Study of extra coordination of bromo-substituted porphyrins to organic bases

Article abstract of DOI:10.1023/A:1020030714257

Extra coordination of zinc complexes of bromo-substituted tetraphenylporphyrins was studied by spectrophotometry. It was found that the stability of complexes thus formed markedly increases upon β- bromosubstitution in tetraphenylporphine. The effects of

Full text of DOI:10.1023/A:1020030714257

Russian Journal of Coordination Chemistry, Vol. 28, No. 9, 2002, pp. 608–612. Translated from Koordinatsionnaya Khimiya, Vol. 28, No. 9, 2002, pp. 648–653.
Original Russian Text Copyright © 2002 by Berezin, Karmanova, Gromova, Syrbu, Semeikin.
Study of Extra Coordination of Bromo-Substituted Porphyrins
to Organic Bases
B. D. Berezin, T. V. Karmanova, T. V. Gromova, S. A. Syrbu, and A. S. Semeikin
Ivanovo State University of Chemical Technology, Ivanovo, Russia
Received December 19, 2000; in final form, March 5, 2002
Abstract—Extra coordination of zinc complexes of bromo-substituted tetraphenylporphyrins was studied by
spectrophotometry. It was found that the stability of complexes thus formed markedly increases upon β-bromo-
substitution in tetraphenylporphine. The effects of electronic and structural factors on the extra coordination
were discussed. The stability constants of complexes formed by zinc porphyrins with organic bases were cal-
culated, and the electronic absorption spectra of porphyrins in toluene and in other organic solvents were stud-
ied.
Among the diverse physicochemical properties of
2
20 L
porphyrins (H2P), the additional coordination (extra
coordination) of organic molecules (L) to metallopor-
phyrins (MP) is especially important [1–3]. As a rule,
this coordination is a stepwise interaction giving extra
complexes (LnMP) due to the donor–acceptor bonding
between the metal atom and the organic base. In addi-
tion, porphyrin macrocycles can act as both electron
donors and acceptors, depending on the nature of the
porphyrin ligand and the metal. Of special interest are
porphyrins with an unusual structure resulting from the
effect of multiple substitution at the β- and meso-posi-
tions on the macrocyclic π-system.
3'
5'
2'
6'
3
18
5
N
4'
17
N
Zn
7
N
N
15
13
8
10
12
(LZnP)
Zinc complexes were prepared as reported previ-
ously [3] by refluxing porphyrins in DMF with analyt-
ical grade Zn(Ac)2 taken in a 1 : 5 molar ratio for
40 min (for H2P1–H2P4) or 10 min (for H2P5).
The formation of zinc porphyrins was monitored by
electronic absorption spectra, the reactions being
stopped when the absorption bands for free porphyrins
disappeared from the spectra. After the synthesis, the
mixture was cooled and the metalloporphyrin was
extracted with chloroform. The chloroform solution of
zinc porphyrin was washed with water to remove
excess salt and DMF and purified by chromatography
on alumina (activity II) or on Silufol plates. Chloroform
was used for elution and visualization.
The extra coordination was studied in toluene. This
was done by the isothermal saturation method. A series
of solutions (12 to 15) with equal metalloporphyrin
concentrations (10–5 mol/l) and with specified concen-
trations of organic bases L were prepared. The elec-
tronic absorption spectra of solutions in hermetically
sealed quartz cells were recorded on Specord M40 and
Hitachi U-2000 spectrophotometers in a chamber
maintained at a constant temperature. An increase in
In this work, we study the effect of substituents
(bromine atoms) in the pyrrole and phenyl fragments of
tetraphenylporphine on the physicochemical properties
of zinc porphyrins.
EXPERIMENTAL
The study was performed for zinc complexes with
5,10,15,20-tetraphenyl-(2'-tetrabromo)porphyrin (H2P1),
5,10,15,20-tetraphenyl-(3'-tetrabromo)porphyrin (H2P2),
5,10,15,20-tetraphenyl-(3',5'-octabromo)porphyrin (H2P3),
5,10,15,20-tetraphenyl-(4'-tetrabromo)porphyrin (H2P4),
and 2,7,12,17-tetrabromo-5,10,15,20-tetraphenylpor-
phyrin (H2P5) and with organic bases (L): pyridine
(Py), piperidine (Pip), and dimethyl sulfoxide
(DMSO). The porphyrins H2P1–H2P4 were prepared by
condensation of substituted pyrroles with benzalde-
hyde and purified by a reported procedure [4]; H2P5 was
synthesized by a known procedure [5]. The porphyrins the L concentration for an invariable metalloporphyrin
and their complexes were identified using electronic concentration brought about changes in the electronic
absorption and IR spectra [4, 5]. The organic bases L absorption spectra (Figs. 1, 2). The optical density (A)
and toluene were dried and purified by known proce- was measured at wavelengths where the difference
dures [6]. The water content in organic solvents was between the electronic absorption spectra of ZnP and
checked using Fischer titration.
LZnP is most pronounced.
1070-3284/02/2809-0608$27.00 © 2002 åÄIä “Nauka /Interperiodica”

