Electrocatalytic Reduction of Carbon Dioxide with a Well-Defined PN3-Ru Pincer Complex

Article abstract of DOI:10.1002/cplu.201500474

A well-defined PN³-Ru pincer complex (5) bearing a redox-active bipyridine ligand with an aminophosphine arm has been established as an effective and stable molecular electrocatalyst for CO₂ reduction to CO and HCOOH with negligible formation of H₂ in a H₂O/MeCN mixture. Done in a pince: A PN³-Ru pincer complex (see figure) has been found to show high activity and stability as a molecular electrocatalyst for CO₂ reduction to CO and HCOOH in MeCN/H₂O.

Full text of DOI:10.1002/cplu.201500474

DOI: 10.1002/cplu.201500474
Communications
Electrocatalytic Reduction of Carbon Dioxide with a Well-
Defined PN3ÀRu Pincer Complex
Shixiong Min,[a, b] Shahid Rasul,[a] Huaifeng Li,[a] David C. Grills,[c] Kazuhiro Takanabe,[a] Lain-
Jong Li,[a] and Kuo-Wei Huang*[a]
A well-defined PN3ÀRu pincer complex (5) bearing a redox-
active bipyridine ligand with an aminophosphine arm has
been established as an effective and stable molecular electro-
catalyst for CO2 reduction to CO and HCOOH with negligible
formation of H2 in a H2O/MeCN mixture.
Transition-metal complexes supported by pincer ligands
have recently emerged as versatile catalysts for a variety of or-
ganic transformation reactions owing to their high stability
and the tunability of stereoelectronic properties by modifying
the moieties on the ligand to afford exceptional chemical reac-
tivities.[7] In particular, pincer catalysts have been developed
and shown to exhibit superior activity and selectivity for the
reversible chemical hydrogenation of CO2 to formate, among
them PCPÀIr,[8a–c] PNPÀRu,[8d,e] PNNÀRu,[8f–i] PNPÀFe,[8j–l] PNPÀ
Co,[8m] and PNPÀNi.[8n] More recently, the utilization of pincer
complexes has also been extended to electrocatalytic water
and CO2 reduction reactions.[9] Crabtree and co-workers report-
ed that tridentate pincer-ligand-supported nickel complexes
are active electrocatalysts for proton reduction to H2 in both
nonaqueous and aqueous solutions.[9a,b] Studies on electrocata-
lytic CO2 reduction by Brookhart and co-workers showed that
POCOP- and PCP-type Ir–dihydride pincer complexes are
highly selective electrocatalysts for CO2 reduction to formate in
the presence of H2O, and the electrocatalytic performance
using this class of complexes was found to be further im-
proved by using a gas-diffusion electrode.[9c–e] These studies
clearly demonstrated a great potential of pincer complexes as
highly efficient electrocatalysts for CO2 reduction under mild
conditions. We have recently developed a new class of dear-
omatized pyridine-based pincer complexes with an imine arm
that exhibit unusual thermodynamic and kinetic proper-
ties.[10,11] This new class of complexes may show improved ac-
tivity and selectivity for CO2 reduction by fine-tuning their
redox-active properties, thereby acquiring deep knowledge of
the structure–performance relationship. In this communication,
we synthesized various PN3ÀRu pincer complexes containing
different redox-active (bi)pyridine-containing ligands, which are
known as effective catalyst precursors for ester hydrogenation
in the presence of H2O.[11d] After optimizing the ligands and
H2O content, highly stable PN3ÀRu-based molecular catalysts
have been identified that are selective for CO2 reduction in
MeCN/H2O mixtures, with negligible H2 formation.
In addition to chemical transformation and reduction process-
es, the electrocatalytic reduction of CO2 into valuable chemi-
cals powered by the input of renewable energy is being pur-
sued as a promising pathway to mitigate the depletion of
carbon resources and environmental problems.[1] The rational
design and development of electrocatalysts that are efficient,
stable, and selective for this reaction is of great importance.
Various metal electrocatalysts have been investigated experi-
mentally and theoretically to rationalize their practical applica-
tion in CO2 electroreduction.[1d,2] However, most of them com-
monly suffer from either low energy efficiency owing to the
need for high overpotentials to activate CO2 and/or low hydro-
carbon selectivity owing to competitive water reduction. De-
spite this, several advances have been made recently to sub-
stantially enhance the activity and selectivity by using nano-
structured noble-metal electrocatalysts and rational modifica-
tion of existing electrocatalysts.[3] In addition, well-defined mo-
lecular electrocatalysts have gained tremendous attention
because of their tunable reactivity and selectivity by varying
the ligand structures and the coordinated metal centers.[4–6]
A
wide variety of transition-metal complexes with nitrogen-
donor ligands have been found to be active for CO2 reduction,
but only a few of them show impressive performances,[5] and
most of them still require quite a high overpotential or exhibit
inferior selectivity and stability.
