Evaluation of the Electrocatalytic Reduction of Carbon Dioxide using Rhenium and Ruthenium Bipyridine Catalysts Bearing Pendant Amines in the Secondary Coordination Sphere

Pendant Amines in the Secondary Coordination Sphere

Article

Evaluation of the Electrocatalytic Reduction of Carbon Dioxide

using Rhenium and Ruthenium Bipyridine Catalysts Bearing

Monica R. Madsen, Joakim B. Jakobsen, Magnus H. Rønne, Hongqing Liang,

Hans Christian D. Hammershøj, Peter Nørby, Steen U. Pedersen, Troels Skrydstrup,*

and Kim Daasbjerg *

Cite This: https://dx.doi.org/10.1021/acs.organomet.9b00815

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sı Supporting Information

ABSTRACT: Carbon dioxide utilization through electrocatalysis

is a promising pathway toward a more sustainable future. In this

work the electrocatalytic reduction of carbon dioxide by Re and

Ru bipyridine complexes bearing pendant amines (N,N′-(([2,2′-

bipyridine]-6,6′-diylbis(2,1-phenylene))bis(methylene))bis(N-eth-

ylethanamine) (dEAbpy)) is evaluated. In both cases, the major

reduction product is carbon monoxide accompanied by some

formic acid, although the yield of the latter never reaches the

predominant level known from the corresponding Mn(dEAbpy)-

(

CO) Br complex. This demonstrates the profound eﬀect of the

identity of the metal center, in addition to the ligand, for the

product distribution. In this work, we report the synthesis

procedures and X-ray diﬀraction studies along with electro-

chemical and infrared spectroelectrochemical studies of Re(dEAbpy)(CO) Cl and Ru(dEAbpy)(CO) Cl to propose a mechanism

for the CO reduction reaction.

3,19

INTRODUCTION

compared to that of the Mn variant.

The same Ru complex

■

has been explored under photochemical conditions yielding

CO selectively from CO₂. In general, Ru catalysts have been

In recent years, electrocatalytic reduction of carbon dioxide to

fuels and C1 and C2 chemicals have received enormous

widely explored in electrochemical and photochemical systems

interest. In particular, this is driven by the need to identify a

−

for CO reduction to CO/HCOO , both as the mono(bpy)

sustainable alternative to fuels that are currently provided from

the petrochemical feedstock. Furthermore, CO is regarded as

and bis(bpy) carbonyl complexes and as poly(bpy) com-

21−26

plexes.

a waste product which makes it a cheap and available carbon

As part of a continued research eﬀort toward CO₂

utilization, we recently introduced benzylic tertiary amines in

the second coordination sphere of Mn-tricarbonyl complexes.

Remarkably, this modiﬁcation increased the activity, prevented

dimerization, and altered the product selectively from CO to

formic acid in the electrochemical reduction of CO₂. This

change in activity and selectivity was attributed to the

capability of the amines to act as proton shuttles leading to

the formation of an intermediate Mn−H species. We now

question if using the dEAbpy ligand (Figure 1) in other metal−

feedstock. Electrochemical transformation of CO to value-

added chemicals can therefore provide one way to close the

−6

carbon cycle.

However, the electrochemical utilization of

CO is challenging due to the stability of the highly oxidized

carbon, requiring multiple protons and electrons to obtain

value-added products. Several molecular catalysts are known

−11

to catalyze the reduction of CO₂.

Some of the most

superior homogeneous systems are of the type Re- and

∧

Mn(N N)(CO) X (N N = polypyridyl, X = halide or weakly

coordinating anion), which under electrochemical conditions

selectively produce CO.

complexes, such as Ru(bpy)(CO) X (bpy = 2,2′-bipyridine),

typically polymerize under reductive conditions.

12−15

In contrast, isoelectronic Ru

Special Issue: Organometallic Chemistry for Enabling

6−18

Carbon Dioxide Utilization

How-

ever, it has been shown that using bulky ligands such as 6,6′-

Received: December 1, 2019

dimesityl-2,2′-bipyridine (mesbpy) circumvents the polymer-

ization and eﬃciently catalyzes the reduction of CO to CO/

−

HCOO , albeit with a lower turnover frequency (TOF)

XXXX American Chemical Society

Organometallics XXXX, XXX, XXX−XXX

ellipsoids at 50% probability.

Article

Figure 1. (a) Metalation of dEAbpy yielding 1 and 2, and (b) molecular structures of 1, dEAbpy, and 2 obtained from single-crystal X-ray

diﬀraction studies (Re = orange, Ru = brown, C = blue, N = orchid, Cl = green, O = red). Hydrogen atoms are omitted for clarity; thermal

−

Figure 2. Cyclic voltammograms recorded on 1 mM (a) 1 and (c) 2 at a GC electrode (d = 1 mm) using ν = 0.1 V s along with the

−

corresponding IR-SEC spectra of (b) 1 and (d) 2 during the course of a voltammetric cycling using ν = 0.05 V s in Ar saturated 0.1 M Bu NBF /

MeCN. IR-SEC spectra acquired at three diﬀerent potentials, i.e., at a resting potential (≥ −1.37 V vs Fc /Fc; black), after the ﬁrst reduction wave

(

red), and after the second reduction wave (blue).

carbonyl complexes, such as Re and Ru , would aﬀord a

similar eﬀect on the activity and product selectivity.

Furthermore, such an extended study would provide insight

into the possible mechanistic change brought about by the

incorporation of a tertiary amine in the ligand system.

Re adopts a facial octahedral geometry having the bpy

ligand as well as two carbonyl groups in the equatorial plane,

while the chloride and one carbonyl group are in an axial

position. In the solid state, the bpy plane is slightly distorted

with a torsion angle of 10.2° and the phenyl rings are near

orthogonal to the bpy ring system. Interestingly, both amine

substituents on the phenyl rings are aligned in the Re−Cl

direction. These observations are in agreement with the

geometry found for the Mn counterpart (3).

