Axial ligation of iron(III) porphyrin with a series of aliphatic bases: Piperazine, piperidine and pyrrolidine

Article abstract of DOI:10.1134/S0036023612010184

The binding of a series of nitrogen donor ligands (Piperazine (Pipz), Piperidine (Pip) and Pyrro- lidine (Pyr)) to iron porphyrin, OEPFeClO ₄, where OEP is octaethylporphyrin, has been characterized by electronic spectroscopy in CH₂Cl₂. In nonaqueous media, in the presence of a neutral ligand, the equilibria observed are: OEPFeClO ₄ + 2L [OEPFeL₂]+ (β₂) where the product is an ion pair and in some cases: OEPFeClO₄ + L OEPFeLClO₄ (K1), where the product may either be the six-coordinate or the five-coordinate [OEPFeL]+ ion pair, that L denotes neutral N-donor ligands. This behavior for the nitrogen donor ligands (L = Pipz, Pip, Pyr) is confirmed by spectrophotometric titrations data and the bind- ing constants for the substitution reaction have been reported. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0036023612010184

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2012, Vol. 57, No. 1, pp. 128–132. © Pleiades Publishing, Ltd., 2012.
PHYSICAL CHEMISTRY
OF SOLUTIONS
Axial Ligation of Iron(III) Porphyrin with a Series of Aliphatic Bases:
Piperazine, Piperidine and Pyrrolidine1
J. Saffari, M. KhorasaniꢀMotlagh, and M. Noroozifar
Department of Chemistry, University of Sistan & Baluchestan, Zahedan, P.O. Box 98155ꢀ147, Iran
eꢀmail: mkhorasani@chem.usb.ac.ir
Received June 23, 2010
Abstract—The binding of a series of nitrogen donor ligands (Piperazine (Pipz), Piperidine (Pip) and Pyrroꢀ
lidine (Pyr)) to iron porphyrin, OEPFeClO4, where OEP is octaethylporphyrin, has been characterized by
electronic spectroscopy in CH2Cl2. In nonaqueous media, in the presence of a neutral ligand, the equilibria
−
observed are: OEPFeClO4 + 2L
[OEPFeL2]+
(β2)
where the product is an ion pair and in some
ClO4
cases: OEPFeClO4 + L
fiveꢀcoordinate [OEPFeL]+
OEPFeLClO4 (K1
−
)
, where the product may either be the sixꢀcoordinate or the
ion pair, that L denotes neutral Nꢀdonor ligands. This behavior for the
ClO4
nitrogen donor ligands (L = Pipz, Pip, Pyr) is confirmed by spectrophotometric titrations data and the bindꢀ
ing constants for the substitution reaction have been reported.
DOI: 10.1134/S0036023612010184
1
Metalloporphyrins have been used to mimic biologꢀ transfer redox centers, e.g., in cytochrome b5. In some
ical systems. A great number of experimental results cases there are additional covalent or coordinate covaꢀ
have been reported over the past decades concerning lent linkages as well, as in the cytochromes b and c, but
the axial ligation properties of metallo– porphyrins the ironꢀimidazole linkage is common to all those
with S, O, P and N bases [1]. The interaction of metꢀ where axial ligands have been identified [5, 6]. One is
alloporphyrins with donor molecules either in their thus led to the question of why some ligands such imiꢀ
ground or excited states can strongly influence the dazole should be the ligand of choice for hemes rather
absorption properties and the efficiency of energy or than some other Lewis base and why some Fe(III)–
electron transfer processes of porphyrin derivatives. porphyrinꢀimidazole complexes are so much more
Thus, understanding the effects of axial ligands on the stable [7]. Several studies of the axial ligation reactions
electronic spectra of metalloporphyrins is a basic but of Fe(III) porphyrins have already been reported [6,
important subject because of its biological relevance 8–13]. Two steps of axial ligand addition to Fe(III)
[2]. Variations in the biological role of the naturally porphyrin are possible. The first step is the formation
occurring hemoproteins are intimately associated with of the 1 : 1 complex:
changes in the axial ligation of the heme moiety. Releꢀ
OEPFeCl + L
OEPFeLCl K1
(1)
vant to our understanding of the mechanism of the action
of the hemoproteins is an understanding of the manner in
which axial ligation affects the electronic structure and
reactivity of a metalloporphyrin system [3].
which may either be the sixꢀcoordinate model comꢀ
plex of ferrihemoglobin or ferrimyoglobin, or the fiveꢀ
coordinate [OEPFeL]+
− ion pair [6]. The distincꢀ
ClO4
In all of the heme proteins investigated to date, the
heme moiety is bound to the protein by at least one
coordinate covalent bond between iron and the “aroꢀ
matic” nitrogen of a histidine residue of the protein.
Iron porphyrin complexes of imidazole are a logical
starting point in the search for appropriate spectroꢀ
scopic models for heme centers in metalloproteins [4],
since the histidyl imidazole side chain in the most
common axial ligand bound to iron in such enzymes.
Sixꢀcoordinate heme centers with two axial imidazole
ligands in metalloproteins are known to act as electron
tion between these two possible coordination numbers
and, indeed, the potential interconversion between
these two forms of the 1 : 1 complex is made difficult
due to the intervention of the second step of axial
ligand addition to form the 2 :1 complex:
OEPFeLCl + L
[OEPFeL2]+Cl– K2
.
(2)
Since, as we shall see, the stepwise constant, K2, is
in general larger than K1. In fact, as we shall also see,
formation of the 1 : 1 complex is not always detectable,
leading to the frequent experimental observation of
addition of two ligands in an apparent single step:
1
The article is published in the original.
128

