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