Synthesis, crystal structure, characterization of pyrazine diaminotriazine based complexes and their systematic comparative study with pyridyl diaminotriazine based complexes for light-driven hydrogen production

Article abstract of DOI:10.1016/j.poly.2020.114412

The present world has a dire need to increase its energy input in order to meet with the demands of growing population. Combining the most abundant renewable energy source such as sunlight with a clean fuel having the capacity to store and carry renewable energy viz. hydrogen is a promising alternative to control the detrimental effects of fossil fuels on our environment. A sunlight-triggered hydrogen evolution reaction (HER) would be an interesting approach for the production of pure hydrogen. Working towards this direction, we have synthesized complexes 8–10 by the reaction of 6-(pyrazin-2-yl)-1,3,5-triazine-2,4-diamine (PzDAT), with Co(II), Ni(II) and Cu(II) respectively. As indicated by single-crystal X-ray diffraction, the molecular organization of all the complexes is mainly dictated by the coordination modes of the ligands and the hydrogen bonds involving the DAT groups with reliable patterns. In conjunction with the synthesis and characterization, we investigated and compared the catalytic activities for HER of 8–10 with 5–7 (complexes of pyridine DAT (PyDAT) with Co(II), Ni(II) and Cu(II)) that we have reported previously. In the presence of triethanolamine (TEOA) as the sacrificial electron donor, Ru^(II)(bpy)₃(PF₆)₂ as the photosensitizer (PS) and aqueous HBF₄ as the proton source under blue light, the highest turnover number amongst 5–10 is observed for PyDAT copper complex 7 with TON of 72 mol of hydrogen per mole of PS. This value is higher compared to some Cu complexes reported in literature.

Full text of DOI:10.1016/j.poly.2020.114412

Journal Pre-proofs
Synthesis, crystal structure, characterization of pyrazine diaminotriazine based
complexes and their systematic comparative study with pyridyl diaminotriazine
based complexes for light-driven hydrogen production
Sanil Rajak, Olivier Schott, Prabhjyot Kaur, Thierry Maris, Garry S. Hanan,
Adam Duong
PII:
S0277-5387(20)30069-3
DOI:
https://doi.org/10.1016/j.poly.2020.114412
POLY 114412
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
1 November 2019
16 January 2020
27 January 2020
Please cite this article as: S. Rajak, O. Schott, P. Kaur, T. Maris, G.S. Hanan, A. Duong, Synthesis, crystal structure,
characterization of pyrazine diaminotriazine based complexes and their systematic comparative study with pyridyl
diaminotriazine based complexes for light-driven hydrogen production, Polyhedron (2020), doi: https://doi.org/
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Submitted to Polyhedron
Version 30-10-2019
Synthesis, Crystal structure, Characterization of Pyrazine
Diaminotriazine Based Complexes and their Systematic
Comparative Study with Pyridyl Diaminotriazine Based
Complexes for Light-Driven Hydrogen Production
[
a]
[b]
[a]
[b]
[b]
Sanil Rajak, Olivier Schott, Prabhjyot Kaur, Thierry Maris, Garry S. Hanan and
Adam Duong*[a]
[a]Département de Chimie, Biochimie et physique and Institut de Recherche sur
l’Hydrogène, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, G9A 5H7,
Canada
[b]Département de Chimie, Université de Montréal, Montréal, Québec, H3C 3J7, Canada
*
To whom correspondence should be addressed. E-mail: adam.duong@uqtr.ca
1

ABSTRACT: The present world has a dire need to increase its energy input in order to
meet with the demands of growing population. Combining the most abundant renewable
energy source such as sunlight with a clean fuel having the capacity to store and carry
renewable energy viz. hydrogen is a promising alternative to control the detrimental effects
of fossil fuels on our environment. A sunlight-triggered hydrogen evolution reaction (HER)
would be an interesting approach for the production of pure hydrogen. Working towards
this direction, we have synthesized complexes 8-10 by the reaction of 6-(pyrazin-2-yl)-
1
,3,5-triazine-2,4-diamine (PzDAT), with Co(II), Ni(II) and Cu(II) respectively. As
indicated by single-crystal X-ray diffraction, the molecular organization of all the
complexes is mainly dictated by the coordination modes of the ligands and the hydrogen
bonds involving the DAT groups with reliable patterns. In conjunction with the synthesis
and characterization, we investigated and compared the catalytic activities for HER of 8-
1
0 with 5-7 (complexes of pyridine DAT (PyDAT) with Co(II), Ni(II) and Cu(II)) that we
have reported previously. In the presence of triethanolamine (TEOA) as the sacrificial
(
II)
electron donor, Ru (bpy)
3
(PF
6
)
2
as the photosensitizer (PS) and aqueous HBF as the
4
proton source under blue light, the highest turnover number amongst 5-10 is observed for
PyDAT copper complex 7 with TON of 72 moles of hydrogen per mole of PS. This value
is higher compared to some Cu complexes reported in literature.
2

