Urease inhibitory activities of ZnBr2 and ZnI2 complexes of terpyridine derivatives: Systematic investigation of aryl substituents on urease inhibitory activities

Article abstract of DOI:10.1016/j.poly.2012.07.035

With the aim of discovering novel urease inhibitors, six new mononuclear complexes of five-coordinated Zn(II) bromide and iodide with terpyridine derivatives have been synthesized and their solid state structures were characterized by X-ray crystallograph

Full text of DOI:10.1016/j.poly.2012.07.035

Polyhedron 45 (2012) 9–14
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Polyhedron
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Urease inhibitory activities of ZnBr2 and ZnI2 complexes of terpyridine
derivatives: Systematic investigation of aryl substituents on urease inhibitory
activities
b
c
c
Ali Nemati Kharat a, , Abolghasem Bakhoda , Giuseppe Bruno , Hadi Amiri Rudbari
⇑
a School of Chemistry, University College of Science, University of Tehran, Tehran, Iran
b Department of Chemistry, Southern Illinois University, Edwardsville, IL 62026, United States
c Dipartimento di Chimica Inorganica, Vill. S. Agata, Salita Sperone 31, Università di Messina, 98166 Messina, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
With the aim of discovering novel urease inhibitors, six new mononuclear complexes of five-coordinated
Zn(II) bromide and iodide with terpyridine derivatives have been synthesized and their solid state struc-
tures were characterized by X-ray crystallography. Among these complexes, the zinc complexes bearing
an electron withdrawing aryl group at the 40-position show medium urease inhibitory activities, with IC50
values much lower than those of previously reported zinc species, at concentrations lower than 100 mM.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 17 April 2012
Accepted 1 July 2012
Available online 20 July 2012
Keywords:
Urease inhibition
Zinc bromide
Zinc iodide
Terpyridine
X-ray crystallography
1. Introduction
istry [17–20]. Studying the influence of the substituents on ligands
may help us to understand and balance the physio-chemical and
Urease (urea amidohydrolase; E.C.3.5.1.5) is a binuclear nickel-
containing metallo-enzyme which catalyzes the hydrolysis of urea
to produce ammonia and carbamate [1,2]. The resulting carbamate
subsequently converts to produce ammonia and carbon dioxide. It
is now apparent that the high concentrations of NH3 generated from
these reactions will result in pH elevation and have significant neg-
ative effects on human health, medicine and agriculture [3–5].
Restriction of the activity of urease using chemical inhibitors possi-
bly will neutralize these negative side effects. Nowadays, urease
inhibitors play a key role in the treatment of the infections caused
by urease producing bacteria [6]. Urease inhibitors could be catego-
rized into two types: (A) organic compounds, such as acetohydroxa-
mic acid (AHA), humic acid and 1,4-benzoquinone [7–9]; (B)
transition metal ion complexes, such as Cu2+, Zn2+, Pb2+ and Cd2+
complexes [10]. Metal complexes as urease inhibitors have been re-
ported rarely. Research has shown that Sn4+, V5+, Bi3+, Mn3+, Zn2+ and
Cu2+ complexes bear interestingureaseinhibitory activities [11–16].
Zinc complexes with N-donor ligands are currently attracting
attention for their interesting molecular topologies and crystal
packing motifs, as well as the fact that they may be designed with
specific applications in bioinorganic chemistry and material chem-
biochemical properties of synthetic compounds. In this paper, six
new zinc(II) bromide and iodide complexes with terpyridine
derivatives ([ZnBr2(40-(4-chlorophenyl)-2,20:60,200-terpyridine)] (1),
[ZnBr2(40-phenyl-2,20:60,200-terpyridine] (2), [ZnBr2(40-(pyridin-2-
yl)-2,20:60,200-terpyridine)] (3), [ZnI2(40-phenyl-2,20:60,200-terpyri-
dine] (4), [ZnI2(40-2-thienyl-2,20:60,200-terpyridine)] (5) and [ZnI2
(40-(4-methoxyphenyl)-2,20:60,200-terpyridine)] (6); Scheme 1) were
synthesized to investigate the influence of 40-position substitution
on the urease inhibitory property.
