Effect of Lewis Basic Amine Site on Proton Reduction Activity of NNN-Co Pincer Complex

Article abstract of DOI:10.1002/bkcs.12373

Electrochemical proton reduction is a promising energy storage method because H₂ molecule has a simple structure with a relatively low potential energy. Current interest in hydrogen catalysts has increased research efforts on synthetic analogs of hydrogenase active sites. In this study, we demonstrated the electrochemical H₂ evolution reactivity of [NNN^R-Co(CH₃CN)₃]²⁺ (R?-?CH₂ (1b), NCH₃ (2b)) complexes and examined a proton-relay process in the H₂ evolution reaction (HER). Upon one-electron reduction, the Co(II) ion center in a high-spin state dissociated a CH₃CN ligand, while opening a reaction site. Cyclic voltammograms of the Co complexes indicated quasi-reversible Co(II/I) redox behaviors, and both complexes 1b and 2b showed catalytic H₂ evolution activity. Interestingly, 2b, assisted by a proton-relaying NCH₃ group, exhibited more efficient catalytic activity than 1b.

Full text of DOI:10.1002/bkcs.12373

Communication
DOI: 10.1002/bkcs.12373
BULLETIN OF THE
S. Song et al.
KOREAN CHEMICAL SOCIETY
Effect of Lewis Basic Amine Site on Proton Reduction Activity of
NNN-Co Pincer Complex
†
†
†
‡
†
Seungjin Song, Jaewhan Cho, Hyeonjeong Jo, Junseong Lee , Jun-Ho Choi, and
†,
*
Junhyeok Seo
†Department of Chemistry, Gwangju Institute of Science and Technology, Gwangju 61005, Republic of
Korea. *E-mail: seojh@gist.ac.kr
‡
Department of Chemistry, Chonnam National University, Gwangju 61186, Republic of Korea
Received July 13, 2021, Accepted July 19, 2021
Electrochemical proton reduction is a promising energy storage method because H molecule has a simple
2
structure with a relatively low potential energy. Current interest in hydrogen catalysts has increased
research efforts on synthetic analogs of hydrogenase active sites. In this study, we demonstrated the elec-
R
2+
trochemical H evolution reactivity of [NNN -Co(CH CN) ] (R
CH (1b), NCH (2b)) complexes
2
3
3
2 3
and examined a proton-relay process in the H evolution reaction (HER). Upon one-electron reduction,
2
the Co(II) ion center in a high-spin state dissociated a CH CN ligand, while opening a reaction site.
3
Cyclic voltammograms of the Co complexes indicated quasi-reversible Co(II/I) redox behaviors, and both
complexes 1b and 2b showed catalytic H evolution activity. Interestingly, 2b, assisted by a proton-
2
relaying NCH group, exhibited more efficient catalytic activity than 1b.
3
Keywords: [FeFe]-hydrogenase, Synthetic model, Cobalt pincer, Proton-relay agent, H evolution
2
reaction
Electrocatalytic and photocatalytic proton reduction are desir-
process and that a cofunctionalized amine pendant enhanced
HER activity by relaying protons to the Co center.
1,2
able methods for H production. Although Pt is a benchmark
2
catalyst for the H evolution reaction (HER), researchers have
The NNN-pincer-type ligand was synthesized by conju-
gating 2,6-bis(bromomethyl)pyridine and piperidine (or a
2
utilized earth-abundant metals to achieve cost-effective cata-
3
lysts. Nature uses [FeFe]-hydrogenase to promote H evolution
piperazine derivative with -NCH ). Metalation of the NNN-
3
2
4
and splitting reactions, and many structural analogs have
shown promising catalytic activities for HER.
pincer ligand with CoBr in dichloromethane afforded
2
5,6,7
R
14
14
NNN -CoBr (R = CH (1a), NCH (2a) ) complexes.