[a] Dr. S. Min, Dr. S. Rasul, Dr. H. Li, Prof. K. Takanabe, Prof. L.-J. Li,
Prof. K.-W. Huang
Division of Physical Science and Engineering and KAUST Catalysis Center
King Abdullah University of Science and Technology
Thuwal 23955-6900 (Kingdom of Saudi Arabia)
E-mail: hkw@kaust.edu.sa
We first investigated the electrocatalytic reduction of CO2
using three PN3ÀRu pincer complexes 1–3 that have different
substituents on one of the arms of the ligand (Scheme 1), by
using cyclic voltammetry (CV) and controlled potential electrol-
ysis (CPE) experiments at constant potential (À1.26 to À1.31 V
versus a standard hydrogen electrode (SHE), depending on the
complex used) on glassy carbon (GC) and carbon paper elec-
trodes in a 3% H2O/MeCN mixture, respectively (see the exper-
imental details in the Supporting Information).
[b] Dr. S. Min
School of Chemistry and Chemical Engineering
Beifang University of Nationalities
Ningxia 750021 (P. R. China)
[c] Dr. D. C. Grills
Chemistry Department
Brookhaven National Laboratory
Upton, NY 11973-5000 (USA)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201500474.
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nature of these ligands in reduced form to accept electrons
and to stabilize the reducing equivalents during the CO2 reduc-
tion.
Previous studies reported that for nitrogen-donor ligand-
supported Re, Mn, and Ru complexes, the incorporation of
redox-active bipyridine ligands was essential for efficient CO2
electroreduction,[6] which in turn influenced the product selec-
tivity.[6g] Consistent with these observations, it was found that
dearomatized bipyridine-containing complex 4 and its aromat-
ized counterpart 5 (Figure 1 and Scheme 1) showed high activ-
ity for CO2 reduction. Complexes 4 and 5 showed similar elec-
trochemical behaviors in the presence of Ar, and there are two
reduction waves with the latter showing quasi-reversibility
during the reverse scan. In the presence of CO2 and the addi-
tion of 3% H2O, a clear enhancement in current was observed
at these two reduction waves, indicative of the catalytic reduc-
tion of CO2, which was also confirmed by experiments with
controlled potentials (Figure 1C,D) at the first reduction waves.
Complex 5 showed formation of CO (3.9%) and HCOOH
(28.3%), even in the absence of H2O. The addition of a small
amount of H2O (3%) significantly enhanced the electrocatalytic
current with much higher Faradaic efficiencies (FEs) for CO
(60.7%) and HCOOH (37.3%) (Figure 1D), with only negligible
formation of H2 (<2.0%) from H2O reduction. No significant
change was recorded in the UV-visible absorption spectra after
the reaction during a two hour reaction (Figure S3 in the Sup-
porting Information). NMR spectroscopy studies also showed
that the chemical structure of 5 remained unchanged even in
Scheme 1. PN3ÀRu pincer complexes 1–5 with different ligands that were in-
vestigated for electrocatalytic CO2 reduction. CPE experimental conditions
for 2–5: Carbon-paper electrode, 2.4 cm2, 3% H2O/MeCN mixture. The FEs
for CO and H2 are the average values and that for HCOOH is a cumulative
value over 12 h of electrolysis.
Dearomatized complex 1 was totally inactive and unstable
under the electrochemical conditions. Complexes 2 and 3 with
an oxazoline group were initially electrochemically active for
CO2 reduction but were deactivated quickly with time
(Scheme 1; Figures S1 and S2 in the Supporting Information).
The low activity and stability of these complexes are presuma-
bly due to the electrochemical ineffectiveness and/or unstable
Figure 1. Cyclic voltammograms of 1.0 mm (A) 4 and (B) 5 in MeCN under Ar, CO2, and CO2 with addition of 3% H2O. Conditions: Glassy carbon electrode,
7.1 mm2, 0.1m nBu4NPF6, 100 mVsÀ1, 296.2 K. Current-time profiles CPE experiments of CO2 reduction over (C) 4 and (D) 5 at À1.26 and À1.28 V (vs. SHE) in
3% H2O/MeCN mixture, respectively. Conditions: carbon-paper electrode, 2.4 cm2, 0.1m nBu4NPF6.
