Kinetics and the potential well in electrochemical hydrogen evolution by [Co(4-tolyl-tpy)2]2+

Article abstract of DOI:10.1016/j.electacta.2020.136000

The ligand 4-tolyl-tpy [4-tolyl-tpy = 2,6-di(pyridin-2-yl)-4-p-tolylpyridine] and the corresponding cobalt complexes like [Co(4-tolyl-tpy)₂]Cl₂ and [Co(4-tolyl-tpy)₂]Br₃ has been synthesized and characterized by

Full text of DOI:10.1016/j.electacta.2020.136000

Journal Pre-proof
Kinetics and the potential well in electrochemical hydrogen evolution by [Co(4-tolyl-
2+
tpy) ]
2
Sumanta Kumar Padhi, Ejaz Ahmad, Surabhi Rai
PII:
S0013-4686(20)30392-3
DOI:
https://doi.org/10.1016/j.electacta.2020.136000
EA 136000
Reference:
To appear in: Electrochimica Acta
Received Date: 6 December 2019
Revised Date: 25 February 2020
Accepted Date: 2 March 2020
Please cite this article as: S.K. Padhi, E. Ahmad, S. Rai, Kinetics and the potential well in
2+
electrochemical hydrogen evolution by [Co(4-tolyl-tpy) ] , Electrochimica Acta (2020), doi: https://
2
doi.org/10.1016/j.electacta.2020.136000.
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II
SYNOPSIS: The [Co (4-tol-tpy)
2
](Cl) complex have been synthesized and employed as an
2
electrocatalyst towards proton as well as water reduction. The Co(III), Co(II) and Co(I) are
independent of pH as inferred from the Pourbaix diagram. The pK value for Co(III)−H species
I
III
2
in DMF was found to be 16.5. rates of formation of [Co (L
2
)] → [Co (H)(L)( L)] and
III
2
II
] are faster whereas the formation of Co(III)-H H+ bond
-
[
Co (L)( L)S] → [Co (L)
2
formation is the rate-determining step in the catalytic cycle.
Table of Contents:
1

Kinetics and the Potential Well in Electrochemical Hydrogen
2
+
Evolution by [Co(4-tolyl-tpy)2]
Sumanta Kumar Padhi*, Ejaz Ahmad, Surabhi Rai[a]
[
a]
[a]
[a]Artificial Photosynthesis Laboratory, Department of Chemistry, Indian Institute of
Technology (Indian School of Mines), Dhanbad, India, 826004. E-mail: sumanta@iitism.ac.in.
KEYWORDS: Cobalt catalyst; Proton reduction; Water reduction; Hydrogen evolution; SVD
Analysis
ABSTRACT: The ligand 4-tolyl-tpy [4-tolyl-tpy = 2,6-di(pyridin-2-yl)-4-p-tolylpyridine] and
its corresponding cobalt complexes like [Co(4-tolyl-tpy)
synthesized and characterized by different spectroscopic along with the electrochemical
method. The electrochemical studies were carried out in 95:5 (v/v) DMF/H O using acetic
acid (AcOH) as a proton source. The faradic efficiency found to be f = (85±4) % using
potential range -1.4 V to -1.7 V for aperiod of 1 h. The i /i value found to be 57.9 for a scan
rate of 100 mV s and TOF was calculated to be 652 s . The electrocatalytic H production
] complex was also employed in phosphate buffer pH 7, and it shows
the Faradic efficiency of (88±4%) within the overpotential range from -1.2 V to -1.5 V. The
TOF of the reaction was found to be 53.58 mol. of H per mole of catalyst per hr. The
2
]Cl
2
and [Co(4-tolyl-tpy)
]Br
2 3
are
2
c
p
-
1
-1
2
2+
by [Co(4-tolyl-tpy)
2
2
compare of electrochemical proton reduction, as well as water reduction by [Co(4-tolyl-
]2+ with the basic scaffold [Co(tpy) ]2+, reveals that it shows higher catalytic activity
tpy)
2
2
and have lower overpotential of 150 mV for proton reduction and 100 mV for water
1

reduction. Various kinetics and spectroscopic studies are utilized to predict the mechanism
for proton reduction. The kinetic and mechanistic studies are depicted based on changes in
absorbance with the addition of Co(Cp)
2
followed by AcOH. The intermediates involved in
the kinetic process was analysed through global fitting and SVD analysis. The formation of
I
III
2
III
2
II
[
Co (L
2
)] → [Co (H)(L)( L)] and [Co (L)( L)S] → [Co (L) ] are faster reactions in
2
δ
δ
-
+
comparison to the formation of Co(III)-H ⋅⋅⋅H to evolve hydrogen. The pK value was
I
I
+
calculated to be 16.5 for Co(III)-H species in DMF generated from Co species i.e. Co + H ꢀ
Co(III)-H.
INTRODUCTION
Artificial photosynthesis serves as a modern tool for solving several problems
encountered in an energy crisis. It mimics the process of photosynthesis; wherein solar
energy is stored in the form of chemical energy. The burning of fossil fuels leads to an
escalation in the concentration of greenhouse gasses resulting in serious environmental
issues like climate change and global warming. Production of hydrogen by electrochemical
and photochemical means provides excellent ways towards sustainable development. [1-3] It
not only serves the issue of our dependence on non-renewable energy sources but also
reduces the number of greenhouse gasses produced due to the combustion of fossil fuels.
Although Pt is known for the generation of hydrogen near its thermodynamic potential, its
[
4]
large-scale use gets limited due to its high cost. Hydrogenase enzyme present in several
prokaryotes also performs reversible conversion of the proton into hydrogen at a very high
rate and efficiency, but it also suffers from the limitation that it remains active in the inert
atmosphere only.[5-6] Several scientists are investigating to develop cheap, robust and
2

