A Thiourea Tether in the Second Coordination Sphere as a Binding Site for CO2 and a Proton Donor Promotes the Electrochemical Reduction of CO2 to CO Catalyzed by a Rhenium Bipyridine-Type Complex

Article abstract of DOI:10.1021/jacs.8b05658

The electrochemical reduction of CO₂ has been extensively investigated in recent years, with the expectation that a detailed mechanistic understanding could achieve the goal of finding a stable catalyst with high turnover frequencies and low re

Full text of DOI:10.1021/jacs.8b05658

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Article
A Thiourea Tether in the Second Coordination Sphere as a Binding Site
for CO2 and a Proton Donor Promotes the Electrochemical Reduction
of CO2 to CO Catalyzed by a Rhenium Bipyridine Type Complex
Eynat Haviv, Dima Azaiza-Dabbah, Raanan Carmieli, Liat Avram, Jan M.L. Martin, and Ronny Neumann
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05658 • Publication Date (Web): 12 Sep 2018
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A Thiourea Tether in the Second Coordination Sphere as a Binding Site
for CO and a Proton Donor Promotes the Electrochemical Reduction
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of CO to CO Catalyzed by a Rhenium Bipyridine Type Complex
2
†
†
‡
‡
†
†
Eynat Haviv, Dima Azaiza-Dabbah, Raanan Carmieli, Liat Avram, Jan M. L. Martin, and Ronny Neumann*
†
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel 76100
‡ Department for Chemical Research Support, Weizmann Institute of Science, Rehovot, Israel 76100
Supporting Information Placeholder
ABSTRACT: The electrochemical reduction of CO
is being extensively investigated in the recent years, where detailed mecha-
2
nistic understanding could lead to the goal of finding a stable catalyst with high turnover frequencies and low reduction potentials.
In the catalytic cycle of the carbon dioxide hydrogenase enzyme it has been suggested that the reduced metal center reacts with
CO
2
to form a carboxylate intermediate that is stabilized by hydrogen bonding using a histidine moiety in the second coordination
Cl complex as a starting point, the bipyridine ligand was modified in the
sphere. Using the well-known fac-Re(I)bipyridine(CO)
3
second coordination sphere with a thiourea tether that is known to form hydrogen bonds with carbonyl moieties. The resulting
-
1
Re(I) catalyst was an excellent electrocatalyst for the selective reduction of CO
2
to CO with a turnover frequency of 3040 s . The
1
binding of CO
2
to the thiourea tether was observable by H NMR, and NOE experiments showed that the hydrogen atoms of the
thiourea group were labile. Further experiments indicated that the thiourea moiety is also a local proton source and addition of an
external proton source actually inhibits catalysis. The absence of a kinetic isotope effect was explained through DFT calculations
that showed that the proton invariably jumps to the nearest CO
through any minimum or transition state. EPR and NMR were used to identify the various reduced intermediates. Thus, the thi-
ourea tether in the second coordination sphere can bind CO , stabilize carboxylic acid reaction intermediates and directly acts as
2
oxygen atom to form a metal-carboxylic acid without going
2
local proton source leading to a significantly more active catalyst.
Introduction
potentials. Recent research to improve electrocatalysis has
mostly focused on the incorporation of functional groups that
can introduce local concentration of protons in the second co-
ordination sphere. Common functional groups that have been
The electrocatalytic reduction of CO
ular catalysts typically proceeds through the reaction of the re-
duced metal center with the carbon atom of CO to form a
metallocarboxylate intermediate. The carboxylate interme-
diate can be stabilized by interactions with an auxiliary func-
tional group that may be present in a second coordination
sphere. An example where such auxiliary functional group has
an apparent role is the CO dehydrogenase enzyme, [NiFe]
CODH, which catalyzes both CO reduction and CO oxida-
tion. The active site of the enzyme consists of two transition
metals, nickel and iron that take part in the activation and the
reduction of CO
reduced and binds CO
metalocarboxylate intermediate. This intermediate can be sta-
bilized by hydrogen bonding of the oxygen atom with a histi-
dine residue (H93).4
2
to CO using molec-
2
1
2-17
18-20
1
-3
introduced include phenols,
amines, , carboxylic ac-
2
1
22
23
ids, ammonium cations, and an imidazolium moiety. Be-
yond the introduction of a local proton source, it has been in-
ferred from the electrochemical measurements and computa-
tional studies that these functional group can bind reaction in-
termediates although this has not been unequivocally sup-
ported by experiment. In fact, most recently DFT calculations
have shown that pendent/proximal amine groups do not di-
rectly supply protons needed for C-O bond cleavage but in-
stead create a hydrogen bond network with the actual proton
2
2
. In the catalytic cycle, the Ni(II) center is
through the carbon atom to form
2
24
donor, trifluoroethanol.
