New heteroleptic 3D metal complexes: Synthesis, antimicrobial and solubilization parameters

Article abstract of DOI:10.3390/molecules25184252

The microbial resistance to current antibiotics is increasing day by day, which in turn accelerating the development of new effective drugs. Several studies have proved the high antimicrobial potential of the interaction of several organic ligands with a variety of metal ions. In the present study, a conventional method has been adopted in the synthesis of twelve new heteroleptic complexes of cobalt (II), nickel (II), copper (II) and zinc (II) using three aldimines, namely, (HL₁ ((E)-2-((4-chloro-2-hydroxybenzylidene)amino)-3,4-dimethyl-5-phenylcyclopent-2-en-1-one), HL₂ ((Z)-3-((4-chlorobenzylidene)amino)-4-hydroxy-5-nitrobenzenesulfonic acid) HL₃ (2,2⁰-((1,2-phenylenebis(azaneylylidene))bis(methaneylylidene))diphenol)) as primary ligands, while phenyl glycine was the secondary ligand. The synthesized compounds were characterized by UV-vis, IR and multinuclear (¹H and ¹³C) NMR spectroscopy, elemental analysis, and electrical conductance. The IR study revealed the coordination of the aldimine derivatives with the -OH and N atom of imine moiety. In contrary to this, the phenyl glycine coordinated to the metal ions via oxygen of carboxylate and nitrogen of the amino group. The spectroscopic analysis unveiled the tetrahedral geometry of the synthesized metal (II) complexes, except for ligand HL₃ which exhibited octahedral geometry. The synthesized compounds generally showed antibacterial activity for all microbes, except Ni (II) complexes lacking sensitivity. Furthermore, to access the bioavailability, the synthesized complexes were screened for their solubilization in the micellar media of sodium lauryl sulphate. The metal complex–surfactant interaction was revealed by UV-vis spectroscopy and electrical conductivity measurements.

Full text of DOI:10.3390/molecules25184252

molecules
Article
New Heteroleptic 3D Metal Complexes: Synthesis,
Antimicrobial and Solubilization Parameters
Muhammad Babar Taj 1
,2, ,†
*
, Muneera D. F. Alkahtani * , Uzma Ali , Ahmad Raheel ,
3,
4,†
4
5
5
5
6,
Walla Alelwani , Afnan M. Alnajeebi
, Nouf Abubakr Babteen , Sadia Noor * and
Heba Alshater 7
1
Department of Chemistry, Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan
Department of Chemistry, University of Sahiwal, Sahiwal 57000, Pakistan
Department of Biology, College of Science, Princess Nourah Bint Abdulrahman University, Riyadh 11675,
Saudi Arabia
Department of Chemistry, Quaid-e-Azam University, Islamabad 44000, Pakistan;
uzmaali@hotmail.com (U.A.); ahmadraheel001@gmail.com (A.R.)
Department of Biochemistry, Faculty of Science, University of Jeddah, Jeddah 80203, Saudi Arabia;
welwani@uj.edu.sa (W.A.); Amalnajeebi@uj.edu.sa (A.M.A.); Nababteen@uj.edu.sa (N.A.B.)
Department of Chemistry, University of Agriculture, Faisalabad 38040, Pakistan
Department of Forensic Medicine and Clinical Toxicology, Menoufia University, Shbien El-Kom 32511, Egypt;
hebaalshater@yahoo.com
2
3
4
5
6
7
*
Correspondence: dr.taj@iub.edu.pk or drbabartaj@gmail.com (M.B.T.); mdf.alkahtani@gmail.com (M.D.F.A.);
sadiaa613@gmail.com (S.N.); Tel.: +92-300-7542669 (M.B.T.); +92-333-6531365 (S.N.)
These authors contributed equally to this work.
†
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Peter J. Rutledge
Received: 7 August 2020; Accepted: 11 September 2020; Published: 16 September 2020
ꢀ
ꢁꢂꢃꢄꢅꢆ
Abstract: The microbial resistance to current antibiotics is increasing day by day, which in turn
accelerating the development of new effective drugs. Several studies have proved the high
antimicrobial potential of the interaction of several organic ligands with a variety of metal ions.
In the present study, a conventional method has been adopted in the synthesis of twelve new
heteroleptic complexes of cobalt (II), nickel (II), copper (II) and zinc (II) using three aldimines, namely,
(
HL ((E)-2-((4-chloro-2-hydroxybenzylidene)amino)-3,4-dimethyl-5-phenylcyclopent-2-en-1-one),
1
0
HL ((Z)-3-((4-chlorobenzylidene)amino)-4-hydroxy-5-nitrobenzenesulfonic acid) HL (2,2 -((1,2-
2
3
phenylenebis(azaneylylidene))bis(methaneylylidene))diphenol)) as primary ligands, while phenyl
glycine was the secondary ligand. The synthesized compounds were characterized by UV-vis,
1
13
IR and multinuclear ( H and C) NMR spectroscopy, elemental analysis, and electrical conductance.
The IR study revealed the coordination of the aldimine derivatives with the -OH and N atom of
imine moiety. In contrary to this, the phenyl glycine coordinated to the metal ions via oxygen of
carboxylate and nitrogen of the amino group. The spectroscopic analysis unveiled the tetrahedral
geometry of the synthesized metal (II) complexes, except for ligand HL which exhibited octahedral
3
geometry. The synthesized compounds generally showed antibacterial activity for all microbes,
except Ni (II) complexes lacking sensitivity. Furthermore, to access the bioavailability, the synthesized
complexes were screened for their solubilization in the micellar media of sodium lauryl sulphate.
The metal complex–surfactant interaction was revealed by UV-vis spectroscopy and electrical
conductivity measurements.
Keywords: mixed chelation; electronic spectroscopy; stability constants; antimicrobial;
micellar solubilization
Molecules 2020, 25, 4252; doi:10.3390/molecules25184252
www.mdpi.com/journal/molecules

