Sitting-atop complex formation of 2,3,7,8,12,13,17,18-octaethylporphyrin with copper(II) ion in acetonitrile

Article abstract of DOI:10.1016/S0020-1693(02)01044-7

The reaction of 2,3,7,8,12,13,17,18-octaethylporphyrin (H₂OEP) with copper(II) triflate and copper(II) perchlorate in acetonitrile was studied using spectrophotometry. The reaction product is the so-called sitting-atop complex where two pyrrolenine nitrogen atoms of the porphyrin coordinate to the incoming metal ion and two protons on the pyrrole nitrogen atoms still remain. The composition of the sitting-atop complex was determined by the mole ratio method, and it was found that the H₂OEP molecule binds two copper(II) ions in the product. The mechanism of the reaction was confirmed to be a series of second-order reactions with the first and second step of the reactions being the outer sphere complex formation between the H₂OEP molecule and copper(II) ion and the rate determining sitting-atop complex formation reaction, respectively, based on the mole ratio method. The reaction is relatively fast, and the second-order rate constants for the reaction of H₂OEP with copper(II) ion was determined to be k = (3.2±0.3) × 10⁶ M^-1 s^-1 (T = 25.0°C) for the copper(II) triflate and k = (3.0±0.2) × 10⁶ M^-1 s^-1 (T = 25.0°C) for the copper(II) perchlorate under the second-order conditions. The pK_a values of the mono- and diprotonated forms of the conjugate acid of several porphyrins including H₂OEP were determined by spectrophotometric titration in acetonitrile. The higher reactivity of H₂OEP toward copper(II) ion as compared with other porphyrins such as 5,10,15,20-tetraphenylporphyrin was attributed to its higher basicity.

Full text of DOI:10.1016/S0020-1693(02)01044-7

Inorganica Chimica Acta 340 (2002) 87ꢃ96
/
www.elsevier.com/locate/ica
Sitting-atop complex formation of 2,3,7,8,12,13,17,18-
octaethylporphyrin with copper(II) ion in acetonitrile
a,
Masahiko Inamo *, Tomoko Kohagura , Ivari Kaljurand , Ivo Leito
a
b
b
a
Department of Chemistry, Aichi University of Education, Igaya, Kariya 448-8542, Japan
b
Institute of Chemical Physics, University of Tartu, Jakobi 2, Tartu 51014, Estonia
Received 23 January 2002; accepted 3 April 2002
Abstract
The reaction of 2,3,7,8,12,13,17,18-octaethylporphyrin (H OEP) with copper(II) triflate and copper(II) perchlorate in acetonitrile
2
was studied using spectrophotometry. The reaction product is the so-called sitting-atop complex where two pyrrolenine nitrogen
atoms of the porphyrin coordinate to the incoming metal ion and two protons on the pyrrole nitrogen atoms still remain. The
composition of the sitting-atop complex was determined by the mole ratio method, and it was found that the H OEP molecule binds
2
two copper(II) ions in the product. The mechanism of the reaction was confirmed to be a series of second-order reactions with the
2
first and second step of the reactions being the outer sphere complex formation between the H OEP molecule and copper(II) ion and
the rate determining sitting-atop complex formation reaction, respectively, based on the mole ratio method. The reaction is relatively
6
fast, and the second-order rate constants for the reaction of H OEP with copper(II) ion was determined to be kꢀ
/
(3.29
25.0 8C) for the copper(II) perchlorate
values of the mono- and diprotonated forms of the conjugate acid of several porphyrins
including H OEP were determined by spectrophotometric titration in acetonitrile. The higher reactivity of H OEP toward
/
0.3)ꢁ/10
2
ꢂ1 ꢂ1
6
ꢂ1 ꢂ1
M
s
(Tꢀ
/
25.0 8C) for the copper(II) triflate and kꢀ
/
(3.09
/
0.2)ꢁ/10 M
s
(Tꢀ
/
under the second-order conditions. The pK
a
2
2
copper(II) ion as compared with other porphyrins such as 5,10,15,20-tetraphenylporphyrin was attributed to its higher basicity.
2002 Elsevier Science B.V. All rights reserved.
#
a
Keywords: Porphyrin metallation; Copper(II) complexes; Reaction intermediate; Sitting-atop complexes; pK of porphyrin in acetonitrile
1
. Introduction
consists of the coordination of pyrrole nitrogen atoms to
metal ion and the release of two pyrrole protons.
Concerning the pyrrole protons, a typical reaction
intermediate was proposed by Fleischer and Wang in
Metalloporphyrins and related compounds play im-
portant roles in biological systems. One of the most
significant processes is the metalloporphyrin formation
reaction with special reference to the heme biosynthesis
1960 [7]. They studied the reaction of protoporphyrin IX
derivatives with several metal ions in chloroform and
observed the formation of the reaction intermediate.
This intermediate is called a sitting-atop complex, in
which two pyrrolenine nitrogen atoms of the porphyrin
coordinate to the metal ion and two pyrrole protons still
remain. Several sitting-atop complexes of naturally
occurring and synthetic porphyrins including such ions
as platinum(II), copper(II), and rhodium(I) were then
[
1ꢃ6]. The terminal step in the heme biosynthesis, the
/
insertion of ferrous iron into protoporphyrin IX to form
protoheme, is catalyzed by the enzyme ferrochelatase.
Although the crystal structures of the ferrochelatases
along with years of biophysical and kinetic studies have
led to a better understanding of the catalytic mechanism
of the ferrochelatase, the mechanism of the complex
formation reaction of porphyrins has not been fully
established not only in biological systems but in solu-
tion. The overall metalloporphyrin formation reaction
reported [8ꢃ10]. Recently, more clear evidence for the
/
sitting-atop complex was spectrophotometrically ob-
tained during the reaction of 5,10,15,20-tetraphenylpor-
phyrin (H TPP) with copper(II) ion in acetonitrile [11].
