Isolating substituent effects in Re(I)-phenanthroline electrocatalysts for CO2 reduction

Article abstract of DOI:10.1016/j.ica.2019.119397

The development of highly-active molecular electrocatalysts for reducing CO₂ to value-added products requires an intimate knowledge of the structural and electronic features surrounding the active site. We previously illustrated how electronic modifications to fac-[Re^I(R₂phen)(CO)₃Cl] electrocatalysts (R₂phen = 2,9-disubstituted-1,10-phenanthrolines) strongly dictate CO₂ reduction; more specifically, introducing methoxy substituents at both the ortho/para positions of a phenyl ring attached to phenanthroline generated high catalytic activity. In the current work, we have prepared four structurally-related Re(I) electrocatalysts to isolate the electronic effects associated with each methoxy group's positioning around the phenyl ring (i.e. none, ortho, meta, or para). The diimine ligands 2,9-diphenyl-1,10-phenanthroline (Ph₂phen), 2,9-bis(2,6-dimethoxyphenyl)-1,10-phenanthroline ((2,6-dmp)₂phen), 2,9-bis(3,5-dimethoxyphenyl)-1,10-phenanthroline ((3,5-dmp)₂phen), and 2,9-bis(4-methoxyphenyl)-1,10-phenanthroline ((4-mp)₂phen) were prepared, and the subsequent fac-[Re^I(R₂phen)(CO)₃Cl] complexes were synthesized, characterized, and studied for electrocatalytic CO₂ reduction. Following 90 min electrolysis at E_app = ?2.55 V vs. Fc^+/0 in DMF solvent, the ortho methoxy-substituted analogue (labeled Re((2,6-dmp)₂phen)) produced 35 μmol CO, corresponding to a turnover number = 14 (TON = mol CO per mol Re catalyst) and a Faradaic efficiency = 84% (F.E. = mol CO per mol electrons). The remaining Re electrocatalysts produced significantly lower levels of CO (Re(Ph₂phen): 1.8 μmol CO; Re((3,5-dmp)₂phen): 2.4 μmol; Re((4-mp)₂phen): 4.2 μmol CO), emphasizing the important electronic contribution that ortho substituted methoxy groups provide towards enhancing electron density at the Re active site to rapidly, and selectively, convert CO₂ substrate to CO product.

Full text of DOI:10.1016/j.ica.2019.119397

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Isolating substituent effects in Re(I)-phenanthroline electrocatalysts for CO2
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Sarah A. Roell, Briana R. Schrage, Christopher J. Ziegler, Travis A. White
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https://doi.org/10.1016/j.ica.2019.119397
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Please cite this article as: S.A. Roell, B.R. Schrage, C.J. Ziegler, T.A. White, Isolating substituent effects in Re(I)-
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Isolating substituent effects in Re(I)-phenanthroline electrocatalysts for CO2
reduction
Sarah A. Roell,† Briana R. Schrage,‡ Christopher J. Ziegler,‡ and Travis A. White*,†
†Department of Chemistry and Biochemistry, Clippinger Laboratories, Ohio University, Athens, Ohio
45701, United States
‡Department of Chemistry, Knight Chemical Laboratory, University of Akron, Akron, Ohio 44325, United
States
Abstract
The development of highly-active molecular electrocatalysts for reducing CO2 to value-added
products requires an intimate knowledge of the structural and electronic features surrounding the
active site. We previously illustrated how electronic modifications to fac-[ReI(R2phen)(CO)3Cl]
electrocatalysts (R2phen = 2,9-disubstituted-1,10-phenanthrolines) strongly dictate CO2
reduction; more specifically, introducing methoxy substituents at both the ortho/para positions of
a phenyl ring attached to phenanthroline generated high catalytic activity. In the current work, we
have prepared four structurally-related Re(I) electrocatalysts to isolate the electronic effects
associated with each methoxy group’s positioning around the phenyl ring (i.e. none, ortho, meta,
or para). The diimine ligands 2,9-diphenyl-1,10-phenanthroline (Ph2phen), 2,9-bis(2,6-
dimethoxyphenyl)-1,10-phenanthroline ((2,6-dmp)2phen), 2,9-bis(3,5-dimethoxyphenyl)-1,10-
phenanthroline ((3,5-dmp)2phen), and 2,9-bis(4-methoxyphenyl)-1,10-phenanthroline ((4-
mp)2phen) were prepared, and the subsequent fac-[ReI(R2phen)(CO)3Cl] complexes were
synthesized, characterized, and studied for electrocatalytic CO2 reduction. Following 90 min
electrolysis at Eapp = −2.55 V vs. Fc+/0 in DMF solvent, the ortho methoxy-substituted analogue
(labeled Re((2,6-dmp)2phen)) produced 35 μmol CO, corresponding to a turnover number = 14
(TON = mol CO per mol Re catalyst) and a Faradaic efficiency = 84% (F.E. = mol CO per mol
1

