Zn(II) carboxylates containing heterocyclic secondary ligands: synthesis and structure manifestation through DFT studies

Article abstract of DOI:10.1080/00958972.2021.1945046

New Zn(II) complexes (1–8) of general formula [Zn(R)₂(L')₂], where R = cinnamic acid (1–7), L' = 3-pyridine carboxamide (1), 2,2'-bipyridine (2), 3-pyridinecarbonitrile (3), 2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole (4), 2-methylimidazole (5), 2-phenylimidazole (6), 2-imidazolidinethione (7) and for (8), R = acetic acid; L' = 2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole)₂, have been synthesized by a stoichiometric reaction between Zn(II) carboxylate(s) and the respective nitrogen-based ligands. The prepared compounds are analyzed by FTIR, NMR spectroscopy and single crystal X-ray diffraction techniques for their structural manifestations along with elemental analyses. The crystallographic data for 6 and 8 exhibit four-coordinated zinc having distorted tetrahedral molecular geometry. The thermogravimetric data demonstrate high stability of the compounds with gradual loss of acetate and thiadiazol fragments that ultimately lead to zinc oxide as the residual mass of 8. Quantum chemical calculations indicate that 7 is the softest in reactivity having the lowest band gap as determined from the energies of frontier molecular orbitals whereas 8 is highly polar with the highest dipole moment value of 12.61 D. The computational data are a complement to the success of the research.

Full text of DOI:10.1080/00958972.2021.1945046

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Zn(II) carboxylates containing heterocyclic
secondary ligands: synthesis and structure
manifestation through DFT studies
Ayesha Zahoor, Imtiaz-ud-Din., Sohaila Andleeb, Ahmad Raheel, Rabia Ayub,
Sumaira Abbas & Muhammad Nawaz Tahir
To cite this article: Ayesha Zahoor, Imtiaz-ud-Din., Sohaila Andleeb, Ahmad Raheel, Rabia Ayub,
Sumaira Abbas & Muhammad Nawaz Tahir (2021) Zn(II) carboxylates containing heterocyclic
secondary ligands: synthesis and structure manifestation through DFT studies, Journal of
Coordination Chemistry, 74:12, 1978-1991, DOI: 10.1080/00958972.2021.1945046
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JOURNAL OF COORDINATION CHEMISTRY
021, VOL. 74, NO. 12, 1978–1991
2
https://doi.org/10.1080/00958972.2021.1945046
Zn(II) carboxylates containing heterocyclic secondary
ligands: synthesis and structure manifestation through
DFT studies
a
a
a
a
Ayesha Zahoor , Imtiaz-ud-Din.
Rabia Ayub
, Sohaila Andleeb , Ahmad Raheel ,
b
a
c
, Sumaira Abbas and Muhammad Nawaz Tahir
a
b
Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan; Department of Chemistry-
c
Ångstr €o m Laboratory, Uppsala University, Uppsala, Sweden; Department of Physics, University of
Sargodha, Sargodha, Pakistan
ABSTRACT
ARTICLE HISTORY
New Zn(II) complexes (1–8) of general formula [Zn(R)
2
(L’)
2
],
Received 13 January 2021
Accepted 23 May 2021
where R ¼ cinnamic acid (1–7), L’ ¼ 3-pyridine carboxamide (1),
2
,2’-bipyridine (2), 3-pyridinecarbonitrile (3), 2-amino-5-(4-pyri-
KEYWORDS
Zinc; carboxylate; nitrogen
donor; X-ray structure; DFT
dinyl)-1,3,4-thiadiazole (4), 2-methylimidazole (5), 2-phenylimi-
dazole (6), 2-imidazolidinethione (7) and for (8), R ¼ acetic acid;
L’ ¼ 2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole)
2
, have been syn-
thesized by a stoichiometric reaction between Zn(II) carboxy-
late(s) and the respective nitrogen-based ligands. The prepared
compounds are analyzed by FTIR, NMR spectroscopy and single
crystal X-ray diffraction techniques for their structural manifesta-
tions along with elemental analyses. The crystallographic data
for 6 and 8 exhibit four-coordinated zinc having distorted tetra-
hedral molecular geometry. The thermogravimetric data demon-
strate high stability of the compounds with gradual loss of
acetate and thiadiazol fragments that ultimately lead to zinc
oxide as the residual mass of 8. Quantum chemical calculations
indicate that 7 is the softest in reactivity having the lowest
band gap as determined from the energies of frontier molecular
orbitals whereas 8 is highly polar with the highest dipole
moment value of 12.61 D. The computational data are a com-
plement to the success of the research.
CONTACT Imtiaz-ud-Din.
drimtiazuddin@yahoo.com
Department of Chemistry, Quaid-i-Azam University,
Islamabad 45320, Pakistan.
Supplemental data for this article can be accessed online at https://doi.org/10.1080/00958972.2021.1945046.
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

