Electrocatalytic H2 Evolution by Supramolecular RuII-RhIII-RuII Complexes: Importance of Ligands as Electron Reservoirs and Speciation upon Reduction

Article abstract of DOI:10.1021/acs.inorgchem.5b01536

The supramolecular water reduction photocatalysts [{(Ph₂phen)₂Ru(dpp)}₂RhX₂](PF₆)₅ (Ph₂phen = 4,7-diphenyl-1,10-phenanthroline, dpp =2,3-bis(2-pyridyl)pyrazine X = Cl, Br) are efficient electrocatalysts for the reduction of CF₃SO₃H, CF₃CO₂H, and CH₃CO₂H to H₂ in DMF or DMF/H₂O mixtures. The onset of catalytic current occurs at -0.82 V versus Ag/AgCl for CF₃SO₃H, -0.90 V for CF₃CO₂H, and -1.1 V for CH₃CO₂H with overpotentials of 0.61, 0.45, and 0.10 V, respectively. In each case, catalysis is triggered by the first dpp ligand reduction implicating the dpp as an electron reservoir in catalysis. A new species with E_pc ～ -0.75 V was observed in the presence of stoichiometric amounts of strong acid, and its identity is proposed as the Rh(H)^III/II redox couple. H₂ was produced in 72-85% Faradaic yields and 95-116 turnovers after 2 h and 435 turnovers after 10 h of bulk electrolysis. The identities of Rh(I) species upon reduction have been studied. In contrast to the expected dissociation of halides in the Rh(I) state, the halide loss depends on solvent and water content. In dry CH₃CN, in which Cl^- is poorly solvated, a [Ru] complex dissociates and [(Ph₂phen)₂Ru(dpp)Rh^ICl₂]⁺ and [(Ph₂phen)₂Ru(dpp)]²⁺ are formed. In contrast, for X = Br^-, the major product of reduction is the intact trimetallic Rh(I) complex [{(Ph₂phen)₂Ru(dpp)}₂Rh^I]⁵⁺. Chloride loss in CH₃CN is facilitated by addition of 3 M H₂O. In DMF, the reduced species is [{(Ph₂phen)₂Ru(dpp)}₂Rh^I]⁵⁺ regardless of X = Cl^- or Br^-. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.inorgchem.5b01536

Article
pubs.acs.org/IC
II
III
II
Electrocatalytic H Evolution by Supramolecular Ru −Rh −Ru
2
Complexes: Importance of Ligands as Electron Reservoirs and
Speciation upon Reduction
Gerald F. Manbeck,* Theodore Canterbury, Rongwei Zhou, Skye King, Geewoo Nam,
,†
and Karen J. Brewer‡
Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States
*
S Supporting Information
ABSTRACT: The supramolecular water reduction photocatalysts
{(Ph phen) Ru(dpp)} RhX ](PF ) (Ph phen = 4,7-diphenyl-1,10-
[
2
2
2
2
6 5
2
phenanthroline, dpp =2,3-bis(2-pyridyl)pyrazine X = Cl, Br) are efficient
electrocatalysts for the reduction of CF SO H, CF CO H, and
3
3
3
2
CH CO H to H in DMF or DMF/H O mixtures. The onset of
3
2
2
2
catalytic current occurs at −0.82 V versus Ag/AgCl for CF SO H, −0.90
3
3
V for CF CO H, and −1.1 V for CH CO H with overpotentials of 0.61,
3
2
3
2
0
.45, and 0.10 V, respectively. In each case, catalysis is triggered by the
first dpp ligand reduction implicating the dpp as an electron reservoir in catalysis. A new species with E ∼ −0.75 V was
observed in the presence of stoichiometric amounts of strong acid, and its identity is proposed as the Rh(H)
pc
III/II
redox couple. H
2
was produced in 72−85% Faradaic yields and 95−116 turnovers after 2 h and 435 turnovers after 10 h of bulk electrolysis. The
identities of Rh(I) species upon reduction have been studied. In contrast to the expected dissociation of halides in the Rh(I)
−
state, the halide loss depends on solvent and water content. In dry CH CN, in which Cl is poorly solvated, a [Ru] complex
3
I
+
2+
−
dissociates and [(Ph phen) Ru(dpp)Rh Cl ] and [(Ph phen) Ru(dpp)] are formed. In contrast, for X = Br , the major
product of reduction is the intact trimetallic Rh(I) complex [{(Ph phen) Ru(dpp)} Rh ] . Chloride loss in CH CN is facilitated
by addition of 3 M H O. In DMF, the reduced species is [{(Ph phen) Ru(dpp)} Rh ] regardless of X = Cl or Br .
2
2
2
2
2
I 5+
2
2
2
3
I 5+
−
−
2
2
2
2
INTRODUCTION
Ru(dπ) → dpp(π*) MLCT (metal-to-ligand charge transfer)
■
excitation, reductive quenching of the triplet state, and
Artificial photosynthesis (AP) for the storage of solar energy in
the form of chemical bonds is a promising route to renewable
+
intramolecular ET to Rh. Similar to [Rh(bpy) Cl ] , sequential
2
2
1
−6
ET presumably triggers halide loss to generate vacant sites for
fuels such as H or reduced carbon dioxide.
A molecular
2
30
catalysis. Subsequent steps in the mechanism are poorly
approach to hydrogen production via AP is attractive since
molecules for light harvesting, charge separation, and catalysis
can first be optimized for each function;
application will require immobilization of each component and
integration into a device. The supramolecular approach, in
which components are covalently linked, is a step toward a
device since diffusion controlled bimolecular electron transfer
ET) reactions are eliminated. Ideally, the supramolecule
should facilitate accumulation of multiple charge carriers
allowing catalysis of multielectron reactions, i.e., the two-
electron reduction of water to hydrogen.
The search for effective hydrogen production chromophore-
understood although recent theoretical work on mononuclear
III
+
7
−12
[Rh (Me -bpy) Cl ] may provide some insight into the
2 2 2
however, practical
31
supramolecule behavior. Key questions include the potentials
of important hydride redox couples, the rate of ET for the
second electron, the influence of dissociated ligands in the
catalytic cycle, the reversibility of H release, and solvent effects
2
on speciation.
(
Hydrogen production catalysts are often evaluated under
electrocatalytic conditions since solar fuels can be produced by
32−58
13
electrolysis powered by photovoltaics.
