Molecular Catalysts with Intramolecular Re-O Bond for Electrochemical Reduction of Carbon Dioxide

Article abstract of DOI:10.1021/acs.inorgchem.0c01181

A new Re bipyridine-type complex, namely, fac-Re(pmbpy)(CO)3Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2′-bipyridine), 1, carrying a single OH moiety as local proton source, has been synthesized, and its electrochemical behavior under Ar and under CO2 has been characterized. Two isomers of 1, namely, 1-cis characterized by the proximity of Cl to OH and 1-trans, are identified. The interconversion between 1-cis and 1-trans is clarified by DFT calculations, which reveal two transition states. The energetically lower pathway displays a non-negligible barrier of 75.5 kJ mol-1. The 1e- electrochemical reduction of 1 affords the neutral intermediate 1-OPh, formally derived by reductive deprotonation and loss of Cl- from 1. 1-OPh, which exhibits an entropically favored intramolecular Re-O bond, has been isolated and characterized. The detailed electrochemical mechanism is demonstrated by combined chemical reactivity, spectroelectrochemistry, spectroscopic (IR and NMR), and computational (DFT) approaches. Comparison with previous Re and Mn derivatives carrying local proton sources highlights that the catalytic activity of Re complexes is more sensitive to the presence of local OH groups. Similar to Re-2OH (2OH = 4-phenyl-6-(phenyl-2,6-diol)-2,2′-bipyridine), 1 and Mn-1OH display a selective reduction of CO2 to CO. In the case of the Re bipyridine-type complex, the formation of a relatively stable Re-O bond and a preference for phenolate-based reactivity with CO2 slightly inhibit the electrocatalytic reduction of CO2 to CO, resulting in a low TON value of 9, even in the presence of phenol as a proton source.

Full text of DOI:10.1021/acs.inorgchem.0c01181

pubs.acs.org/IC
Article
Molecular Catalysts with Intramolecular Re−O Bond for
Electrochemical Reduction of Carbon Dioxide
Laura Rotundo, Dmitry E. Polyansky, Roberto Gobetto, David C. Grills, Etsuko Fujita, Carlo Nervi,*
and Gerald F. Manbeck*
Cite This: https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: A new Re bipyridine-type complex, namely, fac-Re-
(pmbpy)(CO)3Cl (pmbpy = 4-phenyl-6-(2-hydroxy-phenyl)-2,2′-bipyr-
idine), 1, carrying a single OH moiety as local proton source, has been
synthesized, and its electrochemical behavior under Ar and under CO2
has been characterized. Two isomers of 1, namely, 1-cis characterized by
the proximity of Cl to OH and 1-trans, are identified. The interconversion
between 1-cis and 1-trans is clarified by DFT calculations, which reveal
two transition states. The energetically lower pathway displays a non-
negligible barrier of 75.5 kJ mol−1. The 1e− electrochemical reduction of 1
affords the neutral intermediate 1-OPh, formally derived by reductive
deprotonation and loss of Cl− from 1. 1-OPh, which exhibits an
entropically favored intramolecular Re−O bond, has been isolated and
characterized. The detailed electrochemical mechanism is demonstrated by combined chemical reactivity, spectroelectrochemistry,
spectroscopic (IR and NMR), and computational (DFT) approaches. Comparison with previous Re and Mn derivatives carrying
local proton sources highlights that the catalytic activity of Re complexes is more sensitive to the presence of local OH groups.
Similar to Re-2OH (2OH = 4-phenyl-6-(phenyl-2,6-diol)-2,2′-bipyridine), 1 and Mn-1OH display a selective reduction of CO2 to
CO. In the case of the Re bipyridine-type complex, the formation of a relatively stable Re−O bond and a preference for phenolate-
based reactivity with CO2 slightly inhibit the electrocatalytic reduction of CO2 to CO, resulting in a low TON value of 9, even in the
presence of phenol as a proton source.
whenever the intimate molecular mechanism responsible for
the catalysis is initially deeply investigated in the homogeneous
phase. The concept of a local proton source applied to the
electrochemical reduction of CO241 has been recently extended
not only to the Mn-bpy system14,16,23 but also to the Rebpy
complexes.1,11,42−44 Scheme 1 shows a set of Mn and Re
catalysts bearing pendent phenolic groups near the metal
center acting as intramolecular proton sources. We initially
reported fac-Mn(pdbpy)(CO)3Br14,16 (Mn-2OH), and re-
cently we studied the effect of the two OH functionalities on
fac-Re(pdbpy)(CO)3Cl13 (Re-2OH; pdbpy = 4-phenyl-6-
(phenyl-2,6-diol)-2,2′-bipyridine). A similar catalyst, Mn(6-
(2-hydroxyphenol)-2,2′-bipyridine)(CO)3Br (Mn-1OH) was
previously described by Bocarsly and co-workers.45 We already
concluded that the local proton source effects of the pdbpy
ligand altered the selectivity for CO2 electroreduction
INTRODUCTION
■
Carbon dioxide is a key greenhouse gas, and its concentration
in the atmosphere is continuously increasing. The scientific
community is drawing inspiration from natural photosynthesis
in which CO2 and water are converted into glucose and O2 by
various plants and algae just after capturing energy from
sunlight. This process is an excellent model of direct chemical
storage of solar energy. CO2 may be artificially converted into
important chemical industry feedstocks such as CO or
HCOOH and fuels like CH3OH and hydrocarbons (CH4,
C2H4, C2H6).1−6 Another approach consists of converting solar
light into electric energy and employing it for the electro-
chemical reduction of CO2.7 After the pioneer work of Lehn in
the 1980s that first reported the capability of fac-Re(bpy)-
(CO)3Cl (bpy = 2,2′-bipyridine) in homogeneous solution to
selectively reduce CO2 to CO,8 many new transition metal
complexes were discovered.5,9,10 Among them, those derived
from the original Rebpy and the corresponding Mn-bpy type
received considerable interest11−34 due to performances and
stability, especially when chemically bound to the electrode
surface.12,25,35−39 The transition from the homogeneous to the
heterogeneous approach, which in some cases displays superior
stability and durability,40 can find considerable support
Received: April 21, 2020
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
© XXXX American Chemical Society
Inorg. Chem. XXXX, XXX, XXX−XXX
A

Inorganic Chemistry
pubs.acs.org/IC
Article
Scheme 1. Sketches of Sample Mn and Re Bipyridine Electrocatalysts Bearing Local Proton Sources and Proposed
Intermediates Formed during Electrochemical Reduction [a, refs 14 and 16; b, ref 45; c, ref 11; d, ref 13; e, This Work]
depending on the metal involved.13 Indeed, while Mn-2OH
gives the hydride form with subsequent production of formate,
cells using standard Schlenk techniques. Alternatively, acetonitrile was
freshly distilled over calcium hydride. The pmbpy ligand was
46
̈
synthesized by the Krohnke reaction between the pyridinium salt
Re-2OH undergoes a different reduction pathway, and no
formate is afforded. The intermediate Re-OPh (Scheme 1) is
proposed to be produced after the first 1e− reduction. Re-OPh
is characterized by the presence of the intramolecular Re−O
bond and undergoes electrochemical reduction to generate the
actual catalyst in the CO2 conversion. IR-SEC measurements
supported by DFT calculations suggested the formation of this
intermediate following reductive deprotonation; however, its
identity was not corroborated by other experimental
techniques, and its precise effect on catalysis remained unclear.
The aims of the current work are to shed light on the
fundamental aspects of this electrochemical mechanism and to
provide additional spectroscopic evidence for phenolate
coordination to the metal. In order to accomplish this goal, a
novel complex bearing a single hydroxyl group instead of two,
namely, fac-Re(pmbpy)(CO)3Cl (pmbpy = 4-phenyl-6-(2-
hydroxy-phenyl)-2,2′-bipyridine), 1, has been synthesized
(Scheme 1). We hypothesized that a single pendent phenolic
group could simplify the system by eliminating complications
arising from the reductive or chemical deprotonation of the
second phenolic group. Hydroxy-bipyridine-type ligands with
the OH group close to the metal center are known to
experience the electronic effects of the oxyanion formed by
deprotonation to the catalytic site.43 Complex 1 has been
characterized by cyclic voltammetry (CV), infrared spectroe-
lectrochemistry (IR-SEC), and chemical reduction with Na−
Hg. Data have been analyzed through comparison to Re(4,6-
diphenyl-2,2′-bipyridine)(CO)3Cl (RediPh), which does not
bear phenolic groups. The corresponding intermediate 1-OPh
has been isolated and characterized, and 1 has been tested as a
catalyst for CO2 electroreduction with reactivity discussed in
terms of the observed intermediates and compared to literature
data for related complexes.
[N-((2-pyridylacetyl) pyridinium iodide)]47 and [3-(2-methoxyphen-
yl)-1-phenylprop-2-en-1-one]48 (an α,β-unsaturated chalcone). NMR
spectra were recorded on a 400 MHz Bruker Avance spectrometer
(1H operating frequency 400 MHz) at 25 °C or, alternatively, on a
JEOL ECP 400 FT-NMR spectrometer (1H operating frequency 400
MHz) at 25 °C. 13C spectra of 1 and 1-OPh have been recorded on a
JEOL ECZ 600 R. H and 13C chemical shifts are reported in parts
1
per million relative to TMS (δ = 0) and referenced against solvent
residual peaks. UV−vis spectra were measured on an Agilent 8453
spectrophotometer. Electrospray mass spectra (ESI-MS) were
measured using a Thermo Scientific Q Exactive high resolution
mass spectrometer. Samples 1 and 1-CH3CN+ for microanalysis were
dried in a vacuum to constant weight (20 °C, ca. 0.1 Torr). Elemental
analysis (C, H, N) was performed in-house with a Fisons instrument
1108 CHNS-O Elemental Analyzer.
Synthesis of 4-Phenyl-6-(methoxyphenyl)-2,2′-bipyridine.
