Facile hydrogenation of bicarbonate to formate in aqueous medium by highly stable nickel-azatrane complex

Article abstract of DOI:10.1016/j.jcat.2019.12.020

Molecular catalyst-based direct hydrogenation of bicarbonate to formate in aqueous medium is a challenging research topic for the H₂ storage. Finding a green and effective method for the bicarbonate to formate conversion with non-precious metal-based catalyst is vital to the practical application. We report the direct hydrogenation of bicarbonate to formate using a water soluble nickel-azatrane complex. Catalysts 1–5, designed and synthesized, were screened for the hydrogenation of bicarbonate to formate in aqueous medium; the best TON of 121 was obtained for catalyst 4 at 120 °C (60 bar). Introduction of isopropyl (2) and methyl (3 and 4) groups in the coordination environment of the metal center enhances the production of formate. Further, the hydrogenation of bicarbonate with CO₂ promoted the formate production for catalyst 4 with a TON of 92 (3 h). The use of green solvent and non-precious metal catalyst makes this catalytic method environmentally sustainable.

Full text of DOI:10.1016/j.jcat.2019.12.020

Journal of Catalysis 382 (2020) 121–128
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Facile hydrogenation of bicarbonate to formate in aqueous medium by
highly stable nickel-azatrane complex
⇑
Dharmalingam Sivanesan, Bongkuk Seo, Choong-Sun Lim, Hyeon-Gook Kim
Advanced Industrial Chemistry Research Center, Advanced Convergent Chemistry Division, Korea Research Institute of Chemical Technology (KRICT), 45 Jonggaro,
Ulsan 44412, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Molecular catalyst-based direct hydrogenation of bicarbonate to formate in aqueous medium is a chal-
lenging research topic for the H2 storage. Finding a green and effective method for the bicarbonate to for-
mate conversion with non-precious metal-based catalyst is vital to the practical application. We report
the direct hydrogenation of bicarbonate to formate using a water soluble nickel-azatrane complex.
Catalysts 1–5, designed and synthesized, were screened for the hydrogenation of bicarbonate to formate
in aqueous medium; the best TON of 121 was obtained for catalyst 4 at 120 °C (60 bar). Introduction of
isopropyl (2) and methyl (3 and 4) groups in the coordination environment of the metal center enhances
the production of formate. Further, the hydrogenation of bicarbonate with CO2 promoted the formate
production for catalyst 4 with a TON of 92 (3 h). The use of green solvent and non-precious metal catalyst
makes this catalytic method environmentally sustainable.
Received 3 September 2019
Revised 10 December 2019
Accepted 12 December 2019
Keywords:
Bicarbonate reduction
Formate production
Hydrogenation
Non-precious metal
Nickel-azatrane complex
Molecular catalysts
CO2 conversion
Ó 2019 Elsevier Inc. All rights reserved.
Sustainable method
Green solvent
1. Introduction
reactivity was dependent on the added base. Considering that
CO2 is in equilibrium with bicarbonate (HCOÀ3 ) in the aqueous
The ever-increasing concentration of CO2 in the atmosphere has
resulted in global warming and climate change that threatens sus-
tainable development of the environment for human society [1–3].
Various strategies have been developed for the control or reduction
of the amount of CO2 released into the atmosphere [4–6] . Among
these, the utilization of CO2 as a C1 feedstock to produce value-
added chemicals is an attractive method [7–9]. Specifically, the
conversion of CO2 to formic acid (HCOOH) or formate (HCOOÀ) is
desirable; this is because formic acid has numerous applications
such as preservative, insecticide, hydrogen carrier, and industrial
material for synthetic processes [10–12]. However, the thermody-
namic and kinetic stability of CO2 renders its hydrogenation chal-
lenging, as it requires a high amount of energy [13]. In order to
overcome this, in the traditional hydrogenation of CO2, basic addi-
tives such as alkanol amines, KOH, and NaHCO3 are used to
decrease the thermodynamic barrier and promote the forward
reaction [14]. The catalytic hydrogenation of CO2 has been exten-
sively studied; most of the highly active homogeneous catalysts
are based on complexes of Ru [15], Rh [16], Ir [17], and their
medium, the feasibility of the hydrogenation reaction without
any additives has been studied recently [18]. The direct reduction
of bicarbonate to formate has gained great interest because bicar-
bonate is easier to handle than CO2 and could be easily generated
from CO2 using alkanol amines. There are a few examples of the
direct reduction of bicarbonate to formate using non-noble metals.