selective catalysts with a high turnover number and low overpotential. First-row transition
metals are used for developing hydrogen evolving catalysts for many years. Among this
cobalt metal is widely explored for both electrochemical and photochemical proton
reduction due to the higher stability of Co(I) state. [7-12]
It has been seen that various catalysts having different ligand environment follows
different catalytic pathways. Among several reported pathways, one of the influencing
catalytic routes is that it shows the pendant base effect, wherein one of the free nitrogen
atoms of ligand gets protonated and acts as a proton relay for hydrogen production.
Bullock and group have reported cobalt tetraphosphine complexes and studied their
+
catalytic activities for H
2
evolution using (1:1 v/v) [(DMF)H] :DMF. The mechanistic
II
studies reveal that the evolution of hydrogen takes place by protonation of Co -H
intermediate, where the pendant amine present on these complexes facilitates proton
transfer to cobalt metal center.[13] Helm and coworkers have indicated the key role of
pendant base amine that functions as a proton relay during the catalytic cycle for hydrogen
evolution. They have studied electrochemical proton reduction by [Ni(PPh
N )
Ph
](BF
) ,
2
2
4
2
(
PPh NPh = 1,3,6-triphenyl-1-aza-3,6-diphosphacycloheptane), which evolves hydrogen
2
electrocatalytically in dry acetonitrile using protonated DMF along with a TOF value of
3000 s-1.[14] Recently our group has also reported one novel [Ni(DQPD)
] complex, where
3
2
the pendant based N atom of non-coordinated quinolone moiety accounts for internal
proton relay towards the metal center and hydrogen is evolved via hetero coupling
between proton and hydride. It performs electrocatalytic proton reduction in 95:5 (v/v)
DMF/H
O in the presence of acetic acid as a proton source with TOF of 3.25 s-1.[15] Dubois et
2
al. have reported the Fe complexes having various pentadentate amine, wherein
3

mechanistic studies reveal the interaction among metal-hydride and the proton of the
pentadentate base present on phosphine ligand favors the hydrogen production by
intramolecular heterolytic cleavage.[16] Our group also have developed the scaffolds of
[
Co(tpy)
2
]2+ and [Ni(tpy) ]2+ complex and their catalytic properties towards proton and
2
water reduction, wherein the ligand dechelating effect has an important role in the catalytic
process.[17(a-c)]Fontecave and coworkers have reported different cobalt bis-terpyridyl
complexes bearing a different kind of substituent and studied their electrocatalytic activity
towards hydrogen evolution in acetonitrile. It has been inferred from their studies that
substituent present on terpyridine moiety affects their catalytic activity in terms of stability
and overpotential. Among all the complexes studied, the one having electron-donating
moiety containing dimethyl amino terpyridine derivative performs proton reduction with
higher efficiency and low overpotential.[17(d)]
Inspired by the aforementioned literature reports, the [Co(4-tolyl-tpy)
]2+ complex has
2
been utilized for electrocatalytic proton reduction in organic and aqueous media. The
complex shows ligand dechelation for hydride coordination at one of the pyridines of
terpyridine moiety, which in the presence of proton evolves hydrogen. The presence of the
electron-donating group on terpyridine also increases the catalytic behavior of the
complex. The potential well during the catalytic process has been analysed through open
circuit potential study. The intermediates present in the catalytic process are analysed
through global fitting and SVD analysis using the spectroscopic data.
Results and discussion
Synthesis.
4

The ligand 2,6-di(pyridin-2-yl)-4-p-tolylpyridine (4-tolyl-tpy) and the [Co(4-tolyl-
complex was synthesized by the reported procedure.[18a,b] In briefly, the ligand
tpy) ]Cl
2 2
was synthesized in an ethanolic solution by taking (2 equiv.) 2-acetyl pyridine and (1
equiv.) 4-methyl benzaldehyde with the addition of potassium hydroxide and aqueous
ammonia. The reaction mixture was refluxed at 80°C and a light yellow colored precipitate
was obtained. To the methanolic solution of the ligand 4-tolyl-tpy (2 equiv.), CoCl
2
.6H O (1
2
equiv.) in methanol was added slowly resulting in the formation of a deep brown colored
solution. The compound was extracted using the excess amount and diethyl ether, followed
by drying in vacuo. The oxidation of [Co(4-tolyl-tpy)
2
]Cl was carried out in 6 ml of (1:1,
2
v/v) MeOH/H O mixture, with the addition of Br (98%), and the reaction mixture was
2
2
allowed to stir for 12 hours at room temperature. The yellow precipitate was crystallized
from DMF via slow evaporation, the deep yellow crystals were formed after a week from
DMF. Scheme 1, presents the synthesis of the complexes. The UV-Vis, FTIR, NMR and mass
are displayed in Figure S1-S12.
Scheme 1. Synthesis of cobalt complexes of 4-tolyl-tpy.
UV-visible spectroscopy
5

The UV-Vis spectra (Figure S1) of (4-tolyl-tpy) ligand was studied in DMF. The ligand
-
1
–1
-1
exhibits four absorption bands at 276 nm (36231 cm , ε = 36050 M cm ) and 318 nm
-
1
–1
-1
(
31446 cm , ε = 7641 M cm are assigned to πꢁπ* transitions. Whereas the other two
–
1
–1
–1
–1
–1
–1
bands at 376 nm (26595 cm , ε = 82 M cm ) and 446 nm (22421 cm , ε = 32 M cm )
was ascribed to nꢁπ* transitions. The UV-Vis spectra of the [Co(4-tolyl-tpy) ]Cl complex
was studied in DMF (Figure S6). The compound exhibits two MLCT absorption bands at
2
2
-
1
-1
-1
-1
-1
-1
4
45 nm (22471 cm , ε = 1445 M cm ) and 520 nm (19230 cm , ε = 1586 M cm . The
-
1
-1
-1
-1
-1
band at 507 nm (17636 cm , ε = 402 M cm ) which, 650 nm (15,384.61 cm , ε = 85 M
-
1
-1
-1
-1
cm ) and 740 (13513.51 cm , ε = 62 M cm ) nm was assigned to d-d transition. The UV-
3+
Vis spectra of [Co(4-tolyl-tpy) ] complex (Figure S7) was carried out in DMF. The band at
2
-
1
–1
-1
3
48 nm (28735 cm , ε = 50714 M cm ) was assigned to LMCT. Whereas 651 nm (15,360
cm , ε = 46 M cm ) and was assigned for the d-d transition by spin allowed A1g → 1T1g in
-
1
–1
-1
1
low spin Co(III) complex.
X-ray crystallography
Suitable single crystals for X-ray diffraction of the complex [Co(4-tolyl-tpy)
]Br
2 3

