Electron transfer reaction of porphyrin and porphycene complexes of Cu(II) and Zn(II) in acetonitrile

Article abstract of DOI:10.1039/b812575a

The outer-sphere one-electron oxidation reaction of the Cu(ii) and Zn(ii) complexes of nonplanar 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20- tetraphenylporphyrin and planar porphycenes as well as those of 2,3,7,8,12,13,17,18-octaethylporphyrin and 5,10,15,2

Full text of DOI:10.1039/b812575a

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Electron transfer reaction of porphyrin and porphycene complexes of Cu(II)
and Zn(II) in acetonitrile†
Kaori Aoki,a Toshimitsu Goshima,a Yohei Kozuka,a Yukiko Kawamori,a Noboru Ono,b Yoshio Hisaeda,c
Hideo D. Takagid and Masahiko Inamo*a
Received 22nd July 2008, Accepted 24th September 2008
First published as an Advance Article on the web 13th November 2008
DOI: 10.1039/b812575a
The outer-sphere one-electron oxidation reaction of the Cu(II) and Zn(II) complexes of nonplanar
2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin and planar porphycenes as well as those
of 2,3,7,8,12,13,17,18-octaethylporphyrin and 5,10,15,20-tetraphenylporphyrin by Cu2+ giving
corresponding p-cation radicals was investigated spectrophotometrically in acetonitrile. The electron
self-exchange rate constants between the parent porphyrin and porphycene complexes and their
p-cation radicals were determined using the Marcus cross relation for the electron transfer reaction. The
obtained rate constants are in the order of 109 to 1011 M-1 s-1 for the planar porphyrin and porphycene
complexes and 104 to 106 M-1 s-1 for the nonplanar OETPP complexes at T = 25.0 ◦C. The relatively
slow self-exchange reaction of the distorted porphyrin complexes, as compared with those for the planar
porphyrin and porphycene complexes, was ascribed to the significant deformation of the complex
associated with the oxidation reaction from the parent complex to the corresponding p-cation radical.
theory,5,6 the rate constant for an electron transfer reaction can be
expressed by eqn (1)
Introduction
Porphyrins and related macrocyclic tetrapyrroles exist in many
biological systems, and the electron transfer reactions of these
ket = kelZexp[-l(1 + DG◦et/l)2/4RT]
(1)
species play essential roles there.1–4 The electron transfer reaction
of metalloporphyrins has been extensively studied in order to
clarify the factors that control the redox processes in the biological
systems, such as the electron transport system in the respiratory
chain and photosynthesis. The highly conjugated p systems of
the porphyrin molecules are suitable for efficient electron transfer
processes because the release or uptake of an electron causes
minimal structural change of these molecules. Metal-centred
electron transfer reactions are also important in the case where
a redox-active metal ion is included. In these porphyrin-type
cofactors, their reactivity to an electron-transfer depends on the
nature of porphyrin ligand and central metal ion, the axial ligation
of metalloporphyrins, and non-planar distortion of the porphyrin
skeleton. The latter factor is believed to affect the reactivity of
their cofactors as a result of the structural deformation and the
axial ligand affinities.
where kel is the probability that the system passes from the
precursor to the successor state along the adiabatic potential
energy surface, Z is an effective frequency which determines the
rate of transmission along the reaction coordinate, and l is the
reorganization parameter associated with the electron transfer
reaction. The Marcus cross relation is used to estimate the
reorganization energy for the self-exchange reaction of metallo-
porphyrins from the cross reactions of metalloporphyrins with the
outer-sphere oxidizing or reducing reagents. The rate constant of
the electron self-exchange reaction of metalloporphyrins has been
determined in this manner for the metal-centred redox couples
as well as the ligand-centred reactions.4 Relatively slow electron
self-exchange reactions of metalloporphyrins were observed for
the metal-centred Fe2+/3+ and Co2+/3+ couples whose rate constants
fall in the range of 107 to 108 M-1 s-1 and 10-3 to 104 M-1 s-1,
respectively.7–12 These slow self-exchange rate can be ascribed to a
relatively large inner-sphere reorganization energy that originates
from the change in the bond length between the metal and
pyrrole nitrogen atoms. On the other hand, the porphyrin-centred
redox reactions are highly reactive because smaller inner-sphere
reorganization energies are required compared with the metal-
centred redox reactions. We have recently determined the rate
constants of the electron self-exchange reaction between the
metalloporphyrin and its p-cation radical. The rate constants
are in the range 109 to 1011 M-1 s-1 for the Cu(II) complexes of
5,10,15,20-tetraphenylporphyrin (TPP) and 2,3,7,8,12,13,17,18-
octaethylporphyrin (OEP).13 The EPR line-broadening technique
has also been applied to the Zn(II) porphyrins, and high
reactivity of this self-exchange was reported.14 However, the
The electron transfer reactions of synthetic metalloporphyrins
have been widely investigated to gain an insight into biological
functions of this type of species in nature. According to the Marcus
aDepartment of Chemistry, Aichi University of Education, Kariya, 448-
8542, Japan. E-mail: minamo@auecc.aichi-edu.ac.jp; Fax: +81-566-26-
2636; Tel: +81-566-26-2636
bFaculty of Science, Ehime University, Matsuyama, 790-8577, Japan
cDepartment of Chemistry and Biochemistry, Graduate School of Engineer-
ing, Kyushu University, Fukuoka, 819-0395, Japan
dResearch Centre for Materials Science, Nagoya University, Nagoya, 464-
8602, Japan
† Electronic supplementary information (ESI) available: Figures report-
ing the spectral change associated with the oxidations of [Zn(OEPc)],
[Zn(TPP)], [Zn(OEP)], and [Zn(OETPP)] by Cu2+ in acetonitrile, and the
spectrophotometric titration for the first oxidation reaction of [Cu(TPrPc)]
by Cu(II) triflate. See DOI: 10.1039/b812575a
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porphyrin-centred redox reactions have been scarcely studied due
to the highly reactive nature of this type of reaction.