Using a cobalt complex, bicarbonate was hydrogenated with a
turnover number (TON) of 3900 in MeOH [19]. In addition, an Ru
and Fe-based system was proven to be active for the hydrogena-
tion of bicarbonate [20]. More recently, an Fe-based catalyst was
developed for the hydrogenation of bicarbonate in presence of
CO2 with a TON of 4600 [21]. However, the available active cata-
lysts for the direct hydrogenation of bicarbonate are not suitable
for green and sustainable development because they are based
on noble metals or synthesized via complex processes [21]. For
the development of sustainable and green catalysts, the criteria
to be met are mild reaction conditions, simple synthesis of the cat-
alysts, high stability of catalysts, and green solvents.
Herein, we report a non-precious metal based nickel-azatrane
complex for the direct hydrogenation of bicarbonate to formate
in an aqueous medium (Scheme 1). A series of complexes were
synthesized and studied as catalysts for the hydrogenation of
⇑
Corresponding author.
E-mail address: candoit@krict.re.kr (H.-G. Kim).
https://doi.org/10.1016/j.jcat.2019.12.020
0021-9517/Ó 2019 Elsevier Inc. All rights reserved.

122
D. Sivanesan et al. / Journal of Catalysis 382 (2020) 121–128
complexes are summarized in Tables 1S, 2S and 3S and relevant
interatomic distances and angles are listed in Tables S1A, S2A
and S3A.
Scheme 1. General scheme for the generation of formate.
2.4. Synthesis of catalysts 1–5
bicarbonate. To the best of our knowledge these nickel-azatrane
complexes are the first non-precious metal based molecular cata-
lysts without phosphine ligands that are active for the hydrogena-
tion of bicarbonate in aqueous medium.
2.4.1. Synthesis of 1
a solution of Ni(NO3)2Á6H2O (0.994 g, 3.41 mmol) in MeOH
(40.0 mL), was added a solution of tris(2-aminoethyl)amine (L1)
(0.500 g, 3.41 mmol) in MeOH and stirred at RT for 12 h. The purple
solution was evaporated under reduced pressure and the solid was
dissolved in MeOH and diffused with diethyl ether. Purple block-
shaped crystals were obtained after 4 days. Yield (1.10 g, 98%).
2. Materials and methods
2.1. Physical Measurements
FT-IR spectra were recorded using the ATR method with on a
Thermo Scientific FT-IR spectrometer. UV-Vis spectra were
recorded on an OPTIZEN 2120UV spectrophotometer. 1H NMR
and 13C NMR spectra were recorded on a Varian 500 MHz and Var-
ian 125 MHz spectrometer, respectively. Mass data were recorded
on an Agilent 6130 Single Quadrupole LC/MS instrument.
2.4.2. Synthesis of 2
To a solution of Ni(NO3)2Á6H2O (1.067 g, 3.66 mmol) in MeOH
(15.0 mL), was added a solution of tris(2-(isopropylamino)ethyl)a
mine (L2) (1.00 g, 3.66 mmol) in MeOH and stirred at RT for
12 h. The blue solution was evaporated under reduced pressure
and the solid was dissolved in MeOH and diffused with diethyl
ether. Blue block-shaped crystals were obtained after 2 days. Yield
(1.52 g, 91%).
2.2. Electrochemistry
Cyclic voltammetry studies were carried out on a BioLogics
potentiostat. The cell was a standard three-electrode system with
a platinum wire counter electrode, platinum working electrode,
and (3.0 M) Ag/AgCl reference electrode. The measurements were
performed at room temperature in H2O containing KCl (0.1 M)
(supporting electrolytes) along with the synthesized complex
(4 mM).
2.4.3. Synthesis of 3
To a solution of Ni(NO3)2Á6H2O (0.994 g, 0.260 mmol) in MeOH
(15.0 mL), was added a solution of tris(2-(methylamino)ethyl)am
ine (L3) (0.500 g, 0.260 mmol) in MeOH and stirred at RT for
12 h. The blue solution was evaporated under reduced pressure
and the solid was dissolved in MeOH and diffused with diethyl
ether. Blue block-shaped crystals were obtained after 2 days. Yield
(1.01 g, 97%).