Several synthesis protocols for Ru(bpy)(CO) Cl are

available in the literature, starting from RuCl via “the red

RESULTS AND DISCUSSION

■

Synthesis and Characterization. Using Re(CO) Cl as

the Re carbonyl precursor, it was possible to metalate dEAbpy

in reﬂuxing toluene over the course of 16 h to yield fac-

Re(dEAbpy)(CO) Cl (1) as a yellow powder in 89% yield

Figure 1). Single crystals suitable for X-ray crystallographic

analysis were obtained by slow evaporation of CH Cl at −20

C (see the Supporting Information ).

(

carbonyl solution” or from the polymeric Ru carbonyl

precursor [Ru(CO)

were not viable in our hands, even when using a variety of

17,19,29−35

Cl ] .

2 n

However, these pathways

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

solvents and temperatures. Gratifyingly, we found that using

the dimeric precursor [Ru(CO) Cl ] , the metalation of

observed. Since the latter broad band most likely stems from

the overlap of two very closely positioned bands, the number

of carbonyl bands remain unchanged (i.e., three). The large

downshift indicates that a reduction takes place at the central

2 2

dEAbpy proceeded at 110 °C in toluene for 18 h, providing

trans-Cl-Ru(dEAbpy)(CO) Cl (2) (Figure 1) in a low yield

(

21%). Single-crystal X-ray crystallographic analysis was

metal to aﬀord a ﬁve-coordinate Re complex after a rapid loss

−1

performed on yellow crystals obtained by slow evaporation

of chloride. In this case, the value of k_d_i_smust be >400 s

of a saturated solution of 2 in CH Cl at −40 °C (see the

due to the persistent irreversibility of the second reduction

−1

Supporting Information ). The ﬂat bpy ring system is in the

equatorial plane of the octahedral Ru center, while the two

chloride atoms occupy the axial positions. The trans relation-

ship is in agreement with other systems having bulky

wave even at ν = 2 V s . On this basis, we assign the doubly

•−

reduced species to Re (dEAbpy )(CO) .

Furthermore, it should be noted that at high sweep rates in

cyclic voltammetry, a new oxidation peak arises at −1.5 V vs

substituents in the 6,6′-position of the bpy, whereas other

Fc /Fc, which is associated with the second reduction process.

reports using the Ru-dimer as the precursor with simple bpy

Extending the experiment to include a second voltammetric

33,37−40

yield the cis-dicarbonyl-cis-dichloro isomer.

The phenyl

cycle, a small reduction peak at −1.6 V vs Fc /Fc appears

rings of complex 2 in the solid state are found to be

perpendicular to the bpy rings. Furthermore, the benzylic

amines are found to be on the same side of the bpy rings as

(Figure S4 ). These new peaks are assigned to the reversible

•−

oxidation of Re (dEAbpy )(CO) .

In cyclic voltammetry, complex 2 shows two main reduction

reported for the corresponding Mn complex 3. Both

waves with peak potentials at −1.63 and −2.07 V vs Fc /Fc

complexes were characterized by H NMR and C NMR

spectroscopy, attenuated total reﬂection Fourier-transform

infrared spectroscopy (ATR-FTIR), and high-resolution ESI-

MS (see the Experimental Section).

together with a few smaller waves at more extreme potentials

(Figure 2c). The ﬁrst wave is irreversible and approximately

twice as high as that of 1, recorded using the same conditions,

indicating that 2 is initially reduced in a two-electron process.

Electrochemical and Spectroscopic Characterization.

Bulk electrolysis of 2 at −1.77 V vs Fc /Fc supports this

The cyclic voltammetric behavior of 1 in 0.1 M Bu NBF /

assignment as calculated from the consumed charge in

exhaustive electrolysis (Figure S5). With the linear relation

observed between the peak current and square root of ν in the

MeCN was studied under an Ar atmosphere (Figure 2a).

Complex 1 shows two reduction waves on the initial scan, the

ﬁrst one being reversible with E° = −1.82 V vs ferrocenium/

−

range from 0.05 −2 V s , the ﬁrst reduction wave is diﬀusion-

controlled (Figure S6). In addition, there are no changes to the

electron stoichiometry as function of ν. The wave is irreversible

ferrocene (Fc /Fc). The second reduction wave is irreversible

with a peak at −2.3 V vs Fc /Fc. The ﬁrst reduction wave is

−

diﬀusion-controlled with unaltered electron stoichiometry (n =

even at ν = 50 V s (Figure S7 ), which would be consistent

−1

), as evidenced from a linear relation between the peak

with a fast cleavage of the Ru−Cl bond (k > 10 s ) after

dis

current and square root of the sweep rate (ν) varying from

the ﬁrst reduction (assuming an ECE mechanism). Most likely,

−

.05−2 V s (Figure S1 ). In the same range, a unity current

the initial reduction is bpy-based, after which a fast LMCT

ratio for the two reduction waves persists, consistent with two

one-electron reductions. No indication of reversibility of the

takes place, forming Ru (dEAbpy)(CO) Cl upon a single

chloride loss. This Ru complex is easier to reduce than the

parent complex, and Ru (dEAbpy )(CO) Cl is formed,

−1

•−

second reduction wave is achievable, even at ν = 2 V s .