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the presence of H2O and after exposure to air for three days
(Figure S4 in the Supporting Information). These preliminary re-
sults show that a new, high-performance, water- and air-stable
Ru-based pincer electrocatalyst 5 for CO2 reduction has been
established by rationally modulating the redox properties of
the ligand framework. In an effort to obtain more kinetic de-
tails on the electrocatalytic reduction of CO2, cyclic voltammo-
grams of 5 under different reaction conditions were investigat-
ed at a glassy carbon electrode in detail, as shown in Fig-
ure 2A–C. Under Ar in dry MeCN at 100 mVsÀ1(Figure 1B), a re-
duction wave at about À1.30 V versus a standard hydrogen
electrode (SHE) was observed, likely arising from ligand-related
reduction,[6e,j,12] which was consistent with that of recently re-
ported trans-Cl-[Ru(mesbpy)(CO)2Cl2] with a bulky bpy ligand
(mesbpy).[13] The peak current density (ip,c) of this reduction
wave demonstrates a linear dependence on the square root of
the scan rate (u1/2) from 50 to 3000 mVsÀ1, which indicates
a diffusion-limited reduction process of the catalyst at the elec-
trode (Figure 2A, Figures S5 and S6 in the Supporting Informa-
tion). Only at faster scan rates (>400 mVsÀ1), was an oxidation
peak observed at À1.25 V (vs. SHE), with a linear correlation of
the peak current density (ip,a) with u1/2 (Figure S5a in the Sup-
porting Information), also indicative of a diffusion-limited pro-
cess. This observation is consistent with a mechanism in which
an initial electron transfer takes place to form the radical
anion, which can be reversible, followed by an irreversible
chemical step, most likely the loss of chloride to form RuI spe-
cies coordinated with solvent molecule(s). Similar processes
were observed with other bipyridine-related transition-metal
complexes containing Cl ligand(s).[6f,h,j,13] It is also noted that
only one reduction wave is observed for 5 under Ar in the po-
tential range of -1.2 to -1.5 V (vs. SHE), which suggests that
complex 5 is capable of mediating a two-electron reduction
process to produce the neutral species after the chloride loss
(Figures S6 and S7 in the Supporting Information), in agree-
ment with our aforementioned kinetic analysis and the obser-
vation in an NHC-containing Mn molecular electrocatalyst.[6g]
At slower scan rates, the irreversibility is attributed to a fast ir-
reversible chemical step following the initial electron transfer,
whereas at higher scan rates, the reversible behavior of 5 sug-
gests that the loss of chloride is slower than the reoxidation of
the radical anion formed upon the initial reduction.
Under CO2 bubbling in dry MeCN, the reduction peak cur-
rent density at around À1.28 V (vs. SHE) for 5 was nearly dou-
bled from 0.38 to 0.69 mAcmÀ2 (Figure 1B), which suggests
catalytic CO2 turnover. The catalytic peak current density (icat)
for CO2 reduction also shows scan-rate-dependent electro-
chemical behavior (Figure 2B, and Figure S8a in the Supporting
Information). The complete absence of oxidation currents in
the presence of CO2 even at higher scan rates suggests that
the active species (probably a neutral radical) formed by an ini-
tial electron transfer (Figure S7 in the Supporting Information)
is likely transformed immediately by the reaction with CO2. The
similar onset potential for the CO2 reduction wave is consistent
Figure 2. Cyclic voltammograms of 1 mm 5 in MeCN under (A) Ar, (B) CO2, and (C) CO2 with addition of 3% H2O, at various scan rates. Conditions: Glassy
carbon electrode, 7.1 mm2, 0.1m nBu4NPF6, 296.2 K. (D) Long-term CPE at À1.28 V (vs. SHE) in 3% H2O/MeCN mixture (carbon-paper electrode, 1.2 cm2). The
FEs for CO and H2 are average values and that for HCOOH is a cumulative value over 24 h of electrolysis.
