The structures of the synthesized Schiff base ligand and its metal complexes are given in Figs. 1 and 3. The compounds on physicochemical basis remained non-hygroscopic, unchanging at room temperature, mostly insoluble in water, ethanol, methanol fairly soluble in dimethylformamide and then equitably soluble in dimethyl-sulphoxide. Though, the analytical figures indicated 1:1 molar ratio of metal-to-ligand for the metal complexes systems and different shades of colors in expectedly good yields (80.0–83.0%) and were displayed by the compounds synthesized, which were completely different from their precursors. Elemental analyses results for the synthesized compounds in combination with the percentage metal compositions in the metal complexes revealed good agreement among the experimental and theoretical data (Table 1). The Schiff base and its metal complexes showed sharp and higher melting points with distinct deviation from that of the precursors. The Schiff base melted at > 300 oC while its metal complexes had high melting points ranged from 149 oC to > 300 oC.[4]
Table 1
Analytical and physical data of Schiff base ligand (H2L) and its metal complexes.
Compound
(Molecular Formula)
|
Colour
(%yield)
|
M.p.
(°C)
|
% Found (Calcd.)
|
Λm
Ω−1mol−1 cm2
|
µeff
(BM)
|
C
|
H
|
N
|
M
|
H2L
|
yellow
(92)
|
> 300
|
62.41
(62.67)
|
6.79
(7.18)
|
9.02
(9.14)
|
----
|
-----
|
----
|
[Cr(H2L)(H2O)Cl]Cl2.3H2O
|
black
(83)
|
> 300
|
35.23
(35.76)
|
5,34
(5.59)
|
5.00
(5.22)
|
9.47
(9.69)
|
103
|
3.72
|
[Mn(H2L)(H2O)2]Cl2.2H2O
|
black
(81)
|
204
|
37.88
(38.07)
|
5.79
(5.95)
|
5.20
(5.55)
|
10.39
(10.90)
|
104
|
4.97
|
[Fe(H2L) Cl 2]Cl2H2O
|
Dark brown
(83)
|
> 300
|
38.00
(38.03)
|
5.02
(5.15)
|
5.19
(5.55)
|
10.56
(11.09)
|
90
|
5.19
|
[Co(H2L)(H2O)2]Cl2.H2O
|
black
(82)
|
184
|
38.77
(39.15)
|
5.37
(5.71)
|
5.60
(5.71)
|
12.41
(12.03)
|
115
|
5.48
|
[Ni(H2L)(H2O) Cl]Cl.3H2O
|
black
(80)
|
> 300
|
37.73
(37.77)
|
5.42
(5.90)
|
5.24
(5.51)
|
11.28
(11.54)
|
62
|
3.43
|
[Cu(H2L)Cl2]4H2O
|
black
(83)
|
187
|
37.22
(37.43)
|
5.61
(5.85)
|
5.40
(5.55)
|
11.37
(12.38)
|
35
|
1.80
|
[Zn(H2L)Cl2]2H2O
|
black
(80)
|
149
|
40.00
(40.13)
|
5.18
(5.43)
|
5.28
(5.85)
|
19.19
(13.67)
|
38
|
Diamagnetic
|
[Cd(H2L)Cl2]H2O
|
brown
(82)
|
231
|
37.70
(37.81)
|
4.65
(4.73)
|
5.32
(5.51)
|
22.00
(22.14)
|
29
|
Diamagnetic
|
IR spectra
The IR spectra of Schiff base ligand and its metal complexes were represented in Table 2. The IR spectrum of the prepared ligand, which showed a strong band at 1600
Table 2
IR spectra of H2L ligand and its metal complexes.
Assignment
|
ν(OH)
phenolic and H2O
|
ν(C = N)
azomethine
|
V(C-O)
|
ρr(H2O) and ρw(H2O).
|
ν(M-O)
|
ν(M-N)
|
H2L
|
3424br
|
1600sh
|
1128sh
|
------
|
---------
|
-
|
[Cr(H2L)(H2O)Cl]Cl2.3H2O
|
3432br
|
1606 sh
|
1118m
|
816, 697
|
565
|
448
|
[Mn(H2L)(H2O)2]Cl2.2H2O
|
3405br
|
1634sh
|
1102s
|
873, 620
|
579
|
414
|
[Fe(H2L) Cl 2]Cl2H2O
|
3432br
|
1622m
|
1106s
|
841, 670
|
579
|
445
|
[Co(H2L)(H2O)2]Cl2.H2O
|
3430br
|
1633sh
|
1115s
|
838,666
|
578
|
431
|
[Ni(H2L)(H2O) Cl]Cl.3H2O
|
3402br
|
1623sh
|
1119s
|
837, 665
|
563
|
423
|
[Cu(H2L)Cl2]4H2O
|
3438br
|
1622m
|
1132m
|
769, 666
|
534
|
445
|
[Zn(H2L)Cl2]2H2O
|
3432br
|
1632sh
|
1115sh
|
875, 602
|
580
|
417
|
[Cd(H2L)Cl2]H2O
|
3428br
|
1630sh
|
1113sh
|
770, 611
|
520
|
416
|
sh = sharp, m = medium, br = broad, s = small, w = weak |
cm− 1, was assigned to the azomethine (C = N) group. This band was shifted to a higher frequency (1606–1634 cm− 1) in the complexes [8]. This shift indicates coordination of the azomethine nitrogen to the metal ions in the complexes. Also, OH stretching vibration band found at 3424 cm− 1 in the free ligand was appeared as broad band in the range of 3402–3438 cm− 1 in the spectra of metal complexes which assigned to ѵOH group and water molecules, ν(H2O) associated with the complexes. Coordinated water molecules exhibited ρr(H2O) rocking vibration in the range of 769–875 cm− 1 and ρw(H2O) wagging vibration near 602–697 cm− 1 [7] [8]. The etheric ѵ(C—O) stretching vibration band is observed at 1128 cm− 1 in the free ligand. By comparing the IR spectra of metal complexes with the spectrum of the free ligand, this band was found in the spectra of the complexes at 1102–1132 cm− 1 confirming the participation of etheric oxygen atoms in complex formation. However, two new bands in the range of 520–580 and 414– 448 cm− 1 were also observed (Table 2). These two bands were observed in the complexes and not found in the spectrum of the free ligand and they are attributed to M—O and M—N bonds in the complexes, respectively.[1]
Consequently, the IR spectra illustrated a tetradentate ligand that coordinates through two etheric oxygen atoms and two imino nitrogen atoms from azomethine group. The coordination sphere of the metal is completed by the water molecules and chloride atoms and its mononuclear metal complexes are given in the experimental part.
