Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore-Imidazole-Phenol Complex: Stepwise Oxidation and Concerted Reduction

Article abstract of DOI:10.1021/jacs.6b03506

Proton-coupled electron transfer (PCET) reactions were studied in acetonitrile for a Photosystem II (PSII)-inspired [Ru(bpy)₂(phen-imidazole-Ph(OH)(^tBu)₂)]²⁺, in which Ru(III) generated by a flash-quench sequence oxidizes the appended phenol and the proton is transferred to the hydrogen-bonded imidazole base. In contrast to related systems, the donor and acceptor are strongly coupled, as indicated by the shift in the Ru^III/II couple upon phenol oxidation, and intramolecular oxidation of the phenol by Ru(III) is energetically favorable by both stepwise and concerted pathways. The phenol oxidation occurs via a stepwise ET-PT mechanism with k_ET = 2.7 × 10⁷ s^-1 and a kinetic isotope effect (KIE) of 0.99 ± 0.03. The electron transfer reaction was characterized as adiabatic with λ_DA = 1.16 eV and 280 < H_DA < 540 cm^-1 consistent with strong electronic coupling and slow solvent dynamics. Reduction of the phenoxyl radical by the quencher radical was examined as the analogue of the redox reaction between the PSII tyrosyl radical and the oxygen-evolving complex. In our PSII-inspired complex, the recombination reaction activation energy is <2 kcal mol^-1. The reaction is nonadiabatic (V_PCET ≈ 22 cm^-1 (H) and 49 cm^-1 (D)) and concerted, and it exhibits an unexpected inverse KIE = 0.55 that is attributed to greater overlap of the reactant vibronic ground state with the OD vibronic states of the proton acceptor due to the smaller quantum spacing of the deuterium vibrational levels.

Full text of DOI:10.1021/jacs.6b03506

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Article
Proton Coupled Electron Transfer in a Strongly Coupled
Photosystem II-Inspired Chromophore-Imidazole-Phenol
Complex: Stepwise Oxidation and Concerted Reduction
Gerald F. Manbeck, Etsuko Fujita, and Javier J. Concepcion
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b03506 • Publication Date (Web): 18 Aug 2016
Downloaded from http://pubs.acs.org on August 19, 2016
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Journal of the American Chemical Society
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Proton Coupled Electron Transfer in a Strongly Coupled Pho-
tosystem II-Inspired Chromophore-Imidazole-Phenol Com-
plex: Stepwise Oxidation and Concerted Reduction
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Gerald F. Manbeck,* Etsuko Fujita, and Javier J. Concepcion*
Chemistry Division, Brookhaven National Laboratory, Upton, NY 11973−5000, United States.
Supporting Information Placeholder
ABSTRACT: Protonꢀcoupled electronꢀtransfer (PCET) reactions were studied in acetonitrile for a Photosystem II (PSII) inspired
[Ru(bpy)2(phenꢀimidazoleꢀPh(OH)(tBu)2)]2+, in which Ru(III) generated by a flashꢀquench sequence oxidizes the appended phenol and the
proton is transferred to the hydrogen bonded imidazole base. In contrast to related systems, the donor and acceptor are strongly coupled as
indicated by the shift in the RuIII/II couple upon phenol oxidation, and intramolecular oxidation of the phenol by Ru(III) is energetically
favorable by both stepwise or concerted pathways. The phenol oxidation occurs via a stepwise ETꢀPT mechanism with kET = 2.7 × 107 s−1
and a kinetic isotope effect (KIE) of 0.99 ± 0.03. The electron transfer reaction was characterized as adiabatic with λDA = 1.16 eV and 280
< HDA < 540 cm−1 consistent with strong electronic coupling and slow solvent dynamics. Reduction of the phenoxyl radical by the quenchꢀ
er radical was examined as the analogue of the redox reaction between the PSII tyrosyl radical and the oxygen evolving complex (OEC). In
our PSIIꢀinspired complex, the recombination reaction activation energy is < 2 kcal mol−1. The reaction is nonadiabatic (VPCET ~ 22 cm−1
(H) and 49 cm−1 (D)), concerted, and exhibits an unexpected inverse KIE of 0.55 that is attributed to greater overlap of the reactant vibronꢀ
ic ground state with the OD vibronic states of the proton acceptor due to the smaller quantum spacing of the deuterium vibrational levels.
potential chromophore excited state is capable of oxidizing an
INTRODUCTION
appended or freely diffusing phenol via PCET reductive quenchꢀ
ing.26ꢀ30 The chromophore may function as the base and electron
Faced with the exhaustibility of fossil fuel reserves and the risꢀ
acceptor, i.e., Chart 1 complex C,26 or a dissolved base (including
ing level of atmospheric CO2 generated by their combustion, the
need for renewable energy becomes more pressing. In principle,
solvent and buffers) may function as the proton acceptor in a soꢀ
called multidirectional PCET.31,32
solarꢀdriven production of liquid fuels (artificial photosynthesis)
can be achieved by the multielectron, multiproton reduction of
water to H2 and CO2 to CO, HCO2H, CH3OH, CH4, and other
oxygenates or hydrocarbons with water oxidation as the source of
Chart 1 Examples of Photo-Oxidation of Tyrosine-
Histidine Mimics
H+ and electrons. A persistent challenge in artificial photosyntheꢀ
sis is the separation of charges with adequate energy to drive the
pertinent redox reactions while suppressing the highly exergonic
charge recombination. The recombination may occur on the ps
timescale,1ꢀ4 while catalytic reactions such as the oxidation of
water in Photosystem II (PSII) turn over more slowly (1ꢀ2
ms−1).5,6
In PSII,7ꢀ11 charge separation by electron transfer (ET) from
photoexcited *P680 to pheophytin to plastoquinone QA ensues
within a few hundred ps.8,9 The charge recombination from the
[P680+/QA−•] pair is highly exergonic but is sluggish (1ꢀ2 ms) due
to its position in the Marcus inverted region.12ꢀ14 The strongly
oxidizing P680+ is reduced by tyrosineꢀ161 (Tyrz) with multiphasic
kinetics including a fast 20ꢀ40 ns component.15ꢀ18 The Tyrz oxidaꢀ
tion19ꢀ21 occurs by a multisite proton coupled electron transfer
(PCET)22ꢀ25 pathway19 with proton transfer to the nearby His190
residue20 (2.5 Å O‒N distance).21 In turn, the Tyrz radical is the
oxidant in each oxidation step of the oxygen evolving complex
with time constants from ~30 to 1400 ꢁs.6,10,11
The photoꢀdriven Tyrz/His PCET chemistry of PSII has motiꢀ
vated the pursuit of synthetic mimics of this reaction (Chart 1)
with tremendous variability in the mechanism based on structural
and electronic considerations. In one subset of complexes, a highꢀ
(A) Ref. 33 (B) Ref. 34 (C) Ref. 26 (D) Ref. 38 (E)
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In a second subset of tyrosineꢀhistidine mimics, oxidation of
imidazole moiety which can also be oxidized via PCET pathꢀ
ways.55
Here we report a study of the related complex RuꢀphenꢀImꢀ
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the phenol to the phenoxyl radical is driven by a preꢀoxidized
chromophore generated through oxidative quenching of the excitꢀ
ed state33ꢀ36 or charge injection into a semiconductor.37,38 Phenols
with an internal Hꢀbonded base, which are the most accurate repꢀ
resentations of PSII, have been oxidized by appended polypyridyl
RuIII complexes (transiently in a flashꢀquench experiment)34, porꢀ
phyrin radical cations, or excited porphyrin dyads.37ꢀ39 In recent
contributions, the Hꢀbond distance dependence of the rate conꢀ
stant was quantified in photochemical34 and chemical oxidation40
of Hꢀbonded phenols with contrasting results between the two
studies.
PhOH (Chart 1) which was inspired by the Tyrz/His PCET proꢀ
cess in PSII. Our system differs from PSII by direct connection of
the imidazole and phenol to the chromophore through π conjugaꢀ
tion. The kinetics of phenol oxidation and phenoxyl radical reducꢀ
tion reactions are measured by transient absorption spectroscopy
in acetonitrile to avoid proton transfer to the bulk solvent. It is
important to note that the reduction reaction is often ignored in
mimetic studies although it is the analogue of Tyrz+• reduction in
PSII by the OEC, which takes place in micro to milliseconds.56,57
The phenoxyl reduction reaction is highly important in artificial
photosynthetic systems using a phenol/base PCET mediator.
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Precise determination of the mechanism of phenol oxidation is
a persistent challenge in the field. Analysis is complicated beꢀ
cause the PCET reaction is often poised to occur stepwise with
electron transfer preceding proton transfer (ETꢀPT) or vice versa
(PTꢀET), or by a concerted mechanism (EPT) in which the elecꢀ
tron and proton travel simultaneously at an energy minimum on
the multiꢀdimensional potential energy surface which includes
contributions from proton vibrational states, solvent reorganizaꢀ
tion, and the electron coordinate. The theory of PCET is well
developed as an extension of semiꢀclassical electron transfer theoꢀ
ry that includes proton vibrational states in the donor and accepꢀ
tor.41ꢀ46 The electron and proton movement can be discussed in
terms of their adiabaticity. In this context, we will use “adiabatic”
to describe an ET reaction that occurs on the electronic ground
state due to strong coupling between the donor and acceptor and
“nonadiabatic” to describe weak coupling between donor and
acceptor diabatic states. In the nonadiabatic limit, electrons do not
respond instantaneously to the solvent motion and electronic exꢀ
cited states are involved. Likewise, proton transfer may be deꢀ
scribed as vibrationally nonadiabatic or adiabatic depending upon
the involvement of vibrational excited states. An excellent tutorial
is available in the literature.43
Our results show that the properties of RuꢀphenꢀImꢀPhOH are
atypical among other Tyrz/His mimics. The photogenerated RuIII
center is thermodynamically capable of oxidizing the phenol by
concerted or stepwise pathways. Although the driving force for
EPT is much larger, the ETꢀPT pathway (Scheme 1, ꢂGo
=
ET
−0.01 eV) is preferred with rate limiting ET of ~3 × 107 s−1 at 20
ºC and no kinetic isotope effect (KIE). A Marcus theory treatment
of the kinetics reveals an adiabatic mechanism consistent with
strong electronic coupling (λDA = 1.16 eV and 280 < HDA < 540
cm−1). The reduction of the phenoxyl radical by the methyl vioꢀ
logen radical cation (MV+•) is a concerted nonadiabatic (λDA = 1.7
eV, Ea = 1.4 kcal mol−1, and VPCET ~ 22 cm−1) multisite PCET
reaction and exhibits a surprising inverse KIE kH/kD of ~0.54. The
reaction is nearly activationless, and the unusual inverse KIE is
possibly attributed to stronger overlap between the reactant viꢀ
bronic ground state and product vibronic excited states for the
deuterated complex due the smaller deuterium quantum spacing in
phenol. Low frequency heavy atom vibrations are also suggested
as promoting modes for the concerted reaction.
Scheme 1. Intramolecular Proton Coupled Electron Trans-
fer Following Flash Photolysis Showing Possible Stepwise
and Concerted PCET Mechanisms
For multisite PCET reactions, the EPT pathway is favorable
energetically; however, it is often nonadiabatic and may be kinetꢀ
ically inhibited due to the requirement of simultaneous transfer of
the electron and proton. Furthermore, a single system may exhibit
four separate mechanisms depending on the solution medium.33 In
water, the analysis is tedious due to the apparent pHꢀdependent
oxidation potential of the phenol which is an incorrect measure of
driving force (the opposite of the standard free energy of the reacꢀ
tion). Correct analysis requires specification of the exact proton
acceptor (i.e., water, buffer, or OH−) and its concentration.47,48
The thermochemistry of PCET models can be analyzed with a
larger variation of chemical or electrochemicallyꢀsupplied driving
forces.40,49ꢀ53
The expansion of this field beyond intellectual analysis of PSII
mimicry is evident in recent applicationꢀbased studies employing
Hꢀbonded phenols as electron mediators. In one study, the quanꢀ
tum efficiency for photoelectrochemical water splitting with an
iridium oxide water oxidation catalyst coꢀadsorbed onto TiO2 with
a Ru(II) sensitizer was more than doubled by using a benzimidazꢀ
ole phenol mediator adsorbed on the catalyst to facilitate the slow
electron transfer rate of the catalyst.54 In this system there was no
covalent link between the mediator and sensitizer (nonadiabatic
PCET) and complicated kinetics were observed.
