Highly Active Ruthenium CNC Pincer Photocatalysts for Visible-Light-Driven Carbon Dioxide Reduction

Article abstract of DOI:10.1021/acs.inorgchem.9b00791

Five ruthenium catalysts described herein facilitate self-sensitized carbon dioxide reduction to form carbon monoxide with a ruthenium catalytic center. These catalysts include four new and one previously reported CNC pincer complexes featuring a pyridino

Full text of DOI:10.1021/acs.inorgchem.9b00791

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Highly Active Ruthenium CNC Pincer Photocatalysts for Visible-
Light-Driven Carbon Dioxide Reduction
Sanjit Das,†,# Roberta R. Rodrigues,‡,# Robert W. Lamb,§ Fengrui Qu,† Eric Reinheimer,∥
Chance M. Boudreaux,† Charles Edwin Webster,*,§ Jared H. Delcamp,*,‡
and Elizabeth T. Papish*,†
†Department of Chemistry and Biochemistry, University of Alabama, Shelby Hall, Tuscaloosa, Alabama 35487, United States
‡Department of Chemistry and Biochemistry, University of Mississippi, Coulter Hall, Oxford, Mississippi 38677, United States
§Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762, United States
∥Rigaku Oxford Diffraction, 9009 New Trails Drive, The Woodlands, Texas 77381, United States
S
* Supporting Information
ABSTRACT: Five ruthenium catalysts described herein facilitate self-
sensitized carbon dioxide reduction to form carbon monoxide with a
ruthenium catalytic center. These catalysts include four new and one
previously reported CNC pincer complexes featuring a pyridinol derived N-
donor and N-heterocyclic carbene (NHC) C-donors derived from imidazole
or benzimidazole. The complexes have been characterized fully by
spectroscopic and analytic methods, including X-ray crystallography.
Introduction of a 2,2′-bipyridine (bipy) coligand and phenyl groups on the
NHC ligand was necessary for rapid catalysis. [(CNC)Ru(bipy)(CH3CN)]-
(OTf)2 is among the most active and durable photocatalysts in the literature
for CO2 reduction without an external photosensitizer. The role of the structure of this complex in catalysis is discussed,
including the importance of the pincer’s phenyl wingtips, the bipyridyl ligand, and a weakly coordinating monodentate ligand.
Chart 1. Self-Sensitized (1−3) and Prior-Photosensitized (4
INTRODUCTION
■
a
and 5) Complexes Tested for Light-Driven CO2 Reduction
The efficient and selective photocatalytic conversion of carbon
dioxide to a usable fuel remains a grand challenge.1−4 The
addition of an oxygen-bearing group to CNC pincer ligands
(Chart 1) has been transformative for both ruthenium- and
nickel-catalyzed carbon dioxide reduction.5,6 We previously
reported ruthenium(II) (4)5,7 and nickel(II) (5)6 complexes
that catalyze CO2 reduction in the presence of a photo-
sensitizer (PS, e.g., Ir(ppy)3 where ppy = 2-phenylpyridine)
and sacrificial donors (SDs: triethylamine (TEA) and 1,3-
dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH)).
Catalyst (4) selectively generates CO for 40 h (turnover
number (TON) = 250), whereas the unsubstituted analog,
with H in place of the methoxy group, is inactive.5 The oxygen-
bearing groups in both 4 and 5 lower the redox potentials such
that the thermodynamics are favorable for both electron
transfer from the PS to the catalyst and from the catalyst to
CO2 at the first reduction potential.5,6 Thus, visible light is able
to provide the driving force for these reactions. However, one
drawback is that a precious metal photosensitizer (PS, e.g.
Ir(ppy)3) is needed for light-harvesting. In this work, we
demonstrate that with a change to the ligand scaffold CO2
reduction without an additional PS is feasible and can produce
quantities of CO comparable to the sensitized reactions. Such
self-sensitized visible-light-driven CO2 reduction reactions
using mononuclear catalysts have been rare in the literature
with limited ligand scaffolds featuring Ir, Re, Ru, and Fe
a
The structure of BIH is also shown.
