Synthesis, antimicrobial activity and high cell viability of copper derivatives of 2-thiouracil, purine-6-thione and 2,4,6-trimercaptotriazine

Article abstract of DOI:10.1080/00958972.2019.1566539

Reactions of copper(II) acetate with 2-thiouracil (tucH₂) and triphenyl phosphine (1:1:1 or 1:1:2 molar ratios) in methanol-acetonitrile/chloroform mixture yielded a light brown N,S-bridged dinuclear Cu^I complex, [Cu(PPh₃)₂(μ-N,S-tucH)Cu(PPh₃)₂Cl] 1, incorporating uninegative 2-thiouracilate. X-ray crystallography has shown that the crystals of complex 1 comprise a two component twin system and belong to the space group Ia. The major component 1a is about 77% and the minor component 1b is about 23%. One Cu is four-coordinate tetrahedral with center {CuP₂SCl} and the other Cu is three-coordinate distorted trigonal planar with {CuP₂N} center. Crystal data: 1, formula, C₇₆H₆₃ClCu₂N₂OP₄S; space group: Ia; monoclinic, a, 22.2638(9); b, 11.8050(4); c, 27.4455(10) ?; β, 113.358(5); R, 4.80%;173(2) K. The antimicrobial activities of dinuclear 1 as well as that of the previously reported 2-thiouracil, 2,4,6-trimercaptotriazine (tmtH₃) and purine-6-thione (purSH₂) complexes, [CuCl(κ¹-S-tucH₂)(PPh₃)₂] 2, [Cu₂Br₂(μ-S-tucH₂)₂(PPh₃)₂] 3, [Cu(tmtH₂)(PPh₃)₂] 4, [Cu₃Br₂(k¹-N,k¹-S,μ-S-tmtH₂)(PPh₃)₆] 5 and [Cu(k¹-N,k¹-S-purSH)(PPh₃)₂] 6, have been screened against gram negative, Escherichia coli (MTCC 119) and Shigella flexneri (MTCC 1457), gram positive, Enterococcus faecalis (MTCC 439) and Staphylococcus aureus (MTCC 740) microorganism. High percentage of cell viability (96–97%) is observed in some cases.

Full text of DOI:10.1080/00958972.2019.1566539

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, Antimicrobial Activity and High Cell
Viability of Copper Derivatives of 2-Thiouracil,
Purine-6-thione and 2,4,6-Trimercaptotriazine
Jaspreet Kaur Aulakh, Samridhi Sharma, Henna Sood, Daljit S. Arora,
Manpreet Kaur, Jerry P. Jasinski & Tarlok S. Lobana
To cite this article: Jaspreet Kaur Aulakh, Samridhi Sharma, Henna Sood, Daljit S. Arora,
Manpreet Kaur, Jerry P. Jasinski & Tarlok S. Lobana (2019): Synthesis, Antimicrobial Activity
and High Cell Viability of Copper Derivatives of 2-Thiouracil, Purine-6-thione and 2,4,6-
Trimercaptotriazine, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2019.1566539
To link to this article: https://doi.org/10.1080/00958972.2019.1566539
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Synthesis, Antimicrobial Activity and High Cell Viability of Copper
Derivatives of 2-Thiouracil, Purine-6-thione and 2,4,6-
Trimercaptotriazine
Jaspreet Kaur Aulakh,a Samridhi Sharma,a Henna Sood,b Daljit S. Arora,b Manpreet
Kaur,c Jerry P. Jasinskic and Tarlok S. Lobana,a*
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 03435–2001 USA
*Corresponding ng author : Tarlok S. Lobana; e-mail address: tarlokslobana @yahoo.co.in
;
tslasr@gmail.com
A B S T R A C T
Reactions of copper(II) acetate with 2- thiouracil (tucH2) and triphenyl phosphine (1 : 1 : 1 or 1 :
1 : 2 molar ratios) in methanol-acetonitrile / chloroform mixture yielded a light brown N,S-
bridged dinuclear CuI complex, [Cu(PPh3)2(µ-N,S-tucH)Cu(PPh3)2Cl] 1, incorporating
uninegative 2-thiouracilate. X-ray crystallography has shown that the crystals of complex 1
comprise a two component twin system and belong to the space group Ia. The major component
1a is about 77% and the minor component 1b is about 23%. One Cu is four-
coordinate tetrahedral with center {CuP2SCl} and the other Cu is three-coordinate distorted
1