1 H NMR Spectra
The 1H NMR spectra have been recorded for Schiff base ligand, Cd(II) and Zn(II) complexes (Table 3). In the spectra of complexes, a shift of electron density from the ligand to metal had been observed. The OH protons for Schiff base ligand appeared at 16.96 ppm was found in the complexes at 16.95 and 16.94 ppm in the Zn(II) and Cd(II) complexes. Respectively. This region of the hydroxyl proton confirming coordination of oxygen to the central metal atom not occur. In the spectrum of Schiff base ligand, two singlet signals at 7.56 and 8.44 ppm were assigned to the aromatic protons. These singlet signals were slightly shifted upon coordination as showed in Table 3. Protons of Schiff base ligand appeared at δ 2.26–2.88 ppm (–CH3) and at δ 3.22–3.69 ppm (–CH2–). These signals showed a slight shift upon coordination with metal ions. In the spectra of Cd(II) and Zn(II) complexes, a singlet at 2.32–2.51 ppm and 2.31–2.98 ppm was due to (–CH3) groups and a singlet at 3.32–3.59 and 3.13–3.81 ppm was due to (–CH2) groups [9][10] [11].
Table 3
The 1H-n.m.r. chemical shifts (d; p.p.m.) of the Schiff base, and its complexes in DMSO-d6 and their assignments
Compound
|
Chemical shift (δ) ppm
|
Assignments
|
H2L
|
16.96
|
[s, 2H, 2OH]
|
7.56, 8.44
|
[D,2H,2Ar-H]
|
2.26–2.88
|
[M, 6H, 2CH3]
|
3.22–3.69
|
[M, three (–CH2–CH2– groups]
|
[Zn(H2L)Cl2]2H2O
|
16.94
|
[s, 2H, 2OH]
|
7.95–8.14
|
[M,7H,7Ar-H]
|
2.31–2.98
|
[M, 12H, 4CH3]
|
3.13–3.81
|
[M, three (–CH2–CH2– groups]
|
[Cd(H2L)Cl2]H2O
|
16.95
|
[s, 2H, 2OH] a and b
|
8.00, 8.21
|
[D,2H,2Ar-H]
|
2.32–2.51
|
[M, 6H, 2CH3]
|
3.32–3.59
|
[M, three (–CH2–CH2– groups]
|
Molar conductance
The molar conductivity (Λm) of the metal complexes was determined by applying the relation Λm = 1000 × κ/c, where κ and c stands for the specific conductance and molar concentration of metal complexes, respectively. The complexes were dissolved in DMF and their conductivity were measured at room temperature. The molar conductivity data were observed as 103, 104, 90, 115 and 62 S cm− 1mol− 1 for Cr(III), Mn(II), Fe(III), Co(II) and Ni(II), respectively, indicating their electrolytic nature and as 35, 38 and 29
S cm− 1mol− 1 for the Cu(II), Zn(II) and Cd(II) complexes, respectively, indicating their non -electrolytic nature [12].
Mass spectral study
Mass spectrometry has been successfully used to investigate molecular structure of complexes. This method is particularly useful when a poorly crystalline nature of complexes prevents their X-ray characterization. The pattern of mass spectrum (Fig. 2a–b) gives an impression of the successive degradation of the target compound with the series of peaks corresponding to the various fragments. Their intensity gives an idea of stability of fragments. The recorded molecular ion peaks of the metal complexes have been used to confirm the proposed formula.[3] Mass spectrum of Schiff base ligand and its Zn(II) complex is carried out to determine its molecular weight and fragmentation pattern as explained in Table 4 [13].
Table 4
The fragmentation data for the H2L and its Zn(II) complex
Ligand
|
Assignments
|
Peak m/z
|
calculation
|
H2L
|
M= (C16H22N2O4)
M- C3NO = M1
M1 – CNOH4 = M2
M2 – C6H3 = M3
M3 – C2H6 = M4
M4- C2H4 = M5
|
307.01
240.10
194.05
119.10
89.10
61.05
|
306.39
240.39
194.39
119.39
89.39
61.39
|
[Zn(H2L)Cl2]2H2O
|
M=(ZnC16H26N2O6Cl2)
M- Zn H4O2Cl2 = M1
M1 –C4N2O2H4 = M2
M2 –C7OH3 = M3
M3 – CH2 = M4
|
479.00
306.20
194.10
91.00
77.00
|
478.78
306.39
194.39
91.39
77.39
|
Electronic spectral and magnetic moments
The UV-Visible spectrum of Schiff base ligand in DMF appeared two absorption bands at 262 nm which is due to π→π* transition and a broad low intensity band at 547 nm which had been related to n→π* transition. The spectral properties of the metal complexes showed bathochromic shifts of ligand band. The bands assigned to intraligand π→π* for the Cr(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II) complexes were observed at 278, 265, 263, 265, 264, 263, 262 and 267 nm, respectively [14][15].
