Activation of electron transfer reduction of p-benzoquinone derivatives by intermolecular regioselective hydrogen bond formation

Article abstract of DOI:10.1039/b205517a

Electron transfer reduction of p-benzoquinones by cobalt tetraphenylporphyrin is enhanced significantly by the presence of o-bis(phenylcarbamoylmethyl)benzene (o-L) due to the regioselective hydrogen bond formation between the corresponding semiquinone ra

Full text of DOI:10.1039/b205517a

Activation of electron transfer reduction of p-benzoquinone derivatives
by intermolecular regioselective hydrogen bond formation
Shunichi Fukuzumi,* Hironori Kitaguchi, Tomoyoshi Suenobu and Seiji Ogo
Department of Material and Life Science, Graduate School of Engineering, Osaka University, CREST, Japan
Science and Technology Corporation (JST), Suita, Osaka 565-0871, Japan.
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Received (in Cambridge, UK) 7th June 2002, Accepted 22nd July 2002
First published as an Advance Article on the web 5th August 2002
Electron transfer reduction of p-benzoquinones by cobalt
tetraphenylporphyrin is enhanced significantly by the pres-
ence of o-bis(phenylcarbamoylmethyl)benzene (o-L) due to
the regioselective hydrogen bond formation between the
corresponding semiquinone radical anions and o-L, whereas
m- and p-isomers (m-L and p-L) have no effect on the
electron transfer equilibrium or the rate.
transfer between CoIITPP and Cl4Q is expected to be in
equilibrium at 298 K [eqn. (1)].
(1)
Electron transfer reactions are regulated through non-covalent
interactions such as hydrogen bonding which play a crucial role
in a variety of biological redox reactions as well as molecular
recognition to form higher-order structures of proteins, nucleic
acid base pairs and enzyme–substrate complexes.1 Hydrogen
bonding is a specific recognition motif which can selectively
stabilize or destabilize different oxidation states of substrates,
thus modulating the electron transfer processes.2 Rotello and
coworkers have developed excellent examples of recognition-
modulated electron transfer processes in host–guest complexes
in which multi-point hydrogen bondings of diacyldiaminopyr-
idine derivatives to flavin modulate the redox potentials of
flavin.3,4 Strong hydrogen bonding is formed by the multi-point
recognition of flavin which has two oxygen atoms and nitrogen
atom in proximity.3,4 In contrast, p-benzoquinones have two
oxygen atoms far from each other and it may be difficult to form
strong hydrogen bonds by single point recognition. In photo-
synthesis, however, two p-benzoquinones termed Qa and Qb act
in concert to enable efficient charge separation to take place.5,6
Qa and Qb are often identical quinones: plastoquinone in higher
plants and ubiquinone in bacterial systems.7 Differences in the
nature of the hydrogen-bonding interactions of both quinones
have been suggested to result in the differing functions
observed; i.e., specific hydrogen bonds to nearby amino acid
residues are able to tailor the quinone to perform a specific
function.8 Hydrogen bonding is expected to be more favorable
in the one-electron reduced state, semiquinone radical anion,
because of the negative charge present in the oxygen atom of the
radical anion. However, there is no example for strong
hydrogen bond formation of p-benzoquinone or semiquinone
radical anion with external hydrogen bond donors in solution,
which can modulate the electron transfer reduction of p-
benzoquinone.9
We report herein the first example of regiospecific inter-
molecular hydrogen bond formation between the radical anions
of p-benzoquinone derivatives and hydrogen bond donors in a
polar solvent such as dimethyl sulfoxide (DMSO), which can
activate the electron transfer reduction of p-benzoquinone
derivatives by a one-electron reductant. Three regioisomers of
o-, m-, and p-bis(phenylcarbamoylmethyl)benzene (denoted as
o-L, m-L and p-L, respectively) are employed as a hydrogen
bond donor to demonstrate regioselective hydrogen bond
formation. Quite remarkably, only o-L is effective to activate
the electron transfer reduction of p-benzoquinone derivatives.
