Electrocatalytic CO2 Reduction with Cis and Trans Conformers of a Rigid Dinuclear Rhenium Complex: Comparing the Monometallic and Cooperative Bimetallic Pathways

Article abstract of DOI:10.1021/acs.inorgchem.8b01775

Anthracene-bridged dinuclear rhenium complexes are reported for electrocatalytic carbon dioxide (CO₂) reduction to carbon monoxide (CO). Related by hindered rotation of each rhenium active site to either side of the anthracene bridge, cis and trans conformers have been isolated and characterized. Electrochemical studies reveal distinct mechanisms, whereby the cis conformer operates via cooperative bimetallic CO₂ activation and conversion and the trans conformer reduces CO₂ through well-established single-site and bimolecular pathways analogous to Re(bpy)(CO)₃Cl. Higher turnover frequencies are observed for the cis conformer (35.3 s^-1) relative to the trans conformer (22.9 s^-1), with both outperforming Re(bpy)(CO)₃Cl (11.1 s^-1). Notably, at low applied potentials, the cis conformer does not catalyze the reductive disproportionation of CO₂ to CO and CO₃^2- in contrast to the trans conformer and mononuclear catalyst, demonstrating that the orientation of active sites and structure of the dinuclear cis complex dictate an alternative catalytic pathway. Further, UV-vis spectroelectrochemical experiments demonstrate that the anthracene bridge prevents intramolecular formation of a deactivated Re-Re-bonded dimer. Indeed, the cis conformer also avoids intermolecular Re-Re bond formation.

Full text of DOI:10.1021/acs.inorgchem.8b01775

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Electrocatalytic CO2 Reduction with Cis and Trans Conformers of a
Rigid Dinuclear Rhenium Complex: Comparing the Monometallic
and Cooperative Bimetallic Pathways
Weiwei Yang,†,‡ Sayontani Sinha Roy,†,‡ Winston C. Pitts,† Rebekah L. Nelson,† Frank R. Fronczek,§
and Jonah W. Jurss*,†
†Department of Chemistry and Biochemistry, University of Mississippi, University, Mississippi 38677, United States
§Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States
S
* Supporting Information
ABSTRACT: Anthracene-bridged dinuclear rhenium com-
plexes are reported for electrocatalytic carbon dioxide (CO2)
reduction to carbon monoxide (CO). Related by hindered
rotation of each rhenium active site to either side of the
anthracene bridge, cis and trans conformers have been isolated
and characterized. Electrochemical studies reveal distinct
mechanisms, whereby the cis conformer operates via cooper-
ative bimetallic CO2 activation and conversion and the trans
conformer reduces CO2 through well-established single-site and
bimolecular pathways analogous to Re(bpy)(CO)3Cl. Higher
turnover frequencies are observed for the cis conformer (35.3
s−1) relative to the trans conformer (22.9 s−1), with both
outperforming Re(bpy)(CO)3Cl (11.1 s−1). Notably, at low applied potentials, the cis conformer does not catalyze the reductive
disproportionation of CO2 to CO and CO32− in contrast to the trans conformer and mononuclear catalyst, demonstrating that
the orientation of active sites and structure of the dinuclear cis complex dictate an alternative catalytic pathway. Further, UV−vis
spectroelectrochemical experiments demonstrate that the anthracene bridge prevents intramolecular formation of a deactivated
Re−Re-bonded dimer. Indeed, the cis conformer also avoids intermolecular Re−Re bond formation.
systems have been functionalized with phosphonate anchoring
groups for surface attachment to photocathodes, where they
exhibit long-term stability for CO2 reduction.18−21 Successful
translation of these catalysts into working devices emboldens
their continued development, where lower overpotentials and
faster rates are remaining challenges.
INTRODUCTION
■
Roughly 85% of the world’s energy is derived from burning
nonrenewable fossil fuels that generate greenhouse gases.1
Carbon dioxide (CO2) is the chief component of this waste
stream, which has been linked to climate change and other
environmental concerns.2 Renewable energy sources such as
wind and solar are attractive, but they are intermittent, site-
specific, and geographically diffuse. Additionally, they require
energy storage before they can be relied on to power society.3
To address this challenge while reducing CO2 emissions, solar
energy or renewable electricity can be used to drive the
conversion of CO2 and water (H2O) into fuels, whereby
energy is stored indefinitely in the form of chemical bonds for
on-demand use. The efficient catalytic conversion of CO2 into
energy-rich compounds could close the carbon cycle and allow
an underutilized resource to be tapped into.4
Molecular metal-based catalysts for CO2 reduction have
been widely pursued, in part because transition metals
generally have multiple redox states accessible to facilitate
multielectron chemistry. The bulk of these catalysts employ a
single metal center.4 Re(bpy)(CO)3Cl (where bpy is 2,2′-
bipyridine) and related derivatives continue to attract attention
as competent electrocatalysts for CO2 reduction, in addition to
being single-component photocatalysts.5−17 Recently, these
Mechanistic studies of Re(bpy)(CO)3X-type catalysts have
provided evidence for the concentration-dependent formation
of dinuclear intermediates during catalysis.6,7,22−31 Indeed,
bimetallic and monometallic pathways exist as dimer formation
is in competition with CO2 reduction at a single metal.24
Disentangling the contributions of competing pathways, in
addition to the transient nature of their respective
intermediates, has made it difficult to assess the possible
advantages of two metal centers in activating and reducing
CO2 in a cooperative fashion. A beneficial interaction between
two active sites is further clouded because a Re−Re-bonded
dimer has been indicted as a deactivated intermediate.27,28
Several bimetallic catalyst designs have been pursued based
on flexible alkyl tethers32,33 or hydrogen-bonding interac-
tions,34 which suffer from a lack of rigidity and variable
Received: June 26, 2018
© XXXX American Chemical Society
A
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solution compositions. Other examples feature dinucleating
scaffolds that preclude intramolecular bimetallic CO2 activa-
tion and conversion.35,36 Thus, the desirable design parameters
surrounding this approach are poorly understood. In this
context, we sought to develop catalysts with a rigid
dinucleating ligand that positions two rhenium active sites in
close proximity with a predictable intermetallic distance and
orientation (Scheme 1). The ability to isolate and even select
Scheme 1. Synthesis of 1 and Its Bimetallic Rhenium(I)
Conformers, cis-Re2Cl2 and trans-Re2Cl2
Figure 1. 1H NMR spectra (500 MHz, DMSO-d6) showing the
aromatic region of (A) cis-Re2Cl2 and (B) trans-Re2Cl2.
comparison, the CO stretching frequencies of the parent
complex, fac-Re(bpy)(CO)3Cl, are at 2019, 1914, and 1893
cm−1 in DMF.6
X-ray-quality crystals of the asymmetric trans conformer
were obtained by slow isopropyl ether diffusion into a
concentrated DMF solution. The crystal structure of trans-
Re2Cl2 is shown in Figure 2, concurrently lending support for a
from competing mechanisms (i.e., bimetallic vs monometallic
pathways) would allow us to compare and understand
differences in efficiency, activity, and stability. Herein, the
synthesis, characterization, and electrocatalytic activity of
homogeneous cis and trans conformers of a Re2 catalyst
supported by an anthracene-based polypyridyl ligand are
described.
SYNTHESIS AND CHARACTERIZATION
■
The synthesis of 1,8-bis(2,2′-bipyridin-6-yl)anthracene (1) and
its metalation to generate conformers cis-Re2Cl2 and trans-
Re2Cl2 is shown in Scheme 1. As previously described,37 1,8-
dichloroanthraquinone is reduced with zinc to give 1,8-
dichloroanthracene (1a). Next, the 1,8-diboronate ester (1b)
is furnished by a palladium-catalyzed borylation with bis-
(neopentyl glycolato)diboron.38 The boronate ester is reacted
directly by Suzuki cross-coupling with 6-bromo-2,2′-bipyridine
to give new dinucleating ligand 1 in 86% yield. Metalation was
achieved by refluxing the ligand with 2 equiv of Re(CO)5Cl in
anhydrous toluene overnight to give a mixture of two dinuclear
rhenium(I) conformers, cis-Re2Cl2 and trans-Re2Cl2. Despite
the possible formation of up to seven isomers (Figure S1), only
two species are observed in the reaction mixture under these
Figure 2. Crystal structure of trans-Re2Cl2. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are shown at the 70%
probability level.
symmetric configuration of the cis conformer, which is
accessible by rotation (cis ↔ trans), as observed at elevated
1
temperatures by H NMR. However, no interconversion was
observed at room temperature over the course of 2 days.
Our catalyst design aims to channel reactivity through a
single mechanism by allowing both active sites to be in close
proximity for cooperative catalysis or by isolating each metal
center for single-site activity. Importantly, because active sites
are generated following chloride dissociation from the metal,
two symmetric cis conformers are possible, one in which the
chloro ligands face each other (cis-Re2Cl2) and another that
has its chloro ligands oriented outward (cis-2, as labeled in
Figure S1). Density functional theory (DFT) calculations were
used to investigate the barrier to rotation of Re(bpy) moieties
to either side of the anthracene bridge by which the trans
conformer may interconvert to either of the symmetric cis
conformers in solution. Since the structure of trans-Re2Cl2 is
known, the energy barrier associated with rotational isomer-
ization from trans-Re2Cl2 was calculated (Figure S5). By DFT,
the barrier to rotation from the trans conformer to cis-Re2Cl2
(27.2 kcal mol−1) is 4.6 kcal mol−1 lower in energy than the
rotation to cis-2 (31.8 kcal mol−1). These calculations provide
indirect evidence that the isolated cis conformer has its chloro
ligands facing “in” (denoted cis-Re2Cl2, as depicted in Scheme
1
conditions, as shown in the crude H NMR spectrum (Figure
S2). Two narrow, closely spaced bands were separated by silica
gel chromatography to give an overall isolated yield of 61%.39
We note that solvent-dependent isomer formation was
reported previously for di- and trinuclear Re(CO)3-based
compounds.40 1H NMR spectra of the purified cis and trans
conformers are shown in Figure 1. From the NMR spectra,
there is a symmetric species (cis-Re2Cl2) with 12 chemically
distinct protons and an asymmetric species (trans-Re2Cl2)
displaying 22 different proton signals. IR spectroscopy in N,N-
dimethylformamide (DMF) established the presence of fac-
Re(CO)3 fragments with carbonyl stretching frequencies
ν(CO) for cis-Re2Cl2 at 2019, 1915, and 1890 cm−1 and for
trans-Re2Cl2 at 2014, 1910, and 1888 cm−1 (Figure S3). For
B
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1). Toward more direct evidence, the IR spectra of cis-Re2Cl2
and cis-2 were calculated by DFT. Notably, cis-Re2Cl2 is
expected to have three ν(CO) absorption bands, while cis-2 is
calculated to have four ν(CO) vibrational modes (Figure S4).
