Synthesis, structures and antimicrobial activity of 5-nitro-salicylaldehyde-thiosemicarbazonates of zinc(II) coordinated to substituted bipyridines/phenanthrolines

Article abstract of DOI:10.1016/j.poly.2018.03.027

Reactions of zinc(II) with 5-nitro-salicylaldehyde-N¹-substituted thiosemicarbazones {(5-NO₂-2-HO–C₆H₄)C²(H)[dbnd]N³–N²H–C¹([dbnd]S)–N¹HR; R = H, H₂

Full text of DOI:10.1016/j.poly.2018.03.027

Accepted Manuscript
Synthesis, structures and antimicrobial activity of 5-nitro-salicylaldehy- de-thi-
osemicarbazonates of zinc(II) coordinated to substituted bipyridines / phenan-
throlines
Mani Kaushal, Shikha Indoria, Tarlok S. Lobana, Henna Sood, Daljit S. Arora,
Geeta Hundal, Victoria A. Smolinski, Manpreet Kaur, Jerry P. Jasinski
PII:
S0277-5387(18)30152-9
https://doi.org/10.1016/j.poly.2018.03.027
POLY 13093
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
22 January 2018
25 March 2018
Please cite this article as: M. Kaushal, S. Indoria, T.S. Lobana, H. Sood, D.S. Arora, G. Hundal, V.A. Smolinski,
M. Kaur, J.P. Jasinski, Synthesis, structures and antimicrobial activity of 5-nitro-salicylaldehy- de-
thiosemicarbazonates of zinc(II) coordinated to substituted bipyridines / phenanthrolines, Polyhedron (2018), doi:
https://doi.org/10.1016/j.poly.2018.03.027
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Synthesis, structures and antimicrobial activity of 5-nitro-salicylaldehy-
de-thiosemicarbazonates of zinc(II) coordinated to substituted
bipyridines / phenanthrolines
Mani Kaushal,a Shikha Indoria,*a Tarlok S. Lobana,*a† Henna Sood,b Daljit S. Arora,b Geeta
Hundal,a Victoria A. Smolinski,c Manpreet Kaur,c and Jerry P. Jasinskic
aDepartment of Chemistry, Guru Nanak Dev University, Amritsar-143 005, India.
bDepartment of Microbiology, Guru Nanak Dev University, Amritsar-143 005, India
cDepartment of Chemistry, Keene State College, Keene, NH 03435 USA
*†Corresponding author: E-mail: tarlokslobana@yahoo.co.in (Tarlok Singh Lobana); Fax: 91-
183-2258820
A B S T R A C T
Reactions of zinc(II) with 5-nitro-salicylaldehyde-N1-substituted thiosemicarbazones {(5-NO2-2-
HO–C6H4)C2(H)=N3–N2H–C1(=S)–N1HR; R = H, H2L1; Me, H2L2; Et, H2L3; Ph, H2L4} and 4,
4'-dimethyl-2, 2'-bipyridine (dm-bipy), 2, 9-dimethyl-1, 10-phenanthroline (dm-phen) and 3, 4,
7, 8-tetramethyl-1,10-phenanthroline (tm-phen) as co-ligands, have yielded complexes of
stoichiometry, [Zn(Ln)(L)] {n = 1-4; L = dm-bipy, 1, 4, 7, 10; dm-phen, 2, 5, 8, 11; tm-phen, 3,
6, 9, 12) characterized by elemental analysis, infrared and electronic absorption spectroscopy and
single crystal X-ray crystallography. Complexes 9 and 10 have distorted trigonal bipyramidal
geometry ( = 0.529 - 0.580), while complexes 5, 8 and 11 have distorted square pyramidal
geometry ( = 0.004 – 0.250).They displayed fluorescence bands at max = 430-440 nm. In
comparison to unsubstituted bipyridines/ phenanthrolines, these zinc(II) complexes have shown
higher antimicrobial activity with low minimum inhibitory concentration (MIC) against the
clinical isolate methicillin resistant Staphylococcus aureus (MRSA),Gram positive bacteria,
namely, Staphylococcus aureus (MTCC740), Enterococcus faecalis (MTCC439), Gram negative
1

bacteria, namely, Klebsiella pneumonia 1 (MTCC109), Escherichia coli (MTCC119),
Salmonella typhimurium 1(MTCC98) and one yeast strain Candida albicans (MTCC227). These
complexes tested were found to be cytotoxic to microorganisms (bactericidal/fungicidal).
1. Introduction
The biochemistry of zinc(II) is very important due to the role of this metal in a number of
enzymes such as zinc proteinases, carbonic anhydrous, histone deacetylase, alcohol
hydrogenases, alkaline phosphates and amino peptidases which are involved in various metabolic
processes of plants, animals, viruses and bacteria [1-5]. Among various N, S- donor thio-ligands,
thiosemicarbazones (Chart 1) constitute an important category of ligands which have formed a
variety of metal complexes and have also shown promising biochemical activity [6-10]. In the
literature, the co-ordination compounds of thiosemicarbazones with zinc(II) have shown
antimicrobial [11-14], anticancer [15-17] and antioxidant activities [18]. The antimicrobial
activity has been studied when R1 substituent at C2 atom of thiosemicarbazone was 2-
thiopheneyl [11], 2-acetylpyridine [12,13], furan [14], or 2-acetylbutyrolactone [14] and the
substituent R2 was hydrogen, methyl at C2 carbon and finally NR3R4 was NH2, NHR (R = Me,
Et, Pr, Ph etc.) at N1 nitrogen. It was noted from the literature that co-ordination compounds of
zinc(II) investigated have shown poor antimicrobial activity at high minimum inhibitory
concentration (MIC) [11-14].
S
R1
1
C
2
NH
C2 N3
N1R3R4
R2
Chart 1. General structure of thiosemicarbazone
In view of the above mentioned interests and observations, recently from our laboratory
antimicrobial activity of co-ordination compounds of Zn(II) coordinated to 5-nitro-
salicylaldehyde-N-substituted thiosemicarbazones and bipyridine (bipy) / 1,10- phenanthroline
(phen) co-ligands (Chart 2) has been reported [19]. It was found that these co-ordination
compounds have significant antimicrobial activity against Staphylococcus aureus (MTCC740),
methicillin resistant Staphylococcus aureus (MRSA), Klebsiella pneumonia 1 (MTCC109),
2

Shigella flexneri (MTCC1457), Salmonella typhimurium 1(MTCC98) and Candida albicans
(MTCC227) [19]. Notably, the bio-activity against K. pneumoniae and S. typhimurium was an
important outcome of the investigations, while in literature co-ordination compounds reported
were found to be inactive [20]. The in vitro cell viability studied using MTT assay {MTT stands
for 3-[(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl] tetrazolium bromide} was found to be low.
H
13
12
7
14
OH
S
11
12
13
8
H
N2
14
6
11
9
15
16
N3
10
15
C
2
C
10
3
O2N
N
5
N
4
9
N1HR
N
N
16
H
bipy
phen
R = H, Me, Et, Ph
Chart 2. 5-nitro-salicyldehydethiosemicarbazones and co-ligands
In this paper we report the antimicrobial activity of 5-nitro-salicyaldehyde-N1-substituted
thiosemicarbazonates of zinc(II) coordinated to chelating dm-bipy, dm-phen and tm-phen ligands
against Staphylococcus aureus (MTCC740), methicillin resistant Staphylococcus aureus
(MRSA), Klebsiella pneumonia 1 (MTCC109), Enterococcus faecalis (MTCC439), Escherichia
coli (MTCC119), Salmonella typhimurium 1(MTCC98) and Candida albicans (MTCC227)
(Chart 3). The main focus of this study is to observe the effect of substituents (R) at N1 atom of
thiosemicarbazones as well as that of methyl substituents in the rings of bipyridine and 1,10-
phenanthroline on the antimicrobial activity and cell viability of zinc(II) complexes.
13
12
H3C
CH3
14
14
11
9
11
12
13
15
16
15
16
10
H
10
9
7
N
N
N
OH
S
N
8
H
N2
6
H3C
CH3
2
C
N3
dm-bipy
dm-phen
C
3
O2N
5
4
N1HR
13
12
H
CH3
H3C
11
14
10
15
R = H, Me, Et, Ph
H3C
CH3
N
N
16
9
tm-phen
Chart 3. 5-Nitro-salicylaldehyde thiosemicarbazones with dm-bipy, dm-phen
and tm-phen as co-ligands
2. Experimental
3