STUDY OF EXTRA COORDINATION
609
A
A
1.2
1
0.6
2
1.0
0.8
0.6
0.4
0.4
3
2
1
0
0.001
0.002
0.003
logcL
8
0.2
0
Fig. 2. Dependence of A on logcL for the formation of the
LZnTPP(β-Br) complexes in toluene (T = 298 K) for L =
4
(1) Pip, (2) DMSO, (3) Py (λ
= 568 nm).
max
500
550
600
650
700
λ, nm
logcL
1
3
2
4
Fig. 1. Change in the electronic absorption spectrum of
the ZnP –Py system in toluene upon the change in c
0.5
5
Py
–3
–5
from (1) 0 to (8) 3.6 × 10 mol/l; cZnP5 = 6.92 × 10 mol/l,
T = 298 K.
0
–0.5
–1.0
–1.5
5
The extra coordination of metalloporphyrins to the
ligands L in solutions affords both mono- (n = 1) and
biligand (n = 2) species:
MP + nL
LnMP.
(1)
–6.0 –5.5 –5.0 –4.5 –4.0 –3.5 –3.0
The stability constant for the resulting complexes
has the form
A0 – Aeq
--------------------
log
Aeq – A∞
[LnMP]
[MP][L]n
Ks = ----------------------- .
(2)
A0 – Aeq
---------------------
Fig. 3. logcL vs. log
for the formation of PipZnP
A
eq – A∞
2
3
1
Under the experimental conditions, all the zinc por-
phyrins form complexes with one additional ligand
(Fig. 3). It was found that n = 0.97 for the reaction of
Pip with ZnP2 and n = 0.99 for ZnP1 and ZnP3–ZnP5.
extra complexes in toluene for P = (1) P , (2) P , (3) P ,
(4) P , and (5) P ; T = 298 K.
4
5
alloporphyrin solution, and Aeq and A∞ are the optical
densities of the equilibrium mixture with L and the
mixture with the highest content of L. The concentra-
tion of free L in the solution, with allowance for the
extra coordination, was determined using the equation
A change in the organic base concentration or in the
temperature makes it possible to shift the equilibrium
(1) toward either the formation or dissociation of the
LnMP complex. The complex concentration was calcu-
lated from the optical densities of the solution using the
relation for a mixture of two colored substances:
Aeq – A∞
A0 – A∞
cL = c0L – c0MP 1 – ------------------- ,
(4)
Aeq – A∞
A0 – A∞
0
-------------------
cLMP = cMP
,
(3)
where c0L is the initial ligand concentration.
where cLMP is the concentration of the extra complex
Equations (3) and (4) were used to calculate the sta-
bility constants; the resulting values for different tem-
peratures were employed to calculate the changes in
formed, c0MP is the initial concentration of the metal-
loporphyrin, A0 is the initial optical density of the met-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 9 2002

