Silver(I)-pyridinyl Schiff base complexes: Synthesis, characterisation and antimicrobial studies

Article abstract of DOI:10.1016/j.molstruc.2017.01.061

Fifteen new silver(I)-pyridinyl complexes of the general formula [AgL₂]X, where X = ClO₄^?, OTf or NO₃^?, were synthesised by reacting (E)-N-(pyridinylmethylene)aniline ligands and the respective silver

Full text of DOI:10.1016/j.molstruc.2017.01.061

Accepted Manuscript
Silver(I)-pyridinyl Schiff base complexes: Synthesis, characterization and
antimicrobial studies
Eric M. Njogu, Bernard Omondi, Vincent O. Nyamori
PII:
S0022-2860(17)30092-3
10.1016/j.molstruc.2017.01.061
MOLSTR 23366
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 11 August 2016
Revised Date: 18 January 2017
Accepted Date: 20 January 2017
Please cite this article as: E.M. Njogu, B. Omondi, V.O. Nyamori, Silver(I)-pyridinyl Schiff base
complexes: Synthesis, characterization and antimicrobial studies, Journal of Molecular Structure (2017),
doi: 10.1016/j.molstruc.2017.01.061.
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ACCEPTED MANUSCRIPT
Silver(I)-pyridinyl Schiff base complexes: Synthesis, characterization and antimicrobial
studies
Eric M. Njogu, Bernard Omondi and Vincent O. Nyamori
School of Chemistry and Physics, University of KwaZulu-Natal, South Africa
Private bag X54001, Durban, 4000
Email: bjumberic2002@gmail.com; owaga@ukzn.ac.za; nyamori@ukzn.ac.za
Graphical abstract
Scheme showing the synthesis of silver(I)-pyridinyl complexes and the antimicrobial screening

ACCEPTED MANUSCRIPT
Silver(I)-pyridinyl Schiff base complexes: Synthesis, characterization and antimicrobial
studies
Eric M. Njogu, Bernard Omondi and Vincent O. Nyamori
School of Chemistry and Physics, University of KwaZulu-Natal, South Africa
Private bag X54001, Durban, 4000
Email: bjumberic2002@gmail.com; owaga@ukzn.ac.za; nyamori@ukzn.ac.za

ACCEPTED MANUSCRIPT
Abstract
-
Fifteen new silver(I)-pyridinyl complexes of the general formula [AgL2]X, where X = ClO4 ,
-
OTf or NO3 , were synthesised by reacting (E)-N-(pyridinylmethylene)aniline ligands and the
respective silver(I) salts namely AgClO4, AgOTf, or AgNO3. The ligands were obtained by
neat grinding of 2- or 4-pyridincarboaxaldehyde together with aniline, 2,6-dimethylaniline or
2,6-diisopropylaniline. The obtained (E)-N-(pyridinylmethylene)aniline ligands were further
reacted with respective silver(I) salts in a 2:1 ratio in anhydrous ethanol at room temperature
under inert atmosphere using the Schlenk techniques. Chemical structures of complexes
were identified by nuclear magnetic resonance, electrospray ionization mass spectrometry,
elemental analysis, infrared spectroscopy and some by single-crystal X-ray diffraction
analysis. Reactions involving the 2-pyridinyl derivatives resulted in cationic complexes in
which two ligands chelate silver(I) centres through the pyridinyl N and imine N atoms, with
the counter anion out of the coordination sphere. The 4-pyridinyl derivatives conversely gave
complexes in which two ligands coordinate to the silver(I) centre through their pyridinyl N
atoms only, most likely a linear fashion. The newly synthesized silver(I) complexes and the
free ligands were evaluated for their in vitro antimicrobial activity against Escherichia coli,
Salmonella typhimirium, Staphylococcus aureus and Candida albicans. The complexes
showed varied growth inhibitory activity against the test organisms.
Keywords: silver(I) complexes, pyridinyl ligands, Schiff base, antimicrobial activity
2

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1.0
Introduction
There is an urgent and sustained pressure on scientists to develop novel and effective
antimicrobial agents with minimal cytotoxicity on mammalian cells. This is has been
triggered, firstly, by the emergence and rise of multidrug resistant strains of microbes
(superbugs) that are progressively becoming less treatable, especially by first choice
antibiotics and antiseptics [1, 2]. This has resulted in many clinical antibacterial and
antifungal drugs becoming less effective [3] heralding the return of stubborn infections, that
are turning fatal.. Moreover, some of the antibacterial and antifungal agents currently in use
have undesirable and life threatening side effects thus their usage is deemed
counterproductive [4]. The failure and decline of a number of antibiotics available for
tackling multidrug resistant strains has occasioned the return to traditional (pre-antibiotic)
methods of managing infectious diseases. These include focusing on use of antimicrobial
metal based agents, such as silver based products in aiding the fight on infectious diseases [5,
6]. The resurgence on research and application of silver(I) as a premier candidate in fighting
infectious diseases is well on course and has been greatly reviewed [6-10]. Pure silver metal
is biologically inactive, however in moist conditions, it ionizes readily into silver(I) cations
that portray antimicrobial activity. Silver nanoparticles have been hypothesised to show
biological activity not only by releasing silver(I) ions, but also by anchoring on and
penetrating bacterial cell walls and thus causing structural changes on their cell membranes.
This increases permeability into the cell and consequently the cells are destroyed [11].
Moreover, nanosilver particles are also thought to show biological activity by releasing free
radicals that in turn induce membrane damage [12]. Silver is well-known medicinal agent
owing to its broad spectrum activity at low concentrations against stubborn infectious gram-
negative and gram-positive bacteria, fungi and yeast, including antibiotic resistant strains.
Additionally, it has been successfully used as a medicinal agent due to its low toxicity and
low bio-accumulation in the mammalian body making it a potentially safe internalized
therapeutic agent. Silver has been reported to portray multiple modes of action such as; (i)
inhibiting activity of some enzymes e.g. those that mediate respiration [13], (ii) binding with
DNA cells [13] and (iii) rescinding normal biological activity of bacteria protein by reacting
with sensitive thiol groups on the proteins [14]. Specificity on the various modes of action of
silver(I) has been explicitly reviewed by S. L. Percivala et al., [15].
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Traditionally, silver was applied as silver nitrate, however, current research is inclined
towards silver based inorganic antimicrobials and silver nanomaterials [16, 17], due to their
reduced photosensitivity as well as their potential for controlled release of silver ions.
Success of silver based antimicrobials is tied to the ancillary ligands and their ability to
stabilize the oxidation state [18]. The ligands also play a key role in modifying the
lipophilicity, bioavailability and reactivity of the silver based antimicrobials [19]. The main
challenge in designing silver based antimicrobial drugs lies in optimizing slow controlled and
sustained release of silver(I) ions over a period of time. Usage of ligands that bond very
firmly to the silver(I) centre, could hamper timely release of silver(I) ions. This would
essentially result in compromised and reduced antibacterial activity and high risk for silver(I)
resistance by the microbes.
Due to the promise of silver in bolstering the fight against infectious diseases, it has been
employed in production of various silver coated medical devices [20, 21] and silver-based
wound care dressings where elemental silver is incorporated within the dressing rather than
being applied as a separate compound or solution [22, 23]. Silver(I) is still being used in
modern medicine for the prevention and treatment of bacterial infections associated with
second and third degree burns and other open wounds where it is applied as silver
sulfadiazine cream [24]. Colloidal silver proteins are being used as mineral supplements
though their indiscriminate usage is being discouraged in order to reduce risks leading to
agyria [25]. Based on the merits associated with silver(I) and emerging challenges in
treatment of diseases, there is a great interest in developing new silver(I) compounds with
high pharmacological activity such as antitumor [26-28] and anticancer agents [29, 30], anti-
ulcer [31, 32], antibacterial [33, 34], and antifungal [35, 36] properties. This is in the quest to
find new drugs to bolster the existing ones and ramp the fight on diseases.
Antibacterial and antifungal studies of silver(I) complexes dominate other biological
applications [37]. Silver(I) complexes of N-donor ligands have shown spectacularly a wide
spectra antimicrobial activities against bacteria and fungi [38]. For instance, functionalized
pyridine derivatives have been used as bactericides [39-41], fungicides [42]. On the other
hand, metal free Schiff bases exhibit a broad range of biological activities, including
antifungal [43], antibacterial [44, 45], antimalarial [46], anticancer [47, 48], anti-
inflammatory [49] and antiviral [50] properties. The imine group is largely credited for the
biological activities portrayed by Schiff base compounds [51-53]. With astute design and
4