2
The Brønsted basicity of acetonitrile is very weak
*
E-mail address: minamo@auecc.aichi-edu.ac.jp (M. Inamo).
Corresponding author. Tel./fax: ꢄ
/
81-566-26 2636
[12,13], and, therefore, the pyrrole protons of H TPP
2
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 0 4 4 - 7

88
M. Inamo et al. / Inorganica Chimica Acta 340 (2002) 87ꢃ96
/
are not easily released even after forming the intermedi-
ate with the copper(II) ion without additional proton
acceptors. This acid-base property of the solvent is
responsible for the stability of the sitting-atop inter-
mediate. The rate of the sitting-atop complex formation
(Fluka,
98%), 2-chloropyridine and 3-chloropyridine (both
TCI, ꢀ98%) were also used without additional pur-
ification. 8-Aminoquinaldine (REAKHIM, ‘pure’) was
recrystallized from ethyl ether. 2-Methoxypyridine (Al-
ꢀ99.8%), 2,6-dimethoxypyridine (Aldrich,
/
/
reaction of H TPP with several 3d-block metal(II)
2
drich, 98%) was fractionally distilled from MgSO under
4
cations in acetonitrile has also been determined, and
the general mechanism of the reaction has been pro-
posed on the basis of the parallel correlation between
these values and the rate of the solvent exchange
reaction of the corresponding cations [14]. The kinetics
of the sitting-atop complex formation reaction have also
been investigated for other meso-substituted porphyrins,
reduced pressure. Solutions of trifluoromethanesulfonic
acid (Aldrich, 99ꢄ%) and triethylamine (Aldrich, 99%)
/
were used as the acidic and basic titrants, respectively.
Copper(II) perchlorate, Cu(ClO ) , was prepared by
4
2
treating copper(II) oxide (Wako Pure Chemicals, 99.9%)
with perchloric acid (Wako Pure Chemicals, Super
Special Grade), recrystallized from distilled water, and
vacuum dried. Copper(II) triflate, Cu(CF SO ) , was
i.e.
5,10,15,20-tetrakis(4-chlorophenyl)porphyrin
H T(4-CIP)P), and 5,10,15,20-tetramesitylporphyrin
3
3 2
(
(
prepared according to the literature procedure [11].
.2. Measurements
The UVꢃVis absorption spectra were measured using
2
H TMP), and the mechanism of the reaction was
2
discussed in terms of the electronic and steric effects of
the substituents of the porphyrin peripherals on the
reaction rate [15]. By using acetonitrile, having a weak
basicity and a coordination ability to metal ions, as a
solvent [16], it was possible to characterize the sitting-
2
/
a Hitachi U-3000 spectrophotometer. The spectral
change for the reaction of the porphyrin with the
copper(II) ion in acetonitrile was measured by a
stopped-flow apparatus equipped with a rapid-scanning
spectrophotometer (RSP-801, Unisoku, Japan). The
temperature of all the measurements was controlled
atop complex of H TPP and other meso-substituted
2
porphyrins.
In a previous paper, we also studied the sitting-atop
complex formation of 2,3,7,8,12,13,17,18-octaethylpor-
phyrin (H OEP) with copper(II) ion in acetonitrile and
2
within 90.1 8C using a circulating thermostated water
/
attempted to determine the rate constant of the reaction
[15]. However, the rate is too fast to be followed under
pseudo-first-order conditions where the copper(II) ion
bath. The water content of the solutions was determined
by Karl Fischer titration to be between 1.3 and 3 mM
with the one exception of 20 mM for the spectro-
phtometric titration study and less than 15 mM for the
kinetic study.
exists in large excess over H OEP. The higher reactivity
2
of H OEP toward the complex formation reactions may
2
be ascribed to its higher basicity and/or higher flexibility
compared with other meso-substituted porphyrins. In
the present study, we investigated the sitting-atop
2.3. Spectrophotometric titration
complex formation reaction of H OEP with copper(II)
2
The spectrophotometric titration method in a glove
box was very similar to the one used in a previous study
17]. The main difference from the previous study was
ion in acetonitrile and successfully determined the rate
constant under the second-order conditions. We also
determined the pK values of the porphyrins in aceto-
[
a
that the saturated solutions of the porphyrins were made
and these solutions were diluted for titration experi-
ments due to the poor solubility of the porphyrins in
acetonitrile. The dilution factor was from 1.1 to 5,
depending on the porphyrin and the reference com-
nitrile using spectrophotometric titration in order to
discuss the reactivity of the porphyrins towards metal
ions in terms of the basicity of the porphyrins as well as
the molecular structure and substituent effects of the
porphyrins.
pound. The concentrations of the porphyrins in the
ꢂ6
titration solutions were from 0.8 to 2ꢁ
/
10
M. The
concentrations of the other bases were usually in the
2
. Experimental
ꢂ5
10
M range and the total concentration of the bases
ꢂ4
in our experiments never exceeded 1.8ꢁ
/10
M.
2
.1. Chemicals
Acetonitrile (Romil, ꢀ/99.9%, Super Purity Solvent
2.4. Calculation methods
(
far UV), water content B
/
0.005%) was the same as used
in a previous study [17]. Acetonitrile (Wako Pure
Chemical Industries, Japan) was dried over activated 3
The basicity of a polybasic base, H Bmꢄ, in solvent S
is defined using Eq. (1) and is expressed as the
m
˚
A molecular sieves for several days and distilled under
dissociation constant, K , of the conjugate acid,
a
(
mꢄ1)ꢄ
mꢄ
nitrogen atmosphere. H OEP (Tokyo Chemical Indus-
2
Hmꢄ1
monly its negative logarithm, pKa.