[N-((2-pyridylacetyl) pyridinium iodide)] (6.13 mmol), [3-(2-
methoxyphenyl)-1-phenylprop-2-en-1-one] (6.13 mmol), ammonium
acetate (63 mmol), and methanol (40 mL) were combined in a three-
neck flask. The solution was purged with Ar and heated at reflux for 4
h. The progress of the reaction was monitored via TLC. After
complete consumption of the reagents, the solvent was removed in
vacuo to afford a brown oil. The crude product was dissolved in ethyl
acetate and washed three times with aqueous 10% NaHCO3. The oil
collected after evaporation of ethyl acetate was purified on a silica gel
column eluting with petroleum ether/ethyl acetate (7:1) to provide
1
the title complex as a yellow oil in 56% yield. H NMR (400 MHz,
(CD3)2SO): δ 8.75 (d, J = 4.7 Hz, 1H), 8.59 (d, J = 1.8 Hz, 1H), 8.53
(d, J = 7.9 Hz, 1H), 8.18 (d, J = 1.5 Hz, 1H), 7.95−8.02 (m, 2H),
7.89 (d, J = 8.5 Hz, 2H), 7.59 (t, J = 7.0 Hz, 2H), 7.46−7.55 (m, 2H),
7.22−7.29 (m, 2H), 7.16 (t, J = 7.6 Hz, 1H), 3.9 (s, 3H).
Synthesis of 4-Phenyl-6-(2-hydroxyphenyl)-2,2′-bipyridine
(pmbpy). 4-Phenyl-6-(methoxyphenyl)-2,2′-bipyridine (3 mmol)
was dissolved in 30 mL of HBr (48% aqueous solution) and heated
at reflux for 24 h. The mixture was then cooled to room temperature
and the pH adjusted to pH 7 using a NaHCO3 aqueous solution.
When the solution became neutral, pmbpy precipitated as a white
solid, which was filtered and then washed with water and CH2Cl2. No
EXPERIMENTAL DETAILS
■
General. All reagents for synthesis were purchased from Sigma-
Aldrich or Alfa Aesar and used without further purification.
Anhydrous toluene for synthesis was stored over activated molecular
sieves under Ar. Acetonitrile for electrochemical experiments was
dried on a solvent drying tower by passage through activated alumina
then stored in a glovebox or transferred directly to electrochemical
1
further purification was needed (yield 89%). H NMR (400 MHz,
(CD3)2SO): δ 8.82 (d, J = 4.4 Hz, 1H), 8.59 (s, 1H), 8.53 (s, 1H),
8.31 (t, J = 10.0 Hz, 2H), 8.1 (td, 1J = 6.2 Hz, 2J = 1.2 Hz, 1H), 8.05
(d, J = 7.0 Hz, 2H), 7.60 (m, 5H), 7.38 (td, 1J = 7.9 Hz, 2J = 1.16 Hz,
1H), 7.00 (d, J = 7.9 Hz, 1H).
B
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Synthesis of fac-Re(pmbpy)(CO)3Cl. Complex 1 was prepared
according to the typical literature procedure.13 [Re(CO)5Cl] (0.500
mmol, 1 equiv) and the pmbpy ligand (0.501 mmol, 1.01 equiv) were
dissolved in anhydrous toluene (20 mL) in a sealed flask and heated
in a Biotage microwave reactor at a constant temperature of 130 °C
for 1 h. After cooling of the reaction mixture to room temperature,
petroleum ether was added to precipitate the yellow product, which
was then centrifuged, filtered, and washed once with cold diethyl
supporting electrolyte. CVs were collected in a single-compartment
cell with a three-electrode configuration using a glassy carbon (GC)
working electrode (d = 3 mm), a Pt counter electrode, and a solid-
state Ag/AgCl reference electrode (eDAQ) or a silver wire pseudo-
reference. CVs at 195 K were collected in a sealed cell submerged in a
dry ice/acetone bath. All potentials are reported relative to the
ferrocenium/ferrocene couple (Fc+/0). Solutions were saturated with
Ar or CO2 that were passed through an acetonitrile prebubbler to
maintain a constant concentration of analyte. CPE experiments under
N2 or CO2 were performed in two-compartment cells where anodic
and cathodic compartments were separated by a glass frit. Preparative
electrolysis experiments were carried out in an N2-filled glovebox, and
Pt mesh was used as the working electrode or counter-electrode. CPE
under CO2 was performed in a 30 mL cell using a glassy carbon rod
working electrode and an aqueous SCE reference electrode in the
cathodic compartment and a Pt wire counter electrode in the second
compartment. Some experiments under CO2 were performed in the
presence of water or methanol (5% by volume). A controlled flow of
CO2 (50 mL min−1), measured just before arrival into the cell, was
maintained during the CPE measurements by means of a Smart Trak
100 (Sierra) flow controller. The cell was airtight and equipped with a
bubbler that maintained the inner atmosphere but avoided gas
overpressure. Quantitative analysis of CO2 reduction products was
carried out as reported previously.11
Spectroelectrochemistry. A custom reflective infrared spectroe-
lectrochemical (IR-SEC) cell equipped with boron-doped diamond
working, platinum counter, and solid-state leakless miniature Ag/AgCl
reference (EDAQ) electrodes was attached to a Fourier transform
infrared (FTIR) spectrometer (Bruker, IFS 66/S) using a VeeMAX
III Variable Angle Specular Reflectance Accessory (Pike Technolo-
gies), and the IR absorption changes were monitored following
application of fixed potentials.
Na−Hg Reduction. Sodium amalgam reduction was performed in
a homemade vessel equipped with a quartz spectrophotometric cell
separated by a glass frit from a second compartment containing 0.5%
Na in Hg. Samples were prepared under a high vacuum with vacuum
distilled CH3CN and were reduced gradually by introducing small
portions to the amalgam chamber under a vacuum.
1
ether (yield: 92%). H NMR (400 MHz, (CD3)2SO): 9.96−9.89 s
(1H), 9.03−9.08 (m, 2H), 9.00 (s, 1H), 8.37 (t, J = 8 Hz, 1H), 8.12−
8.17 (m, 2H), 8.05 d−7.96 d (J = 1.8 Hz, 1H), 7.59−7.62 (m, 3H),
7.76−7.98 (m, 1H), 7.30−7.37 (m, 2H), 7.04 (t, J = 7.0 Hz, 1H),
6.97 (t, J = 7.5 Hz, 1H). 13C NMR (150 MHz, (CD3)2SO), carbonyl
signals: 198.67, 198.50, 194.67, 193.85, 192.11, 190.78; 162.18 (Cq),
161.55 (Cq), 157.43 (Cq), 157.03 (Cq), 156.65 (Cq), 155.34 (Cq),
155.21 (Cq), 153.13 (−CH), 150.72 (Cq), 149.84 (Cq), 140.36
(−CH), 140.31 (−CH), 135.56 (Cq), 135.41 (Cq), 131.81 (−CH),
131.79 (−CH), 131.68(−CH), 131.33 (−CH), 131.25 (−CH),
131.19 (−CH), 129.91 (−CH), 129.84 (−CH), 129.43 (−CH),
129.13, 128.75 (−CH), 128.35 (−CH), 128.23 (−CH), 128.14
(−CH), 127.66 (−CH), 126.25 (−CH), 125.96 (−CH), 125.85
(−CH), 125.74 (−CH), 125.69 (−CH), 120.49 (−CH), 120.42
(−CH), 119.58 (−CH), 118.96 (−CH), 117.06 (−CH), 116.22
(−CH). UV−vis (CH3CN λmax): 379 nm. IR (CH3CN, νCO
̃
/cm−1):
2022, 1919, 1895 cm−1. HRMS (ESI+) m/z calcd for [M − Cl +
Na]+: 653.0254. Found: 653.0223. [M − Cl + CH3CN]+: 636.0933.
Found: 636.0910. [M−OH]+: 614.0407. Found: 614.0620. Anal.
Calcd (%) for C25H16ClN2O4Re: C (47.66), H (2.56), N (4.45).
Found: C (47.02), H (3.2), N (4.45).
Synthesis of fac-[Re(pmbpy)(CO)3(CH3CN)](PF6). The acetoni-
trile complex was prepared by heating complex 1 (7 mmol, 1 equiv)
and AgPF6 (7 mmol, 1 equiv) in anhydrous acetonitrile (25 mL) at
reflux overnight. After cooling of the mixture, it was filtered through
Celite to remove AgCl, and the solution was then evaporated to
dryness (yield: 60%). 1H NMR (400 MHz, CD3CN): 9.08 d (J = 5.1
Hz, 1H), 8.70−8.72 m (2H), 8.32 t (J = 7.8 Hz, 1H), 8.03 d (J = 2.0
Hz, 1H), 7.97−7.99 m (2H), 7.73 td (1J = 5.4 Hz, 2J = 2.5 Hz, 1H),
7.60−7.63 m (2H), 7.54 d (J = 3.2 Hz, 1H), 7.45 t (J = 7.3 Hz, 1H),
7.35 d (J = 7.6 Hz, 1H), 7.05−7.12 m (2H). 13C NMR (100 MHz,
CD3CN) carbonyl signals: 194.09, 189.85, 184.17; 157.34 (Cq),
156.84 (Cq), 154.36 (Cq), 153.92 (−CH), 152.07 (Cq), 151.71
(Cq), 140.63 (−CH), 135.28 (Cq), 132.03 (−CH), 131.13 (−CH),
130.32 (−CH), 129.59 (−CH), 127.84 (−CH), 126.85 (−CH),
125.14 (−CH), 122.46 (Cq), 120.81 (−CH), 120.36 (−CH), 120.13
(−CH), 117.45 (−CH), 116.68 (−CH), 116.27 (−CH). (Figures S9
and S10). Anal. Calcd (%) for C27H19F6N3O4PRe: C (41.54), H
(2.45), N (5.38). Found: C (40.86), H (3.16) N (5.22). IR (CH3CN,
νCO/cm−1): 2039, 1942, 1927 cm−1 (Figure S19).