electrons). The remaining Re electrocatalysts produced significantly lower levels of CO
(Re(Ph2phen): 1.8 μmol CO; Re((3,5-dmp)2phen): 2.4 μmol; Re((4-mp)2phen): 4.2 μmol CO),
emphasizing the important electronic contribution that ortho substituted methoxy groups provide
towards enhancing electron density at the Re active site to rapidly, and selectively, convert CO2
substrate to CO product.
Keywords: electrocatalytic CO2 reduction, rhenium(I) catalyst, phenanthroline, electronics,
methoxy substituents
1. Introduction
Converting carbon dioxide to value-added products such as fuels (e.g. CO, CH3OH, CH4) and
bulk chemicals (e.g. acetic acid, urea, carbonates) is an attractive method to utilize this abundant
resource [1, 2]. More specifically, CO2 to CO reduction provides a C1 feedstock for carbon-carbon
bond forming, Fischer-Tropsch chemistry to generate liquid hydrocarbon fuels compatible with
current fuel transporting infrastructures. The thermodynamically more accessible 2e−/2H+ pathway
reduces CO2 to CO and H2O at E0′ = −0.53 V vs. NHE (pH 7, 25° C); however, this potential also
accesses the 2e− reduction of protons to H2 (E0′ = −0.41 V vs. NHE) and raises selectivity concerns
for CO2 vs. H+ reduction.
Molecular electrocatalysts benefit from synthetic tunability to optimize catalyst-CO2 binding
affinity and reactivity (i.e. high turnover frequency, TOF), while the applied potential can be varied
to minimize wasted energy to drive CO2 reduction (i.e. low overpotential, η) [3-8]. Relevant to this
manuscript, fac-[MI(NN)(CO)3X]n+ complexes (M = Re(I) or Mn(I); NN = bidentate diimine
ligand; X = Cl−, Br−, CH3CN) possess a catalytically-active metal center and redox-active diimine
ligand functioning to collect multiple electrons for delivery to bound CO2 [6, 9-16]. Using fac-
[ReI(bpy)(CO)3Cl] (bpy = 2,2′-bipyridine) as an example, Scheme 1 illustrates the proposed
2

mechanism whereby multi-electron reduction rapidly dissociates Cl−, rendering the
coordinatively-unsaturated Re metal center a strong nucleophile to attack CO2, thus forming a
metallocarboxylate intermediate. Addition of a weak Brønsted acid (e.g. MeOH, PhOH, 2,2,2-
trifluoroethanol, H2O), or adventitious H2O present, rapidly protonates the carboxylate, forming a
metallocarboxylic acid intermediate which undergoes either a protonation-first pathway to release
H2O (red arrows) or a reduction-first pathway followed by protonation to release H2O (blue
arrows). Finally, the tetracarbonyl intermediate undergoes reduction to release CO and regenerate
the catalyst.
Scheme 1. Proposed catalytic CO2 reduction cycle using fac-[ReI(bpy)(CO)3Cl] electrocatalysts.
Introducing electron-rich substituents onto the diimine ligand generally increases
electrocatalytic activity in Re(I) and Mn(I) catalysts; however, this is often accompanied by an
increased η [9, 13]. To reduce η and yet maintain a high TOF, recent efforts have focused on the
second coordination sphere surrounding the metal by placing hydrogen-bonding-capable
3

substituents (e.g. hydroxy, methoxy, imidazolium) in positions primed to stabilize CO2 adducts
[17-29]. A notable example is the fac-[MnI(6,6′-(MeO2Ph)2bpy)(CO)3(CH3CN)]+ electrocatalyst
(6,6′-(MeO2Ph)2bpy = 6,6′-bis(2,6-dimethoxyphenyl)-2,2′-bipyridine) that features Lewis basic
methoxy groups able to participate in weak hydrogen bonding interactions with the
metallocarboxylic acid intermediate [20]. In the presence of phenol, the authors indicate accessing
the thermodynamically more favorable protonation-first pathway, thus reducing the overpotential
by 0.55 V and maintaining catalytic activity (TOF = 138 ± 4 s−1).
The fac-[ReI(R-phen)(CO)3Cl] architecture (R-phen = substituted-1,10-phenanthroline) has
recently gained attention as a feasible CO2 reduction electrocatalyst given the ability to
synthetically modify the 5,6 positions of the phenanthroline backbone [30-35]. Our previous
manuscript describes electronic contributions to electrocatalytic CO2 reduction using a series of
fac-[ReI(2,9-R2phen)(CO)3Cl] complexes, where 2,9-R2phen represents 2,9-disubstituted-1,10-
phenanthroline (Figure 1) [33]. We found the ortho/para substitution pattern (labeled (2,4,6-
tmp)2phen) displayed significantly enhanced CO2 reduction activity in DMF solvent compared to
the methylated (Mes2phen) and meta/para-substituted methoxy ((3,4,5-tmp)2phen) analogues.
However, the combination of ortho/para and meta/para substitutions made it difficult to decipher
which position(s) contributed most to the enhanced catalysis. Furthermore, ortho substituted
methoxy groups are ideally placed for potential weak hydrogen-bonding with the
metallocarboxylic
acid
intermediate,
analogously
to
the
fac-[MnI(6,6′-
(MeO2Ph)2bpy)(CO)3(CH3CN)]+ electrocatalyst.
Herein we further probe the electronic effects stemming from electron-rich methoxy groups
decorated around the ortho, meta, or para positions of phenyl-substituted 1,10-phenanthroline
ligands, Figure 1. The unsubstituted Re(Ph2phen) complex was included as a structural control
4

to establish a baseline for the observed electrochemical and catalytic CO2 reduction properties in
the absence of methoxy substituents. We expect the ortho-substituted catalyst Re((2,6-
dmp)2phen) to display the highest catalytic activity given the increased electron density on the Re
metal center. Conversely, the meta- and para-substituted analogues Re((3,5-dmp)2phen and
Re((4-mp)2phen), respectively, provide limited electronic contributions to the Re catalytic site,
thus the catalytic activity is expected to be comparable to the unsubstituted Re(Ph2phen). Cyclic
voltammograms and controlled-potential electrolysis confirmed our electronics effect-driven
hypothesis, providing more insight into structure-activity relationships for substituted Re(I)-phen
electrocatalysts.
Figure 1. General structure of the fac-[ReI(R2phen)(CO)3Cl] architecture, along with the ligands
used in previous (top) and current (bottom) studies for electrocatalytic CO2 reduction.
5