B
, of the base, H B , or more com-
m
try) was used without further purification. Pyridine

M. Inamo et al. / Inorganica Chimica Acta 340 (2002) 87ꢃ
/96
89
Ka
ꢄSXH B ꢄHS
(9) and HBꢄ from Eq. (7) and substituting these into
Eq. (6) gives:
(
mꢄ1)ꢄ
mꢄ
ꢄ
Hmꢄ1
B
(1)
(2)
m
mꢄ
ꢄ
a(H B )a(HS )
a(Hmꢄ1B(mꢄ1)ꢄ
m
K ꢀ
a
[B]
C ꢂ [B] Al
l
l
B
)
A ꢀ
A ꢄ
ꢄ
(11)
HB
ꢀ
ꢁ
C
C
a(H Bmꢄ)a(HSꢄ)
m
pK ꢀꢂlog
(3)
a
a(Hmꢄ1B(mꢄ1)ꢄ
After rearrangement and substituting xꢀ
/[B]/C and
)
yꢀ1/(1ꢂ[B]/C), we get for i-th spectrum.
/
/
(
mꢄ1)ꢄ
If mꢀ
/
0, then Hmꢄ1
B
is the conjugate cationic
1, then
is the conjugate dicationic acid of
l
HB (i)
l
l
B
A
ꢄ
ꢀy A ꢂx y A
(12)
acid of the neutral base B and if mꢀ
/
ꢄ
/
i
i
i
(
B
mꢄ1)ꢄ
Hmꢄ1
On the other hand, in the solutions of high acidity, the
concentration of B approaches zero and in a similar
way, we can express the absorbance of HB :
ꢄ
cationic base HB .
To exclude the necessity for measuring the hydrogen
ion activity (see Eq. (3)), we studied the equilibrium
ꢄ
l
HB (i)
l
l
H B
nꢄ
1
mꢄ
A
ꢄ
ꢀy A ꢂx y A
2ꢄ
(13)
between two bases, H B
n
and H B
m
.
i
i
i
2
2
mꢄ
(nꢄ1)ꢄ
(mꢄ1)ꢄ
nꢄ
H B2 ꢄHnꢄ1B1
XHmꢄ1B2
ꢄH B1
(4)
If we have a set spectra for which either Eq. (12) or
Eq. (13) can be applied, we then get the spectrum of
m
n
nꢄ
1
The relative basicity, DpK , of bases H B
H B2 is expressed as:
and
ꢄ
a
n
HB using the least-squares minimization. The mini-
mized quantities are the sums S (Eq. (15)) of dispersions
mꢄ
m
2
l
l
(mꢄ1)ꢄ
(nꢄ1)ꢄ
s (Eq. (14)) of A over the range of wavelengths
HBꢄ(i)
DpK ꢀpK (Hmꢄ1B2
)ꢂpK (Hnꢄ1B1
)
a
a
a
lstart
ꢃlend. This range of wavelengths was chosen so that
/
ꢀa(H
ꢁ
(
2
mꢄ1)ꢄ
nꢄ
mꢄ1B
n
)a(H B1
)
)
it would incorporate the most characteristic absorption
bands of the three forms of base B.
ꢀ
log
(5)
a(H B m2 ꢄ)a(Hnꢄ1B1
(nꢄ1)ꢄ
m
n
The mixture of bases as well as both bases separately
were titrated with an optically transparent acid or base
and the data for the calculations was obtained from the
X
l
HB
l
HB
2
¯
A
(
ꢄ
ꢂ A
ꢄ
(i))
2
l
iꢀ1
s ꢀ
(14)
n ꢂ 1
UVꢃ
/
Vis spectra. From each titration experiment, the
20 values using
mostly the same calculation methods as previously
described [17ꢃ19]. For the complicated cases appearing
in this study, where pK (HB ) and pK (H B ) of the
l
X
end
DpK was determined as the mean of 5ꢃ
/
a
2
l
Sꢀ
s
(15)
lꢀl
start
/
ꢄ
2ꢄ
l
HB
l
is the average of the A
HB (i)
A¯
a
a
2
/
ꢄ
ꢄ
/values at wavelength
two polybasic porphyrins are too close (less than 3 pKa
units) to observe the pure spectrum of the monoproto-
nated form (HB ), a different approach was used. At
wavelength l, the absorbance of a solution (with a 1 cm
path length) containing all forms of the dibasic com-
pound B is expressed as follows:
l. The adjusted quantities are x and y . In addition, if
i i
the isosbestic points were present in the wavelength
range lstart lend, then the spectra were ‘pushed’ through
the isosbestic points by setting higher weights (usually
ꢄ
ꢃ
/
2
100 times) to the dispersions s
ꢄ
in Eq. (15). The
spectrum of HB obtained this way is used in the DpKa
calculation procedure to calculate the difference be-
lisosbestic
l
l
B
ꢄ
l
HB
ꢄ[H B2ꢄ]ol 2ꢄ
A ꢀ[B]o ꢄ[HB ]o
ꢄ
(6)
2
H B
ꢄ
2ꢄ
2
tween pK (HB ) and pK (H B ). If in Eq. (5), mꢀ
/
a
a
2
nꢀ
coefficients does not change during titration experi-
ments. If mꢀ 1 and nꢀ0, then we calculated the
activity coefficients using the DebyeꢃH u¨ ckel equation
as described in [20]. It appears that in very diluted
/
0, then we assumed that the ratio of the activity
The analytical concentration of the base expresses:
C ꢀ[B]ꢄ[HB ]ꢄ[H B2ꢄ]
ꢄ
(7)
2
/
ꢄ
/
/
/
To use the least-squares of the linear combinations
method (see [18]), the spectra of B, HBꢄ and H B are
2ꢄ
2
ꢂ4
solutions (B
accounting activity coefficients are very small (B
pK units).