It occurred to us that a spatially wellꢀdefined and strongly elecꢀ
tronically coupled Ruꢀphenol system could be useful in favoring
an adiabatic mechanism and therefore facilitating a more rapid
PCET process in applicationꢀbased research. A literature search
a The concerted pathway is shown in green, the ETꢀPT pathway in
blue, and the PTꢀET pathway in red. The data are best described
by the ETꢀPT pathway (see text). The recombination reaction,
with rate kBETꢀ2 is concerted.
revealed intramolecular PCET in
a Ru(II)ꢀphenanthrolineꢀ
imidazoleꢀphenol complex, which should be strongly coupled, but
the details of PCET were obscured because excited state quenchꢀ
ing was rate limiting.36 Furthermore, the system contained an NH
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Journal of the American Chemical Society
and a bleach at ca. 460 nm (Figure S20). These features are typiꢀ
RESULTS
3
cal of the MLCT excited state of [Ru(bpy)3]2+ꢀlike complexes,59
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Synthesis and characterization. The reaction of 1,10ꢀ
phenanthrolineꢀ5,6ꢀdione with 3,5ꢀdiꢀtertꢀbutylꢀsalicylaldehyde
and excess NH4OAc in methanol proceeded cleanly in a microꢀ
wave reactor at 150 ºC (Scheme 2). While this reaction is often
carried out in glacial acetic acid at reflux, the reaction time was
shortened in the microwave and highly pure material crystallized
upon cooling. The imidazole and phenol acidic sites were methylꢀ
ated with MeI in the presence of KOH to yield the PCETꢀfree
ligand, phenꢀImꢀPhOMe. Cleavage of the methyl ether58 with
PhSH and catalytic Cs2CO3 provided the desired phenꢀImꢀPhOH
ligand. The Ru(II) complexes were prepared in the usual way by
the reaction of phenꢀImꢀPhOH/OMe with [Ru(bpyꢀd8)2Cl2] in
ethanol/water, and the deuterated bipyridine was chosen only for
and decay with time constants of 1.48 and 1.50 ꢁs, respectively, in
agreement with the emission lifetime. In the presence of methyl
3
viologen (MV2+), the MLCT state is quenched oxidatively (kQ,
Scheme 1) as shown by loss of the 350 nm excited state absorbꢀ
ance and growth of MV+• absorbances at 390 and 610 nm.
At low MV2+ concentrations (e.g. 5 mM), the intramolecular
PCET step is not distinguishable from quenching (see Figure S21
and the detailed explanation); however, the rate of oxidative
3
quenching of the MLCT state by MV2+ can exceed the rate of
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intramolecular PCET if a sufficient excess of MV2+ is used. Sternꢀ
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Volmer quenching constants, kQ, on the order of 1.4 × 109 M−1 s−1
3
were measured indicating that the MLCT lifetime could be reꢀ
duced below the limit of the laser pulse with 100 mM MV2+. Figꢀ
ure 1 shows the 50 ns and 200 ns transient spectra of RuꢀphenꢀImꢀ
PhOH/OMe and kinetic traces at 390 nm and 460 nm. After 50 ns,
the quenching reaction is complete and the spectra show MV+•
absorptions and the RuIII bleach for both molecules. At 200 ns
these features are unchanged in the PhOMe complex, while the
460 nm bleach of the PhOH complex is absent and a new weak
absorption appears at 420 nm. The kinetics at 460 nm are well
modeled by a biexponential fit that includes a fast temperature
independent component of 6 ns, which is the instrument response,
and a temperature dependent component with a time constant of
2.6 × 107 s−1 at 19 ºC. Equivalent rates were also obtained from
the growth of absorptions at 390 or 420 nm indicating the appearꢀ
ance of these features coincides with disappearance of the RuIII.
The system is completely reversible, and a single sample can be
exposed to hundreds of laser pulses with no change in the ground
state UVꢀvis absorption spectrum.
1
the purpose of simplifying the H NMR spectra. The OH proton
of RuꢀphenꢀImꢀPhOH resonates sharply at 11.24 ppm in dry
CD3CN and remains sharp with added methanol or water indicatꢀ
ing a strong NꢀꢀꢀHOPh hydrogen bond resistant to exchange in
protic solvents. Indeed, the strength of this hydrogen bond (estiꢀ
mated as 9 kcal mol−1 in the literature)49 complicated preparation
of the deuterated phenol PhOD complex by passive exchange in
CD3OD at room temperature or 50 ºC. Attempts to prepare the
PhOD complex in CD3CN/D2O in the presence of Et3N also
failed. Complete exchange required deprotonation with excess
sodium deuteroxide, acidification with DCl, and precipitation with
KPF6 in D2O (see Supporting Information for 1H NMR spectra).
Electronic absorption spectra of the RuII complexes in CH3CN
1
exhibit MLCT absorptions at λmax = 455 nm (ε ~ 17,000 M−1
cm−1) and intraligand π→π* bands at 280 nm (ε ~ 90,000 M−1
cm−1) typical for RuII polypyridyl complexes (Figure S17). Steady
state emission spectra exhibit λmax = 608 nm and quantum yields
of emission (Φ) of 0.098, 0.099, and 0.11 for the PhOMe, PhOH,
and PhOD complexes, respectively (Figure S18). The lifetime of
the 3MLCT excited state is 1.5 ꢁs for each molecule. Importantly,
the similar photophysical data for RuꢀphenꢀImꢀPhOMe vs Ruꢀ
phenꢀImꢀPhOH/D eliminates intramolecular electron transfer in
the excited state since such a process would attenuate the emisꢀ
B
A
150
100
50
[(bpyꢀd8)2Ru(phenꢀImꢀPhOMe)]2+
[(bpyꢀd8)2Ru(phenꢀImꢀPhOH)]2+
50 ns
200 ns
50 ns
200 ns
3
0
sion lifetime through quenching of the MLCT state. Apparently,
the excited state RuII*/I potential is insufficiently oxidizing to trigꢀ
ger the electron transfer reaction.
ꢀ50
Scheme 2. Synthesis of the ligands and ruthenium com-
plexes
400
500
600
700
800
400
500
600
700
800
Wavelength, nm
Wavelength, nm
D
C
20.0
100
50
19.8
19.6
19.4
19.2
19.0
18.8
18.6
18.4
390 nm
PhOH
PhOD
PhOH
PhOMe
double exponential fit
0
460 nm
slope: –3.902
intercept: 33.333
ꢀ50
ꢀ100
0.00
0.05
0.10
time, ꢁs
0.15
0.20
3.4
3.5
3.6
3.7
3.8
1000/T, K–1
Figure 1. (A) Transient absorption spectra of RuꢀphenꢀImꢀPhOH
50 and 200 ns after the laser pulse. (B) Transient absorption specꢀ
tra of RuꢀphenꢀImꢀPhOMe 50 and 200 ns after the laser pulse. (C)
Kinetic traces at 390 and 460 nm at 13 ºC. The black trace is a
double exponential fit of the 460 nm decay used to calculate the
rate constant. (D) Plots of ln(ketT1/2) vs T−1 used to determine the
reorganization energy, λ, for electron transfer (see text). Each
experiment was performed in dry CH3CN with [Ru] = 30 ꢁM,
[MV2+] = 0.1 M, and [Bu4NPF6] = 0.1 M.
The disappearance of the RuIII transient bleach at 460 nm and
the appearance of absorptions from 390ꢀ420 nm consistent with
the phenoxyl radical support a net PCET oxidation of the phenol
to regenerate RuII and the Im(H)+ꢀPhO• phenoxyl radical. The
Transient absorption. Excited state transient absorption difꢀ
ference spectra of RuꢀphenꢀImꢀPhOH/OMe in CH3CN are similar
and feature prominent absorptions centered at 350 nm and 600 nm
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phenoxyl radical signature is visible on the difference spectrum
Three oxidative processes are observed for RuꢀphenꢀImꢀPhOH
since the extinction coefficient of the tyrosyl radical of ~3000 M−1
cm−1 is approximately 7% of the value for the MV+• (41800 M−1
cm−1).60,61 The kinetics of the PCET reaction were investigated as
a function of temperature using the protio PhOH and deuterio
PhOD isotopologues and the results are presented as a modified
Arrhenius plot in Figure 1D. A kinetic isotope effect (KIE, kH/kD)
of 0.99 ± 0.03 was obtained over the accessible temperature range
of −13 to +20 ºC, and the linear fit in Figure 1D includes OH and
OD data points. Among synthetic models of the Tyrz/His pair in
PSII, the absence of a KIE is rare. In Tyrz oxidation in PSII, the ns
phase shows no KIE, but the ꢁs component does.17,18 To the best
of our knowledge, the only other model with an internal base and
lacking a KIE is a closely related phenolꢀbenzimidazoleꢀporphyrin
complex.38 However, that result is questionable because the
measurement was performed by cyclic voltammetry (CV) in
CH3CN with 2% CH3OH or CD3OD. We found that the strongly
Hꢀbonded phenolꢀbenzimidazole proton will not exchange in
CH3CN/CD3OD. In certain cases, where bulk solvent is the proton
acceptor, there is no KIE at low pH, rates were ~103 s−1, and a
stepwise ETꢀPT mechanism was proposed.33 Analysis of Figure
1D is essential for elucidating the PCET mechanism (stepwise or
concerted, Scheme 1) and for characterizing the thermodynamic
parameters (see Discussion).
Finally, the recombination reaction between [RuIIIꢀphenꢀImꢀ
PhOMe]3+ or [RuIIꢀphenꢀIm(H)+ꢀPhO•]3+ with MV+• was studied.
In each case, second order equal concentration kinetics are obꢀ
served (Figure 2A). The fastest reaction occurred between the
[RuIIIphenꢀImꢀPhOMe]3+ / MV+• pair near the diffusional limit
with a rate of 1.7 × 1010 M−1 s−1 at 25 ºC. The reaction of MV+•
with the Im(H/D)+ꢀPhO• radicals revealed an inverse KIE of 0.54
with kBET_2 = 5.9 × 109 M−1 s−1 and 1.1 × 1010 M−1 s−1 for the proꢀ
tio and deuterio complexes, respectively, at 25 ºC. The inverse
KIE suggests a stepwise PTꢀET or concerted EPT mechanism in
this case. Figure 2B shows a modified Arrhenius plot of the data
which, as for the intramolecular PCET reaction, was used to anaꢀ
lyze the mechanism and thermodynamics of the intermolecular
PCET reduction of the phenoxyl radicals by the MV+• (see Disꢀ
cussion).
in CH3CN: a highly distorted electrochemically irreversible, but
chemically reversible wave 1 (the original species is regenerated
1
2
3
4
5
6
7
8
a
c
upon reduction) with Ep = 1.13 V and Ep = −0.07 V, an irreꢀ
a
versible wave 2 with Ep = 1.27 V, and a reversible RuIII/II couple
at Eo = 1.35 V that is shifted anodically from the RuꢀphenꢀImꢀ
PhOMe reference with Eo = 1.28 V. The 0.08 V shift is attributed
to strong coupling to the preceding oxidations. The total current
for waves 1 and 2 is roughly equivalent to the current of the
RuIII/II couple as shown in Figure 3A.
9
The effect of strong acid or base on waves 1 and 2 was investiꢀ
gated (Figure 3B). In the presence of triflic acid, the imidazole
ring is protonated, and wave 1 is absent while the current for wave
2 is larger. If Bu4NOH is added, the phenol is deprotonated, and
an irreversible phenoxide oxidation is observed at 0.23 V. These
acid/base reactions and their associated electrochemical responses
are reversible: a CV recorded after acidification of the Bu4NOH
solution was identical to the red trace in Figure 3B.
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3. RuIII/II
1.35 V
A
6
4
⋅
PhO + H+ + e– → PhOH –0.07 V
2
0
ꢀ2
ꢀ4
ꢀ6
ꢀ8
⋅
1. PhOH – e–
→
PhO + H+ 1.13 V
⋅
2. PhOH – e–
→
PhOH+ 1.27 V
1.5
1.0
0.5
0.0
ꢀ0.5
E, V vs SCE
B
C
0
ꢀ5
1.6
1.2
0.8
0.4
1.
in dry CH3CN
with CF3SO3H
ꢀ10
ꢀ15
current ratios
wave 1:2
2.
wave 3:2
3.