INTRODUCTION
The ever-increasing human population demands equally increasing energy resources. Until
now fossil fuels have been the world’s primary energy resource. However, their growing
utility has caused a detrimental effect on our environment. [1-3] In order to achieve a balance
with our environment, the dependency on fossil fuels needs to be reduced. This can be done
by increasing our dependency on renewable and environmentally friendly energy resources
and finding a possible substitute for fossil fuels. [4-7] Among the diverse renewable energy
resources, sunlight is the predominant one which has the potential to fulfil all our energy
demands. Hydrogen is a possible future alternative to fossil fuels as it is a clean fuel that
can be used in near future to store renewable energy. Hence, sunlight and hydrogen are
two clean and pollution free energy source and carrier, respectively, which can be
combined to meet the present-day energy crisis and other environmental issues. [8-10]
Taking inspiration from natural photosynthesis, solar energy can be stored in high-density
chemical bonds of hydrogen molecules. A sunlight-triggered hydrogen evolution reaction
(HER) would be an interesting alternative to produce pure hydrogen.
Currently, one of the key challenges for HER is the development of efficient catalysts.
Many of the studies reported so far have used platinum group metal viz. Pt[11], Rh[12], Ir[13,14]
based complexes as catalysts and PSs for hydrogen evolution. However, photocatalysts
based on first row transition metals are also being increasingly explored because of their
low cost and good electrochemical stability. Although previously, researchers have used
these low-cost metals viz. Co, Ni, Cu, Zn[15-19] as PSs and catalysts for H
production, there
2
is an ever increasing need to synthesize more efficient and better improved catalytic
systems for HER.[20-23]
Considering the potential of transition metal complexes as catalysts for hydrogen
production, we synthesized and studied the photocatalytic activity of complexes 5-10
which comprise of Co(II), Ni(II) and Cu(II) transition metals (denoted by M) in M(2,2-
bipy)
2
(NO
3
2
) framework functionalized with a diaminotriazinyl (DAT) and a pyrazinyl
group by replacing one or two pyridyl rings (Chart 1). DAT-substituted pyridine and
3

pyrazine complexes are called as tectoligands (2 and 3) and their self-assembly with metal
ions forms metallotectons.[24,25] The main advantages associated with the synthesis of
coordination complexes using tectoligands are facile synthetic route and predictable
intermolecular hydrogen bonding interactions along with simultaneous binding with metal
ions to produce reliable patterns (Chart 2).[26-32]
H
N
H
N
H
N
H
N
N
N
H
H
H
H
N
N
N
N
N
N
N
N
N
1
(2,2'-bipy)
2 (pyDAT)
3 (pzDAT)
H
H
H
H
N
N
N
N
N
N
H
H
H
H
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
M
M
2
NO3
2 NO3
H
M
2 NO3
N
H
H
H
N
N
N
N
N
N
H
H
H
H
4
5
6
7
. M = Co(II)
. M = Ni(II)
. M = Cu(II)
8. M = Co(II)
9. M = Ni(II)
10. M = Cu(II)
Chart 1. Molecular structure of ligands 1-3 and complexes 4-10
The investigation to prepare functionalized catalysts is an important task in order to modify
their properties. Herein, we focus our work on the design, synthesis and characterization
of DAT functionalized complexes 8-10 which are easy to obtain. As part of our ongoing
research in the area of energy application, we studied and compared the catalytic activities
of 5-10 for HER.[33,34] In this work, along with the design and characterization of PzDAT
complexes, a systematic comparative photocatalytic study of six complexes comprised of
DAT groups (three PzDAT and three PyDAT) towards hydrogen evolution reaction has
been presented.
4

EXPERIMENTAL SECTION
General Notes and Procedures for the Synthesis of complexes 8-10
6
-pyridin-2-yl-[1,3,5]-triazine-2,4-diamine ligand 2, 6-pyrazin-2-yl-[1,3,5]-triazine-2,4-
diamine ligand 3 and complexes 5-7 were synthesized by reported methods.[35-37]
Complexes 8-10 were prepared by the experimental procedure described below. Other
chemicals were commercially available, purchased and used without any additional
purification. A solid sample of ligand 3 (2.0 equiv) was added in small portions at 25 °C to
the stirred solutions of M(NO
3
)
2
·xH O (1 equiv) in MeOH (25 mL). The mixtures were
2
refluxed for 12 h and the resulting homogeneous solutions were cooled to room
temperature and subjected to crystallization by slow diffusion with diethyl ether.
Compound 8. The reaction of ligand 3 (0.05 g, 0.2644 mmol) with Co(NO
g, 0.1322 mmol) according to the general procedure yielded 87% of pink crystals of
complex 8 with the composition Co(3) ·(NO . FTIR (ATR): 3450.61, 3392.94, 3318.76,
224.71, 3170.05, 1672.10, 1653.75, 1641.17, 1598.87, 1576.29, 1540.28, 1503.21,
)
3 2
.6H
2
O (0.038
2
)
3 2
3
1
1
7
484.83, 1451.17, 1429.80, 1372.96, 1289.24, 1271.33, 1196.52, 1175.07, 1160.91,
066.92, 1038.76, 1013.13, 989.38, 954.49, 910.94, 847.18, 824.22, 808.43, 770.55,
-1
+
37.13, 697.28 cm . HRMS (ESI) for [C14
H
14
N
15
O Co] m/e 499.0732, found 499.0731.
3
Anal. calcd for C14
9.40.
H14CoN16
O
6
: C, 29.96; H, 2.51; N, 39.93. Found: C, 30.05; H, 2.25; N,
3
Compound 9. The reaction of ligand 3 (0.05 g, 0.2644 mmol) with Ni(NO
g, 0.1322 mmol) according to the general procedure yielded 90% of cyan crystals of
complex 9 with the composition [Ni(3) (MeOH)(NO )].NO . FTIR (ATR): 3427.54,
383.71, 3330.85, 3158.68, 1662.55, 1622.82, 1602.10, 1574.52, 1515.64, 1486.09,
459.67, 1378.07, 1277.62, 1212.06, 1175.03, 1166.50, 1043.71, 1014.14, 995.70, 914.10,
3
)
2
.6H O (0.038
2
2
3
3
3
1
8
-1
+
50.20, 807.10, 771.26, 731.53 cm . HRMS (ESI) for [C14
H
14
N
15
O
3
Ni] m/e calcd
5