was tested by elemental analysis. Cu(II) triflate, Cu(CF3SO3)2, was
prepared according to the method in the literature.25 Acetonitrile
˚
In the present study, we focused on elucidation of the factors in-
fluencing the electron transfer reactivity in the aspects of the distor-
tion of the molecular structure of porphyrins as well as the struc-
tural perturbation of the porphyrin skeleton (Chart 1). The struc-
tural deformation was introduced by using a porphyrin ligand that
has bulky peripheral substituents, 2,3,7,8,12,13,17,18-octaethyl-
5,10,15,20-tetraphenylporphyrin (OETPP), and the structure of
the Cu(II) and Zn(II) complexes of OETPP is highly distorted
in an S4 saddle conformation due to the steric crowding of the
multiple peripheral substituents.15,16 The oxidation of the complex
results in additional ruffling imposed on the original saddle
shape conformation in the case of [Cu(OETPP)].17 We previously
reported on the effect of the deformation of the porphyrin
skeleton on the electron transfer reaction for [Cu(OETPP)] as a
preliminary report.18 The structural perturbation of the porphyrin
skeleton was derived by using porphycenes, which are structural
isomers of porphyrin. We investigated the kinetics of the electron
transfer reaction of the Cu(II) and Zn(II) complexes of various
porphyrins and porphycenes with Cu(II) ion as an oxidizing
reagent in acetonitrile. All the one-electron oxidation reactions of
these complexes by Cu(II) are ligand-centred oxidations, and the
oxidation state of the central metal ions in the complexes remains
unchanged. The rate constants for the electron self-exchange
reactions between the porphyrin or porphycene complexes and the
corresponding p-cation radicals were estimated using the Marcus
cross-relation from the kinetics of the cross reaction with Cu(II)
ion whose self exchange reactions are extremely slow. The effect
of the structural deformation and perturbation accompanying the
oxidation on the electron self-exchange reactivity will be discussed
on the basis of the self-exchange rate constant.
(AN, Wako Pure Chemical Industries) was dried over activated 3 A
molecular sieves for several days and distilled under a nitrogen
atmosphere. Tetra-n-butylammonium perchlorate (TBAP) was
purchased from Tokyo Kasei Organic Chemicals, recrystallized
from ethyl acetate, and dried under vacuum at room temperature
for several days prior to use. Doubly distilled water was used for
the preparation of the aqueous acetonitrile solutions.
Caution! Although we have experienced no problems in handling
perchlorate compounds, these salts are potentially explosive and
should be handled in small quantities and with adequate precau-
tions.26,27
Measurements
Redox potentials of the metal complexes were determined by
cyclic voltammetry using a BAS 100B electrochemical analyzer
under deaerated conditions at T = 25.0 ◦C. A three-electrode
electrochemical cell was used consisting of a 3 mm glassy carbon
or a Pt working electrode, a Pt wire auxiliary electrode, and an
Ag/AgCl reference electrode. Each solution contained 0.1 M
TBAP as a supporting electrolyte. Redox potentials in acetonitrile
were referenced to the ferrocenium/ferrocene couple (Fc+/0) as an
external standard.
UV-visible absorption spectra were recorded on a Hitachi U-
3000 spectrophotometer. Kinetic measurements were made using
a stopped-flow spectrophotometer (RSP- 801, Unisoku, Japan).
The temperature of all solutions was maintained at 25.0 0.1 ◦C
using a circulating water bath. The spectral change for the
reactions of the metal complexes of porphyrin or porphycene with
Cu2+ in acetonitrile was measured by a multi-channel detection
system. The absorbance, A, was followed after mixing acetonitrile
solutions containing each of the metal complexes of porphyrin
or porphycene with that of a large excess of Cu(II) triflate to
determine the pseudo-first-order rate constant, kobsd. The kobsd
value was evaluated by fitting the absorbance-time traces with a
non-linear least-squares fitting program. The reported kobsd value is
the average of several runs. Reproducibility of the kobsd value was
within 3%. A Karl Fischer apparatus (Mitsubishi Chemicals
CA-06) was used to determine the concentration of water in the
acetonitrile solution.
Results and discussion
Chart 1
Redox potentials
Experimental
It is well known that free base of porphyrins and their metal
complexes show two reversible one-electron oxidations, as well as
two one-electron reductions all centred on the tetrapyrrolic ring
system.28,29 Additionally, the metal-centred oxidation or reduction
can be observed for some metal complexes, e.g., those of Co(II)
and Fe(III). In the case of Cu(II) and Zn(II) complexes, the central
metal undergoes no oxidation or reduction, and all electron
transfers are tetrapyrrole ligand centred. In the present study, the
oxidation potentials of the metal complexes were determined by
cyclic voltammetry in acetonitrile, and it is difficult to determine
the second oxidation potential accurately for most porphyrin
and porphycene complexes used here due to poor solubility in
Materials
TPP and OETPP were prepared by the literature methods.15,19,20
OEP was purchased from Tokyo Kasei Organic Chemicals. Cu(II)
and Zn(II) complexes of these porphyrins were obtained by
a metal(II) acetate method,21 purified by column chromatog-
raphy, and recrystallized from dichloromethane/methanol or
dichloromethane/heptane. 2,7,12,17-Tetra-n-propylporphycene
(TPrPc), 2,3,6,7,12,13,16,17-octaethlporphycene (OEPc), and
their Cu(II) and Zn(II) complexes were synthesized according to the
published procedures.22–24 The purity of the obtained compounds
120 | Dalton Trans., 2009, 119–125
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Table 1 Redox potentials of the complexes used in the present studya
[Cu(TPP)] and [Cu(OEP)], indicating that the obtained p-cation
radicals may be further oxidized by Cu2+ to give the dication of
the corresponding porphyrin and porphycene complexes under
the conditions where Cu2+ exists in large excess over the porphyrin
or porphycene complex. A consecutive reaction was observed
spectrophotometrically for these reaction systems as described
later.