AXIAL LIGATION OF IRON(III) PORPHYRIN
129
[OEPFeL2] +Cl– β2
.
(3)
We describe here the binding of OEPFeClO4, to a
group of aliphatic nitrogen donor ligands (L = Pipz, Pip,
Pyr). These ligands bind to porphyrin, through 1 : 1 comꢀ
plex, to form a 1 : 2 complex even in the presence of an
excess of the ligands. By using UVꢀvisible absorption we
have elucidated the equilibrium constants for the reacꢀ
tions of these ligands with porphyrin (Scheme 1).
OEPFeCl + 2L
Equation (3) is the sum of equations (1) and (2),
and β2 = KlK2. The product of reactions (2) and (3) is
usually an ion pair [9, 12, 14] which, in the stoichiomꢀ
etry found for the equilibrium constants K2 and β2
behaves as one unit rather than as two ions.
,
+
(1)
ClO4
ClO4
L
Et
Et
Et
Et
Et
Et
Et
Et
K
1
Et
N
N
Et
Et
Et
Et
N
N
N
+ L
N
N
, or
ClO4
Et
Et
Fe
Fe
L
N
N
Fe
N
N
Et
Et
Et
Et
N
Et
Et
Et
Et
Et
β2 = K1K2
(3)
+ 2L
+
ClO4
L
Et
Et
Et
Et
K
Et
Et
2
+ L
ClO–4 (2)
N
N
Et
Et
Et
Et
N
N
N
N
Fe
L
Fe
L
N
N
Et
Et
Et
Et
Et
Et
N
Piperazine (Pipz)
N
N
Piperidine (Pip)
Pyrrolidine (Pyr)
L =
N
Scheme 1.
EXPERIMENTAL
Ligand Substitution Measurement
Equilibrium constants were measured by a spectroꢀ
Material and Instrumentation
photometric titration method. During the titration,
the temperature of the solution was maintained at
[
OEPFeClO4] was prepared by the method of
Ogoshi et al. [15]. All reagents and solvents used in this
study were obtained from Merck and Aldrich Chem.
Co. Dichloromethane was dried by refluxing under an
inert gas over a drying agent, such as phosphorous
pentoxide or calcium hydride, and distilled immediꢀ
ately before use. Piperidine was distilled from zinc dust
and then barium oxide. UVꢀvisible spectra were
recorded on an analytikjena SPECORD S100 specꢀ
trometer with photodiode array detector with thermoꢀ
stat cell compartment, that control the temperature
25.0 0.1
taining ligands were added to a dichloromethane soluꢀ
tion of OEPFe(ClO4) (4.14
10–4 M) in separately
°C and a dichloromethane solution conꢀ
×
steps. In general, the spectra were recorded in the 300
to 650 nm region during the titration. In each step 2 mL
of the porphyrin solution was placed in a cuvette and
the UVꢀvisible absorption spectrum was recorded. 5–
10 μL aliquots of ligand solution were sequentially
added to the porphyrin solution, and the spectrum was
recorded after each addition. The values of the absorꢀ
bance at a fixed wavelength were used in our calculaꢀ
tion to obtain the relevant binding constants. The
around the cell within cuvette at 25.0 0.1 С in the
cell compartment.
°
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 1 2012

