Studies of cobalt-mediated electrocatalytic CO2 reduction using a redox-active ligand

Article abstract of DOI:10.1021/ic403122j

The cobalt complex [Co^IIIN₄H(Br)₂] ⁺ (N₄H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]- heptadeca-1(7),2,11,13,15-pentaene) was used for electrocatalytic CO₂ reduction in wet MeCN with a glassy carbon working electrode. When water was employed as the proton source (10 M in MeCN), CO was produced (f_CO= 45% ± 6.4) near the Co^I/0 redox couple for [Co ^IIIN₄H(Br)₂]⁺ (E_1/2 = -1.88 V FeCp₂^+/0) with simultaneous H₂ evolution (f_H2= 30% ± 7.8). Moreover, we successfully demonstrated that the catalytically active species is homogeneous through the use of control experiments and XPS studies of the working glassy-carbon electrodes. As determined by cyclic voltammetry, CO₂ catalysis occurred near the formal Co^I/0redox couple, and attempts were made to isolate the triply reduced compound ( [Co⁰N₄H] ). Instead, the doubly reduced ( Co^I ) compounds [CoN₄] and [CoN₄H(MeCN)]⁺ were isolated and characterized by X-ray crystallography. Their molecular structures prompted DFT studies to illuminate details regarding their electronic structure. The results indicate that reducing equivalents are stored on the ligand, implicating redox noninnocence in the ligands for H₂ evolution and CO₂ reduction electrocatalysis.

Full text of DOI:10.1021/ic403122j

Article
pubs.acs.org/IC
Studies of Cobalt-Mediated Electrocatalytic CO2 Reduction Using a
Redox-Active Ligand
David C. Lacy, Charles C. L. McCrory, and Jonas C. Peters*
Joint Center for Artificial Photosynthesis, Division of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: The cobalt complex [CoIIIN4H(Br)2]+ (N4H =
2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-
1(7),2,11,13,15-pentaene) was used for electrocatalytic CO2
reduction in wet MeCN with a glassy carbon working electrode.
When water was employed as the proton source (10 M in
MeCN), CO was produced (f CO= 45% 6.4) near the CoI/0
redox couple for [CoIIIN4H(Br)2]+ (E1/2 = −1.88 V FeCp2
)
+/0
with simultaneous H2 evolution (fH2= 30% 7.8). Moreover, we successfully demonstrated that the catalytically active species is
homogeneous through the use of control experiments and XPS studies of the working glassy-carbon electrodes. As determined
by cyclic voltammetry, CO2 catalysis occurred near the formal CoI/0redox couple, and attempts were made to isolate the triply
reduced compound (“[Co0N4H]”). Instead, the doubly reduced (“CoI”) compounds [CoN4] and [CoN4H(MeCN)]+ were
isolated and characterized by X-ray crystallography. Their molecular structures prompted DFT studies to illuminate details
regarding their electronic structure. The results indicate that reducing equivalents are stored on the ligand, implicating redox
noninnocence in the ligands for H2 evolution and CO2 reduction electrocatalysis.
consistent with appreciable current being consumed to
reductively degrade the molecular precursor. The catalytic
role of resulting heterogeneous material must therefore be
considered but was difficult to assess owing to the choice of
mercury as the working electrode. Mercury electrodes are
known to strongly adsorb N4-macrocyclic cobalt complexes,13
and there has been significant discussion about the activity of
dissolved molecular complexes versus mercury adsorbed species
in catalysis.14,15 Reports that some nominally discrete
homogeneous molecular cobalt-based electrocatalysts form
catalytically active heterogeneous deposits, even on glassy
carbon electrode surfaces, encourage added caution to be taken
when defining likely contributors to observed overall electro-
catalysis.16
The present combined synthetic/electrocatalytic study
employs a glassy-carbon working electrode to investigate the
cobalt-N4H system for CO2 reduction. The lead observation
made pertains to preferential CO2 reduction (to produce CO)
relative to H2 evolution when wet organic solvent is used. To
begin to develop a better understanding of this system,
synthetic studies are described wherein reduced and protonated
cobalt species that we presume are relevant to overall
electrocatalysis are characterized. The structural data obtained
for these species are correlated to DFT calculations and suggest
that redox noninnocent ligand behavior is likely operative. Of
course, the molecular studies described are directly relevant to
the overall CO2 reduction electrocatalysis only if homogeneous
INTRODUCTION
■
Transition metal complexes supported by nitrogen-donor
ligands constitute an important class of molecular electro-
catalysts for CO2reduction.1 Whereas the earliest report
featured phthalocyanine as a supporting ligand, a host of
nitrogen-rich donor ligands have since been employed that
include porphyrins,3 polypyridines,4 cyclam, and related
unsaturated N4-macrocycles.5 Convincing evidence has been
provided in support of a hypothesis whereby reducing
equivalents are stored on supporting polypyridine ligands
during electrocatalytic CO2 reduction.6 Redox noninnocence at
the ligand may have a significant impact on both substrate and
product selectivity.7 For example, bipyridyl-supported man-
ganese and rhenium tricarbonyl catalysts reduce CO2 rather
than protons in the presence of water and/or weak acids.8
These findings encouraged our interest in exploring the
importance of redox noninnocent ligand properties in electro-
catalytic CO2 reductions.
In the present study we continue the theme of elucidating a
role for ligand redox noninnocence in the context of
electrocatalytic CO2 reduction by cobalt. We employ the
ligand N4H (N4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo-
[11.3.1]-heptadeca-1(7),2,11,13,15-pentaene), chosen because
it contains a potentially redox-active pyridyldiimine (PDI)
moiety.9,10 Additionally, there is literature precedent for
electrocatalytic CO2 reduction with the PDI platform with
cobalt.11 In particular, three prior studies employed the N4H
ligand for electrocatalytic CO2 reduction and showed
production of large amounts of H2 relative to CO and low
overall current efficiencies.5a,12 The latter observation is
Received: December 30, 2013
Published: April 28, 2014
© 2014 American Chemical Society
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catalysis is operative. Several control experiments support a
dominant role for homogeneous species in the observed CO2
electrocatalysis, despite the fact that some heterogeneous cobalt
material forms on the electrode surface during electrocatalysis.