2 2 3
Pincer ligands, such as an NNN-, PCP-, PNP-, PNN-types,
have been shown to enable the versatile utilization of transi-
tion metal ions in catalytic reactions. The ability of these
Elimination of the bromide ligands by 2 equiv of AgPF in
6
acetonitrile (CH CN) solution provided the catalytically
3
8
R
active species of [NNN -Co(CH CN) ](PF ) (R = CH2
3
3
6 2
ligands to conserve central metal ions has expanded their
applications in electrocatalytic HERs. Early work by Crabtree
showed that Ni(II) coordinated by a redox-active NNN ligand
promoted an HER at a 0.14 V overpotential with a TOF of
(1b), NCH (2b)). Complexes 1a and 2a have been previ-
ously used for the hydrogenation of alkenes, but the
electrocatalytic activity of the NNN-Co site coordinated by
3
CH
vapor into a saturated 1b solution in CH
3
CN has not been reported. Diffusion of diethyl ether
3
2
À1
9
~10 s in aqueous media (Figure 1). Later, other PCP-type
CN afforded
complexes were reported for HERs with overpotentials in the
reddish-brown crystals suitable for single crystal X-ray crys-
tallography. Unfortunately, we could not obtain single crys-
tals for 2b. 1b had an octahedral Co center coordinated by
13
range of 0.6–1 V. The side “arm” element of pincer ligands
affects their catalytic efficiencies. Beweries and coworkers
E1 E1
reported that the overpotential for [P C P-NiCl)] with E = S
an NNN-donor and three CH CN ligands, as shown in
3
1
10
was ~0.3 V lower than with E = O. Solvent-bound [PCP-Ni
Figure 2. The saturated ligand coordination of [NNN-Co(II)
1
+
2+
(
CH CN)] intermediates were suggested to be the active spe-
(CH
from the previous [PNP-Co(II)(CH
center in 1b twisted the N ,N
thereby to increase the asymmetry of the complex. The dihe-
3
CN)
3
]
around the Co center appeared to be different
3
2+
11
cies in the proton reaction. Yang and coworkers synthesized
CN) ]
3 2
case. The Co
E2 E2
2+
[
P N P-Co(CH CN) ] (E = CH , NH, O) complexes and
1
2
,N ,-coordination plane,
3
3
2
2
2
11
examined their electrochemical reactivities.
coworkers reported that air-stable [PNN-Ni(SC H CH )] pro-
Fan and
+
dral angle between the pyridine ring and the N
was measured to be 12.74 . The Co-Npyridyl distance was
,N
,N
-plane
6
4
3
1
2
3
ꢀ
moted the reduction of trifluoroacetic acid at a 0.54 V over-
12
potential with ~75% Faradaic efficiency (FE).
2.051(5) Å, and the bond distance of Co-NCCH (2.055
3
In this study, we demonstrated that a pyridine-based NNN-
pincer ligand stabilized a Co ion during the electrochemical
(6) Å) trans to Co-Npyridyl was ~0.1 Å shorter than the two
cis Co-NCCH bonds (2.178(6) and 2.124(6) Å). The C N
3
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CH2
Figure 2. ORTEP image of [NNN -Co(CH CN) ](PF ) (1b)
3
3
À
6 2
with 50% probability ellipsoids. Counter ions (PF6 ) and hydro-
gen atoms are omitted for clarity. Selected bond distance (Å) and
angles ( ) are as follows: Co N1 2.051(5), Co N2 2.313(6),
ꢀ
Co N3 2.257(5), Co N4 2.178(6), Co N5 2.055(6), Co N6
2
1
.124(6), N1-Co-N5 177.7(2), N6-Co-N4 173.5(2), N3-Co-N2
53.60(19), N1-Co-N2 76.3(2), N1-Co-N3 77.3(2).
Figure 1. Selected pincer-type complexes for HER: (a) [(NNN)
9
E1
E1
10
2
NiBr ] by Crabtree, (b) [(P CP )NiCl] by Beweries,
E2 E2
2+
11
+
(
c) [(P N P)Co(CH
3
CN)
2
]
by Yang, and (d) [(PNP)NiX]
12
by Fan.
ipa) was 2.39 at a 25 mV/s scan rate and approached 1 upon
increasing the scan rate (e.g., i /i = 1.03 at 1000 mV/s)
pc pa
distance in the trans Co-NCCH ligand was measured to be
(Figure 3). The NCH group of 2b induced a slightly posi-
3
3
~
0.1 Å longer than that in the cis Co-NCCH3.
The electronic properties of 1b and 2b were examined
tive shift in the Co(II/I) redox potential (E = À1.34 V
1/2
0/+
vs. Fc ) compared to 1b. These potential values were
by comparing the stretching frequency of Co-NCCH3.