Compound
E◦/V vs. Fc+/0
[Cu(TPP)]+/0
0.600
0.480
-0.011
0.418
0.390
0.314
-0.093
0.337
[Cu(OEP)]+/0
[Cu(OETPP)]+/0
[Cu(TPrPc)]+/0
[Zn(TPP)]+/0
[Zn(OEP)]+/0
[Zn(OETPP)]+/0
[Zn(OEPc)]+/0
Spectral change
a Ionic strength of the solution was adjusted to 0.1 M by tetra-n-
butylammonium perchlorate (TBAP). T = 25.0 ◦C.
The oxidation of porphyrin and porphycene complexes by Cu(II)
ion was followed spectrophotometrically in acetonitrile. The
spectral change in the UV-visible region for the reaction of
[Cu(TPrPc)] is shown in Fig. 1, and the results for the reactions
of other complexes are given in Fig. S1–S4 of the ESI.† In the
case of [Cu(TPrPc)], the spectral change exhibits bleaching of
the porphycene peaks and the formation of broad absorptions,
followed by an additional small spectral change, as shown in
Fig. 1, which is consistent with the results of pulse radiolysis of
[Cu(TPrPc)] in CH2Cl2.36 Similar spectral changes were observed
for [Zn(OEPc)], as shown in Fig. S1. The spectral change for
[Zn(TPP)], [Zn(OEP)], and [Zn(OETPP)] shown in Fig. S2–S4
closely resemble with those for the corresponding Cu(II) porphyrin
complexes reported previously.13,18 Such spectral features of the
products indicate that the observed spectral changes associated
with the first oxidation reaction of these complexes is typical for
the formation of p-cation radical of the porphyrin and porphycene
complexes.30,31,35,36
acetonitrile. The first oxidation potentials of the porphyrin and
porphycene complexes are listed in Table 1 and correspond to the
tetrapyrrole ligand centred oxidation reaction of complexes, giving
p-cation radicals.30–37 The oxidation potential of the porphycene
complexes is comparable to those of the corresponding porphyrin
complexes. On the other hand, the first oxidation potentials of
distorted OETPP complexes are significantly lower than those
for the planar porphyrin complexes. [Zn(OETPP)] is easier to
oxidize (E◦ = -0.093 V vs. Fc+/0 in acetonitrile) than either
1/2
[Zn(TPP)] (0.390 V vs. Fc+/0) or [Zn(OEP)] (0.314 V vs. Fc+/0).
Similarly, the first oxidation potential of [Cu(OETPP)] is -0.011 V
vs. Fc+/0, which is significantly lower than the corresponding value
of [Cu(TPP)] (0.600 V vs. Fc+/0) and [Cu(OEP)] (0.480 V vs. Fc+/0).
These phenomena were observed for the redox reaction in other
solvents such as dichloromethane, i.e., the first oxidation potential
of [Cu(OETPP)] and [Zn(OETPP)] is lower than those of TPP and
OEP complexes by ca. 0.2–0.6 V, which can be attributed to the
destabilization of the porphyrin p system and/or the stabilization
of the p-cation radical caused by the saddle conformation of the
sterically constrained nonplanar porphyrin skeleton.15,17 In agree-
ment with these observations, the theoretical calculations indicate
that the deformation of the porphyrin skeleton destabilizes the p
system of the porphyrin and raises the HOMO level with a smaller
perturbation of the LUMO level.37
The reduction potential of the solvated Cu2+ ion to Cu+ in
acetonitrile is 0.660 V vs. Fc+/0, and this redox potential is higher
than those of the complexes used in the present study. As has been
reported previously, the reaction of the Cu(II)-TPP and Cu(II)-
OEP complexes with Cu2+ in acetonitrile leads to the formation
of the corresponding p-cation radicals.13 The average potential
differences between the first and second ring-centred oxidations of
the Cu(II)- and Zn(II)-porphyrin complexes are 0.28 0.05 V for
Cu(II)-TPP, 0.45 0.05 V for Cu(II)-OEP, 0.31 0.03 V for Zn(II)-
TPP, and 0.34 0.04 V for Zn(II)-OEP in dichloromethane.38
Although the second oxidation step was not clearly observed in
the CV measurements due to poor solubility of these complexes in
acetonitrile, it is probable that the second oxidation potentials of
[Cu(TPP)] and [Cu(OEP)] are higher than the redox potential of
the solvated Cu(II)/(I) couple. Therefore, it is concluded that the
products of the oxidation reactions of these porphyrin complexes
with solvated Cu2+ are the p-cation radicals of the Cu(II)-porphyrin
complexes and [Cu(AN)4]+, the latter of which is known as one of
the most stable Cu(I) species.39,40 Meanwhile, other complexes used
in the present study have oxidation potential lower that those of
Fig. 1 Change of the UV-visible absorption spectrum of [Cu(TPrPc)]
associated with the reaction with Cu(II) triflate in acetonitrile at T =
25.0 ◦C: [Cu(TPrPc)] (A), [Cu(TPrPc)]+ (B), [Cu(TPrPc)]2+ (C). CCu-TPrPc
=
1.2 ¥ 10-6 M, CCu(II) = 7.40 ¥ 10-4 M. The absorbance of the excess amount
of the Cu(II) ion was subtracted from the observed spectrum for each case.
