Rhenium Complexes Based on 2-Pyridyl-1,2,3-triazole Ligands: A New Class of CO2 Reduction Catalysts

Article abstract of DOI:10.1021/acs.inorgchem.6b03078

A series of [Re(N^N)(CO)₃(X)] (N^N = diimine and X = halide) complexes based on 4-(2-pyridyl)-1,2,3-triazole (pyta) and 1-(2-pyridyl)-1,2,3-triazole (tapy) diimine ligands have been prepared and electrochemically characterized. The first ligand

Full text of DOI:10.1021/acs.inorgchem.6b03078

Article
pubs.acs.org/IC
Rhenium Complexes Based on 2‑Pyridyl-1,2,3-triazole Ligands: A
New Class of CO2 Reduction Catalysts
H. Y. Vincent Ching,†,‡,§,∥,⊥,#,∇ Xia Wang,† Menglan He,‡,§ Noemi Perujo Holland,‡,§ Regis Guillot,○
́
Cyrine Slim,∥,⊥,#,∇ Sophie Griveau,∥,⊥,#,∇ Helene C. Bertrand,‡,§ Clotilde Policar,*,‡,§
́
̀
Fethi Bedioui,*,∥,⊥,#,∇ and Marc Fontecave*,†
†Laboratoire de Chimie des Processus Biologiques, PSL Research University, Colleg
et Marie Curie, 11 Place Marcelin Berthelot, 75231 Paris Cedex 05, France
‡Dep
artement de Chimie, Ecole Normale Superieure, PSL Research University, UPMC Univ Paris 06, CNRS, Laboratoire des
Biomolecules, 24 rue Lhomond, 75005 Paris, France
§Sorbonne Universites
, UPMC Univ. Paris 06, Ecole Normale Super
̀ ́
e de France, CNRS UMR8229, Universite Pierre
́
́
́
́
́
ieure, CNRS, Laboratoire des Biomolec
́
ules, 24 rue Lhomond,
75005 Paris, France
∥Chimie ParisTech, PSL Research University, Unite
⊥CNRS, UTCBS UMR 8258, F-75006 Paris, France
de Technologies Chimiques et Biologiques pour la Sante, F-75005 Paris, France
́ ́
#Universite
∇INSERM, UTCBS U 1022, F-75006 Paris, France
○Institut de Chimie Molec
́ ́
ulaire et des Materiaux d’Orsay, Universite Paris-Sud, UMR CNRS 8182, Universite Paris-Saclay, 91405
́ ́
Paris Descartes, Sorbonne-Paris-Cite, UTCBS, F-75006 Paris, France
́
́
Orsay, France
S
* Supporting Information
ABSTRACT: A series of [Re(N^N)(CO)3(X)] (N^N = diimine
and X = halide) complexes based on 4-(2-pyridyl)-1,2,3-triazole
(pyta) and 1-(2-pyridyl)-1,2,3-triazole (tapy) diimine ligands have
been prepared and electrochemically characterized. The first ligand-
based reduction process is shown to be highly sensitive to the nature
of the isomer as well as to the substituents on the pyridyl ring, with
the peak potential changing by up to 700 mV. The abilities of this
class of complexes to catalyze the electroreduction and photo-
reduction of CO2 were assessed for the first time. It is found that
only Re pyta complexes that have a first reduction wave with a peak
potential at ca. −1.7 V vs SCE are active, producing CO as the
major product, together with small amounts of H2 and formic acid. The catalytic wave that is observed in the CVs is enhanced by
the addition of water or trifluoroethanol as a proton source. Long-term controlled potential electrolysis experiments gave total
Faradaic yield close to 100%. In particular, functionalization of the triazolyl ring with a 2,4,6-tri-tert-butylphenyl group provided
the catalyst with a remarkable stability.
functionalized ligands and a variety of available methods for
grafting them at the surface of conductive materials.5−10
Surprisingly only a few classes of molecular electrocatalysts
have been investigated since the 1980s with very limited
structure−activity relationship studies. Compounds that have
been receiving renewed attention include iron porphyrins,11 on
one hand, and polypyridine metal complexes (based on
different metals such as Re, Ru, Ir, Rh, Mn, and Co), on the
other.4 The latter has indeed the ability to store multiple
electrons across the entire molecule, not only at the metal
center but also in the ligand π system, thus facilitating the
reduction of CO2. The prototypical catalyst from this second
INTRODUCTION
■
Carbon dioxide electroreduction into energy-dense carbon-
based liquid or gaseous products is an attractive way to store
renewable energies into chemical energy. However, because of
the high stability of CO2, its transformation implies multiple
electron- and proton-transfers. Thus, catalysts are required for
overcoming the slow kinetics, minimizing overpotentials, and
controlling product selectivities. While solid metal-based
catalysts are currently extensively investigated, homogeneous
molecular metal complexes (coordination and organometallic
complexes) provide unique opportunities to rationally address
some of the issues discussed above, since their reactivity can be
finely tuned via synthetic modifications of the ligands.1−4
Furthermore, solid electrodes decorated with molecular
complexes can be quite easily obtained using appropriately
Received: December 20, 2016
© XXXX American Chemical Society
A
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Article
family is [2,2′-bipyridine]-chlorotricarbonylrhenium(I) ([Re-
(bpy)(Cl)(CO)3]), as it has a high Faradaic efficiency for the
generation of CO in the presence of Brønsted acids.12
However, optimizing the system via modifications of the bpy
ligand has been limited to a small number of functionalized
bpys, with [Re(bpy-tBu)(Cl)(CO)3] (bpy-tBu = 4,4′-di-tert-
butyl-2,2′-bipyridine) being the most efficient catalyst to date.13
This is in part due to the complexity of functionalizing the bpy
ligand with moieties that can tune the electronic and steric of
the metal complex, particularly in a dis-symmetric manner.14,15
To explore other classes of Re diimine complexes, using
original diimines as alternative ligands to bpys, we investigated
[Re(pyta)(X)(CO)3] and [Re(tapy)(X)(CO)3] complexes, in
which pyta designates 4-(2-pyridyl)-1,2,3-triazole derivatives,
while tapy designates 1-(2-pyridyl)-1,2,3-triazole derivatives,
and X = Cl or Br (Scheme 1), as catalysts for CO2
corresponding Re complexes are also reported here. The
complexes have been evaluated for their electrocatalytic activity
for CO2 reduction, via rotating disk electrode (RDE)
experiments, cyclic voltammetry (CV), and controlled potential
electrolysis (CPE), as well as for their photocatalytic properties.
To our knowledge this is the first structure−activity relation-
ship study of a whole new class of electrocatalysts for CO2
reduction based on such a large ensemble of molecules.
EXPERIMENTAL SECTION
■
Synthesis. General Considerations. 1H and 13C{1H} NMR
spectra were recorded on a Bruker Avance 300 or 400 spectrometers
using residual solvent peaks as internal references. The following
abbreviations are used: singlet (s), doublet (d), doublet of doublets
(dd), triplet (t), doublet of triplets (td), quintuplet (quint), sextuplet
(sext), and multiplet (m). Mass spectrometry (MS) services were
́
performed at the ICMMO (Universite Paris Sud, Orsay, France). The
following abbreviations are used: MS, high-resolution mass spectrom-
etry (HRMS), electrospray ionization (ESI), time-of-flight (TOF).
Analytical reversed-phase high-performance liquid chromatography
(HPLC) was performed on a Dionex Ultimate 3000 equipped with a
variable wavelength detector, using a PROTO200 C18 3 μm 100 × 4.6
mm (Higgins Analytical Inc.) column. The results were analyzed using
Chromeleon software. Thin-layer chromatography (TLC) analysis was
performed on silica gel (Merck 60F-254) with visualization at 254 and
366 nm. Preparative flash chromatography was performed with Merck
silica gel (Si 60, 40−63 μm). Reagents and chemicals were purchased
from Sigma-Aldrich, Alfa Aesar, or Strem Chemicals and used without
further purification. Anhydrous solvents (acetonitrile (MeCN),
dichloromethane (DCM), dimethylformamide (DMF), tetrahydrofur-
an (THF), and toluene) were purchased from Sigma and used without
further purification. IR spectra were recorded on a PerkinElmer
Spectrum 100 FT-IR spectrometer. Caution! Though no problem was
encountered in the handling of any of the azides used, these compounds are
potentially explosive and should thus be manipulated with caution. [2-(1-
Dodecyl-1H-1,2,3-triazol-4-yl)-pyridine]-chlorotricarbonylrhenium(I)
(1), [3-(4-(pyridin-2-yl)-1H-1,2,3-triazol-1-yl)propan-1-ol]-
chlorotricarbonylrhenium(I) (4), [2-(1-dodecyl-1,2,3-triazol-4-yl)-
quinoline]-bromotricarbonylrhenium(I) (7), [2-(1-dodecyl-1H-1,2,3-
triazol-4-yl)-pyridine]-bromotricarbonylrhenium(I) (8), [2-(4-dodec-
yl-1H-1,2,3-triazol-1-yl)pyridine]-chlorotricarbonylrhenium(I) (1′),
a n d [2 - ( 4 -d o de c y l - 1H -1 , 2, 3-t ria zo l-1 -y l) py ri din e ]-
bromotricarbonylrhenium(I) (8′) were previously described by
Bertrand et al.17 [2,2′-Bipyridine]-chlorotricarbonylrhenium(I) (9)
and [2,2′-bipyridine]-bromotricarbonylrhenium(I) (10) were pre-
pared according to classical rhenium coordination procedures as
described for the pyta complexes. 4-Azidotoluene was prepared
Scheme 1. Structure of the Ligands and Rhenium Complexes
Synthesized and Investigated in This Study
according to Gjonaj and Roelfes,19 dodecyl azide was prepared
k et al.,20 and 1,3-dimethyl-2-phenyl-2,3-dihydro-
́
according to Jurice
̌
1H-benzo[d]imidazole (BIH) was prepared according to Zhu et al.21
General Procedure A: Preparation of Rhenium Complexes.