Zinc(II) acetate dihydrate, thiosemicarbazide, N-methyl-thiosemicarbazide, N-ethyl-
thiosemicarbazide, N-phenyl-thiosemicarbazide, 5-nitro-salicylaldehyde, 4, 4′-dimethyl-2,2'-
bipyridine (dm-bipy), 2, 9-dimethyl-1,10-phenanthroline (dm-phen) and 3,4,7,8-tetramethyl-
1,10-phenanthroline (tm-phen) were procured from Aldrich Sigma Ltd. The thio-ligands (Chart
1) were prepared as reported earlier [21-24]. Elemental analysis for C, H and N were carried out
with a thermoelectron FLASHEA1112 analyzer. Melting points were determined with a
Gallenkamp electrically heated apparatus. IR spectra of the compounds were recorded in the
4000 – 450 cm-1region with a Perkin Elmer FT-IR Spectrometer by making their KBr pellets.
UV-visible spectra of the compounds (10-3-10-4 M) were recorded in dimethylsulfoxide (dmso)
with the help of a UV-1601 PC Shimadzu spectrophotometer. Fluorescence spectra of the
complexes (10-4 M) were recorded with a Varian Cary Eclipse Fluorescence spectrophotometer.
The 1H NMR spectra were recorded on Bruker Avance 500 MHz NMR spectrometer in CDCl3-
DMSO mixture (9 : 1) with TMS as the internal reference.
2.1. Synthesis of complexes
2.1.1. [Zn(L1)(dm-bipy)] (1). To a pale yellow solution of thio-ligand, H2L1 (0.027 g, 0.11
mmol) in acetonitrile (10 mL) was added white solid Zn(OAc)2·2H2O (0.025 g, 0.11 mmol).
The reaction mixture was stirred for 15 minutes which yielded white precipitate. To the
white precipitate, solution of dm-bipy (0.021 g, 0.11 mmol) in dichloromethane (10mL) was
added and the contents were again stirred for 15 min. A clear orange solution formed was
allowed to evaporate at room temperature which yielded an orange compound. Yield: 0.039g,
71%, m.p. 210-212°C. Anal. Calc. for C20H18N6O3SZn: C 49.24; H 3.69; N 17.23; S 6.57;
Found: C 50.14; H 3.81; N 17.46; S 6.63 %. IR (cm-1, KBr): ν(N1–H) 3421 s; ν(C–H) 3109
w, 3062 w, 2921 w; 1648 s, 1601 s; νas(N-O) 1547 s; 1492 s, 1437 w, 1414 w; νs(N-O) 1320
s; 1234 m, 1189 w, 1093 s, 935 m, 831 m; ν(C–S) 757 s; 726 s, 640 w, 492 w, 421 w.
Electronic absorption spectrum, DMSO, λmax /nm, ε / L mol-1 cm-1 : [10-4 M] 424 (1.00 x 104),
389 (1.74 x 104), 284 (1.45 x 104). Fluorescence spectrum: λmaxem = 434 nm; λmaxex = 300 nm.
1H NMR (, ppm; CDCl3: DMSO; 9:1):  = 8.37 (2H, s, C9Hdm-bipy + C16Hdm-bipy), 8.18(1H,
m, C2H), 8.01 (2H, m, C4H + C7H), 7.80 (2H, s, C12Hdm-bipy + C13Hdm-bipy), 7.72 (2H, s,
4

C10Hdm-bipy + C15Hdm-bipy), 7.33 (2H, d, N1H2), 6.40 (1H, s, C6H), 2.98 (6H, C11, 14H3).
Complexes 2-12 were prepared by a similar method.
2.1.2. [Zn(L1)(dm-phen)] (2). Yield: 0.044g, 70%, m.p. 221-223°C. Anal. Calc. for C22H18N6O3 -
SZnCH3CN: C 52.13; H 3.80; N 17.74; S 5.79; Found : C 51.90; H 4.01; N 17.56; S 6.08 %. IR
(cm-1, KBr) : ν(N1–H) 3432 sb, 3384 s; ν(C–H) 3156 w, 3098 w, 2926 w, 2858 w; 1691 s; νas(N-
O) 1547 m; δ(C−H) 1521 w, 1490 s, 1439 m; νs(N-O) 1306 s; 1243 w, 1191 w, 1169 w, 1097
w, 1008 w, 951 w; 805 w; ν(C–S) 750 m; 654 w, 621 w, 514 w, 440 w, 414 w. Electronic
absorption spectrum, DMSO, λmax / nm, ε / L mol-1 cm-1 : [10-4 M] 421 (1.01 x 104), 380 (1.76
x 104), 301 (1.55 x104), 271 (3.97 x 104). Fluorescence spectrum: λmaxem = 435 nm; λmaxex = 300
nm. 1H NMR (, ppm; CDCl3: DMSO; 9:1):  = 8.95 (2H, s, C9Hdm-phen + C16H2 C9Hdm-phen), 8.54
(1H, s, C2H), 8.11(2H, d, C4H + C7H), 7.83 (1H, s, N1H2), 7.73 (2H, s, C12H C9Hdm-phen
+
C13Hdm-phen), 7.47 (2H, m, C10Hdm-phen + C15Hdm-phen), 6.27 (1H, s, C6H), 2.78 (6H, C9, 16H3).
2.1.3. [Zn(L1)(tm-phen)] (3). Yield: 0.046 g, 75%, m.p. 214-216°C. Anal. Calc. for
C24H22N6O3SZn: C 53.39; H 4.07; N 15.57; S 5.93 %; Found: C 53.51; H 4.23; N 15.69; S 6.04
%. IR (cm-1, KBr) : ν(N1–H) 3477, 3328 m b; ν(C–H) 3156 w, 3062 w, 2930 w; 1609s; νas(N-O)
1547s; δ(C−H) 1500 s, 1422 m; 1383 m; νs(N-O) 1305 s; 1250 w, 1188 w, 1156 m, 1094 w,
953 w, 896 w; ν(C–S) 773 m; 726 w, 680 s, 664 s, 625 w, 547 w, 499 w, 460 w, 390 w.
Electronic absorption spectrum, DMSO, λmax /nm, ε / L mol-1 cm-1 : [10-4 M] 419 (1.04 x 104);
em
387 (1.79 x 104), 298 (1.58 x 104), 271(3.22 x 104). Fluorescence spectrum: λmax = 435 nm;
1
λmaxex = 300 nm. H NMR (, ppm; CDCl3: DMSO; 9:1):  = 8.50 (1H, s, C2H), 8.43(2H, m,
C9Hdm-phen + C16H dm-phen), 8.06(1H, s, N1H), 7.93 (2H, m, C4H + C7H), 7.78 (2H, s, C12H dm-phen
+ C13H dm-phen), 6.28(1H, s, C6H), 2.90 (6H, s, C10, 15H3), 3.07 (6H, s, C11, 14H3).
2.1.4. [Zn(L2)(dm-bipy)]CH2Cl2 (4). Yield: 0.047 g, 70%, m.p. 198-200°C. Anal. Calc. for
C21H20N6O3SZnCH2Cl2: C 45.02; H 3.75; N 14.32; S 5.46; Found:C 44.89; H 3.51; N 14.65; S
5.29%. IR (cm-1, KBr) : ν(N1–H) 3390s; ν(C–H) 3130 w, 3061 s, 2983 m, 2936 w, 2851w;
1609s; νas(N-O) 1559 s; δ(C−H) 1515 s, 1491 s, 1436 m; 1335 m; νs(N-O) 1302 s; 1257 s, 1171
s, 1093 s, 1019 m, 952 m; ν(C–S) 750 m; 710 w, 663 m, 553 m, 499 w, 452 w. Electronic
absorption spectrum, DMSO, λmax /nm, ε / L mol-1 cm-1 : [10-4 M] 425 (1.07 x 104), 386 (1.74
5