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synthesis, metal complexes of Schiff base ligands provide good platform of forming new
medicinal compounds with good therapeutic activity [52].
In light of aforementioned merits of silver(I), N heterocycles and Schiff base functions, we
have designed and synthesised silver(I)-pyridinyl Schiff base complexes for antimicrobial
studies. It was hypothesised that these ligands and silver(I) ions would act synergistically
and hence show pronounced activity against gram-negative and gram-positive bacteria as
well as fungi. The (E)-N-(pyridinylmethylene)aniline ligands were synthesised via
mechanochemistry technique under-solvent free conditions. The antimicrobial potency of the
new discrete silver(I) complexes with (E)-N-(pyridinylmethylene)aniline ligands were
studied. Also, the influence of subtle changes in the electronic and steric properties of the
(E)-N-(pyridinylmethylene)aniline ligands in biological properties of the silver(I) complexes
was investigated. Though silver(I)-pyridinyl Schiff base complexes have been reported
before, a huge chunk of these studies focuses on the photoluminescence and structural aspects
of silver(I) polymeric or discrete helices obtained by complexing the metal with polydentate
pyridinyl Schiff bases [54-62]. Moreover, the pyridinyl Schiff base ligands employed in the
reported studies are often synthesised via the conventional technique (i.e. reflux method).
2.0
2.1
Experimental
Materials, physical measurements and general procedures
All commercially available chemical reagents and gases were procured from local suppliers
and were used without further purification or modification. These were: 2-
pyridinecarboxaldehyde 99% (Aldrich, USA), 4-pyridinecarboxaldehyde 97% (Aldrich,
USA), 2,2-dimethylaniline 99% (Aldrich), 2,6-diisopropylaniline >92% (Merck, Germany),
silver(I) perchlorate 97% (Aldrich USA), silver trifluoromethanesulfonate >99% (Aldrich,
USA), dimethyl sulfoxide-d6 99.8% (Merck, Germany), silver nitrate 99.8% (Aldrich, USA),
, argon gas, 5.0 technical grade, (Airflex Industrial Gases, South Africa), nitrogen, 5.0 gas
technical grade (Airflex Industrial Gases, South Africa). All solvents employed in the
synthesis and work ups i.e. ethanol 99.5% (Associated Chemical Enterprise, South Africa),
diethyl ether > 99% (Aldrich), dichloromethane 99% (Aldrich), were dried by standard
procedures and distilled under nitrogen prior to use. All manipulations in synthesis of the
complexes reported here were performed under an atmosphere of dry argon gas by use of
standard Schlenk techniques.
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1
13
The H and C NMR spectra were recorded in deuterated DMSO-d6, on Bruker AVANCEIII
400 MHz spectrometer. The chemical shifts are reported in ppm relative to the solvent peak
shift (2.5 and 39.5 ppm for 1H and 13C NMR respectively). The splitting patterns in 1H NMR
spectra are reported as follows: s = singlet, d = doublet, m = multiplet and J values are given
in Hertz. FT-IR spectra of the compounds were recorded in the range of 4000 – 400 cm-1
regions with a PerkinElmer spectrum 100 FT-IR spectrometer and the data are reported as
percentage transmittances at given wavenumbers. High resolution electro-spray ionization
(ESI) mass spectrometry spectra were recorded using a Waters Micromass LCT Premier
TOF-MS. Only the molecular ions (M+) and the major fragmentation peaks are reported. All
melting points were determined using Stuart Scientific melting point apparatus (SMP10).
Microanalyses were performed on a Thermoscientific Flash 2000 elemental analyser. Single
crystal analysis were done on Bruker Smart APEX2 diffractometer.
2.2
Synthesis of ligands
The ligands (E)-N-(pyridinylmethylene)aniline numbered L1 – L6 (Scheme 1) were
synthesised by solvent-less grinding (mechanochemical) of the respective aniline together
with appropriate pyridinecarboxaldehyde in a Pyrex tube with a fitted ground joint. Grinding
for 2 – 5 min resulted in a paste (L3, L4 and L6) or an amber viscous oil (L1, L2 and L5).
The pastes were left to dry to the desired products in air while the oils were dried under
reduced pressure overnight. All the physical and spectral data for the known ligands [63-65]
1
i.e. colour, melting points, IR, H and 13C NMR were in line with the ones previously
reported (see supporting information) [66, 67].
2.3
Synthesis of complexes
The complexes were prepared by reacting two equivalents of the respective ligands with the
appropriate silver(I) salt. Generally, a solution of ligand (either of L1 – L6) in dry ethanol
(ca 15 mL) was added slowly to a solution of the respective silver salt (AgClO4, AgOTf or
AgNO3) in anhydrous ethanol (ca 10 ml), while stirring at room temperature. The solution
was further stirred for six hours yield to yield precipitates. The obtained solids were then
isolated by filtration using Buchner system and recrystallized from dichloromethane/diethyl
ether or dichloromethane/hexane. Where precipitates were not obtained, the solvent was
removed under reduced pressure and solid obtained recrystallized in dichloromethane/diethyl
ether solvent.
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2.3.1 [Ag(L1)2]ClO4, 1
Synthesised by use of (E)-N-(pyridin-2-ylmethylene)aniline L1, (0.3866 2.12 mmol) and
silver perchlorate (0.2104 g, 1 mmol). A shiny laser lemon solid obtained (0.5431 g, 95%).
1
Melting point: 149 – 150 °C. H NMR (400 MHz, DMSO-d6, 25℃): δ = 7.34 (d, J = 7.2
Hz, 2 H, Hh-C6H5), 7.42 (t, J = 15.3 Hz, J = 7.3 Hz, 4 H, Hg-C6H5), 7.48 (d, J = 7.9 Hz, 4 H,
Hf- C6H5), 7.78 (m, 2 H, Hc-C5H4N), 8.13 (d, J = 7.6 Hz, 2 H, Hd- C5H4N), 8.23 (td, J = 7.8
Hz, J = 1.7 Hz, 2 H, Hc- C5H4N), 8.80 (d, J = 4.2 Hz, 2 H, Ha- C5H4N), 9.09 (s, 2 H, CH=N)
ppm. 13C NMR (400 MHz, DMSO-d6, 25℃): δ = 122.10 (C6H5), 128.09 (C5H4N), 128.13
(C6H5), 128.75 (C6H5), 129.46 (C5H4N), 139.36 (C5H4N), 148.06 (C5H4N), 149.77 (C5H4N),
151.30 (C6H5), 159.91 (CH=N) ppm. FT-IR: 3068, 1590, 1490, 1072, 902 cm-1. MS (ESI-
TOF): m/z Calcd. for [Ag(L1)2]: 471.0739; found [M+]: 471.0760 (100%), 471.0739
(98%) [AgL1] 289.0114 (28%). Anal. Calcd. (%) for [Ag(L1)2]ClO4: C, 50.42; H, 3.53; N,
9.80; found (%): C, 50.53; H, 3.29; N, 9.80.
2.3.2 [Ag(L1)2]OTf, 2
Synthesis by use of (E)-N-(pyridin-2-ylmethylene)aniline L1, (0.3866 2.12 mmol) and silver
trifluoromethanesulfonate (0.2573 g, 1 mmol) gave a shiny lemon yellow solid obtained,
1
(0.5343 g, 86%). Melting point 136 – 137 °C. H NMR (400 MHz, DMSO-d6, 25℃): δ =
7.34 (t, J = 7.1 Hz, 2 H, Hh-C6H5), 7.41 - 7.49 (m, 8 H, Hf-, Hg- C6H5), 7.76 (td, J = 12.3 Hz,
J = 6.1 Hz, 2 H, Hb-C5H4N), 8.13 (d, J = 7.6 Hz, 2 H, Hd- C5H4N), 8.21 (t, J = 15.3 Hz, J =
7.6 Hz, 2 H, Hc- C6H5), 8.80 (d, J = 4.6 Hz, 2 H, Ha- C5H4N), 9.03 (s, 2 H, CH=N) ppm. 13C
NMR (400 MHz, DMSO-d6, 25℃): δ = 122.18 (C2- C6H5), 128.09 (C4- C6H5), 129.58 (C3-
C6H5), 137.90 (C5- C5H4N), 139.36 (C4- C5H4N), 151.32 (C1- C5H4N, C3- C5H4N, C7-
C6H5), 160.01 (CH=N). FT-IR: 3071, 1589, 1489, 1255, 1151, 1027 cm-1. MS (ESI-TOF):
m/z Calcd. for [Ag(L1)2]; 471.0739 found: 471.0731 (100%); 470.4012 (10%), 472.0765
(30%), 473.0731 (99%), 474.0764 (30%). Anal. Calcd. (%) for [Ag(L2)2]O3SCF3: C,
48.32; H, 3.24; N, 9.02; found (%): C, 48.69; H, 3.24; N, 8.93.
2.3.3 [Ag(L2)2]ClO4, 3
Synthesised by use of (E)-2,6-dimethyl-N-(pyridin-2-ylmethylene)aniline (0.4364 g, 2.08
mmol) and silver perchlorate (0.2108 g, 1.02 mmol. A light khaki solid obtained (0. 0.605 g,
1
94%). Melting point: 202 °C with decomposition. H NMR (400 MHz, DMSO-d6, 25℃): δ
7

