Role of proton-coupled electron transfer in the redox interconversion between benzoquinone and hydroquinone

Article abstract of DOI:10.1021/ja308700t

Benzoquinone/hydroquinone redox interconversion by the reversible Os(dmb)₃^3+/2+ couple over an extended pH range with added acids and bases has revealed the existence of seven discrete pathways. Application of spectrophotometric moni

Full text of DOI:10.1021/ja308700t

Communication
pubs.acs.org/JACS
Role of Proton-Coupled Electron Transfer in the Redox
Interconversion between Benzoquinone and Hydroquinone
Na Song, Christopher J. Gagliardi, Robert A. Binstead, Ming-Tian Zhang, Holden Thorp,
and Thomas J. Meyer*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
S Supporting Information
*
Amino acid oxidation is irreversible, but the Q/H Q couple is
reversible, providing an opportunity for mechanistic inves-
2
ABSTRACT: Benzoquinone/hydroquinone redox inter-
3
+/2+
conversion by the reversible Os(dmb)3
couple over an
tigation in “both directions”. There is an extensive literature on
5
extended pH range with added acids and bases has
revealed the existence of seven discrete pathways.
Application of spectrophotometric monitoring with
stopped-flow mixing has been used to explore the role of
PCET. The results have revealed a role for phosphoric acid
and acetate as proton donor and acceptor in the concerted
electron−proton transfer reduction of benzoquinone and
oxidation of hydroquinone, respectively.
Q/H Q redox interconversion, but very little is known about
2
the role of acid- and base-assisted PCET pathways.
We report here mechanistic details of the redox interconver-
sion between Q and H Q as a function of pH by the couple
2
Os(dmb)33
+/2+
(dmb = 4,4′-dimethyl-2,2′-bipyridine). Remark-
ably, our results provide evidence for seven distinct pathways for
this interconversion, including an important role for concerted
EPT that may be of relevance in biological Q/H Q reactions.
2
Results of extensive electrochemical measurements on Q/
6
H Q interconversion are available, but interpretation is typically
2
erivatives of benzoquinone/hydroquinone (Q/H Q) play
essential roles in biology. An important example appears
complicated by adsorption and mass-transfer effects. Following
2
1
D
Laviron, a potential−pK diagram for the Q/H Q couple under
a
2
7
standard conditions is shown in Figure 1a. From the diagram the
e semiquinone intermediate, HQ , is highly unstable toward
disproportionation, 2HQ → H Q + Q, with ΔG° = −0.7 eV.
The implied importance of PCET and concerted EPT
in photosynthesis, in the reduction of plastoquinone (Q ) to the
B
−
•
1
mobile redox carrier plastoquinol (H Q ), which is transported
2
B
8
•
2
through the thylakoid membrane to cytochrome b f, where it is
6
2a,b,9
2
in
oxidized to Q with proton release to the lumen.
B
Q/H Q reactivity is apparent from the E°′−pK diagram. For
2
a
Interconversion between Q and H Q in photosystem II (PSII)
2
•
•
PCET reduction of Q to HQ , E°′(Q/HQ ) = 0.34 V (vs NHE).
In a mechanism involving initial proton transfer, Q + H ⇌ HQ ,
followed by electron transfer, HQ + e → HQ (PT-ET),
and amino acid redox mediators in biology utilize proton-
coupled electron transfer (PCET) in transferring redox
+
+
+
−
•
1a
equivalents with the transfer of both electrons and protons.
+
protonation is unfavorable, with pK (HQ ) ≈ −6 and ΔG°′ =
a
+
In tyrosine and cysteine oxidation, concerted electron−proton
−
0.059(pH+6) = 0.35 eV at pH 0. HQ is an enhanced oxidant
+
•
transfer (EPT) pathways are utilized to avoid high-energy
with E°′(HQ /HQ ) = 0.70 V (vs NHE). For electron transfer
3,4
−•
protonated radical intermediates. In these reactions, pendant
followed by proton transfer (ET-PT), E°′(Q/Q ) = 0.099 V for
+
bases or solvent molecules enable EPT by acting as H acceptors
the initial electron transfer, with ΔG°′ = −0.24 eV for
protonation of Q− at pH 0.