x 104), 283 (1.59 x 104). Fluorescence spectrum: λmaxem = 437 nm; λmaxex = 300 nm. 1H NMR (,
ppm; CDCl3: DMSO; 9:1):  = 8.41 (2H, d, C9Hdm-bipy + C16Hdm-bipy), 8.36 (2H, d, C12Hdm-bipy
+
C13Hdm-bipy), 8.07 (3H, d, C2H+ C4H + C7H), 7.97(2H, s, C10Hdm-bipy + C15Hdm-bipy), 7.75 (1H, d,
N1H), 6.40 (1H, s, C6H), 2.98 (6H, C11,14H3), 3.12 (3H, m, N1(CH3)).
2.1.5. [Zn(L2)(dm-phen)]CH3CN (5).Yield: 0.046 g, 72%, m.p. 193-194°C. Anal. Calc. for
C23H20N6O3SZnCH3CN: C 52.97; H 4.06; N 17.30; S 5.65; Found: C 52.79; H 3.93; N 16.91; S
5.33 %. IR (cm-1, KBr) : ν(N1–H) 3400 s; ν(C–H) 3070 w, 3000 w, 2930w; 1593s; νas(N-O)
1548s; δ(C-H) 1485s, 1437s; 1398m, 1375m; νs(N-O) 1305 s; 1250 s, 1194m, 1093s, 944w, 919
m, 883 s; ν(C–S) 760s; 646m, 602 m, 497m, 441m. Electronic absorption spectrum, DMSO,
λmax /nm, ε / L mol-1 cm-1 : [10-4 M] 430 (9.38 x 103), 388 (1.46 x 104), 294 (1.05 x 104),
em
263(2.25 x 104). Fluorescence spectrum: λmax = 436 nm; λmaxex = 300 nm. 1H NMR (, ppm;
CDCl3: DMSO; 9:1):  = 8.60 (1H, s, C2H), 8.47 (2H, d, C11Hdm-phen + C14Hdm-phen), 8.10 (1H, m,
N1H), 7.95 (2H, m, C4H+C7H), 7.95 (2H, m, C12Hdm-phen + C13Hdm-phen), 7.79 (2H, d, C10Hdm-phen
C15Hdm-phen), 6.36 (1H, s, C6H), 3.11 (3H, d, (CH3)N1), 2.96 (6H, d, C9,16H3).
+
2.1.6. [Zn(L2)(tm-phen)] (6). Yield: 0.049 g, 78%, m.p. 205-207°C. Anal. Calc. for
C25H24N6O3SZn: C 54.21; H 4.34; N 15.18; S 5.78 %; Found: C 54.18; H 4.12; N 15.30; S 5.53
%. IR (cm-1, KBr) : ν(N1–H) 3390 m b; ν(C–H) 3078w, 2930 w, 2838 w; 1609 s; νas(N-O) 1533
s; δ(C-H) 1485 s, 1430 s; 1398 m; νs(N-O) 1305 s; 1250 s, 1094 s, 1023 w, 952 w, 890 w, 820
m; ν(C–S) 720 s; 687 s, 609 w, 531 w 492 w. Electronic absorption spectrum, DMSO, λmax
/nm, ε / L mol-1 cm-1 : [10-4 M] 427 (1.03 x 104), 387 (1.93 x 104), 303 (1.75 x 104), 270 (2.89
em
x104). Fluorescence spectrum : λmax = 434 nm; λmaxex = 300 nm. 1H NMR (, ppm; CDCl3:
DMSO; 9:1):  = 8.97(2H, s, C9Htm-phen + C16Htm-phen), 8.59 (2H, d, C12Htm-phen+ C13Htm-phen),
8.37 (1H, d, C2H), 8.22 (2H, m, C4H+C7H), 8.10 (1H, m, N1H), 6.70 (1H, s, C6H), 2.98 (6H, d,
C11, 14H3), 2.79 (6H,d, C10,15H3), 3.17 (3H, s, (CH3)N1).
2.1.7. [Zn(L3)(dm-bipy)] (7). Yield: 0.044 g, 75%, m.p. 191-193°C. Anal. Calc. for
C22H22N6O3SZn: C 51.22; H 4.30; N 16.29, S 6.22; Found: C 51.43; H 4.17; N 15.95, S 6.17 %.
IR (cm-1, KBr) : ν(N1–H) 3375 s, ν(C–H) 3075 w, 2992 w, 2922 w,2867 w; 1601 s; νas(N-O)
1554 m; δ(C−H) 1485 s, 1430 m; νs(N-O) 1305 s; 1235 m, 1196 w, 1164 w, 1101 m, 1030 w,
953 w, 820 s, ν(C–S) 757 w; 718 w, 680 m, 640 w, 547 w, 531 w, 421 w. Electronic absorption
6