2. Experimental Details
2.1.
Materials
All materials were used as received unless otherwise indicated. 2,9-Dichloro-1,10-
phenanthroline (95+%), 2,6-dimethoxyphenylboronic acid (98%), 3,5-dimethoxyphenyl boronic
acid (98%), and 4-methoxyphenylboronic acid (98%) were purchased from Ark Pharm. Rhenium
pentacarbonyl chloride (98%) and deuterated dimethyl sulfoxide (DMSO-d6) were purchased from
Acros Organics. Deuterated chloroform (CDCl3) was purchased from Cambridge Isotopes
Laboratories. Tetrakis(triphenylphosphine)palladium(0) (99%), phenylboronic acid (95%), and n-
tetrabutylammonium hexafluorophosphate (98%) were purchased from Sigma-Aldrich. Grade 562
molecular sieves (3Å; 4 – 8 mesh beads), anhydrous MgSO4, CH2Cl2, DMF, hexanes, and toluene
were purchased from Fisher Chemical. Ethanol (190 proof) was purchased from Decon Labs. The
n-tetrabutylammonium hexafluorophosphate (n-Bu4NPF6) supporting electrolyte was
recrystallized using ethanol prior to electrochemical analyses. Ultra-high purity Ar (99.995%), Ar
(for synthesis), CO2, CO, and H2 were purchased from Airgas.
2.2.
Instrumentation
13
1H and C NMR spectra were collected at 298 K using a Bruker Ascend spectrometer (500
MHz for 1H and 125 MHz for 13C). Chemical shifts (ppm) were referenced to the residual solvent
peak; CDCl3 (1H NMR) δ = 7.26 ppm, DMSO-d6 (13C NMR) δ = 39.42 ppm [36]. High-resolution
electrospray ionization mass spectrometry (HR-ESI(+)-MS) was performed using a Thermo Fisher
Scientific Q Exactive Plus hybrid quadrapole – Orbitrap mass spectrometer operating in positive
mode. Samples were dissolved in HPLC-grade CH3CN and a few drops of aqueous formic acid
(0.1%) were added prior to injection. Elemental analysis (C, H, N combustion) was performed by
Atlantic Microlabs, Inc. (Norcross, GA). IR spectra were collected in transmission mode using a
6

Shimadzu IRAffinity-1S FT-IR spectrophotometer (average of 10 scans, 1 cm−1 resolution),
whereby samples were dissolved in CH2Cl2 solvent and added to a liquid IR cell equipped with
CaF2 windows.
2.3.
Single crystal X-ray crystallography
X-ray intensity data were measured on a Bruker CCD-based diffractometer with dual Cu/Mo
ImuS microfocus optics (Cu Kα radiation, λ = 1.54178 Å, Mo Kα radiation, λ =0.71073 Å).
Crystals were mounted on a cryoloop using Paratone oil and placed under a steam of nitrogen at
100 K (Oxford Cryosystems). The detector was placed at a distance of 5.00 cm from the crystal.
The data were corrected for absorption with the SADABS program. The structures were refined
using the Bruker SHELXTL Software Package (Version 6.1), and were solved using direct
methods until the final anisotropic full-matrix, least squares refinement of F2 converged.
Crystallographic data for Re((2,6-dmp)2phen) and Re((3,5-dmp)2phen) is shown in Table S1.
CCDC numbers 1957928-1957929 contain the supplementary crystallographic data for this paper
and can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/structures.
2.4.
Computational Methods
Molecular structure optimizations were performed using density functional theory (DFT) at
the M06 level [37] with the Gaussian 16 program package [38] using the Ohio Supercomputer
Center. The 6-31G* basis set was used for H, C, N, O, and Cl atoms [39] and the Stuttgart-
Dresden (SDD) energy-consistent pseudopotentials were used for Re [40]. All calculations used
the Solvation Model based on Density (SMD) that mimicked the solvation effect of CH3CN and
were performed in Gaussian 16. Following structural optimization, force constant and frequency
analyses were performed to confirm the structure existed at a global minimum on the potential
7

energy surface and the absence of imaginary frequencies. Analysis of the Gaussian 16 outputs
was performed using the Avogadro 1.2.0 software package [41, 42].
2.5.
Electrochemical measurements
Electrochemical measurements were performed using a BASi Epsilon EClipseTM
electrochemical analyzer (Bioanalytical Systems Inc.; West Lafayette, Indiana, USA). Cyclic
voltammograms (CV) and linear sweep voltammograms (LSV) were obtained using a standard
three-electrode configuration with a glassy carbon (3 mm diameter) working electrode, a Pt wire
auxiliary electrode, and a Ag/AgCl (3 M NaCl(aq)) reference electrode encapsulated in a glass
tube with a Vycor® tip. Ferrocene (Fc) was added to the cell following each set of CV experiments,
and potentials were referenced against the ferrocenium/ferrocene (Fc+/Fc) couple (DMF solvent:
E1/2(Fc+/Fc) = +0.55 V vs. Ag/AgCl, ΔEp = 100 mV). Typical voltammograms were recorded with
the following conditions: 0.5 mM Re(R2phen) catalyst in DMF solvent, 0.1 M n-Bu4NPF6
supporting electrolyte, 200 mV/s scan rate, N2 or CO2 atmosphere, and 298 K temperature.
Controlled-potential electrolysis (CPE) was performed in a single-cell, three-electrode
configuration using a carbon graphite rod working electrode, Pt wire auxiliary electrode, and a
Ag/AgCl (3 M NaCl(aq)) reference electrode encapsulated in a glass tube with a Vycor® tip. For
a typical experiment, 0.5 mM Re(R2phen) catalyst in 5 mL of 0.1 M n-Bu4NPF6/DMF solvent
system was vigorously bubbled with CO2 for 15 minutes. The cell was then sealed and while
rapidly stirring, a constant potential was applied (Eapp = −2.55 V vs. Fc+/Fc). Using a Hamilton
GASTIGHT syringe, 100 μL aliquots were periodically removed from the 17 mL headspace and
injected directly into a Shimadzu GC-2014 gas chromatograph (GC). The GC had the following
conditions: ultra-high purity Ar carrier gas flowing at 25 mL/min, a packed ShinCarbon ST
SilicoSmooth stainless steel column (2 m long × 1/8 in. o.d. × 2.0 mm i.d.; 80/100 mesh), a
8

