Proton-coupled electron transfer of ruthenium(III)-Pterin complexes: A mechanistic insight

Article abstract of DOI:10.1021/ja904386r

Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and proton-coupled electron transfer (PCET) processes. Thermodynamic parameters

Full text of DOI:10.1021/ja904386r

Published on Web 07/28/2009
Proton-Coupled Electron Transfer of Ruthenium(III)-Pterin
Complexes: A Mechanistic Insight
†
,†,‡
,§
Soushi Miyazaki, Takahiko Kojima,*
James M. Mayer,* and
,
†
Shunichi Fukuzumi*
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity,
and SORST (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan, and Department of
Chemistry, Campus Box 351700, UniVersity of Washington, Seattle, Washington 98195-1700
Received May 30, 2009; E-mail: kojima@chem.tsukuba.ac.jp; mayer@chem.washington.edu;
fukuzumi@chem.eng.osaka-u.ac.jp
Abstract: Ruthenium(II) complexes having pterins of redox-active heteroaromatic coenzymes as ligands
were demonstrated to perform multistep proton transfer (PT), electron transfer (ET), and proton-coupled
a
electron transfer (PCET) processes. Thermodynamic parameters including pK and bond dissociation energy
(
BDE) of multistep PCET processes in acetonitrile (MeCN) were determined for ruthenium-pterin complexes,
II
II
II
-
II
-
[
Ru (Hdmp)(TPA)](ClO
TPA)]ClO (4) (Hdmp ) 6,7-dimethylpterin, Hdmdmp ) N,N-dimethyl-6,7-dimethylpterin, TPA ) tris(2-
pyridylmethyl)amine), all of which had been isolated and characterized before. The BDE difference between
4 2 4 2 4
) (1), [Ru (Hdmdmp)(TPA)](ClO ) (2), [Ru (dmp )(TPA)]ClO (3), and [Ru (dmdmp )-
(
4
III
-
2+
-1
1
and one-electron oxidized species, [Ru (dmp )(TPA)] , was determined to be 89 kcal mol , which was
large enough to achieve hydrogen atom transfer (HAT) from phenol derivatives. In the HAT reactions from
III
-
2+
phenol derivatives to [Ru (dmp )(TPA)] , the second-order rate constants (k) were determined to exhibit
a linear relationship with BDE values of phenol derivatives with a slope (-0.4), suggesting that this HAT
is simultaneous proton and electron transfer. As for HAT reaction from 2,4,6-tri-tert-buthylphenol (TBP;
-
1
III
-
2+
q
BDE ) 79.15 kcal mol ) to [Ru (dmp )(TPA)] , the activation parameters were determined to be ∆H )
-1
q
-1
-1
1
.6 ( 0.2 kcal mol and ∆S ) -36 ( 2 cal K mol . This small activation enthalpy suggests a hydrogen-
bonded adduct formation prior to HAT. Actually, in the reaction of 4-nitrophenol with [Ru (dmp )(TPA)]
III
-
2+
,
the second-order rate constants exhibited saturation behavior at higher concentrations of the substrate,
and low-temperature ESI-MS allowed us to detect the hydrogen-bonding adduct. This also lends credence
to an associative mechanism of the HAT involving intermolecular hydrogen bonding between the
deprotonated dmp ligand and the phenolic O-H to facilitate the reaction. In particular, a two-point hydrogen
bonding between the complex and the substrate involving the 2-amino group of the deprotonated pterin
ligand effectively facilitates the HAT reaction from the substrate to the Ru(III)-pterin complex.
Introduction
Proton-coupled electron transfer (PCET) is one of the most
bond. With regard to PCET involving transition-metal com-
plexes, a number of systems have been examined, and the
3-6
mechanistic details have been explored.
In particular, metal-
fundamental reactions and plays important roles in energy
conversion in biological systems, including water oxidation at
mediated hydrogen atom transfer (HAT) has attracted much
attention due to the relevance to many chemical conversions of
organic molecules in biological systems, including reactions
1
the oxygen evolving center in photosynthesis and four-electron
2
reduction of dioxygen by cytochrome c oxidase in respiration.
7
catalyzed by soybean lipoxygenase. PCET reactions can
In the course of PCET, the redox reaction occurs in a
synchronized manner with protonation or deprotonation to
regulate the redox potential of the redox-active entity for
accepting and releasing electron(s). PCET reactions involve the
transfer of one electron and one proton from the same chemical
•
proceed in four different patterns: (1) H transfer, (2) proton
transfer (PT) followed by electron transfer (ET), (3) ET followed
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†
Osaka University.
Present address: Department of Chemistry, Graduate School of Pure
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337–369. (b) Stubbe, J.; Nocera, D. G.; Yee, C. S.; Chang, M. C. Y.
Chem. ReV. 2003, 103, 2167–2201. (c) Chang, C. J.; Chang, M. C. Y.;
Damrauer, N. H.; Nocera, D. G. Biochim. Biophys. Acta 2004, 1655,
13–28.
‡
and Applied Sciences, University of Tsukuba, Tennou-dai, Tsukuba, Ibaraki
3
05-8571, Japan.
§
University of Washington.
(
1) (a) Paddock, M. L.; Feher, G.; Okamura, M. Y. Proc. Natl. Acad.
Sci. U.S.A. 2000, 97, 1548–1553. (b) Feher, G. Photosynth. Res. 1998,
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10.1021/ja904386r CCC: $40.75  2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 11615–11624 9 11615

A R T I C L E S
Miyazaki et al.
by PT, and (4) simultaneous ET and PT in a concerted manner.
The reactions in (1) and (4) could be categorized as HAT. For
the PCET reactions, mechanistic insights can be provided
Chart 1. Schematic Description of Structures of 1 (R ) H) and 2
a
(
3
R ) CH ), and Atom Numbering Scheme
through the thermodynamic approach using the thermochemical
8
a
cycle involving redox potentials and pK values of a reagent
and the kinetic study on the reaction. For example, rate constants
II
of hydrogen atom abstraction reactions between Fe -tris-
(
biimidazoline) complexes and substrates are well analyzed in
9
-11
light of the Marcus cross relation.
PCET is the key step for functions of heteroaromatic coenzymes
involved in a variety of biological redox processes, as represented
2
,12
by NADH (dihydronicotinamide adenine dinucleotide),
1
3,14
15,16
flavins,
and pterins.
Those heteroaromatics including
nitrogen atom(s) as proton-accepting site(s) undergo dearoma-
tization in the course of reduction with protonation to recover
their aromaticity during the oxidation with deprotonation.
Among those, pterins can undergo multiple PCET from the fully
oxidized biopterins to fully reduced tetrahydropterins by ac-
a
3
The pterin ligands in 3 (R ) H) and 4 (R ) CH ) are deprotonated
from the N-1 position.