magnitude of the catalytic wave near the CoI/0 couple in the CV
(Figure 1). A different current response was observed when no
CO2 was present (Figure 1, bottom). These findings are
generally consistent with an early study by Tinnemans et al. in
1984, where it was first demonstrated that CVs of
[CoIIIN4H(Br)2]+ were affected by CO2 and that H2O could
enhance the catalytic wave.12a
Tinnemans et al. and Che et al. performed controlled
potential electrolysis (CPE) experiments under various
conditions, and CO was produced in 20−30% Faradaic
efficiency at −1.8 V in MeCN with water concentrates as
high as 3 M (Table S2 in the Supporting Information [SI]). We
obtained similar results and observed turbid solutions after an
extended 40 min CPE in MeCN with water concentrations as
low as 0.035 4 M (measured by Karl Fischer) and likewise
obtained low total current efficiencies for CO (f CO = 24%) and
little to no H2 (f H2 = <1%). However, increasing the water
concentration to 10 M gave substantially higher Faradaic
RESULTS
■
Electrocatalytic CO2 Reduction with [CoIIIN4H(Br)2]+ in
Wet MeCN. Cyclic voltammograms (CVs) of 0.3 mM
solutions of [CoIIIN4H(Br)2]+ with nBu4NPF6 in MeCN were
measured (Figure 1), and three reversible redox couples were
efficiencies for CO (f CO = 45%
6.4), with H2 production
(fH2 = 30% 7.8) (Table 1).17 The solutions with 10 M H2O
Table 1. Results of Controlled Potential Electrolysis (CPE)
Experiments Held at −2.0 V vs a Ag/AgNO3 Reference
a
Electrode for 40 Min
comments
q/ C
f CO/ %
f H2/ %
[CoN4H(Br)2]+ in MeCN (10 M
22 3.2
45 6.4
30 7.8
H2O) w/ CO2
[CoN4H(Br)2]+ in MeCN (10 M
H2O) w/o CO2
40 4.5
0
63 3.5
83 4.0
61 13*
66 17*
CoCl2 in MeCN (10 M H2O) w/ 6.9 0.4
CO2
<1%
background in MeCN (10 M
H2O) w/CO2 w/bare electrode
2.1 1.3
2.9 1.3
<1%
<1%
background in MeCN (10 M
H2O) w/ CO2 w/ used
electrode
a
The reference electrode was externally referenced to a solution
0/+
containing ferrocene in MeCN with 0.1 M nBu4NClO4. The FeCp2
couple occurred at 0.13 V. See Experimental Section for conditions.
*
The hydrogen detected was near the detection limit of the GC
(∼1000 ppm) and accounts for the large error.
did not become turbid, and a UV−vis spectrum of the working
solution after a CPE displayed features that were similar to
those observed for [CoIIN4H(Br)]+ and [CoIIN4H(MeCN)]2+
(vida infra) (Figure S1 in the SI); though quantification was
not possible because the exact speciation could not be
established, the spectrum indicates that a [CoIIN4H]2+ complex
is the majority species present. Over the course of these 40-min
CPE experiments 22 3.2 C were passed, and a plot of current
vs time was relatively constant (Figure 1, inset). These data
correspond to a TONCO of 4.1 0.9 and TONH2 of 2.8 1.0.
In contrast to the original report,12a no formate or oxalate was
detected under any set of conditions. Substitution of CO2 with
NaHCO3 (30 mM, saturated) in bulk electrolysis experiments
did not yield any reduced CO2 products.
Figure 1. (Top) Cyclic voltammograms of 0.3 mM [CoIIIN4H(Br)2]+
in MeCN (black), addition of CO2 (red), and addition of water to
make a 10 M solution in MeCN (blue). Inset shows a plot of current
vs time for a 40-min controlled potential electrolysis experiment
(potential held at −2.13 vs FeCp2+/0). (Bottom) CV of 0.7 mM
[CoIIIN4H(Br)2]+ in MeCN with 10 M H2O with CO2 (blue) and
without CO2 (green); the dashed black line is the background current
in MeCN with 10 M H2O and CO2 with no added cobalt complex.
Conditions: scan rate = 0.100 V/s; supporting electrolyte is 0.1 M
nBu4NPF6; working electrode = glassy carbon; reference electrode =
isolated Ag/AgNO3 (1 mM) with 0.1 M nBu4NPF6; counter electrode
= glassy carbon.
It has been previously reported that, in the case of hydrogen
evolution by a tris(glyoximate) cobalt clathro chelate precatalyst
complex, the catalytic species is a cobaltous material electro-
deposited onto the electrode surface at negative potentials
rather than the molecular complex.16a To test whether the
catalytic species in the present study might be an electro-
deposited film, the electrode surface was probed with XPS, and
several informative control experiments were conducted. After a
observed at −0.40 V, −0.92 V, and −1.88 V (all potentials are
referenced to the FeCp2+/0 couple). These redox events are
assigned as the formal CoIII/II, CoII/I, and CoI/0 couples,
respectively. When MeCN solutions of [CoIIIN4H(Br)2]+ were
saturated with CO2, an increase in the magnitude of the
reductive current appeared near the CoI/0 couple, suggesting
the possibility of catalytic activity. Adding water increased the
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CPE experiment, the electrode was removed and washed with
fresh acetonitrile and water, and the surface was then probed by
XPS. A very low coverage of <0.3 atom % cobalt was found on
the surface (Figure S2 and S3, Table S3 in the SI),
corresponding to a Co/C ratio of <0.004. For comparison, a
monolayer coverage of e-beam deposited Co on SiO2 has a Co/
Si ratio of ∼0.1,19 and a monolayer coverage of electro-
deposited Co oxide on Au has a Co/Au ratio of ∼1.4.20 This
suggests that there is significantly less than a monolayer
coverage of Co deposited onto the glassy carbon electrodes
post-CPE, although rigorous quantification of Co coverage
would require additional experiments beyond the scope and
intent of this manuscript. To test whether the deposited
material was catalytically active, electrodes that were used for
CPE experiments with [CoIIIN4H(Br)2]+ were rinsed with fresh
acetonitrile and water, and an additional 40 min electrolysis was
run under identical conditions, except that the precatalyst
[CoIIIN4H(Br)2]+ was not added to the solution. For these
background experiments with the “used electrodes”, 2.9 1.3
C of charge were passed with f CO = <1%. The results are similar
to those obtained from the background CPE experiments in
which a clean unused electrode was used with no dissolved
X-ray diffraction were obtained by slow diffusion of pentane
into a concentrated benzene solution of [CoN4].
