Effect of the axial halogen ligand on the substitution reactions of chromium(III) porphyrin complex

Article abstract of DOI:10.1016/j.ica.2012.04.001

The thermodynamics and kinetics of the substitution reaction of the chromium(III) complex of 5,10,15,20-tetraphenylporphyrin, [Cr(TPP)(X)(L)] (X = F, Cl, Br), where L represents a non-charged ligand such as H₂O, pyridine, or 1-methylimidazole, was investigated. The present study aimed to elucidate the effect of the axial halogen ligand, X on the substitution reaction of the ligand trans to X. The substitution reaction of the axial pyridine ligand of [Cr(TPP)(X)(Py)] by 1-methylimidazole was studied spectrophotometrically in dichloromethane, and it was found that the reaction proceeds via a limiting dissociative mechanism and the activation enthalpy of the dissociation of pyridine is much smaller for the fluorine complex, [Cr(TPP)(F)(Py)], than for the chlorine complex, [Cr(TPP)(Cl)(Py)], i.e., ΔH^? = 57.5 ± 1.1 and 97.2 ± 1.4 kJ mol^-1, respectively. These results are discussed in terms of the trans effect of the halogen ligand on the axial ligand substitution reaction of [Cr(TPP)(X)(L)], and it is concluded that the strength of the bond shows the tendency of Cr-F > Cr-Cl > Cr-Br, leading to the strong trans effect of the F^- ligand.

Full text of DOI:10.1016/j.ica.2012.04.001

Inorganica Chimica Acta 392 (2012) 473–477
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Note
Effect of the axial halogen ligand on the substitution reactions
of chromium(III) porphyrin complex
⇑
Kikuko Okada, Atsumi Sumida, Rie Inagaki, Masahiko Inamo
Department of Chemistry, Aichi University of Education, Hirosawa 1, Igaya, Kariya 448-8542, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 May 2011
Received in revised form 29 March 2012
Accepted 1 April 2012
Available online 6 April 2012
The thermodynamics and kinetics of the substitution reaction of the chromium(III) complex of
5,10,15,20-tetraphenylporphyrin, [Cr(TPP)(X)(L)] (X = F, Cl, Br), where L represents a non-charged ligand
such as H2O, pyridine, or 1-methylimidazole, was investigated. The present study aimed to elucidate the
effect of the axial halogen ligand, X on the substitution reaction of the ligand trans to X. The substitution
reaction of the axial pyridine ligand of [Cr(TPP)(X)(Py)] by 1-methylimidazole was studied spectrophoto-
metrically in dichloromethane, and it was found that the reaction proceeds via a limiting dissociative
mechanism and the activation enthalpy of the dissociation of pyridine is much smaller for the fluorine
Keywords:
Metalloporphyrin
Chromium(III)
Substitution reaction
Trans effect
complex, [Cr(TPP)(F)(Py)], than for the chlorine complex, [Cr(TPP)(Cl)(Py)], i.e.,
D
Hà = 57.5 1.1 and
97.2 1.4 kJ molꢀ1, respectively. These results are discussed in terms of the trans effect of the halogen
ligand on the axial ligand substitution reaction of [Cr(TPP)(X)(L)], and it is concluded that the strength
of the bond shows the tendency of Cr–F > Cr–Cl > Cr–Br, leading to the strong trans effect of the Fꢀ ligand.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Besides these photochemical studies, we also reported the sub-
stituent effect of the axial ligand on the substitution reaction of the
The chemistry of metalloporphyrins has been extensively stud-
ied for many years in relation to the naturally occurring tetrapyr-
role macrocyclic compounds in various biological systems. One of
the characteristic features of metalloporphyrins caused by porphy-
rin ligation is acceleration of the axial ligand substitution reaction,
which is closely related to biological functions such as oxygen
transport and storage by heme proteins, and this labilization effect
reaches more than several orders of magnitude, especially in reac-
tions with the usually substitution – inert chromium(III) [1–13],
cobalt(III) [14–26], and rhodium(III) [27,28] ions. In order to eluci-
date the mechanism of the labilization effect of the porphyrin
ligand, axial substitution reactions of metalloporphyrins have been
extensively studied. O’Brien et al. studied the axial ligand substitu-
tion reactions of chromium(III) porhyrin complexes in a non-
coordinating organic solvent [6] and was revealed that the axial
substitution reaction proceeds via a limiting dissociative mecha-
nism that includes a coordinately unsaturated complex as a reac-
tive intermediate based on the kinetics of the reaction. It is
desirable to study the chemical properties of a reactive intermedi-
ate directly in elucidating the intrinsic reaction mechanism, and
photochemical studies using the laser photolysis technique have
been successfully applied to these reaction systems, confirming
the extremely high reactivity of the coordinately unsaturated
intermediate [29–36].
