Synthesis, structure, and urease inhibitory activities of three binuclear copper(II) complexes with protocatechuic acid derivative

Article abstract of DOI:10.1080/00958972.2014.910597

Three Cu(II) complexes, [Cu^II₂(L₁) ₄(L₂)₂] (1), [Cu^II ₂(L₁)₄(H₂O)₂]·HL (2), and [Cu^II₂(L₁)

Full text of DOI:10.1080/00958972.2014.910597

Journal of Coordination Chemistry, 2014
Vol. 67, No. 7, 1265–1278, http://dx.doi.org/10.1080/00958972.2014.910597
Synthesis, structure, and urease inhibitory activities of three
binuclear copper(II) complexes with protocatechuic acid
derivative
GUI-HUA SHENG†‡, QUAN-CHENG ZHOU†§, JUAN SUN†‡, XIAO-SHAN CHENG¶,
SHAO-SONG QIAN†, CHUN-YANG ZHANG†, ZHONG-LU YOU*¶ and
HAI-LIANG ZHU*†‡
†State Key Laboratory of Pharmaceutical Biotechnology, Nanjing University, Nanjing, PR China
‡School of Life Sciences, Shandong University of Technology, Zibo, PR China
§School of Agricultural Engineering and Food Science, Shandong University of Technology, Zibo,
PR China
¶Department of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian,
PR China
(Received 21 February 2013; accepted 11 March 2014)
Three Cu(II) complexes, [CuII2(L1)4(L2)2] (1), [CuII2(L1)4(H2O)2]·HL (2), and [CuII2(L1)3(L3)2]ClO4
(3), with a ligand derived from protocatechuic acid (HL1 = C9H8O4=2,3-dihydrobenzo[b][1,4]-
dioxine-6-carboxylic acid, L2 = C7H9N=o-toluidine, L3 = C6H16N2=N,N-diethylethylenediamine)
were synthesized and characterized by C, H, and N elemental analysis and single-crystal X-ray
diffraction, which revealed that the three complexes have similar binuclear structures. Complexes 1
and 3 crystallized in triclinic space group P-1 and 2 in orthorhombic space group P212121. The
urease inhibitory activities of the three complexes were tested. All three complexes showed strong
inhibitory activity against jack bean urease with an IC50 value of 6.8, 5.5, and 3.5 μM compared
with the acetohydroxamic acid (IC50 = 7.5 μM), which was a positive control.
Keywords: Copper(II) complex; Crystal structure; Urease inhibitors; Protocatechuic acid
*Corresponding authors. Email: youzhonglu@lnnu.edu.cn (Z.-L. You); zhuhl@nju.edu.cn (H.-L. Zhu)
G.-H. Sheng and Q.-C. Zhou are contributed equally to this paper.
© 2014 Taylor & Francis