Molecules 2020, 25, 4252
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1
. Introduction
Biomedical specialists aim to develop more potent and aggressive pharmacological alternatives
due to the significant increase in drug-resistant microbial infections [ ]. Researchers have embraced the
potentials of natural products as effective pharmacological substitutes to combat microbial infections [ ].
1
2
The role of transition metal complexes formation is considerably acknowledged in biological
systems due to their ability to construct mix ligand complexes and their role in complicated equilibrium
processes, especially enzyme–substrate interactions [
through one or more coordination sites of ligands having different functional groups, and they play a
significant role in detoxification and remediation of metal pollutants [ ]. The complexation behavior
and stability of many ligands with divalent transition metal ions have been widely investigated [ ].
Copper is an essential component of numerous endogenous antioxidant enzymes as well as oxidative
processes [ ]. The role of cobalt in biochemical processes is extensively studied and is the component
of many coenzymes and vitamin B12 ]. Nickel is an essential metal that performs numerous roles
in biochemistry of microorganisms, animals and plants [ ]. Zinc is important for its catalytic role or
exclusively structural role in maintaining the configuration of protein for most of its contribution [ ].
3]. The chelates are formed in biological fluids
4
5
6
[7
8
9
Zinc ions can adapt to a variety of binding arrangements and different combinations of ligands,
subsequently depicting a broad range of reactivity and functions [10].
In coordination chemistry, nitrogen-donor ligands, with strong redox stability and quite easy
access to functionalization, have been extensively explored for the formation of heteroleptic complexes,
that is, metal species which have more than one type of ligand [11
including cytotoxicity, genotoxicity, antineoplastic, antitumor and bactericidal attributes have been
reported for the heteroleptic chelates encompassing the bidentate ligands [ 13]. The chelation in metal
,12]. The biological potentials
2,
complexes plays an important role in improving their lipophilicity in comparison to their precursor
ligands [14]. The involvement of heteroatoms of ligands during chelation with metallic ions augmented
the biological potentials of biologically active compounds [15]. The metal complexes with aldimine
ligands are becoming key medicinal compounds having a broad range of biological activities [16].
It is well known that amino acids play an important role in the transportation of metal in the blood
through the tissues. Many copper complexes with amino acids have been reported as anti-tumor
and anti-inflammatory drugs. Pt (II) complexes with an L-alanyl-based ligand have been reported
as promising antiproliferative drugs [17]. The antioxidant, antibacterial, antifungal and antitumor
activities of metal complexes have been studied very extensively [18].
Several metal complexes show limited therapeutic efficiency due to the low solubility in aqueous
media, and the surfactants play a vital role in enhancing solubility of drugs and other additives.
Surface active agents possess a polar head and a non-polar tail end, which allow them to
self-aggregate in the form of colloidal size particles called micelles. These micelles mimic the
mechanism of interfacial adsorption of additives on their surface. The critical micellar concentration
(CMC) is the concentration point where the aggregates of surfactants start to form, and it is affected by
changes in concentration, temperature, solvents, electrolytic additives, nature of counter ion, pH etc.
The extreme biological importance of mixed ligand complexes, especially those containing amino
acids as ligands, motivated us to synthesize new heteroleptic 3D metal complexes. To the best of our
knowledge, this is the first study on the synthesis, antimicrobial activity and solubilization factors of
3
2
2
D heteroleptic complexes of aldimine and glycine.
. Results and Discussion
.1. Synthesis of the Heteroleptic Complexes
Three aldimine ligands were prepared by reacting separately three aromatic aldehydes with
three different amines under reflux for 30 min, as reported in Scheme 1. From these reactions,
three precipitates of the aldimines were obtained after cooling by filtration and used directly without
further purification. The subsequent metal complexation was simply obtained by mixing the aldimine

Molecules 2020, 25, 4252
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ligands (HL1–3) with equimolar amounts of phenyl glycine and the chlorides of Co (II), Ni (II), Cu (II)
and Zn (II) (Scheme 1).
Scheme 1. Synthesis of aldimine ligands (HL –HL ) and metal complexes.
1
3
2
.2. Characterization
All the synthesized heteroleptic complexes are colored amorphous, non-hygroscopic and air-stable
solids which were soluble in DMF (dimethylformamide), DMSO (dimethyl sulfoxide) and partially
soluble in methanol, ethanol, and chloroform. All the synthesized complexes were characterized based
on the determination of melting points, molar conductance and spectrochemical analysis. The electrical
−3
molar conductance of the synthesized complexes was measured in DMSO (10 M) at ambient
temperature. The low or high values of molar conductance obtained for the synthesized heteroleptic
complexes of divalent metal ions Co (II), Ni (II), Cu (II) and Zn (II) indicated their nonelectrolyte or
electrolytic behavior [19] (Scheme 1). The selected NMR and FTIR spectra of ligands and complexes
are reported in Figures S1–S16.
2
.3. Infrared Spectroscopic Analysis
The FTIR spectra (4000–400 cm 1) were recorded for the free aldimines,
α-phenyl glycine, and their
−
corresponding metal complexes. The FTIR spectrum of free phenyl glycine ligand has prominent
−
−
peaks at 2975, 3167 for
υ
(-NH ) sym./ asym., 1355 for
υ
(COO ) asym., and 1576 for
υ
(COO ) sym.
2
−
When the amino acid chelated the metal, significant changes were observed in
υ
(NH ) and
υ
(COO )
2
IR regions. It is important to point out here that free amino acids occur as zwitterions, and the IR
spectra of these cannot be matched entirely with those of metal complexes, as amino acids in metal

