Photocatalytic CO2 Reduction by Trigonal-Bipyramidal Cobalt(II) Polypyridyl Complexes: The Nature of Cobalt(I) and Cobalt(0) Complexes upon Their Reactions with CO2, CO, or Proton

Article abstract of DOI:10.1021/acs.inorgchem.8b00433

The cobalt complexes Co^IIL1(PF₆)₂ (1; L1 = 2,6-bis[2-(2,2′-bipyridin-6′-yl)ethyl]pyridine) and Co^IIL2(PF₆)₂ (2; L2 = 2,6-bis[2-(4-methoxy-2,2′-bipyridin-6′-yl)ethyl]pyridine) were synthesized and used for photocatalytic CO₂ reduction in acetonitrile. X-ray structures of complexes 1 and 2 reveal distorted trigonal-bipyramidal geometries with all nitrogen atoms of the ligand coordinated to the Co(II) center, in contrast to the common six-coordinate cobalt complexes with pentadentate polypyridine ligands, where a monodentate solvent completes the coordination sphere. Under electrochemical conditions, the catalytic current for CO₂ reduction was observed near the Co(I/0) redox couple for both complexes 1 and 2 at E_1/2 = -1.77 and -1.85 V versus Ag/AgNO₃ (or -1.86 and -1.94 V vs Fc^+/0), respectively. Under photochemical conditions with 2 as the catalyst, [Ru(bpy)₃]²⁺ as a photosensitizer, tri-p-tolylamine (TTA) as a reversible quencher, and triethylamine (TEA) as a sacrificial electron donor, CO and H₂ were produced under visible-light irradiation, despite the endergonic reduction of Co(I) to Co(0) by the photogenerated [Ru(bpy)₃]⁺. However, bulk electrolysis in a wet CH₃CN solution resulted in the generation of formate as the major product, indicating the facile production of Co(0) and [Co-H]ⁿ⁺ (n = 1 and 0) under electrochemical conditions. The one-electron-reduced complex 2 reacts with CO to produce [Co⁰L2(CO)] with ν_CO = 1894 cm^-1 together with [Co^IIL2]²⁺ through a disproportionation reaction in acetonitrile, based on the spectroscopic and electrochemical data. Electrochemistry and time-resolved UV-vis spectroscopy indicate a slow CO binding rate with the [Co^IL2]⁺ species, consistent with density functional theory calculations with CoL1 complexes, which predict a large structural change from trigonal-bipyramidal to distorted tetragonal geometry. The reduction of CO₂ is much slower than the photochemical formation of [Ru(bpy)₃]⁺ because of the large structural changes, spin flipping in the cobalt catalytic intermediates, and an uphill reaction for the reduction to Co(0) by the photoproduced [Ru(bpy)₃]⁺.

Full text of DOI:10.1021/acs.inorgchem.8b00433

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Photocatalytic CO Reduction by Trigonal-Bipyramidal Cobalt(II)
2
Polypyridyl Complexes: The Nature of Cobalt(I) and Cobalt(0)
Complexes upon Their Reactions with CO , CO, or Proton
2
Tomoe Shimoda,† Takeshi Morishima, Koichi Kodama, Takuji Hirose, Dmitry E. Polyansky,
,‡
‡
‡
‡
†
Gerald F. Manbeck,† James T. Muckerman, and Etsuko Fujita*
†
,†
†Chemistry Division, Brookhaven National Laboratory, Upton, New York 11973-5000, United States
‡Graduate School of Science and Engineering, Saitama University, Saitama, 338-8570, Japan
*
S Supporting Information
II
ABSTRACT: The cobalt complexes Co L1(PF ) (1; L1 = 2,6-
6
2
II
bis[2-(2,2′-bipyridin-6′-yl)ethyl]pyridine) and Co L2(PF ) (2;
6
2
L2 = 2,6-bis[2-(4-methoxy-2,2′-bipyridin-6′-yl)ethyl]pyridine)
were synthesized and used for photocatalytic CO reduction in
2
acetonitrile. X-ray structures of complexes 1 and 2 reveal
distorted trigonal-bipyramidal geometries with all nitrogen atoms
of the ligand coordinated to the Co(II) center, in contrast to the
common six-coordinate cobalt complexes with pentadentate
polypyridine ligands, where a monodentate solvent completes the
coordination sphere. Under electrochemical conditions, the
catalytic current for CO reduction was observed near the
2
Co(I/0) redox couple for both complexes 1 and 2 at E = −1.77 and −1.85 V versus Ag/AgNO (or −1.86 and −1.94 V vs
1
/2
3
+
/0
2+
Fc ), respectively. Under photochemical conditions with 2 as the catalyst, [Ru(bpy)3] as a photosensitizer, tri-p-tolylamine
TTA) as a reversible quencher, and triethylamine (TEA) as a sacrificial electron donor, CO and H were produced under
visible-light irradiation, despite the endergonic reduction of Co(I) to Co(0) by the photogenerated [Ru(bpy) ] . However, bulk
(
2
+
3
electrolysis in a wet CH CN solution resulted in the generation of formate as the major product, indicating the facile production
3
n+
of Co(0) and [Co−H] (n = 1 and 0) under electrochemical conditions. The one-electron-reduced complex 2 reacts with CO to
0
−1
II
2+
produce [Co L2(CO)] with ν = 1894 cm together with [Co L2] through a disproportionation reaction in acetonitrile,
CO
based on the spectroscopic and electrochemical data. Electrochemistry and time-resolved UV−vis spectroscopy indicate a slow
I
+
CO binding rate with the [Co L2] species, consistent with density functional theory calculations with CoL1 complexes, which
predict a large structural change from trigonal-bipyramidal to distorted tetragonal geometry. The reduction of CO is much
2
+
slower than the photochemical formation of [Ru(bpy) ] because of the large structural changes, spin flipping in the cobalt
3
+
catalytic intermediates, and an uphill reaction for the reduction to Co(0) by the photoproduced [Ru(bpy) ] .
3
INTRODUCTION
ethyl]pyridine (L2) (Chart 1) for the investigation of CO2
reduction. L2 is expected to exhibit stronger σ donation by
methoxy groups, making the Co center more electron-rich,
■
In recent years, energy, natural resources, and the environment
have become indisputably important issues. Many scientists
have expended great effort to address these issues through
technologies that convert sunlight into electricity or chemical
leading to higher association constants with CO and other
2
substrates. Here we report the synthesis of new Co(II)
complexes, 1 and 2, their X-ray structures, the spin states of
the Co(II) center, and experimental and theoretical studies of
1
−6
bonds (i.e., solar fuels).
Among these, the development of
carbon dioxide (CO ) reduction to energy-rich C1 chemicals
2
the binding of CO , CO, or a proton to the reduced Co
2
such as carbon monoxide (CO), formic acid, methanol
complexes. Interestingly, these Co(II) complexes have trigonal-
bipyramidal (TBP) geometry without an additional mono-
dentate ligand such as a solvent molecule. For photocatalytic
(
CH OH), and methane using molecular complexes, metal
3
electrodes, and semiconductor materials has attracted signifi-
7
−26
cant attention.