trigonal planar with {CuP2N} center. Crystal data: 1, formula, C76H63ClCu2N2OP4S; space
group : Ia; monoclinic, a, 22.2638(9) ; b, 11.8050(4) ; c, 27.4455(10) Å ; β, 113.358(5)◦; R,
4.80%;173(2) K. The antimicrobial activities of dinuclear 1 as well as that of the previously
reported 2-thiouracil, 2,4,6-trimercaptotriazine (tmtH3) and purine-6-thione (purSH2) complexes,
[CuCl(κ1-S-tucH2)(PPh3)2] 2, [Cu2Br2(μ-S-tucH2)2(PPh3)2] 3, [Cu(tmtH2)(PPh3)2] 4, [Cu3Br2(k1-
N,k1-S,µ-S-tmtH2)(PPh3)6] 5 and [Cu(k1-N,k1-S-purSH)(PPh3)2] 6, have been screened against
gram negative, Escherichia coli (MTCC 119) and Shigella flexneri (MTCC 1457), gram
positive, Enterococcus faecalis (MTCC 439) and Staphylococcus aureus (MTCC 740)
microorganism. High percentage of cell viability (96-97 %) is observed in some cases
KEYWORDS
2-Thiouracil; Copper(II); 2,4,6-trimercaptotriazine; purine-6-thione; Triphenyl phosphine
1. Introduction
Heterocyclic-2- thiones bearing functional moieties such as –N(H)-C(=S)-, -N(H)-C(=S)-
N=, -N(H)-C(=S)-N(H)-, have several coordination modes with metals giving a variety of
compounds of variable nuclearities [1-15]. Another interesting feature of this class of thio-
ligands pertains to the chemical reactivity involving C-S rupture with copper halides forming an
interesting class of compounds [16-21]. The coordination chemistry of thio-ligands, pyridine-2-
thiones, pyrimidine- 2-thiones, 1,3-imidazolidine-2-thiones and their derivatives has been the
main focus of several researchers [1-15], while other multifunctional thio-ligands such as 2-
thiouracil, 2,4-dithiouracil, purine-6-thione and 2,4,6-trimercaptotriazine have found much less
attention [22-31].
2

We have been interested in coordination chemistry with different metals and chemical
reactivity of heterocyclic-2-thiones involving C-S bond rupture under ambient conditions [15-23,
29, 30] and more recently the applications of metal complexes (Cu, Ag) with imidazolidine-2-
thiones as antimicrobial agents have been reported [20, 32, 33]. In this paper, syntheses and
antimicrobial activities of copper based complexes with multidentate thio-ligands as shown in
Chart 1 are examined. A brief summary of the biological applications
of metal 2-thiouracil (tucH2), 2,4,6-trimercaptotriazine (H3tmt) and purine-6-thione (purSH2)
reported previously is presented here. For example, Au(I) and Pt(IV) complexes of 2-thiouracil
are known for their anti-arthritic activities [34] and cytotoxic activities towards A549 and A2780
cell lines [35]. The 5-substituted-2-thiouracil derivatives have been used as anti-cancer [36] and
anti-tumor drugs [37]. These derivatives inhibited the folate mechanism of cancer cells and were
also useful as coronary vasodilators [37]. Similarly, Zn2+, Fe2+ and Mn2+ complexes of H3tmt
have shown in vitro antitumor activities against G-361 (human malignant melanoma), HOS
(human osteogenic sarcoma), K–562 (human chronic myelogenous leukemia) and MCF-7
(human breast adenocarcinoma) tumor cell lines, along with antimicrobial activities against
bacteria Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus and Enterococcus
faecalis [38]. Platinum(II) complexes of purine-6-thione have shown anticancer activity [39].
Thio-ligands, 2-thiouracil and 2,4-dithiouracil have limited coordination chemistry [22-
31]. For example, 2-thiouracil with copper(II) chloride formed a mononuclear trigonal planar CuI
complex, [CuCl(κ1-S-tucH2)2] [28], involving reduction of CuII to CuI. Reactions of copper(I)
halides with tucH2 and triphenylphosphine (PPh3) yielded a tetrahedral complex, [CuCl(κ1-S-
tucH2)(PPh3)2] 2 [29] and a sulfur-bridged dinuclear complex, [Cu2Br2(μ-S-tucH2)2(PPh3)2] 3
3

