Electrocatalytic and photocatalytic conversion of CO2 to methanol using ruthenium complexes with internal pyridyl cocatalysts

Article abstract of DOI:10.1021/ic500051m

The ruthenium complexes [Ru(phen)₂(ptpbα)]²⁺ (Ruα) and [Ru(phen)₂(ptpbβ)]²⁺ (Ruβ), where phen =1,10-phenanthroline; ptpbα = pyrido[2′,3′:5,6] pyrazino[2,3-f][1,10]phenanthroline; ptpbβ = pyrido[3′,4′:5,6] pyrazino[2,3-f][1,10]phenanthroline, are shown as electrocatalysts and photocatalysts for CO₂ reduction to formate, formaldehyde, and methanol. Photochemical activity of both complexes is lost in water but is retained in 1 M H₂O in DMF. Controlled current electrolysis of a solution of Ruβ in CO₂ saturated DMF:H₂O (1 M) yields predominantly methanol over a 6 h period at ～ -0.60 V versus Ag/AgCl, with traces of formaldehyde. After this time, the potential jumped to -1.15 V producing both methanol and CO as products. Irradiation of Ruβ in a solution of DMF:H₂O (1 M) containing 0.2 M TEA (as the sacrificial reductant) yields methanol, formaldehyde, and formate. Identifications of all of the relevant redox and protonated states of the respective complexes were obtained by a combination of voltammetry and differential reflectance measurements. Spectroelectrochemistry was particularly useful to probe the photochemical and electrochemical reduction mechanisms of both complexes as well as the complexes speciation in the absence and presence of CO₂.

Full text of DOI:10.1021/ic500051m

Article
pubs.acs.org/IC
Electrocatalytic and Photocatalytic Conversion of CO to Methanol
2
using Ruthenium Complexes with Internal Pyridyl Cocatalysts
†
‡
,‡
†
́
David J. Boston, Yeimi M. Franco Pachon, Reynaldo O. Lezna,* N. R. de Tacconi,
,
†
and Frederick M. MacDonnell*
†
Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065, United States
‡
Instituto de Investigaciones Fisicoquímicas Teor
B1906ZAA) La Plata, Argentina
́
icas y Aplicadas (INIFTA, UNLP, CCT La Plata-CONICET), Sucursal 4, C.C. 16,
(
*
S Supporting Information
ABSTRACT: The ruthenium complexes [Ru(phen) (ptpbα)]2+ (Ruα) and [Ru(phen) (ptpbβ)] (Ruβ), where phen =1,10-
2+
2
2
phenanthroline ; ptpbα = pyrido[2′,3′:5,6]pyrazino[2,3-f ][1,10]phenanthroline; ptpbβ = pyrido[3′,4′:5,6]pyrazino[2,3-
f ][1,10]phenanthroline, are shown as electrocatalysts and photocatalysts for CO reduction to formate, formaldehyde, and
2
methanol. Photochemical activity of both complexes is lost in water but is retained in 1 M H O in DMF. Controlled current
2
electrolysis of a solution of Ruβ in CO saturated DMF:H O (1 M) yields predominantly methanol over a 6 h period at ∼ −0.60
2
2
V versus Ag/AgCl, with traces of formaldehyde. After this time, the potential jumped to −1.15 V producing both methanol and
CO as products. Irradiation of Ruβ in a solution of DMF:H O (1 M) containing 0.2 M TEA (as the sacrificial reductant) yields
2
methanol, formaldehyde, and formate. Identifications of all of the relevant redox and protonated states of the respective
complexes were obtained by a combination of voltammetry and differential reflectance measurements. Spectroelectrochemistry
was particularly useful to probe the photochemical and electrochemical reduction mechanisms of both complexes as well as the
complexes speciation in the absence and presence of CO2.
3
1−34
Bocarsly,35−39 and Crabtree40 among
INTRODUCTION
Abruna,
others.
■
5
,6,11,16
In light of the increasing concerns about global atmospheric
Of the various strategies toward catalytic CO reduction, the
work of Bocarsly
electrocatalyst, such as pyridine, to produce the 6-electron
reduced product methanol over the more common 2-electron
reduced products CO and formate.
2
CO levels and the dependence of modern society on cheap
2
35−39
stands out for its use of a simple organic
energy, i.e., fossil fuels, technological advances in the photo-
chemical or electrochemical reduction of CO to liquid fuels
2
could be an important component of a carbon neutral fuel
37,39
1
−5
The direct production
cycle.
Reduction of CO by transition metal complexes
2
of a viable liquid transportation fuel like methanol from CO2
under modest (near ambient) conditions has been explored for
decades, with most recent attention focused on coupling this
process to electrochemical and/or photochemical methods.
Some of the seminal early studies include a report by Meyer
and co-workers showing that Rh and Ir polypyridyl complexes
are competent electrocatalysts for CO reduction to formate in
has obvious advantages over other technologies that are only
5
−16
showing a 2-electron reduction leading to CO or formate. It is
theorized that this six-electron process occurs in a sequentially
manner starting with the reduction of the pyridinium cation
(pyH ) to the neutral radical (pyH ), which is followed by
insertion of CO into the NH bond to give a radical carbamate
+
•
2
17
DMF under air-free, ambient conditions. Since then, there
2
have been significant contributions in both electrocatalytic and
photocatalytic CO reduction studies by a number of groups
Received: January 8, 2014
2
4
,18−23
24−28
29,30
12
including Meyer,
Fujita,
Kubiak,
Saveant,
©
XXXX American Chemical Society
A
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Figure 1. Structures of [(phen) Ru(ptpbα)]2 and [(phen) Ru(ptpbβ)] complexes and [(phen) Ru(dppz)] complex.
+
2+
2+
2
2
2
3
7,41
intermediate.
final product.
Subsequent sequential reductions lead to the
with N or CO prior to each measurement, and the atmosphere was
2 2
maintained over the electrochemical solution throughout the course of
the experiment. All experiments were performed at laboratory ambient
temperature (20 ± 2 °C).
We recently showed that pyridine and [Ru(phen) ]2+ can
work in a homogeneous photochemical CO reduction process
to produce both formate and methanol, using ascorbic acid as
the sacrificial donor. While this photochemical process has
poor selectivity for methanol production, favoring formate, it
did demonstrate that the pyridinium-based catalysts can reduce
CO to methanol in the absence of a metal surface, which had
previously been debated as essential or not in some recent
3
2
Spectroelectrochemistry experiments were performed on a mirror-
polished platinum disk as the working electrode. A platinum coil was
used as a counterelectrode and was placed in a separate compartment
with a fritted end. The reference electrode was a Ag/AgCl, saturated
KCl and was used with a Luggin capillary to minimize uncompensated
ohmic resistance in the cell. Oxygen was exhaustively removed from
the working electrode compartment by bubbling N2 or CO2
depending on the particular experiment.