Regarding mecha-
nistic insight, cyclic voltammetry (CV) enables catalyst
screening and scrutiny in the absence of complications
catalyst supramolecules has revealed dinuclear Ru-cobalox-
59
1
4,15
16
associated with photochemical studies. CV data yield rates,
overpotentials, and redox potentials of putative intermediates
unobservable in photocatalysis. Again, the issue of mass
transport has prompted the invention of electrode materials
imes;
self-assembled photosystem I cobaloxime hybrids;
17
tetrametallic Ru −Pt complexes; bimetallic varietals of Ru−
3
1
8
19
20
21
Pt, Ir−Co, Ru−Pd, Ru−Rh; zinc porphyrin-cobalox-
2
2
II
II
II
imes; and Ru −Rh −Ru complexes studied by the Brewer
60−65
2
3−29
modified by molecular catalysts.
This approach provides a
group and others.
dpp)} RhX ] (TL = terminal bidentate ligand, dpp = 2,3-
Trimetallic supramolecules [{(TL) Ru-
2
5
+
remarkable improvement in efficiency, overpotential, and
(
2 2
bis(2-pyridyl)pyrazine, and X = Cl, Br) are promising
photocatalysts operating through photoinitiated electron
collected at Rh via the two cycles of the following sequence:
Received: July 9, 2015
Published: August 6, 2015
©
2015 American Chemical Society
8148
DOI: 10.1021/acs.inorgchem.5b01536
Inorg. Chem. 2015, 54, 8148−8157

Inorganic Chemistry
Article
durability as shown by the 55 000 turnovers in water using
“dark reaction” consistent with previous reports of concen-
tration-dependent turnover numbers. Analysis of reduced
6
0
78
cobaloxime catalysts or 100 000 turnovers for Ni bis-
61,62
diphosphosphine catalysts
grafted on carbon nanotubes.
species reveals an unexpected solvent-dependent speciation in
which the reduced Rh(I) trimetallic remains intact only if halide
dissociation is facilitated by its solubility. If the halide is
Photocathodes modified with molecular catalysts provide a
65−67
method to drive reductive reactions with solar input.
An
2
+
emerging advanced design entails dye-sensitized photocathodes
insoluble in the reaction medium, a [Ru(Ph phen) (dpp)]
2
2
68−71
such as p-type NiO
in which charge injection occurs from
unit dissociates in contrast to halide loss.
the semiconductor valence band into the excited state of the
dye, thereby eliminating the sacrificial electron donors
commonly used for exploratory research.
RESULTS
■
II
III
II
III
II
Cyclic Voltammetry in DMF. The reductive CV of
The supramolecular Ru −Rh or Ru −Rh −Ru moieties
proven efficient for photocatalysis are ideally suited for surface-
confined photoelectrochemical hydrogen production, and the
5
+
[
{(Ph phen) Ru(dpp)} RhBr ] in DMF features overlapping
2 2 2 2
III/II
II/I
irreversible Rh
and Rh couples at −0.34 V versus Ag/
0/−
c
3
2+
AgCl (Figure 2, peak 1) and two dpp couples at E = −0.87
MLCT excited state of the [(TL) Ru(dpp)] chromophore is
p
2
V (quasireversible, peak 2) and E1/2 = −1.22 V (peak 3). Peak 4
a sufficiently strong oxidizing and reducing agent to split water.
To date, no electrocatalytic studies have been undertaken using
these motifs. While electrocatalytic hydrogen production is
known for several Rh catalysts such as [(C Me )Rh(bpy)-
−
/2−
at −1.47 V is assigned as a second reduction (dpp
) of one
bridging ligand based on the ca. 0.5 V difference from the first
79
5+
dpp reduction. CV data for [{(Ph phen) Ru(dpp)} RhCl ]
2
2
2
2
5
5
+
72−75
76
I
are similar with the exception of peak 1 appearing at −0.42 V.
Cl] ,
5
trans-RhCl(CO)(PPh ) , and Rh (TPP) (TPP =
3 2
77
CVs of 1Br with 0, 5, 10, and 25 equiv of CF CO H are
,10,15,20-tetraphenylporphyrin), it is anticipated that the
2
3
2
shown in Figure 2A. Addition of acid did not influence the
unique electronics of the dpp bridging ligand bound to Ru(II)
III/II/I
Rh
appeared at −0.75 V prior to the first dpp reduction. For
5 equiv of acid the current at −0.75 V remained unchanged,
and catalytic current was observed at −0.9 V. The catalytic
current peaked at −1.3 V for 10 equiv of CF CO H and at −1.6
wave or ligand-based couples; however, peak 5
may provide reactivity different from simple models such as
0
/−
+
+
[
Rh(bpy) Cl ] or [Rh(dpp) Cl ] .
2 2 2 2
≥
As an initial study prior to surface confinement, we sought to
verify homogeneous electrocatalytic proton reduction using
5
+
[
{(Ph phen) Ru(dpp)} RhX ] (X = Br (1Br ), Cl (1Cl ),
3
2
2
2
2
2
2
2
V for 25 equiv. CVs for the reduction of CF CO H using 1Cl
Figure 1) in DMF, and to gain understanding of the
3
2
2
(
Figure S3) are nearly identical to those with 1Br . The current
2
enhancement, expressed as the ratio of peak current (i ) to the
c
current of the dpp0 couple in the absence of acid (i ), is 4.1
/−
p
for 1Br and 3.4 for 1Cl complex in the presence of 10 equiv
2
2
of acid. The thermodynamic potential for CF CO H reduction
3
2
80
in DMF is −0.45 V versus Ag/AgCl. Therefore, the onset of
catalysis at −0.9 V for 1Br represents an overpotential of 0.45
2
V.
CVs in the presence of CF SO H (E ° = −0.21 V vs Ag/
3
3
HA
81
5+
Figure 1. Representation of [{Ph phen) Ru(dpp)} RhX ] (X = Cl
AgCl) were studied to examine the catalytic reduction of a
stronger acid (Figure 2B). At low acid concentrations, peak 5
appeared at −0.62 V, and catalysis was triggered at −0.82 V.
The current peaked at −1.3 V for 25 equiv of acid. Comparable
2
2
2
2
(
1Cl ), Br (1Br ); Ph phen = 4,7-diphenyl-1,10-phenanthroline; dpp
2 2 2
=
2,3-bis(2-pyridyl)pyrazine).
mechanism. Catalytic waveforms are strongly dependent on
CVs using 1Cl as the catalyst were obtained. For 10 equiv of
2
acid pK , and suggest a mechanism in which H is released by
CF SO H, i /i = 3.5 and 3.4 for 1Br and 1Cl , respectively.
a
2
3
3
c
p
2
2
protonation of a Rh(II) hydride with one reduced dpp ligand.