/10
M), the corrections arising from the
needed. The absorbances of the solutions containing
2
only B (Eq. (8)) or H B (Eq. (10)) are experimentally
ꢄ
/
0.04
2
ꢄ
obtained and the absorbance of HB (Eq. (9)) is
calculated as described below.
a
l
B
l
B
A ꢀCo
(8)
(9)
3. Results
l
HB
l
HB
A
A
ꢄ
ꢀCo
ꢄ
l
H B
l
H B
2ꢄ ꢀCo
2ꢄ
(10)
2
2
3.1. Basicity of porphyrins
In the solutions of low acidity, the concentration of
2
H B is very low and the third member in Eqs. (6) and
ꢄ
The relative basicity measurements between ten
compounds were carried out and 26 relative acidꢃbase
2
(
7) can be ignored. Expressing the o ꢃ/s from Eqs. (8) and
/

90
M. Inamo et al. / Inorganica Chimica Acta 340 (2002) 87ꢃ96
/
Table 1
Directly measured relative basicity values for neutral and cationic bases and estimated absolute pK
a
values for their conjugate acids in acetonitrile
3
.2. Formation of the sitting-atop complex in acetonitrile
equilibrium constants were found. These values produce
the self-consistent and continuous basicity scale pre-
sented in Table 1. Multiple overlapping measurements
make the results more reliable and help to estimate their
self-consistency. The entire basicity range involves at
least two independent pathways of measurements and
the relative basicity of any two bases can be obtained by
combining at least two independent sets of measure-
ments. The reversibility of the protonation/deprotona-
tion process of all bases was checked. All equilibria were
reached in minutes and were stable.
Fig. 1 shows the spectra of H OEP and its copper(II)
2
complexes in acetonitrile. Upon the addition of cop-
per(II) ion to the porphyrin solution, the spectrum
rapidly changes to spectrum B in Fig. 1 with broader
and weaker Soret band. The characteristic broader Soret
band of the product indicates that the porphyrin ring
should be distorted from the planar structure. The
addition of nitrogen bases such as pyridine to the
obtained solution causes a spectral change to that of
the copper(II)ꢃOEP complex, [Cu(OEP)]. These find-
/
The method used in this study has the disadvantage
that only the relative basicity can be determined. To
assign the absolute pK values for the conjugate acids of
ings indicate that the intermediate species still binds the
pyrrole protons and that the added pyridine acts as an
acceptor of the pyrrole protons to accelerate the
formation of [Cu(OEP)]. Without any bases, acetonitrile
cannot easily accept these pyrrole protons due to its very
weak basicity. A similar phenomenon was previously
a
the bases, the scale has to be anchored to a reference
compound or compounds for which the pK value(s) are
a
known. The absolute pKa values of the bases were
calculated in a way similar to the previous studies
observed for the reaction of H TPP with copper(II)
2
[
17,19]. The scale was anchored to the pKa value
12.33) of pyridine [21]. The precision s of the measure-
triflate in acetonitrile [11]. The reaction intermediate
was ascribed to the so-called sitting-atop complex.
Although the basicity of H OEP is much higher than
(
ments was calculated as in [19]; n ꢀ
/
26, n ꢀ
/
14ꢂ
/
1ꢀ
/
m
c
2
1
3. For our results, sꢀ
/
0.09 pK units.
a
that of H TPP, it is reasonably understood that the
2

M. Inamo et al. / Inorganica Chimica Acta 340 (2002) 87ꢃ
/96
91
H OEPꢄCu X[H OEP ꢀ ꢀ ꢀ Cu2ꢄ]
2
ꢄ
(16)
2
2
2
ꢄ
2ꢄ
2ꢄ
[
H OEP ꢀ ꢀ ꢀ Cu ]ꢄCu X[H OEP ꢀ ꢀ ꢀ (Cu ) ] (17)
The mole ratio dependence of the absorbance A
2
2
2
shown in Fig. 2 was analyzed by the least squares fitting
program employing the following equation:
2
ꢄ
2ꢄ 2
o ꢄ o K [Cu ] ꢄ o K K [Cu
]
1
2
1
3
1
2
Aꢀ
COEP
(18)
2ꢄ
2ꢄ 2
1
ꢄ K [Cu ] ꢄ K K [Cu
]
1
1
2
where o , o , and o represent the molar absorption
1
2
3
2
ꢄ
coefficients of H OEP, [H OEPꢀ ꢀ ꢀCu ], and
2
2
2
ꢄ
[
H OEPꢀ ꢀ ꢀ(Cu ) ], respectively, and K and K2 are
2
2
1
the equilibrium constants of reaction 16 and 17,
respectively. The determined value of o C is
0.021 (393 nm),
which can be considered to be equal to that of o C
2
OEP
Fig. 1. UVꢃ
plexes in acetonitrile. H
OEP with copper(II) ion (B), [Cu(OEP)] (C). Wavelengths of the
/
Vis absorption spectra of H
2
OEP and copper(II) com-
0.0719
/
0.010 (340 nm) and 0.4869
/
2
OEP (A), the product after the reaction of
1
OEP
H
2
(
0.0749
/
0.006 at 340 nm and 0.48190.006 at 393 nm)
/
absorption maximum at the Soret band are 393, 348, and 394 nm for
A, B, and C, respectively.
within experimental error. These findings indicate that
the introduction of the first equivalent amount of the
copper(II) ion to the H OEP solution is spectrophoto-
2
reaction intermediate was observed for both porphyrins
in acetonitrile.
