Synthesis, solid-state structures, and urease inhibition activities of new copper(II) complexes based on O,N,O-tridentate Schiff bases

Article abstract of DOI:10.1080/00958972.2016.1206199

Six new complexes of copper(II) coordinated with O,N,O-tridentate Schiff base dianions were synthesized and structurally characterized. The solid-state structures of 1–6 contain four-coordinate mononuclear copper(II) units with a slightly distorted square planar geometry. Complexes 1 and 4 derived from d-tyrosine have an infinite 1-D, right-handed helical chain, while 5 derived from l-tyrosine has an infinite 1-D, left-handed helical chain. Inhibitions of jack bean urease by 1–6 have been investigated, and potent inhibitory activities with IC₅₀ range of 2.15?±?0.11–32.12?±?0.65?μM have been observed for these copper(II) complexes. A docking analysis using a DOCK program was conducted to position 4 into the jack bean urease active site to determine the probable binding conformation.

Full text of DOI:10.1080/00958972.2016.1206199

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Synthesis, solid-state structures, and urease
inhibition activities of new copper(II) complexes
based on O,N,O-tridentate Schiff bases
Yongming Cui, Lu Qiao, Yi Li, Hairui Jing, Yuguang Li & Qiang Wang
To cite this article: Yongming Cui, Lu Qiao, Yi Li, Hairui Jing, Yuguang Li & Qiang Wang (2016)
Synthesis, solid-state structures, and urease inhibition activities of new copper(II) complexes
based on O,N,O-tridentate Schiff bases, Journal of Coordination Chemistry, 69:15, 2318-2328,
DOI: 10.1080/00958972.2016.1206199
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1206199
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Date: 21 July 2016, At: 23:20

Journal of Coordination Chemistry, 2016
Vol. 69, no. 15, 2318–2328
http://dx.doi.org/10.1080/00958972.2016.1206199
Synthesis, solid-state structures, and urease inhibition activities
of new copper(II) complexes based on O,N,O-tridentate Schiff
bases
Yongming Cui, Lu Qiao, Yi Li, Hairui Jing, Yuguang Li and Qiang Wang
ministry of education, engineering research Center for Clean Production of textile Printing and dyeing, Wuhan
textile university, Wuhan, Pr China
ARTICLE HISTORY
ABSTRACT
received 1 January 2016
accepted 13 may 2016
Six new complexes of copper(II) coordinated with O,N,O-tridentate Schiff
base dianions were synthesized and structurally characterized. The solid-
state structures of 1–6 contain four-coordinate mononuclear copper(II)
units with a slightly distorted square planar geometry. Complexes 1 and
4 derived from d-tyrosine have an infinite 1-D, right-handed helical chain,
KEYWORDS
Copper(ii) complexes; X-ray
structures; urease inhibition;
molecular docking
while 5 derived from l-tyrosine has an infinite 1-D, left-handed helical chain.
Inhibitions of jack bean urease by 1–6 have been investigated, and potent
inhibitory activities with IC50 range of 2.15 0.11–32.12 0.65 μM have been
observed for these copper(II) complexes. A docking analysis using a DOCK
program was conducted to position 4 into the jack bean urease active site
to determine the probable binding conformation.
CONTACT yuguang li
liyg2010@wtu.edu.cn; Qiang Wang
qiang_wang@wtu.edu.cn
supplemental data for this article can be accessed at http://dx.doi.org/10.1080/00958972.2016.1206199.
© 2016 informa uK limited, trading as taylor & francis Group