Quite a few cobalt molecular complexes
27−30
CO reduction, we employed the conditions reported by Shan
have been studied for their binding of CO ,
electro-
and photochemical CO2
especially in nonaqueous media because in
aqueous systems proton reduction competes with CO
2
2
45
2+
1
4,31−38
and Schmehl, which use [Ru(bpy)3] as a photosensitizer,
tri-p-tolylamine (TTA) as a commercially available efficient
reductive quencher, and triethylamine (TEA) as a sacrificial
chemical CO reduction
2
3
9−43
reduction,
2
4
4
reduction. We have prepared new cobalt complexes with
pentadentate ligands 2,6-bis[2-(2,2′-bipyridin-6′-yl)ethyl]-
pyridine (L1) and 2,6-bis[2-(4-methoxy-2,2′-bipyridin-6′-yl)-
Received: February 20, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00433
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Inorganic Chemistry
Article
Chart 1. Structures of the Cations of Various Cobalt Complexes Discussed in This Study
2
+
electron donor. The quenching rate constant of *[Ru(bpy) ]
were prepared under high vacuum with vacuum-distilled CH CN and
3
reduced gradually by introducing small portions to the amalgam
3
8
−1 −1
by 0.04 M TTA was reported as 1.03 × 10 M
s
with a
chamber. NMR samples of Co(I) species were prepared in CD CN
quenching efficiency of 0.77. The quantum yield of the one-
electron-reduced [Ru(bpy) ] of 0.62 is a dramatic improve-
ment over the efficiency for the quenching of *[Ru(bpy) ] by
TEA alone. We envisioned that the high quantum yield for
the formation of [Ru(bpy) ] under those conditions would
enhance the photochemical CO reduction with complexes 1
3
+
using a homemade glassware apparatus equipped with a NMR tube, a
0.1 mm optical cell, and Ha/Hg chamber separated by a glass frit and
flame-sealed in the NMR tube after confirmation of the formation of
the Co(I) species by UV−vis spectroscopy.
Electrochemistry and Spectroelectrochemistry. Electrochem-
ical measurements were carried out with a BASi Epsilon potentiostat
or a CH Instruments 620E electrochemical analyzer. Cyclic
voltammetry (CV) was performed using a glassy carbon disk (3
mm) working electrode, a platinum wire counter electrode, and a
3
2+
3
4
5
+
3
2
and 2.
While Co(II) is reduced to Co(I) more efficiently during
photolysis in the presence of TTA than in its absence, the rates
of CO production are nearly identical in both experiments. The
thermal reaction of CO reduction and the release of CO seem
to be much slower than the formation of [Ru(bpy) ] for
silver/silver nitrate (Ag/AgNO ) reference electrode in CH CN
3
3
containing 0.1 M tetrabutylammonium hexafluorophosphate
2
+
(TBAPF ) as a supporting electrolyte. The scan rate was 100 mV
6
3
−1
s
(
unless otherwise noted. Solutions were purged with argon or CO2
to saturation at 0.28 M) before cyclic voltammograms were taken.
several reasons, which we will discuss.
Before and after CV measurements, the potentials were calibrated with
+
/0
+/0
EXPERIMENTAL DETAILS
reference to the Fc couple (Fc = 90 mV vs Ag/AgNO ). Bulk
3
■
electrolysis was performed in a two-compartment cell with a mercury
pool working electrode and a silver wire pseudoreference separated
from the platinum counter electrode by a fine glass frit. Experiments
were performed for 10 min with 20−30 turnovers of products formed
with no sign of catalyst deactivation except for the experiments in the
absence of water, in which case rapid precipitation of the deactivated
catalyst was observed. Hydrogen and CO were detected by gas
Materials. All chemicals and solvents used for synthesis were of
reagent-grade quality and were used without further purification.
Acetonitrile (CH CN) used for photochemical studies and Na/Hg
reduction was purified using a procedure similar to one published
previously. CH CN used for electrochemical studies was dried by
passage through two alumina columns in a solvent-dispensing system
designed by J. C. Meyer. [Ru(bpy) ]Cl was synthesized according to
literature methods
converted to the [Ru(bpy) ](ClO ) salt using AgClO . (Warning!
3
28
3
3
2
1
46,47
chromatography (GC). Formate was quantified by H NMR using an
internal standard.
or obtained from Strem Chemicals and
3
4
2
4
IR spectroelectrochemical measurements were carried out using a
home-built electrochemical cell equipped with a gold working
electrode, a platinum reference electrode, and a solid Ag/AgCl
reference electrode (LF-1 from Innovative Instruments, Inc.) using a
VeeMAX III Variable Angle Specular Reflectance Accessory (PIKE
Technologies, Inc.) installed in a Bruker IFS 66/s spectrometer.
The perchlorate salts used in this study may be potentially explosive and
hazardous.) L1 was prepared by a method similar to the one published
48
previously. The detailed synthesis procedures of complexes 1 and 2
are shown in the Supporting Information (SI), together with those of
L1 and L2. Certified gas mixtures [3, 10, and 30% CO balanced with
2
N ] from MG Industries were used for electrochemistry measure-
2
1
Solutions containing 100 mM TBAPF electrolyte and ca. 1 mM
ments. The H NMR spectra were recorded with a Bruker Avance 500,
6
cobalt complex that have been purged with argon, CO , or CO were
4
00, or 300 spectrometer at 298 K, and coupling constants are
2
reported in hertz. The effective magnetic moment of complex 2 was
loaded into a gastight syringe (Hamilton Co.) and delivered into the
cell through a gastight fluidic connection. Solutions were exchanged
with at least three cell volumes between runs. A square-wave
voltammetry scan was performed prior to each bulk electrolysis
experiment to determine redox-wave potentials versus reference
electrode. For spectroelectrochemical runs, the cell was biased at a
constant potential and spectral data were collected with 2 cm−
resolution using a mercury−cadmium telluride detector (Kolmar
Technologies) at regular time intervals. The spectra were background-
corrected for the absorption of solvent and electrolyte.
determined by the Evans method in CD CN with 5 vol %
3
4
9−53
benzene.
Mass spectrometry was performed on a LCQ
ADVANTAGE MAX (Finnigan) mass spectrometer using CH OH
3
as the eluent. Elemental analyses were conducted by Robertson
Microlit Laboratories (Ledgewood, NJ). Electronic absorption spectra
were recorded using an Agilent 8454 UV−vis spectrophotometer.
Fourier tranform infrared (FT-IR) spectra were recorded with a Jasco
FT-IR 400 spectrometer or a Bruker IFS 66/S spectrometer. Sodium
reduction was performed in a homemade airtight vessel equipped with
a quartz spectrophotometric cell separated by a fine glass frit from a
second compartment containing 0.5% sodium in mercury. Samples
1
Photocatalysis. Each sample in a CH CN solution with 300 μM
3
[Ru(bpy) ](ClO ) , 40 mM TTA, 0.55 M TEA, and a 180 μM cobalt
3
4 2
B
DOI: 10.1021/acs.inorgchem.8b00433
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Inorganic Chemistry
Article
Figure 1. X-ray structures of the cations of complexes 1 (left) and 2 (right).
complex, unless otherwise noted, was prepared using a homemade
glassware apparatus equipped with a 1 cm × 1 cm rectangular optical
cell and a valve in a glovebox. The total volume of the apparatus was
calculated positions and allowed to ride on the atom to which they
were attached. The isotropic thermal parameters for the hydrogen
atoms were determined from the atom to which they are attached. The
data were corrected for absorption using the multiscan method
1
2.5 mL, and 4 mL of solution was used for photolysis. The system
5
5
was purged with CO for 20 min. Some experiments were carried out
(SADABS). A summary of the crystallographic data is given in Table
2
with 0.59 M TEA without TTA to investigate the effects of TTA. The
sample was irradiated by a 150 W xenon lamp with a UV cutoff filter
S1.
Density Functional Theory (DFT) and Wave-Function-Based
Calculations. DFT calculations (RKS states for closed shells and
UKS states for open shells) were carried out using the Gaussian 09
program package, the B3LYP functional,
basis set and effective core potentials for all atoms.
considered were fully optimized in a SMD continuum model of the
(
410 nm ≤ λ) at room temperature under constant stirring. Gaseous
products such as CO and H were analyzed by gas chromatography on
2
5
6
57−60
and the LANL2DZ
an Agilent 6890N gas chromatograph with HP-MOLSIV (30 m × 0.32
mm × 12 μm) and GS-CARBON-PLOT (15 m × 0.32 mm × 1.5 μm)
columns. CO was quantified via a methanizer using a calibrated flame
ionization detector. The products in the solution were analyzed using a
Dionex ICS-1600 ion chromatography instrument with a 4.5 mM
sodium carbonate and 0.8 mM sodium bicarbonate eluent solution.
The turnover number (TON) was calculated by the ratio of CO
produced to the initial concentration of the cobalt complex catalyst.