Shimadzu TCD-2014 thermal conductivity detector, injector temperature of 41 °C, column
temperature of 30 °C, and detector temperature of 150 °C. The amount of CO and H2 produced
were determined using two separate calibration curves generated with CO and H2 gas standards.
2.6.
General synthesis of phenanthroline ligands
Diaryl-substituted phenanthrolines were prepared by combining the dihalo-substituted
phenanthroline with the appropriate phenylboronic acid in a 100 mL Schlenk flask. The solids
were suspended in 30 mL of toluene and 20 mL of 2 M Na2CO3(aq), then purged with Ar for 15
min. Subsequently, Pd(PPh3)4 (0.175 g, 0.15 mmol) was added to the flask, purged for an
additional 15 min, then heated at reflux under Ar while rapidly stirring. After 48 h, the biphasic
mixture was cooled to room temperature and the two layers were isolated. The aqueous layer was
washed with dichloromethane and all organic fractions were combined, dried using anhydrous
MgSO4, and filtered. The solvent was removed by rotary evaporation, and the solids were
recrystallized with minimal toluene followed by vacuum filtration. If necessary, the volume of the
resulting filtrate was reduced and hexanes added to induce precipitation. The mixture was vacuum
filtered and resulting white solids were collected.
2.6.1. 2,9-Diphenyl-1,10-phenanthroline (Ph2phen) [43]
Starting materials: 2,9-dichloro-1,10-phenanthroline (0.250 g, 1.00 mmol), phenyl boronic
acid (0.428 g, 3.51 mmol), Pd(PPh3)4 (0.175 g, 0.150 mmol). Yield = 0.153 g (0.460 mmol, 46%).
2.6.2. 2,9-Bis(2,6-dimethoxyphenyl)-1,10-phenanthroline ((2,6-dmp)2phen)[44]
Starting materials: 2,9-dichloro-1,10-phenanthroline (0.250 g, 1.00 mmol), 2,6-
dimethoxyphenylboronic acid (0.730 g, 4.01 mmol), Pd(PPh3)4 (0.175 g, 0.150 mmol). Yield =
1
0.315 (0.700 mmol, 70%). H NMR (500 MHz, 298 K, CDCl3): δ (ppm) = 8.24 (d, 2H, J = 8.2
Hz), 7.81 (s, 2H), 7.63 (d, 2H, J = 8.2 Hz), 7.30 (t, 2H, J = 8.4 Hz), 6.66 (d, 4H, J = 8.4 Hz), 3.73
9

13
(s, 12H). C NMR (125 MHz, 298 K, DMSO-d6): δ (ppm) = 157.64, 154.79, 145.40, 135.84,
129.82, 127.20, 126.27, 125.63, 119.27, 104.33, 55.62.
2.6.3. 2,9-Bis(3,5-dimethoxyphenyl)-1,10-phenanthroline ((3,5-dmp)2phen)
Starting materials: 2,9-dichloro-1,10-phenanthroline (0.250 g, 1.00 mmol), 3,5-
dimethoxyphenylboronic acid (0.639 g, 3.50 mmol), Pd(PPh3)4 (0.175 g, 0.150 mmol). Yield =
1
0.195 (0.431 mmol, 43%). H NMR (500 MHz, 298 K, CDCl3): δ (ppm) = 8.31 (d, 2H, J = 8.4
Hz), 8.13 (d, 2H, J = 8.4 Hz), 7.80 (s, 2H), 7.74 (d, 2H, J = 2.3 Hz), 6.61 (t, 2H, J = 2.3 Hz), 3.98
13
(s, 12H). C NMR (125 MHz, 298 K, DMSO-d6): δ (ppm) = 161.51, 154.80, 145.44, 141.04,
137.73, 128.44, 126.66, 120.27, 105.62, 102.00, 55.88. HR-ESI(+)-MS: [M + H]+ m/z = 453.1810
(calc. m/z = 453.1815). Elemental analysis calculated for C28H24N2O4: 74.32% C, 5.35% H, 6.19%
N. Found: 73.04% C, 5.30% H, 6.01% N.
2.6.4. 2,9-Bis(4-methoxyphenyl)-1,10-phenanthroline ((4-mp)2phen) [45]
Starting materials: 2,9-dichloro-1,10-phenanthroline (0.250 g, 1.00 mmol), 4-
methoxyphenylboronic acid (0.533 g, 3.50 mmol), Pd(PPh3)4 (0.175 g, 0.150 mmol). Yield = 0.213
(0.543 mmol, 54%). 1H NMR (500 MHz, 298 K, CDCl3): δ (ppm) = 8.44 (d, 4H, J = 8.9 Hz), 8.27
(d, 2H, J = 8.4 Hz), 8.09 (d, 2H, J = 8.4 Hz), 7.75 (s, 2H), 7.12 (d, 4H, J = 8.9 Hz), 3.93 (s, 6H).
13C NMR (125 MHz, 298 K, DMSO-d6): δ (ppm) = 160.64, 154.98, 145.26, 137.18, 131.37,
128.64, 127.38, 125.74, 119.26, 114.36, 55.33.
2.7.
General synthesis of Re(I)-phenanthroline complexes
Re(R2phen) complexes were prepared by combining Re(CO)5Cl, diaryl-substituted
phenanthrolines, and 5 mL toluene in a 50 mL Schlenk flask. The mixture was purged with Ar for
15 minutes prior to heating at reflux in the dark. After 6 h, the yellow mixture was cooled to room
10