The molecular structure of [CoN4] revealed that the cobalt
ion is four-coordinate and distorted square planar (Figure 2).
Figure 2. X-ray crystal structure of [CoN4] (left) and
[CoN4H(MeCN)]+ (right). Except for the NH group, hydrogen
atoms are removed for clarity. Thermal ellipsoids displayed at 50%
probability. The [BPh4]− counteranion in [CoN4H(MeCN)][BPh4]
has been removed for clarity.
The amido nitrogen N4 is planar (Σ∠ = 360.0°), and no
hydrogen was found in the difference map. The cobalt-pyridine
Co1−N1 bond distance is 1.800(2) Å, a contraction compared
to 1.848(1) Å in [CoIIN4H(MeCN)]2+ (vide infra; Table 2).
precatalyst (2.1
1.3 C, f CO = <1%). For these control
experiments, the amount of charge passed is appreciably less
than those containing dissolved precatalyst complex (22 3.2
C), and more importantly, selectivity for CO2 over H+ is
dramatically enhanced in the presence of [CoIIIN4H(Br)2]+. We
also performed control CPE experiments with dissolved CoCl2
which formed turbid solutions and produced H2 in fH2 = 83%
4.0 and f CO = <1% (6.9 0.4 C). Note that it is known that
CoCl2 deposits cobalt material on glassy carbon.16c These
observations provide evidence that any cobalt material
deposited on the electrode is not responsible for the observed
CO2 reduction catalysis and that a molecular cobalt−N4H
complex is involved in the electrocatalytic process.
Table 2. Comparison of metrical parameters from XRD data
for [CoN4], [CoN4H(MeCN)]+, and [CoIIN4H(MeCN)]2+
[CoN4]
[CoN4H(MeCN)]+ [CoIIN4H(MeCN)]2+
Bond Distances (Å)
Co1−N1
Co1−N2
Co1−N3
Co1−N4
N2−C2
N3−C3
Co1−N5
C1−C2
1.800(2)
1.881(2)
1.888(2)
1.810(2)
1.330(2)
1.330(2)
−
1.807(1)
1.920(1)
1.848(1)
1.957(1)
1.971(1)
1.966(1)
1.303(2)
1.294(2)
2.105(1)
1.476(3)
1.474(3)
1.921(1)
2.023(1)
1.319(2)
Synthesis and Molecular Structure of [CoN4]. Consid-
ering that electrocatalytic CO2 reduction occurred at a potential
very close to the CoI/0 couple, we endeavored to isolate the
reduced species and study its stoichiometric reactions with
CO2. This was accomplished by using [CoIIN4H(Br)]Br as a
synthon.21 Reduction of [CoIIN4H(Br)]Br with 2 equiv KC8 in
THF afforded a dark-purple, benzene soluble product that
1.325(2)
1.998(1)
1.442(2)
1.444(2)
C3−C4
1.436(2)
1.435(2)
Bond Angles (deg)
155.48(2)
160.92(2)
N1−Co−N4
N2−Co−N3
178.81(5)
162.91(5)
172.671(4)
162.101(3)
analyzed as [CoN4] (Scheme 1).22 The H NMR spectrum of
1
[CoN4] in C6D6 is that of a diamagnetic species and contains
four aliphatic and two aromatic C−H resonances but is missing
the resonance expected for the NH group of the N4H ligand
(Figure S4 in the SI).23 Further indication of the absence of an
N−H bond comes from the lack of a ν(NH) in the FTIR-ATR
spectrum of [CoN4] (Figure S5 in the SI). Crystals suitable for
Similarly short Co−Npy bond distances of 1.787(2) Å and
1.797(3) Å were observed for the related [PDI]CoCl
complexes.24,25 For [CoN4], the Co1−N1 contraction is
accompanied by a shortening of the Co1−N4(amido) distance
to 1.810(2) Å, down from 1.966(1) Å in [CoIIN4H(MeCN)]2+.
Compared to other cobalt−amido bond distances, the Co1−
N4 distance in [CoN4] is unusually short. For comparison,
Fryzuk et al. has characterized several high-spin [CoIPNP]
complexes with Co−Namido bond distances ranging from
1.898(3) to 1.904(3) Å.26 Caulton and co-workers have
reported a three coordinate S = 1 [CoIPNP] complex which
has a Co−Namido bond distance of 1.973(2) Å that shortens by
∼0.03 Å when CO is bound.27a Additionally, Mindiola and co-
workers synthesized a different [CoIPNP]2(μ-N2) complex with
a Co−Namido distance of 1.928(2) Å.27b Except for [CoIPNP-
(CO)] and [CoIPNP]2(μ-N2), the difference in spin state
(paramagnetic [CoIPNP] vs diamagnetic [CoN4]) makes it
difficult to meaningfully compare these bond distances to those
in [CoN4]. Finally, the imine C−N bond distances (1.330(2)
Scheme 1. Synthesis of [CoN4], [CoN4H(MeCN)]+, and
a
[CoIIN4H(MeCN)]2+
a
Conditions: (a) 2KC8, THF, RT; (b) NaBPh4, MeCN/H2O (3:1),
RT; (c) [H-DMF][OTf], MeCN, RT; (d) electrochemical reduction,
+/0
E1/2 = −0.92 V vs FeCp2
.
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Å) in [CoN4] are longer than expected for a redox-innocent
PDI moiety, which complicates the oxidation state assignment
of the cobalt ion and is discussed further below.
S y n t h e s i s a n d M o l e c u l a r S t r u c t u r e o f
[CoN4H(MeCN)]+. We investigated the possibility of isolating
the protonated form of [CoN4]. This was accomplished by
dissolving purple [CoN4] in a 3:1 MeCN/H2O mixture,
affording a dark-forest green solution (Scheme 1). The new
species [CoN4H(MeCN)][BPh4] was isolated in 65% yield by
precipitation with NaBPh4. The 1H NMR spectrum of
[CoN4H(MeCN)]+ contains sharp resonances in the diamag-
netic region and includes a resonance at 2.43 ppm (1H, t, JN−H
= 11.5 Hz) that integrates to one proton. This resonance is
greatly diminished in the 1H NMR spectrum of the isotopomer
[CoN4D(MeCN)]+ (Figure S6 in the SI). The FTIR-ATR
spectrum of [CoN4H(MeCN)]+ contains an isotopically
sensitive ν(NH) band at 3250 cm−1 that shifts to 2415 cm−1
when [CoN4D(MeCN)]+ is used (ν(NH)/ ν(ND) = 1.345;
calcd = 1.370) (Figure S5 in the SI).