•
•+
3
avoiding high-energy intermediates like TyrOH . For tyrosine
−
•+
As shown in Figure 1b, E°′ = 0.63 V (vs NHE) for the
oxidation, E° ≈ 1.5 V (vs NHE) for 1e oxidation to TyrOH ,
compared to E° ≈ 1.0 V for oxidation of the tyrosine−histidine
Os(dmb)33
+/2+
couple, and it is pH-independent. The Os
•
+
2a−c
complexes are substitutionally inert and have minimal barriers to
acid−base pair in PSII, TyrOH---His → TyrO --- H-His.
1
0
electron transfer. By contrast, E°′ for the Q/H Q couple is pH-
2
dependent and varies with the Nernst slope of −0.059 V/pH unit
−
+
for a 2e /2H couple. E°′ values for the two couples cross at pH
2+
1
.2. Below this pH, Os(dmb) reduction of Q is spontaneous;
3
3+
above this pH, Os(dmb)3 oxidation of H Q is spontaneous. By
2
varying the pH, the overall reaction can be studied in either
direction.
In our experiments, the kinetics of reduction of Q by
Os(dmb)32+ or oxidation of H Q by Os(dmb)
3+
were
2
3
investigated by stopped-flow mixing with spectrophotometric
monitoring at 20 °C, I = 0.8 M (NaCl). Stock solutions of
Os(dmb)32+ were freshly prepared and oxidized to Os by Cl ,
III
Figure 1. (a) E°′(vs NHE)−pK diagram for the Q/H Q couple.
a
2
2
+
−
Diagonal lines give potentials vs NHE for 1H /1e PCET couples at pH
+
0
in water. pK for HQ was obtained in the present work. (b) Calculated
a
3+/2+
Received: September 1, 2012
Published: November 1, 2012
E°′−pH diagram for Q/H Q (red) and Os(dmb)3
(blue) couples.
2
©
2012 American Chemical Society
18538
dx.doi.org/10.1021/ja308700t | J. Am. Chem. Soc. 2012, 134, 18538−18541

Journal of the American Chemical Society
Communication
D
followed by an argon purge. All solutions were degassed with
argon prior to stopped-flow mixing. p-Benzoquinone was
Figure SI.4b, k /[Q] increased linearly with [D PO ], with k
3
obs
T
3
4
= 654 ± 28 M−
2
s
−1
and k /k = 0.87 ± 0.05.
H
D
3 3
purified by sublimation to give yellow crystals. The purities of
A [H PO ]-dependent pathway is a novel observation,
3 4
1
Q and H Q were checked by H NMR.
consistent with pre-association of H PO (eq 6) followed by
2
3
4
2
+
2a
II
Absorption−time traces for either appearance of Os(dmb)
concerted multiple-site electron−proton transfer (MS-EPT)
3
3+
by Os(dmb)3 oxidation of H Q, or its disappearance by
2
(eq 7) with proton transfer to Q and electron transfer from Os .
It is analogous to related base-catalyzed pathways in the
oxidation by Q, were monitored at the metal-to-ligand charge
transfer absorption, 480 nm. Under pseudo-first-order con-
III
•
+
oxidation of tyrosine, TyrOH---His + Os → TyO + H-His
II
3c,d,16
III/II
ditions in either Q or H Q, with added buffers or acids, both
2
+ Os .
For the EPT pathway, ΔG°′ = E°′(Os ) −
II
oxidation and reduction of Os followed first-order kinetics, with
0/−•
•
E°′(Q ) − 0.059(pKa,HQ − pK
) = 0.39 eV.
a1,H PO
3 4
analysis of the data giving an observed rate constant k . Typical
obs
absorption−time traces and kinetic analyses are shown in the
Supporting Information, Figure SI.1.