The diffused reflectance spectrum of the Cr(III) complex displayed three absorption bands at 25,641, 25,000 and 18,281 cm− 1 assignable to 4A2g → 4T2g, 4A2g→ 4T1g(f) and 4A2g →4T1g(p) transitions, respectively. The magnetic moment of the solid Cr(III) complex is 3.72 B.M., which confirms the high spin octahedral geometry around the Cr(III) ion.[16]
The diffused reflectance spectrum of the Mn(II) complex showed three bands at 33,333, 27,397 and 24,390 cm− 1 which attributed to 6A1g → 4Eg(D), 6A1g → 4T1g(P), 6A1g → 4T2g(D) and 6A1g → 4A1g(G) transitions, respectively. This data consistent with a high spin d5 octahedral system. The complex was found to have magnetic moment value of 4.97 B.M., thus, it assumed to has an octahedral geometry.[17]
The diffused reflectance spectrum of the brown Fe(III) complex exhibits two peaks at 27,777 and 29,585 cm− 1 assigned to the d–d transitions of the type 6A1g → 4T2g and 6A1g → 4T2g, respectively. The complex was found to has magnetic susceptibility value of 5.19 suggesting an octahedral symmetry around the Fe(III) ion.[16]
The diffused reflectance spectrum of Co(II) complex showed three broad peaks at 15,408, 16,420 and 18,281 cm− 1 assigned to 4T1g→4T2g (F), 4T1g→4A2g(F) and 4T1g→4T1g(P) transitions, respectively. The magnetic moment value of the Co(II) (d7) complex is found to be 5.48 B.M. The spectrum resembles those reported for octahedral complexes [15], [18].
The diffused reflectance spectrum of Ni(II) complex showed bands at 14,880 and 16,447 cm− 1 which can be assigned to 3A2g→3T1g (F) and 3A2g→3T1g (P) transitions, respectively. The magnetic moment for the Ni(II) complex (d8) was found to be 3.43 B.M, which fall within the range of octahedral Ni(II) complexes [18][15][19]
The observed magnetic moments for Cu(II) complex was 1.80 B.M and the band observed at 17241 cm− 1 (2Eg→2T2g) in the diffused reflectance spectrum suggested an octahedral geometry.[19]
Zn(II) and Cd(II) complexes are diamagnetic consistent with the (d10) configuration [15].
Geometry optimization
Figure 3 shows the fully optimized geometries of the H2L ligand plus its Cu(II) complex. Distorted octahedral geometry around the Cu(II) ion existed with the selected bond lengths and bond angles calculated for Cu(II) complex (Table 5). A small length elongation was found in bond lengths C(13) - N(23), C(14) - N(24), C(28) - O(31), C(25) - N(23) and O(38) -C39). In the Cu(II) complex, this is noted as the Schiff base ligand coordinated by two azomethine nitrogen, two oxygen, while two chloride ions took up other two positions. As previously stated, the bond angles in the Cu(II) coordination sphere were investigated and confirmed octahedral geometry [6].
Table 5
The different optimized and quantum chemical parameters of H2L and its Cu(II) complex.
Bond lengths (Å)
|
H2L
|
[Cu(H2L)Cl2]4H2O
|
C(13) - N(23)
|
1.29
|
1.64
|
C(14) - N(24)
|
1.30
|
1.64
|
C(25) - N(23)
|
1.46
|
2.09
|
N(23) - Cu(45)
|
-----
|
2.06
|
N(24) - C(42)
|
1.48
|
2.10
|
N(24) - Cu(45)
|
----
|
1.96
|
C(28) - O(31)
|
1.44
|
1.80
|
C(32) - O(31)
|
1.44
|
1.49
|
Cu(45) - O(31)
|
----
|
1.92
|
C(35) - O(38)
|
1.44
|
1.55
|
O(38) -C39)
|
1.44
|
1.84
|
Cu(45) - O(38)
|
---
|
1.76
|
Cu(45) - Cl(46)
|
---
|
2.16
|
Cu(45) - Cl(47)
|
------
|
2.16
|
Bond angles (o)
|
|
|
N(23)- Cu(45)- N(24)
|
----
|
120
|
N(23)- Cu(45)- O(31)
|
----
|
81
|
N(24)- Cu(45)- O(38)
|
----
|
85
|
O(31) - Cu(45)- Cl(46)
|
----
|
111
|
O(31) - Cu(45)- Cl(47)
|
----
|
118
|
O(38) - Cu(45)- Cl(46)
|
----
|
119
|
O(38) - Cu(45)- Cl(47)
|
|
112
|
Cl(47)- Cu(45)- Cl(46)
|
|
115
|
The calculated quantum chemical parameters
|
E (a.u.)