2.3. Single crystal X-ray Data measurement
2.4.4. Synthesis of 4
To
a solution of tris[2-(dimethylamino)ethyl]amine (L4)
The single crystal was mounted on the tips of quartz fibers at
room temperature, coated with Partone-N oil, and cooled under a
stream of cold nitrogen. Intensity data were collected on a Bruker
CCD area diffractometer running the SMART software package,
(0.500 g, 2.17 mmol) in MeOH was added Ni(NO3)2Á6H2O
(0.0.631 g, 2.17 mmol), followed by stirring at RT for 12 h. A green
precipitate was obtained after evaporating the reaction solution
and dried. Yield (0.919 g, 98%).
with Mo K
a radiation (k = 0.71073 Å). The structure was solved
by direct methods and refined on F2 using the SHELXTL software
package [22]. Empirical absorption correction was applied with
the SADABS [23] part of the SHELXTL program package, and the
structure was checked for higher symmetry by the program PLA-
TON [24]. All the non-hydrogen atoms were refined anisotropi-
cally. In general, hydrogen atoms were assigned idealized
positions and given thermal parameters equivalent to 1.2 times
the thermal parameter of the carbon atom to which they
are attached. Data collection and experimental details for the
2.4.5. Synthesis of 5
To a mixture of Cu(NO3)22Á.5H2O (0.504 g, 2.17 mmol) in MeOH
(15.0 mL), was added tris[2-(dimethylamino)ethyl]amine (L4)
(0.500 g, 2.17 mmol) and stirred at RT. The blue solution was evap-
orated under reduced pressure to afford a yellow solid. The solid
was dissolved again in MeOH and diffused with diethyl ether. Sui-
table blue block-shaped crystals were obtained in 2 days. Yield
(0.921 g, 98%).
Table 1
Catalytic hydrogenation of bicarbonate to formate.
Entry
Catalyst
Starting
Material
(1.0 M)
Time (h)
Temperature
(°C)
Formate
conc. (M)
Turnover number (TON)a
Turnover frequency
TOF) (hÀ1
)
1
2
3
4
5
6
7
8
1
2
3
4
5
4
4
4
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
KHCO3
NH4HCO3
K2CO3
NaHCO3
NaHCO3
3
3
3
3
3
3
12
3
3
3
3
3
100
100
100
100
100
120
120
100
100
100
120
120
0.024 (5)
0.031(8)
0.032(11)
0.040 (4)
0.008 (13)
0.078 (9)
0.148 (6)
0.037 (11)
0.030 (14)
–
19
25
26
33
6
57
121
28
22
–
6
8
9
11
2
19
10
9
7
–
9
4
4
10
11
12
Ni(NO3)2Á6H2O
–
–
92
–
30
4
0.125 (18)b
Reaction conditions: catalyst (2.5 mM), reaction medium water (50 mL), Reaction time (3 h), H2 pressure 60 bar at RT, Formate concentration calculated using HPLC, a - mol of
b
product per mol of catalyst. pressure (CO2 30 bar/H2 30 bar) at RT.

D. Sivanesan et al. / Journal of Catalysis 382 (2020) 121–128
123
2.4.6. Hydrogenation reaction procedure
centers. The equatorial positions are occupied by the dimethyl
nitrogens of the azatrane ligand. The distances between the axial
tertiary nitrogen of azatrane and the metal in 4 and 5 are 2.116
(3) and 2.029(2) Å, respectively. From the crystal structures of 1
to 5, it appears that the binding modes of the nitrate anions are
dependent on the substituents on the nitrogens in the azatrane
ligand. Catalyst 1 containing unsubstituted amine groups can
accommodate two nitrate anions in the monodentate binding
mode [26], whereas isopropyl or methyl substituted catalysts
result in the bidentate binding mode of the nitrate anions. Interest-
ingly, the tertiary amine with dimethyl groups in the azatrane
ligands caused the nitrate anions to avoid bonding with the metal
center. These results indicate that changing the substituents has a
profound effect on the binding modes of the nitrate anion; a simi-
lar behavior is expected for the bicarbonate anion binding with the
metal center when the catalyst is introduced into bicarbonate
medium for direct hydrogenation.