Cr(III) porphyrin complex. The effect of b and c substituents of the
pyridine ligand on the dynamics of the substitution reaction was
discussed in terms of the electronic effect caused by the substitu-
ents [12]. On the other hand, the presence of a steric strain of
the methyl group bound at the
a position of the coordinating nitro-
gen atom of imidazole was found to result in acceleration of the
dissociation reaction of the ligand, as well as a decrease in the
binding constant of the imidazole ligand to the metalloporphyrin
[13]. These findings are in accord with the increase in the bond
length between the chromium and nitrogen atoms induced by
the steric strain due to the a-methyl substituent of the imidazole
ligand. As an extension of our study, we investigated the kinetics
of the axial ligand substitution reaction of [Cr(TPP)(X)(L)] (X = F,
Cl, Br; L = H2O, pyridine, 1-methylimidazole) in the dichlorometh-
ane solution spectrophotometrically. The purpose of the present
study is to investigate the dynamics of the substitution reaction
of the non-charged axial ligand trans to the halogen atom of the
Cr(III) porphyrin complex in order to gain insight into the effect
of the trans halogen ligand on the dynamics of the axial substitu-
tion reaction.
2. Experimental
2.1. Reagents
⇑
Pyridine (Py) and 1-methylimidazole (1-MeIm) dried over solid
potassium hydroxide were distilled before use. Spectroscopic
Corresponding author. Tel./fax: +81 566 26 2636.
E-mail address: minamo@auecc.aichi-edu.ac.jp (M. Inamo).
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.04.001

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K. Okada et al. / Inorganica Chimica Acta 392 (2012) 473–477
grade dichloromethane (Nacalai Tesque, Inc.) was used without
further purification.
2.5. Kinetic measurements
UV–Vis absorption spectra were recorded on a Hitachi U-3000
spectrophotometer. Kinetics of the axial substitution reaction of
the porphyrin complex was studied by following the change of
the UV–Vis absorption spectrum with time. The concentrations of
the leaving and entering ligands were kept in large excess over that
of the chromium(III) porphyrin complex to guarantee the pseudo-
first-order conditions. The total concentration of the complex was
in the range of (0.3–1.0) ꢂ 10ꢀ5 mol kgꢀ1. The absorbance, A, was
followed after mixing the solution of the porphyrin complex and
that of the entering ligand to determine the pseudo-first-order rate
constant, kobsd, according to Eq. (1)
2.2. Preparation of [Cr(TPP)(F)(H2O)]
Aquafluoro(5,10,15,20-tetraphenyl-porphyrinato)chromium
(III), [Cr(TPP)(F)(H2O)] was prepared according to a procedure
similar to that of [Cr(TPP)(Cl)(H2O)] [29,37]. Crude [Cr(TPP)(Cl)
(H2O)] (200 mg) was dissolved in chloroform and filtered. The fil-
trate was then applied to a column of activated alumina (Merck,
neutral, 70–230 mesh) and eluted with chloroform. A slowly mov-
ing green band was collected, and the volume of the solution was
reduced to about 50 cm3 on a rotary evaporator. Diluted hydroflu-
oric acid (20%, 5 cm3) was added to the chloroform solution, and
the mixture was vigorously stirred for 30 min. The organic layer
was separated and washed with water three times. Chloroform
was removed on a rotary evaporator. The obtained solid was puri-
fied by column chromatography using silica gel (Merck, 0.040–
0.063 mm) and eluted with chloroform containing 2% methanol.
The green band was collected, and the solvent was evaporated to
dryness. The solid was recrystallized from chloroform/hexane
and dried under vacuum at room temperature for several hours.
