JJC 14, Number 2, June 2019jjc.yu.edu.jo/Issues/Vol14No2PDF/5.pdf · Volume JJC 14, Number 2, June 2019 Pages 81-87 *Corresponding Author: Taghreed M. A. Jazzazi Email: [email protected]

JJC

Volume 14, Number 2, June 2019

Pages 81-87

*Corresponding Author: Taghreed M. A. Jazzazi Email: [email protected]

Jordan Journal of Chemistry

ARTICLE

Zinc(II) Complexes of Symmetrical Tetradentate Schiff Base Ligands

Derived From 2,2'-Diamino-6,6'-dibromo-4,4'-dimethyl-1,1'-

biphenyl-salicylaldehyde: Synthesis, Characterization and

Computational Study

Taghreed M. A. Jazzazi

a*, Taher S. Ababneh

b and Eman K. Abboushi

a

a Department of Chemistry, Yarmouk University, Irbid 21163, Jordan.

b Department of Chemistry and Chemical Technology, Tafila Technical University, Tafila,

Jordan.

Received on: 6th

Aug. 2019; Accepted on: 9th

Sep. 2019

Abstract: Five new complexes (1-5) of the general formula ZnL were prepared by

refluxing the new Schiff base ligands (L1-L5) with Et2Zn in THF. The new ligands (L1-L5)

were prepared by reacting two equivalents of salicyladehyde derivatives (3,5-ditert-butyl-,

3-tert-butyl-, 5-tert-butyl-, 3,5-dinitro- and 3,5-dibromo-salicylaldehyde) with 2,2'-di-

amino-6,6'-dibromo-4,4'-dimethyl-1,1'-biphenyl. The new Schiff base ligands and their zinc

complexes were characterized by 1H-,

13C-NMR and IR spectroscopy as well as elemental

analysis. Additionally, the molecular geometries of all prepared zinc complexes were fully

optimized and examined using density functional theory (DFT) calculations at the

B3LYP/6-31G(d) level of theory. Infrared vibrational analysis was conducted, and the

results are in good agreement with the experimental data.

Keywords Schiff base ligands, Synthesis, Computational study, Zinc complexes, DFT.

Introduction

Schiff bases are compounds with a functional

group that contains a carbon-nitrogen double

bond[1]

. These ligands can coordinate and

stabilize metal ions with different oxidation

states through imine nitrogen and other groups.

Schiff base ligands and their complexes have

many interesting applications. For instance,

several of them have been used in medicinal and

pharmaceutical chemistry with significant bio-

logical activities[2-7]

. Moreover, they are used as

catalysts in various biological systems[8,9]

, in

polymers[10]

, dyes[11]

and as effective corrosion

inhibitors[12]

. They are also used in optical

computers to measure and control the intensity

of radiation in imaging systems[13,14]

. Further-

more, Schiff base metal complexes are used as

catalysts in many chemical reactions[15]

. For

example, divalent metal Schiff base complexes

of Fe(II), Ru(II) and Cu(II) have been used in the

oxidation of alcohols, cyclopropanation and base

hydrolysis of amino acid esters[16]

. In addition,

Zn(II) complexes derived from acetylacetone

and p-anisidine have displayed antimicrobial

activity[17]

. Zinc(II) complexes have also shown

promising applications in organic light-emitting

devices OLEDS[18,19]

.

In this study, five new Schiff base

tetradentate ligands (L1-L5) derived from two

equivalents of salicylaldehyde derivatives with

2,2'-diamino-6,6'-dibromo-4,4'-dimethyl-1,1'-bi-

phenyl and their complexes (1-5) have been

prepared (Figure 1).

Experimental Section

General Remarks

CHN elemental analysis was carried out on a

Perkin Elmer 240 elemental analyzer. 1H- and

13C-NMR spectra were recorded on a Bruker AC

400 spectrometer in CDCl3. Infrared spectra

were recorded using KBr on Bruker FT-IR-4100

Jazzazi et al.