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with the fact that the reduced 5 as a result of the initial elec-
tron transfer is an active species for CO2 reduction catalysis. By
measuring the ratio of peak currents with CO2 (icat) and without
CO2 (ip,c) as a function of uÀ0.5 (Figure S8b in the Supporting In-
formation),[8c–e] the pseudo-first-order rate constant (kcat) for
CO2 reduction under one atmosphere of CO2 ([CO2]=0.28m) is
calculated to be 0.66 sÀ1. Long-term CPE experiments (12 h)
(Table 1, entry 1) confirmed the successful CO2 reduction even
without H2O, but with low FEs to CO (3.2%) and HCOOH
(15.4%). Note that control experiments confirmed a lack of in-
cremental catalytic currents when either no CO2 or 5 was pres-
ent (Figure S9 in the Supporting Information).
expected, when more H2O (10%) or less 5 was present, the re-
duction of water to H2 became more favorable than CO2 reduc-
tion (Table 1, entries 3 and 4). It was reported that for Re– and
Mn–tricarbonyl complexes the presence of water not only
serves as a proton source to lower overpotentials for CO2 re-
duction, but also stabilizes the metal–CO2 intermediate
through protonation, thus facilitating the reduction of CO2.[6]
A
similar beneficial effect of water was also reported for the elec-
trocatalytic reduction of CO2 to formate with an Ir–dihydride
pincer complex.[9c] Kinetic studies showed that the catalytic
peak current, icat, varies linearly with [5] under one atmosphere
of CO2 and linearly with [CO2]0.5 with 1 mm of 5 (Figures S12
and S13 in the Supporting Information), which indicates that
the catalytic reaction is first order in both concentrations of 5
and CO2.[9c,d]
As reported previously, the presence of H2O (or some other
proton sources) is effective to further improve the catalytic
performance for CO2 reduction.[5,6] When the CV of 5 under
CO2 was recorded in CH3CN with increasing amounts of added
H2O, a gradual increase in catalytic current at À1.28 V (vs. SHE)
was observed (Figure S10a in the Supporting Information) and
icat reached a maximum of 1.25 mAcmÀ2 at 2–3% H2O, with an
onset for CO2 reduction appearing at approximately À1.15 V
(vs. SHE), corresponding to overpotentials of 0.629 and 0.549 V
for CO and HCOOH production, respectively, which is compara-
ble to most recently reported Ir–pincer[9c] and Mn–carbonyl
complexes.[6f–h] In contrast, there was no significant current en-
hancement under Ar at À1.28 V (vs. SHE) with added H2O up
to 3%, instead there was a significant contribution to the cur-
rent enhancement at more negative potentials, probably due
to the background water reduction (Figure S10b in the Sup-
porting Information). Diffusion-limited kinetic behavior was
also observed for CO2 reduction with 3% H2O added (Fig-
ure 2C, and Figure S11 in the Supporting Information), and
a much larger kcat of 4.44 sÀ1 was achieved. This value is com-
parable to that of polypyridyl-supported Ru complexes
(ꢀ5.1 sÀ1 with 10% H2O),[6i] but much lower than the value ob-
tained with Ir–dihydride pincer complex (20 sÀ1 with 5%
H2O).[9c] Controlled potential electrolysis experiments showed
that, with 1 mm of 5 and in the presence of 3% H2O, the aver-
age FEs for both CO and HCOOH production over a 12 hour re-
action were considerably higher than those in dry MeCN, with
negligible formation of H2 (Table 1, entry 2, and Figure 2D). As
Scheme 2 shows a plausible electrocatalytic reduction mech-
anism of CO2 reduction over 5. The irreversibility of the CV re-
sponse of 5 (i) at the first reduction potential is a bpy-based re-
duction pathway followed by a fast ligand-to-metal charge
transfer resulting in the ClÀ loss, which yields RuI species (ii)
(step (a)). The RuI species readily accepts an electron and
reacts with H+ to generate the neutral dihydride RuII species
(iii) (step (b)) followed by an insertion reaction of CO2 into one
of the RuÀH bonds of (iii) to give intermediate (v) (step (c))
under an applied potential. In another possible pathway, CO2
may insert into the RuÀH bond of RuI species (ii) directly to
give a neutral RuI species (iv) (step (b’)) under CO2-saturation
conditions and then undergoes a proton-coupled electron-
transfer process to form (v). Subsequently, bulk electrolysis at
À
the first reduction potential may generate HCO2 /HCO2H via
step (d). The CO formation may be a result of decarbonylation
from a metallocarboxylate intermediate (vi) through steps (e)–
(g).[6,14] The formation of intermediate (v) was supported by
a control experiment using complex 4 with the addition of
one equivalent of HCOOH under otherwise identical condi-
tions, which gave a similar activity and selectivity for CO2 re-
duction to that of 4 without the addition of HCOOH (Figur-
es S14 in the Supporting Information).