The quantum yields of Co(I) and CO formation were determined
using a 405 nm laser diode for 60 s (60 mW) and 15 min (25 mW)
irradiation, respectively. The amounts of Co(I) and CO were
determined by optical spectroscopy and GC, respectively.
6
1−64
All species
6
5
CH CN solvent, and frequency analyses were carried out to exclude
3
any transition-state geometries and to confirm a minimum in the
energy.
To provide insight into determination of the spin state of the
cobalt(II) center, we performed higher-level wave-function-based ab
initio electronic structure calculations on complex 1 (to take advantage
of its C symmetry and to simplify the electronic structure) using the
2
5
6
66−71
Gaussian09 program package, in particular its CASSCF facility.
First a converged UHF quartet wave function at the geometry of the
−
experimental crystal structure (omitting the two PF6 counterions)
Determination of the CO Binding Rate to the Photo-
I
+
was obtained. Then CASSCF calculations with an active space of seven
electrons in five orbitals were carried out separately for spin
multiplicities 4 and 2, iteratively exchanging active-space orbitals that
were doubly occupied in all configurations with d orbitals in order to
obtain an active space consisting of the five d orbitals of the cobalt
center. A final CASSCF(7,5) calculation was carried out with the
optimized active space, followed by MP2 calculations on the CASSCF
wave function to include the effect of dynamic correlation on the
relative energy of the quartet and doublet states. To explore the
produced [Co L2] . Samples containing 2 mM p-terphenyl (TP) as
a photosensitizer, 0.1 mM complex 2, and 150 mM TEA as a sacrificial
electron donor were degassed using three freeze−pump−thaw cycles
on a vacuum line and filled with different partial pressures of CO using
a Baratron gauge for pressure measurements. The concentration of
CO in solution was calculated using CO partial pressure and its
solubility in CH CN (0.0083 M under 1 atm of CO). Prepared
3
samples were excited with 266 nm laser pulses (fourth harmonic of
−1
Nd/YAG SpectraPhysics Lab 170, 2 ns pulse width, 1 mJ pulse ), and
the absorbance at 580 nm was probed using the filtered (long pass
filter above 500 nm) output of a 75 W xenon arc lamp in a 90°
configuration detected by a photomultiplier tube (Hamamatsu, R928)
and recorded with a digital oscilloscope (Tektronics DPO 4054B).
One kinetic trace was recorded per each laser pulse, and the sample
was stirred between laser pulses. The final kinetic traces were the
average of eight acquisitions and were fit to a single-exponential
function.
4
II
2+
electronic states of the singly reduced species of [Co (L1)] , a
similar procedure was carried out for states of triplet and singlet
multiplicity. Additional B3LYP/LANL2DZ calculations were per-
formed to explore the effects of geometry change and solvation on the
electronic states of the one-electron-reduced species.
RESULTS AND DISCUSSION
■
Synthesis, Characterization, and X-ray Structures. The
synthesis procedures for ligands L1 and L2 and complexes 1
and 2 along with their characterization data are provided in
Collection and Refinement of X-ray Data. Single crystals of 1
and 2 suitable for X-ray crystallographic analysis were grown by
recrystallization from CH CN/CH OH mixed solvents, respectively.
3
3
Figures S1−S11. For the synthesis of complexes 1 and 2, CoCl
2
X-ray diffraction data were collected using a Bruker Smart APEX II
diffractometer with graphite-monochromated Mo Kα radiation
and ligands L1 and L2, respectively, were dissolved in a
CH OH/CHCl (8/1, v/v) mixture and stirred for 30 min at
5
4
(
0.71073 Å). The structures were solved by direct methods using
3
3
room temperature. A saturated KPF aqueous solution was
SIR-2004 and refined using the SHELXL-2013 program. In the least-
squares refinement, the anisotropic temperature parameters were used
for all of the non-hydrogen atoms. Hydrogen atoms were placed at
6
added, and the precipitate was collected by filtration. Crystals
suitable for the X-ray diffraction study were obtained by
C
DOI: 10.1021/acs.inorgchem.8b00433
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Inorganic Chemistry
Article
recrystallization from CH CN/CH OH to yield complex 1 as
NCS, NO , and ClO ) complexes with TBP geometry are
3
3
3
4
76
dark-brown plates and from CH CN/diethyl ether to yield
4.12−4.55 μ . We measured the effective magnetic moment
3
B
complex 2 as purple plates.
of complex 2 at room temperature by the Evans method as 4.25
μ (Figure S12), which is close to 3.88 μ for three unpaired
ORTEP drawings of the cations of 1 and 2 are shown in
Figure 1. Crystal data and a summary of selected bond
distances and angles of the cations of 1 and 2 are provided in
Tables S1 and S2, respectively. The polypyridine ligand L1
binds to one Co(II) ion in a pentadentate fashion to afford
complex 1 in TBP geometry without coordination of any
solvent molecules. In complex 1, the asymmetric unit contained
half of an L1 ligand, and the nitrogen atoms of the central (N3)
and terminal (N1 and N1*) pyridines occupy the equatorial
positions while the axial positions are occupied by the other
two nitrogen atoms (N2 and N2*). The equatorial Co−N
bond distances (2.032−2.048 Å) are much shorter than those
at the axial positions (2.145 Å). Within the trigonal plane
involving N1, N3, N1*, and the cobalt center, the N1−Co−
N1* angle is about 20° smaller than the N1−Co−N3 and N3−
Co−N1* angles. However, the N2−Co−N2* angle is
B
B
electrons, indicating a high-spin Co(II) complex.
The UV−vis spectrum of complex 2 in CH CN (Figure 2)
3
shows an intense band at 295 nm from a ligand π−π* transition
1
75.14(7)°. Such a geometry is in contrast with the
corresponding Ru(II) complex of L1 previously reported,
where the additional coordination of CH CN on the plane
Figure 2. UV−vis absorption spectra of complex 2 in CH
3
CN using
3
−5
−2
(
a) 5.3 × 10 M and (b) 1.0 × 10 M complex 2 solutions.
formed by the central and two terminal pyridine nitrogen atoms
48
affords a six-coordinate, square-bipyramidal geometry. The
II
Ru L1 geometry deviates largely from octahedral because of
the constraints caused by the ethylene linkers (with N−Ru−N
angles of the ligand nitrogen atoms ranging from 80.33° to as
large as 106.82° except those involving the two nitrogen atoms
of a 2,2′-bipyridine ligand). Complex 2 has a TBP geometry
and a weak shoulder peak at 325 nm likely from a metal-to-
ligand charge-transfer (MLCT) transition. Weak d−d tran-
−1
−1
sitions with molar absorptivity of less than 50 M cm are
observed in the visible region (Figure 2b), similar to a reported
72
similar to that of complex 1 and crystallized with two CH CN
TBP complex. Time-dependent DFT calculations of complex
1 also indicate very weak d−d transitions in the visible region
(see the SI).
3
molecules. The asymmetric unit in complex 2 contains one
molecule of L2. The equatorial Co−N bond distances (2.011−
2.041 Å) are also much shorter than those at the axial positions
Electrochemistry. The cyclic voltammograms of complexes
(2.149−2.159 Å) in complex 2. Despite the different measure-
1 and 2 in CH CN display three reversible redox couples at
3
ment temperatures, the Co−N1 and Co−N5 distances of
complex 2 are shorter than the Co−N1 and Co−N1* distances
of complex 1, respectively. This stronger coordination is
attributable to the electron-donating ability of the methoxy
groups attached to the terminal pyridines of ligand L2. While
E1/2 = −1.10, − 1.77, and −2.27 V for 1 and E = −1.27, −
1/2
1.85, and −2.36 V for 2 (vs Ag/AgNO ), assignable to Co(II/
3
I), Co(I/0), and Co(0/1−) (Figure 3a). Here we write the
doubly and triply reduced species as Co(0) and Co(1−),
respectively, but the actual electronic structures are more
I
•−
0
•−
−
complex 2 does not have C symmetry because of the positions
accurately described as [Co (L )] and [Co (L )] according
to the spectral changes observed for complex 2 and free ligand
L2 upon Na/Hg reduction and DFT calculations of complex 1
(see below). The potentials of complex 2 are cathodically
shifted compared to complex 1 because of the electron-
donating methoxy groups (Figure 3a). Irreversible reduction of
the free ligands occurred at potentials more negative than −2.5
V (Figure S15). However, when these ligands coordinate to the
cobalt center, the reduction potentials seem to shift anodically,
2
of the anion and solvated CH CN, the cation of 2 has near-C2
3
symmetry. A TBP geometry of the Co(II) complexes with a
pentadentate ligand is very rare, and only a few X-ray structures
72,73
have been previously reported.