5H O
2
were grown through slow evaporation from DMF solution at room temperature. An
appropriate crystal was mounted on a Rigaku SuperNova G8910B EosS2 single-crystal X-
ray diffractometer. A perspective view of the cationic unit of the crystal structure has been
displayed in Figure 1. The complex [Co(4-tolyl-tpy)
2 3
]Br
5H
2
O crystallizes in a triclinic
lattice with the space group of Pī(#2). The asymmetric unit consists of [Co(4-tolyl-tpy)
2
]3+
cation along with three bromide ions and five water molecules as a solvent of
crystallization. The total bromide ions are situated at disordered locations with a total site
3+
occupancy factor of 2.96. In the case of [Co(4-tolyl-tpy)
2
] , the cobalt center is coordinated
6

by an octahedral environment with two 4-tolyl-tpy ligands are located at meridional
positions. The terpyridine and the 4-tolyl planes are situated to each other through the
dihedral angles ranging from -15.29° and 21.03°. An inversion center with operation along
-
x,-y,-z is located at [0,0, 0] is present in the molecule. The bond distances of the trans-
coordinating Npy-Co are 1.946 - 1.950 Å, whereas the central Npy-Co distances are 1.863 and
.867 Å. The bite angles at the cobalt center are 81.61-82.31°. The meridionally located 4-
tolyl-tpy ligands are nearly vertical to each other with an angle tending to 89.35°.
1
3
+
Figure 1. The 30% ORTEP of [Co(4-tolyl-tpy)
2
] .
Acid-base equilibria
The acid-base equilibria studies were carried out in Britton–Robinson buffer from (pH
.0 to 7.0) using UV-Vis spectroscopy (Figure S13–S14). The peak appeared at 226 nm at
2
pH 2.0 in Britton–Robinson buffer was due to the formation of protonated species
generated by the protonation of one nitrogen of a dechelated pyridine moiety of one of the
II
3+
4
-tolyl-tpy ligands to generate [Co (4-tolyl-tpy)(4-tolyl-tpyH)] . It has been observed that
7

on increasing the pH, the peak at 226 nm decreases gradually due to the deprotonation of
II
II
one of the N of dechelated pyridine moiety to form [Co (4-tolyl-tpy)
2
]. The pK
[Co (4-tolyl-tpy)(4-tolyl-tpyH)] was found at 4.6 (Scheme 2). This
observation was also supported by MALDI mass spectra (Figure S15-S16) in phosphate
a
for [Co (4-
II
3+
tolyl-tpy) ] ꢀ
2
buffer. At pH 2.0, the mass spectrum shows the presence of [CoC22
H
N
17 3
(HPO
)NaCl]+
4
+
species (calculated for [CoC22
H
17
N
3
(HPO
4
)NaCl] : 535.99, found at m/z 535.98) at pH 2.0
+
and another peak at 324.74 (calculated for [(C22
H
17
N
3
)+H] : 324.15 and found at m/z
3
24.74) was due to the free ligand. This indicates the complete dissociation of one 4-tolyl-
tpy from the [Co(4-tolyl-tpy)
2
]2+ complex. At pH 7.0, a minor intensity of the free ligand
+
appeared at m/z 324.75 (calculated for [(C22
H
17
N
3
)+H] : 324.15 and found at m/z 324.75)
in MALDI mass spectrum. The MALDI mass spectrum at m/z at 382.77 was due to the
+
+ 2+
[
[C22
H
H
17
N
3
CoC22
H
H
17
N
3
(NaOH)OH] +H ]
species
(calculated
for
+
+ 2+
[[C22
N
17 3
CoC22
N
17 3
(NaOH)OH] +H ] : 382.11, found at m/z 382.77) and another peak
+
+
at 346.77 was obtained for free ligand [C22
H
17
N
3
Na] (calculated for [C22
H
17
N
3
Na] : 346.13
and found at m/z 346.77).
8

2+
Scheme 2. Acid-base equilibria of [Co(4-tolyl-tpy) ] complex in phosphate buffer.
2
Electrochemistry
The cyclic voltammogram of the ligand 4-tolyl-tpy was studied in DMF/H O (95:5 v/v)
2
solution using TBAP (0.1 M) as the supporting electrolyte. It shows two irreversible
reduction peaks at -1.94 V and -2.37 V vs. SCE, whose corresponding oxidation peaks
obsewrved at -1.72 and -2.16 V vs. SCE respectively. (Figure S17). The cyclic voltammogram
2+
of 1.0 mM [Co(4-tolyl-tpy)
two metal-based reversible peaks at E1/2 = 0.28 V (Epa = 0.33 V and Epc= 0.23 V vs. SCE) and
1/2 = -0.69 V(Epa = -0.64 V and Epc = -0.74 V vs. SCE) (Figure 2). The peak at E1/2 = 0.28 V vs.
2
] complex was studied in (95:5 v/v) DMF/H
2
O, which displays
E
III
II
SCE was assigned to the metal-based Co /Co redox couple, whereas the peak at E1/2 = -
II
I
0
.69 V vs. SCE was assigned to Co /Co redox couple. Along with the two metal-based peaks
in the complex and two ligand centered reduction peaks were also appeared at a more
negative potential. The first ligand-based one-electron reduction process 4-tolyltpy/4-
9

