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Communication
III
Table 2 Kinetic data for oxidation reactions with 2 in B–R buffer at 297 K
In conclusion, we have synthesized a novel Ru –OH
À1
À1
2
complex, 2, with PY5Me as a pentadentate polypyridyl ligand
Substrate
k
H
(s
)
k
D
(s
)
H D
k /k
II
by electrochemical oxidation of the corresponding Ru –aqua
complex in B–R buffer. We have experimentally elucidated the
H
H
H
H
H
H
2
2
2
2
2
2
Q(OMe)
Q
QCl
QF
56
53
13
10
52
48
13
1.1
1.1
1.0
1.1
1.7
1.7
1.2
III
reactivity of a Ru –OH complex in substrate oxidation in
8.8
water on the basis of kinetic analysis. The oxidation reactions
of substrates by 2 in water involve a pre-equilibrium process
based on adduct formation by hydrogen bonding between the
QCl
QF
2
1.9
1.7
2.1
1.1
1.0
1.7
4
AS
III
20
Ru –OH complex and the substrates. The oxidation reaction
mechanism with 2 in water switches from HAT to ET, depend-
ing on the oxidation potential of substrates employed. The
switching of the reaction mechanisms for substrate oxidation
reactions has never been observed without assistance by
Lewis-acidic metal ions to control the reduction potential of
were utilized as substrates for the oxidation reactions with 2
1
4
(Fig. S12 in the ESI†). The three substrates also exhibited the
pre-equilibrated adduct formation (Fig. S8 in the ESI†) and the
DH1 values also showed a similar dependence on the pKa1 values of
the substrates (Table 1). The larger rate constants for the oxidation
5
the oxidant. Here, mechanistic switching resulting from the
2 2 2
of H Q, H QF and H QCl compared to those for the oxidation of
the other three substrates probably derive from the lower redox
potential of substrates (Table 1 and Fig. S8 in the ESI†). In
remarkable increase of the l value of ET in water is observed,
which enables the slow ET from substrates showing positive
ÀDGET. This work may provide a valuable basis to elucidate the
reactivity of a trivalent metal–hydroxo complex in oxidation
reactions of organic substrates in water in relevance to those
catalysed by lipoxygenases.
addition, the oxidation reactions of H
2 2 2
Q, H QF, H QCl and
H Q(OMe) exhibited no KIE (1.1 for H Q, 1.0 for H QCl, 1.1 for
2
2
2
H QF, and 1.1 for H Q(OMe), and see Fig. S11 in the ESI†),
2
2
indicating that the oxidation reactions of the four substrates
5
proceed through electron transfer (ET) from substrates to 2.
A plot of the logarithm of the rate constants at 297 K relative to Notes and references
the driving force of ET (ÀDGET) was made to shed light on the
1
2
(a) T. M. Jonsson, H. Glickman, S. J. Sun and J. P. Klinman, J. Am.
change of the reaction mechanism from HAT to ET in the substrate
Chem. Soc., 1996, 118, 10319; (b) C.-C. Hwang and C. B. Grissom,
J. Am. Chem. Soc., 1994, 116, 795; (c) E. R. Lewis, E. Johansen and
T. R. Holman, J. Am. Chem. Soc., 1999, 121, 1395.
oxidation reactions by 2 (Table S1 in the ESI†). As shown in Fig. 5,
substrates having smaller ÀDG (o0.5 eV) are oxidized by the HAT
ET
(a) C. R. Goldsmith and T. D. P. Stack, Inorg. Chem., 2006, 45, 6048;
mechanism with small driving-force dependence. In stark contrast,
(
b) C. R. Goldsmith, A. P. Cole and T. D. P. Stack, J. Am. Chem. Soc.,
those having larger ÀDGET (40.5 eV) are oxidized by the ET
2005, 127, 9904.
18
3 P. J. Donoghue, J. Tehranchi, C. J. Cramer, R. Sarangi, E. I. Solomon
and W. B. Tolman, J. Am. Chem. Soc., 2011, 133, 17602.
(a) C. R. Goldsmith and T. D. P. Stack, Inorg. Chem., 2006, 45, 6048;
(b) C. R. Goldsmith, A. P. Cole and T. D. P. Stack, J. Am. Chem. Soc.,
mechanism and the rate constants fit a Marcus plot for intra-
molecular non-adiabatic ET with l of 1.31 eV and the electronic
4
À1 12
coupling matrix element (V) of 0.0011 cm
. The switching point
2
005, 127, 9904.
of the reaction mechanisms is estimated to be located at –DGET
=
5
(a) Y. Morimoto, J. Park, T. Suenobu, Y.-M. Lee, W. Nam and
S. Fukuzumi, Inorg. Chem., 2012, 51, 10025; (b) J. Park,
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Soc., 2011, 133, 5236; (c) J. Park, Y. Morimoto, Y.-M. Lee, W. Nam
and S. Fukuzumi, Inorg. Chem., 2014, 53, 3618.
0
.51 eV. The l value and the ÀDGET of the switching point for the
reaction mechanisms of substrate oxidation with 2 are larger than
those of other high-valent metal complexes that are proposed
5
to oxidize substrates through ET. These results indicate that the
6
7
(a) T. J. Meyer, Acc. Chem. Res., 1989, 22, 163; (b) S. L.-F. Chan,
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l value becomes larger and ET from substrates to an oxidant
19
becomes more difficult in a polar solvent like water.
4
0, 1875.
(a) T. Ishizuka, S. Ohzu and T. Kojima, Synlett, 2014, 1667;
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(
S. Fukuzumi, Angew. Chem., Int. Ed., 2008, 47, 5772; (c) T. Kojima,
Y. Hirai, T. Ishizuka, Y. Shiota, K. Yoshizawa, K. Ikemura, T. Ogura
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T. Ishizuka, Y. Hirai, H. Jiang, M. Sakaguchi, T. Ogura, S. Fukuzumi
and T. Kojima, Chem. Sci., 2012, 3, 3421.
8
9
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(
b) P. Neubold, K. Wieghardt, B. Nuber and J. Weiss, Angew. Chem.,
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3
2
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013, 8, 2046.
1
1
0 S. Perrier, T. C. Lau and J. K. Kochi, Inorg. Chem., 1990, 29, 4190.
Fig. 5 A plot of the logarithm of rate constants relative to the driving
forces of ET (ÀDGET). All the data were determined at 297 K in B–R buffer
1 E. A. U¨ nal, D. Wiedemann, J. Seiffert, J. P. Boyd and A. Grohmann,
Tetrahedron Lett., 2012, 53, 54.
2 See the ESI†.
(
pH 1.8). The blue line is drawn by using eqn (3) in the ESI† with l = 1.31 eV
1
and the red dashed line indicates a linear dependence of log k
H
on ÀDGET
13 R. Wang, J. G. Vos, R. H. Scmehl and R. Hage, J. Am. Chem. Soc.,
1992, 114, 1964.
for HAT reactions.
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