1.4
0.0 0.2 0.4 0.6 0.8 1.0
ν1/2
1.6
1.2
1.0
0.8 0.6
E, V vs SCE
Figure 3. (A) Square wave voltammetry of RuꢀphenꢀImꢀPhOH (1
mM) with a cathodic (top) or anodic (bottom) sweep in CH3CN at
a glassy carbon electrode. (B) Oxidations (2 mM) in CH3CN or in
the presence of CF3SO3H. (C) Scan rateꢀdependent plots of the
ratios of current for peaks 1 and 3 vs peak 2.
3.0x106
B
A
PhOMe
PhOD
PHOH
26.4
26.2
26.0
25.8
25.6
25.4
25.2
25.0
24.8
2.5
PhOMe
slope: –879.9
intercept: 29.3
2.0
1.5
1.0
0.5
Next, the relationships among waves 1, 2, and 3 were investiꢀ
gated by changing the sweep rate (ν). The total current observed
at the peak potential of peak 3 is proportional to ν1/2 as expected
for a diffusion controlled process; however, the ratio of current for
peaks 1:2 or 3:2 varies inversely with ν1/2 (Figure 3C). This obꢀ
servation provides evidence that waves 1 and 2 are not sequential
1e− oxidations. It also negates an associated chemical step accordꢀ
ing to an ECE mechanism since the ratio of current at waves 1:2
would be largest at fast scan rates, the opposite of what is obꢀ
served here. Instead, the data suggest that the distorted response
of wave 1 is due to slow oxidation kinetics, as indicated by the
large separation between anodic and cathodic peaks while wave 2
is a fast oxidation step which dominates at fast sweep rates. Both
oxidations 1 and 2 yield the same product, and after passage
through peak 2, the molecule is oxidized by 1 electron. Assignꢀ
ment of these processes will be provided in the Discussion.
slope: –1125
intercept: 29.7
PhOD
PhOH
slope: –844.4
intercept: 28.1
3.8x10ꢀ3
0
40
80
time, ꢁs
120
3.3
3.4
3.5
3.6
3.7
T–1, K–1
Figure 2. (A) Secondꢀorder kinetics for the recombination reacꢀ
tion between [RuIIIꢀphenꢀImꢀPhOMe]3+ or [RuIIꢀphenꢀIm(H/D)+ꢀ
PhO•]3+ with MV+• at 8.2 ºC in CH3CN monitored by the disapꢀ
pearance of the MV+• absorbance at 607 nm. (B) Temperature
dependence of the recombination reaction.
Electrochemistry. The redox potentials for the RuIII/II
,
PhOH/PhOH+•, and PhOH/PhO• couples are needed to determine
the driving force for PCET oxidation and were determined by
cyclic and square wave voltammetry (Figure 3 and Figures S23ꢀ
27). The CV of the phenꢀImꢀPhOMe ligand shows sequential
DISCUSSION
a
Electrochemistry. An accurate assignment of the electrochemꢀ
ical behavior presented in Figure 3 is needed for understanding
the transient absorption data in Figures 1 and 2. Comparison of
CV data for the free PhOMe and PhOH ligands shows that the
imidazole/imidazolyl oxidation occurs at 1.52 and 1.58 V, respecꢀ
tively. The electrochemically irreversible couple (ꢂEp = 1.07 V)
irreversible oxidations at Ep = 1.52 and 1.86 V. The CV of the
phenꢀImꢀPhOH ligand shows an electrochemically irreversible,
but chemically reversible, couple with Epa = 1.19 V and Epc = 0.12
V and an irreversible couple at 1.58 V (Figure S23).
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Journal of the American Chemical Society
of the phenꢀImꢀPhOH is assigned as concerted EPT oxidation of
The calculated potential of 1.11V for the PhOH+/0 is found in
1
2
3
4
5
6
7
8
the phenol with the Hꢀbonded imidazole serving as the proton
acceptor. The reaction is chemically reversible as shown by the
regeneration of the original species upon reduction. Waves 1 and
2 of RuꢀphenꢀImꢀPhOH are not sequential oxidations or oxidation
by an ECE mechanism as shown by scanꢀrate dependent current
ratios. Wave 1 is highly distorted (ꢂEp = 1.2 V, E1/2 = 0.53 V), is
absent in the presence of a strong acid, and is absent in the Ruꢀ
phenꢀImꢀOMe control complex. These observations and compariꢀ
son to the CV to the phenꢀImꢀPhOH ligand show that waves 1 and
2 are the CV response to phenol oxidation. The similarity between
wave 1 and the free ligand support assignment of wave 1 as the
PCET oxidation of the complex.
reasonable agreement to the experimental value of 1.27 V.
The RuIII/II potential is coupled to phenol oxidation and is thereꢀ
fore shifted relative to Eo of RuIII/IIꢀphenꢀImꢀPhOMe. In the
quenching experiment, RuIII is obtained without prior phenol oxiꢀ
dation. Thus, for analysis of ꢂGo, we assume the RuIII/II potential
is equivalent to the control OMe complex of E0 = 1.28 V, which
was accurately reproduced by DFT calculations. Oxidation of the
phenol by EPT or ETꢀPT pathways is thus favorable given asꢀ
signment of waves 1 and 2 as the PhOH/PhO• and PhOH+/0 couꢀ
ples. Excited state intramolecular quenching is an uphill reaction
(see SI) and does not compete with quenching by MV2+ as shown
by the identical emission lifetimes of RuꢀphenꢀImꢀPhOH/OMe.
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31
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46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
While waves 1 and 2 are undoubtedly related to the phenol oxiꢀ
dation, the appearance of two waves and the large ꢂEp of wave 1
are curious features. The PCET oxidation of a hydrogen bonded
Phenol Oxidation. Assignment of standard potentials to the
RuIII/II couple and phenol oxidations by concerted or stepwise
pathway allows analysis of the kinetic data obtained in the TA
experiment and scrutiny of the mechanism. We propose a stepꢀ
wise ETꢀPT mechanism with rate limiting ET according to the
following arguments. Fast proton loss from a phenoxyl radical
cation is expected since these species are highly acidic in CH3CN
with pKa values < 0.66 The rapid rates measured here are supportꢀ
ive of a stepwise ETꢀPT mechanism since the concerted pathway
comes at a kinetic expense due to simultaneous proton and elecꢀ
tron movement in the transition state. For example, similar pheꢀ
nolꢀbase EPT oxidations are more sluggish (~ 105 s–1) even
though the driving forces of 0.25 to 0.44 eV are up to five times
larger than observed in the present work.34 The absence of a KIE
is also a reasonable argument against the concerted EPT mechaꢀ
nism; however, EPT without a KIE is possible if there is no barriꢀ
er to PT or if excited vibronic states with low energy vibrational
modes contribute significantly to the rate.46 The absence of temꢀ
perature effects on the KIE disfavors involvement of excited
states in EPT oxidation, but does not rule out activationless proton
transfer. Furthermore, analysis of the rate in the Marcus frameꢀ
work yields the most reasonable reorganizational and activation
energies if the stepwise ETꢀfirst pathway is considered and DFT
calculations reveal that the [RuIIꢀphenꢀImꢀPhOH+•]3+species is a
local minimum on the potential energy surface (Scheme 1).
phenol typically exhibits
a reversible or quasiꢀreversible
wave.50,62ꢀ64 For example, the series of phenolꢀbenzimidazoles
differing by protonation exhibit reversible oxidations at 0.21,
0.94, and 1.19 V for the anionic phenolate, the neutral compound,
and the protonated benzimidazolium, respectively.64 The complex
closely related to RuꢀphenꢀImꢀPhOH which differs by the N(H) vs
N(CH3) imidazole exhibits a reversible oxidation at 1.09 V vs
SCE.36 To the best of our knowledge, irreversible oxidation of Hꢀ
bonded phenol was observed in only one paper for a series of four
Ru(II) complexes (Chart 1, B).34 In this particular paper, the oxiꢀ
dative CV data were collected by sweeping through the RuIII/II
couple and ending at 0 V vs Fc+/0 (Fc+/0 = 0.4 V vs SCE). Thereꢀ
fore, it is unclear if a peak associated with reduction of the pheꢀ
noxyl radical could have been detected at a more negative potenꢀ
tial.
a
In the present case, the Ep of wave 1 is found in approximate
agreement with the potentials of Eo for reversible oxidations64 or
a
Ep of irreversible oxidations34 previously assigned as PCET oxiꢀ
dation with proton transfer to Hꢀbonded bases and a similar asꢀ
signment is warranted here. We postulate that the electrochemical
irreversibility observed in this work is attributed in part to the
N(1)ꢀmethyl benzimidazole which: (1) induces a 29º torsion angle
between the imidazole and phenol rings through steric repulsion
with H6 of the phenol according to the optimized DFT geometry
(Figure S28), and may contribute to the reorganization energy for
phenoxyl radical reduction; and (2) raises the pKa of the conjugate
acid of the free nitrogen. A second factor is backꢀbonding from
the Ru(II) which will also increase the pKa in the conjugated sysꢀ
tem. The basicity of the N atom is reflected in the asymmetry of
the Hꢀbond according to DFT calculations of the ν(NH+ꢀꢀꢀO•).
The calculated value of 3454 cm−1 agrees well with the experiꢀ
mentally observed imidazolium stretch of PSII, suggesting that
the harmonic approximation of the calculation is reasonable for a
localized, NH+ stretching frequency favoring the N atom.65 Comꢀ
bined, these factors contribute to the slow kinetics for the concertꢀ
ed EPT redox couple.
The PTꢀET pathway is disfavored according to the following:
(1) the absence of a KIE disfavors PT in the rate limiting step; (2)
the difference in phenol (pKa ~ 27)67 and benzimidazolium (pKa ~
17)68 pKa values suggests an equilibrium constant for PT of
10−ꢂpKa = 10−10 and eliminates PTꢀET with rate limiting ET (this
approximation assumes equivalent Hꢀbond stabilization in the Nꢀꢀ
ꢀHO and NH+ꢀꢀꢀO• forms); and (3) the driving force for ET in a
PTꢀET mechanism is approximately 1.13 eV since Eo
,
(PhO−/PhO•)
the potential of the phenolate couple, is now relevant. Computaꢀ
tion of λ (see below) with ꢂGo = −1.13 eV gives the unreasonable
value of λ > 3.2 eV.
As an initial approach to characterizing the ET reaction, the daꢀ
ta were analyzed using the semiclassical Marcus equation for
nonadiabatic electron transfer, equation 1,
ꢊ
Finally, the identity of the irreversible wave at 1.27 V is asꢀ
signed by considering its current relative to the PhOH/PhO• EPT
wave. As the scan rate is decreased, current for the EPT oxidation
increases at the expense of wave 2 suggesting these two waves
correspond to competing processes. Second, when the protonated
imidazolium cation cannot act as the proton acceptor for phenol
oxidation, the PhOH/PhO• couple is absent, and wave 2 remains
with enhanced current. We then assign wave 2 as a pure electron
transfer oxidation of the phenol (PhOH+/0), with irreversibility due
to the rapid proton loss. In the presence of acid, the bulk medium
is the proton acceptor and no associated cathodic wave was obꢀ
served. Wave 2 is observable in the absence of acid simply beꢀ
cause electron transfer at wave 1 is slow in the EPT oxidation.
ꢀꢁꢂꢃ = νꢄꢅ exp ꢆꢇ(ꢈꢉ ꢋꢌ ꢏꢔ
(1a)
)
ꢍꢎ
ꢐꢌꢑꢒ
ꢓ
ꢏ
ꢐꢕꢏ ꢖꢍꢎ
|
|
νꢄꢅ
=
(1b)
ꢗꢘꢐꢙꢌꢍꢎꢑꢒ
ꢓ
where ꢂGo is the free energy change for electron transfer, λDA
is the total nuclear reorganizational energy (equal to the sum of
λm, the energy required for reorientation of the medium, and λin,
the energy of high frequency distortions of the reactants), HDA is
the electronic coupling element between the donor (RuIII) and
acceptor (the phenol), and νel is the frequency of electron hopping
in the intersection region.69ꢀ71 For completeness, the intercept of
this Arrhenius plot yields an activation enthalpy, so there is an
entropic factor, exp(−ꢂS‡/RT), missing from the prefactor. Given
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Journal of the American Chemical Society
Page 6 of 12
that this is essentially a unimolecular reaction in which the proton
It is apparent from equation 3a that kET = kNAD when ꢚꢺ
and conversely, kET = kAD when ꢻꢄꢅ ≫ ꢚꢺ. Substitution of ꢻꢄꢅ and
ꢛ into equation 3a yields equation 3b.