As shown in Chart 1, those
structures involve cobalt complexes with rigid five-coordinate
polyamine ligands such as the encapsulated octaazamacrocyclic
ligand [4,14,19-tris(methoxymethyl-1,4,6,9,12,14,19,21-
7
2
octaazabicyclo[7.7.7]tricosane], complex 3,
and 3,7-
I
I
•−
diazabicyclo[3.3.1]nonane with four tertiary amine groups
corresponding to the process [Co (L)]/[Co (L )].
73
and one pyridyl group, complex 4. While the geometry of
complex 3 is similar to those of complexes 1 and 2 with long
Co−N axial bond distances, complex 4 has a rather distorted
TBP geometry, probably because of the strained nonsym-
metrical nature of the ligand. The X-ray crystal structures of
Co(II) complexes of 2,6-bis[[methyl(pyrid-2-ylmethyl)amino]-
In solutions containing CO , the first reductions are
2
unchanged, but increased current attributable to catalytic CO2
reduction is observed at the second reductions of complexes 1
and 2 (Figure 3b for 2). The catalytic rate constants (k ) for 1
cat
and 2 were calculated from icat. using eqs 1−3. Here i is the
cat.
catalytic current, i is the peak current in the absence of
p
74
N-methyl]pyridine (5), tetradentate bis[2-(3,5-dimethylpyr-
catalysis, n = 2 is the electrochemical stoichiometry for CO
2
75
2
2
azol-1-yl)ethyl][(pyrazol-1-y)methyl]amine (6), and tris(2-
reduction to CO, F is the Faraday constant, A (0.15 π cm ) is
the electrode surface area, R is the gas constant, T is the
absolute temperature, v is the scan rate, and DCo is the catalyst
43
pyridylmethyl)amine also reveal distorted TBP geometries.
Complex 6 is a high-spin Co(II) complex with effective
magnetic moments of 4.50 μ at 300 K and 3.68 μ at 2 K.
diffusion coefficient (Figures S16 and S17). The CO
B
B
2
77
Complex 3 has an effective magnetic moment of 4.18 μ at 290
concentration at 25 °C is 0.28 M under 1 atm of CO
.
2
B
72
+
K, while the magnetic moments of a series of [Co(NTB)X]
7; NTB = tris(benzimidazolyl-2-methyl)amine and X = Br,
icat. = nFA[Co] D k [CO ]
Co
0
2
(1)
(
D
DOI: 10.1021/acs.inorgchem.8b00433
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Inorganic Chemistry
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Figure 3. (a) Cyclic voltammograms of 0.5 mM complexes 1 and 2 in 0.1 M TBAPF /CH CN under argon. (b) Cyclic voltammograms of 0.5 mM
6
3
−1
complex 2 under argon and CO /N (or CO /argon) gas mixtures. Electrode = glassy carbon, scan rate = 100 mV s , potential = volts versus Ag/
2
2
2
+
/0
AgNO , and E1/2(Fc ) = 0.09 V.
3
the pK of water in CH CN might be balanced to accelerate
icat.
n
RTk [CO ]
k0
v
a
3
0
2
=
= 0.72
[CO2]
CO reduction while precluding proton reduction catalysis. In
2
ip
0.4463
Fv
(2)
(3)
the 2−10% range of water by volume, the catalytic current at
the Co(I/0) couple is enhanced 2-fold compared to that when
no water was added, but the current decreases slightly at the
highest water content (Figure S24). Because the solubility of
CO in CH CN drops from 280 mM in a pure solvent to
kcat. = k [CO ]
0
2
The current increases linearly with the square root of the
2
3
CO concentration (Figures S18−S21). The k values of 0.94
85
2
0
roughly 215 mM at 10% water, the small drop in the catalytic
−1
−1
and 6.8 M
s
for CO reduction with complexes 1 and 2,
2
current might be due to the change of [CO ] without a major
2
respectively, were determined from eq 1 using the measured
diffusion coefficients of 1.0 × 10 and 9.5 × 10 cm s ,
respectively. Using eq 3, the kcat. values of 0.26 and 1.9 s ,
change in the selectivity for CO reduction.
−5
−6
2
−1
2
Bulk electrolysis experiments were used to quantify the
products of catalysis using various water concentrations at an
applied potential equivalent to that of the peak current by CV.
In the absence of water, the catalytic current was not sustained
and a pale-gray precipitate was observed. CO was produced
with low Faradaic efficiency (FE) and in substoichiometric
quantities (Table 1). In the presence of water, catalysis was
−1
respectively, were determined in a CO -saturated solution. The
2
−1
values obtained from eqs 2 and 3 are smaller (0.1 and 0.9 s ,
respectively), but within the experimental error, assuming that
the product may be reactive at more negative applied potentials
because the limiting current was not observed. These rate
constants are smaller than those calculated for the doubly
reduced species of precious metal complexes [Ru(tpy)(bpy)-
a
Table 1. Summary of Bulk Electrolysis Data
2+
−1 78
(
1
CH CN)] (kcat. = 5.5 s ) and [Re(bpy)(CO) Cl] (k
=
3
3
cat.
−
1
79
b
4 s ) under 1 atm of CO in CH CN using a similar
FE,
%
2
3
method.
water
HCO H/
2
Reduction of CO to formate or CO requires an oxide
acceptor, such as a proton or another molecule of CO (to yield
carbonate).
entry
content %
CO
c
H
2
HCO
2
H
CO/H
2
2
2
1
2
3
4
0
2
14
9
,11
These considerations motivate the examination
22 ± 4
9 ± 3
4.2 ± 1.3
5.9 ± 1.0
7.2 ± 1.8
9.5 ± 1.1
64 ± 4
75 ± 12
64 ± 8
2+
10.8:3.7:1
10.4:1.2:1
6.7:0.4:1
80−84
of CO reduction catalysis in the presence of weak acids.
2
5
The cyclic voltammogram of complex 2 under argon in the
presence of acetic acid or trifluoroacetic acid shows a positive
shift of the Co(II/I) couple, followed by a catalytic wave
consistent with the binding of H to the singly reduced species
and catalytic reduction of the acid to H (Figure S22). Given
the high reactivity for proton reduction using strong acids, CO2
reduction was not investigated in this medium. On the other
hand, the Co(II/I) couple is unchanged when a weaker acid
such as phenol or water is added to the solution (Figures S23
10
a
II
Reaction conditions: 0.5−1 mM [Co L2] , 0.1 M Bu NPF ,
mercury pool working electrode, silver wire pseudoreference, platinum
mesh counter electrode, CO -saturated CH CN ([CO ] = 0.28 M),
potential = −1.9 V vs Ag/AgNO . The FEs for CO, H , and formate
were averaged over two runs and were corrected for the background
current. This is an estimated value because under 1 turnover was
obtained and at least 2 equiv of charge was consumed by reduction of
the catalyst.
4
6
+
2
3
2
b
2
3
2
c
and S24). Catalytic H production is observed at the Co(I/0)
couple using phenol and slightly cathodic of the Co(I/0)
couple using water under argon.
2
sustained and no precipitate was observed. The reaction yielded
CO, H , and formate with a constant FE for formate but
2
At low concentrations of phenol, selective reduction of CO2
may be possible (Figure S25), but when a larger amount of
phenol (>0.3 M) is added, the catalytic current does not show
decreasing CO and increasing H as water was increased to
10% by volume.