Molecules 2020, 25, 4252
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complexes do not occur as zwitterions. The amino acid showed symmetric and asymmetric vibrations
−
due to
υ
(COO ), which shifted and changed in intensities in the complexes. Similarly, the symmetric
and asymmetric band in -NH group also migrated to higher frequencies after chelating the metal
2
ion with aldimine and the
α
-phenyl glycine ligand. Further evidence regarding the involvement of
−
both the COO and NH groups in the complexes was assessed by the appearance of some weak IR
2
−
1
−1
(M-N) and 600–700 cm [21] due to
bands in the far region at 500–600 cm
[
20] due to
υ
υ (M-O),
respectively, being absent in the free ligands.
2
.4. IR Analysis of HL1
The characteristic band for carbonyl group (C=O) appeared at 1647 cm and for aldimine group
−
1
−
1
(–CH=N) appeared at 1591 cm in the spectrum of the free aldimine ligand, which was found at
−1
lower frequencies after metal complexation (~10–30 cm ). This shift indicated the involvement of
the nitrogen atom of the aldimine group in the coordination. The O-H stretching band of –COOH
was absent in the IR spectra of all metal complexes after deprotonation. Some new bands with weak
intensities at 646–614 (M-O) and 597–459 cm (M-N) were also seen in the complexes. The less
covalent character in the M-O bond relative to M-N bond caused the appearance of metal–oxygen
stretching bands to bond at high-frequency regions.
−1
2
.5. IR Analysis of HL2
In the case of ligand 2, the -OH stretching vibration was spotted at 3379 cm 1 which vanished
after complex formation, indicating its contribution towards M-O bond formation. The aldimine
−
−1
characteristic –C=N stretching vibration of the free ligand was witnessed at 1625 cm which shifted
to 1651, 1595 and 1622 cm in complexes, indicating the involvement M-O co-ordinate covalent
bond. The symmetric and asymmetric vibrations due to –COO were observed at 1294–1377 and
−1
−
−1
−1
1562–1585 cm correspondingly. The bands at 597–644 and 441–486 cm were designated to the
characteristic M-O and M-N bonds in the complexes.
2
.6. IR Analysis of HL3
The IR analysis of ligand HL demonstrated the characteristic band due to the aldimine
3
−
1
functional group (–CH=N–) at 1609 cm , which then shifted to higher wavenumbers in complexes.
−1
The
complexes. The characteristic bands due to secondary ligand (DL)-
observed. The asymmetric vibration bands due to carboxylic group of the amino acid at 1576 cm
in the free ligand were shifted to a lower frequency (~50 cm ) in the spectra of the complexes.
Similarly, the symmetric and asymmetric bands of the
The evidence regarding contributions of aldimine and carboxylate after complexation was validated
υ
(O-H) band of free ligand was present at 3453 cm , and it disappeared in the spectra of
α
-phenyl glycine were also
−1
−
1
−
υ (–NH ) were observed at 3167–2975 cm .
2
1
−
1
by weak bands at 500–600 and 600–700 cm due to M-N and M-O bonds of complexes.
2
.7. Electronic Spectral Studies
The electronic spectra of synthesized complexes and ligands (10 3 M) were recorded after
−
dissolving them in DMSO, and the results are presented in Table 1.
The free ligand HL exhibited strong absorption at 347 nm due to the n→π* transition of imine
1
moiety. On complexation, the band shifted to higher wavelength regions and possibly due to the
coordination of nitrogen with metals.
The free ligand HL confirmed the presence of a prominent and intense band at 358 nm and the
2
low-intensity band at 441 nm due to the n→π* transition of aldimine and the lone pair electrons of its
nitrogen atom. In cobalt complexes, two bands of same intensity were observed at 456 nm and 476 nm,
4
4
4
4
4
4
7
and another at 690 nm due to A (F)
→
T (P), A (F)
→
T (F) and A (F)
→
T2(F) and transitions for d
2
1
2
1
2
tetrahedral geometry of the complex. The nickel complex shows three bands at 675 nm, 507 nm, 406 nm
3
3
3
3
3
3
2+
8
assigned to T g(F)
→
T g(F), T g(F)
→
A (F) and T (F)
→
T1g(P) for Ni d tetrahedral geometry
1
2
1
2g
1g

Molecules 2020, 25, 4252
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of the complex. Similarly, in the copper complex, three bands, one of which at 680 nm was due to
2
2
2+
9
T2g
→
g
E transition of Cu d tetrahedral geometry of the complex, were generally found in square
planar geometry when tetracoordinated, while other bands were due to the ligand-centered n→π* and
charge transfer transitions. The zinc complex did not show such a d-d transition due to its complete
10
d
configuration, while a band 445 nm was observed due to the n→π* transition of aldimine moiety.
Ligand HL showed two major peaks due to n→ π*, π→π* of aldimine functionality and benzene
3
π
-bonding electrons. The spectra of complexes indicated some prominent peaks for charge transfer as
well as d-d transitions. The brown-colored cobalt complex showed three bands at 332 nm, 419 nm and
4
4
4
4
4
4
7
32 nm assigned to T (F)
→
T (F), T (F)
→
A (F) and T (F)
→
T1g(P) transitions, suggesting
1g
2g
1g
2g
1g
7
d octahedral geometry of the complex. The maroon-colored nickel complex showed three bands
at 377 nm 475 nm and 640 nm assigned to A (F)
transitions, suggesting d octahedral geometry of the complex. The blue-colored copper complex
3
3
3
3
3
→
3T (P), A (F)
→
T (F) and A
→
T (F)
2g
2
g
1g
2g
1g
2g
8
2
2
9
showed charge transfer bands and one d-d band at 657 nm assigned to Eg→ T2g transition due to d
2
octahedral geometry of the complex with the D ground atomic term. In the case of zinc complexes,
filled d10 configuration caused no d-d transitions; however, two peaks at 293 nm and 402 nm were
spotted indicating intra-ligand and metal-to-ligand charge transfer. The selected electronic spectra of
ligands and complexes are given in Figure S17.
Table 1. Electronic spectral data of ligands (HL1–3) and heteroleptic complexes (ML1–3Gly).
Compound
Electronic Absorption Peaks (nm)
Type of Transition
HL1
347
724, 349.2, 262
734, 348.2, 258
734, 348.5, 260
349
n→π*, π→π*
CoL Gly
d-d, CT
1
NiL Gly
d-d, π→π*, n→π
d-d, π→π*
1
CuL Gly
1
ZnL Gly
Intra-ligand transition
n→π*
1
HL2
441, 358
CoL Gly
690, 476, 456
675, 507, 406, 303, 262
680, 532, 441, 426
445
d-d, CT (charge transfer)
d-d, CT, n→π*, π→π*
d-d, CT
2
NiL Gly
2
CuL Gly
2
ZnL Gly
Intra-ligand transition
n→π*, π→π*
2
HL3
334, 276
CoL Gly
732, 419, 332, 256, 246
640, 475, 377, 308, 246, 207
657, 439
d-d, CT, n→π*, π→π*
d-d, CT, n→π*, π→π*
d-d, Intra-ligand, n→π*
Intra-ligand transition
3
NiL Gly
3
CuL Gly
3
ZnL Gly
402, 293
3
2
.8. Stability Constants of Heteroleptic Complexes
The spectrophotometric method was carried out for the determination of the stability constants of
metal complexes of aldimine ligands. There was a 1:2 ratio (metal:ligand) in all complexes. To estimate
the stability constant, the degree of dissociation of complex ( ) was obtained from Job’s plot for the
α
given ligands and the metal ion in solutions. By substituting the value of (
α
) in the equation mentioned
below, the value of the stability constant (Kst) was obtained.
Kst = (1−α)/m × n × (α)m + n(C)
m
n
m + n − 1
(1)
The calculations were performed for the heteroleptic complexes of Co (II), Ni (II), Cu (II) and
Zn (II) with the three aldimine ligands (HL , HL , HL ). The zinc complex of ligand HL had the
1
2
3
3
lowest Kst value among all the synthesized complexes. Exceptionally, Cu (II) complexes of all three
ligands appeared to have lower Kst values as compared to the other ones, although the stability of the
complexes increased as the size decreased. It is well known that a complex with a stability constant
with a smaller value is less stable than those with a higher value.