|
1032.969
|
-1259.304
|
Dipole moment (Debye)
|
1.3445
|
3.5586
|
EHOMO (eV)
|
-5.99
|
-6.41
|
ELUMO (eV)
|
-0.96
|
-1.66
|
Δ E (eV)
|
5.03
|
4.75
|
χ(eV)
|
-3.48
|
-4.04
|
η (eV)
|
2.52
|
2.38
|
σ (eV)−1
|
0.40
|
0.42
|
Pi (eV)
|
3.48
|
4.04
|
S (eV)−1
|
0.20
|
0.21
|
ω (eV)
|
2.40
|
3.43
|
∆Nmax
|
-1.38
|
-1.70
|
Molecular electrostatic potential (MEP)
To determine the electrical charge distribution around the molecular surface and therefore expect sites for these reactions, electrostatic potential V(r) maps, were investigated. These maps have been determined on the same basis as for optimization. 3D plots of MEPs for the ligand and its Cu(II) complex have been drawn in the existing study. On the basis of the MEP, the electron-rich field that is red on the map can be ordered in the majority (a favorite position for electrophilic attack). The neutral electrostatic potential region points outside the green colored region. The H2L is stable and has nearly uniform load density distribution. However, two oxygen and two nitrogen atoms are surrounded by a large negative charge surface, which could lead to electrophilic attacks on these sites (red) (Fig. 4) [6].
In conditions of electron density, the aromatic ring appears neutral. This means that the distribution of potential supports the complexation reaction that is further confirmed through the electrostatic potential distribution of Cu(II) complex where the metal center has a larger negative load (Fig. 4b). In addition to the increase of the electron negative capacity of Cu(II) oxygen and nitrogen as free H2L, the Mulliken electron negative is the favored location for electrical attacks with copper ion. The charge of Cu atom decreased from (+ 2 to + 0.08) due to coordination sphere process (Table 7) [6].
Molecular parameters
Following on from our work [6] before. Table 5 lists the free H2L ligand with its Cu(II) complex. The Cu(II) complex displayed high dipole values compared to the free ligand. The main orbital elements participating in chemical stability were the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO). The HOMO is an electron donation capability, LUMO is an electron accepting device, it is an electron donation capability. It can assume from the achieved data that:
The HOMO and LUMO energies were negative, and more negative than free H2L, confirming lonely complex stability. The Cu(II) component EHOMO and ELUMO values have been calculated and showed a lower Cu(II)–H2L bond strength than the free ligand. The total energy of Cu(II) is larger than the free ligand, and the consistency of the solid lonely complex is highly evident. The little energy divide can be linked to a high chemical reactivity, low kinetic stability, and the highly polarizing molecule mirror to active electronic charge transfer interaction. Azomethine nitrogens and etheric oxygens were usually located at the HOMO levels of the ligand phenol groups, which indicated the preferred places of nuclear attack against the central metal atom.H2Landits Cu(II) complex contains different optimized and quantum chemical parameters (Table 5). [4, 6].
UV-Vis spectra
In addition to appropriate excited states, the photochemistry of transition metal compounds requires information regarding the characteristics of molecular orbits. Frontier orbitals play a major role in such systems as they control the electronic stimuli and the typeset of transitions. With the help of TD-DFT estimates, the charity of the ligands and metal orbitals are likely to be commented on molecular orbitals. All electronic transitions and orbitals are not practical to analysis; thus certain constraints are applied. It has been popularized in favor of theoretical research into the electronic molecules spectrum due to its high accuracy and low computational costs of TD-DFT. The present study was carried out in the TD-DFT/B3LYP/LANL2DZ theory level of 30 single tested states for the small laying excited status of H2L with its Cu(II) complex on optimized ground status structures. Experimental and electronic theoretical spectrums existed. In Table 6, the experimental and theoretical electronic spectra were existing. The TD-DFT calculations have been estimated in the N,N-dimethylformamide solvent background and compared by the experimental data. The transitions aimed interfrontier orbitals for wavelengths matching to maximum oscillator strength of simulated results with contemporary experimental observations were presented in Figure (5a, b). For example, the electronic transitions for H2L obtained at calculated 276 nm match to experimental peak at 262 nm. This transition has been majorly contributed from HOMO to LUMO transitions which were primarily π→π* in nature. The various transitions and their experimental matching part of the free ligand and its Cu(II) complex have been summarized in Table 6 [4, 6].
Table 6. Main calculated optical transition with composite ion in terms of molecular orbitals.
Compound
|
Transition
|
Excitation energy (ev)
|
λmax Calc.
(nm)
|
λmax exp.
(nm)
|
Oscillating strength
|
H2L
|
HOMO ➔ LUMO
|
4.49
|
276
|
262
|
0.064
|
[Cu(H2L)Cl2]4H2O
|
HOMO -1 ➔ LUMO
|
4.39
|
282
|
263
|
0.013
|
Thermogravimetric analysis
TGA was carried out for solid Schiff base ligand and its metal complexes under nitrogen flow. Thermal data of the complexes were shown in Table 8. The heating rate was suitably controlled at 30°C min− 1 and the weight losses were measured from the ambient temperature up to 1000°C.
Table 8
Thermoanalytical results (TG and DTG) of Schiff base ligand (H2L) and its metal complexes.