2
When the cyclic voltammogram in dry CH CN (Figure 3b)
3
any dependency on the CO concentration, indicating that
is compared to that in bulk electrolysis (Table 1, entry 1), the
2
proton reduction is the dominant pathway. A comparison of the
absence of catalysis in the latter suggests a stoichiometric CO
reduction to form [Co CO], which inhibits catalysis owing to
2
0
CV data under argon or CO using 2−10% water suggests that
2
E
DOI: 10.1021/acs.inorgchem.8b00433
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 4. Left panel: Photocatalytic CO production in dry CH CN containing 180 μM 1 (purple diamonds) or 2 (black down triangles), 300 μM
3
[
Ru(bpy) ](ClO ) , 40 mM TTA, and 0.55 M TEA under irradiation 410 nm ≤ λ. Right panel: Photocatalytic CO production in H O/CH CN for
3
4
2
2
3
complex 2 with similar conditions: 2% water (black squares), 5% water (red circles), and 10% water (blue triangles).
a
Table 2. Photocatalytic Reductions in CH CN
3
−
TOFCO, h−1
entry
catalyst
water content, %
catalyst, μM
CO, μmol
H , μmol
2
HCOO , μmol
TONCO
TONH2
1
2
3
4
5
blank
0
180
60
0.04
2.5
0.95
2.2
3.7
5.2
4.3
6.3
8.5
1.2
1.0
0.8
1
2
2
2
2
2
2
2
3.4
4.0
4.6
5.1
7.2
6.0
8.8
12
2.7
4.0
3.5
3.5
2.1
3.8
5.8
8.8
7.9
8.1
120
180
180
180
180
180
b
6
7
8
2
5
28
23
12
2.1
2.3
3.6
3.1
4.0
39
32
17
c
9
10
a
Reaction conditions: 0−180 μM cobalt complex catalyst, 300 μM [Ru(bpy) ](ClO ) as a photosensitizer, 40 mM TTA as a quencher, 0.55 M TEA
3
4 2
as a sacrificial electron donor, irradiation with a xenon lamp (λ ≥ 410 nm) in CO -saturated CH CN ([CO ] = 0.28 M). All of the reactions were
performed at 20 °C for 4 h. TON was calculated based on the cobalt complex. The amounts of CO and H and the TON and TOF values were
averaged over two runs, with errors of less than 20% for CO and 25% for H . In addition, TOF was calculated based on the TON for CO at an
initial 1 h irradiation. After a second run was conducted by the addition of 1.2 μmol of [Ru(bpy) ](ClO ) to the solution after the experiment of
entry 5 was completed. Cloudy solution due to the limited solubility of TTA.
2
3
2
CO
2
CO
2
CO
b
3
4 2
c
4
5
the strong CO binding. To investigate the mechanism, a
Photochemical CO2 Reduction. Shan and Schmehl
1
3
concentrated sample was electrolyzed with CO for analysis
reported efficient reductive quenching of the excited state of
[Ru(bpy)3] when TTA and TEA are used in CH CN as the
quencher and sacrificial electron donor, respectively. We
anticipated that such conditions could improve the efficiency
of a photocatalytic CO reduction due to the high quantum
2
1
2+
by NMR spectroscopy. After electrolysis, the H NMR
3
spectrum was identical with the free ligand and the 13C NMR
showed a single enhanced resonance at 163 ppm, which can be
86
assigned to carbonate. In the absence of protons, CO will
2
2
+
function as the oxide acceptor to produce carbonate and a
cobalt carbonyl complex. The putative Co−CO species was
observed by spectroelectrochemical IR experiments (see
below), indicating that, although some ligand dissociation
occurred in the electrolysis experiment, a portion of the catalyst
remained with a strongly associated CO ligand, consistent with
the low TON for CO release.
efficiency of producing [Ru(bpy) ] (over 60%).
3
Using conditions similar to those of Schmehl’s group, dry
CH CN solutions containing 180 μM 1, 300 μM [Ru(bpy) ]-
3
3
(ClO ) , 40 mM TTA, and 0.55 M TEA were irradiated using a
150 W xenon lamp with a 410 nm long pass filter at 20 °C
(Figure 4). Complex 1 catalyzed the reduction of CO to CO
for 4 h with a TON of 3.4 and a maximum TOF of 2.7. When
4
2
2
The addition of water shifts the major catalytic product to
using complex 2, CO generation was improved with a TON of
formate. A small amount of H is also produced, and its yield is
5.1 and TOF of 3.5. H was produced along with CO with the
2
2
increased at the expense of CO as the water content is
product ratio H /CO ∼ 1.5−2 despite the absence of water. In
2
increased. The production of formate indicates that the doubly
this experiment, the TEA oxidation products are potential
proton sources. To examine the dependence of the CO
production activity on the concentration of the cobalt complex,
we conducted photolysis experiments using catalyst concen-
trations of 60, 120, and 180 μM. As the concentration of the
cobalt complex increases, the amount of CO generation
increases proportionally (Figure S26). The initial TOF values
are almost the same, indicating a first-order reaction. After 5 h
of irradiation, a fresh aliquot of 300 μM [Ru(bpy) ](ClO )
4 2
+
reduced complex preferentially binds H over CO and shows
2
that the insertion of CO into the metal hydride bond is
2
favored over protonolysis of the hydride by water to yield H2.
On the other hand, CV data with other proton sources such as
phenol show large currents under argon, indicating that the
cobalt hydride will react with acids that are stronger than water
in CH CN. The data do not clearly indicate the oxidation state
3
of the cobalt hydride during CO insertion. Given the negative
2
3
potentials, it is likely reduced at least to Co(I), which will
increase the hydricity and selectivity for formate.
(1.2 μmol) was added and the TON increased further to 7.2.
This experiment suggests that the decomposition of [Ru-
F
DOI: 10.1021/acs.inorgchem.8b00433
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
−
5
Figure 5. UV−vis absorption spectra of the cobalt species for 5.3 × 10 M complex 2 upon Na/Hg reduction in CH CN: (a) complex 2 (black)
3
and the one-electron-reduced species (red); (b) the one-electron-reduced species (red) and the two-electron-reduced species (blue).
2
+
(
bpy) ] causes limited durability of our photochemical system
2, 40 mM TTA, and 0.55 M TEA (or 0.59 M TEA without
TTA) under 405 nm irradiation (60 mW power) for 60 s. The
quantum yields for Co(I) formation with and without TTA are
3
and that the cobalt complex remains active as a catalyst for CO2
reduction to CO.
To explore the effect of a proton source on CO generation,
we carried out photolysis experiments with 2, 5, and 10% water.
These experiments yielded 6.0, 8.8, and 12 turnovers of CO,
1.1 × 10− and 7.5 × 10 , respectively. Electron transfer to
2
−3
Co(II) appears to be inefficient considering that the reported
+
quantum yield for the formation of [Ru(bpy) ] via TTA/TEA
3
respectively, and 39, 32, and 17 turnovers of H , which are
reductive quenching under similar conditions is 0.62. In the
presence of CO , the quantum yields for CO and H formation
2
considerably higher than the H TON in the absence of water
2
2
2
(
Table 2 and Figure S27). While we observed some
(ΦCO and Φ , respectively) based on two photons to one
H
2
precipitation of TTA with 10% water, the decreasing combined
turnovers with increasing water are consistent with the lower
current observed by CV above. The increase in the selectivity
molecule of gas were determined using 15 min of 405 nm
−3
−3
irradiation (25 mW) to be 2.8 × 10 and 3.1 × 10 ,
respectively, with the above solution with TTA.
for CO reduction over proton reduction suggests that the
2
UV−Vis Spectra of the Reduced Species. The cyclic
proton might facilitate the formation of a metallocarboxylic acid
Co−COOH intermediate but not enough to completely
suppress the competitive binding of a proton to Co(0). An
increase in the CO TON might imply that proton-induced
cleavage of this Co−COOH intermediate to cobalt carbonyl
and water is accelerated by the higher water content of the
reaction mixture.