Importantly, this mechanism allows speculation regarding the
photochemical reaction since the dpp reduction is accessible by
reductive excited state quenching. Hydrogen release is then a
Catalysis beginning at −0.82 V represents a 0.61 V over-
potential for the reduction of CF SO H.
3
3
Weak acid behavior was examined using acetic acid (E ° =
HA
81
−1.0 V). As shown in Figure 2C, a catalytic wave appeared
5+
Figure 2. Cyclic voltammograms of 1.0 mM [{(Ph phen) Ru(dpp)} RhBr ] in DMF/0.1 M Bu NPF with 0, 5, 10, and 25 equiv of CF CO H
2
2
2
2
4
6
3
2
(
A), CF SO H (B), and CH CO H (C). In each plot, the CV in the absence of catalyst is shown for 22 mM acid, and the arrow indicates the onset
3 3 3 2
5+
+
of catalysis. Plot D compares CVs of 1.0 mM trimetallic [{(Ph phen) Ru(dpp)} RhCl ] (black) and [Rh(dpp) Cl ] (red) in the absence (solid
2
2
2
2
2
2
lines) and presence (dashed lines) of 10 equiv of CH CO H.
3
2
8
149
DOI: 10.1021/acs.inorgchem.5b01536
Inorg. Chem. 2015, 54, 8148−8157

Inorganic Chemistry
Article
0
/−
with an onset potential near −1.1 V. In this case, the dpp
couple was observed as a shoulder on the catalytic wave at
−
1.23 V. The catalytic current appeared to plateau at −1.5 V
for moderate acid concentrations. At 10 or more equiv of acid, a
third peak appeared at −1.9 V. For 25 equiv of acid, i /i = 4.5
c
p
for 1Br and 4.3 for 1Cl . At the onset of catalysis at −1.05 V
2
2
for acetic acid reduction, the overpotential is only 0.05 V.
Catalysis by [Rh(dpp) Cl ]PF . The merit of
2
2
6
II
III
II
Ru −Rh −Ru supramolecules as electrocatalysts lies in the
photophysical properties of the appended Ru(II) chromo-
phores and potential application in photoelectrocatalysis.
III
Nevertheless, electrocatalysis using some cis-Rh Cl mono-
2
metallic precatalysts was examined for comparative purposes.
+
No catalysis was observed for [Rh(bpy) Cl ] . Since the
2
2
electronic properties of dpp are somewhat different from bpy,
+
[
Rh(dpp) Cl ] was also studied. As shown in Figure 2D, 1Cl
2
2
2
exhibits greater activity for CH CO H reduction compared to
3
2
+
[
[
Rh(dpp) Cl ] (i /i = 3.3 and 2.1, respectively). Current for
Rh(dpp) Cl ] did not increase beyond 10 equiv of acid.
2 2 c p
+
2
2
Reduction of CF CO H is compared in Figure S4. For 10 equiv
3
2
of acid, and potentials less negative than −1 V, the
+
[
Rh(dpp) Cl ] exhibited slightly more current than 1Cl .
2
2
2
Beyond a −1 V bias, 1Cl holds clear advantage. While current
2
+
for [Rh(dpp) Cl ] continued to increase beyond 10 equiv in
2
2
this case, the increase is cathodic of −1 V, and the curve-cross
at this potential was maintained for higher acid concentrations.
Cyclic Voltammetry in DMF/H O. Catalytic proton
reduction experiments in pure organic solvents provide a
2
Figure 3. Cyclic voltammograms in DMF/5% H O (A, C, E) or
2
DMF/10% H O (B, D, F) of 1.0 mM [{(Ph phen) Ru-
2
2
2
5+
starting point for analysis; however, water is the ideal medium
(dpp)} RhBr ] . (A and B) 0, 5, and 10 equiv of CF SO H. (C
2
2
3
3
48,49,52,53,82−84
for H2 production.
Recently, efficient photo-
and D) 0, 5, 10, 15, and 25 equiv of CF
CO H. (E and F) 0, 5, 10, and
3 2
1
5 equiv of CH CO H. CVs in the absence of catalyst are shown for
catalysis has been reported in pure water using similar
3
2
8
5
22 mM acid (dashed red line).
multimetallic complexes. Catalysis by [{(Ph phen) Ru-
2
2
5+
(
dpp)} RhX ] was examined using 5% or 10% water in
2 2
DMF acidified with CF SO H, CF CO H, or CH CO H
3
3
3
2
3
2
Kinetics of Catalysis. For a reaction that is first order in
(Figure 4).
catalyst and second order in acid, the catalytic current, i , is
c
In DMF/5% H O, the reductive CV of 1Br was similar to
2
2
given by eq 1 where n = 2 electrons, F is Faraday’s constant, A
the CV in pure DMF. Upon addition of CF SO H, a catalytic
3
3
is the electrode area, D is the diffusion coefficient, and k is the
wave appeared at onset −1 V (Figure 3A). At the peak potential
86−88
observed rate constant.
Equation 1 assumes an ECcat
of −1.45 V, i /i = 5.3 for 10 equiv of acid. Without water, i /i
c
p
c
p
mechanism and applies to conditions in which catalysis is
pseudozeroth order in substrate; i.e., catalytic current exhibits a
plateau. As shown in Figures S1 and S2, plots of catalytic
current versus [acid] or [catalyst] are linear. The peak current,
=
3.5 at the peak potential of −1.3 V. At higher acid
concentrations, direct reduction at the electrode begins to
contribute to the current at −1.4 V. Experiments in DMF/10%
water indicated adsorption of the reduced complex to the
electrode at potentials cathodic of −1.2 V, and significant
background current was observed beyond −1.2 V. Without
i , for a reversible one-electron redox couple is given by eq 2 (ν
p
=
−1
scan rate in V s , R = the ideal gas constant, and T =
temperature). The ratio of i /i in eq 3 provides a convenient
c
p
0
/−
added acid, the dpp couples were obscured by the reduction
of water as the proton source; however, current was low
indicating a slow reaction (Figure 3B). With CF SO H, current
+
method for estimating third order [H ]-dependent rate
constants by plotting the slope of i /i versus [acid] as a
c
p
−1/2
89
3
3
function of ν
.