JOURNAL OF COORDINATION CHEMISTRY
1979
1. Introduction
Zinc(II) is one of the most important metal cations in biological systems as it plays an
essential role in the activity of nearly 300 different enzymes, catalyzing about 50
important cellular biochemical reactions in living beings [1]. The biological functions
of zinc are associated with various proteins. The metal binds to proteins and adopts
distorted tetrahedral, trigonal bipyramidal or octahedral geometries by developing
linkages through the sulfur of cysteine, the nitrogen of histidine or the oxygen of
aspartate and glutamate, or of various combinations [2, 3]. In zinc proteins, the major
role of the metal may be catalytic, co-catalytic or structural. The catalytic activity of
zinc describes its direct participation in the bond making and breaking steps whereas
several metal ions are involved in co-catalytic processes via mutual cooperation. The
structural zinc sites give stability to the tertiary structure of the enzyme [4].
Zinc carboxylates with nitrogen donor ligands have interest because of their poten-
tial biological applications [5]. Studies on the formation of mixed ligand complexes
revealed their growing importance, in particular, their role in biological processes. The
ternary complexes involving carboxylic acids as primary ligand and nitrogen contain-
ing molecules as secondary ligands can serve as useful models in metalloenzyme-cata-
lyzed biochemical reactions [6]. Cinnamic acid and its hydroxyl derivatives are natural
compounds that bind to hemicelluloses and celluloses in the cell wall of plants. They
are also found naturally in products such as coffee beans, tobacco leaves and olive oil
[7–9]. Cinnamic acid and related compounds attain special attention due to their abil-
ity to scavenge free radicals and protect the living species against certain carcinogens
and mutagens [10–12].
The synthesis of mixed metal complexes have shown significant progress and con-
tinuously draw attention of coordination and bioinorganic chemists [13]. Here, compu-
tational chemistry is also used as a tool to rationalize and predict the course of
reactions along with assessing molecular properties of both the electronic ground and

1
980
A. ZAHOOR ET AL.
excited states. In continuation of our previous work exploring various structural and
biological aspects of metal complexes for advanced applications [14–19], we report
here the syntheses and characterizations of eight new mixed ligand complexes of
zinc(II) carboxylates having nitrogen-based secondary ligands in their coordin-
ation sphere.
2. Experimental
2.1. Chemicals/instrumentation
0
All chemicals such as zinc acetate dihydrate, sodium bicarbonate, nicotinamide, 2,2 -
bipyridine, 3-cyanopyridine, 2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole, 2-methylimida-
zole, 2-phenylimidazole, 2-imidazolidinethione, trans-cinnamic acid and acetic acid
were obtained from Sigma Aldrich, Germany and used as received. Solvents (metha-
nol, acetone, chloroform, ethanol, dichloromethane, dimethyl sulfoxide) were pur-
chased from Merck, Germany and used as such. The melting point of the
compounds were determined using a Gallen Kamp (U.K.) electrothermal melting
point apparatus and are uncorrected. FT-IR spectra were recorded on a Thermo
ꢀ
1
Shimadzu FT-IR spectrophotometer from 400 to 4000 cm with the resolution 4[1/
1
13
cm]. The H and C NMR spectra were recorded on a Bruker 300 MHz spectropho-
tometer operating at 300 and 75 MHz, respectively. The Bruker Smart-CCD diffractom-
eter was used to analyze single crystals of the compounds. The source of X-rays was
MoKa which had wavelength 0.71073 Å at 296 K. A Flash 2000 elemental analyzer
was used to perform elemental analyses. The percentage of zinc in the complexes
was determined using inductively coupled plasma optical emission spectroscopy
(
ICP-OES; Thermo iCap 6300). The crystal structures were solved by direct methods
2
followed by final refinement on F with full-matrix least-squares using SHELXL-97.
Thermogravimetric study was conducted by a Perkin-Elmer 7 Series thermal analyzer.
ꢁ
The measurements were carried out under nitrogen at a heating rate of 10 C/min.
2.2. Crystal structure determination
Single crystals for 6 and 8 were mounted on a thin glass fiber at room temperature
and the reflection data were collected on a Bruker Kappa APEXII CCD diffractometer
equipped with graphite monochromated MoKa radiation (k ¼ 0.71073 Å). The data
were also corrected for Lorentz and polarization effects. The structures were solved
2
using SHELXS-97. Final refinement on F was carried out by full-matrix least-squares
techniques using SHELXL-97 [20]. The projection views of the structures were prepared
using ORTEP-3 for Windows [21]. Hydrogens were stereochemically positioned and
refined with the riding model. The crystallographic data collections and experimental
details are summarized in Table 1.
2
.3. Synthesis
Aqueous solution of sodium salt of cinnamic or acetic acid, obtained by reacting
mmole of the respective acid with equimolar amount of sodium bicarbonate, was
2