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= 2.05 (s, 12 H, Hh-CH3), 7.05 (t, J = 7.4 Hz, 2 H, Hg-C6H5), 7.11 (d, J = 7.4 Hz, 4 H, Hf-
C6H5), 7.79 - 7.82 (m, 2 H, Hb-C5H4N), 8.05 (d, J = 7.6 Hz, 2 H, Hc-C5H4N), 8.22 (td, J =
7.8 Hz, J = 1.7 Hz, 2 H, Hd- C5H4N), 8.63 (s, 2 H, He-CH=N), 8.76 (d, J = 4.9 Hz, 2 H, Ha-
C5H4N) ppm. 13C NMR (400 MHz, DMSO-d6, 25℃): δ = 18.19 (CH3), 124.93 (C6H5),
127.06 (C5H4N), 128.09 (C5H4N), 128.46 (C5H4N), 139.45 (C5- C5H4N), 148.80 (C5H4N,
C6H5), 151.11 (C5H4N), 163.94 (CH=N). FT-IR: 3068, 2913, 1639, 1591, 1468, 1442, 1265,
1301, 1061, 900 cm-1. MS (ESI-TOF): m/z Calcd. for [Ag(L2)2]: 527.1365; found:
527.1376 (100%), 528.1409(30%). 529.1375(100%), 530.1525(30%), [AgL2],
317.0292(85%), 319.0288(80%). Anal. Calcd. (%) for [Ag(L2)2]ClO4: C, 53.56; H, 4.49;
N, 8.92; found (%): C, 53.46; H, 4.10; N, 8.92.
2.3.4 [Ag(L2)2]OTf, 4
Synthesised by use of (E)-2,6-dimethyl-N-(pyridin-2-ylmethylene)aniline (0.4224 g, 2.00
mmol) and silver trifluoromethanesulfonate (0.2352 g, 0.91 mmol). Banana yellow solid
(0.5691 g, 92%). Crystals for single crystal X ray diffraction analysis were obtained by
layering diethyl ether onto dichloromethane solution of product. Melting point 183 - 184 °C.
1H NMR (400 MHz, DMSO-d6, 25℃): δ = 2.00 (s, 12 H, Hh-CH3), 7.00 (t, J = 6.3 Hz, 2 H,
Hg- C6H5), 7.07 (d, J = 7.3 Hz, 4 H, H-C6H3), 7.75 (t, J = 6.0 Hz, 2 H, Hb-C5H4N), 8.08 (d, J
= 7.7 Hz, 2 H, Hd-C5H4N), 8.16 (td, J = 7.6 Hz, J = 1.6 Hz, 2 H, Hc-C5H4N), 8.57 (s, 2 H,
He-CH=N), 8.71 (d, J = 4.5 Hz, 2 H, Ha-C5H4N) ppm. 13C NMR (400 MHz, DMSO-d6,
25℃): δ = 17.79 (CH3), 124.73 (C5H4N), 126.85 (C5H4N), 127.14 (C10- C6H5), 127.98 (C8-,
C12- C6H5), 128.07 (C6H5), 139.03 (C5H4N), 148.98 (C5H4N), 149.76 (C6H5), 150.82
(C5H4N), 163.94 (CH=N) ppm. FT-IR: 2942, 1639, 1591, 1474, 1442, 1258, 1149, 1090,
1030 cm-1. MS (ESI-TOF) m/z: Calcd. for [Ag(L2)2]: 527.1365 Found [M+]:
527.1360(100%) 527.1091(98%) 530.1127(30%). Anal. Calcd. (%) for [Ag(L2)2]O3SCF3:
C, 51.41; H, 4.17; N, 8.27; found (%): C, 51.67; H, 3.85; N, 8.17.
2.3.5 [Ag(L2)2]NO3, 5
Synthesised by use of silver nitrate (0.1700 g, 1.0 mmol) and (E)-2,6-dimethyl-N-(pyridin-2-
ylmethylene)aniline L2, (0.4204 g, 2.00 mmol). A golden yellow solid (0.5488 g, 93%).
1
Melting point 190 – 191 °C. H NMR (400 MHz, DMSO-d6, 25℃): δ = 1.97 (s, 12 H, Hh-
CH3), 7.01 (t, J = 8.8 Hz, J = 5.9 Hz, 2 H, Hg-C6H3), 7.06 (d, J = 6.4 Hz, 4 H, Hf-C6H3), 7.77
(t, J = 12.5 Hz, J = 6.0 Hz, 2 H, Hb-C5H4N), 8.06 (d, J = 7.7 Hz, 2 H, Hd-C5H4N), 8.20 (td, J
8

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= 7.6 Hz, J = 1.6, 2 H, Hc-C5H4N), 8.61 (s, 2 H, He-CH=N), 8.69 (d, J = 4.2 Hz, 2 H, Ha-
C5H4N) ppm. 13C NMR (400 MHz, DMSO-d6, 25℃): δ = 17.75 (CH3), 124.87, (C5H4N),
126.94 (C5H4N), 128.08 (C6H3), 128.30 (C6H3), 139.31 (C5H4N), 148.80 (C5H4N), 149.17
(C5H4N), 150.99 (C5H4N), 163.99 (CH=N) ppm. FT-IR: 3061, 2958, 2909, 1646, 1592,
1337, 1187, 772 cm-1. MS (ESI-TOF): m/z Calcd. for [Ag(L2)2]: 527.1365; found
527.1375(10%), 528.1411(30%), 529.1375 (100%), 530.1407(30%), [AgL2] 317.0423
(70%). Anal. Calcd. (%) for [Ag(L2)2]NO3: C, 56.96; H, 4.78; N, 11.86; found (%): C,
56.96; H, 4.50; N, 11.81.
2.3.6 [Ag(L3)2]ClO4, 6
Synthesised by use of (E)-2,6-diisopropyl-N-(pyridin-2-ylmethylene)aniline L3, (0.5321 g,
2.00 mmol) and silver perchlorate (0.2028 g, 0.98 mmol). A laser lemon solid obtained,
1
(0.690 g, 95%). Melting point 250 – 251 °C. H NMR (400 MHz, DMSO, d6, 25℃): δ =
0.99 (d, J = 6.8 Hz, 24 H, Hi-CH3), 2.91 (t, J = 6.8 Hz, 4 H, Hh-CH(CH3)2), 7.17 (s, 6 H,
He,Hf-C6H5), 7.64 - 7.68 (m, 2 H, Hc-C5H4N), 8.11 (d, J = 7.12 Hz, 2 H, Hd-C5H4N), 8.15
(td, J = 7.7 Hz, J = 1.6 Hz, 2 H, Hb-C5H4N), 8.40 (d, J = 4.6 Hz, 2 H, Ha-C5H4N), 8.59 (s, 2
H, CH=N) ppm. 13C NMR: (400 MHz, DMSO-d6, 25℃) δ = 23.10 (CH3), 27.45
(CH(CH3)2), 123.04 (C6H5), 125.25 (C6H5), 127.00 (C5H4N), 127.85 (C5H4N), 137.08
(C5H4N), 139.02 (C6H5), 147.04 (C6H5), 149.60 (C5H4N), 150.50 (C5H4N), 163.36 (CH=N).
FT-IR: 3067, 3026, 2960, 2879, 1636, 1592, 1444, 1061, 900 cm-1. MS (ESI-TOF) m/z:
Calcd. for [Ag(L3)2] 639.2617 found 639.2643(100%), 641.2651(40%), 640.2686(100%)
[AgL3] 373.0828(25%). Anal. Calcd. (%) for [Ag(L3)2]ClO4: C, 58.42; H, 5.99; N, 7.57;
found (%): C, 58.33; H, 5.71; N, 7.78.
2.3.7 [Ag(L3)2]OTf, 7
Synthesised by use of (E)-2,6-diisopropyl-N-(pyridin-2-ylmethylene)aniline L3 (0.474 g,
1.77 mmol) and silver trifluoromethanesulfonate (0.2160 g, 0.84 mmol). An aureolin solid
obtained (0.6244 g, 94%). Crystals for single crystal X-ray diffraction were obtained by slow
1
evaporation of the ethanol solution of the solid. Melting point 240 – 241 °C. H NMR (400
MHz, DMSO-d6, 25℃): δ = 0.99 (d, J = 6.8 Hz, 24 H, Hi-CH3), 2.85 (t, J = 6.8 Hz, 4 H, Hh-
CH(CH3)2), 7.17 (s, 6 H, Hf-, Hg-C6H3), 7.66 (t, J = 6.6 Hz, 2 H, Hb-C5H4N), 8.11 (d, J = 7.2
Hz, 2 H, Hd-C5H4N), 8.15 (t, J = 15 Hz, J = 7.2 Hz, 2 H, Hc- C5H4N), 8.38 (d, J = 4.5 Hz, 2
H, Ha-C5H4N). 8.6 (s, 2 H, He-CH=N) ppm. 13C NMR (400 MHz, DMSO-d6, 25℃): δ =
9