Further optimization of the electrolyte conditions was at-
tempted to improve the electrocatalytic performance of CO2
Table 1. Results of controlled potential electrolysis (CPE) experiments for 12 hours.[a]
Entry
Comments
E vs. SHE [V]
Charge passed [C]
FECO [%] (TON)[f]
FEHCOOH [%] (TON)[f]
FEH [%]
Total FE [%]
2
1
2
3
4
5
6
7
8
9
5 (1.0 mm) in MeCN/CO2
À1.28
À1.28
À1.28
À1.28
À1.75
À1.28
À1.28
À1.28
À1.28
34.8
57.9
10.9
58.2
75.1
67.0 (24 h)
10.7
7.3
38.5
3.2 (0.2)
60.7 (6.1)
20.3 (2.0)
60.9 (6.1)
36.4 (4.7)
72.7 (8.4)
n.d.
15.4 (0.9)
37.3 (4.2)
40.1 (7.5)
13.9 (1.4)
34.3 (4.5)
24.5 (2.8)
18.4 (0.3)
18.3 (0.5)
8.5 (0.3)
n.d.
<2.0
26.8
13.2
5.3
<3.0
35.4
n.d.
18.6
ꢀ100
87.2
88.0
76.0
ꢀ100
53.8
5 (1.0 mm) in MeCN (3% H2O)/CO2
5 (0.2 mm) in MeCN (3% H2O)/CO2
5 (1.0 mm) in MeCN (10% H2O)/CO2
5 (1.0 mm) in MeCN (3% H2O)/CO2
5 (1.0 mm) in MeCN (3% H2O)/CO2
MeCN (3% H2O)/CO2/used electrode[c]
MeCN/CO2/bare electrode[d]
MeCN (3% H2O)/CO2/bare electrode[e]
[b]
n.d.
n.d.
18.3
97.8
89.3
[a] Conditions: Carbon-paper electrode, 2.4 cm2, 1 atm CO2, 0.1m nBu4NPF6, 296.2 K. [b] A 1.2 cm2 carbon-paper electrode was used. [c] Electrode used for
entry 2 experiment was reused after thoroughly rising with acetonitrile and acetone, and recharged with fresh solution without 5 added. [d] Clean carbon
electrode was used without addition of 5 and H2O. [e] Clean carbon electrode was used without addition of 5. [f] The TON was calculated based on the
amount of 5 used. n.d.=not detectable. The FEs for CO and H2 are average values and that for HCOOH is a cumulative value during the electrolysis reac-
tion.
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Scheme 2. Proposed overall mechanism for the reduction of CO2 to CO and HCO2H by 5 in H2O/MeCN mixture.
using 5. When adding 3% aqueous HCl solution with concen-
trations varying from 10À5 to 10À1 m instead of pure water
(Table S1 in the Supporting Information), a slight decrease in
CO selectivity and negligible increase in H2 selectivity were ob-
served compared with the values obtained with pure water
(Table 1, entry 2), together with a slight decrease in HCOOH se-
lectivity, further confirming the high activity of 5 for CO2 re-
duction over proton reduction. When the reaction was carried
out in an aqueous KHCO3 solution in the presence of the re-
quired amount of MeCN to dissolve 5 (Table S1 in the Support-
ing Information), the production of H2 (21.8%) became more
favorable with concurrent HCOOH formation (9.2%), which
might still show potential for applications of 5 in aqueous CO2
reduction.
ucts equals 100%, which suggests a robustness of the catalytic
species from 5 during this period of time. In the next 12 hours
of electrolysis, there was a gradual decrease in both the cata-
lytic current and FE for CO (Figure 2D). An overall turnover
number (TON) of about 11.2 for CO2 reduction was achieved
over 24 hours of electrolysis. The decrease in activity was pre-
sumably due to the decomposition of the catalyst caused by
the hydrolysis of the PÀN bond of the aminophosphine arm
when more formic acid was formed,[15] as evidenced by a visible
color change in both the carbon-paper electrode and electro-
lyte solutions, and the formation of a white precipitate in solu-
tion. X-ray photoelectron spectroscopy (XPS) (Figure S15 in the
Supporting Information) confirmed that the surface of the
carbon electrode contained Ru and N after the long-term elec-
trocatalytic reaction, in which Ru 3p peaks show both oxidized
and metallic forms. The deactivation of 5 during CO2 electroly-
sis also resulted in the change of the CO/HCOOH ratio as the
decomposed catalytic species may be involved in the electroly-
sis.