increased at −1 V, but the high water content decreases
catalytic activity compared to the 5% water solution. In the
mixed 5% water in DMF solution, CVs with CF CO H are
+
2
i = nFA[cat] Dk[H ]
(1)
(2)
c
3
2
similar to those obtained in pure DMF (Figure 3C). As shown
in Figure 3D, the catalyst is effective at similar potentials when
the water content is increased to 10%. A new irreversible wave
at −0.7 V was followed by peaks in current that shifted from
ip = 0.4463FA[cat] FνD/RT
n
RTk[H ]2
+
k[H ]
+ 2
ic
=
= 0.72
ip
0.4463
Fν
ν
(3)
−
0.9 V at 5 equiv of acid to −1.3 V at 10 equiv of acid.
Figure 3E shows CVs of 1Br in DMF/5% water with
Voltammograms for electrocatalytic acid reduction using
2
CH CO H. The CVs are similar to those without H O with the
1Br did not exhibit pseudozeroth order kinetics in acid (no
3
2
2
2
exception of higher i /i ratios at E . Without H O, for 15 equiv
plateau current) under most conditions. Thus, it was not
possible to calculate turnover frequencies at potentials of
highest activity using eq 3. For conditions in which catalytic
c
p
p
2
of CH CO H, i /i = 3.8 for 1Br and 3.5 for 1Cl . With 5%
3
2
c
p
2
2
H O, i /i increased to 6.1 and 6.4, respectively, indicating
2
c
p
+
enhanced catalysis in the presence of water.
current is independent of [H ], eq 3 simplifies to i /i = 0.72
c
p
8
150
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Inorganic Chemistry
Article
a
Table 1. Summary of CV and Bulk Reduction Data
b
c
d
e
f
acid
EHA
°
Ecat
η
faradaic yield, %
turnovers
95 ± 8
CF SO H
−0.21
−0.45
−1.0
−0.82
−0.90
−1.10
0.61
0.45
0.10
72 ± 8
3
3
g
g
CF CO H
3
81 ± 8 (100)
100 ± 10 (415)
116 ± 4
2
CH CO H
85 ± 3
3
2
a
CV experiments were performed using 1.0 mM 1Br or 1Cl in DMF with 0.1 M Bu NPF . Bulk electrolysis was performed in duplicate using 55
2
2
4
6
b
c
μM 1Br in DMF at an applied potential of −1.4 V vs Ag/AgCl. Standard potential for the reduction of acid in DMF vs Ag/AgCl. Onset of
2
d
e
−1
catalytic current. Overpotential in V. Determined using the equation Q = nFN where Q = charge, n = 2, F = 96485 C mol , and N = moles of H .
2
f
g
Moles H per mole catalyst after 2 h. 10 h experiment.
2
1
/2
(
k/v) , and the turnover frequency can be calculated from the
55
ratio of i /i at a given scan rate. This condition was met for
c
p
reduction of CH CO H in DMF/10% water in which the
3
2
−1
turnover frequencies are 4.1 and 3.0 s for 1Br and 1Cl ,
2
2
respectively. It is noted that these values do not reflect the high
activity, and are viewed as a lower limit in which the catalysis is
saturated in acid.
Bulk Electrolysis. Bulk electrolysis experiments were
performed using 1Br to confirm H production and Faradaic
2
2
yields and to access the long-term activity under continuous
electrolysis. For comparative experiments between acids, a
potential of −1.4 V was applied (Table 1). During 2 h
experiments, H was generated with Faradaic yields of 70−85%
2
Figure 5. Spectroelectrochemistry during bulk electrolysis of
5+
and turnovers of 80−116. A lower TOF and decrease in H
2
[{(Ph phen) Ru(dpp)} RhBr ] in the presence of CF CO H with
2 2 2 2 3 2
generation using CF SO H may reflect catalyst decomposition
an applied potential of −1.4 V vs Ag/AgCl.
3
3
during extended exposure to the strong acid.
Rates of H generation and coulometry were similar for
2
intraligand dpp(π → π*) transitions, and Ru(dπ) → Ph phen-
2
CF SO H, CF CO H, and CH CO H (Figure 4). Similar rates
3
3
3
2
3
2
(
π*), and Ru(dπ) → dpp(π*) MLCT transitions, respec-
25
tively. During electrolysis with CF CO H, absorptivity
3
2
decreased at 350 and 520 nm and increased from 420 to 480
nm. After acid consumption, two broad bands at 500 and 680
nm appeared. The final change was reversed by addition of acid,
and the process could be repeated multiple times. Reduction at
−
1.4 V is sufficiently negative to reduce the bridging ligands as
indicated by bleaching of the dpp(π → π*) and Ru(dπ) →
dpp(π*) transitions. The Ru(dπ) → Ph phen(π*), CT
absorptions remain unperturbed in the reduced state since
2
the Ph phen ligand is reduced at more negative potentials. After
2
acid consumption at −1.4 V, the molecule is highly reduced
−
90
with numerous Rh(I)-based (dpp )-based absorptions.
Figure 4. H2 generation and coulometry for bulk electrolysis
After reduction, the purple solution was purged with H for
0 min with no apparent spectroscopic changes. This
2
5+
experiments using 55 μM [{(Ph phen) Ru(dpp)} RhBr ] in DMF
2
2
2
2
3
with 50 mM acid. The applied potential was −1.4 V vs Ag/AgCl.
Experiments are the average of two trials.
demonstrates that H does not react with the reduced catalyst
2
to form a Rh(III) dihydride and implies accumulation of H2
alone should not decrease the activity of hydrogen evolution.
+
of H2 evolution among acid substrates are supported by
Resistance to H addition contrasts with [Rh(bpy) ] which
2
2
voltammetric data. At E , the ratio of i /i is comparable for
readily adds H to form the Rh(III) dihydride
2
91,92
p
c
p
III
+
CF SO H, CF CO H, and CH CO H. Control experiments
[Rh (bpy) (H) ] .
This experiment does not disprove a
3
3
3
2
3
2
2
2
generate H at −1.4 V with low charge efficiency, and activity
Rh(III) dihydride as the active H production species, but it
verifies that the equilibrium reaction between such a dihydride
2
2
ceases after a short time. Catalyst stability was studied during a
1
0 h experiment after which 16.7 mL of H was produced in
and H2 strongly favors H . The spectroscopic changes
2
2
quantitative charge yield and 415 catalytic turnovers.
Spectroelectrochemistry During Catalysis. The Ru −
associated with acid consumption and catalyst regeneration
by acid addition provide evidence for homogeneous catalysis
and stability of the catalyst in a variety of oxidation states.