Received: March 19, 2019
© XXXX American Chemical Society
A
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a
Scheme 1. Synthesis of 2A and Conversion to 2B
a
Bis benzimidazolium salt 1d is used to synthesize complex 2A, Ru-[{BIm(Me)-py(4-OMe)-BIm(Me)}(bipy)Cl]triflate which is then converted to
2B, Ru-[{BIm(Me)-py(4-OMe)-BIm(Me)}(bipy)(CH3CN)]ditriflate. The reagents used are as follows: (a) Ru(bipy)Cl4, ethylene glycol,
saturated aqueous NH4Cl solution; (b) zinc granules, ethanol; (c) silver trifluoromethanesulfonate, acetonitrile.
catalytic centers.8−16 We do note that numerous examples of
exciting work in the area of visible-light-photosensitized CO2
reduction catalysis exist with several review articles avail-
able;2−4,17−21 however, this manuscript is written with a focus
on self-sensitized reactions using visible light and mononuclear
catalysts. The work herein provides an unusual example of a
ruthenium-based photocatalyst system with exceptionally high
performance for CO2 reduction.
Our prior report on photosensitized catalysis with 4 showed
only trace reactivity in the absence of a photosensitizer during
the time period monitored.5 We hypothesized that an
expanded ligand π-system with benzimidazole (in 3; cf.
imidazole in 4) derived NHC rings could increase the intensity
of charge transfer bands, shift light absorption to lower energy,
and lead to a photocatalyst. Altering the wingtip substituents of
3 leads to 1 with phenyl groups, which provides a method for
increasing steric bulk to limit catalyst−catalyst interactions.
Complexes 2A and 2B add a 2,2′-bipyridine (bipy) ligand22−25
to the coordination sphere in place of monodentate ligands in
1. The bidendate bipy ligand is predicted to increase catalyst
stability and accept electron density during a metal-to-ligand
charge transfer (MLCT) event more so than the pincer
ligand.9 Given the stronger electron-accepting ability of
pyridine versus that of NHC ligands, a redshift is expected
to enable broader visible light use. Complexes 2A and 2B differ
by either the incorporation of a Cl− or a MeCN ligand,
respectively. Given the lability of MeCN ligands, 2B is
predicted to be a faster catalyst due to the easy opening of a
free catalytic site.
manner similar to that described previously for complex 4
(Scheme S3).5
Crystal Structures. The crystal structures for complexes 1,
2A, 2B, and 3 are shown in Figure 1. The bond distances and
angles are similar to that observed for complex 4 and for other
related CNC pincer complexes of ruthenium (Tables S1 and
S2, Figure S1). The benzimidazole-derived NHC rings in 1−3
feature slightly shorter Ru−C distances versus those of the
imidazole-derived NHC in 4 (average = 2.039(6) Å in 1−3
versus 2.062(3) Å in 4, respectively). This is likely due to
benzimidazole having an electron-withdrawing effect (cf.
imidazole) that enhances the Ru−C π back-bonding.30 The
Ru−N (acetonitrile) distance in 2B (2.094(6) Å) is elongated
relative to that in 1 (average = 2.032(8) Å) and suggests a
more labile acetonitrile in 2B. Notably, the C−O distance of 1,
2A, 2B, and 3 is contracted relative to a C−O single bond
(average = 1.346(9) Å versus ∼1.42 Å, respectively), indicating
that the methoxy group acts as a π-donor. Furthermore, the
pyridine ring is partially dearomatized with long and short C−
C distances with the greatest differences in C−C bond lengths
being ∼0.03 Å. Thus, the O-donor appears to alter the
electronics of the pyridine ring.