•
-
tolyl-tpy arises at E1/2= -1.5 V (Epa = -1.48 V and Epc = -1.53 V vs. SCE) and the second
•
-
ligand centered reduction peak for 4-tolyltpy/4-tolyl-tpy appears at E1/2 = -1.85 V (Epa = -
1
.79 V and Epc= -1.91 V vs. SCE).
Figure 2. Cyclic voltammogram of 1.0 mM complex [Co(4-tolyl-tpy)
2
]2+ in 95:5 (v/v)
O solution containing 0.1 M TBAP as supporting electrolyte at scan rate of 100
DMF/H
mV/s.
2
Electrochemistry of hydrogen evolution in organic media
The electrocatalytic H
95:5 v/v) using AcOH as a source of the proton. The pK
be 13.3 [19(a)], while in (95:5 v/v) DMF/H value of AcOH at 8.5. [19(b)] In the
O the pK
presence of 2 equiv. of AcOH, the CV curve of complex shows two irreversible peaks at -
.57 V and -1.82 V vs. SCE along with two metal-based reduction peaks (Figure S18). With
2
evolution of [Co(4-tolyl-tpy)
2
]2+ was investigated in DMF/H
2
O
(
a
of AcOH in DMF was reported to
2
a
1
increasing the concentration of AcOH, an enhancement in the catalytic current was
observed. The catalytic reduction peak starts at -1.4 V vs. SCE, with a plateau nearly at -1.80
II
I
V vs. SCE. The Co /Co reversible redox peak gradually loses its reversibility upon
increasing the concentration of acid (Figure 3 and Figure S19).
10

Figure 3. The CV curve of 1 mM complex for variable concentration of AcOH in (95:5, v/v)
-
1
DMF/H O using TBAP (0.1 M), at scan rate 100 mV s , under inert atmosphere.
2
The increase in the current with increase in acid concentration along the cathodic
direction is consistent with electrochemical proton reduction and similar behavior has
been observed for different catalyst concentrations (1.25 mM and 1.5 mM) (Figure S20 and
S21). The controlled potential electrolysis of 0.05 mM complex was conducted in (95:5 v/v)
DMF/H
2
O upon the addition of 30 equiv. of acetic acid in different potential from -1.4 V to -
1
.7 V vs. SCE. The H
2
gas evolution during CPE was quantified through gas chromatography
using a gas-tight syringe injected into the headspace of the cell (Figure S22). Faradaic
efficiency was observed to be f = (86±4) % [(Observed TON/Electrochemical TON ) × 100]
after the time duration of 30 min.
For the determination of the diffusion coefficient of [Co(4-tolyl-tpy)
2
]2+
,
the redox peak
II
I
-1
current of Co /Co varies linearly with the square root of the scan rate from 25 mV s to
-
1
-6
-1/2
s-1/2 was observed by the linear
4
00 mV s (Figure S23). A slope of 33.18 x 10 A V
fitting of i
p
vs. ν1/2 (Figure S24). The linear variation of peak current (i
p
) with the square
root of scan rate (ν1/2) indicates the diffusion-controlled process and also shows that
11

2+
[
Co(4-tolyl-tpy)
been determined using Randles-Sevcik equation 1. Where, i
electrons, F = Faraday constant, A = surface area of the working (glassy carbon) electrode, ν
scan rate, D = diffusion coefficient, R = universal gas constant and T = temperature, and
2
] is a homogeneous species. The diffusion coefficient of the complex has
p
= peak current, n = no. of
=
-
6
2 -1
found to be 3.11 x 10 cm s .
ꢊ
ꢏ
ꢀꢁꢂ = ꢃ. ꢄꢄꢅꢆꢀꢇꢈꢉ[ꢊ](ꢇꢈꢋꢌ/ꢍꢎ)
(1)
The plot of catalytic peak current (i
c
) vs. the concentration of [Co(4-tolyl-tpy)
] 2+
2
st
indicates (Figure S25) that hydrogen evolution follows 1 order kinetics with respect to the
nd
catalyst, whereas i
c
vs. [AcOH] plot suggests a 2 -order rate kinetics. Dependence of the
reduction peak current of the catalyst (i
by equation 2,
c
) with the catalyst concentration and AcOH is given
ꢀ
ꢁꢐ = nFA[Co(4-tolyl-tpy) ]ꢑꢌꢒ[ꢓꢔ]ꢏ (2)
2
+
A representative plot of i
Co(4-tolyl-tpy) ]Cl , at scan rate 100 mV s is displayed in Figure 4(A), where i
current in the presence of AcOH and i = reduction peak current in the absence of AcOH. The
/i ratio vs. different acid concentration (Figure S26) reveals that the catalytic current
c
(with the addition of 30 equiv. of AcOH) and i
p
for the 1 mM
-
1
[
2
2
c
= catalytic
p
i
c
p
increases linearly with the enhancement of the concentration of AcOH (up to 30 equiv.).
There was no significant escalation in the catalytic current observed beyond 30 equiv. of
AcOH and the current was saturated. This could be ascribed to the consumption of catalysts
using a high concentration of acid. Furthermore, the i
concentrations indicates that the i /i value is independent of the concentration of [Co(4-
tolyl-tpy) ]Cl , i /i
value being 57.9 at scan rate 100 mV s-1 in the acid independent
c
/i
p
plot for three different catalyst
c
p
2
2
c
p
12