Atom
|
H2L
|
[Cu(H2L)Cl2]4H2O
|
Mulliken charge
|
1 C
|
0.161390
|
0.084079
|
2 C
|
0.017149
|
-0.289215
|
3 C
|
0.152033
|
0.096566
|
4 C
|
0.117463
|
0.387069
|
5 C
|
0.051311
|
-0.231235
|
6 C
|
0.106932
|
0.330040
|
9 O
|
-0.258556
|
-0.148050
|
11 O
|
-0.250780
|
-0.147352
|
13 C
|
0.169619
|
0.139595
|
14 C
|
0.151273
|
0.126762
|
15 C
|
-0.000299
|
0.033911
|
19 C
|
0.009909
|
0.038200
|
23 N
|
-0.352717
|
-0.228657
|
24 N
|
-0.359838
|
-0.245915
|
25 C
|
0.135447
|
0.006493
|
28 C
|
0.257876
|
0.253436
|
31 O
|
-0.504369
|
-0.397787
|
32 C
|
0.251665
|
0.213836
|
35 C
|
0.250398
|
0.219458
|
38 O
|
-0.503025
|
-0.443001
|
39 C
|
0.260257
|
0.257745
|
42 C
|
0.136862
|
0.020067
|
45 Cu
|
|
0.088369
|
46 Cl
|
|
-0.079872
|
47 Cl
|
|
-0.084543
|
Table8. Thermoanalytical results (TG and DTG) of Schiff base ligand (H2L) and its metal complexes.
Complex
|
TG range
(°C)
|
DTGmax
(°C)
|
n*
|
Mass loss Total mass loss
Estim (Calcd) %
|
Assignment
|
Residues
|
H2L
|
30–295
295–1000
|
132, 253
380, 400
|
2
2
|
39.51(37.86)
60.42(62.14) 99.93 (100.00)
|
-Loss of C6H12O2.
-Loss of C10 H10N2O2.
|
............
|
[Cr(H2L)(H2O)Cl]Cl2.3H2O
|
30–129
129–402
402–1000
|
62
182,396
410, 430, 609
|
1
2
3
|
10.19 (10.06)
29.46 (28.78)
42.40(42.47) 82.05 (81.31)
|
-Loss of 3H2O.
-Loss of 1.5Cl2, H2O and C2H6
-Loss of C12 H16N2O2.5
|
1/2Cr2O3 + 4C
|
[Mn(H2L)(H2O)2]Cl2.2H2O
|
30–156
156–1000
|
125
210, 230
|
1
2
|
13.96 (14.27)
56.66(57.30) 70.62(71.57)
|
-Loss of 4H2O
-Loss of Cl2 and C10H22N2O3.
|
MnO + 6C
|
[Fe(H2L) Cl 2]Cl2H2O
|
30–109
109–178
178–316
316–1000
|
76
153
194
320, 435
|
1
1
1
2
|
6.76 (7.13)
7.94 (7.03)
20.39 (20.00)
47.67 (47.54) 82.76(81.70)
|
- Loss of 2H2O
-Loss of 0.5Cl2
-Loss of Cl2 and C2H6
-Loss of C13 H16N2O2.5
|
1/2Fe2O3 + 3C
|
[Co(H2L)(H2O)2]Cl2.H2O
|
30–159
159–1000
|
127
229, 425
|
1
2
|
11.61 (11.01)
67.78(68.72) 79.39(79.73)
|
-Loss of 3H2O
-Loss of Cl2 and C14H22N2O3.
|
CoO + 2C
|
[Ni(H2L)(H2O) Cl]Cl.3H2O
|
30–165
165–1000
|
66, 135
226, 506, 820
|
2
3
|
21.39 (21.15)
55.94 (54.68) 77.33(75.83)
|
-Loss of 4H2O, 0.5Cl2
-Loss of 0.5Cl2 and C15H22N2O3.
|
NiO + C
|
[Cu(H2L)Cl2]4H2O
|
30–86
86–194
194–1000
|
74
150
199, 269, 805
|
1
1
3
|
6.70(7.02)
21.45(20.86)
46.66 (47.18) 74.81 (75.06)
|
-Loss of 2H2O.
-Loss of 2H2O and Cl2
-Loss of C12H22N2O3.
|
CuO + 4C
|
[Zn(H2L)Cl2]2H2O
|
30–258
258–1000
|
95, 140,212,239
319,498
|
4
2
|
21.36 (22.37)
58.82(58.11) 80.18 (80.48)
|
-Loss of Cl2, 2H2O
-Loss of C15H22N2O3
|
ZnO + C
|
[Cd(H2L)Cl2]H2O
|
30–149
149–247
247–1000
|
67
202
300,571,760,988
|
1
1
4
|
3.60 (3.54)
7.60 (6.99)
64.11(64.10) 74.31 (74.63)
|
-Loss of H2O.
-Loss of 0.5 Cl2.
. -Loss of 0.5Cl2 and C16H22N2O3.
|
CdO
|
* n = number of decomposition steps. |
The TGA and DTG curve of Schiff base ligand indicated that it was decomposed into four main steps. Where, the 1st and 2nd steps involved the removal of C6H12O2 molecule (calculated 37.86%, experimental 39.51%) within the temperature range of 30–295 ºC. The remaining part of the ligand (C10H10N2O2 molecule) was decomposed in the temperature range of 295–1000 ºC (calculated 62.14%, experimental 60.42%) for the 3rd and 4th steps and indicated complete decomposition of the ligand.
The TGA and DTG curve of [Cr(H2L)(H2O)Cl]Cl2.3H2O complex indicated that the complex was decomposed into six main steps. The major fragmentation occurred at 30–129 ºC temperature range and involved the decomposition of three H2O molecules (calculated 10.06%, experimental 10.19%). The other molecules of the complex (1.5Cl2, H2O and C2H6 molecules) were decomposed between temperature range of 129–402 ºC (calculated 28.78%, experimental 29.46%) within the 2nd and 3rd steps of decomposition. The complex is completely decomposed in the temperature range of 402–1000 ºC (calculated 42.47%, experimental 42.40%) at last three steps and 1/2Cr2O3 contaminated by carbon atoms was left as the residue [20].