Thioureas, especially those substituted with electron with-
drawing groups such as PhCF are organic catalysts that pro-
3
mote reactions through the formation of hydrogen bonds
with various functional groups such as nitro groups, imines,
carbonyls and various anions, such as a carboxylate anion.2
We now report on the modification of the long-known elec-
Since the first reports on transition metal complexes for
electrochemical reduction,5 many modifications have
-11
CO
2
5-29
been suggested to increase total turnover numbers, to attain
higher turnover frequencies and to operate at lower reduction
trocatalyst, fac-Re(I)(bipy)(CO)
3
Cl with such a thiourea
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tether for the selective reduction of CO
2
to CO where thiou-
Scheme 1. Synthetic Pathway for the Preparation of the Re
Complex with the Thiourea tethered Bipyridine Ligand
1
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rea moiety can act to bind CO , stabilize carboxylate reaction
2
intermediates, and act as a local proton donor, Figure 1. Com-
pared to other catalysts of this series, very high turnover fre-
-1
quencies of 3040 s were obtained
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The ability of the thiourea functional group in compound
to interact with CO was examined in a NMR experiment.
Thus, a solution of in toluene-d was measured under 1 bar
and then under 1 bar CO . The assignment of the chem-
1
2
1
8
Figure 1. A notional optimized structure using a constrained ge-
ometry from DFT calculations of the Re(Bipy)(CO)
pound with the thiourea tether, the binding of CO , and interac-
tion with the thiourea hydrogen atom to facilitate C-O bond
cleavage. C - black, F - green, N - blue, S - yellow, O - red, Re –
purple, H – pink.
13
N
2
2
3
com-
ical shifts of the thiourea hydrogen atoms was possible by 2D
NMR (see SI). Notably a downfield shift from 9.19 to 9.43
ppm was observed for the hydrogen atom assigned as the one
2
adjacent to the bipyridine moiety, Figure 2. This indicates a
28
direct interaction between the thiourea moiety and CO
2
. In
13
Experimental observations include (1) the binding of CO
2
addition a further C NMR measurement shows the appear-
ance of a peak at 140 ppm assigned to bound CO downfield
1
to the thiourea tether as observed by H NMR measurements,
2) NOE experiments that showed that the hydrogen atom of
2
(
from unbound CO at 125 ppm. (Figure S16).
2
the thiourea group has fast exchange with water, (3) DFT
studies that showed that the thiourea moiety easily protonates
a carboxylate intermediate and (4) EPR and NMR measure-
ments of the various electrochemically reduced catalyst spe-
cies. Thus, the thiourea tether in the second coordination
sphere can bind CO and by inference stabilize other reaction
2
intermediates and act as local proton source in the absence of
an external proton source leading to a significantly more ac-
tive catalyst.
Results and Discussion
The thiourea tethered bipyridine ligand was prepared in
two synthetic steps, Scheme 1. First, 6-bromo-2,2'-bipyridine
was aminated by a new method developed in our lab using
guanidine nitrate as an aminating reagent. The resulting 6-
amino-2,2,'-bipyridine was coupled with 4-(trifluoromethyl)-
phenyl isothiocyanate to yield the ligand, 1-([2,2'-bipyridin]-
1
Figure 2. H NMR spectra of
1
in toluene-d in the presence of
8
6
-yl)-3-(4-(trifluoromethyl)phenyl)thiourea (
L
). The elec-
N and CO showing the binding to the thiourea moiety. The
2
2
tron withdrawing trifluoromethyl substituted phenyl isothio-
cyanate was used to increase the acidity of the hydrogen at-
oms of the subsequently formed thiourea, thereby increasing
the likelihood of interaction with the basic oxygen atoms of
peaks of the H atoms of the thiourea moiety are circled.
The electrocatalytic properties of compound were meas-
ured by cyclic voltammetry and compared with the properties
of the known fac-Re(bpy)(CO) Cl catalyst, Figure 2. Cyclic
voltammograms were recorded in acetonitrile, in the absence
and in the presence of CO . The cyclic voltammogram of
compound under N shows three main reduction events,
Figure 3 left. See below for a discussion on this observation.
The addition of CO shows a large increase in current, Figure
right, and a comparison to the fac-bpyRe(CO) Cl catalyst
reveals that compound has significantly improved catalytic
properties, Figure 3 right.
1
3
CO
2
. The ligand L1 was then reacted with Re(CO)
5
Cl to
yield the complex,
1
. The details of the synthesis and charac-
2
terization of the compounds by NMR, ESI-MS and IR are pre-
sented in the experimental section and supporting infor-
mation, Figures S1-S14. Despite multiple attempts we were
1
2
2
unable to crystallize . It should be noted that is sensitive to
1 1
3
3
oxygen and in its presence the enthiolate tautomer dimerizes
to yield a disulfide that was identified by X-ray diffraction of a
single crystal (Figure S15).
1
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(
prior to C-O bond cleavage) by intramolecular hydrogen
1
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bonding and also act as a proton source. Thus, the addition of
an additional external proton source may disrupt the intramo-
lecular interaction. Indeed, cyclic voltammetry measurement
after the addition of 0.5 M phenol at 1 bar CO
crease in the catalytic current compared to a cyclic voltamme-
try measurement at 1 bar CO without any additive, Figure 4,
left. In a similar manner, the addition of 5 vol % water also led
to a decrease in catalytic current, Figure 4 right.
