Zn(II) Complexes with Quinoline Supported Amidate Ligands: Synthesis, Fluorescence, and Catalytic Activity

Article abstract of DOI:10.1134/S1070363219120302

Zn(II) complexes of N-(quinolin-8-yl)picolinamide (HL¹) (1) and N²,N⁶-di(quinolin-8-yl)pyridine-2,6-dicarboxamide (H₂L²) (2) have been synthesized by deprotonation of the ligands and characterized by IR, NMR, and Single crystal X-ray crystallography. The mononuclear [Zn(L¹)₂] (3) and homodinuclear [Zn₂(L²)₂] (4) complexes are characterized by distorted octahedral geometries stabilized by hydrogen bonding and weak π···π interaction. The complexes demonstrate intense fluorescence bands in comparison with their corresponding ligands with well-distinguished intensity. The complexes act as efficient catalysts in various transesterification reactions. Among those, the best results have been achieved with complex 3 in conversion of 4-nitrophenylacetate into methyl acetate within 3 h.

Full text of DOI:10.1134/S1070363219120302

ISSN 1070-3632, Russian Journal of General Chemistry, 2019, Vol. 89, No. 12, pp. 2516–2521. © Pleiades Publishing, Ltd., 2019.
Zn(II) Complexes with Quinoline Supported Amidate Ligands:
Synthesis, Fluorescence, and Catalytic Activity
F. Rehmana, M. N. Zafara,*, S. Yousufb, M. F. Nazarc, E. U. Mughalc, A. Malikd,
S. H. Sumrrac, M. N. Zafarc, and H. Rafiquec
a Department of Chemistry, Quaid I Azam University, Islamabad, 45320 Pakistan
b H. E. J. Research Institute of Chemistry, International Center for Chemical and Biological Sciences,
University of Karachi, Karachi, 75270 Pakistan
c Department of Chemistry, University of Gujrat, Gujrat, 50700 Pakistan
d Department of Chemistry, COMSATS Institute of Information Technology, Islamabad, 44000 Pakistan
*e-mail: mnzafar@qau.edu.pk
Received June 6, 2019; revised December 11, 2019; accepted December 25, 2019
Abstract—Zn(II) complexes of N-(quinolin-8-yl)picolinamide (HL1) (1) and N2,N6-di(quinolin-8-yl)pyridine-2,6-
dicarboxamide (H2L2) (2) have been synthesized by deprotonation of the ligands and characterized by IR, NMR,
and Single crystal X-ray crystallography. The mononuclear [Zn(L1)2] (3) and homodinuclear [Zn2(L2)2] (4)
complexes are characterized by distorted octahedral geometries stabilized by hydrogen bonding and weak π···π
interaction. The complexes demonstrate intense fluorescence bands in comparison with their corresponding ligands
with well-distinguished intensity. The complexes act as efficient catalysts in various transesterification reactions.
Among those, the best results have been achieved with complex 3 in conversion of 4-nitrophenylacetate into
methyl acetate within 3 h.
Keywords: Zn(II), quinoline, amidate, transesterification, fluorescence
DOI: 10.1134/S1070363219120302
INTRODUCTION
Here in, the synthesis and characterization of two new
Zn(II) complexes with amidates stabilized by additional
pendent quinoline and picoline arms are presented. Their
fluorescence properties as well as catalytic activity in
various transesterification reactions have been tested.
Photoluminescence properties of transition and main
group metal complexes have already been well estab-
lished [1–3]. Quinoline and most of its derivatives are
potentially photoluminescent compounds [4, 5], and their
photoluminescence effect increases upon coordination to
some metals [6, 7]. Such property makes them photolumi-
nescent detectives for selective probe of various metallic
ions in solution both in vivo and in vitro. Many studies of
modification of these compounds have been performed
for enhancing the desired selectivity and luminescence
properties [8–10]. Highly diverse photoluminescent
features and advance applications have been reported
for electronic and steric modifications of the ligand in
quinoline based zinc complexes [11–14].
EXPERIMENTAL
Synthetic precursors including picolinic acid, dipico-
linc acid, 8-aminoquinoline, zinc(II) acetate, and 1,8-di-
azabicyclo[5.4.0]undec-7-ene (DBU) were purchased
from Sigma Aldrich. Triphenylphosphite (TPP) and
tetrabutylammonium bromide (TBAB) were supplied
by Daejung chemicals, Korea. Dichloromethane (DCM)
was dried by anhydrous CaO followed by distillation.
N-(Quinolin-8-yl)picolinamide (HL1) (1) was prepared
according to the reported earlier procedure [20]. IR spec-
tra were recorded on a Thermo Scientific Nicolet-6700
FT-IR spectrophotometer. NMR spectra were measured
on a Bruker Avance 300 NMR spectrometer. A Sanyo
Gallen Kamp MPD350 apparatus was used for deter-
mining melting points. Single crystal X-ray profiles of
compounds were accumulated on a Bruker Kappa Apex
Various metal catalyzed transesterification reactions
have been cited in literature [15-17]. Zn(II) Complexes
of relatively low toxicity characterized by air and mois-
ture stability were found to have real potential for these
reactions [18]. Zinc-amidates under moderately tough
catalytic conditions could be used in transesterification
reactions [19].
2516