130
SAFFARI et al.
from isosbestic behavior during the latter stages of the
titration could be quantitatively related to dilution
effects.
(B)
10
5
y
= 2.003x + 8.446
RESULTS AND DISCUSSION
The origin of the marked differences in electronic
spectra between metalloporphyrin ligand complex and
the metalioporphyrin without axial ligand(s) has been
subjected of several discussions. Corwin and coworkꢀ
ers (1963) have purposed the Soret shift upon ligand
binding arises from the stereoelectronic effect of
ligand upon the porphyrin n system; the shift to longer
wavelength of the metalloporphyrin ligand complex
compared with the metalloporphyrin without axial
ligand(s) was ascribed to sreric interference between
y
= 1.011
x
+ 4.583
0
–10.0
–7.5
–5.0
–2.5
–5
ln[Pipz]
1.2
(A)
the ligand and the ꢀelectron system of the porphyrin
π
ring. Mauzerall (1965) has suggested the shift could be
unrelated to ligand binding but rather could result
from differences in solvent interactions with the porꢀ
phyrin
π
system. Caughey et al. [21] has considered
0.6
the factor most likely to make a dominant contribuꢀ
tion to the spectral shift to be the change in interaction
between porphyrin and metal ion which results upon
binding the axial ligands. The binding of nitrogen
bases as axial ligands can be expected to reduce markꢀ
edly the electronegativity of iron ion; that is, the iron
ion will serve as a less effective electron acceptor from
the porphyrin nitrogens.
0
300
350
400
Wavelength/nm
450
500
UVꢀVis spectrophotometric titration method was
used to determine the equilibrium constants between
OEPFeClO4 and three aliphatic nitrogen donor
ligands (L = Pipz, Pip, Pyr). Addition of several drops
of a nitrogenous ligand to a cuvette containing a
dichloromethane solution of OEPFeClO4 results in an
immediate change in the visible spectrum to one idenꢀ
tical spectrum of the dichloromethane solution of the
no reacted iron porphyrin. In all cases, the red shift of
the Soret band has been observed after addition of
nitrogen donor ligands, due to axial ligand binding.
Figure 1 shows the absorption spectral change of
OEPFeClO4 in the presence of various concentrations
Fig. 1. A—Visible spectral changes observed upon addiꢀ
tion of Pipz to OEPFeClO in CH Cl . B—Plot of ln(A –
4
2
2
0
A)/(A
–
A
)
versus ln[Pipz] to calculate
K
and
β
.
f
1
2
titration shows isosbestic points and the data can be
fitted with equation (4) [16–20]:
A0 − A
(4)
ln
= nln L + ln K
[ ]
A − Af
where A0, initial absorbance (uncomplexed porphyꢀ
rin); Af final absorbance (fully complex porphyrin; and
of Piperazine (2.50
× × .
10–4 to 8.3 10–3 M) in CH2Cl2
A
, absorbance at a specific [L]; and
constant. Values for ln were obtained from they interꢀ
cept of the regression line for a plot of ln(A0 – A) (A
Af vs ln[L]. In all cases, isosbestic points were mainꢀ
tained throughout most of the titration. Deviations
K is the binding
The Soret band at 382 nm decreased and a new band
appeared at 400 nm with clear two isosbestic points:
one at low ratios of amine to Fe(III) (392 nm) and a
second at 408 nm for higher ratios. The spectral patꢀ
terns at high ratios of amine to Fe(III) are similar to
those of the sixꢀcoordinated iron porphyrins [6];
therefore, the reaction corresponds to the axial ligaꢀ
tion of Piperazine (Pipz) to the Fe(III) center.
K
/
–
)
Table 1. Absorption maxima and extinction coefficients (
for addition of ligands to OEPFeClO4 in CH2Cl2
ε
)
The absorption maxima and extinction coefficients
of OEPFeClO4 and its amine adduct in CH2Cl2 are
listed in Table 1. The effects of the axial ligands on the
absorption spectra of iron porphyrins include considꢀ
erable red shifts of the absoiption bands. Visible specꢀ
tral changes observed upon addition of ligands to
OEPFeClO4 in CH2Cl2 are shown in Figs. 1–3.
Ligand
Soret bond/nm
ε
/M–l cm–1
None
Pip
382
399
400
402
2.37
×
×
×
×
103
103
103
103
2.66
2.50
2.93
Pipz
Pyr
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 1 2012