JOURNAL OF COORDINATION CHEMISTRY
1981
Table 1. Crystal data and structure refinement parameters for 6 and 8.
6
8
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
C
36
H
30
N
4
O
4
Zn
18 18 8 4 2
C H N O S Zn
539.89
296(2)
Monoclinic
P2/c
10.7290(5)
5.7506(3)
17.7116(9)
90
98.157(2)
90
1081.72(9)
2
1.658
1.374
552.0
648.01
296(2)
Monoclinic
P2 /c
11.8899(4)
14.0461(5)
19.5051(6)
90
106.9990(10)
90
3115.16(18)
4
1.382
1
b/Å
c/Å
ꢁ
a/
ꢁ
b/
ꢁ
c/
3
Volume/Å
Z
q
3
calc g/cm
-
1
l/mm
F(000)
0.835
1344.0
3
Crystal size/mm
Radiation
0.44 ꢂ 0.36 ꢂ 0.30
MoKa (k ¼ 0.71073)
5.242 to 55.888
–15 ꢃ h ꢃ 15,
0.400 ꢂ 0.360 ꢂ 0.320
MoKa (k ¼ 0.71073)
8.06 to 55.872
–13 ꢃ h ꢃ 14,
–6 ꢃ k ꢃ 7,
ꢁ
2H Range for data collection/
Index ranges
–
–
18 ꢃ k ꢃ 16,
22 ꢃ l ꢃ 25
–23 ꢃ l ꢃ 15
Reflections collected
20,353
7000
Independent reflections
7410 [Rint ¼ 0.0355,
2555 [Rint ¼ 0.0291,
R
sigma ¼ 0.0441]
R
sigma ¼ 0.0352]
Data / restraints / parameters
7410 / 134 / 473
1.017
2555 / 0 / 151
1.069
2
Goodness-of-fit on F
Final R indexes [I> ¼2r (I)]
R
wR
1
¼ 0.0339,
¼ 0.0844
¼ 0.0520,
¼ 0.0935
0.27/–0.29
R
1
¼ 0.0276,
wR
¼ 0.0751
¼ 0.0304,
¼ 0.0766
0.34/–0.27
2
2
Final R indexes [all data]
R
1
R
1
wR
2
wR
2
-
3
Largest diff. peak/hole /e Å
added dropwise into aqueous solution of zinc(II) acetate dihydrate in mole ratio 2:1.
ꢁ
The reaction mixture was stirred for two hours at 50 C, the resultant solid product
was filtered, washed and air dried to get the precursors (I and I ). The precursor was
1
2
dissolved in methanol with subsequent addition of the respective secondary ligand to
it. The reaction mixture was stirred again for another 3–4 h under ambient tempera-
ture. The clear solution, thus obtained, was left for evaporation at room temperature
to obtain 1–8.
2
2
.3.1. Precursors
.3.1.1. Bis(3-phenyl acrylato)Zn(II) (I ). Quantities used; sodium salt of cinnamic acid
1
0
.5 g (3 mmole), zinc(II) acetate dihydrate 0.275 g (1.5 mmole). White crystalline prod-
ꢁ
ꢀ1
uct, Yield: 85%, melting point: 120–122 C. FT-IR (cm ): m (C ¼ C)Ar 1665 m, m (C ¼ O)
1
1
646s, m (C ¼ C) Alkene 1540w, m (Zn-O) 484 m. H NMR data (300 MHz, DMSO-d ,
6
13
ppm) d: ArH (7.62–7.34m), C ¼ CH (6.49, 6.54). C NMR data (75 MHz, DMSO-d , ppm):
6
d C ¼ O (172.8), C H (128.0–135.7), C ¼ C (120.6, 140.0). Anal. Calcd (%) for ZnC H O
6
5
18 14 4
(359.69 g g/mol): C, 60.11; H, 3.92; Zn, 18.18. Found: C, 59.65; H, 3.88; Zn, 18.21.
2
.3.1.2. Bis(acetato)Zn(II) (I ). Quantities; sodium salt of acetic acid 1.7 mL (3 mmole),
2
zinc(II) acetate dihydrate 0.275 g (1.5 mmole). White crystals, Yield: 84%, melting point:

1
982
A. ZAHOOR ET AL.
ꢁ
ꢀ1
1
1
07–108 C. FT-IR data (cm ): m (C ¼ O) 1617s. H NMR data (300 MHz, DMSO-d6,
13
ppm): d CH (1.62). C NMR data (75 MHz, DMSO-d , ppm): d C ¼ O (170.7), CH (20.1).
3
6
3
Anal. Calcd (%) for ZnC H O (183.1 g/mol): C, 26.18; H, 3.30; Zn, 35.65. Found: C,
4
6 4
25.93; H, 3.27; Zn, 35.67.
2
2
.3.2. Target compounds
.3.2.1. Bis(3-phenyl acrylato)bis(3-pyridine carboxamide)Zn(II) (1). Quantities used;
bis(3-phenyl acrylato)Zn(II) 0.269 g (0.75 mmole) and nicotinamide 0.17 g (1.5 mmole).
ꢁ
ꢀ1
White amorphous solid, Yield: 85%, melting point: 160–162 C. FT-IR data (cm ): N(NH)
270w, m (C ¼ C)Ar 1668 m, m (C ¼ O) 1738s, m (C ¼ C)Alkene 1541 m, m (C ¼ N)1558m m
3
(
1
Zn-N) 419 m (Zn-O) 485 m. H NMR data (300 MHz, DMSO-d , ppm): d ArH (7.55–7.34m),
6
13
C ¼ CH (6.45,6.54), CH ¼ N (9.06 s), NH (4.27s). C NMR data (75 MHz, DMSO-d , ppm): d
2
6
acid C ¼ O (173.1), nicotinamide C ¼ O (167.0), CH ¼ N (151.8), C H (124.0–148.8), C ¼ C
6
5
(
115.6, 148.0). Anal. Calcd (%) for ZnC H N O (603.94 g/mol): C, 59.66; H, 4.34; N, 9.28.
30 26 4 6
Zn, 10.83. Found: C, 59.17; H, 4.30; N, 8.97; Zn, 10.86.
2
.3.2.2. Bis(3-phenyl acrylato)bis(2,2’-bipyridine)Zn(II) (2). Quantities used were bis(3-
0
phenyl acrylato)Zn(II) 0.269 g (0.75 mmole) and 2,2 -bipyridine 0.22 g (1.5 mmole). White
ꢁ
ꢀ1
amorphous solid, Yield: 85%, melting point: 155–156 C. FT-IR data (cm ): m (C ¼ C)Ar
1
645 m, m (C ¼ O) 1738s, m (C ¼ C)Alkene 1577 m, m (C ¼ N)1495m, m (Zn-N) 417, m Zn-O)
1
4
84 m. H NMR data (300 MHz, DMSO-d , ppm): d ArH (7.21–8.01m), C ¼ CH (6.49, 6.54),
6
13
CH ¼ N (8.71 s).
C NMR data (75 MHz, DMSO-d , ppm): d C ¼ O (172.9), C H
6
6 5
(124.6–149.2), C ¼ C (122.4, 140.2). Anal. Calcd (%) for ZnC H N O (672.06 g/mol): C,
38 30 4 4
67.91; H, 4.50; N, 8.34; Zn, 9.73. Found: C, 67.46; H, 4.45; N, 8.19; Zn, 9.71.
2.3.2.3. Bis(3-phenyl acrylato)bis(3-pyridinecarbonitrile)Zn(II) (3). Quantities used were
bis(3-phenyl acrylato)Zn(II) 0.269 g (0.75 mmole) and 3-cyanopyridine 0.15 g
ꢁ
(
(
1.5 mmole). White amorphous solid; Yield: 85%; melting point: 158–160 C. FT-IR data
ꢀ
1
cm ): m (C ¼ C)Ar 1631 m, m (C ¼ O) 1640s, m (C ¼ C)Alkene 1577 m, m (C ¼ N)1495m, m
1
Zn-N) 419 m (Zn-O) 480 m. H NMR data (300 MHz, DMSO-d , ppm): d ArH (7.34–8.02m)
6
13
C ¼ CH (6.48, 6.54) CH ¼ N (9.0 s). C NMR data (75 MHz, DMSO-d , ppm): d C ¼ O
6
(
(
177.6), C H (124.7–153.5), C ¼ C (123.5, 140.6). Anal. Calcd (%) for ZnC H N O
6
5
28 18 4 4
539.86 g/mol): C, 62.29; H, 3.36; N, 10.38; Zn, 12.12. Found: C, 61.96; H, 3.32; 9.89;
Zn, 12.09.
2
.3.2.4. Bis(3-phenyl acrylate)bis(2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole)Zn(II) (4).
Quantities used were bis(3-phenyl acrylato)Zn(II) 0.359 g (1 mmole) and 2-amino-5-(4-
pyridinyl)-1,3,4-thiadiazole 0.26 g (2 mmole). White crystalline solid; Yield: 85%; Melting
ꢁ
ꢀ1
point: 209–210 C. FT-IR data (cm ): m (C ¼ C)Ar 1683 m, N(C ¼ O) 1644s, m
(
C ¼ C)Alkene 1555 m, m (C ¼ N) 1497 m, m (NH) 3272w, m (Zn-N) 444w, m (Zn-O) 408 m.
1
H NMR data (300 MHz, DMSO-d , ppm): d ArH (7.76–6.59m), C ¼ CH (6.51, 6.56), NH
6
2
13
(
4.30 s), CH ¼ N (8.66 s). C NMR data (75 MHz, DMSO-d , ppm): d C ¼ O (170.3), C H
6
6
5
(
128.0–154.7) C ¼ C (120.6, 141.0). Anal. Calcd (%) ZnC H N O S (716.12 g/mol): C,
32 26 8 4 2
5
3.67; H, 3.66; N, 15.65; S, 8.96; Zn, 9.13. Found: C, 54.11; H, 3.62; N, 15.07; S, 8.87;
Zn, 9.13.