Synthesis of 1-OPh. In two-compartment electrochemical cell
inside an N2-filled glovebox, a 0.5 mM solution of 1 in CH3CN
containing 0.035 mM Bu4NPF6 was subject to controlled potential
electrolysis at −1.7 V vs Fc+/0 using a Pt mesh electrode. After
complete consumption of 1 as judged by CV, the solution was
evaporated to dryness. The resultant solid was dissolved in d6-DMSO
and analyzed by NMR in a J-Young tube. 1H NMR (400 MHz,
(CD3)2SO): 9.28 d (J = 5.1 Hz, 1H), 8.96 d (J = 8.6 Hz, 1H), 8.84 s
(1H), 8.39 t (J = 8.2 Hz, 1H), 8.24 s (1H), 8.17 d (J = 8.1 Hz, 2H),
8.05 d (J = 8.1 Hz, 1H), 7.77 t (J = 6.7 Hz, 1H), 7.57−7.64 m (3H),
7.26 t (J = 7.5 Hz, 1H), 6.71 d, (J = 7.5 Hz, 2H). 13C NMR (150
MHz, (CD3)2SO): 199.50, 197.80, 193.68, 171.64 (Cq), 157.87
(Cq), 157.70 (Cq), 155.41 (Cq), 153.64 (Cq), 151.53 (−CH),
140.44 (−CH), 136.05 (Cq), 133.56 (−CH), 131.05 (−CH), 130.34
(−CH), 129.72 (−CH), 129.44 (−CH),126.75 (−CH), 124.83
(−CH), 120.87 (−CH), 120.27 (Cq), 119.55 (−CH), 119.45
(−CH), 118.50 (−CH). IR (CH3CN, νCO/cm−1): 2013, 1907,
1884 cm−1.
DFT Calculations. Gaussian 16, Revision B.01 and Gaussian 09,
Revision D.01 packages49 were adopted for all DFT calculations.
Solvent effects were taken into account by the conductor-like
polarizable continuum model (CPCM)50,51 with acetonitrile as a
solvent. No constraints were imposed during geometry optimizations.
The B3LYP functional,52,53 with the optimized def2-TZVP basis set
for Re and Cl and the def2-SVP basis set54,55 for all other atoms was
employed. The D3 version of Grimme’s dispersion method was
applied adopting the Becke−Johnson damping scheme.56 Gibbs free
energies were determined using thermal corrections for entropy and
enthalpy at 298 K to the electronic energies. In these calculations, the
computed harmonic frequencies were scaled by 0.965 to account for
anharmonicity. For radical anions, the unrestricted Kohn−Sham
formalism was adopted. The nature of all stationary points was
confirmed by normal-mode analysis.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The pmbpy ligand was
̈
prepared by the Krohnke reaction similarly to pdbpy by using
appropriately modified chalcone and pyridinium salts as
precursors to cyclization.16 In this work, higher yields were
obtained by introducing the methoxy precursor to the pmbpy
ligand as a part of the chalcone. Complex 1 was obtained as a
pure yellow solid from the reaction of Re(CO)5Cl with the
pmbpy ligand in toluene. The infrared (IR) spectrum confirms
the fac geometry with three strong CO stretching frequencies
at 2022, 1919, and 1895 cm−1.
Electrochemistry. Cyclic voltammetry (CV) and controlled
potential electrolysis (CPE) experiments were performed using a
BASi Epsilon potentiostat. Data were collected in dry acetonitrile with
tetrabutylammonium hexafluorophosphate Bu4NPF6 (0.1 M) as the
The 298 K 1H NMR spectrum of 1 is shown in Figure 1, and
NMR correlation spectroscopy (COSY) with all signal
assignments and the 13C spectrum are provided as Supporting
C
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
1
Figure 1. H NMR spectrum of [Re(pmbpy)(CO)3Cl] (1) in d6-
DMSO.
Information (Figures S1−S4). Notably, there are two sharp
resonances of unequal intensity at 9.96 and 9.89 ppm assigned
to the −OH proton. The total integration of these two peaks is
1, and they are present in the same ratio for each preparation
of the complex. The −OH resonances are shifted downfield
relative to the two resonances of Re-2OH at 9.60 and 9.56
ppm,13 but the similar separation suggests two environments
differing by comparable electronic effects. Additional evidence
for the proposed isomers is found in separate resonances of the
H5 singlets that appear in comparable ratios to the −OH
singlets (Figure S1). The carbonyl region of the 13C NMR
spectrum of 1 reveals six carbonyl resonances instead of three
in agreement with the presence of two isomers, and two sets of
resonances of several other carbon signals are also evident
(Figure S3).
The two possible isomeric structures were investigated by
DFT calculations (Figure 2). The form 1-cis is the most stable,
by 5.1 kJ mol−1, due to the interaction between the −OH
group and the Cl atom.
Figure 3. (A) Cyclic voltammogram of 0.4 mM 1 in Ar-saturated
CH3CN with 0.1 M Bu4NPF6 at a glassy carbon electrode, scan rate =
0.25 V s−1. (B) Current at peaks I and IV. (C) Current at peaks II and
III. (D) Ratios of currents at peaks I/IV and the sum of (II + III)/I.
(E) Peak potentials of reductions II and III.
Figure 2. Representation of the two isomers 1-cis and 1-trans
computed by DFT calculations.
Peak currents at waves I and IV are linearly dependent upon
ν1/2 consistent with diffusional processes (Figure 3B), but the
ratio of iI/iIV decreases from a maximum of ∼1.6 at slow scan
rates to ∼1 as the sweep rate is increased (Figure 3D). The
decreasing current ratio iI/iIV could be a simple effect of
different diffusion coefficients of the oxidized and reduced
species of the I/IV couple; however, further observations
below suggest the participation of a chemical step and an
additional reduction near wave I.
Peak currents at waves II and III are shown in Figure 3C,
and sample CV’s with variable scan rates are shown in Figure
S11. At the lowest limit of 0.01 V s−1, wave II is absent. As the
scan rate is increased, the current at wave II increases at the
expense of the current at wave III, and wave II shifts in
potential from about −2 V to −2.15 V, after which it begins to
overlap with wave III. The sum of peak currents at waves II
and III remains proportional to ν1/2 throughout the entire
range of sweep rates. The ratio of currents at (peaks II + III)/
peak I is ∼0.5 at 0.05 mV s−1 and increases to ∼1 at 4 V s−1.
Above 4 V s−1, it is difficult to differentiate currents from
overlapping processes II and III.
Intrigued by the NMR that evidenced a non-negligible
rotational barrier of the phenolic group, we searched for two
transition states connecting the two isomers. The first, TS1′,
may be thought to be obtained by the counterclockwise
rotation of the 1-cis phenolic group (i.e., the OH group is on
the opposite side of the Re(CO)3 moiety, see Figure 2), while
the second, TS1′′, may be thought to be reached by the
clockwise rotation of the 1-cis phenolic group (i.e., the OH
group is on the same side of the Re(CO)3 moiety). As
expected, the energy barrier of TS1′ (75.5 kJ mol−1, single
negative frequency at −28.8 cm−1) is lower than that of TS1′′
(95.0 kJ mol−1, single negative frequency at −35.0 cm−1),
because the rotation of the phenolic group occurs when the
OH moiety is far from Re(CO)3. The relatively high rotational
barrier accounts for the presence of the two isomers and hence
the splitting of NMR signals observed experimentally.
Electrochemistry. The cyclic voltammogram (CV) of 1 in
anhydrous acetonitrile exhibits three reduction processes
labeled I−III at intermediate sweep rates, ν (0.05 to 2 V s−1;
Figure 3). Reduction I is reversible with an associated
reoxidation wave IV and E1/2 = −1.70 vs Fc+/0. At a scan
rate of 0.25 V s−1 wave II is irreversible and wave III is
quasireversible, with peak potentials at −2.06 and −2.26 V,
respectively.
Multisweep voltammograms (Figure S12) show decreasing
current for reduction I and a complete loss of reduction II
without accumulation of any distinct redox processes. For
comparison, the multisweep CV of [Re(4,6-diphenyl-2,2′-
bpy)(CO)3Cl]11 is also shown. This complex differs only by
the absence of the −OH group and represents the electro-
D
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Scheme 2. Proposed Electrochemical Mechanisms for Reduction of 1 (Species Observed by IR-SEC Labeled in Red)
chemical behavior expected of [Re(bpy)(CO)3Cl]-type
complexes. The dominant reductive pathway follows an EEC
mechanism with a reversible [Re(bpy)(CO)3Cl]0/− couple and
an irreversible [Re(bpy)(CO)3Cl]−/2− couple followed by
chloride dissociation to produce [Re(bpy)(CO)3]−,15 although
a competing ECE pathway with a relatively slow dissociation of
the chloride from the singly reduced radical is possible as
well.57 Dimerization of the singly reduced species is
possible,57,58 but we did not observe characteristic features of
dimerization during reduction or reoxidation, especially at a 0.4
mM analyte concentration. Multiple scans of the [Re(4,6-
diphenyl-2,2′-bpy)(CO)3Cl]0/− couple (Figure S12B) verified
its reversibility while multiple scans through the second
reduction showed the accumulation of the solvento complex
indicated by the appearance of the reversible [Re(4,6-diphenyl-
2,2′-bpy)(CO)3(CH3CN)]+/0 couple, which typically appears
at less cathodic potentials than the reduction of the chloro
complex.11,59 CVs of 1 measured at 195 K showed only the
typical EEC mechanism (Figure S13).
mechanism in which the chemical step is a composite net H
atom loss (reductive deprotonation) and chloride dissociation
to produce the phenolate-ligated 1-OPh, and characterization
of this species will be presented below. As a neutral Re(I)
complex with an anionic ligand, 1-OPh, will have similar
electronic structure and reduction potential to 1, therefore the
high iI/iIV current ratio (Figure 3D) at slow scan rate is
attributed to further reduction to 1-OPh•− after the chemical
step.
At fast or slow scan rates, peak III is present without
significant changes in potential or current. Its assignment is
proposed as the 1-OPh−/2− couple, which is a shared final
electron transfer step on both pathways, namely, EECEC′ and
ECEEC′. The shared chemical step indicates that both
mechanisms provide 1-OPh•− prior to the final step with the
difference being whether the Cl− dissociation and H atom loss
occur after one or two initial reductions of the complex. The 1-
OPh−/2− couple is most accurately described as an EC process
itself because dissociation of the phenolate is proposed (see
below).