Infrared spectroelectrochemistry (IR-SEC) was used to

monitor changes for the carbonyl bands during potential

sweeping on 1 (Figure 2b). At the resting potential (≥ −1.37

V vs Fc /Fc), three distinct carbonyl peaks are observed at

consuming overall two electrons, following an ECE/DISP-

type mechanism. At the second reduction potential (−2.07 V

vs Fc /Fc), a metallic reduction is assumed to take place to

formally aﬀord an 18-electron complex possessing a single Cl⁻

ligand. The second reduction wave is half the size of the ﬁrst

−

021, 1918, and 1891 cm , which is in close proximity of ν

−

reported for Re(tBu-bpy)(CO) Cl (tBu-bpy = 4,4′-di-tert-

one and is only partially reversible for ν > 1 V s , which would

−

butyl-2,2′-bipyridine). Once sweeping the potential to more

be in line with a second Cl loss in an E C mechanism (k

≤

dis

−

•−

negative values than that of the ﬁrst reduction peak, these

12 s ), thus ultimately forming Ru (dEAbpy )(CO) .

−

bands are down shifted by ∼20−30 cm to 1999, 1887, and

IR-SEC was used to analyze 2 at its diﬀerent redox states

−

862 cm . This modest IR downshift, together with the

(Figure 2d). At the resting potential (≥ −1.37 V vs Fc /Fc),

−

observation of a Nernstian behavior of the ﬁrst electron

two peaks at 2057 and 1994 cm are present, almost identical

transfer process in cyclic voltammetry, is in line with a bpy-

to those reported for Ru(mesbpy)(CO) Cl . When sweeping

2 2

•−

42−44

based reduction, generating Re (dEAbpy )(CO) Cl.

to a potential more negative than the ﬁrst reduction wave,

−

−1

Complexes of this type are known to lose Cl after a ligand-

to-metal charge transfer (LMCT). On the longest time scale in

cyclic voltammetry (using ν = 0.02 V s ), reversibility of the

ﬁrst reduction wave remains unperturbed, indicating that the

these carbonyl bands are down shifted by ∼50 cm to 2002

−

and 1940 cm . These peaks are assigned to the complex

−1

•−

Ru (dEAbpy )(CO) Cl. Applying a potential more negative

than the second reduction peak reveals a broadening and a very

−

rate constant, k , for this ﬁrst order chemical step would be

large down shift of ∼100 cm of both carbonyl bands to 1903

dis

−

−1

•−

0.2 s assuming an E C mechanism (see the Experimental

and 1837 cm . We assign these to Ru (dEAbpy )(CO) (i.e.,

Section and Figure S2 ). This is further supported by

a 16-electron complex with a reduced bpy ligand after the

second Cl dissociation). Unfortunately, it was not possible

−

potentiostatic IR-SEC measurements conducted at −1.92 V

vs Fc /Fc, where no changes are observed during a 12 s

within the time frame of IR-SEC to detect the complex before

−

electrolysis, implying a very slow or absent LMCT/Cl

dissociation of Cl . Isomerization of the CO ligands in this

dissociation (Figure S3).

unsaturated complex may cause the broadening of the IR

bands.

During the second reduction process at −2.37 V vs Fc /Fc,

−

an additional, but larger (∼50 cm ), downshift of the

Electrochemical CO Reduction. Figure 3 displays the

−

carbonyl stretch bands to 1948 and 1843 (br) cm is

electrocatalytic performance of 1 and 2 studied by linear sweep

Organometallics XXXX, XXX, XXX−XXX

Article

not changed by addition of TFE and/or CO . However, a

signiﬁcantly higher activity is observed at the second reduction

wave, reaching its maximum at i /i = 15 with 1.0 M TFE.

cat

Table 1 reports the catalytic potentials for both complexes

determined as the half wave potential, E_c_a_t_/₂

along with the

values of i /i determined at the TFE concentration yielding

cat

the highest activity and the maximum turnover frequency

(

TOF_m_a_x).

Equation 1 shows the relationship between TOF_m_a_xand i_c_a_t/

i , where n is the number of electrons transferred to the

catalyst in the ﬁrst reduction wave in absence of CO , n is the

cat

number of electrons in the catalytic transformation, F is

Faraday’s constant, R the universal gas constant, and T is the

50−52

temperature.

According to the electrochemical character-

ization under Ar, n = 1 for complex 1 and n = 2 for complex

. In both cases, two electrons are involved in the catalytic

reduction of CO (i.e., n = 2). A good estimation of TOF_m_a_x

cat

is only obtainable when the catalytic voltammograms are S-

shaped with well-deﬁned plateaus. Unfortunately, the waves of

and 2 are peak-shaped (see Figure 3) which may lead to an

underestimation of TOF_m_a_x

Fνn i

p j 0.4463 z ji z

j cat z

z j

TOF_m_a_x

z j

cat

{

(1)

{

Figure 4 shows the product distribution obtained after 1 h

bulk electrolysis at various potentials for complexes 1 and 2,

following the experimental procedure reported previously by

our group. Both complexes show a good faradaic eﬃciency

(

FE) for CO (∼80% in most cases) with smaller, but still

Figure 3. Linear sweep voltammograms recorded on 1 mM (a) 1 and

signiﬁcant, amounts of H (<2% for 1; 3−6% for 2) and

(

b) 2 with varying amounts of TFE at a GC electrode (d = 1 mm)

HCOOH (5−12%) detected. Complex 1 was tested at low

−

using ν = 0.1 V s in CO -saturated 0.1 M Bu NBF /MeCN. A

(−1.87 V vs Fc /Fc) and highly overpotentials (−2.27 V vs

voltammogram recorded under Ar is shown as a reference.