Figure 3. X-ray crystal structure of [CoIIN4H(MeCN)]2+. Except for
the NH group, hydrogen atoms are removed for clarity. Thermal
ellipsoids displayed at 50% probability. The [BPh4]− and [OTf]−
counteranions in [CoIIN4H(MeCN)][OTf][BPh4] have been re-
moved for clarity.
consistent with the spectroscopic assignment of the product
being a CoII ion bound to the neutral ligand N4H.
Electrochemical Properties of [CoN4H(MeCN)]+ and
[CoIIN4H(MeCN)]2+ and Electrocatalytic H2 Evolution. We
also probed the electrochemical properties of
[CoN4H(MeCN)]+ and [CoIIN4H(MeCN)]2+. The CV of
[CoN4H(MeCN)]+ (Figure 4) exhibits two reversible couples,
The X-ray crystal structure of the cation [CoN4H(MeCN)]+
reveals a five-coordinate distorted square pyramidal (τ = 0.09)
cobalt ion bound to N4H (Figure 2). In contrast to [CoN4], the
amine nitrogen N4 is protonated and pyramidalized (Σ∠ =
338.8°). The Co1−N1 bond distance in [CoN4H(MeCN)]+ is
1.807(1) Å, similar to that found in [CoN4]. However, in
[CoN4H(MeCN)]+ the Co1−N4 bond is elongated to
2.023(1) Å reflecting the protonation of the amido ligand. An
acetonitrile molecule occupies the fifth coordination site of the
cobalt ion. As with [CoN4], the C2−N2 and C3−N3 imine
bond lengths in [CoN4H(MeCN)]+ are elongated to 1.319(2)
Å and 1.325(2) Å, respectively, compared to those in
[CoIIN4H(MeCN)]2+.
S y n t h e s i s a n d M o l e c u l a r S t r u c t u r e o f
[CoIIN4H(MeCN)]2+. We explored the stoichiometric reactivity
of [CoN4H(MeCN)]+ with acid. Treating [CoN4H(MeCN)]+
with one equivalent of [H-DMF][OTf] (pKa = 6.1)28 in MeCN
resulted in an immediate color change from dark-forest green
to orange-red. The headspace of the reaction analyzed by GC
confirmed production of 1/2 equiv H2 (47% 4). A similar
experiment was performed in a sealed J-young tube with [H-
DMF][OTf] and [CoN4D(MeCN)]+, and it was found that the
1H NMR spectrum contained an H2 resonance but showed no
evidence of coupling from incorporation of deuterium. This
result suggests that the proton on the N4H ligand is not
incorporated into the H2 product.
Figure 4. CV of 1 mM [CoN4H(MeCN)]+ in MeCN before (black)
and after (dashed blue) addition of 1 equiv [H-DMF][OTf].
Conditions: 0.1 M nBu4NPF6; reference electrode = silver wire (* =
internal FeCp2).
The orange-red solution resulting from treatment of
[CoN4H(MeCN)]+ with [H-DMF][OTf] contained a para-
magnetic S = 1/2 cobalt complex (μeff = 1.6 μB, RT in
CD3CN). This conclusion is supported by the 77 K EPR
spectrum (Figure S7 in the SI) of a frozen solution of the
purified material, which was isolated in 87% yield and analyzed
as [CoIIN4H(MeCN)][OTf][BPh4]. The UV−vis and EPR
spectra of the product are nearly identical to those very recently
reported by Deronzier and co-workers (Figure S8 in the SI) in
the context of photochemical H2 production.30 The molecular
structure of [CoIIN4H(MeCN)]2+ was determined by X-ray
diffraction and revealed a five-coordinate (τ = 0.18) cobalt
complex with two outer sphere anions (Figure 3). The amine
on N4H is H-bonded to the triflate anion as indicated from the
N4···O1 distance of 2.844(2) Å. Compared to [CoN4] and
[CoN4H(MeCN)]+, the complex [CoIIN4H(MeCN)]2+ has
noticeably shorter imine C−N bond lengths (1.303(2) Å and
1.294(3) Å) (Table 2). These shorter bond lengths are
one oxidation at −0.88 V vs FeCp2 (formally the CoI/II
+/0
+/0
couple) and one reduction at −1.88 V vs FeCp2 (formally
the CoI/0 couple).31 Addition of 1 equiv [H-DMF][OTf]
caused the solution to turn orange red, but there was no change
in the CV except that the resting state potential shifted positive
of the redox couple at −0.88 V, indicative of an oxidation state
change. Notably, addition of excess [H-DMF][OTf] or tosic
acid (TsOH·H2O) resulted in the formation of a catalytic wave
near the CoII/I couple consistent with electrocatalytic H2
evolution (Figure S9 in the SI). CPE experiments were
conducted at two potentials for TsOH·H2O (pKa = 8)32 and
2,6-dichloroanalinium tetrafluoroborate ([2,6-DCA][BF4], pKa
= 5.1),33 the results of which are presented in Table S4 in the
SI. These experiments confirm H2 as the major product with
>85% Faradaic efficiency.
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description for [CoN4] having a CoII ion bound to N4 (and
•−
DISCUSSION
■
N4H•− for [CoN4H(MeCN)]+).
The CVs of [CoIIIN4H(Br)2]+ in the presence of CO2 indicated
that CO2 reacts at the formal “CoI/0” couple. On the basis of
the reversibility of this couple and on the successful isolation of
the [NiN4H] complex,10 we anticipated that treatment of
cobalt−N4H complexes with the appropriate stoichiometric
reductant would produce an analogous [CoN4H] complex.
However, in all attempts the only tractable cobalt-containing
product was [CoN4]. To summarize, treatment of either
[CoIIN4H(Br)]+ or [CoIIN4H(MeCN)]2+ with 2 equiv KC8 or
NaHg only formed [CoN4]. The same occurred when 1 equiv
KC8 was reacted with [CoN4H(MeCN)]+. The mechanism by
which [CoN4] is formed is not known; however, a cobalt-
hydrido species may result from isomerization of [CoN4H] and
subsequently lose H• bimolecularly in the form of H2 (Scheme
2).22 This contrasts the stability of [NiN4H], which does not
appear to lose H2 to form the corresponding [NiN4] complex.