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1. Introduction
The enzyme urease (urea amidohydrolase; e.C.3.5.1.5) contains two nickels at its active site and is known
to hydrolyze urea to ammonia and carbon dioxide at a rate approximately 104 times the rate of the
un-catalyzed reaction [1–4]. Hydrolysis of the reaction products results in an abrupt overall pH increase,
responsible for negative effects of urease activity in human and animal health, as well as in agriculture.
for example, urease has been shown to have virulence determinants in human and animal infections
of the urinary and gastrointestinal tracts, being involved in the formation of infection stones, pyelo-
nephritis, hepatic coma, and urinary catheter encrustation [5–7]. urease in soils can lead to excessive
volatilization of ammonia in urea fertilizer, since it degrades urea too rapidly [8, 9]. urease inhibition
studies have attracted attention in recent years; some urease inhibitors have been reported, such as
phosphorodiamidates, hydroxamic acid derivatives, and imidazoles [10–12]. unfortunately, some urease

2320
Y. CuI eT AL.
inhibitors cannot be used in vivo because of their toxicity or instability [13]. Thus, it is interesting to
synthesize new urease inhibitors with good activity and low toxicity.
recently, a series of Schiff base metal complexes with potent inhibitory activities against urease
have been synthesized by our group [14–16]. As a continuation of our work on Schiff base complexes
as urease inhibitors, six mononuclear copper(II) complexes of Schiff base ligands derived from conden-
sation of amino acids with different aldehyde compounds were synthesized. The inhibitory activities of
the obtained complexes were tested against jack bean urease. The binding model of 4, the strongest
inhibition of urease in these Schiff base Cu complexes, with jack bean urease was simulated to validate
its structure–activity relationship.
2. Experimental
2.1. Materials and physical measurements
urease (from jack beans, type III, activity 22 units/mg solid), HePeS (ultra) buffer and urea (molecu-
lar biology reagent) were from Sigma. 3,5-Dibromosalicylaldehyde and 5-chlorosalicylaldehyde were
purchased from Aldrich and used without purification. Distilled water was used for all procedures. All
experiments were performed at ambient temperature. elemental analyses (C, N, and H) were performed
using an elementar Vario eL III elemental analyzer. Infrared spectra of solid samples were recorded
using KBr pellets on a Nexus 870 fT-Ir spectrophotometer between 4000 and 400 cm−1. electron spin
resonance (eSr) spectra were recorded at room temperature using a Bruker eSr A-300 spectrometer
with the following parameters: center field 3450 G, sweep width 1500 G, microwave frequency 9.86 G,
modulation frequency 100 kHz, microwave power 1 mV. The enzyme inhibitory activity was measured
on a Bio-Tek SynergyTM HT microplate reader.
2.2. Synthesis of the complexes
General procedure for the synthesis of O,N,O-tridentate Schiff bases and copper(II) complexes 1–6:
amino acid (2.0 mmol) was added to the solution of 3,5-dibromosalicylaldehyde (0.56 g, 2.0 mmol)
[5-chlorosalicylaldehyde (345 mg, 2.0 mmol) or 2-hydroxy-1-naphthaldehyde (345 mg, 2.0 mmol)] in
methanol (25 mL). The mixture was stirred for 30 min at ambient temperature to give an orange solu-
tion, which was added to a methanol solution (25 mL) of Cu(NO3)2 (0.340 g, 2 mmol) and two drops
of pyridine/4-methyl-pyridine. The mixture was stirred for another 15 min at ambient temperature to
give a clear solution and then filtered. The filtrate was kept in air for ca. 7 days to yield block crystals.
The crystals were isolated, washed three times with distilled water, and dried in a vacuum desiccator
containing anhydrous CaCl2. H2L1, H2L2, H2L3, and H2L4 are shown in scheme 1.
2.2.1. [Cu(L1)(pyridine)] (1)
Brown solid, X-ray quality single crystals were obtained, yield: 590.2 mg (62%). Ir (KBr, cm−1): 3447, 3246,
3055, 2961, 2930, 1617, 1602, 1588, 1538,1521, 1490, 1454, 1388, 1334, 1306, 1257, 1213, 1189, 1166,
1145, 1098, 1073, 1032, 972, 948, 901, 858, 832, 757, 676, 609, 450, 420. Anal. Calcd for C25H20CuN2O4:
C, 63.03; H, 4.20; N, 5.89. found: C, 63.08; H, 4.15; N, 5.72%.
2.2.2. [Cu(L2)(pyridine)] (2)
red-brown solid, yield: 678.8 mg (71%). Ir (KBr, cm−1): 3398, 3270, 3057, 2962, 2930, 2870, 1603, 1539,
1512, 1487, 1453, 1430, 1396, 1362, 1337, 1274, 1246, 1190, 1166, 1060, 1025, 968, 847, 824, 746, 641, 532,
512, 465, 426. Anal. Calcd for C18H19CuN3O3: C, 63.76; H, 4.60; N, 5.86. found: C, 62.74; H, 4.44; N, 5.91%.
2.2.3. [Cu(L2)(4-methyl-pyridine)] (3)
red-brown solid, yield: 654.1 mg (69%). Ir (KBr, cm−1): 3521, 3194, 3060, 2935, 2851, 1676, 1626, 1541,
1511, 1457, 1435, 1417, 1399, 1363, 1345, 1305, 1284, 1182, 1092, 949, 937, 866, 825, 742, 650, 620,

