–
–
–
DMF
Acetic acid
''
tively. The presence of substituents in β-positions
gives rise to perturbations leading to the appearance of
a great number of configurations. A possible overlap
of bands of Qy and Qx transitions for porphyrin IV and
the appearance of an intensive band (λmax 626 nm,
log ε 4.12) for solutions in methylene chloride are
presumed. A similar relation of absorption bands is
observed in the spectrum of compound IV in toluene
[17].
The complex formation of tetraphenylporphyrins
II–IV with salts of 3d-metals is accompanied by a
change in electronic absorption spectra: the spectra of
the complexes contain the Soret band and two bands in
the visible region (Figs. 1–3), which are caused by
electronic transitions. The decrease in the number of
bands in the electronic absorption spectra results from
a higher molecular symmetry and also from the
suppression of vibration states upon the complex
formation. The position of absorption bands in the
electronic spectra of metal complexes is defined by the
nature and strength of M–N chemical bonds.
Table 1 allows us arranging porphyrins under study
in the following series according to increasing red shift
of absorption band 1 and the Soret band: Н2TPP <
Н2TPP(β-Br) < Н2TPP(β-Br)4 <Н2TPP(β-Br)8.
It was found that the spectral criterion of strength,
physical essence of which is described in [18], is
applicable to metal complexes. The thermodynamic
stability of metal complexes in the series of similar
compounds is the greater, the greater is the blue shift
of band 1 of a metalloporphyrin compared to band 1 of
the ligand, porphyrin Н2P. We can assert on the basis
of electronic absorption spectra and kinetic data that
the spectral criterion of strength is applicable to
complexes of all tetraarylporphyrins under study
irrespective of the extent of their macrocycle
distortion. Thus band 1 of Cu(II) complexes in the
studied series of compounds is always subjected to a
blue shift as compared to the λI value of less stable
Zn(II) complexes, in which an ionic contribution to the
M–N bond is more significant (Table 2).
А
А
500
600 λ, nm
λ, nm
Fig. 1. Electron absorption spectrum of (1) Н2TPP(β-Br)
and (2) its zinc complex in DMF.
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SPECTRAL AND COMPLEX-FORMING PROPERTIES
1281
А
А
А
А
λ, nm
λ, nm
λ, nm
λ, nm
Fig. 2. Electron absorption spectrum of (1) Н2TPP(β-Br)4
Fig. 3. Electron absorption spectrum of (1) Н2TPP(β-Br)8
and (2) its zinc complex in DMF.
and (2) its zinc complex in DMF.
The effect of bromine substitution in β-positions of
compound I on its coordination properties in the
reactions of Н2TPP(X)mС with Zn(OAc)2 in toluene,
pyridine, and acetic acid was studied in [19, 27]. The
reaction of coordination of porphyrins with a low-
active N–H bond (weak acids), compound I among
them, is the fastest in glacial acetic acid and the
slowest in pyridine [19]. For NH-active porphyrins
(strong acids), like compound IV, the reaction rate is
maximal in pyridine and minimal in acetic acid.
mixture were studied by the spectrophotometric stop-
ped-flow method [20]. Complex-formation rate
constants exceed a corresponding value for tetra-phenyl-
porphyrin by 2–3 orders of magnitude. Bhyrappa et al.
attribute this increase not to the increase in the basicity
of tertiary nitrogen atoms (bromine atoms as
substituents reduce the value of protonation constants
рK3 2.6 and рK4 1.75 [19]), but to the macroring
nonplanar structure.
We have studied the kinetics of the complex
formation of porphyrins I–IV with Zn(OAc)2 and
Cu(OAc)2. We have chosen N,N-dimethylformamide
The bromine atom in β-position favors the shift of
π-electrons of nitrogen atoms in a macrocycle, the
increase in the polarity of the N–H bond, and the
change in the complex formation rate as compared to
unsubstituted tetraphenylporphyrin [kv298 7.03×
10–2 l mol–1 s–1 for compound II in a mixed toluene-
pyridine solvent, whereas for compound I kv298 8.70×
10–3 l mol–1 s–1 in pyridine] [19]. The complex-
formation rate in the toluene-pyridine medium is two
orders of magnitude less than in the toluene-acetic acid
mixed solvent [kv298 7.03×10–2 and 6.08 l mol–1 s–1,
respectively]. Orders of magnitude of the true kinetic
constants in Py, kv298 0.85 l mol–1 s–1, and АсНО, kv298
3.46 l mol–1 s–1, for compound III [17] are close to
each other, which reflects a partial polarization of the
N–H bond under the influence of four bromine atoms.
Low values of the activation energy Ea point to the
increase in ionization of the N–H bond under the
action of an organic base. The complex formation with
compound IV in pyridine is instantaneous, whereas in
acetic acid it does not proceed at all.
Table 2. Characteristics of electronic spectra (absorption
bands 1–5) of porphyrins I–IV and their complexes with
Cu(II) and Zn(II) in DMF
Compound
1
2
3
4
5
ΔλIа
645
598
539
651
601
544
696
613
598
783
680
642
591
558
548
513
411
420
412
420
427
419
432
435
430
473
472
473
I
I(Zn)
I(Cu)
II
II(Zn)
II(Cu)
III
III(Zn)
III(Cu)
IV
IV(Zn)
IV(Cu)
–47
–106
595
562
552
536
519
–50
–107
612
569
557
627
624
592
–83
–98
–103
–141
a
ΔλI is the shift of absorption band 1 in the electronic spectra of
tetraarylporphyrins upon the complex formation (ΔλI = ΔλImp
ΔλIH2P).