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23.09 (CH3), 27.45 (CH(CH3)2), 123.05 (C6H3), 125.29 (C5H4N), 127.17 (C5H4N), 127.90
(C6H3), 137.07 (C5H4N), 147.00 (C6H3), 149.52 (C5H4N), 150.52 (C5H4N), 163.35 (CH=N)
ppm. FT-IR: 3067, 3028, 2960, 2868, 1638, 1593, 1461, 1446, 1365, 1273, 1254, 1156,
1145, 1028 cm-1. ESI-TOF m/z: Calcd. for [Ag(L2)2]: 639.2617 Found: 639.3106 (100 %),
642.3143. Anal. Calcd. (%): C, 56.27; H, 5.62; N, 7.09; found: C, 56.71; H, 6.03; N, 7.51.
2.3.8 [Ag(L3)2]NO3, 8
Synthesised by use of silver nitrate (0.1712 g 1 mmol) and (E)-2,6-diisopropyl-N-(pyridin-2-
ylmethylene)aniline L3, (0.2723 g, 1.03 mmol). Golden yellow solid obtained, (0.400 g,
1
92%). Melting point: 156 °C with decomposition. H NMR (400 MHz, DMSO-d6, 25℃): δ
= 1.03 (d, J = 6.9 Hz, 12 H, Hi-CH3), 2.87 (t, J = 13.7 Hz, J = 6.8 Hz, 2 H, Hh-CH(CH3)2),
7.18 (s, 3 H, Hf-, Hg-C6H3), 7.71 (t, J = 12.2 Hz, J = 6.1 Hz, 1 H, Hb-C5H4N), 8.12 (d, J =
7.5 Hz, 1 H, Hd-C5H4N), 8.17 (t, J = 15.1 Hz, J = 7.6 Hz, Hc- C5H4N), 8.52 (d, J = 4.4 Hz, 1
H, Ha-C5H4N), 8.61 (s, 1 H, He-CH=N) ppm. 13C NMR (400 MHz, DMSO-d6, 25℃): δ =
23.18 (CH3), 27.44 (CH(CH3)2), 123.05, (C6H3), 125.26 (C6H3), 127.95 (C5H4N), 137.15
(C6H3), 139.11 (C5H4N), 147.06 (C6H3), 150.68 (C5H4N), 163.36 (CH=N) ppm. FT-IR:
3025, 2964, 1637, 1591, 1407, 1363, 1290, 1176, 781 cm-1. MS (ESI-TOF) m/z: Calcd. for
[Ag(L3)2]: 639.2617 found [M+] 639.2624 (100%), 641.2627; [AgL3] 373.0078(25%). Anal.
Calcd. (%) for [AgL3]NO3: C, 49.56; H, 5.08; N, 9.63; found (%): C, 49.26; H, 4.57; N,
9.53.
2.3.9 [Ag(L4)2]ClO4, 9
Synthesised by use of (E)-N-(pyridin-4-ylmethylene)aniline L4, (0.3585 g, 1.97 mmol) and
silver perchlorate (0.1930 g, 0.93 mmol). An off white solid obtained (0.4820 g 91%).
Melting point 190 - 191 °C. 1H-NMR (400 MHz, DMSO-d6, 25℃): δ = 7.30 (d, 2 H, J =
7.2 Hz, Hf-C6H5), 7.34 (d, 4 H, J = 7.3 Hz, He-C6H5), 7.45 (d, 4 H, J = 7.5 Hz, Hd-6H5), 7.93
(d, 4 H, J = 6 Hz, Hb-C5H4N), 8.71 (s, 2 H, Hc-CH=N), 8.77 (d, 4 H, J = 6.0 Hz, Ha-C5H4N)
ppm. 13C-NMR (400 MHz, DMSO-d6, 25℃): δ = 121.66 (C5H4N), 122.75 (C6H5) 127.94
(C6H5), 129.76 (C6H5), 143.46 (C5H4N), 151.25 (C5H4N), 151.74 (C5H4N), 159.46 (CH=N)
ppm. FT-IR: 3052, 1612, 1424, 1067, 926 cm-1. MS (ESI-TOF) m/z: [Ag(L4)2]: 471.0739
found [M+]: 471.0760 [AgL4] 289.0114(28%). Anal. Calcd. (%) for [AgL2]ClO4. C, 50.42;
H, 3.53; N, 9.80; found (%): C, 50.73; H, 3.36; N, 9.69.
10

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2.3.10 [Ag(L5)2]ClO4, 10
Synthesised by use of (E)-2,6-dimethyl-N-(pyridin-4-ylmethylene)aniline L5, (0.4410 g, 2.10
mmol) and AgCLO4 (0.2131 g, 1.03 mmol). An electric yellow solid (0.530 g, 82%).
1
Melting point 210 °C. H NMR (400 MHz, DMSO-d6, 25℃): δ = 2.07 (s, 12 H, Hf-CH3),
6.97 (t, J = 15 Hz, J = 7.3 Hz, 2 H, He-C6H5), 7.08 (d, J = 7.6 Hz, 4 H, Hd-C6H5), 7.92 (d, J =
5.7 Hz, 4 H, Hb-C6H5), 8.43 (s, 2 H, Hc-CH=N), 8.78 (d, J = 5.6 Hz, 4 H, Ha-C5H4N). 13C
NMR (400 MHz, DMSO-d6, 25℃): δ = 17.82 (CH3), 124.03 (C6H5), 126.18 (C6H5), 127.97
(C6H5), 142.57 (C5H4N), 150.10 (C6H5), 150.83 (C5H4N), 162.08 (CH=N). FT-IR: 2920,
1613, 1469, 1426, 1076, 924 cm-1. MS (ESI-TOF): m/z, Calcd. for [Ag(L5)2]: 527.1365
found: 527.1680(100%), 527.1516(40%), [AgL5] 317.0304(100%), 319.0305(95%). Anal.
Calcd. (%) for [Ag(L5)2]ClO4: C, 53.56; H, 4.49; N, 8.92; found (%): C, 53.63; H, 4.10; N,
8.86.
2.3.11 [Ag(L5)2]OTf, 11
Synthesised by use of (E)-2,6-dimethyl-N-(pyridin-4-ylmethylene)aniline L5, (0.4180 g, 2.00
mmol) and silver trifluoromethanesulfonate (0.2502 g, 0.97 mmol). A lemon precipitate
1
(0.5641 g, 86%). Melting point: 140 -142 °C. H NMR (400 MHz, DMSO-d6, 25℃): δ =
2.06 (s, 12 H, Hf-CH3), 6.96 (t, J = 7.5 Hz, 2 H, He-C6H5), 7.08 (d, J = 7.5 Hz, 4 H, Hd-
C6H5), 7.89 (d, J = 5.8 Hz, 4 H, Hb-C5H4N), 8.42 (s, 2 H, CH=N), 8.78 (d, J = 5.8 Hz, 4 H,
Ha-C5H4N) ppm. 13C NMR (400 MHz, DMSO-d6, 25℃): δ = 17.82 (CH3), 122.05 (C5H4N),
123.99 (C6H3), 126.17 (C6H3), 127.96 (C6H3), 142.38 (C5H4N), 150.14 (C6H3), 150.65
(C5H4N), 162.13 (CH=N). FT-IR: 3061, 2960, 1615, 1284, 1232, 1158, 1026, 825, 630 cm-1.
(MS) ESI-TOF m/z Calcd. for [Ag(L5)2]: 527.1365; found: 527.1385(100%), [AgL5]
317.0272(12%). Anal. Calcd. for [Ag(L5)2]O3SCF3: C, 51.41; H, 4.17; N, 8.27; found:
C, 51.67; H, 3.76; N, 7.88.
2.3.12 [Ag(L5)2]NO3, 12
Synthesised by use of silver nitrate (0.16960, 1.0 mmol) and (E)-2,6-dimethyl-N-(pyridin-4-
ylmethylene)aniline L5 (0.4300 g, 2.04 mmol). Bumblebee yellow solid, (0.500 g, 85%).
1
Melting point 157 °C with decomposition. H NMR (400 MHz, DMSO-d6, 25℃): δ = 2.07
(s, 12 H, Hf-CH3), 6.97 (t, J = 15 Hz, J = 7.5 Hz, 2 H, He-C6H3), 7.09 (d, J = 7.5 Hz, 4 H, hd-
C6H3), 7.91 (d, J = 6.0 Hz, 4 H, Hb-C5H4N), 8.43 (s, 2 H, Hc-CH=N), 8.78 (d, J 6.0 Hz, Ha-
11