In an initial set of experiments, Os(dmb)32+ reduction of Q
was investigated under pseudo-first-order conditions in [Q] with
I = 0.8 M (NaCl) at T = 20 °C. In 0.16 M HCl, reduction
II
II
occurred with the rate law −d[Os ]/dt = k [Os ] (Figure SI.2).
obs
k
varied linearly with added quinone, [Q] , with the slope k =
T
obs
2
70 ± 4 M−
1
s and negligible intercept. The acid dependence of
the reaction was investigated over the pH range 0.6−2.0. As
shown in Figure SI.3a, k /[Q] varied linearly with [H ],
−1
11
II
•
+
III
Os + HQ + H → Os + H Q
rapid
(8)
2
+
obs
T
+
consistent with the expression k /[Q] = k + k [H ], with k =
The appearance of the inverse KIE for the EPT pathway was
obs
T
1
2
−2 −1
s
1
.7 ± 2.6 M−
1
s
−1
and k = (2.2 ± 0.1) × 10 M
The term zero-order in [H ] is consistent with outer-sphere
3
(Table SI.1).
3+
3
9
unexpected. For EPT oxidation of tyrosine by Os(bpy)
with
2
+
histidine as the proton acceptor, k K (H O)/k K (D O) =
EPT
A
2
EPT
A
2
16
electron transfer with k = kET,Q,
3.2. As shown in eq 7, k = k
KA,Q, and the inverse
EPT,Q‑‑H PO4
1
3
3
Os + Q → Os + Q−•
II
III
isotope effect may originate in the pre-equilibrium. Small KIEs
kET,Q = k1 (1)
17a
have been reported for other EPT reactions and discussed by
II
−•
+
III
17b−d
Os + Q + 2H → Os + H Q
rapid
(2)
Hammes-Schifffer and Cukier.
2
The reverse reaction, oxidation of H Q by Os(dmb)33+, was
2
+
and the pathway first-order in [H ] is consistent with pre-
protonation of Q to give HQ , followed by ET,
investigated under the same conditions with H Q in pseudo-first-
2
+
order excess from 0.2 to 4 mM over the pH range 3.5−5.6. As
+
+
shown in Figures SI.1b and SI.5, under these conditions, the
Q + H ⇌ HQ
1/Ka,HQ
kET,HQ = k2Ka,HQ
rapid
+
(3)
(4)
(5)
III
reaction is first-order in both Os and H Q, consistent with the
2
Os + HQ → Os + HQ•
II
+
III
II
III
+
+
rate law d[Os ]/dt = k [Os ]. At pH 4.0, k varied linearly
obs obs
4
−1 −1
with [H Q] , with k /[H Q] = (1.1 ± 0.1) × 10 M
s and
II
•
+
III
2
T
obs
2
T
Os + HQ + H → Os + H Q
2
a negligible intercept.
Evidence for EPT pathways was found with acetate (Ac−)
added as the acceptor base. These experiments were conducted
+
With this interpretation and pKa,HQ = −6, the rate constant for
outer-sphere reduction of HQ , k
s , approaches the diffusion-controlled limit in H O. The
significant rate enhancement compared to reduction of Q is not
+
+
9
−1
= (2.2 ± 0.1) × 10 M
ET,HQ
−
−
1
12
at fixed pH (3.5) and buffer ratio [HAc]/[Ac ] = 10/1, varying
2
the concentrations of both acid and base. As shown in Figure
−
II
+
SI.6a, k /[H Q] varies quadratically with [Ac ] over the buffer
surprising since ΔG°′ = −0.07 eV for Os reduction of HQ and
obs
2
T
•
concentration range 0.05−4 M, consistent with the rate law
0
.53 eV for reduction of Q. By comparison, HQ disproportio-
1
1
nation to / Q + / H Q occurs with ΔG°′ = −0.7 eV and k
=
2
2
2
disp
II
−
− 2
III
9
−1 −1
8
d[Os ]/dt = {k″ + k [Ac ] + k [Ac ] }[Os ][H Q]
(9)
1
.1 × 10 M s .
