A Molecular Ruthenium Electrocatalyst for the Reduction of Carbon Dioxide to CO and Formate

Article abstract of DOI:10.1021/jacs.5b03913

The use of a bulky bipyridine ligand, 6,6′-dimesityl-2,2′-bipyridine (mesbpy), to enable the reduction of carbon dioxide by a Ru-based molecular electrocatalyst is reported. Under catalytic conditions, this compound exhibits turnover frequencies of 1300 s^-1 and 95% Faradaic efficiency for the production of CO and H₂O from CO₂ in the presence of Br?nsted acids. Mechanistic electrochemical and spectroelectrochemical studies, supplemented by the direct synthesis of relevant intermediates, indicate that this behavior is the result of the cooperative redox response of the bipyridine ligand and Ru metal center at negative potentials, as well as the inhibition of Ru-Ru bond formation through steric interactions.

Full text of DOI:10.1021/jacs.5b03913

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Article
A Molecular Ruthenium Electrocatalyst for the
Reduction of Carbon Dioxide to CO and Formate
Charles W. Machan, Matthew D. Sampson, and Clifford P. Kubiak
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b03913 • Publication Date (Web): 18 Jun 2015
Downloaded from http://pubs.acs.org on June 21, 2015
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A Molecular Ruthenium Electrocatalyst for the Reduction of Carbon Dioxide to CO and
Formate
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Charles W. Machan, Matthew D. Sampson, and Clifford P. Kubiak*
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University of California – San Diego, Department of Chemistry and Biochemistry, 9500 Gilman
Drive 0358, La Jolla, CA 92023
* ꢀ ckubiak@ucsd.edu
Abstract
The use of a bulky bipyridine ligand, 6,6ʹꢀdimesitylꢀ2,2ʹꢀbipyridine (mesbpy), to enable the
reduction of carbon dioxide by a Ruꢀbased molecular electrocatalyst is reported. Under catalytic
conditions, this compound exhibits turnover frequencies of 1300 s–1 and 95% Faradaic efficiency
for the production of CO and H2O from CO2 in the presence of Brønsted acids. Mechanistic
electrochemical and spectroelectrochemical studies, supplemented by the direct synthesis of
relevant intermediates, indicate that this behavior is the result of the cooperative redox response
of the bipyridine ligand and Ru metal center at negative potentials, as well as the inhibition of
Ru–Ru bond formation through steric interactions.
Introduction
The eventual depletion of petrochemical feedstocks and rising levels of carbon dioxide
(CO2) continue to drive interest in the use of CO2 as a C1 source for liquid fuels.1ꢀ6
Electrocatalytic reduction offers a way to close the fuel cycle by using CO2 in the synthesis of
energyꢀdense molecules.1,6 If done using renewable energy sources, this process could enable the
storage of electrical energy in chemical bonds in an efficient and recoverable manner. Molecular
catalysts have been shown to be active and selective for the electrocatalytic reduction of CO2 to
CO, but often have limited turnover frequencies (TOFs), high overpotentials, and poor stabilities.
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Recent progress has been made in benchmarking different catalysts for CO2 reduction, however
this is still a work in progress.7ꢀ10 To the limited extent to which the rates of molecular
electrocatalysts can be compared, the ‘stateꢀofꢀtheꢀart’ catalysts reported so far are Re and
Mn(bpy)(CO)3X (bpy = 2,2ʹꢀbipyridine, X = halide or a weakly coordinating anion),
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hydroxyphenylꢀsubstituted Fe porphyrins, and Ni(cyclam)2+ (cyclam
=
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tetraazacyclotetradecane).10ꢀ14 Generally, molecular Ruꢀbased electrocatalysts have shown much
lower activities.11,15,16 Under reducing conditions, complexes of the type Ru(bpy)(CO)2Cl2 have
been shown to polymerize on the cathode through the formation of Ru–Ru bonds.17ꢀ19 The
resultant metallopolymer is highly active for the reduction of CO2 to CO as a heterogeneous
catalyst. Our group has shown that the use of a bulky bpy ligand, 6,6´ꢀdimesitylꢀ2,2´ꢀbipyridine
(mesbpy, 1), inhibited metalꢀmetal bond formation in the case of Mn tricarbonyl complexes,
resulting in catalysts with TOFs in excess of 5000 s–1.13 Recently, Kuramochi et al. also reported
that the use of the same bulky bpy ligand (mesbpy), instead of an unfunctionalized bpy ligand,
for complexes of the type Ru(bpy)(CO)2Cl2 resulted in the selective photocatalytic production of
CO from CO2.20 We reasoned that the use of the mesbpy ligand in the context of Ruꢀbased
molecular catalysts could have a similar effect on their electrocatalytic properties.
