Complexation of sterically hindered cobalt porphyrin with 1-methylimidazole along the axial coordinate and the reaction of reversible binding of oxygen

Article abstract of DOI:10.1007/s11167-005-0084-7

The complexation of a sterically hindered cobalt porphyrin with 1-methyl imidazole and its reaction of reversible binding of oxygen was studied. Spectrophotometry and potentiometry were used to determine the equilibrium constant of the oxygenation process.

Full text of DOI:10.1007/s11167-005-0084-7

Russian Journal of Applied Chemistry, Vol. 77, No. 10, 2004, pp. 1623 1626. Translated from Zhurnal Prikladnoi Khimii, Vol. 77, No. 10, 2004,
pp. 1634 1637.
Original Russian Text Copyright
2004 by Valiotti, Abakumova.
PHYSICOCHEMICAL STUDIES
OF SYSTEMS AND PROCESSES
Complexation of Sterically Hindered Cobalt Porphyrin
with 1-Methylimidazole along the Axial Coordinate
and the Reaction of Reversible Binding of Oxygen
A. B. Valiotti and R. A. Abakumova
St. Petersburg State University, St. Petersburg, Russia
Received December 19, 2003; in final form, June 2004
Abstract The complexation of a sterically hindered cobalt porphyrin with 1-methyl imidazole and its re-
action of reversible binding of oxygen was studied. Spectrophotometry and potentiometry were used to de-
termine the equilibrium constant of the oxygenation process.
In the last decade, methods for synthesis and study
of properties of model compounds of natural porphy-
rins reversibly binding oxygen have been developed
successfully. Synthetic oxygen transporting agents of
this kind find use as blood substitutes, gas sorbents,
and materials for recovery of oxygen from liquid
media [1 4].
spectra. The complexation with 1-methylimidazole was
studied spectrophotometrically. In the experiments, the
dependence of the optical density of [Co(II)TPivPP]
solutions in propyl cyanide on the concentration of
1-methylimidazole was determined at a fixed wave-
length and analyzed. Propyl cyanide was chosen as
the solvent because it has a sufficiently high dielectric
constant ( = 20.3) and does not react with metal por-
phyrins. As supporting electrolyte served tetrabutyl-
ammonium perchlorate (TBAP), c(TBAP) = 0.1 M.
All the measurements were performed in quartz cu-
vettes of thickness 0.1 and 1.0 cm on an SF-18 spec-
trophotometer. The titration with a solution of 1-meth-
ylimidazole was performed directly in the cuvette.
The titrant was added dropwise to a solution of
[Co(II)TPivPP] in the atmosphere of argon and a spec-
trum of the solution was recorded upon each addition.
All the measurements were carried out at a tempera-
ture of 20 0.5 C. Figure 1 shows the experimental
data in the form of a dependence of the optical density
A( 532) of the [Co(II)TPivPP] solution on the concen-
Monitoring the partial pressure of oxygen in gases,
liquids, and semi-liquid media is an important task in
most of biochemical investigations and technological
processes. It is of indubitable interest, in this context,
to develop a potentiometric sensor for measuring the
partial pressure on the basis of a redox system that
contains cobalt porphyrin and reversibly binds oxygen
[5]. A necessary stage in this case is a study of the
properties of the cobalt porphyrin.
The goal of this study was to examine the com-
plexation of sterically hindered cobalt(II) meso-
tetra( , , , -ortho-pivalamidophenyl)porphyrin
[Co(II)TPivPP] with 1-methylimidazole along the ax-
ial coordinate and the reaction of reversible binding
of oxygen.
EXPERIMENTAL
The meso-tetra( , , , -ortho-pivalamidophenyl)-
porphyrin was synthesized by the procedure suggested
by Collman [6]. The compound [Co(II)TPivPP] was
obtained by heating free porphyrin with CoCl2 in a
flow of argon under constant stirring in a solution of
tetrahydrofuran [7]. The purity of the products syn-
thesized was verified chromatographically and using
Fig. 1. Optical density
[Co(II)TPivPP] solution vs. the pc of 1-methylimidazole.
A
(
= 532 nm) of the
L
1070-4272/04/7710-1623 2004 MAIK Nauka/Interperiodica

