Electrocatalytic CO2 reduction using rhenium(I) complexes with modified 2-(2'-pyridyl)imidazole ligands

Article abstract of DOI:10.1016/j.ica.2016.09.019

The reduction of CO₂ to CO is an ongoing chemical challenge of great interest for the development of renewable and sustainable technologies. Coordination complexes of rhenium(I) are widely known as CO₂ reduction electrocatalysts, with emphasis on complexes bearing 2,2'-bipyridine (bpy) ligands. Recent research efforts have focused on incorporation of these catalysts into solid, electroactive supports, which can be challenging due to the requirement for synthetic modification to bpy ligands. Development of related ligand sets with modular syntheses is therefore an attractive goal. We investigated electrocatalytic CO₂ reduction using ReCl(CO)₃(N-N) complexes, where N-N is 2-(2'-pyridyl)imidazole (PyIm), 2-(2'-pyridyl)benzimidazole (PyBIm), 2-(2'-quinolyl)imidazole (QuIm), or 2-(2'-quinolyl)benzimidazole (QuBIm) ligands. In CO₂-saturated MeCN solutions, cyclic voltammograms of the Re-QuBIm complex showed current enhancement at a peak potential of ?2.01 V vs. Cp₂Fe^+/0. The catalytic current was stable upon repeated cycling. Re-QuBIm has an overpotential ～200 mV lower than related Re-bpy catalysts, but also is kinetically slower. The redox reactivity of each complex can be correlated to electronic parameters of the N-N chelating ligands. These results suggest that modular reduction potential tuning of CO₂ reduction catalysts is possible, but associated penalties to activity are important considerations.

Full text of DOI:10.1016/j.ica.2016.09.019

Accepted Manuscript
Research paper
Electr ocatalytic CO reduction using rhenium(I) complexeswith modified
2
2-(2´-pyridyl)imidazoleligands
Soumalya Sinha, Ellan K. Berdichevsky, Jeffrey J. Warren
PII:
S0020-1693(16)30525-4
http://dx.doi.org/10.1016/j.ica.2016.09.019
ICA 17264
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
6 July 2016
7 September 2016
10 September 2016
Please cite this article as: S. Sinha, E.K. Berdichevsky, J.J. Warren, Electrocatalytic CO reduction using
2
rhenium(I) complexes with modified 2-(2´-pyridyl)imidazole ligands, Inorganica Chimica Acta (2016), doi:
http://dx.doi.org/10.1016/j.ica.2016.09.019
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Electrocatalytic CO reduction using rhenium(I) complexes with
2
modified 2-(2´-pyridyl)imidazole ligands
a
Soumalya Sinha, Ellan K. Berdichevsky, and Jeffrey J. Warren *
a
a,b
a
Simon Fraser University
Department of Chemistry
8888 University Drive
Burnaby BC V5A 1S6
Canada
b
Canadian Institute for Advanced Research
Azrieli Global Scholars and Bio-Inspired Solar Energy Programs
Toronto, ON M5G 1Z8
Canada
Email: j.warren@sfu.ca
Abstract
The reduction of CO
development of renewable and sustainable technologies. Coordination complexes of rhenium(I)
are widely known as CO reduction electrocatalysts, with emphasis on complexes bearing 2,2´-
2
to CO is an ongoing chemical challenge of great interest for the
2
bipyridine (bpy) ligands. Recent research efforts have focused on incorporation of these catalysts
into solid, electroactive supports, which can be challenging due to the requirement for synthetic
modification to bpy ligands. Development of related ligand sets with modular syntheses is
therefore an attractive goal. We investigated electrocatalytic CO
N) complexes, where N-N is 2-(2´-pyridyl)imidazole (PyIm), 2-(2´-pyridyl)benzimidazole
PyBIm), 2-(2´-quinolyl)imidazole (QuIm), or 2-(2´-quinolyl)benzimidazole (QuBIm) ligands. In
CO -saturated MeCN solutions, cyclic voltammograms of the Re-QuBIm complex showed
current enhancement at a peak potential of –2.01 V vs. Cp
2 3
reduction using ReCl(CO) (N-
(
2
+
/0
2
Fe . The catalytic current was stable
upon repeated cycling. Re-QuBim has an overpotential ~ 200 mV lower than related Re-bpy
catalysts, but also is kinetically slower. The redox reactivity of each complex can be correlated to
electronic parameters of the N-N chelating ligands. These results suggest that modular reduction
2
potential tuning of CO reduction catalysts is possible, but associated penalties to activity are
important considerations.
2
Keywords: CO reduction, electrocatalysis, ligand design, rhenium