42
2
4
3−45
theoretical works.
Differential reflectivity in the form of δR/R versus potential profiles
was recorded on a 2 mV/s potential scan with a superimposed, small
amplitude sinusoidal, potential perturbation (ca. 50 mVp-p, 11 Hz).
Monochromatic light of a desired wavelength was reflected off the
working electrode, and focused on a photomultiplier operating at a
constant current set by a feedback system and a programmable power
supply. Alternating current voltammetry was used in some cases along
with δR/R measurements. The rectified ac current and optical ac
response (normally the in-phase component) were monitored, after
demodulation with a lock-in amplifier, as a function of the electrode
The success of this initial bimolecular photocatalytic system
led us to examine unimolecular photocatalysts incorporating
both the ruthenium chromophore and the pyridyl subunit. In
this Article, we evaluate the performance of Ruα and Ruβ
(
shown in Figure 1) as electrocatalysts and photocatalysts for
the reduction of CO in mixed DMF:H O (1 M). DMF was
2
2
chosen as main solvent because the photo-excited-state of these
complexes is rapidly quenched in aqueous solvents, but can be
appreciable in mixed solvent systems of water and nonprotic
4
6
49−51
solvents. This has an additional advantage in that the
potential.
Photochemical Reduction. Two types of photochemical experi-
ments were conducted: (i) an absorption experiment looking at the
change of the spectra with time irradiation, and (ii) a photocatalysis
experiment where a more concentrated solution was irradiated and
analyzed for products. For both experiments all solutions were
saturated with N or CO gas prior to irradiation. These solutions were
solubility of CO in DMF is appreciably higher than in water
2
(
0.2 M vs 0.03 M). Using a combination of voltammetry and
differential reflectance measurements, we elaborate on the
electrochemical and photochemical reduction mechanism and
complexes speciation in the absence (N2 saturated) and
2
2
presence of CO . Constant-current electrolyses and constant
42
2
then irradiated with a custom built photoreactor using point source
irradiation photolysis along with gas chromatography deter-
−5
lights emitting at 470 nm ±20 nm with a photon flux of 1 × 10
mined methanol to be one of the CO reduction products.
Einsteins/second, as measured by chemical actinometry using
potassium ferrioxalate. Solution components and concentrations
are described in the text. For the photocatalysis experiments, periodic
2
5
2
EXPERIMENTAL SECTION
■
5
.0 mL aliquots were withdrawn by syringe and placed in airtight vials,
Synthesis. Ruthenium(III) chloride was purchased from Pressure
Chemical (Pittsburgh, PA) and used as received. DMF, ethanol, and
acetone were purchased from VWR and used as received. Ammonium
hexafluorophosphate and 1,10-phenanthroline were purchased from
Alpha Aesar and used as received; 3,4-diaminopyridine, 2,3-
diaminopyridine, and phenylenediamine were purchased from
Sigma-Aldrich and used as received. [Ru(phen) (ptpbβ)](PF ) ,
where they were kept at −20 °C until analyzed by head space GC/MS
analysis as described below.
Constant-Current Electrolysis. A two-compartment cell with a
large area reticulated vitreous carbon (RCV) cylindrical working
electrode (BASI MF-2077) contacted with a platinum wire was placed
in the cell’s main compartment along with the reference electrode.
Platinum foil (2 cm × 8 cm) was used as the counter electrode and
was placed within an inner compartment separated by a glass frit from
the main compartment. The working electrode compartment
contained 0.30 mM of the respective ruthenium complex in
2
6 2
[
Ru(phen) (ptpbα)](PF ) , and [Ru(phen) (dppz)](PF ) were
2 6 2 2 6 2
47,48
synthesized according to standard proceedures.
Electrochemistry and Spectroelectrochemistry. Electrochem-
ical data were obtained by using cyclic (CV) and differential pulse
voltammograms (DPV) on a CHI620C electrochemical analyzer (CH
Instruments, Austin, TX) using a single compartment (3 mL volume)
electrochemical cell. A glassy carbon (1.5 mm diameter disk) working
electrode from Cypress Systems was used. The electrode was polished
to a mirror finish with wet alumina (Buehler, 0.05 μm), followed by
rinsing with Millipore Milli-Q water and sonication. A platinum wire
and a Ag/AgCl reference electrode (Cypress, model EE009) were used
as counter and reference electrodes, respectively. The electrolyte
DMF:H O (1 M) solution with 0.1 M TBAPF as supporting
2
6
electrolyte. The counter electrode compartment contained the
supporting electrolyte solution but no ruthenium complex. The cell
was saturated with CO and kept pressurized and sealed while the
2
electrolyses were performed at an applied current of −0.6 mA.
Product Detection. Gas chromatography (GC) and colorimetric
tests were adopted to analyze the conversion of CO to organic
2
products. A Shimadzu GC with mass spectrometer detector (GC-MS-
2010 Plus chromatograph with a MS TQ8030 detector) and an AOC-
5000 Plus autosampler was used. The chromatographic column
(SHRX105MS, 30-m length and 0.25 mm inner diameter) at an oven
solutions contains the ruthenium complex in DMF:H O (1 M)
2
containing 0.1 M tetrabutylammonium hexafluorophosphate
(
TBAPF ) as the supporting electrolyte. The solutions were saturated
6
B
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temperature of 45 °C was used in combination with a MS detector at
2
4
50 °C; helium was the carrier gas. Detection at m/z = 31, 29, 28, and
5 was chosen to identify methanol, formaldehyde, carbon monoxide,
and formate, respectively, as the reaction products. Sample aliquots
were preheated at 80 °C in a 20 mL headspace vial with a septa cap,
and 2.5 mL of the head space gas was injected from a syringe heated to
8
0 °C and analyzed in the GC/MS instrument. Control samples
containing known concentrations of methanol (in the range 5−1000
μM) were analyzed to obtain a standard curve (Supporting
Information Figures S1 and S2).
A quantitative colorimetric procedure (chromotropic acid test) was
used for formaldehyde and formic acid detection (see Supporting
3
1,35,53
Information Figure S3).
First, liquid samples were treated with
Amberlite IRN-150 ion-exchange resin to remove the ruthenium
complex. As this test is for aldehyde detection, the sample was divided,
and formaldehyde concentration was determined from one-half.
Subsequently, 0.5 mL aliquots of the other half were treated with
magnesium turnings under acidic conditions to reduce the formic acid
to formaldehyde. After determination of the new formaldehyde
concentration, formic acid concentration was determined by differ-
ence.