consistent with those obtained by our density functional the-
0/+
Complex 1b showed three v(CN) Infrared (IR) stretching
ory (DFT) calculation (calculated E1 = À1.37 V vs. Fc
/2
À1
0/+
frequencies at 2277, 2287, and 2313 cm
with similar
for 1b and À1.35 V vs. Fc for 2b) and similar to those of
19
strengths (Supporting Information Figure S1), and 2b
known Co-pyridyl complexes. The large i /i ratio at the
pc pa
showed v(CN) frequencies at 2283 (stronger) and 2313
low scan rate originated from structural distortion along with
À1
(weak) cm
(Supporting Information Figure S2). One
dissociation of the labile CH CN ligand. The initial com-
3
electron-reduction shifted v(CN) values to lower frequen-
cies: the reduction of 1b with ~1 equiv of Na/Hg in
plexes of 1b and 2b had three CH CN ligands, but electron-
3
reduction steps possibly dissociated one CH CN from the
3
THF/CH CN (5:1) solution resulted in the appearance of
two peaks at 2115 and 2202 cm (Supporting Information
Co(I) center, as supported by two v(CN) frequencies. The
DFT calculation results also suggested that dissociation of
3
À1
Figure S1), whereas the corresponding reduction of 2b
resulted in two peaks at 2134 and 2198 cm (Supporting
the CH CN ligand is favored after the reduction of Co(II) to
3
À1
Co(I). Ligand dissociation behavior of Co-CH CN by
3
Information Figure S2). The spin-only magnetic moments
electron-reduction was observed for an analogous [PNP-Co
(CH CN) ] complex.
3 2
2+
11
(
μB) of the Co complexes were measured following the
Evans
Figure S3).
method
(295 K)
(Supporting
Information
CVs of the Co complexes were obtained in the presence
of proton sources. In the extended potential scan for 1b,
additional reduction peak appeared around À2.06 V
15,16
Complex 1b had a solution μB of 4.74,
indicating a high-spin state of Co(II) with an S = 3/2 sys-
tem of three unpaired electrons. Complex 2b had a similar
μB of 4.41. The spin state was dependent on the identity of
a ligand. Unlike the cases of 1b and 2b, [PNP-Co(II)
0/+
20
vs. Fc , which is assignable as a Co(I/0) reduction. The
addition of 50 mM [HDBU]BF (DBU = 1,8-diazabicyclo
4
[5.4.0]undec-7-ene) as a proton source (pK = 24.3 in
a
2+
21
(
(
CH CN) ]
has been reported to be a low-spin state
CH CN) to a solution of 1b (5 mM in CH CN) resulted
3
2
3
3
0/+
μ = 2.0), but PNP-type Co complexes of high-spin states
B
in an Ecat/2 at À2.28 V vs. Fc
(Ecat/2 indicates a half-
wave potential of the maximum catalytic current)
1
7,18
with μ = 4.3–4.7 values are also known.
B
The electrochemical behaviors of the Co complexes were
examined by collecting cyclic voltammograms (CVs) in
(Supporting Information Figure S4). Complex 2b exhibited
0/+
an Ecat/2 at À2.31 V vs. Fc
(Supporting Information
CH CN under an Ar atmosphere. The CV of 1b revealed a
Figure S5), showing similar catalytic activity as 1b.
3
quasi-reversible redox couple at a half-wave potential (E ,
However, under different reaction conditions in which
acetic acid (pK = 22.3 in CH CN) was used as a proton
a 3
source, the two complexes showed different catalytic activi-
ties. Under a low concentration of acetic acid (14 mM), 1b
1/2
22
determined as the average of the cathodic and anodic peak
0
/+
potentials) of À1.43 V vs. Fc
assignable to Co(II/I)
couple. The ratio of the cathodic-to-anodic peak current (i /
pc
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CH2
CH2
Figure 3. Cyclic voltammograms of [NNN -Co(CH
3
CN)
(2b)
4 6
NPF at scan rates of (a) 25 mV/s and
3
]
3 3
Figure 4. Cyclic voltammograms of [NNN -Co(CH CN) ]
NCH3
NCH3
(
(
(
PF
6
)
2
(1b) (black line) and [NNN
3 3 6 2
-Co(CH CN) ](PF )
(
PF
6
)
2
(1b), [NNN
-Co(CH
3
CN)
3
](PF
6
)
2
(2b) in the absence
and (b) 98 mM of
acetic acid. CV of each proton source at the same concentration
was added for comparison (gray line). Reaction conditions: 5 mM
red line) in 0.1 M Bu
b) 1000 mV/s.
and presence of (a) 50 mM of [HDBU]BF
4
3 4 6
of complex in CH CN, Bu NPF as electrolyte, 100 mV/s scan
rate under N atmosphere.
2
and 2b (5 mM) showed comparable activities (E
of
cat/2
0/+
0
/+
1
b = À2.19 V vs. Fc ; Ecat/2 of 2b = À2.05 V vs. Fc ),
whereas under a high concentration (98 mM) of acetic acid,
b exhibited a higher catalytic efficiency than 1b (Figure 4,
Supporting Information Figures S6 and S7). The TOF of 1b
2
À1
À1
was calculated to be 37 s , whereas TOF of 2b was 69 s
see Supporting Information for the calculation details).
