Novel monoanionic diphenate-nicotinamide/N,N-diethylnicotinamide complexes of NiII, ZnII. Synthesis, structural investigations and hydrogen adsorption study

Article abstract of DOI:10.1016/j.molstruc.2020.128514

Mixed ligand complexes containing monoanionic diphenate (Hdiphen^-), dianionic diphenate (diphen^2?)-nicotinamide (nic)/N,N-diethylnicotinamide (denic) ligands of Ni(II) and Zn(II) transition metal cations were synthesized, their structural characterizations and molecular simulation for hydrogen adsorption were performed. It is the salt type coordination compound of the Ni(II) complex, complementing the metal octahedral environment with 2 mol of nic and 4 mol of aqua ligands. Two moles of Hdiphen^- ligand is monoanionic located outside the coordination sphere and provides the charge equivalence of the complex. In the Zn(II) complex structure, denic and diphen^2? ligand is located within the coordination sphere. In the Zn(II) coordination compound structure, diphen^2? ligands, which are located in the structure of a bridge between two different metal centres, make the structure polymeric. The binding properties of complexes to metal cations are also elucidated by infrared spectra. It has also been clarified whether the diphenate ligand is bonded to monovalent or bivalent. The mixed ligand complexes of diphenate [Ni(nic)(H₂O)₄].2(Hdiphen) (I) and [Zn(denic)(diphen)(H₂O)]·H₂O (II) distorted octahedral, and they have P-1 space group and triclinic crystal systems. The thermal decomposition of the complexes begins with dehydration steps and continues with the decomposition of organic ligands. As a result of the thermal analysis, it is determined that the oxides of the relevant metal cations remain as the final decomposition products of the molecules. The hydrogen storage of nickel complex is 0.153 wt% and the zinc complex is 0.289 wt% at 77 K and 10 bars.

Full text of DOI:10.1016/j.molstruc.2020.128514

Journal of Molecular Structure 1218 (2020) 128514
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Novel monoanionic diphenate-nicotinamide/N,N-diethylnicotinamide
complexes of NiII, ZnII. Synthesis, structural investigations and
hydrogen adsorption study
a
b
,
*
a,
Omer Yurdakul , Zarife Sibel S¸ ahin , Dursun Ali Kose **, Onur S¸ ahin c, Fatih Akkurt d
€
€
a Department of Chemistry, Science and Art Faculty, Hitit University, 19100, Çorum, Turkey
b Department of Energy Systems Engineering, Faculty of Engineering and Architecture, Sinop University, 57000, Sinop, Turkey
c Department of Occupat Health & Safety, Faculty of Health Sciences, Sinop University, TR-57000, Sinop, Turkey
d Gazi University, Engineering Faculty, Department of Chemical Engineering, Maltepe, Ankara, Turkey
a r t i c l e i n f o
a b s t r a c t
Mixed ligand complexes containing monoanionic diphenate (Hdiphen-), dianionic diphenate (diphen2ꢀ)-
nicotinamide (nic)/N,N-diethylnicotinamide (denic) ligands of Ni(II) and Zn(II) transition metal cations
were synthesized, their structural characterizations and molecular simulation for hydrogen adsorption
were performed. It is the salt type coordination compound of the Ni(II) complex, complementing the
metal octahedral environment with 2 mol of nic and 4 mol of aqua ligands. Two moles of Hdiphen- ligand
is monoanionic located outside the coordination sphere and provides the charge equivalence of the
complex. In the Zn(II) complex structure, denic and diphen2ꢀ ligand is located within the coordination
sphere. In the Zn(II) coordination compound structure, diphen2ꢀ ligands, which are located in the
structure of a bridge between two different metal centres, make the structure polymeric. The binding
properties of complexes to metal cations are also elucidated by infrared spectra. It has also been clarified
whether the diphenate ligand is bonded to monovalent or bivalent. The mixed ligand complexes of
diphenate [Ni(nic)(H2O)4].2(Hdiphen) (I) and [Zn(denic)(diphen)(H2O)]$H2O (II) distorted octahedral, and
they have P-1 space group and triclinic crystal systems. The thermal decomposition of the complexes
begins with dehydration steps and continues with the decomposition of organic ligands. As a result of
the thermal analysis, it is determined that the oxides of the relevant metal cations remain as the final
decomposition products of the molecules. The hydrogen storage of nickel complex is 0.153 wt% and the
zinc complex is 0.289 wt% at 77 K and 10 bars.