Hydrogenation reactions were performed in a high-pressure
reactor (250 mL). The solution of NaHCO3 was added into the reac-
tor along with the catalyst. The reactor was then purged with N2
before the introduction of H2 (60 bar at room temperature), and
then heated to the chosen temperature accompanied by stirring
at 300 rpm. After a specific period of time, the reactor was cooled
down to room temperature and the pressure released. The product
was analyzed using an Agilent 1200 series (Agilent Technologies,
Germany) LC system. The product separation was performed at
50 °C on a BIORAD AmineX HPX-87H Ion Exclusion Column
(300 mm  7.8 mm). The flow rate was maintained at 0.75
mL/min, run time was 20 min, and the UV detector was set at
220 nm. The injection volume of the sample was 10 lL. The calibra-
tion curve with R2 of 0.999 was obtained with a series of formate
solutions (0.001 to 0.050 M).
The bond distances of the axially coordinated water molecules
are 2.019(2) (4) [26] and 1.969 (3) Å (5), respectively, indicating
that the water molecules could easily dissociate from the metal
centers. Further, the presence of methyl groups in 4 and 5 forms
a small cavity in the catalysts that was visible in the space-filling
model (Fig. S1). The weak intermolecular hydrogen bonding
between the nitrate anions and metal-bound water molecules
forms a three-dimensional network.
3. Results and discussion
The tetradentate phosphine and tridentate ligand systems (two
P-donor and N-donor) combined with different metals (Ru [15], Rh
[16], and Ir [17]) reported for the hydrogenation of bicarbonate to
formate are displayed in Chart 1. Most of the active catalysts
exploited for hydrogenation are based on precious metals and
require complicated synthetic procedures to incorporate the
required ligands. Further, phosphorous-based metal complexes
were found to be very sensitive to moisture and air. We synthe-
sized tetradentate azatrane based nickel-complexes (1–5,
Scheme 1) in a single step with excellent yield (Scheme 2). These
complexes are not sensitive to moisture and air; more importantly,
they are highly soluble in water, even in 3.0 M NaHCO3. The pres-
ence or absence of the secondary amine group (ANH) in the coor-
dination environment of ruthenium complexes plays an important
role in the hydrogenation of CO2 to formate [25]. Hence, we intro-
duced primary (1) and secondary amine groups (2, 3) in the coor-
dination environment of the complexes. In order to clarify the
effect of hydrogen in the coordination environment, tertiary amine
groups were introduced in the metal complexes (4, 5). Excellent
yields (90–97%) were obtained for all the complexes, which were
easily crystallized from a diffusion system containing MeOH and
diethyl ether. Single crystal structures of the complexes 2, 3, and
5 are shown in Fig. 1, and crystal information, bond angles, and
bond distances are listed in Tables S1–S3.
3.2. FT-IR spectra of 1–5
Using FT-IR spectroscopy the presence of functional groups and
hydrogen bonds in the catalysts was identified. The FT-IR spectra
of the five catalysts 1–5 are given in Fig. 2. The metal bound amine
(–NH2) stretching vibration peaks appeared at 3310 cmÀ1 in 1, and
other major peaks are (ACH, 2879 cmÀ1), (ACAN,1598 cmÀ1), and
(ANAO(nitrate),1300 cmÀ1). The metal bound secondary amines
(–NH) stretching vibration peaks appeared at 3202 cmÀ1 and
3219 cmÀ1, respectively for 2 and 3. For the metal bound water
molecule (MAH2O) stretching vibration peaks appear at
3280 cmÀ1 and 3296 cmÀ1, respectively for 4 and 5. The well charac-
terized compounds 1–5 were evaluated as catalysts for the direct
hydrogenation of bicarbonate to formate in aqueous medium.
3.3. Hydrogenation of bicarbonate to formate
The hydrogenation reaction was initially performed with 1
(1.0 M NaHCO3) at 100 °C (60 bar) for 3 h. As expected, the
nickel-azatrane complex successfully hydrogenated the bicarbon-
ate to yield formate in aqueous medium with a TON of 19 (Table 1,
entry 1). The formation and concentration of the obtained formate
was confirmed by HPLC. Interestingly, the catalyst is completely
soluble even at high concentrations of NaHCO3 (up to 3.0 M,
Fig. S2).