The stoichiometry of the reaction was determined by spec-
trophotometric titration experiments for the first oxidation re-
action of the complexes with Cu(II). The solution of the porphyrin
or porphycene complex was titrated with a Cu(II) triflate solution,
and the resulting spectrum was measured. The absorbance at a
given wavelength was plotted as a function of the ratio of the
concentration of Cu(II) triflate over that of the porphyrin or
porphycene complex. The results for [Cu(TPrPc)] are shown in
Fig. S5 as an example. The ratio of Cu(II) triflate consumed for
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the oxidation of the complex was confirmed to be 1 : 1 for each
complex.
Kinetics studies
The kinetics of the oxidation reaction of the porphyrin and por-
phycene complexes by Cu(II) triflate in acetonitrile was studied by
a stopped-flow technique with a multi-channel photodiode array
detection system. Ionic strength of the solution was adjusted to
0.1 M by TBAP. The reaction was followed spectrophotometrically
under the pseudo-first-order conditions in which the Cu(II) triflate
exists in large excess over the Cu(II) and Zn(II) complexes. The
oxidation of these porphyrin and porphycene complexes by Cu2+
is a first-order reaction with respect to the complex in all reaction
systems studied here. In the present report, we will describe the
kinetics of the first stage of the reaction, which gives p-cation
radical of the corresponding porphyrin or porphycene complex
(eqn (2)).
Fig. 3 Dependence of the second-order rate constant kf of the reaction
of [Cu(TPrPc)] with Cu(II) triflate on the concentration of water in
acetonitrile at T = 25.0 ◦C. Broken (A) and dotted (B) lines represent
the contribution of the kS0 and kS1 terms, respectively.
[MII(por)] + Cu2+ → [MII(por)]+ + Cu+
(2)
b
n
[Cu(AN)6 ]2+ + nH O
[Cu(AN)6-n (H2O)n ]2+ + nAN
ꢀꢀꢀꢁ
(3)
ꢂꢀꢀꢀ
2
(M = Cu or Zn; por = porphyrin or porphyrcene). Because the
reaction rate was found to depend on the water concentration in
the acetonitrile solution, we studied the kinetics of the electron
transfer reaction as a function of the water concentration.
The observed pseudo-first-order rate constant, kobsd, was de-
termined by applying the least squares fitting calculation to the
absorbance-time traces of the reactions. The dependence of kobsd
on the concentration of the Cu(II) ion is shown in Fig. 2 for the
reaction of [Cu(TPrPc)] as an example. The pseudo-first-order
rate constant, kobsd, is proportional to the concentration of Cu2+
under a constant concentration of water. The second-order rate
The overall formation constant bn for [Cu(AN)6-n(H2O)n]2+ is
defined as bn = [Cu(AN)6-n(H2O)n2+][Cu(AN)62+]-1[H2O]-n. The
equilibrium of the solvation of the Cu(II) ion in the aqueous ace-
tonitrile solution was previously studied spectrophotometrically
under the conditions of [H2O] < 0.9 M, and the equilibrium
constants were determined to be log(b1/M-1) = 1.19
0.18,
log(b2/M-2) = 1.86 0.35, and log(b3/M-3) = 2.12 0.57.41 The
present electron transfer reaction can be expressed by eqn (4).
k
[Cu(TPrPc)] + [Cu(AN)6-n (H2O)n ]2+ æææÆ [Cu(TPrPc)] + Cu
+
+
Sn
constant, kf, was then determined using the relationship, kobsd
=
kf[Cu2+]. Similar results were obtained for all reaction systems. The
effect of the water concentration on kf is shown in Fig. 3, and the
reaction rate decreases as [H2O] increases. The water concentration
dependence of kf can be interpreted by the solvation of the Cu(II)
ion in acetonitrile containing a small amount of water. It has been
known that some of the acetonitrile molecules on [Cu(AN)6]2+ are
replaced by water molecules (eqn (3)).41
(4)
The dependence of kf on the water concentration can be explained
by the retardation of the reaction due to the reduced concentration
of reactive Cu(II) species such as [Cu(AN)6]2+ through reaction
(3) because it is expected that the oxidation potential of the
solvated Cu(II) species decreases as the coordinated acetonitrile
molecules are successively substituted with H2O molecules. The
concentrations of [Cu(AN)6-n(H2O)n]2+ were estimated by using
the reported values of bn.41 It is probable that the solvent exchange
reaction of [Cu(AN)6-n(H2O)n]2+ is sufficiently fast in this solvent,
and then the second-order rate constant, kf, can be expressed by
eqn (5).
kf = (kS0 + kS1b1[H2O] + kS2b2[H2O]2 + kS3b3[H2O]3)
¥ (1 + b1[H2O] + b2[H2O]2 + b3[H2O]3)-1
(5)
The kinetic results shown in Fig. 3 were analyzed by the least-
squares fitting calculation to determine the values of kSn (n = 0, 1, 2,
and 3). It was revealed that it is not necessary to take into account
the contribution of [Cu(AN)6-n(H2O)n]2+ (n ≥ 2) to the present
electron transfer reaction, indicating that only [Cu(AN)6]2+ and
[Cu(AN)5(H2O)]2+ have redox potential high enough to oxidize
the porphyrin and porphycene complexes. The values of the rate
constants, kS0 and kS1, are listed in Table 2. As shown in Fig. 3, the
calculated curve of kf using obtained kS0 and kS1 values reproduces
observed rate constants well.