JOurNAL Of COOrDINATION CHeMISTrY
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Scheme 1. ligands h2l1, h2l2, h2l3 and h2l4.
567, 517, 487, 461, 415. Anal. Calcd for C26H22CuN2O3: C, 65.82; H, 4.64; N, 5.91. found: C, 65.84; H, 4.60;
N, 5.93%.
2.2.4. [Cu(L3)(pyridine)] (4)
red-brown solid, yield: 852.2 mg (73%). Ir (KBr, cm−1): 3418, 3082, 2925, 2360, 1633, 1510, 1485, 1442,
1384, 1330, 1280, 1190, 1049, 939, 813, 769, 694, 619, 570. Anal. Calcd for C21H16Br2CuN2O4: C, 43.21; H,
2.76; N, 4.80. found: C, 43.10; H, 2.52; N, 4.93%.
2.2.5. [Cu(L3)(4-methyl-pyridine)] (5)
red-brown solid, yield: 800.9 mg (67%). Ir (KBr, cm−1): 3247, 3054, 2962, 2929, 2872, 1618, 1603, 1519,
1491, 1453, 1389, 1366, 1334, 1306, 1256, 1214, 1187, 1163, 1147, 1072, 972, 829, 754, 698, 676, 660, 526,
477, 448. Anal. Calcd for C22H18Br2CuN2O4: C, 44.17; H, 3.01; N, 4.68. found: C, 44.20; H, 2.99; N, 4.64%.
2.2.6. [Cu(L4)(4-methyl-pyridine)] (6)
red-brown solid, yield: 660.1 mg (72%). Ir (KBr, cm−1): 3521, 3186, 3060, 2935, 2851, 1676, 1626, 1541,
1511, 1457, 1435, 1417, 1399, 1363, 1345, 1305, 1284, 1182, 1082, 959, 937, 887, 815, 722, 646, 624,
577, 537, 477, 456, 420. Anal. Calcd for C18H19Br2CuN3O3: C, 57.59; H, 4.15; N, 6.11. found: C, 57.55; H,
4.18; N, 6.13%.
2.3. Crystal structure determinations
X-ray crystallographic data were collected on a Bruker SMArT Apex II CCD diffractometer using graph-
ite-monochromated Mo-Kα (λ = 0.71073 Å) radiation [17, 18]. The collected data were reduced using
SAINT, and empirical absorption corrections were performed using SADABS. The structures were solved
by direct methods and refined against F2 by full-matrix least squares using SHeLXTL version 6.1. All
non-hydrogen atoms were refined anisotropically. All hydrogens were placed in geometrically ideal
positions and constrained to ride on their parent atoms.
2.4. Measurement of jack bean urease inhibitory activity
The measurement of urease activity was carried out according to the procedure reported by Tanaka
[19]. Generally, the assay mixture containing 25 μL of jack bean urease (12 ku L−1) and 25 μL of the
test complexes with the concentration range of 0.5−7.5 μM (dissolved in DMSO :H2O = 1:1 v/v) was