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1. Introduction
Urease (urea amidohydrolase; E.C.3.5.1.5) is a nickel-containing metalloenzyme that
rapidly catalyzes the hydrolysis of urea to ammonia and carbamate [1, 2]. Urease is widely
distributed in a variety of algae, bacteria, fungi and plants. The reaction catalyzed by urease
may cause an accumulation of ammonia and accompanying pH elevation, which has
negative implications in medicine and agriculture [3–6]. Urease in Helicobacter pylori is
accepted as a major cause of peptic ulcers [2]. H. pylori is characterized by very high
urease activity which may act as a virulence or survival factor. Urease inhibitors could
counteract these negative effects through control of the activity of urease, so urease inhibi-
tors are very important in treatment of infections caused by urease-producing bacteria [7].
Current efforts are focused on the discovery of urease inhibitors against H. pylori urease.
Therefore, urease inhibitors have recently attracted attention as potential new anti-ulcer
drugs. Urease inhibitors can be broadly classified into two fields: (1) organic compounds,
such as acetohydroxamic acid, humic acid, and 1,4-benzoquinone [8–10] and (2) metal
ions, such as Cu2+, Zn2+, Pd2+, and Cd2+ [11, 12]. Some metal complexes have been
reported to show urease inhibitory activities [13–17]. Protocatechuic acid (3,4-dihydroxy-
benzoic acid) is a simple phenolic compound widely distributed in nature. It is detected in
almost all plants and is one of the biologically active components of some medicinal plants,
including those used in natural medicine. It is reported that protocatechuic acid has antioxi-
dant, anti-radical activity, and chemopreventive ability in chemically induced carcinogenesis
[18–20]. Some complexes with protocatechuic acid and its derivatives as ligands have been
reported [21, 22]. In this article, we report the synthesis of a protocatechuic acid derivative
and synthesized three new binuclear copper(II) complexes with the derivative. The structure
and urease inhibitory activity of these complexes were evaluated.
2. Experimental
2.1. General methods and materials
Unless otherwise stated all solvents were of reagent grade and purchased commercially. All
chemicals were also commercially available and used without purification. Elemental analy-
ses were performed on a Perkin-Elmer 240C elemental analyzer.
2.2. Synthesis of 2,3-dihydrobenzo[b][1,4]dioxine-6-carboxylic acid
The protocatechuic acid derivative 2,3-dihydrobenzo[b][1,4]dioxine-6-carboxylic acid
(C9H8O4, HL1) was synthesized with protocatechuic acid (3,4-dihydroxybenzoic acid) and
1,2-dibromoethane (scheme 1). Yield 81%. M.p. 133–137 °C. Anal. Calcd for C9H8O4: C,
60.00; H, 4.48%. Found: C, 60.02; H, 4.46%. The synthesis method is different from that
reported in the literatures which synthesized the compound with 2,3-dihydrobenzo[b][1,4]
dioxine or 2,3-dihydrobenzo[b][1,4]dioxine-6-carbaldehyde [23, 24]. Three complexes were
synthesized by the reaction of HL1 with the corresponding metal salts (scheme 2) in a solu-
tion of aqueous ethanol; o-toluidine (C7H9N, L2) and N,N-diethylethylenediamine
(C6H16N2, L3) were added to the solutions for 1 and 3, respectively.

Binuclear Cu(II) complexes
1267
Scheme 1. Synthesis of HL1(2,3-dihydrobenzo[b][1,4]dioxine-6-carboxylic acid). Reagents and conditions: (a)
MeOH, H2SO4, reflux, 8 h; (b) Br(CH2)2Br, acetone, reflux, 24 h; (c) NaOH, H2O, reflux, 5 h; (d) HCl.
Scheme 2. Synthesis of 1–3.
2.3. Synthesis of 1
An ethanol solution (2 mL) of HL1 (0.2 mM, 0.036 g) and CuSO4·5H2O (0.1 mM, 0.0159 g)
was added to H2O solution (4 mL) of o-toluidine (0.1 mM, 0.0107 g). The mixture was
stirred for 30 min at room temperature to give a green clear solution. After keeping the
solution in air for five days, green block-shaped single crystals of 1 suitable for structure
determination were obtained on slow evaporation of the solvent. Crystals were isolated by
filtration and washed with cold ethanol and then dried in air. Yield 81%. Anal. Calcd for
C50H46Cu2N2O16: C, 56.76; H, 4.38; N, 2.65%. Found: C, 56.63; H, 4.36; N, 2.64%.
2.4. Synthesis of 2
An ethanol solution (2 mL) of HL1 (0.2 mM, 0.036 g) was added to a H2O solution (4 mL)
of CuSO4·5H2O (0.1 mM, 0.0159 g). The mixture was stirred for 30 min at room
temperature to give a green clear solution. After keeping the solution in air for five days,
green block-shaped single crystals of 2 suitable for structure determination were obtained