28

N

N

O

OZn

Br

Br

X

X

Y

Y

(X,Y)=

(tert-Butyl, tert-Butyl)(L1) (1);

(tert-Butyl, H)(L2) (2); (H, tert-Butyl)(L3) (3);

(NO2, NO2)(L4) (4); (Br, Br)(L5) (5)

Figure 1. Structure of ligands (L1-L5) and

their zinc complexes (1-5).

spectrometer over the range 4000-400 cm-1

. All

commercially available substrates were

purchased from Sigma Aldrich or Alfa Aesar and

used without further purification. Solvents were

purified and dried according to standard

procedures followed by distillation under nitro-

gen. 2,2'-diamino-6,6'-dibromo-4,4'-dimethyl-

1,1'-biphenyl was prepared according to a

literature procedure[20]

.

General Synthesis of Schiff Bases (L1-L5)

A mixture of 0.81 mmol 2,2'-diamino-6,6'-

dibromo-4,4'-dimethyl-1,1'-biphenyl and 1.62

mmol salicylaldehyde derivatives in absolute

ethanol (10 ml) was stirred and refluxed for 3 hr.

During the reaction, the corresponding Schiff

base was precipitated as a colored solid. The

solid was collected by filtration, washed with

cold ethanol and dried under vacuum.

L1: Yield 0.45 g, 69.2%. Elemental analysis

for C44H54Br2N2O2 calculated (found): %C =

65.84 (65.98), %H = 6.78 (6.76), %N = 3.49

(3.54). MS (EI, m/z): 802 [M]+. IR/cm−1

: 2995

(w, OH), 2867 (s, t-Bu), 1613 (s, C=N).

1H NMR

(400 MHz, CDCl3, 298 K): δ 1.29 (s, 18H,

C(CH3)3), 1.38 (s, 18H, C(CH3)3), 2.42 (s, 6H,

(CH3), 7.04-7.44 (m, 8H, aromatic-H), 8.52 (s,

2H, N=CH), 12.81 (s, 2H, OH). 13

C NMR (400

MHz, CDCl3, 298 K): δ 163.6 (N=CH), 158.4,

148.5, 140.5, 140.1, 136.8, 131.9, 130.7, 128.0,

126.7, 124.5, 118.0, 117.8 (aromatic-C), 35.0,

34.1 (CMe3), 31.4, 29.2 (CMe3) and 21.0 (Me).



62.62 (62.59), %H = 5.55 (5.53), %N = 4.06

(4.01). MS (EI, m/z): 690 [M]+. IR/cm−1

: 3010

(m, OH), 2935 (w, t-Bu), 1619 (s, C=N).

1H

NMR (400 MHz, CDCl3, 298 K): δ 1.32 (s, 18H,

C(CH3)3), 2.39 (s, 6H, (CH3), 6.71-7.28 (m, 10H,

aromatic), 8.44 (s, 2H, N=CH), 12.70 (s, 2H,

OH). 13

C NMR (400 MHz, CDCl3, 298 K): δ

165.4 (N=CH), 156.8, 150.5, 144.2, 142.3,

138.5, 131.7, 129.8, 128.4, 126.2, 123.8, 118.4,

116.5 (aromatic-C), 34.8, 33.9 (CMe3), and 21.5

(Me).



62.62 (62.61), %H = 5.55 (5.52), %N = 4.06

(3.98). MS (EI, m/z): 690 [M]+. IR/cm−1

: 2958

(m, OH), 2863 (w, t-Bu), 1618 (s, C=N).

1H

NMR (400 MHz, CDCl3, 298 K): δ 1.34 (s, 18H,

C(CH3)3), 2.40 (s, 6H, (CH3), 6.69-7.31 (m, 10H,

aromatic), 8.41 (s, 2H, N=CH), 12.78 (s, 2H,

OH). 13


164.2 (N=CH), 158.6, 153.5, 146.3, 143.4,

135.9, 133.2, 128.2, 128.0, 125.8, 124.1, 117.8,

116.1 (aromatic-C), 32.7, 31.6 (CMe3), and 21.3

(Me).



44.35 (44.32), %H = 2.39 (2.36), %N = 11.08

(11.06). MS (EI, m/z): 758 [M]+. IR/cm−1

:

IR/cm−1

: 2991 (m, OH), 1620 (s, C=N), 1524 (s,

NO2), 1341 (s, NO2), 1H NMR (400 MHz,

CDCl3, 298 K): δ 2.41 (s, 6H, (CH3), 6.64-7.29

(m, 8H, aromatic), 8.57 (s, 2H, N=CH), 13.02 (s,

2H, OH). 13


163.1 (N=CH), 155.6, 152.7, 144.1, 142.4,

138.9, 131.2, 129.4, 127.6, 126.3, 125.8, 116.9,

115.7 (aromatic-C), and 21.0 (Me).