Fig. 2 Dependence of the pseudo-first-order rate constant kobsd of the
reaction of [Cu(TPrPc)] with Cu(II) triflate on the concentration of the
Cu(II) ion in acetonitrile at T = 25.0 ◦C. The concentration of water is
0.094 M (A) and 0.20 M (B). I = 0.1 M (TBAP).
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Table 2 Rate constant of electron transfer reaction of porphyrin and
Table 3 Self-exchange rate constant between porphyrin and porphycene
porphycene complexes with Cu(II) ion acetonitrilea
complexes and their p-cation radicals at T = 25.0 ◦
C
Complex
kS0/M-1 s-1
kS1/M-1 s-1
Complex
k11/M-1 s-1
c
c
[Cu(TPP)]b
[Cu(OEP)]b
[Cu(OETPP)]d
[Cu(TPrPc)]
[Zn(TPP)]
[Zn(OEP)]
[Zn(OETPP)]
[Zn(OEPc)]
(5.81 0.12) ¥ 103
(2.01 0.03) ¥ 105
(1.46 0.06) ¥ 105
(6.54 0.13) ¥ 104
(5.37 0.03) ¥ 105
(1.72 0.02) ¥ 106
(3.42 0.11) ¥ 106
(5.77 0.12) ¥ 106
[Cu(TPP)]a
[Cu(OEP)] a
[Cu(OETPP)]b
[Cu(TPrPc)]
[Zn(TPP)]
[Zn(OEP)]
[Zn(OETPP)]
[Zn(OEPc)]
2.7 ¥ 1010
4.3 ¥ 1011
2.4 ¥ 104
5.0 ¥ 109
1.4 ¥ 1011
1.3 ¥ 1011
1.9 ¥ 106
3.4 ¥ 1012
(7.54 0.42) ¥ 104
(3.80 0.65) ¥ 103
(7.01 0.20) ¥ 104
(2.95 0.11) ¥ 105
(2.88 0.10) ¥ 106
(1.51 0.07) ¥ 106
a Ionic strength of the solution was adjusted to 0.10 M by TBAP. T =
rate constant kf was found to be negligible. d Ref. 18.
a Ref. 13. b Ref. 18.
◦
25.0 C. b Ref. 13. c The contribution of the kS1 term to the second-order
from these quite different experimental techniques employed and
the errors inherent in the application of the Marcus cross relation.
It has been well documented that the 24-atom core of the
porphyrin molecule is readily defomed in a direction perpendicular
to the porphyrin skeleton, but to a lesser extent in the radial
direction.47 In the case of the Cu(II)-TPP complex, the porphyrin
core shows a ruffled core conformation in the solid state.48 The
meso-carbon atoms are displaced alternatively above and below
the mean plane of the core. The maximum displacement of the
Electron self-exchange reactions
The rate constant of the electron self-exchange reaction between
the parent complex and its p-cation radical for metalloporphyrin
and metalloporphycene complexes, k11, was calculated from the
cross-reaction rate constant using the Marcus cross relationship
in the form42,43
k12 = (k11k22K12f 12)1/2W12
(6)
˚
meso-carbon atoms is 0.42 A for [Cu(TPP)]. The dihedral angle
where
between the peripheral phenyl group and the porphyrin core is
larger than 60◦. The porphryin core of the p-cation radical is also
deformed. For example, the p-cation radical, [Cu(TPP)]+, has a
saddle-shaped conformation and the pyrrole rings are displaced
alternatively above and below the mean plane of the porphyrin
core.49 The meso-carbon atoms are nearly in the plane of the
core, which is in contrast to the ruffled conformation of the
ln f 12 = [ln K12 + (w12 - w21)/RT]2/4[ln(k11k22/Z2)
+ (w11 + w22)/RT]
(7)
(8)
(9)
W12 = exp[-(w12 + w21 - w11 - w22)/2RT]
wij = 37.9zizj/sij(1 + 0.481sijI1/2
)
neutral complex, [Cu(TPP)]. The average displacement of the b-
+
˚
In the above expressions, wij is the work required to bring ions
i and j (charges zi and zj) to the separation distance sij (taken
as equal to the sum of the radii of the reagents), and Z is the
collision frequency (kBT/h). In eqn (6), k12 represents the rate
constant, kS0, for the cross-reaction between each porphyrin or
porphycene complex and [Cu(AN)6]2+; k22 represents the electron
self-exchange rate constant for the Cu2+/+ couple in acetonitrile;
K12 represents the equilibrium constant for reaction (4) (n =
0), which can be estimated by using the potential values of the
reactants as listed in Table 1; f 12 represents the nonlinear correction
term and W12 represents as electrostatic work term correction.44,45
This electron transfer reaction is expected to proceed through the
ordinary outer-sphere process since no potential bridging ligand
exists in the medium. Using the self-exchange rate constant of
the Cu2+/+ couple (k22 = 4.2 ¥ 10-5 M-1 s-1),13 we determined the
self-exchange rate constant k11 listed in Table 3. In the previous
paper, the self-exchange rate constant of the Cu(II)-TPP and
Cu(II)-OEP complexes w◦ere reported to be 3 ¥ 109 M-1 s-1 and
1 ¥ 1011 M-1 s-1 at 25.0 C, respectively.13,46 The corresponding
values for the [Zn(TPP)]+/0 and [Zn(OEP)]+/0 couples are 1.4 ¥
1011 M-1 s-1 and 1.3 ¥ 1011 M-1 s-1 at 25.0 ◦C, respectively. Fukuzumi
et al. determined the self-exchange rate constant for the Zn(II)-TPP
complex to be 3 ¥ 109 M-1 s-1 by using the line-width broadening in
the EPR spectra.14 Although this value is somewhat smaller than
that determined in the present study, it could be concluded that
these two determinations are consistent with each other, judging
pyrrole carbon atoms is 0.65 A for [Cu(TPP)] . The saddle-shaped
conformation of [Cu(TPP)]+ should result from the necessity of
making the bulky peripheral phenyl groups more nearly coplanar
with the porphyrin core, allowing the close contact of the adjacent
porphyrin molecules in the solid state to form the dimer species.