̅
≫ ꢻꢄꢅ
1
2
3
4
5
6
7
8
in the reactant is poised to transfer, one can argue that the (negaꢀ
tive) entropy of activation is quite small and the entropic factor
should be near unity. An Eyring analysis of the data yields ꢂH‡ =
6.9 kcal mol−1 (0.30 eV) and ꢂS‡ = −0.71 cal mol−1 K−1 (ꢀTꢂS‡ =
0.009 eV) confirming acceptable omission of ꢂS‡ in the Marcus
analysis. The Marcus activation energy of (ꢂGo + λ)2/4λ is 6.8
kcal mol−1 (see below for ꢂGo and λ).
̅
ꢚ̅
ꢏ
ꢏ
ꢼꢽ ꢾꢿ
ꢾ
ꢍꢎ
ꣃ
ꢊ
ꢀꢮꢓ
=
exp ꢶꢇ(ꢈꢉ ꢋꢌ ꢏꢹ (3b)
ꣀ
ꢘ
ꢼꣁꣂ
꣄
)
ꢜ
ꢜ
ꢏ
ꢒ
ꢭ
꣋
ꢬ
꣌
ꣃ꣄
ꢏ
ꢐꢸꢜꢑꢒ
ꢓ
꣆ꢽ ꢾꢿ
ꢾ
꣇
ꢱ꣈꣉꣊
ꢍꢎ
ꢬ
ꢝꢋꣅ
꣍
ꣀꣂ
ꢜ
The kET vs T data were fitted to equation 3b with adjustable
HDA and ꢤꢧꢨ (Figure 4A). Using the lower limit for HDA = 280
cm−1 from the nonadiabatic equation requires ꢤꢧꢨ = 48 fs. Conꢀ
versely, in the adiabatic limit, the rate constant is independent of
HDA as the second term in the denominator becomes >> 1. The
upper limit of ꢤꢧꢨ = 54 fs (equation 2c) requires HDA ≥ 540 cm−1,
which is also acceptable because the barrier lowering due to a
large HDA can be ignored for HDA < λm/2 and ꢂGo ≈ 0.80 Thus, we
conclude 280 cm−1 < HAD < 540 cm−1 but cannot specify an exact
value.
The free energy change was best estimated as the difference beꢀ
tween the RuIII/II and PhOH+/0 couples (0.01 eV). From the slope
and intercept of Figure 1D, λDA = 1.36 eV and HDA = 280 cm−1
(0.035 eV). The λDA value is in reasonable agreement, though
slightly high, with λDA = 1 eV estimated from the sum of half the
reorganization energy for the selfꢀexchange reaction of
[Ru(bpy)3]3+/2+ (0.4 eV72) and the reorganization energy for elecꢀ
trochemical oxidation of phenols hydrogen bonded to amines (~1
eV).50,52 The uncertainty in λDA is approximately 12% from the
error in fitting of Figure 1D. Calculations using a larger ꢂGo, i.e.,
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46
47
48
49
50
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52
53
54
55
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58
59
60
It is useful to visualize nonadiabatic and adiabatic contributions
to the observed rate (equation 5).75 Figures 4B and 4C illustrate
that the reaction is strongly adiabatic using both sets of ꢤꢧꢨ/ HDA
from Figure 4A. In the lower limit of HDA = 280 cm−1, minimal
nonadiabatic contribution is possible. This comparison justifies
using the solvent controlled equation 2c to determine λm from the
slope of Figure 1D.
a
Eo(RuIII/II) – Ep (PhOH/PhO•) = 0.15 eV yield values of λDA >1.63
eV, which appear large considering the above comparisons. One
possible cause of an elevated value for λDA is the omission of
temperature dependence of the reaction free energy. Additionally,
HDA exceeds the approximate 200 cm−1 (0.025 eV) upper limit for
nonꢀadiabatic electron transfer70 and prompted further scrutiny of
the kinetic description.
ꢝ
ꢝ
ꢝ
=
+
(5)
The rate constant for electron transfer in the adiabatic (slow
ꢑ꣋꣄
ꢑꢯꢎꢍ
ꢑꢎꢍ
solvent relaxation) limit is given by
A
ꢊ
exp ꢆꢇ(ꢈꢉ ꢋꢌ ꢏꢔ
(2a)
(2b)
)
ꢜ
τL = 48 fs HDA = 280 cm–1
25x106
20
o
ꢀꢂꢃ = ꢚꢛ
̅
τL = 54 fs HDA = 540 cm–1
ꢐꢌꢜꢑꢒ
ꢓ
o
ꢌꢡ
ꢚꢛ̅
= ꢆ ꢝ ꢔ
ꢠ
ꢞꢟ
ꢝꢢꢕꢑꢣꢓ
15
10
where ꢚꢛ and ꢤꢥare the effective relaxation frequency and the
̅
PhOH
PhOD
longitudinal relaxation time of the surrounding medium, respecꢀ
tively. Contributions from the high frequency innerꢀshell distorꢀ
tions to the reorganization energy are ignored and ꢂGo/λm is less
than 0.3.73ꢀ75 The longitudinal relaxation time, ꢤꢥ = ꢤꢦ(εo/εs)
where ꢤꢦis the dielectric relaxation time and εo and εs are the statꢀ
ic and optical dielectric constants of the solvent76 has a thermally
activated temperature dependence given by ꢤꢥ = ꢤꢧꢨexp(ꢩꢥ/ꢀꢪꢫ)
where ꢩꢥ is the activation energy.77 Estimated values of ꢩꢥand ꢤꢧꢨ
are 4.94 kJ mol−1 (413 cm−1) and 31 fs from the temperature deꢀ
pendence of εs and ꢤꢦ.78 Substitution yields the expression for the
temperature dependence of the solventꢀcontrolled ET rate.
265
270
275
T, K
280
285
290
B
C
τL = 48 fs HDA = 280 cm–1
τL = 54 fs HDA = 540 cm–1
1/kET
o
o
150
100
50
150
1/kET
1/kET_adiabatic
1/kET_nonadiabatic
1/kET_adiabatic
1/kET_nonadiabatic
100
50
0
0
265 270 275 280 285 290
T, K
265 270 275 280 285 290
T, K
ꢌꢡ
ꢊꢋꢌꢜ)ꢏꢇꢐꢌꢜꢮꢬ
ꢀꢂꢃ = ꢆ ꢝ ꢔ
exp ꢆꢇ(ꢈꢉ
ꢔ
(2c)
ꢠ
ꢞꢬꢭ
ꢝꢢꢕꢑꢣ
ꢓ
ꢐꢌꢜꢑꢒ
ꢓ
Figure 4. (A) Kinetic data for the intramolecular oxidation of the
phenol by RuIII in RuꢀphenꢀImꢀPhOH/D fit to equation 3 using
ꢂGo = 0.01 eV, λm = 1.16 eV, and EL = 0.051 eV. (B and C) Visuꢀ
alization of adiabatic and nonadiabatic contributions to the total
time constant for electron transfer at the minimum HDA = 280
cm−1 (B) and adiabatic limit with ꢤꢧꢨ = 54 fs (C).
From the slope of Figure 1D using ꢩꢥ =4.94 kJ mol−1, λm
=
1.16 eV in better agreement with 1.2 eV from the sum of the doꢀ
nor and acceptor reorganizational energies as discussed above.
The prefactor of equation 2c was analyzed using the intercept of
Figure 1D by calculating ꢤꢧꢨ = 54 fs in reasonable agreement with
the estimated value of 31 fs. Agreement here suggests the adiaꢀ
batic treatment is an appropriate description of the ET reaction;
however we sought to determine the magnitude of HDA in the
strongly coupled DꢀA pair.
Table 1 Summary of Electrochemical Potentials and Kinet-
ics of PCET Reactions.
Phenol
PhOH0/+
(V)
kETꢀPT
×
HDA
λDA
A general expression for rate constants for ET controlled by
small electronic coupling and slow solvent dynamics is readily
obtained from a steadyꢀstate treatment of the electron transfer.73ꢀ
Oxidation
10−7 (s−1) (cm−1)
(eV)
1.16
RuꢀphenꢀImꢀ 1.27
PhOH(D)b
2.7
280ꢀ540
75,79
Provided the reaction is in the “narrow reaction window”
limit (ꢂGo / λm << 1),77 the rate constant for ET proceeding via
Phenoxyl
Radical
Reduction
PhOH/PhO• kEPT
×
VPCET
λDA
the competing nonadiabatic and adiabatic pathways is given by
(V)
10−9 (M−1 (cm−1)
(eV)
ꢊ
exp ꢶꢇ ꢷꢉ ꢋ ꢸ ꢏꢹ
(3a)
s−1)
(
)
ꢑꢯꢎꢍ
ꢰꢱꢲ
ꢜ
ꢀꢮꢓ
=
=
ꢝ ꢋ ꢆꢴꢵꢳꢱꢲꢔ
(ꢝꢋ ꢑꢯꢎꢍ/ꢑꢎꢍ
)
ꢐꢸꢜꢑꢒꢓ
ꢡ
RuꢀphenꢀImꢀ 0.53
PhOH(D)b
5.9 (11)
22 (49)
1.7
(1.8)
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Journal of the American Chemical Society
a V vs SCE in CH3CN. b OD values are in parenthesis.
ic peak potential) is considered, λDA is greater than 2.4 eV, which
1
2
3
4
5
6
7
8
9
is larger than typical outer sphere concerted phenol oxidations.
Conversely, if ꢂGo = −0.38 eV (from the cathodic peak potential)
λDA is less than 0.9 eV and is unreasonably small since ~1 eV is
expected for the phenoxyl radical reduction alone. These considꢀ
erations support the choice of E1/2 as Eo for the concerted phenol
oxidation potential despite its abnormally large peakꢀtoꢀpeak sepꢀ
aration as discussed above.
Phenoxyl Radical Reduction. The phenoxyl radical PCET reꢀ
duction is relevant to PSII, to other biological enzymes including
DNA photolyases and ribonucleotide reductases,81 and to artificial
photosynthetic schemes. Both stepwise and concerted pathways
have been observed in various natural or model systems. An EPT
reaction facilitated by a network of strong hydrogen bonds was
proposed for the reduction of Tyrz of PSII by QA− at 190 K.82 The
recombination reaction between the TyrD (tyrosine 160) radical
The activation energies, as determined from the Marcus analyꢀ
sis, are 1.7 and 2.2 kcal mol−1 (590 and 790 cm−1), respectively
for the NH+ and ND+ reactants in good agreement with the Arrheꢀ
nius Ea values of 1.4 and 2.0 kcal mol−1. The log(A) values are
10.7 (NH+) and 11.5 (ND+) with log (AH/AD) = −0.7.
−
and QA in OEC depleted PSII is a multisite concerted reaction
with a normal KIE of 2ꢀ4 and rate constants < 1 × 105 s−1.83 Proꢀ
ton inventory experiments revealed multiple proton donors in the
radical reduction.84
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13
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15
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17
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22
23
24
25
26
27
28
29
30
31
32
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34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The inverse KIE is rare in the PCET literature and deserves furꢀ
ther discussion. An inverse solvent isotope effect of 0.5ꢀ0.7 has
been observed in the concerted recombination of a photodissociꢀ
ated tyrosyl radical and the solvated electron, but was not obꢀ
served in a PSIIꢀinspired βꢀhairpin peptide.98 In this case, the
solvent isotope effect was attributed to the phenolicꢀammonium
group interaction of tyrosine which produces a low reactantꢀstate
fractionation factor (preference for the proton) below pH 11.99 In
our experiment, fractionation factors do not contribute to the obꢀ
served kinetics since the OH/OD exchange was complete prior to
the experiment and there is no proton source in the acetonitrile.