2
showed a de-
2
Figure 3. Cyclic voltammograms of compound
electrochemical experiments support the involvement of hydro-
gen transfer in the CO reduction reaction. (left) 1mM , 1 bar
. (right) 1mM , 1 bar N (black); 1mM , 1 bar CO (red); 1
mM fac-Re(bpy)(CO) Cl (blue), 1 bar CO . Measurements
were carried out at a scan rate of 100mV/s, with 0.1M TBAPF
as supporting electrolyte, glassy carbon as working electrode, Pt
wire as counter electrode and Ag/AgNO as reference electrode
1
in MeCN. The
1
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N
2
1
2
1
2
3
2
6
3
and standardized with ferrocene/ferrocenium.
Figure 4. Cyclic voltammograms of 1 mM
CO (black lines) compared with the cyclic voltammograms after
the addition of a proton source. Left: addition of 0.5 M PhOH
(red). Right: addition of 5 vol% H O (red). Measurements were
carried out at a scan rate of 100 mV/s, with 0.1 M TBAPF as
supporting electrolyte, a glassy carbon as working electrode, a Pt
wire counter electrode and a Ag/AgNO reference electrode and
1
in MeCN at 1 bar
There are several methods which can be used to extract cat-
2
alytic parameters from the cyclic voltammetry measure-
30-36
ments.
We used the equation used by Kubiak et al. to com-
2
pare many Re- based catalysts to derive the maximum turno-
ver frequency (TOF). CV measurements were carried out at
increasing scan rates to achieve the plateau current (Figures
S17-S19) for the determination of kcat (TOF max under kinetic
conditions) according the following equation 1:
36
6
3
standardized with ferrocene/ferrocenium.
(1)
Where R is the universal gas constant, T is the temperature,
F is Faraday constant, ν is the scan rate, n is the number of
unique electron transfer processes that occur at the electrode
per catalyst (n = 2), n´ is the catalyst equivalents required per
catalyst (n´= 1), and kcat is the intrinsic catalytic rate constant.
-
1
The calculated TOF for
1
is 3040 s at 1 bar CO
to 355 s measured for the standard Re(bipy)(CO)
pound.
2
compared
Cl com-
-
1
3
Further, a controlled potential electrocatalytic reduction of
CO was performed in a typical electrochemical cell, using a 3
mm diameter glassy carbon working electrode, a Pt wire sep-
arated by a glass frit as counter electrode and Ag/AgNO as
reference electrode. The reaction was run for 12 h at -1.9 V
versus Ag/AgNO , yielding 12.4 µmol CO as only product
Figure 5. The intermolecular homonuclear NOEs between water
2
(
I) and protons 1 and 2 in compound
1
and its precursor ligand
L1 (I
S
).
3
The characterization of compound
1
by NMR revealed that
3
the protons on the thiourea tether quickly exchange with re-
sidual water in the solvent. This exchange suggests that these
protons will be also available to protonate the carboxylate in-
termediate. In order to qualitatively estimate this exchange,
the intermolecular homonuclear NOEs between these pro-
tons and water were measured. 1D-NOE experiments, with
different mixing times, were performed on a THF-d solution
of 1 (10 µmol ) and also the precursor ligand, L1. The water
peak (I) was integrated and normalized to the irradiated sig-
nal (I ). The results presented in Figure 5 show that the NOE
with a Faradaic efficiency of 89 %. A plot of the current as a
function of time shows that the current is stable over a reac-
tion period of 15 h (Figure S20). In addition, extraction of
and analysis by IR and H-NMR as previously reported
1
1
35
showed mainly the retention of taking into account the loss
1
of Cl during the catalytic cycle and solvent ligation (Figures
S21, S22). Together these measurements show very good sta-
–
8
bility of
Often Brønsted acids such as phenol are added as a proton
source in order to increase the catalytic current for CO reduc-
1
during electrolysis.
S
buildup is much faster for the proton next to the bipyridine
moiety, and the reaction center, which indicates that this pro-
ton exchanges with water. In addition, this proton exchanges
2
3
0
tion. The presence of a thiourea tether in the second coordi-
nation sphere can stabilize the key carboxylate intermediate
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faster before complexation of Re than after complexation. The
proton at position 2 shows only very slight exchange in the
complex but not the ligand (not shown). The NMR and elec-
trochemical experiments presented above suggest interaction
the nitrogen atom. This is also in line with calculations done
1
2
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6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
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2
2
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on a cobalt complex with an amine tether. These calcula-
tions probably explain the inhibition of electrocatalysis by an
external proton donor as shown in Figure 4, due to the intro-
duction of additional, in the thiourea case, of undesired hy-
drogen bonding networks.
between CO and/or a carboxylate intermediate with the thi-
2
ourea group. Since Equation 1 directly links the catalytic cur-
rent observed in the cyclic voltammogram with the kcat and
TOFmax values, there is a remains a question whether there is
a H/D kinetic isotopic effect that should be measureable in
the cyclic voltammogram upon the replacement of the proton
source with deuterium. Compound 1-D was prepared by dis-
Previous research on the reactivity and mechanism of
Re(bipy)(CO) Cl compounds and bipyridine substituted an-
3
1-
alogs suggest that a doubly reduced species, Re(bipy)(CO)
3
0
1
2
3
4
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, is the active species that binds CO
2
to form the key carbox-
33,36
ylate intermediate.