Sinha et al.
Re-pyridylimidazole CO
2
reduction
2
Introduction
2
The development of catalysts that convert CO value-added products (e.g., synthesis of
liquid fuels via Fisher-Tropsch chemistry) is of great interest for sustainable chemistries. There
–
, including the 2e reduction of CO
are several approaches to utilizing CO
2
2
to CO (equation 1)
1
using photochemical or electrochemical catalysts,
,2
3,4,5
6
or copolymerization, to name a few.
Homogeneous electrocatalysts have been extensively explored, including metal-2,2´-bipyridyl-
4
,7,8,9
10,11
12,13,14,15
based compounds,
porphyrinoid compounds,
metal carbene compounds,
and
1
6
metal carbonyl compounds. However, exploration of, and improvements to, catalyst design are
still important goals. In addition, incorporation of molecular catalysts into materials also is an
1
7,18,19
ongoing challenge.
2
Herein, we explore the electrochemical reduction of CO using a series
of molecular Re complexes bearing ligands derived from 2-(2´-pyridyl)imidazole as modular
alternatives to bipyridyl ligands.
–
+
+ 2e + 2H → CO + H
CO
2
2
O
(1)
Electrochemical reduction of CO
2
by ligand bidentate rhenium coordination complexes is
7
well established. For example, ReX(CO)
–
(bpy-R) (X = Cl , CF
–
3
3
SO
3
, bpy = 2,2´-bipyridyl, R =
t
H, Bu, OMe at the 4,4´positions of bpy) complexes have been extensively studied.
7,8
H, Me, CO
2
2
Investigation of electrochemical CO reduction using these compounds showed that the onset of
catalytic currents is correlated to the electronic properties of the ligands. The most active
t
catalyst, with 4,4´- Bu-bpy as the N-N chelating ligand, showed impressive catalytic activity, but
at more negative potentials than other complexes due to the electron donating ability of the tert-
8
butyl group.
1
7,18
Taking inspiration from the above work, others demonstrated that non-covalent
or
1
9
covalent immobilization of Re-diimine catalysts offers another strategy for investigating, and

Sinha et al.
Re-pyridylimidazole CO
2
reduction
3
perhaps improving, their electrochemical CO
data demonstrates that the molecular structures of the catalysts are intact. Electrochemical
measurements show that the materials are active for electrochemical reduction of CO to CO and
O. While these approaches show promise, synthetic modifications to the bipyridyl or
2
reduction properties. In all cases, spectroscopic
2
H
2
phenanthroline ligands can be time consuming or involve low yielding and/or multistep
reactions. In contrast, derivatives of 2-(2´-pyridyl)imidazole and 2-(2´-pyridyl)benzimidazole
can be synthesized in one step and as such are widely used as N-N chelating ligands, analogous
in many ways to bipyridine. Notable examples of relevance here include coordination complexes
2
0,21,22,23
of metal ions such as Re(I), Ru(II), and Pt(II).
imidazole ring can be readily modified (e.g., via N-alkylation
functional groups for attachment to other redox moieties, to photosensitizers, and/or to surfaces.
Here, we explore the electrochemical CO reduction activity of Re complexes of four ligands
without modification to imidazole-NH:
pyridyl)benzimidazole (PyBIm), 2-(2´-quinolyl)imidazole
quinolyl)benzimidazole (QuBIm) (Figure 1).
The externally directed NH group of
2
0,21
) in order to introduce new
2
2-(2´-pyridyl)imidazole
(QuIm),
(PyIm),
and
2-(2´-
2-(2´-
CO
Re
Cl
ReCl(CO)5
OC
OC
N
N
N
N
=
Toluene
heat, 1h
H
N
H
N
N
N
N
N
N
N
PyIm
PyBIm
H
N
H
N
N
N
N
N
QuIm
QuBIm
Figure 1. The chemical structures of PyIm, PyBIm, QuIm, QuBIm ligands and their
corresponding Re-complexes.