AXIAL LIGATION OF IRON(III) PORPHYRIN
131
Typically in the case of Piperazine as ligand, calcuꢀ
(B)
= 1.9092x + 9.185
lations were done on absorbance data taken at 382 nm
and the wavelength maximum of the product, which is
400 nm. For addition of one base molecule, lnK1 is
estimated from the intercept of the portion of the
graph which yields slope of 1.0, and lnβ2 is obtained
from the intercept of the portion of the graph which
yields a slope of 2.0. The values of lnK1 and lnβ2 have
been obtained from the y intercept extrapolation in
Fig. 1B. The assumption made in the determination of
lnK1 in Fig. 1 is that Af for the 1 : 1 complex is similar
to Af, for the 2 : 1 complex. This is probably not true,
except, perhaps, at 382 nm, but in the absence of specꢀ
tral data for the isolated 1 : 1 complexes, it is the only
reasonable assumption which can be made. That the
spectra of the 1 : 1 and 2 : 1 complexes are at least simꢀ
ilar at 382 nm is suggested by the behavior of isosbestic
points. Typically, only two or three data points, for difꢀ
ferent ln[L] values, were available for drawing the
slope = 1.0 line. Thus, the values of lnK1 should be
considered as only rough estimates. The method of
calculation of β2 is based on the stoichiometry of Eq. (3)
that is three moles of reactants forming one mole of
product. Thus the product behaves as one unit, an
“associated” ion pair, and β2 has units of M–2 [6].
Measurement of β2 for addition of nitrogenous ligands
as a function of total [OEPFeClO4], with CH2Cl2 as
solvent, indicates that at concentrations greater than
10–5 M, β2 and K1 are constants, while at lower conꢀ
centrations of OEPFeClO4, β2 varies as the concentraꢀ
tion of base varies. Thus the equilibrium constant for
the second step of complex formation can be estiꢀ
mated from spectral changes which occur when more
9
6
3
y
y
= 0.9064
x
+ 4.925
–6
0
–9
–3
–3
ln[Pip]
1.2
0.6
(A)
0
300
350
400
Wavelength/nm
450
500
Fig. 2. A—Visible spectral changes observed upon addiꢀ
tion of Pip to OEPFeClO in CH Cl . B—Plot of ln(A –
than 1 equiv. of ligands are added to OEPFeClO4
.
4
2
2
0
Also, titration of OEPFeClO4 with Pip and Pyr
were followed by UVꢀvisible absorption spectroscopy
by using the coordination shift of the Soret absorption
(Figs. 2, 3). The Soret band of Pip titration decreased
at 382 nm and a new band appeared at 399 nm and for
Pyr titration the new band appeared at 402 nm (Table 1).
Also, titration with Pip and Pyr results in well defined
isosbestic points at 390 nm. The values of β2 and K1
were obtained from Eq. (4) (Figs. 2B, 3B) and the data
was reported in Table 2. Values of K1 can be estimated
from samples containing less than 1 equiv. of ligand.
The equilibrium constant for the second step of comꢀ
plex formation can be estimated from spectral changes
which occur when more than 1 equiv. of ligands are
added to OEPFeClO4 (The concentration range is
A)/(A
–
A
)
versus ln[Pip] to calculate
K
and
β .
2
f
1
little or no importance in the coordination of aliphatic
ligand to iron(III) porphyrin. In other hand, the basicꢀ
ity of these aliphatic ligands is more than ligands such
as imidazole. The values of β2 are in reasonable agreeꢀ
ment with the result obtained by Walker et al. [22] for
ligation of imidazole and its derivatives with OEPꢀ
FeCl. In comparison, K1 value for these aliphatic
ligands is higher than K1 value for aromatic ligands that
it can be refer to the effect of basicity of these aliphatic
ligands.
1.22
2.58
×
×
10ꢀ4 to 1.01
× ×
10–2 M for Pip and 4.11 10–5 to
Table 2. Equilibrium constants for addition of ligands to
OEPFeClO4 in CH2Cl2 at 25°C
10–3 M for Pyr titrations).
The values of K1 and β2 were reported in Table 2. As
seen in Table 2, the value of β2 for Pyr is significantly
higher than the binding constants for two other ligands
that it can be refer to the effect of basicity of this
ligand.
Ligand
K1
,
M–1
β2, M–2
Pipz
Pip
97.81
137.69
349.46
4.66
9.75
9.11
×
×
×
103
103
105
In addition of basicity of the ligand,
interaction of ligand with metalloporphyrin effects to
the equilibrium constant. It is clear that bonding is of
π
bonding
Pyr
π
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 1 2012

132
SAFFARI et al.
ACKNOWLEDGMENTS
(B)
15
We thank University of Sistan and Baluchestan for
financial support.
y
= 1.974x + 13.723
10
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y
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Fig. 3. A—Visible spectral changes observed upon addiꢀ
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4
2
2
0
A)/(A
–
A
)
versus ln[Pyr] to calculate
K
and
β
.
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CONCLUSIONS
Our results show that the binding processes for aliꢀ
phatic nitrogen donor ligands (L = Pipz, Pip, Pyr) to
OEPFeClO4 are similar. Also, the data available sugꢀ
gested that two steps of axial ligand (L) addition to
Fe(III) porphyrin are possible and there are two equiꢀ
librium constants (K1 and K2) for addition of these
ligands to iron(III) porphyrin, since, as we shall see,
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7, 642
the stepwise constant, K2, is in general larger than K1
.
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 57 No. 1 2012