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G.-H. Sheng et al.
on slow evaporation of the solvent. Crystals were isolated by filtration and washed with
cold ethanol and then dried in air. Yield 75%. Anal. Calcd for C45H40O22Cu2: C, 50.99; H,
3.80%. Found: C, 50.80; H, 3.65%.
2.5. Synthesis of 3
HL1 (0.2 mM, 0.036 g) and Cu(ClO4)2·6H2O (0.1 mM, 0.037 g) were added to 6 mL ethanol
and H2O mixture (v : v = 2 : 4). The mixture was stirred for 30 min; concentrated ammonia
was added to the mixture to promote dissolution to give a blue clear solution. N,N-Diethyl-
ethylenediamine (0.1 mM, 0.0116 g) was added to the solution. The solution was filtered;
after keeping the filtrate in air for 20 days, blue block-shaped single crystals of 3 suitable
for structure determination were obtained on slow evaporation of the solvent. Crystals were
isolated by filtration and washed with cold ethanol and then dried in air. Yield 83%. Anal.
Calcd for C39H53N4O16ClCu2: C, 47.01; H, 5.36; N, 5.62%. Found: C, 46.92; H, 5.32; N,
5.65%.
2.6. Crystal structure determination
X-ray diffraction intensities were collected using a Bruker SMART APEX-II CCD area
detector equipped with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å).
Absorption correction was applied by SADABS [25]. The structure was solved by direct
Table 1. Crystal and experimental data for 1–3.
Complex 1
Complex 2
Complex 3
Molecular formula
Molecular weight
Temperature (K)
Radiation λ
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
50H46Cu2N2O16
C45H40O22Cu2
1059.87
298
Mo Kα (0.7107 Å)
Orthorhombic
P212121
12.5828(13)
13.5193(14)
25.605(3)
90.00
90.00
90.00
4355.6(8)
4
1.616
C39H53N4O16ClCu2
996.40
293
Mo Kα (0.7107 Å)
Triclinic
P-1
9.9262(6)
15.0558(9)
16.4227(11)
94.843(2)
103.908(2)
108.410(2)
2225.9(2)
2
1057.99
293
Mo Kα (0.7107 Å)
Triclinic
P-1
7.6688(5)
12.4183(9)
13.8603(9)
68.489(2)
78.691(2)
72.285(2)
1164.46(14)
1
γ (°)
V (Å3)
Z
DCalcd (g cm−3
)
1.509
0.30 × 0.20 × 0.22
546
1.487
0.26 × 0.25 × 0.19
1036
Crystal size (mm3)
F (0 0 0)
0.24 × 0.22 × 0.16
2176
θ Range (°)
Reflections collected/unique
2.78–26.49
11,287/4787
[Rint = 0.0152]
4353
2.19–27.14
37,830/9604
[Rint = 0.1016]
6270
2.3–25.8
21,169/8456
[Rint = 0.0254]
6902
Refns obs. I > 2σ(I)
Goodness of fit on F2
Data/parameters/restraints
Largest diff. peak and hole (e Å−3
R1, wR2 [I > 2σ(I)]a
R1, wR2 (all data)a
Â
1.071
1.011
1.036
4787/335/0
0.257 and −0.374
0.0304, 0.0773
0.0357, 0.0805
9604/668/69
1.249 and −1.226
0.0688, 0.1648
0.1270, 0.1968
8456/593/34
1.194 and −0.773
0.0496, 0.1355
0.0629, 0.1463
)
Ã
aR1 = Fo – Fc/Fo, wR2 = ∑w(Fo2 –Fc2)/∑w(Fo
)
.
2 2 1/2