Article history:
Received 14 February 2020
Received in revised form
21 May 2020
Accepted 22 May 2020
Available online 25 May 2020
Keywords:
Diphenic acid
Nicotinamide
Metal complexes
Hydrogen adsorption
Energy storage materials
Metal-organic materials (MOMs)
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
centres. In these structures, ligands such as salicylates, dicarbox-
ylate succinate, maleate, terephthalate, polycarboxylate, couma-
The coordination compounds and polymers in which the tran-
sition metals are taken centrally are of interest due to their very
different properties. The studies on the synthesis and structural
properties of these compounds due to their potential applications
for nanotechnology, sensor applications, magnets, optoelectronics,
energy conversation and storage, gas storage, separation, catalysis
and similar industrial applications are of great interest [1e5]. The
side ligands may also be positioned as bridges between the metal
rate, 1,3,5-benzenethicarboxylate, 1,2,4,5-benzenetetracarboxylate,
benzene-1,3,5-tricarboxylate, diphenate, etc. are frequently used as
linkers [6e12]. Such coordination compounds are also referred to
as metal-organic frameworks. Studies on metal-organic frame-
works (MOFs) structures (syntheses, characterizations and appli-
cations) have increased in recent years with a considerable
acceleration. Because of their large surface areas and porosity, they
find many uses, especially gas adsorption, separation and catalysis.
Their structures are mainly based on the fact that an organic linker
settles into a bridge between two metal centres and transforms the
structure into a two-dimensional or three-dimensional framework.
By changing the organic linker, the surface areas and porosities can
be changed easily [13e18].
* Corresponding author.
** Corresponding author.
E-mail addresses: zarifesibel@sinop.edu.tr (Z.S. S¸ ahin), dalikose@hitit.edu.tr
€
(D.A. Kose).
https://doi.org/10.1016/j.molstruc.2020.128514
0022-2860/© 2020 Elsevier B.V. All rights reserved.

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O. Yurdakul et al. / Journal of Molecular Structure 1218 (2020) 128514
Terephthalate, 2,6-naphthalene dicarboxylic and diphenic acid
of the complexes are formed by p e p stacking interactions and
having dicarboxylic groups are amongst the aromatic ligand sour-
ces having two carboxylate groups capable of couple bonding for
bridge ligands forming MOFs by settling in the bridge position. The
diphenic acid (H2diphen) contains two carboxylic acid groups
bonded to the two phenyls, not co-planar rings. The protons of the
carboxyl groups present in the structure can be partially or
completely removed and ionized, making the metal ion more
voluntary. It also makes it possible to form hydrogen bond in-
teractions, thereby strengthening the molecular framework. Most
of the structures of the frameworks constructed with a transition
metal or rare earth metal cations of ionized dianionic (diphen2ꢀ) or
monoanionic (Hdiphen-) anions of diphenic acid have been re-
ported using the synthetic hydrothermal method [19e28]. The
coordination properties of the carboxyl group may vary depending
on the reaction conditions such as temperature, solvent and
auxiliary ligand. It has been shown that the pH value does not have
a significant effect on the synthesized molecule and the conversion
of the carboxyl group to the salt form with NaHCO3 before the pH
effect is eliminated [29e32]. The distortion of the space gap of the
diphenyl group characterizes the formation of macrocyclic struc-
tures [33,34] or helical chains by locating the diphenic acid as a
bridge between metal cations and metal clusters [35]. In the ligands
such as 1,10-phenanthroline and 2,2-bipyridine, which exhibit
hydrogen bonds. Thermal decomposition parameters and analyzes
of the complexes were investigated using TGA/DTA curves. The
structural formulas of ligands used in the synthesis of complexes
are given in Scheme 1.
2. Experimental
2.1. Materials and methods
The diphenic acid ([1,10-biphenyl]-2,20-dicarboxylic acid) and
metal salts of (Ni(CH3COO)2$4H2O, and Zn(CH3COO)2$2H2O and
nicotinamide (Vit. B3) and N,N-diethylnicotinamide (nikethamide)
used in the synthesis of complexes were provided Sigma-Aldrich
company.
2.2. Synthesis
Synthesis of metal-diphenate (Hdiphenꢀ)/(diphen2ꢀ)-Nicotin-
amide (nic)/N,N-diethylnicotinamide (denic) Complexes.
In the initial step, sodium diphenate salt (Na-dphen) was pre-
pared at room temperature in accordance with the reaction equa-
tion in eq. (1). Water was used as the solvent medium of the
reactions.
double-thread binding,
p-p may form interesting supramolecular
structures with stacking interactions [36,37]. The ability of
Bpyridine-like and dicarboxylate ligands to form MOFs constructs
has also been studied [38].
Moreover, novel crystal structures had been used for many
possible applications for the last decade, both theoretically and
experimentally [39e41]. Hydrogen storage application which is the
crucial part of the hydrogen energy system is compatible with the
compounds that have metal centres with organic ligands. Thus the
metal-organic compounds that are synthesized and characterized
in this work were tested for hydrogen storage application. In other
words, the hydrogen storage capacity of the compounds manu-
factured in this work was theoretically examined for hydrogen
storage. Some of our previous works show that the theoretical
calculations could be trustworthy for hydrogen storage capacity
calculations [42,43]. A type of metal-organic compound, named
Metal-Organic Framework (MOF) has great attention on it for
hydrogen storage capacities. Siegel et al. reported that more than
500 000 different kinds of metal-organic compounds investigated
and indexed for hydrogen storage and most effective of them have
the hydrogen storage capability by 40 g hydrogen per litre storing
material, approximately [44]. Department of Energy (DOE) (U.S.)
targets for solid-state hydrogen storage have not been met yet thus
the investigating storage capacities of novel metal-organic com-
pounds have attention. Hydrogen storage by physical adsorption in
hosting materials has better performance in low temperatures such
as easily accessible lowest temperature, liquid nitrogen tempera-
ture, 77K (ꢀ196 ꢁC). But the DOE focuses on room temperature
storage which is shown on the targets [45]. This is why various
temperatures, including room and liquid nitrogen temperatures,
were calculated for the complexes in this work.