53
2+
Figure 2. Comparison of DPV runs for [(phen) Ru(ptpbα)] , [Ru
2
2+
(phen) ] , and pyridine encompassing the electroreduction potential
3
range (−0.15 V to −1.5 V) recorded at a glassy carbon disc electrode
in DMF:H O (1 M) solution saturated with N . Reactant
2
2
n
4
concentration = 0.5 mM, supporting electrolyte = 0.1 M NBu PF .
RESULTS AND DISCUSSION
6
■
Approach for Photo- and Electrocatalytic CO2
Reduction by Ruα and Ruβ. Ruα, and Ruβ share structural
similarities with Rudppz (Figure 1) which are manifest in that
all three complexes are weak or nonemitting in aqueous
solution, but exhibit strong (or bright) emission in nonprotic
solvents, such as MeCN and DMF. Mixed-aqueous solutions
potentials than those depicted in Figure 2. With respect to
complex Ruα, the first redox process is located at ca. −0.61 V
and is followed by a low shoulder at ca. −1.0 V and two next
sharp peaks appearing at −1.22 and −1.43 V, respectively. In
2
+
concurrence with the voltammetric profile of [Ru(phen) ] ,
3
(
i.e., DMF:water) show intermediate behavior with lumines-
the last two processes in Ruα are associated with two sequential
electron uptakes in the two phen ligands, but the two more
positive waves are assigned to electroreduction of the ptpbα
ligand.
cence intensity and lifetimes falling with increasing water
concentration.
1
4
6,54
For example, a Rudppz solution containing
M water in DMF displays a lifetime on the order of 400 ns
compared to 250 ps in pure water; thus, it is possible to access
the photoexcited molecule for photochemical reactions in
solutions with appreciable (up to 1M) amounts of water.
The variability of the luminescence lifetimes in these
complexes is attributed to the presence of two energetically
The cyclic voltammograms (CVs) for Ruα and Ruβ in the
initial reduction region are overlaid in Figure 3. As can be seen,
the first reductive redox process for Ruα and Ruβ in N2
saturated DMF:H O (1 M) shows highly reversible voltam-
2
0
metric waves (v = 50 mV/s) at redox potentials, E , of −0.64
3
3
similar triplet excited-states, the MLCT
and MLCT ,
and −0.57 V versus Ag/AgCl, respectively (Figure 3a). These
reductions are assigned to reduction of the ptpbα and ptpbβ
ligands, and are found at slightly more positive potentials than
observed for Rudppz (located at −0.81 V vs Ag/AgCl) in dry
prox
dist
5−57
5
which formally localize the excited electron in a proximal
bpy-like” MO on the extended ligand or in a distal “phenazine-
like” MO, respectively. In aqueous solution, Rudppz emission is
“
−6
60
weak and red-shifted (∼670 nm, ϕ = 2.5 × 10 ) relative to
DMF and at −0.80 V after addition of water. The potential
lum
52
that in MeCN (610 nm, ϕ = 0.033) due to population of
positive shift in Ruα and Ruβ is reasonable given the extra
nitrogen in these ligands relative to dppz increases the overall
electronegativity of the ligand. Figure 3b shows the effect of
concentration of complex Ruα on CVs spanning the first
electroreduction process. The concentration was varied from
0.05 to 0.30 mM. The shape of the voltammograms and the
peak separation afford a Nernstian (reversible) behavior under
lum
3
3
the MLCT state being favored in water and the MLCT
dist
prox
state being favored in MeCN. As the proportion of water is
increased in the mixed solvent system, the increased polarity of
the solvent is better able to stabilize the large excited-state
3
dipole in the MLCT shifting the excited-state equilibration
dist
3
between the two states more toward the short-lived MLCT
dist
4
6,54−56
state.
As Ruα and Ruβ exhibit largely analogous
diffusion control for this complex in the absence of CO . From
2
luminescent behavior, it is thought that they have similarly
the linear slope of the cathodic current peak with complex
concentration (inset to Figure 3b), it was determined that it is a
one-electron process, assuming a diffusion coefficient of 3 ×
3
3
55,56
behaving MLCT and MLCT excited-states.
prox
dist
Electrochemistry. Figure 2 compares differential pulse
−
6
2
61
voltammograms (DPV) in N saturated DMF:H O (1 M)
10 cm /s.
2
2
encompassing the electroreduction of Ruα (green solid line),
[
A Pourbaix diagram of the first electroreduction process for
Ruβ was obtained in water to determine if and when the
reduction is coupled to protonation (see Supporting
Ru(phen)3]2 (red dash line), and pyridine (blue dash dot
+
line). The Ruα complex shows three main electroreduction+
processes while two are detected for complex [Ru(phen)3]2
and no electrochemical activity is seen for pyridine and in
Information Figure S4). The pK of the conjugate acid
a
3+
[Ru(phen) (ptpbβH)] (RuβH) was determined to be 3.3,
2
58
agreement with previously reported data. For the parent
and the slope (m = −62 mV/pH) of the redox process at pH >
3.3−7.0 indicates a one-electron, one-proton process. We note
that this behavior is different from that observed with pyridine
as the pH is varied, which instead shows a larger peak current
but no significant potential shift (see Supporting Information
Ru(phen)3]2 complex only two sharp peaks are shown at
+
[
−
1.13 and −1.26 V, respectively; they are associated with two
59
sequential 1-electron uptakes, one in each phen-like MO
while the third electron uptake is located at more negative
C
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Figure 3. (a) Comparison of the cyclic voltammetric behavior of Ruα (red dash line) and Ruβ (blue solid line) in N saturated DMF:H O (1 M)
2
2
solutions. Working electrode: glassy carbon disk (1 mm diameter), scan rate = 50 mV/s, complex concentration = 20 μM, supporting electrolyte =
0
.1 M TBAPF . (b) Effect of concentration on the voltammetric behavior of the first electroreduction process of Ruα in solutions saturated with N .
6
2
The increase of the cathodic current peak with the complex concentration follows a linear relationship (shown in the inset plot).
Figure S5a,b). This latter behavior has been suggested to show
62
the reduction of hydrated protons and, importantly, is absent
in our electrocatalysts. A similar Pourbaix analysis for Rudppz
and Ruα revealed the pK ’s of the conjugate acids are below
a
zero and the electroreduction is also a one-electron, one-proton
process out to pH 7. In the mixed solvent, the CV behavior of
Ruβ is similar although cathodically shifted from −0.07 V
(
water pH 4.5) to −0.6 V (DMF:H O 1 M) (see Supporting
2
Information Figure S5).