The experimental Fourier transform infrared (FTIR) spectrum
of the cis conformer (Figure S3) is in good agreement with the
calculated spectrum of cis-Re2Cl2, further corroborating its
specified stereochemistry.
Table 1. Reduction Potentials, Diffusion Coefficients, and
Comproportionation Constants (KC) from Cyclic
Voltammetry in Anhydrous DMF/0.1 M Bu4NPF6 Solutions
catalyst
cis-Re2Cl2
Ep1,c
Ep2,c
Ep3,c
Ep4,c
−1.77
−1.87
−2.34
−2.65
KC = 50
trans-Re2Cl2
Re(bpy)(CO)3Cl
anthryl-Re
−1.78
−1.80
−1.82
−1.88
−2.40
−2.67
KC = 50
−2.25
D = 5.40 × 10−6 cm2 s−1
−2.29 −2.54
D = 5.48 × 10−6 cm2 s−1
ELECTROCHEMISTRY
■
Following characterization, the redox behaviors of cis-Re2Cl2
and trans-Re2Cl2 were analyzed in the absence of CO2 by cyclic
voltammetry in anhydrous DMF containing 0.1 M tetrabuty-
lammonium hexafluorophosphate (Bu4NPF6) supporting elec-
trolyte. Re(bpy)(CO)3Cl and a mononuclear derivative
(anthryl-Re) bearing a pendant anthracene (shown in Figure
3) were studied in parallel to provide insight into the catalytic
On the basis of these results and previous work,34,36,43 we
assign the most positive overlapping one-electron redox events
to ligand-centered bipyridine reductions to form [ReI(bpy•)-
(CO)3Cl]− moieties followed by a third (overall) reductive
process that involves overlapping reductions of each of the
Re(bpy) units and totals two electrons. The most negative
redox event is ascribed to a one-electron, anthracene-based
reduction. A CV of free ligand 1 is shown in Figure S10.
Electrocatalytic CO2 reduction by the dinuclear conformers
and monomers Re(bpy)(CO)3Cl and anthryl-Re was then
investigated by cyclic voltammetry. The CVs of cis-Re2Cl2 and
trans-Re2Cl2 at 1 mM concentrations and of monomers at
concentrations of 2 mM are shown in Figure 4 in DMF
solutions under argon and CO2 atmospheres. The monomer
catalyst concentrations were doubled to maintain a consistent
concentration with respect to rhenium.
Figure 3. CVs of (A) 2 mM Re(bpy)(CO)3Cl and anthryl-Re and
(B) 1 mM cis-Re2Cl2 and trans-Re2Cl2 in DMF/0.1 M Bu4NPF6 under
argon. Glassy carbon disk; ν = 100 mV s−1.
activity and ligand-based redox chemistry. Cyclic voltammo-
grams (CVs) were referenced internally at the end of the
experiments to the ferrocenium/ferrocene (Fc+/0) couple. The
electrochemical properties of Re(bpy)(CO)3Cl have been
studied extensively, most often in acetonitrile solutions. Its CV
in DMF, overlaid with that of anthryl-Re (Figure 3A), exhibits
a quasi-reversible redox process with Ep,c at −1.80 V versus
Fc+/0 and an irreversible reduction at −2.25 V. Notably, the
CV of anthryl-Re has a third reduction at −2.54 V that is
absent in the parent complex. Similar behavior is observed for
the dinuclear catalysts (Figure 3B). The CVs of cis-Re2Cl2 and
trans-Re2Cl2 reveal two closely spaced, quasi-reversible, one-
electron reductions at Ep,c = −1.77 and −1.87 V and at −1.78
and −1.88 V, respectively, followed by reductions at Ep,c
=
−2.34 and −2.65 V and at −2.40 and −2.67 V, respectively,
with a scan rate of 100 mV s−1. The number of electrons
transferred in each redox process based on integrated cathodic
peak areas corresponds to a 1:1:2:1 stoichiometry. From scan
rate-dependent CVs of the dinuclear conformers and both
mononuclear complexes, linear fits were obtained in plots of
the reductive peak current (ip,c) versus the square root of the
scan rate (ν1/2) consistent with freely diffusing systems
(Figures S6−S9).41
Comproportionation constants (KC) for the dinuclear
catalysts were estimated from the overlapping one-electron
reductions as a measure of electronic coupling between
Re(bpy) units.42 Based on the difference in potentials, ΔE,
between Ep1,c and Ep2,c, a KC value of 50 was calculated for both
Re2 conformers, indicative of weak electronic coupling through
the anthracene bridge.42 Weak electronic coupling is not
surprising given the number of bonds between redox sites.
These results are summarized in Table 1.
Figure 4. Cyclic voltammetry in argon- and CO2-saturated DMF/0.1
M Bu4NPF6 solutions with (A) 1 mM cis-Re2Cl2, (B) 1 mM trans-
Re2Cl2, (C) 2 mM Re(bpy)(CO)3Cl, and (D) 2 mM anthryl-Re.
Glassy carbon disk; ν = 100 mV s−1. The background under CO2 is
shown with a dashed blue line.
For an electrocatalytic reaction involving heterogeneous
electron transfers at the electrode surface, the peak catalytic
current is described by eq 1:44−46
icat = ncatFA[cat](Dkcat[S]y)1/2
(1)
where icat is the limiting catalytic current in the presence of
CO2, ncat corresponds to the number of electrons transferred in
the catalytic process (here ncat = 2 as CO2 is reduced to CO), F
is Faraday’s constant, A is the area of the electrode, [cat] is the
C
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molar concentration of the catalyst, D is the diffusion
coefficient, kcat is the rate constant of the catalytic reaction,
and [S] is the molar concentration of dissolved CO2. By
applying eq 1, the catalytic mechanisms of cis-Re2Cl2 and trans-
Re2Cl2 were probed using cyclic voltammetry to determine the
reaction orders with respect to [CO2] and [cat] (Figures S11
and S12, respectively).
First, gas mixtures of argon and CO2 were used to vary the
partial pressure of CO2 from 0 to 1 atm to reveal a linear
increase in the catalytic current for CO2 reduction as a
function of the square root of dissolved CO2 concentration for
both cis-Re2Cl2 and trans-Re2Cl2 catalysts (Figure 5). The
concentration of dissolved CO2 in DMF under saturation
conditions at 1 atm is 0.20 M.47
Our initial inquiry regarding a half-order dependence on
catalyst concentration, specifically for trans-Re2Cl2, emerged
from a previous mechanistic study involving a dinuclear
palladium complex, in which its polydentate phosphine ligand
was designed to prevent cooperative, intramolecular CO2
binding between active sites.48 A half-order dependence on
catalyst concentration was reported and explained by assuming
the metal active sites operate independently of one another,
such that only half of the complex is intimately involved in the
rate-determining step.48b As rationalized, however, a first-order
dependence would be expected. A square-root dependence on
catalyst concentration is typically observed following rate-
limiting dissociation of a dinuclear precatalyst that is in
reversible equilibrium with the active form.49,50
Several plausible modes of reactivity with a dinuclear catalyst
for CO2 reduction are expected to yield a first-order
dependence on the catalyst concentration: (1) both metal
centers participate in the intramolecular activation, or binding,
of CO2 and its conversion; (2) only one metal center reacts
with CO2, while the other acts as a spectator; (3) only one
metal center reacts with CO2, while the other serves as an
electron reservoir capable of intramolecular electron transfer;
(4) both metal centers operate independently and react with
one CO2 molecule each. In cases 2 and 3, the metal fragment
that does not bind CO2 can be thought of more simply as (2)
an elaborate ligand substituent of variable electronics depend-
ing on its redox state or (3) a pendant redox mediator that
supplies electron(s) to the active site, neither of which would
give a half-order dependence on the catalyst concentration.
Figure 5. CO2 concentration dependences for (A) 1 mM cis-Re2Cl2
and (B) 1 mM trans-Re2Cl2 in DMF/0.1 M Bu4NPF6. Glassy carbon
disk; ν = 100 mV s−1. The catalytic current (μA) is plotted versus the
square root of [CO2].
Likewise, the dependence of the catalytic current (icat) on
the concentration of each dinuclear rhenium catalyst ([cat]) in
CO2-saturated solutions was determined (Figure 6).
CATALYTIC RATES AND FARADAIC EFFICIENCIES
■
With these results in hand, we sought to compare the catalytic
rates of the cis and trans conformers along with Re(bpy)-
(CO)3Cl and its anthracene-functionalized derivative, anthryl-
Re. The quantity (icat/ip)2 can serve as a useful benchmark of
catalytic activity that is proportional to the turnover frequency
(TOF), as shown in eq 2, where ν is the scan rate, np is the
number of electrons transferred in the noncatalytic Faradaic
process, R is the ideal gas constant, T is the temperature (in
K), and ip is the peak current observed for the catalyst in the
absence of substrate.34,51
Figure 6. Catalyst concentration dependence for (A) cis-Re2Cl2 and
(B) trans-Re2Cl2 in CO2-saturated DMF/0.1 M Bu4NPF6 solutions.
Glassy carbon disk; ν = 100 mV s−1. Linear fits with slopes of 1 were
obtained in log−log plots of catalytic current (μA) versus catalyst
concentration (mM).
2
2
3
i
y
z
z
z
z
z
z
Fνnp i
y
z
z
z
z
z
ji
0.4463
ncat
j
cat
j
j
j
j
j
j
TOF = kcat[CO2] =
j
j
j
RT
k
ip
{
(2)
k
{
The application of eq 2 requires steady-state behavior, which
can often be established at sufficiently fast scan rates that avoid
substrate depletion and give rise to a limiting catalytic current
plateau. Linear-sweep voltammograms of cis-Re2Cl2 (1 mM),
trans-Re2Cl2 (1 mM), and the mononuclear catalysts (2 mM)
were conducted in DMF/0.1 M Bu4NPF6 solutions under
argon and CO2 as a function of the scan rate (Figure S14).
While scan rate independence was not completely reached,
estimated TOFs using eq 2 are plotted versus the scan rate in
Figure S15, where TOFs of 35.3, 22.9, 11.1, and 19.2 s−1 were
identified for cis-Re2Cl2, trans-Re2Cl2, Re(bpy)(CO)3Cl, and
anthryl-Re, respectively.
For both conformers, the data indicate a first-order
dependence on [Re2Cl2] and a first-order dependence on
[CO2] in the rate-determining step, as expressed by the
following rate law:
[CO2]
dt
rate = −
= kcat[Re2Cl2][CO2]
It was not possible on the basis of reaction orders alone to
distinguish between the reaction mechanisms of the cis and
trans conformers. The log−log plots were critical for the
correct kinetic analysis of these catalysts because plots of both
catalytic current versus [trans-Re2Cl2] and catalytic current
versus [trans-Re2Cl2]1/2 gave reasonable linear fits (Figure
S13). The log−log plots confirm that the reactions are first-
order in catalyst with slopes of 1 (Figure 6).