2. Results and discussion
2.1. Synthesis
The five terpyridine ligands in this study are similar tridentate
ligands, which can coordinate to the metal atom through the pyr-
idyl N-atoms. It is noteworthy that the terpyridine derivatives
were prepared by simple one-pot reactions and their zinc com-
plexes have not been reported previously. The complexes were
synthesized according to the same method: a methanol solution
(10 mL) of ZnX2 (1 mmol) was added with stirring to a chloroform
solution (10 mL) of the corresponding terpyridine derivative
(0.1 mmol). The mixture was stirred at room temperature for
10 min to give a white to yellow precipitate. Colorless to yellow
block-shaped crystals of the complexes were formed by slow diffu-
⇑
Corresponding author. Tel.: +98 21 61112499; fax: +98 21 66495291.
E-mail address: alnema@khayam.ut.ac.ir (A.N. Kharat).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.035

10
A.N. Kharat et al. / Polyhedron 45 (2012) 9–14
sion of methanol into the concentrated DMSO solution of the com-
Ar
plexes. Yield: 87% (1), 89% (2), 92% (3), 85% (4), 89% (5) and 91%
(6)1
.
N
2.2. X-ray crystallography
N
N
Zn
X-ray crystallography2 of complexes 1, 2, 4 and 5 reveals that the
structures of the complexes are similar mononuclear five coordi-
nated zinc complexes, except for the different aryl group on the 40
X
X
Cl
OMe
position of the terpyridine moiety. Complex
1
(Fig. 1),
[ZnBr2(C21H14ClN3)], exists as a distorted trigonal bipyramid and
crystallizes with one independent molecule in the asymmetric unit.
The apical position of the square pyramid is occupied by one of the
Br atoms, with the base of the pyramid consisting of the three N
atoms of the terpyridine ligand, having para-chlorophenyl as a sub-
stituent in the 40-position, and the other Br atom.
Ar =
S
N
In complex 2 (Fig. 2), [ZnBr2(C21H15N3)], the Zn2+ ion is five
coordinated by the three N atoms from a 2,20:60,200 terpyridine li-
gand, containing a phenyl group as a substituent in the 40-position,
and two bromide anions in a distorted trigonal bipyramidal config-
uration. It is noteworthy that complex 2 reported in this paper has
a totally different structure in comparison with the ZnBr2 analogue
X=Br, I
Scheme 1. Structures of the terpyridine derivatives used in the present study.
iodide anions, forming a distorted trigonal bipyramidal environ-
ment again.
For complex 5, Zn2+ is similarly five coordinated with three
nitrogen atoms from the terpyridine skeleton and two iodide
atoms, with the Zn atom being located at the distorted trigonal
bipyramidal center (shown in Fig. 4). In this complex the terpyri-
dine ligand bears a 2-thienyl ring as a substituent.
of 2,20:60,200 terpyridine (
a-terpy) [21]. Fig. 3 presents the crystal
structure of zinc iodide complex of the phenyl-substituted
2,20:60,200 terpyridine (complex 4). In the complex 4, the Zn2+ cation
is coordinated by the N atoms of the terpyridine ligand and by two
1
Further investigation of the structures reveals that the Zn–N
and Zn–Br bond lengths in 1 and 2 are comparable to each other.
The Zn–N bond length averages are in the range of 2.12–2.18 Å.