610
BEREZIN et al.
Table 1. Stability constants and thermodynamic parameters for extra coordination of the ZnP1–ZnP5 complexes (H2P1–H2P5
are bromo-substituted porphyrins)
Ks, l mol–1
ZnP
L
–∆H0, kJ/mol –∆S0, J/(mol K)
288 K
298 K
308 K
318 K
ZnTPP [3] Py
10600 ± 200
5800 ± 200
3600 ± 100
45400 ± 2100
2957 ± 245
12125 ± 600
610 ± 27
2000 ± 200
7200 ± 500
2332 ± 125
10570 ± 750
460 ± 33
43 ± 2
44 ± 1
21 ± 1
12 ± 1
30 ± 2
15 ± 1
20 ± 1
29 ± 1
11 ± 1
27 ± 1
18 ± 1
37 ± 2
21 ± 2
34 ± 2
40 ± 1
32 ± 2
24 ± 1
72 ± 4
52 ± 2
38 ± 2
15 ± 1
75 ± 4
18 ± 1
29 ± 2
71 ± 3
30 ± 2
49 ± 2
32 ± 2
95 ± 5
39 ± 2
88 ± 4
95 ± 5
60 ± 4
48 ± 3
Pip
146000 ± 12000 80900 ± 6500
ZnP1
ZnP2
ZnP3
ZnP4
ZnP5
Py
4996 ± 375
16330 ± 1125
1418 ± 100
6545 ± 490
54000 ± 300
1734 ± 250
8235 ± 450
89720 ± 8125
2748 ± 200
3440 ± 180
8375 ± 530
1617 ± 60
3823 ± 250
14030 ± 995
865 ± 45
Pip
DMSO
Py
5380 ± 300
43610 ± 580
1200 ± 180
7200 ± 400
63030 ± 3500
2205 ± 150
2010 ± 100
5880 ± 480
1096 ± 55
4495 ± 350
3750 ± 300
Pip
32820 ± 1000 25705 ± 750
DMSO
Py
835 ± 50
589 ± 25
6376 ± 380
5558 ± 350
Pip
45145 ± 3000 33010 ± 3000
DMSO
Py
1753 ± 100
1260 ± 85
4465 ± 300
679 ± 24
1410 ± 50
870 ± 40
Pip
3810 ± 150
462 ± 21
DMSO
Py
46177 ± 2035
28360 ± 110
17238 ± 1200 10625 ± 8501
Pip
645480 ± 30560 446245 ± 31105 293736 ± 30000 198532 ± 27050
5820 ± 300 4396 ± 250 3164 ± 180 2460 ± 140
DMSO
enthalpy (∆H) and entropy (∆S) from known chemical [10–12] if the Br atoms enter the ortho-positions in the
benzene rings, for example, in H2TPP(o-Br)4 (H2P1), or
β-positions of each pyrrole ring, for example, H2TPP(β-
Br)4 (H2P5). It follows from the electronic absorption
spectra of the tetraphenylporphine dications H4TPP2+
that the accumulation of positive charge at the reaction
thermodynamics relations:
T1T2
T2 – T1
K2
-----
K1
----------------
∆H = 2.3R
log
,
(5)
∆G = –RT lnK,
∆H – ∆G
(6)
(7)
site (H3N+4, H4N24+, and Mσ+) is favorable for partial
conjugation of electrons of the phenyl groups with elec-
trons of the substituted C6H4X groups, where X is a σ-
or π-electron donor.
----------------------
∆S =
.
T
The bromine atoms in the H2P1–H2P5 porphyrins
exhibit a strong inductive effect (–I), irrespective of
their positions in the macrocycle, as in any other
organic molecule [13]. In addition, the bromine atom
has a nonbonding orbital occupied by a pair of 4pz elec-
trons, which can be conjugated with various π-bonds,
benzene rings, and π-systems of nonbenzene aromatic
molecules.
The electrons of bromine atoms in the ZnP1–ZnP5
molecules can, in principle, be conjugated with the
macrocycle π-system. However, the nonorthogonal
arrangement of the macrocycle π-system and C6H4Br
(or C6H3Br2) precludes pronounced π–π interaction in
the case of ZnP1–ZnP4. In the ZnTPP(β-Br)4 (ZnP5)
The results of calculations are summarized in Table 1.
RESULTS AND DISCUSSION
The porphyrins studied in this work are intermediate
between classical porphyrins with a planar aromatic
π-system in the macrocycle and an inert NH bond (tet-
raphenylporphine, octaalkyl- and octapseudoalkylpor-
phyrins, etc. [7–9]) and nonclassical porphyrins with
nonplanar structures (dodeca-substituted porphyrins)
and reactive, highly polarized NH bond (tetraazapor-
phyrins, phthalocyanine).
The replacement of H atoms in the tetraphenylpor-
phine (H2TPP) by strongly polarizing bromine atoms
markedly distorts the planar structure of the macrocycle molecule, the bromine atoms are located in the plane of
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 9 2002