Results
Metallating mesbpy21 with the Ru(II) carbonyl precursor [Ru(CO)2Cl2]n22 (2) in refluxing
toluene resulted in the isolation of transꢀClꢀRu(mesbpy)(CO)2Cl2 (3) as a pale yellow powder
(Scheme 1).20 Xꢀray crystallographic studies on single crystals grown from THF and pentanes
show an octahedral coordination geometry around the Ru metal center, with the chloride ligands
occupying axial positions, the bpy fully chelated in the equatorial plane, and two CO ligands at
the remaining coordination sites (Figure 1, Table S1).17,18,20 All other characterization data are
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consistent with this assignment. The plane of the bpy is slightly distorted: the N–C–C–N dihedral
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angle is 16.9(4)°.
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Scheme 1. Synthesis of 3 from 1 and 2.
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Figure 1. Molecular structure of 3 obtained from singleꢀcrystal Xꢀray diffraction studies; Ru =
cerulean, C = grey, N = blue, Cl = green, O = red; thermal ellipsoids at 50%, H atoms omitted for
clarity.
An initial electrochemical survey under Ar saturation showed two reduction features at
negative potentials; the first at –1.69 V vs Fc/Fc+ is irreversible, the second at –1.86 V vs Fc/Fc+
is quasiꢀreversible (Figure 2, ꢁEp = 0.61 V; see Supporting Information). The current response
of the first and second reduction features show a dependence on scan rate consistent with a freely
diffusing molecule (Figures S1, S2, and S3). Based on these observations, we assign the first
reduction to a bpyꢀbased reduction of 3 to [RuII(mesbpy●–)(CO)2Cl2]–, followed by Cl loss
through a ligandꢀtoꢀmetal charge transfer (LMCT) to yield [Ru(mesbpy)(CO)2Cl]0, before a
second bpyꢀbased reduction to generate [RuI(mesbpy●–)(CO)2Cl]–. The titration of phenol
(PhOH) under Ar saturation revealed a new reduction feature (Ep = –2.12 V vs Fc/Fc+, 3H+,
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Figures 2 and S4). The potential of the first reduction shows a dependence on PhOH
concentration, which is indicative of a protonation step coupled to electron transfer (Figure
S5).13,23ꢀ25 The observed current response at the third reduction is consistent with secondꢀorder
kinetic dependence on PhOH concentration (Figure S6). Bulk electrolysis experiments suggest
that this new feature (3H+, Figure 2) represents the catalytic reduction of PhOH to H2 (Table
S2). No other gaseous products were detected by GC. Based on these observations, we assign the
first feature (1, Figure 2) to an initial reduction followed by the formation of a cationic Ru–H,
[RuIII(mesbpy)(H)(CO)2Cl]+, through a protonation by PhOH. Using the concentration of added
PhOH and the corresponding shift in potential it is possible to determine a binding constant for
H+ by Ru of KH+ = 4.3 ± 1.4 M–1 under these conditions (Figure S5).13,23ꢀ25 The subsequent
reduction of this cationic species (2, Figure 2) is expected to generate RuII(mesbpy)(H)(CO)2Cl.
The third feature (3H+, Figure 2) corresponds to the reduction of RuII(mesbpy)(H)(CO)2Cl to
generate a species, presumably [RuII(mesbpy●–)(H)(CO)2Cl]–, which has sufficient hydricity to
react with a proton and form H2. We note that catalytic H2 production from PhOH on glassy
carbon electrodes does not occur until –2.60V vs. Fc/Fc+.26
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Figure 2. Cyclic voltammograms of 3 under Ar saturation (black), Ar saturation with added
PhOH (red), and CO2 saturation with added PhOH (blue). First, second, and third reduction
waves labeled for clarity. Conditions: 1 mM 3; 0.1 M TBAPF6/MeCN; glassy carbon working
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electrode, Pt counter electrode, Ag/AgCl pseudoꢀreference electrode; referenced to internal
ferrocene standard; scan rate 100 mV/s.
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Sparging the solution to saturation with CO2 in the presence of added PhOH altered all
three of these reduction features (Figure 2). Although reactivity was observed with CO2 only,
the adventitious addition of trace amounts of H2O through CO2 sparging precluded isolation of
the pure CO2 response (Figure S7). As was the case under Ar saturation, the first reduction wave
showed a positive potential shift with titration of PhOH under fixed CO2 concentration (0.28 M,
Figure S8). The second reduction feature, however, did not shift as the concentration of added
PhOH increased. Varying CO2 concentration under a fixed concentration of PhOH (0.28 M)
showed that only the second reduction potential was dependent on CO2 concentration (Figure
S9). Note that in the presence of CO2, the third reduction wave (3CO2, Ep = –2.05 V vs Fc/Fc+;
Figure 2) is both shifted >100 mV more positive than the feature in the presence of PhOH only
(3H+) and there is a significant increase (~3x) in the catalytic current density. The titration of
PhOH under CO2 saturation (~0.28 M) also revealed that the current dependence of the third
reduction feature was second order in [PhOH] (Figures S10 and S11). Bulk electrolysis under
CO2 saturation with added PhOH confirmed that at the third reduction (~–2.2 V vs Fc/Fc+), this
system was catalytic, showing 95% current efficiency for CO production through 5.2 turnovers
(relative to moles of 3 in solution, Table S2). The formation of H2 was negligible by GC (~1%).