Electrochemistry of Ruα and Ruβ in N - and CO -
2
2
Saturated Solutions. Electrocatalytic reduction of CO by
2
Ruα and Ruβ is observed by simply bubbling CO through the
2
DMF:H O (1 M) solution and examining the resulting CVs,
2
particularly when performed at low potential scans rates (5−10
mV/s). As seen in Figure 4, large increases in the cathodic
current are observed at −0.64 and −0.60 V for Ruα and Ruβ,
respectively, indicating electrocatalytic CO reduction. Rudppz,
2
in contrast, is not electrocatalytically active under identical
conditions when CO is added (data not shown), indicating the
2
need for the extra nitrogen in the ligand structure for catalytic
activity. A control run of CO -saturated solution without
2
Figure 4. Comparison of the voltammetric behavior of Ruα (red solid
addition of any of the ruthenium complexes (Figure 4, black
dot trace) indicates that the glassy carbon electrode is not
line) and Ruβ (blue solid line) in DMF:H O (1 M) solutions saturated
2
with CO . Voltammograms of Ruα before saturation with CO (red
2
2
active for CO reduction. In the absence of a catalyst and in
dash line) and that without Ru complex but with CO
2
(black dot line)
2
are included as reference. All voltammograms were run at 5 mV/s with
a glassy carbon working electrode.
media of low proton availability (such as DMF and DMSO),
electroreduction of CO is reported to occur at −2.16 V versus
2
6
3
Ag/AgCl.
In both Ruα and Ruβ, the presence of a distal nitrogen site
on the ptpb (either α or β) is postulated to be responsible for
the catalytic activity as the Rudppz complex was found
electrocatalytically inactive. This pyridyl-like moiety appears
(
phen) ]2+ both in N and CO saturated solutions, shown in
3 2 2
Supporting Information Figure S6, reveals only the former is
catalytic, at potentials greater than −1.4 V indicating the ptpb
ligand is integral to the catalysis at −0.6 and −1.2 V. Moreover,
the catalytic mechanism for CO2 electroreduction was
corroborated by plotting the Nicholson−Shain current peak
function (i /v ) versus the scan rate (v) for Ruβ complex in
DMF:H O (1 M) saturated with CO (Supporting Information
Figure S7).
to be needed to form the initial CO adduct, presumably in the
2
37
form of a carbamate-type radical intermediate. Figure 4 also
shows a net increase of cathodic current at potentials more
negative than −1.1 V for both complexes, indicating that the
reduction of CO2 is also catalytically occurring at these
potentials. A comparison of DPV runs for Ruα and [Ru-
1/2
2+
p
2
2
64
D
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Figure 5. (a) Potential/time profile recorded during cathodic galvanostatic electrolysis of Ruβ (0.30 mM) in DMF:H O (1 M) saturated with CO .
2
2
(
b) Head space GC/MS analyses of liquid aliquots removed after 6 and 14 h of electrolysis, respectively. MS peak intensities at 1.56 min (m/z = 31)
from CH OH increase ca. 2.5 times from 6 to 14 h of electrolysis. Formation of CO (m/z = 28, at 1.47 min, red trace) is only present after the
3
potential has evolved to ca. −1.2 V. Electrolysis conditions: 0.1 M TBAPF (0.1 M) as supporting electrolyte; working electrode = RVC cylindrical
6
electrode; applied current = −0.6 mA.
Table 1. Comparison of Two Photosystems That Make Methanol. (a)
MeOH
formaldehyde
TON (in
formate
Ru
μmol
TON (in
Φ ×
Φ ×
TON (in
Φ ×
−
−5
−
−3
−
−3
system
μmol
e )
10
μmol
e )
10
μmol
e )
10
Ruβ DMF:water (2.5 μmol)
2.5
6
1.1 ± 0.2 (n = 4)
0.9 ± 0.08
1.7 ± 0.3
0.4 (2.6)
0.15 (0.9)
0.3 (2.0)
15
6.3
11
7.1
2.8 (11.4)
1.6
16.2
55
6.5 (13)
9 (18)
3.6
3
2+
a
[
Ru(phen) ] :py 1:200 (6 μmol Ru)
3
2+
[
Ru(phen) ] :py 1:200 (0.1 M KCl) (6
6
450
76 (152)
25
3
a
μmol Ru)
a
Data from ref 41.
In the absence of substrate (CO ), we observe a second
process of the complex) for a period of time spanning 6 h, and
then the potential is seen to rapidly evolve to more negative
values associated with the second electroreduction of the
complex and remains there for at least 15 h. Comparison of
Figure 5a with the respective DPV run in Figure 2
demonstrates that the two potential arrests in Figure 5a are
associated with the respective electroreduction processes
tracked in the DPV run (green solid line in Figure 2). The
abrupt negative potential shift at 6 h electrolysis signals a
change in the electrocatalytic reduction mechanism which
curiously corresponds with the production of approximately 1
methanol per Ruβ. Coulometric calculation of the number of
electrons consumed in the time period of the first potential
plateau (at −0.6 V) is 5.5 electrons per Ruβ. Headspace GC/
MS analysis of aliquots collected after 5 h electrolysis (Figure
2
reductive redox process for Ruα at −1.05 V which is strongly
attenuated when CO is present (Figure 4 and Supporting
2
Information Figure S6). The magnitude of this follow-up wave
increases at lower scan rates, which is suggestive of dimerization
II
•
+
•
of the radical [Ru (phen) (ptpbα )] , where (ptpbα ) is the
2
one-electron reduced ptpbα ligand, as shown in reaction 1, and
that the new dimer is the resulting electroactive species. Radical
dimerization is not uncommon in these systems and has been
6
0,65−67
reported previously in closely related complexes.
•
•
Ruα + Ruα → (Ruα)
(1)
2
The lack of a similar secondary redox process in the CV of Ruβ
suggests that this complex is not subject to this dimerization
side-reaction and hints that the site of dimerization is the distal
carbons on ptpbα.
5b) revealed that 0.92 CH OH per Ruβ was produced. No CO
3
Product Analysis of Electrocatalytic and Photo-
was detected at this time but some formaldehyde was detected
at m/z 29. While it is unclear what caused this abrupt potential
change, it suggests that the ptpbβ ligand in Ruβ is covalently
modified such that it can no longer participate in the redox
reaction, thus the jump to the next available redox couple. After
14 h of electrolysis, the methanol yield is nearly triple that
found at 6 h (Figure 5b); however, a change in mechanism is
clear as CO is now observed in the GC/MS with a final product
distribution being 75% methanol and 25% CO.
catalytic CO Reduction. In order to determine the products
2
of CO reduction, Ruβ was subjected to controlled-current
2
electrolysis in CO -saturated DMF:H O solution. As CV
2
2
studies revealed typical diffusion control behavior for the first
electron uptake in Ruβ at concentrations ≤0.30 mM, the bulk
electrolysis was performed under similarly dilute concentrations
in order to minimize potential side reactions, such as
dimerization. A representative potential−time plot obtained
during electrolysis is shown in Figure 5a. The electrode
potential is seen to stabilize at ca. −0.6 V (first electroreduction
In a photochemical experiment, 25 mL of a 100 μM Ruβ
solution in DMF:H O (1 M) with 0.1 M TEA was saturated
2
E
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Inorganic Chemistry
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Figure 6. Transient ΔA spectra of Ruα (22 μM) during photolysis in N (a) and in CO (b) saturated DMF:TEA (0.25 M)/H O (1 M) solutions
2
2
2
and Ruβ (22 μM) during photolysis in N (c) and in CO (d) saturated DMF/TEA (0.25 M)/H O (1 M) solutions. Peaks pointing down indicate
2
2
2
bands disappearing while those pointing up correspond to new bands appearing due to photolysis.