All of the coordinate bond lengths can be considered as normal
values by comparison with those reported in the literature [21–
25]. The packing of the complexes (See Figs. S1–S4) are stabilized
by medium to weak hydrogen bonds as well as
(for complexes 1, 2 and 4). There are stacking interactions in
the lattice of 1, where the chlorophenyl ring (Cg1; C16–C21) and
one flanking pyridyl ring (Cg2; N3, C8–C11) of the terpyridine
Anal. Calc. for 1 (C21H14Br2ClN3Zn): C, 44.33; H, 2.48; N, 7.38. Found: C, 44.9; H,
2.5; N, 7.7%. Selected IR data (Polyethylene, cmꢀ1): 3213w, 3091m (
m
C–H), 1619w,
1534m, 1450m, 1317m, 1199m, 1160w, 1064m, 1002w, 921w, 874w, 770m, 260m (
Zn–Br),252m ( Zn–N), 238 (
Zn–N). 1H NMR (DMSO-d6) d: 8.89 (m, 4H), 8.78 (d,
2H), 8.24 (dd, 2H), 7.92 (dd, 2H). ESI-MS of 1 in DMSO: m/z 569. Anal. Calc. for 2(C21
15Br2N3Zn): C, 47.18; H, 2.83; N, 7.86. Found:C, 47.9; H, 2.9; N, 8.0%. Selected IR data
(Polyethylene, cmꢀ1): 3250w, 3203w, 3128w, 3080 (
C–H), 1587m, 1533m, 1480s,
1435m, 1392m, 1223m, 1159m, 1112m, 1032w, 882m, 799m, 770s, 667m, 261m (
Zn–Br), 248m ( Zn–N), 236 (
Zn–N). 1H NMR (DMSO-d6) d: 8.78 (2H, d), 8.39 (2H, s),
m
m
m
H
m
m
p. . .p stacking
m
m
p–p
8.02 (2H, dd), 7.96 (2H, dd), 7.88 (2H, d), 7.82 (2H, s,), 7.79 (1H, dd), 7.65 (2H, d). ESI-
MS of 2 in DMSO: m/z 533. Anal. Calc. for 3 (C20H14Br2N4Zn): C, 44.85; H, 2.63; N,
10.46. Found:C, 45.2; H, 2.7; N, 10.7%. Selected IR data (Polyethylene, cmꢀ1): 3208w,
3079m (
878w, 766m, 251m (
m
C–H), 1653w, 1511m, 1474m, 1306m, 1212m, 1155w, 1079m, 998w, 914w,
backbone are stacked with
3.924 Å. (Fig. S1). In addition, in the crystal packing of 2, the pyridyl
rings are parallel (Cg1 and Cg 2 where Cg1 is N1,C1–C5 and Cg2 is
N3, C8–C12) and p–p stacking interactions are found between each
side ring of a pyridyl ring and another contiguous counterpart,
a centroid–centroid distance of
m
Zn–Br), 249m ( Zn–N), 231 (
m
m
Zn–N). 1H NMR (DMSO-d6) d:
8.88 (2H, d), 8.76 (1H, d), 8.13 (1H, dd), 8.02 (2H, dd), 7.93 (2H, dd), 7.89 (1H, dd), 7.86
(2H, dd), 7.78 (2H, s,), 7.64 (1H, dd). ESI-MS of 3 in DMSO: m/z 536. Anal. Calc. for 4
(C21H15I2N3Zn): C, 40.13; H, 2.41; N, 6.68. Found: C, 40.8; H, 2.5; N, 6.9%. Selected IR
data (Polyethylene, cmꢀ1): 3198w, 3115w, 3076w (
m
C–H), 1594m, 1565m, 1494s,
1451m, 1396m, 1231m, 1179m, 1146m, 1049w, 898m, 768m, 748s, 692m, 171m (
Zn–I), 245m ( Zn–N), 234 ( Zn–N). 1H NMR (DMSO-d6) d: 8.88 (2H, d), 8.34 (2H, dd),
m
with a centroid–centroid separation of 3.930 Å. Thus, a 3-D frame-
m
m
work sustained by weak
p–p stacking interactions is formed
8.05 (2H, dd), 7.93 (1H, d), 7.76 (1H, dd), 7.71 (2H, dd), 7.66 (2H, s), 7.61 (1H, d). ESI-
MS of 4 in DMSO: m/z 627. Anal. Calc. for 5(C19H13I2N3SZn): C, 35.96; H, 2.06; N, 6.62.
