Redox properties of a bis-pyridine rhenium carbonyl derived from an anthracene scaffold

Article abstract of DOI:10.1016/j.inoche.2015.10.012

We report the synthesis of a novel bis-pyridine chelate derived from an anthracene scaffold. Complexation of the N2 ligand with rhenium carbonyl affords the Re tricarbonyl bromide complex [(Anth-N2)Re(CO)₃Br]. In THF solution, the title complex exhibits two quasi-reversible reductions located near - 2.1 and - 2.5 V vs Fc/Fc⁺, which are similar to those observed in the free Anth-N2 ligand, suggesting the non-innocent behavior of the complexed anthracene scaffold. Under CO₂ atmosphere, the title complex exhibits an electrocatalytic response consistent with CO₂ → CO reduction, and the presence of electrocatalytically generated CO was confirmed by bulk electrolysis. These results suggest that alternate locations of redox activity (other than at the Re metal center, pyr donors, or bpy framework) can lead to interesting electrochemical behavior.

Full text of DOI:10.1016/j.inoche.2015.10.012

Inorganic Chemistry Communications 61 (2015) 221–224
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Redox properties of a bis-pyridine rhenium carbonyl derived from an
anthracene scaffold
⁎
Taylor A. Manes, Michael J. Rose
The University of Texas at Austin, Austin, TX 78712, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis of a novel bis-pyridine chelate derived from an anthracene scaffold. Complexation of the
N2 ligand with rhenium carbonyl affords the Re tricarbonyl bromide complex [(Anth-N2)Re(CO)3Br]. In THF so-
lution, the title complex exhibits two quasi-reversible reductions located near −2.1 and −2.5 V vs Fc/Fc+, which
are similar to those observed in the free Anth-N2 ligand, suggesting the non-innocent behavior of the complexed
anthracene scaffold. Under CO2 atmosphere, the title complex exhibits an electrocatalytic response consistent
with CO2 → CO reduction, and the presence of electrocatalytically generated CO was confirmed by bulk electrol-
ysis. These results suggest that alternate locations of redox activity (other than at the Re metal center, pyr donors,
or bpy framework) can lead to interesting electrochemical behavior.
Received 21 September 2015
Accepted 8 October 2015
Available online 13 October 2015
Keywords:
Rhenium
Electrochemistry
Redox
© 2015 Elsevier B.V. All rights reserved.
Scaffold
X-ray
NMR
Communication
conversion by employing new modifications to the catalytic system by
means of ionic liquids [15,16], as well as employing novel ligand designs
In an ideal energy utilization paradigm, the carbon dioxide (CO2)
produced by biological and industrial processes would be balanced
with the CO2 consumed by biological processes and industrial needs
[1–5]. With industrial processes increasing in size and output, and glob-
al population continually growing, the amount of CO2 in the atmosphere
continues to increase and climate change continues to be a pressing
issue in science and society. To be a viable contributor to a ‘greener’ en-
ergy landscape, a renewable energy source must be able to produce
large quantities of electricity or fuel and have long term storage capabil-
ities [6]. Wind power and solar power produce large amounts of elec-
tricity but only intermittently and do not have sustainable long term
storage options and since grid storage is not available there is a trade-
off between batteries and supercapacitors. Batteries have a high energy
density but limited lifetimes, and supercapacitors have low energy den-
sity [7]. A solution to this problem would be chemical storage. The com-
bination of processes such as water oxidation, hydrogen generation,
solar power, and CO2 reduction could produce an artificial photosyn-
thetic device to generate a carbon neutral fuel source [8]. Such a CO2
to CO process could be beneficial for production of syngas, but direct re-
duction to methanol is preferable to generate liquid fuels [9–12]. Cur-
rent challenges to the electrochemical reduction of CO2 include large
overpotentials [13] and poor selectivity [14]. Thus on the molecular
catalyst front, the focus of much recent work has been to address both
the efficiency and selectivity of the catalytic process for CO2 → CO
to support rhenium and other metal centers for electrocatalysis.
Rhenium bis-pyridine carbonyl complexes (see Fig. 1) have become
the ‘gold standard’ for the CO2 reduction catalysts due to their high turn-
over rates, turnover numbers and faradaic efficiencies [1,8,20]. Lehn
et al. [17] studied the reduction of CO2 to CO using the [fac-
(bpy)Re(CO)3Cl] catalyst that has since been the focus of many studies
regarding the mechanism elucidation and selectivity optimization dur-
ing the catalytic process. More recently Kubiak et al. [21] reported sev-
eral common degradation pathways in the catalytic process, one of
which is the formation of the Re2 dimer, which occurs slowly once the
halide is reductively eliminated from the metal center. To solve this
problem, more recent work from the same group has successfully
employed the use of bulky substituents at the 2 and 2′ positions on
the bipyridine framework [12]. Substitution studies have been per-
formed related to Re-bpy complexes to determine if similar catalysis oc-
curs upon modulating the strength of the ligand field [9]. However,
there are presently no reports that investigate the effect of orientation
of the pyridine donors (with respect to the equatorial plane of the Re
center), nor are there reports regarding alternate modes of displaying
the bidentate array of pyridine donors. In this Communication, we re-
port an isoelectronic bis-pyridine Re carbonyl complex with interesting,
non-innocent redox features that may participate in the CO2 → CO con-
version process.
The synthesis of the bis-pyridine ligand was performed in three
steps (see Scheme 1). Starting from 1,8-dichloroanthraquinone, succes-
sive reductions using NaBH4 afforded 1,8-dichloroanthracene in good
yields [22,23]. Suzuki coupling catalyzed by XPhos/[Pd(dba)2] with
⁎
Corresponding author.
E-mail address: mrose@cm.utexas.edu (M.J. Rose).
http://dx.doi.org/10.1016/j.inoche.2015.10.012
1387-7003/© 2015 Elsevier B.V. All rights reserved.