2322
Y. CuI eT AL.
preincubated for 1 h at 37 °C in a 96-well assay plate. After preincubation, 200 μL of 100 mM HePeS
(4-(2-hydroxyerhyl)piperazine-1-erhanesulfonic acid) buffer pH 6.8 containing 500 mM urea [20] and
0.002% phenol red was added and incubated at 37 °C. The reaction was measured with a microplate
reader (560 nm), where an increase in the pH value from 6.8 to 7.7 for the HePeS buffer was determined
by the color change of the phenol red indicator [21].
2.5. Docking simulations
Molecular docking of the inhibitor with the 3-D structure of jack bean urease (entry 3LA4 in the Protein
Data Bank) was carried out using the DOCK 4.2 program suite [22–24]. The graphical user interface
AutoDockTools (ADT 1.4.5) was performed to setup inhibitor–enzyme interactions, where all hydrogens
were added. Gasteiger charges were calculated, and nonpolar hydrogens were merged to carbons. The
Ni initial parameters were set as r = 1.170 Å, q = +2.0, and van der Waals well depth of 0.100 kcal mol−1
[25]. As performed by the graphical user interface AutoDockTools, the catalytic center and the peripheral
anionic site of the target protein were scanned to evaluate the binding mode of the inhibitor–urease
complex. The flexible docking of the ligand structures was done by the Lamarckian genetic algorithm,
searching for favorable bonding conformations of the ligands at the sites of the target protein. The
docking procedure of complex with the enzyme active site of jack bean urease was performed as
described previously [26].
3. Results and discussion
3.1. Synthesis
All the complexes were prepared from reactions of 3,5dibromosalicylaldehyde/5-chlorosalicylalde-
hyde or 2-hydroxy-1-naphthaldehyde with l-tyrosine/phenylalanine, pyridine/4-methyl-pyridine and
copper(II) chloride. The obtained complexes were microcrystalline solids which were stable in air with
melting points above 200 °C. They were soluble in organic solvents such as methanol, DMf, and DMSO.
Copper ions in 1–6 were coordinated by oxygen and nitrogen donors from two Schiff base ligands. The
elemental analyses were in agreement with the chemical formula proposed for 1–6.
3.2. IR analysis and ESR spectra
The Schiff base complexes 1–6 were characterized by single-crystal X-ray diffraction, fT-Ir spectroscopy,
and elemental analysis. Ir spectra of these complexes exhibit strong absorptions at 1633–1603 cm−1
,
assignable to v(C=N) [27]. The C=N groups in 1–6 show strong absorptions at 1617, 1603, 1626, 1633,
1618, and 1626 cm−1, respectively. Due to the impact of metal atoms, the amino-absorption peaks of the
Schiff base ligands shift to lower wavenumber. In addition, the broad strong absorption between 1521
and 1512 could be attributed to the C=C backbone stretching vibration in the benzene ring. Several
weak bands at 3000–2800 cm−1 were most likely due to aromatic C–H groups. These Schiff base com-
plexes were confirmed by additional weak bands at 487–415 and 577–512 cm−1, which were attributed
to v(M–O) [28]. The ePr spectrum of 5 clearly indicates a slightly distorted square planar geometry of
the copper(II) center (g∥ = 2.144, g⊥ = 2.043, see figure S7), which is compared with that of a copper(II)
center (g∥ = 2.226, g⊥ = 2.064) in the literature [29].
3.3. Crystal structure description
The molecular structures of 1–6 determined by single-crystal X-ray diffraction are shown in figures 1–3.
These copper(II) complexes adopt square planar coordination geometries and comprise mononuclear
four-coordinate copper(II) units with a general formula of CuLX, in which X is a monodentate ligand
(X = pyridine for 1, 2, and 4, and 4-methylpyridine for 3, 5, and 6). L is the O,N,O-tridentate Schiff base