Collectively, these observations indicate reductive mecha-
nisms differing from the usual EEC pathway. Scan-rate-
dependent currents at waves I, II, and III suggest the
involvement of one or more chemical steps proposed in
Scheme 2, and evidence for the assignments are provided by
CV (Figures 3, S11−S13), IR-SEC (Figures 5, 6, S18, S21),
chemical reduction with sodium amalgam (Figure S17), and
chemical deprotonation of 1 (Figure S16). The potential of the
first reduction and its reversible nature are similar to related
complexes,11,59,60 allowing assignment of the I/IV couple as
[Re(pmbpy)(CO)3Cl]0/− (or 10/−). Subsequent competing
mechanisms can be isolated at the fast and slow scan rate
extremes and assignments made by comparison to [Re(4,6-
diphenyl-2,2′-bpy)(CO)3Cl] (RediPh). At 1 V s−1, peak II is
well-resolved from peak III, and the current ratio iII/iI of ∼0.5
is similar to the ratio observed for the irreversible RediPh−/2−
reduction at −2.1 V vs Fc+/0. This process in the reference
compound also shifts positive by ∼100 mV as the scan rate is
decreased 100 mV s−1 due to its irreversible nature. Peak III is
over 150 mV more negative than the RediPh−/2− reduction,
ruling out an analogous assignment to peak III. Therefore, the
irreversible peak II is assigned to the reduction of [Re-
(pmbpy)(CO)3Cl]− followed by fast chloride dissociation to
produce [1 − Cl−]− (Scheme 2) by the standard EEC
mechanism through wave II.
The proposed reductive mechanism was simulated using
Digisim61 software, and parameters were adjusted until
reasonable models of the data collected at 100 and 1000 mV
s−1 were produced (Figure 4 and S14). A perfect simulation
Figure 4. Experimental (black) and simulated (red) reductive
voltammograms of 1 according to the mechanism in Scheme 2 with
kCa = 920 s−1 for the EECaEC′ pathway and kCb = 0.43 s−1 for the
ECbEEC′ pathway. See Figure S14 for additional details.
was not expected for several reasons. The initial state of 1
exists as two isomers with potentially different rates of
chemical steps. The low solubility of 1 may cause a more
pronounced effect of a slightly sloped baseline since the
software models capacitance as a constant value. Finally, the
chemical steps of chloride dissociation, net H atom loss (1/2
H2), and phenolate coordination are modeled as a single step
The diminished CV current of peak II as the scan rate is
slowed suggests that a chemical reaction of modest rate
consumes the singly reduced complex, 1•−, on the time scale of
the CV experiment (Figure S11). At 10 mV s−1, peak II is
absent showing complete consumption of 1•− by the alternate
pathway. The nature of this pathway is proposed as an ECE
E
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
with equilibrium Keq and a rate determining “k”, while in reality
heterogeneous or homogeneous electron transfers of any
putative intermediates are possible and may contribute to wave
distortions. Nevertheless, the model shown in Figure 4
captures the experimental properties well, and the EEC
portion of the fast scan pathway in Scheme 2 (kCa = 920
s−1) was simulated in good agreement with the reference
compound [(4,6-diphenyl-2,2′-bipyridine)Re(CO)3Cl] (Fig-
ure S15) for which kf_Cl‑loss = 900 s−1.
The simulated mechanism reproduces the scan rate
dependent potential and current of peak II. The potential
shifts due to the irreversible reaction that drives the
equilibrium toward the product, and the current is low at
slow scan rates due to the competing ECEEC′ pathway, with
kCb = 0.43 s−1. Both chemical steps represent the same
processes occurring at species differing by an additional
reduction, and the extra charge accelerates the reaction rate by
over 2000-fold. The simulated mechanism shows that E1/2 for
the reduction of 1-OPh along the ECEEC′ pathway is only 30
mV more negative than that of 1. The implications of these
comparable potentials will be discussed in the context of
spectroelectrochemistry and preparative electrolysis.
Infrared Spectroelectrochemistry (IR-SEC). The reduc-
tive electrochemical mechanism was investigated by infrared
spectroelectrochemistry (IR-SEC) in N2-saturated CD3CN
where the improved solvent window relative to CH3CN allows
observation of the changes in carbonyl stretching frequencies
as well as low energy bands associated with aromatic
vibrational modes.
Electrolysis at the onset of the first reduction (−1.5 V vs
Fc+/0) resulted in an immediate bleach of the vibrational bands
of 1 and the simultaneous appearance of a small band at 2002
cm−1 and a pronounced set of peaks at 2013, 1907, and 1884
cm−1 (Figure 5 and Table 1). These peaks are convoluted with
the bleach of 1; however, the clean spectrum was obtained by
other means discussed below. With reference to previous work,
the band at 2002 cm−1 is tentatively assigned to the radical
anion, 1•−, which should have additional bands near 1890 and
1970 cm−1.13,62,63 The species with νCO = 2013, 1907, and
1884 cm−1 is assigned as 1-OPh along the proposed ECEEC′
pathway. The complex is a neutral Re(I) species, and these
data are in good agreement with the singly reduced form of
[Re(pdbpy)(CO3)Cl], which underwent the same proposed
reaction. The frequencies of this species are similar to the
reference data63 of neutral radical solvento complexes,
[Re(bpy)(CO)3(CH3CN)]•, but this possibility was ruled
out by an acid/base reaction of 1 with (iPr)2NH, which cleanly
produced 1-OPh (Figure S16). Moreover, solvento radicals
often exhibit two broad IR bands experimentally, and the
computed IR frequencies of the corresponding neutral radical
solvento complex [Re(pmbpy)(CO)3(CH3CN)]• (2014,
1918, 1899 cm−1) do not fit. The IR spectrum of 1-OPh in
the low energy IR region matches well the DFT computed
spectrum of this species (Figure 6). Vibrations around 1600
cm−1 can be assigned to C−C stretches of the bpy ligand. The
band at 1532 cm−1 is mainly due to C−C stretching modes of
the bpy-Ph ligand. Two peaks at 1479 and 1469 cm−1 are in-
plane C−H bending modes of bpy-Ph, and the 1400 cm−1 peak
is due to strongly coupled C−C and C−O stretching modes of
the phenolate ligand. While overall charge and the oxidation
state of the metal center in 1 and 1-OPh are the same,
observed shifts of ligand vibrational modes are consistent with
significant distortion of phenyl-bpy ligand due to the binding
Figure 5. Difference IR spectra (expanded metal carbonyl region)
measured during controlled potential electrolysis of 1 in CD3CN
containing 0.1 M Bu4NPF6 under a N2 atmosphere. Applied
potentials: (A) −1.5 V vs Fc+/0; (B) −1.7 V vs Fc+/0; and (C)
−2.3 V vs Fc+/0
.
of the phenolate to the Re metal center. This observation is
also consistent with significant shifts of 1H NMR signals of the
bpy ligand upon the formation of 1-OPh (see below).
Stepping the electrolysis potential by only −100 mV to −1.6
V immediately produced a mixture of 1•−, 1-OPh, and a new
species with νCO = 1990, 1872, and 1858. The new species,
assigned as 1-OPh•−, is the sole product of electrolysis at −1.7
V, which is very close to E1/2 of the 10/− couple in agreement
with simulations. Here, the parallel between 1-OPh, 1-OPh•−,
and the reductively deprotonated imidazolate complex and its
subsequent radical in the literature is noteworthy (Table 1).64
F
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Table 1. Selected Experimental and Calculated νCO
Vibrational Frequencies of 1, Its Reduced Forms, and
a
Related Complexes in Acetonitrile
νCO/cm−1
experimental
calculated
[Re(pmbpy)(CO)3Cl] (1)
2022, 1919,
1895
2022, 1921,
1904
cis
2018, 1917,
1895
2001, 1893,
1877
trans
cis
[Re(pmbpy)(CO)3Cl]•−
2002
(1•−
)
1997, 1889,
1868
trans
1-OPh
2013, 1907,
1884
1990, 1872,
1858
2011, 1903,
1891
1989, 1873,
1864
1-OPh•−
1-PhO(CH3CN)2−
1-PhO2−
1995, 1873,
1858
1970, 1874,
1843
1935, 1844,
1823
cis
cis
[Re(bpy)
2018, 1908a
1995, 1878a
(CO)3(imidazolate)]
[Re(bpy)
(CO)3(imidazolate)]•−
1-CH3CN+
2039, 1942,
1926
2022, 1912,
1893
2000, 1909,
1876
2036, 1946,
1929
2014, 1918,
1899
2005, 1909,
1881
cis
cis
cis
1-CH3CN•
1-PhO(CH3CN)•−
a
All assignments are also made in analogy with species already
Figure 6. (A) Difference IR spectra (expanded ligand region)
measured during controlled potential electrolysis of 1 at −1.5 V vs
Fc+/0 in CD3CN containing 0.1 M Bu4NPF6 under a N2 atmosphere.
(B) Difference IR spectrum obtained from subtraction of DFT
calculated spectrum of 1-OPh from calculated spectrum of 1.
Wavenumber axis is scaled by a factor of 0.965.
reported in the literature.13,14,65
The reductive chemistry was further probed by briefly
electrolyzing the sample, stopping electrolysis, and continuing
IR data collection. These experiments show that 1•−
dissociates chloride and converts to 1-OPh, without
electrolysis verifying the chemical nature of this step.
for the reduction to 1-PhO(CH3CN)2−. Both 1-OPh•− and 1-
PhO(CH3CN)2− are expected to have ligand radical character,
and similar transitions are observed for both species. Identities
of each step were confirmed by stopping the experiment at
appropriate stages and transferring the sample to a sealed IR
cell under an inert atmosphere. The νCO’s were identical to
those obtained during electrolysis. 1-PhO(CH3CN)2− is
strongly reducing and slowly oxidized back to 1-OPh•−.
Separately, 1-OPh•− was oxidized to 1-OPh by exposure to air.
The IR and amalgam data support the proposed “slow scan”
EECEC′ pathway of Scheme 2 in which 1•− undergoes
reductive deprotonation and dissociation of Cl− to produce 1-
OPh. The mechanism of reductive deprotonation is unclear
and is counterintuitive because a reduced complex should be
more basic, not more acidic, but there are now several
According to CV data, 1-OPh•− is reduced at −2.26 V (peak
III). IR-SEC at this potential induced small blue shifts in νCO
to 1995, 1873, and 1858 cm−1 inconsistent with calculated
values of 1960, 1839, and 1830 cm−1 for 1-OPh2− with
retention of the chelate or of 1935, 1844, and 1826 cm−1 for
the 5-coordinate complex 1-PhO2− with a dissociated Re−O
bond. Instead, we propose that the product of peak II is the
solvento complex 1-PhO(CH3CN)2− with a dangling
phenolate group. While dissociation of the anionic ligand is
expected, the affinity for solvent in this highly reduced state is
unexpected and has implications regarding catalysis as will be
discussed below. Computed frequencies of the final product
are an imperfect match. However, it is worth mentioning that
even a single CH3CN molecule explicitly included in the
calculations has a significant effect on the computed
frequencies, and an extensive survey of other possibilities did
not provide promising alternatives.
precedents of this reaction in the literature.13,14,39,43,66
A
theoretical approach to understanding the mechanism in Ru
and Re complexes proposed a pathway involving protonation
of the bpy ligand at carbon followed by disproportionation
with a net loss of 1/2 H2.43,67 In agreement with the loss of H2
and not H+, electrolysis of 1 under Ar produced ∼0.45 equiv of
H2.