Fc /Fc) in the presence of 0.3 M TFE, resulting in the latter

case in a slight increase in FE_H_C_O_O_Hand FE . With the

observation in cyclic voltammetry of a signiﬁcant current

increase at even higher overpotentials in the presence of CO₂

voltammetry in CO -saturated 0.1 M Bu NBF /MeCN with

,2,2-triﬂuoroethanol (TFE) as proton source. Both complexes

show a catalytic wave under CO -saturation, even in the

and in the absence of TFE, we conducted an electrolysis at

absence of a proton source (see also Figure S8 ). In particular,

−

2.37 V vs Fc /Fc without addition of a proton source.

complex 1 is remarkably active for CO reduction under such

now increases to 11.8 ± 3.5%, while FE_C_Odrops to

6 ± 0.4%. In comparison, complex 2 was tested at a narrower

HCOOH

conditions (i.e., no TFE) reaching i /i = 6.7 at the ﬁrst

cat

plateau at −2.4 V vs Fc /Fc. Here, i deﬁnes the maximum

cat

potential interval close to E_c_a_t_/₂, as only a small pre-peak at low

overpotentials was present. FE_H_C_O_O_H(7.5−8.7%) is found to

be independent of the potential, while FE_C_Oincreases and

^F^EH2 decreases at higher overpotentials. The total FE is

signiﬁcantly lower than 100% at −1.87 V; it has not been

possible to account for the remaining charge in this case.

To evaluate the stability of the two catalysts, 4 h electrolysis

experiments were conducted on complexes 1 and 2 (Figure

S9 ). In accordance with the observations in linear sweep

voltammetry, complex 2 shows the highest initial current

density. However, it decreases rapidly during the ﬁrst hour,

current of the catalytic wave in the presence of CO (and

TFE), and i is the peak current under Ar. A further increase of

i /i to 8.4 is observed at more extreme potentials (−2.6 V vs

cat

Fc /Fc). This behavior is similar to that seen for many other

∧

Re(N N)(CO) X complexes, showing signiﬁcant activity even

5,46

in the absence of purposely added proton sources.

This

eﬀect has previously been assigned to the ability of the [Re−

−

CO ] adducts to be reduced at these negative potentials

followed by a proton abstraction from the solvent to drive the

6,47

catalytic process.

Upon addition of TFE, an increase in current is seen for

complex 1. Ultimately, this leads to a two-electron reduction

wave and an ECE process, where the proton donor assists in

−2

after which it stabilizes at 1−3 mA cm . In contrast, complex

_t_h_e_d_i_s_s_o_c_i_a_t_i_o_n_o_f_C_l− and formation of Re (dEAbpy )-

•−

Table 1. Key Figure of Merits for Complexes 1 and 2 in the

Electrochemical Reduction of CO₂

(

CO) , the same intermediate as formed at the second

reduction wave in the absence of TFE. Further addition of

TFE increases the electrocatalytic performance at the second

E_c_a_t_/₂(V vs Fc /Fc)

[TFE] (M)

i /i

TOF_m_a_x

cat

−2.12

−2.08

0.4

1.0

9.4

340

wave, reaching its maximum activity of i /i = 9.4 with 0.4 M

cat

TFE. For complex 2, the ﬁrst reduction wave already follows

an ECE mechanism with fast Cl⁻dissociation and

consumption of two electrons. This process is, by and large,

At maximal activity. May be underestimated due to the lack of a

plateau in the catalytic voltammograms.

Organometallics XXXX, XXX, XXX−XXX

Article

complex(es) giving rise to the remaining ν_C_Ostretches is

uncertain, as diﬀerent constitutional isomers can be formed

−

17,19

upon Cl loss.

Figure 5a shows the eﬀect of adding TFE to the CO₂

containing electrolyte solutions of 1. Noteworthy, once the

potential is stepped to the second reduction wave, strong

−

bands at 1708 and 1680 cm are increasing with time. The

−

former is assigned to CF CH OC(O)O , being an alkyl

carbonate formed during a reaction between TFE and CO ,

and the latter to formate. The remaining part of the IR-SEC

spectra seems to be a superposition of the parent complex (i.e.,

−1

Re (dEAbpy)(CO) Cl with ν = 2021, 1918, and 1891 cm )

•−

and the singly reduced complex (i.e., Re (dEAbpy )(CO) Cl

with ν_C_O= 1999, 1887, and 1862 cm ). The ratio of these two

Figure 4. Product distribution after 1 h of electrolysis at diﬀerent

−1

applied potentials in CO -saturated solutions of (a) 1 mM 1 and 0.3

complexes changes only slightly during the 60 s electrolysis

M TFE and (b) 1 mM 2 and 1.0 M TFE in 0.2 M Bu NBF /MeCN.

−1

(Figure S12), although an additional peak at 2008 cm slowly

No TFE was present in the electrolysis conducted at −2.37 V vs Fc /

−

grows at the expense of the one at 2021 cm . As previously

Fc (*).

−1

discussed, the peak at 2008 cm could indicate the presence

of a metal-carboxylate, which is formed upon oxidative

starts at a lower current density, but it remains fairly stable

•−

addition of CO to the active catalyst, Re (dEAbpy )(CO) .

and decreases only slightly during the course of the

experiment.

During the CO insertion and subsequent protonation, the

molecules diﬀuse away from the electrode. However, two

In Table S1 , the performance of 1 and 2 is compared to that

of similar bipyridinic Re and Ru catalysts with respect to

turnover number (TON), TOF_m_a_x, and FE. The activity of

complex 1 is relatively low compared to that of other Re bpy

catalysts, both with respect to TON and TOF_m_a_x. The lower

performance can, most likely, be attributed to the bulky

dEAbpy ligand, shielding the metal center, while the other Re

complexes have substituents pointing away from the metal

center. Nevertheless, 1 stands out by being the only catalyst

capable of producing signiﬁcant amounts of HCOOH.

electron transfers are required to drive the electrochemical

reduction of CO . If these electrons are provided by the singly

•−

reduced complex, Re (dEAbpy )(CO) Cl, instead of the

electrode, then this complex itself is oxidized, explaining the

presence of IR bands corresponding to the parent complex at

Complex 2 shows similar TON and TOF as those of the

max

reported Ru catalyst. However, 2 is distinguished by its

ability to produce HCOOH in a large potential window, while

the reported Ru(mesbpy) complex only produces a detectable

amount of HCOOH at low overpotentials.