Fifty-five milligrams of a dark violet microcrystalline solid was ob-
tained. Anal. Calc. for C46H32N4O2CrF ([Cr(TPP)(F)(H2O)]ꢁH2O: C,
73.43; H, 4.48; N, 7.78. Found: C, 73.42; H, 4.43; N, 7.79%.
A ¼ A1 ꢀ ðA1 ꢀ A0Þ expðꢀkobsdtÞ
ð1Þ
where A and A0 are the absorbances at t = 1 and 0, respectively.
1
The reproducibility of the kobsd values was better than 5%. The
temperature of the reaction solutions was controlled to within
0.1 °C using a circulating water bath with a thermostat. The
concentration of water in the dichloromethane solution, which
was determined by a Karl-Fischer titrator (CA-06, Mitsubishi Kasei,
Japan), was kept as low as possible (usually less than 3 ꢂ
10ꢀ3 mol kgꢀ1). The absorption spectra obtained were analyzed to
determine the equilibrium constant of the axial ligand substitution
reaction using the SPECFIT/32 program [38]. The unit of mol kgꢀ1 is
used in the present work instead of mol dm–3 because the value
of the concentration given on the latter unit depends on the
temperature.
2.3. Preparation of [Cr(TPP)(Br)(H2O)]
Aquabromo(5,10,15,20-tetraphenyl-porphyrinato)chromium
(III), [Cr(TPP)(Br)(H2O)] was prepared according to a method sim-
ilar to that for [Cr(TPP)(F)(H2O)] using hydrobromic acid instead of
hydrofluoric acid. A mixture of toluene and hexane was used as the
3. Results
3.1. UV–Vis absorption spectra of chromium(III) porphyrin complexes
solvent for recrystallization. Anal. Calc. for
([Cr(TPP) (Br)(H2O)]: C, 69.30; H, 3.96; N, 7.35. Found: C, 69.08;
H, 4.40; N, 6.98%.
C44H30N4OCrBr
It has been demonstrated that the Cr(III) porphyrin complex
having a Clꢀ ligand at the axial coordination site exists as [Cr(por-
phyrin)(Cl)(L)] in a non-coordinating organic solvent such as
dichloromethane and toluene, where L represents a non-charged
ligand such as H2O, pyridine, and 1-methylimidazole [6]. The
UV–Vis absorption spectra of [Cr(TPP)(X)(H2O)] (X = F, Cl, Br) in
dichloromethane are shown in Fig. S1. The chemical bond between
the Cr and Cl atoms is strong enough to not be broken by adding a
strongly coordinating ligand such as 1-methylimidazole. However,
under more vigorous conditions, such as chromatographic purifica-
tion on activated alumina, there exists some evidence for the dis-
sociation of the Clꢀ ligand from [Cr(TPP)(Cl)(H2O)]. The treatment
of the eluted green band from the column of the activated alumina
with perchloric acid could transfer the Cr(III)–TPP complex to the
perchlorate salt, which is further converted to [Cr(TPP)(Py)2]+
through the reaction with pyridine [39]. Therefore, it is possible
for the Cr–X bond (X = halogen atom) to dissociate during the reac-
tion with a strong Lewis base when the Cr–X bond is not very
strong, and this phenomenon was observed for the bromine com-
plex, [Cr(TPP)(Br)(H2O)], as described below.
As in the case of the chlorine complex, [Cr(TPP)(Cl)(H2O)], the
addition of pyridine to the dichloromethane solution of
[Cr(TPP)(F)(H2O)] causes the red shift of the absorption spectrum
of the complex as shown in Fig. 1. The reaction product is stable
in solution for at least 1 day. A similar phenomenon was observed
for the reaction with 1-methylimidazole in the same solvent. These
facts are in accord with the mechanism by which the axial H2O li-
gand is replaced by pyridine or 1-methylimidazole to produce
[Cr(TPP)(F)(L)] (L = Py, 1-MeIm). On the other hand, the spectral
change occurs via consecutive reactions after the pyridine ligand
is added to the dichloromethane solution of [Cr(TPP)(Br)(H2O)].