Reduction of Q was investigated in D O with added DCl with
pD varied from 0.6 to 2.0. Variation of k /[Q] with [D ] was
4
5
2
T
2
3
−1
13
+
As determined from the intercept, k″ = (4.8 ± 0.2) × 10 M
1
obs
−1 −1
T
−
D
1
D
2
s . This is consistent with the value obtained by direct
linear (Figure SI.3b), with k = 8.8 ± 1.6 M
s and k = (1.5 ±
+
3
−1
measurement (k″ = k + k /[H ], see below). As shown in
0
(
.1) × 10 s , yielding H O/D O solvent kinetic isotope effects
6
7
2
2
Figure SI.6a, the rate constants k and k were obtained by fitting
the extended data set to give k = (2.6 ± 0.1) × 10 M
(8.2 ± 0.1) × 10 M s .
The experiments with added HAc/Ac were repeated in D O
KIE; kH2O/kD2O) of 1.1 ± 0.4 for k and 1.4 ± 0.1 for k (Table
SI.1). Although the magnitude of KIE for k implies sequential
ET-PT, a contribution by concerted EPT with water as the
proton donor cannot be ruled out.
4
5
1
2
5
−2 −1
s and k5
4
1
5
−3 −1
=
−
2
at pD 4.1 with the same rate law behavior (Figure SI.6b). Analysis
We also searched for a possible EPT pathway for reduction
with added H PO at fixed pH (1.3). In these experiments, the
3
−1 −1
of the results gave k″(D O) = (1.8 ± 0.1) × 10 M s ,
2
3
4
−
−
consistent with the value obtained in D O with no added Ac
buffer ratio was held constant at [H PO ]/[H PO ] = 4, and
2
3
4
2
4
D
4
−2 −1
D
(
0
see below), with k = (9.9 ± 0.3) × 10 M s and k = (2.9 ±
[
H PO ] was varied by increasing the total buffer concentration.
4
5
3
4
14
5
−3 −1
.2) × 10 M s . Based on these results and those obtained in
O, the H O/D O KIE values are 2.6 ± 0.1 for k and 2.8 ± 0.2
for k
The most straightforward interpretation of the term first-order
in Ac is that, as found for tyrosine oxidation by Os(bpy) , pre-
association occurs between Ac and H Q, followed by MS-EPT:
Ionic strength was adjusted to 0.8 M by adding NaCl. As shown
in Figure SI.4a, kobs increased linearly with [H PO ], with no sign
H
3
4
2
2
2
4
of saturation up to [H PO ] = 0.48 M. From a plot of k /[Q] =
.