ZN(II) COMPLEXES WITH QUINOLINE SUPPORTED AMIDATE LIGANDS
2517
Scheme 1. Synthetic approach to compounds 1–4.
O
O
N
O
O
N
NH
N
HN
N
OH
OH
0.5 eq.
Zn(OAc)2
DBU
2
Zn
2(L )2
4
TBAB, TPP
2
2, H L
2
N
O
NH2
N
O
N
1 eq.
HN
N
OH
Zn(OAc)2
DBU
Zn(L1
3
)
2
1
1, HL
II X-ray diffractometer. Steady-state fluorescence (SSF)
Zn(L1)2 (3). A mixture of Zn(OAc)2.2H2O (100 mg,
measurements were carried out on a Shimadzu RF-6000
spectrofluorophotometer using a low-noise photomulti-
plier detector. The slit width (ex 5.0 nm; em 5.0 nm) was
adjusted to obtain good resolution of the bands. Fluores-
cence spectra of all samples were recorded in DCM in the
range of 350−800 nm with excitation at 330 nm.
0.46 mmol) with HL1 (227 mg, 0.92 mmol) and DCM
(10 mL) was stirred for 10 min at room temperature fol-
lowed by the addition of DBU (0.206 mL, 1.38 mmol).
The resulting mixture was refluxed for 2 h. After cooling
down to room temperature, n-hexane was added slowly
to induce precipitation. The precipitate was filtered off
and washed three times with n-hexane and diethyl ether
to give the product 3 as light green solid, yield 58%, mp
290°C. IR spectrum, ν, cm-1: 3057, 1613, 1585, 1495,
1382, 1208, 1150, 1041, 829, 749. 1H NMR spectrum, δ,
ppm: 9.68 d (1H, Ar-H), 8.56 d (1H, Ar-H), 8.19 d (1H,
Ar-H), 8.09 d (1H, Ar-H), 7.98 d (1H, Ar-H), 7.85–7.73
m (2H, Ar-H), 7.44 d (1H, Ar-H), 7.13–7.08 m (2H, Ar-
H). Found, %: C 63.90; H 3.35; N 14.50. C30H20N6O2Zn.
Calculated, %: C 64.10; H 3.60; N 14.95.
N2,N6-di(quinolin-8-yl)pyridine-2,6-dicarbox-
amide (H2L2) (2). The reaction mixture of 8-amino-
quinoline (1.0 g, 6.94 mmol) with dipicolinic acid (0.58 g,
3.47 mmol), triphenylphosphite (TPP) (2.15 g, 6.94
mmol), and tetrabutylammonium bromide (TBAB)
(1.12 g, 3.47 mmol) was heated for 30 min at 120°C till
the homogeneous solution was formed. The solution
was cooled down to room temperature, and the prod-
uct was precipitated out upon addition of cold water
(10 mL). The precipitate formed was filtered off and
washed with cold ethanol to give pure compound 2. Light
green solid, yield 83.0%, mp 285°C. IR spectrum, ν, cm-1:
3320, 3296, 3015, 2970, 1738, 1677, 1592, 1487, 1366,
Zn2(L2)2 (4). A mixture of Zn(OAc)2.2H2O (100 mg,
0.46 mmol) with H2L2 (193 mg, 0.46 mmol) and DCM
(10 mL) was stirred for 10 min at room temperature prior
to addition of DBU (0.206 mL, 1.38 mmol). The reaction
mixture was refluxed for 2 h. Upon cooling down to ambi-
ent temperature the precipitate was filtered off and washed
three time with diethyl ether to give the product 4 as a
parrot green solid, yield 74%, mp 310°C. IR spectrum, ν,
cm–1: 3053, 1631, 1605, 1568, 1401, 1317, 1238, 1157,
1004, 825, 759. 1H NMR spectrum, δ, ppm: 9.03 d (2H,
1
1022, 852s, 527. H NMR spectrum, δ, ppm: 12.39 s
(2H, N-H), 9.05 d (2H, Ar-H), 8.61 d (2H, Ar-H), 8.28 d
(2H, Ar-H), 8.25–8.19 m (2H, Ar-H), 7.72–7.63 m (2H,
Ar-H), 7.37–7.33 m (1H, Ar-H). Found %: C 71.35; H
3.75; N 16.25. C25H17N5O2. Calculated, %: C 71.70; H
4.10; N 16.70.