C
14H10O4 þ NaHCO3 / C14H9O4Na þ CO2 þ H2O
eq. 1
In the next step, the derivation of the dipenate salts of the
transition metal cations was carried out by replacing the transition
metal cation with the sodium metal cation. The transition metal salt
solutions obtained according to the given reaction concentration
below were stored for the next step.
2C14H9O4Na þ M(CH3COO)2.nH2O /
M(C14H9O4)2.nH2O þ 2NaCH3COO
eq. 2
(M: NiII, ZnII; n:3-6)
For the synthesis of complex ligand complex structures,
neutralized ligand solutions of nicotinamide or N,N-
diethylnicotinamide were added at room temperature to the
transition metal diphenate salt solutions prepared above. The
resulting solution was allowed to crystallize at room temperature
for 20-22 days. The crystals of salt-type complex ligand complexes
[C12H20N4NiO6$2(C14H9O4) (I) and C24H24N2O6Zn$H2O (II)] where
the difenate cation was located outside the coordination sphere as
counter-ion were collected and washed with cold acetone and
dried in vacuo. The equation for the synthesis reaction of the mixed
ligand-containing complexes of transition metals is shown in eq.
(3).
M(C14H9O4)2.nH2O þ 2C6H6N2O / C10H14N2O / C12H20N4NiO6$2(-
C14H9O4) / C24H24N2O6Zn$H2O
eq. 3
In this report, we synthesized and investigated structures and
theoretical hydrogen storage capacity of two novel mixed ligand
complexes of a flexible diphenic acid ligand. While one of them is
salt type metal-organic materials (MOMs) complex containing
diphenic acid/nicotinamide ligands [Ni(nic)2(H2O)4].2Hdphen-
(nic:nicotinamide, Hdphen-:monoanionic diphenate), the other one
is polymeric type metal-organic frameworks (MOFs) complex
(M: CoII, ZnII; n:3-6; )
The structural properties of non-metal cation pentaborate
compounds were investigated with FT-IR (PerkinElmer Spectrum
One B), elemental analysis (LECO, CHNS-932), magnetic measure-
ments Mꢂ1 Sherwood Scientific model magnetic susceptibility
balance and examined according to Gouy Method, TGA/DTA/DTG
analysis (Shimadzu DTG60H) and P-XRD analysis (Rigaku Ultime-
containing
diphenic
acid/N,N-diethylnicotinamide
ligands,
[Zn(denic)(diphen)(H2O)]$H2O (denic: N,N-diethylnicotinamide,
IV) using a copper target (Cu K
a) forming an X-ray of the wave-
diphen2ꢀ: monoanionic diphenate). The supramolecular structures
length of 1.54 Å. The data were recorded from 20ꢁ to 100ꢁ.

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O. Yurdakul et al. / Journal of Molecular Structure 1218 (2020) 128514
3
Scheme 1. Structures of ligands. (a) diphenic acid, (b) nicotinamide, (c) N,N-diethylnicotinamide.
Table 2
2.3. Crystallography
Crystal data and structure refinement parameters for complexes I-II.
Crystal data
NiII complex
ZnII complex
The H atoms of C atoms were treated as riding atoms with
distances of 0.93e0.97 Å. We used these procedures for our anal-
ysis: solved by direct methods; SHELXS-2013 [46]; refined by full-
matrix least-squares methods; SHELXL-2013 [47]; data collection:
Bruker APEX2 [48]; molecular graphics: MERCURY [49]; solution:
WinGX [50]. Crystallographic data for the structures reported in
this paper have been deposited in the Cambridge Crystallographic
Data Center with CCDC numbers 1853969 for I and 1853970 for II.
Table 2 contains crystallographic data information of the structures.
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C40H38N4NiO14
857.45
Triclinic
C24H26N2O7Zn
519.84
Triclinic
P-1
P-1
8.3516 (14)
10.656 (2)
11.510 (2)
78.021 (6)
88.738 (5)
78.971 (6)
983.3 (3)
1
8.8027 (6)
10.9590 (9)
13.5806 (11)
112.333 (3)
97.703 (2)
96.774 (2)
1180.34 (16)
2
b (Å)
c (Å)
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
V (Å3)
Z
2.4. Computational details
Diffractometer
BRUKER D8-QUEST
0.71073 (Mo Ka)
ꢃ
l
ðAÞ
Temperature (K)
Dc (g cmꢀ3
range (ꢁ)
296
1.448
3.0e28.4
48447
4910
0.044
1.13
0.043/0.092
0.35/-0.28
Adsorption isotherms calculated for 10-10000 kPa pressure
range within 30 equilibration steps in logarithmic scale for different
four temperatures which are 253K, 273K, 298K and 313K. Besides,
low temperature (77K) adsorption isotherms calculated in 30 log-
arithmic equilibrium steps for both crystals in 0.01e10000 kPa
pressure range. Materials Studio suite academic software collection
used for computation. The sorption module of the software suite
was the primary calculation tool for hydrogen adsorption calcula-
tions. Supercells created for calculation accuracy (3 ꢂ 3 ꢂ 3 for Zn
complex and 4 ꢂ 4 ꢂ 4 for Ni complex) by using cif (crystal infor-
mation file) data to have geometrical coordinates and crystal
parameters.