Homogeneity and Catalyst Stability. Determining the
homogeneous or heterogeneous nature of the catalyst is
II
III
II
Rh −Ru supramolecules are strong visible light absorbers,
and color changes accompanying oxidation state changes are
clearly visible. During bulk electrolysis experiments, a color
change from yellow to purple was observed after acid
consumption, coincident with a plateau in H2 evolution.
Catalysis resumed upon addition of acid, and the yellow color
reappeared. This process was monitored by spectroelectro-
93
important for mechanistic considerations. Formation of
nanoparticle catalysts from soluble precursors should be
considered especially under the reducing or oxidizing
94−99
conditions of artificial photosynthetic reactions.
In the
chemistry (Figure 5). The absorption spectrum of 1Br exhibits
transitions at 350, 420, and 515 nm which are assigned as
electrolysis experiments, a small deviation from linear kinetics
toward sigmoidal behavior might hint at heterogeneous
2
8
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Article
catalysis; however, the apparent lag period could also result
from slow detection of H2 in the headspace. Control
experiments without catalyst using new carbon cloth electrodes
exhibit similar curvature in H detection. The spectroelec-
2
trochemistry experiments and the reversible spectral changes
associated with catalysis, substrate consumption, and substrate
addition are more consistent with homogeneous behavior. To
probe this question further, several basic tests were performed.
CVs were recorded in pure DMF followed by two consecutive
CVs with 15 equiv of acid. A third CV was recorded after
carefully polishing the electrode (Figure S5). For CH CO H
3
2
and CF CO H, all CVs in the acidic solution were identical
3
2
regardless of polishing supporting the molecular and not
precipitated identity of the catalyst. In the presence of
CF SO H, slight deviation was observed on the second scan
without polishing the electrode. To test for acid-induced
absorption or decomposition, a CV was recorded in pure DMF
with 1 mM catalyst, and the electrode was transferred to
5
+
Figure 6. Cyclic voltammograms of [{(Ph phen) Ru(dpp)} RhBr ]
2
2
2
2
3
3
in CH CN prior to reduction (a) and after electrolysis at −0.55 V (b).
3
1
.03 V were followed by two reversible couples at E1/ 2− = 1.54
catalyst-free DMF/CF CO H. Current was slightly greater than
3
2
and 1.38 V. The irreversible waves are oxidation of Br to Br
2
background current supporting minor adsorption of the
catalyst. To confirm adsorption of the supramolecular catalyst
confirmed by literature data and by a CV with added
Bu NBr. The reversible couples at 1.54 and 1.38 V suggest
103
4
III/II
(
as opposed to electrodeposition) a polished electrode was
Ru
processes: the wave at 1.54 V remained unchanged from
III/II
soaked in DMF/catalyst for 5 min after which a CV was
recorded in pure solvent. Current similar to that of the previous
experiment confirms the adsorbed nature of the molecular
catalyst to the electrode. Bulk electrolysis experiments
employed carbon cloth working electrodes. Identical results
were obtained for sequential catalytic experiments with the
same electrode and fresh solutions. Control experiments with a
previously used or fresh electrode showed the same low activity
1Br while the couple at 1.38 V is identical to the Ru
couple
2
2+
of [(Ph phen) Ru(dpp)] in CH CN (the couple at −1 V is
also identical to the first reduction of this species). The
presence of [(Ph phen) Ru(dpp)] and Br after reduction in
dry CH CN is evidence for a mixture of electrogenerated Rh(I)
species in dry CH CN: the expected [{(Ph phen) Ru-
dpp)} Rh ] and [(Ph phen) Ru(dpp)Rh Br ] in contrast
to the reported and widely assumed halide loss for [Rh (α-
2
2
3
25
2+
−
2
2
3
3
2
2
I 5+
I
+
(
2 2 2 2
I
+
23−27,30,102
for H production. These experiments support the notion of
2
diimine) ] complexes.
2
molecular catalysis with minor adsorption of these supra-
molecules being possible under selective conditions. Electro-
deposition is disfavored, but formation of dissolved nano-
particles is not ruled out.
Electrochemical reduction and CV analysis was repeated for
1Cl (Figure 7). In dry CH CN, redox processes (trace a) are
2
3
−
similar to 1Br . After 2e reduction at −0.55 V, a limited
amount of Cl was detected in solution as indicated by the
small 2Cl /Cl couple at 1.04 V (b). Two reversible oxidations
2
−
−
Analysis of Reduced Species. Knowledge of active
species and intermediates is necessary to formulate a working
mechanism for the catalytic reaction. The [{(Ph phen) Ru-
2
were observed at 1.54 and 1.38 V and an irreversible reduction
appeared at −0.63 followed by a reversible couple at −1.0 V.
The experiment was repeated with 1 M H O in CH CN (traces
2
2
5+
(
dpp)} RhX ] supramolecules are inherently difficult to
2 2
2
3
1
−
analyze by NMR due to overlapping H resonances and
mixtures of coordination isomers. Likewise, absorption spectra
characterized by broad transitions provide no definitive
identification of species. In contrast, the unique redox units
c and d), and little Cl was observed after reduction. Further
electrolysis at −0.85 V, consumed 1e and the product CV (e)
showed appreciable current for Cl oxidation. With 3 M H O
−
−
2
−
−
in CH CN (f and g) Cl was detected after 2e reduction.
3
I
provide an opportunity to analyze Rh speciation since the
Bulk reduction and CV analysis in CH CN reveal varied
3
−
ligand and metal redox potentials are sensitive to coordination
product distribution with dependence on halide. For 1Br , Br
2
1
00,101
environment but insensitive to geometric isomerization.
was detected upon reduction in dry CH CN, whereas 3 M H O
3
2
III
+
−
The reductive electrochemistry of [cis-Rh (α-diimine) X ]
was necessary to observe Cl dissociation after reduction of
1Cl . Loss of X implies formation of the intact supramolecule
[{(Ph phen) Ru(dpp)} Rh ] . When Cl does not dissociate
after reduction, [(Ph phen) Ru(dpp)Rh Cl ] and
2 2 2
2
2
−
complexes is known to proceed via an ECEC mechanism to
2
I
+
30,102
I 5+
−
provide [Rh (α-diimine) ] .