region. Using the i
c
/i
p
data, the rate constant for the evolution of H was calculated using
2
the following equation 3 and found to be 652 s-1.[20]
ꢁꢐ
ꢇ
ꢍꢎꢒ
ꢈꢖ
=
ꢀ
ꢕ
(3)
ꢁꢂ
ꢃ.ꢄꢄꢅꢆ
The electrocatalytic activity of [Co(4-tolyl-tpy)
2
]2+ was compared to its parent
]2+ in the presence of 28 equiv. of AcOH. It was
observed that during electrocatalytic proton reduction, [Co(4-tolyl-tpy)
]2+ shows the
]2+. The [Co(4-
]2+ exhibits 150 mV and 310 mV of lesser overpotential respectively in
2+
[
Co(tpy)
2
] scaffold as well as Co(4Ql-tpy)
2
2
2+
greater catalytic current as compared to the [Co(tpy)
tolyl-tpy)
comparison to [Co(tpy)
2
] and Co(4Ql-tpy)
2
2
2+
]2+ (Figure 4(B)). The (4-tolyl-tpy)
]2+ complex
2
] and Co(4Ql-tpy)
2
2
-
1
2+
-1
shows a TOF of 652 s , which is better than [Co(4Ql-tpy)
2
] (TOF = 28 s ) and [Co(QCl-
]2+ (TOF = 22.2 s ) in 95:5 (v/v) with rate of using 28 equivalent of acetic acid as
-1
tpy)
2
proton source.
Figure 4. (A) The i
c
(with the addition of 30 equiv. of AcOH) and i
p
of 1 mM [Co(4-tolyl-
]2+ (black), Co(4Ql-tpy) ]2+
] (blue), and using (95:5 v/v) DMF/H O and TBAP (0.1 M), at
scan rate 100 mV s , with the addition of 28 equiv. of AcOH.
-
1
2 2
tpy) ]Cl , at scan rate 100 mV s . (B) LSV curves of [Co(tpy)
2
2
2+
(red) and [Co(4-tolyl-tpy)
2
2
-
1
13

Stability analysis of the Catalyst
2+
Time-dependent UV-vis spectroscopy of [Co(4-tolyl-tpy) ] complex was carried out in
2
(
95:5, v/v) DMF/H O and also by the addition of 30 equiv. of AcOH to the solution. No
2
significant spectral changes were observed up to 8 h, indicating the stability of the complex
in reaction conditions (Figure S27-S29). The stability of the complex has also been
verified by UV-Vis. spectroscopy and cyclic voltammetric investigations after the
electrolysis (Figure 5). The CV curves of complex upon adding 30 equivalents of AcOH
was compared before and after bulk electrolysis of 1 mM complex at -1.7 V vs. SCE for
half an hour. It was inferred from the plot that the complex remains stable after
performing the catalytic activity. A similar inference was also observed in the case of
UV-Vis. spectra as there even no substantial change in the spectra were observed before
and after electrolysis.
Figure 5: (a) CV plot of 1 mM complex in (95:5, v/v) DMF/H
on the addition of 30 equivalent before and after bulk electrolysis at the potential of -1.7
V vs. SCE. (b) UV-Vis. spectra of 1 mM complex in 95:5 (v/v) DMF/H O and spectra of
2
O, plot of the complex
2
14

the complex on the addition of 30 equivalent of acetic acid and spectra of the complex
after bulk electrolysis at the potential of -1.7 V vs. SCE.
SEM and EDX analysis
SEM along with EDX studies of the glassy carbon plate was performed before and after of
bulk electrolysis of 0.05 mM [Co(4-tolyl-tpy)
2
]2+ in (95:5 v/v) DMF/H
O with 28
2
equivalents of AcOH at the potential of –1.6 V vs. SCE after a time span of 1 h. The EDX
information about glassy carbon plate before electrolysis shows the presence of C/O/Co as
1
00: 0.00: 0.00, atomic %. Whereas after electrolysis, a small amount of cobalt deposited
on the glassy carbon plate with relative atomic % of these elements C/O/Co is
approximately 99.89: 0.00: 0.11 %. (Figure S30). It indicates a minor decomposition of the
cobalt complex during the catalytic process.
Potential Well and Kinetics of Proton Reduction
The cyclic voltammogram for proton reduction indicates that the catalytic current starts
II
I
next to Co /Co redox couple indicating that the process of catalysis starts involving Co(I)
species. Therefore, the kinetics study of the complex was carried out using UV Visible
spectroscopy.[21] In this experiment excess amount of cobaltocene was added to the 0.05
mM complex solution for reducing Co(II) species to Co(I) intermediate in completely inert
III
II
condition. Since Co(Cp)
2
has lower redox potential (-1.09 V vs. SCE) for Co /Co couple in
comparison to Co /Co redox couple of [Co(4-tolyl-tpy)
]2+. The process was evident from
UV-Visible spectra by rapid enhancement in the band at 518 nm as well as the decrease in
II
I
2
15

the resting potential of Co(II) to Co(I) during the open circuit potential (OCP, Figure 6A)
2+
test upon the addition of cobaltocene to the solution [Co(4-tolyl-tpy) ] in DMF under inert
2
condition. The addition of 20 equiv. of acetic acid in the inert environment leads to the
formation of the transient Co(III)-H species, which subsequently converts to Co(II)-H upon
one-electron reduction due to the presence of excess cobaltocene (Figure 6B-C). The OCP
test reveals that Co(III)-H species slowly reacts with a proton to produce hydrogen and
revert to Co(III) species, which converts into Co(II) at a faster rate under electrochemical
reduction. The potential realm for the Co(II) species tends from +0.23 V to -0.74 V vs. SCE.
16

Figure 6. (A) The open circuit potential (OCP) was observed using [Co(4-tolyl-tpy)
]2+
2
complex in DMF followed by the addition of cobaltocene and addition of 20 equiv. AcOH
under an inert atmosphere. (B) spectra of intermediate species obtained from React Lab
bundle during the catalytic process for the addition of 30 equiv. of acetic acid to Co(I) and
(C) concentration of intermediate species.
17