JOurNAL Of COOrDINATION CHeMISTrY
2323
Figure 1. (a) Ball-and-stick representation of molecular structure for 1; (b) Ball-and-stick representation of molecular structure for 2;
(c) Ball-and-stick representation of molecular structure for 3.
Figure 2. (a) Ball-and-stick representation of molecular structure for 4; (b) the 1-d right-handed helical chain of 4 (symmetric code:
(i) 1/2 − x, −1/2 + y, 7/4 − z).
Figure 3. (a) Ball-and-stick representation of molecular structure for 5; (b) Ball-and-stick representation of molecular structure for 6.
dianion which was derived from D-tyrosine (L1 in 1, L3 in 4), l-tyrosine (L3 in 5), and l-phenylalanine
(L2 in 2 and 3, L4 in 6), respectively. Crystallographic data of 1–6 are given in table 1. Selected bond
distances and angles are given in table 2.
Single-crystal X-ray diffraction analysis reveals that 1 crystallizes in the monoclinic space group
P21/c (No. 14). H2L1 derived from condensation of 2-hydroxy-1-naphthaldehyde and d-tyrosine, in the
presence of pyridine, forms the four-coordinate Cu(L1)(pyridine), where L1 is the tridentate Schiff base
dianion (figure 1(a)). In the Cu(L1)(pyridine), the coordination environment of the copper(II) is square
planar, with O,N,O-donors of the L1 dianion and a N of pyridine. Complex 1 has a τ4 value of 0.05 in
a CuN2O2 chromophore, supporting the assignment of square planar geometry [30, 31]. The Cu(L1)