This intermediate has a purple color
solving compound 1-H in 1:1 mixture of dry THF and D
O
2
with a lmax = 570 nm and is diamagnetic (open shell singlet)
and after stirring for 30 min the solvents were removed under
reduced pressure. The cyclic voltammograms of 1-H and 1-D
1
with a commensurate H NMR spectrum. EPR spectra of one-
37,38
electron reduced species have also been measured
but the
under 1 bar CO
effects are minimized are presented in Figure S23. No differ-
ence in current was observed between 1-H and 1-D
2
at a fast scan rate, 500 mV/s, where diffusion
species formed have not been clearly assigned although possi-
ble one-electron species and the pathway for their formation
.
35
have been proposed and computationally elucidated. For
compound as noted above in Figure 3 left, three main reduc-
1
tion events were observed with onset potentials at -1.42, -1.66,
1.93 V versus. Ag/AgNO . The 1-, 2- and 3-electron reduced
analogs of compound were prepared by electrochemical re-
duction -2.0 V versus Ag/AgNO with the appropriate charge.
The electrochemically irreversible singly reduced compound
has a definitive NMR spectrum, Figure 7.
-
3
1
3
1
Figure 6. A DFT calculated stable structure of the
plex showing the “jump” of the thiourea proton, the binding of
CO , and interaction with the thiourea hydrogen atom to facili-
1
-CO
2
com-
2
tate C-O bond cleavage. C – black, F - green, N - blue, S - yellow,
O - red, Re – purple, H – silver.
In order explain the lack of a kinetic isotope effect and to
better understand the influence of the thiourea tether on the
electrocatalytic reaction DFT calculations were carried out on
the reduced species as indicated by the notional structure
found from a constrained geometry calculation in Figure 1.
Despite numerous attempts to obtain an actual structure with
an –NH-C(=S) moiety, the proton invariably jumps to the
1
Figure 7. H-NMR spectra of compound
1
in CD CN in the pres-
3
–
ence of TBAPF (0.1M). green - before reduction; red - 1e re-
duced; light blue - 2e reduced; dark blue – 3e reduced. Reduc-
6
–
–
tion was carried out at -2.0 V versus. Ag/AgNO . Note that the
3
formation of small amounts of impurities in CD CN may be as-
3
–
sociated with some exchange of Cl with the solvent. Very similar
results were obtained by reduction at -1.6 V, -1.80 V and -2.0 V
nearest CO
2
oxygen atom to form a metal-carboxylic acid with
versus Ag/AgNO for the first, second and third electrons, re-
3
spectively, with the appropriate charge.
a O-H bond distance of 0.983 Å, Figure 6. We attempted to
locate a transition state between “NH” and “OH” structures
by calculating the second derivative matrix at the equilibrium
geometry and “walking uphill” along the normal mode most
closely corresponding to the proton exchange, but were una-
ble to find either a minimum with an “NH” structure or a first-
order saddle point (“transition state”) that would connect the
two structures. The lack of a minimum with an “NH” struc-
ture or transition state, support the absence of a kinetic iso-
tope effect. It should be noted a local minimum was found for
the enthiolate structure, which is, however, much higher in en-
ergy, DDG298 = 35.7 Kcal/mol, Figure 6. In contrast, calcula-
The apparent disappearance of the peak at 10.0 ppm (it ac-
tually may be broadened or strongly shifted) associated with
the hydrogen atom of the thiourea moiety proximal to the 4-
19
CF
3
-Ph group as well as a down field shift in the F NMR peak
from -63.71 to -62.67 ppm indicates that the first reduction
event occurs distal to the CO reaction center and is related to
the strongly electron withdrawing properties of the 4-CF -Ph
2
3
moiety capable of stabilizing a negative charge. Alternatively,
the change in the spectrum could also be related to the loss of
–
Cl .
Considering that the first reduction event is not directly in-
tions on a species with a simple NH-CH
3
tether shows that the
volved in the CO reduction cycle, the second quasi-reversible
2
lowest energy species has the hydrogen atom remaining on
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reduction wave can be assigned to the formation of an anion
that the original “NH” adjacent to the bipyridine proton com-
pletely resides on the oxygen atom of carboxylate intermedi-
ate without any local minima between the high energy “NH”
form the very significantly more stable “OH” structure. Thus,
the NH proton of the thiourea moiety is the active proton do-
nor without a free energy barrier. In fact, the electrochemical
measurements support this conclusion since the addition of
water or a Bronsted acid inhibits the reaction in contrast to
1
2
3
4
5
6
7
8
9
radical on the bipyridine moiety. The third quasi-reversible
reduction wave then can be associated with the reduction of
39
Re(I). Indeed, the doubly reduced compound
1
has a broad-
1
ened H NMR spectrum, Figure 7, associated with a paramag-
netic species, and a six-line EPR spectrum, Figure 8.
what is typically observed in other Re(bipy)(CO) X catalysts
3
and others having, for example, an amine tether. Finally, NMR
and EPR experiments indicate that the catalytic cycle at the Re
center is likely similar to the cycle previously proposed for
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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0
3
9
such Re compounds with the exception that one electron ap-
pears to reduce compound distal to the catalytic site, before
1
the typical 2-electron process begins.