Molecules 2020, 25, 4252
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The results followed Irving and Williams’s order of stability constants for transition metal ions
and are summarized in Table 2.
Table 2. Stability Constants of heteroleptic complexes (ML1–3Gly).
Compound
Kst (kJ/mol)
5.47 × 101
Compound
Kst (kJ/mol)
Compound
Kst (kJ/mol)
2
12
13
CoL Gly
CoL Gly
1.65 × 10
CoL Gly
3.34 × 10
1
2
3
11
12
12
NiL Gly
1.78 × 10
NiL Gly
1.75 × 10
NiL Gly
4.58 × 10
1
2
3
9
7
5
CuL Gly
1.9 × 10
CuL Gly
7.5 × 10
CuL Gly
6.25 × 10
1
2
3
2.95 × 1010
8.21 × 10
8
1.22 × 10
5
ZnL Gly
ZnL Gly
ZnL Gly
1
2
3
2
.9. Applied Studies on Synthesized Complexes
2
.9.1. Antibacterial Activity
The results of the antibacterial analysis of the ligands HL1–3, glycine and their corresponding
heteroleptic complexes with Co, Ni, Cu and Zn are summarized in Table 3. The synthesized compounds
generally showed antibacterial activity for all microbes, except Ni (II) complexes lacking sensitivity
against Klebsiella oxytoca and Pseudomonas aureginosa. As expected, the complexes were significantly
more sensitive than glycine and their corresponding ligands HL1–3. All compounds exhibited
exceptional potential against E. coli and P. mirabilis, with inhibition zones in the ranges of 13.2–29.7 and
1
6.0–30.0 mm. Furthermore, NiL Gly, CoL Gly, NiL Gly demonstrated much-enhanced susceptibility
1 3 3
as compared to ligands and glycine against B. cereus. These improved antibacterial activities of
complexes may be attributed to the involvement of hyper-conjugation and induction of greater lipo
solubility [22]. The polarity of metal ions diminished and caused delocalization of
in turn, favored the penetration of complexes through the bacterial membrane, subsequently enhancing
their activity [23 24]. In addition to this, cellular enzymes were neutralized by chelation, which plays a
crucial role during metabolic activities of bacteria [24]. The cobalt complexes, particularly CoL Gly
π-electrons, which,
,
1
(Figure 1), among all other counterparts, exhibited significant potential as antibacterial agents in
comparison to the standard drug, making them suitable candidates for antibiotic drug research [25,26].
Table 3. Antibacterial data of HL1–3, Gly and their heteroleptic complexes at 1 mg/mL concentration.
Minimum Zone of Inhibition (mm)
Compound
Bacillus
cereus
Escherichia Klebsiella
Pseudomonas
aeruginosa
Staphylococcus Proteus
coli
oxytoca
aureus
mirabilis
HL1
HL2
HL3
Gly
24.5
22.3
24.1
14.9
31.0
29.0
21.0
27.7
28.0
26.9
19.2
26.1
32.2
30.3
23.1
27.4
33.0
R
17.3
16.2
15.5
13.2
19.5
21.0
28.5
23.0
18.0
20.3
24.1
22.4
21.6
23.6
29.7
24.4
32.0
R
27.3
24.9
23.6
23.0
24.4
R
21.5
23. 6
21.9
R
22.9
22.2
24.4
R
23.1
25.1
36.0
R
17.3
16.5
18.4
11.5
18.6
R
23.1
30.3
17.3
R
24.0
28.8
18.6
R
25.0
32.1
26.0
R
13.9
14.6
16.1
13.7
20.1
23.0
23.0
14.1
19.1
21.5
23.0
17.1
20.1
25.4
26.2
16.4
29.0
R
16.8
19.4
20.3
17.0
26.0
18.5
27.3
18.3
21.0
27.6
26.0
16.0
28.1
30.0
29.2
18.3
23.0
R
CoL Gly
1
NiL Gly
1
CuL Gly
1
ZnL Gly
1
CoL Gly
2
NiL Gly
2
CuL Gly
2
ZnL Gly
2
CoL Gly
3
NiL Gly
3
CuL Gly
3
ZnL Gly
3
+
Ciprofloxacin
–
(CH ) SO
3 2
R: No bacterial growth inhibition was observed; +: standard drug.

Molecules 2020, 25, 4252
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Figure 1. Antibacterial activity of CoL Gly against different bacterial strains.
1
2
.9.2. Antifungal Activity
The outcomes of antifungal activities of the ligands HL1–3, glycine and their corresponding
heteroleptic metal complexes are summarized in Table 4. The activity against A. niger, A. flevus,
and R. stolonifer specified good to moderate inhibitory growth zones. The ligands inhibited the fungal
strains and resulted in inhibitory zones of 14.3–29.4 mm in diameter. However, CoL1–3Gly complexes
exhibited comparable activities to precursor ligands and other complexes against all three fungal
strains, and cobalt ions were adsorbed to a greater extent onto the surface of fungal cell walls.
Table 4. Antifungal data of HL, Gly and Gly’s heteroleptic complexes at 1 mg/mL concentration.
Minimum Zone of Inhibition (mm)
Compound
A. niger
A. flevus
R. stolonifer
HL1
HL2
HL3
Gly
22.7
23.2
24.7
16.0
25.6
14.5
22.2
17.3
24.6
14.4
24.2
13.0
25.1
16.5
23.9
18.0
36.0
–
28.7
29.1
27.6
19.8
24.1
24.2
24.1
25.3
29.3
23.1
24.5
26.0
27.1
24.1
27.5
28.1
29.0
–
26.3
29.4
28.6
13.6
26.5
–
24.7
–
27.9
–
26.1
–
28.6
–
25.5
–
39.3
–
CoL Gly
1
NiL Gly
1
CuL Gly
1
ZnL Gly
1
CoL Gly
2
NiL Gly
2
CuL Gly
2
ZnL Gly
2
CoL Gly
3
NiL Gly
3
CuL Gly
3
ZnL Gly
3
+
Fluconazole
–
(CH ) SO
3 2
+:
standard drug.
2
.10. Interaction of Metal Complexes with Anionic Surfactant
The interaction of metal complexes with anionic surfactant was assessed by electrical conductivity
and UV-visible spectroscopic analysis. In the first step, stock solutions of SLS (sodium lauryl sulfate) was
prepared in water, and, then, conductivity measurements were performed with variable concentrations
of surfactant at 298 K. These conductivity data were then plotted against SLS concentrations, and the
value of CMC was determined from the point of inflection (Figure 1). The accurate value of CMC
was assured by plotting the differential conductance against differential SLS concentration, and a
reverse curve gave a more accurate value of CMC. There was a significant change in conductance
in the pre-micellar region due to the presence of free anions and cations of surfactant but not in the
post-micellar region, possibly due to micelle formation. CMC values for SLS were enhanced in all cases.