When bulk electrolysis was repeated at the second reduction potential (~–1.7 V vs Fc/Fc+), the
observed Faradaic efficiency for CO dropped to 63 ± 16% (H2 accounted for 1.8 ± 0.8% of
current passed under these conditions) and HCO2H was observed by NMR (Figure S12).
Insufficient current was observed at more positive potentials, precluding accurate product
analysis. This indicates that the relative production of CO or HCO2H is dependent on the applied
potential. Similar effects on the ratio of CO to HCO2H production have been observed for the
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Ru(bpy)ꢀbased metallopolymer when the applied potential was varied.27 Similar to the results
under Ar, these data are consistent with the assignment of the first feature to an initial reduction
followed by a protonation by PhOH to form [RuIII(mesbpy)(H)(CO)2Cl]+. The shift in potential
at the first reduction feature when PhOH was varied approximates to a binding constant for H+
by Ru of KH+ = 32 ± 9.2 M–1 (Figure S8).23ꢀ25 A Pourbaix diagram generated from this data
exhibits a slope of approximately 60 mV/pKa unit of PhOH under CO2 saturation, which is also
indicative of a 1e–/1H+ process (Figure S13).28ꢀ30 The increase in this binding constant under
CO2 from the value obtained under Ar is the result of Nernstian shift from the favorable
subsequent insertion of CO2 into the Ru–H bond, vide infra. The second reduction is then
expected to generate RuII(mesbpy)(H)(CO)2Cl, which the data suggests undergoes a reaction
with CO2. The product of this reaction is expected to be a species similar to RuII(mesbpy)(η1ꢀ
OCOH)(CO)2Cl.31 A binding constant for CO2 can be calculated from the change in potential at
the second reduction under a fixed concentration of PhOH: KCO2 = 28 ± 5.0 M–1 with 0.28 M
PhOH (Figure S9). Based on a comparison of icat/ip at the third reduction, which is proportional
to the electrocatalytic rate constant and TOF, this catalyst is estimated to have a TOF for CO2
reduction to CO of 1300 s–1 with 0.91 M PhOH (see Supporting Information for details).13 The
third reduction presumably corresponds to the reduction of RuII(mesbpy)(η1ꢀOCOH)(CO)2Cl to
[RuII(mesbpy●–)(η1ꢀOCOH)(CO)2Cl]–, which we did not expect to catalytically produce CO. In
order to test these assignments more carefully, we explored the behavior of 3 further using
spectroelectrochemistry.
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Infrared spectroelectrochemistry (IRꢀSEC) experiments, which allow for characterization
of changes in the IR signature with respect to potential and time, were used to investigate the
behavior of 3 at negative potentials with and without substrate present.32ꢀ34 At resting potentials
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under N2 saturation, two IR bands are observed for 3 in TBAPF6/MeCN at 2058 and 1995 cm–1
(Figure 3).17,18,20 When the applied negative potential is increased stepwise to the first reduction
observed by CV (~–1.5 V vs Fc/Fc+), these bands are replaced by two new bands at 2014 and
1939 cm–1 (with other minor species, vide infra). We assign this primary species to
[Ru(mesbpy)(CO)2(MeCN)Cl]0, which results from a singleꢀelectron reduction, chloride loss
from the parent species 3, RuII(mesbpy)(CO)2Cl2, and MeCN solvent coordination. This
assignment is consistent with the irreversible reduction observed by CV. The other minor bands
observed at this potential likely correspond to two species: the sixꢀcoordinate anion,
[RuII(mesbpy●–)(CO)2Cl2]–, which forms before Cl dissociation to form the major species, and
the fiveꢀcoordinate, neutral complex, [Ru(mesbpy)(CO)2Cl]0, which forms before solvent
coordination. These assignments are supported by chemical reduction experiments with
potassiumꢀintercalatedꢀgraphite (KC8, Figure S14). Other constitutional isomers, i.e. CO ligands
at axial positions and solvent coordination in various positions, can occur upon losing the Cl
ligand.17ꢀ19,35ꢀ37 Increasing the applied negative potential again to that of the second reduction (~–
1.7 V vs Fc/Fc+) resulted in the appearance of two major bands at 1904 and 1884 cm–1. These
bands are consistent with a fiveꢀcoordinate, anionic complex, [RuI(mesbpy●–)(CO)2Cl]– (see
Figure S14 for more information on this assignment). This bpyꢀbased reduction is consistent
with the reversible redox feature observed by CV. The other minor bands observed at this second
reduction potential could be results of the following: incomplete reduction to the doublyꢀreduced
complex, other isomers (as previously discussed), and MeCN solvent coordination.35ꢀ37
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Figure 3. IRꢀSEC of 3 under N2 saturation showing the species at resting potential (black), the
first reduction (red), and the second reduction PhOH (blue). Conditions: 3.5 mM 3; 0.1 M
TBAPF6/MeCN; Pt working electrode, Pt counter electrode, Ag pseudoꢀreference electrode.
A second set of experiments with added PhOH (0.5 M) under N2 saturation conditions
indicated that at applied potential a reaction with PhOH was occurring. At resting potential,
bands consistent with the starting material 3 were observed at 2058 and 1995 cm–1 (Figure S15).
