Coordination of nitrogen-containing ligands by Co complexes with tetra-and dodeca-substituted porphyrins

Article abstract of DOI:10.1134/S1070328407020078

Capabilities of complexes with tetraphenylporphyrin, the derivative of tetrapyridiniumporphyrin, and with β-octabromine-meso-tetraphenylporphyrin to coordinate additional nitrogen-containing ligands are compared. The equilibrium constants of addition of e

Full text of DOI:10.1134/S1070328407020078

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2007, Vol. 33, No. 2, pp. 116–119. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © L.Zh. Guseva, S.G. Pukhovskaya, A.S. Semeikin, O.A. Golubchikov, 2007, published in Koordinatsionnaya Khimiya, 2007, Vol. 33, No. 2, pp. 121–124.
Coordination of Nitrogen-Containing Ligands by Co Complexes
with Tetra- and Dodeca-Substituted Porphyrins
L. Zh. Gusevaa, S. G. Pukhovskayaa, b, A. S. Semeikinb, and O. A. Golubchikova, b
a Institute of Solution Chemistry, Russian Academy of Sciences, ul. Akademicheskaya 1, Ivanovo, 153045 Russia
b Ivanovo State University of Chemical Technology, Ivanovo 153000, Russia
Received January 30, 2006
Abstract—Capabilities of complexes with tetraphenylporphyrin, the derivative of tetrapyridiniumporphyrin,
and with β-octabromine-meso-tetraphenylporphyrin to coordinate additional nitrogen-containing ligands are
compared. The equilibrium constants of addition of extra ligands, namely, pyridine, piperidine, and N-meth-
ylimidazole, to the Co complexes with these porphyrins in ethanol solutions at 298 K are determined.
DOI: 10.1134/S1070328407020078
Numerous fermentation processes occurring with tral parameters of the obtained compounds agree with
participation of porphyrin complexes include coordina-
tion of substrate by metalloporphyrin cations. This
reaction was called extra coordination and has been
extensively studied in recent years [1–3]. Of particular
interest among metalloporphyrins are compounds with
nonplanar structure of a macrocycle. Obviously, physi-
cochemical, coordination, and catalytical properties of
metalloporphyrins can be monitored within wide limits
through aromatic ring deformation as a result of a steric
repulsion of the peripheral substituents [4]. It is difficult
or almost impossible to separate the electronic effects
of substituents that induce deformation of the porphy-
rin ring from the electronic effects produced by intrin-
sic deformation of a macrocycle. This problem can be
solved by systematic study of the properties of spatially
distorted compounds with their planar analogs.
the data [7–9].
The ç2ê3 porphyrin, containing four pyridinium
groups in meso-positions of porphyrin, was synthesized
in two stages. At the first stage, a solution of pyridine-
4-carboxaldehyde (7.7 g, 0.072 mol) in 5 ml of pyrrole
(0.072 mol) was added in drops to 200 ml of boiling
propionic acid. The mixture obtained was refluxed for
45 min and then, propionic acid was distilled off in
water aspirator vacuum. The residue was mixed with
600 ml of benzene, washed with ammonia solution,
water, and dried with Na sulfate. The benzene solution
was chromatographed on a column with Brockman
activity III Al2O3, using benzene as eluent and a red
porphyrin band was collected. Eluate was concentrated
and porphyrin was precipitates with methanol. The pre-
cipitate was filtered off, washed with methanol, and
In this study, capabilities of coordinating nitrogen-
containing extra ligands by the Co complexes with tet- dried in air at 75°ë. The yield was 0.85 g (7.6%). Rf =
raphenylporphyrin (ç2ê1), β-octabromine-meso-tet-
raphenylporphyrin (ç2ê2) (used in biological systems
[5]), and the derivative of tetrapyridiniumporphyrin
(ç2ê3) are compared. Pyridine (Py), piperidine (Pip),
and N-methylimidazole (MeIm) were used as extra
ligands.
0.71 (silufol, chloroform : methanol = 5 : 1). The elec-
tronic absorption spectrum (CH3Cl), λmax, nm (logε ):
643 (3.43); 589 (3.82); 546 (3.79); 514 (4.30); 417
(5.62).
At the second stage, a solution of 5,10,15,20-tetra-
4-pyridylporphyrin (0.5 g, 0.808 mmol) and bromoace-
tic acid (3.0 g, 21.6 mmol) in 30 ml of DMF was
refluxed for 1 h. After cooling, 30 ml of benzene was
added and the precipitate formed was filtered off,
washed with acetone, and dried in air at 75°ë. The yield
was 0.9 g (91%). The electronic absorption spectrum
(ç2é), λmax, nm (logε ): 630 (3.49); 585 (3.91); 556
(3.86); 519 (4.23); 423 (5.41).The 1H NMR (D2O, TMS
as an external standard) δ, ppm: pyridine α-ç 8.91 d,
EXPERIMENTAL
Benzene, chloroform, ethyl alcohol, Py, Pip, and
MeIm were purified and dehydrated according to a
known procedure [6]. Water content determined by the
Fischer method did not exceed 0.01%.
Tetraphenylporphyrin was synthesized as described
in [7], octabromine derivative of tetraphenylporphyrin
was obtained following the procedure in [8]. The spec- J = 6.6 Hz, β-ç 9.25 d, J = 6.6 Hz; pyrrole β-ç 9.07 s.
116