The TGA and DTG curve of [Mn(H2L)(H2O)2]Cl2.2H2O complex indicated that the complex was decomposed into three main steps. The major fragmentation occurred within the 30–156 ºC temperature range and involved the decomposition of four water molecules as a part of complex (calculated 14.27%, experimental 13.96%). The other part of the complex (Cl2 and C10H22N2O3 molecules) was decomposed between temperature range 156–1000 ºC (calculated 57.30%, experimental 56.66%) at 2nd and 3rd steps of decomposition where complete decomposition and MnO contaminated by carbon atoms was left as a residue.
The TGA and DTG curve of [Fe(H2L)Cl2]Cl.2H2O complex indicated that the complex was decomposed into 5 main steps. The major fragmentation occur at 30–109 ºC temperature range involves the decomposition of the part of complex 2H2O (calculated 7.13%, experimental 6.76 % weight). The other part of complex − 0.5Cl2- was decomposed between temperature range 109–178 ºC (calculated 7.03%, experimental 7.94 % weight) at 2nd Step of decomposition. The other part of complex - Cl2 and C2H6- was decomposed between temperature range 178–316 ºC (calculated 20.00%, experimental 20.39 % weight) at 3rd Step of decomposition The complex is completely decomposed between temperature range 316–1000 ºC (calculated 47.54 %, experimental 47.67 % weight)at last two Steps and removed as 1/2Fe2O3 contaminated by carbon atoms as a residue was left.[20]
The TGA and DTG curve of [Co(H2L)(H2O)2]Cl2.H2O complex indicated that the complex was decomposed into 3 main steps. The major fragmentation occur at 30–159 ºC temperature range involves the decomposition of the part of complex 3H2O (calculated 11.01%, experimental 11.61 % weight). The other part of complex - Cl2 and C14H22N2O3- was decomposed between temperature range 159–1000 ºC (calculated 68.72%, experimental 67.78% weight) at 2nd and 3rd Steps of completely decomposition and removed as CoO contaminated by carbon atoms as a residue was left.
The TGA and DTG curve of [Ni(H2L)(H2O) Cl]Cl.3H2O complex indicated that the complex was decomposed into 5 main steps. The major fragmentation occur at 30–165ºC temperature range involves the decomposition of the part of complex 4H2O, 0.5Cl2 (calculated 21.15%, experimental 21.39 % weight). The other part of complex 0.5Cl2 and C15H22N2O3.- was decomposed between temperature range 165–1000 ºC (calculated 54.68 %, experimental 55.94 % weight) at last three Steps of completely decomposition and removed as NiO contaminated by carbon atoms as a residue was left.
The TGA and DTG curve of [Cu(H2L)Cl2]4H2O complex indicated that the complex was decomposed into 5 main steps. The major fragmentation occur at 30–86 ºC temperature range involves the decomposition of the part of complex 2H2O (calculated 7.02%, experimental 6.70% weight). The other part of complex − 2H2O and Cl2- was decomposed between temperature range 86–194ºC (calculated (20.86%, experimental 21.45% weight) at 2nd Step of decomposition. The complex is completely decomposed between temperature range 194–1000 ºC (calculated 47.18%, experimental 46.66 % weight)at last three Steps and removed as CuO contaminated by carbon atoms as a residue was left.[20]
The TGA and DTG curve of [Zn(H2L)Cl2]2H2O complex indicated that the complex was decomposed into 6 main steps. The major fragmentation occur at 30–258ºC temperature range involves the decomposition of the part of complex 2H2O, Cl2 (calculated 22.37%, experimental 21.36 % weight). The other part of complex -C15H22N2O3.- was decomposed between temperature range 258–1000 ºC (calculated 58.11%, experimental 58.82% weight) at last two Steps of completely decomposition and removed as ZnO contaminated by carbon atoms as a residue was left.
The TGA and DTG curve of [Cd(H2L)Cl2]H2O complex indicated that the complex was decomposed into 6 main steps. The major fragmentation occur at 30–149 ºC temperature range involves the decomposition of the part of complex H2O (calculated 3.54%, experimental 3.60 % weight). The other part of complex − 0.5 Cl2.- was decomposed between temperature range 149–247 ºC (calculated 6.99%, experimental 7.60% weight) at 2nd Step of decomposition. The complex is completely decomposed between temperature range 247–1000 ºC (calculated 64.10%, experimental 64.11% weight)at last four Steps and removed as CdO as a residue was left.[20]
X-ray powder diffraction
XRD pattern of Schiff base ligand (H2L) and the Co(II) complex are shown in Fig. 6. The XRD pattern of the Co(II) complex showed well defined crystalline peaks indicating that the sample was crystalline in phase. The metal complexes showed sharp crystalline XRD patterns, which differ considerably from that of the ligand. The grain size of the metal–Schiff base complexes, dXRD was calculated using Scherre’s formula by measuring the full width at half maximum of the XRD peaks.
dXRD = 0.9λ/β(Cosθ), where ‘λ’ is the wavelength, ‘β’ is the full width at half maximum and ‘θ’ is the peak angle. The ligand and complex have the average crystallite size of 26 and 332 nm suggesting that Schiff base ligand (H2L) and the complex are nanocrystalline, respectively [21, 22].