Photoionization of phenol, phenolate, tyrosine, and tyrosinate
yields the phenoxyl radical, a proton, and a solvated electron on
the picosecond timescale.85ꢀ87 Recombination occurs with multiꢀ
phasic kinetics and time constants on the order of 500 ps with the
ETꢀ first mechanism favored on entropic arguments,85 but a conꢀ
certed reaction has been postulated in βꢀhairpin peptides where
tyrosine and histidine residues interact through a crossꢀstrand πꢀπ
interaction.87
In the absence of an exogenous reductant, the tyrosyl radical of
A. nidulans DNA photolyase is reduced by FADH− (flavin adeꢀ
nine dinucleotide) with t1/2 = 76 ms in H2O or 198 ms in D2O,
suggesting a concerted mechanism, while the radical can be reꢀ
duced by 2ꢀmercaptoethanol with a rate of 2 × 104 M−1 s−1.88,89
Class Ia and Ib ribonucleotide reductases facilitate radical propaꢀ
gation over 35 Å through multiple tyrosine residues via PCET.90ꢀ
93 The first observation of radical intermediates and reduction of a
tyrosyl radical was achieved in a 3ꢀnitro tyrosine mutant.94 In this
case, the radical decayed to the phenolate in an ETꢀfirst mechaꢀ
nism with rate constants of 283 and 67 s−1. Photoionization of
tryptophanꢀtyrosine dipeptides at pH > 12 yields the tryptophan
radical which reduced the tyrosyl radical with k = 1.2 × 105 s−1.95
To the best of our knowledge, the only other example of related
behavior is a Zn(II) porphyrin Hꢀbonded to a naphthalene diimide
acceptor in which normal and inverse KIEs were observed at high
and low temperatures, respectively.100 While the proton promotes
ET in the Zn(II)ꢀnaphthalene system, the proton is not formally
transferred. The KIE crossover was explained considering thermal
averaging of higher frequency proton vibrational states that proꢀ
mote the electronic coupling in the high temperature regime. At
low temperature, a transition from the vibrational ground state
was invoked in which the deuterium wave function better samples
the fluctuations needed to promote the EPT reaction.
The phenoxyl radical reduction in this work is a multiꢀsite
PCET with intermolecular electron transfer and intramolecular
proton transfer. The free energy change for the recombination
reaction between [RuIIIꢀphenꢀImꢀPhOMe]3+ and MV+• is −1.73 eV
(MV2+/+ = −0.45 V)96 and the rate of 1.7 × 1010 M−1s−1 at 25 ºC is
In the RuꢀphenꢀImꢀPhOH/D complexes the opposite behavior is
observed. At low temperature a normal KIE is predicted although
our experiment was limited to data far above the predicted crossꢀ
over temperature of 177 K, and the crossover temperature, while
considered possible, is not overemphasized. Possible explanations
for the KIE <1 are proposed considering the vibronic overlap
between donor and acceptor, the near activationless EPT reaction,
and the involvement of heavy atom low frequency vibrations as
promoting modes.
likely diffusion controlled. For the [RuIIꢀphenꢀIm(H)+ꢀPhO•]3+
+
MV+• reaction, ꢂGo = −0.98 eV if the halfꢀwave potential of the
PhOH/PhO• couple is chosen as Eo. With a lower driving force the
rate is significantly slower and not limited by diffusion. Assuming
a diffusionꢀlimited rate from the PhOMe + MV+• data at all temꢀ
peratures (kBET_1), the activated recombination rate, kBET_2, is
extracted from the measured rate, kobs according to equation 6.
The analytical rate expression for a PCET reaction is given by
equation 7
ꢝ
ꢝ
ꢝ
ꢀ꣑꣒ꢮꢓ
=
=
+
(6)
ꢾꢖꢍꢎ꣕꣖꣗ꢾꢏ
ꢒꢓ꣙꣖ꢏ꣗
꣚꣛ꢏ
꣖꣗ꢋꢌꢍꢎ)
ꢑꢭ꣎꣏
ꢑꢒ꣋꣄_ꢏ
ꢑꢒ꣋꣄_꣐
ꢊ
exp ꢆ꣘ꢑ
ꢔ
ꢓ exp ꢆꢇ(ꢷꢉ
ꢠꢌꢑ
ꢐꢌꢑꢒ
ꢒ
ꢏꢔ (7)
ꢕ
For the PhOH and PhOD complexes, kBET_2 is 5.9 × 109 M−1s−1
and 1.1 × 1010 M−1s−1, respectively, at 25 ºC. For comparison to
PSII, reduction of the tyrosyl radical by the OEC occurs with rates
from 700 to 33000 s−1 and driving forces of ~0.1 to 0.2 eV.6 The
inverse KIE of kH/kD = 0.55 suggests proton movement in the rate
determining step. A PTꢀfirst mechanism is disfavored based on
the pKa difference between the phenoxyl radical cation (< 2) and
the imidazolium (~17) while an ETꢀfirst mechanism should be
less sensitive to the isotope.
∑
∑
꣜
꣔ ꣓
꣔
ħ
ꢓ
where ꢀ and ν are reactant and product vibronic states, Pꢀ is the
Boltzmann population of a reactant state, ꢂGꢀν is the free energy
of reaction for ꢀ and ν, Sꢀν is the overlap between wavefunctions
of initial and final states, αꢀν describes the exponential decay of
the overlap with respect to donorꢀacceptor distance, R, and M and
ꢃ are the effective Rꢀmode mass and frequency, respectively.43 A
detailed calculation is beyond the scope of this manuscript; howꢀ
ever, the calculated O–H stretches of phenolꢀbase compounds of
2313 cm−1 in the diabatic potential energy surface show that the
Hꢀbond of the phenol is highly symmetric and shifted to low freꢀ
quency relative to phenol (ν(OH) ~3600 cm−1).101 As observed
here, the harmonic approximation overꢀestimates the stretching
frequency of the phenol; however the calculated values of ν(NH+)
= 3450 cm−1 and ν(ND+) = 2540 cm−1 are notably similar to the
experimental values of 3250ꢀ3350 cm−1 and 2600ꢀ2400 cm−1
As a first approximation of the reorganization energy and elecꢀ
tronic coupling element, the data were treated in the nonadiabatic
electron transfer limit (equation 2), which is appropriate for EPT
reactions,97 replacing HDA with VPCET as the product of electronic
and vibronic coupling. From the slopes and intercepts of Figure
2B using ꢂGo = −0.98 eV, λDA = 1.7 eV and VPCET = 22 cm−1 for
the OH complex and λ = 1.8 eV and VPCET = 49 cm−1 for the OD
complex. For arguments sake, if ꢂGo = −1.58 eV (using the anodꢀ
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measured for the imidazolium NH and ND stretches of Hisꢀ189 in
CONCLUSIONS
PSII65 supporting the appropriateness of the harmonic approximaꢀ
tion for the phenoxyl radical NH/ND stretch (see SI). Since the
reaction is highly exergonic, PhOH/D vibronic excited states of
the product will be involved in the reduction reaction.
1
2
3
4
5
6
7
8
The complex [(bpy)2RuꢀphenꢀImꢀPhOH]2+ was prepared to
study the intramolecular proton coupled electron transfer oxidaꢀ
tion of the appended phenol by RuIII generated by a flashꢀquench
experiment with methyl viologen as the oxidative quencher. The
electrochemical analysis revealed interesting aspects of the phenol
oxidation. Both the concerted EPT PhOH/PhO• and ETꢀPT
PhOH+/0 couples were observed due to the sluggishness of the
concerted reaction. Importantly, the concerted oxidation was elimꢀ
inated if the internal imidazole base was protonated by a strong
acid. Temperatureꢀ and isotopeꢀdependent experiments revealed
the absence of a kinetic isotope effect for the rapid (>107 s−1)
PCET reaction suggesting strongly that the reaction occurs via a
stepwise ETꢀPT mechanism despite its smaller driving force of
only ca 0.01 eV. The mechanistically simple ETꢀPT dominates
due to the sluggishness of the multisite concerted reaction in
which the electron and proton must necessarily transfer on the
same potential energy minimum.
The contributions of excited acceptor modes to EPT oxidation
of phenols with preꢀformed hydrogen bonds has been illustrated in
the literature.101,102 In these examples, normal kinetic isotope efꢀ
fects were measured and predicted by theory; however, the greater
contribution of deuterium vibrational excited modes in the prodꢀ
uct are shown in agreement with greater accessibility due to the
smaller energy spacing of ND vs NH. The activation energy difꢀ
ference between the phenoxyl radical reduction measured here
and the phenol oxidation measured is significant. For outer sphere
PCET oxidation of phenolꢀbase molecules, typical Ea values are
greater than 5 kcal mol−1, in which case the zero point energy
difference of the donor will contribute to the normal KIE.103 In
our case, the PCET radical reduction reaction is nearly activationꢀ
less with Ea = 1.7 and 2.2 kcal mol−1 (580 and 780 cm −1) for the
protio and deuterio complexes which is well below the zero point
energy of an NH+ or ND+ vibration. Further support for the
stronger overlap of OD states of the acceptor with the reactant is
ascertained in the difference between VPCET for the protio and
deuterio complexes (Table 1). As the product of electronic and
vibronic coupling, VPCET reveals stronger coupling in the deuteratꢀ
ed complex as a contributor to the enhanced EPT rate.
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10
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16
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22
23
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25
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59
60
A detailed analysis of the electron transfer parameters revealed
that the reaction is adiabatic due to the strong electronic coupling
(280 < HDA < 540 cm−1) and slow solvent relaxation. The reorganꢀ
ization energy, λm, was determined as 1.16 eV in good agreement
with the sum of ½ λm for the RuIII/II selfꢀexchange and λm for oxiꢀ
dation of model phenol compounds with hydrogen bonded proton
acceptors.
As a final study, we examined the recombination reaction beꢀ
tween the phenoxyl radical and the methyl viologen radical catiꢀ
on. The reaction follows equal concentration second order kinetics
and occurs over several hundred milliseconds due to the low conꢀ
centration of reactants formed by the laser pulse. The bimolecular
rate constant is greater than 109 M−1 s−1 due to the large driving
force. In this case an inverse kinetic isotope effect was observed
unexpectedly and suggests an EPT mechanism. The reaction is
nearly barrierless with Ea < 2 kcal mol−1, and we interpret the KIE
< 1 as an indication of stronger overlap of the ν = 0 vibrational
state of the donor with vibrational excited states in the product for
OD. The data suggest a possible normal KIE at low temperature
which is interpreted due to the depopulation of low energy heavy
atom promoting vibrations. We are currently investigating the
roles of solvent, imidazole pKa, and phenol standard potentials on
the oxidation and reduction reactions. Detailed calculations of the
concerted recombination reaction will be a fruitful endeavor.
Low energy vibrational modes of the heavy atoms (C, N, O)
can promote EPT oxidation of phenols and can reduce the KIE
below that which is expected based on pure tunneling consideraꢀ
tion in the transition state geometry.104 These particular vibrations
decrease the NꢀO distance and have been discussed as heavy atom
“gating”.101 The bulk of PCET oxidation occurs for NꢀO distances
less than the calculated transition state distance where the proton
potential barrier decreases. Conversely, for large separations,
pathways to excited state vibrational levels are more important.
Interestingly, DFT calculations of the [RuIIꢀphenꢀIm(H/D)+ꢀ
PhO•]3+ (see the SI for coordinates) reveal rocking modes at ~400
cm−1 that clearly decrease the distance between the Nꢀand O atꢀ
oms suggesting these vibrations as promoting the EPT reaction. If
a normal KIE occurs at low temperature, as possibly predicted
from the data, it can be attributed to decreased population of exꢀ
cited states in the low energy heavy atom vibrations.
A KIE < 1 is also possible if the difference in zero point enerꢀ
gies of the transition states is smaller than in the reactants. This
could occur if stronger force constants in the transition state sigꢀ
nificantly resembled the product as in the hydride transfer from a
tungsten hydride to the trityl cation105 or in an equilibrium isotope
effect in the rate determining step of a reaction.106 Both possibiliꢀ
ties suggest a larger activation energy for the proton which is
inconsistent with our calculated ꢂEa (D−H) of 0.55 kcal mol−1.
EXPERIMENTAL SECTION
General. [Ru(bpyꢀd8)2Cl2] was prepared from bpyꢀd8 and
[Ru(COD)Cl2]n according to the literature.107 The PF6 salt of meꢀ
thyl viologen (MV2+) was prepared by anion metathesis of MVCl2
(Aldrich) with NH4PF6 in water, collected by filtration, and dried
under vacuum. 1,10ꢀphenanthrolineꢀ5,6ꢀdione and 3,5ꢀdiꢀtertꢀ
butylꢀ2ꢀhydoxybenzaldehyde were purchased from Ark Pharm
Inc. Acetonitrile was Chromosolv® grade purchased from Aldrich
and was dried first over an activated alumina column and then
over 3Å molecular sieves activated at 350 °C under high vacuum.