Sinha et al.
Re-pyridylimidazole CO
2
reduction
4
Materials and methods
All reagents were purchased from Sigma-Aldrich unless otherwise noted and used
without further purification. Chemicals obtained from other sources are as follows: 2-
5
quinolinecarboxyaldehyde (TCI), o-phenylenediamine (Fisher), and ReCl(CO) (Strem). Gases
1
were obtained from Praxair Canada. H NMR were recorded on a Bruker Ultrashield Plus 400 or
1
3
5
00 MHz instruments and C NMR were recorded on a Bruker AVANCE II 600 MHz
instruments equipped with a 5 mm QNP cryoprobe. Solvent residual signals (CDCl and DMSO-
) used as references. Mass spectrometry experiments were carried out using a Bruker
3
2
4
d
6
microTOF instrument equipped with an electrospray ionization source. UV-visible spectra were
recorded in MeCN solvent using a Cary100Bio UV-Vis spectrophotometer. All emission spectra
and lifetime measurements were recorded in deoxygenated MeCN solution using a Fluorolog
Jobin Yvon fluorometer with time-resolved single photon counting capability (TBX picosecond
detector). The detector rise time is 1.0 ns and the LED (335 nm) pulse width is 1.4 ns. IR spectra
were recorded on Thermo Nicolet NEXUS 670 FT-IR.
2
5
Basal plane graphite electrodes were prepared according to the literature. Pyrolytic
graphite was from www.graphitestore.com, Loctite Hysol 9460 epoxy was obtained from
McMaster-Carr, and silver paint was from SPI Supplies. Electrochemical measurements were
performed on a Gamry Instruments Interface1000 potentiostat, using a conventional three-
electrode cell with a basal plane graphite working electrode (3 mm by 3 mm surface area), Pt
3
wire counter electrode, and non-aqueous Ag/0.01M AgNO in MeCN reference electrode (CH
Instruments). The working electrode surfaces were prepared by lightly abrading with 4000 SiC
paper, washing thoroughly with deionized water, followed by sonication (120 s) in isopropyl
alcohol, and briefly drying with a heat gun. Ferrocene was used as an external standard for all

Sinha et al.
Re-pyridylimidazole CO
2
reduction
5
electrochemical experiments and all potentials are reported with respect to the ferrocenium-
+
/0
ferrocene couple (Cp
concentration) in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate as
supporting electrolyte and were purged with argon or CO before CVs were taken.
2
Fe ). All Re-complexes or ligands were dissolved (1mM final
2
2
6
27
Ligand Syntheses. PyIm and QuIm. An ice-cold solution of the appropriate aldehyde (2-
pyridinecarboxyaldehyde (4.668 mmol) or 2-quinolinecarboxyaldehyde (3.181 mmol) in 5 mL
ethanol was added to an ice-cold solution of 40% aqueous glyoxal (0.2 mL) in 5 mL ethanol, and
4
then ice-cold concentrated aqueous NH OH solution (0.15 mL) was added immediately. This
solution was stirred at 0°C for 1 h, warmed to room temperature, and stirred for an additional 5
h. The solvent was removed under reduced pressure, and the resulting solution was extracted
several times with diethyl ether. The combined organic extracts were evaporated under reduced
pressure and crystalline solids were obtained upon recrystallization with diethyl ether.
1
PyIm. Pale yellow solid. Yield: 41%. H NMR (400MHz, CDCl
3
): δ (ppm) 11.7 (s, 1H) 8.51
(
ddd, 1H, 4.9, 1.8, 1.0 Hz), 8.24 (dt, 1H, 8.0, 1.1 Hz), 7.80 (td, 1H, 7.8 1.7 Hz), 7.26 (ddd, 1H,
.6, 4.9, 1.2 Hz), 7.17 (br s, 2H).
7
1
QuIm. Brown solid. Yield: 52%. H NMR (400MHz, CDCl
.6 Hz), 8.25 (d, 1H, 8.6 Hz), 8.02 (d, 1H, 8.6 Hz), 7.84 (d, 1H, 8.4 Hz), 7.70 (td, 1H, 7.6, 1.1
Hz), 7.53 (td, 1H, 7.6, 1.1 Hz), 7.31 (s, 1H), 7.19 (s, 1H).
3
): δ (ppm) 11.13 (s, 1H) 8.35 (d, 1H,
8
2
8
PyBIm and QuBIm. To a mixture of o-phenylenediamine (4.62 mmol for PyBIm and
.59mmol for QuBIm) and 2-pyridinecarboxyaldehyde (4.62 mmol) or 2-
quinolinecarboxyaldehyde (1.59mmol) in 5 mL of ethanol was added NH Cl (0.075g). In both
cases, the resulting solution was stirred for 4h at 80°C. The reaction mixture was then poured
1
4