Comparison of IR-SEC experiments in the presence and

absence of CO reveals some interesting features about the

reactivity of the individual species. First, no diﬀerences in the

recorded spectrum of 1 are observed, until the second

reduction potential at −2.37 V vs Fc /Fc is reached (in the

absence of TFE, Figure S10). Complex 1 usually shows peaks

−

at 1948 and 1843 cm at this potential, which are assigned to

•−

Re (dEAbpy )(CO) (Figure 2b, blue). However, once CO

is introduced, these peaks are gone, indicating a fast reaction

•−

between Re (dEAbpy )(CO) and CO . Instead, a new

−

carbonyl band at 2008 cm , accompanied by overlapping

−

bands in the range 1950−1850 cm , appears. A distinct band

−

at 1684 cm is assigned to formate, while the peaks at 1999,

892, and 1866 cm⁻are close to those found for the singly

−

reduced complex (Figure 2b, red). The peak at 2008 cm

could be indicative of a metal-carboxylate complex, which is a

41,53

known intermediate in the CO -to-CO reduction.

Similarly, complex 2 shows the same IR signals in the

presence (Figure 2d) and absence of CO (Figure S11) at

neutral and mildly reducing conditions (> −1.92 V vs Fc/Fc ),

while diﬀerences are observed at more negative potentials

−2.27 V vs Fc/Fc ). In the presence of CO , new peaks at

018 and 1956 cm appear together with bands associated

with Ru (dEAbpy )(CO) Cl (∼2000 and ∼1945 cm ). The

major band seen at 1678 cm is assigned to catalytically

(

Figure 5. IR-SEC spectra recorded of (a) 1 mM 1 and 50 mM TFE at

−

2.37 V vs Fc /Fc and (b) 1 mM 2 and 70 mM TFE at −2.07 V vs

•−

−1

Fc /Fc in the presence of CO (half-saturated) in 0.1 M Bu NBF /

MeCN. The potential was kept at −1.37 V vs Fc /Fc for 5 s before

applying the denoted potential.

−

formed formate (vide supra). The exact structure(s) of the

Organometallics XXXX, XXX, XXX−XXX

Article

Figure 6. Proposed catalytic cycle involving complexes 1 and 2 in the electrochemical reduction of CO to CO and HCOOH. “H ” most likely is

present as the alkyl carbonate (i.e., CF CH OC(O)OH), and “e” can indicate heterogeneous as well as homogeneous electron transfers.

−

021, 1918, and 1891 cm . Immediately after stopping the

where X is Cl or a solvent molecule and n is the charge.

electrolysis, IR signals from the singly reduced complex

decrease rapidly (half-life ∼ 2 s), while the intensity of peaks

originating from the parent complex increases. This substan-

Such complexes are formed via CO insertion in a Ru−H

species, and complex 2 might follow a similar mechanism to

form a formato-intermediate.

•−

tiates that Re (dEAbpy )(CO) Cl indeed is oxidized to form

Evaluating the Eﬀect of Diﬀerent Metal Centers.

Re (dEAbpy)(CO) Cl (Figure S12 ). In addition, the observa-

Using 1 and 2 as electrocatalysts in the reduction of CO , the

tion of IR bands from the parent complex indicates that the

reduction potential of the metal-carboxylate is not far from the

ﬁrst standard potential of 1. A similar electron-mediating role

of another Re bipyridine complex has been reported in the

photocatalytic reduction of CO₂.

Figure 5b displays the corresponding IR-SEC spectra

major product is CO (FE > 50%), accompanied by some

formic acid formation with FE’s of 11.8 ± 3.5% at −2.37 V vs

Fc /Fc for 1 (without proton source) and 8.7 ± 0.1% at −1.97

V vs Fc /Fc for 2 (with 1.0 M TFE present). In comparison,

the corresponding Mn complex, 3, gives almost exclusively

HCOOH as reported recently by us. The formation of

HCOOH is suggested to go through a manganese hydride as a

key intermediate, as it could be detected by IR-SEC during the

reduction process in the presence of TFE. With 1 there was no

sign of Re−H, which was further conﬁrmed by a control

experiment using deuterated TFE (TFE-OD), giving no

recorded for complex 2. Here, the initial reduction at −2.07

−1

V vs Fc /Fc yields new IR peaks at 2001 and 1941 cm , which

after a few seconds change to 2042, 2017, 1992, 1978, and

−

945 cm . In general, the peaks are less well-deﬁned, making

it diﬃcult to do speciﬁc assignments. Some of these peaks

slightly shift during the electrolysis, and a new peak at 1890

additional changes to the IR spectrum (Figure S13). Overall,

this is in accordance with the low yield of HCOOH in this

case.

−

cm slowly grows. Interestingly, the position of these peaks is

close to peaks reported for a Ru(mesbpy)-complex after

reduction at its second reduction peak in the presence of CO

Figure 6 outlines the mechanism suggested for the

and phenol. The authors hypothesize that the species giving

electrochemical CO reduction using complexes 1 and 2. For

•−

rise to these peaks are isomers of the type [Ru (mesbpy )(η -

both complexes initial reductions are required before the

•−

OCOH)(CO) X] or [Ru (mesbpy)(η -OCOH)(CO) X] ,

catalytically active complex M (dEAbpy )(CO) X (A) is

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Article

formed. This complex may react with either CO or H to

as a proton shuttle for the formation of the metal-hydride

species, it is also clear that even more speciﬁc proton shuttle

ligands would be necessary to achieve the same control toward

HCOOH formation for Re and Ru as for Mn.

aﬀord the metal-carboxylate (B) or metal-hydride species (D),

respectively, noting at the same time that the presence of the

ammonium moiety (in C), serving as a proton shuttle, is a key

to the formation of the latter. The competition between these

two pathways will decide the product selectivity of CO/

CONCLUSION

■

−

HCOOH. Computational studies on [Re(bpy)(CO)₃] has

The synthesis and electrochemical characterization of two

−

shown that CO binding is exergonic (−3.4 kcal mol ),

CO -reducing catalysts, Re(dEAbpy)(CO) Cl and Ru-

−

whereas it is slightly endergonic (2.2 kcal mol ) for the

(

dEAbpy)(CO) Cl , were described. The two complexes

2 2

corresponding Mn complex. Assuming the same trend is

valid for our complexes, we suggest that the reaction between

Re and CO is faster than the reaction between Mn and CO .