When the negative potential of the IRꢀSEC cell was increased stepwise to that of the first
reduction observed by CV (~–1.5 V vs Fc/Fc+), these bands diminished in intensity and were
replaced by a species with absorbances at 2051 and 1984 cm–1, with a minor species also
observable at 2031 and 1960 cm–1. The former is tentatively assigned to
[RuIII(mesbpy)(H)(CO)2Cl]+, vide infra, and the latter is assigned to [RuII(mesbpy●–)(CO)2Cl2]–
(Figure S14).35,36 If the cell potential is increased again to more negative potentials
corresponding to the second reduction (~–1.7 V vs Fc/Fc+), the major species observable by IR
becomes RuII(mesbpy)(H)(CO)2Cl, with intense bands at 2029 and 1948 cm–1, consistent with
previous reports.35,38 Although the shift in the observed carbonyl frequencies from 2051 and
1984 cm–1 to 2029 and 1948 cm–1 is not necessarily diagnostic of a change in oxidation state, it is
clear from these experiments that the former is a product of a RuI reaction with H+ and from
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literature precedent and direct synthesis that the latter represents a RuII–H species, vide infra (see
Experimental).
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When these experiments were repeated under CO2 saturation with added PhOH (0.5 M),
bands were observed which are consistent with the initial formation of a Ru–H species followed
by rapid CO2 insertion (Figure 4). At the first reduction potential (~–1.5 V vs Fc/Fc+), two
species are observed, with the first set of bands at 2050 and 1983 cm–1 and the second set of
bands at 2032, 1958, and 1949 cm–1. Over the course of 5 minutes the system equilibrates almost
entirely to the second of these two species at this potential. Based on the previously described
experiments, the former is tentatively assigned to [RuIII(mesbpy)(H)(CO)2Cl]+ and the latter is
assigned to RuII(mesbpy)(η1ꢀOCHO)(CO)2Cl (Figure S16).31 We attribute the observation of
products from an additional reduction at this potential to the Nernstian shift of the second
reduction potential from the favorable binding of CO2 by RuII–H. The region where Ru–(η1ꢀ
OCHO) adducts are expected (1500ꢀ1600 cm–1) is obscured by the strong solvent
absorbance.39,40 At the second reduction potential (~–1.7 V vs Fc/Fc+) new bands appear at
2016, 1989, 1945, and 1890 cm–1, which are the only observable IR features at the second
reduction potential and after the third reduction potential (~–2.0 V vs Fc/Fc+, Figure 2). These
species are likely isomers of the type [RuII(mesbpy●–)(η1ꢀOCOH)(CO)2L]X or [RuI(mesbpy)(η1ꢀ
OCOH)(CO)2L]X, where L = MeCN or Cl (Figure S17).35ꢀ37
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Figure 4. IRꢀSEC of 3 under CO2 saturation with added PhOH (0.5 M) showing the species at
resting potential (black) and those which grow in at the first reduction (red to blue) over the
course of 5 minutes. Conditions: 8.6 mM 3; 0.1 M TBAPF6/MeCN; glassy carbon working
electrode, Pt counter electrode, Ag pseudoꢀreference electrode.
Discussion
–
In general, the reduction of CO2 proceeds by either a hydroxycarbonyl (a) or a HCO2 (b)
bound intermediate (1).
(1)
Mechanistic studies show that a hydroxycarbonyl complex is usually obtained by direct
–
interaction of a reduced metal center with CO2, followed by protonation.41 A HCO2 adduct is
usually produced by a metal hydride reacting with CO2 to produce the C–H bond, followed by
rearrangement to the Oꢀbonded species, b.42,43 To our knowledge, there exists no detailed
discussion concerning the potential isomerization between a and b. The results described here
suggest that the relative thermodynamic stabilities of a and b can depend on the redox states of
the metal ions involved. Indeed, Ru(bpy) complexes have previously been reported to
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electrocatalytically or photocatalytically produce mixtures of HCO2 /HCO2H and CO from CO2
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with varying efficiencies.17,18,20,27,44ꢀ47 Our proposed reduction mechanism for the electrocatalytic
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process to HCO2 is shown in Scheme 2. The irreversibility of the observed CV response of 3 (i)
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at the first reduction is indicative of a bpyꢀbased reduction followed by a fast ligandꢀtoꢀmetal
charge transfer (LMCT) resulting in Cl loss, yielding [Ru(mesbpy)(CO)2Cl(L)]0 (ii). This
interpretation of the reduction feature is also supported by DFT calculations, which suggest that
the LUMO of i is localized on the π* orbital of the bpy ligand (Figure S18).
[Ru(mesbpy)(CO)2Cl(L)]0 reacts with H+ to generate [RuIII(mesbpy)(H)(CO)2Cl]+ (iii).