Reduction of 1 to 1-OPh, 1-OPh•−, and 1-PhO(CH3CN)2−
was also investigated using sodium amalgam with UV−vis
detection (Figure S17). The first reaction cleanly produced 1-
OPh as a pale-yellow solution after reaction of the phenol with
sodium and substitution of chloride for phenoxide. Reduction
to 1-OPh•− was indicated by intense structured bands at 500
nm and a broad band at 650 nm (the complex is dark purple).
A final change indicated by a shift in isosbestic points and
characterized by growth of radical absorptions was observed
To investigate the mechanism and any role of Cl− in the
reductive deprotonation, CV’s and IR-SEC of the solvento
complex, 1-CH3CN+, were measured. The CV (Figure S20)
shows a reversible 1-CH3CN+/0 couple at −1.48 V, and the 1-
OPh0/− couple expected at −1.65 V confirms formation of the
chelate in the absence of chloride. The IR-SEC (Figure S21)
G
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
confirmed the existence of the neutral solvent radical 1-
CH3CN• and interestingly showed that this species is present
at a much higher concentration than 1-Cl•− upon initial
reduction, although 1-OPh is also present immediately. This
observation suggests that the rate of reductive deprotonation is
attenuated by the less cathodic standard potential for the
solvento complex and may support the previously proposed
pathway involving ligand-based protonations. Stepping the
potential resulted in the presence of a minor species with red-
shifted νCO consistent with deprotonation prior to coordina-
tion, i.e., 1-PhO(CH3CN)•− (2000, 1909, 1876 cm−1),
although this species was observed as a mixture, and no clear
CV wave was isolated. Changes during further reduction were
identical to the experiment starting with 1.
During reduction of 1, the immediate deprotonated species
with a dangling phenolate was not observed by IR. Therefore,
we cannot determine whether formation of the phenolate or
chloride dissociation is rate limiting; however, it is clear that a
combination of H2 loss and the chelate effect of 1-OPh
destabilize the singly reduced 1•−, which is observed only in
small concentrations by IR-SEC. The IR data also confirm that
1-OPh is reduced at a very similar potential to 1, which is
logical considering their identities as neutral Re(I) complexes.
The final reduction observed by CV(III) produces 1-
PhO(CH3CN)2−. This couple is observed at fast and slow
rates confirming that the chemical step of the fast scan
EECEC′ is a composite Cl− dissociation, CH3CN binding, and
reductive deprotonation.
In summary, the electrochemical mechanisms as observed by
CV and characterized by IR-SEC and Na-Hg reduction occur
by competing EECEC′ (waves I, II, C, III, C′; isolated at fast
scans) and ECEEC′ (I, C, I′, III, C′; isolated at slow scans)
pathways with the chemical steps in the slow limit being a
composite of reductive deprotonation, chloride dissociation,
and oxygen coordination to the metal center. Importantly, the
I/IV couple of 10/− includes the 1-OPh0/− couple at slow scan
rates and after a full three-electron sweep, peak IV is the
reoxidation of the 1-OPh•−, not 1•−.
Isolation of Complex 1-OPh. As discussed in the
Introduction, one hypothesis of this work was that a simplified
system with a single pendent −OH group could confirm the
validity of the mechanism shown in Scheme 1 by obtaining
additional characterization of the species assigned as 1-OPh.
The IR-SEC data, supported by DFT calculations, suggest the
presence of this intermediate after reduction of 1 similarly to
the reduction of [Re(pdbpy)(CO)3Cl]. As a neutral Re(I)
complex, 1-OPh should be sufficiently stable for preparation
by electrolysis and analysis by NMR.
In order to accomplish this goal, we took advantage of the
clean formation of 1-OPh as ascertained by IR-SEC and
increased the reaction volume to the preparative scale. Upon
bulk electrolysis in CH3CN at the first reduction potential in a
N2-filled glovebox, the solution changed color from yellow to
dark green and the flow of current ceased. It is known from the
amalgam experiment that the dark green solution is an
indication of slight over reduction. Once the electrolysis was
completed, an IR spectrum showed the majority presence of 1-
OPh with a small amount of 1-OPh•− (Figure S18). The CV
recorded after bulk electrolysis also confirmed completion of
the reaction (Figure 7A). The second reduction peak, assigned
as the irreversible reduction of [Re(pmbpy)(CO)3Cl]− is
absent, and the 1-OPh0/− couple is observed to be nearly
identical to the 10/−couple. The solvent was removed under
Figure 7. (A) CVs of 1 in acetonitrile/Bu4NPF6 before and after
controlled potential electrolysis (CPE at −1.8 V vs Fc+/0) at 100 mV
s−1. The working electrode was a 5 mm glassy carbon electrode. (B)
1H NMR spectra of 1-OPh in d6-DMSO.
reduced pressure to afford a solid, which was poorly soluble in
CH3CN but sufficiently soluble in d6-DMSO for NMR
characterization.
1
The H NMR spectrum of the resultant product 1-OPh
(Figure 7) reveals the absence of the −OH resonances and
distinct changes in the chemical shifts of the protons of the
phenolate ring (see SI for additional data, Figures S5−S8).
Chemical shifts of the protons of the bipyridine rings also shift
significantly, presumably due to the strained puckered
conformation of the tridentate chelate relative to the planar
orientation in 1. The carbonyl region of the 13C spectrum
(Figure S7) shows three unique CO resonances relative to
those of 1. A new and distinct resonance is observed at 172
ppm in a frequency region where the 13C spectrum of 1 has no
signals. This new resonance is tentatively assigned to the
quaternary carbon atom connected to the phenolate oxygen,
which now coordinates the rhenium metal. Its appearance at
172 ppm is consistent with CO character of the phenolate
resonance structure. Another significant difference from
compound 1 is the absence of signals below 118.50 ppm due
to the different chemical environment experienced by CH
groups in the phenolate ring of 1-OPh.
Reactivity toward CO2. Complex 1 was evaluated as a
homogeneous electrocatalyst for CO2 reduction in CH3CN by
CV and bulk electrolysis. In the presence of CO2, the CV
shows a catalytic current beginning at wave II and an increase
in slope at wave III (Figure 8). The solution of 1-OPh
prepared in situ shows similar behavior in Figure 7. The nature
of this current enhancement was investigated at various scan
rates. Eq 1 is derived for the condition of kinetic control with
no substrate consumption (i.e., the rate of consumption is
countered by the rate of diffusion), where the catalytic current
is “S-shaped” and ic is read as the plateau current.68 According
to eq 1, where n = 2, F is Faraday’s constant, A is the electrode
area, D is the diffusion coefficient, k is the rate constant, and a
is the reaction order with respect to CO2; an electrocatalytic
current should be scan-rate independent provided the
necessary conditions are met.
H
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Table 2. Bulk Electrolysis Data of 0.5 mM Solution of 1 in
Acetonitrile
a
b
c
E/V
time/min
acid
TONCO
FECO%
−2.0
−2.0
−2.0
−2.0
−2.3
75
160
60
150
100
1.4
3
1.2
4
100
98
100
104
102
H2O
CH3OH
phenol
phenol
9
a
b
c
V vs Fc+/0. Time before activity ceased. 5% by volume.
PhO(CH3CN)2−. Evidence for this reaction was found during
chemical reduction in which samples partially reduced from 1-
OPh to 1-OPh•− showed small shoulders assignable to 1-
PhO(CH3CN)2−. This equilibrium is uphill by 51.7 kJ mol−1
and inhibits catalysis at peak II. Direct reduction of 1-OPh•− at
peak III produces 1-PhO(CH3CN)2−, which explains the
increased catalytic current at peak III; however, overall
catalysis is inefficient for two reasons: (1) the strong affinity
of the dianion for solvent and (2) reactivity of CO2 at the
oxyanion. The latter is predicted by the Mulliken and Hirshfeld
atomic charges of 1-PhO2−, which predict that the phenolate is
the most nucleophilic site. The reaction with CO2 is mildly
endergonic, but the rearrangement to the chelating carbonate
1-OCOOPh2− is exergonic by 21.2 kJ mol−1. Identification of
the precise pathway for CO production is computationally
uncertain; however, we propose that CO2 which interacts with
the Re center of 1-PhO2− may convert to CO via standard
pathways.29 The 1-OCOOPh2− may release CO; however, this
species is likely stable and may reduce catalytic rates. The
“slow one-electron pathway”29 is also characterized by an
intermolecular Re−O bond (which is stronger than Re−C
bond), but 1-OCOOPh2− displays an entropically stabilized
intramolecular Re−O bond. The IR-SEC of 1 under a CO2
atmosphere produces spectra very similar to those observed
under N2, indicating that the steady state concentration of any
CO2 adduct intermediate is below the detection limit of the
experiment.
Figure 8. (A) CVs of 0.4 mM 1 in Ar or CO2-saturated CH3CN with
0.1 M Bu4NPF6 at a glassy carbon electrode, scan rate = 50 mV s−1
.
(B) Current at peaks IICO and IIICO
.