Sinha et al.
Re-pyridylimidazole CO
2
reduction
6
into ice-cold water and the product was precipitated as a yellow solid. The solids were filtered
and washed with water thoroughly and dried.
1
PyBIm. Off-white solid. Yield: 78%. H NMR (400MHz, CDCl
3
): δ (ppm) 10.76 (s, 1H), 8.66
(
ddd, 1H, 4.8, 1.7, 0.9 Hz,), 8.46 (dt, 1H, 7.9, 1.1 Hz), 7.89 (td, 1H, 7.7, 1.7 Hz), 7.88 (br, 1H)
7
.50 (br, 1H), 7.39 (ddd, 1H, 7.6, 4.8, 1.2 Hz), 7.33 (m, 2H).
1
QuBIm. Pale yellow solid. Yield: 86%. H NMR (400MHz, CDCl
3
): δ (ppm); 10.96 (s, 1H), 8.59
(
d, 1H, 8.5 Hz), 8.34 (d, 1H, 8.7 Hz), 8.14 (d, 1H, 8.7 Hz), 7.91 (m, 1H), 7.90 (d, 1H, 8.7 Hz)
7
.78 (ddd, 1H, 8.5, 6.8, 1.5 Hz), 7.60 (ddd, 1H, 8.1, 6.8, 1.2 Hz), 7.50 (m, 1H), 7.33 (m, 2H).
(N-N) complexes. ReCl(CO) and a requisite N-N
General synthetic procedure for ReCl(CO)
3
5
ligand were dissolved in a 1:1 molar ratio in 50 mL of hot toluene and the reaction mixture was
stirred at reflux for 1 hour, after which the mixture was allowed to cool to room temperature. A
8
,29
yellow precipitate formed during cooling.
Storing this solution at –20°C for an additional 1
hour promoted further precipitation. The mixture was then filtered and washed with cold toluene.
Note that the numbering system for each ring is given with the NMR spectra shown in the
t
(4,4´- Bu-bpy) was prepared and characterized according to
Supporting Information. ReCl(CO)
3
8
the literature.
2
Re-PyIm. Pale yellow solid. Yield: 92%. H NMR (400 MHz, DMSO-d
0
1
6
): δ (ppm) 14.28 (s,
br, 1H, imidazole-NH) 8.91 (dt, 1H, 5.5, 1.3 Hz, H4), 8.28 (m, 2H, H1 and H2), 7.70 (d, 1H, 1.4
1
3
Hz, H5/H6), 7.63 (ddd, 1H, 7.4, 5.5, 2.0 Hz, H3), 7.51 (d, 1H, 1.4 Hz, H5/H6); C NMR (600
MHz, DMSO-d ): δ (ppm) 197.85, 197.40, 189.88, 153.27, 148.37, 146.88, 140.43, 130.48,
6
-
1
-1
-
1
26.27, 121.98, 121.25. λmax/nm (ε/M cm ): 265 (10564), 302 (12115). IR (CH CN) υ(CO) (cm
3
1
+
+
+
)
3
: 2017, 1909, 1886. ESI-MS(+): 416.009 (M ), 444.015 (M + CO), 457.035 (M + CH CN).