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13
C5H4N) ppm. C NMR (400 MHz, DMSO-d6, 25℃): δ = 17.82 (CH3), 122.11 (C6H3) 124.00
(C5H4N), 126.18 (C6H3), 127.96 (C6H3), 142.48 (C5H4N), 150.13 (C6H3), 150.74 (C5H4N),
162.11 (CH=N) ppm. FT-IR: 3064, 2960, 1612, 1328, 1175, 764 cm-1. MS (ESI-TOF): m/z
calcd. for [Ag(L5)2]: 527.1365; found: 527.1365 (100%), 529.1370(100%), 530.1405(30%)
Anal. Calcd. (%) for [Ag(L5)2]NO3: C, 56.96; H, 4.78; N, 11.86; found (%): C, 56.82; H,
4.60; N, 11.77.
2.3.13 [Ag(L6)2]ClO4 13
Synthesised by use of (E)-2,6-diisopropyl-N-(pyridin-4-ylmethylene)aniline L6 (0.5390 g,
2.02 mmol) and silver perchlorate (0.2100 g, 1.01 mmol). A cream solid obtained, (0.7116 g,
1
95%). Melting point: 241 °C with decomposition. H NMR (400 MHz, DMSO-d6, 25℃): δ
= 1.11 (d, J = 6.9 Hz, 24 H, Hg-CH3), 2.81 (t, J = 13.7 Hz, J = 6.9 Hz, 4 H, Hf-CH(CH3)2),
7.11 (t, J = 8.8, J = 6.2 Hz, 2 H, Hd -C6H3), 7.16 (d, J = 6.6 Hz, 4 H, Hc-C6H3), 7.89 (d, J =
5.9 Hz, 4 H, Hb-C6H3N), 8.40 (s, 2 H, CH=N), 8.78 (d, J = 5.8 Hz 4 H, Ha-C6H3N). 13C
NMR (400 MHz, DMSO-d6, 25℃): δ = 23.15 (CH3), 27.48 (CH(CH3)3), 122.27 (C5H4N),
122.92 (C6H3), 124.50 (C6H3), 136.49 (C6H3), 142.37 (C6H3), 148.28 (C5H4N), 151.00
(C5H4N), 161.73(CH=N). FT-IR: 2962, 1639, 1614, 1526, 1462, 1427, 1320, 1061, 934 cm-
1
.
MS (ESI-TOF) m/z: calcd. for [Ag(L6)2]: 639.2617; found: 639.2623 (98%),
640.2673(30%), 641.2650(100%), 642.2075(30%). Anal. Calcd. (%) for [Ag(L6)2]ClO4: C,
58.42; H, 5.99; N, 7.57; found (%): C, 58.32; H, 5.77; N, 7.12.
2.3.14 [Ag(L6)2]OTf, 14
Synthesised by use of silver trifluoromethanesulfonate (0.2300 g, 0.90 mmol) and (E)-2,6-
diisopropyl-N-(pyridin-4-ylmethylene)aniline L6 (0.5040 g, 1.89. A lemon yellow solid,
1
(0.6511 g, 92%). Melting point 238 °C with decomposition. H NMR (400 MHz, DMSO-d6,
25℃): δ = 1.12 (d, J = 6.8 Hz, 24 H, Hg-CH3), 2.82 (t, J = 13.7 Hz, J = 6.9 Hz, 4 H, Hf-
CH(CH3)2), 7.11 (t, J = 8.8 Hz, Hz, 6.2 Hz, 2 H, He-C6H3), 7.17 (t, J = 8.8 Hz, J = 2 Hz, 4 H,
Hd-C6H3), 7.91 (d, J = 5.9 Hz, 4 H, Hb-C5H4N), 8.41 (s, 2 H, Hc-CH=N), 8.78 (d, J = 5.9
Hz, 4 H, Ha-C5H4N) ppm. 13C NMR (400 MHz, DMSO-d6, 25℃): δ = 22.58 (CH3), 27.45
(CH(CH3)2), 122.88 (C6H3), 124.46 (C5H4N), 136.47 (C6H3), 142.23 (C6H3), 148.26 (C6H3),
150.76 (C5H4N), 151.18 (C5H4N), 161.72 (CH=N) ppm. FT-IR: 3062, 2963, 2863, 1638,
1610, 1441, 1422, 1287, 1237, 1159, 1026 cm-1. Anal. Calcd. (%) for [Ag(L6)2]O3SCF3: C,
12

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56.27; H, 5.62; N, 7.09; found (%): C, 56.70; H, 5.80; N, 7.27. MS (ESI-TOF): m/z calcd.
for [Ag(L6)2]: 639.2617; found: 639.2640 (98%), 641.2637 (100%), 642.2671(40%).
2.3.15 [Ag(L6)2]NO3, 15
Synthesised by use of (E)-2,6-diisopropyl-N-(pyridin-4-ylmethylene)aniline L6, (0.2712 g,
1.02 mmol), and silver nitrate (0.0852 g, 0.5 mmol). Banana yellow solid, (0.31 g, 88%).
1
Melting point: 231 °C with decomposition. H NMR (400 MHz, DMSO-d6, 25℃): δ = 1.10
(d, J = 6.8 Hz, 24 H, Hg-CH3), 2.81 (t, J = 13.7 Hz, J = 6.8 Hz, 4 H, Hf-CH(CH3)2), 7.11 (t, J
= 8.8 Hz, J = 6.2 Hz, 2 H, He-C6H3), 7.16 (d, J = 6.6 Hz, 4 H, Hd-C6H3), 7.93 (d, J = 6.0 Hz,
4 H, Hb-C5H4N), 8.42 (s, 2 H, Hc-CH=N), 8.78 (d, J = 6.0 Hz, Ha-C5H4N) ppm. 13C NMR
(400 MHz, DMSO-d6, 25℃): 23.15 (CH3), 27.48 (CH(CH3)2), 122.26 (C6H3), 122.92 (C6H3),
124.50 (C5H4N), 136.49 (C6H3), 142.37 (C6H3), 148.28 (C5H4N), 151.0 (C5H4N), 161.73
(CH=N) ppm. FT-IR: 3050, 2958, 1609, 1305, 817, 765 cm-1. MS (ESI-TOF): m/z calcd.
for [Ag(L6)2]: 639.2617; found: 639.2624(98%), 641.2627(100%), 642.2661(40%), [AgL6]
373.0078. Anal. Calcd. (%) for [Ag(L6)2]NO3: C, 61.54; H, 6.31; N, 9.97; found (%): C,
61.92; H, 6.30; N, 9.95.
2.4
Crystal structure determination and refinement
Single crystals suitable for analysis by X-ray diffraction were selected and glued onto the tips
of glass fibre mounted on brass holders. Crystal evaluation and data collection were done on
a Bruker Smart APEX2 diffractometer with graphite monochromated Mo Kα radiation (λ =
0.71073 Å) equipped with an Oxford Cryostream low temperature apparatus operating at 100
K. The initial cell matrix was determined from three series of scans containing twelve frames
collected at intervals of 0.5° in a 6° range with the exposure time of ten seconds per frame
and the reflections indexed using the APEX2 program suite [68]. Data collection involved
the use of scans of 0.5° width with 20 seconds exposure time per frame. The total number
of images was based on results from the program COSMO, whereby the expected redundancy
was to be 4.0 and completeness of 100% out to 0.75 Å. Cell parameters were retrieved using
APEX2 and refined using SAINT on all observed reflections. Data reduction was performed
using the SAINT [68] software and the scaling and absorption corrections were applied using
SADABS [69] multi-scan technique. The structure were solved by the direct method using the
SHELXS program and refined [69]. The visual crystal structure analysis was performed using
ORTEP-3 system software [70]. Non-hydrogen atoms were first refined isotropically and
13