UV−Vis Spectroscopy and Electrochemistry. With
complexes 1, 2A, 2B, and 3 in hand, the suitability of these
complexes for the photocatalytic CO2 reduction with BIH as a
sacrificial electron donor was probed by UV−vis absorption
spectroscopy and electrochemical analysis (Figures 2 and S33−
S44; Table 1). UV−vis spectra show that the change from
imidazole to benzimidazole-derived NHC ligands (in 4 versus
3) results in a 12 nm redshift in the lowest energy absorption
feature. This feature is a broad peak for 3 with a molar
absorptivity of 7600 M−1 cm−1, which is more intensely
absorbing than the low energy transition at 6400 M−1 cm−1 for
4. Changing the wingtip methyls (3) to phenyls in complex 1
led to very similar electronic spectra in terms of the wavelength
absorbed and the molar absorptivity. Replacement of the
MeCN ligands of 1 with bipy leads to a longer wavelength
absorption (24 nm comparing 2A to 1), which may be
attributed to a new MLCT event to the bipy ligand. Finally,
removal of the chloride and replacement with MeCN to give
2B from 2A gave a blueshift; however, 2B still has significant
light absorption in the visible spectrum. Computationally, the
assertion that 2A and 2B have MLCT events involving the
electron transfer to the bipy ligand is supported by the
presence of a metal-based HOMO and a bipy-ligand-based
LUMO for 2B (Figure 3). This differs from complex 1 which
RESULTS AND DISCUSSION
■
Synthesis. The synthesis of the pincer ligand of complex 1
starts with a Buchwald−Hartwig coupling between 2,6-
dibromo-4-methoxypyridine and N1-phenylbenzene-1,2-dia-
mine followed by imidazole ring cyclization with triethylor-
thoformate (Scheme S1).26−28 Dichloro(p-cymene)ruthenium-
(II) dimer was then used to metalate the ligand to give 1 in
22% overall yield. Attempts to synthesize complex 2A from 1
through addition of bipy resulted in either a mixture of 2A and
2B under thermal conditions or a complex reaction mixture
under photolysis conditions. However, complex 2A was
successfully synthesized by modifying a literature procedure
and treating (bipy)RuIVCl4 with the pincer precursor followed
by reduction to RuII (Scheme 1).29 Complex 2B was obtained
by salt metathesis upon treating 2A with silver triflate in
acetonitrile (Scheme 1). Complex 3 was synthesized in a
B
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Figure 2. Electronic spectra for each complex in MeCN.
Table 1. Electronic Spectral Features and Electrochemical
Redox Potentials for 1−4
−
E(S/S )
λmax
ε
E
(S/S−) N2 E(S−/S2−)
CO2
ac
,
a
b
cat (nm) (M−1 cm−1
)
(V)
N2 (V)
(V)
(icat/ip)2
c
1
428
452
410
417
405
7300
9800
8500
7600
6400
−1.95,
−2.16
n/a
−1.80
−1.90
−1.85
−2.10
−2.20
2.0
d
d
d
d
d
c
c
2A
2B
3
−1.90,
−2.15,
2.8
3.7
d
d
−1.97
−2.18
c
c
−1.85,
−2.15,
−1.94
−2.16
c
−2.15,
n/a
7.8
−2.26
c
4
−2.30,
n/a
21.4
−2.35
a
b
E(S/S−) is the first reduction potential. E(S−/S2−) is the second
reduction potential. From cyclic voltammetry reduction onset. See
the Supporting Information for example illustrations of how onset was
c
d
determined. Differential pulse voltammetry peak potential. These are
peak values. See the Supporting Information for voltammograms.
Figure 1. Molecular diagrams of complexes 1, 2A, 2B, and 3 based on
crystallographic data with hydrogen atoms and counteranions
removed for clarity. Thermal ellipsoids are drawn at the 50%
probability level.
Figure 3. Frontier molecular orbitals for 1 and 2B calculated at the
SMD-PBE0/BS1 level (see the Supporting Information for details).
Electrochemical studies on complexes 1−4 were conducted
under N2 and CO2 atmospheres (Table 1, Figures S35−S44).
Via cyclic voltammetry (CV) under N2 both of the bipy
complexes, 2A and 2B, show two reductions within the solvent
shows the LUMO positioned primarily on the pyridine ring of
the pincer ligand.
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window, while the remaining complexes show just one
reduction. All of the reductions were irreversible with complex
1 being the easiest to reduce at −1.95 V vs Fc+/Fc.