Chloropentacarbonylrhenium(I) (1.0 equiv) was dissolved in hot
toluene (110 °C) under argon. The diimine ligand (1.1 equiv) was
added, and the solution was refluxed overnight. The mixture was
cooled to room temperature, and if a precipitate was formed, it was
isolated by centrifugation, washed with cold toluene and cold diethyl
ether, and dried in vacuo; otherwise, the solvent was evaporated, and
the crude compound was purified by column chromatography. The
purification protocols for each complex are described below.
electroreduction. Not only can such ligands be easily prepared
in good yields via a clean and efficient copper-catalyzed
alkyne−azide cycloaddition (CuAAC) reaction (a widely used
example of “click” chemistry)16,17 but also this synthetic
strategy allows controlled functionalization of both the pyridine
and the triazole moieties and production of a great variety of
ligands and complexes with finely tuned stereoelectronic
properties. While some of the complexes studied here have
been previously reported by us and found to display interesting
luminescent properties,17,18 a set of new pyta ligands and
2-(1-(p-Tolyl)-1H-1,2,3-triazol-4-yl)pyridine. 4-Azidotoluene (333
mg, 2.5 mmol, 1.0 equiv) and 2-ethynylpyridine (250 mg, 2.5 mmol,
1.0 equiv) were dissolved in 1:1 DCM/water (10 mL) mixture.
Copper(II) sulfate pentahydrate (62 mg, 0.25 mmol, 0.1 equiv) and
sodium ascorbate (100 mg, 0.5 mmol, 0.2 equiv) in water (1 mL) were
added to the mixture. After it was stirred overnight at room
temperature under argon, more copper(II) sulfate pentahydrate (62
mg, 0.25 mmol, 0.1 equiv) and sodium ascorbate (100 mg, 0.5 mmol,
0.2 equiv) in water (1 mL) were added. After it was stirred for 48 h,
DCM (50 mL) was added to the mixture. The organic phase was
B
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decanted, and aqueous ammonium hydroxide (20 mL, 0.1 M) and
sodium ethylenediaminetetraacetate (20 mL, 0.1 M) solutions were
added to the aqueous phase. The aqueous phase was extracted with
DCM (3 × 50 mL). The organic phases were combined, dried over
sodium sulfate, filtered, and evaporated. The residue was purified by
column chromatography on silica gel (eluent 3:2 cyclohexane/ethyl
acetate) to yield a pale yellow solid (270 mg, 48% chemical yield).
Characterizations were in accordance with those published in Bolje et
al.22
[ 2 - ( 1 - ( p - T o l y l ) - 1 H - 1 , 2 , 3 - t r i a z o l - 4 - y l ) p y r i d i n e ] -
chlorotricarbonylrhenium(I) (2). This complex was obtained follow-
ing the general procedure A starting from 2-[1-(p-tolyl)-1H-1,2,3-
triazol-4-yl]pyridine (74 mg, 0.31 mmol, 1.1 equiv). The desired
product precipitated as a pale yellow solid (154 mg, quantitative).
1H NMR (300 MHz, CD3CN): δ (ppm) 9.06 (s, 1H), 9.00 (ddd, J
= 5.5, 1.4, 0.8 Hz, 1H), 8.18 (ddd, J = 7.9, 7.8, 1.4 Hz, 1H), 8.08 (ddd,
J = 7.9, 1.4, 0.8 Hz, 1H), 7.78 (m, 2H), 7.55 (ddd, J = 7.8, 5.5, 1.4 Hz,
1H), 7.49 (m, 2H), 2.46 (s, 3H). 13C{1H} NMR (75 MHz, CD3CN):
δ (ppm) 154.2, 150.3, 150.0, 142.2, 141.5, 131.6, 127.4, 124.2, 123.5,
121.9, 21.3. MS (ESI+) m/z (%): 525.0544 (100) [M − Cl + water]+,
507.0435 (75) [M − Cl]+, 565.0011 (25) [M + Na]+. HRMS (ESI+)
calcd for C17H12ClN4NaO3Re 565.0039, found 565.0011. HPLC (50
153.3, 153.0, 148.2, 147.8, 147.0, 146.8, 140.9, 132.0, 129.9, 126.9,
124.0, 123.9, 122.8, 36.5, 36.4, 35.0, 31.9. MS (ESI+) m/z (%):
719.1761 (100) [M
+
Na]+. HRMS (ESI+) calcd for
C28H34ClN4NaO3Re 719.1761, found 719.1761. HPLC (80 to 100%
MeCN in 10 min): rt 4.77 (>95%). IR: νmax/cm−1 2023, 1925, 1883,
1868 (CO).
4-Nitro-2-(trimethylsilyl)ethynyl)pyridine. 2-Bromo-4-nitropyri-
dine (437.0 mg, 2.15 mmol, 1.0 equiv) was dissolved with DMF
(3.0 mL) and Et3N (1.0 mL). The mixture was bubbled with argon for
15 min. Bis(triphenylphosphine)palladium(II) dichloride (16.6 mg,
0.0236 mmol, 1.1 mol %) and CuI (1.7 mg, 0.0086 mmol, 0.4 mol %)
were added under an atmosphere of argon, followed by trimethylsi-
lylacetylene (365 μL, 2.58 mmol, 1.2 equiv). The mixture was stirred
at 60 °C under argon for 4 h. The mixture was diluted with DCM and
washed with saturated aqueous ammonium chloride solution (2×) and
saturated brine. The organic phase was dried over sodium sulfate,
filtered, and evaporated. The residue was purified by column
chromatography on silica gel (eluent 9:1 cyclohexane/ethyl acetate)
to afford the product as a pale brown solid (395.0 mg, 83% chemical
yield). Characterizations were in accordance with those published in
Carlsson et al.23
2-Ethynyl-4-methoxypyridine. 4-Nitro-2-(trimethylsilyl)ethynyl)-
pyridine (375.0 mg, 1.7 mmol, 1.0 equiv) was dissolved in methanol
(30.0 mL), and potassium carbonate (247.0 mg, 1.79 mmol, 1.05
equiv) was added. The mixture was stirred at room temperature under
argon for 3 h. The mixture was diluted wih DCM and water and
extracted with DCM (2×). The combined organic phases were dried
over sodium sulfate, filtered, and evaporated. The residue was purified
by column chromatography on silica gel (eluent 7:3 cyclohexane/ethyl
acetate) to afford 2-ethynyl-4-methoxypyridine as a brown oil (166.0
mg, 73% chemical yield). Characterizations were in accordance with
those published in Carlsson et al.23
to 100% MeCN in 10 min): retention time (rt) 4.94 (>95%). IR: νmax
/
cm−1 2027, 1943, 1920, 1892 (CO).
2,4,6-Tri-tert-butylphenylazide. 2,4,6-Tri-tert-butylaniline (400 mg,
1.5 mmol, 1.0 equiv) was dissolved in anhydrous MeCN (4 mL) under
argon. The solution was cooled to 0 °C, and tert-butyl nitrite (274 μL,
2.3 mmol, 1.5 equiv) was added dropwise, followed by azidotrime-
thylsilane (238 μL, 1.8 mmol, 1.2 equiv) dropwise. After the solution
was stirred overnight at room temperature, the solvent was evaporated
to dryness, and the residue was purified by column chromatography on
silica gel (eluent 2:1 cyclohexane/DCM) to yield a white crystalline
solid (313 mg, 75% chemical yield)).