Binuclear Cu(II) complexes
1269
methods and refined on F2 by full-matrix least-squares using Bruker’s SHELXTL-97 pro-
gram [26]. All non-hydrogen atoms were refined anisotropically. The water hydrogens were
located from a difference Fourier map and refined isotropically, with O–H and H⋯H dis-
tances restrained to 0.85(1) and 1.35(2) Å, respectively. The remaining hydrogens were
placed in calculated positions and constrained to ride on their parent. There are 69 and 34
restraints used to deal with the structure disorder during the refinement of 2 and 3, respec-
tively. The details of the crystallographic data are summarized in table 1. Selected bond
lengths and angles are listed in table 2. Geometrical parameters for hydrogen bonds are
shown in table 3.
Table 2. Selected distances (Å) and angles (°) for 1–3.
Complex 1 (#1: −x + 1, −y + 1, −z)
Cu(1)–O(1)
1.9769(13)
1.9861(14)
1.9509(13)
Cu(1)–O(5)
Cu(1)–N(1)
1.9608(13)
2.2271(16)
2.6377(4)
90.32(6)
87.91(6)
89.59(6)
Cu(1)–O(2)#1
Cu(1)–O(6)
Cu1…Cu1#1
O2–Cu1–N1#1
O5–Cu1–O1
O5–Cu1–O2#1
O6–Cu1–N1
O1–Cu1–N1
O5–Cu1–O6
O6–Cu1–O1
O6–Cu1–O2#1
O1–Cu1–O2#1
O5–Cu1–N1
168.32(6)
91.16(6)
88.99(6)
168.33(5)
94.56(6)
97.05(6)
101.24(6)
Complex 2
Cu1–O8
Cu1–O12
Cu1–O1
Cu1–O16
1.958(5)
1.977(4)
2.162(4)
1.963(5)
1.973(4)
2.5789(10)
89.9(2)
89.9(2)
171.2(2)
93.2(2)
93.7(2)
89.8(2)
88.5(2)
99.1(2)
94.98(19)
167.7(2)
Cu2–O15
Cu2–O4
Cu2–O2
Cu2–O7
Cu2–O11
1.936(5)
1.955(4)
2.189(4)
1.945(5)
1.987(4)
Cu1–O3
Cu1…Cu2
O8–Cu1–O12
O8–Cu1–O3
O12–Cu1–O3
O16–Cu1–O1
O3–Cu1–O1
O16–Cu1–O12
O16–Cu1–O3
O8–Cu1–O1
O12–Cu1–O1
O8–Cu1–O16
O15–Cu2–O11
O4–Cu2–O11
O7–Cu2–O2
O11–Cu2–O2
O15–Cu2–O4
O7–Cu2–O4
O7–Cu2–O11
O15–Cu2–O2
O4–Cu2–O2
O15–Cu2–O7
89.1(2)
168.83(19)
93.86(19)
97.04(18)
89.3(2)
89.7(2)
90.3(2)
94.56(19)
94.11(19)
171.6(2)
Complex 3
Cu1–O9
Cu1–O14
Cu1–N2
Cu1–N1
1.937(2)
2.032(2)
2.198(3)
Cu2–O13
Cu2–O10
Cu2–N4
Cu2–O6
Cu2–N3
1.974(3)
1.954(2)
2.272(3)
1.962(2)
2.011(3)
1.989(3)
Cu1–O5
2.035 (2)
2.9069(6)
177.54(13)
87.06(13)
89.44(13)
94.75(12)
105.75(13)
91.95(11)
92.53(11)
150.06 (11)
83.36 (14)
103.35 (13)
Cu1…Cu2
O9–Cu1–N1
N1–Cu1–O14
N1–Cu1–O5
O9–Cu1–N2
O14–Cu1–N2
O9–Cu1–O14
O9–Cu1–O5
O14–Cu1–O5
N1–Cu1–N2
O5–Cu1–N2
O10–Cu2–O6
O10–Cu2–O13
O10–Cu2–N3
O13–Cu2–N3
O6–Cu2–N4
N3–Cu2–N4
O6–Cu2–O13
O6–Cu2–N3
O10–Cu2–N4
O13–Cu2–N4
90.53(12)
90.13(11)
175.67(13)
87.65(14)
100.11(12)
84.27(13)
168.04(11)
90.87(14)
99.51(11)
91.56(11)