The mole ratio method was employed in order to
clarify the composition of the sitting-atop complex. The
spectra of solutions containing a constant amount of
metrically silent. The equilibrium constants are calcu-
ꢂ1
lated to be log(K (M ))ꢀ
/
9.290.4 and log(K2
/
1
ꢂ1
(M
triflate system based on the assumption that o is equal
))ꢀ
/
7.89
/
0.4 (Tꢀ25.0 8C) for the copper(II)
/
2
to o . The solid lines in Fig. 2 are calculated curves based
1
H OEP and different concentrations of copper(II) ion
2
on the obtained equilibrium constants and explain the
results very well. For copper(II) perchlorate, similar
were measured using the rapid scanning spectrophot-
ometer. As shown in Fig. 2, the plot of the absorbance
of the solution at 340 and 393 nm as a function of the
ratio of the total concentrations of copper(II) ion and
results were obtained, and the equilibrium constants
ꢂ1
were determined to be log(K (M ))ꢀ
/
9.490.3 and
/
1
ꢂ1
log(K (M ))ꢀ
/
8.29
/
0.3 (Tꢀ25.0 8C). These findings
/
2
H OEP clearly demonstrates that the introduction of the
2
indicate that the counter anion of copper(II) has little
effect on the sitting-atop complex formation reaction of
first equivalent amount of copper(II) ion to the H OEP
2
solution does not cause any spectral change and that the
produced sitting-atop complex contains two copper(II)
H OEP.
2
ions based on the inflection point at CCu/COEP
ꢀ2.
/
3.3. Kinetics of the sitting-atop complex formation
reaction
These findings indicates the two-step reaction for the
sitting-atop complex formation reaction:
The reaction of H OEP with the copper(II) ion is too
2
fast to be followed by a stopped-flow apparatus under
the pseudo-first-order conditions where the copper(II)
ion exists in large excess over the porphyrin in acetoni-
trile. The reaction rate was, therefore, measured under
second-order conditions. The reaction was spectropho-
tometrically followed by monitoring the absorbance
change associated with the formation of the sitting-
atop complex under the conditions where the reaction
quantitatively proceeds. Taking into account the results
of the mole ratio method, several reaction mechanisms
should be considered to analyze the reaction curve. The
first candidate is the series of second-order reactions in
which the first step (reaction 16) is the rate-determining
step. In this case, the rate equation can be expressed by:
ꢂ1
ꢂ1
ꢂ1
Fig. 2. Dependence of the absorbance on the ratio of the total
concentrations of copper(II) ion and H
(bꢂ2a) lnfa(bꢂ2x)b (aꢂx) gꢀkt (19)
where a and b represent the initial concentrations of
2
OEP for the reaction of
H
2
OEP and copper(II) triflate at Tꢀ25.0 8C. Total concentration of
/
ꢂ6
the porphyrin is 2.94ꢁ
/10
M.
H OEP and copper(II) ion, respectively, x is the
2

92
M. Inamo et al. / Inorganica Chimica Acta 340 (2002) 87ꢃ
/96
ꢂ1
ꢂ1
ꢂ1
2
(2aꢂb) f(bꢂ2x) ꢂb
g
ꢂ2
ꢂ1
ꢂ1
ꢄ2(2aꢂb) lnfa(bꢂ2x)b (aꢂx) gꢀkt
In Fig. 3 are shown plots of the left-hand sides of Eqs.
(21)
(19), (20) and (21) as a function of time t. If the equation
was correct, the plot in Fig. 3 should be a straight line. It
is clear that the reaction mechanisms based on Eqs. (19)
and (21) are not the case for the present reaction. On the
other hand, a fairly linear plot was obtained for the
reaction mechanism using Eq. (20). The second-order
rate constant thus obtained under various conditions is
listed in Table 2. The average value is kꢀ
/
(3.29
/
0.3)ꢁ
/
6
0 M
ꢂ1 ꢂ1
1
s
(Tꢀ
/
25.0 8C) for copper(II) triflate and
6
10 M
ꢂ1 ꢂ1
kꢀ(3.09
/
/
0.2)ꢁ
/
s
(Tꢀ25.0 8C) for cop-
/
Fig. 3. Kinetic plots for the reaction of H
at Tꢀ25.0 8C. Each plot is based on Eq. (19) (A), Eq. (20) (B), and
Eq. (21) (C). CCu
2
OEP and copper(II) triflate
per(II) perchlorate. The activation enthalpy and activa-
tion entropy were determined from the temperature
dependence of the second-order rate constant in the
temperature range between 15.0 and 35.0 8C for the
/
ꢂ6
ꢂ6
ꢀ/6.83ꢁ/10
M, COEP
ꢀ/2.94ꢁ/10
M.
%
ꢂ1
concentration of the produced sitting-atop complex, and
k is the second-order rate constant of reaction 16. The
second possible mechanism is the series of second-order
reactions in which the second step (reaction 17) is the
rate-determining step, i.e. the mechanism including the
reaction of copper(II) triflate: DH ꢀ
/
589
/
2 kJ mol
,
mol . Almost the same rate
%
ꢂ1
ꢂ1
DS ꢀ
/
7496 J K
/
constant was obtained for both copper(II) compounds
indicating that the counter anions (triflate and perchlo-
rate ions) of the copper(II) salts do not affect the rate of
the sitting-atop complex formation reaction. Since both
anions have a weak coordination ability toward metal
ions, they at most exist as a solvent-separated ion pair
with the copper(II) ion and do not directly coordinate to
the copper(II) ion even in acetonitrile. These findings are
consistent with the results for the solution chemistry of
the copper(II) ion in acetonitrile, where the copper(II)
fast pre-association equilibrium between H OEP and
2
copper(II) ion. In this case the rate equation can be
expressed by:
ꢂ1
ꢂ1
ꢂ1
(
bꢂa) lnfa(bꢂx)b (aꢂx) gꢀkt
(20)
where k is the second-order rate constant of reaction 17.