Powder XRD pattern of Schiff base ligand (H2L) and Co(II) complex recorded in the range (2ϴ = 0–80) were shown in Fig. 6a–b. XRD patterns showed the sharp crystalline peaks indicating their crystalline phase. The average crystallite size (dXRD) of them was calculated using Scherer’s formula. The Schiff base ligand (H2L) and Co(II) complex have crystallite size of (27, 15, 37, 33 and 15) nm and (1287, 12, 13 and 16) nm, respectively [23, 24].
X-ray diffraction study of ligand showed a clear crystalline peaks with maxima at 2θ = 15, 10, 22, 24 and 26° and d = 5.68, 9.13, 4.10, 3.70 and 3.46 Å, respectively(Fig. 6a) [25].
Unlike Schiff base, the Co(II) complex had sharp peaks at 31 ° and d = 2.90 Å that might be assigned to coordination moiety [26]. X-ray diffraction study of Co(II) complex showed a clear crystalline peaks with maxima at 2θ = 9, 14, 16 and 31 ° and d = 9.62, 6.41, 5.35 and 2.9 Å, respectively (Fig. 6b). Powder XRD of all the other compounds exhibited their crystalline nature [25, 21, 27 ]
The obtained Co(II) complex shows sharp crystalline XRD patterns, which differ considerably from that of the ligand. The appearance of crystallinity in Co(II) Schiff base complex is due to the inherent crystalline nature of the metallic compound. [28]
Structural interpretation
The structures of the metal complexes of the tetradentate Schiff base ligand (H2L) were characterized through elemental analyses, molar conductivity, IR, 1HNMR, UV–Vis, mass and thermal analyses after that the suggested structures of transition metal complexes were given in Fig. 7.
Antimicrobial Studies
The mean inhibitory activities of the ligand and its complexes against the tested microbes are shown in Table 9. Broad-spectrum antibacterial activities against pathogenic microbes have been reported for Schiff base ligand (Fig. 8). Coordination between biologically active Schiff base and metal ions are important components in the design of new metal-based therapeutic agents. The synthesized compounds exhibited significant activities against the screened microbes with variable grades of inhibitory properties. The metal(II) complexes were generally more active than the ligands and in some cases had comparable activity to those of the positive control drugs. The Co(II) complex had inhibitory zones of 21–22 mm against all the tested microbes with the exception of Aspergillus flavus. Additionally, Cd(II) complex showed inhibitory effects greater than that of the ligand and other complexes with inhibitory zones of 27 mm against only Aspergillus flavus. The increased sensitivity of the complexes might be attributed to hyper conjugation of the coordinated aromatic system and enhanced liposolubility which leads to a decrease in the polarity of metal ions and raises delocalization of π-electrons over the complex ring. Permeation of the metal(II) complexes through the lipid layers of the microbial membrane was favored by the latter, thus improving antimicrobial activity. Furthermore, chelation also deactivated various cellular enzymes, essential for metabolic pathways in the microorganisms. The Cd(II), Co(II) and Zn(II) complexes displayed the best antibacterial activity of all other synthesized compounds. The compound could be an antibiotic drug research interest in the near future.[5]
Table9. Biological activity of Schiff base ligand (H2L) and its metal complexes.
The activities of the prepared Schiff base ligand and its metal complexes were confirmed via calculating the activity index according to the following relation and that showed in Fig. 9.[29–30] Activity index (A) = (Inhibition zone of compound (mm)/ Inhibition zone of standard drug (mm))×100
Molecular docking studies with DNA