2-(1-Dodecyl-1H-1,2,3-triazol-4-yl)-4-methoxypyridine. 2-Ethyn-
yl-4-methoxypyridine (21.0 mg, 0.16 mmol, 1.0 equiv) and 1-
azidododecane (36.0 mg, 0.17 mmol, 1.05 equiv) were dissolved in
a mixture of DCM (1.0 mL) and water (0.6 mL). Copper(II) sulfate
pentahydrate (4.0 mg, 0.016 mmol, 0.1 equiv) and sodium ascorbate
(9.0 mg, 0.048 mmol, 0.3 equiv) were added under argon, and the
mixture was stirred at room temperature for 23 h. The mixture was
diluted with DCM and water and decanted. The organic phase was
dried over sodium sulfate, filtered, and evaporated. The residue was
purified by column chromatography (eluent ethyl acetate) to afford 2-
(1-dodecyl-1H-1,2,3-triazol-4-yl)-4-methoxypyridine as a white solid
(30.3 mg, 55% chemical yield).
Rf (2:1 cyclohexane/DCM) = 0.93. 1H NMR (300 MHz, CDCl3): δ
(ppm) 7.35 (s, 2H), 1.48 (s, 18H), 1.32 (s, 9H). 13C{1H} NMR (75
MHz, CDCl3): δ (ppm) 148.3, 145.3, 134.9, 122.5, 36.2, 35.0, 31.4,
31.3. MS (ESI+) m/z (%): 260.2370 (100) [M-N2 + H]+. HRMS (ESI
+) calcd for C18H30N 260.2378, found 260.2370.
2-(1-(2,4,6-Tri-tert-butylphenyl)-1H-1,2,3-triazol-4-yl)pyridine.
2,4,6-Tri-tert-butylphenylazide (50.0 mg, 0.17 mmol, 1.0 equiv) and 2-
ethynylpyridine (18.0 mg, 0.17 mmol, 1.0 equiv) were dissolved in 1:1
DCM/water (4 mL) mixture. Copper(II) sulfate pentahydrate (4.3
mg, 0.017 mmol, 0.1 equiv) and sodium ascorbate (6.8 mg, 0.035
mmol, 0.2 equiv) in water (0.5 mL) were added to the mixture. After it
was stirred for 48 h, DCM (10 mL) was added to the mixture. The
organic phase was decanted out and aqueous ammonium hydroxide
(10 mL, 0.1 M) and sodium ethylenediaminetetraacetate (10 mL, 0.1
M) solutions were added to the aqueous phase. The aqueous phase
was extracted with DCM (3 × 10 mL). The organic phases were
combined, dried over sodium sulfate, filtered and evaporated. The
residue was purified by column chromatography on silica gel (eluent
4:1 cyclohexane/ethyl acetate) to yield a white crystalline solid (30
mg, 45% chemical yield).
1
Rf (eluent ethyl acetate) = 0.71. H NMR (300 MHz, CDCl3): δ
(ppm): 8.37 (d, J = 5.8 Hz, 1H), 8.21 (s, 1H), 7.76 (d, J = 2.6 Hz,
1H), 6.78 (dd, J = 5.8, 2.6 Hz, 1H), 4.40 (t, J = 7.2 Hz, 2H), 3.94 (s,
3H), 1.94 (quint, J = 7.6 Hz, 2H), 1.51−1.10 (m, 18H), 0.86 (t, J = 6.0
Hz, 3H). 13C{1H} NMR (75 MHz, CDCl3): δ (ppm): 166.9, 151.8,
150.0, 147.8, 122.4, 110.5, 105.3, 55.6, 50.7, 32.0, 30.4, 29.7, 29.6, 29.5,
29.45, 29.1, 26.6, 22.8, 14.2. MS (ESI+) m/z (%): 345.2652 (100) [M
+ H]+. HRMS (ESI+) calcd for C20H33N4O 345.2654, found
345.2652.
[2-(1-Dodecyl-1H-1,2,3-triazol-4-yl)-4-methoxypyridine]-
chlorotricarbonylrhenium(I) (5). This complex was obtained follow-
ing the general procedure A starting from 2-(1-dodecyl-1H-1,2,3-
triazol-4-yl)-4-methoxypyridine (24 mg, 0.07 mmol, 1.0 equiv). The
residue was dried under reduced pressure and purified by column
chromatography on silica gel (eluent ethyl acetate) to yield the
product as a pale yellow solid (41.7 mg, 92% chemical yield).
1H NMR (300 MHz, CDCl3): δ (ppm): 8.70 (d, J = 6.4 Hz, 1H),
8.41 (s, 1H), 7.35 (d, J = 2.7 Hz, 1H), 6.86 (dd, J = 6.4, 2.7 Hz, 1H),
4.39−4.23 (m, 1H), 4.14−3.98 (m, 1H), 3.91 (s, 3H), 1.93−1.82 (m,
2H), 1.46−1.16 (m, 18H), 0.87 (t, J = 7.5 Hz, 3H). 13C{1H} NMR (75
MHz, CDCl3): δ (ppm): 167.8, 153.8, 151.3, 149.1, 124.4, 113.1,
107.3, 57.4, 52.3, 32.0, 30.1, 29.7, 29.6, 29.5, 29.4, 29.0, 26.5, 22.8,
14.3. MS (ESI+) m/z (%): 673.1538 (100) [M + Na]+. HRMS (ESI
+): calcd for C23H32ClN4NaO4Re 673.1567, found 673.1538. HPLC
(80 to 100% ACN in 10 min): rt 6.15 (>95%). IR: νmax/cm−1 2019,
1885 (CO).
1
Rf (4:1 cyclohexane/ethyl acetate) = 0.53. H NMR (300 MHz,
CDCl3): δ (ppm) 8.61 (ddd, J = 4.8, 1.7, 1.0 Hz, 1H), 8.33 (s, 1H),
8.29 (dt, J = 7.9, 1.0 Hz, 1H), 7.81 (td, J = 7.9, 1.7 Hz, 1H), 7.57 (s,
2H), 7.27 (td, J = 4.8, 1.0 Hz, 1H), 1.38 (s, 9H), 1.01 (s, 18H).
13C{1H} NMR (75 MHz, CDCl3): δ (ppm) 152.0, 150.3, 149.5, 147.5,
147.1, 136.9, 131.7, 129.1, 123.7, 122.9, 120.4, 36.9, 35.3, 32.2, 31.3.
MS (ESI+) m/z (%): 391.2857 (100) [M + H]+, 413.2671 (30) [M +
Na]+. HRMS (ESI+) calcd for C25H35N4 391.2856, found 391.2857.
[2-(1-(2,4,6-Tri-tert-butylphenyl)-1H-1,2,3-triazol-4-yl)pyridine]-
chlorotricarbonylrhenium(I) (3). This complex was obtained follow-
ing the general procedure A starting from 2-[1-(2,4,6-tri-tert-
butylphenyl)-1H-1,2,3-triazol-4-yl]pyridine (14 mg, 0.036 mmol, 1.1
equiv). The desired product precipitated as a cream solid (20 mg, 85%
chemical yield).
1H NMR (300 MHz, DMSO-d6): δ (ppm) 9.76 (s, 1H), 9.01 (m,
1H), 8.33 (m, 2H), 7.77 (m, 1H), 7.63 (s, 2H), 1.37 (s, 9H), 1.07 (s,
9H), 1.05 (s, 9H). 13C{1H} NMR (75 MHz, DMSO-d6): δ (ppm)
C
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4-Nitro-2-(triisopropylsilyl)ethynyl)pyridine. 2-Bromo-4-nitropyri-
dine (150.0 mg, 0.74 mmol, 1.0 equiv) was dissolved with DMF
(2.0 mL) and Et3N (0.1 mL). The mixture was bubbled with argon for
15 min. Bis(triphenylphosphine)palladium(II) dichloride (11.0 mg,
0.015 mmol, 2 mol %) and CuI (1.4 mg, 0.0071 mmol, 1 mol %) were
added under an atmosphere of argon, followed by triisopropylsilyla-
cetylene (0.2 mL, 0.89 mmol, 1.2 equiv). The mixture was stirred at
room temperature under argon for 30 h. The mixture was filtered and
evaporated to dryness, and the residue was purified by column
chromatography on silica gel (eluent 10:1 cyclohexane/ethyl acetate)
to afford 4-nitro-2-(triisopropylsilyl)ethynyl)pyridine as a yellow
liquid, (99.8 mg, 44% chemical yield), which was used directly in
the next step.
2-Ethynyl-4-nitropyridine. 4-Nitro-2-(triisopropylsilyl)ethynyl)-
pyridine (99.8 mg, 0.33 mmol, 1.0 equiv) was dissolved in 2.0 mL
of THF and cooled at 0 °C. Tetrabutylammoniumfluoride, (TBAF;
0.31 mL, 0.31 mmol, 0.95 equiv, 1 M in THF) was added, and the
mixture was stirred at 0 °C under argon for 30 min. The mixture was
diluted wih water and extracted with DCM. The organic phase was
dried over sodium sulfate, filtered, and evaporated. The residue was
purified by column chromatography on silica gel (eluent 7:3
cyclohexane/ethyl acetate) to afford 2-ethynyl-4-nitropyridine as a
beige solid (37.2 mg, 76% chemical yield). Characterizations were in
accordance with those published in Carlsson et al.23
diatomaceous earth was washed with Et2O (2 × 25 mL). The filtrate
was washed with water (3 × 25 mL) and saturated brine (3 × 25 mL).