[30]. The related thio-ligand, 2,4-dithiouracil with copper(I) halides formed a mononuclear
[CuBr(κ2-P,P-dppbz)(κ1-S-dtucH2)] {dppbz
=
1,2-bis(diphenylphosphanyl)benzene}[31],
dinuclear [Cu2(μ-X)(κ1-S,κ1-S-dtucH)(PPh3)4] (X = Cl, Br) [29] and polynuclear [Cu(μ-S,S-
dtucH2)(PPh3)X]n (X = Cl, Br, I) complexes [22]. Silver(I) halides with dtucH2 yielded dinuclear
complexes, [Ag2(μ-X)(κ1-S,κ1-S-dtucH)(PPh3)4] (X = Cl, Br) [23]. The multidentate thio-
ligands, tmtH3 and purSH2, with copper(I) formed complexes, [Cu(tmtH2)(PPh3)2] 4 [29] ,
[Cu3Br2(k1-N,k1-S,µ-S-tmtH2)(PPh3)6] 5 [29] and [Cu(k1-N,k1-S-purSH)(PPh3)2] 6 [29].
In this paper, synthesis of a dinuclear complex, [Cu(PPh3)2(µ-N,S-tucH)Cu(PPh3)2Cl] 1,
its antimicrobial activities, along with that of the previously reported 2-6 [29, 30] are reported.
Complexes have been screened against gram negative, Escherichia coli (MTCC 119) and
Shigella flexneri (MTCC 1457), gram positive Enterococcus faecalis (MTCC 439) and
Staphylococcus aureus (MTCC 740) microorganism.
2. Experimental
2.1. Materials and techniques
2-Thiouracil, copper(II) acetate dihydrate and triphenyl phosphine were procured from Aldrich
Sigma Ltd. Elemental analyses (CHNS) were carried out using the THERMO
FINNIGAN FLASH technique. Infrared spectra were recorded using KBr pellets from 4000-
200 cm-1 on a Pye–Unicam SP-3-300 spectrophotometer. Melting points were determined with
1
an electrically heated Gallenkamp apparatus. H NMR spectra were recorded on a Bruker
4

Avance II 400 NMR FT spectrometer operating at 400 MHz using DMSO-d6 as the solvent with
TMS as the internal standard. ESI-mass spectra were recorded in DMSO using a Bruker Daltonik
LS-MS high resolution microTOF-Q II 10356.
2.2. Synthesis of [Cu(PPh3)2(µ-N,S-tucH)Cu(PPh3)2Cl] (1)
A solution of 2- thiouracil (0.016 g, 1 mmol) in methanol (15 mL) was added to a solution of
copper(II) acetate (0.025 g, 1 mmol) in acetonitrile (8 mL). The contents were stirred at room
temperature overnight and the black precipitate formed was separated. To this precipitate was
added triphenylphosphine (0.060 g, 2 mmol) dissolved in chloroform (10 mL) and the contents
were refluxed for one hour; addition of a few milliliters of methanol made the solution clear.
This solution was left at room temperature for two weeks when light brown crystals were
0
formed. Yield: 0.070 g, 66%, m. p., 190 -195 C, C, H, N, S analysis for C76H63ClCu2N2OP4S
(1338.73): C, 68.00; H, 4.70; N, 2.09; S, 2.39. Found: C, 67.83; H, 4.96; N, 2.14; S, 2.21. IR
data (KBr, cm-1) : 3176 (w, N-H); 3070 (m), 3051 (m), 2933 (w), 2882 (m), 2796(m) (C-H);
1648 (s, C=O), 1585 (s, C-N); 1532 (s), 1480s, 1434 w, 1388 w (C-C + C-H); 1329w, 1283s;
1157 (m, C-S); 1094 (s, P-C); 1027 w, 998 w; 922w, 848 w, 812 w, 744 s, 695 s, 618 w, 572 w,
1
452 w. H NMR (DMSO-d6, , ppm): 12.23 (sb, 1H, NH); 8.2(sb, H, H5), 7.58 (m, 24H, o-H,
PPh3), 7.50 (m, p-H, 12H, PPh3); 7.34 (m, m-H, 24H, PPh3 + H6, Htuc-). 1 is soluble in solvents
such as DMSO, dichloromethane and methanol.
2.3. Preparation of previously reported complexes. Complexes [CuCl(κ1-S-tucH2)(PPh3)2] (2),
[Cu2Br2(μ-S-tucH2)2(PPh3)2] (3), [Cu(tmtH2)(PPh3)2] (4), [Cu3Br2(k1-N,k1-S, µ-S-tmtH2)(PPh3)6]
(5) and [Cu(k1-N,k1-S-purSH)(PPh3)2] (6) were prepared as reported earlier [29,30] and their
5