4
98.0765, found 498.052. Anal. calcd for C15
H
18
N
16NiO : C, 30.38; H, 3.06; N, 37.79.
7
Found: C, 29.82; H, 2.67; N, 38.12.
Compound 10. The reaction of ligand 3 (0.05g, 0.2644 mmol) with Cu(NO
0.030 g, 0.1322 mmol) according to the general procedure yielded 92% green crystals of
complex 10 with the composition Cu(3) ·(NO . FTIR (ATR): 3481.68, 3403.44, 3321.96,
221.84, 3150.14, 1653.76, 1617.58, 1580.80, 1559.45, 1507.24, 1486.40, 1462.61,
429.85, 1375.55, 1295.82, 1279.38, 1205.88, 1178.18, 1157.85, 1064.20, 1041.40,
3
)
2
. 2.5H O
2
(
2
)
3 2
3
1
9
-1
94.75, 954.68, 944.62, 911.26, 871.13, 830.49, 818.92, 804.82, 714.27 cm . HRMS (ESI)
+
for [C14
H
14
N
15
O
3
Cu] m/e calcd for 503.0800, found 503.0684. Anal. calcd for
C
14
H14CuN14
6
O : C, 29.71; H, 2.49; N, 39.60. Found: C, 29.98; H, 2.50; N 39.41.
Instrumentation
Crystallographic data were collected using a Bruker Venture Metaljet diffractometer with
Ga Kα radiation. The structures were solved by intrinsic phasing using SHELXT,[38] and
non-hydrogen atoms were refined anisotropically with least-squares minimization.[39]
Hydrogen atoms were treated by first locating them from different Fourier maps,
recalculating their positions using standard values for distances and angles, and then
refining them as riding atoms. Microcrystalline powders were analyzed in transmission-
mode geometry using a Bruker D8-Discover instrument (θ-θ geometry) equipped with a
XYZ platform and a HI-STAR gas detector. X-rays were generated using a conventional
sealed-tube source with a copper anode producing Cu Kα radiation (λ = 1.54178 Å). The
samples were gently ground and then mounted on a flat Kapton sample holder. The data
collection involved acquisition of two different sections with increasing angular position,
giving two different 2D frames. These frames were integrated and combined to produce
the final one-dimensional powder X-ray diffraction pattern. Calculated powder X-ray
diffraction patterns were generated from the structural data in the corresponding CIF
resulting from single crystal analyses. The calculation was performed using MERCURY[40]
software of the Cambridge Crystallographic Data Center. A unique value of the full width
at half maximum for the diffraction peaks was adjusted to get a better match between the
6

resolution of the experimental and the calculated patterns. The determination of the total
carbon, hydrogen, nitrogen and sulfur (C, H, N, and S) contents in the compounds was
performed by using EA 1108 Fisons CHNS Element analyser by quantitative “dynamic
flash combustion” method. The UV−Vis spectra were recorded on a Cary 5000
spectrometer. The crystals were gently ground and placed on quartz holders. The
ATR−FTIR spectra were collected with a Nicolet iS 10 Smart FT-IR Spectrometer within
−
1
6
00−4000 cm . The thermogravimetric analysis was performed using Mettler Toledo at
0o C/min, under N
gas.
1
2
Electrochemical measurements
Electrochemical measurements were performed with a BAS SP-50 potentiostat, in pure
dimethylformamide purged with argon at room temperature. Glassy carbon electrode was
used as a working electrode, the counter electrode was a Pt wire and silver wire was the
pseudo-reference electrode. The reference of electrochemical potential was set using 1 mM
ferrocene as an internal standard and the values of potentials are reported vs SCE.[41] The
concentrations of samples were 1mM. Tetrabutylammonium hexafluorophosphate (TBAP)
(0.1 M) was used as supporting electrolyte. Cyclic voltammograms were obtained at a scan
rate of 100 mV/s and current amplitude of 100 µA.
Photocatalytic experiments
A PerkinElmer Clarus-480 gas chromatograph (GC) was used to measure hydrogen
evolution for our complexes. The assembly of this chromatograph consists of a thermal
conductivity detector, a 7-inch HayeSep N 60/80 pre-column, a 9-inch molecular sieve
1
3*45/60 column, a 2 mL injection loop and argon gas as carrier and eluent. DMF was the
solvent of choice for our experiments. Three separate solutions of 1) sacrificial donor and
proton source, 2) photosensitizer [Ru(bpy) ] (PF and 3) catalyst were prepared in order
3
)
6 2
to obtain 5 mL of sample solutions in standard 20 mL headspace vials. In DMF, the
resulting molar concentration of photocatalytic components were: 1 M for triethanolamine
(
TEOA), 0.1 M for HBF
4
(proton source), 0.56 M for water, 0.1 mM for the photosensitizer
7

[
Ru(bpy)
3
](PF
6
)
2
and 1 mM catalysts (pH apparent= 8, 9). (Aqueous HBF solution acts as
4
the proton source and TEOA as the sacrificial electron donor). This was followed by
placing the vials on a panel of blue LED 10 W center at 445 nm in a thermostatic bath set
at 20°C which was sealed with a rubber septum and two stainless-steel tubes pierced in it.
-1
Argon was carried in the first tube at a flow rate of 10 ml min (flow rate adjusted with a
manual flow controller (Porter, 1000) and referenced with a digital flowmeter (Perkin
Elmer Flow Mark)). The second stainless steel tube carried the flow to the GC sample loop
passing through a 2 mL over flow protection vial and an 8-port stream select valve (VICCI).
Timed injections were done by a microprocessor (Arduino Uno) coupled with a custom PC
interface. Corresponding to a specific argon flow, H
2
production rate was calibrated. For
calibration of H production rate at a specific argon flow, a syringe pump (New Era Pump)
2
equipped with a gas-tight syringe (SGE) and a 26s-gauge needle (Hamilton) was used to
bubble different rates of pure hydrogen gas into the sample, to a minimum of 0.5
μL/minute. For calibration testing, stock cylinders of known concentration of H in argon
2
replaced the argon flow (inserted at the pre-bubbler, to keep the same vapor matrix). The
measured results independent of flow rate (under same pressure) can be easily converted
into the rate of hydrogen using equation 1. The errors associated to the TON and TOF are
estimated to be 10 %.[42]
Equation 1
(μL min ) = [H
standard] (ppm) x Ar flow rate (L min-1)
-
1
Rate of production of H
2
2
RESULTS AND DISCUSSION
In comparison to a pyridine ring, the DAT group present in our synthesized compounds
consists of a triazinyl ring and two –NH groups. Located in ortho position of a
2
coordination site, the DAT group can participate by binding with metal ions to produce
functionalized transition metal complexes. As the catalytic properties can be severely
altered depending upon the metal ion used and the functional groups attached to the
catalysts, we examined the effect of DAT group for HER. We have previously reported the
synthesis and the characterization of compounds M(PyDAT)
2
(NO
3
2
) 5-7 obtained by
coordination of 6-(pyridin-2-yl)-1,3,5-triazine-2,4-diamine (PyDAT) with earth-abundant
8