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then by anisotropic refinement with full-matrix least squares method based on F2 using
SHELXL [69]. All hydrogen atoms were positioned geometrically, allowed to ride on their
parent atoms and refined isotropically. All the data was subjected to the online checkCIF
evaluation criteria. The crystal data and structural refinement information are summarized in
Table 1.
Table 1. Crystal data and structure refinement for complexes 4 and 7
[Ag(L2)2]OTf, 4
[Ag(L3)2]OTf, 7
Empirical formula
Formula weight
Temperature
C18 H22 Ag F3 N2 O3 S
511.30
C37 H44 Ag F3 N4 O3 S
789.69
173(2) K
173(2) K
Wavelength
0.71073 Å
0.71073 Å
Crystal system
Space group
Monoclinic
P 21/c
Triclinic
P -1
Unit cell dimensions
a = 16.7539(7) Å
b = 15.6733(7) Å
c = 22.8836(10) Å
5994.1(5) Å3
8
a = 9.041(5) Å
b = 11.109(5) Å
c = 19.039(5) Å
1870.5(14) Å3
2
Volume
Z
1.700 Mg/m3
1.162 mm-1
3096
1.402 Mg/m3
0.650 mm-1
816
Density (calculated)
Absorption coefficient
F(000)
Crystal size (mm3)
Theta range for data collection
Index ranges
0.430 x 0.280 x 0.260
1.576 to 27.355°
-21 ≤ h≤21,
-20 ≤k ≤ 20,
-29 ≤ l ≤ 29
100039
0.380 x 0.332 x 0.305
1.083 to 28.538°
-11 ≤h ≤ 12,
-14 ≤ k ≤ 14,
-25 ≤ l ≤ 25
40390
Reflections collected
Independent reflections
Completeness to theta
25.242°
13184 [R(int) = 0.0252]
98.3 %
9333 [R(int) = 0.0256]
100.0 %
=
Refinement method
Full-matrix least-squares Full-matrix
least-
on F2
squares on F2
9333 / 0 / 442
1.127
Data / restraints / parameters
Goodness-of-fit on F2
13184 / 0 / 747
1.153
Final R indices [I > 2 sigma (I)] R1 = 0.0294, wR2 = R1 = 0.0233, wR2 =
0.0695 0.0649
R indices (all data)
R1 = 0.0360, wR2 = R1 = 0.0258, wR2 =
14

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0.0754
n/a
0.0718
Extinction coefficient
n/a
Largest diff. peak and hole (Å-
0.869 and -0.614
0.496 and -0.520
3
)
2.5
Biological studies
2.5.1 Inoculation procedure
Inoculation of the microorganism used was done in nutrient broth after sterilization. Nutrient
broth (1.3 g) was dissolved in distilled water (100 mL). The nutrient broth, ca 10 mL, was
poured into test tubes. The test tubes were plugged with cotton wool and then wrapped with
aluminium foil. The broth in test tubes was sterilized by autoclaving at 121 °C for 15 min
and then allowed to cool to 37 ℃. Thereafter the microorganisms were inoculated using
sterilized cotton swabs. The inoculums were incubated at 37 ℃ for 24 hr.
2.5.2 Preparation of Müeller-Hinton agar test plates
Müeller-Hinton agar (38 g) was dissolved in distilled water (1 L) and the resulting agar
medium was sterilized by autoclaving at 121 °C for 15 min. The medium was allowed to
cool in a water bath set at 45 °C. Approximately 25 mL of the cool agar medium were
poured into sterile flat-bottomed glass petri dishes to give a uniform depth of approximately 4
mm. The agar medium was allowed to cool to room temperature (ca. 25 °C) and set. The
test organisms were then struck on the surface of the agar medium using sterile wire loops.
The paper discs containing the 20 µg of compound under investigation were then loaded onto
the already-inoculated agar plates using sterilized forceps and then incubated at 37 ℃ for 24
hr. The diameters of the inhibition zones around the disc were measured. Each compound
was screened in triplicate and three independent experiments were performed. The minimum
inhibitory concentrations were determined through the agar dilution method.
A representative sample of each batch of plates without any microorganism was also
incubated at the same temperature for 24 hr to examine for sterility. Viability of the
organisms was tested by incubating test plates without the compounds being investigated.
Chloramphenicol was used as the standard antibacterial agent whereas fluconazole was used
in the technique as the standard antifungal agent.
15

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3.0
3.1
RESULTS AND DISCUSSION
Synthesis and characterisation of ligands
A series of pyridinyl derived Schiff bases (L1 – L6) were synthesised by an equimolar
condensation reaction between substituted aromatic anilines and pyridinealdehydes via
mechanochemical approach (Scheme 1) where, the appropriate reactants were ground
together neat for approximately 2 – 5 min then products dried under reduced pressure or in air
to remove the water produced in the reaction. The ligands were obtained as amber oils or
light yellow solids. The ligands were characterised by melting point (where applicable), IR,
1H NMR, 13C NMR, and mass spectrometry. The absence of the aldehydic carbonyl
stretching bands (1720 – 1740 cm-1) and primary amine N-H stretching bands (3400 – 3500
cm-1), and a corresponding appearance of the characteristic azomethine ѴC=N sharp bands
ranging from to 1630 to 1640 cm-1, confirmed the formation of the Schiff bases. This
conforms well to reported data of the relevant compounds [67]. The relatively weak
absorption bands ranging from 3020 - 3070 cm−1 in the ligands are due to the C–H modes
involving the aromatic ring hydrogen atoms. The C–H modes involving the aliphatic
hydrogen atoms in L2, L3, L5 and L6 appear at 2915, 2959, 2917 and 2960 cm-1,
respectively. The absorption bands with variable intensity in the frequency range 1410–1590
cm-1 correspond to ring vibrations of the pyridinyl rings of the ligands. Other bands with
various intensity below 1400 cm-1 are due to the C–C vibrations, C–H bending and C–N
stretching.
16

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Scheme 1: Solvent-free synthesis of the ligands in air at room temperature via
mechanochemistry technique. Yields are provided in parentheses.
1
The H NMR spectra of each ligand L1 – L6, in deuterated DMSO-d6 exhibited a singlet at
8.59, 8.29, 8.26, 8.67, 8.40 and 8.40 ppm respectively. These peaks are assigned to the
resonance of the azomethine (CH=N) proton. In general, the aromatic proton signals appear
1
at ca 7.00 – 8.78 ppm in the H NMR spectra of these ligands. A singlet at 2.06 ppm
indicates the presence of methyl groups in L2 and L5; a doublet at ca 1.10 ppm and a triplet
at ca 2.86 (L3) and 2.82 (L6) indicates presence of the isopropyl groups in L3 and L6. The
protons on alpha carbons with respect to N atoms are summarized in Table 2.
1
Table 2: H NMR chemical shifts of some protons in the L1 – L6 in DMSO-d6
Compound CH=N Ha-C5H4N Hb-C5H4N
8.59
8.29
8.26
8.67
8.40
8.40
8.71
8.71
8.72
8.75
8.77
8.77
7.50
753
L1
L2
L3
L4
L5
L6
7.56
7.85
7.87
7.88
17

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3.2
Synthesis and characterisation of the of complexes
Coordination of the silver(I) by the ligands was achieved by stirring one equivalent of a
silver(I) salt with 2 equivalents of a ligand in anhydrous ethanol at room temperature under
dry argon for 6 to 12hr. All complexes from silver perchlorate formed precipitates within 10
min of the reaction. The FT-IR spectrum of the complexes show some fundamental
differences when compared with those of the free ligands. The stretching frequency bands at
1590, 1589, 1591, 1591, 1592, 1592, 1593, 1591 in the FTIR spectra of complexes 1 – 8,
respectively, corresponds C=N imine bond stretching. These bands in the complexes are
Ѵ
red shifted to lower wavenumbers relative to comparable bands in free ligands. This is a
strong indicator that the Nim donor participates in coordination of the silver(I) atoms. The red
shift upon coordination is in conformity to what has been observed elsewhere for silver(I)-
Nim coordination [55, 71]. The FTIR spectral data results provide strong evidences for the
complexation of the pyridinyl Schiff bases L1 – L3 coordinate via both pyridinyl and imine
N atoms and also suggests that the complexes exist as discrete cationic complexes in the solid
state [72]. The stretching peaks for C=N imine bond in complexes 9 – 15 appear at the
Ѵ
same position like in the corresponding free ligands (either L4 – L6). The fact that these
bands do no shift indicates that the imine N atoms in complexes 9 – 15 are not involved in the
coordination of silver(I) ions. The peaks at 1072, 1061, 1083, 1067, 1076 and 1061 cm-1 in
complexes 1, 3, 6, 9, 10 and 13 correspond to the ѵ3 asymmetric stretch of free perchlorate
anions whereas respective peaks at 902, 900, 932, 926, 924 and 934 cm-1 correspond to the
Ѵ1 asymmetric stretching in the free perchlorate anions [73]. The peaks at 1271, 1258, 1254
and 1237 cm-1 in the spectra of complexes 2, 4, 7 and 11, respectively, are for the degenerate
antisymmetric stretch of the SO3 moiety in free trifluoromethanesulfonate anions. The peaks
at 1027, 1030, 1028 and 1026 cm-1 are assigned the symmetric stretching in SO3 moiety in
the triflate anion accompanying the respective complexes.
Also, the degenerate
antisymmetric stretch of the CF3 moiety are observed at 1151, 1149, 1156 and 1159 cm-1,
respectively [74]. The sharp strong peaks at 1290 – 1337 cm-1 in the IR spectra of complexes
5, 8, 12 and 15 are assigned to the nitrate anion vibration modes which agrees well with
related compounds [56, 75-77]. The absorption bands with variable intensity in the
frequency range 1422 – 1490 cm-1 correspond to the pyridinyl ring vibrations [57].
18