1624
VALIOTTI, ABAKUMOVA
by Eq. (1). The logarithm of the stability constant for
[Co(II)TPivPP] with 1-methylimidazole, calculated
by Eq. (2), is 3.3 0.2. The data obtained suggest
that, at a 1-methylimidazole concentration of 0.1 M,
the complexation in the system constituted by por-
phyrin and 1-methylimidazole is complete. This result
is of primary importance for a study of the reaction
in which oxygen is reversibly bound in the presence
of 1-methylimidazole.
The reversible binding of O2 was studied by means
of spectrophotometry in a hermetically sealed quartz
cuvette. The partial pressure of O2 was set by mixing
argon and oxygen in various volume ratios at 1 atm
in calibrated gas meters. Each mixture of gases, pre-
liminarily dried over KOH, was bubbled through the
cuvette for 20 min, and the spectrum of the solution
was recorded after that. All the measurements were
performed at a temperature of 20 0.5 C.
Fig. 2. Spectra of [Co(II)TPivPP] solutions in propyl
cyanide in the presence of 1 M of 1-methylimidazole and
0.1 M of TBAP at different partial pressures of oxygen.
It is known that, in the presence of an axial ligand,
1-methylimidazole, and O2 in a [Co(II)TPivPP] solu-
tion, there exist the equilibrium
(A) Optical density and ( ) wavelength. P
(mm Hg):
O2
(1) 0, (2) 38, (3) 84, (4) 114, (5) 190, (6) 380, and (7) 760.
[Co(II)TPivPPL] + O2 [Co(II)TPivPPLO2 ], KO . (3)
tration of 1-methylimidazole (pcL). The first stage of
the reaction of axial complexation of [Co(II)TPivPP]
with 1-methylimidazole can be represented as
2
The fact that logarithm of the stability constant of
[Co(II)TPivPP] with 1-methylimidazole has a value
of 3.3 0.2 shows that the complex [Co(II)TPivPPL]
is formed to an extent exceeding 99% at a 1-meth-
[Co(II)TPivPP] + L = [Co(II)TÞivPPL],
(1)
ylimidazole concentration of 1 M. To determine KO ,
where L is 1-methylimidazole.
2
spectra of a [Co(II)TPivPP] solution in propyl cyanide
were recorded in the presence of 0.1 M of 1-methyl-
imidazole and 0.1 M of TBAP at different partial
pressures of O2. Typical changes in the spectrum of
a [Co(II)TPivPPL] solution in propyl cyanide in the
presence of O2 are shown in Fig. 2. It can be seen
that, as the concentration of O2 increases, the peak at
534 nm, associated with the deoxygenated form, is
shifted to longer wavelengths, and a new peak asso-
ciated with the oxygenated form appears at 552 nm.
Introduction of 1-methylimidazole into a porphyrin
solution leads to a decrease in the optical density at
= 532 nm. It can be shown, with account of this
circumstance, that the dependence of the optical den-
sity of the solution on the ligand concentration cL
can be expressed as
A0 + A1K1cL
(2)
A =
,
1 + K1cL
where A is the optical density measured in the exper-
iment; A0, optical density of a solution without ligand;
A1, optical density of a solution in which the com-
plexation is complete; and K1, complexation constant
found from the equation
Two clearly pronounced isobestic points are ob-
served at 540 and 470 nm, which points to the pres-
ence of two interconvertible species in the solutions.
In deoxygenation of the solution by bubbling of
argon, the spectrum changes in the opposite direc-
tion and virtually coincides with the initial spec-
trum, i.e., the starting cobalt porphyrin is oxidized
only slightly. It should be noted that the oxygenated
form Co(II)TPivPPLO2 and the oxidized species
[Co(II)TPivPPL]
K1
=
.
[Co(II)TPivPP][L]
The experimental dependence is described by a
smooth single-step curve, which is due to the occur-
rence of the reaction of 1-methylimidazole addition
Co(II)TPivPPL+ have the same spectrum. KO was
2
calculated using the Drago equation [6]
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 10 2004