Additional DFT calculations on the presumptive 5ꢀcoordinate intermediate generated by L loss
from ii, [Ru(mesbpy)(CO)2Cl]0, indicate a cooperative metalꢀligand redox response; the added
charged in the SOMO is distributed between the dz2 orbital of the metal center and the π* orbital
of the bpy ligand (Figure S19). Under applied potential under CO2 saturation, however, this
species is rapidly reduced to RuII(mesbpy)(H)(CO)2Cl, iv. An insertion reaction involving CO2
and iv generates RuII(mesbpy)(η1ꢀOCHO)(CO)2Cl, v. Bulk electrolysis at this potential shows
–
that both CO and HCO2 /HCO2H are generated. For CO production to occur, a dehydration
reaction must take place involving the formato species: Ru complexes have previously been
observed to catalyze the dehydration of HCO2H.22,48ꢀ50 Interestingly, the synthesis of the Ru(II)
precursor used in these studies seems to rely on a similar mechanism to generate [Ru(CO)2Cl2]n
from RuCl3, HCl (aq.), and HCO2H.22
Scheme 2. Proposed overall mechanism for the reduction of CO2 to CO and H2O or HCO2H by
3, Ar = mesityl, L = MeCN.
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If an additional reduction reaction involving v takes place, the dehydration reaction is
accelerated, as evidenced by the increased current response for CO production observed at the
third reduction feature by CV. Current efficiency for CO is almost quantitative at this potential,
suggesting an efficient interꢀconversion of HCO2 and H+ to CO and H2O. Generating CO from
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solvated HCO2H is predicted to be favored over a decarboxylation reaction at room temperature
as a result of the favorable solvation effects of producing H2O as a coꢀproduct.51 The necessity of
additional electrochemical driving force to enhance the catalytic response in a similar manner
has been previously observed in other molecular electrocatalysts for CO2 reduction.13,52 It is also
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noteworthy that a RuII(EDTA) complex has even been shown to convert HCO2 and HCO2H to
CO.53
A mechanism for the Ruꢀcatalyzed dehydration reaction of HCO2H has not yet been
proposed. Previous reports have shown that a carboxylate ester of an analogous compound,
RuII(bpy)(η1ꢀC(O)OCH2CH2OH)(CO)2Cl, could be converted to [RuII(bpy)(CO)3Cl]+ in the
presence of HCl.36 A carboxylate intermediate would be consistent with other observations and
proposed mechanisms involving Ru(bpy) complexes, although a Ru–[η1ꢀC(O)OH] species is not
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observed here experimentally.20,27,54,55 Although many mechanisms could allow for this, we
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favor the one proposed below for the conversion of the HCO2 adduct to CO (Scheme 3).
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The Oꢀbound HCO2 species vi is capable of a hydride elimination reaction, to yield a Ru
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hydride with a [CO2] molecule in close proximity, vii. A concomitant LMCT generates a RuI
metal center that can immediately reduce CO2 to form a hydroxycarbonyl species, viii. Species
viii is expected to be weakly hydridic as a result of the RuIII oxidation state. It is therefore likely
for a proton shift to occur, especially when considering the expected basicity of the bound
hydroxycarbonyl. Such a protonation reaction would generate a RuI–[η1ꢀC(O)OH] species, ix,
which would favor protonation by an external acid to cleave the C–O bond and lose H2O. As an
octahedral 19ꢀelectron complex the resulting RuI species, x, should rapidly lose CO. The loss of
CO as an axial ligand would regenerate ii, which is the entry point to the catalytic cycle that was
discussed in Scheme 2. Consecutive protonation and reduction steps will generate a hydride,
which can undergo a CO2 insertion reaction to generate RuII–(η1ꢀOCHO) again.
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Scheme 3. Proposed dehydration mechanism for production of CO from HCO2 by 3.
To test certain elements of this proposed mechanism, a series of experiments were
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performed with HCO2H and tetraethylammonium formate (TEA+[HCO2 ]).56 Bulk electrolysis
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experiments at ~–2.2 V vs Fc/Fc+ with 3 and an excess of either HCO2H or TEA[HCO2 ]
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showed that CO was produced in both circumstances.53,57 Heating complex 3 to reflux in MeCN
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with TEA+[HCO2 ] under a static N2 blanket with a gas bubbler attached via tubing to a reflux
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condenser allowed us to confirm by GC that CO was produced under these conditions as well as
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CO2 and H2. Any attempts to abstract or displace a chloride with a HCO2 salt resulted in similar
behavior. Interestingly, the preparation of a partially chlorideꢀabstracted species with
trifluoromethanesulfonic acid (HOTf), RuII(mesbpy)(CO)2[OTf]Cl, 4, did not allow for the
isolation of a stable formato species. Electrochemical characterization of the directly synthesized
presumptive Ru–H intermediate, RuII(mesbpy)(CO)2(H)Cl 5, by CV was also consistent with the
proposed mechanism. Under Ar saturation a single irreversible wave was observed at –1.98 V vs
Fc/Fc+ (Figures S20 and S21). Similar to the observed behavior of 3 in the presence of a fixed
concentration of PhOH, an increased current response consistent with catalysis is observed under
CO2 saturation at potentials ~100 mV more positive than Ar saturation for 5 (Figures 5 and
S21). Additionally, the voltages at peak current under CO2 saturation (Ep = –2.07 V vs Fc/Fc+)
and Ar saturation (Ep = –2.14 V vs Fc/Fc+) for 5 are also comparable to those observed for 3
(Figure 5).