́
Alternatively, Costentin and Saveant’s foot-of-the-wave
analysis (FOWA)46,52,53 can be applied to find intrinsic
catalytic rates via eq 3:
D
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RT
Fνnp
2.24
3 2kcat[CO2]
i
ip
=
Ä
Å
Å
Å
É
Ñ
Ñ
Ñ
Ñ
F
RT
1 + exp (E − Ecat/2
)
Å
(3)
Å
Ç
Ñ
Ö
This electroanalytical method provides access to a TOF
maximum, in which the observed catalytic rate constant kcat can
be determined at the foot of the catalytic wave, where
competing factors (i.e., substrate depletion, catalyst inhibition,
or decomposition) are minimal. Indeed, scan rate-independent
TOFs were obtained by FOWA (Figure S16). In eq 3, Ecat/2 is
the half-wave potential of the catalytic wave. From the slope of
the linear portion of a plot of i/ip versus 1/{1 + exp[(F/
RT)(E−Ecat/2)]}, kcat can be calculated.54 The maximum TOF
under saturation conditions is then found from TOF =
kcat[CO2].
The maximum TOFs determined via FOWA for the
dinuclear catalysts (11.0 s−1, cis-Re2Cl2; 4.4 s−1, trans-Re2Cl2)
are higher than that of the mononuclear parent catalyst (1.5
s−1) at the same effective concentrations. Likewise, the cis
conformer is considerably more active than its trans counter-
part, consistent with the estimated TOFs from eq 2. With
regard to anthryl-Re (4.1 s−1), the pendant anthracene results
in a higher TOF relative to unsubstituted Re(bpy)(CO)3Cl,
whereas very similar rates (via both electroanalytical
equations) are observed relative to trans-Re2Cl2. Comparable
TOFs of anthryl-Re and trans-Re2Cl2 are consistent with the
isolated active sites of the dinuclear complex on opposite sides
of the anthracene bridge. Notably, slow catalysis begins after
the first overlapping one-electron reductions at around −1.9 V
with the dinuclear catalysts, while no activity is observed until
the second reduction at −2.1 V for the monomers.
Brønsted acids are frequently added to polar aprotic solvents
to promote the proton-coupled reduction of CO2. Indeed,
significant enhancements in the electrocatalytic CO2 reduction
activity of mononuclear rhenium catalysts have been observed
with various proton source additives.13,55,56 The influence of
four different acids [H2O, methanol (MeOH), 2,2,2-trifluor-
oethanol (TFE), and phenol (PhOH)] on the CO2 reduction
activity of cis-Re2Cl2 in DMF was investigated (Figure S17).
This selection of proton sources affords a good range of
acidity, with the following pKa values reported in dimethyl
sulfoxide (DMSO):57 31.4 (H2O),58 29.0 (MeOH),58 23.5
(TFE),59 18.0 (PhOH).60 The catalyst cis-Re2Cl2 shows a
decrease in the current upon addition of these acids. However,
the onset of fast catalysis is significantly more positive after the
addition of a proton source relative to anhydrous DMF. From
cyclic voltammetry, added H2O produces the most promising
catalytic activity for CO2 reduction with cis-Re2Cl2 of the four
Brønsted acids investigated, taking into account its nominal
activity under argon. Accordingly, the amount of added H2O
was optimized for cis-Re2Cl2 by plotting icat versus H2O
concentration where catalysis reached a maximum with 3 M
H2O (Figure S18). The catalytic activity of each catalyst in the
presence of 3 M H2O is shown in Figure 7 for comparison.
A thermodynamic potential of −0.73 V vs Fc+/0 for the 2e−/
2H+ CO2/CO couple in pure DMF has been determined via a
thermochemical approach.61 In the presence of a Brønsted
acid, this standard potential shifts according to eq 4, in which
the pKa value of the acid must be taken into account.61
Figure 7. CVs of (A) 1 mM cis-Re2Cl2, (B) 1 mM trans-Re2Cl2, (C) 2
mM Re(bpy)(CO)3Cl, and (D) 2 mM anthryl-Re with 3 M H2O in
DMF/0.1 M Bu4NPF6 under argon (black) and CO2 (red). Glassy
carbon disk; ν = 100 mV s−1.
As previously described, carbonic acid (H2CO3, pKa = 7.37)
is more acidic than H2O in DMF, so its pKa was used in eq 4 to
give an estimated reduction potential of −1.17 V for the CO2/
CO couple in DMF/H2O solutions.36 Using these standard
potentials, a summary of the TOFs from anhydrous DMF and
observed overpotentials from Figures 4 and 7, based on Ecat/2
,
is presented in Table 2. Ecat/2 is a reliable metric that
corresponds to the steepest point, or nearly so, in the catalytic
wave of the current−potential profile.62 It is worth mentioning
that overpotentials cited in Table 2 correspond to applied
potentials and/or conditions that promote the 2e−/2H+
reduction of CO2 to CO + H2O. To the best of our
knowledge, a standard potential for the reductive disproportio-
2−
nation of CO2 to CO + CO3 has not been reported.
Controlled potential electrolyses were carried out to identify
the products and Faradaic efficiencies in an airtight electro-
chemical cell using a glassy carbon rod working electrode and
CO2-saturated solutions. The applied potential (Eappl
)
corresponds to the potential at which the maximum current
was observed in CVs taken in the same setup prior to
electrolysis. Headspace gases and electrolytic solutions were
1
analyzed by gas chromatography and H NMR and FTIR
spectroscopies.63−65 CO was found to be the only detectable
product under these conditions, where Eappl is −2.4 or −2.5 V
(Table 3). No other products were observed. In addition to IR
spectroscopy, titration with barium triflate was also conducted
to investigate carbonate as a potential product. Formation of
BaCO3 was not apparent.10,63,64 The absence of carbonate
indicates that the dinuclear catalysts do not mediate reductive
disproportionation of CO2, i.e., 2CO2 + 2e− → CO + CO32− at
these potentials, consistent with earlier results with Re(bpy)-
(CO)3Cl in DMF solutions.22 It is well-established that under
anhydrous conditions protons can be generated by Hofmann
degradation of the quaternary alkylammonium cation of the
supporting electrolyte.22,66 This degradation is thought to arise
from a highly basic metal carboxylate intermediate that is
capable of deprotonating the tetrabutylammonium ion,66
consistent with IR stopped-flow measurements in the absence
of an added proton source.67
0
E = E
− 0.0592pKa
CO2/CO(DMF)
(4)
E
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Table 2. Summary of Electrocatalysis Obtained from Cyclic Voltammetry
a
b
catalyst
cis-Re2Cl2
trans-Re2Cl2
Re(bpy)(CO)3Cl
anthryl-Re
TOF (s−1) eq 2
TOF (s−1) eq 3
E
cat/2(η)
Ecat/2(η)
35.3
22.9
11.1
19.2
11.0
4.4
1.5
−2.29 (1.56)
−2.34 (1.61)
−2.22 (1.49)
−2.26 (1.53)
−2.09 (0.92)
−1.95 (0.78)
−2.04 (0.87)
−2.18 (1.01)
4.1
a
b
Anhydrous DMF. DMF with 3 M H2O. Ecat/2 is the potential at which the current is equal to icat/2. Overpotentials reported here were calculated
from the expression η = |Ecat/2 − E0CO /CO|.
2
Table 3. Summary of Controlled Potential Electrolyses and Faradaic Efficiencies for CO2 Reduction under Various Conditions
catalyst
time (min)
Eappl (V)
charge passed (C)
CO (%)
cis-Re2Cl2
trans-Re2Cl2
60
60
60
60
60
−2.5
−2.5
−2.5
−2.4
−2.4
−2.4
−2.0
−2.0
−2.0
−2.0
−2.0
1.52 0.32
0.60 0.08
1.17 0.15
0.96 0.22
1.01 0.09
0.92 0.29
2.24 0.37
5.94 0.82
1.01 0.12
6.27 0.03
4.46 0.11
81
89
78 10
1
6
Re(bpy)(CO)3Cl
cis-Re2Cl2 (3 M H2O)
trans-Re2Cl2 (3 M H2O)
Re(bpy)(CO)3Cl (3 M H2O)
cis-Re2Cl2
cis-Re2Cl2
trans-Re2Cl2
cis-Re2Cl2 (3 M H2O)
trans-Re2Cl2 (3 M H2O)
59
74
65
52
52
49
47
49
5
1
6
5
5
5
3
6
60
180
600
180
600
600
At lower applied potentials, Re(bpy)(CO)3Cl is known to
catalyze the reductive disproportionation of CO2 through the
so-called “one-electron pathway”.22 Following its one-electron
reduction, the parent catalyst reduces CO2 in a bimolecular
process to give CO and CO32−. In this mechanism, two singly
reduced catalysts are proposed to bind CO2 to generate a Re2
carboxylate-bridged intermediate that undergoes insertion with
a second equivalent of CO2 before reductive disproportiona-
tion. Here, CO2 acts as an oxide acceptor to facilitate a proton-
independent conversion of CO2 to CO. By inference, less basic
intermediates are generated at these less reducing potentials
and oxide transfer to CO2 is favored over Hofmann
degradation. We note that the hydrogen-bonded Re(bpy)
dimers developed by Kubiak and co-workers also promote this
bimolecular pathway with enhanced rates.34
Figure 8. FTIR spectra of (A) trans-Re2Cl2 and (B) cis-Re2Cl2
samples before and after electrolysis. Electrolyzed solutions are
evaporated to dryness and washed with dichloromethane to remove
the Bu4NPF6 electrolyte. The resulting solid is analyzed.
CO2 insertion step and favors protonation of the CO2 adduct
by slow Hofmann degradation.
In this context, we investigated the cis and trans conformers
at applied potentials immediately following the first over-
lapping one-electron reductions. Controlled potential elec-
trolyses were conducted in anhydrous DMF/0.1 M Bu4NPF6
as well as solutions containing 3 M H2O. Unsurprisingly, trans-
Re2Cl2 was found to catalyze the reductive disproportionation
of CO2 at an applied potential of −2.0 V in anhydrous DMF.