This is evidenced by the molecular structure of the p-cation radical
of the 5,10,15,20-tetramesitylporphyrin complex, [Cu(TMP)]+,
that has meso-aryl groups with ortho methyl groups on the aryl
groups.50 The 24-atom core of TMP is almost planar, and the
peripheral mesityl groups are nearly perpendicular to the mean
plane of the porphyrin core due to these ortho substituents.
The peripheral mesityl groups inhibit the approach of a second
radical, resulting in a planar conformation of the porphyrin core.
These findings indicate that the ruffled and saddle-shaped core
conformations for the neutral molecules and p-cation radicals
of tetraarylporphyrins are not due to an intrinsic demand but
are a consequence of the interaction with adjacent porphyrin
molecules in the solid state. In the case of OEP complex in which
peripheral substituents are less sterically demanding than those
of the tetraarylporphyrin complexes, the inter-ring interactions
cause these molecules to form dimers or higher aggregates more
easily. The OEP and related complexes have an almost planar
core conformation.47,51 The p-cation radical of the Zn(II) complex
forms a very strongly coupled dimer, and the unpaired electrons
on the two porphyrin rings are so strongly coupled that the dimer
is diamagnetic.52 The porphyrin core is essentially planar, with the
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largest deviation of an atom from the mean plane of the 24-atom
should be deformed not only in the solid state but also in solution.
Therefore, the origin of the distorted core conformation of OETPP
is different from that of TPP, which can be ascribed to the
minimization of the intermolecular distance between the adjacent
molecules by rotating the phenyl ring more coplanarly with the
porphyrin core for the effective intermolecular interactions in
the solid state, as mentioned above. The one-electron oxidation
of this distorted metalloporphyrin causes further conformational
change to give the p-cation radical, in which the b-pyrrole carbon
˚
core equal to 0.05 A.
The porphycene molecule that has no peripheral substituents is
centrosymmetric and virtually planar with a maximum distance
of the core C and N atoms from the mean plane of 0.04 A,53 as in
˚
the case of porphyrins having alkyl substituents at the peripheral
positions such as OEP. n-Propyl groups in TPrPc little affect the
planarity of the porphycene core due to the lack in the nonbonding
interaction between these substituents.23 On the other hand, the
porphycene skeleton of OEPc is twisted to some extent because
of the steric repulsion between the peripheral ethyl groups in the
3,6- and 13,16-positions, and the maximum deviation of the core
atoms are further displaced from the mean plane of the porphyrin
17
˚
core about 1.2–1.5 A. Such an additional deformation of the
porphyrin skeleton should be associated with a larger inner-
sphere reorganization energy, l, for the one-electron oxidation
of the OETPP complex, resulting in the retardation of the electron
self-exchange rate by a factor of ca. 105–106 compared with
the corresponding TPP complexes. This structural perturbation
caused by the steric crowding of the peripheral substituents
amounts to an enhancement in the l value of ca. 110–140 kJ mol-1.
The schematic representation of the potential energy curves for the
self-exchange reactions is shown in Fig. 4. The intrinsic energy
barrier, l, is significantly larger for the reactions of distorted
porphyrin complexes because of the very steep slope along the
reaction coordinate. The free energy barrier of the reaction is
approximated by l/4 in the Marcus theory.
23
˚
atoms from the mean plane of the porphycene core is 0.27 A.
However, the incorporation of a Zn(II) ion into the OEPc core
leads to the contraction of the bond angle in the NCCN segment
of the porphycene core, resulting in the better alignment of the
lone-paired electrons of nitrogen atoms and bonds towards the
central metal ion.23 The metalation of OEPc is, thus, associated
with a reduction in the nonbonding interactions between the ethyl
substituents and a structural change to a virtually planar core
structure. Therefore, both complexes of OEPc and TPrPc studied
in the present work show considerably planar core conformations.
The rate of the electron self-exchange reaction between the
porphycene complex and its p-cation radical is extremely fast
for both Cu(II) and Zn(II) complexes, as shown in Table 3.
These findings are similar to those of the corresponding TPP
and OEP complexes. Although the TPP complexes of Cu(II)
and Zn(II) and their p-cation radicals have a ruffled or saddle-
shaped conformation in the solid state, these deformation from the
planar core conformation could be ascribed to the intermolecular
interaction between the porphyrin cores in the solid state. In the
acetonitrile solution where metalloporphyrin concentration is as
quite low, in the order of 10-6 M, it is probable that no aggregation
of the porphyrin molecules exists and that the core conformation
of the TPP and OEP complexes and their p-cation radicals may be
planar, as in the case of [Cu(TMP)]+. Therefore, the inner-sphere
reorganization energy associated with the electron self-exchange
reactions between the parent porphyrin complex and its p-cation
radical may be small, resulting in a very fast electron self-exchange
reaction. Such a fast electron self-exchange rate is consistent with
the mechanism of the ligand-centred redox reaction because the
structural parameters show little difference between the parent
complex and the p-cation radical species, e.g., the average Cu–N
Fig. 4 Reaction profiles described by diabatic surfaces of the precursor
and successor complexes for two different types of the self-exchange
reactions, where the diabatic energy surfaces are chiefly made up with
the inner-sphere reorganization barrier.