Combined organic phases were dried over sodium sulfate and filtered,
and the solvent was removed in vacuo. The crude product was purified
by column chromatography on silica gel (eluent 7:3 cyclohexane/ethyl
acetate) to afford the product as pale yellow crystals (159 mg, 42%
chemical yield).
1
Rf (7:3 cyclohexane/ethyl acetate) = 0.64. H NMR (400 MHz,
CDCl3): δ (ppm): 8.75 (d, J = 4.8 Hz, 1H), 8.66 (d, J = 8.0 Hz, 1H),
8.57 (d, J = 8.8 Hz, 1H), 8.23 (d, J = 8.8 Hz, 1H), 8.19 (d, J = 8.4 Hz,
1H), 7.92−7.84 (m, 2H), 7.74 (t, J = 7.6 Hz, 1H), 7.56 (t, J = 7.6, 1H),
7.39−7.33 (m, 1H). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm):
156.2, 156.0, 149.0, 147.8, 136.8, 136.6, 129.7, 129.4, 128.1, 127.4,
126.6, 123.8, 121.7, 118.8. HRMS (ESI+): calcd for C14H11N2 ([M +
H]+) 207.0917, found 207.0194; calcd for C14H10N2Na ([M+Na+]+)
229.0736, found 229.0732.
[2-Pyridin-2-yl-quinoline]-chlorotricarbonylrhenium(I) (11). This
complex was obtained following the general procedure A starting from
2-pyridin-2-yl-quinoline (30.3 mg, 0.155 mmol, 1.0 equiv). The
residue was dried under reduced pressure and purified by column
chromatography on silica gel (eluent 5:5 cyclohexane/ethyl acetate) to
yield a bright orange solid (44.2 mg, 56% chemical yield).
Rf (eluent ethyl acetate) = 0.40. 1H NMR (300 MHz, acetone d-6):
δ (ppm): 9.29 (d, J = 5.1 Hz, 1H), 8.97−8.81 (m, 3H), 8.75 (d, J = 8.7
Hz, 1H), 8.41 (t, J = 7.6 Hz, 1H), 8.23 (d, J = 8.4 Hz, 1H), 8.13 (t, J =
8.4 Hz, 1H), 7.88 (q, J = 7.4 Hz, 2H). 13C{1H} NMR (75 MHz,
acetone d-6): δ (ppm): 159.7, 158.3, 154.1, 148.4, 142.2, 141.0, 133.6,
131.4, 130.2, 128.7, 126.7, 121.0. HRMS (ESI+): calcd for
C17H10ClN2NaO3Re ([M + Na]+) 534.9821, found 534.9807. HPLC
(50 to 100% ACN in 10 min): rt 4.19 (>95%). IR: νmax/cm−1 2015,
1891, 1860 (CO).
Crystallography. X-ray diffraction data were collected by using a
Kappa X8 APPEX II Bruker diffractometer with graphite-monochro-
mated Mo Kα radiation (λ = 0.710 73 Å). Crystals were mounted on a
CryoLoop (Hampton Research) with Paratone-N (Hampton
Research) as cryoprotectant and then flash-frozen in a nitrogen-gas
stream at 100 K. The temperature of the crystal was maintained at the
selected value (100 K) by means of a 700 series Cryostream cooling
device to within an accuracy of 1 K. The data were corrected for
Lorentz polarization and absorption effects. The structures were solved
by direct methods using SHELXS-9724 and refined against F2 by full-
matrix least-squares techniques using SHELXL-9725 with anisotropic
displacement parameters for all non-hydrogen atoms. Hydrogen atoms
were located on a difference Fourier map and introduced into the
calculations as a riding model with isotropic thermal parameters. All
calculations were performed by using the Crystal structure crystallo-
graphic software package WINGX.26 Additional crystallographic
information is available in the Supporting Information.
Electrochemistry. Rotating disk and cyclic voltammetry were
performed with a classical three-electrode single compartment cell at
room temperature, using a glassy carbon disk working electrode (3
mm diameter, Origalys, France), a Pt wire counter electrode, and a
Ag/AgCl wire pseudoreference electrode. The experiments were
performed using a VSP Potentiostat (Bio-Logic, France). Fc/Fc+ (0.46
V vs saturated calomel electrode (SCE)) was used as an internal
standard. The working electrode was polished with 1200 grit and then
2400 grit SiC papers, and at the start of each experiment with 3 and
0.3 μm diamond pastes followed by extensive rinsing and sonication
with ultrapure water and ethanol. With the same set up, micro-
electrode voltammetry was performed using a microcarbon fiber
electrode (33 μm dia., IJ Cambria Scientific Ltd., U.K.) instead of the 3
mm disk electrode. IR drop was compensated to 75% using the ZIR
built-in compensation method of the Bio-Logic potentiostat. MeCN
solutions containing 0.1 M Bu4NPF6 as the supporting electrolyte were
deaerated with Ar or saturated with CO2 for at least 10 min before
cyclic voltammograms (CVs) were recorded.
2-(1-Dodecyl-1H-1,2,3-triazol-4-yl)-4-nitropyridine. 2-Ethynyl-4-
nitropyridine (37.0 mg, 0.25 mmol, 1.0 equiv) and 1-azidododecane
(58.0 mg, 0.28 mmol, 1.05 equiv) were dissolved in a mixture of DCM
(2.0 mL) and water (1.5 mL); copper(II) sulfate pentahydrate (8.0
mg, 0.03 mmol, 0.12 equiv) and sodium ascorbate (16.0 mg, 0.08
mmol, 0.36 equiv) were added under argon, and the mixture was
stirred at room temperature for 16 h. The mixture was diluted with
DCM and water and decanted. The organic phase was dried over
sodium sulfate, filtered, and evaporated. The residue was purified by
column chromatography (eluent 7:3 cyclohexane/ethyl acetate) to
afford 2-(1-dodecyl-1H-1,2,3-triazol-4-yl)-4-nitropyridine (79.0 mg,
88% chemical yield.)
1
Rf (eluent 7:3 cyclohexane/ethyl acetate) = 0.39. H NMR (300
MHz, CDCl3): δ (ppm): 8.89 (d, J = 2.2 Hz, 1H), 8.85 (d, J = 5.4 Hz,
1H), 8.21 (s, 1H), 7.93 (dd, J = 5.4, 2.2 Hz, 1H), 4.45 (t, J = 7.2 Hz,
2H), 1.97 (quint, J = 7.0 Hz, 2H), 1.52−1.11 (m, 18H), 0.87 (t, J = 6.0
Hz, 3H). 13C{1H} NMR (75 MHz, CDCl3): δ (ppm): 154.9, 153.6,
151.6, 146.7, 123.0, 115.0, 113.0, 50.9, 32.0, 30.4, 29.7, 29.6, 29.5,
29.46, 29.1, 26.6, 22.8, 14.25. MS (ESI+) m/z (%): 382.2222 (100)
[M + Na]+. HRMS (ESI+) calcd for C19H29N5NaO2 382.2219, found
382.2222.
[2-(1-Dodecyl-1H-1,2,3-triazol-4-yl)-4-nitropyridine]-
chlorotricarbonylrhenium(I) (6). This complex was obtained follow-
ing the general procedure A starting from 2-(1-dodecyl-1H-1,2,3-
triazol-4-yl)-4-nitropyridine (30 mg, 0.08 mmol, 1.0 equiv). The
residue was dried under reduced pressure and purified by column
chromatography on silica gel (eluent 1:1 cyclohexane/ethyl acetate) to
yield the product as an orange solid (55.5 mg, quantitative chemical
yield).
1H NMR (300 MHz, acetone-d6): δ (ppm): 9.42 (dd, J = 6.1, 0.7
Hz, 1H), 9.39 (s, 1H), 9.00 (dd, J = 2.4, 0.7 Hz, 1H), 8.35 (dd, J = 6.1,
2.4 Hz, 1H), 4.76 (t, J = 7.2 Hz, 2H), 2.21−2.06 (m, 2H), 1.53−1.13
(m, 18H), 0.86 (t, J = 6.0 Hz, 3H). 13C{1H} NMR (75 MHz, acetone-
d6): δ (ppm): 156.6, 156.2, 153.3, 148.6, 127.5, 119.7, 116.6, 53.1,
32.6, 30.4, 30.2, 30.1, 30.06, 29.6, 26.9, 23.3, 14.35. MS (ESI+) m/z
(%): 688.1285 (100) [M + Na]+. HRMS (ESI+): calcd for
C22H29ClN5NaO5Re 688.1312, found 688.1285. HPLC (80 to 100%
ACN in 10 min): rt 6.03 (>95%). IR: νmax/cm−1 2023, 1897 (CO).