To supplement the crystallography, we also performed DFT
calculations on [CoN4] and [CoN4H(MeCN)]+ and found that
in both cases the open-shell singlet configuration was 4.4 and
4.6 kcal/mol, respectively, lower in enthalpy than the closed-
shell configuration. A surface plot of the atomic spin density
(Figure 5) supports the hypothesis that an anion radical (N4
•−
,
Figure 5. Mulliken atomic spin-density surface of [CoN4] (left) and
[CoN4H(MeCN)]+ (right) (isovalue = 0.003). Hydrogen atoms
removed for clarity.
Scheme 2. Plausible Outline for CO2 Reduction Involving
the Putative “[CoN4H]” Complex That Is Formed at the
Formal CoI/0 Redox Couple; the “[CoN4H]” Complex Is
Unstable to Loss of H2 but Can Be Intercepted by CO2
N4H•−) is antiferromagnetically coupled to a low-spin CoII ion.
For [CoN4] and [CoN4H(MeCN)]+, the atomic spin-density
on the cobalt ion is 0.75 and 0.81, respectively, with the
remaining spin density distributed throughout the ligand.
The redox noninnocence of N4H may have implications on
selectivity between H2 evolution and CO2 reduction electro-
catalysis. This is important because the difficulty of decoupling
CO2 reduction from H2 evolution remains an outstanding
challenge in electrocatalysis. For example, some of the better
H2-evolving molecular electrocatalysts are cobalt N4-macrocyles
with imino-glyoximate ligands.34 However, to our knowledge
there are no reports of successful electrocatalytic CO2 reduction
using cobalt complexes with these ligands.35 Recent inves-
tigations into the electronic structure of reduced cobalt and
nickel complexes with imino-glyoximate ligands have suggested
redox noninnocence, albeit with deleterious side reactions
resulting in ligand modification.36 Including this report,
previous investigations from Peters,34i Deronzier30 and Lau37
have demonstrated that [CoIIIN4H(X)2]+ (X = halogen) is a
competent precatalyst for H2 evolution. It was therefore
surprising that [CoIIIN4H(Br)2]+ is a competent precatalyst
for CO2 reduction under the appropriate conditions. The
stability of the reduced N4H•− ligand radical anion, and perhaps
the ability to accommodate a second redox equivalent as
demonstrated by [NiN4H], likely contributes to preferential
CO2 reduction in wet MeCN. Although the putative [CoN4H]
complex is unstable to loss of H2, the electrogenerated
[CoN4H] intermediate is at least sufficiently stable under
electrocatalytic conditions to be intercepted by CO2 (Scheme
2). The mechanism by which CO and H2 are formed, either by
two competing or one synchronous path, is still undetermined
and currently under investigation.
This loss of H2 at the CoI/0 redox event may be a
consequence of ligand noninnocence and its interaction with
the cobalt ion. To probe this possibility, we explored the
electronic structures of the reduced complexes we prepared. On
the basis of the charge of [CoN4] and [CoN4H(MeCN)]+, the
cobalt ions are in the +1 oxidation state. However, the imine
C−N bond distances suggest a more complicated description of
the electronic structures. Several research groups have
investigated [PDI]CoX (X = halide, alkyl, H) com-
plexes.9,10,24,25 From these studies, the sum of the results
suggest that the electronic environments of [PDI]CoX species
(formally CoI) are usually best described as having an open-
shell singlet configuration in which a low-spin CoII ion is
antiferromagnetically coupled to a ligand-based anion radi-
cal.24,25 The [NiN4H] complex, that Wieghardt and co-workers
have characterized, formally contains a “Ni0” ion but has
elongated imine C−N bond lengths (1.353(7) Å and 1.351(8)
Å).10 Along with DFT studies, the complex [NiN4H] was
interpreted as containing a NiII ion bound to the dianionic
ligand N4H2−. The imine C−N bond lengths of [CoN4] and
[CoN4H(MeCN)]+ are in between those of N4H2−and N4H,
and close to those in the [PDI]CoX complexes, suggesting a
CONCLUSIONS
■
In this report, we have explored the CO2 reduction activity of a
cobalt complex with a redox active pyridyldiimine moiety. In
particular, we have shown that the formally “CoI” complex
[CoN4H(MeCN)]+ is a precatalyst for the reduction of CO2 to
CO in CO2-saturated solutions of MeCN with 10 M H2O (fCO
= 45% 6.4), and does so preferentially to H+ reduction even
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though the complex is known to be competent for electro-
catalytic H2-evolution. XPS measurements of glassy carbon
electrodes post-CPE and other control experiments support the
assertion that the dissolved molecular complex, rather than an
electrodeposited film, is responsible for the observed electro-
catalysis. In addition, we have isolated the formally “CoI”
complex [CoN4H(MeCN)]+ by protonation of the “CoI”−
amido complex [CoN4], both of which were characterized by
X-ray crystallography. Along with the molecular structures,
broken symmetry DFT calculations suggest they are nominally
described as low-spin CoII ions antiferromagnetically coupled to
a ligand radical-anion (N4H•− and N4•−), a result consistent
with the [PDI]CoX literature.9 The stability of the N4H•−
ligand radical anion and ability to accommodate a second redox
equivalent may contribute to the preferential reduction of CO2
over H+ in the presence of large concentrations of water.
from the solution by a Vycor frit (Bioanalytical Systems, Inc.)
and contained 0.1 M nBu4NClO4. The cell was prepared with
degassed solvent on a Schlenk line with N2 or CO2 for 30 min
and then sealed before the beginning of each controlled-
potential electrolysis experiment. Each controlled-potential
electrolysis experiment was conducted for 40 min at the
specified potential (−2.0 V vs the Ag/AgNO3 reference
electrode) under vigorous stirring (the stir plate was set to
900 rpm). The amount of CO and H2 evolved was quantified
from an analysis of the headspace with an Agilent 7890A gas
chromatograph using a thermal conductivity detector. Faradaic
efficiencies were determined by dividing the measured CO and
H2 produced by the amount of CO and H2 expected on the
basis of the charge passed during the controlled-potential
electrolysis measurement.