COMPLEXATION OF STERICALLY HINDERED COBALT PORPHYRIN
1625
[CoP]tot
l
1
2
(4)
P1/2 = KO = PO
1 ,
2
A
where [CoP]tot is the total concentration [Co(II)TPivPP]
in solution; l, cuvette thickness; , difference of
the absorption coefficients of the oxygenated and
deoxygenated species; and A, difference of the op-
tical densities at a certain PO and in the absence of
2
oxygen.
This equation is convenient in that it does not re-
quire any knowledge of the optical density of the ox-
ygenated form, because the oxygenation is not com-
plete even at PO = 1 atm and 0 C. Transformation
of Eq. (4) yields 2
Fig. 3. P / A vs. the partial pressure of oxygen, P
.
O2
O2
= 530 nm, c(TBAP) = 0.1 M, T = 25 C.
PO = [CoP]tot
l
PO / A
P1/2
.
(5)
2
2
and cO and cR are the total concentrations of the oxi-
dized and reduced forms of cobalt porphyrin.
The plot of PO against (PO / A) is a straight
2
2
line with a slope equal to [CoP]totl . This plot
With account of (7), the equation for the oxidation
potential is transformed into
(Fig. 3) was used to determine KO .
2
At a temperature of 20 0.5 C in propyl cyanide in
0
=
+
log cO/cR
+
log 1 + KO PO ,
2 2
the presence of 0.1 M of TBAP, P1.2 = 140 7 mm Hg
3
(KO = 7.2 10 ).
2
and then, at constant concentrations cO and cR,
log 1 + KO PO (8)
The reversible binding of O2 was also studied
by means of potentiometry and, in particular, by
the method of the oxidation potential [8]. The equa-
=
+
,
2
0
2
0
tion for the oxidation potential
of the system
where
=
+ log cO cR
.
/
0
Co(II)TPivPPL Co(III)TPivPPL+ can be written as
It follows from Eq. (8) that at high PO , when
KO PO >> 1, the dependence
linear with a slope equal to
2
[Co(III)TPivPPL+]
[Co(II)TPivPPL]
0
log PO must be
=
+
log
,
(6)
2
2
2
.
where = 2.3RT/F and the concentrations of the oxi-
dized and reduced forms of the redox system are given
in the square brackets.
The experimental study consisted in finding
the dependence log PO . The oxidation potential
2
was determined by measuring the electromotive force
of the galvanic cell Hg |Hg2Cl2, LiCl(sat.) ||solution
under study |Pt.
It was shown preliminarily that the oxidation po-
tential of this system at a 1-methylimidazole concen-
tration c 0.1 M is independent of pcL, which indi-
cates the formation of complexes of the same compo-
sition for the oxidized and reduced forms. If reaction
(1) is regarded as complexation of Co(II)TPivPPL
with O2, then the material balance equations can be
written as
All the measurements were performed at 25 C.
The concentration of 1-methylimidazole was 0.1 M.
The ionic strength was maintained with 0.1 M of
TBAP. The cobalt porphyrins Co(II)TpivPP and
Co(III)TPivPP+ were taken in a 1 : 1 molar ratio.
The experimental dependence E log PO is shown
in Fig. 4. The oxidation potential is giv2en relative
to a saturated calomel electrode. It can be seen that
this dependence is represented by a straight line at high
cO = [Co(III)TPivPPL+ ],
cR = [Co(II)TPivPPL] + [Co(II)TPivPPLO2 ]
PO , in agreement with Eq. (8). At PO = 0,
=
.
0
2
made it pos2sible to calcu-
= [Co(II)TPivPPL] 1 + KO PO
,
2
(7)
The graphically found
0
2
late, using Eq. (8), KO (KO = 1/P1/2) and, accord-
2
ingly, P1/2. The calculat2ion was performed using the
where
[Co(II)TPivPPLO2]
values of PO falling within the range in which the
KO
=
,
2
2
[Co(II)TPivPPL]PO
2
dependence log PO is linear. The averaged value of
2
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 10 2004

1626
VALIOTTI, ABAKUMOVA
on the partial pressure of oxygen was determined on
the basis of experimental data.
These results can be used to design a potentiomet-
ric sensor for molecular oxygen in aqueous solution
and to assess the region and limits of its applicability.
REFERENCES
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Izv. Ros. Akad. Nauk, Ser. Khim., 2003, no. 2,
pp. 377 380.
Fig. 4. Oxidation potential E of a solution of the redox
system Co(II)TPivPPL Co(III)TPivPPL in propyl cy-
anide in the presence of 1-methylimidazole (1-MeIm) vs.
+
log P . c(1-MeIm) = 0.1 M, c(TBAP) = 0.1 M, T = 25 C.
O2
4. Komatsu Teruyuki, Okado Tomoyuki, Moritake Miho,
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vol. 74, no. 9, pp. 1695 1702.
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P1/2 is 160 mm Hg, i.e., the results of spectropho-
tometric and potentiometric studies are in agreement
within the experimental error.
CONCLUSION
7. Collman, J.P., Brauman, J.J., Doxsee, K.M., et al.,
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The functional dependence of the oxidation potential
of the redox system Co(II)TPivPPL Co(III)TPivPPL+
8. Oksredmetriya (Redoxmetry), Nikol’skii, B.P. and
Pal’chevskii, V.V., Eds., Leningrad: Khimiya, 1975.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 10 2004