metal ions (M= Co(II), Ni(II) and Cu(II)).[35] These complexes can be prepared easily in
high yield and purity (Chart 1). Herein, we discuss the synthesis and the structural study of
three novel complexes 8-10 obtained by the reaction of 6-(pyrazin-2-yl)-1,3,5-triazine-2,4-
II
II
II
diamine (PzDAT) with Co , Ni and Cu . Moreover, we have studied the physicochemical
properties of complexes 5-10 by thermogravimetric analysis (TGA), UV-Vis, infrared and
cyclic voltammetry (CV). In this work, we also investigated the catalytic activities of 5-10
for HER.
Structural Analysis and Characterization of Complexes 8-10
Based on the coordination of 2,2’-bipy and related bidentate ligands, we expect ligand 3 to
yield well established coordination motifs with transition metal ions and to further associate
in solid-state by hydrogen bonding of DAT groups (Chart 2) to generate crystalline
molecular materials. Complexes 8-10 were obtained by mixing ligand 3 (2 equiv) with
M(NO
3
)
2
.x(H O) (1 equiv) in methanol and purified by crystallization. Analyses by
2
electrospray ionization mass spectrometry (ESI-MS) measurements confirm the presence
of the metal ions in the complexes with the masses and isotope distribution patterns
+
corresponding to that of [M(pzDAT)
2
(NO
3
)] for complexes 8-10. The infrared spectra of
8
-10 (Fig. S3) indicate the presence of the ligand 3 with typical broad bands characteristic
-1
of symmetric and asymmetric N-H stretching of the NH group between 3500-3200 cm .
2
-1
The band appearing between 1300-1200 cm that is not observed in the IR spectrum of the
-
free ligand is assigned to NO
3
group (Table S1). The compositions confirmed by elemental
(NO for complexes 8 and 10 and
for complex 9 (see experimental section). Except for 8, we
have studied the molecular organization in solid state by single-crystal X-ray diffraction
SCXRD).
analysis (EA) for the samples are M(pzDAT)
M(pzDAT) (MeOH)(NO
2
)
3 2
2
)
3 2
(
9

H
H
N
N
N
H
H
N
N
N
H
H
H
N
N
N
N
N
N
N
H
H
N
N
H
H
H
N
N
N
H
H
H
N
N
H
N
N
H
H
H
N
N
N
H
N
H
N
H
N
N
H
N
H
I
II
III
=
aromatic fragment
Chart 2. Cyclic hydrogen bonding motifs I-III of diamino-1,3,5-triazinyl group (DAT).
1
. Structure of 6-(pyridin-2-yl)-1,3,5-triazine-2,4-diamine 2 with M(NO
3
)
2
.6(H O)
2
(Complexes 5-7, M = Co(II), Ni(II) or Cu(II))
In the previous work, we have reported the single-crystal structures of compounds 5-7.[35]
In order to facilitate a comparison of these known structures with new ones we summarized
succinctly their supramolecular organization. Molecular structures of 5-7 are shown in Fig.
1
. The chemical compositions of 5-7, with 2:1 pyDAT -to- M ratio, are M(PyDAT)
2
3 2
(NO )
(M = Co(II), Ni(II) or Cu(II)). Isostructural crystal structures were determined for 5 and 6
by single-crystal X-ray diffraction. A strongly distorted octahedral coordination geometry
was observed for complexes 5-7. The observed structures of 5-7 consist of enantiomeric
…
metallotectons that are linked alternatively via N-H N hydrogen bonds of DAT groups to
produce the three-dimensional supramolecular networks.
10

A
b
Fig. 1. Molecular structures of (a) 5 and 6 and (b) 7. Unless stated otherwise, carbon atoms
are shown in grey, hydrogen atoms in white, oxygen atoms in red, nitrogen atoms in blue,
cobalt or nickel atoms in pink and copper atoms in green.
2
. Structure of 6-(pyrazin-2-yl)-1,3,5-triazine-2,4-diamine 3 with Ni(NO
3
)
2
.6(H
2
O)
(Complex 9). Ligand 3 chelates with Ni(II) in the ratio 2:1 to form a cationic complex 9.
Crystals of this complex are grown in MeOH/Et O and belong to the triclinic space group
2
̅
P1. Perspective views of the crystal structure are shown in Fig. 2 and other crystallographic
data is provided in Table 1. As expected, the pyrazinyl-substituted diaminotriazine ligand
3
reacts with Ni(II) to form a mononuclear complex, which then engages in multiple
hydrogen bonding directed in part by the DAT groups resulting in
Ni(pzDAT) (NO )(MeOH)].(NO ) structure for complex 9 (Fig. S1). Ni(II) displays a
[
2
3
3
distorted octahedral coordination geometry with the two DAT groups in trans
conformation, and the nitrato and methanol in cis conformation. The Ni atom and one pair
o
of coordinated N atoms from different DAT groups gives a N-Ni-N angle of 169.3(2) . The
o
other coordinated pair of N atoms of pyridine rings has N-Ni-N angle of 96.6(1) . The
pyrazinyl and DAT rings of one of the ligand is nearly coplanar and the other is distorted
o
…
at an angle of 14.1(1) . As anticipated, DAT groups engage in intermolecular N-H N
hydrogen bonding according to type I (Chart 2) (average distance = 3.058 Å) to produce
tapes of alternating complexes ( and ) (Fig. 2a). These tapes are further connected to
form the three-dimensional structure by multiple hydrogen bonds involving bridging of
…
nitrate and N-H Omethanol (Fig. 2b). Selected hydrogen bonds and angles are given in Table
11