2-Pyridin-2-yl-quinoline. 2-Chloroquinoline (303.0 mg, 1.852
mmol, 1.0 equiv) and 2-(tributylstannyl)pyridine (600.0 μL, 1.852
mmol, 1.0 equiv) were added to degassed anhydrous DMF (3.9 mL).
The mixture was degassed with argon for
5 min, bis-
(triphenylphosphine)palladium(II) dichloride (130.0 mg, 0.185
mmol, 0.1 equiv) was added, and the mixture was stirred at 110 °C
for 24 h under argon. The mixture was diluted with water (20 mL) and
Et2O (20 mL) and filtered over diatomaceous earth, and the
Controlled-potential electrolysis experiments were performed at
room temperature in a custom-built, gas-tight two-compartment
electrochemical cell. The cathodic compartment, which contained a
fitting for a 15 mm diameter glassy carbon disk working electrode, was
D
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a
Scheme 2. Synthesis of Complexes 2, 3, 5, and 6
a
(i) Trimethylsilylacetylene (1.2 equiv), CuI (0.4 mol %), Pd(PPh3)Cl2 (1.1 mol %), DMF, Et3N, 60 °C, 4 h, 83%; (ii) K2CO3 (1.05 equiv), MeOH
73%; (iii) triisopropylsilylacetylene (1.2 equiv), CuI (1 mol %), Pd(PPh3)Cl2 (2 mol %), DMF, Et3N, room temperature, 30 h, 44%; (iv) TBAF
(0.95 equiv), THF, 0 °C, 76%; (v) CuSO4.5H2O (0.1 equiv), sodium ascorbate (0.2 or 0.3 equiv) 1:1 DCM/water; (vi) [Re(CO)5Cl] (1.0 equiv),
toluene, reflux.
separated from the anodic compartment via a glass frit of fine porosity.
The counter electrode was a platinum wire gird (10 × 40 mm), and
the reference electrode was a saturated Ag/AgCl/KCl electrode that is
separated from the solution by a Vycor tip. MeCN solutions
containing 0.1 M Bu4NPF6 as the supporting electrolyte were
saturated with CO2 for at least 15 min prior to bulk electrolysis and
was stirred rigorously during the experiment. Typically, the solution
volume of the cathodic compartment was 9 mL with a headspace
volume of 50 mL.
Photochemistry. Photochemical reactions were performed using a
300 W, high-pressure Xe arc lamp (Oriel Instruments). The beam was
passed through an infrared filter, a collimating lens, and a filter holder
equipped with a 415 nm band-pass filter. Samples were prepared in a 1
cm path length quartz cuvette (Starna), which was placed in a
temperature-controlled cuvette holder (Quantum Northwest) main-
tained at 20 °C with a circulated water bath. For the entirety of the
study, a MeCN/triethanolamine (TEOA) mixture (5:1 v/v) was used
as the solvent. Samples containing 50 μM of Re complex (catalyst),
500 μM of [Ru(bpy)3]Cl2 (photosensitizer), and 100 mM of BIH
(electron donor) were saturated with CO2 by bubbling CO2 directly
into the solution mixture for 10 min. All samples were assayed under
an atmosphere of CO2. Control experiments in the absence of Re
complexes were also performed, and the evolution of the products was
subtracted from the experiments containing Re complexes.
Chemical Analysis. H2 and CO evolution were monitored by gas
chromatography during bulk electrolysis and photocatalysis by
periodically sampling 50 μL of the headspace. Measurements for H2
were performed by gas chromatography on a Shimadzu GC-2014
equipped with a Quadrex column, a thermal conductivity detector, and
using N2 as a carrier gas. CO was measured using a Shimadzu GC-
2010 Plus gas chromatography, fitted with a Restek Shin Carbon
column, helium carrier gas, a methanizer, and a flame ionization
detector. Gas chromatography calibration curves were made by
sampling known volumes of CO and H2 gas, respectively. Formate
concentration was determined using a Metrohm 883 Basic IC plus
ionic exchange chromatography instrument, using a Metrosep A Supp
5 column and a conductivity detector. A typical measurement consists
of sampling 200 μL of the reaction mixture, which is diluted 100-fold
in deionized 18 MΩ water, followed by a 20 μL injection into the
instrument.
numbered x, the corresponding tapy-based complex is
numbered x′. Some have already been reported and were
thus synthesized according to literature procedures.17,18 The
synthesis and characterization of the new ligands and
complexes are described in detail in the Experimental section.
The pyta-based complexes have different alkyl and aryl
substituents at N1triazole (complexes 1−4), differently para-
substituted pyridine moieties (complexes 1, 5, 6), or different
halide terminal ligands (X = Cl or Br) (complexes 1, 8). The
tapy-based compounds all contain a dodecyl chain at C4triazole
and different X (Cl or Br) ligands (complexes 1′ and 8′). For
comparison we also synthesized and studied the reference
[Re(bpy)(X)(CO)3] complexes 9 and 10, with X = Cl and Br,
respectively, as well as [Re(quinpy)(Cl)(CO)3] (11), where
quinpy is a 2-(2-quinolyl)-pyridine ligand.
The ligands were synthesized following the pathways
described in Scheme 2. The pyta ligands were synthesized by
copper-catalyzed azide−alkyne cycloadditions (CuAAC) using
the corresponding azido intermediates and 2-ethynylpyridine
derivatives. The different azido intermediates were prepared
using standard azidation conditions from the corresponding
anilines or by direct substitution of halogen-substituted
precursors.19,20 2-Ethynyl-4-nitropyridine was prepared from
2-bromo-4-nitropyridine by Sonogashira coupling with triiso-
propylsilylacetylene followed by triisopropylsilane (TIPS)
cleavage using TBAF (33% yield over two steps). Sonogashira
coupling with trimethylsilylacetylene afforded instead the
tetramethylsilane (TMS)-protected 2-ethynyl-4-nitropyridine
in good yield (83%). Subsequent TMS deprotection by
treatment with potassium carbonate in methanol led to 2-
ethynyl-4-methoxypyridine as the major product (73% isolated
yield). The same reactivity was reported by Carlsson et al.23
during the course of our studies. CuAAC reactions on these
derivatives with dodecyl azide afforded ligands for complexes 6
and 7. 2-Pyridin-2-yl-quinoline, the ligand of complex 11, was
synthesized in one step by modifying a previously described
Stille coupling procedure using 2-chloroquinoline and 2-
(tributylstannyl)pyridine.27,28 The synthesis of the new
complexes followed classical procedures by refluxing the desired
ligand with rhenium pentacarbonyl chloride in toluene
(Scheme 2).
RESULTS AND DISCUSSION
■
Synthesis of Ligands and Complexes. Scheme 1 shows
the nine [Re(pyta)(X)(CO)3] and [Re(tapy)(X)(CO)3]
complexes, as well as one [Re(quinta)(Br)(CO)3] complex
(7), where quinta is a 4-(2-quinolyl)-1,2,3-triazole ligand, which
have been investigated. When a pyta-based complex is
Single crystals suitable for X-ray diffraction were isolated in
the case of complex 3 by slow evaporation of a chloroform
E
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solution of the complex. A summary of the crystal data
collection and refinement parameters are listed in Table S2.
Selected interatomic bond lengths and angles are listed in Table
S3. An ORTEP diagram of complex 3 is shown in Figure 1. The
CVs are shown in the Table S1 and referenced to SCE using
Fc/Fc+ (0.46 V vs SCE) as an internal standard.
Scanning toward positive potentials, a first irreversible
oxidation is observed at ∼1.45 V vs SCE, associated with the
Re(II)/Re(I) couple, followed by a second oxidation at ∼1.9 V
vs SCE, assigned to the oxidation of the ligand.16 The redox
potential values of these features are given in Table S1. Both are
only very slightly sensitive to modifications of the ligands: as
expected, more positive values are obtained with electron-
withdrawing ligands, while less positive values are obtained with
electron-donating ligands (compare 5 and 6, e.g.). Scanning
toward negative potentials, an irreversible feature is observed in
the case of Re pyta complexes. This feature has been previously
assigned to the multielectron reduction of the ligand in DCM.16
This redox process is more significantly affected by the
electronic properties of the ligands (ca. −1.65 V for 1−4;
−1.79 V for 5; −1.08 V for 6 vs SCE). The reduction peak
current displays a linear relation to the square root of the scan
rate (e.g., Figure S1 for complexes 2 and 3), indicating that the
active species are molecular in nature and that they function in
a diffusion-controlled regime. This reduction process was
further investigated by rotating disk and microelectrode
voltammetry on complex 2 (Figure S2). Solving the Levich−
Koutecky equation and the equation for limiting current at a
microelectrode simultaneously,32,33 the diffusion coefficient was
found to be 1.6 × 10−5 cm2·s−1, which is comparable to related
complexes.13,34 The number of electrons for this peak was 1.3
suggesting that this feature in the CV is predominately a one-
electron process.