voltammograms of complex 2 in CH CN exhibited three
3
reversible redox couples (Figure 3a). Among them, the Co(II/
I) and Co(I/0) couples at potentials E1/2 = −1.27 and −1.85 V
vs Ag/AgNO , respectively, indicate that complex 2 can be
3
chemically reduced by a sodium amalgam (Na/Hg; E = −1.96
0
87
V vs NHE ). UV−vis spectra of the one- and two-electron-
reduced ligand L2 species prepared by Na/Hg reduction
A small amount of formate was also generated in photolysis
(
Figure S28) are distinctly different from the UV−vis spectra of
2+
experiments (formate/CO ≤ 0.5); however, [Ru(bpy) ] is
known to decompose to active CO reduction catalysts during
3
the one- and two-electron-reduced cobalt complex species
2
(Figure 5). The CH CN solution of complex 2 is pale pink with
3
reductive photolysis, and a comparable amount of formate was
produced in a control experiment without the cobalt catalyst
under our conditions. The different selectivities between
electrochemical and photochemical experiments are note-
visible absorptions, as shown in Figure 2b, but almost colorless
for a 5.3 × 10− M solution (Figure 5a, black line). Upon Na/
Hg reduction, the color changed to blue for the one-electron-
reduced species and to brown for the two-electron-reduced
species. Both reduced species have MLCT bands with high
molar absorptivities in the visible region. It is of interest to
5
worthy. The dominance of H in the photochemical reaction
2
shows that the mixture contains an acid that is stronger than
+
1
water and is likely TEAH derived from the deprotonation of
investigate the electronic structure of these species. The H
+
TEA oxidation products. TEAH may react with a Co(II)
NMR spectrum of the one-electron-reduced species in Figure
S29 shows sharp aliphatic and aromatic proton signals of the
hydride, whereas CO insertion likely requires further reduction
2
8
of the hydride.
ligand, suggesting that this is a low-spin Co(I) d species and
II
•−
We also carried out photochemical CO reduction using 0.59
not the Co(II) ligand radical anion [Co (L2 )], indicating
spin flipping during the reduction. These chemical shifts are
different from those of the free ligand, as shown in Figure S11.
The pattern of the NMR signal indicates that the ligand has a
symmetrical configuration around the cobalt center. The two-
2
M TEA without 40 mM TTA in CH CN solutions containing
3
2% water and found that the amounts of both CO and H2
formation are almost the same as those with TTA. However, in
CH CN solutions containing 10% water, the formation of both
3
I
•−
CO and H with only TEA decreased to less than half of the
electron-reduced species can be assigned as [Co (L2 )]
because a characteristic ligand radical anion absorption at
∼350 nm appears, accompanied by a decrease in the broad
π−π* absorption at 298 nm (Figure 5), as observed in the
reduction of L2 (Figure S28).
2
amounts compared to those in the system with TTA. Because
the system with TTA contains a considerable amount of
precipitate, a comparison of these results may not be
meaningful.
As an initial step toward investigating the mechanism of CO2
reduction, the quantum yield for the formation of Co(I) in the
In order to investigate the reactivity of the reduced species
with CO , the solution was saturated with CO after the
2
2
absence of CO2 was determined using CH CN solutions
preparation by Na/Hg (Figure S30). The singly reduced
3
2
+
I
+
containing 2% water, 300 μM [Ru(bpy) ] , 180 μM complex
[Co L2] did not react with CO in 24 h of observation. In
3
2
G
DOI: 10.1021/acs.inorgchem.8b00433
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
contrast, when CO was added to the two-electron-reduced
investigated under a 10% and 100% CO atmosphere (1 atm
2
I
•−
species [Co (L2 )], a notable color change from brown to
almost colorless was observed. The spectrum of this final
product was similar to the spectrum of 2; however, λmax is
shifted from 298 nm in 2 to 278 and 286 nm with a shoulder at
of CO in CH CN: 0.0083 M). A reversible peak with E
=
3
1/2
−1.27 V vs Ag/AgNO , assigned as Co(II/I) under argon
3
changed to an irreversible two-electron wave, and the Co(I/0)
couple at −1.85 V was absent under 100% CO (Figure 7).
When the concentration of CO was reduced 10-fold, the CVs
∼298 nm in the product (Figure 6). In the absence of
(
Figure S31) showed the current at −1.3 V, intermediate
between a one- or two-electron wave and a residual reversible
couple at −1.85 V with a current less than that under argon.
The ratio of the current at each peak is scan-rate-dependent
(
−
Figures 7, right panel, and S31), with the highest current at
1.3 V at slow scan rates. This response indicates that the
bimolecular reaction of Co(I) with CO is slow at this
concentration relative to the sweep rate, and the residual
current at −1.85 V is the Co(I/0) couple without CO binding.
The reaction of Co(I) with CO was probed further by laser
flash-quenching experiments using CH CN solutions contain-
3
ing TP, TEA, and complex 2 under various CO concentrations.
Upon 266 nm laser excitation, the singlet excited state of TP is
•−
rapidly quenched by TEA to form TP , which has a reduction
40,88
•−
II
2+
I
+
potential of −2.4 V (vs SCE in dimethylamine).
TP
Figure 6. UV spectra of [Co L2] (black), [Co L2] CO (green),
I
+
0
rapidly reduces Co(II) to Co(I), and the disappearance of
Co(I) could be monitored. Figure S32 shows that the rate of
Co(I) decay is linear with the concentration of CO (1 atm of
[
Co L2] + CO + one electron (blue), and [Co L2] + CO (red) in
2
CH CN.
3
CO in CH CN: 0.08 M) and k is 890 M− s , indicating a
1
−1
continued reduction, Co(II) carbonyl is a plausible inter-
mediate. To investigate this possibility, the reduced complex
was exposed to CO after Na/Hg reduction. The Co(I) species
reacted with CO, as shown by the spectral shift in Figure 6;
however, the resulting spectrum does not match either that of
complex 2 or that obtained by the reaction of [Co (L2 )] with
CO . Instead, the spectrum seems to be that of a mixture of the
two species. The reduction of this mixture by Na/Hg produced
a spectrum identical with that obtained by the reaction of
Co (L2 )] with CO . This finding was rather puzzling,
3
CO
6
8
very slow reaction compared to the typical rates of 10 −10
−1
−1
12
M
s
for the binding of CO to transition-metal complexes.
IR spectroelectrochemistry was carried out to detect
I
+
0
[
Co L2] and [Co L2] species in CH CN under argon, CO,
3
I
•−
and CO . The results are shown in Figures S33 and S34. The
2
−
1
CC stretching frequencies around 1600 cm shift to lower
2
II
2+
energy upon the reduction of [Co L2] under argon. Under a
CO atmosphere and an applied potential of 100 mV more
negative than that of the Co(II/I) couple, νCO was observed at
I
•−
[
2
I
•−
−1
because the reaction of [Co (L2 )] with CO should yield a
1894 cm . In order to confirm the identity of observed species,
2
0
product describable as Co(II): either as the metallocarboxylate
or the carbonyl if C−O bond cleavage occurs (carbonate is the
other product of this reaction). Therefore, we further
investigated the reaction of the reduced complex 2 with CO
using electrochemistry, laser flash photolysis, and IR
spectroelectrochemistry.
the [Co L2] complex was prepared by Na/Hg reduction of the
II
2+
parent [Co L2] in dry CH
CO gas. The IR spectrum of the resulting [Co L2(CO)]
species featured a single band at νCO = 1894 cm , indicating
that the same species was produced during the electrolysis
experiment. This finding indicates that the Co(I/0) reduction
CN, followed by saturation with
3
0
−1
I
+
Reaction of the Reduced Complex 2 with CO. The
cyclic voltammograms of complex 2 in CH CN were
potential of [Co L2(CO)] is less negative than that of the
I
+
[Co L2] complex, in agreement with the two-electron wave
3
Figure 7. Left: Cyclic voltammograms of [CoL2]2+ under argon (red) and 100% CO (black) with a scan rate of 100 mV s . The small peak around
−1
−
1600 mV is due to an Fe(CO) contaminant from the CO cylinder. Right: Current changes associated with scan rate changes under 10% CO. See
the selected cyclic voltammograms in Figure S31.