Molecules 2020, 25, 4252
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The values of CMC determined by conductivity data were crosschecked with UV-visible
measurements. Initially, the λmax of each complex was determined, and an absorbance was plotted
against the concentration surfactant, as represented in Figure 2. Both techniques showed close
agreement in terms of CMC values.
Figure 2. Plots of conductivity and differential conductivity vs. (sodium lauryl sulfate) SLS concentrations.
2
.11. Electrical Conductivity Measurements
The micellar parameters of selected complexes and their CMC values were explored from simple
and differential conductivity data. On gradual dilution, a noticeable variation in conductivity was
observed. The value of critical micelle concentration (CMC) was determined from plots of electrical
conductivity (EC) against SLS concentrations (CS) (Figure 2) [27,28].
A significant interaction was observed in the complexes of cobalt with all ligands and nickel with
HL in the micellar media of SLS, and their micellar parameters are presented in Table 5. The negative
3
m
values of ∆G indicated exothermicity of micellization as well as stability and spontaneity of this
system. CoL Gly/SLS was the most stable system, and its micellization was spontaneous as compared
3
to the other complexes.
Table 5. Calculated micellar parameter for the interaction of complexes with SLS.
System
CMC (mM)
∆Gm (kJ/mol)
β
SLS
8.3
10.52
9
9.56
10.83
−38.39
−33.79
−28.11
−27.11
−31.072
0.243
0.426
0.7
0.737
0.532
CoL Gly/SLS
1
CoL Gly/SLS
2
CoL Gly/SLS
3
NiL Gly/SLS
3

Molecules 2020, 25, 4252
9 of 17
2
.12. UV-Visible Spectroscopic Measurements
Interaction of complexes with SLS micelles was assessed from simple and differential UV-visible
spectroscopic data. The maximum absorbance (λmax) of the complexes in the absence and presence of
various SLS concentrations was measured, and the extent of interaction was determined, as presented
in Figure 3, whereas the partitioning and binding parameters are reported in Table 6.
Figure 3. Plots of simple and differential absorbance vs. SLS concentrations.
Table 6. Partitioning and binding parameters of selected complexes in micellar media.
3
3
Complex
Kx (dm /mol)
∆Gp (kJ/mol)
K (dm /mol)
∆G (kJ/mol)
b
b
CoL Gly
137
27,400
50,000
956
−12.19
−25.31
−26.8
−17
190
325
58.5
80
−13
1
CoL Gly
−14.25
−10.08
−10.85
2
CoL Gly
3
NiL Gly
3
The location of metal complexes in the surfactant depended on the hydrophobic and electrostatic
interactions, and the results confirmed the partitioning in the proximal regions of the micelles.
The molecules of metal complexes interacted with micelles and absorbance increase with increasing
concentration of the surfactant. A bathochromic shift in the maximum absorbance was mainly due to
significant interactions between complexes and surfactant molecules [27–30].
The SLS structure breaking effect caused an increment in the CMC of surfactant in the presence
of the complexes [27]. The penetration of complex molecules into the micelles increased the values
of CMC in comparison to that of pure SLS. The involvement of intermolecular hydrogen bonding
between water and metal complexes caused adsorption in the periphery of micelles [27]. The molecules

Molecules 2020, 25, 4252
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of CoL Gly resided near the outer region of the micelles, and those of CoL Gly were positioned near
3
1
the inner core region of micelles, as depicted in Figure 4.
Figure 4. Proposed locus of metal complexes in the micelles of SLS.
. Materials and Methods
3
3
.1. Chemicals and Instruments
All analytical grade chemicals were purchased from Sigma Aldrich and used as such during this
investigation, and all the solvents were used after distillation and purification. Spectrophotometric
measurements were taken on a Shimadzu UV-1700 Double Beam Scanning UV-Vis Spectrophotometer
with 1.00 cm cell and a pH-meter (Sartorius Professional Meter PP-15, Duisburg Germany).
Infrared spectra were recorded on Perkin Elmer FTIR spectrophotometer (Perkin-Elmer, USA).
1
13
The H-NMR (300 MHz) and C-NMR (75.43 MHz) spectra were recorded on Bruker AM-250
spectrometer (Bruker, Billerica, MA, USA) in CDCl and DMSO solution using TMS as the internal
3
standard. Elemental analysis was performed by PerkinElmer 7300 DV and Leco (PerkinElmer, Inc.,