with CO and irradiated with 470 nm light for up to 6 h at 25
if we could detect any of the likely intermediates by examining
2
°
C. A color change is apparent after 90 min, and the solution is
the evolution of the UV−vis absorption spectrum in the range
300−900 nm as a function of irradiation time in the presence
appreciably darker after 6 h. Product analysis at the 1 h time
point yielded 16.2 μmol formate, 7.1 μmol formaldehyde, and
and absence of CO . The differential absorption (ΔA) data for
2
1
.1 μmol methanol solution, as shown in Table 1. At longer
irradiation periods (ca. 8 h), the methanol has only increased to
.5 μmol indicating that the system deactivation is apparently
coupled to the color change, which is likely due to ligand
Ruα and Ruβ under N or CO atmosphere as a function of the
2
2
irradiation time are shown in Figure 6. The initial absorption
1
spectra for Ruα and Ruβ in a CO -saturated solution containing
2
0
.25 M TEA (sacrificial donor) before and after visible light
6
8
labilization. A comparison of products for the bimolecular
irradiation are shown in Supporting Information Figure S8a,b,
respectively. Observe in Figure 6 that Ruα and Ruβ show
2+
42
[
Ru(phen) ] −pyridine photosystem and Ruβ after 1 h
3
irradiation is given in Table 1 and reveals that Ruβ is slightly
superior as a methanol producing photocatalyst in terms of
quantum yield. The turnover number (TON) in the first hour
for Ruβ is 27 on an electron basis, which is broken down to 0.4
differences in the evolution of their ΔA spectra in N but are
2
nearly identical in their evolution in the presence of CO .
2
Moreover, there is little noticeable difference in the evolution
of the ΔA spectra for Ruβ under N or CO (Figure 6c,d,
−
−
2
2
for MeOH (6 e product), 2.8 formaldehyde (4 e product),
and 6.5 formate (2 e product), compared to 154 on an
electron basis for the [Ru(phen) ] −pyridine system. While
Ruβ is apparently less active than the [Ru(phen) ] −pyridine
system, it is more selective for methanol, producing more in the
first hour on both an absolute quantum yield basis and relative
basis (9% of the reducing equivalents end up in MeOH for Ruβ
vs 1% for the Ru:pyridine system). These comparisons need to
be considered with the caveat that solvent systems are different
in the two cases.
−
respectively) upon irradiation. The changes seen under a N
atmosphere are consistent with the formation of a one-electron
2
2
+
3
II
•
2+
2
+
reduced, protonated complex [(phen) Ru (ptpbβH )]
(
2
3
•
RuβH ). The observed spectral changes are characteristic of
ligand reduction in ruthenium(II) coordinated dppz complexes
and in related complexes with extended phenazine-like
55−57,60,69,70
ligands.
Reduction is associated with a ligand
bleach around 350 nm and a broad new absorption band
•
around 560 nm. From Figure 6d, it would appear that RuβH
Effect of CO on the Photochemistry of Ruα and Ruβ
(detected at 564 nm) is also the product of photoreduction
2
complexes. As the ability of Ruβ to function as a CO2
under a CO atmosphere except for two subtle differences. The
2
reduction photocatalyst was established, we attempted to see
final spectrum obtained under CO2 is slightly attenuated
F
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Inorganic Chemistry
Article
relative to that seen under N , and the growth of the peak at
2
5
64 nm is slightly slower under CO2.
The more complicated evolution of the ΔA spectra for Ruα
under N (shown in Figure 6a) is seen more clearly in Figure 7
2
The existence of a fast equilibrium between the complex
radical anion and the CO adduct is more clearly seen in the
2
differential reflectance data as a function of potential and is
shown in Figure 8. The data for Ruα (at 575 nm, Figure 8a)
and Ruβ (at 564 nm, Figure 8b) under N (black lines) and
2
CO (red lines) atmospheres are shown for the forward and
2
Figure 7. Absorbance versus time for selected wavelengths for Ruα
under nitrogen atmosphere upon irradiation. Data taken from Figure
reverse scan. The vertical scale is such that the appearance of
electrochemically generated species gives rise to positive δR/R
signals at ∼575 nm for Ruα and 564 nm for Ruβ, respectively,
regardless of the direction of the scan. While this is a
6
a.
in which the absorbance at 364 nm (ptpbα bleach), 460 nm
intermediate band), 517 nm (final new product band), and
75 nm (radical anion band) are plotted versus irradiation time.
(
5
The intermediate is assigned to formation of the dimer, (Ruα)2,
which itself can be further reduced to yield a product with
•
•
similar spectral features to Ruα or RuαH . Formation of this
final product has the broad absorption feature between 500 and
8
5
00 nm, plus it introduces a distinct new absorption peak at
17 nm which is not seen in the radicals of the monomeric
•
•
RuαH or RuβH . We have similar behavior in the radical
dimerization reactions of the radicals of [(bpy) Ru(tatpp)] ,
where tatpp is tetraazatetrapyridopentacene. Under a CO2
atmosphere, the dimerization reaction (shown by a band at 517
nm) is inhibited under both electrochemical (Figure 4) and
photochemical (Figure 6b) conditions. The final ΔA spectrum
in Figure 6b is very similar to that seen for Ruβ in Figure 6c,d,
2+
2
65
•
suggesting that the final product here is RuαH .
From the spectral data seen in Figure 6 b−d, it appears that
•
CO is not interacting with the radical products RuαH and
2
•
RuβH , as there are no obvious spectral differences in the 300−
00 nm window. However, since Ruα does not show significant
9
signs of dimerization when CO is present, it is clear that CO2
2
has some interaction with the reduced complexes. These
contradictory data can be interpreted if we posit that there is
fast and reversible formation of a CO -radical adduct, as shown
2
in reaction 2, and the rate of photoreduction is significantly
greater than the subsequent CO reduction reactions of the
2
adduct. This initial equilibrium inhibits dimer formation in the
case with Ruα.
To examine the photocatalyst capability to continuously
affect CO reduction, samples of Ruβ prepared as described in
2
Figure 8. δR/R vs potential curves of Ruα at 575 nm (a) and Ruβ at
Figure 6d were irradiated for 30 s to fully photoreduce the
complex; the samples were then kept in the dark for 5 min
during which time the original spectrum was regenerated. Re-
exposure to light for 30 s showed the process could be repeated
without issue (see Supporting Information Figure S9).