1
stoichiometry is confirmed through analytical data and H-NMR data given in supporting
information (see supporting information, Figure S 6a-f).
2.4. 2-Thiouracil Ligand
IR data (KBr, cm-1) : 3200m, 3135 m (N-H); 3086m, 3050m, 2931m (C-H); 1708 (s, C=O),
1567 (s, C-N); 1450s, 1423 s, 1394 s (C-C + C-H); 1176 (m, C-S); 1097 (m), 1003 m, 964 w,
912 m, 896 m, 834 m, 761m, 737 w, 710 w, 549 s, 472 w. 1H NMR (DMSO-d6, , ppm) :
12.37(s, 2H, NH), 7.36 (d, J = 8 Hz; 1H, H5), 5.78 (d, J = 8; 1H, H6).
2.5. X-ray crystallography
The crystal structure of 1 was determined by mounting a single crystal of the complex on a glass
fiber and data were measured with Rigaku-Oxford Diffraction equipped with graphite
monochromated Mo-Kα (λ = 0.71073 Å) radiation at 173(2) K. The structure was solved by
direct methods and refined using full-matrix least-squares based on F2 using ShelXL-2014. In
this structure, all non-hydrogen atoms were refined anisotropically and the hydrogens have been
fixed geometrically [40].
2.6. Antimicrobial studies
Test organisms. The reference strains of bacteria and yeast were obtained from Microbial Type
Culture Collection (MTCC), Institute of Microbial Technology (IMTECH), Chandigarh, India.
Reference strains included Gram positive bacteria: Staphylococcus aureus (MTCC 740),
Enterococcus faecalis (MTCC 439), Gram negative bacteria Shigella flexneri (MTCC 1457),
Escherichia coli (MTCC 119). A loopful of broths with bacterial and yeast colonies were
inoculated into 5 mL of their respective medium and incubated at 37 °C and 25 °C, respectively,
6

for 4 h. This was used as inoculum after adjusting the turbidity as per the McFarland turbidity
standard. This turbidity is equivalent to approximately (1 to 2) ×108 colony forming units per mL
(CFU/mL). The inoculums thus prepared were used for antimicrobial screening. The
experimental details for antimicrobial screening and a test to check the level of cellular toxicity
of the test compounds, MTT [3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium bromide]
assay is given in supporting information.
3. Results and Discussion
3.1. Synthesis, IR and NMR spectroscopy
In a reaction of copper(I) chloride with 2-thiouracil and triphenylphosphine in 1 : 1 : 2
molar ratio in acetonitrile-methanol mixture, the product obtained was a tetrahedral complex,
[CuCl(κS1-tucH2)(PPh3)2], as reported earlier [29]. When this reaction was carried out with 2-
thiouracil and PPh3 in the same molar ratio in acetonitrile-methanol mixture containing a few mL
of DMSO, the product is [Cu2(PPh3)4(tucH)Cl] (tucH is 2-thiouracilate anion) [41]. Reaction
of copper(II) acetate with 2-thiouracil in acetonitrile-methanol mixture followed by reaction
with triphenylphosphine (1: 1 : 2 molar ratio) in chloroform-methanol mixture yielded a product
[Cu(PPh3)2(µ-N,S-tucH)Cu(PPh3)2Cl] 1, as confirmed by the analytical data and X-ray
crystallography. The formation of 1 involves abstraction of chlorine from chloroform. Chart 2
shows the structure of 1 as well as those of previously reported 2 [29], 3 [30], 4-6 [29], which
form part of the antimicrobial study in the present investigation.
IR spectra of the uncoordinated tucH2 and 1 have been recorded from 4000 – 400 cm-1 and
the major bands are reported in the experimental section. The thio-ligand showed two medium
intensity ν(N-H) bands at 3200, 3135 cm-1 in 1, one weak ν(N-H) band at 3176 cm-1 was
7