…
S2. It is worth noticing that intramolecular N-H O hydrogen bonds involving the free
hydrogen atom from –NH group and oxygen atom from methanol reinforce the geometry
2
of coordination of the complex. Moreover, the observed structure reflects intra- and inter-
complex interactions which are strongly directed by the multiple hydrogen bonds involving
DAT groups and nitrate counterions. However, unpredictably, a methanol molecule is
coordinated to the nickel atom during the crystallization process.
a
b
Fig. 2. Views of the crystal structure of the bis[6-(pyrazin-2-yl)-1,3,5-triazine-2,4-
diamine](methanolato)(nitrato-O)Nickel(II) nitrate 9 grown from MeOH/Et O. Hydrogen
2
bonds are represented by broken lines. Unless stated otherwise, carbon atoms are shown in
grey, hydrogen atoms in white, oxygen atoms in red, nitrogen atoms in blue and nickel
atoms in cyan. (a) Alternating arrangement of complex 9 and its enantiomer joined together
…
by hydrogen bonding of DAT groups according to motif I and by free N-H Onitrate. to
generate chains (b) Chains are maintained together by hydrogen bonding involving
bridging nitrates to form the three-dimensional structure. For clarity a chain is marked in
green.
12

Table 1. Crystallographic Data for 9 and 10
9
10
(NO
Formula
Mr
[(Ni(C
7
H
7
N
7
)
2
(MeOH)(NO
3
)](NO
3
)
Cu(C
7
H
7
N
7
)
2
3 2
)
593.16
565.95
Crystal size/mm3
Crystal system
radiation
λ (Å)
0.15 × 0.14 × 0.1
Triclinic
GaKα
0.17 × 0.11 × 0.06
Triclinic
GaKα
1.34139
1.34139
F(000)
608.0
574.0
̅
̅
Space group
a (Å)
P1
P1
7.9534(3)
10.3562(4)
14.6063(6)
100.637(2)
104.771(2)
97.685(2)
1122.45(8)
2
8.3550(4)
10.4080(5)
12.2948(5)
93.120(2)
96.497(2)
97.972(2)
1049.33(8)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
ρ
calcg/cm3
1.755
1.791
T (K)
100
100
-
1
μ (mm )
5.175
6.058
Measured reflns
independent reflns
26335
36360
5040
3582
R
int
0.0691
0.0681
3410
observed reflns I > 2σ (I) 4308
R
R
1
, I > 2σ (I)
0.0478
0.0594
0.1144
0.1243
1.038
0.0623
0.0642
0.1645
0.1677
1.097
1
, all data
ωR
ωR
2
, I > 2σ (I)
2
, all data
GoF
2
. Structure of 6-(pyrazin-2-yl)-1,3,5-triazine-2,4-diamine 3 with Cu(NO
3
)
2
.(H O)
2
(Complex 10). In complex 9, nitrate plays a structural role by serving as a counterion and
by forming multiple hydrogen bonds to stack the tapes together. Moreover, we have seen
that methanol molecule is not only playing as a solvent of crystallization but also as a
ligand. To probe the importance of these effects, we prepared the 2:1 complex of ligand 3
with Cu(NO
3
)
2
.(H
2
O) under the same conditions. Crystals of 10 were grown from
̅
MeOH/Et
2
O and proved to belong to the triclinic space group P1. Views of the crystal
structure are shown in Fig. 3, and other crystallographic data are summarized in Table 1.
The chemical composition of complex 10, with 2:1 pzDAT-to-Cu ratio, is
13

[
Cu(pzDAT)
2
(NO
3
)].(NO ) (Fig. S2). In the structure, DAT groups of ligand 3 are
3
positioned in trans conformation as expected and the nitrato is in an equatorial position
creating a typical Cu(II) coordination geometry that can be described as a distorted
bipyramid. The other nitrate ion is present outside the coordination sphere. The Cu atom
and two coordinated N atoms from different DAT groups are almost linear with N-Cu-N
o
angle of 177.5(2) . The other coordinated pair of N atoms forms N-Cu-N angle of
o
1
28.7(6) . The pyrazinyl and DAT rings of both ligands are nearly coplanar with an average
o
torsional angle of 2.76 . As anticipated, complex 10 self-assembles to form tapes via
hydrogen bonding of DAT groups according to the modified version of standard motif I
…
(
average distance N-H N = 3.109 Å) (Fig. 3a). The overall molecular organization
…
involves N-H O hydrogen bonding involving bridging of nitrato ligands and nitrate
counterions and - stacking (3.835 Å) of pyrazinyl and DAT rings to generate the three-
dimensional structure (Fig. 3b). Details of the hydrogen bonds and their angles are provided
in Table S3. Again, the results confirm that complex 10 can be obtained with predictable
number of ligands 3 as well as a single well-defined constitution. The DAT groups and the
counterions play a key role in directing molecular association by engaging in multiple
hydrogen bonds. However, the topology of complexes can not be easily predicted owing
to the diverse coordination geometries associated with Cu(II). As can be seen, in the
structure of complex 10, methanol molecules used in the crystallization are not involved in
coordination with the Cu(II).
14