Found:C, 36.5; H, 2.2; N, 6.7%. Selected IR data (Polyethylene, cmꢀ1): 3239w, 3128w,
(Fig. S2). Also, in the packing of 4, the pyridyl rings (Cg1 and Cg 2
where Cg1 is N1,C1–C5 and Cg2 is N3, C8–C12) are parallel and
3069w
1104m, 1088w, 957m, 879m, 844 (
Zn–N). 1H NMR (DMSO-d6) d: 8.86 (2H, d), 8.81 (2H, dd), 8.11 (2H, dd), 7.84 (1H, d),
(
m
C–H), 1560m, 1524m, 1476s, 1421m, 1339m, 1249m, 1195m, 1144m,
p–p stacking interactions are found between neighboring pyridyl
m
C–S), 638m, 178m ( Zn–I), 245m ( Zn–N), 239
m
m
rings with a centroid–centroid distance of 3.955 Å. These interac-
tions have been shown in Fig. S3. Table 1 summarizes the most sig-
nificant bond lengths and angles for complexes 1, 2, 4 and 5.
(m
7.73 (1H, dd), 7.68 (2H, dd), 7.61 (2H, s), 7.53 (1H, d). ESI-MS of 5 in DMSO: m/z 633.
Anal. Calc. for 6 (C22H17I2N3OZn): C, 40.12; H, 2.60; N, 6.38. Found: C, 40.5; H, 2.7; N,
6.5%. Selected IR data (Polyethylene, cmꢀ1): 3235w, 3196w, 3114w, 3068w (
m
C–H),
C–H, Me) 1601m, 1541m, 1484s, 1440m, 1374m, 1236m, 1167m, 1128m,
1036w, 874m, 771m, 763s, 649m, 161m ( Zn–I), 242m ( Zn–N), 237 ( Zn–N). 1H
2853(
m
2.3. Urease inhibitory activity
m
m
m
NMR (DMSO-d6) d: 8.63 (2H, d), 8.62 (2H, dd), 8.58 (2H, dd), 7.87 (2H, d), 7.77 (2H,
The measurement of jack bean urease inhibitory activity was
carried out in triplicate according to the phenol-red literature
dd), 7.31 (2H, dd), 7.12 (2H, s), 3.84 (3H, s). ESI-MS of 6 in DMSO: m/z 657.
2
Crystal data for 1 (C21H14Br2ClN3Zn): Mw = 568.99, Monoclinic, space group P21/
n, a = 9.0278(4), b = 15.3710(6), c = 14.6727(5) Å, V = 1981.87(14) Å3, Z = 4,
method [26]. The assay mixture, containing 25
lL of jack bean
calc = 1.907 Mg/m3, ) = 5.419 mmꢀ1, R1 = 0.0465, wR2 = 0.0938 (all data),
l (Mo Ka
urease (10 kULꢀ1) and 25
l
L of the test materials of various
q
T = 296(2) K. Crystal data for 2 (C21H15 Br2ClN3Zn): Mw = 534.55, Monoclinic, space
concentrations (dissolved in DMSO:H2O solution 1:1 (V/V)), was
pre-incubated for 1 h at 37 °C in a 96-well assay plate. Aliquots of
0.2 mL of 100 mM Hepes (N-[2-hydroxyethyl]piperazine-N0-[2-eth-
anesulfonic acid]) buffer at pH 6.8 containing 500 mM urea and
0.002% phenol red were added and incubated at 37 °C. The reaction
time, which was required to produce enough ammonium carbonate
to raise the pH of the Hepes buffer from 6.8 to 7.7, was measured by
a micro-plate reader (at 570 nm) with the end-point determined by
group C2/c, a = 13.6700(2), b = 14.9655(2), c = 18.7708(3) Å, V = 3839.73(10) Å3, Z = 8,
q
calc = 1.849 Mg/m3, ) = 5.452 mmꢀ1, R1 = 0.0435, wR2 = 0.0881 (all data),
l (Mo Ka
T = 296(2) K. Crystal data for 4 (C21H15I2N3Zn): Mw = 628.559, Monoclinic, space
group C2/c, a = 13.7769(4), b = 15.2209(4), c = 19.4317(6) Å, V = 4074.3(2) Å3, Z = 8,
q
calc = 2.049 Mg/m3, ) = 4.247 mmꢀ1, R1 = 0.0459, wR2 = 0. 1162 (all data),
l (Mo Ka
T = 296(2) K. Crystal data for 5 (C19H13I2N3SZn): Mw = 634.55, Monoclinic, space
group P21/n, a = 10.1344(2), b = 17.9080(4), c = 11.2606(2) Å, V = 2025.58(7) Å3, Z = 4,
q
calc = 2.081 Mg/m3,
T = 296(2) K.
l (Mo Ka
) = 4.372 mmꢀ1, R1 = 0.0308, wR2 = 0.0913 (all data),

A.N. Kharat et al. / Polyhedron 45 (2012) 9–14
11
Fig. 1. Molecular structure of 1 and the atomic numbering scheme, thermal ellipsoids are drawn at 30% probability.