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REHMAN et al.
15
C
36
C
1
O
14
13
C
35
C
3
12
37
O
16
C
C
C
1
O
5
11
C
C
34
C
7
C
C
C
17
C
38
26
C
12
11
C
C
28
8
C
C
4
6
33
N
18
C
C
C
C
0M
39
N
5
C
C
10
C
2
N
3
2
C
N
3
19
21
N
C
0L
C
6
N
40
2
1
C
C
2
Zn
10
N
31
C
41
C
1
30
Zn
24
42
C
C
1
C
Zn
22
9
C
C
C
29
C
8
3
N
N
43
9
C
C
7
5
N
N
13
C
23
24
C
C
4
N
23
4
4
8
C
28
N
C
C
1
14
C
N
44
C
15
26
C
C
22
C
12
O
14
C
C
6
N
15
27
7
11
2
C
C
C
6
C
O
C
45
C
25
C
21
48
C
3
C
17
C
C
16
46
5
C
C
C
29
C
2
O
47
C
51
C
4
C
18
C
19
C
Fig. 1. ORTEP of [Zn(L1)2].
Fig. 2. ORTEP of [Zn2(L2)2] molecule.
Ar-H), 8.95 d (2H, Ar-H), 8.86 d (2H, Ar-H), 8.36 d (2H,
Ar-H), 8.11–8.06 m (2H,Ar-H), 8.00–7.52 m (1H,Ar-H).
Found, %: C 61.75; H 3.00; N 14.10. C50H30N10O4Zn2.
Calculated, %: C 62.20; H 3.15; N 14.50.
of triphenylphosphite that acted as a peptide coupling
agent. The new and improved approach to the synthesis
of N2,N6-di(quinolin-8-yl)pyridine-2,6-dicarboxamide
(2) involved addition of tetrabutylammonium bromide
(TBAB) [22,23] (Scheme 1). Two new zinc complexes,
[Zn(L1)2] (3) and [Zn2(L2)2] (4), were prepared by depro-
tonation of compounds 1 and 2 respectively. In the course
of these processes 1,8-diazabicyclo(5.4.0)undec-7-ene
(DBU) was added. It acted as a base but also improved
the solubility of ligands.
The IR spectrum of [Zn(L1)2] demonstrated absence of
the band at 3421 cm–1 that indicated N-H deprotonation
of the ligand. The shift of the carbonyl band from 1738 to
1613 cm–1 also indicated formation of the deprotonated
ligand. Elemental analysis suggested that the metal to
ligand ratio was 1 : 2. 1H NMR spectrum of compound
3 demonstrated absence of the singlet between 10 and
12 ppm confirming deprotonation of amide. Suitable
crystal of 3 for single crystal XRD was obtained by slow
evaporation in chloroform solution.According to ORTEP
diagram of complex 3 (Fig. 1), it had distorted octahedral
geometry in which Zn(II) was surrounded by two L1.
Transesterification. To a solution of an ester
(0.05 mmol) dissolved in methanol/chloroform (1 : 1)
(1 mL) the catalyst (2.0 μmol) was added, and the mixture
was stirred at 50°C (500 rpm). Reaction progress was
monitored by GC–MS of 20-μL aliquots. All reactions
were carried out not less than three times and the average
conversion was calculated.
RESULTS AND DISCUSSION
N-(Quinolin-8-yl)picolinamide (1) was prepared ac-
cording to the literature procedure [21] in the presence
Table 1. Crystal data for [Zn(L1)2] and [Zn2(L2)2]
Parameter
[Zn(L1)2]
C30H21N5O3Zn
564.89
[Zn2(L2)2]
C52H32Cl6N10O4Zn2
1204.31
Formula
Formula weight
Temperature, K
Wavelength, Å
Crystal system
Space group
100(2)
100(2)
IR spectrum of [Zn2(L2)2] exhibited the absence
of N–H band of H2L2 ligand at 3320 cm–1. Shifting of
carbonyl band from 1738 to 1631 cm–1 indicated the
ligand coordination to Zn(II). H1 NMR spectrum also
demonstrated absence of the singlet between 10 and
1.54178
1.54178
Monoclinic
P21/c
Trigonal
P32
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 12 2019