Molecules 2020, 25, 4252
11 of 17
Shelton, CT, USA); CHN-932 elemental analyzer (Leco CHNS 932, Calgary, Canada) and the electronic
sensitive balance (Shimadzu, Kyoto, Japan) were used during the study to find the stoichiometry and
formation constant.
3
.2. Synthesis of Aldimine Ligands (HL1–3
Three aldimine ligands were prepared and subsequently used to synthesize complexes with Co
II), Ni (II), Cu (II) and Zn (II). The detailed physico-chemical analysis is given in Tables S1–S3.
)
(
3
.2.1. (E)-2-((4-chloro-2-hydroxybenzylidene)amino)-3,4-dimethyl-5-phenylcyclopent-2-en-1-one (HL1)
This ligand was prepared by mixing the ethanolic solution (25 mL) of 4-amino-2,3-dimethyl-1
-phenyl-3-pyrozoline-5-one (2.03 g, 10 mmol) with an ethanolic solution of (25 mL) 4-chloro-2-hydroxy
benzaldehyde (1.56 g, 10 mmol) in methanol (5 mL). The mixture was placed in a 100 mL round
bottom flask connected with an external condenser and refluxed for 30 min. Upon cooling on ice,
yellow precipitates were formed, filtered, washed with ethanol, and stored in vacuo over silica gel.
◦
Yellow amorphous solid, yield, 75%; m.p. 250–254 C. Elemental analysis of CHN for
1
C H ClNO : C 70.69; H 5.34; N 4.12%. Found: C 70.65; H 5.28; N 3.90%. H-NMR spectrum
20
18
2
(
7
1
300 MHz, DMSO-d6)
.8 (2H, d, Ph J = 8.7 Hz), 9.56 (1H, s, –CH=N–); 13C-NMR spectrum (75 MHz, DMSO-d6)
4.3 (C , Me), 15.9 (C , Me), 58.1 (C ), 126.1 (C , Ph), 128 (C , C , Ph), 132, 129 (C , C , C , C ,
δ
H, ppm: 2.46 (1H, s, Ph), 3.196 (s, Me), 3.35 (s, Me), 7.5 (3H, t, Ph J = 8.1 Hz),
δC, ppm:
20
19
10
14
12
16
4
6
13
15
Ph), 130.6 (C , C , Ph), 131.9 (C , Ph), 132.9 (C ), 136.6 (C , Ph), 139.3 (C , Ph), 146.8 (C ), 163.7 (C
1
7
3
2
8
5
11
18
9
C=N), 187.6 (C , C=O). Electrical Conductance (µS/cm) 3.55.
3
.2.2. (Z)-3-((4-chlorobenzylidene)amino)-4-hydroxy-5-nitrobenzenesulfonic acid (HL2)
For preparing ligand HL , the ethanolic solution (25 mL) 5-nitro-2-aminophenol-4-sulfonic acid
2
(2.34 g, 10 mmol) was mixed with an ethanolic solution of (25 mL) of 4-chlorobenzaldehyde (1.4 g,
10 mmol) in methanol (5 mL). The mixture was placed in a 100 mL round bottom flask connected with
an external condenser and refluxed for 30 min. Upon cooling on ice, brown precipitates were formed,
filtered, washed with ethanol, and stored in vacuo over silica gel.
◦
Brown amorphous solid, yield, 79%; m.p. 125–128 C. Elemental analysis of CHN for
1
C H ClN O S: C 43.77; H 2.54; N 7.85%. Found: C 43.55; H 2.50; N 7.67%. H-NMR spectrum
13
9
2
6
(
300 MHz, DMSO-d6) δ , ppm: 3.51 (1H, s, SO H), 6.9 (1H, d, J = 8.7 Hz Ph), 7.2 (1H, t, J = 4.2 Hz,
H 3
13
Ph), 7.4 (2H, d, J = 8.4 Hz, Ph), 7.8 (2H, d, J = 8.4 Hz, Ph), 8.5 (1H, s, C=N), 10 (1H, s, OH); C-NMR
spectrum (75 MHz, DMSO-d6) , ppm: 116.19 (C ), 116.23 (C ), 125.08 (C , C ), 128.81 (C , C ),
δ
C
12
13
4
6
3
7
1
29.31 (C ), 130.11 (C-Cl, C ), 133.86 (C-NO , C ), 135.82 (C , C-SO H), 138.22 (C ), 150.96 (C , C-OH,
2 5 2 10 12 3 8 9
Ph), 156.75 (C=N). Electrical Conductance (µS/cm) 2.3.
0
3
.2.3. 2,2 -((1,2-phenylenebis(azaneylylidene))bis(methaneylylidene))diphenol (HL )
3
The ethanolic solution (25 mL) of salicylaldehyde (2.44 gm; 20 mmol), with an ethanolic solution
of (25 mL) (1.08 gm; 10 mmol), was dissolved in methanol (5 mL) in the presence acetic acid as catalyst.
The mixture was placed in a 100 mL round bottom flask connected with an external condenser and
refluxed for 30 min. Bright yellow precipitates were obtained on cooling the reaction mixture, and the
precipitates were filtered, washed with ethanol, and stored in vacuo over silica gel.
◦
Bright yellow amorphous solid, yield, 82%; m.p. 161–163 C. Elemental analysis of CHN for
1
C H N O : C 75.93; H 5.10; N 8.86%. Found: C 75.82; H 4.95; N 8.71%. H-NMR spectrum (300 MHz,
20
16
2
2
DMSO-d6)
δ
, ppm: 6.92 (1H, m, Ph), 7.06 (1H, d, Ph, J = 8.1 Hz), 7.24 (1H, m, Ph), 7.35 (3H, m, Ph),
H
13
8
.65 (1H, s, –CH=N–), 13.08 (1H, s, Phenolic); C-NMR spectrum (75 MHz, DMSO-d6)
δC, ppm: 117.58,
1
18.5 (C , Ph), 119 (C , Ph), 119.24 (C , Ph), 119.73 (C , Ph), 132.36 (C , Ph), 133.4 (C , Ph), 142.58 (C ,
2
4
9
10
3
5
8
Ph), 161.36 (C , –CH=N–), 163.73 (C Ph). Electrical Conductance (µS/cm) 1.06.
1
7

Molecules 2020, 25, 4252
12 of 17
3
.3. General Method for the Preparation of the Heteroleptic Metal Complexes (ML1–3Gly)
To well stirred ethanolic solutions (20 mL) of aldimine ligands (HL1–3), equimolar amounts of metal
chlorides and phenyl glycine were gradually added, maintaining the reaction mixture temperature at
◦
6
0–70 C. The resulting precipitated products were filtered, washed, and dried in vacuum over silica gel.
◦
CoL Gly: Velvety brown amorphous solid, yield, 80%; m.p. 185–190 C. Elemental analysis
1
of CHN for C H CoN O : C 64.25; H 4.24; N 8.03%. Found: C 64.04; H 4.12; N 7. 89%. Electrical
28
22
3
4
Conductance (µS/cm) 18.56.
◦
NiL Gly: Light green amorphous solid, yield, 78%; m.p. 312–316 C. Elemental analysis of
1
CHN for C H ClNiN O : C 61.41; H 4.60; N 5.12%. Found: C 60.87; H 4.55; N 5.02%. Electrical
2
8
25
2
4
Conductance (µS/cm) 31.4.
◦
CuL Gly: Light blue amorphous solid, yield, 77%; m.p. >330 C. Elemental analysis of
1
CHN for C H ClCuN O : C 60.87; H 4.56; N 5.07%. Found: C 60.58; H 4.50; N 5.16%. Electrical
28
25
2
4
Conductance (µS/cm) 13.42.
◦
ZnL Gly: White amorphous solid, yield, 81%; m.p. >350 C. Elemental analysis of CHN for
1
C H ClZnN O : C 60.67; H 4.55; N 5.05%. Found: C 60.63; H 4.32; N 5.00%. Electrical Conductance
28
25
2
4
(µS/cm) 27.9.
◦
CoL Gly: Dark brown amorphous solid, yield, 65%; m.p. >250 C. The elemental analysis
2
of CHN for C H ClCoN O S: C 44.66; H 2.86; N 7.44%. Found: C 44. 47; H 2.76; N 7.40%. Electrical
21
16
3
8
Conductance (µS/cm) 30.7.
◦
NiL Gly: Dark Green amorphous solid, yield, 74%; m.p. 333–338 C. Elemental analysis of
2
CHN for C H ClCoN O S: C 44.68; H 2.86; N 7.44%. Found: C 44.59; H 2.85; N 7.31%. Electrical
2
1
16
3
8
Conductance (µS/cm) 4.69.
◦
CuL Gly: Brown amorphous solid, yield, 76%; m.p. 217–222 C. Elemental analysis of
2
CHN for C H ClCuN O S: C 44.30; H 2.83; N 7.38%. Found: C 44.28; H 2.76; N 7.21%. Electrical
21
16
3
8
Conductance (µS/cm) 3.42.
◦
ZnL Gly: Light brown amorphous solid, yield, 80%; m.p. 280–282 C. Elemental analysis of
2
1
CHN for C H ClCuN O S: C 44.15; H 2.82; N 7.36%. Found: C 43.89; H 2.65; N 7. 31%. H-NMR
2
1
16
3
8
spectrum (300 MHz, DMSO-d6)
δ , ppm: 3.50 (1H, s, SO H), 3.34 (s, Me), 6.9 (1H, d, J = 8.7 Hz Ph), 7.1
H 3
(
1H, t, J = 4.2 Hz, Ph), 7.3 (2H, d, J = 8.4 Hz, Ph), 7.6 (2H, d, J = 8.4 Hz, Ph), 8.7 (1H, s, C=N), 9.3 (1H, s,
OH); 13C-NMR spectrum (75 MHz, DMSO-d6)
δ
C
, ppm: 117.93 (C ), 119.37 (C ), 123.36 (C , C ),
12
13
4
6
1
1
27.34 (C , C ), 129.29 (C ), 131.16 (C-Cl, C ), 135.41 (C-NO , C ), 136.60 (C , C-SO H), 139.25 (C ),
3
7
2
5
2
10
12
3
8
50.75 (C , C- OH, Ph), 160.07 (C=N). Electrical Conductance (µS/cm) 27.9.
9
◦
CoL Gly: Velvety brown amorphous solid, yield, 79%; m.p. 185–190 C. Elemental analysis
3
of CHN for C H CoN O : C 64.25; H 4.24; N 8.03%. Found: C 64.04; H 4.12; N 7.89%. Electrical
28
22
3
4
Conductance (µS/cm) 31.7.
◦
NiL Gly: Maroon amorphous solid, yield, 78%; m.p. >346 C. Elemental analysis of CHN
3
for C H NiN O : C 64.28; H 4.24; N 8.03%. Found: C 63.75; H 3.99; N 8.01%. Electrical Conductance
28
22
3
4
(µS/cm) 25.4.
◦
CuL Gly: Dark blue amorphous solid, yield, 90%; m.p. 225–235 C. Elemental analysis of
3