spectrum, DMSO, λmax /nm, ε / L mol-1 cm-1 : [10-4 M] 426 (1.02 x 104), 387 (1.82 x 104), 275
em
ex
1
(1.29 x 104). Fluorescence spectrum: λmax = 437 nm; λmax = 300 nm. H NMR (, ppm;
CDCl3: DMSO; 9:1):  = 8.41 (2H, m, C9Hdm-bipy + C16H dm-bipy), 8.36 (2H, m, C12H dm-bipy+ C13H
dm-bipy), 8.16 ( 1H, s, C2 H), 8.01 (2H, d, C4H + C7H), 7.75 (1H, d, N1H), 7.69 (2H, d, C10H dm-bipy
+ C15H dm-bipy), 6.32 (1H, s, C6H), 3.41 (2H, m, N1(CH2), 3.28(6H, m, C11,14H3), 1.03 (3H, m,
CH3).
2.1.8. [Zn(L3)(dm-phen)]CH3CN (8).Yield: 0.049 g, 74%, m.p. 196-198°C. Anal. Calc. for
C24H22N6O3SZnCH3CN: C 53.75; H 4.34; N 16.88; S 5.52 %.; Found: C 53.51; H 4.07; N
16.43; S 5.43 %. IR (cm-1, KBr) : ν(N1–H) 3398 m b; ν(C–H) 3053 w, 2975 w, 2931 w, 2853 w;
1600 s; νas(N-O) 1541m; δ(C-H) 1490 s, 1417 m; 1375 w, νs(N-O) 1304 s; 1234 m, 1195 w,
1151 w, 1100 w, 958 w, 862 s; ν(C–S) 758 m; 683 s, 657 s, 535 w, 497 w, 439 w. Electronic
absorption spectrum, DMSO, λmax /nm, ε / L mol-1 cm-1 : [10-4 M] 429 (1.18 x 104), 388 (2.10 x
em
104), 308 (7.22 x 103), 284 (1.58 x 104). Fluorescence spectrum: λmax = 440 nm; λmaxex = 300
nm.  = 8.59 (2H, s, C11Hdm-phen+ C14Hdm-phen), 8.50 (3H, d, C2H+ C12 Hdm-phen+ C13Hdm-phen),
8.13 (2H, d, C4H+C7H), 7.99 (1H, s, N1H), 7.79 (2H, m, C10Hdm-phen+ C15Hdm-phen), 6.35 (1H, d,
C6H), 3.47 (2H, m, N1(CH2), 3.12 (6H, m, C9,16H3), 1.19 (3H, m, CH3).
2.1.9. [Zn(L3)(tm-phen)] (9). Yield: 0.049 g, 71%, m.p. 204-206°C. Anal. Calc. for
C26H26N6O3SZnCH3CN: C 55.22; H 4.80; N 16.10; S 5.26 %; Found: C 55.31; H 4.58; N 16.24;
S 5.23 %. IR (cm-1, KBr) : ν(N1–H) 3375 m,b; ν(C–H) 3054 w, 2985 w, 2921 w; 1617 s; νas(N-
O) 1554 m; 1537 w; δ(C-H) 1476 s, 1422 m; νs(N-O) 1327 s; 1250 m, 1127 w, 1093 m, 1023 w,
952 w, 882 w, 820 s; ν(C–S) 735 s; 671 s, 515 w, 429 w. Electronic absorption spectrum,
DMSO, λmax /nm, ε / L mol-1 cm-1 : [10-4 M] 430 (8.38 x 103), 385 (1.20 x 104), 305 (1.40 x
em
1
104), 275 (2.49 x 104). Fluorescence spectrum: λmax = 440 nm; λmaxex = 300 nm. H NMR (,
ppm; CDCl3: DMSO; 9:1):  = 8.64 (2H, s, C9Htm-phen+ C16Htm-phen), 8.57 (1H, d, C2H), 8.47
(2H, d, C12Htm-phen+ C13Htm-phen), 8.21 (2H, s, C4H+C7H), 8.10 (1H, m, N1H), 6.70 (1H, s, C6H),
3.11 (6H, d, C11,14H3), 2.81 (6H,d, C10,15H3), 3.51 (2H, s, CH2N1), 1.21 (3H, m, CH3).
2.1.10. [Zn(L4)(dm-bipy)] (10). Yield: 0.045 g, 70%, m.p. 221-223 °C. Anal. Calc. for
C26H22N6O3SZn: C 55.37; H 3.93; N 14.90; S 5.69 %; Found: C 55.46; H 3.62; N 15.12; S 5.80
%. %. IR (cm-1, KBr) : ν(N1–H) 3390 m b; ν(C–H) 3062 w, 2992 w, 2862 w; 1609 m; νas(N-O)
7

1547s; δ(C-H) 1476 s, 1437 s; νs(N-O) 1312 s; 1250 m, 1171 s, 1101s, 1015 w, 966 w, 836 w;
ν(C–S) 757 m; 695 s, 648 m, 562 w, 515 w, 438 m. Electronic absorption spectrum, DMSO,
λmax /nm, ε / L mol-1 cm-1 : [10-4 M] 394 (1.37 x 104), 380 (1.26 x 104), 284 (1.79 x 104).
1
Fluorescence spectrum: λmaxem = 438 nm; λmaxex = 300 nm. H NMR (, ppm; CDCl3):  = 9.66
(2H, s, C9Hdm-bipy + C16Hdm-bipy), 8.66 (2H, d ,C12Hdm-bipy+ C13Hdm-bipy), 8.41 (1H, s, C2H), 8.14
(3H, m, C4H + C7H +N1 H), 7.71 (2H, d, C10Hdm-bipy+ C15Hdm-bipy), 7.44 (2H, d, m-Hph),
7.27(2H, m, m-Hph), 6.94 (1H, m, p-Hph), 6.77 (1H, d, C6H), 3.00 (6H, s, C11,14H3) .
2.1.11. [Zn(L4)(dm-phen)]CH3CN (11).Yield: 0.052g, 73%, m.p. 216-218°C. Anal. Calc. for
C28H22N6O3SZnCH3CN: C 57.28; H 4.01; N 15.59; S 5.10 %; Found: C 57.33; H 4.07; N 15.61;
S 5.29 %. IR (cm-1, KBr) : ν(N1–H) 3360 br; ν(C–H) 3078 w, 3023 w, 2921 br, 2831 w; 1648 s,
1601 s; νas(N-O) 1547 s; δ(C−H) 1492 s, 1414 s; 1312 s; νs(N-O) 1312 s; 1257 m, 1164 m, 1101
m, 945 s, 870 s; ν(C–S) 749 m; 735 s, 694 m, 655 w, 554 w, 515 w, 414 w. Electronic
absorption spectrum, DMSO, λmax /nm, ε / L mol-1 cm-1 : [10-4 M] 425 (2.06 x 104), 376 (1.89 x
em
104), 298(1.07 x 104), 271(2.39 x 104). Fluorescence spectrum: λmax = 438 nm; ex = 300 nm.
1H NMR (, ppm; CDCl3:DMSO, 9:1):  = 9.93 (2H, s, C11Hdm-phen + C14Hdm-phen), 8.75 3H, m,
C2H +C12Hdm-phen+ C13Hdm-phen), 8.51 (2H, m, C12Hdm-phen+ C13Hdm-phen), 8.01 (2H, m, C4H +
C7H), 7.82 (1H, m, N1 H), 7.68 (2H, m, C10Hdm-phen+ C15Hdm-phen), 7.41 (2H, d, m-Hph), 7.24(2H,
m, m-Hph), 6.91 (1H, m, p-Hph), 6.34 (1H, d, C6H), 3.05 (6H, s, C9,16H3).
2.1.12. [Zn(L4)(tm-phen)]CH3CN (12). Yield: 0.051 g, 69%, m.p. 224-226°C. Anal. Calc. for
C30H26N6O3SZnCH3CN: C 58.49; H 4.45; N 14.92; S 4.88 %; Found: C 58.67; H 4.58; N 15.03;
S 5.02 %. IR (cm-1, KBr) : ν(N1–H) 3382 m, b; ν(C–H) 3061 w, 2936 w, 2861 w; 1639 s, 1609
s; νas(N-O) 1562 s; δ(C−H) 1494 s, 1429 s, 1406 s; νs(N-O) 1328 s; 1242 m, 1171 s, 1101 s,
1093 s, 921 s; 865 s; ν(C–S) 757 m; 725 m, 631 m, 491 m, 452 w. Electronic absorption
spectrum, DMSO, λmax /nm, ε / L mol-1 cm-1 : [10-4 M] 421 (1.89 x 104), 371(1.66 x 104), 305
em
1
(1.12 x 104), 280(2.68 x 104). Fluorescence spectrum: λmax = 439 nm; ex = 300 nm. H NMR
(, ppm; CDCl3:DMSO, 9:1):  = 9.52 (2H, s, C9Htm-phen + C16Htm-phen), 8.82 (1H, s, C2H), 8.68
(2H, s, C4H + C7H), 8.60 (2H, m, C12Htm-phen+ C13Htm-phen), 8.31(1H, m, N1 H), 8.11 (2H, d, m-
Hph), 7.71 (2H, m, o-Hph), 7.19 (1H, m, p-Hph), 6.64 (1H, d, C6H), 2.92 (6H, m, C11,14H3), 2.72
(6H, m, C10,15H3).
8