observed, suggesting deprotonation of one (N-H) moiety probably the one close to C4=O group
which was confirmed by the x-ray crystal structure of 1. The thio-ligand tucH2 showed one
strong ν(C=O) band at 1708 cm-1 which shifted to 1648 cm-1 probably as a result of coordination
of N3 to Cu. Complex 1 has shown a ν(C-S) of medium intensity at 1157 cm-1 which occurs in
the low energy region relative to free tucH2 [ν(C-S)), 1176 cm-1] and it supports the formation of
metal-sulfur bonds. The appearance of ν(P-CPh) at 1094 cm-1 (strong in intensity) confirmed the
presence of coordinated PPh3 in 1 (see supporting information for complete spectra, Figure S1
and Figure S2).
The 1H NMR spectrum of uncoordinated tucH2 showed a broad signal at  = 12.37 ppm,
which corresponded to the combined N1-H and N3-H protons and in 1 a broad signal at  =
12.23 ppm corresponds to N1-H proton after deprotonation of N3-H moiety. The uncoordinated
tucH2 ring protons showed NMR signals due to C6H and C5H protons at  7.36 and 5.78 ppm,
respectively (see SI: Figure S3a, S3b). The C6H ring proton underwent a downfield shift upon
coordination to Cu(I) and gave a single broad signal at  = 8.28 ppm, whereas the C5H ring
proton merges with the absorption due to m-H protons of PPh3 ligands. The o-H, m-H and p-H
protons of coordinated PPh3 are multiplets at 7.34 -7.58 ppm.
3.2. Crystal structure of [Cu(PPh3)2(µ-N,S-tucH)Cu(PPh3)2Cl] 1
The X-ray crystal structure determination revealed that monoclinic crystals of 1 comprise a two
component twin system and belong to the space group Ia. The major component is about 77%
and the minor component is about 23%. The crystal data are placed in Table 1 and the bond
parameters are placed in Table 2. [Cu2(PPh3)4(tucH)Cl] (tucH is 2-thiouracilate anion) with
similar stoichiometry formed triclinic crystals in Pī space group and exists as a single component
8

[41], unlike the two component twin system of 1. Figure 1 shows the molecular structure of
major component (1a; 77%), Figure 2 shows molecular structure of minor component (1b; 23%)
and finally Figure 3 shows the overlapping major and minor components of 1. Here the detailed
discussion of major and minor components is presented.
In the major component 1a (Figure 1), Cu1 is bonded to one N1 and P1 and P2 at bond
distances of 1.928(5), 2.2335(14) and 2.2666(14) Ǻ, respectively, with bond angles of N-Cu-P2,
112.81(16), N-Cu-P1, 122.78(16) and P-Cu-P, 122.91(5)o. These bond parameters support a
distorted trigonal planar geometry of Cu1. Cu2 is bonded to P3, P4, S1 and one Cl1 donor at
bond distances of 2.2658(13), 2.2912(15); 2.368(2) and 2.4471(18) Ǻ, respectively, with bond
angles of 120.16(5) -102.03(6)o. These bond parameters support a distorted tetrahedral geometry
of Cu2 (Figure 1).
In the minor component 1b, the tucH- anion changes orientation leading to change of donor
atoms of the two metal centers (Figure 2). Here Cu1 is bonded to S1A, P1, P2 and Cl1A at bond
distances of 2.431(8); 2.2335(14), 2.2666(14) and 2.597(7) Ǻ, respectively, with bond angles of
97.87(14) – 122.91(5)o and these bond angles support distorted tetrahedral geometry of Cu1; Cu2
is bonded to N1A, P3 and P4 at bond distances of 1.768(14), 2.2658(13) and 2.2912(15) Ǻ,
respectively, with bond angles of 108.7(6), 120.15(5) and 129.9(6)o. These bond angles support a
distorted trigonal planar geometry of Cu1. In the two components, 1a and 1b, the distortions of
two tetrahedral geometries as well as two trigonal planar geometries are different.
The coordination pattern of [Cu2(PPh3)4(tucH)Cl] (tucH is 2-thiouracilate anion) [41] is
similar to say 1a, one Cu is four coordinate (CuClP2S, tetrahedral) and the second Cu is three-
coordinate (CuP2N, trigonal planar). The interesting feature is formation of a twin system in the
9