Given its solid-state structure, catalytic activity analogous to
that of Re(bpy)(CO)3Cl was expected for the trans conformer
and carbonate was identified as a coproduct by IR spectros-
copy, with strong C−O stretching frequencies observed at
∼1450 and 1600 cm−1 (Figure 8A) and subsequent
precipitation of BaCO3 from DMF solutions.64,68 Conversely,
carbonate was not observed with cis-Re2Cl2. In order to bolster
this result and ensure that carbonate is not simply sequestered
in a stable species (i.e., bound between rhenium centers), long-
term electrolyses were performed in which the evolved CO
amounts to greater than two turnovers with respect to total
moles of catalyst in solution. On this basis, cis-Re2Cl2 does not
catalyze the reductive disproportionation of CO2, consistent
with the IR spectra in Figure 8B. Presumably, restricted
rotation of the Re(bpy) fragments in cis-Re2Cl2 impedes the
With 3 M H2O, at the lower applied potential (−2.0 V vs
Fc+/0), carbonate or bicarbonate is not produced by cis-Re2Cl2
or trans-Re2Cl2. Authentic samples of tetrabutylammonium
carbonate and tetrabutylammonium bicarbonate were synthe-
sized68,69 and detected by FTIR using the established protocol
and barium triflate titration to validate these results. Consistent
with the enhanced catalysis at initial reductions of both Re2
catalysts (Figure 7) and the concentration dependence in
Figure S18, the addition of water clearly promotes the proton-
coupled reduction of CO2, i.e., CO2 + 2H+ + 2e− → CO +
H2O, at lower overpotentials. Representative charge-time
profiles from electrolyses in DMF solutions are overlaid in
Figure S19. Experiments were performed in duplicate, and the
results are summarized in Table 3.
UV−VIS SPECTROELECTROCHEMISTRY
■
Additional insight into the electrocatalytic CO2 reduction
mechanisms of the cis and trans conformers was gained from
UV−vis spectroelectrochemical (SEC) measurements. Data
were collected stepwise at increasingly negative applied
potentials under an argon atmosphere using a thin-layer cell
with a “honeycomb” working electrode. Representative
F
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absorption spectra versus time (Figure 9) are shown for cis-
Re2Cl2 and trans-Re2Cl2.
Figure 10. Possible intermediates following reduction and chloride
dissociation from cis-Re2Cl2. DFT-optimized structures of a neutral
dinuclear rhenium species and its one-electron-reduced radical anion.
It is clear from the structure of trans-Re2Cl2 and the
observed reactivity that catalysis occurs by well-established
pathways known for Re(bpy)(CO)3Cl (catalytic cycles are
shown in Figure S20), including the “one-electron” bimolec-
ular reductive disproportionation mechanism at applied
potentials just after the initial overlapping one-electron
reductions of the Re2 complex under anhydrous conditions.
Carbonate is a coproduct of this bimolecular pathway for trans-
Re2Cl2. However, in the presence of 3 M H2O or at more
negative potentials, CO2-to-CO conversion occurs but with
H2O as the coproduct.
Figure 9. UV−vis spectroelectrochemistry with (A) 0.5 mM cis-
Re2Cl2 at −1.7 V versus Ag/AgCl, (B) 0.5 mM trans-Re2Cl2 at −1.7 V
versus Ag/AgCl, (C) 0.5 mM cis-Re2Cl2 at −2.0 V versus Ag/AgCl,
(D) 0.5 mM trans-Re2Cl2 at −2.0 V versus Ag/AgCl in argon-
saturated DMF/0.1 M Bu4NPF6 solutions.
Different behavior is observed for cis-Re2Cl2. Proposed
catalytic cycles for the two applied potential regimes at −2.0
and ≤−2.4 V are given in Figures 11 and S21, respectively. In
contrast to the mononuclear catalyst and trans-Re2Cl2, the cis
conformer does not generate carbonate at applied potentials
immediately following the initial overlapping one-electron
reductions. This difference in reactivity is presumably a
consequence of the proximal active sites being confined by
At an applied potential of −1.7 V versus Ag+, both cis-Re2Cl2
and trans-Re2Cl2 catalysts display a new absorption peak at 517
nm, consistent with the one-electron-reduced species.70 The
terminology used here refers to reduction at a single rhenium
site to facilitate a comparison with earlier studies on
mononuclear catalysts. The catalysts have been reduced by
two electrons in total. At a more negative applied potential of
−2.0 V versus Ag+, absorption bands at 562 nm (cis-Re2Cl2)
and 570 nm (trans-Re2Cl2) appear, indicative of the two-
electron-reduced species.7
Upon closer inspection of the absorption spectra obtained
with Eappl = −1.7 V versus Ag+, a broad band centered around
850 nm also emerges for the trans conformer. This feature is
characteristic of a Re−Re-bonded dimer,7 presumably a Re4
species in this case formed from 2 equiv of trans-Re2Cl2.
Characteristic absorption features of a Re−Re dimer, however,
were not observed in experiments with cis-Re2Cl2. We
reasoned that an added benefit of the rigid anthracene
backbone is constraining the metal centers from Re−Re
bond formation, a known deactivation pathway.
DFT calculations were conducted to probe the likelihood of
forming the neutral Re−Re dimer or its one-electron-reduced
product with cis-Re2Cl2. Optimized geometries are shown in
Figure 10. Calculated distances between rhenium centers are
3.38 Å for the neutral species and 3.52 Å for the radical anion.
Solid-state structures of the corresponding dimers prepared
from Re(bpy)(CO)3Cl reduction have been characterized by
X-ray crystallography.27 A Re−Re bond distance of 3.0791(13)
Å was found in the neutral dimer and a longer distance of
3.1574(6) Å was determined in the reduced dimer.27 Given the
calculated intermetallic distances for the anthracene-based
catalyst in conjunction with the SEC data, the Re−Re-bonded
dimer is likely inaccessible following the reduction of cis-Re2Cl2
or is limited to a transient interaction. These results are
consistent with the improved stability observed in controlled
potential electrolyses with cis-Re2Cl2.
Figure 11. Proposed mechanism for cis-Re2Cl2 at Eappl = −2.0 V
versus Fc+/0 in DMF/0.1 M Bu4NPF6.
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Anhydrous N,N-dimethylformamide (DMF) was purchased from Alfa
Aesar and packaged under argon in ChemSeal bottles. The rhenium
precursor Re(CO)5Cl was purchased from Strem and stored in the
glovebox. All other chemicals were reagent- or ACS-grade, purchased
hindered rotation about the anthracene bridge. With this
catalyst, CO2 is unable to act as an oxide acceptor by CO2
insertion into the Re−O bond of the bridging carboxylate;
instead, CO2 reduction is governed by protonation via
Hofmann degradation of the supporting electrolyte. Along
these lines, Kubiak’s dynamic hydrogen-bonded dinuclear
system mediates reductive disproportionation at low applied
potentials, analogous to the “one-electron” pathway of
Re(bpy)(CO)3Cl and consistent with its flexibility to
accommodate a CO2 insertion step.34 The proposed
mechanism of cis-Re2Cl2 at more negative potentials (Figure
S21) is similar to Figure 11 but with fast catalysis coinciding
with further reduction of the catalyst.
1
from commercial vendors, and used without further purification. H
and 13C NMR spectra were obtained using a Bruker Advance DRX-
500 spectrometer operating at 500 MHz (1H) or 126 MHz (13C).
Spectra were calibrated to residual protiated solvent peaks; chemical
shifts are reported in ppm. Electrospray ionization mass spectrometry
(ESI-MS) spectra were obtained with a Waters SYNAPT HDMS Q-
TOF mass spectrometer. UV−vis spectra were recorded on an
Agilent/Hewlett-Packard 8453 UV−vis spectrophotometer with a
diode-array detector. IR spectra were obtained on an Agilent
Technologies Cary 600 series FTIR spectrometer equipped with a
PIKE GladiATR accessory.
Electrochemical Measurements. Electrochemistry was per-
formed with a Bioanalytical Systems, Inc. (BASi), Epsilon
potentiostat. Cyclic voltammetry studies employed a three-electrode
cell equipped with a glassy carbon disk (3 mm diameter) working
electrode, a platinum wire counter electrode, and a silver wire quasi-
reference electrode that was referenced using ferrocene as an internal
standard at the end of the experiments. Electrochemistry was
conducted in anhydrous DMF containing 0.1 M Bu4NPF6 supporting
electrolyte. Solutions for cyclic voltammetry were thoroughly
degassed with argon or CO2 before data collection. All CVs were
cycled from the most positive potential to the most negative potential
and back.
Controlled potential electrolyses were carried out in a three-neck
glass cell with a glassy carbon rod (2 mm diameter, type 2, Alfa Aesar)
working electrode, a silver wire quasi-reference electrode, and a
platinum mesh (2.5 cm2 area, 150 mesh) counter electrode that was
separated from the other electrodes in an isolation chamber
comprised of a fine glass frit. Solutions were degassed with CO2 for
30 min before data collection. The applied potential values were
determined by cyclic voltammetry before controlled potential
electrolysis was performed. Evolved gases during electrolysis measure-
ments were quantified by gas chromatographic analysis of the
headspace samples using an Agilent 7890B gas chromatograph and an
Agilent PorapakQ (6 ft length and 1/8 in. o.d.) column. CO was
measured using a flame ionization detector equipped with a
methanizer, while dihydrogen was quantified at the thermal
conductivity detector. Constant stirring was maintained during
electrolysis. Integrated gas peaks were quantified with calibration
curves obtained from known standards purchased from BuyCalGas.-
com. From the experimental amount of product formed during
electrolysis, the Faradaic efficiencies were calculated against the
theoretical amount of product possible based on the accumulated
charge passed and the stoichiometry of the reaction.
CONCLUSION
■
We have described a novel ligand platform that gives access to
isolable cis and trans conformers comprised of two rhenium
active sites. Indeed, the conformers were purified by silica gel
chromatography and characterized. Consistent with its NMR
spectra, a crystal structure of trans-Re2Cl2 confirmed the
asymmetric orientation of rhenium sites on opposite sides of
the anthracene bridge. In contrast, the combined data (1H
NMR, FTIR, and reactivity studies) indicate that the cis
conformer is a symmetric species in which the rhenium sites
are on the same side of the anthracene backbone with their
chloro ligands directed toward one another. Both Re2
compounds are active electrocatalysts for reducing CO2 to
CO. From mechanistic studies, a pathway involving bimetallic
CO2 activation and conversion was identified for cis-Re2Cl2,
whereas well-established single-site and bimolecular pathways,
analogous to that of Re(bpy)(CO)3Cl, were observed for trans-
Re2Cl2.
The catalytic rates were measured by cyclic voltammetry for
cis-Re2Cl2, trans-Re2Cl2, Re(bpy)(CO)3Cl, and anthryl-Re with
estimated TOFs of 35.3, 22.9, 11.1, and 19.2 s−1, respectively.
The maximum TOFs of trans-Re2Cl2 and anthryl-Re (at twice
the concentration) are approximately equal, showing the effect
of the pendant anthracene on catalytic activity in comparison
to Re(bpy)(CO)3Cl and with each having single-site reactivity.