Sinha et al.
Re-pyridylimidazole CO
2
reduction
7
2
Re-PyBIm. Pale yellow solid. Yield: 89%. H NMR (400 MHz, DMSO-d
2
1
6
): δ (ppm) 9.04 (d,
1
H, 5.3 Hz, H4), 8.51 (d, 1H, 7.5 Hz, H1), 8.37 (td, 1H, 7.8, 1.5 Hz, H2), 7.86-7.72 (m, 3H, H5,
1
H8, and H3), 7.54-7.48 (m, 2H, H6 and H7); C NMR (600 MHz, DMSO-d
3
6
): δ (ppm) 198.37,
1
1
1
97.88, 189.68, 154.67, 153.66, 147.82, 141.34, 140.53, 135.69, 127.49, 125.12, 124.47, 123.27,
-
1
-1
-1
17.23, 114.48. λmax/nm (ε/M cm ): 321(14127), 333 (14127). IR (CH
3
CN) υ(CO) (cm ): 2019,
+
907, 1893. ESI-MS(+): 466.025 (M ), 494.032 (M + CO), 507.052 (M + CH
+
+
3
CN).
1
Re-QuIm. Brown solid. Yield: 81%. H NMR (400 MHz, DMSO-d
6
): δ (ppm) 14.54 (s, 1H,
imidazole-NH), 8.92 (d, 1H, 8.7 Hz, H2), 8.65 (dd, 1H, 8.7, 0.7 Hz, H3), 8.40 (d, 1H, 8.3, 1.4
Hz, H1), 8.21 (dd, 1H, 8.2, 1.5 Hz, H6), 8.11 (ddd, 1H, 8.7, 6.9, 1.5 Hz, H4), 7.85 (ddd, 1H, 8.0,
1
.9, 1.0, H5), 7.82 (d, 1H, 1.4 Hz, H7/H8), 7.68 (d, 1H, 1.4 Hz, H7/H8); C NMR (600 MHz,
3
6
DMSO-d ): δ (ppm) 198.53, 196.54, 189.73, 149.75, 149.21, 146.42, 141.62, 132.90, 131.20,
6
-
1
-1
1
29.52, 128.61, 128.51, 128.18, 122.85, 118.14. λmax/nm (ε/M cm ): 283 (17181), 347 (11299).
-
1
+
+
IR (CH
3
CN) υ(CO) (cm ): 2017, 1909, 1887. ESI-MS(+): 466.025 (M ), 494.031 (M + CO),
+
07.052 (M + CH
5
3
CN).
1
Re-QuBIm. Orange solid. Yield: 83%. H NMR (400 MHz, DMSO-d
imidazole-NH) 9.05 (d, 1H, 8.5 Hz, H2), 8.77 (d, 1H, 8.8 Hz, H3), 8.62 (d, 1H, 8.5 Hz, H1), 8.29
d, 1H, 8.2 Hz, H6), 8.19 (ddd, 1H, 8.7, 6.9, 1.5 Hz, H4), 7.99 – 7.89 (m, 3H, H5 and BIm-H),
.64 – 7.59 (m, 2H, BIm-H). Note that BIm-H indicates the protons (H – H10) from
benzimidazole ring (see Supporting Information). C NMR (600 MHz, DMSO-d ): δ (ppm)
98.13, 197.22, 189.37, 155.01, 149.65, 146.80, 141.99, 141.39, 134.11, 133.35, 129.68, 129.31,
6
): δ (ppm) 15.0 (s, br, 1H,
(
7
7
1
3
6
1
1
-1
-1
28.92, 128.82. 126.31, 125.42, 119.51, 117.71, 114.00. λmax/nm (ε/M cm ): 250 (22123), 297
-
1
+
(
11084), 364 (14657). IR (CH
3
CN) υ(CO) (cm ): 2017, 1905, 1891. ESI-MS(+): 516.041 (M ),
+
44.047 (M + CO), 557.068 (M + CH
+
5
3
CN).

Sinha et al.
Re-pyridylimidazole CO
2
reduction
8
Result
Synthesis and Characterization. Each of the N-N chelating ligands was synthesized in
moderate to good yields in a single step from commercially available starting materials. In all
1
cases, H NMR characterization data matched those reported in the literature. The corresponding
3 5
ReCl(CO) (N-N) complexes were synthesized from a 1:1 mixture of ligand and ReCl(CO) in
1 13
refluxing toluene and were isolated in good yields. NMR ( H and C) spectra are presented in
1
the Supporting Information. Of note is a downfield resonance (ca. 10-14 ppm) in most H NMR
spectra that is attributed to the imidazole or benzimidazole NH.
The electronic absorption and the emission spectra (λex = 375 nm) for each complex in
MeCN solvent are shown in Figures 2 and 3, respectively. In all absorption spectra, characteristic
2
9
dπ-π* (MLCT) and π-π* transitions are observed. In analogy to the related bpy compounds, the
broad bands extending to ~400 nm (Re-PyIm and Re-PyBIm) and ~450 nm (Re-QuIm and Re-
3
–1
–1
QuBIm) are attributed to the MLCT transitions, with ε ~ 10 M cm . The higher energy,
sharper and more intense features, arise from intraligand transitions. For reference, spectra for
each of the ligands are shown in the Supporting Information.
All four complexes also are luminescent, with λem, max between 550 and 650 nm at 295 K.
At the same concentrations, Re-PyIm and Re-PyBIm appear (qualitatively) more luminescent
than Re-QuIm and ReQuBIm. Both the weak intensity and the band position of the Re-quinolyl
2
9
3
complexes are similar to those reported for ReCl(CO) (2,2´-biquinoline). The luminescence
lifetimes (τ) in deoxygenated MeCN are: 3.5 µs (Re-PyIm), 3.3 µs (Re-PyBIm), 2.7 µs (Re-
QuIm), and 5.6 µs (Re-QuBIm). These values also are in accord with those reported for related
2
9,30,31
bipyridyl complexes