2
2
ꢄ
ꢄ
ion exists as a hexasolvated ion [Cu(AN)6]
[14,22].
Because the intermediate species [H OEPꢀ ꢀ ꢀCu ] is
2
produced immediately after mixing the solution of
H OEP and copper(II) ion, the value of b is set at
2
CCu
Ã
/
COEP. Another reaction mechanism is the third-
4. Discussion
order reaction with two reactants, i.e. ꢂd[H OEP]/dtꢀ
/
/
2
2
k[Cu ] [H OEP]. In this case the rate equation can be
ꢄ 2
2
The kinetics of the metalloporphyrin formation reac-
tion has been extensively studied for many kinds of
porphyrins and metal ions in various solvents in order to
expressed as:
Table 2
Second-order rate constant of the sitting-atop complex formation of
clarify the reaction mechanism [23ꢃ26]. Usually the
/
reaction of the free-base porphyrin (H por) with a metal
ion occurs via a single step reaction to form the
metalloporphyrin as expressed in Eq. (22), and two
2
2
H OEP in acetonitrile at Tꢀ25.0 8C
6
ꢂ1
sꢂ1
a
Copper(II)
compound
C
OEP (10ꢂ6 M)
C
Cu (10ꢂ6 M) k (10 M
)
pyrrole protons in H por will be released after the
2
complexation.
Copper(II) 2.94
triflate
6.83
3.6
Mnꢄ ꢄH porX[M(por)]
(nꢂ2)ꢄ
ꢄ
ꢄ2H
(22)
2
2
2
2
2
2
2
2
.94
.34
.94
.34
.94
.34
.39
8.14
8.14
9.54
9.58
3.5
3.1
3.5
3.1
3.0
3.0
3.0
3.1
Metalloporphyrin formation reactions are generally
interpreted to proceed via a dissociative-interchange
mechanism. The metal ion and porphyrin first form an
outer sphere complex and the porphyrin core deforms to
present the reactive pyrrolenine nitrogen atoms having
lone-paired electrons to the incoming metal ion. The
metal ion then dissociates a solvent molecule to form a
bond between the nitrogen atom of the porphyrin and
metal ion. A sitting-atop (or activated) complex is thus
formed. In the solvents with a higher Brønsted basicity,
10.8
11.2
12.9
copper(II)
perchlorate
2.81
7.32
2
2
2
.81
.81
.81
8.68
10.1
11.6
3.1
3.0
2.9

M. Inamo et al. / Inorganica Chimica Acta 340 (2002) 87ꢃ
/96
93
the following deprotonation should occur from the
activated complex. The trend in the rate constant
variation for a series of divalent metal ions including
the 3d-block metal ions for the solvent exchange of
the planar porphyrin core, and the second to incorpo-
rate from the opposite side of the porphyrin [30]. This
proposal was supported by many kinetic studies.
Tanaka et al. investigated the kinetics of the reaction
DMF and for the metallation of H TPP and its N-
2
of H TPP with copper(II), zinc(II), and cadmium(II)
2
methylated derivative in DMF is very similar, although
the metallation rate is slower than the solvent exchange
ions in DMF, and evidence for the involvement of the
two metal ions in the reaction was obtained from the
kinetics of the reaction [27]. The metalloporphyrin
formation reaction is first-order with respect to the
porphyrin, and the pseudo-first-order rate constant was
obtained as:
rate by several orders of magnitude [27ꢃ29]. Therefore,
/
the rate-determining step for the metallation of the
porphyrin is thought to be the dissociation of coordi-
nated solvent molecules from the metal ion in the outer-
sphere association complex. The overall rate constant
kobsd ꢀ(k K[M ]ꢄk K[M ] )(1ꢄK[M ])ꢂ1
2
ꢄ
2ꢄ 2
2ꢄ
(23)
1
2
for the metalloporphyrin formation, k , is given by the
f
product of the equilibrium constant for the outer-sphere
complex formation (KOS), the equilibrium constant for
On the basis of the rate law, the following reaction
mechanism was proposed.
the porphyrin ring deformation (K ), and the rate
D
K
H TPPꢄCu X[H TPP ꢀ ꢀ ꢀ Cu2ꢄ]
2
ꢄ
(24)
(25)
constant for the dissociation of the coordinating solvent
2
2
molecules in the inner-sphere of the metal ions (k): k ꢀ
/
k1
f
2ꢄ
ꢄ
[
H TPP ꢀ ꢀ ꢀ Cu ] 0[Cu(TPP)]ꢄ2H
2
KOSK k. The slow metalation rate of the porphyrin can
be explained by the small value of KD.
D
k
2
ꢄ
2ꢄ
2
2ꢄ
[H TPP ꢀ ꢀ ꢀ Cu ]ꢄCu 0[Cu(TPP)]ꢄCu
2
The transient species, which includes both the incom-
ing metal ion and two pyrrole protons, can exist as an
intermediate of the complex formation reaction of the
porphyrin, and this intermediate is assumed to be the
sitting-atop complex first proposed by Fleischer and
Wang [7]. However, in solvents such as water and DMF
where the pyrrole protons of the intermediate may be
easily released, the sitting-atop intermediate can hardly
be observed and the metalloporphyrin is simply pro-
duced as the product during the metallation reaction.