Fig. 2. Molecular structure of 2 and the atomic numbering scheme, thermal ellipsoids are drawn at 30% probability.
the color of the phenol red indicator. The complexes remained in-
tact under these conditions (in the presence of DMSO and Hepes
buffer) as we checked their solutions by 1H NMR.1
The results are summarized in Table 2. Complexes 1 and 3 show
medium urease inhibitory activity with IC50 values being margin-
ally higher than those of acetohydroxamic acid (AHA), co-assayed
as a standard urease inhibitory agent, while complexes 2, 4, 5 and
6 show poor activity. The results in this paper are not in accordance
with those reported formerly, in which it was stated that zinc(II)
complexes have no urease inhibitory activity [27,28].
As is clearly shown in Table 2, the electron withdrawing aryl
substituted terpyridine ligands (40-(4-chlorophenyl)-2,20:60,200-ter-
pyridine and 60-(pyridin-2-yl)-2,20:40,200-terpyridine) could en-
hance the inhibition property of the complexes, while electron
donating substituted aryl rings have a minimal effect on the urease
inhibitory activity. Control experiments were carried out and the

12
A.N. Kharat et al. / Polyhedron 45 (2012) 9–14
Fig. 3. Molecular structure of 4 and the atomic numbering scheme, thermal ellipsoids are drawn at 30% probability.
Fig. 4. Molecular structure of 5 and the atomic numbering scheme, thermal ellipsoids are drawn at 30% probability.
results are presented in Table 2, as well. Both zinc(II) halides as
well as the free ligands displayed negligible activities. The results
of the control experiments are in accordance with those reported
in the literature [28].
The mechanism of this inhibition has not been elucidated. Con-
sidering the effect of Zn compounds on urease [10], the following
suggestion is probable. The active site of urease contains two Ni
ions, which is essential to the enzymatic activity. If those Ni ions
are substituted by Zn ions, urease is significantly deactivated. We
propose that the exchange of Ni by Zn happens considering the
comparable complex-forming ability of these two transition metal
ions and the deactivation of urease. Hence, the electron withdraw-

A.N. Kharat et al. / Polyhedron 45 (2012) 9–14
13
Table 1
Bond lengths (Å) and angles (°) for complexes 1, 2, 4 and 5.
Complex (1)
Complex (2)
Complex (4)
Complex (5)
Zn1–Br1
Zn1–Br2
Zn1–N1
Zn1–N2
Zn1–N3
2.3934(5)
2.3899(4)
2.212(3)
2.109(2)
2.214(3)
Zn1–Br1
Zn1–Br2
Zn1–N1
Zn1–N2
Zn1–N3
2.3826(4)
2.4050(4)
2.198(2)
2.098(2)
2.224(2)
Zn1–I1
Zn1–I2
Zn1–N1
Zn1–N2
Zn1–N3
2.6030(6)
2.5714(6)
2.205(4)
2.090(4)
2.229(4)
Zn1-I1
2.6067(5)
2.5957(5)
2.192(3)
2.079(3)
2.200(3)
Zn1–I2
Zn1–N1
Zn1–N2
Zn1–N3
Br1–Zn1–Br2
Br1–Zn1–N1
Br1–Zn1–N2
Br1–Zn1–N3
Br2–Zn1–N1
Br2–Zn1–N2
Br2–Zn1–N3
N1–Zn1–N2
N1–Zn1–N3
N2–Zn1–N3
112.467(18)
101.71(7)
110.79(6)
100.01(7)
96.62(6)
136.74(6)
98.96(6)
73.91(9)
Br1–Zn1–Br2
Br1–Zn1–N1
Br1–Zn1–N2
Br1–Zn1–N3
Br2–Zn1–N1
Br2–Zn1–N2
Br2–Zn1–N3
N1–Zn1–N2
N1–Zn1–N3
N2–Zn1–N3
114.091(17)
97.96(6)
138.99(6)
96.33(6)
I1–Zn1–I2
112.62(2)
97.68(10)
106.34(10)
105.45(10)
97.83(10)
140.95(10)
97.22(10)
73.99(13)
144.86(14)
74.32(14)
I1–Zn1–I2
114.320(17)
98.24(9)