2
2
ic = nFA[cat] D(k)[CO2]a
(1)
Data in Figure 8, panel B, show that the additional current
observed under CO2 is proportional to ν1/2 over the entire
range of data collection, suggesting that the electrochemical
currents here are controlled by electron transfers (eq 2), and
that catalysis is slow on the CV time scale. For comparison, the
same experiment was performed on the reference compound,
[(4,6-diphenyl-2,2′-bipyridine)Re(CO)3Cl], which shows a
catalytic current that becomes scan rate-independent above 2
V s−1 (Figure S22). The total current at IICO and IIICO
Catalytic performances of Re-2OH with respect to 1 are
only marginally better, yielding a TONCO of 14, whereas the
presence of three OH groups in meta and para phenolic
positions decrease the catalytic activity toward CO2 reduc-
tion,13 thus confirming that despite the close proximity of the
OH group to the metal center in which chelation of a
carbonate can be inhibitive, the close metal−OH proximity
also has beneficial effects. Similar proximity conclusions were
drawn by Bocarsly on Mn-1OH, which showed enhanced
reactivity attributed to better hydrogen bonding compared to
related congeners with distal OH groups.23 Mn(bpy)(CO)3Br
reported by Deronzier et al.17 displayed a TONCO of 13, while
Mn-1OH showed a reduced TONCO value of 2.7.45
Conversely, our Mn-2OH14 yielded a higher TONCO (28)
2
2
relative to peak I is a maximum of 4 at slow scan rates but
converges to 3 as the sweep rate exceeds 250 mV s−1,
suggesting that CO2 may bind to the metal in a reduced state,
but a competing reaction is limiting the turnover frequency.
FνD
RT
ip = 0.4463FA[cat]
(2)
Bulk electrolysis in CO2-saturated CH3CN was performed to
examine the products of catalysis. The potential was cathodic
of peak II or at peak III, and experiments were performed in
dry solvent or with 5% Brønsted acids (water, methanol and
phenol). Table 2 summarizes results obtained by these CPE
experiments. Fewer than three turnovers were obtained in dry
solvent or in the presence of water or methanol. An experiment
in the presence of phenol performed slightly better, yet
turnovers were underwhelming. Electrolysis at wave III (−2.3
V vs Fc+/0) yielded nine turnovers; however, the catalyst
ceased activity faster at this strongly reducing potential.
The catalytic reactivity is interpreted according to Scheme 3.
When 1-OPh is reduced to 1-OPh•−, a small amount of the
solvento complex, 1-PhO(CH3CN)•−, may exist in equili-
brium. At the potential of the 1-OPh0/− couple, the solvento
complex will be reduced immediately to the dianion, 1-
−
together with formate production (TONHCOO 12). Interest-
ingly their predicted catalytic mechanism involves the
formation of a hydrogen bond between the phenolic proton
and the Mn-bound CO2 molecule, followed by facile proton-
assisted C−O bond cleavage of the CO2 to form CO.23,45
Contrarily, our investigations on Mn-2OH show that the
reductive deprotonation of phenolic OH takes place before
CO2 binding to the Mn center, similar to complex 1 in the
present study. Meanwhile, the effect of a second coordination
sphere69 has been investigated for a Mn complex,
{MnI([(MeO)2Ph]2bpy)(CO)3(CH3CN)}(OTf), reported by
I
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Scheme 3. Proposed Electrochemical Mechanisms for Reduction of 1 in the Presence of CO2
Rochford et al.,70 where [(MeO)2Ph]2bpy = 6,6′-bis(2,6-
dimethoxyphenyl)-2,2′-bipyridine. Together with the steric
influence preventing dimerization,71 the four pendent methoxy
groups exhibit weak hydrogen bonding interaction, playing a
significant role during CO2 reduction. Complexes 1, Re-2OH,
and Mn-2OH prevent the dimerization too, by forming the
intramolecular chelating M−O (M = Re, Mn) bond, but the
resulting Re−O bond appears to be strong enough to limit the
catalytic activity toward CO2 reduction, or formation of a
carbonate may also limit catalysis. On the other hand, Mn-
based catalysts provide better TON values than their precious
metal congeners, probably because greater strain in the Mn−O
bond leads to less stability of the intermediate.
electrocatalytic properties of these two complexes were deeply
investigated in our previous works. As we previously
concluded, the effect of the local proton source is more
beneficial in terms of electrocatalytic turnovers by Mn
compared to Re and also changes the selectivity for CO and
formate. Previous conclusions are consistent with the low CO
turnover numbers of 1 and the absence of formate as a
reduction product. We can conclude that a reductive
deprotonation of 1 affords 1-OPh, evidencing the capability
of the pendent OH group to behave as a chelating ligand. Two
subsequent reductions are needed to provide the active
catalyst; however, a small equilibrium dissociation of the
Re−O bond after the first reduction provides a minor pathway
for catalysis at a lower overpotential. The active catalyst is a
dianion with a dangling phenolate that strongly favors solvent
coordination over a five-coordinate intermediate. This
equilibrium disfavors catalysis, and calculations suggest that
the oxyanion is more nucleophilic toward CO2 than the Re(0)
center. Formation of a stable carbonate and metal chelation
lead to diminished CO2 interaction with the reduced
complexes consistent with small CV catalytic currents and a
low TONCO under CO2. The mechanistic characterization and
comparison to related complexes highlight the importance of
considering deleterious reactivity by coordination or ligand-
based reactivity of CO2 when designing catalysts with pendent
acid/base functionality in the second coordination sphere;
however, the greater reactivity of the same ligand set on Mn is
a promising observation considering that the (L)M(CO)3X
motif is one of a few examples where first and third row metal
congeners with the same ligand set undergo similar CO2
conversion reactions.
CONCLUSIONS
■
This study provides an intimate understanding of the
electrochemical behavior of fac-Re(pmbpy)(CO)3Cl (1).
NMR data suggest the existence of two different isomers, 1-
cis characterized by the proximity of Cl and OH moieties and
1-trans, where Cl and OH are oriented trans to each other.
DFT calculations confirmed that a non-negligible energy
barrier separates the two isomers and reveal two transition
states, TS1′ and TS1′′, with activation energy of 75.5 and 95.0
kJ mol−1, respectively. By experimental evidence, we confirmed
the tendency toward the formation of 1-OPh bearing a Re−O
intramolecular bond. 1-OPh has been synthesized by
controlled potential electrolysis and characterized by 1H
NMR spectroscopy, which revealed the absence of the OH
resonances and distinct changes in the chemical shifts of the
protons of the phenolate and bpy rings when compared to 1
due to coordination of the phenolate and a puckered distortion
of the bpy backbone. The IR spectrum of isolated 1-OPh
exhibits three new CO stretching bands (2013, 1907, 1884
cm−1) which exactly overlap with the bands obtained after the
first 1e− reduction during IR-SEC, in agreement with DFT
calculations. The CV recorded after bulk electrolysis added
further evidence of the formation of 1-OPh and confirmed
competition between EECEC′ and ECEEC′ pathways for the
reduction of 1. After electrolysis, the peak assigned to the
irreversible reduction of 1•− is absent, and the 1-OPh0/−
couple is observed at a potential nearly identical to the 10/−
redox couple. Similar intermediates, as depicted in Scheme 1,
were already proposed to be formed during electrochemical
reductions of the closely related Re-2OH and Mn-2OH. The
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01181.
Data include NMR spectra, cyclic voltammograms, UV−
vis absorption and IR spectra, sodium amalgam
reduction and DFT calculations (PDF)
J
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
CO2 reduction: reaction of Re(dmb)(CO)3COOH with CO2. Chem.
Commun. 2012, 48 (54), 6797−6799.
AUTHOR INFORMATION
Corresponding Authors
■
(7) Armaroli, N.; Balzani, V. Solar Electricity and Solar Fuels: Status
and Perspectives in the Context of the Energy Transition. Chem. - Eur.
J. 2016, 22 (1), 32−57.
Carlo Nervi − Chemistry Department, University of Torino,
10125 Torino, Italy; CIRCC (Bari), University of Bari, 70126
Bari, Italy; orcid.org/0000-0002-3712-7369;
Email: carlo.nervi@unito.it
Gerald F. Manbeck − Chemistry Division, Brookhaven National
Laboratory, Upton, New York 11973-5000, United States;
orcid.org/0000-0002-6632-3895; Email: gmanbeck@
bnl.gov
(8) Hawecker, J.; Lehn, J. M.; Ziessel, R. Electrocatalytic reduction
of carbon dioxide mediated by Re(bpy)(CO)3Cl (bipy = 2,2’-
bipyridine). J. Chem. Soc., Chem. Commun. 1984, No. 6, 328−330.
(9) Costentin, C.; Robert, M.; Saveant, J.-M. Catalysis of the
electrochemical reduction of carbon dioxide. Chem. Soc. Rev. 2013, 42
(6), 2423−2436.
(10) Ronge, J.; Bosserez, T.; Martel, D.; Nervi, C.; Boarino, L.;
Taulelle, F.; Decher, G.; Bordiga, S.; Martens, J. A. Monolithic cells
for solar fuels. Chem. Soc. Rev. 2014, 43 (23), 7963−7981.
(11) Rotundo, L.; Azzi, E.; Deagostino, A.; Garino, C.; Nencini, L.;
Priola, E.; Quagliotto, P.; Rocca, R.; Gobetto, R.; Nervi, C. Electronic
Effects of Substituents on fac-M(bpy-R)(CO)3 (M = Mn, Re)
Complexes for Homogeneous CO2 Electroreduction. Front. Chem.
2019, 7, DOI: 10.3389/fchem.2019.00417.
(12) Rotundo, L.; Filippi, J.; Gobetto, R.; Miller, H. A.; Rocca, R.;
Nervi, C.; Vizza, F. Electrochemical CO2 reduction in water at carbon
cloth electrodes functionalized with a fac-Mn(apbpy)(CO)3Br
complex. Chem. Commun. 2019, 55 (6), 775−777.
(13) Rotundo, L.; Garino, C.; Priola, E.; Sassone, D.; Rao, H.; Ma,
B.; Robert, M.; Fiedler, J.; Gobetto, R.; Nervi, C. Electrochemical and
Photochemical Reduction of CO2 Catalyzed by Re(I) Complexes
Carrying Local Proton Sources. Organometallics 2019, 38 (6), 1351−
1360.
(14) Franco, F.; Cometto, C.; Nencini, L.; Barolo, C.; Sordello, F.;
Minero, C.; Fiedler, J.; Robert, M.; Gobetto, R.; Nervi, C. Local
Proton Source in Electrocatalytic CO2 Reduction with [Mn(bpy-
R)(CO)3Br] Complexes. Chem. - Eur. J. 2017, 23 (20), 4782−4793.