Prior to use, the CH3CN was vacuum transferred and stored in an
N2 filled glovebox. NMR spectra were recorded on a 400 MHz
Bruker Avance spectrometer at room temperature, and chemical
shifts in ppm are referenced to residual protio solvent peaks. Peak
assignments are given based on analysis of 1H and 13C 1D spectra
and COSY, HMBC, HSQC, and NOE 2D experiments. Elecꢀ
trospray mass spectra (ESIꢀMS) were measured using a Thermo
Finnigan mass spectrometer. Cyclic voltammograms were recordꢀ
ing using a threeꢀelectrode configuration with a glassy carbon or
Pt working electrode and Pt counter electrode. The reference elecꢀ
trode was Ag/AgCl (3 M KCl) separated from the working soluꢀ
tion by two Vycor frits and the supporting electrolyte was 0.1 M
Finally, it is interesting to consider the implications of our reꢀ
sults in artificial photosynthesis research. Due to the strong couꢀ
pling and the adiabatic electron transfer, quenching of a photoꢀ
generated hole at a RuIII center by the appended phenol will be
rapid, thus preventing charge recombination of RuIII with a
quencher or an electron injected into a semiconductor. Regeneraꢀ
tion of the phenol through recombination is nonadiabatic in the
case of the methyl viologen radical cation; however, this reaction
should be adjustable based on the distance between the phenoxyl
radical and its redox partner and on the driving force. The recomꢀ
bination of a semiconductor electron with the phenoxyl radical
should be excessively slow due to the diffusion of charge in the
semiconductor, while the reaction with an adjacent catalyst or
charge carrier would be more rapid. Experiments to probe these
hypotheses are underway.
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Journal of the American Chemical Society
Bu4NPF6. Potentials are reported versus the SCE electrode using
were performed to ensure that each optimized structure is a local
ferrocene as an internal standard (Fc+/0 = 0.40 V vs SCE in
CH3CN and 0.46 V vs SCE in CH2Cl2).108 The potential of the
Ag/AgCl electrode was −0.017 V vs SCE in CH3CN and + 0.021
V vs SCE in CH2Cl2. Analyte solutions (1 mM) were prepared in
the glovebox and transferred to the electrochemical cell under Ar.
A constant flow of Ar was passed through a CH3CN bubbler and
blanketed over the solution during measurements. Electronic abꢀ
sorption spectra were measured in a 1 cm quartz cell using an
Agilent 8453 diode array spectrophotometer. Infrared spectra
were recorded in KBr using a Thermo Nicolet Nexus 670 FTIR in
absorption mode. Emission spectra were measured in dry, air free
CH3CN using optically dilute samples in which the absorbance at
the excitation wavelength (440 nm) was less than 0.1. Quantum
yields of emission were determined relative to [Ru(bpy)3]Cl2 in
minimum on the potential energy surface.
1
2
3
4
5
6
7
8
2,4-di-tert-butyl-6-(1H-imidazo[4,5-f][1,10]phenanthrolin-
2-yl)phenol. A mixture of 1,10ꢀphenanthrolineꢀ5,6ꢀdione (1.00 g,
4.7 mmol), 3,5ꢀdiꢀtertꢀbutylꢀ2ꢀhydoxybenzaldehyde (1.11 g, 1
equiv.), and NH4OAc (3.66 g, 10 equiv.) were dissolved in 40 mL
CH3OH and heated in a microwave reactor at 150 °C for 1h with
constant stirring. Upon cooling, the solution was held overnight at
5 °C. A crop of yellow crystals was collected by vacuum filtration
(1.16 g, 57% yield). Additional product remained in the filtrate
1
9
but was not collected. H NMR data were identical to the literaꢀ
ture.36
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2-(3,5-di-tert-butyl-2-methoxyphenyl)-1-methyl-1H-
imidazo[4,5-f][1,10]phenanthroline (phen-Im-PhOMe).
slurry of 2,4ꢀdiꢀtertꢀbutylꢀ6ꢀ(1Hꢀimidazo[4,5ꢀ
f][1,10]phenanthrolinꢀ2ꢀyl)phenol (1.1 g, 2.6 mmol) in acetone
was treated with crushed KOH (3 equiv) and stirred under Ar at
room temperature for 1 h. Methyl iodide (3 equiv) was added by
syringe and the mixture was stirred for 24 h. The precipitate that
formed was collected by vacuum filtration and dried to provide
A
꣟꣠ ꢂ꣡
CH3CN (Φem = 0.062)109 using the equation: ꣝ꢄꢺ = ꣝꣞
꣟꣡ ꢂ꣠
where the subscripts “S” and “R” refer to the sample and referꢀ
ence respectively, I is the integrated steady state emission intensiꢀ
ty, and A is the absorbance at the excitation wavelength. Sternꢀ
Volmer quenching experiments were performed by measuring the
emission lifetime of optically dilute samples in the absence (I0)
and presence (I) of varied concentrations of MV2+. Aliquots of a
concentrated MV2+ solution containing the same concentration of
the analyte as used for Io measurements were titrated into a sepꢀ
tumꢀsealed quartz cuvette of analyte.
1
pure phenꢀImꢀPhOMe (0.91 g, 78% yield). H NMR (CDCl3, 25
°C): δ 9.18‒9.16 (m, 2H), 9.08 (dd, J = 8.1, 1.6, 1H), 8.83 (dd, J =
8.4, 1.3, 1H), 7.74‒7.68 (m, 2H), 7.54 (2s, overlapping, 2H), 4.12
(s, 3H, NCH3), 3.27 (s, 3H, OCH3), 1.45 (s, 9H), 1.37 (s, 9H). 13
C
NMR (CDCl3, 25 °C): δ 156.2 (q), 152.9 (q), 148.9, 147.9, 146.4
(q), 144.9 (q), 144.2 (q), 142.5 (q), 136.5 (q), 130.5, 128.2, 127.5,
126.6, 125.7 (q), 124.3 (q), 123.6, 122.7 (q), 122.6, 120.5 (q),
60.9 (OCH3), 35.4 (CtBu3), 35.2 (NCH3), 34.9 (CtBu3), 31.6
(tBu3), 30.8 (tBu3).
Transient Absorption. Samples for transient absorption were
prepared such that the ground state absorbance at wavelengths
above 350 nm were less than 1 and the absorbance at 440 nm was
approximately 0.5‒0.7. All samples were prepared in dry CH3CN
in airꢀtight Teflon sealed cuvettes and contained 0.1 M Bu4NPF6.
Samples with quencher contained 0.1 M methyl viologen. Altꢀ
hough passive OH to OD exchange was difficult as discussed in
the text, the OD to OH exchange appeared more facile and rigorꢀ
ously dried solvents were used for kinetic experiments. Samples
were pumped with a continuously tunable OPO (Opotek) pumped
by a Qꢀswitched Nd:YAG at 355 nm. The instrument was operatꢀ
ed at 5 Hz with 2.5‒3 mJ per pulse and ~10 ns pulse width. The
sample was probed at 90° with a 75 W Xe arc lamp which was
pulsed for time domains under 20 ꢁs and not pulsed for experiꢀ
ments up to 2 ms. The probe light was filtered where appropriate
and focused on an ISA monochromator. The signal was detected
using a Hamamatsu R955 PMT and digitized with an oscilloꢀ
scope. Spectra were compiled from transients which were collectꢀ
ed every 5 nm from 350 to 800 nm and each data point is the avꢀ
erage of 32 kinetic traces. For temperature dependent measureꢀ
ments, the sample was held in a water cooled sample holder with
an available temperature range of −10 to 30 °C. At least 64 laser
shots were averaged for the temperatureꢀdependent kinetics. Kiꢀ
netics of the recombination reaction were monitored at 607 nm,
the λmax of the methyl viologen radical,61 and were determined by
plotting [MV+•]−1 vs. time. Data were fit to single or double expoꢀ
nentials decays as appropriate with Origin® or Igor Pro® softꢀ
ware.
2,4-di-tert-butyl-6-(1-methyl-1H-imidazo[4,5-
f][1,10]phenanthrolin-2-yl)phenol (phen-Im-PhOH). The phenꢀ
ImꢀPhOH was prepared from phenꢀImꢀPhOMe by cleavage of the
methyl ether according to a literature procedure.58 A flask was
charged with phenꢀImꢀPhOMe (0.2 g, 0.44 mmol), NMP (3 mL),
benzene thiol (0.044 mL, 1 equiv), Cs2CO3 (8 mg, 5%), sealed
with a septum and purged with argon. The mixture was then heatꢀ
ed to 185 °C for 30 minutes. Upon cooling, 5 mL of water was
added to induce precipitation. The solid was collected by vacuum
filtration and purified by flash chromatography on basic alumina
eluting with chloroform. The material collected (0.09 g, 46%
yield unꢀoptimized) was > 95% pure and contained a small
amount of starting material. Final purification was achieved by
1
recrystallization from hot ethanol. H NMR (CDCl3, 25 °C): δ
11.74 (s, OH), 9.16 (m, 2H), 8.95 (dd, J =8.0, 1.6, 1H), 8.84 (dd, J
= 8.4, 1.6, 1H), 7.72‒7.63 (m, 2H), 7.49 (d, J = 1.4, 1H), 7.45 (d, J
= 2.4, 1H), 4.39 (s, 3H, NCH3), 1.54 (s, 9H), 1.40 (s, 9H). 13C
NMR (CDCl3, 25 °C): δ 154.4 (q), 153.1 (q), 149.0, 147.9, 144.7
(q), 144.1 (q), 140.6 (q), 137.8 (q), 134.3 (q), 130.2, 128.2, 126.3,
125.9 (q) 123.5, 123.0 (q), 122.9, 122.4, 119.7 (q), 112.2 (q), 37.4
(NCH3), 35.5 (CtBu3), 34.5 (CtBu3), 31.7 (tBu3), 29.7 (tBu3).
[(bpy-d8)2Ru(phen-Im-PhOMe)](PF6)2. A mixture of 2:1 ethꢀ
anol/water (10 mL) containing [Ru(bpyꢀd8)2Cl2] (55.3 mg, 0.11
mmol) and phenꢀImꢀPhOMe (50 mg, 0.11 mmol) was heated at
reflux under air for 4 h. Upon cooling, 5 mL of water containing
~100 mg sodium dithionite was added and the mixture was stirred
for 30 minutes. Addition of 10 mL water with ~0.5 g KPF6 inꢀ
duced precipitation of a red solid which was collected by vacuum
filtration and washed with water. After air drying, the solid was
dissolved in 3 mL CH2Cl2 (slurry) and 20 mL ether was added.
The precipitate was triturated, sonicated, and filtered to provide
95 mg of pure product in 69% yield. 1H NMR (CD3CN, 25 °C): δ
9.11 (d, J = 8.8, 1H), 9.08 (d, J = 8.4, 1H), 8.04 (d, J = 5.2, 2H),
7.80ꢀ7.75 (m, 2H), 7.66 (d, J = 2.4, 1H), 7.48 (d, J = 2.4, 1H),
4.15 (s, 3H, NCH3), 3.26 (s, 3H, OCH3), 1.48 (s, 9H), 1.37 (s,
9H). 13C NMR (solvent, 25 °C): δ 158.1 (q), 158.0 (q), 157.3 (q),
155.6 (q), 151.5, 150.8, 147.1 (q), 146.8 (q), 146.6 (q), 143.6 (q),
DFT Calculations. Theoretical calculations were carried out
using Density Functional Theory (DFT) as implemented in Gaussꢀ
ian09, revision D.01.110 Becke’s threeꢀparameter hybrid functionꢀ
al111ꢀ114 with the LYP correlation functional115 (B3LYP) was used.