Experimental Section
Synthesis
6
-amino-2,2’-bipyridine: Under an inert atmosphere, 6-bromo-2,2’-bi-
pyridine (0.53 g, 1eq.), guanidinium nitrate (0.825 g, 3 eq.) and KOt-Bu
(1.512 g, 6 eq.) were dissolved in 20 mL dry dimethyl sulfoxide. The reaction
solution was heated to 120 °C and stirred for 15 h. After the reaction mixture
cooled down, about 10 mL of water were added during continuous stirring.
The product was extracted 8-10 times using diethyl ether. An additional pos-
sibility is to evaporate DMSO at high vacuum at 80 °C followed by dissolving
the crude in water and extraction of the product with CH Cl 4-6 times. The
Figure 8. X-band EPR spectra of 2-electron (top) and 3-electron
(bottom) reduced compound
tion was carried out at -2.0 V versus. Ag/AgNO
spectrum (red line) gave hyperfine constants of, 4.0 G, 3.5 G and
1
at ambient temperature
.
Reduc-
.The simulated
3
1
4
85
3.0 G for three N atoms and 21.8 G for Re. The same results
were obtained by reduction at -1.6 V, -1.80 V and -2.0 V versus
2
2
organic layers were combined, washed with brine, dried over Na
2
SO and
4
evaporated under reduced pressure. The compound was purified by column
Ag/AgNO for the first, second and third electrons, respectively,
3
chromatography over silica gel, using 30 to 100% ethylacetate/n-hexane as
with the appropriate charge.
1
eluent. Yield: 42%. H-NMR (CDCl
3
, 300MHz): δ= 8.645 (dd, 1H), 8.25
This spectrum is very similar to the spectrum of the one-
(dd, 1H), 7.70 (d, 1H), 7.75 (t, 1H), 7.27 (m, 1H), 6.52 (d, 1H), 4.56 (bs,
H).
-([2,2'-bipyridin]-6-yl)-3-(4-(trifluoromethyl)phenyl)thiourea
2
electron reduced fac-Re(bpy)(CO) Cl, in which the radical is
3
1
(
L1):
-
located on the bipyridine ligand and the Cl ligand is dissoci-
Equimolar amounts of 6-amino-2,2’-bipyridine and 4-trifluoromethyl- phe-
nyl- isothiocyanate were dissolved in 5 mL N, N’-dimethyl formamide
(DMF) and transferred to a 20 mL glass pressure tube, that was sealed under
an inert atmosphere. The reaction was stirred at 120 °C for 20 h. After the
reaction mixture cooled down, the solvent was evaporated under vacuum.
The compound was purified by column chromatography over silica gel, us-
ated (Figure S24). After reduction of compound
1
by three
1
electrons one can see the reappearance of non-broadened H
NMR signals associated with an open shell singlet, Figure 7
and the almost complete disappearance of the EPR signal, Fig-
ure 8. It should be noted that the 3-electron reduced com-
pound 1 does not yield a purple compound, but rather an am-
ber colored compound as can be observed in the relevant UV-
vis spectrum (Figure S25).
1
ing 30 to 100% ethylacetate/n-hexane as eluent. Yield: 38%. H-NMR
(CDCl
1H), 7.945 (d, 1H), 7.88 (t, 1H), 7.84 (t, 1H), 7.68 (d, 2H), 7.39 (t, 1H),
.815 (d, 1H). ESI-MS: positive ion m/z = 397 [M+Na]; negative ion m/z
373.2 [M-H]. HMRS (negative ion): Calculated mass:373.0711, Found
mass: 373.0723, Formula: C18 H12 N4 O3 F3 S.
ReL1(CO)
Cl ( ): Ligand L1 (56 mg, 0.15 mmol) and Re(CO)
3
, 300MHz): δ= 8.77 (d, 1H), 8.37 (bs, 1H), 8.06 (d, 2H), 8.00 (d,
6
=
Conclusions
3
1
5
Cl (54
It has been shown that a thiourea tether in the second coor-
mg, 0.15 mmol) were reacted in 5 mL dry THF under Argon 80 °C for 15 h
in a sealed tube. After cooling, the yellow precipitate was collected, re-dis-
solved in 5 mL THF and transferred to a falcon tube. 35 mL n-hexane was
added as a non-solvent, and the tube was centrifuged for 20 min at 5000 rpm.