JOURNAL OF COORDINATION CHEMISTRY
1983
2
.3.2.5. Bis(3-phenyl acrylato)bis(2-methylimidazole)Zn(II) (5). Quantities used were
bis(3-phenyl acrylato)Zn(II) 0.269 g (0.75 mmole) and 2-methylimidazole 0.12 g
ꢁ
(
(
(
1.5 mmole). White crystalline solid; Yield: 85%; Melting point: 220–221 C. FT-IR data
ꢀ
1
cm ): m (NH) 3235w, m (C ¼ C)Ar 1626 m, m (C ¼ O) 1638s, m (C ¼ C)Alkene 1558 m, m
1
C ¼ N) 1472 m, m (Zn-N) 430w, m (Zn-O) 472 m. H NMR data (300 MHz, DMSO-d ,
6
1
3
ppm): d ArH (8.262–6.66m), C ¼ CH (6.48, 6.54), CH (1.83 s), NH (12.04). C NMR data
3
(
75 MHz, DMSO-d , ppm): d C ¼ O (176.4), C H (124.0–151.0), C ¼ C (121.8,146.5), imid-
6
6 5
azole CH (23.6 s). Anal. Calcd (%) for ZnC H N O (523.90 g/mol): C, 59.61; H, 5.00; N,
3
26 26 4 4
10.69; S, 10.69; Zn, 12.49. Found: C, 59.13; H, 4.94; N, 10.14; Zn, 12.46.
2.3.2.6. Bis(3-phenyl acrylato)bis(2-phenylimidazole)Zn(II) (6). Quantities used were
bis(3-phenyl acrylato)Zn(II) 0.269 g (0.75 mmole) and 2-phenylimidazole 0.21 g
ꢁ
ꢀ1
(
1.5 mmole). White crystalline solid; Yield: 85%; Melting point 182–184 C. FT-IR (cm ):
m (NH) 3245w, m (C ¼ C)Ar 1665 m, m (C ¼ O) 1639s, m (C ¼ C)Alkene 1546 m, m C ¼ N)
1
1
(
492 m, m (Zn-N) 435w, m (Zn-O) 485 m. H NMR data (300 MHz, DMSO-d , ppm): d ArH
6
13
7.89–6.54m), C ¼ CH (6.45, 6.54), CH ¼ N (8.46 s), NH (12.66 s). C NMR data (75 MHz,
DMSO-d , ppm): d C ¼ O (176.1), C H (125.6–129.0), C ¼ C (146.2). Anal. Calcd (%) for
6
6 5
ZnC H N O (648.04 g/mol): C, 66.72; H, 4.67; N, 8.65; Zn, 10.09. Found: C, 66.97; H,
3
6 30 4 4
4.61; N, 8.58; Zn, 10.13.
2.3.2.7. Bis(3-phenyl acrylato)bis(2-imidazolidinethione)Zn(II) (7). Quantities used were
bis(3-phenyl acrylato)Zn(II) 0.269 g (0.75 mmole) and 2-imidazolidinethione 0.15 g
ꢁ
(
(
(
(
1.5 mmole). White amorphous solid; Yield: 85%; Melting point: 24–250 C. FT-IR data
ꢀ
1
cm ): m (NH) 3328w, m (C ¼ C)Ar 1659 m, m (C ¼ O) 1637s, m (C ¼ C)Alkene 1570 m, m
1
C ¼ N) 1492 m, m (Zn-O) 496 m. H NMR data (300 MHz, DMSO-d , ppm): d ArH
6
13
7.72–6.59m), C ¼ CH (6.48, 6.54), CH ¼ N (8.29 s), NH (1.81 s), thione CH (3.34, 3.49).
C
2
NMR data (75 MHz, DMSO-d , ppm): d C ¼ O (178.3), C H (128.0–135.8m), C ¼ C (115.6,
6
6 5
1
48.0), thione CH (44.5). Anal. Calcd (%) for ZnC H N O S (564.01 g/mol): C, 51.11; H,
2 24 26 4 4 2
4.65; N, 9.93; S, 11.37; Zn, 11.60. Found: C, 50.67; H, 4.60; N, 9.67; S, 10.98; Zn, 11.58.
2.3.2.8. Bis(acetate)bis(2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole)Zn(II) (8). Quantities
used were bis(acetato)Zn(II) 0.181 g (1 mmol) and 2-amino-5-(4-pyridinyl)-1,3,4-thiadia-
ꢁ
zole 0.4 g (2 mmole). White crystalline solid; Yield: 85%; melting point: 213–214 C. FT-IR
ꢀ
1
(
cm ): m (C ¼ C)Ar 1455 m, m (C ¼ O) 1619s, m (C ¼ N) 1494 m, m (NH) 3232w, m (Zn-N)
1
4
(
42w, m (Zn-O) 506 m. H NMR data (300 MHz, DMSO-d , ppm): d Py H (7.77–8.67m), CH
6
3
1
3
1.85 s). C NMR (75 MHz, DMSO-d , ppm): d C ¼ O (170.6), Py C (120.9, 150.7), CH (22.7),
6
3
C-NH (154.0). Anal. Calcd (%) for ZnC H O N S (539.91 g/mol): C, 40.04; H, 3.36; N,
2
18 18 4 8 2
20.75; S, 11.88; Zn, 12.12. Found: C, 39.69; H, 3.32; N, 20.23; S, 11.41; Zn, 12.08.
2.4 DFT studies
Quantum calculation and geometry optimization of the compounds have been carried
by using density function theory (DFT). The Gaussian 16 program package, revision
B.01, is employed for all the quantum chemical calculations [22]. The B3LYP/LANL2DZ
[23–27] was used for the geometry optimizations. The optimized structures were