Sinha et al.
Re-pyridylimidazole CO
2
reduction
9
Figure 2. Electronic absorption spectra for Re-PyIm, Re-PyBIm (A), Re-QuIm, and Re-QuBIm
B) in MeCN.
(
Figure 3. Emission spectra (λex = 375 nm) for Re-PyIm, Re-PyBIm (A), Re-QuIm, and Re-
QuBIm (B) in deoxygenated MeCN.
The carbonyl infrared spectra for each Re-complex are given in the Supporting
Information and the band positions for the three observed carbonyl stretches are set out in Table
1
. The bands are consistent with the pseudo-C
s
symmetry about Re, and are similar to those
2
reported for the same or related complexes (e.g., Re-PyBIm recorded in the solid state and the
2
3
N-ethyl analog of Re-PyBIm recorded in KBr ). Also shown in Table 1 are characteristic data
2
8
for related 2,2´-bipyridyl derivatives for comparison. The slightly lower IR stretching
frequencies for Re-PyIm, Re-PyBIm, Re-QuIm, and Re-QuBIm are in qualitative agreement with

Sinha et al.
Re-pyridylimidazole CO
2
reduction
10
the differences in electronic properties of imidazole-containing ligands versus bpy ligands (e.g.,
3
PyIm is a somewhat weaker π-acceptor ), that results in greater electron density on the metal.
3
a
3
Table 1. Carbonyl IR stretching frequencies for ReCl(CO) (N-N) compounds
8
t
8
8
bpy
Bubpy
bpyCO
2
H
PyIm
2017
1909
1886
PyBIm QuIm QuBIm
2
1
1
025
918
902
2023
1916
1898
2037
1935
1911
2019
1907
1893
2017
1909
1887
2017
1905
1891
a
t
bpy = 2,2´-bipyridyl, Bubpy = 4,4´-tert-butyl-2,2´-bipyridyl, bpyCO
bipyridyl. Data for all complexes collected in MeCN solvent.
2
H = 4,4´-dicarboxylicacid-2,2´-
Electrochemical behavior under inert atmosphere. Electrochemical studies of the series of
n
Re-complexes (1 mM) were carried out in MeCN solution with 0.1 M Bu
4
NPF
6
supporting
electrolyte. The working electrode was basal plane graphite and potentials are reported with
+
/0
2
respect to Cp Fe . Other experimental details are given above. In argon saturated solution, Re-
PyIm and Re-QuIm show one irreversible reduction wave at –1.74 and –1.72 V (peak potentials),
respectively. RePyBIm and Re-QuBIm each show two irreversible waves. The first irreversible
wave appears at –1.74 and –1.57 V, respectively followed by an additional wave at –2.20 and –
2
.01V, respectively. Oxidative scans also showed irreversible waves. All observed peak
potentials are summarized in Table 2. The potentials measured for Re-PyBIm are close to those
reported in MeCN solvent in two independent reports of the electrochemical properties of the
3
closely related N-methyl-PyBIm Re complex (–1.83, –2.27, 0.88 V and –1.79, –2.14, 0.87 V,
4
35
+
/0
respectively; potentials are versus Cp
2
Fe ).
+
Fe ) for Re-complexes under Ar
/0
Table 2. Peak reduction potentials (in MeCN versus Cp
2
1
2
Compound
Re-PyIm
E
red (V)
E
red (V)
Eox (V)
–1.74
–1.74
–1.72
–1.57
--
0.93
0.96
0.98
0.95
Re-PyBIm
Re-QuIm
Re-QuBIm
–2.20
--
–2.01

Sinha et al.
Re-pyridylimidazole CO
2
reduction
11
Electrochemical behavior under CO
2
. Cyclic voltammograms for all four complexes were
reproducibly shifted the
collected in CO -saturated MeCN solution. The addition of CO
2
2
reduction peak potential to –2.05 V for Re-PyIm, but did not increase the intensity. Addition of
methanol, as a proton source, also did not affect the peak position or current. The origin of this
unusual behavior is not yet known. In the case of Re-PyBIm, ≤5% current enhancement at the
2
second reduction peak (–2.24 V) is observed under CO . A steeply rising wave is observed for
this complex, but it is convoluted with signal from the electrochemical window. Addition of
MeOH has no effect on the observed currents.
The two quinolyl-containing complexes (Re-QuIm and Re-QuBIm) were more active for
2 2
reduction of CO . CVs collected under CO atmosphere for ReQuIm showed current
–
2
–2
enhancement (160 µA cm to 630 µA cm at –2.24 V) after the first reduction wave (Figure 4).
The response is near the limit of the electrochemical window, but is not attributable to
background alone. Re-QuBIm also is active for CO
2
reduction, exhibiting 2-fold current
enhancement (285 µA cm to 560 µA cm at –2.01 V) at the second reduction potential in the
presence of CO (Figure 4). In controlled potential experiments under CO atmosphere, Re-
–
2
–2
2
2
QuBIm remains active with –2 V applied potential for about 40 minutes (see Supporting
Information). Furthermore, with the addition of 1M MeOH, the reduction potential shifted by
about 20mV anodically with the negligible change in current intensity. For reference, CVs of
2
ligands alone (under Ar and under CO ) are presented in the Supporting Information. Addition of
CO causes negligible increases in current upon ligand reduction.
2