Substitution of monodentate ligands (in 1) with bipy led to
a slightly less negative reduction potential for 2A and 2B
(−1.85 to −1.90 V). Replacement of the phenyl group
wingtips with methyl groups led to significantly more negative
reduction potentials at −2.15 V for 3 and −2.30 V for 4.
Differential pulse voltammetry (DPV) was also conducted on
these complexes under N2, and the results are in close
agreement to the onset potentials observed with CV (Figures
S35−S44). Under CO2, the reduction potential onset does not
change dramatically for any of the complexes. A current
enhancement was observed for all of the catalysts at the first
reduction peak under CO2 when compared with N2. The
catalytic rate (kcat) for CO2 reduction is proportional to (icat/
ip)2 where icat and ip are the peak currents observed under CO2
and nitrogen, respectively.31,32 Through this estimate (Table
1), electrocatalytic CO2 reduction rates increase in the
following order: 1< 2A < 2B< 3 ≪ 4.
Figure 5. Catalyst TON versus time for the first hour of CO
production.
reported results.5 This result can be better understood by
analyzing the initial turnover frequency (TOF) values at 20
min which range from 8.3 to 0 h−1 (inactive) and decrease in
the following order: 2B > > 2A > 1 = 3 = 4 (Figure 5, Table
2). Interestingly, this order changes when the maximum TOF
values observed are analyzed with 2B > 1 > 2A > 4 > 3.
Precatalysts 1, 3, and 4 have significant periods of slow
catalysis possibly due to either an induction period or not
enough product accumulating for detection, with catalysis
clearly detectable after 1−20 h (clearly detectable defined as
>1 TON). Precatalysts 1 and 3 ultimately reach 8 and 12 TON
of CO and are durable for 22 and 95 h, respectively; thus, both
perform sluggishly even after activation (Table 2). Precatalyst
4 has the slowest initial rate, but after activation, catalysis
continues for 200 h producing the second highest TON value
(45) before catalysis ceases. In contrast, rapid initial catalysis is
observed for 2A and 2B, and Figure 5 shows an expanded view
of the early time points from Figure 4. These early data points
for the first hour show a constant rate of CO production
apparent from a linear fit with an R2 value of >0.99 and a zero
intercept (Figures 4 and 5). No change in slope occurs until
catalyst death after several hours for these complexes. Thus, 2A
and 2B appear to be true catalysts.
Photocatalysis. Having found that electrochemical CO2
reduction occurs in the presence of catalysts 1-4, these
catalysts were next tested under photocatalytic reaction
conditions. Remarkably, complexes 1−4 are catalysts for self-
sensitized visible-light-driven CO2 reduction to CO (Figures 4
Figure 4. Catalyst TON versus time plot for CO production from 1−
4. Reactions are run after vigorous bubbling with CO2 for 15 min with
0.2 μmol of catalyst and 0.2 mmol of BIH in 2 mL of a 5% TEA/
MeCN solution. Solutions are irradiated with a solar simulated
spectrum set to 1 sun (150 W Xe lamp, AM 1.5G filter).
During the photoreaction of 1 and 3, a white precipitate was
observed to form after extended reaction times with CO
production. We reasoned this white solid could be
accumulation of carbonate from a disproportionation reaction
of 2 CO2 molecules to CO and CO32−. Through the use of
and 5, Table 2). All photocatalysis experiments reported herein
were performed with TEA and BIH as sacrificial donors in
anhydrous acetonitrile with no added PS and quantified via
GC (see the Supporting Information for details). In terms of
durability, catalyst 2B was found to be the most durable at 55
TON over a 20 h irradiation period before ceasing CO
production. To test for the presence of a heterogeneous
catalyst, mercury was added to a photocatalytic reaction with
2B, and there was no significant difference in catalyst
performance (Figure S49).6 This suggests 2B is a homoge-
neous catalyst, although we note that proving homogeneity is
challenging.33−35
13CO2, 13CO3 could be observed in these reactions by 13C
2−
NMR (Figure S46). The white solid certainly attenuates the
photon flux reaching 1; thus, photocatalysis experiments were
performed without stirring to allow the solids to settle. With
this protocol, the TON and TOF values increased significantly
from 8 and 1.1 h−1 with stirring to 33 and 7.0 h−1 without
stirring.