Similar experiments were also repeated for the addition of 20 and 30 equiv. of acetic acid
2+
to the Co(I) species generated through the addition of cobaltocene to [Co(4-toly-tpy) ] .
2
The kinetic plot in (Figure S31) was utilized to calculate equilibrium constant K for the
protonation of Co(I) species to form Co(III) hydride. The value of K for Co + H+
I
ꢀ
Co(III)-
3
-1
H, was found to be 1.5 x 10 M (Figure S32), and corresponding pK value was calculated to
be 16.5 for Co(III)-H species in DMF considering equation 4.[22]
ꢂꢒ ꢌꢗ ꢘꢈ(ꢙꢚ(ꢛꢛꢛ) − ꢓ) = ꢂꢒ ꢌꢗ ꢘꢈꢀ(ꢉꢐꢜꢓ) − ꢂꢒ
(4)
The kinetic studies were analyzed utilizing the changes in the absorbance vs. time in UV-Vis
spectroscopy with the addition of cobaltocene to [Co(4-toly-tpy)
2
]2+ (Figure S33).
I
Considering three consecutive reactions A → B → C → D (Scheme 3), where, A is [Co (L
2
)], B
III
2
III
2
II
is [Co (H)(L)( L)], C is [Co (L)( L)S] and D is [Co (L)
2
] species formed during catalytic
I
process for the addition of 30 equiv. of acetic acid to [Co (L
2
)]. The concentration and the
changes in the spectra of the intermediates, different are provided in (Figure 5B-C). The
rate of the involved reactions was analyzed through global fitting and SVD analysis using
-
1
ReactLab Bundle version 1.1. The rate of k
, k
1 2
and k was found to be 1.23 x 10 ± 5.10 x
3
-
4
-3
-5
-2
-4
1
0 , 5.47 x 10 ± 3.87 x10 and 2.01 x 10 ±1.85 x 10 respectively. This indicates that the
I
III
2
III
2
II
formation of [Co (L
2
)] → [Co (H)(L)( L)] and [Co (L)( L)S] → [Co (L)
2
] are faster reactions
δ
δ
+
-
in comparison to the formation of Co(III)-H ⋅⋅⋅H to evolve hydrogen. Hence the results
δ
δ
-
+
suggest that H ⋅⋅⋅H bond formation is the rate-determining step in the catalytic cycle.
18

Scheme 3. For mechanistic studies for hydrogen evolution.
The active species and plausible mechanism for proton reduction.
2+
The co-ordination number of Co in [Co(4-tolyl-tpy) ] complex is six, and there are rare
2
reports on Co center having more than six coordination numbers.[23] Therefore there could
be two possible ways, either dechelation of one of the pyridine ring moiety from 4-tolyl-tpy
ligand followed by its rotation or removal of one of the 4-tolyl-tpy ligand from the metal
center (Figure 7). There were no significant peaks observed in the CV curve of 2 mM 4-
tolyl-tpy ligand in DMF/H
2
O (95:5 v/v) within the potential window of -1.0 V to 0.4 V vs.
SCE. Upon addition of one equiv. of CoCl
2
.6H O, the two reversible peaks at E1/2 = 0.71 V
2
III
II
II
I
and 0.24 V vs. SCE was observed due to Co /Co and Co /Co redox couple respectively. On
the addition of another 1 equiv. of CoCl
the formation of mono-[Co(4-tolyl-tpy)S
CoCl .6H O indicates that the peak at -0.93 V vs. SCE increases with a decrease in peak
intensity of Co /Co and Co /Co redox couple.
2
.6H O, a new peak arises at E1/2 = -0.93. It is due to
2
2+
3
] complex (S = solvent). The further addition of
2
2
III
II
II
I
The decomposition of [Co(4-tolyl-tpy)
2
] 2+ along with [Co(tpy)
2
]2+ complex was also
carried out in accordance with decay studies reported by Hayward and group (Figure S34-
19

S35).[24] They have suggested that the dissociation constant obtained for the bis terpyridine
complexes were solvent dependent and increased with an increase in polarity of the
solvent. Herein, we have studied the dissociation chemistry of the complexes [Co(4-tolyl-
2+
]2+ in (95:5 v/v) DMF/H
tpy)
2
] and [Co(tpy)
2
2
O, and it was observed that the rate constant
] was found to be (7.2 ± 0.013) x 10 s and for [Co(tpy)
]2+ it was
2
+
-1 -1
for [Co(4-tolyl-tpy)
2
2
-
1
-1
(
11.7 ± 0.044) x 10 s on addition of 5 equiv. of CoCl
2
.6H O.
2
Figure 7. The change in CV curve of (2 mM) 4-tolyl-tpy ligand in DMF/H
2
O (95:5 v/v)
-
1
having TBAP (0.1 M), at scan rate 100 mV s , upon addition of 1 mM to 5 mM CoCl
iteratively.
2
·6H
2
O
2
+
Pourbaix diagram of [Co(4-tolyl-tpy)
The Pourbaix plot of [Co(4-tolyl-tpy)
buffer at pH 2.0 to pH 7.0 (Figure 8). The two peaks at potential 0.05 V along with -0.8 V vs.
2
] Complex
2+
2
] complex was carried out in Britton Robbinson
III
II
II
I
SCE for Co /Co and Co /Co were observed respectively in differential pulse voltammetry
DPV) (Figure S36). On the variation of the pH of the solution from 2.0 to 7.0 and scanning
the peaks in a cathodic direction no significant changes in the potential were noticed. The
(
III
II
II
I
results reveal that Co /Co and Co /Co are not pH-dependent. The two parallel lines
20