Scheme 1. Multistep Proton-Coupled Electron Transfer Process of
1
7
cepting and releasing up to four electrons and four protons.
Ruthenium Complexes with Redox-Noninnocent Pterin Ligand
(
HP ) Pterin)
Pterins locate in the vicinity of metal ions to conduct many
biologically important redox reactions including dioxygen
activation in collaboration with metal ions by manipulating
18
PCET. So far, metal complexes of pterins have gathered much
(
7) (a) Hatcher, E.; Soudackov, A. V.; Hammes-Schiffer, S. J. Am. Chem.
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17a,19
complexes having redox-noninnocent ligands.
Among
D. A.; Borden, W. T.; Mayer, J. M. J. Am. Chem. Soc. 2009, 131,
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exhibit PCET with clear reversibility for both the metal center
4
729–4743. (b) Wu, A.; Mayer, J. M. J. Am. Chem. Soc. 2008, 130,
1
4745–14754.
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(
12) (a) Fukuzumi, S. In AdVances in Electron Transfer Chemistry; Mariano,
P. S., Ed.; JAI Press: Greenwich, CT, 1992; pp 67-175. (b) Fukuzumi,
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M., Eds.; Elsevier: Amsterdam, 1988; Part C, Chapter 10, pp 578-
and the pterin ligands. We have reported on the synthesis and
characterization of the ruthenium-pterin complexes as shown
II
21
II
4 2
in Chart 1, [Ru (Hdmp)(TPA)](ClO ) (1), [Ru (Hdmdmp)-
22
2
1
II
-
6
35. (c) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am.
(TPA)](ClO
)
(2),
[Ru (dmp )(TPA)]ClO
(3),
and
4
2
4
Chem. Soc. 1987, 109, 305–316.
II
-
22,23
[
Ru (dmdmp )(TPA)]ClO (4),
4
in light of their structures
(
13) (a) De Colibus, L.; Mattevi, A. Curr. Opin. Struct. Biol. 2006, 16,
and redox properties including PCET. No report has appeared,
however, to demonstrate any reactivity of transition-metal
complexes of pterins toward external entities, including PCET
reactions with substrates.
7
22–728. (b) Alberts, B.; Bray, D.; Lewis, J.; Raff, M.; Roberts, K.;
Watson, J. D. Molecular Biology of the Cell, 3rd ed.; Garland
Publishing: New York, 1994; pp 659-662.
(
14) (a) Tollin, G. In Electron Transfer in Chemistry; Balzani, V., Ed.;
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S.; Tanaka, T. In Photoinduced Electron Transfer; Fox, M. A., Chanon,
M., Eds.; Elsevier: Amsterdam, 1988; Part C, pp 636-687.
We report herein the thermochemical study of the PCET
system of those Ru-pterin complexes, as presented in Scheme
(
15) (a) Bobst, A. M. Nature 1968, 220, 164–165. (b) Mastropaolo, D.;
Camerman, A.; Camerman, N. Science 1980, 210, 334–336.
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1
a
, on the basis of the determination of pK values and redox
(
potentials in acetonitrile. We also report the detailed kinetic
analysis including the kinetic isotope effects on HAT reactions
from phenol derivatives to Ru(III)-pterin complexes, which
reveals the formation of a hydrogen-bonded adduct prior to the
HAT reactions. Thus, the present study provides valuable
1
1
989, 816–818. (b) Fukuzumi, S.; Tanii, K.; Tanaka, T. Chem. Lett.
989, 35–38.
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17) (a) Burgmayer, S. J. N. Struct. Bonding (Berlin) 1998, 92, 67–119.
(
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39–950. (g) Crane, B. R.; Arval, A. S.; Ghosh, D. K.; Wu, C.; Getzoff,
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1616 J. AM. CHEM. SOC. 9 VOL. 131, NO. 32, 2009

Proton-Coupled Electron Transfer of Ru-Pterin Complexes
A R T I C L E S
mechanistic insights into HAT reactions of transition-metal
complexes of pterins for the first time.
electrochemical quartz cell (path length of 0.5 mm) using a three-
electrode arrangement: light transparent platinum gauze (100 mesh,
0.07 mm diameter) as a working electrode, a platinum wire counter
Experimental Section
electrode, and an Ag/AgNO (0.01 M) reference electrode. Poten-
3
tials were applied with a HOKUTO DENKO HSV-100 electro-
chemical analyzer. The electrochemical reaction was monitored with
a Shimadzu UV-3100 spectrophotometer at room temperature.
Kinetic Measurements. Kinetic measurements were performed
by a UNISOKU RSP-601 stopped-flow spectrophotometer equipped
with a MOS type highly sensitive photodiode array using a double-
mixing mode except for the reaction with 4-nitrophenol. Ru(II)-pterin
General. All chemicals available were purchased from the
appropriate commercial sources. MeCN was purified by distillation
over CaH . For some of experiments, the MeCN used was Burdick
2
and Jackson low water brand, which was stored in an argon-
pressurized steel drum plumbed directly into a glovebox. Phenols
2
4
were further purified by the standard methods. 2,4,6-Tri-tert-
2
5
21
buthylphenol-d, [Ru(Hdmp)(TPA)](ClO
)
4 2
(1), [Ru(Hdmdmp)-
-5
complexes (3 and 4; 2.5 × 10 M) were first mixed with
2
1
-
22
-
(
(
TPA)](ClO
4
)
(4)
2
(2), [Ru(dmp )(TPA)]ClO
4
(3), and [Ru(dmdmp )-
were prepared according to literature methods.
III
-5
[
Fe (bpy)
3
](PF
6
)
3
(2.5 × 10 M) to generate Ru(III)-pterin
22,23
TPA)]ClO
4
III
-
2+
III
-
2+
complexes ([Ru (dmp )(TPA)] and [Ru (dmdmp )(TPA)] ).
After an aging time of 1.0 s, the solution was further mixed with
phenols (5.0 × 10 -2.0 × 10 M). The spectral changes were
monitored by the rise in the absorption band at 500 nm due to
formation of Ru(II)-Hpterin complexes (1 and 2) in MeCN. The
kinetic measurements were carried out under pseudo first-order
conditions where the concentrations of phenols were maintained
NMR measurements were performed on JEOL AL-300 and Bruker
AV-300 spectrometers. UV-vis absorption spectra were recorded
on Jasco V-570 and Hewlett-Packard HP8453 diode array spec-
trophotometers at room temperature unless otherwise noted. ESI-
MS spectra were measured on a Perkin-Elmer API-150 spectrometer.
Safety Note. Perchlorate salts of metal complexes with organic
ligands are potentially explosive. They should be handled with great
care in small quantities.