2324
Y. CuI eT AL.
Table 1. Crystal data for 1–6.
Complex
1
2
3
4
5
6
empirical formula
C25h20Cun2o4
C25h22Cun2o4
C26h22Cun2o3
C21h16Br2Cu-
n2o4
C22h18Br2Cu-
n2o4
C22h19ClCu-
n2o3
molecular weight
Crystal system
space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
475.97
monoclinic
P21/c
8.5871(7)
12.8439(10)
19.2723(13)
90.00
107.071(3)
90.00
477.99
monoclinic
P21
10.2706(4)
5.2969(3)
20.1325(9)
90.00
474.00
monoclinic
P21
10.0945(10)
5.2955(6)
20.322(2)
90.00
583.72
tetragonal
P43212
10.7451(11)
10.7451(11)
36.473(4)
90.00
597.74
monoclinic
P21
9.6228(10)
12.4415(13)
18.1188(19)
90.00
95.647(2)
90.00
458.38
orthorhombic
P21212
14.789(2)
9.3378(14)
14.398(2)
90.00
102.307(2)
90.00
100.883(2)
90.00
90.00
90.00
90.00
90.00
T (K)
291(2)
2031.9(3)
4
1.556
980
1.112
3985/0/289
291(2)
1070.09(9)
2
1.483
494
1.737
2563/1/289
291(2)
1066.79(19)
2
1.476
490
1.055
3917/1/290
291(2)
4211.0(7)
8
1.841
2296
4.864
3568/0/275
291(2)
2158.7(4)
4
1.839
1180
4.746
7282/1/563
291(2)
1988.3(5)
4
1.531
940
1.259
3898/0/263
V (Å3)
Z
ρCalcd (g cm−3
F (0 0 0)
)
μ (mo-Kα) (mm−1
data/restraint/
parameters
)
Goodness of fit on F2
final R1, wR2
1.006
1.010
1.017
1.012
1.015
1.012
0.0566, 0.1047 0.0284, 0.0725 0.0213, 0.0567 0.0463, 0.1114 0.0278, 0.0620 0.0193, 0.0522
[I > 2σ(I)]
Table 2. selected bond lengths (Å) and angles (°) for 1–6.
1
Cu1−o1
Cu1−o2
1.874(2)
1.924(3)
Cu1−n1
Cu1−n2
1.926(3)
2.024(3)
o1−Cu1−o2
176.41(12)
n1−Cu1−n2
175.90(13)
2
Cu1−o1
Cu1−o2
1.900(2)
1.954(2)
Cu1−n1
Cu1−n2
1.932(2)
2.014(3)
o1−Cu1−o2
163.72(10)
n1−Cu1−n2
174.64(12)
3
Cu1−o1
Cu1−o2
o1−Cu1−o2
1.9110(14)
1.9528(14)
162.29(6)
Cu1−n1
Cu1−n2
n1−Cu1−n2
1.9359(14)
2.0155(15)
175.51(7)
4
Cu1−o1
Cu1−o2
1.879(5)
1.936(5)
Cu1−n1
Cu1−n2
1.907(6)
1.997(6)
o1−Cu1−o2
166.8(2)
n1−Cu1−n2
169.1(3)
5
Cu1−o1
Cu1−o2
1.903(3)
1.938(3)
Cu1−n1
Cu1−n2
1.924(4)
1.994(4)
Cu2−o5
Cu2−o6
1.896(3)
1.952(3)
Cu2−n3
Cu2−n4
1.936(4)
1.984(4)
o1−Cu1−o2
o5−Cu2−o6
167.56(13)
165.66(14)
n1−Cu1−n2
n3−Cu2−n4
169.73(15)
167.17(15)
6
Cu1−o1
Cu1−o2
o1−Cu1−o2
1.9043(12)
1.9506(11)
169.59(5)
Cu1−n1
Cu1−n2
n1−Cu1−n2
1.9403(14)
2.0319(15)
171.55(5)
(pyridine) molecules are interconnected through intermolecular O–H⋯O hydrogen bonds between
the phenol OH of the tyrosine and two carboxylic oxygens (O2 and O3) of the adjacent molecule,
{O4–H4⋯O2#=3.033(4) Å, 118.7° and O4–H4⋯O3#=2.803(4) Å, 159.2°}. This leads to an infinite 1-D,
right-handed helical chain.