As the CV scan of 5 was expanded to more negative poten-
tials (Figure 1B), one more reductive process was observed
under an atmosphere of argon, with an additional catalytic re-
duction wave appearing at À1.75 V (vs. SHE) under CO2 in the
presence of H2O. Electrolysis of CO2 at this potential, however,
only showed lower CO selectivity and total FE for CO2 reduc-
tion (CO and HCOOH), due to the background proton reduc-
tion and/or decomposition of the catalyst (Table 1, entry 5).
The product selectivity was also affected by the applied poten-
tials (Table S2 in the Supporting Information). The CO/HCOOH
ratio increased with an increase in applied potential from
about À1.15 to À1.35 V (vs. SHE). The catalytic current was
highest at about À1.28 V (vs. SHE), consistent with the CV
result (Figure 1B), with the highest total FE of CO2 reduction
products reaching as high as 98%.
To investigate whether the deposited material was catalyti-
cally active, the used electrode was subjected to an additional
12 hours of electrolysis after removing 5 and recharging with
fresh solution, under otherwise identical conditions. As
a result, H2 appeared to be the main product, with no CO de-
tected (Table 1, entry 7). These results are similar to those
obtain from background CPE experiments with a clean carbon-
paper electrode without 5 irrespective of H2O presence
(Table 1, entries 8 and 9). From these control experiments, the
presence of 5 drastically improved not only the amount of
charge passed but specifically the CO2 reduction over H2 evolu-
tion. These observations indicated that any Ru species deposit-
ed on the electrode was not responsible for the observed CO2
electrocatalysis and 5 indeed worked as a homogeneous cata-
lyst for electrocatalytic reduction of CO2.
At the optimal potential (À1.28 V vs. SHE), the current for
CO2 reduction electrolysis was sustained without significant de-
crease for at least 12 hours using a carbon-paper working elec-
trode (Figure 2D). The FE of CO approaches an average value
of 85.2% (Table 1, entry 6) without a change in the ratio of CO
and HCOOH by assuming that the total FE of the main prod-
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In summary, the PN3ÀRu pincer complex 5 has been demon-
strated as an effective and stable electrocatalyst for CO2 reduc-
tion. When an appropriate amount of water is present, CO2 is
reduced to CO and HCOOH at high FEs. Importantly, formation
of H2 from background water reduction during CO2 reduction
is only negligible, which is indicative of the high activity of this
complex toward CO2 reduction over water reduction. More-
over, a lack of CO2 reduction activity with a used carbon elec-
trode confirmed that the dissolved molecular complex in ho-
mogeneous phase is solely responsible for the observed CO2
electrocatalysis. This study reveals a new ligand platform for
the development of high-performance electrocatalysts of
pincer-based metal complexes with versatile functionality and
reactivity for CO2 conversion. Further studies involving pulse
radiolysis with time-resolved infrared detection to elucidate
the detailed mechanism are currently underway.[16]
Bourrez, F. Molton, S. Chardon-Noblat, A. Deronzier, Angew. Chem. Int.
Ed. 2011, 50, 9903; Angew. Chem. 2011, 123, 10077; c) J. Agarwal, T. W.
Shaw, C. J. Stanton, G. F. Majetich, A. B. Bocarsly, H. F. Schaefer III,
Angew. Chem. Int. Ed. 2014, 53, 5152; Angew. Chem. 2014, 126, 5252;
d) M. Bourrez, M. Orio, F. Molton, H. Vezin, C. Duboc, A. Deronzier, S.
Chardon-Noblat, Angew. Chem. Int. Ed. 2014, 53, 240; Angew. Chem.
2014, 126, 244; e) Z. Chen, P. Kang, M.-T. Zhang, T. J. Meyer, Chem.
Commun. 2014, 50, 335; f) D. C. Grills, J. A. Farrington, B. H. Layne, S. V.
Lymar, B. A. Mello, J. M. Preses, J. F. Wishart, J. Am. Chem. Soc. 2014,
136, 5563; g) M. D. Sampson, A. D. Nguyen, K. A. Grice, C. E. Moore, A. L.
Rheingold, C. P. Kubiak, J. Am. Chem. Soc. 2014, 136, 5460.
[7] a) M. E. van der Boom, D. Milstein, Chem. Rev. 2003, 103, 1759; b) J.
