[RuII(tpy)(bpy)Cl]+-Catalyzed reduction of carbon dioxide. Mechanistic insights by carbon-13 kinetic isotope effects

Article abstract of DOI:10.1039/c8cc03009j

In this work, we examine the use of competitive¹³C kinetic isotope effects (¹³C KIEs) on CO₂ reduction reactions that produce CO and formic acid as a means to formulate reaction mechanisms. The findings reported here mark a further advancement in the combined¹³C KIE measurements and theoretical calculations methodology for probing CO₂ conversion reactions.

Full text of DOI:10.1039/c8cc03009j

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II
+
[
Ru (tpy)(bpy)Cl] -Catalyzed Reduction of Carbon Dioxide.
Mechanistic Insights by Carbon-13 Kinetic Isotope Effect
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
,b
,a,c
T. W. Schneider, M. Hren, M. Z. Ertem,* and A. M. Angeles-Boza,*
DOI: 10.1039/x0xx00000x
www.rsc.org/
1
In this work, we examine the use of competitive C kinetic KIEs at natural abundance levels have been used to probe the
3
1
3
isotope effects ( C KIEs) on CO
CO and formic acid as a means to formulate reaction mechanisms. complexes during turnover conditions.
The findings reported here mark a further advancement in the gained from natural abundance KIEs measurements is
2
reduction reactions that produce mechanisms of small molecule activation by transition metal
1
1-15
The information
1
3
combined C KIE measurements and theoretical calculations especially powerful when coupled with theoretical
1
calculations. For example, O KIE determinations have been
8
methodology for probing CO
2
conversion reactions.
1
1-14
used to formulate mechanisms of O-O bond formation.
recently expanded the scope of this technique by probing the
mechanism of the photocatalytic CO reduction by the well-
known Lehn’s catalyst, Re(bpy)(CO) Cl (bpy = 2,2’-bipyridine).
We
Conversion of CO into useful chemicals such as CO, formic
2
acid, or methanol by activation/reduction is one of the most
important and interesting topics of research given modern
1
energy and environmental concerns. Since the 1970s, when
2
15
3
13
We now focus on using C KIEs in reactions in which more
than one product is formed, a very common outcome among
Aresta and Nobile synthesized and crystallographically
2
-4
characterized several metal complexes containing CO2,
number of catalytic systems capable of performing the
a
CO reduction catalysts.
2
As a proof-of-concept, we sought to study a system
5
activation of CO have been explored. These early results,
-7
2
capable of producing two distinct products using a robust
metal catalyst. A perusal of the literature indicated that the
combined with the fact that metal complexes are among the
most effective catalysts for many chemical reactions, has led
to extensive research effort toward developing metal-based
CO2 activation systems. Understanding the formation and
properties of activated metal−CO2 complexes will not only
unravel the basis of their complex reactivities but will also
guide further synthetic modifications for the development of
metal-based catalysts that are more efficient, inexpensive and
environmentally friendly.
II
catalyst precursor [Ru (tpy)(bpy)Cl]
+
(
1, tpy = 2,2’:6’,2”-
II
+
Fig.
1
Schematic representations of [Ru (tpy)(bpy)Cl] (1) and
A
variety of experimental tools and computational
II
+
[Ru (tpy)(bpy)H] (2)
methods have been applied to study CO activation; however,
2
fast reaction rates and elusive reaction intermediates have terpyridine) is the ideal model as
1 and its derivatives have
made it difficult to establish rate-determining steps (rds) and been shown to produce CO in electrochemical reductive
16-18
probe catalytic mechanisms. Heavy atom isotope effects is a processes,
2
and it is very likely that 1 can reduce CO under
technique that, historically, has been routinely used to afford photochemical conditions in the presence of a sacrificial
5
insight into the nature of the transition state for the rds in electron donor similarly to other ruthenium(II) photocatalysts.
8
-10
II
+
enzyme catalysis.
However, in recent years, competitive In addition, the hydride derivative of
1, [Ru (tpy)(bpy)H] (2)
1
9-
can produce formate upon CO
2
2
insertion into the Ru-H bond.
2
Moreover, Matsubara et al. demonstrated that
II
2 can be
+
formed by irradiation of [Ru (tpy)(bpy)(DMF)] (DMF = N,N-
dimethylformamide) in the presence of triethylamine with
2
3
moderate yields. Therefore, we hypothesized that a solution
of in the presence of triethanolamine (TEOA) and CO will
produce both CO and formate.
1
2
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Photochemical formation of CO and formate. In a typical run, reactions under “dry” conditions and with 1% added water
1
3
a solution of
containing 1 equivalent of [Ru(bpy)
with 100% CO was irradiated under visible light. RuTB is a
photosensitizer with an outer sphere electron transfer rate
1
in a 5:1 acetonitrile/TEOA solvent mixture resulted in C KIE values of 1.052 ± 0.004 and 1.044 ± 0.006,
3
]Cl (RuTB), and saturated respectively.