(15) Franco, F.; Cometto, C.; Garino, C.; Minero, C.; Sordello, F.;
Nervi, C.; Gobetto, R. Photo- and Electrocatalytic Reduction of CO2
by [Re(CO)3{a,a’-Diimine-(4-piperidinyl-1,8-naphthalimide)}Cl]
Complexes. Eur. J. Inorg. Chem. 2015, 2015 (2), 296−304.
(16) Franco, F.; Cometto, C.; Ferrero Vallana, F.; Sordello, F.;
Priola, E.; Minero, C.; Nervi, C.; Gobetto, R. A local proton source in
a [Mn(bpy-R)(CO)3Br]-type redox catalyst enables CO2 reduction
even in the absence of Brønsted acids. Chem. Commun. 2014, 50,
14670−14673.
(17) Bourrez, M.; Molton, F.; Chardon-Noblat, S.; Deronzier, A.
Mn(bipyridyl)(CO)3Br: An Abundant Metal Carbonyl Complex as
Efficient Electrocatalyst for CO2 Reduction. Angew. Chem., Int. Ed.
2011, 50 (42), 9903−9906.
(18) Bourrez, M.; Orio, M.; Molton, F.; Vezin, H.; Duboc, C.;
Deronzier, A.; Chardon-Noblat, S. Pulsed-EPR Evidence of a
Manganese(II) Hydroxycarbonyl Intermediate in the Electrocatalytic
Reduction of Carbon Dioxide by a Manganese Bipyridyl Derivative.
Angew. Chem., Int. Ed. 2014, 53 (1), 240−243.
(19) Walsh, J. J.; Neri, G.; Smith, C. L.; Cowan, A. J. Water-Soluble
Manganese Complex for Selective Electrocatalytic CO2 Reduction to
CO. Organometallics 2019, 38 (6), 1224−1229.
(20) Franco, F.; Pinto, M. F.; Royo, B.; Lloret-Fillol, J. A Highly
Active N-Heterocyclic Carbene Manganese(I) Complex for Selective
Electrocatalytic CO2 Reduction to CO. Angew. Chem., Int. Ed. 2018,
57 (17), 4603−4606.
(21) Sinopoli, A.; La Porte, N. T.; Martinez, J. F.; Wasielewski, M.
R.; Sohail, M. Manganese carbonyl complexes for CO2 reduction.
Coord. Chem. Rev. 2018, 365, 60−74.
Authors
Laura Rotundo − Chemistry Department, University of Torino,
10125 Torino, Italy; CIRCC (Bari), University of Bari, 70126
Bari, Italy
Dmitry E. Polyansky − Chemistry Division, Brookhaven
National Laboratory, Upton, New York 11973-5000, United
States
Roberto Gobetto − Chemistry Department, University of
Torino, 10125 Torino, Italy; CIRCC (Bari), University of Bari,
70126 Bari, Italy; orcid.org/0000-0002-2431-8051
David C. Grills − Chemistry Division, Brookhaven National
Laboratory, Upton, New York 11973-5000, United States;
orcid.org/0000-0001-8349-9158
Etsuko Fujita − Chemistry Division, Brookhaven National
Laboratory, Upton, New York 11973-5000, United States;
orcid.org/0000-0002-0407-6307
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01181
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.N., R.G., and L.R. gratefully acknowledge MIUR for financial
support (PRIN 2015, Towards a Sustainable Chemistry:
Design of Innovative Metal−Ligand Systems for Catalysis
and Energy Applications). Work performed at Brookhaven
National Laboratory was supported by the U.S. Department of
Energy (DOE), Office of Science, Office of Basic Energy
Sciences, Chemical Sciences, Geosciences, and Biosciences
Division under contract DE-SC0012704. A part of L.R.’s stay
at BNL was also supported by the above DOE grant.
REFERENCES
■
(1) Nakada, A.; Ishitani, O. Selective Electrocatalysis of a Water-
Soluble Rhenium(I) Complex for CO2 Reduction Using Water As an
Electron Donor. ACS Catal. 2018, 8 (1), 354.
(2) Kuramochi, Y.; Ishitani, O.; Ishida, H. Reaction mechanisms of
catalytic photochemical CO2 reduction using Re(I) and Ru(II)
complexes. Coord. Chem. Rev. 2018, 373, 333−356.
(3) Takeda, H.; Cometto, C.; Ishitani, O.; Robert, M. Electrons,
Photons, Protons and Earth-Abundant Metal Complexes for
Molecular Catalysis of CO2 Reduction. ACS Catal. 2017, 7 (1),
70−88.
(4) Yamazaki, Y.; Takeda, H.; Ishitani, O. Photocatalytic reduction
of CO2 using metal complexes. J. Photochem. Photobiol., C 2015, 25,
106.
(5) Wang, W. H.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.;
Fujita, E. CO2 Hydrogenation to Formate and Methanol as an
Alternative to Photo- and Electrochemical CO2 Reduction. Chem. Rev.
2015, 115 (23), 12936−73.
(6) Agarwal, J.; Sanders, B. C.; Fujita, E.; Schaefer, H. F.; Harrop, T.
C.; Muckerman, J. T. Exploring the intermediates of photochemical
(22) Hu, X.-M.; Hval, H. H.; Bjerglund, E. T.; Dalgaard, K. J.;
Madsen, M. R.; Pohl, M.-M.; Welter, E.; Lamagni, P.; Buhl, K. B.;
Bremholm, M.; Beller, M.; Pedersen, S. U.; Skrydstrup, T.; Daasbjerg,
K. Selective CO2 Reduction to CO in Water using Earth-Abundant
Metal and Nitrogen-Doped Carbon Electrocatalysts. ACS Catal. 2018,
8 (7), 6255−6264.
(23) Tignor, S. E.; Shaw, T. W.; Bocarsly, A. B. Elucidating the
origins of enhanced CO2 reduction in manganese electrocatalysts
K
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
bearing pendant hydrogen-bond donors. Dalton Trans. 2019, 48 (33),
12730−12737.
electrochemical CO reduction using immobilized manganese
complexes. Faraday Discuss. 2015, 183, 147−60.
(40) Cosnier, S.; Deronzier, A.; Moutet, J. C. Electrocatalytic
Reduction of CO2 on Electrode Modified by fac-Re(2,2’-bipyridine)-
(CO)3Cl Complexes Bonded to Polypyrrole Films. J. Mol. Catal.
1988, 45 (3), 381−391.
(41) 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.
(42) Nakada, A.; Koike, K.; Nakashima, T.; Morimoto, T.; Ishitani,
O. Photocatalytic CO2 Reduction to Formic Acid Using a Ru(II)-
Re(I) Supramolecular Complex in an Aqueous Solution. Inorg. Chem.
2015, 54, 1800−1807.
(43) Manbeck, G. F.; Muckerman, J. T.; Szalda, D. J.; Himeda, Y.;
Fujita, E. Push or Pull? Proton Responsive Ligand Effects in Rhenium
Tricarbonyl CO2 Reduction Catalysts. J. Phys. Chem. B 2015, 119
(24), 7457−7466.
(24) Madsen, M. R.; Jakobsen, J. B.; Rønne, M. H.; Liang, H.;
Hammershøj, H. C. D.; Nørby, P.; Pedersen, S. U.; Skrydstrup, T.;
Daasbjerg, K. Evaluation of the Electrocatalytic Reduction of Carbon
Dioxide using Rhenium and Ruthenium Bipyridine Catalysts Bearing
Pendant Amines in the Secondary Coordination Sphere. Organo-
metallics 2020, 39, 1480.
(25) Rønne, M. H.; Cho, D.; Madsen, M. R.; Jakobsen, J. B.; Eom,
́
́
S.; Escoude, E.; Hammershøj, H. C. D.; Nielsen, D. U.; Pedersen, S.
U.; Baik, M.-H.; Skrydstrup, T.; Daasbjerg, K. Ligand-Controlled
Product Selectivity in Electrochemical Carbon Dioxide Reduction
Using Manganese Bipyridine Catalysts. J. Am. Chem. Soc. 2020, 142
(9), 4265−4275.
(26) Jensen, M. T.; Rønne, M. H.; Ravn, A. K.; Juhl, R. W.; Nielsen,
D. U.; Hu, X.-M.; Pedersen, S. U.; Daasbjerg, K.; Skrydstrup, T.
Scalable carbon dioxide electroreduction coupled to carbonylation
chemistry. Nat. Commun. 2017, 8 (1), DOI: 10.1038/s41467-017-
00559-8.
(44) Matsubara, Y.; Shimojima, M.; Takagi, S. A Bi-functional
Second Coordination Sphere for Electrocatalytic CO2 Reduction: The
Concerted Improvement by a Local Proton Source and Local
Coulombic Interactions. Chem. Lett. 2020, 49 (3), 315−317.
(45) Agarwal, J.; Shaw, T. W.; Schaefer, H. F.; Bocarsly, A. B. Design
of a Catalytic Active Site for Electrochemical CO2 Reduction with
Mn(I)-Tricarbonyl Species. Inorg. Chem. 2015, 54 (11), 5285−5294.
(27) Taylor, J. O.; Neri, G.; Banerji, L.; Cowan, A. J.; Hartl, F.
Strong Impact of Intramolecular Hydrogen Bonding on the Cathodic
Path of [Re(3,3′-dihydroxy-2,2′-bipyridine)(CO)3Cl] and Catalytic
Reduction of Carbon Dioxide. Inorg. Chem. 2020, 59 (8), 5564−5578.
(28) Fokin, I.; Denisiuk, A.; Wurtele, C.; Siewert, I. The Impact of a
̈
̈
(46) Krohnke, F. The specific synthesis of pyridines and
Proton Relay in Binuclear α-Diimine-Mn(CO)3 Complexes on the
CO2 Reduction Catalysis. Inorg. Chem. 2019, 58 (16), 10444−10453.
(29) Du, J.-P.; Wilting, A.; Siewert, I. Are Two Metal Ions Better
than One? Mono- and Binuclear α-Diimine-Re(CO)3 Complexes with
Proton-Responsive Ligands in CO2 Reduction Catalysis. Chem. - Eur.
J. 2019, 25 (21), 5555−5564.
oligopyridines. Synthesis 1976, 1976 (01), 1−24.
(47) Sandroni, M.; Volpi, G.; Fiedler, J.; Buscaino, R.; Viscardi, G.;
Milone, L.; Gobetto, R.; Nervi, C. Iridium and ruthenium complexes
covalently bonded to carbon surfaces by means of electrochemical
oxidation of aromatic amines. Catal. Today 2010, 158 (1−2), 22−28.