STUDY OF EXTRA COORDINATION
611
Table 2. Band positions and intensities of the electronic absorption spectra of solutions of ZnP1–ZnP5 complexes in organic
solvents
λ, nm (logε)
*
ZnP
Solvent
∆λI , nm
Band I
Band II
Soret
ZnTPP
Toluene
Py
588(3.44)
606(3.98)
610(4.01)
596(3.55)
602(3.65)
603(3.69)
597(3.62)
588(3.67)
601(4.18)
602(4.29)
597(3.95)
588(3.88)
603(4.32)
605(4.33)
598(4.24)
589(3.57)
601(4.00)
603(4.07)
599(4.03)
609(3.85)
618(3.97)
618(4.11)
617(3.95)
549(4.28)
566(4.27)
568(4.27)
551(4.30)
563(4.27)
564(4.29)
560(4.29)
549(4.69)
562(4.66)
565(4.67)
558(4.42)
549(4.64)
563(4.63)
565(4.63)
559(4.63)
550(4.30)
562(4.26)
565(4.28)
559(4.29)
568(4.14)
575(4.16)
576(4.31)
572(4.18)
423(5.68)
431(5.81)
432(5.72)
423(5.67)
430(5.84)
431(5.85)
429(5.83)
420(5.64)
427(5.85)
431(5.89)
427(5.80)
422(5.02)
428(5.07)
429(5.08)
426(5.09)
423(5.32)
429(5.60)
430(5.76)
428(5.64)
429(5.12)
437(5.29)
441(5.68)
430(5.20)
18
22
Pip
ZnP1
ZnP2
ZnP3
ZnP4
ZnP5
Toluene
Py
6
7
1
Pip
DMSO
Toluene
Py
13
14
9
Pip
DMSO
Toluene
Py
15
17
10
Pip
DMSO
Toluene
Py
12
14
10
Pip
DMSO
Toluene
Py
9
9
8
Pip
DMSO
* ∆λ = λ (LZnP) – λ (ZnP).
I
I
I
the macrocycle π-system; in this case, the pπ(Br)–pπ(P) involved in the π–π conjugation with either the benzene
interaction becomes quite possible. The possibility of ring or the macrocycle π-system. The meta-dibromo-
this interaction can be evaluated, to a certain extent, substitution in H2TPP is responsible for not only elec-
from the electronic absorption spectra and the stability tronic effects in H2P3 (–I effect) but also for the change
constants of the complexes formed by ZnP with elec- in the geometric structure of the porphyrin (structural
tron-donating Py, Pip, and DMSO molecules (Table 2). effects).
Analysis of the data presented in Table 2 leads to the
These two effects act in opposite directions. There-
following conclusions. The extra coordination of Py
fore, the positions of bands in the electronic absorption
and Pip causes a bathochromic shift of all three bands
spectra of ZnTPP and ZnP3 are close and the molar
in the electronic absorption spectrum of ZnTPP. Similar
extinction coefficient (ε) of bands I and II increases
upon meta-octabromo-substitution.
shifts, although less pronounced, occur in the spectra of
similar complexes formed by bromoporphyrins. The
compound Zn(TPP)(m-Br2)4 (ZnP3), in which the –I
The introduction of four bromine atoms in the
effect of the eight bromine atoms results in the lowest ortho-position of the benzene rings induces a batho-
electron density on Zn, is close to ZnTPP in its ∆λI chromic shift of the λI band in the ZnP1 spectrum in tol-
value. Since all the bromine atoms in ZnP3 occur in the uene. The energy characteristics and the probabilities of
meta-positions of the benzene rings, they cannot be transitions characterized by λII and λSoret do not change.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 28 No. 9 2002

612
BEREZIN et al.
PipZnTPP(β-Br)4 reaches a value of 4.46 × 105 mol–1 l
(this is the most stable extra complex known to date [2]).
On the basis of the obtained results, one should
expect some violation of the planar structure of
H2TPP(β-Br)4 in solutions, substantial polarization of
the NH bond, an increase in the acid dissociation con-
stant, and an increase in the complexation rate of
H2TPP(β-Br)4 with d-metal salts.
The LZnTPP(o-Br)4 complexes are 2 to 5 times less sta-
ble than the corresponding LZnTPP complexes. The
electron-withdrawing effect of the Br atoms is expected
to stabilize complexes with organic bases due to the
favorable decrease in the electron density on the central
Zn atom. However, bromination has the opposite effect
on Ks. Thus, there is at least one more factor that influ-
ences the properties of the MN4 coordination unit upon
bromination.
It was found previously [14–17] that the introduc-
tion of a halogen atom in any position of the benzene
rings, especially in the ortho-position, markedly stabi-
lizes MTPP complexes (M = Zn, Cu, Ni, Mn) due to
steric shielding of the MN4 reaction site. However,
steric restrictions caused by the substituted benzene
rings cannot significantly influence the formation and
dissociation of extra omplexes with organic bases.
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case of ZnP1, namely, a bathochromic shift of the λI
band with invariable positions of the λII and λSoret
bands. Among the LZnTPP(m-Br)4 complexes, only the
complex with L = Pip is three times more stable than
the corresponding complex with the ortho-substituted
porphyrin (PipZnP2), whereas the stability of the other
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(ZnP5). This shows itself as the greatest bathochromic
shifts of all bands in the spectrum observed on passing
from ZnTPP to ZnTPP(β-Br)4, in particular, ∆λI = 21,
∆λII = 19, and ∆λSoret = 6 nm (Table 2).
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chromophore by the substituents and redistribution of
the π-electron density between the macrocyclic (C12N4)
and benzene (C6H5) chromophores and the Cβ=Cβ dou-
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withdrawing action of the Br atom on the ZnN4 reaction
site (–I effect) is so pronounced that Ks of the
PyZnTPP(β-Br)4 complex (T = 298 K) increases by
4.8 times with respect to that of PyZnTPP, while Ks of
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