The LANL2 relativistic effective core potential116 and associated
uncontracted basis set was used for Ru and the 6ꢀ311G(d,p) basis
set for C, N, O, and H. The solvent (acetonitrile) was modeled
with the Integral Equation Formalism Polarizable Continuum
Model (IEFꢀPCM),117ꢀ120 as implemented in Gaussian09. Univerꢀ
sal Force Field radii (UFF) were used in all cases. The position of
the unpaired electron was determined by the position of the proton
in the input geometry before optimization. Frequency calculations
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Journal of the American Chemical Society
Page 10 of 12
138.1 (q), 131.5, 131.0, 128.8, 128.0 (q), 127.8, 127.4 (q), 127.3,
critical reading of the manuscript. The work carried out at
126.4, 123.6 (q), 123.3 (q), 61.5 (OCH3), 36.1 (CtBu3), 35.9
(NCH3), 35.4 (CtBu3), 31.6 (tBu3), 30.9 (tBu3). ESIꢀMS(+): m/z
calcd for m/z calcd for C49H32D16F6N8OPRu, (M−PF6)+ 1027.36,
found 1027.3. Anal. Calcd for C49H32D16F12N8OP2Ru·H2O: C,
49.45; H, 4.24; N, 9.42. Found: C, 49.05; H, 3.73; N, 9.22. UVꢀ
vis, CH3CN, λmax 455 nm (ε 18500 M−1 cm−1).
Brookhaven National Laboratory was supported by the U.S. Deꢀ
partment of Energy, Office of Science, Division of Chemical Sciꢀ
ences, Geosciences, & Biosciences, Office of Basic Energy Sciꢀ
ences under contract DEꢀSC00112704.
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REFERENCES
6
7
8
9
[(bpy-d8)2Ru(phen-Im-PhOH)](PF6)2. A mixture of 2:1 ethaꢀ
nol/water (10 mL) containing [Ru(bpyꢀd8)2Cl2] (42.9 mg, 0.086
mmol) and phenꢀImꢀPhOMe (37.3 mg, 0.09 mmol) was heated at
reflux under argon for 4 h. Upon cooling, 5 mL of water containꢀ
ing ~100 mg sodium dithionite was added and the mixture was
stirred for 30 minutes. Addition of 10 mL water with ~0.3 g
NH4PF6 induced precipitation of a red solid which was collected
by vacuum filtration and washed with water. After air drying, the
solid was dissolved in 3 mL CH2Cl2 (slurry) and 20 mL ether was
added. The precipitate was triturated, sonicated, and filtered to
(1) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001,
34, 40ꢀ48.
(2) Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem.
1996, 100, 13148ꢀ13168.
(3) Yonemoto, E. H.; Saupe, G. B.; Schmehl, R. H.; Hubig, S.
M.; Riley, R. L.; Iverson, B. L.; Mallouk, T. E. J. Am. Chem. Soc.
1994, 116, 4786ꢀ4795.
(4) AlstrumꢀAcevedo, J. H.; Brennaman, M. K.; Meyer, T. J.
Inorg. Chem. 2005, 44, 6802ꢀ6827.
(5) Kok, B.; Forbush, B.; McGloin, M. Photochem. Photobiol.
1970, 11, 457ꢀ475.
(6) Vinyard, D. J.; Ananyev, G. M.; Dismukes, G. C. Annu. Rev.
Biochem. 2013, 82, 577ꢀ606.
(7) Renger, G.; Renger, T. Photosynth. Res. 2008, 98, 53ꢀ80.
(8) Schatz, G. H.; Brock, H.; Holzwarth, A. R. Proc. Natl. Acad.
Sci. U. S. A. 1987, 84, 8414ꢀ8418.
(9) Holzwarth, A. R.; Muller, M. G.; Reus, M.; Nowaczyk, M.;
Sander, J.; Rogner, M. Proc. Natl. Acad. Sci. U. S. A. 2006, 103,
6895ꢀ6900.
(10) Vrettos, J. S.; Limburg, J.; Brudvig, G. W. Biochim.
Biophys. Acta. Energetics 2001, 1503, 229ꢀ245.
(11) Yachandra, V. K.; Sauer, K.; Klein, M. P. Chem. Rev. 1996,
96, 2927ꢀ2950.
(12) Vass, I.; Cser, K. Trends Plant Sci. 2009, 14, 200ꢀ205.
(13) Rutherford, A. W.; Osyczka, A.; Rappaport, F. FEBS Lett.
2012, 586, 603ꢀ616.
(14) Rappaport, F.; GuergovaꢀKuras, M.; Nixon, P. J.; Diner, B.
A.; Lavergne, J. Biochemistry 2002, 41, 8518ꢀ8527.
(15) Tommos, C.; Babcock, G. T. Biochim. Biophys. Acta.
Energetics 2000, 1458, 199ꢀ219.
(16) Ahlbrink, R.; Haumann, M.; Cherepanov, D.; Bogershausen,
O.; Mulkidjanian, A.; Junge, W. Biochemistry 1998, 37, 1131ꢀ
1142.
(17) Schilstra, M. J.; Rappaport, F.; Nugent, J. H. A.; Barnett, C.
J.; Klug, D. R. Biochemistry 1998, 37, 3974ꢀ3981.
(18) Christen, G.; Renger, G. Biochemistry 1999, 38, 2068ꢀ2077.
(19) Kuhne, H.; Brudvig, G. W. J. Phys. Chem. B 2002, 106,
8189ꢀ8196.
(20) Hays, A. M. A.; Vassiliev, I. R.; Golbeck, J. H.; Debus, R. J.
Biochemistry 1999, 38, 11851ꢀ11865.
(21) Umena, Y.; Kawakami, K.; Shen, J.ꢀR.; Kamiya, N. Nature
2011, 473, 55ꢀU65.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
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47
48
49
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1
provide 65 mg of pure product in 65% yield. H NMR (CD3CN,
25 °C): δ 11.24 (s, 1H, OH), 9.13 (m, 2H), 8.07 (d, J = 5.2, 2H),
7.82‒7.77 (m, 2H), 7.63 (d, J = 2.4, 1H), 7.59 (d, J = 2.4, 1H),
4.45 (s, 3H, NCH3), 1.51 (s, 9H), 1.41 (s, 9H). 13C NMR (solvent,
25 °C): δ 158.1 (q), 158.0 (q), 155.8 (q), 155.0 (q), 151.9, 151.2,
147.0 (q), 146.8 (q), 142.2 (q), 138.4 (q), 136.2 (q), 131.9, 131.1,
128.7 (q), 128.0, 127.3, 126.6, 126.3 (q), 124.9, 123.3 (q), 113.2
(q), 38.4 (NCH3), 36.1 (CtBu3), 35.2 (CtBu3), 31.7 (tBu3), 29.8
(tBu3). ESIꢀMS(+): m/z calcd for C48H30D16F6N8OPRu, (M−PF6)+
1013.35,
found
1013.3.
Anal.
Calcd
for
C48H30D16F12N8OP2Ru·2H2O: C, 48.28; H, 4.16; N, 9.38. Found:
C, 48.35; H, 3.93; N, 9.77. UVꢀvis, CH3CN, λmax 455 nm (ε 16700
M−1 cm−1).
[(bpy-d8)2Ru(phen-Im-PhOD)](PF6)2. A 40 mg portion of
[(bpyꢀd8)2Ru(phenꢀImꢀPhOH)](PF6)2 was dissolved in 1 mL
CD3CN. D2O (~3 mL) was added and the mixture was purged
with Ar. Three drops of 40% NaOD in D2O were then added. The
red solution darkened slightly and a handꢀheld UV lamp showed
no orange phosphorescence. The solution was stirred for 30 min
then quenched with 5 drops of conc. DCl whereupon the color
lightened and phosphorescence was evident. A saturated solution
of KPF6 in D2O (~4 mL) was added to induce precipitation. The
filtrate was collected on a glass frit by vacuum filtration, wash
quickly with 4:1 D2O/CH3OD (5 mL) then diethyl ether. The solid
was dried in a vacuum desiccator on the glass frit and transferred
1
directly to a glovebox for storage without weighing. H NMR
(CD3CN, 25 °C): δ 9.19 (d, J = 8.4, 1H), 9.07 (d, J = 8.0, 1H),
8.21 (d, J = 5.2, 2H), 7.94‒7.88 (m, 1H), 7.76 (s, 1H), 7.56 (s,
1H), 4.37 (s, 3H), 1.51 (s, 9H), 1.40 (s, 9H).
ASSOCIATED CONTENT
Supporting Information
(22) In this context, “PCET” is the net transfer of a proton and
electron. Stepwise mechanisms are denoted "PTꢀET or ETꢀPT",
and a concerted reaction is designated as "EPT".
(23) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.;
Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D.
G.; Meyer, T. J. Chem. Rev. 2012, 112, 4016ꢀ4093.
(24) Migliore, A.; Polizzi, N. F.; Therien, M. J.; Beratan, D. N.
Chem. Rev. 2014, 114, 3381ꢀ3465.
(25) Costentin, C.; Robert, M.; Saveant, J. M. Chem. Rev. 2010,
110, PR1ꢀPR40.
(26) Concepcion, J. J.; Brennaman, M. K.; Deyton, J. R.;
Lebedeva, N. V.; Forbes, M. D. E.; Papanikolas, J. M.; Meyer, T.
J. J. Am. Chem. Soc. 2007, 129, 6968ꢀ6969.
NMR spectra, additional transient absorption data and tables of
raw data, cyclic voltammograms, and coordinates of DFT strucꢀ
tures. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Email: gmanbeck@bnl.gov, jconcepc@bnl.gov
Notes
The authors declare no competing financial interests.
(27) Wenger, O. S. Acc. Chem. Res. 2013, 46, 1517ꢀ1526.
(28) Irebo, T.; Reece, S. Y.; Sjodin, M.; Nocera, D. G.;
Hammarstrom, L. J. Am. Chem. Soc. 2007, 129, 15462ꢀ15464.
ACKNOWLEDGMENT
We are grateful to Drs. Norman Sutin, James T. Muckerman, and
Marshall Newton for their assistance in data interpretation and for
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(29) Reece, S. Y.; Seyedsayamdost, M. R.; Stubbe, J.; Nocera, D.
(58) Chakraborti, A. K.; Sharma, L.; Nayak, M. K. J. Org. Chem.
1
2
3
4
5
6
7
8
G. J. Am. Chem. Soc. 2006, 128, 13654ꢀ13655.
2002, 67, 6406ꢀ6414.
(59) Bensasson, R.; Salet, C.; Balzani, V. J. Am. Chem. Soc.
1976, 98, 3722ꢀ3724.
(60) Feitelson, J.; Hayon, E. J. Phys. Chem. 1973, 77, 10ꢀ15.
(61) Watanabe, T.; Honda, K. J. Phys. Chem. 1982, 86, 2617ꢀ
2619.
(62) Maki, T.; Araki, Y.; Ishida, Y.; Onomura, O.; Matsumura,
Y. J. Am. Chem. Soc. 2001, 123, 3371ꢀ3372.
(63) Benisvy, L.; Blake, A. J.; Collison, D.; Davies, E. S.;
Garner, C. D.; McInnes, E. J. L.; McMaster, J.; Whittaker, G.;
Wilson, C. Dalton Trans. 2003, 1975ꢀ1985.
(64) Moore, G. F.; Hambourger, M.; Kodis, G.; Michl, W.; Gust,
D.; Moore, T. A.; Moore, A. L. J. Phys. Chem. B 2010, 114,
14450ꢀ14457.
(65) Kim, S.; Liang, J.; Barry, B. A. Proc. Natl. Acad. Sci. U. S.
A. 1997, 94, 14406ꢀ14411.
(66) Bordwell, F. G.; Cheng, J. P. J. Am. Chem. Soc. 1991, 113,
1736ꢀ1743.
(67) Chantooni, M. K.; Kolthoff, I. M. J. Phys. Chem. 1976, 80,
1306ꢀ1310.
(68) Izutso, K. AcidꢁBase Dissociation Constants in Dipolar
Aprotic Solvents Blackwell: London, 1990.
(69) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta. 1985, 811,
265ꢀ322.
(70) Newton, M. D.; Sutin, N. Annu Rev. Phys. Chem. 1984, 35,
437ꢀ480.
(71) Sutin, N. Prog. Inorg. Chem. 1983, 30, 441ꢀ498.
(72) Young, R. C.; Keene, F. R.; Meyer, T. J. J. Am. Chem. Soc.
1977, 99, 2468ꢀ2473.
(73) Rips, I.; Jortner, J. J. Chem. Phys. 1987, 87, 6513ꢀ6519.
(74) Bixon, M.; Jortner, J. Chem. Phys. 1993, 176, 467ꢀ481.
(75) Rips, I.; Jortner, J. J. Chem. Phys. 1987, 87, 2090ꢀ2104.
(76) Maroncelli, M.; Macinnis, J.; Fleming, G. R. Science 1989,
243, 1674ꢀ1681.