temperature and 25 mL of hexanes were added to induce further precipitation. The yellow solids
were collected by vacuum filtration and rinsed with hexanes.
2.7.1. fac-[Re(Ph2phen)(CO)3Cl] (Re(Ph2phen)) [46]
Starting materials: Re(CO)5Cl (0.050 g, 0.14 mmol), Ph2phen (0.045 g, 0.14 mmol). Yellow
solid; yield = 0.049 g (0.077 mmol, 56%). 1H NMR (500 MHz, 298 K, CDCl3): δ (ppm) = 8.57 (d,
2H, J = 8.3 Hz), 8.06 (s, 2H), 7.94 (d, 2H, J = 8.3 Hz), 7.82 – 7.74 (broad singlet, 4H), 7.62 – 7.58
(m, 6H). FTIR (CH2Cl2) ν(CO): 2024 cm−1, 1924 cm−1, 1888 cm−1.
2.7.2. fac-[Re((2,6-dmp)2phen)(CO)3Cl] (Re((2,6-dmp)2phen))
Starting materials: Re(CO)5Cl (0.050 g, 0.14 mmol), (2,6-dmp)2phen (0.062 g, 0.14 mmol).
Yellow solid; yield = 0.087 g (0.11 mmol, 83%). 1H NMR (500 MHz, 298 K, CDCl3): δ (ppm) =
8.44 (d, 2H, J = 8.3 Hz), 7.96 (s, 2H), 7.74 (d, 2H, J = 8.3 Hz), 7.46 (t, 2H, J = 8.5 Hz), 6.75 (d,
2H, J = 8.3 Hz), 6.67 (d, 2H, J = 8.3 Hz), 3.82 (s, 6H), 3.69 (s, 6H). 13C NMR (125 MHz, 298 K,
DMSO-d6): δ (ppm) = 194.18, 190.55, 159.03, 157.71, 157.31, 147.49, 138.64, 131.95, 129.32,
128.09, 127.11, 119.18, 105.31, 103.60, 56.19, 55.26. FTIR (CH2Cl2) ν(CO): 2018 cm−1, 1913
cm−1, 1886 cm−1. HR-ESI(+)-MS: [M + Na]+ m/z = 781.0712 (calc. m/z = 781.0729). Elemental
analysis calculated for C31H24N2O7ClRe·C7H8: 53.68% C, 3.79% H, 3.29% N. Found: 52.70% C,
3.71% H, 3.29% N. Crystals suitable for X-ray diffraction were obtained by vapor diffusion of
diethyl ether into an CH3CN solution of Re((2,6-dmp)2phen) to obtain yellow crystals.
2.7.3. fac-[Re((3,5-dmp)2phen)(CO)3Cl] (Re((3,5-dmp)2phen))
Starting materials: Re(CO)5Cl (0.051 g, 0.14 mmol), (3,5-dmp)2phen (0.063 g, 0.14 mmol).
1
Dark yellow solid; yield = 0.100 g (0.13 mmol, 95%). H NMR (500 MHz, 298 K, CDCl3): δ
(ppm) = 8.54 (d, 2H, J = 8.3 Hz), 8.05 (s, 2H), 7.92 (d, 2H, J = 8.3 Hz), 6.93 (s, 2H), 6.83 (s, 2H),
13
6.67 (t, 2H, J = 2.3 Hz), 3.88 (s, 6H), 3.87 (s, 6H). C NMR (125 MHz, 298 K, DMSO-d6): δ
11

(ppm) = 193.78, 192.73, 163.21, 160.58, 160.19, 147.19, 143.32, 139.50, 130.00, 127.44, 126.83,
108.03, 102.17, 55.63, 55.38. FTIR (CH2Cl2) ν(CO): 2022 cm−1, 1923 cm−1, 1892 cm−1. HR-
ESI(+)-MS: [M + H]+ m/z = 764.1406 (calc. m/z = 764.1407). Elemental analysis calculated for
C31H24N2O7ClRe: 49.11% C, 3.19% H, 3.69% N. Found: 47.57% C, 3.16% H, 3.79% N. Crystals
suitable for X-ray diffraction were obtained by slow evaporation of a 1:1 toluene/CH2Cl2 solution
Re((3,5-dmp)2phen) to obtain golden crystals.
2.7.4. fac-[Re((4-mp)2phen)(CO)3Cl] (Re((4-mp)2phen)) [47]
Starting materials: Re(CO)5Cl (0.050 g, 0.14 mmol), (4-mp)2phen (0.054 g, 0.14 mmol).
Yellow solid; yield = 0.073 g (0.10 mmol, 76%). 1H NMR (500 MHz, 298 K, CDCl3): δ (ppm) =
8.53 (d, 2H, J = 8.3 Hz), 8.02 (s, 2H), 7.93 (d, 2H, J = 8.3 Hz), 7.73 (d, 4H, J = 8.3 Hz), 7.11 (d,
4H, J = 8.4 Hz), 3.91 (s, 6H). 13C NMR (125 MHz, 298 K, DMSO-d6): δ (ppm) = 193.72, 193.45,
163.27, 160.93, 147.63, 139.25, 134.28, 130.95, 129.62, 127.05, 126.66, 113.66, 55.50. FTIR
(CH2Cl2) ν(CO): 2024 cm−1, 1924 cm−1, 1886 cm−1. HR-ESI(+)-MS: [M – Cl + CN3CN]+ m/z =
704.1191 (calc. m/z = 704.1196).
3. Results and Discussion
3.1.
Synthesis and structural characterization
Diaryl-substituted phenanthrolines (labeled R2phen) were prepared in good yield using
Suzuki-Miyaura, Pd(0)-catalyzed cross-coupling between 2,9-dichloro-1,10-phenanthroline and
1
arylboronic acids, Scheme 2 [44, 48]. Following H and 13C NMR characterization (see
Supplementary Information), R2phen ligands were reacted with Re(CO)5Cl (1:1 mol ratio) in
toluene to afford fac-[Re(R2phen)(CO)3Cl] as yellow solids. For the newly reported Re((2,6-
dmp)2phen) and Re((3,5-dmp)2phen) complexes, HR-ESI(+)-MS and elemental analysis
confirmed the proposed mass and elemental composition, respectively.
12