Sinha et al.
Re-pyridylimidazole CO
2
reduction
12
-
4
4
4
4
-
4
4
4
-
8.0x10
-
6.0x10
Re-QuBIm under Ar
Re-QuBIm under CO
Re-QuIm under Ar
Re-QuIm under CO
-
2
2
-5.0x10
-
-
-
-
6.0x10
-
-
-
-
-
4.0x10
-4
-
4.0x10
3.0x10
-
4
2.0x10
-
2.0x10
-
4
1.0x10
0.0
0.0
0.0
-0.5
-1.0
-1.5
-2.0
/0
-2.5
0.0
-0.5
-1.0
-1.5
-2.0
+/0
-2.5
+
Potential (V) vs. Cp Fe
Potential (V) vs. Cp Fe
2
2
Figure 4. Cyclic voltammograms for 1mM Re-QuIm (left) and Re-QuBIm (right) under Ar
n
(
arrow indicates the scan direction.
2 4 6
black) and CO (red) saturated 0.1M Bu NPF MeCN solution at 100 mV/sec scan rate. The
Discussion and Conclusions
Modifications of the 2-(2´-pyridyl)imidazole (PyIm) backbone provide a facile approach
for tuning the electronic properties of transition metal complexes. Here, we investigated the
spectroscopic properties and CO
ReCl(CO) (N-N) (N-N = 2-(2´-pyridyl)imidazole (PyIm), 2-(2´-pyridyl)benzimidazole (PyBIm),
-(2´-quinolyl)imidazole (QuIm), and 2-(2´-quinolyl)benzimidazole (QuBIm)). Each of the
complexes displays properties consistent with related coordination complexes bearing a fac-
Re(CO) core, including IR features and luminescence. As for 2,2´-bipyridyl derived Re
complexes, the four complexes described in this work also have reduction potentials at > 1V
2
reduction behavior of four Re(I) complexes of the general form
3
2
3
II/I
Re ) and < –1 V (ligand-centered reduction). As for related Re and Ru pyridyl complexes,
29
36
(
coordination of the ligand to a metal shifts the ligand-centered reduction to potentials more
positive than the ligand alone (see Supporting Information).
3
7,38
Lever’s parameterization of ligand properties and metal complex reduction potentials
offers one way in which to rationalize the electrochemical behavior of our four Re complexes.
The parameters (E ) are: 0.12 (imidazole), 0.17 (bibenzimidazole), 0.25 (pyridine), and 0.29
L

Sinha et al.
Re-pyridylimidazole CO
2
reduction
13
(
biquinoline). Note that values are not known individually for benzimidazole and quinolone, but
values are given for each coordinating atom (i.e., ΣE for biquinoline is 2•0.17). Bearing this
in mind, the ΣE values for PyIm, PyBIm, QuIm, and QuBIm are 0.37, 0.42, 0.41, and 0.46,
respectively. For comparison, ΣE
butyl analog are not reported. The E
E
L
L
L
3
7
L
for 4,4´-dimethly-2,2´-bipyridine is 0.46; values for the tert-
–
L
values for CO and Cl are 0.99 and –0.24, respectively.
II/I
values are correlated to higher Re potentials. In theory, this also should qualitatively
Larger E
L
I/0
correspond to higher (less negative) Re potentials. Here, this would correspond to potentials at
which CO
catalysis. The calculated Re couples are 0.78, 0.81, 0.81, and 0.84 V versus Cp
Lever’s approach gives values with respect to NHE, which we converted using known
2
is reduced, since input of two electrons (ligand + Re reduction) is required for
II/I
+/0
2
Fe . Note that
3
parameters. The calculated values do not quantitatively reproduce the observed peak potentials,
9
as expected for irreversible waves. However, the trend in Ep,c is in agreement with the calculation
2
and indicates that the potentials of Re-based CO reduction catalysts can be rationally tuned with
ligands other than bpy.
The ability of each ligand to support radical anion character (non-innocence) increases
with increasing aromatic ring size. We do not observe reduction of the PyIm ligand itself, and the
corresponding Re complex has the most negative potential (Table 2). The slightly larger.
aromatic system in PyBIm supports some additional catalytic activity. In contrast, the QuBIm
0
ligand has the highest reduction potential (QuBIm , see Supporting Information), as does the
/–
corresponding Re complex. Re-QuIm also exhibits catalytic activity. These data demonstrate that
–
that the ability of a system to accept 2e is an important feature for molecular catalysis in these
complexes. Similar behavior is observed for related Re and porphyrin CO
2
reduction
3
,5,7
catalysts.
We note that it was recently demonstrated that ligand non-innocence is not a