1
984
A. ZAHOOR ET AL.
Scheme 1. Synthetic strategy for 1–8.
confirmed to display a real minimum with no imaginary frequencies (Nimg ¼ 0) by cal-
culation of the harmonic vibrational analytical frequency calculations. Geometry opti-
mizations were carried out in the gas phase. Optimized geometries and cartesian
coordinates are given as Supporting Information (SI-data). The time-dependent density
functional theory (TD-DFT), using TD-RB3LYP/LANL2DZ in the gas phase, was also used
to study the properties of frontier molecular orbitals. Ionization energies (IE), electron
affinities (EA), band gap (DEHOMO-LUMO), electronegativity (v) and other theoretical
parameters were determined by using the following equations:
DE ¼ ELUMO―EHOMO
IEðIÞ ¼ ꢀEHOMO
EAðAÞ ¼ ꢀELUMO
v ¼ ðI þ AÞ=2
g ¼ ðI―AÞ=2
r ¼ 1=g
(1)
(2)
(3)
(4)
(5)
(6)
3. Results and discussion
The target ompounds (1–8) were synthesized in three steps and the resultant product was
separated and purified with various organic solvents in each step. The first step involved
the formation of sodium salt of the respective carboxylic acid and was prepared by react-
ing the acid with two equivalents of sodium bicarbonate in a water/methanol solution.
Then the salt was used in metathesis by reacting it with zinc acetate dihydrate. The zinc(II)
cinnamate (I ) and zinc(II) acetate (I ), thus, formed, were further reacted with the appro-
1
2
priate secondary heterocyclic ligands to synthesize the desired metal complexes, the
details of which have been delineated in Scheme 1. All the complexes were characterized
by various analytical techniques such as spectroscopy and single crystal XRD. Melting point
determinations, at certain time interval, revealed that all the synthesized complexes exhibit
high degree of stability in moist air and are not sensitive to hydrolysis.

JOURNAL OF COORDINATION CHEMISTRY
1985
3.1. Spectroscopic data
The synthesized products (1–8) were characterized by spectroscopic techniques of which
IR was employed to indicate the various functional groups present in the compounds. The
appearance of Zn-N and Zn-O absorption bands in FTIR spectra of all the compounds at
ꢀ
1
4
90 and 420cm , respectively, indicate the establishment of the relevant bond to the
metal center. The difference between asymmetric and symmetric carboxylate stretching
frequencies (Dꢀ) is often used to correlate the data with the binding mode of the carboxyl-
ate functionality to the metal center in elucidating the structure [28]. The literature reports
ꢀ
1
suggest that Dꢀ above 180 cm is typical for monodentate coordination, a value of
ꢀ
1
ꢀ1
1
20–180 cm indicates bridging behavior while a difference less than 120 cm corre-
sponds to bidentate coordination for carboxylate [28]. The data show that carboxylate is
monodentate with zinc which was further confirmed by crystallographic data. Broad
ꢀ
1
stretching band for m(N-H) is observed at 3300cm in these compounds.
The formation of the mixed ligands’ complexes was further authenticated by NMR
spectroscopic data. All the distinct protons in the compounds were identified by their
chemical shift values, multiplicity pattern and integration values. No significant shifting
of peaks was observed for alkene protons of zinc cinnamate as two distinct signals
appeared at around 6.5 ppm which correspond to trans-hydrogens and almost similar
resonances for these protons occur for 1–7. The linkage of secondary ligands to the
metal center does not occur through deprotonation, rather by sharing the charge
density of loan electron pair of nitrogen with zinc as indicated by the retention of
well-defined NH functionality in the NMR data for all the synthesized complexes. The
aromatic protons appear in their usual region but with slight up-field shift after com-
plexation with the metal. The number of magnetically nonequivalent carbons in the
13
newly synthesized complexes were determined by C NMR data for which the num-
ber of signals, found in their spectra, were in accordance with the proposed structures.
The characteristic resonance for C ¼ O appeared around 172.0 ppm that indicated for-
mation of precursors I and I and this peak remains at its parent position with slight
1
2
shift according to the substituent on the heterocyclic ring in all the synthesized com-
plexes. The electropositive nature of zinc and circulation of pi-electrons are the main
factors that cause shielding and up-field shift of the carbons of (-C–N-C-), the moiety
which is present in most of the complexes having imidazole as the secondary ligand.
All the alkenyl carbons of cinnamate, as well as the aromatic carbons, resonate in their
respective regions, depending upon the substituents attached to the ring; the values
are in agreement with those reported [29]. Based on the spectroscopic data zinc
behaves as an electrophilic center that coordinates with the electron lone pair of nitro-
gen for I and I . This is supported by the XRD data for 6 and 8.
1
2
3.2. X-ray structural analysis
The structural authentication for 1–8 was further achieved through the X-ray diffrac-
tion studies. The single crystals were grown in acetone/toluene solution by slow evap-
oration at room temperature to get the X-ray data. The ORTEP diagrams for 6 and 8
along with the atomic numbering schemes are delineated in Figures 1 and 2, respect-
ively, along with their packing diagrams. The crystallographic and structural refinement