)
1.463
3.1e28.3
55461
5879
0.036
1.18
0.042/0.089
0.65/-0.54
q
Measured refls.
Independent refls.
Rint
S
R1/wR2
Drmax/ )
Drmin (eÅꢀ3
3.2. Structural analysis
Complex (I): The molecular structure of complex I, with the
atom numbering scheme, is shown in Fig. 1. The asymmetric unit of
I contains one Ni(II) ion, one nicotinamide ligand, one non-
coordinated diphenic acid ligand and two aqua ligands. The Ni(II)
ion is located on a centre of symmetry and coordinated by two
nitrogen atoms [Ni1eN1 ¼ 2.1050(15) Å] of two different nicotin-
amide ligands and four oxygen atoms [Ni1eO2 ¼ 2.0516(14) Å and
Ni1eO3 ¼ 2.0673(13) Å] from aqua ligands, thus showing a dis-
torted octahedral coordination geometry. The selected bond pa-
rameters of the structures are given in Table 3. The crystal packing
in I is provided by OeH/O, NeH/O and CeH/O hydrogen bonds
The amino N2 atom acts as a hydrogen-bond donor to atom O1ii, so
forming a centrosymmetric R22(8) ring centred at (nþ1, nþ1, 1/2)
[n ¼ zero or integer] [(ii) -xþ2, -yþ2, -zþ1]. Similarly, aqua O3 atom
acts as hydrogen-bond donor to atom O1iii, so forming
Hydrogen loading changes the electrostatic field of the host
structure. Thus the Ewald summation method [51] used to calculate
the electrostatic field for each equilibration step. And, Lennard
Jones (LJ) 12-6 potentials that are explained by Rappe et al. [52]
used for adsorbate-adsorbate and adsorbate-adsorbent in-
teractions. A particular parameter set called “universal force field”
consist of interaction parameters that responses accurately in
comparison to experimental data [53,54].
3. Results and discussion
3.1. Elemental analysis
(see Table 1)
Table 1
Analytical data of complexes.
Complex
MW g/mol
Yield
Contents % found(calcd.)
Colour
d.p.
ꢁC
meff. BM
C
H
N
[Ni(nic)(H2O)4].2(Hdiphen)
857.45
519.84
92
89
55.88
(56.03)
55.69
4.65
(4.47)
4.82
6.49
(6.53)
5.35
green
112
123
2.63
C
40H38N4NiO14 (I)
[Zn(denic)(diphen)(H2O)]$H2O
24H26N2O7Zn (II)
pale white
diamag.
C
(55.45)
(5.04)
(5.39)
d.p: decomposition point.

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O. Yurdakul et al. / Journal of Molecular Structure 1218 (2020) 128514
Fig. 1. The molecular structure of complex I showing the atom numbering scheme. [(i) -xþ1, -yþ1, -zþ1.].
Table 3
Selected bond distances and angles for complexes I-II (Å, ꢁ).
Complex I
Ni1eN1
Ni1eO3
2.1050(15)
2.0673(13)
Ni1eO2
2.0516(14)
O2eNi1eO3
O2eNi1eN1
O3eNi1eN1
90.29(6)
88.61(6)
91.88(6)
O2eNi1eO3i
O2eNi1eN1i
O3eNi1eN1i
89.71(6)
91.39(6)
88.12(6)
Complex II
Zn1eN1
2.098(2)
Zn1eO2
2.2374(16)
2.1191(16)
1.9784(18)
102.90(8)
99.27(7)
Zn1eO3
2.0959(19)
2.3281(19)
100.20(8)
145.48(7)
60.28(6)
93.89(7)
95.02(7)
152.31(7)
83.23(8)
Zn1eO4ii
Zn1eO5ii
Zn1eO6
O6eZn1eO3
O3eZn1eN1
O3eZn1eO2
O6eZn1eO4i
N1eZn1eO4i
O6eZn1eO5i
N1eZn1eO5i
O6eZn1eN1
O6eZn1eO2
N1eZn1eO2
O3eZn1eO4i
O4i-Zn1-O2
O3eZn1eO5i
O2eZn1eO5i
90.73(7)
108.68(7)
164.09(6)
88.11(7)
107.70(6)
Symmetry code: (i) -xþ1, -yþ1, -zþ1 for I; (ii) xꢀ1, y, z for II.
centrosymmetric R22(16) ring centred at (nþ1, 1/2, 1/2) [n ¼ zero or
integer] [(iii) -xþ2, -yþ1, -zþ1]. The combination of these hydrogen
bonds produces edge-fused R22(8)R42(20)R22(16) rings which are
running parallel to the ab plane (Fig. 2). In I, the OeH/O hydrogen
bonds (Table 4) are observed between aqua ligands and carboxylate
oxygen atoms, with the combination of these hydrogen bonds
produces R22(8)R64(16) rings which is running parallel to the [001]
direction (Fig. 3). All of these intermolecular interactions give
three-dimensional framework results.