2
2
ꢁꢂ
1
ꢇꢈꢉ
2
4
ln ꢀ ꢄ ꢅ ꢀ1 ꢆ
ꢁꢃ
ꢄ lnꢊ1 ꢆ ꢋꢌꢍꢍꢍꢍꢍꢍꢍꢍꢍꢍꢍꢍꢍꢍꢍꢍꢍꢊ1ꢌ
near that of the diffusion limit. We observed that this
photosensitizer was needed for to exhibit catalytic behavior
1
under the reaction conditions. The irradiation times ranged
from 2 to 24 hours. Formation of CO was determined using gas
chromatography (GC) (Figure S1). As hypothesized, we
observed the formation of formate which was quantified by
Theoretical Investigation of the Photocatalytic Reduction of
CO by 1. Density Functional Theory (DFT) calculations at the
2
27
M06 level of theory with the SMD continuum solvation
28
model were performed to investigate the photocatalytic CO2
1
H-NMR, using a method involving Verkade’s Base, as was
recently described by Kubiak and coworkers (Figures S2 and 13
reduction mechanism by 1, and to compute the corresponding
C isotope effects of the optimized structures in the catalytic
cycle. The proposed catalytic cycle and the computed free
2
5
S3).
The yields for both CO and formic acid were calculated
and are presented in Figure 2a in terms of turnover number
2
energy changes (ꢀG) for the reduction of CO by 1 under
(TON). Experiments were also ran in the presence of 1% H O
2
catalytic conditions are presented in Scheme 1. Optimized
structures are in Table S1 in the Supporting Information. The
various branching pathways which could lead to potential side
products are shown in Scheme S1.
(
Figure S4).
1
3
Experimental Kinetic Isotope Effect. The C KIEs for CO2
reduction were determined using a previously established
1
5,26
competitive methodology.
photocatalytic conditions involved a solvent mixture that was
:1 acetonitrile/TEOA. An additional set of experiments
CO2 reduction under
The proposed mechanism for the reduction of CO
starts with a pair of one-electron reductions, the first with a
potential of 1.51 V, and the second with a potential of 1.99
2
to CO
5
−
−
contained 1% H O in a 5:1 acetonitrile/TEOA solution. The
2
V vs SCE (Scheme 1). The second reduction leads to the
solutions were saturated with a gas mixture that was 3% CO2
dissociation of chloride ion with an activation free energy
in N . This mixture was then injected into a reaction vessel
2
‡
G ) of 5.4 kcal/mol, and a free energy change (
(
ꢀ
ꢀ
kcal/mol, generating the neutral two-electron reduced [Ru]
G) of
−
7.5
0
containing equimolar amounts of
mixture was irradiated under a visible light source (λ ≥ 400 nm,
000 V FEL bulb with band-pass filter) while stirring. After
1
and RuTB. The reaction
.
0
‡
The binding of CO
2
molecule to [Ru] proceeds with ꢀG = 9.3
1
kcal/mol and this process involves net two electron transfer
16
catalysis had been performed, the remaining CO was isolated.
2
from the complex to CO
0
2
,
resulting in the formation of
The isolation process involved a series of cold traps designed
to separate the CO from such impurities as N , CO, or solvent
[
Ru
−CO ] . We also considered the possibility of the chloro
2
0
Cl]
2
2
ligand dissociating from the one-electron reduced [Ru
−
+
vapor. The pressure of the remaining CO was measured with a
2
species, and the subsequent binding of CO
‡
2
to the [Ru] but
digital manometer, and then frozen and sealed into a glass
the calculated
ꢀG s were significantly higher than those steps
involving their two-electron reduced counterparts, and their
products were much less stable (Scheme S1). The generated
0
[
Ru
−
CO
2
]
can follow two reactions pathways. The first one is
molecule to the complex, which
would result in the formation of one equivalent of CO and
the addition of a second CO
2
2
-
‡
CO
the protonation of [Ru
process is barrierless, and has
). Due to the fact that the addition of a proton both has lower
3
(ꢀG = 24.1 kcal/mol). The second possible pathway is
0
+
−
CO
2
]
forming [Ru
−C(O)OH] . This
ꢀ
G of 20.9 kcal/mol (Scheme
−
1
Fig. 2 (A) CO (red circles) and formic acid (blue triangles) are formed
activation energy requirements and results in a more stable
product, we are confident that this protonation step is the
during the photocatalytic reduction of CO
are shown with error bars representing standard deviations from two
measurements each. (B) Isotope fractionation of CO during its
2
catalyzed by 1. Data points
more likely of the two.