5
64 nm (b) in DMF:H O containing 0.1 M TBAPF . These runs were
2
6
recorded as a function of potentials in degassed (black trace) and CO2
saturated solutions (red trace). The working electrode was a mirror-
polished Pt disc subjected to a sine potential wave (11 Hz, 50 mVp‑p
)
superimposed on a 2 mV/s potential scan.
G
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Inorganic Chemistry
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spectroelectrochemical and not a photochemical experiment, it
clearly shows an attenuation and a positive shift of the peak
associated with formation of RuαH• and RuβH• in the
S11); the highest pH is chosen because of its proximity to the
natural pH of a CO2 saturated aqueous solution. The
voltammetric waves shifting to more positive potentials as the
pH decreases and following a Pourbaix plot indicated a 1-
electron and 1-proton reaction during the first electroreduction
presence of CO . This can be interpreted in terms of the
2
•
equilibrium expression in reaction 2 for RuβH or a similar
expression for RuαH , if we assume that the 517 nm peak is
•
process for both Ruα and Ruβ. The pK of both complexes (3.3
a
associated with formation of the dimer species and this peak is
for Ruβ and <0 for Ruα) show definitively that, at pH > 3,
either in organic or aqueous media, the first process occurring
in both is the electron uptake (reaction 3), and that they are
not protonated prior to the electroreduction reaction and in
absent in the CO adduct (Supporting Information Figure
2
S10). The attenuation is indicative of the equilibria shown in
reaction 2, showing the consumption of the electroreduced
38
species (via formation of the CO adduct) in the two cases. The
contrast with the case of the pyridine as reported by Bocarsly.
2
attenuation is more pronounced for Ruα suggesting the
The first electroreduction process in Ru complexes
containing dppz-like ligands has been shown to be 1-electron
and 1-proton processes localized largely on the dppz-like ligand
(as indicated in reactions 3 and 4 for our present system) both
equilibrium with the CO adduct (and subsequent follow-up
2
reduction reactions) lies further to the right. The decrease in
absorbance, measured as δR/R, shows that some of the
•
•
65
•
•
electroreduced species, RuβH and RuαH , are intercepted by
in organic and water media. For RuβH and RuαH , the
CO in an ECC′-type (catalytic) mechanism, and if we assume
additional pyridyl-type nitrogen couples the site of electron
2
a fast equilibration for adduct formation (i.e., reaction 2), then
storage with the site of CO adduct formation (reaction 5). The
2
−1
the attenuation reflects the relative equilibrium constants for
rate constant (k′) for reaction 5 can be estimated at 0.14 M
•
•
−1 −1 −1
RuαH than RuβH with CO , which are estimated at 3 and 1,
s
and 0.33 M
s
for Ruα and Ruβ, respectively. These rate
64
2
respectively (assuming a pCO2 of 1 atm). The larger
constants were calculated from the respective λ values for
each complex (λ = (RT/nFv)k′CCO2) as obtained from
voltammetric profiles as those in Figure 4. The product of
reaction 5 needs five additional reduction cycles to form one
molecule of methanol, the details of which are not yet
understood.
•
•
equilibrium constant seen for RuαH than RuβH is in keeping
with the relative basicities of the two radical anions (pK
a
•
•
(
RuαH ) < pKa (RuβH )). This interpretation is also
supported by the changes in magnitudes seen in the reverse
scans. Under a N atmosphere, both Ruα and Ruβ show typical
2
behavior in that the return peak amplitude is larger as the
concentration of the electroreduced species has built up during
the time spent in the cathodic regime. In contrast, the
In a photochemical cycle, reaction 3 represents reductive
quenching of the photoexcited chromophore by the sacrificial
donor and from there can similarly react via reactions 4 and 5.
Subsequent reductions may occur via excitation of the now
modified Ru complex or via electron-transfer from another
amplitude of the return peak under a CO atmosphere has
2
diminished relative to the forward scan indicating that this
•
species has been further depleted in an ECC′ mechanism,
Ruφ . While Ruβ is only a slight improvement over the
2+
which would follow if the CO adducts were being consumed
bimolecular [Ru(phen) ] −pyridine system as a photocatalyst
2
3
on a slower time scale in further reduction reactions, thus
shifting the equilibrium position further away from the radical
in terms of methanol production, it appears to be more
selective for methanol production.
•−
•−
anions, of Ruα and Ruβ . The positive ca. 50 mV shift seen
in the potentials in the presence of CO is due, in part, to the
CONCLUSIONS
2
■
consumption of some of the product in the CO adduct
2
Ruα and Ruβ show that a ruthenium(II) polypyridyl
chromophore can be covalently modified with a pendant
pyridyl function so as to act as a unimolecular electro- and
formation and possibly, in part, to small decrease in pH as the
presence of CO acidifies the solution. δR/R/potential curves
2
for complex Ruα were also obtained at 517 nm to explain the
photochemical mechanism in the absence and presence of CO2
photocatalyst for CO reduction to deeply reduced products,
2
such as methanol and formaldehyde, in addition to formate.
While the extent of photocatalysis is modest, these complexes
represent a small but growing family of photocatalysts able to
(
Supporting Information Figure S10). In the absence of CO2,
the species tracked at 517 nm is clearly seen to be formed in the
same potential range as that at 575 nm.
From the electrochemical and photochemical data, Ruβ is the
better catalyst for CO2 reduction because it lacks the
dimerization side reaction seen for Ruα. Therefore, a catalytic
ECC′-mechanism of Ruα and Ruβ complexes to drive CO2
reduction is perceived as
drive CO reduction beyond a simple 2-electron reduction to
2
CO or formate to yield arguably more valuable products such as
methanol. Unlike the situation with pyridine as an electro-
catalyst, there is nothing unusual about the reduction potentials
for Ruα (−0.64 V) and Ruβ (−0.57 V), which are typical for
dppz-like ligands when coordinated to the Ru(II) ion.
Interestingly, the electrocatalysis is far more selective for
methanol production than photocatalysis, perhaps reflecting the
ability of the electrode to rapidly provide multiple electrons to
the catalyst compared to the photocatalytic cycle. Differential
absorbance studies versus time reveal initial products in the
−
•
Ruφ + e → Ruφ
(Ruφ = Ruαor Ruβ)
E
(3)
(4)
(5)
•
+
•
Ruφ + H O → RuφH + H O
C
3
2
•
RuφH + CO → RuφCO H C′
2
2
•
•
photochemical process to be the radicals RuαH and RuβH ,
and differential reflectance spectroelectrochemistry shows these
products are in rapid equilibration with their CO adducts. The
with reaction 5 competing with dimerization (reaction 1) in the
case of Ruα complex.