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G.-H. Sheng et al.
Table 3. Geometrical parameters for hydrogen bonds.
Hydrogen bonds
Symmetry code
D–H (Å)
H···A (Å)
D···A (Å)
D–H···A (°)
Complex 1
N1–H1B…O2
[x + 1, y, z]
0.90
2.59
3.353(4)
143.44
Complex 2
O1–H1A…O21
O2–H2A…O12
O20–H20…O14
O2–H2B…O19
O1–H1B…O2
O1–H1B…O11
[−x + 1, y − 1/2, −z + 1/2]
[−x, y + 1/2, −z + 1/2]
[x + 1/2, −y + 1/2, −z + 1]
[−x + 1, y + 1/2, −z + 1/2]
[−x, y − 1/2, −z + 1/2]
[ −x, y − 1/2, −z + 1/2]
0.850
0.820
0.820
0.849
0.846
0.846
2.073
2.231
1.916
2.040
2.266
2.567
2.802
2.909
2.699
2.838
2.985
3.127
143.50
140.25
159.22
156.38
142.96
124.67
Complex 3
N3–H3B…O4
N3–H3A…O2
N1–H1B…O2
N1–H1A…O3
N1–H1A…O4
0.896(10)
0.896(11)
0.895(10)
0.896(10)
0.896(10)
2.28(3)
3.105(5)
3.210(7)
3.099(7)
3.233(7)
3.249(6)
154(6)
170(7)
170(6)
150(6)
154(6)
[−x + 1, −y + 1, −z + 1]
[−x + 1, −y + 1, −z + 1]
2.324(19)
2.214(18)
2.43(4)
2.42(4)
2.7. Measurement of inhibitory activity against jack bean urease
Jack bean urease was purchased from Sigma Aldrich Co. (St. Louis, MO, USA). The mea-
surement of urease was carried out according to the literature [27, 28]. Generally, the assay
mixture, containing 25 μL of jack bean urease (10 kU/L) and 25 μL of the tested samples
(complexes, ligands, and metal salts) of various concentrations [dissolved in solution of
DMSO : H2O = 1 : 1 (v : v)], was preincubated for 1 h at 37 °C in a 96-well assay plate.
After preincubation, 0.2 mL of 100 mM phosphate buffer at pH 6.8 containing 500 mM urea
and 0.002% phenol red were added and incubated at 37 °C. The reaction time was mea-
sured by micro-plate reader (570 nm), which was required to produce enough ammonium
carbonate to raise the pH of a phosphate buffer from 6.8 to 7.7, the end-point being deter-
mined by the color of phenol red indicator.
3. Results and discussion
3.1. Crystal structure description of the complexes
Single-crystal X-ray diffraction reveals that [CuII2(L1)4(L2)2] (1), [CuII (L1)4(H2O)2]·HL
2
(2), and [CuII (L1)3(L3)2]ClO4 (3) have similar binuclear structures. There are two copper
2
ions in each complex, linked by four (for 1 and 2) or three (for 3) bridging bidentate L1.
The carboxylate bridged Cu…Cu distances of 1–3 are 2.6377(4), 2.5789(10), and 2.9069
(6) Å, respectively, comparable to those reported in similar binuclear carboxylate copper
complexes [29–32].
3.1.1. Structure of 1. Complex 1 crystallizes in the triclinic space group P-1. Perspective
views of the crystal structure of 1 are shown in figure 1. Each molecule consists of two
copper ions, four 2,3-dihydrobenzo[b][1,4]dioxine-6-carboxylic acid (L1) anions, and two

Binuclear Cu(II) complexes
1271
Figure 1. A perspective view of 1 with atom-labeling scheme (symmetry code: A −x + 1, −y + 1, −z). The thermal
ellipsoids are drawn at the 30% probability level (hydrogens are omitted for clarity).
o-toluidine (L2) molecules. Each copper is five coordinate with four oxygens of L1 and one
N of L2 in square–pyramidal arrangement (τ = 0.0002 for Cu1 and Cu1A) [33], thus form-
ing a [CuO4N1] chromophore. Two copper ions are linked by four bridging bidentate
carboxylic groups of four L1. Four oxygens of four L1 anions form the basal plane and one
N of L2 is in the axial position. The in-plane Cu–O bond distances average 1.97 Å and axial
Cu–N bond distance is 2.23 Å, in the normal ranges (figure 1 and table 2) [34]. In the crys-
tal, intermolecular N1–H1B…O2 [x + 1, y, z] hydrogen bonds serve as bridges to link adja-
cent molecules into 1-D chains along the a axis (figure 2 and table 3).
3.1.2. Structure of 2. Complex 2 is in the orthorhombic space group P212121. Perspective
views of the crystal structure of 2 are shown in figure 3. Each complex consists of one
[CuII (L1)4(H2O)2] and one uncoordinated HL. In each [CuII (L1)4(H2O)2], there are two
2
2
copper ions, four L1 anions, and two H2O molecules. Each copper is five coordinate with
four oxygens from L1 and one H2O in the distorted square–pyramidal arrangement
(τ = 0.058 for Cu1 and 0.046 for Cu2) [33], forming a [CuO5] chromophore. Two copper
ions are linked by four bridging bidentate carboxylates of L1. Four L1 anions form the basal
plane and H2O is axial. The in-plane Cu–O bond distances average 1.96 Å and axial Cu–O
bond distance is 2.18 Å, in the normal ranges [34]. O1–Cu1–O8 and O1–Cu1–O16 bond
angles (99.1(2) and 93.2(2) Å, respectively) reflect the distorted square–pyramidal geometry
surrounding Cu1. O11–Cu2–O2 and O7–Cu2–O2 bond angles (97.04(18) and 93.86(19) Å,
respectively) reflect the distorted square–pyramidal geometry surrounding Cu2 (figure 3 and
table 2). The binuclear units form a 1-D chain-like structure extended along the