X-ray Photoelectron Spectroscopy. The surface compo-
sition of the carbon electrode surface after a 40-min bulk
electrolysis in the presence of [CoIIIN4H(Br)2]+ and CO2 was
determined via XPS on a Kratos Axis Nova spectrometer with
DLD (Kratos Analytical; Manchester, UK). The excitation
source for all analysis was monochromatic Al Kα1,2 (hv =
1486.6 eV) operating at 30 mA and 15 kV. The X-ray source
was directed 45° with respect to the sample normal. A base
pressure of 1 × 10−9 Torr is maintained in the analytical
chamber, which rises to 5 × 10−9 Torr during spectral
acquisition. All spectra were acquired using the hybrid lens
magnification mode and slot aperture, resulting in an analyzed
area of 700 μm × 400 μm. Survey scans were collected using
160 eV pass energy, while narrow region scans used 20 eV;
charge compensation via the attached e−-flood source was not
necessary in this study. The following sequence of scans was
performed: Survey (−5−1200 eV), Na 1s (1068−1076 eV), O
1s (528−538 eV), Ag 3d (364−378 eV), C 1s (280−292 eV),
Si 2s (146−161 eV) and Co 3p (52−70 eV).
Subsequent peak fitting and composition analysis was
performed using CasaXPS version 2.3.16 (Casa Software Ltd.;
Teignmouth, UK). Energy scale correction for the survey and
narrow energy regions was accomplished by setting the large
component in the C 1s spectrum, corresponding to a C 1s
C(C) transition, to 284.8 eV. All components were fitted
using a Gaussian 30% Lorentzian convolution function. For
quantification, Shirley baselines were employed where there
was a noticeable change in CPS before and after the peak in the
survey spectrum; otherwise, linear was chosen. Atomic
percentages were calculated using the CasaXPS packages for
regions and/or components and are reported herein.
Calculations were performed using region or component
areas normalized to relative sensitivity factors specific to the
instrument conditions with deconvolution from the spectrom-
eter transmission function.
EXPERIMENTAL SECTION
■
General and Physical Methods. All reagents were
purchased from commercial sources and used as received
unless otherwise noted. Solvents were sparged with nitrogen
and dried over columns containing molecular sieves or alumina.
The deuterated solvents were degassed and dried over activated
3 Å sieves prior to use. NMR spectra were recorded on Varian
300, 400, and 500 MHz spectrometers. H and 13C chemical
1
shifts are reported in ppm relative to residual solvent as internal
standards and are singlets unless otherwise stated. Identification
of 13C shifts were made on the basis of standard 2D methods
(HSQC and HMBC). Elemental analyses were performed on a
Perkin Elmer 2400 CHNS analyzer. Electronic absorbance
spectra were recorded with a Cary 50 spectrometer. Fourier
transform infrared ATR spectra were collected on a Thermo
Scientific Nicolet iS5 spectrometer with diamond ATR crystal
(utilized iD5 ATR insert). GC measurements were collected
using an Agilent Technologies 7890A GC system with front
and back TCD channels. X-band EPR spectra were recorded on
a Bruker EMX spectrometer. Electrochemical experiments were
conducted using a Biologic VSP-300 five-channel potentiostat
using the EC Lab Express version 5.53 software package.
Electrochemical Methods. Cyclic Voltammetry. Unless
otherwise stated, the working electrode was a 0.071 cm2
diameter glassy carbon disk electrode (CH instruments), and
the counter electrode was carbon rod (99.999% Strem). The
reference electrode was a Ag/AgNO3 (1.0 mM)/MeCN
nonaqueous reference electrode (also contained 0.1 M
nBu4NPF6) separated from the solution by a Vycor frit
(Bioanalytical Systems, Inc.) and externally referenced to
ferrocene. Alternately, the reference electrode was a silver
wire with an internal ferrocene standard (internally referenced
+/0
CVs contain an asterisk indicating the FeCp2 couple).
Synthetic Methods. [CoI I N4 H(Br)]Br and
[CoIIIN4H(Br)2]Br were synthesized on a Schlenk line
according to literature methods.21 Remaining manipulations
and syntheses were conducted in a Vacuum Atmospheres, Co.
drybox under a nitrogen atmosphere. Solvents were dried using
a JC-Meyer solvent system and otherwise degassed with N2
before use. KC8 and [H-DMF][OTf] were synthesized
according to literature procedures.38,39
Controlled-Potential Electrolysis. Controlled-potential elec-
trolysis experiments were conducted at ambient pressure in a
sealed two-chamber cell where the first chamber held the
working and reference electrodes in 40 mL of 0.1 M
nBu4NClO4 in MeCN with 0.3 mM catalyst, and the second
chamber held the auxiliary electrode in 19 mL of 0.1 M
nBu4NClO4 in MeCN with 20 mM FeCp2. The two chambers
were separated by a fine porosity glass frit. A 6 cm × 1 cm × 0.3
cm glassy carbon plate (Tokai Carbon U.S.A.) was used as the
working electrode, about a quarter of which was submerged in
the solution. The auxiliary electrode was a nichrome wire
(EISCO scientific). The reference electrode was a Ag/AgNO3
(1 mM)/MeCN nonaqueous reference electrode separated
Preparation of [CoIIN4H(Br)]Br. Following a slightly
modified procedure from Busch,21 2,6-diacetylpyridine (1.00
g, 6.13 mmol) and CoBr2 (1.35 g, 6.17 mmol) were dissolved in
20 mL of degassed EtOH and treated with 0.5 mL of H2O.
Dropwise addition of 3,3′-diaminodipropylamine to the blue-
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green solution caused the solution to become dark-red and
opaque. After complete addition of 3,3′-diaminodipropylamine,
the solution was treated with glacial acetic acid (1 μL), and the
resulting dark-purple heterogeneous mixture was stirred for 12
h at 50 °C and afterward cooled to room temperature. The
purple solid was collected on a glass fritted funnel and washed
with Et2O and dried over P2O5 for 24 h (2.5 g, 84%). FTIR-
ATR: (solid powder, cm−1) 3193, 3056, 2930, 2862, 1585,
1567. The elemental analysis match literature but are reported
here for convenience: Anal. Calcd (found) for [CoIIN4H(Br)]-
Br·0.5H2O (C15H23Br2CoN4O0.5): %C 37.06, (37.22); %H
4.77, (4.48); %N 11.53, (11.39). Post drying with P2O5 Anal.