The liquids were removed, and the precipitate was re-dissolved/re-precipi-
dination sphere to a classic Re(bipy)(CO)
3
X catalyst for elec-
acts both as a hydrogen bond
trochemical reduction of CO
2
promoter and a proton donor active in the C-O bond cleavage
step of the catalytic cycle. This leads to a significant rate accel-
tated in 5 mL CHCl
3
and 35 mL n-hexane. After centrifugation (20 min,
eration of one order of magnitude for compound
to Re(bipy)(CO) X as shown by the electrochemical meas-
urements. Compound is stable over 15 h in a controlled po-
tential electrolysis experiment. Hydrogen bond formation of
the thiourea group to CO is concluded from the observation
1
compared
1
5
000 rpm) the product was collected as a yellow powder. Yield: 90%. H-
3
NMR (500MHz, THF-d8): δ= 10.49 (bs, 1H), 9.31 (bs, 1H), 9.055 (d, 1H),
1
8.45 (d, 1H), 8.325 (d, 1H), 8.195 (d, 1H), 8.13 (m, 2H), 7.87 (d, 2H), 7.65
+
-
(
[
d, 2H), 7.615 (t, 1H). ESI- MS: 703 [M+Na ], 645.05 [M-Cl ], 1382
+
2M+Na ]. HMRS: Calculated mass: 645.0218, Found mass: 645.0229,
2
Formula: C21 H13 N4 O3 F3 S Re. Elemental Analysis exp (calcd): C –
1
13
of chemical shifts in both the H and C NMR spectra. In ad-
dition, the lability of the hydrogen atoms on the thiourea
group has been verified by a 1D NOE experiment. There was
no kinetic isotope effect by replacement of the thiourea hy-
drogen atom with a deuterium atom. DFT calculations show
3
5.52 (37.09); H – 1.78 (1.93); N – 7.99 (8.24)
Electrochemistry: All electrochemical experiments were performed in a
20 mL Pyrex cell, with 3 mm diameter glassy carbon as working electrode, Pt
wire as counter electrode, and Ag/AgNO as reference electrode. All meas-
urements were done under an inert atmosphere or CO as appropriate with
a dry solvent containing 0.1 M tetrabutylammonium hexafluorophosphate
TBAPF ) as electrolyte and 1 mM in MeCN. Bulk electrolysis was done
3
2
(
6
1
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under the same conditions with additional separation of the Pt counter elec-
trode by a glass frit.
5. Hawecker, J.; Lehn, J.-M.; Ziessel, R. Electrocatalytic Reduction of Car-
bon Dioxide Mediated by Re(bipy)(CO) Cl (bipy = 2,2′-bipyridine). J.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
Preparation of the reduced compounds for EPR, NMR and UV-VIS
measurements: The preparation of all the samples was carried out in a glove-
box. The samples were reduced in a 20 mL electrochemical cell using Pt-
gauze as a working electrode, Pt-wire separated by glass frit as a counter elec-
Chem. Soc. Chem. Commun. 1984, 328–330.
6. Fisher, B.; Eisenberg, R. Electrocatalytic Reduction of Carbon Dioxide
by using Macrocycles of Nickel and Cobalt. J. Am. Chem. Soc. 1980, 7361–
7363.
7. Beley, M.; Collin, J.; Ruppert, R.; Sauvage, J. Nickel(II)-Cyclam: An
Extremely Selective Electrocatalyst for Reduction of CO
Soc. Chem. Commun 1984, 1315–1316.
8. Costentin, C.; Robert, M.; Saveant, J. Catalysis of the Electrochemical
Reduction of Carbon Dioxide. Chem. Soc. Rev. 2013, 42, 2423–2436.
9. Costentin, C.; Robert, M.; Saveant, Current Issues in Molecular Catal-
trode and Ag/AgNO
acetonitrile containing 1mM of the Re complexes was used. For the NMR
measurements- we used CD CN (dried over molecular sieves) instead of
CH CN. Compound was reduced at a potential of -1.6 V versus
Ag/AgNO for the 1-electron reduction, and at -2.0 V versus Ag/AgNO for
the second and third electron reduction. Re(bipy)(CO) Cl was reduced at a
potential of -1.7 V versus Ag/AgNO for the 1-electron reduction, and at -
2.0 V versus Ag/AgNO for the second electron reduction. Samples were
3
reference electrode. A 3 mL solution of TBAPF
6
in dry
2
in Water. J. Chem.
3
3
1
3
3
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
ysis Illustrated by Iron Porphyrins as Catalysts of the CO -to-CO Electro-
2
3
chemical Conversion. Acc. Chem. Res. 2015, 48, 2996–3006.
taken once the desired number of electrons had passed, and then introduced
into an NMR tube (NMR), flat cell (EPR) or a cuvette (UV- VIS).
DFT calculations: The geometry was optimized using the Gaussian 16
10. Takeda, H.; Cometto, C.; Ishitani, O.; Robert, M. Electrons, Photons,
Protons and Earth-Abundant Metal Complexes for Molecular Catalysis of
CO
11. Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Electrocata-
2
Reduction. ACS Catal. 2017, 7, 70–88.
4
0
program system at the PBE0 level with the def2-SVP basis set/ECP com-
41
42
bination and Grimme’s D3BJ empirical dispersion correction using a con-
lytic and homogeneous approaches to conversion of CO
Chem. Soc. Rev. 2009, 38, 89–99.
2
to liquid fuels.
43
tinuum solvent model for tetrahydrofuran.
12. Costentin, C.; Drouet, S.; Savéant, J. A Local Proton Source Enhances
CO
Electroreduction to CO by a Molecular Fe Catalyst. Science 2012, 338,
0–94.
3. Costentin, C.; Passard, G.; Robert, M.; Savéant, J. Ultraefficient Ho-
mogeneous Catalyst for the CO -to-CO Electrochemical Conversion. Proc.
014, 111, 14990–14994.
ASSOCIATED CONTENT
2
9
Supporting Information. The Supporting Information is availa-
ble free of charge on the ACS Publications website. A detailed ex-
perimental section, NMR and mass spectra, additional electro-
chemical measurements and EPR spectra combined in one PDF
1
2
Natl. Acad. Sci.