JOurNAL Of COOrDINATION CHeMISTrY
2325
In contrast to 1, both 2 and 3 crystallize in the monoclinic space group P21 (No. 4). Complex 2 with
pyridine crystallizes with water of crystallization when compared with 3 with 4-methylpyridine. The
molecular structures of 2 and 3 consist of mononuclear four-coordinate copper(II) units, [Cu(L2)(pyri-
dine)]·H2O and Cu(L2)(4-methylpyridine), respectively, as depicted in figures 1(b) and (c). The coordi-
nation geometry around copper in 2 is strikingly similar to that in 3, both showing slightly distorted
square planar geometry with the L2 dianion a O,N,O-tridentate ligand derived from l-phenylalanine.
Complexes 2 and 3 have the corresponding τ4 values of 0.15 and 0.16 in a CuN2O2 chromophore, sup-
porting the assignment of a distorted square planar geometry. The four-coordinate copper centers
in 2 and 3 are further linked through the carboxyl O from an adjacent molecule occupying the apical
site, Cu1–O3# = 2.466(3) Å in 2 and 2.501(3) Å in 3. The coordination sphere around copper(II) in 2
and 3 can be described as a slightly distorted square pyramidal with the respective τ5 values of 0.18
and 0.22 [32]. This contact leads to an infinite 1-D zigzag chain in the solid-state structures of 2 and
3. In addition, the lattice water molecules in 2 are connected by hydrogen bonding interactions {O4–
H4B⋯O4# = 2.900(4) Å, 111.3°}, which leads to an infinite 1-D water chain. Such an infinite water chain
is further linked with two neighboring zigzag chains by hydrogen bonds between the carboxylate O3 of
the tridentate L2 dianion and lattice water molecules, {O4–H4A⋯O3# = 2.818(3) Å, 140.1°}, to generate
an infinite 1-D ladder-like structure of 2.
Complexes 4 and 5 crystallize in the tetragonal space group P43212 (No. 96) and the monoclinic space
group P21 (No. 4), respectively. The crystal structures of 4 and 5 are shown in figures 2(a) and 3(a). In
the asymmetry unit, 4 presents a [Cu(L3)(pyridine)] molecule, while 5 contains two independent [Cu(L3)
(4-methylpyridine)] molecules. X-ray crystallographic studies have shown that both 4 and 5 contain
copper in a slightly distorted square planar geometry with a CuN2O2 chromophore. The coordination
sites are occupied by O,N,O-donors from L3 dianion and a pyridine (4) or 4-methylpyridine (5). The τ4
values for the CuN2O2 chromophore are 0.17 in 4 and 0.16 and 0.19 in 5, respectively. This supports
an assignment of the distorted square planar geometry of 4 and 5. Similar to 1, the Cu(L3) (pyridine)
molecules of 4 derived from d-tyrosine are linked into an infinite 1-D, right-handed helical chain by inter-
molecular O–H⋯O hydrogen bonds between the phenol OH groups of d-tyrosines and one carboxylic
O3 of adjacent molecules, {O4–H4A⋯O3# = 2.690(9) Å, 140.2°} (figure 2(b)). [Cu(L3)(4-methylpyridine)]
molecules of 5 derived from l-tyrosine are linked into an infinite 1-D, left-handed helical chain by
intermolecular O–H⋯O hydrogen bonds between the phenol OH groups of the l-tyrosines and the
carboxylate oxygens (O3 and O8) of adjacent molecules, {O4–H4A⋯O3# = 2.698(5) Å, 164.7° for Cu1
and O8–H8A⋯O7# = 2.744(4) Å, 165.1° for Cu2}.
The space group of 6 was determined to be P21212 (No. 18). The crystal structure of 6, shown in figure
3(b), is structurally very similar to 3. The tridentate Schiff base ligand (H2L4) derived from condensation
of 5-chlorosalicylaldehyde and l-phenylalanine, in the presence of 4-methylpyridine, forms the four-co-
ordinate Cu(L4)(4-methylpyridine), where L1 is a tridentate dianion. This leads to a square planar CuN2O2
chromophore with τ4 value of 0.13. In the solid state, an infinite zigzag chain of 6 is also achieved, owing
to the apical position occupied by a carboxylate O3 from an adjacent molecule, Cu1–O3# = 2.449(3) Å.
3.4. Inhibitory activity against jack bean urease
The urease inhibiting abilities of 1–6 were studied by testing their IC50 values against jack bean urease
(see table 3). Compared with the standard inhibitor acetohydroxamic acid (AHA, IC50 = 63.70 μM), the
copper(II) complexes 1–6 display potent inhibitory activities (IC50 = 2.15−32.12 μM) against jack bean
urease. The strong urease inhibiting ability exhibited by the copper(II) complexes is due to the strong
Lewis acid properties of copper ion [33].
Inhibitory strength of the six copper(II) complexes toward jack bean urease decreases in the order
4 > 5 > 6 > 2 > 3 > 1. Among the six copper(II) complexes tested, 4, 5, and 6 derived from salicy-
laldehyde with halogen substitution are much more potent than those of 1, 2, and 3 derived from
2-hydroxy-1-naphthaldehyde. This defines the pattern of halogen (Br or Cl) substitution in the aromatic
ring for obtaining potent activities.

