A negative temperature dependence of the electron self-exchange rates of zinc porphyrin π radical cations

Article abstract of DOI:10.1021/ja026089l

The one-electron oxidation of ZnT(t-Bu)PP (T(t-Bu)PP²- = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin dianion) by one equivalent of Ru(bpy)₃³⁺ (bpy = 2,2′-bipyridine) results in quantitative formation of ZnT(t-Bu)PP^?+ which is detected by ESR. In the presence of excess ZnT(t-Bu)PP, the ESR line width becomes broader with increasing ZnT(t-Bu)PP concentration due to the electron self-exchange between ZnT(t-Bu)PP^?+ and ZnT(t-Bu)PP. The line width of the ESR signal of ZnT(t-Bu)PP^?+ becomes broader as the temperature is decreased from 313 to 233 K. This indicates that the electron self-exchange reaction becomes faster at a lower temperature. The substituent and solvent effects on such a negative temperature dependence of the electron self-exchange rates are reported. Copyright

Full text of DOI:10.1021/ja026089l

Published on Web 08/23/2002
A Negative Temperature Dependence of the Electron Self-Exchange Rates of
Zinc Porphyrin π Radical Cations
Shunichi Fukuzumi,* Yoshito Endo, and Hiroshi Imahori†
Department of Material and Life Science, Graduate School of Engineering, CREST, JST,
Osaka UniVersity, Suita 565-0871, Japan
Received March 4, 2002
In general the rate of any chemical reaction becomes faster with
increasing reaction temperature, since an activation is required to
reach the reaction transition state. However, there is a case where
the rate of reaction becomes slower with increasing reaction
temperature, provided that the reaction proceeds via an intermediate,
the energy of which is lower than that of the reactant pair, and the
energy difference between the reactant pair and the intermediate is
larger than the activation energy from the intermediate. The first
definitive case for a negative temperature dependence was reported
by Kiselev and Miller who have shown that a negative activation
enthalpy arises when the charge transfer (CT) complex formed
between 9,10-dimethylanthracene and tetracyanoethylene lies along
the reaction pathway of the Diels-Alder reaction.1 In such a case,
the observed rate constant (kobs) is given by
kobs ) k1KCT
[∆Hobsq ) ∆H1q (> 0) + ∆HCT (<0)]
where k1 is the intracomplex rate constant and KCT is the formation
constant of the CT complex. The necessary condition to observe a
negative activation enthalpy for reactions involving CT complexes
is that the heat of formation of the CT complex (∆HCT < 0) is of
greater magnitude than the activation enthalpy for the passage of
q
the CT complex to the transition state (∆H1 > 0), that is, -∆HCT
q 1
> ∆H1 . Such a CT complex has been regarded as a key
intermediate in a number of important chemical reactions.2-5
A
negative temperature dependence has also been reported for the
rates of hydride transfer reactions from 10-methyl-9,10-dihydroacri-
dine to 2,3-dichloro-5,6-dicyano-p-benzoquinone in chloroform to
q
afford the negative activation enthalpy (∆Hobs ) -32 kJ mol-1 6,7
)
Figure 1. (a) ESR spectrum of ZnT(t-Bu)PP•+ (5.0 × 10-4 M) in toluene
at 313 K and (b) the computer simulation spectrum (∆Hmsl ) 0.240 G). (c)
ESR spectrum of ZnT(t-Bu)PP•+ in the presence of ZnT(t-Bu)PP (5.0 ×
10-4 M) in toluene at 313 K and (d) the computer simulated spectrum (∆Hmsl
) 0.305 G). (e) ESR spectrum of ZnT(t-Bu)PP•+ in the presence of ZnT-
(t-Bu)PP (5.0 × 10-4 M) in toluene at 233 K and (f) the computer simulated
spectrum (∆Hmsl ) 0.390 G).
However, there has been no report on a negative temperature effect
caused by the presence of an intermediate for a simple electron-
transfer reaction.8
Porphyrin π radical cations are known to play a crucial role in
biological electron-transfer systems such as respiration and pho-
tosynthesis.9 We report herein a negative temperature dependence
of the electron self-exchange between a zinc porphyrin π radical
cation and the neutral form for the first time. Fine-tuning of the
substituent on the porphyrin ring and the proper choice of the sol-
vent has enabled us to observe negative activation enthalpies for
the simple electron self-exchange reactions based on the ESR line
width variation of the ESR spectra of zinc porphyrin radical cations.