122.24(8)
98.64(8)
96.40(9)
123.41(8)
99.35(8)
74.92(11)
149.65(11)
74.75(11)
I1–Zn1–N1
I1–Zn1–N2
I1–Zn1–N3
I2–Zn1–N1
I2–Zn1–N2
I2–Zn1–N3
N1–Zn1–N2
N1–Zn1–N3
N2–Zn1–N3
I1–Zn1–N1
I1–Zn1–N2
I1–Zn1–N3
I2–Zn1–N1
I2–Zn1–N2
I2–Zn1–N3
N1–Zn1–N2
N1–Zn1–N3
N2–Zn1–N3
98.44(6)
106.90(6)
104.66(6)
74.20(8)
144.87(8)
73.96(8)
145.91(9)
73.95(9)
Table 2
Inhibition of urease by the tested materials.
a
a
Tested materials
IC50
(
lM)
Tested materials
IC50
(
lM)
>100
72.96 0.12
>100
92.84 0.50
>100
56.12 0.72
>100
91.72 0.48
>100
92.36 0.24
96.42 0.36
45.32 0.27
(continued on next page)

14
A.N. Kharat et al. / Polyhedron 45 (2012) 9–14
Table 2 (continued)
a
a
Tested materials
IC50
(
lM)
Tested materials
IC50 (lM)
Acetohydroxamic Acid (AHA)
ZnBr2
>100
ZnI2
>100
a
All IC50 values were expressed as mean S.D. values of the triplicate tests.
ing substituents on the terpyridyl rings can help the metal center to
be separated and coordinate the Ni centers of the urease. This phe-
nomenon is reported by another research group elsewhere. [29].
Appendix A. Supplementary data
CCDC Nos. 846606–846610 contains the supplementary crystal-
lographic data for complexes 1, 2, 4 and 5, respectively. These data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/con-
ts/retrieving.html, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223–
336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.poly.2012.07.035.
3. Conclusion
The present paper reports the syntheses, structures and urease
inhibitory activities of a series of structurally similar zinc(II) bro-
mide and iodide complexes with
a-terpyridine derivatives. The
structure–activity relationship specifies that electron withdrawing
aryl groups on the 2,20:60,200-terpyridine ligands could improve the
urease inhibitory activities of the zinc(II) complexes. Bearing in
mind that zinc complexes have interesting biological activities
and have been used in medicine, the complexes reported in this pa-
per may possibly be used in the treatment of toxicities caused by
urease producing bacteria.
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The ligand was synthesized according to the Wang and Hanan
procedure, with a slight modification [30]. 2-Acetylpyridine and
the corresponding carboxaldehyde were mixed together all at
once. Addition of 28–30% ammonia solution resulted in the synthe-
sis of crude 40-(aryl)- 2, 20; 60, 200-terpyridine (see Scheme 1 of the
paper). After crystallization of the impure ligand from a CHCl3:Et2O
(1:1) mixture, the analytically pure 40-(aryl)- 2, 20; 60, 200-terpyri-
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The assay mixture, containing 25
lL (4U) of jack bean urease and
25
l
L (100 g) of the test compound, was pre-incubated for 30 or
l
180 min at room temperature in a 96-well assay plate. After pre-
incubation, 0.2 mL of 100 mM phosphate buffer pH 6.8 containing
500 mM urea and 0.002% phenol red indicator were added and
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Acknowledgments
This work was financially supported by the University of Teh-
ran. The authors are also grateful to Prof. Sadegh Khazaeli for his
useful comments.