As mentioned previously, catalysts 2A and 2B have initial
TOF values matching the maximum TOF values (Table 2).
This suggests that they enter the catalytic cycle by a facile
process such as MeCN dissociation in the case of 2B. Since
rapid initial catalysis is observed for these catalysts a pincer or
bipy ligand dissociation process is highly unlikely. Catalyst 2B
significantly outperforms 2A both in terms of durability (55
versus 15 TON) and rate (8.3 versus 0.6 h−1 TOF). This
The remaining complexes were found to have TON values
ranging from 55 to 8 TON in the following order under
identical conditions: 2B > 4 > 2A > 3 > 1 (Figure 4, Table 2).
The high performance from 4 is surprising given our previously
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Table 2. Comparison of Catalyst Performance Using New Complexes 1−4 and Literature Benchmarks 6−12
a
b
entry
cat.
conc.
TON (CO)
TOF (h−1) @20 min
TOF (h−1) max
ϕCO (%)
1
2
3
4
1
1
0.1 mM
0.1 mM
0.1 mM
0.1 mM
0.1 mM
1.0 nM
0.1 mM
0.1 mM
0.1 mM
0.1 mM
0.1 mM
0.1 mM
0.5 mM
0.1 mM
2.0 μM
0.9 mM
40 μM
8
33
15
0.0
0.0
0.6
8.3
2.0
250
0.0
0.0
11.2
31.6
3.2
1.1
7.0
0.7
8.3
7.0
250
0.2
0.5
15.9
31.6
3.2
96.7
24.1
0.2
1.4
12
14.5
0.4 × 10−1
9.1 × 100
c
2A
2B
1
2B
3
0.2 × 10−1
2.6 × 10−1
19.3 × 100
5.4 × 10−5
0.1 × 10−1
0.2 × 10−1
4.8 × 10−1
10.2 × 10−1
1.0 × 10−1
26.6 × 10−1
55
c,d
5
6
7
8
9
10
11
12
13
14
15
16
17
158
33,000
12
4
45
14
32
3
f
e
6
f
e
7
f
e
8
g
h
h
h
h
h
e
8
e
97
80
20
96.7
24.1
1.5
1.4
i
i
i
8
e
j
i
i
9
e
i
i
10
11
12
101
48
160
e
e
i
h
14.5
a
b
TOF values were calculated from the fastest reaction rates observed over a 1 h period of photocatalysis. ϕCO is calculated using a full spectrum
solar simulator at 1 sun irradiation intensity, not an isolated wavelength with reduced photon flux as is sometimes reported. ϕCO is reported as the
c
d
e
highest value observed between two time points. Without stirring. Using 1 equiv of bipy added relative to the complex. 6 =
Re(bipy)(CO)3Br;10,11 7 = Re(pyNHC-PhCF3)(CO)3Br;9,13 8 = [Ir(ppy)(terpy)Cl]+;8 9 = [Ir(terpy)(bipy)Cl]2+;14 10 = Fe-p-TMA;12,36 11 =
Re(bipy)(CO)3Cl;15 12 = [Ru(terpy)(pqn)(MeCN)]2+.37 See Figure S53 for the catalyst structures. These entries use the benchmark catalysts
under the same conditions as entries 1−5. This entry uses the conditions reported in the manuscript with a solar simulated spectrum as the
irradiation source. These entries are reported with the highest values observed in the referenced reports of these catalysts. The conditions vary
f
g
h
significantly from those in our manuscript in many cases, especially with regard to irradiation wavelengths and filters which can have dramatic
i
j
effects.38 Estimated from data in the manuscript. Value is for HCO2 .