which is detected by ESR at 313 K as shown in Figure 1a. The
hyperfine structure is well resolved as compared to the ESR spectra
of porphyrin radical cations reported thus far.10 The hyperfine
coupling constants (hfc) are determined by comparison of the
observed spectrum with the computer simulated spectrum as shown
in Figure 1b.11 When an excess of ZnT(t-Bu)PP (1.0 × 10-3 M) is
3+
used for the oxidation with Ru(bpy)3 (5.0 × 10-4 M) at 313 K,
The one-electron oxidation of ZnT(t-Bu)PP (T(t-Bu)PP2-
)
the ESR line width becomes broader than the spectrum obtained
in the equimolar condition (Figure 1c,d). The line broadening results
from the electron self-exchange between ZnT(t-Bu)PP•+ and ZnT-
(t-Bu)PP which is left in solution. To our surprise, the line width
of the ESR signal of ZnT(t-Bu)PP•+ becomes broader as the
temperature is decreased from 313 to 233 K (Figure 1e,f). This
indicates that the electron self-exchange reaction becomes faster
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin dianion, 5.0 ×
3+
10-4 M) by one equivalent of Ru(bpy)3 (bpy ) 2,2′-bipyridine:
5.0 × 10-4 M) results in quantitative formation of ZnT(t-Bu)PP•+
* To whom correspondence should be addressed: Fax: +81(0)6-6879-7370.
E-mail fukuzumi@ap.chem.eng.osaka-u.ac.jp.
† Present address: Department of Molecular Engineering, Graduate School of
Engineering, Kyoto University, Kyoto 606-8501, Japan.
9
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J. AM. CHEM. SOC. 2002, 124, 10974-10975
10.1021/ja026089l CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
t-Bu substitution on the phenyl group of ZnTPP, affecting the ∆Hq
obs
value. The negative ∆Sq values in Table 1 are consistent with
obs
the complex formation prior to electron transfer. In the electron
self-exchange reaction, there is no net change in solvation before
and after the electron transfer, when the solvent reorganization
energy of electron-transfer becomes smaller as the solvent polarity
decreases.17 This may be the reason a negative activation enthalpy
is observed for the ZnT(t-Bu)PP•+/ZnT(t-Bu)PP system in less polar
solvents such as CH2Cl2, CHCl3, and toluene as compared to the
case in MeCN (Table 1).
Acknowledgment. This work was partially supported by Grants-
in-Aid for Scientific Research on Priority Area (Nos. 11228205
and 13440216) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Supporting Information Available: Data for electron self-
exchange for the ZnT(t-Bu)PP/ZnT(t-Bu)PP•+ and ZnTPP/ZnTPP•+
systems (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 2. Arrhenius plots of electron self-exchange reaction between ZnT-
(t-Bu)PP•+ and ZnT(t-Bu)PP in different solvents.
Table 1. Activation Parameters (∆Hq and ∆Sqobs) for the
obs
References
ZnT(t-Bu)PP/ZnT(t-Bu)PP•+ and ZnTPP/ZnTPP•+ Systems
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K. AdV. Phys. Org. Chem. 2000, 35, 193. (c) Kim, J. H.; Lindeman, S.
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(3) (a) Fukuzumi, S. In AdVances in Electron-Transfer Chemistry; Mariano,
P. S., Ed.; JAI Press: Greenwich, CT, 1992; pp 67-175. (b) Fukuzumi,
S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am. Chem. Soc. 1987, 109,
305.
•+
•+
ZnT(t-Bu)PP
ZnTPP
q
a
q
a
q
a
q
a
∆H obs
∆S obs
kJ mol-1
∆H obs
∆S obs
kJ mol-1
kJ mol-1
kJ mol-1
MeCN
CH2Cl2
CHCl3
toluene
0.6
-0.2
-1.0
-0.9
-30
-14
-17
-32
2.2
0
-
2.2
-21
-22
-
(4) Mulliken, R. S.; Person, W. B. Molecular Complexes; A Lecture and
Reprint Volume; Wiley-Interscience: New York, 1969.
-24
(5) (a) Chanon, M.; Tobe, M. Angew. Chem., Int. Ed. Engl. 1982, 21, 1. (b)
Chanon, M.; Rajzmann, M.; Chanon, F. Tetrahedron 1990, 46, 6193. (c)
Jones, G., II. In Photoinduced Electron Transfer; Fox, M. A., Chanon,
M., Eds.; Elsevier: Amsterdam, 1988; Part A, p 245.
(6) Fukuzumi, S.; Ohkubo, K.; Tokuda, Y.; Suenobu, T. J. Am. Chem. Soc.
2000, 122, 4286.
(7) (a) Zaman, K. M.; Yamamoto, S.; Nishimura, N.; Maruta, J.; Fukuzumi,
S. J. Am. Chem. Soc. 1994, 116, 12099. (b) Yamamoto, S.; Sakurai, T.;
Liu, Y. J.; Sueishi, Y. Phys. Chem. Chem. Phys. 1999, 1, 833.
(8) For other examples of negative temperature effects, see: (a) Sergeev, G.
B.; Serguchev, Yu. A.; Smirnov, V. V. Russ. Chem. ReV. 1973, 42, 697.
(b) Kim, H.-B.; Kitamura, N.; Kawanishi, Y.; Tazuke, S. J. Am. Chem.