Figure 5. Cyclic voltammograms of 3 and 5 under Ar saturation (red, maroon) and CO2
saturation (blue, navy) with added PhOH (0.91 M, red/blue; 0.5 M, maroon/red). (A) current
response and detail (B) at negative potentials. Conditions: 1 mM 3 or 5; 0.1 M TBAPF6/MeCN;
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glassy carbon working electrode, Pt counter electrode, Ag/AgCl pseudoꢀreference electrode;
referenced to internal ferrocene standard; scan rate 100 mV/s.
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Based on these experimental results, we note several key points about the observed
behavior. Because of the thermodynamic considerations and factors that relate the production of
CO and HCO2H from CO2, it is remarkable that such a delicate balance can be achieved in this
system. It is possible to rationalize this by considering the equilibrium that relates CO and
HCO2H. The common intermediate shared by CO2 and the potential products (CO, HCO2H) in
this system is a Ru–H species. In the absence of applied potential, the RuII metal center
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decomposes HCO2 and HCO2H to CO2 and H2, which suggests that the RuII–H is less hydridic
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than HCO2H or HCO2 . Under electrochemical conditions, intermediate applied potential is
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enough to disrupt this equilibrium, favoring the production of HCO2H and HCO2 from CO2 by
increasing the relative hydricity of the Ru–H species. At high applied potential, the observed rate
of catalysis increases, which is consistent with the expected increase in the hydricity of the Ru–
H. At these potentials it also appears that this reduced Ru species is capable of reacting directly
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with CO2 through hydride elimination from HCO2 /HCO2H. Further study is in order to increase
our understanding of the branching ratio between CO and HCO2H.
Conclusion
Mechanistic electrochemical and spectroelectrochemical studies, supplemented by the
synthesis of relevant intermediates, indicate that this catalytic behavior is the result of the
cooperative redox response of the bpy ligand and Ru metal center at negative potentials, as well
as the inhibition of Ru–Ru bond formation through sterics.13,20 In this system CO2 binding is the
result of an insertion reaction, preceded by Ru–H bond formation, which is observed by IRꢀSEC.
The resulting species is likely similar to RuII(mesbpy)(η1ꢀOCOH)(CO)2Cl. In similar Ru systems
where more than one hydride is available, high pressure is used, or relevant substrate molecules
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are added, HCO2 is observed from this insertion product.58ꢀ64 In this case, however, an
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additional reduction reaction seems to accelerate a thermal process: the dehydration of HCO2 in
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the presence of H+ to CO and H2O. The results of these two studies suggest that the equilibria
relating HCO2H, CO2 and H2, and CO and H2O can have significant consequences on the
catalytic functionalization of CO2 under electrochemical conditions. We are currently exploring
additional modifications of the overall ligand structure and primary coordination sphere of the
Ru metal center to further investigate this behavior.
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Experimental and Procedures
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General. 1H NMR spectra were recorded on a Varian 400 or Jeol 500 MHz spectrometer at 298
K and referenced to residual solvent shifts. Data manipulations were completed using ACD.
Infrared spectra were taken on a Thermo Scientific Nicolet 6700 or a Bruker Equinox 55
spectrometer. Microanalyses were performed by Midwest Microlab for C, H, and N.
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Solvents and chemicals. All solvents were obtained from Fisher Scientific. All solvents were
dried in house by storing in a moisture free environment and dried on a custom drying system
running through two alumina columns under an argon atmosphere prior to use. All compounds
were obtained from Fisher Scientific, Alfa Aesar, or SigmaꢀAldrich and used as obtained unless
otherwise specified. Tetrabutylammonium hexafluorophosphate (TBAPF6, Aldrich, 98%) was
recrystallized from CH3OH twice and dried at 90oC overnight before use in electrochemistry.