Methyl, isopropyl, and dimethyl groups were introduced into
the catalysts (2, 3, and 4) in order to enhance the direct hydrogena-
tion of bicarbonate to formate. Formate generation was found to
increase upon the introduction of electron releasing functional
groups. The order of reactivity for the catalysts is 1 < 2 < 3 < 4. Thus,
4 is the best among the screened catalysts with a TON of 33
(Table 1, entry 4). Hence, catalyst 4 was used to optimize the tem-
perature and pressure conditions. The temperature was varied
from 80 to 120 °C and maximum formate was obtained at 120 °C
with a TON of 121 after 12 h. Similarly, the pressure of H2 was var-
ied from 10 to 60 bar and maximum formate (TON – 57) was gen-
erated at 120 °C under 60 bar (Table 4S).
3.1. Single crystal X-ray structures of 1–5
The Ni(II) metal center in 2 adopts a slightly distorted octahe-
dral coordination formed by four nitrogen atoms from the azatrane
ligand and a nitrate anion. The tertiary nitrogen atom of the aza-
trane ligand and one oxygen of the nitrate anion occupy the axial
positions, while the equatorial positions are occupied by three sec-
ondary amine groups of the azatrane ligand and another oxygen of
the nitrate anion. The distance between the secondary nitrogen
atoms and the metal center range from 2.045(3) to 2.170(3) Å.
The bidentate binding of the nitrate anion to the metal center
yields bond distances in the range 2.137(3)–2.168(3) Å.
An additional nitrate anion is present in the crystal lattice to
maintain charge neutrality. Similar to catalyst 2, the metal center
in 3 has a distorted octahedral geometry with four nitrogen atoms
from the azatrane ligand and a nitrate anion. In 4 and 5, the Ni(II)
and Cu(II) metal centers adopt a trigonal bipyramidal geometry
involving the nitrogen atoms from azatrane and a water molecule.
Here, the axial positions are occupied by water and the tertiary
nitrogen of the azatrane ligand. Two nitrate anions are present in
the crystal lattice to neutralize the overall charge on the metal
At a pressure of 10 bar, traces of formate were generated and
the production increased with H2 pressure. To evaluate the effect
of copper metal, catalyst 5 was screened under the optimized

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D. Sivanesan et al. / Journal of Catalysis 382 (2020) 121–128
Chart 1. Structure of the Catalysts.
Scheme 2. Structure of the catalysts 1–5.
conditions; after 3 h, traces of formate were generated. Addition-
ally, catalyst 5 was found to decompose (by demetallation) under
the experimental conditions (Table 1, entry 5) to give L4. Thus,
the nickel-azatrane complexes are more stable than the copper-
azatrane complex. It was found that the metal precursor Ni(NO3)2-
Á6H2O did not yield any formate when investigated as a catalyst for
the hydrogenation of bicarbonate (Table 1, entry 11). Thus, the
specific coordination environment of the nickel-azatrane complex
drives the hydrogenation of bicarbonate in the aqueous medium.
In order to extend the possible use of nickel-azatrane com-
plexes to other bases such as KHCO3, NH4HCO3, and K2CO3, hydro-
genation reactions were carried out in the presence of the catalyst
with each of the bases (1.0 M). Similar concentrations of formate
were obtained for KHCO3 and NaHCO3 (Fig. 3), suggesting that
the cations did not affect formate generation in the presence of
nickel-azatrane complexes. Indeed, formate was obtained even in
the case of NH4HCO3 with a TON of 44, which is lower than that
obtained with NaHCO3 and KHCO3. Formate was not generated in
the presence of K2CO3; to understand this better, the pH of each
reaction mixture was tested before and after the hydrogenation
(Fig. 3). For NaHCO3, NH4HCO3, and KHCO3, the pH is in the range
7.5–8.0, while the pH of the K2CO3 reaction medium is 11.5. This
suggests that the difference in the hydrogenation of bicarbonate
by catalyst 4 is a result of the change in pH of the reaction medium.