–
The kinetics of Н2TPP(β-Br)8 complex formation
with metals (Сo, Cu, Zn) in the CHCl3–MeOH (1:1)
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1282
MAL’TSEVA et al.
Table 3. Kinetic parameters of the complex formation
reaction of porphyrins II–IV with Cu(OAc)2 and Zn(OAc)2
in DMFa
kv[Zn(OAc)2] for Н2TPP(Br) in DMF is equal to four,
whereas for Н2TPP(Br)4 it is equal to two. The
formation of ZnTPP(Br)8 and CuTPP(Br)8 in the same
medium at 298 K proceeds instantly. It points to a
greater strength of zinc - dimethylformamide solvates
as compared with copper solvates and is in agreement
with the published data [22]. Complex-forming proper-
ties of β-bromo-substituted porphyrins II–IV in DMF
were studied in our work for the first time.
Comp.
no.
kv,
l mol–1 s–1 kJ mol–1 J mol–1 K–1
Еа,
ΔSа≠,
Salt
T, K
Zn(OAc)2 298b
0.106
0.735
1.634
3.920
75±5
–20±19
II
318
328
338
Cu(OAc)2 298
0.495
1.245
2.906
0.892
2.285
5.047
70±1
69±4
–24±3
Thus, according to the data on the kinetics of the
complex formation of 2-bromo-5,10,15,20-tetra-
phenylporphyrin, 2,3,12,13-tetrabromo-5,10,15,20-tetra-
phenylporphyrin, and 2,3,7,8,12,13,17,18-octabromo-
5,10,15,20-tetraphenylporphyrin with zinc(II) and
copper(II) acetates in N,N-dimethylformamide, and
also on the basis of spectral characteristics of por-
phyrin ligands and zinc and copper porphyrinates, we
have revealed structural and electronic effects on the
reactivity and chromophore properties of tetrapyrrole
macrocycles modified by bromine atoms. Sequential
bromo-substitution of pyrrole positions in tetra-
phenylporphyrin changes the macrocycle electronic
structure toward polarization of the N–H bond, which
is not suppressed in the DMF medium. These data are
of interest for the development of the chemistry of
macroheterocyclic compounds capable of selective
binding cations of metals various in nature.
308
318
Zn(OAc)2
298
308
318
–22±15
III
IV
Cu(OAc)2 288
1.525
2.541
3.906
67±4
–20±12
293
298
Zn(OAc)2
298
Reaction proceeds instantly
The same
Cu(OAc)2 298
a csalt 2.5×10–3 M, c(H2P) 2.5×10–5 M. b Found by extrapolation.
(DMF) as the organic solvent, which has occupied
non-bonding φN-orbitals and shows σ-donor properties.
The 100-fold molar excess of a salt in relation to a
porphyrin allows us to describe the reaction by the
equation of the pseudo-first order. The linear character
of the dependences log [c0(H2P)/c(Н2P)] – f(τ) points
to the reaction proceeding according to the first order
with respect to porphyrin. The true reaction rate
constant [kv, l/(mol·s)] was calculated as kv = Kef/сs
(Kef was an effective rate constant of the process, s–1;
сs was a molar concentration of a salt, M).
EXPERIMENTAL
Porphyrin ligands II–IV were synthesized by
procedures similar to those in [13, 14, 16]. Synthesized
compounds were identified by electronic absorption,
1Н NMR, and IR spectra, which completely cor-
responded to the characteristics described in the
literature. Kinetic measurements were carried out on a
Сary 100 spectrophotometer according to the proce-
dure described in [23]. Solvents were purified by the
standard procedures [24, 25]. Analytical-grade zinc
and copper(II) acetates were recrystallized from glacial
АсОН and stored in a desiccator above concentrated
Н2SO4.
Sequential β-bromine substitution results in an
increase in the complex formation rate for compound
I, whereas values of activation energy (Еа) and
activation entropy (ΔS≠) remain constant.
Complex formation of compounds II–IV (Table 3)
has its special features. The comparison of the com-
plex formation ability of Н2TPP(β-Br) and unsub-
stituted tetraphenylporphyrin in standard conditions
(298 K, strongly dilute solutions of a porphyrin and
metal salt) shows that kv for Zn(OAc)2 in DMF
increases by the factor of 30 when only one bromine
atom is introduced: kv298 0.0034 l mol–1 s–1 for Н2TPP
[21]; on passing from Н2TPP(β-Br) to Н2TPP(β-Br)4
the rate increases by the factor of 8.5, and with Н2TPP
(β-Br)8 the reaction proceeds instantly.
ACKNOWLEDGMENTS
This work was financially supported by the Russian
Foundation for Basic Research (grants nos. ×10–03-
90000-Bel_а and 11-03-00003_а), by the Seventh
Framework Program of the European Community for
Research, Technological Development, and Demon-
stration Activities, IRSES-GA-2009-247260, and by
the Federal target program “Scientific and educational-
On passing from Н2TPP(Br)1 to Н2TPP(Br)4 the
dependence of the complex-formation rate on the
metal nature becomes weaker. The ratio kv[Сu(OAc)2]/
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SPECTRAL AND COMPLEX-FORMING PROPERTIES
1283
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