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Scheme 2: Synthesis of complexes 1 – 15 via magnetic stirring in anhydrous ethanol under
inert atmosphere at room temperature. Yield are provided in parentheses.
The NMR studies of the complexes were performed in deuterated d6-DMSO at room
temperature and the results are in agreement with the proposed structures. The resonances
associated with pyridinyl and phenyl moieties are unambiguously assigned from their
distinctive splitting patterns. For all the complexes reported, only one ligand pattern was
1
observed in the H NMR spectra an indication that only one type of complex species was
present in solution [78]. The spectra also indicate that in the complex, the ligands
coordinating to the metal centres are symmetrical on the NMR timescale. From the 1H NMR
spectra of the complexes, there are significant downfield shifts in the resonances of the
protons in the vicinity of N atoms involved in the coordination on comparison with free
ligands. In complexes 1 – 8, the azomethine protons and the alpha protons with respect to the
N atom in the pyridine rings are shifted downfield when compared to their resonance position
in the free ligands. This indicates that the 2-pyrdinyl derivatives, L1 – L3 coordinate the
silver(I) centres in a chelating mode. This is consistent with other reported complexes of
similar ligands [79]. Factors that contribute substantially to the change in chemical shift
between the free ligands and the coordinated ligands are the transfer of electrons from the
19