-4
-1
III
-
at more than 10-fold excess of concentrations of [Ru (dmp )-
TPA)] at 298 K. Pseudo first-order rate constants were deter-
2+
(
2
5
Synthesis of 2,4,6-Tri-tert-buthylphenol-d. Under N
0.03 g) was added into 2,4,6-tri-tert-buthylphenol (0.39 g) in
DMSO-d (9 mL), and the solution was stirred overnight. D O (15
mL) was added, and then the precipitate formed was collected by
filtration. The white solid was washed with D O.
Spectroscopic Titrations in MeCN. UV-vis titrations were
2
, NaH
mined by least-squares curve fits. Reactions between Ru(III)-pterin
complexes and 4-nitrophenol were performed by adding an MeCN
(
6
2
-4
-1
solution (1.5 mL) of 4-nitrophenol (2.1 × 10 -3.6 × 10 M)
into an MeCN solution (1.5 mL) of Ru(II)-pterin complexes (3
2
-5
III
-5
and 4; 5.0 × 10 M) and [Fe (bpy)
3
](PF
6
)
3
(5.0 × 10 M). The
change in the absorption at 420 nm was measured with use of diode
array spectrophotometers and analyzed by using biexponential curve
fitting, because the decomposition of Ru(III)-pterin complexes (ca.
2
6
performed by using 2,6-dichlorobenzoic acid (Hdcba; pK
a
) 17.6)
27
for the first protonation step and HClO (pK ) 2.1) for the second
4 a
protonation step as acids. Hdcba in MeCN was added to an MeCN
solution (3.0 mL) of 1 (0.1 mM) in a quartz cuvette, and UV-vis
spectra were taken after each addition. The obtained spectra were
-2 -1
III
-
2+
-3 -1
1
.0 × 10
s
for [Ru (dmp )(TPA)] and ca. 1.0 × 10
s
III
-
2+
for [Ru (dmdmp )(TPA)] ) affects the rate constants of the HAT
reactions in this time scale.
analyzed using the absorbance at 430 nm, yielding [1]/[3] ) (A
and A are the absorbances for 1 and 3 at
30 nm: [1] ) [dcba ] ) {[Ru]total (A - A)/(A - A )}/{1 + ((A
)). A slope of the linear fit to a plot of [3][dcba ]/[1]
1
-
ESR Spectroscopy. ESR spectra in MeCN were recorded on a
A)/(A - A
3
), where A-
1
3
JEOL JEX-REIXE spectrometer at room temperature for radical
4
-
1
3
1
2+
species, and g values were calibrated using an Mn marker.
-
A)/(A - A
3
versus [Hdcba] gave an acid-base equilibrium constant, Keq ) 0.15.
Results and Discussion
The pKa1 value of 1 was determined from pKa1(1) ) pK
log Keq ) 16.7 using the pK
a
(Hdcba)
+
a
value of 17.6 for Hdcba. The pKa2
value for 1 and also the pKa1 and pKa2 values for 2 were determined
by a similar procedure.
1
Spectroscopic Characterization of 1 and 2. The H NMR
spectrum of 1 in CD
3
h
CN exhibited a σ -symmetric pattern for
the TPA proton signals. As compared to that of 3, all of the
resonances showed downfield shifts (ca. +0.1 ppm) except that
assigned to the H6 of the axial pyridine of the TPA ligand
Electrochemical Measurements. Cyclic voltammetry was per-
formed in MeCN in the presence of 0.1 M [(n-butyl)
TBAPF ) as an electrolyte under N at room temperature, with
use of a glassy carbon electrode as a working electrode, Ag/AgNO
4 6
N]PF
(
6
2
(
-0.02 ppm). A broad singlet of the 2-amino N-H protons
3
as a reference electrode, and Pt wire as an auxiliary electrode. All
potentials were calibrated with respect to the ferrocene/ferricenium
redox couple as 0 V.
exhibited a large downfield shift from 5.10 ppm for 3 to 6.38
ppm for 1. This is indicative of the protonation at the N1
position, which is next to the C-2 position connected to the
amino group (see Chart 1). The N-H proton at N1 was not
observed at room temperature because of the fast exchange with
Controlled-potential electrolysis was carried out in a three-
electrode cell by using a BAS CV-27 voltammetry controller,
modified for ESR and electronic absorption measurements. The
3
the proton of the water molecule in CD CN. At 233 K, however,
2
working electrode was a Pt net of 0.3 mmφ (1 × 1 cm ). The
a broad signal assigned to the proton at N1 was observed at
auxiliary and reference electrodes were the same as those adopted
in CV measurements. The reference electrode was separated from
1
1
0.7 ppm. In the H NMR spectrum of 2, similar downfield
shifts were observed for all of the signals except that assigned
to the H6 of the axial pyridine of the TPA ligand (-0.03 ppm).
The signal of N,N-dimethylamino group protons exhibited a
upfield shift (-0.11 ppm) as compared to that of 4. A broad
signal assigned to the N-H proton at N1 was also observed at
6
the solution by a salt bridge of a saturated TBAPF solution in
+
MeCN. All potentials were measured relative to Ag/Ag and then
+
corrected to be referenced to the ferrocene/ferricenium (Fc/Fc )
couple as 0 V as an internal standard.
Spectroelectrochemical Experiments. UV-vis spectroelectro-
chemical experiments were performed with a thin-layer spectro-
1
0.0 ppm at 233 K for 2.
pK Determination in MeCN. The reversible two-step pro-
a
(
(
(
24) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory
Chemicals, 5th ed.; Butterworth-Heinemann: Oxford, UK, 2003.
25) Goldsmith, C. R.; Jonas, R. T.; Stack, T. D. P. J. Am. Chem. Soc.
tonation/deprotonation behavior was observed in absorption
spectra of 3 and 4 in aqueous buffer solutions and in MeCN by
2
2,23
adding HClO
scopic titrations were carried out to determine the pK
in MeCN by using Hdcba (pK
4
or base as reported previously.
The spectro-
of 3 and
) 17.6) for the first protonation
2
002, 235, 83–96.
26) (a) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
SolVents; Blackwell Scientific: Boston, MA, 1990. (b) Coetzee, J. F.;
Padmanabhan, G. R. J. Phys. Chem. 1965, 69, 3193–3196.
27) Felton, G. A. N.; Glass, R. S.; Lichtenberger, D. L.; Evans, D. H.
Inorg. Chem. 2006, 45, 9181–9184.
a
4
a
steps. The absorption spectrum of 3 changed into that of 1 by
(
addition of an excess amount of Hdcba as shown in Figure 1a.
J. AM. CHEM. SOC. 9 VOL. 131, NO. 32, 2009 11617

A R T I C L E S
Miyazaki et al.