More importantly, the pH dependent nature of the catalyst
suggests that the nickel-azatrane complex could even possibly

D. Sivanesan et al. / Journal of Catalysis 382 (2020) 121–128
125
Fig. 1. Single crystal X-ray structures of 2, 3, and 5.
Fig. 3. Effect of different bases on the pH and generation of formate by catalyst 4.
Fig. 2. FT-IR spectra of 1–5.
combine with CO2 and bicarbonate. Further, it was reported that
changing the pressure of CO2 with bicarbonate altered the pH of
the reaction medium. Thus, when catalyst 4 was combined with
CO2 (30 bar)/H2 (30 bar) and bicarbonate, maximum formate pro-
duction with a TON of 92 was obtained after 3 h (Table 1, entry 12).
Furthermore, this result strongly indicates that the nickel-azatrane
complexes could be used to reduce CO2 with NaHCO3. Catalysts 1
and 4 were studied under the optimized reaction conditions, while
varying the bicarbonate concentration from 0.25 to 1.0 M (Fig. 4);
formate production increased along with the concentration of
bicarbonate.
Fig. 4. Effect of bicarbonate concentration on the generation of formate by catalysts
1 and 4.
production rose linearly from 10 bar to 60 bar and it clearly indi-
cates that the production of formate by complex is dependent on
the H2 pressure. Next, with constant temperature and H2 pressure
the bicarbonate concentration was varied from 0.25 to 3.0 M. The
formate production was linearly increased (Fig. S4) and the maxi-
mum TON for the formate production was obtained with 2.0 M of
3.4. Kinetics studies for the bicarbonate hydrogenation
In the optimized reaction conditions, the bicarbonate concen-
tration (1.0 M) was kept constant and varied the pressure of H2
from 10 bar to 60 bar with complex 4 (Fig. S3). The formate

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D. Sivanesan et al. / Journal of Catalysis 382 (2020) 121–128
bicarbonate. However, the production of formate was decreased
with 3.0 M of bicarbonate and this could possibly result from the
low solubility of the H2 in the high concentration of bicarbonate.
Similar behavior has been observed for the homogeneous ruthe-
nium complexes [27].
Based on the time course reaction of formate production at
three temperatures (353, 373, and 396 K) rate constant values
were obtained. Followed by, the activation energy was calculated
using the Arrhenius plot (Fig. 5). The calculated activation
energy (Ea) of the bicarbonate reduction in the aqueous medium
is 62.0 kJ/molÀ1. This value is slightly higher than the Ea obtained
for the Rh and Ru complexes and could be compared with the lit-
erature values [27–29]. The reported activation energy for the
reduction of CaCO3 and CO2 mixture in aqueous medium by a Rh
(I)-complex of monosulfonated triphenylphosphine (mtppms),
[RhCl(mtppms)3][27] is 36.0 kJ/molÀ1. Similarly, the activation
energy was reported for the hydrogenation of CO2 in aqueous solu-
tion with K[RuCl(edta-H)][28] (31 kJ molÀ1), and with [RhCl
(mtppts)3] in the presence of amines (25 kJ molÀ1) [29].
3.5. Electrochemical studies for 1–5
Electrochemical studies were carried out for catalysts 1–5 in an
aqueous medium with KCl as the electrolyte to know the change in
redox potential for the catalysts with different ligands (Fig. 6). Cat-
alysts 1–4 showed an anodic oxidation peak between À0.852 and
1.139 V vs. Ag/AgCl, while a cathodic peak was not observed for
any of the catalysts (Fig. S4). Complex 5 exhibited a redox couple
at À0.511 V vs. Ag/AgCl and a cathodic peak at À0.299 V (Fig. S5).
Two anodic peaks at 0.672 and 1.142 V were observed for 1 in
aqueous medium corresponding to the [Ni(II)(L1)]2+ to [Ni(III)
(L1)]3+ and [Ni(II)(L1)(NO3)2] to [Ni(III)(L1)(NO3)2] stages, respec-
tively; the corresponding reduction peaks were not observed in
the reverse sweep. Introduction of isopropyl and methyl groups
in the coordination environment of complexes (2) and (3) leads
to an anodic shift from 0.672 (1) to 0.887 V (2) and 0.865 V (3)
for [Ni(II)(L2)]2+ to [Ni(III)(L2)]3+ and [Ni(II)(L3)]2+ to [Ni(III)
(L3)]3+, respectively. Further anodic shift was observed for 4 after
introduction of the dimethyl groups, from 0.865 (3) to 0.932 V
(4). These results demonstrate that the incorporation of electron
releasing functional groups changes the oxidation potential of
the complexes 1–4. Further, these results explain the changes in
formate production in the presence of complexes 1–4 based on
the available electron density at the metal center.