Choi, A. H. R. MacArthur, M. Brookhart, A. S. Goldman, Chem. Rev. 2011,
111, 1761; c) N. Selander, K. J. Szabo, Chem. Rev. 2011, 111, 2048; d) C.
Gunanathan, D. Milstein, Chem. Rev. 2014, 114, 12024; e) H. Li, B. Zheng,
K.-W. Huang, Coord. Chem. Rev. 2015, 293–294, 116–138.
[8] a) R. Tanaka, M. Yamashita, K. Nozaki, J. Am. Chem. Soc. 2009, 131,
14168; b) R. Tanaka, M. Yamashita, L. W. Chung, K. Morokuma, K. Nozaki,
Organometallics 2011, 30, 6742; c) T. J. Schmeier, G. E. Dobereiner, R. H.
Crabtree, N. Hazari, J. Am. Chem. Soc. 2011, 133, 9274; d) G. A. Filonen-
ko, M. P. Conley, C. CopØret, M. Lutz, E. J. M. Hensen, E. A. Pidko, ACS
Catal. 2013, 3, 2522; e) G. A. Filonenko, R. van Putten, E. N. Schulpen,
E. J. M. Hensen, E. A. Pidko, ChemCatChem 2014, 6, 1526; f) G. A. Filo-
nenko, E. J. M. Hensen, E. A. Pidko, Catal. Sci. Technol. 2014, 4, 3474;
g) C. A. Huff, M. S. Sanford, J. Am. Chem. Soc. 2011, 133, 18122; h) C. A.
Huff, M. S. Sanford, ACS Catal. 2013, 3, 2412; i) N. M. Rezayee, C. A. Huff,
M. S. Sanford, J. Am. Chem. Soc. 2015, 137, 1028; j) R. Langer, Y. Diskin-
Posner, G. Leitus, L. J. W. Shimon, Y. Ben-David, D. Milstein, Angew.
Chem. Int. Ed. 2011, 50, 9948; Angew. Chem. 2011, 123, 10122; k) O.
Rivada-Wheelaghan, A. Dauth, G. Leitus, Y. Diskin-Posner, D. Milstein,
Inorg. Chem. 2015, 54, 4526; l) T. Zell, D. Milstein, Acc. Chem. Res. 2015,
48, 1979; m) X. Z. Yang, ACS Catal. 2011, 1, 849; n) M. Vogt, O. Rivada-
Wheelaghan, M. A. Iron, G. Leitus, Y. Diskin-Posner, L. J. W. Shimon, Y.
Ben-David, D. Milstein, Organometallics 2013, 32, 300.
Acknowledgements
We are grateful for the generous financial support from King Ab-
dullah University of Science and Technology and the National
Natural Science Foundation of China (grant no. 21463001). Work
at Brookhaven National Laboratory was supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, Division of Chemical Sciences, Geosciences& Biosciences
under contracts DE-AC02-98CH10886 and DE-SC0012704.
[9] a) O. R. Luca, S. J. Konezny, J. D. Blakemore, D. M. Colosi, S. Saha, G. W.
Brudvig, V. S. Batista, R. H. Crabtree, New J. Chem. 2012, 36, 1149;
b) O. R. Luca, J. D. Blakemore, S. J. Konezny, J. M. Praetorius, T. J. Schmei-
er, G. B. Hunsinger, V. S. Batista, G. W. Brudvig, N. Hazari, R. H. Crabtree,
Inorg. Chem. 2012, 51, 8704; c) P. Kang, C. Cheng, Z. Chen, C. K. Schauer,
T. J. Meyer, M. Brookhart, J. Am. Chem. Soc. 2012, 134, 5500; d) P. Kang,
T. J. Meyer, M. Brookhart, Chem. Sci. 2013, 4, 3497; e) P. Kang, S. Zhang,
T. J. Meyer, M. Brookhart, Angew. Chem. Int. Ed. 2014, 53, 8709–8713;
Angew. Chem. 2014, 126, 8853.
[10] D. Benito-Garagorri, E. Becker, J. Wiedermann, W. Lackner, M. Pollak, K.
Mereiter, J. Kisala, K. Kirchner, Organometallics 2006, 25, 1900.
[11] a) L.-P. He, T. Chen, D. Gong, Z. Lai, K.-W. Huang, Organometallics 2012,
31, 5208; b) L.-P. He, T. Chen, D.-X. Xue, M. Eddaoudi, K.-W. Huang, J. Or-
ganomet. Chem. 2012, 700, 202; c) G. Zeng, T. Chen, L. He, I. Pinnau, Z.