Scheme 2. Thermochemical Parameters for Multistep
Proton-Coupled Electron Transfer of (a) dmp Complexes and (b)
dmdmp Complexes in MeCN, with E1/2 Values Referenced to
+
Fc/Fc
Figure 1. Absorption spectral changes in the course of spectroscopic
titration of 3 in MeCN at 298 K: (a) the first step with Hdbca; (b) the
second step with HClO .
4
The first protonation led to the increase of absorbance at 311, 344,
and 498 nm and the decrease of that at 383 and 461 nm with two
isosbestic points at 368 and 486 nm. The pKa1 value of 3 was
determined to be 16.7 by following absorption at 430 nm (see
Supporting Information, Figure S1a). The spectroscopic titration
for the second protonation steps was performed by adding HClO
4
in MeCN at 25 °C. The protonation of 1 allowed us to observe
another absorption spectral change showing an increase in absor-
bance at 319, 378, and 555 nm concomitant with its decrease at
344, 437, and 498 nm, keeping five isosbestic points at 310, 332,
360, 391, and 527 nm as depicted in Figure 1b. The pKa2 value
was determined to be 1.8 by following the change in absorbance
at 600 nm (see Supporting Information, Figure S1b). Ru-dmdmp
complexes also exhibited a similar spectral change for the proton-
Thermochemical Square Schemes for 1 and 2. Based on the
pK values and the redox potentials (E1/2) of 1 and 2, bond
a
ation, and the pK
a
values are determined: pKa1 ) 15.7 and pKa2
)
dissociation enthalpies (BDEs) were calculated on the basis of
2.0 (see Figure S2). The pK
a
values determined in MeCN are much
28
the following equation:
22a
higher than those in an aqueous solution; however, the order of
the pKa1 and pKa2 values in MeCN is consistent with those in an
aqueous solution.
BDE ) 2.303RT(pK ) + F(E ) + C
(1)
a
1/2
Redox Potentials in MeCN. Redox potentials of 1, 2, 3, and
where R is the gas constant, F is the Faraday constant, and C
is a constant in a certain solvent. In the case of MeCN, the C
2
1-23
4
had been measured previously.
Reversible one-electron
-1
28a,29
redox couples assigned to Ru(II)/Ru(III) were observed at 0.55
value has been estimated to be 59.5 kcal mol .
As for the
+
V (vs Fc/Fc , ∆E ) 60 mV) for 1 and 0.52 V (∆E ) 64 mV)
Ru complexes, we adopted BDE rather than BDFE (bond
dissociation free energy) because of a small contribution of the
for 2. Those values are 0.28 and 0.26 V higher than those
observed for 3 (0.27 V) and 4 (0.26 V), respectively; however,
they are only 0.2 V lower than those of the corresponding doubly
3
0
entropy term. Thus, we considered the energetics of HAT
reactions described in this Article in terms of BDE values of
phenols. Four BDE values for the formal HAT reactions of the
Ru-dmp complexes were calculated. The BDE for the reaction
2
2a
protonated species (see Scheme 2).
In contrast to the
reversible redox behavior of deprotonated and monoanionic
pterin ligands in 3 and 4 and that of the corresponding doubly
protonated and monocationic species, monoprotonated and
neutral pterin ligands in 1 and 2 exhibited irreversible redox
III
-
2+
-1
of 1 to [Ru (dmp )(TPA)] was calculated to be 89 kcal mol ,
(28) (a) Parker, V. D.; Handoo, K. L.; Roness, F.; Tilset, M. J. Am. Chem.
Soc. 1991, 113, 7493–7498. (b) Wayner, D. D. M.; Parker, V. D. Acc.
Chem. Res. 1993, 26, 287–294. (c) Skagestad, V.; Tilset, M. J. Am.
Chem. Soc. 1993, 115, 5077–5083. (d) Tilset, M.; Parker, V. D. J. Am.
Chem. Soc. 1989, 111, 6711–6717.
2
1
behavior due to “proton shift” upon one-electron reduction.
Thus, we applied the DPV (differential pulse voltammetry)
method under the same conditions to determine the one-electron
reduction potentials for the neutral pterin ligands in 1 and 2:
(29) Tilset, M. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim, Germany, 2001; Vol. 2, pp 677-713.
-
1.36 for 1 and -1.39 V for 2. Those potentials are ca. 0.7 V
(
30) (a) Wu, A.; Mayer, J. M. J. Am. Chem. Soc. 2008, 130, 14745–14754.
-
-
higher than those of the dmp and dmdmp ligands in 3 and 4,
(
b) Mader, E. A.; Manner, V. W.; Markle, T. F.; Wu, A.; Franz, J. A.;
respectively.
Mayer, J. M. J. Am. Chem. Soc. 2009, 131, 4335–4345.
1
1618 J. AM. CHEM. SOC. 9 VOL. 131, NO. 32, 2009

Proton-Coupled Electron Transfer of Ru-Pterin Complexes
A R T I C L E S
III
Figure 2. UV-vis spectral change of 1 upon addition of [Fe (bpy)
3 6 3
](PF ) .
Each spectrum was monitored upon adding 0.1 equiv of the oxidant. Inset:
Plot of the ratio of the [Ru (dmp )(TPA)] concentration to the initial
III
-
2+
-
5
III
-
concentration of 3 ([3]
0
) 2.6 × 10 M), [Ru (dmp )]/[3]
0
, relative to
III
the ratio of the [Fe (bpy) ](PF concentration to the initial concentration
3
6 3
)
III
3 0
of 3, [Fe (bpy) ]/[3] .
-
1
II
•-
2+
5
1 kcal mol for that of [Ru (Hdmp )(TPA)] to 3, 75 kcal
-
1
II
+
2+
III
+
2
mol for that of [Ru (H dmp )(TPA)] to [Ru (Hdmp )-
TPA)] , and 48 kcal mol for that of [Ru (H
3+
-1
II
+
2+
(
2
dmp )(TPA)]
to 1 in MeCN at 298 K. Similarly, the BDE values for Ru-
-
1
dmdmp complexes were calculated to be 87 kcal mol for the
III
-
2+
formal HAT reaction of 3 to [Ru (dmdmp )(TPA)] , 49 kcal
-1
II
•-
2+
-1
mol for that of [Ru (Hdmdmp )(TPA)] to 4, 75 kcal mol
II
+
2+
III
+
2
for that of [Ru (H dmdmp )(TPA)] to [Ru (Hdmdmp )-
TPA)] , and 47 kcal mol for that of [Ru (H
III
-
2+
Figure 3. (a) Difference spectrum of [Ru (dmp )(TPA)] (solid line)
3
+
-1
II
+
(
(
2
dmdmp )-
II
2+
obtained by subtraction of the spectrum of [Fe (bpy)
3
]
(dotted line) from
TPA)]2 to 3 at 298 K.