a
b
Fig. 3. Views of the crystal structure of the bis[6-(pyrazin-2-yl)-1,3,5-triazine-2,4-
diamine](nitrato-O)Copper(II) nitrate 10 grown from MeOH/Et O. Hydrogen bonds are
2
represented by broken lines. (a) Alternating arrangement of complex 10 and its
enantiomer joined together by hydrogen bonding of DAT groups according to the
modified version of motif I to generate tapes. Tapes are maintained together by hydrogen
bonding involving bridging nitrate counterion and nitrato ligand to form the 3D-
networks. Furthermore, -stacking reinforce the structure. (b) View showing stacking of
tapes to form the three-dimensional structure. Carbon atoms are shown in grey, hydrogen
atoms in white, oxygen atoms in red, nitrogen atoms in blue and copper atoms in green.
Comparing the structures of compounds 5-7 and 8-10, we note that their structures are
similar to each other and the association of these complexes is mainly dictated by the self-
assembly via hydrogen bonds of the DAT groups with typical association motifs. In the
conditions of preparation of complexes 8-10, the nitrogen atom of the pyrazinyl ring in the
DAT group does not participate in coordination to metal ions. Within the six crystal
structures analyzed by SCXRD, methanol molecules are observed only in the crystal
structure of complex 9. In this case, methanol not only acts as a solvent of crystallization
but also as a ligand which participates in the first nickel coordinating sphere.
The phase purity of complexes 9 and 10 were evaluated by powder X-ray diffraction
(PXRD) which confirmed the absence of secondary phase. Fig. S4 shows the comparison
of the measured PXRD (in black) and the simulated patterns (in red) obtained from the
15

SCXRD. As can be seen the two patterns are matching well which confirms the phase
purity of the bulk samples 9 and 10. Together, the SCXRD and PXRD demonstrate that
samples 9 and 10 can be prepared in a single phase.
Thermal behaviours of complexes 8-10 were studied by thermogravimetric analysis
(TGA). The TG curves of these compounds are shown in Fig. S5. Compounds 8-10 show
similar decomposition curves with two main weight loss steps in the range of ~32-39% at
o
o
3
00-306 C and 36-46% at 305-795 C which are attributed to the de-coordination of the
nitrates and ligands to produce the final residue. In the case of complex 9, an additional
o
step is observed in the range 45-107 C with net mass loss of ~3 % that is assigned to the
loss of methanol.
UV-visible Spectroscopy and Cyclic Voltammetry
For successful utilization of the synthesized complexes as photocatalysts in HER, they
must show absorption in the visible region of the spectrum and must be able to accept
an electron from the excited photosensitizer. To better understand the electronic
properties of compounds 2-10, we measured their UV-Vis and cyclic voltammetry (CV).
UV-Vis spectra of these compounds were measured at room temperature in DMF
-6
solutions at concentration 8.8 x 10 M (Fig. 4). The UV-Vis spectra in DMF solution of
the free ligands 2 and 3 displayed two absorption bands between 200-350 nm which can
be attributed to the π-π* electronic transitions of aromatic rings. In the case of complexes
5
-10, the presence of transition metal ions produced significant variation in the intensities
of the absorption bands in the UV region of the spectra but only a slight shift towards lower
energy as compared to the absorption bands of free ligands. This red shift is well supported
by literature.[43] The transitions in the visible region (400-900 nm) are attributed to weak
d-d transitions. Selected data (maximum absorption wavelengths, λmax, molar absorption
coefficients, ε) are summarized in Table 2. As d-d transitions are weak, we also recorded
-3
the UV-Vis of 5-10 at room temperature in DMF solutions at concentration 8.8 x 10 M
to be able to see clearly the absorption bands (see inset Fig. 4).
16

Table 2. UV-Vis spectroscopy data of 2 and 3 and 5-10 in DMF solution
Parameters
DMF
Conc = 8.8 x 10-6
Conc = 8 x 10-3
From 250 to 900 nm
From 350 to 900 nm
Sample
2
-
1
3
-1
-1
3
-1
λ
max (nm)  (mol dm cm )
268
λ
max (nm)
 (mol dm cm )
4
1.21x 10
6.17 x 10
3
2
88(sh)
4
3
3
5
274
20(sh)
2.49 x 10
4.19 x 10
3
4
4
3
273
10(sh)
50(sh)
3.21 x 10
1.35 x 10
4.97 x 10
404
444
60
49
3
3
4
4
6
7
8
9
275
11(sh)
2.90 x 10
1.09 x 10
492
685
6
2
10
19
3
3
8
82
4
3
4
4
3
275
04(sh)
2.17 x 10
8.59 x 10
4.09 x 10
1.95 x 10
2.36 x 10
658
2
75
405
553
26
3
302(sh)
93(sh)
3
3
4
4
279
14(sh)
4.39 x 10
1.87 x 10
514
701
4
1
4
1
0
275
4.24 x 10
1.50 x 10
412
663
4
5
4
3
08
sh: shoulder
17

a
b
Fig. 4. UV-Vis spectra of 2-3 and 5-10 in DMF solution at room temperature at a
-6
concentration of 8.8 x 10 M. Inset is shown the visible region (450-800 nm) at a
-3
concentration 8 x 10 M (a) 2 and 5-7 and (b) 3 and 8-10.
The electrochemical behavior of ligands 2 and 3 and complexes 5-10 were measured using
CV in DMF solutions at a concentration of 1 mM for ligands and complexes with 1 M
TBA-PF (tetrabutylammonium hexafluorophosphate) as supporting electrolyte at a scan
6
rate 100 mV/s (Fig. 5). A summary of the wave peaks of these compounds are provided in
Table 3. Cyclic voltammograms of the complexes exhibit oxidation and reduction peaks,
which are attributed to the different redox states of the metal ions and reduction of the
ligands.
Cyclic voltammograms (CV) of ligands 2 and 3 shows two reversible redox waves at -2.22
and -2 V, and -2.31 and -1.72 V versus the Saturated Calomel Electrode (SCE),
respectively. These redox signals can be assigned as the two consecutive one electron redox
reaction of ligands. The CV of complex 5 shows three sequential redox waves at -2, -1.34
and at -1.15 V out of which, the first and the third redox waves are reversible. The first
I/II
redox signal can be assigned to be ligand based and the third signal can be of a Co couple.
In the CV of 8, there are four reversible redox waves occurring at -1.89, -1.71, -1.54 and -
0
.93 V, from which the second wave is indistinguishable due to overlapping of waves. The
first and the second redox signal can be assigned as ligand based and the third and fourth
18