1
986
A. ZAHOOR ET AL.
Figure 1. (a) ORTEP diagram and (b) packing diagram for 6.
Figure 2. (a) ORTEP diagram and (b) packing diagram for 8.
data are presented in Table 1 and selected metrical parameters are illustrated in Table
S1 (as Supporting Information). The packing diagrams show six and two formula units
are enclosed in a crystal lattice for 6 and 8, respectively. The units are connected
through H-bonding while 6 and 8 exist as discrete molecules. The existence of
H-bonding and the presence of aromatic rings contribute stability to the crystal struc-
tures of the compounds (Supporting Information Table S3). The crystallographic data
suggest that 6 crystallizes in monoclinic space group P2 /c with four-coordinate zinc
1
through two oxygens of the two cinnamate species and nitrogens of 2-phenylimida-
zole with the resultant molecular geometry as distorted tetrahedral because of steric
hindrance. The cinnamate is monodentate with the Zn-O1 and Zn-O3 bond lengths
1.975 and 1.960, respectively. The distance of the uncoordinated cinnamate oxygen
from zinc is too long to be considered an effective interaction. The most important
deviation from the ideal value is observed for O3-Zn1-O1 and O3-Zn1-N3 (117.9(2) and
117.31(15)), respectively, for 6. The acetate coordinates to zinc monodentate assuming
a distorted tetrahedral molecular geometry in 8 and the most important deviations
from ideal values were observed for O1-Zn1-O1 and O1-Zn1-O1 with angles 102.99(8)
and112.61(6), respectively. Thus, crystallographic data manifest the linkages for all the

JOURNAL OF COORDINATION CHEMISTRY
1987
coordinating species to zinc, confirming the formation of these mixed ligand
zinc complexes.
Complexes 6 and 8 also exhibit various weak interactions in their structures with the
formation of 2 D networks as depicted in Figures 1 and 2. The packing diagram of 6
shows intermolecular hydrogen bonds as well as other weak interactions with neighbor-
ing molecules. Its single unit is connected to the adjacent neighboring units through H-
bonding between the O4 of the 3-phenyl acrylate-group and N2 of phenyl imidazole moi-
ety (of neighboring molecules). Similarly, N2 of the first molecule form hydrogen bonds
with O4 of the neighboring molecules and O4 … .N2 separation was 1.832 Å. Moreover,
carbons of phenyl acrylate weakly interact with the C-H bonds of adjacent imidazole
groups (C … H-C separation of 2.821Å). The oxygen of 3-phenyl acrylate weakly interacts
with the H-C bond of 3-phenyl acrylate group of the neighboring molecule having
O … H-C separation of 2.5821 Å. The C33 (of the phenyl imidazole group) weakly interacts
with carbons, C13 and C14, of 3-phenyl acrylate group. All these interactions demonstrate
formation of a network of molecules as depicted in the packing diagram.
The geometrical parameters for 8 indicate that the single molecule develops inter-
molecular H-bonds and various types of weak interactions with the neighboring mole-
cules as revealed in its packing diagram. Its single unit is connected to the adjacent
molecular units through H-bonding between N4 (of amine) and O2 (of the acetate
groups) of the neighboring complex molecules (O2-H-N4). The N3 (of thiadiazole
group) is also found to show H-bonds with amine N3 of the other neighboring mole-
cules (N3 … .H-N4). The N4 … .O2 and N3 … .N4 separations were 2.104 Å and 2.189 Å,
respectively. N2 of thiadiazole interacts weakly with C–H bonds of an adjacent carbon
(C2) of the acetate group with a separation of 2.588 Å. Furthermore, O1 of acetate
weakly interacts with pyridinyl carbons C1 and C2, establishing O1 … .C1 and O1 … .C2
weak linkages, separated by 3.201 Å and O2 … .C2 Å, respectively.
3.3. Thermal study
Thermal stability of 1–8 could be assessed by thermogravimetric techniques. To find out
the thermal stabilities of the compounds as well as to get a clue about their structural
composition, the thermogram for 8 is shown in Figure 3. The temperature ranges, peak
temperature, probable assignment and total mass loss data, as acquired from TG curves,
are summarized in Table 2. The data suggest five-stage decomposition pattern for 8,
ꢁ
[
Zn(CH COO) (C H N S) ]. The complex exhibited mass loss of 4.54% at 135 C, indicat-
3
2
7
6
4
2
ing the presence of coordinated solvent molecules in it and described the first stage of
decomposition. The second, third, fourth and fifth stages correspond to the removal of
both the ligands, i.e. imidazole and the acetate moieties that are attached to Zn(II) spe-
cies. The overall weight loss from the curve is 89.74%, while the theoretical weight loss
for the complex was calculated to be 90.90%, thus, verifying its structural composition.
The mass of the final residue corresponds to the stable zinc(II) oxide as anticipated.
3.4. DFT studies
The density functional theory (DFT) results at RB3LYP/LANL2DZ level for 1–8 are sum-
marized in Table 3. The quantum chemical calculations (RB3LYP and TD-RB3LYP) and