2.2. X-ray crystallography
X-ray data of complexes were collected on a Bruker’s Apex-II CCD diffractometer (5, 8, 9;
296(2) K) using Mo Kα (λ = 0.71069 Å) and with an Agilent Eos (Gemini) diffractometer (10,
11; 173(2) K) using Cu-Kα (λ = 1.54178 for 10, 11) radiation. The data for 5, 8 and 9 were
processed by SAINT and that for 10 and 11 were processed with CrysAlisPro (data collection)
and CrysAlisPro RED (cell refinement and data reduction). Lorentz and polarization effects and
empirical absorption corrections were applied using SADABS. The structures of 5, 8 and 9 were
solved by direct methods, using SIR-92 [25] and refined by full-matrix least squares refinement
methods based on F2, using SHELX-2017 [26]. The hydrogen atoms of nitrogen were located
from the difference Fourier synthesis and were refined isotropically with a distance of 0.84 Å
with Uiso values 1.2 times that of their carrier nitrogen atoms. All non-hydrogen atoms were
refined anisotropically and all hydrogen atoms were fixed geometrically with their Uiso values
1.2 times of phenyl carbons and in case of methyl carbons Uiso values were fixed 1.5 times of
methyl carbons. WE couldn’t resolve disorder in some atoms of complexes 8 and 9 (acetonitrile
molecule in complex 8, and terminal methyl and nitro group groups in 9). All calculations were
performed using Wingx package [27]. The structures of 10 and 11 was solved by direct methods
using Superflip (compound 10) or SHELXS-97(compounds 11), refined by full-matrix least-
squares techniques against F2 using SHELXL-97 and molecular graphics from SHELXTL.
Atomic scattering factors were taken from the International Tables for Crystallography. All non-
hydrogen atoms were refined anisotropically and the hydrogens were fixed geometrically [28-
32].
2.3. Antimicrobial studies
2.3.1. Test organisms and Inoculum preparation : The reference strains of bacteria and yeasts
were obtained from the Microbial Type Culture Collection (MTCC), Institute of Microbial
Technology (IMTECH), Chandigarh, India and the clinical isolate methicillin resistant
Staphylococcus aureus (MRSA) was obtained from the Post graduate Institute of Medical
Education and Research, (PGIMER), Chandigarh, India. Reference strains included Gram
9

positive bacteria, Staphylococcus aureus (MTCC740), Enterococcus faecalis (MTCC439), Gram
negative bacteria, Klebsiella pneumonia 1 (MTCC109), Escherichia coli (MTCC119),
Salmonella typhimurium 1(MTCC98), and one yeast strain,Candida albicans (MTCC227). A
loopful of isolated bacterial and yeast colonies were inoculated into 5 mL of their respective
medium and incubated at 37 °C and 25 °C, respectively, for 4 h. This was used as inoculums
after adjusting the turbidity as per the Mc Farland turbidity standard. This turbidity is equivalent
to approximately 1 to 2 × 108 colony forming units per ml (CFU/ml). The inoculums thus
prepared was used further for further testing.
2.2.2. Screening of Compounds for antimicrobial activity by agar well diffusion assay:
Sensitivity of different bacterial strains to different compounds was measured in terms of zone of
inhibition using an agar well diffusion assay [33]. All the compounds were dissolved in dimethyl
sulfoxide (dmso) to a final concentration of 0.1%. The plates containing Muller Hinton agar
medium, yeast malt agar and Sabouraud agar were spread with 50 µL of the bacterial and yeast
inoculums, respectively. Wells (6 mm diameter) were cut out from agar plates using a sterilized
stainless steel cork borer and filled with 50 µL of the compound. The plates were incubated at
37˚C and 25˚C for 24 h and the diameter of the resultant zone of inhibition was measured.
Experiments were run in duplicate for each combination of extract and microbial strains. The
results were compared with a control in which compounds were replaced with dmso. In order to
compare the effectiveness of the compounds, their activities were compared with a standard
antibiotic, namely, Gentamicin (1 mg/ mL) for bacterial culture and Amphotericin (1 mg/ mL)
for yeast culture. Gentamicin acts as a positive control against bacteria (MRSA, S. aureus, K.
pneumoniae , E. faecalis, E. coli, S. typhimurium) and Amphotericin acts as a positive control
against yeast (Candida albicans).
2.2.3. Minimum inhibitory concenteration (MIC): Minimum inhibitory concentration of the
selected compounds was worked out by the agar dilution method [34]. A stock solution of (5
mg/mL) concentration was prepared and incorporated into a Muller Hinton agar medium for
bacteria and yeast malt extract medium for yeast. The final concentrations of the compound in
the medium containing plates ranged from (0.005-3 mg/mL). These plates were then inoculated
with 0.1 mL of the activated bacterial and yeast strains by streaking with a sterile tooth pick. The
plates were incubated at 37 °C for bacteria and 25 °C for yeast for 24 h. The minimum
10

concentration of the extract causing complete inhibition of the microbial growth was taken as
MIC. The results were compared with that of control in which the sample was replaced with
DMSO.
2.2.4.Cellular toxicity testing using MTT assay.The calorimetric measurements of 3-[(4, 5-
dimethylthiazol-2-yl)-2, 5-diphenyl] tetrazolium bromide (MTT) cytotoxicity assay is based on
the capacity of mitochondrial succinate dehydrogenase enzymes in living cells (sheep blood
used) to reduce the yellow water soluble substrate (MTT) into an insoluble purple colored
formazan product that are dissolved in DMSO [19, 35].
3. Results and discussion
3.1. Synthesis, IR and 1H NMR spectroscopy
Chart
4
shows coordination compounds of 5-nitro-salicylaldehyde-N1-substituted
thiosemicarbazonates of Zn(II) with dm-bipy, dm-phen or tm-phen as co-ligands. Here, the
acetate anions of the zinc salt has removed OH and N2H protons of the thiosemicarbazones and
thus the thio-ligands are coordinated to Zn(II) as O,N,S- terdentate dianions. Further, each of the
co-ligands occupies two coordination sites leading to the formation of five coordinated
compounds of general stoichiometry, [Zn(Ln)(L)] {n = 1-4; L = dm-bipy, 1, 4, 7, 10; dm-phen,
2, 5, 8, 11; tm-phen, 3, 6, 9, 12). These compounds are soluble in dimethylsulfoxide and partially
soluble in dichloromethane, methanol and acetonitrile.
N
N1HR
CH3
CH3
S
C
H3C
H3C
CH3
CH3
N
Zn
=
2
N
N
N
N
N
N3
N
N
H3C
CH3
N
N
O
1, 4, 7, 10
2, 5, 8, 11
3, 6, 9, 12
H
C
R = H, H2L1 1 -3; Me, H2L2 4- 6; Et, H2L3 7 - 9; Ph,
H2L4 10 -12
1- 12
O2N
Chart 4. Chem draw structral view of complexes.
11