(77) Sumi, H.; Marcus, R. A. J. Chem. Phys. 1986, 84, 4894ꢀ
4914.
(78) Mansingh, K.; Mansingh, A. J. Chem. Phys. 1964, 41, 827ꢀ
&.
(79) Hartmann, L.; Goychuk, I.; Hanggi, P. J. Chem. Phys. 2000,
113, 11159ꢀ11175.
(80) Brunschwig, B. S.; Sutin, N. Coord. Chem. Rev. 1999, 187,
233ꢀ254.
(81) Warren, J. J.; Mayer, J. M. Biochemistry 2015, 54, 1863ꢀ
1878.
(82) Keough, J. M.; Jenson, D. L.; Zuniga, A. N.; Barry, B. A. J.
Am. Chem. Soc. 2011, 133, 11084ꢀ11087.
(83) Jenson, D. L.; Evans, A.; Barry, B. A. J. Phys. Chem. B
2007, 111, 12599ꢀ12604.
(84) Jenson, D. L.; Barry, B. A. J. Am. Chem. Soc. 2009, 131,
10567ꢀ10573.
(85) Oliver, T. A. A.; Zhang, Y. Y.; Roy, A.; Ashfold, M. N. R.;
Bradforth, S. E. J. Phys. Chem. Lett. 2015, 6, 4159ꢀ4164.
(86) Chen, X. Y.; Larsen, D. S.; Bradforth, S. E.; van Stokkum, I.
H. M. J. Phys. Chem. A. 2011, 115, 3807ꢀ3819.
(87) Pagba, C. V.; Chi, S. H.; Perry, J.; Barry, B. A. J. Phys.
Chem. B 2015, 119, 2726ꢀ2736.
(88) Aubert, C.; Brettel, K.; Mathis, P.; Eker, A. P. M.; Boussac,
A. J. Am. Chem. Soc. 1999, 121, 8659ꢀ8660.
(89) Aubert, C.; Mathis, P.; Eker, A. P. M.; Brettel, K. Proc.
Natl. Acad. Sci. U. S. A. 1999, 96, 5423ꢀ5427.
(90) Cotruvo, J. A.; Stubbe, J. In Annual Review of Biochemistry,
Vol 80; Kornberg, R. D., Raetz, C. R. H., Rothman, J. E., Thorner,
J. W., Eds.; Annual Reviews: Palo Alto, 2011; Vol. 80, p 733ꢀ
767.
(30) Cohen, B. W.; Polyansky, D. E.; Achord, P.; Cabelli, D.;
Muckerman, J. T.; Tanaka, K.; Thummel, R. P.; Zong, R. F.;
Fujita, E. Faraday Discuss. 2012, 155, 129ꢀ144.
(31) KussꢀPetermann, M.; Wolf, H.; Stalke, D.; Wenger, O. S. J.
Am. Chem. Soc. 2012, 134, 12844ꢀ12854.
(32) Pizano, A. A.; Yang, J. L.; Nocera, D. G. Chem. Sci. 2012,
3, 2457ꢀ2461.
(33) Irebo, T.; Zhang, M.ꢀT.; Markle, T. F.; Scott, A. M.;
Hammarstrom, L. J. Am. Chem. Soc. 2012, 134, 16247ꢀ16254.
(34) Zhang, M.ꢀT.; Irebo, T.; Johansson, O.; Hammarstrom, L. J.
Am. Chem. Soc. 2011, 133, 13224ꢀ13227.
(35) Irebo, T.; Johansson, O.; Hammarstrom, L. J. Am. Chem.
Soc. 2008, 130, 9194ꢀ9195.
(36) Lachaud, F.; Quaranta, A.; Pellegrin, Y.; Dorlet, P.; Charlot,
M.ꢀF.; Un, S.; Leibl, W.; Aukauloo, A. Angew. Chem., Int. Ed.
2005, 44, 1536ꢀ1540.
(37) Moore, G. F.; Hambourger, M.; Gervaldo, M.; Poluektov, O.
G.; Rajh, T.; Gust, D.; Moore, T. A.; Moore, A. L. J. Am. Chem.
Soc. 2008, 130, 10466ꢀ10467.
(38) Megiatto, J. D.; MendezꢀHernandez, D. D.; TejedaꢀFerrari,
M. E.; Teillout, A. L.; LlansolaꢀPortoles, M. J.; Kodis, G.;
Poluektov, O. G.; Rajh, T.; Mujica, V.; Groy, T. L.; Gust, D.;
Moore, T. A.; Moore, A. L. Nature Chem. 2014, 6, 423ꢀ428.
(39) Megiatto, J. D.; AntoniukꢀPablant, A.; Sherman, B. D.;
Kodis, G.; Gervaldo, M.; Moore, T. A.; Moore, A. L.; Gust, D.
Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 15578ꢀ15583.
(40) Markle, T. F.; Rhile, I. J.; Mayer, J. M. J. Am. Chem. Soc.
2011, 133, 17341ꢀ17352.
(41) Soudackov, A. V.; Hazra, A.; HammesꢀSchiffer, S. J. Chem.
Phys. 2011, 135, 144115.
(42) HammesꢀSchiffer, S.; Stuchebrukhov, A. A. Chem. Rev.
2010, 110, 6939ꢀ6960.
(43) HammesꢀSchiffer, S. Energy Environ. Sci. 2012, 5, 7696ꢀ
7703.
(44) HammesꢀSchiffer, S. Acc. Chem. Res. 2009, 42, 1881ꢀ1889.
(45) Cukier, R. I. J. Phys. Chem. 1996, 100, 15428ꢀ15443.
(46) Edwards, S. J.; Soudackov, A. V.; HammesꢀSchiffer, S. J.
Phys. Chem. A. 2009, 113, 2117ꢀ2126.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(47) Costentin, C.; Robert, M.; Saveant, J. M. J. Am. Chem. Soc.
2007, 129, 5870ꢀ5879.
(48) Bonin, J.; Costentin, C.; Robert, M.; Routier, M.; Saveant,
J.ꢀM. J. Am. Chem. Soc. 2013, 135, 14359ꢀ14366.
(49) Markle, T. F.; Rhile, I. J.; DiPasquale, A. G.; Mayer, J. M.
Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 8185ꢀ8190.
(50) 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, 128, 6075ꢀ6088.
(51) Bonin, J.; Costentin, C.; Robert, M.; Saveant, J.ꢀM.; Tard, C.
Acc. Chem. Res. 2012, 45, 372ꢀ381.
(52) Costentin, C.; Robert, M.; Saveant, J.ꢀM. J. Am. Chem. Soc.
2007, 129, 9953ꢀ9963.
(53) Costentin, C.; Louault, C.; Robert, M.; Saveant, J. M. Proc.
Natl. Acad. Sci. U. S. A. 2009, 106, 18143ꢀ18148.
(54) Zhao, Y.; Swierk, J. R.; Megiatto, J. D., Jr.; Sherman, B.;
Youngblood, W. J.; Qin, D.; Lentz, D. M.; Moore, A. L.; Moore,
T. A.; Gust, D.; Mallouk, T. E. Proc. Natl. Acad. Sci. U. S. A.
2012, 109, 15612ꢀ15616.
(55) Quaranta, A.; Lachaud, F.; Herrero, C.; Guillot, R.; Charlot,
M. F.; Leibl, W.; Aukauloo, A. Chem. Eur. J. 2007, 13, 8201ꢀ
8211.
(56) Haumann, M.; Liebisch, P.; Muller, C.; Barra, M.; Grabolle,
M.; Dau, H. Science 2005, 310, 1019ꢀ1021.
(57) Razeghifard, M. R.; Pace, R. J. Biochim. Biophys. Acta.
Energetics 1997, 1322, 141ꢀ150.
(91) Minnihan, E. C.; Seyedsayamdost, M. R.; Stubbe, J.
Biochemistry 2009, 48, 12125ꢀ12132.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 12 of 12
(92) Seyedsayamdost, M. R.; Chan, C. T. Y.; Mugnaini, V.;
Stubbe, J.; Bennati, M. J. Am. Chem. Soc. 2007, 129, 15748ꢀ
15749.
(93) Stubbe, J.; Nocera, D. G.; Yee, C. S.; Chang, M. C. Y.
Chem. Rev. 2003, 103, 2167ꢀ2201.
(94) Yokoyama, K.; Uhlin, U.; Stubbe, J. J. Am. Chem. Soc.
2010, 132, 15368ꢀ15379.
(95) Reece, S. Y.; Stubbe, J.; Nocera, D. G. Biochim. Biophys.
Acta. Energetics 2005, 1706, 232ꢀ238.
(96) Tsukahara, K.; Wilkins, R. G. J. Am. Chem. Soc. 1985, 107,
2632ꢀ2635.
(97) Schrauben, J. N.; Cattaneo, M.; Day, T. C.; Tenderholt, A.
L.; Mayer, J. M. J. Am. Chem. Soc. 2012, 134, 16635ꢀ16645.
(98) Pagba, C. V.; McCaslin, T. G.; Chi, S. H.; Perry, J. W.;
Barry, B. A. J. Phys. Chem. B 2016, 120, 1259ꢀ1272.
(99) Schowen, K. B.; Schowen, R. L. Methods Enzymol. 1982,
87, 551ꢀ606.
(100) Hodgkiss, J. M.; Damrauer, N. H.; Presse, S.; Rosenthal, J.;
Nocera, D. G. J. Phys. Chem. B 2006, 110, 18853ꢀ18858.
(101) Markle, T. F.; Tenderholt, A. L.; Mayer, J. M. J. Phys.
Chem. B 2012, 116, 571ꢀ584.
(109) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444ꢀ
2453.
1
2
3
4
5
6
7
8
(110) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G.
E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.;
Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li,
X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.;
Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.;
Nakai, H.; Vreven, T.; Montgomery Jr., J. A.; Peralta, J. E.;
Ogliaro, F.; Bearpark, M. J.; Heyd, J.; Brothers, E. N.; Kudin, K.
N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari,
K.; Rendell, A. P.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi,
M.; Rega, N.; Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann,
R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V.
G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.;
Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.;
Cioslowski, J.; Fox, D. J.; Gaussian, Inc.: Wallingford, CT, USA,
2009.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(111) Becke, A. D. Phys. Re., Ser. A 1988, 38, 3098ꢀ3100.
(112) Becke, A. D. J. Chem. Phys. 1993, 98, 1372ꢀ1377.
(113) Becke, A. D. J. Chem. Phys. 1993, 98, 5648ꢀ5652.
(114) Stevens, P. J.; Devlin, F. J.; Chablowski, C. F.; Frisch, M.
J. J. Phys. Chem. 1994, 98, 11623ꢀ11627.
(102) Ishikita, H.; Soudackov, A. V.; HammesꢀSchiffer, S. J. Am.
Chem. Soc. 2007, 129, 11146ꢀ11152.
(103) Markle, T. F.; Mayer, J. M. Angew. Chem., Int. Ed. 2008,
47, 738ꢀ740.
(104) Johannissen, L. O.; Irebo, T.; Sjodin, M.; Johansson, O.;
Hammarstrom, L. J. Phys. Chem. B 2009, 113, 16214ꢀ16225.
(105) Cheng, T. Y.; Bullock, R. M. J. Am. Chem. Soc. 1999, 121,
3150ꢀ3155.
(106) Churchill, D. G.; Janak, K. E.; Wittenberg, J. S.; Parkin, G.
J. Am. Chem. Soc. 2003, 125, 1403ꢀ1420.
(107) Norris, M. R.; Concepcion, J. J.; Glasson, C. R. K.; Fang,
Z.; Lapides, A. M.; Ashford, D. L.; Templeton, J. L.; Meyer, T. J.
Inorg. Chem. 2013, 52, 12492ꢀ12501.
(108) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877ꢀ
910.
(115) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37,
785ꢀ789.
(116) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299ꢀ310.
(117) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005,
105, 2999ꢀ3093.
(118) Cances, E.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997,
107, 3032ꢀ3041.
(119) Mennucci, B.; Cances, E.; Tomasi, J. J. Phys. Chem. B
1997, 101, 10506ꢀ10517.
(120) Cances, E.; Mennucci, B. J. Math. Chem. 1998, 23, 309ꢀ
326.
PCET
stepwise
kET = 3 ×107 s−1
kH/kD = 1
concerted
kPCET >5 ×109 M−1s−1
kH/kD = 0.55
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