The protonation of the tertiary amines is most likely equally

fast for 2 and 3; hence, the proton shuttling to form a metal

hydride must be crucial when diﬀerentiating the two pathways.

have the same bipyridine-modiﬁed ligand, where two benzylic

amines are positioned in the second coordination sphere of the

metal center. This ligand was earlier studied with Mn as the

catalytic center, where a high selectivity toward HCOOH in

the electrochemical CO reduction was observed. However,

with either Re or Ru as the metal center, the amounts of

Since binding of CO to Re is thermodynamically favored, we

HCOOH produced, were at most 11.8 ± 3.5% and 8.7 ± 0.1%,

hypothesize that this reaction is faster than the formation of a

Re−H, explaining the selectivity toward CO rather than

HCOOH. This is, furthermore, consistent with the selectivity

observed for other Re electrocatalysts (Table S1). Following

respectively. The major CO reduction product was CO, but

minor amounts of H were produced as well. Evidence for a

metal-carboxylate was found for the Re complex and a

formato-intermediate was proposed for the Ru-catalyst, using

IR-SEC. Although formato-intermediates often are involved in

the same analogy, we believe CO binding is slower for Mn

complex 3, meaning that Mn−H is formed instead of a metal-

carboxylate. This pathway was evaluated computationally in

our previous work, and the presence of Mn−H was evidenced

CO -to-HCOOH reductions, rearrangement to a hydroxyl-

carbonyl adduct is believed to take place, yielding CO as the

major product. These studies highlights not only the unique

selectivity of the Mn catalyst toward HCOOH formation but

also that by a combined tuning of both the metal center and

the ligand it will be possible to control the product selectivity.

using IR-SEC and NMR.

In the case of 2, the IR characterization under catalytic

conditions implied formation of a formato-intermediate,

although the competitive formation of a metal-carboxylate

cannot be ruled out. Hence, we suggest that active catalyst A is

protonated, yielding ammonium intermediate C, which can

rearrange and transfer the proton to the Ru metal center,

forming Ru−H (i.e., D). CO is inserted herein, after a

reduction, to give the formato-intermediate (E). Since CO is

quantiﬁed as the major CO reduction product and E is key to

formate production, we propose an interconversion to a

hydroxyl-carbonyl adduct (F), as also suggested for other Ru

The tricarbonyl complex (G) is then

formed upon protonation followed by expulsion of water, and

the catalytic cycle is closed with a ﬁnal reduction, releasing

CO. If the reduced hydride reacts with H instead of CO ,

then hydrogen is formed in a competing reaction. Hence, the

signiﬁcant amounts of H produced in electrolysis experiments

support the presence of a Ru−H intermediate. To support the

rearrangement from E to F, a bulk electrolysis with complex 2,

TFE, and excess HCOOH instead of CO was conducted at

2.07 V vs Fc /Fc. The experiment showed that CO was

produced together with substantial amounts of H . While

hydrogen evolution can be explained by the lack of a

competing CO insertion, by which protons react with the

hydride, CO production is explained by the interconversion of

EXPERIMENTAL SECTION

■

General Methods. All commercially available reagents and

analytical-grade solvents were purchased from Fluorochem, Sigma-

Aldrich, or TCI Chemicals and used as received, unless otherwise

noted. Dry solvents were obtained from a MBRAUN MB SP-800

puriﬁcation system, and further drying was performed over 3 Å

molecular sieves. All air-sensitive reactions were either performed in

an Ar-ﬁlled glovebox or using standard air-free techniques.

9,25,57,58

complexes.

Bu NBF was synthesized by mixing sodium tetraﬂuoroborate

(98%, Fluka) and tetrabutylammonium hydrogensulfate (Chemische

Fabrik Berg) in a 1:1 molar ratio. High purity CO (99.999%) was

supplied by Air Liquide. ATR FTIR was recorded with a PerkinElmer

Spectrum Two instrument (abbreviation of intensity assignments: w,

weak; m, medium; s, strong). Quantiﬁcation of gaseous products

generated during bulk electrolysis was performed on an Agilent 7890B

Gas Chromatograph equipped with TCD and FID detectors. The

liquid phase was analyzed using a Dionex ICS-1100 Ion

Chromatography System. NMR spectra were recorded using a Bruker

−

500 MHz spectrometer running at 500 MHz for H NMR and 126

MHz for C NMR. Chemical shifts (δ) are reported in parts per

million (ppm) relative to the residual solvent signals (CD CN, 1.94

ppm for H NMR; CD CN, 118.26 ppm for C NMR; CD Cl , 5.32

ppm for H NMR; CD Cl , 54.00 ppm for C NMR). Abbreviations

used to indicate the multiplicity in NMR spectra are as following: s,

the formato-intermediate (E) to the hydroxylcarbonyl adduct

F). However, it should be underlined that the metal-

carboxylate (B) pathway may be partially responsible for the

singlet; d, doublet; t, triplet; m, multiplet. C NMR spectra were

(

acquired in broad band decoupled mode. High-resolution mass

spectra were recorded using electrospray ionization (ESI ) (refer-

production of CO when CO is present in the solution.

enced to the mass of the charged species) on a Bruker Maxis Impact

mass spectrometer. The obtained data were analyzed using Data

Analysis by Bruker Daltonics to give the mSigma value as a measure

for the isotope pattern. Thin-layer chromatographic analysis (TLC)

was performed using precoated aluminum-backed plates (Merck

The high selectivity toward HCOOH using the dEAbpy

ligand seems to be unique for the Mn complex (i.e., 3): One of

the reasons is the less favorable interaction with CO , leading

to formation of the ammonium intermediate, that then further

protects the metal center from attack by CO , and favors the

Mn−H adduct formation. For 3, there is no evidence of a

hydroxylcarbonyl interconversion after CO insertion, as was

Kieselgel 60 F254) and visualized by UV radiation or KMnO stain.