Scheme 2. General synthesis of 2,9-diaryl-1,10-phenanthroline ligands (a) and corresponding fac-
[ReI(R2phen)(CO)3Cl] complexes (b).
1H NMR spectra of Re(R2phen) display three resonance signals (doublet-singlet-doublet
splitting pattern) between δ = 8.60 – 7.60 ppm which correspond to protons from the
phenanthroline backbone. The presence of ortho methoxy-substituted phenyl groups in Re((2,6-
dmp)2phen) shift these signals farthest upfield, as observed with the previously reported
Re((2,4,6-tmp)2phen) analogue [33]. Worth noting in the aliphatic region is the larger Δδ value
between the two –OCH3 singlets for Re((2,6-dmp)2phen) compared to Re((3,5-dmp)2phen) (Δδ
= 0.13 vs 0.01 ppm, respectively), emphasizing the different chemical environments experienced
by the phenyl ring’s ortho and meta substituted groups proximal to a fac-ReI(CO)3Cl core. Treating
the phenanthroline backbone as a plane containing two equatorial Re-CO bonds, ortho –OCH3
above and below this plane are closer to either a –Cl or –CO group which possess different
through-space shielding effects. Conversely, meta –OCH3 are further away from the –Cl or –CO
groups, thus any spatially-related influences on chemical environment will be minimal and this is
reflected in the small Δδ value.
13

The Re(R2phen) complexes display three intense IR absorption bands between ν = 2050 –
1850 cm−1 corresponding to the symmetric and asymmetric stretching vibrational modes of the
facial ReI(CO)3 arrangement (Figure 2) [47]. In agreement with previously reported FTIR spectra,
ortho methoxy-substituted phenyl rings in Re((2,6-dmp)2phen) increase electron density on the
Re(I) metal center, causing enhanced π-backbonding to the CO π* molecular orbitals and a shift
to lower frequency for the symmetric A′(1) CO vibrational mode (Table S2). Interestingly, the
para methoxy-substituted Re(4-mp)2phen) displays a smaller red-shift of the A′(1) band,
suggesting that electron density at the Re(I) metal is comparable to the unsubstituted Re(Ph2phen)
and meta methoxy-substituted Re((3,5-dmp)2phen). The similar electronic character between the
latter three complexes implies that similar redox and catalytic activity will occur if solely based
on electronic effects.
Figure 2. FTIR spectra of Re(Ph2phen) (─), Re((2,6-dmp)2phen) (─), Re((3,5-dmp)2phen) (─),
and Re((4-mp)2phen) (─) measured in CH2Cl2.
14

Molecular structures of Re((2,6-dmp)2phen) and Re((3,5-dmp)2phen) were determined by
single crystal X-ray diffraction and their thermal ellipsoid plots are shown in Figure 3. As with
many fac-[ReI(NN)(CO)3Cl] complexes, both complexes possess distorted octahedral
environments and facial CO arrangements at the Re center, with electronic contributions from the
coordinated R2phen ligands dictating the average Re-CO bond length (Re-COavg). The shorter Re-
COavg bond length in Re((2,6-dmp)2phen) compared to Re((3,5-dmp)2phen) (Re-COavg = 1.90
vs. 1.93 Å, respectively) is attributed to increased π-backbonding with CO, resulting in decreased
Re-C bond length. Corroborating this assessment is the longer average carbonyl C-O bond length
in Re((2,6-dmp)2phen) compared to Re((3,5-dmp)2phen) (C-Oavg = 1.16 vs. 1.13 Å,
respectively), which is in agreement with the red-shifted A′(1) CO stretching frequency described
above.
Figure 3. Crystal structures of Re((2,6-dmp)2phen) (a) and Re((3,5-dmp)2phen) (b) with thermal
ellipsoids set at a 50% probability level. Counter ions and solvent molecules are omitted for clarity.
The Re-Cl···OCH3 distances (dashed red lines) are written in red.
A final structural note worth mentioning is the different through-space distances of the Re-
Cl···OCH3 components for the ortho- and meta-substituted complexes. While the initial chloro-
coordinated complexes are considered pre-catalytic species, they provide an estimation of the
15

distance between the coordinated carboxylic acid intermediate and the proximal (or distal)
methoxy group, and whether weak hydrogen-bonding is capable or not during the catalytic cycle.
The shorter Cl···O distance for Re((2,6-dmp)2phen) compared to Re((3,5-dmp)2phen) (Cl···Oavg
= 4.18 vs. 6.71 Å, respectively) is expected given the close proximity of ortho substituted phenyl
rings, and DFT/M06-level of theory calculated structures corroborate this finding (see Figures
S17 and S18). When –Cl is substituted with –C(O)OH (Figure 4), the calculated C(O)OH···OCH3
distance (2.94 Å for Re((2,6-dmp)2phen)) is within the range of potential hydrogen bonding.
Conversely, the same analysis for Re((3,5-dmp)2phen) provides a much longer calculated
C(O)OH···OCH3 distance (5.69 Å), thereby removing the possibility of weak hydrogen bonding
between the metallocarboxylic acid intermediate and meta methoxy substituent. In addition, future
synthetic modifications to convert –OCH3 to –OH are warranted for the Re((2,6-dmp)2phen)
analogue in an effort enhance hydrogen bonding with the metallocarboxylate intermediate.
Figure 4. DFT/M06-level of theory calculated structure for the proposed metallocarboxylic acid
intermediates, Re((2,6-dmp)2phen)(COOH) (a) and Re((3,5-dmp)2phen)(COOH) (b),
illustrating the spatial orientation and distance between Re-C(O)OH and methoxy O. The
calculated C(O)OH···OCH3 through space distance is written in red.
16