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Complexes 4 and 5 exhibit stronger ability to inhibit urease than 6 although they all have halogen
substitution. This indicates that Br-substitution in the aromatic ring is advantageous for obtaining potent
activity compared to Cl-substitution. This result is consistent with our previous study [34]. Although the
crystal structures of 2, 3, 4, and 5 are very similar except for the 4-methyl substitution on pyridine, the
urease inhibiting abilities of 4 and 2 are higher than those of 5 and 3, respectively. The results suggest
that methyl substitution will make the molecular chain longer and decrease the activity.
3.5. Molecular docking study
X-ray structure of the native jack bean urease (entry 3LA4 in the Protein Data Bank) shows that the
two nickels are surrounded by His407, His409, Lys490, His492, Asp494, His519, His545, Cys592, His593,
Arg 609, Asp633, and Ala636 [35]. Molecular docking of 4 into the active site of jack bean urease was
performed on the binding model based on the jack bean urease structure (3LA4.pdb). This provides
understanding of the present inhibitory activity of the complex against jack bean urease.
The binding model of 4 at the active site of jack bean urease is depicted in figure 4. All the
amino acid residues which had interactions with urease are shown in figure 4. In the 4-urease
Table 3. inhibition of jack bean urease by 1–6 and aha.
Tested materials
IC50 (μM)
1
32.12 0.65
22.50 1.01
27.32 0.31
2.15 0.11
4.25 0.54
8.80 1.02
60.22 0.85
2
3
4
5
6
acetohydroxamic acid
Figure 4. the binding model of 4 at the active site of jack bean urease.

JOurNAL Of COOrDINATION CHeMISTrY
2327
Figure 5. the docking model of enzyme surface structure of 4-urease complex.
model, three hydrogen bonds were formed. One formed between the oxygen of 4 and the amino
hydrogen of His593 (length of the hydrogen bond: His593N–H⋯O4 = 3.111(7) Å, energy of the
hydrogen bond: His593 N–H⋯O4 = −2.1864). The other two hydrogen bonds formed between the
hydroxyl group of 4 and oxygens of Met588 with Leu589, respectively (length of the hydrogen
bonds: Met588O⋯OH4 = 2.944(3) Å, Leu589O⋯OH4 = 2.5042 Å; energy of the hydrogen bonds:
Met588O⋯OH4 = −2.5, Leu589O⋯OH4 = −1.7016). Moreover, 4 might form hydrophobic inter-
actions with Leu 589, Ala 440, and Ala 636, respectively. The urease inhibitory property may be
attributed to the above hydrogen bonds and hydrophobic interactions formed between these
ligands and urease.
The docking model of the enzyme surface structure of 4-urease complex is shown in figure 5.
Complex 4 filled in the active pocket of the urease. The results of molecular docking study could explain
the inhibitory activity of the complex against jack bean urease.
4. Conclusion
This paper describes the synthesis, crystal structures, and urease inhibitory activities of six new cop-
per(II) complexes with O,N,O-tridentate Schiff base ligands. These copper(II) complexes exhibited good
inhibitory activity against jack bean urease. The docking calculations revealed that the Schiff base-cop-
per(II) complexes have potential as urease inhibitors. The results in this work are in accord with studies
reported earlier, where some Cu(II) complexes have stronger urease inhibitory activities to urease, with
IC50 ranging from 0.1 to 50 μM [15, 33, 36–39]. Compared with the data reported before, the complexes
reported in this study exhibit strong inhibitory activity to urease and may be used as urease inhibitors.
Detailed investigations are continuing to study the mechanisms of the inhibitory activity.
Supplementary material
CCDC numbers 887876 (1), 887877 (2), 887878 (3), 1416455 (4), 887880 (5), and 887879 (6) contain the supplementary
crystallographic data (CIf) for this article. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html or from the Director, CCDC, 12 union road, Cambridge CB2 1eZ, uK (fax: +44-1223-336-033; e-mail: deposit@
ccdc.cam.ac.uk).

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Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the National Natural Science foundation of China [grant number 21101122], [grant number
21277106]; the Collaborative Innovation Plan of Hubei Province for KeyTechnology of eco-ramie Industry; State Scholarship
fund of China [grant number 201408420061].
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