As shown in Fig. 2, the absorption spectrum changes rapidly
2.4. Preparation of [Cr(TPP)(Py)2](ClO4)
The pyridine adduct of the Cr(III)–TPP complex, [Cr(TPP)(Py)2]
(ClO4), was synthesized by reaction of the perchlorate salt of the
Cr(III)–TPP complex. The chloride ion in [Cr(TPP)(Cl)(H2O)] was
substituted by the perchlorate ion by the following method. Crude
[Cr(TPP)(Cl)(H2O)] (100 mg) was dissolved in 10 cm3 of chloroform
and filtered. The filtrate was then applied to a column of activated
alumina (Merck, neutral, 70–230 mesh) and eluted with chloro-
form. A green band was collected, and the volume of the solution
was reduced to about 50 cm3 on a rotary evaporator. Perchloric
acid (20%, 6 cm3) was added to the chloroform solution, and the
mixture was vigorously stirred for 20 min. The organic layer was
separated, and chloroform was removed on a rotary evaporator.
The solid was recrystallized from the mixed solvent of methanol
and water and dried under vacuum at room temperature for sev-
eral hours. Thirty-five milligrams of a dark violet microcrystalline
solid was obtained. Anal. Calc. for C46H38N4O7CrCl ([Cr(TPP)]
(ClO4)ꢁ2CH3OHꢁH2O): C, 65.29; H, 4.53; N, 6.62. Found: C, 65.09;
H, 4.79; N, 6.49%. Two methanol molecules or methanol and water
molecules may coordinate to the central chromium atom at the ax-
ial coordination sites. The perchlorate compound thus obtained
(30 mg) and pyridine (60 mg) was dissolved in 6 cm3 of chloro-
form, and toluene (12 cm3) was then added to the solution. Dark
violet crystals of [Cr(TPP)(Py)2]ClO4 were obtained by slow evapo-
ration of the chloroform–toluene solution. Anal. Calc. for
[Cr(TPP)(Py)2](ClO4): C, 70.32; H, 4.15; N, 9.11. Found: C, 70.04;
H, 4.13; N, 8.93%.

K. Okada et al. / Inorganica Chimica Acta 392 (2012) 473–477
475
the heterolysis of the latter bond should generate an energetically
unfavorable charged species in an organic solvent. The mechanism
of the two-phase reaction observed here thus can be explained by
Chart 1. A similar two-phase reaction was also observed for the
reaction of [Cr(TPP)(Br)(H2O)] with 1-methylimidazole. These find-
ings indicate that the Cr–Br bond is not so strong that coordination
of a strong donor ligand at the axial coordination site trans to the
bromine atom in [Cr(TPP)(Br)(L)] (L = Py, 1-MeIm) may weaken
the Cr–Br bond, leading to the dissociation of the bromine atom
as Brꢀ ion.
In the case of the chlorine complex, [Cr(TPP)(Cl)(H2O)], the axial
H2O ligand substitution by the entering ligand such as pyridine is
too fast to follow using a stopped-flow spectrophotometer. This
is due to the labile nature of the axial H2O ligand, i.e., the rate con-
stant for the dissociation of H2O is k
¼ 1:57 ꢂ 105 sꢀ1 at
ꢀH2
O
T = 25.0 °C [33]. Because a similar situation could be expected for
the fluorine complex, [Cr(TPP)(F)(H2O)], we studied the kinetics
of the substitution of the axial pyridine ligand by 1-methylimidaz-
ole in [Cr(TPP)(F)(Py)]. The axial ligand substitution reaction of
[Cr(TPP)(Br)(L)] is, on the other hand, complicated due to the disso-
ciation of the axial bromine atom, and the kinetics of the ligand
substitution reaction was not investigated in the bromine complex.
Fig. 1. UV–Vis absorption spectra of the dichloromethane solution of the Cr(III)–
TPP complex. [Cr(TPP)(F)(H2O)] (black), [Cr(TPP)(F)(Py)] (blue), [Cr(TPP)(F)(1-
MeIm)] (red). (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
3.2. Equilibria and kinetics of the axial substitution reaction
It is known that the substitution reaction of the axial ligand of
the Cr(III) porphyrin complex by the entering ligand occurs rela-
tively fast in non-coordinating organic solvents [6]. In the present
study, we investigated the reaction represented by Eq. (2) in
dichloromethane by means of the spectrophotometric method.