5
3
4
obs
T
−
2 −1
k′ + k [H PO ], k = 570 ± 20 M
s with an intercept, k′ = k +
3
3
4
3
1
+
−1 −1
−
3+
k [H ], of 123 ± 4 M s . The experiment was repeated in D O
by adding varying concentrations of D PO . As shown in
2
2
3
15
−
3
4
2
1
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Journal of the American Chemical Society
Communication
−
−
H Q + Ac ⇌ Ac ‐‐‐H Q
KA (10)
2
2
OsIII + H O‐‐‐H Q →
k
EPT,H Q
2
= k6 (19)
2
2
II
•
OsIII + Ac ‐‐‐H Q → Os + HQ + HAc kEPT (11)
−
II
•
Os + HQ + H O
3
2
OsIII + HQ → Os + Q + H
•
II
+
rapid
(20)
OsIII + HQ → Os + Q + H
•
II
+
rapid (12)
This pathway is kinetically indistinguishable from outer-sphere
Given the similarity in KIE values and the high concentrations of
•
+
−
−
oxidation of H Q to H Q followed by proton equilibration
Ac used, the squared term in [Ac ] may arise from a parallel
mechanism, but with ion-pairing with the tri-cationic metal
2
2
•
+
•+
from H Q with Ka,H2Q = 10 (eqs 21 and 22):
2
−
complex oxidant by a second Ac , followed by MS-EPT
OsIII + H Q → Os + H O
II
•+
k = k6 (21)
ET,H Q
2
−
2
2
oxidation of H Q---Ac (eqs 13 and 14):
2
•
+
•
+
•+
(22)
(23)
−
−
H Q
2
→ HQ + H
K
a,H Q
2
H Q + Ac ⇌ Ac ‐‐‐H Q
KA (10)
KIP (13)
2
2
•
1
1
III 3+
−
III 3+
−
HQ → / H Q + / Q
rapid
(
Os ) + Ac → (Os ) , Ac
2
2
2
III 3+
−
−
However, the magnitude of the KIE points to a dominant role for
MS-EPT with the solvent as the proton acceptor. Once again,
EPT is energetically favored. For the initial ET step in eq 21,
ΔG°′ = +0.47 eV based on E°′ values for the two couples. For the
(
Os ) , Ac + Ac ‐‐‐H Q →
k′
(14)
2
EPT
II 2+
•
−
(
Os ) + HQ + HAc + Ac
OsIII + HQ → Os + Q + H
•
II
+
rapid (12)
III/II
•+
MS-EPT step (eq 19), ΔG°′ = −[E°′(Os ) − E°′(H Q /
2
Other interpretations are possible, including formation of a
doubly H-bonded Ac adduct with H Q.
+
•+
21
H Q)] − 0.059(pK (H O ) − pK (H Q ) = 0.41 eV.
−
2
a
3
a
2
2
This result highlights an important role for an EPT pathway in
With this interpretation, k = K k and k = K K k′EPT, with
4
A
EPT
5
IP A
the oxidation of H Q in water, in this case with a solvent
−
2
K the association constant between Ac and H Q and K the
A
2
IP
molecule or water cluster acting as the proton acceptor as
3
+
−
ion pair constant between Os(dmb)3 and Ac . The observed
KIEs include contributions from the pre-equilibria but are
presumably dominated by the KIEs for the EPT steps.
An additional pH-dependent term appears in the rate law from
oxidation of HQ . This term was investigated by stopped-flow
measurements over the pH range 3.5−5.6 with added 0.05 M
Ac buffer at I = 0.8 M. Under these conditions, there are
contributions to k from the pathways first- and second-order in
Ac ] (eq 9). A correction was made to k
22a
reported earlier for phenol oxidation by Stanbury
and
22b
III
Saveant.
By comparison, oxidation of H Q by the Ru
oxidant, Ru (bpy) (py)(OH) , occurs by direct EPT with
both electron and proton transfer to the Ru −OH acceptor,
3
b−d
2
III
2+
2
III
2+
−
III
2+
III
2+
Ru OH + H Q ⇌ Ru OH ‐‐‐H Q
(24)
(25)
(26)
2
2
−
III
2+
II
2+
+ HQ•
obs
Ru OH ‐‐‐H Q → Ru OH
2
2
−
[
for their
obs
III
2+
•
II
2+
contributions by using the known values for k and k with
Ru OH + HQ → Ru OH
+ Q
rapid
4
5
2
−
+
[
Ac ] = K
[buffer]/([H ] + Ka,HAc). As shown in Figure
a,HAc
5c
This reaction occurs with a KIE of 9.7 ± 0.1.