Cyclic Voltammetry. Cyclic voltammograms were recorded with

a CH Instrument (601D) potentiostat using 0.1 M Bu NBF /MeCN

as electrolyte and a three-electrode setup. The working electrode

(WE) was a homemade glassy carbon disk electrode (Sigradur G,

HTW, diameter = 1 mm, unless otherwise stated) embedded into an

the case for 2. Whereas the HCOOH that we observe for both

and 2 underlines the signiﬁcance of the second-sphere amine

Organometallics XXXX, XXX, XXX−XXX

Organometallics

The Supporting Information is available free of charge at

Article

epoxy resin similar to our previous studies. As reference electrode

RE), a nonaqueous Ag/AgI electrode was used, and the potentials

spectroscopic data were in accordance with literature data. A crystal

suitable for X-ray crystallographic analysis was obtained by layering a

saturated CH Cl solution with heptane at −20 °C. The crystals were

(

were referred to the ferrocenium/ferrocene (Fc /Fc) redox couple

using Fc as an internal reference. A Pt wire was employed as the

counter electrode (CE).

obtained as slightly tainted colorless hexagonal crystals.

fac-Re(dEAbpy)(CO) Cl (1). In a 50 mL round-bottomed ﬂask

Bulk Electrolysis. An Autolab PGSTAT302 potentiostat was

employed. Electrolysis experiments were conducted in a homemade

two-chamber cell with a glass frit separating the two chambers as

described elsewhere. Carbon paper (Toray Paper 060 from

FuelCellStore) was used as WE, and the desired electroactive area

equipped with a magnetic stir bar Re(CO) Cl (123 mg, 0.34 mmol,

1.0 equiv) and dEAbpy (163 mg, 0.34 mmol, 1.0 equiv) were

dissolved in toluene (25 mL) and degassed with Ar. The solution was

heated to reﬂux for 16 h under an Ar atmosphere. Hereafter, the

solvent was removed by rotatory evaporation and the residue was

redissolved in CH Cl (0.5 mL). The product was crystallized by

(

0.5 cm × 0.27 ± 0.02 cm) was demarcated using Teﬂon tape. The

RE was a saturated Ag/AgCl, attached to the WE with Teﬂon tape

during electrolysis to maintain the same distance in all experiments. A

Pt mesh served as CE. Each chamber was equipped with a magnetic

stir bar and 0.2 M Bu NBF /MeCN. The catalyst was added in 1 mM

vapor diﬀusion of pentane at −18 °C. After 16 h, additional pentane

3 mL) was added and the solution was cooled at −20 °C for 1 h. The

product was collected by vacuum ﬁltration and washed with pentane

3 × 5 mL) yielding the pure Re complex as a yellow powder (236

(

concentration to one of the chambers, while TFE (0.3 M with

complex 1 and 1.0 M with complex 2) was added to both chambers.

The ﬁnal volume in each chamber was 5 mL. The cell was equipped

with electrodes and sealed. The WE/RE assembly was placed in the

catalyst-containing chamber, while the CE was placed in the chamber

mg, 89%). H NMR (500 MHz, CD CN) (δ) ppm: δ 8.47 (dd, J =

.2, 1.2 Hz, 2H)*, 8.12 (t, J = 8.1 Hz, 2H)*, 7.68−7.28 (m, 10H),

3.64−3.26 (m, 4H), 2.57−2.16 (m, 8H), 0.90 (t, J = 7.1 Hz, 12H)*.

C NMR (126 MHz, CD CN) (δ) ppm: 194.3 (2C), 191.8, 163.3

(2C), 158.3 (2C), 142.7 (2C), 139.7 (2C), 139.1 (2C), 131.4 (2C),

without catalyst. The electrolyte was saturated with CO by purging

30.8 (2C), 130.4 (2C), 128.3 (2C), 127.2 (2C), 124.2 (2C), 56.5

with the gas for 10−15 min prior to electrolysis. The electrochemical

cell was placed in a water bath at room temperature during the 1−4 h

electrolysis to avoid overheating of the membrane, and the solutions

were stirred during experiment.

(

2C), 47.5 (4C), 12.1 (4C). HRMS(ESI ) m/z: Calcd for

C H ClN O Re [M + H] : 785.2255. Found: 785.2275 (mSigma

35 39

−

14.9). IR (ATR, cm ) ν = 2957 (m), 2932 (m), 2808 (m), 2012

Infrared Spectroelectrochemistry. A Nicolet 6700 (Thermo

Fisher Scientiﬁc) instrument was used for recording IR spectra, and a

CH Instruments (601C) potentiostat controlled and monitored the

electrochemical processes during the experiments. The IR-SEC cell

(s), 1879 (s), 1606 (m), 1557 (m), 1458 (m), 1412 (w), 1384 (w),

1227 (w), 803 (w), 765 (m), 633 (w), 532 (w). For peaks marked

with “*”, 2−3 additional minor peaks with the exact same splitting

were observed next to or underneath these peaks, presumably due to

rotamers.