Complex (II): The X-ray single-crystal study shows that complex
II has 1D coordination polymer. The asymmetric unit of II contains
one Zn(II) ion, one nikethamide ligand, one diphenic acid ligand,
one aqua ligand and one non-coordinated aqua ligand (Fig. 4). The
Zn(II) ion is coordinated by one nitrogen atom [Zn1eN1 ¼ 2.098(2)
Å] of nikethamide ligand, four oxygen atoms [range from
2.0959(19) to 2.3281(19) Å] of two different diphenic acid ligands
and one oxygen atom [Zn1eO6 ¼ 1.9784(18) Å] from the aqua
ligand, thus showing a distorted octahedral coordination geometry.
For charge neutralization, the diphenic acid ligand is deprotonated.
The Zn(II) ions are bridged by diphenic acid ligands to generate 1D
coordination polymer which is running parallel to the [100] di-
rection, with the Zn∙∙∙Zn separation is 8.803 Å. The 1D polymers
are further joined by OeH$$$O hydrogen bonds (Table 4), gener-
ating 2D supramolecular network, which is running parallel to the
ab plane (Fig. 5).

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O. Yurdakul et al. / Journal of Molecular Structure 1218 (2020) 128514
5
Fig. 2. The formation of edge-fused R22(8)R42(20)R22(16) rings in I.
Fig. 3. The formation of edge-fused R22(8) and R64(16) rings in I.
3.3. Electronic spectra
monoanionic Hdiphenꢀ ligand settle as a counter ion outside of the
coordination sphere. The Zn(II) complex (II) includes 1-mol neutral
denic ligand coordinated to metal cation as monodentate via pyri-
dine nitrogen and 1-mol dianionic-bidentate diphen2ꢀ bonded to
metal cation and 1-mol coordination aqua and 1-mol aqua locate
outside of the coordination sphere. The structure of the Zn(II)
complex is thought to be polymeric, so the octahedral coordination
The Bohr Magneton data of complexes (2.63 for I and diamag-
netic for II) are an agreement with literature values for similar
compounds [22,55e57]. According to elemental analysis results,
the Ni(II) complex (I) contains 2 mol neutral nic ligand and 4 mol
crystalline aqua inside of the coordination sphere and 2 mol
Fig. 4. The molecular structure of complex II showing the atom numbering scheme. [(i) xþ1, y, z; (ii) x-1, y, z].

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O. Yurdakul et al. / Journal of Molecular Structure 1218 (2020) 128514
Table 4
Table 5
Hydrogen-bond parameters for complexes I-II (Å, ꢁ).
Some influential FT-IR bands of complexes.
D-H$ $ $A
Complex I
D-H
H$$$A
D$$$A
D-H$$$A
Groups
NiII
ZnII
n
n
n
n
n
n
n
(OH)H2O
(NH2)
(C]O)amid
(C]O)anionic carbonyl
(C]O)neutral carbonyl
(COO-)sym
3550e2950
3059
1601
1624
1683
1552
1396
156
1567; 1535
1243/1174
1297
1125e761
677
441w
533w
595s
3400e2900
e
C5eH5/O5
0.93
2.48
3.353 (2)
2.992 (2)
3.227 (2)
2.686 (2)
2.736 (2)
2.572 (2)
2.650 (2)
2.739 (2)
157
175
146
177
154
173
178
163
1595
1623
N2eH2A$$$O1ii
N2eH2B/O5
O3eH3A$$$O5i
O3eH3B/O1iii
O6eH6/O4
0.87 (2)
0.85 (2)
0.83 (2)
0.80 (2)
0.84 (2)
0.83 (2)
0.82 (2)
2.13 (2)
2.48 (2)
1.86 (2)
2.00 (2)
1.73 (2)
1.83 (2)
1.94 (2)
e
1546
1307
239
1560; 1531
1242/1175
1281
1133e751
683
(COO-)asym
Dnas-s
O2eH2C/O4
n
n
n
n
n
n
n
n
(C]C; CeC)ring
(C9eO1eC1)
(CC)þ (CCH)ring
(Ring)
(M ꢀ N)nic/denic
(M ꢀ O)carboxyl
(M ꢀ O)carbonyl
(M ꢀ O)H2O
O2eH2D$$$O7iv
d
Complex II
O6eH6A$$$O2iii
O6eH6B/O4iii
O7eH7C/O5ii
O7eH7D$$$O1iv
0.90 (3)
0.83 (4)
0.89 (4)
0.97 (5)
1.75 (3)
1.83 (4)
1.99 (4)
1.91 (5)
2.651 (3)
2.658 (2)
2.867 (3)
2.863 (3)
178
178
167
168
470s
540s
592s
Symmetry codes: (i) ꢀxþ1, ꢀyþ1, ꢀzþ1; (ii) ꢀxþ2, ꢀyþ2, ꢀzþ1;
(iii) ꢀxþ2, ꢀyþ1, ꢀzþ1; (iv) ꢀxþ1, ꢀyþ1, ꢀzþ2 for I; (ii) xꢀ1, y, z;
(iii) ꢀxþ1, ꢀyþ1, ꢀzþ1; (iv) x, yꢀ1, z for II.