4. Costentin, C.; Passard, G.; Robert, M.; Savéant, J. M. Pendant Acid–
Base Groups in Molecular Catalysts: H-Bond Promoters or Proton Relays?
Mechanisms of the Conversion of CO to CO by Electrogenerated
2
1
file and a cif file for oxidized compound .
1
2
Iron(0)Porphyrins Bearing Prepositioned Phenol Functionalities. J. Am.
Chem. Soc. 2014, 136, 11821–11829.
AUTHOR INFORMATION
1
5. Agarwal, J.; Shaw, T. W.; Schaefer, H. F.; Bocarsly, A. B. Design of a
Corresponding Author
Catalytic Active Site for Electrochemical CO Reduction with Mn(I)-Tri-
carbonyl Species. Inorg. Chem. 2015, 54, 5285–5294.
16. Costentin, C.; Robert, M.; Savéant, J.-M.; Tatin, A. Efficient and se-
2
Ronny.Neumann@weizmann.ac.il
ORCID
lective molecular catalyst for the CO
water. Proc. Natl. Acad. Sci. 2015, 112, 6882-6886.
7. Ngo, K. T.; McKinnon, M.; Mahanti, B.; Narayanan, R.; Grills, D. C.;
Ertem, M. Z.; Rochford, J. Turning on the Protonation-First Pathway for
2
-to-CO electrochemical conversion in
Ronny Neumann: 0000-0002-5530-1287
Jan M. L. Martin: 0000-0002-0005-5074
1
Electrocatalytic CO Reduction by Manganese Bipyridyl Tricarbonyl Com-
2
Funding Sources
plexes. J. Am. Chem. Soc. 2017, 139, 2604-2618.
No competing financial interests have been declared.
18. Chapovetsky, A.; Do, T. H.; Haiges, R.; Takase, M. K.; Marinescu, S.
C. Proton-Assisted Reduction of CO
2
by Cobalt Aminopyridine Macrocy-
ACKNOWLEDGMENT
cles. J. Am. Chem. Soc. 2016, 138, 5765–5768.
1
9. Seu, C. S.; Appel, A. M.; Doud, M. D.; DuBois, D. L.; Kubiak, C. P.
This research was supported by the Minerva Foundation (RN,
JMLM) and by Israel Science Foundation grant 1358/15
(JMLM). RN is the Rebecca and Israel Sieff Professor of Organic
Chemistry. JMLM is the Baroness Thatcher Professor of Chem-
istry. Dr. Gregory Leitus is thanked for the X-ray crystallography.
R
R′
2+
Formate oxidation via β-deprotonation in [Ni(P
plexes. Energy Environ. Sci. 2012, 5, 6480-6490.
20. Schmeier, T. J.; Dobereiner, G. E.; Crabtree, R. H.; Hazari, N. Sec-
ondary Coordination Sphere Interactions Facilitate the Insertion Step in an
2
N
2
)
2
(CH
3
CN)] Com-
Iridium(III) CO
277.
1. Neri, G.; Aldous, I. M.; Walsh, J. J.; Hardwick, L. J.; Cowan, A. J. A
2
Reduction Catalyst. J. Am. Chem. Soc. 2011, 133, 9274-
9
2
REFERENCES
Highly Active Nickel Electrocatalyst Shows Excellent Selectivity for CO
duction in Acidic Media. Chem. Sci. 2016, 7, 1521-1526.
22. Azcarate, I.; Costentin, C.; Robert, M.; Saveant, J.-M. Through-Space
Charge Interaction Substituent Effects in Molecular Catalysis Leading to the
2
Re-
1
. 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.
Design of the Most Efficient Catalyst of CO -to-CO Electrochemical Con-
2
version. J. Am. Chem. Soc. 2016, 138, 16639-16644.
23. Sung, S.; Kumar, D.; Gil-Sepulcre, M.; Nippe, M. Electrocatalytic CO
Reduction by Imidazolium-Functionalized Molecular Catalysts. J. Am.
Chem. Soc. 2017, 139, 13993-13996.
24. Chapovetsky, A.; Welborn, M.; Luna, J. M.; Haiges, R.; Miller, T. F.
III; Marinescu, S. C. Pendant Hydrogen-Bond Donors in Cobalt Catalysts
2
2
. Riplinger, C.; Sampson, M. D.; Ritzmann, A. M.; Kubiak, C. P.; Carter,
E. A. Mechanistic Contrasts between Manganese and Rhenium Bipyridine
Electrocatalysts for the Reduction of Carbon Dioxide. J. Am. Chem. Soc.
2
014, 136, 16285–16298.
. Francke, R.; Schille, B.; Roemelt, M. Homogeneously Catalyzed Elec-
3
Independently Enhance CO
Reduction. ACS Cent. Sci. 2018, 4, 397−404.
2
troreduction of Carbon Dioxide—Methods, Mechanisms, and Catalysts.
Chem. Rev. 2018, 118, 4631-4701.
25. Schreiner, P. R.; Wittkopp, A. H-Bonding Additives Act Like Lewis
Acid Catalysts. Org. Lett. 2002, 4, 217–220.
4
. Jeoung, J.-H.; Dobbek, H. Carbon Dioxide Activation at the Ni,Fe-
Cluster of Anaerobic Carbon Monoxide Dehydrogenase. Science, 2007
318, 1461–1464.