The uncoordinated thiosemicarbazones have shown IR bands due to (N1H), (OH) and
(N2H) vibrational groups in the ranges, 3341-3435, 3302-338 and 3131-3221 cm-1 respectively
(see SI). In complexes only IR bands due to (N1H) were observed in the range, 3328 -3477
cm-1 and it supported deprotonation of N2H and OH moieties and consequently the thio-
ligands coordinated to the metal center as dianions, (Ln)2-. The nitro group (NO2) of the
aromatic ring at C2 carbon of the coordinated thio-ligand showed medium to strong bands in the
ranges, as(NO), 1533 -1562; s(NO), 1302 -1328 cm-1 [36, 37]. Further, the diagnostic ν(C-S)
(intense bands) of the free ligands at 1042-1012 cm-1shift to the low energy region, 773–720 cm-1
(as medium to strong bands) in their coordination compounds supporting co-ordination by S-
donor atoms [19].
The proton NMR spectra of the uncoordinated thio-ligands H2L1, H2L2, H2L3 and H2L4
showed the signals due to OH and N2H moieties in the ranges, 11.10 – 11.89, and 8.50 – 10.32
ppm, respectively and these signals disappeared in the complexes, 1-12. It supports that the thio-
ligands coordinate to zinc(II) metal center as dianions. The proton signals due to C2H and N1H
moieties appeared in the ranges, 8.07 -8.82 and 7.33-8.31 ppm respectively in complexes, and
were low / upfield relative to the free thio-ligands. The signals due to other protons of thio-
ligands and co-ligands are listed in the experimental section (see SI).
3.2. Crystal and molecular structures
Coordination compounds 5, 8-11 formed single crystals which have been studied using X-ray
crystallography. Compounds 5, 8-10 crystallized in the triclinic crystal system with space groups
P in each case while compound 11 crystallized in the monoclinic crystal system with space
group P21/n. Crystal data are given in Table 1 while bond parameters of the co-ordination
polyhedron are given in Table 2. The crystal structure of compound, [Zn(L4)(dm-bipy)] 10,
showed the presence of two independent molecules, namely, molecule I and molecule II (Fig.1)
with small differences in various bond parameters. In molecule I, zinc(II) is bonded to O(1A),
N(1A) and S(1A) donor atoms of the thio-ligand at bond distances of 2.0134(13), 2.3441(6) and
2.0976(17) Ǻ, respectively, and to N(4A)ax and N(5A)eq atoms of dm-bipy at bond distances of
2.1286(17) and 2.0895(15) Ǻ, respectively (ax = axial; eq = equatorial). The angles around
12

zinc(II) vary in the range 77-175º (Table 2). The chelating angle, N(4A)ax-Zn(1A)-N(5A)eq,
made by the co-ligand dm-bipy is the shortest angle of 77.78(6)º. The distortion value of a five
co-ordinated polyhedron is calculated by the two largest bond angles using the relationship, τ =
(β-α)/60 (where α, β are the two largest bond angles). The value of τ = 1 supports an ideal
trigonal bipyramid structure and the value of τ =0 supports a square pyramidal environment [38].
In molecule I, the trans bond angles, namely, N(1A)-Zn(1A)-N(5A)eq, 175.55(6)º and O(1A)-
Zn(1A)-S(1A), 140.74(5)º give a τ value of 0.580 which suggests that the geometry around
zinc(II) is distorted from a square pyramid. Molecule II has similar bond parameters and a τ
value of 0.529 suggests that this molecule has a similar distorted structure. In the literature, the
analogous compound [Zn(L4)(bipy)] also showed a similar structure with a τ value of 0.567 [19].
Fig 2 shows a simplified structural view of molecules I and II and it can be seen that N(5)eq-Zn-
N(1) form the largest bond angles in both of the molecules. Two molecules are inversely
positioned in the unit cell.
Fig.1. Molecular structure of compound, [Zn(L4)(dm-bipy)] (10).
13

CH3
N4
CH3
CH3
CH3
N5
N5
O
N4
S
Zn
N1
Zn
N1
S
O
NHPh
C
NHPh
C
N
O2N
CH
O2N
N
CH
Molecule II
Molecule I
Fig. 2. A simple structural view of molecules I and II of compound 10.
The crystal structure of co-ordination compound, [Zn(L2)(dm-phen)]CH3CN 5 having dm-phen
as the co-ligand also showed the presence of two independent molecules (Fig. 3). In molecule I,
zinc(II) is coordinated to O(1A), N(4A) and S(1A) of the thio-ligand at bond distances of
1.992(2), 2.101(3) and 2.3870(10), respectively, and to N(1A)ax and N(2A)eq atoms of dm-phen
at bond distances of 2.165(3) and 2.110(3), respectively. Atoms O(1A), N(4A), S(1A) and
N(2A)eq occupy a square basal plane and N(1A)ax occupies an axial site. The trans bond angles in
the square plane, namely, N(4)-Zn(1)-N(2)eq, 164.00(10)º and O(1)-Zn(1)-S(1), 153.54(7)º give a
τ value of 0.174 which suggests that the geometry around zinc(II) is close to a square pyramid.
Molecule II has similar bond parametersand geometry with τ = 0.185. Compound 5 contains co-
crystallized acetonitrile in the crystal. Fig. 4 shows a simplified structural view of molecules I
and II.
14

Fig. 3. Molecular structure of [Zn(L2)(dm-phen)]CH3CN(5).
N1
N2
N10
N8
CH3
H3C
H3C
O2N
CH3
S
C
Zn
N4
Zn
N7
S
O
O
NHPh
NHPh
C
N
O2N
CH
N
CH
Molecule II
Molecule I
Fig. 4. A structural view of molecules I and II of complex 5.
The molecular structures of compounds, [Zn(L3)(dm-phen)]CH3CN 8 and [Zn(L4)(dm-
phen)]CH3CN 11 are given in Fig. 5 and Fig. 6 respectively.The co-ordination pattern of both
these compounds is similar to that of compound 5. The τ values of compounds 8 and 11 are
0.004 and 0.250, respectively, which suggests that the geometry of each compound is similar to
that of compound 5. Compound [Zn(L3)(tm-phen)] 9 with tm-phen as the co-ligand has a co-
15

ordination pattern similar to that of compound 8 (Fig. 7). The τ value of 0.546 suggests that the
geometry around zinc(II) is distorted from a square pyramid and the structure is similar to that of
compound 10. The donor atom N1 occupies axial sites in compounds 8 and 9 while N4 donor
atom occupies the axial sites in compound 11. Fig. 8 shows simplified the structural views of
compounds 8, 9 and 11.
Fig. 5. Molecular structure of [Zn(L3)(-dm-phen)]CH3CN (8)
16

Fig. 6. Molecular structure of complex, [Zn(L4)(dm-phen)]CH3CN (11).
Fig.7. Molecular structure of [Zn(L3)(tm-phen)] (9).
17

H3C
H3C
CH3
CH3
N5
N4
N1
N2
CH3
NH
N1
N2
H3C
H3C
CH3
Et
Zn
S
O
S
Zn
N4
Ph
S
C
Zn
N4
N1
O
NH
C
O
Et
NH
C
O2N
N
CH
N
O2N
CH
N
O2N
CH
8
11
9
Fig. 8. A simplified structural view of molecules of complexes 8, 9 and 11.
18