Cobalt tetraphenylporphyrin (CoIITPP) is used as an electron
donor to reduce p-benzoquinone derivatives.10 Since the one-
electron oxidation potential of CoIITPP in DMSO (E0ox vs. SCE
= 0.03 V) is quite close to the one-electron reduction potentials
of p-chloranil (Cl4Q: E0red vs. SCE = 20.01 V),11 the electron
In fact, addition of an excess amount of Cl4Q to a deaerated
DMSO solution of CoIITPP results in partial conversion of
CoIITPP to CoIIITPP+ as indicated by the appearance of the
absorption band due to CoIIITPP+ at 434 nm.12 Further addition
of Cl4Q causes further increase of the absorption band due to
CoIIITPP+ to approach a constant value. Formation of Cl4Q·2 is
confirmed by a clear isotropic ESR signal with no hyperfine
splitting at g = 2.0058 in deaerated DMSO.13
According to eqn. (1), the equilibrium constant (K) is
obtained from the plot of (A 2 A0)21 vs. [Cl4Q]21 where A0 is
the absorbance of CoIITPP at 434 nm. From the slope and
intercept the electron transfer equilibrium constant (K = 3.7 3
104 M21) is determined. Similarly the K value for p-fluoranil
(F4Q) was determined as 2.2 3 104 M21. The rate of formation
of CoIIITPP+ obeys pseudo-first-order kinetics in the presence
of a large excess Cl4Q and the pseudo-first-order rate constant
(kobs) increases linearly with increasing Cl4Q concentration.
According to eqn. (1), kobs is expressed by eqn. (2),
(2)
kobs = k2et + ket [Cl4Q]
where ket and k2et are the forward and back electron transfer rate
constants, respectively. From the slope and intercept are
determined the ket and k2et values from which the K ( = ket/k2et
)
value was obtained as 4.1 3 104 M21. This value agrees with
the K value (3.7 3 104 M21) determined independently from the
spectral titration.
When a hydrogen bond donor (o-L) is added to the solution,
the electron transfer equilibrium is shifted to the product side
due to the stabilization of semiquinone radical anion (Cl4Q·2)
by hydrogen bonding with o-L. The K value increases linearly
with increasing o-L concentration as shown in Fig. 1, indicating
that Cl4Q·2 forms a 1+1 complex with o-L as expressed by eqn.
(3).
(3)
In contrast, m- and p-L do not affect the equilibrium at all even
at high concentrations of L. Similarly, only o-L shifted the
electron transfer equilibrium between CoIITPP and F4Q. It has
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Fig. 3 Optimized structure of the Cl2Q·2–o-L complex.16
Fig. 1 Plots of K vs. [L] for electron transfer from CoIITPP to Cl4Q, in the
presence of o- (5), m- (:) and, p-L (-) in deaerated DMSO at 298 K.
The ESR spectrum of the Cl2Q·2–o-L complex produced in
the electron transfer reduction of Cl2Q by CoIITPP in the
presence of o-L (0.10 M) was observed at 298 K. The g value
(2.0056) and the proton hyperfine coupling constants (aH
=
been confirmed that a mono-chelating compound (benzanilide)
has no effect as the hydrogen bond donor in the present system.
This indicates that the hydrogen bond formation is quite
sensitive to the geometry of interaction between the dichelating
hydrogen bond donor and the semiquinone radical anion.
When Cl4Q is replaced by a weaker electron acceptor,
0.268 mT) are virtually the same as observed in the absence of
o-L (g = 2.0056, aH = 0.268 mT). This indicates the
interaction between Cl2Q·2 and o-L is largely electrostatic.
This work was partially supported by a Grant–in–Aid for
Scientific Research Priority Area (No. 13031059) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
2,6-dichloro-p-benzoquinone (Cl2Q: E0 vs. SCE = 20.19
red
V), electron transfer from CoIITPP to Cl2Q becomes en-
dergonic, and the electron transfer equilibrium is shifted to the
reactants side.14 When o-L is added to the CoIITPP–Cl2Q
system, the equilibrium is drastically shifted to the product side
and the observed first-order rate constant of electron transfer
(kobs) increases linearly with increasing o-L concentration as
shown in Fig. 2.15 No such enhancement of electron transfer
was observed with m-L or p-L.
Notes and references
1 (a) S. Scheiner, Hydrogen Bonding, Oxford University Press, Oxford,
1997; (b) S. Fukuzumi, in Electron Transfer in Chemistry, ed. V.
Balzani, Wiley-VCH, Weinheim, 2001, vol. 4, pp. 3–67.