Synthetic Methods
The ligand 6,6´ꢀdimesitylꢀ2,2´ꢀbipyridine, tetraethylammonium formate, and [Ru(CO)2Cl2] were
prepared according to literature methods.13,20ꢀ22,56
trans-Cl-Ru(6,6'-dimesityl-2,2'-bipyridyl)(CO)2Cl2, 3. A round bottom flask was charged with
[Ru(CO)2Cl2]n (0.288 g, 1.26 mmol) and 6,6´ꢀdimesitylꢀ2,2´ꢀbipyridine (0.500 g, 1.26 mmol) in
dry toluene (25 mL). A reflux condenser was attached to the flask and the mixture heated to
reflux and left overnight (18 h) in the dark. After this time the solution was allowed to cool to
room temperature and placed in a freezer at –20°C for 1 h. Over the course of an hour a pale
yellow powder precipitated, which was isolated by vacuum filtration and washed with pentanes
(2 x 10 mL). Yield (recovered) 580 mg, 74%. Removing the solvent from the filtrate under
reduced pressure isolated a pale yellow solid from which further material could be obtained by
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recrystallization from THF solution with pentanes. H NMR (d2ꢀCD2Cl2, 400 MHz): δ 8.32 (d,
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2H, ArH), 8.13 (dꢀd, 2H, ArH), 7.45 (d, 2H, ArH), 6.98 (s, 4H, ArH), 2.34 (s, 6H, –CH3), 2.14
(s, 12H, –CH3). 13C{1H} NMR (d2ꢀCD2Cl2, 500 MHz): δ 191.26, 165.19, 157.98, 141.38,
140.21, 139.75, 138.09, 130.40, 129.58, 129.39, 123.51, 115.96, 21.96, 21.74. IR (CH2Cl2) νCO:
2062, 1999 cm–1. ESIꢀMS (m/z) [M+Na+]+: Calcd. 643.05. Found: 643.17. Elemental Analysis
for C30H28Cl2N2O2Ru Calc’d: C 58.04, H 4.55, N 4.51; Found: C 58.21, H 4.60, N 4.32.
Ru(6,6'-dimesityl-2,2'-bipyridyl)(CO)2(CF3SO3)Cl, 4. A Schlenk flask was charged with 4
(0.100 g, 0.16 mmol) in dry CH2Cl2 (2 mL) under N2 atmosphere. An excess of
trifluoromethanesulfonic acid (0.30 mL, 3.2 mmol) was added slowly via syringe. The mixture
was stirred 1 h in the dark under N2 atmosphere. After this time diethyl ether (20 mL) was added
to the solution and a white precipitate formed which was collected by filtration. Yield
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(recovered) 84 mg, 61%. H NMR (d2ꢀCD2Cl2, 400 MHz): δ 8.69 (br m, 2H, ArH), 8.58 (br m,
2H, ArH), 7.95 (br m, 2H, ArH), 7.05 (s, 4H, ArH), 2.36 (s, 6H, –CH3), 2.12 (s, 12H, –CH3). IR
(CH2Cl2) νCO: 2086, 2014 cm–1. Elemental Analysis for C31H28ClF3N2O5RuS Calc’d: C 50.72, H
3.84, N 3.82; Found: C 50.38, H 4.34, N 3.84.
Ru(6,6'-dimesityl-2,2'-bipyridyl)(CO)2(COOMe)Cl. A roundꢀbottom flask was charged with
mesbpy (0.193 g, 0.49 mmol) and dichlorotricarbonylruthenium (II) dimer (0.125 g, 0.24 mmol)
in dry methyl formate (10 mL) under N2 atmosphere. The mixture was stirred at reflux for 2 h in
the dark. After this time the solution was condensed under reduced pressure and recrystallized
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from a mixture of CH2Cl2 and pentanes. Yield (recovered) 163 mg, 52%. H NMR (d2ꢀCD2Cl2,
400 MHz): δ 8.68 (br m, 2H, ArH), 8.32 (br m, 2H, ArH), 7.53 (br m, 2H, ArH), 6.99 (s, 4H,
13
ArH), 3.10 (s, 3H, –CH3), 2.34 (s, 6H, –CH3), 2.06 (s, 12H, –CH3). C{1H} NMR (d2ꢀCD2Cl2,
500 MHz): δ 189.07, 186.65, 185.04, 177.67, 177.55, 177.30, 158.36, 145.52, 136.72, 129.53,
123.57, 121.34, 115.98, 21.67, 20.84. IR (CH2Cl2) νCO: 2134, 2127, and 2059 cm–1; νOCO 1611,
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1606 cm–1. ESIꢀMS (m/z) [M–Cl]+: Calcd. 609.13. Found: 609.26. Elemental Analysis for
C32H31ClN2O4Ru ꢂ 0.5 CH2Cl2 Calc’d: C 56.85, H 4.70, N 4.08; Found: C 56.54, H 4.45, N 4.14.
Ru(6,6'-dimesityl-2,2'-bipyridyl)(CO)2(H)Cl. A roundꢀbottom flask was charged with mesbpy
(0.193 g, 0.49 mmol) and dichlorotricarbonylruthenium (II) dimer (0.250 g, 0.49 mmol) in dry
methyl formate (10 mL) under N2 atmosphere. The mixture was stirred at reflux for 30 minutes
in the dark. After this time the solution was condensed under reduced pressure loaded onto
neutral alumina with CH2Cl2. Running a column on silica gel with ethyl acetate (100%)
recovered a single fraction containing mesbpy ligand. Switching to CH2Cl2 as the eluent with a
MeOH gradient (0 to 10%) recovered a single fraction, which was condensed under reduced
pressure and found to contain Ru(mesbpy)(CO)2(H)Cl as a tan powder. Yield (recovered) 75
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mg, 26%. H NMR (d2ꢀCD2Cl2, 500 MHz): δ 8.27 (m, 2H, ArH), 8.10 (m, 2H, ArH), 7.44 (m,
2H, ArH), 6.97 (m, 4H, ArH), 2.33 (d, 6H, –CH3), 2.18 (d, 6H, –CH3), 1.94 (d, 6H, –CH3), –
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11.69 (s, 1H, Ru–H). C{1H} NMR (d2ꢀCD2Cl2, 500 MHz): δ 198.14, 196.55, 165.22, 163.55,
158.53, 156.81, 141.44, 140.06, 139.81, 139.59, 139.44, 137.28, 136.98, 135.71, 135.13, 129.20,
128.83, 128.47, 127.91, 127.84, 122.41, 122.32, 21.84, 21.54, 20.81, 20.53. IR (CH2Cl2) νCO:
2032, 1951 cm–1. ESIꢀMS (m/z) [M–Cl]+: Calcd. 551.13. Found: 551.19. Elemental Analysis for
C30H29ClN2O4Ru Calc’d: C 61.48, H 4.99, N 4.78; Found: C 61.18, H 4.91, N 4.63.