Table 1
Crystallographic data for compounds 5, 8-11.
5
8
9
Empirical formula
C23H20N6O3SZn·
C2H3N
C26H26N6O3SZn·
C2H3N
C24H22N6O3SZn
C2H3N
M
566.96
580.99
609.04
T(K)
296(2)
296(2)
296(2)
Crystal system
Triclinic
P
Triclinic
P
Triclinic
P
Space group
Unit cell dimensions
a(Å)
13.699(5)
13.915(3)
15.304(3)
116.069(9)
94.562(10)
107.541(10)
2420.9(12)
4
8.0073(12)
12.2681(19)
14.628(2)
108.360(8)
104.303(8)
97.679(8)
1286.2(3)
2
8.5786(16)
12.192(3)
15.426(3)
72.473(7)
84.978(7)
82.119(7)
1522.2(6)
2
b(Å)
c(Å)
()
()
()
V(Å3)
Z
Dcalcd(g cm-3)
1.556
1.500
1.329
19

(mm-1)
1.145
1.079
0.915
F(000)
1168
600
632
Reflections collected
Unique reflections
80144
16145
26216
10688,
4726,
7141,
(Rint= 0.0843)
10688 / 0/ 675
(Rint= 0.0470)
4726 /2/350
(Rint= 0.0366)
7141/3/ 370
Data/restraints/
parameters
8695
3677
4643
Reflens.with [I2(I)]
R Indices
R1
0.0509
0.1206
0.0495
0.1238
0.0626
0.1790
WR2
R indices (all data)
R1
0.0675
0.1343
0.0689
0.0974
WR2
0.1330
0.2144
Largest diff. Peak and 0.570 and
hole
1.668 and
-0.552 e.Å-3
11
0.875 and
-0.513 e. Å-3
-0.463 e. Å-3
10
Empirical formula
C26H22N6O3SZn
C28H22N6O3SZn
C2H3N
M
563.92
173(2)
629.00
173(2)
T(K)
20

Crystal system
Triclinic
Monoclinic
P21/n
Space group
P
Unit cell dimensions
a(Å)
10.4125(3)
12.7046(6)
18.9301(7)
101.526(3)
91.235(3)
91.318(3)
2452.18(16)
4
10.8178(2)
14.3720(3)
18.1175(4)
90
b(Å)
c(Å)
()
98.325(2)
90
()
()
V(Å3)
2787.12(10)
4
Z
Dcalcd(g cm-3)
(mm-1)
1.527
1.499
1.129
1.003
F(000)
1160
1296
Reflections collected
Unique reflections
30382
37683
16213,
9595,
(Rint = 0.0284)
16213 / 0/ 671
(Rint =0.0370)
9595/ 0/ 382
Data/restraints/
parameters
12389
7524
Reflens.with [I2(I)]
21

R Indices
R1
0.0437
0.0947
0.0351
0.0791
WR2
R indices (all data)
R1
0.0648
0.1049
0.0539
WR2
0.0893
Largest diff. Peak and 1.044 and
0.418 and
-0.372 e. Å-3
hole
-0.543e. Å-3
Table 2
Important bond distances (Å) and bond angles (°) in complexes 5, 8-11.
5
8
9
10
11
Zn –O
Zn–N
1.992(2)I
2.022(2)II
2.101(3)I
2.081(3)II
1.994(3)
1.986(3)
2.0134(13)I
1.994(1)II
2.0976(17)I
2.1038(16)II
2.3441(6)I
2.3142(5)II
2.1286(17)I
2.1207(17)II
2.0895(15)I
1.9972(11)
2.123(3)
2.3874(10)
2.138(3)
2.119(3)
2.085(3)
2.3498(11)
2.142(3)
2.0902(13)
2.3817(4)
2.1294(12)
2.1141(13)
Zn–S
2.387(1)I
2.375(1)II
Zn-Neq
Zn–Nax
2.110(3)I
2.107(3)II
2.165(3)I
2.103(3)
22

2.115(3)II
2.0947(16)II
O-Zn-N
N-Zn-S
88.38(10)I
88.77(10)II
81.14(8)I
82.59(8)II
153.54(7)I
163.92(8)II
91.74(10)
89.89(10)
91.88(7)
86.95(11)
81.00(8)
156.48(9)
89.15(11)
93.99(8)
88.47(11)
81.91(8)
144.91(10)
91.60(11)
96.74(8)
177.69(11)
78.45(12)
0.546
86.53(6)I
87.10(6)II
81.79(5)I
83.24(4)II
140.74(5)I
143.77(5)II
95.53(6)I
94.68(6)II
88.86(5)
80.82(4)
150.21(4)
95.61(5)
88.48(3)
165.25(5)
79.21(5)
0.250
O-Zn-S
O-Zn-Neq
Neq-Zn-S
N-Zn-Neq
94.16(4)I
97.63(5)II
91.64(8)
164.00(10)I 156.73(12)
152.83(11)II
175.55(6)I
175.51(6)II
77.78(6)I
78.13(6)II
0.580I
Neq-Zn-Nax 79.09(10)I
79.87(11)II
79.15(12)
τ value
0.174I
0.185II
0.004
0.529II
N = N(thio-ligand); Neq = Nitrogen atom of co-ligand forming the largest bond angle; Nax =
second nitrogen atom of co-ligand; I = Molecule 1, II = Molecule 2.
3.3. Electronic absorption and fluorescence spectroscopy
The electronic absorption spectra of 10-4 M solutions of 1-12 in dimethylsulfoxide display
absorption bands in the UVVisible region as shown in Figs. 6-9. The electronic spectral bands
in the region, 270-308 nm are assigned to π→π* transitions, 371-389 nm are assigned to n→π*
transitions and the bands in the region 421-430 nm are due to MLCT electronic absorption bands
[39]. In order to check the change, if any, in the electronic absorption spectra of complexes, the
23

spectra were repeated after 24 h and it can be seem from the Figures S3.1-S3.4 that the spectra
remain unchanged and it supports that complexes are stable in dmso. It supports the view that
thio- and co-ligands are chelating strongly and are not replaced by dmso in its solutions.
Complexes were found to be non-conducting in dmso (see supporting information).
4
3.5
1
2
3
2.5
2
3
1.5
1
0.5
0
255
305
355
405
455
505
Wavelength (nm)
Fig. 9. UV-visible spectra of complexes 1-3
4
4
5
6
3.5
3
2.5
2
1.5
1
0.5
0
255
305
355
405
455
505
Wavelength (nm)
Fig.10. UV-visible spectra of complexes 4-6
24

3.5
3
7
8
9
2.5
2
1.5
1
0.5
0
255
305
355
405
455
505
Wavelength (nm)
Fig.11. UV-visible spectra of complexes 7-9
3.5
10
3
2.5
2
11
12
1.5
1
0.5
0
255
305
355
405
455
505
Wavelength (nm)
Fig.12. UV-visible spectra of complexes 10-12
These complexes exhibit fluorescence in the range, 368-570 nm with max at 430-440 nm
corresponding to the excitation wavelength,  = 300 nm (Fig. 13). The origin of fluorescence
appears to be linked to the {Zn(N,N)}(N, N = substituted bipy/phen) moiety involving
intraligand transitions [19]. It may be noted that compounds with dm-phen/tm-phen complexes
display more intense fluorescent bands (Fluorescence Intensity, F.I. = 160-180 a.u.) in
comparison to dm-bipy complexes (F.I. = 150 a.u.; Fig. 13). The difference in fluorescence
intensity may be attributed to the fact that there occurs more conjugation in complexes with phen
as co-ligand than that in complexes with bipy as co-ligand. In addition, zinc(II) complexes with
methyl and phenyl substitution at N1 nitrogen have shown less intense fluorescent bands.
25