Soc. 1987, 109, 2506.
a The experimental error is (10%.
at a lower temperature. The rate constants (kex) of the electron
exchange reactions between ZnT(t-Bu)PP•+ and ZnT(t-Bu)PP were
determined using eq 1,
kex ) 1.52 × 107(∆Hmsl - ∆H0msl)/{(1 -
Pi)[ZnT(t-Bu)PP]} (1)
(9) (a) Dolphin, D.; Felton, R. H. Acc. Chem. Res. 1974, 7, 26. (b) Fukuzumi,
S. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: San Diego, 2000; Vol. 8, pp 115-151.
(10) The ESR resolution of porphyrin π-radical cations is known to be affected
by a variety of factors: the peripheral substitution, axial ligands, and
solvents. See: (a) Felton, R. H.; Dolphin, D.; Borg, D. C.; Fajer, J. J.
Am. Chem. Soc. 1969, 91, 196. (b) Ichimori, K.; Ohya-Nishiguchi, H.;
Hirota, N. Bull. Chem. Soc. Jpn. 1988, 61, 2753. (c) Huber, M.; Galili,
T.; Mo¨bius, K.; Levanon, H. Isr. J. Chem. 1989, 29, 65. (d) Dave, P. C.;
Srinivas, D. Eur. J. Inorg. Chem. 2000, 447.
(11) The well-resolved ESR spectrum enabled the determination of hfcs due
to four nitrogens, eight â-pyrrole protons, eight o-phenyl protons, four
p-phenyl protons as shown in Figure 1. For more accurate assignment of
hfcs of ZnTPP•+ by ENDOR, see ref 10c. It should be noted that the
ESR resolution is highly dependent on the excess porphyrin concentration
due to the rapid electron exchange.
where ∆Hmsl and ∆H0msl are the maximum slope line widths of the
ESR spectra in the presence and absence of excess ZnT(t-Bu)PP,
respectively, and Pi is a statistical factor, which can be taken as
nearly zero.12 From the slopes of the linear plots of ∆Hmsl and [ZnT-
(t-Bu)PP] are obtained the electron self-exchange rate constants
(kex) in different solvents at various temperatures.13 The activation
parameters are determined from the Arrhenius plots in Figure 2,14,15
where the positive slopes for the data in toluene, CH2Cl2, and CHCl3
afford the negative activation enthalpies. In MeCN, however, a
normal negative slope is obtained to afford the positive ∆Hq
obs
value. The activation parameters were also determined for electron
self-exchange between ZnTPP•+ and ZnTPP (TPP2- ) tetraphen-
ylporphyrin dianion, see Supporting Information, S4). The results
(12) (a) Chang, R. J. Chem. Educ. 1970, 47, 563. (b) Cheng, K. S.; Hirota, N.
In InVestigation of Rates and Mechanisms of Reactions; Hammes, G. G.,
Ed.; Wiley-Interscience: New York, 1974; Vol. VI, p 565.
(13) The ∆H0 value in the absence of excess porphyrin slightly varies,
are summarized in Table 1. The ∆Hq values of the ZnTPP•+/
msl
obs
depending on temperature and solvent (see Supporting Information S2).
The same hyperfine coupling constants were used to fit well the ESR
spectra using different ∆Hmsl values irrespective of temperature or solvent.
Thus, the observed line broadening due to electron self-exchange was in
ZnTPP system are larger than those of the ZnT(t-Bu)PP•+/ZnT-
(t-Bu)PP system.
the slow-exchange region.12 At each temperature, the ∆Hmsl - ∆H0
The negative activation enthalpy indicates that electron self-
exchange reaction proceeds via an intermediate, the energy of which
is lower than that of the reactant pair, and the energy difference
between the reactant pair and the intermediate is larger than the
activation energy from the intermediate. In general, an electron-
transfer reaction proceeds via a precursor complex formed between
an electron donor and an acceptor.15 In the case of electron self
exchange, an electron donor is ZnT(t-Bu)PP which may form a
charge-transfer π complex with an electron acceptor, ZnT(t-Bu)-
PP•+.16 The formation of such a π complex may be sensitive to the
msl
value is plotted against excess porphyrin concentration (S3).
(14) The diffusion is taken into account in the Arrhenius plot, based on the
relation: (kex-1 - kdiff
)
-1 ) Z exp(-∆Gq/RT), where kdiff is the diffusion
-1
rate constant and Z is the collision frequency.15
(15) (a) Marcus, R. A. Annu. ReV. Phys. Chem. 1964, 15, 155. (b) Kavarnos,
G. J. Fundamentals of Photoinduced Electron Transfer; Wiley-VCH:
New York, 1993.
(16) No charge-transfer (CT) band between ZnT(t-Bu)PP•+ and ZnT(t-Bu)-
TPP was observed under the present experimental conditions.
(17) Fukuzumi, S.; Ohkubo, K.; Suenobu, T.; Kato, K.; Fujitsuka, M.; Ito, O.
J. Am. Chem. Soc. 2001, 123, 8459.
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