high-intensity peaks were detected at 245 nm and 261 nm peaks in
the spectrum. These peaks were interpreted to be the peaks in the
assemblies, respectively, showing transitions due to metal-ligand
and ligand-metal charge transfer. The results were consistent
with the literature [22,56e58].
of metal cation is complemented by the double-threaded bonding
of the adjacent diphenic ligand. The coordination of the metal
cations in both complexes is thought to be octahedral.
The electronic spectra showed three spin-allowed d-d transi-
tions of complex (I) correspond to the wavelengths of
813 nm (A2g/T1g), 611 nm (A2g/T1g) and 366 nm (A2g/T2g).
These transition bands indicate that the d orbitals of the NiII metal
cation are supposed to support the octahedral geometry. The ZnII
complex (II) was expected to show no magnetic vibration due to
full d orbitals. Since the complex did not show any vibration on the
Gouy magnetic scale and was diamagnetic, no peaks were observed
for d-d transitions in the UVevisible region spectrum. However,
3.4. Infrared spectra
The infrared graphics of mixed ligands complexes were shown
in Fig. 6 and some significant peaks of them were summarized in
Table 5. The range of 3500-2650 cmꢀ1 stretching peaks can be
attributed to eOH bands of coordinated aqua ligands. The
stretching peak due to C]O group observed at 1753 cmꢀ1 for
diphenic acid was shifted to the region at 1624 cmꢀ1 for (I) and
1645 cmꢀ1 for (II). The salt type ionic bonding in the complex (II)
Fig. 5. The 2D supramolecular network in II.

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O. Yurdakul et al. / Journal of Molecular Structure 1218 (2020) 128514
7
and the covalent bonding in the compound (I). In the salt-type
metal complex, electron transfer is carried out from the carbonyl
group to the metal. At the same time, in the salt form (I) complex
structure, the diphenic acid exhibits two types of carbonyl
stretching. The first one is the carbonyl group of the ionic carbox-
ylate group, which provides coordination of the metal by trans-
ferring electrons to the metal and provides a charge of the metal.
This neutral stretching vibration carbonyl group occurs at
1683 cmꢀ1, which is a value close to the tensile wave of pure
diphenic acid. The peak was observed at the 3059 cmꢀ1 belongs to
stretching vibrations of eNH2 group of nicotinamide ligand in the
structure (I). The carbonyl groups of nic and denic ligands (amide
groups) stretching peaks were detected in the range of 1601 and
1595 cmꢀ1
.
The numerical difference between symmetrical and asymmetric
stretching vibrations (COOꢀsymeCOO-asym) of the carboxylic group
of the ligand is the most important indicator of whether the
carboxyl group is bound to monodentate or bidentate. It can be
observed that these differences are at 152 for the salt type mono-
anionic-monodentate diphenate complex (I) and 239 for coordina-
tion type dianionic-bidentate diphenate complex (II).
The three type absorption bands were observed corresponding
to M ꢀ O, and M ꢀ N bonding is the stretching vibrations in salt
type complex (I). The absorption peak of M-Nnic-pyridine bonding was
obtained 677 cmꢀ1, and the M-OH2O was 595 cmꢀ1 (strong peak),
Fig. 6. FT-IR spectra of the metal complexes Ni(II) complex is (I), and Zn(II) complex is
(II).
Fig. 7. TG-DrTG/DTA curves of complexes (a) Ni(II) complex (b) Zn(II) complex.
Fig. 8. The powder-XRD patterns of NiO (a) and ZnO (b).

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O. Yurdakul et al. / Journal of Molecular Structure 1218 (2020) 128514
and the M-Ocarboxyl was 441 cmꢀ1 (weak peak). But for the com-
plex (II) has four types M ꢀ O and M ꢀ N bonding peaks that are
M-Ndenic-pyridine at 683 cmꢀ1 and M-OH2O at 592 cmꢀ1 (strong
peak) and M-Ocarboxyl at 540 cmꢀ1 (strong peak) and M-Ocarbonyl at
470 cmꢀ1 (strong peak). The results obtained are sufficient to
explain the coordination types and regions of ligands to metal,
and they are compatible with the literature data [22,55e59].
3.5. Thermal analysis
[Ni(C6H6N2O)2(H2O)4]·2C14H8O4 (I): The thermal decomposi-
tion curves (TG-DTG/DTA) of complex I were showed Fig. 7a. The
metal complex doesn’t contain any hydrated water out of the
coordination sphere. The four moles coordination aqua remove
with one step at the range of 101e156 ꢁC (exp.8,75%; calc.8.40).