+
C(O)OH] can then either (i) react with another CO
2
[
Ru−
2
-
‡
reduction catalysed by 1. Data points are shown with error bars
molecule, leading to the production of CO and HCO
3
(ꢀG =
representing standard errors.
2
8.6 kcal/mol) or (ii) react with a proton donor with
+
concomitant cleavage of C—OH to generate [Ru
−
CO] and a
G = 16.3 and 24.1 kcal/mol respectively for
2
O as the proton source ) or (iii) alternatively
‡
water molecule (
ꢀ
ampule. The isotope ratio of each CO
with an isotope ratio mass spectrometer (IR-MS).
Figure 2b shows the plot of ln(R /R ) vs ln(1-f) for CO
reduction by the catalytic system, in which f is the fraction of
CO which has been catalytically reduced, and R and R are the
before and after fractionation,
2
sample was measured
+
TEOAH and H
further reduced to generate [Ru
reaction conditions (e.g., the pK
competition between pathways (ii) and (iii) is predicted to
0
−
C(O)OH] . Depending on the
of proton source) a
f
0
2
a
‡
2
0
f
exist whereas the
ꢀG associated with (i) is prohibitively high
1
2
13
C/ C isotope ratios of CO
2
for this reaction route to be relevant.
respectively. Using equation 1, we determined that the
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+
0
13
Similarly to [Ru−C(O)OH] , [Ru−C(O)OH] can react with a weighted average of the C KIEs for different pathways of CO
2
-
‡
CO
2 3
molecule to generate CO and HCO (ꢀG = 24.0 kcal/mol), reduction leading to the formation of CO and formic acid as
3
0
or C—OH bond cleavage could occur assisted via either represented in equation 2, where x is the fraction of CO
2
that
+
‡
‡
TEOAH (ꢀG = 9.7 kcal/mol) or H O (ꢀG = 17.4 kcal/mol) as was reduced to CO and (1-x) is the fraction reduced to formic
2
the proton source. Again, C—OH bond breakage facilitated by acid.
+
TEOAH is the most likely step based on computed activation
+
free energies. The final common product [Ru—CO] for all
ꢇꢈꢉꢎꢏꢐ ꢅ ꢑꢍꢇꢈꢉꢂꢎꢒꢓꢔꢕꢍꢖꢕꢔꢗ ꢘ ꢊ1 ꢆ ꢑꢌꢇꢈꢉꢙꢚꢍꢍꢍꢍꢍꢍꢍꢍꢊ2ꢌ
different pathways is thought to evolve CO via further
0
reduction reactions regenerating the reactive [Ru] species
The fraction of formic acid under the employed reaction
(Scheme 1).
conditions is approximately 80% (Table S2).
In addition to the reduction of CO
2
to CO, production of
Natural abundance level competitive KIEs include the
formic acid was also observed in our experiments and the isotope effects in mechanistic steps up to first irreversible step
proposed catalytic cycle for the production of formic acid is in catalytic reactions during turnover conditions. The proposed
presented in Scheme 1. The proposed mechanism starts with catalytic cycles indicate that for formic acid generation via the
0
+
the protonation of vacant site on ruthenium center of [Ru] by electrophilic attack of CO to [Ru
2
−
H] is the first irreversible
0
+
‡
+
TEOAH (
ꢀ
G = 5.1 kcal/mol) to generate [Ru
G =
pK for [Ru] is 31.2. The second step involves an electrophilic
−
H] species. This step whereas CO2 binding to [Ru] species is the first
step is highly exergonic (
ꢀ
−28.3 kcal/mol) and computed irreversible step for the CO evolution pathway. The computed
0
13
C KIEs are KIEformic acid = 1.055 and KIECO = 1.068 for the above-
a
+
H]
‡
attack by CO to [Ru
2
−
(
ꢀ
G
= 13.2 kcal/mol) to form mentioned steps. The weighted average according to equation
+
[
Ru−OCHO] (ꢀG = −0.2 kcal/mol), which represents the rate 2 is KIEcalc = 1.058 which is in good agreement with the
limiting step for the formate production pathway. Subsequent experimentally observed value of KIEexpt = 1.052 ± 0.004 and
1
reduction steps will release formate and result in the 1.044 ± 0.006. We also computed C KIEs for several
3
0
formation of the reactive [Ru] species (Scheme 1).