ACCEPTED MANUSCRIPT
pyridinyl rings or the imine bonds to the silver(I) ion and polarization effects [80]. These
effects are extended to the entire ligand moiety, but the alpha protons are more influenced by
coordination than the rest, hence there is a change in the resonance of the alpha protons in
going from the free ligands to the complexes. Coordination of silver(I) to the N atoms causes
a shift in the electrons from the ligands towards the metal centre, thus the alpha protons are
deshielded in the complexes hence resonate further downfield. The downfield shift is
consistent with an increase in double bond character of the C–N bond [80]. These shifts
point towards coordination of the silver(I) ions by the ligands through both the pyridinyl and
the imine N atoms. The shift in resonance of the azomethine protons in ligands L4 – L6 is
insignificant when compared to the same protons in complexes 9 – 15 an indication that the
imine N atoms in these complexes are not involved in the coordination of silver(I) centres
[81]. The chemical shifts of significant protons in the complexes are tabulated (Table 3).
The spectra of the analogues complexes with perchlorate, nitrate and
trifluoromethanesulfonate salts are all very similar, implying they have comparable solution
species, that are not affected by the counter anions [82]. The 13C NMR spectra of complexes
and free ligands show only slight changes in resonances. The azomethine carbon resonates at
159 – 163 ppm in the complexes.
Table 3: 1H NMR chemical shifts of some protons in the silver(I) complexes 1 – 15 in
DMSO-d6
complex
Ha-Py
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
8.80 8.80 8.76 8.71 8.69 8.40 8.38 8.52 8.77 8.78 8.78 8.78 8.78 8.78 8.78
9.09 9.03 8.63 8.57 8.61 8.59 8.60 8.61 8.71 8.43 8.42 8.43 8.40 8.41 8.42
CH=N
CH3
-
-
-
-
2.05 2.00 1.97 0.99 0.99 1.03
2.91 2.85 2.87
-
-
2.07 2.06 2.07 1.11 1.12 1.10
2.81 2.82 2.81
CH(CH3)2
-
-
-
-
-
-
The formation of the complexes was further confirmed by mass spectrometry. The mass
spectra of the complexes show cation molecular peaks of moieties having 1:2 metal to ligand
ratio. For instance, mass spectra for complexes 1 and 2 have a molecular cation peaks at m/z
471 [Ag(L1)2]+ (≈100%) and 289 [AgL1]+ (≈30%). Mass spectra for complexes 3 – 5 and 10
– 12 all have molecular cation peaks at m/z 527 (~100%) corresponding to [Ag(L2)2]+ in
complexes 3 – 5 and [Ag(L5)2]+ in complexes 10 – 12. Complexes 3, 5, 10 and 11 also have
a peak with m/z 317 that correspond to fragment bearing one metal and one ligand i.e.
[AgL]+. Complexes 6 – 8 have molecular cation peaks at m/z 639 (≈100%) for [Ag(L3)2]+.
Compound 6 and 8 have m/z 373 for the fragment bearing one metal and one ligand
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molecule. The molecular cation peaks at m/z 639 (≈100%) in spectra of complexes 13 – 15
correspond to [Ag(L6)]+ fragment. The purity and elemental composition of the complexes
was confirmed via elemental analysis. All experimental data obtained is consistent with the
proposed structure. The percentage composition of the elements found is within acceptable
limits (deviations <0.4%) in all complexes. The elemental analyses of the complexes are
given in Table 4. The structure of complexes 4 and 7 was further established via single
crystal X-ray diffraction experiments.
Table 4: Physical and chemical data of the silver(I) complexes 1 – 15
Anal. Calcd.
Anal. Found
ESI-TOF, [M]+
M.p
(°C)
Chemical formula
C
H
N
C
H
N
Calcd.
Found
1
2
C24H20AgClN4O4 50.42 3.53 9.80 50.53 3.29 9.80 149 471.0739 471.0760
–
150
C25H20AgF3N4O3S 48.32 3.24 9.02 48.69 3.24 8.93 136 471.0739 471.0731
–
137
3
4
C28H28AgClN4O4 53.56 4.49 8.92 53.46 4.10 8.92 202 527.1365 527.1376
C29H28AgF3N4O3S 51.41 4.17 8.27 51.67 3.85 8.17 183 527.1365 527.1360
-
184
5
6
7
C28H28AgN5O3
56.96 4.78 11.86 56.96 4.50 11.81 190 527.1365 527.1375
–
191
C36H44AgClN4O4 58.42 5.99 7.57 58.33 5.71 7.78 250 639.2617 639.2643
–
251
C37H44AgF3N4O3S 56.27 5.62 7.09 56.71 6.03 751
639.2617 639.3106
240
–
241
8
9
C18H22AgN3O3
49.56 5.08 9.63 49.26 4.57 9.53 244 639.2617 639.2624
-
245
C24H20AgClN4O4 50.42 3.53 9.80 50.73 3.36 9.69 190 471.0739 471.0760
10 C28H28AgClN4O4 53.56 4.49 8.92 53.63 4.10 8.86 210 527.1365 527.1680
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11 C29H28AgF3N4O3S 51.41 4.17 8.27 51.67 3.76 7.88 140 527.1365 527.1385
-
142
12 C28H28AgN5O3
56.96 4.78 11.86 56.82 4.60 11.77
527.1365 527.1365
157
13 C36H44AgClN4O4 58.42 5.99 7.57 58.32 5.77 7.12 241 639.2617 639.2623
14 C37H44AgF3N4O3S 56.27 5.62 7.09 56.70 5.80 7.27 238 639.2617 639.2640
15 C36H44AgN5O3
61.54 6.31 9.97 61.92 6.30 9.95 235 639.2617 639.2624
3.3 Crystal structures of complexes 4 and 7
Complexes 4 and 7 are discrete mononuclear cationic complexes with their counter anions
out of the coordination sphere. The ORTEP diagrams along with the crystallographic
numbering scheme for complexes 4 and 7, are shown in Figure 1. The asymmetric unit of
-
complex 4 consists of two mononuclear complex cations, [Ag(L2)2]+ and two CF3SO3 anions
that are out of the coordination sphere. The two cationic molecules in 4 are not related by
symmetry and an overlay of the two give an RMSD of 0.961 Å. In contrast, the asymmetric
unit in complex 7 has one cationic complex molecule [Ag(L3)2]+ and one free CF3SO3 anion.
-
This type of coordination and arrangement in the asymmetric units has been reported for
silver(I) complexes with bis(pyridyl) ligands [83, 84]. In both complexes 4 and 7, two ligand
molecules chelate the Ag(I) centres through the pyridine N (Npy) and imine N atoms (Nim).
The coordination generates two five-member rings connected by the Ag(I) centres and having
Npy—Ag—Nim angles averaging at 73.30° (Table 5), in agreement to similar Ag(I)
complexes [85]. Each of the five membered ring in the molecules is planar and are
orthogonal to each other with dihedral angles of 82.31(5) and 82.44(5)° in the two molecules
of complex 4. The angle is slightly acute, 75.72(3)°, in complex 7 but all three are
comparable to those of similar complexes [85]. Besides, the pyridine ring and the
metallacycle planes including the imine C atoms form one plane while the phenyl rings
including the imine N atoms form another plane. These sets of planes are nonplanar and their
dihedral angles in complex 4 are 80.80(7) and 77.59(5)° in one molecule while in the second
molecule the angles are 83.98(5) and 78.88(4)°. Similarly, the two sets of planes in the
molecule of complex 7 have dihedral angles of 62.76(3) and 86.15(5)°. The noticeable
differences between the two molecules in complex 4 suggest that each belong to the C1 point
group and therefore highly unsymmetrical.
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The silver(I) centres in the two compounds are tetra-coordinate via the Npy and Nim atoms
with a distorted tetrahedral geometry, with the N-Ag-N bond angles ranging from 72.51(4) to
139.07(6)° (Table 5), similar to other reported silver(I) complexes with N,N-bis(pyridyl)
ligands [59]. In the two complexes, the Ag-Npy bond lengths range from 2.2527(16) to
2.3758(16) Å and are shorter than the Ag-Nim bond distances which range from 2.2560(16) to
2.3738(16) Å (Table 3) but all fall within the expected ranges and are as good as other similar
Ag-N bond lengths of related compounds [59].
Table 5: Selected bond lengths and bond angles in complexes 4 and 7
4
7
4
7
Bond distances
Ag—Npy
2.2527(16)
2.3428(16)
2.3758(16)
2.2771(16)
2.2777(14) Ag—Nim
2.2995(14)
2.3646(16) 2.3738(15)
2.2699(16) 2.3355(12)
2.2560(16)
2.3411(16)
Bond angles
Npy—Ag—Npy 139.07(6)
135.44(6)
128.30(4) Npy—Ag—Nim 73.16(6)
73.41(6)
72.91(4)
72.51(4)
135.25(4)
138.67(4)
Nim—Ag—Nim 127.28(6)
130.95(6)
120.29(4)
73.13(6)
72.74(6
132.31(6)
120.32(6)
133.19(6
120.90(6)
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(a)
(b)
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Figure 1: The ORTEP diagrams showing the atom numbering scheme with the thermal
ellipsoids drawn at the 50% probability level for; (a) one of the two molecules in complex 4
and (b) molecule of complex 7. Hydrogen atoms have been omitted for clarity.
3.4
In vitro anti-bacteria susceptibility tests
The metal-free ligands (L1 – L6) and their silver(I) complexes (1 – 15) were screened for
their in vitro antibacterial activity against gram-negative bacteria Escherichia coli and
Salmonella typhimirium, gram-positive bacterium Staphylococcus aureus and the fungus
Candida albicans using the agar disc diffusion method. Chloramphenicol was used as a
standard for the bacteria and fluconazole used as a standard for the fungus. The antimicrobial
activities of the compounds 1 – 15 (2 mg mL-1) were obtained from the screening studies are
reported in Table 6 as averages of three measurements. Minimum inhibitory concentrations
(MIC, ꢀg mL-1) for the compounds against Escherichia coli were also determined and are
listed in Table 6.
These complexes showed mild activity against gram negative bacterium Escherichia coli.
The complexes 9 – 6, that were obtained from 4-pyridinyl ligands (L4 – L6), showed higher
activity than complexes 1 – 8 obtained from 2-pyridinyl derivatives. This could be due to the
higher stability of cationic complexes 1 – 8, which reduced the bioavailability of silver(I)
ions, hence compromising and plummeting their potency in inhibition of bacterial growth.
This stems from the proposition that biological activity of silver(I) complexes is
fundamentally dependent on the simplicity of displacing the synthetic ligands from the
complexes by ligands in biological systems such as enzymes, proteins, and membranes [37].
The mild activities recorded here indicate that these complexes are highly stable.
Consequently, their ligands (that essentially play the role of delivering silver(I) ions to the
biological system) could not be easily displaced by biological ligands like proteins and DNA,
albeit the presence Ag-N bonds [38, 86, 87]. These complex molecules are also relatively
bulky hence it’s hard for them to be inserted into DNA helical chains in the bacterial
molecules thus unavailable to disrupt the microorganisms biological processes, as a
consequence they show lesser inhibition [88].
Surprisingly, the silver(I) complexes were generally ineffective towards inhibiting bacterial
growth against the gram positive bacteria used in this study. However, inactivity of silver(I)
complexes against these microorganisms is not entirely unusual and has been reported in
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other studies [77]. The compounds studied are selective against Gram-negative bacteria as
portrayed by their good inhibition against Escherichia coli. Silver complexes have been are
reported to show relatively higher selectivity against Escherichia Coli compared to the Gram-
positive bacteria species [89]. This selectivity could have resulted from the structural
dissimilarities between Gram-positive and negative bacterial cells, though the precise causes
for the discrepancy are not yet clear. Also, Salmonella typhimirium could have shown
resistance to the complexes studied here, as has been reported in other silver(I) complexes
[15, 90, 91].
Table 6: Antimicrobial activity of the complexes 1 - 15
MIC
Diameter of inhibition zone, Ø (mm)
(
ꢀgmL-1)
complex
(2mgmL-1)
Escherichia Salmonella
Staphylococcus Candida
Escherichia
Coli
120
coli
11
11
11
10
10
7
typhimirium aureus albicans
1
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
0
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
-ve
0
+ve
-ve
2
125
3
-ve
140
4
-ve
130
5
-ve
130
6
-ve
200
7
7
-ve
180
8
11
12
12
13
14
11
8
-ve
120
9
8
80
10
11
12
13
14
15
DMSO
+ve
+ve
+ve
+ve
+ve
+ve
0
100
80
80
130
120
13
0
100
0
Chloramphenicol 20
Fluconazole Not tested
23
20
Not tested
22
30
Not tested
Not tested
Not tested
Key: ‘-ve’ no activity; ‘+ve’ low activity with an inhibition zone of diameter 6 mm
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Conclusion
The work reported herein demonstrates how pyridinyl Schiff base ligands can be synthesised
effortlessly via mechanochemistry techniques. The reactions were achieved within record
time and the target (E)-N-(pyridinylmethylene)aniline ligands (L1 - L6) obtained in excellent
yields. The (E)-N-(pyridinylmethylene)aniline ligands L1 – L6 were successfully employed
in synthesis of fifteen new structure-related silver(I)-pyridinyl complexes. The complexes
were successfully characterised using NMR and FTIR spectroscopies, mass spectrometry,
melting points, microanalysis and some by single crystal X ray diffraction technique. The
proposed molecular structures are unambiguous and perfectly agree with the analytical data
obtained. The 2-pyridinyl derivatives ligands L1 – L3 yielded complexes in which the
silver(I) ions were chelated via the pyridinyl and imine N donors. On the other hand, the 4-
pyridinyl ligands L4 – L6 coordinated only using the pyridinyl N atoms. Structure of
complexes 4 and 7 of silver trifluoromethanesulfonate with pyridin-2-ylmethyleneaniline
derivatives L2 and L3, respectively were determined by single crystal X-ray diffraction
technique. The structural motifs obtained show the complexes are mononuclear and cationic
with well separated anions. This coordination pattern corresponds to known modes of this
type of ligand towards silver(I), in which the ligands chelate the metal centres via the
pyridinyl and imine N atoms. The complexes showed moderate antibacterial activity against
bacterium Escherichia Coli, but were inactive against bacteria Staphylococcus aureus and
Salmonella typhimirium. The activities of the silver(I)-pyridinyl Schiff base complexes
reported here are attributable to ligand substitution with the biological ligands such as
proteins and DNA. The moderate activities portray high stability of the complexes especially
the chelates. Thus, their ligands were thus difficult to dislodge by the biological ligands and
silver(I) ions were not readily bioavailable. The complexes showed high selectivity against
gram Escherichia coli. The antimicrobial results underpin the need to balance between
ambience and photo stability of silver(I) complexes against their dissolution and transport in
biological systems. Synthesis of enzyme targeted ligands and their silver(I) complexes as
well as structure activity relationships is desired to further understand the bioactivity and
resistance portrayed.
Supplementary data
CCDC 1498656 and 1498657 contains the supplementary crystallographic data for
compounds 4 and 7 respectively. This data is available free of charge from the CCDC.
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Acknowledgement
The authors acknowledge the financial support by University of KwaZulu-Natal and National
Research Foundation (South Africa). We are grateful to Mr Sizwe Joshua Zamisa for his
vital assistance in single X-ray experiments.
28