SI.7a, under these conditions k /[H Q] varied linearly with
H ] with k = (3.5 ± 0.1) × 10 M
obs
2
T
+
3
−1 −1
−1
Our results highlight a remarkable versatility in the redox
[
s
and k = 0.54 ± 0.01 s :
6
7
interconversion between quinone and hydroquinone by the
kobs
−
− 2
k7
3+/2+
outer-sphere Os(dmb)3
couple. This versatility arises from−
=
k [Ac ] + k [Ac ] + k +
4
5
6
+
[
H Q]
[H ]
(15)
the nature of the reagents themselves with accessibility to 1e
2
T
intermediates Q− and H Q by 1e reduction of Q or oxidation
•
•+
−
2
The pD dependence was also investigated for this pathway in
D O solutions dilute in added Ac (0.01 M) free of contributions
from the MS-EPT pathways. A fit of a plot of k /[H Q] vs pD
•
of H Q, or to HQ and by their use of EPT pathways with
concerted e /H transfer to Q or from H Q. A summary is given
−
2
2
−
+
2
obs
2
in Scheme 1 for the reduction of Q and in Scheme 2 for the
D
D
+
(
Figure SI.7b) to the expression k /[H Q] = k + k /[D ]
obs 2 T 6 7
and k = (9.8 ± 0.2) × 10
7
+
13
oxidation of H Q.
D
6
3
−1 −1
D
−2
2
gave k = (1.0 ± 0.1) × 10 M
s
Important insights also emerge for the individual pathways:
−1
D
D
6
D
7
s . As noted above, k″ = k + k /[D ]. Based on these values,
(
i) Specific acid and base catalysis occur for both reduction of
H O/D O KIEs were 3.5 ± 0.2 for k and 5.5 ± 0.1 for k .
2
2
6
7
Q and oxidation of H Q. This is due to the relatively high energy
2
For the pathway through k , the appearance of the inverse first-
−
−•
•+
7
of the 1e intermediates Q and H Q , which favors pathways
involving PT-ET or ET-PT with prior formation of HQ or HQ .
+
2
order dependence in [H ] is consistent with deprotonation of
H Q to give HQ , followed by ET:
+
−
−
2
−
+
H Q ⇌ HQ + H
K
(16)
(17)
(18)
Scheme 1
2
a1,H Q
2
Os HQ → O + HQ•
III
−
II
kET,HQ
= k /K
−
S
7
a1,H Q
2
III
•
II
+
Os HQ → Os + Q + H
rapid
1
8
−1
−
With pK
= 9.82 and k = 0.54 ± 0.01 s , k
= (3.6 ±
ET,HQ
a1,H
9
2
Q
7
−1 −1
0.1) × 10 M
s
was obtained, near the diffusion-controlled
9
−1 −1 19
20
limit of 7 × 10 M s . Given pKa1,D2Q ≈ 10.4 in D O,
2
−
Ka1,H2Q (H O)/K
(D O) ≈ 4.0, and the KIE for k
is
2
a,1,D
2
Q
2
ET,HQ
∼
1.4, consistent with outer-sphere oxidation as in eq 17.
The rate law and KIE for the k term are consistent with
6
electron-transfer oxidation of H Q but with simultaneous proton
2
transfer to the solvent (eq 19):
1
8540
dx.doi.org/10.1021/ja308700t | J. Am. Chem. Soc. 2012, 134, 18538−18541

Journal of the American Chemical Society
Communication
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Electroanal. Chem. 2001, 500, 12. (c) Astudillo, P. D.; Tiburcio, J.;
́
Gonzalez, F. J. J. Electroanal. Chem. 2007, 604, 57. (d) Gupta, N.;
(
ii) General acid and base catalysis appears with the acid
Linschitz, H. J. Am. Chem. Soc. 1997, 119, 6384.
(7) Laviron, E. J. Electroanal. Chem. 1984, 169, 29.
−
(
H PO ) or base (Ac ) forms of added buffers due to the
3 4
•
intervention of concerted EPT pathways which give HQ directly
(8) Adams, G. E.; Michael, B. D. Trans. Faraday Soc. 1967, 63, 1171.
by reduction of Q---HA or oxidation of Q---H Q. This is, no
doubt, a general phenomenon and, as for tyrosine, will appear
generally with added proton acceptor bases including use of these
(9) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Chem. Rev. 2010, 110,
6961.