optimized transition state structures associated with different
The competition between protonation versus binding of pathways and alternative assumptions for the first irreversible
0
CO to [Ru] species will determine the product selectivity of
1
step for CO evolution pathway which are presented in the
2
1
towards generation of formic acid and CO respectively. The supporting information. Interestingly, the C KIE for the CO
3
‡
computed ΔG s are close for the protonation (ΔG = 5.1 pathway resembles the values found for the photochemical
‡
kcal/mol) and CO binding (ΔG = 9.3 kcal/mol) steps favoring CO reduction catalyzed by Re(bpy)(CO) Cl (~1.07) determined
‡
2
2
3
1
5
the former pathway. Moreover, the relative concentration of under similar conditions to those used in the current study.
the proton source and CO as well as the pK of the proton Within the context of Transition State Theory, the
significant influence on relative calculated KIEs can be further analyzed to provide insights into
production yields of formic acid and CO. Under the current the contributions to the isotopic discrimination by the various
2
a
source will impose
a
1
3
13
13
15,30
experimental conditions, the yields of formic acid are larger terms, namely, νRC and KTS ( K = ZPE × EXC × VP).
The
computed isotope effects and individual terms indicate that
Interpretation of the competitive carbon-13 kinetic isotope for both the CO -binding (CO pathway) and CO addition to
TS
than the yields of CO.
2
2
2
1
3
13
13
effect. The experimentally observed C KIE represents a (HCO H pathway) νRC represents half of the C KIEs observed
2
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1
3
(
ν
RC ≈ 1.03) and large ZPE terms are responsible for the 13. A. M. Angeles-Boza and J. P. Roth, Inorg. Chem., 2012, 51,
1
magnitude of the pseudoequilibrium constant ( KTS) values,
3
4
722-4729.
1
1
1
4. Khan, S.; Yang, K. R.; Ertem, M. Z.; Batista, V. S.; Brudvig, G.
W. ACS Catal. 2015, 5, 7104−7113.
5. T. W. Schneider, M. Z. Ertem, J. T. Muckerman and A. M.
Angeles-Boza, ACS Catalysis, 2016, 6, 5473-5481.
6. Z. Chen, C. Chen, D. R. Weinberg, P. Kang, J. J. Concepcion, D.
P. Harrison, M. S. Brookhart and T. J. Meyer, Chem. Commun.,
1
.038 and 1.026 for the CO pathway and HCO
1
2
H pathway,
3
respectively. Remarkably, slightly larger
KTS values, c.a. 1.05,
were calculated for CO
electron reduced species generated from Re(bpy)(CO)
originating from a large contribution from the ZPE term.
2
binding by the one-electron and two-
3
Cl also
1
3
In summary, we demonstrated that C KIEs in combination
with theoretical calculations can be used to study CO
reduction reactions in which two products are formed, and
2
011, 47, 12607-12609.
2
1
7. T. A. White, S. Maji and S. Ott, Dalton Transactions, 2014, 43,
15028-15037.
more importantly, we showed the detailed analysis of the 18. B. A. Johnson, S. Maji, H. Agarwala, T. A. White, E. Mijangos
determined values. We found that the first irreversible step in
2
the CO pathway involves substrate binding to . In the HCO H
and S. Ott, Angew. Chem. Int. Ed., 2016, 55, 1825-1829.
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Saitoh and O. Ishitani, Inorg. Chim. Acta, 2000, 299, 155-163.
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1
2
2
1
1
pathway, the results produced a large normal C KIE for CO
3
2
insertion into the Ru-H bond with a more reactant-like
transition state structure. This study serves as a reference
point for mechanisms associated with Ru-catalyzed CO
1
2
3
transformations and demonstrates that C KIEs coupled with
theoretical calculations is a powerful method to investigate
2
2
2
the mechanism of CO reduction reactions.
2
4. R. C. Young, R. Keene and T. J. Meyer, J. Am. Chem. Soc., 1977,
99, 2468-2473.
AMAB acknowledges the financial support from the National Science
Foundation CAREER grant (CHE-1652606). The work at BNL (M.Z.E) was 25. P. L. Cheung, C. W. Machan, A. Y. Malkhasian, J. Agarwal and C.
P. Kubiak, Inorg. Chem., 2016, 55, 3192-3198.
carried out with support from the U.S. Department of Energy, Office of
2
6. T. W. Schneider and A. M. Angeles-Boza, Dalton Trans, 2015,
4, 8784-8787.
7. Y. Zhao, D. G. Truhlar, Acc. Chem. Res. 2008, 41, 157-167.
Science, Division of Chemical Sciences, Geosciences & Biosciences,
Office of Basic Energy Sciences under contract DE-SC0012704
4
2
2
8. A. V. Marenich, C. J. Cramer, D. G. Truhlar, J. Phys. Chem. B
2
009, 113, 6378-6396.
Conflicts of interest
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3
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There are no conflicts to declare.
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13
C kinetic isotope effect determinations combined with DFT calculations provide insight
on the CO reduction reaction catalyzed by a ruthenium complex.
2