In agreement with tapy ligands having poorer electron-
donating ability than the corresponding pyta ligands,31 the
redox features of Re tapy complexes are in general anodically
shifted with respect to their corresponding Re pyta complexes
(e.g., 1 vs 1′). In particular, the irreversible cathodic feature was
shifted up to almost 200 mV. The same remark must be applied
to the [Re(quinta)(Br)(CO)3] complex (7), which has a
cathodic feature at −1.29 V vs SCE, in agreement with the
quinta ligand being less electron-donating than the correspond-
ing pyta ligand. Furthermore, CVs of Re tapy complexes display
an additional feature when scanning further cathodically at ca.
−1.64 V vs SCE, which is assigned to a further reduction of the
complex. In the case of 6 an additional reversible signal at −0.5
V is assigned to the reversible reduction of the nitro group.
Figure 1. ORTEP plot of 3, with 50% probability thermal ellipsoids
and the atom numbering scheme. (Only one of the disordered
positions is represented by clarity).
Re(I) ion attains the usual distorted octahedral coordination
geometry with bond lengths and angles within the typical range
found for tricarbonyl rhenium complexes.13,16,17,29−31 Besides
the bidentate pyta ligand, the Re(I) ion has three carbonyl
ligands adopting the expected facial arrangement, while the
coordinated chloride occupies the other apical position. The
Re, C(27), Cl(1), and O(2) atoms are disordered over two sites
with occupancies 0.867:0.133. Compared to the structures of
related compounds,30 the dihedral angle between the triazole
cycle and the 2,4,6-tri-tert-butylphenyl ring is close to 90° and is
likely due to the steric influence of the tert-butyl groups. This
steric bulk also hinders the rotation around the triazole-phenyl
1
bond as reflected by the solution H NMR of 3, where the
protons of the tert-butyl groups at the 2- and 6-positions of the
phenyl ring are no longer equivalent.
Electrochemical Characterization of the Re Complexes in
the Absence of CO2. The electrochemical behavior of the
various complexes, at a concentration of 1 mM, was
investigated by cyclic voltammetry (CV) at 100 mV s−1 in
argon-saturated MeCN in the presence of Bu4NPF6 (0.1 M) as
the supporting electrolyte, using a glassy carbon electrode. All
Figure 2. CVs of 1 mM of compounds 1, 2, 3, and 1′ (left to right) in MeCN with 0.1 M Bu4NPF6 under argon or CO2 and with or without water
recorded at 0.1 V·s−1 at a glassy carbon disk electrode at room temperature.
F
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Reduction of the complexes is also observed to occur at lower
potential with X = Cl vs X = Br (compare 1 and 8).
further. Complex 6 also did not display any catalytic activity,
because its reduction potential is too positive, due to the strong
electron-withdrawing power of the nitro substituent on the pyta
ligand. Similarly complex 7 does not show any catalytic activity
in the range of potentials studied here. Note that this is also the
case for compound 11, indicating that substituting a quinoline
moiety for a pyridine in a diimine ligand has a drastically
adverse effect on the catalytic activity. Typical experiments are
shown in Figure 2, and in all cases (complexes 1−3), in the
presence of CO2, the catalytic wave is observed at a potential
more negative (−1.8 V vs SCE) than that corresponding to
ligand reduction (ca. −1.65 V vs SCE; Table 1). Interestingly
The well-defined redox events described above are observed
in the same potential range as in the CVs of the reference
[Re(bipy)(X)(CO)3] complexes 9−10. However, the reduc-
tion of the bipy ligand occurs at much less negative potentials
(ca. −1.3 V vs SCE), allowing, as in the case of Re tapy
complexes, a second reduction of the complex to occur at ca.
−1.5 V vs SCE, in agreement with previous reports.12,13 Finally,
complex 11, which has a ligand where one of the bipy pyridines
has been changed to a quinoline, displays greatly anodically
shifted features in its CV (Table S1).
Electrochemical Characterization of the Re Complexes in
the Presence of a Proton Source. For Re pyta and Re tapy
complexes, the addition of increasing amounts of water results
in more complex CVs, notably in the −1.5/−1.8 V vs SCE
region (Figure 2, Figures S3), without showing a significant
catalytic process. In all cases the first reduction feature occurs at
more positive potential, shifting anodically with the amount of
water. Furthermore, a new feature appears at ca. −1.8 V vs SCE
and increases in intensity upon addition of water. This change
was most prominent and resolved for 3. The titration end point
occurred at ∼4%, 6%, and 14% v/v water for 1, 2, and 3,
respectively. At saturation, the peak potential of the first
reduction wave has shifted by ca. +150 mV in the case of 3 and
slightly less for 1 and 2. The size and shape of the new feature
at −1.8 V vs SCE are consistent with a second one-electron
process favored by the addition of water. These observations
are tentatively interpreted as reflecting first a displacement of
the anionic chloride ligand by a neutral aqua ligand thus making
the complex more easily reduced (Scheme S1). The same trend
has been observed with [Re(bipy)(X)(CO)3] complexes.29,35
The irreversibility of the first reduction wave suggests that the
aqua ligand decoordinates from the complex as is the case for
the halide analogues. Then, the singly reduced complex is
protonated, most likely at the triazolyl ligand, favoring a second
one-electron reduction. A similar process involving protonation
and electron transfer has recently been postulated for a
ruthenium polypyridyl complex containing accessible Brønsted
base sites.36 Protonation of the coordinated triazolyl ligand is
also suggested by the different water titration end points that
were observed for complexes 1−3.
Table 1. Comparison of the Catalytic Peak Current (Ec),
Overpotential (η), and (ic/ip)2 with Proton Sources for
Selected Complexes
proton source (v/v)
Ec (V vs SCE)
η (mV)
(ic/ip)2
1
2
3
3
9
H2O (4%)
H2O (4%)
H2O (14%)
TFE (4.6%)
−1.67
−1.64
−1.69
−1.87
−1.58
690
670
640
760
570
14
15
12
176
75
the peak potential corresponds to the second reduction of the
complex in the presence of a proton source (see above).
Consistently, addition of water has a stimulating effect on the
catalytic wave. As seen in Figure 2 and Figure S5, in the case of
complexes 1, 2, and 4, the catalytic peak increases upon the
addition of more water up to ∼4−5%. Further addition of water
has no further effect. In contrast, maximal catalytic peak is
obtained with 14% water for complex 3 (Figure 2 and S5). In
all cases a noncatalytic prefeature shifting anodically upon the
addition of more water is observed. Thus, the reaction occurs
via two successive one-electron processes. A first reduction,
facilitated by Cl ligand substitution with a molecule of water,
generates a one-electron reduced [Re(L)(CO)3] species, which
is not catalytic. Because of protonation of the ligand, which
facilitates further reduction, a second electron transfer to the
complex, which is required to achieve catalysis, occurs at −1.8
V vs SCE. Thus, the Re pyta complexes can accumulate two
electrons and bind CO2 to achieve a two-electron reduction of
CO2.
In Table 1 the most active compounds are compared in
terms of catalytic peak potential (Ec), overpotential (η), and
(ic/ip)2, where ic is the catalytic peak current and ip is the peak
current associated with the catalyst in the absence of substrate.
The last quantity, (ic/ip)2, is proportional to the turnover
frequency (TOF),34 thus providing a useful benchmark of the
relative TOF. These terms are potential-independent in cases
where the catalytic current is measured at its plateau value. To
calculate the overpotential we compared the standard potential
for the CO2/CO couple in MeCN (−0.89 V vs SCE)11 to the
onset potential extrapolated from the CVs, since CO is to be
the major product of the reaction (see below). Thus, Table 1
shows that, in the presence of water, complexes 1−3 have
similar TOFs and η, with 2 having the best TOF ((ic/ip)2 = 15),
while 3 has the lowest overpotential (640 mV). In comparison,
reference complex 9 is ∼5 times more efficient at ∼100 mV
lower overpotential.
The proposed mechanism shown in Scheme S1 is also
supported by CV experiments using trifluoroethanol (TFE) at
increasing concentrations up to 1.2% v/v (Figure S4). While
the peak potential of the first reduction wave has shifted only
slightly, because TFE is a poorer ligand than water, thus less
efficient for chloride ligand exchange, a second reduction wave
is observed at −1.8 V vs SCE even at this low TFE
concentration. This is also consistent with protonation of the
complex, facilitating reduction, since TFE (estimated pKaMeCN
≈ 33.637) is a stronger Brønsted acid than water (pKaMeCN
≈
38−4137).
Electrochemical Characterization of the Re Complexes in
the Presence of CO2. Catalytic activity with respect to the
electroreduction of CO2 was first evaluated using CV under
conditions described above, but in a CO2-saturated electrolyte.