2
Reactions 3 and 4 are certainly occurring at the glassy carbon
electrode interface, as indicated by cyclic voltammograms of
both complexes in N saturated DMF:H O as well as in
stability of these ruthenium(II) photocatalysts with respect to
photolabilization and degradation appears to be the most
significant initial challenge in further developing this technol-
ogy. Projects addressing this aspect of the technology are in
progress.
2
2
aqueous media at different pH values in the range 6 to 0
Figure 3 and Supporting Information Figures S4, S5, and
(
H
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Article
(
21) O’Toole, T. R.; Sullivan, B. P.; Bruce, M. R. M.; Margerum, L.
ASSOCIATED CONTENT
Supporting Information
Figures regarding quantitative detection and analyses of
methanol, formaldehyde, and formate, as well as electro-
chemical controls and comparisons. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
D.; Murray, R. W.; Meyer, T. J. J. Electroanal. Chem. Interfacial
Electrochem. 1989, 259, 217−239.
(
*
S
22) Pugh, J. R.; Bruce, M. R. M.; Sullivan, B. P.; Meyer, T. J. Inorg.
Chem. 1991, 30, 86−91.
23) Chen, Z.; Concepcion, J. J.; Brennaman, M. K.; Kang, P.; Norris,
M. R.; Hoertz, P. G.; Meyer, T. J. Proc. Natl. Acad. Sci. U.S.A. 2012,
09, 15606−11.
24) Chen, J.; Szalda, D. J.; Fuita, E.; Creutz, C. Inorg. Chem. 2010,
49, 9380−9391.
25) Doherty, M. D.; Grills, D. C.; Muckerman, J. T.; Polyansky, D.
E.; Fujita, E. Coord. Chem. Rev. 2010, 254, 2472−2482.
26) Fukushima, T.; Fujita, E.; Muckerman, J. T.; Polyansky, D. E.;
Tanaka, K. 238th ACS National Meeting, 2009; p PHYS-012.
27) Grodkowsik, J.; Dhanasekaran, T.; Neta, P.; Hambright, P.;
(
1
(
AUTHOR INFORMATION
Corresponding Authors
E-mail: rolezna@inifta.unlp.edu.ar.
E-mail: macdonn@uta.edu.
Notes
■
(
*
*
(
The authors declare no competing financial interest.
(
Brunschwig, B. S.; Shinozaki, K.; Fuita, E. J. Phys. Chem. A 2000, 104,
11332−11339.
ACKNOWLEDGMENTS
■
The authors thank the NSF for Grants CHE-1301332 and
CHE-0840509 (for the purchase of a 500 MHz NMR), the
Robert A. Welch Foundation [Grant Y-1301], and BID1728/
OC-RA, CONICET PIP #00246 (to R.O.L.), for financial
support. We also thank the DOE EERC for financial support to
the UTA Center for Renewable Energy Science and
Technology (CREST) 2011−2013.
(28) Morris, A. J.; Meyer, G. J.; Fujita, E. Acc. Chem. Res. 2009, 42,
1
(
983−1994.
29) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Chem.
Soc. Rev. 2009, 38, 89−99.
30) Haines, R. J.; Wittrig, R. E.; Kubiak, C. P. Inorg. Chem. 1994, 33,
723−4728.
31) Arana, C.; Yan, S.; Keshavarz-K, M.; Potts, K. T.; Abruna, H. D.
Inorg. Chem. 1992, 31, 3680−3682.
32) Chiericato, G.; Arana, C. R.; Casado, C.; Cuadrado, I.; Abruna,
(
4
(
(
REFERENCES
■
H. D. Inorg. Chim. Acta 2000, 300−302, 32−42.
(
1) Kumar, B.; Llorente, M.; Froehlich, J.; Dang, T.; Sathrum, A.;
Kubiak, C. P. Annu. Rev. Phys. Chem. 2012, 63, 541−569.
2) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
(33) Hurrell, H. C.; Mogstad, A. L.; Usifer, D. A.; Potts, K. T.;
Abruna, H. D. Inorg. Chem. 1989, 28, 1080−1084.
(
1
(
(
(34) Ramos, S. J. A.; Arana, C. R.; Hernandez, L.; Potts, K. T.;
Keshavarz-K, M.; Abruna, H. D. Inorg. Chem. 1995, 34, 3339−3348.
(35) Barton, E. E.; Rampulla, D. M.; Bocarsly, A. B. J. Am. Chem. Soc.
5729−15735.
3) Armaroli, N.; Balzani, V. Angew. Chem., Int. Ed. 2007, 46, 52−66.
4) Song, W.; Chen, Z.; Brennaman, M. K.; Concepcion, J. J.;
2
(
008, 130, 6342−6344.
Patrocinio, A. O. T.; Iha, N. Y. M.; Meyer, T. J. Pure Appl. Chem. 2011,
3, 749−768.
5) Boston, D. J.; Huang, K.-L.; De Tacconi, N.; Myung, N.;
MacDonnell, F. M.; Rajeshwar, K. In Photoelectrochemical Water
Splitting: Materials, Processes and Architectures; Lewerenz, H. J., Peter,
L., Eds.; Royal Society of Chemistry: London, 2013; p 289.
6) Kelly, C. A.; Mulazzani, Q. G.; Venturi, M.; Blinn, E. L.; Rodgers,
M. A. J. J. Am. Chem. Soc. 1995, 117, 4911−4919.
7) Jacquet, O.; Frogneux, X.; Das Neves Gomes, C.; Cantat, T.
Chem. Sci. 2013, 4, 2127−2131.
8) Costentin, C.; Drouet, S.; Robert, M.; Saveant, J. M. Science 2012,
38, 90−94.
9) Darensbourg, D. J.; Rokicki, A.; Darensbourg, M. Y. J. Am. Chem.
Soc. 1981, 103, 3223−3224.
10) Cokoja, M.; Bruckmeier, C.; Rieger, B.; Herrman, W. A.; Kuhn,
F. E. Angew. Chem., Int. Ed. 2011, 50, 8510−8537.
11) Sato, S.; Koike, K.; Inoue, H.; Ishitani, O. Photochem. Photobiol.
36) Cole, E. B.; Bocarsly, A. B. In Carbon Dioxide as a Chemical
8
(
Feedstock; Aresta, M., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA:
Weinheim, 2010; pp 291−316.
(37) Cole-Barton, E.; Lakkaraju, P. S.; Rampulla, D. M.; Morris, A. J.;
Abelev, E.; Bocarsly, A. B. J. Am. Chem. Soc. 2010, 132, 11539−11551.
38) Morris, A. J.; McGibbon, R. T.; Bocarsly, A. B. ChemSusChem
011, 4, 191−196.