+
the final spectrum (dashed line) shown in Figure 2. (b) Absorption spectrum
III
-
2+
The thermochemical cycle enables us to calculate the pK
a
of [Ru (dmp )(TPA)] generated by bulk electrolysis with a controlled
potential of 0.5 V (solid line) and absorption spectrum of 3 (dotted line) in
MeCN at 293 K.
values of the two-step protonation/deprotonation processes for
Ru(III)-pterin complexes and Ru(II)-pterin radical complexes.
All thermodynamic parameters for multiple PCET processes of
Ru-pterin complexes are determined as shown in Scheme 2.
The thermodynamic parameters of the two complexes differ only
slightly. The importance of these thermochemical square
schemes is that the introduction of a redox-active ligand with
proton accepting site(s) to a redox-active metal complex allows
us to achieve extended multistep PCET processes showing
different BDEs.
comparison of a difference spectrum obtained by subtracting
the spectrum of [Fe (bpy)
Figure 3a with its absorption spectrum observed in the bulk
electrolysis of 3 in MeCN at 293 K with a controlled potential
of 0.5 V as depicted in Figure 3b. The solid lines in Figure 3a
and b are identical to exhibit absorption maxima at 385 and
97 nm, confirming that the chemical oxidation affords
Ru (dmp )(TPA)] . The titration curve based on the spec-
tral change exhibited the linear saturation behavior at the
addition of 1 equiv of the oxidant as shown in the inset of Figure
. In addition, those oxidized species were characterized by
II
2+
3
]
from the spectrum shown in
4
[
III
-
2+ 33
On the basis of the thermochemical square schemes as
III
-
described in Scheme 2, we can expect that [Ru (pterin )-
TPA)]2+ (pterin ) dmp and dmdmp ) can perform hydrogen
atom transfer from substrates having a bond with a BDE lower
-
-
-
(
2
2
2a
ESR spectroscopy as described previously.
Hydrogen Atom Transfer from Phenol Derivatives to
Ru (dmp )(TPA)] . First, we used 2,4,6-tri-tert-butylphenol
TBP) having the bond dissociation enthalpy (BDE) of 79.15
than those to form 1 and 2, respectively.
III
-
2+
Formation of [Ru (dmp )(TPA)] . As described in Scheme
III
-
2+
[
2
, among four steps of PCET processes of Ru-pterin com-
(
plexes, the most reactive species toward HAT reactions should
-1
34
III
-
2+
-1
kcal mol for the O-H bond as a substrate. The HAT reaction
III
be [Ru (dmp )(TPA)] having BDE ) 89 kcal mol and
-
2+
III
-
2+
-1
from TBP to [Ru (dmp )(TPA)] should be downhill by ca.
[
Ru (dmdmp )(TPA)] having BDE ) 87 kcal mol . Thus,
III
-
1
-
10 kcal mol . The reaction was revealed to give 1 and the
corresponding phenoxyl radical (TBP ) as described in Scheme
. Stopped-flow measurements for the reaction of [Ru (dmp )-
(TPA)] with various concentrations of TBP in MeCN were
performed to determine second-order rate constants (k). In the
course of the reaction, the absorption derived from 1 increased
(λmax ) 436, 498 nm; Figure 4a). In addition, TBP was detected
in the ESR measurement under the same conditions as stopped-
flow measurements (Figure S3), and the double integral of the
we examined HAT from phenol derivatives to [Ru (dmp )-
•
TPA)]2+ and [Ru (dmdmp )(TPA)] in MeCN to shed some
III
-
2+
(
III
-
3
light on their reactivity and to gain mechanistic insights.
2+
III
-
2+
III
-
The formation of [Ru (dmp )(TPA)] and [Ru (dmdmp )-
TPA)]2+ was successfully achieved by one-electron oxidation
(
III
0
+
of 3 and 4 with [Fe (bpy
MeCN).
3
)](PF
The absorption spectral change of 3 in the course
of the one-electron oxidation is shown in Figure 2. The increase
6 3
) (E red ) 0.70 V vs Fc/Fc in
•
3
1,32
II
2+
of absorbance at 520 nm is due to the formation of [Fe (bpy)
The formation of [Ru (dmp )(TPA)] was confirmed by
3
] .
•
III
-
2+
ESR signal indicates that TBP was converted to TBP quanti-
tatively. The absorption spectral change obeyed pseudo first-
order kinetics in the presence of excess amount of substrate as
shown in Figure 4b. A plot of the pseudo first-order rate
constants (kobs) versus concentrations of TBP was linear, and
(
31) Rhile, I. J.; Markle, T. F.; Nagao, H.; DiPasquale, A. G.; Lam, O. P.;
Lockwood, M. A.; Rotter, K.; Mayer, J. M. J. Am. Chem. Soc. 2006,
1
28, 6075–6088.
(
(
32) Fukuzumi, S.; Wong, C. L.; Kochi, J. K. J. Am. Chem. Soc. 1980,
1
02, 2928–2939.
33) [Ru (dmp )(TPA)]3+, λmax [nm] (ε (M-1 cm-1)): 380 (6.0 × 10 ),
III -
3
(34) Wright, J. S.; Johnson, E. R.; DiLabio, G. A. J. Am. Chem. Soc. 2001,
123, 1173–1183.
3
5
00 (2.1 × 10 ).
J. AM. CHEM. SOC. 9 VOL. 131, NO. 32, 2009 11619

A R T I C L E S
Miyazaki et al.
III
-
2+
Scheme 3. Reaction of [Ru (dmp )(TPA)] with Tri-t-butyl Phenol
4
36
III
-
2+
the k value at 293 K was determined to be (1.6 ( 0.2) × 10
reactioin from TBP to [Ru (dmp )(TPA)] could be at-
tributed to the offset with the activation enthalpy of the
formation of hydrogen-bonded intermediate.
-1
-1
M
s
from the slope of the linear correlation. In the same
manner, we determined the second-order rate constants for
reactions of [Ru (dmp )(TPA)] with various phenol deriva-
tives in MeCN at 293 K as listed in Table 1. All of these phenols
III
-
2+
To gain more insight into the HAT mechanism, we gave
scrutiny into the dependence of first-order rate constants on
the concentration of phenolic substrates. Linear relationships
between kobs and concentrations of substrates were observed
in the HAT reactions of 4-phenylphenol, 4-tert-butylphenol,
4-chlorophenol, and 4-cyanophenol, except for the reaction
III
-
2+
III
-
were reacted with [Ru (dmp )(TPA)] and [Ru (dmdmp )-
+
(
TPA)]2 to give 1 and 2, respectively. The formation of 1 and
was quantitative; however, the oxidized products derived from
phenols could not be identified by GC-MS and NMR analysis,
2
3
5
III
-
2+
except for TBP and 4-nitrophenol.
of 4-nitrophenol. The reaction between [Ru (dmp )(TPA)]
An Eyring plot was obtained from the temperature depen-
dence of the second-order rate constants as shown in Figure 5.