present case and medium might have played a role. An analogous thio-ligand, namely, 2,4,6-
trimercaptotriazine (tmtH3) with Cu(OAc)(PPh3)2 has one coordination pattern (CuNP2), similar
the three-coordination shown by 1a or 1b. This tmtH3 with copper(I) bromide and PPh3 showed a
four-coordination pattern (CuSP2Br) similar to the one shown by 1a or 1b [29].
In formation of 1, the 2-thiouracilate anion (tucH-) binds to one Cu centre through its N
donor near C=O and two PPh3 ligands. The electronegative N and two P donors stabilize CuI of
CuNP2. Further, tucH- also binds to another Cu via its S donor, two PPh3 ligands and one
chloride. The origin of chloride is traced to CHCl3 and is necessary for stabilization of CuI when
bonded to one S and two P donors, i.e. the core CuSP2 abstracts halogen from CHCl3 to generate
CuSP2Cl in 1. The 2-thiouracil with PPh3 resulted in stabilization of CuI and no complex in CuII
oxidation state could be obtained. Direct reaction of copper(II) chloride with 2-thiouracil yields,
a three coordinate CuI complex, [CuCl(κ1-S-tucH2)2] [28] in which 2-thiouracil is a neutral
ligand.
3.3. ESI-mass studies
The ESI-mass spectrum of 1 shows the formation of two types of species, [Cu2(PPh3)3(µ-N,S-
tuc)]+ (A- type) and [Cu2(PPh3)2(µ-N,S-tuc)]+ (B-type). Figures 4 and 5 depict ESI-mass spectral
peaks of A-type and B-type species, respectively, which are consistent with their isotopic
patterns. The formation of A-type species reveals loss of one halide, one PPh3 and that of B-type
involves loss of one halogen and two PPh3 molecules from 1.
3.4. Antimicrobial activity and biosafety evaluation
10

[Cu2(PPh3)4(µ-N,S-tucH)Cl] 1, [CuCl(k1S-tucH2)(PPh3)2] 2, [Cu2Br2(μ-S-tucH2)2(PPh3)2] 3,
[Cu(tmtH2)(PPh3)2] 4, [Cu3Br2(k1-N,k1-S,µ-S-tmtH2)(PPh3)6] and [Cu(k1-N,k1-S-
5
purSH)(PPh3)2] 6 [complexes 2-6; 29, 30] show activity against Staphylococcus aureus from 12-
18 mm (ZOI), the highest activity is shown by 5. H3tmt has activity of 14 mm against S. aureus,
unlike 2-thiouracil and purine-6-thione, which were inactive. However, the structurally
uncharacterized CuII complex, [Cu(tucH2)(NH3)4]Cl2 showed activity against S. aureus with a
ZOI of 14 mm [42], while another CuII complex, [Cu(tucH-6-NH2)2(H2O)]·2H2O showed low
activity [43]. Complexes 1-3 show higher activity (ZOI of 16-21 mm) against Shigella flexneri
as compared to 2-thiouracil (ZOI of 15 mm). There is no report with which comparison could be
made [42, 43]. Only 1 showed somewhat higher activity against E. faecalis with ZOI of 16 mm
in comparison to 2-thiouracil which showed activity of 14 mm, but 2-6 and free ligands, H3tmt
and purSH2 were inactive. There is no report with which comparison could be made [42, 43].
Only 2-thiouracil showed antimicrobial activity (ZOI, 16 mm) against E. Coli bacteria, while all
other complexes tested were inactive. The structurally uncharacterized CuII complexes showed
either comparable activity, {[Cu(tucH2)(NH3)4]Cl2, ZOI of 14 mm} [42] or very poor activity
{[Cu(tucH-6-NH2)2(H2O)]·2H2O [43] against E. Coli bacteria. Activity of complexes/2-
thiouracil as described above was less than that of the standard Gentamicin (Table 3).
3.4.1. Cellular toxicity using MTT assay
All of the complexes 1-6, which were tested for their antimicrobial activity against various
microorganisms, were evaluated for cellular toxicity using MTT assay[44]. This cytotoxicity
assay is based on the capacity of mitochondrial succinate dehydrogenase enzymes in blood cells
11