The effect of the substituents such as ethyl, phenyl, and propyl
groups may contribute to this type of electron transfer reactions
through the steric and electronic aspects, where the former was
demonstrated clearly in the present study. On the other hand, the
electronic effect of the substituents is taken into account in the
Marcus cross relation, which was originally derived by assuming
that the intrinsic energy barrier for a cross reaction is approximated
by the arithmetic mean of the intrinsic energy barriers for
two independent self-exchange reactions. It was reported that
the identical relation is derived on the purely thermodynamic
basis, by assuming (1) the activation process for each reactant
is independent of the counter reagent and (2) the activated species
are the same for the self-exchange and cross reactions.54 With
another independent development concerning the electronic work
terms between reactants and products, the Marcus cross relation
can be utilized to examine the independent activation process
and the nature of the activated species involved in outer-sphere
electron transfer reactions. Since the difference in the types of the
substituents is accounted for as the ground-state (thermodynamic)
˚
˚
bond distances are 1.981(7) A and 1.988(4) A for [Cu(TPP)] and
[Cu(TPP)]+, respectively.48,49
In contrast to the planar porphyrin and porphycene complexes,
the distorted porphyrin complexes show considerable retardation
in electron self-exchange rate. According to an X-ray structural
study, the Cu(II)-OETPP complex is highly distorted due to the
steric interaction between the peripheral phenyl and ethyl groups
so as to minimize the steric crowding between the peripheral
substituents, and the geminal b-carbons of successive pyrrole
rings are displaced alternatively above and below the average
16
˚
nitrogen plane by 1.1–1.2 A. The OETPP complex of Zn(II) is
also severely saddle-shaped with the b-carbons of adjacent pyrrole
˚
rings displaced by 1.0–1.2 A relative to the plane of the four
pyrrole nitrogen atoms.15 The deformation of the porphyrin core
of the OETPP complexes is mainly caused by the intramolecular
steric demand, and it is expected that the porphyrin core structure
124 | Dalton Trans., 2009, 119–125
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property of the complexes such as the difference in the redox
potentials, discussions in this article is valid as far as the Marcus
cross relation holds.
21 J. W. Buchler, In Porphyrins and Metalloporphyrins, ed. K. M. Smith,
Elsevier, Amsterdam, 1975, ch. 5.
22 E. Vogel, M. Balci, K. Pramod, P. Koch, J. Lex and O. Ermer, Angew.
Chem., Int. Ed. Engl., 1987, 26, 928–931.
In conclusion, the effect of the deformation of the porphyrin
skeleton on the electron self-exchange reaction does not originate
from the deformation itself but from the enhancement of the
conformational deformation during the oxidation of the OETPP
complexes of Cu(II) and Zn(II). These features are in contrast
with the planar porphyrin and porphycene complexes that tend
to maintain their molecular structures during electron transfer
reactions.
23 E. Vogel, P. Koch, X.-L. Hou, J. Lex, M. Lausmann, M. Kisters, M. A.
Aukauloo, P. Richard and R. Guilard, Angew. Chem., Int. Ed. Engl.,
1993, 32, 1600–1604.
24 M. W. Renner, A. Forman, W. Wu, C. K. Chang and J. Fajer, J. Am.
Chem. Soc., 1989, 111, 8618–8621.
25 Y. Inada, Y. Sugimoto, Y. Nakano, Y. Itoh and S. Funahashi, Inorg.
Chem., 1998, 37, 5519–5526.
26 W. C. Wolsey, J. Chem. Educ., 1973, 50, A335–A337.
27 K. N. Raymond, Chem. Eng. News, 1983, 61, 4.
28 C. Bernard, J. P. Gisselbrecht, M. Gross, E. Vogel and M. Lausmann,
Inorg. Chem., 1994, 33, 2393–2401.
29 K. M. Kadish, E. van Caemelbecke and G. Royal, in The Porphyrin
Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic
Press, New York, 2000, vol. 8, pp. 1–114.
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Research
(No. 18550052) from the Ministry of Education, Science, Sports,
and Culture of Japan.
30 J.-H. Fuhrhop and D. Mauzerall, J. Am. Chem. Soc., 1969, 91, 4174–
4181.
31 A. Wolberg and J. Manassen, J. Am. Chem. Soc., 1970, 92, 2982–2991.
32 C. Mengersen, J. Subramanian and J.-H. Fuhrhop, Mol. Phys., 1976,
32, 893–897.
33 S. Konishi, M. Hoshino and M. Imamura, J. Am. Chem. Soc., 1982,
104, 2057–2059.
34 R. S. Czernuszewicz, K. A. Macor, X.-Y. Li, J. R. Kincaid and T. G.
Spiro, J. Am. Chem. Soc., 1989, 111, 3860–3869.
35 F. D’Souza, P. Boulas, A. M. Aukauloo, R. Guilard, M. Kisters, E.
Vogel and K. M. Kadish, J. Phys. Chem., 1994, 98, 11885–11891.
36 D. M. Guldi, J. Field, J. Grodkowski, P. Neta and E. Vogel, J. Phys.
Chem., 1996, 100, 13609–13614.
37 K. M. Barkigia, L. Chantranupong, K. M. Smith and J. Fajer, J. Am.
Chem. Soc., 1988, 110, 7566–7567.
References
1 H. Sigel and A. Sigel, Metal Ions in Biological Systems Volume 27:
Electron Transfer Reactions in Metalloproteins, Marcel Dekker, New
York, 1991.