Previous work in the Brewer
2
2 2 2
I
+
group has assumed identical behavior in supramolecular
III
2+
complexes with a cis-Rh X catalytic center. In this section
[(Ph phen) Ru(dpp)] are formed selectively. The appearance
2
2 2
II
I
−
we present electrochemical evidence for the intact Ru −Rh −
Ru supramolecule and discuss the effect of solvent and water
of Cl after electrolysis at −0.85 V allows assignment of the
0/−
II
irreversible couple at −0.63 V as the dpp
couple in the
II
I
on product distribution: namely, preferential dissociation of
Ru −Rh species (Figure 7d), which triggers dissociative
decomposition upon reduction. Scheme 1 summarizes the
product dependence on solvent and anion.
2
+
[
(Ph phen) Ru(dpp)] when the bound halide is poorly
2 2
soluble.
−
−
The CV of 1Br in freshly distilled CH CN is shown in
The simplest interpretation for Br but not Cl dissociation
upon 2e reduction in dry CH CN is poor solvation of the
2
3
−
Figure 6 before (trace a) and after electrolysis at −0.55 V (trace
b). There were 2 equiv of charge consumed during the
electrolysis, consistent with reduction from Rh(III) to Rh(I).
After electrolysis, the reductive CV showed an irreversible wave
at −0.63 V and a reversible couple at −1 V. Oxidative processes
were more complicated. Two irreversible waves at 0.74 and
3
chloride ion in acetonitrile. The solubilities of KCl and KBr in
−
5
−3
CH CN were reported as 8 × 10 and 1.6 × 10 molal,
3
respectively, and small amounts of water greatly increased
The two-electron reduction of 1Cl triggers
dissociation of one [(Ph phen) Ru(dpp)] moiety which is
104
solubility.
2
2+
2
2
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Inorganic Chemistry
Article
5
+
Figure 8. Cyclic voltammograms of [{(Ph phen) Ru(dpp)} RhCl ]
2
2
2
2
in DMF (a) and after controlled potential electrolysis at −0.55 V
scanning sweeping through +1.3 V (b) or +1.8 V (c and d). Arrows
indicate direction of initial scan.
5
+
Figure 7. Cyclic voltammograms of [{(Ph phen) Ru(dpp)} RhCl ]
2
2
2
2
−
in CH CN (a), after 2e reduction at −0.55 V (b), in 1 M water/
3
−
−
CH CN (c) after 2e reduction in 1 M water at −0.55 V (d) or 3e
3
reduction at −0.85 V (e). In traces f and g, the experiment was
−
repeated with 3 M water and 2e reduction at −0.55 V.
Information Figure S6). The RuIII/II couple appeared near 1.5
V.
Observation of Cl− or Br− in solution after reduction of
−
highly soluble in CH CN, and two Cl ligands are retained by
5+
3
[
{(Ph phen) Ru(dpp)} RhX ] in DMF demonstrates facile
2
2
2
2
Rh(I) unless water is added to solvate the Cl−.
halide dissociation from Rh(I). The solubilities of KCl and KBr
Reduction and product analysis were repeated in DMF with
significantly different outcomes compared to those in CH CN.
Figure 8 shows the CV of 1Cl (a) and after reduction (b). The
−3
−2
in DMF are reported as 2.44 × 10 and 7.6 × 10 molal,
3
104
respectively. The major Rh(I) species should be
[{(Ph phen) Ru(dpp)} Rh ] . The presence of this species
2
2
I 5+
III/II/I
−
2
2
absence of the Rh
wave confirmed 2e reduction, and
oxidation of Cl− was observed at +1 V demonstrating
and its stability are supported by irreversible redox waves at
−
−
−
−0.34 and −0.51 V after anodic sweeps of the Cl and Br
labilization of the Cl in DMF. A CV obtained by sweeping
containing solutions. These potentials are comparable to the
the same solution through +1.8 V (c) provided a new
irreversible reduction at Epc = −0.34 V. This wave was not
apparent when scanning first reductively (d) or sweeping only
through +1.3 V (b). 1Br displayed identical behavior with the
exception that oxidation through 1.3 V was sufficient to
generate a new reduction wave at −0.51 V (Supporting
III/II/I
Rh
reduction of the starting materials and are assigned as
n+
Rh-based reductions of [{(Ph phen) Ru(dpp)} RhL ] com-
2
2
2
2
−
plexes where L = X and/or solvent. Different potentials
depending on the presence of Cl− or Br− support
recoordination of at least one anion when Rh(I) is reoxidized.
2
5
+
Scheme 1. Solvent-Dependent Speciation upon Two-Electron Reduction of [{(Ph phen) Ru(dpp)} RhX ] Complexes
2
2
2
2
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Inorganic Chemistry
Article
In Figure 2, there is no catalysis at the RhIII/II/I couple.
DISCUSSION
■
Therefore, reduction of putative Rh(III) hydrides must occur
Results in this work demonstrate the applicability of supra-
molecular complexes [{(Ph phen) Ru(dpp)} RhX ] as elec-
trocatalysts for proton reduction. Although catalysis using the
III
5
+
cathodic of the cis-Rh X potential. Figure 2A,B shows new
2
2
2
2
2
reduction couples (peak 5) at −0.75 and −0.62 V. Peak 5 does
not trigger catalysis, and its nature is proposed as reduction of
complicated supramolecule instead of the simpler
III
II
+
Rh (H) to Rh (H), eq vi. Consistent with [Rh(Me bpy) (H)-
2 2
[
RhX (dpp) ] core may appear counterintuitive, we propose
2
2
+
Cl] , this couple appears ca. 400 mV more negative than the cis-
surface-confined photoelectrocatalysis as a future research
direction in which the applied potential can be alleviated if
solar energy is directly utilized by the appended Ru(II)
chromophore. Furthermore, the exact potentials of redox
couples of interest are different when the dpp ligands bridge Rh
centers. This is shown in Figure 2D in which reduction of
III
71
III/II
Rh X potential. The Rh(H)
couple was observed for
2
77
[
[
Rh(H)(TPP)] at −1.3 V versus Ag/AgCl, at −1.33 V for
Rh(Me C )(L)(H)] (L = substituted bipyridine) in water,
and at −1.34 V for [Rh(Me C )(bpy)(H)] in CH CN. The
+
108
5
5
+
109
5
5
3
anodic shift for this reduction in the present work is attributed
to the weak σ-donor and strong π-acceptor properties of the
dpp bridging ligands which render Rh electron deficient and
easily reduced. Slight differences in the potential for eq vi in the
presence of CF CO H or CF SO H support conjugate base
Rh(III) in the trimetallics is more facile than that for
+
[
RhCl (dpp) ] .