−
illustrates that chloride removal greatly enhances self-sensitized
catalysis (2B versus 2A). Presumably, the acetonitrile ligand in
2B is more labile than a chloride and can be more easily
displaced during the catalytic cycle. It appears that the bipy
coligand is needed for self-sensitized catalysis without an
activation period which may be due to a difference in the
MLCT state as suggested by the UV−vis absorption data
above. Catalyst 1 differs from 2A in only a bipy ligand
substitution for two acetonitrile ligands, and during synthesis,
some 2A and 2B are observed upon mixing 1 with bipy.
Therefore, an experiment was conducted with 1 and added
bipy in the reaction mixture without stirring. In this case, a
TON value of 158 could be observed, and the catalyst is active
from the earliest time points (in contrast to 1 alone). This
combination does increase activity with time (from 2.0 to 7.0
h−1), presumably due to a slow displacement of monodentate
ligands in 1 for bipy, and ultimately reaches a similar maximal
TOF to 2B. Interestingly addition of more than 1 equiv of bipy
resulted in diminished reactivity to 10 TON and a TOF of 0.3
(see Table S6). From the synthetic attempts to make 2A from
1 and bipy under irradiation, the result is a mixture of
complexes with the exact structures of the active catalyst(s)
unknown. However, the observed rapid initial catalysis and a
significant increase in quantum yield (see discussion below)
suggests that the species formed is the active catalyst and that a
discrete photosensitizer is likely not present. In all cases, after
irradiation the photocatalytic activity is found to stop after 150
h, and in the case of 2B, catalytic activity ceases abruptly at 16
h even though ample BIH, TEA, and CO2 remain (see Figure
S51 for an NMR of a completed reaction with 2B showing BIH
remaining).
equation: ϕCO = [(number of CO molecules × 2)/(number of
incident photons)] × 100%, where ϕCO is the quantum yield
for CO production, the number of incident photons is taken as
the total number of photons striking the reaction vessel, and a
factor of 2 is used to account for the two electrons needed to
reduce CO2 to CO.39 Using this method, catalyst 2B shows
approximately an order of magnitude higher ϕCO than that of
the remaining complexes, 0.26% versus 0.04−0.01%. The more
efficient use of photons by 2B relative to that of the other
complexes highlights the importance of each catalyst structural
change from 4 to 2B with the exchange of the chloride atom
for a more labile MeCN ligand being critical (0.02% ϕCO for
2A versus 0.26% ϕCO for 2B). Interestingly, a high quantum
yield of 19.3% is observed for the reaction employing 1 with 1
equiv of added bipy. Importantly, quantum yields in excess of
∼1 × 10−2 are often indicative of mononuclear photocatalysts
at this concentration since PS based systems commonly show
lower quantum yields.39
Comparison to Literature Catalysts. Under identical
conditions, 2B was found to be a more durable catalyst than
the high-performance literature benchmarks Re(bipy)(CO)3Br
(6), Re(pyNHC-PhCF3)(CO)3Br (7), and [Ir(ppy)(terpy)-
Cl]+ (8) when tested in our laboratory (Table 2, entries 3, 7−
9). A near doubling of TON values is observed compared to
the next highest performing catalysts tested with 2B at 55
TON versus 32 TON for 7. The TOF values were found to be
lower for 2B than for rhenium-based complexes 6 and 7 (8.3
versus 11.2 and 31.6, respectively). ϕCO measurements show a
similar trend when 2B, 6, and 7 are compared with values of,
respectively, 2.6 × 10−1, 4.8 × 10−1, and 10.2 × 10−1 observed.
Iridium complex 8 was found to perform substantially lower
Quantum Yields for CO Production. The quantum
yields of these reactions were estimated during the maximum
TOF period within at least a 20 min time period via the
under identical conditions in terms of TON, TOF, and ϕCO.
The conditions employed in this manuscript differ in SD
choice with 2B using BIH/TEA and 8 using triethanol amine
E
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(TEOA) in the original report. Holding all conditions constant
and replacing the BIH/TEA SD with TEOA, complex 8 is
observed to regain a high performance similar to that reported
in literature with 97 TON and a TOF of 97 h−1. This
observation serves to highlight that conditions have a very large
impact on catalyst performance;7 thus, direct comparisons of
catalyst structure with performance are best made under
identical conditions, as in Table 2 (entries 1−11).
diminished concentration on the performance of 2B was
conducted. Accordingly, the catalyst concentration was
dramatically reduced to 1 nM to analyze the lowest
concentration anticipated to provide significant CO amounts
needed for accurate detection. For 2B, we observe 33 000
TON (TOF = 250 h−1) at 1 nM concentration (Figure S48).