Fig. 6. Cyclic voltammograms of catalysts 1–4. Scan rate 100 mV/s.
3.6. 1H NMR studies for 4 with bicarbonate in D2O
In order to investigate the stability of the catalyst and under-
stand the possible mechanism, 1H NMR spectra of catalyst 4 were
recorded in D2O (Fig. 7A). In the spectrum, the dimethyl protons
appear at 2.56 ppm, while the methylene protons are observed at
2.73 and 2.93 ppm. This solution was heated to 120 °C for 10 h
and the NMR spectra recorded again (Fig. 7B). This spectrum is
almost identical to that at RT (Fig. 7A), indicating that the catalyst
is stable even at 120 °C. Next, NaHCO3 (10 equivalents) was added
along with the catalyst in D2O and stirred for 3 h before recording
the NMR spectra (Fig. 7D). Here it was seen that the signals from
the dimethyl (2.74 ppm) and methylene groups (2.87 and
3.08 ppm) moved downfield. This change in chemical shift is pos-
sibly a result of the dissociation of water and binding of bicarbon-
ate with the metal center in 4. The solution of NaHCO3 and catalyst
4 in D2O was heated to 120 °C for 10 h and NMR spectra recorded
at room temperature (Fig. 7C). Interestingly, the spectrum resem-
bles that of catalyst 4 in D2O (Fig. 7A). These results indicate that
bicarbonate binds with the metal center of the catalyst to form a
bicarbonate bound species (4A, Fig. 8). When this solution was
heated, the bicarbonate dissociated and the catalyst regained its
initial structure.
3.7. Possible mechanism for the hydrogenation of bicarbonate to
formate
Based on the literature reports20a,b and our experimental
results; single crystal X-ray analysis, the NMR stability studies,
and change in concentration of NaHCO3, CO2 and H2 pressure, we
propose a plausible mechanism (Fig. 8). The dissociation of water
from 4 and binding of bicarbonate could yield species 4A, which
was confirmed by 1H NMR spectroscopy. In the next step, the
bicarbonate dissociates from the metal center, possibly generating
a [Ni(L4)H]+ intermediate (4B). This is followed by the binding of
Fig. 5. Variation of ln k vs 1/T for the formate production by 4.

D. Sivanesan et al. / Journal of Catalysis 382 (2020) 121–128
127
Fig. 7. 1H NMR spectra of 4 in D2O under various conditions: (A) 4, (B) 4 after heating at 120 °C for 10 h, (C) 4 with 10 equivalent of NaHCO3, after heating at 120 °C for 10 h
and (D) 4 with 10 equivalent of NaHCO3.
4. Conclusions
We designed and synthesized water-soluble nickel-azatrane
based catalysts which are not sensitive to moisture and air and
do not contain precious metals. The best TON of 121 was obtained
using catalyst 4 for the direct hydrogenation of bicarbonate to for-
mate in aqueous medium. Introduction of electron releasing
groups enhances the generation of formate. The pH dependent nat-
ure suggested that the catalyst could be bound to CO2 at some
stage; indeed, formate production was enhanced when CO2 (TON
– 92, after 3 h) was introduced in the reaction medium. We expect
that these studies will open new avenues for the development of
non-precious metal based catalysts for the direct hydrogenation
of bicarbonate to formate.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
This study was supported by the Korea Research Institute of
Fig. 8. A plausible mechanism for the catalytic hydrogenation of bicarbonate to
Chemical Technology program [SI-1941-20].
formate by catalyst 4.
Appendix A. Supplementary material
CO2 with [Ni(L4)H]+ to yield the [Ni(L4)(HCO2)]+ species (4C).
Finally, bicarbonate binds with (4C) to release the formate and
generate 4A again, and this cycle continues.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2019.12.020.

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D. Sivanesan et al. / Journal of Catalysis 382 (2020) 121–128
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