Calcd (found) for [CoIIN4H(Br)]Br (C15H22Br2CoN4): %C
37.76, (37.28); %H 4.75, (4.93); %N 11.74, (11.35).
inky-purple. After stirring for 5 min, 2 mL of pentane was
added and allowed to stir for an additional 1 min after which
the solution was passed through a filter pipet containing glass-
fiber filter paper and a Celite pad, removing white/gray solid
(KBPh4 and C). The resulting homogeneous inky-purple
filtrate was reduced to dryness in vacuo, and the resulting solid
was treated with 5 mL benzene followed by 1 mL of pentane
and again filtered through a filter pipet with glass wool. The
resulting filtrate was reduced to dryness in vacuo (this process
was repeated at least one more time). The resulting product
was identical to [CoN4] and analytically pure.
Preparation of [CoN4H(MeCN)][BPh4]. Solid [CoN4] (107
mg, 0.339 mmol) was dissolved in MeCN (15 mL) with
NaBPh4 (118 mg, 0.345 mmol), and degassed H2O (2 mL) was
added causing a color change from dark purple to dark green
and was brought into an oxygen-free “wet” box. A small amount
of water was added (∼0.5−5 mL) until crystallization was
induced, and the resulting mixture was allowed to rest at room
temperature for several days. The dark-green crystals were
isolated on a glass fritted filter funnel and dried under reduced
Preparation of [CoIIIN4H(Br)2]Br. The salt [CoIIIN4H(Br)2]
Br was prepared according to literature procedures.18 Aerobic
oxidation of in situ prepared [CoIIN4H(Br)2] in the presence of
1 equiv HBr(aq) overnight afforded a green solution.
Recrystallization was accomplished by diffusion of Et2O into
MeOH solutions of [CoIIIN4H(Br)2]Br. The solid was dried
1
1
under reduced pressure with P2O5. H NMR (MeOD-d3, 400
pressure for at least 12 h (149 mg, 65%). H NMR (MeCN-d3
MHz): δ 8.57 (3H, m, Ar-H), δ 6.43 (1H, t, J = 11.5, NH), δ
4.20 (2H, d, J = 16.4, CH), δ 3.65 (2H, t, J = 13.5, CH), δ 3.48
(2H, q, J = 11.5, CH), δ 3.12 (2H, d, J = 12.4, CH), δ 2.92 (6H,
s, CH3), δ 2.28 (4H, m, CH2). Anal. Calcd (found) for
[CoIIIN4H(Br)2]Br (C15H22Br3CoN4): %C 32.34, (32.07); %H
3.98, (4.04); %N 10.06, (9.78).
400 MHz): δ 8.48 (1H, t, J = 7.7 Hz, Ar-H), δ 7.71 (2H, d, J =
7.7 Hz, Ar-H), δ 7.27 (8H, br s, Ar-H), δ 6.99 (8H, t, J = 7.3
Hz), Ar-H), δ 6.84 (4H, t, J = 7.1 Hz, Ar-H), δ 4.47 (2H, d, J =
14.2 Hz, CH), δ 3.29 (2H, t, J = 13.4 Hz, CH), δ 2.86 (2H, t, J
= 11.3 Hz, CH), δ 2.75 (2H, q, J = 11.5 Hz, CH), δ 2.42 (1H, t,
J = 11.5, NH), δ 2.22 (2H, d, J = 14.9, CH), δ 1.96 (s,
coordinated MeCN), δ 1.82 (2H, q, J = 12.9, CH), δ 1.63 (6H,
s, CH3).13C NMR (C6H6, 100 MHz) δ 164.8 (q, J = 49.5,
BPh4), δ 148.2 (ortho-Cpy) δ 136.7 (q, J = 1.34, BPh4), δ 126.6
(q, J = 2.65, BPh4), δ 124.6 (meta-Cpy), δ 122.7 (BPh4), δ 116.6
(para-Cpy), δ 54.2 (CH2), δ 52.5 (CH2), δ 29.6 (CH2), δ 16.6
(CMe). UV−vis: λmax (MeCN, nm (ε, M−1 cm−1)) 331 (6900),
430 (4750), 687 (1300), 845 (sh). FTIR-ATR: (cm−1) 3251,
3047, 2979, 2935, 2920, 2873, 2840, 1579, 1475, 1426, 1386,
1306, 1257, 1142, 1060. Anal. Calcd (found) for
[CoN4H(MeCN)][BPh4], C41H45BCoN5: %C 72.68 (72.67);
%H 6.69 (6.63); %N 10.34 (10.26).
Yield of H2 from Treatment of [CoN4H(MeCN)][BPh4] with
[H-DMF][OTf]. Solid [CoN4H(MeCN)][BPh4] (20.0 mg, 0.030
mmol) was dissolved in 5 mL of MeCN in a 250 mL round-
bottom flask and sealed. A stock solution of [H-DMF][OTf] in
MeCN was added (1.0 mL, 30 mM), causing the immediate
color change from dark green to orange-red. After 10 min, the
headspace was sampled with a gas-tight syringe and injected
into a GC. The yield was based on a calibrated method
experiment (47% yield ( 4%)).
Preparation of [CoN4]. Solid [CoIIN4H(Br)]Br (294 mg,
0.616 mmol) and KC8 (178 mg, 1.32 mmol) were placed in a
20 mL scintillation vial with a stir bar (the stir bar had been
previously stirred over KC8 in THF). THF (10 mL) was added
at room temperature, and the vigorously stirring solution
immediately became inky-purple and warm. Small amounts of
bubbles were observed. After 30 min, the solution was filtered
through Celite, and the filtrate was dried in vacuo. The solid was
washed with 10 mL of toluene and passed through a medium
porosity glass fritted funnel to remove insoluble material. The
toluene was removed in vacuo to yield an analytically pure dark-
purple solid (161 mg, 83%). X-ray quality crystals were grown
by pentane vapor diffusion into a saturated benzene solution.