Then in the field of 203e958 ꢁC diphenate and nicotinamide li-
gands start to decompose. The decomposition of organic parts
takes place in five steps, and the intermediate products are
formed in each step. The decomposition proceeds with the output
of CO/CO2/NO2/N2O gases. The final decay product was deter-
mined by the powder XRD spectrum (Fig. 8) in which the black
NiO residue was present in the reaction crucible. The black colour
of the residual product is an indication that the combustion is not
complete due to the low oxygen in the structure of the complex in
degradation reactions in the inert nitrogen atmosphere. Accord-
ingly, some carbon atoms remain carbonized carbon on the metal
oxide surface. Theoretical and experimental data are compatible
with each other (Table 6).
[Zn(C14H8O4)(C10H14N2O)(H2O)]·H2O (II): The TG-DTG/DTA
curves of mixed ligand complex II was given Fig. 7b. According
to thermal analysis results, ZnII coordination compound has five
decomposition steps. Firstly, outside of the coordination sphere,
hydrated aqua removes from the main structure by one step. Then
one mole coordinated aqua ligand decomposes and leaves from
the complex. The neutral ligand N,N-diethylnicotinamide ligand,
starts to crumble from an anhydrous complex structure with one
step at the 234 ꢁC. Lastly, the diphenate ionic and bidentate ligand
decomposes in the range of 382e893 ꢁC by two steps. It was
found that black ZnO residues remained in the reaction vessel as a
thermal decomposition product. Experimental and computa-
tional per cent weight losses of all degradations are given in
Table 6. It has been found that the details of the proposed
degradation steps are consistent with the literature data
[22,55e59].
Created supercells used for hydrogen adsorption calculations
that are given in Fig. 9. It is seen in orthographic representation
that the holes exist in the structures. That means the hydrogen
could diffuse inner sites of the structure in this way. The phe-
nomena were also discussed as an accessible solvent surface area
previously in detail [60]. The supercells created because the
adsorbate-adsorbate and adsorbate-adsorbent interactions get
more realistic at the cells consist of more atoms; also, the elec-
tronic field makes sense while the electrostatic summation
method ignores some of the interactions at small structures. The
reason for creating small structures in comparison to much more
significant supercells was the computation constraints.
Hydrogen adsorption capacities of the adsorbents collected in
two temperature ranges which are low (77K) and relatively high
(253K, 273K, 298K and 313K) temperatures. Because the calcu-
lated hydrogen capacities have meaningful differences to give the
same plot, even each complex differs in hydrogen uptake capac-
ities, so the following Figures (Fig. 10a and b) created. It is clear
from the previous works [61,62] and the present work (Figs. 10
and 11) that the low-temperature hydrogen storage is more
effective. But the main reason was to investigate if the complexes

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O. Yurdakul et al. / Journal of Molecular Structure 1218 (2020) 128514
9
Fig. 9. Created supercells for a) Zn complex and b) Ni complex.
work nicely in the room temperature region (253 Ke313 K).
Both complexes had the highest hydrogen uptake at 253K for
the room temperature region as it is expected. And the values are
0.0435 and 0.00254 wt % at 253K for the zinc and nickel complexes
respectively. Also, the lowest hydrogen storage capacities calcu-
lated at 313K for both compounds. The capacities were 0.0200 and
0.0012 wt % at 313 K and 100 bars for complexes. Zinc complex had
better uptake performance, so the calculated capacities were bigger
than ten times in comparison to nickel complex at the same con-
ditions (adsorption temperature and equilibrium pressure). There
are two possible reasons which are two metal centres and aqua
molecule existence in zinc complex. In the experimental mea-
surements, degas procedure being applied to the host materials, by
this way moist and free aqua molecules exhaust from the adsor-
bent. Thus the free aqua molecules that are existing in the zinc
complex removed from the structure results in more surfaces that
the hydrogen could be stored. Another possible reason for storing
more hydrogen at the zinc complex is the two metal centre exis-
tence. Metal centres change the electrical charges of the molecule
or crystal structures. So the charged structures allow to placing of
hydrogen closer to the host material surfaces.
For better understanding, hydrogen molecules represented in-
side the host materials in Fig. 11. The positions of the hydrogen
molecules were calculated by using simulated annealing algo-
rithms that attend to the lowest total energy of the final structure.
The positions of the hydrogens being change by the atomic coor-
dinate shifting and rotating molecules. At last final locations being
determined by the lowest total energy.