To further prudence into the interaction of Schiff base ligand and its metal complexes with DNA, molecular docking studies were conducted using DNA duplex of sequence (PDB ID: 6NE0) to portend the selected binding sites together with the chosen orientation of the molecule. The minimum energy conformation (Fig. 10) of Co(II) complex with DNA shows that it is located inside the minor groove slightly deeper inside the G–U rich region. The complex was comfortably fitted in a manner to effectively formed hydrogen bonding with the DNA phosphate back bone and the closer proximity of oxygen moiety of ligand allows suitable stacking interactions (partial intercalation) amongst DNA base pairs. The minimum energy docked pose of Co(II) complex also reveals the more negative binding energy which confirms its stronger binding affinity towards the DNA molecule which is in corroboration with the findings obtained from in vitro DNA binding experiments for Pseudomonas aeruginosa. Figure 10 and Table 10 showed the molecular docked model of Schiff base ligand and its metal complexes. They showed the electrostatic and partial intercalation between them and base pairs of PDB code 6NE0 (structure of double-stranded target DNA engaged Csy complex from Pseudomonas aeruginosa (PA-14)) [31]
Table 10
Energy values obtained in docking calculations of H2L and its metal complexes with to PDB code 6NE0 Structure of double-stranded target DNA engaged Csy complex from Pseudomonas aeruginosa (PA-14)
COMPOUND
|
moiety
|
Receptor site
|
Interaction
|
Distance
(Aο)
|
E (kcal/mol)
|
Ligand H2L
|
O 38
|
OP2 U 21
|
H-donor
|
3.18
|
-2.7
|
C11 26
|
6-ring G 19
|
H-pi
|
3.92
|
-0.8
|
C2 33
|
5-ring G 19
|
H-pi
|
4.28
|
-0.6
|
[Cr(H2L)(H2O)Cl]Cl2.3H2O
|
O 38
|
OP1 C 20
|
H-donor
|
2.91
|
-4.8
|
C11 26
|
6-ring G 19
|
H-pi
|
4.34
|
-0.6
|
[Mn(H2L)(H2O)2]Cl2.2H2O
|
O 43
|
OP2 U 21
|
H-donor
|
2.91
|
-5.0
|
[Co(H2L)(H2O)2]Cl2.H2O
|
O 43
|
OP1 U 22
|
H-donor
|
2.89
|
-6.2
|
O 47
|
OP1 G 23
|
H-donor
|
2.79
|
-7.6
|
[Fe(H2L) Cl 2]Cl2H2O
|
C11 4
|
O2' G 19
|
H-donor
|
3.58
|
-0.7
|
C112 10
|
O2' G 19
|
H-donor
|
3.65
|
-0.5
|
CL 46
|
C5' C 20
|
H-acceptor
|
4.00
|
-1.8
|
CL 46
|
C3' C 20
|
H-acceptor
|
3.83
|
-2.9
|
CL 47
|
C3' C 20
|
H-acceptor
|
3.63
|
-3.7
|
[Ni(H2L)(H2O) Cl]Cl.3H2O
|
O 43
|
OP2 U 21
|
H-donor
|
2.76
|
-3.8
|
C113 29
|
5-ring DG 21
|
H-pi
|
4.59
|
-1.2
|
[Cu(H2L)Cl2]4H2O
|
O 38
|
OP1 G 23
|
H-donor
|
2.90
|
-5.3
|
O 43
|
OP1 U 22
|
H-donor
|
3.01
|
-2.1
|
CL 46
|
N4 C 20
|
H-acceptor
|
3.37
|
-4.8
|
[Zn(H2L)Cl2]2H2O
|
O 43
|
OP1 C 20
|
H-donor
|
2.95
|
-5.6
|
[Cd(H2L)Cl2]H2O
|
O 38
|
OP1 U 22
|
H-donor
|
3.08
|
-4.3
|
On other hand the interaction of Schiff base ligand and its metal complexes with PDB code 6IY0 Crystal structure of conserved hypothetical protein SAV0927 from Staphylococcus aureus subsp. aureus Mu50, The minimum energy docked pose of Zn(II) complex also reveals the more negative binding energy which confirms its stronger binding affinity towards 6IY0. Figure 11 and Table 11 showed the molecular docked model of Schiff base ligand and its metal complexes. They showed the electrostatic and partial intercalation between them and base pairs of PDB code 6IY0. By comparing the experimental with the theoretical docking calculation, we found the results to be near to each other.
Table 11
Energy values obtained in docking calculations of H2L and its metal complexes with to PDB code 6iyo
COMPOUND
|
moiety
|
Receptor site
|
Interaction
|
Distance
(Aο)
|
E (kcal/mol)
|
Ligand H2L
|
C113 7
|
O VAL 86
|
H-donor
|
3.15
|
-0.8
|
[Cr(H2L)(H2O)Cl]Cl2.3H2O
|
O 43
|
O ASN 84
|
H-donor
|
3.16
|
-2.7
|
[Mn(H2L)(H2O)2]Cl2.2H2O
|
O 46
|
OE2 GLU 75
|
H-donor
|
2.76
|
-7.1
|
O 47
|
OE1 GLN 47
|
H-donor
|
2.98
|
-5.7
|
[Co(H2L)(H2O)2]Cl2.H2O
|
C11 4
|
OD1 ASP 77
|
H-donor
|
3.13
|
-0.9
|
O 46
|
OE1 GLU 61
|
H-donor
|
2.65
|
-8.9
|
[Fe(H2L) Cl 2]Cl2H2O
|
C113 7
|
OG1 THR 41
|
H-donor
|
2.90
|
-1.8
|
CL 46
|
CA LEU 83
|
H-acceptor
|
3.42
|
-2.4
|
CL 46
|
CB VAL 86
|
H-acceptor
|
3.50
|
-1.7
|
CL 47
|
CA LEU 83
|
H-acceptor
|
3.42
|
-2.4
|
CL 47
|
CB VAL 86
|
H-acceptor
|
3.50
|
-1.7
|
[Ni(H2L)(H2O) Cl]Cl.3H2O
|
C11 4
|
OE1 GLN 47
|
H-donor
|
3.36
|
-0.7
|
C2 17
|
OD1 ASN 45
|
H-donor
|
3.22
|
-0.8
|
O 47
|
OG1 THR 48
|
H-donor
|
2.65
|
-4.3
|
O 47
|
O GLY 70
|
H-donor
|
2.73
|
-7.4
|
[Cu(H2L)Cl2]4H2O
|
O 43
|
OE2 GLU 75
|
H-donor
|
2.92
|
-7.5
|
CL 46
|
ND2 ASN 72
|
H-acceptor
|
3.69
|
-4.1
|
CL 47
|
ND2 ASN 72
|
H-acceptor
|
3.95
|
-0.6
|
[Zn(H2L)Cl2]2H2O
|
CL 46
|
NZ LYS 40
|
H-acceptor
|
3.17
|
-1.7
|
CL 47
|
NZ LYS 40
|
H-acceptor
|
3.30
|
-14.4
|
[Cd(H2L)Cl2]H2O
|
O 38
|
OD1 ASP 58
|
H-donor
|
3.31
|
-1.5
|
CL 46
|
NZ LYS 40
|
H-acceptor
|
3.42
|
-2.1
|
CL 47
|
NZ LYS 40
|
H-acceptor
|
3.60
|
-9.7
|