used herein was described in detail in our previous work. The WE

was a 5 mm glassy carbon disk, which was surrounded by a Pt wire

functioning as CE. A commercial Ag/AgCl RE was positioned in

between the WE and CE. The electrolyte was 0.1 M Bu NBF /

trans-Cl-Ru(dEAbpy)(CO) Cl (2). In an Ar-ﬁlled glovebox [Ru-

(CO) Cl ] (54 mg, 0.21 mmol, 0.5 equiv), dEAbpy (100 mg, 0.21

2 2

mmol, 1.0 equiv), and dry toluene (5 mL) were charged into a ﬂame-

dried Schlenk tube equipped with a magnetic stir bar. The tube was

transferred out and ﬁtted with a reﬂux condenser, and the mixture was

stirred at reﬂux overnight shielded from light and under an

atmosphere of Ar. After 18 h the tube was transferred to the

glovebox, and approximately half of the red solvent was removed in-

vacuo. The ﬂask was placed at −35 °C for 20 h. After this time, a pale

yellow solid had precipitated which was isolated by vacuum ﬁltration

and washed with pentane (3 × 1 mL) to give the complex as a pale

yellow solid (31 mg, 21%). Adding pentane to the ﬁltrate allowed a

pale red solid to crash out, from which further product could be

MeCN, which also was used for background subtractions. A catalyst

concentration of 1 mM was used in all experiments. Half-saturation of

CO was obtained by equal mixing of Ar and CO with the use of two

variable area ﬂow meters (Key Instruments). In electrolysis

experiments, the potential was kept at −1.37 V vs Fc /Fc for 5 s

before stepping to the denoted potential. For cyclic voltammetric

−

experiments in this part the sweep rate was 0.05 V s .

Single-Crystal X-ray Diﬀraction. Crystallographic single-crystal

X-ray data for 1 and 2 was collected using an Oxford Diﬀraction

Supernova instrument equipped with a Mo microfocus X-ray source,

an Atlas charge-coupled device detector, and a four-circle goniometer.

The crystal was cooled to 100(1) K using an Oxford Cryosystems

liquid nitrogen Cryostream device. The intensities were empirically

corrected for absorption using SCALE3 ABSPACK implemented in

obtained by recrystallization from heptane. H NMR (500 MHz,

CD Cl ) (δ) ppm: δ 8.28 (t, J = 8.2 Hz, 2H), 8.05 (t, J = 8.0 Hz, 2H),

.66−7.33 (m, 10H), 3.77−3.31 (m, 4H), 2.55−2.23 (m, 8H), 0.87

(t, J = 7.1 Hz, 12H)*. C NMR (126 MHz, CD

) (δ) ppm: 191.2

CrysAlisPRO. The unit cell parameters were determined, and the

Bragg intensities were integrated using CrysAlisPRO. Crystallographic

single crystal X-ray data for N,N′-(([2,2′-bipyridine]-6,6′-diylbis(2,1-

phenylene))bis(methylene))bis(N-ethylethanamine) (dEAbpy) was

collected on a Bruker Kappa Apex2 diﬀractometer equipped with a Ag

source. Absorption correction was done with SADABS. Cell

(2C), 163.9 (2C), 156.6 (2C), 142.2 (2C), 139.4 (2C), 138.3 (2C),

32.2 (2C), 130.9 (2C), 130.6 (2C), 130.5 (2C), 127.1 (2C), 123.1

(2C), 56.4 (2C), 47.2 (4C), 12.2 (4C). HRMS (ESI ) m/z: Calcd for

C H Cl N O Ru [M + H] : 707.1488. Found: 707.1496 (mSigma =

−

2.3). IR (ATR, cm ) ν

1568 (m), 1459 (m), 1229 (m), 803 (m), 766 (s), 729 (m), 581 (m),

31 (m). For peaks marked with “*”, one additional minor peak with

3062 (w), 2964 (m), 2050 (vs), 1987 (s),

reﬁnement and data reduction were done in SAINT-plus. All

structures were solved and reﬁned with SHELXT and SHELXL,

61−63

the exact same splitting was observed next to or underneath these

peaks, presumably due to rotamers.

respectively, in Olex2.

Determination of Rate Constants. For an E C mechanism, the

rate constant was determined using λ = k RT/nFν, where k is the

dis

rate constant for the chemical reaction, n is the number of electrons

involved, ν is the sweep rate, and R, T, and F are the universal gas

constant, the temperature, and Faraday’s constant, respectively. The

value of λ was estimated from the shape of the cyclic voltammograms

ASSOCIATED CONTENT

■

sı Supporting Information

https://pubs.acs.org/doi/10.1021/acs.organomet.9b00815.

64,65

by comparing with simulated voltammograms of known λ values.

A reversible voltammogram indicates that λ < 0.3 and k_d_i_s< 12ν_m_i_n

whereas a completely irreversible voltammogram indicates that λ > 5

Additional ﬁgures (Figures S1−S13), including addi-

tional cyclic voltammograms, IR-SEC data, and elec-

trolysis data; comparison of catalyst performances

^aⁿ^d^kdis ^>²⁰⁰^νmax. Likewise, for E C E or DISP mechanisms, a two-

i r

electron irreversible wave will be indicative of λ > 5 and k_d_i_s> 200ν_m_a_x

Synthesis of Starting Materials. Ligand dEAbpy. N,N′-(([2,2′-

(

Table S1); NMR data (Figures S14−S17); and X-ray

Bipyridine]-6,6′-diylbis(2,1-phenylene))bis(methylene))bis(N-ethyl-

crystallographic data (Tables S2−S4) (PDF)

ethanamine) was synthesized according to a known procedure. All

Organometallics XXXX, XXX, XXX−XXX

Organometallics

Sargent, E. H. What would it take for renewably powered

Article

Accession Codes

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ACKNOWLEDGMENTS

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We gratefully acknowledge the ﬁnancial support from the

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