222
T.A. Manes, M.J. Rose / Inorganic Chemistry Communications 61 (2015) 221–224
Fig. 1. The original Lehn catalyst [17] [Re(bpy)(CO)3Cl] with 4,4′ R group substitutions re-
lated to recent reports [18,19].
two equivalents of 3-pyridinyl boronic acid in THF/H2O generated the
final product, Anth-N2 (1), in 80% yield as a yellow powder.
Under inert atmosphere, a 1:1 mixture of Anth-N2 and [Re(CO)5Br]
was heated in THF (Scheme 1) to afford pure product as a bright yellow
powder in good yield (48%). The infrared spectrum of the product con-
firmed formation of the {Re(CO)3}+ moiety, evidenced by bound CO
features at 2025, 1886, and 1872 cm−1. The CO stretches observed in
2 coincide reasonably with the stretches seen at 2019 and 1907 cm−1
in the Re(bpy) congeners.
Yellow needles of [(Anth-N2)Re(CO)3Br] (2) were grown using
vapor effusion of a DCM solution of the complex into n-hexane, and
the resulting crystal structure is depicted in Fig. 2. The binding of the
Re tricarbonyl fragment to the Anth-N2 is analogous to that of the Re-
bpy family of complexes. A key difference in the present complex is
the torsion angles exhibited by the pyridine donors. The anthracene-
appended pyridine donors are displaced from the plane of the anthra-
cene backbone and when complexed with the Re carbonyl fragment
the torsion ranges from 45.0–48.1°. The close positioning of the donor
atoms promotes the cis binding motif rather than a trans orientation,
and this facilitates sufficient rotation of the pyridine moiety for mono-
metallic binding. The bite angle of the Anth-N2 ligand is 86.9° compared
to the more acute ~75° reported in Re(bpy) complexes. The Re–N dis-
tances of 2.247(3) and 2.242(3) Å are similar to the ~2.20 Å distances
seen in the Re(bpy) system [24].
Electrochemical studies were performed on 1 (N2 ligand) and 2 (Re
complex) to determine their redox activities. The cyclic voltammogram
of the free ligand 1 (Fig. 3, black line) exhibits two quasi-reversible fea-
tures −2.20 and −2.64 V vs Fc/Fc+. The cyclic voltammogram of 2 (Fig.
3, red line) exhibits similar reversible features at −2.10 and −2.58 V vs
Fc/Fc+ that are quite similar to those of the free ligand. The peak cur-
rents in the CV follow a scan rate dependence that is linear with √ν (in-
sets, Figs. 3 and 4). The more positive features in the CVs of both the free
ligand and the rhenium complex show redox events similar to that
Fig. 2. ORTEP diagram (30% ellipsoids) of the molecular structure of [Re(Anth-
N2)(CO)3Br] (2); H atoms have been omitted for clarity. Selected bond distances (Å):
Re1–N1 = 2.247(3), Re1–N2 = 2.242(3), Re1–C1 = 1.931(4), Re1–C2 = 1.922(5), Re1–
C3 = 1.923(5), C1–O1 = 1.138(5), C2–O2 = 1.115(5), C3–O3 = 1.135(5). Selected angles
(deg): N1–Re1–N2 = 87.23(12), N1–Re1–C1 = 92.52(15), N2–Re1–C3 = 89.91(15),
C1–Re1–C3 = 90.32(18). Selected torsion angles (deg): C7–C8–C9–C14 = 48.2(6),
C18–C22–C23–C24 = 48.0(6).
reported for anthracene [25]. For comparison, a cyclic voltammogram
of 1,8-dichloroanthracene was obtained (Fig. 3, gray line; and Fig. S3,
supporting information). A single redox event at −2.23 V vs Fc/Fc+
occurs, suggesting that the more positive peaks observed for free ligand
and metal complex emanate from an anthracene-based redox event.
The more negative peak that occurs in the free ligand and metal com-
plex is not observed for 1,8-dichloroanthracene, suggesting that the
feature is due to a pyridine-based redox event. Rhenium-based redox
events were not observed. The more negative wave observed in the
CV of free ligand 1 exhibits greater irreversibility (ipa/ipc` = 0.39),
Fig. 3. Cyclic voltammograms of 1 mM 1,8-dichloroanthracene (line), (Anth-N2) (1, black
line) and [(Anth-N2)Re(CO)3Br] (2, red line) in THF with 0.1 M TBAPF6 as supporting electro-
lyte under N2 atmosphere. Inset: scan rate dependence of the reversible reduction wave of (1)
at −2.34 V. Experiment conditions: scan rate, 100 mV/s; WE, glassy carbon (3 mm); CE, Pt
wire; Re, Ag wire pseudo-reference; all potentials were converted to Fc/Fc+ by inclusion of
Fc in the final scan (not shown).
Scheme 1. Synthesis of the ligand Anth-N2 (1) and the corresponding complex [(Anth-
N2)Re(CO)3Br] (2).