On the other hand, the existence of the sitting-atop
complexes was evidenced for the protoporphyrin IX
ꢄ
ꢄ2H
(26)
The k path involving two metal ions is consistent
2
with Hambright’s proposal. Similar kinetic features
were observed for the other metalloporphyrin formation
reaction [31,32]. On the other hand, it was demonstrated
that the rate of the metalloporphyrin formation reaction
of the one-face-hindered fenced porphyrin with cop-
per(II), zinc(II), and cadmium(II) ions is about the same
as for the sterically unhindered porphyrin in DMF or
pyridine [33], and these findings are considered to be the
evidence against the Hambright mechanism. Neverthe-
less, the Hambright mechanism should apply to the
complex formation reaction of many sterically unhin-
dered porphyrins.
derivatives, H TPP, and other porphyrins including
2
H OEP in the solvents such as chloroform and aceto-
2
nitrile [7ꢃ11,14,15]. It is obvious that the low basicity of
/
these solvents is ascribed to the stability of the sitting-
atop intermediate. Although the sitting-atop complex
The two-step reactions of the sitting-atop complex
formation of H OEP observed in the present study
2
nꢄ
was claimed to have
a
1:1 for many cases, the results of the
composition of
M
:
corresponded to the mechanism in which two metal ions
interact with the porphyrin molecule from the opposite
side of the porphyrin core as proposed by Hambright.
This type of sitting-atop complex should be regarded as
the intermediate of the metalloporphyrin formation
reaction in the solvents such as DMF and water. What
is significant is the extremely large K value for the
formation of the outer sphere association complex of
porphyrinꢀ
/
present study clearly suggest that two copper(II) ions
are included in the sitting-atop complex of H OEP as
2
evidenced by the mole ratio method. The proposed
structure of this sitting-atop complex is shown in Chart
1
.
Concerning the stoichiometry and mechanism of the
metalloporphyrin formation reaction, Hambright has
proposed that two metal ions are always required for the
metal ion incorporation into porphyrins, one to distort
H
2
OEP with copper(II) ion in acetonitrile. The corre-
sponding value of the pre-association constant K was
4
ꢂ1
reported to be 1.6ꢁ10
reaction of H TPP with copper(II) ion in DMF [27].
/
M
at 25.0 8C for the
2
Formation of the outer sphere complex is usually
postulated as the primary step for the ligand substitu-
tion reaction of the metal complexes. The formation
constant of the outer sphere complex can be theoreti-
cally estimated to be 0.3 for the reaction of the non-
charged ligand at the distance of closest approach of two
˚
species of 5 A [34,35]. Although the pre-association
Chart 1. Proposed structure of the sitting-atop complex.

94
M. Inamo et al. / Inorganica Chimica Acta 340 (2002) 87ꢃ96
/
2
constant for the reaction of H TPP with Cu in DMF
ꢄ
Table 1 are shown the pK values of the mono- and
a
2
is much greater than the predicted value for the outer-
sphere complex, the first step of the metalation reaction
diprotonated porphyrins. It seems reasonable to assume
that the higher the degree of delocalization of the charge
introduced by the first proton from the center of the
(
Eq. (24)) can be ascribed to the outer sphere complex
formation due to the following spectral features. Under
the conditions where the outer-sphere pre-associated
porphine moiety, the closer the second pK will be to the
a
first. This explains why H OEP has the greatest DpK of
2
a
2
ꢄ
complex [H TPPꢀ ꢀ ꢀCu ] is quantitatively formed, the
the porphyrins, because the delocalizing power of the
phenyl rings of TPP is greater than that of the ethyl
groups of OEP. Although the phenyl groups in the free
base of TPP are almost perpendicular to the porphyrin
core, this does not hold for the mono- and diprotonated
forms, for which the porphyrin core is also quite
distorted. Such a situation can be seen in the molecular
2
spectral change for the metallation reaction was re-
ported to be from the spectrum of H TPP to that of
2
[
Cu(TPP)], and no spectral evidence was obtained for
the formation of the outer-sphere complex [27]. This fact
indicates that the kinetically detected intermediate
2
ꢄ
[
that of H TPP. A similar spectral feature was observed
H TPPꢀ ꢀ ꢀCu ] has the same absorption spectrum as
2
2
structure of the porphyrin diacid species [41ꢃ48]. The
/
for the sitting-atop complex formation reaction of
H OEP in the present study. The pre-association con-
crystal structure of the porphyrin diacid species,
[H OEP](ClO ) , [H TPP](ClO ) , [H TMP](ClO ) , in-
2
4
4 2
4
4 2
4
4 2
stant of H OEP with copper(II) ion in acetonitrile is
2
dicates that the distortion of the porphyrin core from
planarity should result from the severe transannular
crowding between the four (or three for the monoacid
form) hydrogen atoms of the pyrrole nitrogen atoms
much greater than that of the H TPP case in DMF.
2
Taking into account the low solubility of H OEP in
2
ꢂ5
acetonitrile being on the order of 10 M, the large pre-
association constant can be rationalized by the stabiliza-
[
48]. The crystal structures of the D -symmetry distor-
2d
tion of the H OEP molecule by forming the outer sphere
2
tion of the porphyrin core depend on the nature of the
peripheral substituents. The phenyl and 4-Cl-phenyl
substituents can rotate to some extent under the D -
complex with copper(II) ion in the solvent.