1
988
A. ZAHOOR ET AL.
Figure 3. Thermogram for 8.
Table 2. Thermal decomposition data for 8.
Mass loss from
TG (%)
Theoretical mass
loss (%)
Assigned
fragments
ꢁ
ꢁ
Stage
I
Temp. range ( C)
Peak temp. ( C)
36.05–135.79
135.79–248.47
247.51–344.85
344.85–496.37
496.37–998.40
48.66
212.21
312.60
384.90
0.00
4.454
10.383
25.21
4.56
10.54
25.36
27.48
22.96
90.90
Loss of
moisture/
solvent
Loss of fragment
of acetate
and thidiazole
Loss of fragment
of acetate
and thidiazole
Loss of fragment
of acetate
and thidiazole
Loss of fragment
of acetate
II
III
IV
V
27.268
22.432
and thidiazole
Traces of
8
9.747
metal oxides
frontier molecular orbital analysis show that HOMO is located on one side (terminal)
of the molecule containing the styrene moiety and there is a possibility of removal of
an electron from this site. The LUMO is located on the side towards the pyridine side
of 1 which in turn indicates the addition of electron to this site (Figure 4). This result
is also in agreement with the charge distribution within the molecule. The highest
DEHOMO-LUMO has been observed for 5 with a value of 4.54 eV and the lowest value of
2
.80 eV is for 7. Global hardness (g) and softness (r) values range from 1.40 ꢀ 2.27 to
0.44 ꢀ 0.71, that is, the highest hardness was found for 4 and the softest for 7 in terms
of their reactivity. Moreover, the highest dipole moment value has been observed for

JOURNAL OF COORDINATION CHEMISTRY
1989
a
Table 3. The quantum chemical calculation data for 1–8.
Compounds
DE (eV)
I (-EHOMO) (eV)
A (-ELUMO) (eV)
v
g
r
l (Debye)
1
2
3
4
5
6
7
8
a
4.33
3.81
3.62
3.29
4.54
4.41
2.80
3.51
6.50
6.04
6.34
5.96
5.85
5.82
4.49
6.20
2.18
2.23
2.72
2.67
1.31
1.41
1.69
2.69
4.34
4.14
4.53
4.31
3.58
3.62
3.09
4.45
2.16
1.90
1.81
1.65
2.27
2.20
1.40
1.76
0.46
0.52
0.55
0.61
0.44
0.45
0.71
0.57
5.21
7.29
6.39
11.22
7.54
10.68
7.75
12.61
Data obtained at B3LYP/LANL2DZ level.
Figure 4. Mulliken charge density and molecular orbitals, population analysis for
1
(ISOsurface: 0.02).
8
which is 12.61 Debye and the lowest is 5.21 Debye for 1. The data show that 8 is
the most polar while 1 is the least polar compound. Thus, computational modelling
provides complementary information to the experiment and a guide to future
researchers for understanding the synthetic procedure for such types of compounds.
4. Conclusion
A series of mixed ligand Zn(II) complexes (1–8) have been synthesized and character-
ized. The carboxylate is monodentate with zinc as suggested by IR data and subse-
quently confirmed by single-crystal XRD analysis. Single-crystal XRD data suggest that
6
and 8 adopt distorted tetrahedral geometries. The melting point determination and

1
990
A. ZAHOOR ET AL.
thermogravimetric data for 8 suggest that all the complexes possess stability and
insensitivity to atmospheric moisture. The quantum chemical calculations indicate that
the lowest band gap is for 7 and it is the softest compound in terms of reactivity. The
highest hardness and dipole moment values have been observed for 4 and 8, respect-
ively. The discovery of mixed ligand metal complexes helps in understanding various
structural aspects for such compounds and may serve as synthons for modified frame-
works to be employed in future drug/material development processes.
Data availability
CCDC 1562551 for 6 and 1562553 for 8 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.
uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax:
þ44 1223 336033.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
The authors are thankful to QAU, Islamabad, Pakistan, for providing financial assistance to carry
out the research work. The computations were enabled by resources provided by the Swedish
National Infrastructure for Computing (SNIC) at the National Supercomputer Center (NSC),
Link o€ ping with project number SNIC 2020/13-45.
ORCID
Imtiaz-ud-Din.
Rabia Ayub
http://orcid.org/0000-0001-8510-4551
http://orcid.org/0000-0003-2128-6733
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