37.62 (37.55), %H = 2.03 (2.00), %N = 3.13

(3.14). MS (EI, m/z): 894 [M]+. IR/cm−1

: 2996

(w, OH), 1615 (s, C=N). 1H NMR (400 MHz,

CDCl3, 298 K): δ 2.40 (s, 6H, (CH3), 6.35-7.41

(m, 8H, aromatic), 8.36 (s, 2H, N=CH), 12.88 (s,

2H, OH). 13


162.7 (N=CH), 157.2, 157.5, 147.5, 146.1,

136.8, 133.5, 127.8, 126.5, 124.8, 124.3, 116.4,

115.0 (aromatic-C), and 21.4 (Me).

General Synthesis of Complexes 1-5

To a stirred solution of 0.62 mmol Schiff

base in 20 ml THF, Et2Zn (0.62 ml, 1.0 M

solution in hexane) was added at room

temperature under inert atmosphere. The mixture

was stirred at room temperature overnight. The

Zinc(II) Complexes of Symmetrical Tetradentate Schiff Base Ligands …

28

zinc complex was obtained during the

evaporation of the solvent under reduced

pressure.

1: Yield 0.46 g, 85.7%. Elemental analysis

for C44H52Br2N2O2Zn calculated (found): %C =

61.02 (60.89), %H = 6.05 (6.12), %N = 3.23

(3.18). MS (EI, m/z): 866 [M]+. IR/cm−1

: 2865

(s, t-Bu), 1593 (s, C=N).

1H NMR (400 MHz,

CDCl3, 298 K): δ 1.21 (s, 18H, C(CH3)3), 1.28

(s, 18H, C(CH3)3), 2.92 (s, 6H, (CH3), 6.77-7.41

(m, 8H, aromatic-H), 8.27 (s, 2H, N=CH). 13

C

NMR (400 MHz, CDCl3, 298 K): δ 169.3

(N=CH), 168.5 (CO), 147.6, 141.3, 140.6, 134.9,

130.8, 130.0, 128.9, 128.6, 125.9, 120.3, 115.9

(aromatic-C), 34.7, 32.8 (CMe3), 30.0, 28.4

(CMe3), and 19.8 (Me).



57.36 (56.95), %H = 4.81 (5.03), %N = 3.72

(4.01). MS (EI, m/z): 754 [M]+. IR/cm−1

: 2958

(w, t-Bu), 1591 (s, C=N).

1H NMR (400 MHz,

CDCl3, 298 K): δ 1.42 (s, 18H, C(CH3)3), 2.42

(s, 6H, (CH3), 6.89-7.50 (m, 10H, aromatic),

8.28 (s, 2H, N=CH). 13

C NMR (400 MHz,

CDCl3, 298 K): δ 171.4 (N=CH), 169.7 (CO),

148.2, 141.9, 140.1, 136.0, 132.3, 131.2, 130.7,

129.7, 127.6, 122.4, 117.2 (aromatic-C), 31.3,

29.6 (CMe3), and 21.3 (Me).



57.36 (57.06), %H = 4.81 (4.93), %N = 3.72

(3.91). MS (EI, m/z): 754 [M]+. IR/cm−1

: 2883

(w, t-Bu), 1588 (s, C=N).

1H NMR (400 MHz,

CDCl3, 298 K): δ 1.19 (s, 18H, C(CH3)3), 2.29

(s, 6H, (CH3), 6.72-7.34 (m, 10H, aromatic),

8.23 (s, 2H, N=CH). 13

C NMR (400 MHz,

CDCl3, 298 K): δ 169.4 (N=CH), 168.5 (CO),

147.1, 140.7, 136.5, 133.7, 130.8, 130.3, 128.9,

125.6, 122.8, 120.7, 116.0 (aromatic-C), 30.0,

24.8 (CMe3), and 19.9 (Me).



40.93 (40.32), %H = 1.96 (1.86), %N = 10.23

(10.16). MS (EI, m/z): 822 [M]+. IR/cm−1

:

IR/cm−1

: 1521 (s, NO2), 1338 (s, NO2), 1601 (s,

C=N). 1H NMR (400 MHz, CDCl3, 298 K): δ

2.33 (s, 6H, (CH3), 6.90-8.29 (m, 8H, aromatic),

8.38 (s, 2H, N=CH). 13

C NMR (400 MHz,

CDCl3, 298 K): δ 167.7 (N=CH), 166.1 (CO),

145.8, 141.7, 135.2, 132.7, 132.2, 127.2, 125.3,

120.3, 119.6, 117.9, 116.3 (aromatic-C), and

20.0 (Me).