½CrðTPPÞðXÞðPyÞꢃ þ 1-MeIm
ꢀ ½CrðTPPÞðXÞð1-MeImÞꢃ þ Py ðX ¼ F; ClÞ
ð2Þ
We previously studied the substitution reaction of pyridine in
[Cr(TPP)(Cl)(Py)] [11–13], and therefore we focused on the substi-
tution reaction of [Cr(TPP)(F)(Py)] in the present study. The equi-
librium constant K for reaction 1 (Eq. (2)) was determined by a
conventional spectrophotometric method. Spectrophotometric
titration of the [Cr(TPP)(F)(Py)] solution with 1-methylimidazole
in the presence of a large excess of pyridine over the complex
shows that the initial pyridine complex with a Soret band at
443.4 nm was converted into a species with a Soret band at
440.4 nm with isosbestic points as shown in Fig. S2 as an example.
Equilibrium constants were determined by simultaneously analyz-
ing the UV–Vis absorption spectra between 300 and 800 nm as a
function of the concentrations of leaving and entering ligands
using the SPECFIT/32 program [38]. The obtained equilibrium con-
stant for reaction 1 (Eq. (2)) is (1.97 0.15) ꢂ 102 at 15.0 °C,
(1.87 0.11) ꢂ 102 at 25.0 °C, and (1.57 0.12) ꢂ 102 at 35.0 °C.
Fig. 2. UV–Vis absorption spectra of the dichloromethane solution of the bromine
complex of Cr(III)–TPP. [Cr(TPP)(Br)(H2O)] (black), the spectrum measured imme-
diately after adding pyridine (blue, [Py] = 4.0 ꢂ 10ꢀ2 mol kgꢀ1), and the spectrum
measured at 120 min after adding pyridine (red). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version
of this article.)
The thermodynamic parameters
Hoff plot of lnK versus Tꢀ1 (Kꢀ1) were
and
S0 = 15.0 5.3 J Kꢀ1 molꢀ1
D D
H0 and S0 determined by van’t
D
H0 = ꢀ8.4 1.6 kJ molꢀ1
D
.
Rate constants of the substitution reaction of pyridine on
[Cr(TPP)(F)(Py)] by 1-methylimidazole were determined using a
stopped-flow spectrophotometer. The progress of the ligand sub-
stitution reaction was followed by monitoring the absorbance
change after mixing the solution of 1-methylimidazole with that
of [Cr(TPP)(F)(Py)], which was prepared by dissolving [Cr(TPP)(F)
(H2O)] into the dichloromethane solution of pyridine. The rate con-
stant was measured under various conditions where 1-methylim-
idazole and pyridine were present in large excess over the
porphyrin complex. The reaction was first order with respect to
the porphyrin complex. As shown in Fig. S3, the pseudo-first-order
rate constant, kobsd, increased with an increase in the concentration
of 1-methylimidazole and then leveled off at higher concentrations
immediately after adding pyridine, followed by a slow spectral
shift that occurs over several hours. The initial spectral change
resembles that observed for the reaction of pyridine with chlorine
and fluorine complexes, while the succeeding spectral change to
the final product was observed only for the bromine complex.
The spectrum of the final product was identical with that of
[Cr(TPP)(Py)2]+, which was prepared independently by the reaction
of the perchlorate complex of Cr(III)–TPP with pyridine. It is prob-
able that the bond between the chromium atom and the axial H2O
ligand in [Cr(TPP)(Br)(H2O)] dissociates more easily in dichloro-
methane than the bond between the Cr and Br atoms because

476
K. Okada et al. / Inorganica Chimica Acta 392 (2012) 473–477
Chart 1. Reaction of [Cr(TPP)(Br)(H2O)] with Py.
Table 1
Kinetic parametersa for the pyridine substitution reaction of [Cr(TPP)(X)(Py)] (X = F,
Cl) by 1-methylimidazole at T = 25.0 °C.
[Cr(TPP)(F)(Py)]
[Cr(TPP)(Cl)(Py)]b
k1 (sꢀ1
)
(7.24 0.06) ꢂ 10ꢀ1
57.5 1.1
1.66 0.05
97.2 1.4
85.1 4.5
1.68 0.04
2.3 1.2
12.0 3.9
4.13 ꢂ 10ꢀ3c
109.8 1.3
77.7 4.3
à
D
D
H1
S1
à
ꢀ54.3 3.5
1.49 0.03
ꢀ2.9 2.3
k2/k3
à
à
D
D
H2
S2
ꢀ
DH3
à
à
ꢀ
D
)
S3
ꢀ6.3 7.6
k4 (sꢀ1
2.6 ꢂ 10–3c
68.9 3.0
à
D
D
H4
à
S4
ꢀ63.0 9.9
a
b
c
D
H (kJ molꢀ1) and
Reference [13].