(10) (a) Chan, M. S.; Wahl, A. C. J. Phys. Chem. 1978, 82, 2542.
2
(
b) Song, N.; Stanbury, D. M. Inorg. Chem. 2012, 51, 4909.
couples in biology.
(11) pH = −log [H ] − log γ . In this expression, γH+ is the activity
+
+
III
H
(
iii) In the oxidation of H Q by Os , the dominant
2
coefficient of the proton at I = 0.8 M.
mechanism is EPT with concerted proton transfer to the solvent.
(12) (a) Alberty, R. A.; Hammes, G. G. J. Phys. Chem. 1958, 62, 154.
(
b) Hammes, G. G.; Amdur, I. Principles of chemical kinetics; Academic
ASSOCIATED CONTENT
Supporting Information
Experimental details and analyses. This material is available free
of charge via the Internet at http://pubs.acs.org.
■
Press: New York, 1978. (c) Espenson, J. H. Chemical kinetics and reaction
mechanisms; McGraw-Hill: New York, 1995.
*
S
(13) pD = −log [D ] − log γD , assuming that γD = γ +
+
+
+
.
H
(14) pKa1,H3PO4 = −1.79 at I = 0.8 M was converted from the reported
values, 2.127 at zero ionic strength; see SI.
AUTHOR INFORMATION
Corresponding Author
tjmeyer@email.unc.edu
■
(15) pKa1,D PO = 2.06 at I = 0.8 M in D O was converted from the
2
3 4
reported values, 2.3981 at zero ionic strength with γH+ = γ +. pD = pH
D
meter reading + 0.4; see SI.
(
16) Murphy, C. F. Ph.D. Dissertation, University of North Carolina at
Chapel Hill, 2009.
17) (a) Biczok, L.; Gupta, N.; Linschitz, H. J. Am. Chem. Soc. 1997,
19, 12601. (b) Edwards, S. J.; Soudackov, A. V.; Hammes-Schiffer, S. J.
Phys. Chem. A 2009, 113, 2117. (c) Cukier, R. I. J. Phys. Chem. 1994, 98,
377. (d) Hazra, A.; Soudackov, A. V.; Hammes-Schiffer, S. J. Phys.
Chem. Lett. 2011, 2, 36.
18) pKa1,H2Q = 9.82 in H O at I = 0.8 M was corrected from 9.85 at I =
Notes
The authors declare no competing financial interest.
(
1
́
ACKNOWLEDGMENTS
This work has been supported by the National Science
Foundation under grant CHE-0957215, supporting M.-T.Z.
■
2
(
T.J.M.) and C.J.G. (T.J.M. and H.T.). N.S. (T.J.M.) is
(
2
supported through the Richard T. and Hugh G. Chatham
Fund for Faculty Research and Development. R.A.B. (T.J.M.) is
supported as part of the UNC EFRC: Center for Solar Fuels, an
Energy Frontier Research Center funded by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences,
under Award No. DE-SC0001011. We thank Dr. Zhen Fang
0
(
.65 M; see SI.
9
−1 −1
19) The bimolecular diffusion control limit, k = 3 × 10 M s , was
D
III 3+
−
corrected for the electrostatic interaction between (Os ) and HQ ;
see SI.
(20) Martin, R. B. Science 1963, 139, 1198.
(
21) Under standard conditons, theoretical thermodynamic calcu-
+
lation requires [H O] = 1 M, which gives pK (H O ) = 0.
(
UNC EFRC) for synthesis of the Os(II) complex.
2
a
3
(22) (a) Song, N.; Stanbury, D. M. Inorg. Chem. 2008, 47, 11458.
(
b) Bonin, J.; Costentin, C.; Louault, C.; Robert, M.; Routier, M.;
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