On the other hand, the cis conformer clearly outperforms the
trans conformer in terms of catalytic rate and stability,
indicating that the structure of cis-Re2Cl2 enables improved
performance. Indeed, a change in mechanism is observed at
low applied potentials in anhydrous conditions where
carbonate is not formed as a coproduct. Synergistic catalysis
by cooperative active sites in cis-Re2Cl2 is hypothesized. SEC
measurements indicate that cis-Re2Cl2 does not form a
deactivated Re−Re-bonded species. The catalyst structure
delivers a unique form of protection for catalyst longevity
because intermolecular deactivation pathways are limited by the
cofacial arrangement of active sites, while the rigid anthracene
backbone prevents intramolecular Re−Re bond formation.
The cis and trans conformers reported here exhibit distinct
mechanisms for electrochemical CO2-to-CO conversion.
Photocatalytic CO2 reduction combined with photophysical
studies exploring the role of anthracene as a pendant organic
chromophore is in progress.
UV−vis SEC measurements were conducted with a commercial
“honeycomb” thin-layer SEC cell from Pine Research Instrumenta-
tion, employing a gold working electrode, a silver wire quasi-reference
electrode, and a platinum wire counter electrode.
X-ray Crystallography. Single crystals were coated with Para-
tone-N hydrocarbon oil and mounted on the tip of a MiTeGen
micromount. The temperature was maintained at 100 K with an
Oxford Cryostream 700 during data collection at the X-ray
Crystallography Facility, Department of Chemistry and Biochemistry,
University of Mississippi. Samples were irradiated with Mo Kα
radiation with λ = 0.71073 Å using a Bruker Smart APEX II
diffractometer equipped with a Microfocus Sealed Source (Incoatec
IμS) and APEX-II detector. The Bruker APEX2, version 2009.1,
software package was used to integrate raw data, which were corrected
for Lorentz and polarization effects.71 A semiempirical absorption
correction (SADABS) was applied.72 The structure was solved using
direct methods and refined by least-squares refinement on F2 and
standard difference Fourier techniques using SHELXL.73 The thermal
parameters for all non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were included at ideal positions. Poorly resolved
outer-sphere DMF molecules could not be modeled successfully in
EXPERIMENTAL SECTION
■
Materials and Methods. Unless otherwise noted, all synthetic
manipulations were performed under a dinitrogen atmosphere using
standard Schlenk techniques or in an MBraun glovebox. Toluene was
dried with a Pure Process Technology solvent purification system.
H
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the difference map. The data were treated with the SQUEEZE
procedure in PLATON;74 details are provided in the CIF file.
Computational Methods. All calculations were performed in
Gaussian 09.75 All energies discussed in this work contain zero-point-
energy corrections.76 Global minima of the cis and trans conformers
were located using DFT and the ωB97X-D76 functional.77 The 6-
311+G* basis set was used for all coordinating atoms, which include
CO. The 6-311G* basis was used for all hydrogen and carbon atoms
of the ligand manifold. LanL2TZ(f) was used for the two rhenium
atoms. The minima were confirmed by calculating the harmonic
vibrational frequencies and finding all real values. To obtain a
structure close to the transition state (TS) of conformational
isomerization (via rotation), an initial relaxed scan was performed
using semiempirical PM6 and parametrized by the dihedral angle
C(14)−C(15)−C(23)−N(39) or C(6)−C(5)−C(29)−N(40) for
TS1 and TS2, respectively, as labeled in Figure S5. From these
scans, the maximum was used as the starting point for a TS
optimization using the Berny algorithm at the level of theory and basis
sets mentioned previously.
Structural optimization and IR spectral calculations were
performed using the BP86-D3/def2-TZVP functional−basis set
combination with Beck’s and Johnston’s dispersion corrections. The
rhenium atoms were described with the LanL2TZ(f) effective core
potential with the added polarization function. These calculations also
included DMF solvent-induced corrections using the COSMO
solvation model. Frequency analysis confirmed the global minima.
Global minima for the Re−Re dimers (Figure 10) were found using
the ωB97X-D functional with basis sets LanL2TZ(f) for rhenium
atoms and 6-31++G* for all light atoms. An ultrafine numerical
integration grid was employed for both species. Cartesian coordinates
and energies are given in Tables S2−S11.
2H), 7.83 (s, 1H), 7.74 (m, 4H), 7.61 (d, J = 7.6 Hz, 2H), 7.54 (d, J =
6.6 Hz, 2H). 13C NMR (126 MHz, DMF-d7): δ 199.47, 195.09,
195.07, 161.87, 158.22, 157.57, 154.10, 141.11, 140.77, 133.41,
131.90, 131.57, 131.42, 131.02, 128.77, 128.71, 128.48, 128.32,
126.82, 126.10, 125.93, 124.94, 124.04. ATR-FTIR: ν(CO) at 2015,
1915 (with shoulder at 1924), and 1871 cm−1. ESI-MS. Calcd for [cis-
Re2Cl2 + Cs+] (M+): m/z 1230.9. Found: m/z 1230.9.
1
trans-Re2Cl2. H NMR (500 MHz, DMSO-d6): δ 9.01 (d, J = 5.4
Hz, 1H), 8.98 (d, J = 5.4 Hz, 1H), 8.87 (s, 1H), 8.77 (d, J = 8.3 Hz,
1H), 8.73 (d, J = 8.2 Hz, 1H), 8.61 (d, J = 8.1 Hz, 1H), 8.57 (d, J =
8.1 Hz, 1H), 8.42 (t, J = 8.0 Hz, 1H), 8.39−8.32 (m, 3H), 8.02 (t, J =
7.9 Hz, 1H), 7.83 (t, J = 6.6 Hz, 1H), 7.77−7.68 (m, 3H), 7.65 (d, J =
7.1 Hz, 2H), 7.53 (t, J = 7.8 Hz, 1H), 7.49 (d, J = 6.8 Hz, 1H), 7.44
(d, J = 7.7 Hz, 1H), 7.33 (s, 1H). 13C NMR (126 MHz, DMF-d7): δ
199.36, 198.83, 195.14, 194.84, 191.95, 190.96, 162.48, 162.45,
157.90, 157.83, 157.60, 157.06, 154.20, 154.09, 141.80, 141.36,
141.12, 140.91, 140.59, 139.59, 133.31, 133.22, 131.91, 131.60,
131.50, 130.92, 130.57, 130.37, 129.28, 128.81, 128.61, 128.51,
126.67, 126.12, 126.01, 125.90, 124.42, 124.39, 123.92. ATR-FTIR:
ν(CO) at 2015 and 1891 cm−1. ESI-MS. Calcd for [trans-Re2Cl2 +
Cs+] (M+): m/z 1230.9. Found: m/z 1230.9.
1-(2,2′-Bipyridine)anthracene (2). To an oven-dried, two-neck,
round-bottomed flask were added 1-(neopentylglycolatoboryl)-
anthracene (1.0 g, 3.46 mmol), 6-bromo-2,2′-bipyridine (1.0 g, 4.15
mmol), and Pd(PPh3)4 (0.2 g, 5 mol %) under an inert atmosphere.
Degassed toluene (50 mL), ethanol (5 mL), and 2 M aqueous
solution of K2CO3 (5 mL) were added to the reaction vessel under
dinitrogen and refluxed for 2 days. After cooling to room temperature,
25 mL of saturated NH4Cl and 25 mL of H2O were added to the
mixture. The crude product was extracted with dichloromethane and
purified by silica gel chromatography (5:1 hexanes/ethyl acetate) to
yield a pure product (0.6 g, 52%). 1H NMR (500 MHz, DMSO-d6): δ
8.84 (s, 1H), 8.76 (dd, J = 88.9 and 4.7 Hz, 1H), 8.72 (s, 1H), 8.52
(d, J = 7.8 Hz, 1H), 8.41 (d, J = 8.0 Hz, 1H), 8.24 (d, J = 8.4 Hz, 1H),
8.18 (t, J = 7.8 Hz, 1H), 8.13 (d, J = 8.5 Hz, 1H), 8.00 (d, J = 8.4 Hz,
1H), 7.93 (td, J = 7.8 and 1.6 Hz, 1H), 7.85 (d, J = 7.6 Hz, 1H), 7.73
(d, J = 6.5 Hz, 1H), 7.66 (dd, 1H), 7.58−7.52 (m, 1H), 7.51−7.44
(m, 2H). 13C NMR (126 MHz, DMSO-d6): δ 158.47, 155.82, 155.49,
149.84, 138.87, 138.32, 137.89, 132.16, 131.98, 131.49, 129.73,
129.41, 129.06, 128.26, 127.78, 127.13, 126.43, 126.26, 125.69,
125.50, 124.86, 124.79, 121.07, 119.57. ESI-MS. Calcd for [2 + H+]
(M+): m/z 333.1. Found: m/z 333.1.
Synthetic Procedures. Re(bpy)(CO)3Cl was prepared as
previously described.78 Ligand precursors 6-bromo-2,2′-bipyridine79
and 1,8-bis(neopentylglycolatoboryl)anthracene38 were prepared
according to literature procedures.
1,8-Bis(2,2′-bipyridine)anthracene (1). In an oven-dried, two-
neck, round-bottomed fl ask were added 1, 8-bis-
(neopentylglycolatoboryl)anthracene (1.0 g, 2.49 mmol), 6-bromo-
2,2′-bipyridine (1.4 g, 5.97 mmol), and Pd(PPh3)4 (0.172 g, 6 mol %)
under an inert atmosphere. Degassed toluene (50 mL), ethanol (5
mL), and 2 M aqueous K2CO3 (5 mL) were added to the reaction
vessel under dinitrogen and refluxed for 2 days. After cooling to room
temperature, 25 mL of saturated aqueous NH4Cl and 25 mL of H2O
were added to the mixture. The crude product was extracted with
dichloromethane and purified by silica gel chromatography (1:1
[1-(2,2′-Bipyridine)anthracene][Re(CO)3Cl] (anthryl-Re). To an
oven-dried, two-neck, round-bottomed flask were added 1-(2,2′-
bipyridine)anthracene (100 mg, 0.300 mmol) and Re(CO)5Cl (109
mg, 0.300 mmol) under an inert atmosphere. Anhydrous toluene (5
mL) was added, and the reaction mixture was refluxed overnight. The
mixture was cooled to room temperature, and the precipitate was
collected by filtration with a glass frit and washed with toluene and
diethyl ether several times. A pure product was obtained by
crystallization from dichloromethane/hexanes (161 mg, 84%). Two
1
hexanes/ethyl acetate) to yield a pure product (1.04 g, 86%). H
NMR (500 MHz, DMSO-d6): δ 9.80 (s, 1H), 8.83 (s, 1H), 8.62 (ddd,
J = 4.7, 1.8, and 0.9 Hz, 2H), 8.27 (d, J = 8.4 Hz, 2H), 7.98 (dt, J =
7.8 and 1.0 Hz, 2H), 7.83 (dq, J = 7.9 and 1.0 Hz, 2H), 7.78−7.73
(m, 4H), 7.71−7.65 (m, 4H), 7.57 (td, J = 7.7 and 1.8 Hz, 2H), 7.33
(ddt, J = 6.9, 4.8, and 1.1 Hz, 2H). 13C NMR (126 MHz, DMSO-d6):
δ 157.60, 154.89, 154.52, 149.01, 137.99, 137.85, 137.01, 131.55,
129.14, 129.06, 127.38, 126.91, 125.50, 124.92, 123.84, 123.61,
119.96, 118.61. ESI-MS. Calcd for [1 + H+] (M+): m/z 487.2. Found:
m/z 487.2.