to reduce the yellow water soluble substrate MTT into an insoluble purple formazan product
which is measured spectrophotometrically. [Cu(tmtH2)(PPh3)2] (4) and [Cu3Br2(k1-N,k1-S,µ-S-
tmtH2)(PPh3)6] (5), showed purple color with optical density (OD) of 0.651 (4) and 0.642 (5),
respectively, at 590 nm wavelength. The OD of the control (untreated cells in DMSO) was
0.672. Therefore, these compounds were non-cytotoxic with 97% (4) and 96% (5) cell viability.
4. Conclusion
2-Thiouracil with copper(II) acetate yielded a dinuclear complex [Cu2(PPh3)4(µ-N,S-tucH)Cl] 1
involving abstraction of chlorine from methanol-chloroform during re-crystallization; 1 has
unsymmetrically bridged 2-thiouracilate ligand. The crystals of this complex are a two
component twin system and belong to the space group Ia. The major component 1a is about 77%
and the minor component 1b is about 23%. One Cu is four-coordinate tetrahedral with
{CuP2SCl} and the second Cu is three-coordinate {CuP2N} with distorted trigonal planar
geometry. The antimicrobial activities of 1 as well as those of previously reported 2-6 [29, 30]
reveal activity against S. aureus, Shigella flexneri and E. Faecalis. On examining 4 and 5 for
their in vitro cellular toxicity using MTT [3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltrazolium
bromide] assay, showed high cell viability of 97 % (4) and 96 % (5).
Disclosure statement
No potential conflict of interest was reported by the authors.
Acknowledgement
Financial assistance from the Council of Scientific and Industrial Research (CSIR), New Delhi
for Emeritus Scientist Grant [21(0904)/12-EMR-II] to TSL is gratefully acknowledged. JPJ
12

acknowledges the NSF--MRI program (Grant No. CHE-1039027) for funds to purchase the X-
ray diffractometer.
Appendix A. Supplementary data
Crystallographic data for the structure in this paper have been deposited with the Cambridge
Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of the
data can be obtained free of charge on quoting the depository numbers CCDC – 1838783 (1)
(Fax: +44-1223-336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk). IR and
NMR data of ligand and complex 1, ESI-MS data of complex 1, experimental details for
antimicrobial activity of 2-thiouracil and complexes.
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16

Graphical abstract
Chart 1. 1-Thiouracil and related thio-ligands: tucH2 = 2-thiouracil (I); dtucH2 = 2,4 – dithiouracil (II);
tmtH3 = 2,4,6-trimercaptotriazien (III); purSH2 = purine-6-thione (IV)
17

O
NH
HN
PPh3
PPh3
Cu
Ph3P
Cl
Ph3P
Br
PPh3
Br
Ph3P
Cu
S
S
HN
Cu
Cu
Cu
S
N
O
Ph3P
PPh3
Cl
H
S
3
2
N
NH
2
HN
4
O
HN
O
1
3
5
6
S
N
S
S
N
S
1
Ph3P
HN
NH
NH
PPh3
S
S
Cu
Cu
HN
Ph3P
S
Bond isomers
PPh3
PPh3
4a, 4b
PPh3
PPh3
Ph3P
Cu
Cu
N
S
Cu
PPh3
N
Ph3P
Ph3P
Cu
Br
NH
S
S
N
PPh3
N
HN
Br
HN
5
6
Chart 2. The structures 1, 2-6[29,30]
18

Figure 1. Molecular structure of [Cu(PPh3)2(µ-N,S-tucH)Cu(PPh3)2Cl] 1a, the major
component.
19

Figure 2. Molecular structure of [Cu(PPh3)2(µ-N,S-tucH)Cu(PPh3)2Cl] 1b, the minor
component.
20

Figure 3. The overlapping major- minor components of [Cu(PPh3)2(µ-N,S- tucH)Cu(PPh3)2Cl]
1.
21

Intens.
[%]
+MS, 0.0-0.3min #1-19
1+
1040.7114
1038.7104
30
20
10
0
1+
1041.7134
1039.7123
1+
1042.7129
1+
1043.7146
1036
1038
1040
1042
1044
1046
1048
m/z
[%]
Cu₂C₅₈H₄₇N₂OP₃S, M, 1038.1208
1+
1+
1040.1205
100
1038.1208
1+
1+
1039.1241
1041.1228
1+
50
0
1042.1209
1+
1+
1043.1219
1044.1227
1036
1038
1040
1042
1044
1046
1048 m/z
Figure 4. ESI mass spectrum of [Cu2(PPh3)3(µ-N,S-tuc)]+ species (A) (Cu2C58H47N2OP3S; m/z =
calc. 1038.12, obsd 1038.71) with isotopic pattern (1).
22