Fig. 13. Fluorescence spectra of zinc(II) complexes, 1-12 {maxem = 434 (1), 435 (2), 435 (3),
437 (4), 436 (5),434 (6), 437 (7),440 (8), 440 (9), 438 (10), 438 (11), 439 (12); ex = 300 nm}
3.4. Antimicrobial activity
3.4.1. Discussion of antimicrobial activity and its comparison with literature
Tables 3 and 4 contain data of zone of inhibition (zoi) and minimum inhibitory
concentration (mic) and some important observations are delineated below. Complexes 2, 4, 7
and 8 have shown higher activity against Methicillin resistant Staphylococcus aureus (MRSA)
with zoi in the range, 29-32 mm, which is comparable with that of the commercially available
Gentamicin (zoi, 33 mm). Interestingly, the mic of complexes 2, 4, 7 and 8 (5-7 g/mL, Table 4)
is less than that of Gentamicin (mic, 10 g/mL). It was noted that the bio-activity of these
complexes is higher at low mic values as compared with that of the analogous Zn(II) complexes
26

with bipy and phen as co-ligands [19]. The antimicrobial activity of complexes 2, 4, 10 and 11
against Staphylococcus aureus, with zoi in the range 25-27 mm, is comparable with that of
commercially available Gentamicin (zoi, 26 mm, mic, 0.5 g/mL), but complexes 2, 4 and 11
have mic of 7 g/mL which is higher than that of Gentamicin. The bioactivity of these
complexes against Staphylococcus aureus is low as compared with that of the analogous Zn(II)
complexes with bipy and phen as co-ligands [19].
Eight complexes (2, 4-6, 8-11) have shown antimicrobial activity (zoi, 30-34 mm) against
Klebsiella pneumoniae 1 which is close to that of Gentamicin (zoi, 35 mm). Complexes 8 and 9
have mic of 1 g/mL which is close to that of the reference compound (mic, 0.3 g/mL), while
other complexes 2, 4-6, 10 and 11 have higher mic of 5-7 g/mL. The bio-activity of these
compounds is found to be more than that of analogous Zn(II) complexes with bipy and phen as
co-ligands reported in literature [19]. The activity of complexes against Salmonella typhimurium
1 and Escherichia coli is poor (zoi, 17-22 mm; mic high) as compared to that of the standard
compounds (Salmonella typhimurium : zoi, 40 mm, mic 1g/mL; Escherichia coli : zoi, 30.5
mm, mic 5g/mL). Five complexes (4, 7, 8, 10 and 11) have shown antimicrobial activity against
Enterococcus faecalis in the range, 23-28 mm zoi, which is comparable to that shown by
Gentamicin (zoi, 27 mm). Interestingly, complexes 4, 7, 8 and 11 have mic of 7 g/mL which is
much less than that of Gentamicin (mic, 30 g/mL). In the case of Candida albicans, among
complexes 1-12, only complex 11 showed bio-activity (zoi, 31 mm; mic, 5 g/mL) comparable
to that of Amphotericin (zoi, 34 mm; mic, 0.1 g/mL). Further, the bio-activity of these
compounds against Candida albicans is more (zoi, 18 - 31 mm) than that of the previously
reported analogous compounds [19].
In the literature only a few Zn(II) complexes of thiosemicarbazones have been studied for
their antimicrobial activity, which are summarized as follows. It was noted from the literature
that co-ordination compounds of zinc(II) investigated have shown poor antimicrobial activity at
high minimum inhibitory concentration (mic) [11-14]. For example, zinc complexes of 2-
furanaldehyde and 2-acetyl-γ-butyrolactone thiosemicarbazone, namely, [Zn(N, S-L)2] and
[Zn(N, S-L)Cl2], showed low antimicrobial activity against Methicillin resistant Staphylococcus
aureus (MRSA), Staphylococcus aureus and Candida albicans and with high mic value of more
27

than 200 µg mL-1. Further these complexes were inactive against Klebsiella pneumoniae 1 and
Salmonella typhimurium [14]. The 2-acetylpyridine thiosemicarbazone complexes, [Zn(N,N, S-
HL)2], showed activity against Staphylococcus aureus with a mic of 125 µg mL-1 and against
Candida albicans with a mic of 65 µg mL-1; other complexes of this ligand, namely, [ZnCl2(N,N,
S-HL)] and [Zn(OSO3)(N,N, S-HL)(H2O)] were active against Staphylococcus aureus with a mic
of 250 µg mL-1 and showed no activity against Candida albicans [12, 13]. Finally, the
thiophene-2-carbaldehyde thiosemicarbazone complex, namely, [Zn(S-HL)2Cl2], showed
antimicrobial activity against Candida albicans with a zoi of 17 mm [11]. Higher antimicrobial
bio-activity of complexes under present investigation may be attributed to the fact that the use of
polypyridyl heterocyclic bases increases lipophilicity [40, 41], which enhances the cell
membrane penetrating ability. The cause of inhibitory action of complexes can be due to their
interaction with the enzyme prosthetic group in microbial cells, which inhibits the replication of
DNA [42, 43]. In vitro cellular toxicity examined using MTT assay showed low cell viability .
Time kill studies was thought to be not necessary as we noted previously [19] that time
needed to kill all bacteria by analogous complexes was maximum of 24 h. Similar time is
expected for complexes under investigation.
Table 3
Antimicrobial activitya,b,c of complexes 1-12 (zone of inhibition)
Complexes
No.(R3)
MRSA S.
K.
S.
E.
E.
C.
Aureus pneumonia 1 typhimurium1 Coli
Faecalis Albicans
[Zn(Ln)(dm-bipy)] (n =1-4)
1 (H)
20
29
32
16
25
24
23
34
28
NA
18
13
17
14
15
27
27
20
25
26
4 (Me)
7 (Et)
15
28

10(Ph)
27
25
34
NA
NA
28
27
[Zn(Ln)(dm-phen)] (n =1-4)
2 (H)
30
27
29
26
27
23
23
26
30
30
33
30
21
15
15
16
22
13
15
13
21
21
25
28
20
25
25
31
5(Me)
8 (Et)
11 (Ph)
[Zn(Ln)(tm-phen)] (n =1-4)
3 (H)
22
21
22
21
22
26
27
33
30
25
35
NA
17
NA
14
23
19
20
22
23
20
18
22
6 (Me)
27
24
25
33
9 (Et)
NA
14
NA
NA
12 (Ph)
Gentamicind
Amphotericine
40
30.5 27
34
a) All measurements are in mm diameter of the inhibition zone; b) The standard deviation varied
c)
d,e)
in the range 0-1 based on three readings; Studies were made in DMSO;
Commercially
available anti-microbial agents. d) Gentamicin acts as positive control against bacteria (MRSA, S.
aureus, K. pneumonia 1, S. typhimurium 1, E. coli, E. Faecalis ) and e) Amphotericin acts as
positive control against yeast (Candida albicans); NA-not active
Table 4
Minimum Inhibitory concentration (MIC in g/mL) of zinc(II) complexes, 1-12
Complex
No.
MRSA
S.
K.
S.
E.
E.
C.
aureus pneumonia 1 typhimurium1 Coli
faecalis albicans
[Zn(Ln)(dm-bipy)] (n =1-4)
1 (H)
50
7
750
7
10
5
NA
1000 1000
50
7
4 (Me)
7 (Et)
500
500
7
7
5
10
50
7
1000
N.A.
1000
7
10 (Ph)
10
5
N.A. 50
500
29