1
conformers are present by H NMR with slow rotation observed at
room temperature. ESI-MS. Calcd for [anthryl-Re + Cs+] (M+): m/z
770.9453. Found: m/z 770.9452. Elem anal. Calcd for
C28H16N2O4Re·1.5H2O: C, 48.76; H, 2.88; N, 4.21. Found: C,
49.09; H, 2.88; N, 4.22.
1,8-Bis(2,2′-bipyridine)anthracene(Re(CO)3Cl)2 (Re2Cl2). To an
oven-dried, two-neck, round-bottomed flask were added 1,8-bis-
(2,2′-bipyridine)anthracene (100 mg, 0.205 mmol) and Re(CO)5Cl
(148 mg, 0.41 mmol) under an inert atmosphere. Anhydrous toluene
(5 mL) was added, and the reaction mixture was refluxed overnight.
The mixture was cooled to room temperature, and the precipitate was
collected by filtration with a glass frit and washed with toluene and
diethyl ether several times. The crude product was purified by silica
gel chromatography with gradient elution from dichloromethane to
2:3 acetone/dichloromethane using a Biotage automated flash
purification system to give pure compounds, cis-Re2Cl2 (41% yield)
and trans-Re2Cl2 (20% yield).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01775.
Structures of seven possible isomers, 1H NMR,
experimental and theoretical IR spectra, calculated
energy barrier, CVs, plots of i/ip from linear sweep
voltammograms, plot of TOF versus scan rate,
concentration dependence, accumulated charge versus
cis-Re2Cl2. 1H NMR (500 MHz, DMSO-d6): δ 9.04 (d, J = 5.4 Hz,
2H), 8.87 (s, 1H), 8.77 (d, J = 8.3 Hz, 2H), 8.68 (d, J = 8.1 Hz, 2H),
8.39 (d, J = 8.5 Hz, 2H), 8.35 (t, J = 7.9 Hz, 2H), 7.90 (t, J = 7.9 Hz,
I
DOI: 10.1021/acs.inorgchem.8b01775
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
time, catalytic cycles, crystallographic data, and
Article
(f) Takeda, H.; Cometto, C.; Ishitani, O.; Robert, M. Electrons,
photons, protons, and earth-abundant metal complexes for molecular
catalysis of CO2 reduction. ACS Catal. 2017, 7, 70−88. (g) Francke,
R.; Schille, B.; Roemelt, M. Homogeneously catalyzed electro-
reduction of carbon dioxide − methods, mechanisms, and catalysts.
Chem. Rev. 2018, 118, 4631−4701.
(5) (a) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Efficient photochemical
reduction of CO2 to CO by visible light irradiation of systems
containing Re(bipy)(CO)3X or Ru(bipy)32+-Co2+ combinations as
homogeneous catalysts. J. Chem. Soc., Chem. Commun. 1983, 536−
538. (b) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Electrocatalytic
reduction of carbon dioxide mediated by Re(bipy)(CO)3Cl (bipy =
2,2′-bipyridine). J. Chem. Soc., Chem. Commun. 1984, 328−330.
(c) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Photochemical and
electrochemical reduction of carbon dioxide to carbon monoxide
mediated by (2,2′-bipyridine)tricarbonylchlororhenium(I) and re-
lated complexes as homogeneous catalysts. Helv. Chim. Acta 1986, 69,
1990−2012.
Cartesian coordinates (PDF)
Accession Codes
CCDC 1578651 contains the supplementary crystallographic
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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: jwjurss@olemiss.edu.
■
ORCID
Winston C. Pitts: 0000-0001-8964-8441
Frank R. Fronczek: 0000-0001-5544-2779
Jonah W. Jurss: 0000-0002-2780-3415
(6) Johnson, F. P. A.; George, M. W.; Hartl, F.; Turner, J. J.
Electrocatalytic reduction of CO2 using the complexes [Re(bpy)-
(CO)3L]n (n = + 1, L = P(OEt)3, CH3CN; n = 0, L = Cl−, Otf−; bpy
= 2,2′-bipyridine; Otf− = CF3SO3) as catalyst precursors: Infrared
spectroelectrochemical investigation. Organometallics 1996, 15,
3374−3387.
Author Contributions
‡Contributed equally.
Notes
The authors declare no competing financial interest.
(7) Breikss, A. I.; Abruna, H. D. Electrochemical and mechanistic
̃
studies of [Re(CO)3(dmbpy)Cl] and their relation to the catalytic
reduction of CO2. J. Electroanal. Chem. Interfacial Electrochem. 1986,
201, 347−358.
(8) Kurz, P.; Probst, B.; Spingler, B.; Alberto, R. Ligand variations in
[ReX(diimine)(CO)3] complexes: Effects on photocatalytic CO2
reduction. Eur. J. Inorg. Chem. 2006, 2006, 2966−2974.
(9) Smieja, J. M.; Kubiak, C. P. Re(bipy-tBu)(CO)3Cl-improved
catalytic activity for reduction of carbon dioxide: IR-spectroelec-
trochemical and mechanistic studies. Inorg. Chem. 2010, 49, 9283−
9289.
(10) Machan, C. W.; Chabolla, S. A.; Kubiak, C. P. Reductive
disproportionation of carbon dioxide by an alkyl-functionalized
pyridine monoamine Re(I) fac-tricarbonyl electrocatalyst. Organo-
metallics 2015, 34, 4678−4683.
(11) Stanton, C. J., III; Machan, C. W.; Vandezande, J. E.; Jin, T.;
Majetich, G. F.; Schaefer, H. F., III; Kubiak, C. P.; Li, G.; Agarwal, J.
Re(I) NHC complexes for electrocatalytic conversion of CO2. Inorg.
Chem. 2016, 55, 3136−3144.
(12) Huckaba, A. J.; Sharpe, E. A.; Delcamp, J. H. Photocatalytic
reduction of CO2 with Re-Pyridyl-NHCs. Inorg. Chem. 2016, 55,
682−690.
(13) Liyanage, N. P.; Dulaney, H. A.; Huckaba, A. J.; Jurss, J. W.;
Delcamp, J. H. Electrocatalytic reduction of CO2 to CO with Re-
Pyridyl-NHCs: Proton source influence on rates and product
selectivities. Inorg. Chem. 2016, 55, 6085−6094.
ACKNOWLEDGMENTS
■
We thank the University of Mississippi for generous start-up
funds and a seed grant from the National Science Foundation
(Grant OIA-1539035). The computational work is supported
by the Mississippi Center for Supercomputing Research and
the National Science Foundation (Grant CHE-1338056). We
also thank Dr. Javier Concepcion for helpful discussions and
Dr. Steven Davis and Sarah Johnson for guidance on the DFT
calculations.
REFERENCES
■
(1) Lewis, N. S.; Nocera, D. G. Powering the planet: Chemical
challenges in solar energy utilization. Proc. Natl. Acad. Sci. U. S. A.
2006, 103, 15729−15735.
(2) (a) Feely, R. A.; Sabine, C. L.; Lee, K.; Berelson, W.; Kleypas, J.;
Fabry, V. J.; Millero, F. J. Impact of anthropogenic CO2 on the
CaCO3 system in the oceans. Science 2004, 305, 362−366.
(b) Houghton, J. Global warming. Rep. Prog. Phys. 2005, 68, 1343−
1403. (c) IPCC. Climate Change 2014: Synthesis Report. Contribution
of Working Groups I, II and III to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change; Core Writing Team,-
Pachauri, R. K., Meyer, L. A., Eds.; IPCC: Geneva, Switzerland, 2014;
151 pp.
(14) Ching, H. Y. V.; Wang, X.; He, M.; Perujo Holland, N.; Guillot,
R.; Slim, C.; Griveau, S.; Bertrand, H. C.; Policar, C.; Bedioui, F.;
Fontecave, M. Rhenium complexes based on 2-pyridyl-1,2,3-triazole
ligands: A new class of CO2 reduction catalysts. Inorg. Chem. 2017,
56, 2966−2976.
(3) Concepcion, J. J.; House, R. L.; Papanikolas, J. M.; Meyer, T. J.
Chemical approaches to artificial photosynthesis. Proc. Natl. Acad. Sci.
U. S. A. 2012, 109, 15560−15564.
(4) (a) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M.
Electrocatalytic and homogeneous approaches to conversion of CO2
to liquid fuels. Chem. Soc. Rev. 2009, 38, 89−99. (b) Yui, T.; Tamaki,
Y.; Sekizawa, K.; Ishitani, O. Photocatalytic reduction of CO2: from
molecules to semiconductors. Top. Curr. Chem. 2011, 303, 151−184.
(c) Finn, C.; Schnittger, S.; Yellowlees, L. J.; Love, J. B. Molecular
approaches to the electrochemical reduction of carbon dioxide. Chem.
Commun. 2012, 48, 1392−1399. (d) Appel, A. M.; Bercaw, J. E.;
Bocarsly, A. B.; Dobbek, H.; DuBois, D. L.; Dupuis, M.; Ferry, J. G.;
Fujita, E.; Hille, R.; Kenis, P. J. A.; Kerfeld, C. A.; Morris, R. H.;
Peden, C. H. F.; Portis, A. R.; Ragsdale, S. W.; Rauchfuss, T. B.; Reek,
J. N. H.; Seefeldt, L. C.; Thauer, R. K.; Waldrop, G. L. Frontiers,
opportunities, and challenges in biochemical and chemical catalysis of
CO2 fixation. Chem. Rev. 2013, 113, 6621−6658. (e) Kang, P.; Chen,
Z.; Brookhart, M.; Meyer, T. J. Electrocatalytic reduction of carbon
dioxide: Let the molecules do the work. Top. Catal. 2015, 58, 30−45.
(15) Sung, S.; Kumar, D.; Gil-Sepulcre, M.; Nippe, M. Electro-
catalytic CO2 reduction by imidazolium-functionalized molecular
catalysts. J. Am. Chem. Soc. 2017, 139, 13993−13996.