The material could be further purified by saturating a solution
in toluene and adding pentane and storing the solution at −35
°C for several days and washing the crystals with cold pentane
on a glass fritted funnel (10% yield). To determine gaseous
products, the reaction was conducted in a sealed flask, and THF
solvent was added via syringe. By GC analysis, the amount of
H2 produced corresponded to a 20% yield based on [CoN4]
product. 1H NMR (C6H6, 300 MHz): δ 8.15 (1H, t, J = 7.5 Hz,
Ar-H), δ 7.62 (2H, d, J = 7.5 Hz, Ar-H), δ 4.28 (4H, br s, CH2),
δ 3.55 (4H, br s, CH2), δ 2.25 (4H, br s, CH2), δ 0.99 (6H, s,
CH3).13C NMR (C6H6, 100 MHz) δ 117.3 (meta-Cpy), δ 114.2
(para-Cpy), δ 57.7 (CH2), δ 53.4 (CH2), δ 34.3 (CH2), δ 17.0
(CMe). FTIR-ATR: (cm−1) 3105, 3070, 2905, 2808, 2745, 2663,
1571, 1530, 1510, 1485, 1446, 1380, 1355, 1337, 1317, 1260,
1194, 1177, 1152, 1136, 1125, 1013. UV−vis: λmax (THF, nm
(ε, M−1 cm−1)) 375 (8300), 452 (shoulder, 2100), 544 (8300),
650 (1400), 773 (1750). Anal. Calcd (found) for [CoN4]
(C15H21CoN4): %C 56.96 (56.69); %H 6.69 (6.79); %N 17.71
(17.51). Alternatively, solid [CoIIN4H(MeCN)2][BPh4]2 (100
mg, 0.1 mmol) was suspended in 5 mL of THF and stirred with
a stir bar that had been previously stirred over KC8. The
solution was treated with KC8 (28 mg, 0.2 mmol) in two
roughly equal portions causing the reaction to become dark
Preparation of [CoIIN4H(MeCN)][OTf][BPh4]. Solid
[CoIIN4H(MeCN)][BPh4] (74.1 mg, 0.109 mmol) was
dissolved in 5 mL of MeCN in a 20 mL vial. [H-DMF][OTf]
(25.0 mg, 0.112 mmol) in 1 mL MeCN was added causing the
immediate color change from dark green to orange-red. After
0.5 h the solution was layered under Et2O affording red crystals
(78.8 mg, 87%). μeff = 1.60 μB (MeCN-d3, 20 °C). UV−vis: λmax
(MeCN, nm (ε, M−1 cm−1)) 358 (990), 445 (1220), 450
(1210). FTIR-ATR: (cm−1) 3235, 3082, 3048, 3026, 2995,
2982, 2940, 2924, 2886, 2873, 1582, 1478, 1464, 1426, 1366,
1326, 1286, 1251, 1237, 1220, 1170, 1157, 1144, 1093, 1077,
1064, 1051, 1024. Anal. Calcd (found) for [CoIIN4H(MeCN)]-
[OTf][[BPh4], C42H45BCoF3N5O3S: %C 61.02 (60.79); %H
5.49 (5.66) %N 8.47 (8.47).
Preparation of [CoIIN4H(MeCN)2][BPh4]2. Under a flow of
N2, solid Co(BF4)2·6H2O (1.04 g, 3.1 mmol) and diacetyl-
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Smieja, J. M. Chem. Soc. Rev. 2009, 38, 89. (d) Costentin, C.; Robert,
pyridine (0.50 g, 3.1 mmol) were dissolved in 30 mL of
degassed MeCN in a 500 mL Schlenk round-bottom flask. The
pale-orange solution was treated dropwise with 3,3′-diamino-
dipropylamine (0.41 g, 3.1 mmol) in 5 mL of water (degassed)
via addition funnel, and the solution turned slightly darker
orange. The addition funnel was replaced under a stream of N2
with a reflux condenser and heated to boiling upon which it
turned from pale orange to very dark purple-red. The solution
was refluxed overnight and afterward cooled to room
temperature. The reflux condenser was replaced under a stream
of N2 with a new addition funnel and was treated dropwise with
NaBPh4 (2.00 g, 5.8 mmol) in 20 mL MeCN/H2O (1:3)
solution. The reaction mixture was exposed to vacuum and
brought into a glovebox (wet-box) and treated with 40 mL of
water, causing the precipitation of purple-red crystals which
were isolated on a frit and washed one time with water and
dried until a free-flowing powder. The solid was used to prepare
a saturated MeCN solution (160 mL) of the compound and
was recrystallized by slow diffusion of Et2O, which yielded the
pure compound as dark purple-black needles. The crystals were
isolated on a glass fritted funnel and dried to constant weight at
room temperature and analyzed as [CoIIN4H(MeCN)2]-
[BPh4]2 (yield 2.47 g = 78%). Anal. Calcd (found) for
[CoIIN4H(MeCN)2][BPh4]2, C67H68B2CoN6: %C 77.54
(77.05); %H 6.60 (6.55) %N 8.10 (8.06). When one equivalent
(rather than excess) of NaBPh4 was used in the synthesis, the
product was obtained in 33% crystalline yield. Anal. Calcd
(found) for [CoIIN4H(MeCN)2][BPh4]2, C67H68B2CoN6: %C
77.54 (77.40); %H 6.60 (6.47) %N 8.10 (8.17). Spectroscopic
m e a s u r e m e n t s m a t c h e d t h o s e o b t a i n e d f o r
[CoIIN4H(MeCN)][OTf][BPh4].
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ASSOCIATED CONTENT
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* Supporting Information
Additional experimental details, UV−vis and FTIR and NMR
spectra, computational details, and X-ray crystallographic
information. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jpeters@caltech.edu
Andernach, L.; Angersbach, F.; Nuckel, S.; Schoffel, J.; Susnjar, N.;
̈
̈
Notes
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ACKNOWLEDGMENTS
■
This material is based upon work performed by the Joint
Center for Artificial Photosynthesis, a DOE Energy Innovation
Hub, supported through the Office of Science of the U.S.
Department of Energy under Award Number DE-SC0004993.
D.C.L. would also like to acknowledge the National Institutes
of Health (Award Number F32GM106726). The authors
would also like to thank Tzu-Pin Lin, Michael Takase, and
Lawrence Henling for help with crystallography, and Kyle
Cummins and Slobodan Mitrovic for help with XPS. Clifford
Kubiak is also thanked for many insightful discussions.
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