The highest storage capacities calculated for the compounds at
77 K as it is expected. Hydrogen uptake versus equilibrium pressure
values given in Fig. 12. And the compounds had been storing
hydrogen almost full capacity up to 2 bars for zinc complex and
10 bars for nickel complex. The differences between the complexes
could be the same reasons, explained previously for high-
temperature region calculations, which are the extra surfaces af-
ter molecular aqua removal and two metal centre existence in zinc
complex in comparison to nickel complex. Additional surfaces that
the molecular hydrogen could be filled in the zinc complex could
result from the two times more considerable hydrogen uptake at
the maximum capacity (~10 bars for 77 K calculations). On the
other side, two metal atom existence in the unit cell (the smallest
unit of the crystal) could result in extra electrical charges which
enable additional electrical attraction of molecular hydrogen. Thus
the adsorbent could be filled relatively lower pressures (It is not
meant fast filling because the time was not the parameter for cal-
culations). Besides, a gap is also shown in Fig. 12.b. that the uptake
trends differ for the complexes up to 10 bars.
The highest hydrogen uptake calculated for the zinc complex
that is 0.289 wt % at 77 K and 10 bars. The other one, nickel com-
plex, could uptake 0.153 wt % hydrogen at the same conditions.
Calculated values are 0.0274 and 0.020 wt % at 77 K and 1 bar
equilibrium pressure for the zinc and nickel complexes,
respectively.
In consequence, the highest uptake capacities are shown that
the complexes have mean and lower uptake capacities in compar-
ison to previously examined calculations and experiments [44,62].
The hydrogen storage capacities of our complexes compared to the
hydrogen storage values of the calculated MOF compounds studied
in the literature can be evaluated as low [63e69]. But is not the
concern that calculating hydrogen storage capacities such com-
plexes is not meaningful. On the contrary, such complexes prove
and show what kind of differences result in more hydrogen uptake.
Fig. 10. Hydrogen storage capacities of the complexes at the high-temperature region,
a) Zn complex and b) Ni complex.

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O. Yurdakul et al. / Journal of Molecular Structure 1218 (2020) 128514
Fig. 11. Molecular hydrogen adsorbed complexes (green ellipsoids are hydrogen molecule), a) Zn complex and b) Ni complex at 2 x 2 x 2 supercells.
Fig. 12. Hydrogen storage capacities of the complexes at 77 K and different pressures at logarithmic scale for the complexes.
4. Conclusions
stabilizing ion outside the coordination sphere, while one of the
two acidic carboxylate groups was ionized. Therefore, 2 mol of
monoanionic diphenate ligand to balance the 2þ charge equiva-
lence of the metal in the structure were included in the structure.
The octahedral coordination of the metal is provided by 2 mol of
nicotinamide and 4 mol of aqua ligands involved in the coordina-
tion sphere. In its complex structure II, it is involved in coordination
In this study, two different metal cations and diphenic acid
complexes were synthesized, and their structural characterizations
were studied. The structures with neutral nicotinamide/N,N-
diethylnicotinmaid ligands were obtained as mixed ligand com-
plexes. In structure, I, the 2 mol diphenic acids, was placed as a

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O. Yurdakul et al. / Journal of Molecular Structure 1218 (2020) 128514
11
in both organic ligands and provides the octahedral coordination of
Bruker D8 QUEST diffractometer.
the metal cation with 1-mol of N,N-diethylnicotinamide ligand,
1 mol of monoanionic-bidentate diphenate ligand, one mol of aqua
ligand and the adjacent diphenate ligand that bind to the bidentate.
In this structure, the diphenate ligand is located in the position of a
bridge between two metal centres and transforms the structure
into a polymeric coordinating compound. While structures I and II
are considered as metal-organic materials (MOMs) growing with
covalent and hydrogen bonds, structure II can be called metal-
organic frameworks (MOFs) structure developed by coordinated
covalent bonds. The binding properties of the molecules were
supported by the infrared spectra, while the elemental analysis
results explained the per cent compositions. The thermal decom-
position steps of the structures were examined, and their thermal
characterizations were explained. Thermal decomposition steps of
coordination compounds begin with the degradation of the aqua
ligands. Both complexes in the final decomposition products were
found in the reaction vessel as determined NiO and ZnO residue.
The highest hydrogen uptake calculated for the zinc complex
that is 0.289 wt % at 77 K and 10 bars. The other one, nickel com-
plex, could uptake 0.153 wt % hydrogen at the same conditions.
Calculated values are 0.0274 and 0.020 wt % at 77 K and 1 bar
equilibrium pressure for the zinc and nickel complexes,
respectively.
In consequence, the highest uptake capacities are shown that
the complexes have mean and lower uptake capacities in compar-
ison to previously examined calculations and experiments. But is
not the concern that calculating hydrogen storage capacities such
compounds is not meaningful. On the contrary, such complexes
prove and show what kind of differences result in more hydrogen
uptake.
Thermal degradation is noticeable due to the different second-
ary ligands (nicotinamide and N,N-diethylnicotinamide) present in
the complexes. Besides, depending on the complexes having a
different metal centre, structural differences come to the fore.
These differences are the most important reasons for the differ-
ences both in thermal analysis curves and in other spectroscopic
analyzes. The salt type of the basic structure of the complex I, the
crystallization of the compound II in the polymeric coordination
compound structure was reflected in both thermal decomposition
curves and FT-IR spectra. It is determined that the complex number
I in the salt type structure shows low hydrogen storage capacity
due to the lower molecular spaces due to more frequent stacking.
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