(16) Nganga, J. K.; Samanamu, C. R.; Tanski, J. M.; Pacheco, C.;
Saucedo, C.; Batista, V. S.; Grice, K. A.; Ertem, M. Z.; Angeles-Boza,
A. M. Electrochemical reduction of CO2 catalyzed by Re(pyridine-
oxazoline)(CO)3Cl complexes. Inorg. Chem. 2017, 56, 3214−3226.
(17) Nakada, A.; Ishitani, O. Selective electrocatalysis of a water-
soluble rhenium(I) complex for CO2 reduction using water as an
electron donor. ACS Catal. 2018, 8, 354−363.
(18) (a) Ha, E.-G.; Chang, J.-A.; Byun, S.-M.; Pac, C.; Jang, D.-M.;
Park, J.; Kang, S. O. High-turnover visible-light photoreduction of
CO2 by a Re(I) complex stabilized on dye-sensitized TiO2. Chem.
Commun. 2014, 50, 4462−4464. (b) Won, D.-I.; Lee, J.-S.; Ji, J.-M.;
J
DOI: 10.1021/acs.inorgchem.8b01775
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Jung, W.-J.; Son, H.-J.; Pac, C.; Kang, S. O. Highly robust hybrid
photocatalyst for carbon dioxide reduction: Tuning and optimization
of catalytic activities of dye/TiO2/Re(I) organic-inorganic ternary
systems. J. Am. Chem. Soc. 2015, 137, 13679−13690.
(34) (a) Machan, C. W.; Chabolla, S. A.; Yin, J.; Gilson, M. K.;
Tezcan, F. A.; Kubiak, C. P. Supramolecular assembly promotes the
electrocatalytic reduction of carbon dioxide by Re(I) bipyridine
catalysts at a lower overpotential. J. Am. Chem. Soc. 2014, 136,
14598−14607. (b) Machan, C. W.; Yin, J.; Chabolla, S. A.; Gilson, M.
K.; Kubiak, C. P. Improving the efficiency and activity of
electrocatalysts for the reduction of CO2 through supramolecular
assembly with amino acid-modified ligands. J. Am. Chem. Soc. 2016,
138, 8184−8193.
(35) (a) Shakeri, J.; Hadadzadeh, H.; Tavakol, H. Photocatalytic
reduction of CO2 to CO by a dinuclear carbonyl polypyridyl
rhenium(I) complex. Polyhedron 2014, 78, 112−122. (b) Rezaei, B.;
Mokhtarianpour, M.; Ensafi, A. A.; Hadadzadeh, H.; Shakeri, J.
Electrocatalytic reduction of CO2 using the dinuclear rhenium(I)
complex [ReCl(CO)3(μ-tptzH)Re(CO)3]. Polyhedron 2015, 101,
160−164.
(36) (a) Wilting, A.; Stolper, T.; Mata, R. A.; Siewert, I. Dinuclear
rhenium complex with a proton responsive ligand as a redox catalyst
for the electrochemical CO2 reduction. Inorg. Chem. 2017, 56, 4176−
4185. (b) Wilting, A.; Siewert, I. A dinuclear rhenium complex with a
proton responsive ligand in the electrochemical-driven CO2-reduction
catalysis. ChemistrySelect 2018, 3, 4593−4597.
(37) Guilard, R.; Lopez, M. A.; Tabard, A.; Richard, P.; Lecomte, C.;
Brandes, S.; Hutchison, J. E.; Collman, J. P. Synthesis and
characterization of novel cobalt aluminum cofacial porphyrins. First
crystal and molecular structure of a heterobimetallic biphenylene
pillared cofacial diporphyrin. J. Am. Chem. Soc. 1992, 114, 9877−
9889.
(38) Wada, T.; Muckerman, J. T.; Fujita, E.; Tanaka, K. Substituents
dependent capability of bis(Ru-dioxolene-tpy) complexes toward
H2O oxidation. Dalton Trans. 2011, 40, 2225−2233.
(39) The modest yield is due to incomplete separation of the two
conformers, resulting in a subset of fractions that were not purified.
The reported yield could be higher because these fractions were taken
to dryness, accumulated with those of subsequent reactions, and later
purified as described.
(40) Fraser, M. G.; Clark, C. A.; Horvath, R.; Lind, S. J.; Blackman,
A. G.; Sun, X.-Z.; George, M. W.; Gordon, K. C. Complete family of
mono-, bi-, and trinuclear ReI(CO)3Cl complexes of the bridging
polypyridyl ligand 2,3,8,9,14,15-hexamethyl-5,6,11,12,17,18-hexaaza-
trinaphthalene: Syn/anti isomer separation, characterization, and
photophysics. Inorg. Chem. 2011, 50, 6093−6106.
(19) Windle, C. D.; Pastor, E.; Reynal, A.; Whitwood, A. C.;
Vaynzof, Y.; Durrant, J. R.; Perutz, R. N.; Reisner, E. Improving the
photocatalytic reduction of CO2 to CO through immobilization of a
molecular Re catalyst on TiO2. Chem. - Eur. J. 2015, 21, 3746−3754.
(20) (a) Sahara, G.; Abe, R.; Higashi, M.; Morikawa, T.; Maeda, K.;
Ueda, Y.; Ishitani, O. Photoelectrochemical CO2 reduction using a
Ru(II)-Re(I) multinuclear metal complex on a p-type semiconducting
NiO electrode. Chem. Commun. 2015, 51, 10722−10725. (b) Sahara,
G.; Kumagai, H.; Maeda, K.; Kaeffer, N.; Artero, V.; Higashi, M.; Abe,
R.; Ishitani, O. Photoelectrochemical reduction of CO2 coupled to
water oxidation using a photocathode with a Ru(II)-Re(I) complex
photocatalyst and a CoOx/TaON photoanode. J. Am. Chem. Soc.
2016, 138, 14152−14158.
̈
(21) Schreier, M.; Luo, J.; Gao, P.; Moehl, T.; Mayer, M. T.; Gratzel,
M. Covalent immobilization of a molecular catalyst on Cu2O
photocathodes for CO2 reduction. J. Am. Chem. Soc. 2016, 138,
1938−1946.
(22) Sullivan, B. P.; Bolinger, C. M.; Conrad, D.; Vining, W. J.;
Meyer, T. J. One- and two-electron pathways in the electrocatalytic
reduction of CO2 by fac-Re(bpy)(CO)3Cl (bpy = 2,2′-bipyridine). J.
Chem. Soc., Chem. Commun. 1985, 1414−1416.
(23) Paolucci, F.; Marcaccio, M.; Paradisi, C.; Roffia, S.; Bignozzi, C.
A.; Amatore, C. Dynamics of the electrochemical behavior of diimine
tricarbonyl rhenium(I) complexes in strictly aprotic media. J. Phys.
Chem. B 1998, 102, 4759−4769.
(24) Hayashi, Y.; Kita, S.; Brunschwig, B. S.; Fujita, E. Involvement
of a binuclear species with the Re-C(O)O-Re moiety in CO2
reduction catalyzed by tricarbonyl rhenium(I) complexes with
diimine ligands: Strikingly slow formation of the Re-Re and Re-
C(O)O-Re species from Re(dmb)(CO)3S (dmb = 4,4′-dimethyl-2,2′-
bipyridine, S = solvent). J. Am. Chem. Soc. 2003, 125, 11976−11987.
(25) Fujita, E.; Muckerman, J. T. Why is Re-Re bond formation/
cleavage in [Re(bpy)(CO)3]2 different from that in [Re(CO)5]2?
Experimental and theoretical studies on the dimers and fragments.
Inorg. Chem. 2004, 43, 7636−7647.
(26) Agarwal, J.; Fujita, E.; Schaefer, H. F., III; Muckerman, J. T.
Mechanisms for CO production from CO2 using reduced rhenium
tricarbonyl catalysts. J. Am. Chem. Soc. 2012, 134, 5180−5186.
(27) Benson, E. E.; Kubiak, C. P. Structural investigations into the
deactivation pathway of the CO2 reduction electrocatalyst Re(bpy)-
(CO)3Cl. Chem. Commun. 2012, 48, 7374−7376.
(28) Liang, W.; Church, T. L.; Zheng, S.; Zhou, C.; Haynes, B. S.;
D’Alessandro, D. M. Site isolation leads to stable photocatalytic
reduction of CO2 over a rhenium-based catalyst. Chem. - Eur. J. 2015,
21, 18576−18579.
(29) Machan, C. W.; Sampson, M. D.; Chabolla, S. A.; Dang, T.;
Kubiak, C. P. Developing a mechanistic understanding of molecular
electrocatalysts for CO2 reduction using infrared spectroelectrochem-
istry. Organometallics 2014, 33, 4550−4559.
(30) Kou, Y.; Nabetani, Y.; Masui, D.; Shimada, T.; Takagi, S.;
Tachibana, H.; Inoue, H. Direct detection of key reaction
intermediates in photochemical CO2 reduction sensitized by a
rhenium bipyridine complex. J. Am. Chem. Soc. 2014, 136, 6021−
6030.
(31) Grice, K. A.; Kubiak, C. P. Recent studies of rhenium and
manganese bipyridine carbonyl catalysts for the electrochemical
reduction of CO2. Adv. Inorg. Chem. 2014, 66, 163−188.
(32) Bruckmeier, C.; Lehenmeier, M. W.; Reithmeier, R.; Rieger, B.;
Herranz, J.; Kavakli, C. Binuclear rhenium(I) complexes for the
photocatalytic reduction of CO2. Dalton Trans. 2012, 41, 5026−5037.
(33) Tamaki, Y.; Imori, D.; Morimoto, T.; Koike, K.; Ishitani, O.
High catalytic abilities of binuclear rhenium(I) complexes in the
photochemical reduction of CO2 with a ruthenium(II) photo-
sensitizer. Dalton Trans. 2016, 45, 14668−14677.
(41) (a) Randles, J. E. B. A cathode ray polarograph. Part II.-The
current-voltage curves. Trans. Faraday Soc. 1948, 44, 327−338.
(b) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals
and Applications, 2nd ed.; John Wiley and Sons: New York, 2001.
(42) Richardson, D. E.; Taube, H. Mixed-valence molecules:
Electronic delocalization and stabilization. Coord. Chem. Rev. 1984,
60, 107−129.
(43) Manes, T. A.; Rose, M. J. Redox properties of a bis-pyridine
rhenium carbonyl derived from an anthracene scaffold. Inorg. Chem.
Commun. 2015, 61, 221−224.
́
(44) Saveant, J. M.; Vianello, E. Potential-sweep chronoamperom-
etry theory of kinetic currents in the case of a first order chemical
reaction preceding the electron-transfer process. Electrochim. Acta
1963, 8, 905−923.