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G.-H. Sheng et al.
Figure 2. View of the hydrogen bond-driven 1-D chain of 1 running along the a axis. Hydrogen bonds are shown
as dashed lines.
Figure 3. A perspective view of 2 with the atom-labeling scheme. The thermal ellipsoids are drawn at the 30%
probability level (hydrogens are omitted for clarity).
crystallographic b axis via five hydrogen bonds: O1–H1A…O21 (−x + 1, y − 1/2, −z + 1/2;
2.802 Å), O1–H1B…O2 (−x, y − 1/2, −z + 1/2; 2.985 Å), O1–H1B…O11 (−x, y − 1/2, −z +
1/2; 3.127 Å), O2–H2A…O12 (−x, y + 1/2, −z + 1/2; 2.909 Å), O2–H2B…O19 (−x + 1,
y + 1/2, −z + 1/2; 2.838 Å). One free HL1 is linked with two [CuII (L1)4(H2O)2] and
2
involved in the formation of the 1-D chain along the b axis (figure 4 and table 3). Free HL1
molecules link the chains (running along the b axis, highlighted by different color) into a
2-D network (figure 5 and table 3) via three intermolecular H bonds: O1–H1A…O21
(−x + 1, y + 1/2, −z + 0.5), O2–H2B…O19 (−x + 1, y − 1/2, −z + 0.5), and O20–H20…O14
(x + 1/2, −y + 1/2, −z + 1). In free HL1, O19, and O21 are H acceptors linked with one 1-D
chain which along the b axis and O20 is an H donor to O14 of another 1-D chain linking

Binuclear Cu(II) complexes
1273
Figure 4. View of the hydrogen bond-driven 1-D chain of 2 running along the b axis. Hydrogen bonds are shown
as dashed lines.
adjacent 1-D chains into the 2-D network. Free HL1 also links 2-D structures into a 3-D
network in the same way via the aforementioned three intermolecular H bonds (figure 6
and table 3).
3.1.3. Structure of 3. Complex 3 is in the triclinic space group P-1. Perspective views of
the crystal structure of 3 are shown in figure 7. Each complex consists of one
[CuII (L1)3(L3)2]+ and one ClO4 . In each [CuII (L1)3(L3)2]+, there are two copper ions,
−
2
2
three L1, two N,N-diethylethylenediamine (L3), and one perchlorate. Each copper is five
coordinate with three oxygens of L1 and two nitrogens of L3 in distorted square–pyramidal
arrangement (τ = 0.46 for Cu1 and 0.13 for Cu2), thus forming a [CuO3N2] chromophore.
Two copper ions are linked by three bridging bidentate carboxylates. Three oxygens of
three L1 and one nitrogen from L3 form the basal plane, and the other N of L3 is in the axial
position. Cu–O and Cu–N bond distances are in the normal range [34]. N2–Cu1–O14 and
N1–Cu1–N2 bond angles (105.75(13) and 83.36(14) Å, respectively) reflect the distorted
square pyramidal geometry surrounding Cu1. O6–Cu2–N4 and N3–Cu2–N4 bond angles
(100.11(12) and 84.27(13) Å, respectively) reflect the distorted square–pyramidal geometry
surrounding Cu2. Two complex molecules are linked forming a dimer by intramolecular
and intermolecular hydrogen bonds (figure 8 and table 3): N3–H3B…O4 (3.105(5) Å),
N1–H1A…O3 (3.233(7) Å), N1–H1A…O4 (3.249(6) Å), N3–H3A…O2 [−x + 1, −y + 1,
−z + 1] (3.210(7) Å), N1–H1B…O2 [−x + 1, −y + 1, −z + 1] (3.099(7) Å).