Molecules 2020, 25, 4252
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CHN for C H CuN O : C 63.69; H 4.20; N 7.83%. Found: C 63.58; H 4.01; N 7. 83%. Electrical
28
22
3
4
Conductance (µS/cm) 13.5.
◦
ZnL Gly: Yellow amorphous solid, yield, 89%; m.p. 220–222 C. Elemental analysis of CHN
3
1
for C H ZnN O : C 63.47; H 4.19; N 7.59%. Found: C 63.40; H 3.98; N 7. 93%. H-NMR spectrum
28
22
3
4
(
300 MHz, DMSO-d6) δ , ppm: 6.5 (1H, m, Ph), 6.9 (1H, d, Ph, J = 8.1 Hz), 7.27 (1H, m, Ph), 7.4 (3H, m,
H
13
Ph), 7.9 (1H, s, –CH=N–), 9.06 (1H, s, Phenolic); C-NMR spectrum (75 MHz, DMSO-d6)
δC, ppm:
1
1
13.42, 116.93 (C , Ph), 119.89 (C , Ph), 123.55 (C , Ph), 119.73 (C , Ph), 134.78 (C , Ph), 136.69 (C , Ph),
2
4
9
10
3
5
39.83 (C , Ph), 163.29 (C , –CH=N–), 172.73 (C Ph). Electrical Conductance (µS/cm) 3.57.
8
1
7
FTIR data of all the complexes and aldimine ligands are presented in Tables S4–S6 of the
Supplementary Material.
3
.4. The Stability Constants of Heteroleptic Complexes
Buffer solutions were prepared by using different volumes of 0.2 M sodium hydrogen phosphate
prepared form 2.83g in (100 mL) in distilled water, and (0.1) M citric acid prepared by dissolving
−
10 mol. The standard solutions ligands (HL , HL , HL )
1 2 3
3
1
.92 gm in (100) mL distilled water [16]. 1
×
and metal salt (CoCl2 6H O, NiCl2 6H O, CuCl
·
·
·
2H O, ZnCl2 2H O) were prepared by dissolving the
·
2
2
2
2
2
appropriate amounts in 100 mL of absolute ethanol. Metal stock solutions were further diluted with
distilled water to prepare other standard solutions.
3
.5. Mechanistic Study
The reaction for the formation of the complex can be assumed as
MmLn → mM + nL
(2)
The equation for the dissociation of the complex, and stability constant (Kst) can be expressed as:
MmLn
C
→
mM
0
+
nL
0
Initial concentration
(
1− ∝)C
m ∝ C
n ∝ C
Equilibrium concentrations
Kst = (1− ∝)/m × n × (∝)m+n(C)
m
n
m+n−1
(3)
(4)
m
n
m
n
m+n
Kst = (1− ∝)C/(m ∝ C) × (n ∝ C) = (1− ∝)C/m × n × (∝ C)
where ‘∝’ represents the degree of dissociation of the compound. ‘C’ is the initial concentration.
To find the stoichiometry and formation constant of the complex, metal ion solution and reagent
solutions were mixed to give the following nine set of solutions:
Metal ion solution (mL) 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 and 4.5
Reagent solution (mL) 4.5, 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0 and 0.5.
The solutions were suitably diluted with distilled water up to 100 or 250 mL in volume to record
the spectra. Job’s method was applied for stoichiometric ratio and stability constant determination of
the complexes. The curve is obtained with a maximum corresponding to the specific value of mole
fraction, which gives the stoichiometric mole fraction indicating the stoichiometry of the complex
(M: L).
A
max is the absorbance value of the maxima at experimental curve represents the maximum
), and A is absorbance value
is the absorbance value of the part of a
quantity of the complex formed with a degree of dissociation (
corresponding to intersect point of theoretical straight lines. A
α
o
α
dissociated concentration of the complex that represents the difference between Ao and Amax;
Aα = Ao − Amax
(5)