3.2.
Electrochemistry under N2 atmosphere
The redox properties of fac-[ReI(NN)(CO)3Cl] complexes are well-established in the literature,
whereby cathodic scans display a quasi-reversible diimine0/− couple followed by an irreversible
ReI/0 wave, the latter of which is due to rapid halide loss [11]. Potentials required to access this
two-electron reduced, five-coordinate intermediate are strongly dictated by the diimine ligand’s
substituents, with electron-donating and electron-withdrawing groups destabilizing and
stabilizing, respectively, the frontier molecular orbitals [9, 13, 33]. Re(Ph2phen), Re((3,5-
dmp)2phen), and Re((4-mp)2phen) display similar redox potentials for the R2phen0/− and ReI/0
processes (Table 1, Figure 5), highlighting the similar electronic contributions arising from an
unsubstituted, meta methoxy-substituted, or para methoxy-substituted phenyl ring. Conversely,
both R2phen0/− and ReI/0 potentials are cathodically-shifted (~200 mV) for Re((2,6-dmp)2phen)
due to the ortho methoxy substituents adding electron density to the phenyl and phenanthroline
ring systems. The latter compound agrees well with the previously reported Re((2,4,6-tmp)2phen)
which possesses both ortho and para methoxy substituents and displays the most negative
c
potentials for R2phen0/− (E = −1.99 V) and ReI/0 (Ep = −2.58 V).
½
Regardless of methoxy substitution pattern, all four Re(R2phen) complexes undergo rapid Cl−
a
dissociation following the ReI/0 wave, as evident from the lack of an Ep return wave, to access the
proposed catalytically-active five-coordinate intermediate, [Re(R2phen)(CO)3]−. In the presence
of CO2 substrate, we expect current enhancement to ensue as a result of catalytic CO2 reduction to
CO and the extent of activity should be influenced by the location of methoxy substituents on the
phenyl rings.
17

Table 1. Redox properties of Re(R2phen) electrocatalysts a
c
Complex
E (R2phen0/−) / V
Ep (ReI/0) / V b
½
Re(Ph2phen)
−1.82
−2.01
−1.81
−1.85
−2.39
−2.60
−2.37
−2.43
Re((2,6-dmp)2phen)
Re((3,5-dmp)2phen)
Re((4-mp)2phen)
a Reportedvalues were measured using 0.5 mM Re(R2phen) catalyst, 0.1 M n-Bu4NPF6 electrolyte
in DMF solvent, room temp., N2 atm, and 200 mV/s scan rate unless otherwise stated. Potentials
c
referenced against the Fc+/Fc couple. b Reported Ep for an irreversible reductive process.
Figure 5. Cyclic voltammograms isolating each redox couple of 0.5 mM Re(R2phen) complexes.
Conditions: 0.1 n-Bu4NPF6 electrolyte, DMF solvent, N2 atm., room temp., 200 mV/s scan rate.
18

3.3. Electrocatalytic CO2 reduction
Cyclic voltammograms collected under a CO2 atmosphere display catalytic current
enhancement at the ReI/0 wave for all four complexes (dashed colored lines in Figure 6), indicating
two-electron reduction and halide loss occur prior to sufficient catalysis. Comparing the catalytic
activity in Table 2, Re(Ph2phen), Re((3,5-dmp)2phen), and Re((4-mp)2phen) display similar
catalytic current enhancement (icat/ip) as measured by the ratio between the current response in the
presence (icat) and absence (ip) of CO2. The comparable CO2 reduction activity is in agreement
with the R2phen0/− and ReI/0 reduction potentials under N2 atmosphere, indicating similar electronic
contributions to the Re catalytic site regardless of methoxy substitution at the meta or para
positioning, as well as their complete absence. The ortho-substituted analogue, Re((2,6-
dmp)2phen), displays a 2x-increased icat/ip ratio, suggesting more active CO2 reduction. However,
this current enhancement is observed at more negative potentials, therefore a larger overpotential
must be applied to access the increased activity. For structurally-related complexes, a larger
overpotential often results in higher catalytic CO2 reduction activity. Using the potential whereby
half the catalytic current enhancement is observed (Ecat/2) as a thermodynamic metric relating to
rapid catalysis, an additional ~250 mV are required for Re((2,6-dmp)2phen). An additional feature
for Re((2,6-dmp)2phen) under CO2 atmosphere is the trace crossover occurring upon switching
scan direction. Such features generally indicate a sluggish chemical reaction following an electron
transfer step. At the moment, we do not know the origins of the trace crossover observed near
−2.50 V; however, a rinse test was performed to rule out Re((2,6-dmp)2phen) functioning as a
heterogeneous catalyst adhered to the electrode surface (Figure S19).
19

Table 2. Electrocatalytic activity of Re(R2phen) electrocatalysts a
b
e
Complex
icat/ip
Ecat/2 / V c CO / μmol d TONCO
FECO / % f H2 / μmol d
Re(Ph2phen)
5.0
−2.32
−2.59
−2.32
−2.37
1.8
35
0.7
14
15
84
17
40
4.3
0.1
2.0
0.4
Re((2,6-dmp)2phen) 10.3
Re((3,5-dmp)2phen)
Re((4-mp)2phen)
4.7
5.0
2.4
4.2
1.0
1.7
a Reported values measured using 0.5 mM Re(R2phen) catalyst, 0.1 M n-Bu4NPF6 electrolyte in
b
DMF solvent, room temp., and CO2 atm. Potentials referenced against Fc+/Fc couple. Ratio of
the peak current response under CO2 atmosphere (icat) and current response for ReI/0 reduction
c
under N2 atmosphere (ip) at 200 mV/s scan rate. Potential corresponding to half the catalytic
current enhancement. d Amount of product after 90 min controlled-potential electrolysis at Eapp
=
e
−2.55 V. Turnover numbers, after 90 min electrolysis, corresponding to ratio between mol CO
f
produced and mol Re(I) catalyst used. Faradaic efficiency for CO produced after 90 min
electrolysis considering two electrons required per mol CO produced.
Figure 6. Cyclic voltammograms of 0.5 mM Re(R2phen) complexes under N2 (solid colored line)
and CO2 (dashed colored line). Conditions: 0.1 n-Bu4NPF6 electrolyte, DMF solvent, room temp.,
200 mV/s scan rate.
20