2 S. K. Chapman, S. Daff, and A. Munro, in Struct. Bonding, 1997, 88,
39-70.
3 S. Fukuzumi, In The Porphyrin Handbook: Electron Transfer, ed.
K. M. Kadish, K. M. Smith and R. Guillard, Academic Press, New
York, 2000, vol. 8, pp. 115–151.
38 K. M. Kadish, G. Royal, E. van Caemelbecke and L. Gueletti, in The
Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard,
Academic Press, New York, 2000, vol. 9, pp. 1–67.
39 D. T. Richens, The Chemistry of Aqua Ions, Wiley, New York, 1994.
40 J.-H. Fuhrhop, Struct. Bonding, 1974, 18, 1–67.
41 M. Inamo, N. Kamiya, Y. Inada, M. Nomura and S. Funahashi, Inorg.
Chem., 2001, 40, 5636–5644.
4 Electron Transfer in Chemistry, ed. V. Balzani, Wiley-VCH Verlag,
Weinheim, Germany, 2001.
5 R. A. Marcus, J. Chem. Phys., 1956, 24, 966–978.
6 R. A. Marcus, Annu. Rev. Phys. Chem., 1964, 15, 155–196.
7 D. W. Dixon, S. Woehler, X. Hong and A. M Stolzenberg, Inorg. Chem.,
1988, 27, 3682–3685.
8 A. Shirazi, M. Barbush, S. Ghosh and D. W. Dixon, Inorg. Chem., 1985,
24, 2495–2502.
9 D. W. Dixon, M. Barbush and A. Shirazi, Inorg. Chem., 1985, 24, 1081–
1087.
10 D. W. Dixon, M. Barbush and A. Shirazi, J. Am. Chem. Soc., 1984, 106,
4638–4639.
42 N. Sutin, Acc. Chem. Res., 1982, 15, 275–282.
43 N. Sutin, Prog. Inorg. Chem., 1983, 30, 441–498.
44 M. J. Martin, J. F. Endicott, L. A. Ochrymowycz and D. B. Rorabacher,
Inorg. Chem., 1987, 26, 3012–3022.
45 R. A. Marcus and N. Sutin, Biochim. Biophys. Acta, 1985, 811, 265–322.
46 In the previous paper, the redox potentials of the complexes used for
the calculation in the Marcus cross relation were those measured in the
mixed solvent of acetonitrile and chloroform (9 : 1 v/v) so as to attain
higher solubility of the porphyrin complexes.13,18 In the present study,
we measured the redox potentials of the complexes in pure acetonitrile,
and the self-exchange rate constants for Cu(II)-TPP and Cu(II)-OEP
were reevaluated.
11 R. D. Chapman and E. B. Fleischer, J. Am. Chem. Soc., 1982, 104,
1575–1582.
12 H. Sun, V. V. Smirnov and S. G. DiMagno, Inorg. Chem., 2003, 42,
6032–6040.
13 M. Inamo, H. Kumagai, U. Harada, S. Itoh, S. Iwatsuki, K. Ishihara
and H. D. Takagi, Dalton Trans., 2004, 1703–1707.
14 S. Fukuzumi, T. Hasobe, Y. Undo, K. Ohkubo and H. Imahori,
J. Porphyrins Phthalocyanines, 2003, 7, 328–336.
15 K. M. Barkigia, M. D. Berber, J. Fajer, C. J. Medforth, M. W.
Renner and K. M. Smith, J. Am. Chem. Soc., 1990, 112, 8851–
8857.
16 L. D. Sparks, C. J. Medforth, M.-S. Park, J. R. Chamberlain, M. R.
Ondrias, M. O. Senge, K. M. Smith and J. A. Shelnutt, J. Am. Chem.
Soc., 1993, 125, 581–592.
47 W. R. Scheidt and Y. J. Lee, Struct. Bonding, 1987, 64, 1–70.
48 E. B. Fleischer, L. K. Miller and L. E. Webb, J. Am. Chem. Soc., 1964,
86, 2342–2347.
49 B. S. Erler, W. F. Scholz, Y. J. Lee, W. R. Scheidt and C. A. Reed, J. Am.
Chem. Soc., 1987, 109, 2644–2652.
50 H. Song, C. A. Reed and W. R. Scheidt, J. Am. Chem. Soc., 1989, 111,
6865–6866.
51 M. O. Senge, C. W. Eigenbrot, T. D. Brennan, J. Shusta, W. R. Scheidt
and K. M. Smith, Inorg. Chem., 1993, 32, 3134–3142.
52 H. Song, R. D. Orosz, C. A. Reed and W. R. Scheidt, Inorg. Chem.,
1990, 29, 4274–4282.
53 E. Vogel, M. Kocher, H. Schmickler and J. Lex, Angew. Chem., Int. Ed.
Engl., 1986, 25, 257–259.
17 M. W. Renner, K. M. Barkigia, Y. Zhang, C. J. Medforth, K. M. Smith
and J. Fajer, J. Am. Chem. Soc., 1994, 116, 8582–8592.
18 M. Inamo, K. Aoki, N. Ono and H. D. Takagi, Inorg. Chem. Commun.,
2005, 8, 979–982.
19 A. D. Adler, F. R. Longo, J. D. Finarelli, J. Goldmacher, J. Assour and
L. Korsakoff, J. Org. Chem., 1967, 32, 476.
54 M. A. Ratner and R. D. Levine, J. Am. Chem. Soc., 1980, 102, 4898–
4900.
20 G. H. Barnett, M. F. Hudson and K. M. Smith, J. Chem. Soc., Perkin
Trans. 1, 1975, 1401–1403.
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