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G.-H. Sheng et al.
Figure 5. View of free HL1 connecting adjacent 1-D chains (running along the b axis, highlighted by different
color) into a 2-D network. Hydrogen bonds are shown as dashed lines.
Figure 6. View of free HL1 connecting 1-D chains (running along the b axis, highlighted by different color) into
a 3-D network. Hydrogen bonds are shown as dashed lines.

Binuclear Cu(II) complexes
1275
Figure 7. A perspective view of 3 with the atom-labeling scheme. The thermal ellipsoids are drawn at the 30%
probability level (hydrogens are omitted for clarity).
Figure 8. View of dimer of 3 via hydrogen bonds (shown as dashed lines).

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Table 4. Inhibition of urease by the tested materials.
Tested materials
IC50 (μm)
HL1
L2
>10
>10
L3
>10
CuSO4·5H2O
>10
Complex 1: [CuII2(L1)4(L2)2]
Complex 2: [CuII2(L1)4(H2O)2]·HL
Complex 3: [CuII2(L1)3(L3)2]ClO4
Acetohydroxamic acid
6.8 0.4
5.5 0.4
3.5 0.3
7.5 0.6
3.2. IR and UV–vis spectra
Infrared spectra of HL1 and the three complexes provide information about the metal–ligand
bonding. For HL1, the strong absorption band at 1686 cm−1 is assigned to C=O(Ar–COOH)
stretch and the broad absorptions at 2500–2663 cm−1 are assigned to O–H(Ar–COOH)
stretch. These two absorptions are absent in the three complexes, indicating complete depro-
tonation and coordination of L1 in the complexes. The weak and moderate bands around
3400 cm−1 of 2 are assigned to O–H vibrations of water. The absorptions at 3200–3340 cm−1
of 1 and 3 are assigned to N–H stretch of N,N-diethylethylenediamine and o-toluidine,
respectively. UV–vis [DMSO, λmax (nm)] for HL1 and the three complexes are 250.
3.3. Inhibitory activity against jack bean urease
The abilities of the ligands, Cu2+, and complexes to inhibit urease were studied by the IC50
values of the material tested against jack bean urease according to the literature phenol-red
method. The results are summarized in table 4. The IC50 value of the ligand and the Cu2+
are >10 μM. Under the same conditions, 1–3 show much stronger inhibitory activity against
jack bean urease with an IC50 value of 6.8, 5.5, and 3.5 μM respectively, with the acetohy-
droxamic acid (IC50 = 7.5 μM) as a standard reference against urease.
4. Conclusion
The present study reports the synthesis, structures, and urease inhibitory activities of three
copper(II) complexes with protocatechuic acid ligands. Complexes 1–3 exhibit stronger
urease inhibitory activities than their parent ligands and metal ion and 3 with N,N-diethyl-
ethylenediamine co-ligand exhibited higher activity than 1 and 2 with toluidine and H2O as
co-ligands. The results indicate that the inhibitory efficiency of the complex towards urease
may be influenced by the transition metal and ligands, and the inhibitory activity probably
is due to strong Lewis acid properties of copper ions and the ligands strengthen the inhibi-
tory activity of the complexes. The results are in accord with those reported previously,
where some Cu(II) complexes have stronger urease inhibitory activities to urease than their
parent ligands and metal ion, with IC50 ranging from 1 to 50 μM [15, 28, 35–39]. Com-
pared with the data reported before, the complexes reported in this study exhibit fairly
strong inhibitory activity to urease and may be used as urease inhibitors. Detailed investiga-
tions are continuing to study the inhibitory mechanism.

Binuclear Cu(II) complexes
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Supplementary material
Crystallographic data for the structural analysis has been deposited with the Cambridge
Crystallographic Data Center (CCDC-925293 for 1, CCDC-925294 for 2, and
CCDC-925295 for 3). Copy of this information can be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: (+44) 1223-336033;
E-mail: deposit@ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
Funding
We thank the financial support from National Natural Science Foundation of China [grant number
31200090]; Scientific Research Foundation for Returned Scholars, Ministry of Education of China
and China Postdoctoral Science Foundation National [grant number 1302006A].
Supplemental data
Supplemental data for this article can be accessed here. [http://dx.doi.org/10.1080/
00958972.2014.910597]
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