5
H
DOI: 10.1021/acs.inorgchem.8b00433
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Inorganic Chemistry
Article
2
0
5
2
0
3
Figure 8. DFT-calculated structures of [Co (κ -L1)(CO )], left, and [Co (κ -L1)(CO)], right. The CO adduct might be more correctly described
2
2
2
II
5
2−
as [Co (κ -L1)(CO )].
2
observed electrochemically and the observation of an
approximately 1:1 mixture of species when the Na/Hg-
prepared Co(I) was exposed to CO. In that experiment,
[CoL1] complex’s HOMO indicates ligand-based electron
density; see Figure S41) and UV−vis spectrum and its CO and
CO adducts here.
2
I
+
disproportionation of the [Co L2(CO)] species yields
The CO2 adduct is distorted octahedral, and CO2
coordinates on the opposite side of the terminal N2 with a
Co−C distance of 2.032 Å. The distances between cobalt and
the coordinating nitrogen atoms are between 2.014 and 2.404 Å
(Figure 8 and Table S6). The adduct can be described as a 19-
II
2+
0
[
Co L2] and [Co L2(CO)].
No evidence for the reaction between [Co L2] and CO was
I
+
2
1
2
found during bulk electrolysis in CO -saturated solutions at a
2
potential of about 100 mV more negative than the Co(II/I)
couple, which is consistent with CV measurements (see above).
However, at potentials more negative than Co(I/0), the 1894
2
II
5
2−
electron system, [Co (κ -L1)(CO )]. The CO adduct
2
0
5
cannot have a [Co (κ -L1)(CO)] coordination geometry with
−1
0
cm peak of [Co L2(CO)] was observed. Normalizing the
intensities to the absorption of the CC stretching shows that
about 30% of the catalyst exists as Co(0) carbonyl during
continued electrolysis. In addition, vibrational frequencies for
a 21-electron system; therefore, it changes to a distorted
0
3
tetragonal [Co (κ -L1)(CO)] geometry with a dangling
bipyridine moiety, as shown in Figure 8.
Proposed Mechanism of Photocatalytic Reactions.
The extensive characterization and mechanistic experiments
discussed above can be compiled to illustrate likely mechanisms
−
2−
−1 86
HCO /CO were evident at 1685, 1646, and 1304 cm .
3
3
This observation indicates that at this potential (−2.05 V vs
+/0
Fc ) complex 2 has entered the catalytic cycle, during which
for CO reduction catalysis. The Co(II) precatalyst is reduced
2
II
2+
CO functions as the oxide acceptor to yield [Co L2(CO)] ,
inefficiently as shown above, consistent with the requisite spin
flip from the high-spin quartet Co(II) to the low-spin singlet
2
0
which subsequently releases CO or converts to [Co L2(CO)]
by reduction. No clear spectroscopic signatures of other forms
Co(I). CV data and CO binding experiments showed that the
2
0
of the catalyst, e.g., [Co L2(CO )], were observed, suggesting
doubly reduced “Co(0)” oxidation state is needed to initiate
2
catalysis; however, the [Ru(bpy)3]2
+/+
couple, relevant in
that the reactions of these intermediates are not rate-limiting.
The C−O vibrational frequencies of [CoL2(CO)] are found at
photochemical electron transfer, is about 200 mV more positive
849 cm− under CO (Figure S40). The result clearly shows
1
13
1
than the Co(I/0) potential of complex 2 (Figure S39). The
2
+
that the generated CO originates from the reduction of CO .
moderately endergonic reduction of Co(I) by [Ru(bpy) ] can
2
3
0
Exposure of [Co L2(CO)] to air resulted in a loss of
be driven by fast subsequent steps in the reaction; however, the
radical products of the one-electron oxidation of triethylamine.
absorbance of the CO stretch, but the UV−vis spectrum
remained unchanged. Because the CO adduct is unlikely to be a
•+
•+
•
TEA (Et N ), and Et NC HCH can also reduce the
3 2 3
2
1-electron species, dissociation of one or more pyridyl ligands
complex in the photochemical milieu. The standard potentials
of these radicals are not known. The following reactions might
occur to initiate catalysis.
is possible. The nature of these species is investigated below by
DFT calculations.
DFT Calculations on Complex 1 and Its CO , CO, and
2
[Ru(bpy)3]2 + hν → [*Ru(bpy)3]2+
+
Hydride Complexes. DFT calculations on complex 1 and its
reduced species were carried out at the B3LYP/LANL2DZ
level of theory and the SMD continuum solvation model for
(4)
(5)
(6)
(7)
*Ru(bpy)3]2+ + TTA → [Ru(bpy) ] + TTA
+
•+
[
3
CH CN as the solvent. Because complexes 1 and 2 are very
3
similar in terms of their electronic and geometric structures, we
carried out calculations only for complex 1, but we believe our
results also to be applicable to complex 2. The detailed
computational procedure is shown in the SI, and calculated
•
+
•+
TTA + Et N → TTA + Et N
3
3
•+
+
•
4
II
5
2+
3
I
5
+
1
I
5
+
Et
3
N
+ Et
3
N → Et
3
NH + Et NC HCH
2 3
structures of [Co (κ -L1)] , [Co (κ -L1)] , [Co (κ -L1)] ,
2
1
I
5
•−
2
0
5
2
0
3
[
[
Co (κ -L1 )], [Co (κ -L1)(CO )], [Co (κ -L1)(CO)],
2
III
5
2+
2
II
5
+
+
4
II
2+
Co (κ -L1)(H)] , and [Co (κ -L1)(H)] are summarized
[
Ru(bpy) ] + [Co L2]
3
2
0
5
in Table S4. We discuss [Co (κ -L1)], which is actually
+
→ [Ru(bpy)3] + + [Co L2]
2
1
I
2
I
5
•−
(8)
[
Co (κ -L1 )] based on the DFT calculations (i.e., the
I
DOI: 10.1021/acs.inorgchem.8b00433
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
+
+
•
1
I
2
II
5
+
Et NC HCH + [Co L2]
[Co (κ ‐L)(COOH)] + H
2
3
2
+
+
2
I
•−
2
II
3
→
Et N CHCH + [Co (L2 )]
(9)
→ [Co (κ ‐L)(CO)] + H O
(17)
2
3
2
2[
Co (L2 )] + CO → [Co L2(CO )]
I
•−
2
II
2−
Under electrochemical or photochemical conditions, the
availability of protons to function as oxide acceptors must be
considered. In this case, the protonation of Co(II) metal-
locarboxylate (eq 15) is followed by C−O bond cleavage
induced by CO (eq 16) or H (eq 17). Both reactions yield
the same short-lived Co(II) carbonyl species. Spectroelec-
trochemical data revealed the formation of bicarbonate during
electrolysis as evidence for eq 16. The IR experiments also
show the occurrence of eq 14 during catalytic conditions. While
the Co(II) complex releases CO in exchange for CH CN,
Co(0) coordinates to CO, representing a mechanism of
(10)
2
2
As mentioned in the theoretical characterization above, the
structures of [Co (L1 )] and [Co (κ -L1)(CO )] are
distorted TBP and distorted octahedral, respectively, and these
results should agree for complex 2. Therefore, the coordination
of CO can proceed with minimal geometric constraints. On
the other hand, [Co (κ -L1)(CO)] (not observed exper-
imentally) and [Co (κ -L1)(CO)] (observed experimentally)
are distorted tetragonal, and generating [Co (κ -L1)(CO)]
from [Co (κ -L1)(CO )] requires cleavage of a C−O bond
and significant structural rearrangement.
In the Na/Hg preparation of [Co (L2 )] and its reaction
with CO , there are no protons available, and CO must
2
I
•−
2
II
5
2−
2
+
2
2
2
II
3
2+
2
0
3
2
II
3
2+
3
2
II
5
2−
2
product inhibition because CO addition is precluded by this
2
2
I
•−
strong equilibrium.