(48) Mellado, M.; Madrid, A.; Reyna, M.; Weinstein-Oppenheimer,
(30) Sung, S.; Kumar, D.; Gil-Sepulcre, M.; Nippe, M. Electro-
catalytic CO2 Reduction by Imidazolium-Functionalized Molecular
Catalysts. J. Am. Chem. Soc. 2017, 139 (40), 13993−13996.
(31) Cheung, P. L.; Machan, C. W.; Malkhasian, A. Y. S.; Agarwal, J.;
Kubiak, C. P. Photocatalytic Reduction of Carbon Dioxide to CO and
HCO2H Using fac-Mn(CN)(bpy)(CO)3. Inorg. Chem. 2016, 55 (6),
3192.
(32) Machan, C. W.; Stanton, C. J.; Vandezande, J. E.; Majetich, G.
F.; Schaefer, H. F.; Kubiak, C. P.; Agarwal, J. Electrocatalytic
Reduction of Carbon Dioxide by Mn(CN)(2,2′-bipyridine)(CO)3:
CN Coordination Alters Mechanism. Inorg. Chem. 2015, 54 (17),
8849−8856.
(33) Vollmer, M. V.; Machan, C. W.; Clark, M. L.; Antholine, W. E.;
Agarwal, J.; Schaefer, H. F.; Kubiak, C. P.; Walensky, J. R. Synthesis,
Spectroscopy, and Electrochemistry of (α-Diimine)M(CO)3Br, M =
Mn, Re, Complexes: Ligands Isoelectronic to Bipyridyl Show
Differences in CO2 Reduction. Organometallics 2015, 34 (1), 3−12.
(34) Machan, C. W.; Sampson, M. D.; Chabolla, S. A.; Dang, T.;
Kubiak, C. P. Developing a Mechanistic Understanding of Molecular
Electrocatalysts for CO2 Reduction using Infrared Spectroelectro-
chemistry. Organometallics 2014, 33, 4550−4559.
(35) Sun, C.; Rotundo, L.; Garino, C.; Nencini, L.; Yoon, S. S.;
Gobetto, R.; Nervi, C. Electrochemical CO2 Reduction at Glassy
Carbon Electrodes Functionalized by MnI and ReI Organometallic
Complexes. ChemPhysChem 2017, 18 (22), 3219−3229.
́
C.; Mella, J.; Salas, C. O.; Sanchez, E.; Cuellar, M. Synthesis of
chalcones with antiproliferative activity on the SH-SY5Y neuro-
blastoma cell line: Quantitative Structure−Activity Relationship
Models. Med. Chem. Res. 2018, 27 (11−12), 2414−2425.
(49) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H.
P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;
Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Montgomery, J. A., Jr.; Peralta, J. R.; Ogliaro, F.; Bearpark, M.;
Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi,
R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar,
S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J.; Klene, M.; Knox, J.
E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth,
G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.;
Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2009.
̌
(50) Miertus, S.; Scrocco, E.; Tomasi, J. Electrostatic interaction of a
solute with a continuum. A direct utilizaion of AB initio molecular
potentials for the prevision of solvent effects. Chem. Phys. 1981, 55
(1), 117−129.
(36) Sun, C.; Prosperini, S.; Quagliotto, P.; Viscardi, G.; Yoon, S. S.;
Gobetto, R.; Nervi, C. Electrocatalytic reduction of CO2 by
thiophene-substituted rhenium(I) complexes and by their polymer-
ized films. Dalton. Trans. 2016, 45 (37), 14678−88.
(51) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. New
developments in the polarizable continuum model for quantum
mechanical and classical calculations on molecules in solution. J.
Chem. Phys. 2002, 117 (1), 43−54.
(37) Sun, C.; Gobetto, R.; Nervi, C. Recent advances in catalytic
CO2 reduction by organometal complexes anchored on modified
electrodes. New J. Chem. 2016, 40 (7), 5656−5661.
(52) Becke, A. D. Density-functional thermochemistry. III. The role
of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652.
(53) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-
Salvetti correlation-energy formula into a functional of the electron
density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789.
(54) Weigend, F. Accurate Coulomb-fitting basis sets for H to Rn.
Phys. Chem. Chem. Phys. 2006, 8 (9), 1057−1065.
(38) Walsh, J. J.; Forster, M.; Smith, C. L.; Neri, G.; Potter, R. J.;
Cowan, A. J. Directing the mechanism of CO2 reduction by a Mn
catalyst through surface immobilization. Phys. Chem. Chem. Phys.
2018, 20 (10), 6811−6816.
(39) Walsh, J. J.; Smith, C. L.; Neri, G.; Whitehead, G. F.;
Robertson, C. M.; Cowan, A. J. Improving the efficiency of
(55) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence,
triple zeta valence and quadruple zeta valence quality for H to Rn:
L
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7
(18), 3297−3305.
(56) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping
Function in Dispersion Corrected Density Functional Theory. J.
Comput. Chem. 2011, 32 (7), 1456−1465.
NOTE ADDED AFTER ASAP PUBLICATION
■
This paper was published on August 17, 2020. Figures 3, 4, and
8 have been updated and the revised version was re-posted on
August 18, 2020.
(57) Breikss, A. I.; Abruna, H. D. Electrochemical and Mechanistic
Studies of Re(CO)3(dmbpy)Cl and Their Relation to the Catalytic
Reduction of CO2. J. Electroanal. Chem. Interfacial Electrochem. 1986,
201 (2), 347−358.
(58) Sullivan, B. P.; Bolinger, C. M.; Conrad, D.; Vining, W. J.;
Meyer, T. J. One-Electron and 2-Electron Pathways in the
Electrocatalytic Reduction of CO2 by fac-Re(2,2’-Bipyridine)-
(CO)3Cl. J. Chem. Soc., Chem. Commun. 1985, No. 20, 1414−1415.
(59) Paolucci, F.; Marcaccio, M.; Paradisi, C.; Roffia, S.; Bignozzi, C.
A.; Amatore, C. Dynamics of the Electrochemical Behavior of Diimine
Tricarbonyl Rhenium(I) Complexes in Strictly Aprotic Media. J. Phys.
Chem. B 1998, 102 (24), 4759−4769.
(60) 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.
(61) BASi DigiSim Simulation Software for Cyclic Voltammetry.
http://www.basinc.com/products/ec/digisim/ (accessed April 2020).
(62) Christensen, P.; Hamnett, A.; Muir, A. V. G.; Timney, J. A. An
in-situ Infrared Study of CO2 Reduction Catalyzed by Rhenium
Tricarbonyl Bipyridine Derivatives. J. Chem. Soc., Dalton Trans. 1992,
No. 9, 1455−1463.
(63) Stor, G. J.; Hartl, F.; Van Outersterp, J. W. M.; Stufkens, D. J.
Spectroelectrochemical (IR, UV/Vis) Determination of the Reduc-
tion Pathways for a Series of [Re(CO)3(α-diimine)L’]0/+ (L’ =
Halide, OTf−, THF, MeCN, n-PrCN, PPh3, P(OMe)3) Complexes.
Organometallics 1995, 14 (3), 1115−1131.
̌
(64) Zeng, Q.; Messaoudani, M.; Vlcek, A., Jr.; Hartl, F.
Electrochemical Reductive Deprotonation of an Imidazole Ligand in
a Bipyridine Tricarbonyl Rhenium(I) Complex. Eur. J. Inorg. Chem.
2012, 2012 (3), 471−474.
(65) Johnson, F. P. A.; George, M. W.; Hartl, F.; Turner, J. J.
Electrocatalytic reduction of CO2 using the complexes [Re(bpy)-
(CO)3L]n (n = +1, L = P(OEt)3, CH3CN; n = 0, L = Cl−, Otf−; bpy =
2,2’-bipyridine; Otf−=CF3SO3) as catalyst precursors: Infrared
spectroelectrochemical investigation. Organometallics 1996, 15,
3374−3387.
(66) Wilting, A.; Stolper, T.; Mata, R. A.; Siewert, I. Dinuclear
Rhenium Complex with a Proton Responsive Ligand as a Redox
Catalyst for the Electrochemical CO2 Reduction. Inorg. Chem. 2017,
56 (7), 4176−4185.
(67) Duan, L.; Manbeck, G. F.; Kowalczyk, M.; Szalda, D. J.;
Muckerman, J. T.; Himeda, Y.; Fujita, E. Noninnocent Proton-
Responsive Ligand Facilitates Reductive Deprotonation and Hinders
CO2 Reduction Catalysis in [Ru(tpy)(6DHBP)(NCCH3)]2+
(6DHBP = 6,6′-(OH)2bpy. Inorg. Chem. 2016, 55 (9), 4582−4594.
(68) Andrieux, C.P.; Blocman, C.; Dumas-Bouchiat, J.M.; M'Halla,
F.; Saveant, J.M. Homogeneous redox catalysis of electrocheical
reactions 5. Cyclic voltammetry. J. Electroanal. Chem. Interfacial
Electrochem. 1980, 113 (1), 19−40.
(69) Nichols, A. W.; Machan, C. W., Secondary-Sphere Effects in
Molecular Electrocatalytic CO2 Reduction. Front. Chem. 2019, 7,
DOI: 10.3389/fchem.2019.00397.
(70) Ngo, K. T.; McKinnon, M.; Mahanti, B.; Narayanan, R.; Grills,
D. C.; Ertem, M. Z.; Rochford, J. Turning on the Protonation-First
Pathway for Electrocatalytic CO2 Reduction by Manganese Bipyridyl
Tricarbonyl Complexes. J. Am. Chem. Soc. 2017, 139 (7), 2604−2618.
(71) Sampson, M. D.; Nguyen, A. D.; Grice, K. A.; Moore, C. E.;
Rheingold, A. L.; Kubiak, C. P. Manganese Catalysts with Bulky
Bipyridine Ligands for the Electrocatalytic Reduction of Carbon
Dioxide: Eliminating Dimerization and Altering Catalysis. J. Am.
Chem. Soc. 2014, 136 (14), 5460−5471.
M
https://dx.doi.org/10.1021/acs.inorgchem.0c01181
Inorg. Chem. XXXX, XXX, XXX−XXX