III
II
which signifies one-electron reduction process between [Co L(LH)]
ꢀ [Co L(LH)] ꢀ
I
[
Co L(LH)] within pH 2.0 to 4.6 and beyond pH 4.6 it signifies the one-electron transfer
III
II
I
between [Co (L)
2
]
ꢀ
[Co (L)
2
]
ꢀ
[Co (L) ] species.
2
2+
Figure 8. Pourbaix diagram of [Co(4-tolyl-tpy) ] complex in Britton Robinson buffer.
2
Electrochemistry for water reduction
2+
The electrocatalytic hydrogen generation of [Co(4-tolyl-tpy)
phosphate buffer. The cyclic voltammetry of 1.0 mM [Co(4-tolyl-tpy)
phosphate buffer, exhibits a quasi-reversible peak at E = 0.2 V vs. SCE, which was assigned
2
] complex was studied in
2+
2
] complex in 0.1 M
c
III
II
II
I
to Co /Co redox couple. However, a quasi-reversible peak for Co /Co redox couple was
observed in the cathodic potential of -0.872 V vs. SCE. The higher catalytic current was
observed for [Co(4-tolyl-tpy)
solution, as well as with the presence of CoCl
conditions (Figure S37-S38). The cyclic voltammogram of [Co(4-tolyl-tpy)
in phosphate buffer at a varying concentration of complex from (0.25 mM to 1.5 mM). It
2
]2+ complex with respect to the blank phosphate buffer
.6H O under the same experimental
] 2+ was studied
2
2
2
21

indicates that catalytic current increases (i
of the complex (Figure S39).
c
) linearly with an increase in the concentration
The study of electrocatalytic water reduction was conducted in phosphate buffer at
different pH from 4.0 to 7.0 (Figure 9). It was found that on decreasing the pH of the
phosphate buffer, catalytic current increases with a decrease in overpotential. By
decreasing the pH value, the number of protons in the medium increases which result in
the greater catalytic current. The increase in catalytic current (i
c
) was plotted with respect
to the change in pH of the phosphate buffer solution (Figure S40).
2+
Figure 9. The CV curve of 1 mM [Co(4-tolyl-tpy) ] was studied in (0.1 M) phosphate
2
buffer through varying pH (pH 4.0–7.0).
The bulk electrolysis was studied using 0.05 mM of [Co(4-tolyl-tpy)
]2+ solution in
2
phosphate buffer at pH 7.0, and the analysis was carried out in the three-component
system for 1800 sec in various potential ranges from -1.2 V to -1.5 V vs. SCE (Figure S41).
The faradaic efficiency of hydrogen evolution was found to be (88±4%) within the potential
range of -1.2 V to -1.5 V vs. SCE, indicating that total charge increases with an increase in
22

the potential. The formation of a gas bubble was observed on the surface of the working
electrode whereas the evolution of hydrogen gas was confirmed through gas
chromatography. Furthermore, the control potential electrolysis was studied at -1.3 V vs.
SCE in different pH (Figure S42), and it has been observed that catalytic current increases
+
with a decrease in the pH which is due to the increase in H concentration. The
electrocatalytic water reduction of [Co(4-tolyl-tpy)
2
]2+ was compared with the basic
2+
scaffold [Co(tpy)
2
] at pH 7.0 in phosphate buffer (Figure 10), and it was observed that
2+
2+
[
Co(4-tolyl-tpy)
2
] complex shows higher catalytic current as compared to [Co(tpy) ] and
2
also have 100 mV of lower overpotential. The TOF of the reaction was found to be 53.58
/mol catalyst/h using equation 5.[25-26]
mol H
2
∆
ꢝ
TOF =ꢀ
(5)
ꢞ.ꢟ .ꢟ ꢀ.ꢠ
ꢊ
ꢏ
2+
Figure 10. Cyclic voltammogram of 1.0 mM [Co(tpy)
tpy) ]Cl complex (red) in phosphate buffer at pH 7.0.
2
] complex (black) and [Co(4-tolyl-
2
2
23

Where ∆ꢝ is the difference in charge generated from controlled potential electrolysis in
presence and absence of catalyst as calculated from graph displayed in (Figure S43) after
two hours, F is the Faraday constant, n
1
is the number of moles of electrons required for the
generation of one mol of H , n is the number of moles of catalyst present in the solution, t
2
2
is the duration of the Electrolysis.
Plausible Mechanism
The mechanism for electrocatalytic proton reduction of the complex [Co(4-tolyl-tpy)
]2+
2
proceeds via two-electron reduction followed by the addition of two protons, leading to the
formation of one molecule of hydrogen along with the regeneration of the catalyst as
shown in Scheme 4. As inferred from the Pourbaix diagram and the mass data, there is no
PCET step is involved in the catalytic cycle. At pH 7.0, a minor intensity of the free ligand
appeared at m/z 324.75 whereas, the MALDI mass spectrum in Figure S16, at m/z at
+
+ 2+
3
82.77 was due to the [[C22
H
17
N
3
CoC22H
17
N (NaOH)OH] +H ] species (calculated for
3
+
+ 2+
[[C22H
17 3
N CoC22H
17
N (NaOH)OH] +H ] : 382.11, found at m/z 382.77). The mechanism
3
for proton reduction, as well as water reduction, has been considered on the basis of pH
II
]2+ (L = 4-tolyl-tpy) species gets reduced by one
higher than the pK
a
higher. The [Co (L)
2
I
+
I
]+ species in the presence of proton
electron to generate [Co (L)
2
] species. The [Co (L)
2
III
2
+
2
III
2
+
converts into [Co (H)L( L)] ( L = bidentate 4-tolyl-tpy). The [Co (H)L( L)] species
+
III
2
+
evolves one mole of H
2
in the presence of one H , and revert to [Co (S)L( L)] in the
presence of the solvent, which is the rate-determining step in the reaction. The
III
2
+
-
[
Co (S)L( L)] upon subsequent 1e reduction and removal of solvent converts into
II
]2+ for the completion of the catalytic cycle.
[
Co (L)
2
24