The reason for this significant increase in TON and TOF at
low concentration can be explained at least in part by an
increased number of photons available throughout the
solution. At higher concentrations, a significant loss in
transmission is evident before and during the reaction with
as little as 10% transmittance at ∼425 nm (Figure S54). This
indicates that at 0.1 mM photons are not in large excess
throughout the reaction, whereas at 1 nM transmittance is
measured at >95% during the entire reaction time. With
greater than 95% of the photons striking the vessel passing
though unabsorbed, a lower ϕCO value is expected. Notably,
the ϕCO value diminished dramatically (5.4 × 10−5 versus 2.5 ×
10−1) as is expected with a lower amount of chromophore to
absorb photons.
Recently, low concentration studies are appearing more
commonly in literature and provide a method to demonstrate
catalyst performance with photons in large excess and catalyst
molecules isolated from one another by solvent. For the case of
a practical system where SDs are eliminated and CO2
reduction catalysts are coupled with oxidative catalysts, a
promising direction is to use surface-bound molecular catalysts
in photoelectrochemical cells.46,47 In literature reports of such
systems, catalysts are immobilized, and TON values show
dramatic increases when the surface bound system is compared
with high-concentration homogeneous studies.48−54 Low-
concentration homogeneous studies could likely become a
key parameter in catalyst selection for immobilization. To the
best of our knowledge, the TON and TOF values observed
herein are record-setting for a mononuclear photocatalyst
driving CO2 reduction. The expansion of comparison to other
conditions shows that catalyst 2B has comparable performance
to the best self-sensitized catalysts at high concentration (0.1
mM) and exceptional performance at low concentration (1.0
nM). This indicates that 2B is unusually robust and high-
performing among mononuclear visible-light-driven systems,
and 2B serves as the second example using ruthenium and
visible light to reduce CO2.
Comparing Photocatalysis to Other Methods of CO2
Reduction. In general, when comparing these reactions to
other CO2 reduction methods, homogeneous photocatalytic
CO2 reduction proceeds at significantly slower reaction rates
than those commonly observed for electrocatalysis and
thermal-based approaches to CO2 reduction. However, we
stress that studies within the homogeneous photocatalysis field
are analyzing half-reactions with sacrificial electron donors and
are not predictive for rates in future large-scale practical
applications which will likely exclude reagents such as BIH,
TEA, and MeCN due to cost constraints. Rather, practical
applications will likely be restricted to water as both the solvent
and the source of electrons. Importantly, the choice of electron
donor can have a profound influence on rates and even
product distributions obtained from CO2 reduction.7 Signifi-
cant caution is warranted in comparing reaction rates between
different systems where the rate in question could be more
dependent on the sacrificial donor rather than on CO2 bond
cleavage. Nonetheless, studies of these half-reactions with
sacrificial donors allow for the rapid analysis of catalyst
However, an expanded comparison to catalysts under
different conditions does have merit in that practically a
system that maximizes TON, TOF, and ϕCO is desired. Thus, a
comparison to literature observed values is shown in Table 2,
entries 13−17, which are under different conditions. The
comparison in Table 2 for the complexes 6−12 has been
limited to the basic parent homogeneous complexes for
mononuclear catalysts and to only photocatalysts using visible
light to avoid an excessive number of entries. However, we
note that many interesting additional examples exist with
mononuclear, visible-light-driven, homogeneous photocatalysts
(see reviews2,20,21,40 with ligand derivatives demonstrating
TON values of up to 530 at 0.5 mM are obtainable).38
When comparing additional catalysts under varying
conditions, one of the most important parameters to assess
is concentration since this has been observed in several
instances to have a dramatic effect on TON, TOF, and ϕCO in
literature and with 2B as discussed below.41,42 As reported in