ZN(II) COMPLEXES WITH QUINOLINE SUPPORTED AMIDATE LIGANDS
Table 2. Selected bond angles in [Zn(L1)2] and [Zn2(L2)2]
2519
Table 3. Selected bond lengths in [Zn(L1)2] and [Zn2(L2)2]
structures
Bond
d, Å
Bond
d, Å
Bond angle
φ, deg
Bond angle
φ, deg
[Zn(L1)2]
[Zn(L1)2]
Zn1–N6
Zn1–N1
Zn1–N2
2.064(4)
2.198(4)
2.063(5)
Zn1–N3
Zn1–N4
Zn1–N5
2.157(5)
2.174(4)
2.197(4)
N6Zn1N1
N6Zn1N2
N6Zn1N3
98.4(2)
173.9(2)
106.6(2)
N1Zn1N5
N2Zn1N3
N2Zn1N4
91.2(2)
77.9(2)
106.7(2)
[Zn2(L2)2]
[Zn2(L2)2]
Zn1–N1
Zn1–N0L
Zn1–N0M
2.014(8)
2.480(1)
2.552(7)
Zn2–N0L
Zn2–N0M
Zn2–N5
2.549(7)
2.480(1)
2.011(8)
N1Zn1N0L
N1Zn1N0M
N1Zn1N2
74.8(3)
94.1(3)
79.6(3)
N0LZn2N0M
N0LZn2N5
N0LZn2N6
93.0(3)
94.1(3)
80.7(3)
12 ppm attributed to deprotonated amide. Elemental
analysis indicated the metal to ligand ratio 1 : 1. ORTEP
(Fig. 2) showed that the complex 4 consisted of ho-
modinuclear Zn(II) dimer along with two chloroform
molecules as a solvent of crystallization. Zn(II) was
bounded with ligands forming helical like structure.
All N-Zn-N bond angles were deviate from octahedral
geometry and formed distorted octahedral structure. The
most important structural data of both complexes are
presented in Tables 1–3.
Transesterification. Transesterification ability of var-
ious esters in the presence of zinc complexes 3 and 4 was
tested with methanol in chloroform medium (Table 4).
The best results were achieved for 4-nitrophenyl acetate
that reacted with methanol completely in the presence of
catalysts 3 and 4, respectively. This could be attributed
to the presence of the electron withdrawing group in
the substrate ester. Similar results were observed with
vinyl acetate and phenyl acetate. According to the data
presented in Table 4, electron donating groups on a sub-
strate could slow down the transesterification reaction,
for example, the methyl group prolonged the reaction
time from 5 to 10 h. Similar trends were observed with
Fluorescence. Fluorescence characteristics of com-
plexes and ligands were studied in dichloromethane
because of low solubility of these compounds in other
solvents at room temperature. The spectra demonstrated
fluorescence in the range between 350 and 800 nm
(Fig. 3). The complexes exhibited more intense fluores-
cence than their corresponding ligands. Among those
compound, complex 4 was characterized by the maximum
fluorescence at 549 nm.
Table 4. Transesterification of esters by methanol in the pres-
ence of catalysts 3 and 4a.b
Homogeneous catalyst
Entry
Substrate
Zn(L1)2
Zn2(L2)2
time, h
1
2
3
4
5
6
7
Vinyl acetate
4
3
5
4
4-Nitrophenyl acetate
Phenyl acetate
1800
5
6
4
1400
3
4-Methyl phenyl acetate
4-Chlorophenyl benzoate
Phenyl benzoate
10
7
28
8
1000
1
2
600
200
9
11
32
4-methyl phenyl benzoate
20
350
450
550
Ȝ, nm
650
750
a
All esters were completely converted into the corresponding
products.
b Reaction conditions: esters, 0.05 mmol, catalyst 2 μmol, solvent
Fig. 3. Steady-state fluorescence (SSF) spectra of the
synthesized compounds (1–4) 1–4.
CH3OH/CHCl3 (1:1) (1 mL) at 50°C.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 12 2019

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REHMAN et al.
https://doi.org/10.1039/C6TC04989C
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in literature [24, 25].
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CONCLUSIONS
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Two new complexes, [Zn(L1)2] and [Zn2(L2)2], have
been prepared by deprotonation of N-(quinolin-8-yl)-
picolinamide (HL1) and N2,N6-di(quinolin-8-yl)pyri-
dine-2,6-dicarboxamide (H2L2), respectively. FT-IR, 1H
NMR and single crystal XRD studies confirm formation
of the complexes. The crystal structures of [Zn(L1)2]and
[Zn2(L2)2] are characterized by the distorted octahedral
geometry around Zn(II). Both complexes are fluorescent
withthe higher quantum yield determined for [Zn2(L2)2].
Both complexes act as active catalysts in transestrification
reactions. [Zn(L1)2] Demonstrates the superior catalytic
activity.
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No conflict of interest was declared by the authors.
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