Intens.
[%]
+MS, 0.0-0.3min #1-19
1+
778.6656
1+
776.6671
100
1+
777.6728
1+
779.6701
1+
780.6669
50
1+
781.6737
1+
782.6814
0
776
777
778
779
780
781
782
783
784
785
m/z
[%]
Cu₂C₄₀H₃₂N₂OP₂S, M, 776.0297
1+
778.0286
1+
776.0297
100
50
0
1+
777.0329
1+
779.0313
1+
780.0282
1+
781.0298
1+
782.0298
775
776
777
778
779
780
781
782
783
784
m/z
Figure 5. ESI mass spectrum of [Cu2(PPh3)2(µ-N,S-tuc)]+ species (B) (Cu2C40H32N2OP2S; m/z =
calc, 776.02, obsd. 776.66) with isotopic pattern (1)
23

Table 1. Crystal data for 1.
T(K)
1
173(2) K
Empirical formula
C76H63ClCu2N2OP4S V(Å3)
6622.2(5)
4
M
1338.75
0.71073
Z
Dcalcd(g cm-3)
1.343
0.857
(Å)
(mm-1)
Crystal system
Monoclinic
Ia
Space group
Unit cell Dimensions
a(Å)
F(000)
2768
Reflns collected
Unique reflns
22103
22.2638(9)
11.8050(4)
15703
(Rint= 0.0302)
15703 / 110 /
688
b(Å)
Data / restraints /
parameters
Reflns.with [I2(I)]
R Indices, R1
wR2
c(Å)
()
27.4455(10)
90
12328
0.0480
0.1042
113.358(5)
R indices (all data)
R1
()
0.0699
wR2
0.1192
90
Largest diff. peak and
hole
0.567, -0.598
()
e.Å-3
.
24

Table 2. Important bond lengths (Ǻ) and angles (o) of 1
Major component (77%)
1.928(5)
2.2658(13)
2.2912(15)
2.368(2)
Cu1  N1
Cu1  P1
Cu1  P2
O1  C2
Cu2  P3
Cu2  P4
Cu2  S1
Cu2 Cl1
S1  C1
2.2335(14)
2.2666(14)
1.202(7)
2.4471(18)
1.708(5)
122.91(5)
112.81(16)
102.03(6)
111.08(6)
107.29(6)
122.78(16)
P1Cu1P2
N1Cu1P2
P3Cu2Cl1
P3Cu2S1
P4Cu2S1
N1Cu1P1
120.16(5)
108.28(6)
107.26(6)
P3Cu2P4
P4Cu2Cl1
S1Cu2Cl1
Minor component (23%)
2.431(8)
2.2658(13)
2.2912(15)
1.768(14) .
1.203(17)
Cu1S1A
Cu2  P3
Cu2  P4
Cu2 N1A
O1AC2A
2.2335(14)
2.2666(14)
2.597(7)
Cu1  P1
Cu1  P2
Cu1Cl1A
S1AC1A
P2Cu1Cl1A
P1Cu1S1A
1.697(14)
112.98(15)
109.35(18)
122.91(5)
97.86(14)
P1Cu1P2
P1Cu1Cl1A
25

109.17(19)
120.15(5)
108.7(6)
102.3(2)
129.9(6)
P2Cu1S1A
P3Cu2P4
S1ACu1Cl1A
N1ACu2P3
N1ACu2P4
Table 3. Antimicrobial activity(a-c) of 1-6.
Ligand/complex
S. aureus
S. flexneri
E. faecalis
E. coli
(MTCC 740) (MTCC 1457) (MTCC 439)
(MTCC 119)
NAe
[Cu2(PPh3)4(µ-N,S-
15
21
17
16
16
tucH)Cl]1
[This work]
[CuCl(k1S-tucH2)(PPh3)2]
2 [29]
12
NAe
NAe
NAe
NAe
[Cu2Br2(μ-S-tucH2)2(PPh3)2] 14
·2CHCl3 3 [30]
[Cu(tmtH2)(PPh3)2] 4 [29]
[Cu3Br2(k1-N,k1-S,µ-S-
tmtH2)(PPh3)6] 5 [29]
[Cu(k1-N,k1-S-
purSH)(PPh3)2] 6 [29]
2-Thiouracil
15
18
ND
ND
NAe
NAe
ND
ND
15
ND
NAe
ND
NAe
14
15
14
16
H3tmt
ND
ND
34.5
NAe
NAe
27
ND
ND
30.5
purSH2
NAe
26
Gentamicind
26

a All measurements are in mm diameter of the inhibition zone. b The standard deviation varied in
d
the range 0-1 based on three readings. c Studies were made in dmso. Commercially available
e
antimicrobial agent and it acts as positive control against bacteria . NA- not active; ND-not
determined.
27