2 (a) A. Niemz and V. M. Rotello, Acc. Chem. Res., 1999, 32, 44; (b) V.
M. Rotello, in Electron Transfer in Chemistry, ed. V. Balzani, Wiley-
VCH, Weinheim, 2001, vol. 4, pp. 68–87.
3 E. Breublinger, A. Niemz and V. M. Rotello, J. Am. Chem. Soc., 1995,
117, 5379.
4 A. O. Cuello, C. M. McIntosh and V. M. Rotello, J. Am. Chem. Soc.,
2000, 122, 3517.
5 (a) B. G. Malmström, Acc. Chem. Res, 1993, 26, 332; (b) S. Ferguson-
Miller and G. T. Babcock, Chem. Rev., 1996, 96, 2889; (c) C. W.
Hoganson and G. T. Babcock, Science, 1997, 277, 1953.
6 M. Y. Okamura and G. Feher, Annu. Rev. Biochem., 1992, 61, 861.
7 R. A. Isaacson, E. C. Abresch, F. Lendzian, C. Boullais, M. L. Paddock,
C. Mioskowski, W. Lubitz and G. Feher, in The Reaction Center of
Photosynthtic Bacteria: Structure and Dynamics, ed. M.-E. Michel-
Beyerle, Springer Verlag, Berlin, 1996, pp. 353–367.
8 (a) P. J. OAMalley, J. Am. Chem. Soc., 1998, 120, 5093; (b) P. J.
OAMalley, J. Phys. Chem. A, 1998, 102, 248.
9 For the intramolecular hydrogen bond formation of p-benzosemiqui-
none radical anion, see: S. Fukuzumi, Y. Yoshida, K. Okamoto, H.
Imahori, Y. Araki and O. Ito, J. Am. Chem. Soc., 2002, 124, 6794.
10 S. Fukuzumi and K. Ohkubo, Chem. Eur. J, 2000, 6, 4532.
11 Electrochemical measurements were performed on a BAS 100B
electrochemical analyzer in deaerated DMSO containing 0.01 M
Fig. 2 Plots of kobs vs. [Cl2Q] for electron transfer from CoIITPP (5.0 3
1026 M) to Cl2Q in the presence of o-L (0.10 (5), 0.30 (:) M) in DMSO
at 298 K.
2
Ph4As+ClO4 as a supporting electrolyte at 298 K. A three-electrode
system was used with a platinum working electrode, a platinum wire
counter electrode, and an Ag/AgNO3 (0.01 M) reference electrode.
12 Observation of the Cl4Q·2 absorption band is obscured due to the
overlap with the absorption band of CoIIITPP+ which has a much larger
absorption coefficient than Cl4Q·2.
The ortho-selective activation of electron transfer from
CoIITPP to p-benzoquinone derivatives by L described above is
rationalized by the computed structure of the Cl2Q·––o-L
complex in which two hydrogens of o-L interact with the
carbonyl oxygen of Cl2Q·– as shown in Fig. 3, where the
hydrogen bond distance is 1.64 Å.16,17 A similar structure was
obtained for the Cl2Q·––m-L complex, but the hydrogen bond
distance (1.82 Å) is significantly longer than the distance in the
Cl2Q·––o-L complex. In the case of the Cl2Q·2–p-L complex,
only one hydrogen interacts with the carbonyl oxygen of Cl2Q·2
because of the geometrical restriction. In such a case, only o-L
is effective in accelerating the electron transfer reduction of
Cl2Q by regioselective hydrogen bond formation in DMSO.17
13 No superhyperfine due to the Co nucleus was observed, indicating no
covalent interaction in the CoIIITPP+–Cl4Q·2 complex.
14 The K value of Cl2Q obtained is 78 M21 which is more than 400 times
smaller than that of Cl4Q.
15 From the plot of kobs vs. [Cl2Q], the K ( = ket/k2et) value obtained at
[Cl2Q] = 0.3 M is 1.9 3 104 M21 which is ca. 240 times larger than the
value of the o-L free system.
16 Theoretical calculations were performed on a COMPAQ DS20E
computer using the Amsterdam Density Functional (ADF) program
version 1999.02 developed by Baerends et al.17 The two phenyl groups
in L were replaced by methyl groups in the calculation.
17 G. Te Velde and E. J. Baerends, J. Comput. Phys., 1992, 99, 84.
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