To determine the gaseous products formed, controlled-potential electrolysis (CPE) was
performed at an applied potential (Eapp) of −2.55 V vs. Fc+/Fc for all four Re(R2phen) complexes,
and the headspace was sampled periodically over the course of 90 minutes by injecting 100 μL
into a gas chromatograph (Table 2). Figure 7 illustrates that CO evolution increased over the
entirety of the experiment; however, the Re((2,6-dmp)2phen) electrocatalyst displayed
significantly higher levels of CO after just 30 minutes. Comparing the turnover numbers (TON)
and Faradaic efficiencies (F.E.) in Table 2, it becomes apparent that the ortho-substituted methoxy
substituents contribute to the higher activity and selectivity for CO2-to-CO reduction. The applied
potential of −2.55 V is near the Ecat/2 value for Re((2,6-dmp)2phen), while the same applied
potential is ~200 mV more negative than the Ecat/2 values for Re(Ph2phen), Re((3,5-dmp)2phen),
and Re((4-mp)2phen). The intrinsically slower catalysis for the latter three complexes was not
overcome by applying the more negative potential, providing further support that the electron-
donating character associated with the ortho-substituted methoxy groups strongly affects activity.
Figure 7. Controlled-potential electrolysis data illustrating the catalytic activity of 0.5 mM
Re(R2phen) complexes at Eapp = −2.55 V vs. Fc+/Fc in 0.1 M n-Bu4NPF6/DMF electrolyte-solvent
system under a CO2 atmosphere.
21

Although levels of CO evolved were significantly less for the Re(Ph2phen), Re((3,5-
dmp)2phen), and Re((4-mp)2phen) complexes, we were quite surprised to observe such low
Faradaic efficiencies for CO as well. This suggests competing reactions are occurring, and we
confirmed the evolution of H2 in the gas chromatograms as the main culprit (Figure S20).
Performing electrolysis under CO2 atmosphere without any Re(R2phen) catalysts present
produced 7.8 μmol H2 after 90 min. While the level of H2 produced decreased in all cases when
any Re(R2phen) catalysts were present, it is worth noting that the slowest CO-evolving catalysts,
Re(Ph2phen) and Re((3,5-dmp)2phen), still produced considerable H2. This indicated their
inability to rapidly reduce CO2 to CO and consume more electrons; therefore, the charge delivered
to the cell was used to generate H2 at the electrode. Conversely, the Re((4-mp)2phen) and Re((2,6-
dmp)2phen) catalysts were more rapid at turning over CO2 such that more charge consumed
resulted in CO evolution, with the latter ortho-methoxy substituted analogue achieving high TON
(14) and F.E. (84%) for CO.
4. Conclusions
Four structurally-related Re(I)-phen electrocatalysts were synthesized and fully characterized
to better understand the structural and electronic contributions that electron-rich methoxy
substituents have on the electrocatalytic reduction of CO2 to CO. Supporting and further explaining
our previous observations, the presence of ortho-substituted methoxy phenyl groups increase
electron density on the phen ligand and Re metal center as evidenced by the cathodic shift of the
R2phen0/− and ReI/0 reduction potentials in Re((2,6-dmp)2phen). In the presence of CO2, formation
of this two-electron reduced, five-coordinate intermediate results in rapid catalytic CO2 reduction
with TON = 14 (F.E. = 84%; 35 μmol CO produced) after just 90 min electrolysis. Conversely,
the Re(Ph2phen) (no methoxy groups), Re((3,5-dmp)2phen) (meta methoxy groups), and Re((4-
22

mp)2phen) (para methoxy groups) electrocatalysts display significantly lower amounts of CO
evolved, as indicated by the lower observed catalytic current (i.e. slow catalysis) and TON values
for CO (TON = 0.7, 1.0, 1.7, respectively). By isolating the methoxy substituents at different
locations around a phenyl ring, we have been able to determine how small structural and electronic
modifications to a redox-active catalytic metal center can greatly impact the observed catalysis.
We hypothesize that the electron-donating nature of the ortho methoxyphenyl substituents
produces a more nucleophilic Re metal center to bind and activate CO2 en route to evolving CO,
albeit at a larger overpotential. Current efforts involve converting the methoxy substituents to
hydroxy substituents to covalently attach strong hydrogen bonding groups onto the ligand
structure, as well as attaching our structurally-modified Re(I)-phenanthroline electrocatalysts onto
electrode surfaces to heterogenize our molecular catalysts.
Appendix A. Supplementary Information.
Supplementary data including HR-ESI(+)-MS data, 1H NMR spectra, 13C NMR spectra, FTIR
tabulated data, LSV rinse test, H2 evolution, calculated structures, and X-ray crystallographic
data associated with this article can be found, in the online version, at
*Corresponding Author
Mailing Address: Department of Chemistry & Biochemistry, 100 University Terrace, 136
Clippinger Laboratories, Athens, Ohio 45701, USA
E-mail address: whitet2@ohio.edu (T. A. White)
ORCID
Travis A. White: 0000-0002-6200-4416
Christopher J. Ziegler: 0000-0002-0142-5161
Author Contributions
23

The manuscript was written through contributions of all authors and all authors have given
approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
Declarations of interest: none
Acknowledgements
T.A.W. acknowledges the College of Arts & Sciences and the Department of Chemistry &
Biochemistry at Ohio University for start-up funds supporting this research.
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Credit Authorship Contribution Statement:
Sarah A. Roell: Conceptualization, Investigation, Visualization, Writing – Original draft,
Briana R. Schrage: Investigation, Visualization
Prof. Christopher J. Ziegler: Writing – Review & Editing, Supervision
Prof. Travis A. White: Conceptualization, Investigation, Visualization, Supervision, Project
Administration, Writing – Review & Editing
Highlights:
 Re(I)-phenanthroline electrocatalysts were synthesized and fully characterized
 Location of methoxy substituents on phenyl rings strongly dictate CO2 reduction activity
26

 Ortho methoxy phenyl groups display increased CO formation
27