signal are for Co and Co couple respectively. [44] The CV of complexes 6 and 9 with
I/0
II/I
nickel (II) metal ion shows two quasi-reversible redox waves at -1.30 and -1.05 V, and -
1
.54 and -0.88 V versus SCE, respectively. These redox signals can be assigned to a first
I/II
ligand reduction and a Ni couple, respectively. In case of the complexes 7 and 10 with
copper (II) metal ion, only the ligand based redox waves at -1.96 and -1.19 V respectively,
appears to be quasi reversible but its not completely distinct due to overlapping of the
waves. Whereas in case of the metal ion, no quasi reversible redox waves were observed.
However, Cu (II) metal ions gets reduced at 0.0063 and 0.089 V for 7 and 10 respectively,
it further undergoes oxidation at 0.32 and 0.42 V respectively, Vs SCE. This might be due
to the dimerization of copper complexes. [45] These values have been found comparable to
those of 2,2’-bipyridine complexes reported in literature (Table S4).[44, 46-47] The possibility
of multiple redox states for our complexes indicates their ability to be employed as redox
catalysts especially for HER.
Table 3. Redox data of the ligands 2 and 3 and the complexes 5-10 in DMF solution
Compound
Eox1/2 [V] Ered11/2 [V] Ered21/2 [V]
Ered31/2 [V] Ered41/2 [V]
2
3
5
6
7
8
9
-2
-2.22
-1.72(74)
-1.15(18)
-1.05(20)
-1.04(20)
-0.93(96)
-0.88(91)
-0.57(27)
-2.31(38)
-1.34(02)
-1.30(32)
-1.55(52)
-1.54(04)
-1.54(39)
-0.85(80)
-1.74(56)
-1.99(68)
-1.89(02)
0.13(24)
0.09(95)
-1.71(99)
-1.90(61)
1
0
19

b
a
c
e
g
d
f
h
Fig. 5. Cyclic voltammetry of 1 mM (a-b) 2-3 and (c-h) 5-10 respectively in DMF vs SCE.
20

Conditions: 1 M TBA-PF as supporting electrolyte, scan rate: 100 mV/s, glassy carbon
6
working electrode (1 mm diameter), Pt counter electrode and Ag wire as the pseudo-
reference electrode.
Catalytic Hydrogen Evolution
In order to study the effect of substituent and metal ion on the suitability of complexes 5-
1
0 as photocatalysts for hydrogen production, we examined their photocatalytic properties.
The hydrogen production displayed by 1 mM of each complex 5-10 in DMF using 1 M
(
II)
triethanolamine (TEOA) as the sacrificial electron donor, 0.1 mM Ru (bpy)
3
(PF
6
2
) as the
photosensitizer (PS) and aqueous 0.1M HBF as the proton source at an apparent pH
4
between 8-9 was recorded (Fig. 6). The experiment was conducted for 15 hours (after
which no further change in photocatalytic activity is observed) and the hydrogen
production rate, TON and TOF have been reported in millimoles per hour (mmol/h), moles
of hydrogen per moles of PS and mmol of hydrogen per mole of PS per minute[33,48]
,
respectively, for each compound (Table 4).
In the visible region, under blue irradiation (445 nm) the production of H starts after
2
turning on the light. This irradiation wavelength is sufficient to excite the photosensitizer
II
. The proposed mechanism for the process is shown in Fig. S7. [49,50]
[
Ru (bpy)
3
6 2
](PF )
Benjamin Probst and group studied the effect of pH on the photocatalytic production of
hydrogen, under different pH conditions using reference catalyst [Co(dmgH) ]. It was
2
observed that although acetic acid was the main proton source, but photocatalysis also
occurred without acetic acid, at a very slow rate, due to the residual protons coming from
water present in the system and the decomposition of TEOA. Similarly, we can deduce that
in the current photocatalysis experiment, HBF
and TEOA are acting as a subsidiary proton source.[51] Control experiments were conducted
in the presence of PS/TEOA alone with and without light and no H production was
recorded which was consistent with the previous results.[52] We also conducted blank
experiments with Co(NO . 6H O, Ni(NO . 6H O, Cu(NO . 2.5H O, 2 (pyDAT) and
(pzDAT) which displayed TON’s of 9.96, 18.54, 0.58, 5.25 and 0.12 molH2 molPS-1
respectively (Fig. S9, S10). Amongst the synthesized complexes, Cu complex 7 and Co
4
is the major source of proton while water
2
3
)
2
2
3
)
2
2
3
)
2
2
3
21