2+
Scheme 4. A plausible mechanism for proton reduction by [Co(4-tolyl-tpy) ] complex.
2
Conclusion
It is concluded that the ligand (4-tolyl-tpy) [2,6-di(pyridin-2-yl)-4-p-tolylpyridine] and its
corresponding cobalt complexes like [Co(4-tolyl-tpy) ]Cl and [Co(4-tolyl-tpy) ](PF
were synthesized and characterized by various techniques. The [Co(4-tolyl-tpy) ]Cl was
utilized to carry out electrochemical proton reduction in both organic ((95:5, v/v)
DMF/H O, using AcOH as proton source) and aqueous (0.1 M phosphate buffer, pH 7.0) and
it was observed that catalyst showed fair catalytic activity towards hydrogen evolution in
2
2
2
)
6 3
.
2
2
2
-
1
both the media and shows TOF value of 652 s and 53.58 mol. of H per mole of catalyst per
2
h in organic and aqueous medium, respectively. The kinetics of the reaction was studied
using UV-Visible and OCP experiments and concentration and spectra of various
intermediates were obtained using SVD analysis. The electrocatalytic activity of [Co(4-tolyl-
tpy)
2
]2+ was compared with parent [Co(tpy) ]2+ with addition of 28 equiv. of AcOH. The
2
25

2+
[
Co(4-tolyl-tpy) ] complex shows greater catalytic activity and also has less overpotential
2
of 150 mV for proton reduction than the parent complex. The similar results were also
obtained in the case of phosphate buffer pH 7. The global fitting and SVD analysis shows
I
III
2
III
2
II
the rates of formation of [Co (L
2
)] → [Co (H)(L)( L)] and [Co (L)( L)S] → [Co (L)
] are
2
δ
δ
+
-
faster whereas the formation of Co(III)-H ⋅⋅⋅H bond formation is the rate-determining
step in the catalytic cycle.
Experimental
Materials and Methods
Materials
The chemicals used in this work are of reagent grade. 2-acetyl pyridine was purchased
from Spectrochem. Potassium hydroxide, ammonia, CoCl
2
.6H O were purchased from
2
Merck. All these chemicals were used without further purification. Some solvents such as
DMF, ethanol, diethyl ether, and methanol were bought from Merck and dried before the
use. The ligand 2,6-di(pyridin-2-yl)-4-p-tolylpyridine and the [Co(4-tolyl-tpy)
2
]Cl complex
2
was synthesized by the reported procedure.[18a,b]
Optical Spectra
All the UV-Vis spectra were carried out in Agilent Technologies Cary 8454 UV-Visible
spectrophotometer. FT-IR spectra were recorded on FT-IR Affinity 1spectrophotometer
[Shimandzu-2100].
26

NMR Spectroscopy
1
13
The H NMR and C NMR spectra were recorded in Bruker 400 MHz NMR spectrometer
at room temperature.
Mass Spectroscopy
The high-resolution ESI mass spectra were studied in Q-TOF LCMS-Agilent Technology-
6
530 and Flexar SQ 300 MS (PerkinElmer, USA) mass spectrometers The MALDI-TOF MS
was carried out by Bruker Daltonics FlexAnalysis.
Cyclic Voltammetry
All the cyclic voltammetric analysis was recorded in inert condition using ALS/CHI model
-
1
1
140C model at scan rate 100 mV s in DMF medium containing TBAP (0.1 M) as a
supporting electrolyte, with glassy carbon utilized as a working electrode, counter
electrode as a platinum wire and saturated calomel electrode as reference electrode
respectively using a complex concentration of 1mM. The electrochemical hydrogen
evolution was studied in (95:5 v/v) DMF/H O using AcOH for three different complex
2
concentration (1.0 mM, 1.25 mM, and 1.5 mM) under a nitrogen atmosphere.
Bulk Electrolysis
The bulk electrolysis of [Co(4-tolyl-tpy)
cell using glassy carbon as working electrode, Pt mesh as a counter electrode and calomel
electrode acting as a reference electrode in 0.05 mM complex in DMF/H O (95:5 v/v) in
2
]Cl
2
complex was carried out in a custom-made
2
27

presence of 30 equiv. of acetic acid under an inert condition of different potential from -1.4
V to -1.7 V vs. SCE.
Gas Evolution
The H-type customized electrochemical cell was used for the purpose of gas evolution.
The hydrogen gas was detected through Agilent 7890B Gas chromatography with a TCD
detector and Helium was used as a carrier gas. In the H-type cell where a working section
2
was joined through (1 cm ) of glassy carbon plate, working electrode as a saturated
calomel electrode (SCE) and reference electrode respectively. All experiments were taken
in a minimum three times for calculation of TON and TOF. The quantification of gas was
done from the calibration plot obtained from GC by injecting a various known amounts of
hydrogen gas in the system. Herein, the amount of gas produced in H type cell during the
electrolysis process was collected by gas-tight syringe and injected periodically into GC
column to get the peak of hydrogen having peak area x. With reference to the calibrated
graph plotted in Figure S44 the slope and the intercept values using the linear equation y =
mx + C (Where m = 0.00604 and C = 9.285) are determined. The quantitative determination
of the volume of H was determined using the equation y = 0.00604x + 9.285 = volume of
2
2
H .
X-ray crystallographic analysis
A single crystal of [Co(4-tolyl-tpy)
]Br
2 3


5H O complex was mounted on a Rigaku
2
SuperNova G8910B EosS2 single-crystal X-ray diffractometer having using Mo Kα radiation
λ = 0.71073 Å). The collected data were processed by CrysAlisPro software and the EosS2
CCD detector was used for the collection of Frames.[27] The data collected for the crystal at
(
28