In the present study, we determined the second-order
2
d
rate constant for the reaction of H OEP with the
2
symmetry distortion, so that they will be more coplanar
with the porphyrin core. The mean porphyrin-phenyl
group dihedral angles in [H TPP](ClO ) are 27(2)8.
copper(II) ion. The rate of the sitting-atop complex
formation of H OEP is faster than that of H TPP (kꢀ
/
2
2
4
4 2
5
10 M
ꢂ1 ꢂ1
%
3
.6ꢁ
/
s
) [11]. The activation enthalpy, DH ,
for the H OEP and H TPP is almost the same, i.e. 5892
This enables the resonance interaction that is very
efficient in charge delocalization. As a consequence,
/
2
2
ꢂ1
ꢂ1
kJ mol
for the former and 569
/5 kJ mol
for the
the DpK of TPP and T(4-ClP)P is far less than the OEP.
a
latter and the activation entropy for the H OEP (749
/
6 J
2
TMP is in a different situation from TPP, because it has
two o-methyl substituents on each aryl substituent. The
eight o-methyl groups of [H TMP](ClO ) are posi-
ꢂ1
ꢂ1
ꢂ1
K
mol ) is greater than that for H TPP (469
/
19 J
2
K
molꢂ1) [11]. The activation enthalpy for these
4
4 2
reactions is much greater than the usual ligand substitu-
tion and solvent exchange reactions of the copper(II)
tioned above and below the plane of the porphyrin ring,
approximately over the meso-carbons. Such a steric
hindrance significantly hinders the rotation of the aryl
%
ion. For example, DH for the reaction of the copper(II)
ion with ammonia in aqueous solution is 18.8 kJ mol
ꢂ1
groups about the CmÃ
Cp bond. The porphyrin-aryl
/
%
ꢂ1
[
for water [37], 24.3 kJ mol for DMF [38], and 17.2 kJ
mol
36]. The DH for the solvent exchange is 11.5 kJ mol
ꢂ1
ꢂ1
for methanol [39,40]. The larger activation
enthalpy for the reaction of the present sitting-atop
complex formation can be attributed to the energetic
requirement due to the distortion of the porphyrin core
prior to the rate-determining step of the binding
between the metal ion and porphyrin.
The difference in the reactivity of the porphyrins
towards a metal ion has been often discussed in terms of
the acidꢃbase properties of the porphyrin as well as the
/
structural properties of the molecule, especially con-
cerning the planarity of the porphyrin core. Before
discussing the relationship between the reactivity of the
porphyrin with the copper(II) ion and the porphyrin
Fig. 4. Relationship between the second-order rate constant k of the
reaction of the sitting-atop complex formation with copper(II) triflate
and the basicity of the porphyrin for H
2
OEP (A), H
2 2
TPP (B), H T(4-
basicity, we will first consider the acidꢃ
/
base properties
of H OEP and some meso-substituted porphyrins. The
ClP)P (C), and H TMP (D). The pK
2
a
value represents that of the
2
conjugate acid of the corresponding free base porphyrin in acetonitrile.
The values of the rate constants for the porphyrins other than H OEP
are from [15].
pK value of the conjugate acid of the porphyrins was
a
2
spectrophotometrically determined in acetonitrile. In

M. Inamo et al. / Inorganica Chimica Acta 340 (2002) 87ꢃ
/96
95
group dihedral angles of [H TMP](ClO ) are less acute
4
skeleton due to the steric interaction between the methyl
groups of the aryl rings and the hydrogen atoms on the
pyrrole rings. Due to this steric hindrance, the electro-
4 2
than those of [H TPP](ClO ) , averaging 63(13) and
4
4 2
7
Consequently, [H TMP](ClO ) show substantially
5(15)8 in the two forms of molecules in the crystal.
static approach between the copper(II) ion and H TMP
2
4
4 2
smaller core distortions than [H TPP](ClO ) , though
4
is also inhibited and the more distant rupture in the
bonding between the copper(II) ion and the dissociating
acetonitrile molecule is also required since it is hard for
the pyrrolenine nitrogen to approach closer to the
copper(II) ion in the transition state. The correlation
between the porphyrin reactivity and the acid-base
properties of the porphyrins shown in Fig. 4 can thus
be interpreted by the electronic and steric factors of the
porphyrins.
4 2
both
[
are
considerably
more
saddled
than
H OEP](ClO ) . These steric effects cause a smaller
4 4 2
charge delocalization which is given by the resonance
effect than in the case of TPP. However, even without
the resonance effect, the mesityl groups are more
efficient in charge delocalization than the ethyl groups.
As a result, the DpK of TMP is intermediate between
a
OEP on one hand and TPP on the other hand.
The dependence of the second-order rate constant of
the sitting-atop complex formation reaction on the
basicity of the porphyrins is shown in Fig. 4. The rate
5. Supplementary material
constant decreases in the order of H OEPꢁ
/
H TPPꢀ
/
2
2
H T(4-ClP)Pꢁ
/
H TMP. The difference in the reactivity
Figure showing the temperature dependence of the
second-order rate constant for the sitting-atop complex
formation reaction of H OEP with copper(II) triflate in
acetonitrile (Fig. S1). The supplementary material is
available from the authors on request.
2
2
can be interpreted by the electronic and steric factors. It
has been demonstrated how the reactivity of the
porphyrins with metal ions is influenced by the por-
phyrin basicity. A linear relationship between the log k
2
(
phyrin formation reaction) and the pK of the conjugate
k is the second-order rate constant of the metallopor-
a
acid of the monoprotonated porphyrin has been demon-
strated, for example, for the reaction of the water-
soluble porphyrins with the copper(II) and zinc(II) ions
Acknowledgements
M.I. gratefully acknowledges the helpful discussion of
Professor Shigenobu Funahashi of Nagoya University,
Japan. I.L. and I.K. gratefully acknowledge the support
from grant 4376 from the Estonian Science Foundation.
[
49,50]. Also, the correlation between the porphyrin
reactivity and the reduction potential of the porphyrins
has been explored [26,51]. The faster metalloporphyrin
formation reaction was observed for the porphyrin with
a stronger basicity and a more negative reduction
potential for the metal ion incorporation reaction into
the water-insoluble porphyrins. The reduction potentials
parallel the basicity scale of the porphyrins, and thus the
porphyrin bearing the stronger basicity reacts with the
zinc(II) ion faster than the less basic porphyrins [51]. As
shown in Fig. 4, a similar correlation was observed in
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