On the basis of the Eyring plot, the activation parameters were
and 4-nitrophenol was monitored by an increase in absorption
at 440 and 500 nm derived from 1 (Figure 6). The pseudo first-
order rate constant exhibited a saturation behavior at higher
concentration of the substrate as can be seen in Figure 7a. This
result clearly indicates the existence of a pre-equilibrium prior
q
-1
q
determined to be ∆H ) 1.6 ( 0.2 kcal mol and ∆S ) -36
(
2 cal K- mol . These activation parameters suggest a fairly
1
-1
36
stabilized and organized transition state for the HAT reaction
from TBP to [Ru (dmp )(TPA)] . Recently, a negative activa-
tion enthalpy of the hydrogen-abstraction reaction was reported
in the reaction between [Fe (H
rahydropyrimidine)) and large excess TEMPO, and formation
of a hydrogen-bonded adduct was suggested as an intermediate
to the HAT reaction. In this case, kobs is given by the following
equation:
III
-
2+
II
2+
k1K[substrate]
2
bip)
3
]
2
(H bip ) 2,2′-bi(tet-
kobs
)
(2)
1
+ K[substrate]
3
6
in a pre-equilibrium step, as has been observed in organic
HAT reactions. The relatively small ∆H of the HAT
where K is the association constant. The data are fit well to this
3
7
q
-1 -1
equation, giving values of k
1
) 1.6 × 10
s
and K ) 13
III
-
2+
Figure 4. (a) Absorption spectral change in the course of the reaction of [Ru (dmp )(TPA)] with TBP monitored by stopped-flow in MeCN at 293 K.
III
-
2+
-5
-4
[
Ru (dmp )(TPA)] ) 2.5 × 10 M; [TBP] ) 5.9 × 10 M. (b) The change of absorbance at 430 nm (b) and the pseudo first-order fit (red line).
a
Table 1. pK , Oxidation Potential, BDE Values of Phenols, and Second-Order Rate Constants of Hydrogen Abstraction Reactions by
- 2+
III
[
Ru (dmp )(TPA)] in MeCN at 293 K
-1
-1
k, M
s
0
a
+
b
a
c
BDE, kcal mol
-1
substrate
E
ox, V (vs Fc/Fc )
pK
dmp
dmdmp
4
3
3
2
2
2
2
4
4
4
4
4
,4,6-tri-tert-butylphenol
-phenylphenol
-tert-butylphenol
-chlorophenol
-cyanophenol
1.02
1.14
1.28
1.32
1.81
1.86
27.3
26.6
27.5
22.8
22.8
20.7
79.15
80.52
84.55
85.65
89.25
91.65
(1.6 ( 0.2) × 10
(8.4 ( 0.3) × 10
(1.9 ( 0.1) × 10
(3.7 ( 0.4) × 10
(2.6 ( 0.2) × 10
(7.3 ( 0.2) × 10
(5.1 ( 0.2) × 10
(9.4 ( 0.2) × 10
(2.9 ( 0.2) × 10
(9.9 ( 0.2) × 10
(2.7 ( 0.2) × 10
-
1
d
-1
-nitrophenol
1.6 ( 0.2
a
b
c
d
Determined by SHACV in MeCN. Jover, J.; Bosque, R.; Sales, J. QSAR Comb. Sci. 2007, 26, 385-397. Reference 34. The second-order rate
constant was determined in the lower-concentration region.
1
1620 J. AM. CHEM. SOC. 9 VOL. 131, NO. 32, 2009

Proton-Coupled Electron Transfer of Ru-Pterin Complexes
A R T I C L E S
Taken together with the arguments described above, we
propose that intermolecular hydrogen bonding to form an adduct
occurs prior to HAT as shown in Scheme 4. The adduct structure
is related to the crystal structures of 1 and 2 that form hydrogen
21
bonds with perchlorate of counteranion. The hydrogen bonding
can be formed between the phenolic O-H group and the N-1
-
III
-
2+
of the dmp ligand in [Ru (dmp )(TPA)] , which are the most
acidic and the most basic site in each entity.
The 2-amino group can additionally form a hydrogen bond
with the phenolic oxygen trapped in the vicinity. This two-point
interaction may polarize the O-H bond and stabilize the
transition state to facilitate the HAT reaction via an associative
mechanism exhibiting the negative activation entropy.
III
-
2+
The hydrogen-bonded adduct between [Ru (dmp )(TPA)]
III
-
Figure 5. Eyring plot for the HAT reaction from TBP to [Ru (dmp )-
(
TPA)]2 in MeCN.
+
and 4-nitrophenol was detected by ESI-MS measurement in
MeCN at 233 K. The ESI-MS spectrum of the mixture of 3
-
5
III
-5
-1
(2.5 × 10 M), [Fe (bpy) ](PF ) (2.5 × 10 M), and
3
6 3
M . This adduct formation is consistent with the negative
activation entropy observed in the HAT reactions from TBP to
Ru (dmp )(TPA)] as mentioned above. In sharp contrast to
-2
4
-nitrophenol (1.2 × 10 M) in MeCN exhibited overlapped
III
-
2+
peaks derived from two dicationic species around m/z ) 361 at
[
III
-
2+
233 K (Figure 8a). The ion peaks are in good agreement with
the case of [Ru (dmp )(TPA)] , concentration dependence of
III
-
2+
III
-
2+
the superposition of simulated spectra of [Ru (dmp )(TPA)] -4-
nitrophenol adduct and 1-4-nitrophenol adduct (Figure 8b-d).
This result provides clear evidence for the adduct formation
between 1 and 4-nitropenol even in the low concentration of
the reactants; however, the adducts of the other substrates with
k
obs for the reaction between [Ru (dmdmp )(TPA)] and
4
7
[
-nitrophenol showed a linear relationship as shown in Figure
b. This is probably due to the fact that it is difficult for
Ru (dmdmp )(TPA)] to form hydrogen bonding with phe-
nols due to the steric hindrance of the dimethylamino group of
the dmdmp ligand. This difference indicates the importance
of the 2-amino group of the pterin ligand as a substrate-trapping
site by forming the hydrogen bond with the phenols in the HAT
reactions.
III
-
2+
III
-
2+
-
[Ru (dmp )(TPA)] could not be observed.
III
-
The HAT reactions from deuteriated TBP to [Ru (dmp )-
(TPA)]2 were performed in MeCN at 243-293 K to determine
the kinetic isotope effects. Temperature dependence of the
+
III
-
2+
Figure 6. (a) Absorption spectral change in the course of the reaction of [Ru (dmp )(TPA)] with 4-nitrophenol monitored by UV-vis spectrometer in
III
-
2+
-5
-1
MeCN at 293 K. Inset: Difference spectra for the last spectrum. [Ru (dmp )(TPA)] ) 2.5 × 10 M; [4-nitrophenol] ) 1.2 × 10 M. (b) The change
of absorbance at 430 nm (O) and the biexponential fit (red line).