35.13 (35.55), %H = 1.68 (1.57), %N = 2.93

(3.06). MS (EI, m/z): 957 [M]+. IR/cm−1

: 1596

(s, C=N). 1H NMR (400 MHz, CDCl3, 298 K): δ

2.30 (s, 6H, (CH3), 6.81-7.67 (m, 8H, aromatic),

8.12 (s, 2H, N=CH). 13

C NMR (400 MHz,

CDCl3, 298 K): δ 168.3 (N=CH), 164.3 (CO),

146.4, 141.3, 139.8, 136.0, 131.5, 127.8, 125.7,

123.2, 120.5, 124.3, 118.1, 116.2 (aromatic-C),

and 19.9 (Me).

Computational Method

All DFT calculations were performed using

the Wavefunction Spartan'18 Parallel Suite[21]

.

Schiff base complexes were fully optimized in

the gas phase at the B3LYP/6-31G(d) level of

theory[22-26]

without any geometry or symmetry

constraints. The absence of imaginary

frequencies in the vibrational analysis was taken

as evidence that the optimized complexes

represent stable minimal-energy geometries.

Results and Discussion

The synthesis of Schiff base 2,2'-diamino-

6,6'-dibromo-4,4'-dimethyl-1,1'-biphenyl-salicyl-

aldehyde tetradentate ligands (L1, L2, L3, L4

and L5) was carried out by the condensation

reaction of 2,2'-diamino-6,6'-dibromo-4,4'-di-

methyl-1,1'-biphenyl with two equivalents of

3,5-ditert-butyl-, 3-tert-butyl-, 5-tert-butyl-, 3,5-

dinitro-, and 3,5-dibromo-salicylaldehyde. Com-

plexes 1–5 were prepared by the reaction of

univalent of ZnEt2 with ligands L1-L5,

respectively, in dry THF solvent (Scheme 1).

The obtained complexes are stable at room

temperature.

The obtained Schiff base ligands and the

corresponding zinc complexes have been

characterized by 1H-,

13C-NMR, IR spectroscopy

as well as elemental analysis. The 1H-NMR

spectra of zinc complexes (1–5) show a shift in

the characteristic peak of (-CH=N-) proton

which appears at 8.27, 8.28, 8.23, 8.38 and 8.12

ppm, respectively, compared to 8.52, 8.44, 8.41,

8.57 and 8.36 ppm in the free ligands,

respectively. A shift in the position of the

protons in (-CH=N-) group indicates the bonding

of this group to the metal. And while the OH

proton peaks appear at 12.81, 12.70, 12.78,

13.02 and 12.88 ppm in the free ligands (L1-L5),

they are absent in the complexes indicating loss

Jazzazi et al.

28

Scheme 1. Synthesis of 2,2'-diamino-6,6'-dibromo-4,4'-dimethyl-1,1'-biphenyl-salicylaldehyde

Schiff base ligands (L1-L5) and the corresponding complexes.

of protons from the two hydroxy groups in the

free ligand and formation of new bonds between

the metal and the two oxygen atoms from the

tetradentate ligand.

These 1

H-NMR values for both free ligands

and zinc complexes are compatible with those

reported for similar compounds such as 2,2-

bis(salicylideneamino)-4,4-dimethyl-6,6-

dibromo-1,1-biphenyl with different methoxy-

substituted derivatives and their corresponding

zinc complexes [27]

.

The infrared spectra of prepared zinc

complexes show a shift in the characteristic

peaks of the imine (-C=N-), where they appear at

1593, 1591, 1588, 1601 and 1596 cm-1

in 1, 2, 3,

4 and 5, respectively, compared to the corres-

ponding bands in the free ligands that appear at

1613, 1619, 1618, 1620 and 1615 cm-1

in L1-L5,

respectively. A shift in the position of ν(C=N) is

usually an indication of bonding of this group to

the metal[28]

. The broadband seen in the free

ligands (L1-L5) at 2995, 3010, 2958, 2991 and

2996 cm-1

, respectively, is attributed to that the

OH group disappeared in the case of complexes

due to deprotonation from the OH group and

coordination bond formation between oxygen

and the metal ion.