2
2
function as the oxide acceptor via eq 11 or 12. Interestingly, the
2
II
5
CONCLUSIONS
reaction of CO (g) with the two-electron-reduced [Co (κ -
■
2
2−
4
5
L1)(CO )] complex to form the spin-forbidden [Co(κ -
We have presented in this work a new type of cobalt complex
with TBP geometry that can be a CO reduction catalyst for
both electrochemical and photochemical approaches when
2
L1)]2 along with CO(g) and CO3 (solv) (eq 11) is predicted
+
2−
2
−1
by DFT calculations to be only ca. 2.3 kcal mol endergonic.
Equation 12 is more plausible because the resultant UV−vis
water is added as a proton source. Efficient reductive quenching
4
II
2+
2+
spectrum is different from the initial [Co L2] spectrum. The
of the excited state of [Ru(bpy)3] by the reversible TTA
−
1
45
net DFT free-energy change for eq 12 is −138.4 kcal mol
electron donor previously investigated by Shan and Schmehl
with L1, i.e., extremely exoergic, but also requiring a lot of
has been applied to photocatalytic CO reduction. Unfortu-
2
5
3
rearrangement of the complex (κ to κ ) and reactants, which,
consistent with experimental data, would probably be quite
slow. The Co(II) carbonyl complex was not observed by IR,
nately, the benefits of the TTA/TEA reductive quenching
system were not realized to their fullest potential because the
reduction of CO is much slower than the photochemical
2
+
consistent with the rapid exchange for CH CN, which is
formation of [Ru(bpy) ] . Nevertheless, we were able to
3
3
−1
calculated to be exergonic by 10.6 kcal mol (eq 13). Further
evidence for exchange via eq 13 is found in the similarity
between the UV spectra after stoichiometric reactions of
observe several stoichiometric reactions and elucidate many of
the mechanistic details of catalysis by a variety of spectroscopic
techniques supported by DFT calculations. Knowledge of the
reaction bottlenecks is particularly useful in advancing the next
generation of catalysts, and several were determined. (1) A spin
flip occurs during the reduction from Co(II) to Co(I), and the
reaction is inefficient. (2) The reduction to “Co(0)” (more
2
I
•−
−1
[
Co (L2 )] with CO or CO [ΔG° = −5.6 kcal mol with
2
L1 (eq 14) and confirmed by IR data]. These spectra are
identical and are characterized by three π−π* transitions
consistent with a dangling L2 ligand in contrast to the single
4
II
5
2+
•−
+
broad π−π* absorbance of [Co (κ -L2)] . The spin-
strictly [Co(I)L ]) by [Ru(bpy) ] is endergonic and may
3
2
II
3
2+
•
forbidden conversion of [Co (κ -L2)(NCCH )]
Co (κ -L2)] and CH CN was not observed in the
stoichiometric reaction of Co L2 with CO , in agreement
with the calculated ΔG° of 33.8 kcal mol .
to
require the potent reductant Et NC CH that is formed by a
secondary reaction of Et N . (3) A small structural change
3
2
3
4
II
5
2+
•+
[
3
3
0
from a TBP to a distorted octahedral geometry occurs during
2
−
1
the addition of CO to “Co(0)”. (4) A significant geometrical
2
change involving the dissociation of two Co−N bonds is
2[
2[
2[
Co L(CO )] + CO
II
2−
2
2
required during CO reduction to form the distorted
2
II
3
2+
2
+
tetrahedral [Co (κ -L)(CO)] complex. (5) The CO formed
may recoordinate to the reduced complex and act as an
inhibitor.
2
[Co (κ ‐L)] + CO + CO32−
II
5
→
(11)
(12)
II
2−
Co L(CO )] + CO2
2
2
+
2
[Co (κ ‐L)(CO)] + CO32−
II
3
→
ASSOCIATED CONTENT
■
*
S
Supporting Information
2
+
II
3
Co (κ ‐L)(CO)] + CH CN
3
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00433.
2
+
2
II
3
→
[Co (κ ‐L)(NCCH )] + CO
(13)
(14)
3
2[
2[
Co (L )] + CO → [Co (κ ‐L)(CO)]
I
•−
2
0
3
Detailed synthetic procedures, NMR spectra, summary of
the crystal data, bond distances and angles, cyclic
voltammograms, UV−vis and FT-IR spectra of various
cobalt species, additional photochemical data and tables
of DFT, CASSCF, and CASSCF+MP2 results, Cartesian
+
II
2−
+
2
II
5
Co L(CO )] + H → [Co (κ ‐L)(COOH)]
2
(15)
+
2[
Co (κ ‐L)(COOH)] + CO
II
5
coordinates of the C -symmetrized crystal structure of 1,
2
2
and DFT-calculated structures of key reduced species
and their adducts (PDF)
2
+
2
[Co (κ ‐L)(CO)] + HCO −
II
3
→
(16)
3
J
DOI: 10.1021/acs.inorgchem.8b00433
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Accession Codes
(11) Morris, A. J.; Meyer, G. J.; Fujita, E. Molecular Approaches to
the Photocatalytic Reduction of Carbon Dioxide for Solar Fuels. Acc.
Chem. Res. 2009, 42 (12), 1983−1994.
CCDC 1825268−1825269 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(
12) Schneider, J.; Jia, H. F.; Muckerman, J. T.; Fujita, E.
+
Thermodynamics and kinetics of CO , CO, and H binding to the
2
metal centre of CO reduction catalysts. Chem. Soc. Rev. 2012, 41 (6),
2
2
(
036−2051.
13) Yamazaki, Y.; Takeda, H.; Ishitani, O. Photocatalytic reduction
of CO using metal complexes. J. Photochem. Photobiol., C 2015, 25,
2
AUTHOR INFORMATION
1
(
06−137.
■
14) Manbeck, G. F.; Fujita, E. A review of iron and cobalt
Corresponding Author
E-mail: fujita@bnl.gov.
porphyrins, phthalocyanines and related complexes for electrochemical
and photochemical reduction of carbon dioxide. J. Porphyrins
Phthalocyanines 2015, 19 (1−3), 45−64.
*
ORCID
Koichi Kodama: 0000-0002-8567-9360
Gerald F. Manbeck: 0000-0002-6632-3895
Etsuko Fujita: 0000-0002-0407-6307
(15) Berardi, S.; Drouet, S.; Francas, L.; Gimbert-Surinach, C.;
Guttentag, M.; Richmond, C.; Stoll, T.; Llobet, A. Molecular artificial
photosynthesis. Chem. Soc. Rev. 2014, 43 (22), 7501−7519.
(
16) Qiao, J.; Liu, Y.; Hong, F.; Zhang, J. A review of catalysts for the
Notes
electroreduction of carbon dioxide to produce low-carbon fuels. Chem.
Soc. Rev. 2014, 43 (2), 631−675.
The authors declare no competing financial interest.
(17) Das, S.; Wan Daud, W. M. A. A review on advances in
ACKNOWLEDGMENTS
photocatalysts towards CO
0856−20893.
18) Tu, W.; Zhou, Y.; Zou, Z. Photocatalytic Conversion of CO2
2
conversion. RSC Adv. 2014, 4 (40),
■
2
(
We thank Drs. David Shaffer and M. Zahid Ertem for useful
discussions. The work carried out at Brookhaven National
Laboratory (BNL) was supported by the U.S. Department of
Energy, Office of Science, Division of Chemical Sciences,
Geosciences, & Biosciences, Office of Basic Energy Sciences,
under Contract DE-SC0012704. T.S. thanks the Japan Public−
Private Partnership Student Study Abroad Program “Tobitate!
Young Ambassador Program” for partial financial support
during her stay at BNL.
into Renewable Hydrocarbon Fuels: State-of-the-Art Accomplishment,
Challenges, and Prospects. Adv. Mater. 2014, 26 (27), 4607−4626.
(19) Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K.
Photocatalytic Reduction of CO on TiO and Other Semiconductors.
2
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Angew. Chem., Int. Ed. 2013, 52 (29), 7372−7408.
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electrochemical reduction of carbon dioxide. Chem. Soc. Rev. 2013, 42
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