2
2
Kinetic analysis of the CVs is difficult owing to multiple
underlying redox couples of [{(Ph phen) Ru(dpp)} RhX ]
5+
3
2
3
3
2
2
2
2
III
coordination to the Rh (H) species; however, this notation is
omitted for consistency in the equations.
within the electrocatalytic response. Analysis according to an
ECcat mechanism is an oversimplification since each successive
reduction leads to a new species which may increase (or
decrease) the rate of catalysis. Under most conditions, the CVs
do not exhibit the canonical S-shaped plateau response typical
of systems in which an ECcat simplification is appropriate. The
absence of plateau current may result from substrate
consumption; however, the catalytic response may be perturbed
by other phenomena including catalyst decomposition or
inhibition by product. The foot-of-the-wave analysis avoids
complications in the low potential region by considering only
III
6+
−
II
5+
[
(dpp)Rh (H)(dpp)] + e → [(dpp)Rh (H)(dpp)]
(vi)
II
(H)]+,
In stark contrast to the protonolysis of [Rh (Me
bpy)
2
2
which reacts with formic acid, or the behavior of related group
3
1,71,110−114
9 cobaloxime catalysts,
here catalysis is not observed
couple. No catalysis at the
III/II
at the proposed Rh(H)
III/II
Rh(H)
couple shows slow or negligible protonolysis of
the Rh (H) (eq vii) and is likely attributed to the anodic shift
and therefore weaker hydricity) of this couple relative to
monometallic Rh complexes.
II
105
(
the initial CV catalytic response.
For [{(Ph phen) Ru-
2 2
5+
(
dpp)} RhX ] , this method is not appropriate because the
2 2
specific redox couple which triggers catalysis is unknown,
further reductions generate new species, and a large deviation
from ideal behavior is observed.
(dpp)Rh (H)(dpp)] + H+
→
II
5+
[
II
6+
[(dpp)Rh (dpp)] + H2
(vii)
Despite the complications of kinetic analysis, experiments
with acids of varied strength provide insight into important
redox couples and plausible intermediates. Catalyst activation
An electrocatalytic wave is triggered near the first dpp0
couple in the presence of CF CO H or CF SO
CF CO H, the slopes of the voltammogram increase at the
/−
H. Using
3
3
2
3
−
III
proceeds through 2e reduction of the cis-Rh X precatalysts
3
2
2
30
0/−
via an ECEC mechanism in DMF eqs i−iv. This reaction was
verified by bulk electrolysis and CV analysis. The
(Ph phen) Ru] units are omitted from the following
2 2
second dpp couple. On the basis of this observation, eqs viii
and ix are formulated in which the Rh(II) hydride becomes
increasingly hydridic and prone to protonolysis as the adjacent
ligands are reduced. The product of eq ix is written as
Rh(I),dpp(0) since the protonolysis reaction must not
necessarily happen within the diffusion layer.
2
+
[
equations for clarity.
III
5+
4+
−
II
4+
[
[
[
[
(dpp)Rh Br (dpp)] + e → [(dpp)Rh Br (dpp)]
2
2
(
i)
II
5+
−
II
−
4+
[
(dpp)Rh (H)(dpp)] + e → [(dpp)Rh (H)(dpp )]
(dpp)Rh Br (dpp)] → [(dpp)Rh Br(dpp)] + Br−
II
II
5+
2
(viii)
(ii)
[
(dpp)Rh (H)(dpp )] + H+
II
−
4+
II
5+
−
I
4+
(dpp)Rh Br(dpp)] + e → [(dpp)RhBr(dpp)]
I
5+
→
[(dpp)Rh(dpp)] + H2
(ix)
(
iii)
The preceding discussion omitted analysis of CH CO H
since reduction of this weaker acid was not observed until
−1.05 V. On the basis of the range of possible pK values for
(dpp)RhBr(dpp)] → [(dpp)Rh(dpp)] + Br−
I
4+
I
5+
3
2
(iv)
I
5+
The [(dpp)Rh (dpp)] should be strongly basic considering
a
Rh(III) hydrides, the Rh(I) is unlikely to be protonated by
acetic acid until one or more dpp ligands are reduced, thereby
generating a more basic Rh(I) center. At the more negative
potentials, eqs viii and ix apply.
the pK values of related Rh(III) hydrides in H O, i.e.,
a
2
III
2+
III
2+
[
(
Rh (bpy) (H)(H O)] (9.5), [Rh (NH ) (H)(H O)]
2 2 4 2 2
106
III
>14), and [Rh (dmgH) (H) (PPh )].
Protonation by
2
3
CF CO H and CF SO H (eq v) provides the Rh(III)−H
3
2
3
3
The proximity of electrocatalysis to the dpp0 couple(s) has
an interesting mechanistic implication; namely, reduction of
dpp is possible through reductive quenching of the Ru(II)
excited state. Photocatalytic experiments have shown catalysis is
/−
81,107
whereas the reaction with acetic acid (pK ∼ 13.2−13.5)
a
perhaps favors the reverse equilibrium. The Rh(III)−H is most
−
likely octahedral with solvent or A as the sixth ligand.
I
5+
3
[
(dpp)Rh(dpp)] + HA
⇄
initiated by reductive quenching of the MLCT state followed
by thermal electron transfer from dpp− to Rh X .
III
115
The
[(dpp)Rh (H)(dpp)] + A−
III
6+
2
(v)
present data reveals that ligand-based reductions increase the
8
154
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Inorganic Chemistry
Article
basicity of Rh(I) (i.e., the reaction of Rh(I) with CH CO H)
potentiostat. The working electrode was a glassy carbon disk, and the
counter electrode was a Pt wire. The reference electrode was a Ag wire
in a glass tube equipped with a porous Vycor tip and filled with 3 M
KCl (aq). This electrode was calibrated against the Fc+ couple in
DMF at −0.56 V. Bu NPF (0.1 M) was used as the supporting
3
2
and increase the rate or proton attack on putative Rh(II)
hydride intermediates. Although photocatalytic experiments
were performed in basic conditions (CH CN or DMF, H O,
/0
3
2
4
6
N,N-dimethylaniline pH ∼ 9), protonation of the Rh(I) species
electrolyte. Samples were purged with Ar prior to scans and blanketed
with Ar during CVs. Aliquots of acid (0.1 M) were added to 1 mL
portions of solutions of 1.0 mM [{(Ph phen) Ru(dpp)} RhX ]-
III
is feasible considering that the pK for [Rh (bpy) (H)-
a
2
2+
106
(
H O)] is 9.5 in water.