39) Seshadri, G.; Lin, C.; Bocarsly, A. B. J. Electroanal. Chem. 1994,
72, 145−150.
40) Crabtree, R. H. In Carbon Dioxide as Chemical Feedstock; Aresta,
M., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2010; pp
01−306.
41) Yan, Y.; Zeitler, E. L.; Gu, J.; Hu, J.; Bocarsly, A. B. J. Am. Chem.
Soc. 2013, 135, 14020−14023.
42) Boston, D. J.; Xu, C.; Armstrong, D. W.; MacDonnell, F. M. J.
Am. Chem. Soc. 2013, 135, 16252−16255.
43) Lim, C.-H.; Holder, A. M.; Musgrave, C. B. J. Am. Chem. Soc.
013, 135, 142−154.
44) Keith, J. A.; Carter, E. A. J. Am. Chem. Soc. 2012, 134, 7580−
583.
45) Ertem, M. Z.; Konezny, S. J.; Araujo, C. M.; Batista, V. S. J. Phys.
Chem. Lett. 2013, 4, 745−748.
46) Nair, R. B.; Cullum, B. M.; Murphy, C. J. Inorg. Chem. 1997, 36,
62−965.
47) Hartshorn, R. M.; Barton, J. K. J. Am. Chem. Soc. 1992, 114,
919−5925.
48) Tan, L.; Xiao, Y.; Liu, X.; Zhang, S. Spectrochim. Acta, Part A
009, 73, 858−864.
49) Singh, S.; de Tacconi, N. R.; Diaz, N. R. G.; Lezna, R. O.;
Munoz Zuniga, J.; Abayan, K.; MacDonnell, F. M. Inorg. Chem. 2011,
(
2
(
3
(
(
(
(
3
(
3
(
(
(
(
(
2
(
7
Sci. 2007, 6, 454−461.
(
(
4
(
12) Saveant, J.-M. Chem. Rev. 2008, 108, 2348−2378.
13) Costentin, C.; Robert, M.; Saveant, J. M. Chem. Soc. Rev. 2013,
2, 2423−2436.
(
14) Das Neves Gomes, C.; Jacquet, O.; Villiers, C.; Thuery, P.;
Ephritikhine, M.; Cantat, T. Angew. Chem., Int. Ed. 2012, 51, 187−190.
(
9
(
5
(
2
(
(
(
15) Jeoung, J.-H.; Dobbek, H. Science 2007, 318, 1461−1464.
16) Hori, H.; Ishihara, J.; Koike, K.; Takeuchi, K.; Ibusuki, T.;
Ishitani, O. J. Photochem. Photobiol., A 1999, 120, 119−124.
17) Bolinger, C. M.; Story, N.; Sullivan, B. P.; Meyer, T. J. Inorg.
Chem. 1988, 27, 4582−4587.
18) Bruce, M. R. M.; Megehee, E.; Sullivan, B. P.; Thorp, H. H.;
O’Toole, T. R.; Downard, A.; Pugh, J. R.; Meyer, T. J. Inorg. Chem.
992, 486−4873.
19) Chen, Z.; Chen, C.; Weinber, R.; Kang, P.; Concepcion, J. J.;
Harrison, D. P.; Brookhart, M. S.; Meyer, T. J. Chem. Commun. 2011,
7, 12607−12609.
20) O’Toole, T. R.; Meyer, T. J.; Sullivan, B. P. Chem. Mater. 1989,
, 574−576.
(
(
1
(
̃
̃
50, 9318−9328.
(50) de Tacconi, N. R.; Lezna, R. O.; Chitakunye, R.; MacDonnell, F.
M. Inorg. Chem. 2008, 47, 8847−8858.
4
(
1
(51) Wolcan, E.; Feliz, M. R.; Ruiz, G. T.; Juliarena, M. P.; Lezna, R.
O. J. Electroanal. Chem. Interfacial Electrochem. 2002, 533, 101−106.
I
dx.doi.org/10.1021/ic500051m | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
52) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of
Photochemistry, 3rd ed.; CRC Press: Boca Raton, FL, 2012.
(
(
53) Grant, W. M. Anal. Chem. 1948, 20, 267−269.
54) Jenkins, Y.; Friedman, A. E.; Turro, N. J.; Barton, J. K.
Biochemistry 1992, 31, 10809−10816.
55) Sun, Y.; Liu, Y.; Turro, C. J. Am. Chem. Soc. 2010, 132, 5594−
595.
(
5
(
(
(
56) Sun, Y.; Turro, C. Inorg. Chem. 2010, 49, 5025−5032.
57) Bates, W. D.; Meyer, T. J. Inorg. Chem. 1995, 34, 6421−6422.
58) Yasukouchi, K.; Taniguchi, I.; Yasukouchi, K.; Taniguchi, I.;
Yamaguchi, H.; Shiraishi, M. J. Electroanal. Chem. 1979, 105, 403−408.
59) Skarda, V.; Cook, M. J.; Lewis, A. P.; McAuliffe, G. S. G.;
Thomson, A. J. J. Chem. Soc., Perkin Trans. 2 1984, 8, 1309−1311.
60) Fees, J.; Kaim, W.; Moscherosch, M.; Matheis, W.; Klima, J.;
Krejcik, M.; Zalis, S. Inorg. Chem. 1993, 32, 166−174.
61) Flynn, C. P. Point Defects and Diffusion; Clarendon Press:
Oxford, 1972.
62) Costentin, C.; Canales, J. C.; Haddou, B.; Saveant, J. M. J. Am.
Chem. Soc. 2013, 135, 17671−17674.
63) Amatore, C.; Saveant, J.-M. J. Am. Chem. Soc. 1981, 103, 5021−
023.
64) (a) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706−723.
b) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals
and Applications, 2nd ed.; Wiley: New York, 2001; p 501.
65) de Tacconi, N. R.; Chitakunye, R.; MacDonnell, F. M.; Lezna, R.
O. J. Phys. Chem. A 2008, 112, 497−507.
66) Ruiz, G. T.; Juliarena, M. P.; Lezna, R. O.; Feliz, M. R.; Ferraudi,
G. J. Photochem. Photobiol., A 2006, 179, 289−297.
67) Juliarena, M. P.; Lezna, R. O.; Feliz, M. R.; Ruiz, G. T.; Thomas,
S.; Ferraudi, G.; Carmichael, I. J. Org. Chem. 2006, 71, 2870−2873.
68) Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J. Am. Chem.
Soc. 1982, 104, 4803−4810.
69) Majewski, M. B.; de Tacconi, N. R.; MacDonnell, F. M.; Wolf,
M. O. Chem.Eur. J. 2013, 19, 8331−8341.
70) Staffilani, M.; Belser, P.; Hartl, F.; Kleverlaan, C. J.; De Cola, L.
J. Phys. Chem. A 2002, 106, 9242−9250.
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