Chronoamperometry for 2b was conducted at an applying
(
0/+
potential of À1.98 V vs. Fc , which resulted in the FE of
3
9% (Supporting Information Figure S8). For complex 1b,
0
/+
the reduction current began to increase at À1.28 V vs. Fc
,
0
/+
corresponding to an E
2
at À1.77 V vs. Fc . For complex
/+
cat/2
b, the reduction current increased at a more positive poten-
0
tial of À1.07 V vs. Fc , corresponding to an Ecat/2 at
0/+
À1.55 V vs. Fc . The standard reduction potential of acetic
0
/+ 23
acid in CH CN is À1.46 V vs. Fc
.
As acetic acid is used
3
as a proton source, the conjugate base (acetate) can bind to a
Co ion during the catalytic cycle. A separate reaction of 2b
with 1 equiv of [Bu N](CH COO) shifted the reductive
4
3
potential negatively by ~0.3 V (Supporting Information
Figure S9). The obtained J–V curve shapes of (2b + acetate)
and (2b + acetic acid) appeared similar with a slight shift of
potentials, which suggested probable coordination of ace-
tate to the Co center. Nonetheless, the onset potential for
the HER was shifted positively by ~0.5 V at higher concen-
trations, thus we assigned the active species to be
Figure 5. Proposed reaction mechanism for the electrochemical
evolution by complex 2b. The relative Gibbs free energy (ΔG,
kcal/mol) and transition state energy (ΔG , kcal/mol) are given
together. The ΔG was set to be zero after one-electron reduction.
H
2
‡
NCH3
2+
[NNN
-Co(CH CN) ] . Acetic acid is known to cause
3 2
22
homoconjugation in CH CN. The solution acidity may have
We assumed a proton reduction pathway for 2b based on
3
been lowered while increasing the acetic acid concentration,
which possibly caused the positive shift in the onset potential
the DFT calculation results. After one-electron reduction of
Co(II/I), one CH CN ligand dissociates from the Co center,
3
than that expected based on the original pK of acetic acid.
leaving two CH CN ligands. The open coordination site
a
3
For comparison, a CV of acetic acid (98 mM) in the absence
of catalyst was added in Figure 4(b). We attempted to follow
allows proton-relay between the NCH3 pendant and Co
center. DFT calculations provided the optimal electronic
structures and free energy values for the reaction intermedi-
ates during the catalytic reaction (Figure 5).
Two pathways are possible depending on the proton transfer
routes. One pathway involves direct proton-transfer to the Co
center from a proton source in solution but is disfavored in the
presence of a proton-relay agent because of a relatively higher
energy barrier. The second pathway consists of 2b first being
the effect of the pK of a proton source using a stronger acid,
a
such as p-toluenesulfonic acid, but it was unsuccessful
because the complexes were decomposed by the acid.
However, the trend of a positive shift in the onset poten-
tial by increasing the acid strength is commonly known for
24,25
HER catalysts.
The positive reduction potential of 2b
relative to 1b suggested that the NCH group of 2b played
3
a proton-relaying role that enhanced the catalytic activity.
one-electron reduced, after which the NCH amine site is
3
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protonated. Upon protonation of the NCH site, the proton
remains ~4.745 Å away from the Co center. As the piperazine
ring moiety changes to a twisted boat form, the proton at the
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3
distance of 2.759 Å. Given that a proton is relayed from the
NCH site to the Co(I) center, the Gibbs free energy appears
3
III
to increase by 23.97 kcal/mol for the formation of the Co -H
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7
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3
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3
+
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1
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NNN ligand successfully stabilized the high-spin Co ion during
the electrochemical process. The opened coordination site
enabled protons to access the Co site. The presence of an NCH3
amine pendant promoted proton reduction by relaying protons
to the Co center under acidic conditions. The catalytic efficiency
was enhanced by the proton-relay agent. These results provide a
novel example of the utilization of NNN pincer-type ligands for
electrochemical HER and emphasize the essential role of a
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Acknowledgment. This work was supported by the National
Research Foundation of Korea (NRF) grant funded by the
Korea Government (MSIT) (No. 2020R1C1C1007106) and
GIST Research Institute grant funded by the GIST in 2021.
Supporting Information. Additional supporting informa-
tion may be found online in the Supporting Information
section at the end of the article.
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