2
2
2
2
2
A consistent trend in photocatalysis has been higher
[(PF ) in DMF or DMF/water mixtures. The working electrode was
6 5
efficiency for Rh(III) dibromide precursors compared to
polished between successive CVs. To determine the reaction order
with respect to catalyst, a 1 mL solution of 24 mM CF CO H and 100
mM TBAPF6 was titrated with a stock solution of 20 mM
{(Ph phen) Ru(dpp)} RhBr ](PF ) containing 24 mM CF CO H
Rh(III) dichloride complexes. Furthermore, catalysis is more
3
2
2
5,78
efficient in DMF versus acetonitrile.
A number of
[
explanations have been proposed including inhibition through
2
2
2
2
6
5
3
2
and 100 mM electrolyte.
coordination of CH CN to Rh, and a greater driving force for
3
Bulk reduction experiments were performed using a custom-built
airtight two-compartment cell equipped with a threaded joint for
attachment of a HY-OPTIMA 700 in-line process solid state hydrogen
sensor from H scan for real-time monitoring of of H produced. A Pt
Rh(I) formation in the cis-RhBr complexes due to a slightly
2
III/II/I
more anodic Rh
couple. The latter hypothesis should only
apply if halide remains active in the catalytic cycle. In the
2
2
present work, we have analyzed the Rh(I) species formed upon
mesh auxiliary electrode was separated from the catalyst solution by a
medium porosity frit. The working electrode was a carbon cloth
attached to a copper wire using an alligator clip. For all experiments,
both chambers were deoxygenated with argon at equal pressures.
Appropriate solutions were deoxygenated separately by purging with
Ar, and then 28 mL was injected simultaneously into each chamber via
syringe. During the injection, pressure was vented through a bubbler.
The HY-OPTIMA sensor was calibrated by injecting known amounts
−
II
I
2
e reduction in DMF and CH CN. Data suggest a Ru −Rh −
3
II
Ru trimetallic species is exclusively formed in DMF, whereas
reduction in acetonitrile generates a mixture of the trimetallic
2
+
and [(Ph phen)Ru(dpp)] . The 1Cl yields a reduced
2
2
trimetallic in CH CN only with >3 M H O suggesting the
3
2
solubility of the anion controls the concentrations of the active
catalyst. These studies present a cautionary warning for analysis
of supramolecular devices for AP. Properties expected of model
compounds may not hold true for complicated photo- and
electroactive supramolecular structures of large size and high
charge.
of H gas into the working electrode chamber containing 28 mL of
2
solution. Under these conditions, transfer of solution through the frit
to the auxiliary chamber did not occur until the hydrogen content in
the headspace reached ∼10−12% or ca. 3 mL. For a 10 h experiment,
the working compartment was purged with Ar after accumulating 10−
1
2% hydrogen in the headspace.
CONCLUSIONS
■
(
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01536.
The supramolecular complexes [{(Ph phen) Ru-
■
2
2
5+
−
−
S
dpp)} RhX ] with X = Cl and Br function as electro-
2
2
catalyts for the production of H from acids in DMF and DMF/
2
water mixtures. This provides impetus for the design of
supramolecular photoelectrocatalysts which combine light and
electrochemical energy to reduce the overpotential for
Plots of catalytic current versus [acid] and [catalyst];
CVs for electrocatalytic reduction of CF CO H,
production of H from water. Surface confinement may also
2
3
2
influence redox couples in these systems. Voltammetry
CF SO H, and CH CO H using [{(Ph phen) Ru-
3
3
3
2
2
2
experiments with varied acids suggest protonation of a ligand-
(
(
dpp)} RhCl ](PF ) ; CVs comparing [{(Ph phen) Ru-
2 2 6 5 2 2
II
−
4+
reduced [(dpp)Rh (H)(dpp )] species as a key step in
catalysis. In the presence of weak acids, the Rh(I) is not
+
dpp)} RhCl ](PF ) and [RhCl (dpp) ] ; CVs showing
2
2
6 5
2
2
effect of polishing the electrode on catalytic response,
protonated until after a ligand-based reduction. The important
and 2e− reduction of [{(Ph phen) Ru(dpp)} RhBr ]-
2
2
2
2
III/II
Rh(H)
couple is estimated as −0.75 V; therefore, the Rh(II)
(
PF ) in DMF (PDF)
6 5
hydride can be generated photochemically through reductive
quenching of the Ru(II)* excited state followed by ground state
electron transfer. Analysis of reduction products provided
insight into the influence of solvent on product distribution.
AUTHOR INFORMATION
Corresponding Author
*E-mail: gmanbeck@bnl.gov.
■
−
The solubility of the X is an important consideration for
generating the intact reduced photocatalyst. Studies regarding
surface immobilization and catalysis in pure water are now
underway.
Present Address
Department of Chemistry, Brookhaven National Laboratory,
Upton, NY 11973.
†
Author Contributions
EXPERIMENTAL SECTION
■
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
The [{(Ph phen) Ru(dpp)} RhX ](PF ) catalysts were prepared by a
2
2
2
2
6 5
published procedure and purified by chromatography on Sephadex
25
LH-20. RuCl ·nH O, RhBr ·nH O, and Ph phen were purchased
3
2
3
2
2
from Alfa Aesar. DMF was high purity Burdick & Jackson grade.
Bu NBr was purchased from Acros Organics. CF SO H, CF CO H,
and dpp were purchased from Aldrich. Glacial acetic acid was
Notes
4
3
3
3
2
The authors declare no competing financial interest.
‡
Deceased October 24, 2014.
purchased from Crescent Chemical Co. Bu NPF was prepared by the
4
6
metathesis reaction of Bu NBr and HPF , crystallized thrice from
ethanol, and dried at 100 °C.
Cyclic voltammograms were recorded using a standard three-
electrode cell controlled by a Bioanalytical System (BAS) Epsilon
4
6
ACKNOWLEDGMENTS
■
This material is based upon work supported by the Department
of Energy under award number DE-FG02-05ER15751. The
8
155
DOI: 10.1021/acs.inorgchem.5b01536
Inorg. Chem. 2015, 54, 8148−8157

Inorganic Chemistry
Article
authors would like to thank Dr. Etsuko Fujita for helpful
comments during the preparation of this manuscript.
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(
31) Kayanuma, M.; Stoll, T.; Daniel, C.; Odobel, F.; Fortage, J.;
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