Electrochemistry. Electrochemical experiments were carried out using a BAS Epsilon
potentiostat. For all experiments, a single compartment cell was used with dry stir bar and a dry
needle was connected to control the atmosphere. A 3 mm diameter glassy carbon electrode from
BASi was employed as the working electrode. The counter electrode was a platinum (Pt) wire
and the pseudoꢀreference electrode was a silver/silver chloride (Ag/AgCl) electrode separated
from solution by a Vycor tip. Experiments were run with and without an added internal reference
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of ferrocene. All solutions were in dry MeCN and contained 1 mM of catalyst and 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) as the supporting electrolyte, unless
otherwise noted. Scan rates were 100 mV/s unless otherwise indicated. Experiments were purged
with N2 or CO2 (to saturation at 0.28 M) before CVs were taken and stirred in between
successive experiments. All experiments were reported referenced an internal ferrocene standard
except for the bulk electrolysis experiments, which used the pseudoꢀreference electrode Ag/AgCl
behind a Vycor tip.
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Infrared Spectroelectrochemistry. The experimental setup and design of the IRꢀSEC cell has
been published previously by our lab.33,34 A Pine Instrument Company model AFCBP1
bipotentiostat was employed. As the potential was scanned, thin layer bulk electrolysis was
monitored by FourierꢀTransform Reflectance IR off the electrode surface. All experiments were
conducted in 0.1 M TBAPF6/MeCN solutions with catalyst concentrations of ~5 mM (unless
otherwise noted) prepared under a nitrogen atmosphere. For IRꢀSEC experiments under catalytic
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conditions, airꢀfree samples were sparged briefly with CO2 (10ꢀ20 seconds). The IRꢀSEC cells
used (working electrode/counter electrode/reference electrode ) were either glassy carbon/Ag/Pt
or Pt/Ag/Pt, meaning that all potentials were in reference to a pseudoꢀRE, Ag/Ag+ (~+200 mV
from the Fc/Fc+ couple).
X-ray Crystallography. Single crystal Xꢀray diffraction studies reported herein were carried out
on a Bruker Kappa APEXꢀII CCS diffractometer equipped with MoKα radiation (λ = 0.71073
Å). The crystals were mounted on a Cryoloop while in Paratone oil. The data was collected
under a stream of N2 gas at 100(2) K using ω and ϕ scans. Data were integrated using the Bruker
SAINT software program and scaled using SADABS software. A complete phasing model
consistent with the molecular structure was produced by SHELXS direct methods. Nonꢀ
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hydrogen atoms were reined anisotropically by full matrix least squares (SHELXLꢀ97).65 All
hydrogen atoms were determined using a riding model with positions constrained to their parent
atom using the appropriate FHIX command. Crystallographic data is in supplementary
information (Tables S1).
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Calculations. Complexes were geometryꢀoptimized using the BP86 functional with TZ2P basis
sets. The resulting structures were confirmed as minima through analytical frequency
calculations at the same level of theory. KohnꢀSham orbital representations were generated using
the ADF GUI. All calculations were performed with the ADF 2012.01 software suite in
collaboration with the research group of Prof. Joshua S. Figueroa.
ASSOCIATED CONTENT
Supporting Information. A selection of cyclic voltammograms, Xꢀray crystallographic
information tables, IRꢀSEC plots, and chemical reductions are available online at
http://pubs.acs.org.
Corresponding Author
*ckubiak@ucsd.edu
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Funding Sources
AFOSR Basic Research Initiative (BRI) Grant (FA9550ꢀ12ꢀ1ꢀ0414).
ACKNOWLEDGMENT
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We acknowledge support for this work from the AFOSR through a Basic Research Initiative
(BRI) grant (FA9550ꢀ12ꢀ1ꢀ0414) and the UCSD Department of Chemistry and Biochemistry for
analytical instrumentation. We also acknowledge the assistance of Dr. Curtis Moore in obtaining
and solving the molecular structure presented here through single crystal Xꢀray diffraction
studies. We acknowledge the UCSD Chemistry and Biochemistry Molecular MS facility for use
of their facilities and Steven A. Chabolla for his assistance with ESIꢀMS. All calculations were
performed in collaboration with Prof. Joshua S. Figueroa.
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