III
-
2+
Figure 7. Concentration dependence of kobs for the reaction (a) between [Ru (dmp )(TPA)] and 4-nitrophenol (b) and curve fitting (solid line) based on
III
-
2+
eq 2. (b) Concentration dependence of kobs for the reaction between [Ru (dmdmp )(TPA)] and 4-nitrophenol (b) and curve fitting (solid line).
J. AM. CHEM. SOC. 9 VOL. 131, NO. 32, 2009 11621

A R T I C L E S
Scheme 4. Hydrogen Atom Abstraction from Phenols to [Ru (dmp )(TPA)] through Hydrogen-Bonded Adduct
Miyazaki et al.
III
-
2+
second-order rate constants was observed, and the kinetic isotope
effects (k /k ) were determined to be 2.2 at 293 K and 4.3 at
43 K as listed in Table 2. The values are not so large, unlike
the reactions exhibiting larger KIE values, such as that for
shown in Scheme 4. The kinetic isotope effects of the reaction
of the Ru(III)-pterin complexes exhibit relatively large tem-
perature dependence as compared to C-H abstraction reactions
with use of Cu(II)-phenoxyl radical complexes reported by
H
D
2
1
8a
39
lipoxygenases.
The values fall in the range of “linear
Stack and co-workers.
H-transfer unsymmetrical” (KIE ) 2-5) or “nonlinear H-
Mechanistic Insights into HAT. First, we considered relation-
ships among thermodynamic parameters to gain mechanistic
insights into the HAT reactions from the phenols to the
Ru(IIII)-pterin complexes. If the rate-limiting step of the HAT
reactions is proton transfer, an R value of the Polanyi equation
1
/2
38
transfer bent” (KIE > 2 ) as a transition state. Based on the
argument described above, the HAT reactions described here
would experience a “nonlinear H-transfer bent” transition state,
and it is consistent with the proposed intermediate in an
associative mechanism with intermolecular hydrogen bonds as
obtained from the plot of log k relative to pK
a
is expected to be
0.5. However, the plot of log k relative to pK values of the
phenols exhibits a linear relationship with a positive slope (R
0.48) as depicted in Figure 9a. The more acidic phenols afford
slower rates, suggesting proton transfer (PT) from the substrates
4
0
-
a
)
III
-
2+
to [Ru (dmp )(TPA)] is not rate-limiting. It has been reported
that the pK values and the oxidation potentials of phenol
derivatives exhibit a linear relationship: the more acidic (lower
pK ), the less easily oxidized (higher oxidation potentials). In
a
41
a
the case of the plot of log k relative to oxidation potentials of
the phenols, it shows a linear relationship with a negative slope
as shown in Figure 9b. The R value for the plot is -0.25, which
is smaller than the ideal R value for a pure electron transfer
4
2
process, -0.5. This result indicates that the ET from the
substrates to the Ru(III) complex should be involved in the rate-
determining step of the HAT; however, the energy barrier of
ET is thermodynamically uphill more than 0.47 eV. Thus, the
ET needs to be aided by PT as PCET. In addition, the
relationship between the rate constants and BDEs of the O-H
bonds in the phenol derivatives also exhibits a linear relationship
as can be seen in Figure 9c: the stronger are the O-H bonds,
the harder it is to be transferred. It is surprising that the O-H
-
1
bond in 4-nitrophenol having 92 kcal mol can be broken in
this process, because hydrogen abstraction from a C-H bond
with such a large bond energy cannot easily occur even by
(
(
(
35) The lack of ESR signals in the reaction mixtures including other
phenols suggests the formation of diamagnetic coupling products.
36) Mader, E. A.; Davidson, E. R.; Mayer, J. M. J. Am. Chem. Soc. 2007,
129, 5153–5166.
III
-
2+
Figure 8. (a) ESI-MS spectrum of [Ru (dmp )(TPA)] and 4-nitrophenol
in MeCN at 233 K. (b) Superposition of simulated spectrum of the adduct
of [Ru (dmp )(TPA)] with 4-nitrophenol (c) and that of the adduct of 1
III
-
2+
37) Foti, M. C.; Daquino, C.; Mackie, I. D.; DiLabio, G. A.; Ingold, K. U.
J. Org. Chem. 2008, 73, 9270–9282.
and 4-nitrophenol (d).
(
(
(
38) Kwart, H. Acc. Chem. Res. 1982, 15, 401–408.
39) Pratt, R. P.; Stack, T. D. Prog. Inorg. Chem. 2005, 44, 2367–2375.
40) Fecenko, C. J.; Thorp, H. H.; Meyer, T. J. J. Am. Chem. Soc. 2007,
129, 15098–15099.
Table 2. Kinetic Isotope Effects of the HAT Reactions from TBP to
III
-
2+
[
Ru (dmp )(TPA)] in MeCN
-1
-1
-1 -1
H
k , M
s
k
D
, M
s
H D
k /k
(
41) Bordwell, F. G.; Cheng, J.-P. J. Am. Chem. Soc. 1991, 113, 1736–
4
3
3
3
2
2
93 K
43 K
(1.6 ( 0.1) × 10
(7.3 ( 0.4) × 10
(7.5 ( 0.6) × 10
(1.7 ( 0.1) × 10
2.2
4.3
1
743.
(42) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985, 811, 265–
22.
3
1
1622 J. AM. CHEM. SOC. 9 VOL. 131, NO. 32, 2009

Proton-Coupled Electron Transfer of Ru-Pterin Complexes
A R T I C L E S
Figure 9. Correlations of second-order rate constants for HAT reactions with thermodynamic parameters: (a) log k versus pK
a
, (b) log k versus oxidation
III
2+
potentials of phenols, and (c) log k versus bond dissociation energy (BDE) of the O-H bond in a phenol derivative for [Ru (dmp)(TPA)] (black) and
III
2+
[
Ru (dmdmp)(TPA)] (red).
4
3
metal-oxo species. The Ru(III)-pterin complexes have N-H
correlation also seems to hold for abstraction by the nitrogen-
-1
-1
•
48
BDE values of only 89 kcal mol for 1 and 87 kcal mol for
(as described in Scheme 2), both of which are slightly lower
than the value of the BDE of the O-H bond in 4-nitrophenol.