A catalytic wave is observed in the case of selected Re pyta
complexes (Figure 2). In contrast, for reasons requiring further
investigations, this wave was much less intense with Re tapy
complexes (Figure 2). For this reason, we exclusively studied
the former. Furthermore, as the reduction of complex 5 occurs
at a more negative potential (−1.8 V), it was not studied
Controlled Potential Electrolysis. To further characterize
the catalyzed reaction, long-term controlled potential elec-
trolysis (CPE) of CO2-saturated MeCN/water with 0.1 M
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Bu4NPF6 was performed using complexes 1−3 as well as
complex 9 as a reference catalyst. Two different potentials were
applied: −1.75 and −1.85 V. While only CO was formed in the
case of 9, as previously reported,12,13 both CO and H2 were
detected by gas chromatography during CPE in the case of
complexes 1−3. No formate was observed by ion-exchange
chromatography. Figure S6 shows that, for all the Re pyta
complexes, the current density remained approximately
constant for 6 h under these conditions. As expected, the
current density value is dependent on the applied potential
(Figure S6). For complex 1 the current density decreases after
∼6 h, while complex 3 remains active for at least 19 h (Figure
S7). The stability of the complexes was assessed by electro-
chemical characterization of the solution during CPE. As shown
in Figure S8, the electrochemical signatures of the complexes
and the catalytic waves of 1 and 2 have decreased after 8 h
CPE, coupled with the appearance of a wave ∼1.5 V vs Ag/
AgCl. The origins of this feature will require further
investigations; nevertheless, its appearance suggests some
degradation of complex 1 and 2 during CPE. In contrast, the
CVs of complex 3 recorded during CPE were essentially the
same, suggesting that it is more stable than 1 and 2 under these
conditions.
■
●
▲
Figure 3. CO ( ), H2 ( ) and formate ( ) production during CPE
(top) at −1.75 V vs Ag/AgCl catalyzed by 1 mM of complex 3, in
CO2-saturated MeCN/H2O (86:14 v/v) with 0.1 M Bu4NPF6, using a
glassy carbon disk electrode (15 mm diameter) at room temperature;
and irradiation (bottom) catalyzed by 0.05 mM of complex 1 in CO2
saturated MeCN/TEOA (5:1 v/v) with 0.5 mM [Ru(bpy)3]Cl2
(photosensitizer) and 100 mM BIH (electron donor) using a 300
W Xe arc lamp equipped with a 415 nm filter at 20 °C.
Faradaic yields (FY) and TONs values for CPE are reported
in Table 2. The results show that total FY in most cases are
a
Table 2. Controlled Potential Electrolysis of 1, 2, 3, and 9
at Different Potentials with Different Proton Sources
The class of catalysts reported here might be interesting in that
respect, since slight modifications of the pyta ligand are
significantly impacting the CO/H2 ratio.
TON
FY (%)
proton source
(v/v)
V vs Ag/
AgCl
t
(h)
CO
H2
CO H2
Another way to modulate the CO/H2 ratio is to change the
proton source;39 thus, the reduction of CO2 by complex 3 was
also studied with the addition of TFE. Figure S9 shows the CV
of complex 3 in a CO2-saturated electrolyte with increasing
concentration of TFE. The current of the catalytic wave
increases with TFE concentration, reaching a maximum at
∼4.6% TFE with a current density of 7.3 mA cm−1. At this
concentration of TFE, the (ic/ip)2 value is about a magnitude
higher than in the case of 14% water, but the overpotential is
120 mV more negative (Table 1). CPE of CO2-saturated
MeCN/TFE with 0.1 M Bu4NPF6 was performed using
complex 3 at −1.75 and −1.85 V vs Ag/AgCl. Although the
(ic/ip)2 value derived from the CV with TFE as the proton
source is significantly higher, CPE at the less negative potential
using 4.6% TFE or 14% water has almost the same TON (5.0
vs 4.7) for CO after 8 h (Table 2), and this can be attributed to
the higher overpotential when TFE is used. In contrast, the
TON for CO after 8 h of CPE using TFE at the more negative
potential is ∼50% higher than when water was used (7.6 vs
5.2). Moreover, at a given CPE potential when TFE is used
instead of water, a higher CO/H2 ratio is observed (Table 2),
with CPE at −1.75 V vs Ag/AgCl using TFE giving exclusively
CO at 92% FY. Finally, in the case of complex 3, no matter the
proton source, it appears that the CO/H2 can be controlled by
the CPE potential, with more H2 being consistently produced
at the more negative potential. Figure 4 summarizes the control
one can have over the activity of 3 by changing the potential
and the proton source.
1
2
3
H2O (4%)
H2O (4%)
H2O (14%)
−1.75
−1.85
−1.75
−1.85
−1.75
8
6
3.3
3.3
3.3
4.7
4.7
11.5
5.2
5.0
7.6
9.2
2.5
1.3
1.7
2.4
0.6
1.6
1.2
<0.1
0.6
<0.1
56
65
43
26
31
27
11
11
18
<1
7
8
61
8
52
8
89
19
8
81
−1.85
−1.75
−1.85
−1.85
78
3
9
TFE (4.6%)
H2O (4%)
8
92
8
88
4
>99
<1
a
1 mM complexes 1−3 in CO2 saturated MeCN with 0.1 M Bu4NPF6
using a glassy carbon disk electrode (15 mm dia.) at room
temperature.
close to 100%. However, significant differences are observed
with respect to CO/H2 ratios between the three Re pyta
complexes. While complexes 1 and 2 gave significant amounts
of H2 (FY ≈ 25−45% and CO/H2 ≈ 1.5−3), complex 3 was
much more selective for CO (FY ≈ 80−90% and CO/H2 ≈
10), producing both gases at a steady linear rate for 19 h
(Figure 3). In comparison, besides being very selective for CO,
reference complex 9 was also more efficient. Under similar CPE
conditions, complex 9 produced ∼3 times more products than
the Re pyta complexes.
However, note that systems that produce a mixture of CO
and H2, whose proportions can be controlled, have some value:
indeed a mixture of H2 and CO, also known as syngas, is an
attractive feedstock for the chemical industry.38 Coupling the
electrochemical CO2 reduction to syngas to a subsequent gas-
phase catalytic Fischer−Tropsch process step constitutes an
alternative strategy to produce synthetic fuels from waste CO2.
Photochemistry. The photoreduction of CO2 in the
presence of 1−3 and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-
benzo[d]imidazole (BIH), as the electron donor, in MeCN/
TEOA was also assessed. Unlike the reference complexes 9 and
H
DOI: 10.1021/acs.inorgchem.6b03078
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
receives an electron from another OERS to form a doubly
reduced CO2 adduct.44 A binuclear intermediate is implied by
this mechanism.45,46 The inactivity of complex 3 in the
photochemical experiments supports this proposed mechanism,
as the steric hindrance of the tri-tert-butyl phenyl moiety near
the Re active site would hamper the formation of the binuclear
intermediate.
Previous studies have also shown that [Re(bpy)
(CO)3{R2N−CH2CH2O−COO}] (where R = CH2CH2OH)
is an important adduct in photocatalytic systems using
[Re(bpy) (CO)3X] in CO2-saturated DMF/TEOA solutions
and is believed to be applicable to Re(I)-diimine complexes in
general.47 Although MeCN and not DMF is used as the solvent
in the present work, it is likely that the formation of the TEOA-
CO2 adduct is still an important factor. Therefore, the steric
hindrance in complex 3 may also have impeded the formation
of the active TEOA-CO2 adduct and resulted in inactivity.
Figure 4. TON for CO (black) and H2 (white) after 8 h of CPE at
−1.75 V or −1.85 V vs Ag/AgCl catalyzed by 1 mM of complex 3, in
CO2 saturated MeCN with H2O (14% v/v) or TFE (4.6% v/v) with
0.1 M Bu4NPF6, using a glassy carbon disk electrode (15 mm
diameter) at room temperature.
CONCLUSION
1040 and other mononuclear Rebpy systems,29,41 no photo-
catalytic activity was observed for the Re pyta complexes.