,
ACS Paragon Plus Environment

Page 7 of 8
Journal of the American Chemical Society
2
6. Lippert, K. M.; Hof, K.; Gerbig, D.; Ley, D.; Hausmann, H.; Guenther,
S.; Schreiner, P. R. Hydrogen‐Bonding Thiourea Organocatalysts: The Priv-
ileged 3,5‐Bis(trifluoromethyl)phenyl Group. Eur. J. Org. Chem. 2012
919–5927.
7. Schreiner, P. R. Metal-free Organocatalysis Through Explicit Hydro-
gen Bonding Interactions. Chem. Soc. Rev. 2003, 32, 289 –296.
8. Zhang, Z.; Lippert, K. M.; Hausmann, H.; Kotke, M.; Schreiner, P. R.
37. Scheiring, T.; Klein, A.; Kaim, W. EPR Study of Paramagnetic Rhe-
•–
1
2
3
4
5
6
7
8
9
nium(I) Complexes (bpy )Re(CO)
3
X Relevant to the Mechanism of
,
Electrocatalytic CO
2571.
2
Reduction. J. Chem. Soc., Perkin Trans 2 1997, 2569–
5
2
38. Kaim, W.; Kohlmannt, S. EPR Evidence for Related Electronic Struc-
2+
tures of α-Diimine Complexes with [Ru(bpy)
Fragments. Chem. Phys. Lett. 1987, 139 , 365–369.
39. Keith, J. A.; Grice, K. A.; Kubiak, C. P.; Carter, E. A. Elucidation of the
Selectivity of Proton-Dependent Electrocatalytic CO Reduction by fac-
Re(bpy)(CO)
Cl. J. Am. Chem. Soc. 2013, 135, 15823−15829.
2
] and Re(CO) (halide)
3
2
Cooperative Thiourea–Brønsted Acid Organocatalysis: Enantioselective
Cyanosilylation of Aldehydes with TMSCN. J. Org. Chem. 2011, 76, 9764–
2
9
776.
9. Zhang, Z.; Schreiner, P. R. (Thio)urea organocatalysis—What can be
learnt from anion recognition? Chem. Soc. Rev. 2009, 38, 1187–1198.
0. Costentin, C.; Drouet, S.; Robert, M.; Saveant, J.-M. Turnover Num-
3
2
40. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji,
H.; Li, X.; Caricato, M.; Marenich, A. V; Bloino, J.; Janesko, B. G.; Gomperts,
R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V; Izmaylov, A. F.; Sonnenberg,
J. L.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng,
B.; Petrone, A.; Henderson, T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.;
Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda,
R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.;
Vreven, T.; Throssell, K.; Montgomery Jr., J. A.; Peralta, J. E.; Ogliaro, F.;
Bearpark, M. J.; Heyd, J. J.; Brothers, E. N.; Kudin, K. N.; Staroverov, V. N.;
Keith, T. A.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A. P.;
Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.;
Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.;
Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 16 Rev. B01. Gaussian, Inc.:
Wallingford, CT 2016.
41. Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple
Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and
Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305.
42. Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function
in Dispersion Corrected Density Functional Theory. J. Comput. Chem.
2011, 32, 1456–1465.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
bers, Turnover Frequencies, and Overpotential in Molecular Catalysis of
Electrochemical Reactions. Cyclic Voltammetry and Preparative-Scale Elec-
trolysis. J. Am. Chem. Soc. 2012, 134, 11235–11242.
3
1. Sampson, M. D.; Kubiak, C. P. Manganese Electrocatalysts with Bulky
Bipyridine Ligands: Utilizing Lewis Acids To Promote Carbon Dioxide Re-
duction at Low Overpotentials. J. Am. Chem. Soc. 2016, 138, 1386-1393.
3
Evaluation of Homogeneous Electrocatalysts by Cyclic Voltammetry. Inorg.
Chem. 2014, 53, 9983–10002.
2. Rountree, E. S.; McCarthy, B. D.; Eisenhart, T. T.; Dempsey, J. L.
3
3. 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. 2012, 109, 15646–15650.
3
4. Smieja, J. M.; Kubiak, C. P. Re(bipy-tBu)(CO) Cl−improved Cata-
3
lytic Activity for Reduction of Carbon Dioxide: IR-Spectroelectrochemical
and Mechanistic Studies. Inorg. Chem. 2010, 49, 9283-9289.
3
nium Electrocatalyst for the Reduction of Carbon Dioxide to CO and For-
mate. J. Am. Chem. Soc. 2015, 137, 8564–8571.
5. Machan, C. W.; Sampson, M. D.; Kubiak, C. P. A Molecular Ruthe-
43. Mennucci, B. Polarizable Continuum Model. Wiley Interdiscip. Rev.
Comput. Mol. Sci. 2012, 2, 386–404.
3
6. Clark, M. L.; Cheung, P. L.; Lessio, M.; Carter, E. A.; Kubiak, C. P.
Kinetic and Mechanistic Effects of Bipyridine (bpy) Substituent, Labile Lig-
and, and Brønsted Acid on Electrocatalytic CO
Complexes. ACS Catalysis, 2018, 8, 2021-2029.
2
Reduction by Re(bpy)
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