the literature,8 8 at 0.5 mM shows estimated TON and TOF
values of 80 and 24.1 h−1 which are higher than the values for
2B at 0.1 mM. Iridium complex 9 is a structural analogue of 8
with a different charge (2+) and is reported to give 20
turnovers of formate and only 2 turnovers of CO at the same
concentration as our studies with 2B.14
Recently, Fe-porphyrin complex 10 has gained significant
attention as a photocatalyst with TON and TOF values of 101
and 1.4 h−1 reported at 2.0 μM concentration.12,36 The TON
value is higher than that we observe with 2B, although we note
that 2B has a considerably higher TOF and that the study with
10 was done at significantly lower concentration. The original
report on the first visible-light-driven mononuclear homoge-
neous photocatalyst (11), which is now a ubiquitous
benchmarking standard in the literature, was done at a similar
concentration and shows slightly fewer TONs than 2B but
with a slightly higher TOF than 2B.15 Very recently after our
work was submitted, [Ru(terpy)(pqn)(MeCN)]2+ at 40 μM
was shown to self-sensitize CO2 reduction producing 160
TON of CO with BIH as the sacrificial donor in DMA/H2O
solvent, which appears to be the first report of a mononuclear,
visible-light-driven, homogeneous Ru CO2 reduction catalyst.37
Effect of Catalyst Concentration on Photocatalysis.
The above comparisons to literature have several variables
changed, and concentration is a critical variable when analyzing
homogeneous photocatalytic reactions. A reduction in catalyst
concentration can often significantly increase catalyst durability
by reducing catalyst−catalyst interactions and is potentially a
good indicator of how a catalyst may perform when
immobilized.41,43 Additionally, low concentration experiments
also allow probing of mechanistic pathways. Bimolecular
mechanistic pathways are known in which one complex acts
as a photosensitizer and the second as a catalyst.44,45 This
pathway access is reduced at very low concentration as we have
previously shown.42 Since the two highest TON values in
literature were reported at much lower concentrations than our
original studies with 2B, an experiment on the effects of
F
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Inorg. Chem. XXXX, XXX, XXX−XXX

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Article
Notes
durability. Several examples are discussed above which were
heterogenized onto semiconductors to produce faster reaction
rates and increased durability. There is certainly appeal in
studying homogeneous catalysis as it allows for rapid access to
simplified systems where modifications lead to predictable
changes for trend establishment in many cases. Since
immobilization onto an electrode surface results in significantly
higher durability in many cases, low-concentration studies
limiting catalyst−catalyst interactions with excess photons
present are desirable to get a reasonable prediction in a
simplified system as to how the catalyst will behave in a more
practical setting.
The authors declare the following competing financial
interest(s): E.T.P. and J.H.D. have filed a patent application
related to this chemistry.
ACKNOWLEDGMENTS
■
We thank the NSF (awards CHE-1800281, 1800214, and
1800201) for funding this research. Preliminary data was also
obtained with NSF OIA-1539035 support. S.D. thanks the
University of Alabama’s Graduate Council Fellowship (GCF),
and C.M.B. is grateful for a GAANN fellowship (Department
of Education P200A150329) for partial support.
CONCLUSION
■
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00791.
■
S
Experimental details on synthesis and characterization,
single crystal X-ray diffraction, photocatalysis, and
computations (PDF)
Crystallographic information (XYZ)
Accession Codes
CCDC 1879281−1879284 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: ewebster@chemistry.msstate.edu (C.E.W.)
*E-mail: delcamp@olemiss.edu (J.H.D.).
*E-mail: etpapish@ua.edu (E.T.P.).
■
ORCID
Roberta R. Rodrigues: 0000-0003-2930-2451
Fengrui Qu: 0000-0002-9975-2573
Chance M. Boudreaux: 0000-0003-1322-9878
Charles Edwin Webster: 0000-0002-6917-2957
Jared H. Delcamp: 0000-0001-5313-4078
Elizabeth T. Papish: 0000-0002-7937-8019
Author Contributions
#S.D. and R.R.R. contributed equally to this work.
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