COORDINATION OF NITROGEN-CONTAINING LIGANDS
117
A
0.36
0.32
0.28
0.24
0.20
0.16
0.12
0.08
0.04
0
R
R'
R'
R'
R
R'
N
HN
N
R
NH
R'
R'
R'
R'
R
H P1: R = C6H5, R' = H;
H2P2: R = C6H5, R' = Br;
400 410 420 430 440 450 460 470 480 490
2
λ, nm
N+ CH2COOH · Br–
,
Fig. 1. The change in the electronic absorption spectrum of
H2P3: R =
R' = H.
–3
–1
CoP3 in ethanol atc = 4 × 10 –2 × 10 mol/l.
Pip
The Co complexes (CoP1 and CoP2) were obtained
on refluxing porphyrins with an excess of Co acetate on
passing a nitrogen flow through a preliminarily
degassed ethanol–benzene mixture for 3 h. The mixture
was cooled and twice chromatographed on a silica gel
(with chloroform as eluent) and on activity III Al2O3
(with benzene as eluent).
ëÓP + L
LëÓP.
(1)
The optical density on analytical wavelengths
(Table 1) were used to calculate the stability constants
(äst) of reaction (1)
äst = [LCoê]/([CoP][L])
(2)
and the equilibrium concentration of extra complex
[LCoê]/[CoP] = (A0 – Aeq)/(Aeq – Af),
The electronic absorption spectrum (ë2ç5éç),
(3)
λ
max, nm (logε ): CoP1—426 (5.68), 543 (4.61);
where A0, Aeq, and Af are the starting, equilibrium, and
final optical densities of a solution, respectively.
CoP2—435 (4.93). 574 (4.17), 614 (4.06).
To obtain CoP3, a freshly prepared low-soluble Co
hydroxide [10] (the Co(OH)2 solubility in water is
All the experiments were performed with significant
(more than 50-fold) excess of L as compared to metal-
loporphyrin. Therefore, the equilibrium concentration
of extra ligand was taken equal to the initial concentra-
tion. The plots of (A0 – Aeq)/(Aeq – Af) vs. cL in the sys-
tems under study turned linear (Fig. 2). This confirms
the conclusion that the process of extra coordination
terminates at the stage of formation of monoligand
complexes LMP. The equilibrium constants of addition
of the nitrogen-containing ligands to Co porphyrinates
were calculated from the equation
about 2 × 10–4 mass % in a ratio 1 : 100) was added to
an aqueous solution of porphyrin H2P3 and stirred with
a magnetic stirrer for 8 h at 50°ë. The formation of
metalloporphyrin was monitored by spectrophotomet-
ric method. The product was purified by double filtra-
tion of the solution and recrystallization of a complex
with benzene. The precipitate formed was washed with
acetone and dried at 75°ë. The yield was 96%. The
electronic absorption spectrum (ç2é), λmax, nm
(logε ): 540 (3.78); 429 (5.00); (ë2ç5éç)—440
(4.33), 550 (3.75). 607 (3.63).
äÛ = (A0 – Aeq)/(Aeq – Af)ÒL],
and tabulated (Table 1).
(4)
The extra coordination processes were investigated
spectrophotometrically on a Specord M400 instrument
using 50-ml cells (l = 10 cm) at 293.15 0.1 K. An
exact volume of extra ligand solution in ethanol was
added to a solution of metalloporphyin in ethanol using
a microsyringe. An increase in the volume of a solution
after termination of the process did not exceed 0.05%.
Table 1. Stability constants of CoP1, CoP2, and CoP3 com-
plexes with nitrogen-containing bases in ethanol at 298 K
Kst, mol–1
l
Metal-
loporphy-
rin
λ
max, nm
Py
N-MeIm
Pip
RESULTS AND DISCUSSION
CoP1 429, 432, 425 31
CoP2 435 17
CoP3 438, 430, 440 39
4
3
27
7
230 60
After the addition of extra ligands (L), the electronic
absorption spectra of solutions of all investigated met-
alloporphyrins in ethanol retained distinct isosbestic
points (Fig. 1), which suggests the formation of mono-
ligand complexes:
47 12 17
15 14
2
2
5
3
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 33 No. 2 2007