However, with the addition of [Ru(bpy)3]2+ as a photo-
sensitizer, CO2 reduction products were readily observed for
homogeneous systems containing complexes 1 or 2. Table 3
■
A new series of well-characterized Re pyta and Re tapy
tricarbonyl complexes have been studied for their ability to
catalyze the electro- and photoreduction of CO2. We
specifically investigated the influences of the two isomeric
pyridyl−triazolyl ligands and of functionalizing the pyridyl and
triazolyl rings on their catalytic activity. On the one hand,
although the Re tapy derivatives (1′ and 8′) displayed similar
CV features as the reference complexes 9 and 10, they do not
show appreciable activity for the reduction of CO2. On the
other hand, for Re pyta derivatives (1−4) that display a first
reduction wave at ∼1.7 V vs SCE, a catalytic wave is observed
in the presence of CO2. Changing the potential of this
reduction wave by the introduction of electron-donating (5, 7)
or electron-withdrawing (6) groups on the pyridyl ring results
in decreased activity. The first reduction wave is also influenced
by the auxiliary halide ligand (1 vs 8), and from the CVs of 1−3
in the presence of water it is likely that an aqua ligand displaces
the coordinated halide. The added water also acts as a proton
source facilitating a second reduction of the complex at slightly
more negative potential. For the active complexes, addition of a
proton source also enhanced the catalytic wave that is observed
in the presence of CO2. While by no means outperforming
complex 9, the catalytic rates for complexes 1−3 in the
presence of water (4 or 14%) were only ∼5 times slower, with
∼100 mV higher overpotential. For complex 3 the addition of
4.6% TFE increases the catalytic rate by over a magnitude but
at a cost of another 100 mV overpotential. CPE experiments
using 1−3 in the presence of a proton source show that CO is
the major product of the electrocatalytic reaction, with complex
3 displaying the best stability (19 h) and selectivity for CO (up
to 92% FY). In contrast, complex 3 does not catalyze the
reduction of CO2 in photocatalytic experiments using [Ru-
(bpy)3]2+ as a photosensitizer, whereas complexes 1 and 2
display good activity, producing ∼13 TON of CO as the major
product over the course of 5 h. Currently, we are exploiting our
ability to dis-symmetrically furnish functional groups on the
pyta scaffold to (i) better understand and improve the catalytic
efficiency and selectivity of this new class of CO2 reduction
catalyst; and (ii) graft the catalyst onto the surface of
conductive materials for the manufacturing of solid electrodes,
the results of which will be reported in due course.
a
Table 3. Photolytic CO2 Reduction Using 1−3 as Catalysts
TON
TOF (h−1
)
time plateau (h) CO
H2 formate
CO
H2
formate
1
2
3
5
5
5
15
12
0
4
2
3
5
3
0
3.0
2.6
0.7
0.6
0.6
1.0
0.8
a
0.05 mM complexes 1−3, with 0.5 mM [Ru(bpy)3]Cl2 and 100 mM
BIH in CO2 saturated MeCN/TEOA (5:1 v/v) irradiated with a 300
W Xe arc lamp equipped with a 415 nm filter at 20 °C.
shows that upon visible light irradiation, CO, H2, and formate
are produced in the cases of 1 and 2, while only H2 is observed
for 3. Complex 1 is the most active, steadily producing CO, H2,
and formate at TOF of 3.0, 0.7, and 1.0 h−1, respectively, for up
to 5 h, after which the production levels off (Figure 3). Control
experiments in the absence of Re pyta complexes suggest
noticeable degradation or deactivation of the photosensitizer
after 5 h.
Similar to the electrochemical experiments, CO is the major
reduction product in the photochemical experiments involving
complexes 1 and 2 (CO/H2 ≈ 3.8−6). Formate is also
produced; however, these values should be considered with
caution, because [Ru(bpy)2]2+, a decomposition product of
[Ru(bpy)3]2+, can photocatalyze the reduction of CO2 to
formate,42 and a significant amount is observed in our control
experiments. In contrast, complex 3 is inactive in the
photoreduction of CO2 even in the presence of a photo-
sensitizer. This is surprising when considering the similarities
between the electrochemical properties of 1−3, and it provides
possible insights into the mechanism. The potential of the
[Ru(bpy)3]2+/[Ru(bpy)3]+ couple (E1/2 = −1.59 V vs SCE43) is
just negative enough to reduce complexes 1−3 by one electron
but is insufficient to doubly reduce them. Therefore, the activity
that is observed for complexes 1 and 2 is likely to proceed via a
similar mechanism as [Re(bpy)(CO)3X] complexes, in which a
one-electron reduced species (OERS) reacts with CO2 and
I
DOI: 10.1021/acs.inorgchem.6b03078
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Electrodes for Fuel Production. J. Am. Chem. Soc. 2013, 135 (49),
18288−18291.
ASSOCIATED CONTENT
■
S
* Supporting Information
(8) Chen, X.; Ren, H.; Peng, W.; Zhang, H.; Lu, J.; Zhuang, L. Highly
Efficient Molecular Cobalt Electrode for (Photo)electrochemical
Hydrogen Evolution. J. Phys. Chem. C 2014, 118 (36), 20791−20798.
(9) Elgrishi, N.; Griveau, S.; Chambers, M. B.; Bedioui, F.; Fontecave,
M. Versatile Functionalization of Carbon Electrodes with a
Polypyridine Ligand: Metallation and Electrocatalytic H + and CO 2
Reduction. Chem. Commun. 2015, 51 (14), 2995−2998.
(10) Rosser, T. E.; Windle, C. D.; Reisner, E. Electrocatalytic and
Solar-Driven CO 2 Reduction to CO with a Molecular Manganese
Catalyst Immobilized on Mesoporous TiO 2. Angew. Chem., Int. Ed.
2016, 55 (26), 7388−7392.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b03078.
X-ray crystallographic information (CIF)
Proposed reduction mechanism, CVs, illustrated struc-
tures, selected potentials, tabulated crystallographic data
and structure refinement details, selected experimental
bond lengths and angles, plots of current densit as a
function of time, photo of experimental setup with parts
labeled (PDF)
́
(11) Costentin, C.; Drouet, S.; Robert, M.; Saveant, J.-M. A Local
Proton Source Enhances CO2 Electroreduction to CO by a Molecular
Fe Catalyst. Science 2012, 338 (6103), 90−94.
(12) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Electrocatalytic Reduction
of Carbon Dioxide Mediated by Re(bipy) (CO)3Cl (Bipy = 2,2?-
Bipyridine). J. Chem. Soc., Chem. Commun. 1984, 984 (6), 328.
(13) Smieja, J. M.; Kubiak, C. P. Re(bipy-tBu) (CO)3Cl-Improved
Catalytic Activity for Reduction of Carbon Dioxide: IR-Spectroelec-
trochemical and Mechanistic Studies. Inorg. Chem. 2010, 49 (20),
9283−9289.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: clotilde.policar@ens.fr. (C.P.)
*E-mail: fethi.bedioui@chimie-paristech.fr. (F.B.)
*E-mail: marc.fontecave@college-de-france.fr. (M.F.)
ORCID
(14) Smith, A. P.; Fraser, C. L. Bipyridine Ligands. In Comprehensive
Coordination Chemistry II; Elsevier, 2003; Vol. 1, pp 1−23.
(15) Bodapati, R.; Sarma, M.; Kanakati, A.; Das, S. K. Asymmetrically
Substituted and π-Conjugated 2,2′-Bipyridine Derivatives: Synthesis,
Spectroscopy, Computation, and Crystallography. J. Org. Chem. 2015,
80 (24), 12482−12491.
Clotilde Policar: 0000-0003-0255-1650
Marc Fontecave: 0000-0002-8016-4747
Notes
The authors declare no competing financial interest.
CCDC 1518528 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via http://
www.ccdc.cam.ac.uk/Community/Requestastructure.
(16) Kim, T. Y.; Elliott, A. B. S.; Shaffer, K. J.; John McAdam, C.;
Gordon, K. C.; Crowley, J. D. Rhenium(I) Complexes of Readily
Functionalized Bidentate Pyridyl-1,2,3-Triazole “click” ligands: A
Systematic Synthetic, Spectroscopic and Computational Study.
Polyhedron 2013, 52, 1391−1398.
ACKNOWLEDGMENTS
■
̀
(17) Bertrand, H. C.; Clede, S.; Guillot, R.; Lambert, F.; Policar, C.
We acknowledge PSL Research University (“PSL-Chimie”
Project CATACARB under the Framework Agreement ANR-
10-IDEX-0001-02) for funding. The China Scholarship Council
for a CSC scholarship for M.H. The authors would like to
thank Dr. P. Simon (College de France) for assistance with gas
and ion chromatography, and Dr. V. Mougel (College de
France), Dr. Maria Gomez-Mingot (College de France), and T.
Forgeron (College de France) for fruitful discussions.
Luminescence Modulations of Rhenium Tricarbonyl Complexes
Induced by Structural Variations. Inorg. Chem. 2014, 53 (12), 6204−
6223.
(18) Cled
̀
e, S.; Lambert, F.; Saint-Fort, R.; Plamont, M. A.; Bertrand,
H.; Vessier
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es, A.; Policar, C. Influence of the Side-Chain Length on the
Cellular Uptake and the Cytotoxicity of Rhenium Triscarbonyl
Derivatives: A Bimodal Infrared and Luminescence Quantitative
Study. Chem. - Eur. J. 2014, 20 (28), 8714−8722.
(19) Gjonaj, L.; Roelfes, G. Selective Chemical Modification of DNA
with Alkoxy- and Benzyloxyamines. Org. Biomol. Chem. 2015, 13 (21),
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