Calculated by using values of k1, k2/k3, and K.
D
S (J molꢀ1 Kꢀ1).
The obtained rate constants are k1 = (3.46 0.05) ꢂ 10ꢀ1 sꢀ1 at
15.0 °C, (7.24 0.06) ꢂ 10ꢀ1 sꢀ1 at 25.0 °C, 1.76 0.02 sꢀ1 at
35.0 °C, and k2k3ꢀ1 = 1.54 0.05 at 15.0 °C, 1.49 0.03 at 25.0 °C,
1.42 0.03 sꢀ1 at 35.0 °C. Eyring plots of k1 and k2k3
proved
ꢀ1
linear within experimental error for all reactions. The activation
Fig. 3. Dependence of the pseudo-first-order rate constant kobsd on the ratio of [1-
MeIm]/[Py] at T = 15.0 °C (A), 25.0 °C (B), and 35.0 °C (C). The concentration of the
Cr(III)–TPP complex is (0.3–1.0) ꢂ 10ꢀ5 mol kgꢀ1. Total concentrations of 1-meth-
ylimidazole and pyridine are in the range of 1 ꢂ 10ꢀ3–5 ꢂ 10ꢀ2 mol kgꢀ1. The solid
curves were calculated by using the activation parameters obtained.
parameters,
D
Hà and Sà, were determined by simultaneously fit-
D
ting the variable-temperature data to Eq. (6) and the Eyring equa-
tion. Kinetic parameters thus obtained are summarized in Table 1.
Based on the thermodynamic and kinetic parameters, the effect
of the axial halogen ligand on the chemical reactivity of the axial
ligand trans to the halogen atom will be discussed in terms of
the trans effect. It was revealed that the reactivity of the dissocia-
tion reaction of the axial pyridine ligand in [Cr(TPP)(X)(Py)] (X = F,
Cl) depends on the nature of the halogen ligand, i.e., the activation
enthalpy was determined to be 57.6 kJ molꢀ1 for X = F and
97.2 kJ molꢀ1 for X = Cl. Coordination of the fluorine atom to the
chromium atom decreased the activation enthalpy for the pyridine
dissociation reaction by ca. 40 kJ molꢀ1 as compared with that of
the chlorine complex. A similar effect was observed for the disso-
ciation of 1-methylimidazole. These findings can be ascribed to
the stronger bond between the Cr and F atoms than between the
Cr and Cl atoms, i.e., the stronger the chemical bond between the
metal and coordinating ligand, the weaker the bond trans to it.
On the other hand, the activation entropy of the dissociation of
pyridine is ꢀ54.4 J molꢀ1 Kꢀ1 for X = F, which is significantly differ-
ent from the corresponding value of 85.1 J molꢀ1 Kꢀ1 for X = Cl. Be-
cause the positive value of the activation entropy would be
expected for the dissociation of a non-charged ligand such as pyr-
idine from the parent complex, the dissociation of pyridine from
the Fꢀ complex should be accompanied with some chemical pro-
cess that is ascribed to the decrease in activation entropy. Although
in the hexacoordinate complex, [Cr(TPP)(X)(Py)], the polarization
of the bond between the chromium and halogen atoms should be
compensated by the Cr–N bond trans to the Cr–X bond, the disso-
ciation of pyridine causes the larger polarization of the Cr–X bond
in the pentacoordinate intermediate for the Fꢀ complex because
the electronegativity of the fluorine atom is much larger than that
at a constant concentration of pyridine, while the rate decreased
with an increase in the concentration of pyridine at a constant con-
centration of 1-methylimidazole. The pseudo-first-order rate con-
stant is plotted against the ratio of the concentrations of entering
and leaving ligands in Fig. 3. These features of the dependence of
the rate constant of the substitution reaction on the ligand concen-
trations can be interpreted by a dissociative mechanism shown by
Eqs. (3) and (4) [13].
k1
½CrðTPPÞðFÞðPyÞꢃ ꢀ ½CrðTPPÞðFÞꢃ þ Py
ð3Þ
ð4Þ
k2
k3
½CrðTPPÞðFÞꢃ þ 1-MeIm ꢀ ½CrðTPPÞðFÞð1-MeImÞꢃ
k4
When the steady-state approximation is applied to the pentaco-
ordinate intermediate [Cr(TPP)(F)], the pseudo-first-order rate con-
stant can be expressed as Eq. (5).