Lai, K.-W. Huang, Chem. Eur. J. 2012, 18, 15940; d) T. Chen, H. Li, S. Qu,
B. Zheng, L. He, Z. Lai, Z.-X. Wang, K.-W. Huang, Organometallics 2014,
33, 4152.
[12] J. R. Pugh, M. R. M. Bruce, B. P. Sullivan, T. J. Meyer, Inorg. Chem. 1991,
30, 86.
[13] C. W. Machan, M. D. Sampson, C. P. Kubiak, J. Am. Chem. Soc. 2015, 137,
8564.
[14] D. H. Gibson, Y. Ding, J. G. Andino, M. S. Mashuta, J. F. Richardson, Orga-
nometallics 1998, 17, 5178.
[15] D. Benito-Garagorri, K. Mereiter, K. Kirchner, Eur. J. Inorg. Chem. 2006,
4374.
Keywords: electrocatalysts · electrochemistry · N,P ligands ·
redox · ruthenium
[1] a) W. Wang, S. P. Wang, X. B. Ma, J. L. Gong, Chem. Soc. Rev. 2011, 40,
3703; b) A. M. Appel, J. E. Bercaw, A. B. Bocarsly, H. Dobbek, D. L.
DuBois, M. Dupuis, J. G. Ferry, E. Fujita, R. Hille, P. J. A. Kenis, C. A. Ker-
feld, R. H. Morris, C. H. F. Peden, A. R. Portis, S. W. Ragsdale, T. B. Rauch-
fuss, J. N. H. Reek, L. C. Seefeldt, R. K. Thauer, G. L. Waldrop, Chem. Rev.
2013, 113, 6621; c) S. Saeidi, N. A. S. Amin, M. R. Rahimpour, J. CO2 Util.
2014, 5, 66; d) T. Sakakura, J.-C. Choi, H. Yasuda, Chem. Rev. 2007, 107,
2365; e) E. V. Kondratenko, G. Mul, J. Baltrusaitis, G. O. Larrazabal, J.
Perez-Ramirez, Energy Environ. Sci. 2013, 6, 3112; f) J. Qiao, Y. Liu, F.
Hong, J. Zhang, Chem. Soc. Rev. 2014, 43, 631.
[2] a) K. P. Kuhl, T. Hatsukade, E. R. Cave, D. N. Abram, J. Kibsgaard, T. F. Jara-
millo, J. Am. Chem. Soc. 2014, 136, 14107; b) H. A. Hansen, J. B. Varley,
A. A. Peterson, J. K. Nørskov, J. Phys. Chem. Lett. 2013, 4, 388.
[3] a) C. W. Li, M. W. Kanan, J. Am. Chem. Soc. 2012, 134, 7231; b) Y. Chen,
C. W. Li, M. W. Kanan, J. Am. Chem. Soc. 2012, 134, 19969; c) Q. Lu, J.
Rosen, Y. Zhou, G. S. Hutchings, Y. C. Kimmel, J. G. Chen, F. Jiao, Nat.
Commun. 2014, 5, 3242; d) S. Rasul, D. H. Anjum, A. Jedidi, Y. Minenkov,
L. Cavallo, K. Takanabe, Angew. Chem. Int. Ed. 2015, 54, 2146; Angew.
Chem. 2015, 127, 2174.
[4] a) E. E. Benson, C. P. Kubiak, A. J. Sathrum, J. M. Smieja, Chem. Soc. Rev.
2009, 38, 89; b) C. D. Windle, R. N. Perutz, Coord. Chem. Rev. 2012, 256,
2562.
[5] a) C. Costentin, S. Drouet, M. Robert, J.-M. SavØant, Science 2012, 338,
90; b) C. Costentin, M. Robert, J.-M. Saveant, Chem. Soc. Rev. 2013, 42,
2423.
[16] D. C. Grills, J. A. Farrington, B. H. Layne, J. M. Preses, H. J. Bernstein, J. F.
Wishart, Rev. Sci. Instrum. 2015, 86, 044102.
Manuscript received: October 14, 2015
Accepted Article published: October 29, 2015
Final Article published: November 13, 2015
[6] a) Z. Chen, C. Chen, D. R. Weinberg, P. Kang, J. J. Concepcion, D. P. Harri-
son, M. S. Brookhart, T. J. Meyer, Chem. Commun. 2011, 47, 12607; b) M.
ChemPlusChem 2016, 81, 166 – 171
www.chempluschem.org
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