118
log (A0 – Aeq)/(Aeq – Af)
GUSEVA et al.
the π-donor properties of the Co cation. Therefore, on
going from CoP1 to CoP3, the stability of pyridinates
remains almost unchanged.
–1.2
–1.4
–1.6
–1.8
–2.0
–2.2
–2.4
–2.6
Taking into account the key role of σ-interactions in
the process of extra coordination of Pip and MeIm, one
could expect that the CoP3 complex should be more
stable than the CoP1 complex. However, in reality, the
situation is quite opposite: the stabilities of complexes
with Pip decrease ten times, while the stabilities of
complexes with MeIm decrease two times. Obviously,
the reasons for this are not due to the electronic effects
of substituents.
In ethanol solutions, the Br– anions of H2P3 are
likely to give weakly dissociated ionic pairs with posi-
tively charged N atoms of pyridinium meso-substitu-
ents. Because of their large sizes, they shield the coor-
dination center of Co porphyrinate. The steric hin-
drances are inessential for pyridine, which I on
coordination is arranged perpendicular to a macrocycle
plane (having the sp2-hybrid state of the N atom). On
the contrary, in the case of Pip, arranged at some angle
to a macrocycle plane and due to the sp3-hybridization
of the donor atom, the steric hindrances are essential. A
large size of the MeIm molecule is most likely to be the
reason for steric repulsion between its methyl group
and the Br– anions. The scheme of coordination of the
nitrogen-containing ligands to CoP3 is shown in Fig. 3.
–1.4 –1.2 –1.0 –0.8 –0.6 –0.4 –0.2
logcL
Fig. 2. The plot of log[(A0 – Aeq)/(Aeq – Af)] vs. logcL
for extra coordination of CoP1 with N-methylimidazole.
Piperidine is a base, which is million times as strong
as pyridine and therefore, the stability of extra complex
CoP1 with piperidine is one order higher than that of a
complex with pyridine. The CoP1–Pip interaction is of
the σ-type. Pyridine contains low unoccupied π-orbit-
als and thus can enter the π-dative interaction with the
Co cation of metalloporphyrin. As compared to pyri-
dine, MeIm is a stronger σ-donor, but a weaker
π-acceptor. As a result of the compensation of these two
factors, the stabilities of extra complexes CoP1 with Py
and MeIm are equal within the limits of the error of
determination.
The data in [11] indicate that dodeca-substituted
porphyrins have strongly distorted structure of a tet-
rapyrrole ring, which brings about a decrease in the aro-
maticity of porphyrins and separation of the pyrrole
fragments. As a result, the electronic density on the N
atoms increases.
The replacement of eight β-H atoms by the electron-
acceptor Br atoms results not in the increase but in the
decrease in the effective positive charge on the coordi-
nating N atom. The stability of extra complexes on
The degree of dissociation of the carboxyl groups of
the CoP3 complex in ethanol is low due to low polarity
of a solvent. The electron-acceptor pyridinium groups
of this complex increase the σ-acceptor and decrease going from CoP1 to CoP3 decreases, this decrease
CH3
N
Br–
HOOCH2N+
Br–
N+CH2COOH
Br–
HOOCH2N+
Br–
N+CH2COOH
N
N
Co
Co
1
2
N
Br–
HOOCH2N+
Br–
N+CH2COOH
Co
3
Fig. 3. The scheme of coordination of nitrogen-containing ligands by CoP3: (1) Py, (2) MeIm, and (3) Pip.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 33 No. 2 2007

COORDINATION OF NITROGEN-CONTAINING LIGANDS
Table 2. The change in the position of maxima of absorp-
119
ACKNOWLEDGMENTS
tion bands (∆λ) in the electronic absorption spectra during
the formation of extra complexes of Co porphyrinates with
nitrogen-containing molecules in ethyl alcohol
This work was supported by RFFI grant (no. 03-03-
96458-r2003-tschr-a) “Theoretical principles and
experimental modeling of depositing and recycling of
NO in biological systems by porphyrin complexes”.
Metalloporphyrin
Extra ligand
∆λ, nm
5 (435
CoP2
CoP2
CoP1
CoP1
CoP3
CoP3
CoP3
N-MeIm
Pip
440)
445)
432)
430)
449)
448)
446)
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10 (435
6 (426
4 (426
9 (440
8 (440
6 (440
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RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 33 No. 2 2007