Kobsd ¼ ðk1k3½1-MeImꢃ þ k2k4½PyꢃÞðk3½1-MeImꢃ þ k2½PyꢃÞꢀ1
ð5Þ
Because the k4 value that corresponds to the intercept of the
plot in Fig. 3 is too small to be determined accurately, its value
was estimated using the relationship of K = k1k2ꢀ1k3k4ꢀ1. Values
ꢀ1
of k1 and k2k3 at each temperature were determined by fitting
the kobsd obtained at various concentrations of pyridine and 1-
methylimidazole to Eq. (6) using a least-squares fitting program.
Kobsd ¼ k1½1-MeImꢃð½1-MeImꢃ þ k2kꢀ1½PyꢃÞꢀ1
ð6Þ
3

K. Okada et al. / Inorganica Chimica Acta 392 (2012) 473–477
477
of the chromium atom. Such enhanced polarization for the Fꢀ com-
Appendix A. Supplementary material
plex during the pyridine dissociation reaction should cause the
structure-making effect on the surrounding solvent and/or the
association of the water molecules that are included in the
dichloromethane solution at the concentration of less than
3 ꢂ 10ꢀ3 mol kgꢀ1 with the fluorine ligand of the complex through
a hydrogen bond, leading to the negative value of the activation en-
tropy. The strength of the bond between the chromium and halo-
gen atoms showed the tendency of Cr–F > Cr–Cl > Cr–Br judging
from the findings that the activation enthalpy for the dissociation
of the axial pyridine and 1-methylimidazole ligands trans to the
F or Cl atom strongly depends on the halogen atom, as well as
the two-step reaction observed for the reaction of [Cr(TPP)(Br)
(H2O)] with pyridine, where the cleavage of the bond between
the Cr and Br atoms was induced by the introduction of pyridine
into the solution. Since the chromium(III) ion is a hard Lewis acid,
the tendency of the bond strength between Cr and X (X = F, Cl, and
Br) is reasonable.
The trans effect is a kinetic phenomenon and depends on fea-
tures of both transition and ground states, i.e., the stabilization of
the transition state and the weakening of the bond between the
metal and the leaving group due to the remaining ligand trans to
the leaving group in the ground state, respectively. The trans effect
was originally discussed with regard to square planar complexes
such as those of Pt(II), where the ligand substitution reactions oc-
cur via an associative mechanism [40]. In such a case, the labiliza-
tion of the leaving ligand may not necessarily be accompanied by
the weakening of the bond between the metal ion and the leaving
group [41]. On the other hand, in the case of the dissociatively acti-
vated ligand substitution reaction, the strengthening of the chem-
ical bond between the metal atom and the ligand trans to the
leaving group causes both the weakening of the bond to the leaving
group and the stabilization of the transition state of the reaction,
and a close relationship between these two factors can be expected
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2012.04.001.
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tion is enhanced with increasing
r-electron donating ability of
the ligand trans to the leaving methanol ligand in the order of p-
ꢀ
CH3C6H4SO3ꢀ < (CH3)2PO < SO32ꢀ < C6H5ꢀ < CH3
. In such cases
the trans effect is usually observed in the rate constant of the sub-
stitution reaction. On the other hand, the rate constant of the dis-
sociation reaction of pyridine in [Cr(TPP)(X)(Py)] (X = F, Cl) shows
similar values of k1 = 1.66 sꢀ1 for X = Cl and k1 = 0.724 sꢀ1 for
X = F (T = 25.0 °C). The coincidence of these k1 values for both reac-
tion systems may be by chance, and a large difference in the acti-
vation enthalpy of the k1 value for X = Cl and F as mentioned above
(ꢀ39.6 kJ molꢀ1) is compensated by the difference in the activation
entropy (ꢀ139.5 J molꢀ1 Kꢀ1), leading to a small difference in the
activation energy of 2.0 kJ molꢀ1 at T = 25.0 °C.