(45) Nicholson, R. S.; Shain, I. Theory of stationary electrode
polarography. Single scan and cyclic methods applied to reversible,
irreversible, and kinetic systems. Anal. Chem. 1964, 36, 706−723.
(46) Rountree, E. S.; McCarthy, B. D.; Eisenhart, T. T.; Dempsey, J.
L. Evaluation of homogeneous electrocatalysts by cyclic voltammetry.
Inorg. Chem. 2014, 53, 9983−10002.
(47) Huang, D.; Makhlynets, O. V.; Tan, L. L.; Lee, S. C.; Rybak-
Akimova, E. V.; Holm, R. H. Kinetics and mechanistic analysis of an
extremely rapid carbon dioxide fixation reaction. Proc. Natl. Acad. Sci.
U. S. A. 2011, 108, 1222−1227.
(48) (a) Steffey, B. D.; Curtis, C. J.; DuBois, D. L. Electrochemical
reduction of CO2 catalyzed by a dinuclear palladium complex
containing a bridging hexaphosphine ligand: Evidence for coopera-
K
DOI: 10.1021/acs.inorgchem.8b01775
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
tivity. Organometallics 1995, 14, 4937−4943. (b) Raebiger, J. W.;
Turner, J. W.; Noll, B. C.; Curtis, C. J.; Miedaner, A.; Cox, B.; DuBois,
D. L. Electrochemical reduction of CO2 to CO catalyzed by a
bimetallic palladium complex. Organometallics 2006, 25, 3345−3351.
(49) Liu, S.; Senocak, A.; Smeltz, J. L.; Yang, L.; Wegenhart, B.; Yi,
[PF6]. Electrocatalytic reduction of carbon dioxide. Organometallics
1992, 11, 1986−1988.
(65) Fei, H.; Sampson, M. D.; Lee, Y.; Kubiak, C. P.; Cohen, S. M.
Photocatalytic CO2 reduction to formate using a Mn(I) molecular
catalyst in a robust metal-organic framework. Inorg. Chem. 2015, 54,
6821−6828.
(66) (a) Bolinger, C. M.; Sullivan, B. P.; Conrad, D.; Gilbert, J. A.;
Story, N.; Meyer, T. J. Electrocatalytic reduction of CO2 based on
polypyridyl complexes of rhodium and ruthenium. J. Chem. Soc.,
Chem. Commun. 1985, 796−797. (b) Bolinger, C. M.; Story, N.;
Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1988, 27, 4582−4587.
(67) Sampson, M. D.; Froehlich, J. D.; Smieja, J. M.; Benson, E. E.;
Sharp, I. D.; Kubiak, C. P. Direct observation of the reduction of
carbon dioxide by rhenium bipyridine catalysts. Energy Environ. Sci.
2013, 6, 3748−3755.
(68) Christensen, P. A.; Hamnett, A.; Muir, A. V. G.; Freeman, N. A.
CO2 reduction at platinum, gold, and glassy carbon electrodes in
acetonitrile. An in-situ FTIR study. J. Electroanal. Chem. Interfacial
Electrochem. 1990, 288, 197−215.
(69) Cheng, S. C.; Blaine, C. A.; Hill, M. G.; Mann, K. R.
Electrochemical and IR spectroelectrochemical studies of the
electrocatalytic reduction of carbon dioxide by [Ir2(dimen)4]2+
(dimen = 1,8-diisocyanomenthane). Inorg. Chem. 1996, 35, 7704−
7708.
(70) Takeda, H.; Koike, K.; Inoue, H.; Ishitani, O. Development of
an efficient photocatalytic system for CO2 reduction using rhenium(I)
complexes based on mechanistic studies. J. Am. Chem. Soc. 2008, 130,
2023−2031.
(71) APEX2, version 2009.1; Bruker Analytical X-ray Systems Inc.:
Madison, WI, 2009.
̈
J.; Kenttamaa, H. I.; Ison, E. A.; Abu-Omar, M. M. Mechanism of
MTO-catalyzed deoxydehydration of diols to alkenes using sacrificial
alcohols. Organometallics 2013, 32, 3210−3219.
́
́
(50) Soler-Yanes, R.; Arribas-Alvarez, I.; Guisan-Ceinos, M.; Bun
̃
uel,
́
E.; Cardenas, D. J. NiI catalyzes the regioselective cross-coupling of
alkylzinc halides and propargyl bromides to allenes. Chem. - Eur. J.
2017, 23, 1584−1590.
(51) Ngo, K. T.; McKinnon, M.; Mahanti, B.; Narayanan, R.; Grills,
D. C.; Ertem, M. Z.; Rochford, J. Turning on the protonation-first
pathway for electrocatalytic CO2 reduction by manganese bipyridyl
tricarbonyl complexes. J. Am. Chem. Soc. 2017, 139, 2604−2618.
́
(52) (a) Costentin, C.; Drouet, S.; Robert, M.; Saveant, J.-M.
Turnover numbers, turnover frequencies, and overpotential in
molecular catalysis of electrochemical reactions. Cyclic voltammetry
and preparative-scale electrolysis. J. Am. Chem. Soc. 2012, 134,
́
11235−11242. (b) Costentin, C.; Robert, M.; Saveant, J.-M.;
́
Costentin, C.; Robert, M.; Saveant, J.-M. Catalysis of the electro-
chemical reduction of carbon dioxide. Chem. Soc. Rev. 2013, 42,
2423−2436.
(53) Wiedner, E. S.; Bullock, R. M. Electrochemical detection of
transient cobalt hydride intermediates of electrocatalytic hydrogen
production. J. Am. Chem. Soc. 2016, 138, 8309−8313.
(54) No significant differences were observed in the estimated TOFs
from FOWA when using E0cat rather than Ecat/2 in eq 3, except the
TOF versus scan rate plots (Figure S16) were more well-behaved
before reaching the scan-rate-independent TOF maximum when
using Ecat/2. Because of the slow catalysis following the initial
overlapping one-electron reductions of the dinuclear catalysts, the
linear portion of the FOWA plots was used to estimate the TOFs.
(55) Wong, K.-Y.; Chung, W.-H.; Lau, C.-P. The effect of weak
(72) Sheldrick, G. M. SADABS, version 2.03; Bruker Analytical X-ray
Systems Inc.: Madison, WI, 2000.
(73) (a) Sheldrick, G. M. Phase annealing in SHELX-90: Direct
methods for larger structures. Acta Crystallogr., Sect. A: Found.
Crystallogr. 1990, 46, 467−473. (b) Sheldrick, G. M. A short history
of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64,
112−122. (c) Sheldrick, G. M. SHELXL-2014/7: Program for crystal
̈
Bronsted acids on the electrocatalytic reduction of carbon dioxide by
a rhenium tricarbonyl bipyridyl complex. J. Electroanal. Chem. 1998,
453, 161−170.
̈
̈
structure determination; University of Gottingen: Gottingen, Germany,
2014.
(56) Smieja, J. M.; Benson, E. E.; Kumar, B.; Grice, K. A.; Seu, C. S.;
Miller, A. J. M.; Mayer, J. M.; Kubiak, C. P. Kinetic and structural
studies, origins of selectivity, and interfacial charge transfer in the
artificial photosynthesis of CO. Proc. Natl. Acad. Sci. U. S. A. 2012,
109, 15646−15650.
(57) It has been shown that the pKa values of weak acids are very
similar between DMSO and DMF. Maran, F.; Celadon, D.; Severin,
M. G.; Vianello, E. Electrochemical determination of the pKa of weak
acids in N,N-dimethylformamide. J. Am. Chem. Soc. 1991, 113, 9320−
9329.
(74) (a) Spek, A. L. PLATON: A Multipurpose Crystallographic Tool;
Utrecht University: Utrecht, The Netherlands, 2007. (b) Spek, A. L.
PLATON SQUEEZE: A tool for the calculation of the disordered
solvent contribution to the calculated structure factors. Acta
Crystallogr., Sect. C: Struct. Chem. 2015, 71, 9−18.
(75) Frisch, M. J.; et al. Gaussian 09, revision D.01; Gaussian, Inc.:
Wallingford, CT, 2009.
(76) Lin, Y.-S.; Li, G.-D.; Mao, S.-P.; Chai, J.-D. Long-range
corrected hybrid density functionals with improved dispersion
corrections. J. Chem. Theory Comput. 2013, 9, 263−272.
(77) Minenkov, Y.; Singstad, Å.; Occhipinti, G.; Jensen, V. R. The
accuracy of DFT-optimized geometries of functional transition metal
compounds: A validation study of catalysts for olefin metathesis and
other reactions in the homogeneous phase. Dalton Trans. 2012, 41,
5526−5541.
(58) Olmstead, W. N.; Margolin, Z.; Bordwell, F. G. Acidities of
water and simple alcohols in dimethyl sulfoxide solution. J. Org. Chem.
1980, 45, 3295−3299.
(59) Taft, R. W.; Bordwell, F. G. Structural and solvent effects
evaluated from acidities measured in dimethyl sulfoxide and in the gas
phase. Acc. Chem. Res. 1988, 21, 463−469.
(78) Caspar, J. V.; Meyer, T. J. Application of the energy gap law to
nonradiative, excited-state decay. J. Phys. Chem. 1983, 87, 952−957.
(60) Bordwell, F. G.; McCallum, R. J.; Olmstead, W. N. Acidities
and hydrogen bonding of phenols in dimethyl sulfoxide. J. Org. Chem.
1984, 49, 1424−1427.
̈
(79) Bottger, M.; Wiegmann, B.; Schaumburg, S.; Jones, P. G.;
Kowalsky, W.; Johannes, H.-H. Synthesis of new pyrrole-pyridine-
based ligands using an in situ Suzuki coupling method. Beilstein J. Org.
Chem. 2012, 8, 1037−1047.
(61) Pegis, M. L.; Roberts, J. A. S.; Wasylenko, D. J.; Mader, E. A.;
Appel, A. M.; Mayer, J. M. Standard reduction potentials for oxygen
and carbon dioxide couples in acetonitrile and N,N-dimethylforma-
mide. Inorg. Chem. 2015, 54, 11883−11888.
(62) Appel, A. M.; Helm, M. L. Determining the overpotential for a
molecular electrocatalyst. ACS Catal. 2014, 4, 630−633.
(63) Lee, G. R.; Maher, J. M.; Cooper, N. J. Reductive
disproportionation of carbon dioxide by dianionic carbonylmetalates
of the transition metals. J. Am. Chem. Soc. 1987, 109, 2956−2962.
(64) Ratliff, K. S.; Lentz, R. E.; Kubiak, C. P. Carbon dioxide
chemistry of the trinuclear complex [Ni3(μ3-CNMe)(μ3-I)(dppm)3]-
L
DOI: 10.1021/acs.inorgchem.8b01775
Inorg. Chem. XXXX, XXX, XXX−XXX