T.A. Manes, M.J. Rose / Inorganic Chemistry Communications 61 (2015) 221–224
223
In conclusion, we have constructed a novel anthracene scaffold that
displays a meta-linked, cis-coordinating bis-pyridine motif in alternate
fashion compared to the standard bpy ligand. The Anth-N2 ligand sup-
ports a stable Re tricarbonyl complex, which exhibits redox behavior
not unlike the free ligand. The observed potentials suggest that the an-
thracene moiety is redox active, but not directly involved in the
electrocatalysis. Compared to the ‘gold standard’ Re(bpy) system, the
present system exhibits an inefficiency that may be due to the distance
from the metal center to the scaffold, and/or a break in conjugation be-
tween the ligating pyridine moieties. Future work will endeavor to bet-
ter align the redox features of the supporting scaffold and the
catalytically active event, with the aim of elaborating the scope and ef-
ficiency of available multi-electron transformations. Enhancing the ro-
tational flexibility of the pyridine moieties may also lead to better
orbital overlap with the metal center.
Acknowledgments
Fig. 4. Cyclic voltammograms of 1 mM [(Anth-N2)Re(CO)3Br] (2) under argon (black line)
or CO2 (blue line) in THF with 0.1 M TBAPF6 as supporting electrolyte. Inset: scan rate de-
pendence of the reversible reduction wave of (2) at −2.74 V. Experiment conditions: scan
rate, 100 mV/s; WE, glassy carbon (3 mm); CE, Pt wire; RE, Ag wire pseudo-reference; all
potentials were converted to Fc/Fc+ by inclusion of Fc in the final scan (not shown).
The authors gratefully acknowledge funding from the Robert A.
Welch Foundation (F-1822), the ACS Petroleum Research Fund
(53542-DN13) and the US Office of Naval Research (N00014-13-
10530). We also thank Prof Allen Oliver (Notre Dame Univ) and Prof
Jeanette Krause (Univ of Cincinnati) for the X-ray data collection at
the Advanced Light Source, and we acknowledge Dr. Vince Lynch for
the assistance with analysis of single crystal X-ray diffraction data.
Crystallographic data for 2 were collected through the SCrALS (Service
Crystallography at the Advanced Light Source) program at Beamline
11.3.1 at the Advanced Light Source (ALS), Lawrence Berkeley National
Laboratory. The Advanced Light Source is supported by the Director,
Office of Science, Office of Basic Energy Sciences, of the U.S. Department
of Energy under contract no. DE-AC02-05CH11231.
compared to the analogous redox process in the Re complex 2 (ipa/ipc
=
0.72). This change in reversibility may indicate enhanced electronic
communication between two pendant pyridine moieties due to the
conjoining Re metal center.
Given the novel redox properties of 2 compared with Re-bpy
complexes, CV experiments were performed under CO2 atmosphere
(Fig. 4). First, a shift of the peak near −2.12 V to a more negative poten-
tial of −2.24 V vs Fc/Fc+ is observed. Second, a marked increase in cur-
rent is observed at −2.62 V vs Fc/Fc+, along with a complete loss of
reversibility. Both of these features are hallmarks of electrocatalytic
CO2 reduction by a molecular catalyst of this type [8,26]. This phenom-
enon provided a sound basis to continue the investigation by controlled
potential electrolysis (CPE).
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2015.10.012.
References
Bulk electrolysis experiments under CO2 (1 atm) were carried out in
a two compartment H-cell cell with a glassy carbon rod working elec-
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(compared to the Re-bpy system) may be due to a lack of facile electron-
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alignment with the Re dπ orbitals may allow for facile movement back
to the metal center. Lastly, the anthracene electron reservoir may be
too geometrically far from the metal center to act as an efficient electron
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