The uphill reactions may be followed by faster subsequent
reactions to form stable coupling products. Actually, we could
observe the formation of a dimeric product that should be
derived from radical coupling reaction of two molecules of
centered 2,2-diphenyl-1-picrylhydrazyl radical (dpph ). The
rate constants found in this study for 4-tert-butylphenol (a close
model for phenol) appear to fall very close to Lau’s correlation
2
3
-1 -1
-1
line: k ) (1.9 ( 0.1) × 10 M
s
at BDE ) 89 kcal mol
44
III
-
2+
-1 -1
for [Ru (dmp )(TPA)] , and k ) 94 ( 2 M
87 kcal mol for [Ru (dmdmp )(TPA)] .
s
at BDE )
-
1
III
-
2+
4
-nitrophenol as represented by the peak cluster at m/z ) 310
Summary
+
(C H N O ) in GC-MS analysis of the products (see
12 10 2 8
45
The redox-active ruthenium complexes bearing pterins that
are related to redox-noninnocent heteroaromatic coenzymes
allowed us to demonstrate multistep PT, ET, and PCET
processes. Their thermochemical square schemes have been
established, as described in Scheme 2, by determining pK values
a
and redox potentials to estimate the BDE values.
Supporting Information, Figure S4). The R value of the rate
constants to the BDEs of the phenolic O-H bonds shows a
best correlation among these three plots (-0.42). This result
III
-
2+
suggests that the HAT reaction between [Ru (dmp )(TPA)]
and phenols proceeds via concerted proton and electron transfer,
that is, PCET.
III
-
A BDEs versus log k plot for the reactions of [Ru (dmdmp )-
On the basis of the BDE values, we examined HAT reactions
from phenol derivatives to [Ru (dmp )(TPA)] as a hydrogen
atom acceptor to afford [Ru (Hdmp)(TPA)] and the corre-
sponding phenoxyl radicals that decomposed rapidly or gave a
dimer as a radical coupling product, except TBP that could be
TPA)]2 shows a nearly identical slope (R ) -0.38), as
+
III
-
2+
(
II
2+
depicted in Figure 9c, where the k values of the HAT reaction
III
-
2+
between [Ru (dmdmp )(TPA)] and phenols are 6-22 times
smaller than those of the reactions using [Ru (dmp )(TPA)] .
The smaller rate constants are likely due to the 2 kcal mol
lower BDE of [Ru (dmdmp )(TPA)] (Scheme 2) and to the
dimethylamino group of the dmdmp ligand inhibiting the
formation of two-point hydrogen bonds with phenols (Scheme
III
-
2+
•
-
1
observed by ESR spectroscopy in quantitative yield. The linear
relationships of log k values with the pK , oxidation potentials,
a
III
-
2+
and BDEs of the O-H bonds in the phenols allowed us to
conclude that the HAT reactions from the substrates to the
Ru(III)-pterin complexes are PCET dominantly governed by
the BDEs of the substrates. The saturation behavior of the
pseudo first-order rate constants with concentration of nitro-
phenol and the negative activation entropy indicate that the
reaction proceeds via an associative mechanism involving
intermolecular hydrogen bonding between the Ru(III)-pterin
complex and the phenolic substrates prior to HAT. The HAT
4
).
The rate constants for phenol oxidation are similar to those
observed in other systems with similar driving force. Such
comparisons are complicated for phenols because there are often
significant solvent effects, steric effects, and effects of hydrogen-
4
6,47
bonded precursor adducts (as in Scheme 4).
Still, Lau et
al. have shown that rate constants of hydrogen atom abstraction
from phenol correlate well with the oxidant BDE of oxyl radicals
and a ruthenium(VI) dioxo complex (Figure 7 of ref 47a). This
III
-
2+
reactions of [Ru (dmp )(TPA)] were accelerated as compared
III
-
2+
to that of [Ru (dmdmp )(TPA)] due to more stable hydrogen-
bond formation in the pre-equilibrium step to trap the substrates
and also in stabilization of the transition state. The observations
described herein will provide valuable insight into the new
development of transition-metal complexes having heteroaro-
matic coenzymes as ligands to exhibit multistep PCET.
(
43) For RudO: (a) Che, C.-M.; Tang, W.-T.; Wong, K.-W.; Li, C.-K.
J. Chem. Soc., Dalton Trans. 1991, 3277–3280. (b) Seok, W. K.;
Meyer, T. J. J. Am. Chem. Soc. 1988, 110, 7358–7367. For FedO:
(
c) Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde, J.-U.; Song, W. J.;
Stubna, A.; Kim, J.; M u¨ nck, E.; Nam, W.; Que, L., Jr. J. Am. Chem.
Soc. 2004, 126, 472–473.
Acknowledgment. This work was supported by Grants-in-Aid
(
(
(
44) Yiu, D. T. Y.; Lee, M. F. W.; Lam, W. W. Y.; Lau, T. C. Inorg.
Chem. 2003, 42, 1225–1232.
(
nos. 19205019 and 20108010), a Global COE program, “The
45) Napolitano, A.; Palumbo, A.; d’Ischia, M. Tetrahedron 2000, 56, 5941–
Global Education and Research Center for Bio-Environmental
Chemistry” from the Japan Society of Promotion of Science (JSPS)
and The Ministry of Education, Science, Technology of Japan, by
KOSEF/MEST through WCU project (R31-2008-000-10010-0), and
5
945.
46) See, for instance: (a) Valgimigli, L.; Banks, J. T.; Ingold, K. U.;
Lusztyk, J. J. Am. Chem. Soc. 1995, 117, 9966–71. (b) Snelgrove,
D. W.; Lusztyk, J.; Banks, J. T.; Mulder, P.; Ingold, K. U. J. Am.
Chem. Soc. 2001, 123, 469–477.
(
47) (a) Yiu, D. T. Y.; Lee, M. F. W.; Lam, W. W. Y.; Lau, T.-C. Inorg.
Chem. 2003, 42, 1225–1232. (b) Lam, W. W. Y.; Man, W.-L.; Lau,
T.-C. Coord. Chem. ReV. 2007, 251, 2238–2252.
(48) For HAT from phenol to dpph , k ) 0.1 M s-1 at BDE ) 79 kcal
•
-1
-1
37
mol
.
J. AM. CHEM. SOC. 9 VOL. 131, NO. 32, 2009 11623

A R T I C L E S
Miyazaki et al.
by the U.S. National Institutes of Health (GM50422). S.M.
appreciates support from a JSPS postdoctoral fellowship (no.
hydrogen atom transfer reaction from 2,4,6-tri-tert-butyl phenol
III
-
2+
to [Ru (dmp )(TPA)] , and the mass spectrum of the reaction
III
-
2+
1
0727).
product of 4-nitrophenol with [Ru (dmp )(TPA)] . This
material is available free of charge via the Internet at http://
pubs.acs.org.
Supporting Information Available: UV-vis spectra in the
spectroscopic titrations to determine the pK
a
values, the ESR
spectrum of 2,4,6-tri-tert-butyl phenoxyl radical obtained by the
JA904386R
1
1624 J. AM. CHEM. SOC. 9 VOL. 131, NO. 32, 2009