Sinha et al.
Re-pyridylimidazole CO
2
reduction
14
1
6
2
requirement for metal-mediated CO reduction, but complexes that are able to accept multiple
reducing equivalents are generally superior catalysts, exhibiting lower overpotentials and more
1
0,11
rapid turnover frequencies.
The activity of Re-QuBIm is interesting to compare to that of ReCl(CO)
under similar reaction conditions. The overall electrochemical properties (based on Lever’s
t
8
3
(4,4´- Bu-bpy)
parameters) are similar, but the activities are different. Re-QuBIm has current onset for CO
2
t
(4,4´- Bu-bpy). This is a
reduction at potentials about 200 mV more positive than ReCl(CO)
notable shift in onset potential, but the overpotential (η) for CO
3
2
reduction by Re-QuBIm is still
4
high. However, analysis of the peak current response for Re-QuBIm (or Re-QuIm) shows
0
41
–
1
–1 42
CO
2
reduction kinetics, with a rate constant of about 5 M s . Note that our rate constant is
. This value is
(4,4´- Bu-bpy). We were surprised
approximate due to the irreversibility of the CV response in the absence of CO
2
3
t
8
almost 10 -fold more sluggish than reported for ReCl(CO)
3
by this large difference in activity for such similar compounds, so we collected CVs of
t
(4,4´- Bu-bpy) under CO
ReCl(CO)
3
2
using experimental conditions identical to those we used
for Re-QuIm and Re-QuBIm (see Supporting Information). Based on a comparison the intensity
of the catalytic waves, Re-QuIm and Re-QuBIm are only ca. 15 times more sluggish CO
2
t
(4,4´- Bu-bpy). We conclude that the ~200 mV decrease in
reduction catalysts than ReCl(CO)
3
overpotential is coupled to modest losses in CO
In sum, we synthesized, characterized, and investigated the electrochemical reduction of
using four fac-Re(CO) complexes with N-N chelating ligands derived from 2-(2´-
2
reduction kinetics.
CO
2
3
pyridyl)imidazole. The modular framework that comprises the four N-N chelating ligands (PyIm,
PyBIm, QuIm, and QuBIm) allows for the electronic properties of the complex to be tuned and
for catalytic overpotentials to be consequently altered, although our results suggest that there can

Sinha et al.
Re-pyridylimidazole CO
2
reduction
15
2
be a cost with respect to CO reduction kinetics. Re-QuBIm, with the largest aromatic system,
+
/0
2
showed good catalytic activity at a peak potential of –2.01V versus Cp Fe . Conversely, the
complex with the smallest aromatic system, Re-PyIm, showed the least catalytic activity under
the identical electrochemical conditions. In ongoing work, we are investigating methods of
4
3,44
17
and non-covalent attachment of these catalysts to carbon electrodes via the
covalent
external imidazole-NH.
Acknowledgment
The Simon Fraser University President’s Research Startup Grant, the National Sciences and
Engineering Research Council (RGPIN05559 to J. J. W.), and the CIFAR Azrieli Global
Scholars – Bioinspired Solar Energy Program supported this work. E.K.B. and J.J.W. also
acknowledge support from the Canada Summer Jobs Program (Project # 013745021). Dr.
1
Andrew Lewis assisted with C NMR acquisition.
3
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The rate constant was calculated using the following equation: i
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Electrocatalytic CO reduction using rhenium(I) complexes with
2
modified 2-(2´-pyridyl)imidazole ligands
Soumalya Sinha, Ellan K. Berdichevsky, and Jeffrey J. Warren*
Simon Fraser University
Department of Chemistry
8888 University Drive
Burnaby BC V5A 1S6
Canada
Email: j.warren@sfu.ca
Highlights:
•
•
•
A series of Re(I) coordination complexes with bidentate ligands were prepared.
Complexes with quinolone-derived ligands are active for electrochemical CO
2
reduction.
Reactivity can be correlated to ligand electronic properties.

Sinha et al.
Re-pyridylimidazole CO
2
reduction
18