Molecules 2020, 25, 4252
14 of 17
To estimate the stability constant, the degree of dissociation of complex (
α
) is calculated in the
following manner
Aα = ε × b × (αC)
α = Aα/ε × b × C
(6)
(7)
where
ε is molar absorptivity (calculated for each compound by using the reported method [31]), b is a
cell thickness, and C is a concentration of the complex at stoichiometry point.
3
.6. Antibacterial Studies
Studies on antibacterial activity of synthesized ligands and metal complexes were performed with
slight changes in the reported protocol [22]. Six microbial strains viz. B. cereus, S. aureus, K. oxytoca,
E. coli, P. mirabilis, and P. aeruginosa were used to study the antibacterial activity of ligands and their
◦
heteroleptic metal complexes. The microbes were grown in nutrient agar broth medium at 35 C
for 24 h using the agar-well diffusion method [13]. Muller Hinton’s agar plates were homogenized
superficially by injecting 0.2 mL microbial cultures using pasteurized cotton plugs, followed by digging
the wells into solidified agar with a sterilized cork borer (7 mm). Afterwards, 10 mg/mL solutions
of test compounds dissolved in dimethylsulfoxide were shifted to the wells and left undisturbed for
◦
3
0 min at 35 C for 24 h before incubation. The activities were assessed from the inhibition zones,
whereas the bactericidal ciprofloxacin was the reference drug.
3
.7. Antifungal Studies
The in vitro antifungal activities of all ligands and metal complexes were evaluated by employing
the disc technique [22]. The antifungal screening against A. niger, A. flevus and R. Stolonifer were
determined by potato dextrose agar (PDA) media prepared from washed-sliced skinned potatoes
(250 g), dextrose (25 g), and agar (25 g) mixed in 1250 mL of distilled water. The pure fungal cultures
were inoculated uniformly on the PDA solution in a Petri dish. Afterwards, 15 g stock solutions were
µ
transferred to test samples (1 mg/mL) by liquefying 10 mg of the compound in 10 mL dimethylsulfoxide.
◦
These plates were inoculated followed by incubation for 48 h at 35 C; then, the growth inhibition zone
was observed as the sensitivities of fungal isolates with antibiogram. Diflucan (fluconazole) was the
reference standard, while DMSO served as the negative control, and all the values < 7 mm were taken
as inactive.
3
.8. Solubilization Studies
3
.8.1. Solutions Preparation and Procedures for the Solubilization Studies
The solutions of the complexes were prepared in distilled water to make primary solutions, while
secondary solutions were prepared by dissolving SLS in pre-micellar to a post-micellar concentration
range (7–15 mM).
3
.8.2. Electrical Conductivity (EC)
Electrical conductivity parameters were studied with the Hanna Cond. meter (HI-99301),
ensuring the accuracy of up to
with KCl(sol) to maintain the concentration in a particular range according to molar conductivity [32,33].
±0.5% and ∼±0.5 K. The electrode of conductivity meter was calibrated
The differential conductivity was also measured.
The dissociation and free energy of micellization were calculated from conductivity data using
Equations (8) and (9).
S2
(Post−micellar slope)
(Pre−micellar slope)
Extent of dissociation (β) =
(8)
S1

Molecules 2020, 25, 4252
15 of 17
◦
m
Free Energy of Micellization (∆G ) = (2 − β)RT ln X
(9)
CMC
In Equation (9), X
= mole fraction of SLS at CMC, R = Universal gas constant
CMC
−1
−1
(
~8.314 Jmol ·K ), and T = absolute temperature.
The results of the micellar parameters of complexes are summarized in Table 5.
3
.8.3. UV-Visible Spectroscopy
The interaction of metal complexes with SLS was estimated by measuring UV-visible absorbance
of aqueous solutions of the metal complexes (1 mM). The extent and nature of the interaction between
complexes and surfactant were revealed from the absorbance data at 298 K maintaining the accuracy
control at 0.5 K. In differential absorbance, the solution of the surfactant was taken as reference.
±
These values of absorbance were then used for measuring partitioning and binding parameters using
Equations (10)–(13).
The distribution of metal complexes in micellar and aqueous media was governed by Partition
law according to Kawamura et al.
The differential absorbance (∆A) was calculated using with relationship presented in Equation (10).
1
1
1
=
ꢀ
ꢁ +
(10)
m
◦
∆
A
∆A∞
K ∆A∞ Ca + C
C
s
In Equation (10): K
c
= partition constant; C
differential absorbance on infinite dilution. In the expression
a
= concentration of metal complexes; and
∆
A =
∞
m
s
◦
C
= Cs − CMC
o
; CMC
= K
o
= CMC of SLS
in aqueous medium and C
moles of water).
s
is SLS concentration. Partitioning coefficient K
x
c
n
w
(n
w
represent
(11)
Change in Free energy of partition (∆Gp) = −RT ln Kx
The binding constant (K ) was computed with the help of Equation (12).
b
C Ca
CS
1
S
=
+
(12)
∆
A
∆ ∈ l K ∆εl
b
In Equation (12), Ca is the concentration of the metal complex.
Standard binding free energy change (∆G ) = −RT ln K
(13)
b
b
The results of binding and partitioning parameters using the above equations are presented in
Table 6.
4
. Conclusions
Heteroleptic 3D metal complexes having cobalt (II)/nickel (II)/copper (II)/zinc (II) with aldimine
derivatives as a primary ligand and phenyl glycine as a secondary ligand were synthesized and
characterized by different physico-chemical spectroscopic techniques. Elemental analysis and
spectroscopic techniques allowed proposing the tetrahedral and octahedral geometry to the heteroleptic
complexes synthesized. The spectroscopic analysis unveiled the tetrahedral geometry of the synthesized
metal (II) complexes, except for ligand HL which exhibited octahedral geometry. The stability constants,
3
antimicrobial and micellar solubilization studies were also conducted with synthesized heteroleptic
complexes. The synthesized compounds generally showed antibacterial activity for all microbes,
except Ni (II) complexes lacking sensitivity. The complexes of cobalt showed the highest stability as
compared to other complexes. The antifungal activity of ligand (HL , HL , HL ), especially against
1
2
3
A.niger and R. stolonifer fungal strains, was enhanced upon complexation to form heteroleptic cobalt
complexes. All the complexes were screened for their solubilization studies in anionic micellar media,

Molecules 2020, 25, 4252
16 of 17
and only four complexes showed interaction with sodium lauryl sulphate (CoL1–3Gly and NiL Gly).
3
The CMC values obtained from the conductivity and UV-vis data were in good agreement.
Supplementary Materials: The detailed physico-chemical analysis of the ligands and complexes is reported in
Tables S1–S3. The FTIR analyses details are given in Tables S4–S6. Selected NMR and FTIR spectra of ligands and
complexes are reported in Figures S1–S16, while selected electronic spectra of ligands and complexes are given in
Figure S17.
Author Contributions: Conceptualization, M.B.T., W.A.; methodology, M.B.T., U.A. and S.N.; software, A.R. and
H.A.; antimicrobial analysis, U.A., A.M.A.; Investigation, N.A.B., U.A.; writing—original draft preparation and
supervision, M.B.T.; review and editing, M.D.F.A. All authors have read and agreed to the published version of
the manuscript.
Funding: This research was funded by the Deanship of Scientific Research at Princess Nourah Bint Abdulrahman
University though the Fast-Track Research Funding Program Saudi Arabia.
Acknowledgments: The authors are grateful to The Islamia University Bahawalpur, Pakistan and Prince Nourah
Bint Abdulrahman University, Saudi Arabia for providing basic lab facilities.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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