complex 8 display TON’s of 72 and 37 molH2 molPS-1. However, the remaining complexes
don’t prove to be efficient photocatalysts towards HER.
The replacement from the pyDAT (complex 5) to pzDAT (complex 8) significantly
modifies the capacity of Co(II) complexes in the production of H with TONs that almost
2
increase to three times. However, for copper complexes 7 and 10, pyDAT based Cu(II)
complex 7 has TON almost thirty five times higher, than its pzDAT analogue complexes
1
0. Since, different combinations of metals and ligands result in different TONs and TOFs,
we can suggest that the combined effect of the metal ion and the ligand leads to the overall
hydrogen production. Hence, the variation of substituent/ligand and the metal ion affects
the hydrogen production rate in photocatalytic experiments. The maximum TON and
-
1
hydrogen production rate of 72 molH2 molPS and 0.48mmol of H
2
per hour, respectively,
production rate of
observed for complex 7, is moderately higher than the TON and the H
2
the literature standard cobaloxime catalyst, which has 68 and 0.45 mmol/h values,
respectively (Fig. 6).
Amid all the tested complexes, Cu and py-DAT proved to be the most effective
combination as a photocatalyst to produce hydrogen. This can be due to the inherent nature
of Cu metal which displays diverse valence states, well-defined coordination chemistry and
special d–d spin-allowed transitions. [53,54] However, Cu complex 10 with pzDAT doesn’t
display good H
2
production. This indicates that for H production along with the metal, the
2
organic part of the complex also plays a major role and the combined effect of the two
leads to the final TON and TOF values. The values obtained for hydrogen production are
better than some of the previously reported copper based photocatalysts. For instance, Wu
I
II
and coworkers synthesized {[Cu Cu
DCTP = 4’-(3,5-dicar-boxyphenyl)-4,2’:6’,4’’-terpyridine and observed its photocatalytic
activity in UV-visible light in the presence of methanol as sacrificial agent and H PtCl as
co-catalyst. 0.032 mmol/g/h was the observed hydrogen production rate.[55] Song and group
synthesized C42 compound, the hydrogen production rate of 0.006 mmol/h
was observed in visible light using TEOA as sacrificial agent.[56] The same group later
synthesized C21 metal organic framework and observed a hydrogen
production rate of 0.073mmol/h using TEOA as sacrificial electron donor and platinum as
co-catalyst in visible light.[57] Majee and coworkers synthesized [Cu(DQPD)]
2
-(DCTP)
2
]NO
3
·1.5DMF}
n
3D framework where
2
6
6 21 4
H46CuN O S
H21CuN
3
O S
10 2
2
22

photocatalyst (where DQPD = deprotonated N2,N6-di(quinolin-8-yl)pyridine-2,6-
dicarboxamide) that produced 427 μmol H /h/μmol PS, using a PS (fluorescein)
2
concentration of 2 mM and TEA as a sacrificial electron donor.[58]
Table 4 Turnover number and Turnover frequency maximal for complexes 5-10
Compound Molecular Formula
H production TONmax TOFmax
2
-1
(
mmol/hour)
( min )
1892
307
std.*
[Co(dmgH)
Co(pyDAT)
2
(py)Cl]
0.45
0.11
0.14
0.48
0.25
0.06
0.01
68
16
21
72
37
9.1
2.1
5
6
7
8
9
2
(NO
(NO
(NO
(NO
(MeOH)(NO
(NO
3
)
2
Ni(pyDAT)
Cu(pyDAT)
2
3
)
2
793
314
2
3
)
2
Co(pzDAT)
2
3
)
2
841
215
90
Ni(pzDAT)
Cu(pzDAT)
2
3
)
2
1
0
2
3
)
2
*
std. stands for literature standard viz. cobaloxime; TON is reported in moles of hydrogen
per mole of PS and TOF in mmol of hydrogen per mole of PS per minute.
Fig. 6. Turn-over number with respect to PS for hydrogen evolution reaction of complexes
5
-10 and literature standard (std: [Co(dmgH) (py)Cl]).
2
23

CONCLUSIONS
A series of functionalized transition metal ion complexes 8-10, M(PzDAT)(NO
3
2
) , with
molecular structures that resemble M(bpy)
2
(NO
3
2
) was easily prepared by a facile
procedure. The solid-state structures of complexes 8-10 reveals that all cation complexes
have 2:1 ligand to metal ion ratio. In the solid-state the molecular organization of the
complexes is mainly dictated by the coordination modes of the ligands and the hydrogen
bonds involving the DAT groups with reliable patterns. An analysis of the thermal stability
and electronic properties of complexes 8-10 synthesized herein and pyridine DAT based
complexes 5-7, (pyDAT)(NO
3
2
) previously synthesized indicates the complexes to be
o
stable up to ~300 C and their ability to be employed as redox catalysts for HER
respectively. An investigation on the effect of substituent and metal ion on the catalytic
activity of complexes 5-10 for hydrogen evolution reaction designates copper complex 7
to the best displaying the highest TON viz. 72 moles of hydrogen per mole of PS suggesting
Cu and py-DAT to be the best amongst the synthesized combinations of ligands and metal
ions. This value observed for complex 7 (0.48mmol/h) is higher compared to some
previously reported Cu based catalysts (0.032 mmol/g/h, 0.006 mmol/h, 0.073mmol/h, 427
μmol H .
/h/μmol PS) [55-58]
2
ASSOCIATED CONTENT
Appendix A. Supplementary data
CCDC 1918901-1918902 contains the supplementary crystallographic data for this paper.
These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-
mail: deposit@ccdc.cam.ac.uk.
Supporting Information. Supporting Information. The supporting Information is
available free of charge via the internet at Elsevier Publications website at DOI:
24

Infrared data, additional crystallographic details, PXRD patterns, TGA curves, Cyclic
voltammograms and Emission spectrum.
AUTHORS INFORMATION
Corresponding Author
*
E-mail: adam.duong@uqtr.ca.
ORCID
Adam Duong: 0000-0002-4927-3603
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the Natural Sciences and Engineering Research Council of Canada, the
Canadian Queen Elizabeth II Diamond Jubilee Scholarships, the Fonds de recherche du
Québec – Nature et technologies, the Université du Québec à Trois-Rivières and the
Université de Montréal for financial supports.
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Graphical Abstract - Pictogram
28

Graphical Abstract Synopsis
Design, synthesis and characterization of pyrazinyl diaminotriazine complexes and their
comparison with pyridyl diaminotriazine based complexes for photocatalytic activity
towards hydrogen evolution reaction.
Credit Author Statement
Adam Duong: Supervision, Project administration, Conceptualization, Methodology,
Writing- original draft
Sanil Rajak: Synthesis, Investigation, Interpretation of results
Olivier Schott: Supervision of Hydrogen Evolution Reaction (HER) study and analysis of
the associated data
Prabhjyot Kaur: Writing- Reviewing and Editing, Analysis of results
Thierry Maris: Single Crystal X-ray Diffraction (SCXRD) data collection
Garry S. Hanan: Provided his laboratory facility to conduct HER and CV (Cyclic
Voltammetry) studies
29