In order to obtain a better insight into the

structural features of the prepared monoligated

tetradentate zinc complexes, DFT computational

study was performed to fully optimize the

ground-state geometries of the title compounds.

Due to similarities in coordination environment

around the Zn(II) ion in the complexes, all

optimized structures exhibit the same tetra-

hedral-based geometry around the metal ion with

only small variations in bond distances and

angles across structures. For example, calculated

distances of Zn-O1 bond in going from 1Zn to

5Zn are 1.921, 1.921, 1.918, 1.923 and 1.922 Å,

respectively, with an average length of 1.921 Å.

Similarly, variations in calculated bond angles

around the central metal are rather small. For

instance, the bond angle in O1-Zn-O2 ranges

from 112.44 to 114.07 and averages 112.89 in

all complexes. The optimized ground-state

geometries for all complexes showing the

atom‐numbering scheme around the metal ion

are depicted in Figure 2. Selected parameters of

the optimized complexes at the B3LYP/6-31G(d)

level of theory are listed in Table 1, where bond

lengths are in Å and bond angles are in degrees.

It is clear that the revealed geometries of the

coordination environments are the result of the


28

relatively large ligands (L1-L5) and their

imposed steric constraints, suggesting the

significant role played by such multidentate

ligands in determining the geometry around the

metal ion. Computed IR spectra of complexes

showed very strong imine fingerprint υ(C=N)

peaks at 1657, 1659, 1657, 1681 and 1664 cm-1

for 1Zn, 2Zn, 3Zn, 4Zn and 5Zn, respectively,

which are comparable to the experimentally

determined values. Additional absorption bands

attributed to υ(C-H) appear at 3124, 3114 and

3116 cm-1

in 1Zn, 2Zn and 3Zn, respectively,

while, two characteristic peaks assigned to NO2

modes of vibration appear at 1386 and 1640 cm-1

in 4Zn (exp. 1338 and 1521 cm-1

). In Figure. 2

the atom-numbering scheme around the metal

ion is shown (red=O, blue=N and grey=C).

1 2

3 4

5 atom-numbering

Figure 2. Perspective views of the optimized ground-state geometries for the 1Zn, 2Zn, 3Zn, 4Zn

and 5Zn complexes at the B3LYP/6-31G(d) level of theory.

Jazzazi et al.

28

Table 1. Selected calculated bond lengths (Ǻ) and angles (°) of the complexes.

Bond (Å) 1 2 3 4 5

Zn-O1 1.921 1.921 1.918 1.923 1.922

Zn-O2 1.916 1.915 1.920 1.923 1.921

Zn-N1 2.002 2.003 2.015 2.007 2.010

Zn-N2 2.006 2007 2.014 2.005 2.010

Angle () 1 2 3 4 5

Q1-Zn-O2 112.44 112.45 112.73 112.76 114.07

N1-Zn-N2 99.05 98.87 98.24 100.23 99.26

O1-Zn-N1 93.97 93.94 94.80 93.79 93.95

O2-Zn-N2 93.59 93.55 94.60 93.96 94.10

O1-Zn-N2 130.82 130.86 130.06 130.02 129.47

O2-Zn-N1 131.15 131.40 129.99 129.94 129.33

Conclusions

Five new Schiff base tetracoordinate ligands

(L1-L5) and their zinc complexes (1-5) have

been successfully prepared with different

substituents of nitro, bromo and tert-butyl on the

aromatic ring in the salicylaldehyde subunit.

Subsequently, these Schiff base ligands were

reacted with diethyl zinc to produce the

corresponding zinc complexes (1-5). The title

ligands and their zinc complexes were fully

characterized by 1H-,

13C- NMR and IR-

spectroscopy, as well as elemental analysis. The

optimized ground-state geometries and IR

spectral data for the complexes were reported

using DFT calculations at the B3LYP/6–31G(d)

level of theory and the obtained results were in

good agreement with the experimental data.

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Documents

JJC 14, Number 2, June 2019jjc.yu.edu.jo/Issues/Vol14No2PDF/5.pdf · Volume JJC 14, Number 2, June 2019 Pages 81-87 *Corresponding Author: Taghreed M. A. Jazzazi Email: [email protected]